Abstract:
The regeneration phase of the adsorption cycle includes a depressurization step, a heating/elution step, during which the bed is purged with a hot heating/elution gas, and a cooling/elution step, during which the bed is purged with a cold cooling/elution gas. The cooling/elution step is terminated while the cooling/elution gas leaving the bed is at a temperature markedly higher than the temperature of the gas to be treated and the adsorption phase comprises an initial adsorption step during which the bed is cooled down to the low adsorption temperature due to the effect of the gas to be treated.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for the treatment of a gas by Temperature Swing Adsorption (TSA), of the type in which each bed of adsorbent is subjected to a cycle which comprises, in succession: an adsorption phase which itself comprises an adsorption step at a low adsorption temperature; an adsorbent regeneration phase which itself comprises a depressurization step; a heating/elution step, during which the bed is purged with a hot heating/elution gas, and a cooling/elution step, during which the bed is purged with a cold cooling/elution gas; and a repressurization phase. 
     The invention applies in particular to the purification of atmospheric air intended to be distilled, especially by stripping out the water and the carbon dioxide. 
     The adsorbents used may be activated alumina, doped alumina, zeolites (A, X, LSX, etc.), silica gel or a succession of several of these products. 
     Adsorption of CO 2 , water and other impurities is promoted by low temperatures. It is therefore important for the adsorption to be carried out at low temperature and, consequently, the operation universally adopted involves cooling the entire adsorbent down to approximately the low adsorption temperature, which is close to the temperature of the gas to be treated, before starting the next adsorption phase. This way of carrying out the process also has the advantage, in the case of a cryogenic plant located downstream, of minimizing the heat flux sent into this plant when the bed is switched to the adsorption phase. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to make it possible to reduce the investment and/or to increase the productivity of the plant located downstream, while maintaining satisfactory adsorption performance. 
     For this purpose, the subject of the invention is a treatment process of the aforementioned type, characterized in that the cooling/elution step is terminated while the cooling/elution gas leaving the bed is at a markedly higher temperature than the temperature of the gas to be treated and in that the adsorption phase comprises an initial adsorption step during which the bed is cooled down to the low adsorption temperature due to the effect of the gas to be treated. 
     The process according to the invention may include one or more of the following characteristics: 
     the cooling/elution step is terminated when the cooling/elution gas leaving the bed is at a temperature at least 30° C. greater, preferably at least 50° C. greater, than the temperature of the gas to be treated; 
     during at least a first part of the initial adsorption step, the bed is connected up in parallel with another bed at the end of the adsorption phase and the two gas streams coming from these two beds are combined. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described with regard to the appended drawings, in which: 
     FIG. 1 shows schematically an air distillation plant to which the process of the invention is applied; 
     FIG. 2 is a diagram which illustrates the heating/elution and cooling/elution steps according to the invention; 
     FIG. 3 is a similar diagram to FIG. 2, which illustrates the start of the adsorption phase according to the invention; and 
     FIGS. 4 and 5 are diagrams which illustrate two embodiments of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The pressures indicated below are absolute pressures. 
     Shown schematically in FIG. 1 is a plant for producing gaseous oxygen and gaseous nitrogen at low pressure by the distillation of atmospheric air. This plant comprises essentially an air compressor  1  equipped at its output with an air or water chiller  2 , an apparatus  3  for purifying the compressed air, especially by stripping out the water and CO 2 , a double distillation column  4  and a heat exchange line  5  intended to cool the purified air by indirect heat exchange with the cold gas streams coming from the double column  4 . 
     The double column  4  essentially comprises a medium-pressure lower column  6  surmounted by a low-pressure column  7 . A reboiler/condenser  8  brings the vapour (almost pure nitrogen) from the top of the column  6  into heat-exchange relationship with the liquid (almost pure oxygen) from the bottom of the column  7 . The latter is surmounted by a section of smaller diameter or “minaret”  9  with which its top communicates freely. 
