A method for separating helium and neon

Abstract

The present invention relates to a method for separating helium and neon, which comprises adsorbing and separating a mixed gas containing helium and neon with an adsorbent containing a metal organic framework material, wherein the metal organic framework material comprises a metal ion and an organic ligand, wherein the organic ligand comprises a compound shown in Formula I; and the metal ion is selected from transition metal ions. The metal organic framework material of the present invention has low raw material price, mild synthesis conditions, simple operation, easy post-treatment, and good stability. At the same time, the metal organic framework material is used for low-temperature adsorption separation with a high neon adsorption capacity. Compared with the existing helium-neon low-temperature separation process, it has significant advantages such as low energy consumption, low equipment requirements, and simple process, and has broad industrial application prospects.

Description

Technical Field

[0001] The invention relates to the field of adsorption separation, and in particular to a method for separating helium and neon. Background Art

[0002] Helium (He) and neon (Ne) are important rare strategic resources. Their content in the air is extremely low (He: 5.24ppm; Ne: 18.21ppm), but they have irreplaceable applications in high-tech fields such as military industry, electronics, medical, semiconductors, aerospace, and basic research. Among them, helium is the lightest inert rare gas other than hydrogen, and it is also the gas with the lowest known boiling point. It has the properties of low density, good stability, strong diffusivity, and high thermal conductivity. It is indispensable in liquid fuel rocket launches, ultra-low temperature scenes, and deep diving. Neon is widely used in lighting and as an important filling medium in the electronics industry. In the electronics field, extremely high-purity helium is a necessary protective gas for chip manufacturing, and neon is an important lithography raw gas. In the mixed gas used in excimer lasers, neon accounts for more than 96%. It is estimated that the consumption of neon in semiconductor factories exceeds 75% of the total global neon consumption. The interruption of neon supply will lead to the shutdown of the semiconductor industry chain. In recent years, with the rapid development of domestic chip, high-tech industry, national defense science and technology and other application industries and the progress of large-scale air separation technology, the market demand for rare gases has continued to increase, greatly promoting the localization of the specialty gas industry. It can be seen that the separation and purification technology of neon and helium faces new opportunities and challenges, which is also a major challenge and research frontier in the field of chemical separation in recent years.

[0003] In industry, neon is mainly produced from cryogenic air separation waste gas, while helium mainly relies on natural gas with a higher content. In the extraction of air separation equipment, helium and neon are both non-condensable gases at 77K. The enriched crude neon-helium fraction needs to face the separation process of various gases (N2, H2, He, Ne, etc.), including oxygenation and dehydrogenation, condensation and nitrogen removal, etc., but the extremely low boiling point makes helium (4.30K) and neon (27.07K) difficult to liquefy and separate. Although China is one of the countries with the most air separation equipment in the world, most air separation equipment does not have a neon-helium extraction section and a crude neon-helium distillation tower in the lower tower of the cold box, and cannot extract neon and helium gases. Only a few manufacturers that can extract crude neon-helium are also facing the blockade of neon-helium refining technology and extremely high costs. The current mainstream helium-neon separation technologies are liquid hydrogen condensation method, liquid neon condensation method, etc. The liquid hydrogen condensation method uses reduced pressure hydrogen as a cold source, and the temperature is maintained at 13-15K to freeze neon to achieve separation. The purity is about 99.9%, with high energy consumption and poor safety. The liquid neon condensation method uses a high-pressure throttling cold cycle to liquefy the neon in the raw gas. Due to the phase equilibrium limitation, the purity of neon is only 99.9% at most, and it is difficult to directly obtain high-purity helium. In addition to the outstanding purity stability and poor safety problems, since the separation requires an ultra-low temperature control system, throttling expansion is required to gradually cool to the target temperature, which consumes huge energy. Therefore, the development of a new neon-helium separation technology with low energy consumption and high product purity has important industrial value and is of great strategic significance for breaking the foreign technology monopoly.

