Abstract:
A process and a device for the cryogenic separation of a mixture of hydrogen and carbon monoxide, in particular of a mixture having, as main components, hydrogen and carbon monoxide is presented.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a §371 of International PCT Application PCT/FR2009/050484, filed Mar. 20, 2009. 
     FIELD OF INVENTION 
     The present invention relates to a process and a device for the cryogenic separation of a mixture of hydrogen and carbon monoxide, in particular of a mixture having, as main components, hydrogen and carbon monoxide. 
     BACKGROUND 
     The units for the production of carbon monoxide and hydrogen can be separated into two parts:
         generation of synthesis gas (mixture comprising H 2 , CO, CH 4 , CO 2 , Ar and N 2  essentially). Among the various industrial routes for the production of synthesis gas, that based on coal gasification appears to become increasingly more prevalent, in particular in countries rich in coal deposits, such as China. The design of this unit, which comprises a reactor for the gasification of coal with oxygen and steam, is based on the required production of CO and the required production of hydrogen.   purification of the synthesis gas. The following are found:   a unit for scrubbing with a liquid solvent, in order to remove the larger part of the acid gases present in the synthesis gas,   a unit for purification over a bed of adsorbents,   a unit for separation by the cryogenic route, “cold box”, for the production of CO.       

     Generally, the synthesis gas comprises a mixture at high pressure (generally between 30 and 60 bar) and is very rich in CO (approximately 50 mol %). Another advantage of the coal gasification process is the low content of impurities (CH 4 , argon and nitrogen) present in the synthesis gas at the inlet of the cold box for the production of pure CO. 
     This makes it possible to envisage a relatively simplified flowchart for the cold box, the cryogenic separation being limited to a separation between CO and hydrogen. The contents of inert substances in the synthesis gas are compatible with the CO purity required by the client in the majority of cases. 
     This flowchart does not comprise a cycle dedicated to separation. 
     The hydrogen separated from the CO is required at high pressure in order to be able to make economic use of it, either in a PSA or in a unit for the synthesis of methanol or other. 
     A portion of the separation energy of said cold box is provided by free expansion between the synthesis gas and the pure CO produced at low pressure but, in the majority of cases, this free expansion is not sufficient to complete the refrigeration balance of the unit. A supply of liquid nitrogen is necessary in order to keep the cold box cold and to complete the refrigeration balance. 
     The synthesis gas at a pressure generally of between 30 and 60 bar coming from a pretreatment unit (CO 2  and MeOH separation) is cooled in the main exchange line and partially condensed before feeding a one-stage partial condensation separator pot. The hydrogen-rich vapor is in the majority of cases conveyed to an MeOH unit or to a PSA after reheating in the exchange line. The bottom liquid is conveyed to a medium-pressure stripping column (about 14 bar) after expansion. The top vapor, known as flash gas, exits from the cold box after reheating and is conveyed as fuel or recycled material to the system via a compressor. 
     A stream withdrawn from the column at a level above the vessel bottom is subcooled to a certain temperature level, expanded, conveyed to a thermosiphon pot and then evaporated in the exchange line before being conveyed to the suction port of the CO compressor. 
     The bottom liquid from the stripping column is subcooled in the exchange line to a temperature level less cold than that of the stream mentioned above before being expanded, evaporated and reheated in the main exchange line and finally conveyed to an intermediate stage of the CO compressor. 
     A third stream can be withdrawn at another level of the column (a level above the bottom), subcooled to a temperature level different from the two preceding levels, expanded, evaporated and reheated in the exchange line. 
     The CO compressor makes it possible to compress the CO produced (which is the sum of all the liquid withdrawals from the stripping column) to the pressure required by the downstream unit (acetic acid or other). 
     The advantage of subcooling the flows of carbon monoxide to different temperature levels is to reduce the KS and the heat load and thus the capital cost of the main exchange line. The medium-pressure flow(s) is (are) subcooled to (a) temperature level(s) which is (are) less cold than if they were all mixed. 
     This makes it possible to reduce the electrical energy of the CO compressor  33 ,  35 ,  37 . Less energy is consumed for the subcooling of the medium-pressure levels. Each of the CO-rich flows is subcooled only to a certain temperature level which prevents the creation of gas after expansion, which makes it possible to avoid having to install two-phase (liquid-gas) introduction pots for the main exchange line. 
     This makes it possible to have a low-pressure fluid rich in carbon monoxide (lowest pressure level at the thermosiphon pot  27 ) which can comprise more hydrogen than the bottom of the stripping column and thus makes it possible to have a low evaporation temperature for an identical pressure level and thus makes it possible to further cool the synthesis gas (for an identical ΔT) and to improve the CO output of the unit. Alternatively, at an identical output, this makes it possible to reduce the electrical energy of the compressor as it is possible to increase the suction pressure of the compressor. 
     SUMMARY OF THE INVENTION 
     According to one subject matter of the invention, a process as claimed in claim  1  is provided for. 
     Optionally:
         the first and second flows have different purities;   the first and second flows have the same purity;   at least one of the flows is withdrawn at the bottom of the stripping column;   the first flow is a flow withdrawn at the stripping column bottom which is cooled to a first temperature in the exchange line, is expanded and is then reheated in the exchange line and the second flow is withdrawn at an intermediate level of the stripping column, is cooled to a second temperature lower than the first temperature, is expanded and is then reheated in the exchange line;   the second flow, cooled to the second temperature, is conveyed to the separator pot;   a third flow is withdrawn at an intermediate level of the stripping column above or below that of the second flow, is cooled to a third temperature lower or greater than the first temperature and/or lower or greater than the second temperature, is expanded and is then reheated in the exchange line.       

