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 dissolved impurity in a cryogenic liquid taken, for example, from an air gas separation system, as well as a corresponding determination method.

[0002] An air gas separation system comprises low and medium pressure distillation columns for separating the various constituents of the air. Although the air is purified before being admitted to the columns, impurities remain in the air fed to the columns and are concentrated in particular in an oxygen vaporizer in one of the columns. Indeed, most of these impurities have a liquid / vapor equilibrium coefficient such that almost all of these impurities remain present in the liquid phase of the vaporizer, with an infinitesimal portion of the impurities leaving the vaporizer in the gas phase. The impurity content in the liquid phase therefore increases as the vaporizations progress and ends up accumulating in the form of a liquid or solid deposit in the aluminum matrix of the vaporizer.

[0003] Adsorption processes, implemented before 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 air fed to the columns. The impurities which are not or only slightly stopped 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 create a pure phase which can react with oxygen in the vaporizer. However, propane is relatively slightly soluble in oxygen and can therefore create a pure phase whose contact with liquid oxygen in the vaporizer can generate an explosive situation, in particular when the energy released by this impurity is sufficient to start the 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 phenomenon of all other impurities, therefore also impurities reactive with oxygen. This is particularly the case for 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 create a solid phase in liquid oxygen from the vaporizer, which can clog the vaporization channels of the vaporizer. This mechanism, called "dead end boiling" or "dead-end boiling", accelerates the concentration of all the impurities contained 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 limit quantities of impurities and to ensure the operational safety 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, in particular for carbon dioxide and nitrous oxide. In order to be able to control acceptable limit quantities of impurities, it is necessary to measure levels of less than 100 ppb (parts per billion), and preferably between 10 and 50 ppb. These measurements can be continuous or carried out at a frequency allowing intervention well before reaching a critical level of impurities.

[0007] Currently, the techniques for determining the impurity content in the air to be distilled or in the oxygen in the vaporizer use complex devices and require significant operational skill. These techniques use gas analyzers only, which requires sampling the cryogenic liquid entering the distillation columns or the vaporizer, and vaporizing it in specific equipment so that it can then be analyzed. Preferably, the sampling is carried out at the inlet of the vaporizer, or at the outlet of the vaporizer, or even between different vaporization stages if necessary, given that the vaporizer is a critical point in the air gas separation system.

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

[0009] In order to facilitate the measurement of the impurity content in the 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 capacity, the gas thus vaporized being evacuated in an open circuit, a quantity of residual cryogenic liquid in which the impurities of the sample have concentrated, being preserved at the end of this vaporization. Then the capacity is kept closed and the residual quantity of cryogenic liquid is completely vaporized in the capacity, 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, but also to know precisely the quantity of residual cryogenic liquid, in order to accurately evaluate the impurity concentration factor between the volume of the cryogenic liquid sample and the volume of 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, the impurity content in the cryogenic liquid sampled in the air gas 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 amount of cryogenic liquid withdrawn is easily measurable with less than 2% error, but the amount of residual cryogenic liquid is more difficult to measure, the latter measurement being made with more than 10% error in some cases.

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

[0014] A first method is based on measuring the heating power used to vaporize the sampled cryogenic liquid. This first method requires evaluating the heating power used solely for this vaporization, and therefore also evaluating the heat losses through the capacity. In addition, this heating power is such (compared to the power that would be necessary to vaporize the quantity of residual liquid) that it is difficult to know whether 98.9% or 99.1% of the sampled cryogenic liquid has been vaporized, which corresponds to an error of 10% in the measurement.

[0015] A second method consists of directly measuring the volume of residual cryogenic liquid, but this volume being small compared to the volume of the cryogenic liquid sample, the precision of this measurement is low, especially since the vaporization system developed by the inventors uses a heating surface of complex geometry to allow as few impurities as possible to escape into the vaporized gas in order to concentrate them in the residual liquid. This heating surface prevents the formation of over-concentrated impurity zones in the cryogenic liquid as well as dry vaporization on this heating surface, so as not to modify the liquid / vapor equilibrium coefficients in the capacity. In addition, the vaporization system has inertia and continues to vaporize during the measurement, which further distorts it.

