Abstract:
The invention relates to an apparatus for cryogenic separation of a mixture of carbon monoxide, hydrogen and nitrogen, including a stripping column and a denitrogenation column, a pipe for sending the mixture in liquid form to the head of the stripping column, a pipe for removing a liquid depleted of hydrogen connected to the stripping column, a pipe for removing a gas enriched with hydrogen from the stripping column, means for sending the liquid depleted of hydrogen or a fluid derived from said liquid to the denitrogenation column, a pipe for drawing a liquid enriched with carbon monoxide from the denitrogenation column, a pipe for drawing a gas enriched with nitrogen from the head of the denitrogenation column and means for sending at least one portion of the gas enriched with hydrogen to the denitrogenation column.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a §371 of International PCT Application PCT/FR2014/052368, filed Sep. 23, 2014, which claims the benefit of FR1359155, filed Sep. 24, 2013, both of which are herein incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and to an apparatus for the cryogenic separation of a mixture containing at least carbon monoxide, hydrogen and nitrogen. The nitrogen content may fluctuate. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known practice to separate such a mixture by cryogenic distillation, using at least one stripping column which serves to reduce the H 2  content of the CO, followed by a nitrogen-removal column which is a CO/N 2  separation column. 
         [0004]    Head condensers at the top of the CO/N 2  columns are of the “water bath vaporizer” type. 
         [0005]    When the N 2  content in the mixture (which means to say the N 2  content entering the CO/N 2  column) is far lower than the content used as a basis for designing the CO/N 2  column, the installed condenser has, de facto an excess heat-exchange surface area which leads to all of the CO/N 2  mixture becoming condensed at the bottom of the tank, the result of this being that the column operating pressure can no longer be maintained. It is then no longer possible to maintain the purity of the CO that is to be produced, and that results in a risk of the unit shutting down. One way of reducing the exchange surface area of the condenser is to lower the level of the bath (liquid CO—partially “unflood” the condenser) but there is a limit to this principle caused by the fact that the condenser is of the “thermosiphon” type, namely is designed for a recirculation rate that requires a minimum liquid charge in order to operate correctly, the minimum head of liquid required not necessarily being aligned with the head to which it needs to be set in order to manage very low N 2  contents. 
         [0006]    The risks associated with excessively low N 2  contents in the CO/N 2  column are generally compensated for through the possibility of importing and injecting gaseous N 2  into the impure-CO line feeding the column in order to guarantee the operational stability of the “water bath vaporizer” condenser and the stability of the operating pressure in the column. 
         [0007]    FR-A-2895067 and WO-A-2008/099124 describe solutions using an addition of nitrogen if the nitrogen content in the mixture drops. 
         [0008]    EP-A-0928936 and “Cryogenic Gas Separation: The most economic and experienced separation process for production of carbon monoxide and hydrogen from raw synthesis gas” by Linde AG describe methods according to the closest prior art. 
       SUMMARY OF THE INVENTION 
       [0009]    One subject of the invention provides a method for the cryogenic separation of a mixture containing at least carbon monoxide, hydrogen and nitrogen in which: 
         [0010]    the mixture, in liquid form, is sent to the top of a stripping column where it separates to form a hydrogen-lean liquid and a hydrogen-enriched gas and the hydrogen-lean liquid or a fluid derived from this liquid is sent to a nitrogen-removal column where it separates to form a carbon monoxide-enriched liquid and a nitrogen-enriched gas containing the majority of the nitrogen present in the mixture wherein:
       if the nitrogen content of the mixture and/or of the nitrogen-enriched gas and/or, in the case of variant ii), of a carbon monoxide-enriched gas, is below a threshold,       
 
         [0012]    i) at least part of the hydrogen-enriched gas coming from the stripping column is sent to an intermediate point of the nitrogen-removal column, or 
         [0013]    ii) if the fluid is derived from the hydrogen-lean liquid by distillation in a separation column, at least part of the hydrogen-enriched gas from the stripping column is sent to an intermediate point of the column to the separation column, the carbon monoxide-enriched gas is removed from the top of the separation column and an uncondensed gas from a head condenser at the top of the separation column is sent to the nitrogen-removal column. 
