EP0130122B1 - Device for evaporating a liquid by heat exchange with a second fluid and air distillation unit comprising such a device - Google Patents

Abstract

Liquid oxygen of a low pressure column forms a bath 5 at the top of a heat exchanger 2 of the type having plates defining vertical passages 17, 18. This liquid is predistributed along every other passage 17 by a series of apertures 27 formed for example in the plates, and then is uniformly distributed in a fine manner in the same passages by a packing 24 so as to form a continuous running liquid film. The gaseous nitrogen of a medium pressure column is introduced into the remaining passages 18 and condensed by heat exchange with the oxygen which is vaporized.

Description

The present invention relates to a heat exchanger intended for the vaporization of a liquid by heat exchange with a second fluid, as defined in the preamble of claim 1. It applies in particular to installations for the distillation of l 'air.

In air distillation installations of the double column type, the liquid oxygen which is in the bottom of the low pressure column is vaporized by heat exchange with the nitrogen gas taken off at the head of the medium pressure column. For a given operating pressure of the low pressure column, the temperature difference between oxygen and nitrogen made necessary by the structure of the heat exchanger imposes the operating pressure of the medium pressure column. It is therefore desirable that this temperature difference is as small as possible, in order to minimize the expenses linked to the compression of the air to be treated injected into the medium pressure column.

To achieve this goal, it has been proposed to supply the heat exchanger with liquid oxygen from above, ensuring the flow of this liquid along very long tubes (up to approximately 6 m).

Remarkable performance has thus been obtained from the point of view of heat exchange, but this at the cost of serious technological difficulties. In fact, in particular when large oxygen flow rates have to be treated, problems arise in producing a multitude of long tubes resistant to the external pressure of nitrogen, as well as other problems linked to the presence of thick stainless steel end plates.

There is also known, in a neighboring technical field, a heat exchanger of the type comprising a parallelepiped body formed by an assembly of parallel vertical plates defining between them a multitude of flat passages, means for distributing the liquid in two stages in a first set of passages, provided at the upper end of each of these passages, and means for sending the second fluid into the remaining passages (FR-A-2 007 887). In this document, the two distribution stages are obtained the first thanks to an oblique wave, the second thanks to holes. A double distribution of the liquid comprising a final distribution by means of holes is also provided in FR-A-2 017 807.

The object of the invention is to provide a heat exchanger of the same type which is better suited to the actual operating conditions of air distillation installations.

To this end, the subject of the invention is a heat exchanger of the aforementioned type, characterized in that said distribution means comprise openings for predistribution of the liquid in the passages of said first assembly and over their entire length, these openings opening onto a lining for fine distribution of the liquid over the entire length of the same passages.

The invention also relates to an air separation installation by distillation, of the type comprising a first distillation column operating at a relatively high pressure, a second distillation column operating at a relatively low pressure, and a heat exchanger allowing to put the liquid oxygen in the tank of the second column in heat exchange relation with the nitrogen gas at the top of the first column, characterized in that the heat exchanger is as defined above, and in that that the installation comprises supply means for supplying liquid oxygen to said predistribution openings, and means for supplying gaseous nitrogen to said remaining passages.

• - la figure 1 est un schéma partiel d'une installation de distillation d'air conforme à l'invention;
• - la figure 2 est une vue schématique partielle en perspective, avec arrachement, d'un échangeur de chaleur équipant l'installation de la figure 1.
• - la figure 3 est une vue analogue d'une variante de l'échangeur de chaleur de la figure 2;
• - les figures 4 et 5 représentent respectivement en perspective deux variantes d'un détail de l'échangeur de la figure 3; et
• - la figure 6 est une vue partielle en perspective, avec arrachements, d'un autre échangeur de chaleur conforme à l'invention.

