FR2929369A1 - METHOD FOR VAPORIZING A CRYOGENIC LIQUID BY EXCHANGING HEAT WITH A CALORIGENE FLUID - Google Patents

Abstract

A method of vaporizing cryogenic liquid, for example liquefied natural gas, by heat exchange with a calorigenic fluid, for example gaseous nitrogen is provided.

Description

The present invention relates to a method for vaporizing a cryogenic liquid, for example liquefied natural gas, by heat exchange with a heat-generating fluid, for example nitrogen gas. In order to heat and vaporize liquefied natural gas (LNG) or equivalent cryogenic liquids against a circulating fluid, in order to recover the LNG frigories, one of the three following possibilities has been used in the past: • a technology of winding up in the form of pancakes a system of two tubes interconnected by a ligament. The tubes are welded or dug on collectors transverse to the slabs; • brazed heat exchangers with plates and fins; • coiled tubular exchangers. In the case where it is desired to recover the frigories for liquefying gases from the air, it is absolutely necessary to avoid accidental pollution of nitrogen or oxygen by a hydrocarbon gas, in particular when natural gas circulates in the atmosphere. exchanger at a pressure greater than that of the air gas. Tubular geometries are not very thermally efficient and often lead to expensive oversizing. Furthermore, the LNG terminals as well as the existing air separation plants do not always have the equipment to avoid sudden thermal transients during shutdowns and restart, which leads to thermal shocks and therefore damage to the air. exchangers. According to one object of the invention, there is provided a method of heating a first fluid by heat exchange with a second fluid in a plate and fin heat exchanger in which the first fluid is heated in a first series of passages. separated and the second fluid cools in a second series of separate passages characterized in that each passage of the first series is separated from the passage closest to the second series by an auxiliary passage containing fins which circulates an inert gas. Optionally: the first fluid is liquefied natural gas that vaporizes or heats in the first series of separate passages; 2929369 - the second fluid consists of nitrogen gas which cools or liquefies in the second series of separate passages; the inert gas is at a pressure at least 0.1, or even at least 0.5 bars greater than that of the first fluid and the second fluid; The inert gas is at a pressure at least 0.1 or at least 0.5 bar lower than that of the first fluid and the second fluid; the inert gas is nitrogen gas; the inert gas sent to at least some auxiliary passages is then sent to the atmosphere or flared; at least one inlet and / or outlet box of one of the first and second fluids is separated from the passages in which the other of the first and second fluids circulates by means of a system of double bars, the bars possibly being separated by a dead zone; the first fluid is heated to a pressure of at least 60 bar abs.

According to another object of the invention, there is provided a method for starting a plate and fin heat exchanger in which at full speed a first fluid is heated by heat exchange with a second fluid in a plate heat exchanger and with fins, the first fluid heating in a first series of separate passages and the second fluid cooling in a second series of separate passages characterized in that each passage of the first series is separated from the passage closest to the second series by an auxiliary passage containing fins and wherein during start-up an inert gas at a temperature below room temperature, optionally at cryogenic temperature, is sent to at least one auxiliary passage to accelerate the cold setting. The invention will be described in more detail with reference to the figures. Figures 1 to 3 show a section taken in the direction of the length of the exchanger of each type of passage for an exchanger 30 operating according to the invention. Figure 1 shows an auxiliary passage of inert gas, Figure 2 a passage of LNG and Figure 3 a passage of nitrogen to be heated. Figures 4 to 6 show another exchanger operating according to the invention. Fig. 4 shows a section through the exchanger parallel passages 299369 in the width direction of the exchanger, Fig. 5 shows a passage of low pressure nitrogen cut along its length and Fig. 5 shows a passage of LNG cut in the direction of its length. According to the invention, a passage of the type of FIG. 1 will be placed between each passage of the type of FIG. 2 and each type of FIG. 3. Thus each passage of series of the type of FIG. 2 is separated from each passage of FIG. the type series of Figure 3 by a passage of the type of Figure 1 to form a brazed plate and fin exchanger made of aluminum, or other material. The fins are not illustrated to simplify the drawing. Figure 1 is the auxiliary passage of low pressure nitrogen gas inert whose inlet 9 is bottom right and outlet 11 in the upper left. In Figure 2 is illustrated a liquefied natural gas (LNG) warming passage that enters the lower left passage 1 and exits at the top right 3. A double bar isolates the top and bottom of the LNG passage 15 from the passage of inert nitrogen. Figure 3 shows a high pressure nitrogen gas cooling passage that enters the top of the passage through the inlet 7 and exits at the bottom through the outlet 5. The passage of high pressure nitrogen gas is narrower than the passages of FIG. 2 shows the low pressure nitrogen of FIG. 1 and of liquefied natural gas. In order to avoid nitrogen pollution by the liquefied natural gas, an auxiliary passage is interposed between each pair of nitrogen and LNG passages. The thermal exchange between the nitrogen and LNG passages will be through the fins of the auxiliary passage by conduction. Obviously the wave chosen for the auxiliary passage will have an optimum height / thickness ratio. In the illustrated case, the auxiliary passages will be swept with low pressure nitrogen gas (lower pressure than that of the LNG of Figure 2 and that of the nitrogen of Figure 3) and collected to the atmosphere or possibly a torch. The boxes that cover the stack and can therefore be sources of pollution will therefore be isolated from the other fluid by means of dead zones Z. Dead zones Z will be collected and possibly flushed with low pressure nitrogen.

