Evaluation of processes for subsea dehydration and glycol regeneration
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The worlds remaining oil and gas resources are often found at vast sea depths and locations with harsh weather. This has created an incentive for developing subsea technology, enabling production and processing at the seafloor, which is unaffected by the rough conditions. Many fields are already operating with subsea pressure boosting, compression, separation and so on. The technology will soon enable complete subsea natural gas processing, the missing link being subsea gas dehydration technology, which is yet to be developed. This work is a study of methods for subsea natural gas dehydration through glycols and subsea glycol regeneration. The report includes a literature study establishing available subsea technology, and a brief presentation of common dehydration and regeneration methods. Subsea applicable dehydration and glycol regeneration solutions have been developed and tested through simulations in Aspen HYSYS. The results have been used to design and roughly dimension a subsea processing plant processing the gas to meet the rich gas transport specifications. The literature study found that available subsea technology includes subsea separation, boosting, pumping, cooling, power supply, and subsea dry and wet gas compression. Natural gas dehydration to meet rich gas transport specifications is commonly done through absorption using a glycol as absorbent. The glycol regeneration is achieved in distillation columns. A possible alternative for dehydration is absorption through ionic liquids, and the regeneration can alternatively be obtained by pervaporation membranes. The dehydration unit developed includes a static mixer and subsequent separator. Results showed that the process could be improved by changing the single dehydration unit to a double dehydration unit, introducing a second stage of static mixer and subsequent separator. Three regeneration alternatives has been developed, and includes an internal regeneration alternative operating at two different pressures, an electric heater and subsequent separator alternative, and an alternative for regeneration through two different pervaporation membranes. All alternatives were tested for both MEG and TEG, and the first two alternatives were also tested for several process plant inlet pressures. The two internal regeneration versions had varying results, and only six of the twelve cases tested met the required dew point specifications. The six electric heater and separator cases all met the required water specifications, but had large glycol losses. The membrane alternative showed the most promising results, and of all nine membrane cases, one was selected for further design and weight/volume estimation. The regeneration method chosen for design includes a two-stage dehydration unit and regeneration through the membrane PVA/PP with zeolite, implemented with sweep gas for optimization purposes. The complete simulated process requires six compressors, two pumps, four separators, four scrubbers, eight coolers, two electric heaters, two mixers, and a membrane module. The plant will in addition require pipelines, process control equipment, power supply, and support structures. The separators, scrubbers, coolers, and membrane module has been size estimated. The largest volume contributors are the separators, the first two coolers in the process, and the membrane module. The largest weight contributions are the two coolers with the largest volumes.