System to determine ratio at least one impurity in cryogenic liquid

Abstract

To provide a system for determining a content of impurities sample-collected from a system for separating gas from air and dissolved in cryogenic liquid, and a corresponding determination method.SOLUTION: A determination system 2 for determining a ratio of impurities dissolved in cryogenic liquid comprises: a container 3 for accepting an initial volume of cryogenic liquid; evaporation means 84 for evaporating the initial volume of cryogenic liquid until a certain volume of residual cryogenic liquid with impurities condensed therein can be obtained (the evaporation means is arranged at a bottom of the container and has a heating surface for evaporating the cryogenic liquid); and means for determining the ratio of impurities in the residual cryogenic liquid. The evaporation means comprises a liquid intake duct, and a liquid discharge duct. The evaporation means gives a thermal siphoning effect between the intake duct and the discharge duct, and the evaporation means is configured to keep the heating surface in a wet state using a certain volume of the residual cryogenic liquid.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to the chemical industry and to a system for determining the proportion of at least one impurity dissolved in a cryogenic liquid sampled, for example from a system for separating gases from air, and to a corresponding determination method.

[0002] The system for separating gases from air comprises a low pressure and a medium pressure distillation column for separating various components from the air. Although the air is purified before entering the columns, impurities remain in the feed air of the columns and are especially concentrated in the oxygen vaporizer in one of the columns. In particular, most of these impurities have a vapor-liquid equilibrium coefficient such that almost all of these impurities remain in the liquid phase of the vaporizer and very little of the impurities leave the vaporizer again in the vapor phase. Therefore, the proportion of impurities in the liquid phase increases with successive vaporization cycles and eventually accumulates in the form of liquid or solid deposits on the aluminum matrix of the vaporizer.

[0003] The absorption process carried out before the distillation makes it possible to remove from the feed air of the column heavy hydrocarbons, that is to say those with more than four carbon atoms and those with unsaturated bonds. Impurities that are barely or not captured at all by these absorption processes are in particular light hydrocarbons and propane. Light hydrocarbons with one or two carbon atoms have a high solubility in oxygen and therefore do not give rise to a pure phase capable of reacting with the oxygen in the vaporizer. However, propane has a relatively low solubility in oxygen and therefore can give rise to a pure phase when in contact with the liquid oxygen in the vaporizer, which can create an explosive situation, especially when the energy released by this impurity is sufficient to trigger the combustion of the aluminum matrix of the vaporizer.

[0004] Moreover, even when the impurities present in the vaporizer do not react with oxygen, it accelerates the accumulation of all other impurities, and therefore also the accumulation of impurities that react with oxygen. This is especially true for carbon dioxide and nitrous oxide, which have a freezing temperature above the operating temperature of liquid oxygen. It can happen that these impurities are not captured by the absorption process and give rise to a solid phase in the liquid oxygen of the vaporizer, which can block the vaporization ducts of the vaporizer. This mechanism, called "dead end boiling", accelerates the concentration of all impurities contained in the liquid being vaporized, especially hydrocarbons, and therefore increases the risk of combustion of the vaporization matrix.

[0005] Therefore, in order to maintain the maximum permissible amount of impurities in the oxygen bath of the distillation column and / or vaporizer and to ensure the operational safety of the system for separating gases from air, it is necessary to monitor the proportion of impurities entering there.

[0006] One problem to be overcome is that the levels of the impurities to be measured are extremely low, especially given the very low solubility and gas-liquid equilibrium coefficients for carbon dioxide and nitrous oxide. In order to be able to control the maximum permissible amount of impurities, it is necessary to measure proportions of less than 100 ppb (parts per billion), preferably between 10 and 50 ppb. These measurements may be continuous or carried out at a frequency that makes it possible to take action well before a critical proportion of the impurity is reached.

[0007] Currently, techniques for determining the content of impurities in the air to be distilled or in the oxygen of the vaporizer use complex equipment and require considerable operating skills. These techniques use only gas analyzers, which require sampling the cryogenic liquid entering the distillation column or vaporizer and vaporizing it in a special device to be able to then analyze it. Depending on preference, the samples are taken at the vaporizer inlet, or at the vaporizer outlet, or between the various vaporization stages as needed, considering that the vaporizer is the critical point of the system for separating gas from air.

[0008] These prior art techniques involve complete vaporization of the sample and its impurities to obtain the gas to be analyzed, where the percentage of impurities is the same as that in the sample of the cryogenic liquid, and the vaporization is carried out under conditions that prevent the accumulation of impurities in the device for taking the sample or vaporizing the sample, so as not to distort the measurements obtained by the gas analyzer.

