EP0566435B1 - Trickle heat-exchanger and an air destillation comprising such a heat-exchanger - Google Patents

Description

The present invention relates to a liquid flow heat exchanger for vaporizing a liquid by heat exchange with a second fluid, of the type comprising a parallelepipedic body formed by an assembly of parallel vertical plates defining between them a multitude of flat passages distributed in a set of vaporization passages and in a set of heating passages, each passage, in its current heat exchange part, containing a spacer wave with vertical generators, means for distributing the liquid being provided at the upper end of the exchanger for distributing the liquid over the entire length of the vaporization passages, and means being provided for sending the second fluid into the heating passages. Such an exchanger is known, for example, from document EP-A-0 130 122. It applies in particular to air distillation installations.

In air distillation systems 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 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 by benefiting from the very advantageous technology of brazed plate heat exchangers, EP-A-0 130 122 in the name of The Applicant has proposed a particularly efficient method of distributing liquid oxygen.

However, whatever the distribution method adopted, current technology has certain limits. These are due to the fact that, while the liquid oxygen is at a pressure which is only slightly higher than atmospheric pressure, the gaseous oxygen resulting from the vaporization must evacuate itself from l 'exchanger. The pressure drop in the path of the gaseous oxygen must therefore be very low. In all known solutions, this constraint limits the height of the exchanger, and more generally its performance.

The object of the invention is to make it possible to increase the height of such a heat exchanger or, at a given height, to reduce the pressure drop of the flow of vaporized oxygen. To this end, it relates to a heat exchanger of the aforementioned type, characterized in that said distribution means are arranged in compartments closed at their upper end and each located above a heating passage, of which it is separated by a horizontal bar, in that a horizontal slot, extending over the entire length of the exchanger, just above the bar, puts the lower part of the compartment in free communication with an adjacent vaporization passage, and in that the spray passages are open at their two upper and lower ends, over their entire length, and contain at most one wave-spacer with vertical generators at any point of their height.

• les passages de vaporisation sont dépourvus de toute onde-entretoise en regard des fentes;
• la surface supérieure de la barrette est inclinée latéralement vers la fente;
• les moyens de distribution du liquide comprennent, d'une part, une barre horizontale s'étendant sur toute la longueur de chaque compartiment, à un niveau intermédiaire de celui-ci, cette barre ayant une épaisseur égale à l'espacement mutuel des plaques et comportant des ouvertures de prédistribution du liquide, et d'autre part, au-dessous de cette barre, un garnissage de distribution fine du liquide sur toute la longueur horizontale du compartiment;
• lesdites ouvertures forment une rangée horizontale de trous équidistants les uns des autres;
• la barre comporte sur une face verticale un ou plusieurs évidements arrière fermés en bas et ouverts vers le haut et sur son autre face verticale un ou plusieurs évidements avant ouverts vers le bas et fermés vers le haut, et en ce que lesdites ouvertures sont ménagées à travers une paroi verticale commune aux évidements avant et arrière;
• la barre comporte plusieurs évidements arrière espacés les uns des autres, et plusieurs évidements avant espacés les uns des autres;
• les évidements avant ont une forme évasée vers le bas;
• le garnissage est une onde à génératrices horizontales dont les flancs sont munis de crevés;
• le garnissage est espacé de la surface supérieure de la barrette;
• l'échangeur comporte une boîte d'entrée latérale de liquide dans lesdits compartiments, le point bas de cette boîte se trouvant au-dessous du point bas de la fenêtre d'entrée de ces compartiments.

According to particular embodiments of the invention:

• the spray passages are devoid of any wave-spacer facing the slots;
• the upper surface of the bar is inclined laterally towards the slot;
• the liquid distribution means comprise, on the one hand, a horizontal bar extending over the entire length of each compartment, at an intermediate level thereof, this bar having a thickness equal to the mutual spacing of the plates and comprising liquid predistribution openings, and on the other hand, below this bar, a lining for fine distribution of the liquid over the entire horizontal length of the compartment;
• said openings form a horizontal row of holes equidistant from each other;
• the bar comprises on one vertical face one or more rear recesses closed at the bottom and open at the top and on its other vertical face one or more front recesses open at the bottom and closed at the top, and in that the said openings are arranged at through a vertical wall common to the front and rear recesses;
• the bar has several rear recesses spaced from each other, and several front recesses spaced from each other;
• the front recesses have a flared shape down;
• the lining is a wave with horizontal generatrices whose flanks are provided with punctures;
• the lining is spaced from the upper surface of the bar;
• the exchanger comprises a lateral liquid inlet box in said compartments, the bottom point of this box being below the bottom point of the inlet window of these compartments.

