Ind. Eng. Chem. Res. 1992,31, 238-243
238
tynediol to Cis-Butenediol Catalyzed by Pd-Zn-CaC03: Reaction Kinetics and Modelling of a Batch Slurry Reactor. AIChE J. 1985,31, 1891-1903. Chaudhari, V. R.; Shah, Y. T.; Foster, N. R. Novel Gas-Liquid-Solid Reactors. Catal. Rev. 1986,223(4),431-518. Chriitoffel, E. G. Laboratory Reactors and Heterogeneous Catalytic Reactors. Catal. Rev. 1982,24 (2),159-232. Doraiswamy, L. K.; Tajbl, D. G. Laboratory Catalytic Reactors. Catd. Rev. 1974,10 (2),177-219. Duda, R. 0. “The PROSPECTOR System for Mineral Exploration”; Final Report, SRI Project 8172;SRI International, Artificial Intelligence Center: Menlo Park, CA, 1980. Firebaugh, M. W. Artificial Intelligence: A Knowledge-Base Approach; Bond & Fraser: Boston, 1988. Fishburn, P. C. The Foundations of Expected Utility; Reidel: London, 1982. Hanratty, P. J.; Joseph, B. Decision-makingin Chemical Engineering and Expert Systems: Application of the Analytic Hierarchy Process to Reactor Selection. Submitted for publication in CAChE, 1991. Harrison, D. P.; Hall, J. W.; Rose, H. F. Automatic Precision Microreactor. Ind. Eng. Res. 1965,57,51-60. Hill, C. G. An Introduction to Chemical Engineering Kinetics and Reactor Design; Wiley: New York, 1977. Lee, N. S.; Grize, Y. L.; Dehnab, K. Quantitative Models for Reasoning under Uncertainty in Knowledge-Based Expert Systems. Int. J. Intell. Syst. 1987,2,15-24. Levenspiel, 0.The Chemical Reactor Omnibook; OSU Book Stores: Corvallis, OR, 1984.
Maiers, J.; Sherif, Y. S. Application of Fuzzy Set Theory. IEEE Syst. Cyb. 1985,15, 175-189. Mills, P. L.; Ramachandran, P. A.; Dudukovic, M. P. Reaction Engineering for Multi-Phase Catalyzed Systems: Class Lecture Notes; Washington University: St. Louis, 1989. Minsky, M. A Framework for Representing Knowledge. In The Psychology of Computer Vision; Winston, P., Ed.; McGraw-Hilk New York, 1975;pp 217-277. Peniwati, K.; Hsiao, T. Ranking Countries According to Economic, Social, and Political Indicators. Math. Model. 1987,9,203-209. Ramachandran, P. A.; Chaudhari, R. V. Three Phase Catalytic Reactors; Gordon and Breach New York, 1983. Robinson, K. K.; Mahoney, J. A. Gradientless Reactors for Process and Kinetic Studies and Catalyst Screening. Presented at the Symposium on Laboratory and Bench Scale Reactor DesignAmerican Chemical Society, Chicago, 1977. Rolston, D. W. Principles of Artificial Intelligence and Expert Systems Development; McGraw-Hill: New York, 1988. Saaty, T. L. The Analytic Hierarchy Process; McGraw-Hill: New York, 1980. Shah, Y. T. Gas-Liquid-Solid Reactor Design; McGraw-Hilk New York, 1979. Weekman, V. W. Laboratory Reactors and Their Limitations. AIChE J. 1974.20.833-fM0. Zadeh, L. Fuzzy Sets: Inf. Control 1965,8,338-353. Received for review May 15, 1991 Revised manuscript received August 29, 1991 Accepted September 13, 1991
Process Synthesis for Waste Minimization James M. Douglas Department of Chemical Engineering, University of Massachusetts, --.nherst, Massachusetts 01003
The hierarchical decision procedure described by Douglas provides a simple way of identifying potential pollution problems early in the development stages of a design. The procedure focuses on the decisions required to complete a design. If these decisions are changed, then process alternatives are generated. Some of the decisions affect the exit streams from (and the feeds to) the process, and in some cases these exit streams have an adverse environmental impact. Hence, if we can make decisions, i.e., find alternatives, that do not lead to pollution problems, we can develop “cleaner” processes. The hierarchical decision procedure also can be used to decompose existing designs, and then process alternatives that eliminate existing pollution sources can be identified. Introduction There have been several papers published indicating how solutions to pollution treatment problems have also led to significant cost savings (Huisingh et al., 1986; Huisingh, 1989; Martin, 1989). However, a recent paper by Friedlander (1989) clearly describes the implication of environmental issues, both for the process industries and for chemical engineering education. The paper stresses the need to eliminate pollution problems at the source by using different processing routes rather than using endof-pipe treatments and disposal. Friedlander notes, however, that a design procedure that eliminates pollution at the source is not available. This paper shows that the hierarchical decision procedure for process synthesis developed by Douglas (1985) can be used for this purpose. Only minor modifications of Douglas’ procedure are needed to identify or solve waste minimization problems, but a recognition of the waste minimization implications of the decisions made in the procedure should be useful for environmental studies. Moreover, by understanding how pollution problems arise from decisions made at the various levels of detail of the