[0016] A third, more precise method consists of measuring the quantity of residual liquid after vaporization thereof in the capacity isolated from any material inlet and outlet. Indeed, the volume V of the capacity being known, by measuring the temperature T and the pressure P in the capacity after vaporization of all the residual liquid, the number n of moles of residual liquid is obtained by the relation PV= ZnRT, where R is the constant of ideal gases (in Joules per mole and per Kelvin) and Z the compressibility factor of the gas obtained by vaporization. However, this measurement implies a homogeneity of the temperature in the capacity including in the pipes connecting the body of the capacity to the inlet and outlet valves thereof.However, since the temperature is measured immediately after vaporization, during which the container is brought to 70°C (degrees Celsius), in order to be able to continue with a new sampling cycle and determine the impurity content of the sampled cryogenic liquid, the temperature within the container is in fact not 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 while being sufficiently precise to ensure the operational safety 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 dissolved impurities 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 overcoming the problem of measuring a volume of residual cryogenic liquid.

[0019] To this end, the invention proposes a method for determining the content of at least one dissolved impurity in a cryogenic liquid, the at least one impurity being less volatile than the cryogenic liquid, comprising steps of: • filling a capacity with an initial volume of cryogenic liquid, • vaporization within the capacity of all the cryogenic liquid present in the capacity, the pressure in the capacity during vaporization being less than or equal to the pressure in the capacity during the filling step, and the gas resulting from the vaporization being evacuated from the capacity, thus promoting the formation of a solid or liquid phase of the impurity in the capacity, • isolation of the capacity from any material outlet, • sending a determinable volume of gas into the capacity, and heating the gas present in the capacity so as to vaporize or sublimate the liquid or solid phase of the impurity in the gas present in the capacity, which thus becomes charged with the impurity, • isolation of the capacity from any material input, • sending the gas loaded with the impurity from the capacity to a gas analyzer, And • determination of the impurity content in the cryogenic liquid from an impurity content measured in the gas loaded 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 taken, corresponding to the initial volume of cryogenic liquid, is vaporized. This initial volume is predetermined or in any case easily measurable. In order not to modify this initial volume when filling the capacity, this is of course carried out at a pressure and a temperature within the capacity, preventing vaporization of the cryogenic liquid.

[0021] The gas then dissolving the impurity crystals and / or the liquid phases of impurities is of determinable volume, either because the volume of gas with which the capacity 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 open circuit.

[0023] Thanks to the invention, the impurities present in the initial volume of cryogenic liquid are therefore 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, 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 of the determinable volume of gas is for example chosen from nitrogen, helium, or unpolluted dry air. In other words, any gas free from impurities and capable of being charged with the impurity to be measured can be used.

[0025] The determinable volume of gas sent into the capacity preferably corresponds to a concentration factor greater than 30, and preferably greater at 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 gaseous 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 been 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 comes from a reservoir of known volume, and the determination method 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 capacity, • a second step of measuring the pressure and temperature within the tank, after the step of sending the determinable volume of gas into the capacity, 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 reservoir. Alternatively, not the determinable volume of gas or its mass is determined, but a corresponding quantity, for example the number of moles of corresponding gas sent into the capacity 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 a little piping into the determination system previously developed by the inventors. One of the valves at the outlet of the tank allows it to be fluidically connected to the capacity while the other valve at the inlet of the tank allows a gas such as nitrogen to be brought into the tank, each time the determination method according to the invention is implemented.

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

[0029] Before the step of sending the determinable volume of gas into the capacity, the gas in the tank is for example at room temperature and at a pressure greater than 5 bars, and preferably between 5 and 10 bars. Thus, sending the gas from the tank to the capacity does not require complex processing upstream of this step. In addition, since the gas is at a pressure greater than 5 bars in the tank, the temperature of the gas in the tank is homogenized, which makes it possible to make fewer errors in determining the determinable volume of gas introduced into the capacity.

