Method for determining the amount of at least one impurity accumulated in an air separation unit

Abstract

The invention relates to an air separation unit comprising means (FIC1) for measuring the average air flow rate coming into the unit over a period of time, means (A1) for determining the content of the at least one impurity in the incoming air over the period of time, means (FIC6, FIC9) for measuring the average flow rate of outgoing product for any outgoing product containing at least 90 mol.% of oxygen over the period, means (A2) for determining the content of the at least one impurity in at least one outgoing product containing at least 90 mol.% of oxygen over the period, means (C) for calculating the amount of the at least one accumulated impurity or the plurality of accumulated impurities over the period and means (C) for comparing the amount of the accumulated impurity with a threshold.

Description

[0001] Description

[0002] Title of the invention: Method for determining the quantity of at least one impurity accumulated in an air separation unit. The present invention relates to a method for determining the quantity of at least one impurity accumulated over time in an air separation unit. In particular, it relates to a method for determining the quantity of at least one impurity, present in the air, that may be heavier than oxygen and accumulated over time in a cryogenic distillation air separation unit.

[0003] A cryogenic distillation air separation unit usually comprises a first distillation column operating at a first pressure and a second distillation column operating at a second pressure lower than the first, the columns being thermally connected by a vaporizer, located in the tank of the second column and heated by nitrogen from the first column.

[0004] This vaporizer is used to vaporize the liquid oxygen that forms in the tank of the second column.

[0005] This equipment is a place where many impurities from the column feed air accumulate, impurities that could not be removed upstream. Impurities can also become highly concentrated at the coldest end of a heat exchanger where the air to be separated is cooled by heat exchange with at least one nitrogen-enriched or oxygen-enriched fluid from the first and / or second column, for example, an oxygen-enriched liquid. These impurities can solidify at the low operating temperatures of the air separation unit and / or be dissolved in liquid oxygen, while being heavier than oxygen.

[0006] Most airborne impurities have a liquid / vapor equilibrium coefficient such that almost all of the impurity is present in the liquid phase, and only a minute portion returns to the gaseous phase. These impurities are heavier than nitrogen and often heavier than oxygen. Their concentration in the liquid phase therefore increases during vaporization. It is known from FR2730172 to determine the nitrous oxide concentration in the liquid oxygen bath of the second column of a double column, in order to monitor the operation of the means to prevent the accumulation of impurities.

[0007] Numerous studies have shown that the higher the concentration of an impurity in the liquid phase, the more that impurity accumulates as a solid or liquid deposit in the aluminum vaporization matrix. Therefore, an excessively high concentration of impurities is unacceptable because:

[0008] • Either the impurity is directly reactive with oxygen, and can generate situations with a high risk of explosion, particularly when the energy released by this reactive impurity is sufficient to initiate combustion of the aluminum matrix. This is the case with hydrocarbons. Heavy hydrocarbons (C4+) or those with unsaturated bonds are known to be stopped upstream of the distillation system by an adsorption process. Light saturated hydrocarbons (C1, C2) are stopped only slightly or not at all, but have high solubility in liquid oxygen, which makes the presence of a nearly pure phase of hydrocarbons of this type very unlikely. The most critical hydrocarbon is propane, given its only partial removal in the adsorption systems that purify the air to be separated, and its relatively low solubility in liquid oxygen.Thus, propane entering through the air can accumulate in the liquid oxygen bath until it reaches a concentration above its solubility limit and therefore generate a pure phase in contact with oxygen.

[0009] • Either the impurity is not reactive, but accelerates the accumulation of all other impurities, including reactive ones. This is the case with compounds whose solidification point is higher than the operating temperature of liquid oxygen. CO2 and N2O are the most critical compounds in this respect, because they may, accidentally or otherwise, not be completely stopped by the adsorption system. Once they reach the oxygen, a solid phase forms and can clog the vaporization channels of the exchanger vaporizing the liquid oxygen. This mechanism, known as "dead-end boiling," is known to accelerate the concentration of all impurities contained in the vaporizing liquid, and therefore the risk associated with hydrocarbons and combustion of the aluminum matrix is ​​increased.WO2018 / 211299 teaches that it is necessary and critical to monitor the impurity content in the air entering the distillation system and / or the oxygen bath in order to control acceptable limit quantities of impurities and ensure the safe operation of the unit.

