Abstract:
A method for air distillation with production of argon using an air distilling installation (1) comprising an air distilling apparatus (2) in particular with double column, and at least one column for producing impure argon. The installation has dimensions for supplying argon with a nominal yield ρ n  of argon extraction at the impure argon producing column output. For reduced argon production requirements corresponding to a required yield ρ of argon extraction at the impure argon producing column output, with ρ≦ρ o ≦ρ n  where ρ o  is a predetermined optimal yield, the argon extraction yield in the impure argon producing column is maintained at the value ρ o .

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is the 35 USC 371 national stage of International application PCT/FR99/00931 filed on Apr. 20, 1999, which designated the United States of America. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an air distillation process with production of argon by means of an air distillation plant comprising an air distillation apparatus and at least one column for production of impure argon, the plant being intended to deliver argon with a nominal argon extraction yield ρ n  at the outlet of the said impure-argon production column. 
     BACKGROUND OF THE INVENTION 
     The invention applies in particular to the production of argon by means of air distillation plants having a double distillation column. 
     In such a plant with a double air distillation column, medium-pressure nitrogen is generally removed from the top of the medium-pressure column of the double column. This medium-pressure nitrogen is generally used, after expansion in a turbine, as a source of refrigeration, especially to cool the air to be distilled. Thus, a portion of the refrigeration energy supplied to the air to be distilled can be recovered and therefore the operating costs of such a plant can be reduced. 
     Such a plant is designed to meet nominal argon production requirements with a nominal argon extraction yield ρ n  at the outlet of the impure-argon production column, called the mixture column. In general, it is sought to have a maximum yield ρ n . 
     Hitherto, when the argon production requirements decrease, for example during periods of less load on a consuming plant or when storage tanks to be filled are full, the argon extraction yield ρ at the outlet of the impure-argon production column is correspondingly reduced in order to meet at the very most these reduced argon-proauction requirements. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide an air distillation process with production of argon allowing the operating costs to be optimized when the argon-production requirements are less than the nominal requirements. 
     For this purpose, the subject of the invention is an air distillation process with production of argon by means of an air distillation plant comprising an air distillation apparatus and at least one column for production of impure argon, the plant being designed to deliver argon with a nominal argon extraction yield ρ n  at the outlet of the said impure-argon production column, characterized in that, for reduced argon production requirements corresponding to a necessary argon extraction yield ρ at the outlet of the impure-argon production column with ρ≦ρ o  ≦ρ n  , where ρ o  is a predetermined optimum yield, the argon extraction yield at the outlet of the impure-argon production column is maintained at approximately the value ρ o . 
     Depending on the particular embodiments, the process may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination: 
     the argon extracted in excess with respect to the necessary extraction yield ρ is used as a source of refrigeration in the air distillation plant, for example to cool the air to be distilled; 
     the said excess argon is at least partially withdrawn in gas and/or liquid form at the top of the impure-argon production column and this withdrawn portion is sent into at least one heat exchanger of the plant or into the air distillation apparatus; 
     the said at least partially withdrawn portion is mixed with a residual fluid withdrawn from one of the columns of the plant before it is sent into the said heat exchanger; 
     the said at least partially withdrawn portion is mixed with a fluid intended for one of the columns of the plant; 
     since the plant also comprises a column for production of nearly pure argon by argon removal connected to the said impure-argon production column, at least a portion of the excess argon is withdrawn in gas and/or liquid form at the bottom or at the top of the pure-argon production column and this withdrawn portion is sent into at least one heat exchanger of the plant or into the air distillation apparatus; 