     In operation, the air to be treated, compressed at 1 to a medium pressure MP of about 5 to 6 bar, precooled at 2, purified at 3 and cooled at 5 down to close to its dew point, is introduced into the bottom of the column  6 . The “rich liquid” LR (oxygen-enriched air) from the bottom of this column is, after expansion in an expansion valve  10 , introduced at an intermediate point into the column  7 . “Upper lean liquid” LP 1  (impure nitrogen) withdrawn from an intermediate level of the column  6  is, after expansion in an expansion valve  11 , introduced into the top of the column  7 . “Upper lean liquid” LP 2  (almost pure nitrogen) withdrawn from the top of the column  6  is, after expansion in an expansion valve  12 , introduced into the top of the minaret  9 . 
     The oxygen production gas and the nitrogen production gas are withdrawn from the bottom of the column  6  and from the top of the minaret  9 , respectively. The waste gas W of the double column, consisting of impure nitrogen, is withdrawn from the top of the column  7 . These three gas streams, at close to atmospheric pressure AP, are warmed in the countercurrent exchange line  5  by the incoming air. 
     The purification apparatus  3  comprises two bottles  13 A,  13 B, each of which contains a bed  14 A,  14 B of an adsorbent which is, for example, doped alumina. Each bottle is subjected to the following cycle: 
     (1) an adsorption phase (a) at the medium pressure MP. The air coming from the chiller  2  is introduced into the inlet (lower end) of the bottle, passes through the bed  14  from the bottom up, emerges, purified, from the outlet of the bottle and is then sent into the exchange line  5 ; 
     (2) a purification phase comprising several steps: 
     a decompression step (b) down to atmospheric pressure; 
     a countercurrent (i.e. downward) heating/elution step (c) by means of waste gas W coming from the exchange line and then heated to 150° C. by a heater  15 . During this step, as illustrated by the dotted lines in FIG. 2, in which the distance along the bottle from its inlet end is plotted on the x-axis while the temperature of the adsorbent is plotted on the y-axis, a heat front moves from the outlet towards the inlet of the bottle (successive times t 1  to t 5 ). This step terminates at a predetermined time t 2  for which the heat front is entirely contained in the bottle; 
     a countercurrent cooling/elution step (d) by means of waste gas W coming from the exchange line and introduced into the bottle  13  at the temperature of the hot end of the exchange line, the heater  15  not being involved. This step (d) starts at the aforementioned time t 2 . During this step, as illustrated by the solid lines in FIG. 2, a cooling front moves from the outlet towards the inlet of the bottle (successive times t 3  to t 8 ). This step terminates substantially when, at the time t 8 , the cooling front reaches the inlet of the bottle and well before the adsorbent lying near this inlet has returned to close to the normal or low adsorption temperature. This time is, for example, determined by a temperature of the emerging elution gas of 70° C.; 
     (3) a repressurization phase, comprising an initial step (e) in which the pressure rises to the medium pressure, followed by a final safety step (f) at this medium pressure. 
     In general, the start of steps (b) and (e) takes place by the balancing of the pressures between the two bottles, whereas step (e) continues by introducing the air to be treated into the inlet of the bottle, the outlet of the latter being closed. 
     When the bottle is returned to the adsorption phase, part of the adsorbent is thus markedly above the normal or low adsorption temperature, which is close to a temperature of the air to be treated, precooled at 2, for example close to the ambient temperature. Consequently, at the start of the adsorption phase, a heat front moves along the bottle as far as its outlet, so that the incoming air cools the entire bed down to the normal adsorption temperature. 
     This is illustrated in the diagram in FIG. 3, which is similar to that in FIG. 2, for successive times t 9  to t 13  after the time t 8 . 
     The advantages resulting from the process described above will be apparent from the following two examples. 
     Comparative Example 
     The absorbent is activated alumina. 
     The operating conditions are used as follows: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Normal adsorption temperature 
                 19° 
                 C. 
               
