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Effect of Condensable Impurities in CO2/CH4 Gas Feeds on Carbon Molecular Sieve Hollow-Fiber Membranes De Q. Vu,†,‡ William J. Koros,*,§ and Stephen J. Miller‡ Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062, School of Chemical Engineering, Georgia Institute of Technology, 778 Atlantic Drive NW, Atlanta, Georgia 30332-0100, and ChevronTexaco, Energy Research & Technology Company, 100 Chevron Way, Richmond, California 94802-0627
The presence of vapor impurities in gas streams presents problems for both polymeric and carbon materials. For natural gas processing applications, it is suspected that condensable hydrocarbons can cause significant performance declines for polymer membranes. This paper investigates the effect of these condensable vapor impurities on the performance of carbon molecular sieve (CMS) hollow-fiber membranes. Toluene and n-heptane were separately used as representative aromatic and paraffinic impurities, respectively, in 10% CO2/90% CH4 gas feed streams. Shell-side feed pressures of up to 900 psia and temperatures of 35 and 50 °C were used. Experimental results reveal that the CMS membranes maintained CO2/CH4 selectivity with a maximum of 20% reduction in CO2 permeance during exposure to gas feeds containing these impurities, in comparison to “clean” gas feeds without these impurities. Furthermore, a simple in situ regeneration procedure of moderate heating (70-90 °C) with dry N2 purge gas resulted in almost complete recovery of CO2 permeance without loss of CO2/CH4 selectivity. Comparisons with polyimide membrane performance in similar adverse environments are also presented. The robustness and regeneration capabilities of the CMS fibers offer noteworthy advantages over polyimide fibers, which can display significant losses in membrane performance under similar conditions. 1. Introduction Carbon molecular sieve (CMS) membrane materials are attractive gas separation membranes because they offer very high selectivities and productivities and have advantages in high-temperature and high-pressure applications. CMS materials exhibit high internal surface areas and adsorptive capacities and have been used successfully in adsorption processes where kinetic separation can be achieved through diffusivity differences of gases.1-4 There is much ongoing research into using carbon molecular sieves as size- and shape-selective membrane materials that can capitalize on separations of differently sized gas molecules. Several reviews in the literature discuss CMS membranes and their applications.5-13 CO2/CH4 separation is gaining increasing attention because of many immediate industrial applications (e.g., CO2 removal in natural gas processing and enhanced oil recovery). Furthermore, work by many researchers has demonstrated the excellent performance of CMS membranes, with permeation performances (CO2/CH4 selectivity and CO2 permeability) exceeding those of existing conventional polymeric membranes. Previous work in the literature has examined various gas separations using carbon molecular sieve membranes, such as O2/N2, H2/N2, CO2/N2, and more recently CO2/CH4 separation. However, most of the CO2/CH4 * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 404-385-2845. Fax: 404-8942866. † The University of Texas at Austin. ‡ ChevronTexaco. § Georgia Institute of Technology.
studies deal with pure gas permeation experiments; use dense, flat membranes; and/or predominantly involve testing at low pressures (generally less than 200 psia). For industrial application, it is important to examine the mixed-gas environment with interactions with other components in natural gas streams and to evaluate performance at relevant elevated pressures (up to 1000 psia). With these objectives, this work considers the high-pressure separation of CO2/CH4 using asymmetric carbon molecular sieve hollow-fiber membranes. In a previous publication,8 experimental results of CMS fibers tested under these conditions were presented for CMS fibers pyrolyzed from different polymer precursors and different pyrolysis protocols. The results in that paper demonstrated encouraging evidence for CMS fiber use in high-pressure applications (such as natural gas processing), especially in terms of their mechanical, permeance, and selectivity stabilities. The previous findings are important for industrial implementation considerations as research work continues to examine membrane performance under “realistic” environments. In a continuation of this effort, this paper investigates the performance of these same CMS fibers in the presence of condensable vapor impurities with high-pressure, mixed-gas (CO2/CH4) feeds. An important objective of this work is to evaluate the interaction of condensable natural gas components on CMS membrane fibers. For the experimental work, toluene and n-heptane were used as typical aromatic and paraffinic hydrocarbons, respectively. The extent to which these components affect CMS membrane selectivity and productivity at varying concentrations were explored.
