separation system synthesis liquid mixtures separations

Ind. Eng. Chem. Res. 1990,29,421-432 42 1

Separation System Synthesis: A Knowledge-Based Approach. 1. Liquid Mixture Separations

Scott D. Barnicki and James R. Fair* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062

A description is given for a task-oriented, or problem decomposition, approach to the selection and sequencing of methods for separating multicomponent liquid mixtures. The design knowledge of the expert is organized into a structured query system, the separation synthesis hierarchy (SSH). This hierarchy divides the overall separation synthesis problem into subproblems or “tasks”. These tasks can be solved essentially independently from each other. Each task consists of a series of ordered heuristics based on pure component properties and on process characteristics. In its current implementation, SSH is limited to the sequencing of multicomponent mixture separations using eight industrially significant separation methods: simple distillation, azeotropic/extractive distillation, liquid-liquid extraction, stripping, adsorption, membrane permeation, and crystallization.

During the past 15 years, considerable effort has been expended on developing systematic methods for the sequencing of distillation columns. Evolutionary and ordered heuristic methods have been notably successful for this type of space search problem and require relatively little expert design knowledge (Nishida et al., 1981; Kelley, 1987).

Although distillation is the mainstay of the separation industry, a considerable number of situations exist in which distillation is a poor choice. The more general industrial problem of separation synthesis, using a number of different separation methods, has received little attention. Such a knowledge-intensive problem is not suited to solution solely by the ordered heuristic methods developed thus far.

This paper describes a task-oriented, or problem decomposition, approach to the selection and sequencing of separation methods for multicomponent liquid mixtures. The design expert’s knowledge is organized into a structured query system, the separation synthesis hierarchy (SSH). This hierarchy divides the overall separation synthesis problem into subproblems or “tasks”. Each task can be solved essentially independently from the other tasks. The separation synthesis hierarchy presented here is being developed explicitly for implementation in a knowledge-based expert system, the separation synthesis advisor (SSAD). SSAD is currently in the prototype stage of development. In its current implementation, SSAD is limited to the preliminary sequencing of multicomponent liquid mixtures using one of the following methods: (1) simple distillation, (2) azeotropic/extractive distillation, (3) liquid-liquid extraction, (4) stripping, (5) adsorption, (6) membrane permeation, and (7) melt crystallization.

In this work, methods requiring an extraneous substance to effect the separation are called mass separating agent (MSA) processes. All methods on the list above except simple distillation and melt crystallization (and sometimes azeotropic distillation) are MSA processes. Both simple distillation and melt crystallization require only the addition or removal of energy. Mass separating agent processes are further divided into methods requiring physical solvents or entrainers (PSE processes-azeotropic/extractive distillation, liquid-liquid extraction, and stripping), and methods requiring solid-phase agents (SPA processes-adsorption and membrane permeation).

The term azeotropic distillation is commonly used to refer to two different types of fractionation involving azeotropes. The first type relies on the azeotrope(s) in- herently present in the mixture to effect the separation; only the addition of energy is required. The second type of azeotropic distillation is a PSE process. An extraneous

0888-5885/90/2629-0421$02.50/0

substance, called an entrainer, which forms an azeotrope with one or more components is added to the mixture. The fractionation of the resulting azeotrope(s) achieves the desired separation.

Problem Statement The development of an expert system for the synthesis

of separation sequences is an interdisciplinary endeavor, combining aspects of both chemical engineering and artificial intelligence (AI). These two diverse fields con- tribute very different, but deeply interrelated, perspectives to the separation synthesis problem. In broad terms, the chemical engineering separation synthesis problem for liquid mixtures can be stated as follows:

Given (1) an n-component liquid mixture, (2) physical property data on the mixture, (3) product specifications, and (4) a portfolio of potential separation techniques, find the method(s) and sequence(s) of separations that (1) produce the desired products with the desired purities, (2) result in minimum separation costs, and (3) result in a limited number of feasible, reliable process designs.

The synthesis of separation sequences is a classical chemical engineering design problem. Such work has been done successfully for decades. However, due to the inherent uniqueness and complexity of each new design problem, a comprehensive and systematic approach to process synthesis has remained elusive; process design still resides in the domain of the “expert”. As such, the following questions remain largely unanswered:

(1 j What knowledge is needed to determine which separation techniques should be used and in what order they should be accomplished?

(2) How does an expert organize this knowledge to make design decisions?

The synthesis of separation sequences encompasses three basic categories of AI problem types: (1) space search, (2) selection, and (3) design.

The space search problem arises from the need to ef- ficiently and systematically explore the potential separation sequences. Parallel to the sequencing is the selection of a separation method for a given split in a multicomponent mixture. The need for short-cut process modeling and economic evaluation bring into play design problems. Moreover, the situation is further complicated by the need to manipulate a large data base of physical/chemical properties.

The important questions from an AI/ knowledge engineering viewpoint are (1) can the searchselection-design problems be decoupled or decomposed into tractable subproblems and (2) what is the most effective way to represent and structure the separation design knowledge

0 1990 American Chemical Society

422 Ind. Eng. Chem. Res., Vol. 29. No. 3, 1990

STMT

s m m Selector

Dlstillatlon Designer

/

I

Adsorbent Selector

1 Designer

Figure 1. Separation synthesis hierarchy.

for use in an expert system environment.

Task-Oriented Expert System Design In the past, a number of highly successful rule-based

expert systems (e.g., DENDRAL, (Feigenbaum et al., 1971), and MYCIN (Buchanan and Shortliffe, 1984)) have been constructed. In a rule-based system, the knowledge base and the inference mechanisms are typically separate from each other. The rules themselves often do not explicitly indicate the order in which they should be used. Large- scale rule-based systems usually resort to metarules, an implicit grouping of rules. Metarules guide the problem solving locally, by allowing specific rules to be used only under certain circumstances. The separation of knowledge and inference mechanisms promotes general, domain-independent programming but does not take advantage of the inherent structure of many problems.

The task-oriented approach to expert system design represents a strategy of explicit knowledge organization (Chandrasekaran, 1986). This method is based on the following premises:

(1) A complex problem can be decomposed in terms of “generic” problem types or “tasks”. A large problem may be composed of scores of interrelated tasks.

( 2 ) The domain knowledge is available to encode into blocks of knowledge, each of which solves a single task.

(3) The tasks can be built into a structured hierarchy which solves the overall complex problem.

