EP1008826B1 - Falling film vaporizer and air distillation plant - Google Patents

Description

The present invention relates to a vaporizer of the type comprising a heat exchanger body which has main passages placed in heat exchange relationship, means for forming a bath of the liquid to be sprayed so that it circulates in at least a first of said passages main, and means for introducing circulating fluid into the minus a second of said main passages so that it ensures the vaporization of the liquid, the heat exchanger body being formed from an assembly of substantially parallel parallel plates similar alternately delimiting the first and second passages main which are flat. Such a vaporizer is already known from EP-A1-0130122.

The invention applies, for example, to a vaporizer-condenser for a double column air distillation installation.

In such a vaporizer-condenser, oxygen-rich liquid, from the low pressure column tank, is vaporized in the evaporator-condenser by condensation of a nitrogen-rich gas, taken off at the head of the medium pressure column.

More generally, an air separation device such as a double distillation column includes several types of heat exchanger heat.

A main heat exchanger is used to cool the supply air of the apparatus at distillation temperature by heat exchange with a or more fluids from the distillation apparatus. In some cases, these are pressurized liquids from the device that vaporize against the air at distill in the exchanger. These exchangers are normally made entirely of aluminum or copper or alloys of these metals (WO95 / 28610).

For safety reasons, these liquids sometimes vaporize in a dedicated exchanger, or vaporizer, against a single fluid such as air or nitrogen.

The apparatus also comprises at least one vaporizer-condenser which is a heat exchanger placed inside or outside of the column. These vaporizers-condensers are usually made entirely in copper, stainless steel, nickel or aluminum and are made up of at least two circuits which are connected to the rest of the installation by means of pipes welded to the equipment.

Exchangers used in air separation devices include heat exchanger bodies which are often made in parallel aluminum plates having a similar contour brazed between they.

Generally in exchangers that serve as vaporizers, a oxygen-rich liquid vaporizes against a current rich in gas nitrogen (such as air or nitrogen with a purity greater than 80%).

To improve the performance of these vaporizers, one can use so-called falling or dripping film vaporizers, that is to say of the aforementioned type and in which the oxygen-rich liquid of the bath is distributed at the top of the vaporizer in the form of a very thin film which flows vertically in the first main passages and part of which vaporizes by heat exchange with passages dedicated to nitrogen-rich gas common way.

EP-0795349 describes the case where such a vaporizer is combined with a thermosiphon type vaporizer (so-called bath vaporizer, i.e. a vaporizer completely immersed in the liquid where the recirculation of the liquid rich in oxygen is made thanks to the hydraulic thrust due to the difference in density between the bath and the liquid vaporizing in the passages).

In the bodies of brazed plate heat exchangers used in vaporizers of the aforementioned type with trickling film such as that of EP-A-0130122, the liquid is distributed between many passages made up of waves vertical inserted between two so-called separating sheets and thus constituting thermal fins, and due to the pitch of these waves the bodies brazed plate heat exchangers have surfaces very large exchange.

So when the whole surface is wet, the liquid film will be very thin and to avoid dry spraying at the bottom of the first passages main or in the event of a distribution fault, liquid is poured in excess in the heat exchanger body. This excess liquid forces in general to recycle liquid by means of a pump.

In vaporizers of the aforementioned type called bath, recirculation of the liquid is also maintained to avoid dry vaporization in the top of the first main passengers.

US-A-5699671 further describes a vaporizer with an exchanger body vertically arranged tubular in which nitrogen gas condenses at contact of its tubes.

It has been found, especially in film vaporizers-condensers dripping, that solid pollutants such as for example, hydrocarbons or nitrous oxide, can accumulate in passages dedicated to the oxygen-rich fluid, which can lead to clogging of these.

Such a blockage then deteriorates the operation of the vaporizer-condenser.

An object of the invention is to solve this problem by providing a vaporizer of the aforementioned type which limits the risk of clogging of the passage (s) dedicated to the liquid to be vaporized.

Another object of the invention is to minimize the recirculation of the liquid to be vaporized in vaporizers of the aforementioned type and ensure the safety of the optimal operation and performance.

