BACKGROUND OF THE INVENTION 
     The present invention relates to an integrated air-separation/energy-generation process and to a plant for implementing such a process. 
     It is well known to send a nitrogen-enriched gas from an air separation unit upstream of a turbine for expanding combustion gas. The combustion chamber is fed with compressed air coming from an air compressor which may deliver all or some of the air needed for the air separation unit (ASU) as illustrated in EP-A-0 538 118. Alternatively, as in the case of GB-A-2 067 688, all the air may come from a dedicated compressor. 
     If it were desired to produce argon, EP-A-568 431 describes the use of an integrated system. 
     The difficulties in regulating this kind of system are explained in EP-A-0 622 595. 
     In general, for reasons of reliability, on the same site there are two gas turbines and two air separation units, which are substantially identical, producing both impure oxygen, needed for the gasification of fuels, and nitrogen. Each separation unit is fed from a gas turbine compressor and sends nitrogen only to this same gas turbine. 
     It is one object of the invention to remedy the defects of the known systems. 
     SUMMARY OF THE INVENTION 
     In particular, one object of the invention is to allow greater flexibility in the choice of products coming from an integrated air-separation/gas-turbine system. According to one aspect of the invention, this provides an integrated air-separation process producing an oxygen-enriched fluid and, optionally, a nitrogen-enriched fluid in a plant comprising at least two air separation units, each comprising at least two distillation columns, a first air compressor, a first combustion chamber and a first expansion turbine, in which process compressed air is delivered to the first air separation unit at least by the first air compressor which also delivers compressed air to the first combustion chamber, compressed air is delivered to the second air separation unit at least by an auxiliary compressor, the second separation unit: 
     i) not receiving air from a compressor which feeds a combustion chamber or 
     ii) receiving air which it treats by means of at least one compressor which also feeds a combustion chamber, the percentage of total air, treated in the second unit, which comes from the compressor feeding a combustion chamber, being less than the percentage of air, treated in the first unit, coming from the first air compressor (or coming from the first air compressor and a second air compressor also feeding a second combustion chamber), a nitrogen-enriched gas is sent from the first air separation unit upstream of at least one expansion turbine fed with combustion gases from at least one of the combustion chambers and optionally a nitrogen-enriched gas is sent from the second air separation unit upstream of at least one expansion turbine fed with combustion gases from at least one of the combustion chambers. 
     Preferably, the percentage of the total air, treated in the second unit, which comes from the compressor feeding a combustion chamber represents at most 80%, or at most 50% or even at most 30% of the percentage of air, treated in the first unit, coming from the first air compressor (or coming from the first air compressor and from a second air compressor which also feeds a second combustion chamber). 
     In certain methods of implementing the invention, an oxygen-enriched gas is sent from the first unit and/or the second unit to a gasifier or several gasifiers. This or these gasifiers deliver fuel to the combustion chamber (combustion chambers). 
     According to optional aspects of the invention: 
     the nitrogen-enriched gas coming from the first air separation unit is sent upstream of the first expansion turbine fed with combustion gases from a combustion chamber and a nitrogen-enriched gas sent from the second air separation unit is sent upstream of at least one expansion turbine fed with combustion gases from at least one combustion chamber, optionally the first turbine; 
     the percentage of cryogenic liquid produced as final product by the second unit with respect to the flow of air treated by the second unit is greater than the percentage of cryogenic liquid produced as final product by the first unit with respect to the flow of air treated by the first unit, or in which the second unit produces cryogenic liquid while the first unit produces none. For example, the second unit may produce a liquid richer in oxygen and/or a liquid richer in nitrogen and/or a liquid richer in argon than air; 
     the second air separation unit receives at most 50%, optionally at most 30%, of the compressed air which it treats from one or more compressors feeding one or more combustion chambers with compressed air, optionally the first compressor; 
     the first air separation unit is fed with air from a second air compressor which also feeds a second combustion chamber, the combustion gases from the second combustion chamber being sent to a second expansion turbine; 
     the first separation unit produces one or more oxygen-enriched fluids, this fluid containing at most 98 mol % oxygen and/or at least 80% of these products consisting of a fluid containing at most 98 mol % oxygen, preferably at most 97 mol %; 
     the first separation unit produces oxygen-enriched products, at least 90% of these oxygen-enriched fluids consisting of one or more fluids containing at most 98 mol % oxygen; 
     the second separation unit produces one or more oxygen-enriched fluids, this fluid containing at least 98 mol % oxygen or at least 50% of these oxygen-enriched fluids consisting of one or more fluids containing at least 98 mol % oxygen; 
     the first separation unit produces oxygen-enriched products, at least 70% of these products consisting of a fluid containing at least 98 mol % oxygen; 
     the first air separation unit is also fed with compressed air by a compressor which does not feed a combustion chamber and/or which feeds only the first air separation unit; 
     the second air separation unit is fed with compressed air by a compressor which does not feed a combustion chamber and/or which feeds only the second air separation unit; 
     the second air separation unit produces an argon-enriched final product; 
     only the second air separation unit produces an argon-enriched final product or in which the second air separation unit produces more argon-enriched final product(s) than the first unit; 
     the first air separation unit comprises a blowing turbine and/or the second air separation unit comprises a Claude turbine; 
     a compressor feeds both air separation units and does not feed a combustion chamber; 
     the first unit and/or the second unit comprise/comprises a low-pressure column from which an oxygen-enriched product fluid is derived, this low-pressure column operating at at least 1.3 bara, optionally at least 3 bara; 
     the first and/or second unit comprises a low-pressure column and a high-pressure column, and optionally a column operating at intermediate pressure between the low and high pressures; 
     the air sent from the first compressor to the first and/or the second air separation unit is compressed or expanded and/or the air sent from the second compressor to the first and/or second air separation unit is compressed or expanded. 
