Method And Apparatus For Obtaining A Compressed Nitrogen Product

Abstract

A method and apparatus to obtain a compressed nitrogen product by low-temperature fractionation of air in a distillation column system. The system has a high-pressure column, a low-pressure column, a main condenser, and a low-pressure column top condenser. Bottoms liquid from the low-pressure column is evaporated in the top condenser and the gas formed is decompressed to perform work that drives a cold compressor. A gaseous first compressed nitrogen product stream from the high-pressure column is warmed in the main heat exchanger. A further gaseous nitrogen stream from the low-pressure column is compressed in the cold compressor and warmed as a second compressed nitrogen product stream in the main heat exchanger. The cold compressor overcomes a pressure differential which is at least equal to two thirds of the pressure differential between the top of the high-pressure column and the top of the low-pressure column.

1 . A method for obtaining a compressed nitrogen product by low-temperature fractionation of air in a distillation column system having a high-pressure column and a low-pressure column and also a main condenser and a low-pressure-column top condenser which are both designed as condenser-evaporators, wherein
 compressed, pre-cooled and cleaned feed air is cooled in a main heat exchanger and is at least in part introduced into the high-pressure column,   top gas of the high-pressure column is introduced into the liquefaction space of the main condenser and at least part of the liquid nitrogen formed in the liquefaction space of the main condenser is introduced into the high-pressure column,   top gas of the low-pressure column is introduced into the liquefaction space of the low-pressure-column top condenser and at least part of the liquid nitrogen formed in the liquefaction space of the low-pressure-column top condenser is introduced into the low-pressure column,   bottoms liquid of the low-pressure column is introduced into the evaporation space of the low-pressure-column top condenser,   gas formed in the evaporation space of the low-pressure-column top condenser is warmed as tail gas (in the main heat exchanger to an intermediate temperature, and at least a first part thereof is expanded in a work-performing manner in a first tail-gas turbine, reintroduced into the main heat exchanger, and warmed as far as the warm end of the main heat exchanger,   a nitrogen stream is drawn off in gaseous form from the top of the low-pressure column, and   a first compressed nitrogen product stream is drawn off in gaseous form from the top of the high-pressure column and is warmed in the main heat exchanger,   
       characterized in that
 the mechanical energy produced in the first tail-gas turbine is at least in part used for driving a cold compressor, 
 the nitrogen stream which has been drawn off in gaseous form from the top of the low-pressure column is compressed in the cold compressor to a pressure which is at least equal to the pressure of the first compressed nitrogen product stream, when the latter is drawn off from the high-pressure column minus 2 bar, and is subsequently warmed as a second compressed nitrogen product stream in the main heat exchanger, and 
 the nitrogen stream which has been drawn off in gaseous form from the top of the low-pressure column is compressed in the cold compressor to a pressure which is at least equal to the pressure of the first compressed nitrogen product stream when the latter is drawn off from the high-pressure column minus 2 bar, and is subsequently warmed as a second compressed nitrogen product stream in the main heat exchanger, wherein 
 the cold compressor overcomes a pressure differential which is at least equal to two thirds of the pressure differential between the top of the high-pressure column and the top of the low-pressure column. 
 
     
     
         2 . The method as claimed in  claim 1 , characterized in that the first compressed nitrogen product stream and the second compressed nitrogen product stream are mixed upstream of the main heat exchanger. 
     
     
         3 . The method as claimed in  claim 1 , characterized in that the first tail-gas turbine is mechanically coupled to the cold compressor via a common shaft or a gear mechanism. 
     
     
         4 . The method as claimed in  claim 3 , characterized in that the first tail-gas turbine is also mechanically coupled to an electrical generator or to an oil brake. 
     
     
         5 . The method as claimed in  claim 1 , characterized in that the first tail-gas turbine is mechanically coupled to an electrical generator, the cold compressor is driven by an electric motor, and the energy produced in the generator is at least partially electrically transferred to the motor. 
     
     
         6 . The method as claimed in  claim 1 , characterized in that a second part of the tail gas (warmed to the intermediate temperature is expanded in a work-performing manner in a second tail-gas turbine which is connected in parallel with the first tail-gas turbine. 
     
     
         7 . The method as claimed in  claim 6 , characterized in that the first tail-gas turbine is mechanically coupled to the cold compressor and the second tail-gas turbine is mechanically coupled to a generator or to a dissipative brake. 
     
     
         8 . The method as claimed in  claim 1 , characterized in that the first, the second or both compressed nitrogen streams are further compressed downstream of the main heat exchanger in a nitrogen compressor. 
     
     
         9 . The method as claimed in  claim 8 , characterized in that the feed air is compressed in a main air compressor which is formed by the first i stages of a combined n-stage compressor, where n≧2, i<n, and in that the nitrogen compressor is formed by the n−i last stages of the combined n-stage compressor. 
     
     
         10 . The method as claimed in  claim 1 , characterized in that the nitrogen stream which has been drawn off in gaseous form from the top of the low-pressure column is compressed in the cold compressor to a pressure which is at least equal to the pressure of the first compressed nitrogen product stream when the latter is drawn off from the high-pressure column. 
     
     
         11 . The method as claimed in  claim 1 , characterized in that the first compressed nitrogen product stream and the second compressed nitrogen product stream are warmed in separate passages and are in particular merged afterwards. 
     
     
         12 . The method as claimed  claim 1 , characterized in that at least one, more than one or all of the following measures are applied:
 design of the main condenser as a forced-flow evaporator,   design of the main condenser as a falling-film evaporator,   design of the low-pressure-column top condenser as a forced-flow evaporator.   
     
     
         13 . The method as claimed in  claim 1 , characterized in that
 the low-pressure column is arranged next to the high-pressure column,   the main condenser is arranged above the high-pressure column, and   the low-pressure-column top condenser is arranged above the low-pressure column.   
     
     
         14 . An apparatus for obtaining a compressed nitrogen product by low-temperature fractionation of air, comprising
 a distillation column system having a high-pressure column and a low-pressure column and also a main condenser and a low-pressure-column top condenser which are both designed as condenser-evaporators, comprising   a main heat exchanger for cooling compressed, pre-cooled and cleaned feed air, wherein   the liquefaction space of the main condenser is flow-connected to the top of the high-pressure column,   the liquefaction space of the low-pressure-column top condenser is flow-connected to the top of the low-pressure column, and the evaporation space of the low-pressure-column top condenser is flow-connected to the bottom of the low-pressure column,   
       and comprising
 means for extracting gas formed in the evaporation space of the low-pressure-column top condenser as tail gas, 
 means for warming the tail gas in the main heat exchanger to an intermediate temperature, 
 a first tail-gas turbine for expanding the partially-warmed tail gas in a work-performing manner, 
 means for introducing the expanded tail gas into the main heat exchanger and means for extracting the warmed tail gas from the hot end of the warm heat exchanger, 
 means for using mechanical energy, produced in the first tail-gas turbine, for driving a cold compressor, and 
 a first compressed nitrogen product line for drawing off a gaseous first compressed nitrogen product stream from the top of the high-pressure column and for warming the first compressed nitrogen product stream in the main heat exchanger, 
 
       characterized by
 means for drawing off a gaseous nitrogen stream from the top of the low-pressure column, 
 means for introducing the gaseous nitrogen stream into the cold compressor, and 
 means for introducing the cold-compressed nitrogen stream, as the second compressed nitrogen product stream, into the main heat exchanger, 
 wherein the cold compressor is designed to overcome a pressure differential which is at least equal to two thirds of the pressure differential between the top of the high-pressure column and the top of the low-pressure column.