Process for separating HCl from acid gas mixtures using ionic liquids

Abstract

The present invention relates to a method for separating HCl from an acidic gas mixture, which further contains CO2, using an ionic liquid, the method comprising: an absorption step of bringing an ionic liquid into contact with the acidic gas mixture, the absorption temperature being 10-50 DEG C and the absorption pressure being 0.1-4.0 MPa; and a desorption step for discharging the acidic gas from the ionic liquid having absorbed the acidic gas at a desorption temperature of 60-120 DEG C and a desorption pressure of 0.1-1.0 MPa. According to the method, selectivity improvement on removal of HCl from an acid gas mixture containing CO2 and HCl can be achieved, efficient absorption and separation of hydrogen chloride can be achieved, and the cycle stability of the ionic liquid is improved while the absorption amount of HCl is increased.

Description

Technical Field

[0001] The present invention relates to the field of waste gas treatment, and in particular to a method for separating HCl from an acidic gas mixture by using ionic liquid. Background Art

[0002] This section provides background information related to the present application which does not necessarily constitute prior art.

[0003] Exhaust gases emitted from industrial production processes such as petroleum, chemical, pharmaceutical, and metallurgical industries often include mixtures of various acidic gases. Examples of acidic gases include carbon dioxide (CO2), hydrogen sulfide, sulfur dioxide, sulfur trioxide, nitrogen dioxide, and hydrogen chloride (HCl). Many different technical solutions can be used to treat acidic gases, such as absorption separation, cryogenic distillation (the Ryan Holmes method), and membrane separation. Absorption separation is a commonly used method for treating acidic gases. This method relies on the selective absorption of specific gases by an absorbent, which then undergoes absorption and desorption regeneration as the temperature and pressure change. Ionic liquids (ILs) can be used as absorbents. Ionic liquids are liquid "salts" composed solely of anions and cations.

[0004] Patent CN101671259A discloses an alcoholamine-functionalized ionic liquid (the ionic liquid is composed of cations such as ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, and diglycolamine, and anions such as chloride, bromide, iodide, tetrafluoroborate, hexafluorophosphate, trifluoromethylsulfamide, trifluoromethylsulfonate, hexafluoroantimonate, sulfate, nitrate, and trifluoromethylbutyrate). The liquid can be used for the absorption and separation of acidic gases such as carbon dioxide, sulfur dioxide, hydrogen chloride, and hydrogen sulfide.

[0005] Patent CN102019128A discloses an ionic liquid for absorbing hydrogen chloride gas. The cation of the ionic liquid is N, N'-dialkyl substituted imidazole, N-alkyl substituted pyridine, alkyl quaternary ammonium salt, alkyl quaternary phosphonium salt cation, and the anion of the ionic liquid is BF4 - , PF6 - , Cl - .

[0006] Patent JP2016083623A discloses an ionic liquid that can be used to absorb acidic gases such as carbon dioxide, sulfur dioxide, hydrogen chloride and hydrogen sulfide. The cation of the ionic liquid is preferably an imidazolium ion having a primary or secondary hydroxyl group, or an ammonium ion having a primary or secondary hydroxyl group, and the anion of the ionic liquid is preferably a carboxylate ion having a primary or secondary hydroxyl group.

[0007] Hydrogen chloride is a highly corrosive gas with a pungent odor. Direct emission poses a significant environmental risk. HCl is more toxic and corrosive than CO2. Therefore, for acidic gas mixtures containing both CO2 and HCl, it is necessary to selectively remove HCl from the mixture. However, existing solutions for separating HCl from acidic gas mixtures containing both CO2 and HCl using ionic liquids have the following technical problems: 1. The selectivity of ionic liquids for HCl absorption is reduced by the presence of CO2, and the selectivity of ionic liquids for HCl / CO2 is significantly reduced under high pressure; 2. Ionic liquids used for HCl absorption have poor cyclic stability. Summary of the Invention

[0008] In response to the technical problems existing in the prior art, the present invention aims to provide a method for separating HCl from an acidic gas mixture using an ionic liquid. This method can achieve improved selectivity for removing HCl from an acidic gas mixture containing both CO2 and HCl. Furthermore, this method can achieve efficient absorption and separation of hydrogen chloride, thereby increasing the amount of HCl absorbed and improving the cyclic stability of the ionic liquid.

