Maxwell–Stefan modeling of high flux tubular silicalite-1 membranes for CO2 removal from CO2/H2 gas mixtures

Abstract

In this work, a Maxwell–Stefan model for high flux tubular silicalite-1 membranes for separation of CO2 from a CO2/H2 mixture was developed. The model concerns tubular membranes operating in a counter flow module and includes transport through flow-through defects in the silicalite-1 film and pressure drop over the graded alumina support. Adsorption and diffusion parameters for perfect silicalite-1 crystals were taken from the literature. The flux and selectivity predicted by the model were in reasonably good agreement with experimentally observed data for a ZSM-5 membrane without any fitting of the model. However, the CO2 flux and selectivity measured experimentally for the ZSM-5 membrane were higher than that predicted by the model for a silicalite-1 membrane.The model was used to investigate a case with a 20,000Nm3/d feed comprised of a 50/50 mixture of CO2/H2 at pressure of 25bar and a membrane temperature of 296K. The permeate pressure was 1bar and 90% of the CO2 permeated the membrane. In this case, the membrane permselectivity and CO2 flux varied along the length of the tubes between 20–26 and 950–396kg/(m2h), respectively. Further, both defects and pressure drop over the support were shown to have an adverse effect on the selectivity, which indicates that membrane selectivity can be improved by reducing the flow-through defects and/or by preparing supports with less flow resistance. For a one-stage process, the required membrane area is as small as ca. 0.85m2 and the hydrogen loss through the membrane was 12.4%. For a two-stage process the required membrane area almost doubled to 1.6m2, however the hydrogen loss through the second membrane is reduced to as little as 2.5%. In summary, this work shows that high flux zeolite membranes may be an interesting option for CO2 removal from synthesis gas.