1211 V VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS The toxic gases produced during combustion and other chemical processes may be removed by destructive dis- posal, dispersive dilution or as recoverable side products. The removal path chosen is at the present time motivated primarily by economics, but public pressure and aware- ness of environmental problems also influence the choice. This section will concern itself with destructive dis- posal and/or various recovery processes, the subject of dis- persion being ably handled in the sections on Air Pollution Meteorology and Urban Air Pollution Modeling. The main emphasis will be on principles of gaseous reaction and removal with the description of equipment for air pollu- tion abatement covered by pollutant. Problems specifi- cally concerned with the automobile can be found under Mobile Source Pollution. Although the control principles to be described below are general, it is usually necessary to design equipment for each installation because of varia- tions in physical and chemical properties of effluents; also, in general, the cost of adding pollution devices to an existing unit (retrofit) will be higher than if they were placed in the original design, because of construction difficulty and downtime. Although the majority of effluent material from com- bustion occurs in the gaseous state, it is important to char- acterize the total effluent stream for control purposes. For example, the effluent may be condensible at operating temperature (a vapour) or noncondensible (a gas), but it usually is a mixture of the two. Particulate matter (solids) and mists (liquids) are often suspended in the gaseous stream; if the particles do not separate upon settling they are called aerosols. The considerations in this section deal with gas or vapor removal only and not with liquid or solid particle removal. SULFUR DIOXIDE, SO 2 , AND TRIOXIDE, SO 3 Sulfur dioxide is generated during combustion of any sulfur- containing fuel and is emitted by industrial processes that use sulfuric acid or consume sulfur-containing raw material. The major industrial sources of SO 2 are sulfuric acid plants, smelt- ing of metallic ores, paper mills, and refining of oil. Fuel com- bustion accounts for roughly 75% of the total SO 2 emitted. Associated with utility growth is the continued long term increase in utility coal consumption from some 650 million tons/year in 1975 to between 1400 and 1800 million tons/year in 1990. Also the utility industry is increasingly converting to coal. Under the current performance standards for power plants, national SO 2 emissions are projected to increase approx- imately 15 to 16% between 1975 and 1990 (Anon. 1978). The SO 2 emitted from power plants is usually at low concentration (0.5% by volume). However, a 900 MW unit will emit over 13,000 pounds of SO 2 per hour for a 1% sulfur coal. The SO 2 emitted from industrial processes is at higher concentrations and lower flow rates. The emitted SO 2 combines readily with mists and aerosols, thus compounding the removal problem. Information concerning emissions standards is essential to pollution control engineering design. The current US fed- eral SO 2 emissions limits for a stack are 1.2 lb/10 6 BTU for new oil and gas fired plants. Also, uncontrolled SO 2 emis- sions from new plants firing solid, liquid, and gaseous fuels are required to be reduced by 85%. The percent reduction requirement does not apply if SO 2 emissions into the atmo- sphere are less than 0.2 lb/10 6 BTU. Flue gas desulfurization (FGD) methods are catego- rized as nonregenerable and regenerable. Nonregenerable processes produce a sludge that consists of fly ash, water, calcium sulfate and calcium sulfite. In regenerable pro- cesses, SO 2 is recovered and converted into marketable by-products such as elemental sulfur, sulfuric acid or con- centrated SO 2 . The sorbent is regenerated and recycled. The US Environmental Protection Agency believes the following types of FGD systems are capable of achieving the emissions limit standards: lime, limestone, Wellman- Lord, magnesium oxide and double alkali. Due to the pro- cess economics, utility industry prefer the lime/limestone systems. Limestone processes constitute about 58% of the current calcium-based capacity in service and under con- struction, and 69% of that planned, which amounts to 63% of C022_001_r03.indd 1211 11/18/2005 2:32:34 PM © 2006 by Taylor & Francis Group, LLC 1212 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS the total (De Vitt et al., 1980). The following reactions take place in the limestone systems: CaCO(s)CaCO(aq) 