Stream Costs

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Ind. Eng. Chem. Res. 2000, 39, 2410-2417

Stream CostssA First Screening of Reaction Pathways Michael A. Schultz† and James M. Douglas* Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts, 01003

The recent trend in the chemical industry is the rapid growth in the production of specialty chemicals, that is, specialty monomers, agricultural chemicals, pharmaceuticals, and so forth (high-value-added materials). The products produced often are very large molecules, and normally there are a very large number of reaction paths that can be used to produce them. Another feature of these multistep reaction problems is that the reaction chemistry usually is not completely known (a single reaction having a yield of less than unity is given or a single reaction that is not stoichiometrically balanced is reported for each reaction step). A first screening of these reaction paths can be accomplished by introducing fictitious reactions and/or fictitious components to account for the chemical uncertainties, and then using these approximations to calculate the raw materials costs and waste costs. Because the chemistry is not complete and the waste loads do not include separation system waste costs, all reaction paths within about 50% of the best should be retained for further consideration. 1. Introduction There is an accelerating trend for the chemical industry to increase their production of specialty chemicals. For instance, one major company indicated that their goal is to obtain 50% of their revenues from commodities and 50% from specialties. Examples of the types of molecules considered are shown in Figure 1 for agricultural chemicals, in Figure 2 for specialty monomers, and in Figure 3 for some pharmaceuticals. If a company makes a new molecule, similar to those given above, the physical properties of that molecule generally are not known. Moreover, with the numerous functional groups encountered in many of these molecules, group contribution estimates are very uncertain. For instance, in some cases melting points are 100 K in error. Minimization of the time-to-market is also a dominant feature of the process development. 1.1. Using Design Estimates To Guide Experiments. Experience has indicated that initiating a preliminary design project within a few weeks after a chemist discovers a new molecule can reduce the timeto-market. The older approach of having a chemist spending 1-2 years developing a complete chemical database for a design and then having designers determine that any process based on this reaction chemistry would not be profitable was extremely wasteful. Hence, the reaction kinetics, crystallization rates, filterability, and so forth are not known initially. Nevertheless, an engineer should develop the best possible design with the available information. These economic estimates can be used to help the chemist establish priorities for additional experimental work in the ranges of the design variables that are likely to give the most profitable process, rather than necessarily operating at the maximum yield. 1.2. Incomplete Reaction Chemistry and Multiple Reaction Paths. Another feature of these processes is that normally the chemistry of the multistep reaction system is not completely known. The dominant * To whom correspondence should be addressed. † Current address: UOP LLC, 25 East Algonquin Rd., Des Plains, IL 60017-5017. E-mail: [email protected]..

Figure 1. Some agricultural chemical molecules.

Figure 2. Some specialty polymer molecules.

Figure 3. Some pharmaceutical molecules.

reactions for each step may be given, although these reactions often have a yield less than unity, and in many cases are not stoichiometrically balanced. Still another feature is that normally there are a very large number of reaction paths that can be used to make any particular large molecule. For instance, fenvalerate (shown in Figure 1) has 56 alternative paths. Because each reaction path will have a different set of flowsheet alternatives, it is far from a trivial matter to decide which reaction path to pursue in more detail. It should be

10.1021/ie990272e CCC: $19.00 © 2000 American Chemical Society Published on Web 05/24/2000

Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2411

noted that the complete reaction chemistry for some petrochemical processes also is not known; that is, a component called “heavies”, “other”, or “impurities” often is formed. One of the goals of early designs then is to determine how sensitive the flowsheet structure and the economic estimates are to this unknown “component”. In particular, the economic sensitivity associated with both the reactor and separation system recycle loops is of primary interest, rather than the cost of a particular unit. 1.3. Batch versus Continuous Processes. Chemists have traditionally dominated the specialty chemical industry. New reaction systems normally were carried out in existing batch plants, and the process economics focused on only the raw material and waste costs. However, the production rates of specialty monomers and agricultural chemicals usually are in the range from about 0.5 to 10 MMkg/year, which is the range where a continuous process, a dedicated batch process, or some combination might be the most profitable. Hence, the batch versus continuous process decisions should always be considered. In contrast, the capacity of pharmaceutical processes normally are less than 50 000 kg/year, so they are almost always batch processes and equipment design and costs are normally neglected.

