Patent Publication Number: US-2007122272-A1

Title: Gas compressor, unit for separating a gas mixture incorporating such a compressor, and method of separating a gas mixture incorporating such a compressor

Description:
The present invention relates to a gas compressor and to a unit for separating a gas mixture incorporating such a compressor.  
      In a conventional multistage compressor, the gas compressed in one stage is cooled in an inter-stage cooler or intercooler before being sent to the next stage, so that the gas remains at an acceptable temperature for the next compression stage. The gas compressed in the final stage is also cooled downstream of this stage. The gas cooled by the cooler undergoes a pressure drop. According to the prior art, the same pressure drops undergone by the compressed gas are the same for each cooler, whatever the pressure of the compressed gas that they cool.  
      The term “compressor” includes blowers, boosters and compressors followed by boosters forming a single machine. The compressors in question may be centrifugal, axial, radial or reciprocating compressors or combinations of these types of compressors. The compressors may have intermediate inlets and/or outlets.  
      The air passes into the compressor from the upstream into the downstream end. One stage of a compressor is upstream of another stage if the air passes into this stage before passing into the other stage.  
      All the pressures are absolute pressures.  
      In so far as the same pressure drops cost only little energy at high pressure compared with at medium or low pressure, instead of having a compressor having intermediate coolers with the same pressure drops at each of the stages, using higher gas pressure drops on the stages compressing the gas to the highest pressure it is possible to save on the investment cost of the compressor.  
      One object of the invention is to provide a gas compressor having n stages connected in series, where n is equal to at least 3, each stage being followed by a cooler characterized in that at least two coolers have different pressure drops for the compressed gas, the cooler having the lower pressure drop being upstream of that having the higher pressure drop.  
      According to other optional aspects of the invention: 
          the cooler of the final stage of the compressor has a higher pressure drop than that of the first stage;     the compressor comprises at least four stages, in which the final stages of the compressor have a higher pressure drop than the first ones;     at least two coolers have pressure drops differing by at least 30%, or at least 50% or even at least 100%, the cooler having the lower pressure drop being upstream of that having the higher pressure drop;     at least two coolers have pressure drops different by at least 100%, the cooler having the lower pressure drop being upstream of that having the higher pressure drop.        

      Another aspect of the invention provides for a method of compressing a gas in a gas compressor having n stages connected in series, where n is equal to at least 3, each stage being followed by a cooler characterized in that at least two coolers have different pressure drops for the compressed gas, the cooler having the lower pressure drop being upstream of that having the higher pressure drop.  
      Another aspect of the invention provides for a unit for separating a gas mixture, which includes at least one compressor as defined above and means for sending a gas coming from and/or intended for the unit to this compressor.  
      Preferably, an air separation unit as defined above is provided, comprising a cryogenic distillation unit comprising at least one distillation column, means for sending compressed air to a column of the unit, means for withdrawing a liquid from a column of the unit, means for vaporizing the liquid by heat exchange with a compressed gas, the compressed gas having been compressed by at least one of the last stages (or by the last stage) of the compressor and/or the compressed air having been compressed in the compressor.  
      According to other optional aspects, the unit includes means for vaporizing the liquid by heat exchange with air coming from one of the final stages (or from the final stage) of the compressor.  
      Another aspect of the invention provides for a method of separating a gas mixture by cryogenic distillation in a system of columns in which a gas intended for the system of columns or the gas coming from the system of columns is compressed in a compressor as defined above, the gas leaving the final stage of the compressor being at a pressure above 5 bar, preferably above 10 bar.  
      Another optional aspect provides for a method in which: 
          i) a stream of air is compressed to a first pressure;     ii) one portion of the air at the first pressure is boosted to a second pressure of greater than 10 bar;     iii) one portion of the air at the first pressure is sent for distillation in one column of the system of columns;     iv) a liquid stream is withdrawn from one column of the system;     v) the liquid stream is vaporized by heat exchange with air at the second pressure, characterized in that     vi) the stream of air at the first pressure is compressed and/or a portion of the air is boosted up to the second pressure in at least one compressor as defined above.       

    
    
      The invention will be described in greater detail with reference to the figures. The figures represent very simplified separation units incorporating at least one compressor according to the invention.  
      In  FIG. 1 , a stream of air  3  at atmospheric pressure is sent to a compressor  1 . 
    
    
      The compressor consists of stages C 1 , C 2 , C 3 , C 4  and C 5  and their respective coolers. The air is: 
          compressed in the first stage C 1 , cooled by the intermediate cooler R 1 ;     compressed in the second stage C 2 , cooled by the intermediate cooler R 2 ;     compressed in the third stage C 3 , cooled by the intermediate cooler R 3 ;     compressed in the fourth stage C 4 , cooled by the intermediate cooler R 4 ; and     compressed in the fifth stage C 5  and cooled by the final cooler R 5  before leaving at a pressure of between 20 and 40 bar.        