2CaCO(aq)2HCaCHCOCa SO(g)SO(aq 33 33 2 22 � � �   )) SO(aq)HOHSOHSOH HSOO(aq)SOH CaSO 22233 3 1 2 24 2 2       → → � 44 2 44 CaSO(aq)CaSO(s)  �→↓ Or Ca2ClCaCl(aq)CaCl(s) 2HSOHSO HClHCl Ca(H 2 22 4 2 24       � � � →↓ CCO)2HCa2HCO HCOCOHO 32 2 23 2322    � � The dissolution rate of limestone depends on the pH values. The pH values encountered in practical operations of limestone systems is in most cases between 5.5 and 6.5. If the limestone systems operate at too low pH, SO 2 removal efficiency will decrease. At too high pH, the scale formation will be promoted. Other factors affecting the performance of limestone systems include solids content, liquid-gas ratio, and corrosion. A discussion can be found elsewhere (De Vitt et al., 1980). In selecting the FGD processes, the following should be considered: 1) process type: wet or dry, regenerable or non- regenerable. 2) chemical reagent used. 3) end-product produced: saleable product or dis- posable waste. The reagent, end product, principle of operation, and SO 2 removal efficiency of major FGD processes are shown in Table 1 (Princiotta, 1978). For details of SO 2 removal refer to the Stack gas cleaning sections. Sulfur dioxide reacts slowly with a large excess of oxygen in the presence of sunlight to form trioxide. Gerhard (1956) showed that the process occurs with O 2 at a rate of 0.1–0.2% per hour and Cadle (1956) with ozone, O 3 , at 0.1% per day. Niepenburg (1966) illustrates the effects of oxygen in the waste gas during combustion of oil. The conversion of SO 2 to SO 3 is believed to be possible at realistic rates because of the presence, on diverse surfaces, of Fe 2 O 3 which acts as a catalyst. The SO 3 has a short lifetime since it readily combines with water vapor in the atmosphere to form sulfuric acid. Oxides of Nitrogen, NO x NO x is produced in all combustions which take place using air as an oxygen supply and in those chemical industries employ- ing nitric acid. More than 55% of the total NO x emissions of 20 million tons originate from stationary sources as shown in Figure 1, and 93% of all stationary source NO x emissions are from combustion of fossil fuels for utilities. Direct industry- related emissions account for only 5% of the stationary source total. Approximately 30% of all stationary source NO x is emit- ted by coal-fired utility boilers. Uncontrolled NO x emissions from coal-fired sources have been measured in the range 0.53 to 2.04 lb/10 6 BTU at full load (Ziegler and Meyer, 1979). The NO x formed in combustion is from fixation of atmospheric nitrogen and/or fuel nitrogen. Ermenc (1956) found that at high temperature nitrogen and oxygen combine to form both NO and NO 2 . The yield of NO increases from 0.26% at 2800F to 1.75% at 3800F. If the temperature is reduced slowly the reverse reaction will take place, but if the products are quenched by rapid heat exchange, the reverse reaction rate becomes small and the oxides remain in the exhaust stream. The oxide NO can usually be oxidized to form NO 2 according to: 2NOO2NO .2→2 Because this is a tri-molecular gas phase reaction, the con- centration of NO and NO 2 tremendously affects the rate at which the oxidation takes place. At low concentration, for example 1–5 ppm in air, the reaction is so slow that it would be negligible except for the photochemical reactions which take place in the presence of sunlight. The dioxide also reacts with oxygen to form ozone. The existence of nitrogen trioxide at low concentration in polluted atmospheres is postulated (Hanst, 1971) to form by the reaction with ozone. NOONOO2332� and remain in equilibrium with N 2 O 5 NONONO.2523� The NO 3 formation reaction only takes place after NO is sub- stantially depleted in the atmosphere and O 3 begins to appear. Without more stringent control of new sources, the NO x emissions by 1995 are projected to be 66% higher than than in 1985. Even with application of the best control method to all new sources there is still a projected 24% increase over 10 year emissions (McCutchen, 1977). The typical NO x emis- sion from nitric acid plants is 1000–3000 ppm. The federal standard for new nitric acid plants is 3 lb NO x /ton 100% HNO 3 —that is, about 200 ppm (Ricci, 1977). The most widely used process for nitric acid plant tailgas cleanup is catalytic decomposition of NO x to nitrogen and oxygen. The current and projected values of the New Source Performance Standards (NSPS) for NO x are discussed later in this article. During recent years N 2 O formation rates have been the subject of controversy, especially in fuel NO x mechanisms. C022_001_r03.indd 1212 11/18/2005 2:32:38 PM © 2006 by Taylor & Francis Group, LLC VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1213 Oxides of Carbon The carbon dioxide, CO 2 , concentration threshold for humans is 5% (5000 ppm) for an 8-hour exposure. This compares with a normal atmospheric CO 2 concentration of 0.03% (300 ppm). With a perfect stoichiometric combination of pure carbon in air, a CO 2 concentration of about 21% could be attained. Considering the usual dispersion of combustion gases it would take an unusual isolation to produce a CO 2 health hazard. A more detailed description of CO 2 consequences may be found in the Appendix. Incomplete combustion of fuels is the more serious problem, since carbon monoxide, CO, will form. This rarely happens in stationary furnaces for which efficiencies of combustion are high and oxygen is available in excess of the theoretical requirements. It has been estimated (Anon., 1970) that slightly more than 100 million tons of CO are emitted annually in the USA, of which the major sources are automobiles (59%), various open burnings (16%), chemical industry (10%) and other transpor- tation means (5%). New York City, with its acute urban traffic problem, has established a first alert at 15 ppm of CO over TABLE 1 Status of Commercial FGD Processes (Adapted from Princiotta, 1978) FGD Process Reagent End Product Principle of Operation SO 2 Removal Efficiency(%) Limestone Scrubbing Limestone (LS) CaSO 3 /CaSO 4 Sludge LS slurry reacts in scrubber absorbing SO 2 and producing insoluble sludge. 80–90 Lime Scrubbing Lime CaSO 3 /CaSO 4 Sludge Lime slurry reacts in scrubber absorbing SO 2 and producing insoluble sludge. 85–95 Wellman Lord Sodium carbonate (regenerated) Sulfuric Acid Sulfur Soluble sodium sulfite absorbs SO 2 in scrubber, the sodium bisulfite produced is thermally regenerated, yielding sodium sulfite and SO 2 for either acid or S production. 85–95 Double Alkali Sodium carbonate (regenerated) CaSO 3 /CaSO 4 sludge Soluble sodium sulfite absorb SO 2 in scrubber; the sodium bisulfite produced is reacted with lime precipitating CaSO 3 /CaSO 4 . 85–95 Magnesium Oxide Magnesium oxide (regenerated) Sulfuric Acid Magnesium oxide slurry absorbs SO 2 in scrubber; the magnesium sulfite produced is thermally treated, yielding MgO and SO 2 for acid production. 85–95 YIT XY  OUSSYX -EC USTTSYX xA XY  OUSSYX PYU I SYXI YMO -DDBD USTTSYX xA XY  OUSSYX PYU I SYXI MYULSYX YMO -DCBF USTTSYX xA RI LSXO UYLSTO YMO HG GC EG C HG GC EG C HG GC EG C I SYXI YMO MYULSYX SXNSIT xYMOO PSOTN L XSXR SM OXRSXO LYSTO SXMSXOIY FIGURE 1 Sources of NO x emissions. C022_001_r03.indd 1213 11/18/2005 2:32:39 PM © 2006 by Taylor & Francis Group, LLC 1214 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS at 8-hour period. High temperatures favor the equilibrium dissociation of CO 2 to CO, with the latter being very stable at high temperatures. Thus is a CO 2 –CO mixture is quenched from its high temperature zone the percentage CO may remain high, since at lower temperatures longer times are required to reach equilibrium. Rich fuel-air mixtures favor the formation of CO over CO 2 . A complete description of CO control meth- ods may be found in the section Mobile Source Pollution. Miscellaneous Gases Compounds of fluorine are known to have negative effects (Fluorisis) at concentrations as low as 5 × 10 −3 ppm. They are generated as waste gases of fertilizer aluminum and ceramic processes, but are present to a lesser extent in most flue gases. A concentration of 0.1 ppm (vol.) of fluorine has been set as a maximum permissible value by the American Conference of Governmental Industrial Hygienists; USSR standards are roughly one tenth as stringent. Ozone, O 3 , is one of the strongest gaseous oxidants and is formed naturally from oxygen during electrical discharges in the atmosphere and at the high temperatures of combustion. O 2O O O O 2 2 3 → ⋅ ⋅ → Taken as oxidants, New York City classified an ozone level above 0.03 ppm as unsatisfactory and above 0.07 ppm as unhealthy over a 6 hour period. Eye irritation commences at concentrations of about 0.1 ppm. Interestingly enough ozone in the lower stratosphere affords part of the protective shield against ultraviolet radiation from the sun, which could destroy land vegetation. O O + O . 