4 2929369 The above dead zones can be isolated from the LNG and nitrogen circuits by means of a double bar system 2 in order to perfect the watertightness. The play between the double bars 2 can itself be collected to enhance the intrinsic safety. This is explained in more detail for the process of Figures 5 and 6 but also applies to the process of Figures 1 to 3. The passages of Figure 1 are used during starts to cool the heat exchanger in a progressive manner and controlled by means of a low pressure nitrogen flow from an auxiliary capacity. According to another aspect of the invention, illustrated in FIG. 4, each passage of nitrogen to be heated (N2 BP) is isolated from the LNG passages to be vaporized by a passage containing an inert process gas at high pressure (N 2 HP). in this case nitrogen at higher pressure than nitrogen to be heated (35 bar) and liquefied natural gas to vaporize (15 bar).

As can be seen in FIGS. 5 and 6, the bars which separate a nitrogen circuit to be heated from an LNG circuit are doubled, so that the space between them forms a dead zone Z open to the atmosphere by a vent V, so that any leakage of liquefied natural gas can escape. The passages of Figures 5 and 6 are separated by a high pressure inert gas passage. 5

Claims (10)

REVENDICATIONS1. A method of heating a first fluid by exchanging heat with a second fluid in a plate and fin heat exchanger in which the first fluid is heated in a first series of separate passages and the second fluid is cooled in a second series of passages characterized in that each passage of the first series is separated from the passage closest to the second series by an auxiliary passage containing fins in which circulates an inert gas. io The method of claim 1 wherein the first fluid is liquefied natural gas that vaporizes or heats in the first series of separate passages. 3. A process according to claim 1 or 2 wherein the second fluid is nitrogen gas which cools or liquefies in the second series of separate passages. 4. Method according to one of claims 1 to 3 wherein the inert gas is at a pressure at least 0.1 or even at least 0.5 bars greater than those of the first fluid and the second fluid. 5. Method according to one of claims 1 to 3 wherein the inert gas is at a pressure at least 0.1 or at least 0.5 bar lower than that of the first fluid and the second fluid. 6. Method according to one of the preceding claims wherein the inert gas is nitrogen gas. 7. Method according to one of the preceding claims wherein the inert gas fed into at least some auxiliary passages is then sent to the atmosphere or flared. 8. Method according to one of the preceding claims wherein at least one inlet and / or outlet box of one of the first and second fluids is separated from the passages in which the other of the first and second fluids circulates by means of a system of double bars (2), the bars optionally being separated by a dead zone (Z). 9. Method according to one of the preceding claims wherein the first fluid is heated to a pressure of at least 60 bar abs. 6 2929369 10. A method of starting a plate and fin heat exchanger wherein in full speed a first fluid is heated by heat exchange with a second fluid in a plate heat exchanger and fins, the first fluid heating in a first series of separate passages and the second fluid cooling in a second series of separate passages characterized in that each passage of the first series is separated from the passage closest to the second series by an auxiliary passage containing fins and wherein during Starting an inert gas at a temperature lower than the ambient temperature, optionally at cryogenic temperature, is sent to at least one auxiliary passage to accelerate the cold setting.