               
                   
                 Adsorption pressure 
                 23 
                 bar 
               
               
                   
                 Adsorption rate 
                 15,000 
                 Sm 3 /h 
               
               
                   
                 Hot regeneration temperature 
                 150° 
                 C. 
               
               
                   
                 Regeneration pressure 
                 1.17 
                 bar 
               
               
                   
                 Regeneration rate 
                 2650 
                 Sm 3 /h 
               
               
                   
                 End-of-cooling temperature of the 
                 19° 
                 C. 
               
               
                   
                 adsorbent 
               
               
                   
                   
               
             
          
         
       
     
     The cooling/elution step (d) is conventionally continued until the adsorbent has been almost completely cooled. At this moment, the elution gas leaves the bottle at close to the low adsorption temperature (19° C. in this example). 
     Example 1 
     According to the Invention 
     The above operating conditions are retained, but step (d) is stopped at a 70° C. outlet temperature of the elution gas. The following comparative table can then be drawn up. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Comparative example 
                 Example 1 
               
               
                   
                 Step 
                 Duration (min) 
                 Duration (min) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 (a) Adsorption 
                 200 
                 100 
               
               
                   
                 (b) Depressurization 
                 9 
                 9 
               
               
                   
                 (c) Heating/elution 
                 45 
                 21 
               
               
                   
                 (d) Cooling/elution 
                 115 
                 45 
               
               
                   
                 (e) Repressurization 
                 17 
                 11 
               
               
                   
                 (f) Safety 
                 14 
                 12 
               
               
                   
                 Total Cycle 
                 400 
                 200 
               
               
                   
                   
               
             
          
         
       
     
     Thus, the partial cooling of the adsorbent in step (d) results, for a constant elution gas flow rate, in a significant shortening of this step, and therefore of the entire regeneration phase, which determines the duration of the adsorbent phase (a). The latter may be correspondingly reduced, so that the volume of adsorbent is decreased. In turn, this allows the durations of phases (c), (e) and (f) to be shortened. 
     Overall, it may be seen that it is possible to reduce the cycle time by a half, and therefore a significant saving on the investment can be made. 
     The above results are illustrated by the two curves in solid lines in FIGS. 4 and 5 relating to the Comparative example and to Example 1, respectively. These curves show the variation in the pressure of a bed as a function of time. The cycle of Example 1 thus comprises, in succession: an adsorption phase (a 1 ), which itself comprises an initial adsorption step (a′ 1 ) at a relatively high temperature and a final adsorption step (a″ 1 ) substantially at the normal low adsorption temperature; a depressurization step (b 1 ) down to atmospheric pressures; a heating/elution step (c 1 ); a partial cooling/elution step (d 1 ); a repressurization step (e 1 ); and a safety step (f 1 ). 
     FIG. 4 also shows, by the dot-dash lines, an alternative embodiment of the invention, which constitutes Example 2. 
     In this alternative embodiment, the volume of adsorbent and the durations of steps (a), (b), (e) and (f) of the Comparative Example are retained, as is also the total elution duration (c)+(d). 
     However, the duration of the cooling/elution step (step (d 2 )) for partially cooling the adsorbent is reduced from 115 min to 98 min; the heating/elution step (step (c 2 )) is extended accordingly, from 45 min to 62 min; and the flow rate of the elution gas is correspondingly reduced, from 2650 to 1475 Sm 3 /h, since the volume of adsorbent has not changed. 
     As regards the adsorption phase (a), this is subdivided into an initial adsorption step (a′ 2 ) at a relatively high temperature and a final adsorption step (a″ 2 ) at approximately the normal low adsorption temperature. 
     This alternative embodiment makes it possible to save a substantial fraction of the elution gas, and therefore to increase the total output of production gas, namely the output of nitrogen in the plant of FIG.  1 . In other words, the invention makes it possible, with this alternative embodiment, to increase the productivity of the plant. 
     Of course, other embodiments of the invention may be envisaged which combine the advantage of reduced investment with the advantage of increased productivity. 
     The heat front which passes through the bed at the start of the adsorption phase, illustrated in FIG. 3, moves more rapidly than the incoming air. Consequently, steps (a′ 1 ) and (a′ 2 ) are short and the adsorbent rapidly reaches a temperature close to its normal adsorption temperature, so that the performance of the adsorbent is hardly affected by the partial cooling described above. 
     If it is desired to reduce the heat peak sent into the cold box (essentially consisting of the double column  4  and the exchange line  5 ) when passing into the adsorption phase, a short time interval may be provided during which the two bottles  13 A and  13 B are in adsorption mode at the same time, and the two production gas streams (purified air) are combined before being sent into the exchange line, as illustrated by the dotted lines in FIG.  1 .