Baixe Air and Water Pollution e outras Notas de estudo em PDF para Engenharia Química, somente na Docsity! Adsorption, lon Exchange and Catalysis Design of Operations and Environmental Applications Preface In this book, three important processes, namely, adsorption, ion exchange, and hetero- geneous catalysis, are presented along with environmental issues. Specifically, this book is essentially a mixture of environmental science (Chapters 1 and 2) and chemical reactor engineering (Chapters 3 to 6). The question is why environmental issues are being presented in a chemical engineer- ing book containing heterogeneous processes. First of all, the processes discussed (adsorp- tion, ion exchange, and catalysis) are largely employed in our effort to minimize the emissions of various pollutants into the environment. The use of catalytic converters in the after-treatment of automobile exhaust gases (catalysis), a subject known to almost every- one, the treatment of VOC-containing gases (catalysis, adsorption), and the removal of toxic metals from industrial wastewater (ion exchange, adsorption) are some important examples of the environmental applications of the presented heterogeneous processes. Moreover, the environment is a field of continuous research since its protection is essen- tial in the context of sustainable development, and generally for a planet worth living on. Consequently, the environmentally oriented presentation of adsorption, ion exchange, and catalysis is of substantial importance. Engineers are often provided with a lot of informa- tion regarding heterogeneous processes, chemical phenomena, and waste treatment tech- niques, but lack know-ledge of the very basics of environmental problems. The first two chapters of the book do not attempt to cover all environmental subjects; they are an effort of the authors to provide the reader with a basic knowledge of the major environmental problems and connect them to chemical engineering. Why adsorption, ion exchange and heterogeneous catalysis in one book? The basic sim- ilarity between these phenomena is that they all are heterogeneous fluid–solid operations. Second, they are all driven by diffusion in the solid phase. Thus, mass transfer and solid- phase diffusion, rate-limiting steps, and other related phenomena are common. Third, the many aspects of the operations design of some reactors are essentially the same or at least similar, for example, the hydraulic analysis and scale-up. Furthermore, they all have impor- tant environmental applications, and more specifically they are all applied in gas and/or water treatment. In connection with the engineering content of the book, a large number of reactors is analyzed: two- and three-phase (slurry) agitated reactors (batch and continuous flow), two- and three-phase fixed beds (fixed beds, trickle beds, and packed bubble beds), three-phase (slurry) bubble columns, and two-phase fluidized beds. All these reactors are applicable to catalysis; two-phase fixed and fluidized beds and agitated tank reactors concern adsorption and ion exchange as well. Apart from the analysis of kinetics, mass transfer, and equilibrium of the processes at a fundamental level, the analysis of material, and in fixed beds energy balances in the reactors, as well as a number of analytical solutions of the reactors models are presented. Furthermore, the hydraulic behavior of the reactors is presented in detail. Hydraulic analysis is basically vii Else_AIEC-INGLE_pREFACE.qxd 7/1/2006 6:10 PM Page vii –1– Air and Water Pollution 1.1 INTRODUCTION The reason for the presence of this chapter in this book is more than apparent to its authors. In the last century, human activities in combination with a lack of respect for nature, expressed by the view that raw material resources are inexhaustible and the toler- ance of ecosystems to pollutant release is unlimited, led to the pollution and degradation of the environment. Chemical engineering is the field of science that combines chemistry with technology and is able to give solutions to most environmental problems. The envi- ronment is going to set the pace in chemical engineering evolution, since we have reached the point where if we do not stop destroying and polluting the environment, life on Earth will be in danger—at least life as we know it today. Sustainability is the proposal for a better future, where economic development can coexist with social cohesion and envi- ronmental protection. Human activities harmfully influence the environment and nature in many ways. The production of undesirable wastewater, waste gas, and liquid plus solid residues seems to be inevitable during chemical processes. The public is more sensitive to pollution of the aquatic environment and the depletion of clean water resources, because they have an immediate impact on daily routine and recreational activities. However, air pollution has an adverse impact on our health in the short and long term, and the problems of the green- house effect and the destruction of stratospheric ozone could extinguish life from the face of the Earth. These problems are enhanced by overpopulation and urbanization. Today, urban areas can be seen as “monsters” that consume large amounts of energy, matter, and freshwater and release all kinds of waste into the environment. The first chapter of this book is devoted to air and water pollution. Although much attention will be given to releases from chemical processes, environmental problems arising from other activities will also be dealt with. Issues concerning pollutants, emis- sion sources, and treatment methods are going to be presented. Moreover, before focus- ing on the processes of adsorption, ion exchange, catalysis, and the design principles of the relevant operations in the following chapters, it is useful to show their connection with environmental protection. Emphasis is given on the current environmental situation in Europe. 1 Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 1 1.1.1 Air pollution Clean air is an important prerequisite for sustainable development and is a basic require- ment for human health and welfare. In addition, air pollutants contribute to atmospheric problems such as acidification and global climate change, which have impacts on crop pro- ductivity, forest growth, biodiversity, buildings, and cultural monuments. The benefits from the progress made in the areas of waste gas treatment and environmental legislation are partially offset by industrialization, an increase in the number of private cars in use, and overpopulation. Air pollutants are divided into two broad categories: primary and secondary. Primary pollutants are those emitted directly into the air, in contrast to secondary pollutants, which are created in the atmosphere by the reactions among the primary pollutants, usually in the presence of sunlight. Specifically, a variety of chemical or photochemical reactions (cata- lyzed by light) produce a wide range of secondary pollutants, especially in urban air. A prime example is the formation of ozone in smog. There is a variety of problems associated with air pollution, starting from photochemi- cal smog, ozone formation, and acid rain at a regional level, to the greenhouse effect and ozone-layer depletion at a global level. These problems have an adverse impact on both environment and public health (Table 1.1); the last two problems are a threat to life on Earth generally. Agriculture, energy plants, road transport, and industry are the most important sources of pollutants of the atmosphere. Agriculture, for example, charges air with acidifying gases that may lead to acid rain formation with a dramatic impact on lakes, rivers, and marine life. Air pollution is a problem at a local as well as a global level. For purposes of studying, it is useful to catagorize air pollution according to the levels at which it appears: 2 1. Air and Water Pollution Table 1.1 The main health effects of the most important air pollutants (Source: Parliamentary Office of Science and Technology, 2002; UNEP, 1992) Pollutant Main Health Effects Sulfur dioxide Irritation of lungs, shortness of breath, increased susceptibility to infection Nitrogen dioxide Irritation or damage of lungs Particulate matter Eye and nasal irritation, long-term exposure associated with coronary heart disease and lung cancer Carbon monoxide Interferes with oxygen transport by blood, resulting in the reduction of oxygen supply to the heart (chronic anoxia), heart and brain damage, impaired perception Ozone and other photochemical Pain on deep breathing, irritation and inflammation oxidants of lungs, heart stress or failure Benzene Cause of cancer 1,3-Butadiene Cause of cancer Lead Kidney disease and neurological impairments, primarily affecting children Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 2 • local • urban • regional • transboundary • global. Local level At the local level, air pollution concerns a region within a 5-km radius. It is characterized by high concentrations of specific pollutants that may come from automobiles or industrial activities in that region. For example, emissions from vehicles can lead to high concentra- tions of carbon monoxide near traffic-jammed roads. High buildings and the terrain can also contribute to high local concentrations of pollutants. Urban areas In urban areas, there are three major types of air pollution found (EEA, 2003): • High annual average concentration levels of various pollutants, e.g. benzene, lead, sulfur dioxide (SO2), and particulate matter (PM). As in the case of air pollution at the local level, this type of pollution is linked to specific pollutants resulting from either large industrial and power plants or automotive vehicles. • Winter-type smog, characterized by high concentrations of SO2 and PM that arise mainly from the combustion of coal and fuels with a high content of sulfur. This kind of pollution occurs in urban areas with many power plants or industrial units clustered together, where low temperatures and mist are observed in the year. It has been also termed “industrial pollution.” • Summer-type smog, characterized by high concentrations of carbon monoxide, volatile organic compounds (VOCs), and nitrogen oxides (NOx). It is also called “photochemi- cal smog” or “LA smog”, since it first appeared in Los Angeles. This type of pollution is closely connected to automotive vehicles, and its formation is favored by sunlight and high temperatures. The first two types are associated with primary pollutants, namely, compounds such as nitrogen oxide (NO), carbon monoxide (CO), and sulfur dioxide (SO2) that are emitted directly into the atmosphere from various sources, whereas the third type of urban air pollution is a more complex phenomenon associated with secondary pollutants, which are formed from reactions between primary pollutants in the atmosphere, usually in the pres- ence of sunlight and heat. Specifically, various volatile organic compounds react in the atmosphere with nitrogen oxides by means of the ultraviolet radiation of sunlight, produc- ing the so-called photochemical smog consisting of nitrogen dioxide, various oxidized forms of organic compounds, and ozone. The pollution of primary pollutants is easier to treat than the one associated with secondary pollutants, because the latter are produced by various organic compounds participating in numerous photochemical reactions. Approximately half of the world’s population now lives in urban areas, and half of these people suffer an atmosphere containing harmful amounts of substances such as sulfur dioxide, ozone, and particulate matter. Approximately 4000 people died from lung and heart conditions during a London smog episode in December 1952. Globally, around 50% of cases of chronic respiratory illness are now connected with air pollution. The most 1.1 Introduction 3 Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 3 emissions. Between 1990 and 1999, the emissions of these pollutants have been significantly reduced (SO2 by 70%, NOx by 25%, NMVOCs by 28%, and CO by 32%), mainly due to the introduction of automobile catalytic converters with the exception of ammonia, whose emis- sions due to agriculture are very difficult to control. The energy sector, and specifically its combustion processes, plus road transport are the main sources of air pollutants except ammonia (NH3). The most significant sources of some pollutants in EU-15, in 1999, are shown in Table 1.3. 6 1. Air and Water Pollution Table 1.2 Air pollution problems in association with the most important pollutants Pollutant Smog Urban air Acid Global Ozone Health quality deposition warming depletion Ozone Sulfur dioxide Carbon monoxide Carbon dioxide CFCs Nitrogen oxides Volatile organic compounds Toxicsa Particles Total reduced sulfur compounds aToxic metals and organic compounds. Table 1.3 The most significant sources of atmospheric pollutants in Europe in 1999 Pollutant Emission Sources CO Road transport (57%), industry (16%), other transport (7%) NMVOC Road transport (31%), solvent and other product use (32%), industry (10%), agriculture (7%), energy (6%) NOx Transport (64%), energy sector (16%), industry (13%) NH3 Agriculture (94%) SO2 Energy sector (61%), industry (24%), commercial and domestic combustion (7%), transport (7%) PM Road transport (28%), energy industries (24%), industry (16%), agriculture (13%) Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 6 Despite the benefits from stringent legislation and advances in environmental techno- logy, the increase in the fleet of automotive vehicles and overpopulation in urban areas results in bad air quality. It is estimated that up to 45% of Europe’s urban population remains exposed to particulate concentrations above limit values, and up to 30% to ozone concentrations above target levels that assure human health protection. The concentrations of various pollutants in the atmosphere in various cities across Europe are shown in Table 1.4. The data in the table are from WHO’s Healthy Cities Air Management Information System and the World Resources Institute, which relies on various national 1.1 Introduction 7 Table 1.4 The concentrations of various pollutants in the atmosphere in various cities in Europe, in 1995 Country City City Total Sulfur Nitrogen population suspended dioxide dioxide (1000) particulates (g/m3) (g/m3) (g/m3) Austria Vienna 2060 47 14 42 Belgium Brussels 1122 78 20 48 Bulgaria Sofia 1188 195 39 122 Croatia Zagreb 981 71 31 – Czech Republic Prague 1225 59 32 23 Denmark Copenhagen 1326 61 7 54 Finland Helsinki 1059 40 4 35 France Paris 9523 14 14 57 Germany Frankfurt 3606 36 11 45 Berlin 3317 50 18 26 Munich 2238 45 8 53 Greece Athens 3093 178 34 64 Hungary Budapest 2017 63 39 51 Iceland Reykjavik 100 24 5 42 Ireland Dublin 911 20 – Italy Milan 4251 77 31 248 Rome 2931 73 – – Turin 1294 151 – – Netherlands Amsterdam 1108 40 10 58 Norway Oslo 477 15 8 43 Poland Katowice 3552 .. 83 79 Warsaw 2219 .. 16 32 Lodz 1063 .. 