Method and system for determining the content of at least one impurity in cryogenic liquid

Abstract

To provide a method for determining the content of an impurity in a cryogenic liquid.SOLUTION: The invention comprises the following steps: filling a vessel (3) with an initial volume of cryogenic liquid; vaporizing all the cryogenic liquid in the vessel (3), and forming a solid or liquid phase of the impurity in the vessel (3); isolating the vessel (3) against any exit of material; sending a determinable volume of gas into the vessel (3), capable of dissolving the liquid or solid phase of the impurity in the determinable volume of gas; isolating the vessel (3) against any entry of material; sending the gas loaded with the impurity to a gas analyser (6); and determining the content of the impurity in the cryogenic liquid from a content of the impurity measured, by the gas analyser (6), in the gas loaded with the impurity.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] A system for separating gases in air comprises low-pressure and medium-pressure distillation columns for separating the various components of air. Although the air is purified before entering these columns, impurities remain in the air fed to the columns and are concentrated in particular in the oxygen vaporizer in one of these columns. In particular, most of these impurities have a liquid / vapour equilibrium coefficient, which causes almost all of these impurities to remain in the liquid phase of the vaporizer, and a small part of the impurities to leave the vaporizer again in the gas phase. Thus, the content of impurities in the liquid phase increases with successive vaporization cycles and eventually accumulates in the form of liquid or solid deposits in the aluminum matrix of the vaporizer.

[0003] Adsorption processes carried out before the distillation make it possible to remove heavy hydrocarbons, in other words those with more than four carbon atoms and those with unsaturated bonds, from the air fed to these columns. Impurities that are hardly or not captured by these adsorption processes are in particular light hydrocarbons and propane. Light hydrocarbons with one or two carbon atoms are highly soluble in oxygen and therefore do not give rise to a pure phase capable of reacting with oxygen in the vaporizer. Propane, however, is relatively insoluble in oxygen and therefore may give rise to a pure phase which, when in contact with liquid oxygen in the vaporizer, may result in an explosive situation, especially when the energy released by this impurity is sufficient to cause combustion in the aluminum matrix of the vaporizer.

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

[0005] It is therefore necessary to monitor the content of impurities entering the oxygen bath of the distillation column and / or vaporizer in order to maintain the maximum permissible amount of impurities therein and ensure the operational safety of the system for separating gases in air.

[0006] One problem to be overcome is that the levels of impurities to be measured are very low, especially for carbon dioxide and nitrous oxide, taking into account the very low solubility and gas-liquid equilibrium coefficients of these impurities. In order to be able to control the maximum permissible amount of impurities, it is necessary to measure contents of less than 100 ppb (parts per billion), preferably between 10 and 50 ppb. These measurements can be continuous or can be made at a certain frequency, making it possible to take action well before a critical content of the impurity is reached.

[0007] Currently, techniques for determining the content of impurities in the air to be distilled or in oxygen in the vaporizer use complex equipment and require considerable operating skills. These techniques use only gas analyzers, which require that the cryogenic liquid entering the distillation column or vaporizer is sampled and vaporized in a special device so that it can then be analyzed. Preferably, the samples are taken at the vaporizer inlet or at the vaporizer outlet, or between the various vaporization stages, as appropriate, considering that the vaporizer is a critical point in the system for separating gases in air.

[0008] These prior art techniques involve the complete vaporization of the sample together with its impurities to obtain the gas to be analyzed, where the content of the impurities is identical to 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 sampling or vaporizing the sample.

[0009] In order to facilitate the determination of the content of impurities in a cryogenic liquid, the inventors have developed an apparatus and a method for analyzing this content, which are 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 thus vaporized being discharged in an open circuit and a certain amount of residual cryogenic liquid enriched in the impurities of the sample being retained at the end of this vaporization. The container is then left closed and this residual amount of cryogenic liquid is completely vaporized in the container, which results in a gas with a much higher content of impurities than the initial sample and therefore much easier to measure by a gas analyzer.

[0010] This innovative approach requires not only that the amount of impurities evaporated during vaporization in an open circuit is negligible relative to the amount of impurities remaining in the residual cryogenic liquid, but also that the exact amount of the residual cryogenic liquid is known in order to accurately evaluate the impurity concentration factor between the volume of the sample of cryogenic liquid and the volume of the residual cryogenic liquid after vaporization of almost all of the sampled cryogenic liquid. By multiplying the impurity content measured by the gas analyzer by this concentration factor, it is possible to determine the impurity content in the cryogenic liquid sampled in a system for separating gases in air.

[0011] The accuracy of this determination therefore depends on the accuracy of the measurements of the amount of sampled cryogenic liquid, the amount of residual cryogenic liquid and the impurity content in the residual cryogenic liquid, the latter being performed by a gas analyzer.

[0012] While the amount of sampled cryogenic liquid can be easily measured with an error rate of less than 2%, the amount of residual cryogenic liquid is more difficult to measure, in some cases resulting in an error rate of more than 10%.

[0013] Specifically, impurities are concentrated enough in the residual cryogenic liquid that this liquid represents only about 1% of the volume of the sampled cryogenic liquid. To measure this volume accurately, several methods are possible.