[0030] In this embodiment of the invention, the determination method according to the invention preferably comprises a step of filling the reservoir with the gas intended to be 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 capacity, which still allows the temperature of the gas in the tank to equilibrate and homogenize.

[0031] The determination method according to the invention is preferably repeated in cycles, in a manner sufficiently close to be able to detect a high content of impurities sufficiently quickly to be able to intervene in time on the air gas separation system. 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 for example carried out in parallel with a step of the determination method corresponding to the current cycle or to the previous cycle, for example during a step of sending the gas loaded with impurities to the gas analyzer.

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

[0033] Furthermore, preferably, the vaporization step is controlled so that very few impurities escape into the gas 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 bars, the quantity of nitrous oxide, or carbon dioxide, or propane, escaping into the gas 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 capacity 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 makes it possible to lower the vaporization temperature and consequently to further reduce the liquid / vapor equilibrium coefficients. In addition, this increases the temperature difference between the heating surface and the cryogenic liquid, which makes it possible to reduce the duration of the vaporization step. The vaporization step is carried out, for example, under controlled vacuum through a vacuum production 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 dissolved impurity 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 for vaporizing all the cryogenic liquid present in the capacity in open circuit, capable of promoting the formation of a solid or liquid phase of at least one impurity in the capacity, • means of isolating the capacity of any material outlet once all the cryogenic liquid has been vaporized by the vaporization means, • means for sending, into the capacity insulated by the insulation means, a determinable volume of gas, and means for vaporizing or sublimating the liquid or solid phase of the at least one impurity so as to charge the gas present in the capacity with the impurity, by heating the gas present in the capacity, • means of isolating the capacity from any entry of material once the solid or liquid phase has dissolved in the gas present in the capacity, • means for sending the gas loaded with at least one impurity from the capacity to a gas analyzer, 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 loaded 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 comprise a reservoir of known volume and piping connecting an outlet of the reservoir to an access to the capacity, as well as a valve capable of isolating the capacity from the reservoir.

[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 discharge ducts, which open on the one hand onto a face of the body delimiting a lower part of the capacity, and on the other hand onto a face of the body opposite the face delimiting a lower part of the capacity, the vaporization means also comprising a collector arranged on the opposite face and fluidly connecting the inlet duct with the discharge ducts.

[0039] This thermosiphon effect makes it possible to agitate the cryogenic liquid and therefore to avoid the formation of areas over-concentrated in impurities in the cryogenic liquid, which also avoids excessive vaporization of these impurities.

[0040] In this embodiment of the vaporization means, the discharge conduits surround, for example, the intake conduit and are of smaller diameter than the intake conduit.

[0041] The vaporization means further comprise, in this embodiment, at least one electric heating member arranged in a recess of the body proximal to the evacuation conduits. For example, the vaporization means comprise several electric heating elements in the form of heating cartridges, the body comprising spaces arranged around the exhaust 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 part of the height of the body, to increase the heat exchange surface with a cooling liquid during the cooling step.

[0042] To enable this cooling, an envelope surrounds, for example, the vaporization means, the envelope being configured to contain a cooling liquid coming into contact with the vaporization means. In this case, the opposite face opens out of the envelope, the intake and exhaust ducts extending from the face delimiting the lower part of the capacity, through the envelope. This makes it possible to easily supply the heating cartridges, and to insert them just as easily into the body, the opposite face including in particular orifices for inserting the heating cartridges into the spaces.

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

[0045] The system for determining according to the invention the content of at least one dissolved impurity 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 according to the invention the content of at least one impurity in a cryogenic liquid.