Table 1. Waste Disposal Costs6

2. Stream Costs

or not (i.e., whether they must be chemically modified before they exit the process). (3) The raw materials costs and the values of the product (in some cases a best guess), and the valuable byproducts that are sold or used as fuel. (4) The waste treatment costs for the other components. The costs we use for waste streams are given in Table 1.6 It should be noted that because we are considering only the reaction chemistry and not the separations in a complete flowsheet, our guesses of the component destinations, and therefore their waste costs, might need to be revised if a more detailed design can be economically justified. The procedure is applicable to both the batch and continuous processes because the overall material balances for a year’s operation are the same. This is a major subset of the data that the EPA uses for screening the environmental risks associated with new chemicals7 in terms of potential releases and environmental dispersions, their environmental persistence, uptake by organisms, human uptake, toxicity, and other health effects. The EPA is encouraging chemical companies to evaluate these risks as soon as a new reaction system is discovered. However, because only about 1% of new discoveries ever lead to a commercial process, it seems to be more reasonable to complete a conceptual design first to assess if the idea for a new process should be terminated or whether additional work can be justified.

In this paper we consider a very simple procedure for the initial screening of alternate reaction paths. The procedure is based on the introduction of fictitious reactions in each reaction step to account for yield losses and fictitious components to account for imbalanced reactions. Then, a first estimate of the stream costs (the economic potential at level 2)

EP2 ) product value + byproduct values raw material costs - waste costs (1) can be made by proceeding through level 0 (input data), level 1 (number of plants and plant connections), and level 2 (input-output structure) of Douglas’ Hierarchical Decision Procedure1-3 or an equivalent means-end analysis.4,5 It should be noted that the raw materials costs for a multistep reaction process include only the fresh feed costs and do not use transfer prices for any of the intermediates. Also, the waste costs we consider here include only those associated with the reaction chemistry and not those associated with the separation systems for each plant (a potential major error). Of course, if the reaction chemistry is not complete, then material balances based on fictitious components will not be correct. Similarly, in some cases the waste loads associated with the separation system are much larger than those associated with the reaction chemistry. Thus, we recognize that our screening procedure is very approximate and that all reaction paths within 50%, or so, of the best should be retained for further consideration. Nevertheless, any simplification of the problem is helpful. A great advantage of these partial estimates is that a minimum of input data is needed. The data needed include the following: (1) The reaction phase (as given by the chemist). (2) No physical property data other than the molecular weights of the known components, their ambient phase, whether they are organic or inorganic, whether they are water soluble or not, and whether they are toxic

Air Emissions Abatement

hydrocarbon VOCs chlorocarbon VOCs chlorocarbon VOCs + solids

1994 investment ($/cfm)

1996 operating costs (($/year)/scfm)

40 60 75

6 10 13

Minimum Incremental Costs for Aqueous Waste Treatment (Assumes 1 lb of BOD/lb of Organic)

1994 investment ($) 1996 operating costs ($/year)

water flow (gpm)

organic loading (lb/h)

3000 300

6000 2000

Solid Waste Disposal 1994 operating costs ($/lb) bulk organic liquids sludges with organics sludges with inorganics

0.32 1.05 0.40

solids (with organics) solids (with trace organics)

0.80 0.12

disposal method incineration incineration stabilization + landfill incineration secure landfill

3. General Structure of the Flowsheet First, we consider the general structure of the flowsheet (level 1, number of plants and plant connections) using the available chemistry. Douglas’ rules2 indicate that the number of plants is simply the number of reaction steps (the number of groupings of reactions that take place at the same temperature and pressure and use the same catalyst and the same solvent). Also, any byproducts produced in one plant that are reactants in another plant should be recovered/recycled to that plant. It is useful to see if these rules work for specialty

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Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000

Figure 5. PEI plant connections.

Figure 4. Molecules for polyetherimide reactions. Table 2. Polyetherimide Reactions plant

reaction

1

PA + MA f MPI + H2O

2

MPI + HNO3 f MNPA + H2O

3

BPA + 2NaOH f BPA-Na

4 5

BPA-Na + 2MNPA f 2 NaNO2 + BI PA + BI T 2MPI + DA

6

DA + PDA f PEI + 2H2O

reaction conditions 540 K, 20 psia 160 °F, 15 psia 220 °F, 15 psia 170 °F, 15 psia 395 °F, 400 psia 600 °F, extrusion

