Abstract:
A carbon dioxide-rich mixture is cooled in a first brazed aluminum plate-fin heat exchanger, at least one fluid derived from the cooled mixture is sent to a purification step having a distillation step and/or at least two successive partial condensation steps, the purification step produces a carbon dioxide-depleted gas which heats up again in the first exchanger, the purification step produces a carbon-dioxide rich liquid which is expanded, then sent to a second heat exchanger where it is heated by means of a fluid of the method, the exchanger carrying out an indirect heat exchange only between the carbon dioxide-rich liquid and the fluid of the method, the carbon dioxide-rich liquid at least partially vaporizes in the second exchanger and the vaporized gas formed heats up again in the first exchanger to form a carbon dioxide-rich gas which can be the end product of the method.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a §371 of International PCT Application PCT/FR2013/051649, filed Jul. 10, 2013, which claims the benefit of FR1256778, filed Jul. 13, 2012, 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 to a device for purifying a mixture rich in carbon dioxide at low temperature. 
       BACKGROUND 
       [0003]    A low temperature is below 0° C., preferably below −40° C. 
         [0004]    A mixture rich in carbon dioxide contains at least 60 mol % of carbon dioxide or even at least 80 mol % of carbon dioxide. 
         [0005]    The remainder of the mixture may contain one or more of the following components: oxygen, nitrogen, argon, nitrogen oxide (NO, NO 2  or N 2 O), carbon monoxide, hydrogen, mercury. 
         [0006]    The purification may be carried out by one or more successive partial condensation steps and/or by distillation. 
         [0007]    US-A-2010/0326134 describes a process according to the preamble of claim  1 . WO-A-2012/030223 describes a process according to the preamble of claim  1  except that it does not mention the technology used for the first heat exchanger. 
         [0008]    In the prior art, the process used for purifying CO 2  in a large amount consists in cooling the CO 2 -rich gas mixture to the temperature as close as possible to the freezing of the CO 2  (−56.5° C.) in order to condense the maximum amount of CO 2 . The cold is provided in general by the vaporization of the CO 2  recovered and the heating of the various fluids, for example the vaporized CO 2 , the noncondensable gases, or various recycling streams. Advanced thermal integration makes it possible to reduce the energy consumption of the process. Thus, brazed aluminum plate exchangers are particularly well suited for cooling the CO 2 -rich gas mixture since they permit heat exchanges between numerous fluids (6 is a common number) with a small temperature difference between the hot fluids and the cold fluids (a difference of 2° C. is common practice also). 
         [0009]    The coldest temperature of the exchange is provided by the vaporization of CO 2  close to the triple point. This vaporization thus has the risk of making a solid CO 2  phase appear if the pressure drops to the triple point value. Indeed, below the pressure of the triple point (5.185 bar a), liquid CO 2  cannot exist and it is then converted to a solid phase (for around 60% of the molecules) and a gas phase. The vaporization of the liquid provides the cold necessary for solidifying the rest. 
         [0010]    This solid phase, which may appear upon a fluctuation of the pressure of the vaporized gas, for example if the compressor which recovers it sucks up more molecules than vaporized in the exchanger, may block the exchanger and damage it during heating phases. This is a limitation for the use of a brazed aluminum exchanger. 
       SUMMARY OF THE INVENTION 
       [0011]    There are several ways to reduce the risk of damaging the unit: 
         [0012]    1. Operate at higher pressure, that is to say heat the cold end, and therefore condense less CO 2  in the CPU. This will increase the energy cost of the CO 2 . 
         [0013]    2. Raise the position of the liquid supply pot above the exchanger, so as to ensure that the pressure of the liquid in the exchanger is always greater than the pressure of the liquid in the pot owing to the hydrostatic height and therefore that it cannot form a solid in the exchanger. This solution, more elegant since it avoids in almost all cases the formation of solid in the exchanger (which point 1 does not permit), all the same degrades the energy efficiency since the vaporization of CO 2  will be carried out at higher pressure (the hydrostatic height) and therefore higher temperature. 
