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

Abstract

Title: Method and System for Determining the Content of At Least One Impurity in a Cryogenic Liquid. The invention relates to a method for determining the impurity content in a cryogenic liquid, comprising the steps of: 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) from any material exit, introducing 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) from any material entry, introducing the impurity-laden gas to a gas analyzer (6), and determining the impurity content in the cryogenic liquid from an impurity content measured in the impurity-laden gas by the gas analyzer (6). Abstract Figure 2

Description

Title of the invention: Method and system for determining the content of at least one impurity in a cryogenic liquid

[0001] The present invention relates to the chemical industry and concerns a system for determining the content of at least one impurity dissolved in a cryogenic liquid taken, for example, from an air gas separation system, as well as a corresponding determination method.

[0002] An air separation system comprises low- and medium-pressure distillation columns for separating the various components of air. Although the air is purified before entering the columns, impurities remain in the column feed air and concentrate, in particular, in an oxygen vaporizer within one of the columns. Indeed, most of these impurities have a liquid / vapor equilibrium coefficient such that almost all of them remain in the liquid phase of the vaporizer, with only a minute fraction returning from the vaporizer to the gaseous phase. The impurity content in the liquid phase therefore increases with each vaporization and eventually accumulates as a liquid or solid deposit in the aluminum matrix of the vaporizer.

[0003] Adsorption processes, implemented prior to distillation, make it possible to remove heavy hydrocarbons, i.e., those with more than four carbon atoms, as well as hydrocarbons containing unsaturated bonds, from the column feed air. Impurities that are not, or only minimally, removed by these adsorption processes include light hydrocarbons and propane. Light hydrocarbons with one or two carbon atoms are highly soluble in oxygen and therefore do not form a pure phase capable of reacting with oxygen in the vaporizer. However, propane is relatively poorly soluble in oxygen and can therefore form a pure phase whose contact with liquid oxygen in the vaporizer can generate an explosive situation, particularly when the energy released by this impurity is sufficient to initiate combustion of the aluminum vaporizer matrix.

[0004] Furthermore, even when the impurity present in the vaporizer is not reactive with oxygen, it accelerates the accumulation of all other impurities, including those reactive with oxygen. This is particularly true of carbon dioxide and nitrous oxide, whose solidification temperature is higher than the operating temperature of liquid oxygen. These impurities may not be stopped by the adsorption processes and may create a solid phase in The liquid oxygen in the vaporizer can clog the vaporization channels. This mechanism, known as "dead end boiling," accelerates the concentration of all impurities in the vaporizing liquid, particularly 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 distillation columns and / or the oxygen bath of the vaporizer in order to maintain acceptable limits of impurities and to ensure the safe operation of the air gas separation system.

[0006] One difficulty to overcome is that the levels of impurities to be measured are extremely low, given the very low solubilities and liquid / vapor equilibrium coefficients of these impurities, particularly for carbon dioxide and nitrous oxide. In order to control acceptable limits for impurities, it is necessary to measure concentrations below 100 ppb (parts per billion), and preferably between 10 and 50 ppb. These measurements can be continuous or carried out at a frequency that allows intervention well before a critical impurity concentration is reached.

[0007] Currently, techniques for determining the impurity content in the air to be distilled or in the oxygen of the vaporizer use complex equipment and require significant operational expertise. These techniques use only gas analyzers, which necessitates collecting the cryogenic liquid entering the distillation columns or the vaporizer and vaporizing it in specific equipment for subsequent analysis. Preferably, sampling is carried out at the vaporizer inlet, at the vaporizer outlet, or between different vaporization stages as appropriate, given that the vaporizer is a critical point in the air separation system.

[0008] These prior art techniques include complete vaporization of the sample with its impurities to obtain a gas to be analyzed, in which the content of impurities is identical to the content of impurities in the cryogenic liquid sample, the vaporization being carried out under conditions preventing an accumulation of impurities in the sampling or vaporization device of the sample, so as not to distort the measurements of the gas analyzer.

[0009] To facilitate the measurement of impurity content in cryogenic liquid, the inventors have designed an apparatus and a method for analyzing this content, described in documents FR3066596 and FR3066597. The apparatus allows the vaporization of a large quantity of a sample of cryogenic liquid to be analyzed in a container, the vaporized gas being discharged in an open circuit, a quantity of residual cryogenic liquid in which the impurities from the sample have concentrated, being retained at the end of this vaporization. Then the container is kept closed and the residual quantity of cryogenic liquid is completely vaporized in the container, so as to obtain a gas with a much higher impurity content than that of the initial sample, therefore easier to measure by a gas analyzer.

[0010] This innovative approach requires that the quantity of impurities evaporating during open-circuit vaporization be negligible compared to the quantity of impurities remaining in the residual cryogenic liquid. It also requires precise knowledge of the quantity of residual cryogenic liquid in order to accurately assess the impurity concentration factor between the sampled cryogenic liquid volume and the residual cryogenic liquid volume after vaporization of almost all of the sampled cryogenic liquid. By multiplying the impurity content measured by the gas analyzer by this concentration factor, the impurity content in the cryogenic liquid sampled from the air separation system is determined.