     since the air distillation apparatus comprises a double column which itself comprises a medium-pressure column, a low-pressure column and a reboiler for bringing the top of the medium-pressure column into heat-exchange relationship with the bottom of the low-pressure column, medium-pressure nitrogen is withdrawn from the top of the medium-pressure column, ρ o  is the yield for which medium-pressure nitrogen can be withdrawn at a maximum flow rate D(ρ o ) and, for a necessary extraction yield ρ of less than ρ o , medium-pressure nitrogen is withdrawn at a flow rate of greater than D(ρ); 
     for a necessary extraction yield ρ of less than ρ o , medium-pressure nitrogen is withdrawn at the maximum flow rate D(ρ o ); 
     the medium-pressure nitrogen withdrawn as source of refrigeration in the plant is used by sending it, especially after expansion in a turbine, in a heat exchanger of the plant, for example to cool the air to be distilled; 
     since the air distillation apparatus comprises a double column which itself comprises a medium-pressure column, a low-pressure column and a reboiler for bringing the top of the medium-pressure column into heat-exchange relationship with the bottom of the low-pressure column, medium-pressure nitrogen is withdrawn from the top of the medium-pressure column, ρ o  is the yield for which air can be expanded to the low pressure at a maximum flow rate D′ (ρ o ) with the performance of external work, for the purpose of blowing it into the low-pressure column, and, for a necessary extract-ion yield ρ of less than ρ o , air is expanded to the low pressure, with the performance of external work, at a flow rage greater than D′ (ρ o ), and especially a flow rate equal to D′ (ρ o ) 
     The subject of the invention is also a plant for the implementation of the process as defined above, characterized in that it comprises an air distillation apparatus, at least one impure-argon production column, a heat exchanger, especially one through which a feed line for air to be distilled passes, and at least one branch line for sending at least one portion of the argon extracted in excess into the said heat exchanger. 
     Depending on the particular embodiments, the plant may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination: 
     an inlet of the said branch line is connected to an outlet for liquid or gaseous argon from the top of the impure-argon production column; 
     the plant comprises a nearly-pure argon production column connected to the impure-argon production column, and an inlet of the said branch line is connected to an outlet for gas or liquid from the bottom or from the top of the nearly-pure argon production column; 
     the said branch line is connected to an outlet for residual fluid from one of the columns of the plant in order to mix a residual fluid with the argon conveyed in the said branch line; 
     the said branch line is connected to an inlet for fluid into one of the columns of the plant in order to mix the fluid with the argon conveyed in the said branch line; 
     the distillation apparatus comprises a double distillation column which itself comprises a medium-pressure column, a low-pressure column and a reboiler for bringing the top of the medium-pressure column into heat-exchange relationship with the bottom of the low-pressure column, the top of the medium-pressure column has a medium-pressure nitrogen outlet and a line connects the said medium-pressure nitrogen outlet to a heat exchanger of the plant, especially one through which a feed line for air to be distilled passes; 
     the said line is provided with a turbine for expanding the medium-pressure nitrogen withdrawn; and 
     the plant comprises a turbine for blowing purified air into the low-pressure column. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly 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 diagrammatic view of an air distillation plant with production of argon according to the invention; 
     FIG. 2 is an enlarged partial view of a variant of the plant in FIG. 1, illustrating the region around the impure-argon production column; 
     FIG. 3 is a view similar to FIG. 1, illustrating a second embodiment of an air distillation plant according to the invention; 
     FIG. 4 is a diagrammatic partial view of another embodiment of an air distillation plant according to the invention; 
     FIG. 5 is a view similar to FIG. 1, illustrating a third embodiment of an air distillation plant according to the invention; and 
