(407a) Influence of Propane on CO2/CH4 and N2/CH4 Separations in CHA Zeolite Membranes
Two types of zeolite membranes with the CHA structure (SAPO-34, SSZ-13) were used for CO2/CH4 and N2/CH4 separations. These zeolite structures have pore diameters of 0.38 nm, and thus CO2 (kinetic diameter = 0.34 nm) diffuses faster through the CHA pores than CH4 (0.38 nm) or N2 (0.36 nm). These membranes can be prepared with low defect concentrations on porous alumina tubes, as shown by CO2/i-butane separation selectivities greater than 500,000 for SAPO-34 membranes; i-butane, with a kinetic diameter of 0.50 nm, is too large to enter the CHA pores. Both membranes selectively permeated CO2 in CO2/CH4 mixtures for feed pressures up to 4.5 MPa. For applications in natural gas purification, these membranes must operate not only at high pressures, but also in the presence of low concentrations of higher alkanes. Thus, the influence of propane on CO2/CH4 separations was studied for a range of feed pressures and propane concentrations. Although the kinetic diameter of propane (0.43 nm) is larger than the CHA pores, propane adsorbs in the CHA pores, and thus propane significantly influenced separations by decreasing permeances. Although propane caused permeances to decrease significantly over time, selectivities decreased much less because both propane decreased permeances of CO2, CH4, and N2. Also, propane decreased permeances more for SAPO-34 than SSZ-13 membranes, and in contrast to the behavior at low pressures, propane initially increased CO2 permeances and decreased CH4 permeances in SSZ-13 membranes at high feed pressures. Thus, propane significantly increased CO2/CH4 selectivities at high pressure initially. For both types of membranes, the permeances did not reach steady state in the presence of propane, even after seven days. These studies demonstrate that propane concentrations need to be low in feeds to CHA-type membranes in order to remove CO2 or N2 from natural gas, or the membranes must be periodically regenerated. The amounts of CO2 and propane adsorbed in CHA crystals from CO2/CH4 mixtures was measured using temperature-programmed desorption with CO2 and propane in the gas phase during desorption. These TPD measurements show that at shorter times, CO2 preferentially adsorbs, whereas over many hours the amount of CO2 adsorbed decreases and the amount of propane adsorbed increases until propane is the dominant species adsorbed. The slow approach to steady state in the membrane separations is thus due to the slow diffusion of propane into the CHA pores.