Abstract:
A method and air separation plant for the distillative cryogenic separation of feed air in a distillation column system of an air separation plant at different distillation pressures, wherein all of the feed air in a total air quantity is compressed to a first pressure level that is at least 4 to 5 bar above the highest of the distillation pressures, and from the total air quantity a first part air quantity is first cooled to a first temperature level of 130 to 170 K and then compressed to a second pressure level that is at least 10 bar above the first pressure level, and a second part air quantity is first cooled to a second temperature level of 110 to 150 K and then expanded to a third pressure level that is below the first pressure level.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from European Patent Application EP 15001668.1 filed on Jun. 3, 2015. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method and to a plant for the cryogenic separation of air by the distillative cryogenic separation of feed air in a distillation column system of an air separation plant at different distillation pressures, wherein all of the feed air in a total air quantity is compressed to a first pressure level that is at least 4 to 5 bar above the highest of the distillation pressures, and from the total air quantity
       a first part air quantity is first cooled to a first temperature level of 130 to 170 K and then compressed to a second pressure level that is at least 10 bar above the first pressure level, and   a second part air quantity is first cooled to a second temperature level of 110 to 150 K and then expanded to a third pressure level that is below the first pressure level, wherein   a compression/expansion arrangement with a gearing, in which a driving wheel engages with a gear wheel and the gear wheel engages with a driven wheel is provided, wherein   the rotor blades of two or more turbo expanders are coupled to the driving wheel and the rotor blades of two or more turbo compressors are coupled to the driven wheel, wherein both couplings are rotationally fixedly coupled, and   the first part air quantity is fed, for compression to the second pressure level, in series through the turbo compressors and the second part air quantity is fed, for expansion to the third pressure level, in parallel through the turbo expanders.       
 
         [0008]    The production of air products in liquid or gaseous form by cryogenic separation of air in air separation plants is known and described for example in H.-W. Häring (Ed.), Industrial Gases Processing, Wiley-VCH, 2006, in particular section 2.2.5, “Cryogenic Rectification”. The present invention is particularly well-suited to air separation plants with internal compression, as explained in section 2.2.5.2, “Internal Compression”, of the above document. 
         [0009]    Air separation plants have distillation column systems which can for example take the form of two-column systems, in particular conventional Linde two-column systems, but also three- or multi-column systems. In addition to the distillation columns for the production of nitrogen and/or oxygen in liquid and/or gaseous form (for example liquid oxygen, LOX, gaseous oxygen, GOX, liquid nitrogen, LIN and/or gaseous nitrogen, GAN), that is to say the distillation columns for nitrogen-oxygen separation, it is also possible to provide distillation columns for the production of other air components, in particular the noble gases krypton, xenon and/or argon. 
         [0010]    The distillation column systems of air separation plants are run with various operating pressures in their distillation columns. Known two-column systems have, for example, a so-called (high-)pressure column and a so-called low-pressure column. The operating pressure of the high-pressure column is for example 4.3 to 6.9 bar, in particular approximately 5.5 bar. The low-pressure column is run at an operating pressure of for example 1.2 to 1.7 bar, in particular approximately 1.4 bar. The pressures indicated here are absolute pressures at the bottom of corresponding distillation columns. The specified pressures are referred to hereinbelow also as “distillation pressures” because it is at those pressures that the fractional distillation of the air fed in each case takes place within the distillation columns. This does not exclude the possibility of other pressures also prevailing at other points in a distillation column system. 
         [0011]    Cooled compressed air (feed air), which has been pressurized by means of various compressors or combinations of various compressors (for example main air compressors and booster air compressors), is fed into the distillation column systems. Products obtained in a corresponding air separation plant can also be pressurized with compressors (product compressors) or corresponding combinations of compressors. 
         [0012]    The operating costs (OPEX) of an air separation plant are determined essentially by the energy consumption which in turn is primarily dependent on the energy consumption of the compressors (main air compressor, booster air compressor and product compressor if present). The investment costs (CAPEX) are also determined essentially by the costs to be borne for the provision of the compressors. 
         [0013]    In air separation, use can be made of methods in which the feed air is pressurized by means of a main air compressor (MAC) to approximately the pressure of the high-pressure column and only part of the feed air is boosted by means of a booster air compressor (BAC) and is used for internal compression of oxygen (see below) or for refrigeration. These rather conventional methods are also termed MAC/BAC methods. 
         [0014]    Latterly, instead of the MAC/BAC methods, use is increasingly being made of so-called HAP (high air pressure) methods since these can offer advantages over the MAC/BAC methods. HAP methods are advantageous in particular in coal gasification or coal liquefaction (coal to gas, coal to liquids, CTX) or in gasification of heavy oil. The air separation plants used here must in general provide exclusively or almost exclusively gas products (gaseous, internally compressed oxygen, GOXIC, or compressed nitrogen, PGAN), there being no requirement for substantial flexibility in the execution of the process. 