[0009] In order to facilitate the measurement of the proportion of impurities in a cryogenic liquid, the inventors have developed an apparatus and a method for analyzing this proportion, described in documents FR3066596 and FR3066597. This apparatus allows the vaporization of a large sample of the cryogenic liquid to be analyzed in a container, the gas so vaporized being discharged in an open circuit, and a residual amount of cryogenic liquid being retained at the end of this vaporization, in which the impurities of the sample are concentrated. The container is then kept closed and the residual amount of cryogenic liquid is completely vaporized in the container, thereby obtaining a gas with a much higher proportion of impurities than that of the initial sample and therefore much easier for the gas analyzer to measure.

[0010] This innovative approach requires that the impurities whose concentration is to be measured have a lower volatility than the cryogenic liquid in which they are contained, but also that the amount of impurities that evaporates during open circuit vaporization is negligible compared to the amount of impurities that remains in the residual cryogenic liquid. These conditions depend on the values ​​of the thermodynamic equilibrium between the impurities and their solvent, which themselves depend on the vaporization pressure and temperature. The lower the vaporization pressure, the more favorable the vaporization is for the concentration of the impurities in the residual cryogenic liquid.

[0011] Based on these thermodynamic equilibrium values, the concentration of impurities in the residual cryogenic liquid should not present a problem. More specifically, depending on these values, when the residual cryogenic liquid is oxygen maintained at 15 bar, the amount of nitrous oxide or carbon dioxide or propane escaping as vapor during open circuit vaporization remains less than 2% of the amount of impurities remaining in the cryogenic liquid.

[0012] However, the inventors have noted that the amount of impurities escaping to the gas phase during open circuit vaporization can actually be much higher when the vaporized cryogenic liquid has regions of over-concentration of impurities, or when vaporization of the cryogenic liquid to dryness occurs on the walls of the device that transfer the heat of vaporization. In particular, if these walls are overheated, the thermodynamic equilibrium is distorted and a higher amount of impurities escapes to the gas phase. To avoid such over-heating, the concentration of impurities can be carried out at pressures higher than atmospheric pressure, but this does not prevent regions of over-concentration of impurities from forming in the liquid.

[0013] It should be noted that prior art techniques proposing total vaporization of a sample of a cryogenic liquid do not allow the cryogenic liquid to vaporize at the bubble temperature without the cryogenic liquid vaporizing to dryness locally on parts of the heated walls of the vaporization device and without causing deposits of impurities on these heated walls that impurify the gas formed by vaporization.

[0014] Finally, another problem with the approach proposed by the inventors is that it is necessary to vaporize 99% of the sample of the cryogenic liquid in an open circuit in order to significantly concentrate the impurities in the residual cryogenic liquid, with a concentration factor of 100 or more, which makes it difficult to have a heated wall to vaporize the cryogenic liquid to dryness during this vaporization step in an open circuit. This becomes even more difficult when the duration of the analysis cycle is limited to less than one hour to guarantee the safety of the system for separating gas from air. Specifically, the analyzer proposed by the inventors works in a cycle in which the container in which the sample is vaporized must be cooled after all the residual cryogenic liquid has been vaporized and delivered to the gas analyzer in order to receive another sample. Summary of the Invention

[0015] The present invention aims to overcome at least some of the above drawbacks by providing a system and method for determining the proportion of at least one impurity in a cryogenic liquid, as well as a system for separating gas from air, which allows the impurity to be concentrated in a portion of the liquid with a concentration factor that is not distorted by excessive vaporization of the impurity while it is concentrated in the portion of the cryogenic liquid, before vaporizing the portion of the liquid and delivering the resulting vapor to a gas analyzer.

[0016] For this purpose, the invention proposes a system for determining the proportion of at least one impurity dissolved in a cryogenic liquid, the at least one impurity being less volatile than the cryogenic liquid, the system comprising: a vessel capable of receiving an initial volume of cryogenic liquid; a vaporizing means for vaporizing an initial volume of the cryogenic liquid until a volume of residual cryogenic liquid is obtained in which the impurities are concentrated, the vaporizing means being disposed in a lower portion of the vessel and comprising a heated surface capable of vaporizing the cryogenic liquid; means for determining the proportion of impurities in the residual cryogenic liquid; The determination system is characterized in that the vaporization means comprises at least one cryogenic liquid intake duct and a plurality of cryogenic liquid exhaust ducts, the vaporization means being capable of creating a thermosiphon effect between the intake duct and the plurality of exhaust ducts, and the heated surface comprises an inner surface of the exhaust duct.

[0017] In other words, the vaporization means is configured to use a volume of residual cryogenic liquid to keep said heated surface wet.