     According to another subject matter of the invention, provision is made for a device for the cryogenic separation of a mixture of hydrogen and carbon monoxide optionally comprising low contents of methane, argon and nitrogen for the production of pure carbon monoxide by partial condensation in a stage comprising an exchange line, a stripping column, a first and a second separator pot, a carbon monoxide compressor comprising at least two stages, means for conveying the mixture to be cooled into the exchange line, means for conveying the cooled mixture to the first separator pot, means for withdrawing at least a portion of the liquid resulting from the partial condensation from the first pot, means for conveying this liquid to the top of the stripping column, means for diverting at least a first flow rich in carbon monoxide and a second flow rich in carbon monoxide from the stripping column, means for cooling the first and the second flows to a first and a second temperature respectively in the exchange line, means for conveying the first flow at the first temperature or the second flow at the second temperature to the second separator pot after expansion, means for reheating the gas from the separator pot in the exchange line and means for conveying the reheated gas to the first stage of the carbon monoxide compressor, means for reheating the second flow or the first flow in the exchange line and means for conveying the reheated second flow or the reheated first flow to a stage of the compressor downstream of the first stage. 
     Optionally, the device comprises:
         means for withdrawing the at least first and second flows at the same level of the stripping column;   means for withdrawing the at least first and second flows at levels separated by at least one theoretical stage of the stripping column;   means for withdrawing at least the first flow at the bottom of the stripping column;   means for withdrawing the first flow at the bottom of the stripping column and/or means for withdrawing a second flow at a first intermediate level of the stripping column;   means for withdrawing a third flow at a level below or above the first intermediate level of the stripping column;   means for conveying the second flow, cooled in the exchange line, or the first flow, cooled in the exchange line, to a separator pot.       

    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates one embodiment of the present invention. 
         FIG. 2  illustrates another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For a further understanding of the nature and objects for the present invention, reference should be made to the detailed description, taken in conjunction with the accompanying drawing, in which like elements are given the same or analogous reference numbers and wherein: 
     The invention will be described in more detail with reference to the figures, which are diagrams of processes according to the invention. 
     In  FIG. 1 , a flow of synthesis gas  1  at a pressure generally of between 30 and 60 bar comprises hydrogen and carbon monoxide and optionally small amounts of methane, argon and nitrogen. It is cooled down to an intermediate level of the exchange line  3 , is withdrawn and is used to reboil the bottom of the column  15  by means of an exchanger  5 . Subsequently, the cooling of the synthesis gas is continued in the exchange line  3  and the synthesis gas is conveyed to a separator pot  7  where it is partially condensed. The top vapor  9  is reheated in the exchange line in order to form a hydrogen-rich flow. The bottom liquid  11  is expanded to  14  bar in a valve  13  and conveyed to the top of the stripping column  15 , also fed at the top with injected liquid nitrogen. The liquid  19  withdrawn at an intermediate level of the stripping column  15  is subcooled in the exchange line  3  before being expanded and sent to a thermosiphon pot  27 . The bottom liquid  29  from the pot  27  is evaporated in the exchange line  3  with a thermosiphon operation and the top gas  31  is conveyed to the suction port of the first stage  33  of the CO compressor. 
     The bottom liquid  21  from the stripping column  15  is subcooled in the exchange line  3  to a temperature level less cold than that of the withdrawn liquid set out above, expanded and then evaporated in the exchange line  3  to feed the compressor at an intermediate stage  35  of the CO compressor. 
     A third flow  17  is withdrawn at an intermediate level of the column different from the flow  19  (can be withdrawn at a level higher or lower than the flow  19 ) and is subcooled to a temperature level less cold than that of the flow  19 . Subsequently, this flow is expanded and then evaporated in the exchange line  3  to feed the compressor at a stage  37  of the CO compressor. 
     The three flows  17 ,  19 ,  21  are expanded to different pressures before being evaporated. 
     The three mixed flows  17 ,  19 ,  21 , after evaporation and reheating, form the carbon monoxide product  39 . 
     In  FIG. 2 , a flow of synthesis gas  1  at a pressure generally of between 30 and 60 bar comprises hydrogen and carbon monoxide and optionally small amounts of methane, argon and nitrogen. It is cooled down to an intermediate level of the exchange line  3 , is withdrawn and is used to reboil the bottom of the column  15  by means of an exchanger  5 . Subsequently, the cooling of the synthesis gas is continued in the exchange line  3  and the synthesis gas is conveyed to a separator pot  7  where it is partially condensed. The top vapor  9  is reheated in the exchange line in order to form a hydrogen-rich flow. The bottom liquid  11  is expanded to a pressure of about 14 bar in a valve  13  and conveyed to the top of the stripping column  15 , also fed at the top with injected liquid nitrogen. The bottom liquid  21  from the stripping column  15  is divided into three fractions  41 ,  43 ,  45 , which are each subcooled to a different temperature in the exchange line  3  before being expanded. The first flow  45  is conveyed to a thermosiphon pot  27 . The bottom liquid  47  from the pot is evaporated in the exchange line  3  with a thermosiphon operation and the top gas  31  is conveyed to the suction port of the first stage  33  of the CO compressor. 
     The other flows  41 ,  43  are expanded to different pressures and are then evaporated in the exchange line  3  to feed the compressor at intermediate stages  35  and  37  of the CO compressor. 
     The three flows  41 ,  43 ,  45  are expanded to different pressures before being evaporated. 
     The three mixed flows  41 ,  43 ,  45  form the carbon monoxide product  39 . 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.