[0004] In contrast, adsorption separation technology with adsorbent materials as the core has the advantages of short process, low equipment investment and low operating energy consumption, and is considered to be a strong competitor to replace distillation separation. As early as the 1950s, attempts at low-temperature adsorption separation began at home and abroad, but traditional adsorbent materials such as activated carbon, molecular sieves, and silica gel were mainly used. Under a pressure of 3 to 15 MPa, pure neon and helium were obtained through two adsorption processes at temperatures of 77K and 68K respectively. After desorption by activated carbon and molecular sieves, the purity of neon and helium was 98-99%. The limitations of traditional materials are still very obvious. For example, the Ne adsorption capacity of activated carbon at 77K and a pressure of 13.3 kPa is 12 cm 3 / g, the separation selectivity is only 1.1-2.5; while the Ne adsorption capacity of molecular sieve 13X at 77K and pressure of 26.6kPa is as low as 3.5cm 3 / g. It can be seen that the adsorption capacity of neon and the separation selectivity of neon and helium by traditional adsorbents are relatively poor, and it is impossible to produce high-purity neon and helium in one step. It requires extremely low temperatures to achieve practical separation, which is even more difficult than the adsorption of hydrogen. At present, the adsorption efficiency of adsorbents has become a bottleneck problem for improving the purity of neon and helium.

[0005] In recent years, metal-organic frameworks (MOFs), as a new type of porous materials, have shown great prospects in the field of gas adsorption and separation due to their advantages such as easy surface functionalization and controllable pore size. They are widely used in the selective separation of gas molecules with similar structures. Barreto et al. studied the adsorption of Ne by two star materials, HKUST-1 and UiO-66, at low temperature and high pressure. The results showed that HKUST with open metal sites and extremely high porosity can adsorb 22.21mmol / g of Ne at 77K and 49bar, while the Ne capacity of UiO-66 at 77K and 86bar is relatively low, at 17.63mmol / g. Rui et al. studied Co3(ndc)3(dabco) at 77K and 84bar, and the Ne adsorption amount was 26.47mmol / g. However, at normal pressure, the adsorption capacity was less than 2mmol / g even at 77K, which is not possible for practical application. It is still challenging to develop adsorbent materials with high Ne adsorption capacity at normal pressure. Summary of the invention

[0006] In view of the above problems, the present invention proposes a method for separating helium and neon, which has significant separation advantages and overcomes the disadvantages of high energy consumption of traditional liquid hydrogen condensation method and liquid neon condensation method.

[0007] Specifically, the present invention provides a method for separating helium and neon, which comprises separating a mixed gas containing helium and neon by adsorption using an adsorbent containing a metal organic framework material, wherein the metal organic framework material comprises metal ions and organic ligands,

[0008] Wherein, the organic ligand includes a compound shown in formula I,

[0009]

[0010] In formula I, R is selected from hydrogen, halogen or C1-C4 alkyl;

[0011] The metal ions are selected from transition metal ions.

[0012] In some embodiments, in Formula I, R is selected from hydrogen, fluorine, chlorine, bromine, methyl, ethyl, or propyl.

[0013] In some embodiments, the organic ligand is 2,5-dihydroxy-1,4-benzoquinone. In some embodiments, the metal ion is selected from one or more of iron ions, manganese ions and zirconium ions.

[0014] Transition metal ions such as iron ions are coordinated with 2,5-dihydroxybenzoquinone through coordination bonds. Each metal center is formed by octahedral coordination of six oxygen atoms of three different deprotonated 2,5-dihydroxybenzoquinone ligands to form a layered structure, and three independent layered structures are interlaced to form a metal organic framework material with a one-dimensional pore structure. The oxygen-rich polar pores of the metal organic framework material have a high adsorption capacity for neon and high neon-helium separation selectivity, and the material structure and adsorption performance are stable, with good prospects for industrial application.

[0015] In some embodiments, the temperature of the adsorption separation is 77K-298K, for example, 80K, 85K, 90K, 100K, 110K, 120K, 130K, 140K, 150K, 160K, 170K, 180K, 190K, 200K, 210K, 220K, 230K, 240K, 250K, 260K, 270K, 180K, 290K or any value therebetween. In some embodiments, the temperature of the adsorption separation is 77K-87K.