[0046] Other characteristics and advantages of the invention will become apparent from the following description on the one hand, and from several examples of embodiment given for informational and non-limiting purposes with reference to the appended schematic drawings on the other hand, in which:

[0047] [Fig.l] represents steps of a method according to the invention for determining the content of at least one dissolved impurity in a cryogenic liquid, in one embodiment of the invention,

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

[0049] [Fig.3] represents in perspective an element of the vaporization means represented 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 determining mination 100 according to the invention of the content of at least one impurity dissolved in a cryogenic liquid, is implemented by a system 2 according to the invention for determining the content of at least one impurity dissolved in a cryogenic liquid shown [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] The at least one impurity whose content is determined is, for example, propane. The contents of impurities other than propane in the cryogenic liquid are of course also preferably determined by the determination method 100, the determination system 2 allowing these multiple determinations.

[0053] The air gas separation system comprises 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 comprises in particular a capacity 3 in the form of a cylindrical tank, capable of containing liquid oxygen, and arranged vertically on feet not shown. The capacity 3 comprises a liquid inlet 32, a liquid outlet 34 and a gas access 36, allowing the entry of gas into the capacity 3 or the exit of gas from the capacity 3.

[0055] The determination system 2 also comprises a reservoir 7 with a volume VR of, for example, 2 liters, fluidically connected to the gas access 36 of the capacity 3 by a first conduit comprising a valve 74 making it possible to cut off the circulation of fluid between the reservoir 7 and the capacity 3. A second conduit, for supplying gas, is connected to the reservoir 7 by means of a valve 72. It should be noted that in this application, the volume VR includes the volume of the pipes up to the valves 72 and 74.

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

[0057] Prior to a first implementation of a filling step 110 of the capacity 3 of the determination method 100 according to the invention, the tank 7 is filled, during a step 105 of the determination method, with nitrogen in the gaseous state, at room temperature and at a pressure of 6 bars absolute. Of course, as a variant, another type of gas and / or other storage conditions for this gas in the tank 7 can be chosen.

[0058] A few tens of minutes after step 105 of filling the tank 7, the temperature of the gas is balanced 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 tank 7 between the steps 105 and 115 just described, the first step 110 of the determination method 100 is for example implemented.

[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 capacity 3, via the liquid inlet 32 ​​on the capacity 3, during this first step 110 of the determination method 100, this first step 110 corresponding to the filling of the capacity 3.

[0061] This initial volume of cryogenic liquid is predetermined. In order to obtain it precisely, the capacity 3 is filled until the cryogenic liquid overflows from the capacity 3 through the liquid outlet 34, the position of which on the capacity 3 is determined so that the capacity 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 capacity 3 is then hl. This height is measured vertically relative to the ground along a vertical axis Z. For information purposes, in this embodiment of the invention, the capacity 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 it from vaporizing. The temperature of the cryogenic liquid is notably lower than its vaporization temperature at the pressure to which it is subjected during this step.

[0064] The next step of the determination method 100 is a step 120 of vaporization of the cryogenic liquid down to its last drop. In this step, the liquid inlet 32 ​​and outlet 34 on the capacity 3 are closed, while the gas access 36, arranged at the top of the capacity 3, is open. Indeed, the cryogenic liquid transformed into vapor is evacuated 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 capacity 3, which bring the cryogenic liquid to its vapourization temperature. porization, the vaporization pressure in the capacity 3 being pulled 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. In addition, this low pressure reduces the liquid / vapor equilibrium coefficients, avoiding the escape of a significant quantity of impurities into the gas 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 oxygen phase present in the capacity 3, until the latter disappears and impurity crystals or liquid impurity phases 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 form of a cylinder of height H. The body 4 is arranged along this height H vertically 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 constituted 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 proximal to the ground relative to the face 46 forming the lower part of the capacity 3.

