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Ind. E n g . Chem. Res. 1992,31, 1502-1509
Khan, A. R.; Hunt, A. The Propagation of Faults in Process Plants: Integration of Fault Propagation Technology into Computer-aided Design. Znst. Chem. Eng. Symp. Ser. 1989, 114, 35. Kramer, M. A.; Palowitch, B. L., Jr. A Rule-Based Approach to Fault Diagnosis Using the Signed Directed Graph. AZChE J. 1987,33, 1067. Kumamoto, H.; Henley, E. J. Safety and Reliability Synthesis of Systems with Control Loops. AZChE J . 1979, 20, 376. Kumamoto, H.; Henley, E. J.; Inoue, K. Signal-Flow-Based Graphs for Failure-Mode Analysis of Systems with Control Loops. ZEEE Trans. Reliab. 1981, R-30, 110. Lambert, H. E. Comments on the Lapp-Powers ‘Computer-Aided Synthesis of Fault Trees.’ ZEEE Trans. Reliab. 1979, R-28, 6. Lapp, S . A,; Powers, G. J. Computer-Aided Synthesis of Fault Trees. IEEE Trans. Reliab. 1977, R-26, 2. Lapp, S. A.; Powers, G. J. Update of Lapp-Powers Fault-Tree Synthesis Algorithm. ZEEE Trans. Reliab. 1979, R-28, 12. Lee, W. S.; Grosh, D. L.; Tillman, F. A,; Lie, C. H. Fault Tree Analysis, Method, and Applications-A Review. ZEEE Trans. Reliab. 1985, R-34, 194. Oyeleye, 0. 0.;Kramer, M. A. Qualitative Simulation of Chemical Process Systems: Steady State Analysis. A K h E J. 1988,34,1441. Powers, G. J.; Lapp, S. A. A Short Course on Risk and Reliability
Assessment by Fault Tree Analysis; Post College Professional Education Center, Carnegie-Mellon University: Pittsburgh, PA, 1987. Qian, D. Q. An Improved Method for Fault Location of Chemical Plants. Comput. Chem. Eng. 1990,14,41. Salem, S. L.; Apostolakis, G. E.; Okrent, D. A New Methodology for the Computer-aided Construction of Fault Trees. Ann. Nucl. Energy 1977,4, 417. Shaeiwitz, J. A.; Lapp, S. A.; Powers, G. J. Fault Tree Analysis of Sequential Systems. Znd. Eng. Chem. Process Des. Dev. 1977,16, 529. Shafaghi, A,; Andow, P. K.; Lees, F. P. Fault Tree Synthesis Based on Control Loop Structure. Chem. Eng. Res. Des. 1984,62,101. Shiozaki, J.; Matsuyama, H.; O’Shima, E.; Iri, M. An Improved Algorithm for Diagnosis of System Failures in the Chemical Process. Comput. Chem. Eng. 1985, 9, 285. Tarjan, R. Depth-First Search and Linear Graph Algorithms. SZAM J. Comput. 1972, 1 , 146. Received for review August 15, 1991 Revised manuscript received February 5, 1992 Accepted March 4,1992
Avoiding Accumulation of Trace Components Sanjay K. Joshi and James M. Douglas* Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003
The accumulation of trace components in a recycle loop can cause a process to become inoperable. To prevent this problem, it is often necessary t o add additional exit points to the flowsheet, a t the conceptual stage of a process design, in order to remove the trace components. In this paper we present a systematic procedure for identifying exit points which are needed in order to ensure that the design will be operable.