To this end, the invention relates to a vaporizer of the aforementioned type, characterized in that the or each first main passage has, in current section transverse to the direction of circulation of the liquid at spray, at least one region of free flow continues sufficiently extended to allow the liquid to bypass a deposit of impurities, or at least a first main passage is either narrower than a second main passage and contains neither exchange wave nor passage auxiliary, either contains one or more closed auxiliary passage (s) which span most of the dimension of the heat exchanger body parallel to the direction of flow of the liquid to be vaporized, the walls of the (of) auxiliary passage (s) touching the plates defining the passage main.

Preferably, all the first main passages contain at least minus a closed auxiliary passage.

Thus the liquid sent into the auxiliary passage crosses the vaporizer without contacting the plates defining the first passages key. As far as possible, the liquid should be avoided between the exterior of the auxiliary passage and the passages defined by the plates.

One way to avoid this problem is to form the passages auxiliaries in a material block (e.g. aluminum, nickel or copper). If the block has substantially the dimensions of a first pass main, the liquid will not be able to flow outside the passages auxiliaries which are cylindrical holes passing through the block.

Ideally, the maximum width of an auxiliary passage is greater than 50% of the distance between two adjacent plates.

In order to avoid the accumulation of impurities, the inner surface of the auxiliary passage or each auxiliary passage includes only curved surfaces and possibly convexities. The absence of cavities in the passages of the first set ("liquid" passages) never has been proposed in the prior art.

Alternatively, at least one, and preferably all, of the first main passages contain several auxiliary passages formed by a series of cylindrical tubes parallel to each other and each having a diameter at least equal to 50% of the separation between two adjacent plates.

According to another variant, at least one and preferably all of the first main passages contain several auxiliary passages consisting of tubes, each having an inner surface with at least three identical convexities and curved surfaces connecting the convexities.

The adjacent tubes may or may not be contiguous.

Preferably, there are means for directing liquid into the or each auxiliary passage and / or liquid distribution means constituted by predistribution openings, these openings leaving drop this liquid on a lining located above the means for direct liquid into one or each auxiliary passage.

Alternatively, the means for directing the liquid into the passages are inclined points whose ends are above inside the auxiliary passage (or passage).

The vaporizer can be a main exchanger which is used to cool the air purified at its distillation temperature, a sub-cooler or the vaporizer-condenser of a double column.

The invention also relates to an air distillation installation comprising at least one vaporizer as defined above.

• la figure 1 est un schéma partiel d'une installation de distillation d'air selon l'invention,
• la figure 2 est une vue schématique éclatée du vaporiseur-condenseur de l'installation de la figure 1,
• la figure 3 est une vue partielle, schématique et en coupe d'un passage du vaporiseur-condenseur de la figure 1 dédié à la circulation du liquide à vaporiser,
• la figure 4 est une vue partielle, schématique et en coupe prise suivant la ligne IV-IV de la figure 3,
• la figure 5a est une section partielle schématique illustrant la structure des passages du vaporiseur-condenseur de la figure 1 dédiés au liquide à vaporiser et au gaz à condenser, et
• les figures 5b à 5d sont des vues analogues à la figure 5a illustrant des variantes de l'invention.

The invention will be better understood on reading the description which follows, given only by way of example, and made with reference to the appended drawings, in which:

• FIG. 1 is a partial diagram of an air distillation installation according to the invention,
• FIG. 2 is an exploded schematic view of the vaporizer-condenser of the installation of FIG. 1,
• FIG. 3 is a partial schematic view in section of a passage of the vaporizer-condenser of FIG. 1 dedicated to the circulation of the liquid to be vaporized,
• FIG. 4 is a partial schematic view in section taken along the line IV-IV of FIG. 3,
• FIG. 5a is a schematic partial section illustrating the structure of the passages of the vaporizer-condenser of FIG. 1 dedicated to the liquid to be vaporized and to the gas to be condensed, and
• Figures 5b to 5d are views similar to Figure 5a illustrating variants of the invention.

