Ind. Eng. Chem. Res. 1992,31, 1679-1694
T,= sampling time U = matrix derived from the SVD of X V = matrix derived from the SVD of X W = projection matrix of X W,+ = weights used for SSV analysis described in Figure 4b; 1=1,2
Wd = disturbance weight W, = performance weight X = input data, each row corresponds to one input sample; each input sample consists of 12 temperatures and 2 manipulated variables X, = lower dimensional data set obtained from the projection ofXonW y = measured concentration yn = nominal concentration defined in (28) 6 j = deviation for the variable j A" = uncertainty matrix Ai* = individual uncertainty elements A = IMC filter constant p = structured singular value (SSV) Z = matrix of the singular values of X T~ = controller integral reset time in (46)
Literature Cited Brosilow, C.; Joseph, B. Inferential control of processes. AZChE J. 1978,24,485-509. Doyle, J. Analysis of feedback systems with structured uncertainties. ZEEE Roc. Part D 1982,129,242-250. Geladi, P.; Kowalski, B. Partial least squares regression: A tutorial. Anal. Chim. Acta 1986,185,1-17. Gulandoust, M. T.; Morris, A. J.; Tham, M. T. Adaptive estimation algorithm for inferential control. Znd. Eng. Chem. Res. 1988,27, 1658. Harris, T.; MacGregor, J.; Wright, J. Optimal sensor location with an application to a packed bed tubular reactor. AZChE J. 1980, 26. Hill, C. Chemical Engineering Kinetics and Reactor Design; Wiley: New York, 1977. Holt, B.; Morari, M. Design of Resilient processing planta-V The effect of deadtime on dynamic resilience. Chem. Eng. Sci. 1985, 40, 1229-1237.
1679
Hoskuldsson, A. Partial least squares regression methods. J. Chemom. 1988,2,211-228. Jorgensen, S.; Goldschmidt, L.; Clement, K. A sensor-location procedure for chemical processes. Comput. Chem. Eng. 1984,8, 195-204. Kumar, S.; Seinfeld, J. Optimal location of measurements in tubular reactors. Chem. Eng. Sci. 1978,33,1507-1516. Laughlin, D.; Jordan, K.; Morari, M. Internal model control and process uncertainty: Mapping uncertainty regions for SISO controller design. Znt. J. Control 1986,44, 1675-1698. Lee, J.; Morari, M. Robust control of nonminimum-phase systems through the use of secondary measurements: Inferential and inferential cascade control. Automatica 1992,submitted for publication. Lee, J.; Morari, M. Robust measurement selection. Automatica 1991,27(3),519-527. Luyben, W. L. Parallel cascade control. Znd. Eng. Chem. Fundam. 1973,12 (41,463-467. Mandler, J. A. Robust Control System Design for a Fixed-Bed Catalytic Reactor. Ph.D. Thesis, California Institute of Technology, 1987. Mejdell, T.; Skogestad, S. Estimate of process outputs from multiple secondary measurements. Proc. Am. Control Conf. 1989, 2112-2121. Morari, M.; Zdiriou, E. Robust Process Control; Prentice Hall: Englewood Cliffs, NJ, 1989. Tham, M. T.; Montague, G. A.; Morris, A. J.; Lant, P. A. Soft-sensors for process estimation and inferential control. J. Process Control. 1991,l (3). Van Herwijnen, T.; Van Doesburg, H.; De Jong, W. Kinetics of the methanation of carbon monoxide and carbon dioxide on a nickel catalyst. J. Catal. 1972,28,391-402. Webb, C. Robust Control Strategies for a Fixed Bed Chemical Reactor. Ph.D. Thesis, California Institute of Technology, 1990. Webb, C.; Budman, H.; Morari, M. Identifying frequency domain uncertainty bounds for robust controller design-theory with application to a fixed-bed reactor. Proc. Am. Control Conf. 1989, 1528-1533. Wold, S.; Ruhe, A.; Wold, H.; Dunn, W. The collinearity problem in linear regression: The partial least squares approach to generalized inverses. SZAM J. Sci. Stat. Comput. 1984,5(3), 753-743. Received for review March 24, 1992 Accepted April 13,1992
Separation System Synthesis: A Knowledge-Based Approach. 2. Gas/Vapor Mixtures Scott
D.Barnicki and James R.Fair*
Separations Research Program, Department of Chemical Engineering, The University of Texas at Austin, Austin, Teras 78712-1062
A description is given for a prototype knowledgebased expert system, the separation synthesis advisor (SSAD),for synthesis of separation sequences for gas/vapor mixtures. The core of the SSAD is
the separation synthesis hierarchy (SSH),a highly structured, taak-oriented framework for representing separation knowledge. The hierarchy, based on interviews and information from the literature, emulates the approach that an expert process engineer follows. In ita current implementation, the SSH is limited to the preliminary sequencing of multicomponent gas/vapor mixtures using the following separation methods: (1)physical absorption; (2) chemical absorption; (3) cryogenic distillation; (4) membrane permeation; (5) molecular sieve adsorption; (6) equilibrium-limited absorption. Several examples of practical industrial separation problems are included. Introduction This paper is the second of a series on the development of a prototype expert system for the syntheais of separation sequences for fluid mixtures; the system is called the separation synthesis advisor (SSAD).Part 1 concentrates on separation system synthesis for liquid mixtures (Barnicki and Fair, 1990). Part 2 focuses on the parallel
problem for gaslvapor mixtures. The SSAD is a preliminary process design tool. Ita purpose is to formulate a limited number of feasible separation systems for a given multicomponent mixture. Final comparisons and optimization must be carried out with the aid of a process simulator, as the SSAD currently does not have the capability to perform a detailed economic analysis.