[0011] The accuracy of this determination therefore depends on the accuracy of the measurements of the quantity of cryogenic liquid taken, the quantity of residual cryogenic liquid and the content of impurities in the residual cryogenic liquid, the latter measurement being carried out by the gas analyzer.

[0012] The quantity of cryogenic liquid taken is easily measurable with less than 2% error, but the quantity of residual cryogenic liquid is more difficult to measure, the latter measurement being carried out with more than 10% error in some cases.

[0013] Indeed, in order to sufficiently concentrate the impurities in the residual cryogenic liquid, the latter represents only about 1% of the quantity of cryogenic liquid collected. Several methods are possible to precisely measure this volume.

[0014] A first method is based on measuring the heating power used to vaporize the collected cryogenic liquid. This first method requires evaluating the heating power used solely for this vaporization, and therefore also evaluating the heat losses through the vessel. Furthermore, this heating power is so high (compared to the power that would be required to vaporize the remaining quantity of liquid) that it is difficult to determine whether 98.9% or 99.1% of the collected cryogenic liquid was vaporized, which corresponds to a 10% error in the measurement.

[0015] A second method consists of directly measuring the volume of residual cryogenic liquid, but since this volume is small compared to the volume of the cryogenic liquid sample, the accuracy of this measurement is low, especially since the vaporization system developed by the inventors uses a heating surface. The complex geometry of the heating surface minimizes the escape of impurities into the vaporized gas, concentrating them instead in the residual liquid. This heating surface prevents the formation of highly concentrated areas of impurities in the cryogenic liquid, as well as dry vaporization on the heating surface itself, thus avoiding alterations to the liquid / vapor equilibrium coefficients within the vessel. Furthermore, the vaporization system exhibits inertia and continues to vaporize during measurement, further skewing the results.

[0016] A third, more precise method consists of measuring the quantity of residual liquid after its vaporization in the container, which is isolated from any inlet or outlet of material. Indeed, knowing the volume V of the container, by measuring the temperature T and the pressure P in the container after vaporization of all the residual liquid, the number n of moles of residual liquid is obtained using the relation PV = ZnRT, where R is the ideal gas constant (in Joules per mole per Kelvin) and Z is the compressibility factor of the gas obtained by vaporization. However, this measurement requires a homogeneous temperature throughout the container, including in the piping connecting the container body to its inlet and outlet valves.However, since the temperature is measured immediately after vaporization, during which the vessel is brought to 70°C (degrees Celsius) in order to proceed with a new cycle of sampling and determining the impurity content of the cryogenic liquid, the temperature within the vessel is not actually homogeneous. This third measurement method is therefore not as precise as desired.

[0017] There therefore remains a need for a determination of the impurity content of a cryogenic liquid taken from an air gas separation system, which is inexpensive yet sufficiently accurate to ensure the safe operation of the separation system.

[0018] The present invention aims to remedy at least in part the aforementioned drawbacks by providing a method for determining the content of impurities dissolved in a cryogenic liquid and a corresponding determination system, which make it possible to concentrate the impurities before their analysis by a gas analyzer, while avoiding the problem of measuring a residual volume of 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, the at least one impurity being less volatile than the cryogenic liquid, comprising the steps of: • filling a container with an initial volume of cryogenic liquid, • vaporization within the container of all the cryogenic liquid present in the container, the pressure in the container during vaporization being less than or equal to the pressure in the container during the filling step, and the gas resulting from vaporization being evacuated from the container, thus promoting the formation of a solid or liquid phase of the impurity within the container, • isolation of the capacity from any material output, • sending a determinable volume of gas into the vessel, and heating the gas present in the vessel so as to vaporize or sublime the liquid or solid phase of the impurity in the gas present in the vessel, which thus becomes charged with the impurity, • isolation of the capacity from any incoming material, • Sending the impurity-laden gas from the tank to an analyzer gas, and • determination of the impurity content in the cryogenic liquid from an impurity content measured in the gas charged with the impurity by the gas analyzer.

[0020] The determination method according to the invention differs from the method previously developed by the inventors in that all the cryogenic liquid withdrawn, corresponding to the initial volume of cryogenic liquid, is vaporized. This initial volume is predetermined or at least easily measurable. In order not to alter this initial volume during the filling of the container, the filling is carried out at a pressure and temperature within the container that prevents vaporization of the cryogenic liquid.

[0021] The gas which then dissolves the impurity crystals and / or the liquid phases of impurities, is of determinable volume, either because the volume of gas which is filled after the vaporization step is known, or because it is easily measurable.

[0022] The impurity crystals and / or the liquid phases of impurities correspond to almost all of the impurities present in the initial volume of cryogenic liquid, thanks to the vaporization conditions, in an open circuit.

[0023] Thanks to the invention, the impurities present in the initial volume of cryogenic liquid are thus concentrated in a determinable volume of gas. 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. This impurity content corresponds to a number of moles of impurities in the determinable volume of gas that is almost identical to 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 for the determinable volume of gas is chosen, for example, from nitrogen, helium, or dry, unpolluted air. In other words, any gas free of impurities and capable of absorbing the impurity to be measured is usable.

