Patent Publication Number: US-2016223254-A1

Title: Method and apparatus for separation of a gaseous mixture at sub-ambient temperature

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a §371 of International PCT Application PCT/FR2014/052103, filed Aug. 20, 2014, which claims the benefit of FR1358668, filed Sep. 10, 2013, both of which are herein incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a method and to a device for separating a gaseous mixture, for example air, at sub-ambient, or even cryogenic, temperature. 
     BACKGROUND OF THE INVENTION 
     In order to produce a pressurized air gas it is known practice to vaporize a pressurized liquid withdrawn from a distillation column by exchange of heat with another pressurized gas from the process, generally high-pressure pressurized air. This vaporization generally takes place by sending the pressurized liquid into at least one passage of an exchange line, the other pressurized gas being sent to cool in at least one other passage of this exchange line, the transfer of latent heat from the other pressurized gas to the pressurized liquid being indirect because it takes place across the wall of the passage. 
     If the liquid is pressurized at a supercritical pressure, pseudo-vaporization replaces vaporization. In what follows the term “vaporization” also covers pseudo-vaporization. If the other gas is pressurized to a supercritical pressure, pseudo-condensation replaces condensation. In what follows, the term “condensation” also covers pseudo-condensation. 
     The percentages regarding purity given in this document are molar percentages. 
     Separation may take place in at least one distillation column and/or at least one absorption column and/or at least one separating pot and/or at least one membrane and/or by dephlegmation. 
     Magnetic refrigeration relies on the use of magnetic materials that exhibit a magnetocaloric effect. Reversibly, this effect is manifested by a variation in their temperature when they are subjected to the application of an external magnetic field. The optimum ranges within which these materials are used lie in the vicinity of their Curie temperature (Tc). This is because the greater the variation in magnetization and, therefore, the changes in magnetic entropy, the greater the changes in temperature. The magnetocaloric effect is said to be direct when the temperature of the material increases when placed in a magnetic field, and indirect when it cools when placed in a magnetic field. The remainder of the description will be given for the direct case, but it is obvious to a person skilled in the art how to reapply this to the indirect case. There are many thermodynamic cycles based on this principle. A conventional magnetic refrigeration cycle consists i) in magnetizing the material in order to increase its temperature, ii) in cooling the material in a constant magnetic field in order to dissipate heat, iii) in demagnetizing the material in order to cool it, and iv) in heating the material in a constant (generally zero) magnetic field in order to absorb heat. 
     A magnetic refrigeration device employs elements made of magnetocaloric material, which generate heat when magnetized and absorb heat when demagnetized. They may employ a magnetocaloric material regenerator in order to amplify the temperature difference between the “hot source” and the “cold source”: the magnetic refrigeration is then said to be magnetic refrigeration employing active magnetic regeneration. This effect is described in the Lebouc 2005 Techniques de l&#39;Ingénieur [Engineering techniques] article entitled “Réfrigération magnétique [Magnetic refrigeration]”. 
     It is known practice—see EP-A-2551005 or even U.S. Pat. No. 6,502,404—to use the magnetocaloric effect to supply cold to a method for separating at sub-ambient temperature. 
     SUMMARY OF THE INVENTION 
     The present invention tackles the problem of how to vaporize a liquid derived from the separation while reducing the pressure ratio between the gas that is to be condensed and the liquid that is to be vaporized that is normally required for an exchange of heat through an exchanger. 
     According to certain embodiments of the present invention, at least part of the heat required for vaporizing a liquid from a separation comes from a heat pump using the magnetocaloric effect. 
     One subject of the invention is a method for separating a gaseous mixture by separation at sub-ambient, or even cryogenic, temperature, in which a gaseous mixture at a first pressure is cooled, then separated in a separation unit, for example a system of columns comprising at least one column. A liquid is withdrawn from the separation unit and vaporized to form a pressurized gaseous product, possibly following pressurization to a higher pressure or following depressurization to a pressure lower than the pressure at which it is withdrawn, characterized in that at least part of the heat of vaporization of the liquid is supplied by a heat pump using the magnetocaloric effect of which the hot source exchanges heat, indirectly or directly, with the liquid which vaporizes. 