[0010] However, a substantial portion of the impurities leave the distillation system by dissolving in the liquid oxygen drawn from the bath around the heat exchanger vaporizing the liquid oxygen. An analysis based solely on the air flow rates entering the system, as suggested by WO2018 / 211299, would be overly pessimistic, as it would conclude that the amount of accumulated impurities is greater than it actually is, since it would not take into account the impurities that are removed from the system.

[0011] The impurity or impurities whose content needs monitoring may include CO2, NOx (for example, N2O), and hydrocarbons such as propane. Propane is poorly soluble in liquid oxygen, and CO2 and N2O solidify at temperatures above the operating temperature of the liquid oxygen bath.

[0012] Furthermore, to ensure operational safety, the unit undergoes periodic defrosting to clean it of all accumulated impurities. During this operation, the unit is shut down, and therefore the time between successive defrosting cycles is increased.

[0013] Similarly, the concentration of an impurity in the air can vary over a period of time, for example due to a change in wind direction or during the life of the separation device, for example due to the construction of a factory next to the device emitting the impurity in large quantities.

[0014] It is also necessary to take into account that the accumulation of an impurity in the separation unit can increase suddenly, due to a blockage occurring in an exchanger or elsewhere.

[0015] In "Instrument Engineer's Handbook" by Liptak et al, 4 ème edition, it is explained that defrosting is carried out preventively, typically every 3-5 years or as planned when operating conditions deteriorate, for example if an excessive pressure drop is observed in the exchangers.

[0016] However, it is not suggested to precisely measure the impurities accumulated in the cold part of a cryogenic separation device in order to trigger defrosting based on these values.

[0017] According to one object of the invention, a method is provided for determining the quantity of at least one impurity dissolved in air, the at least one impurity representing, for example, at most 0.5 mol% of the air, the quantity being accumulated over time in a cryogenic distillation air separation unit, comprising a heat exchanger for cooling the air to a cryogenic temperature at one end of the heat exchanger and a column system comprising at least a first column operating at a first pressure and a second column operating at a second pressure, lower than the second, a vessel of the second column being thermally connected to the head of the first column through a vaporizer, at least one air stream being sent to the column system after cooling in the heat exchanger and at least one stream containing at least 90 mol% oxygen being withdrawn from the second column in liquid form.at least one impurity being heavier than oxygen and soluble in air and possibly in liquid oxygen and / or having a solidification temperature lower than an operating temperature of the end of the heat exchanger where the air exits and:

[0018] A) The average content of at least one impurity in the air entering the separation unit is determined for each of a plurality of n time periods totaling a total duration D, the duration D starting at a time t1 and ending at a time t2. The average flow rate(s) of air entering the separation unit to be separated is measured for a single flow rate, or several average flow rates are each measured, for each of the n periods. B) The average content of at least one impurity exiting is determined in the tank of the second column or in each liquid stream containing at least 90 mol% oxygen for a plurality of n' time periods totaling the total duration D starting at time t1 and ending at time t2.and the flow rate of each liquid stream containing at least 90 mol% oxygen is measured for each liquid stream containing at least 90 mol% oxygen exiting the second column for each of the n' periods. From the average concentrations determined and the average flow rates measured during steps A) and B), the quantity of at least one impurity accumulated in the air separation apparatus during time D is determined. Depending on other optional features, the process may include the following steps:

[0019] • All the air destined for the separation unit is sent there in a single flow rate

[0020] • The air destined for the separation unit is sent there at several flow rates

[0021] • The quantity Q of at least one impurity entering the separation unit, or of each impurity entering the separation unit during the time period D, is determined by calculating the quantity entering for each of the n periods p and by totaling the quantities of at least one impurity for the n periods p to equal the quantity of the impurity entering the air.