A problem decomposed in this manner can be thought of as a group of “specialists” each working on a separate task. Higher level “managers” ensure that the hierarchy of specialists works toward resolution of the overall problem. Tasks at the upper levels of the hierarchy are more abstract in nature, while those at the lower levels are more concrete. This behavior is reflected in the expert who

Membrane Selector

Adsorbent Selector

focuses on broader issues in the problem and delays consideration of the low level details until much later. The task-oriented method has proven useful in malfunction diagnosis (Davis et al., 1987; Ramesh et al., 1988), equipment design (Myers et al., 1988), and equipment selection problems (Gandikota, 1988) in chemical engineering.

The Separation Synthesis Hierarchy

The key to the task-oriented approach is problem decomposition and knowledge structuring. Expert process engineers are able to select and combine successfully independent process steps into a coherent problem solution. Clearly an extensive body of information on separation processes is available, albeit much of it in a form unsuitable for direct coding into tasks. The separation synthesis hierarchy represents our approach to problem decomposition and knowledge organization for separation synthesis. The hierarchy emulates the approach that an expert process engineer follows. It is based on interviews with expert designers and supplemented by information from the literature.

Figure 1 presents the complete selection/sequencing hierarchy in its present form. Each block represents a clearly defined and essentially independent subtask of the overall separation selection and desequencing problem. The SSH consists of three types of task specialists. Each type of specialist deals with a specific task type: (1) manager-separation sequencing; (2) selector-separation method selection, MSA selection; (3) designer-separation equipment design.

Previously published heuristic methods have dealt with a simplified version of one of these blocks, the liquid split manager. These methods deal almost exclusively with split sequencing.

Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 423

INPUT MIXTURE One of the heuristics that is repeatedly referred to in the literature (e.g., King, 1980; Rudd, 1973) states that the method of separation should be chosen first. In terms of the concepts used here, the heuristic states that all selection tasks should be done first, (i.e., all selector specialists should be at the top of the separation hierarchy). In most cases, this has meant that distillation is assumed to be the best method for all separations.

The use of the method selection heuristic, in principle] greatly reduces the magnitude of the remaining separation synthesis problem. By eliminating the selection problem all at once, one is left with only a split sequencing problem. In other words, the selection and sequencing problems can be completely decoupled.

However, we have found the method selection heuristic to be too restrictive. The selection and sequencing problems cannot be completely decoupled in this manner. Although one can gain some early insight into the most favorable separation method(s) for a given split, the final choice cannot be made until much later. This is especially true for methods requiring mass separation agents. A judgement on separation method cannot be made until a list of potential solvents or adsorbents is available. In turn, the choice of solvent/adsorbent is influenced by the composition of the mixture to which it is to be added. Thus, the method selection problem is dependent on both the solvent/adsorbent selection task and the split sequencing problem. The separation synthesis hierarchy reflects this observation; selection and sequencing tasks are distributed throughout.

The form of the hierarchy is guided by two principles. First of all, calculations are done as little as possible. Most decisions in the upper levels of the hierarchy are based solely on qualitative relationships. Detailed quantitative information is used primarily for final comparisons at the level of the designer specialists.

The second principle is that distillation is the bench- mark separation method to which all other methods must be compared. Distillation should always be the first method considered for any separation. Moreover, when other methods give comparable results to distillation, the reliability and efficiency of distillation make it the likely choice. This is reflected in the hierarchy by the continued com- parison to distillation. The following sections describe in more detail the structure of the tasks needed for the preliminary analysis of liquid mixtures.

Phase Separation Selector At the highest level of the hierarchy is the phase sepa-

ration selector (Figure 2). The phase separation selector uses equilibrium data and normal boiling point informatior. to determine whether a liquid or gas separation system (or possibly both) is necessary. There are two purposes of this task.

The first purpose of the task is to divide the input stream into substreams of low volatility components and of high volatility components reducing the magnitude of the sequencing problem. Although some components may distribute, two smaller, independent sequencing problems are created. These reduced problems will typically require considerably less effort to solve. Removal of one component from a multicomponent mixture will generally reduce the number of possible separation sequences by an order of magnitude or more.

The second purpose of the task is to reduce the method selection problem. For gaseous mixtures, the number of potential separations is reduced to only four: absorption, adsorption, membrane permeation] and cryogenic distillation. Similarly for liquids, one need only consider simple

Rank component8 by 1 normal boiling points I

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All components I

gases?

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NO All componenta liquids 9 1 __-

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A

1 Perform flash to

maxlmm separation baween ' O..a + more I key components upulda + In@

voI~111e G-L componema vomlk 0.L sompononta

Figure 2. Phase separation selector.

distillation, extractive/azeotropic distillation, liquid-liquid extraction, adsorption, membrane permeation, stripping, and crystallization.

A simple example illustrates the utility of the phase separation task. Thompson and King (1972) developed an equation relating the number of components, N , to be separated by M potential separation methods to the number of possible sequences, S:

For a 6-component mixture using the 10 potential separation methods mentioned above, there are 4200 000 possible separation sequences. Now assume four components appear in each of the liquid and gas substreams (i.e., two nonkey components distribute to both the liquid and gas). Considering four potential separation methods for the gas mixture and eight methods for the liquid stream, the number of possible sequences is 320 and 2560, re- spectively. Thus, for this case, the number of possible sequences is reduced by 99.9%.

The grouping of components into liquid and gas substreams is based on the relationship of the normal boiling point of a component to the pressure needed to perform a separation of that component by distillation. Theoret- ically, distillation can be used over the entire range that vapor and liquid phases coexist (i.e., from the freezing point to the critical point). However, in practice, distillation a t extremes of temperature and pressure are often prohibitively expensive. At these limits, other separation methods compete favorably with distillation.

424 Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990

Table I . Distillation Conditions LIQUID MIXTURE

distillation component normal bp pressure range, condenser

type _- group range, atm gas T R p > -20 P > 25 refrigeration gas-liquid -56 < TBP < 0.0 14.5 < P < 25 partial

0.0 < T B p < 50.0 P < 14.5 total liquid 7 u p > 50 P < 14.5 total

Gases are taken as those components with normal boiling points less than -20 "C. Distillations of such components typically require high pressures (greater than 25 atm) and refrigeration. Components that can be condensed by cooling water (normal boiling point of 50 "C or more) are considered to be liquids. Distillation pressures are usually less than 14.5 atm and total condensers can be used. Table I summarizes the distillation conditions for gases and liquids.

Components with normal boiling points between -20 and 50 "C require further evaluation. These gas-liquid transition components may require either partial or total condensers with distillation pressures between 25 and 14.5 atm. At this point in the decision process, one cannot make a clear judgement on the appropriate separation method for transition compounds. It may be best to condense these components so as to use liquid separation methods. On the other hand, gas separations may be more economical.

