Patent Application
20180112913
The invention relates to a plate heat exchanger condenser-evaporator and a process for low-temperature air separation as per the preambles of the independent claims.
In air separation plants, combined evaporation and condensation units can be used for differing purposes, inter alia as main condensers or as overhead condensers of (crude) argon columns.
As explained, for example, in H.-W. Hiring (editor), Industrial Gases Processing, Wiley-VCH, Weinheim: 2006, in the section 2.2.5.6, “Apparatus” and the subsection “Combined Evaporator/Condenser—Heat Transfer Units” on page 52 and 53, respectively, these are typically special types of plate heat exchangers (plate fin heat exchangers). Hereinafter, the expression plate heat exchanger condenser-evaporator is therefore used for such units.
With respect to the understanding of an expert of names for the elements explained hereinafter, in particular reference is made to the publication “The Standards of the Brazed Aluminium Plate-Fin Heat Exchanger Manufacturers' Association”, 2nd edition, 2000, in particular section 1.2.1, “Components of an Exchanger” (hereinafter termed “ALPEMA standards” for short).
A plate heat exchanger condenser-evaporator, as underlies the present invention, has a number of heat-exchanger plates lying one above the other that form a usually cubic, heat-exchanger block (block, core). Each of the heat-exchanger plates, in the central region thereof, usually comprises an embossed metal sheet having heat-transfer fins. The heat-transfer fins can be, for example, straight (plain fins) and optionally perforated (plain-perforated fins) or else designed to be interrupted and offset to one another (serrated fins), in wave form (herringbone fins) or in some other form.
The embossed metal sheet is enclosed by an edging of side elements (side bars). The embossed metal sheets and side elements of the individual heat-exchanger plates are separated from one another by flat metal sheets (parting sheets). The uppermost and lowermost heat-exchanger plates each have a covering metal sheet (cap sheet). The heat-transfer fins and any fluid-distribution and fluid-collecting structures present (see hereinafter) of each heat-exchanger plate define “fluid-conducting structures” in the language used here.
In a plate heat exchanger condenser-evaporator of a conventional type, the fluid-conducting means of a first group of the heat-exchanger plates end at a first and a second outer surface of the heat-exchange block, wherein the first and second outer surfaces are situated opposite to one another and parallel to one another. According to the function thereof, the first outer surface is also termed “exit surface” and the second outer surface is termed “intake surface” for fluid. The fluid conductors of the first heat-exchange plate group each have not-covered openings on the exit surface and the intake surface, wherein here a “not-covered” opening is taken to mean an opening that does not opens out either into a fluid feed line constructed on the heat-exchange block or into a corresponding fluid-collecting line (header). The fluid conducting means of the first heat-exchanger plate group therefore open in operation to a fluid space surrounding the heat-exchange block. In particular, the fluid channels of the first heat-exchanger plate group in a plate heat exchanger condenser-evaporator can run parallel through the entire heat exchange block and either no fluid-distribution and liquid-collecting structures (distributor fins) are provided, or they are constructed so as to be open-ended (open end distributors).
In a plate heat exchanger condenser-evaporator, in contrast, the fluid conductors of a second group of heat-exchanger plates run, as they also do in standard plate heat exchangers, between a fluid feed line and a fluid-collecting line. The fluid feed line is connected to fluid distribution structures constructed in the heat-exchanger plates, via which fluid fed via the fluid feed line is distributed over the entire width of the heat-transfer fins. Correspondingly, the fluid-collecting line is connected to fluid-collecting structures constructed in the heat-transfer plates. Through the latter, fluid is collected over the entire width of the heat-transfer fins and fed to the fluid-collecting line. Corresponding fluid-distribution structures and fluid-collecting structures are known in various designs, inter alia, from the abovementioned ALPEMA standard.
