FR2951242A1 - Supplying sub-cooled cryogenic liquid from a reservoir to a consumer equipment, comprises placing a liquid nitrogen bath in a heat exchanger immersed in a bath of (non)cryogenic liquid, and controlling the bath at a predetermined level - Google Patents

Abstract

The method of supplying sub-cooled cryogenic liquid from a reservoir to a consumer equipment, comprises placing a liquid nitrogen bath in a heat exchanger (22) immersed in a bath of (non)cryogenic liquid, controlling the bath at a predetermined level, and passing the liquid through the heat exchanger before introducing it to the consumer device. The reservoir contains the cryogenic liquid in liquid phase at its bottom and gaseous phase at its top. A content of pressure in the reservoir is higher than atmospheric pressure. An independent claim is included for an installation for supplying sub-cooled cryogenic liquid from a reservoir to a consumer equipment.

Description

The invention relates to methods of supplying a user site with cryocooled liquid.

A cryogenic fluid is commonly understood as a fluid that at atmospheric pressure is liquid at a temperature much below 0 ° C. Among the many industrial applications using such cryogenic liquids, we are particularly interested in applications of food cryogenics (cryogenic tunnels, kneaders, churns etc ...).

Cryogenic liquid is traditionally supplied with consumer equipment (for example in liquid nitrogen) from a cryogenic fluid reservoir connected to the consumer equipment of this fluid, which reservoir contains, under a storage pressure greater than atmospheric pressure, a cryogenic fluid in the liquid phase at the bottom of the tank and in the gaseous phase at the top of the tank, this tank being adapted to firstly feed the consumer equipment with liquid which is drawn off at the bottom of the tank, and secondly to be supplied from outside with fluid. The so-called "low storage pressure" tanks are most commonly used, that is to say the maximum pressure reached at the top of the tank is generally less than about 4 bar, the pressure indicated here and the pressures indicated by the rest being in absolute bars. Since the storage pressure of the tank is greater than the atmospheric pressure, the opening of a valve placed on the line connecting the tank to the consumer equipment causes the liquid to move from its point of drawdown to its point of use, without forced drive means and despite pressure losses on the line (valves, bent portions ...). To ensure that the entrainment of the cryogenic liquid is always effective regardless of the liquid level in the tank, the pressure of the gas at the top of the tank is conventionally regulated so that this pressure remains substantially equal to a predetermined value. fixed, generally of the order of 2 to 4 bars.

However, the pressure of the liquid at the bottom of the tank varies according to the height of the liquid inside the tank, so that as the liquid level drops, the pressure of the liquid withdrawn decreases and tends to get closer to the gas pressure at the top. For example, in the case of nitrogen, a liquid height of about 10 meters implies a pressure differential of the order of 0.6 bar between the gas pressure at the top and the liquid pressure at the bottom of the tank, at the level of the drawing. This variation in pressure of the liquid at the point of drawing necessarily leads to a variation in the flow of liquid withdrawn, causing operating disturbances for downstream consumer equipment. A symmetrical effect occurs when the reservoir is re-supplied with fluid.

For well-known reasons of better "cryogenic quality" in terms of available frigories, the literature and this industry have been interested in the means of supplying these users with free or frank liquids or undercooled liquid, that is to say to say in liquid at lowered pressure, and therefore at a lower temperature than when it was at higher pressure. As an illustration, this approach has been adopted by the prepared foods industry, which uses processes using immersion systems to produce so-called "IQF" products. This IQF name (Individually Quick Frozen) covers the growing demand for individual quick freezing of products that are normally difficult to deep-freeze (fruits, vegetables, shrimps, ...), thus preserving their original appearance, texture and taste.

The manufacturers here sought to reduce the consumption of cryogenic liquid and thus to limit the proportion of gas in the cryogenic distribution pipes of these immersion systems. Still as an example, the ice cream dessert manufacturing industry uses cryogenic liquids to freeze the toppings and make overlays. The level regulations of these systems require a free-flowing liquid so as not to disturb the measurement, and this by reducing the rate of gas present in the pipes. The use of a free and undercooled liquid also allows these fabrications to obtain a certain reproductivity.

