The Decompression Behaviour of Carbon Dioxide in the Dense Phase

Abstract

Pipelines can be expected to play a significant role in the transportation infrastructure required for the successful implementation of carbon capture and storage (CCS). National Grid is undertaking a research and development programme to support the development of a safety justification for the transportation of carbon dioxide (CO2) by pipeline in the United Kingdom. The ‘typical’ CO2 pipeline is designed to operate at high pressure in the ‘dense’ phase. Shock tube tests were conducted in the early 1980s to investigate the decompression behaviour of pure CO2, but, until recently, there have been no tests with CO2-rich mixtures. National Grid have undertaken a programme of shock tube tests on CO2 and CO2-rich mixtures in order to understand the decompression behaviour in the gaseous phase and the liquid (or dense) phase. An understanding of the decompression behaviour is required in order to predict the toughness required to arrest a running ductile fracture. The test programme consisted of three (3) commissioning tests, three (3) test with natural gas, fourteen (14) tests with CO2 and CO2-rich mixtures in the gaseous phase, and fourteen (14) tests with CO2 and CO2-rich mixtures in the liquid (or dense) phase. The shock tube tests in the liquid (dense) phase are the subject under consideration here. Firstly, the design of the shock tube test rig is summarised. Then the test programme is described. Finally, the results of the dense phase tests are presented, and the observed decompression behaviour is compared with that predicted using a simple (isentropic) decompression model. Reference is also made to the more complicated (non-isentropic) decompression models. The differences between decompression through the gaseous and liquid phases are highlighted. It is shown that there is reasonable agreement between the observed and predicted decompression curves. The decompression behaviour of CO2 and CO2-rich mixtures in the liquid (dense) phase is very different to that of lean or rich natural gas, or CO2 in the gaseous phase. The plateau in the decompression curve is long. The following trends (which are the opposite of those observed in the gaseous phase) can be identified in experiment and theory: • Increasing the initial temperature will increase the arrest toughness. • Decreasing the initial pressure will increase the arrest toughness. • The addition of other components such as hydrogen, oxygen, nitrogen or methane will increase the arrest toughness.