[0069] The body 4 is traversed along its height by conduits 43 arranged vertically, namely an intake conduit 42, in the center of the body 4, and discharge conduits 44 surrounding the intake conduit 42. The discharge conduits 44 are of smaller diameter than the intake conduit 42. A copper collector 8 is attached to the opposite face 48 so as to provide fluid communication between the intake conduit 42 and the discharge conduits 44. The collector 8 is 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 comprise 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, hollow out the cylindrical surface of the body 4 between the spaces 41, in particular in order to increase the heat exchange surface. between the body 4 and a cooling liquid intended to circulate in an envelope 5, surrounding the lower half of the capacity 3 and in particular a part of the vaporization means 84. More precisely, the envelope 5 takes the form of an annular envelope of which a first circular edge surrounds the body 4 by bordering the opposite face 48 and of which a second circular edge surrounds the capacity 3 slightly below the liquid outlet 34 on the capacity 3. The usefulness of this envelope 5 will be described later.

[0073] The vaporization means 84 operate like a bath vaporizer with a thermosiphon effect in the discharge conduits 44. Indeed, during the vaporization step 120, the heating cartridges are powered, and bring the temperature of the cryogenic liquid present in the discharge conduits 44 to its vaporization temperature, allowing the vaporized oxygen to escape with very few impurities towards the gas outlet 36. A circulation is created due to the thermal flows, the cryogenic liquid circulating in the intake conduit 42 from the face 46 to the opposite face 48 of the body 4, then passing through the collector 8 to supply the discharge conduits 44 with cryogenic liquid.

[0074] This circulation allows good agitation and good homogeneity of the cryogenic liquid in the vaporization means 84 and in particular at the level of its heating surface formed by the internal surfaces of the evacuation conduits 44, thus making it possible to maintain a liquid / vapor equilibrium coefficient favorable to the conservation of impurities in the liquid oxygen phase. In addition, the heating surface remains wet thanks to this circulation of liquid, for as long as possible before the crystallization or liquefaction of the impurities, which also makes it possible 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 body 4 makes it possible, through its material, to homogenize the temperature of the internal surfaces of the evacuation conduits 44.

[0077] These have a circular section in order to promote good wetting of their internal surfaces, which are smooth or textured, for example porous or having fins in order to improve the exchange coefficient, increase the heat flow and reduce the duration of this vaporization step 120. The low pressure applied in the capacity 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 letting too large a quantity of impurities escape into the gas phase.

[0078] The configuration of the body 4 and in particular the arrangement of its conduits 43, makes it possible to have 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 liquid oxygen taken.

[0080] The determination method then implements a step 130 of isolating the capacity 3 from any material outlet, 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. Furthermore, in this step 130, the heating cartridges are no longer electrically powered.

[0081] The next step 140 is the sending of a determinable volume of gas into the capacity 3, this gas being nitrogen in this embodiment of the invention. For this, the valve 74 is opened, which sends a portion of the nitrogen present in the tank 7 into the capacity 3, the latter still being isolated from the other circuits of the determination system. The pressure in the tank 7 being higher than that of the capacity 3, this sending of gas occurs naturally. Thanks to the pressure difference between the tank 7 and the capacity 3, of the order of several bars, and the relatively comparable sizes of the tank 7 and the capacity 3, the gas which fills this capacity 3 is fairly homogeneous in temperature, which makes it possible to make less error on the average measurement of the quantity of gas introduced into the capacity 3, as explained below.

[0082] This step 140 comprises 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 greater than -70°C (preferably greater than 0°C).

[0083] The injected gas, here nitrogen, must be "dry", in other words without humidity, that is to say containing less than 1 ppm (part per million) of water and preferably less than 100 ppb (part per billion) of water, and without the presence of impurities to be analyzed or other impurities which could disturb the analysis of the impurities to be measured. Of course, other types of dry gases and without the impurities to be analyzed can be used, for example unpolluted dry air, argon, etc.

