In addition, increasingly stringent regulation of CO2 emissions has rendered release of CO2 to the atmosphere more difficult. There is growing pressure for the captured CO2 to be re-compressed before it may be re-injected and re-used on site, or transported for further use elsewhere. This step is energy intensive, and could potentially change the economic outlook of the separation process, and consequently the entire natural gas production operation.
A process for the separation of gaseous CO2/CH4 has been developed, capable of economically treating natural gas feeds at the high pressures and CO2 concentrations mentioned. The process has been modelled for a system comprising methane, CO2 and alkane solvent. The separation process model includes absorption and desorption units, as well as heat exchangers and a CO2 re-compression stage. An advanced equation of state, SAFT-VR, has been employed in this study. This facilitates accurate representation of the complex thermodynamic and fluid-phase equilibrium behaviour of high pressure/ temperature CO2-hydrocarbon mixtures. The work has been carried out using the modelling-optimisation software gPROMS. The process is optimised by considering an economic objective over a 15-year lifetime. In addition to finding the best equipment sizes and operating conditions, the alkane chain length of the physical solvent is also treated as an optimisation variable, to achieve the most cost effective absorption for differing CO2 content.
Several case studies are examined. In particular, the impact of including further impurities in the feed is assessed. The effect of recompression costs on the optimal design is also considered. Given the large variability in feedstock over the 15-year lifetime of the plant, process flexibility is quantified.