EP0035444B1 - Process and installation for reheating a cold fluid - Google Patents

Description

The present invention relates to a method and an installation for heating a cryogenic fluid by heat exchange with a circulating fluid, the solidification temperature of which is higher than the temperature of the cryogenic fluid before its final heating, and firstly a method for heating d a cryogenic fluid, such as liquefied natural gas, by heat exchange with a circulating fluid, such as water, the solidification temperature of which is higher than the temperature of said cryogenic fluid, according to which said cryogenic fluid is led into the at least one module comprising two vertical tube elements with fins connected in series, and the circulating fluid is made to flow by gravity in the form of a free sheet along the periphery of said tube elements.

It is particularly applicable to the re-vaporization and reheating of liquefied natural gas with water available in large quantities (river, sea ...), as opposed to reheating by burners (see for example FR-A- 1,393,641), which is suitable for economic reasons only at relatively low gas flow rates.

Various solutions have already been proposed, but none of them has been entirely satisfactory.

FR-A-2 096 919 describes a process for heating natural gas by countercurrent exchange in a plurality of elements of vertical tubes mounted in parallel, the natural gas always circulating in an upward direction inside the elements of tubes and circulating water naturally flowing by gravity outside these tube elements, which are provided with longitudinal fins. In order to optimize the heat exchange, that is to say to maximize the heat flow while avoiding icing of the water at the external periphery of the tubes, an internal section of tube is provided for the passage natural gas which is increasingly reduced, which leads to successive increases in the speed of natural gas flowing in the tubes. These successive decreases in cross-section have in particular been achieved by the installation of an internal lining constituted by a blind tube with variable section, which constitutes a fairly complex technology.

JP-A-54-7403 describes the heating of natural gas by first co-current exchange with the natural gas flowing from bottom to top in a tube bundle and the water flowing from bottom to top in a calender according to a forced flow, then an exchange against the current with the gas circulating from top to bottom in another tube bundle and the water circulating from bottom to top in the corresponding grille. This way of doing things is quite complex and leads to significant deterioration, in particular calenders in the event of accidental freezing of the heating water.

JP-A-52-144 006 describes a process of the type indicated above. According to this method, a heating is carried out comprising a first stage in countercurrent exchange with the natural gas flowing from bottom to top in a first plurality of upstream tube elements and the water naturally flowing outside, then a second stage in countercurrent exchange also, the natural gas flowing from bottom to top in a second plurality of downstream tube elements and the water flowing naturally outside, with the particularity that the second plurality of elements tubes offers a smaller cross section for natural gas than the first plurality. This arrangement does not allow the objective of the present invention to be fulfilled either, because the icing around the upstream tube elements cannot be avoided.

The object of the invention is to ensure that water or other fluid at relatively cold temperature can be used as circulating fluid while avoiding any risk of solidification of this fluid.

The essential characteristic of the invention resides in the fact that, in a heating process of the aforementioned type, the cryogenic fluid is made to circulate firstly co-current with said circulating fluid and then counter-current with said circulating fluid, the element upstream tube being supplied with cryogenic fluid at its upper end, and in that the heat exchange between the cryogenic fluid and the circulating fluid at this end is slowed down by means of an insulating sleeve.

• D'une part, l'existence d'un premier échange à co-courant, freiné au début au moyen d'un manchon isolant, est décisive en raison de la limitation du flux thermique pour éviter la prise en glace externe. En effet, si la température de l'eau à l'entrée, c'est-à-dire à l'extrémité supérieure de l'élément de tube amont, est par exemple de + 4 °C et de + 2 °C à la sortie, c'est-à-dire à l'extrémité inférieure de ce même élément de tube, le débit de gaz naturel liquéfié à une température de - 160 °C pouvant entrer dans un tube fonctionnant à co-courant est plus de deux fois supérieur à celui qui peut entrer dans ce même tube fonctionnant à contre-courant.

