Inherently Safer Design for low-temperature process during depressurization: An industrial case study

Abstract

The exploitation of a low-quality gas field with high CO2 concentration is more viable through liquid CO2 produced from cryogenic distillation technology. Despite the bright potential of the technology, there are deficiencies in handling high concentration of CO2 at low temperature and high pressure during the blowdown condition. This study focuses on the CO2 blowdown at a cryogenic pilot plant designed to manage high concentrations of CO2 in the feed gas, high pressures, and low temperatures. A comprehensive design review and risk assessment using Inherent Safer Design (ISD) indexes were carried out in this study. The ISD was performed to identify the current risk level, and the critical parameters that may cause solid CO2 formation in the piping or equipment as well as to identify mitigation measures to avoid the temperature to drop below the CO2 freezing point during blowdown. The present findings confirmed that the initial pressure and temperature, as well as CO2 concentration are key parameters towards significant impact on blowdown conditions. Therefore, the reduction of the feed gas pressure from 80 bar to 70 bars has minimized the Joule Thomson (JT) effect during blowdown and avoided the CO2 solid formation in the system. Moreover, the relocation of the blowdown valve at the downstream heater resulted in a higher final temperature above the CO2 freezing point. The ISD indexes confirmed that the cryogenic facilities are inherently safer during blowdown with the mitigation measures adopted.