     According to another aspect of the invention, this provides an integrated plant comprising a first air separation unit, a second air separation unit, a first compressor, a combustion chamber, an expansion turbine, an auxiliary compressor, means for sending air from the first compressor to the combustion chamber and to the first air separation unit, means for sending air from the auxiliary compressor to the second separation unit and no means for sending air from the first compressor or another compressor associated with a combustion chamber to the second air separation unit. 
     Preferably, the plant comprises means for sending a nitrogen-enriched gas from the first separation unit upstream of the expansion turbine and/or means for sending a nitrogen-enriched gas from the second separation unit upstream of the expansion turbine and/or means for sending an oxygen-enriched gas from the first unit and/or from the second unit to one or more gasifiers which delivers or deliver fuel for (at least) one (the) combustion chamber. 
     According to other aspects of the invention, the plant may comprise: 
     means for sending air from the dedicated compressor to the first unit; 
     the first unit does not include a means of producing liquid as final product and/or the second unit does include a means of producing liquid as final product; 
     the first unit does not include an argon production column and/or the second unit does include an argon production column; 
     the first unit includes a blowing turbine and/or the second unit includes a Claude turbine and optionally does not include a blowing turbine. 
     According to another aspect of the invention, this provides an integrated air separation process producing an oxygen-enriched fluid and optionally a nitrogen-enriched fluid in a plant comprising at least two air separation units, each comprising at least two distillation columns, a first air compressor, a first combustion chamber and a first expansion turbine, in which process compressed air is delivered to the first air separation unit at least by the first air compressor, which also delivers compressed air to the first combustion chamber, compressed air is delivered to the second air separation unit at least by an auxiliary compressor which does not feed a combustion chamber but which also feeds the first air separation unit. 
     According to another aspect of the invention, this provides an integrated plant comprising a first air separation unit, a second air separation unit, a first compressor, a combustion chamber, an expansion turbine, an auxiliary compressor, means for sending air from the first compressor to the combustion chamber and to the first air separation unit, means for sending air from the auxiliary compressor to the first air separation unit and to the second air separation unit, this auxiliary compressor not feeding a combustion chamber. 
     Preferably, the auxiliary compressor is not connected with a unit which consumes compressed air, apart from the first and second air separation units. 
     Thus, the first air separation unit receives proportionally more air from a gas turbine than the second air separation unit. This second unit may even not be integrated at all with a gas turbine or else it may produce a nitrogen-enriched stream which is sent to the gas turbine. 
     Thus, the first air separation unit receives more air from a gas turbine than the second air separation unit. This second unit, which may even not be integrated at all with a gas turbine. 
     The degree of integration determines what products may be output by each unit, in general the products purest in oxygen and/or in argon coming from the second unit, the operation of which will be more stable, thanks to the low degree of integration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Processes and plants according to the invention will now be described with reference to FIGS. 1 and 2, which are schematic drawings of integrated plants. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a compressor  13  is fed with air and sends a first stream of air to a combustion chamber  17  with fuel, a second stream of air to a first air separation unit  1  and, optionally, a third stream of air to a second air separation unit  101 , the third stream being in general less than the second stream. 
     The means for cooling the air from the outlet temperature of the compressor  13  to a temperature close to ambient upstream of the air separation unit  1  and upstream of the air separation unit  101  are not illustrated. 
     The first air separation unit  1  is also fed with air by a compressor  21 , which feeds only the latter, and the second air separation unit  101  is also fed with air by a compressor  121 , which only feeds the latter. Alternatively, each of the compressors  21 ,  121  may feed the first and the second air separation units or only one of the compressors  21  or  121  may feed the first and second air separation units (not illustrated). 
