Method for cryogenic separation of air, and air separation plant

Abstract

The invention relates to a method for cryogenic separation of air, in which an air separation plant (100-300) having a rectification column arrangement (10) which comprises a pressure column (11), a low-pressure column (12) and a raw argon column (13) is used. In the method, evaporation gas from a head gas condensation device (13.10) associated with the raw argon column (13) is partly or completely fed into the low-pressure column (12) via a gas line (13G). The gas line (13G) contains a first throttle valve (13V1) which is adjusted by means of a control device so that argon is prevented from freezing out in the first head gas condensation arrangement (13.10). This creates, at least temporarily, a pressure drop of at least 50 mbar across the throttle valve (13V1). The present invention also relates to a corresponding air separation plant (100-200).

Description

Description Process for low-temperature separation of air and air separation plant The present invention relates to a process for the low-temperature separation of air and an air separation plant according to the respective preambles of the independent patent claims. Background of the invention The production of air products in liquid or gaseous state 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, especially Section 2.2.5, "Cryogenic Rectification." Air separation plants have rectification column arrangements that can be designed in different ways. In addition to rectification columns for the recovery of nitrogen and / or oxygen in the liquid and / or gaseous state, i.e., rectification columns for nitrogen-oxygen separation, which can be combined in a known double column, rectification columns can be provided for the recovery of other air components, in particular noble gases, or pure oxygen. The rectification columns of typical rectification column arrangements are operated at different pressure levels. Common double columns comprise a so-called pressure column (also referred to as a high-pressure column, medium-pressure column, or lower column) and a so-called low-pressure column (upper column). The high-pressure column is typically operated at a pressure level of 4 to 7 bar, in particular approximately 5.3 bar, while the low-pressure column is typically operated at a pressure level of 1 to 2 bar, in particular approximately 1.4 bar. In certain cases, higher pressure levels can also be used in these rectification columns. The pressures specified here and below are absolute pressures at the top of the respective rectification columns. Air separation plants with crude and pure argon columns can be used to produce argon. An example is illustrated by Häring (see above) in Figure 2.3A and described starting on page 26 in the section "Rectification in the Low-pressure, Crude and Pure Argon Column" and starting on page 29 in the section "Cryogenic Production of Pure Argon." As explained there, argon accumulates in such plants at a certain height in the low-pressure column. At this or another favorable location, possibly even below the argon maximum, argon-enriched gas with an argon concentration of typically 5 to 15 mol percent can be withdrawn from the low-pressure column and transferred to the crude argon column. Such a gas typically contains approximately 0.05 to 500 ppm nitrogen and otherwise essentially oxygen. It should be expressly emphasized that the values ​​given for the gas withdrawn from the low-pressure column are merely typical examples.The crude argon column essentially serves to separate the oxygen from the gas withdrawn from the crude argon column. The oxygen separated in the crude argon column, or a corresponding oxygen-rich fluid, can be recycled in liquid form to the low-pressure column. A gaseous fraction remaining in the crude argon column during the separation, which essentially contains argon and nitrogen, can be further separated in a pure argon column to obtain pure argon. The crude argon column and optionally the pure argon column have top condensers that can be cooled, in particular, with a portion of an oxygen-enriched and nitrogen-depleted liquid (so-called "enriched liquid") withdrawn from the pressure column, which partially evaporates during this cooling. This is also the case within the scope of the present invention.The gas phase formed during partial evaporation and the corresponding remaining liquid are also fed into the low-pressure column at different feed points, the selection of which will be explained below. In conventional processes, the pressure in the gas space of the top condenser is the same as at the gas phase feed point into the low-pressure column. "Same pressure" in this case is understood to mean a pressure range in which the two pressures differ by no more than 25 mbar, preferably no more than 10 mbar. The oxygen or the oxygen-rich fluid from the crude argon column is typically fed several theoretical or practical plates below the Feed points for the partially evaporated liquid used in the cooling process are fed back from the pressure column into the low-pressure column. The present invention has for its object to provide means for improving the operation of an air separation plant with an argon recovery system comprising a crude argon column and a pure argon column. Disclosure of the invention Against this background, the present invention proposes a process for the low-temperature separation of air and an air separation plant having the features of the respective independent patent claims. Further embodiments are the subject of the dependent patent claims and the following description. In the following, some of the terms used in describing the present invention and its advantages as well as the underlying technical background are explained in more detail. The devices used in an air separation plant are described in the cited technical literature, for example, in Häring's Section 