Abstract:
In this process for purifying a gas by adsorption of a first impurity and of a second impurity, at least two main adsorbers ( 5 A,  5 B) and at least one auxiliary adsorber ( 6 A,  6 B) are used, the main adsorbers comprising a packing ( 8 A,  8 B,  9 A,  9 B) for adsorbing the first and second impurities. During at least a first step, the gas is purified by adsorbing the two impurities by passing through at least a first ( 5 A,  5 B) of the main adsorbers without passing through a first auxiliary adsorbers ( 6 A,  6 B), and simultaneously the second main adsorber ( 5 A,  5 B) and the or each auxiliary adsorbers ( 6 A,  6 B,  6 ) is regenerated in parallel, then, during a second step, at least some of the gas flow is purified by adsorption of the two impurities by passing in series through the first main adsorber ( 5 A,  5 B) and through the first auxiliary adsorber ( 6 A,  6 B). Application, for example, to the purification of air for the purpose of its distillation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for purifying a gas by adsorption of a first impurity and of a second impurity, of the type in which several adsorbers are used cyclically and selectively in the adsorption phase and in the regeneration phase. 
     The invention is applicable, for example, to the purification of air for the purpose of distilling it. 
     BACKGROUND OF THE INVENTION 
     For such an application, it is known to use a purification system comprising two identical adsorbers, the operation of which alternates, i.e. one is in the adsorption phase while the other is in the regeneration phase. 
     When the air flow to be treated is large, it is also known to use four identical adsorbers coupled in pairs. The two adsorbers of one and the same pair operate in parallel. The operation of the two pairs of adsorbers alternates, so that one pair of adsorbers is in the adsorption phase while the other is in the regeneration phase. Such a parallel operation makes it possible to treat large flows while limiting the manufacturing constraints on the adsorbers. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a process of the aforementioned type which makes it possible, especially, to reduce even more the costs of manufacturing or of operating an air distillation plant in which the process is implemented. 
     To this end, the subject of the invention is a process of aforementioned type, characterized in that at least two main adsorbers and at least one auxiliary adsorber are used, in that, during at least a first step, the gas is purified by adsorbing the two impurities by passing through at least a first of the main adsorbers without passing through a first auxiliary adsorber, and in that simultaneously the second main adsorber and the or each auxiliary adsorber are regenerated in parallel, then, during a second step, at least some of the gas flow is purified by adsorption of the two impurities by passing in series through the first main adsorber and through the first auxiliary adsorber. 
     According to particular embodiments, the process may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination: 
     during the second step, the pressure of the gas between the first main adsorber and the first auxiliary adsorber is changed; 
     during the second step, the gas between the first main adsorber and the first auxiliary adsorber is compressed; 
     only one auxiliary adsorber is used; 
     between the first step and the second step, the gas is purified by adsorbing the two impurities by passing through a second main adsorber; 
     at least two auxiliary adsorbers are used; 
     the first step is interrupted when the first main adsorber is substantially saturated with the second impurity; 
     each auxiliary adsorber comprises an adsorption packing comprising a single adsorbent material; 
     the first impurity is H 2 O and the second impurity CO 2 ; 
     the gas is air. 
     The subject of the invention is also a system for purifying a gas in order to implement a process as defined hereinabove, characterized in that it comprises a line for supplying the gas to be purified, a line for discharging the purified gas, a line for supplying a regeneration gas, a line for discharging the regeneration gas, at least two main adsorbers and at least one auxiliary adsorber, the main adsorbers comprising a packing for adsorbing the first and second impurities, and each auxiliary adsorber comprising a packing for absorbing at least the second impurity, and in that the system furthermore comprises first connection means, in order to connect the main adsorbers to the line for discharging the purified gas without passing through the auxiliary adsorber or without passing through any of the auxiliary adsorbers, second connection means, in order to connect each main adsorber in series with an auxiliary adsorber, and third connection means, in order to connect the or each auxiliary adsorber and at least one main adsorber in parallel with the line for supplying the regeneration gas. 