[0009] To achieve the above object, the present invention adopts the following technical solution: a method for separating HCl from an acidic gas mixture using an ionic liquid, wherein the acidic gas mixture also contains CO2, the method comprising the following steps: an absorption step of contacting the ionic liquid with the acidic gas mixture, wherein the absorption temperature is 10-50° C. and the absorption pressure is 0.1 MPa-4.0 MPa; The desorption process is to make the ionic liquid that absorbs the acid gas discharge the acid gas, the desorption temperature is 60-120 ° C, and the desorption pressure is 0.1 kPa-1.0 MPa.

[0010] The ionic liquid comprises at least one of the compounds represented by the following structural formulas (1) to (6): Structural formula (1); Structural formula (2); Structural formula (3); Structural formula (4); Structural formula (5); Structural formula (6).

[0011] Preferably, the volume ratio concentration of HCl in the acidic gas mixture is 0.5%VOL-80%VOL. Within this range, the selectivity of the ionic liquid for HCl / CO2 is ideal.

[0012] Compared with the prior art, the present invention has the following beneficial effects: The method provided by the present invention is used to treat the acidic gas mixture, and has a strong ability to selectively absorb HCl, an excellent absorption amount of HCl, and is easy to desorb, and the ionic liquid has excellent cyclic stability.

[0013] The following describes the details in conjunction with specific embodiments. DETAILED DESCRIPTION

[0014] It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0015] Existing ionic liquids disclosed to have high CO2 absorption and desorption rates do not necessarily have high HCl absorption and desorption rates. CO2 in ionic liquids primarily dissolves physically or undergoes weak chemical adsorption without completely dissociating. Therefore, the anions of ionic liquids used for CO2 absorption and desorption are preferably those that can bind to CO2 through weak interactions (such as hydrogen bonds, Lewis acid-base interactions, or physical dissolution), and the cations are preferably those with strong structural tunability and that can cooperate with the anions to form a low-viscosity solvent environment, thereby promoting CO2 diffusion. HCl may dissociate into H⁺ and Cl⁻ in ionic liquids and be fixed through ion exchange or protonation reactions, which involve stronger chemical bonds. Therefore, the anions of ionic liquids used for HCl absorption and desorption need to have strong interactions with H⁺ or Cl⁻, and the cations need to be acid-resistant to avoid decomposition in acidic environments. Therefore, if ionic liquids (ILs) used for absorbing carbon dioxide (CO2) are directly used for hydrogen chloride (HCl) treatment, there will be at least the following adverse consequences: (1) Poor acid resistance of anions: The anions of ILs in the CO2 system are easily protonated with H⁺ in HCl under acidic conditions, resulting in structural destruction; (2) The cations of ILs in the CO2 system are easily protonated or decomposed in an acidic environment, resulting in increased viscosity of the ionic liquid and even phase separation; (3) The addition of CO2 can reduce the viscosity of ILs, but the H⁺ and Cl⁻ produced by the dissociation of HCl will increase the ionic strength of the system, resulting in increased viscosity, hindering gas diffusion and reaction rate.

[0016] The ionic liquid used to absorb HCl should have a hydrogen bond acceptor and a hydrogen bond donor, so that HCl forms a strong hydrogen bond with the ionic liquid and is absorbed, and after the ionic liquid absorbs hydrogen chloride, it can quickly desorb the HCl gas by heating and / or reducing pressure. The ionic liquid after desorption needs to be reusable, that is, it needs to meet the requirements of good cyclic stability. Therefore, the cations of the existing ionic liquids used to absorb HCl are groups containing one or more hydroxyl groups, and their anions are mostly conventional acid ions. The ionic liquids of this structure achieve the absorption and separation of HCl by forming multi-site hydrogen bonds with HCl. However, it was found that when HCl and CO2 are mixed, the selectivity of the ionic liquid for HCl decreases. It is speculated that this is due to the O-carbonation of the hydroxyl groups of the ionic liquid, especially at higher absorption pressures. CO2 will react at the hydroxyl sites of the ionic liquid to form intermediates such as bicarbonate / carbonate, which inhibits the HCl / CO2 selectivity of the ionic liquid. The selectivity of the ionic liquid for HCl gradually loses as the absorption pressure increases. Therefore, it is necessary to improve the absorption amount and selectivity of the ionic liquid for HCl by designing a specific ionic liquid and defining the process parameters for absorption and desorption.