procedure, we can develop a classification of waste minimization problems. Hierarchical Decision Procedure for Process Synthesis A systematic procedure for the synthesis of processes has been developed in recent years (Douglas, 1985,1988, 1990; Rossiter and Douglas, 1986; Kirkwood et al., 1988; Rajagopal et al., 1988),and it is a simple matter to use this procedure to identify potential pollution problems and to identify process alternatives that can be used to eliminate these problems. The systematic procedure develops a design by proceeding through a series of hierarchical levels, where additional details are added at each level. The decisions required to develop a flowsheet at each level of detail are identified, and if these decisions are changed, all of the process alternatives at each level of detail are identified. An economic analyais is carried out at each level to make it efficient to terminate poor design projects early. Some of the decisions made at each level of detail might change the streams leaving (and/or entering) the process. In some cases these exit streams cause pollution problems.
0888-588519212631-0238~03.00/ 0 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 239 Table I. Hierarchical Procedure for Process Synthesis level 1. input information: type of problem level 2. input-output structure of the flowsheet level 3. recycle structure of the flowsheet level 4. specification of the separation system level 4a. general structure: phase splits level 4b. vapor recovery system level 4c. liquid recovery system level 4d. solid recovery system level 5. energy integration level 6. evaluation of alternatives level 7. flexibility and control level 8. safety
Hence, if we can make decisions that do not introduce exit streams to the flowsheet that have an adverse environmental impact, then we will have accomplished our waste minimization goal. It should be recognized, however, that the "cleanest" process will not always be the least expensive process. For example, one of the decisions at the (level 2) input-output level of detail is whether to recover and recycle or to remove from the process by-product components formed by secondary reversible reactions, i.e., reactions of the type A + B product (1) A + product + by-product (2) where the second reaction is reversible, but the first reaction is not. If the by-product is removed from the process, we add a new exit stream, whereas if it is recycled, we do not. The removal of the by-product increases the raw material cost, because of the selectivityloss of the more expensive raw materials to the lower value by-product. Moreover, if the by-product is a waste material, it must be sent to treatment or disposal, so that it has a negative value (i.e., we must pay to have it removed from the process). On the other hand, if we recycle the by-product and let it build up to its equilibrium level (sothat the by-product decomposes at the same rate that it is formed), then the recycle costs of the process will increase (although there will be no selectivity loss). Thus,recycle of the by-product will always lead to a "cleaner" process, although it might not be the least expensive. The sensitivity of the total design costs to the pollution treatment cost must also be considered, because treatment cwts are increasing rapidly in most locations. A number of waste minimization problems of this type can be identified. An efficient way of identifying these problems is to reexamine the hierarchical decision procedure of Douglas and to see which decisions might add new exit streams to the flowsheet. After identifying the waste minimization decisions, we can use the results to develop a classification scheme for waste minimization problems. A slightly modified list of the hierarchical decision levels is given in Table I. The early descriptions of the procedure were limited to vapor-liquid processes, but the revised list also includes solid operations (Rossiter and Douglas, 1986;Rajagopal et al., 1988). By considering some case studies, we can identify where pollution problems might be encountered at each level. Here we consider only individual plants, but a procedure for decomposing plant complexesor multistep reaction processes into single plants also is available (Douglas, 1989). The general structure we use to represent a single plant is shown in Figure 1. Normally we assume that we recover and recycle all reactants (except for gaseous reactants, where normally we use a gas recycle and purge stream), reaction intermediates, and by-products produced by
-
(Gas Recycle) m I
p
u
=
I
c
Feed Streams
O
r
b
Reactor
Product
+
A
Liquid
Separa System
c
* t
Byproducts
Recycles
4
Figure 1. General structure of a flowsheet.