yield Y ) 0.99 Y ) 0.84 Y ) 1.00 Y ) 0.97 Y ) 0.97 Y ) 0.995

chemicals as well as for petrochemical processes. Moreover, it is of interest to see how the flowsheet structures and waste loads for specialty chemicals compare to those for petrochemicals. It should be noted that the reaction systems considered below are quite different than those discussed in the reaction path synthesis literature. Most of the reactions in the systems we consider are irreversible liquid-phase or liquid-solid reactions, inorganic components are present, solvents and catalysts are required, and the desired intermediates and product are solid. Some examples are presented for each of the specialty areas. 3.1. Specialty MonomerssPolyetherimide. One reaction system that can be used to make polyetherimide is given in Table 2. The molecules used in these reactions are shown in Figure 4. We see from the table that there are six reaction steps (so there will be six plants), each reaction is balanced, and the yields of most of the reactions are less than unity (the side reactions are missing). The production rates for specialty monomers are in the range from 0.5 to 10 MMkg/year, so that these processes might be continuous, batch, or some

combination. However, initially, we ignore the type of operation because the annual overall balances are the same. To develop a flowsheet, we begin with the final product reaction (plant 6) because the production rate of the product is the only flow that we know, and we see from the chemistry that the feeds to this plant are DA and PDA. The DA is produced in plant 5, so that these plants should be connected, and since the PDA is not produced in any other plant, it must be a fresh feed stream. Water is a waste byproduct from plant 6. The DA produced in plant 5 is made from PA and BI, where BI is produced in plant 4 and PA is a fresh feed. MPI is an exit stream from plant 5, and we do not assign a destination to it as of yet. The BI produced in plant 4 is made from BPA-Na and MNPA, where BPA-Na is made in plant 3 and MNPA is made in plant 2. NaNO2 is a waste byproduct from plant 4, which we might assume is water soluble. If we continue in this way, we find the structure corresponding to the forward reaction chemistry. Next, we consider the byproducts from each plant, and we note that MPI is produced in plant 5 and is a reactant in plant 2, so we connect these plants. Hence, we obtain the general structure of the complete process flowsheet shown in Figure 5, and we note that the plants are coupled by a recycle stream from plant 5 to plant 2. 3.2. Agricultural ChemicalssFenvalerate. A reaction system that can be used to produce fenvalerate is given in Table 3. Actually, there are 55 other routes that can be synthesized based on patented reaction steps. The production rates for agricultural chemicals are again in the range from 0.5 to 10 MMkg/year, but we delay any batch versus continuous implementation decision until later. We expect that there will be 10 plants required for this 10-step reaction process, and we note from the chemistry that again the side reactions are missing. We also note that inorganics are present in many of the reaction steps and that we produce NaCl, NaBr, SO2, HCl, NH3, and H2O as byproducts, so the waste costs are likely to be very high. It should be pointed out that we have ignored any solvents and catalysts at this point. The H2SO4 catalyst that we use in plant 5 will react with the NH3 byproduct to produce an ammonium sulfate waste stream, instead of NH3. Heptane is used

Table 3. Fenvalerate Reactions plant

reaction

yield

1 2 3 4 5 6 7 8 9 10

p-ClC6H4CH3 + Cl2 f p-ClC6H4CH2CN + HCl p-ClC6H4CH2Cl + NaCN f p-ClC6H4CH2CN + NaCl CH2dCHCH3 + HCl f CH(CH3)2Cl p-ClC6H4CH2CN + CH(CH3)2Cl + NaOH f H2O + NaCl + p-ClC6H4CH[CH(CH3)2]CN p-ClC6H4C[CH(CH3)2]HCN + 2H2O f NH3 + p-ClC6H4CH[CH(CH3)2]COOH p-ClC6H4CH[CH(CH3)2]COOH + SOCl f SO2 + HCl + p-ClC6H4CH[CH(CH3)2COCl C6H5CHO + BrCl f m-BrC6H4CHO + HCl C6H5OH + NaOH f C6H5ONa + H2O m-BrC6H4CHO + C6H5ONa f C6H5OC6H4CHO + NaBr m-C6H5OC6H4CHO + p-ClC6H4CH[CH(CH3)2]COCl + NaCN f fenvalerate + NaCl

Y ) 0.97 Y ) 0.99 Y ) 1.00 Y ) 0.95 Y ) 0.98 Y ) 0.98 Y ) 0.89 Y ) 1.00 Y ) 0.85 Y ) 0.97


Figure 6. Fenvalerate plant connections.

