Abstract:
In a method for purifying a flow rich in carbon dioxide and containing at least one impurity lighter than carbon dioxide, the flow is cooled in a heat exchanger ( 7 ) and partially condensed, the partially condensed flow is sent to a first phase separator ( 9 ) operating under a first pressure, a gas from the first phase separator is compressed and sent to a second phase separator ( 31 ) operating under a second pressure higher than the first pressure, a first liquid ( 11 ) is sent from the first phase separator to a housing ( 15 ) operating under a pressure lower than the first pressure, and a second liquid ( 33 ) is sent to the housing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a §371 of International PCT Application PCT/FR2011/052631 filed Nov. 14, 2011, which claims §119(a) foreign priority to French patent application 1059389, filed Nov. 16, 2010. 
     FIELD OF THE INVENTION 
     The present invention relates to a process and to an appliance for the purification of a flow rich in carbon dioxide. 
     BACKGROUND 
     A flow rich in carbon dioxide comprises at least 20% vol. of carbon dioxide, indeed even at least 40% vol. of carbon dioxide, indeed even at least 50% vol. of carbon dioxide or even at least 60% vol. or at least 70% vol. of carbon dioxide. 
     The flow is cooled and partially condensed in an appliance for the purification of a flow rich in carbon dioxide. The liquid phase thus formed is enriched in carbon dioxide and the gas phase is enriched in at least one lighter component which can be oxygen, nitrogen, argon, carbon monoxide, hydrogen, methane, and the like, depending on the composition of the flow to be purified. 
     An appliance for the purification of a flow rich in carbon dioxide known from WO-A-20090007937 comprises several phase separators, two being connected in series. 
     SUMMARY OF THE INVENTION 
     According to the invention, the appliance can comprise at least two phase separators operating at different pressures in order to improve the efficiency of the separation. 
     According to a subject matter of the invention, provision is made for a process for the purification of a flow rich in carbon dioxide and comprising at least one impurity which is lighter than carbon dioxide, in which:
     i) the flow is cooled in a heat exchanger and partially condensed   ii) the partially condensed flow is sent to a first phase separator operating at a first pressure   iii) a gas from the first phase separator is reheated, compressed, cooled and sent to a second phase separator operating at a second pressure greater than the first pressure   iv) a first liquid is reduced in pressure and sent from the first phase separator to a chamber operating at a pressure lower than the first pressure   v) a second liquid originating from the second phase separator or a third liquid derived from the second liquid is reduced in pressure and sent to the chamber and   vi) a purified liquid rich in carbon dioxide exits from the chamber.   

     The liquid sent from the first phase separator to the chamber can be composed of the first liquid mixed with the third liquid, as illustrated for  FIG. 3 . The third liquid is derived from the second liquid by separation in the first phase separator. 
     According to other optional characteristics:
         the second liquid is reheated, optionally in the heat exchanger, then reduced in pressure in a valve down to the pressure of the chamber and sent to the chamber,   the second phase separator operates at a lower pressure than the inlet of the second liquid into the exchanger, because of a hydrostatic pressure due to the position of the second phase separator above the inlet of the second liquid into the exchanger,   the second liquid is reduced in pressure first to an intermediate pressure between the second pressure and the pressure of the chamber and subsequently down to the pressure of the chamber,   the second liquid is reduced in pressure down to the first pressure and sent to the first phase separator, and the third liquid derived from the second liquid is sent from the first phase separator to the chamber,   the process produces a liquid final product rich in carbon dioxide,   the chamber is a phase separator,   the chamber is a distillation or washing column,   the inlet temperature of the compressor is substantially equal to the inlet temperature of the flow to be cooled in the heat exchanger.       

     According to another subject matter of the invention, provision is made for an appliance for the purification of a flow rich in carbon dioxide and comprising at least one impurity which is lighter than carbon dioxide, comprising a chamber, a compressor, a first phase separator, a second phase separator, a heat exchanger, a pipe for sending the flow rich in carbon dioxide to be cooled into the heat exchanger, a pipe for conveying the cooled flow from the exchanger to the first phase separator, means for conveying a gas from the first phase separator to the heat exchanger in order to be reheated, means for conveying this gas from the heat exchanger to the compressor, a pipe for conveying the gas from the compressor to the heat exchanger, a pipe for conveying the compressed gas from the heat exchanger to the second phase separator, a pipe for conveying a first liquid from the first phase separator to the chamber, a valve for reducing the first liquid in pressure upstream of the chamber, a pipe for bringing about the exit of a purified liquid rich in carbon dioxide from the chamber and
     i) means for withdrawing a second liquid from the second phase separator and for conveying the second liquid to the chamber and a valve for reducing the second liquid in pressure upstream of the chamber or   ii) a pipe for withdrawing the second liquid from the second phase separator and for sending it to the first phase separator, a pipe for conveying a third liquid derived from the second liquid from the first phase separator to the chamber and a valve for reducing the third liquid in pressure upstream of the chamber.   