         [0014]    The invention proposed here aims to make it possible to operate as close as possible to the triple point, or even with the liquid CO 2  at the triple point. 
         [0015]    According to one subject of the invention, a process is provided for purifying a mixture containing carbon dioxide at low temperature in order to produce a fluid rich in carbon dioxide wherein: 
         [0016]    i) the mixture rich in carbon dioxide is cooled in a first brazed aluminum plate heat exchanger, consisting of corrugated sheets separated by plates, 
         [0017]    ii) the cooled mixture or at least one fluid derived from the cooled mixture is sent to a purification step comprising a distillation step and/or at least two successive steps of partial condensation, 
         [0018]    iii) the purification step produces a gas depleted in carbon dioxide which is heated in the first exchanger, 
         [0019]    iv) the purification step produces a liquid rich in carbon dioxide which is expanded then sent to a second heat exchanger where it is heated using a process fluid, the exchanger carrying out an indirect heat exchange only between the liquid rich in carbon dioxide and the process fluid, and 
         [0020]    v) the liquid rich in carbon dioxide is at least partially vaporized in the second exchanger and the vaporized gas formed is heated in the first exchanger in order to form a gas rich in carbon dioxide, that may be the final product of the process, 
         [0021]    characterized in that the process fluid which heats the liquid rich in carbon dioxide in the second exchanger consists of a gas derived from the mixture cooled in the first exchanger and in that the cooled mixture is at least partially condensed in the first exchanger and is sent to a first phase separator and the gas from the first phase separator is sent to the second exchanger where it is at least partially condensed in order to form condensed liquid. 
         [0022]    According to other optional aspects:
       the gas rich in carbon dioxide is compressed and condensed in order to form a liquid rich in carbon dioxide which is a final product of the process;   the liquid from the first phase separator is expanded and sent to a second phase separator;   the condensed liquid is sent to a (the) second phase separator;   the liquid from the second phase separator is expanded and is sent to the top of a distillation column where the distillation takes place, in order to supply the distillation column;   the gas from the second phase separator is heated in the first exchanger;   the liquid from the first phase separator is expanded and sent to the top of a distillation column where the distillation takes place, in order to supply the distillation column;   the cooled mixture is at least partially condensed in the first exchanger and is sent to a first phase separator, the liquid from the first phase separator being expanded and sent to a second phase separator, the gas from the first phase separator is sent to the second exchanger where it is condensed in order to form condensed liquid, the condensed liquid is sent to the second phase separator and the liquid from the second phase separator is sent to the top of the distillation column in order to supply the distillation column;   the second exchanger is a shell and tube exchanger, the liquid rich in carbon dioxide being sent to be heated in the shell and the process fluid being sent to be cooled in the tubes;   a second liquid rich in carbon dioxide is produced by the purification step and sent to the first exchanger without passing through the second exchanger and preferably without having been expanded, this second liquid is vaporized in the first exchanger in order to form a gas rich in carbon dioxide;   a purge liquid from the second exchanger is vaporized in order to provide frigories to the process;   the liquid rich in carbon dioxide is sent to the second exchanger after expansion at a pressure corresponding to that of the triple point of carbon dioxide and at a temperature equal to its triple point;   the density of the liquid phase rich in carbon dioxide used in the second exchanger for cooling the gas mixture rich in carbon dioxide is between 1171 kg/m 3  and 1562 kg/m 3 .       