[0014] The first method is based on the measurement of the heating power used to vaporize the sampled cryogenic liquid. The first method requires that the heating power used for this vaporization is evaluated alone, and therefore also the heat losses through the container. Furthermore, this heating power is such that it is difficult to know whether 98.9% or 99.1% of the sampled cryogenic liquid has been vaporized (with respect to this power required to vaporize the amount of residual liquid), which corresponds to an error rate of 10% in the measurement.

[0015] The second method consists in directly measuring the volume of the residual cryogenic liquid, but this volume is small compared to the volume of the sample of the cryogenic liquid, so that the accuracy of this measurement is low, especially since the vaporization system developed by the inventors uses a heated surface of complex geometric shape in order to prevent as far as possible impurities from escaping in the vaporized gas so that they are concentrated in the residual liquid. This heated surface prevents the formation of areas in which the impurities in the cryogenic liquid are excessively concentrated and from vaporizing to dryness on this heated surface, so as not to modify the gas-liquid equilibrium coefficient in the container. Moreover, the vaporization system has an inertia, which continues to vaporize during the measurement, further falsifying the measurement.

[0016] A third, more accurate method consists in measuring the amount of residual liquid after its vaporization, in a vessel isolated against any inflow or outflow of material. In particular, by measuring the temperature T and pressure P in the vessel after vaporization of all the residual liquid, with the volume V of the vessel known, the number of moles n of residual liquid is obtained by the relationship PV=ZnRT, where R is the ideal gas constant (in joules per mole and joules per kelvin) and Z is the compressibility coefficient of the gas obtained by vaporization. However, this measurement requires a uniform temperature in the vessel, including in the piping connecting the body of the vessel to the inlet and outlet valves, which is not in fact uniform in the vessel, since the temperature is measured immediately after vaporization and the vessel is brought to 70° C. (degrees Celsius) so that it is possible to start a new cycle of sampling and determination of the content of impurities in the sampled cryogenic liquid. This third method of measurement is therefore not as accurate as desired.

[0017] There is therefore a need to be able to determine the content of impurities in a sampled cryogenic liquid in a system for separating gases from air, which is inexpensive and at the same time sufficiently accurate to ensure the operational safety of the separation system. Summary of the Invention

[0018] The present invention aims at least in part to overcome the above-mentioned drawbacks by providing a determination method and a corresponding determination system for determining the content of at least one impurity dissolved in a cryogenic liquid, which makes it possible to concentrate the impurities before they are analyzed by a gas analyzer, while avoiding the problem of measuring the volume of the residual cryogenic liquid.

[0019] To this end, the invention proposes a method for determining the content of at least one impurity dissolved in a cryogenic liquid, this at least one impurity being less volatile than the cryogenic liquid, said method comprising the following steps: Filling a vessel with an initial volume of cryogenic liquid; a step of vaporizing in the container all the cryogenic liquid present in the container, wherein the pressure in the container during the vaporization is equal to or less than the pressure in the container during the filling step, and the gas resulting from the vaporization is exhausted from the container, thus promoting the formation of a solid or liquid phase of the impurities in the container; isolating the container against any spillage of material; a step of feeding a determinable volume of gas into a vessel and heating the gas present in the vessel so as to vaporize or sublimate the liquid or solid phase of the impurities in the gas present in the vessel, thus the gas contains impurities; isolating the vessel against any inflow of material; - delivering the impure gas from the vessel to a gas analyzer; - Determining the impurity content in the cryogenic liquid from the impurity content in the impurity-containing gas measured by a gas analyzer.

[0020] The method of determination according to the invention differs from the conventional methods developed by the inventors in that all the sampled cryogenic liquid is vaporized, which corresponds to an initial volume of cryogenic liquid, which is either predetermined or in any case easy to measure. In order to avoid this initial volume changing when filling the container, the filling is of course carried out at the pressure and temperature inside the container, preventing vaporization of the cryogenic liquid.

[0021] The gas that dissolves the impurity crystals and / or the liquid phase of the impurity then has a determinable volume in that the volume of gas with which the container is filled after the vaporization step is either known or it can be easily measured.

[0022] The impurity crystals and / or the impurity liquid phase correspond, in an open circuit, to substantially all of the impurity present in the initial volume of cryogenic liquid, depending on the conditions of vaporization.

[0023] By means of the invention, the impurities present in the initial volume of cryogenic liquid are thus concentrated in the determinable volume of gas, and the gas analyzer therefore receives a gas whose impurity content is easier to determine than if it received a gas formed by vaporization of the initial volume of cryogenic liquid, which impurity content corresponds to a number of moles of impurities in the determinable volume of gas that is approximately the same as the number of moles of impurities in the initial volume of cryogenic liquid, which makes it possible to determine the impurity content in the initial volume of cryogenic liquid.

[0024] The gas of the determinable volume of gas is selected, for example, from nitrogen, helium or uncontaminated dry air, in other words, any gas can be used that is free of impurities and that can contain the impurities to be measured.

[0025] The determinable volume of gas delivered into the vessel preferably corresponds to a concentration factor between the content of impurities measured by the gas analyzer and the corresponding content of impurities in the initial volume of cryogenic liquid of more than 30, preferably more than 50. This concentration factor is the ratio, under the same temperature and pressure conditions, between the gas volume corresponding to the initial volume of cryogenic liquid (when the latter is completely vaporized in a closed volume) and the volume of gas in which the impurities in the liquid or solid phase are dissolved after vaporization of all the cryogenic liquid.

