EP1146300A1 - Thermosiphon boiler-condenser - Google Patents

Abstract

The exchanger comprises an external enclosure (20) containing a first fluid (F1) to be vaporized. Inside the enclosure is a heat exchange module (24) which defines passages (26) for the circulation of a second fluid (F2) to be condensed which has an outlet temperature from the passages of T2,2. The first fluid which enters the passages from the bottom has an inlet temperature T1,1 and an outlet pressure P1,2. The inlet pressure of the first fluid, P1,1, is greater than a minimum pressure PmS of about 1.3 bar. The height of the passages, h, is 2.5 m and the temperature T2,2 is less than T1,1 + 1.2oC.

Description

The present invention relates to an exchanger bath type evaporator-condenser and a heat exchange process in a bath type exchanger.

More specifically, the invention relates to an exchanger bath type vaporizer-condenser between a first fluid to be vaporized and a second fluid to condense as well as the use of this type heat exchange. By vaporization is meant vaporization partial or total and by condensation is meant condensation partial or total.

This provision is used in particular, but not exclusively in air distillation systems of the type double column where, for example, the liquid oxygen in the tank of the low pressure column is vaporized in a bath vaporizer by heat exchange with nitrogen gas taken from the top of the column medium pressure.

The operation of bath exchangers imposes by its specific characteristics of the limitations on the height of exchange between the first and the second fluid or as for the temperature difference between the primary fluid and the secondary fluid.

This problem will be better understood by referring to Figures 1 and 2 attached which represent on the one hand an example of diagram functional of a bath exchanger and on the other hand an example of heat exchange diagram between the primary fluid and the fluid secondary.

In Figure 1, the tank is shown in a simplified manner external 10 of the heat exchanger inside which is contained a set of passages 12 for the second "hot" fluid F2 which enters the upper part of these passages at 14 and comes out at the part lower in 16. With regard to the first “cold” fluid F1 to vaporize, it is contained in the external enclosure 10 and circulates by thermosyphon of the lower end 12a of the passages for the second fluid F2 up to its upper end 12b, the height of this heat exchange zone being equal to h.

As best shown in the diagram in FIG. 2, the first fluid F1 at the entrance to the exchange zone is at a temperature T _1.1 and at a pressure P _1.1 . This temperature T _1.1 and this pressure P _1.1 correspond to a sub-cooling state, that is to say to a temperature below the bubble temperature T _b1 of the fluid F1 at the pressure P _1.1 due to hydrostatic pressure due to the height of liquid F1 fluid. This is what we will represent on this diagram. Note T _b the temperature (bubble) at which the first gas bubble appears in the fluid F1 during the heat exchange (at intermediate pressure between P _1.1 and P _1.2 ). It is understood that the energy used to bring the primary fluid to the bubble temperature T _b is "lost" energy to obtain the vaporization of the first liquid. In this FIG. 2, the second fluid F2 has also been shown with its inlet temperature in the exchange zone 12 which is equal to T _2.1 and its outlet temperature equal to T _2.2 . We see that the phenomenon of subcooling causes a "pinch effect" in the heat exchange between the two fluids.

In addition, the thermosyphon effect which allows the circulation of first fluid F1, is made possible by the formation of bubbles of the first fluid. If the height in the exchanger corresponding to the phase too much "subcooling", the thermosyphon effect will be insufficient.

We understand that the higher the height h of the exchange zone the greater the effect of the hydrostatic pressure on the first fluid at the entrance to the exchange zone will be important and therefore more the sub-cooling zone must also be. To allow maintenance of the thermosiphon effect ensuring circulation of the first fluid, the phenomenon of "pinching" must therefore be limited. In the bath type heat exchange systems, this height is therefore limited to 2.5 meters.

