Abstract:
The invention relates to a method for separating air by means of cryogenic distillation in a system of columns, in which two single-stage air superchargers are connected in series and coupled to two turbines, which expand the air that was not supercharged. The superchargers supercharge the cooled high-pressure air in an exchange line in which the oxygen from the system of columns is vaporized.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a §371 of International PCT Application PCT/FR2012/050701, filed Mar. 30, 2012, which claims the benefit of FR1152734, filed Mar. 31, 2011, both of which are herein incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    This invention relates to a method and an apparatus for producing a pressurised air gas by means of cryogenic distillation. 
       BACKGROUND 
       [0003]    The industrial state of the art, for apparatuses producing pressurised oxygen of about 15 bars, is comprised of “pump” apparatuses, which use a main air compressor at a pressure of about 6 bars, and an air booster compressing a portion of the airflow at a pressure of about 35-40 bars. But this solution is not available for apparatuses of small size, for which the combination of a small flow to be boosted and a very high discharge pressure leads to an actual flow at the outlet of the booster that is too small to be carried out technologically. 
         [0004]    So, for small apparatuses, oxygen compressors have to be used, which are expensive. 
         [0005]    The solution proposed makes it possible to reduce the costs for such apparatuses, through the use of a single air compressor, at a moderately high discharge pressure, which proposes a competitive advantage in relation to the two preceding solutions: the use of a single compressor and avoiding an expensive oxygen compressor. 
         [0006]    US-A-20050126221 describes a method for separating air according to the preamble of claim  1 . To produce pressurised oxygen, two boosters in series compress the air at intermediate temperatures of the main exchanger, with the intake temperature for the first booster being hotter than the outlet temperature of the second booster. A cooling unit is used to lower the intake temperature of the second booster, which as such increases the complexity of the method. 
         [0007]    US-A-20060010912 describes a method for separating air wherein air at a medium pressure is boosted in two cold boosters in series. 
         [0008]    The two boosters must not be coupled to a turbine, because the turbines of the method only operate during a particular operation making it possible to manufacture liquid. In nominal operation, the method is kept cold by adding cryogenic liquid. 
         [0009]    All of the pressures are absolute pressures. 
       SUMMARY OF THE INVENTION 
       [0010]    One purpose of the invention is to propose an alternative for creating process schemes which make it possible to improve the installation costs of devices for separating air for a production of oxygen between 10 and 16 bars, more preferably between 14 and 16 bars, therefore about 15 bars. 
         [0011]    According to certain embodiments of the invention, all of the air is brought to high pressure (substantially higher than the pressure of the medium-pressure column) and purified at this pressure, then divided into at least two portions. Only one fraction of the air, the fraction that is liquefied later at the cold end of the main exchange line, undergoes a succession of cryogenic compressions in such a way as to bring this flow to a pressure that is enough to allow for the vaporisation of oxygen at the desired pressure. The rest of the air is expanded in at least one turbine to the pressure of the medium-pressure column. At least a portion of the work generated by the expansion of the air is used for the cryogenic compression. 
         [0012]    According to an object of the invention, a method is provided for separating air by means of cryogenic distillation in an installation comprising a system of columns, including a column operating at the highest pressure called the medium pressure wherein: 
         [0013]    all of the air is brought to a high pressure, at least  3  bars higher than the medium pressure, purified at this pressure in a purification unit and air is sent at the output temperature from the purification unit to an exchange line; 
         [0014]    all of the purified air is cooled in the exchange line and a portion constituting between 10% to 35% of the purified air is boosted by means of at least one first single-stage booster with an inlet at an intermediate temperature of the exchange line; 
         [0015]    at least one portion of the boosted air in the first booster is cooled in the exchange line, boosted by means of at least one second single-stage booster and with an inlet at a second intermediate temperature of the exchange line and is sent back into the exchange line where it is cooled, then is liquefied, possibly at the cold end of the exchange line and is sent into the system of columns after expansion; 
         [0016]    another portion of the high-pressure purified air, constituting possibly between 65% and 90% of the high-pressure purified air, is cooled in the exchange line then at least partially expanded in at least two turbines having one or more intake temperatures which is an intermediate temperature or which are intermediate temperatures of the exchange line and then sent to the system of columns in order to be separated; 
         [0017]    the work generated by the expansion of the air is used at least partially for the cryogenic compression carried out by the first and/or the second booster by coupling the first booster to one of the two turbines and the second booster to the other of the two turbines; 
         [0018]    liquid oxygen is vaporised in the exchange line characterised in that all of the air purified in the purification unit is sent at the output temperature of the purification unit to an exchange line, the liquid oxygen that has been pressurised at a pressure less than or equal to 16 bars, more preferably between 10 and 16 bars, is vaporised in the exchange line, an energy dissipating device is coupled to at least one of the boosters, the first temperature differs from the second temperature by at most 10° C. and the first and second temperatures are between −145° C. and −165° C. 