Introduction Just about every company has had the unfortunate experience during the startup of a new process to observe the continual buildup of trace components in a recycle loop, so that eventually the process fails to meet its design specifications. It is significantly more expensive to shut down the process, empty it, purge the system, add new exit points, and restart the process than it is to add additional exit pointa before construction. For this reason, many experienced designers add an exit point in each recycle loop. However, the addition of an exit point in each recycle stream has the disadvantage that the exit points that are used during operation to prevent the accumulation of trace components often will cause waste treatment problems, which is in conflict with the new EPA priority of eliminating pollution problems at the source. Similarly, the exit points that are not used during operation correspond to wasted capital required to install the exit point. Hence, it would be advantageous to have a systematic procedure available that would indicate when additional exit points are needed, and to be able to estimate the increased recycle flows caused by trace Components. Another advantage of a procedure of this type is that it can be automated. We present a procedure of this type below. By defmition, a trace component is present in only small amounts in an input stream or produced in a reactor, so that trace components have a negligible effect on the overall material balances. However, these trace components can accumulate in recycle loops. If there is no way that the trace component can exit from the recycle loop, it will continue to accumulate until the process cannot 0888-5885/92/2631-1502$03.00/0
Table I. Primary Classification of Recycle Loops 1. reactant recycle loops 2. separation unit recycle loops 3. separation system recycle loops
meet the original design specifications. Even if the trace component can build up to a level where it can leave the recycle loop, if the design of the equipment in the recycle loop is not based on the increased recycle flow, the process might not be able to meet the overall design specifications. Since we are primarily concerned with the accumulation of trace components in recycle loops, we can simplify the problem if we classify the types of recycle loops that are common in flowsheets. The primary classifications are given in Table I. Separation unit recycle loops can then be classified as absorber recycles, extraction recycles, cake washing recycles, etc. Simple, common-sense, heuristics can be used to identify the need for exit pointa for reactor recycle loops and separation unit recycle loops. However, separation system recycle loops, i.e., where there are interconnected separation unit recycle loops in a separation system, are much more complicated. Hence, we present a new notation for treating these problems. Previous Work Douglas (1985,1988) described a hierarchical synthesis procedure for the conceptual design of chemical processes. However, while developing alternative flowsheets, the synthesis procedure does not consider the presence of trace component impurities. There does not seem to have been any previous attempt to develop a systematic procedure for adding new exit points to a flowsheet in order to avoid 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992 1503 Table 11. Comwnent Destinations gas recycle and purge or vent gas-phase reactants recycle liquid- or solid-phase reactants recycle reaction intermediates recycle or treat as byproduct byproducts produced by secondary reversible reactions feed impurities treat as byproduct primary product exit byproducts exit valuable byproduct exit fuel byproduct exit to waste byproduct wastewater treatment incineration landfill Table 111. Component Destinations for the Butadiene Sulfone Process component normal bp, K destination butadiene 264 recycle 268 recycle so* 424 product butadiene sulfone
the accumulation of trace components.
Reactor Recycle Loops One of the early steps in Douglas’ hierarchical decision procedure is to specify the destinations of all of the components that appear in the reaction chemistry and the raw material streams. These classifications are shown in Table 11. We can use this component classification scheme to propose some common-sense heuristics for the accumulation of trace components in reactor recycle loops. Heuristic 1. If the “lightest” component (lowest volatility) is a liquid-phase reactant (or a solid-phase reactant that can be melted and recycled as a liquid), then any trace component that is lighter than this “lightest” component will accumulate in this recycle loop. Heuristic 2. If two reactants with neighboring boiling pointa are recycled to two different reactors, then any trace component with an intermediate boiling point will accumulate on the feed stage of the distillation column used to separate the components (there will be an unexpected distributed key). Heuristic 3. If the “heaviest” component is recycled, then any trace component having a lower volatility will accumulate. Example: Butadiene Sulfone Production (AIChE Student Contest Problem, McKetta (1977)). Butadiene sulfone can be made at 90 O F and 150 psia by the liquidphase reaction. butadiene + SO2 butadiene sulfone (1) The destinations of the components are shown in Table 111,and a flowsheet is shown in Figure 1 (since the reverse reaction rate is significant at the boiling point of butadiene sulfone, distillation is not used for the separation). From Table 111,we see that if there is a trace component having a boiling point less than butadiene, it will accumulate. Moreover, if a trace component has a boiling point between butadiene and SO2,it will accumulate. Thus,we could add exit points in both recycle streams, and send the exit streams to a fuel supply. Example: Acetic Anhydride Production (AIChE Student Contest Problem, McKetta (1976)). Acetic anhydride can be made by the reactions acetone = ketene + CHI 700 “C, 1 atm (2) ketene = CO + (1/2)C2H4 700 O C , 1 atm (3) ketene + acetic acid = acetic anhydride 80 “C, 1 atm (4)
-
.--FT-P FLASH
BUTAD FLASH
PRODUCT
Figure 1. Flowsheet for the production of butadiene sulfone from butadiene and SO2.
i ACETIC ACID
ACETIC ACID INERTS
7 0::
4
ACETONE -
1
ACETONE
ANHYDRIDE
Figure 2. Flowsheet for the production of acetic anhydride by acetone cracking. Table IV. Component Destinations for the Acetic Anhydride Process component normal bp, K destination 82 vent co 112 vent CH, 170 vent CzH4 ketene 232 complete conversion acetone 329 recycle to reactor 1 acetic acid 391 recycle to reactor 2 acetic anhydride 412 product
The destinations of the Components are listed in Table N, and a flowsheet is shown in Figure 2. We see from Table IV that any trace component that boils between acetone and acetic acid will accumulate in the distillation column used to separate acetone from acetic acid and will accumulate in the reactor recycle loops. Hence, we would add an exit point near the feed tray of the acetone-acetic acid distillation column, and we would send the exit stream to a fuel supply. Adding New Exit Points to the Flowsheet. In order to avoid the accumulation of trace components in a reactor recycle loop, there are several alternatives for adding new exit points, including (1)purge from the recycle streams, a reboiler, a condenser, or near the feed tray in a distillation column and (2) addition of an additional separation
1604 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992 COMP
H z G L ]
BENZENE
REACT
‘45 H,4
FLASH
e l
+
CYCLOHEXANE
Figure 3. Flowsheet for cyclohexane production.