[0025] The determinable volume of gas sent into the vessel preferably corresponds to a concentration factor greater than 30, and preferably greater than 50, between the impurity content measured by the gas analyzer and the corresponding impurity content of the initial volume of cryogenic liquid. This concentration factor is the ratio between a gas volume corresponding to the initial volume of cryogenic liquid (if it were completely vaporized in a closed volume) and the volume of gas in which the liquid or solid phases of impurities have dissolved after vaporization of all the cryogenic liquid, under identical temperature and pressure conditions.

[0026] In one embodiment of the invention, the determinable volume of gas is drawn from a reservoir of known volume, and the determination process comprises: • a first step of measuring the pressure and temperature within the tank, before the step of sending the determinable volume of gas into the tank, • a second step of measuring the pressure and temperature within the tank, after the step of sending the determinable volume of gas into the tank, and • a step of determining the determinable volume of gas or its mass based on the measurements taken during the first and second measurement steps, and based on the known volume of the tank. Alternatively, instead of determining the determinable volume of gas or its mass, a corresponding quantity is determined, for example, the number of moles of the corresponding gas to be sent into the tank to dissolve the impurity.

[0027] This embodiment of the invention is simple to implement, since it only requires the integration of a tank, two valves, and some piping into the determination system previously developed by the inventors. One of the valves at the tank outlet allows the tank to be fluidly connected to the vessel, while the other valve at the tank inlet allows a gas, such as nitrogen, to be introduced into the tank each time the determination process according to the invention is carried out.

[0028] The known volume of the tank includes of course the internal volume of the tank as well as the internal volume of the piping connecting the tank to each of the valves.

[0029] Before the step of sending the determinable volume of gas into the tank, the gas in the reservoir is, for example, at ambient temperature and at a pressure greater than 5 bar, and preferably between 5 and 10 bar. Thus, sending the gas from the reservoir to the tank does not require complex processing upstream of this step. Furthermore, since the gas is at a pressure greater than 5 bar in the tank, the temperature of the gas in the tank is homogenized, which reduces the error in determining the determinable volume of gas introduced into the tank.

[0030] In this embodiment of the invention, the determination process according to the invention preferably comprises a step of filling the tank with the gas intended to become charged with the impurity, before or during the vaporization step. Thus, the tank is filled with gas a few tens of minutes before the step of sending the determinable volume of gas into the vessel, which still allows the temperature of the gas in the tank to equilibrate and become homogenized.

[0031] The determination method according to the invention is preferably repeated in cycles, with sufficient frequency to detect a high impurity content quickly enough to allow timely intervention on the air-gas separation system. To reduce the duration of a cycle for determining the impurity content in a cryogenic liquid sample according to the invention, the tank filling step is, for example, carried out in parallel with a step of the determination method corresponding to the current or previous cycle, for example, during a step of sending the impurity-laden gas to the gas analyzer.

[0032] Also, in order to be able to restart as quickly as possible a cycle of determining the impurity content of a new sample of cryogenic liquid, the step of sending the gas loaded with the impurity from the capacity to a gas analyzer is followed by a step of cooling the capacity.

[0033] Moreover, preferably, the vaporization step is controlled so that very few impurities escape into the gaseous phase during this step.

[0034] It should be noted that this vaporization step can be carried out at a pressure greater than or equal to atmospheric pressure. Indeed, based on the thermodynamic equilibrium values ​​of the compounds present, when the cryogenic liquid is maintained at 15 bar, the quantity of nitrous oxide, carbon dioxide, or propane escaping into the gaseous phase during open-circuit vaporization remains less than 2% of the quantity of impurities remaining in the liquid phase during this vaporization step.

[0035] However, preferably in the determination method according to the invention, the pressure within the vessel during the vaporization step is brought to a value between 0.2 bar and 0.3 bar, and preferably equal to 0.2 bar. This lowers the vaporization temperature and consequently reduces the liquid / vapor equilibrium coefficients even further. Moreover, it increases the temperature difference between the heating surface and the cryogenic liquid, thus reducing the duration of the vaporization step. The vaporization step is carried out, for example, under controlled vacuum using a vacuum generation system such as a vacuum pump or an ejector.

[0036] The invention also relates to a 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, comprising: • a capacity capable of receiving an initial volume of cryogenic liquid, • means of vaporizing all the cryogenic liquid present in the open-circuit capacity, capable of promoting the formation of a solid or liquid phase, with at least one impurity present in the capacity. • means of isolating the capacity from any material exit once all the cryogenic liquid has been vaporized by the vaporization means, • means for sending, into the container insulated by the insulating means, a determinable volume of gas, and means for vaporizing or sublimating the liquid or solid phase of at least one impurity so as to charge the gas present in the container with the impurity, by heating the gas present in the container, • means of isolating the capacity from any ingress of material once the solid or liquid phase has dissolved in the gas present in the capacity, • means of sending gas containing at least one impurity from the vessel to a gas analyzer, and • means of determining the content of at least one impurity in the cryogenic liquid, from a content of at least one impurity measured in the gas charged with the impurity by the gas analyzer.

[0037] In one embodiment of the invention, the means for sending a determinable volume of gas into the capacity include a tank of known volume and piping connecting an outlet of the tank to an access to the capacity, as well as a valve suitable for isolating the capacity from the tank.

