System and method for liquefying natural gas using single mixed refrigerant as refrigeration 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 ) driven by a motor, two coolers ( 21, 22 ), a liquid pump ( 4 ), three gas-liquid separators ( 31, 32, and 6 ), two throttling devices ( 51, 52 ), a plate-fin heat exchanger group ( 7 ) and a LNG storage tank ( 8 ). The method comprises the following steps: the mixed refrigerant is compressed step by step by using the two-stage mixed refrigerant compressor ( 1 ), separated step by step, and gas-phase and liquid phase mixed refrigerant streams obtained from the separation flow into different passages of the heat exchanger group ( 7 ) respectively for throttling and heat exchanging. Heat exchange curves of cold fluid and hot fluid in the whole heat exchange process match with each other better by using two-stage heat exchange, thereby flow of the mixed refrigerant is reduced effectively.

1 . A system for liquefying natural gas using single mixed refrigerant as refrigeration medium, the system 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 a first stage and a 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 gas-liquid separator,   and the cold box system comprises:   a plate-fin heat exchanger group comprising at least five heat exchange passages including a first, second, third, fourth and fifth heat exchange passages, the first and second heat exchange passages are connected to a gas phase port and a liquid phase port of the second gas-liquid separator, respectively, and the third heat exchange passages is connected to the first stage of the two-stage mixed refrigerant compressor;   a first throttling device connected to the second and third heat exchange passages of the plate-fin heat exchanger group;   a second throttling device connected to the first and third heat exchange passages of the plate-fin heat exchanger group; and   a heavy hydrocarbon separator connected to the fourth 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,   a liquid phase discharge line of the first gas-liquid separator is converged via the liquid pump with the discharge line of the second stage of the two-stage mixed refrigerant compressor, and then connected to the second cooler,   a gas phase port and a liquid phase port of the second gas-liquid separator are connected to the first and second heat exchange passages of the plate-fin heat exchanger group respectively,   a natural gas line is connected to the heavy hydrocarbon separator via the fourth heat exchange passage of the plate-fin heat exchanger group, and   a gas phase port at a top of the heavy hydrocarbon separator is connected to an LNG storage tank after passing through the fifth 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 fifth and a sixth heat exchange passages of the plate-fin heat exchanger successively. 
     
     
         3 . A method for liquefying natural gas using single mixed refrigerant as refrigeration medium, the method comprising:
 passing purified natural gas as a raw material firstly through a fourth heat exchange passage of a plate-fin heat exchanger group, cooled to −30° C.˜60° C. and then entering the natural gas into a heavy hydrocarbon separator for gas-liquid separation,   passing a gas phase component separated from a top of heavy hydrocarbon separator further through a fifth heat exchange passage of the plate-fin heat exchanger group for heat exchanging, cooled to −130° C.˜166° C., and thus obtaining LNG and delivering the LNG to an LNG storage tank for storing;   
       feeding a mixed refrigerant composed of C1˜C5 alkanes and N 2  into an inlet of a compressor, compressed to 0.6˜1.8 MPa by first stage compression, entering into a first cooler and cooling to 30˜40° C., and then introducing into a first gas-liquid separator for gas-liquid separation;
 feeding gas separated from a top of the first gas-liquid separator to an inlet of a second stage compressor, compressed to 1.2˜5.4 MPa by second stage compression; 
 pressurizing liquid separated from a liquid phase port at a bottom of the first stage gas-liquid separator by a liquid pump, mixing with hot gas from an outlet of the second stage compressor, further introducing to a second cooler and cooling to 30˜40° C., and then feeding the mixed refrigerant after cooling to a second gas-liquid separator for gas-liquid separation; 
 passing gas obtained at a top of the second gas-liquid separator through a first heat exchange passage of the plate-fin heat exchanger group for heat exchanging, and passing liquid separated from a bottom of the second gas-liquid separator through the second heat exchange passage of the plate-fin heat exchanger group for heat exchanging; 
 precooling liquid separated from the bottom of the second gas-liquid separator to about −30° C.˜80° C. in a second heat exchange passage of the plate-fin heat exchanger group, throttling to 0.25˜0.75 MPaA by the first throttling device, converging with a mixed refrigerant stream which is passed through the first heat exchange passage of the plate-fin heat exchanger group and returned via the second throttling device, and entering reversely into a third heat exchange passage for providing cold energy for the heat exchanger group and then returning to a first compressor stage; 
 cooling a gas stream of mixed refrigerant separated from a top of the second gas-liquid separator to −135° C.˜169° C. in the first heat exchange passage of the plate-fin heat exchanger group, throttling to 0.25˜0.75 MPaA by a second throttling device, and entering reversely into a third heat exchange passage for providing cold energy for the plate-fin heat exchanger group. 
 
     
     
         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. 
     
     
         5 . The method according to  claim 3 , 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 fifth and a sixth heat exchange passages of the plate-fin heat exchanger successively.