Natural Gas Liquefaction System for Producing LNG and Merchant Gas Products

Abstract

Pipeline natural gas is dried to remove water and carbon dioxide and liquefied. This gas and nitrogen are fed into a first heat exchanger for cooling against colder flash and expanded gases. High pressure nitrogen gas is expanded producing refrigeration to liquefy the natural gas passing through further heat exchangers and throttled via a control valve where the resulting liquid natural gas (LNG) exits into a separator. A portion of the LNG is fed to a nitrogen liquefaction unit with a major portion of LNG stored as product. The LNG portion is vaporized and heated to ambient against a pressurized liquefied nitrogen stream, a portion of which is stored as product. The remaining portion of the liquid nitrogen is optionally sent to an air separation unit (ASU) as “liquid assist” producing liquid and gaseous oxygen, liquid argon and gaseous nitrogen merchant gases. A portion of the natural gas stream selectively regenerates one of two dryers. If a letdown station is employed, a smaller booster compressor may be used. Medium and low pressure nitrogen is recovered and recycled through compression. The ASU separates and produces merchant gases to be liquefied wherein the liquid is generally produced by expanding nitrogen gas to generate refrigeration producing the liquid products while consuming nitrogen molecules. Natural gas feed streams richer in ethane, propane and heavier hydrocarbons enables the production of natural gas liquids as additional products in a further embodiment. Other embodiments are also disclosed.

What is claimed is: 
     
         1 . A system for producing liquid natural gas (LNG), and liquid nitrogen utilizing a source of a methane rich stream of natural gas and an air separation unit (ASU) to produce merchant gases comprising:
 an LNG generator including at least one heat exchanger responsive to applied processed pipeline natural gas for producing liquid natural gas;   a nitrogen liquefaction section for liquefying nitrogen received from the ASU;   a liquid nitrogen storage unit and an LNG storage unit;   a first control valve for applying a predetermined amount of LNG in a selected first portion of the produced LNG to the LNG storage unit as product;   a second control valve for applying a second portion of the produced LNG by the LNG generator to the nitrogen liquefaction section for assisting in the liquefaction of nitrogen as determined by the selected first portion amount; the nitrogen liquefaction section for producing nitrogen to assist in the production of the merchant gases as products by the ASU; and   a third control valve for applying liquid nitrogen from the nitrogen liquefaction section to the liquid nitrogen storage unit for storing the liquefied nitrogen as product.   
     
     
         2 . The system of  claim 1  wherein the LNG generator is arranged to be responsive to applied pressurized nitrogen and includes a nitrogen gas expansion arrangement for causing the pressurized nitrogen to produce refrigeration to assist in liquefying the natural gas into LNG in the LNG generator. 
     
     
         3 . The system of  claim 1  including a nitrogen feed compressor responsive to nitrogen gas fed from the ASU to provide compressed nitrogen to the LNG generator. 
     
     
         4 . The system of  claim 1  including a compander for supplying compressed and then expanded natural gas (NG) to the LNG generator. 
     
     
         5 . The system of  claim 1  wherein the LNG generator includes a control valve and separator for producing vaporized low pressure and flashed natural gas and including a bank of heat exchangers for heating to ambient temperature the flashed natural gas passed through the heat exchangers and at least one compressor for compressing the passed natural gas to pipeline pressure. 
     
     
         6 . The system of  claim 1  including an arrangement for recovering medium and low pressure nitrogen from the liquefaction section and recycling the recovered nitrogen through compression into the LNG generator. 
     
     
         7 . The system of  claim 6  wherein the LNG generator includes a bank of heat exchangers, the arrangement for recovering nitrogen includes a series of compressors for sequentially compressing the nitrogen and for then selectively applying the compressed nitrogen to the LNG generator to selected ones of the heat exchangers. 
     
     
         8 . The system of  claim 7  including a series of expanders each for expanding the compressed nitrogen applied to a different one of the selected ones of the heat exchangers. 
     
     
         9 . The system of  claim 1  wherein the LNG generator produces liquid natural gas LNG wherein the liquid is produced by a series of nitrogen expanders and a series of nitrogen compressors for generating nitrogen refrigeration to assist in the production of the LNG. 
     
     
         10 . The system of  claim 1  wherein the nitrogen liquefaction section comprises a bank of heat exchangers, a compressor arrangement and a gas flash arrangement for recycling nitrogen gas supplied by the ASU through the heat exchangers for the liquefying of the nitrogen in cooperation with the liquefied natural gas and for selectively feeding the liquefied nitrogen to a selected one or both of the ASU and nitrogen storage unit. 
     