10.1021/ie020698k CCC: $25.00 © 2003 American Chemical Society Published on Web 01/24/2003
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2. Background The presence of impurities in a feed stream can often affect the permeation behavior of membranes. After simple dehydration and removal of particulates and entrained liquids, typical natural gas processing streams can contain well over 500 trace components of paraffinic and aromatic hydrocarbons. It is suspected that the presence of even trace quantities of condensable, especially aromatic, hydrocarbons (C6 and higher) in a process stream can seriously alter the performance of polymeric membranes. There is experimental work suggesting a “competition of mixed penetrants for sorption sites and transport pathways associated with unrelaxed volume in glassy polymers” that tends to depress permeability and selectivity.14 One study examining glassy polymeric (polyimide) membrane films reports harsh performance declines in the range of 50% reduction in CO2/CH4 selectivity due to saturated concentrations of toluene or n-hexane in mixed-gas feeds of CO2/CH4.15 It appears that the rigidity and highly ordered structure that gives high-performance glassy polymers their superior permeation properties might also make them more susceptible to the harmful effects of condensable agents. Consequently, condensable hydrocarbons have been suspected of causing membrane performance declines in the field as well.16-19 As with glassy polymeric membranes, sorption of these condensable agents or impurities in CMS membranes is an important concern and determines how and to what extent their permeation properties are affected. For carbon materials, surface adsorption is often divided into two general cases: physical adsorption and chemical sorption. Physical adsorption involves intermolecular interactions (e.g., van der Waals, dipole-dipole) between the penetrant and surface and is analogous to simple condensation, whereas chemical adsorption involves chemical bonding (i.e., transfer of electrons) between the penetrant and surface.20 Physical adsorption is a reversible process, whereas chemical adsorption might or might not be reversible but would require very high temperatures to break chemical bonds to desorb penetrants. For example, carbon materials are susceptible to chemical adsorption of water or oxygen.21-25 For gas separation applications (e.g., pressure-swing adsorption), it is well-documented that carbon materials experience severe transport losses under humid and/or oxygen environments.26,27 Often, the drastic loss in transport rates is accompanied by no change or a slight increase in selectivity. Water vapor in air has often been suspected as the primary culprit in these performance declines. Water molecules can “chemisorb” onto the carbon surface and accumulate at the initial sorption sites through hydrogen bonding (physical sorption), resulting in bulk pore filling. In addition to water vapor, however, oxygen can also chemisorb to reactive groups on the carbon surface to form surface oxides and make the carbon surface more hydrophilic and more susceptible to hydrogen bonding with water molecules.21 In fact, oxygen chemisorption might pose an even greater problem than the physical and chemical sorption of water vapor. Work with our CMS hollow fibers also encountered this same phenomenon. When stored in low-humidity air, our CMS fibers suffered permeance declines of approximately 50% over a 2-month period with negligible change in selectivity. Other researchers have observed similar results, with decreases in gas per-
meance as great as 50-90% when exposed to air for several days to several months (even in very lowhumidity storage environments).26-29 Recently, Menendez and Fuertes26 examined the effect of several storage environments on permeation properties. Over a period of over 100 days, they showed that storage in dry air did not prevent the permeance declines and, in fact, gave results similar to those obtained with CMS membranes stored in laboratory air. They hypothesized that a large fraction of water sorbed by the membranes when stored in laboratory air is physically sorbed and can be removed by evacuation of the CMS membrane. They further concluded that the presence of oxygen was the controlling factor over the long term. The work of Jones and Koros27,30 examined permeance declines in the presence of humid gas feeds and showed significant permeance losses that might possibly be due to competitive sorption effects and pore blockage by the water molecules. To counter permeance losses from water and oxygen chemisorption, typical regeneration efforts have attempted heat treatments of CMS membranes. Moderate heating and evacuation can successfully remove the fraction of water molecules physisorbed to the carbon surface, but removal of chemisorbed water and oxygen requires significantly higher heat treatments. Even so, temperature treatments as high as 600 °C yielded only partial restoration of permeance performance, still giving less than 5% of the original performance.26 In particular, removal of chemisorbed oxygen from the carbon surface is reported to require very high temperatures of around 700-800 °C,26 but these high temperatures would also possibly change the morphology and porosity of the CMS material and its transport properties.31 Additionally, such high-temperature treatments are industrially unattractive. Other promising alternatives for pretreating the CMS materials have also been examined. For example, Jones and Koros32 showed that coating the membrane surface with a highly hydrophobic polymer barrier (i.e., Teflon) is successful in lowering the water activity on the surface of the carbon membrane and mitigating the effects of permeance losses with only small losses in selectivity and permeance from the resistance contribution of the added polymeric layer. Rao and Anand33 proposed a passivation procedure for carbon materials used for adsorption processes in which a passivating gas, such as carbon dioxide or even moist air, is used to selectively oxidize the carbon surface at high temperatures (200-400 °C). They did not offer an explanation of this mechanism or its fundamental reasons for passivating the surface but showed improved effectiveness for hydrocarbon separations in humid environments for passivated carbon materials in comparison to unpassivated carbon materials. Nevertheless, as evidenced by the work of Jones and Koros,27 the presence of water vapor in gas feeds certainly can cause immediate permeance losses in CMS membranes. Certainly, water vapor poses significant problems with gas feeds containing water vapor, as well as longterm permeance declines due to high oxygen exposure. The low oxygen content (
