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Choosing the appropriate technology for control of VOC emissions
CHOOSING THE APPROPRIATE TECHNOLOGY FOR CONTROL OF VOC EMISSIONS
by Victor S. Engleman and Linda L. Hunter Science Application International Corp., San Diego. Calif. VOCs are used in the metal-finishing and other U.S. industries in production, manufacture, and surface coating. With stricter air pollution regulations being promulgated, it is necessary for industry to minimize its use of solvents and/or control solvent emissions. In some cases it may be possible to substitute other compounds; in other cases control technologies will be required to minimize emissions of the regulated compounds. A number of control technology choices are available to control the emissions of VOCs. Two possible approaches to control VOC emissions are recovery and destruction (see Table I). Recovery methods include adsorption and condensation. Destruction methods include incineration and biodegradation, The choice of control technology will depend on economic and/or regulatory factors. Recovery technologies provide the benefit of both cost and energy savings. Recovery saves the energy for production of new material and the cost of purchasing additional material for the same or a similar application. Destruction technologies provide the benefit of not requiring subsequent disposal of the VOC. High-value VOCs favor recovery methods. Mixtures of VOCs or low-value solvents favor destruction methods.
RECOVERYTECHNOLOGIES Adsorption Adsorption has been used to control VOC emissions from paint spraying; metal foil coating; paper coating; plastic film coating; printing; fabric impregnation; dry cleaning; degreasing; solvent extraction; and manufacturing of plastics, chemicals, pharmaceuticals, rubber, linoleum, and transparent wrapping. Adsorption can treat airstreams with VOC concentrations lower than 100 ppm by volume, where combustion or condensation is neither feasible nor economical. Stable compounds are preferred for adsorption. Unstable compounds may decompose, leading to a reduced value of the recovered solvent and to decomposition products that may damage the bed. High-boiling compounds may not be suitable for adsorption because they may be difficult to remove from the bed during regeneration. During adsorption, the components of the solvent-laden airstream (SLA) are adsorbed physically or chemically onto the surface of a solid. The adsorbent must be selected to provide physical adsorption because in chemical adsorption, the solvent molecule is chemically bonded to the surface of the adsorbent. Chemical adsorption is not readily reversible and is therefore not suitable for solvent recovery. Activated carbon is the most common adsorbent for VOC emission control. Carbon is highly porous, resulting in a very large surface/volume ratio. Adsorption is rapid at first but gradually decreases as the carbon particles become saturated with organic material. As the carbon particles become saturated, the VOCs begin to break through and to be emitted from the adsorbent bed. To prevent unwanted emissions, the carbon bed must be regenerated before the carbon particles become completely saturated. To avoid shutdown a minimum of two carbon beds are used in parallel. One is on-stream while the second is being regenerated. Carbon can undergo many regenerations, and the typical life of carbon used as an adsorbent is three to five years. The adsorption capacity of carbon varies for different organics, concentrations, and adsorption temperatures. In general the adsorption capacity is inversely proportional to volatility and adsorption temperature and is directly proportional to concentration.
420
Table I. Control Technologies for VOC Emissions Recovery Methods Adsorption Technologies with RegenerationlRecovery Carbon adsorption With off-site regeneration With on-site steam regeneration With on-site Rankine regeneration With on-site Brayton regeneration With decoupled Brayton regeneration Condensation Technologies with Regeneration/Recovery Reverse Rankine cycle Reverse Brayton cycle Cryogenic liquid Destruction Methods Incineration Technologies Thermal incineration Without heat recovery With heat recovery Catalytic incineration With heat recovery Regenerative incineration Adsorption with incineration Biodegradation Biofiltration
Adsorption systems pose two potential secondary pollution problems: contaminated wastewater from the steam condensate, when steam is used to regenerate the carbon beds, and waste carbon. If steam regeneration is used, and there are compounds that are miscible with water, distillation is necessary to separate the water-miscible compounds from the steam condensate. The used or spent carbon can be returned to the manufacturer for screening and regeneration at high temperatures in an inert atmosphere, thereby rendering it suitable for further service in the adsorber. The primary sizing parameter for adsorbers is the total carbon requirement, which depends on adsorption time, flow rate of the waste gas, inlet concentration of the VOC, adsorption capacity of the carbon for the VOCs in the waste gas, and the working capacity of the adsorption bed. The number of beds in the system depends on the total carbon requirement, the size of the individual beds, and the desorption time. Continuously operated, on-site regenerated bed systems have a minimum of two adsorption beds: One set of beds is desorbing while the other set is adsorbing. The on-line set must be large enough to handle the entire gas stream while the other set is desorbing.
Carbon Adsorption with Off-Site Regeneration Adsorbers that are not regenerated on-site are termed modular or canister units. Modular adsorbers are regenerated off-site for a fee and are used primarily to control low VOC concentrations at low flow rates. Carbon Adsorption with On-Site Steam Regeneration This technology uses on-site steam to regenerate adsorption beds (Fig. I). If steam regeneration is used, compounds that are miscible with water will require distillation to separate them for recovery. Carbon Adsorption with On-Site Rankine Regeneration Adsorption with Rankine regeneration uses an inert gas to regenerate the carbon beds,
421
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J FiR· I. Carbon adsorptionlsteam regeneration.
thus eliminating the need for additional control equipment to separate water-miscible compounds that are desorbed from the adsorption beds (Fig. 2). The Rankine regeneration operates like a home refrigeration unit, but the commercial systems can be designed to use the heat for regeneration and can be designed to produce lower temperatures for condensation. The adsorption bed for this technology works as a concentrator, making the solvent concentration of the regenerator flow higher than the solvent concentration of the inlet SLA to the adsorber. This means that the required condensation temperature for recovery of the VOC in the regenerator is not as low as that required for direct recovery of the VOC in the inlet air. Carbon Adsorption with On-Site Brayton Regeneration This technology is similar to carbon adsorption with on-site Rankine regeneration, but instead the adsorption beds are regenerated by the Brayton cycle (Fig. 2). The Brayton cycle operates by using adiabatic compression and expansion to produce the hot gases for regeneration and the cold gases for condensation. When a very low condensation temperature (below -1 QOoF) is required to recover the solvents desorbed from the adsorption bed, the Brayton cycle is more economical than the Rankine cycle. Carbon Adsorption with Decoupled Brayton Regeneration This method is similar to on-site regeneration elfadsorption beds by the Brayton cycle, but it uses a mobile, decoupled Brayton cycle unit (possibly provided by an outside vendor for a fee) to regenerate the adsorption beds. It has a lower capital cost than on-site Brayton regeneration because only the carbon adsorption beds are purchased. It is viable for low VOC concentrations and flow rates when the fee paid to regenerate the adsorption beds is Jess than the annualized capital and operating cost of the Brayton regeneration equipment.