With the grouping of the components identified, the next step is to calculate adjacent relative volatilities at the input mixture temperature and pressure. In most cases, there will be at least one large adjacent relative volatility value between two components in the gas-liquid transition region. This will certainly be true if there are no transition compounds; the relative volatility between the least volatile gas and most volatile liquid will undoubtedly be large. The components with the largest adjacent relative volatilities in the gas-liquid transition region are chosen as the key components.

The mixture is divided into gas and liquid streams by a simple equilibrium flash. The flash is conducted a t an appropriate temperature and pressure so that the split between the key components is reasonably sharp. (See Example 2: Purification of Acetic Acid.) The liquids with some gas-liquid transition compounds proceed to the liquid split manager. Similarly, the gases go to the gas split manager.

Liquid Split Manager The next phase in the synthesis process involves a

preliminary effort at split sequencing. The sequencing method emphasizes the use of distillation for as many splits as possible and the early use of distillation. The primary purpose of the liquid split manager (LSM) is to make the best distillation sequence possible out of those separations where simple distillation is the favored method. Separa- tions that require mass separating agent processes or crystallization are deferred to a lower level manager. The four-step procedure is outlined below (see also Figure 3).

(1) Identify product streams and product specifications. This ensures that no unnecessary separations are done.

(2) Rank components in order of decreasing adjacent relative volatilities. Relative volatility gives a strong indication of the ease of separation and the favorability of simple distillation.

(3) Identify all azeotropic mixtures that may interfere with product specification. Azeotropes require special processing considerations and should be dealt with when

L I Rank components by

adjacent relative volatilities

L l Identify products and 1 1 product stream speclfications - I Identify all known azeotropes, I

1 potential azeotropes rf information , IS lacking 1

I order speclfied by heuristics

l Gas-Liquid Azeotropic Split Selector ' Zeotropic Split

Selector

1 Repeat until all

Figure 3. Liquid split manager.

spins examined ~.

further information is available. (4) Perform splits in the order specified by a set of

sequential heuristics. Each potential split is evaluated by one of the mixture selector specialists (see next section, Zeotropic/ Azeotropic Mixture Selectors). If simple distillation is the favored method, then the separation is performed, and the resulting substreams are analyzed further by the LSM. If simple distillation is inappropriate, the separation is not performed, but other potential separation methods are identified. The next split specified by the LSM is now checked for the applicability of simple distillation.

The LSM is guided by the assumption that all simple distillations should be performed first. This is based on the premise that simple distillation, when suitable, is the easiest and most reliable method for multicomponent separations. The presence of nonkey components tends to complicate the design of MSA processes and crystal- lizers. Moreover, as mentioned previously, the removal of a component from the mixture reduces the number of possible sequences by an order of magnitude or more.

Azeotropic separations are typically difficult to perform. They should be performed in the absence of other components if possible. It is important to identify these mixtures as early as possible. When data are available, the azeotropes can be easily identified. However, for cases when incomplete information is available, the potential of azeotrope formation can still be determined. The following set of five questions, in decreasing order of certainty, are used to indicate the likelihood of azeotropes. An affirm- ative response indicates unlikelihood. In other words, an answer of yes to question 1 is a stronger indication that azeotropes are not present than an answer of yes to question 5 .


LIQUID SPLIT (1) Are the components homologous or isomers of the

(2) Is the difference in normal boiling points greater than

(3) Are the components members of chemical families

(4) Are the carbon numbers of the compounds greater

(5) Is the ratio of vapor pressures less than the infinite

same chemical family?

15 “C?

unlikely to form azeotropes?

than six?

dilution activity coefficient?

p?%/p!& < %K (2)

(This is a semiquantitative relationship, based on the as- sumptions that the binary solution is regular and the activity coefficient curves are symmetric (Martin, 1975)).

Once the azeotropes have been identified, a list of ordered heuristics is used to obtain a preliminary split sequencing. The list of heuristics is based on the work of Nadgir and Liu (1983). Their list has been modified to account for azeotropes. The heuristics are applied se- quentially. If a heuristic is inapplicable, the next one on the list is considered.

(1) Remove corrosive and hazardous materials first. (2) Remove reactive components first. (3) Perform separations between azeotropes last.

Azeotropic separations tend to be difficult, and they should be done in the absence of other components.

(4) Perform difficult zeotropic (nonazeotropic) separations last, but before azeotropic separations. This is a modification of the heuristic of Rudd et al. (1973) and King (1980) stating that separations of low relative volatilities should be done in the absence of other components.

(5) Remove components in order of decreasing per- centage of the feed. If the relative volatility is reasonable, a component that is a large fraction of the feed should be removed first to decrease the size of later separation equipment.

(6) Favor 50/50 splits. If feed percentages do not vary widely, favor sequences that give equimolar product and residue streams provided the relative volatility is reasonable.

(7) All things being equal, perform the separation with the smallest coefficient of difficulty of separation (CDS) first (Nath and Motard, 1981). The CDS quantifies the last three heuristics:

cns =

The first term is the number of minimum stages for distillation. The second and third terms penalize uneven distributions and overly large distillates. In essence, the CDS is a measure of the applicability of distillation.

It must be emphasized that the split sequence specified at this point is preliminary. The LMS determines the best sequence for the separations that can be done by simple distillation. Separations requiring MSA methods or crystallization are identified. Sequencing of these separations is done at a lower level of the hierarchy.

Zeotropic/Azeotropic Mixture Selectors For each split selected by the LSM, one must determine

a list of potential separation methods. This task is accomplished by one of three mixture selector specialists. The mixture selectors do not indicate a ranking of sepa-

-1-- -

NO Arefhe componems YES 1 temperature sensnwe 7 +---

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~ -- -* BULK Is IhlS a DILUTE IS this a BULK -- bulk ordilute - bulk or dilute -

separation 7 -~ reparaion 7 I ----J I

i T t SEE SEE SEE

Figure 7: Temperalurw Figure 8: Zwlroplc Figure 6: Dilute Swn8nlve Sapiratlons MlXture . Bulk. Tmnrmrlture Sepiritlons

Insen8nlve Sep8mlons

Figure 4. Zeotropic mixture selector.

ration methods from most favored to least favored but rather an unordered list of all possible processes.

The zeotropic selector is used for separations between nonazeotropic (zeotropic) components (Figures 4 and 6-8).

The azeotropic selector is used for separations between azeotropic components (Figures 5-7 and 9).

The gas-liquid transition selector determines whether a group of components identified as gas-liquid transition components by the phase separation selector should be condensed or vaporized (Le., determines whether gas or liquid separation methods should be used). This task will be described in a future paper.

Qualitative information is still quite useful a t this level of analysis. The mixture selectors employ criteria based on pure component data, process characteristics, and whether azeotropes are present. The results of these simple comparisons generally reduce the number of potential separation methods to four or less.