     The first air separation unit, typically of the double-column or triple-column type, produces at least one nitrogen-enriched gas  3  and an oxygen-enriched high-pressure gas  5  containing at most 98 mol. % oxygen, possibly at most 95 mol. % oxygen or even at most 93 mol. % oxygen, which is sent to a gasifier  31 . The nitrogen-enriched gas is sent to the combustion chamber  17  or to another point upstream of the turbine  19 . 
     The first unit may optionally produce a small amount of liquid. 
     In the example, argon is not produced. 
     Some of the air sent to the air separation unit  1  may be sent through a blowing turbine (which supplies the low-pressure column of the double or triple column). 
     The second air separation unit produces oxygen  105  containing at least 98 mol. % oxygen, gaseous argon and/or liquid and, optionally, liquids rich in nitrogen or oxygen, together with a stream of impure nitrogen  103  which may optionally be sent to the combustion chamber  17 . 
     Optionally, some of the oxygen  105  may be sent to the gasifier  31 . 
     The second unit  101  is preferably of the type under pressure, therefore with a low-pressure column, from which the oxygen-enriched fluid is withdrawn, operating above 1.5 bara, preferably above 3 bara. 
     The second unit may include a column for purifying an argon-enriched stream. 
     Preferably, some of the air sent to the second unit  101  is expanded in a Claude turbine before being sent to the air distillation column operating at the higher pressure. 
     Preferably, the ratio of the flow of air sent from the compressor  121  to the unit  101  to the flow of air (if there is one) sent from the compressor  13  to this unit  101  is greater than the ratio of the flow of air sent from the compressor  21  to the unit  1  to the flow of air sent from the compressor  13  to this unit  1 . 
     Optionally, the two compressors  21 ,  121  may be replaced with a single compressor feeding the units  1 ,  101 . 
     In FIG. 2, a first air compressor  13  delivers air to the first air separation unit  1  and to a first combustion chamber  17 , the combustion gases of which feed a first expansion turbine  19  which allows electricity to be generated. 
     A second air compressor  15  delivers air to the air separation unit  1  and to a second combustion chamber  23 , the combustion gases of which feed a second expansion turbine  25  which allows electricity to be generated. A third air compressor  21  delivers air exclusively to the air separation unit. 
     The means for cooling the air from the outlet temperature of the compressors  13 ,  15  to a temperature close to ambient upstream of the first air separation unit  1  and upstream of the second air separation unit  101  have not been illustrated. 
     The waste gas  3  from the separation unit  1  may be sent upstream of the first and/or the second turbine, for example to the first and/or to the second combustion chamber and/or to the inlet of the first and/or the second turbine. 
     The oxygen-enriched gas  5  under pressure is preferably sent to one or more gasifiers  31 ,  131 , where it is used to produce fuel for at least one of the combustion chambers  17 ,  23 . 
     The compressors  13 ,  15 ,  21  may deliver air at different pressures, for example pressures which differ by at least 0.5 bar from one another. The streams at the highest pressures may be expanded to the lowest pressure so as to purify all the air streams together. 
     Otherwise, the streams may be sent to columns of the ASU operating at different pressures with adapted purification. 
     In the plant in FIG. 2, there are two air separation units  1 ,  101 , each having at least two distillation columns and each having, optionally, its own cold box. 
     The unit  1  produces the same products as those described above: the unit  101  produces at least waste nitrogen  103  and oxygen-enriched gas, optionally at several pressures, or at least at high pressure. 
     The waste nitrogen  103  may be sent to the first and/or the second combustion chamber or, alternatively, may be discharged into the atmosphere, used for purification regeneration of first and/or second units  1 ,  101 , or used for other purposes. 
     The oxygen  105  may be sent to another gasifier  131 , the gasifier  31  or another point of use, particularly if its purity is different from that of the oxygen  5 . As described above, the unit  101  may deliver mainly or only pure oxygen containing more than 98 mol % oxygen, whereas the first unit may produce only or mainly impure oxygen containing less than 95 mol % oxygen. 
     The unit  101  is fed with air from a dedicated compressor  121  and, optionally, very partially from the first compressor  13  and/or the second compressor  15  and/or the dedicated compressor  21  and/or a dedicated compressor which sends air to both air separation units.
Integrated air-separation process and plant, including at least two air separation units ( 1, 101), an air compressor ( 13), which feeds a combustion chamber and at least one of the air separation units with compressed air, and at least one dedicated air compressor ( 21, 121) feeding one or both of the air separation units, so that, if both air separation units receive air from the compressor ( 13), the proportions of air coming from the air compressor are different in the case of the two air separation units.