Method and apparatus for the cryogenic separation of a synthesis gas containing a nitrogen separation step

Abstract

A method for separating a gas mixture comprising carbon monoxide, nitrogen and hydrogen involves sending a hydrogen-depleted fluid to a denitrification column (K2) having a top condenser (C1) and a bottom reboiler (R2) in order to produce a nitrogen-enriched gas at the top of the column and a nitrogen-depleted liquid at the bottom of the column, cooling the condenser of the denitrification column by means of a nitrogen cycle using a nitrogen compressor (V1, V2, V3), vaporising, in the heat exchanger of the condenser, the liquid nitrogen (53) from the nitrogen cycle, and returning the nitrogen (55) vaporised in the heat exchanger to the nitrogen compressor.

1 .- 13 . (canceled) 
     
     
         14 . A process for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen, and methane, the method comprising:
 i) cooling the gas mixture in a heat exchanger,   ii) separating the gas mixture cooled in the heat exchanger by at least one scrubbing and/or distillation and/or partial condensation step, to form a hydrogen-depleted fluid containing carbon monoxide and nitrogen,   iii) introducing the hydrogen-depleted fluid into a denitrification column comprising a column top, a top condenser, a column bottom, and a bottom reboiler, thereby producing a nitrogen-enriched gas at the column top and a nitrogen-depleted liquid at the column bottom,   iv) cooling the top condenser by means of a nitrogen cycle using a nitrogen compressor comprising at least a first stage and a second stage, the first stage comprising a first entry pressure and the second stage comprising a second entry pressure, the first entry pressure being lower than that of the second entry pressure,   v) expanding the nitrogen-depleted liquid and sending the expanded nitrogen-depleted liquid to the top condenser wherein it is at least partially vaporized by heat exchange in the condenser with the nitrogen-enriched gas, thereby condensing the expanded nitrogen-depleted liquid,   vi) vaporizing a liquid nitrogen from the nitrogen cycle in the condenser and returning the vaporized nitrogen to the heat exchanger at an inlet of the second stage of the nitrogen compressor, and
 a) sending the bottom liquid from the denitrification column to a methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid, or 
 b) the separation in step ii) comprises a distillation step in a methane and carbon monoxide separation column for separating a methane-depleted flow from a methane-enriched flow, and at least a portion of the methane-depleted flow constitutes the hydrogen-depleted fluid supplying the denitrification column, the methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid, 
   the bath of liquid of a) or b) being supplied with liquid nitrogen from the nitrogen cycle.   
     
     
         15 . The process of  claim 14 , wherein liquid nitrogen from the top condenser of the methane and carbon monoxide separation column is sent to vaporize in the top condenser of the denitrification column. 
     
     
         16 . The process of  claim 14 , wherein the gas mixture cooled in the heat exchanger is separated by at least one partial condensation step so as to form a hydrogen-depleted gas, the hydrogen-depleted gas is sent to an intermediate level of a stripping column comprising a bottom reboiler, and bottom liquid of the stripping column is sent to a denitrification column in case a) or to the methane and carbon monoxide separation column in case b). 
     
     
         17 . The process of  claim 14 , wherein the reboiler of the stripping column and/or the reboiler of the methane and carbon monoxide separation column is reheated with at least a portion of the gas mixture. 
     
     
         18 . The process of  claim 14 , wherein the operating pressure of the denitrification column is at least 7 bar abs and/or the operating pressure of the methane and carbon monoxide separation column is at least 5 bar abs. 
     
     
         19 . The process of  claim 14 , wherein the top condenser of the CO/CH 4  column is cooled solely by cycle nitrogen. 
     
     
         20 . The process of  claim 14 , wherein the reboiler of the denitrification column is reheated by means of the cycle nitrogen. 
     
     
         21 . The process of  claim 20 , wherein the nitrogen used for reheating the reboiler of the denitrification column is at the maximum pressure of the nitrogen cycle. 
     
     
         22 . The process of  claim 14 , wherein the nitrogen sent to the bath of the condenser of the column CO/CH 4  is condensed at the maximum pressure of the nitrogen cycle. 
     
     
         23 . The process of  claim 14 , wherein the operating pressure of the methane and carbon monoxide separation column is at least 5 bar. 
     
     
         24 . An apparatus for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen and methane, comprising a heat exchanger for cooling the gas mixture, means for separating the gas mixture cooled in the heat exchanger by at least one scrubbing and/or distillation and/or partial condensation step, to form a hydrogen-depleted fluid containing carbon monoxide and nitrogen, a denitrification column comprising a top condenser and a bottom reboiler, a pipe for sending the hydrogen-depleted fluid to the denitrification column, to produce a nitrogen-enriched gas at the column top and a nitrogen-depleted liquid at the column bottom, a nitrogen cycle using a nitrogen compressor comprising at least a first stage and a second stage, the entry pressure of the first stage being lower than that of the second stage, means for sending liquid of the nitrogen cycle to the condenser of the denitrification column, means for expanding bottom liquid from the denitrification column, means for sending the expanded liquid to the top condenser of the denitrification column for at least partial vaporization by heat exchanger in a heat exchanger of the condenser with the nitrogen-enriched gas, which is thereby condensed, means for sending nitrogen vaporized in the heat exchanger of the condenser to the inlet of the second stage of the nitrogen compressor, a methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid,
 a) means for sending the bottom liquid from the denitrification column to the methane and carbon monoxide separation column, or   b) the methane and carbon monoxide separation column, means forming part of the means for separating the gas mixture cooled in the heat exchanger by at least one distillation step,   the apparatus further comprising means for sending liquid nitrogen from the nitrogen cycle to the top condenser of the methane and carbon monoxide separation column.   
     
     
         25 . The apparatus of  claim 24 , comprising means for sending liquid nitrogen from the top condenser of the methane and carbon monoxide separation column to the top condenser of the denitrification column. 
     
     
         26 . The apparatus of  claim 24 , further comprising at least one phase separator for separating the gas mixture cooled in the heat exchanger by a partial condensation step, to form a hydrogen-depleted gas, a stripping column, and means for sending the hydrogen-depleted gas to an intermediate level of the stripping column.