(A) Component Properties. ( 1 ) Relative Volatility. The relative volatility, a , between two components indicates the ease of separation by simple distillation. For a > 1.5, simple distillation is generally the most economical process (see below, Process Characteristics. (1) Separation Type, for a possible exception to this rule). If a < 1.1, distillation requires high refluxes and large numbers of stages. For these cases, distillation is ruled out. For the large gray area (1.1 < a < 1.5), other separation methods may be competitive with simple distillation. No firm judgment can be made by these qualitative comparisons.

(2) Slope of Vapor Pressure Curve. If the slopes of the vapor pressure curves of two components differ sig- nificantly within an acceptable temperature and pressure range, then the relative volatility can generally be altered, possibly to greater than 1.5. The “acceptable” temperature and pressure ranges will depend on available heating medium and cooling water temperatures and on the temperature sensitivity of the materials being processed.

(3) Freezing Point Differences. Feasible crystallization processes typically require 20-30 “C differences in pure component freezing points. In addition, the freezing points should be at or above ambient temperatures if the added expense of refrigeration is to be avoided.

(4) Chemical Family Similarity. Selective physical solvents for PSE processes will achieve separations only for chemically dissimilar components. Homologues of similar size and isomers in the same chemical family generally cannot be separated by PSE methods. Com- pounds of close molecular weight and shape in the same chemical family tend to exhibit similar physical properties and thus similar selectivity and solubility in solvents.

As the size and shape differences increase, the physical properties may differ considerably, even for homologues. Typically, the boiling points of compounds of largely

426

varying sizes in the same chemical family will be suffi- ciently different to allow the use of simple distillation. However, when simple distillation cannot be used for other reasons (e.g., temperature sensitivity of the compounds), PSE processes should not be eliminated as potential separation methods for chemically similar compounds of widely varying sizes.

The effectiveness of a given membrane for a separation depends both on the diffusivity and solubility of the various components in the membrane. Solubility can be related roughly to the interaction between the functional groups in the membrane material and those of the components to be separated. The differences in solubilities of two given components will be significant only if the components themselves contain different functional groups. Thus, membrane permeation may be a feasible separation method if the components to be separated are in different chemical families.

(5) Structure and Size Characteristics. Membrane permeation based on diffusion effects and molecular sieve adsorption both require structural and/or size differences between components to be separated. The effect of structure and size on selectivity can be especially dramatic for adsorption using zeolites and carbon molecular sieves. Certain sizes and shapes of molecules may be excluded completely from the micropores of the adsorbent due to the extremely narrow distribution of pore sizes. A number of industrially important bulk adsorptive separations are based on this molecular sieving effect, notably Union Carbide’s IsoSiv processes (Cusher, 1986) and certain Sorbex processes of UOP (Mowry, 1986). Even if the size and structural differences are insignificant, adsorption may still be a feasible alternative if polarities vary.

(6) Polarity Differences. Commercial adsorbents can be divided into polar and nonpolar types. Polar adsorbents, such as silica gel, activated alumina, and zeolites, tend to bind the polar compounds in a mixture more strongly. Nonpolar adsorbents, such as activated carbon, are more useful for removing less polar materials from a mixture of more polar compounds. For both polar and nonpolar adsorbents, higher selectivity is achieved when there is a large difference in polarity between the desired adsorbates and the unadsorbed liquid. However, adsorption may still be a viable option if polarities are similar when size and structural differences are large.

(7) Boiling Point Range. The boiling range of the component to be separated may indicate the favored method. For example, stripping is favored for separations of low boilers. Liquid-liquid extraction and extractive distillation are better for high boilers.

(8) Temperature Sensitivity. Some components may decompose or react unfavorably at the temperature needed for distillation. Moreover, the freezing point of a component may be too high for distillation to be carried out at an acceptable temperature and pressure. Simple, extractive, or azeotropic distillation cannot be used for these separations.

(B) Process Characteristics. ( 1 ) Separation Type (Bulk or Dilute). As the ratio of distillate to bottoms moves away from unity, other separation methods compete more favorably with distillation. In general, a dilute distillation is uneconomical. A separation is considered dilute when the total distillate or bottoms of a potential distillation operation is less than 5% of the feed.

In addition, a large distillate-to-bottoms (D/B) ratio has a greater effect on the econimics of a distillation than a small D/B ratio. Mixtures composed of mostly low-value, low-boiling components to be separated from a small

Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990

amount (less than 10-1570) of a low-value, high-boiling component require large amounts of energy to vaporize the 85-9070 of the feed that will appear in the distillate. All forms of distillation (simple, extractive, and azeotropic) can be eliminated as potential methods for dilute separations.

During the past 10 years, adsorption has gained a place as a bulk separation method in addition to its continued use as a dilute purification tool. Union Carbide’s vapor- phase IsoSiv processes and UOP’s liquid-phase Sorbex technology have proven economical for the separation of what are considered here as liquid compounds (see phase separation selector for the definition of liquid compounds). Thus, adsorption is a potential method for both dilute and bulk separations.

Membrane permeation can generally be used only for dilute liquid mixtures. No bulk liquid separations are done commercially. Melt crystallization is limited to bulk separations. The low reliability and low recovery typically associated with crystallization processes make its use as a dilute purification tool unfeasible. Liquid-liquid extraction and stripping can be used for either dilute or bulk separations if an appropriate solvent can be found.

(2) Purity. In practice, both simple distillation and crystallization can achieve high-purity separations (99+ YO pure). The purity of the products obtained by PSE processes depends to a large extent on the solvent chosen. However, in principal, PSE processes can achieve high- purity separations if a selective solvent can be found. Adsorption is much the same. If a selective adsorbent can be found, high purity is possible.

Membrane permeation, on the other hand, tends to give only an incremental increase in purity with each passage through the membrane. As a result, a high-purity product will not result from membrane permeation unless one re- sorts to a multistage scheme. Depending on the selectivity of the membrane, typically a t least four stages are needed to achieve greater than 90% purity, with a correspondingly low recovery rate. Thus, if a high purity is essential, membrane permeation can be eliminated as a potential separation method.

(3) Recovery. Recovery is defined here as the degree of separation obtained between product streams. In other words, a high recovery separation results in two high-purity products. As is the case with purity, simple distillation and PSE processes (with a selective solvent) can achieve high recovery separations. Adsorption recovery can be high for both bulk and dilute solutions, depending on the adsorbent. Bulk adsorptive separations using IsoSiv or Sorbex technology are claimed to have recoveries of 95-98% (Mowry, 1986; Cusher, 1986).