As an example, the industry of coated products, such as country frying pans implements cryogenic barrages, and here again it is important for these manufacturers to have nitrogen in its most liquid form possible, and therefore of have free and uncooled nitrogen. In particular, we may be interested in EP-711 511 in the name of BOC, recommending the use of phase separation means (degassing) on the line connecting the reservoir to the consumer equipment.

A second technical solution would be to couple two tanks and use them alternately after filling and depressurization. The disadvantages of this solution are obviously the very strong manipulation induced and the mobilization of two tanks.

Another technical solution is to insert a heat exchanger (plate) just upstream of the point of use. In one of the channels of the exchanger (main circuit) circulates the liquid nitrogen to be cooled (typically originally at 3 bar and high temperature close to -185 ° C). In another way of the exchanger circulates a depressurized nitrogen, typically at a pressure of about 1 bar and at a low temperature, close to -196 ° C. It is the exchange between these two channels, co-current or against current, which will allow to sub-cool the nitrogen of the main circuit. But the control of the temperature here is difficult to control and to stabilize, in particular when the downstream consumer equipment operates in a discontinuous manner (for example a churn), forcing the exchanger to go through phases of reheating, re-cooling etc ... It will be noted moreover that in the uses that interest us here, this temperature control is all the more difficult to control that the pressures 30 and therefore the temperatures are close in both circuits. There is a risk of liquid overflow at the outlet of the low pressure circuit and thus overconsumption of liquid nitrogen. Recall in a few words the explanation of this phenomenon of overflow: the regulation of liquid nitrogen flow obtained is carried out on the control of the temperature of the cold gases obtained from the cooling fluid side. However, to obtain the maximum efficiency it is preferable that this temperature is as close as possible to the boiling temperature of the liquid nitrogen at atmospheric pressure, at about 2 ° C (for example, When the liquid nitrogen boils at -195.8 ° C. at atmospheric pressure, a cold gas control temperature of about 194 ° C. to 193 ° C. will be advantageously adopted. Thus, in view of the small difference between the two temperature values, liquid droplet entrainments may occur, and if the regulation does not react correctly or quickly enough, the liquid can invade the exchanger very quickly and overflow to outside, lost fluid that does not participate in the heat exchange.

It is also possible to consult the document WO2004 / 00 5791 in the name of the Applicant, who recommends varying the pressure of the gas at the top of the tank according to the operating state of this tank (consumption phase of the downstream user installation, waiting phase, or supply phase of the reservoir in cryogenic liquid), and which precisely advocates according to one of its embodiments, venting the reservoir during the waiting periods. In other words, when the tank is not loaded for withdrawal and will not be a priori for a significant period of time, for example several hours (for example at night), a control unit controls the opening of the tank. a vent valve of the upper part of the tank. The gas pressure at the top of the tank then changes from a storage value to a value substantially equal to atmospheric pressure (residual pressure of a few hundred grams). Thus, by lowering the nitrogen storage pressure in this way, the enthalpy variation of the latter tends to increase, which amounts to having a lower temperature fluid than when it was under pressure. The fluid thus stored during these periods of non-use of the reservoir therefore has a lower temperature than usual, guaranteeing a better cryogenic quality in terms of available frigories. And in fact, a rapid re-pressurization - using for example its own atmospheric heater or other.- allows to use the destabilized liquid (under cooled).

Nevertheless this solution is not without drawbacks, this setting necessarily leads to losses, and moreover the paradox of this procedure lies in the need to re-pressurize to be able to use the nitrogen, thus to bring in the heat. The experience of this solution has notably demonstrated a vaporization of 4 to 9% of the stored volume. This vaporization is not valued, the cost directly impacts the user site. In summary, two major disadvantages of this venting solution are deduced: 1) the use of non-recoverable nitrogen for the repressurization. 2) The entry of a hot gas into the storage for the depressurization and the creation of thermal bridge.

In this context, one of the objectives of the present invention is to propose a new method of supplying a user site with cryogenic free or sub-cooled liquid, avoiding the drawbacks of the prior art and making it possible to stabilize the pressures. injection (ie availability) of the cryogenic liquid.