21 43 Portugal Lisbon 1863 61 8 52 Romania Bucharest 2100 82 10 71 Slovak Republic Bratislava 651 62 21 27 Spain Madrid 4072 42 11 25 Barcelona 2819 117 11 43 Sweden Stockholm 1545 9 5 29 Switzerland Zurich 897 31 11 39 United Kingdom London 7640 – 25 77 Manchester 2434 – 26 49 Birmingham 2271 – 9 45 Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 7 sources. For reasons of comparison, the population of each city is also presented. Total sus- pended particulates refer to smoke, soot, dust, and liquid droplets from combustion that are in the air. It has to be noted that particulate levels are an indicator of the quality of the air and the state of a country’s technology and pollution controls. Although pollutant concen- trations are sensitive to local conditions, and therefore, the data presented should be con- sidered as a general indication of air quality in each city, some comparisons can be conducted. WHO annual mean guidelines for air quality standards are 90 g/m3 for total suspended particulates, 50 g/m3 for sulfur dioxide, and 50 g/m3 for nitrogen dioxide. Sofia, Athens, Turin, and Barcelona, all in south Europe, exhibit the highest concentra- tions of total suspended particulates, above the WHO standard value. Concerning sulfur dioxide, all the cities presented exhibit concentration values below the standard set by WHO. In contrast, nitrogen dioxide is a cause for concern in many European cities, since values close to or above WHO standards are common. For this compound, cities in north- ern Europe also have to be on alert. The combination of Tables 1.2 and 1.3 reveals the responsibility for each kind of air pol- lution. Specifically, road transport is the main source of nitrogen dioxide, whereas the increased sulfur dioxide levels should be attributed to the energy sector. Both sources con- tribute to increased levels of total suspended particulates in the atmosphere. It is obvious that this type of information is a valuable asset for environmental policy makers. Measurements in relation to air quality and exhaust emissions from automotive vehicles have been conducted for many years, and as a result, the evolution of air quality and the contribution of vehicles are known to authorities. However, this is not the case for indus- try. Even if the contribution of industry to air pollution could be roughly estimated, it was very difficult to connect each kind of industrial process to each pollutant. This lack of information led to the idea of establishing a Pollutant Release and Transfer Register (PRTR), which first emerged in the United States following the tragic accident in Bhopal, India, in 1984. Shortly thereafter, the United States Congress approved the Emergency Planning and Community Right-to-Know Act, establishing a register called the Toxic Release Inventory (TRI), which tracks releases to all media (air, water, and land) and off- site transfers of more than 600 chemicals. A look into the origins of U.S. TRI In 1984, a deadly cloud of methyl isocyanate killed thousands of people in Bhopal, India. Shortly thereafter, there was a serious chemical release at a sister plant in West Virginia. These incidents underscored demands by industrial workers and communities in several states for infor- mation on hazardous materials. Public interest and environmental organizations around the country accelerated demands for informa- tion on toxic chemicals being released. Against this background, the Emergency Planning and Community Right-to-Know Act (EPCRA) was enacted in 1986. EPCRA’s primary purpose is to inform communities and citizens of chemical hazards in their areas. Sections 311 and 312 of EPCRA require businesses to report the locations and quantities of chemicals stored on-site to state and local governments in order to help communities prepare to respond to chemical spills and similar emergencies. EPCRA 8 1. Air and Water Pollution Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 8 measures that will be taken have to be focused not only on road transport but also on com- bustion installations. A special case: Suppose that your country has occasions of acid rain due to sulfuric acid. The main source of SOx, which leads to sulfuric acid formation, is combustion instal- lations. However, there are not large combustion installations in your land. Can you find the source of air pollution and the possible position of your country in Europe, using Table 1.6? The answer is simple. You have been the victim of air pollution “traveling”. Acid rain is among the problems connected to air pollution that may appear at a transboundary level. So, your country has suffered the results of the combination of the elevated SOx emissions from a neighboring country with favoring climatic conditions. Air pollution is not a problem only in Europe but constitutes a reason to worry all over the world. The concentrations of total suspended particulates, sulfur dioxide, and nitrogen dioxide in the atmosphere in various cities in 1995, in America, Asia, Africa, and Australia are presented in Tables 1.7, 1.8, 1.9, and 1.10, respectively. It is useful to recollect the limits set by WHO: 90 g/m3 for total suspended particulates, 50 g/m3 for sulfur dioxide, and 50 g/m3 for nitrogen dioxide. It is apparent that the world has a long way to go till compliance with these numbers is achieved. Mexico City is notorious for bad air quality. Pollution levels exceed WHO standards 350 days per year. More than half of all children in the city have lead levels in their blood sufficient to lower intelligence and retard development. The 130,000 industries and 2.5 million motor vehicles spew out more than 5500 metric tons of air pollutants every day, which are trapped by the mountains ringing the city. 1.1 Introduction 11 Table 1.6 SOx releases (annual) to air for each country in Europe (2001) Country Total emission (t) Percentage of European total Austria 12,321 0.3 Belgium 105,539 2.5 Denmark 12,433 0.3 Finland 59,436 1.4 France 369,051 8.6 Germany 370,590 8.6 Greece 408,222 9.5 Ireland 91,498 2.1 Italy 509,126 11.8 Luxembourg 604 0.0 Netherlands 51,777 1.2 Portugal 166,147 3.9 Spain 1,169,999 27.2 Sweden 23,403 0.5 United Kingdom 948,488 22.1 Total 4,298,634 100.0 Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 11 Most of the Third World megacities (those with populations greater than 10 million peo- ple) experience similar problems. Air quality in Cairo, Bangkok, Jakarta, Bombay, Calcutta, New Delhi, Shanghai, Beijing, São Paulo, and many lesser known urban areas regularly reach dangerous levels. In the following sections, the most important problems of the atmospheric environment across the Earth are presented briefly. North America Electric power plants are the major source of toxic air pollutants in North America, accounting for almost half of all industrial air emissions in 2001 (CEC, 2004). According to the data provided by industrial facilities, 46 of the top 50 air polluters in North America were power plants. The sector is responsible for the 45% of the 755,502 t of toxic air releases in 2001, with hydrochloric and sulfuric acids being most commonly released from the burning of coal and oil. Power plants also accounted for 64% of all mercury emissions to the air. However, air quality in Canada and the United States shows the clearest trend of improvement among all environmental categories during the last two decades. The reports predicting that there would be a sharp decline in air quality after the signing of the North America Free Trade Agreement (NAFTA) were incorrect. For example, The Environmental Implications of Trade Agreements, released by the Ontario Ministry of Environment and Energy in 1993, predicted that pollutants such as sulfur dioxide would increase by more than 4.5% annually in North America as a direct result of NAFTA. However, data from Environment Canada, the United States, and the Organization for Economic Cooperation and Development (OECD) show that sulfur dioxide levels in North 12 1. Air and Water Pollution Table 1.7 The concentrations of various pollutants in the atmosphere in various cities in America, in 1995 Country City City Total Sulfur Nitrogen population suspended dioxide dioxide (1000) particulates (g/m3) (g/m3) (g/m3) Argentina Córdoba City 1294 97 – 97 Brazil São Paulo 16,533 86 43 83 Rio de Janeiro 10,181 139 129 – Canada Toronto 4319 36 17 43 Montreal 3320 34 10 42 Vancouver 1823 29 14 37 Chile Santiago 4891 – 29 81 Colombia Bogotá 6079 120 – – Cuba Havana 2,241 – 1 5 Ecuador Guayaquil 1831 127 15 – Quito 1298 175 31 – Mexico Mexico City 16,562 279 74 130 United States New York 16,332 – 26 79 Los Angeles 12,410 – 9 74 Chicago 6844 – 14 57 Venezuela Caracas 3007 53 33 57 Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 12 1.1 Introduction 13 Table 1.8 The concentrations of various pollutants in the atmosphere in various cities in Asia, in 1995 Country City City Total Sulfur Nitrogen population suspended dioxide dioxide (1000) particulates (g/m3) (g/m3) (g/m3) China Shanghai 13,584 246 53 73 Beijing 11,299 377 90 122 Tianjin 9415 306 82 50 India Bombay 15,138 240 33 39 Calcutta 11,923 375 49 34 Delhi 9948 415 24 41 Indonesia Jakarta 8621 271 – – Iran, Islamic Tehran 6836 248 209 – Rep. Japan Tokyo 26,959 49 18 68 Osaka 10,609 43 19 63 Yokohama 3178 – 100 13 Korea, Rep. Seoul 11,609 84 44 60 Pusan 4082 94 60 51 Taegu 2432 72 81 62 Malaysia Kuala Lumpur 1238 85 24 – Philippines Manila 9286 200 33 – Russian Moscow 9269 100 109 – Federation Omsk 1199 100 9 30 Singapore Singapore 2848 – 20 30 Thailand Bangkok 6547 223 11 23 Turkey Istanbul 7911 – 120 – Ankara 2826 57 55 46 Izmir 2031 – – – Ukraine Kiev 2809 100 14 51 Table 1.9 The concentrations of various pollutants in the atmosphere in various cities in Africa, in 1995 Country City City Total Sulfur Nitrogen population suspended dioxide dioxide (1000) particulates (g/m3) (g/m3) (g/m3) Egypt, Arab Rep. Cairo 9690 – 69 – Ghana Accra 1673 137 – – Kenya Nairobi 1810 69 – – South Africa Capetown 2671 – 21 72 Johannesburg 1849 – 19 31 Durban 1149 – 31 – Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 13 occurrence of bushfires or hazard reduction burning in the vicinity of the cities. Hopefully, even then, the current standards are not actually expected to be exceeded (Australian Academy of Technological Sciences and Engineering, 1997). 1.1.2 Water pollution Water covers 71% of the planet’s surface, mainly in the form of salty water in the oceans. It is a vital substance for supporting life on Earth. For example, a tree contains 60% water, most animals are composed of about 65% water, while our bodies contain around 55% water (Tyler Miller, 1999). Everyone needs fresh water everyday to cover the daily demand in food, domestic use, etc. Fresh water is used in agriculture, construction, transport, the chemical industry, and numerous other activities of human beings. The use of abstracted water in Europe is presented in Figure 1.2. It has to be noted that in many regions on Earth, where the struggle for existence of population is continuous, it is the lack of fresh and clean water that limits the production of food. According to the United Nations, the first priority of poor countries, especially in Africa, should be not financial support or techno- logical knowledge but clean water supply to the population. Unfortunately, despite the fact that most of the planet is covered by water, only a small amount of this water is available as fresh water. Almost 97.5% of the total is in oceans in the form of salty water and is not suitable for drinking, watering, or industrial use as is. The remaining 2.5% is fresh water. However, not even that small amount is easily acces- sible or exploited, because it is stored as ice on the poles and on mountaintops. Furthermore, a significant amount of the rest lies so deep in the ground that it is very dif- ficult to extract. In Figure 1.3, the distribution of water on Earth is presented. According 16 1. Air and Water Pollution Industry* 14% Public water supply 18% Agriculture 30% Energy sector 38% *excluding cooling water Figure 1.2 The use of abstracted water in Europe (Nixon et al., 2004). Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 16 to the EC, less than 1% of the planet’s water is available for human consumption and more than 1.2 billion people in the world have no access to safe drinking water. Fresh water is purified and reallocated through the hydrological cycle in nature. Nowadays, this natural process is inadequate due to human activities, and specifically because of the thoughtless wasting of water and discharge of various pollutants into the aquatic environment. These activities of human beings threaten not only the fresh water supply but also marine life. Moreover, with an ever-increasing world population, the situation is expected to worsen in the near future, especially in densely populated or industrial areas. These areas consume large amounts of fresh water, and at the same time produce and release large amounts of wastewater into the environment. Water-quality deterioration can be attributed to water pollution or contamination. Water pollution is generally defined as any physical, chemical, or biological alteration in water quality that has a negative impact on living organisms. In the stricter sense, pollution can be defined as the transfer of any substance to the environment. However, there is a toler- ance limit for each pollutant, since zero-level pollution is economically and technically unpractical. The most important kinds of water quality deterioration are the following. Thermal Pollution The discharge of warm wastewaters into a surface receiver may have many adverse effects on aquatic life. The increase in temperature results in a decrease in the oxygen concentra- tion in water and the elimination of the most sensitive species. Temperature changes may also cause changes in the reproductive periods of fishes, growth of parasites and diseases, or even thermal shock to the animals found in the thermal plume. Biological Pollution by Oxygen-Demanding Wastes The release of oxygen-demanding wastes into water (mainly biodegradable organic compounds) results in the decrease in oxygen dissolved in water due to its consumption by the aquatic microorganisms that decay the organic pollutants. A minimum of 6 mg of oxygen per liter of water is essential to support aquatic life. A few species, like carp, can survive in low-oxygen waters. Each biodegradable waste is characterized by the 1.1 Introduction 17 Total water (100%) Salt (97.5%) Fresh (2.5%) Ice (1.7%) Liquid (0.8%) Ground (0.79%) Lakes (0.008%) Rivers (0.0002%) Soil (0.001%) Atmosphere (0.0009%) Organisms (0.0001%) Figure 1.3 Water on Earth (Nixon et al., 2004). Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 17 biological oxygen demand (BOD), which is a measure of the amount of dissolved oxy- gen needed by aquatic microorganisms for the degradation of waste. Pollution by Persistent Organic Chemicals (POPs) Besides biodegradable organic compounds, there are also organic substances that show great resistance and high lifespan in the environment, thus constituting a long-term danger to life. Dioxins, polychlorinated biphenyls (PCBs), and pesticides (DDT and others) are man-made compounds that remain intact for months in the environment. Consequently, people and animals at the top of the food chain eventually consume food containing these compounds. DDT, a popular compound that helped in the elimination of malaria, was proved to have many adverse effects on natural life. Paul Muller, who discovered the effec- tiveness of DDT as an insecticide in 1939 was awarded the Nobel Prize in medicine and physiology in 1948 for this discovery. Today, DDT is banned in most developed countries. Eutrophication by Nitrates and Phosphorus Eutrophication is the rapid depletion of dissolved oxygen in a body of water because of an increase in biological productivity. It is connected to the excess presence of plant nutrients in the environment, mainly nitrates and phosphorus. These compounds are connected to the excessive use or production of fertilizers. Inorganic Pollutants Metals, nonmetals, and acids/bases released by human activities severely deteriorate water quality, since they are toxic even at concentrations of parts per million. It has to be noted that heavy metals are extremely dangerous to human health and aquatic life. But what is worse is that there is nocycle of natural treatment of these substances. Inevitably, heavy metals remain intact in the environment and finally, they are accumulated in the food chain (bioaccumulation). Unless we take the right course of action, problems associated with both quality and quantity of water are going to be encountered, even in regions that seem to have sufficient amounts of clean water today. After the disputes and wars over the possession of oil in the past, water may be the next conflict territory between adjacent countries, even in Europe. According to the EC, 20% of all surface water in the European Union is seriously threat- ened with pollution. Furthermore, water is far from being evenly distributed in Europe and this is one major reason for resource problems. For example, whereas freshwater avail- ability is more than 100,000 m3 per capita per year in Iceland, it is less than 10,000 m3 per capita per year in Switzerland, Portugal, and Spain. The available water in a country or region depends on the rainfall and on the net result of flows from and to its neighbors, mainly through rivers. For example, in Iceland, Norway, Sweden, Denmark, the United Kingdom, Italy, Spain, France, and Finland, more than 98% of freshwater is generated within the country, whereas in Hungary and the Netherlands more than 50% of freshwater is due to river flows from other countries (Nixon et al., 2004). So, most European countries rely more on surface water than groundwater. For exam- ple, Finland and Lithuania take more than 90% of their total supply from surface waters 18 1. Air and Water Pollution Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 18 areas, 5 million m³ of drinking water is transported into the area every year, because groundwater is no longer potable. 