EP2863103A2 - Device and method for supercooling carbon dioxide - Google Patents

Abstract

According to the invention, the liquid carbon dioxide to be supercooled is brought into thermal contact with a cooling bath (18) within a container (2) in a heat exchanger (4). The cooling bath (18) comprises a cold mixture of solid carbon dioxide (20) and a carrier medium. When exchanging heat with the liquid carbon dioxide to be subcooled, a part of the carbon dioxide present in the cooling bath (18) sublimes and is subsequently removed as carbon dioxide gas. The carbon dioxide sublimated from the cooling bath (18) is replaced by removing a partial stream or a predetermined amount of the liquid carbon dioxide to be supercooled and depressurized on a flash nozzle (12) arranged in the tank and the carbon dioxide snow produced during the expansion is added to the cooling bath (18). is added. In this way, the cooling bath (18) can be kept at a constant temperature over an almost arbitrarily long period of time and used to subcool the liquid carbon dioxide.

Description

The invention relates to a device for supercooling liquid carbon dioxide, comprising a conduit for the liquid carbon dioxide to be supercooled, which is flow-connected to a heat exchanger, which is arranged in a thermally insulated container, and is in heat exchange with a cold bath received in the container. The invention further relates to a corresponding method.

Low-boiling liquefied gases can only be kept liquid by means of particularly good insulation of the storage tanks and the pipelines. Even the slightest heat or frictional heat can lead to partial evaporation, depending on the boiling state. The boiling bubbles collect, except in the headspace of the storage container, e.g. also in vertical pipe bends. These so-called gas cushions in the supply line lead to disruptions at the sampling point when a reproducible dosing of the liquefied gas is required. It is easy to see that due to the density difference between the gas and the liquid, different amounts flow through an opening of the same size at equal time intervals. In order to have reliably present pure liquid before the metering, the liquefied gas is "undercooled" thus brought to a temperature which is lower than the boiling point at the prevailing pressure.

Such supercooling can be accomplished, for example, by the isobarically cooling the liquefied cryogenic medium by means of an electric cooling unit to a temperature below its boiling point so that partial evaporation does not occur during circulation in a loop system due to heat radiation and friction losses. However, the necessary aggregates are very expensive to buy and operate due to their high power requirements.

As simple as effective alternative is a supercooling means own medium into consideration, as for example from the DE 2 929 709 A1 , of the US 5,123,250 A1 and the US 5 214 925 A1 are known. The devices described therein for subcooling cryogenic liquids, such as Liquid nitrogen, liquid carbon dioxide or freon, by means of own medium comprise a heat exchanger through which the under-cooling liquid cryogenic medium flows under pressure, which is arranged in an insulated container. The heat exchanger is designed as a cooling coil, which is surrounded by a bath of the same liquid cryogenic medium. In the headspace of the container, a gas outlet valve is provided, which ensures that the pressure in the container corresponds to the ambient pressure. Since the pressure of the liquid bath is reduced compared to the pressure of the gas to be undercooled, its boiling temperature is below the boiling point of the gas to be supercooled. By the bath, therefore, the cryogenic medium in the cooling coil is supercooled and there already occurred gas bubbles are liquefied again. Incidentally, such apparatuses are also suitable as gas phase separators. By varying the pressure conditions, the temperature difference, by which the liquid to be cooled is to be cooled, can be adjusted within a wide range; in particular, the pressure of the cooling bath can also be brought to a value below ambient pressure (eg 1013 mbar) by means of a vacuum pump and applied thereto Way temperatures of supercooled cryogenic medium can be achieved, which are below its boiling point at ambient pressure.

When subcooling of liquid carbon dioxide, however, the problem arises that this merges with a relaxation to a pressure of below 5.18 bar in a mixture of gaseous carbon dioxide and carbon dioxide snow. It is no longer possible in the known subcoolers to maintain a liquid cooling bath below this pressure when using carbon dioxide, which limits the extent possible to undercool carbon dioxide with the known from the prior art devices.

The invention is therefore based on the object to provide a device for subcooling of liquid carbon dioxide, with a simple way, an increased bandwidth of the temperature setting can be achieved.