2.2.5.6, "Apparatus." Unless the following definitions deviate from this, the terminology used in this application expressly refers to the cited technical literature. A "condenser-evaporator" refers here to a heat exchanger in which a first, condensing fluid stream enters into indirect heat exchange with a second, evaporating fluid stream. Each condenser-evaporator has a condensing chamber and an evaporating chamber. The condensing and evaporating chambers have condensation and evaporation passages, respectively. The condensation (liquefaction) of the first fluid stream takes place in the condensing chamber, and the evaporation of the second fluid stream takes place in the evaporating chamber. The evaporation and condensing chambers are formed by groups of passages that are in heat exchange relationship with each other. Condenser evaporators are also referred to as "head condensers" and "bottom evaporators" according to their function, whereby a head condenser is a A condenser-evaporator is one in which the top gas of a rectification column is condensed, and a bottom evaporator is one in which the bottom liquid of a rectification column is evaporated. However, bottom liquid can also be evaporated in a top condenser, for example, as used in the present invention. In particular, the so-called main condenser, which connects a high-pressure column and a low-pressure column of an air separation plant in a heat-exchanging manner, is designed as a condenser-evaporator. The main condenser or other condenser-evaporators can be designed as single- or multi-level bath evaporators, in particular as cascade evaporators (as described, for example, in EP 1 287 302 B1), or as falling-film evaporators. A corresponding condenser-evaporator can be formed, for example, by a single heat exchanger block or by several heat exchanger blocks arranged in a common pressure vessel. In a "forced-flow" condenser-evaporator or condenser-evaporator with forced flow on the evaporation side, which can also be used in the present invention, a liquid stream is forced through the evaporation chamber by means of its own pressure and partially evaporated there. ("Forced-flow" evaporators are sometimes also referred to as "once-through evaporators"). This pressure is generated, for example, by a liquid column in the supply line to the evaporation chamber, which results from the appropriate positioning of a liquid reservoir. The height of this liquid column corresponds at least to the pressure loss in the evaporation chamber. The gas or gas-liquid mixture emerging from the evaporation chamber, i.e., a two-phase stream, is conveyed in a "once-through" / "forced-flow" condenser-evaporator directly to the next process step or process.to a downstream device and, in particular, is not introduced into a liquid bath of the condenser-evaporator from which the remaining liquid portion would be sucked in again, as is the case, for example, in a conventional bath evaporator operating on the basis of the known thermosiphon effect. As used herein, fluids, i.e., liquids and gases, can be rich or poor in one or more components, where "rich" can mean a content of at least 50%, 75%, 90%, 95%, 99%, 99.5%, 99.9%, or 99.99%, and "poor" can mean a content of at most 50%, 25%, 10%, 5%, 1%, 0.1%, or 0.01% on a molar, weight, or volume basis. The term "predominantly" can correspond to the definition of "rich." Fluids can also be enriched or depleted in one or more components, where these terms refer to a content in a source fluid from which the fluid was derived. The fluid is "enriched" if it contains at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times or 1,000 times the content of a corresponding component, and "depleted" if it contains at most 0.9 times, 0.5 times, 0.1 times, 0.01 times or 0.001 times the content of the starting fluid.For example, if we talk about "oxygen" or "nitrogen", this also means a fluid that is rich in oxygen or nitrogen, but does not necessarily have to consist exclusively of these. The present disclosure uses the terms "pressure range" and "temperature range" to characterize pressures and temperatures. This is intended to express that corresponding pressures and temperatures in a corresponding system do not have to be used in the form of exact pressure or temperature values ​​to implement the inventive concept. For example, different pressures exist at different positions within the high-pressure and low-pressure columns, but these pressures vary within a specific pressure range, also referred to as the operating pressure range. Corresponding pressure ranges and temperature ranges can be disjoint ranges or overlapping ranges. Absolute and / or relative spatial terms used below, such as "above", "below", "above", "below", "next to" and "side by side", refer here in particular to the spatial orientation of the correspondingly designated elements of an air separation plant, for example rectification columns, sub-columns of multi-part rectification columns, or rectification sections of rectification columns during normal operation. An arrangement of two elements "one above the other" is understood here in particular to mean that the upper end of the lower of the two elements is at a lower or the same level. geodetic height as the lower end of the upper of the two elements and the projections of the two elements onto a horizontal plane overlap. In particular, the two elements can be arranged exactly one above the other, i.e. the vertical center axes of the two elements run on the same vertical line. A "side by side" arrangement should be understood in particular to mean that the projections of the two elements onto a horizontal plane do not overlap. In the case of a multi-part rectification column, terms such as "functionally below" or "functionally above" refer to the arrangement of rectification areas or sub-columns that these would have if the rectification column were a single-part one. State-of-the-art air