AIR POLLUTION CONTROL TECHNOLOGY HANDBOOK - CHAPTER 14 docx

Control of VOC and HAP by Condensation 14.1 INTRODUCTION Condensation of a vapor from an air stream can take place as a film of the condensed material on the wall of the condenser tube or as a series of drops that form at various points on the surface. Film-type condensation is the more common mechanism encountered in a condenser. The film uniformly coats the surface, and the thickness of the film increases with the extent of the surface. In dropwise condensation, the surface is not uniformly covered. The individual drops form and grow on the surface and tend to coalesce with neighboring drops. Adhesion of the drops is then overcome by gravitational forces, and the coalesced drops run off the surface. Impurities in the vapor stream promote dropwise condensation which results in higher heat transfer coefficients. Unfortunately there is not much information available on dropwise condensation. Therefore, design methods are limited to the film-type case. Condensers are best applied for removal of VOC and HAP from emission streams when the concentration is greater than 5000 ppmv. Removal efficiencies range from 50 to 90%. The upper end of efficiencies are practically achievable for concentrations in the range of 10,000 ppmv or greater. With high concentrations of pollutant, condensers are frequently employed as preliminary air-pollution-control devices prior to other devices such as incinerators, absorbers, or adsorbers. Flows up to 2000 scfm can be handled in condensers. In condensation, one or more volatile components of a vapor mixture are sepa- rated from the remaining vapors through saturation followed by a phase change. The phase change from gas to liquid can be achieved by increasing the system pressure at a given temperature, or by lowering the temperature at a constant pressure. The lower the normal boiling point, the more volatile the compound, the more difficult to condense, and the lower the temperature required for condensation. Refrigeration must often be employed to obtain the low temperatures required for acceptable removal efficiencies. 14.2 VOC CONDENSERS The two most common types of condensers used are surface and contact condensers. In surface condensers, the coolant does not contact the gas stream. Most surface condensers are the shell and tube type as shown in Figure 14.1. Shell and tube condensers circulate the coolant through tubes. The VOCs condense on the outside of the tubes within the shell. Plate and frame type heat exchangers are also used as condensers in refrigerated systems. In these condensers, the coolant and the vapor 14 9588ch14 frame Page 209 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC flow separately over thin plates. In either design, the condensed vapor forms a film on the cooled surface and drains away to a collection tank for storage, reuse, or disposal. In contrast to surface condensers where the coolant does not contact either the vapors or the condensate, contact condensers cool the volatile vapor stream by spraying either a liquid at ambient temperature or a chilled liquid directly into the gas stream. Spent coolant containing the VOCs from contact condensers usually cannot be reused directly and can be a waste-disposal problem. Furthermore, spent coolant is then contaminated with the VOC, and therefore, must undergo further treatment before disposal. 14.2.1 C ONTACT C ONDENSERS In contact condensers, a coolant, frequently water, is sprayed into the gas stream. Condensation proceeds as a heat-exchange process where the air stream containing the condensable materials is first cooled to its condensation temperature, then loses its heat of condensation. The coolant first gives up its sensible heat, then its heat of vaporization. The balancing of the heat exchange between the two streams will determine the amount of coolant needed. Design of contact condensers is based on the gas–liquid stage concept. However, spray systems operate with a high degree of back mixing of the phases. This practically limits spray chamber performance to a single equilibrium stage. For a direct contact device, this means that the temperatures of the exiting gas and liquid would be the same. Backmixing results because the chief resistance to flow is only the liquid drops. There is no degree of stabilization of the flow such as would happen in a packed tower. Anything less than perfect liquid distribution will induce large eddies and bypass streams. Thus, special care must be taken to obtain a uniform spray pattern. FIGURE 14.1 Shell and tube type surface condenser schematic. 