0888-5885/92/2631-1679$03.00/00 1992 American Chemical Society
1680 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 THE SEPARATION SYNTHESIS HIERARCHY
Split
!wuamQQ mass
siwa9mm Distiilatbn
Azeototropic Zeotropic
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Simpb Distillation Azeotrapic Disliilatlon Extractive Distillation Liquid-Liquid Earaction Equilibrium-Limited Adsorption Mokcular Sieve Adsorption Melt Cystalliutbn Siripping Membrane Penneation
Bulk, Sharp Enrichment Purification
l2wiamM Phyaicai Abrorptbn Chemical Absorption Equiibuhn-Limited Adsorption Mokcular Sbvo Adsorption Cryogenk DlSlltrtbn Membrana Permeation Condensation Catalytic Comembn
Figure 1. Separation synethesis hierarchy, showing each manager with its complement of selectors and designers.
The separation of gas and vapor mixtures is a significant part of many key activities in the chemical process industries, ranging from the recovery of carbon dioxide in enhanced oil recovery to environmental concerns over the removal of solvents and acid gases from exhaust and process streams. In spite of its obvious importance, the synthesis of separation sequences for gas/vapor mixtures has been completely neglected in the process design literature. In the 23 years since the first proposals of Rudd and Masso advocating a systematic approach to separation system synthesis (Rudd, 1968; Masso and Rudd, 1969),not one article has appeared on any aspects of gas/vapor separation system synthesis. As with liquid mixture separation synthesis, the general gas/vapor synthesis problem involves method selection and sequencing subproblems. However, beyond these superficial similarities the specifics of the synthesis problem for gas/vapor mixtures are fundamentally different from the corresponding problem for liquids. Whereas liquid method selection is clearly biased toward simple distillation, no such dominant method exists for gases. Several methods can often compete favorably. Moreover, the appropriateness of a given method depends to a large extent on specific process requirements, such as the quantity and extent of the desired separation. The situation contrasts markedly with liquid mixtures in which the chemical characteristicsof the components to be separated are often the dominant factors (Barnicki and Fair, 1990). This paper addresses the complexities of separation method selection and sequencing for gas/vapor mixtures. The design expert’s knowledge is organized as an autonomous problem-solving entity, called the gas split manager (GSM). The purpose of the GSM is to provide control over the overall synthesis activity. The GSM is further subdivided into three essentially independent subproblems, referred to as separation method selectors. Each selector pertains to a distinct aspect of the gas mixture separation method problem. The GSM and its complement of selectors are part of a larger, highly structured framework for representing separation knowledge, the separation synthesis hierarchy (SSH)(Figure 1). The hierarchy emulates the approach that an expert process engineer follows. The overall sep-
aration problem for fluid mixtures can be divided into four distinct synthesis phases composed of unique selection, sequencing, and design subproblems, but all with parallel problem-solving structure. The four managers which comprise the SSH are the phase split manager (PSM), the distillation split manager (DSM), the liquid split manager (LSM), and the gas split manager (GSM). Each manager, selector, and designer represents a clearly defined and essentially independent subtask of the overall separation synthesis activity. The first three of these (the PSM, DSM,and LSM) have been described previously (Barnicki and Fair, 1990). This paper concentrates on the manager and selector subtasks for gas/vapor mixtures. The paper is organized into three sections. The first summarizes briefly the structure of the manager subtasks, concentrating on the particulars of the gas split manager. The basic task-oriented problem-solving philosophy of the SSH and the SSAD has been described in part 1 of this series of papers; it will be outlined only briefly here to orient the reader. The second section presents the concepts involved in separation method selection for gas/vapor mixtures. The process attributes which can be used to categorize gas/vapor separations are described, together with a discussion of the conditions under which the individual separation methods are feasible. Emphasis is placed on identifying the component properties and process attributes which determine the utility of a particular technique. The final section presents several industrially significant gas/vapor separation examples which illustrate the capabilities of the SSH. T h e Gas Split Manager Structure of the Gas Split Manager (GSM). A schematic overview of the problem-solving strategy employed by the SSH is shown in Figure 2. An initial separation problem (Le., a fluid mixture) is formulated from the problem specifications pertaining to the feed components and desired products. This first mixture is placed on an “agenda”, which is initially empty. The agenda is expanded as separations of the initial and succeeding submixtures are specified. Only those submixtures that require further separation (Le., those that do not match a desired product directly) are added to the agenda.
Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 1681 GAS MIXTURE
I
Pmbkm spsclflution: creme inltbl midun.. r w - nda
Generate ranked lists for a11 pertinent propertks
I
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”
’
11 ‘j
Analyze all submixtures
ANALYSIS COMPLETE. PROCEED TO NEXT MIXTURE ON AGENDA
Figure 3. Logic diagram for the gas split manager.
Figure 2. Overview of the problem-solving strategy of the separation synthesis hierarchy.
For each mixture accessed from the agenda, one must select the appropriate manager. This choice is trivialized here, as all example separations described here require only the GSM. However, more complicated examples requiring manager selection are given in Barnicki and Fair (1990) and Barnicki (1991). A manager oversees five problemsolving activities: 1. Split generation: Possible separation points (splits) are identified. The possible splits of a given mixture depend on product specifications and on the order of the components in ranked property lists. 2. Selector Analysis: The appropriate separation methods (if any) for each possible split are determined. A potential split is defined as a split which may be accomplished by at least one separation method. 3. Split sequencing: The potential splits are compared to determine which are the most appropriate to perform next. This analysis is guided by well-known design heuristics. 4. Separation design: Each potential split chosen in step 3 is subjected to a shortcut separation design procedure to find the distribution of components in the resulting submixtures. 5. Submixture analysis: The submixtures generated by each separation must be analyzed to determine if they meet product specifications. Those that require further separation are added to the agenda. The procedure is repeated for each mixture on the agenda and continues until the agenda is empty (i.e., all product specifications are met).