     According to particular embodiments, the system may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination: 
     the said second connection means include means for changing the gas pressure; 
     the said second connection means include compression means; 
     the purification system comprises a single auxiliary adsorber; 
     the purification system comprises at least two auxiliary adsorbers; 
     each auxiliary adsorber comprises an adsorption packing comprising a single adsorbent material; 
     the first impurity is H 2 O and the second impurity CO 2 ; and 
     the gas is air. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood on reading the description which follows, given solely by way of example, and with reference to the appended drawings, in which: 
     FIG. 1 is a schematic view of a purification system according to the invention, and 
     FIGS. 2 to  5  are views similar to FIG. 1, illustrating two variants of the system for the process of FIG.  1  and two variants of another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a purification system  1  of an air distillation plant. This system  1  can operate by varying the temperature and/or pressure. This system  1  is designed to remove the majority of the impurities, and especially the H 2 O and CO 2 , contained in a stream of compressed air at a pressure of between 4 and 50 bar and supplied by a line  2 , in order to feed, via a line  3 , a main heat exchange line and then an air distillation apparatus. The latter components are not shown in order not to overload FIG.  1 . The air distillation apparatus may be, for example, a medium-pressure column of a double air distillation column. 
     The purification system  1  comprises two identical main adsorbers  5 A and  5 B and two identical auxiliary adsorbers  6 A and  6 B. 
     Each main adsorbers  5 A, B comprises a container or bottle  7 A, B containing successively, in the adsorption direction which is vertical and directed upwards, a layer  8 A, B of a material capable of adsorbing H 2 O, for example alumina, and a layer  9 A, B of a material capable of adsorbing CO 2 , for example a molecular sieve. The layer  8 A, B has a thickness which is clearly greater than that of the layer  9 A, B. In variants not shown, the material of the layers  8 A, B and  9 A, B may be similar. The object is then to obtain a material capable of adsorbing the two impurities. Thus, the material of the layer  8 A, B generally represents between 60% and 100% of the adsorption packing loaded in the container  7 A, B. 
     Each auxiliary adsorber  6 A, B comprises a container  10 A, B in which a single layer  11 A, B of a material capable of adsorbing CO 2 , for example, the same material as that of the layers  9 A and  9 B, is placed. 
     The purification system  1  furthermore comprises a certain number of valves and connecting pipes, the position of which will now become apparent during the description of the process implemented in the purification system  1 . 
     This process is carried out by repetition of a cycle comprising four successive steps I to IV. 
     During step I, the main adsorber  5 A is in the adsorption phase, while the main adsorber  5 B and the auxiliary adsorbers  6 A and  6 B are in the regeneration phase. 
     The air of the line  2  is then supplied through an open valve  13 A to the main adsorber  5 A. The air successively passes through the layer  8 A, where H 2 O is completely adsorbed, then the layer  9 A, where CO 2  is completely adsorbed. The purified air, i.e. the dried and decarbonated air, is then sent via two open valves  15 A and  16 A directly to the line  3 , i.e. without passing through another adsorber. 
     During this time, waste nitrogen which is possibly heated and channelled by a line  18  and coming, for example, from the top of the low-pressure column of the air distillation plant, feeds in parallel: 
     the auxiliary adsorbers  6 A and  6 B, via two open valves  20 A and  20 B, and 
     the main adsorber  5 B via an open valve  24 B. 
     This waste nitrogen flows through the adsorbers  5 B,  6 A and  6 B in the regeneration direction, i.e. in the opposite direction to the adsorption direction, regenerating these adsorbers, the layers  8 B,  9 B,  11 A and  11 B of which have been substantially saturated during a previous cycle. 
     The waste nitrogen transporting the desorbed H 2 O and CO 2  is then sent, on the one hand, from the auxiliary adsorbers  6 A and  6 B via open valve  26 A and  26 B and, on the other hand, from the main adsorber  5 B via an open valve  30 B, to a discharge line  32 . 
     This step I is continued until the layer  9 A is substantially saturated with CO 2  and until the adsorber  6 A is regenerated. 
     During step II, the valves  16 A,  20 A and  26 A are closed and the air, dried by the layer  8 A and exiting the main adsorber  5 A, is sent to the auxiliary adsorber  6 A via an open valve  34 A. The air purification is then continued therein by adsorption of the CO 2  in the layer  11 A. The dried and decarbonated air exiting the auxiliary adsorber  6 A is then sent via an open valve  36 A directly to the line  3 . 
     During this step II, the auxiliary adsorber  6 A is therefore in the adsorption phase in order to purify the air in series with the main adsorber  5 A. 