9588ch14 frame Page 210 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC 14.2.2 S URFACE C ONDENSERS In the shell and tube heat exchangers, the coolant typically flows through the tubes and the vapors condenser on the outside of the tubes. In these units the pollutant gas stream must be cooled to the saturation temperature on the material being removed. The problem of design is complicated by the fact that most pollutant gas streams are essentially air with a small amount of VOC or HAP included. Therefore, condensation takes place from a gas in which the major component is noncondens- able. In the case of a simple air stream where the other component is condensable, condensation occurs at the dew point when the partial pressure of the condensable equals its vapor pressure at the temperature of the system. Since the coolant from surface condensers does not contact the vapor stream, it is not contaminated and can be recycled in a closed loop. Surface condensers also allow for direct recovery of VOCs from the volatile gas stream. This chapter addresses the design of surface condenser systems only. Figure 14.2 shows some typical vapor pressure curves. The more volatile the component, i.e., the lower the normal boiling point, the larger the amount that will remain uncondensed at a given temperature, hence the lower the temperature that is required to reach saturation. Condensation for this type of system typically occurs nonisothermally. The assumption of constant temperature conditions in the design of surface condensers does not introduce large errors into the calculations. FIGURE 14.2 Typical vapor pressure curves. 9588ch14 frame Page 211 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC 14.2.2.1 An Example — Condensation Temperature Consider an air stream flowing at 771 scfm containing 13.00 mol% benzene in which 90% removal of the benzene is required. The air flow entering the condenser is at 1.000 atm. What temperature is necessary to achieve this percent removal? The condenser is depicted in Figure 14.3 which is Figure 14.1 labeled with the conditions of operation specific to this example. The partial pressure of the benzene at the outlet of the condenser can be calculated as follows: Basis: 1.000 moles of air stream including the benzene. Assumption: Condenser operates at 1.000 atmosphere or 760 mm of Hg Moles of benzene entering = 0.1300 Moles of benzene leaving = (1 – .90) × 0.1300 = 0.0130 Moles of air + benzene leaving = 1.000 – (0.1300 – 0.0130) = 0.8830 Partial pressure of benzene leaving = (0.0130/0.8830) × 760 = 11.19 mm of Hg Refer to Figure 14.2, the vapor pressure curve. For benzene the value of the abscissa Solving for t CON = 6.80°F Therefore the condenser temperature must be below 6.80°F. FIGURE 14.3 Shell and tube type surface condenser schematic, example calculation. 1 459 67 0 00214 t R CON + [] = ° . . 9588ch14 frame Page 212 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC 14.3 COOLANT AND HEAT EXCHANGER TYPE The next step is to select the coolant based on the condensation temperature required. Table 14.1 summarizes some possible coolants. In the case of water, chilled water, and brine solutions, all remain in the liquid phase as they condense the VOC. Refrigerants usually condense the VOC by absorbing the heat as they change phase from liquid to vapor. The usual refrigeration cycle is used for these refrigerants. The vapor is compressed and condensed at a higher pressure and a higher temperature by a fluid at a temperature lower than the condensation temperature. Frequently water can be used to condense the high pressure refrigerant. Most thermodynamics textbooks contain a