separation plants with a crude column (and optionally with a pure argon column), in which the top condenser of the crude argon column is designed as a force-flow condenser-evaporator as described above, often exhibit unsatisfactory operating stability, particularly under low-load conditions (partial load operation), i.e., when less feed air is introduced into the plant than in normal operation, for example, at least 5%, preferably at least 10% less, and / or at most 60% or 40% less. A reduction in the air flow of 5 to 50% is usually considered an underload condition. Advantages of the invention The invention deviates from the usual operating method for forced-flow condensers, according to which the first evaporation gas is introduced into the low-pressure column with the lowest possible pressure loss, i.e., without any pressure-altering measures. This is fundamentally efficient. Then, in the evaporation chamber of the first head gas condensation arrangement, a pressure is established that corresponds to the operating pressure of the low-pressure column plus line losses. This ensures stable operation of the system under normal conditions. In the effort to find the cause of the undesirably fluctuating operation, it has been discovered within the scope of the invention that in special operating situations, for example, during partial load operation, the liquid argon can be so severely subcooled that the risk that the condensation passages are blocked by freezing argon (triple point of argon: 83.8 K). This problem is solved according to the invention by passing the first evaporation gas between the first overhead gas condensation arrangement and the low-pressure column through a first throttle valve. By partially closing the throttle valve, the pressure and thus the temperature in the evaporation chamber can be increased in the case of underload, thus effectively preventing the condenser from blocking due to freezing. Preferably, the valve is designed as an automatic valve; alternatively, a manual valve can be used. Overall, this results in particularly stable operation of the first overhead gas condensation arrangement and the crude argon column. The term "throttle valve" is used here in the general sense of "throttle device" and includes, for example, throttle flaps. The pressure drop across the first throttle valve can, for example, be between 300 and 50 mbar in at least one operating case, preferably between 250 and 80 mbar. Generally, this throttling of the first evaporation gas is performed underload. Depending on the extent of the underload, a lower or higher pressure drop, and thus a lower or higher temperature, is set. In an example case, the following values ​​result for a gas composition of approximately 57.4% nitrogen, 1.8% argon, and 40.8% oxygen: (Table 1) Pressure at the evaporator inlet, bara 1 ,49 1 ,39 1 ,29 1 ,19 Boiling temperature, K 84.20 83.52 82.80 82.03 System load, % (approx.) 100% 83% 64% 40% To prevent the freezing of argon, the first throttle valve is adjusted by means of the control device according to the invention so that the temperature of the first cooling liquid upon entry into the first head gas condensation arrangement is above the triple point temperature of argon. This entry temperature is preferably at least 0.1, in particular at least 0.25 K above the triple point of Argon. The temperature difference is preferably between 0.1 and 2.0 K, most preferably between 0.2 and 1.0 K. In principle, a throttle valve in place of the first throttle valve is known from the prior art (see also Figure 1), but only in systems with a bath evaporator at the top of the crude argon column. In a bath evaporator, however, the valve has a completely different function, namely a flow control for the gas flow at the column inlet. In a forced-flow evaporator, however, this flow control is achieved by backing up the liquid at the evaporator outlet (on the condensation side); in this case, a valve between the condenser and the low-pressure column would only produce undesirable pressure loss and is unnecessary for flow control. Within the scope of the invention, a throttle valve is used again, which is completely open in many operating cases, but in certain operating cases, the stability of the operation of the The condenser evaporator's performance has been surprisingly effectively improved. Of course, this throttling also causes an avoidable pressure loss and thus tends to reduce the energy efficiency of the process. However, the invention has shown that the effect of this throttling does not result in the expected disadvantage, but rather, surprisingly, in an energy saving. The condenser is usually designed and dimensioned for the design case (normal operation). It offers a comparatively large heat exchange surface in underload conditions. Therefore, it must be "braked" in underload cases to set the appropriate performance. In systems with bath evaporators, this is done, for example, by increasing the evaporation pressure (usual control of an evaporator). In systems with forced-flow evaporators, however, the performance (the load on the crude argon column) is controlled by backing up the liquid and covering part of the heat exchange surface, since there is no valve between the evaporation space of the condenser and the low-pressure column. The operating pressure in the low-pressure column in underload cases is noticeably lower than in the design case; therefore, the pressure (or temperature) in the evaporation space of the forced-flow evaporator is also lower, and freezing of the argon can occur. To counteract this, the pressure in the The low-pressure column can be artificially raised. However, this requires energy. This energy loss is prevented by the process according to the invention. The control device according to the invention can be analog or digital and, in particular, integrated into an operating control system. It ensures, without human intervention, that