Apparatus And Method For Liquefying Natural Gas By Refrigerating Single Mixed Working Medium

Abstract

A system and a method for liquefying natural gas using single mixed refrigerant as refrigeration medium are provided. The system comprises a two-stage mixed refrigerant compressor ( 1 ), coolers ( 21, 22 ), gas-liquid separators ( 31, 32 ), throttling devices ( 51, 52 ), a plate-fin heat exchanger group ( 8 ) and a LNG storage tank ( 9 ). The method of the present invention reduces the power consumption for gas compression by compressing and separating the mixed refrigerant stage by stage. The heat exchange curves of cold fluid and hot fluid in the total heat exchange process match with each other better by the aid of using multiple-stage heat exchange, which can reduce the flow of the mixed refrigerant. Further, the system of the present invention has a good adaptability to load-variable operation of the apparatus, and thus can effectively avoid abnormal liquid-flooding at the bottom of the cold box.

What is claimed is: 
     
         1 . A system for liquefying natural gas using single mixed refrigerant as refrigeration medium comprising a mixed refrigerant compressor system and a cold box system, wherein:
 the mixed refrigerant compressor system comprises:   a two-stage mixed refrigerant compressor;   a first cooler and a second cooler respectively connected to the first stage and the second stage of the two-stage mixed refrigerant compressor;   a first gas-liquid separator and a second gas-liquid separator respectively connected to the first cooler and the second cooler; and   a liquid pump connected to the first stage gas-liquid separator,   and the cold box system comprises:   a plate-fin heat exchanger group comprising at least six heat exchange passages, i.e., the first, second, third, fourth, fifth and sixth heat exchange passages, wherein the inlet ends of the first and second heat exchange passages are respectively connected to the gas phase port and liquid phase port of the second gas-liquid separator via two pipelines, and the outlet end of the third heat exchange passage are connected to the first stage compressor by pipeline;   a first throttling device connected to the outlet end of the second heat exchange passage of the plate-fin heat exchanger group;   a second throttling device connected to the outlet end of the first heat exchange passage and the inlet end of the fourth heat exchange passage of the plate-fin heat exchanger group;   a refrigerant separator connected to the inlet end of the third heat exchange passage, the outlet end of the fourth heat exchange passage of the plate-fin heat exchanger group and the first throttling device;   a heavy hydrocarbon separator connected to a separate heat exchange passage, i.e., the fifth heat exchange passage of the plate-fin heat exchanger group,   the gas phase port of the first gas-liquid separator is connected to the second stage of the two-stage mixed refrigerant compressor,   the liquid phase discharge line of the first gas-liquid separator is converged via a liquid pump with the discharge line of the second compressor stage, and then connected to the second cooler,   the gas phase port and liquid phase port of the second gas-liquid separator are respectively connected to the inlet ends of two heat exchange passages, i.e., the first and second heat exchange passages of the plate-fin heat exchanger group,   wherein the first throttling device connected to the outlet end of the second heat exchange passage is additionally connected to the refrigerant separator,   the gas phase discharge line at the top of the refrigerant separator is converged with the liquid phase discharge line at the bottom of the refrigerant separator, and then connected to the inlet end of the third heat exchange passage, and the outlet end of the third heat exchange passage is connected to the first stage of the two-stage mixed refrigerant compressor,   the fourth heat exchange passage, which is connected on its inlet end to the second throttling device, is further connected on its outlet end to the refrigerant separator,   a natural gas line is connected to the heavy hydrocarbon separator via said separate heat exchange passage, i.e., the fifth heat exchange passage of the plate-fin heat exchanger group, and   the gas phase port at the top of the heavy hydrocarbon separator is connected to the LNG storage tank after passing through a heat exchange passage, i.e., the sixth heat exchange passage of the plate-fin heat exchanger.   
     
     
         2 . The system according to  claim 1 , characterized in that the gas phase port at the top of the heavy hydrocarbon separator is connected to the LNG storage tank after passing through the sixth and further seventh heat exchange passages of the plate-fin heat exchanger successively. 
     
     
         3 . A method for liquefying natural gas using single mixed refrigerant as refrigeration medium comprises:
 a natural gas cycle wherein:   Purified natural gas is firstly passed through a plate-fin heat exchanger group, precooled to −30° C.˜−80° C. and then entered to a heavy hydrocarbon separator for gas-liquid separation,   The gas phase stream separated from the top of the heavy hydrocarbon separator is further passed through the other stages of the heat exchanger group for heat exchanging, cooled to −130° C.˜−166° C., and thus obtained LNG is delivered to a LNG storage tank for storing;   a mixed refrigerant cycle wherein:   The mixed refrigerant composed of C1˜C5 alkanes and N 2  is fed into the inlet of a two-stage mixed refrigerant compressor, compressed to 0.6˜1.8 MPa by first stage compression, entered into a first stage cooler and cooled to 30˜40° C., and then introduced into a first stage gas-liquid separator for gas-liquid separation;   the gas separated from the top of the first stage gas-liquid separator is fed to the inlet of the second stage compressor, compressed to 1.2˜5.4 MPa by the second stage compression;   the liquid separated from the liquid phase port at the bottom of the first stage gas-liquid separator is pressurized to 1.2˜5.4 MPaA by a liquid pump, mixed with the hot gas from the outlet of the second stage compressor, further introduced to a second cooler and cooled to 30˜40° C., and then the mixed refrigerant after cooling is fed to a second stage gas-liquid separator for gas-liquid separation;   the gas obtained at the top of the second stage gas-liquid separator is passed through the first heat exchange passage of the main heat exchanger group for heat exchanging, and the liquid separated from the bottom of the second stage gas-liquid separator is passed through the second heat exchange passage of the main heat exchanger group for heat exchanging;   the liquid separated from the bottom of the second stage gas-liquid separator is precooled to about −30° C.˜−80° C. in the second heat exchange passage of the heat exchanger group, throttled to 0.2˜0.8 MPaA by a first throttling device, and then introduced to the middle part of a refrigerant separator;   the gas phase stream of the mixed refrigerant separated from the top of the second gas-liquid separator is passed through the gas phase passage, i.e., the first heat exchange passage of the heat exchanger group, cooled to −135° C.˜−169° C. and then throttled to 0.2˜0.8 MPaA by a second throttling device; the throttled gas stream is reversely passed through the fourth heat exchange passage of the heat exchanger group for providing cold energy and reheated to −30° C.˜−80° C., and then introduced into the middle part of the refrigerant separator after exiting from the heat exchanger group, and converged, in the refrigerant separator, with the cooled and throttled liquid refrigerant which is discharged from the first throttling device and entered likewise into the above-mentioned refrigerant separator,   the converged two refrigerants are separated into two phases, i.e., gas phase and liquid phase, by the refrigerant separator, and the two phases exited from the refrigerant separator are joined together, returned to the third heat exchange passage of the heat exchanger group for providing cold energy and then introduced into the first compressor stage as the mixed refrigerant.   
     
     
         4 . The method according to  claim 3 , characterized in that the mixed refrigerant comprises four or five or six components selected from C1, C2, C3, C4, C5 alkanes and N 2 , and these components are mixed in any volume ratio or in substantially equal ratio.