Melt crystallization recovery is limited in practice by the presence of eutectic points. In all crystallization opera- tions, only one pure component crystal can be obtained at a time. For simple systems, a second component will not crystallize until all of the first component is removed from solution. However, if the system in question forms a eutectic, the second component will begin to simultaneously crystallize at some intermediate composition (see Walas (1985) for a review of solid-phase thermodynamics). Although the two crystals can sometimes be separated by density differences, this is usually not a reasonable industrial option. Thus, the eutectic point represents a practical limit on the recovery of crystallization processes. The maximum fractional recovery, R, of a component can be related to the eutectic point composition:

(4)


Table 11. Special Processing Situations favored method condition

liquid-liquid extraction

stripping

azeotropic distillation

a dilute solution (between 1% and 5%) of a high boiling, polar compound; distillation would require vaporization of large amounts of the feed

a dilute solution (<5%) of a low-boiling component; if a highly selective solvent is available, stripping may compete favorably with distillation

a low concentration (<10-15%) of the component which forms a minimum boiling azeotrope with the entrainer; in this case, the minimum boiling azeotrope will go overhead; a low concentration of this component reduces the vapor load

extractive distillation a close boiling mixture in which the product is the less volatile component; the extractive agent, which is introduced a t the top of the column, will alter the relative volatility throughout the column

molecular sieve adsorption when both polarity and size/differences are large, adsorbent selectivity is enhanced by the presence of molecular sieving and polarity effects

Table 111. Physical Properties of Xylene Isomers and Ethylbenzene m-xylene o-xylene p-xylene ethylbenzene

composition, mol % 43 23 19 15 M W- normal freezing pt, K normal bp, K dipole moment, D slope of vapor pressure curve, max diameter, A chemical family

azeotropes

106.16 225.4 412.3 0.40

mmHg/K 26.5 8.33 dialkyl-substituted

none aromatic

where xfc is the feed mole fraction of the crystallizing component and x,, is the eutectic mole fraction of the crystallizing component. In addition, the eutectic composition determines which component can be obtained as a pure crystal for a given feed composition. The component for which xfe > x,, crystallizes first.

Membrane permeation is typically an enrichment process. The relative ratio of component mole fractions is shifted, but neither the permeate nor the residual liquid is highly pure. Thus, if high recovery is essential, membrane permeation and melt crystallization (depending on the eutectic point) can be eliminated as potential separation methods.

(C) Azeotropic Separations. Azeotropic separations require additional analysis (see Figure 5). Systems con- taining homogeneous azeotropes cannot be separated by simple distillation unless the azeotrope composition varies with pressure. When the composition changes a t least 4-570 over a nominal change of total pressure, then it is possible to use two-column simple distillation schemes (Smith, 1963). If the azeotrope composition is not pressure sensitive, then simple distillation can be immediately eliminated as a potential separation method.

As is apparent from Figures 6-9, a number of situations exist in which several separation methods feasibly could be used for a given split. The mixture selectors determines which separation methods may be feasible for a given split. Only those methods that are clearly inappropriate are eliminated.

For example, assume that two key components are in different chemical families, indicating that PSE processes should be considered as potential separation methods. However, after a more detailed analysis, a solvent cannot be found with a high enough selectivityfcapacity to make liquid-liquid extraction competitive. In this case, liquid- liquid extraction can be eliminated as a potential method, although in theory it could be used.

Analysis at this highly qualitative level cannot indicate with further certainty whether one method is favored over another. Although not conclusive, some methods are more appropriate for certain special situations. A list of these special considerations is given in Table 11. The principles explored here are further illustrated by two examples of

106.16 248.1 417.3 0.62 23.3 7.80 dialkyl-substituted

none aromatic

106.16 286.6 411.8 0.0 26.8 8.67 dialkyl-substituted

none aromatic

106.16 178.4 409.6 0.58 28.0 9.00 alkyl-substituted

none aromatic

UQUID SPLIT

7--

NO Are the components YES temperame sensme 7

-.a__ ----Y_---. . BULK Isthisa DILUTE Isthis a BULK - bulk or diilite -- bulk or d w e -- -

separation 7 1

'I 1 1 .~ - separation 7 ,

SEE SEE SEE

Mixture. Bulk. Temperature Inrensnlve Separatlons

Flgure 0: kzwtroplc Flgure 6: Dllule Flgure 7: Temperature Separatlons Sensnlve Separatlons

Figure 5. Azeotropic mixture selector.

industrially significant separation problems.

Example 1: Xylene Isomer Purification Xylene isomers and ethylbenzene are important raw

materials in the plastics industry (Debreczeni, 1977). The typical composition and physical properties of the isomer product stream from a naphtha reformer are given in Table 111.

According to eq 1, for a 4-component mixture with 10 potential separation methods, the number of possible sequences is 5000. Examination of the normal boiling points reveals that all components can be considered liquids. This eliminates the gas separation methods, reducing the number of possible sequences to 2560. The complete mixture goes to the liquid splits manager.

For this example, it is assumed that the products are four 99% pure streams of one component each. No azeotropes are present in the mixture nor are there any gas-liquid transition components. Therefore, all splits can be handled by the zeotropic mixture specialist (ZMS). Calculated values of adjacent relative volatilities for this mixture at 1 atm and 450 K are q2 = 1.60, = 1.06, and a31 = 1.02, where 1 is m-xylene, 2 is o-xylene, 3 is p-xylene, 4 is ethylbenzene.

The first round of sequencing can now be accomplished by application of the list of ordered heuristics as follows: heuristic 1, not applicable; heuristic 2, not applicable;


).

Eliminate LIOUID-LIOUID EXTRACTION

STRIPPING

FAVORED SEPARATION Is the dltference Conslder

In polarities large 7 METHOD(S)

Follow ALL of the branches. not In any

necessary order Are the component.

oi stmllar slze/shape 7 NO

f YES

\

Are tho components Con c I d e r In Ihe same LIOUID-LIOUID EXTRACTION

STRIPPING

A Consider

Ellmlnate MOLECULAR SIEVE

ADSORPTION

YES

Eliminate ADSORPTION

t NO ?

Figure 6. Dilute separations.

EYn*u ( . UOUlDUOUlD EXlRlCTlON

STRIPPINO

Cons1d.r UOUlCbLlOUlD EXTMCTION

STRIPPING

Ellmlrul. MOLECULAR SIEVE

.r FAVORED

k SEPARATION YETHOO(S)

A

Ar. lh. eomponea. of .Imllar r1i.l.h.p. ?