         [0015]    In an HAP method, all of the feed air is compressed in a main air compressor to a pressure substantially greater than the distillation pressure in the high-pressure column. The pressure difference which is used is at least 4 bar and preferably between 6 and 16 bar. Such HAP methods are known for example from EP 2 466 236 A1, EP 2 458 311 A1 and U.S. Pat. No. 5,329,776 A, MAC/BAC methods for example from the technical literature cited in the introduction. 
         [0016]    U.S. Pat. No. 5,901,579 A shows a turbo compressor and turbo expander coupled via a gearing for use in an HAP method. EP 2 634 517 A1 and EP 2 520 886 A1 disclose arrangements which use so-called cold compressors or cold boosters (see below). 
         [0017]    As mentioned, air separation plants can be operated with so-called internal compression. In internal compression, for example for providing the above-mentioned gaseous, internally compressed oxygen, a liquid stream is extracted from the distillation column system and is pressurized at least partially in liquid form. The stream pressurized in liquid form is heated and evaporated against a heat transfer medium in a main heat exchanger of the air separation plant. The liquid stream can in particular be liquid oxygen, but can also be nitrogen or argon. In that context, internal compression is used to obtain corresponding gaseous pressurized products. The advantage of internal compression methods is, inter alia, that corresponding fluids need not be compressed in the gaseous state outside the air separation plant, which frequently proves to be very onerous and/or requires onerous safety measures. 
         [0018]    As also explained below, the term “evaporate” in the context of internal compression includes cases in which a supercritical pressure prevails and thus no phase transition proper takes place. The stream pressurized in liquid form is then “pseudo-evaporated”. Against a (pseudo-)evaporating stream, a heat transfer medium is liquefied (or pseudo-liquefied if it is at supercritical pressure). In that context, the heat transfer medium commonly consists of a part stream of the compressed feed air, which is termed throttle stream. In a hotter region of the heat exchanger used for evaporation or pseudo-evaporation, additional evaporation heat can be supplied by the so-called turbine stream (or by multiple turbine streams). 
         [0019]    In order to be able to efficiently heat and evaporate the stream pressurized in liquid form, this heat transfer medium must, due to thermodynamic conditions, be at a relatively high pressure. For that reason, a correspondingly highly-compressed stream must be provided. This is especially the case if, for example, internally compressed oxygen at high or very high pressures (for example 50 bar or more) is to be provided for use in the above-mentioned CTX methods or in gasification of heavy oil, but also in other scenarios. 
         [0020]    This can lead to problems which cannot, or can only disadvantageously, be remedied with conventional measures, as explained in detail below. The present invention therefore has the object of providing possibilities which allow, in an HAP method, a corresponding compression to be effected in a simple and efficient manner. 
       SUMMARY OF THE INVENTION 
       [0021]    Against this background, the present invention proposes a method and a plant for the cryogenic separation of air, by the distillative cryogenic separation of feed air in a distillation column system of an air separation plant at different distillation pressures, wherein all of the feed air in a total air quantity is compressed to a first pressure level that is at least 4 to 5 bar above the highest of the distillation pressures, and from the total air quantity
       a first part air quantity is first cooled to a first temperature level of 130 to 170 K and then compressed to a second pressure level that is at least 10 bar above the first pressure level, and   a second part air quantity is first cooled to a second temperature level of 110 to 150 K and then expanded to a third pressure level that is below the first pressure level, wherein   a compression/expansion arrangement with a gearing, in which a driving wheel engages with a gear wheel and the gear wheel engages with a driven wheel is provided, wherein   the rotor blades of two or more turbo expanders are coupled to the driving wheel and the rotor blades of two or more turbo compressors are coupled to the driven wheel, wherein both couplings are rotationally fixedly coupled, and   the first part air quantity is fed, for compression to the second pressure level, in series through the turbo compressors and the second part air quantity is fed, for expansion to the third pressure level, in parallel through the turbo expanders.       
 
         [0027]    Configurations form the subject matter of the respective dependent claims and of the following description. 
         [0028]    Prior to explanation of the features and advantages of the present invention, the basic principles thereof and the terms used will be explained. 
         [0029]    In air separation plants, turbo compressors are used to compress the air. This is the case for example for the “main (air) compressor” which is distinguished in that it compresses all of the quantity of air fed into the air separation plant, that is to say all of the “feed air”. A “booster air compressor”, in which, in MAC/BAC methods, part of the air quantity compressed in the main air compressor is raised to a still higher pressure, is typically also designed as a turbo compressor. For the (subsequent) compression of part air quantities, provision is typically made of further turbo compressors which are also termed boosters. 
         [0030]    Furthermore, at multiple points in air separation plants, air is expanded, to which end use can be made, inter alia, of turbo expanders. This is the case in particular for the expansion of a so-called turbine stream, as explained below. Turbo expanders can also be coupled to turbo compressors (boosters) and drive these. In the case of one or more turbo compressors being driven without externally supplied energy, i.e. only by one or more turbo expanders, the term “turbine booster” is also used for such an arrangement. 