[0026] In one embodiment of the invention, a determinable volume of gas is drawn from a tank of known volume, the method comprising: a first step of measuring the pressure and temperature in the tank prior to the step of pumping a determinable volume of gas into the vessel; a second step of measuring the pressure and temperature in the tank after the step of pumping a determinable volume of gas into the vessel; □ Determining a determinable volume of gas or its mass as a function of the measurements obtained during the first and second measurement steps and as a function of the known volume of the tank. In a variant, it is not a determinable volume of gas or its mass that is determined, but a corresponding amount, for example the corresponding number of moles of gas that are pumped into the container to dissolve the impurities.

[0027] This embodiment of the invention is easy to implement since it only requires the integration of a tank, two valves and a small amount of piping into a determination system previously developed by the inventors. One of the tank's outlet valves allows the tank to be fluidly connected to a container, while the other tank's inlet valve allows the supply of a gas, such as nitrogen, into the tank whenever the determination method according to the invention is performed.

[0028] The known volume of the tank, of course, includes not only the internal volume of the tank, but also the internal volume of the piping connecting the tank to each of these valves.

[0029] Prior to the step of pumping the determinable volume of gas into the container, the gas in the tank is, for example, at ambient temperature and at a pressure greater than 5 bar, preferably between 5 and 10 bar. Pumping the gas from the tank to the container therefore does not require any complex procedures upstream of this step. Furthermore, due to the gas being at a pressure greater than 5 bar in the tank, the temperature of the gas in the tank is homogenized, which makes it possible to reduce the error rate in determining the determinable volume of gas introduced into the container.

[0030] In this embodiment of the invention, the method of determination according to the invention preferably comprises a step of filling the tank with the gas intended to contain the impurity before or during the vaporization step. The tank is therefore filled with gas a few tens of minutes before the step of feeding a determinable volume of gas into the container, which further allows the temperature of the gas in the tank to equilibrate and be homogenized.

[0031] The determination method according to the invention is preferably repeated in cycles quite close to each other so as to be able to determine a high content of impurities quickly enough so that measures can be taken in time in a system for separating gases in air. In order to reduce the duration of a cycle for determining the content of impurities in a sample of cryogenic liquid according to the invention, the step of filling the tank is carried out, for example, in parallel with a step of the determination method corresponding to the ongoing or preceding cycle, for example during the step of feeding the gas containing impurities to the gas analyzer.

[0032] The step of transferring the impurity-laden gas from the container to the gas analyzer is followed by a step of cooling the container so as to be able to resume the cycle of determining the impurity content in a new sample of cryogenic liquid as quickly as possible.

[0033] Furthermore, the vaporization step is preferably controlled so that only small amounts of impurities escape into the gas phase during this step.

[0034] It should be noted that this vaporization step can be carried out at atmospheric pressure or higher. Specifically, based on the thermodynamic equilibrium values ​​of the compounds involved, when the cryogenic liquid is maintained at 15 bar, the amount of nitrous oxide, or carbon dioxide, or propane that escapes to the gas phase during vaporization in an open circuit is less than 2% of the amount of impurities remaining in the liquid phase during this vaporization step.

[0035] However, preferably, in the method of determination according to the invention, the pressure in the container during the vaporization step is brought to a value between 0.2 bar and 0.3 bar, preferably equal to 0.2 bar. This makes it possible to lower the vaporization temperature and thus further reduce the vapor-liquid equilibrium coefficient. 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. The vaporization step is carried out under vacuum, controlled for example by a vacuum generating system such as a vacuum pump or an evacuation device.

[0036] The invention also relates to a determination system for determining the content of at least one impurity dissolved in a cryogenic liquid, the at least one impurity being less volatile than the cryogenic liquid, the determination system comprising: A container capable of receiving an initial volume of cryogenic liquid; A means for vaporization of any cryogenic liquid present in the vessel in an open circuit, capable of promoting the formation of a solid or liquid phase of at least one impurity in the vessel; means for isolating the vessel against any escape of material once all the cryogenic liquid has been vaporized by the vaporizing means; A means for feeding a determinable volume of gas into a container isolated by the isolating means, and a means for vaporizing or sublimating a liquid or solid phase of at least one impurity by heating the gas present in the container, so as to include the impurity in the gas present in the container; A means for isolating the vessel against any inflow of material once the solid or liquid phase has been dissolved in the gas present in the vessel; means for delivering the gas containing at least one impurity from the vessel to a gas analyzer; • A means for determining the content of at least one impurity in the cryogenic liquid from the content of at least one impurity in the impurity-containing gas measured by a gas analyzer.

[0037] In one embodiment of the invention, the means for delivering a determinable volume of gas to the container comprises a tank of known volume, piping connecting an outlet of the tank to an access to the container, and a valve capable of isolating the container from the tank.

[0038] The evaporation means is preferably capable of producing a thermosiphon effect and comprises, for example, a thermally conductive body of generally cylindrical shape hollowed out over its height by an intake duct and an exhaust duct, which ducts open on one face of the body defining the bottom of the container and on the other face of the body opposite the face defining the bottom of the container, the evaporation means also comprising a manifold arranged on the opposite face and fluidly connecting the intake duct to the exhaust duct.