[0016] In some embodiments, the pressure of the adsorption separation is 0.01MPa-10MPa, for example, 0.05MPa, 0.1MPa, 0.3MPa, 0.5MPa, 0.7MPa, 1MPa, 3MPa, 5MPa, 7MPa or any value therebetween. In some embodiments, the pressure of the adsorption separation is 0.1MPa-1MPa.

[0017] In some embodiments, the temperature of the adsorption separation is 77 K and the total pressure of the mixed gas is 100 kPa. The separation in the method provided by the present invention can be carried out at normal pressure, which exceeds most existing adsorbents.

[0018] In some embodiments, the pore size of the metal organic framework material is Above, preferably More preferably

[0019] In some embodiments, the specific surface area of ​​the metal organic framework material is 300-2000m 2 / g.

[0020] In some embodiments, the metal organic framework material is in the shape of a cube, a needle or a rod. The metal organic framework material used in the present invention can be prepared into spherical, columnar, granular and other adsorption and separation materials through different processing techniques.

[0021] In some embodiments, the method for preparing the metal organic framework material comprises: reacting a soluble transition metal salt and a compound represented by Formula I in a solvent, and washing and drying the reaction product.

[0022] In some embodiments, the transition metal soluble salt is selected from one or more of soluble iron salts, soluble manganese salts and soluble zirconium salts. In some embodiments, the transition metal soluble salt is selected from one or more of ferric chloride, ferric nitrate, ferric sulfate, ferric perchlorate, manganese chloride, manganese nitrate, manganese sulfate, manganese perchlorate, zirconium chloride, zirconium nitrate and zirconium sulfate.

[0023] In some embodiments, the compound represented by Formula I is 2,5-dihydroxy-1,4-benzoquinone.

[0024] In the preparation process of the metal organic framework material in the present invention, readily available organic ligands such as 2,5-dihydroxybenzoquinone ligands are reacted with transition metal salts such as metal iron salts in a solvent. The raw material price of the prepared material is low, the synthesis conditions are mild, the operation is simple, the post-treatment is easy, and the material synthesis cost is low.

[0025] In some embodiments, the molar ratio of the transition metal soluble salt to the compound of Formula I is 1:(1-5), for example, 1:1.3, 1:1.5, 1:1.7, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5 or any value therebetween, based on the transition metal ion. In some embodiments, the molar ratio of the transition metal soluble salt to the compound of Formula I is 1:(1-2).

[0026] In some embodiments, the reaction temperature is 10° C.-120° C., for example, 20° C., 30° C., 40° C., 50° C., 60° C., 70° C., 80° C., 90° C. or 100° C. In some embodiments, the reaction temperature is 10° C.-50° C. In some embodiments, the reaction temperature is room temperature.

[0027] In some embodiments, the reaction time is 0.5h-48h, for example, 1h, 5h, 10h, 15h, 20h, 25h, 30h, 35h, 40h or 45h. In some embodiments, the reaction time is 3h-24h.

[0028] In some embodiments, the reaction is carried out under stirring. In some embodiments, the stirring speed is 300rpm-1000rpm, for example, 400rpm, 500rpm, 600rpm, 700rpm, 800rpm or 900rpm. In some embodiments, the stirring speed is 400rpm-700rpm. Too fast or too slow stirring speed will affect the yield and particle size.

[0029] In some embodiments, the solvent is selected from one or more of methanol, ethanol, acetonitrile, acetone, N,N-dimethylformamide, N,N-dimethylacetamide and water, preferably water.

[0030] In some embodiments, the molar ratio of the solvent to the soluble transition metal salt is (5-30):1, calculated on a transition metal ion basis.

[0031] In some embodiments, the drying is vacuum drying. In some embodiments, the temperature of vacuum drying is 30° C.-120° C. In some embodiments, the time of vacuum drying is 6 h-24 h.

[0032] In some embodiments, the washing comprises washing and filtering with water for several times to replace the residual unreacted ligand and inorganic salt.

[0033] In some embodiments, the adsorptive separation is performed using a fixed bed, wherein the adsorbent is filled in the fixed bed adsorption column.