[0038] The vaporization means are preferably capable of producing a thermosiphon effect, the vaporization means comprising for example a thermally conductive body in the overall shape of a cylinder hollowed out in its height by an inlet duct and outlet ducts, which open on one side onto a face of the body delimiting a lower part of the capacity, and on the other side onto a face of the body opposite the face delimiting a lower part of the capacity, the vaporization means also comprising a collector disposed on the opposite face and fluidly connecting the inlet duct with the outlet ducts.

[0039] This thermosiphon effect allows the cryogenic liquid to be agitated and thus prevents the formation of areas of over-concentration of impurities in the cryogenic liquid, which also prevents excessive vaporization of these impurities.

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

[0041] In this embodiment, the vaporization means further comprise at least one electric heating element disposed in a recess in the body proximal to the discharge ducts. For example, the vaporization means comprise several electric heating elements in the form of heating cartridges, the body having spaces arranged around the discharge ducts and housing the heating cartridges, the spaces not opening onto the face of the body delimiting the lower part of the capacity. The spaces are, for example, separated by recesses on the circumference of the body, over at least a portion of the body's height, to increase the heat exchange surface with a coolant during the cooling stage.

[0042] To enable this cooling, an enclosure surrounds, for example, the vaporization means, the enclosure being configured to contain a coolant that comes into contact with the vaporization means. In this case, the opposite face opens outside the enclosure, with the inlet and outlet channels extending from the face delimiting the lower part of the capacity, through the enclosure. This allows for easy supply of the heating cartridges and their easy insertion into the body, the opposite face notably having insertion holes for the heating cartridges.

[0043] Finally, the invention also relates to a system for separating air gases 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 fluid circulating in the separation system, means for liquefying the sampled fluid if it is gaseous, and means for sending the sampled fluid, possibly liquefied, into the capacity to determine its content of the impurity.

[0044] Of course, the system and method for determining the content of at least one impurity dissolved in a cryogenic liquid, the invention, are applicable to other systems, for example to a system for separating another type of gas, from which one would want to separate carbon dioxide, for example.

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

[0046] Other features and advantages of the invention will become apparent from the following description on the one hand, and from several illustrative and non-limiting examples of embodiments given with reference to the accompanying schematic drawings on the other hand, in which:

[0047] [Fig-1] represents steps of a determination method according to the invention of the content of at least one impurity dissolved in a cryogenic liquid, in one embodiment of the invention,

[0048] [Fig.2] represents a system for determining the content of at least one impurity dissolved in a cryogenic liquid according to the invention, in one embodiment of the invention, the system comprising in particular a capacity and vaporization means shown in cross-section in the lower part of the capacity,

[0049] [Fig.3] represents in perspective an element of the vaporization means shown in [Fig.2], and

[0050] [Fig.4] is a perspective view of a section of the element of [Fig.3].

[0051] According to an embodiment of the invention shown [Fig. 1], a method of determination 100 according to the invention of the content of at least one impurity dissolved in a cryogenic liquid, is implemented by a determination system 2 according to the invention of the content of at least one impurity dissolved in a cryogenic liquid represented [Fig.2], this determination system 2 being part, in this embodiment of the invention, of a system for separating air gases by cryogenic distillation.

[0052] At least one impurity whose content is determined is, for example, propane. The contents of other impurities besides propane in the cryogenic liquid are, of course, also preferentially determined by determination method 100, determination system 2 allowing these multiple determinations.

[0053] The air separation system includes means for sampling fluid circulating in the separation system. In this embodiment of the invention, it is assumed that these sampling means sample liquid oxygen at the inlet of an oxygen vaporizer of the gas separation system, this oxygen vaporizer being arranged in a distillation column of the gas separation system. The liquid oxygen thus sampled is sent to the determination system 2.

[0054] This device includes, in particular, a cylindrical tank 3, suitable for holding liquid oxygen, and arranged vertically on feet (not shown). The tank 3 has a liquid inlet 32, a liquid outlet 34, and a gas inlet 36, allowing gas to enter or exit the tank 3.

[0055] The determination system 2 also includes a reservoir 7 with a volume VR, for example, of 2 liters, fluidly connected to the gas inlet 36 of the capacity 3 by a first conduit including a valve 74 allowing the fluid circulation between the tank 7 and the capacity 3 to be shut off. A second conduit, for gas supply, is connected to the tank 7 via a valve 72. It should be noted that in this application, the volume VR includes the volume of the piping up to the valves 72 and 74.

[0056] Furthermore, the determination system 2 includes means for measuring temperature and pressure in the tank 7.

[0057] Prior to the initial implementation of a filling step 110 of the capacity 3 of the determination process 100 according to the invention, the tank 7 is filled, during a step 105 of the determination process, with nitrogen in gaseous form, at ambient temperature and at a pressure of 6 bar absolute. Alternatively, another type of gas and / or other storage conditions for this gas in the tank 7 may be chosen.

[0058] A few tens of minutes after step 105 of filling the tank 7, the temperature of the gas is equilibrated in the tank 7. A first measurement of temperature and pressure in the tank 7 is then carried out by means of measurement and pressure in the tank 7, during a measurement step 115, providing a temperature Ti and a pressure Pi.