description of a refrigeration cycle. The problem now is to determine the size and design of the particular type of heat exchanger that is required to carry out the heat transfer needed. Figure 14.1 illustrates a horizontal shell and tube heat exchanger with the coolant inside the tubes and the condensing vapor outside the tubes. Vertical shell and tube heat exchanger arrangements are shown in Figure 14.4. Advantages and disadvantages of each type are listed in Table 14.2. The design of this type of equipment requires the knowledge of suitable heat transfer coefficients. These coefficients are highly dependent on the condensing material, the coolant used, and the particular arrangement of the heat exchanger. They range from 10 to 300 BTU/(h-ft 2 -°F). Finally the design procedure would include determining the amount of coolant needed. 14.3.1 A N E XAMPLE — H EAT E XCHANGER A REA AND C OOLANT F LOW R ATE For the heat exchanger discussed previously where the flow is 771.0 scfm, the number of moles would be 2.0 moles/min or 120 moles/h. Data: TABLE 14.1 Coolant Selection Required Condensation Temperature (°F) Coolant 80 to 100 Water 45 to 60 Chilled water –30 to 45 Brine solutions –90 to –30 Refrigerants Heat of condensation of benzene - at 1.0 atm, 176 F Specific heats; at 77 F Heat transfer medium C - PM =° °= ° == ° == ° =° 13 230 25 696 19 65 065 , . . . BTU lb mole C Air C BTU lb mole F Benzene C BTU lb mole F BTU lb F PA PA 9588ch14 frame Page 213 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC FIGURE 14.4 Vertical shell and tube heat exchangers arrangements: (a) condensation inside tubes, downflow vapor; (b) condensation inside tubes, upflow vapor; (c) condensation outside tubes, downflow vapor. 9588ch14 frame Page 214 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC © 2002 by CRC Press LLC On the basis of 1 h of operation, Refer to Figure 14.3 for temperatures. Condenser heat load, assuming no heat loss from the heat exchanger to the atmosphere, TABLE 14.2 Advantages and Disadvantages of Shell and Tube Heat Exchanger Types Used in Condensation Shell and Tube Condenser Types Advantages Disadvantages Horizontal Exchanger Condensate outside tubes May be operated partially flooded Free draining Condensate inside tubes Liquid builds up causing slugging Vertical Exchanger Condensate inside tubes, vertical downflow Positive venting of noncondensables Wet tubes retain light- soluble components Low pressure may require large tubes Condensate inside tubes, vertical upflow Used for refluxing Usually partially condensing Liquid and vapor remain in intimate contact Condensate outside tubes, vertical downflow High coolant side heat transfer coefficient Requires careful distribution of coolant Ease of cleaning Benzene in moles Air in and out moles Benzene condensed moles Benzene out moles Air Benzene out moles =×= =×= =×= =−= +=+= 0 13 120 15 60 0 87 120 104 40 090 156 1404 15 60 14 04 1 56 104 4 1 56 105 96 . WQ HH H H H BTU h H BTU h S uncon voc air cond voc uncon voc air == =++ =× ×− () = =××− () = 00 1 56 19 65 80 6 8 2244 104 4 6 96 80 6 8 53 189 ., . . , therefore H∆ ∆∆ ∆ ∆ ∆ ∆ 9588ch14 frame Page 215 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC To estimate the heat of condensation at T 2 = (6.8 + 460) = 466.8°R, use the Watson Equation. 1 where T C = 1012°R, the critical temperature of benzene, T 1 = (176 + 460) = 636°R, and ∆ H voc at T 2 = 13,236 BTU/lb-mole. Calculate the heat transfer area, U = heat transfer coefficient U = 40 BTU/h-ft 2 -°F ∆ T log mean = log mean temperature difference For a derivation and discussion of the log mean temperature difference see Perry and Green 2 or a text on heat transfer. Calculate the coolant flow. ∆∆HH TT TT voc at T voc at T C C 21 1 1 2 1 038 = − −       . ∆ ∆∆ ∆ ∆ ∆ ∆ H BTU lb mole HH H H H BTU h H BTU h voc at T cond voc voc to cond temp cond cond voc cond voc 2 13 236 1 466 8 1012 1 636 1012 15243 14 04 19 65 80 6 8 14 04 15 243 234 207 2244 53 180 234 207 289 640 038 = − −       = =+ =××− () +× = =+ + = . . , , ,, , . - QUAT mean =∆ log ∆TF Aft meanlog . ln . . , . = − () −−− () () − −− () ()               =° = −− () () = 80 16 8 6 8 8 2 80 16 8 68 82 33 51 289 640 06568 82 216 2 QWC T T W lbs h cool PM in out cool =− () = −− () () = 289 640 065 68 82 29 707 , . . 