[0034] In some embodiments, the adsorptive separation comprises the following steps:

[0035] (1) passing a mixed gas containing helium and neon through a fixed bed adsorption column, wherein strongly adsorbed neon is adsorbed on the adsorbent and weakly adsorbed helium penetrates the adsorption column, thereby obtaining weakly adsorbed helium;

[0036] (2) Desorbing the strongly adsorbed neon gas from the adsorbent to obtain the strongly adsorbed neon gas.

[0037] In some embodiments, the flow rate of the mixed gas comprising helium and neon through the fixed bed adsorption column is 1-100 mL / min / g adsorbent, for example, 5 mL / min / g adsorbent, 10 mL / min / g adsorbent, 20 mL / min / g adsorbent, 30 mL / min / g adsorbent, 40 mL / min / g adsorbent, 50 mL / min / g adsorbent, 60 mL / min / g adsorbent, 70 mL / min / g adsorbent, 80 mL / min / g adsorbent or 90 mL / min / g adsorbent.

[0038] In some embodiments, the adsorption separation includes the following specific steps: loading the metal organic framework material into a packing column; passing a mixture of neon and helium through the packing column. Helium has a weak interaction force with the adsorbent, and is almost not adsorbed. It flows out from the tail end of the packing column quickly, and high-purity helium can be obtained at the outlet of the adsorption bed, thereby achieving helium separation. However, neon has a strong interaction force with the adsorbent, and slowly flows out from the tail end of the packing column after the adsorption reaches saturation. During the desorption process, neon is introduced from the inlet of the adsorption bed to replace the helium in the bed. After the replacement is completed, the adsorption bed is depressurized to desorb the neon in the gaps in the bed to achieve neon separation; the different interaction forces between the material and the two gases can achieve effective separation of the neon and helium mixed gas.

[0039] In some embodiments, the method further comprises regenerating the adsorbent after the adsorptive separation is completed.

[0040] In some embodiments, the regeneration comprises heating the adsorbent to 50-120° C. under vacuum or inert atmosphere conditions for 10-72 hours. In some embodiments, the regeneration comprises maintaining the adsorbent at 5° C.-30° C. under vacuum or inert atmosphere conditions for 10-72 hours. In some embodiments, the regeneration comprises maintaining the adsorbent at room temperature under vacuum or inert atmosphere conditions for 10-72 hours.

[0041] If the heating temperature is too high or the time is too long, the adsorbent structure will be destroyed; if the temperature is too low or the time is too short, the adsorbate remaining in the adsorbent will not be completely removed.

[0042] Compared with the prior art, the present invention has the following advantages:

[0043] (1) The 2,5-dihydroxybenzoquinone compound and metal salt used in the preparation of the metal organic framework material of the present invention are both cheap and readily available, the synthesis conditions are mild, the purification steps are simple, and the operation and scale-up are easy. The metal organic framework material of the present invention has a stable structure and stable performance, has a very high adsorption selectivity for neon / helium, and after repeated adsorption and regeneration, the adsorption performance still maintains the original effect.

[0044] (2) In terms of neon / helium adsorption separation, a high neon adsorption capacity can be achieved at 77K and normal pressure. The adsorbent prepared by the present invention is far superior to most solid adsorbents and has significant separation advantages, overcoming the high energy consumption disadvantages of the traditional liquid hydrogen condensation method and liquid neon condensation method. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 The metal organic framework material prepared in Example 1 and its XRD patterns after being immersed in solutions with different pH values ​​for two weeks.

[0046] Figure 2 The neon adsorption isotherm of the metal organic framework material prepared in Example 1 at 77K is shown.

[0047] Figure 3 The neon adsorption isotherm of the metal organic framework material prepared in Example 2 at 77K is shown.

[0048] Figure 4 The neon adsorption isotherm of the metal organic framework material prepared in Example 3 at 77K is shown.

[0049] Figure 5 The neon adsorption isotherms of activated carbon and molecular sieves in the prior art at 77K are shown.

