Study of CO₂ Solid Formation During Blowdown of Cryogenic CO₂–CH₄ Distillation Process

Abstract

Solidification of CO₂ is one of the critical issues during blowdown in the cryogenic natural gas distillation process. Cryogenic distillation process for separation of CO₂ from natural gas usually works at low temperature and high pressure that produces higher purity natural gas and CO₂ in the vapor-liquid phase. However, in case there is an emergency, the mixture inside distillation column is passed through blowdown valve for sudden depressurization. This rapid depressurization would cause sudden expansion of CO₂ mixture resulting in a dramatic drop in the fluid temperature that could solidify CO₂ and cause pipe blockage. This solidification phenomenon is not fully understood that makes the solidification of CO₂ during blowdown process difficult to control. Therefore, the research study presents investigations of the thermodynamic study for the depressurization of highly pressurized vessel containing CO₂–CH₄ binary mixture from cryogenic conditions. Simulation is performed for 20% concentrations of CO₂–CH₄ mixture over different orifice size e.g. 1.00 and 1.25 mm using BLOWDOWN software package in Aspen HYSYS^® V 9.0.