High-pressure natural gas liquefaction and adsorption pretreatment : theoretical work and experimental field study

Abstract

The objective of this work has been to study offshore natural gas liquefaction processes with emphasis on utilization of the high reservoir pressure. A natural gas liquefaction process specially developed for offshore conditions is proposed and investigated. Pre-treatment of the liquefaction feed at elevated pressure was found to be a bottlenecking operation, and several possibilities of natural gas treatment by adsorption have been examined experimentally.

The liquefaction process proposed in this work utilizes the feed gas pressure in the natural gas liquefaction to reduce energy consumption. High-pressure feed gas is pre-cooled in a gas/gas heat exchanger using an expander type cooling cycle. The natural gas is then expanded directly from dense to liquid phase in an expansion turbine. The process is simple and compact, and uses a nonflammable refrigerant (nitrogen). The process has been simulated using Pro II simulation software provided by Simulation Sciences Inc.

Because of the low temperatures involved in natural gas liquefaction, relatively strict limits are set to impurities and water vapor levels in the process feed. Adsorption is on of the methods used in natural gas liquefaction feed treatment. This thesis contributes with experimental field study of natural gas adsorption. A small-scale test plant for adsorption was constructed and used to investigate breakthrough curves for various adsorbents and adsorbates at pressures up to 117 bara. The test plant was located at the Kårstø gas processing plant in Norway. The test facility was connected to the Statpipe natural gas transport pipeline and was limited to the pipeline gas composition and conditions. Breakthrough experiments were carried out for simultaneous adsorption of CO2 and H2S on various industrial molecular sieves. Breakthrough experiments were also executed for removal of Triethylene glycol (TEG) contamination descending from absorption dehydration processes.

Silica gel proved to be a suitable adsorbent for TEG vapor removal from natural gas streams at high pressure and low temperature. A concept with a multi-layer bed of silica gel and molecular sieve is suggested, in order to remove both TEG and water vapour from the natural gas stream.

Hydrocarbon dew point control can be accomplished using a special type of silica gel. Some initial experiments for this type of adsorption have been performed.