ADSORPTION AND DIFFUSION OF LIGHT

Many of the untapped deposits of natural gas are in remote locations and contain substantial concentrations of CO2 and H2S as impurities. Current upgrading technology is energy and capital intensive and requires a large footprint, making it unsuitable for off shore well heads or installation at remote locations. One potentially attractive solution is the use of zeolite materials as “molecular sieves” to separate out CO2 (also found in bio-gas) by a size selective adsorption process.
Diffusivities of methane were measured using the ZLC method in the presence and absence of CO2 in order to quantify the ability of DDR zeolite to separate methane from CO2. Counter to recent computer based predictions, it was found that the diffusivity of methane is increased (rather than decreased) by the presence of CO2. These studies were then extended to larger hydrocarbons (ethane and ethylene). The results show that the presence of CO2 does not affect either the kinetic or equilibrium behavior of these molecules in DDR. There are many possible barriers to commercialization of a molecular sieve separation process but the results of the present study suggest that the interference between CO2 and CH4 is unlikely to be a serious problem.
Some of the samples showed evidence of significant surface resistance to mass transfer. This led to a further development of the mathematical model used to analyze the ZLC response curves making it possible to extract both the internal diffusion coefficient and the surface resistance from measurements made over a range of flow rates.