Abstract:
The invention relates to a method for separating air by cryogenic distillation in a set of columns including a first column operating at a first pressure, a second column operating at a second pressure which is lower than the first pressure, and a third column operating at a third pressure, which is lower than the second pressure, wherein the third column includes first and second evaporator-condensers, and nitrogen from a cold compressor is sent to one of the evaporator-condensers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a §371 of International PCT Application PCT/FR2014/052607, filed Oct. 14, 2014, which claims the benefit of FR1360002, filed Oct. 15, 2013, both of which are herein incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a process and unit for separating air by cryogenic distillation. 
       BACKGROUND OF THE INVENTION 
       [0003]    Within the context of supplying oxygen to gasification processes, the consumption of impure oxygen (typically of the order of 95 mol %) which is pressurized (typically 50 bara and above) is accompanied by a consumption of pressurized nitrogen for the gasifier and for the associated gas turbine. 
         [0004]    When the client recycles all of the available pressurized nitrogen, layouts with a pressurized low-pressure column make it possible to obtain a good oxygen separation energy. A low-pressure column is said to be “pressurized” when it operates at a pressure of greater than 2 bar abs. 
       SUMMARY OF THE INVENTION 
       [0005]    It is proposed to improve the efficiency of such a system via a more thorough thermal integration between columns. 
         [0006]    The invention is particularly advantageous in layouts which naturally have a disparate hot end of the exchange line or if there is refrigerating capacity to be recycled. 
         [0007]    A typical hot-end difference for a process in which this invention would be used would be between 6 and 10° C. 
         [0008]    U.S. Pat. No. 5,341,646 describes a separation unit comprising three columns, a high-pressure column, a low-pressure column and an intermediate-pressure column operating at a pressure between the low pressure and the high pressure. 
         [0009]    Air is sent to the high-pressure column and nitrogen from the top of the high-pressure column is condensed in an intermediate condenser of the intermediate-pressure column. A cycle nitrogen stream is condensed in the bottom condenser of the intermediate-pressure column. Gaseous nitrogen is produced at the top of the intermediate column. 
         [0010]    The nitrogen from the top of the intermediate-pressure column is condensed in the bottom of the low-pressure column and liquid oxygen originating from the low-pressure column is pressurized and is vaporized in the exchange line. 
         [0011]    U.S. Pat. No. 6,286,336 and U.S. Pat. No. 5,675,977 describe the prior art closest to this invention. 
         [0012]    According to one subject of the invention, a process is provided for separating air by cryogenic distillation in a set of columns comprising a first column operating at a first pressure, a second column operating at a second pressure lower than the first pressure and a third column operating at a third pressure lower than the second pressure, wherein:
       i) compressed, purified and cooled air is sent to the first column where it is separated in order to form an oxygen-enriched liquid and a nitrogen-enriched gas,   ii) a portion of the nitrogen-enriched gas from the first column is condensed in a bottom vaporizer-condenser of the second column,   iii) oxygen-enriched liquid is sent from the first column to the second column,   iv) an oxygen-enriched liquid is sent from the bottom of the second column to the third column,   v) a nitrogen-enriched gas is sent from the top of the second column to a first vaporizer-condenser of the third column where it is condensed, the condensed liquid being sent back to the second column,   vi) a nitrogen-rich gas is withdrawn from the top of the third column,   vii) a liquid containing at least 85% oxygen is withdrawn from the bottom of the third column, pressurized and vaporized in order to form a gaseous product containing at least 85% oxygen,
 
wherein the first vaporizer-condenser of the third column is a vaporizer-condenser positioned at an intermediate point of the third column, the third column has a second vaporizer-condenser which is a bottom vaporizer-condenser, a nitrogen-enriched gas from the second column is compressed in a compressor having an inlet temperature below ambient temperature and is sent to the second vaporizer-condenser in order to be condensed therein, the vaporizer-condenser of the second column is a bottom vaporizer-condenser which is the only vaporizer-condenser present in the second column, the overhead gas from the first column is sent to the vaporizer-condenser of the second column without having been compressed, optionally a nitrogen-rich liquid is pressurized then sent from the top of the second column to the top of the first column and gaseous nitrogen is withdrawn from the top of the first column and reheated to form a product of the process.