[0039] This thermosiphon effect makes it possible to agitate the cryogenic liquid and thus prevent the formation of regions of excessive concentration of impurities in the cryogenic liquid, which also prevents excessive evaporation of these impurities.

[0040] In this embodiment of the vaporization means, these exhaust ducts, for example, surround the intake ducts and have a smaller diameter than the intake ducts.

[0041] The vaporization means further comprises, in this embodiment, at least one electric heating element arranged in a recess in the body proximal to the exit duct. For example, the vaporization means comprises several electric heating elements in the form of heating cartridges, the body comprising spaces arranged around these exit ducts and accommodating the heating cartridges, the spaces not opening in the face of the body defining the bottom of the container. The spaces are separated by recesses on the periphery of the body, for example over at least part of the height of the body, in order to increase the surface area for heat exchange with the cooling liquid during the cooling step.

[0042] To allow this cooling, for example, a jacket surrounds the evaporation means, said jacket being configured to contain a cooling liquid in contact with the evaporation means, the facing face then opening to the outside of the jacket and the inlet and outlet ducts extending through the jacket from the face defining the bottom of the container, this allows for an easier supply of power to the heating cartridge and for the latter to be inserted into the body as well, the facing face being in particular provided with a hole for inserting the heating cartridge into the space.

[0043] Finally, the present invention also relates to a system for separating gases in air by cryogenic distillation, comprising a system for determining the content of at least one impurity dissolved in a cryogenic liquid according to the invention, means for sampling the fluid circulating in the separation system, means for liquefying the sampled fluid if it is in gaseous form, and means for pumping the sampled, optionally liquefied, fluid into a container in order to determine its impurity content.

[0044] Of course, the system and method for determining the content of at least one impurity dissolved in a cryogenic liquid of the present invention can also be applied to other systems, for example systems for separating other types of gases, from which carbon dioxide is to be separated.

[0045] The system according to the invention for determining the content of at least one impurity dissolved in a cryogenic liquid and the system according to the invention for separating gases in air have similar advantages as those of the method according to the invention for determining the content of at least one impurity in a cryogenic liquid.

[0046] Other characteristics and advantages of the invention will become more apparent, on the one hand, from the following description and, on the other hand, from some exemplary embodiments given by way of non-limiting indication and with reference to the attached schematic drawings. [Brief description of the drawings]

[0047] [Figure 1] FIG. 1 illustrates, in one embodiment of the present invention, the steps of a method according to the invention for determining the content of at least one impurity dissolved in a cryogenic liquid. [Diagram 2] FIG. 2 illustrates, in one embodiment of the invention, a system for determining the content of at least one impurity dissolved in a cryogenic liquid according to the invention, which system comprises in particular a vessel and evaporation means shown in section at the bottom of the vessel. [Diagram 3] FIG. 3 is a perspective view of an element of the vaporization means shown in FIG. [Figure 4] FIG. 4 is a perspective view of a section through the element of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] According to the embodiment of the invention shown in FIG. 1, the determination method 100 according to the invention for determining the content of at least one impurity dissolved in a cryogenic liquid is implemented by a determination system 2 according to the invention for determining the content of at least one impurity dissolved in a cryogenic liquid shown in FIG. 2, which in this embodiment of the invention forms part of a system for separating gases in air by cryogenic distillation.

[0049] The at least one impurity whose content is determined is, for example, propane. Of course, the content of impurities other than propane in the cryogenic liquid is preferably also determined by the determination method 100, the determination system 2 allowing such multiple determinations.

[0050] The system for separating gases in air comprises means for sampling the fluid circulating in the separation system. In this embodiment of the invention, it is assumed that these sampling means sample the liquid oxygen at the inlet of an oxygen vaporizer of the gas separation system, which oxygen vaporizer is arranged in a distillation column of the gas separation system. The liquid oxygen thus sampled is fed to the determination system 2.

[0051] The system comprises in particular a container 3 in the form of a cylindrical vat capable of containing liquid oxygen and arranged vertically on a foot, not shown, which is provided with a liquid inlet 32, a liquid outlet 34 and a gas access 36 allowing gas to enter or leave the container 3.

[0052] The determination system 2 may also determine a volume V of, for example, 2 liters. R The present application comprises a tank 7 having a volume V, which is fluidly connected to the gas access 36 of the container 3 by a first duct equipped with a valve 74 making it possible to cut off the fluid circulation between the tank 7 and the container 3. A second duct for supplying gas is connected to the tank 7 via a valve 72. R Note that includes the volume of the piping up to the extent of valves 72 and 74.

[0053] Furthermore, the determination system 2 comprises means for measuring the temperature and pressure in the tank 7 .

[0054] Before the first implementation of step 110 of filling the container 3 of the determination method 100 according to the invention, the tank 7 is filled with dinitrogen in the gaseous state at ambient temperature and at a pressure of 6 bar (absolute) during step 105 of the determination method. Of course, as a variant, another type of gas and / or other conditions for the storage of this gas in the tank 7 can be selected.

[0055] A few tens of minutes after the step 105 of filling the tank 7, the temperature of the gas equilibrates in the tank 7. A first measurement of the temperature and pressure in the tank 7 is then performed by the means for measuring the temperature and pressure in the tank 7 during a measurement step 115, giving the temperature Ti and the pressure Pi.

[0056] For example, the first step 110 of the determination method 100 is carried out while the temperature of the dinitrogen in the tank 7 is being equalized between steps 105 and 115 described above.

