Method for capturing nitrogen in liquid argon

Abstract

The present invention relates to a method for purifying liquid argon by removing nitrogen using the adsorbent (molecular sieve) Li-FAU. More specifically, the invention aims to describe a method for capturing dissolved nitrogen to produce liquid argon with a high degree of purity, for the purpose, among other applications, of studying the behaviour of neutrinos in experiments that require massive quantities of ultra-pure liquid argon, for which there is as yet no industrial process.

Description

METHOD OF CAPTURING NITROGEN IN LIQUID ARGON Field of Invention

[0001] The present invention falls within the technical field of physics and materials engineering, more specifically in technologies for purifying cryogenic liquids and gases using adsorbents, and relates to a method for purifying liquid argon by removing dissolved nitrogen using a specific adsorbent (molecular sieve). Background of the Invention

[0002] Gaseous argon is used in special lighting systems and for protecting industrial electrical welding. Liquid argon, in turn, has extensive applications in high-energy physics, as the argon nucleus serves as an important mass scattering center for various particles.

[0003] Currently, liquid argon is the cryogenic liquid of choice for the most important experiments and for the study of neutrino properties, in particular in the Deep Underground Neutrino Experiment (DUNE), which will use approximately 70,000 tons of ultrapure liquid argon, for which there is still no industrial process.

[0004] The use of liquefied noble gases as targets for neutrino and dark matter experiments requires extremely high purity of the liquids in terms of oxygen contamination (<100 ppt). Therefore, ultrapure liquid argon (LAr) is the target of choice for the world's most important particle physics experiments, including DUNE.

[0005] Previous studies have explored the effect of nitrogen and oxygen contamination on LAr to detect a reduction in LAr's scintillation emission as a function of the concentration of these contaminants. In particular, the decreasing behavior in the lifetime and relative amplitude of the slow component is found to be appreciable above 1 ppm nitrogen concentrations and 0.1 ppm oxygen concentrations.

[0006] To achieve the required purity, Gaseous Argon (GAr) and / or LAr is circulated through filters filled with special adsorbents commonly used (BASF Cu-0226 and Mol Sieve 4A) for the capture of O2 and water. On the other hand, N2 capture does not occur through these adsorbents and there are no precedents for liquid argon purification methods with nitrogen removal for concentrations below 10 ppm.

[0007] However, N2 removal is a highly desirable additional feature to increase LAr purity control in case of unwanted leaks and to enable advanced use of LAr scintillation light beyond neutrino detection. Ionizing particle interactions in LAr cause the formation of Ar * 2 excited dimers, whose relaxation leads to the emission of Vacuum Ultraviolet (VUV) photons with λ = 127 nm.

[0008] In view of the above, and taking into consideration that there are no methods in the state of the art for the industrial production of liquid argon with a high degree of purity, the objective of this present invention is to describe a method for the production of liquid argon with a high degree of purity, focused on the capture of nitrogen dissolved in argon. liquid. In the present invention, the purification of liquid argon in terms of the capture of nitrogen dissolved in liquid argon occurs through the capture of nitrogen by a specific adsorbent, the Li-FAU zeolite molecular sieve, in which its molecular formula is Li 87 Al 96 Yes 96 THE 384 , with a Si / Al atomic ratio of 1 and a lattice length of 24.55 Å.

[0009] The method allows the capture of nitrogen at concentrations lower than 10 ppm in industrial liquid argon, and becomes a great alternative for argon purification in the event of an unexpected leak in liquid argon tanks, followed by nitrogen contamination. State of the Art

[0010] Document US5706674A describes a process for recycling impure argon effluent from a silicon crystal growth furnace using cryogenics. Argon purification from this process occurs in two steps: fractional distillation followed by adsorption by another type of adsorbent (non-Li-FAU). However, the purity level of this product is not specified.

[0011] The ultimate goal of this document is a purified gaseous Ar stream, and it is not explicit that N2 removal occurs in liquid Argon, but rather at low (cryogenic) temperatures. The purification of liquid Argon by the present invention occurs in a single step, at near-ambient pressure and cryogenic temperature, by adsorption of small concentrations of Nitrogen on a distinct zeolite, Li-FAU. The production is ultrapure Argon: 99.999%.