         [0019]    According to other optional characteristics: 
         [0020]    the two turbines have equal or different intake temperatures, constituted by the third intermediate temperature and a fourth intermediate temperature of the exchange line;
       the third temperature is lower than the first temperature.   the third temperature differs from the fourth temperature by at most 20° C., or even by at most 10° C.;   the first temperature is higher than the second temperature;   the first temperature is lower than or equal to the second temperature;   a portion of the energy generated by at least one of the turbines is dissipated;   a portion of the energy is dissipated by means of a hydraulic brake system connected to the turbine;   a portion of the air is liquefied at high pressure, preferably in the exchange line;   the air from at least one of the turbines is sent to the column operating at the highest pressure;   all of the air boosted in the first booster is sent to the second booster;   all of the air purified in the purification unit is sent to the exchange line at the output pressure of the purification unit;   the system comprises a double column for separating air comprising a first column and a second column operating at a lower pressure than the first, with the air expanded in the two turbines being sent to the first column;   the first temperature is colder than the output temperature of the second booster;   the output temperature or temperatures of the first and/or of the second booster is/are between −110° C. and −150° C.;   the output temperature or temperatures of the first and/or of the second booster is/are between −125° C. and −145° C.       
 
         [0035]    According to another object of the invention, an apparatus is provided for separating air by means of cryogenic distillation comprising a system of columns, of which one column operating at the highest pressure called the medium pressure, a compressor for compressing all of the air at a high pressure, greater by at least  3  bars than the medium pressure, a purification unit connected to the compressor to purify all of the high-pressure air, a pipe for sending a portion constituting between 10% to 35% of the high-pressure purified air to be cooled in an exchange line, a first single-stage booster, a second single-stage booster, a pipe for sending the portion constituting between 10 and 35% of the air to be purified in the first booster at a first intermediate temperature of the exchange line, a pipe for sending at least one portion of the air boosted in the first booster to be cooled in the exchange line, a pipe for sending this cooled portion to the second booster at a second intermediate temperature from the exchange line, a pipe for sending air from the second booster to the exchange line in order to be cooled, a pipe for sending the cooled air coming from the second booster from the exchange line to a means for expanding and then into the system of columns, with the exchange line being designed in such a way that the first temperature differs from the second temperature by at most 10° C. and the first and second temperatures being between −145° C. and −165° C., at least two turbines, a pipe for sending another portion of the high-pressure purified air, constituting possibly between 65% and 90% of the high-pressure purified air, from the exchange line to the two turbines having one or more intake temperatures which is an intermediate temperature or which are intermediate temperatures of the exchange line, pipes for sending air from the two turbines to the system of columns, with the first booster being coupled to one of the two turbines and the second booster to the other of the two turbines, a pipe for sending liquid oxygen, pressurised at a pressure lower than or equal to 16 bars, more preferably between 10 and 16 bars, is vaporised in the exchange line and an energy dissipating device coupled to at least one of the boosters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    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. 
           [0037]      FIG. 1  shows a method according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    The invention shall be described in more detail by referring to the figure which shows a method for separating air according to the invention. 