         [0031]    The mechanical construction of turbo compressors and turbo expanders will in principle be known to a person skilled in the art. In a turbo compressor, the air is compressed by means of rotor blades which are arranged on an impeller wheel or directly on a shaft. In that context, a turbo compressor forms a structural unit which can however have multiple “compressor stages”. A compressor stage generally comprises an impeller wheel or a corresponding arrangement of rotor blades. All of these compressor stages can be driven by a common shaft. A turbo expander is fundamentally of comparable design, the rotor blades being however driven by the expanding air. Here, too, multiple expansion stages can be provided. Turbo compressors and turbo expanders can be designed as radial- or axial-flow machines. 
         [0032]    If the following text states that “the rotor blades” of a turbo compressor or turbo expander are “rotationally fixedly coupled” to another element, for example a driving wheel or driven wheel, this means that there is a mechanical connection between the rotor blades and the other element. As mentioned, the rotor blades are attached to one or more impeller wheels which is/are rotationally fixedly connected to a shaft, or are directly attached to a shaft, thus permitting a direct transfer of torque between the shaft and the rotor blades. In this manner, each element that is rotationally fixedly coupled to the shaft is also rotationally fixedly coupled to the rotor blades. Such a “rotationally fixed coupling” causes the rotor blades and the other element, for example the driving wheel or the driven wheel, to rotate about the axis of the shaft with the same rotational speed and in the same direction of rotation. Hence, coupling via gear wheel engagement is not a “rotationally fixed coupling” in the stated sense. 
         [0033]    In the following, a “driving wheel” is to be understood as a gear wheel to which a shaft imparts a torque, in particular by said gear wheel being coupled to the rotor blades of a turbo expander. A “driven wheel”, by contrast, is a gear wheel which in turn imparts a torque to a shaft, in particular the shaft of a turbo compressor. 
         [0034]    The present application uses the terms “pressure level” and “temperature level” to characterize pressures and temperatures, these being intended to express that corresponding pressures and temperatures need not be used in the form of exact pressure or temperature values in order to realize the inventive concept. However, such pressures and temperatures typically move within certain ranges which lie for example ±1%, 5%, 10%, 20% or even 50% about a central value. In that context, corresponding pressure levels and temperature levels can lie in disjoint ranges or in overlapping ranges. In particular, pressure levels for example include unavoidable or expected pressure losses. The same holds for temperature levels. The pressure levels indicated here in bar are absolute pressures. 
         [0035]    In order to be able to efficiently configure an HAP method, or in order to be able to efficiently provide internally compressed gaseous compressed oxygen at high pressures, a relatively high-pressure air stream is required, as mentioned in the introduction. This pressure is once again substantially higher than the already-high pressure, in HAP methods, of the total compressed air and is typically generated by means of so-called cold boosters. A cold booster is understood as a turbo compressor which is supplied with low-temperature air, typically below 0° C. It is advantageously driven by means of a turbo expander. 
         [0036]    Single-stage compression of this stream, which is also termed throttle stream, is shown in the appended  FIG. 1  merely as a hypothetical example by way of illustration. Single-stage compression is not sufficient to achieve the required pressures, since the pressure ratio that can be achieved in the cold booster is limited (the pressure ratio π in corresponding turbo compressors is generally no higher than 1.8 to 2.0). In addition, the mass flow of the air stream which is correspondingly to be highly compressed (in an energy-optimized configuration, a second air stream at a lower pressure level is used) is much lower than the mass flow of the air which is expanded in a turbo expander which can be used for driving. This would lead to large differences in specific rotational speeds, and hence a corresponding unit cannot currently be built. 
         [0037]    This problem could in principle be circumvented by using, instead of a single-stage cold booster, a two-stage arrangement which is driven at least partially externally, for example with electrical energy. However, this is generally not desired since this would give rise to additional costs both for the required provision of an electric motor and also for dimensioning the local medium-voltage grid. In particular, HAP methods can, as explained below, be carried out entirely without the use of electric motors for compression, such that the provision of an electric motor and the necessary infrastructure would be particularly disadvantageous here. In particular in the context of large CTX projects, or in methods for the gasification of heavy oil, in which air separation plants are run using HAP methods, and for which the present invention is particularly well-suited, it is in any case necessary, for the high air quantities used, to use multiple turbo expanders (in the case of an HAP method, the total gaseous air for rectification is expanded via corresponding turbo expanders), such that it is recommended to use these to compress the throttle stream, i.e. without external electrical energy. 