9588ch14 frame Page 216 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC 14.4 MIXTURES OF ORGANIC VAPORS The condensation of mixtures of organic vapors occurs over a range of temperatures, thereby complicating the design of heat exchangers for condensing these mixtures. The process for a binary mixture is illustrated in Figure 14.5 where it is presumed the pressure remains constant. A vapor at point A is cooled until it reaches its dewpoint at point B. Further reduction of temperature will cause the mixture to form two phases. At the temperature at point C, the vapor composition is given by point D and the liquid composition by point E. A constant temperature flash calculation could determine not only the compositions at this temperature but also the quantity of vapor and liquid. Continued coolant to the bubble point temperature F will produce 100% liquid with the same composition as the initial vapor. Therefore, as an organic mixture cools from its dew point to its bubble point, the condensing liquid is changing composition. This results in the heat of condensation varying throughout the cooling process. This variation in the heat of condensation should be accounted for in the determination of the area for heat exchange and will result in a greater area than would be calculated from the log mean temperature difference method. In some cases it can make a major difference in the area and, if not accounted for, can result in poor performance of the heat exchanger. For a more detailed description of the dew point, bubble point, and flash calculation methodology, refer to a thermo- dynamics textbook like Smith et al. 1 The original model for sizing a condenser for mixtures of vapors was presented by Colburn and Hougen. 3 This method was elab- orated upon by Silver 4 and Bell and Ghaly. 5 FIGURE 14.5 Equilibrium dew point–bubble point curve for a binary mixture. 9588ch14 frame Page 217 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC 14.4.1 A N E XAMPLE — C ONDENSATION OF A B INARY M IXTURE As an example to illustrate the methodology, consider the condensation of an iso- propyl alcohol (IPA) — water mixture in a vertical, countercurrent, upflow condenser at 1 atm. The heat exchanger is to be sized to totally condense the mixture. The total flowrate is 608 lb/h at 214.4°F (101.33°C) and atmospheric pressure at a mole fraction of IPA = 0.128. Cooling water is available at 80°F with a 10°F temperature rise allowed. In the vertical, countercurrent, upflow heat exchanger the vapor is condensing inside the tubes. Figure 14.7 is the bubble point–dew point curve for the IPA–water system. It shows this system to be an azeotrope. The mixture composition we are considering is to the left or the lower IPA composition side. From Figure 14.6 for the IPA mole fraction of 0.128, the dew point is 95.8°C (204.4°F), and the bubble point is 82.6°C (180.7°F). The overall heat transfer coefficient U o = 100 BTU/h-ft 2 -°F, and the gas film heat transfer coefficient h g = 7 BTU/h-ft 2 -°F. First calculate the heat exchange area from the log-mean temperature difference (LMTD) method with the overall heat transfer coefficient. The total heat transferred can be approximated from the latent heats of vaporization and the molar composition. On a mole basis, both latent heats of vaporization for IPA and water are about equal FIGURE 14.6 Vertical upflow total condenser, example calculation. 9588ch14 frame Page 218 Wednesday, September 5, 2001 9:57 PM © 2002 by CRC Press LLC [...]...9588ch14 frame Page 219 Wednesday, September 5, 2001 9:57 PM FIGURE 14. 7 Bubble point–dew point curve for the isopropyl alcohol (IPA)–water system at 1 atm to 17,400 BTU/lb-mol The 608 lb/h flowrate is 25.9758 lb-moles/h Therefore, the heat transferred is Q T = 17, 400 × 25.9758 = 451, 979 BTU h Figure 14. 6 is a schematic of the vertical heat exchanger From Figure 14. 