This object is achieved by a device having the features of patent claim 1 and by a method having the features of patent claim 7.

A device according to the invention is characterized in that the cold bath is formed from a cold mixture of solid carbon dioxide and a liquid or pasty carrier medium and that the line for the liquid carbon dioxide to be sub-cooled is flow-connected to a flash-out nozzle in the container. To generate the cryogen mixture, a partial stream or a certain amount of liquid carbon dioxide to be supercooled is removed before or after passing through the heat exchanger and expanded at the expansion nozzle to form carbon dioxide snow and carbon dioxide gas. The carbon dioxide snow produced in the process is admixed with the carrier medium already present in the container or still to be supplied thereto and / or completely or partially dissolved in it. As a result of the expansion, the temperature of the carbon dioxide drops drastically, for example to a value of -78 ° C. at an internal container pressure of 1 bar. In the admixture and / or redemption resulting endothermic effects, the temperature of the cold bath can additionally reduce and thus enhance the cooling effect of the cold bath, it should be ensured that the under-cooling liquid carbon dioxide by applying a sufficiently high pressure even after the heat transfer the cooling bath remains in the liquid state. Since the cooling bath is always in the liquid state, good heat transfer takes place at the heat exchanger surfaces of the heat exchanger through which the liquid carbon dioxide to be supercooled flows. Furthermore, the cold bath consisting of a mixture of a carrier medium with the carbon dioxide snow keeps its temperature constant until the carbon dioxide snow supplied is at least substantially sublimated. By supplying liquid carbon dioxide to the expansion nozzle and mixing the carbon dioxide snow produced by the expansion of this liquid carbon dioxide into the carrier medium, however, the sublimated carbon dioxide can be easily replaced, thus keeping the temperature of the cold bath constant over a virtually unlimited period of time.

An advantageous development of the invention provides that a branch line, which connects the line for the carbon dioxide to be cooled with the expansion valve, is equipped with a valve and a controlled system, by means of which the amount of the expansion valve supplied liquid carbon dioxide in Depending on a temperature measured at a temperature sensor temperature value for the cooling bath and / or for the supercooled liquid carbon dioxide can be controlled. In this embodiment, therefore, the temperature of the cold bath and / or the temperature of the supercooled liquid carbon dioxide is detected continuously or at predetermined time intervals, and depending on the difference between the measured temperature and a target temperature of the cold bath or supercooled liquid carbon dioxide the cooling bath fresh carbon dioxide fed.

Conveniently, the branch line leading to the expansion nozzle discharges downstream of the heat exchanger from the conduit for the liquid carbon dioxide to be supercooled, i. it is already supplied to the expansion valve supercooled liquid carbon dioxide, whereby the proportion of the resulting during the expansion of solid carbon dioxide increases compared to the gas content.

A yet further advantageous embodiment of the invention is characterized in that the container is equipped with an exhaust pipe in which a pressure valve is arranged, by means of which the pressure within the pressure-resistant in this case formed container to a predetermined value below the triple point pressure of carbon dioxide (5, 18 bar) is adjustable. By setting the pressure to which the liquid carbon dioxide is released, the temperature of the solid carbon dioxide is set at the same time after the relaxation. Since the temperature of the solid carbon dioxide co-determines the temperature of the entire cryogen mixture, the temperature of the cryogen can be adjusted in a wide range in this way. Incidentally, by connecting a suction pump to the exhaust pipe, it is also possible to achieve pressures in the tank of less than 1 bar and thus temperature values below -78 ° C.

The preferred carrier medium used in the cold bath is an alcohol or an organic carrier medium. In the context of the present invention, a "carrier medium" is to be understood as meaning a substance which remains in a liquid or pasty state during mixing and / or when solid carbon dioxide is redissolved. In particular, it does not have to be a substance in which the solid carbon dioxide is soluble. Furthermore, the carrier medium can in turn consist of a mixture of two or more liquids. In particular, dual-substance mixtures may be used, in which the temperature of the mixed with carbon dioxide refrigerant mixture depend on the proportion of the respective substances. Thus, for example, the temperature of a mixture consisting of solid carbon dioxide and ethylene glycol and ethanol depending on the proportion of ethanol over that of ethylene glycol in the support medium between - 78 ° C (at 100% ethanol) and -17 ° C (at 100% ethylene glycol) varies become. The same applies to a cryogen mixture of solid carbon dioxide and o-xylene and p-xylene, in which the temperature of the cryogen can be varied by adjusting the ratio of o-xylene to p-xylene between -78 ° C and -26 ° C.