[0022] • The quantity Q' of at least one impurity exiting, or of each impurity exiting, from the second column during the time period D is determined by calculating the quantity exiting for each period p' and by totaling the quantity of at least one impurity for the n' periods p' for the quantity of the impurity exiting in the at least one liquid stream drawn off, possibly from the tank, from the second column and

[0023] • The quantity of the impurity accumulated or of each impurity accumulated during the time period D is calculated by subtracting Q' from Q.

[0024] • an alarm is triggered and / or the air supply to the separation unit is stopped and / or a message is sent if Q-Q' is greater than a threshold and possibly the air separation unit is defrosted only if Q-Q' is greater than a threshold.

[0025] • n is equal to n' and

[0026] • The quantity q of at least one impurity entering the separation unit, or of each impurity entering the separation unit, during a plurality of n periods whose total duration is equal to D, is determined for the quantity of at least one impurity, or of each impurity, arriving in the air for each of the periods

[0027] o The quantity q' of at least one impurity exiting or of each impurity exiting the second column during the first period for the quantity of the impurity exiting in the at least one liquid withdrawn from the tank of the second column for each of the n periods and

[0028] • The quantity of the impurity accumulated or of each impurity accumulated during the time D is calculated by subtracting q' from q for the first of the n periods and then for the n-1 periods that follow and then the sum of the n quantities q-q' is totaled to give the quantity of the impurity accumulated during the n periods p totaling D.

[0029] • if the sum of the quantities q-q' for at least two consecutive periods is greater than a threshold, the defrosting of the unit is triggered.

[0030] • the content of at least one impurity in the air and / or in oxygen is determined by taking a sample of the air and / or in the oxygen and concentrating the impurity, the concentration process possibly including a liquefaction step.

[0031] • the measurement of the content of at least one impurity is carried out using an analyzer of at least one impurity have a measurement range greater than 0.1 ppm, or even 1 ppm or even 10 ppm for that impurity.

[0032] • the content of at least one impurity is measured or deduced during each period p or p' with a measurement or acquisition frequency of less than 2h, preferably less than 1h.

[0033] • the frequency of determining the content of at least one impurity in the incoming air is the same as, higher than or lower than the frequency of detecting at least one impurity on the outgoing product.

[0034] • at least one impurity is CO2, a hydrocarbon such as propane or a NOx such as N2O.

[0035] • The pressure under which partial vaporization of the liquid takes place is different between air and oxygen.

[0036] • the concentration ratio between the liquid initially introduced into the enclosure and the residual liquid after the partial vaporization step is greater than 10, preferably close to 100.

[0037] • The quantity of at least one impurity accumulated during several consecutive periods throughout the time since a defrosting of the unit is determined, the measured quantities are added together to obtain the quantity of at least one impurity accumulated since this defrosting and a defrosting of the unit is initiated if the quantity of at least one impurity accumulated since the previous defrosting is between two predetermined values.

[0038] • We determine the quantities of at least two impurities accumulated, during several consecutive periods and throughout the duration since a defrosting of the unit, we add the measured quantities to obtain the quantities of each impurity accumulated since this defrosting and we initiate a defrosting of the unit if the quantity of at least one of the impurities accumulated since the previous defrosting is between two predetermined values.

[0039] • No liquid containing more than 90 mol% oxygen is sent from the second column to a mixing column

[0040] • Impurities accumulate mainly in the columns and the heat exchanger

[0041] These optional features can be combined with each other in any way that is consistent with logic and science.

[0042] According to another object of the invention, a cryogenic distillation air separation unit is provided, comprising a heat exchanger for cooling the air to a cryogenic temperature at one end of the heat exchanger and a column system comprising at least a first column operating at a first pressure and a second column operating at a second pressure, lower than the second, a vessel of the second column being thermally connected to the head of the first column through a vaporizer, means for supplying the heat exchanger with air containing at least one impurity, dissolved in air, heavier than oxygen and soluble in liquid oxygen, means for sending cooled air from the heat exchanger to the column system, means for measuring the average incoming air flow rate over a period of time, means for determining the content of at least one impurity in the incoming air during the period of time,means for measuring the average flow rate of outgoing product in any outgoing product containing at least 90% mol of oxygen during the period; means for determining the average content of at least one impurity in each outgoing liquid product containing at least 90% mol of oxygen during the period; means for calculating the quantity of at least one impurity accumulated or of several impurities accumulated during the period; and means for comparing the quantity of the accumulated impurity with a threshold or the quantities of the accumulated impurities with a threshold corresponding to each accumulated impurity.