[0018] According to the invention, the vaporization of an initial volume of cryogenic liquid, which allows the concentration of impurities in a volume of residual cryogenic liquid, is carried out without any significant amount of impurities escaping into the gas phase during this vaporization. In particular, the heated surface of the vaporization means which allows this vaporization is totally immersed or totally wetted by the volume of residual cryogenic liquid. There is therefore no evaporation to dryness of the cryogenic liquid, which may cause deposits on this heated surface. It is defined as allowing the vaporization of the cryogenic liquid and therefore releasing heat which allows this vaporization. The other surfaces of the vaporization means may be relatively hot compared to the cryogenic liquid, but do not vaporize it. The heated surface is for example formed in the lower part of the container.

[0019] The thermosiphon effect provided by the vaporization means agitates the cryogenic liquid and thus makes it possible to avoid the formation of regions of excessive concentration of impurities in the cryogenic liquid, which also avoids excessive vaporization of these impurities.

[0020] In the present invention, the exhaust duct allows the vaporization of the cryogenic liquid since its internal surface heats the cryogenic liquid to a temperature that allows this vaporization. The vaporization means are, for example, arranged in the lower part of the vessel so as to at least partially immerse the exhaust duct in a volume of the residual cryogenic liquid under the influence of gravity, and even the internal surfaces of the exhaust duct that are not immersed by the residual cryogenic liquid remain wet due to the circulation of the residual cryogenic liquid in the exhaust duct by the thermosiphon effect.

[0021] In one embodiment, the exhaust ducts, for example, surround the intake ducts. Furthermore, the exhaust ducts have a smaller diameter than the intake ducts. Distributing the cryogenic liquid in the exhaust ducts of smaller diameter in this way helps the cryogenic liquid to evaporate and keeps the heated surfaces of these ducts wet. The intake ducts and exhaust ducts preferably have a circular cross section, and the exhaust ducts are preferably evenly distributed angularly around the intake duct.

[0022] The vaporization means comprise, for example, a thermally conductive body of general cylindrical shape, over the height of which the inlet and outlet ducts are made and which emerge in the face of the body which defines the bottom of the container.

[0023] The bottom of the container forms part of the lower part of the container, for example having the same surface area as the face of the body corresponding to the cylindrical base of the body. This face is therefore flat, if we ignore the openings of the ducts appearing in this face. It is for example flush with the lower part of the container, in order to allow the evaporation means to contain, by gravity, all the volume of the residual cryogenic liquid, which for example forms one percent or at least one percent of the initial volume of the cryogenic liquid. The ducts are therefore arranged vertically with respect to the ground and the face defining the bottom of the container is arranged horizontally.

[0024] The thermally conductive body is a good heat conductor, for example made of copper or aluminum, preferably forming a one-piece component, the surface of this body being for example smooth or textured, in particular the internal surface of the exhaust duct having for example a surface which improves the evaporative heat exchange coefficient and / or wetting of the wall by its unevenness, textured geometry, the presence of micro-fins or other means.

[0025] The body preferably comprises an insulating lining arranged between the intake duct and the exhaust duct, so that the cryogenic liquid in the intake duct remains at a fairly low temperature so as not to vaporize there and form deposits on the inner surface of the intake duct, the insulating lining being made, for example, from polytetrafluoroethylene (also known as "Teflon").

[0026] The vaporization means in particular comprise at least one electric heater arranged in a recess in the body close to the exhaust duct, for example a heating resistor or a heating cartridge arranged between the intake duct and the exhaust duct or around the exhaust duct.

[0027] Optionally, the vaporization means comprises a plurality of electric heaters in the form of heating cartridges, the body having a plurality of spaces arranged around the exhaust duct, the spaces housing the heating cartridges, the spaces not appearing in the face of the body defining the bottom of the container, the spaces extending parallel to the exhaust duct in the thermally conductive body and at a distance from it, but in sufficient proximity to one another to keep the internal surface of the exhaust duct at a temperature allowing the vaporization of the cryogenic liquid, the spaces being preferably evenly arranged in angular direction around the exhaust duct.

[0028] The spaces are separated, for example, by a number of recesses around the periphery of the body, over at least a portion of its height, which limit the mass of the body that is heated by the heating cartridge during the vaporization of the cryogenic liquid, but also the mass of the body that is cooled during a subsequent step for bringing the walls of the container to the filling temperature, which is necessary to carry out a new determination of the proportion of impurities in the new initial volume of cryogenic liquid, which must be able to be accurately evaluated when filling the container.

[0029] The recesses between the spaces for accommodating the heating cartridges are open on the outer cylindrical wall of the body, thereby giving the entire body a cylindrical shape, for example in the form of a barrel. The recesses make it possible to reduce the cost of the determination system according to the invention.

[0030] The body has a face opposite the face defining the bottom of the vessel, and the vaporization means in particular comprise a manifold arranged on the opposite face, fluidly connecting the inlet duct to the outlet duct, for example a copper component mounted on the opposite face of the body, comprising a fluid circulation chamber, the manifold being arranged vertically below the body so that it is always filled with residual cryogenic liquid and therefore does not allow deposits of impurities to form.