Another drawback present in this type of bath heat exchanger is that the "pinching phenomenon" described above makes it necessary to provide a temperature difference between the inlet temperature T _1.1 of the cold fluid F1 to be vaporized and the temperature T _{2 , 2} of the hot fluid F2 higher than about 1.2 ° C to allow the operation of the heat exchanger by thermosiphon due to the "pinching effect". However, it is understood that the increase in this temperature difference increases the thermodynamic irreversibilities, and, consequently, decreases the energy efficiency of the entire installation. For example, in the case of the distillation of air gases using a double column, the pressure of the so-called medium pressure column and, consequently, the pressure of the supply air compressor , must be increased, which increases the energy consumption of the entire installation.

There is therefore a real need for heat exchangers bath-type vaporizer-condenser or exchange processes thermal in a bath type installation which allow either increase the vertical heat exchange height to decrease the footprint of the installation, i.e. to reduce the temperature between the first fluid and the second fluid, i.e. still allow a combination of these two characteristics of the exchanger vaporizer-condenser.

des moyens pour définir une pluralité de passages d'échange thermique entre les deux fluides pour faire circuler ledit deuxième fluide, ledit deuxième fluide ayant une température T_2,2 à la sortie desdits passages,

des moyens formant enceinte contenant les moyens formant passage pour faire circuler par thermosiphon ledit premier fluide entre lesdits passages du bas vers le haut sur une hauteur h, ledit premier fluide ayant une température d'entrée T_1,1 avec T_1,1 < T_2,2, ledit premier fluide vaporisé ayant une pression de sortie P_1,2,

des moyens pour donner à la pression d'entrée P_1,1 dudit premier fluide une valeur telle que la pression P_1,2 soit supérieure à ladite pression minimale P_mS et des moyens pour qu'au moins une des deux conditions suivantes soit remplie :

la hauteur h des passages d'échange thermique est au moins égale à 2,5 m ; et

la température T_2,2 dudit deuxième fluide est inférieure à T_1,1 + 1,2°C.

To achieve this object according to the invention, the evaporator-condenser exchanger of the bath type between a first fluid (F1) to vaporize and a second fluid (F2) to condense, said exchanger having a minimum outlet pressure P _m S of said first fluid to allow the operation of the installation in which said exchanger is mounted, comprises:

means for defining a plurality of heat exchange passages between the two fluids for circulating said second fluid, said second fluid having a temperature T _2.2 at the outlet of said passages,

enclosure means containing the passage means for circulating by thermosyphon said first fluid between said passages from bottom to top over a height h, said first fluid having an inlet temperature T _1.1 with T _1.1 <T _2.2 , said first vaporized fluid having an outlet pressure P _1.2 ,

means for giving the inlet pressure P _{1.1 of} said first fluid a value such that the pressure P _1.2 is greater than said minimum pressure P _m S and means so that at least one of the following two conditions is fulfilled:

the height h of the heat exchange passages is at least equal to 2.5 m; and

the temperature T _{2.2 of} said second fluid is less than T _1.1 + 1.2 ° C.

We have shown that if we increase the pressure output of the first fluid, the pinching effect is modified, which allows either to increase the height h of heat exchange, or to decrease the difference of temperature between the two fluids.

• on fait circuler un deuxième fluide (F2) dans des passages verticaux d'échange, ledit deuxième fluide ayant une température de sortie T_2,2 ;
• on fait circuler de bas en haut sur une hauteur h par thermosiphon ledit premier fluide entre lesdits passages d'échange thermique, ledit premier fluide ayant une température d'entrée T_1,1 (T, < T_2,2), la fraction vaporisée dudit premier fluide ayant une pression de sortie P_1,2 ;
• on donne à ladite pression P_1,2 une valeur supérieure à la pression minimale de sortie de la fraction vaporisée du premier fluide nécessaire pour permettre le fonctionnement de l'installation dans laquelle est monté ledit échangeur ; et
• on choisit la hauteur h des passages d'échange thermique et la température T_2,2 dudit deuxième fluide de telle manière qu'au moins une des deux conditions suivantes soit remplie :
  • la hauteur h desdits passages d'échange thermique est au moins égale à 2,5 m ; et
  • la température T_2,2 dudit deuxième fluide est inférieure à T_1,1 + 1,2°C.