3 2→ Some scientists are concerned that nitric oxide formed by supersonic jets may deplete the ozone supply in the lower stratosphere, eroding the barrier to the destructive rays. High temperature processes involving metal recovery from ores emit mercury vapor in addition to sulfur dioxide. Mercury is available at concentrations up to a few hundred ppm (Kangas et al., 1971) during zinc sulfide ore process- ing for example. Hydrochloric and hydrofluoric acids also appear in the roaster gases of such processes. No danger levels for mercury vapor have been officially established in ambient air quality standards. A few limits have been established for less common pollut- ants of the process industries in USSR standards given below. The US ambient air quality standards call for hydrocar- bon concentrations below 160 mg/m 3 (0.24 ppm) between 6–9 am. Aldehydes and other oxygenated hydrocarbons are formed by the action of ozone on unburned hydrocarbons in the presence of sunlight. For example, O 1-3 Butadiene A crolein Formaldehyde.3  → In the above reaction both products have been linked to the severe eye irritation encountered in urban environments. TRANSPORT OF POLLUTANTS The feed and waste materials of any combustion or chemi- cal process travel through ducts or pipes. Control devices may be placed at various stages of the process, depend- ing on the separation technique to be employed. It will be valuable to review the flow and transport behavior for fluids and then the separation methods. The important pro- cess variables to be considered are the mass flow rate of the waste gas, its temperature, pressure and composition. The raw material feed rate variables may also be of sig- nificance, as in the desulfurization of fuel oil. Control devices may broadly be classified according to the physi- cal separation process being used, adsorption: absorption: extraction: distillation: or to the chemical process, homoge- nous or heterogeneous catalytic reaction. In each instance, equations which account for the transport of material and energy must be developed. In a sense almost any process may be considered as taking place in a pipeline. The simplest model of flow is called plug flow and assumes that no mixing takes place along the axis of the pipeline, but that lateral mixing is com- plete. Also, this assumes a flat velocity profile exists at each D BE BF BG BH BI E ED EDD BL ED I ED H ED G ED F ED E XPYRPN OSxxUNSMSUT -ECYA  ED G FIGURE 2 C022_001_r03.indd 1214 11/18/2005 2:32:41 PM © 2006 by Taylor & Francis Group, LLC VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1215 TABLE 2 Properties of Selected Gaseous Pollutants Name Formula Molec WT Sense Properties Boiling Pt, C Solubility, CC per 100 GMS Cold H 2 O Warm H 2 O Other Ammonia NH 3 17.03 Colorless Pungent 33.4 Very soluble  1000 (99) — Carbon monoxide CO 28.01 Colorless Odorless 192 3.5 (0) 2.32 (20) Alcohol, Cu 2 Cl 2 Chlorine Cl 2 70.91 Gn-yellow Pungent 34.6 310 (10) 177 (30) Aq. NaOH or KOH Fluorine F 2 38.00 Gn-yellow 187 decomposes — — Hydrochloric acid HCl 36.47 Colorless 85 very soluble  1000 (99) Alcohol, Ethylether Hydrafluoric acid HF 20.01 Colorless 19.4 very soluble  1000 (99) — Hydrogen sulfide H 2 S 34.08 Colorless Decay odor 59.6 437 (0) 186 (40) Alcohol, CS 2 Mercury Hg 200.61 — 356.9 — — — Nitric oxide NO 30.01 Colorless 151 7.34 (0) 0 (100) Alcohol, H 2 SO 4 Nitrogen dioxide NO 2 46.01 Red-brown 21.3 decomposes — HNO 3 , H 2 SO 4 , CS 2 Ozone O 3 48.00 Faint blue 112 0.494 (0) 0 (50) Oil turp., oil cinn. Sulfur dioxide SO 2 64.07 Colorless Choking 10 8000 (9) 1600 (50) H 2 SO 4 , alcohol, acetic acid Sulfur trioxide SO 3 80.66 Colorless 44.6 decomposes — H 2 SO 4 C022_001_r03.indd 1215 11/18/2005 2:32:41 PM © 2006 by Taylor & Francis Group, LLC 1216 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS longitudinal position, or that the average velocity is the same at each lateral position. Continuity If the density, k, of the fluid at a distance along the pipe, Z, of cross section, S, changes, the velocity must also change as seen by an elemental mass balance across d Z distance, i.e. setting the mass accumulation rate equal to the sum of net input and generation rates (see Figure 3). ∂ ∂ ∂ ∂   t S vS Z  1 ( ) . (1) For steady state results, vS  const.  W o and the mass flow rate becomes the same at all axial positions. If the fluid is incompressible,  const., as for most liquids, vS, the vol- umetric flow rate, does not vary with position even during transient conditions. Motion In a comparable manner an elemental momentum (force) balance may be made over length dZ, which for incompress- ible flow reduces to ∂ ∂ ∂ ∂ ∂ ∂ v t v Z g p Z g F F S g c c w o z      1 2 2   ( ) . (2) TABLE 3 Daily Instantaneous mg/m 3 ppm/wt mg/m 3 ppm/wt Cl 2 0.03 (0.024) 0.10 (0.081) H 2 S 0.01 (0.0081) 0.03 (0.024) CS 2 0.15 (0.122) 0.50 (0.406) P 2 O 5 0.05 (0.049) 0.15 (0.122) Phenol 0.10 (0.081) 0.30 (0.24) UDC U SU dU rnO rnO d-r OSUA d-rnOA dT B FMPFNIMH GLRIE SU FIGURE 3 For both steady and incompressible flow dp dZ F S g g const o z c           . (3) The equation describes the relation between velocity and pressure along the pipe. The quantities F and F w are the magni- tudes of skin frictional force and force doing work on external surfaces, respectively, both per unit length of pipe. ENERGY The First Law of Thermodynamics may be written for the differential element of length, dz, at steady state dH dz g g v g dv dz Q W c c s    d d . (4) For unsteady behaviour where temperature gradients are desired the equation of thermal energy may be applied assum- ing a uniform temperature at any cross-section and no axial conduction.   