Patent Publication Number: US-2011067445-A1

Title: Method And Apparatus For Separating Air By Cryogenic Distillation

Description:
The present invention relates to a method and to an apparatus for producing oxygen by separating air using cryogenic distillation. 
     One object of the invention is to reduce the specific energy of separation of low-pressure low-purity oxygen, particularly in schemes where the pressurized nitrogen is not realized as an asset in its own right by the end-customer. 
     The object of the invention is achieved via the use of a scheme employing three vaporizers in the low-pressure column, in which:
         the bottom vaporizer operates on cold compressed medium-pressure nitrogen   the upper intermediate vaporizer operates on medium-pressure nitrogen   the lower intermediate vaporizer operates either on medium-pressure air or on medium-pressure nitrogen which is cold compressed.       

     Depending on variables, the specific energy saving represents between 0.5% and 7%. 
     U.S. Pat. No. 5,006,139 describes a method for producing nitrogen using a low-pressure column the bottom vaporizer of which is heated by a flow of medium-pressure nitrogen compressed in a cold compressor. According to the invention, there is also a proposal to use several turbines in order better to optimize the exchange line. 
     The method of the present invention produces gaseous oxygen and preferably does not produce nitrogen from the medium-pressure column. 
     One subject of the invention provides a method for producing oxygen by separating air using cryogenic distillation in an installation comprising a double column comprising a medium-pressure and a low-pressure column, the low-pressure column containing a bottom vaporizer, an intermediate vaporizer and an upper vaporizer, in which method:
         a) compressed air is purified in a purification unit, cooled in an exchange line and fed to the medium-pressure column of the double column   b) an oxygen-rich fluid is withdrawn from the low-pressure column, heated up and sent to the customer   c) nitrogen tapped off from the medium-pressure column is split into at least three parts   d) a first part of the nitrogen is expanded in a first turbine   e) a second part of the nitrogen has its pressure raised in a cold compressor and is sent to the bottom vaporizer, the nitrogen thus condensed being sent to at least one column of the double column   f) a third part of the nitrogen is sent to the upper vaporizer, with no pressure-modifying step downstream of the column from which it is tapped off and upstream of the upper vaporizer, the nitrogen thus condensed being sent to at least one column of the double column   g) a gaseous flow is sent to the intermediate vaporizer, this flow consisting of purified and cooled compressed air or of the nitrogen tapped off from the medium-pressure column and compressed in a cold compressor.       

     According to other optional aspects:
         the gaseous flow sent to the intermediate vaporizer, is nitrogen from the medium-pressure column   the gaseous flow sent to the intermediate vaporizer is purified and cooled compressed air   the first part of the nitrogen expanded in the first turbine is used for regenerating the purification unit   the first turbine drives the cold compressor in which the pressure of the second part of the fluid is raised and provides substantially all the cold energy for the method   the low-pressure column has no top condenser   all the gaseous nitrogen from the top of the medium-pressure column is split into just the three parts   all the gaseous nitrogen from the top of the medium-pressure column is split into just four parts, the fourth part being sent to the intermediate vaporizer   substantially all the cold energy is produced by expanding nitrogen from the medium-pressure column in at least one turbine   a liquid oxygen flow from the low-pressure column is vaporized, possibly after pressurizing it, to produce the fluid sent to the customer.       

     Another aspect of the invention provides an apparatus for producing oxygen by separation of air using cryogenic distillation comprising a double column comprising a medium-pressure column and a low-pressure column, the low-pressure column containing a bottom vaporizer, an intermediate vaporizer and an upper vaporizer, a purification unit, an exchange line, at least one first turbine, at least one cold compressor, means for sending compressed air, purified in the purification unit and cooled in the exchange line, to the medium-pressure column of the double column, means for withdrawing an oxygen-rich fluid from the low-pressure column, means for possibly heating it up, said means consisting at least in part of the exchange line and means for sending the heated-up oxygen-rich fluid sent to the customer, means for splitting the nitrogen tapped off from the medium-pressure column into at least three parts, means for sending a first part of the nitrogen to a first turbine, means for sending a second part of the nitrogen to a cold compressor where its pressure can be raised, means for sending the raised-pressure second part to the bottom vaporizer, means for sending the nitrogen thus condensed to at least one column of the double column, means for sending a third part of the nitrogen to the upper vaporizer, without a pressure modifying means downstream of the column from which it was tapped off and upstream of the upper vaporizer, means for sending the nitrogen thus condensed being sent to at least one column of the double column, means for sending a gaseous flow to the intermediate vaporizer, this flow consisting of purified and cooled compressed air or of nitrogen tapped off from the medium-pressure column and compressed in the second cold compressor. 
     The apparatus may possibly comprise:
         means for connecting the intermediate vaporizer to the top of the medium-pressure column   means for connecting the outlet of the first turbine to the purification unit   the first turbine is coupled to the cold compressor in which the pressure of the second part of the fluid is raised and constitutes the only turbine in the apparatus   the low-pressure column has no top condenser   means for vaporizing liquid oxygen, possibly in the exchange line, and possibly means for pressurizing liquid oxygen upstream of the means for vaporizing the liquid oxygen.       

