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COMMENTARIES Past Progress and Future Challenges in Adsorption Research When I received a letter from Donald Paul inviting me to prepare a commentary summarizing past progress and future challenges in the general area of adsorption, my first reaction was to make my excuses and decline as politely as possible. Such opinion pieces inevitably reflect mainly the experience and biases of their authors and, as regards the future, my crystal ball is just as opaque as anyone else’s. Nevertheless, the invitation was not without its attractions. In research it is generally necessary to focus exclusively on the particular problem under investigation. We rarely stand back and examine the “big picture” to identify the broader challenges and to see how the pieces of the puzzle fit together. Despite my initial reservations, I concluded that such an exercise may indeed be useful, especially if it prompts others to think about these issues and perhaps to challenge the opinions expressed here. It is important to emphasize that my perspective is that of an academic chemical engineer whose major interests are in the application of adsorption as a separation process. No doubt the perspectives of a pure scientist or a practicing process engineer would be different, although I like to think that there would be substantial common ground. To facilitate the discussion, it is convenient to consider progress under four different headings: fundamentals, modeling/simulation, adsorbent materials, and process development. A tabular summary in which several key developments in each of these categories are identified by decade is given in Table 1. Fundamentals Fundamental understanding of an adsorption process requires a detailed mechanistic knowledge of the kinetics and equilibria of sorption and their impact on the dynamic response of an adsorption column. Sorption kinetics and equilibria were widely studied during the 20 years following World War II, notably in the U.K. by Barrer (University of Aberdeen and then Imperial College, London), in the U.S.S.R. by Dubinin at the Academy of Sciences and Kiselev at the Lomnosov State University, in Canada by Habgood at the Research Council of Alberta, and in East Germany by Schirmer at the Institute of Physical Chemistry in Berlin. It is remarkable that, despite the pioneering work of Irving Langmuir during the prewar years and the excitement generated by the development of synthetic zeolites and gas and liquid chromatography in the late 1950s and early 1960s, during this period, the study of the fundamentals of adsorption was largely ignored in the U.S. Some important developments from this era include the Dubinin-Polanyi potential theory,1 the first predictive calculations of Henry constants and heats of adsorption (Barrer and Kiselev2), recognition of the importance of considering chemical potential gradient,
rather than concentration gradient, as the driving force for transport in (thermodynamically nonideal) adsorption systems (Habgood3), and practical and theoretical demonstration of the separating power of microporous membranes (Barrer4). From the chemical engineering perspective, the 1965 paper of Myers and Prausuitz5 setting forth the principles of the “ideal adsorbed solution theory” must also be considered as a landmark because this provided the first reasonably realistic approach for prediction of binary and multicomponent equilibria from single-component isotherms. The measurement of micropore diffusion in zeolite crystals and other microporous solids has proved a far more challenging task than might have been anticipated. The development during the 1970s of the pulsed field gradient nuclear magnetic resonance (PFGNMR) technique for measuring self-diffusion under equilibrium conditions proved a major milestone. Remarkably, the key developments were made by Professor Pfeifer and his students at the University of Leipzig in East Germany, working under very difficult conditions with a home-built spectrometer and an improvised power supply (a room full of car batteries!).6,7 For many experimental systems, the PFGNMR measurements yielded diffusivities that were several orders of magnitude larger than the values derived from traditional macroscopic measurements, thus calling into question the validity of the earlier measurements. Over the past 25 years there have been several comparative studies in which diffusion has been studied by both macroscopic and NMR methods in the same well-characterized adsorbent samples.8,9 For several systems the results have been shown to agree, at least within a factor of 2 or 3. This can perhaps be considered as acceptable agreement because it is inherent in the techniques that PFGNMR measurements tend to overestimate the diffusivity while most macroscopic methods tend to yield underestimates. However, there remain a few systems such as benzene-NaX zeolite and propane-AlPO45which, despite careful study, still show discrepancies of more than an order of magnitude. Most