CO2 Freezing in the Low-Temperature Separation of Carbon Dioxide from Natural Gas- By Distillation

Natural gas is a cleaner alternative to coal and oil. This increases the demand for natural gas and creates an incentive to exploit CO₂-rich gas reserves. Extracted gas is increasingly transported as liquefied natural gas (LNG) over long distances, which is more cost-effective than pipelines. This study explores solutions to remove CO₂ using low-temperature distillation to meet the 50 ppm requirement for liquefaction of CO₂-rich natural gas reserves. Distillation can produce high-purity natural gas and liquid CO₂ suitable for sequestration but low temperatures can cause CO₂ to freeze, creating operational difficulties for the distillation.

This study investigates the freezing behaviour of CO₂ in distillation operations considering a mixture of methane and impurities to understand the range of conditions and compositions where CO₂ freezing occurs. Phase equilibrium predictions using Aspen HYSYS are verified against the published experimental data available and a published thermodynamic model.

Simulations of the process consisting of a sequence of two distillation columns considering binary, ternary and multicomponent mixtures have been carried out in Aspen HYSYS, using the CO₂ freeze-out tool to predict freezing. This study also explores approaches to avoid CO₂ freezing in both columns including varying pressure and intermediate CO₂ mol fraction to alter freezing temperatures.

This study finds that the condenser of the first column and the stripping section of the second column are most vulnerable to freezing. The presence of nitrogen exacerbates freezing in the second column, while the presence of heavier hydrocarbons lowers the CO₂ freezing temperature. When considering a natural gas feed system, the simulations found that CO₂ freezing in the first column can be avoided by specifying the process at high pressures and high intermediate CO₂ mole fraction in the distillate.

Future work aims to adopt optimal design and operation of the distillation process with process intensification for offshore applications.