The gas split manager (GSM) is the only split manager of the SSH devoted exclusively to predominantly gaseous mixtures. No clearly dominant separation method exists for gas separations. However, the problem-solving approach of the GSM still follows the general strategy outlined above. The logic diagram for the GSM is presented in Figure 3. Split Generation. Every separation method is based on a difference between one or more physical or chemical properties of the components in a mixture (King, 1980, Rudd et al., 1973). A given method can achieve a separation between any components in which these characteristic properties differ significantly. Since the term “differ significantly” is difficult to quantify, one could conceivably separate between any two components of a mixture. We will call these two components the keys of the separation. For a large multicomponent mixture the number of possible pairs of keys clearly precludes a detailed examination of all options. A workable approach requires a compromise between thoroughness and the need to eliminate infeasible splits with minimal effort. The strategy adopted in the SSH utilizes ranked property lists. A ranked property list orders the components by the magnitude of the characteristic property of the separation method (e.g., relative volatility for cryogenic distillation). Thus, a ranked list determines, in a qualitative sense, the possible distribution of components for a given separation method. The possible separation points are further restricted by product specifications. Splits between components appearing in the same product are prohibited. Not all separation methods (especially those requiring mass separating agents, MSA) are well-characterized by a single, easily-calculable property. For example, once the mass separation agent is known, the distribution of components can be determined readily, and from this the possible splits. However, the selection of the MSA, which
1682 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 Table I. Separation Methods and Their Corresponding Characteristic Properties seDaration method properties relative volatility cryogenic distillation chemical family physical absorption chemical family chemical absorption chemical family catalytic conversion critical temperature, membrane permeation van der Waals volume kinetic diameter molecular sieve adsorption equilibrium-limited adsorption equilibrium loading relative volatility condensation
Table 11. Sequencing Heuristics for the Gas Split Manager 1. Remove corrosive and hazardow materials first. 2. Remove troublesome trace impurities first. 3. Favor separations which match the desired products directly. If a separation resulta in a substream which requires no further separation, and is a desired product, and if that product is the most plentiful in the mixture, remove it next. 4. Favor separations which give equimolar splits. When ease of separation and compositions are similar, perform the separation which divides the feed as equally as possible.
itself is nontrivial, is entirely dependent on the separation method chosen. One cannot determine definitely the proper separation method until the possible separation points are known. In these situations (i.e., for essentially all processes except distillation), ranked property lists are still used to generate the possible splits. Although these characterizations are not perfect, they are generally accurate enough in a qualitative sense for a preliminary analysis. The selection of the appropriate property (or properties) which can reliably predict the component distribution for a given separation method is open to some debate. Table I presents a list of the gas/vapor separation methods currently implemented in the SSH along with their corresponding characteristic property or properties. Split Sequencing. Separation sequencing is a critical step in the generation of optimal to near-optimal separation system designs. Limited aspects of the topic have received serious consideration in the literature. This effort has resulted in some relatively simple, reliable heuristic methods of generating near-optimal distillation sequences (Kelly, 1987; Liu, 1987; Nishida et al., 1981). These methods generally make use of a series of ranked heuristics, which are applied sequentially. If a heuristic is not applicable, the next one on the list is considered. For these techniques, sequencing is based primarily on process characteristics, such as the relative magnitudes of the desired products and the ratio of the expected distillate to bottoms flow rates. Since simple distillation typically haa been the only separation method dealt with in previous studies, relative volatility is an adequate measure of how easy it is to accomplish a particular separation method. Thus, there is no need to consider separation method characteristics as well as process characteristics; the sequencing and separation method selection problems become decoupled. When a variety of separation methods are available, however, one must now compare the relative ease of separation of competitive techniques. For example, if molecular sieve adsorption and cryogenic distillation processes yield similar product distributions (and would thus trigger the same process characteristic heuristics), how does one compare the relative volatility and the difference in kinetic diameters to determine which separation method results in a more efficient separation? The sequencing of gas separations does not lend itself as readily to the highly qualitative approach used for distillation sequencing. However, several of the general separation sequencing heuristics formalized in previous studies are still applicable to gas separations, albeit in rather weak forms. Table I1 shows the sequencing heuristics as modified for gas separations. The presence of corrosive or hazardous materials tends to increase the expense of equipment. Therefore these Components should be removed as early as practical. Many gas-phase separation processes are affected adversely by the presence of trace impurities. Small amounts of
Table 111. Typical Special Processing Conditions for the Gas Split Manager 1. Favor condensation for the removal of high boilers from noncondensable gases when cooling water can be used as the condensing medium. Condensation is one of the simplest and cheapest unit operations. 2. Favor catalytic conversion when the impurities can be converted into 4 desired product. Further purification and/or separation steps may be unnecessary. 3. Favor adsorption for small-scale desiccation operations. Solid-phase desiccant systems are relatively simple to design and operate. They are generally the lowest cost alternative for processing small quantities of gas. 4. Favor adsorption for processes which require essentially complete removal of water vapor. Adsorptive dehydration is capable of achieving dew point depressions of 80 O F or more. 5. Favor glycol absorption for large-scale desiccation operations required dew point depressions of 50 O F or less. The initial and operating costa of high-volume glycol absorbers are typically much lower for small to medium dew point depressions than the corresponding costa of solid-phase desiccation.
freezable components (water, carbon dioxide) may foul cryogenic units. High gas humidity or moisture content reduces the effectiveness of many adsorption processes (particularly the adsorption of low molecular weight compounds). Other components, high boilers and polymerizable compounds, may permanently foul an adsorbent. Such components should be removed first, downstream of any important, larger-scale separations. It is obviously advantageous to perform a separation which removes a component directly as a product. This generally will improve overall recovery and purity. The specification of equimolar splits tends to reduce the downstream separation load more effectively. In terms of energy usage, two smaller separation units are generally more efficient than one very large unit. Some processing conditions are especially favorable for certain separation methods. These special circumstances may override the more general sequencing and selection heuristics. A list of some typical special processing conditions is given in Table III. See also the sections Physical Absorption, Adsorption, and Condensation. The sequencing procedure presented above does not guarantee that only one potential separation will be found in all cases. Because of their highly qualitative nature, the heuristics often cannot differentiate between several alternatives. When such a situation arises, all alternatives are considered to be equally feasible at this stage of the process design; a detailed economic analysis is often necessary to determine which method is actually preferred. The qualitatively equivalent separations are propagated through the remainder of the manager activity and lead to unique submixtures. If such branching does occur, the final output of the gas split manager will contain several competing separation system designs. On the other hand, if no equivalent separations are found at any stage of the
Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 1683 uimlm
PHYSCAL ABSORPTION No
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Figure 4. Logic diagram for the selection of separation methods for gas-phase enrichment separations, enrichment split selector.
synthesis process, only one final separation system design will result.