[0057] The liquid oxygen sampled in the separation system corresponds to an initial volume of cryogenic liquid, i.e., in this case, liquid oxygen, which is pumped into container 3 via liquid inlet 32 ​​of container 3 during this first step 110 of determination method 100, which first step 110 corresponds to filling container 3.

[0058] This initial volume of cryogenic liquid is pre-determined. To obtain this exact volume, the container 3 is filled until the cryogenic liquid overflows from the container 3 through the liquid outlet 34, and the position of the liquid outlet 34 on the container 3 is determined such that when this liquid reaches the liquid outlet 34, the container 3 is filled with the pre-determined initial volume of cryogenic liquid.

[0059] In this case, the height of the cryogenic liquid in the container 3 is h1. This height is measured perpendicular to the ground along the vertical axis Z. 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.

[0060] During the filling step 110, the pressure and temperature of the cryogenic liquid prevent its vaporization. The temperature of the cryogenic liquid is specifically below its vaporization temperature at the pressures to which it is exposed during this step.

[0061] The subsequent step of the method 100 is the vaporization 120 of the cryogenic liquid down to its last drop. In this step, the liquid inlet 32 ​​and the liquid outlet 34 on the container 3 are closed, while the gas access 36 located at the top of the container 3 is open. In particular, the cryogenic liquid converted to vapor is discharged through the gas outlet 36 during this vaporization step 120.

[0062] This vaporization is carried out by vaporization means 84 placed in the lower part of the vessel 3, which bring the cryogenic liquid to its vaporization temperature and apply a vacuum to obtain a vaporization pressure in the vessel 3 until a pressure of about 0.2 bar (absolute) is reached. Due to this low pressure, the vaporization temperature (or boiling point) is lower than the vaporization temperature at atmospheric pressure, which reduces the duration of the vaporization step 120. Furthermore, this low pressure reduces the vapor-liquid equilibrium coefficient, preventing large amounts of impurities from escaping into the gas phase.

[0063] The vaporization step 120 therefore makes it possible to gradually concentrate the impurities present in the initial volume of cryogenic liquid in the liquid phase of oxygen present in the container 3 until the liquid phase disappears and crystals of the impurities or a liquid phase of the impurities form in the container 3.

[0064] The vaporizer means 84 can be seen in more detail in Figure 3. The vaporizer means 84 comprises a copper body 4 having a generally cylindrical shape of height H. The body 4 is arranged vertically along this height H in the lower part of the container 3, such that a face 46 of the body, which corresponds to the base of the cylinder, is horizontally disposed and forms the bottom of the container 3. In other words, the base of the cylindrical vat forming the container 3 is constituted in part by the face 46 of the body 4, which is flush with the aluminium wall of the cylindrical vat.

[0065] The opposing surface 48 of the body 4 , which corresponds to the other base of the cylinder, is therefore positioned horizontally proximal to the ground relative to the surface 46 forming the bottom of the container 3 .

[0066] The body 4 has vertically arranged ducts 43 penetrating 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 added to the facing face 48 so as to put the intake duct 42 and the exhaust duct 44 in fluid communication. The manifold 8 forms part of the vaporisation means 84.

[0067] Naturally, the lower part of the vessel 3 and the vaporizing means 84 form a sealed means for containing the cryogenic liquid.

[0068] The vaporization means 84 also comprise heating cartridges housed in spaces 41 (seen 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 opening onto the face 46 forming the bottom of the container 3. However, these spaces 41 open onto the opposite face 48 to allow the insertion of heating cartridges into these cavities 41 and the supply of power to these heating cartridges.

[0069] A recess 45 in the form of a groove is made in the cylindrical surface of the body 4 between the spaces 41 so as to in particular increase the surface area for heat exchange between the body 4 and the cooling liquid intended to circulate in the jacket 5, which surrounds the lower half of the vessel 3, in particular a 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 opposing face 48, and a second circular edge of which surrounds the vessel 3 slightly below the liquid outlet 34 on the vessel 3. The purpose of this jacket 5 will be explained below.

[0070] The vaporization means 84 operates as a bus vaporizer with a thermosiphon effect in the discharge duct 44. In particular, during the vaporization step 120, power is supplied to the heating cartridge, which brings the temperature of the cryogenic liquid present in the discharge duct 44 to its vaporization temperature, allowing the vaporized oxygen to escape to the gas outlet 36 with very few impurities. The circulation is caused by the heat flow, and the cryogenic liquid circulates in the intake duct 42 from the face 46 to the opposite face 48 of the body 4, then passes through the manifold 8 and supplies the cryogenic liquid to the discharge duct 44.

[0071] This circulation allows good mixing and good homogeneity of the cryogenic liquid in the vaporization means 84, in particular on its heated surface formed by the internal surface of the discharge duct 44, and therefore makes it possible to maintain a favorable gas-liquid equilibrium coefficient for keeping the impurities in the liquid phase of oxygen. Moreover, the heated surface remains wetted by this circulation of liquid for as long as possible before crystallization or liquefaction of the impurities, which also makes it possible to limit the impurities escaping into the gas phase of oxygen.

[0072] To avoid the transfer of heat of vaporization to the cryogenic liquid in the intake duct 42, the inner surface of the intake duct is lined with a thermal insulating lining 47, for example made of Teflon.