In preferred embodiments of the invention, the carrier medium contains one or more of the following: ethanol, ethylene glycol, 3-heptanone, acetonitrile, cyclohexanone, diethylcarbitol, chloroform, acetone, diethyl ether, o-xylene, p-xylene, m-xylene, benzyl alcohol, n Octane, isopropyl ether, tetrachlorethylene.

The object of the invention is also achieved by a method for subcooling liquid carbon dioxide with the features of claim 7.

The liquid carbon dioxide is fed under a pressure of at least 5.18 bar arranged in a thermally insulated container heat exchanger and placed there with a container disposed in the cooling bath in thermal contact and subcooled, that is brought to a temperature below its boiling temperature at the pressure in which it is passed through the heat exchanger. For example, the liquid carbon dioxide previously present at a temperature of -20 ° C. at a pressure of approximately 20 bar after the heat exchange with the cooling bath has a temperature between -25 ° C. and -30 ° C. and a substantially constant pressure of approx 20 bar. The cold bath is formed from a cold mixture of solid carbon dioxide and a carrier medium. When exchanging heat with the liquid carbon dioxide to be supercooled, the solid carbon dioxide in the cooling bath sublimates successively and enters a gas phase above the cooling bath, from which it is subsequently removed. The sublimated from the cold bath solid carbon dioxide is replaced by a partial flow or a predetermined amount of subcooled liquid carbon dioxide is removed before or after passing through the heat exchanger and is expanded at a arranged in the container expansion nozzle, wherein the resulting during the expansion solid carbon dioxide is then fed to the cooling bath and mixed with this and / or dissolved in this. During the heat exchange, the carbon dioxide to be cooled always remains in the liquid state.

An advantageous development of the method according to the invention provides that the temperature in the cooling bath and / or the temperature of the supercooled liquid carbon dioxide after passing through the heat exchanger detected continuously or at predetermined time intervals and the supply of liquid carbon dioxide to the expansion nozzle in dependence on the measured temperature is regulated. In this way, therefore, due to the heat exchange with the liquid carbon dioxide to be supercooled gradually sublimated from the cooling bath solid carbon dioxide by an equal amount of solid carbon dioxide, which was formed at the expansion, replaced and kept the cold bath permanently at a constant temperature.

Reference to the drawings, an embodiment of the invention will be explained in more detail. The only drawing ( Fig. 1 ) schematically illustrates an embodiment of the device according to the invention.

The inventive device 1 for subcooling liquid carbon dioxide comprises a pressure-resistant, closed container 2 with thermally insulated walls 3, within which a cooling coil 4 is arranged. The cooling coil 4 is used for cooling of liquid, pressurized carbon dioxide, which is stored from a pressure vessel, not shown here, for example, a standing tank in which the liquid carbon dioxide at a pressure of 20 bar and a temperature of -20 ° C, via a thermally insulated supply line 6 is guided into the cooling coil 4 and leaves the cooling coil 4 in the direction of a consumer not shown here via a likewise thermally insulated lead 7. In the head space 8 of the container 2 opens a supply line 9 for supplying a liquid carrier medium and an exhaust pipe 10 for discharging gaseous carbon dioxide into the container 2 a. From the exit 7 branches, downstream of the container 2 and the cooling coil 4, a branch line 12 from. The branch line 12 leads into the head space 8 of the container 2 and opens at a relaxation nozzle 13 into the interior of the container 2. In the branch line 12, a flow valve 14 is arranged that is connected via a control controller 15 with a temperature sensor 16 in the discharge line 7 in data connection.

