Abstract:
A method for operating a typical cyclic adsorption unit that is easily implemented in both new and existing treatment plants, wherein fluctuations in the stream compositions due to the adsorption and regeneration phase transitions of the cycles are minimized.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an adsorption process for treatment of a gas mixture comprising at least one main constituent to be produced and impurities to be separated from said mixture, especially for the production of carbon monoxide with streams having predetermined hydrogen/carbon monoxide ratios. 
   2. Related Art 
   Throughout the text, the gas pressures indicated are in bar absolute. 
   Such a treatment process is widely used to separate “noble” constituents to be produced, that are contained in the gas mixture, from undesirable constituents, generally denoted by the term “impurities”. 
   The typical process is cyclic and involves at least two adsorbers, of at least two adsorption units having respectively several adsorbers operating in common, which follow in an offset manner the same operating cycle. This cycle conventionally comprises an adsorption phase, during which the corresponding adsorber is subjected to the gas mixture and adsorbs the impurities thereof, and a regeneration phase, during which the adsorber is subjected to a regeneration gas and is desorbed of the impurities that it had previously adsorbed. 
   Depending on whether or not the regeneration phase is accompanied by heating of the regeneration gas, it is common practice to distinguish cycles called TSA (Temperature Swing Adsorption) cycles from cycles called PSA (Pressure Swing Adsorption) cycles. 
   It is also known that the adsorbers may be subjected to depressurization and repressurization steps and to a step of paralleling the adsorbers, during which the total stream of treated gas is obtained both by the treatment of a first flow of gas by at least one adsorber terminating its adsorption phase and by the treatment of a second flow of gas to be treated by at least one other adsorber starting its adsorption phase. This paralleling is conventionally intended to prevent pressure surges in the stream of treated gas during passage in production from one adsorber to another, especially in order to take into account the operating time of the valves that implement the paralleling operation. 
   However, adsorption treatment cycles have drawbacks during transient periods at the start of the adsorption and regeneration phases, as partly explained in document EP-A-00 748 765. 
   That document describes a carbon monoxide production plant comprising a cryogenic production unit and, upstream of the latter, a treatment unit that employs a process of the type defined above. This plant is intended to retain the water and carbon dioxide of a gas mixture rich in carbon monoxide and in hydrogen coming from a hydrocarbon steam reforming unit. Fixing the carbon monoxide by the adsorbent of the adsorber that is starting its adsorption phase causes an appreciable reduction in the carbon monoxide content of the stream output by this adsorber, together with fluctuations in the flow rate of this stream. The solution proposed in EP-A-0 748 765 consists in interposing, between the adsorption treatment unit and the cryogenic carbon monoxide production unit, a tank filled with an adsorbent having an affinity for carbon monoxide. 
   This solution proves to be particularly expensive in terms of investment, is not very modular and attains only to the transient period when each adsorber returns to production, whereas similar transient phenomena occur at the start of the regeneration phase of each adsorber, the stream leaving the adsorbers exhibiting large fluctuations in content and in flow rate. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to propose a process of the type defined above, that is easily implemented in the treatment plants of the prior art and that makes it possible to obviate stream perturbations due to the absorption and regeneration phase transitions of the cycles of known processes. 
