Systems and Methods for Improving the Efficiency of Open-Cycle Cascade-Based Liquified Natural Gas Systems

Abstract

Systems and methods for improving the efficiency of open-cycle cascade-based liquified natural gas systems by utilizing one or more ejectors to reduce and/or eliminate compression stages. The systems and methods may thus, be used to improve the efficiency of new and preexisting open-cycle cascade-based liquified natural gas systems to reduce in the flow rate through each compressor, which reduces the energy consumption of the overall process.

1 . A liquefied feed gas system, which comprises:
 a first heat exchanger enclosing a first portion of a feed gas line and a portion of a first chilled refrigerant line;   a first flash drum in fluid communication with the first chilled refrigerant line for receiving a two-phase refrigerant from the first heat exchanger, the first flash drum having a first vapor outlet line and a first liquid outlet line;   a second heat exchanger enclosing a second portion of the feed gas line and a portion of a second chilled refrigerant line;   a second flash drum in fluid communication with the second chilled refrigerant line for receiving a two-phase refrigerant from the second heat exchanger, the second flash drum having a second vapor outlet line and a second liquid outlet line;   a third heat exchanger enclosing a portion of the first vapor outlet line, a portion of the second vapor outlet line and a portion of a third chilled refrigerant line; and   a first ejector in fluid communication with the second vapor outlet line, the first chilled refrigerant line and the third chilled refrigerant line.   
     
     
         2 . The system of  claim 1 , further comprising a pump in fluid communication with the third chilled refrigerant line and positioned between the third heat exchanger and the first ejector. 
     
     
         3 . The system of  claim 1 , wherein the first ejector is connected to the second vapor outlet line downstream from the third heat exchanger. 
     
     
         4 . The system of  claim 1 , wherein the second liquid outlet line is in fluid communication with the second vapor outlet line. 
     
     
         5 . The system of  claim 1 , further comprising a first expansion valve positioned between the first heat exchanger and the first ejector for producing a chilled refrigerant in the first chilled refrigerant line. 
     
     
         6 . The system of  claim 1 , further comprising a second expansion valve positioned between the second heat exchanger and the third heat exchanger for producing a chilled refrigerant in the second chilled refrigerant line. 
     
     
         7 . The system of  claim 6 , wherein the third heat exchanger encloses a portion of the first liquid outlet line between the first flash drum and the second expansion valve. 
     
     
         8 . The system of  claim 1 , further comprising:
 a third flash drum in fluid communication with the feed gas line, the third flash drum having a third vapor outlet line and a third liquid outlet line;   a second ejector in fluid communication with the third liquid outlet line, a boil-off gas line connected to a boil-off gas tank and a chilled mixed feed gas line that is in fluid communication with at least one of a fourth flash drum and a fifth flash drum; and   a third ejector in fluid communication with the third liquid outlet line, a fifth vapor outlet line from the fifth flash drum and the chilled mixed feed gas line.   
     
     
         9 . The system of  claim 8 , further comprising a third expansion valve positioned downstream from the second ejector for producing a chilled mixed feed gas in the chilled mixed feed gas line. 
     
     
         10 . The system of  claim 8 , further comprising a fourth expansion valve positioned downstream from the third ejector for producing a chilled mixed feed gas in the chilled mixed feed gas line. 
     
     
         11 . A method for liquifying a feed gas, which comprises:
 introducing a feed gas stream though a first heat exchanger and a second heat exchanger;   chilling the feed gas stream in the first heat exchanger by circulating a first chilled refrigerant stream adjacent the feed gas stream in the first heat exchanger;   chilling the feed gas stream in the second heat exchanger by circulating a second chilled refrigerant stream adjacent the feed gas stream in the second heat exchanger using a first liquid refrigerant stream from a first flash drum;   pumping a third chilled refrigerant stream from a third heat exchanger to an ejector for converting the third chilled refrigerant stream to the first chilled refrigerant stream; and   returning at least a portion of a first vapor refrigerant stream from the first flash drum to the first ejector.   
     
     
         12 . The method of  claim 11 , further comprising using a first expansion valve positioned between the first heat exchanger and the first ejector to convert the third chilled refrigerant stream into the first chilled refrigerant stream. 
     
     
         13 . The method of  claim 12 , further comprising using a second expansion valve positioned between the second heat exchanger and the third heat exchanger to convert the first liquid refrigerant stream into the second chilled refrigerant stream. 
     
     
         14 . The method of  claim 11 , further comprising chilling the first vapor refrigerant stream from the first flash drum in the third heat exchanger before returning the portion of the first vapor refrigerant stream to the first ejector. 
     
     
         15 . The method of  claim 13 , further comprising chilling the first liquid refrigerant stream from the first flash drum in the third heat exchanger before converting the liquid refrigerant stream into the second chilled refrigerant stream. 
     
     
         16 . The method of  claim 11 , further comprising:
 transferring a vaporized portion of the feed gas stream to a second flash drum;   transferring a portion of a second liquid refrigerant stream from the second flash drum to a second ejector to convert the portion of the second liquid refrigerant stream into a chilled mixed feed gas stream; and   transferring another portion of the second liquid refrigerant stream from the second flash drum to a third ejector to convert the another portion of the second liquid refrigerant stream into another chilled mixed feed gas stream.   
     
     
         17 . The method of  claim 16 , further comprising:
 transferring the chilled mixed feed gas stream to at least one of a fourth flash drum and a fifth flash drum; and   transferring the another chilled mixed feed gas stream to at least one of the fourth flash drum and the fifth flash drum.   
     
     
         18 . The method of  claim 17 , further comprising:
 returning a fifth vapor refrigerant stream from the fifth flash drum to the third ejector; and   returning a boil-off gas stream from a boil-off gas tank to the second ejector.   
     
     
         19 . The method of  claim 18 , further comprising:
 using a third expansion valve positioned downstream from the second ejector to convert the portion of the second liquid refrigerant stream into the chilled mixed feed gas stream; and   using a fourth expansion valve positioned downstream from the third ejector to convert the another portion of the second liquid refrigerant stream into the another chilled mixed feed gas stream.   
     
     
         20 . The method of  claim 11 , further comprising chilling the second liquid refrigerant stream in a fourth heat exchanger.