Table 11. Input Information 1. desired product, production rate, product value, and product purity 2. reactions and reaction conditions 3. raw material streams, conditions, and costa 4. data on the product distribution 5. data concerning the reaction rates and catalyst deactivation 6. any processing constraints 7. plant and site data and physical property data 8. data concerning the safety, toxicity, and environmental impact of each of the materiale 9. cost data for by-products produced secondary reversible reactions, and that we remove all
other by-products and feed impurities. We classify the by-products by their destinations,i.e., valuable by-product that we sell, fuel by-product where we recover the fuel value, aqueous waste by-products that are sent to pollution treatment, waste by-products that are incinerated, and solid wastes that are sent to a landfill, because if components have different destinations they must be separated and they have different values or costs. In level 1 the input information, including the reaction chemistry and the feed streams is supplied. In level 2, we consider only the feed, product, and by-product streams entering and leaving the plant, in order to get some idea of the difference between the product values and the raw material costs as a function of the design variables that affect the product distribution (reactor conversion,reactor temperature, reactor pressure, and molar ratio of reactants at the reactor inlet), as well as the reactant composition in the gas purge stream. Moreover, we identify the decisions that need to be made, and what alternative decisions we could make. Then, we consider the reactor recycle streams (level 3), and the decisions that affect these streams, in order to estimate the reactor exit flows. The reactor exit flows and the desired component destinations is the information we require to synthesize the separation system (level 4). If we neglect thermal pollution (level 5 ) , then we have a complete description of all the streams that can cause pollution problems. Hence, we only need to consider levels 1-4 for waste minimization studies. A discussion of the waste minimization decisions and several examples of pollution problems are presented below. Level 1. Input Information The information required to initiate a first design is listed in Table 11. It is important to note that it is necessary to have some information concerning the safety, toxicity, and environmental impact of all the components that will be present. It should also be noted that all of the information described in the list is seldom available at the outset of a deaign study, and that in practice one develops as much of the design as one can with the data available and then uses the results to justify taking additional data.
240 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
Table 111. Input-Output Decisions 1. purify the feed streams? 2. do not recover and recycle some reactants? 3. use a gas recycle and purge stream or vent gaseous reactants? 4. recover and recycle or remove a by-product formed by a secondary reversible reaction?
Level 2. Input-Output Structure This level of detail will describe all the streams that are fed to the process and that leave the process that are associated with the reaction chemistry and feed impurities. Hence, we can get a quick understanding of possible pollution problems caused by the reaction system. However, if we add new components to the process at later levels, then we will have to remove these materials, and we might encounter pollution problems that are associated with reactor diluents or heat carriers, separation system solvents, filter cake wash liquids, etc. The decisions we need to consider at the input-output level of detail are given in Table 111. Obviously, the decision not to recover and recycle some reactant and the decision to remove a by-product formed by a secondary reversible reaction will affect the streams leaving the plant and the possible need to send these streams to a waste treatment facility. Hence, we can identify process alternatives that will change the pollution treatment requirements. More serious problems are encountered, however, when the basic nature of the chemistry introduces difficult treatment problems. Some examples of this type are presented below. The only solution to these problems is to look for another chemical route for making the desired product. Air Oxidations. Numerous chemical intermediates are made by air oxidation of hydrocarbons. Some C02 and H20 are also produced during the oxidation. Economics dictates that we want to minimize the amount of C02 formed, but some NO, compounds may also be produced, which leads to a pollution problem. We could eliminate the NO, production by simply using pure oxygen for the oxidation. However, if this is attempted, the temperature rise in the reactor usually becomes excessive; i.e., the nitrogen in air moderates the temperature rise. An alternative would be to use pure O2for the oxidation, but to recycle C02 around the reactor, so that the C02 would moderate the temperature rise. Still another alternative would be to supply the oxygen from the surface of a solid, similar to a process for the ammoxidation of p-xylene to product terephthalonitrile: TPN plant: 860 O F , 25 psia p-xylene + NH3 + 3v205= p-TN + 3H20 + 3V204 p-TN + NH3 + 3VzOb = TPN + 3H20 + 3v204 p-xylene + 21V205 = 8C02 + 5H20 + 21V20, regeneration: 860 O F , 25 psia V2O4 + f/ZO, = V205 where p-TN is p-toluonitrile, TPN is terephthalonitrile, and the V204oxidation is carried out in a separate reactor system. Thus, process alternatives that avoid the production of NO, can be evaluated. Specialty Chemicals. Specialty chemicals normally are large molecules. A common technique for joining together smaller molecules to produce these products is to use caustic-chlorinationreactions. However, this chemistry often leads to the formation of salt, which c a w a disposal problem. Some examples of this type are described below.