[0031] To cool the vessel between the two evaporation steps for the two separate determinations according to the invention, a jacket surrounds the evaporation means, the jacket being configured to contain a cooling liquid in contact with the evaporation means. Of course, other cooling means are conceivable.

[0032] The jacket is, for example, an annular jacket that surrounds at least the lower portion of a vessel, such as a cylindrical tank.

[0033] Optionally, the facing surface emerges from the jacket, and the inlet and outlet ducts extend from the surface defining the bottom of the container through the jacket. This makes it easy to power the heater or heaters. The facing surface has openings, for example, for inserting heating cartridges into the space. This also makes it easy to install and replace these cartridges.

[0034] Another subject of the invention is a system for separating gases from air by cryogenic distillation, comprising a system for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to the invention, means for taking samples of a fluid circulating in the separation system, means for liquefying the sampled fluid if it is a gas, and means for delivering the sampled, and optionally liquefied, fluid to a container so as to determine the proportion of its impurities.

[0035] Of course, the system and method for determining the content of at least one impurity dissolved in a cryogenic liquid of the present invention is applicable to other systems, for example systems for separating other types of gases, for example in which carbon dioxide is separated.

[0036] The present invention also relates to a method for determining the proportion of at least one impurity dissolved in a cryogenic liquid using a system for determining the proportion of at least one impurity according to the present invention, the at least one impurity being less volatile than the cryogenic liquid, the method comprising the steps of: filling the container with an initial volume of cryogenic liquid maintained at a fill temperature and a fill pressure that prevents vaporization of the cryogenic liquid during the filling step; vaporizing the cryogenic liquid by a vaporizing means which during the vaporizing step brings the cryogenic liquid to its vaporization temperature under a pressure of the cryogenic liquid which is equal to or lower than the filling pressure, and the gas resulting from the vaporization is discharged from the container; determining the proportion of impurities in the residual cryogenic liquid.

[0037] The charging temperature and the charging pressure are within the respective temperature and pressure ranges corresponding to the liquid state of the elements or compounds forming the cryogenic liquid, which is air or oxygen in the liquid state downstream of the place where the cryogenic liquid is sampled in the system for separating gases from air according to the invention. This sample is in particular taken, for example, at the inlet of the distillation column of the system for separating gases from air, or at the inlet of the oxygen vaporizer of one of these columns, or at the outlet of the oxygen vaporizer, or if necessary between the different vaporization stages.

[0038] The vaporization step is carried out at atmospheric pressure or under vacuum controlled by a vacuum generating system such as a vacuum pump or an ejector. For example, the pressure in the vessel during the vaporization step is brought to a value ranging from 0.2 bar to 0.3 bar, preferably equal to 0.2 bar. This makes it possible to reduce the vaporization temperature and, as a consequence, to reduce the gas-liquid equilibrium coefficient even more. Furthermore, this makes it possible to increase the temperature difference between the heated surface and the cryogenic liquid, which makes it possible to reduce the duration of the vaporization step.

[0039] The amount of cryogenic liquid vaporized during the vaporization step is controlled so that the concentration factor, which corresponds to the ratio between the initial volume of cryogenic liquid and the volume of the residual cryogenic liquid, is precisely known by monitoring the duration of the vaporization step and the temperature and pressure parameters during this vaporization step.

[0040] The determining step then comprises taking a sample of the residual cryogenic liquid, then vaporizing the sampled liquid to result in a gas, or vaporizing all of the residual cryogenic liquid to result in a gas, and then delivering the gas obtained by vaporization of the sampled liquid or all of the residual cryogenic liquid to a gas analyzer. As a result, the gas analyzer should detect the proportion of concentrated impurities and therefore does not need to be very accurate or very expensive.

[0041] In order to tightly control the proportion of impurities in the cryogenic liquid in the gas separation system according to the invention, the method of determination according to the invention comprises a step of emptying the container, followed by a step of cooling it by introducing liquid at a temperature below the filling temperature of the cryogenic liquid into the jacket of the determination system. The step of emptying is followed by a sampling of the remaining cryogenic liquid or a delivery of gas obtained by vaporization of all the remaining cryogenic liquid, so as not to disturb the determination of the proportion of impurities carried out by the gas analyzer. Once cooled, the container receives a new initial volume of cryogenic liquid sampled from the system for separating gas from air, and a new determination of the proportion of its impurities can be carried out.