c T t T p T v z q w vT z v & s v                         d d ( ) (5) in which q and w s are the volumetric thermal energy input rate (produced for example by an electrical or chemical phe- nomenon) and the work output rate, respectively. For a con- stant density fluid equation (5), the left hand side represents the accumulation of internal energy, and the right hand terms represent the influence of pressure on the energy transport rate, the combined energy input rate per unit volume by gen- eration and forces and the net energy input rate by flow (force convection), respectively. Component Balance The equations of continuity, motion and energy often may be applied to describe the situation in stacks of power plants, in the flow of fuels and effluents, and in the analy- sis of material, momentum and energy requirements of a C022_001_r03.indd 1216 11/18/2005 2:32:41 PM © 2006 by Taylor & Francis Group, LLC VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1217 pollution producing process. To analyze the concentrations of pollutant it still remains to make component material balances of n − 1 species within the system (for which n components exist). For separation processes an additional phase equation is usually required for transfer of pollutants between the rich and lean phases. The mass balance on a particular species may be found for component A by examining the imaginary stationary dif- ferential element of thickness dz. Assuming plug flow we may derive an expression for c A : Accum.Net InputGeneration rate of rate of rate of  AAA (6) c A —molar concentration F a —flux of A at position Z, moles A flowing/(time) (cs. area)  vc A R A —production rate of component A, by chemical or nuclear reaction, moles A formed/(time) (vol.) v—average fluid velocity. Also, r A is usually some empirical function of c A such as kc A n for an irreversible decomposition reaction of nth order. Gas Adsorption Adsorption is the process by which a solid surface attracts fluid phase molecules and forms a chemical or physical bond with them. The mechanism of adsorption includes: 1) diffusion of the pollutant from bulk gas to the external surface of the particles, 2) migration of the adsorbate molecules from the external surface of the absorbent to the surface of the pores within each particle, 3) adsorption of the pollutant to active sites on the pore. The attraction for a specific gas phase component will depend on properties such as the concentration of the gas phase com- ponent, the total surface area of absorbent, the temperature, polarity of the component and adsorbent, and similar prop- erties of competing gas molecules. Adsorption is used to concentrate (30–50 fold) or store pollutants until they can be recovered or destroyed in the most economical manner. Adsorption is an exothermic process. The heat of adsorp- tion for chemical adsorption is higher than that for physical adsorption. In the former, if the amount of pollutants to be removed large, it is necessary to remove the heat of adsorp- tion, since the concentration of adsorbed gas decreases with increasing temperature at a given equilibrium pressure. For chemical adsorption, properties which affect reaction kinet- ics will also come into play (see section on Gas Reaction). Activated carbon, silicon, aluminum oxides, and molecular sieves make up the majority of commercially significant adsor- bents. Activated carbon is the least affected by humidity and physically adsorbs nonpolar compounds since it has no great electrical charge itself. The adsorption rate of activated carbon can be increased with chemical impregnation. For instance, activated carbon impregnated with oxides of copper and chro- mium are found very useful to remove the hydrogen sulfide in gas streams where oxygen is not present (Lovett and Cunniff, 1974). Alumina and silica materials preferentially adsorb polar compounds. Molecular sieves have greater capture efficiencies than activated carbons but they often have a lower retention