Method and installation for separating air by cryogenic distillation

Abstract

An installation for separating air by cryogenic distillation comprises a compressor, a heat-exchange line, a system of columns, an electric motor for driving the compressor, a pipe for taking off a liquid from the system of columns, a pump for pressurising the liquid drawn off and means for affording an exchange of heat between the air compressed by the compressor and the liquid pressurised by the pump and means for reducing the output pressure of the pump according to the frequency of the electricity supplying the electric motor.

1 - 15 . (canceled) 
     
     
         16 . A method of separating air by cryogenic distillation, the method comprising the steps of:
 i) compressing a flow of air in a compressor, cooling said flow of air in a heat-exchange line, and then sending said flow of air to a system of columns where the flow of air separates in order to form a nitrogen-enriched flow and an oxygen-enriched flow;   ii) driving the compressor by a motor supplied by electricity having a first frequency;   iii) withdrawing a liquid flow from the system of columns, pressurizing said liquid flow to a first pressure by a pump and either vaporizing said liquid flow by indirect heat exchange with the flow of air coming from the compressor to produce a gaseous product substantially at the first pressure, or, in the case of an oxygen-rich liquid flow, sending the oxygen-rich liquid flow at the first pressure to the top of an auxiliary column supplied at the bottom by the flow of air coming from the compressor, withdrawing gaseous oxygen from the top of the auxiliary column as a product, and sending an intermediate liquid from the auxiliary column to the system of columns,   
       wherein if the frequency of the electricity passes below a given threshold below the first frequency, the pressurization pressure of the liquid flow is reduced to a second pressure lower than the first pressure. 
     
     
         17 . The method according to  claim 16 , in which the compressor compresses the flow of air from atmospheric pressure. 
     
     
         18 . The method according to  claim 16 , in which the compressor compresses the flow of air from a pressure above 2 bar. 
     
     
         19 . The method according to  claim 16 , in which the liquid flow vaporizes in the heat-exchange line. 
     
     
         20 . The method according to  claim 16 , in which the oxygen-rich liquid flow is sent at the first pressure to the top of the auxiliary column supplied in the bottom with the flow of air coming from the compressor, gaseous oxygen being drawn off at the top of the column as a product and at least one liquid being sent from the auxiliary column to the system of columns. 
     
     
         21 . The method according to  claim 16 , wherein the frequency is equal to the first frequency, the liquid flow has a flow rate V and if the frequency is below the first frequency the liquid flow rate is at least equal to 0.9 V, or even at least equal to 0.95 V or even equal to V. 
     
     
         22 . The method according to  claim 16 , wherein the pressurization pressure of the liquid is lowered if the air flow rate measured passes below a threshold with respect to the air flow rate required to achieve the nominal value of the liquid flow rate at the first pressurization pressure. 
     
     
         23 . The method according to  claim 16 , wherein the pressurization pressure of the liquid is lowered if the air pressure measured passes below a threshold with respect to the air pressure required to achieve the nominal value of the liquid flow rate at the first pressurization pressure. 
     
     
         24 . The method according to  claim 16 , wherein the pressurization pressure of the liquid is reduced only by modifying the operation of the pressurization pump. 
     
     
         25 . The method according to  claim 16 , wherein the output pressure of the pump is equal to the vaporization pressure of the liquid pressurization by the pump. 
     
     
         26 . The method according to  claim 16 , wherein the vaporised liquid serves as a product at the vaporization pressure, without undergoing any pressure reduction downstream of the vaporization. 
     
     
         27 . An installation for separating air by cryogenic distillation, the installation comprising a compressor, a heat-exchange line, a system of columns, an electric motor configured to drive the compressor, a pipe configured to draw off liquid from the system of columns, a pump configured to pressurize the liquid drawn off and means for affording a direct or indirect exchange of heat between the air compressed by the compressor and the pressurized liquid, and means for regulating the pressure of the liquid pressurized in the pump as a function of the frequency of the electricity supplying the electric motor. 
     
     
         28 . The installation according to  claim 27 , comprising a means for regulating the pressure of the pressurized liquid by action of the control system on the flow rate and/or the output pressure of the pump. 
     
     
         29 . The installation according to  claim 27 , comprising a means for measuring the frequency supplying the electric motor, the means for regulating the pressurization pressure of the liquid being capable of being started up if the frequency goes below a threshold. 
     
     
         30 . The installation according to  claim 27 , comprising a supercharger and means for sending air from the compressor to the supercharger and from the supercharger to the heat-exchange line, the supercharger being driven by the or a motor supplied by electricity having a or the frequency. 
     
     
         31 . The installation according to  claim 27 , further comprising means for affording an exchange of mass if the pressurized liquid is rich in oxygen.