The advantages of the invention can be explained as follows:

• On the one hand, the existence of a first co-current exchange, braked at the start by means of an insulating sleeve, is decisive due to the limitation of the heat flow to avoid setting in external ice. In fact, if the temperature of the water at the inlet, that is to say at the upper end of the upstream tube element, is for example + 4 ° C and + 2 ° C at the outlet, that is to say at the lower end of this same tube element, the flow of liquefied natural gas at a temperature of - 160 ° C which can enter a tube operating in co-current is more than two times greater than that which can enter this same tube operating against the current.

The existence of at least a second countercurrent heat exchange is also decisive because of the small temperature difference between the natural gas leaving the downstream tube element and the water cooling this element. Indeed, if the temperature of the water at the upper end, that is to say at the upper outlet of this element, is + 4 ° C and + 2 ° C at the lower inlet, the The length of this downstream tube element operating in counter-current is 30% less than that which would be necessary for similar operation in co-current.

On the other hand, as the critical temperature of natural gas is generally close to - 60 ° C, its density in the vicinity of this temperature varies rapidly with temperature, even under a pressure higher than the critical pressure (T kg / m3 / ° C under 75 bars). However, the flow rate of natural gas in the downstream tube element is still necessarily low at this temperature to avoid setting in external ice. Under these conditions, a downward flow of natural gas would lead to flow disturbances due to the untimely influence of gravity and generating thermodynamic irreversibilities. On the other hand and according to the invention, an upward flow in the downstream element leads to a natural stratification according to the density and the temperature of the natural gas which therefore does not create any disturbance of flow. Since the intermediate temperature between the upstream and downstream tube elements is close to the critical temperature, it is therefore preferable to provide for an upward circulation of natural gas in the downstream tube element in order to ensure a final heating of the natural gas. without untimely irreversibilities, which should then be compensated for by a noticeable increase in the exchange surface.

The present invention also relates to an installation for heating a cryogenic fluid, such as liquefied natural gas, by heat exchange with a circulating fluid, such as water, the solidification temperature of which is higher than the temperature of said cryogenic fluid, of the type comprising at least one heat exchange module comprising two vertical tube elements with fins connected in series and in which the cryogenic fluid circulates, and means for causing the circulating fluid to flow by gravity in the form of a free ply along the periphery of said tube elements, characterized in that the two tube elements are connected by their lower ends, and in that said means comprise a runoff liquid distributor disposed at the upper end of each element of tube and an insulating sleeve disposed at the upper end of the single element of upstream tube and interposed between the two fluids.

• la figure 1 est une vue partielle en coupe verticale d'une installation de réchauffement de liquide cryogénique selon l'invention ;
• la figure 2 est un détail, à échelle agrandie, d'une partie de la figure 1 ;
• la figure 3 est une vue, à échelle agrandie, en coupe selon la ligne III-III de la figure 2 ;
• les figures 4, 5, 6, 7, 8 et 9 sont des variantes de réalisation d'une installation selon l'invention.

The characteristics and advantages of the invention will emerge from the description which follows with reference to the appended drawings in which:

• Figure 1 is a partial view in vertical section of a cryogenic liquid heating installation according to the invention;
• Figure 2 is a detail, on an enlarged scale, of part of Figure 1;
• Figure 3 is a view, on an enlarged scale, in section along the line III-III of Figure 2;
• Figures 4, 5, 6, 7, 8 and 9 are alternative embodiments of an installation according to the invention.

Referring to Figures 1 to 3, we see that an installation comprises a plurality of heating tubes 1 forming heat exchange passages, made of aluminum, each consisting of a tube element "upstream ₂ 2 and a downstream tube element 3, connected by a lower bend 4. The upstream tube element 2 2 is connected to a pipe 5 to a source of cryogenic fluid to be heated via a connection box 10, while that the downstream tube element 3 is connected directly to a pipe 6 for withdrawing heated fluid: the tube elements 2 and 3 are suspended so as to extend substantially vertically, and all around and along these tube elements, which have external fins 7, flow streams of heating liquid in the form of sheets 8 and 9 which are previously formed by upper distribution devices 11.