     According to other optional subjects of the invention:
         the cold source of the heat pump exchanges heat with at least part of the gaseous mixture and/or of a gas derived from the separation method which cools, or even condenses, at least partially;   the vaporized liquid contains at least 70% oxygen, or at least 80% nitrogen, or at least 60% carbon dioxide, or at least 60% methane or at least 60% carbon monoxide;   the separation is performed by distillation and the system comprises at least one distillation column;   a fluid, which may or may not participate in the separation, is brought into direct contact with a magnetocaloric material of the heat pump;   the exchange of heat is performed at least in part between at least a fluid which may or may not participate in the separation and a heat-transfer fluid in contact with a magnetocaloric material of the heat pump through an exchanger;   the exchange of heat is performed at least in part between at least a fluid which may or may not participate in the separation and the heat-transfer fluid in contact with a magnetocaloric material of the heat pump through an intermediate heat-transfer circuit;   the gaseous mixture is air, the pressurized liquid is rich in oxygen or in nitrogen, all of the gaseous mixture is compressed to a single pressure and at least part of the gaseous mixture is at least partially condensed, transferring heat to the cold source of the heat pump;   the gaseous mixture is air, the pressurized liquid is rich in oxygen or in nitrogen, all of the gaseous mixture is compressed to a first pressure, part of the gaseous mixture is compressed from the first pressure to a second pressure higher than the first pressure, and at least part of the compressed gaseous mixture at the second pressure is at least partially condensed, transferring heat to the cold source of the heat pump.       

     Another subject of the invention is a device for separating a gaseous mixture by separation at sub-ambient, or even cryogenic, temperature, comprising cooling means for cooling a gaseous mixture at a first pressure, a separation unit, for example a system of columns comprising at least one column, which is connected to the cooling means, and a pipe for withdrawing a liquid from the separation unit, means for vaporizing the liquid to form a pressurized gaseous product, possibly downstream of means of pressurizing to a pressure that is higher or of depressurizing to a pressure that is lower than the pressure at which it is withdrawn, characterized in that it comprises using a heat pump using the magnetocaloric effect capable of supplying at least part of the heat of vaporization of the liquid and means allowing the hot source of the heat pump to exchange heat, directly or indirectly, with the liquid which vaporizes. 
     The device may comprise
         means for allowing an exchange of heat between the cold source of the heat pump and at least part of the gaseous mixture and/or of a gas derived from the separation method which becomes cooled, or even condenses at least partially;   means for withdrawing a liquid containing at least 70% oxygen, or at least 80% nitrogen, or at least 60% carbon dioxide, or at least 60% methane or at least 60% carbon monoxide;   the separation takes place by distillation and the system comprises at least one distillation column;   means for placing a fluid, which may or may not participate in the separation, into direct contact with a magnetocaloric material of the heat pump;   the exchange of heat is performed at least in part between at least a fluid which may or may not participate in the separation and a heat-transfer fluid in contact with a magnetocaloric material of the heat pump through an exchanger;   the exchange of heat is performed at least in part between at least a fluid which may or may not participate in the separation and the heat-transfer fluid that has been in contact with a magnetocaloric material of the heat pump through an intermediate heat-transfer circuit;   the gaseous mixture is air, the pressurized liquid is rich in oxygen or in nitrogen, a compressor for compressing all the gaseous mixture to a single pressure and means for transferring heat from at least part of the at least partially condensed gaseous mixture to the cold source of the heat pump;   the gaseous mixture is air, the pressurized liquid is rich in oxygen or in nitrogen, all the gaseous mixture is compressed to a first pressure;   a compressor for compressing part of the gaseous mixture from the first pressure to a second pressure higher than the first pressure and means for transferring heat from at least part of the compressed gaseous mixture at the second pressure to the cold source of the heat pump.       

     A heat pump is a thermodynamic device that allows a quantity of heat to be transferred from a medium considered to be the “emitter” and referred to as the “cold source” from which heat is extracted, to a medium considered to be the “receiver” and referred to as the “hot source” to which the heat is supplied, the cold source being at a colder temperature than the hot source. 
     The conventional cycle used in the prior art for this type of application is a thermodynamic cycle of compressing—cooling (condensing)—expanding—reheating (vaporizing) a refrigerating fluid. 
       FIG. 12  of the document entitled “TECHNIQUES DE L&#39;INGENIEUR—Réfrigération magnétique” [Engineering techniques—Magnetic refrigeration] 2005” shows a twofold improvement in the coefficient of performance of a refrigeration system using a magnetic cycle as compared with the conventional cycle. 
     An ambient temperature is the temperature of the ambient air in which the method is situated or, alternatively, a temperature of a cooling water circuit connected with the air temperature. 
     A sub-ambient temperature is at least 10° C. below the ambient temperature. 
     A cryogenic temperature is below −50° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  represents a process flow diagram in accordance with an embodiment of the present invention. 
         FIG. 2  represents a process flow diagram in accordance with an embodiment of the present invention. 
         FIG. 3  represents a process flow diagram in accordance with an embodiment of the present invention. 