According to a second aspect of the invention, the method of vaporizing a first fluid using a vaporizer-condenser bath exchanger comprises the following steps:

• a second fluid (F2) is circulated in vertical exchange passages, said second fluid having an outlet temperature T _2.2 ;
• said first fluid is circulated from bottom to top over a height h by said thermosyphon between said heat exchange passages, said first fluid having an inlet temperature T _1.1 (T, <T _2.2 ), the vaporized fraction said first fluid having an outlet pressure P _1.2 ;
• said pressure P _{1.2 is given} a value greater than the minimum outlet pressure of the vaporized fraction of the first fluid necessary to allow the operation of the installation in which said exchanger is mounted; and
• the height h of the heat exchange passages and the temperature T _{2.2 of} said second fluid are chosen so that at least one of the following two conditions is met:
  • the height h of said heat exchange passages is at least equal to 2.5 m; and
  • the temperature T _{2.2 of} said second fluid is less than T _1.1 + 1.2 ° C.

We understand that this process improves the characteristics of the bath exchanger as already explained in connection with the previous definition of the bath exchanger conforming to the invention.

According to a preferred embodiment, the outlet pressure of the first fluid P _1.2 is of the order of 4 bars absolute or greater.

According to another characteristic, preferably, the height of the heat exchange passages between the two fluids is at least equal at 3 m.

Preferably, the heat exchange passages between the two fluids are limited by parallel plates, which can be of the type with brazed fins.

According to an implementation variant, the passages can be made up of tubes.

According to a first embodiment, the means forming enclosure include a single enclosure containing said passages heat exchange and in which the first fluid circulates by thermosyphon.

According to a second embodiment, the means forming enclosure include a first enclosure defining a lower volume of entry of the first fluid and a higher volume of first fluid outlet and a second enclosure connected respectively to the upper and lower volumes, this second enclosure that can be reduced to piping.

la figure 1, déjà décrite, est une vue simplifiée d'un échangeur à bain connu ;

la figure 2, déjà décrire, montre le diagramme d'échange thermique de l'échangeur à bain de la figure 1 ;

la figure 3 montre un premier exemple de réalisation d'un échangeur à bain conforme à l'invention utilisé dans la distillation de l'air ;

la figure 4 est un diagramme d'échange thermique de fonctionnement de l'échangeur à bain de la figure 3 ;

la figure 5 montre une variante de réalisation de l'échangeur à bain selon l'invention ; et

la figure 6 montre la courbe de variation du sous-refroidissement en fonction de la pression du liquide, pour une hauteur hydrostatique de 1 mètre.

Other characteristics and advantages of the invention will appear better on reading the following description of several embodiments of the invention given by way of nonlimiting example. The description refers to the appended figures in which:

Figure 1, already described, is a simplified view of a known bath exchanger;

Figure 2, already described, shows the heat exchange diagram of the heat exchanger of Figure 1;

Figure 3 shows a first embodiment of a bath exchanger according to the invention used in the distillation of air;

Figure 4 is a heat exchange diagram of operation of the heat exchanger of Figure 3;

Figure 5 shows an alternative embodiment of the bath heat exchanger according to the invention; and

FIG. 6 shows the variation curve of the sub-cooling as a function of the pressure of the liquid, for a hydrostatic height of 1 meter.

Referring first to Figures 3 and 4, we will describe a first embodiment of the bath exchanger according to the invention. In the description which follows, we will more particularly consider the case where the cold fluid to be vaporized is liquid oxygen and where the hot fluid is nitrogen gas, which is for example the case during distillation air gas cryogenic with a double column type scheme. However, it goes without saying that the present invention can be applied to heat exchange between two other fluids for example at separation cryogenic synthesis gas such as methane, carbon monoxide carbon, hydrogen ...