     FIG. 6 is a view similar to FIG. 3, illustrating a fourth embodiment of an air distillation apparatus according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an air distillation plant  1  with production of argon. This plant  1  essentially comprises a double air distillation column  2 , an impure-argon production column  3 , called a mixture column, a pure-argon production column  4 , called a nitrogen-removal column, a main heat-exchange line  5 , a main compressor  6  for the air to be distilled and an apparatus  7  for purifying the air to be distilled. 
     The double column  2  comprises a medium-pressure column, operating at a medium pressure of, for example,  6  bar absolute, a low-pressure column  9 , operating at a low pressure of less than the medium pressure, for example a pressure slightly above  1  bar absolute, and a main reboiler  10 . 
     The impure-argon production column  3  comprises a top condenser  12  for partially condensing the impure argon from the top of the column  3 . 
     The pure-argon production column  4  comprises a top condenser  13  and a bottom boiler  14 . 
     A gas line  16 , called an argon bleed line, connects an intermediate point in the low-pressure column  9  to the bottom of the impure-argon production column  3 , from the base of which column  3  a liquid return line  17  rejoins the column  9 , at approximately the same level as the line  16 . 
     A gas line  19  connects an outlet of the top condenser  12  of the column  3  to an intermediate level in the nearly-pure argon production column  4 . This line draws off that portion of the impure argon at the top of the column  3  that has not condensed in the condenser  12 . This line  19  passes from the column  3  in succession through a heat exchanger  20 , in order to condense the gaseous impure argon, and an expansion valve  21 , in order to expand this condensed impure argon. 
     The gaseous air to be distilled, compressed by the compressor  6  and purified of water and of CO 2 , for example by adsorption, in the apparatus  7  is divided into two primary streams. The first primary air stream is cooled in the main heat-exchange line  5  and then split into two secondary streams. The first secondary stream is injected into the bottom of the medium-pressure column near its dew point. The second secondary stream is sent into the bottom vaporizer  14  of the pure-argon production column  4 , where this second secondary stream is liquefied, by vaporizing the argon in the bottom of this column  4 . The liquid thus produced is sent via a line  23  into the bottom of the medium-pressure column  8 . 
     The second primary air stream, compressed and purified, is pressurized by a compressor  230 , then liquefied on passing through the main heat-exchange line  5  and expanded in an expansion valve  231  down to approximately the pressure prevailing in the medium-pressure column  8 . A first portion of this stream is then injected into the medium-pressure column  8  at an intermediate level. The other portion of this stream is supercooled on passing through a heat exchanger  24 , then expanded in an expansion valve  240  and injected into the low-pressure column  9  at an intermediate level. 
     The reboiler  10  vaporizes the liquid oxygen in the bottom of the low-pressure column  9  by condensation of the nitrogen at the top of the medium-pressure column  8 . 
     “Rich liquid” LR (i.e. liquid enriched with oxygen) is withdrawn from the bottom of the medium-pressure column  8 , then supercooled in the heat exchanger  24  and finally split into two streams. The first stream is sent, after expansion in an expansion valve  25 , into the low-pressure column  9  at an intermediate level. The second stream is sent, after expansion in an expansion valve  26  into the top condenser  12  of the impure-argon production column  3 , where this second stream is vaporized, by condensing the impure argon at the top of the column  3 . The gas thus produced is sent back, via a line  27 , into the low-pressure column  9  at an intermediate level below that at which the first stream of rich liquid was injected. 
     “Depleted liquid” (almost pure nitrogen) LP is bled from the upper part of the medium-pressure column  8 , then supercooled in the heat exchanger  24  and finally split into three streams. The first stream is expanded in an expansion valve  30  and then injected into the top of the low-pressure column  9 . The second stream is expanded in an expansion valve  31 , then vaporized in the heat exchanger  20 , by condensing the impure argon conveyed by the line  19 , and then this vaporized stream is again expanded in an expansion valve  32 . Next, this second stream is sent back via a residue line  33  into the heat exchanger  24  where this second stream is warmed by cooling the liquids LP and LR passing through the exchanger  24 . Finally, this second stream is sent into the main heat-exchange line  5 , where this second stream is warmed, thus helping to cool the air to be distilled. The third stream of depleted liquid is expanded in an expansion valve  34  before being sent into the top condenser  13  of the pure-argon production column  4 , where this third stream is vaporized by condensing the impure nitrogen at the top of the column  4 . The gas thus produced is sent, after expansion in an expansion valve  35 , into the residue line  33  in order to be warmed, on the one hand, in the heat exchanger  24 , thus cooling the liquids LP and LR, and, on the other hand, in the main heat-exchange line  5 , thereby helping to cool the air to be distilled. 