         [0014]    According to other optional subjects:
       in variant ii), the mixture contains methane and the hydrogen-lean liquid is sent to the separation column, the latter having a head condenser at the top, a methane-enriched liquid being produced at the bottom and a carbon monoxide-enriched gas being tapped off at the top of the column.   it is a detector of the flow rate of the nitrogen-enriched gas that triggers the sending of hydrogen-enriched gas to the separation column or to the nitrogen-removal column.   in the case of variant ii) it is a detector of the nitrogen-enriched flow rate at the top of the nitrogen-removal column that triggers the sending of gas not condensed in the condenser to the nitrogen-removal column.   in the case of variant ii) the separation column is decoupled in terms of pressure from the nitrogen-removal column by a pressure detector on the carbon monoxide-enriched gas at the top of the separation column.   in the case of variant ii) in which the gas not condensed in the condenser is mixed with the carbon monoxide-enriched gas.   in the case of variant ii) in which only if the nitrogen content of the mixture and/or of the carbon monoxide-enriched gas and/or of the nitrogen-enriched gas is below a threshold is at least part of the hydrogen-enriched gas sent to the separation column at an intermediate point of the separation column and is gas from the head condenser which has not condensed there sent to the nitrogen-removal column.   in the case of variant ii), the mixture is a bottom liquor from a methane scrubbing column, the scrubbing liquid coming from the bottom of the separation column.   the mixture is a liquid from a phase separator.   in variant i) the fluid derived from the hydrogen-lean liquid is produced by expanding the liquid through a valve.       
 
         [0024]    Another subject of the invention provides an apparatus for the cryogenic separation of a mixture of carbon monoxide, hydrogen and nitrogen and possibly methane, comprising a stripping column, a nitrogen-removal column and possibly a separation column with a head condenser at the top, a pipe for sending the mixture in liquid form at the top of the stripping column, a pipe for removing a hydrogen-lean liquid, which pipe is connected to the stripping column, a pipe for removing a hydrogen-enriched gas from the stripping column, means for sending the hydrogen-lean liquid or a fluid derived from this liquid to the nitrogen-removal column, a pipe for tapping a carbon monoxide-enriched liquid off the nitrogen-removal column and a pipe for tapping a nitrogen-enriched gas off the top of the nitrogen-removal column, characterized in that it comprises means for sending at least part of the hydrogen-enriched gas or a fluid derived from this gas to the nitrogen-removal column. 
         [0025]    The apparatus may comprise a separation column that has a head condenser at the top and in which the means for sending a fluid derived from the hydrogen-enriched gas to the nitrogen-removal column comprise a pipe for sending at least part of the hydrogen-enriched gas to a hydrogen-lean liquid separation column, the pipe being connected to an intermediate point of the separation column, and a pipe for sending a gas from the head condenser which has not condensed there to the nitrogen-removal column. 
         [0026]    The apparatus may comprise a separation column having a head condenser at the top, the pipe for removing the hydrogen-lean liquid from the stripping column being connected to the separation column, the pipe for removing a hydrogen-enriched gas being connected to the separation column and comprising a pipe for removing carbon monoxide-enriched gas from the top of the separation column and a pipe for sending an uncondensed gas from the head condenser (6) at the top of the separation column to the nitrogen-removal column. 
         [0027]    The apparatus may comprise means for detecting the nitrogen content of the mixture and/or of the nitrogen-enriched gas and/or of the carbon monoxide-enriched gas and means for regulating the flow rate of the at least part of the hydrogen-enriched gas or of the fluid derived from this gas that is sent to the nitrogen-removal column. 
         [0028]    The apparatus may comprise means for sending at least part of the hydrogen-enriched gas to the separation column at an intermediate point of the separation column or to the nitrogen-removal column using a detector of the nitrogen content of the mixture and/or of the carbon monoxide-enriched gas and/or of the nitrogen-enriched gas and/or a detector of the flow rate of the mixture and/or of the carbon monoxide-enriched gas and/or of the nitrogen-enriched gas. 
         [0029]    One preferred feature of the present invention is that it can compensate for the reduction in nitrogen by importing some of the head gas from the stripping column upstream of the CO/N 2  column into the CO/N 2  column: the head gas from the stripping column contains hydrogen, making it possible to increase the flow rate of incondensables heading toward the CO/N 2  column. 