[0012] Document US20220387923A1 essentially discloses the use of a zeolitic adsorbent material based on faujasite zeolite (FAU) crystals for the non-cryogenic separation of industrial gases by variable pressure adsorption (VPSA), particularly of gases from air. This process occurs in the gas phase and aims to capture part of the nitrogen present in the air for enrichment of hospital oxygen.

[0013] Compared to the present invention, the objectives of both processes are different: in the aforementioned document: the production of enriched oxygen, and in the present invention: the production of ultrapure argon. Furthermore, the process occurs in the gas phase. As previously described, the purification of liquid argon by the present invention occurs in a single step, at near-ambient pressure and cryogenic temperature, by adsorption of small concentrations of nitrogen on a distinct zeolite, Li-FAU. The production is ultrapure argon: 99.999%.

[0014] Document BR1120180017230 develops a method and apparatus for argon recovery, in which argon is separated from air within a cryogenic air separation facility. It should be noted that the purification of argon in the gas phase of this process occurs in two stages: fractional distillation followed by adsorption by another type of adsorbent.

[0015] The degree of purity of this product is not specified. As previously described, the purification of liquid argon by the present invention occurs in a single step, at near-ambient pressure and cryogenic temperature, by adsorption of small nitrogen concentrations in a distinct zeolite, Li-FAU. The production is ultrapure argon: 99.999%.

[0016] Patent document PI07006411 protects a method for producing argon and equipment for producing high-purity argon. The method seeks to improve argon recovery from air using a cryogenic rectification column in combination with a pressure swing adsorber. Argon purification in the gas phase of this process occurs in two steps: pressure-swing fractional distillation (PSA) followed by adsorption onto carbon, another type of adsorbent. Note that the purity level of this product is not specified.

[0017] Patent document US4477265A describes a method for purifying argon (including nitrogen retention) in which argon is recovered from a gas stream containing argon mixed with oxygen and nitrogen. In this process, the nitrogen present as an impurity in the argon is removed at 0°C in the gas phase by pressure-varying gas adsorption (PSA). It should be noted that the adsorbent used is not specified, nor is the degree of purity of the argon produced, as is the case in the present invention.

[0018] And finally, document US6244071B1 is also part of the general state of the art, and discusses a process for purifying cryogenic fluid (liquid state) containing impurities such as N2O, CnHm and / or NOx. The process for purifying especially argon, helium or oxygen, establishes that the impurities (such as nitrogen) are removed by placing the cryogenic fluid to be purified in contact with particles of an adsorbent having an average size less than or equal to 1.5 mm, preferably from 0.8 mm to 1.1 mm.

[0019] In this process, the removal of various gases present as impurities in argon occurs at temperatures below -120^C (still in the gas phase), by adsorption at a variable pressure (PSA). According to the inventors, some adsorbents are used; however, there is no explicit mention of Li-FAU as an adsorbent.

[0020] Thus, a general analysis of prior art documents reveals no methods demonstrating that the adsorbents used are effective at cryogenic temperatures and at low nitrogen concentrations (100 ppm) dissolved in liquid argon. In the present invention, in contact with the Li-FAU zeolite, nitrogen has a greater affinity for it than argon. This information was obtained from the adsorption isotherms of both substances with Li-FAU and subsequently confirmed in experiments in a liquid argon cryostat. Brief description of the invention

[0021] The present invention relates to a method for purifying liquid argon by removing nitrogen using the Li-FAU adsorbent (molecular sieve). More specifically, the invention aims to describe a method for capturing dissolved nitrogen to produce highly pure liquid argon, aiming, among other applications, at studying neutrino behavior in experiments requiring massive quantities of ultrapure liquid argon, for which there is currently no industrial process. Ultrapure liquid argon is obtained by capturing small amounts of nitrogen, less than 100 ppm, dissolved in industrial argon, through the capture of nitrogen by a specific adsorbent, the molecular sieve, Li-FAU zeolite. Brief description of the figures

[0022] Figure 1 shows an illustrative diagram of the main stages of the process of capturing nitrogen dissolved in liquid argon.

[0023] Figure 2 presents a plot of the LAr scintillation light emission amplitude as a function of time for selected photomultiplier tube (PMT) signals acquired during LAr purification.

[0024] Figure 3 shows a graph of the PMT signal of the LAr scintillation light as a function of time during N2 contamination.

