WO2002046669A1

WO2002046669A1 - Evaporator-condenser and air distillation plant comprising same

Info

Publication number: WO2002046669A1
Application number: PCT/FR2001/003877
Authority: WO
Inventors: Marc Wagner; Claire Szulman
Original assignee: L'air Liquide, Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2000-12-08
Filing date: 2001-12-07
Publication date: 2002-06-13
Also published as: FR2817952A1

Abstract

The invention concerns an evaporator-condenser comprising a stack of parallel plates, closure bars and optionally spacer-folds, which define a first series of passages (1) for a source-supplied fluid to be evaporated, and a second series of passages adjacent to the first for at least a fluid heating said fluid to be evaporated. The invention is characterised in that said passages (1) of the first series are divided into three successive zones, from the bottom upwards of the evaporator-condenser: a first zone (2) configured so as to promote heat exchanges by convection and lined with heat-exchanging folds having a density not less than 10 folds per inch (3.9 folds/cm); a second zone (3) configured so as to promote nucleate boiling and comprising heat-exchanging surfaces treated so as to promote therein nucleation the of fluid bubbles; a third zone (4) configured so as to promote convective boiling and lined with heat-exchanging folds having a density not less than 10 fpi (3.9 corrugations/cm) and not more than the heat-exchanging folds used in the first zone (2) and, optionally, in the second zone (3).

Description

CONDENSER VAPORIZER AND AIR DISTILLATION SYSTEM COMPRISING SUCH A VAPORIZER-CONDENSER

The present invention relates to a vaporizer-condenser, of the type comprising a stack of parallel aluminum or copper plates, closing bars and spacer waves, which define a first series of passages for a vaporized fluid supplied " in source ”, and at least a second series of passages for at least one heating fluid.

The invention applies in particular to the main vaporizer-condensers of air distillation apparatus. These vaporizers-condensers vaporize the liquid oxygen under low pressure (typically slightly higher than atmospheric pressure) collected at the bottom of a column, by condensation of medium pressure nitrogen (typically from 5 to 6 bars absolute) circulating in neighboring passages. passages dedicated to the circulation of oxygen. Medium pressure nitrogen is most often taken in the gaseous state at the top of a medium pressure air distillation column to which the low pressure column mentioned above is connected. After its passage and at least partial condensation in the vaporizer-condenser, this nitrogen is returned to the medium pressure column. It is more specifically in the context of this application that the invention will be described below, it being understood that its application can be envisaged in other contexts.

The cryogenic air separation systems of the double column type include an air compressor whose energy consumption is conditioned in particular by the temperature difference between the oxygen vaporized in the low pressure column and the nitrogen present in the form condensed in the medium pressure column. This temperature difference is itself linked to the pressure difference between the two columns. A reduction in this temperature difference makes it possible to considerably improve the energy consumption of the air compressor, the latter then having to supply air under a lower pressure than in the case where the temperature difference would be higher.

To achieve this result, it is necessary to obtain as good heat exchanges as possible inside the vaporizer-condenser, in other words to obtain in its different parts high heat transfer coefficients.

This optimization of the heat transfer coefficients has led to the design of relatively complex vaporizers-condensers, since the fluids which pass through them are not in the same physical state at all levels of the device. In particular, the oxygen is found in the entirely liquid state in the lower part of the vaporizer-condenser, then sees its proportion of vapor gradually increase as it rises in the device by thermosyphon effect under the effect of its heating by nitrogen gas. We thus imagined to play on the configuration of the waves-spacers lining the passages where oxygen circulates.

Document US-A-4,700,771, for example, proposes to provide two zones of very different configurations distributed over the height of each oxygen vaporization passage. The first zone, located in the lower part of the passage, contains waves whose configuration makes it possible to privilege the exchanges of heat by convection, so as to quickly bring the liquid to its saturation temperature, even if this causes significant in this zone pressure losses. For this purpose, waves are used having a large exchange surface and defining corridors of small section for the passage of oxygen. A second zone, located above the previous one, contains waves defining clearly wider section corridors, or even no waves at all, so as to cause low pressure losses, and thus not to hinder the ascent of the gaseous oxygen bubbles generated there. This generation of bubbles is accentuated by a rough or porous surface state of the waves and / or of the walls of the vaporizer-condenser with which the oxygen is in contact. The first convection heat exchange zone can itself be divided into several sub-zones, each imposing different pressure drops on liquid oxygen, these pressure drops tending to decrease from the bottom to the top of the first zone.