[0084] The impurities in the liquid and / or solid state thus pass into the gaseous state and dissolve in the nitrogen thus introduced into the capacity 3, to which is mixed a remainder of gaseous oxygen not escaped from the capacity 3 at the end of step 120 of va porization. This dilution of impurities in nitrogen is rapid, in the order of a few seconds. The impurities thus diluted in nitrogen can then be easily analyzed later 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 reservoir 7 during the step 105 or 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 nitrogen loaded with impurities.

[0086] The following step 150 is the isolation of the capacity 3 from any entry and exit of material, by closing the valve 74. The determination method 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 nitrogen gas loaded with impurities into a gas analyzer 6, during a step 170 of the determination method 100.

[0088] In parallel with this step 170 of sending the nitrogen gas loaded with impurities into the gas analyzer 6, the tank 7 is filled again 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 the temperature in the tank 7. Alternatively, the step 105 takes place only 5 to 10 minutes before the step 120 of vaporization of the cryogenic liquid in the capacity 3.

[0089] Still in parallel with this step 170 of sending the nitrogen gas loaded with impurities into the gas analyzer 6, the determination method 100 determines, during a step 165, the volume or mass of nitrogen introduced into the capacity 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 capacity 3 is in fact:

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

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

[0092] VR * density of nitrogen (at initial pressure Pi and temperature Ti measured) / normal density of nitrogen (under normal pressure and temperature conditions of 1.013 bar and 0°C) - VR * density of nitrogen (at the final measured pressure Pf and temperature Pf) / normal density of nitrogen (under normal pressure and temperature conditions of 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 then determine the impurity contents in the cryogenic liquid taken, as explained below.

[0094] Once the impurity contents in the nitrogen have been determined by the analyzer 6 during step 170, and the quantity of nitrogen introduced into the capacity 3 determined during step 165, the determination method 100 determines during a step 180, the impurity contents 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 taken, the remaining volume or mass of gas in the capacity 3 just after the latter has been isolated from any material outlet, and before sending the gas present in the tank 7 into the capacity 3, is added to the volume V or to the mass M of nitrogen introduced into the capacity 3. This remaining volume / mass of gas is calculated in a similar manner by temperature and pressure measurements in the capacity 3 just after the latter has been isolated from any material outlet, and before sending the gas present in the tank 7 into the capacity 3. In the case where this remaining volume / mass of gas is calculated, the determination system 2 therefore comprises means for measuring the pressure and the temperature within the capacity 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 the remaining volume is added, 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 propane content in the initial volume of cryogenic liquid taken.

[0098] It is sufficient to multiply the content measured by the gas analyzer during step 180, by a concentration factor equal to the volume V of nitrogen introduced into the capacity 3 to which the remaining volume of gas in the capacity 3 was added 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 a total vaporization of the initial volume of cryogenic liquid taken in a closed volume, under temperature and pressure conditions of course identical to those corresponding to the volume V of nitrogen introduced into capacity 3 added to the remaining volume of gas in capacity 3, to which this gaseous volume is reported.

[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 capacity 3 in step 140, this being carried out with less than 2% error.

[0100] Indeed, the precision of this determination depends directly on the precision of the measurements of the pressure, temperature and volume of the tank, given the calculations carried out during 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 the step 180 of determining the impurity contents in the cryogenic liquid taken from the inlet of the vaporizer of the air gas separation system, the determination method 100 implements a step 190 of cooling the capacity 3, during which a liquid at a temperature lower than the filling temperature of the cryogenic liquid is sent into the casing 5 via a liquid inlet 52 with which the casing 5 is provided in its lower part. This liquid is for example liquid oxygen. A gas outlet 56 in an upper part of the casing 5 makes it possible to release a gaseous phase produced by evaporation of the liquid in the casing 5 in contact with the hot wall of the capacity 3.

[0102] Then the cooling liquid is evacuated 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 method 100 and in particular a new step 110 of filling the capacity 3 with cryogenic liquid taken from the air gas separation system.