,ao-@+u
+
n ci-@
+
n
CE-QCcl 1 I
I1
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lsophthaloyl Chloride
Terephthaloyl Chloride
Na salt Bisphenol A
r
L 1
-0Gr4 +4n
NaCl
cn,
Polyarylate
J
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Figure 2. Polyarylate chemistry. Table IV. Fenvalerate Reactions 1 p-ClCeH4CH3 + Clz = J J - C ~ C ~ H ~ C+HHC1 ~C~ 2 D-CICA~CHX~ + NaCN = D-CIC,H,CH,CN + NaCl 3 ^CHz&HCH3 + HCl = CH(CH3);Cl4 p-C1C6H4CH2CN+ CH(CH3)&1 + NaOH = p-ClC6H4CH[CH(CH3)z]CN + HzO + NaCl 5 PCIC~H~C[CH(CH~)Z]HCN + 2Hz0 = ~-CIC~H&H[CH(CH~)Z]COOH + NH3 6 p-ClC6H4CH[CH(CH3)2]COOH + SOCl2 = p-ClCpH4CH[CH(CH&]COCl 7 C,H&HO + BrCl = m-BrC6H4CH0+ HBr 8 CsH50H + NaOH = C6H,0Na + H20 9 m-BrC6H4CH0+ C6H60Na = C6HbOC6H4CH0 + NaBr 10 m-C&60C&&HO + p-ClC6H4CH[CH(CH3)zCOC1 + NaCN = fenvalerate + NaCl
Agricultural Chemicals. A set of reactions that can be used to produce fenvalerate, a synthetic pyrethroid, is shown in Table IV. A separate plant is used for each reaction. We see that NaCl (and NaBr) is formed in several of the reaction steps, and we expect that the salt will leave the reactors as a solid if water or any strong hydrogen bonding molecule is not present (reactions 2 and 10) or as a brine solution if water is present (reaction 4). The solid salt will be contaminated with organics, so that there will be a solids disposal problem. In reaction 4, both NaCl and water are formed so we would expect the NaCl to dissolve in the water, which will phase split from the organics. Thus,we will have a contaminated water stream to treat. Hence, we see that the basic chemistry can lead to significant waste disposal problems. Specialty Polymers. The chemistry used to produce the specialty polymer polyarylate is shown in Figure 2. We see that 4 mol of NaCl is produced per mer in the polymer (Le., for 60 repeat units, we would obtain 240 mol of NaC1). Hence, specialty polymer procesees exhibit many of the same waste treatment difficulties as are encountered in agricultural chemicals. Chemistry Alternatives. Two alternative routes for making alachlor are given in Table V. All of the reactions take place at different temperatures and pressures, so that for the first route two plants are required, whereas for the alternative route we need four plants. In the first route we make NaCl as a by-product, which will pose a pollution problem, whereas in the alternative route, we can sell the concentrated HC1 by-product. However, for the second alternative we need four plants, which probably will increase the capital investment. Stream Costs. In the hierarchical decision procedure we calculate the stream costs, which we call the economic potential at level 2 EP2 = product value + by-product values raw material costs (3)
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 241 Table V. Alternate Chemical Routes for Making Alachlor, a Herbicide A. Reactions CaHdC,Hd,NH, + ClCHpCOCl = HCl + "C~H3~C~Hs)2NHCOCH2~l C&3(C2Hs)2NHCOCH2Cl + NaNHz + ClCHzOCH3= C~H3(C2Hs)2N(COCH2C1)(CH20CH3) + NaCl + NH3
Table VI. Recycle Decisions I. excess of a reactant at the reactor inlet? 2. reactor heat effects-adiabatic, isothermal, heat carrier? 3. shift equilibrium conversion? how? 4. diluent to improve the product distribution? 5. reactor or product solvents? 6. complete conversion to avoid a separation? 7. reactor-separator (distillation, extraction, etc.)