1.2 POLLUTANTS AND EMISSION SOURCES Considerable information about pollutants, emission sources, and treatment techniques has been given in the reference document on best available techniques (BATs) in common waste- water and waste gas treatment released by the European Commission in 2003 (EC, 2003). 1.2.1 Air The sources of air emissions can be divided into two categories: • mobile sources, such as vehicles and ships • stationary sources, such as chemical industries. The exhaust air emissions are classified as • ducted emissions—process emissions released through a pipe • diffuse emissions—emissions that are not released via specific emission points (e.g. emissions during filling storage equipment, emissions from agriculture) • fugitive emissions—emissions due to leaks It is easily understood that whereas ducted emissions can be rather easily treated, the other two kinds of emissions can only be prevented or minimized. For example, agricul- tural emissions are very difficult to control. The main air pollutants are the following. • Carbon Dioxide Description: It is a nontoxic gas and the final product of complete combustion. Actually, it is the desirable and inevitable product of combustion. However, it is recognized as the main greenhouse gas, whose increased levels in the atmosphere play a large role in global warming. Source: Any combustion of fossil fuels. Combustion installations are responsible for 955 106 t/yr released into the air and 64.9% of the total emissions from industry in Europe. Impact: Global warming, climate change. • Sulfur Oxides and other sulfur compounds (H2S, CS2 , COS) Description: Sulfur dioxide (SO2) is a gas resulting from the combustion of coal, mainly in power plants, and certain types of liquid fuels that contain sulfur. In addition, it is pro- duced during the manufacture of paper and smelting of metals. It may cause respiratory problems and permanent damage to the lungs when inhaled at high levels. It plays a major role in the production of acid rain (EPA site). Carbon disulfide (CS2) has many industrial 1.2 Pollutants and Emission Sources 21 Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 21 applications. It is released into the air from industries producing, using, or handling CS2, sanitary landfills, and natural-gas production and distribution. Source: Any combustion of sulfur-containing fuels. Combustion installations are responsible for 2.9 106 t/yr released into the air and 68.3% of the total emissions from industry in Europe. Impact: Winter-type smog, acidification. • Nitrogen Oxides (NOx, N2O) and other nitrogen compounds (NH3, HCN) Description: Nitrogen oxides (NOx) are produced when fuel is burned at high tempera- tures. The main anthropogenic sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. Nitrogen oxide (NO) is easily oxidized in the atmosphere to nitrogen dioxide, which reacts with volatile organic compounds in the atmosphere, thus contributing to photochemical smog. Nitrogen dioxide (NO2) can also react with hydroxyl radicals in the atmosphere forming nitric acid, a major component of acid rain. It can cause lung damage and illnesses of the respiratory system. Nitrous oxide (N2O) is one of the most drastic compounds in greenhouse effect. Moreover, it is stable for almost 120 years and can reach the strato- sphere, where it participates in a reaction cycle catastrophic for ozone. Generally, nitro- gen oxides may contribute to the greenhouse effect, acid rain, photochemical smog, and ground-level ozone formation. Source: Combustion of nitrogen-containing fuels (fuel NOx) or oxidation of atmos- pheric nitrogen during combustions at high temperatures. Transport is the main contribu- tor, whereas in the industry sector, combustion installations are responsible for 1.5 106 t/yr released to air and 58.3% of the total emissions from industry in Europe. Impact: Global warming, acidification, photochemical smog, ozone layer depletion. • Incomplete combustion compounds, such as CO and CxHy Description: Carbon monoxide (CO) is a colorless, odorless, and poisonous gas. It is the product of any incomplete combustion of fossil fuels and many natural and synthetic prod- ucts. After it is inhaled, it enters the blood through the lungs and reacts with hemoglobin, reducing the capacity of blood to carry oxygen to cells, tissues and organs. The body’s parts need oxygen for energy, so high-level exposures to carbon monoxide can cause serious health effects, with death possible from massive exposures. Symptoms of exposure to carbon monoxide can include vision problems, reduced alertness, and general reduction in mental and physical functions. Carbon monoxide exposures are especially harmful to people with heart, lung, and circulatory system diseases. It may cause chest pain, vision problems, diffi- culties in the learning ability of young children, and generally reduction in mental and phys- ical functions. It also has a significant role in ozone production in the troposphere. Compounds in of the form CxHy are hydrocarbons, which may be unburned fuel compounds or produced by incomplete combustion. Their impact on environment and public health depends on the exact structure of the compound. Source: Road transport is the main source of carbon monoxide and unburned hydro- carbons in the atmosphere. Among industrial processes, the metal industry is responsible for 2.8 106 t/yr released into the air and 71.2% of the total emissions from industry in Europe. Impact: Health problems like chest pain and vision problems, photochemical smog. 22 1. Air and Water Pollution Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 22 • Volatile Organic Compounds (VOCs) and organosilicon compounds Description: Volatile organic compounds are released from burning fuels (gasoline, oil, wood coal, natural gas) and volatile liquid chemicals, such as solvents, paints, and glues. Automotive vehicles are also an important source of VOCs. The list of VOCs is lengthy and includes chemicals such as benzene, toluene, and methylene chloride. Depending on the specific compound, they can take part in ozone formation or may cause serious health problems such as cancer and other undesirable effects. Source: Road transport is the main source, whereas mineral oil and gas refineries are responsible for 0.2 106 t/yr released into the air, reaching 39.2% of the total industrial emissions. Impact: Depends on the exact compound; from health effects (cancer) to photochemi- cal smog formation. • Particulate Matter (such as dust, soot, alkali, and heavy metals) Description: The sources of particulate matter are many: burning of wood, diesel, and other fuels, industrial plants, and agriculture. It leads to decrease in visibility in urban areas and poses a threat to health, since it enters the organism through the respiratory system. It has to be noted that compounds of low volatility that are formed secondarily in the atmos- phere may be condensed on particulates, and as a result, the inhaled particles constitute a complex mixture of hazardous chemical compounds. The highest concentrations of air- borne metal particles occur near mines, smelters, and metal processing/heavy engineering works. These particles are so small that they can be carried enormous distances by the wind. Mercury in particular, which largely occurs in gaseous form in the atmosphere, can be dispersed a very long way indeed. Source: Road transport and the energy sector. Combustion installations are responsible for 77 103 t/yr in the atmosphere, this being 47.9% of the total emissions from industry in Europe. Impact: Respiratory diseases, winter-type smog. It is easily concluded from the above that road transport and combustion installations are the main sources of air pollutants. In Table 1.11, the releases of the main air pollu- tants in connection to the main activities as covered in EPER are presented (transport is not covered). 1.2.2 Water The sources of water pollution are divided into • point sources, such as chemical industries and human communities • nonpoint sources, such as agricultural activities and landfill leachates. Point sources are mainly responsible for the pollution of surface waters (rivers, lakes, seas), whereas nonpoint sources mainly contribute to the pollution of groundwater resources. Moreover, releases from point sources can be treated by wastewater treatment plants, whereas nonpoint source releases can only be minimized. 1.2 Pollutants and Emission Sources 23 Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 23 Source: Agriculture, industrial processes. Specifically, industrial plants for the production of pulp from timber or other fibrous materials, and paper or board production are responsi- ble of 70% of the total organic carbon released directly into water from industry per year. Impact: Depends on the exact type of the pollutants; from adverse long-term effects to immediate danger to human and biotic life. • Pathogenic Microorganisms Description: Pathogenic microorganisms include bacteria, viruses, and protozoans. Source: Untreated sewage, storm drains, run-off from farms, and particularly, boats that dump sewage. Impact: Many adverse effects on health. Typhoid, dysentery, and skin diseases are among the possible health effects. Commonly, wastewater contains numerous compounds and its exact composition is very difficult to determin or is even unknown, and therefore, its impact on the environment is characterized by • the concentration of specific substances, such as NH4 , NO3 , NO 2 , and PO4 3 ions, and heavy metals • sum parameters, such as TSS (total solids suspended), BOD (biological oxygen demand), COD (chemical oxygen demand), pH, conductivity and temperature. • its toxicity to organisms in the receiver • its hydraulic load. In Table 1.12, the direct releases of the most important pollutants into water are presented in association with the corresponding main industrial source. For purposes of comparison, it is useful here to recall that road transport and combustion installations, mainly of the 26 1. Air and Water Pollution Table 1.12 The releases of the main pollutants released directly into water from the industrial sector in Europe (2001) Compound t/yr Main source Phenols 1,419,344 Basic inorganic chemicals or fertilizers (47%) Total organic carbon 246,524 Industrial plants for pulp from timber or other fibrous materials, and paper or board production (70%) Nitrogen 22,317 Basic inorganic chemicals or fertilizers (29%) Phosphorus 1662 Basic inorganic chemicals or fertilizers (25%) Chromium 864 Metal industry (87%) BTEX 82.5 Basic organic chemicals (56.1%) Nickel 71.5 Metal industry (45%) Copper 45.8 Metal industry (23%) Lead 41.7 Metal industry (40%) Polycyclic aromatic 10.3 Metal industry (74%) hydrocarbons Cadmium 8.1 Metal industry (66%) Arsenic 5.1 Metal industry (22%) Mercury 0.5 Metal industry (23%) Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 26 energy sector, are the most important sources of air pollutants. In the case of water bodies, it is obvious that agriculture and the metal industry, plus the activities in the production of inorganic chemicals and fertilizers, constitute the major polluters. 1.3 TREATMENT METHODS The minimization of the releases into the environment can be largely achieved by • pollution prevention measures • waste treatment (end-of-pipe techniques). The first approach may involve cleaner synthesis processes, improved technology, recy- cling of residues, improved use of catalysts, and generally, every technique integrated into the process that leads to less waste; whereas the second one is an end-of-pipe treatment of the waste that is inevitably produced by a chemical process. Both approaches have to be com- bined so that our releases into the environment are as minimal and harmless as possible. Waste-treatment techniques are classified by the type of contaminant. The main tech- niques concerning waste gas treatment are the following. VOCs and inorganic compounds: membrane separation, condensation, adsorption, wet scrubbing, biofiltration, bioscrubbing, biotrickling, thermal oxidation, catalytic oxidation, and flaring. Particulate matter: separator, cyclone, electrostatic precipitator, wet dust scrubber, fab- ric filter, catalytic filtration, two-stage dust filter, absolute filter, high-efficiency air filter, and mist filter. Gaseous pollutants in combustion exhaust gases: dry sorbent injection, semidry sorbent injection, wet sorbent injection, selective noncatalytic reduction of NOx (SNCR), selective catalytic reduction of NOx (SCR). In Table 1.13, the conditions for the application of some treatment processes are shown. 1.3 Treatment Methods 27 Table 1.13 Evaluation of alternative treatment processes used to control industrial vapor-phase pollutants Case Activated Thermal Scrubbers Particulate Catalytic carbons oxidation filters oxidation Low VOC levels High VOC levels Continuous load Intermittent loads Halogenated organics T 150 °F T150 °F High flows Low flows High humidity Inorganic particles Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 27 The main wastewater treatment techniques are Separation or clarification techniques: grit separation, sedimentation, air flotation, fil- tration, microfiltration and ultrafiltration, and oil–water separation. Physico-chemical treatment techniques: precipitation, sedimentation, air flotation, fil- tration, crystallization, chemical oxidation, wet air oxidation, super-critical water oxida- tion, chemical reduction, hydrolysis, nanofiltration, reserve osmosis, adsorption, ion exchange, extraction, distillation, rectification, evaporation, stripping, and incineration. Biological treatment techniques: anaerobic digestion processes, aerobic digestion processes, nitrification, denitrification, and central biological wastewater treatment. Adsorption, ion exchange, and catalysis share a great portion of environmental applica- tions, as shown in the next section, and more extensively, in Chapter 2. Specifically, adsorption and catalysis are extensively used for the removal or destruction of air pollu- tants in gas streams as well as for purifying wastewaters or fresh water. Ion exchange has a special position among other techniques in the removal of heavy metals from wastewater. 