In operation, a plate heat exchanger condenser-evaporator is immersed with the intake surface in a liquid bath of a fluid that is condensed and is to be evaporated, for example an oxygen-rich fluid in the sump of the low-pressure column of an air separation plant. Gaseous fluid that is to be condensed, for example a nitrogen-rich overhead product of the high-pressure column, is fed into the fluid feed line. The fluid conductors of the first heat-exchanger plate group can be oriented so as to be perpendicular, at least the openings thereof in the exit surface are arranged, however, above the openings thereof in the intake surface, in order to effect what is termed the thermosiphon effect:
The fluid that is condensed and is to be evaporated enters via the openings in the intake surface into the fluid conductors of the first heat-exchanger plate group and there experiences a heat exchange with the gaseous fluid that is to be condensed in the fluid conductors of the second heat-exchanger plate group and/or a (partial) condensate of this fluid formed there. By this means, partial evaporation occurs in the fluid that is condensed and is to be evaporated. The resultant two-phase mixture has in total a lower density than the condensed fluid that is to be evaporated from which it was formed. Therefore, it ascends in the fluid conductors of the first heat-exchanger plate group and exits via the openings in the exit surface. A continuous flow thereby results in the fluid conductors of the first heat-exchanger plate group. The fluid that is evaporated in the fluid conductors of the first heat-exchanger plate group transfers into a gas space above the exit surface. Fluid that is not evaporated flows back into the liquid bath on the outside of the heat-exchange block.
At the same time, a (partial) condensation occurs owing to the heat exchange in the fluid channels of the second heat-exchanger plate group, as described above. In this manner, a (partial) condensate of the fluid fed in via the fluid feed line, i.e. a liquid stream or a two-phase stream, can be taken off from the fluid-collecting line of this second heat-exchanger plate group.
In plate heat exchanger condenser-evaporators, when a gas mixture is fed into the fluid feed line of the second heat-exchanger plate group, owing to selective condensation and other effects, an unmixing can occur in the fluid-distribution structures of the heat-exchanger plates, i.e. a gas component of the gas mixture can accumulate in certain regions. This is disadvantageous, in particular, when one of the gas components is an inert gas. In such regions, the fluid passage can be impeded and/or an effective heat exchange can be prevented. Ultimately, this leads to a reduction of the overall available heat-exchange surface area.
The object of the invention is to specify measures that eliminate the said disadvantages, in particular to prevent unmixing in a plate heat exchanger condenser-evaporator in a simple and effective manner.
This object is achieved by a plate heat exchanger condenser-evaporator and a process for low-temperature air separation having the features of the independent claims. Embodiments are subject matter of the dependent claims and also of the description hereinafter.
The present invention proposes a plate heat exchanger condenser-evaporator having a heat-exchanger block which comprises a number of heat-exchanger plates having fluid-conducting structures. The heat-exchanger block has outer surfaces. If the heat-exchanger block is constructed so as to be cuboidal, in total six outer surfaces are present. Hereinafter, a first, a second, a third and a fourth outer surface of the heat-exchanger block are considered. The first outer surface is situated opposite to the second outer surface, the third outer surface is situated opposite the fourth. The first outer surface and the second outer surface are situated in particular parallel to one another, likewise the third and fourth outer surfaces. The distance between the first and second outer surfaces corresponds to the height of the heat-exchanger block. The first outer surface, during operation of the plate heat exchanger condenser-evaporator, is situated on the top side of the heat-exchanger block, the second on the bottom side thereof. The second outer surface and in each case part of the third and fourth outer surfaces are, during operation of the plate heat exchanger condenser-evaporator, immersed in a liquid bath of a condensed fluid. The heat-exchanger plates comprise a plurality of first and a plurality of second heat-exchanger plates which in principle have different functions and are constructed differently.
The fluid-conducting structures of the first heat-exchanger plates, and thereby the first heat-exchanger plates overall, are equipped for evaporating the condensed fluid in part or completely, in such a manner that said fluid ascends upwardly in the fluid-conducting structures of the first heat-exchanger plates, for which purpose the described thermosiphon effect is used. The fluid-conducting structures of the first heat-exchanger plates comprise openings which are not covered by fluid feed lines or by fluid-collecting lines, in such a manner that a direct entry of the condensed fluid and a direct exit of the partially or completely evaporated fluid can be ensured.