[0059] While the nitrogen is homogenizing in temperature in the reservoir 7 between the steps 105 and 115 just described, the first step 110 of the determination process 100 is carried out, for example.

[0060] The liquid oxygen taken from the separation system corresponds to an initial volume of cryogenic liquid, i.e. here liquid oxygen, which is sent into the tank 3, through the liquid inlet 32 ​​on the tank 3, during this first step 110 of the determination process 100, this first step 110 corresponding to the filling of the tank 3.

[0061] This initial volume of cryogenic liquid is predetermined. In order to obtain it precisely, the chamber 3 is filled until the cryogenic liquid overflows from the chamber 3 through the liquid outlet 34, the position of which on the chamber 3 is determined so that the chamber 3 is filled with the predetermined initial volume of cryogenic liquid when it reaches the liquid outlet 34.

[0062] The height of the cryogenic liquid in the container 3 is then hl. This height is measured vertically from the ground along a vertical axis Z. For the sake of 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.

[0063] During the filling step 110, the pressure and temperature of the cryogenic liquid prevent its vaporization. In particular, the temperature of the cryogenic liquid is lower than its vaporization temperature at the pressure to which it is subjected during this step.

[0064] The next step in the determination process 100 is a vaporization step 120 of the cryogenic liquid down to the last drop. In this step, the liquid inlet 32 ​​and outlet 34 on the tank 3 are closed, while the gas inlet 36, located at the top of the tank 3, is open. The cryogenic liquid, transformed into vapor, is discharged during this vaporization step 120 through the gas outlet 36.

[0065] 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. The vaporization pressure in the vessel 3 is reduced under vacuum to a pressure of approximately 0.2 bar absolute. Thanks to this low pressure, the vaporization temperature (or bubble temperature) is lower than at atmospheric pressure, which reduces the duration of the vaporization step 120. Furthermore, this low pressure reduces the liquid / vapor equilibrium coefficients, preventing a significant amount of impurities from escaping into the gaseous phase.

[0066] The vaporization step 120 thus makes it possible to progressively concentrate the impurities present in the initial volume of cryogenic liquid, in the liquid phase of oxygen present in the capacity 3, until the latter disappears and crystals of impurities or liquid phases of impurities form in the capacity 3.

[0067] The vaporization means 84 are more particularly visible in [Fig. 3]. They comprise a copper body 4, generally in the shape of a cylinder of height H. The body 4 is arranged vertically along this height H in the lower part of the capacity 3, so that a face 46 of the body, corresponding to a base of the cylinder, is arranged horizontally and forms a lower part of the capacity 3. In other words, the bottom of the cylindrical tank forming the capacity 3 is formed in part by the face 46 of the body 4, which is flush with the aluminum walls of the cylindrical tank.

[0068] An opposite face 48 of the body 4 corresponding to the other base of the cylinder, is therefore arranged horizontally proximally to the ground with respect to the face 46 forming the lower part of the capacity 3.

[0069] The body 4 is traversed along its height by vertically arranged conduits 43, namely an inlet conduit 42, in the center of the body 4, and outlet conduits 44 surrounding the inlet conduit 42. The outlet conduits 44 are of smaller diameter than the inlet conduit 42. A copper manifold 8 is attached to the opposite face 48 so as to allow fluid communication between the inlet conduit 42 and the outlet conduits 44. The manifold 8 forms part of the vaporization means 84.

[0070] Of course the lower part of the capacity 3 and the vaporization means 84 form sealed means for retaining the cryogenic liquid.

[0071] The vaporization means 84 also include heating cartridges housed in spaces 41 (visible [Fig. 4]) arranged in the body 4 around the evacuation conduits 44. These spaces 41 extend parallel to the evacuation conduits 44 in the body 4, without opening onto the face 46 forming the lower part of the capacity 3. They do, however, open onto the opposite face 48 in order to allow the insertion of the heating cartridges into these cavities 41 as well as the electrical supply of these heating cartridges.

[0072] Recesses 45 in the form of grooves cut into the cylindrical surface of the body 4 between the spaces 41, in particular to increase the heat exchange surface between the body 4 and a coolant intended to circulate in a casing 5, surrounding the lower half of the capacity 3 and in particular part of the vaporization means 84. More precisely the casing 5 takes the form of an annular casing, the first circular edge of which surrounds the body 4 by bordering the opposite face 48 and the second circular edge of which surrounds the capacity 3 slightly below the liquid outlet 34 on the capacity 3. The purpose of this casing 5 will be described later.

[0073] The vaporization means 84 function as a bath vaporizer with a thermosiphon effect in the discharge ducts 44. Indeed, during the vaporization step 120, the heating cartridges are powered, raising the temperature of the cryogenic liquid present in the discharge ducts 44 to its vaporization temperature, allowing the vaporized oxygen, with very few impurities, to escape towards the gas outlet 36. A circulation is created due to the heat fluxes, with the cryogenic liquid flowing in the inlet duct 42 from the face 46 to the opposite face 48 of the body 4, then passing through the manifold 8 to supply the discharge ducts 44 with cryogenic liquid.