As will be seen in more detail in the following, the method according to the present invention is based on the following principles: - we have a heat exchanger but which is here immersed in a bath of liquid nitrogen, which one bath can control the level (one could use another cryogenic liquid but it is conceivable that the liquid nitrogen is the most advantageous, especially in terms of cost of implementation). The heat exchanger, preferably a plate heat exchanger, (as will be seen below other types and configurations of exchanger are possible according to the invention such microchannel exchangers) is disposed in an isothermal vessel or other Dewar and immersed in liquid nitrogen which ensures the liquid nitrogen which will pass therein a liquid nitrogen temperature undercooled, substantially equal to the temperature of the liquid nitrogen at atmospheric pressure (-196 ° C.). VS). - There is no risk of overflow of liquid nitrogen, the level of the bath being controlled. - By way of illustration, according to one of the possible embodiments, there is a "direct" solenoid valve (A in appended FIG. 2) which ensures the first filling of the bath of liquid nitrogen to a given level, predefined and there is a second solenoid valve (B in Figure 2 attached), with calibrated orifice, which puts the heat exchanger cold and once the exchanger cold ensures the maintenance of the liquid level of the bath.

The invention thus relates to a method for supplying sub-cooled cryogenic liquid of equipment consuming this fluid, from a storage tank, which reservoir, under a storage pressure greater than atmospheric pressure, cryogenic fluid in the liquid phase at the bottom of the tank and in the gaseous phase at the top of the tank, said tank being adapted to feed the consumer equipment with liquid drawn off at the bottom of the tank, as well as to be supplied from the outside with fluid, characterized in that: - a heat exchanger is available, immersed in a bath of cryogenic liquid, identical or not to the cryogenic liquid stored in said storage, preferably a bath of liquid nitrogen, - the level of the bath is monitored. a predetermined level; - The heat exchanger is passed through the cryogenic liquid from the storage tank before it arrives at the consumer equipment.

The invention also relates to an installation for supplying sub-cooled cryogenic liquid of equipment consuming this fluid, comprising a storage tank, which reservoir, under a storage pressure greater than atmospheric pressure, the cryogenic fluid in liquid phase at the bottom of the tank and in the gaseous phase at the top of the tank, said tank being adapted to supply the consumer equipment with liquid drawn off at the bottom of the tank, as well as to be supplied from the outside with fluid, the installation being characterized in that it comprises: a heat exchanger, immersed in a bath of cryogenic liquid, identical or not to the cryogenic liquid stored in said storage, preferably a bath of liquid nitrogen, means for controlling the level of the bath at a temperature of predetermined level; - A piping system, able to pass through the heat exchanger cryogenic liquid from the storage tank before arriving at the consumer equipment.

Furthermore, the invention can adopt one or more of the following characteristics: the heat exchanger is a plate heat exchanger; - It has a phase separator, installed upstream of the exchanger, in the path of the cryogenic liquid to be cooled; the exchanger is integrated in the cryogenic bath of a phase separator.

Other characteristics and advantages will emerge from the following description, examples of embodiments of the invention, made in particular with reference to the appended figures: FIG. 1 is a schematic representation of a traditional storage tank of a cryogenic liquid; FIG. 2 illustrates one of the embodiments of the invention. - Figure 3 illustrates an embodiment of the invention wherein a plate heat exchanger is immersed in the cryogenic liquid bath of a phase separator. - Figure 4 illustrates an embodiment of the invention wherein the heat exchanger is integrated in the bath of a phase separator (exchange with the surface of a coil immersed in the bath).