 
         [0035]    According to another subject of the invention, a device is provided for purifying a mixture containing carbon dioxide at low temperature in order to produce a fluid rich in carbon dioxide comprising a first brazed aluminum plate heat exchanger, consisting of corrugated sheets separated by plates, a second exchanger other than a brazed aluminum plate exchanger capable of permitting an indirect heat exchange between only two fluids, a purification unit comprising at least one distillation column and/or at least two phase separators connected in series, a duct for sending the mixture rich in carbon dioxide to be cooled in the first exchanger, a duct for sending the cooled mixture or at least one fluid derived from the cooled mixture to the purification unit, a duct for extracting from the purification unit a gas depleted in carbon dioxide connected to the first exchanger, a duct for extracting from the purification unit a liquid rich in carbon dioxide connected to a valve, the valve being connected to the second heat exchanger and a duct for sending a process fluid to the second exchanger in order to vaporize the liquid rich in carbon dioxide and a duct for sending the gas produced by the vaporization of the liquid rich in carbon dioxide in the second exchanger to the first exchanger in order to be heated, characterized in that it comprises means for sending a gas, derived from the mixture cooled in the first exchanger, as process fluid that heats the liquid rich in carbon dioxide in the second exchanger, a first phase separator, means for sending the cooled mixture from the first exchanger to the first phase separator and means for sending a gas from the first phase separator to the second exchanger. 
         [0036]    The second exchanger is optionally a shell and tube exchanger. 
         [0037]    There may be means for sending a liquid from the first phase separator and/or from a second phase separator to the distillation column. 
         [0038]    According to other optional subjects, the device comprises:
       a compressor for compressing the gas rich in carbon dioxide and a cooler for condensing the compressed gas in order to form a liquid rich in carbon dioxide which is a final product of the process;   the process fluid which heats the liquid rich in carbon dioxide in the second exchanger consists of a gas derived from the mixture cooled in the first exchanger;   a second phase separator, means for expanding the liquid from the first phase separator and for sending it to the second phase separator, means for sending the gas from the first phase separator to the second exchanger where it is condensed in order to form condensed liquid, means for sending the condensed liquid to the second phase separator and means for sending the liquid from the second phase separator to the top of the distillation column in order to supply the distillation column;   a duct for sending the gas from the second phase separator to be heated in the first exchanger;   the second exchanger is a shell and tube exchanger, means for sending the liquid rich in carbon dioxide to be heated in the shell and means for sending the process fluid to be cooled in the tubes;   means for sending a second liquid rich in carbon dioxide produced by the purification step to the first exchanger without passing through the second exchanger and preferably without having been expanded;   means for sending a purge liquid from the second exchanger to be vaporized in an exchanger in order to provide frigories to the process.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]    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. 
           [0047]      FIG. 1  provides an embodiment of the present invention. 
           [0048]      FIG. 2  provides an embodiment of the present invention. 
           [0049]      FIG. 3  provides an embodiment of the present invention. 
           [0050]      FIG. 4  provides an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0051]    The principle according to the invention is to revert to an exchanger that enables an indirect heat exchange between only two fluids, for example a “shell and tube” exchanger where the CO 2  is vaporized in the shell and the flow to be partially condensed is found in the tubes. Since the boiling liquid is found in a shell, it is no longer to be feared that the appearance of a solid phase will block the passages and lead to local overpressures when some solid sublimes between two plugs of ice. 
         [0052]    It is therefore possible to operate the pot at the triple point pressure for the boiling liquid; the solid CO 2  that appears periodically will melt when the pressure goes back up above the triple point or when the liquid supplying the exchanger is slightly overheated. 
         [0053]    The impurities optionally present in the liquid CO 2  vaporized will be discharged from the exchanger:
       either in the gas phase for the lightest impurities;   or in a liquid purge for the heaviest impurities.       
 
         [0056]    This liquid purge will be discharged from the exchanger. 
         [0057]    The invention will be described in greater detail by referring to  FIGS. 1 to 4  which represent processes according to the invention. 
         [0058]    In  FIG. 1 , a mixture  1  contains at least 60 mol % of carbon dioxide, or even at least 80 mol % of carbon dioxide. 