Figure 1 illustrates a vaporizer-condenser 2 (see description of Figure 1 in EP-A-0130122). The vaporizer-condenser 2 includes a heat exchanger body formed by a sealed envelope 3 and a series of parallel vertical plates 4 made of aluminum, which define a multitude of main flat passages intended alternately for one of two fluid flows, for example, a gas flow containing 98% nitrogen at around 5 bar and a liquid flow rate containing 98% oxygen at around 1.5 bars. Obviously pressures and purities can take other values.

The passages dedicated to the liquid to be vaporized are called first main passages and are marked with the letter L in the figures, while the passages dedicated to the gas to be condensed are called second passages main and are identified by the letter G in the figures.

The space above the plates 4 contains a bath 5 of the liquid to be vaporized from a line 6.

As illustrated in Figures 1 to 4, the liquid from this bath enters each first pass L through a series of perforations in a upper distribution bar 27. It then falls on a wave 26 which is a non-perforated aluminum sheet with horizontal generators (layout so-called hardway with respect to the flow of liquid oxygen) and offset partial vertical (partial vertical offset is not illustrated so as not to overload the figures) and which ensures the fine distribution of the liquid.

The liquid falls from the wave 26 onto an upper drip 25 constituted by an aluminum strip folded with a series of triangular points 29 forming an angle of 135 ° with the plane of one of the plates 4 of the passage L considered.

The end of each point 29 of the upper drip 15 is located above with a point of a lower drip 24, identical to the first but whose tips are oriented towards the other plate 4 of passage L considered.

The liquid to be vaporized then flows on the plates 4 of the first passage L considered in the form of a film streaming downwards.

The gas to be condensed enters the second passages G at by means of a pipe 9 welded in the middle of a head 8 (sometimes called "Box" or in English "headline") semi-cylindrical.

The gas then flows downwards in the second passages G to cocurrent of the liquid in the first passages L, the condensation of the gas ensuring the vaporization of the liquid in the first passages L.

As illustrated in FIG. 5a, only the second passages G each contain a spacer wave 21 consisting of a sheet corrugated perforated aluminum with vertical generators (available in "Easy-way").

Conventionally, these spacer waves 21 fill also the function of thermal fins.

The first passages L have a thickness less than that of second passages G.

In particular, the thickness of the first passages L is included between 2.5 mm and two thirds of the thickness of the second G passages.

The first passages L are each delimited by two plates 4 neighbors and by closing bars 30 situated between them on their side edges. The plates 4, between which a first passage L is located, therefore define between them a free and continuous space practically over their entire width, this width being measured in a direction transverse to that of the flow of the streaming film.

The first passages L are narrower than the second passages G and contain neither exchange waves nor passages Auxiliary. The distance between the adjacent plates 4 of the first passages L varies between 2.5 mm and two-thirds of the separation between the plates 4 of the second passages G.

Consequently, the first passages L have on their whole length a rectangular cross section free of any obstacle and keep on going. This section has a width substantially equal to the width of the plates 4 and therefore the width of the heat exchanger body, that is to say a width of about 1 meter.

Due to the transverse extent of the passages L, the risks of clogging of these are therefore limited.

Indeed, if a local deposit of substances resulting from vaporization liquid occurs on the plates 4 of a first passage L, the liquid to spraying can bypass this deposit.

In addition, it can be seen that the structure of the first passages L limits the recirculation of liquid required in the vaporizer 2.

In general, the risks of blockage can be limited using first passages L having in cross section current, that is to say over most of their length, a region unobstructed and continuous flow that extends along a guide curve C of length greater than approximately 10 cm. In the case Figures 1 to 5a, this guide curve C is a straight line parallel to plates 4, located between them, and about 1 m long. The right C is shown in dotted lines in FIG. 5a.

According to the variant of FIG. 5b, the distance separating the two plates 4 associated with a first passage L is greater than that of the variant of Figures 1 to 5a.

