ECONOMIC AND ENVIRONMENTAL BENEFITS OF TWO- PHASE LNG EXPANDERS

摘要

LNG liquefaction plants have a complex structure with numerous systems interacting to produce the desired output. Capital investment, operational costs and environmental burden of these plants are relatively high. These high economic demands have initiated new and additional efforts to reduce the costs and the environmental impact of natural gas liquefaction plants. The conventional liquefaction process for natural gas is to operate at high pressure through the condensation phase, after which the high pressure of the liquefied natural gas is reduced by expansion across a Joule-Thomson valve. All LNG plants commissioned before 1996 are operating with an inefficient expansion valve. By replacing the existing JT-valve with a cryogenic LNG liquid turbine to expand the condensed liquefied natural gas from high pressure to low pressure the thermodynamic efficiency of the existing refrigeration process is substantially improved resulting in an increase of the total LNG output between 3 to 5%. These innovations are already applied to new LNG liquefaction plants in Oman and Malaysia and the attained economic and technical benefits are convincing. The next step of economical improvement in LNG liquefaction is the most recent development of two-phase LNG expanders. Since the early days of the cryogenic industry it is known that two-phase cryogenic liquid expanders would improve significantly the thermodynamic efficiency of a gas liquefaction process. Only in recent years has the mechanical and electrical technology become available to build and operate reliable cryogenic two-phase expanders capable of expanding LNG partially into the vapor phase. This Paper Presents a new generation of cryogenic two phase LNG expanders. It discusses the economic and environmental benefits, the interaction of these two-phase expanders with the liquefaction process, and the added advantage for nitrogen removal. Two-phase LNG expanders compared to conventional JT valve expansion increase the total LNG output between 5 to 8%.