422
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Condensation
Condensers, often in conjunction with other control equipment, have been used to reduce organic emissions from the surface coating industry, petroleum refining, petrochemical manufacturing, asphalt manufacturing, coal tar dipping operations, degreasing operations, the pharmaceutical industry, and dry cleaning units. Condensers work best on airstreams that have high concentrations of condensable VOCs. Condensation may not be viable for airstreams thai are dilute because considerable cooling is required to bring the stream to the saturation point, and additional cooling is then needed to actually condense the VOCs. Low-volatility VOCs have higher condensing temperatures, making them easier to condense than high-volatility VOCs. Volatility therefore determines whether it is economically feasible to use condensation. Condensation produces very little decomposition of VOCs, so either stable or unstable VOCs can be recovered. Condensation units are sized primarily according to the flow rate and' the required condensation temperature. The condensation temperature depends on the volatility and the concentration of the VOCs in the SLA. In general, the condensation temperature is directly proportional to the concentration of the VOCs in the airstream. Reverse Rankine Cycle Condensation The reverse Rankine cycle is a commercial version of the refrigeration system used in most household refrigerators (Fig. 3). The Rankine cycle heat pump can recover VOCs from process airstreams without degrading the solvents. Usually a closed-cycle Rankine heat pump, with a separate working fluid, is used to condense VOCs. This makes the condensing and evaporating temperatures of the working fluid fixed and dictates the temperature range over which the VOCs must condense. At low temperatures the Rankine cycle is slow to achieve operating temperatures. Typical condensing temperatures for the reverse Rankine heat pump are -20 to -30°F, but it can reach temperatures down to -lOO°F.
423
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Fig.B. Reverse Rankine cycle condensation,
Reverse Brayton Cycle Condensation The reverse Brayton cycle uses the cooling caused by expansion to produce the low temperatures for voe condensation (Fig. 4). Theoretically, the only limit on condensation temperatures the reverse Brayton cycle can reach is the temperature at which oxygen liquefies (-297°F), but process limitations restrict the lowest condensation temperature to approximately -150°F. The reverse Brayton cycle heat pump has several advantages over the Rankine cycle heat pump: (l) it operates efficiently at temperatures below -100°F; (2) it handles fluctuations in temperature, composition, or flow rate in the voe contaminant airstream; (3) it achieves low condensation temperatures rapidly, thus making startups and shutdowns less of a problem; and (4) it can produce lower effluent VOC concentrations (because it can reach lower condensation temperatures). Cryogenic Liquid Condensation This method uses the cryogenic fluid (liquid nitrogen) as a refrigerant to reduce
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424
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Fi[?5.Direct condensation. emissions (Fig. 5). Cryogenic liquid condensation can achieve low condensation temperatures easily. Because of the cost of liquid nitrogen, high VOC concentrations (3-5%) are necessary for this technology to be economical. DESTRUCTION TECHNOLOGIES
Incineration Incineration can control emissions from a variety of industries: surface coating, printing, varnish cookers, foundry coke ovens, filter paper processing ovens, plywood veneer dryers, and gasoline bulk loading stations. Incineration is usually most applicable at VOC concentrations of 1000 ppm or greater. Incineration can remove VOCs from low-concentration airstreams (less than 100 ppm by volume), but it can require considerable additional fuel to heat the large volume of air (relative to the solvent volume). Incineration may not be practical for the control of low-concentration VOC airstreams unless there is a need for the large amount of heat produced. Incineration is a rapid, exothermic oxidation process that can convert VOCs to carbon dioxide and water. Organic material in the off-gas is destroyed by incineration; therefore, recovery and reuse of solvents are not possible. Thermal energy released by combustion devices can be recovered as heat for recuperative heat exchangers or waste heat boilers. Combustion of waste gas with a low heating value may require auxiliary fuel to maintain the desired combustion temperature. Waste gas with a heating value sufficient to bum may still require auxiliary fuel for flame stability and to maintain a sufficiently high furnace temperature to ensure complete combustion of VOCs. Preheating the combustion air by means of a heat exchanger will decrease the auxiliary fuel requirement. Insurance regulations usually require that the VOC concentration be maintained at or below 25% of the lower explosive limit and that instrumentation be installed to prevent risk of fire or explosion. Problems can occur if a VOC is relatively noncombustible and its decomposition leads to noxious or toxic gases. An example is chlorofluorocarbons that are noncombustible or require high incineration temperatures to bum, thus raising the operating cost. Combustion of halogenated VOC emissions may result in the release of halogenated combustion products to the environment. High incineration temperatures and wet scrubbers are required to prevent the release of halogenated combustion products. The use of a wet scrubber will cause an increase in wastewater that may contain small quantities of organic compounds.
425
PREHEATED AIR
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FUEL WASTE
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Fig. 6. Thermal incineration.