the first throttle valve is partially closed in the corresponding operating conditions in order to set the desired pressure difference. This measure is preferably integrated into an automatic load adjustment and thus ensures consistently stable operation of the system, for example, during the transition from normal load operation to underload operation. In the invention, a first liquid pressure stream is formed using a first portion of an oxygen-enriched liquid from the pressure column, which first liquid pressure stream is expanded to obtain a first flash gas and to remain a first low-pressure liquid. The crude argon column is operated using a first overhead gas condensation arrangement in which the overhead gas of the crude argon column is subjected to condensation with partial evaporation of a first cooling liquid, which is provided using the first low-pressure liquid or a portion thereof. The first overhead gas arrangement is also referred to below as the overhead condenser of the crude argon column. The process and the plant can also include a pure argon column operated using a second overhead gas condensation arrangement in which the overhead gas of the pure argon column is subjected to condensation with partial evaporation of a second cooling liquid provided using the second low-pressure liquid or a portion thereof. The second overhead gas arrangement is also referred to below as the overhead condenser of the pure argon column. A first evaporation gas formed during the partial evaporation of the first cooling liquid or a part thereof and a first excess liquid remaining during the partial evaporation of the first cooling liquid or a part thereof are fed into the low-pressure column. A second evaporation gas formed during the partial evaporation of the second cooling liquid or a part thereof and a second excess liquid remaining during the partial evaporation of the second cooling liquid or a part thereof are fed into the low-pressure column. The term "evaporation gas" refers to the vaporized portion which is formed by heat transfer from the respective head gases of the crude and pure argon column in the head gas condensation arrangements or Condenser evaporators form within them. Any remaining liquid residue is referred to here as "excess liquid." In contrast to the term "evaporation gas," the term "flash gas" refers to the gas or vapor portion formed solely through expansion. Preferably, in the invention, the liquid level on the evaporation side of the head gas condensation arrangement (13.10) is regulated by a second throttle valve (13V2), by means of which the first cooling liquid upstream of the head gas condensation arrangement (13.10) can be throttled. As is also common with forced-flow evaporators, the first evaporation gas is preferably withdrawn from the top gas condensation arrangement together with the first excess liquid as a first two-phase stream, without any part of the excess liquid being circulated via the evaporator. Within the scope of a particularly advantageous embodiment of the invention, one or more "forced-flow" condenser-evaporators of the type described can be used in the first top gas condensation arrangement. Reference is made to the above explanations. In particular, the first low-pressure liquid or a portion thereof is thus passed as the first cooling liquid through one or more condenser-evaporators, which are or are part of the first Head gas condensation arrangement is or are designed, forcibly guided and thereby subjected to partial evaporation to the first evaporation gas and the first excess liquid. "Forcibly guided" here means a feed into the evaporation chamber under pressure, for example, via a pipeline. The throttle valve may be fully open at least temporarily during operation, particularly during normal operation (first operating mode). However, in at least one underload condition (second operating mode), a pressure drop of at least 50 mbar is generated. In addition to the first evaporation gas, the first excess liquid is preferably also fed into the low-pressure column within the scope of the invention. In a first variant of the invention, the gas and the liquid can be fed together as a first two-phase stream, partially or completely, into the low-pressure column, in particular in a first feed zone. In this case, the gas line is designed as a two-phase line, and the first throttle valve is designed as a two-phase valve, and the two-phase stream, or the portion flowing into the low-pressure column, is guided through the first throttle valve. In a second variant, the first two-phase stream is fed into a phase separator. In this case, the gas line between the phase separator and the low-pressure column is designed as a pure gas line, and the first throttle valve as a pure gas valve. The first excess liquid is fed between the overhead gas condensation arrangement and the first throttle valve through a phase separator, in which the first evaporation gas and the first excess liquid are separated from one another. The first evaporation gas is then introduced into the low-pressure column separately from the first excess liquid. The pressure in the evaporation space is not controlled by the valve in the line for the first excess liquid, but by the first valve in the pure gas line, which connects the phase separator to the low-pressure column on the gas side. The liquid level in the phase separator can be measured. Depending on the measured value, the amount of first cooling liquid introduced into the first overhead gas condensation arrangement is preferably adjusted. The amount of liquid accumulating in the phase separator is preferably volume-controlled. The invention can, in principle, be applied to all process cycle topologies with argon recovery, regardless of the type of refrigeration or the type of product compression. These include, in particular, so-called MAC / BAC or HAP processes, as described, for example, in paragraphs