[0074] This circulation ensures good agitation and homogeneity of the cryogenic liquid in the vaporization means 84, and particularly at its heating surface formed by the internal surfaces of the discharge ducts 44, thus maintaining a liquid / vapor equilibrium coefficient favorable to the retention of impurities in the liquid oxygen phase. Furthermore, thanks to this liquid circulation, the heating surface remains wetted for as long as possible before the crystallization or liquefaction of the impurities, which also helps to limit the escape of impurities into the gaseous oxygen phase.

[0075] In order not to transmit heat of vaporization to the cryogenic liquid in the intake duct 42, the internal surface of the latter is covered with a thermally insulating jacket 47, for example made of Teflon®.

[0076] Thanks to the heating cartridges and the good conductivity of the body 4, the heat flow in the vaporization means is controlled, which makes it possible to control the temperature of the heating surface. In particular, the material of the body 4 helps to homogenize the temperature of the internal surfaces of the exhaust ducts 44.

[0077] These have a circular cross-section to promote good wetting of their internal surfaces, which are smooth or textured, for example porous or with fins to improve the heat transfer coefficient, increase the heat flux and reduce the duration of this vaporization step 120. The low pressure applied in the capacitance 3 makes it possible in particular to increase the temperature difference between the temperature of the heating surface and the vaporization temperature of the liquid without risking the release of too much impurity into the gaseous phase.

[0078] The configuration of the body 4 and in particular the arrangement of its conduits 43, allows for a large wetted heating surface for as long as possible during the vaporization step 120, although the volume of residual cryogenic liquid is very small.

[0079] When the last drop of liquid oxygen is vaporized, a sudden increase in temperature is detected by a temperature probe present in the vaporization means 84, and which is of course part of the determination system 2. The impurities are then present in the form of crystals or one or more liquid phases of impurities in the capacity 3, depending on the impurities initially present in the oxygen liquid taken.

[0080] The determination process then implements a step 130 of isolating the capacity 3 from any material output, in particular the liquid inlet 32, the liquid outlet 34 and the access 36 are closed, in order to keep the impurities in the capacity 3. In addition, in this step 130, the heating cartridges are no longer electrically supplied.

[0081] The next step 140 is to send a determinable volume of gas into the tank 3, this gas being nitrogen in this embodiment of the invention. To do this, the valve 74 is opened, which sends a portion of the nitrogen present in the reservoir 7 into the tank 3, the latter remaining isolated from the other circuits of the determination system. Since the pressure in the tank 7 is higher than that of the tank 3, this gas is sent in naturally. Thanks to the pressure difference between the tank 7 and the tank 3, on the order of several bars, and the relatively comparable sizes of the tank 7 and the tank 3, the gas that fills the tank 3 is fairly homogeneous in temperature, which reduces the error in the average measurement of the quantity of gas introduced into the tank 3, as explained below.

[0082] This step 140 includes a heating phase, for example reusing the heating cartridges of the vaporization means, to vaporize, sublimate and heat the gas present in the capacity 3 to a temperature at least above -70°C (preferably above 0°C).

[0083] The injected gas, here nitrogen, must be "dry", in other words without moisture, i.e., containing less than 1 ppm (parts per million) of water and preferably less than 100 ppb (parts per billion) of water, and free from the impurities to be analyzed or other impurities that could interfere with the analysis of the impurities being measured. Of course, other types of dry gases free from the impurities to be analyzed can be used, for example, dry, unpolluted air, argon, etc.

[0084] The impurities in liquid and / or solid form thus transition to a gaseous state and dissolve in the nitrogen introduced into the vessel 3, which is mixed with any remaining gaseous oxygen that did not escape from the vessel 3 at the end of the vaporization step 120. This dilution of the impurities in the nitrogen is rapid, on the order of a few seconds. The impurities thus diluted in the nitrogen can then be easily analyzed subsequently by a gas analyzer 6 of the determination system.

[0085] The volume of nitrogen introduced into the capacity 3 is optionally controlled, in particular as a function of the pressure measured in the tank 7 during step 105 or of this step 140 of sending the determinable volume of gas, in order to obtain approximately the desired concentration factor and / or a volume of gas necessary for the analysis of the impurity content of the impurity-laden nitrogen.

[0086] The next step 150 is the isolation of the capacity 3 from any inflow and outflow of material, by closing the valve 74. The determination process 100 then proceeds to a new measurement of temperature and pressure in the tank 7 during a step 160, providing a temperature Tf and a pressure Pf.

[0087] Then the gas access 36 is opened again in order to send the impurity-laden nitrogen gas into a gas analyzer 6, during a step 170 of the determination process 100.

[0088] In parallel with this step 170 of sending the impurity-laden nitrogen gas into the gas analyzer 6, the tank 7 is refilled with nitrogen at ambient temperature, to reach the initial pressure Pi of 6 bars, during a new step 105, which will be followed a few tens of minutes later by a new step 115 of measuring the pressure and temperature in the tank 7. Alternatively, step 105 takes place only 5 to 10 minutes before the step of vaporization 120 of the cryogenic liquid in the capacity 3.

[0089] Also in parallel with this step 170 of sending the impurity-laden nitrogen gas into the gas analyzer 6, the determination process 100 determines, during a step 165, the volume or mass of nitrogen introduced into the tank 3 as a function of the measurements carried out during the measurement steps 115 and 160 of temperature and pressure in the tank 7, and of the known volume VR of the tank 7. The mass M of nitrogen introduced into the tank 3 is in fact:

[0090] M = (Pi / Ti - Pf / Tf) * VR * Mmol / R, where Mmol is the molar mass of dinitrogen, * is the multiplication operator and / is the division operator.