         [0038]    Another arrangement is shown in  FIG. 2  and explained below, also merely for illustrative purposes. Although a corresponding arrangement has hitherto not been considered in the cold part of a corresponding plant, such an arrangement is known per se and has already been used in “hot” turbine boosters, although the volume flows there are very different. Were such an arrangement to be used in the “cold” part of a corresponding plant, as shown in  FIG. 2 , the conditions of the volume flows, in particular in the case of the second cold booster where a final pressure of approximately 57 bar (at a pressure similar to that of the internally compressed oxygen) is generated, would however remain too unequal, with the result that such a unit can also currently not be built due to excessive differences in specific rotational speeds. In order to make the described units possible to build, it would be necessary to increase the quantity of the throttle stream and reduce the quantity of the turbine stream. However, this would reduce the efficiency of the method since the final pressure of the throttle stream would be lower. 
         [0039]    Therefore, the present invention was based on the search for an arrangement by means of which two-stage compression of the throttle stream at optimum quantity ratios is possible using a turbine drive. 
         [0040]    It has been recognized, according to the invention, that a compression/expansion arrangement, in which multiple (that is to say two or more) driving turbo expanders are connected in parallel and two or more turbo compressors are connected in series, solves the above-mentioned problems. Instead of direct drive or direct transmission via a shaft, in the context of the invention use is made of an intermediate gearing as explained below. The use of such an intermediate gearing permits the decoupling of specific rotational speeds and permits the desired drive. The arrangement proposed according to the invention is then similar to a so-called compander, in which a turbo compressor is coupled to a turbo expander via an intermediate gearing. However, according to the invention, neither a generator nor a motor is required. 
         [0041]    The present invention proposes a method for the distillative cryogenic separation of feed air in a distillation column system of an air separation plant at different distillation pressures. Within the context of the method according to the invention, all of the feed air in a total air quantity is compressed to a first pressure level that is at least 4 to 5 bar above the highest of the distillation pressures. Within the context of the present invention, use is therefore made of an HAP method as mentioned in the introduction. As mentioned, the highest of the operating pressures in a corresponding distillation column system, i.e. the operating pressure in the (high-)pressure column, can be for example between 4.3 and 6.9 bar, for example 5.2 bar. Therefore, within the context of the method according to the invention, all of the feed air is compressed to a pressure level that is at least 4 to 5 bar above such a pressure, thus for example at least 11, 12, 13, 14, 15 or 16 bar. Specific values are listed below. 
         [0042]    Within the context of the method according to the invention, from the mentioned total air quantity a first part air quantity is first cooled to a first temperature level of 130 to 170 K, typically in a main heat exchanger of a corresponding air separation plant, and then compressed to a second pressure level that is at least 10 bar above the first pressure level. The present invention is therefore used in HAP methods in which relatively high pressures are generated, for example, as explained below, in order to be able to provide internally compressed pressurized products at correspondingly high pressures. 
         [0043]    Furthermore, within the context of the present invention, a second part air quantity is first cooled to a second temperature level of 110 to 150 K and then expanded to a third pressure level that is below the first pressure level, and can for example be at the highest of the distillation pressures, that is to say the operating pressure of the high-pressure column. The cooling to the second temperature level can also take place in a main heat exchanger of the air separation plant. 
         [0044]    As mentioned above, the simultaneous compression and expansion of corresponding part air quantities gives rise to problems in particular if, in this context, the compression is to be carried out using work released during expansion, i.e. if it is to be performed without the supply of external (electrical) energy. This is in particular the case if the first and the second part air quantity differ markedly because, as mentioned, this leads to marked differences in rotational speed. 
         [0045]    The present invention provides using a compression/expansion arrangement with a gearing, in which a driving wheel engages with a gear wheel and the gear wheel engages with a driven wheel. The rotor blades of two or more turbo expanders are coupled to the driving wheel and the rotor blades of two or more turbo compressors are coupled to the driven wheel, in both cases rotationally fixedly within the meaning explained above. The first part air quantity is fed, for compression to the second pressure level, in series through the turbo compressors and the second part air quantity is fed, for expansion to the third pressure level, in parallel through the turbo expanders. “Parallel” feeding through multiple turbo expanders is understood, in that context, as meaning that the second part air quantity is split into two or more part streams and each of the part streams is fed through one of the turbo expanders. The above-mentioned problem can be solved by using the compression/expansion arrangement in that the gearing used makes it possible to match the rotational speeds of the respective turbo compressors and turbo expanders used. Using multiple turbo compressors in series makes it possible to generate very high pressure differences which could not be achieved with just one compressor unit. The invention also permits the use of first and second part air quantities in markedly different orders of magnitude since the use of the gearing makes it possible to compensate for differences in rotational speed. The first part air quantity fed through the turbo compressors is at the above-mentioned first temperature level. For that reason, the turbo compressors are operated as cold boosters. 