Separation Method Selection for Gas Mixtures SeparationSpes. Gas-phase separations are classified into three categories based on the purity, recovery, and magnitude of the pertinent Separation. Each category is the basis for a dietinct separation selector in the SSH (1) enrichment separation; (2) sharp separation; (3)purification separation. The classification system allows for a certain amount of synergy, as several separation methods may be combined in order to achieve the desired result. Each separation category is organized as a distinct selector, with its own favored separation methods. The applicable separation techniques for gas mixtures are shown in Table I. A. Enrichment Separations. An enrichment is defined as a separation process that results in the increase
in concentration of one or more species in one of the product streams and the depletion of the same species in the other product stream. Neither high purity nor high recovery of any components is achieved. Because of a lack of stringent purity and recovery specifications, enrichments are the most general gas separation type. They can be accomplished with a wide variety of separation methods: physical absorption, molecular sieve adsorption, equilibrium adsorption, cryogenic distillation, condeneation, and membrane permeation. The logic diagram for the enrichment split selector (ESS) is shown in Figure 4. B. Sharp Separations. A sharp separation results in two high-purity, high-recovery product streams. No restrictions are placed on the mole fraction(s) of the component(s) to be separated. A separation is considered to be sharp in the present work when the key component
1684 Ind. Eng. Chem. Res., Vol. 31,No. 7, 1992 mimiPMVSlCAL ABSORPTION
\
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Figure 5. Logic diagram for the selection of separation methods for gas-phase sharp separations, sharp split selector.
splita are greater than 9.0-9.5 or less than 0.111-0.105. A key component split is defined as: Skey
Skey
= c1/c2 1 9.0-9.5 for c1 > c2 cl/c2 5 0.105-0.111 for
c2
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where Shy= split of light or heavy key, c1 = flow rate of key component in product 1, and c2 = flow rate of key component in product 2. The separation methods that can potentially obtain a sharp separation in a single step are physical absorption, molecular sieve adsorption, equilibrium adsorption (for componenta which comprise less than 10% of feed mixture), and cryogenic distillation. The sharp split selector (SSS)is illustrated in Figure 5. Chemical absorption is often used to achieve sharp separations, but is generally limited to situations in which the componenta to be removed are present in low concentrations. These special cases of low mole fraction, high-recovery, high-purity separations are treated as a distinct separation type, purification separation. C. Purification Separations. A purification separation involves the removal of one or more low concentration impurities from a feed stream. A low concentration impurity is arbitrarily defined here as a component or group
of componenta which comprise less than 2 mol 9% of the parent mixture. A purification separation typically resulta in a product of very high purity (e.g., >99% impurity removal, depending on the separation method used). It may or may not be desirable to recover the impurities. The separation methods considered here as applicable to purifications are limited to equilibrium adsorption, molecular sieve adsorption, chemical absorption, and catalytic conversion. Physical absorption is not included in this list. With low inlet concentrations of the impurity (characteristic of a purification separation), physical absorption processes are typically not able to achieve extremely high purities (Tennyson and Schaaf, 1977). One notable exception to this rule is the absorption of water vapor by glycol solutions. Glycol dehydration processes are able to achieve dew point depreasiom of 200 OF or more (Valerius, 1974). However, adsorptive desiccation is generally more economical when dew point depreasions of 80 OF or more are neceesary (Kohland Riesenfield, 1986) (refer also to sections Physical Absorption and Adsorption). In some cases an enrichment may be coupled with a purification step in order to achieve the desired separation sharpness (e.g., physical absorption followed by chemical absorption, condensation followed by a purification op-
Ind. Eng. Chem. Res., Vol. 31, NO. 7, 1992 1685
Figure 6. Logic diagram for the selection of separation methods for gas-phase purification separations, purification split selector.