as a function of the design variables that affect the product distribution (reador conversion, reactor temperature, reactor pressure, and molar ratio of reactants at the reactor inlet) and the purge composition of gaseous reactants. If we produce waste by-products, then we have negative by-product values. For solid waste streams, we need to know the land fill costa per pound of waste. For contaminated water waste streams, normally we must pay a sewer charge, given in terms of dollars per 1000 gal of the total waste stream (such as $0.20/1000 gal), as well as a waste treatment charge. An initial estimate of this cost can be determined by measuring the pounds of BOD (biological oxygen demand) per pound of organic compounds, and then using a treatment cost in terms of dollars per pound of BOD (such as $0.25/lb of BOD). Incineration costa are currently about $0.65/lb. These cost factors include both the annualized capital and the operating costa of the treatment facility. Hence, we consider pollution treatment costa associated with the chemistry at the very beginning of the design study. However, it is also important to consider the sensitivity of the design to these cost factors, because we expect that these costa will escalate rapidly in the near future.
Level 3. Recycle Structure The design decisions that are encountered when we consider the recycle structure of the flowsheet (level 3) are given in Table VI. If we can accomplish the reaction and separation in a single vessel, obviously we obtain a simpler flowsheet. However, some of the decisions involve introducing a new component into the flowsheet, i.e., adding a new component to shift the equilibrium conversion, to shift the product distribution, or to act as a heat carrier. This will require that we also remove the component from the process (unless it can be recovered and recycled), and this may cause a waste treatment problem. Some examples of this type are discussed below. Ethylene Production. A simplified set of reactions for ethane cracking to produce ethylene is given below. If this set of reactions properly describes the product distribution, CzHs = C2H4 + Hz (4) CzH6+ Hz = 2CH4 (5) we see that we can produce less by-product methane by adding a diluent to the reactor feed (or possibly along the length of the reactor). Steam is normally chosen as the
diluent, because the steam is easy to condense and separate. However, this introduction of steam to shift the product distribution, and to act as a heat carrier for the endothermic reactions, leads to an exit water stream that must be treated. If the steam is an inert, an alternative procedure would be to choose a different diluent that could be sent to a fuel supply or recycled less expensively than water. Of course, we could recover and recycle the water, rather than letting it exit the process. However, the high heat of vaporization of water normally makes the recycle after evaporation too expensive. Most other components have a lower heat of vaporization and therefore are less expensive to recover and recycle. Other alternatives are to recycle water after treatment in a bio pond or to use an impure water stream that is available somewhere else in the process (rather than to use fresh water everywhere). Styrene Production. A similar problem is encountered in styrene production. A simple representation of the chemistry is given below, and we see that the reaction that EB f styrene + H2 (6) EB benzene CzH4 (7) EB + H2 toluene CHI (8)
--
+
+
produces the product is reversible, while the two reactions that produce by-products are not. Hence, if we add a diluent to the reactor feed (or along the length of the reactor), we decrease the tendency of the first reaction to act in the reverse direction and we obtain an improved product distribution. Again, steam is normally chosen as the diluent (it also acta as a heat carrier),because when the reactor exit stream paases through a partial condenser and the gases have been removed in a flash drum,the water-hydrocarbon mixture will phase split. Thus, the water is easy to remove. Unfortunately, we again obtain a large stream of contaminated water that must be treated. We can avoid this treatment problem by choosing a different diluent or considering the alternatives for the ethylene process. Equilibrium-Limited Reactions. There are some processes where we add an inert component to shift the equilibrium conversion. Any time we add a new component, it either must exit from the process or be recycled. If we recycle the component, normally there still will be about a 1% loss directly to the environment or to another exit stream (i.e., complete recovery and recycle is impossible). Hence, when we select an inert component to shift the equilibrium conversion,we need to consider where that component will leave the process. Reactor Solvents. If we have a solid-phasefeed stream and a liquid-phase reaction, then we must either melt or dissolve the solid feed. We prefer to melt the feed in order to avoid a new separation, but if the melting point of the feed is greater than the reactor temperature, we need a solvent. We prefer to use another feed stream or a byproduct stream as the solvent in order to avoid adding a new separation unit, but in many cases we need to add a new component to the process. Depending on the rate of dissolution vs the reaction rate, we might dissolve the feed in the reactor or we might need to add a dissolver to the flowsheet. If we use water as a solvent, the common practice is to discard the water, whereas if we use some other component it usually is necessary to recover and recycle the solvent, although there will always be a 13'% ,or so, loss. Thus, whenever we add a new solvent, we need to consider where this solvent will leave the process, and whether it will lead to a pollution treatment problem.