During operation of the device 1 is in the container 2 to the level of a level 17, a cooling bath 18, consisting of a liquid carrier medium, for example ethanol or acetone, and mixed therein solid carbon dioxide. Through the line 6 liquid carbon dioxide is introduced, passes through the cooling coil 4 and is then fed via the line 7 to a consumer. The pressure inside the container 2 is smaller than the pressure at which the liquid carbon dioxide is introduced through line 6, for example, the pressure in the line 6 is between 10 and 100 bar and the internal pressure of the container 1 bar. In this case, there is a thermal contact with the cooling bath 18 via the walls of the cooling coil 4. In order to produce the cooling bath 18, the support medium is introduced via line 9 into the container 2. At the same time a predetermined amount of flowing in the derivative 7 liquid carbon dioxide is removed and fed via the branch line 12 of the expansion nozzle 13, where it relaxes on the container internal pressure and thereby passes into a mixture of gaseous carbon dioxide and carbon dioxide snow 20. The carbon dioxide snow 20 mixes with the carrier medium already present in the container 2 to form a cryogenic mixture, wherein a mixer 19 ensures a uniform mixing of the two substances. The resulting during the expansion of the liquid carbon dioxide gaseous carbon dioxide is removed via the exhaust pipe 10 and optionally supplied to a further use. In this case, arranged at the entrance of the exhaust pipe 10 filter 21 ensures that carbon dioxide snow 20 is not entrained or only in small quantities from the flow of gaseous carbon dioxide and escapes through the exhaust pipe 10.

The cooling bath 18 has a temperature corresponding to the cooling mixture. For example, for a mixture of ethanol and solid carbon dioxide, the temperature is -78 ° C, for a mixture of acetone and solid carbon dioxide up to -86 ° C, for one of diethyl ether and solid Carbon dioxide existing cold mixture up to -100 ° C. As the liquid carbon dioxide flows through the cooling coil 4, its walls act as heat transfer surfaces, via which heat is released from the liquid carbon dioxide in the cooling coil 4 to the cooling bath 18. There, the heat absorption leads to the sublimation of present in the cooling bath 18 in dissolved or mixed form solid carbon dioxide. The resulting in the sublimation carbon dioxide gas is removed via the exhaust pipe 10. As long as there is still solid carbon dioxide in the cooling bath 18, the temperature of the cooling bath 18 remains substantially constant, then it rises. The increase in temperature in the cooling bath 18 leads to an increase in temperature of the liquid carbon dioxide in line 7, which in turn is detected at the temperature sensor 16. Due to the deviation from a setpoint value of the temperature stored in the control unit 15, the valve 14 is opened and liquid carbon dioxide is supplied via the branch line 12 to the expansion nozzle 13, the carbon dioxide snow 20 formed during the expansion being admixed with the cooling bath 18. In this case, the temperature of the cooling bath 18 and thus of the liquid carbon dioxide in line 7 decreases again. When falling below a predetermined temperature in line 7, the valve 14 is closed. When operating the device 1, moreover, care must be taken to ensure that the pressure in the line 7 is sufficiently high to keep the supercooled carbon dioxide in the liquid state; if appropriate, the line 6 can be preceded by a device for increasing the pressure.

By a suitable choice of the carrier medium in the container 2, the temperature of the cryogen in the cooling bath 18 and thus the supercooling of the conduit 7 durströmenden liquid carbon dioxide can be adjusted. In particular, endothermic effects in the solution of carbon dioxide in the carrier medium can also be used, and temperatures of below -78 ° C. (at a pressure of 1 bar in the head space 8) can thus be achieved. A very accurate setting of a cold bath temperature is achieved in particular by the choice of a binary mixture as a carrier medium, wherein the respective mixing ratio determines the temperature of the cryogen.

Another way to influence the temperature of the cold bath, is the pressure in the headspace 8 of the container 2 to a predetermined value of below 5.18 bar, since the temperature of the carbon dioxide snow, after its relaxation, depends on the pressure to which it relaxes. The pressure in the headspace 8 can be maintained at a value between 1 bar and 5.18 bar, for example, by a corresponding adjustment of a valve disposed in the exhaust line 10 pressure valve 22. Pressure values below 1 bar can be achieved by connecting a suction pump, not shown here, to the exhaust pipe 10. The temperature of the carbon dioxide snow and thus of the cryogen can thus be varied to values between about -58 ° C and -100 ° C.