         [0030]    The flow rate of this new line can be controlled by a flow rate control means (FIC). Fitting an incondensables bypass pipe between the stripping column and the CO/N 2  affords the following advantages:
       The operating pressure of the CO/N 2  column is no longer dependent on the nitrogen content in the impure CO (resulting from the natural gas feeding the synthesis gas generation unit).   There is no importing of gaseous nitrogen when the N 2  content in the cold box charge gas proves to be too low.       
 
         [0033]    The invention will be described in greater detail with reference to the figures which show apparatuses according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    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. 
           [0035]      FIG. 1  represents a process flow diagram in accordance with an embodiment of the present invention. 
           [0036]      FIG. 2  represents a process flow diagram in accordance with another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    In  FIG. 1 , a mixture  10  of carbon monoxide, nitrogen and hydrogen is cooled in an exchanger  9  and sent to a distillation column  1  (or separator). A hydrogen-enriched head gas  12  leaves the top of the column  1  and is heated up in the exchanger  9 . The bottom liquor  13  is expanded and then sent to the top of a stripping column  2  which is heated at the bottom by a reboiler  8 . The hydrogen-enriched head gas  14  is heated up in the exchanger  9  and then burnt. The bottom liquor  15  containing mainly nitrogen and carbon monoxide is expanded and sent to the middle of a nitrogen-removal column  4 . The carbon monoxide-enriched bottom liquor  22  is sent to the exchanger  18  where it is used to condense the head gas from the nitrogen-removal column. In the nitrogen-removal column  4 , a nitrogen-enriched gas  21  leaves the top of the column. 
         [0038]    If the flow rate of nitrogen  21  drops below a given threshold, the valve of the pipe  26  opens and hydrogen-enriched gas is sent from the top of the stripping column to the nitrogen-removal column  4 , preferably mixing it with the gas formed by expanding the liquid from the pipe  16  through a valve. 
         [0039]    According to  FIG. 2 , the mixture that is to be separated contains nitrogen, hydrogen, methane and carbon monoxide. 
         [0040]    It is known practice to separate such a mixture using cryogenic distillation, using a CO/CH 4  separation column followed by a CO/N 2  separation column. 
         [0041]    The incondensables (H 2 —N 2 ) from the CO/CH 4  column are not put to use: they are sent directly to the flare or to a fuel network. 
         [0042]    The CO/CH 4  and CO/N 2  columns are operated at equal pressures. 
         [0043]    The innovative feature consists in compensating for the reduction in nitrogen by importing head gas (which contains hydrogen) coming from the stripping column upstream of the CO/CH 4  column, to the CO/CH 4  column and then by importing incondensables coming from the CO/CH 4  column to the CO/N 2  column. 
         [0044]    All or part of the head gas from the stripping column (which contains hydrogen) is collected so as to be routed toward the feed of the CO/CH 4  column. 
         [0045]    Thanks to this new line, the flow rate of incondensables heading toward the CO/CH 4  column can be increased. 
         [0046]    All or some of the incondensables are then recovered at the top of the CO/CH 4  column (which contains hydrogen and does not contain methane) so as to be directed toward the CO/N 2  column via the impure-CO line that feeds the CO/N 2  column. Thanks to this other new line, the pressure in the CO/N 2  column can also be maintained if there is a nitrogen deficit in the CO: the presence of hydrogen compensates for the lack of nitrogen. 
         [0047]    The flow rates of each of the new lines are controlled by an FIC. 
         [0048]    The pressure in the CO/CH 4  column needs to be raised slightly as a result in order to be sure that the flow of incondensables will indeed be directed toward the CO/N 2  column. 
         [0049]    The flow rate of impure CO is controlled by a PIC added to the CO/CH 4  column so that the two operating pressures of the columns can be decoupled from one another. 
         [0050]    The incondensables at the top of the CO/N 2  column are then directed toward the fuel network via the N 2  purge line which passes via the exchange line. 