1.4 ENVIRONMENTAL APPLICATIONS OF ADSORPTION, ION EXCHANGE, AND CATALYSIS Adsorption, ion exchange, and catalysis are discussed in this book. The first two are among the end-of-pipe techniques, whereas catalysis has a role to play in either preventing pollu- tion during the process or as an end-of-pipe technique of waste treatment. Moreover, ion exchange is mainly used in wastewater treatment, whereas adsorption and catalysis can be found in both wastewater and gas management. Specifically, ion exchange is one of the best available techniques (BAT) suggested by EC for heavy metal and inorganic salts removal from wastewaters. As shown in Table 1.12, metal industry is the major source of metal releases into the environment and ion exchange can be employed as the main pollution abatement technology. We should again mention that nature does not have any efficient way of coping with these substances, and as a result, their discharge into the environment should be minimized. Adsorption is suggested as the BAT for both the minimization of contami- nants in water that are unsuitable for biological treatment and the removal of VOCs and inorganic compounds from normal waste gas streams. Catalysis is also considered as the BAT for the destruction of water pollutants that are resistant to biological treatment (as cat- alytic wet-air oxidation) and for the oxidation of VOCs and inorganic compounds in gas streams (as catalytic oxidation). Generally, VOCs, sulfur oxides, nitrogen oxides, and vari- ous forms of hydrocarbons can be effectively treated by means of these processes. MULTIPLE CHOICE QUESTIONS 1. CFCs are involved in (a) photochemical smog (b) ozone depletion (c) greenhouse effect (d) both ozone depletion and greenhouse effect 28 1. Air and Water Pollution Else_AIEC-INGLE_Ch001.qxd 7/13/2006 1:53 PM Page 28 –2– Adsorption, Ion Exchange, and Catalysis 2.1 DEFINITIONS 2.1.1 Adsorption The term “sorption” is used to describe every type of capture of a substance from the external surface of solids, liquids, or mesomorphs as well as from the internal surface of porous solids or liquids (Skoulikides, 1989). Depending on the type of bonding involved, sorption can be classified as follows. (a) Physical sorption. In physical sorption (or physisorption), no exchange of electrons is observed; rather, intermolecular attractions between favorable energy sites take place and are therefore independent of the electronic properties of the molecules involved. Physisorption is characterized by interaction energies comparable to heats of vaporization (condensation). The adsorbate is held to the surface by relatively weak van der Waals forces and multiple layers may be formed with approximately the same heat of adsorption. The heat of adsorption for physisorption is at most a few kcal/mole and therefore this type of adsorption is stable only at temperatures below 150 °C. (b) Chemical sorption. Chemical sorption (or chemisorption) involves an exchange of electrons between specific surface sites and solute molecules, and as a result a chemical bond is formed. Chemisorption is characterized by interaction energies between the sur- face and adsorbate comparable to the strength of chemical bonds (tens of kcal/mol), and is consequently much stronger and more stable at high temperatures than physisorption. Generally, only a single molecular layer can be adsorbed. (c) Electrostatic sorption (ion exchange). This is a term reserved for Coulomb attrac- tive forces between ions and charged functional groups and is commonly classified as ion exchange. The most important characteristics of physical and chemical sorption are presented in Table 2.1. The term “adsorption” includes the uptake of gaseous or liquid components of mixtures from the external and/or internal surface of porous solids. In chemical engineering, adsorp- tion is called the separation process during which specific components of one phase of a fluid are transferred onto the surface of a solid adsorbent (McCabe et al., 1993). 31 Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 31 32 2. Adsorption, Ion Exchange, and Catalysis When the species of the adsorbate travel between the atoms, ions, or the molecules of the adsorbent, the phenomenon of “absorption” takes place and this discriminates absorp- tion from the main phenomenon of adsorption that takes place on the interface. The adsorption of various substances from solids is due to the increased free surface energy of the solids due to their extensive surface. According to the second law of ther- modynamics, this energy has to be reduced. This is achieved by reducing the surface ten- sion via the capture of extrinsic substances. Consider a molecule above a surface with the distance from the surface being normal to the surface. There are two competitive types of influence occuring: (a) repulsion between the cloud of electrons in the atoms that form the surface and those of the molecule and (b) van der Waals nuclear attraction force. The nuclear attraction has a much shorter radius of influence and as a result of the balance of these two forces, there is a “well” in the poten- tial energy curve at a short distance from the surface, as shown in Figure 2.1. Molecules or atoms that reach this “well” are trapped or “adsorbed” by this potential energy “well” and cannot escape, unless they obtain enough kinetic energy to be desorbed. The surface can be characterized either as external when it involves bulges or cavities with width greater than the depth, or as internal when it involves pores and cavities that have depth greater than the width (Gregg and Sing, 1967). All surfaces are not really smooth and they exhibit valleys and peaks at a microscopic level. These areas are sensitive to force fields. In these areas, the atoms of the solid can attract atoms or molecules from a fluid nearby. The most important property of adsorbent materials, the property that is decisive for the adsorbent’s usage, is the pore structure. The total number of pores, their shape, and size determine the adsorption capacity and even the dynamic adsorption rate of the material. Generally, pores are divided into macro-, meso- and micropores. According to IUPAC, pores are classified as shown in Table 2.2. Porosity is a property of solids that is attributed to their structure and is evident by the pres- ence of pores between internal supermolecular structures (Tager, 1978). It is not considered Table 2.1 Physical versus chemical sorption Chemisorption Physisorption Temperature range over Virtually unlimited; however, Near or below the which adsorption occurs a given molecule may be condensation point of the gas effectively adsorbed only (e.g. CO2 200 K) over a small range Adsorption enthalpy Wide range, related to the Related to factors like chemical bond strength— molecular mass and polarity typically 40–800 kJ/mol but typically 5–40 kJ/mol (i.e. heat of liquefaction) Nature of adsorption Often dissociative and may be Nondissociative and reversible irreversible Saturation uptake Limited to one monolayer Multilayer uptake is possible Kinetics of adsorption Very variable; often is an Fast, because it is a activated process nonactivated process Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 32 to be an intrinsic property of the solids, but depends on the treatment of the materials. The porosity can be developed by the aggregation of particles as well as by the detachment of a part of the mass of the solid. The pores shaped during the second process are comparable in shape and size with the particles detached. Adsorptive molecules transport through macropores to the mesopores and finally enter the micropores. The micropores usually constitute the largest portion of the internal sur- face and contribute the most to the total pore volume. The attractive forces are stronger and the pores are filled at low relative pressures in the microporosity, and therefore, most of the adsorption of gaseous adsorptives occurs within that region. Thus, the total pore vol- ume and the pore size distribution determine the adsorption capacity. 2.1.2 Ion exchange Ion exchangers are solid materials that are able to take up charged ions from a solution and release an equivalent amount of other ions into the solution. The ability to exchange 2.1 Definitions 33 Table 2.2 The classification of pores according to their size (Rodriguez–Reinoso and Linares–Solano, 1989) Type Pore diameter d (nm) Macropores d 50 Mesopores 2 d 50 Micropores d 2 Ultramicropores d 0.7 Supermicropores 0.7 do a 2 ad0 is the pore width for slit-type pores or the pore diameter for cylindrical pores. molecule di st an ce x surface distance x po te nt ia l e ne rg y Figure 2.1 The potential energy versus distance. Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 33 to acetaldehyde via the third reaction, dehydrated to ethylene via the fourth reaction, dehy- drated to diethylether via the fifth reaction, and finally decomposed to methane via the last reaction. It is obvious that what could be misunderstood as a simple oxidation is really a complex scheme of reactions. The presence of a catalyst may enhance one or more of these reactions or even all of them by various degrees, leading to a different overall selectivity. Its selection would be made on the basis of the desired products, and catalyst selectivity is the key characteristic to practical applications. So, catalysts enable reactions to occur much faster and allow the use of milder condi- tions of temperature for achieving reaction rates of practical use. They achieve this by pro- viding an alternative pathway of lower activation energy for the reaction to proceed. As shown in Figure 2.2, a catalyst lowers the energy of the transition state without changing the energy of the reactants and products. For example, the uncatalyzed value of the acti- vation energy of the decomposition of nitrogen oxide to nitrogen and oxygen is 1240 kJ/mol, whereas with a gold catalyst this becomes 120 kJ/mol. Since the catalyzed path requires lower activation energy, more molecules will have suffi- cient energy to react effectively than in the case of the uncatalyzed path. In homogeneous catalysis, this is generally achieved by the reaction between the catalyst and one or more reac- tants to form an unstable chemical intermediate, which subsequently reacts to produce the final product. The catalyst is regenerated in the final step. For example, if reactant A reacts with B to form the product (P) in the presence of a catalyst (C), a possible reaction scheme is An example of great environmental interest is the catalytic mechanism for ozone destruc- tion by the hydroxyl radical, which is believed to be The hydroxyl radical is regenerated in the second reaction and may continue its action. HO O OH 2O2 3 2 OH O HO O3 2 2 A C AC B AC P C A B PC  → 36 2. Adsorption, Ion Exchange, and Catalysis po te nt ia l e ne rg y Reaction coordinate uncatalysed reaction path catalysed reaction path Figure 2.2 The catalyzed and the uncatalyzed reaction path. Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 36 In heterogeneous catalysis, the catalyst provides a surface on which the reactants are adsorbed. The chemical bonds of the reactants become weakened on the catalytic surface and new compounds are formed. These compounds (products) have weaker bonds with the catalyst and consequently are released. An example of heterogeneous catalysis is the industrial synthesis of ammonia, which requires solid catalysts to obtain significant rates of reaction between nitrogen and hydrogen: It has to be noted that the adsorption of reactants is generally not uniform across the cata- lyst surface. Adsorption, and therefore catalysis, takes place mainly at certain favorable locations on a surface called active sites. In environmental chemistry, catalysts are essen- tial for breaking down pollutants such as automobile and industrial exhausts. 2.2 HISTORICAL ASPECTS 2.2.1 Adsorption The first known use of adsorption was made in 3750 B.C. by Egyptians and Sumerians who used charcoal for the reduction of copper, zinc, and tin ores for the manufacture of bronze. Around 1550 B.C., Egyptians applied charcoal for medicinal purposes, whereas around 460 B.C., Hippocrates and Pliny introduced the use of charcoal to treat a wide range of infections. Around the same age, Phoenicians used charcoal filters to treat drink- ing water. So, this must have been the first use of adsorption for environmental purposes. In 157 B.C., Claudius Galen introduced the use of carbons of vegetable and animal origin to treat a wide range of complaints. These early applications of adsorption were based on intuition and not on a systematic study. It was in 1773 that Scheele made the first quantitative observations in connection with adsorption, whereas F. Fontana in 1777 reported his experiments on the uptake of gases from charcoal and clays. However, the modern application of adsorption is attributed to Lowitz. Lowitz used charcoal for the decolorization of tartaric acid solutions in 1788. The next systematic studies were published by Saussure in 1814. He concluded that all types of gases can be taken up by a number of porous substances and this process is accompanied by the evolution of heat (Dabrowski, 2001). NH (ads) NH (g) (desorption of ammonia)3 3 N(ads) 3H(ads) NH (ads) (reaction to form ammonia adsorbed)3 H (ads) 2H(ads) (dissociation of hydrogen)2 H (g) H (ads) (adsorption of hydrogen on catalyst surface)2 2 N (ads) 2N(ads) (dissociation of nitrogen)2 N (g) N (ads) (adsorption of nitrogen on catalyst surface)2 2 2.2 Historical Aspects 37 Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 37 The term “adsorption” was first used by H. Kayser in 1881. J. W. McBain introduced a similar term in 1909, i.e. “absorption”, to determine an uptake of hydrogen by carbon much slower than adsorption. He proposed the term “sorption” for adsorption and absorp- tion (Dabrowski, 2001). In 1903, Tswett was the first to study selective adsorption. He investigated the separa- tion of chlorophyll and other plant pigments using silica materials. This technique pro- posed by Tswett has been called “column solid–liquid adsorption chromatography.” However, there was no sound theory that enabled the interpretation of adsorption isotherm data untill 1914. Despite the fact that the Freundlich equation was used, there was no theo- retical justification for it. It was an empirical equation, proposed actually by van Bemmelen in 1888. However, it is today known as the Freundlich equation because Freundlich assigned great importance to it and popularized its use. Langmuir was the first to have introduced a clear concept of the monomolecular adsorption on energetically homogeneous surfaces in 1918 and derived the homonymous equation based on kinetic studies (Dabrowski, 2001). The first practical applications of adsorption were based on the selective removal of individual components from their mixtures using other substances. The first filters for water treatment were installed in Europe and the United States in 1929 and 1930, respec- tively. Activated carbon was recognized as an efficient purification and separation material for the synthetic chemical industry in the 1940s. By the late 1960s and early 1970s, acti- vated carbon was used in many applications for removing a broad spectrum of synthetic chemicals from water and gases. In Table 2.3, the history of adsorption is presented briefly. 