[0022] until

[0025] EP 3 196 573 A1, processes with nitrogen cycle, as described in EP 2 235 460 A2 or in H. Hausen and H. Linde, "Low-temperature technology: generation of very low temperatures, gas liquefaction and decomposition of gas mixtures", 2nd edition 1985, Springer-Verlag, Heidelberg, Section 4.5.2.2, and / or air separation plants with internal compression, as described in Hausen / Linde, Section 4.5.1.6 or Häring (see above), Section 2.2.5.2, "Internal Compression". Regarding the features of the air separation plant also proposed according to the invention, express reference is made to the corresponding independent patent claim. The air separation plant is particularly designed to carry out a process as previously explained in embodiments. Therefore, express reference is made to the above explanations regarding the process according to the invention and its advantageous embodiments. The invention will be explained in more detail below with reference to the accompanying drawings, which illustrate the preferred embodiments of the present invention. Character description Figure 1 illustrates an air separation plant according to a non-inventive embodiment with a bath evaporator in a simplified representation. Figures 2 to 11 illustrate air separation plants according to embodiments of the invention in a simplified representation. In the figures, structurally or functionally corresponding elements are indicated with identical reference symbols and are not explained repeatedly for the sake of clarity. Explanations concerning systems and system components apply equally to corresponding processes and process steps. Detailed description of the drawings In Figure 1, an air separation plant according to a non-inventive embodiment of the present invention is illustrated in the form of a simplified process flow diagram and is designated overall by 90. Air separation plants of the type shown have been described in numerous other places, for example in (see above), Industrial Gases Processing, Wiley-VCH, 2006, particularly Section 2.2.5, "Cryogenic Rectification," and in connection with Figure 2.3A. For detailed explanations of their design and operation, please refer to the relevant specialist literature. An air separation plant for implementing the present invention can be designed in a variety of ways. As mentioned, the present invention can, in principle, be applied to all process cycle topologies with argon recovery, regardless of the type of refrigeration or product compression. The air separation plant 90 shown as an example in Figure 1 comprises, among other things, a main air compressor 1, a pre-cooling device 2, a cleaning system 3, a secondary compressor arrangement 4, a first booster turbine 5, a second booster turbine 6, a main heat exchanger 7, pumps 8 and 9, and a rectification column system 10. In the example shown, the rectification column system 10 comprises a classic double column arrangement consisting of a pressure column 11 and a low-pressure column 12, as well as a crude argon column 13 and a pure argon column 14. The crude argon column 13 and the pure argon column 14 have a top gas condensation arrangement 13.10 and 14.10, referred to here as "first" and "second" top gas condensation arrangements, which here each comprise a reflux and bath condenser evaporator, respectively. In the air separation plant 90, a feed air stream is sucked in by the main air compressor 1 through a filter (not designated) and compressed. The compressed feed air stream is fed to the pre-cooling device 2 operated with cooling water. The pre-cooled feed air stream is cleaned in the cleaning system 3. In the cleaning system 3, which typically comprises a pair of The pre-cooled feed air stream is largely freed of water and carbon dioxide by adsorber vessels used in alternating operation. Downstream of the cleaning system 3, the feed air stream is divided into substreams. The air in the feed air stream is cooled in the main heat exchanger 7 in a generally known manner. In the example illustrated here, two so-called turbine streams are formed in corresponding turbines. The booster unit of the turbine booster 6 is designed as a so-called cold booster, i.e., it is fed with already cooled air from the main heat exchanger 7. Air completely cooled in the main heat exchanger 7 is expanded in a liquefied state via throttle valves (not specifically designated) and fed into the rectification column system as so-called throttle streams. In pressure column 11, an oxygen-enriched liquid bottom fraction and a nitrogen-enriched gaseous top fraction are formed. The oxygen-enriched liquid bottom fraction is withdrawn from pressure column 11 and fed in portions into the evaporation chambers of the reflux and reflow columns. The bath condenser evaporators in the overhead gas condensation arrangements 13.10 and 14.10 expand the pressure. Gas fractions formed by the expansion and evaporation against the overhead gas of the crude or pure argon columns 13 and 14, as well as the unevaporated liquid here, are fed into the low-pressure column 12. The operation of the air separation plant 90 illustrated here is standard practice, so reference is made to the cited technical literature. The crude argon column 13 is fed in the usual way from the low-pressure column 11, and the pure argon column 14 is fed in the usual way from the crude argon column 13. In Figures 2 to 8, air separation plants according to embodiments of the invention are shown and designated 100, 200 and 300, respectively. In all cases, an oxygen-enriched liquid withdrawn from the pressure column 11 is designated by A. Using a first portion thereof, a first liquid pressure stream B is formed, which is expanded in a valve (not separately designated) to obtain a first flash gas and a first low-pressure liquid. The first evaporation gas from the Head gas condensation arrangement 13.10 is introduced into the low-pressure column 12 via a gas line 13G, which contains a first throttle valve 13V1. In the embodiments 100 and 200 according to Figures 2 to 4, in which identical reference numerals are used as before for the sake of simplicity, a previously described "forced-flow" condenser-evaporator 13.12 is used in the first top gas condensation arrangement 13.10, next to which a separate phase separator 13.11 is arranged. The first low-pressure liquid is forced from this via the pressure of the developing liquid column through evaporation passages of the "forced-flow" condenser-evaporator 13.12; the first flash gas can be withdrawn as illustrated by C. The embodiments 100 and 200 differ essentially in that the turbine booster 6 is not present in the embodiment 200 of Figure 3. In all cases, using a second portion of the oxygen-enriched