efficiency and are considerably more expensive. The ease of adsorbent regeneration depends on the mag- nitude of the force holding the pollutants on the surface of adsorbent. The usual methods for the adsorbent regeneration include stripping (steam or hot inert gas), thermal desorp- tion, vacuum desorption, thermal swing cycle, pressure swing cycle, purge gas stripping, and in situ oxidation. In many respects the equilibrium adsorption characteris- tics of a gas or vapor upon a solid resemble the equilibrium solubility of a gas in a liquid. For simple systems, a single curve can be drawn of the solute concentration in the solid phase as a function of solute concentration or partial pressure in the fluid phase. Each such curve usually holds at only one specific temperature, and hence is known as an isotherm. Five types of commonly recognized isotherms are shown by the curves in Figure 4. There are three commonly used mathemat- ical expressions to describe vapor or gas adsorption equilib- rium: the Langmuir, the Brunauer-Emmett-Teller (BET), and the Freundlich isotherm. The Langmuir isotherm applies to adsorption on completely homogeneous surfaces, with neg- ligible interaction between adsorbed molecules. It might be surmised that these limitations correspond to a constant heat of adsorption. The Freundlich isotherm can be derived by assuming a logarithmic decrease in heat of adsorption with fraction of coverage. Gas adsorption is an unsteady state pro- cess. The curve of effluent concentration as a function of time is commonly referred to as the break-through curve. It usually has an S shape. The break-through curve may be steep or rela- tively flat, depending on the rate of adsorption, the adsorption isotherm, the fluid velocity, the inlet concentration, and the column length. The time at which the break-through curve first begins to rise appreciably is called breakpoint. The design of an adsorption column requires prediction of the breakthrough curve, and thus the length of the adsorp- tion cycle between elutions of the beds, given a bed of certain length and equilibrium data. Because of the different forms of equilibrium relationship encountered, and the unsteady nature of the process, prediction of the solute break-through curve can be quite difficult. At present, detailed design of adsorption columns is still highly dependent on pilot scale evaluations of simulated or real systems. Before discussing the method of predicting the break- through curves, one should consider the isotherm. For Langmuir isotherm (Langmuir, 1917), if it is assumed that A 1 reacts with an unoccupied site X 0 to form adsorbed component X 1 , AXX 1 k k  01 1 1 � − (7) C022_001_r03.indd 1217 11/18/2005 2:32:43 PM © 2006 by Taylor & Francis Group, LLC 1218 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS Amount adsorbed Amount adsorbed p p p p p II III IV V I FIGURE 4 Types of adsorption isotherms. the equilibrium adsorption concentration, C, is obtained in terms of the gas phase concentration C 1 and the total adsorp- tion site concentration, C 0 C C KC KC 0 11 11 1   . (8) Here, K 1 is the adsorption equilibrium constant which varies only if temperature varies. Diatomic molecules such as chlorine might be expected to simultaneously adsorb and dissociate on adjacent sites. Such an adsorption might be described symbolically by AXX k k 101 22 1 1 � − in which case the equilibrium isotherm expression can readily be shown to be C C KC KC 0 11 12 11 12 1   () () . / / (9) If more than one pollutant is being adsorbed, each compo- nent, A j , undergoes AXXjj+ 0 �. The equilibrium for single site adsorption of component A j becomes C C KC KC jj j n 0 11 1 1   = ∑ . (10) The latter may be referred to as competitive adsorption. Figure 5 depicts the different dependence on gas phase con- centration for the adsorption types described thus far. Another isotherm finding wide use, particularly in multi- layer adsorption, is that of Brunauer, Emmett and Teller (1938), the BET equation C C QX XQX s max ()[()]   2 2 111 (11) in which: C s , C max —amount of gas absorbed per gm. of solid, and maximum amount, respectively Q 2 —term exponentially dependent on heat of adsorption X—ratio of equilibrium gas phase concentration of saturation Value. If neither the Langmuir or BET equations are satisfactory, a plynomial fit to adsorption data may be required. C022_001_r03.indd 1218 11/18/2005 2:32:43 PM © 2006 by Taylor & Francis Group, LLC VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1219 For adsorption the particles are usually dumped into a column as a packed bed (Figure 6). In commercial adsorp- tion