Figure 8. Molecules for the reaction system in Table 5. Table 5. Another Pharmaceutical Reaction System

Figure 7. Schematic flowsheet for the reaction system in Table 4.

as a solvent in plants 6 and 11, while toluene is used as a solvent in plants 8 and 9, so that these plants are coupled by the solvent flows. We develop a general flowsheet in the same way as for specialty polymers, again starting from the final product molecule. It is straightforward to show that the general flowsheet has the form of a converging tree, as shown in Figure 6. All of the agricultural molecules given in Figure 1 have this same converging tree structure. It would not have made any difference if we had ignored the plant coupling caused by the solvents to estimate the stream costs, just as we ignored solvent recycles in the individual plants for the PEI process. 3.3. Pharmaceutical Processes. Ali and Stephanopoulos5 have recently developed a systematic procedure (a means-end analysis with nonmonotonic planning) for the production of pharmaceuticals. The production rates of the processes are normally less than 50 000 kg/year, so they are almost always batch. The simplest chemical path they considered consisted of the multistep reaction system presented in Table 4. Although the precise yield information is not available, and thus not listed in the table, it is known that the yields of each reaction step are less than unity. In addition, the reactions are not stoichiometrically balanced. A schematic flowsheet is shown in Figure 7, and it is interesting to note that the process has only serial reaction steps, which is the most common case for pharmaceutical processes (i.e., when branches occur, these intermediates often are outsourced). Another example they considered involves much more complex molecules, where the reaction system is given in Table 5. Molecular structures for the components in these reactions are shown in Figure 8. Again, we would distrust group contribution estimates of the component physical properties. The yields of each of the eight reaction steps are less than unity (although again, the

(I) + CH3(CH2)3COCl + (AlCl3 + CH2Cl2) ) (II) + HCl (II) + ((CH3)2N)2CH2 + (CH3CO)2O ) (III) (III) + (H2SO4 + CH2Cl2) ) (IV) A + B + (toluene) ) Cat (IV) + C + (cat. + Triton X-405 + toluene + 50% NaOH) ) (V) (V) + (H2SO4 + toluene) ) (VI) + HCl (VI) + (H2SO4 + toluene + H2O) ) (VII) + H2O (VII) + AlCl3 + (toluene) ) (VIII)

data are not available) and the reactions are not stoichiometrically balanced. A schematic flowsheet is shown in Figure 9, and again we see a serial structure. 4. Waste Loads With the uncertain chemistry and the presence of so many inorganics, we would expect very high waste loads for these processes. To gain a better “feeling” for the magnitudes of these loads, we made a study of the waste loads for 20 single and multistep reaction processes (which included 79 individual plants). The results are shown in Table 6, where the number in parentheses after the process name refers to the number of plants in the process. Also shown are the wastewater reductions that would be possible by reusing almost pure water obtained overhead in distillation columns, evaporators, flash and evaporative crystallizers, and so forth. Another generic waste problem observed was the use of dryers for each intermediate in a multistep reaction process and then redissolution of the dried solid in the next plant. If the plants are on the same site, the washed cake could be transferred to the next plant as a slurry or in solution, thereby eliminating the dryer. The number in parentheses in the “no. of dryers” column of Table 6 indicates the number of dryers remaining after the elimination of unnecessary dryers. The waste loads are highly dependent on the reaction chemistry. It is well-known that serial flowsheet structures have a very detrimental effect on the raw material efficiencies, leading to large waste loads. If a 10-step serial process has a yield of 0.9 in each step, the overall yield is only 0.35. Hence, the converging tree structures used in agricultural (“ag.”) products are much more efficient than the serial structures encountered in pharmaceuticals. Because of this difference in flowsheet structures,

Table 4. Pharmaceutical Reaction System ClCH2CN + NaOCH3 + (MeOH) f ClCH2C(OCH3)dNH ClCH2C(OCH3)dNH + (CH3)2NH-HCl f ClCH2C[N(CH3)2])NH-HCl ClCH2C[N(CH3)2])NH-HCl + CH3COSH + NaOCH3 f CH3C(O)SCH2C([N(CH3)2])NH-HCl CH3C(O)SCH2C([N(CH3)]2dNH-HCl + HCl + (MeOH) f HSCH2C([N(CH3)2])NH-HCl

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Table 6. Process Waste Loads

process

solid aqueous no. of waste waste (kg/kg of % H2O gas no. of (kg/kg of product) saved exits dryers product)