2.2.2 Ion exchange The first citation of an application of ion exchange can be found in Aristotle’s Problematica, where it is mentioned that sand filters were used for the purification of sea and impure drinking waters. That is also the first environmental application. In the same book, Aristotle suggested that desalination resulted from density effects. It seems that practical applications of ion exchange were well recognized before the 19th century. However, the underlying physical phenomenon was not known. Credit for the identifica- tion of the ion-exchange phenomenon is attributed to two agriculture chemists, Thomson and Way. In 1848, Thomson reported to Way that he had found that urine was decolorized and deodorized during the filtration of liquid manure through a bed of an ordinary loamy soil. It was Way, who illustrated the basic characteristics of ion exchange after conducting several experiments (Lucy, 2003). After soil and clays, natural and synthetic aluminum silicates and synthetic zeolites were tested as ion-exchange materials by other scientists. However, the first practical applications of ion exchange took place in the early 20th century. The first synthetic organic resins were synthesized in 1935. This spectacular evolution began with the finding of two English chemists, Adams and Holmes, who found that crushed phonograph records exhibited ion-exchange properties (Helfferich, 1962). Much progress was made during World War II in the field of ion exchange, but the results 38 2. Adsorption, Ion Exchange, and Catalysis Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 38 clay plates many millennia ago. A few examples of the utilization of catalysis in ancient civilizations are the following: • 6000 B.C.—beer brewing by malting procedure (malt enzymes) • 3000 B.C.—wine making by fermentative conversion of grape juice sugars • 2000 B.C.—making alcohol by fermentation of various carbohydrate sources • 800 B.C.—cheese making by casein hydrolysis with calf stomach extract (calf rennet) The phenomenon under consideration was studied systematically in the beginning of the 19th century. In 1815, Davy performed experiments that dealt with catalytic com- bustion on platinum gauzes. The term “catalysis”, however, was introduced by Berzelius in 1836. He first defined a catalyst (Berzelius, 1836) as “a compound, which increases the rate of a chemical reaction, but which is not consumed during the reaction.” This def- inition was later amended by Ostwald (1853–1932) in 1895 to involve the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly decreased: “A catalyst is a substance that increases the rate of approach to equi- librium of a chemical reaction without being substantially consumed in the reaction.” It was more than a century after Berzelius’ first definition that Marcel Prettre’s introduced the notion of yield: “The catalyst is a substance that increases the rate of a chemical transformation without modifying the yield, and that is found intact among the final products of the reaction.” It is fascinating that even today, heterogeneous catalysis still remains an empirical sci- ence. Although the application of catalysts in the chemical industry is a fact for at least 150 years, the experimental techniques for investigation of catalysis at the atomic level did not become routine until less than 25 years ago; the computational techniques are even younger and have hardly become routine yet. For this reason, a vast amount of empirical knowledge exists and awaits systematic investigation. A short history of heterogeneous catalysis is presented in Table 2.5. 2.2 Historical Aspects 41 Table 2.4 (Continued) Scientist(s) name(s) Breakthrough Year H. P. Gregor Invention and development of chelating polymers. 1952–1971 K. W. Pepper L. R. Morris M. A. Peterson, H. A. Sober Development of cellulose ion exchangers. 1956 1956-58 Preparation and studies of nonsiliceous inorganic ion 1956 exchangers—insoluble salts, heteropolyacids F. Helfferich Foundations laid for the new theoretical treatment of 1959 ion exchange. T. R. E. Kressmann Invention and development of isoporous ion-exchange 1960 J. R. Millar resins. J. Weiss Thermally regenerable ion-exchange resins and water 1964 desalination based on them. Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 41 42 2. Adsorption, Ion Exchange, and Catalysis Table 2.5 Brief history of heterogeneous catalysis Scientist(s) name(s) Breakthrough Year von Marum Studies the dehydrogenation of alcohols using metals. 1796 J. Dalton Suggests that chemical compounds consist of molecules 1808 and molecules consist of atoms. H. Davy Studies the oxidation of methane on platinum wires. 1817 W. Henry Studies oxidations catalyzed by platinum adsorbed 1824 on clay pellets. M. Faraday Studies the ignition of hydrogen in air at platinum surfaces. 1825 J. Berzelius Formulates the definition of catalysis. 1836 E. Frankland Formulates the concept of valency. 1852 C. W. Guldberg Formulation of the law of mass action. 1867 P. Wage von Hoffmann Develops Ag as a catalyst for the oxidation of CH3OH to 1869 HCHO. R. Messel Develops the industrial oxidation of SO2 1875 catalyzed by Pt J. W. Gibbs Publishes “On the equilibrium of heterogeneous 1876 substances”,which contains Gibbs’ phase law C. Winkler Invention of the contact process for the synthesis of 1879 sulfuric acid. Badische Anilin and Industrial synthesis of sulfuric acid using a platinum 1889 Soda Fabrik Germany catalyst W. Ostwald Discovers that the reaction 2NH3 5/2O2 = 2NO 3H2O 1901 is catalyzed by Pt. S. Sabatier Studies hydrogenation of alkenes catalyzed by Ni 1902 (1902–1905). F. Haber Reports the production of small amounts of NH3. 1905 from N2 3H2 using an iron catalyst W. Ostwald Receives the Nobel prize in chemistry for his work 1909 on catalysis, chemical equilibrium, and the rate of chemical reactions. P. Sabatier Receives the Nobel prize in chemistry for the 1912 development of the hydrogenation of organic compounds catalyzed by small metal particles. I. Langmuir Formulates a theory of adsorption. 1915 Chemical Builds an industrial nitric acid plant based on the 1917 Construction Co. Ostwald process. J. Frenkel Publishes a theory of adsorption. 1924 H. S. Taylor Theory of catalysis. 1925 I. Langmuir Formulate the principles of Langmuir– 1927 W. Hinschelwood Hinschelwood kinetics. I. Langmuir Receives the Nobel prize in chemistry for his work 1932 on surface chemistry G. Damköhler Introduces the Damköhler group. 1937 E. W. Thiele Introduces the effectiveness factor and the Thiele 1939 modulus. H. Kramers Publishes the definitive treatment of kinetics. 1940 (Continued) Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 42 2.3 ADSORPTION, ION EXCHANGE, AND CATALYSIS:THREE RELATED PHENOMENA At a first glance, adsorption, ion exchange, and catalysis are three different phenomena with diverse characteristics. However, despite these differences, there are many common features among these processes. In the following sections, a relationship between them will be attempted. 2.3.1 Adsorption and ion exchange Ion exchange is similar to adsorption, since mass transfer from a fluid to a solid phase is common in both processes, i.e. they are basically diffusion processes. Ion exchange is also a sorption process, but ions are the sorbed species in contrast to adsorption, where electri- cally neutral species are sorbed (Noble and Terry, 2004; Perry and Green, 1999). It is gen- erally accepted that adsorption and ion exchange can be grouped together as sorption for a unified treatment in practical applications. Most of the mathematical theories and approaches have been developed originally for sorption rather than ion exchange. However, they are sufficiently general to be applicable with minor, if any, modifications to a number of similar phenomena such as ion exclusion and ligand exchange. According to Helfferich (1995), the applicability of a simplified theory depends more on the mode of operation than on the particular mechanism of solute uptake. A significant feature of physical adsorption is that the rate of the phenomenon is generally too high and consequently, the overall rate is controlled by mass (or heat transfer) resistance, rather than by the intrinsic sorption kinetics (Ruthven, 1984). Thus, sorption is viewed and termed in this book as a “diffusion-controlled” process. The same holds for ion exchange. 2.3.2 Catalysis and adsorption As discussed earlier, the first step in heterogeneous catalysis is the adsorption of the mole- cules of the reactants on the surface of the adsorbent or of the catalyst (inner and outer sur- faces). Then, molecular dissociation of at least one or two reacting components takes place, usually preceded by surface diffusion. The next step is a surface reaction, which is 2.3 Adsorption, Ion Exchange, and Catalysis: Three Related Phenomena 43 Table 2.5 (Continued) Scientist(s) name(s) Breakthrough Year G. Natta Continues the study initiated by Karl Ziegler on 1953 metal-organic catalysts for polymerization of alkenes P. Kisliuk Publishes a theory of precursor kinetics for 1957 chemisorption. Catalytic converters are introduced in new cars 1975 in the United States. R. Kelley, Measure the rate of a reaction catalyzed by a single 1982 D. Goodman crystal (methanation, Ni single crystals) Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 43 Catalysis with ion-exchange resins provides effective and efficient answers to a number of catalytic problems: • the resins do not introduce counterions (which would have to be removed from the final product made under homogeneous catalysis conditions), • they may be regenerated and reused over relatively long periods, • corrosion arising from the presence of strong acids in the bulk phase is eliminated, • resins may be tailored in particle size, pore volume, surface area, swelling in solvents, etc. However, it should be noted that the maximum operating temperature recommended for ion-exchange resin catalysts is in the range 137–145°C. Thus, the use of resins as catalysts is limited to systems that operate at relatively low temperatures. In the case of elevated temperatures, zeolites could be used instead, because they exhibit higher stability for tem- peratures as high as 800°C (e.g. clinoptilolite). In many applications, acid-treated clays could be used as an alternative. Clays, being naturally occurring aluminosilicates, are read- ily available and inexpensive compared to other types of heterogeneous acid catalysts, e.g. ion-exchange resins (Chitnis and Sharma, 1997). Clays, in general, are thermally stable up to 200 °C and they can be greatly improved by the pillaring process. Furthermore, in many cases, they exhibit higher selectivity than resins. The disadvantage of clays is that their activity is lower than resins, their use is restricted to nonaqueous reactions systems only, and their mechanical strength is low. 2.4 ENVIRONMENTAL APPLICATIONS OF ADSORPTION, ION EXCHANGE, AND CATALYSIS 2.4.1 Adsorption There are many environmental applications of adsorption in practice and many others are being developed (Noble and Terry, 2004). Activated carbons and clays are frequently used for the removal of organic contaminants, such as phenol and aniline, both of which are prevalent in industry wastewaters and are known to have a significant negative impact on marine life and human health (IRIS, 1998; Dabrowski et al., 2005). Moreover, the adsorp- tion on inexpensive and efficient solid supports has been considered a simple and eco- nomical viable method for the removal of dyes from water and wastewater (Forgacsa et al., 2004). Activated carbon, clays, coal, vermiculite, and other adsorbents have been used for this purpose. Specifically, adsorption can be employed in (Noble and Terry, 2004; Dabrowski, 2001): • the removal of water from organic solvents • the removal of organics from water • taste and odor regulation in wastewater treatment • the removal of radon, hydrogen sulfide, and other sulfur compounds from gas streams • mercury removal from chlor-alkali-cell gas effluent 46 2. Adsorption, Ion Exchange, and Catalysis Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:32 AM Page 46 • heavy-metal removal in clay barriers • nitrogen and phosphorus removal from wastewater, i.e. removal and recovery of nutrients • solvent recovery and solvent vapor fractionation • volatile organic compounds recovery from gas streams and groundwater • water removal from gas streams containing acid gases Other important applications of adsorption are the control of “greenhouse” gases (CO, CH4, N2O), the utilization of CH4, the flue gas treatment (SOx, NOx, Hg removal), and the recovery of the ozone-depleting CFCs (Dabrowski, 2001). Activated carbons and hydrophobic zeolites are used for the adsorption of HCFCs (Tsai, 2002). The most commonly used adsorbents are shown in Table 2.7. The adsorption process can be used for substance recovery as well as for the abatement of undesirable emissions in wastewaters (Table 2.8) and gas streams (Table 2.9). 2.4 Environmental Applications of Adsorption, Ion Exchange, and Catalysis 47 Table 2.7 The most common adsorbents Wastewater treatment VOC removal Activated carbon (mainly as granulates) Granular activated carbon Lignite coke Zeolites Activated aluminum oxide Macroporous polymer particles Adsorber resins Silica gel Zeolites Sodium–aluminum silicates Table 2.8 Representative commercial liquid-phase adsorption separations Liquid bulk separations Adsorbent (adsorbate concentration in the feed 10% wt.) Fructose/glucose Zeolites p-Xylene/o-xylene, m-xylene Zeolites Detergent-range olefins/paraffins Zeolites Normal paraffins/isoparaffins, aromatics Zeolites p-Diethyl benzene/isomer mixture Zeolites Liquid Purifications Adsorbent (adsorbate concentration in the feed 3% wt.) Sulfur compounds/organics Zeolites Organics/H2O Activated carbon Odor, taste/drinking H2O Activated carbon H2O/organics Silica, alumina, zeolite Decolorizing petroleum fractions, sugar syrups, vegetable oils, etc. Activated carbon Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:32 AM Page 47 As any process, adsorption has both some advantages and disadvantages: Advantages • high removal efficiency • enables removal of refractory and/or toxic organic compounds • possibility of compounds recovery (preferably with zeolites) • simple installation and maintenance • capability of systems for fully automatic operation • a large variety of adsorbents available Disadvantages • adsorbents deteriorate in capacity gradually • particulates in the feed can cause problems • high content of macromolecular compounds decreases efficiency and may cause irre- versible blockage of active sites • risk of bed fires in the VOC abatement • spent adsorbent has to be regenerated (high energy consumption) or disposed (causing waste) • relatively high capital cost 48 2. Adsorption, Ion Exchange, and Catalysis Table 2.9 Representative commercial gas-phase adsorption separations Gas bulk separations Adsorbent (adsorbate concentration in the feed 10% wt.) N2/O2 Zeolite O2/N2 Carbon molecular sieve H2O/ethanol Zeolite CO, CH4, CO2, N2, NH3/H2 Zeolite, activated carbon Acetone/vent streams Activated carbon C2H4/vent streams Activated carbon Gas purifications Adsorbent (adsorbate concentration in the feed 3% wt.) H2O/olefin-containing cracked gas, natural gas, Silica, alumina, zeolite air, synthesis gas SO2/vent streams Zeolite CO2/C2H4, natural gas Zeolite Organics/vent streams Activated carbon and others Sulfur compounds/natural gas, hydrogen, liquefied Zeolite petroleum gas (LPG) Solvents/air Activated carbon Odors/air Activated carbon NOx/N2 Zeolite Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:32 AM Page 48 nitrogen oxides by over 70%. Today, about one-third of the world market for catalysts involves environmental catalysis. Generally, catalysts are called into action to eliminate emissions from mobile (cars) and stationary (industry) sources, to take part in liquid and solid waste treatment, and con- tribute to the effort to reduce volatile organic compounds and gases that pose major envi- ronmental problems such as photochemical