liquid from the pressure column 11, a second liquid pressure stream D is formed, which is expanded to obtain a second flash gas and to remain a second low-pressure liquid, the second flash gas being designated E in each case. The crude argon column 13 is therefore operated here using a first head gas condensation arrangement 13.10, in which the head gas of the crude argon column 13 is subjected to condensation with partial evaporation of a first cooling liquid, which is provided using the first low-pressure liquid or a part thereof, The pure argon column 14 is operated using a second overhead gas condensation arrangement 14.10, in which overhead gas of the pure argon column 14 is subjected to condensation with partial evaporation of a second cooling liquid, which is provided using the second low-pressure liquid or a part thereof. A first evaporation gas formed during the partial evaporation of the first cooling liquid or a part thereof and a first excess liquid remaining during the partial evaporation of the first cooling liquid or a part thereof are in both embodiments 100, 200 according to Figures 2 to 4, fed into the low-pressure column 12, as illustrated by F and G. Likewise, a second evaporation gas formed during the partial evaporation of the second cooling liquid or a part thereof and a second excess liquid remaining during the partial evaporation of the second cooling liquid or a part thereof are fed into the low-pressure column 12, as illustrated by H and I. The first evaporation gas F or the part thereof fed into the low-pressure column 12 is always partially or completely fed into a first feed region into the low-pressure column 12, in particular at a common position with the first excess liquid G. The second evaporation gas H, or the portion thereof fed into the low-pressure column 12, is partially or completely fed into the low-pressure column 12 in a second feed region. Likewise, the second excess liquid I, or the portion thereof fed into the low-pressure column 12, is partially or completely fed into the low-pressure column 12 in the second feed region. The first flash gas C, or a portion thereof, is partially or completely fed into the low-pressure column 12 in the second feed region, separately from the first evaporation gas F. A transfer stream from the crude argon column 13 into the pure argon column is additionally designated T in Figure 4 and is also present in the other embodiments. Figure 5 shows a very schematic view of the upper ends of columns 10, 13, and 14. The process is the same as in Figure 2 or Figure 3, except that no separate separator is used as the phase separator for the first pressurized liquid stream B, but rather simply the evaporation space of the second top gas condensation arrangement 14.10 (pure argon top condenser). The gas line 13G is designed here as a two-phase line, and the first throttle valve 13V1 is designed as a two-phase valve. For this purpose, the two liquid pressure streams B and H are expanded together downstream of the bottom evaporator 600 of the pure argon column 14 in valve 601 and via line 602 together into this evaporation space of the second The first flash gas C is introduced into the top gas condensation arrangement 14.10, which acts as a common phase separator. The first flash gas C is withdrawn via line 603, together with the second evaporation gas E generated in the condenser-evaporator 14.10. The first cooling liquid K is withdrawn from the evaporation space of the second top gas condensation arrangement 14.10, together with the second excess liquid I, via line 604 and separately introduced into the evaporation space of a first top gas condensation arrangement (13.10) for partial evaporation. The first top gas condensation arrangement (13.10) is designed as a forced-flow evaporator on the evaporation side. The remaining fluids are conducted to and from the first top gas condensation arrangement (13.10) as shown in Figures 2 and 3. Compared to Figures 2 to 4, this results in reduced manufacturing costs for the system and also a reduced footprint, thus also reducing the number of boxes for the insulating cold box and its filling with insulating material such as perlite. Figure 6, also schematically, illustrates a further development based on Figure 5. However, this further development can also be applied to Figures 2 to 4, in which the first top gas condensation arrangement (13.10) also has a forced-flow evaporator. The gas line 13G is designed here as a two-phase line, and the first throttle valve 13V1 as a two-phase valve. In Figure 6, the first throttle valve 13V1 is installed in the downcomer 702 of the two-phase stream 701. The downcomer represents part of the gas line 13G. The first throttle valve 13V1 is typically fully open during normal operation. During special operating situations, for example during partial load operation, the two-phase stream can be throttled according to the invention in order to increase the pressure and thus the temperature in the first top gas condensation arrangement (13.10). This effectively prevents the freezing of argon and achieves particularly stable operation.The valve can be pressure-controlled (or alternatively temperature-controlled). The fluid 604 is divided into streams K and I as shown in Figure 5; the corresponding proportions are adjusted by valve FIC1. Figure 6 also shows the corresponding control elements. The following are: FIC - Flow Indication and Control - Flow measurement and adjustment LIC - Liquid Indication and Control - Measurement and adjustment of a liquid level PIC - Pressure Indication and Control - Pressure measurement and adjustment The data lines between the measuring and actuating elements are shown in dashed lines in Figure 7 (and also in Figures 8 and 9). FIC1 controls the supply of second excess liquid I to the low-pressure column 12, i.e., the division of liquid stream 604. FIC2 controls the supply of condensate from the first overhead gas condensation arrangement (13.10) as a function of the feed quantity for the crude argon column. PIC1 controls the pressure on the evaporation side of the second overhead gas condensation arrangement (14.10). LIC1 controls the amount of first cooling liquid flowing into the first overhead gas condensation arrangement (13.10). LIC2 controls the total amount of cooling liquid via the bottoms level measurement in the pressure column. FIC2 controls the evaporator capacity (by backing up the liquid into block 13.10 and covering part of the condensing surface). The liquid fraction in stream 701 is calculated and adjusted if necessary by FIC1. Alternatively, particularly stable operation can be achieved by using an additional phase separator 804 to separate the two-phase stream 701 into the