Liquid nitrogen generator and process

Abstract

A process for producing liquid nitrogen utilizes a distillation system comprising higher-pressure and lower-pressure columns, a first condenser in the lower-pressure column bottom, and a second condenser disposed on top of the lower-pressure column. Feed air is compressed in a main air compressor above 15 bar(a) and cooled in a main heat exchanger. A first cooled air portion is expanded in a cold turbine before separation in the higher-pressure column. A second cooled air portion is liquefied before entering the higher-pressure column. Liquid nitrogen streams condensed by both the first and second condensers are combined to yield the final product. Another feature involves warming and expanding a lost air stream (e.g., from the cold turbine outlet and/or higher-pressure column) to below 2 bar(a) in a warm turbine.

We claim: 
     
         1 . A process for production of liquid nitrogen from a system of columns comprised of a higher-pressure column, a lower-pressure column, a first condenser disposed in a bottom portion of the lower-pressure column, and a second condenser, the process comprising the steps of:
 purifying a feed air in a purification unit at a pressure greater than 15 bar(a) to yield a compressed dry air feed;   cooling the compressed dry air feed in a heat exchanger;   expanding a first portion of the compressed dry air feed in a cold turbine to form an expanded fluid;   sending at least a portion of the expanded fluid to the higher-pressure column for separation therein;   liquefying a second portion of the compressed dry air feed within the heat exchanger before sending at least a portion of the second portion of the compressed dry air feed to the higher-pressure column;   sending an oxygen-rich liquid from a bottom portion of the higher-pressure column to an intermediate portion of the lower-pressure column; and   sending a second oxygen-rich liquid from a bottom portion of the lower-pressure column to the second condenser, wherein at least a portion of the second oxygen-rich liquid is vaporized at less than 2 bar(a) to form a top gas, and wherein at least a portion of the top gas is warmed in the heat exchanger against the compressed dry air feed,   mixing a first liquid nitrogen stream that was condensed by the first condenser with a second liquid nitrogen that was condensed by the second condenser to yield a liquid nitrogen product,   wherein a lost air stream is warmed and expanded to less than 2 bar(a) in a warm turbine, wherein the lost air stream is selected from the group consisting of a portion of the expanded fluid from the cold turbine, a stream withdrawn from the higher-pressure column, and combinations thereof.   
     
     
         2 . The process as claimed in  claim 1 , wherein the warm turbine has an inlet pressure substantially similar to an outlet pressure of the cold turbine. 
     
     
         3 . The process as claimed in  claim 1 , wherein the higher-pressure column operates at a first pressure that is greater than 6.5 bar(a), wherein the second condenser operates at a third pressure that is less than 2 bar(a), wherein the lower-pressure column operates at a second pressure that is between the first pressure and the third pressure. 
     
     
         4 . The process as claimed in  claim 1 , wherein the warm turbine has an inlet temperature and an outlet temperature and the cold turbine has an inlet temperature and an outlet temperature, wherein a temperature difference of the inlet and outlet temperatures of the warm turbine is greater than a temperature difference of the inlet and outlet temperatures of the cold turbine. 
     
     
         5 . The process as claimed in  claim 1 , wherein the cold turbine drives a first compressor that is configured to further pressurize the compressed dry air feed prior to cooling in the heat exchanger. 
     
     
         6 . The process as claimed in  claim 2 , wherein the warm turbine drives a second compressor that is configured to further pressurize the compressed dry air feed prior to cooling in the heat exchanger. 
     
     
         7 . The process as claimed in  claim 3 , wherein the first and second compressors are arranged in parallel. 
     
     
         8 . The process as claimed in  claim 3 , wherein the first and second compressors are arranged in series. 
     
     
         9 . The process as claimed in  claim 3 , wherein the compressed dry air feed exiting at least one of the first and second compressors is at a pressure above 50 bar(a) prior to cooling in the heat exchanger. 
     
     
         10 . The process as claimed in  claim 1 , wherein a main air compressor is disposed upstream of the purification unit and is configured to produce the feed air purified in the purification unit, wherein the main air compressor is the only compression means used in the process that relies on an external energy source for compression. 
     
     
         11 . The process as claimed in  claim 7 , wherein the external energy source includes an electrical motor, steam turbine, and/or gas turbine. 
     
     
         12 . The process as claimed in  claim 1 , wherein a ratio of fluid expanded in the warm turbine to compressed dry air feed is between 0.4 and 0.6, preferably between 0.45 and 0.5, and recovery of nitrogen as liquid nitrogen product as compared to total nitrogen sent to the higher-pressure column is between 0.55 and 0.65. 
     
     
         13 . The process as claimed in  claim 1 , wherein a pressure ratio of the warm turbine is greater than 5.2 and less than 8, and wherein a ratio of a flow rate of the warm turbine to a flow rate of the cold turbine is greater than 0.6.