The said openings are hereinafter also called “first” and “second” openings, wherein the condensed liquid is drawn in by suction via the second openings and given off in partially or completely evaporated form via the first openings. In order that it is possible to draw in the condensed liquid by suction from the liquid bath, the second openings used for this purpose are arranged below the liquid surface of the liquid bath, or the liquid surface of the liquid bath is brought to a corresponding height. The exit of the partially or completely evaporated fluid via the first openings can, in contrast, either proceed completely above, partially above and partially below, or else completely below, the liquid surface of the liquid bath. Also in the two last-mentioned cases, a liquid circulation can be caused by the thermosiphon effect, if, owing to the evaporation, a sufficient pressure difference between the respective openings can be produced that overcomes the hydrostatic pressure drop.
The fluid-conducting structures of second heat-exchanger plates which are equipped for the (partial) condensation of a fluid run, in contrast, between a fluid feed line arranged on the heat-exchanger block and a fluid-collecting line arranged on the heat-exchanger block, in such a manner that a fluid that is to be condensed is fed via the fluid feed line and can be withdrawn in (partially) condensed form via the fluid-collecting line.
The present invention provides for the fluid feed line to be arranged on the first outer surface, i.e. in operation on the upper side of the plate heat exchanger condenser-evaporator. Via the arrangement of the fluid feed line and the specific embodiment of corresponding fluid-distribution structures in the fluid-conducting structures of the second heat-exchanger plates, the present invention permits the disadvantageous effect mentioned at the outset of a partial unmixing of a corresponding mixed fluid to be avoided. By the use of the present invention, the homogeneous distribution of a corresponding fluid fed in is improved and thereby the heat-exchange efficiency of a corresponding plate heat exchanger condenser-evaporator is increased overall.
In the context of the present invention, it has been found in this case that such a fluid distribution offers distinct advantages, even though the openings of the fluid-conducting structures of the first heat-exchanger plates (as mentioned here called “first” openings) that are customarily arranged on the first outer surface of the heat-exchanger block in this case must be displaced from a region covered by the fluid feed line. It has been recognized that the complexity of a corresponding displacement and the possible disadvantages owing to the effect on the evaporation section in this case are outweighed by the improved fluid distribution in the second heat-exchanger plates. Overall, a markedly improved heat-exchange efficiency results.
According to the invention, the first openings of the fluid-conducting structures of the first heat-exchanger plates are either arranged on the first outer surface in a region not covered by the fluid feed line, or are respectively arranged in a first region of the third and fourth outer surfaces. The first openings can therefore be displaced on the first outer surface from a region covered by the fluid feed line or to the third and fourth outer surfaces. The “first regions” of the third and fourth outer surfaces are respectively situated above the liquid surface of the fluid that is to be evaporated by the plate heat exchanger condenser-evaporator.
Advantageously, in this case, the first openings of the fluid-conducting structures of the first heat-exchanger plates on the first outer surface are formed by fluid-collecting structures and are displaced thereby in the manner just indicated from the region in which the fluid feed line is situated. The fluid-collecting structures are connected to heat-transfer fins in a central region of the fluid-conducting structures. This permits a fluid feed line to be arranged on the first outer surface without impairing the (partial) evaporation of a corresponding fluid in the first heat-exchanger plates or in the fluid-conducting structures thereof.
Particular advantages result when the fluid feed line in a central region of the first outer surface runs at right angles to the heat-exchanger plates, that is to say a central feed-in of fluid into the second fluid-conducting structures proceeds. By this means, a particularly homogeneous distribution of corresponding fluid may be achieved and a horizontal fluid feed or corresponding fluid-distribution structures can be completely dispensed with.
If the first openings of the first heat-exchanger plates that are formed by the fluid-collecting structures are, as is provided for according to the invention, situated on the first outer surface in a region not covered by the fluid feed line or respectively situated in a first region of the third and fourth outer surfaces, they can be constructed as what are termed double-exit distributors, as are known in principle from the field of heat-exchange technology, and are described in the ALPEMA standard mentioned at the outset. However, as mentioned they do not open out into fluid-distribution or fluid-collecting lines like conventional double-exit distributors.