         [0046]    By virtue of the fact that, within the context of the present invention, multiple turbo expanders are connected in parallel, the mechanical loads which arise can be spread symmetrically across these and the individual turbo expanders can be made smaller and more cost-effective. Particular advantages result when use is made of two turbo compressors of which the rotor blades are respectively coupled to a first shaft coupled to the driven wheel, on either side of the driven wheel. This makes it possible to reduce asymmetric loads and to reduce wear. 
         [0047]    Advantageously, a torque transmitted by means of the driving wheel to the gear wheel is greater than or equal to a torque transmitted by means of the gear wheel to the driven wheel. Thus, there is no need to provide additional drives such as an electric motor; the turbo compressors can be driven simply by means of the turbo expanders. 
         [0048]    Another advantage of the compression/expansion arrangement according to the invention lies in the fact that the gear wheel makes it possible for other driving or driven gear wheels to be brought into engagement flexibly, and thus the method can be expanded as desired. It is for example possible, in this manner, to use one or more further turbo expanders driving via a driving wheel and/or one or more turbo compressors driven by the turbine wheel via a driven wheel. The gear wheel itself is preferably not rotationally fixedly coupled to turbo expanders and/or turbo compressors, but rather preferably merely transmits torques between driving and driven wheels. In other words, in a compression/expansion arrangement according to the present invention, the gear wheel on one hand and the driving or driven units on the other (for example the rotor blades of turbo expanders and turbo compressors) are run at different rotational speeds. However, this does not exclude the possibility of the driving and/or driven units being run at the same rotational speed as one another. 
         [0049]    Advantageously, within the context of the present invention, the first pressure level is 10 to 17 bar, in particular 13 to 16 bar, and/or the second pressure level is 40 to 70 bar, in particular 50 to 60 bar. Thus, the second pressure level is at relatively high values which preclude the use of a single booster since, as mentioned, the pressure difference that can be achieved therein is too small. This problem is solved with the use in series of multiple turbo compressors which, according to the invention, are coupled by means of a gearing. 
         [0050]    As mentioned, the third pressure level which is below the first pressure level can for example be at the highest of the distillation pressures in the distillation column arrangement, for example the pressure level at which a high-pressure column is run. The statement that the third pressure level is “at” the highest of the distillation pressures is intended to mean that the third pressure level deviates by no more than 1 bar from the highest of the distillation pressures in the distillation column arrangement. 
         [0051]    As explained, the present invention is particularly well-suited if the respective part air quantities differ markedly from one another. This is in particular the case if the first part air quantity corresponds to 0.2 to 0.6 times the second part air quantity, and/or if the first and second part air quantities together correspond to 0.3 to 0.6 times the total air quantity. The stated ratios relate, respectively, to normal volume per unit of time, that is to say normal volume flow rates, for example normal cubic metres per hour (Nm 3 /h). The differences in the volume flow rates actually present prove to be much higher since the pressures which are present differ markedly. In particular, in the case of markedly differing first and second part air quantities, which lead to a relatively small quantity of air having to be compressed, but a relatively large quantity of air being available for expansion, the method of the present invention is well-suited due to the use of the specified gearing. 
         [0052]    Advantageously, within the context of the present invention, the second part air quantity is compressed, prior to cooling to the second temperature level, from the first pressure level to an intermediate pressure level below the second pressure level. To that end, another turbo compressor can be used, for example. This can be driven by a turbo expander (which is present for the purpose of providing the refrigeration power necessary for the process as a whole) which expands another stream. 
         [0053]    The method according to the invention provides particular advantages if a liquid, oxygen-rich air product is extracted from the distillation column system, is pressurized in liquid form and is then converted, by heating, from the liquid to the supercritical or gaseous state, that is to say for internal compression methods. In such internal compression methods, it is possible, within the context of the present invention, for the liquid, oxygen-rich air product to be pressurized in liquid form to the first pressure level or another high pressure level. The invention is thus particularly well-suited to methods in which corresponding internal compression products are to be provided at high pressures. 
         [0054]    Combining an internal compression method with an HAP method, within the context of the present invention, achieves particular advantages. Conventional MAC/BAC methods sometimes come up against their capacity limits at large air quantities. Thus, in the case of air separation plants, the limits of conventional plant components are reached for oxygen capacities above 130,000 Nm 3 /h. Due to the low feed air pressure prevailing here, for example adsorber stations with diameters greater than 6 metres are required and are run partially at the limit. In addition, installation work for pipes and valves of very large nominal widths, which are necessary for such high air throughputs, leads to high costs. Using an HAP method substantially mitigates such problems due to substantially lower volume flow rates; the entire “hot” part of an air separation plant proves to be markedly smaller. 
         [0055]    Both in MAC/BAC methods and in HAP methods, the air compressors are typically driven by steam turbines. The steam turbines used in this context are typically two-shaft turbines which are set up to simultaneously drive the main and booster air compressors (MAC and BAC in MAC/BAC methods) by means of respectively one of the shafts. In the case of an MAC/BAC method, a typical nitrogen product compressor, which is additionally necessary, requires its own drive in the form of an electric motor. This gives rise to additional costs, as explained above. For that reason, HAP methods are particularly advantageous because here the product compressor can be driven directly via one of the shafts of the steam turbine (the booster air compressor is omitted). Plants of this type therefore benefit particularly from solutions which require no electrical drives. 