AIR
Incinerators are sized according to the combined SLA flow rate and the auxiliary fuel flow rate. Often the dominant operating cost for incineration is not the annualized equipment cost but the auxiliary fuel cost. Thermal Incineration without Heat Recovery Thermal incineration without heat recovery uses high incineration temperatures to destroy YOCs in a SLA (Fig. 6). This method does not reclaim any of the heat generated by the process. Thermal incineration ofYOCs can produce secondary emissions, such as nitrogen oxides. Factors affecting the rate of nitrogen oxide formation during combustion include the following: the amount of excess air available, the peak flame temperature, the length of time that the combustion gases are at a peak temperature, and the cooling rate of the combustion products. Thermal Incineration with Heat Recovery This technology recovers some of the heat generated by the thermal incineration process to preheat the incoming SLA (Fig. 6). The heat recovery equipment raises the capital cost but lowers the auxiliary fuel requirement. Regenerative Incineration Regenerative incineration thermally oxidizes YOCs and recovers up to 95% of the heat generated by the process (Fig. 7). This method warms the entering waste gas by passing it through a preheat chamber filled with several feet of ceramic or stoneware packing. Some of the YOC burns in this preheat chamber, and the rest is burned in a central combustion chamber. Following combustion, the waste gas exits through another packed chamber, where it exchanges most of its heat. The beds are then switched, so that the next increment of waste gas is preheated by passing it through the previous exit chamber. The equipment costs for regenerative incinerators are higher than for thermal incinerators, but the heat recovery greatly reduces the auxiliary fuel requirement. Catalytic Incineration without Heat Recovery Catal ysts can cause oxidation to occur at a lower temperature than is required for thermal oxidation (Fig. 8). Typical combustion catalysts usually operate over a temperature range of 600-1 200°F. Because of the lower combustion temperature, the additional fuel required is less than that for thermal oxidation. Catalysts can be deactivated by compounds containing sulfur, bismuth, phosphorus, arsenic, antimony, mercury, lead, zinc, tin, or halogens. Catalytic Incineration with Heat Recovery Catalytic incineration with heat recovery uses heat generated during the incineration process to preheat the SLA before it enters the catalytic incinerator (Fig. 8). The equipment cost will increase over catalytic incineration without heat recovery, but the auxiliary fuel requirement will decrease.
426
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Fi8. 7. Regenerative incineration. Adsorption with Incineration When the concentration of VOCs in the SLA is too low for incineration to be economical, it may be feasible to increase the concentration by using adsorption. This also reduces the flow rate of the waste gases entering the incinerator. Concentration factors of 3 to 10 or more are possible. In some cases the operating cost of such a combined system is reduced by a factor of 2 or more. If the higher concentration of the VOC approaches the explosive limit, consideration must be given to the use of inert gas for regeneration of the adsorbent.
Biodegradation Biodegradation utilizes the capacity of microorganisms to oxidize organic components to carbon dioxide and water. This is similar to incineration, but instead of using heat to destroy VOCs, biodegradation uses microorganisms. The microorganisms live in the water phase. The amount of water that the microorganisms live in must be controlled to maintain the efficiency of the degradation process. Too little water means that there is less living space for the bacteria. Too much water increases the chance of anaerobic zones. The temperature of the SLA must be PREHEATED AIR
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427
between 40 and 100°F because the bacteria are not sufficiently active outside this temperature range. The types of microorganisms required depend on the VOCs being controlled. Biofiltration This method utilizes a biofilter containing an enzymatic catalyst for each VOC in the SLA, an organic carrier material to provide the necessary nutrient salts for the microorganisms, expanded polystyrene spheres to keep the pressure drop over the biofilter low, and calcium carbonate to neutralize possible acids from the degradation of chlorinated VOCs. The size of the biofilter depends on the concentration of the VOC, the biodegradation ability of the VOC, and the SLA flow rate. VOC SCREENING MODEL
A screening model was developed to help with preliminary comparisons of options for VOC emission control technologies. This model looks at operating conditions to provide economic and technical information for both recovery and destruction technologies. The results from the model aid in making preliminary selections of technologies that are potentially applicable and help to eliminate technologies that are not practical. The following discusses the control technologies implemented in the model and how the model is used to screen possible control technologies for systems with a VOC emission problem. The VOC screening model currently has 14 VOC emission control options and a choice of 25 compounds (other.VOCs may be substituted) to be controlled. The model allows the choice of the concentration and compounds to be controlled, flow rates, operating cycles, SLA temperature and pressure, recovery value of each VOC, and percent recovery required. Utility, chemical and labor rate costs can also be specified for each case. These inputs allow the model to consider several factors that are specific to each VOC emission problem to choose viable VOC emission control options. The model uses the input information for each case to calculate key parameters for sizing emission control equipment. Some of the key parameters that the model calculates are the condensation temperature, the heating value of the waste gas, the lower explosive limit of the waste gas, and the adsorption capacity of carbon for the VOCs in the waste gas. Each of these parameters depends heavily on the operating conditions of the SLA, the inlet flow rate, the type of VOCs in the SLA, the concentration of each solvent, and the recovery percentage. The model utilizes the input information and the calculated parameters to determine the capital and operating costs of each technology in the model. The model also flags potential concerns with regard to using certain technologies under the operating conditions of a specific emission problem. The output of the screening model consists of capital costs, including equipment, installation, and annualized operating costs, which include energy, labor, maintenance, capital recovery, taxes/insurance, overhead, and solvent recovery value/disposal cost. The annualized costs are reported in both dollars per year and dollars per Ion of VOC controlled. Because this is a screening model the results are reported as ranges rather than as absolute numbers. This represents the probable range of costs because not all site-specific factors are considered by the model. Examples of the use of the model to screen potential VOC emission control technologies are discussed below. CASE STUDIES
The screening model was used for the initial evaluation of control technologies for a selected solvent that is pertinent to several industries (methyl ethyl ketone (MEK), a reactive compound in common use]. The results of the calculations from the screening model are presented in Figs. 9-13, to illustrate the regions where control technologies are cost
428
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vac Concentration (ppm) Fig. 9. Examples ofranges for control technologies. Case for MEK (including adsorption) (one ofthree): regions for adsorption-based technologies (calculation basis: solvent value,$020/lb; 24 hriday, 7 dayiwk, 48 wktyr; 90% recovery; 10 yr depreciation; 10% interest.
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WHEN ADSORPTION IS INCLUDED FOR THIS CASE, INCINERATION IS NOT COST COMPETITIVE USING THE GIVEN ASSUMPTIONS
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competitive for this particular case and illustrates some very important points in VOC control technologies. They show how much difference there can be at various concentrations and flow rates and when constraints are placed on the choice of control technologies. Figures 9 to 13 show the regions in which each of the control technologies represents the lowest cost emission control option given the input used for the particular test. The model outputs include capital (equipment and installation) and operating (labor, energy, maintenance, supplies, taxes/insurance, overhead, and solvent recovery credit) costs. Total annualized costs are also calculated and were used as the basis for Figs. 9to 13. Each of the calculations is based
429
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Fig, II. Examples of ranges for control technologies. Case for MEK (including adsorption) (three of three): regions for condensation-based technologies (calculation parameters as in Fi~.9).