     According to other optional characteristics:
         the means for withdrawing the second liquid from the second phase separator and for conveying the second liquid to the chamber are composed of a pipe connected to an inlet point of the exchanger and to the second phase separator and a pipe connected to an intermediate point of the exchanger and to the chamber,   the inlet point of the exchanger is below the withdrawal point of the second liquid from the second phase separator,   the appliance comprises means for reducing the second liquid in pressure to an intermediate pressure lower than the operating pressure of the second phase separator and means for reducing in pressure the second liquid or a third liquid derived from the second liquid to the pressure of the chamber,   the appliance comprises means connecting the second phase separator to the first phase separator in order to make possible the passage of liquid,   the chamber is a distillation or washing column,   the chamber is a third separation vessel.       

     The gas from a first phase separator can be compressed to a higher pressure and recondensed, optionally at the same temperature. 
     When the carbon dioxide is required at high purity (more than 98% vol.), a distillation column may be necessary. In this case, all the liquid flows originating from the phase separators are reduced in pressure and conveyed to a phase separator or the distillation column. In this case, during the reduction in pressure of the flow at the higher pressure, it may be desirable to operate at a temperature close to the solidification temperature in order to increase the output of pure carbon dioxide. The liquid which is cooled during the reduction in pressure may then solidify. Even if the partial pressure is such that the carbon dioxide does not solidify, the temperature reached might be too low for the other fluids present in the separator or the distillation column; thus, it would be the mixture in the separator or the column which might partially freeze. Alternatively, a liquid pipeline installed in the cold box close to a carbon dioxide pipeline might freeze. 
     The main risk is not so much the complete solidification of the liquids rich in carbon dioxide but rather the formation of needles of carbon dioxide which might damage the pipelines (in particular in the bends) and the instrumentation (valves, sensors, and the like). 
     The basic solution is to avoid excessively cooling the flow at higher pressure so that the liquid phase can be reduced in pressure without risk. 
     This approach reduces the carbon dioxide output of the process as it reduces the pressure and the temperature of a partial condensation. 
     One solution is to gently heat at least one liquid at higher pressure upstream of the reduction in pressure, so that it remains above the solidification point. This approach complicates the heat exchanger which cools the liquid. 
     In this case, it is envisaged to install the phase separators and the heat exchanger so that there is sufficient hydrostatic height to prevent the evaporation of the liquid. If the liquid from the separator is heated, even a little, at the same pressure, it will immediately begin to evaporate. A higher pressure is required in order for the liquid to remain liquid at the higher temperature. 
     Yet another solution is to reduce in pressure at least one of the liquids at the higher pressure in stages. 
     One possibility is to reduce in pressure the liquid at higher pressure in an intermediate phase separator, the liquid of which is sent to the column. 
     One advantage of this solution is that it reduces the number of pipes in the cold box and the number of connections to the column and the number of connections on the main exchanger and, finally, the arrangement constraints related to hydrostatic height requirements. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic representation of one embodiment of the present invention. 
         FIG. 2  is a schematic representation of one embodiment of the present invention. 
         FIG. 3  is a schematic representation of one embodiment of the present invention. 
         FIG. 4  is a schematic representation of one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , a compressor  1  compresses a flow rich in carbon dioxide, comprising at least 20% vol. of carbon dioxide, indeed even at least 40% vol. of carbon dioxide, optionally at least 60% vol. of carbon dioxide, and at least one light impurity which can be oxygen, nitrogen, argon, carbon monoxide, hydrogen, methane or several of these impurities. The flow can originate, for example, from an oxy-combustion, from a steelworks, from a cement works, from an SMR, and the like. 
     After the compression to a pressure between 8 and 40 bar abs, the flow is cooled in the cooler  4 , purified from water in the adsorption unit  5  and then sent to be cooled in the exchange line  7 , which can be composed of a plate and fin exchanger made of brazed aluminum. 
     The cooled and partially condensed flow is sent to a first phase separator  9 . The first liquid  11  from the first phase separator  9  is reduced in pressure in a valve  13  and then sent to a chamber operating at lower pressure than the first phase separator, which can be a third phase separator  15 . 
     A liquid very rich in carbon dioxide  17 , comprising less in the way of impurities than the flow compressed in the compressor  1 , is produced in the third phase separator  15 . 
     A gas rich in at least one impurity  19  exits from the third phase separator  15  and can be reheated in the exchange line  7 . 
     The gas  25  from the first phase separator  9  is reheated in the exchange line  7  and compressed in the compressor  27  to form a compressed gas  29  at a pressure between 5 and 50 bar higher than the preceding compression pressure. The gas  29  is cooled in the exchange line  7  and is sent to a second phase separator  31 . The second liquid  33  from the second phase separator is reduced in pressure in a valve  35  down to the pressure of the chamber  15 . The gas  36  from the second phase separator  31  is reheated in the exchange line  7 , is reduced in pressure in a turbine  37  and exits from the appliance as gas  39 . 