Common Monomers styrene (3) 66.3 100 caprolactam (9) 4.9 63 adipic acid (BASF) (5) 1.6 70 HMDA(ADA) (2) 0.8 17 BHET(TPN) (2) 1.8 23 TPA(phthal. anhy.) (4) 0.0 N/A TPA(PXY-Br) (1) 0.4 57 TPA(PXY-HNO3) (1) 3.7 96 TPA(PXY-MEK) (1) 0.3 11 DMT(p-xyl) (4) 0.6 0 DMT(TPA) (1) 4.7 96 PEI (6) polyethersulfone (2) polysulfone (2)

3 3 2 0 1 2 2 2 1 2 1

0 1 1 0 1(0) 1 1 0 1 0 1

0.00 1.80 0.00 0.00 0.00 0.05 0.09 0.01 0.08 0.00 0.00

Specialty Polymers 18.0 33 1 16.2 65 0 2.5 10 1

5(1) 1 1

0.00 0.00 0.00

itaconic acid (1)

Fermentation 17.3 20

1

0

0.00

fenvalerate (10) methomyl (8) alachor (7) phenmedipham (6) benomyl (4)

Ag. Chemicals 36.3 60 22.5 26 0.7 88 5.7 4 12.0 60

3 1 0 2 3

0 2(1) 0 3(1) 1

0.00 0.00 0.00 0.00 2.40

it is not surprising that the waste loads for specialty polymers are about 12 lb of waste/lb of product (fewer reactions steps and sometimes recycle between plants), for ag. chemicals about 20 lb of waste/lb of product, and for pharmaceuticals5 about 100 lb of waste/lb of product, in contrast to petrochemicals where the loads are in the range of 0-5 lb of waste/lb of product. As shown in Figure 3 some pharmaceutical molecules are relatively small, whereas others are very complicated. An early engineering analysis helps to demonstrate to the chemist the value of developing converging tree reaction paths, at least for the small pharmaceutical molecules. 5. Other Differences among Various Specialty Processes Although of not immediate concern for our purposes, it is interesting to note some of the other differences among the various specialty processes. Ag. chemical processes normally recycle unconverted reactants in the early plants that produce the small molecules in the pathway. As the molecules become larger (and for large molecules in the specialty monomer pathways) they are often solids, and so the use of solvents is common and the solvents are recycled in the individual plants. Because the separation of large molecules is difficult, every attempt is made to operate the reactors at close to complete conversion. This means that an excess of the nonlimiting reactant is used, and this excess is often discarded, thus increasing the waste loads. In contrast, recycle of reactants in pharmaceutical processes is almost never used, and common practice is to crystallize and recover each intermediate as a solid. This practice leads to very robust processes, where impurities have little chance of being passed to a downstream plant. Another commonly accepted practice in pharmaceuticals processes is to operate every reactor at 300 K (laboratory temperature), so that the commercial facility usually has a very large refrigeration system to cope with reactor cooling. Hence, an early

engineering analysis might help to convince pharmaceutical chemists to carry out laboratory reactions at slightly higher temperatures, which would reduce the required cooling load. 6. Introducing Fictitious Reactions and ComponentssLevel 2 Analysis Tables 2 and 3 present the reaction chemistry in terms of single reactions having a yield less than unity. Tables 4 and 5 imply this same behavior, but in addition the reactions are not stoichiometrically balanced. Initially, we attempt to compensate for these errors by introducing fictitious reactions to account for yield losses and fictitious components to balance the reactions. For the case of the addition of a fictitious reaction, we assume that the same reactants as those that appear in the primary reaction are used and that the fictitious waste byproduct has the same molecular weight as the sum of the reactants (weighted by the stoichiometric coefficients). Obviously, this assumption will give us incorrect material balances. Nevertheless, these estimates are useful for demonstrating to the chemist the economic incentive for determining the actual chemistry. (A more profound difficulty is encountered later in the development of a design, when we try to figure out how to separate the missing byproducts when we do not know what they are). It is important to recognize that any additional experiments will delay the time-to-market, so there must be a significant incentive for making more measurements. Moreover, an early interaction between an engineer and a chemist is essential to convince the chemist to consider alternative pathways before too much investment of time has been made on any particular pathway. We might try to use the reaction path synthesis procedures to determine the unknown side reactions, but Stephanopoulos8 found that for pharmaceuticals as many as 10-20 side reactions all seemed reasonable. A recent publication by Wayhu et al.9 indicates that 18 byproduct reactions could be considered for ethylene dichloride production in a vinyl chloride process. If we have a 10-step reaction process with 10 or so possible side reactions for each step, the problem becomes too complex to consider. Hence, we stay with the introduction of fictitious components, but we would like to get some “feeling” for the magnitude of the error we introduce. 7. Error Estimate in Approximate Material Balances If we consider the first reaction in the fenvalerate process, Table 3, assume that the side reactions involve the formation of a dichloro compound; that is, the reactions should be given as below:

