LEAN LNG PLANTS - HEAVY ENDS REMOVAL AND OPTIMUM RECOVERY OF LIGHT HYDROCARBONS FOR REFRIGERANT MAKE-UP

摘要

Strong demand for LNG means that many operators are developing projects based on lean gas with high methane content, in locations such as North America, East Africa and Queensland Australia. The pre-treatment section of LNG plant designs varies widely to suit feed gas composition. In particular, the choice of technology for heavy ends removal and recovery of light hydrocarbons for refrigerant make up can greatly influence the operability of a liquefaction train. The usual practice in the LNG industry is to use a scrub column, operating at the same pressure as the liquefier. Although a C_(2+) recovery unit with a turbo-expander is higher in CAPEX compared to a scrub column this paper will show that there are savings when the impact on the overall train are taken into account. Failing to consider savings in the MR refrigerant cycle brought by the liquefaction at high pressure, a scrub column is often selected. For a lean gas this can be a bad choice due to difficulties with ensuring stable operation in this column and in some cases the impossibility of extracting sufficient refrigerant make-up. Through a CAPEX analysis including the surrounding units, the above-mentioned C_(2+) recovery processes are compared. This paper shows that a turbo-expander and booster compressor process brings technical and economic benefits, providing operational stability and flexibility.