Combination of membrane separation and gas condensation for advanced natural gas conditioning

Abstract

Membrane separation and gas condensation are combined to reveal an advanced method for the separation of alkanes. First, the applicability of MFI membranes for alkane separation is principally demonstrated by means of realistic adsorption isotherms computed by configurational biased Monte Carlo (CBM) simulations. Next, dew point curves of mixtures comprising different ratios of n-butane (C4) and methane (C1) were calculated according to the thermodynamic methods of Soave–Redlich–Kwong (SRK) and Peng Robinson (PR). From that, isothermal phase boundaries in dependence on the composition of the gas mixture were derived and process parameters under which condensation of the alkane mixture occurs were predetermined. Experimentally, the separation performance of MFI membranes was recorded during separation of n-butane from methane. It was found that liquefied n-butane in the feed and a further liquefaction in the permeate enhance the separation selectivity of MFI zeolite membranes under sweeping conditions tremendously. At the dew point of the feed mixture a sudden rise of the separation factor α is observed. At a temperature of 258K a mixture with χC4=0.5 can be separated with a separation factor αC4/C1=174 due to liquefaction. Experiments without sweeping show a similar behaviour. When forming a two phase mixture in the feed an increase in overall condensation efficiency ηC4 is detected in the permeate. At 258K and pfeed=2bar and ppermeate=1bar 29.6% liquefied n-butane was isolated in the permeate from a mixture comprising χC4=0.5.