         [0056]    The present invention further relates to an air separation plant which is set up for the distillative cryogenic separation of feed air in a distillation column system at different distillation pressures. This plant has means which are set up to compress all of the feed air in a total air quantity to a first pressure level that is at least 4 to 5 bar above the highest of the distillation pressures, from the total air quantity to first cool a first part air quantity to a first temperature level of 130 to 170 K and then to compress this to a second pressure level that is at least 10 bar above the first pressure level, and to first cool a second part air quantity to a second temperature level of 110 to 150 K and then to expand this to a third pressure level that is below the first pressure level. 
         [0057]    The plant is characterized by a compression/expansion arrangement with a gearing, in which, according to the invention, a driving wheel engages with a gear wheel and the gear wheel engages with a driven wheel. The rotor blades of two or more turbo expanders are coupled to the driving wheel and the rotor blades of two or more turbo compressors are coupled to the driven wheel. There are provided means which are set up to feed the first part air quantity, for compression to the second pressure level, in series through the turbo compressors and to feed the second part air quantity, for expansion to the third pressure level, in parallel through the turbo expanders. 
         [0058]    Advantageously, within the context of the present invention, use is made of an arrangement in which at least one other driving wheel and/or at least one other driven wheel engage(s) with the gear wheel. It is thus possible to simply and flexibly couple further driving or driven units to a corresponding compression/expansion arrangement. 
         [0059]    The present invention is particularly well-suited to air separation plants in which the compression/expansion arrangement comprises two turbo compressors of which the rotor blades are coupled to a first shaft coupled to the driven wheel, on either side of the driven wheel, and/or two turbo expanders of which the rotor blades are coupled to a second shaft coupled to the driving wheel, on either side of the driving wheel. It is thus possible, as already explained, to reduce asymmetric loads. An arrangement “on either side” is intended to mean here, as above, that the driving or driven wheel is arranged on a shaft which extends axially on either side of the driving or driven wheel. A corresponding turbo compressor or turbo expander, or the rotor blades thereof, can be arranged on each of the two sides. 
         [0060]    For features and advantages of such air separation plants, which are in particular set up for carrying out a method as explained in detail beforehand, reference is made to the above explanations. Corresponding plants benefit from the advantages explained in relation to the method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0061]    The invention will be explained in greater detail below with reference to the appended drawings which show preferred embodiments of the invention. 
           [0062]      FIG. 1  illustrates, by way of illustration of the problem upon which the invention is based, an air separation plant in the form of a schematic process flow diagram. 
           [0063]      FIG. 2  illustrates, by way of illustration of the problem upon which the invention is based, an air separation plant in the form of a schematic process flow diagram. 
           [0064]      FIG. 3  schematically illustrates a compression/expansion arrangement according to one embodiment of the invention. 
           [0065]      FIG. 4  schematically illustrates a compression/expansion arrangement not according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0066]    In the figures, corresponding elements bear identical reference signs and, for the sake of clarity, will not be explained anew. 
         [0067]      FIG. 1  illustrates, by way of illustration of the problem upon which the invention is based, an air separation plant in the form of a schematic process flow diagram, labelled  100  as a whole. 
         [0068]    Feed air (AIR) is fed to the air separation plant  100  via a filter  1  and is compressed by means of a main air compressor  2 . In that context, the feed air is compressed in the main air compressor  2  to a pressure level which, within the context of this application, is termed the “first” pressure level and is markedly higher than the maximum operating pressure of a distillation column system  10 , explained below, of the air separation plant  100 . The method carried out in the air separation plant  100  is thus an HAP method, as explained in the introduction. The first pressure level is for example approximately 14.5 bar. The quantity of air in stream c, compressed by the main air compressor  2 , is here termed “total air quantity”. This is for example approximately 655,000 Nm 3 /h. 
         [0069]    A compressed air stream a provided in this manner is pre-cooled in a direct contact cooler  3  which is supplied, inter alia, with a cooled water stream b from an evaporative cooler  4 . The operation of the direct contact cooler  3  and of the evaporative cooler  4  will not be described further. After cooling in the direct contact cooler  3 , a correspondingly cooled compressed air stream, now labelled c, is fed to an adsorber set  5  which, in the example shown, comprises two adsorber containers which are filled with a suitable adsorption material and are operated in alternation, and the operation of which will also not be described further. In the evaporative cooler  4  and the adsorber set  5 , use can for example be made, for cooling or regeneration, of a stream d which is extracted from the distillation column system  10  as so-called impure nitrogen and suitably prepared. In this context, use is made, for example, of a steam heater  6 . 