[0025] Figure 4 shows a graph of the PMT signal of the LAr scintillation light as a function of time during the LAr purification procedure.

[0026] Figure 5 shows a graph of N2 concentration as a function of time for two rounds of LAr circulation through the Li-FAU molecular sieve adsorbent, and a control experiment for a similar circulation through the Molecular Sieve 4A adsorbent (Mol Sieve 4A).

[0027] Figure 6 presents a graph with two sets of experimental data of adsorption curves for the three media considered: Ca-LTA (5A), Li-FAU and H-FAU (HSZ390), in which Figure 6A represents the results for Argon adsorption, and 6B the results for Nitrogen adsorption.

[0028] Figure 7 presents a graph with the experimental data of argon adsorption in Figure 7A and Nitrogen in Figure 7B, for the Li-FAU material, and the curves fitted from several equilibrium isotherms.

[0029] Figure 8 shows a graph with simulated data from the argon-nitrogen purification process using Li-FAU under the following conditions: T = 90 K (-183.15 ºC), CN 2,0 =0.1374 kg / m 3(100 ppm), Q=1 L / min, Li-FAU. Figure 8A shows the concentration profile of argon and nitrogen over time at the column outlet, while Figure 8B shows the adsorbed mass of argon and nitrogen over time in the Li-FAU column. Detailed description of the invention

[0030] The present invention falls within the field of physics and materials engineering and comprises a method for producing high-purity liquid argon, focused on capturing nitrogen dissolved in liquid argon. Figure 1 describes the main steps of the process for capturing nitrogen dissolved in liquid argon, which occurs in a system (called PuLArC) that essentially comprises a few components, namely a reservoir containing industrial liquid argon, contaminated with small amounts (~100 ppm) of dissolved nitrogen.

[0031] In addition, a pump injects liquid argon into the adsorption column. The column contains the adsorbent consisting of the Li-FAU molecular sieve, which captures part of the nitrogen contained in liquid argon, and a detector, which analyzes the purity of the liquid argon leaving the adsorption column.

[0032] The partially purified liquid argon returns to the reservoir and is recycled several times into the column, where the dissolved nitrogen is recaptured. The liquid argon is recycled repeatedly, and its purity is measured by the detector. The liquid argon is recycled several times until the required purity level is reached.

[0033] Due to the quadrupolar nature of nitrogen molecules, they interact with lithium cations present in the zeolitic adsorbent. Consequently, nitrogen adsorption is preferred over argon.

[0034] However, among all the adsorbents analyzed, Li-FAU zeolite presents a highly favorable attraction for the nitrogen molecule due to the smaller ionic radius of Li+ and the polarity of nitrogen, presenting the best performance in N2 capture. The molecular formula of Li-FAU zeolite is Li 87 Al 96 Yes 96 THE 384 with a Si / Al atomic ratio of 1 and a lattice length of 24.55 Å.

[0035] Additionally, the purification of liquid argon by the present invention occurs at pressure P = 1 to 3 Bar, cryogenic temperature 89 K (-184.15 o C), by adsorption of small concentrations of nitrogen on a distinct zeolite, Li-FAU. The production is ultrapure argon: 99.999%. Methodology

[0036] In summary, the process comprises the steps of weighing and drying the adsorbent in granular form. (step a), preparation of the adsorption column, with the insertion of the adsorbent (step b), filling the reservoir with liquid argon containing impurities (step c), activating the system so that the pump injects the liquid argon into the adsorption column, containing an adsorbent (step d), returning the liquid argon to the system and repeating step d until adequate purity is obtained (step e).

[0037] Before preparing and activating the system, as an example, the amount of adsorbent is weighed and dried (step a), such as approximately 1.1 to 1.3 kg of commercial molecular sieve Li-FAU, in granular form (0.1 mm in diameter), which are dried by heating the adsorbent to a temperature of T = 245 o C to T = 255 oC for 14 to 16 hours with a N2 gas flow rate of 3.9 to 4.1 L / min. Furthermore, for comparison purposes, approximately 1.9 to 2.1 kg of BASF's commercial Cu-S0226 products in granular form (0.1 mm diameter) were activated at a temperature of T = 185 o C to T = 195 o C for 14 to 16 hours with a flow of 3.9 to 4.1 L / min of 2.45 to 2.55% H2 in argon gas.