   For this purpose, the subject of the invention is a process of the aforementioned type, in which N adsorbers are used, where N is greater than or equal to 2, each following in an offset manner the same cycle of period T, during which there are in succession an adsorption phase and a regeneration phase using a regeneration gas, and in that each adsorber at the start of the phase and/or at the start of the use of the regeneration gas is subjected to only a portion of the nominal flow of the gas mixture to be treated, or alternatively of the nominal flow of the regeneration gas, until said adsorber is substantially saturated with, or alternatively substantially discharged of, at least one of the main constituents to be produced, while maintaining at least one other adsorber in adsorption phase. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood on reading the description that follows, given solely by way of example and with reference to the appended drawings, in which: 
       FIG. 1  is a schematic view of a carbon monoxide production plant according to the invention combined with a pure hydrogen production unit; 
       FIG. 2  is a diagram illustrating the operating cycle of the adsorbers of the plant shown in  FIG. 1 ; 
       FIGS. 3 and 4  are schematic views of the plant shown in  FIG. 1  for time intervals indicated by the numerals I and VI on the cycle shown in  FIG. 2 ; and 
       FIG. 5  is a schematic view of a plant for producing a stream with a predetermined hydrogen/carbon monoxide ratio according to the invention, combined with a pure hydrogen production unit. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Thus, one subject of the invention is a first adsorption process for the treatment of a gas mixture comprising at least one main constituent to be produced and impurities to be separated from said mixture, in which N adsorbers are used, where N is greater than or equal to 2, each following in an offset manner the same cycle of period T, during which there are in succession an adsorption phase and a regeneration phase, and each adsorber ( 11 A) at the start of the adsorption phase (step I or step IV) is subjected to only a portion of the nominal flow of the gas mixture to be treated, until said adsorber is substantially saturated with at least one of the main constituents to be produced, while maintaining at least one other adsorber ( 11 B) in adsorption phase. 
   According to other features of this process:
         in order to form the treated gas mixture, the stream coming from the adsorber subjected to said portion is mixed with the stream coming from said at least one other adsorber in adsorption phase;   the duration of the adsorption phase of each adsorber is between T/N inclusive and 2T/N noninclusive;   the adsorption treatment of the gas mixture is carried out—for most of the time—by a single adsorber in adsorption phase; and   after said adsorber has been subjected at the start of the adsorption phase to said portion of the nominal flow of the gas mixture to be treated, said adsorber is subjected to a paralleling step, during which the flow of treated gas is obtained half by said adsorber and half by said at least one other adsorber in adsorption phase.       

   Another subject of the invention is a second adsorption process for the treatment of a gas mixture comprising at least one main constituent to be produced and impurities to be separated from said mixture, characterized in that N adsorbers are used, where N is greater than or equal to two, each following in an offset manner the same cycle of period T, during which there are in succession an adsorption phase and a regeneration phase using a regeneration gas, and in that each adsorber is subjected at the start of use of the regeneration gas to only a portion of said nominal flow of the regeneration gas, until said absorber is substantially discharged of at least one of the main constituents to be produced. 
   According to other features of this second process:
         in order to form a discharged gas stream, the stream coming from the adsorber subjected to said portion is mixed with the rest of the nominal flow of the regeneration gas;   the stream coming from the adsorber subjected to said portion and the rest of the nominal flow of the regeneration gas are mixed together directly;   the stream coming from the adsorber subjected to said portion is mixed with the stream coming from another adsorber that terminates its regeneration phase and that is subjected to at least a portion of the rest of the nominal flow of regeneration gas;   the regeneration phase of each adsorber comprises a step of depressurizing and a step of repressurizing said adsorber; and   the phase of regenerating each adsorber comprises a step of heating the regeneration gas.       

   Shown in  FIG. 1  is a carbon monoxide production plant  1  connected upstream, via a line  2 , to a hydrogen production unit  4 . 
   The plant  1  comprises, upstream, an adsorption treatment unit  11  suitable for removing most of the impurities, especially water and carbon dioxide, that are contained in a gas mixture fed by a feed line  12  and compressed to a pressure of between 15 and 45 bar. This gas mixture is compressed, for example, to below 15.5 bar and has a nominal flow rate, namely the total flow rate in the line  12 , of between a few hundred and several tens of thousands of Sm 3 /h. This gas mixture includes, as main constituents, hydrogen and carbon monoxide, at 73.5 and 21.6 mol % respectively, and possibly secondary constituents, such as nitrogen and methane, for example with respective contents of 1.1 and 3.8 mol %, and it also contains, as impurities, between 10 and 200 molar ppm (parts per million) of carbon dioxide, and also water, generally to saturation. 