The invention also relates to an air separation installation by distillation, of the type comprising a first distillation column. operating under relatively high pressure, a second distillation column operating under relatively low pressure, and a heat exchanger enabling the liquid oxygen in the tank of the second column to be placed in heat exchange relationship with the top nitrogen gas of the first column, characterized in that the heat exchanger is as defined above, and in that the installation comprises supply means for supplying liquid oxygen to said liquid distribution means, and means for supplying the heating passages with gaseous nitrogen.

• la Figure 1 est un schéma partiel d'une installation de distillation d'air conforme à l'invention;
• la Figure 2 représente en coupe verticale, à plus grande échelle, la région II de la Figure 1, la coupe étant prise suivant la ligne II-II de la Figure 4;
• la Figure 3 est une vue partielle en plan prise suivant la flèche III de la Figure 2;
• la Figure 4 est une vue prise en coupe suivant la ligne IV-IV de la Figure 2; et
• la Figure 5 est une vue analogue d'une variante.

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 shows in vertical section, on a larger scale, the region II of Figure 1, the section being taken along line II-II of Figure 4;
• Figure 3 is a partial plan view taken along arrow III of Figure 2;
• Figure 4 is a sectional view taken along line IV-IV of Figure 2; and
• Figure 5 is a similar view of a variant.

Figure 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 low pressure column, which operates slightly above atmospheric pressure. The nitrogen gas which is at the head of column 1 is brought into indirect 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 two distillation columns can in particular be of the packed type, which also contributes to the energy gain by lowering the operating pressure of the installation, which is that of column 1.

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 exchanger 2 consists of a sealed envelope 3, the main height of which contains a set of parallel plates 4 of rectangular aluminum shape, with a length of the order of 1 to 1.5 m and a height of the order of 3 to 7 m, between which waves also made of aluminum are fixed by brazing.

A space under a pressure slightly higher than that of the low pressure column (for example of the order of 1.4 bar), located at the upper end of the plates 4, opposite one of their vertical edges , contains a liquid oxygen bath 5 supplied with rain 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 bath level regulator 5, or, alternatively, by a flow regulator. At the top of the exchanger 2, the casing 3 forms a dome 7 which contains the bath 5. From this dome leaves a pipe 8 for returning to the bottom of the low pressure column of the vaporized oxygen coming from the bath 5, resulting from the heat inputs at the level of the pump and the pipes, and part of the oxygen vaporized in the exchanger 2.

The set of plates 4 is supplied at its upper part with nitrogen gas at 6 bars by a horizontal supply box 9, located under the bath 5, which communicates by a pipe 10 with the head of the medium pressure column. The evacuation of the condensed nitrogen is carried out at the base of the plates 4 by a horizontal collecting box 11 which communicates by a pipe 12 with a guarded channel 13 arranged at the head of the column 1. On the box 11 is stuck a pipe 14 evacuation of uncondensable rare gases.

A pipe 15 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 16. From the bottom of the casing 3 also leaves a pipe 17 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 FIGS. 2 to 4.

In this region, the exchanger has a parallelepiped shape, and the casing 3 is defined by the edges of the plates 4 and by spacer bars which close the passages that these plates define, except at the entry and exit locations of the fluids. The plates 4 define a multitude of passages intended alternately for the flow of oxygen (passages 18) and for the flow of nitrogen (passages 19). Over most of their height, the passages 18 and 19 each contain a spacer wave 20 made of a corrugated perforated aluminum sheet with vertical generators.

The waves 20 of the nitrogen passages end, at the top as well as the bottom, before the waves 20 of the oxygen passages. At the bottom of the plates 4, these waves of the passages 20 are extended by oblique waves of nitrogen collection (not shown) which lead to the entry of the manifold 11. At their upper end, these same waves 20 are extended by oblique waves 21 of nitrogen distribution which emerge, through a side window 21A of the exchanger, at the outlet of the feed box 9. Above waves 21, the nitrogen passages 19 are closed by horizontal bars 22. Other horizontal bars (not shown) close the lower end of the nitrogen passages below the nitrogen collection areas. Above the bars 22, each nitrogen passage is extended by a compartment 23 for distributing liquid oxygen closed at the upper end of the exchanger by a horizontal bar 24. The compartment 23 contains, from top to bottom: an oblique spacer wave 25 (or, as a variant, a perforated wave with horizontal generators) for rough distribution of liquid oxygen over the entire length of the compartment, this wave opening out laterally, via a side window 26 of the exchanger, in bath 5 (Figure 2); a perforated bar 27 for predistribution of liquid oxygen; and a lining 28 for fine distribution of liquid oxygen. A free space 29 is provided between this lining and the upper surface of the bar 22.