         [0051]    A mixture  10  of carbon monoxide, nitrogen, hydrogen and methane is cooled in an exchanger  9  and sent to a methane scrubbing column  1  fed at the top with liquid methane  11  or to a separator. A hydrogen-enriched head gas  12  leaves the top of the column  1  and is heated up in the exchanger  9 . The bottom liquor  13  is expanded and then sent to the top of a stripping column  2  reheated at its bottom by a reboiler  8 . The hydrogen-enriched head gas  14  is heated up in the exchanger  9  and then burnt. The bottom liquor  15  containing mainly methane and carbon monoxide is expanded and sent to the middle of a distillation column  3  having a bottom reboiler  7  and a head condenser  6 . A flow of incondensable gases  22  leaves this condenser  6 . A carbon monoxide-enriched head gas  16  leaves the column  3  and is sent to the nitrogen-removal column  4 . The methane-enriched bottom liquor is pumped by the pump  5 , and split into two. One part  20  is vaporized in the exchanger  9  and the rest is fed to the scrubbing column  1  as flow  11 . 
         [0052]    In the nitrogen-removal column  4 , a nitrogen-enriched gas  21  leaves the top of the column and a liquid  22  leaves the bottom enriched in carbon monoxide. The carbon monoxide vaporizes in the head condenser  18  which is a water bath vaporizer. 
         [0053]    There is a pipe  26  connecting the head gas outlet  14  of the column  2  to the liquid pipe  15  entering the column  3 . There is also a pipe  24  that allows incondensable gas to be sent from the condenser  6  to the nitrogen-removal column, particularly mixing it with the gas  16 . 
         [0054]    These two pipes operate only if there is a reduction in nitrogen in the flow  10  or  16 . When a reduction in nitrogen is detected, all or part of the head gas from the top of the stripping column  2  is sent toward the feed of the CO/CH 4  column  3 . By virtue of this new pipe, the flow rate of incondensables heading toward the CO/CH 4  column can be increased. 
         [0055]    All or part of the incondensables from the head condenser  6  at the top of the CO/CH 4  column  3  are then recovered to be routed toward the CO/N 2  column via the impure CO line  16  that feeds the CO/N 2  column. By virtue of this other new pipe  26 , the pressure in the CO/N 2  column  4  can thus be maintained if there is a deficit of nitrogen in the CO: the presence of hydrogen compensates for the lack of nitrogen. 
         [0056]    The flow rates in each of the new pipes  24 ,  26  are controlled by a detector detecting minimal purity of nitrogen in the nitrogen purge  21  from the head of the CO/N 2  column  4 , generally after passing through the exchange lines  9 . 
         [0057]    The pressure of the CO/CH 4  column  3  needs to be raised slightly as a result in order to be sure that the flow of incondensables will indeed be directed toward the CO/N 2  column  4 . 
         [0058]    The flow rate of impure CO  16  is controlled by a pressure controller added to the CO/CH 4  column so that the two operating pressures of the columns can be decoupled from one another. 
         [0059]    The incondensables, including the hydrogen, at the top of the CO/N 2  column, are then directed toward a fuel network via the nitrogen purge pipe  21  which passes via the exchange line. 
         [0060]    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. 
         [0061]    The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0062]    “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. 
         [0063]    “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. 
         [0064]    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. 
         [0065]    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. 
         [0066]    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. 
       LIST OF ELEMENTS IN THE DRAWINGS 
       [0067]      1  scrubbing column or separator 
         [0068]      2  stripping column 
         [0069]      3  methane/carbon monoxide separation column 
         [0070]      4  carbon monoxide/nitrogen separation column 
         [0071]      5  methane pumps 
         [0072]      6  methane/carbon monoxide separation column condenser 
         [0073]      7  methane/carbon monoxide separation column reboiler 
         [0074]      8  stripping column reboiler 
         [0075]      9  heat exchanger 
         [0076]      10  synthesis gas feed 
         [0077]      11  scrubbing methane feed 
         [0078]      12  hydrogen-rich gas 
         [0079]      13  stripping column feed 
         [0080]      14  stripping column head gas 
         [0081]      15  methane/carbon monoxide separation column feed 
         [0082]      16  carbon monoxide/nitrogen separation column feed 
         [0083]      17  liquid carbon monoxide reflux 
         [0084]      18  carbon monoxide/nitrogen separation column cooler 
         [0085]      19  pumped liquid methane 
         [0086]      20  liquid methane purge 
         [0087]      21  nitrogen purge 
         [0088]      22  low-pressure carbon monoxide 
         [0089]      23  incondensables 
         [0090]      24  pipe carrying hydrogen-rich gas to the column  3   
         [0091]      26  pipe carrying incondensables to the column  4   
         [0092]      30  cold box