         [0059]    The remainder of the mixture  1  may contain one or more of the following components: oxygen, nitrogen, argon, nitrogen oxide (NO, NO 2  or N 2 O), carbon monoxide, hydrogen. 
         [0060]    The mixture is filtered in a filter F in order to remove the dust, then compressed in a first compressor stage C 1  in order to form a compressed flow  3 . The compressed flow  3  is compressed in a second compressor stage C 2 , cooled in a cooler R 2 , compressed in a third compressor stage C 3 , cooled in a cooler R 3 , compressed in a fourth compressor stage C 4 , cooled in a cooler R 4 , compressed in a fifth compressor stage C 5  and cooled in a cooler R 5  in order to form a flow  5  at between 6 and 20 bar abs. This flow of the mixture  5  is purified of water in a bed of adsorbent A 2  in order to form a purified flow  7 . The purified flow  7  is partially condensed in a first exchanger  9 , which is an aluminum exchanger consisting of a stack of corrugated layers separated by plates. The partially condensed flow is sent to a first phase separator  11 . The gas formed  13  is sent to the tubes of a second exchanger  35 , enabling an indirect exchange of heat between only two fluids, of shell and tube type. The figure does not illustrate the multiplicity of tubes where the gas  13  derived from the mixture is condensed. The liquid formed  43  is sent to a second phase separator  17 , where the liquid  15  from the first phase separator  11  is also sent. 
         [0061]    A gas  45  from the second phase separator  17  is heated in the first heat exchanger. The liquid  19  from the second separator  17  is expanded in a valve  21  and sent to the top of the distillation column  23 . 
         [0062]    An overhead gas  25  depleted in carbon dioxide but enriched in at least one of the impurities (oxygen, nitrogen, argon, nitrogen oxide (NO, NO 2  or N 2 O), carbon monoxide, hydrogen) is heated in the first heat exchanger  9 . 
         [0063]    A bottoms liquid  27  is withdrawn from the bottom of the column and contains at least 80 mol % of carbon dioxide. The liquid  27  is divided into two; one flow  29  is vaporized in the first heat exchanger  9  without having been expanded. A portion  30  of the gas formed is sent to the bottom of the distillation column. The remainder  32  forms part of the product of the process. 
         [0064]    The liquid  33  originating from the bottom of the column is expanded in a valve  31  up to a pressure equivalent to the triple point of the carbon dioxide that it contains. The liquid is then sent to the shell of the second exchanger  35  where it is partially vaporized. The gas  37  formed is heated in the first exchanger  9  and sent to an intermediate vessel for absorbing the pressure variations  39 . From there it is compressed by a compressor stage C 6  and cooled in a cooler R 6  before being mixed with the vaporized liquid  32 . The gas thus formed is compressed by the stages C 7 , C 8 , C 9  and cooled by the coolers R 7 , R 8 , R 9 , R 10  in order to form a condensed gas. This condensed gas is mixed with the liquid purge  41  from the second exchanger and partly pumped by a pump P 1  in order to form a pressurized liquid product  51 , at at least 50 bar. The liquid purge  41  has previously been pumped in a pump P 2 . A portion  49  of the liquid is used as cycle liquid, is expanded at the triple point in the valve  53  and sent to the second exchanger  35 , mixed with the flow  33 . 
         [0065]    A portion of the gas  45  is heated in an exchanger E 1  and is used to regenerate the adsorbent bed A 1  which is in the regeneration phase. The flow  55 , having been used for the regeneration, is mixed with the flow  3  downstream of the stage C 1 . 
         [0066]    It is of course possible to vaporize the liquid  33  in the second exchanger  35  by indirect heat exchange with another fluid from the process, for example a portion of the vaporized bottoms liquid. 
         [0067]    The purge liquid  41  is pumped by the pump P 2  up to the pressure where the remainder of the CO 2  is condensed (against air or water in the cooler R 10 ) (around 80 bar a) in order to be mixed therewith. 