unit or units in a recycle loop, for example, a flash drum, a pasteurization section, or a side stream to a distillation column, a new distillation column, etc. Estimating the Recycle Flows of Trace Components. It is useful to be able to estimate the recycle flow corresponding to the accumulation of a trace component in order to ensure that the capacity of the equipment in the recycle loop is adequate to handle this increased flow. Some industrialists calculate these flows using simulators, but since the trace component flows are much smaller than the other flows, convergence problems often are encountered. Hence, it is useful to be able to estimate the recycle flows using approximate material balances; Le., we superimpose the trace component flows on the dominant flows. Many separations are based on phase splits (flash drums and decanters) or the transfer of one or more components from one phase to another (absorbers, adsorbers, extractors,crystdizers, etc.). A trace component will accumulate in a recycle loop until it reaches a level where the thermodynamic equilibrium allows it to escape. The exit flow of the trace component from the phase-transfer device (directed away from the recycle loop) must be equal to the feed flow of the trace component to the recycle loop or the rate at which it is produced in the reactor. Thus,we fin the inert feed rate as 0.1 % of the feed flow (or the amount of product produced in the reactor), we set this equal to the exit flow from the phase splitter and calculate the mole fraction based on the dominant flows, and we use thermodynamics to calculate the mole fraction in the recycle loop. Then, knowing the recycle flows of the dominant components in the recycle loop and the mole fraction of the trace component, we estimate the recycle flow of the trace component. In order to illustrate this procedure, consider the production of cyclohexane by the hydrogenation of benzene (see Figure 3). benzene + 3Hz cyclohexane (5)
-
If we want to produce 100 lb-mol/h cyclohexane, and there are no separation system losses, we need to supply 100 lb-mol/h benzene. If a very high purity H2 stream is available, we might assume that we do not need a purge stream from the H2recycle stream; i.e., any impurity will eventually leave with the flash liquid. Hence, the hydrogen feed rate needed is 300 lb-mol/h. Benzene and cyclohexane are difficult to separate, so we want to operate close to complete conversion of the benzene (there is no benzene recycle). The reaction is reversible, but by operating at high molar ratios (MR = 6) of H2 at the reactor inlet, low pressures and at high values of the equilibrium constant (400O F , 335 psia) we can obtain a high conversion. If the molar ratio of H2/
benzene at the reactor inlet is 6.0, then the recycle flow of H2is 300 lb-mol/h. We separate H2from the cyclohexane in a flash drum, recycle the hydrogen, and distill any H2dissolved in the flash liquid using a stabilizer. Thus, the flash liquid flow is primarily 100 lb-mol/h cyclohexane. If we assume that the composition of the trace component (say CHI) in the H2 feed stream is 0.1%, then the impurity feed rate is 0.3 lb-mol/h, which must also be the steady-state flow in the flash liquid. Then, the CHI mole fraction in the flash liquid is 0.003. If the flash drum operates at 100 OF and 330 psi4 the K-value of CH4is && = 7.5, and so the gas-phase mole fraction is 7.5(0.003) = 0.0225. Then, the recycle flow of CHI is approximately 0.0225(300) = 6.75 lb-mol/h, which does not cause a large increase in the gas recycle flow.
Separation Unit Recycles Most separation units (absorbers, extractors, etc.) require solvents, and normally these solvents must be recovered and recycled. If the solvent is a component in the process, then we can recover and recycle the solvent from the same distillation train used to separate unconverted reactants, the product, and byproducts. However, in some cases extraneous components are selected as solvents. If an extraneous component is used as a solvent, then the design heuristics indicate that the solvent should be recovered and recycled in the next unit. However, trace components can accumulate in these recycle loops. Gas Absorbers. Gas absorbers are used to remove valuable “heavy” components from a gas stream into a solvent. We prefer to use a reactant or a byproduct (or a feed impurity) as the solvent because we would not add new separation units to the flowsheet. However, if we choose to use a new component as a solvent, then we can chose this solvent to be “lighter” than the lightest component we want to recover or “heavier” than the heaviest component. With the first choice, we recover the solvent overhead from a distillation column and recycle it to the absorber, while we send the other components to the liquid separation system for the reactants, product, and byproducts. If we choose a heavier component as a solvent, then we can recover and recycle the solvent from the bottom of a local distillation column. Thus,for a gas absorber, we can propose two additional heuristics. Heuristic 4. If an extraneous component is selected as a solvent for a gas absorber and is lighter than the lightest component we want to recover, any trace component lighter than the solvent will accumulate in the solvent recycle loop. Heuristic 5. If an extraneous