     Impure or residual nitrogen NR, withdrawn from the top of the low-pressure column  9 , are [sic] sent into the residue line  33  where this impure nitrogen is warmed on passing through the heat exchanger  24  and then the main heat-exchange line  5 . 
     Liquid oxygen OL, withdrawn from the bottom of the low-pressure column  9 , is pumped by a pump  37  and then sent via a line  38  into the main heat-exchange line  5  where this liquid oxygen is vaporized, thereby helping to cool the air to be distilled. 
     Medium-pressure gaseous nitrogen NGMP is bled from the top of the medium-pressure column  8  and then sent via a line  39  to the heat-exchange line  5  in order to help to cool the air to be distilled. In an intermediate region of this heat-exchange line  5 , the medium-pressure gaseous nitrogen is split into two streams. The first stream passes through the rest of the line  5 , where it is warmed and then delivered via a production line  40 , for example in order to feed a plant  140  where it is consumed. The second stream is expanded in a turbine  41  and then sent into the residue line  33  at the cold end of the heat-exchange line  5  in order again to help to cool the air to be distilled. 
     Medium-pressure liquid nitrogen NLMP is withdrawn from the top of the medium-pressure column  8  and then sent via a line  43  into the heat exchanger  24 , where this liquid nitrogen is supercooled by warming the residual gases conveyed by the residue line  33 . Next, this liquid nitrogen is delivered, for example, by feeding, after expansion in an expansion valve  143 , a storage tank  144 . 
     Nearly pure liquid argon ArL is withdrawn from the bottom of the column  4  and then delivered via a production line  45 . Impure or residual nitrogen is bled off from the top of the column  4  and then removed via a line  46 . 
     The plant  1  furthermore comprises a branch line  48 , the inlet  49  of which is connected to the line  19 , between the heat exchanger  20  and the expansion valve  21 , and the outlet  50  of which emerges in the residue line  33  just upstream of the heat exchanger  24 . The role of this branch line  48  will be described later. 
     The medium-pressure column  8  has, for example, 40 theoretical trays and the low-pressure column  9  has, for example, 65 theoretical trays. The plant  1  is designed, for example, to handle an air flow rate of 1000 Nm 3 /h and to extract 207.4 Nm 3 /h of pure oxygen, 6.4 Nm 3 /h of pure argon and 160 Nm 3 /h of medium-pressure gaseous nitrogen. 
     These numbers correspond to the nominal operation of the plant  1 . The nominal argon extraction yield ρ n  at the outlet of the impure-argon production column  3  is approximately 69%. This yield ρ n  is in this case equal to the optimum argon extraction yield ρ o  with respect to the amount of medium-pressure nitrogen that can be withdrawn from the top of the medium-pressure column  8 . 
     When the argon supply requirements decrease, for example for constant oxygen supply requirements, the argon extraction yield ρ at the outlet of the column  3  necessary to meet these reduced requirements is less than ρ o . However, the extraction yield is maintained at the value ρ o  and the excess argon thus extracted at the outlet of the impure-argon production column  3  is sent back into the residue line  33  via the branch line  48 . 
     Thus, by maintaining the extraction yield ρ at the value ρ o , the flow rate D at which medium-pressure nitrogen can be withdrawn from the top of the medium-pressure column  8  is maintained at the maximum value D(ρ o ) 
     In contrast, if, as in the prior art, argon were to be produced with an argon extraction yield ρ&lt;ρ o  corresponding to the argon supply requirements, the flow rate D(ρ) at which medium-pressure nitrogen can be withdrawn would be less than D(ρ o ) Table I below illustrates this observation. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Case 
                 Case 
                 Case 
                 Case 
                 Case 
               
