System and Method for Separating Nitrogen from Methane with Ultra-Low Greenhouse Gas Emissions

Abstract

A system and method for removing nitrogen from natural gas using two fractionating columns to achieve an ultra-low greenhouse gas content in a nitrogen vent/product stream, while also producing three sales gas streams at different pressures and with low nitrogen content within pipeline specifications. A portion of a low pressure column overhead stream may be compressed and cooled and recycled back to provide reflux to the low pressure column. A system feed stream is cooled upstream of feed a high pressure column, but preferably not separated into streams with varying compositions. A portion of the high pressure column bottoms stream and the low pressure column bottoms stream provides refrigerant to the high pressure column to produce a reflux stream. An amount of methane in a nitrogen vent/nitrogen product stream may be less than 0.01%.

I claim: 
     
         1 . A system for producing a methane product stream and a nitrogen stream from a feed stream comprising nitrogen, methane, and other components, the system comprising:
 a first fractionating column wherein the feed stream is separated into a first column overhead stream and a first column bottoms stream;   a first splitter for splitting the first column bottoms stream into a first portion, a second portion, and a third portion;   a second fractionating column wherein the first column overhead stream is separated into a second column overhead stream and a second column bottoms stream;   a second splitter for splitting the second column overhead stream into a first portion and a second portion;   a first mixer to mix the second column bottoms stream and the first portion of the first column bottoms stream to form a refrigerant stream;   a first heat exchanger wherein the feed stream is cooled upstream of the first fractionating column through heat exchange with the refrigerant stream, the second portion of the first column bottoms stream, the third portion of the first column bottoms stream, and the first portion of the second column overhead stream;   a second heat exchanger for cooling a vapor stream from an upper fractionation section of the first fractionating column to produce the first column overhead stream and a reflux stream for the first fractionating column through heat exchange with the refrigerant stream prior to the refrigerant stream undergoing heat exchange in the first heat exchanger;   wherein the methane product stream comprises the refrigerant stream, the second portion of the first column bottoms stream, and the third portion of the first column bottoms stream each after undergoing heat exchange in the first heat exchanger; and   wherein the nitrogen stream comprises the first portion of the second column overhead stream and comprises less than 0.05% methane.   
     
     
         2 . The system of  claim 1  wherein the first fractionating column is operated at a pressure between 300 and 500 psig and the second fractionating column is operated at a pressure between 75 and 125 psig. 
     
     
         3 . The system of  claim 2  wherein the second portion of the first column bottoms stream is a high pressure sales gas stream having a pressure between 600 and 1300 psig;
 wherein the third portion of the first column bottoms stream is an intermediate pressure sales gas stream having a pressure between 175 and 275 psig; and 
 wherein the refrigerant stream is a low pressure sales gas stream having a pressure between 60 and 150 psig. 
 
     
     
         4 . The system of  claim 1  further comprising a third splitter for splitting the feed stream into a first portion and a second portion downstream of the feed stream undergoing heat exchange in the first heat exchanger; and
 wherein the first portion of the feed stream is cooled in the first heat exchanger prior to feeding into a mid-upper level of the first fractionating column. 
 
     
     
         5 . The system of  claim 4  further comprising a third heat exchanger for warming a liquid stream from a bottom section of the first fractionating column to produce the first column bottoms stream and a first column returning vapor stream for the first fractionating column through heat exchange with the second portion of the feed stream prior to the second portion of the feed stream feeding into a lower level of the first fractionating column. 
     
     
         6 . The system of  claim 1  further comprising a third splitter for splitting the second portion of the second column overhead stream into a third portion and a fourth portion; and
 a third heat exchanger wherein the third portion and the fourth portion of the second column overhead stream are warmed through heat exchange with a recycled stream; 
 a series of one or more compressors and one or more coolers to compress and cool the fourth portion of the second column overhead stream after heat exchange in the third heat exchanger to form the recycled stream; and 
 wherein the nitrogen stream further comprises the third portion of the second column overhead stream after undergoing heat exchange in the third heat exchanger. 
 
     
     
         7 . The system of  claim 6  further comprising a first expansion valve to expand and cool the recycled stream after undergoing heat exchange in the third heat exchanger; and
 wherein the recycled stream feeds into the second fractionating column as a reflux stream after passing through the first expansion valve. 
 
     
     
         8 . The system of  claim 7  further comprising a fourth heat exchanger for cooling the first column overhead stream prior to feeding into the second fractionating column through heat exchange with the second column bottoms stream and the first portion of the second column overhead stream. 
     
     
         9 . The system of  claim 8  further comprising a second expansion valve to expand and cool the first column overhead stream after undergoing heat exchange in the fourth heat exchanger and prior to feeding into the second fractionating column. 
     
     
         10 . The system of  claim 8  wherein the second fractionating column comprises an internal separation chamber configured to receive heat from the fourth heat exchanger to separate a liquid stream from a lower level of a fractionation section of the second fractionating column into a second column returning vapor stream and the second column bottoms stream prior to the second column bottoms stream undergoing heat exchange in the fourth heat exchanger. 
     
     
         11 . The system of  claim 1  further comprising a first expansion valve for expanding and cooling the first portion of the first column bottoms stream upstream of the first mixer. 
     
     
         12 . The system of  claim 1  further comprising a pump to pump the second portion of the first column bottoms stream prior to undergoing heat exchange in the first heat exchanger; and
 a first expansion valve to expand and cool the third portion of the first column bottoms stream prior to undergoing heat exchange in the first heat exchanger. 
 
     
     
         13 . The system of  claim 1  wherein the second heat exchanger comprises a shell and tube heat exchanger. 
     
     
         14 . The system of  claim 13  wherein the shell and tube heat exchanger comprises a knockback condenser. 
     
     
         15 . The system of  claim 1  wherein the second heat exchanger comprises a knockback condenser. 
     
     
         16 . The system of  claim 15  wherein the knockback condenser comprises:
 a plurality of heat exchange tubes disposed inside a shell space; 
 a headspace zone disposed above and in fluid communication with the plurality of heat exchange tubes; 
 a riser tube configured to allow fluid communication of the vapor stream from the upper fractionation section of the first fractionating column to the headspace zone; and 
 a refrigerant inlet and a refrigerant outlet to allow fluid communication of the refrigerant stream through the shell space. 
 
     
     
         17 . The system of  claim 1  further comprising a second column reflux stream that feeds into an upper level of the second fractionating column and comprises at least 99.5% nitrogen. 
     
     
         18 . The system of  claim 1  wherein the nitrogen stream comprises 0.01% or less methane. 
     
     
         19 . The system of  claim 6  wherein the nitrogen stream comprises 0.01% or less methane. 
     
     
         20 . The system of  claim 8  wherein the first column overhead stream is further cooled in the fourth heat exchanger through heat exchange with a liquid stream withdrawn from the second fractionating column; wherein the liquid stream withdrawn from the second fractionating column is partially vaporized in the fourth heat exchanger and returned to the second fractionating column.