    
    
     The invention will be described in greater detail with reference to  FIGS. 1 ,  2  and  3  which are schematic diagrams of air separation apparatuses according to the invention. 
       FIG. 1  shows an air separation apparatus in which a pressurized and purified air flow  1  is split into three flows  3 ,  5 ,  7 . The flow  3  is cooled as it passes from one end of the exchange line  11  to the other and is sent to the medium-pressure column  15  of a double column. The flow  5  is cooled as it passes from one end of the exchange line  11  to the other and is sent to an intermediate vaporizer  21  of the low-pressure column  17  of the double column  15 ,  17 . The other air flow  7  has its pressure raised in a hot pressurizer  9 , is cooled by passing from one end of the exchange line  11  to the other and is sent to a vaporizer  13  where it condenses at least partially by exchanging heat with pressurized liquid oxygen. The condensed air is either sent in its entirety to the medium-pressure column or split between the medium-pressure column and the low-pressure column  17 . 
     Rich liquid  25 , lean liquid  61  and possibly a liquid  27  somewhat like liquid air are sent from the medium-pressure column  15  to the low-pressure column  17  as reflux flows after supercooling in the exchanger  29 . 
     A flow of liquid oxygen  33  is tapped off from the low-pressure column, pressurized by the pump  35  and vaporized in the vaporizer  13  upstream of the exchange line  11 . The fluid  33  may also be compressed by the effect of a hydrostatic head, without the pump  35 . 
     Low-pressure nitrogen  31  is tapped off from the top of the low-pressure column  17  and is heated up in the exchangers  29 ,  11 . 
     A medium-pressure gaseous flow of nitrogen  39  is tapped off from the top of the medium-pressure column  15  and split into two. One part  53  is sent to an upper vaporizer  23  of the low-pressure column  17  where it condenses before being returned to the medium-pressure column by way of reflux. The remainder of the nitrogen  41  is split into two, one portion  43  being sent to a cold compressor  51  to form a flow  55 , and this flow  55  is sent to the bottom vaporizer  19  of the low-pressure column  17 . In this vaporizer  19  it condenses and then acts as reflux for at least one of the columns. 
     The remainder  45  of the nitrogen is sent to the exchange line, is heated up to an intermediate level, and is sent to a turbine  47 . The nitrogen expanded in the turbine  47  is sent to the cold end of the exchange line and heats up, becoming the flow  49 . 
     This yields a potential energy saving of 0.5% over the scheme disclosed in WO-A-2007/129152. 
       FIG. 2  shows an air separation apparatus in which a pressurized and purified air flow  1  is split into two flows  3 , 7 . The flow  3  is cooled as it passes from one end of the exchange line  11  to the other and is sent to the medium-pressure column  15  of a double column. The other air flow  7  has its pressure raised in a hot pressurizer  9 , is cooled as it passes from one end of the exchange line  11  to the other and is sent to a vaporizer  13  where it condenses at least in part by exchange of heat with pressurized liquid oxygen. The condensed air is either sent in its entirety to the medium-pressure column or split between the medium-pressure column and the low-pressure column  17 . 
     Rich liquid  25 , lean liquid  61  and possibly a liquid  27  somewhat like liquid air are sent from the medium-pressure column  15  to the low-pressure column  17  as reflux flows after supercooling in the exchanger  29 . 
     A flow of liquid oxygen  33  is tapped off from the low-pressure column, pressurized by the pump  35  and vaporized in the vaporizer  13  upstream of the exchange line  11 . The fluid  33  may also be compressed using a hydrostatic head, without the pump  35 . 
     Low-pressure nitrogen  31  is tapped off from the top of the low-pressure column  17  and heats up in the exchangers  29 ,  11 . 