Figure 1 allows to visualize and explain the structure and the traditional operation of a storage tank of a cryogenic liquid. FIG. 1 shows the presence of a nitrogen tank 1, containing liquid nitrogen at the bottom, also called "tank", and nitrogen gas at a pressure of about 2 bars at the top, called also "head". The level of liquid inside the tank is marked by the reference N. The bottom of the tank 1 is connected to a consumer equipment 2, for example a freezing tunnel, via a connecting pipe 3 provided with a closing valve. 4. The point of connection of line 3 to the tank 1, which is noted P, is commonly called "point of draw". The tank 1 comprises means 5 for pressurizing the gas at the top of the tank. These means 5 comprise a line 6 connecting the bottom of the tank to its top, and is provided, upstream to downstream, of a device 7 for measuring the pressure of the liquid nitrogen, for example a manometer, a closing valve 8 (preferably a solenoid valve) and a vaporizer 9. The tank 1 also comprises means 10 for venting the gas at the top of the tank. These means 10 comprise a line 11 for outward evacuation, provided upstream downstream of a manometer 12, a closure valve 13 and possibly an air exhaust member, not shown, commonly called "silent". A unit 15, controlling the means 5 for pressurizing the top and the means 10 for venting the top, is connected, for example by electrical connections, on the one hand to the pressure measuring devices 7 and 12, and on the other hand to the valves 8 and 13. The control unit 15 is in this way adapted, on the one hand, to know, continuously or at regular intervals, the liquid nitrogen pressure at the bottom of the reservoir 1 and nitrogen gas at the top of the tank and, secondly, to compare the value of the bottom pressure to a predetermined value selected, modifiable by the user. The unit 15 is also able to control the opening, total or partial, as well as the closing of the valves 8 and 13 so as to regulate the pressures of the bottom and the top of the tank 1.

Means 16 for supplying nitrogen are also provided, so as to regularly, possibly continuously, supply liquid nitrogen to the tank 1. In a conventional manner, these means 16 comprise a feed pipe 17 through the bottom of the tank 1 allowing a filling said "source", and possibly a feed pipe 18 by the top of the tank for filling said "rain". The tank 1 also comprises an overflow member 19 known per se, for limiting the height of the liquid inside the tank. In the single figure, the level of the liquid N is represented at its maximum. By way of illustration, if we consider the example of a tank of 50000 liters, giving rise to a height N of about 10 meters, this creates in such a case a pressure differential between the top and the bottom of the tank about 0.6 bar.

Figure 2 provides a better understanding of the invention through an implementation mode. The following describes a mode of operation of the installation of Figure 2, which will better identify the constituent elements. - Start-up phase: The solenoid valve A opens to bring the liquid nitrogen bath level. It is controlled by the level sensor on a first threshold. The exchanger is completely immersed in liquid nitrogen at atmospheric pressure The solenoid valve B is also controlled by the level sensor on a second threshold. A calibrated orifice is placed at the outlet of the solenoid valve B to adjust the flow of liquid nitrogen to the need for heat transfer and allow the maintenance of the liquid nitrogen level. The inside of the exchanger is put in cold by the admission of liquid, which enters at 23 and leaves at 24. A porous placed at the exit of this way 24 avoids the splashing, the eddies, and facilitates the regulation of level. The liquid nitrogen height above the exchanger will be sufficient to compensate for the various thermal loads of the system and prevent the top of the exchanger is no longer immersed. The nitrogen gas generated by the cooling and expansion of the liquid nitrogen is removed by the gas outlet shown in the figure "S".

Operation phase: When the downstream user station is in demand for liquid nitrogen, "hot" liquid enters the exchanger (via 22). The liquid nitrogen in the cold side exchanger and in the bath will logically vaporize causing a drop in the level of the bath, the solenoid valve B will then open to maintain the exchanger cold and maintain the liquid nitrogen level in the bath. The liquid nitrogen level will go up to the high threshold and the solenoid valve B will close. In the case where the continuous level to be lowered, solenoid valve B remains open. The solenoid valve A of larger flow opens to bring the level to the first threshold then it closes. The solenoid valve B is always open, it will close when the liquid nitrogen level reaches the high threshold.

Figure 3 illustrates an embodiment of the invention wherein a plate heat exchanger is immersed in the cryogenic liquid bath of a phase separator. A single equipment then provides phase separation and undercooling. The gas is separated in the phase separator: It enters the free liquid into the inlet 31 of the exchanger, it comes out under cooling at 32 to go to the user station. The inlet 31 is thus taken directly into the bath of the phase separator.

The undercooled liquid is taken in the phase separator bath (at the sampling point P). It is expanded by the regulating valve to a pressure as close as atmospheric pressure to benefit from the maximum of subcooling, and from there it passes through the exchanger (33, 34) and yields its frigories by vaporization to the liquid to cool.