[0043] Preferably, means for determining the content of at least one impurity in the air are located upstream of the heat exchanger.

[0044] The unit may include means for concentrating at least one impurity in the air and / or the outgoing product upstream of means for determining the content of at least one impurity in the air or the outgoing product respectively.

[0045] WO2018211229 and WO2018211230 describe a method for analyzing the content of at least one contaminant dissolved in a cryogenic liquid having a principal component, the contaminant being heavier than the principal component. In this method:

[0046] • A known quantity L of the initial liquid is introduced at pressure P into a chamber, previously heated to a temperature equal to or lower than the vaporization temperature at pressure P of the liquid to be analyzed, such that no vaporization of the liquid occurs in this step,

[0047] • A specific fraction of the liquid present in the chamber is vaporized by heating it; the vapor thus generated is evacuated from the vaporization chamber to maintain a pressure equal to, or preferably lower than, the pressure P during the vaporization phase.

[0048] • The quantity of liquid thus vaporized, less than L, is precisely regulated so that the quantity of contaminant in the residual liquid present in the enclosure is substantially equal to that in the initial liquid, which implies that its concentration is multiplied by a previously determined factor and

[0049] either

[0050] i. A sample of residual liquid is taken, and the contaminant content is measured after complete vaporization of the residual liquid sample.

[0051] either

[0052] ii. The contaminant content of the residual liquid is measured by completely vaporizing the residual liquid with at least one contaminant it contains, the vaporization chamber being isolated from any input or output of material, and then the contaminant content in the vaporized residual liquid is analyzed.

[0053] and• The contaminant content in the initial liquid is deduced from the measurement of contaminant in the residual liquid.

[0054] The process according to the invention can use the technique of WO2018211229 and WO2018211230 to concentrate the impurity in a fluid in order to determine its content, or another concentration method. It is also possible to omit the concentration step.

[0055] Preferably, a measurement of impurities in the incoming air is carried out:

[0056] • Either by sampling in gaseous form, the sample taken then being liquefied and subcooled to a temperature below its equilibrium temperature at pressure Pconc in a heat exchanger against an external source of cooling (for example liquid nitrogen)

[0057] • Either by sampling already in liquid form in the air separation unit, in such a way that there is no change in impurity composition between the air entering the air separation unit and the liquefied air sample. If necessary, this liquid sample will be subcooled, without altering its impurity composition, to a temperature below the equilibrium temperature at Pconc, possibly against an external cooling source (e.g., liquid nitrogen).

[0058] • The impurity measurement in this sample obtained from one of the two previous methods will be determined according to the technique described in WO2018211229 and WO2018211230, Pconc represents the pressure at which the sample is introduced into the capacity, and before the partial vaporization phase.

[0059] Preferably, a measurement is taken of the least amount of impurity leaving the body, primarily through the oxygen produced:

[0060] • By measuring impurities in the vaporizer bath, for example, according to the technique described in WO2018211229 and WO2018211230

[0061] Alternatively, the method according to the invention may use the technique of FR3163406 or FR315341 to determine the content of at least one impurity in a cryogenic liquid. This method allows for the determination of the content of at least one impurity dissolved in a cryogenic liquid, the at least one impurity being less volatile than the cryogenic liquid. It comprises the steps of:

[0062] • 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 in the container,

[0063] • isolation of the capacity from any material output,

[0064] • 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 saturated with the impurity, • Isolating the vessel from any ingress of matter,

[0065] • Sending the impurity-laden gas from the tank to a gas analyzer, and

[0066] • 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.