[0017] In the design of ionic liquids, the selection of cations and anions is a systematic process based on target properties (such as melting point, viscosity, conductivity, thermal / chemical stability, and gas solubility). The number of ionic liquids that can be used is extremely large. The cation and anion pairing of an ionic liquid must at least meet the following requirements: volume and symmetry (affecting lattice energy and melting point), charge distribution (affecting stability), and hydrogen bonding ability (affecting viscosity and solubility). Furthermore, the cation and anion pairing of the ionic liquid must meet the requirements of the application scenario. For example, ionic liquids for lithium-ion batteries require a wide electrochemical window and high ionic conductivity; ionic liquids for CO2 capture require high CO2 solubility and low regeneration energy consumption; ionic liquids for catalytic reactions require acidic sites and high stability; and ionic liquids for biomass dissolution require strong hydrogen bonding, alkalinity, and low viscosity. Furthermore, in different reaction systems, some substances can serve as both cations and anions in ionic liquids, resulting in the design and synthesis of two completely different absorbents with significantly different gas absorption performance. The ionic liquid absorbent used in the method provided by the present invention absorbs HCl not only by utilizing the hydrogen bonding between the hydroxyl groups in the cations and HCl, but also by selecting anions containing atoms with high electronegativity and small radius (such as oxygen and nitrogen) to form strong hydrogen bonds with HCl. Only with the dual synergistic effect of anions and cations can the optimal absorption selectivity of HCl be achieved.

[0018] The treatment of acidic gas using ionic liquids includes two stages: absorption and desorption. The effects of the absorption and desorption process parameters on the two stages are as follows: (1) Absorption stage Low temperatures promote absorption: Low temperatures (e.g., 10-30°C) significantly increase the absorption of acidic gases (such as CO₂, SO₂, and H₂S) by ionic liquids. This is attributed to the enhanced van der Waals forces and hydrogen bonding during physical adsorption at low temperatures. For HCl absorption by ionic liquids, low temperatures shift the equilibrium of chemical absorption (e.g., the acid-base reaction between the anion Cl⁻ and H⁺) toward the formation of an HCl-ionic liquid complex. High temperatures inhibit absorption; increasing temperatures weaken physical absorption and accelerate the desorption of absorbed HCl.

[0019] High pressure enhanced absorption: Increasing the pressure can increase the partial pressure of the gas phase acid gas, thereby increasing the solubility of the ionic liquid.

[0020] (2) Desorption stage High temperature driven desorption: The desorption of acidic gases in ionic liquids requires the destruction of hydrogen bonds or acid-base complex structures by high temperature (90-120°C).

[0021] Low pressure promotes desorption: Reduced pressure (such as vacuum or low-pressure flash) can quickly release physically absorbed acid gases.

[0022] Surprisingly, in the method provided by the present invention, when the absorption temperature of the absorption step is 30-50°C and the absorption pressure is 0.1 MPa-1.0 MPa, the ionic liquid still maintains excellent absorption of HCl in the acidic gas mixture (containing both CO2 and HCl), and the HCl / CO2 selectivity of the ionic liquid is greater than 5.

[0023] Surprisingly, in the method provided by the present invention, when the desorption temperature of the desorption process is 60-90°C, the desorption rate of HCl by the ionic liquid still remains excellent.

[0024] Example 1 This embodiment provides an ionic liquid, and the preparation method of the ionic liquid is as follows: Step 1: Weigh 0.20 mol (55.15 g) of choline hydroxide solution into a 250 mL round-bottom flask, weigh 0.21 mol (14.30 g) of imidazole, add it to the round-bottom flask, and stir at room temperature for 4 h to complete the reaction; Step 2: The reactant obtained in step 1 is subjected to vacuum rotary evaporation at 70° C. for 6 h to remove most of the water, and then vacuum dried at 60° C. for 40 h to remove trace water to obtain an ionic liquid, wherein the cation of the ionic liquid is a choline cation and the anion of the ionic liquid is an imidazolium anion.