Advantageously, the fluid-conducting structures of the second heat-exchanger plates on the first outer surface are provided with openings that open out into the fluid feed lines and are formed by the fluid-distribution structures, wherein the fluid-distribution structures are connected to heat-transfer fins in a central region of the fluid-conducting structures. Via a suitable design of corresponding fluid-distribution structures, a particularly homogeneous distribution of fluid onto the corresponding heat-transfer fins is ensured. Advantageously, in this case the fluid-distribution structures of the second heat-exchanger plates are constructed as central distributors, as are likewise known in principle from the specialist literature mentioned, which results in the abovementioned advantages.
According to a particularly preferred embodiment of the present invention, the position of the fluid-collecting line is also changed in comparison with the arrangement in conventional plate-exchanger condenser-evaporators, that is to say the fluid-collecting line is arranged on the second outer surface of the heat-exchanger block. In this case, the openings of the fluid-conducting structures of the first heat-exchanger plates that, in a conventional plate heat exchanger condenser-evaporator, are situated on the second outer surface (as mentioned here called “second” openings) are arranged in a region not covered by the fluid-collecting line, in such a manner that intake of condensed fluid by suction is permitted.
In the mentioned particularly preferred embodiment of the invention, in this case the second openings of the fluid-conducting structures of the first heat-exchanger plates on the second outer surface in a region that is not covered by the fluid-collecting line and/or respectively in second regions of the third and fourth outer surfaces, are formed by fluid-distribution structures which are connected to the heat-transfer fins in the central region of the fluid-conducting structures. A partial evaporation of fluid that is drawn in by suction via corresponding openings is not impaired hereby, even if the openings of the fluid-conducting structures of the first heat-exchanger plates are correspondingly displaced.
Advantageously, in such a case, the fluid-collecting line is arranged in a central region of the second outer surface and arranged at right angles to the heat-exchanger plates, in such a manner that fluid can be collected centrally and therefore, here also, no unmixing effects can occur. As explained above with respect to the fluid-collecting structures of the first heat-exchanger plates, in this case advantageously the fluid-distribution structures of the first heat-exchanger plates are constructed as what are termed double-entry distributors, as described in the previously explained standard literature, if the openings of the fluid-conducting structures of the first heat-exchanger plates are arranged on the second outer surface.
In the case explained of the arrangement of the fluid-collecting line in a central region of the second outer surface and at right angles to the heat-exchanger plates, the fluid-conducting structures of the second heat-exchanger plates on the second outer surface form openings which open out into the fluid-collecting lines and are formed by fluid-collecting structures that are connected to the heat-transfer fins in the central region of the fluid-conducting structures. As mentioned, advantageously, the fluid-collecting structures of the second heat-exchanger plates can be constructed as central distributors, which reliably prevent unmixing.
The mentioned first regions of the third and fourth outer surfaces adjoin the first outer surface, and the second regions of the third and fourth outer surfaces adjoin the second outer surface. The first and second regions of the third and fourth outer surfaces respectively comprise at most 50% of the area of the respective outer surface, in particular at most 40%, 30%, 20% or 10%. The first regions of the third and fourth outer surfaces, during operation of the plate heat exchanger condenser-evaporator, are situated completely above, partially above and partially below, or completely below, the surface of the fluid that is to be evaporated, the second regions are situated completely beneath. Details have already been explained.
The present invention also extends to a process for low-temperature air separation, as is described in principle in the specialist literature mentioned at the outset. In such a process, using compressed, cooled feed air, an oxygen-enriched sump product and a nitrogen-enriched overhead product are generated in a first distillation column. Using the oxygen-enriched sump product from the first distillation column, an oxygen-rich sump product and a nitrogen-rich overhead product are generated in a second distillation column.