eration). A logic diagram for the purification split selector (PSS) appears in Figure 6. Separation Methods. A. Membrane Permeation. The ease of separation of two gaseous components by membrane permeation is characterized by the ratio of their permeabilities in the membrane material. This permselectivity is often represented in the literature as consisting of solubility and diffusivity contributions:
The ability of a polymer membrane to act as a selective separatingagent for a particular mixture of gaseous species is a function of the physical properties of the polymer as well as those of the components to be separated. The magnitude of the diffusivity ratio is dependent on the size and shape of the molecules, while the solubility ratio is an expression of their relative condensability (Koros and Hellums, 1989). In general, an aij* 2 15 is required for a membrane permeation process to be commercially feasible (Hogsett and Mazur, 1983). Moreover, permeate purity is relatively unaffected by an aij* > 20 (Stookey et al., 1986). Permeability data are readily available for many common gaseous systems, such as C02/CH4and 02/N2(Koros et al., 1988; Walker and Koros, 1991; Teplyakov and Meares, 1990). However, when experimental data are nonexistent or unavailable, a preliminary screening of potential membrane processes is possible by examining the combination of component properties which would result in a permselectivity of 15 or greater. Barnicki (1991) presents a generalized method for predicting whether a favorable permselectivity (e.g., aij* 2 15) can be obtained for a given gaseous separation using any of 51 glassy or rubbery polymers. One needs only a knowledge of a diffusion-related property (effective kinetic diameter or van
der Waals volume) and a solubility-related parameter (effective Lennard-Jones well depth or critical temperature) of the components in question. B. Catalytic Conversion. Catalytic conversion is not a separation method in the conventional sense. Impurities or other objectionable Components are not removed, but rather chemically transformed on the surface of a solid catalyst into less objectionable species. The new component(s) may then require further separation. Because of its destructive nature, catalytic conversion can be eliminated from further consideration i f the impurities are a desired product. Catalytic conversion is especially favorable for separations in which extremely high purity is needed. In fact, almost complete removal of the objectionable components is possible (down to 1-10 ppm). Conversion is best for a stream with a low concentration of impurities (less than about 5000 ppm), for high temperature, and for low pressure, e.g., flue gases and purges (Kohl and Riesenfeld, 1985; McInnea et al., 1990). The removal of small amounta of the leas volatile components (i.e., liquid-type compounds is also a favored situation. The utility of catalytic conversion hinges on a difference in reactivity of impurities and bulk stream components. Industrially significant purification conversions can be classified as either combustion or hydrogenation reactions. Other conversion methods specific to particular compounds, (e.g., the catalytic reduction of nitrogen oxides with ammonia) are not considered in this work. Combustion (an oxidation-reduction reaction) entails the addition of oxygen and heat, often over a precious metal catalyst, to yield water, carbon dioxide, and sometimes sulfur dioxide, depending on the composition of the impurities: impurities + O2 + heat H 2 0 + N2 + COP+ SO2
-
Typical industrial applications of this technique involve
1686 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 Table IV. Hydrogenation Reactivity reacting chemical family" hydrogenation products alkynes olefins olefins paraffins acyl fluorides aldehydes, hydrogen fluoride acyl chlorides aldehydes, hydrogen chloride nitriles-aromatic nitros-oximes primary, secondary amines aliphatic nitros primary, secondary amines alcohols aldehydes ketones alcohols cycloalkanes aromatic rings cyclic ethers, n-alkanes heterooxygen aromatics heteronitrogen aromatics nitrogen cyclics anhydrides polyesters esters alcohols, acids, ethers organic acids alcohols amides primary amines thiole-sulfides hydrogen sulfide, hydrocarbons heterosulfur aromatics hydrogen sulfide, n-alkanes a
Ease of hydrogenation decreases from top to bottom of table.
the removal of species that are environmentally objectionable or detrimental to downstream processes, (e.g., hydrogen, elemental sulfur, hydrogen sulfide, and organics). Combustion is not recommended for halogenated organics, as the reaction products are generally as objectionable as the original compounds. For safety reasons the concentration of the impurities should be no more than 10% of their lower explosive limit. The selectivity of the combustion reaction is generally poor; almost all organic compounds will catalytically oxidize. Close control of the reaction temperature generally improves selectivity, but the most favorable temperature for oxidation is difficult to predict. Reaction temperatures are usually between 250 and 700 "C, assuming an impurity composition of approximately 10% of the lower explosive limit (Suter, 1955). From the standpoint of preliminary process analysis catalytic combustion is feasible for purification processes only when the impurities are at concentration levels below 10% of the lower flammability limit and when the bulk stream already consists of oxidation products, e.g., air stream, off-gases,and other inerts. In addition, catalytic oxidation should not be used when the process stream contains halogenated organics. The hydrogenation reaction involves the addition of hydrogen to specific functional groups. Conversions of 9599% are typical for the reaction impurities H, addition products Hydrogenation requires much milder conditions than combustion, typically at temperatures lower than 100 "C. High selectivity is possible by a controlled addition of hydrogen, depending on the functional groups present. The order of reactivity of various functional groups as well as their hydrogenation products is listed in Table IV (Rylander, 1985; Streitwieser and Heathcock, 1981). Groups higher in the table hydrogenate more easily than those lower down. Catalytic hydrogenation is a feasible purification operation only when the impurities contain functional groups listed in Table IV. Moreover, the reactivity of the functional groups in the impurities must be higher than that of the bulk stream species, i f the bulk stream is to be unaltered by hydrogenation. Hydrogenation is especially favorable for processes in which the impurities can be converted into desired products. The conversion of acetylene to ethylene during the production of ethylene is an excellent industrial example of the use of hydrogenation for product purification (Reitmeier and Fleming, 1958).
+
-
C. Physical Absorption. Physical absorption is characterizsd by specific nonchemical interactions between the absorbent liquid and the solute gas. These interactions are t y p i d y a linear function of the solute partial pressure in the gas phase and the solute concentration in the liquid phase. Consequently, a physical solvent maintains its absorptive properties even when the partial pressure of the solute in the feed is high (England, 1986). This contrasts markedly to a chemical solvent which typically loses its effectiveness as the solubility limit of the solute is approached. However, unless the solute-solvent solubility is extremely large, the product stream concentration generally cannot be reduced much below 100 ppm with a physical solvent (Tennyson and Schaaf, 1977). Thus the best applications of physical absorption involve sharp and enrichment separations. One exception to this rule is the widespread use of glycol absorption for the dehydration of natural gas and other process streams. For large-scale operations with dew point depression requirements of 50 O F or less, glycol absorption is generally the most economical alternative. When dew point depressions of 50-80 O F are necessary, glycol absorption and adsorption are competitivetechnologies (Kohl and Riesenfeld, 1985). Selective physical absorption is based on a difference in solubility resulting from the intermolecular forces between the gaseous solutes and the absorptive liquid. Fundamental intermolecular force calculations, involving the species' dipole moments and polarizabilities (Kaliszan, 1987), are not accurate enough to be even qualitative indicators of the feasibility of physical absorption. Therefore, one is forced to turn to bulk thermodynamic measurements of the solubility selectivity. The selectivity exhibited by a particular absorbent can be expressed in terms of the ratio of the liquid-phase mole fractions of two gaseous solutes in the liquid solvent. (4)
For purely physical absorption at low to moderate pressures, gas-phase solute-solute interactions are generally small and tend to cancel (Le., = Moreover, as a first approximation the activity coefficient ratio can be replaced with infinite dilution values. Upon substitution eq 4 becomes
4).