[0067] A determination of the quantity of at least one impurity accumulated in the air separation unit is carried out according to a first variant based on concentrations measured in the incoming air and in the oxygen bath of the vaporizer, substantially by the relationship:

[0068] [MATH1]

[0069] Qi acc = Qair in*np * Yi - QOL out * Xi*n'p'

[0070] In which

[0071] Qi acc = quantity of impurity i accumulated during time D

[0072] Qair in = average incoming airflow rate during period p

[0073] Yi = average concentration of impurity i measured in the incoming air during period p; QOL out: Flow rate of oxygen extracted in liquid form from the vaporizer bath

[0074] Xi: average content of impurity i measured in the vaporizer bath during period p'

[0075] p: time interval between two successive measurements of impurities in the incoming air; p': time interval between two successive measurements of impurities in the vaporizer bath

[0076] n: number of periods p in the duration D (integer)

[0077] rï: number of periods p' in the duration D (integer)

[0078] knowing that p can be equal to, greater than or inside p'.

[0079] D will be a multiple of p and also a multiple of p'.

[0080] If p is equal to p', n is equal to n'.

[0081] Thus the sum of the quantities of impurity I accumulated during the time D, starting at tO and ending at t1, is equal to the sum of quantities of impurity i in the supply air during the time D, starting at tO and ending at t1, the time D corresponding to a sum of n periods p less the sum of quantities of impurity i leaving in the liquid oxygen during the time D, starting at tO and ending at t1, the time D corresponding to a sum of n' periods p'.

[0082] In this case, the periods p and p' in the duration D all have the same value. It is also possible to increase the measurement frequency and thus use shorter periods between consecutive measurements. It is prudent to increase the measurement frequency if the amount of accumulated impurity i is observed to be approaching a threshold.

[0083] It is also possible to measure the quantity of each impurity accumulated during the same period for at least two impurities present in air soluble in liquid oxygen.

[0084] Concentrations and flow rates are measured over several consecutive periods without interruption for a series of periods constituting a total duration.

[0085] The quantities of at least one impurity accumulated during each period are therefore added together, to find the total quantity of impurity accumulated from the commissioning of the unit after defrosting (or first start-up), in order to determine the total quantity of impurity accumulated in the unit since its last defrosting.

[0086] The interval between two defrost cycles is therefore determined according to a maximum quantity of accumulated impurity, this maximum quantity being predefined for each impurity, this quantity constituting a threshold. A defrost cycle of the air separation unit is initiated if the quantity of at least one impurity accumulated since the previous defrost cycle exceeds the threshold between two predetermined values. Preferably, when the quantity of accumulated impurity exceeds a threshold, a signal is sent, which can be an electronic message or an audible or visual signal.

[0087] The process can measure the accumulated quantities for at least two different impurities, heavier than oxygen and soluble in liquid oxygen and air, and trigger a signal indicating that defrosting is necessary if the amount of accumulated impurities exceeds the threshold for any one of the impurities.

[0088] The invention will be described in more detail with reference to the figure where:

[0089] [FIG.1] illustrates a method according to the invention.

[0090] A cryogenic distillation air separation unit includes an air compressor 20, an adsorption purification unit 21 for removing water and CO2 from the compressed air, a heat exchanger 25 for cooling the air to a cryogenic temperature at one end of the heat exchanger and a column system including at least a first column 29 operating at a first pressure and a second column 30 operating at a second pressure, lower than the second, a vessel of the second column being thermally connected to the head of the first column through a vaporizer 28.

[0091] Columns 29 and 30 are arranged in a thermally insulated cold box 50, which can also contain the heat exchanger 25 and instrumentation components. Air containing at least one impurity dissolved in air that is heavier than oxygen and soluble in liquid oxygen is separated; the impurity can be a light saturated hydrocarbon (C1, C2), propane, CO2, or N2O.

[0092] The apparatus includes a means FIC1 for measuring the average flow rate of incoming air over a period of time and a means A1 for determining the average content of at least one impurity in the incoming air over the period of time, disposed upstream of the heat exchanger 25.

