Integrated process for purifying and liquefying natural gas

Abstract

A process for liquefying and simultaneously purifying natural gas from acidic compounds is provided. The process involves pre-cooling a natural gas flow in a cryogenic exchanger, pre-treating the pre-cooled natural gas flow inside a pre-treatment unit and obtaining a purified flow, heat recovering and obtaining a higher temperature flow, compressing the higher temperature flow and obtaining a first compressed recirculation flow, cooling the first compressed recirculation flow and obtaining a compressed and cooled flow, and separating from the compressed and cooled flow a recirculation flow portion of natural gas. One or more cooling steps are carried out by a flow of nitrogen circulating inside a closed nitrogen refrigeration cycle.

What is claimed is: 
     
         1 . A process for purifying and liquefying natural gas, the process comprising the steps of:
 1) pre-cooling a natural gas flow in a cryogenic exchanger obtaining a pre-cooled natural gas flow,   2) pre-treating the pre-cooled natural gas flow inside a pre-treatment unit obtaining a purified flow of natural gas,   3) heat recovering inside the cryogenic exchanger obtaining a higher temperature flow,   4) compressing said higher temperature flow by a first compressor obtaining a first compressed recirculation flow,   5) cooling said first compressed recirculation flow in a first natural gas cooler obtaining a compressed and cooled flow,   6) separating from said compressed and cooled flow of a first portion, which is subjected to the steps of:
 6a) further cooling, liquefying and possibly sub-cooling, inside said cryogenic exchanger obtaining a flow of liquefied natural gas, 
 6b) expanding said flow of liquefied natural gas by a first valve obtaining a flow of liquefied natural gas at a lower pressure, 
   7) separating from said compressed and cooled flow a recirculation flow portion of the natural gas, which is subjected to the steps of:
 7a) cooling, inside said cryogenic exchanger, obtaining a first cooled recirculation flow of the natural gas, 
 7b) expanding said first cooled recirculation flow of the natural gas in an expander obtaining a second recirculation flow of the natural gas, and 
 7c) subjecting said second recirculation flow of the natural gas to heat recovery inside said cryogenic exchanger obtaining a third natural gas recirculation flow at a higher temperature, which is reunited with said higher temperature flow, to form an overall recirculation flow to be subjected to step 4), wherein 
   one or more of the cooling steps 1), 6a), 7a) are carried out by a flow of nitrogen that circulates inside a closed nitrogen refrigeration cycle.   
     
     
         2 . The process of  claim 1 , wherein, after step 1) and/or after step 2) the pre-cooled natural gas flow and/or said purified flow of natural gas are expanded, obtaining an expanded pre-cooled flow and/or a purified and further expanded flow, respectively. 
     
     
         3 . The process of  claim 1 , wherein step 4) is carried out on the higher temperature flow, with which the third natural gas recirculation flow is reunited, forming a second overall recirculation flow, which is then compressed in the first compressor, obtaining a first compressed recirculation flow. 
     
     
         4 . The process of  claim 1 , wherein step 4) and step 5) are carried out in a plurality of successive partial compression steps in further compressors obtaining further compressed recirculation flows and respective cooling in further natural gas coolers obtaining further compressed and cooled recirculation flows. 
     
     
         5 . The process of  claim 1 , wherein, in step 2) a first high-pressure flow is used, which is separated from the first compressed recirculation flow obtained from step 4). 
     
     
         6 . The process of  claim 5 , wherein portions of said further compressed recirculation flows are separated from one or more of the further compressed recirculation flows, which are reunited together with the first high-pressure flow, forming a high-pressure treatment flow, which is used in step 2). 
     
     
         7 . The process of  claim 1 , wherein, the first portion separated in step 6) has a capacity of about 10-40% of the capacity of said compressed and cooled flow or of said further compressed and cooled recirculation flows or, in any case, from each compressed and cooled flow. 
     
