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Simulation Modelling of the Otway Co2/Ch4 Adsorption Separation Process and Optimization

Natural gas is a critical transition fossil fuel for the clean energy transition. According to the statistics, however, roughly half of the global proven and probable natural hydrocarbon gas reservoirs have a CO2 content greater than 2%, and most of them are in the range 15% to 80%. To transport natural gas through a pipeline, the concentration of CO2 has to be reduced to about 2-4% in different countries. In the case of liquefied natural gas (LNG) production, less than 100 ppm CO2 content is required to prevent the formation of dry ice when compressing. Thus the presence of sour components makes these reservoirs less desirable because of business considerations and technical difficulties of removing CO2. As the demand for natural gas is soaring globally, novel and economic processes of removing CO2 from natural gas are increasingly needed. Adsorptive CO2 separation is a promising process for CO2 removal from gases in a variety of applications. Pressure swing adsorption (PSA) is considered to be suitable for CO2 removal from raw natural gas with high CO2 partial pressure as it uses the high pressure inherent in the feed stream. It has a potential to cut the energy requirement and operational costs because of its inherent low energy consumption. A two-bed PSA demonstration pilot plant has been designed, constructed, and operated in the Australian CO2CRC limited Otway site. This capture plant uses PSA or membrane technology to capture CO2 from CO2/CH4 mixtures over a range of CO2 partial pressures and feed pressures. Preliminary positive results have been obtained from the plant and showed that with appropriately selected adsorbents, CO2 can be effectively separated from CO2/CH4 gas mixtures under high pressure. To improve our understanding of the process and gain further insight, we have developed a simulation model of the two-bed PSA process using Matlab, Aspen Adsorption and MINSA. The parameters of the simulations were tuned by fitting the results to the Otway demonstration plant data. Then with the validated models, further optimization of the adsorption process was conducted. An optimization run showed that when the desorption pressure of the process decreased from 5 bar, which was the actual pressure used in Otway process, to 1 bar, the purity of the CH4 product would increase from 64% to 96%, while the purity of the CO2 product improved from 62% to 79%. This implies that by changing the desorption pressure of Otway PSA process a much better separation performance could be expected. A multi-stage PSA process had been designed based on the simulation model of the Otway plant. With two PSA units rather than one PSA unit, as well as