[0025] The structural formula of the ionic liquid is as follows: [Example 2] This embodiment provides an ionic liquid, and the preparation method of the ionic liquid is as follows: Step 1: Weigh 0.20 mol (55.17 g) of choline hydroxide solution into a 250 mL round-bottom flask, weigh 0.21 mol (17.89 g) of 2-pyrrolidone solution, add the weighed 2-pyrrolidone solution, and stir for 1 hour at 10°C. Stir at room temperature for 4 hours until the reaction is complete. Step 2: The reactant obtained in step 1 is subjected to vacuum rotary evaporation at 70°C for 6 hours to remove most of the water, and then vacuum dried at 60°C for 40 hours to remove trace water to obtain an ionic liquid, wherein the cation of the ionic liquid is a choline cation and the anion of the ionic liquid is a pyrrolidone anion.

[0026] The structural formula of the ionic liquid is as follows: [Example 3] This embodiment provides an ionic liquid, and the preparation method of the ionic liquid is as follows: Step 1: Weigh 0.20 mol (55.17 g) of choline hydroxide solution into a 250 mL round-bottom flask, weigh 0.21 mol (20.81 g) of succinimide, add the weighed succinimide at 10 °C, stir for 1 hour, and then stir at room temperature for 4 hours to complete the reaction; Step 2: The reactant obtained in step 1 is subjected to vacuum rotary evaporation at 70° C. for 6 h to remove most of the water, and then vacuum dried at 60° C. for 40 h to remove trace water to obtain an ionic liquid, wherein the cation of the ionic liquid is a choline cation and the anion of the ionic liquid is a succinimide anion.

[0027] The structural formula of the ionic liquid is as follows: [Example 4] This embodiment provides an ionic liquid, and the preparation method of the ionic liquid is as follows: Step 1: Weigh 0.20 mol (55.17 g) of choline hydroxide solution into a 250 mL round-bottom flask, weigh 0.21 mol (15.97 g) of glycolic acid solution, add the glycolic acid at 10°C, stir for 1 hour, and then stir at room temperature for 4 hours to complete the reaction; Step 2: The reactant obtained in step 1 is subjected to vacuum rotary evaporation at 70°C for 6 hours to remove most of the water, and then vacuum dried at 60°C for 40 hours to remove trace water to obtain an ionic liquid, wherein the cation of the ionic liquid is a choline cation and the anion of the ionic liquid is a glycolate ion.

[0028] The structural formula of the ionic liquid is as follows: [Example 5] This embodiment provides an ionic liquid, and the preparation method of the ionic liquid is as follows: Step 1: Weigh 0.20 mol (55.17 g) of choline hydroxide solution into a 250 mL round-bottom flask, weigh 0.21 mol (15.97 g) of glycolic acid solution, add the glycolic acid at 10°C, stir for 1 hour, and then stir at room temperature for 4 hours to complete the reaction; Step 2: The reactant obtained in step 1 is subjected to vacuum rotary evaporation at 70°C for 6 hours to remove most of the water, and then vacuum dried at 60°C for 40 hours to remove trace water to obtain an ionic liquid, wherein the cation of the ionic liquid is a choline cation and the anion of the ionic liquid is a glycolate ion.

[0029] The structural formula of the ionic liquid is as follows: [Example 6] This embodiment provides an ionic liquid, and the preparation method of the ionic liquid is as follows: Step 1: Weigh 0.20 mol (16.80 g) of N-hydroxyethylpiperazine into a 250 mL round-bottom flask, weigh 0.21 mol (19.75 g) of 67% nitric acid aqueous solution into a constant pressure dropping funnel, slowly add the acid dropwise while controlling the flow rate. Add the weighed nitric acid solution and stir for 1 hour at 10°C. Stir at room temperature for 6 hours until the reaction is complete. Step 2: The reactant obtained in step 1 is subjected to vacuum rotary evaporation at 70°C for 6 hours to remove most of the water, and then vacuum dried at 60°C for 24 hours to remove trace water to obtain an ionic liquid, wherein the cation of the ionic liquid is a piperazine cation and the anion of the ionic liquid is a nitrate.