     The main  5 B and auxiliary  6 B adsorbers are, as in step I, regenerated in parallel. This step II continues until the layer  8 A is substantially saturated with H 2 O or until the layer  11 A is substantially saturated with CO 2,  and until the main adsorber  5 B is regenerated. 
     During step III, the adsorber  5 B is in the adsorption phase, purifying the air of the line  2  on its own. 
     The adsorbers  5 A,  6 A and  6 B are regenerated in parallel. The path of the air and of the waste nitrogen can be deduced from the description of step I by reversing the suffices A and B. 
     This step III continues until the layer  9 B is substantially saturated with CO 2  and until the adsorber  6 B is regenerated. 
     During step IV, the main  5 B and auxiliary  6 B adsorbers purify the air of the line  2  in series, the main  5 A and auxiliary  6 A adsorbers being in the regeneration phase. The path of the air and of the waste nitrogen can be deduced from description of step II by reversing the suffices A and B. 
     Step IV continues until the adsorber  5 A is regenerated and until the layer  8 B is substantially saturated with H 2 O or until the layer  11 B is substantially saturated with CO 2 . 
     During steps I and III, the head loss between the lines  2  and  3  is limited since the air is purified only by the main adsorber  5 A or  5 B which are small in size. 
     Moreover, the main adsorbers  5 A and  5 B are regenerated during half of the cycle, but the auxiliary adsorbers  6 A and  6 B are regenerated during three steps of the cycle, i.e. steps I, III and IV in respect of the auxiliary adsorber  6 A and steps I, II and III in respect of the adsorber  6 B. Because of the relatively long regeneration time of these auxiliary adsorbers  6 A and  6 B, the waste nitrogen flow, needed for regeneration and flowing in the line  18 , is small. Consequently, the head losses upstream of the line  18  are also small. 
     Thus, the costs associated with the compression of the air in the distillation plant are small. 
     Moreover, the air flowing through the auxiliary adsorbers  6 A and  6 B is dry. There is therefore no H 2 O to desorb from the adsorbers  6 A and  6 B. Thus the regeneration direction in the auxiliary adsorbers  6 A and  6 B may be directed upwards. The adsorption direction in the adsorbers  6 A and  6 B may therefore be directed downwards, which makes it possible to increase the adsorption rate and therefore to reduce the dimensions of the containers  10 A and  10 B. 
     According to the variant in FIG. 2, a compressor  38  is placed between, on one hand, the valves  15 A and  15 B and, on the other, the valves  16 A,  16 B,  34 A and  34 B, in order to compress the air feeding the line  3 . 
     This compressor  38  is, for example, coupled to a turbine placed downstream of an intermediate outlet of the main heat exchange line of the air distillation plant, as described in the applications FR-2 674 011, FR-2 701 553 and FR-2 723 184. 
     This compressor  38  compresses the purified air coming from the main adsorber  5 A during step I, the dry air coming from the main adsorber  5 A and feeding the auxiliary adsorber  6 A during step II, the purified air coming from the main adsorber  5 B during step III and the dried air coming from the main adsorber  5 B and feeding the auxiliary adsorber  6 B during step IV. 
     The compression of the air by the compressor  38  before its passage through the auxiliary adsorbers  6 A and  6 B during steps II and IV makes it possible to improve the adsorption of CO 2  in these adsorbers. 
     Taps  39  and  40 , placed on one side between the valves  15 A and  15 B and the compressor  38  and on the other side between the compressor  38  and the valves  16 A,  16 B,  34 A and  34 B, make it possible to feed systems, not shown, with dry air. Thus, only some of the air dried by the main adsorbers  5 A and  5 B can be decarbonated in the auxiliary adsorbers  6 A and  6 B. 
     According to another variant, not shown, the compressor  38  is replaced with a turbine. This turbine expands the purified air coming from the adsorber  5 A during step I, the dried air coming from the adsorber  5 A and feeding the adsorber  6 A during step II, the purified air coming from the adsorber  5 B during step III and the dried air coming from the adsorber  5 B and feeding the adsorber  6 B during step IV. 
     The expansion of the air by the turbine, and therefore its cooling, before it passes through the auxiliary adsorbers  6 A and  6 B during steps II and IV makes it possible to improve the adsorption of CO 2  in these adsorbers. 
     According to the variant in FIG. 3, the main and auxiliary adsorbers  5 A and  6 A are formed in the same container  40 A, fitted with an intermediate internal wall  41 A isolating the main adsorber  5 A from the auxiliary adsorber  6 A. The auxiliary adsorber  6 A surmounts the main adsorber  5 A. The wall  41 A is domed and its concavity is directed towards the main adsorber  5 A. 