         [0070]    A compressed air stream which has been dried in the absorber set  5  is labelled d. In the example shown, this is split into two part streams e and f. The part stream e is then split again into two part streams g and h and is fed to the hot side of a main heat exchanger  7 . The part stream g is the above-described turbine stream, and the part stream h is a (second) throttle stream at lower pressure. The part stream f is further compressed in a booster turbine  8 , is cooled in an intercooler (which is not separately designated), is split again into two part streams i and k, and is fed to the hot side of the main heat exchanger  7 . The part stream i is a (first) throttle stream which is at higher pressure and is to be boosted, and the part stream k is a stream which is to be expanded in order to provide refrigeration power. 
         [0071]    Thus, all of the part streams e to k each comprise part air quantities of the total air quantity of the stream a, c and, respectively, d. The part air quantity in stream i, for example approximately 102,000 Nm 3 /h, is here termed the “first” part air quantity, and the part air quantity in stream g, for example approximately 307,000 Nm 3 /h, is termed the “second” part air quantity of the total air quantity. The part air quantity, of the total air quantity, in stream h is for example approximately 55,000 Nm 3 /h. The division is discretionary and can also be implemented in a different sequence, deviating from the specific example. 
         [0072]    The part streams g, i and k are in each case extracted from the main heat exchanger  7  at intermediate temperature levels, wherein the intermediate temperature level at which the part stream i is extracted from the main heat exchanger  7  is termed “first” temperature level here and the intermediate temperature level at which the part stream g is extracted from the main heat exchanger  7  is termed “second” temperature level here. The part stream h is drawn from the cold side of the main heat exchanger  7 . 
         [0073]    The air separation plant shown in  FIG. 1  is set up for the provision of internally compressed oxygen at a high pressure level, for example at approximately 57 bar, and at a rate of approximately 135,000 Nm 3 /h, for example. To that end, a liquid, oxygen-rich stream l is extracted from the distillation column system  10 , is pressurized by means of a pump  9  and is converted in the main heat exchanger  7  from the liquid state to the supercritical state at the above-mentioned pressure. 
         [0074]    Due to the relatively high pressure at which, in the main heat exchanger  7 , the internally compressed oxygen of the stream l is converted to the supercritical state or evaporated, there is accordingly a need for a heat transfer medium at high pressure. This heat transfer medium is in this case formed by the part stream i, which to that end must be further compressed. 
         [0075]    It is to be understood that the booster turbine  101  used in  FIG. 1  for this purpose and for the simultaneous expansion of the part stream g could be used only in theory due to the above-mentioned limits of the pressure ratios that can be achieved in such a booster, and cannot currently be structurally realized. At the same time, it is to be expected that the driving turbo expander cannot currently be structurally realized. 
         [0076]    The part stream i, after extraction from the main heat exchanger  7  at the first intermediate temperature level, would have to be compressed by means of the booster turbine  101  from the pressure level achieved in the booster turbine  8 , for example approximately 17 bar, to a pressure level of for example approximately 57 bar. A corresponding pressure level is here termed “second” pressure level. 
         [0077]    In the example shown, the part stream i is fed to the main heat exchanger  7  at an intermediate temperature level, and is extracted therefrom at the cold side. In the example shown, the part streams h and i are expanded downstream of the main heat exchanger  7  to a lower pressure level, for example the pressure level of a pressure column in the distillation column arrangement  10 , of approximately 5.2 bar. To that end, use can be made for example of valves or so-called dense liquid expanders which are shown in  FIG. 1  but not separately designated. Expansion to such a pressure level can also take place for part streams g and k in the respective expansion turbines of the booster turbines  8  or, respectively,  101 . 
         [0078]    The part streams g to k are fed into the distillation column system  10 , which has been mentioned many times, is shown here in highly schematic form and at reduced scale and typically comprises multiple distillation columns run at different operating pressures. The illustrated example shows a high-pressure column  11  and a low-pressure column  12  which are connected such that they exchange heat via a main condenser  13 . The high-pressure column  11  is for example run at the pressure level to which the streams g to k are expanded. The streams g to k are typically fed into the high-pressure column  11 , although they might also be fed in part into the low-pressure column  12 . The connections of the high-pressure column  11  and of the low-pressure column  12  are not shown in detail, nor are additional columns, valves, pumps, heat exchangers and the like. 
         [0079]    The distillation column system  10  can comprise any number of corresponding columns and can be set up for the production of various air products. In addition to the already-mentioned liquid, oxygen-rich stream l for the provision of internally compressed oxygen (GOX IC), it is for example possible to extract from the distillation column system  10  a nitrogen-rich, liquid stream m which can also be pressurized by means of a pump (no reference sign) and converted in the main heat exchanger  7  to the gaseous or supercritical state. Other nitrogen-rich streams n and o can for example be extracted from the distillation column system  10  in gaseous form from the high-pressure column  11 , heated in the main heat exchanger  7  and used as gaseous nitrogen product (GAN) or seal gas for pumps. The stream d, which can also be partially vented to the atmosphere, has already been mentioned. 