   The unit  11  comprises two adsorbers  11 A,  11 B placed alternately in line in order to purify the gas mixture by adsorption. Each adsorber contains an adsorbent placed either in the form of a single bed, formed from a zeolite or from activated alumina optionally doped in order to increase its carbon dioxide stopping capacity, or in the form of a plurality of beds formed respectively from activated alumina or from silica gel in order essentially to stop water, and of a zeolite (for example of the A, X or LSX type) in order essentially to stop carbon dioxide. The adsorbent may also consist of mixtures of adsorbents or of composite adsorbents. 
   The treatment unit  11  also includes valves and connecting pipes which are not shown in  FIG. 1 , but the arrangement of which will become more clearly apparent during the description of the operation of this unit. 
   The plant  1  comprises, connected via a line  13  downstream of the treatment unit  11 , a cryogenic separation unit  14  that includes a substantially pure carbon monoxide production line  15  and a line  16  outputting a stream with a high hydrogen content. For the composition of the gas mixture indicated above, the stream in the line  16  may contain 97.4 mol % hydrogen, 0.3 mol % nitrogen, 0.3 mol % carbon monoxide and 2 mol % methane, at about 14.5 bar. Since this separation unit  14  is known per se, it will not be explained in detail further. 
   The line  16  is connected to the treatment unit  11  in order to allow regeneration of the adsorber  11 A,  11 B that is not in the production line, the stream having a high hydrogen content in line  16  being used, at least partly, as gas for regenerating the adsorbent of this adsorber. The total flow rate of the line  16  forms, for the example shown, the nominal flow rate of the regeneration gas. 
   The regeneration gas output by the adsorption treatment unit is conveyed by the line  2  to the hydrogen production unit  4 , known per se. This unit  4  may, for example, comprise six adsorbers operating cyclically and suitable for producing a substantially pure hydrogen stream. 
   The process employed by the adsorption treatment unit  11  is obtained by repeating the cycle illustrated in FIG.  2 . Each of the two adsorbers  11 A,  11 B follows the cycle shown in  FIG. 2 , with a time shift relative to the other adsorber corresponding to a time interval equal to substantially one half of the period T of the cycle. 
   In  FIG. 2 , in which the times t are plotted on the x-axis and the absolute pressures P are plotted on the y-axis, the lines headed by arrows indicate the movements and destinations of the gas currents, and also the direction of circulation in the adsorbers  11 A and  11 B, respectively. When an arrow parallel to the y-axis points upwards (toward the top of the diagram), the current is said to be co-current in the adsorber, if the arrow pointing upward lies below the line indicating the pressure in the adsorber, the current enters the adsorber at the inlet end of the adsorber, if the arrow pointing upward lies above the line indicating the pressure, the current leaves the adsorber via the outlet end of the adsorber, the inlet and outlet ends being those for the gas to be treated and for the gas withdrawn in the production phase, respectively. When an arrow parallel to the y-axis points downward (toward the bottom of the diagram), the current is said to be countercurrent in the adsorber; if the arrow pointing downward lies below the line indicating the pressure of the adsorber, the current leaves the adsorber via the inlet end of the adsorber if the arrow pointing downward lies above the line indicating the pressure, the current enters the adsorber via the outlet end of the adsorber, the inlet and outlet ends again being those for the gas to be treated and for the gas withdrawn in the production phase. 
   The cycle shown in  FIG. 2  comprises eight successive steps, denoted by I to VIII, which will be described in succession by considering, for example, that the absorber  11 A starts its adsorption phase at time t 0 =0. The period T of the cycle is, for example, equal to 960 minutes for an adsorption pressure P ads  of about 15.5 bar. 