Referring first to Figures 3 and 4, we will describe a first embodiment of the bath exchanger. We represented the external enclosure 20 containing the first fluid F1 which is in the example considered pure oxygen. At the top of enclosure 20, we finds the interface 22 between the first fluid F1 in liquid form and the F1 fluid in the form of vapor recovered at the top of the enclosure. Inside this enclosure, there is an exchange module thermal 24 which defines in a manner known per se passages 26 for the second "hot" fluid F2 which in the example considered is nitrogen pure, these passages extend between an inlet box 28 connected to the inlet pipe 30 and an outlet box 32 connected to the exit 34. These passages, as is known, can be constituted by tubes or by parallel plates defining the circuit of the second fluid. These passages can be vertical as is shown in Figure 3, horizontal or oblique. The exchange module thermal 24 also defines vertical passages for circulation of the first fluid F1, that is to say oxygen.

As already indicated, in this type of bath heat exchanger, the fluid to be vaporized F1 circulates by thermosyphon in the vertical heat exchange passages. The fluid F1 has at its inlet, that is to say at the lower end 24a of the exchange module, a temperature T _1.1 and a pressure P _1.1 , and a temperature T _1.2 and a pressure P _1.2 at the upper end 24b of the exchange module. The total height of the exchange module is called h, that is to say the length of circulation of the first fluid between the inlet end 24a and the outlet end 24b.

The second fluid, which is nitrogen gas in the example under consideration, enters at temperature T _2.1 through line 30 and leaves the exchange module in liquid form at temperature T _2.2 .

In Figure 4, there is shown the heat exchange between the fluid F1 (pure oxygen) and fluid F2 (pure nitrogen). Curve A, substantially vertical due to the fact that the fluid F2 is pure nitrogen, shows the evolution of this fluid between its entry and its exit from the exchange module. Curve B shows the evolution of the first fluid (pure oxygen). She has a first part B1 corresponding to the "sub-cooling" of oxygen and part B2 of vaporization partial oxygen from the bubble temperature Tb of oxygen.

As has already been explained, by increasing the outlet pressure P _1.2 of the first fluid, it is possible to decrease the "pinching effect" which makes it possible to increase the height h of exchange and / or to decrease the temperature difference T _2.2 -T _1.1 .

In the case of cryogenic distillation of air gases with a double column type diagram, the pressure P _1.2 at the outlet of the first fluid (oxygen) depends on the pressure at the outlet of the complete installation containing the exchanger bath taking into account the pressure drop due to the switchgear between the outlet of the exchanger and the outlet of the complete installation. If the outlet of the installation is at atmospheric pressure, the pressure at the outlet of the bath exchanger is of the order of 1.3 bar absolute.

It goes without saying that in order to increase the outlet pressure P _1.2 of the first fluid, it is necessary to increase the pressure of the hot fluid F2 and consequently the pressure of the gas at the inlet of the installation (by example of air).

If a pressure P _1.2 of 4 bar absolute is allowed, a bath heat exchanger can be constructed, the height h of the exchange module being equal to 3 or 4 meters, while maintaining a temperature difference of the order 1.2 ° C.

With the same outlet pressure of 4 bar absolute and in now a height h of 2 meters, we can reduce the difference of temperature at 0.4 or 0.5 ° C.

FIG. 5 shows an alternative embodiment of the bath exchanger.

The exchanger comprises a main enclosure 40 in which is mounted the exchange module 42. The enclosure 40 defines also a lower inlet chamber 44 of the first fluid and a upper chamber 46 for the outlet of the first fluid with a sampling 48 of the first vaporized fluid. The exchanger includes also a chamber 50 for recirculating the first fluid in the state essentially liquid which is connected to the upper chambers and lower by lines 52 and 54. This enclosure could be reduced to simple piping.