     The exchange line  7  and the phase separators  9 ,  15 ,  31  occur inside an isolated chamber (not illustrated) in order to make possible the operation at a temperature below ambient temperature. 
     The cold behavior of the appliance is provided by a refrigeration cycle  23  involving three compressors in order to compress a cycle gas to three pressures, the cycle gas being cooled and reheated in the exchange line. Other methods for producing cold can be envisaged. 
       FIG. 2  differs from  FIG. 1  in that it shows a means for preventing the solidification of carbon dioxide. The second liquid  33  exiting from the second phase separator  31 , operating at the highest pressure than the first phase separator, is reheated in the exchange line  7  and exits from the latter at a warmer temperature than the cold end of the exchange line (indicated by dotted lines, in order to show that the reheated second liquid  33  is not cooled in the exchanger). 
     In addition, the second phase separator  31  can be positioned at a height H above the inlet of the second liquid into the exchange line  7  in order to ensure that the pressure of the liquid  33  is sufficient to prevent it from evaporating in the exchange line  7 . 
     If the pressure of the liquid  33  is reduced in pressure in the valve  35  down to 10 bars abs, it is necessary to reheat the liquid in the exchange line  7  beforehand, in order to avoid falling below −54.5° C., and, in order to prevent the formation of gas on reducing in pressure, the hydrostatic height corresponding to a height H between 2.9 m and 44 m is necessary, according to the composition of the liquid. 
     If the pressure of the liquid  33  exiting from the valve  35  is at 20 bar abs, the reduction in pressure brings about formation of gas but it is not necessary to send this liquid to the exchange line  7  beforehand as the temperature is sufficiently high to prevent the formation of solids. 
       FIG. 3  differs from  FIG. 1  in that the liquid  33  from the second phase separator is not sent directly to the third phase separator  15  after it has been reduced in pressure in the valve  35  but is sent to the first phase separator. Thus, the valve  35  reduces the liquid  33  in pressure to an intermediate pressure between that of the second separator  31  and that of the chamber, thus reducing the fall in temperature. 
     The liquid sent from the first phase separator to the chamber  15  is thus in this case composed of the first liquid and of the third liquid. The third liquid is derived from the second liquid by separation in the first phase separator. 
     The chamber, which operates at lower pressure than the first pressure, can be the third phase separator  15  or, if not, a distillation or washing column if the liquefied product  17  has to be purer. 
     In  FIG. 4 , a compressor  1  compresses a flow rich in carbon dioxide, comprising at least 20% vol. of carbon dioxide, indeed even at least 40% vol. or at least 50% vol. of carbon dioxide, optionally at least 60% vol. or at least 70% vol. of carbon dioxide, and at least one light impurity which can be oxygen, nitrogen, argon, carbon monoxide, hydrogen, methane or several of these impurities. The flow can originate, for example, from an oxy-combustion, from a steelworks, from a cement works, from an SMR, and the like. 
     After the compression to a pressure between 8 and 40 bar abs, the flow is cooled in a cooler, purified from water in the adsorption unit and then sent to be cooled in the exchange line  7 , which can be composed of a plate and fin exchanger made of brazed aluminum. 
     The cold and partially condensed flow is sent to a first phase separator  9 . The first liquid  11  from the first phase separator  9  is reduced in pressure in a valve  13  and then sent to a chamber  15  operating at lower pressure than the first phase separator, this chamber being a distillation column. 
     A liquid very rich in carbon dioxide  17 , comprising less in the way of impurities than the flow compressed in the compressor  1 , is produced in the distillation column  15 . 
     A gas (not illustrated) rich in at least one impurity exits from the top of the column  15  and can be reheated in the exchange line  7 . 
     The gas  25  from the first phase separator  9  is reheated in the exchange line  7  and compressed in the compressor  27  to form a compressed gas  29  at a pressure between 5 and 50 bar higher than the preceding compression pressure. The gas  29  is cooled in the exchange line  7  and is sent to a second phase separator  31 . The second liquid  33  from the second phase separator is reduced in pressure in a valve  35  down to the pressure of the column  15 . The gas  36  from the second phase separator  31  is reheated in the exchange line  7 , is reduced in pressure in at least one turbine  37  and exits from the appliance as gas. 
     The exchange line  7 , the column  15  and the phase separators  9 ,  31  are found inside an isolated chamber (not illustrated) in order to make possible the operation at a temperature below ambient temperature. 
     The cold behavior of the appliance is provided by evaporation of the liquid  17  from the column  15  at three different pressures. The evaporated liquid is subsequently compressed in a compressor  116  and acts as product  118 . Other methods for the production of cold can be envisaged. 
     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.