p-ClC6H4CH3 + Cl2 ) p-ClC6H4CH2Cl + HCl

(2)

p-ClC6H4CH2Cl + Cl2 ) p-ClC6H4CHCl2 + HCl (3) which is a series-parallel scheme. Hence, Cl2 should be the limiting reactant and p-ClC6H4CH3 (p-chlorotoluene, PCT) should be used in excess and recycled. The yield is defined as


yield ) Y )

mol of desired product mol of limiting reactant converted

(4)

If we choose 100 mol/h (or 100 mol for a batch process) as a basis, we can calculate the component flows versus the yield. The results are given in Table 7. Now, if we consider a case where the side reaction is not known, and we introduce a fictitious component to account for the yield loss, the reaction chemistry becomes

p-ClC6H4CH3 + Cl2 ) p-ClC6H4CH2Cl + HCl p-ClC6H4CH2Cl + Cl2 ) W

(5)

Figure 9. Schematic flowsheet for the reaction system in Table 5.

(6)

where W has a molecular weight equal to the sum of the reactant molecular weights. The results are again shown in Table 7. As the table indicates, the results are more accurate at higher yields, which is not surprising. Although not correct, the use of fictitious waste components allows us to make a first estimate of the waste loads, and the stream costs, for a complete process. These results then allow a more informative discussion with a chemist. 7.1. Including Fictitious Waste Components of the Schematic Flowsheet. For the chemistry shown in Table 3, we can redraw the general structure of the flowsheet and include a waste component for each reaction step that has a yield less than unity. This flowsheet is shown in Figure 10. 7.2. Estimating the Streamflows. If we consider a continuous process that produces 0.909 MM kg/year of fenvalerate and operates with a stream factor of 0.850 (7446 h/year), then the production rate is 122 kg/h (0.291 kg mol/h). Note that if we were operating on a much smaller scale, producing 122 kg in 1 year in a batch process, the calculations will be the same except for the units. From Table 3, the reaction that produces the final product is reaction (10), with a yield of 0.970:

m-C6H5OC6H4CHO + p-ClC6H4CH[CH(CH3)2]COCl + NaCN f fenvalerate + NaCl and so the NaCl produced is 0.291 kg mol/year (16.9 kg/h). For a yield of 0.970, the consumption of the reactants in the dominant reaction are

m-C6H5OC6H4CHO ) 0.291/0.970 ) 0.300 kg mol/h (59.4 kg/h) p-ClC6H4CH[CH(CH3)2]COCl ) 0.300 kg mol/h (69.3 kg/h) NaCN ) 0.300 kg mol/h (14.7 kg/h) Because the yield is only 0.970, we assume a waste reaction (see Figure 10),

m-C6H5OC6H4CHO + p-ClC6H4CH[CH(CH3)2]COCl + NaCN f W7 where the molecular weight of W7 is 478 (198 + 231 + 49). The exit flow of this organic waste component is

W7 ) (1-0.970) × (0.291/0.970) ) 0.009 00 kg mol/h (4.30 kg/h)

Figure 10. Fenvalerate flowsheet including fictitious waste streams. Table 7. Comparison of Component Flows for Different Chemistries yield

FPCT (mol/h)

0.8 0.9 0.95

112.5 105.6 102.6

0.8 0.9 0.95

125.0 111.1 105.3

FCl2 (mol/h)

PHCl (mol/h)

Correct Chemistry 125.0 125.0 111.1 111.1 105.3 105.3 Fictitious Waste Component 125.0 100.0 111.1 100.0 105.3 100.0

Pwaste (mol/h) 12.5 5.6 2.6 25.0 11.1 5.3

An estimate of the waste loads then includes the NaCl produced (16.9/122 ) 0.138 kg/kg of product and the fictitious organic waste (4.30/122 ) 0.0353 kg/kg of product). Now that we have an estimate for the production rates of m-C6H5OC6H4CHO and p-ClC6H4CH[CH(CH3)2]COCl, we can make similar balances for plants 6 and 9. If we continue in this way, we can estimate all of the waste flows given in Table 8. 7.3. Excess Reactants. The estimates of the waste loads given above assume that the feeds to any individual plant are the stoichiometric amounts. However, this is seldom the case in practice. For example, a chemist’s recipe for reaction (10) might indicate that the molar ratio of the reactants are 1:1:1.2, so that an excess of NaCN is used to try to force the other reactants to complete conversions. Normally, this excess is not recovered and recycled for the plants that produce the large molecules. Hence, the inorganic waste load would increase to (2.94 + 16.9)/122 ) 0.16 kg of waste/kg of product. Therefore, if a chemist’s recipe is available, we can screen the reaction paths more accurately. However, it is essential to remember at this stage of a design that accuracy is only important in terms of deciding whether