component selected as a solvent for a gas absorber is heavier than the heaviest component we want to recover, any trace component heavier than the solvent will accumulate in the solvent recycle loop. Strippers. Strippers are used to remove “light” components from liquid streams. Thus, they behave in a way opposite from that of gas absorbers. If we select an extraneous component as a stripping agent (which we classify as a solvent for the purpose of the present discussion), and we recover and recycle this stripping agent in a local distillation column, then the heuristics for the accumulation of trace components are similar to those for a gas absorber. Azeotropic and Extractive Distillation. For liquid mixtures that are highly nonideal, azeotropes are often encountered and simple distillation will not produce pure products. In order to break these azeotropes, entrainers (solvents) are added to change the relative volatilities of
Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992 1505 the original components. If we consider homogeneous mixtures, we normally recover and recycle these entrainers in an additional distillation column. If the entrainer is the lightest component in the separation or the heaviest, then the accumulation of trace components might cause an operability problem. If the entrainer is an intermediate boiler and one or more of the other components are reactants that are recycled, then trace components might act as distributed keys and it would become impossible to achieve the design splits, e.g., similar to the acetic anhydride process. The heuristics for these cases are similar to those discussed above. Extraction. Extraction involves the addition of a solvent to cause a phase split and to transfer one or more Components from one liquid phase to another. The solvent is often recovered for recycle in a local distillation column. Hence, depending on whether the solvent is lighter or heavier than the other components recovered, trace components can also accumulate in the solvent recycle loop. Again, the heuristics are similar to those discussed above. Crystallization/Filtration. Crystallization is often used to recover a component from a liquid mixture, particularly for cases where a product or byproduct is a solid at ambient conditions, but where the product or byproduct is produced by a liquid-phase reaction. After crystallization, the slurry is sent to a filter (or centrifuge) to separate the cake from the mother liquor. The mother liquor normally contains some uncrystallized material,so we want to recycle this stream to the crystallizer (or somewhere upstream of the Crystallizer. However, even if the other liquids are removed by evaporation in an evaporative or flash Crystallizer, any other trace component that does not evaporate will accumulate in the crystallizer recycle loop. Thus, we add a new heuristic. Heuristic 6. If a component is recovered in a flash or an evaporative crystallizer and the mother liquor from a subsequent filter or centrifuge is recycled to the crystalher (or upstream from the crystallizer), trace components might accumulate in the mother liquor recycle loop. A purge stream from this loop often causes waste treatment problems when the evaporated solvent is water. Cake Washing. If a filter or centrifuge appears in a flowsheet, the wet filter cake will usually contain valuable product, raw materials, or byproducts in the void space in the cake. Hence, we need to wash the cake to recover these valuable materials. If we choose a new component as a wash liquid (which we loosely refer to as a solvent), then we want to recover and recycle this wash liquid. Often we use a local distillation column for this purpose, and depending on whether the wash liquid has a higher or lower boiling point than the components recovered, we again can encounter accumulation problems for trace components. The heuristics are similar to 4 and 5 above. Dissolvers and Reactor Solvents. In some cases more than one solid crystallizes either in a reactor or in a crystallizer, and sometimes we can separate the mixture of solids by adding a dissolver (Ng, 1991). Similarly, when crystallization is used to recover a product, for high-purity products it often is necessary to redissolve and recrystallize the product. If we add a dissolver, we need to specify a solvent, and usually we recover and recycle this solvent. Depending on how we recover the solvent, trace components might accumulate in the dissolver recycle loop. If we have a solid feed stream and a liquid-phase reaction, then we usually need to add a melter or a dissolver to the flowsheet (in some cases we can dissolve the feed in the reactor). The recovery and recycle of a reactor solvent introduces the same problems as when a dissolver
Table V. Boiling Point List for the m-Aminophenol Process normal bp, K destination component 240 recycle "3 solvent DIPE 341 recycle 373 exit reaction H 2 0 373 recycle solvent H 2 0 rn-aminophenol 502 product 506 recycle resorcinol 662 exit DHDPA
is included in a separation system. Hence, the same potential problems with the accumulation of trace components can occur. If a dissolver solvent is recovered for recycle in a local distillation column, the heuristics are the same as for 4 and 5, whereas if a crystallizer is used, the appropriate heuristic is 6.