The perforated bar 27 is machined from a rectangular blank whose thickness is equal to the spacing of the plates 4, ie of the order of 5 to 15 mm, and whose length is equal to that of these plates. In one from its large faces are machined a series of rear recesses (considering Figure 2) 30 in a U, open upwards, and on its other face are machined a series of front recesses 31 roughly in a semicircle, open down. Each recess 31 is located longitudinally opposite a recess 30 and overlaps the latter in height, so that there is, roughly halfway through the thickness of the bar (FIG. 4), a thin vertical wall 32 common to the two recesses. This wall is pierced with a circular hole 33. The holes 33 are distributed at regular intervals along the perforated bar 27.

The lining 28 is constituted by a wave with horizontal generators (so-called "hard way" arrangement with respect to the flow of liquid oxygen) not perforated but of the "serrated" type. This means that at regular intervals, each horizontal or pseudo-horizontal facet of the wave is provided with a flat offset upward by a quarter of a wave step. The width of the punctures, 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 located on the same facet.

The oxygen vaporization passages 18 are open at their upper and lower ends. They contain the wave 20 from the lower end to the level of the bars 22, are devoid of any wave opposite the space 29, then, from the upper level of this space 29 to their upper end, they contain another spacer wave 20A similar to wave 20 but not larger. The region of each wave-free passage 18 communicates freely with the space 29 of an adjacent passage 19 through a horizontal slot 34 of the same height extending over the entire length of the exchanger. Thus, one plate out of two is continuous over the whole the extent of the exchanger, while one plate out of two is in fact made up of a rectangular plate 4A which extends upwards only to the bar 22, and of a rectangular plate 4B which delimits compartment 23 for dispensing liquid oxygen. The upper surface 35 of the bar 22 is inclined laterally so as to descend from the adjacent plate 4 to the upper edge of the facing plate 4A. By virtue of a recess of the bar 22, this surface extends slightly beyond the face of the plate 4A which delimits the passage 18.

In operation, the liquid oxygen bath 5 is maintained at an approximately constant level, without exceeding the upper face of a vertical plate 5A welded on the exchanger above the windows 26. Thus, the liquid oxygen penetrates laterally in the compartments 23, by one of their ends through the windows 26. Simultaneously, the nitrogen gas at 6 absolute bars enters the upper part of the passages 19, by one end of these passages, via the box 9 and the waves distribution 21.

The liquid oxygen thus forms a column of liquid of practically uniform height above all the holes 33. It is predistributed over the entire length of the passages 18 into a number of jets 36 by these holes 33, then falls freely on the lining 28, which, by its constitution and its arrangement, ensures a fine distribution of the liquid oxygen all along the passages 18. The liquid oxygen therefore falls uniformly on the inclined surface 35 of the bars 22, then pours through the slots 34 in the passages 18.

A film of liquid oxygen thus flows over all the metal surfaces contained in the passages 18, that is to say on the plates 4 and 4A and on waves 20, and it partially vaporizes by indirect heat exchange with nitrogen being condensed from top to bottom in the alternate passages 19.

As indicated above, the passages 18 are not only open upwards and downwards, but also free as much as possible, over their entire height, of obstacles to the flow of oxygen gas. Indeed, at any point of their height, these passages are either empty (opposite the slot 34), or provided with a simple wave 20, 20A with vertical generators and with relatively large pitch. Wave 20 improves the heat exchange with nitrogen by fin effect, while wave 20A only serves as a spacer and can even possibly be partially eliminated.

It follows from this that part of the vaporized oxygen can leave the exchanger from above and therefore add, in the upper dome 7, to the evaporations of the bath 5 (FIG. 1), the rest of the oxygen vaporized leaving the exchanger from below at the same time as the excess liquid oxygen then being evacuated via line 15. The two vapor oxygen outlet paths are traversed by a reduced gas flow, and each path requires in addition a minimum pressure drop at the flow of this gas. Finally, the height of the exchanger can be increased.