               
                   
                 1 
                 2A 
                 2B 
                 3A 
                 3B 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 D (air) 
                   
                 1000 
                 1000 
                 1000 
                 1000 
                 1000 
               
               
                 (Nm 3 /h) 
               
               
                 D (oxygen) 
                 207.5 
                 207.5 
                 207.5 
                 207.5 
                 207.5 
               
               
                 (Nm 3 /h) 
               
               
                 D (argon) 
                 Extracted from 
                 6.4 
                 2.8 
                 6.4 
                 0 
                 6.4 
               
               
                 (Nm 3 /h) 
                 the mixture 
               
               
                   
                 column 
               
               
                   
                 Amount 
                 −6.4 
                 −2.8 
                 −2.8 
                 0 
                 0 
               
               
                   
                 produced (ArL) 
               
               
                 D (NGMP) 
                 Total 
                 160 
                 130 
                 160 
                 100 
                 160 
               
               
                   
                 Expanded in 
                 80 
                 52 
                 52 
                 30 
                 30 
               
               
                   
                 the turbine 
               
               
                   
                 Amount 
                 80 
                 78 
                 108 
                 70 
                 130 
               
               
                   
                 remaining 
               
               
                 Gain (NGMP) 
                 NGMP 
                   
                   
                 30 
                   
                 60 
               
               
                   
                 Energy 
                   
                   
                 −3% 
                   
                 −6% 
               
               
                   
               
             
          
         
       
     