[0091] The volume V of nitrogen introduced into the container 3 is equal to:

[0092] VR * density of dinitrogen (at initial pressure Pi and temperature Ti) measured) / normal density of dinitrogen (under Normal conditions of pressure and temperature i.e. 1.013 bar and 0°C) - VR * density of dinitrogen (at the final measured pressure Pf and temperature Pf) / normal density of dinitrogen (under Normal conditions of pressure and temperature i.e. 1.013 bar and 0°C).

[0093] This determination of the quantity of nitrogen introduced into the capacity 3 during step 140 is necessary to subsequently determine the impurity levels in the cryogenic liquid sampled, as explained below.

[0094] Once the impurity levels in the nitrogen have been determined by the analyzer 6 in step 170, and the quantity of nitrogen introduced into the capacity 3 determined in step 165, the determination process 100 determines in a step 180, the impurity levels in the cryogenic liquid taken from the inlet of the vaporizer of the air gas separation system, and in particular its propane content.

[0095] The quantity of impurities initially present in the initial volume of cryogenic liquid taken is almost identical to the quantity of impurities present in the determinable volume of gas introduced into the capacity 3 during step 140, thanks to the very low proportion of impurities vaporized during step 120 of vaporization of the cryogenic liquid.

[0096] To refine the determination of the impurity content in the cryogenic liquid sampled, the volume V or mass M of nitrogen introduced into the tank 3 is added to the volume V or mass M of nitrogen introduced into the tank 3. This volume / mass of remaining gas is calculated similarly by measuring the temperature and pressure in the tank 3 immediately after it has been isolated from any material exit, and before the gas from the tank 7 is sent into the tank 3. In the case where this volume / mass of remaining gas is calculated, the determination system 2 therefore includes means for measuring the pressure and temperature within the tank 3.

[0097] Thus the concentration of propane in this determinable volume of gas, determined by the analyzer 6, makes it possible, thanks to the knowledge of the determinable volume of gas and the remaining volume of gas in the capacity, to calculate the number of moles of propane in this determinable volume of gas to which we add the remaining volume, this number of moles being almost identical to the number of moles of propane in the initial volume of cryogenic liquid taken, and therefore to determine the content of propane in the initial volume of cryogenic liquid taken.

[0098] It is sufficient to multiply the concentration measured by the gas analyzer during step 180 by a concentration factor equal to the volume V of nitrogen introduced into the container 3 plus the remaining volume of gas in the container 3 just before this introduction, divided by the gaseous volume corresponding to the initial volume of cryogenic liquid taken. The gaseous volume corresponding to the initial volume of cryogenic liquid taken corresponds to the gaseous volume resulting from the complete vaporization of the initial volume of cryogenic liquid taken in a closed volume, under temperature and pressure conditions identical, of course, to those corresponding to the volume V of nitrogen introduced into the container 3 plus the remaining volume of gas in the container 3, to which this gaseous volume is then reduced.

[0099] Compared to the prior art, this determination of the impurity content in the initial volume of cryogenic liquid taken is much more precise, since the determination of the number of moles of propane in the initial volume of cryogenic liquid taken is itself more precise, thanks to the determination of the determinable volume of nitrogen introduced into the vessel 3 in step 140, the latter being carried out with less than 2% error.

[0100] Indeed, the accuracy of this determination depends directly on the accuracy of the measurements of the pressure, temperature, and volume of the tank, given the calculations performed in step 165. However: • Pressure measurements are easily performed with less than 0.05% error, • Temperature measurements are easily performed with less than 0.17% error, and • The measurement of the volume VR of the tank 7 is easily carried out with less than 0.05% error, this measurement being able to be carried out by measuring the mass of a liquid completely filling this volume.

[0101] In parallel with step 180 of determining the impurity levels in the cryogenic liquid sampled at the inlet of the vaporizer of the air-gas separation system, the determination process 100 implements a step 190 of cooling the chamber 3, during which a liquid at a temperature lower than the filling temperature of the cryogenic liquid is sent into the casing 5 through a liquid inlet 52 provided in the lower part of the casing 5. This liquid is, for example, liquid oxygen. A gas outlet 56 in an upper part of the casing 5 allows the release of a gaseous phase produced by evaporation of the liquid in the casing 5 upon contact with the hot wall of the chamber 3.

[0102] Then the coolant is discharged from the casing 5 through an outlet 54 located in the lower part of the casing 5, and the determination system 2 is ready for a new implementation of the determination process 100 and in particular a new step 110 of filling the vessel 3 with cryogenic liquid taken from the air gas separation system.

[0103] Preferably, however, the cooling step takes place after the measurement of the impurity content through the analyzer and after the scanning of the volume of the capacity 3, i.e. after replacement of its contents with a so-called clean gas or after evacuation of its contents by applying a pressure at least less than 0.25 bar absolute.

[0104] Thanks to the invention, each cycle of determination of the impurity content carried out by the determination process according to the invention, can be carried out in 30 to 60 minutes depending on the operating conditions, and generally in less than 40 minutes.