first evaporation gas F and the first excess liquid G. This variant is shown in Figure 7. The gas line 13G is designed here as a pure gas line, extends from the phase separator 804 to the low-pressure column 12, and contains the throttle valve 13V1. The first evaporation gas F separated in the phase separator 804 then flows, according to the invention, via this gas line 13G and through the first throttle valve 13V1 into the low-pressure column 12. The control system is also shown in Figure 7. PIC1 and LIC2 have the same function as in Figure 7. The pressure on the evaporation side of the second top gas condensation arrangement (14.10) can be controlled with PIC2. Alternatively, a TIC (Temperature Indication and Control) controller can be used instead of PIC2, ​​controlling the temperature of the first cooling liquid entering the first overhead gas condensation arrangement (13.10). LIC3 controls the amount of first cooling liquid flowing into the first overhead gas condensation arrangement (13.10), but in this case, it is dependent on the measured fill level in phase separator 804. The amount of second excess liquid I flowing to the low-pressure column is adjusted by LIC4 depending on the liquid level on the evaporation side of the pure argon condenser. Furthermore, there are controllers FIC3 and FIC4 in the lines for the second excess liquid G and the crude argon, which is passed on to the pure argon column 14. Controller FIC3 is particularly important. This allows the liquid content in stream 701 to be directly controlled (and not calculated), thus avoiding dry evaporation in the condenser. Figure 8 shows a simplified illustration of a particular apparatus embodiment of the invention according to Figure 7. Here, the heat exchanger block of the first head gas condensation arrangement 13.10 is arranged inside the phase separator 804, in which the first evaporation gas and the first excess liquid are separated from one another. The first head gas condensation arrangement does not lose its character as a forced-flow evaporator. Rather, the liquid to be evaporated continues to flow in a forced-flow manner through the line at LIC3 and the header on the heat exchanger block into the evaporation passages and is not drawn from the liquid bath of the separator 804, as would be the case with a bath evaporator. The special measures of Figures 6 to 8, in particular the phase separator 804, can also be applied to the overall processes of Figures 2 to 5, both with a separate phase separator for the first liquid pressure stream and with one integrated into the overhead gas condensation arrangement, as shown in Figure 8. The previous examples are optimized for thermodynamic efficiency or maximum yield of argon product. In some cases, however, this is not the decisive criterion, but rather, for example, the equipment costs or the height of the columns, or similar factors. In this case, it may be more advantageous to minimize the feed points into the low-pressure column in the manner shown in Figures 9 to 11, which otherwise correspond to Figures 6 to 8. In Figure 9, for example, line I according to Figure 6 and valve FIC1 are omitted. Instead, the entire liquid stream 604, which is taken from the evaporation space of the top condenser 14.10 of the pure argon column 14, flows through the top condenser 13.10 of the crude argon column 13. The control system also functions as in Figure 6, except that valve FIC1 for dividing the first pressurized liquid stream is omitted. Similarly, Figure 10 differs from Figure 7. The valve LIC4 and the corresponding line are omitted. Furthermore, the vapor 901 from the separator 804 and the vapor 902 from the top condenser 14.10 of the pure argon column 14 are combined and fed into the low-pressure column via a common line 903, preferably at the same point as the liquid G from the separator 804. The entire liquid stream 604 withdrawn from the evaporation space of the top condenser 14.10 of the pure argon column 14 flows through the top condenser 13.10 of the crude argon column 13. The control system also functions differently in some respects. LIC1, like LIC2 previously in Figure 7, is responsible for level control in the bottom of the pressure column (not shown here). The level in the evaporation chamber of the top condenser 14.10 of the pure argon column 14 is controlled by adjusting the amount of liquid withdrawn via valve 13V2 (LIC2). Valve FIC2 thus indirectly controls the amount of gas at the column inlet by backing up the liquid (and thus covering part of the heat exchange surface). This gives the top condenser 13.10 a higher cooling capacity (at a relatively low liquid level) or a lower one (at a relatively high liquid level). Accordingly, more or less top gas is condensed on the condensation side; a corresponding amount of gas is drawn in from the low-pressure column via the argon transition line (not fully shown in Figure 10, but shown in Figures 1 to 4).According to the invention, valve 13V1 is pressure-controlled (PIC2) and thus adjusts the temperature of the head condenser 13.10 and thus its output. The pressure is controlled via LIC3. Liquid outflow from the separator 804 is adjusted and thus the liquid level in the separator is regulated. A particularly preferred embodiment is shown in Figure 11, which is closely based on Figure 8. In particular, here the heat exchanger block of the top condenser 13.10 of the crude argon column 13 is incorporated into the separator 804. Otherwise, the entire liquid stream 604, which is taken from the evaporation space of the top condenser 14.10 of the pure argon column 14, is introduced into the evaporation space of the top condenser 13.10 of the crude argon column 13. Also analogous to Figure 10, the vapor 901 from the separator 804 and the vapor 902 from the top condenser 14.10 of the pure argon column 14 are combined and fed into the low-pressure column via the common line 903, preferably at the same point as the liquid G from the separator 804. LIC1, like LIC2 previously in Figure 8, is responsible for level control in the bottom of the pressure column (not shown here). The level in the evaporation chamber of the top condenser 14.10 of the pure argon column 14 is adjusted by adjusting the amount of liquid withdrawn via valve 13V2 (LIC2). The flow on the liquefaction side of the top condenser 13.10 of the crude argon column 13 is adjusted as shown in Figure 10. Valve 13V1 is pressure-controlled according to the invention (PIC2) and thus adjusts the temperature of the top condenser 13.10 and thus its output. LIC3 adjusts the liquid outflow from the separator 804 and thus regulates the liquid level in the separator. PC1 adjusts the pressure in the evaporation chamber of the top condenser 14.10.

