Gas separation of steam and hydrogen mixtures using an ?-alumina-Alumina supported NaA membrane
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In this study, the feasibility of a NaA zeolite membrane for the gas phase separation of steam and hydrogen mixtures was determined. The Fischer-Tropsch (FT) process, which produces high value fuels and chemicals from coal and natural gas, can be greatly improved upon by the selective removal of water from the FT reactor product stream. According to the FT reaction kinetics, the rate of reaction increases with the partial pressure of hydrogen but is adversely affected the presence of water in the reactor product stream. Chemisorbed water on the surface of the metal catalysts also enhances deactivation due to sintering and fouling. The use of a zeolite membrane reactor is well equipped to serve the purpose of in-situ water removal as it can facilitate the separation of chemical components from one another in the presence of catalytic reactions. The LTA type zeolite membrane NaA or zeolite 4A, in particular, is well suited for the separation of polar (H2O) from non-polar (H2) molecules because of its high hydrophilicity. NaA has also been identified as an excellent candidate for selective water removal applications due its high adsorption affinity and capacity for water. The NaA membrane used in this study was manufactured by means of the in-situ crystallisation method where the growth of crystals on the inside surface of a centrifugally casted a-alumina support was favoured. Scanning electron microscopy (SEM) analyses performed on the membrane after a double hydrothermal synthesis indicated that the surface topology was rough and that the zeolite crystals formed were not uniform in size. Overall, the membrane thickness varied between 6.5 and 8.0 μm. An evaluation of the membrane quality was made possible through permeation experiments involving SF6 and Hz. The calculated Hz/SF6 permselectivity in this study was found to be 9.78, which despite being higher than the Knudsen diffusion selectivity of 8.54, confirmed the presence of intercrystalline defects or non-zeolitic pores in the membrane. Experiments concerning pure component and binary mixture permeation of steam and hydrogen through the supported NaA membrane were conducted over a temperature range of 115°C to 160 °c for binary hydrogen/steam mixtures, 25°C to 160°C for pure hydrogen and 130°C to 170°C for pure steam. For the permeation of pure component hydrogen, a local maximum in its permeance having a value of 224 x 10-08 mol.m,z.s-1.Pa-1was reached at a system pressure and temperature of 6.875 bar and 75°C respectively. For the permeation of pure component steam through NaA, the effects of capillary condensation in the pores and defects of the zeolite membrane resulted in a decrease in steam permeance as a function of absolute pressure for temperatures lower than 160 °c. Once the effects of capillary condensation had receded, maxima in the steam permeances as a function of temperature corresponding to values of 70 x 10,08, 65 X 10,08 and 75 x 10,08 mol.m-2.Pa-1 were found for the 182.5, 197.5 and 222.5 kPa isobars respectively. These observations collaborated well with the description of surface diffusion with permeation taking place in the Langmuir (strong adsorption) regime. Permeation experiments through NaA as function of temperature were conducted for a 90 mol% steam -10 mol% hydrogen (90-10) binary mixture as well as for a 60-40 mixture of these two. At low temperatures the permeation of hydrogen was completely suppressed by the condensed steam resulting in an almost perfect separation. The Kelvin equation was used to estimate the pore size of the defects which was found to range between 1.86 and 2.45 nm. The temperature range over which these defects in the membrane were assumed to become unblocked (i.e. assuming when the first breakthrough of hydrogen occurred), were determined to be between 140 to 148 °c and between 128 to 130 °c for the 90-10 and 60-40 mixtures respectively. The mixture selectivities (towards water) between 115 °c and 130 °c were found to be immensely high (much greater than 1000) for both the 90-10 and 60-40 mixtures, while the ideal selectivities were calculated to be less than lover the same temperature range. At 140 °c, the selectivity towards water for the 9010 mixture was still greater than 1000; however for the 60-40 mixture at this temperature, an inversion of selectivity towards H2 had already taken place. The breakthrough in H2 permeance occurs at a much lower temperature when the feed mixture contains a lower concentration of water. Since the partial pressure of steam will be reduced, larger pores will become unblocked at lower temperatures according to the Kelvin equation.
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