The increasing importance of natural gas as a source of energy poses difficult gas separation design challenges, as the streams recovered from gas fields are at high pressures (typically about 10 MPa) and can contain a high proportion of CO2 (up to 70%). In addition, as the implementation of the Kyoto protocol would require the capture of large quantities of CO2, its injection in depleted or near-depleted reservoirs for enhanced oil/gas recovery operations will become increasingly frequent. This is likely to result in natural gas streams which are even richer in CO2. Conventional separation techniques are usually restricted to low CO2 content or low-pressure feeds, and consequently there is a pressing need for an alternative process that is appropriate for such a scenario.

A new CO2/CH4 gas separation process is presented. Capture of CO2 from natural gas is carried out using an n-alkane solvent in a conventional physical absorption process. A methodology to find optimal process condition and solvent has been developed using an advanced equation of state (SAFT-VR), process modelling-optimisation software (gPROMS), and detailed process cost estimation. Natural gas feeds containing up to 70% mol/mol of CO2 can be treated economically using this process.