     In this table, Case  1  corresponds to the nominal operating conditions of the plant  1 . 
     Cases  2 A and  2 B correspond to the operation of the plant for argon supply requirements of less than the nominal requirements, these corresponding to a necessary argon extraction yield ρ at the outlet of the column  3  of approximately 30%. 
     Cases  3 A and  3 B correspond to the operation of the argon plant  1  for zero argon supply requirements, these therefore corresponding to a necessary argon extraction yield p of 0%. 
     The letters A and B correspond to the implementation of a process according to the prior art and to the implementation of a process according to the invention, respectively. In these cases, it is assumed that the medium-pressure liquid nitrogen is bled off at a constant flow rate. 
     It may therefore be seen that the process according to the invention makes it possible to maintain the amount of medium-pressure gaseous nitrogen bled off at its maximum level. The excess medium-pressure gaseous nitrogen thus extracted, i.e. D(ρ o )−D(ρ), makes it possible to reduce the energy necessary to operate the plant  1  by approximately 3% in Case  2 B compared with Case  2 A and by approximately 6% in Case  3 B compared with Case  3 A. 
     More generally, the excess medium-pressure nitrogen obtained by implementing the process may be used in various ways. Thus, this excess may be bled off in liquid and/or gas form from the top of the medium-pressure column  8 , utilized by delivering it to a plant where it is consumed, or used as a source of refrigeration in the plant  1 . It is thus possible, for example, to increase the amount of medium-pressure gaseous nitrogen expanded in the turbine  41  and therefore, for example, to reduce the amount of liquid oxygen passing through the main heat-exchange line  5 . Thus, a line  52  (shown dotted in FIG. 1) may allow liquid oxygen to be produced directly. 
     As a variant, it is possible, during periods in which excess argon is extracted, to withdraw medium-pressure nitrogen at a flow rate D which is such that D(ρ)&lt;D&lt;D(ρ o ), where ρ is the necessary extraction yield. 
     The branch line  48  makes it possible to recover the refrigerating energy of the argon extracted in excess from the outlet of the impure-argon production column  3 . This argon produced in excess is in fact used as a source of refrigeration in the heat exchanger  24  and in the heat-exchange line  5 . 
     As a variant, this branch line  48  may be omitted, the excess argon extracted then being vented, or the inlet of this branch line  48  may thus be connected to other points in the plant  1 . The inlet  49  of the line  48  may be connected to the bottom or to the top of the pure-argon production column  4  in order to bleed off the excess argon extracted via the column  3 . The inlet  49  of the line  48  may also be connected to the top of the impure-argon production column  3  in order to bleed off the gaseous impure argon, as illustrated in FIG.  2 . 
     According to other variants, the branch line  48  may pass independently through the heat exchanger  24  and/or the main heat-exchange line  5 , without the excess argon extracted being mixed with a residual gas. 
     Depending on the variants, and on the characteristics of the air distillation apparatus  2  used, the optimum yield ρ o  may be different from the nominal yield ρ n . This yield ρ o  is generally less than ρ n . 
     In this case, the argon extraction yield is maintained at the value ρ o  for argon supply requirements corresponding to a necessary yield ρ&lt;ρ o &lt;ρ n . 
     In the plant  1  described, the extraction yield ρ o  is the optimum with respect to the amount of medium-pressure nitrogen that can be withdrawn from the top of the medium-pressure column  8 . 
     However, depending on the type of plant and in particular on the nature of the air distillation apparatus  2  used, this extraction yield may be the optimum with respect to other quantities. 
     A first example, illustrated in FIG. 3, relates to air distillation plants in which the refrigeration is produced by an air-blowing turbine. As is known, this turbine  501  is placed in a line  502  which connects the outlet of the air purification apparatus  7  to the low-pressure column  9  at an intermediate level, and which passes at least partially through the heat-exchange line  5 . The turbine  501  expands air, purified by the apparatus  7  and then compressed by an auxiliary compressor  503  coupled to the turbine  501 , to the low pressure to within the pressure drops. This air-blowing turbine  501  provides the refrigeration of the plant  1  instead of the turbine  41  in FIG.  1 . In such a case, the yield ρ o  may be the optimum yield for a predetermined amount of medium-pressure gaseous nitrogen withdrawn from the top of the medium-pressure column  9  with respect to the amount of air expanded in the air-blowing turbine. Thus, by keeping the argon extraction yield ρ at the value ρ o , a maximum amount of air is expanded in the air-blowing turbine, thereby making it possible, as previously, to maximize the amount of refrigeration produced. 
     FIG. 4 illustrates a second example in which the air distillation apparatus  2  is a single distillation column. 
     In this case, impure nitrogen NC is withdrawn from the top of the column  2 , then warmed in a heat exchanger  51 , compressed in a compressor  52  and cooled in the exchanger  51  by heat exchange with the nitrogen NC to be compressed. Next, this compressed and cooled nitrogen is liquefied, by vaporizing the oxygen in the bottom of the column  2 . Next, the liquefied nitrogen is expanded in an expansion valve  53  and then reintroduced into the top of the column  2 . The yield ρ o  then corresponds approximately to the minimum flow rate of impure nitrogen NC at the top that has to be used to vaporize the oxygen at the bottom. Thus, maintaining the argon extraction yield at ρ o  during periods of reduced argon supply requirements makes it possible to reduce the compression energy delivered to the cycle compressor  52  and therefore the operating costs of the plant  1 . 
     According to the example in FIG. 5, the liquid argon from the condenser  20  is sent to the point  50  where it is mixed with impure nitrogen (lower depleted liquid) withdrawn from the medium-pressure column  8  at an intermediate level and sent into the line  133 . The mixture is partly sent into the top of the low-pressure column  9  after expansion in the valve  30 . 
     One portion of the mixture is sent after expansion in the valve  31  to the condenser  20  and another portion is sent after expansion in the valve  34  to the condenser  13 . 
     The rest of the apparatus is identical to that in FIG.  1 . 
     According to the example in FIG. 6, the gas produced by vaporization in the condenser  13  is expanded in the valve  35  and mixed with the residual nitrogen from the low-pressure column  9 . The liquid argon from the bottom of the column  4  is sent partly into the line  33 . The gas vaporized by the condenser  20  is expanded in  32  and optionally mixed with the liquid argon in the branch line  48 . Next, the liquid argon is mixed with the lower depleted liquid of the medium-pressure column and sent into the top of the low-pressure column after expansion. Any impure argon from the line  19  is sent into the pure-argon production column  4 . 
     The rest of the apparatus is identical to that in FIG.  3 . 
     More generally, the process according to the invention makes it possible to reduce the energy to be delivered to air distillation plants with production of argon. 
     The refrigerating capacity of the apparatus may be produced partly by a Claude turbine or a hydraulic turbine. 
     The process may also produce pressurized nitrogen by withdrawing liquid nitrogen from the medium-pressure column, pressurizing it and vaporizing it in the exchange line. 
     Nevertheless, the process does not necessarily include the pressurization of a liquid before it is vaporized in the exchange line. 
     The air separation apparatus may be a triple column or may include a mixing column.