Two identical sheets 29 and 31, made of aluminum, and of section transverse epicyloid, are arranged between the two plates 4 associated with each first pass L and extend over their entire length. Each sheet 29, 31 therefore comprises a series of semi-cylindrical sections joined at the ends to form a curved line.

Each sheet 29, 31 is carried by a plate 4. The concavities of the sheets 29, 31 are directed towards each other. The sheets 29 and 31 are offset transversely from each other so that the tips of each sheet are located opposite a hollow of the other sheet. So the two sheets 29 and 31 form a single auxiliary passage between them, in which all the fluid flowing in the first passage L considered.

The sheets 29 and 31 play the role of thermal fins and delimit therefore between them the flow region of the liquid to be vaporized.

The auxiliary passage of each first passage L extends, in its cross section, continuously and freely practically over the entire width of the heat exchanger body. The mentioned guideline C above then extends between the sheets 29 and 31 following their contours. The directing curve is then sinuous and has a length greater than 1 m.

Here again, the first passages L make it possible to limit the risks closure thanks to a sufficient transverse extent so that the liquid to be sprayed bypasses any deposits.

In the variant of FIG. 5c), the auxiliary passages of the first passages L are formed by contiguous aluminum tubes 23. In the second passages G we find the generating waves 21 classic verticals.

In the variant of FIG. 5d), the auxiliary passages of the first passages L are non-contiguous tubes having a cross section shape of clover leaves.

In each case of FIGS. 5b to 5d, the auxiliary passage (s) includes only curved surfaces or convexities thus preventing the accumulation of impurities in the passages and making it possible to limit the liquid recirculation required in the vaporizer 2.

The invention is not limited to dripping film vaporizers but also applies to so-called bath vaporizers.

Claims (30)