         [0068]    One drawback is that this cold liquid represents a significant heat loss for the cold box; the advantage is that since this fluid does not pass through the product compressor, the latter is of reduced size. 
         [0069]    It is possible to heat the liquid  41  thus pumped in the pump P 2  before mixing it with the liquid CO 2  at ambient temperature in order to recover the frigories of the sensible heat of the purge, for example in order to reduce the compression energy of the CO 2  produced by the exchanger  9  in  FIG. 2 . 
         [0070]    Thus, the liquid pumped is used as refrigerant in a cooler E 10  between the cooler R 8  and the compressor stage C 9 . 
         [0071]    Another variant will produce iced water in the exchanger E 10  or any other reuse of this cold that can be envisaged. 
         [0072]    In  FIG. 3 , the purge liquid  41  is pressurized in a pump P 3  up to a higher pressure at which the CO 2 , produced by the partial condensation(s) and/or the distillation, is vaporized. The purge  41  is then mixed with this liquid CO 2    29  and it is vaporized in the higher pressure flow. Since this vaporization is carried out in the main brazed aluminum exchanger  9 , the purge may be vaporized completely and there is then no longer any loss of cold and it is possible to cancel the CO 2    49  recycled in order to provide frigories from the preceding figures. 
         [0073]    In  FIG. 4 , the purge flow  41  is vaporized outside of the cold box in an exchanger E 4  in order to produce iced water  55 . Otherwise, the flow  41  may be used to precool the cold box supply gas or to cool the CO 2  produced or for any other reuse of the cold contained. The gas thus obtained by vaporizing the liquid  41  may then be vented if it contains impurities that it is not desired to reuse by some other means, the gas could be treated in order to reduce the content of impurities (washing, adsorption, catalytic bed, etc.). Otherwise the gas may be recycled to the boiler or to the desulfurization unit where the mixture  1  comes from in order to reduce the content of NOx or other content of combustible impurity (hydrocarbons, alcohol, etc.) or else used for any other application (recovery of the NOx contained, hydrocarbons, alcohols, etc.). 
         [0074]    The pump P 2 , P 3  will have to be able to face solid phases (solid CO 2 , crystallization of N 2 O 4  or of other impurities) since all the heavy elements should be concentrated therein. 
         [0075]    Finally, it should be noted that the brazed aluminum exchanger  9 , although only cooling the feed gas down to −40° C. approximately, concentrates all the fluids that may make it possible to recover frigories in order to carry out this first cooling. Thus, the use of a solely two-fluid exchanger  35  for the cold box does not degrade the thermal performance of the assembly too much. It will be advisable to suitably arrange the passages in the exchanger in order to prevent excessively large heat differences, especially with the coldest fluids, for example CO 2  at low pressure vaporized in the “shell and tube” exchanger. The principle is to position the cold gaseous fluids at the ends, solely in contact (indirectly of course) with the “HP” CO 2  flow that is vaporized. The latter will be the only fluid in (indirect) contact with the hot fluid that is partially condensed. 
         [0076]    This point is even more important for the application where it is necessary to avoid accumulating mercury in this exchanger. Thus, it is necessary to prevent the hot fluid from being in contact with fluids colder than the triple point of mercury, i.e. −38° C. Specifically, it would then be possible to locally drop below −38° C. and deposit mercury in solid form in the exchanger. 
         [0077]    In all the figures, the column  23  may be replaced by a series of phase separators which separate the liquid  19  in order to form a gas  25  depleted in carbon dioxide and a liquid  33  enriched in carbon dioxide. 
         [0078]    The column  23 , the exchanger  9 , the phase separators  11 ,  17  and the exchanger  35  are contained inside a thermal chamber since they operate at low temperature. 
         [0079]    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. 
         [0080]    The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0081]    “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. 
         [0082]    “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. 
         [0083]    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. 
         [0084]    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. 
         [0085]    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.