(5)
The standard-state liquid fugacity of a component can be determined by any of the commonly used methods listed below: 1. Extrapolation of the vapor pressure curve to a hypothetical liquid state. This is the simplest approach, but it can be extremely unreliable for temperatures much above the critical temperature (Prausnitz et al., 1986). 2. Use of semiempirical fugacity correlations. Prausnitz and Shair (1961)and Yen and McKetta (1962) present correlations for nonpolar and polar solvent systems. Accuracy is varied. 3. Use of the 'ideal" solubility concept (Gjalbaek, 1952, England, 1986). The expression for ideal solubility is derived form the Clausius-Clapeyron equation and Raoult's law. Results are best for simple non-polar gases well above their critical temperatures. 4. Use of Henry's constants. Sander et al. (1983) describe the use of a modified version of the UNIFAC group contribution method for calculating Henry's constants.
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1687 Table V. Estimated H2S/C02 Selectivities method Selectivity vapor pressure extrapolation 6.56 Shair correlation 4.15 ideal solubility 2.59 Henry’s constants 4.87
Only the activity coefficients in eq 5 are solvent-dependent; the standard-state liquid fugacities depend only on the properties of the individual solutes. For regular solutions, the infinite dilution activity coefficient is given by the Scatchard-Hildebrand-Flory-Huggins equation as (Walas, 1985) vi vi Vi In T~~= -(a, - (si)2 In - - - + 1 (6) RT v, v, Note that eq 6 is a function only of the properties of component i and the solvent, not those of component j . This is consistent with the assumption of negligible solute-solute interactions. Substitution of eq 6 into eq 5 yields
+
v:
(V:- V:) I’
(7)
At the preliminary process design stage, the choice of the “best” solvent cannot be known a priori (Barnicki, 1991). In general, a desirable solvent for selective physical absorption will form an ideal solution with some of the solutes and not the others. AB a rough approximation, one can assume that this yet unknown solvent will have properties very similar to one or more of the solutes (and therefore form an ideal solution with these species). For the binary case of components i and j in solvent s, if the solvent is similar to component i , then 6, = (si and V , = Vi. Component i will be preferentially absorbed:
.* Vj I + -((si PI .* RT
f.01
In
= In
f.01 1
-
vj - vj + In vi vi + 1
(8)
One can now estimate the selectivity of a potential absorption separaton solely from the properties of the two competing solutes. The selectivity calculated from eq 8 tends to be within 20-40% of experimental values, depending on the method chosen for calculating the standard-state liquid fugacity and on how different the chosen solvent is from the solutes. The selectivity achievable with a physical solvent that is not an exact analog of one of the solutes will rarely exceed a value of 10 and is generally in the range of 3-8 (Astarita et al., 1983). Taking into consideration the inaccuracies of the above analysis one can formulate a general heuristic on the feasibility of physical absorption for a given separation: If the selectivity calculated from eq 8 is 3 or greater for an enrichment process or 4 or greater for a sharp separation, then physical absorption should be considered as a feasible separation method. The utility of eq 8 is illustrated by predicting the selectivity for a mixture of hydrogen sulfide and carbon dioxide, common industrial gas components. The selectivities obtained by each of the four methods of correlating the standard-state liquid fugacity are shown in Table V. The solvent is assumed to be similar to H2S,and the partial pressures of the gases are assumed to be equal. The magnitudes of the estimated selectivities indicate that physical absorption is a feasible separation option for these two gases. The ideal solubility method, however, does not
Table VI. Acid-Base Functional basic ~TOUDS ammonia amines water alcohols aromatic amines heteronitrogen aromatics thiols
Groups and Molecules acid groups carbon dioxide sulfur dioxide hydrogen sulfide thiols hydrogen bond donors (see Kaliszan, 1987)
indicate a favorable selectivity. This method is probably inappropriate, as the system temperature is close to the critical temperatures of both components. Astarita et al. (1983) report an experimental H2S/C02 selectivity in methanol of 5.50 for the commercial Rectisol process. D. Chemical Absorption. Chemical absorption is characterized by nonlinear interactions that are particularly strong at low concentrations or partial pressures. These interactions tend to weaken considerably as one approaches the solubility limit of the solute; the solvent loses ita absorptive properties. In general, chemical absorption is favored when the partial pressure in the feed of the components to be removed is low and when the desired removal is high (purities at the p p m level are not uncommon) (Astarita et al., 1983; Tennyson and Schaaf, 1977). Although results have been published for selected systems (Astarita et al., 1983; Kohl and Riesenfeld, 1985), a generalized predictive method for chemical absorption equilibrium is not currently available. Without selectivity information, determining the feasibility of chemical absorption is difficult but not hopeless. Chemical absorption often involves the complexing of the acid-base functional groups of the solvent and solute. Table VI lists common acid-base functional groups (Ho, 1977). Note that only a limited number of functional groups exhibit acid-base behavior. Thus i f the species to be separated contain different acid-base functional groups (or i f one contains neither),then chemical absorption (based on an acid-base reaction) may be a feasible alternative. The above rule is a crude indication of potential utility only; it does not categorically ensure that an appropriate chemical solvent can be found. E. Cryogenic Distillation. The feasibility of a cryogenic distillation can be determined from the relative volatility, a,of the key components in much the same way as high-temperature distillation. The relative volatilities of condensed gaseous systems tend to be larger than those of liquid systems because of the wide boiling point ranges of the gases normally encountered. For typical industrial applications 2.0 I CY I 5.0 (Timmerhaus and Flynn, 1989), and in general cryogenic distillation can be considered as a feasible bulk separation alternative when CY 12.0. Although comparatively high relative volatilities are common for cryogenic distillation separations, one cannot categorically state that such a process will be the clearly favored separation method as is the case for high-temperature distillation (see Barnicki and Fair (1990)). The economics of a cryogenic separation are dominated by the scale of the process as well as the thermodynamics. Cryogenic distillation is rarely cost-efficient for small-scale separations or purification operations which produce less than 10-20 tons/day of product gas. For example, energy consumption for air separations drops from approximately 500 kW-h/ton of gas to less than 300 kW-h/ton of gas as the process scale increases from 10 tons/day to 100 tons/day (Springmann, 1985). Cryogenic distillation is feasible only for bulk, sharp, or enrichment separations involving high throughput.