Separation System Recycles Many of the processes currently under development (new routes to make monomers, specialty polymers, agrochemicals, etc.) involve vapor/liquid/liquid/solid mixtures. The separation systems for these processes are much more complex than those previously discussed. Often several types of liquid separation techniques requiring solvents are present, and the separation unit recycle loops can interact with one another. This interconnection of reactor and separation system recycle loops introduces new possibilities for the accumulation of trace components. That is, trace components can be passed from one separation system recycle loop to another, where they might exit or accumulate in that loop. We consider these problems below in terms of a specific example. Example: Production of Aminophenol from Ammonia and Resorcinol. In an aminophenol process the reactions of interest are C6H4(OH)2 + 3" resorcinol
BC,H,(OH),
+ 3"
= C6H4(OH)(NH2)+ H2O rn-aminophenol
160 OF (6)
=
"[C6H4(OH)212 + H2O (7) (dihydroxydiphenylamine)
Resorcinol is melted using steam and is reacted with vaporized ammonia in a two-phase reactor to produce rn-aminophenol and the byproducts, dihydroxydiphenylamine (DHDPA) and water. The vapor phase leaving the reactor contains ammonia and water, and the liquid phase contains unreacted resorcinol, aminophenol, and DHDPA. Unreacted reactants, ammonia and resorcinol, are eventually recycled back to the reactor from the separation systems on the vapor and the liquid streams leaving the reactor. A flowsheet with a complete separation system is shown in Figure 4, and a component destination list is given in Table V. Resorcinol and the product are close boilers, so that they cannot be separated by distillation. The vapor stream from the reactor is first cooled and then distilled to recover ammonia from the water. Ammonia is recycled to the reactor, and the water is discharged to waste treatment. The liquid phase leaving the reactor is sent to a dissolver, where water is added as a solvent to dissolve the rn-aminophenol. Then, the aqueous mixture (water, aminophenol, and some resorcinol and DHDPA) is contacted with diisopropyl ether (DIPE) in an extractor. rn-Aminophenol is more soluble in water
1506 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992
I
1111 "2"
"I"
I
RESORCINOL
r--
--1
4- - iRECYCLE-NH,
H20 WASTE
I EXIT-H20
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I
'
r - - --1
I I I I I I I 1 I
I RECYCLE-Resw k - - - -J I EXIT-DHDPA I i EXIT-Prod I
L-----J
Figure 5. Separation tree originating a t the two-phase reactor. At this stage the two reactant recycles (one in each phase) are denoted a [RI, Rl and [R,, RI.
t
DHDPA G
Figure 4. Flowsheet for the production of rn-aminophenol from resorcinol and ammonia.
than resorcinol and dihydroxydiphenylamine, whereas resorcinol and dihydroxydiphenylamineare more soluble in DIPE than m-aminophenol. The m-aminophenol/water solution from the extractor is chilled to crystallize m-aminophenol. m-Aminophenol crystals are separated from the mother liquor in a filter or centrifuge. The wet cake is conveyed first to a dryer and finally to a packing unit. The mother liquor leaving the filter, which is a dilute solution of m-aminophenol in water, is recycled back to the dissolver. The solution of resorcinol and DHDPA in DIPE, from the solvent phase of the extractor, is sent to a sequence of distillation columns. Diisopropyl ether is recovered from the top of the fmt distillation column and is recycled back to the extractor, and the resorcinol is distilled from the DHDPA and is recycled back to the reactor. The DHDPA is sent to a fuel supply. Small amounts of the solvents, water and DIPE, are lost during the processing, and therefore, make-up streams are also shown in the flowsheet. The flowsheet contains two reactant recycle loops for the reactants resorcinol and ammonia and two separation-system recycle loops for the solvents DIPE and water. Again, some impurities will always be present in the reactant and/or the solvent streams, or possibly be produced by the reaction system. Trace components lighter than ammonia entering with the reactant streams or produced in the reactor would accumulate in the ammonia recycle loop (see design heuristic 1). If a component that boils between DIPE and NH3 enters with the reactant streams, it would remain in the vapor phase of the reactor and would, eventually, exit with the water stream. However, if the same impurity enters with the solvent, DIPE, then there are two possibilities. 1. If the impurity remains with the DIPE stream in the extractor, then it would accumulate in the DIPE recycle loop. 2. However, if the impurity passes to the water phase in the extractor system, then the impurity would be passed to the water recycle loop around the dissolver. It would accumulate in that loop and eventually affect the operability of the crystallizer and the product quality. Similarly, if an impurity that boils between the boiling point of DIPE and the reactor temperature (433K)enters with the reactants, it would remain in the vapor phase of