       
 
         [0020]    According to other optional features:
       an intermediate liquid is sent from the first column to the third column;   all the nitrogen-enriched gas from the top of the second column is sent to the first and second vaporizer-condensers;   the liquid containing at least 85% oxygen is pressurized to a pressure greater than 30 bar abs, preferably greater than 40 bara before being vaporized or pseudo-vaporized;   the third pressure is greater than 2 bar abs;   the compressor having an inlet temperature below ambient temperature has an inlet temperature above the vaporization temperature of the liquid containing at least 85% oxygen minus 5° C.;   the liquid containing at least 85% oxygen is vaporized in a heat exchanger where the compressed air is cooled, the difference in temperatures at the hot end of the heat exchanger being less than 10° C., preferably less than 6° C.       
 
         [0027]    According to another subject of the invention, a unit is provided for separating air by cryogenic distillation comprising a set of columns comprising a first column operating at a first pressure, a second column operating at a second pressure lower than the first pressure and a third column operating at a third pressure lower than the second pressure, the second column having a bottom vaporizer-condenser, the third column having a first vaporizer-condenser, a line for sending compressed, purified and cooled air to the first column where it is separated in order to form an oxygen-enriched liquid and a nitrogen-enriched gas, a line for sending a portion of the nitrogen-enriched gas from the first column to be condensed in the bottom vaporizer-condenser of the second column, a line for sending oxygen-enriched liquid from the first column to the second column, a line for sending an oxygen-enriched liquid from the bottom of the second column to the third column, a line for sending a nitrogen-enriched gas from the top of the second column to a first vaporizer-condenser of the third column where it is condensed, a line for sending the condensed liquid from the first vaporizer-condenser to the second column, a line for withdrawing a nitrogen-rich gas from the top of the third column, a line for withdrawing a liquid containing at least 85% oxygen from the bottom of the third column, means for pressurizing the liquid and a heat exchanger for vaporizing the pressurized liquid in order to form a gaseous product containing at least 85% oxygen, characterized in that the first vaporizer-condenser of the third column is a vaporizer-condenser positioned at an intermediate point of the third column, the third column has a second vaporizer-condenser which is a bottom vaporizer-condenser, a nitrogen-enriched gas from the second column is compressed in a compressor having an inlet temperature below ambient temperature and is sent to the second vaporizer-condenser in order to be condensed therein, the vaporizer-condenser of the second column is a bottom vaporizer-condenser which is the only vaporizer-condenser present in the second column, the unit comprises means for sending the overhead gas from the first column to the vaporizer-condenser of the second column without having been compressed, optionally means for pressurizing a nitrogen-rich liquid and means for sending the pressurized rich liquid from the top of the second column to the top of the first column, and a line for withdrawing a nitrogen enriched gas from the top of the first column and means for reheating it in order to form a product of the process. 
         [0028]    The unit may comprise means for sending an intermediate liquid from the first column to the third column. 
         [0029]    The unit may comprise means for sending all the nitrogen-enriched gas from the top of the second column to the first and second vaporizer-condensers. 
         [0030]    The use of the cold compressor makes it possible to greatly reduce, by cascade effect, the pressure of the first column, which allows a substantial energy saving. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [0031]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it can admit to other equally effective embodiments. 
         [0032]    The Figure represents a process flow diagram in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION 
       [0033]    The invention will be described in greater detail with reference to the figure. 