With the aid of the device according to the invention, the liquid carbon dioxide in the conduit 7 is very efficiently supercooled, wherein a very good heat transfer to the walls of the cooling coil 4 takes place on the surrounding liquid cooling bath 18. The introduction of a portion of the liquid carbon dioxide in the cooling bath 18 via the branch line 12 thereby enables an automatic refreshment of the cooling bath 18 due to a controlled supply of carbon dioxide snow. Incidentally, it is also conceivable within the scope of the invention to recirculate the supercooled liquid carbon dioxide from line 7 back to the carbon dioxide tank connected to line 6, whereby supercooled carbon dioxide can be stored in this tank.

LIST OF REFERENCE NUMBERS

1.

contraption

Second

container

Third

walls

4th

cooling coil

5th

-

6th

supply

7th

derivation

8th.

headspace

9th

feed

10th

exhaust pipe

11th

-

12th

branch line

13th

expansion nozzle

14th

Valve

15th

regulatory control

16th

Temperature sensor

17th

level

18th

cold bath

19th

mixer

20th

carbon dioxide snow

21st

filter

22nd

pressure valve

Claims (8)

Device for subcooling liquid carbon dioxide, comprising a conduit (6, 7) for the liquid carbon dioxide to be sub-cooled, which is in fluid communication with a heat exchanger (4) arranged in a thermally insulated container (2) and with a reservoir (2). 2) recorded cold bath (18) is in heat exchange,
characterized,
in that the cooling bath (18) is formed from a cold mixture of solid carbon dioxide (20) and a carrier medium, and the line (6, 7) for the liquid carbon dioxide to be supercooled is flow-connected to a flash-out nozzle (13) opening out into the tank. Apparatus according to claim 1, characterized in that the expansion nozzle (13) via a branch line (12) with the conduit (6, 7) for the undercooling liquid carbon dioxide is flow-connected, which branch line (12) with a valve (14) and a controlled system (15) is equipped, by means of which the amount of the expansion valve (13) supplied liquid carbon dioxide in dependence on a measured at a temperature sensor (16) temperature value for the cooling bath (18) and / or for the supercooled liquid carbon dioxide is controllable. Apparatus according to claim 1 or 2, characterized in that the branch line (12) downstream of the heat exchanger (4) to the conduit (6, 7) is fluidly connected to the subcooling liquid carbon dioxide. Device according to one of the preceding claims, characterized in that the container (2) is equipped with an exhaust pipe (10) in which a pressure valve (22) for setting a predetermined pressure in the container (2) is arranged. Device according to one of the preceding claims, characterized in that the carrier medium used in the cooling bath (18) is an alcohol or an organic carrier medium. Device according to one of the preceding claims, characterized in that the carrier medium of the cold bath (18) contains one or more of the following substances: ethanol, ethylene glycol, 3-heptanone, acetonitrile, cyclohexanone, diethylcarbitol, chloroform, acetone, diethyl ether, o-xylene, p-xylene, m-xylene, benzyl alcohol, n-octane, isopropyl ether, tetrachlorethylene. A process for subcooling liquid carbon dioxide, wherein the liquid carbon dioxide from a tank to a in a thermally insulated container (2) arranged heat exchanger (4) and brought there with a in the container (2) arranged cooling bath (18) in thermal contact and is cooled by
characterized,
in that the cooling bath (18) is formed from a cold mixture of solid carbon dioxide (20) and a carrier medium and during the heat exchange with the liquid carbon dioxide to be subcooled at least part of the solid carbon dioxide (20) present in the cooling bath (18) is sublimated and subsequently removed, then a partial flow of the liquid carbon dioxide removed and on a in the container (2) arranged expansion nozzle (13) relaxed and the resulting solid carbon dioxide is fed to the cooling bath (18). A method according to claim 7, characterized in that the temperature in the cooling bath (18) and / or the temperature of the supercooled liquid carbon dioxide after passing through the heat exchanger (4) detected continuously or at predetermined intervals and the supply of liquid carbon dioxide to the expansion nozzle (13). is regulated as a function of the measured temperature.

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