[0073] The good conductivity of the heating cartridge and of the body 4 allows the heat flow in the vaporizing means to be controlled, which makes it possible to manage the temperature of the heating surfaces. In particular, the body 4, by virtue of its material, makes it possible to homogenize the temperature of the inner surface of the exhaust duct 44.

[0074] The exhaust ducts 44 have a circular cross section to promote good wetting of their internal surface, smooth or textured, for example porous or with fins, to improve the exchange coefficient, increase the heat flow and reduce the duration of this vaporization step 120. The low pressure applied in the vessel 3 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 the risk of too much impurity escaping into the gas phase.

[0075] The configuration of the body 4, and in particular the arrangement of its ducts 43, makes it possible to have a large size of wetted heated surface for as long as possible during the vaporization step 120, even if the volume of residual cryogenic liquid is very small.

[0076] When the last drop of liquid oxygen is vaporized, a sudden increase in temperature is detected by the temperature probe present in the vaporization means 84, which necessarily forms part of the determination system 2. The impurities are then present in the container 3 in the form of crystals or as one or more liquid phases of impurities, depending on the impurities originally present in the sampled liquid oxygen.

[0077] The method then performs a step 130 of isolating the vessel 3 against any escape of material, in particular the liquid inlet 32, the liquid outlet 34 and the access 36 are closed in order to keep the impurities within the vessel 3. Furthermore, in this step 130, the heating cartridge is no longer supplied with power.

[0078] The next step 140 is to pump a determinable volume of gas into the container 3, which in this embodiment of the invention is dinitrogen. For this purpose, the valve 74 is opened to pump part of the dinitrogen present in the tank 7 into the container 3, which is still isolated from the other circuits of the determination system. This pumping of gas occurs naturally, since the pressure in the tank 7 is higher than the pressure in the container 3. Due to the pressure difference of the order of a few bars between the tank 7 and the container 3, and the relatively similar sizes of the tank 7 and the container 3, the gas filling this container 3 has a relatively uniform temperature, which makes it possible to reduce the error rate in the average measurement of the amount of gas introduced into the container 3, as will be explained below.

[0079] This step 140 comprises a heating stage, for example reusing the heating cartridge of the vaporization means, to vaporize, sublimate and heat the gas present in the container 3 to a temperature at least above -70°C (preferably above 0°C).

[0080] The injected gas, in this case dinitrogen, must be "dry", that is to say, free of moisture, i.e. containing less than 1 ppm (parts per million) of water, preferably less than 100 ppb (parts per billion), and free of the impurity to be analyzed or other impurities that may interfere with the analysis of the impurity to be measured. Of course, other types of dry gases free of the impurities to be analyzed can also be used, e.g. uncontaminated dry air, argon, etc.

[0081] The impurities in liquid and / or solid state are thus transferred to the gaseous state and dissolve in the dinitrogen thus introduced into the container 3, which mixes with the residues of gaseous dioxygen still remaining in the container 3 at the end of the vaporization step 120. This dilution of the impurities in the dinitrogen is rapid, on the order of a few seconds. The impurities thus diluted in the dinitrogen can then be easily analyzed by the gas analyzer 6 of the determination system.

[0082] The volume of dinitrogen introduced into the vessel 3 is optionally controlled, in particular depending on the pressure measured in the tank 7 during step 105 or during this step 140 of pumping in a determinable volume of gas, in order to obtain approximately the desired enrichment factor and / or volume of gas required for the analysis of the impurity content in the impurity-containing dinitrogen.

[0083] The next step 150 is to isolate the vessel 3 against any inflow or outflow of material by closing the valve 74. The determination method 100 then performs another measurement of the temperature and pressure in the tank 7 during step 160, giving the temperature Tf and pressure Pf.

[0084] Next, during step 170 of the determination method 100 , the gas access 36 is reopened to deliver the impure gaseous dinitrogen to the gas analyzer 6 .

[0085] In parallel with this step 170 of pumping the impure gaseous dinitrogen into the gas analyzer 6, the tank 7 is refilled with dinitrogen at ambient temperature during a new step 105 so as to reach an initial pressure Pi of 6 bar, followed a few tens of minutes later by a new step 115 of measuring the pressure and temperature in the tank 7. As a variant, step 105 is carried out only 5 to 10 minutes before the step 120 of vaporization of the cryogenic liquid in the container 3.

[0086] Again, in parallel with this step 170 of feeding the impure dinitrogen gas to the gas analyzer 6, the method 100, during step 165, measures the temperature and pressure in the tank 7, taken during steps 115 and 160, and the known volume V of the tank 7. R Determine the volume or mass of dinitrogen introduced into the vessel 3 as a function of . The mass M of dinitrogen introduced into the vessel 3 is in fact: M=(Pi / Ti-Pf / Tf)*V R *Mmol / R, where Mmol is the molar mass of dinitrogen, * is the multiplication operator and / is the division operator.

[0087] The volume V of dinitrogen introduced into the vessel 3 is equal to: V R *Density of dinitrogen (at the initially measured pressure Pi and temperature Ti) / Normal density of dinitrogen (under normal conditions of pressure and temperature, i.e., 1.013 bar and 0°C) - V R *Density of dinitrogen (at the measured final pressure Pf and final temperature Tf) / normal density of dinitrogen (under normal conditions of pressure and temperature, i.e., 1.013 bar and 0° C.).