1688 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
Moreover, when cryogenic distillation is considered as an alternative, one must ensure that components with high melting points are removed before the distillation is carried out (i.e,,species that may freeze at processing conditions). Any solids formed may foul reboilers, condensers, and other piping. The nitrogen-oxygen distillation of air is a good example of a separation in which freezable compounds cause problems. The inlet air typically contains carbon dioxide and water that freeze at the temperature and pressure at which nitrogen and oxygen liquify. Isalski (1989) lists other freezable impurities that are commonly present in cryogenic plant feed gases. F. Adsorption. F.1. Adsorbent Fouling and Chemical Damage. The ultimate lifetime and capacity of an adsorption bed depends to a large extent on the types of components that are processed. High-boiling organics (those with normal boiling points above 150-180 “C) tend to be preferentially adsorbed and are extremely difficult to remove during the regeneration cycle. Under favorable conditions, low molecular weight organics may polymerize on the surface of the adsorbent. Dialkenes, 1-alkenes, alkynes, and epoxides are especially susceptible to this behavior. Highly acidic or alkaline moieties may also cause permanent chemical alterations in the adsorbent. Aluminas are sensitive to acid solutions, while silica gels are strongly attacked by alkalies and hydrogen fluoride. Zeolites are generally resistant to chemical attack when the pH is kept in the range of 5-12 (Ullmann’s, 1988). When possible, adsorbent-fouling and adsorbent-damaging components should be removed upstream of the adsorber inlet. F.2. Molecular sieve Adsorption. The effect of differences in adsorbate molecular structure and size on selectivity can be eapecially dramatic when 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 vapor-phase (and liquid-phase) 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; Johnson and Kabza, 1990). The molecular dimension of importance in sieve-baaed adsorption processes is the minimum kinetic diameter. It is a combined measure of the cross-sectional area and shape characteristics of a molecule (see Barnicki (1991)for methods of estimating kinetic diameter). Commercially available molecular sieves fall into five distinct categories according to their nominal aperture sizes (Le., pore size distribution). Thus gaseous species can only be separated by molecular sieving effects when their kinetic diameters fall into different zeolite aperture size categories. Table VI1 presents the nominal aperture size and corresponding zeolite types for each category. This classification system was developed by Barrer (1959) and is repeated in modified form in many other references (e.g., Collins, 1968; Yang, 1987; Kovach, 1988). There is considerable disagreement in the literature on the subject of kinetic diameters of gas molecules. Breck (1974) presents one set of values, whereas several other authors report considerably different figures (Barrer and Brook, 1959; Collins,1968,Ullmann’s, 1988). Barnicki (1991)describes methods of estimating kinetic diameters for limited classes of compounds when no experimental data are available. These estimates are consistent with the results of Barrer
Table VII. Aperture Size Categories for Major Commercial Zeolites category nomind aperture size (A) zeolite type 5 3 3A Linde 3A Davison 4 4 4A Linde 4A Davison 5 5A Linde 3 5A Daviaon 2 a 1OX Linde 1 10 13X Linde 13X Daviaon
and Brook (1959). In spite of the inconsistencies in the reported values of kinetic diameters of individual molecules, there is general agreement on which molecules are excluded from the pores of a given zeolite type. Recent advances in the understanding of zeolite morphology have enabled the fabrication of molecular sieves with aperture sizes tailor-made for a specific separation application (Vaughan, 1988; Ruthven, 1988). However, the use of custom-made sieves adds considerably to the cost of the adsorption process and is not considered as an option here. Molecular sieves are extremely effective desiccants because of their highly polar surface environment. Because of this high affinity for water, molecular sieve drying processes can achieve essentially complete dehumidification of gas streams. Dew point depressions of 80 O F or more are readily obtainable (Kohl and Risenfeld, 1985). If water vapor is present in a gas stream, it typically will be the most strongly adsorbed species. Thus i f the objective is to recover adsorbed components which are free of water vapor, then the inlet gas stream should be dried before the molecular sieve adsorption process occurs. F.3. Equilibrium-LimitedAdsorption. As stated in the section Separation Types, the primary uses of equilibrium-limited adsorption are restricted to purifications and the separation of dilute components from bulk streams (i-e., for components consisting of less than 10% of the feed). In order to limit the necessary size of the adsorbent bed and to facilitate the subsequent regeneration steps, it follows that equilibrium-limited adsorption will be a favorable alternative only when the adsorbent affinity is greater for the impurities or dilute components than for the bulk stream. The mutual affinity of a given adsorbate-adsorbent pair is typically reported in terms of equilibrium loading on the adsorbent. The equilibrium loading is expressed as a function of adsorbate partial pressure at a single temperature (i.e., an isotherm expression (Yang, 1987; Ruthven, 1984)). Once the isotherm expression is known, the design of an adsorber is a relatively simple task (Fair, 1969; Kovach, 1988; Wankat, 1990). The ultimate utility and cost of an adsorption process is closely related to the interrelation between the amount of time that the product gaa(es) can be collected (i.e., the cycle time) and the size of the required adsorption unit. As the cycle time increases, the adsorber length (and separation cost) increases correspondingly. For a largescale industrial process a cycle time of 2 h is typical. Depending on the magnitude of the equilibrium loading of the preferentially adsorbed components, the length of the adsorber needed to achieve such a cycle time may result in an uneconomical process. Thus, the required adsorber length is a criterion of the feasibility of an adsorption separation. In general, for a standard cycle time of 2 h, i f the desired separation or purification requires an adsorber that is longer than 20 f t , then equilibrium-limited adsorption
Ind. Eng. Chem. Res., Vol. 31,No. 7,1992 1689 Table VIII. Favorable Components and Chemical Families for Desiccation by Equilibrium-Limited Adsorption gases chemical families argon aliphatics helium hydrocarbon aromatics hydrogen chlorides chlorine fluorides hydrogen chloride oxygenated compounds sulfur dioxide ammonia
