Experimental and simulation studies of hybrid membrane-absorption systems

Membrane process has been widely applied in chemical and petroleum industries for removing acid gases such as CO2. The implementation of hybrid system for natural gas purification has been studied by several researchers in recent years. Systems based on combining membrane with absorption technologies are the ones that have attracted more interest as industrial applications. The system will provide a critical data for a variety of operational parameters and test industrial scenarios for the carbon capture. Aspen Custom Modeller (ACM) is applied as the modelling tool for building and integrating it into a process simulator as Aspen Plus. It was validated by existing experimental data generated in the laboratory. The membrane model also predicted similar results regarding the stage-cut against CH4 concentration in retentate to the published data with a maximum error of 5.8%. An experimental apparatus based on a powerful in situ perturbation method known as Flux Response Technology (FRT) was designed and modified for making flow measurements of Polydimethylsiloxane (PDMS) micro-scale membrane. A perturbation flow of one component of the gas mixture is added to a main flow of known composition flowing through the capillary at ~20ml/min and 1 bar gauge. Two different sets of data were collected of CO2 into N2 and N2 into CO2 at varying the feed pressure on the membrane side from 2 to 0.1 bar. Composition changes measured by FRT are in good agreement to the Mass Spectrometry results with a maximum error of 4%. This experimental apparatus was also used to validate the ACM membrane model and was found be in a good agreement was observed with 5% error at 2 bar gauge. The membrane-absorption hybrid system for natural gas stream containing 30 mole% CO2 is investigated and economically analysed. Membrane separation is to be applied for the bulk removal of CO2, while final purification to U.S. pipeline specifications (< 2 mole% CO2) is to be achieved by gas absorption. It was found that the separation cost for the hybrid process ($/MMSCF feed) lies between that of membrane and absorption process. The amine absorption process with MDEA is almost 50% higher than that of the hybrid system, while membrane gives the lowest separation costs. For this study, the hybrid system appeared to the be the optimum design as it gave low energy cost, low carbon emissions and high gas purity.