[0038] The two adsorbents were placed on the two filters of the PuLArC system, generating a 28 to 30 cm adsorption column for both media (adsorption column preparation – step b). For the experiments reported in this method, the PuLArC reservoir was filled (step c) with approximately 89 to 91 L of commercial Liquid Argon 5.0 (Purity: 99.999%), which, according to the supplier, contains: Total Hydrocarbon Content (THC) < 0.5 ppm, Oxygen < 1 ppm, H2O < 2 ppm, Nitrogen < 3 ppm, Carbon Dioxide < 1 ppm, and Carbon Monoxide < 1 ppm.

[0039] In summary, in the process that occurs within the PuLArC system, first, the industrial liquid argon, contaminated with 100 ppm or more of dissolved nitrogen, is placed in the reservoir to be purified (step c), then the pump injects the liquid argon into the adsorption column (step d), containing a specific adsorbent, which may be the Li-FAU molecular sieve, to capture the nitrogen dissolved in it.

[0040] The liquid argon is then returned to the reservoir, and steps c) and d) continue until the desired purity or Li-FAU saturation is reached. In this case, the liquid argon was circulated for 200 min, so that the applied method achieved the production of ultrapure liquid argon with nitrogen concentrations lower than 0.1 ppm.

[0041] The purity of LAr inside the PuLArC container was monitored by direct acquisition of detector signals, in the implementation of the present invention a photomultiplier (PMT), which allows extracting with high precision the main characteristics of the scintillation light emission of liquid argon as a function of its contaminants.

[0042] Thus, during the various runs performed, liquid argon (LAr) from the PuLArC system vessel was circulated at a flow rate of 4 L / min through a column with the Li-FAU molecular sieve and another with the BASF Cu-S0226 oxygen capture material for a few hours until the time constant of the slow component of the LAr scintillation light reached a saturation value of ^ T = 1400 ns, which indicates contamination of less than 1 ppm of nitrogen and less than 0.1 ppm of oxygen.

[0043] The highest purity LAr was then spiked with 50 ppm N2 and circulated at the same 4 L / min flow rate, passing only through the filter containing the Li-FAU molecular sieve adsorbent. These results clearly demonstrate that circulating LAr through the Li-FAU adsorbent for approximately 2 hours reduced the N2 from 20-40 ppm to 2-0.5 ppm.

[0044] Figure 2 shows the reduction in the amplitude of the LAr scintillation light emission as a function of time acquired by signals from a PMT during the LAr purification procedure performed in the PuLArC, when LAr was circulated at a flow rate of 4 L / min through the filters with Li-FAU molecular sieve and BASF Cu-S0226.

[0045] Figure 3 shows the PMT signal associated with the amplitude of LAr scintillation light emission as a function of time, during LAr contamination with approximately 50 ppm N2. The PMT was acquired at different times during N2 contamination.

[0046] As can be seen, the decay time of the slow component of the LAr scintillation light emission amplitude clearly decreases during contamination from τT ≈ 1400 ns for t - 0 to τT ≈ 200 ns for t - 50s. Consistently, τT ≈ 200 ns which corresponds to ≈ 40 ppm N2.

[0047] The PMT signal of the LAr scintillation light as a function of time during the N2 purification procedure, performed in the PuLArC, is shown in Figure 4. Here, LAr was circulated at a flow rate of 4 L / min only through the Li-FAU molecular sieve filter.

[0048] Surprisingly, the decay time of the slow component of the LAr scintillation light increases rapidly during the LAr purification procedure, changing from τT ≈ 200 ns to t = 0 to τT ≈ 900 ns for a measurement time of 3 h. Interestingly, the purification time of approximately 2 h was predicted by continuum fluid dynamics (CFD) simulations for the capture of 100 ppm N2 in LAr using Li-FAU adsorbent.

[0049] Subsequently, a second LAr purification run with controlled N2 contamination was performed using the Li-FAU adsorbent, followed by a control experiment under the same conditions using the 4A molecular sieve. Figure 4 summarizes the results recently obtained using PuLARC regarding the capture of N2 from LAr.

[0050] The N2 concentration as a function of time for two runs using 4 L / min circulations of LAr for 200 min through the Li-FAU adsorbent filter and a control experiment for a similar circulation through the 4A molecular sieve is shown in Figure 5.