It should be noted that thanks to the structure of the perforated bars 27, the holes 33 have a horizontal axis and there is a dead end 37 on the rear face of the bar, below these holes. Any solid impurities contained in the liquid oxygen can thus be deposited in these dead ends, which protects the holes 33 against the risks of clogging.

Likewise, the configuration of the envelope 3 in the region of the liquid oxygen bath 5 forms a cul-de-sac 38 adjacent to the entry windows 26 and located under these, which allows the largest solid impurities to settle in this dead end, leaving the supply line 6, as indicated in 39 in Figure 2.

If, in a particular application, this settling is considered sufficient to avoid any risk of plugging of the holes 33, we can use the variant of Figure 5. This differs from the previous one only by the simplified construction of the bar perforated 27, which is a simple rectangular section bar provided at regular intervals with holes 33 with a vertical axis. These holes can have an enlarged diameter over most of their height from the bottom, as explained in the aforementioned EP-A-0 130 122.

Claims (12)

1. Heat exchanger with trickle flow for vaporizing a liquid by heat exchange with a second fluid, of the type comprising a parallelepiped shaped body formed from an assembly of parallel vertical plates (4) defining between them a multiplicity of flat passages (18,19) distributed in an assembly of vaporization passages (18), and in an assembly of heating passages (19), each passage, in its through-flow heat exchange part, containing a corrugated separator (20) with vertical generating lines, means for distributing the liquid being provided at the upper end of the exchanger (2) so as to distribute the liquid throughout all the length of the vaporization passages (18), and means (9) being provided for conveying the second fluid through the heating passages (19), characterized in that the said means of distribution are arranged in compartments (23) closed at their upper end and each situated above a heating passage (19), from which it is separated by a horizontal bar (22), in that a horizontal slit (34), extending over all the length of the exchanger, just above the bar (22), puts the lower part of the compartment (23) in free communication with an adjacent vaporization passage (18), and in that the vaporization passages (18) are open at their two upper and lower ends, throughout their length, and contain no more than one corrugated separator (20A) with vertical generating lines at any point of their height.
2. Exchanger according to claim 1, characterized in that the vaporization passages (18) have no corrugated separator opposite the slits (34).
3. Exchanger according to claim 1 or 2, characterized in that the upper surface (35) of the bar (22) is inclined laterally towards the slit (34).
4. Exchanger according to any one of claims 1 to 3, characterized in that the means for distributing the liquid comprises, on the one hand, a horizontal member (27) extending over all the length of each compartment, at an intermediate level of this, this member having a thickness equal to the mutual spacing of the plates (4) and having openings (33) for pre-distributing the liquid, and on the other hand, below this member, a packing (28) for finely distributing the liquid over all the horizontal length of the compartment (23).
5. Exchanger according to claim 4, characterized in that the said openings (33) form a horizontal row of holes equidistant from each other.
6. Exchanger according to claim 4 or 5, characterized in that the member (27) has, on one vertical face, one or more rear recesses (30) closed at the bottom and open at the top and on its other vertical face one or more front recesses (31) open at the bottom and closed at the top, and in that the said openings (33) are provided through a vertical wall (32) common to the front and rear recesses.
7. Exchanger according to claim 6, characterized in that the member (27) has several rear recesses (30) separated from each other, and several front recesses (31) separated from each other.
8. Exchanger according to claim 7, characterized in that the front recesses (31) have a flared shape at the bottom.
9. Exchanger according to any one of claims 4 to 8, characterized in that the packing (28) is a corrugation with horizontal generating lines, the sides of which are provided with elongated openings.
10. Exchanger according to any one of claims 4 to 9, characterized in that the packing (28) is separated from the upper surface (35) of the bar (22).
11. Exchanger according to any one of claims 1 to 10, characterized in that it includes a side chamber for liquid to enter the said compartments (23), the lower point of this chamber being below the lower point of the inlet window (26) of these compartments.
12. Installation for separating 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 liquid oxygen from the vessel of the second column to be put into a heat exchange relationship with gaseous nitrogen at the head of the first column, characterized in that the heat exchanger is in accordance with any one of claims 1 to 11, and in that the installation comprises means of supply (6) for providing liquid oxygen to the said means for distributing liquid, and means (9) for supplying the heating passages with gaseous nitrogen.

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