smog and (at a global level) the greenhouse effect. The use of catalysts for exploiting renewable energy sources, producing clean fuels in refineries, and minimizing the by-product formation in industry also fall within the defi- nition of environmental catalysis. In the future, the continuous effort to control transport emissions, improve indoor air quality, and decontaminate polluted water and soil will fur- ther boost catalytic technology. All in all, catalysts will continue to be a valuable asset in the effort to protect human health, the natural environment, and the existence of life on Earth. There are, however, some distinctive differences between the environmental and the other aspects of catalysis. First, the feed and operation conditions of environmental cata- lysts cannot be changed in order to increase conversion or selectivity, as commonly done for chemical production catalysts. Second, environmental catalysis has a role to play not only in industrial processes, but also in emission control (auto, ship, and flight emissions), and even in our daily life (water purifiers). Consequently, the concept of environmental catalysis is vital for a sustainable future. Last but not least, environmental catalysts often operate in more extreme conditions than catalysts in chemical production. There are also cases, such as automotive vehicles, where they have to operate efficiently for a continu- ously varying feed flow rate and composition. The most important catalytic production processes are the following: • the Haber process for ammonia synthesis • steam reforming of hydrocarbons to produce synthesis gas • methanol synthesis • Fischer–Tropsch synthesis • hydrogenation/dehydrogenation of organic compounds, • sulfuric acid production • nitric acid production • maleic anhydride production • petroleum refining and processing In the area of environmental application of catalysis, the most important processes are • catalytic reduction of NOx • catalytic oxidation of SO2 • catalytic oxidation of CO, VOC, and hydrocarbons • catalytic denitrification of drinking water • catalytic oxidation of persistent organic pollutants in wastewater 2.4 Environmental Applications of Adsorption, Ion Exchange, and Catalysis 51 Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:32 AM Page 51 The main advantages and disadvantages of catalysts are Advantages • high efficiency in the treatment of gas emissions • large variety of catalysts • simple installation Disadvantages • deactivation phenomena (mainly in the wastewater treatment) • spent catalysts have to be appropriately disposed • a specific temperature is required for their operation • for low concentrations of VOCs, the heating of large volumes of emissions to the tem- perature required for catalytic activity is expensive A look into three-way catalysis The need for controlling the exhaust emissions from automotive vehicles has been recognized since 1975. The most effective and tested method proved to be the installation of three-way catalysts at the exhaust emission system of cars. The development and the improvement of such catalysts was and will be a complicated effort, since a cat- alyst placed in a vehicle should simultaneously accelerate oxida- tion and reduction reactions, under continuously changing conditions of temperature and space velocity, in contrast to industrial applications where catalysts operate under fixed and controlled conditions. Generally, the catalytic converter of a vehicle has to satisfy the following requirements: • facilitate the oxidation reactions of carbon monoxide and unburned hydrocarbons and the reduction of nitrogen oxides (three reactions to perform; hence they are named “three-way catalysts”) • start its operation at the lowest possible temperature, since the emissions are high dur- ing the first minutes of engine operation, where the temperature is still low • show resistance for a short time at temperatures up to 1000 °C • exhibit a satisfactory operation for at least 150,000 km • be highly active in order to achieve the desired conversions for high volumetric feed of emissions that take place at the engine exhaust • all the above have to take place at continuously changing air-to-fuel ratios. The presence of a three-way catalyst is mandatory for every car produced in the United States and Europe since 1981 and 1993, respectively. It is the most massive and one of the most successful stories in the history of catalysis. The demand for occupying less space, operation at high volumetric feed, and low loss of power led to the adoption of monoliths for the automobile catalyst. A monolith 52 2. Adsorption, Ion Exchange, and Catalysis Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:32 AM Page 52 is a ceramic support structured in many channels and shapes that achieve large catalytic surface at small volume. The search for the appropriate active catalytic components ended with the use of Pt, Pd, and Rh. These metals proved to have the required activity, durability at high temperatures, and sufficient resistance to poisoning from the lead traces present in fuels. However, it was the development of electronics and the installation of the so-called “lambda” sensor that allowed the sound operation of catalytic converters by adjusting the air-to-fuel ratio at a specific range of values (Figure 2.3). As mentioned earlier, the oxidation of carbon monoxide and hydrocarbons should be achieved simultaneously with the reduction of nitrogen oxides. However, the first reaction needs oxygen in excess, whereas the second one needs a mixture (fuel-oxygen) rich in fuel. The solution was found with the development of an oxygen sensor placed at exhaust emis- sions, which would set the air-to-fuel ratio at the desired value in real time. So, the com- bination of electronics and catalysis and the progress in these fields led to better control of the exhaust emissions from automotive vehicles. A special application: The Earth itself takes advantage of catalytic processes. It seems that catalysis plays a very important role in the global chemistry of Earth atmosphere. Photocatalytic processes may occur in the troposphere on aerosol particles containing Fe2O3, TiO2, and ZnO under the action of the near-ultraviolet, visible, and near-infrared solar light. Photocatalysis is anticipated to affect the intensity of acid rains, the concentra- tion of some greenhouse gases, and free the atmosphere from harmful compounds. Thus, desert areas where continental dust is generated may, perhaps, serve as “kidneys” for the Earth (Zamaraev, 1997). 2.4 Environmental Applications of Adsorption, Ion Exchange, and Catalysis 53 0 10 20 30 40 50 60 70 80 90 100 13.5 14 14.5 15 15.5 air to fuel ratio % c on ve rs io n CO HC NOx Figure 2.3 The activity of catalytic converter in relation with air to fuel ratio (Shelef and McCabe, 2000). Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:32 AM Page 53 17. Catalysts are appropriate (a) for industrial processes (b) for VOC abatement from stationary sources (c) for both industrial production and environmental applications 18. For the efficient operation of a catalytic converter placed in a car, it is important to adjust the air-to-fuel ratio (a) according to the current engine operating conditions (b) around a specific value (c) so that oxygen excess is achieved 19. Monoliths are used in catalytic converters of automobiles mainly because (a) they provide large surface at small volume and low loss of power (b) of their low cost (c) of their activity 20. Catalytic oxidation is preferable to adsorption, in VOC abatement, if (a) the temperature is below 150°F (b) intermittent loads are involved (c) high concentrations of gaseous pollutants have to be dealt with ANSWERS TO MULTIPLE CHOICE QUESTIONS 1. (a) physical sorption 2. (a) the competitiveness between repulsion and the van der Waals force 3. (d) its pore structure 4. (b) mesopore 5. (c) take up charged ions from a solution and release an equivalent amount of other ions to the solution 6. (a) accelerate chemical reactions 7. (b) affecting each reaction to a different extent 8. (a) weakening the chemical bonds of the reactants on its surface 9. (a) Keyser and Berzelius 10. (c) grouped together as sorption for a unified treatment in practical applications 11. (a) possible for temperatures up to 200 °C 12. (c) activated carbons 13. (a) a risk of bed fires in the VOC abatement 14. (b) if materials being removed are mostly metals 15. (b) activated carbon 16. (c) for the treatment of nuclear waste solutions 17. (c) for both industrial production and environmental applications 18. (b) around a specific value 19. (a) they provide large surface at small volume and low loss of power 20. (c) high concentrations of gaseous pollutants have to be dealt with 56 2. Adsorption, Ion Exchange, and Catalysis Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:32 AM Page 56 – 3 – Heterogeneous Processes and Reactor Analysis 3.1 INTRODUCTION TO HETEROGENEOUS PROCESSES In the relevant literature, many definitions of reaction rates can be found, especially in the case of catalytic systems. Depending on the approach followed, a catalytic reaction rate can be based on catalyst volume, surface, or mass. Moreover, in practical applications, rates are often expressed per volume of reactor. Each definition leads to different manipulations and special attention is required when switching from one expression to another. In the follow- ing, the various forms of catalytic reaction rates and their connection is going to be presented. Starting from the fundamental rate defined per active site, the reader is taken step –by step to the rate based on the volume of the reactor and the concept of the overall rate in two- and three-phase systems. The analysis in this chapter mainly concerns catalytic reactions. However, the basic prin- ciples are applicable to any heterogeneous process, though with different terminology and levels of importance. Concerning adsorption and ion exchange, only the reaction rate per unit mass of solid phase (rm) and per unit volume of reactor (R) are used in practice, whereas the concepts analyzed in the overall rate and rate-controlling sections are equally applicable to ion exchange and adsorption. 3.1.1 Reaction rate in heterogeneous catalysis: from active sites to reactor level Fundamental—Active site level As mentioned in Chapter 2, a catalytic reaction is not catalyzed over the entire surface of the catalyst but only at certain active sites (Fogler, 1999). Then, the reaction rate of any reaction component i at a fundamental level for catalytic reactions can be defined with respect to active sites as follows: (3.1)r n N tt 1 d d 57 Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 57 where: N the moles of reactant that appear in the reaction t time n the number of active sites on the catalyst surface. This rate is usually referred to as the turnover frequency and it is the number of mole- cules reacting per active site per unit time at the conditions of the experiment (Boudart, 1985; McNaught and Wilkinson, 1997; Fogler, 1999). Boudart (1995) used the term “turnover frequency” to define the number of revolutions of the catalytic cycle per unit time and active site. In each revolution, one mole of reactant is consumed. For example, the revolution of a catalytic cycle for SO2 oxidation is shown in Figure 3.1. Frequently, the number of active sites is expressed in mole units (the number of active sites divided by the Avogadro number) and thus, turnover frequency is found in s-1 units. For a specific reaction, the turnover frequency depends on the nature of the catalytic active site, the temperature, and the reactants’ concentration. The above-defined catalytic rate could be described as an “active-site level” rate. Following the reaction rate definition of the form given in eq. (3.1), if component i is a reaction product the rate is positive; if it is a reactant that is being consumed, the rate is negative; thus, the rate of disappearance of the reactant is –rt. In environmental applica- tions, as we are interested in the disappearance of a pollutant, the rate is expressed as –r, which is positive. The rate of disappearance is used in Chapters 3 and 5, where for sim- plicity it is referred to as the reaction rate. Catalyst level—active site plus support The rate of a catalytic reaction as defined above exhibits a great disadvantage: the number of the active sites is unknown and cannot be easily determined from common experiments. The difficulties associated to the measurement of active sites leads, for the time being, to the use of “catalyst level rates,” in most practical applications. Specifically, the most common reaction rate types used are expressed per unit vol- ume of the solid phase (rvs), per unit surface of the solid surface (rs) or per unit mass of the solid 58 3. Heterogeneous Processes and Reactor Analysis Figure 3.1 Revolution of a catalytic cycle for SO2 oxidation. metal oxidation metal reduction SO2½ O2 SO3 Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 58 considered a given property for a specific catalyst. Then, if the rate coefficient kt is con- stant, the rate coefficients km, kvs and ks are constants too. The previously reported relationship (eq. (3.10)) manifests one more important charac- teristic of the catalyst level rate coefficients—for the same reaction, temperature, catalytic agent, and support but for different surface arrangement, i.e. active site concentration, these coefficients will be different; and this is an advantage of the usage of kt, and in gen- eral, of turnover frequency (active site level reaction description). In the case of non-porous spherical particles, (3.11) (3.12) where Sex denotes the external surface area. Thus, (3.13) Then (3.14) This result means that the number of active sites per unit mass of catalyst is not constant since it depends on the particle size. Again, the term nSex can be considered to be a con- stant property for a given catalyst, prepared by the same technique, for all catalyst sizes. Then (3.15) Then, if the rate coefficient kt is constant, ks is constant too, whereas km and kvs are not con- stants and are dependent on the particle size of the catalyst (eq. 3.14). The anatomy of rate coefficient In reactions where the rate is expressed as ri ki f (C), the rate coefficient will often depend on the concentrations, because the latter expression does not take into account the interactions between molecules in a reaction mixture that is thermodynamically nonideal (Froment and Bishoff, 1990). In such a case, if the concentrations are substituted by activ- ities, the rate coefficient is merely independent of the concentration of the reacting species, but one should keep in mind that it is still not truly a constant (Fogler, 1999). 