[0093] In this example, the average flow rate and average concentration are measured in the air immediately after purification step 21, but either measurement can be taken at another location. It is obviously more practical to perform the measurements before any subdivision of the air. However, it is also possible to subdivide the air and then measure the average flow rate and average concentration for each of the airflows.

[0094] The air in this example is then divided into two parts. Part 3 is compressed in a blower 22, cooled by a cooler 24, cooled in the heat exchanger 25, then expanded in a Claude turbine 23 and sent to the tank of column 29. The other part of the air 5 is cooled and liquefied in the heat exchanger 25 and sent to column 29 in liquid form. The air separates in column 29, forming an oxygen-enriched liquid in the tank of column 29 and a nitrogen-enriched liquid at the top. These liquids are sent to column 30 where they separate, forming a gas 8 collected at the top of column 30 and a tank liquid surrounding the vaporizer 28.

[0095] An oxygen-rich liquid flow containing at least one impurity 9 exits column 30 to form a liquid product, and an oxygen-rich liquid flow 6 containing at least one impurity also exits the column 30 vessel. These two liquids are the only ones containing at least 90 mol% oxygen in which at least one impurity concentrates and leaves the column. Obviously, the process can produce a single liquid product containing at least 90 mol% oxygen in which at least one impurity concentrates, or at least three such products.

[0096] It is therefore important to measure their flow rates using means FIC9 and FIC6 to measure the average flow rate of the outgoing product. To determine their average impurity content, liquid is taken from the column 30 vessel and analyzed with analyzer A2. In other cases, the content of each product 6.9 can be measured by a dedicated analyzer that takes a sample of each product 6.9. The values ​​measured by elements A1, A2, FIC1, FIC6, and FIC9 are sent to means C to calculate the quantity of at least one impurity accumulated or of several impurities accumulated during the period. Unit C also includes means for comparing the quantity of the accumulated impurity with a threshold, which may be a hazard threshold. If there are several impurities analyzed, unit C compares the quantities of the accumulated impurities with a threshold corresponding to each accumulated impurity.

[0097] Unit C can also increase the frequency of taking measurements as and when during the measurement period or increase the frequency if an intermediate threshold is reached that is lower than the danger threshold.

[0098] The threshold value will depend on the nature of the impurity being analyzed, the size of the columns, and the type of vaporizer, among other things.

[0099] An alarm is triggered and / or the air supply to the separation unit is stopped and / or a message is sent if the quantity of at least one accumulated impurity exceeds a threshold. The air separation unit may be defrosted depending on whether the alarm is triggered and / or the message is sent. In some cases, it is necessary to concentrate the air or liquid oxygen flow to determine the concentration of the impurity to be measured. The techniques described in WO2018 / 211229 and FR2309982 can be used.

Claims

AMENDED CLAIMS received by the International Bureau on June 4, 2026 (04.06.2026) 1. A method for determining the quantity of at least one impurity dissolved in air, the at least one impurity representing, for example, at most 0.5 mol% of the air, the quantity being accumulated over time in a cryogenic distillation air separation unit, comprising a heat exchanger (25) for cooling the air to a cryogenic temperature at one end of the heat exchanger and a column system comprising at least a first column (29) operating at a first pressure and a second column (30) operating at a second pressure, lower than the second, a vessel of the second column being thermally connected to the head of the first column through a vaporizer (28), at least one air stream being sent to the column system after cooling in the heat exchanger and at least one stream (6, 9) containing at least 90 mol% oxygen being withdrawn from the second column in liquid form,at least one impurity being heavier than oxygen and soluble in air and possibly in liquid oxygen and / or having a solidification temperature lower than an operating temperature of the end of the heat exchanger where the air exits and: A) The average content of at least one impurity in the air entering the separation unit is determined for each of a plurality of n time periods p totaling a total duration D, the duration D starting at a time t1 and ending at a time t2. The average flow rate(s) of air entering the separation unit to be separated are measured for a single flow rate or, for several average flow rates, are each measured for each of the n periods. B) The average content of at least one impurity exiting is determined in the tank of the second column or in each liquid stream containing at least 90 mol% oxygen for a plurality of n' time periods totaling the total duration D starting at time t1 and ending at time t2.and the flow rate of each liquid stream containing at least 90% mol of oxygen is measured for each liquid stream containing at least 90% mol of oxygen exiting the second column for each of the n' periods p' and C) from the average concentrations determined and the average flow rates measured during steps A) and B), the quantity of at least one impurity accumulated in the air separation apparatus during the time D is determined.