Patent application EP-A-1088578 in the name of the Applicant describes a vaporizer-condenser of a type similar to those which have just been described, in which the heat exchanges are optimized inside the passages where the vaporization of the liquid by giving them a section which increases from bottom to top. It is also proposed to play on the orientations of the flows of the fluid, by imposing on the liquid circulating in the lower zone of the passage a transverse course, even in zigzag, compared to the general orientation of the passage.

The object of the invention is to provide a vaporizer-condenser configuration of the type described above providing thermal transfers between the fluid to be vaporized and the heating fluid further improved compared to known vaporizers-condensers. To this end, the subject of the invention is a vaporizer-condenser, of the type comprising a stack of parallel plates, closing bars and possibly spacer waves, which define a first series of passages. for a source-supplied fluid to be vaporized and a second series of passages contiguous to the first for at least one fluid for heating said fluid to be vaporized, said passages of the first series being divided into three successive zones, from the bottom to the top of the vaporizer-condenser: - a first zone configured so as to favor thermal exchanges by convection;

- a second zone configured so as to favor the nucleated boiling phenomenon;

- a third zone configured so as to favor the phenomenon of convective boiling, characterized in that:

- Said first zone is filled by heat exchange waves with a density greater than or equal to 10 fpi (3.9 waves / cm);

- Said second zone includes heat exchange surfaces treated so as to promote nucleation of the fluid bubbles therein;

- Said third zone is filled by heat exchange waves with a density greater than or equal to 10 fpi (3.9 waves / cm) and less than or equal to the density of the heat exchange waves used in the first zone and, optionally , in the second zone. Said first zone can be filled by heat exchange waves of the partial offset type.

The waves have, for example, a density of 14 to 30 fpi (5.5 to 11.8 waves / cm).

Alternatively, a metal foam padding may be used. Said second zone can be filled by heat exchange waves of a density greater than or equal to 10 fpi (3.9 waves / cm) and less than the density of the waves of said first zone.

Said second zone may also not include heat exchange waves, and may be provided with a metallic foam. Said third zone can be filled by heat exchange waves of the 5% perforated straight wave type and with a density of 10 to 14 fpi (3.9 to 5.5 waves / cm).

In an example of such a vaporizer-condenser:

- Said first zone represents between 10% and 40% of the total height of said vaporizer-condenser;

- Said second zone represents between 10% and 60% of the total height of said vaporizer-condenser; - said third zone represents between 40 and 60% of the total height of said vaporizer-condenser

Preferably:

- Said first zone represents between 20% and 35% of the total height of said vaporizer-condenser;

- Said second zone represents between 20% and 40% of the total height of said vaporizer-condenser;

- Said third zone represents between 40 and 60% of the total height of said vaporizer-condenser. A vaporizer-condenser according to the invention may comprise in the second series of passages, facing the second zone of the first series of passages, means making it possible to locally increase the temperature difference between the fluid to be vaporized and the heating fluid .

Said means can be constituted by means for pressurizing the heating fluid, or by means for circulating in the second series of passages, opposite said second zone of the first series of passages, a heating fluid different from that traveling opposite the first and third zones of the first series of passages.

The invention also relates to an air distillation installation of the type comprising a column or part of a column in the tank from which an oxygen-rich liquid to be vaporized is collected and a vaporizer-condenser installed in said tank, characterized in that that said vaporizer-condenser is of the type previously described.

Said heating fluid can be nitrogen, which can be replaced by air facing the second zone of the first series of passages.

As will be understood, the invention consists in dividing the passages of the vaporizer-condenser in which the vaporization of the fluid takes place into three zones, arranged in this order from bottom to top of the device:

- A first zone where the fluid is normally in the liquid state, and where the intensity of the heat exchanges with the heating fluid circulating in the neighboring passages is favored, so as to quickly bring the fluid to vaporize to its saturation temperature ; convection transfers are highly sought after here,

- a second zone where one endeavors to favor the nucleation of gas bubbles within the fluid;

- a third zone where heat exchange and convection transfers are again favored, while taking care not to cause losses of too high a charge in the fluid which is then, at least in large part, in the gaseous state.

The invention will be better understood on reading the description which follows, given with reference to the single appended figure, which schematically represents a passage of a vaporizer-condenser according to the invention, in which oxygen circulates in the state liquid and gaseous.

For more details concerning the overall design of the evaporator-condenser according to the invention applied to the distillation of air, reference may be made in a non-limiting manner to EP-A-1088578 already cited. A passage 1 in which the oxygen circulates from bottom to top by thermosyphon effect, first in the purely liquid state then in the mixed liquid-gas state is shown in the single figure.