MOLECULAR SIEVE ADSORPTION

Follou ALL d Ih. bfanehn . no1 In m y

n.sns.ry Oldel

Ellmhul. CRYSTALLIZATION

4 YES I

I* th . dltl.,.nC. I" fnrlnp pal"1. *In.ll?

con.M.r CRVSTALLIZATION

NO I Figure 7 . Temperature-sensitive separations.

heuristic 3, not applicable; heuristic 4, the relative volatilities between ethylbenzene and p-xylene (4-3) and between p-xylene and m-xylene (3-1) are very small; these separations should be done last; heuristic 5 , m-xylene is the largest component in the feed, but the low relative volatility overrides this consideration; heuristic 6, a 50-50 split cannot be accomplished with these relative volatilities;

heuristic 7 , by comparing the coefficients of difficulty of separation, the favored first split is between o-xylene and m-xylene

split CDS 4-3 42.3 3-1 277 1-2 18.5


Ellmln.1. SIMPLE DISTILLATION

LIQUID SPLIT

I No

Consider SIMPLE

Is Ih. ielaI1Y. - - -. . . . / " O l ~ l l l l l y 1.10 and ktw"" 1.50 7 ~ O I S T I L L A T I O N

Ellmlnal. LIOUID.LlOUID EXTRACTION EXTRACTIVE OlSTlLIATlON AZEOTROPIC DISTILIATION

STRIPPINO

Are Ih. canpownls LIQUIC-LIQUID EXTR*CTION

1, ._ 1 - - - _ -.

vol.lIlly > 1" OISTILLATION UaeSIMPLE ,/ f i s k ch.mk.1 Iamlly 7 U E O T R O P I C EXTRICTNE Ca*ld'r D l S n U T l O N O l S n U T l O H - ~ . .. _.._ --------.__--__

STRIPPINO -_ 1 ~\ ,

FAVORED SEPAPATION

Follow ALL al vr b1anch.i. no1 In my

necnsmry w w Ellmlnml. Ellmlnal. METHDD(S)

MOLECULAR SIEVE ADSORPTION

,Q ADSORPTION

I , , ,',' , ;' I I

_.' I I _ _ - - ,' , ,' I

I

_ - I

._-- ..-- Consider ._ ..~ .-- Are lh. eomponenls h Ihe dm.imc. 01 slmll., .Iz.lihap. 7 In poiarnin I~,W 7 ADSORPTION

Wns1d.r _ _ - - - MOLECULAR SIEVE .._._.._ .--------

ADSORPTION

Figure 8. Zeotropic mixture: bulk, temperature-insensitive separations.

U W I D SPLIT

Ellmlnal. LIOUIDLIOUID EXTRACTION EXTRACTIVE DISTILLATION AZEOTROPIC DISTILLATION

STRIPPINO

YES

Consld., I. Ih. .*~110p10 compo.llla

P I . Y Y I 1 ..n.m. 7

No

Ar. Ih. components LIOUIDLIOUID EXTRACTION EXTPACTIVEDISTILLATION - -~----... .-

STRIPPING YES 1

EllmIn.1.

DISTILLATION Ellmln.1. ADSORPTION

Ellmln.1. MOLECULAA SIEVE

ADSORPTION

b 2.COLUMN SIMPLE

Is Ih. dlflrrmc. C o n * m ...... ~ ~ ~ ....... -..., in poimmi.. I.I~. 7 ADSORPTION

YES

YES Follow ALL Z the

branch.. . not 10 any n.s.0.a~ Otdei

Are Ih. components 01 SImllar . I zmh.p 7

b lh. ,.I.U". rd.!lllP+ > I . = ?

Us. I.COLUMN SIMPLE

DISTILLATION

A 4 Consldrr MOLECULAR SIEVE

ADSORPTION

NO

Ellmlnil$ Eliml",,. CRYSTALLIZATION CRYSTALLIZATION

YES DOLh~1Iops01lh~ *.pa p,n.ur. SYW..

dm.r *ipnme.ntty 7

YES

Consldar CRYSTALLIZATION

I. !he dmeianc. NO

I" I..llng pol"!. .m.ll?

NO b e w e n 1 10.nd1.507 2-COLUMN SIMPLE

DISTILLATION

v FAVORED

SEPAUATION METHOD(S1

A

NO Ellm1n.h W2.COLUMN SIMPLE

DISTILIATION

Figure 9. Azeotropic mixture: bulk, temperature-insensitive separations.

For the m-xylene-o-xylene separation, the analysis by the ZMS is relatively uncomplicated. This is a bulk separation of temperature-insensitive compounds with a relative volatility greater than 1.5. Simple distillation can be used. The bottoms will contain o-xylene; the distillate will contain the remaining three components.

Reapplication of the heuristics to the remaining mixture reveals that the ethylbenzene-p-xylene split should be done next. For this to be true, simple distillation must be

the favored separation method. Proceeding through the ZMS, one determines that simple distillation is uncom- petitive ( a C 1.1 and the slopes of the vapor pressure curves are similar). Similarly, for the meta-para separation, simple distillation is not the clear-cut choice. At this point, one must determine what other methods have potential for the desired separations.

Referring to Figure 8, for a bulk, temperature-insensitive mixture, one must answer all the questions on each of the


Favored Methods Ethylbenzene ~ CRYSTALLIUTION

SPlR ADSORPTION m Xyiene

~uwred Methods ADSORPTION

p Xylene sp't' c / /f

Separaoon by m Xf le -e 5 P L 1 T 1 SIMPLE DISTILLATION

\ o.Xyiene -PRODUCT 1

Figure 10. Xylene isomer purification.

four branches to determine the favored separation method(s). Since the relative volatilities for both the meta- para and p-xylene-ethylbenzene splits are less than 1.1, simple distillation can be eliminated as a potential method.

All three compounds can be considered to be in the same chemical family (alkyl-substituted benzenes) and are of very similar size and shape. Most likely, a solvent cannot be found that will selectively separate one of these components from the other two. Thus, liquid-liquid extraction, azeotropic/extractive distillation, and stripping can be eliminated.

With a less than 1-8, size difference between the three components, adsorption based solely on molecular sieving effects is infeasible. However, the polarity differences as measured by the dipole moments are large. Between p- xylene and m-xylene, the difference is 0.4 D. Between m-xylene and ethylbenzene, the difference is 0.18 D. In addition, p-xylene is completely nonpolar (dipole moment is 0.0 D). Therefore, adsorption should be considered as a potential separation method for both splits.

In this mixture, the freezing points of the components differ considerably (47 K between m-ethylbenzene and 61.2 K between meta-para). Egan and Luthy (1955) showed that the binary system of m-/p-xylene forms an eutectic a t 15 mol 73 p-xylene. The maximum fractional recovery for the feed conditions is 0.67 (from eq 4). Such a low recovery does not allow one to meet the product specifications (99% pure streams of one component each). Consequently, melt crystallization cannot be used for the meta-para split as the problem is stated here.