         FIG. 4  represents a process flow diagram in accordance with an embodiment of the present invention. 
         FIG. 5  represents a process flow diagram in accordance with an embodiment of the present invention. 
         FIG. 6  represents a process flow diagram in accordance with an embodiment of the present invention. 
         FIG. 7  represents a generic figure illustrating the at least partial vaporization of liquid in accordance with an embodiment of the present invention. 
         FIG. 8  represents a process flow diagram in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will be described in greater detail with reference to  FIGS. 1 to 8 . 
       FIG. 1  shows a device for separating air by cryogenic distillation. The device comprises a heat exchange line  17  and a double air separation column comprising a medium-pressure column  23  and a low-pressure column  25  which are thermally connected by means of a vaporizer-condenser  27 . 
     Air  1  is compressed in a compressor  3  to a pressure of 5.5 bara. 
     The compressed air is cooled in the cooler  5  to form a cooled flow  7  which is purified to remove the water and carbon dioxide and other impurities in an adsorption unit  9 . 
     The purified air is split into two. One part  8  is cooled as it passes completely through the exchange line  17  down to a temperature of around −170° C. It is then split into two. One part  19  acts as a cold source for the heat pump  31  using the magnetocaloric effect. The rest  21  is sent to separate in gaseous form in the bottom of the medium-pressure column  23 . 
     The part  19  cools and liquefies through exchange of heat in the heat pump  31  to form the flow  37 . The flow  37  is split into a part  39  which is sent to the medium-pressure column  23  and a part  41  which is cooled in the subcooler  43 , expanded then sent to the low-pressure column  25 . 
     An oxygen-enriched liquid  33  is withdrawn from the bottom of the medium-pressure column  23 , cooled in the subcooler  43  and sent to the low-pressure column  25 . A nitrogen-enriched liquid  35  is withdrawn from the top of the medium-pressure column  23 , cooled in the subcooler  43  and sent to the top of the low-pressure column  25 . 
     Air  11  is pressure-boosted in a pressure booster  13 , cooled partially in the exchange line  17 , expanded in the inlet turbine  15  and sent to the low-pressure column  25 . 
     A nitrogen-rich gas  45  is withdrawn from the top of the low-pressure column  25 , heated in the subcooler  43  and in the exchange line  17  to serve at least in part as gas for regenerating the adsorption unit  9 . Nitrogen-rich gas  49  is withdrawn from the top of the medium-pressure column  23 , heated in the exchange line  17  and serves as product. Liquid oxygen  47  is withdrawn from the low-pressure column  25 , pressurized by a pump  29  and partially heated in the exchange line  17 . Next, the heated liquid is discharged from the exchange line  17 , vaporized at least partially in the heat pump using the magnetocaloric effect  31  where it acts as a hot source and sent back to the exchange line  17 , either to complete vaporization and warm up or only to warm up. The oxygen thus obtained serves as product. 
     In  FIG. 2 , unlike in  FIG. 1 , all of the air  8  is cooled in the exchange line  17  to form the flow  19  which condenses partially in the heat pump using the magnetocaloric effect  31  to form the flow  37 . All of the flow  37  is sent to the bottom of the medium-pressure column  23 . 
     In  FIG. 3 , the purified air is split into three parts. One part  11  is sent to the pressure booster  13  as in  FIGS. 1 and 2 . Another part  8  is cooled as it passes completely through the exchange line  17  and is then sent to the bottom of the column  23 . The rest of the air  12  has its pressure boosted in a pressure booster  14  and is sent to the exchange line  17  where it cools to an intermediate level. Thereafter, the partially cooled air  12  is condensed at least partly in the heat pump using the magnetocaloric effect  31  where it acts as a cold source. The at least partially condensed air is reintroduced into the exchange line  17  where it cools further. The air cooled further in the exchange line leaves the cold end thereof and is split into two parts. The first part  16  is expanded and sent to the bottom of the medium-pressure column  23 . The second part  18  is cooled in the subcooler  43 , expanded and sent to the low-pressure column. 
     In  FIG. 4 , unlike in  FIG. 1 , liquid oxygen  51  is also withdrawn from the low-pressure column  25 , cooled in the subcooler  43  and serves as liquid product. The proportion of oxygen produced in liquid form may represent up to half of the gaseous oxygen produced under pressure. 
     In  FIG. 5 , unlike in the other figures, the liquid  47  vaporizes through exchange of heat with nitrogen  53  from the low-pressure column  23  with the aid of the heat pump using the magnetocaloric effect  31 . The gaseous nitrogen  53  which acts as a cold source liquefies and is sent to the top of the column  23  to provide reflux. In that case, all the purified air is either sent to the pressure booster  13 , cooled and expanded, or cooled and sent for distillation. 