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Table 8. Waste Summary for the Fenvalerate Process

plant plant 1 (PCBC)

type of waste

aqueous inorganic organic sludge plant 2 (CPAN) aqueous inorganic organic sludge plant 3 (IPC) vapor to fuel plant 4 (MCPBN) aqueous inorganic organic sludge plant 5 (IPCPA) aqueous inorganic organic sludge plant 6 (IPCAC) aqueous inorganic organic sludge plant 7 (BrBenz) aqueous inorganic organic sludge plant 8 (NaPHN) aqueous inorganic plant 9 (MPB) aqueous inorganic organic sludge plant 10 (Fenv) aqueous inorganic organic sludge

estim. actual waste waste total (kg/kg of (kg/kg of waste product) product) estim. 1.55 0.02 0.16 0.00 0.02 0.97 0.04 1.28 0.04 3.99 0.02 0.30 0.01 0.17 0.28 0.13 0.16 0.04

1.55 0.00 0.17 0.02 0.02 4.83 0.00 3.11 0.00 4.06 0.00 9.19 0.00 0.17 11.35 0.00 0.20 0.00

100% 84% 100% 21% 40% 99% 3% 100% 4% 100%

we should do more work or abandon the project and look for a better reaction path. 7.4. Separation System Waste Loads. A comparison of the approximate waste load estimates to a complete design calculation for one alternative for each plant is also given in Table 8. The plants where there are large errors (plants 4, 5, 7, and 9) correspond to situations where there are aqueous-inorganic and organic liquid-phase mixtures, and so large amounts of water are added to accomplish the separations. Our experience indicates that the presence of inorganics in the reaction system always leads to large waste loads, and so we would like to find reaction pathways that include as few inorganics as possible. 8. An Alternative Reaction Path for Producing m-Phenoxybenzaldehyde (MPB) From an inspection of the results in Table 8, it is apparent that the highest waste loads are created in plants 7, 8, and 9, used to make m-phenoxybenzaldehyde (MPB, C6H5OC6H4CHO). Another reaction path that has been proposed to make this intermediate is given in Table 9. This path starts with cresol rather than benzaldehyde (a more expensive reactant). Unfortunately, no yield data could be found for this reaction path, but even by inspection it appears to have large potential waste loads. There are numerous other reaction paths, as complicated or more so, to make this intermediate, including a bioreaction of the m-phenoxytoluene (m-CH2BrC6H4OC6H5) with a culture of a methyotropic microorganism grown in methane or methanol.10 Screening on the basis of stream costs alone is not as simple as it sounds, but it is a reasonable first step before any attempt is made to develop a complete flowsheet or conceptual (short-cut) design for any alternative reaction path. 9. Implementations of the Procedure The procedure for introducing fictitious components described above has been implemented by Schultz and Douglas in a code called StreamCosts ((levels 0, 1, and 2 of Douglas’ Hierarchical Decision Procedure). It will be available to academics through the CAChE Corp. (the code creates the flowsheet structure and estimates the material balances). It has also been implemented by