     
         11 . The system of  claim 1  wherein the nitrogen liquefaction section integrates with the ASU for generating liquid nitrogen. 
     
     
         12 . The system of  claim 1  wherein the nitrogen liquefaction section comprises a series of heat exchanger, a separator, and at least one nitrogen gas flash valve which cooperates with the ASU to produce liquid nitrogen. 
     
     
         13 . The system of  claim 1  wherein the system includes a letdown station in the pipeline to assist in reducing power required and for providing refrigeration to the LNG liquefaction generator. 
     
     
         14 . The system of  claim 13  including a cooling heat exchanger comprising a refrigerant and an expander for expanding and cooling the natural gas output of the cooling heat exchanger coupled to the LNG generator. 
     
     
         15 . The system of  claim 1  wherein the LNG generator comprises a series of nitrogen expanders for cooling nitrogen gas applied to a cascaded series of heat exchangers in the LNG generator. 
     
     
         16 . The system of  claim 1  wherein the ASU supplies nitrogen to a series bank of compressors whose output of compressed nitrogen is applied to a series bank of heat exchangers in the LNG generator. 
     
     
         17 . The system of  claim 16  wherein the LNG generator comprises a series of nitrogen expanders for cooling nitrogen gas applied to a cascaded series of heat exchangers in the LNG generator, the expanders comprising corresponding companders forming the compressors. 
     
     
         18 . The system of  claim 1  wherein the LNG generator includes at least one expander and at least one cooling heat exchanger comprising an ammonia refrigerant for cooling at least one of nitrogen and natural gas. 
     
     
         19 . The system of  claim 1  wherein the LNG generator includes at least one expander for expanding natural gas for cooling natural gas in the generator and at least one cooling heat exchanger comprising an ammonia refrigerant for cooling nitrogen gas used to cool the natural gas in the generator. 
     
     
         20 . The system of  claim 1  wherein the LNG generator includes at least one expander for expanding and cooling natural gas applied thereto. 
     
     
         21 . The system of  claim 1  including at least one nitrogen expander for expanding nitrogen gas applied thereto. 
     
     
         22 . The system of  claim 20  further including at least one natural gas expander for expanding recycled natural gas applied thereto. 
     
     
         23 . The system of  claim 1  wherein the LNG generator further includes at least one natural gas expander and at least one nitrogen gas expander. 
     
     
         24 . The system of  claim 1  wherein the LNG generator includes three different closed gas systems utilizing corresponding three different refrigeration gases. 
     
     
         25 . The System of  claim 1  wherein the LNG generator utilizes at least one ammonia gas refrigeration cooler, a nitrogen gas refrigeration cooling system and a mixed gas cooling refrigeration system. 
     
     
         26 . The system of  claim 1  wherein the LNG generator includes a control valve and separator for producing vaporized low pressure and flashed natural gas and including a bank of heat exchangers for heating to ambient temperature the flashed natural gas passed through the heat exchangers whereby the passed natural gas can be partially or completely fed to an electrical generator set to generate electricity. 
     
     
         27 . A system utilizing a source of a methane rich natural gas for producing 1) liquid natural gas (LNG), 2) natural gas liquids (NGL) predominately comprising propane, butane and heavier hydrocarbons, 3) liquid nitrogen and 4) an air separation unit (ASU) to produce merchant gases comprising:
 an LNG generator including at least one heat exchanger responsive to an applied processed source of natural gas for producing liquid natural gas LNG;   a nitrogen liquefaction section for liquefying nitrogen received from the ASU;   a liquid nitrogen LN storage unit, an NGL storage unit and an LNG storage unit;   a first control valve for applying a predetermined amount of LNG in a selected first portion of the produced LNG to the LNG storage unit as product; and   a second control valve for applying a second portion of the produced LNG by the LNG generator to the nitrogen liquefaction section for assisting in the liquefaction of nitrogen as determined by the selected first portion amount; the nitrogen liquefaction section for assisting in the production of the merchant gases as products by the ASU;   a third control valve for applying liquid nitrogen from the nitrogen liquefaction section to the liquid nitrogen storage unit for storing the liquefied nitrogen as product; and   an arrangement for producing NGL product from the applied processed source of natural gas and for storing the NGL in the NGL storage unit.