2. A method according to claim 1 in which A) The quantity Q of at least one impurity entering the separation unit, or of each impurity entering the separation unit during the time period D, is determined by calculating the quantity entering for each of the n periods p and by totaling the quantities of at least one impurity for the n periods p to equal the quantity of the impurity entering the air. B) The quantity Q' of at least one impurity exiting or of each impurity exiting the second column during the time D is determined by calculating the quantity exiting for each period p' and by totaling the quantity of at least one impurity for the n' periods p' for the quantity of the impurity exiting in the at least one liquid stream withdrawn from the second column and C) The quantity of the impurity accumulated or of each impurity accumulated during the time period D is calculated by subtracting Q' from Q.

3. Method according to claim 2 wherein an alarm is triggered and / or the sending of air to the separation unit is stopped and / or a message is sent if Q-Q' is greater than a threshold and optionally the air separation unit is defrosted only if Q-Q' is greater than a threshold.

4. A method according to any one of the preceding claims wherein the content of at least one impurity in the air and / or in oxygen is determined by taking a sample of the air and / or in the oxygen and concentrating the impurity, the concentration method optionally comprising a liquefaction step.

5. Method according to claim 1 wherein the measurement of the content of at least one impurity is carried out using an analyzer of at least one impurity have a measurement range greater than 0.1 ppm, or even 1 ppm or even 10 ppm for that impurity.

6. A method according to claim 1 or 2 wherein the content of at least one impurity is measured or deduced during each period p or p' with a measurement or acquisition frequency of less than 2h, preferably less than 1h.

7. A method according to any one of the preceding claims wherein the frequency of determining the content of at least one impurity in the incoming air is the same as, greater than or less than the frequency of detecting at least one impurity on the outgoing product.

8. A method according to any one of the preceding claims, wherein the quantity of at least one impurity accumulated is determined over several consecutive periods throughout the time since a defrosting of the unit, the measured quantities are added together to obtain the quantity of at least one impurity accumulated since that defrosting, and a defrosting of the unit is initiated if the quantity of at least one impurity accumulated since the previous defrosting is between two predetermined values.

9. A method according to claim 6, wherein the quantities of at least two accumulated impurities are determined over several consecutive periods and throughout the duration since a defrosting of the unit, the measured quantities are added together to obtain the quantities of each impurity accumulated since that defrosting, and a defrosting of the unit is initiated if the quantity of at least one of the impurities accumulated since the previous defrosting is between two predetermined values.

10. A cryogenic distillation air separation unit comprising a heat exchanger (25) for cooling air to a cryogenic temperature at one end of the heat exchanger and a column system comprising at least a first column (29) operating at a first pressure and a second column (30) operating at a second pressure, lower than the second, a vessel of the second column being thermally connected to the head of the first column through a vaporizer (28), means for supplying the heat exchanger with air containing at least one impurity, dissolved in air, heavier than oxygen and soluble in liquid oxygen, means for sending cooled air from the heat exchanger to the column system, means (FIC1) for measuring the average incoming air flow rate over a period of time, means (A1) for determining the average content of at least one impurity in the incoming air over the period of time,means (FIC6, FIC9) for measuring the average flow rate of outgoing product for any outgoing product containing at least 90% mol of oxygen during the period, means (A2) for determining the average content of at least one impurity in each outgoing liquid product containing at least 90% mol of oxygen during the period, means (C) for calculating the quantity of at least one impurity accumulated or of several impurities accumulated during the period, and means (C) for comparing the quantity of the accumulated impurity with a threshold or the quantities of the accumulated impurities with a threshold corresponding to each accumulated impurity.

Images