[0042] Further features and advantages of the present invention will become more apparent both from the following description and from a number of non-limiting exemplary embodiments, given by way of indication with reference to the accompanying schematic drawings, in which: [Brief description of the drawings]

[0043] [Figure 1] 1 illustrates steps of a method for determining the percentage of at least one impurity dissolved in a cryogenic liquid according to the present invention, in one embodiment of the present invention. [Diagram 2] In one embodiment of the present invention, a system for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to the present invention is shown, which system specifically comprises a vessel and a vaporization means shown in the bottom section of the vessel. [Diagram 3]FIG. 3 shows a perspective view of an element of the vaporization means shown in FIG. 2. [Figure 4] FIG. 4 is a perspective view of a cross section of the element of FIG. [Diagram 5] 2 shows the steps of a method for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to the invention in a variant of the embodiment of the invention shown in FIG. 1 . DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] According to one embodiment of the present invention shown in FIG. 1, the inventive determination method 100 for determining the proportion of at least one impurity dissolved in a cryogenic liquid is implemented by an inventive determination system 2 for determining the proportion of at least one impurity dissolved in a cryogenic liquid shown in FIG. 2, which determination system 2 forms part of a system for separating gas from air by cryogenic distillation according to the present invention.

[0045] The at least one impurity whose proportion is determined is, for example, propane. The proportion of impurities other than propane in the cryogenic liquid is of course also preferably determined by the determination method 100, the determination system 2 enabling such a plurality of determinations. The impurities determined in this embodiment of the invention are less volatile than the cryogenic liquid.

[0046] The system for separating gas from air comprises means for taking samples of the fluid circulating in the separation system. In this embodiment of the invention, it is envisaged that these sampling means take samples of the liquid oxygen at the inlet of an oxygen vaporizer of the gas separation system, the oxygen vaporizer being arranged in a distillation column of the gas separation system. The liquid oxygen thus sampled is delivered to the determination system 2.

[0047] The system comprises a vessel 3 in the form of a cylindrical tank capable of containing liquid oxygen and arranged vertically on legs not shown. The vessel 3 has a liquid inlet 32, a liquid outlet 34 and a gas outlet 36.

[0048] Liquid oxygen sampled from the separation system is delivered to vessel 3 through liquid inlet 32 ​​of vessel 3 during a first step 110 of method 100, which is a step of filling vessel 3 with an initial volume of cryogenic liquid, in this case liquid oxygen.

[0049] This initial volume of cryogenic liquid is predetermined, and to obtain this precisely, the container 3 is filled until the cryogenic liquid overflows from the container 3 through the liquid outlet 34, the position of this in the container 3 being determined such that when this liquid reaches the liquid outlet 34, the container 3 is filled with the predetermined initial volume of cryogenic liquid.

[0050] The height of the cryogenic liquid in the vessel 3 is in this case h1. This height is measured perpendicular to the ground along the vertical axis Z.

[0051] During the filling step 110, the pressure and temperature of the cryogenic liquid prevent the cryogenic liquid from vaporizing. The temperature of the cryogenic liquid is below its vaporization temperature, particularly at the pressures applied during this step.

[0052] The next step of the method 100 is to vaporize the cryogenic liquid until a volume of residual cryogenic liquid is obtained, step 120. In this step, the liquid inlet 32 ​​and outlet 34 of the vessel 3 are closed, but the gas outlet 36 disposed at the top of the vessel 3 is open. Specifically, the cryogenic liquid that has turned into vapor is discharged through the gas outlet 36 during this vaporization step 120.

[0053] This vaporization is carried out by vaporization means 84 arranged in the lower part of the vessel 3, which bring the cryogenic liquid to its vaporization temperature, and a vacuum is applied to obtain a vaporization pressure in the vessel 3, until a pressure of about 0.2 bar absolute is reached. This low pressure makes the vaporization temperature (or bubble temperature) lower than at atmospheric pressure, which shortens the duration of the vaporization step 120. Moreover, this low pressure reduces the vapor-liquid equilibrium coefficient, preventing large amounts of impurities from escaping into the gas phase.

[0054] The vaporization step 120 makes it possible to concentrate the impurities present in the initial volume of cryogenic liquid in a volume of residual cryogenic liquid left at the end of the vaporization step 120. The volume of this residual cryogenic liquid is predetermined by controlling the amount of gas vaporized during this vaporization step 120, or by controlling the duration of this step and the temperature and pressure parameters in the container 3 during this vaporization step 120, or by measuring the variations in the level of the liquid and / or the mass in the container 3. In this way, the concentration factor of the impurities in a volume of residual cryogenic liquid is precisely determined as the ratio between the initial volume of cryogenic liquid and the volume of the residual cryogenic liquid. As an indication, in this embodiment of the invention, the container 3 has a capacity of 1.3 liters and the initial volume of cryogenic liquid is 0.8 liters.