[0088] This determination of the amount of dinitrogen introduced into the vessel 3 during step 140 is necessary to then determine the content of impurities in the sampled cryogenic liquid, as will be explained below.

[0089] Once the content of impurities in the dinitrogen has been determined by the analyzer 6 during step 170 and the amount of dinitrogen introduced into the container 3 during step 165, the determination method 100 determines the content of impurities in the cryogenic liquid sampled at the inlet of the vaporizer of the system for separating gases in air, in particular its propane content, during step 180.

[0090] The amount of impurities initially present in the sampled initial volume of cryogenic liquid is approximately the same as the amount of impurities present in the determinable volume of gas introduced into vessel 3 during step 140, due to the very low proportion of impurities vaporized during step 120 of vaporizing the cryogenic liquid.

[0091] To improve the determination of the content of impurities in the sampled cryogenic liquid, the remaining volume or remaining mass of gas in the container 3 immediately after isolating the container 3 against any outflow of material and before feeding the gas present in the tank 7 into the container 3 is added to the volume V or mass M of dinitrogen introduced into the container 3. This volume / this mass of remaining gas is likewise calculated by measuring the temperature and pressure in the container 3 immediately after isolating the container 3 against any outflow of material and before feeding the gas present in the tank 7 into the container 3. If this volume / this mass of remaining gas is calculated, the determination system 2 therefore comprises means for measuring the pressure and temperature in the container 3.

[0092] Thus, the concentration of propane in this determinable volume of gas, determined by the analyzer 6, by knowing the determinable volume of gas and the remaining volume of gas in the container, makes it possible to calculate the number of moles of propane in this determinable volume of gas plus the remaining volume, which number of moles is approximately identical to the number of moles of propane in the initial sampled volume of cryogenic liquid, thus making it possible to determine the propane content in the initial sampled volume of cryogenic liquid.

[0093] In particular, it is sufficient to multiply the content measured by the gas analyzer during step 180 by an enrichment factor equal to the volume V of dinitrogen introduced into the container 3 plus the remaining volume of gas in the container 3 immediately before this introduction, divided by the gas volume corresponding to the initial volume of cryogenic liquid sampled. The gas volume corresponding to the initial volume of cryogenic liquid sampled corresponds to the gas volume resulting from the complete vaporization of the initial volume of cryogenic liquid sampled in the enclosed volume, under temperature and pressure conditions which are of course identical to those corresponding to the volume V of dinitrogen introduced into the container 3 plus the remaining volume of gas in the container 3, to which this gas volume is added.

[0094] With respect to the prior art, this determination of the impurity content in the sampled initial volume of cryogenic liquid is much more accurate since the determination of the number of moles of propane in the sampled initial volume of cryogenic liquid is itself more accurate due to the determination of the determinable volume of dinitrogen introduced into the vessel 3 in step 140, and this is done with an error rate of less than 2%.

[0095] In particular, the accuracy of this determination depends directly on the accuracy of the measurements of the pressure, temperature and volume of the tank, taking into account the calculations performed during step 165. In practice: □ Pressure measurements are easily performed with an error rate of less than 0.05%. Temperature measurements are easily performed with an error rate of less than 0.17%, and □ Volume V of Tank 7 R Measurements of are easily performed with an error rate of less than 0.05% and can be performed by measuring the mass of liquid that completely fills the volume.

[0096] In parallel with step 180 of determining the content of impurities in the cryogenic liquid sampled at the inlet of the vaporizer of the system for separating gases in air, the method 100 performs a step 190 of cooling the container 3, during which liquid at a temperature lower than the filling temperature of the cryogenic liquid is pumped into the jacket 5 via a liquid inlet 52 provided in the lower part of the jacket 5. This liquid is, for example, liquid oxygen. A gas outlet 56 in the upper part of the jacket 5 allows to release the gas phase generated by evaporation of the liquid in the jacket 5, which comes into contact with the warm wall of the container 3.

[0097] The cooling liquid is then discharged from the jacket 5 via the outlet 54 located at the bottom of the jacket 5 and the determination system 2 is ready for a new implementation of the determination method 100, in particular a new step 110 of filling the container 3 with the cryogenic liquid to be sampled in a system for separating gases in air.

[0098] However, preferably, the cooling step is carried out after the determination of the content of impurities by an analyzer and after flushing the volume of vessel 3, i.e. after replacing its contents with "clean" gas or after withdrawing its contents under vacuum by applying thereto a pressure at least less than 0.25 bar (absolute).

[0099] According to the invention, each cycle of impurity content determination carried out by the determination method according to the invention can be carried out within 30-60 minutes, typically within less than 40 minutes, depending on the operating conditions.

[0100] Naturally, the invention is not limited to the examples given above, and many modifications can be made to these without departing from the scope of the invention, in particular, as a variant, a sample of the cryogenic liquid can be taken at the inlet of a distillation column of a system for separating gases in air, or at the outlet of an oxygen vaporizer, or indeed between the various vaporization stages as required, and the gaseous oxygen can be pre-liquefied before being analyzed.

[0101] Further, the order in which the steps of the determination method 100 are performed may be varied, particularly when certain steps may be performed in parallel with other steps, for example when a step of cooling the container 3 may be performed in parallel with a step of filling the tank 7.