[0050] Figure 6 This is the penetration diagram of the neon-helium mixed gas of the metal organic framework material prepared in Example 1 at 77K. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0052] Example 1

[0053] A deionized water solution containing 15 mmol of 2,5-dihydroxybenzoquinone was added dropwise to an equal volume of an aqueous solution containing 10 mmol of ferric chloride, and the reaction was stirred at room temperature at a stirring rate of 500 r / min for 24 hours. The solid obtained by the reaction was washed with deionized water to obtain a purified metal organic framework material (Fe2dhbq3). The purified metal organic framework material was vacuum degassed at 120°C for 12 hours to obtain a desolventized adsorbent, which was then subjected to gas adsorption.

[0054] XRD characterization data of metal organic framework materials can be found in Figure 1 .

[0055] In order to test the acid-base stability of the sample, the synthesized metal organic framework material was immersed in solutions of different pH values ​​for 2 weeks, and then the crystal structure was analyzed. It was found that the obtained solid structure still maintained a good crystal form and had good acid-base stability. The specific characterization data can be found in Figure 1 .

[0056] In order to test the adsorption and separation performance of the synthesized metal organic framework material, the single component adsorption isotherm of neon was carried out using the metal organic framework material as an adsorbent. Specifically, 100 mg of the metal organic framework material was taken as the adsorbent and the adsorption temperature was set to 77 K. After testing, as shown in FIG. Figure 2 and Figure 5 As shown, at 77K and 1 bar, the adsorption of neon reaches 92 cm 3 / g, which is much higher than the adsorption capacity of conventional activated carbon and molecular sieve (activated carbon: 53.6cm 3 / g; ZSM-5: 41.9cm 3 / g; 13X: 24.6cm 3 / g; 5A:23.6cm 3 / g). At the same time, Figure 2 It can also be seen that at low pressure of 20kPa, the adsorption capacity of neon can reach 41.5cm 3 / g.

[0057] In order to test the actual effect of the metal organic framework material on the separation of neon and helium, a penetration experiment of a neon-helium mixed gas was carried out using the above-synthesized metal organic framework material as an adsorbent. In this embodiment, the neon-helium mixed gas was adsorbed and separated, with a volume ratio of 20:80, a penetration temperature of 77K, and a pressure of 0.1MPa. The specific penetration curve is shown in Figure 6 .

[0058] from Figure 6 It can be seen that when the volume ratio of neon and helium mixed gas is 20:80, helium penetrates immediately, while neon begins to penetrate after 33 minutes. The two mixed gases are effectively separated.

[0059] In addition, the adsorption performance of the metal-organic framework material remained stable after five adsorption-regeneration cycles.

[0060] Example 2

[0061] A deionized water solution containing 10 mmol of 2,5-dihydroxybenzoquinone was added dropwise to an equal volume of an aqueous solution containing 10 mmol of manganese chloride, and the reaction was stirred at room temperature at a stirring rate of 500 r / min for 24 hours. The solid obtained by the reaction was washed with deionized water to obtain a purified metal organic framework material (Mndhbq). The purified metal organic framework material was vacuum degassed at 120°C for 12 hours to obtain a desolventized adsorbent, which was then subjected to gas adsorption.

[0062] In order to test the adsorption and separation performance of the synthesized metal organic framework material, the single component adsorption isotherm of neon was carried out using the metal organic framework material as an adsorbent. Specifically, 100 mg of the metal organic framework material was taken as the adsorbent and the adsorption temperature was set to 77 K. After testing, as shown in FIG. Figure 3 As shown, at 77K and 1 bar, the adsorption capacity of neon is 77cm 3 / g, which is lower than the adsorption capacity of Fe2dhbq3.

[0063] Example 3

[0064] A deionized water solution containing 10 mmol of 2,5-dihydroxybenzoquinone was added dropwise to an equal volume of an aqueous solution containing 5 mmol of zirconium chloride, and the reaction was stirred at room temperature at a stirring rate of 500 r / min for 24 hours. The solid obtained by the reaction was washed with deionized water to obtain a purified metal organic framework material (Zr(dhbq)2). The purified metal organic framework material was vacuum degassed at 120°C for 12 hours to obtain a desolventized adsorbent, which was then subjected to gas adsorption.