6, 124.4 − 100.7 =... the heat exchanger © 2002 by CRC Press LLC 9588ch14 frame Page 221 Wednesday, September 5, 2001 9:57 PM VALUE OF INTEGRAND 0.0020 0.0019 0.0018 0.0017 0.0016 0.0015 0.0 014 0.0013 0 100000 200000 300000 400000 HEAT TRANSFERRED IN BTU/HR FIGURE 14. 10 Area integral to evaluate Equation 14. 1 made according to the algorithm in Appendix B of this chapter Figure 14. 10 presents the numerical results of the calculation... 178 ft2, considerably different from the LMTD result of 40.31 ft2 14. 5 AIR AS A NONCONDENSABLE A significant problem with polluted air streams is the large amount of noncondensable included in the air In the case where water is in the stream, the problem is even more complex The total pressure is now the sum of the partial pressures of the air, the miscible organics, and the water The equilibrium of the... derived in Appendix A of this chapter Area = ∫ QT 0   zU    1 + O   hg      dQ T   U O (TG − TL )  (14. 1) The temperature difference at a mole fraction of IPA = 0.128 between the entering temperature of 214. 4°F (101.33°C) and the final temperature of 180.7°F (82.6°C) at the top of the condenser, is divided into seven segments as shown in © 2002 by CRC Press LLC 9588ch14 frame Page 220 Wednesday,... Press LLC 9588ch14 frame Page 220 Wednesday, September 5, 2001 9:57 PM FIGURE 14. 8 Temperature profiles through heat exchanger TG is the gas and TL is the liquid coolant Figure 14. 7 To evaluate the area, the argument of the integral is calculated over each segment and plotted as a function of the heat transferred, QT Figure 14. 8 shows gas, TG, and liquid coolant, TL, temperature profiles as a function... algorithm shown in Appendix B of this chapter These temperature profiles seem to suggest that the LMTD should suffice because they look almost linear However, examination of the temperature difference as a function of QT on Figure 14. 9 shows a large variance from a linear curve Therefore, the LMTD approach would not give reasonable results Further calculations are FIGURE 14. 9 Gas–liquid temperature difference... condenser Since the water air and miscibles air mixtures attain their separate equilibrium relationships, the system may have two dew points, one for water and one for the miscible organics For multicomponent mixtures, the calculation could become quite complex REFERENCES 1 Smith, J., vanVess, H C., and Abbott, M M., Introduction to Chemical Engineering Thermodynamics, 6th ed., McGraw-Hill Inc., New York,... Engineers’ Handbook, 7th ed., McGraw-Hill, New York, 1997 3 Colburn, A P and Hougan, O A., I&E Chem., 26, 1178–1182, 1934 4 Silver, L., Gas cooling with aqueous condensation, Trans Inst Chem Eng., 25, 30–42, 1947 5 Bell, K G and Ghaly, M A., An approximate generalized design method for multicomponent partial condensers, Chem Eng Prog Symp Ser., 131, 72–79, 1973 © 2002 by CRC Press LLC 9588ch14 frame... 0   zU    1 + O   hg      dQ T   U O (TG − TL )  (14. 1) APPENDIX B: ALGORITHM FOR THE AREA MODEL FOR A MIXTURE CONDENSING FROM A GAS • This algorithm applies specifically to binary mixtures without air but could be extended to multicomponent mixtures Consult Bell and Ghaly5 for extensions to include noncondensables like air • Data required: 1 Binary vapor liquid equilibria 2 Latent heat... Chem Eng Prog Symp Ser., 131, 72–79, 1973 © 2002 by CRC Press LLC 9588ch14 frame Page 222 Wednesday, September 5, 2001 9:57 PM FIGURE 14. 11 Heat transfer through a condensing film APPENDIX A: DERIVATION OF THE AREA MODEL FOR A MIXTURE CONDENSING FROM A GAS Refer to Figure 14. 11 for a schematic of heat transfer through a condensing film dQSV dQT TG TL Ti hg Uo = sensible heat of vapor transferred through . With high concentrations of pollutant, condensers are frequently employed as preliminary air- pollution- control devices prior to other devices such as incinerators, absorbers, or adsorbers. Flows. result of 40.31 ft 2 . 14. 5 AIR AS A NONCONDENSABLE A significant problem with polluted air streams is the large amount of noncondens- able included in the air. In the case where water. cleaning Benzene in moles Air in and out moles Benzene condensed moles Benzene out moles Air Benzene out moles =×= =×= =×= =−= +=+= 0 13 120 15 60 0 87 120 104 40 090 156 140 4 15 60 14 04 1 56 104 4 1