[0051] Figure 5 shows the decrease in nitrogen concentration, in ppm, present as a contaminant in liquid argon, as a function of the recycling time in the adsorption column, described in Figure 1. The points indicated with squares (black line), show the nitrogen concentration in the column effluent when using the 4A molecular sieve, showing that the nitrogen content is practically constant (around 40 ppm), and that it is not efficient for capturing nitrogen.

[0052] The blue curve shows the drop in the concentration of nitrogen present as a contaminant in liquid argon, as a function of the recycling time in the adsorption column (Test 1), containing Li-FAU, showing that, after 180 minutes of recycling, it drops from 40 ppm to approximately 1.8 ppm of N2.

[0053] The green curve shows the decrease in the concentration of nitrogen present as a contaminant in liquid argon as a function of recycling time in the adsorption column (Test 2) containing Li-FAU, showing that, after 200 minutes of recycling, it drops from 20 ppm to approximately 0.6 ppm N2. The black curve shows a control experiment using the 4 A molecular sieve adsorbent, for which there is no appreciable capture of N2. Predictive Method

[0054] Algorithms were important as a way to predict certain development parameters, such as temperature, filter dimensions, runtime, and other important optimization parameters. The mathematical model presented consists of a system of three partial differential equations, after some simplifying assumptions.

[0055] The methodology for solving the mathematical model is implemented in MATLAB language, and the model was validated with state-of-the-art experimental data, for mono and multicomponent cases.

[0056] Since information on the separation of nitrogen from argon is rare, it is necessary experimental procedures to determine adsorption isotherms and find suitable parameters for simulations.

[0057] The experimental analysis related to the adsorbed amount of nitrogen and argon in the solid media was performed, reporting the equilibrium data for each material considered, Ca-LTA 5A, Li-FAU and H-FAU.

[0058] Based on the results obtained, it was possible to classify the equilibrium isotherms according to their shape. The most appropriate isotherm must be defined and adjusted to feed the mathematical model.

[0059] The main adsorption isotherms found in the literature are those presented by Langmuir, Freundlich and Redlich-Peterson, reference: MA Al-Ghouti and DA Da'ana, Guidelines for the use and interpretation of adsorption isotherm models: A review, Journal of Hazardous Materials 393, 122383 (2020).

[0060] Seven isotherm models were used to find the best fit to the experimental data, with their mathematical representations given by: ^ Langmuir: ^ Redliche-Perterson: S: ^^^^^^ SIP ^ = ^ ^ ೄ ^ (^ା^ ೄ ^ ^ ) ^ BET: ^^ ^ ^ ^ =^ಳಶ^ భ,ಳಶ^^ ൫^ା^ మ,ಳಶ^ ^ ൯൫ ^ି^ మ,ಳಶ^ ^ା^ భ,ಳಶ^ ^ ൯ ^Jovanovich: ^ Kahn:

[0061] With ^^ being the adsorbate concentration (kg / m 3 ) ^^ ^ is the equilibrium adsorbed concentration (kg adsorbent / kg solid) and ^^ ^ is the maximum adsorbed capacity (kg adsorbent / kg solid). ^^ ^ is the Langmuir constant (m 3 / kg), ^^ ோ^ (m 3 / kg) ^^ ோ^ (1 / kg), ^^ (-) being constants of the Redliche-Perterson model. ^^ ௌ (m 3 / kg) and ^^ (-) are the constants for the SIPS model. ^^ ^^ா் refers to the adsorption capacity for the BET model (kg / kg), ^^ ^,^ா் (m 3 / kg), ^^ ଶ,^ா் (m 3 / kg) are equilibrium parameters of the BET isotherm. ^^ ^ is the constant for the Jovanovich model (m 3 / kg). ^^ ் (kg / kg) ^^ ் (m z / kg z ) are constants of the Toth model and ^^ is the constant that describes the degree of heterogeneity of the adsorption system of the Toth isotherm. ^^ ^ (m 3 / kg) and ^^ ^ are constants for the Kahn isotherm.

[0062] Figure 6 shows an example of data fitting used in the mentioned models for the adsorption of argon and nitrogen on the Ca-LTA 5A adsorbent. It is clear that some models represent the experimental data accurately, while others fail to do so.