Claims

A process for the low-temperature separation of air, in which an air separation plant (100, 200) with a rectification column arrangement (10) is used, which has a pressure column (11), a low-pressure column (12) and a crude argon column (13), wherein with direct or indirect use of at least a portion of an oxygen-enriched liquid from the pressure column (11) a first liquid pressure stream is formed, which is expanded to produce a first low-pressure liquid, the crude argon column (13) is operated using a first overhead gas condensation arrangement (13.10), in which overhead gas of the crude argon column (13) is subjected to condensation with partial evaporation of a first cooling liquid, which is provided using the first low-pressure liquid or a portion thereof, the first overhead gas condensation arrangement (13.10) has a forced-flow condenser evaporator and a first evaporation gas formed during the partial evaporation of the first cooling liquid or a part thereof is fed into the low-pressure column (12) via a gas line (13G), characterized in that the gas line (13G) contains a first throttle valve (13V1), the first throttle valve (13V1) is adjusted by means of a control device such that freezing of argon in the first top gas condensation arrangement (13.10) is avoided and in the process a pressure drop across the throttle valve (13V1) of at least 50 mbar is generated at least temporarily.

2. Method according to claim 1, wherein the liquid level on the evaporation side of the head gas condensation arrangement (13.10) is regulated by a second throttle valve (13V2) by means of which the first cooling liquid upstream of the head gas condensation arrangement (13.10) can be throttled.

3. A process according to claim 1 or 2, wherein the first evaporation gas or a portion thereof is withdrawn from the first top gas condensing arrangement (13.10) as a first two-phase stream together with the first excess liquid or a portion thereof without recycling the first excess liquid or a portion thereof to the one or more condenser evaporators.

4. Process according to one of claims 1 to 3, wherein the first two-phase stream is passed between the first top gas condensation arrangement (13.10) and the low-pressure column (12) through the first throttle valve (13V1).

5. Method according to one of the preceding claims, in which the first throttle valve (13V1) - is fully open in a first operating mode (normal operation) and is set in a second operating mode (underload case) so that a pressure drop across the throttle valve (13V1) of at least 50 mbar is generated.

6. Method according to one of the preceding claims, in which the throttle valve (13V1) is adjusted so that the temperature of the first cooling liquid upon entry into the first head gas condensation arrangement (13.10) is preferably at least 0.1 K above the triple point temperature of argon.

7. Method according to one of the preceding claims, in which - during the relaxation of the first fluid pressure flow in addition to the first Low-pressure liquid a first flash gas is formed, - using at least part of the oxygen-enriched Liquid from the pressure column (11) a second liquid pressure stream which is expanded to produce a second flash gas and to retain a second low-pressure liquid, - the pure argon column (14) is operated using a second top gas condensation arrangement (14.10), in which top gas of the pure argon column (14) is subjected to condensation with partial evaporation of a second cooling liquid which is provided using the second low-pressure liquid or a part thereof, and in which - a second evaporation gas from the second head gas condensation arrangement (14.10) is withdrawn and mixed with the first evaporation gas and introduced into the low-pressure column and - a second excess liquid from the second The liquid is withdrawn from the top gas condensation arrangement (14.10) and used to form the first cooling liquid for the first top gas condensation arrangement (13.10). The method according to claim 7, wherein the liquid level in the bottom of the pressure column (11) is measured, and the amount of second cooling liquid introduced into the second top gas condensation arrangement (14.10) is adjusted (LIC1, LIC2) depending on the measured value. The method according to claim 7 or 8, wherein the liquid level in the evaporation space of the second top gas condensation arrangement (14.10) is measured and kept constant by adjusting (LIC2) the feed amount to the first condenser arrangement (13.10).Process according to one of the preceding claims, in which the first evaporation gas or the portion thereof, together with the first excess liquid or the portion thereof, are fed together as a two-phase stream via the gas line (13G) and the first throttle valve (13V1) into the low-pressure column. Process according to one of claims 1 to 9, in which the first two-phase stream is fed between the overhead gas condensation arrangement (13.10) and the low-pressure column (12) into a phase separator (804), in which the first evaporation gas and the first excess liquid are separated from one another. wherein the separated evaporation gas is passed between the phase separator and the low-pressure column (12) via the gas line (13G) through the throttle valve (13V1).

12. The method according to claim 11, wherein the liquid level in the phase separator (804) is measured and kept constant with the amount of first excess liquid introduced into the low-pressure column.

13. Method according to one of the preceding claims, in which the head gas condensation arrangement (13.10) has a heat exchanger block and this heat exchanger block is arranged in the interior of the phase separator (804) in which the first evaporation gas and the first excess liquid are separated from one another.

14. Air separation plant (100, 200) with a rectification column arrangement (10) which has a pressure column (11), a low-pressure column (12) and a crude argon column (13), wherein the air separation plant (100, 200) is designed to form a first liquid pressure stream by directly or indirectly using a first portion of an oxygen-enriched liquid from the pressure column (11) and to expand this to produce a first flash gas and to leave a first low-pressure liquid, to operate the crude argon column (13) using a first top gas condensation arrangement (13.10) and to subject the top gas of the crude argon column (13) to a condensation in this arrangement with partial evaporation of a first cooling liquid which is provided using the first low-pressure liquid or a portion thereof, wherein the first top gas condensation arrangement (13.10) has a Forced-flow condenser evaporator, and feeding a first evaporation gas formed during the partial evaporation of the first cooling liquid or a part thereof into the low-pressure column (12), and characterized in that - the gas line (13G) contains a first throttle valve (13V1) and the first throttle valve (13V1) is designed to be adjusted by means of a control device so that freezing of argon in the first head gas condensation arrangement (13.10) is avoided and, at least temporarily, to generate a pressure drop across the throttle valve (13V1) of at least 50 mbar.

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