[0055] The vaporization means 84 can be seen more specifically in Figure 3. The vaporization means 84 comprises a copper body 4 having the general shape of a cylinder of height H. The body 4 is arranged vertically along this height H at the bottom of the container 3, with a face 46 of the body that corresponds to the base of the cylinder and is arranged horizontally so as to form the bottom of the container 3. In other words, the base of the cylindrical tank forming the container 3 is formed in part by the face 46 of the body 4 that is flush with the aluminium wall of the cylindrical tank.

[0056] Thus, the opposing surface 48 of the body 4 corresponding to the other base of the cylinder is disposed horizontally, close to the ground, relative to the surface 46 forming the bottom of the container 3 .

[0057] The body 4 has vertically arranged ducts 43 passing through it over its height: an intake duct 42 in the centre of the body 4 and an exhaust duct 44 surrounding the intake duct 42. The exhaust duct 44 has a smaller diameter than the intake duct 42. A copper manifold 8 is attached to the facing surface 48 so as to establish fluid communication between the intake duct 42 and the exhaust duct 44. The manifold 8 forms part of the vaporisation means 84.

[0058] Naturally, the lower part of the vessel 3 and the vaporizing means 84 form a leak-tight means for containing the cryogenic liquid.

[0059] The vaporization means 84 also comprise heating cartridges housed in spaces 41 (visible in FIG. 4) arranged in the body 4 around the exhaust ducts 44. These spaces 41 extend parallel to the exhaust ducts 44 in the body 4 without appearing on the face 46 forming the bottom of the container 3. However, the spaces 41 are visible on the opposing face 48 in order to make it possible to insert heating cartridges in these cavities 41 and to supply them with electrical power.

[0060] Recesses 45 in the form of grooves are made in the cylindrical surface of the body 4 between the spaces 41, in particular to increase the surface area for heat exchange between the body 4 and the cooling liquid intended to circulate in a jacket 5 surrounding the lower half of the vessel 3, in particular the part of the evaporation means 84. More specifically, the jacket 5 takes the form of an annular jacket, a first circular edge of which surrounds the body 4 and borders the counter face 48, and a second circular edge of which surrounds the vessel 3 shortly below the liquid outlet 34 of the vessel 3. The purpose of this jacket 5 will be explained below.

[0061] The vaporization means 84 acts as a bath vaporizer with a thermosiphon effect in the exhaust duct 44. Specifically, during the vaporization step 120, the heating cartridge is powered to bring the temperature of the cryogenic liquid present in the exhaust duct 44 to its vaporization temperature, allowing the vaporized oxygen and trace impurities to escape to the gas outlet 36. The heat flow creates a circulation, causing the cryogenic liquid to circulate in the intake duct 42 from the face 46 to the opposite face 48 of the body 4, and then across the manifold 8 to supply the cryogenic liquid to the exhaust duct 44.

[0062] This circulation allows good mixing and good homogeneity of the cryogenic liquid in the vaporizing means 84 , in particular on its heated surface formed by the internal surface of the exhaust duct 44 .

[0063] The vaporization means 84 are configured such that at the end of the evaporation step 120, when the volume of non-vaporized cryogenic liquid reaches a predetermined volume of residual cryogenic liquid, this heated surface is still immersed in or wetted with the cryogenic liquid. As an indication, in this embodiment of the invention, the body 4 has a height H of 7 mm (millimeters) and the height h reached by the volume of residual cryogenic liquid in the container is at least 50% of the height H of the body 4. The circulation caused by the thermosiphon effect in the exhaust duct 44 thus makes it possible to keep their internal surfaces wet.

[0064] In this way, the heated surface, which is the vaporizing surface, transfers the heat of vaporization without localized evaporation, and since the heated surface is entirely wetted by the cryogenic liquid, there is no gas-liquid interface on the heated surface, no impurity deposits form there and the vapor escapes in thermodynamic equilibrium with a negligible amount of impurities compared to the amount of impurities remaining in the liquid phase.

[0065] To avoid the transfer of heat of vaporization to the cryogenic liquid in the intake duct 42, its internal surface is covered with a thermal insulating lining 47, for example made of Teflon.

[0066] The good conductivity of the heating cartridge and of the body 4 allows to control the heat flow in the vaporizing means and thus to manage the temperature of the heating surfaces. In particular, the material of the body 4 allows to homogenize the temperature of the internal surface of the exhaust duct 44.

[0067] The exhaust duct 44 has a circular cross section to promote good wetting of its internal surface, which is smooth or textured, for example porous or with fins, to improve the heat exchange coefficient, increase the heat flow and reduce the duration of this vaporization step 120. A low pressure is applied inside the vessel 3, which makes it possible in particular to increase the temperature difference between the temperature of the heated surface and the vaporization temperature of the liquid, without running the risk of escaping excessive amounts of impurities into the gas phase.