[0102] The invention has been described in the context of cryogenic liquids resulting from the separation of air, such as oxygen, nitrogen or argon. It goes without saying that the invention applies to any cryogenic liquid, for example carbon dioxide, carbon monoxide, hydrogen, helium, methane, krypton, xenon, neon.

[0103] Finally, the features of the various variant embodiments of the invention contemplated in this application may be combined to practice the invention, unless those variants are mutually inconsistent.

Claims

1. A method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid, said at least one impurity being less volatile than said cryogenic liquid, Filling (110) a container (3) with an initial volume of cryogenic liquid; a step (120) of vaporizing in said container (3) all of said cryogenic liquid present in said container (3), wherein the pressure in said container (3) during vaporization (120) is equal to or less than the pressure in said container (3) during said filling step, and the gas resulting from vaporization (120) is exhausted from said container (3), thus promoting the formation of a solid or liquid phase of impurities in said container (3); - isolating (130) said container (3) against any escape of material; - pumping (140) a determinable volume of gas into said container (3) and heating the gas present in said container (3) so as to vaporize or sublimate the liquid or solid phase of impurities in the gas present in said container (3), thus allowing said gas to contain said impurities; - isolating (150) said container (3) against any inflow of material; - sending (170) the impure gas from said container (3) to a gas analyzer (6); determining (180) the impurity content in said cryogenic liquid from the impurity content in the impurity-containing gas measured by said gas analyzer (6); 1. A method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid, comprising:

2. 2. The method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid according to claim 1, wherein the gas of the determinable volume of gas is selected from nitrogen, helium, or uncontaminated dry air.

3. The determinable volume of gas is drawn from a tank (7) of known volume, the method (100) comprising: a first step (115) of measuring the pressure and temperature in said tank (7) prior to a step (140) of pumping said determinable volume of gas into said container (3); a second step (160) of measuring the pressure and temperature in the tank (7) after the step (140) of pumping the determinable volume of gas into the container (3); - determining (165) said determinable volume of gas or its mass depending on the measurements obtained during said first and second measuring steps (115, 160) and depending on the known volume of said tank (7); A method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid according to claim 1 or 2, comprising:

4. 4. The method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid according to claim 3, wherein, prior to the step (140) of feeding the determinable volume of gas into the container (3), the gas in the tank is at ambient temperature and at a pressure greater than 5 bar, preferably between 5 and 10 bar.

5. 5. A method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid according to claim 3 or 4, comprising a step (105) of filling the tank (7) with a gas intended to contain the impurity before or during the vaporization step (120).

6. Method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid according to any one of claims 1 to 5, wherein the pressure in the container (3) during the vaporization step (120) is brought to a value between 0.2 bar and 0.3 bar, preferably equal to 0.2 bar.

7. Method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid according to any one of claims 1 to 6, wherein the determinable volume of gas pumped into the container (3) corresponds to a concentration factor of more than 30 between the content of the impurity measured by the gas analyzer (6) and the corresponding content of the impurity in the initial volume of cryogenic liquid.

8. A method (100) for determining the content of at least one impurity dissolved in a cryogenic liquid according to any one of claims 1 to 7, wherein the step (170) of sending impurity-laden gas from the container (3) to the gas analyzer (6) is followed by a step (190) of cooling the container (3).

9. A determination system (2) for determining the content 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; vaporization means (84) for vaporization of all the cryogenic liquid present in said vessel (3) in an open circuit, capable of promoting the formation of a solid or liquid phase of said at least one impurity in said vessel (3); - isolation means for isolating said vessel (3) against any escape of material when all said cryogenic liquid is vaporized by said vaporization means (84); means for feeding a determinable volume of gas into said container (3) isolated by said isolating means, and means for vaporizing or sublimating a solid or liquid phase of said at least one impurity by heating the gas present in said container (3) so as to include the impurity in the gas present in said container (3); - means (74) for isolating said vessel (3) against any inflow of material when the solid or liquid phase of said at least one impurity is dissolved in the gas present in said vessel (3); - means for conveying said gas containing at least one impurity from said container (3) to a gas analyzer (6); means for determining the content of said at least one impurity in said cryogenic liquid from the content of said at least one impurity in the impurity-laden gas measured by said gas analyzer (6); A determination system (2) for determining the content of at least one impurity dissolved in a cryogenic liquid, comprising:

10. 10. A system (2) for determining the content of at least one impurity dissolved in a cryogenic liquid according to claim 9, wherein the means (140) for feeding a determinable volume of gas into the container (3) comprises a tank (7) of known volume, a pipe connecting an outlet of the tank (7) to an access (36) to the container (3), and a valve (74) capable of isolating the container (3) from the tank (7).

11. 11. A determination system (2) for determining the content of at least one impurity dissolved in a cryogenic liquid according to claim 9 or 10, wherein the vaporization means (84) are capable of producing a thermosiphon effect, the vaporization means (84) comprising a thermally conductive body (4) of generally cylindrical shape hollowed out over its height (H) by an inlet duct (42) and an outlet duct (44) opening into one face (46) of the body (4) defining the bottom of the container (3) and into another opposing face (48) of the body (4) opposite the face (46) defining the bottom of the container (3), the vaporization means (84) also comprising a manifold (8) arranged on the opposing face (48) and fluidly connecting the inlet duct (42) to the outlet duct (44).

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