      The pressure drops of the compressed air for the intermediate coolers R 1 , R 2  and R 3  are substantially identical. The pressure drop for the intermediate cooler R 4  is at least 30%, preferably at least 50% or even at least 100% higher than that of the preceding coolers R 1 , R 2  and R 3 . The pressure drop for the final cooler R 5  is at least 30%, preferably at least 50% or even at least 100% higher than that of the cooler R 4 .  
      As a variant, the cooler R 4  may have the same pressure drop as the coolers R 1 , R 2  and R 3 .  
      Again as a variant, the pressure drop for the cooler R 3  may be higher than that of the coolers R 1  and R 2  by at least 30%, preferably at least 50% or even 100% and the pressure drop for the cooler R 4  may be substantially equal to that of R 3  or higher than the latter by at least 30%, preferably at least 50% or even at least 100%.  
      The air cooled in the cooler R 5  is sent to a purification step  5  and then to an air separation unit  7  operating by cryogenic distillation or another technique, in order to produce a product  9  that may be oxygen and/or nitrogen and/or argon.  
      In  FIG. 2 , a stream of air  3  at atmospheric pressure is sent to a compressor  1 .  
      The compressor consists of the stages C 1 , C 2 , C 3 , C 4  and C 5  and their respective coolers. The air is: 
          compressed in the first stage C 1 , cooled by the intermediate cooler R 1 ;     compressed in the second stage C 2 , cooled by the intermediate cooler R 2 ;     compressed in the third stage C 3 , cooled by the intermediate cooler R 3  and then sent to a purification step. A purified stream  6  leaves the purification step before being sent to the air separation unit  7  or elsewhere. The rest of the purified air  8  is     compressed in the fourth stage C 4 , cooled by the intermediate cooler R 4 ; and     compressed in the fifth stage C 5  and cooled by the final cooler R 5  before leaving at a pressure of between 20 and 40 bar. This gas may then serve to vaporize pumped liquid under pressure in an exchanger of the air separation unit  7 .        

      The pressure drops on the air for the intermediate coolers R 1 , R 2  and R 3  are substantially identical. The pressure drop for the intermediate cooler R 4  is at least 30%, preferably at least 50% or even 100% higher than that of the preceding coolers R 1 , R 2  and R 3 . The pressure drop for the final cooler R 5  is at least 30%, preferably at least 50% or even at least 100% higher than that of the cooler R 4 .  
      As a variant, the cooler R 4  may have the same pressure drop as the coolers R 1 , R 2  and R 3 .  
      Again as a variant, the pressure drop for the cooler R 3  may be higher than that of the coolers R 1  and R 2  by at least 30%, preferably at least 50% or even 100% and the pressure drop for the cooler R 4  may be substantially equal to that of R 3  or higher than the latter by at least 30%, preferably at least 50% or even at least 100%.  
      In this example, the stages C 1 , C 2  and C 3  compress all the air to an intermediate pressure and only one portion of the air is compressed to the maximum pressure in the stages C 4 , C 5  that form a booster. All the stages C 1 , C 2 , C 3 , C 4  and C 5  are on the same shaft and form part of the compressor  1 .  
      The air  8  cooled in the final cooler R 5  is sent to the air separation unit.  
      In  FIG. 3 , air  3  is compressed in a compressor  1 , which may be that described in  FIG. 1  or  FIG. 2 , and the compressed air is purified and sent to the air separation unit  7 . Gaseous nitrogen  9  is withdrawn from the unit  7  and sent to a compressor  11 , consisting of three stages CA 1 , CA 2  and CA 3 . The pressure of the nitrogen is above atmospheric pressure, preferably between 1.5 and 10 bar.  
      The nitrogen is: 
          compressed in the first stage CA 1 , cooled by the intermediate cooler RA 1 ;     compressed in the second stage CA 2 , cooled by the intermediate cooler RA 2 ; and     compressed in the third stage CA 3  and cooled by the final cooler RA 3 .        

      The pressure drop on the nitrogen of the final cooler RA 3  is at least 30%, preferably at least 50% or even at least 100% higher than that of the cooler RA 2  and of the cooler RA 1 .  
      The invention applies in particular to separation by cryogenic distillation, but it can be used in separation units operating at higher temperatures. The gas mixture to be separated described in the examples is air, but it may for example consist of carbon monoxide and/or hydrogen and/or methane and/or nitrogen and/or helium as main components.  
      The compressor may be a compressor for air, nitrogen, oxygen, argon, a synthesis gas, hydrogen, carbon monoxide, helium, methane or any other gas.