     
         8 . The process of  claim 1 , wherein said closed nitrogen refrigeration cycle comprises the steps of:
 A) subjecting a low-pressure nitrogen recirculation flow to a compression in a first compressor of the closed nitrogen refrigeration cycle, obtaining a first high-pressure nitrogen recirculation flow, and to a successive step of cooling in a first cooler of the closed nitrogen refrigeration cycle, obtaining a first cooled high-pressure nitrogen flow,   B) subjecting said first cooled high-pressure nitrogen flow to a step of heat exchange in the cryogenic exchanger obtaining a further cooled high-pressure nitrogen flow,   C) subjecting said further cooled high-pressure nitrogen flow to a step of expansion in an expander of the closed nitrogen refrigeration cycle, obtaining a low-pressure nitrogen recirculation flow, and   D) subjecting said low-pressure nitrogen recirculation flow to a step of heat recovery in the cryogenic exchanger, obtaining the low-pressure nitrogen recirculation flow to be subjected to step A).   
     
     
         9 . The process of  claim 8 , wherein the compression in step A) of the low-pressure nitrogen recirculation flow is carried out in a plurality of successive partial steps, obtaining a further high-pressure nitrogen recirculation flow, which is then subjected to a corresponding cooling step, obtaining a further cooled high-pressure nitrogen recirculation flow. 
     
     
         10 . The process of  claim 9 , wherein a portion of the first high-pressure nitrogen recirculation flow and a portion of the further high-pressure nitrogen recirculation flow are separated from said first high-pressure nitrogen recirculation flow and/or from said further high-pressure nitrogen recirculation flow, which are reunited together forming a treatment nitrogen flow, which is used in step 2). 
     
     
         11 . The process of  claim 4 , wherein the higher temperature flow obtained from step 3) is sent directly to one of the further compressors. 
     
     
         12 . The process of  claim 1 , wherein a portion of the second recirculation flow of the natural gas obtained from step 7b) is used in step 2). 
     
     
         13 . The process of  claim 1 , wherein a portion of said flow of liquefied natural gas at a lower pressure obtained from step 6b) is used in step 2). 
     
     
         14 . The process of  claim 9 , wherein step 2) is carried out by using one or more flows selected from the group consisting of: a first high-pressure flow, a high-pressure treatment flow, a portion of the second recirculation flow of the natural gas, a portion of the first high-pressure nitrogen recirculation flow, a portion of the further high-pressure nitrogen recirculation flow, a nitrogen treatment flow, one or more external currents. 
     
     
         15 . The process of  claim 13 , wherein a recovery flow is obtained from step 2), which is subjected to a step of expansion by a fourth valve, obtaining a flow of expanded natural gas, which is reunited with the higher temperature flow to be used in step 4). 
     
     
         16 . The process of  claim 1 , wherein a flow of removed compounds is obtained from step 2). 
     
     
         17 . A plant for purifying and liquefying natural gas in which the process for purifying and liquefying natural gas of  claim 1  is carried out, the plant comprising:
 (I) a natural gas circuit inside which flows of natural gas circulate, the flows of natural gas being low-pressure, high-pressure, compressed or expanded, cooled or heated flows, the natural gas circuit comprising:
 a pre-treatment unit for pre-treating said natural gas, 
 a section for compressing, cooling and expanding said natural gas, 
 a point for recovering liquefied natural gas for storage or introduction thereof into a convenient distribution network; 
 
 (II) a nitrogen circuit inside which several flows of nitrogen circulate, the flows of nitrogen being low-pressure, high-pressure, compressed or expanded, cooled or heated flows, the nitrogen circuit comprising:
 a tank of nitrogen, 
 a section for compressing, cooling and expanding the nitrogen; and 
 
 (III) a cryogenic exchanger comprising:
 a section for cooling, liquefying and sub-cooling the natural gas. 
 
 
     
     
         18 . The plant of  claim 17 , wherein said cryogenic exchanger is configured to allow thermal exchanges between one or more of said flows of natural gas with one or more of said flows of nitrogen and/or natural gas.