The connection box 10 here comprises (see FIG. 2) welded as an extension of the upstream element 2, an envelope tube 12 having a constant wall thickness in a lower section 12 ′ and increasing radially in a middle part 12 ″, with constant internal diameter, at the upper end, this casing 12 extends at 13 to a connection end 14 of the pipe 5 for the cryogenic fluid All of these parts are made of aluminum to be suitably welded between them and with the heat exchange tube element 1. The end piece 14 has an internal bore of small diameter 16 in which is welded a conduit element 17 leading- largely inside the upstream tube element 2 Between the conduit element 17 on the one hand and on the other hand the envelope-tube 13-12 and the upper part of the tube element 2 is placed a thermal insulation product 18. The assembly which comes to be described is housed inside a repair well rtition 20 having a ring of perforations 21. This well 20 is fixed to the distribution device 11 enveloping at a short distance the tube element 2 with its fins 7 and the perforations 21 are located at the upper level of the part 12 "thick oversized. In practice, and as noted in the drawings, the runoff caloric liquid, which is intended to flow in layers such as 8 and 9 along the tube elements "upstream 2 2 and" downstream 3, comes from a general liquid reserve 25, which itself is supplied by a source 25 '.

In operation, the circulating circulating liquid is transferred into a lower part of the distribution well 20 in the form of a plurality of veins or liquid jets 26 coming from the reserve 25 and formed from the perforations 21. Thanks to the arrangement which comes to be described, the cryogenic fluid which circulates inside the pipe 5 and the tube 17 to reach the upstream tube element 2 is radially isolated from the outside by the insulating body 18. Furthermore, the flow significant longitudinal refrigeration, which essentially arises, on the "upstream" side, at the end piece 14 and which propagates downstream along the casing-tube 13-12 towards the tube element 2, is substantially derived radially outwards at the location of the wall thickness of the envelope tube 12 progresses sively increasing upstream. In fact, in the very thick middle part 12 "of the tube-shell 12, the longitudinal refrigerant flow is transferred as much as possible to the water, which is in the form of jets 26 in free and rapid gravitational flow. This maximum transfer effect radially outwardly thermal results on the one hand from the arrangement, at the level of the jets 26, of a significant excess wall thickness of the part 12 "of the tube-envelope 12, which offers increased thermal conductance in the radial direction, on the other hand, a rapid flow of water in free fall, which has the effect of bringing the coefficient of heat exchange to its maximum value. This arrangement therefore allows a diversion towards the liquid jets 26 of a substantial part of the refrigerating flow with longitudinal propagation, which thereby alleviates the residual refrigerating flow thereby continuing its longitudinal propagation in the weaker walled portion 12 ′ and especially towards the upper part 2 ′ of the upstream tube element 2 which is immersed in an individual reserve of distribution water 29 of a substantially stagnant nature, therefore with a low coefficient of heat exchange with the wall of the tube element 2. Without the arrangement described above, we would witness the arrival of a significant refrigeration flow with longitudinal wall propagation at the level of the part 2 ′ of the tube element 2, enveloped in a reserve of standing water 29, which would not fail to cause detrimental surface solidifications of water at the level of the part 2 ′ since these solidifications, by propagating radially, could reach the entire water reserve 29 and render e thus inoperative the heat exchange of the tube 2-3. On the contrary, thanks to the arrangement described, it is possible to very precisely control the heat flux which reaches the level of the part 2 ′ of the element of the upstream tube 2 since this heat flux is the sum of a residual heat flux at longitudinal propagation and of a radially propagating heat flux which is itself weak thanks to the interposition of the insulating product 18. Moreover, in certain cases, it is on the contrary possible to slightly increase the heat exchange coefficient between the part 2 ′ of the tube element 2 with the water reserve 29, giving it a certain convection movement thanks to the presence of relief perforations 21 ′ practiced in the low position in the distribution chimney 20, thus promoting a certain additional water intake coming directly from the main reserve 25.