242 Ind. Eng. Chem. Res., Vol. 31,No. 1, 1992 Table VII. Vapor Recovery System 1. condensation: low temperature and/or high pressure 2. gas absorption 3. gas adsorption 4. reactive absorption 5. membranes Table VIII. Liquid Separation Systems 1. stripping 2. distillation 3. azeotropic, extractive, and reactive distillation 4. extraction 5. crystallization 6. adsorption 7. membranes
Level 4a. Separation System Synthesis Once we know the reactor exit flows, as functions of the design variables, and the component destinations, we can synthesize a separation system. We first attempt to phase split the reactor exit stream. If we add a new component to cause a liquid-liquid split in the reactor exit stream or we add a new component that causes one or more of the other components to crystallize, then we again must consider whether the component will be recovered and recycled (with a small loss) or whether it will be discarded (water). The exit point for a recycled solvent will indicate whether or not pollution treatment is required. Level 4b. Vapor Recovery System Vapor recovery systems are used when we try to prevent valuable components from leaving the process with a gas stream. The most common types of units are listed in Table VII. Gas separation units, i.e., pressure swing adsorption, cryogenic condensation, and cryogenic distillation, are not as common in most petrochemical processes, and we do not consider them here. Gas absorbers require a solvent, and we prefer to use a component in the process as a solvent in order to avoid adding a new separation unit. However, if a new component is needed, and if water is selected, then it is common practice to discard the water, which leads to a pollution treatment problem. If a component other than water is selected as the solvent, then normally it must be recovered and recycled, although there will be a small loss. Gas
adsorption beds require regeneration, and it is common to find that stream is used to remove the adsorbed component from the bed. Again, the common practice has been to discard the condensed steam after the desired component has been recovered by distillation. In addition, spent adsorbents are usually sent to landfills. In cases where we must remove toxic materials, such as phosgene or HCN, from a gas stream, it is common to use reactive absorption. Thus, the toxic components are neutralized with NaOH solutions (or some equivalent), and then the solvent is sent to pollution treatment. The only way of avoiding this type of treatment problem is to change the reaction chemistry and to avoid the use of phosgene, HCN, etc. as a reactant.
Level 4c. Liquid Recovery System Liquid recovery systems are used to transfer components between phases or to separaM liquid mixtures. The most common types of units are listed in Table VIII. Distillation is preferred because it provides sharp separations of liquid mixturesand does not lead to significant pollution treatment problems. Azeoptropic, extractive, and reactive distillation require an entrainer or a catalyst, although normally these components are recovered and recycled. Stripping is used to remove "light" components from a liquid stream, and if steam is used as a stripping agent and the steam is discarded, then waste treatment is required. If water is used as an extraction solvent and the water is discarded, then a pollution problem is encountered. Adsorption is often used to remove color-forming materials, and spent adsorbents are usually sent to landfills. Crystallization separations to recover a desired product are usually operated at low degrees of supersaturation in order to prevent too rapid growth of the crystals and the inclusion of mother liquor within the crystals. For most crystallizer processes, it is necessary to recycle the mother liquor back to the crystallizer or back somewhere upstream of the crystallizer. Since trace components can accumulate in this recycle loop, normally it is necessary to include a purge stream, which often leads to a treatment problem. In cases where a product is crystallized from a mixture containing more soluble components that are solids at ambient conditions, then multistage crystallization procemes are used and the other by-producta are removed in
Table IX. Waste Minimization Problems level 2. input-ouput structure of the flowsheet a. problems caused by the reaction chemistry: change the chemistry b. problems caused by air oxidation to NO,: change to 02 in recycle C02 oxidations c. problems caused by spent catalysts regenerate the catalyst level 3. recycle structure of the flowsheet a. problems caused by adding reactor diluents to shift the product distribution or to shift the equilibrium conversion: change the diluent b. problems caused by adding heat carriers: change the heat carrier c. problems caused by adding reactor solvents: change the solvent level 4. specification of the separation system level 4a. general structure: phase splits level 4b. vapor recovery system a. problems caused by absorber