columns, equilibrium concentrations are not attained uniformly, but for convenience the rate of adsorption is assumed to be proportional to C. See below under Reaction. The time t 3 in which breakthrough (C ig at the exit equals the permissible set amount) occurs may be established by analyzing the differential component mass balance assuming that the transport of material again is governed by diffusion through a film N k C C i g ig ig  ( ) * (12) in which C ig * is the gas phase concentration in equilibrium with the absorbed concentration C is at the same elevation. RL OLX x RL OLX TY PTUX xUTN SLO RL SLO  C  U   A A A     M MHM-BH C A  G I F I E I D  G  F  E  D M   C FIGURE 6 (a) Pictorial representation. (b) Schematic model showing a differential element over which a mass balance is made. (c) Pollutant concentration as a function of time at various heights. B C D E F G H I L BA B-A   A SXUP UMYRXS OSxNSMSP UMYRXS NxXPSSP UMYRXS T B N B FIGURE 5 C022_001_r03.indd 1219 11/18/2005 2:32:44 PM © 2006 by Taylor & Francis Group, LLC 1220 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS The equilibrium concentrations have been determined for most commercially available adsorbents and typical pollutants and are presented as either Langmuir or BET isotherms. In general such equations take the form CfC isig () * (13) The generation term is excluded, as it is assumed that chemi- cal reaction is not taking place in the system. To develop an expression for the net rate of accumulation of the mass of A i in the column, that is, the rate expression for the adsorption process, one also needs the function of the concentration of A i in the fluid phase. Considering a differential column segment and writing the continuity relationship for pollutant A i in each phase for this differential section, one gets for the solid phase in this section. 1 1 M P C t kCC a s is gi ()() *           (14) where: k g : is the gas phase mass transfer coefficient in units of (time – 1). M a : is the molecular weight of A 1 . P s : is the averaged density of the solids. : is the fractional voidage in the bed. For the gas phase in this differential segment,        C t V C z kCC ifif gig () * (15) where: V z : is the superficial velocity of the fluid. These partial differential equations (13)–(15) may be solved simultaneously by numerical analysis using difference formulas to approximate the partial derivatives. In such a way the breakthrough curves of hazardous organic vapors may be predicted for a given adsorbent. Smoothed computerized results were plotted on Figure 7 for five different compounds having Langmuir type behavior on activated carbon under the same hypothetical operating conditions. If one wishes to attain a 90% removal of certain organic vapor, one could easily see from Figure 7 that diethyl ether requires the shortest re-cycle time and methyl isobutyl ketone the longest among the five materials on the graph. Properties of Adsorbents Figures 8 and 9 are adsorption isotherms for activated carbon with nitrous oxide and carbon dioxide respectively. A more sophisticated correlation of adsorption data is pre- sented in Figures 10–12 for pure CO, C 2 H 4 and CO 2 gases. Here (RT/V s )ln f s /f g is plotted versus N s (in which:—gas constant, T—temperature, K, V s —molar volume of adsor- bate, cc/mole, f s and f g —fugacites of adsorbate and gas and N—amount of gas adsorbed, g—moles/gm. adsorbent. Hydrocarbons and SO 3 adsorb readily on activated carbon. SO 2 has a maximum retention of 10 wt.% on carbon at 20C, 760 torr. Ozone decomposes to oxygen on carbon (Ray and Box, 1950). Figure 13 has comparable results plotted for CO 2 adsorp- tion on silica gel. Activated carbon has significantly better equilibrium properties than does silica gel (vis Figure 9 vs. Figure 13). Other results for activated carbon and zeolites may be found in the book by Strauss (1968). Basic facts about adsorption properties of activated charcoal, system types and components and applications are discussed by Lee (1970). He tabulated data on the air purification applications for inexpensive, non- regenerative, thin bed adsorbers and for regenerative systems, and discusses the design of a solvent vapor recovery system. I. Diethyl ether II. Acetone III. Carbon disulfide IV. MEK V. Methyl isobutyl ketone V IV III III II I 500 0.5 1.0 100 150 t X= c c 0 FIGURE 7 Break through curves for various compounds at 20C and 1 atm with C 0  0.00548 mole/liter. C022_001_r03.indd 1220 11/18/2005 2:32:47 PM © 2006 by Taylor & Francis Group, LLC [...]... 