Natural gas liquefaction system with turbine expander and liquefaction method thereof

A liquefaction system for converting natural gas into liquefied natural gas by using mixed refrigerants, comprising:

• a compressor for providing a compressed refrigerant stream by compressing the mixed refrigerants;
• first and second phase separators for separating the compressed refrigerant stream transferred from the compressor into vapor phase and liquid phase refrigerant streams (i.e. vapor and liquid streams);
• a turbine expander for converting the vapor stream in a superheated state into an expanded-cold refrigerant stream;
• a plurality of heat exchangers connected by connecting lines to the first and second phase separators, the turbine expander, a natural gas source and a liquefied natural gas tank so as to cool the natural gas by indirect heat exchange with the expanded refrigerant stream provided by the first and second phase separators and the turbine expander;
• first and second JT valves that provide expanded refrigerant stream by expanding liquid stream from the first and second phase separators, respectively, wherein the expanded refrigerant stream converted to a low temperature by the first and second JT valves is supplied to one of the plurality of heat exchangers connected to the first and second JT valves so as to cool the natural gas by indirect heat exchange,
• wherein the plurality of heat exchangers includes:
• a first heat exchanger which cools the natural gas supplied from the natural gas source and the vapor stream supplied from the first phase separator by indirect heat exchange with the expanded-cold refrigerant stream passed through the first JT valve;
• a second heat exchanger which further cools the cooled natural gas from the first heat exchanger by indirect heat exchange with the vapor stream supplied from the second phase separator;
• a third heat exchanger which further cools the cooled natural gas from the second heat exchanger by indirect heat exchange with the expanded-cold refrigerant stream passed through the second JT valve and the expanded refrigerant stream that is supplied from the turbine expander;
• a fourth heat exchanger which is arranged between the turbine expander and the third heat exchanger, and further cools the cooled natural gas from the third heat exchanger by indirect heat exchange with the expanded refrigerant stream supplied from the turbine expander;
• and
• a fifth heat exchanger which pre-cools the vapor stream supplied from the first phase separator via the first heat exchanger by indirect heat exchange with expanded refrigerant stream passed through the third heat exchanger;
• wherein the connecting line includes:
• a pipe (P1) that is connected continuously through the first, second, third and fourth heat exchangers so as to transfer natural gas,
• a pipe (P2) that is connected from the vapor-phase portion of the first phase separator to the first heat exchanger and from the first heat exchanger via the fifth heat exchanger to the second phase separator so as to transfer the vapor stream supplied from the first phase separator,
• a pipe (P3) that is connected from the liquid-phase portion of the first phase separator via the first heat exchanger to the external compressor so as to transfer the liquid stream supplied from the first phase separator, wherein the first JT valve for expanding the liquid stream supplied from the first phase separator is arranged on pipe (P3) in the place adjacent to the liquid-phase portion of the first phase separator,
• a pipe (P4) that is connected from the vapor-phase portion of the second phase separator to the second heat exchanger and from the second heat exchanger to the turbine expander so as to transfer the vapor stream supplied from the second phase separator,
• a pipe (P5) that is connected from the turbine expander sequentially via the fourth, third and fifth heat exchangers to one end of the pipe (P3), and
• a pipe (P6) that is connected from the liquid-phase portion of the second phase separator to the pipe (P5) between the third heat exchanger and the fourth heat exchanger so as to transfer the liquid stream supplied from the second phase separator, wherein the second JT valve for expanding the liquid stream supplied from the second phase separator is arranged on pipe (P6) adjacent to the liquid-phase portion of the second phase separator.