   During step I, from t 0  to t 1 =35 minutes, the adsorbers  11 A and  11 B are in adsorption phase as shown in  FIG. 3 , the adsorber  11 A receiving only 5% of the flow of the gas mixture in the line  12 , via a valve  111  for regulating the flow that passes through it, and the adsorber  11 B receiving the remaining 95% of the nominal flow, via a regulating valve  112 . 
   During this step, the freshly regenerated adsorber  11 A stops, in addition to the impurities (water and carbon dioxide), the carbon monoxide contained in the gas mixture owing to the chemical affinity of its adsorbent with carbon monoxide. Thus, the purified stream coming from the adsorber  11 A, that passes through an open valve  113 , is virtually free of carbon monoxide. For the composition of the gas mixture indicated above, the hydrogen content of this stream leaving the adsorber  11 A may reach more than 90 mol %. Concomitantly, the adsorbent of the adsorber  11 B, which was saturated with carbon monoxide prior to step I, adsorbs only the impurities from the 95% of the gas mixture that are sent to it and produces a purified stream via an open valve  114 . The streams from the valves  113  and  114  mix in the connection line  13  so that the carbon monoxide and hydrogen contents of this mixture are very similar to their nominal values, that is to say close, for example, to the content of the stream in this line  13  during the step that precedes step I, the stream coming from the adsorber  11 A, with a low flow rate and depleted in carbon monoxide, being diluted in the stream coming from the adsorber  11 B. 
   This step I is complete when most, if not all, of the adsorbent of the adsorber  11 A is saturated with carbon monoxide. 
   During step II, from t 1  to t 2 =10 minutes, wherein the total elapsed time from t 0  to t 2 =45 minutes, the adsorbers  11 A and  11 B remain in adsorption phase, but are subjected to about 50% of the nominal flow of the gas mixture to be purified, respectively, the valves  111  and  112  being operated accordingly. This step II is akin to a paralleling operation with a symmetrical distribution of the feed gas mixture, as mentioned in the preamble of the application. This paralleling advantageously allows thermal adjustment of the purified stream in the line  13 , the stream leaving the freshly regenerated adsorber  11 A having a tendency to be hotter than that of the adsorber  11 B at the end of the adsorption phase. 
   During step III, from t 2  to t 3 =T/2=435 minutes, wherein the total elapsed time from t 0  to t 3 =480 minutes, only the adsorber  11 A is in adsorption phase, the valve  111  being completely open, and the adsorber  11 B switches to the regeneration phase, the valves  112  and  114  being completely closed. Thus, during most of the operating time of the treatment unit  11  (in this case, for more than 90% of this operating time), the gas mixture is treated by a single adsorber  11 A. 
   During step IV, from t 3  to t 4 =35 minutes, wherein the total elapsed time from t 0  to t 4 =515 minutes, the adsorber  11 A is at the end of the adsorption phase and the adsorber  11 B switches to the adsorption phase, the adsorbers  11 A and  11 B being subjected to 95% and to 5% of the nominal flow of the gas mixture, respectively, by means of the corresponding adjustment of the valves  111  and  112 , and by opening the valve  114 . Step IV is therefore similar to step I, the function of the adsorbers  11 A and  11 B being reversed. 
   In the same way, step V, the interval between t 4  and t 5 =10 minutes, wherein the total elapsed time from t 0  to t 5 =525 minutes, is similar to step II, the function of the adsorbers  11 A and  11 B being reversed. 
   During step VI, from t 5  to t 6 =105 minutes, wherein the total elapsed time from t 0  to t 6 =630 minutes, the adsorber  11 A switches to the regeneration phase, the gas mixture being purified completely by the adsorber  11 B, as shown in FIG.  4 . The adsorber  11 A is connected upstream to the line  2  for connection to the hydrogen production unit  4 , via an expansion valve  115 . The pressure in the adsorber  11 A then swings from the pressure P ads  to a lower, elution pressure, denoted P elu , the value of which depends on the type of process employed in the cryogenic separation unit  14 . This elution pressure will, for example, be 1 to 2 bar below the adsorption pressure P ads . It may also be of substantially lower value, for example around 3 bar absolute. 