         [0034]    The air separation unit comprises a heat exchanger  13 , a heat exchanger  49 , a first column  21  operating at a first pressure between 11 and 20 bara, a second column  23  operating at a second pressure lower than the first pressure and between 1 and 11 bara and a third column  25  operating at a third pressure lower than the second pressure. The third pressure is between 2 bar abs and 6 bara. 
         [0035]    The air  1  is compressed to the first pressure and then split into two. One portion  5  at the first pressure is cooled in the exchanger  13  and is sent to the first column  21  in gaseous form. The remainder  3  is boosted in the booster  7  up to a pressure of 49 bara and split into two. One portion  15  is sent to the exchanger  13  where it is cooled to an intermediate temperature of the exchanger then expanded in a Claude turbine  11  and sent to the column  21  after being mixed with the stream  5  in order to form the stream  14 . The remainder  17  of the boosted air is again boosted in a booster  9  coupled to the turbine  11  and sent to the exchanger  13  where it is cooled. The cooled and pseudo-liquefied stream  17  is expanded in a turbine  19  in order to form an at least partially liquid stream which is sent to the column  21 . All the air  1  is sent to the column  21  where it is separated. 
         [0036]    An oxygen-enriched stream  33  is sent from the first column to the middle of the second column  23  after expansion. An intermediate stream  35  is sent from the first column to the third column  25  after subcooling in  49 , then expansion. Liquid nitrogen from the top of the first column  21  is subcooled, then expanded and sent to the top of the third column  25  after subcooling in  49 , then expansion. Gaseous nitrogen  65  is withdrawn from the first column and reheated in the exchanger  13  in order to form a nitrogen product which is pressurized between 11 and 20 bara. 
         [0037]    Another portion of the nitrogen is condensed in the bottom vaporizer-condenser  27  of the second column and is sent back to the top of the first column. 
         [0038]    A bottom liquid  51  from the second column  23  is subcooled, then expanded and sent to an intermediate level of the third column  25 . An overhead liquid  39  from the second column  23  is split into two, one portion  55  being subcooled, then expanded and sent to the top of the third column  25  and the remainder  53  being pressurized by a pump  57  in order to be sent back to the top of the first column  21 . An overhead gas  41  from the second column  23  is split into two. One portion  43  is sent to a first vaporizer-condenser  31  which is located at an intermediate level of the third column  25 . The portion  43  is condensed therein and is sent to the top of the second column  23 . The other portion  45  of the gas  41  is sent back to the heat exchanger  13  where it is reheated to a temperature of −120° C. The portion  45  is reheated to a temperature above the vaporization temperature of the liquid  59  minus 5° C. In this example, the portion  45  is at a temperature level no more than 5° C. below the vaporization plateau of the pressurized oxygen. The portion  45  may also be at a temperature level above this plateau. Next, the gas  45  is compressed in a compressor  47 , sent back to the exchanger  13  where it is cooled down to the cold end and sent to the second vaporizer-condenser  29  which is a bottom vaporizer-condenser of the third column  25 . The portion  45  is condensed in the vaporizer-condenser  29  and the condensed stream is expanded and sent to the top of the second column  23 . 
         [0039]    An overhead gas  63  is withdrawn from the top of the third column  25  and is reheated in the exchangers  49 ,  13  in order to act as waste gas. The bottom liquid  59  from the third column  25  contains at least 85 mol % oxygen, or even at least 95 mol % oxygen but less than 98% oxygen. This liquid  59  is pressurized by the pump  61  to a pressure of at least 30 bar abs and is then vaporized (or pseudo-vaporized if its pressure is supercritical) in the exchanger  13  in order to form a pressurized oxygen stream to be sent to the gasifier. 
         [0040]    The difference in temperatures at the hot end of the heat exchanger  13  being less than 10° C., preferably less than 6° C., for example between 2° C. and 3° C. 
         [0041]    While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step. 
         [0042]    The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0043]    “Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein. 
         [0044]    “Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary. 
         [0045]    Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
         [0046]    Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range. 
         [0047]    All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.