[0105] Of course, the invention is not limited to the examples just described, and many modifications can be made to these examples without departing from the scope of the invention. In particular, the cryogenic liquid sampling is alternatively carried out at the inlet of the distillation columns of the air separation system, or at the outlet of the oxygen vaporizer, or even between different vaporization stages as appropriate, the gaseous oxygen being able to be liquefied beforehand before being analyzed.

[0106] Furthermore, the order of execution of the steps of the determination process 100 is modifiable, in particular when certain steps can be carried out in parallel with other steps, for example the step of cooling the capacity 3 can be carried out in parallel with the step of filling the tank 7.

[0107] The invention is described in the context of a cryogenic liquid obtained from air separation, such as oxygen, nitrogen, or argon. It is understood that the invention applies to any cryogenic liquid, for example, carbon dioxide, carbon monoxide, hydrogen, helium, methane, krypton, xenon, neon.

[0108] Finally, the characteristics of the different variants of embodiment of the invention envisaged in this application can be combined to carry out the invention, insofar as these variants are not incompatible with each other.

Claims

Demands

1. A method for determining (100) the content of at least one impurity dissolved in a cryogenic liquid, the at least one impurity being less volatile than the cryogenic liquid, comprising the steps of: • filling (110) a vessel (3) with an initial volume of cryogenic liquid, • vaporizing (120) within the vessel (3) of all the cryogenic liquid present in the vessel (3), the pressure in the vessel (3) during vaporization (120) being less than or equal to the pressure in the vessel (3) during the filling step, and the gas from vaporization (120) being vented from the vessel (3), thereby promoting the formation of a solid or liquid phase of the impurity in the vessel (3), • isolating (130) the vessel (3) from any material exit, • sending (140) a determinable volume of gas into the vessel (3),and heating the gas present in the vessel (3) so as to vaporize or sublime the liquid or solid phase of the impurity in the gas present in the vessel (3), which thus becomes charged with the impurity, • isolation (150) of the vessel (3) from any ingress of matter, • sending the gas (170) charged with the impurity from the vessel (3) to a gas analyzer (6), and • determination (180) of the content of the impurity in the cryogenic liquid from a content of the impurity measured in the gas charged with the impurity by the gas analyzer (6).

2. Method for determining (100) 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 chosen from nitrogen, helium, or unpolluted dry air.

3. A method for determining (100) the content of at least one impurity dissolved in a cryogenic liquid according to claim 1 or 2, wherein the determinable volume of gas is obtained from a tank (7) of known volume, and in which the determination process (100) comprises: • a first step of measuring (115) the pressure and temperature within the tank (7), before the step of sending (140) the determinable volume of gas into the capacity (3), • a second step of measuring (160) the pressure and temperature within the tank (7), after the step of sending (140) the determinable volume of gas into the capacity (3), and • a step of determining (165) the determinable volume of gas or its mass as a function of the measurements carried out during the first and second measurement steps (115, 160), and as a function of the known volume of the tank (7).

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

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

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

7. Method for determining (100) 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 sent into the capacity (3) corresponds to a concentration factor greater than 30 between the impurity content measured by the gas analyzer (6) and the corresponding impurity content of the initial volume of cryogenic liquid. 21

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

9. A system (2) 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, comprising: • a vessel (3) capable of receiving an initial volume of the cryogenic liquid, • means for vaporizing (84) all the cryogenic liquid present in the vessel (3) in an open circuit, capable of promoting the formation of a solid or liquid phase of the at least one impurity in the vessel (3), • means for isolating the vessel (3) from any material exit once all the cryogenic liquid has been vaporized by the vaporization means (84), • means for sending, into the vessel (3) isolated by the isolation means, a determinable volume of gas, and means for vaporizing or sublimating the solid or liquid phase of the at least one impurity so as to charge the gas present in the vessel (3) with the impurity,by heating the gas present in the container (3), • means for isolating the container (3) from any ingress of material once the solid or liquid phase has dissolved in the gas present in the container (3), • means for sending the gas containing at least one impurity from the container (3) to a gas analyzer (6), and • means for determining the content of at least one impurity in the cryogenic liquid, from a content of at least one impurity measured in the gas containing the impurity by the gas analyzer (6) characterized in that the means for sending (140) a determinable volume of gas into the container (3) comprise a tank (7) of known volume and piping connecting an outlet of the tank (7) to an access point (36) to the container (3), as well as a valve (74) suitable for isolating the container (3) from the tank (7).

10. System for determining (2) the content of at least one impurity dissolved in a cryogenic liquid according to claim 9, wherein the vaporization means (84) are capable of producing an effect of thermosiphon, the vaporization means (84) comprising a thermally conductive body (4) in the overall shape of a cylinder hollowed in its height (H) by an inlet duct (42) and outlet ducts (44), which open on one side onto a face (46) of the body (4) delimiting a lower part of the capacity (3), and on the other side onto a face (48) of the body (4) opposite the face (46) delimiting a lower part of the capacity (3), the vaporization means (84) also comprising a collector (8) disposed on the opposite face (48) and fluidly connecting the inlet duct (42) with the outlet ducts (44).

Images