No information is available on ethylbenzene-xylene eutectics. For this preliminary analysis, one can assume that the desired recovery can be achieved. Thus, crystallization can be considered as a potential method for the m-xylene-ethylbenzene split.

The analysis possible by the current SSH is now complete. The next step is to determine a list of candidate adsorbents for the ethylbenzene-m-xylene and m-xylene- p-xylene splits. Once the adsorbent list is available, one can compare the favorability of adsorption to crystallization. Although the synthesis problem is far from complete, this simple structured analysis has reduced the number of potential sequences by 99.970, from 5000 to 4. Figure 10 summarizes the results.

Example 2: Purification of Acetic Acid Direct oxidation of n-butane to produce acetic acid has

been practiced in the United States since the early 1950's. A typical reactor effluent from oxidation over a manga- nese(II1) catalyst is shown in Table IV (Prengle and Ba- rona, 1970). For this example, it will be assumed that the

Table IV. Acetic Acid Product Mixturea comDonent mol % comuonent mol 9i

Liquid Phase acetic acid 30.5 methanol 1.5 formic acid 11.5 ethanol 3.5 formaldehyde 0.5 acetaldehyde 0.5 acetone 1.0 water 50.0 MEK 1.0

Gas Phase

a Source: Prengle and Barona, 1970.

CO 73.0 CO2 27.0

Table V. Boiling Points of Acetic Acid and Bmroducts component normal bp, K grouping

GO COZ formaldehyde acetaldehyde acetone methanol ethanol MEK water formic acid acetic acid

81.7 194.7 254 294 329.2 337.7 351.4 352.7 373.2 373.8 391.1

gas gas gas-liquid transition gas-liquid transition liquid liquid liquid liquid liquid liquid liquid

Table VI. Liquid-Phase Component Properties

MW acetone 58.8 methanol 32.0 ethanol 46.1 MEK 72.1 water 18.0 formic acid 46.0 acetic acid 60.0

freezing pt, K 178.2 175.5 159.1 186.5 273.2 281.5 289.8

dipole kinetic moment, diameter,

D A 1.71 5.2 1.71 4.1 1.67 5.1 2.70 6.1 1.83 3.0 1.52 5.5 1.74 5.3

products of interest are pure acetic acid and pure formic acid.

A t the level of the phase separator selector, the components are first ordered by normal boiling points as shown in Table V. Calculation of relative volatilities shows that there is a clear separation point between acetaldehyde and acetone. A flash separates the mixture into a vapor phase consisting of the gases, gas-liquid transition components, some acetone and methanol, and traces of other components, plus a liquid phase consisting of the liquid components. Table VI lists the physical properties of the liquid-phase components.

The liquid split manager examines the liquid mixture for the possibility of azeotrope formation. A number of azeotropes are present in this mixture (see Figure 11 (Horsley, 197311, but only the binary azeotrope of formic acid-water and the ternary azeotrope of formic acid- water-acetic acid may interfere with product specifications. The ternary azeotrope is pressure sensitive and is not present at atmospheric conditions. The binary azeotrope, on the other hand, is not pressure sensitive.

Examining the list of heuristics reveals that acetic acid should be separated first because of its corrosiveness. Since the ternary azeotrope is not a problem, the separation of acetic acid is analyzed by the zeotropic mixture selector. Acetic acid is not temperature sensitive and is present in the mixture a t bulk concentrations. The relative volatility between acetic acid and water is fairly low at 1.21. The slopes of the vapor pressure curves are similar; the relative volatility cannot be altered. The polarities and sizes are similar, eliminating adsorption. The freezing points are too close for crystallization (16 "C), but the components

Acetic Acid 1

Formic Acid 2

Fwmaldehyde 3

Acetone 4

Methyl Ethyl Ketone 5

Methanol 6

Ethanol 7

Acetaldehyde 8

Water D

-Q Ivpe ComDoritlon

maximum P 6% water A 10765

B 55 5 minimum 12 wb methanol

c 635 minimum 70 6% mathano1

D 74 0 minimum 39 wb ethanol

E 734 minimum 11 3?4 water

F

0

78 17 minimum 4 wb water

698 ternF$& 23 4% water 45 7% Formic Acid

Figure 11. Azeotropes in acetic acid product mixture.

are chemically dissimilar. Therefore, one of five methods could potentially be used for this separation, simple distillation, or one of the PSE methods. Of these five, stripping is inappropriate because the product is not a dilute, low boiler. One cannot discriminate further between azeotropic, extractive, and simple distillation or liquid-liquid extraction until a list of potential solvents is chosen.

The separation of formic acid is handled by the azeotropic mixture selector because of the formic acid-water azeotrope. This azeotrope cannot be eliminated by changing pressure, ruling out simple distillation. As with the acetic acid separation, this is a bulk, temperature-insensitive separation. Moreover, the freezing point difference is small (9 "C) and the polarities and sizes are similar. Thus, the potential separation method is quickly reduced to only PSE processes. Again, the remaining methods can only be evaluated after a list of solvents is selected.

Although the problem is not complete, the number of potential separation sequences has been considerably reduced. The results are summarized in Figure 12.

Conclusions This paper has presented a qualitative, task-oriented

approach to the separation synthesis problem. The overall synthesis problem is decomposed into a series of essentially independent sequencing and selection subproblem tasks. The tasks are organized into a structured hierarchy, the separation synthesis hierarchy, based on the approach followed by expert process designers. Four the subtasks, the phase separation selector, the liquid split manager, and the zeotropicfazeotropic mixture selectors, are described in detail.

The tasks described here represent a qualitative method of rapidly reducing the magnitude of the overall separation synthesis problem. The current version of the SSH is not a complete solution to the synthesis problem. Considerable work is required on solvent selection, gas separations, and

Ind. Eng. Chem. Res., Vol.

Carbon Monoxide

CarWn Dioxide

Formaldehyde

Acetaldehyde

Acetone

Methanol

Elhanol

Methyl Ethyl Ketone

Formic Acid

Water

Acetic Acid

. . .

Carbon Monoxide

Carton Dioxide

Formaldehyde

Acetaldehyde

Acetone

Methanol

GASES

PHASE SEPARATION

\ LlOUlDS

29, No. 3, 1990 431

Acetone \ Methanol

Figure 12. Acetic acid purification.

sequencing of MSA processes. These topics will be covered in future papers. In spite of these limitations, the number of possible separation sequences can be reduced by 90% or more for most liquid mixture cases.

Acknowledgment

This work was supported in part by a grant from the Exxon Education Foundation. Support was also provided by the Separations Research Program at The University of Texas at Austin. We appreciate very much the gener- osities.