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Fig.l ZExamptes ofranges for control technologies. Case for MEK (eliminating adsorption) (one oftwo j: regions for incineration-based technologies (calculation parameters as in Fig, 9). on seven basic parameters: (I) MEK is the solvent being controlled; (2) 90% of the input solvent is recovered; (3) the process operates 24 hr/day, 7 days/week, 48 weeks/yr; (4) the input stream to the control technology is 100°F; (5) the system is at atmospheric pressure; (6) the value for the recovered solvent is $0.20/1b; and (7) depreciation is set at 10 years, with a 10% interest rate. Figure 9 indicates the regions in which adsorption-based technology is the lowest cost emission control method. Figure 10 indicates the regions in which incineration-based technology is the lowest cost emission control method. Figure I 1 indicates the regions in which condensation-based technology is the lowest cost emission control method.
430
100,000
10,000
1,000
100
10
100
1,000
10,000
100,000
vac Concentration (ppm) Fig. 13.Examples ofranges for control technologies. Case for MEK (eliminating adsorption) (/lrO of two): regions for condensation-based technologies (calculation parameters as in Fig. 9).
MEK is controversial with regard to the application of emission control technologies. There are those who claim that they have had no problem applying adsorption-based technologies with MEK.There are those who suggest that MEK should never be used with adsorption-based technologies. In the following discussion, control methods for MEK presented two options: one includes adsorption and the other eliminates adsorption. For the case of MEK including adsorption, Fig. 9 indicates the regions for adsorption-based technologies. Adsorption with off-site regeneration is most favorable at low concentrations and low flow rates. The region covered is smaller than the region for heptane. Adsorption with on-site steam regeneration is most favorable at medium to high flow rates and low to medium concentrations. The region covered has lower concentrations than heptane. Adsorption with mobile inert gas regeneration covers the region of low to medium flow rates and low to medium concentrations. The region covered is almost the same as the region for heptane. Adsorption with on-site Brayton regeneration overlaps the region for steam regeneration, and although it becomes more expensive than steam at low flow rates, it becomes less expensive at higher concentrations. The region of higher concentrations is also covered by adsorption with on-site Rankine regeneration. Because the Brayton system can achieve lower temperatures more readily than the Rankine system, it tends to be more favorable for streams requiring very low condensation temperatures. For the case of MEK including adsorption, Fig. 10 indicates the regions for incineration-based technologies. Because of the good recovery value used for MEK, incineration technologies were not among the lowest cost emission control options when adsorption was included as an option. If technical factors in the system make adsorption technically unfeasible, incineration would cover a wide range. Incineration is technically feasible across a broad range. The range for incineration when adsorption is excluded is discussed below. For the case of MEK including adsorption, Fig. I I indicates the regions for condensation-based technologies. Both direct Brayton (also called open-cycle Brayton) and indirect Rankine (closed-cycle Rankine) condensation technologies were the lowest cost emission control options at high concentrations across the entire range of flow rates. The direct Brayton system is slightly better than the indirect Rankine system at lower concentrations.
431
For the case of MEK when adsorption is excluded, Fig. 12 indicates the regions for incineration-based technologies. Thermal incineration, with and without heat recovery, is the lowest cost emission control option at low to medium flow rates and low to medium concentrations. Catalytic incineration becomes cost competitive at medium flow rates for low to medium concentrations. Regenerative incineration becomes the low-cost option at medium to high flow rates with low to medium concentrations. As the concentrations become higher, it begins to pay to recover the solvent by direct condensation, using the recovery value of $O.20/lb for MEK. Figure 13 indicates the regions for condensation-based technologies, excluding adsorption. Both direct Brayton (open-cycle Brayton) and indirect Rankine (closed-cycle Rankine) condensation technologies were the lowest cost emission control options at high concentrations across the entire range of flow rates. The direct Brayton system is one of the lowest cost options at lower concentrations than the Rankine system because of its ability to achieve lower concentration temperatures. As stated above, the direct Brayton system becomes technically limited at higher concentrations than shown in Fig. 13, but the indirect Brayton or Rankine system can be used in those circumstances, provided that safety requirements are met. Figures 9 to 13 do not apply universally. Higher or lower boiling points, differences in reactivity, and differences in recovery value, among other factors, influence the regions in which control technologies are most favorable. Each case needs to be evaluated on its own merits, taking case-specific factors into account. The screening model can be used to help in this evaluation process.
CONCLUSIONS VOC emissions in the metal-finishing industry will require an increasing degree of control in the coming years. The selection of control technologies is influenced by a number of factors in the particular application. Although a number of technologies are technically feasible in a range of applications, there are major differences in the economics of control technologies. Differences of more than a factor of 2 in cost are possible in some cases. There may be trade-offs between capital and operating costs. A screening model was developed to assist in the selection of control technologies for specific applications. Case studies indicate thai small changes in critical parameters can result in changes in selection of the optimum technology.
Coatings Encyclopedic Dictionary edited by S. LeSota 391 pages 5110.00
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432
by Victor S. Engleman and Linda L. Hunter Science Application International Corp., San Diego. Calif. VOCs are used in the metal-finishing and other U.S. industries in production, manufacture, and surface coating. With stricter air pollution regulations being promulgated, it is necessary for industry to minimize its use of solvents and/or control solvent emissions. In some cases it may be possible to substitute other compounds; in other cases control technologies will be required to minimize emissions of the regulated compounds. A number of control technology choices are available to control the emissions of VOCs. Two possible approaches to control VOC emissions are recovery and destruction (see Table I). Recovery methods include adsorption and condensation. Destruction methods include incineration and biodegradation, The choice of control technology will depend on economic and/or regulatory factors. Recovery technologies provide the benefit of both cost and energy savings. Recovery saves the energy for production of new material and the cost of purchasing additional material for the same or a similar application. Destruction technologies provide the benefit of not requiring subsequent disposal of the VOC. High-value VOCs favor recovery methods. Mixtures of VOCs or low-value solvents favor destruction methods.
