The removal of water from gas streams is important in many industrial processes in which the presence of water has an adverse effect on downstream process steps. Common examples include the dehydration of compressed air used for instrumentation systems and the pretreatment of compressed air feed to cryogenic air separation systems to remove water in combination with other contaminants. Pressure swing and temperature swing adsorption processes are well-known and widely-used for gas dehydration. Membrane-based processes which utilize water-selective permeable membranes are also known in the art and are used for gas dehydration, either alone or in combination with adsorption processes.
Most membranes used for gas dehydration are hydrophilic polymeric membranes which preferentially permeate water. Representative art describing the use of polymeric dehydration membranes includes U.S. Pat. Nos. 4,497,640, 4,783,201, 4,885,086, 4,981,498, and 5,067,971. Alternatively, inorganic ceramic or silica/alumina membranes can be used for gas dehydration as described for example in U.S. Pat. Nos. 4,692,354 and 5,240,472, and in an article entitled "Characteristics of a Zirconia Composite Membrane Fabricated by a Laser Firing Method" by H. Ohya et al in J. Membr. Sci. (1996), 110(2), 2499-52.
Porous carbonaceous adsorbent membranes can be used to separate gas mixtures by selective permeation. Separation occurs in these membranes by several mechanisms depending upon the sizes of the membrane pores relative to the sizes of the molecules in the gas mixtures. When the membrane pore diameters are generally larger than the diameters of some molecules but smaller than the diameters of other molecules in a gas mixture, separation is effected by size exclusion or molecular sieving in which smaller molecules permeate while larger molecules do not permeate. Such membranes are described in U.S. Pat. No. 4,685,940 and in representative articles entitled "Mechanism of Permeation through Molecular-sieve Carbon Membrane" by J. E. Koresh et al in J. Chem. Soc., Faraday Trans. 1, 1986, 82, 2057-2063, "The Carbon Molecular Sieve Membranes: General Properties and the Permeability of CH.sub.4 /H.sub.2 Mixtures" by J. E. Koresh et al in Separation Science and Technology, 22(2&3), 973-982 (1987), "Asymmetric Molecular Sieve Carbon Membranes" by W. Shusen et al in J. Membrane Science 109 (1966) 267-270, and "Effects of Polyimide Pyrolysis Conditions on Carbon Molecular Sieve Properties" by V. C. Geiszler et al in Ind. Eng. Chem. Res. 1996, 35, 2999-3003.
When the pore diameters of a carbonaceous adsorbent membrane are generally larger than the diameters of all molecular species in a gas mixture but less than about 3 to 4 times the diameter the largest molecules in the gas mixture, surface diffusion or selective surface flow occurs in which more strongly adsorbed molecules selectively permeate the membrane by a predominantly surface diffusion mechanism and less strongly adsorbed molecules are selectively rejected in the nonpermeate gas. The preparation and use of such membranes are described in U.S. Pat. Nos. 5,104,425, 5,431,864, and 5,507,860, and in an article entitled "Nanoporous Carbon Membranes for Separation of Gas Mixtures by Selective Surface Flow" by M. B. Rao et al in J. Membrane Science, 85 (1993) 253-264. Carbon membranes which exhibit molecular sieving or surface diffusion are described in articles entitled "Carbon Molecular Sieve Membrane Prepared from Phenolic Resin" by H. Kita et al in Chemistry Letters 1997, pp. 179-180 and "Gas Permeation through Micropores of Carbon Molecular Sieve Membranes Derived from Kapton Polyimide" by H. Suda et al in J. Phys. Chem B 1997, 101, 3988-3994.
When the pore diameters of a carbonaceous adsorbent membrane are significantly larger than the diameter of the largest molecules in a gas mixture, wherein the pore diameters typically are greater than about 50-100 Angstroms, permeation occurs predominantly by Knudsen diffusion and separation is effected by differences in the mean free paths of the molecules in the mixture. Components with lower molecular weights permeate preferentially over components with higher molecular weights. Carbonaceous adsorbent membranes of this type are described in U.S. Pat. Nos. 4,919,860 and 5,089,135.
Carbonaceous adsorbent membranes are desirable because the more strongly adsorbed components in a gas mixture permeate the membrane with high selectivity over the less strongly adsorbed components. In addition, these membranes can operate with relatively low feed gas pressures and exhibit high gas flux compared to polymeric membranes. Carbonaceous adsorbent membranes have generally hydrophobic surface characteristics when initially prepared by the usual pyrolysis method, and do not have high permeation selectivity for water. For this reason, carbonaceous adsorbent membranes have not been used for gas dehydration. Because of the advantages described above, carbonaceous adsorbent membranes which preferentially permeate water would be desirable in many gas dehydration applications. The invention disclosed below and defined in the claims which follow pertains to a carbonaceous adsorbent membrane which preferentially removes water from water-containing gas mixtures.