As explained above, the runoff water forms in a runoff layer on the finned external wall of the upstream tube element 2 and gradually cools down to the lower end of this tube element. "Upstream 2, where the runoff water is then evacuated at 30 with besides that which comes from the runoff against the current on the tube element" downstream 3. It is noted that at the level of this tube element " downstream 3, the risks of freezing of the runoff liquid are significantly reduced, the fluid being heated circulating in the tube 1 has seen its temperature increase until it is close to that of the runoff liquid. so that the evacuation of the heated fluid from the “downstream” tube element 3 can be carried out, without using a connection box as described with reference to FIG. 2, by a simple pipe of racking 6, however, of course, the distribution device 11 allowing the formation of a uniform run-off ply 9, as shown in FIG. 3.

Instead of using a heating tube whose inlet end “upstream receives the raw fluid to be heated and whose downstream end delivers the fluid to the desired heating temperature (or more precisely a plurality of such tubes arranged in parallel and connected directly to intake manifolds 31A and withdrawal 31 B), it is possible to arrange the tube elements "upstream and the tube elements" downstream in a number of combinations.

Referring to Figure 4, we see that a plurality of tube elements "upstream 42a, 42b, ... 42n are all connected between an upper distribution manifold 50 and a lower connection manifold 51 supplying another plurality 43a, 43b ... 43n of downstream tube elements thus forming a first multi-tubular module whose upper end is connected by a manifold 52 to a second multi-tubular module consisting of another plurality of elements "upstream" tube 44a, 44b ... 44q, the final module having a plurality of "upstream" tube elements 45a, 45b ... 45r and a plurality of "downstream" tube elements 46a, 46b ... 46s, delivering the heated liquid to a 52 "final manifold.

According to FIG. 5, monotubular modules as described with reference to FIG. 1, each consisting of an upstream tube element (54a, 54b, etc.) are supplied at their upper end by a common supply collector 55, and are connected by individual connectors 58a, 58b ... to a downstream tube element (56a, 56b, etc ...), themselves connected at their upper end to a common withdrawal manifold 57.

According to Figure 6, several lines 61 and 62, such as those described in Figure 4, that is to say each incorporating several multi-tubular modules in series 63, 64 ... 63 ', 64' ... , are connected in parallel between a main intake manifold 68 and a main withdrawal manifold 69.

According to FIG. 7, several lines 70, 71, each consisting of several multi-tubular modules 72, 73, ... 72 ', 73' ... are not only connected between a main supply collector 74 and a main collector racking 75, but intermediate equalization collectors 77 connect the homologous modules of several lines in parallel.

According to FIG. 8, a bundle of tube elements is formed of a first set of lines 81a, 81b, 81c (for example three in number). killed by a multi-tubular module (or several multi-tubular modules in series) between a supply collector 83 and an intermediate collector 84 which supplies a second set of lines 82a and 82b (for example two) between this intermediate collector 84 and the final withdrawal manifold 85.

According to FIG. 9, a first set of a plurality of lines 91 a, 91 b, 91 c (for example three) supplied by a supply collector 93 and withdrawn by a withdrawal collector 95a is connected via a pipe 96 with expansion valve 97 to a second set of another plurality of lines 92a, 92b connected between a supply manifold 95b and a withdrawal manifold 94. This arrangement can be used for example if the network is 40 bars and the gas available under higher pressure, for example 80 bars, and it is noted that this delayed expansion which causes a refrigeration release is not detrimental to the pipes, since the natural gas is then in a state already partially warmed up. If necessary, a separator can be placed at the outlet of the expansion valve 97 for withdrawing and removing the heaviest condensates, such as ethane, propane or butane, while the gaseous fraction is only warmed up.