[0065] In order to test the adsorption and separation performance of the synthesized metal organic framework material, the single component adsorption isotherm of neon was carried out using the metal organic framework material as an adsorbent. Specifically, 100 mg of the metal organic framework material was taken as the adsorbent and the adsorption temperature was set to 77 K. After testing, as shown in FIG. Figure 4 As shown, at 77K and 1 bar, the adsorption capacity of neon is 64 cm 3 / g, which is lower than the adsorption capacity of Fe2dhbq3 and Mndhbq.

[0066] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for separating helium and neon, comprising separating a mixed gas containing helium and neon by adsorption using an adsorbent containing a metal organic framework material, wherein the metal organic framework material comprises a metal ion and an organic ligand, in, The organic ligand includes a compound shown in Formula I, In formula I, R is selected from hydrogen, halogen or C1-C4 alkyl; The metal ions are selected from transition metal ions.

2. The method according to claim 1, characterized in that The metal ions are selected from one or more of iron ions, manganese ions and zirconium ions; and / or The organic ligand is 2,5-dihydroxy-1,4-benzoquinone.

3. The method according to claim 1 or 2, characterized in that: The temperature of the adsorption separation is 77K-298K, preferably 77K-87K.

4. The method according to any one of claims 1 to 3, characterized in that The pressure of the adsorption separation is 0.01 MPa-10 MPa, preferably 0.1 MPa-1 MPa.

5. The method according to any one of claims 1 to 4, characterized in that The pore size of the metal organic framework material is Above, preferably More preferably The specific surface area of ​​the metal organic framework material is 300-2000m 2 / g.

6. The method according to any one of claims 1 to 5, characterized in that The preparation method of the metal organic framework material comprises: reacting a transition metal soluble salt and a compound represented by formula I in a solvent, and washing and drying the reaction product; Preferably, the transition metal soluble salt is selected from one or more of soluble iron salts, soluble manganese salts and soluble zirconium salts, such as one or more of ferric chloride, ferric nitrate, ferric sulfate, ferric perchlorate, manganese chloride, manganese nitrate, manganese sulfate, manganese perchlorate, zirconium chloride, zirconium nitrate and zirconium sulfate; Preferably, the compound represented by formula I is 2,5-dihydroxy-1,4-benzoquinone.

7. The method according to claim 6, characterized in that The molar ratio of the soluble transition metal salt to the compound of formula I is 1:(1-5), preferably 1:(1-2), calculated as transition metal ions; and / or The reaction temperature is 10°C-120°C, preferably 10°C-50°C; and / or The reaction time is 0.5h-48h, preferably 3h-24h; and / or The reaction is carried out under stirring conditions, preferably with a stirring speed of 300 rpm-1000 rpm, preferably 400 rpm-700 rpm; and / or The solvent is selected from one or more of methanol, ethanol, acetonitrile, acetone, N,N-dimethylformamide, N,N-dimethylacetamide and water, preferably water; and / or The molar ratio of the solvent to the soluble transition metal salt is (5-30):1 based on the transition metal ion; and / or The drying is vacuum drying. Preferably, the vacuum drying temperature is 30° C.-120° C. and the time is 6 h-24 h.

8. The method according to any one of claims 1 to 7, characterized in that The adsorption separation is carried out using a fixed bed, wherein the adsorbent is filled in the fixed bed adsorption column, and preferably comprises the following steps: (1) passing a mixed gas containing helium and neon through a fixed bed adsorption column, wherein strongly adsorbed neon is adsorbed on the adsorbent and weakly adsorbed helium penetrates the adsorption column, thereby obtaining weakly adsorbed helium; (2) Desorbing the strongly adsorbed neon gas from the adsorbent to obtain the strongly adsorbed neon gas.

9. The method according to claim 8, characterized in that The flow rate of the mixed gas containing helium and neon through the fixed bed adsorption column is 1-100 mL / min / g adsorbent.

10. The method according to any one of claims 1 to 9, characterized in that: The method further comprises regenerating the adsorbent after the adsorption separation is completed. Preferably, the regeneration comprises heating the adsorbent to 50-120°C under vacuum or inert atmosphere conditions and maintaining it for 10-72 hours; or the regeneration comprises maintaining the adsorbent at 5°C-30°C for 10-72 hours under vacuum or inert atmosphere conditions.

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