Several examples of implementation of the invention will now be described with reference to the accompanying drawings. In these drawings:

• - Figure 1 is a partial diagram of an air distillation installation according to the invention;
• - Figure 2 is a partial schematic perspective view, with parts broken away, of a heat exchanger fitted to the installation of Figure 1.
• - Figure 3 is a similar view of a variant of the heat exchanger of Figure 2;
• - Figures 4 and 5 show respectively in perspective two variants of a detail of the exchanger of Figure 3; and
• - Figure 6 is a partial perspective view, with parts broken away, of another heat exchanger according to the invention.

In the various embodiments which will be described below, the same or corresponding elements will be designated by the same references.

FIG. 1 illustrates a possibility of installing an oxygen-nitrogen heat exchanger in an air distillation installation of the double column type. This installation comprises a medium pressure column 1 at the bottom of which the air to be treated is injected, under a pressure of the order of 6 bars absolute. The oxygen-enriched liquid which is collected in the tank of column 1 is sent under reflux in the middle of the height of a second column (not shown), called the low pressure column, which operates slightly above atmospheric pressure. The nitrogen gas which is at the head of column 1 is brought into heat exchange relationship with the liquid oxygen collected in the bottom of the low pressure column; the resulting condensed nitrogen serves as reflux in column 1 and in the low pressure column, while the resulting vaporized oxygen is returned to the bottom of the low pressure column.

The heat exchange between oxygen and nitrogen takes place in an exchanger 2 which is mounted above the column 1, while the low pressure column is juxtaposed to the latter.

The heat exchanger 2 consists of a sealed envelope 3, the main height of which contains a set of parallel plates 4 of shape rectangular aluminum, with a length of the order of 1 to 1.5 m and a height of the order of 3 to 6 m, between which waves also made of aluminum are fixed by brazing.

The space located above the plates 4 contains a liquid oxygen bath 5 supplied by a line 6 coming from the tank of the low pressure column and provided with a pump (not shown). The latter can be controlled by a regulator of the level of the bath 5, which is shown diagrammatically by a tube 6A for level measurement, or, as a variant, by a flow regulator. At the top of the exchanger 2 is provided a pipe 7 for returning oxygen vaporized above the bath 5 to the bottom of the low pressure column, resulting from the heat inputs at the pump and the pipes.

The set of plates 4 is supplied at its upper part with gaseous nitrogen by a horizontal feed box 8 which communicates by a pipe 9 with the head of the medium pressure column 1. The evacuation of the condensed nitrogen takes place at the base of the plates 4 by a horizontal collecting box 10 which communicates by a pipe 11 with a guarded channel 12 disposed at the head of the column 1. On the box 10 is stuck a pipe 13 for discharging incondensable rare gases.

A pipe 14 connects the tank of the low pressure column to the space located in the casing 3 below the plates 4. This pipe penetrates vertically into this space through the bottom point of the casing 3, and its upper end is surmounted by a conical deflector 15. From the bottom of the casing 3 also leaves a pipe 16 intended to return to the bottom of the low pressure column the excess liquid oxygen.

The structure of the active part of the exchanger 2, that is to say of the set of plates 4, will now be described with reference to FIG. 2.

In this region of the exchanger, the casing 3 has a parallelepiped shape. The plates 4 define a multitude of passages intended alternately for the flow of oxygen (passages 17) and for the flow of nitrogen (passages 18). Over most of their height, the passages 17 and 18 each contain a wave 19 consisting of a corrugated perforated aluminum sheet with vertical generators.

The waves 19 of the nitrogen passages end, above and below, before the waves 19 of the oxygen passages. At the bottom of the plates 4, these waves of the passages 18 are extended by oblique waves of nitrogen collection (not shown) which lead to the inlet of the manifold 10. At their upper end, these same waves 19 are extended by oblique waves 20 of nitrogen distribution which open at the outlet of the feed box 8. Above the waves 20, the nitrogen passages 18 are closed by horizontal bars 21.