RECOVERYTECHNOLOGIES Adsorption Adsorption has been used to control VOC emissions from paint spraying; metal foil coating; paper coating; plastic film coating; printing; fabric impregnation; dry cleaning; degreasing; solvent extraction; and manufacturing of plastics, chemicals, pharmaceuticals, rubber, linoleum, and transparent wrapping. Adsorption can treat airstreams with VOC concentrations lower than 100 ppm by volume, where combustion or condensation is neither feasible nor economical. Stable compounds are preferred for adsorption. Unstable compounds may decompose, leading to a reduced value of the recovered solvent and to decomposition products that may damage the bed. High-boiling compounds may not be suitable for adsorption because they may be difficult to remove from the bed during regeneration. During adsorption, the components of the solvent-laden airstream (SLA) are adsorbed physically or chemically onto the surface of a solid. The adsorbent must be selected to provide physical adsorption because in chemical adsorption, the solvent molecule is chemically bonded to the surface of the adsorbent. Chemical adsorption is not readily reversible and is therefore not suitable for solvent recovery. Activated carbon is the most common adsorbent for VOC emission control. Carbon is highly porous, resulting in a very large surface/volume ratio. Adsorption is rapid at first but gradually decreases as the carbon particles become saturated with organic material. As the carbon particles become saturated, the VOCs begin to break through and to be emitted from the adsorbent bed. To prevent unwanted emissions, the carbon bed must be regenerated before the carbon particles become completely saturated. To avoid shutdown a minimum of two carbon beds are used in parallel. One is on-stream while the second is being regenerated. Carbon can undergo many regenerations, and the typical life of carbon used as an adsorbent is three to five years. The adsorption capacity of carbon varies for different organics, concentrations, and adsorption temperatures. In general the adsorption capacity is inversely proportional to volatility and adsorption temperature and is directly proportional to concentration.
420
Table I. Control Technologies for VOC Emissions Recovery Methods Adsorption Technologies with RegenerationlRecovery Carbon adsorption With off-site regeneration With on-site steam regeneration With on-site Rankine regeneration With on-site Brayton regeneration With decoupled Brayton regeneration Condensation Technologies with Regeneration/Recovery Reverse Rankine cycle Reverse Brayton cycle Cryogenic liquid Destruction Methods Incineration Technologies Thermal incineration Without heat recovery With heat recovery Catalytic incineration With heat recovery Regenerative incineration Adsorption with incineration Biodegradation Biofiltration
Adsorption systems pose two potential secondary pollution problems: contaminated wastewater from the steam condensate, when steam is used to regenerate the carbon beds, and waste carbon. If steam regeneration is used, and there are compounds that are miscible with water, distillation is necessary to separate the water-miscible compounds from the steam condensate. The used or spent carbon can be returned to the manufacturer for screening and regeneration at high temperatures in an inert atmosphere, thereby rendering it suitable for further service in the adsorber. The primary sizing parameter for adsorbers is the total carbon requirement, which depends on adsorption time, flow rate of the waste gas, inlet concentration of the VOC, adsorption capacity of the carbon for the VOCs in the waste gas, and the working capacity of the adsorption bed. The number of beds in the system depends on the total carbon requirement, the size of the individual beds, and the desorption time. Continuously operated, on-site regenerated bed systems have a minimum of two adsorption beds: One set of beds is desorbing while the other set is adsorbing. The on-line set must be large enough to handle the entire gas stream while the other set is desorbing.
Carbon Adsorption with Off-Site Regeneration Adsorbers that are not regenerated on-site are termed modular or canister units. Modular adsorbers are regenerated off-site for a fee and are used primarily to control low VOC concentrations at low flow rates. Carbon Adsorption with On-Site Steam Regeneration This technology uses on-site steam to regenerate adsorption beds (Fig. I). If steam regeneration is used, compounds that are miscible with water will require distillation to separate them for recovery. Carbon Adsorption with On-Site Rankine Regeneration Adsorption with Rankine regeneration uses an inert gas to regenerate the carbon beds,
421
EXHAUST
ADSORBER 1
AIR
I
-.
!
PROCE~S IPROCESS AIR
AIR COOLER
STEAM . .- - - - - - - -. .
~~ONDENSOR
SO LV EN T-....I--..
DISTILLA TION
J FiR· I. Carbon adsorptionlsteam regeneration.
thus eliminating the need for additional control equipment to separate water-miscible compounds that are desorbed from the adsorption beds (Fig. 2). The Rankine regeneration operates like a home refrigeration unit, but the commercial systems can be designed to use the heat for regeneration and can be designed to produce lower temperatures for condensation. The adsorption bed for this technology works as a concentrator, making the solvent concentration of the regenerator flow higher than the solvent concentration of the inlet SLA to the adsorber. This means that the required condensation temperature for recovery of the VOC in the regenerator is not as low as that required for direct recovery of the VOC in the inlet air. Carbon Adsorption with On-Site Brayton Regeneration This technology is similar to carbon adsorption with on-site Rankine regeneration, but instead the adsorption beds are regenerated by the Brayton cycle (Fig. 2). The Brayton cycle operates by using adiabatic compression and expansion to produce the hot gases for regeneration and the cold gases for condensation. When a very low condensation temperature (below -1 QOoF) is required to recover the solvents desorbed from the adsorption bed, the Brayton cycle is more economical than the Rankine cycle. Carbon Adsorption with Decoupled Brayton Regeneration This method is similar to on-site regeneration elfadsorption beds by the Brayton cycle, but it uses a mobile, decoupled Brayton cycle unit (possibly provided by an outside vendor for a fee) to regenerate the adsorption beds. It has a lower capital cost than on-site Brayton regeneration because only the carbon adsorption beds are purchased. It is viable for low VOC concentrations and flow rates when the fee paid to regenerate the adsorption beds is Jess than the annualized capital and operating cost of the Brayton regeneration equipment.
422
AIR
_E_X_"f
1_ _. . . .
COOL] DOWN AIR i=ILTER
PROC': [PROC .. E.S.S AIR AIR COOLER!
~---
INERT ....... GAS
ST
ADSORBER 1
r
r~-,==n I _
I
I
, ADSORBER 2
r
>i CONDENSOR
SOLVENT
Fig, 2. Carbon adsorption/inert gas regeneration.
