Acid gas removal using polymeric membranes: Economical study

  • Sama Jamal Taha AlHallaq

Student thesis: Master's Thesis

Abstract

With the continuous increase in demand of natural gas as a fuel, countries worldwide have directed their production towards the low quality natural gas reserves. The development of these low quality gas reserves requires a series of more complex processes to meet sales gas specifications. Therefore, development of new economical technologies to cope with the increased level of impurities for highly sour gas plants is desirable. A major step of natural gas conditioning and processing is the removal of acid gases (carbon dioxide and hydrogen sulfide). There are various technologies for acid gas removal. The most commonly used method is the chemical absorption of acid gases process using aqueous amine solutions combined with using stream to strip acid gas and regenerate the amine solution. This absorption-regeneration cycle is relatively complex and very energy intensive. This thesis focuses on performing an economical study for the use of membrane separation technology for acid gas removal from natural gas. Therefore, the goals of the study are: (1) study the effect of process parameters on the cost of the membranes process, (2) make a cost comparison between the optimum membrane process and the conventional amine process, (3) study the economics of hybrid membranes-amine process, and (4) select the optimum process configuration based on gas flow rate and acid gas content. Two gas feed cases are studied in this thesis. Both with a flow rate of 250 MMSCFD and acid gas content of 23 mole% H2S and 10 mole% CO2 for cases I and 34 mole% H2S and 10 mole% CO 2 for case II. For the membrane process, three different configurations were initially analyzed and the optimum configuration, two stage cascade configuration, was selected. Each membrane stage was modeled as a hollow fiber membrane module with cross counter-current flow. The selected membrane material is a commercial H2S selective Poly(ether urethane urea). Moreover, the membrane process was simulated using the ProMax membrane add-on and the simulation results were validated using a developed Matlab code that solves the mathematical model of membranes. The feed gas was simulated as a ternary components mixture: methane, CO2 and H 2S. Nevertheless, the effect of other components on the process was also studied. The amine process was also simulated using ProMax. Various scenarios were simulated and studied to find the optimum process configuration for each gas case. The three considered processes are a standalone membranes process, a standalone amine process, and a hybrid membrane-amine process. The cost of the standalone membrane process is more than 45% lower than the conventional amine process; however, it cannot achieve the required sale gas specifications. The hybrid membrane-amine process, however, is not only capable of achieving the sale gas specification but also has a lower cost than standalone conventional amine process. The percentage reduction in cost of hybrid process compared to conventional amine for the two studied cases is 44% and 59%, respectively. Total processing cost of the hybrid membrane-amine process is estimated to be $0.95/MSCF of product for case I and $1.1/MSCF of product for case II. The effects of various process parameters on the final optimum process configuration are analyzed. Because the permeability of different gas components in the membrane material is expected to highly affect the performance and the economics of the membrane process, two additional other membrane materials with lower permeability and selectivity are also studied. Cellulose acetate resulted in higher but acceptable processing cost, while polyimide resulted in much higher and unacceptable processing cost. Additionally, presence of other components in the feed showed no effect on economics, although it may affect the stability of the membrane material. The specification in the membrane unit of the hybrid system was set on the H2S only. If specifications on both H2S and CO 2 are needed, three stages membrane unit is recommended. The first two stages are in series where the first stage material is H2S selective and the second material is CO2 selective, and the permeate of both is combined, compressed and fed to the third stage with H2S selective material.
Date of Award2014
Original languageAmerican English
SupervisorVikas Mittal (Supervisor)

Keywords

  • Applied sciences
  • Chemical engineering
  • 0542:Chemical engineering

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