Nomenclature

Symbols B = bottoms flow rate D = distillate flow rate M = number of potential separation methods N = number of components in a multicomponent mixture P = vapor pressure, total pressure S = number of possible separation sequences Xi = mole fraction of component i a = relative volatility y = activity coefficient Superscripts m = infinite dilution sat = saturation pressure Subscripts LK = light key HK = heavy key Abbreviations AI = artificial intelligence CDS = coefficient of difficulty of separation

432 Ind. Eng. Chem. Res. 1990,29, 432-436

LSM = liquid split manager MSA = mass separating agents (MSA processes include all

PSE and SPA processes) SSAD = separation synthesis advisor SSH = separation synthesis hierarchy ZMS = zeotropic mixture selector PSE = physical solvents/entrainers (PSE processes include

azeotropic/extractive distillation, liquid-liquid extraction, and stripping)

SPA = solid-phase agents (SPA processes include adsorption and membrane permeation)

Literature Cited Buchanan, B. G.; Shortliffe, E. H. Rule-Based Expert Systems;

Addison-Wesley: New York, 1984. Chandrasekaran, B. Generic Tasks in Knowledge-Based Reasoning:

High-Level Building Blocks for Expert System Design. IEEE Expert 1986, Fall, 23.

Cusher, N. A. UCC IsoSiv Process. In Handbook of Petroleum Refining Processes; Meyers. R. A,, Ed.; McGraw-Hill: New York. 1986.

Davis, J. F.; Shum, S. K.; Chandrasekaran, B.; Punch, W. F., 111. A Task-Oriented Approach to Malfunction Diagnosis in Complex Processing Plants. Presented a t NSF-AAAI Workshop in AI in Process Engineering, Columbia University, New York, March 1987.

Debreczeni, E. J. Future Supply and Demand for Basic Petrochem- icals. Chem. Eng. 1977, 84 (June 6), 135.

Egan, C. J.; Luthy, R. V. Separation of Xylenes: Selective Solid Compound Formation with Carbon Tetrachloride. Ind. Eng. Chem. 1955; 17, 250.

Feigenbaum, E. A.; Ruchanan, B. G.; Lederberg, J. On Generality and Problem Solving: A Case Study Involving the DENDRAL Program. Mach. Intelligence 1971, 6, 165.

Gandikota, M. S. Expert Systems for Selection Problem Solving Using Classification and Critique. Masters Thesis, The Ohio State University. Columbus, 1988.

Horsley, L. H. Azeotropic Data-HI; ACS Advances in Chemistry Series 116; American Chemical Society: Washington, DC, 1973.

Kelley, R. M. General Process Considerations. In Handbook of Separation Process Technology; Rousseau, R. W., Ed.; John Wiley: New York, 1987; Chapter 4.

King, C. -7. Separation Processes, 2nd ed.; McGraw-Hill: New I’ork, 1980.

Martin, G. Q. Guide to Predicting Azeotropes. Hydrocarbon Process. 1975, 54 (111, 241.

Mowry, J. R. UOP Sorbex Separations Technology. In Handbook of Petroleum Refining Processes; Meyers, R. A,, Ed.; McGraw- Hill: New York, 1986.

Myers, D. R.; Davis, J. F.; Herman, D. J. Still: An Expert System for Distillation Design. Computers in Chemical Engineering The Ohio State University: Columbus, 1988; Vol. 12, Nos. 9 and 10, p 959.

Nadgir, V. M.; Liu, Y. A. Studies in Chemical Process Design and Synthesis, Part 5. AIChE J . 1983, 29 (6), 926.

Nath, R.; Motard, R. L. Evolutionary Synthesis of Separation Pro- cesses. AIChE J . 1981, 27 (41, 578.

Nishida, N.; Stephanopoulos, G.; Westerbert, A. W. A Review of Process Synthesis. AIChE J . 1981, 27 (31, 321.

Prengle, W. H.; Barona, N. Make Petrochemicals by Liquid Phase Oxidation. Hydrocarbon Process. 1970, 49 (31, 106.

Ramesh, T . S.; Shum, S. K.; Davis, J. F. A Structured Framework for Efficient Problem Solving Diagnostic Expert Systems. Com- put. Chem. Engl. 1988, 12, 891.

Rudd, D.; Powers, G.; Sirola, J. J. Process Synthesis: Prentice Hall: Englewood Cliffs, NJ, 1973.

Smith, B. D. Design of Equilibrium Stage Processes; McGraw-Hill: New York, 1963.

Thompson, R. W.; King, C. J. Systematic Synthesis of Separation Schemes. AIChE J . 1972, 28 (5), 941.

Walas, S. M. Phase Equilibria in Chemical Engineering; Butter- worths: Stoneham, MA, 1985.

Received f o r review June 2, 1989 Revised manuscript received October 17, 1989

Accepted November 20. 1989

Waste Lubricating Oil Rerefining by Extraction-Flocculation. 2. A Method To Formulate Efficient Composite Solvents

M. Alves dos Reis* and M. Silva Jeronimo Faculdade de Engenharia, Depar tamento d e Engenharia Quimica, Rua dos Bragas, 4099 Porto Codex, Port uga I

A method to design efficient composite solvents is described. The method consists of selecting one of the components miscible with base oil as the “basic component”. The hydrocarbons and butanone are possible examples of basic components. The other component or solution of components is globally treated as the “polar addition”. This is, for example, an alcohol, a ketone, or a solution of two or more of these compounds. A ternary diagram of waste oil/basic component/polar addition, where the phase envelope and the curves of constant sludge removal are plotted, summarizes all information necessary to select the best solvent composition. In all cases studied, addition of 1-3 g/L KOH to the alcohols has increased the sludge and/or additive removal from waste and virgin oils. Using this method, we have concluded that solvents based on n-hexane and 2-propanol with 3 g/L KOH are very efficient. The weight composition 0.25 waste oil, 0.20 n-hexane, 0.55 2-propanol is proposed for industrial use.

1 . Introduction Treatment of waste oils with polar solvents may be an

interesting alternative to the classical sulfuric acid treatment. This process was named extraction-flocculation by Reis (1982) because the solvent dissolves the base oil and simultaneously promotes the fast flocculation of the un- desirable impurities. Since the sludge produced in this

* To whom correspondence should be addressed.

08S8-58S5/90/2629-0432$02.50/0

process is organic, it may be mixed with liquid fuels and burned or, better, find more noble applications. For example, it may be used as a component of offset inks (Reis and Jeronimo, 1982). This seems to overcome the major problem of the sulfuric acid treatment: the production of an acid sludge, which is a source of pollution and causes very difficult disposal problems.

In our previous paper (Reis and Jeronimo, 1988) it was shown that the flocculating action and subsequent sludge removal promoted by the one-component solvents studied

0 1990 American Chemical Society