The process according to the present invention is distinguished in that one or more plate heat exchanger condenser-evaporators are used in one, or in a plurality, of the abovedescribed embodiments, in such a manner that a particularly good heat exchange efficiency results in such a process. Reference is explicitly made here to the respective advantages of said embodiments, the process according to the invention for low-temperature air separation also profits from said advantages.
The present invention, according to a particularly preferred embodiment of the process, provides for using the, or one of the, plate heat exchanger condenser-evaporators as what is termed main condenser, that is to say a condenser that heat-exchangingly connects the first distillation column and the second distillation column. In this case, in the fluid-conducting structures of the first heat-exchanger plates, the oxygen-rich sump product of the second distillation column is partially evaporated and in the fluid-conducting structures of the second heat-exchanger plates, the nitrogen-enriched overhead product of the first distillation column is partially liquefied. The plate heat exchanger condenser-evaporator for this purpose is immersed in a liquid bath which is formed from the oxygen-rich sump product of the second distillation column. Owing to the thermosiphon effect, this sump product ascends in the fluid-conducting structures of the first heat exchanger plates and is partially evaporated in the course of this. Also, the nitrogen-enriched overhead product of the first distillation column is partially liquefied here.
A further preferred embodiment of the process according to the invention comprises using a corresponding plate heat exchanger condenser-evaporator as overhead condenser of an argon column, it therefore comprises withdrawing a fluid from the second distillation column and feeding it into a third distillation column. The third distillation column in this case comprises an overhead condenser in which by means of the, or one of the, plate heat exchanger condenser-evaporator(s), a part of the oxygen-enriched sump product of the first distillation column is partially evaporated in the fluid-conducting structures of the first heat-exchanger plates, and in which an argon-enriched overhead product of the third distillation column is partially liquefied in the fluid-conducting structures of the second heat-exchanger plates. The invention is suitable in this case for use in what are termed crude argon columns in which an argon-enriched product is obtained, but also in what are termed argon discharge columns, which are only provided to reduce an argon content in the first and/or second distillation column, in such a manner that, therein, an oxygen product of greater purity may be obtained.
An argon discharge column here designates a separation column for argon-oxygen separation that does not serve for obtaining a pure argon product, but for discharging argon from the air that is to be separated in the first and second distillation columns. The interconnection thereof differs only slightly from that of a classical crude argon column, but it contains markedly fewer theoretical plates, namely fewer than 40, in particular between 15 and 30. As in a crude argon column, the sump region of an argon discharge column is connected to an intermediate site of the second distillation column and the argon discharge column is cooled by an overhead condenser, to the evaporation side of which, expanded sump liquid from the high-pressure column is introduced. An argon discharge column does not have a sump evaporator. The overhead condenser can, as mentioned, be constructed as a plate heat exchanger condenser-evaporator.
The invention will be described in more detail hereinafter with reference to the accompanying drawings in which preferred embodiments of the invention are illustrated in comparison with the prior art.
In the figures, elements corresponding to one another are given identical reference signs and for the sake of clarity, are not explained repeatedly.
A heat-exchanger block 10 of the plate heat exchanger condenser-evaporator is formed of alternately arranged heat-exchanger plates 20 and 30, wherein the heat-exchanger plates 20 are hereinafter called “first” heat-exchanger plates, and the heat-exchanger plates 30 are hereinafter called “second” heat-exchanger plates. Corresponding heat-exchanger plates are in part also explained in still more detail in the subsequent figures. Only two corresponding heat-exchanger plates 20, 30 are provided with reference signs, but the entire heat-exchanger block 10 is made up in entirety of corresponding heat-exchanger plates 20, 30.