[0063] Figure 7 shows an example of data fitting using the isotherms for argon and nitrogen adsorption for the Li-FAU adsorbent. It is clear that some models accurately represent the experimental data, while others fail to do so.

[0064] It is necessary to quantify the quality of the fit using statistical parameters. To do this, the Residual Sum of (SSE), R2, and the root mean square error mean (RMSE). These parameters can be calculated as follows. Squared error: Coefficient of determination (R-squared): Mean square error:

[0065] With ^^ ^௫^ and ^^ ^^^^ being the experimental and calculated evaluated value respectively, N the number of samples. Example of implementation of the prediction algorithm

[0066] Using the isotherm parameters, it was possible to simulate the experimental test setup. The column dimensions are 0.0955 m in diameter and 0.48 m in height. The solid material density is assumed to be 1400 kg / m³, with a particle diameter of 0.1 cm, bed porosity of 0.63, and particle porosity of 0.4.

[0067] Liquid argon at a constant temperature of 90 K (-183.15 ºC) has a density of 1378 kg / m 3 . The nitrogen impurity at the inlet is 100 ppm, equivalent to a concentration of 0.1374 kg / m 3 .

[0068] In this case study, the fixed bed column is initially considered empty, with a volumetric flow rate of 1 L / min. The input concentration for both phases remains constant throughout all simulations.

[0069] For this simulation, the Li-FAU material was chosen. Result: Argon with nitrogen impurities flows through the column, and a certain amount of argon and nitrogen is adsorbed by the medium.

[0070] Figure 8 shows the simulation results for the test case. The main behavior of the adsorption process can be seen by observing the time dependence of the dimensionless concentration of each phase in the outlet stream (C i / C0), and the amount of each phase that is adsorbed on the solid phase (q i ).

[0071] Argon with nitrogen impurity flows through the column, and a certain amount of argon and nitrogen is adsorbed. Figure 8 shows that after 5 minutes, the argon concentration at the column outlet is close to that at the inlet. At this point, there is no more argon trapped by the solid phase (saturation), as seen in the qi analysis.

[0072] After this time, due to a change in the Ar-N2 concentration, nitrogen is slowly adsorbed by the solid phase and occupies the adsorption sites on the particle surfaces. During this process, argon is desorbed, and after 400 minutes, the concentrations in the system remain constant. The total amount of argon adsorbed (m Ar ) and nitrogen (m N2 ) by the solid phase, under these conditions, are close to 0.5278 and 0.0194 kg, respectively.

Claims

1 / 2 CLAIMS 1) Method of producing liquid argon by capturing nitrogen dissolved in liquid argon, characterized by comprising the steps of: a) weighing and drying the adsorbent in granular form; b) preparing the adsorption column, with the insertion of the adsorbent; c) filling the reservoir with liquid argon containing impurities; d) activating the system so that the pump injects the liquid argon into the adsorption column, containing an adsorbent; e) returning the liquid argon to the system and repeating step d until obtaining adequate purity. 2) Method, according to claim 1, characterized by the fact that the argon obtained has adequate purity of at least 99.999%. 3) Method, according to claim 1 or 2, characterized by the fact of capturing nitrogen in concentrations lower than 100 ppm. 4) Method according to any one of claims 1 to 3, characterized in that it occurs at a cryogenic temperature of 89 K (-184.15 oC). 5) Method according to any one of claims 1 to 4, characterized in that the adsorbent is preferably Li-FAU zeolite, formula Li 87 Al 96 Yes 96 THE 384 , with a Si / Al atomic ratio of 1 and a lattice length also of 24.55 Å. 2 / 2 6) Method according to any one of claims 1 to 5, characterized in that in step a) the adsorbent is heated and dried at a temperature of 245 ºC to 255 ºC for 14 to 16 hours with a N2 gas flow of 3.9 to 4.1 L / min. 7) Method according to any one of claims 1 to 6, characterized in that in step a) 1.1 to 1.3 kg of Li-FAU adsorbent in granular form are weighed and dried. 8) Method according to any one of claims 1 to 7, characterized in that in step b) the adsorption column comprises 28 to 30 cm for both media. 9) Method according to any one of claims 1 to 8, characterized in that in step c) the reservoir is filled with 89 to 91 L of argon.

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