air
can be eliminated as a potential separation method. Two distinct applications for equilibrium-limited adsorption are possible. The first entails the removal of adsorbable components from an inert carrier gas (e.g., removal of organics from air, oxygen, nitrogen, helium, etc). In thia case, the equilibrium loading of the inert ~ 8 on 9 the adsorbent is negligible and can be ignored. The second application of equilibrium-limited adsorption involves the separation between adsorbable components. The objective here is to collect the less adsorbed component in pure form for a period of time (typically about 2 h) until the adsorber bed is exhausted. At that point the more adsorbed component will break through and will begin to contaminate the product. Such a process will be feasible only under the following conditions: 1. The more adsorbed component must be in the minority in the feed (less than 10 mol %). If the majority feed component(s) were to be adsorbed, the adsorber bed would fill rapidly or would be impractically long. 2. For a cycle time of 2 h, the adsorber length required to achieve breakthrough of the more ahorbed component should be less than 20 f t . The length of an adsorber can be found by several methods such as those given by Fair (1969)or by Wankat (1990). 3. The ratio of the equilibrium loadings of the two components should be at least 2, and preferably higher (Chu, 1991). A high loading ratio ensures that simultaneous adsorption will be minimal. Because of its high concentration in the feed, the less adsorbed component may displace the more adsorbed component if the loading ratio is too low. A limited number of bulk enrichment separations (i.e., adsorbed components consist of 10 mol % of more of the process stream) are now routinely performed with pressure swing adsorption cycles. Examples include hydrogen recovery, methane enrichment from biogases, oxygen enrichment, carbon dioxide recovery, and natural gas recovery. Further details are available in Richter (1987). These cases currently are not covered by the SSAD. The use of equilibrium-limited adsorption for desiccation operations has been notably successful. Silica gels, zeolite molecular sieves, and activated aluminas have high affinities for water. The following heuristic reflects current industrial applications (Keller et al., 1987;Yang, 1987): I f the process stream to be dried contains less than 3 wt % water and is composed of gases or organic species which are members of the chemical families listed in Table VIII, then equilibrium-limited adsorption will be a feasible (and probably the best) alternative. The appropriate adsorbent (some type of silica gel, zeolite molecular sieve, or activated alumina) for the particular application in question cannot be determined at this stage. As is the case with other separation techniques requiring mass separating agents, the appropriate adsorbent for a given separation is not known in the early stages of process development. With hundreds of commercial adsorbents available, the examination of each potential adsorbate-
Table IX. Mixed Solvent Recovery Specifications mol mol boiling point component component % wt (K) type nitrogen 70.645 28.0 77.4 gas oxygen 28.855 32.0 90.2 gas 0.256 88.1 ethyl acetate 350.3 liquid toluene 0.244 92.1 383.6 liquid
adsorbent pair would be prohibitively time-consuming. Moreover, even if an exhaustive search could be done quickly, the available isotherm data are relatively limited (Valenzuela and Myers, 1989). When experimental isotherm data are unavailable, adsorption affinity can be estimated for activated carbon adsorbents from a generalized Dubinin-Polanyi characteristic curve developed by Barnicki (1991). The method described by Barnicki requires only molar volume and fugacity data. G. Condensation. Condensation is a basic separation technique in which a gas stream is brought to its saturation (dew) point where the low volatility components begin to liquefy. As these Components condense out, the dew point rises and the temperature must be lowered further to continue the process. A condenser is typically equivalent to only one or two theoretical equilibrium separation stages. Consequently, condensation processes exhibit poor selectivity unless the relative volatility or boiling point temperature difference of the components is extremely large. Condensation should be explored as a potential separation method for enrichment operations when the relative volatility between key components is greater than approximately 7 or the boiling point difference is greater than 40 "C. Condensation is most favorable for the separation of high-boiling organic vapors from noncondensable gases, especially when cooling water can be used as the condensing medium. In such situations, extreme purity (e.g., ppm levels) cannot be achieved, but generally greater than 95% removal is possible. Example Separations Mixed Solvent Recovery from a n Air Stream. In order to comply with strict environmental regulations on the extent of toxic emissions, many chemical synthesis processes include one or more steps involving the removal of trace amounts of organics from process off-streams. Fair (1967)presented a detailed study of such a process for the removal of toluene and ethyl acetate from an air stream. Table IX gives the input specifications for the solvent recovery problem. The objective is to recover 99% of the ethyl acetate and essentially all of the toluene. Note that the organics are to be separated and recovered, rather than removed and possibly destroyed. Air is considered to be 71 mol % nitrogen and 29 mol % oxygen for this problem. The separation analysis starts with the phase split manager (PSM) as described in part 1 of this series (Barnicki and Fair, 1990). The input stream includes both gas and liquid compounds. However, due to the extremely low concentration of liquids (0.5 mol %), no phase separation is required. The analysis proceeds directly to the purification split selector (PSS) of the gas split manager (GSM). For purification operations, the possible separation methods are chemical absorption, catalytic conversion, molecular sieve adsorption, and equilibrium-limited adsorption. The ranked component property lists are given in Table X. Examining the ranked lists, the possible key component pairs are nitrogen-ethyl acetate using chemical
1690 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 Table X. Ranked Property Lists for Mixed Solvent Recovery ranked Property method DroDertv components values inorganic gas chemical absorption chemical family oxygen inorganic gas catalytic oxidation nitrogen acetate ethyl acetate alkylbenzene toluene