       FIG. 6  can be likened to  FIG. 3 . Unlike in  FIG. 3 , the fluid  12 , or, respectively,  47 , which is indirectly thermally connected with the cold source or, respectively, the hot source, of the heat pump using the magnetocaloric effect  31  does not leave the exchange line  17 . A heat-transfer fluid A transfers heat from the air  12  coming from the pressure booster  14  (at an intermediate level of the exchange line  17  near the point at which the air  12  at least partially condenses), is cooled in the heat pump using the magnetocaloric effect  31  at the level of the cold source and is returned to the exchange line  17 , in closed circuit. A heat-transfer fluid B transfers heat to the oxygen  47  (at an intermediate level in the exchange line  17  near the point at which the oxygen  47  at least partially vaporizes), heats up in the heat pump using the magnetocaloric effect  31  at the level of the hot source and is returned to the exchange line  17 , in closed circuit. 
     The heat-transfer fluids A and B may be the same or different. 
     The invention could also be applied to methods for separating other mixtures. For example, in  FIGS. 1 to 6 , the air could be replaced by a mixture containing methane and/or nitrogen and/or carbon dioxide and/or carbon monoxide and/or hydrogen as its main components. 
       FIG. 7  is a generic figure illustrating the at least partial vaporization of liquid  47  according to the invention. The liquid  47  may come from a separation unit, for example a distillation or absorption column, from a phase separator, a dephlegmator or a membrane. It may be vaporized in the exchanger  17  following a pressurizing (for example in a pump or using hydrostatic head) or depressurizing (for example in a valve or a turbine). It may for example contain at least 70% oxygen, at least 80% nitrogen, at least 60% carbon dioxide or at least 60% methane or at least 60% carbon monoxide. The fluid  12  which supplies the heat directly or indirectly to the cold source may be the fluid that is to be separated in the separation unit, a fluid separated in the separation unit or some other fluid. This fluid  12  at least partially condenses. 
     The exchanger  17  may also be used to heat and/or cool at least one other fluid  8 ,  45 . 
     The heat pump using the magnetocaloric effect  31  allows the exchange of heat between the fluid  12  (for example air) which acts as a cold source and the liquid  47  (for example a liquid containing at least 70% oxygen), which acts as a hot source. 
       FIG. 7  may be modified to use at least one heat-transfer fluid in closed circuit which transfers heat to and/or from the heat pump using the magnetocaloric effect  31 . 
       FIG. 8  shows a device for the cryogenic separation of a mixture of methane and nitrogen (typically 85% methane). The device comprises a heat exchange line  17  and a double separation column comprising a medium-pressure column  23  and a low-pressure column  25  which are thermally connected by means of a vaporizer-condenser  27 . 
     The high-pressure mixture of methane and nitrogen  8  is cooled and condenses partially in the exchange line  17 . It is then expanded to a medium-pressure distillation column  23 . This expansion contributes to keeping the device cold. 
     A methane-enriched liquid  33  is withdrawn from the bottom of the medium-pressure column  23 , cooled in the subcooler  43  and sent to the low-pressure column  25 . A nitrogen-enriched liquid  35  is withdrawn from the top of the medium-pressure column  23 , cooled in the subcooler  43 A and sent to the top of the low-pressure column  25 . 
     A nitrogen-rich gas  45  is withdrawn from the top of the low-pressure column  25  and heated in the subcoolers  43 A,  43  and in the exchange line  17 . Liquid methane  47  is withdrawn from the low-pressure column  25 , pressurized by a pump  29  then heated and then vaporized in the exchange line  17 , then the vaporized liquid methane continues to heat up in the exchange line  17 . The gaseous methane may be used directly as a product without additional compression in a compressor. 
     A heat-transfer fluid A transfers heat from the mixture  12  (at an intermediate level of the exchange line  17  in the vicinity of the point at which the mixture  12  is at least partially condensed), cools in the heat pump using the magnetocaloric effect  31  at the level of the cold source and is returned to the exchange line  17 , in closed circuit. A heat-transfer fluid B transfers heat to the methane  47  (at an intermediate level in the exchange line  17  near the point at which the methane  47  is at least partially vaporized), heats up in the heat pump using the magnetocaloric effect  31  at the level of the hot source and is returned to the exchange line  17 , in closed circuit. 
     Throughout the figures, the liquid that is to be vaporized is not necessarily heated up in the exchange line  17  beforehand before effecting an exchange of heat with the heat pump using the magnetocaloric effect. 
     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. 
     The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
     “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. 
     “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. 
     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. 
     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. 
     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.