solvents change the solvent b. problems caused by regeneration of adsorption beds: change the bed stripping agent c. problems with removing spent adsorbents change to absorption or Condensation d. problems caused by the use of reactive absorbers to remove toxic materials level 4c. liquid recovery system a. problems caused by stripping agents change the agent b. problems caused by extraction solvents: change the solvent c. problems caused by crystallizer (recycle and) purge streams (almost pure water): reuse the purge water elsewhere in the process d. problems caused by crystallizer purge streams (not almost pure water): remove the contaminants and recycle the water or look for a different separation system e. problems caused by reactive crystallization by-products: look for a different separation technique f. problems caused by spend adsorbents regenerate the adsorbent level 4d. solid recovery system a. problems caused by cake washing: same as for crystallizer; filter mother liquor streams
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 243 solution in a purge stream, which requires treatment. Reactive crystallization is a technique often used to recover organic acids. Materials, such as citric acid, are reacted with an inorganic compound, such as Ca(OHI2,to produce the calcium salt of the acid. This salt is extracted into a water phase, and the remaining organic compounds are purged from the process, which causes a treatment problem. The calcium salt is then hydrolyzed with H2S04 solution to recover the original product. The product is then extracted back into an organic phase, and the HzO, H2S04,and CaS04 are purged from the process and sent to a treatment facility.
Level 4d. Solid Recovery and Separation System Normally it is necessary to wash wet cakes leaving a filter or a centrifuge in order to recover valuable materials. Water is often used as a wash liquid, so that the wet cake sent to a dryer contains mostly water. Often the wash water that is not held up in the cake is discarded to a treatment facility, or a purge stream is sent for treatment if the wash liquid is recycled. In cases where cocrystallization leads to a mixture of solids, then dissolver-crystallizer processes can be used to separate the components (Ng, 1991). This type of process normally includes purge streams to avoid the accumulation of impurities in the recycle loop, and therefore treatment normally is required. Segregation and Recycle of Water: Water Integration From the discussion above, it is apparent why there are signifcant wastewater treatment problems in the chemical industry. If we segregate mildly contaminated water streams from more contaminated water streams, it might be possible to use these as solvents or wash liquids somewhere else in the process or in neighboring processes. With this approach we decrease the amount of water added to the process and thereby decrease the total exit flow of wastes. For example, in a process used to produce 300 X lo6 lb/year of caprolactam (the monomer for nylon 6), which is complex process with seven connected plants, about 1.5 X lo9 lb/year of wastewater is generated (about 5.5 lb of wastewater per lb of product.) Many of these waste streams are essentially pure water. Since about 1.5 X lo9 lb/year of fresh water is added to the process, we can reduce the waste load by reusing the high purity water streams. Classification of Waste Minimization Problems From the discussion above, we see that we can classify waste minimization problems according to the type of decision that causes them to occur and the level in the
hierarchy of decisions where the particular decision is encountered. "'his classification is given in Table M. The list given in Table M is probably not complete, but other types of problems can be included in this classification framework. It should also be noted that the alternatives suggested are often "easier said than done". In particular, the remedies of changing the chemistry or looking for a new separation system usually will not be practical for the retrofit of an existing process and might even be difficult for a new process. Nevertheless, the list should be useful for recognizing the origin of pollution problems.
Conclusion Using the hierarchical decision procedure of Douglas, it is a simple matter to identify waste minimization problems as a design is being developed and to identify process alternatives that often can be used to avoid these problems. A variety of examples of this type were presented. The same procedure can also be used to identify the source of pollution problems in existing processes. It is important to develop "cleaner processes", and the list of alternatives discussed in this paper might be useful in this endeavor. Acknowledgment Partial support for this work was received from DOE under Grant No. DE-FG02-87ER13676.
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Received for review February 25, 1991 Revised manuscript received June 25, 1991 Accepted July 8,1991