Table 9. Alternate Reaction Path To Make C6H5OC6H4CHO plant A: cresol to m-cresylates m-CH3C6H4OH + NaOH f m-CH3C6H4ONa + H2O m-CH3C6H4OH + KOH f m-CH3C6H4OK + H2O plant B: m-cresylates to m-phenoxytoluene m-CH3C6H4ONa + ClC6H5 f m-CH3C6H4OC6H5 + NaCl m-CH3C6H4OK + ClC6H5 f m-CH3C6H4OC6H5 + KCl plant C: bromination of m-phenoxytoluene m-CH3C6H4OC6H + Br2 f m-CH2BrC6H4OC6H5 + HBr m-CH3C6H4OC6H + 2Br2 f m-CH2Br2C6H4OC6H5 + 2HBr plant D: production of C6H5OC6H4CHO (very complex reactions!) Primary HCHO + 4NH3 T (CH2)6N4 + 6H2O HCHO + NH3 T CH2dNH + H2O m-CH2BrC6H4OC6H5 + (CH2)6N4 f [C6H5OC6H4CH2(CH2)6N4]+Br- (salt) (salt) + CH2dNH f C6H5OC6H4CH2NdCH2 + (CH2)6N4 + HBr C6H5OC6H4CH2NdCH2 + H2O f C6H5OC6H4CH2NH2 + HCHO C6H5OC6H4CH2NH2 + CH2dNH f C6H5OC6H4CHdNH + CH3NH2 C6H5OC6H4CHdNH + H2O f C6H5OC6H4CHO + NH3 m-CH2Br2C6H4OC6H5 + H2O f C6H5OC6H4CHO + HBr Others (CH2)6N4 + HBr f NH4 Br + ? ?? + ?? f C6H5OC6H4CH2NHCH3

Stephanopolous and co-workers11 in the Batch Design Kit, which is available from Hyprotech Corp. The StreamCost code only includes these estimates for batch and continuous processes, whereas the Batch Design Kit is for a complete design of the process. Conclusions Dramatic changes are taking place within the chemical industry, which can either be perceived as profound threats or fascinating challenges.12 The current thrust of design research for petrochemical processes emphasizes being as rigorous as possible (it is assumed that the chemistry is completely known, all of the physical properties are known, it is simple to synthesize a complete superstructure that contains all of the process alternatives, all of the reaction and crystallization kinetic data are known, information on crystal habit and filter cake porosity as a function of the design variables are known, accurate models are available for all solid processing units, etc.) However, for specialty chemicals the time-to-market becomes a dominant consideration, which implies a minimum number of experiments and basing a design on incomplete data. Resolving the apparent conflict between these two sets of goals, particularly for the batch versus continuous process decision for many specialty chemicals, provides a fertile area for future research. Acknowledgment The author’s are grateful to the National Science Foundation for partial support under Grant NSF-CTS9313015 and to the Design and Control Center at the University of Massachusetts. Literature Cited (1) Douglas, J. M. Conceptual Design of Chemical Processes; McGraw-Hill: New York, 1988. (2) Douglas, J. M. Synthesis of Multistep Reaction Processes. In Foundations of Computer-Aided Process Design, Proceedings

Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2417 of the Third International Conference on Foundations of ComputerAided Process Design, Snowmass Village, CO, July 10-14, 1989; Siirola, J. J., Grossmann, I. E., Stephanopoulos, G., Eds; Elsevier: Amsterdam, 1990; p 79. (3) Douglas, J. M. Process Synthesis for Waste Minimization. Ind. Eng. Chem. Res. 1992, 31, 238. (4) Siirola, J. J.; Rudd, D. F. Computer-Aided Synthesis of Chemical Process Designs. Ind. Eng. Chem. Fundam. 1971, 10, 353. (5) Ali, S.; Stephanopoulos, G. Synthesis of Batch Processing Schemes as Synthesis of Operating Procedures: A Means-Ends Analysis and Non-Monotonic Planning Approach. Presented at the 1998 Annual AIChE Meeting, Miami Beach, FL, Nov 15-20, 1998; paper 216c. (6) Dyer, J. A.; Taylor, W. C. Presented at the Air and Waste Management Association Annual Meeting, Cincinnati, OH, June 1994. (7) Allen, D. T.; Rosselot, K. S. Pollution Prevention for Chemical Processes; John Wiley & Sons: New York, 1997.

(8) Stephanopoulos, G. MIT. Personal communication, 1995. (9) Wayhu, H.; Lakshmanan, R.; Ponton, J. W. Automated Generation of Reaction Products. Presented at FOCAPD ‘99, Breckenridge, CO, July 18-23, 1999. (10) Dalton, H. Microbiological Production of Aromatic Alcohols and Aldehydes. British Patent 2,149,783, 1985. (11) Stephanopoulos, G.; Linninger, A.; Salomone, E. Batch Process Development: Challenging Traditional Approaches. Presented at FOCAPD ‘99, Breckenridge, CO, July 18-23, 1999. (12) Fowler, A. E. Process Development in the New Millennium...Hands on or Modeled, In-sourced or Out-sourced, Solo or Shared. Presented at FOCAPD ‘99, Breckenridge, CO, July 1823, 1999.

Received for review April 12, 1999 Revised manuscript received October 11, 1999 Accepted February 18, 2000 IE990272E

Stream Costs

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