of the more obvious explanations have been considered and shown to be inapplicable.9 There have been a number of recent macroscopic measurements for these systems that purport to show close agreement with the NMR values,10,11 but these claims do not withstand careful scrutiny.12,13 Resolution of this issue therefore remains a challenge for the future. During the 1980s and 1990s, as a result of the dramatic increases in the speed and power of computers, emphasis has shifted toward more detailed theoretical calculations aimed at the a priori prediction of adsorption equilibria and micropore diffusivities by the Monte Carlo and molecular dynamic approaches.14-17 Such studies have provided many useful insights, but the inadequacy of our understanding of intermolecular
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Table 1. Major Milestones and Future Challenges in Adsorption Separation Processes pre-1970 fundamentals
modeling/ simulation
materials
process development
isothermal kinetics chromatography first theoretical predictions of K and ∆U IAST Glueckauf approx. Van Deemter eqn. basic equilib. theory Thomas eqn. first generation molecular sieve adsorbents: zeolites A, X, Y heatless drier PSA H2
1970s
1980s
1990s
PFG NMR diffusion measurements nonisothermal kinetics
M.C. equilib. predictions
M.C. and M.D. predictions MRI Stefan-Maxwell
multicomponent and adiabatic equilib. theory rigorous modeling of single column dynamics Si-rich MFI zeolites carbon mol. sieves
process modeling: PSA, TSA, SMB
direct prediction of cyclic steady state development of process simulations: ADSIM, gPROMS
AlPO4 and SAPO zeolite analogues
large-pore zeolites: VPI5, cloverite, MCM 41 titanosilicates: ETS4 and ETS10 2nd generation VSAO2 (LiX) small-scale SMB processes adsorbent “wheels”
Sorbex process PSA O2
PSA N2(CMS) VSA O2
forces remains a significant problem. The attractive potential field can be calculated with fairly good accuracy, but estimates of the repulsive contribution to the force field are much less reliable. Small differences in the values assumed for the radii of the framework atoms or ions and the kinetic diameters of the sorbate molecules lead to large differences in the calculated force fields and therefore to large differences in the predicted diffusivities, especially under sterically hindered conditions. The impact of errors in the repulsive potentials on adsorption equilibrium calculations is much less severe because the adsorbed molecules spend only a small fraction of their life in the sterically restricted regions of relatively high potential energy. The problem of predicting interference effects in multicomponent diffusion in an adsorbed phase is both academically interesting and practically important. The first practically useful approach came from Habgood et al.,18 who showed, for a binary Langmuir system, how the sorption kinetics of component A are affected by the presence of component B as a result of the modification of the equilibrium isotherm (and therefore the gradient of chemical potential), even when the intrinsic mobilities of both components are constant. The Habgood model has proved to be a practically useful approximation for many systems, provided that the adsorbed phase concentration is not too high. To develop a more realistic model, it is necessary to account for the interference effects between the diffusing species. The formalism of irreversible thermodynamics is qualitatively helpful but provides no guidance as to how to estimate the magnitude of the “cross coefficients”. Krishna and his students have shown that the generalized Stefan-Maxwell equation, coupled with the Vignes correlation, provides a tractable approach to this problem.19,20 A rich variety of behavior is predicted, and some of these predictions have been verified experimentally. When the cross coefficient (which measures the interference between diffusing species) is zero, the Stefan-Maxwell formulation reduces to the Habgood model, which thus emerges as a simplified limiting case of the more general theory. As regards the future, it is safe to predict that the emphasis on theoretical calculation of kinetics and
future challenges MRI development NMR/sorption rate discrepancy
selective adsorbent for N2 over CH4 zeolite membranes miniaturization novel contactors olefin seps. H2 purification for fuel cells high T zeolite membrane processes