the reactor. Since the impurity is heavier than ammonia, it would eventually exit with the wastewater. However, if the same impurity enters with the DIPE make-up stream, then the following possibilities might occur. 1. If the impurity remains in the water phase in the extractor, then it would be passed to the water recycle loop. It would accumulate there and would eventually affect the operability of the separation units in the loop, i.e., the crystallizer, dissolver, and the extractor. 2. On the other hand, if the impurity remains with DIPE in the extractor/dissolver system, then at fist the impurity would accumulate near the feed tray of the fmt distillation column (applying design heuristics 2 and 3, a situation similar to that in the acetic anhydride process) and, eventually, would be passed to the reactant recycle loop for resorcinol. The resorcinol reactant recycle loop would carry the impurity to the reactor where it would remain in the vapor phase of the reactor and would eventually exit with the wastewater stream. Therefore, the interconnection of the recycle loops, the equilibrium distribution of trace components in different separation units, and the entrance points for these components generate different possibilities for the trace components to accumulate or to exit. In order to explore all the possibilities systematically, we will first represent the interconnections of the recycle loops with a tree of all the separation units. We introduce a new notation to simplify the automation of the procedure. Separation System Tree. The separation tree can be easily generated (see Figure 5 ) from the flowsheet. Branches "1"and "2" originate at the two-phase reactor. Branch "1" represents the vapor phase of the reactor and branch "2" the liquid phase. Figure 5 also shows two reactant recycle loops associated with these two branches. Let us denote each of these reactant recycle loops with the sequence of separations they go through. NH3 recycle loop: [ R L R ] resorcinol recycle loop: [R2, R] The nomenclature [R1,represents ] a recycle loop which originates at the vapor phase of the reactor (R1)and comes back to the reactor. Next, we will put more detail in the separation system blocks for the liquid and vapor phases of the reactor. Figure 6 shows a distillation column (denoted as DistA) for the vapor phase of the reactor to separate ammonia and water. A dissolver (denoted as Dsol), with water as solvent, is introduced into the separation system for the liquid phase of the reactor; the water is recovered and recycled thereby creating a separation system recycle loop. When we add this additional detail to the separation tree, we denote the recycle loops as follows (the underlined part of the loop denotes the actual starting and ending points of the loops, whereas the part which is not underlined
Ind. Eng. Chem. Res., Vol. 31, NO.6, 1992 1507
7l
I
I
RECYCLE-“, 1 EXIT-KO i--:---1
I I
I
I
I
i
:,_-i__,I L4
I
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“1“
I I
, ,
,
Z
1 --I 5 : “0 11
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I I
j
LIQUID SEPARATION 1
r--
1
I
---
1
I I
RECYCLE- Resor k - - - 4 EXIT-DHDPA 1
t------+
- IRECYCLE- H20 I EXIT- Prod
1
L------J
Figure 6. Development of separation tree develops as more details are added to the separation syetema for the vapor and liquid phases of the reactor. A solvent recycle, around the dissolver, is identified inside the separation system at the liquid phase of the reactor. The solvent recycle stream is denoted as [&,Deol, Dsol].
denotes the connection between the loop and the upstream units). NH3 recycle loop: [Rl, DistA, R1 resorcinol recycle loop: [R2. Dsol, R] water (solvent) recycle: [R2, Dsol, Dsol] The above recycle loops show two types of interactions, one due to the connection between a loop and the upstream unit(s) and the other between the loops due to a common separation unit(s). For example, [R2, Dsol, Dsol] represents a separation recycle loop around the dissolver (the underlined portion of the string) and shows the poseibility that the components which are present in the liquid phase of the reactor (R2, i.e., upstream) may pass into this recycle loop. In addition, if we compare the recycle loops [R2, Dsol, Dsol] and [R2, Dsol, R], we find (by comparing the underlined part of the loops) that the diesolver is common to both recycle loops. This common separation point creates a possibility that the components which are present in the water recycle loop may get passed to the reactant recycle loop, or vice versa. Let us now consider the separation tree for the complete flowsheet (see Figure 7). One more separation system recycle loop of DIPE around the extractor (denoted as Extr),is introduced. The solvent phase of the extractor goes through a sequence of distillation columns (denoted as DistB) to recover and recycle resorcinol and DIPE and to discharge DHDPA. The raffinate phase of the extractor is chilled in a crystallizer (denoted as Crys) to crystallize the product and to recycle water to the dissolver. With the complete separation tree shown in Figure 7,each of the recycle loops can be defined as follows. NH3 recycle loop: [Rl, DistA, R] resorcinol recycle loop: [R2, Dsol, Extrl, DistB, R] water (solvent) recycle: [R2, Dsol, Extra, Crys2, Dsol] DIPE (solvent) recycle: [R2, Dsol, Extrl, DistB, Extr] The above nomenclature for the recycle loops contains all the information which is necessary to analyze the effect of trace component impurities on the operability of the f l d e e t a with interconnected recycle loops. For example, the nomenclature for the DIPE recycle loop, i.e., [R2, Dsol, Extrl, DistB. Extr] contains the following information.