[0068] The configuration of the body 4, and in particular the arrangement of its ducts 43, makes it possible to have a large heating surface which is always wetted, even with a very small volume of residual cryogenic liquid.

[0069] As an indication, the heat output of these vaporization means 84 makes it possible to vaporize 99% of the initial volume of cryogenic liquid in less than 15 minutes.

[0070] At the end of the vaporization step 120, the gas outlet 36 is closed and the vessel is isolated and contains a predetermined volume of remaining cryogenic liquid.

[0071] The next step is then to determine 130 the percentage of impurities in the remaining cryogenic liquid.

[0072] This determination step 130 comprises vaporizing 132 all the volume of residual cryogenic liquid in the container 3 that remains closed, and then delivering 134 the gas thus vaporized and enriched in impurities to a gas analyzer 6 (shown in FIG. 2), which determines the proportion of propane in the gas and then divides this proportion by an enrichment factor to determine the propane content in the liquid oxygen sampled from the system for separating gases from air. Of course, it is also possible to determine in the same way in this determination step 130 the proportions of other types of impurities, in particular the proportions of nitrous oxide and carbon dioxide in the liquid oxygen sampled from the system for separating gases from air.

[0073] The next step is 140, which involves emptying the container 3, for example by delivering an inert gas that is free of impurities to the container 3. In a variant, a vacuum is applied to the gas remaining in the container, for example by an ejector or a vacuum pump.

[0074] The vessel 3 is cooled further during the cooling step 150, during which a liquid at a temperature below the filling temperature of the cryogenic liquid is delivered to the jacket 5 through a liquid inlet 52 provided in the jacket 5 at its lower part. This liquid is for example liquid oxygen. A gas outlet 56 at the upper part of the jacket makes it possible to release the gas phase resulting from the evaporation of the liquid in the jacket 5 in contact with the hot wall of the vessel 3.

[0075] The cooling liquid is then discharged from the jacket 5 through the outlet 54 located at the bottom of the jacket 5 and the determination system 2 is ready for a new execution of the determination method 100 .

[0076] A variant of the decision-making method 100 according to the invention will now be presented in relation to FIG. 5, which shows the steps of a decision-making method 200 according to the invention.

[0077] The method 200 according to the invention comprises a step 210 of filling the container 3 and a step 220 of vaporizing the cryogenic liquid, which are respectively identical to the filling step 110 and the vaporizing step 120 described above.

[0078] In this variant, during a subsequent step 230 of determining the proportion of impurities in the residual cryogenic liquid, not all of the volume of the residual cryogenic liquid is vaporized, but a sample of a predetermined volume of this volume of residual cryogenic liquid is taken 232, which sample is totally vaporized 234 and delivered 236 to a gas analyzer 6, which determines from this the proportion of impurities in the same way as in the determination step 130.

[0079] In this variant, it is possible to carry out a step 240 of emptying the container 3 at the same time that a sample of the cryogenic liquid is vaporized in a separate container. During this emptying step 240, the cryogenic liquid remaining in the container 3 is, for example, vaporized and discharged through the gas outlet 36 of the container 3.

[0080] The vessel 3 is then cooled during a cooling step 250 which is identical to the cooling step 150 of the determination method 100. Once the cooling liquid has been drained from the jacket 5, the determination system 2 is ready for a new execution of the determination method 200.

[0081] The invention is described in the context of cryogenic liquids resulting from the separation of air, such as oxygen, nitrogen or argon, although it goes without saying that the invention applies to any cryogenic liquid, for example carbon dioxide, carbon monoxide, hydrogen, helium, methane, krypton, xenon, neon.

[0082] Naturally, the invention is not limited to the examples just described, to which numerous modifications can be made without departing from the scope of the invention. In particular, the features of the variants of the various embodiments of the invention contemplated in this application can be combined to realize the invention, provided that these variants are not mutually incompatible.

Claims

1. A determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid, said at least one impurity being less volatile than said cryogenic liquid; a container (3) capable of receiving an initial volume of said cryogenic liquid; a vaporization means (84) for vaporizing said initial volume of cryogenic liquid until a volume of residual cryogenic liquid enriched in said impurities is obtained, said vaporization means being arranged in the lower part of said container (3) and comprising a heated surface capable of vaporizing said cryogenic liquid; means for determining the proportion of impurities in the residual cryogenic liquid; Equipped with 1. A determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid, characterized in that the vaporization means (84) comprises at least one cryogenic liquid intake duct (42) and a plurality of cryogenic liquid exhaust ducts (44), the vaporization means (84) being capable of creating a thermosiphon effect between the at least one intake duct (42) and the plurality of exhaust ducts (44), and the heated surface comprises an inner surface of the exhaust ducts (44).

2. 2. The determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid as claimed in claim 1, wherein said exhaust duct (44) surrounds said intake duct (42).