equilibria will continue and even accelerate. What is needed is a judicious combination of experimental measurements carried out in conjunction with modeling studies and designed to test key elements of the theory. However, because few research groups are involved in both theoretical modeling and experimental measurements, it seems more probable that the present dichotomy will persist. Another area where significant progress can be expected is the application of NMR imaging methods to the study of sorption kinetics and adsorber dynamics. Although such methods have been widely used for imaging of soft tissue, etc., the application to adsorption systems has been limited by the short relaxation times of many adsorbed molecules. The problem of imaging in systems with a short relaxation time has recently been addressed,21 and a new imaging technique SPRITE (Single Point Ramped Imaging with TI Enhancement) has been developed and appears to provide a workable solution.22 However, this approach has so far been applied only to very few systems, and further studies are therefore needed to explore its versatility. Modeling/Simulation Earlier research during the 1950s and 1960s was directed toward understanding the dynamic behavior of an adsorption column when subjected to a welldefined change in feed composition. This requires solution of the differential mass balance equation for a column element together with the appropriate rate expressions for mass and heat transfer and the energy balance. This general problem, for a multicomponent nonisothermal system, is too complex for analytical solution, and its solution had to await the development of appropriate numerical methods and the availability of sufficient computational power. The earlier studies, which were directed toward understanding the behavior of simpler systems, followed two main approaches. For single-component linear isothermal trace systems (and even for certain nonlinear systems), analytical solutions that show the general nature of the dynamic response and the interplay
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between kinetic and equilibrium effects can be obtained. Notable papers from this era are those of Thomas,23 Glueckauf,24 Rosen,25 Amundson,26 and Vermeulen,27 who generalized the Thomas solution to a practically useful form. In a parallel development, Glueckauf28 also established the conditions under which an adsorption column shows “constant pattern” and “proportional pattern” behavior and showed how equilibrium theory can be used to predict the general form of the column response for binary and multicomponent systems, thus laying the groundwork for the later studies of Basmadjian,29 Rhee, Amundson, and Aris,30-32 and Helfferich,33 in which the predictions of multicomponent equilibrium theory were explored in considerable detail for both isothermal and adiabatic systems. By the early 1980s access to powerful computers had become widely available, and this, together with the development of efficient numerical procedures such as orthogonal collocation and the method of lines for the solution of sets of coupled partial differential equations, made it possible to solve reasonably realistic dynamic models to predict dynamic behavior, even for multicomponent, nonlinear, and nonisothermal systems. The paper by Liapis and Crosser34 is noteworthy as one of the earliest examples of this approach. With these developments it became feasible to simulate an entire cyclic process by coupling together the simulations for the individual steps and running the simulation until it converged to the cyclic steady state. The study of pressure swing adsorption (PSA), temperature swing adsorption (TSA), and countercurrent simulated moving bed (SMB) processes therefore made rapid progress during the 1980s,35 leading eventually to the development of generalized numerical process simulations such as ADSIM and gPROMS, which are now widely available and are routinely used in the development and optimization of adsorption processes.36 In this context two more recent developments are noteworthy. Croft and LeVan showed that it is possible to solve directly for the cyclic steady state by simultaneous solution of both the equations and the boundary conditions for all steps in the process.37 Their procedure was not particularly efficient from the perspective of computational time, but Pantelides36 showed that combining this approach with the double collocation procedure used by Raghavan and Ruthven39 yielded a computationally efficient way to establish the cyclic steady state. The major problems in the area of process modeling and simulation appear now to have been solved. It is always possible to extend these studies to include more complex mass-transfer rate expressions or equilibrium isotherms, but the basic tools are now available. Future progress appears more likely to come from the use of numerical simulation for process optimization rather than from further refinement of the simulations themselves. Materials In earlier adsorption processes the choice of adsorbent was limited to activated carbon, silica gel, and activated alumina. Natural zeolites were known as a scientific curiosity but were never used as adsorbents on the industrial scale. The development of synthetic “molecular sieve” zeolites by Robert Milton and his team at the Union Carbide laboratories during the late 1950s38 greatly broadened the available range of adsorbents and