Condensation
Condensers, often in conjunction with other control equipment, have been used to reduce organic emissions from the surface coating industry, petroleum refining, petrochemical manufacturing, asphalt manufacturing, coal tar dipping operations, degreasing operations, the pharmaceutical industry, and dry cleaning units. Condensers work best on airstreams that have high concentrations of condensable VOCs. Condensation may not be viable for airstreams thai are dilute because considerable cooling is required to bring the stream to the saturation point, and additional cooling is then needed to actually condense the VOCs. Low-volatility VOCs have higher condensing temperatures, making them easier to condense than high-volatility VOCs. Volatility therefore determines whether it is economically feasible to use condensation. Condensation produces very little decomposition of VOCs, so either stable or unstable VOCs can be recovered. Condensation units are sized primarily according to the flow rate and' the required condensation temperature. The condensation temperature depends on the volatility and the concentration of the VOCs in the SLA. In general, the condensation temperature is directly proportional to the concentration of the VOCs in the airstream. Reverse Rankine Cycle Condensation The reverse Rankine cycle is a commercial version of the refrigeration system used in most household refrigerators (Fig. 3). The Rankine cycle heat pump can recover VOCs from process airstreams without degrading the solvents. Usually a closed-cycle Rankine heat pump, with a separate working fluid, is used to condense VOCs. This makes the condensing and evaporating temperatures of the working fluid fixed and dictates the temperature range over which the VOCs must condense. At low temperatures the Rankine cycle is slow to achieve operating temperatures. Typical condensing temperatures for the reverse Rankine heat pump are -20 to -30°F, but it can reach temperatures down to -lOO°F.
423
I REFRIGI
AIR AND RESIDUAL VAPOR
'I I I I
I
I
~~~- -
PRO~
---
I
---
I
VAPOR
AIR IEHUJDIF~
R
CONDENSO~
RECUPERATO
-
RECOVERED VOC
WATER
Fig.B. Reverse Rankine cycle condensation,
Reverse Brayton Cycle Condensation The reverse Brayton cycle uses the cooling caused by expansion to produce the low temperatures for voe condensation (Fig. 4). Theoretically, the only limit on condensation temperatures the reverse Brayton cycle can reach is the temperature at which oxygen liquefies (-297°F), but process limitations restrict the lowest condensation temperature to approximately -150°F. The reverse Brayton cycle heat pump has several advantages over the Rankine cycle heat pump: (l) it operates efficiently at temperatures below -100°F; (2) it handles fluctuations in temperature, composition, or flow rate in the voe contaminant airstream; (3) it achieves low condensation temperatures rapidly, thus making startups and shutdowns less of a problem; and (4) it can produce lower effluent VOC concentrations (because it can reach lower condensation temperatures). Cryogenic Liquid Condensation This method uses the cryogenic fluid (liquid nitrogen) as a refrigerant to reduce
voe
IIJ------------..
TO PROCESS ..... OR EXHAUST I :
TO
I
ADSORBE~
E
'"""'
FROM PROCESS
r :
FROM
\0
I
~DSOR_BEH:
RECUPERA TOR WATER
Fig. 4, Reverse Brayton cycle condensation.
424
RECOVERED SOLVENT
..._ _....~ AIR, MOISTURE, AND RESIDUAL VAPOR
T~~l
REFRIGERA LIQUID
RECOVERED LIQUID
_
_-----~--_
_-- ---
INPUT
Fi[?5.Direct condensation. emissions (Fig. 5). Cryogenic liquid condensation can achieve low condensation temperatures easily. Because of the cost of liquid nitrogen, high VOC concentrations (3-5%) are necessary for this technology to be economical. DESTRUCTION TECHNOLOGIES
Incineration Incineration can control emissions from a variety of industries: surface coating, printing, varnish cookers, foundry coke ovens, filter paper processing ovens, plywood veneer dryers, and gasoline bulk loading stations. Incineration is usually most applicable at VOC concentrations of 1000 ppm or greater. Incineration can remove VOCs from low-concentration airstreams (less than 100 ppm by volume), but it can require considerable additional fuel to heat the large volume of air (relative to the solvent volume). Incineration may not be practical for the control of low-concentration VOC airstreams unless there is a need for the large amount of heat produced. Incineration is a rapid, exothermic oxidation process that can convert VOCs to carbon dioxide and water. Organic material in the off-gas is destroyed by incineration; therefore, recovery and reuse of solvents are not possible. Thermal energy released by combustion devices can be recovered as heat for recuperative heat exchangers or waste heat boilers. Combustion of waste gas with a low heating value may require auxiliary fuel to maintain the desired combustion temperature. Waste gas with a heating value sufficient to bum may still require auxiliary fuel for flame stability and to maintain a sufficiently high furnace temperature to ensure complete combustion of VOCs. Preheating the combustion air by means of a heat exchanger will decrease the auxiliary fuel requirement. Insurance regulations usually require that the VOC concentration be maintained at or below 25% of the lower explosive limit and that instrumentation be installed to prevent risk of fire or explosion. Problems can occur if a VOC is relatively noncombustible and its decomposition leads to noxious or toxic gases. An example is chlorofluorocarbons that are noncombustible or require high incineration temperatures to bum, thus raising the operating cost. Combustion of halogenated VOC emissions may result in the release of halogenated combustion products to the environment. High incineration temperatures and wet scrubbers are required to prevent the release of halogenated combustion products. The use of a wet scrubber will cause an increase in wastewater that may contain small quantities of organic compounds.
425
PREHEATED AIR
EXHAUST
FUEL WASTE
GAS
Fig. 6. Thermal incineration.
AIR
Incinerators are sized according to the combined SLA flow rate and the auxiliary fuel flow rate. Often the dominant operating cost for incineration is not the annualized equipment cost but the auxiliary fuel cost. Thermal Incineration without Heat Recovery Thermal incineration without heat recovery uses high incineration temperatures to destroy YOCs in a SLA (Fig. 6). This method does not reclaim any of the heat generated by the process. Thermal incineration ofYOCs can produce secondary emissions, such as nitrogen oxides. Factors affecting the rate of nitrogen oxide formation during combustion include the following: the amount of excess air available, the peak flame temperature, the length of time that the combustion gases are at a peak temperature, and the cooling rate of the combustion products. Thermal Incineration with Heat Recovery This technology recovers some of the heat generated by the thermal incineration process to preheat the incoming SLA (Fig. 6). The heat recovery equipment raises the capital cost but lowers the auxiliary fuel requirement. Regenerative Incineration Regenerative incineration thermally oxidizes YOCs and recovers up to 95% of the heat generated by the process (Fig. 7). This method warms the entering waste gas by passing it through a preheat chamber filled with several feet of ceramic or stoneware packing. Some of the YOC burns in this preheat chamber, and the rest is burned in a central combustion chamber. Following combustion, the waste gas exits through another packed chamber, where it exchanges most of its heat. The beds are then switched, so that the next increment of waste gas is preheated by passing it through the previous exit chamber. The equipment costs for regenerative incinerators are higher than for thermal incinerators, but the heat recovery greatly reduces the auxiliary fuel requirement. Catalytic Incineration without Heat Recovery Catal ysts can cause oxidation to occur at a lower temperature than is required for thermal oxidation (Fig. 8). Typical combustion catalysts usually operate over a temperature range of 600-1 200°F. Because of the lower combustion temperature, the additional fuel required is less than that for thermal oxidation. Catalysts can be deactivated by compounds containing sulfur, bismuth, phosphorus, arsenic, antimony, mercury, lead, zinc, tin, or halogens. Catalytic Incineration with Heat Recovery Catalytic incineration with heat recovery uses heat generated during the incineration process to preheat the SLA before it enters the catalytic incinerator (Fig. 8). The equipment cost will increase over catalytic incineration without heat recovery, but the auxiliary fuel requirement will decrease.