Similar bars close the lower end of the nitrogen passages below the nitrogen collection areas. Above the bars 21, each nitrogen passage comprises a liquid oxygen tank 22 containing a vertical wave 23 of perforated aluminum sheet, with vertical generators, the thickness and pitch of which are clearly greater than those of waves 19. The waves 23 have only a function of spacers between the plates 4, so as to allow the assembly of the exchanger by a single brazing operation. The tanks 22 are open upwards to communicate with the liquid oxygen bath 5. The waves 19 of the oxygen passages 17 extend downwards to the lower end of the plates 4, so that these passages are open down. These waves extend upwards to the upper edge of the bars 21, then are extended by a lining 24. The latter consists of a wave of the "serrated _" type which is illustrated in more detail in FIG. 6.

As can be seen in this FIG. 6, the wave 24 is an unperforated aluminum sheet with horizontal generators (so-called "hard way" arrangement with respect to the flow of liquid oxygen). At regular intervals, each horizontal or pseudo-horizontal facet 25 of the wave 24 is provided with a puncture 26 offset upwards by a quarter of a wave step. The width of the punctures 26, measured along a generatrix of the wave, is of the same order as the distance which separates each of them from the two adjacent punctures situated on the same facet 25.

Returning to FIG. 2, each plate 4 comprises, above the lining 24, a horizontal row of holes 27 arranged at regular intervals over the entire length of the exchanger, the holes of the successive plates being arranged at the same height but in a staggered arrangement. As a variant, these holes could moreover be provided only in every second plate. Just above these holes, the oxygen passages are closed by horizontal bars 28, arranged at the upper end of the plates 4. To avoid the risk of obstruction of certain holes 27 by waves 23, these are interrupted for a short height at the level of said holes.

In operation, the regulating device of the pump for supplying the exchanger 2 with liquid oxygen maintains above the plates 4 a level of the bath 5 sufficient to overcome the various pressure drops which oppose the flow of oxygen. The height of liquid oxygen above the plates 4 is for example of the order of 20 cm.

Liquid oxygen fills the reservoirs 22 and passes through the holes 27, at a flow rate defined by the passage section of the latter and by the height of liquid which overcomes it. As this height is constant in steady state, the flow of liquid oxygen is that supplied by the pump for raising this liquid. The holes 27 therefore ensure a rough predistribution of the liquid oxygen along the passages 17-, and the liquid oxygen thus pre-distributed reaches the lining 24, which ensures a fine distribution over the entire length of each passage 17 Liquid oxygen thus approaches waves 19 by flowing in a perfectly uniform manner over all the walls (waves 19 and plates 4) of the passages which are assigned to it, that is to say by forming on these walls a continuous descending film.

At the same time, the nitrogen gas arrives in the exchanger through the box 8 and the distribution waves 20, then flows down along the passages 18. In so doing, it gradually gives off heat to the oxygen. liquid which is in the adjacent passages 17, so that the oxygen vaporizes and that, simultaneously, the nitrogen condenses.

The condensed nitrogen is collected in the box 10 and flows in the pipe 11 as far as the channel 12. When the height of liquid nitrogen in the pipe 11 is sufficient to overcome the pressure prevailing in the medium pressure column 1, this liquid overflows from the channel and falls into reflux in the medium pressure column after a portion has been withdrawn through a pipe 11A to ensure the reflux of the low pressure column. This creates suction in the passages 17, which ensures the circulation of nitrogen.

The liquid oxygen flow rate is adjusted so as to guarantee an excess of liquid oxygen over the entire height of the plates 4. In fact, a total vaporization of the oxygen in a region of the passages 17 would lead to this location at a concentration of acetylene dissolved in liquid oxygen, which could cause a local explosion. Independently of this risk of explosion, it would also result in a drop in performance of the exchanger by neutralization of the non-wetted surface. This risk is limited thanks to the high efficiency of the fine distribution provided by the lining 24. However, for safety, it is preferable to work with an excess of liquid oxygen, generally of the same order as the flow of vaporized oxygen.

Consequently, a two-phase mixture of gaseous oxygen and liquid oxygen leaves through the lower end of the passages 17; this mixture separates in the lower part of the casing 3, the liquid and vapor phases returning respectively to the tank of the low pressure column by the lines 16 and 14.