Claims (14)

1. Process for reheating a cryogenic fluid, such as liquefied natural gas, by heat exchange with a calorific fluid, such as water, the temperature of solidification of which is higher than the temperature of said cryogenic fluid, according to which said cryogenic fluid is conducted in at least one module comprising two vertical tube elements (2, 3) having wings (7) serially connected, and one causes that the calorific fluid trickles under gravity in the form of a free nappe (8, 9) along the periphery of said tube elements (2, 3), characterized in that the cryogenic fluid is caused to circulate first cocurrently (at 2) with said calorific fluid, then (at 3) countercurrently with said calorific fluid, the upstream tube element (2) being fed with cryogenic fluid at its upper end, and the heat exchange between the cryogenic fluid and the calorific fluid is braked at said end by means of an insulating sleeve (18).

2. Process for reheating according to claim 1, characterized in that the calorific liquid is passed over an upper part (2') of each upstream tube element (2) provided with said insulating sleeve (18) in the form of a crown of jets (26), and a reserve (29) of this liquid is formed around the said upper part (2') before distributing as nappe around said upstream tube element.

3. Process for reheating according to claim 2, characterized in that in the region of the jets (26) the radial derivation to the exterior of the longitudinal frigorific flux is augmented, preferably as much as the said reserve (29) is exceeded.

4. Installation for reheating a cryogenic fluid, such as a liquified natural gas, by heat exchange with a calorific fluid, such as water, the temperature of solidification of which is higher than the temperature of said cryogenic fluid, of the type comprising at least one module of heat exchange comprising two vertical tube elements (2, 3) with wings (7) serially connected and in which the cryogenic fluid is circulated, and means (11) for trickling the calorific fluid under gravity in the form of a free nappe (8, 9) along the periphery of said tube elements (2, 3), characterized in that the two tube elements (2, 3) are connected by their lower ends (at 4) and that the said means comprise a distributor for trickling liquid (11) arranged at the upper end of each tube element (2, 3) as well as an insulating sleeve (18) arranged at the upper end (2') of the same upstream tube element (2) and interposed between the two fluids.

5. Installation for reheating according to claim 4, characterized in that the said means comprise a shaft (20) which surrounds the upstream tube element (2) above said distributor (11), delimitat- ing with it a reserve (29) of calorific liquid, and a crown of holes (21) for the formation of jets (26) pierced in said shaft.

6. Installation for reheating according to claim 5, characterized in that the upper end (12") of the upstream tube element (2) has a thickening in the region comprised between the crown of holes (21) and the reserve (29), preferably a thickening growing upwardly.

7. Installation for reheating according to one of the claims 4 to 6, characterized in that it comprises a plurality of modules mounted parallely between an upstream feed collector (50 ; 55 ; 68 ; 74; 83) and a downstream withdrawal collector (52 ; 57 ; 69 ; 75 ; 84).

8. Installation for reheating according to claim 7, characterized in that the modules define a first plurality of upstream tube elements (42a, 42b, ... 42n) connected by their lower ends by a common « low collector (51) to lower ends of a second plurality of « downstream tube elements (43a, 43b, ... 43n).

9. Installation according to claim 8, characterized in that the upstream tube element (54a, 54b) of each module is connected individually to the downstream tube element (56a, 56b) of the same module.

10. Installation according to one of the claims 7 to 9, characterized in that it comprises a plurality of modules connected serially by high collectors (52, 52") to form a line.

11. Installation according to claim 10, characterized in that several lines (61, 62 ; 70, 71) are arranged and branched at one end of said lines at a main admission collector (68) and at the other end of said lines at a main withdrawal collector (69).

12. Installation according to claim 11, characterized in that certain homologous modules (72, 72', 73, 73') of several parallel lines (70, 71) are connected by intermediate collectors (77).

13. Installations according to claim 11, characterized in that several lines are arranged, certain (81a, 81b, 81c ; 91 a, 91 b, 91c) parallely, certain serially (81a, 82a ; 91a, 92a) with intermediate collectors (84).

14. Installation according to claim 13, characterized in that the intermediate collectors incorporate means of expansion (97), optionally followed by a condensate separator.

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