426
I I
CERAMIC BED PREHEATER
G~
I
INLET
COMBUSTION
~D I I I I I I I I I I
"7
CHAMBER
CERAMIC BED HEAT ABSORBER
-
EXHAUST
Fi8. 7. Regenerative incineration. Adsorption with Incineration When the concentration of VOCs in the SLA is too low for incineration to be economical, it may be feasible to increase the concentration by using adsorption. This also reduces the flow rate of the waste gases entering the incinerator. Concentration factors of 3 to 10 or more are possible. In some cases the operating cost of such a combined system is reduced by a factor of 2 or more. If the higher concentration of the VOC approaches the explosive limit, consideration must be given to the use of inert gas for regeneration of the adsorbent.
Biodegradation Biodegradation utilizes the capacity of microorganisms to oxidize organic components to carbon dioxide and water. This is similar to incineration, but instead of using heat to destroy VOCs, biodegradation uses microorganisms. The microorganisms live in the water phase. The amount of water that the microorganisms live in must be controlled to maintain the efficiency of the degradation process. Too little water means that there is less living space for the bacteria. Too much water increases the chance of anaerobic zones. The temperature of the SLA must be PREHEATED AIR
FUEL
WASTE
[Xf!/d)SI -"~L
--l
GAS
Fi8. 8. Catalytic incineration.
AIR
427
between 40 and 100°F because the bacteria are not sufficiently active outside this temperature range. The types of microorganisms required depend on the VOCs being controlled. Biofiltration This method utilizes a biofilter containing an enzymatic catalyst for each VOC in the SLA, an organic carrier material to provide the necessary nutrient salts for the microorganisms, expanded polystyrene spheres to keep the pressure drop over the biofilter low, and calcium carbonate to neutralize possible acids from the degradation of chlorinated VOCs. The size of the biofilter depends on the concentration of the VOC, the biodegradation ability of the VOC, and the SLA flow rate. VOC SCREENING MODEL
A screening model was developed to help with preliminary comparisons of options for VOC emission control technologies. This model looks at operating conditions to provide economic and technical information for both recovery and destruction technologies. The results from the model aid in making preliminary selections of technologies that are potentially applicable and help to eliminate technologies that are not practical. The following discusses the control technologies implemented in the model and how the model is used to screen possible control technologies for systems with a VOC emission problem. The VOC screening model currently has 14 VOC emission control options and a choice of 25 compounds (other.VOCs may be substituted) to be controlled. The model allows the choice of the concentration and compounds to be controlled, flow rates, operating cycles, SLA temperature and pressure, recovery value of each VOC, and percent recovery required. Utility, chemical and labor rate costs can also be specified for each case. These inputs allow the model to consider several factors that are specific to each VOC emission problem to choose viable VOC emission control options. The model uses the input information for each case to calculate key parameters for sizing emission control equipment. Some of the key parameters that the model calculates are the condensation temperature, the heating value of the waste gas, the lower explosive limit of the waste gas, and the adsorption capacity of carbon for the VOCs in the waste gas. Each of these parameters depends heavily on the operating conditions of the SLA, the inlet flow rate, the type of VOCs in the SLA, the concentration of each solvent, and the recovery percentage. The model utilizes the input information and the calculated parameters to determine the capital and operating costs of each technology in the model. The model also flags potential concerns with regard to using certain technologies under the operating conditions of a specific emission problem. The output of the screening model consists of capital costs, including equipment, installation, and annualized operating costs, which include energy, labor, maintenance, capital recovery, taxes/insurance, overhead, and solvent recovery value/disposal cost. The annualized costs are reported in both dollars per year and dollars per Ion of VOC controlled. Because this is a screening model the results are reported as ranges rather than as absolute numbers. This represents the probable range of costs because not all site-specific factors are considered by the model. Examples of the use of the model to screen potential VOC emission control technologies are discussed below. CASE STUDIES
The screening model was used for the initial evaluation of control technologies for a selected solvent that is pertinent to several industries (methyl ethyl ketone (MEK), a reactive compound in common use]. The results of the calculations from the screening model are presented in Figs. 9-13, to illustrate the regions where control technologies are cost
428
100,000 ADSORPTIO
E
~ 3= o
10,000
'E
< C Ql
16 ...J E Ql
1,000
1 1 1 1
_
_
RAYTON REGENERATION
ADSORPTIONI
\
'Ol't SITE
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REGENERA'1'I~_._
I
!
1 I"~
1
ADSORPTION! RANKINE REGENERATION
I
I~DSORPTION!MOBILE INERT REGENERATIO~
>
IOFF
(5
C/)
100
I
11SITE
10
I
.... 100
1,000
10,000
100,000
vac Concentration (ppm) Fig. 9. Examples ofranges for control technologies. Case for MEK (including adsorption) (one ofthree): regions for adsorption-based technologies (calculation basis: solvent value,$020/lb; 24 hriday, 7 dayiwk, 48 wktyr; 90% recovery; 10 yr depreciation; 10% interest.