     A medium-pressure gaseous flow of nitrogen  39  is tapped off from the top of the medium-pressure column  15  and split into two. One part  53  is sent to an upper vaporizer  23  of the low-pressure column  17  where it condenses before being returned to the medium-pressure column by way of reflux. The remainder of the air is once again split into two. One fraction is sent to the cold compressor  151  to become the flow  155  which heats the intermediate vaporizer  21  before being sent to the columns by way of reflux. The remainder of the nitrogen  41  is split into two, one portion  43  being sent to a cold compressor  51  to form a flow  55 , and this flow  55  is sent to the bottom vaporizer  19  of the low-pressure column  17 . In this vaporizer  19  it condenses and then serves as reflux for at least one of the columns. 
     The remainder  45  of the nitrogen is sent to the exchange line, is heated up to an intermediate level and is sent to a turbine  47 . The nitrogen expanded in the turbine  47  is sent to the cold end of the exchange line and heats up, becoming the flow  49 . Having the two cold compressors  51 ,  151  on the medium-pressure nitrogen line allows the distribution of power across the compressors to be adjusted to best suit the low-pressure column reboiling requirement. 
     An energy saving of 1.7% over the scheme of WO-A-2007/129152 can be achieved. 
       FIG. 3  shows an air separation apparatus in which an air flow  1 , pressurized by a compressor M and purified in a purification unit  2  is split into two flows  3 ,  7 . The flow  3  is cooled as it passes from one end of the exchange line  11  to the other and is sent to the medium-pressure column  15  of a double column. The other  7  has its pressure raised in a hot pressurizer  9 , is cooled by passing from one end of the exchange line  11  to the other and is sent to a vaporizer  13  where it condenses at least in part by exchange of heat with pressurized liquid oxygen. The condensed air is either sent in its entirety to the medium-pressure column or split between the medium-pressure column and the low-pressure column  17 . 
     Rich liquid  25 , lean liquid  61 , and possibly a liquid  27  somewhat like liquid air are sent from the medium-pressure column  15  to the low-pressure column  17  by way of reflux flows following supercooling in the exchanger  29 . 
     A flow of liquid oxygen  33  is tapped off from the low-pressure column, pressurized by the pump  35  and vaporized in the vaporizer  13  upstream of the exchange line  11 . The fluid  33  may also be compressed using a hydrostatic head, without the pump  35 . 
     Low-pressure nitrogen  31  is tapped off from the top of the low-pressure column  17  and is heated up in the exchangers  29 ,  11 . 
     A medium-pressure gaseous flow of nitrogen  39  is tapped off from the top of the medium-pressure column  15  and split into two. One part  53  is sent to an upper vaporizer  23  of the low-pressure column  17  where it condenses before being returned to the medium-pressure column by way of reflux. The remainder of the air is once again split into two. One fraction is sent to the cold compressor  151  where it becomes the flow  155 , the flow  155  being cooled in the exchange line  11  before being used to heat the intermediate vaporizer  21  before being sent to the columns by way of reflux. The remainder of the nitrogen  41  is split into two, one portion  43  being sent to a cold compressor  51  to form a flow  55 , and this flow  55  is sent to the bottom vaporizer  19  of the low-pressure column  17  after having been cooled in the exchange line  11 . In this vaporizer  19  it condenses and then serves as reflux for at least one of the columns. 
     The remainder  45  of the nitrogen is sent to the exchange line, heats up to an intermediate level and is split into two. One part  49  of the nitrogen  45  is sent to a turbine  47 . The nitrogen expanded in the turbine  47  is sent to the cold end of the exchange line and is heated up before being used periodically to regenerate the purification unit  2 . The remainder of the nitrogen  46  continues to be heated up in the exchange line  11  and is split into two, one part  149  being sent to a turbine  147  at a higher inlet temperature than the turbine  47 . This part of the nitrogen is expanded, heated up and discharged into the atmosphere. The remainder  249  of the nitrogen is sent to a turbine  247  at a higher inlet temperature than the turbines  47 ,  147 . This part  249  of the nitrogen is expanded, heated up and discharged into the atmosphere. 
     An energy saving of 7% over the scheme of WO-A-2007/129152 can be achieved.