1 d d 1 p S vs S ex S S mn N t r M n k kt C S n k kt C M n k kt t S     CS n M S M n S 6 d n Ss ex s ex p p ex S S M ds ex S p p 6 M V d S p s p p 3 6 S S d ex p 2 3.1 Introduction to Heterogeneous Processes 61 Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 61 The rate coefficient is almost always dependent on temperature. However, it can be influ- enced by total pressure, in both gas and liquid systems, plus ionic strength and solvent in liq- uid systems. Following Fogler (1999), in the present book, the rate coefficient is considered to be a function of only temperature, assuming that the effect of other variables is much less. Reactor level—Catalyst plus reactor arrangement The principal difference between homogeneous and heterogeneous reaction rates is that the latter is based on mass, volume, or more rarely, on the area of the solid and not on the fluid- phase volume or reactor volume. The reactor volume or liquid-phase volume is of second- ary significance in heterogeneous reactions since the reaction takes place on the solid rather than throughout the reactor volume. Moreover, the mass of the solid is usually used instead of the solid volume or surface, because it is the most easily measured property. However, for purposes of mass balance in reactors, the following rates have to be also considered: the rate of reaction per unit volume of the fluid phase (ru) and per unit volume of reactor (R), defined as follows (Levenspiel, 1972): (3.16) (3.17) where: VL the fluid volume VR the reactor volume. So, rm, rvs, rs, and rt are the appropriate rates for expressing the intrinsic catalytic reac- tion rate, whereas ru and R are phenomenological rates, used for reactor design. More specifically, ru is also called the “pseudo-homogeneous rate” (Schmidt, 2005). For these rates, the following is valid: (3.18) The overall rate of reaction (R) per unit volume of the reactor is (Levenspiel, 1972) (3.19) The design of a reactor is connected to certain preferred parameters and it is useful to know how they are related to each other. For instance, it is very important to use the appropriate terms in order to correlate the reactor volume to the fluid and solid volumes. In Table 3.1, the most important ratios per reactor are presented. VR denotes the total volume of the reactor, VS denotes the volume of the solid, and VL is the fluid volume in two-phase systems and the liquid-volume in three phase systems. R V N t V V r S V r V V r M V r 1 d dR S R vs R s L R u S R m r V N t V V r S V r M V ru L S L vs L s S L m 1 d d R V N t 1 d dR r V N tu L 1 d d 62 3. Heterogeneous Processes and Reactor Analysis Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 62 Another critical issue that definitely needs to be clarified is the surface of the catalyst per unit volume of reactor or fluid, which is used in reactor analysis. The total surface area of a catalyst includes the internal and the external surface areas. Thus For fixed-beds: • for porous particles, (3.20) • for nonporous particles, (3.21) S V ex R u S V S M V S R s s R s b 3.1 Introduction to Heterogeneous Processes 63 Table 3.1 The most important ratios per reactora Reactor Subtype Reaction Other rateb Fixed Two-phase R 1 – b bed and Trickle bed Slurry Bubble ru hs hL Columns and Agitated Vesselsc Bubble R b bubb bub – Fluidized phase beds (Two-phase Emulsion R (1 – bub) (1 – fm) (1 – bub) fm – model) phase – – Fluidized All phases ru i – – – beds (L-K model) For all – – – – – Reactors a is the fixed-bed porosity (voidage), b is the bulk density of solids, p is the particle density, ms is the mass of solid per unit volume of bubble-free liquid in slurry reactors, hS and hL are the fractional solid and liquid hold- up in slurry reactors, i is the volume of a specific phase per unit volume of the fluid bubbles phase in fluidized beds, bub and fm are the fraction of the bed occupied by fluid bubbles and the bed voidage at minimum flu- idization state in fluidized beds, respectively. bThe reaction rate that is most commonly used in the analysis of the corresponding reactor type or model. cThree- (slurry) and two-phase systems. Ms VR VL VR Vs VR Vs VL 1 ms p M V mS L s 1 fm fm M V S S p Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 63 metals, if the same value of turnover frequency is obtained for a specific reaction at fixed conditions on two catalytic samples containing different amounts of metal on the porous support, the kinetic data are not obscured by heat or mass transfer phenomena. 3.1.2 The concept of the overall reaction rate in heterogeneous reactions General In heterogeneous reactions, phase boundaries exist between phases and transport processes; the intrinsic rate of reaction should be taken into account simultaneously in reactor design. The combination of mass transfer rates and reaction rates leads to the so- called overall rate. The goal is to express the global rate in terms of the bulk properties of the phases, eliminating the interphase properties. If the overall phenomenon requires that a number of steps take place in series, then, at steady state, all these steps will proceed at the same rate, which is equal to the overall rate (Levenspiel, 1972): (3.28) There are cases, as in catalysis, where some steps are in parallel. In these systems, the overall rate is greater than the rate of each individual step. If these steps are independent of each other, the overall rate is the sum of all individual rates (Levenspiel, 1972). (3.29) The elimination of the interphase concentrations could be done easily if the rate expres- sions of all steps are linear in concentration. However, for nonlinear expressions, it is difficult to evaluate and handle the overall rate. We will examine some simple cases in two- and three-phase systems. Two-phase systems In the case of two fluids, two films are developed, one for each fluid, and the corresponding mass-transfer coefficients are determined (Figure 3.2). In a fluid–solid system, there is only one film; whereas the resistance within the solid phase is expressed by the solid-phase dif- fusion coefficient, however, in many cases an “effective” mass-transfer coefficient is used in the case of solids as well. Consider the irreversible catalytic reaction of the form Here, we consider the general case of a porous catalyst, where the internal diffusion effect is included in the effectiveness factor (s). The intrinsic rate of reaction per unit mass of catalyst is (in mol/m2s) (3.30)( ) 1 d ds s s s r S N t k C A(g) B(g) r ri n n overall 1 ∑ r r r rnoverall 1 2 ... 66 3. Heterogeneous Processes and Reactor Analysis Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 66 This is the rate of disappearance of the reactant A and S the total surface of the catalyst, m2 ks the rate coefficient, m 3/m2s Cs the concentration of reactant at the outer surface of the catalyst particle, mol/m3. The rate of mass transfer from the bulk of the gas to the catalyst surface is: (3.31) where: kg the mass transfer coefficient in the gas film, m/s CG the concentration of reactant in the bulk gas phase, mol/m 3. At steady state, (3.32) From the equality of the rates, the concentration of reactant at the outer surface of the cat- alyst particle can be expressed in terms of bulk concentration: (3.33) Then, the overall reaction rate (rov) can be also expressed in terms of the bulk concentra- tion of the reactant: (3.34) where kov is the overall coefficient in m/s. r r r k k k k C k Cov s g s s g g s s G ov G( ) C k k k Cs g g s s G ( ) 1 d d ( )s g s s s g G s r r S N t k C k C C r k C Cg g G s( ) 3.1 Introduction to Heterogeneous Processes 67 Figure 3.2 Two-phase system. R δ C Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 67 The reciprocal of this coefficient is referred to as total resistance: (3.35) It should be noted that the overall coefficient (kov) has been derived under the assumption that the reaction is taking place mainly in the internal surface of the catalyst. See Chapter 5 for a more rigorous analysis on the subject. The concept of the rate-controlling step When a process consists of several individual steps in series, the concept of the rate- controlling step simply states that one of the several steps involved provides the major resistance to the overall process. In such a case, this slow step is termed the “rate- controlling step” and can be considered as acting alone (Levenspiel, 1972). Instead of molecules flowing, consider water flowing through a pipe with a number of partially opened valves. The flow of the water in the pipe will be determined by the valve that offers the largest resistance to the flow. Actually, we could come up with a fairly accu- rate estimate of the flow by calculating the resistance to the flow in this valve, neglecting all the others. Consider the first-order reaction analyzed in the previous paragraph. In the limiting case where ks → ∞ or ks kg, the resistance to the overall rate is due to the gas film around the catalyst and Cs → 0. The rate-controlling step is the diffusion in the gas film and the overall rate is (3.36) On the other hand, if kg → ∞ or kg ks, the resistance to the overall rate is owing to the intrinsic reaction rate and CG → C s. The rate-controlling step is the reaction rate and the overall rate is (3.37) Note that due to the equality of the individual rates, if ks → ∞ then Cs → 0, and if kg → ∞ then (CG – Cs) → 0 or CG → Cs; and so, the individual rates are finite and equal to the over- all rate. It is the resistance of the individual step and not the corresponding rate that could be zero under certain operating conditions. The concept of the rate-controlling step is much more useful in complex kinetic expres- sions, where the overall rate is nonlinear and cannot be obtained by following a simple pro- cedure as presented above for the case of a first-order reaction. For example, consider a second order reaction. In this case, the intrinsic reaction rate is (3.38) while the rate of mass transfer from the bulk of the gas to the catalyst surface is (3.39)r k C Cg g G s( ) ( )s s s 2 r k C ( )s s s G r k C ( )s g G r k C 1 1 1 ov g s sk k k 68 3. Heterogeneous Processes and Reactor Analysis Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 68 where Kov is an overall coefficient (in m/s): (3.52) It is noteworthy that not all the resistances are significant in every case. If only a pure gas constitutes the gas phase and for slightly soluble gases, the resistance to the mass transfer on the liquid side of the interface is predominant. Under these conditions, CG CG,i and the above equation reduces to (3.53) Even when the gaseous reactant is in a mixture with other components in the bubbles, kg appears to be much larger than kfg/H and thus, the last equation is applicable. Derivation of an overall gas transfer rate In many three-phase systems, the two resistances in the gas–liquid interface are combined in one overall gas mass transfer coefficient KL. To do this, we combine the following rates: (3.54) (3.55) If equilibrium exists at the bubble–liquid interface, CG,i and CL,i are related by Henry’s law: (3.56) Then (3.57) The rate becomes (3.58) Defining an overall gas-phase mass transfer coefficient KL (in m/s), (3.59)K Hk k k Hk K Hk kL g fg fg g L g fg 1 1 1 r k C C Hk k k Hk C H Cfg fg L,i L g fg fg g G L( )     C k C k C k k H L,i g G fg L fg g C HCG,i L,i r k C Cfg fg L,i L( ) (bubble interface to bulk liquid) r k C Cg g G G,i( ) (bulk gas to bubble interface) 1 1 1 1 ov fg f sK H k k ks       1 1 1 1 ov g fg f s sK k H k H k k     3.1 Introduction to Heterogeneous Processes 71 Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 71 we have (3.60) where CL,eq is the equilibrium concentration in the liquid: (3.61) This is the concentration in equilibrium with the bulk gas concentration CG. It is important to note that in the general case, (3.62) (3.63) Note that (3.64) (3.65) In the case of kg → ∞, or in other words, when there is no resistance in the gas phase (gas phase consists of a pure gas), (3.66) (3.67) (3.68) Finally, if the liquid is saturated with gas, (3.69) 3.2 HETEROGENEOUS REACTORS 3.2.1 Introduction Chemical reactors vary widely in shape and in the mode of operation. Consequently, there are various ways of classifying them. The first classification is based on the number of the C CL,eq L K kL fg C CG,i G C C H CL,i G L,eq C C HG,i L,i C C k k C k k H L,i G fg g L fg g ( ) C CG G,i C CL,eq L,i C C HL,eq G r r K C Cfg g L L,eq L( ) 72 3. Heterogeneous Processes and Reactor Analysis Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 72 involved reacting phases— • homogeneous reactors, where one phase such as liquid or gas exists in the reactor • heterogeneous reactors, where two or three distinct phases coexist in the reactor. A further classification of homogeneous and heterogeneous reactors is based on the nature of the involved reacting phases— • homogeneous reactors • liquid-phase reactors • gas-phase reactors • heterogeneous reactors • liquid–solid (L–S) reactors • gas–solid (G–S) reactors • liquid–gas (L–G) reactors • liquid–gas–solid (L–G–S) reactors Finally, classification could be based on the contacting pattern of the involved reacting phases, as it is described in the following sections. 3.2.2 Homogeneous reactors Plug-flow tubular reactor (PFTR): This reactor is operated under steady-state condition. The reactor is of tubular shape, the reactants enter at the inlet and the composition is a function of the distance from the inlet. However, the composition is not a function of time. The ideal plug-flow reactor is characterized by the absence of mixing in the direction of flow and complete mixing in the transverse direction. Continuous-stirred tank reactor (CSTR): This reactor is operated under steady-state condition. The reactants flow continuously in and out of the vessel at a constant flow rate and are perfectly mixed by mechanical means, and thus the composition is the same throughout the reactor. The result is that the exit concentration is the same as the one in the reactor. The concentration is constant, i.e. is not time-dependent. Batch-stirred tank reactor (BSTR): In this type of reactor, the reactants are fed into the container, they are well mixed by means of mechanical agitation, and left to react for a cer- tain period of time. This is an unsteady-state operation, where composition changes with time. However, the composition at any instant is uniform throughout the reactor. 3.2.3 Heterogeneous reactors Gas–liquid heterogeneous reactors Gas–liquid continuous-stirred tank reactor: This is a CSTR, where the liquid and gas phases are mechanically agitated (Figure 3.4). 3.2 Heterogeneous Reactors 73 Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:44 PM Page 73 Fluidized-bed reactor (FLBR): The up-flow gas or liquid phase suspends the fine solid particles, which remain in the reactor (Figure 3.8). This reactor is of tubular shape with a relatively low aspect ratio of length to diameter. The most common application of FLBR is the classical FCC process. Entrained flow reactor (riser): This is a fluidized-bed reactor in which the solid is entrained by the fluid phase and is recycled throughout the operation (Figure 3.9). Some applications of this reactor type are the modern FCC process and the calcination of alumina hydrate. Three-phase heterogeneous reactors Three-phase reactors are generally needed in cases where there are both volatile and non- volatile reactants, or when a liquid solvent is necessary with all reactants in the gas-phase (Smith, 1981). Some examples are 76 3. Heterogeneous Processes and Reactor Analysis fluid solid particles Figure 3.8 The fluidized-bed reactor. gas Figure 3.9 The entrained flow reactor. Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 76 • hydrogenation of oils • oxidation of liquids • oxidation of pollutants dissolved in liquids • polymerization reactions. Fixed-bed reactors Trickle-flow reactor (TFR): This is a tubular flow reactor with a concurrent down-flow of gas and liquid over a fixed-bed of catalyst (Figure 3.10). Liquid trickles down whereas the gas phase is continuous. This reactor is mainly used in catalytic applications. Typical application examples of this reactor type are the following: HDS of heavy oil fractions and catalytic hydrogenation of aqueous nitrate solutions. Packed Bubble Bed Reactor (BBR): This is a tubular flow reactor with concurrent up- flow of gas and liquid (Figure 3.11). The catalyst bed is completely immersed in a contin- uous liquid flow while gas rises as bubbles. Some applications of BBR are the catalytic denitrification of aqueous nitrate solutions and the hydrogenation processes. Reactors with moving solid phase Three-phase fluidized-bed (ebullated-bed) reactor: Catalyst particles are fluidized by an upward liquid flow, whereas the gas phase rises in a dis- persed bubble regime. A typical application of this reactor is the hydrogenation of residues. 