In FIG. 6, the variations ΔTb of the sub-cooling induced by a hydrostatic height of 1 m have been shown as a function of the pressure P for pure oxygen (curve I) and for pure methane (curve II). It can be seen that the higher the pressure (P), the weaker the sub-cooling effect. These curves make it possible to better understand the favorable effect of the increase in the pressure of the first fluid on the "pinching effect". In fact, the higher the outlet pressure P _1.2 , the more it will be possible to increase the height h of exchange, that is to say the hydrostatic pressure (P _1.2 -P _1.1 ) while retaining the same variation in sub-cooling ΔTb.

Claims (12)

Bath-type evaporator-condenser exchanger between a first fluid (F1) to vaporize and a second fluid (F2) to condense, said exchanger having a minimum outlet pressure P _m S of said first fluid to allow operation of the installation in which said exchanger is mounted, said exchanger comprising: means (24) for defining a plurality of heat exchange passages (26) between the two fluids for circulating said second fluid, said second fluid having a temperature T _2.2 at the outlet of said passages; enclosure means (20) containing the passage means (24) for circulating by thermosiphon said first fluid between said passages from bottom to top over a height h, said first fluid having an inlet temperature T _1.1 with T _1.1 <T _2.2 , said first vaporized fluid having an outlet pressure P _1.2 ; means for giving the inlet pressure P _{1.1 of} said first fluid a value such that the pressure P _1.2 is greater than said minimum pressure P _m S and means so that at least one of the following two conditions is fulfilled: the height h of the heat exchange passages is at least equal to 2.5 m; and the temperature T _{2.2 of} said second fluid is less than T _1.1 + 1.2 ° C. Vaporizer-condenser exchanger according to claim 1, characterized in that said minimum pressure P _m S being of the order of 1.3 bar absolute, the outlet pressure P _{1.2 of} said first fluid to be vaporized is of the order of 4 bars absolute or higher. Vaporizer-condenser exchanger according to claims 1 and 2, characterized in that the height of said exchange passages (26) is at least equal to 3 meters. Exchanger according to any one of claims 1 to 3, characterized in that said temperature T _2.2 of the second fluid is between T _1.1 + 1.2 ° C and T _1.1 + 0.4 ° C. Exchanger according to any one of claims 1 to 4, characterized in that said heat exchange passages (26) are limited by parallel plates. Exchanger according to claim 5, characterized in that said parallel plates are of the type with brazed fins. Exchanger according to any one of claims 1 to 4, characterized in that said heat exchange passages (26) are tubes. Exchanger according to any one of claims 1 to 7, characterized in that said enclosure means comprise a single enclosure (20) containing said heat exchange passages (26) and in which said first fluid circulates by thermosyphon. Exchanger according to any one of claims 1 to 7, characterized in that said enclosure means comprise a first enclosure (40) defining a lower inlet volume (44) of the first fluid and an upper outlet volume (46) of the first fluid and a second enclosure (50) connected respectively to said upper and lower volumes. Method for vaporizing a first fluid (F1) using a bath vaporizer-condenser exchanger comprising the following steps: a second fluid (F2) is circulated in vertical exchange passages, said second fluid having an outlet temperature T _2.2 ; said first fluid is circulated from bottom to top over a height h by said thermosyphon between said heat exchange passages, said first fluid having an inlet temperature T _1.1 (T _1.1 <T _2.2 ), the vaporized fraction of said first fluid having an outlet pressure P _1.2 ; said pressure P _{1.2 is given} a value greater than the minimum outlet pressure of the vaporized fraction of the first fluid necessary to allow the operation of the installation in which said exchanger is mounted; and the height h of the heat exchange passages and the temperature T _{2.2 of} said second fluid are chosen so that at least one of the following two conditions is met: the height h of said heat exchange passages is at least equal to 2.5 m; and the temperature T _{2.2 of} said second fluid is less than T _1.1 + 1.2 ° C. Application of the exchanger according to any one of claims 1 to 9 to cryogenic separation of gases from air. Application of the exchanger according to any one of Claims 1 to 9 for the cryogenic separation of synthesis gas.

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