1. It is a separation-system recycle loop around the extractor. 2. The loop interacts with the upstream units via the liquid phase of the reactor (R2) and the dissolver (Dsol). Therefore, impurities which are introduced (either with the feed streams or as a reaction byproduct) at the reactor or with the water fed to the dissolver may pass into this loop. For example, if an impurity, lighter than DIPE, enters with the water fed to the dissolver and remains in the solvent phase of the extractor, it will accumulate in this recycle loop. 3. This recycle loop interacts with the water recycle loop at the extractor since the extractor (Extr) is a common separation unit in both of the loops. Therefore, there is a possibility that impurities may pass between these two loops in either direction. For example, if a trace component enters with the DIPE stream and remains with the raffinate phase (Extr2) leaving the extractor, it will pass into the water recycle loop. 4. The DIPE recycle loop also interacts with the resorcinol recycle loop at the distillation column (firstcolumn in DistB), since this distillation column is a common separation point between these two loops. For example, a trace component which enters with DIPE and which boils between DIPE and resorcinol will first accumulate (similar to the acetic anhydride process) near the feed tray of the first distillation column in DistB, and if this column is designed to account for this accumulation, it will eventually pass into the resorcinol recycle loop. However, once the impurity passes into the resorcinol recycle loop, all the other possibilities open up due to similar interactions between the resorcinol recycle loop and the other recycle loops which are present in the flowsheet. For example, if the impurity which is passed to the resorcinol recycle loop from the DIPE recycle loop remains in the vapor phase in the reactor, it will get passed to the ammonia recycle loop (since the resorcinol recycle loop interacts with the ammonia recycle loop at the reactor), and depending on whether it is heavier than ammonia or not, it will either accumulate or exit with the wastewater. Likewise, we can also denote the exit streams which are present in the flowsheet as follows. wastewater: [Rl, DistA, Exit]
DHDPA [R2, Dsol, Extrl, DistB, Exit] product: [R2, Dsol, Extr2, Crysl, Exit] The nomenclature [Rl, DistA, Exit] represents that any impurity which is either introduced with the reactants, or produced in the reactor, or passed on to the reactor through a reactant recycle loop will have an exit if it remains in the vapor phase (R1)in the reactor and if it is heavier than ammonia.
Component Classifications In the discussion above, we have generated a more general classification of the components involved in the process than the one which was used for earlier examples (see the component classifications in Table I1 and Table III). We have associated the sequence of separations that a component goes through before it exits or gets recycled. However, we can still use the heuristics (1-6) developed during the discussion of reactor recycles and separation unit recycles (where separations are carried out by absorption, condensation, distillation, and stripping and the solvent is recycled in a local column) in order to make a qualitative decision regarding the distribution of the impurity in these units. For example, we can still use the
1508 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992 Table VI. Tracing the Impurities That Enter with the Solvent Stream (DIPE) at the Extractor To Identify All the Possibilities for the Trace Components to Accumulate or Exit stane Dossibilities comments since the impurity is introduced at the extractor, all the recycle loops and all the stage 1 possible accumulation exit points which contain the string 'Extr" would generate possibilities resorcinol loop: [R2, Dsol, Extrl, DistB, R] water loop: [R2, Dsol, Extra. Crvs2. Dsol] DIPE loop: [R2, Dsol, Extrl, DistB, Extr] possible exit points DHDPA [R2, Dsol, Extrl, DistB, Exit] product: [R2, Dsol, Extra, Crysl, Exit] the impurities are soluble in DIPE and do not dissolve with water in the stage 2 possible accumulation resorcinol loop: [R2, Dsol, Extrl, DistB R] extractor/dissolver system DIPE loop: [R2, Dsol, Extrl, DistB, Extr] possible exit points DHDPA [R2, Dsol, Extrl, DistB, Exit] the impurities that boil lighter than DIPE will accumulate in the DIPE recycle stage 3 possible accumulation loop, whereas the impurities which boil between DIPE and resorcinol (design resorcinol loop: [R2, Dsol, Extrl, DistB, R] heuristics 1, 2, 3) will pass to the reactor and create a possibility for the ammonia loop: [Rl, DistA, R] impurity to get passed to the ammonia recycle loop DIPE loop: [R2, Dsol, Extrl, DistB, Extr] possible exit points wastewater: [Rl, DistA, Exit] all the impurities that enter the reactor via the resorcinol loop would remain in stage 4 possible accumulation the vapor phase at the reactor temperature, due to their boiling points; ammonia loop: [Rl, DistA, R] therefore, they would be passed to the ammonia recycle loop from the DIPE loop: [R2, Dsol, Extrl, DistB, Extr] resorcinol recycle loop possible exit points wastewater: [Rl, DistA, Exit] since the impurities which are passed to the ammonia loop are heavier than stage 5 possible accumulation ammonia, they will exit with the wastewater stream DIPE loop: [R2, Dsol, Extrl, DistB, Extr] possible exit points wastewater: [Rl, DistA, Exit]
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recycle loops in flowsheeta with interconnected recycle loops, the designer (or a computer code) can systematically trace all the possibilities and identify potential trace component operability problems. Example: Production of m -Aminophenol (Continued). For flowsheeta with separation-system recycle loops, we have discussed a method to identify when trace components will pass from one recycle loop to the other and then whether the trace component will accumulate in the new recycle loop or will find an exit point. The resorcinol feed might contain some organic impurity heavier than resorcinol. Also, some organic impurities which boil between 300 and 400 K might be present with the solvent, DIPE. However, we assume that no trace components are produced in the reactor. With this information about the feed streams and the chemistry, we carry out the analysis for identifying the possibility of the accumulation of impurities. Any impurity in the resorcinol feed enters at the reactor and therefore might accumulate in any of the following recycle loops. NH, recycle loop: [Rl, DistA, R] resorcinol recycle loop: [R2, Dsol, Extrl, DistB, R] water (solvent) recycle: [R2, Dsol, Extra, crys2, Dsol] DIPE (solvent) recycle: [R2, Dsol, Extrl, DistB, Extr] On the other hand, the possibilities that the impurity will exit the process include wastewater: [Rl, DistA, Exit] DHDPA: [R2, Dsol, Extrl, DistB, Exit] product: [R2, Dsol, Extr2, Crysl,Exit] However, if the impurity is heavier than resorcinol, it will remain in the liquid phase at the reactor exit (i.e., R2). This narrows down the list to the following options. Note, the dissolver and extractor act like one separation unit (there is only one stream that comes out of the dissolver).
Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992 1509 possible accumulation resorcinol recycle loop: [R2, Dsol, Extrl, DistB, R] water (solvent) recycle: [R2, Dsol, Extr2, Crys2, Dsol] DIPE (solvent) recycle: [R2, Dsol, Extrl, DistB, Extr] possible exit points DHDPA: [R2,Dsol, Extrl, DistB, Exit] product: [R2, Dsol, Extr2, Crysl, Exit] Now, we need to know more about the distribution of the impurity between the solvent and the raffinate phases in the dissolver/extractor system, If the impurity is equally soluble in water and DIPE, the list of options does not change. possible accumulation resorcinol recycle loop: [R2,Dsol, Extrl, DistB, R] water (solvent) recycle: [R2, Dsol, Extr2, Crys2, Dsol] DIPE (solvent) recycle: [R2, Dsol, Extrl, DistB, Extr] possible exit points DHDPA: [R2, Dsol, Extrl, DistB, Exit] Product: [R2, Dsol, Extr2, Crysl, Exit] We have traced the impurity to the distillation sequence (DistB) and the crystallizer. For the distillation sequence we can apply the design heuristics 1-3. If the impurity boils between resorcinol and DHDPA, from the design heuristic 2 we see that the impurity that enters the distillation sequence (DistB) would initially accumulate near the feed tray of the second column which separates resorcinol from DHDPA. If the column is designed to accommodate this buildup, the impurity would eventually exit with DHDPA. On the other hand, the other fraction of the impurity that has been passed to the water recycle loop would either accumulate or exit with the product, depending on whether the impurity crystallizes or not. If the impurity will crystallize with the product, it will exit with the product. Thus,the list of options is reduced to two exit points that show that the impurity will exit the process, and the flowsheet does not need additional exit points to accommodate the trace component impurities considered. The impurity would, eventually, exit partly with the product and partly with the DHDPA stream. possible accumulation: none
exit points DHDPA [R2, Dsol, Extrl, DistB, Exit] product: [R2, Dsol, Extr2, Crysl, Exit] Similarly, possibilities for the impurities entering with DIPE at the extractor are summarized in Table VI. F’rom Table VI we conclude impurities which are lighter than DIPE will accumulate in the DIPE recycle loop, while heavier impurities will find an exit point with the wastewater stream leaving the distillation column on the vapor stream from the reactor. Therefore, we need an additional exit point in the DIPE recycle loop. The only option is to take a vapor stream from the condenser of the column which separates DIPE from resorcinol and DHDPA.
Conclusions A procedure has been developed to identify potential operability problems caused by trace components, which may enter either with a feed stream or with a solvent stream, or may be produced by the reaction system. Heuristics are proposed to identify the likely buildup of trace components in the reactant or separation-system recycle loops for the case of vapor-liquid systems (no azeotropes)where the ordering of the normal boiling points of the components indicates the order in which the components can be removed. In addition, a systematic procedure has been developed to analyze the effect of trace components in processes with more complex separation systems having interconnected separation-system recycle loops. The analysis indicates where additional exit points should be added to a flowsheet in order to avoid the accumulation of trace components in the recycle loops. Acknowledgment We are grateful to the U.S.Department of Energy for providing financial support under Grant DE-FG02-87-ER 13676.
Literature Cited Douglas, J. M. A Hierarchical Decision procedure for Process Synthesis. AIChE, J . 1985,31, 353-362. Douglas, J. M. Conceptual Design of Chemical Processes; McGrawHill: New York, NY, 1988. McKetta, John J., Ed. Encyclopedia of Chemical Processing and Design; Marcel Dekker: New York, NY, 1976; Vol. 1. McKetta, John J., Ed. Encyclopedia of Chemical Processing and Design; Marcel Dekker: New York, NY, 1977; Vol. 5. Ng, K. M. Systematic Separation of a Multicomponent Mixtures of Solids Based on Selective Crystallization and Dissolution. Sep. Technol. 1991, 1 , 108-120. Received for review June 25, 1991 Revised manuscript received October 30, 1991 Accepted January 13, 1992