1224 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS The design of absorbers involves the estimation of column diameter, height, and pressure drop The column diameter is fixed by the contaminated gas flow rate The determination of the height of the two-phase contacting zone involves an estimation of the mass transfer coefficients, the alternating use of equilibrium concentration relationship, and the law of. .. combustion sources: combustion modification and flue gas denitrification Combustion modification involves change of either operating or design conditions VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS CBB- R x S CBB- P X CBB- R x A S CB MB DB LB P EB IB FB HB O GB GB HB FB IB EB LB DB MB CB CBB- T N Y T S 1235 T P N A S X S CBB- X N CBB- U FIGURE 18 CBB- U CBB- X Flammability diagram (a) Hydrogen–oxygen...  4.6 × 1015 e−2400/RT cc mole−1 sec−1 POLLUTANT CONTROL METHODS Gases containing compounds of sulfur such as SO2, SI3, H2S and mercaptans have received the widest attention for the purpose of control of all noxious gases For this reason the 1232 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS TABLE 9 Combustion constants of pure gases (Schmidt and List, 1962) Heat of Combustionc Btu/ft3 No Substance Formula... flux in each phase must be the same Thus the rate of transfer per unit area is Ni  jg(Cig  CigI)  hL(CiLI  CiL) 1222 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS - CBB PN OxY XxYx TRS A LB HB EIE-D U DMH-M U DIE-D U CMF-I U CLB U CGB U CBB U II-F U FB DB B C CB E × CB Y CBBB CBB A FIGURE 10 C B EDD OD S U XSYS ND x MD LD x ID X P EDD T FDD T HDD T HD E - R FD R LD R OD R A D R FI R HD R ED Y G × ED C... have the capability of handling particulates, dispersed in huge quantities of air Phosphoric acid production is a typical fluoride producer The process begins with the grinding of rock phosphate (typically Ca10F2(PO4)6) This is naturally a source of particulates, as is the drying of the pulverized phosphate Acidulation 1238 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS OXIDATION-ABSORPTION-REDUCTION YES ARE... depending on the method of distribution, may be 5 to 20lb/sq in gauge.” 1240 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS Mercury Vapor Activated carbon, impregnated with elemental sulfur, has been found effective for removing mercury vapor from air or other gas streams (Lovett and Cunniff, 1974) Impregnating the sulfur on activated carbon can increase the reaction rate of mercury vapor and sulfur due to the... EA TYX -L- xR M-NTX PP G A HA FIGURE 9 Absorption isotherms for the system CO2-carbon (note that tc  31C) “Activated carbon filters were used to concentrate atmospheric mixtures of acrolein, methyl sulfide, and n-propyl mercaptan Removal efficiency and carbon capacity for each of the odor compounds were investigated using two different carbones, Cliffchar (4–10 mesh) and Barnebey–Cheney (C-4) A closed... (See Color Plate III)) T VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS Fluidized Bed Combustion (FBC) According to the operating pressure of the beds, there are two classifications of FBC—atmospheric pressure and pressurized The former is appropriate for both utility and industrial heating applications The latter is for electricity-generating plants In FBC, crushed coal is fed to a bed of fine limestone or... Converted to mean Bru/lb (1/180 of the heat per pound of water from 32F to 212F) from data by Frederick D Rossini, National Bureau of Standards, letter of April 10, 1937, except as noted † © 2006 by Taylor & Francis Group, LLC 1233 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS TABLE 9 (continued) ft3/ft3 of combustible Required for combustion No O2 N2 lb/ lb of combustible Flue products Air CO2 H2O Required... NOx 25 22.7 3 Shell/UOP selective NOx and SO2 2 1.8 4 Shell/UOP selective reduction-adsorption NOx and SO2 2 1.8 5 Wet oxidation-adsorption reduction NOx and SO2 9 8.2 6 Wet adsorption-reduction NOx and SO2 4 3.6 110 100 with sulphuric acid produces the gaseous pollutants associated with this process, most notably hydrogen fluoride, and silicon tetrafluoride Both of these gases can be scrubbed in aqueous . VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS C CB CBB CBBB Y  × CB E A         A - B DB FB HB LB CBB   EIE-DU DMH-MU DIE-DU CMF-IU CLBU CGBU CBBU II-FU. Group, LLC VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1227 The simplifications in the above expressions come about because of differences in the order of magnitudes of various rate and equilibrium. to describe the situation in stacks of power plants, in the flow of fuels and effluents, and in the analy- sis of material, momentum and energy requirements of a C022_001_r03.indd 1216 11/18/2005