[0103] Preferably, however, the cooling step takes place after measuring the content of the impurity through the analyzer and after sweeping the volume of the capacity 3, that is to say after replacing its contents with a so-called clean gas or after drawing a vacuum from its contents by applying a pressure at least lower than 0.25 bar absolute.

[0104] Thanks to the invention, each cycle of determination of the impurity content carried out by the determination method 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 which have just been described and numerous adjustments 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 gas separation system, or at the outlet of the oxygen vaporizer, or even between different vaporization stages if necessary, the gaseous oxygen being able to be previously liquefied before being analyzed.

[0106] Furthermore, the order of carrying out the steps of the determination method 100 can be modified, 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 reservoir 7.

[0107] The invention is described in the context of a cryogenic liquid from air separation, 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.

[0108] Finally, the characteristics of the different variant embodiments of the invention envisaged in this application can be combined to achieve the invention, to the extent that these variants are not incompatible with each other.

Claims

Claims

1. Method for determining (100) the content of at least one dissolved impurity in a cryogenic liquid, the at least one impurity being less volatile than the cryogenic liquid, comprising steps of: • filling (110) a capacity (3) with an initial volume of cryogenic liquid, • vaporization (120) within the capacity (3) of all the cryogenic liquid present in the capacity (3), the pressure in the capacity (3) during the vaporization (120) being less than or equal to the pressure in the capacity (3) during the filling step, and the gas resulting from the vaporization (120) being evacuated from the capacity (3), thus promoting the formation of a solid or liquid phase of the impurity in the capacity (3), - isolation (130) of the capacity (3) from any material outlet, • sending (140) a determinable volume of gas into the capacity (3),and heating the gas present in the capacity (3) so as to vaporize or sublimate the liquid or solid phase of the impurity in the gas present in the capacity (3) which thus becomes charged with the impurity, • isolation (150) of the capacity (3) from any entry of material, • sending the gas (170) charged with the impurity from the capacity (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. A method for determining (100) the content of at least one dissolved impurity 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. Method for determining (100) the content of at least one dissolved impurity in a cryogenic liquid according to claim 1 or 2, in which the determinable volume of gas comes from a reservoir (7) of known volume, and in which the determination method (100) comprises: • a first step of measuring (115) the pressure and the 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 the 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 measuring steps (115, 160), and as a function of the known volume of the tank (7).

4. Method for determining (100) the content of at least one dissolved impurity in a cryogenic liquid according to claim 3, in which, 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 bars, and preferably between 5 and 10 bars.

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

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

7. Method for determining (100) the content of at least one dissolved impurity in a cryogenic liquid according to any one of claims 1 to 6, in which 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.

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

9. System for determining (2) the content of at least one dissolved impurity in a cryogenic liquid, the at least one impurity being less volatile than the cryogenic liquid, comprising: • a capacity (3) capable of receiving an initial volume of the cryogenic liquid, • means for vaporizing (84) all the cryogenic liquid present in the capacity (3) in an open circuit, capable of promoting the formation of a solid or liquid phase of the at least one impurity in the capacity (3), • means for isolating the capacity (3) from any exit of material once all the cryogenic liquid has been vaporized by the vaporization means (84), • means for sending, into the capacity (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 capacity (3) with the impurity,by heating the gas present in the capacity (3), • means (74) for isolating the capacity (3) from any entry of material once the solid or liquid phase has dissolved in the gas present in the capacity (3), • means for sending the gas loaded with the at least one impurity from the capacity (3) to a gas analyzer (6), and • means for determining the content of the at least one impurity in the cryogenic liquid, from a content of the at least one impurity measured in the gas loaded with the impurity by the gas analyzer (6).,

10. System for determining (2) the content of at least one dissolved impurity in a cryogenic liquid according to the preceding claim, in which the means for sending (140) a determinable volume of gas into the capacity (3) comprise a reservoir (7) of known volume and piping connecting an outlet of the reservoir (7) to an access (36) to the capacity (3), as well as a valve (74) capable of isolating the capacity (3) from the reservoir (7).

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

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