   Concomitantly with this depressurization, or once the latter has been completed, the adsorber  11 A is subjected to the hydrogen-rich stream (regeneration gas) in the line  16 , via a valve  116  for regulating the flow rate flowing in it. 
   This valve  116  is operated so that only 10% of the flow of the regeneration stream coming from the line  16  is sent countercurrently into the adsorber  11 A, the remaining 90% of the nominal flow being conveyed directly to the connecting line  2  via a branch line  117  provided with a regulating valve  118 . 
   During application of the regeneration gas of this step VI, the adsorbent of the adsorber  11 A that starts its regeneration is saturated with impurities (water and carbon dioxide) and with carbon monoxide. The first moments of regeneration are accompanied by a strong desorption of the carbon monoxide, the carbon monoxide content of the stream coming from the adsorber  11 A possibly reaching more than ten times that of the regeneration stream in line  16 . Applied as such to the unit  4 , especially if the latter operates by adsorption, this sudden and intense blast of carbon monoxide would result in considerable operating perturbations that would lead to a loss of hydrogen yield and/or to contamination of the production by the unit  4 . On the other hand, by mixing the stream coming from the adsorber  11 A that has a high carbon monoxide content with the regeneration stream of the branch line  117 , in respective proportions of 10 and 90%, the carbon monoxide content of the stream in the connecting line  2  remains at a value compatible with the operating tolerances of the production unit  4 . 
   This step VI continues until the adsorbent of the adsorber  11 A is completely discharged of most of the carbon monoxide. 
   Advantageously, this step may continue so as to damp out the thermal effects of the start of regeneration. This is because, since the stream coming from the adsorber  11 A tends to be cooler than the feed standard for the unit  4 , mixing it with the hotter stream, for example 20° C., in the branch line  117  makes it possible to smooth out the temperatures of the stream in the connection line  2 . 
   During step VII, from t 6  to t 7 =160 minutes, wherein the total elapsed time from t 0  to t 7 =790 minutes, the elution of the adsorbent of the adsorber  11 A continues by means of all of the regeneration stream conveyed by the line  16 , the valve  116  being completely open and the valve  118  being closed. Advantageously, the regeneration gas is heated by a heater  119 . 
   During step VIII, from t 7  to t 8 =T=170 minutes, wherein the total elapsed time from t 0  to t 8 =960 minutes, the elution of the adsorbent of the adsorber  11 B terminates with all of the unheated regeneration stream, then the valve  116  is closed in order to allow the adsorber to be repressurized. Step VIII is completed when the pressure of the adsorber  11 A reaches the value P ads . 
   The process according to the invention thus makes it possible to greatly limit the perturbations in carbon monoxide contents both of the stream of treated gas coming from the treatment unit  11  when an adsorber starts its adsorption phase (steps I and IV), and of the stream of waste gas output by this treatment unit when an adsorber starts its phase of using the regeneration gas (step VI). 
   This process is easy to implement in a plant according to the prior art, that it is necessary to equip with regulating valves, such as the valves  111 ,  112 ,  116  and  118 , and with at least one branch line such as the line  117 . 
   Of course, although based on the same idea of diluting the stream coming from the adsorber that has just started its adsorption phase or started to be subjected to the regeneration gas with the stream coming from the adsorber terminating its regeneration phase, or alternatively with the regeneration gas directly, the implementation of steps I and IV and that of step VI are independent, the example of the process according to the invention described above advantageously combining both these. 
   During steps I or IV, the percentage of the flow of gas mixture to be sent to the adsorber that is starting its adsorption phase is not limited to 5% of the flow of the feed line  12 , as in the example developed above. This percentage is generally strictly less than 50% of the flow in the line  12 , advantageously less than one third of the flow in the line  12 , preferably between 5 and 20% of the flow in the line  12 . 