The Applicant has found that such an exchanger can function perfectly reliably with a very small temperature difference, of the order of 0.5 ° C., between nitrogen and oxygen, which consequently makes it possible to compress the air entering the distillation plant under very economical conditions.

In the embodiment of Figure 2, we see that the distribution of liquid oxygen is fully achieved when the fluid arrives in the heat exchange zone with nitrogen. In the variant of FIG. 3, on the contrary, the oxygen is put into heat exchange relation with the nitrogen from the start of the fine distribution operation.

For this, the bars 21 which limit the upper passages 18 are arranged at the upper end of the plates 4, like the bars 28. In addition, the holes 27 are eliminated and replaced by vertical holes 29 drilled at regular intervals in the bars 28, all along these.

In this variant, the liquid oxygen in the bath 5 flows through the holes 29, at a flow rate corresponding to that of the pump for raising the liquid oxygen, and is thus predistributed over the entire length of the passages 17; these liquids then fall onto the lining 24 located just below (this lining has been shown very schematically in FIG. 3). As before, the lining 24 ensures a uniform fine distribution of the liquid oxygen over the entire length of the passages 17, and this liquid then flows along the waves 19 and the corresponding walls 4. The heat exchange between oxygen and nitrogen begins during the passage of liquid oxygen through the linings 24, which are at the same level as the nitrogen gas distribution waves.

As illustrated in FIG. 4, the holes 29 of the bars 28, instead of being of a constant diameter over the entire height of these bars, can have a diameter widened in the greater part of their height by a counterbore 29A made from the bottom.

FIG. 5 shows that similar holes can also be obtained by perforating the upper core 30 of U-shaped profiles constituting the bars 28. The advantage of these two embodiments lies in the fact that the useful part of the holes 29, which defines the liquid oxygen passage section is short and therefore less subject to the appearance of blockages or undesirable vaporization.

In the heat exchangers of Figures 2 and 3, the vaporized oxygen is evacuated from below at the same time as the excess liquid oxygen. In the embodiment of FIG. 6, on the contrary, the vaporized oxygen is free to escape both from the top and from the bottom.

The exchanger of FIG. 6 is identical to that of FIG. 2 from the bottom of the plates 4 to the level of the upper edge of the bars 21 which limit the nitrogen passages 18 above.

Just above these bars 21, each plate 4 has a horizontal row of holes 31. Above these, the plates 4 extend over a significant height, up to a level higher than that of the surface free from the liquid oxygen bath 5. In the intervals above the bars 21 are arranged waves - spacers 32 with vertical generators similar to the waves 23 of FIG. 2. In the remaining intervals, a free space 33 is provided at the holes 31, above the waves 19, and this space is overcome, from bottom to top by the lining 24 previously described, by a bar 28 with holes 29 similar to those of FIG. 3, and by a wave-spacer 34 similar to waves 32 but with horizontal generatrices .

The bath 5 is fed laterally by a feed box 35 located above the box 8 and opening into the spaces occupied by the waves 34. For this, the bars 36 which close on this side the oxygen passages 17 extend upwards only up to the level of the upper edge of the bars 28.

In operation, an appropriate constant liquid oxygen level is maintained in the box 35. The bath 5 overcomes the bars 28 and, as in FIG. 1, the liquid oxygen flows through the holes 29 in the lining 24, which distributes it uniformly in a fine manner, then trickles through the passages 17 in exchange for heat with the nitrogen contained in the passages 18. The vaporized oxygen can be evacuated either downwards, as previously, or upwards through the holes 31 and the spaces containing the waves 32, as indicated by arrows on the figure 6.

In this embodiment, it is also possible, as a variant, to close the passages 17 at their lower end and to collect the liquid oxygen by means of a collection oblique wave and a horizontal collecting box connected by a pipe to the bath. liquid oxygen located in the bottom of the low pressure column. In this case, all of the vaporized oxygen leaves the exchanger from above, as described above.

In each embodiment of the exchanger according to the invention, the nitrogen circuit is conventional. It can therefore be replaced by other known types of nitrogen circuits, in particular by those described in patent FR-A-2431 103 of the applicant.