3. 3. The determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to claim 1 or 2, wherein the exhaust duct (44) has a smaller diameter than the intake duct (42).

4. 4. A determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to any one of claims 1 to 3, wherein the vaporization means (84) comprises a thermally conductive body (4) of general cylindrical shape, over a height (H) of the body (4) the inlet duct (42) and the outlet duct (44) are made, the inlet duct (42) and the outlet duct (44) emerging in a face (46) of the body (4) defining the bottom of the container (3).

5. 5. The determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to claim 4, wherein the body (4) comprises a thermal insulating lining (47) arranged between the inlet duct (42) and the exhaust duct (44).

6. 6. The determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to claim 4 or 5, wherein the vaporization means comprises at least one electric heater arranged in a recess of the body (4) close to the exhaust duct (44).

7. 7. A determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to claim 6, wherein the vaporization means (84) comprise a plurality of electric heaters in the form of heating cartridges, and the body (4) has a plurality of spaces (41) arranged around the exhaust duct (44), the plurality of spaces (41) accommodating the heating cartridges, the plurality of spaces (41) not appearing on the face (46) of the body (4) defining the bottom of the container (3).

8. 8. The determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to claim 7, wherein the plurality of spaces (41) are separated by a plurality of recesses (45) around the periphery of the body (4) over at least a portion of the height (H) of the body (4).

9. 9. A determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to any one of claims 4 to 8, wherein the body (4) has an opposing surface (48) to the surface (46) defining the bottom of the container (3), and the vaporization means (84) comprises a manifold (8) arranged on the opposing surface (48), the manifold (8) fluidly connecting the intake duct (42) to the exhaust duct (44).

10. A determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to any one of claims 1 to 9, wherein a jacket (5) surrounds the vaporization means (84), the jacket (5) being configured to contain a cooling liquid in contact with the vaporization means (84).

11. 11. A determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid as claimed in claim 10 when dependent on claim 9, wherein the opposing surface (48) emerges from the jacket (5) and the inlet duct (42) and the exhaust duct (44) extend through the jacket (5) from the surface (46) defining the bottom of the container (3).

12. A determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to claim 11 depending on claim 7 or 8, wherein the facing surface (48) has an opening for inserting the heating cartridge into the space (41).

13. A system for separating gases from air by cryogenic distillation, comprising a determination system (2) for determining the content of at least one impurity dissolved in a cryogenic liquid according to any one of claims 1 to 12, means for taking samples of a fluid circulating in said system for separation, means for liquefying said sampled fluid if it is gaseous, and means for delivering the sampled and possibly liquefied fluid to said container (3) for determining the proportion of its impurities.

14. A method (100, 200) for determining the proportion of at least one impurity dissolved in a cryogenic liquid using a determination system (2) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to any one of claims 1 to 12, said at least one impurity being less volatile than said cryogenic liquid, said method comprising the steps of: - filling (110, 210) the container (3) with the initial volume of cryogenic liquid, the filling temperature and pressure being maintained at a filling temperature and pressure that prevent the cryogenic liquid from vaporizing during the filling step (110, 210); a step (120, 220) of vaporizing said cryogenic liquid by said vaporizing means, which during said vaporizing step (120, 220) brings said cryogenic liquid to a vaporization temperature at the pressure of said cryogenic liquid, until said volume of residual cryogenic liquid is obtained, said pressure being equal to or lower than said filling pressure, and the gas resulting from said vaporization being discharged from said container (3); - determining (130, 230) the proportion of said impurities in said residual cryogenic liquid; 1. A method (100, 200) for determining the proportion of at least one impurity dissolved in a cryogenic liquid, comprising:

15. 15. Method (100, 200) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to claim 14, wherein the pressure in the vessel (3) during the vaporization step (120, 220) is brought to a value ranging from 0.2 bar to 0.3 bar, preferably equal to 0.2 bar.

16. The method (100, 200) for determining the percentage of at least one impurity dissolved in a cryogenic liquid according to claim 14 or 15, wherein the step (230, 130) of determining the percentage of the impurity comprises taking a sample of the residual cryogenic liquid (232), then vaporizing the sampled residual cryogenic liquid (234) to provide a gas, or vaporizing all of the residual cryogenic liquid (132) to provide a gas, and then delivering the gas obtained by vaporization of the sampled or all of the residual cryogenic liquid to a gas analyzer (236, 134).

17. A method (100, 200) for determining the proportion of at least one impurity dissolved in a cryogenic liquid according to claim 16, using a determination system (2) according to any one of claims 10 to 12, comprising a step (240, 140) of emptying the container (3) followed by a step (250, 150) of cooling the container (3) by introducing liquid at a temperature below the filling temperature of the cryogenic liquid into the jacket (5) of the determination system (2).

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