led to a corresponding upsurge in the development of adsorption processes. The first generation synthetic zeolites comprised the Al-rich X and A materials which are still in widespread use. Zeolites Y and L were discovered by Breck and Flanigen during the early 1960s.39 The 1970s saw the development of carbon molecular sieves and another important family of much higher silica materials, based on the pentasil structure developed by Mobil Corp.40 The best known of these materials is ZSM-5 (and its pure silica analogue silacalite), which is now in widespread use in several important adsorption processes. The development by Bergbau Forschung, during the late 1970s, of carbon molecular sieves for kinetic separation of air (to produce N2) also had a major technological impact.41 The pace of adsorbent research accelerated during the 1980s and 1990s, leading to the development of AlPO4 and SAPO zeolite analogues (Union Carbide) and several large-pore materials such as VPI-5, Cloverite, and MCM-41,42 as well as the microporous titanosilicates (ETS-4 and ETS-10) developed by Engelhard. This period also saw intensive research aimed at tailoring adsorbents for particular separations. Noteworthy developments include improved adsorbents for xylene separation (Heavy Parex Process)43 and the development of highly Li+-exchanged low silica X as the adsorbent of choice for PSA oxygen production.44 Current material challenges include the development of a nitrogen-selective adsorbent for upgrading of low-BTU natural gas (ETS-4 shows some promise), development of a high-affinity adsorbent for CO for purification of the feed hydrogen for fuel cells, and the fabrication of coherent zeolite membranes. Interest in alternatives to the gasoline engine has stimulated a great deal of research into the adsorptive storage of gaseous fuels such as methane and hydrogen. Given an adsorbent with sufficiently high capacity, such an approach has obvious advantages. However, despite some surprising claims, the possibilities of a breakthrough appear remote. In an important contribution to this subject, Matranga et al.45 used molecular simulation to calculate the maximum methane storage capacity of an ideally constructed carbon adsorbent. The resulting values of about 197 v/v based on the solid volume or 140 v/v based on the vessel volume are smaller than the values for compressed gas at 3000 psi (215 v/v). Nevertheless, physically unrealistic claims of extremely high capacity adsorbents are still reported from time to time. Process Development Upon review of the industrial-scale development of adsorption processes, the long time delay between laboratory-scale demonstration and reduction to an industrial process is striking. For example, the first PSA patents date from the 1930s, but it was only during the 1970s and 1980s that PSA processes gained widespread industrial acceptance. The reasons for this are not completely clear but no doubt reflect inter alia the innate conservatism and capital intensity of the process industries and the relatively low frequency with which facilities are developed on green field sites. It is difficult to identify the really important process developments because any such assessment must consider both technical novelty and economic impact. The Sorbex process, introduced by Donald Broughton at UOP in the late 1960s46 and commercialized in the early
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1970s, is my own first choice, because this process rates very highly on both criteria. PSA processes for hydrogen purification have probably had a similar economic impact because this technology, first introduced in the early 1970s, is still in widespread use today. More recently, the kinetic PSA separation of air over a specifically tailored carbon molecular sieve adsorbent to produce relatively pure nitrogen41 scores highly on technical novelty but perhaps less highly on economic impact. The introduction of highly Li+-exchanged low-silica LiX zeolite as the adsorbent of choice for PSA air separation to produce oxygen44 has challenged the economic supremacy of the cryogenic process for oxygen production at rates of up to about 250-300 tons/day. To take advantage of the improved capacity and selectivity of the LiX adsorbent required a change from a supra-atmospheric process to a vacuum desorption system of the type originally suggested by de Montguereuil and Domine in their 1964 patent.47 The case history of the modern PSA oxygen process thus provides a classic example of the need for close collaboration between adsorbent materials and process development. The 1990s have seen the widespread application of adsorption for removal of trace levels of contaminants such as VOCs from air and for desiccant cooling systems. These applications involve large gas (air) flow rates so the costs associated with pressure drop become important. The traditional packed-bed contactor is therefore replaced by a parallel passage contactor, commonly in the form of a slowly rotating wheel. Future Challenges Because process innovation is driven primarily by economics, a prediction of future process developments requires an assessment of current economic challenges. The purification of hydrogen for fuel cells is one such