Furthermore, one or more heat exchangers according to the invention can be installed inside a double air distillation column whose low pressure column is superimposed on the medium pressure column.

Claims (16)

1. Heat exchanger for vapourising a liquid by heat exchange with a second fluid, of the type comprising a parallelepiped body formed by an assembly of parallel vertical plates (4) defining between them a plurality of flat passages (17, 18), means for distributing the liquid in two stages in a first set of passages (17), situated at the upper end of each of these passages, and means for conveying the second fluid into the remaining passages (18) characterised in that the said distribution means comprise openings (27; 29, 29A) for pre-distribution of the liquid in the passages (17) of the said first set and and over their whole length, these openings leading to a lining (24) for fine distribution of the liquid over the whole length of the same passages (17).

2. Heat exchanger according to claim 1, characterised in that the said openings (27; 29, 29A) comprise a horizontal row of holes.

3. Heat exchanger according to one of claims 1 and 2, characterised in that it comprises retaining means (21, 28) for forming a bath of liquid (5) above the said openings.

4. Heat exchanger according to claim 3, characterised in that the said retaining means comprise bars (21) limiting the passages (18) of the said second set at their upper end at a certain distance from the upper end of the vertical plates (4), the said openings (27) being made in the plates above these bars.

5. Heat exchanger according to claim 3, characterised in that the said retaining means comprise bars (28) limiting the passages (17) of the said first set at their upper end, the said openings (29, 29A) being formed vertically in these bars.

6. Heat exchanger according to claim 5, characterised in that each opening (29, 29A) is formed by a counter-bored hole.

7. Heat exchanger according to claim 5, characterised in that each bar (28) is a downwardly open U-shaped section and comprises a series of holes (29) in its web (30).

8. Heat exchanger according to any one of claims 3 to 7, characterised in that it comprises means (31) of placing a region of the passages (17) of the said first set, situated below the fine distribution lining (24) in communication with a free space situated above the bath (5).

9. Heat exchanger according to claims 5 and 8 taken together, characterised in that the plates (4) extend as far as above the level of the bath (5), in that additional bars (21) upwardly limit the passages (18) of the said second set, and in that the said communication devices comprise openings (31) made in the plates (4) above these additional bars and below the fine distribution lining (24).

10. Heat exchanger according to any one of claims 1 to 9, characterised in that the lining (24) is formed by a corrugation comprising horizontal generatrices and a partial vertical offset.

11. Heat exchanger according to any one of claims 1 to 10, characterised in that the fine distribution lining (24) is situated at the same level as a device (20) for distribution of the second fluid into the passages (18) of the said second set.

12. Heat exchanger according to any one of claims 1 to 10, characterised in that the fine distribution lining (24) is situated wholly above a device (20) for distribution of the second fluid into the passages (18) of the said second set.

13. Installation for separation of air by distillation, of the type comprising a first distillation column (1) operating under a relatively high pressure, a second distillation column operating under a relatively low pressure, and a heat exchanger (2) enabling the liquid oxygen at the bottom of the second column to be placed in a heat exchange relationship with the gaseous nitrogen at the head of the first column, characterised in that the heat exchanger is of the nature specified in any one of the claims 1 to 12, and in that the installation comprises delivery means (6, 6A; 35) for supplying the liquid oxygen to the said pre-distribution openings (27; 29, 29A) and means (8) for supplying the said remaining passages (18) with gaseous nitrogen.

14. Installation according to claim 13, characterised in that the said delivery means (6, 6A; 35) comprise means for forming a bath (5) of liquid oxygen at the top of the exchanger (2).

15. Installation according to claim 14, characterised in that the said delivery means (6, 6A; 35) comprise means (6A) for adjusting the level of the said bath.

16. Installation according to any one of claims 13 to 15, characterised in that the openings (27; 29, 29A) are adapted to provide a surplus of liquid oxygen of the same order as the flow of vapourised liquid.

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