The vaporizer-condenser in which passage 1 is included (and the other passages dedicated to the vaporization of oxygen) is almost completely immersed in the liquid oxygen collected in the tank of the low pressure column of a air distillation. Passage 1 is therefore supplied "in source" with liquid oxygen. This liquid oxygen first enters a first zone 2 of the passage 1 to be heated there by the nitrogen circulating in the contiguous passages of the vaporizer-condenser. In this first zone 2, preference is given to heat exchange by convection and the materials which constitute it are given a configuration which maximizes this type of exchange. Typically, this first zone 2 is filled by heat exchange waves having a large exchange surface without however providing too high pressure losses, such as waves with partial offset (called "serrated waves"), or waves straight perforated or not, or herringbone type waves defining numerous and narrow corridors for the passage of liquid oxygen. A density of at least 10 fpi (10 waves per inch in width, ie 3.9 waves per cm) is recommended, preferably from 14 to 30 fpi (5.5 to 11.8 waves / cm). As an example, we can use serrated waves of 26 fpi (10.2 waves per cm) offset every 1/8 of an inch (3.18 mm). In this first zone 2, the main aim is to obtain rapid heating of the liquid oxygen, so as to bring it to its saturation temperature. This first zone can extend over approximately 1/3 of the total height of the vaporizer-condenser, for example over a height of 40 cm for a vaporizer-condenser of 1.20 m high, dimension which is conventional for the apparatuses of air separation. As a variant, the heat exchange waves could be replaced by a lining made of metal foam or a material such as aluminum. The oxygen rising in passage 1 then enters a second zone 3 where it is sought to promote a phenomenon of boiling nucleated by the formation of bubbles of gaseous oxygen on the walls located in passage 1. To this end, care is taken porosities and / or micro-reliefs on these walls which have the effect of multiplying the possible priming sites for the formation of bubbles. These porosities or micro-reliefs can be provided both on the walls of the plates of the exchanger delimiting the passage 1 as on the walls of the heat exchange waves which line this second zone 3 with passage 1 or on the walls of any other material filling the second zone 3. In this regard, a metallic foam (aluminum for example) can be used as the filling material. Again, it is possible to use heat exchange waves with a density greater than or equal to 10 fpi (3.9 waves / cm), but preferably less than the density of the waves present in the first zone 2. In fact, even more than in the first zone 2, it is important to limit the pressure losses of the fluid so as not to hinder the ascent of the liquid oxygen-oxygen gas mixture present. It is also conceivable to dispense with heat exchange waves in this second zone 3, if the treatment of the surface of the plates of the vaporizer-condenser proves sufficient. As the elements with a surface specially treated to promote nucleated boiling are expensive, it is important to minimize as much as possible the height of this second zone 3. It can, for example, extend over approximately 1/6 of the total height of the vaporizer-condenser, that is to say a height of 20 cm for a vaporizer-condenser of 1.20 m in height.

In addition to or in place of the treatment of the heat exchange surfaces which has been described, it is possible to use, to promote nucleation, means making it possible to increase the temperature difference between the liquid oxygen-oxygen gas mixture and the fluid which reheats it by crossing the contiguous passages to passage 1. These means may consist of localized means for pressurizing the nitrogen circulating in these passages, or for local replacement of the nitrogen with another fluid capable of bringing more calories from oxygen, such as air. The oxygen in liquid and gaseous form rising in passage 1 finally enters a third zone 4 where it is again sought to promote heat exchanges with the fluid passing through the contiguous passages. It aims to be in a convective boiling regime. There is no need to provide specially treated exchange surfaces to initiate the formation of bubbles. The bubbles of gaseous oxygen present increase in size under the effect of heat transfers until they have a diameter roughly equal to the width of the corridors defined by the heat exchange waves present in this third zone 4. The walls of the waves and the plates are covered with a layer of liquid oxygen through which heat exchange takes place. Its thickness depends mainly on the flow conditions of the liquid oxygen-oxygen gas mixture. The heat exchanges are all the more favored as the speed of the fluid is high. It is therefore important to limit as much as possible the pressure drop of the oxygen during its ascent in this third zone 4. To this end, to obtain a satisfactory compromise between low pressure drop and good heat transfers, it is advisable to garnish this third zone with straight waves, possibly perforated, with a density greater than 10 fpi (3.9 waves / cm), but less than or equal to the density of the waves used in the first (2) and, possibly, the second (3) zone of passage 1. Straight waves perforated at 5% and with a density of 10 to 14 fpi (3.9 to 5.5 waves / cm) would be consistent with the examples previously given. Serrated waves are not recommended here because of the fairly large pressure drops they would generate.