100,000
E
R 3= o
10,000
WHEN ADSORPTION IS INCLUDED FOR THIS CASE, INCINERATION IS NOT COST COMPETITIVE USING THE GIVEN ASSUMPTIONS
'E
16
-7 'E
1,000 SEE THE MEK EXAMPLE WHICH ELIMINATES ADSORPTION
Ql
>
(5 C/)
100
10
100
1,000
10,000
100,000
vac Concentration (ppm) Fig. 10.Examples ofranges for control technologies. Case for MEK (including adsorption) (two of three): regions for incineration-based technologies (calculation parameters as in Fig. 9).
competitive for this particular case and illustrates some very important points in VOC control technologies. They show how much difference there can be at various concentrations and flow rates and when constraints are placed on the choice of control technologies. Figures 9 to 13 show the regions in which each of the control technologies represents the lowest cost emission control option given the input used for the particular test. The model outputs include capital (equipment and installation) and operating (labor, energy, maintenance, supplies, taxes/insurance, overhead, and solvent recovery credit) costs. Total annualized costs are also calculated and were used as the basis for Figs. 9to 13. Each of the calculations is based
429
rr / ,
E
R ;:
o 't
/ /
10,000
~
/
C Ql
-g
-7
1,000
/ DIRECT
C Ql
I
> "0
(f)
100 10
1,000
100
vac
I
10,000
'
INDIREC 'RANKINE
I
i"j 100,000
Concentration (ppm)
Fig, II. Examples of ranges for control technologies. Case for MEK (including adsorption) (three of three): regions for condensation-based technologies (calculation parameters as in Fi~.9).
100,000
fI="'=====~~-=--~-----~~-" REGEN ERA T1VE INCINERATION
;: o 't
10,000 CATALYTIC INCINERATION
~
C Ql
-g
-7
1,000
C
OJ
> "0
(f)
100 10
100
1,000
vac
10,000
100,000
Concentration (ppm)
Fig.l ZExamptes ofranges for control technologies. Case for MEK (eliminating adsorption) (one oftwo j: regions for incineration-based technologies (calculation parameters as in Fig, 9). on seven basic parameters: (I) MEK is the solvent being controlled; (2) 90% of the input solvent is recovered; (3) the process operates 24 hr/day, 7 days/week, 48 weeks/yr; (4) the input stream to the control technology is 100°F; (5) the system is at atmospheric pressure; (6) the value for the recovered solvent is $0.20/1b; and (7) depreciation is set at 10 years, with a 10% interest rate. Figure 9 indicates the regions in which adsorption-based technology is the lowest cost emission control method. Figure 10 indicates the regions in which incineration-based technology is the lowest cost emission control method. Figure I 1 indicates the regions in which condensation-based technology is the lowest cost emission control method.
430
100,000
10,000
1,000
100
10
100
1,000
10,000
100,000
vac Concentration (ppm) Fig. 13.Examples ofranges for control technologies. Case for MEK (eliminating adsorption) (/lrO of two): regions for condensation-based technologies (calculation parameters as in Fig. 9).
MEK is controversial with regard to the application of emission control technologies. There are those who claim that they have had no problem applying adsorption-based technologies with MEK.There are those who suggest that MEK should never be used with adsorption-based technologies. In the following discussion, control methods for MEK presented two options: one includes adsorption and the other eliminates adsorption. For the case of MEK including adsorption, Fig. 9 indicates the regions for adsorption-based technologies. Adsorption with off-site regeneration is most favorable at low concentrations and low flow rates. The region covered is smaller than the region for heptane. Adsorption with on-site steam regeneration is most favorable at medium to high flow rates and low to medium concentrations. The region covered has lower concentrations than heptane. Adsorption with mobile inert gas regeneration covers the region of low to medium flow rates and low to medium concentrations. The region covered is almost the same as the region for heptane. Adsorption with on-site Brayton regeneration overlaps the region for steam regeneration, and although it becomes more expensive than steam at low flow rates, it becomes less expensive at higher concentrations. The region of higher concentrations is also covered by adsorption with on-site Rankine regeneration. Because the Brayton system can achieve lower temperatures more readily than the Rankine system, it tends to be more favorable for streams requiring very low condensation temperatures. For the case of MEK including adsorption, Fig. 10 indicates the regions for incineration-based technologies. Because of the good recovery value used for MEK, incineration technologies were not among the lowest cost emission control options when adsorption was included as an option. If technical factors in the system make adsorption technically unfeasible, incineration would cover a wide range. Incineration is technically feasible across a broad range. The range for incineration when adsorption is excluded is discussed below. For the case of MEK including adsorption, Fig. I I indicates the regions for condensation-based technologies. Both direct Brayton (also called open-cycle Brayton) and indirect Rankine (closed-cycle Rankine) condensation technologies were the lowest cost emission control options at high concentrations across the entire range of flow rates. The direct Brayton system is slightly better than the indirect Rankine system at lower concentrations.
431
For the case of MEK when adsorption is excluded, Fig. 12 indicates the regions for incineration-based technologies. Thermal incineration, with and without heat recovery, is the lowest cost emission control option at low to medium flow rates and low to medium concentrations. Catalytic incineration becomes cost competitive at medium flow rates for low to medium concentrations. Regenerative incineration becomes the low-cost option at medium to high flow rates with low to medium concentrations. As the concentrations become higher, it begins to pay to recover the solvent by direct condensation, using the recovery value of $O.20/lb for MEK. Figure 13 indicates the regions for condensation-based technologies, excluding adsorption. Both direct Brayton (open-cycle Brayton) and indirect Rankine (closed-cycle Rankine) condensation technologies were the lowest cost emission control options at high concentrations across the entire range of flow rates. The direct Brayton system is one of the lowest cost options at lower concentrations than the Rankine system because of its ability to achieve lower concentration temperatures. As stated above, the direct Brayton system becomes technically limited at higher concentrations than shown in Fig. 13, but the indirect Brayton or Rankine system can be used in those circumstances, provided that safety requirements are met. Figures 9 to 13 do not apply universally. Higher or lower boiling points, differences in reactivity, and differences in recovery value, among other factors, influence the regions in which control technologies are most favorable. Each case needs to be evaluated on its own merits, taking case-specific factors into account. The screening model can be used to help in this evaluation process.
CONCLUSIONS VOC emissions in the metal-finishing industry will require an increasing degree of control in the coming years. The selection of control technologies is influenced by a number of factors in the particular application. Although a number of technologies are technically feasible in a range of applications, there are major differences in the economics of control technologies. Differences of more than a factor of 2 in cost are possible in some cases. There may be trade-offs between capital and operating costs. A screening model was developed to assist in the selection of control technologies for specific applications. Case studies indicate thai small changes in critical parameters can result in changes in selection of the optimum technology.
Coatings Encyclopedic Dictionary edited by S. LeSota 391 pages 5110.00
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