High pressure co2 separation using membranes membrane selection and process modeling

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Date
2015-05-01
Authors
Jimoh Kayode Adewole
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Separation of CO2 from natural gas (NG) has attracted research interest due to increasing demand for energy and the need for more energy efficient and environmental friendly gas purification techniques. Most of the NG is coproduced with CO2 which need to be removed in order to increase its calorific value. Membrane separation is one of the widely used technologies for CO2 removal. However, its market share is still very small as compared to other gas separation processes. This is due to the use of membrane materials with poor separation performance and the use of non-optimum module operating conditions. Systematic optimization of every stage of membrane preparation and high pressure module operation was proposed to solve this problem. One major challenge of high pressure operation is penetrant-induced plasticization phenomenon which is caused by increasing the feed pressure. Commercial polysulfone polymer was modified to optimize its separation performance. High pressure experimental studies and mathematical modeling were performed to evaluate the separation performance of the membrane. To establish the highest possible feed pressure which can be attained during CO2 removal without plasticization, transport properties of the membrane were evaluated using permeation tests at pressure up to 57 bar. Also, dynamic evaluation of membrane performance was performed using timedependent permeation experiments over a period ranging from 5 hours to 1080 hours (45 days) at various pressures between 6 and 57 bar. Mathematical model was developed based on the theory of dual-sorption and the total immobilization models. The optimization for membrane selection was achieved using a multi-objective optimization method while that of module operating conditions was achieved using non-linear programming constraint optimization model and a Golden search algorithm which was implemented using MATLAB. The plasticization pressure of the prepared membrane is 41.07 bar while the permeability and selectivity at this pressure are 5.99 Barrer, and 44.19 respectively. This is equivalent to a 17.65% and 66.39% increase in plasticization pressure and permeability, respectively. However, the membrane lost about 79.65% of its permeability at this pressure while its selectivity increased by 91.13% as compared to the value at 5 bar. The timedependent permeability tests revealed plasticization pressure as possible equilibrium point which can be used as constraint during membrane gas separation process optimization. The mathematical model developed showed an excellent predictive capability for plasticization pressure. It was also shown that membrane materials selection can be efficiently optimized using the multi – objective optimization approach.
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