Use Thermodynamics to Optimize Energy Consumption and Gas Separation in LNG Processes

Natural gas is an important clean energy source to sustain human civilization in this century.  Normally, natural gas contains valuable heavy hydrocarbons such as ethane, propane, butane, and pentanes.  These hydrocarbons can be recovered as natural gas liquids (NGL) through cryogenic separations.  The heavy components may have higher marketing values than the majority of natural gas, which is methane and mainly used as a fuel gas.  Due to large quantities of natural gas processing in normal cases, NGL recovery can be very profitable for oil gas industries; meanwhile, those recovered heavy components from natural gas are important feed stocks that will significantly shape the downstream petrochemical/chemical industrials.

Cryogenic separation process is an effective way for NGL recovery, which generally employs a refrigeration system to partially liquefy the natural gas before a fractionation process.  Conceivably, a cryogenic separation process consumes lots of cold energy for partial liquefaction as well as some hot utilities for stream fractionation.  Hence, energy consumption minimization is a major concern for NGL recovery.

In this report, thermodynamic methods were used to evaluate the efficiencies of natural gas liquefication and cryogenic separation processes.  Moreover, an integrated method was proposed to simultaneously increase energy and separation efficiencies.  Rigorous simulations were made for component cases of the method.  Exergy analysis was adopted to evaluate energy efficiency.