challenge. Hydrogen, which is generally produced by reforming natural gas, contains significant quantities of CO which must be removed to a very low concentration level. There has been a good deal of recent research aimed at developing improved adsorbents for CO removal, but this problem has not yet been fully resolved. The separation of light olefins from cat-cracker offgas is another example. World demand for ethylene and propylene now exceeds the supply. These materials are traditionally produced by steam cracking of naphtha, but that is a costly and relatively inefficient process. The off-gases from catalytic crackers contain light olefins at relatively low concentration, but because of the very large scale of operation, the total quantities of these gases would be more than sufficient to meet world demand. The challenge is to develop a sufficiently selective adsorbent that will allow the light olefins to be extracted efficiently from a stream containing higher concentrations of saturated hydrocarbons. Process miniaturization and improvement of process efficiency are other areas where important developments are likely in the foreseeable future. In the past the development of adsorption processes has been focused mainly on relatively large-scale operations. Considerations of capital cost and operating cost have generally been given greater weight than size and portability. However, in the development of fuel cell powered vehicles and for the production of medical oxygen, size and portability are the dominant issues. The most obvious way to miniaturize an adsorption
process is to reduce the cycle time, and PSA processes with cycle times of a few seconds are now well established. This is about the limit for a traditional packedbed adsorber; further progress depends on the development of a novel contactor based on a monolithic or parallel-plate structure with narrow and closely spaced channels. Theoretical considerations suggest that such a contactor would have a substantial advantage over a packed adsorbent bed although it is clearly more difficult to manufacture (and therefore more costly).48 Nevertheless, preliminary results appear promising, and the commercial development of a new generation of ultrarapid-cycle PSA processes seems likely to occur within the next few years. The development of zeolite membranes is another area where significant success has been achieved on a laboratory scale. However, the problems of fabrication and scale-up remain formidable. It seems likely that these problems will eventually be overcome but probably not within a short time frame. Acknowledgment The author is most grateful to Prof. Douglas LeVan for his constructive comments on the initial draft of this review and for several important suggestions that have been included in the final version. Literature Cited (1) For a useful short review, see: Dubinin, M. M.; Astakhov, Y. A. Adv. Chem. Ser. 1971, 102, 69-85. (2) For a useful short review of the early predictive calculations of Henry constants and adsorption energies, see: Kiselev, A. V.; Lopatkin, A. A. Molecular Sieves; Society of Chemical Industry: London, 1968; pp 252-266. (3) Habgood, H. W. Can. J. Chem. 1958, 36, 1384. (4) Barrer, R. M.; Ash, R.; Pope, C. G. Proc. R. Soc. London 1963, A271, 1-18, 19-31. (5) Myers, A. L.; Prausnitz, J. M. AIChE J. 1965, 11, 121. (6) Pfeifer, H. NMR: Basic Principles and Progress; SpringerVerlag: Berlin, 1972; Vol. 7. (7) Ka¨rger, J.; Pfeifer, H. Z. Chem. 1976, 16, 85-90. (8) Ka¨rger, J.; Ruthven, D. M. J. Chem. Soc., Faraday Trans. I 1981, 77, 1485. (9) Ka¨rger, J.; Ruthven, D. M. Zeolites 1989, 9, 267. (10) Bu¨low, M.; Micke, A. Adsorption 1995, 1, 29-48. (11) Nijhuis, T. A.; van den Broeke, L. J. P.; van de Graaf, J. M.; Kapteijn, F.; Makkee, M. Chem. Eng. Sci. 1997, 52, 3401. (12) Brandani, S. Adsorption 1998, 4, 17-24. (13) Brandani, S.; Ruthven, D. M. Chem. Eng. Sci. 2000, 55, 1935-1937. (14) Tan, Z.; Gubbins, K. E.; van Swol, F.; Marini, U.; Maverin, B. In Fundamentals of Adsorption; Mersmann, A. B., Scholl, S. E., Eds.; United Engineering Trustees: New York, 1991; pp 919928. (15) Pickett, S. D.; Novak, A. K.; Cheetham, A. K.; Thomas, J. M. In Recent Advances in Zeolite Science; Klinowski, J., Barrie, P. J., Eds.; Elsevier: Amsterdam, The Netherlands, 1989; p 253. (16) Gravert, B.; Fiedler, K.; Stach, H.; Ja¨nchen, J. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen, R. A., Eds.; Elsevier: Amsterdam, The Netherlands, 1989; p 711. (17) Cohen de Lara, E.; Kahn, R.; Goulay, A. M.; Lebars, M. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen, R. A., Eds.; Elsevier: Amsterdam, The Netherlands, 1989; p 753. (18) Round, G. F.; Habgood, H. W.; Newton, R. Sep. Sci. 1966, 1, 219. (19) Krishna, R.; Wesselingh, J. A. Chem. Eng. Sci. 1997, 52, 861. (20) Van den Broeke, J. P.; Krishna, R. Chem. Eng. Sci 2000, 50, 2507-2522. (21) Balcom, B. J. SPRITE Imaging of Short Relaxation Time Nuclei. In Spatially Resolved Magnetic Resonance Methods; Bluemler, P., et al., Eds.; Wiley-VCH: Weinheim, Germany, 1998.
Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2131 (22) Prado, P. J.; Balcom, B. J.; Jama, M. J. Magn. Reson. 1999, 137, 59-66. (23) Thomas, H. C. J. Am. Chem. Soc. 1944, 66, 1664. (24) Glueckauf, E. Trans. Faraday Soc. 1955, 51, 1540. (25) Rosen, J. B. J. Chem. Phys. 1952, 20, 387. (26) Lapidus, L.; Amundson, N. R. J. Phys. Chem. 1952, 56, 984. (27) Heister, N. K.; Vermeulen, T. Chem. Eng. Prog. 1952, 48, 505. (28) Glueckauf, E. Discuss. Faraday Soc. 1949, 7, 12. (29) Pan, C. Y.; Basmadjian, D. Chem. Eng. Sci. 1970, 25, 1653; 1971, 26, 45. (30) Amundson, N. R.; Aris, R.; Swanson, R. Proc. R. Soc. London 1965, A286. (31) Rhee, H. K.; Aris, R.; Amundson, N. R. Philos. Trans. R. Soc. London 1970, A267, 419. (32) Rhee, H. K. Ph.D. Thesis, The University of Minnesota, Minneapolis, MN, 1968 (see also Chem. Eng. J. 1970, 1, 241, 279; 1972, 3, 22, 122). (33) Helfferich, F.; Klein, G. Multicomponent Chromatography; Marcel Dekker: New York, 1970. (34) Liapis, A. I.; Crosser, O. K. Chem. Eng. Sci. 1982, 37, 958. (35) For example, see: Raghavan, N. S.; Ruthven, D. M. AIChE J. 1985, 31, 2017. (36) An excellent review of this work is included in the following paper: Nilchan, S.; Pantelides, C. C. Adsorption 1998, 4, 113147. (37) Croft, D. T.; LeVan, M. D. Chem. Eng. Sci. 1994, 49, 1821, 1831. (38) Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J. Am. Chem. Soc. 1956, 78, 5963.
(39) Breck, D. W.; Flanigen, E. M. Molecular Sieves; Society of Chemical Industry: London, 1968; p 47. (40) Kokotailo, G. T.; Lawton, S. L.; Olson, D. A.; Meier, W. M. Nature 1978, 272, 437, and 275, 119. (41) A useful short review of the development of CMS adsorbents has been given: Schro¨ter, H. J.; Ju¨ntgen, H. In Adsorption Science and Technology; Rodrigues, A. E., LeVan, M. D., Tondeur, D., Eds.; NATO Advanced Study Institute Series E158; Kluwer: Dordrecht, The Netherlands, 1989; p 269. See also: Ju¨ntgen, H. Carbon 1977, 15, 273. (42) A historical review of this subject has been given: Flannigen, E. An Introduction to Zeolite Science and Practice; van Bekkum, H., Flannigen, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, The Netherlands, 1991; Chapter 2. (43) Neuzil, R. W. (to UOP). U.S. Patent 3,997,620, 1975. (44) Chao, C. C. (to UOP). U.S. Patent 4,859,217, 1989. (45) Matranga, K.; Myers, A. L.; Glandt, E. D. Chem. Eng. Sci. 1992, 47, 1569. (46) For example, see: Broughton, D. B.; Neuzil, R. W.; Pharis, J. M.; Brearley, C. S. Chem. Eng. Prog. 1970, 66 (9). Also: de Rosset, A. J.; Neuzil, R. W.; Broughton, D. B. In Percolution Processes: Theory and Applications; Rodrigues, A. E., Tondeur, D., Eds.; NATO Advanced Study Institute Series 33; Sijthoff and Noordhoff: Alphen van Rijn, Holland, 1981; p 249. (47) de Montgureuil, P. G.; Domine, D. (to Air Liquide). U.S. Patent 3,155,468, 1964. (48) Ruthven, D. M.; Thaeron, C. Gas Sep. Purif. 1996, 10, 6373.
Douglas M. Ruthven
Department of Chemical Engineering, University of Maine, Orono, Maine 04469-5737 IE000060D