     The structure of the main  5 B and auxiliary  6 B adsorbers is similar and is deduced from that of adsorbers  5 A and  6 A by substituting the suffix B for the suffix A. 
     This variant makes it possible to reduce the cost of manufacturing the adsorbers  5 A, SB,  6 A and  6 B and therefore the cost of manufacturing the air distillation plant. 
     FIG. 4 illustrates another embodiment of an air purification system  1  which differs from that of FIG. 1 by the fact that the purification system  1  comprises only a single auxiliary adsorber  6 . The references of the components relating to this auxiliary adsorber  6  will be the same as those relating to the adsorbers  6 A and  6 B of FIG. 1, the suffices A and B being removed. 
     The cycle of the purification process implemented by this purification system  1  also comprises four steps I to IV described hereinbelow. 
     During step I, the main adsorber  5 A is in the adsorption phase, while the main  5 B and auxiliary  6  adsorbers are in the regeneration phase. 
     The air of the line  2  is then supplied by the open valve  13 A to the main adsorber  5 A, where it is completely decarbonated and dried. This purified air is then sent, via the open valves  15 A and  16 , directly to the line  3 . 
     The waste nitrogen of the line  18  feeds, in parallel, the auxiliary adsorber  6  via the open valve  20  and the main adsorber  5 B via the open valve  24 B. 
     The waste nitrogen, transporting the CO 2  and H 2 O that have accumulated in the layers  8 B,  9 B and  11  during a previous cycle and desorbed, is sent from the adsorbers  6  and  5 B, via the open valves  26  and  30 B, to the line  32 . 
     This step I continues until the layer  9 A is substantially saturated with CO 2  and until the adsorber  5 B is regenerated. 
     During step II, the air is purified only by the adsorber  5 B. The air of the line  2  is then sent to the adsorber  5 B via the open valve  13 B. The dried and decarbonated air is then sent directly to the line  3  via the open valves  15 B and  16 . 
     The valves  13 A,  15 A,  24 A and  30 A are closed so that the adsorber  5 A is neither in the adsorption phase nor in the regeneration phase. 
     The auxiliary adsorber  6  is in the regeneration phase. 
     This step II is continued until the layer  9 B is substantially saturated with CO 2  and until the auxiliary adsorber  6  is regenerated. 
     During step III, the air coming from the line  2  is sent via the open valve  13 A to the main adsorber  5 A, where it is dried. Next, the dried air is sent via the open valves  15 A and  34  to the auxiliary adsorber  6  which decarbonates the air. The purified air is then sent directly to the line  3  via the open valve  36 . 
     The adsorbers  5 A and  6  are therefore in the adsorption phase in order to purify the air of line  2 , in series. 
     The valves  13 B,  15 B,  24 B and  30 B are closed so that the auxiliary adsorber  5 B is neither in the adsorption phase nor in the regeneration phase. 
     This step III continues until the layer  8 A is substantially saturated with H 2 O. 
     During step IV, the adsorbers  5 B and  6  purify the air in series. The path of the waste air can be deduced from the description of step III by substituting the suffix B for the suffix A. 
     Moreover, the adsorber  5 A is in the regeneration phase. The waste nitrogen of the line  18  is then sent via the open valve  24 A to the main adsorber  5 A. The waste nitrogen transporting the desorbed H 2 O and CO 2  is then sent via the open valve  30 A to the line  32 . 
     Step IV continues until the layer  8 B is substantially saturated with H 2 O. 
     The process hereinabove makes it possible to purify a relatively large air flow with only three adsorbers  5 A,  5 B and  6 . Consequently, the cost of manufacturing the air purification system  1 , and therefore the air distillation plant, is relatively low. 
     According to the variant of FIG. 5, a compressor  38  is placed between, on one hand, the valves  15 A and  15 B and, on the other, the valves  16  and  34 . This compressor  38  corresponds to that of the variant of FIG.  3 . 
     The compressor  38  compresses the dried and decarbonated air coming from the main adsorbers  5 A and  5 B during steps I and II, and the dried air coming from the main adsorbers  5 A and  5 B and feeding the auxiliary adsorber  6  in steps III and IV. 
     As previously, the compressor  38  may also be replaced with a turbine.