Parameter determination of dual nitrogen expansion technology for gas liquefaction in FLNG unit

In order to confirm the feasibility to apply FLNG (Floating Liquefied Natural Gas) development model in offshore gas fields, the technological process and main parameters of dual nitrogen expansion for gas liquefaction in domestic offshore gas fields were simulated and analyzed. After the gas composition, temperature and pressure of domestic aimed deepwater gas field were reviewed, the effects of outlet temperature at three stages (precooling, liquefaction and subcooling) and circulating pressure of refrigerant at two stages (liquefaction and subcooling) on compression power of liquefaction process, power of heat exchanger, flow rate of refrigerant, specific power and refrigerator loading were analyzed by HYSYS software while the gas composition, the flow rate and the temperature and pressure at the initial period of liquefaction were kept constant. It is shown that the compression power and the specific power of compressor set in circulating system are the minimum when the gas outlet temperature at the liquefaction stage is set at －75 ℃ and the refrigerant inlet pressure is 3 450 kPa at the liquefaction stage and 1 710 kPa at subcooling stage. The temperature at the precooling stage has no effect on the total compression power of refrigerant, but it can adjust the distribution of heat exchanging load between the precooling stage and the liquefaction stage. The research results can provide the basis for the equipment selection and technical and economic evaluation of FLNG development program.