FIG. 4 illustrates, for its part, a different embodiment of the "exchanger" according to the invention since here the exchanger is "integrated" in the bath of a phase separator: the exchange takes place with the surface of a simple coil (coiled tube) immersed in a bath of cryogenic liquid (here of liquid nitrogen).

The bath is supplied with liquid nitrogen from the main reservoir (not shown but located upstream of the valve on the left of the figure).

A coil is immersed in this bath as it is shown in the figure. The sub-cooling of the liquid nitrogen of this bath takes place by heat exchange with the outer surface of the coil since liquid nitrogen is taken from the liquid bath of the phase separator at the point P, it cools the bath through the coil and spring in gaseous form 47. To maintain the pressure artificially in the phase separator, can be injected in 45 nitrogen gas from the main reservoir (as has been said, not shown but located upstream of the valve on the left of the figure) or alternatively use a heater (as shown) which takes liquid at 44, vaporizes it and reinjects it at 45 in gaseous form. The outlet 43 is used to collect the liquid undercooled to direct it to the user station in demand. The mode of FIG. 4 thus represents a mode of implementation of the invention, in which the liquid nitrogen passes from the reservoir to a heat exchanger immersed in a bath (consisting of the same fluid, serpentine surface / bath), and there is taken to feed the user station downstream.

It will be understood that the advantage of an immersed (FIG. 3) or integrated (FIG. 4) heat exchanger at the bath of a phase separator is that it is not necessary to isolate the exchanger and that it does not need to be isolated. There is thus no external heat loss of this exchanger. -----------------------------------------

Claims (7)

REVENDICATIONS1. Method for supplying cryocene liquid undercooled of a device consuming this fluid, from a storage tank, which reservoir, under a storage pressure greater than atmospheric pressure, the cryogenic fluid in the liquid phase at bottom of the tank and in the gaseous phase at the top of the tank, said tank being adapted to supply the consumer equipment with liquid drawn off at the bottom of the tank, as well as to be supplied from the outside with fluid, characterized in that: has a heat exchanger, immersed in a bath of cryogenic liquid, identical or not to the cryogenic liquid stored in said storage, preferably a liquid nitrogen bath, - one carries out the control of the level of the bath to a predetermined level; - The heat exchanger is passed through the cryogenic liquid from the storage tank before it arrives at the consumer equipment. 2. Installation for supplying cryogenic liquid of equipment consuming this fluid, comprising a storage tank, a tank which contains, at a storage pressure greater than atmospheric pressure, the liquid phase cryogenic fluid at the bottom of the tank and in the gaseous phase at the top of the tank, said tank being adapted to supply the consumer equipment with liquid drawn off at the bottom of the tank, as well as to be supplied from the outside with fluid, characterized in that it comprises: an exchanger thermal, immersed in a bath of cryogenic liquid, identical or not to the cryogenic liquid stored in said storage, preferably a bath of liquid nitrogen, - bath level control means at a predetermined level; - A piping system, able to pass through the heat exchanger cryogenic liquid from the storage tank before arriving at the consumer equipment. 3. Installation according to claim 2, characterized in that the heat exchanger is a plate heat exchanger. 4. Installation according to claim 2 or 3, characterized in that it comprises a phase separator, installed upstream of the exchanger and the bath, in the path of the cryogenic liquid to be cooled. 5. Installation according to claim 2 or 3, characterized in that the exchanger is immersed in the cryogenic liquid bath of a phase separator. 6. Installation according to claim 2 or 3, characterized in that the exchanger is made in the following way: it has a coil immersed in the cryogenic liquid bath of a phase separator, the heat exchange being achieved between the cryogenic liquid coming from the reservoir and supplying the bath of the separator, and the surface of the submerged coil, a coil in which cryogenic liquid taken from the bath is circulated, from which it leaves in gaseous form to be discharged to the outside . 7. Installation according to claim 2 or 3, characterized in that it comprises a first so-called "direct" solenoid valve capable of ensuring the first filling of the bath of cryogenic liquid to a given level, predefined, and a second solenoid valve coupled with a calibrated orifice in its downstream, able to allow the cooling of the heat exchanger and once the exchanger cold to ensure the maintenance of the cryogenic liquid level of the bath.