         [0080]      FIG. 2  illustrates, by way of illustration of the problem upon which the invention is based, an air separation plant in the form of a schematic process flow diagram, labelled  200  as a whole. 
         [0081]    The air separation plant  200  which is illustrated in  FIG. 2 , and which incidentally corresponds to the air separation plant  100  illustrated in  FIG. 1 , serves to explain that even using serial boosters or parallel turbo expanders does not on its own solve the problems explained above, or is technically impossible. 
         [0082]    According to  FIG. 2 , the part stream i would be compressed, in the boosters of two booster turbines  201  and  202 , via an intermediate pressure level of, for example, approximately 31 bar to the second pressure level, defined above, of approximately 57 bar, for example. What  FIG. 2  does not illustrate is that the part stream i can be cooled, for example in the main heat exchanger  7 , after exiting from the boosters of the booster turbine  201  and prior to entry into the boosters of the booster turbine  202 , such that its inlet temperature, on entering the boosters of the booster turbines  201  and  202 , is the same or similar. Accordingly, the part stream g would be split into two part streams and expanded in the turbo expanders assigned to the booster turbines  201  and  202 . In that context, the turbo expanders would each have to process only half of the “second” part air quantity of the stream g, thus in the example shown for example approximately 153,000 Nm 3 /h each. Even so, the volume flow through the booster of the booster turbine  202  would still be too small for the volume flow through the corresponding turbo expander, and thus the specific rotational speeds are too different, such that this solution is also impossible. 
         [0083]      FIG. 3  schematically illustrates a compression/expansion arrangement according to one embodiment of the invention, labelled  30  as a whole. The compression/expansion arrangement  30  can be tied into an air separation plant  100  or  200  according to  FIGS. 1 and 2  instead of the booster turbine  101  or the booster turbines  201  and  202 . This tying-in results from the corresponding designation of the part streams g and i. These are respectively the streams g and i downstream of the main heat exchanger  7 . 
         [0084]    The specified first part air quantity of the total air quantity, as defined for example approximately 102,000 Nm 3 /h at a pressure level of approximately 17 bar, is fed in the form of the part stream i in series through two turbo compressors  31  and  32  and thereby compressed to the defined second pressure level of for example approximately 57 bar. In that context, the pressure of the part stream i between the turbo compressors  31  and  32  is for example approximately 31 bar. Between the turbo compressors  31  and  32 , the stream i can be cooled, in the main heat exchanger  7  or otherwise. The second part air quantity of the total air quantity, as defined for example approximately 307,000 Nm 3 /h at a pressure level of approximately 14.5 bar, is split, in the form of the part stream g, into two part streams and is expanded in parallel in two turbo expanders  33  and  34 , as specified to for example approximately 5.2 bar. 
         [0085]    The turbo compressors  31  and  32  and the turbo expanders  33  and  34  are respectively connected to one another via shafts  35  and  36 . A driven wheel  37  is attached to the shaft  35  of the turbo compressors  31  and  32 , and a driving wheel  38  is attached to the shaft  36  of the turbo expanders  33  and  34 . A gear wheel  39  engages with both the driven wheel  37  and the driving wheel  38 . 
         [0086]    A torque imparted to the shaft  36  by the parallel expansion of the part streams of the stream g in the turbo expanders  33  and  34  can be transmitted via the driving wheel  38  to the gear wheel  39  and thence via the driven wheel  37  into the shaft  35 . With a suitable choice of tooth count and geometry of the driving wheel  38 , the gear wheel  39  and the driven wheel  37 , it is possible to ensure that the, as explained, very different volume flows in the turbo expanders  31  and  32  on one hand and in the turbo compressors  33  and  34  on the other hand can be overcome without any problem. 
         [0087]      FIG. 4  schematically illustrates a compression/expansion arrangement not according to the invention, labelled  40  as a whole. The compression/expansion arrangement  40  can also be tied into an air separation plant  100  or  200  according to  FIGS. 1 and 2  instead of the booster turbine  101  or the booster turbines  201  and  202 . Here, too, this tying-in results from the corresponding designation of the part streams g and i. These are respectively the streams g and i downstream of the main heat exchanger  7 . 
         [0088]    Elements of the compression/expansion arrangement  40  which have already been explained with reference to the compression/expansion arrangement  30  of  FIG. 3  are indicated with identical reference signs as in that figure. However, in contrast to the compression/expansion arrangement  30  illustrated in  FIG. 3 , the compression/expansion arrangement  40  of  FIG. 4  has just one turbo expander  33 . This reduces the number of components to be provided, but presupposes that a corresponding turbo expander  33 , with the necessary size and against the background of the mechanical loads which arise, is technically feasible.