3.2 Heterogeneous Reactors 77 L G G L Figure 3.10 The trickle-bed reactor. L L G G Figure 3.11 The packed bubble bed reactor. Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 77 Slurry Bubble Column Reactors (SBCR): This reactor is tubular (Figure 3.12). The liquid is agitated by means of dispersed gas bubbles. Gas bubbles provide the momentum to suspend the catalyst particles. The gas phase flows upward through the reactor at a constant rate. This reactor could be of continuous type or of semibatch type. This type is used only in catalysis. Agitated slurry reactor (ASR): This is a mechanically agitated gas–liquid–solid reactor (Figure 3.13). The liquid is agitated by a mechanical apparatus (impeller). The fine solid particles are suspended in the liquid phase by means of agitation. Gas is sparged into the liquid phase, entering at the bottom of the tank, normally just under the impeller. This reac- tor can also be of continuous type or of semibatch type. This type is used only in catalysis. 3.3 TWO-PHASE AGITATED REACTORS The analysis of this type of reactor requires a uniform composition of fluid phase through- out the volume. While this is easily achieved by standard agitation devices for liquid–solid systems, i.e. impellers, it requires special design to be achieved for gas–solid systems. This type of reactor is basically used for laboratory experimentation. 78 3. Heterogeneous Processes and Reactor Analysis L L G G Figure 3.12 The slurry bubble column reactor. Figure 3.13 The agitated slurry reactor. L L G G Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 78 heel in solid suspension applications. Moreover, they are effective in laminar flow applica- tions, especially when impeller Reynolds numbers drop below 50. Disc-type turbines In this type of turbines, blades can be straight or curved. In the related literature, the term “straight” is some times replaced by the term “flat.” The most popular turbine of this type is the straight-blade disc turbine, which is better known as the “Rushton turbine” (Figure 3.18). The same turbine is also called “flat-blade turbine, vaned disc” or simply “flat-blade turbine.” This type is a good cost-effective impeller for low concentrations of immiscible liquid or gas. Two very strong trailing vortices are shed from each blade. These areas of high shear are responsible for breaking the larger droplets to smaller droplets. Maximum aera- tion numbers should be limited to 0.1. Like all radial-flow impellers, the Rushton turbine is designed to provide the high shear conditions required for breaking bubbles and thus increasing the oxygen transfer rate. 3.3 Two-Phase Agitated Reactors 81 Figure 3.16 Pitched-blade turbine. Figure 3.17 Left: Straight-blade open turbine. Right: Curved-blade open turbine. Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 81 Vertical blade disc turbines can have “curved” blades, and in this case they are called “Smith” turbines. Flow patterns in agitated vessels Unbaffled tanks If a low-viscosity liquid is stirred in an unbaffled tank by an axially mounted agitator, there is a tendency for a swirling flow pattern to develop regardless of the type of impeller (Perry and Green, 1999). Figure 3.19 shows a typical flow pattern. A vor- tex is produced owing to the centrifugal force acting on the rotating liquid. In spite of the presence of a vortex, satisfactory process results often can be obtained in an unbaffled ves- sel. However, there is a limit to the rotational speed that may be used, since once the vortex reaches the impeller, severe air entrainment may occur. The so-called surface aeration is undesirable due to its negative effect on the mass transfer coefficients (see Section 3.5.3). In addition, the swirling mass of liquid often generates an oscillating surge in the tank, which coupled with the deep vortex, may create a large fluctuating force acting on the mixer shaft. The drawing of gas into liquid is frequently undesirable, in addition, vortex formation leads to difficulties in scaling up, so that steps are usually taken to prevent vor- tices (Treybal, 1980) (Figure 3.20). Baffled tanks In this case, the tank is supplied with baffles that are flat vertical strips placed radially along the tank wall so that adequate agitation of thin suspensions can be achieved, as shown in Figure 3.21. Usually, four baffles are enough. A common baffle width is one-tenth to one-twelfth of the tank diameter (radial dimension). In the agitation of slur- ries, the accumulation of solids near the walls or baffles has to be avoided. It can be pre- vented by placing the baffles at a distance that is half their width, from the vessel wall . For Reynolds numbers greater than 2000, baffles are commonly used with turbine impellers and with on-centerline axial-flow impellers. The use of baffles results in a large top-to-bottom circulation without vortexing or severely unbalanced fluid forces on the impeller shaft. In the transition region (10 NRe 10,000), the width of the baffle may be reduced to one-half the standard width. In the case that the circulation pattern is satisfactory in an 82 3. Heterogeneous Processes and Reactor Analysis Figure 3.18 Rushton impellers. Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 82 3.3 Two-Phase Agitated Reactors 83 Figure 3.20 Typical flow pattern for a noncentered impeller. liquid level Figure 3.19 Typical flow pattern in an unbaffled tank. Figure 3.21 Typical flow pattern in a baffled tank. Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 83 where: (3.74) and ru is the overall rate of reaction (disappearance) per unit volume of the fluid phase. Note that this is identical to the definition of the (homogeneous) reaction rate. This is because the restrictions of uniform concentration and temperature are satisfied in an ideal BSTR and the volume has been assumed to be constant. Since BSTR is used mainly for liquid-phase reactions, the latter assumption usually holds. It should be noted here that while in catalytic systems the rate is based on the moles dis- appearing from the fluid phase – dC/dt, and the rate has the form (ru) f (k, C), in adsorption and ion exchange the rate is normally based on the moles accumulated in the solid phase and the rate is expressed per unit mass of the solid phase dq/dt where q is in moles per unit mass of the solid phase (solid loading). Then, the rate is expressed in the form of a partial differential diffusion equation. For spherical particles, mass transport can be described by a diffusion equation, written in spherical coordinates r : (3.75) where Ds is the solid diffusion coefficient and q is the solid-phase concentration of the solute. Finally, the rate of change of a species is related to the stoichiometry. For the general reaction of the form the rates are (Fogler, 1999) (3.76) Continuous flow reactors In the ideal CSTR, the fluid concentration is uniform and the fluid flows in and out of the reactor. Under the steady state condition, the accumulation term in the general material balance, eq. (3.70), is zero. Furthermore, the exit concentration is equal to the concentra- tion in the reactor. For a volume element of fluid (VL), the mass balance for the limiting reactant becomes (Levenspiel, 1972) (3.77) where F is the molar feed rate of the limiting reactant. Subscripts i and o denote the inlet and outlet parameters, respectively. In analogy to the batch reactor, (3.78)x F F F i o i F F r Vi o u L( ) 0 1 1 1 1 A B C Da r b r c r d r a b c dA B C D r q t D q t r q rm s 2 2 d d 2     x C C C i t i 86 3. Heterogeneous Processes and Reactor Analysis Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 86 then (3.79) Variable-volume systems A variable-volume batch reactor is a constant-pressure (piston-like) closed tank. On the other hand, a variable-pressure tank is a constant-volume batch reactor (Fogler, 1999). Thus, in batch reactors, the expansion factor is used only in the case of a constant-pressure tank whereas and not in a constant-volume tank, even if the reaction is realized with a change in the total moles. However, in continuous-flow reactors, the expansion factor should be always considered. In the following section and for the continuous-flow reac- tors, the volume V can be replaced by the volumetric flow rate Q, and the moles N by the molar flow rate F in all equations. Change in the total moles in gas–solid reactions Consider a reaction of the form where all the reactants and products are gases and A is the limiting reactant. Then, based on the conversion level of A, initial moles, (3.80) final moles after a conversion level x of A, (3.81) (3.82) (3.83) (3.84) and thus, (3.85) where xA is the conversion of the limiting reactant A, (3.86)x N N NA A,i A,o A,i N N x Ntot tot,i A A,i N N d a x ND D,i A A,i N N c a x NC C,i A A,i N N b a x NB B,i A A,i N N x NA A,i A A,i N N N N Ntot,i A,i B,i C,i D,i a b c dA B C D F x r Vi A u L( ) 3.3 Two-Phase Agitated Reactors 87 Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 87 and is defined as (3.87) This parameter can be termed as “fractional mole change.” Note that in the calculation of , inerts that are involved in the gas phase are not taken into account. They are taken into account only in the determination of the total moles of the reacting system. The expansion factor R is defined as (Fogler, 1999) (3.88) Then (3.89) Since (3.90) where Z is the compressibility factor. In practice, the compressibility factor does not change significantly during the course of reaction, and thus Z ≈ Zi. Then, the volume change in the case of nonisothermal and nonisobaric operation is (Fogler, 1999) (3.91) For a constant volume container (batch reactor), V Vi and thus, eq. (3.91) can be used to calculate the pressure inside the reactor as a function of temperature and conversion. Under constant P and T, eq. (3.91) becomes (3.92) In this relationship, Vi is the initial (feed) volume of the gas. This is the case of Levenspiel’s simplification where the volume of the reacting system varies linearly with conversion (Levenspiel, 1972). The last equation shows that even if we have a change in moles (R 0), if the conversion of the limiting reactant is very low, the volume of the reaction mixture could be taken as constant and R is not involved in the solutions of the models (since RxA can be taken as approximately zero). V V x i R A1 V V x P P T Ti R A i i (1 ) N PV ZRT N N xtot tot,i R A1 R tot,( 1) tot,i tot,i A,i tot,i N N N N N x c d a b a 88 3. Heterogeneous Processes and Reactor Analysis Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 88 Then Example 3 Consider the gas-phase reaction which is carried out isothermally and isobarically. The reaction rate is first order in A and first order in B. The feed concentration of A and B is 0.5 mol/L. Express the rate of reac- tion –rA solely as a function of conversion, evaluating all possible parameters involved. Solution First of all, we have to determine which the limiting reactant is. The way to determine which reactant is limiting is to divide the moles of each reactant by the coefficient from the balanced equation associated with that reactant. The smallest number that comes out indicates which reactant is the limiting one. This reactant limits how much of every other species made or needed for the reaction. For A, this calculation gives 0.25 and for B, 0.5. Thus, A is the limiting reactant and the calculation of , R, and x should based on it. The parameter is The expansion factor R is The moles of A and B after a conversion level of A equal to x are The concentrations of A and B are C N V x C x x x x C N V A A i R A A,i A R A A A B B i R (1 ) 1 1 0.5 1 1 0.25 (1 x C x C x x x xA B,i R A A,i A R A A A) 1 2(1 ) 0.5 1 0.5 1 0.25 N N x N N N x A A,i B B,i A,i (1 ) 2 R N N A,i tot,i 0.5 1 1 2 0.25     c d a b a 2 0 2 1 2 1 2 2A B 2 C C C x x C x C SO2 SO2,i SO2 SO2 O2 O2,i SO2 SO2,i 1 1 0.015 C 1 0.015 0.5 x x SO2 SO21 0.015 3.3 Two-Phase Agitated Reactors 91 Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 91 where xA the conversion of A Then Expansion factor in gas–solid adsorption systems The meaning of the expansion factor is the same in gas adsorption. Consider the adsorption of a species (A) from the gas phase. where A(s) denotes that the solute (A) is in the solid phase. Although this is not a reaction, it has the same result; the removal of the solute from the gas phase is equivalent to the con- sumption of a species in a reaction. For such a case, A(s) is not taken into account for the volume change in the gas, and thus R 1, and in the absence of inerts, R 1; whereas if the inerts are in great excess, R ≅ 0. This result is similar to the one in three-phase systems, as presented in Section 3.4.5. 3.3.4 Space-time and space velocity in flow reactors Space-time in flow reactors The time required to process one reactor volume of feed at specified conditions is called “space-time” and is defined normally at actual entering conditions (Levenspiel, 1972). (3.98) where: Qi the volumetric flow rate in the entrance of the reactor VR the reactor volume. Space-time is commonly referred to as “mean residence time,” “holding time,” or simply “residence time.” However, for a system with expansion (variable density system), these quantities are not equal and the residence time is a variable (Levenspiel, 1972): (3.99)t V Q V Q xm,z R z R i R(1 ) V Q R i A (g) inerts A (s) ( ) 0.5 1 1 0.25 0.5 1 0.5 1 0.25A A B A A A A r kC C k x x x x             0.25 (1 )(1 0.5 ) (1 0.25 ) A A A 2 k x x x x N N NA A,i A,o A,i 92 3. Heterogeneous Processes and Reactor Analysis Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 92 where: Qz the volumetric flow rate in the reactor at length z x the conversion of the limiting reactant at the same length R the expansion factor. In fixed-beds, the space-time as defined above is the superficial space time, as it is based on the total volume of the bed. The real space-time for a bed of bed voidage is (3.100) Another important feature related to mean residence time in fixed-beds is the fluid holdup based on the empty bed volume he,t. If the fluid occupies the whole empty bed volume, then he,t . However, this is not the case when he,t , i.e. when there is a bypass of the fluid from some regions in the bed, most commonly in the upper section of the bed in a downflow operation, and the fluid is a liquid. In this case, the real residence time is (3.101) The actual residence time of a reactor is measured by employing residence time distribu- tion (RTD) experiments utilizing tracing techniques. Furthermore, several correlation forms estimating the fluid holdup can be found in the related literature. Space velocity in flow reactors The number of reactor volumes of feed at specified conditions, which can be treated in a unit time is called “space velocity” and is (Levenspiel, 1972) (3.102) This parameter is frequently used in ion-exchange and adsorption operations in fixed-beds and it is frequently called “relative volumetric flow rate”: (3.103) The most common unit of Qrel is bed volumes per hour (BV/h). Space velocity is also used in catalytic reactors, especially in three-phase fixed-beds, and is referred to as liquid hourly space velocity (LHSV) for the liquid phase, and gas hourly space velocity (GHSV) for gas phase. As mentioned above, space-time and space velocity are measured under the entrance conditions. However, for space velocity, other conditions are frequently used (Fogler, 1999). For example, the LHSV is measured at 60 to 75 °F, and GHSV at standard temperature and pressure. s Q V Q V V Q i R rel R R rel s Q V 1 i R t h V Qm e,t R i V Q R i 3.3 Two-Phase Agitated Reactors 93 Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:45 PM Page 93