   Similarly, during step VI, the percentage of the flow of regeneration gas to be sent to the adsorber that is starting to be subjected to the regeneration gas is not limited to 10% of the flow in the discharge line  16 , as in the example described above. This percentage is generally strictly less than 50% of the flow in the line  16 , advantageously less than one third of the flow in the line  16 , preferably between 5 and 20% of the flow in the line  16 . 
   As a variant of the process and independently of the value of the percentage of the abovementioned flows, the duration of step I or of step IV may be predetermined so that it is greater than about 1% of the duration of the adsorption phase of an adsorber (that is to say the period extending from step I to step V in the cycle shown in FIG.  2 ), advantageously greater than about 5% of the duration of this adsorption phase, preferably between 10 and 20% of the duration of this adsorption phase. 
   Similarly, the duration of step VI may be predetermined so that it is greater than about 1% of the duration of the regeneration phase of an adsorber (that is to say the duration extending from step VI to VIII), advantageously greater than about 5% of the duration of this regeneration phase, preferably between 10 and 20% of the duration of this regeneration phase. 
   Moreover, although described above with adsorbers  11 A and  11 B suitable for retaining, as impurities, water and carbon dioxide, the process according to the invention applies to treatment units with an adsorbent suitable for preferably fixing only water. 
   In addition, although described with a treatment unit  11  having only two adsorbers, the process according to the invention applies to units comprising a larger number of adsorbers, operating individually or in groups, for example operating in pairs. Thus, the term “adsorber” must be understood as meaning either an adsorber with its own operation, or a group of adsorbers operating in common. 
   In the case of a treatment unit comprising more than two adsorbers, each respectively operating individually, for example three adsorbers that follow the same cycle with an offset substantially equal to one third of the cycle period, the process according to the invention proves to be particularly advantageous when the adsorption treatment is carried out—for most of the time—by a single adsorber in adsorption phase (as during step III of the cycle shown in FIG.  2 ). More generally, for a treatment unit comprising N adsorbers, where N is greater than or equal to 2, which follow a cycle of period T, the process according to the invention proves to be advantageous when the duration of the adsorption phase of each adsorber is between T/N inclusive and 2T/N noninclusive. 
   Again in the case of a unit comprising more than two adsorbers, and on condition that, over a given time interval of the cycle, at least two adsorbers are in regeneration phase, the stream coming from the adsorber that is starting to be subjected to a portion of the flow of the regeneration gas may be mixed with the rest of this flow, either directly as described above, or after the rest of this flow has been sent to another adsorber that is terminating its regeneration phase. This is because, as described during step VII and at the start of step VIII of the cycle shown in  FIG. 2 , the stream coming from an adsorber that is in regeneration for a certain time no longer has sufficient perturbations of its contents and of its flow rate. Thus, this steady stream may be used to dilute the stream coming from an adsorber that is starting to be subjected to the regeneration gas. 
   As shown in  FIG. 5 , the cryogenic separation unit may, as a variant, be replaced with a permeation unit  20  suitable for producing a permeate with a predetermined hydrogen/carbon monoxide ratio, while forming a non-permeate sent to the treatment unit  11  in the same way as the stream in the line  16  for the plant  1  shown in FIG.  1 . The plant  21  thus formed makes it possible to produce a stream with a hydrogen/carbon monoxide ratio that is particularly stable over time, the process according to the invention ensuring that the unit  20  is fed via the line  13  correctly in terms of flow rate stability, hydrogen and carbon monoxide contents and temperature. 
   As a variant (not shown) of the process according to the invention, the stream coming from the adsorber that has just started its adsorption phase or that has just been subjected to the regeneration gas may be, at least partly, sent to a waste network to be reutilized, for example as combustion gas (fuel gas), especially if a loss of hydrogen and/or carbon monoxide yield by the downstream unit  14  or  20  is acceptable. 
   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.