Both in the first heat-exchanger plates 20 and in the second heat-exchanger plates 30, in each case fluid-conducting structures are constructed which, for the first heat-exchanger plates 20, are designated 21, and for the second heat-exchanger plates 30, are designated 31. As mentioned, “fluid-conducting structures”, in the language used here, is taken to mean not only the heat-transfer fins typically arranged in a central region of corresponding heat-exchanger plates 20, 30, but also the terminal fluid-distribution or fluid-collecting structures (distributors). In the view of
In
The first heat-exchanger plates 20 have, in contrast to the second heat-exchanger plates 30, no or at all events open-end fluid-distribution structures, which, however, are not shown in
By the arrangement of the fluid feed line 40 and the fluid-collecting line 50, in contrast, the fluid-conducting structures 31 of the second heat-exchanger plates 30 run between the fluid-feed line 40 and the fluid-collecting line 50. If a gaseous fluid that is to be condensed, for example a nitrogen-rich overhead product of a high-pressure column of an air separation plant, is fed in via the fluid feed line 40, and if the heat-exchanger block 10 is immersed up to a certain height in a liquid bath of a fluid that is to be evaporated, for example in an oxygen-rich liquid in the sump of a low-pressure column of an air separation plant, the thermosiphon effect mentioned at the outset occurs.
In
As mentioned, in operation, fluid is fed via the fluid feed line 40 and exits via the openings 311 into the fluid-conducting means 31 of the second heat-exchanger plates 30. The fluid in this case is first distributed by means of fluid-distribution structures 313 onto the entire width of the heat-exchanger plate 30. In the example shown, the fluid-distribution structures 313 and the subsequently explained collecting structures 314 are in each case shown as diagonal distribution or collecting structures of type B, but other types of fluid-distribution and fluid-collecting structures can also be used, for example the types shown in the ALPEMA standard mentioned at the outset. Fluid distributed onto the entire width of the heat-exchanger plate 30 by means of the fluid-distribution structures 313 ideally enters with the same flow rate and composition on the entire width into the region of the heat-transfer fins 315 and is ideally collected over the entire range of the heat-transfer fins 315 by means of the fluid-collecting structures 314. The fluid collected by means of the fluid-collecting structures 314 exits from the second heat-exchanger plate 30 via the openings 312 and is fed in this manner to a fluid-collecting line 50.
As mentioned, in second heat-exchanger plates 30, as are shown in
In contrast to the plate heat exchanger condenser-evaporator 110 of
The fluid-collecting structures of the heat-exchanger plates 30 can be constructed, for example, in addition, as shown in
According to
In
In contrast to the heat-exchanger plate 30 shown in
As mentioned, it is also possible, however, to provide the fluid-collecting line 50 on a second outer surface 12, or the bottom side of the heat-exchanger block 10. A second heat-exchanger plate 30 used for this case is illustrated in
As described many times, the second heat-exchanger plates 30 according to the present figures are equipped to condense a fluid partially or completely, but the first heat-exchanger plates 20 are equipped to evaporate a fluid partially or completely. The first heat-exchanger plates 20 are operated in this case on the basis of the mentioned thermosiphon effect.
Hereinafter, first heat-exchanger plates 20 are illustrated, as can be used in the above explained embodiments, namely, when a fluid feed line is arranged on the top side and a fluid-collecting line either on the side or on a bottom side of the heat-exchanger block 11. In both cases, the fluid feed line 40 and the fluid-collecting line 50 are illustrated, wherein the fluid-conducting structures 21, however, are not connected to such fluid feed- or fluid-collecting lines 40, 50.
The first heat-exchanger plates 20 are, as mentioned, equipped to evaporate a fluid partially or completely, and thereby induce the thermosiphon effect. For this purpose, the fluid-conducting structures 21 must have openings that are not covered by fluid feed lines and also fluid-collecting lines. If the fluid feed line 40 is arranged on the first outer surface of the heat-exchanger block 10, no openings of fluid-conducting structures 21 of the heat-exchanger plates 20 (such as the first openings 211 illustrated in
If, however, an arrangement is selected in which the fluid-collecting line 50 is arranged on an underside or on the second outer surface 12 of the heat-exchanger block 10, the second openings of the fluid-conducting structures 21 must also be displaced into a region which is not covered by the fluid-collecting line. For this purpose, a fluid-distribution structure 214 is used which is constructed, for example, in the manner of what is termed a double-entry distributor. This is shown in