This third zone 4 can represent approximately half of the total height of the passage, or 60 cm for a vaporizer-condenser of 1.20 m high.

At its exit from the third zone 4, the gaseous oxygen emerges from the vaporizer-condenser and rises towards the head of the low pressure column, while the liquid oxygen descends in the tank of this same column.

It goes without saying that the examples which have been given are not limiting, and that other configurations can be imagined. In particular, each of the zones described above can be divided into several sub-zones having exchange surfaces configured in different ways, provided that in each of these sub-zones the phenomenon to which the corresponding zone is dedicated is actually favored: convective exchange for the first zone, nucleated boiling for the second zone, convective boiling for the third zone.

The invention can also be applied in vaporizers-condensers treating gases other than oxygen if the advantages which it presents can be exploited there.

Claims

1. Vaporizer-condenser, of the type comprising a stack of parallel plates, closing bars and possibly spacer waves, which define a first series of passages (1) for a fluid to be vaporized supplied at source, and a second series of contiguous passages to the first for at least one fluid for heating said fluid to be vaporized, said passages (1) of the first series being divided into three successive zones, from the bottom to the top of the vaporizer-condenser:

- a first zone (2) configured so as to favor thermal exchanges by convection;

- a second zone (3) configured so as to favor the nucleated boiling phenomenon;

- a third zone (4) configured so as to favor the phenomenon of convective boiling, characterized in that:

- Said first zone (2) is filled by heat exchange waves with a density greater than or equal to 10 fpi (3.9 waves / cm);

- Said second zone (3) comprises heat exchange surfaces treated so as to promote nucleation of the fluid bubbles therein; - said third zone (4) is filled by heat exchange waves with a density greater than or equal to 10 fpi (3.9 waves / cm) and less than or equal to the density of the heat exchange waves used in the first zone (2) and, optionally, in the second zone (3).

2. Vaporizer-condenser according to claim 1, characterized in that said first zone (2) is filled by heat exchange waves of the partial shift type.

3. Vaporizer-condenser according to claim 1 or 2, characterized in that said waves have a density of 14 to 30 fpi (5.5 to 11.8 waves / cm).

4. Vaporizer-condenser according to claim 1, characterized in that said first zone is filled with a metallic foam.

5. Vaporizer-condenser according to one of claims 1 to 3, characterized in that said second zone (3) is filled by heat exchange waves with a density greater than or equal to 10 fpi (3.9 waves / cm) and less than the density of the waves of said first zone (2).

6. Vaporizer-condenser according to one of claims 1 to 4, characterized in that said second zone (3) does not include heat exchange waves.

7. vaporizer-condenser according to claim 6, characterized in that said second zone (3) comprises a lining of metal foam.

8. Vaporizer-condenser according to one of claims 1 to 7, characterized in that said third zone (4) is filled by heat exchange waves of the straight wave type perforated at 5% and a density of 10 to 14 fpi (3.9 to 5.5 waves / cm).

9. vaporizer-condenser according to one of claims 1 to 8, characterized in that:

- said first zone (2) represents between 10% and 40% of the total height of said vaporizer-condenser;

- said second zone (3) represents between 10% and 60% of the total height of said vaporizer-condenser;

- Said third zone (4) represents between 40 and 60% of the total height of said vaporizer-condenser.

10. Vaporizer-condenser according to claim 9, characterized in that:

- said first zone (2) represents between 20% and 35% of the total height of said vaporizer-condenser;

11. Vaporizer-condenser according to one of claims 1 to 10, characterized in that it comprises in the second series of passages, facing the second zone (3) of the first series of passages, means making it possible to increase locally the temperature difference between the fluid to be vaporized and the heating fluid.

12. A vaporizer-condenser according to claim 11, characterized in that said means consist of means for pressurizing the heating fluid.

13. vaporizer-condenser according to claim 11, characterized in that said means are constituted by means for circulating in the second series of passages, facing said second zone (3) of the first series of passages, a heating fluid different from that circulating opposite the first (2) and third (4) zones of the first series of passages.

14. Air distillation installation of the type comprising a column or part of column in the tank from which an oxygen-rich liquid to be vaporized is collected and a vaporizer-condenser installed in said tank, characterized in that said vaporizer-condenser is of the type according to one of claims 1 to 13.

15. Installation according to claim 14, characterized in that said heating fluid is nitrogen.

16. Installation according to claim 14, characterized in that said vaporizer-condenser is of the type according to claim 13 and in that said heating fluid circulating in the second series of passages facing the second zone (3) of the first series of passages is air.