         [0039]    An airflow is compressed in a main compressor  3  to a pressure at least  3  bars above the pressure of the column  31 , which is the medium-pressure column of a double column for separating air by means of cryogenic distillation. The compressed air is purified in a purification unit  7  in order to form the purified flow  9 . The purified flow is sent to the exchange line  11  without having been cooled and in the exchange line it is cooled to a first intermediate temperature. At this temperature, the air is divided into a portion  13  and a portion  14 . The portion  13  enters into a single first single-stage booster  15  at the first intermediate temperature wherein it is boosted. The boosted air is sent to the exchange line  11  wherein it is cooled again to a second intermediate temperature, lower than the first intermediate temperature. At this second intermediate temperature, at least one portion of the air boosted in the booster  15 , even all of the air  13 , is boosted in a single second single-stage booster  25 . 
         [0040]    The first intermediate temperature differs from the second temperature by at most 10° C. and the first and second temperatures are between −145° C. and −165° C. 
         [0041]    The first intermediate temperature can possibly be higher or equal to the second intermediate temperature. 
         [0042]    Each of the output temperatures of the boosters  15 ,  25  is between 110° C. and −150° C., more preferably between −125° C. and −145° C. 
         [0043]    The doubly boosted flow  13  is sent to the exchange line at the pressure required for the vaporisation of a flow of pressurised oxygen. The boosted flow  13  cooled to this pressure to the cold end of the exchange line  11  and is condensed. At the output of the exchanger, the flow is expanded, and is sent to the medium-pressure column  31 . 
         [0044]    The rest of the air  14  is divided into two or three portions. According to an alternative, all of the air  14  is divided into two portions. One portion  19  is sent to a turbine  17  having an intake temperature which is a third intermediate temperature of the exchange line, then is sent in gaseous form to the medium-pressure column  31 . 
         [0045]    Another portion  21  is sent to a turbine  27  having an intake temperature which is a fourth intermediate temperature of the exchange line, higher than the third temperature, then is sent in gaseous form to the medium-pressure column  31 . More preferably the portions  19 ,  21  are mixed in order to form a single flow  23 . 
         [0046]    Otherwise in addition to the portions  19 ,  21 , a portion  26  of the high-pressure air can possibly continue to be cooled to the cold end of the exchange line  11  and is condensed. At the output of the exchanger, it will be expanded in a valve and sent to the system of columns, for example to the medium-pressure column  31 . 
         [0047]    The double column comprises a medium-pressure column  31  and a low-pressure column  33 , thermally connected together with reflux flows  39 ,  41  in a known manner. 
         [0048]    The low-pressure column  33  produces a flow of nitrogen  43  which is heated in the exchange line  11 . It also produces liquid oxygen  35  in the tank which is pressurised at a pressure between  10  and  16  bars and is vaporised in the exchange line in order to form pressurised gaseous oxygen. 
         [0049]    It can be considered to vaporise liquid oxygen at two different pressures in this way or to vaporise liquid nitrogen or liquid argon, possibly pressurised at the same time as the liquid oxygen. 
         [0050]    In the case where two products are vaporised in the exchange line (or one product at two different levels of pressure), a portion of the flow  13  can continue to be cooled to the cold end of the exchanger and not be boosted by the booster  25 . This fraction of flow will condense. At the exchanger output, it will be expanded in a valve and sent to the system of columns, for example to the medium-pressure column  31 . 
         [0051]    The booster  15  is driven at least in part by one of the two turbines  17  or  25 , and the booster  25  by the other turbine  25  or  17 . In each case, there can also be a motor or a generator coupled to the compressor. An energy dissipating device  22 ,  24 , for example a valve, preferably an oil valve system, will be integrated into at least one of the two turbine/compressor systems  15 / 17 ,  25 / 27 . 
         [0052]    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. 
         [0053]    The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0054]    “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. 
         [0055]    “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 a range is expressed, it is to be understood that another embodiment is from the one. 
         [0056]    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. 
         [0057]    Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range. 
         [0058]    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.