1. Reboiler (2), of the type comprising a heat-exchanger body that has main passages (G, L) placed in a heat-exchange relationship, means for forming a bath (5) of the liquid to be vaporized, in order for it to flow in at least a first (L) of the said main passages, and means (8, 9) for introducing a heat-transfer fluid into at least one second (G) main of the said main passages, in order to ensure that the liquid vaporizes, the heat-exchanger body being formed from an assembly of parallel plates (4) of substantially similar outline defining, alternately, the first (L) and second (G) main passages which are flat, characterized in that the or each first main passage has, in working cross section transverse to the direction of flow of the liquid to be vaporized, at least one continuous free-flow region of sufficient extent to allow the liquid to go around a deposit of impurities, or at least one first main passage (L) is either narrower than a second main passage and contains neither an exchange corrugation nor an auxiliary passage, or contains one or more closed auxiliary passages (23, 50) that extend over most of the length of the heat-exchanger body parallel to the direction of flow of liquid to be vaporized, the walls of the auxiliary passage(s) touching the plates (4) that define the main passage.
2. Reboiler according to Claim 1, characterized in that the said flow region extends along a directrix curve (C) of length greater than about 10 cm.
3. Reboiler according to Claim 2, characterized in that the said directrix curve (C) has a length of greater than or equal to about 30 cm.
4. Reboiler according to any one of Claims 1 to 3, characterized in that the said flow region extends approximately over one dimension of the heat-exchanger body transverse to the direction of flow of the liquid to be vaporized.
5. Reboiler according to Claim 4, characterized in that each first main passage (L) has no corrugated spacer between the two plates (4) between which it is situated.
6. Reboiler according to Claim 5, characterized in that each first main passage (L) is bounded by the two parallel plates (4) between which it is located, these two plates defining between them a substantially free and continuous space over most of their width transverse to the direction of flow of the liquid to be vaporized.
7. Reboiler according to Claim 4 or 5, characterized in that each first main passage (L) contains at least one corrugation (29, 31) forming a thermal fin.
8. Reboiler according to Claim 7, characterized in that each first passage (L) contains two corrugations (29, 31) forming thermal fins arranged opposite each other and between them defining an approximately free and continuous auxiliary passage over most of their width transverse to the direction of flow of the liquid to be vaporized.
9. Reboiler according to Claim 7 or 8, characterized in that the or each corrugation (29, 31) forming a thermal fin of a first passage (L) has an approximately epicycloidal cross section transverse to the direction of flow of the liquid to be vaporized.
10. Reboiler according to Claims 8 and 9 taken together, characterized in that the epicycloidal corrugations of a one and the same first passage (L) are offset from one another transversely to the direction of flow of the liquid to be vaporized.
11. Reboiler according to any one of Claims 1 to 10, characterized in that the means for forming a bath of the liquid to be vaporized and means for introducing the heat-transfer fluid are arranged so that the heat-transfer fluid and the liquid to be vaporized flow cocurrently through the heat-exchanger body.
12. Reboiler according to any one of Claims 1 to 11, characterized in that the reboiler is a reboiler-condenser, the means for introducing the heat-transfer fluid being means (8, 9) for introducing a gas to be condensed and the reboiler furthermore including means (10, 11) for extraction of the condensed gas.
13. Reboiler according to Claim 1, characterized in that the or each auxiliary passage (23, 50) prevents the liquid to be vaporized from getting back into contact with the plates (4) of the corresponding first main passage (L).
14. Reboiler according to Claim 1 or 13, in which each first main passage (L) contains at least one closed auxiliary passage (23, 50).
15. Reboiler according to Claim 1, 13 or 14, in which the maximum width of an auxiliary passage is greater than 50% of the distance between two adjacent plates (4).
16. Reboiler according to any one of Claims 1 and 13 to 15, in which the internal surface of the or each auxiliary passage (23, 50) has only curved surfaces and optionally convexities.
17. Heat reboiler according to any one of Claims 1 and 13 to 16, in which at least one, and preferably each, first main passage (L) contains several auxiliary passages formed by a series of cylindrical tubes (23) parallel to one another and each having a diameter at least equal to 50% of the separation between two adjacent plates (4).
18. Reboiler according to any one of Claims 1 and 13 to 17, in which at least one, and preferably each, first main passage contains several auxiliary passages formed by tubes (50) each having an internal surface with at least three identical convexities and curved surfaces that connect the convexities.
19. Reboiler according to Claim 17 or 18, in which the adjacent tubes (23, 50) are contiguous.
20. Reboiler according to Claim 17 or 18, in which the adjacent tubes (23, 50) are not contiguous.
21. Reboiler according to any one of Claims 1 and 13 to 20, comprising means (24, 25) for directing the liquid of the bath towards the or each auxiliary passage.
22. Reboiler according to Claim 21, comprising means for distributing the liquid of the bath, comprising predistribution apertures, these apertures allowing this liquid to fall over a lining (26) lying above the means (24, 25) for directing the liquid into one or each auxiliary passage.
23. Reboiler according to Claim 21 or 22, in which the means (24, 25) for directing the liquid into the auxiliary passages are inclined points, the end of which lies above the inside of the auxiliary passage (or an auxiliary passage).
24. Reboiler according to Claim 1 or 13, in which the auxiliary passages are formed in a block of material placed in the first main passage (L) and having substantially the same dimensions as the latter.
25. Reboiler according to any one of Claims 1 to 24, characterized in that the means for forming a bath of the liquid to be vaporized are means for forming a bath (5) above the main passages (G, L), the reboiler furthermore having means for introducing the liquid of the bath (5) into the or each first main passage so that it flows through in the form of a falling film.
26. Air distillation unit comprising at least one reboiler (2) according to any one of Claims 1 to 25.
27. Unit according to Claim 26, in which the reboiler (2) is a main exchanger that is used to cool purified air down to its distillation temperature.
28. Unit according to Claim 26 or 27, in which the reboiler (2) is a subcooler.
29. Unit according to any one of Claims 26 to 28, comprising a first column fed with air and thermally coupled to a second column by means of the reboiler (2).
30. Air distillation unit according to Claim 29, in which the first column is a medium-pressure column (1), the second column is a low-pressure column and the reboiler (2) is a reboiler-condenser for bringing oxygen from the bottom of the low-pressure column into heat-exchange relationship with the nitrogen from the top of the medium-pressure column.

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