[0030] The structural formula of the ionic liquid is as follows: [Comparative Examples 1-10] The only difference between the ionic liquids provided in Comparative Examples 1-5 and the ionic liquid provided in Example 1 is that: In Comparative Example 1, the cation of the ionic liquid is tetramethylammonium; In Comparative Example 2, the cation of the ionic liquid is tetrabutylphosphine; In Comparative Example 3, the anion of the ionic liquid is a 2,6-dihydroxybenzoic acid anion; In Comparative Example 4, the anion of the ionic liquid is a hydrogen sulfate ion; In Comparative Example 5, the anion of the ionic liquid is a trifluoromethanesulfonic acid anion.

[0031] Comparative Example 6 provides an ionic liquid, specifically [MDEA][BF4]: N-methyldiethanolamine tetrafluoroborate.

[0032] Comparative Example 7 provides an ionic liquid, specifically [Emim][NTf2]: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt.

[0033] Comparative Example 8 provides an ionic liquid, specifically [OHEtmim][BF4]: 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate.

[0034] Comparative Example 9 provides an ionic liquid, specifically [Bmin][PF6]: 1-ethyl-3-methylimidazolium hexafluorophosphate.

[0035] Comparative Example 10 provides an ionic liquid, specifically [BPy][BF4]: N-butylpyridinium tetrafluoroboric acid.

[0036] [Absorption-desorption test of HCl] The ionic liquids provided in Examples 1-6 and Comparative Examples 1-10 were tested for their ability to absorb HCl gas using the following method: 2.00 g of ionic liquid was added to a 10 mL absorption flask. Pure HCl gas (at a flow rate of 50 mL / min) was introduced into the absorption flask under the conditions shown in Table 1. The mass change of the absorption flask was recorded at regular intervals until absorption equilibrium was reached. The ionic liquid, after HCl absorption, was then desorbed under the conditions shown in Table 1. The test results are shown in Table 1.

[0037] Table 1 Test results of the absorption-desorption effect of ionic liquids on hydrogen chloride gas According to the results in Table 1, the ionic liquids provided in Examples 1-6 can achieve efficient absorption and separation of hydrogen chloride gas. The cations in the ionic liquids provided in Examples 1-6 are groups containing hydroxyl groups, and the anions are groups containing electron-withdrawing ability. The ionic liquids achieve efficient absorption and separation of HCl by forming multi-site hydrogen bonds with HCl. That is, in order to achieve the technical effects described in this application, the synergistic effect of anions and cations in the ionic liquid is required to achieve high absorption and desorption performance.

[0038] [Cycling stability test] The ionic liquids provided in Example 1, Example 3, Example 6 and Comparative Examples 1-10 were subjected to cyclic absorption-desorption tests, specifically: Add 5.00 g of ionic liquid to a 20 mL absorption flask. Flow HCl gas (at a flow rate of 50 mL / min) at 10°C and 4.0 MPa. Record the mass change of the absorption flask at regular intervals until absorption reaches equilibrium. Desorb the saturated ionic liquid by heating and reducing pressure (at 120°C and 0.1 kPa). Repeat the above absorption-desorption process 10 times, record the HCl absorption amounts (HCl g / g IL) corresponding to the first and tenth absorptions, and calculate the ratio (%) of the HCl absorption amount corresponding to the tenth absorption to the HCl absorption amount corresponding to the first absorption; Record the desorption rate (%) corresponding to the 10th desorption.

[0039] The results are shown in Table 2. As can be seen from Table 2, the ionic liquid of the present invention has good absorption stability for hydrogen chloride gas for 10 cycles, and the desorption rate corresponding to the 10th absorption-desorption of the ionic liquid of the present invention is ≥99%, indicating that the regeneration performance of the ionic liquid provided by the present invention is good.

[0040] Table 2 Absorption-desorption cycle stability test of ionic liquids for hydrogen chloride gas [Testing the selectivity of HCl / CO2] 5.00 g of ionic liquid was added to an absorption bottle with a volume of 20 mL. Under the conditions shown in Table 3, 100 mL of an acidic gas mixture consisting of CO2 and HCl was introduced into the absorption bottle (the flow rate was 50 mL / min, and the ratio of CO2 and HCl in the acidic gas mixture was shown in Table 3). After 2 minutes of closed absorption, the gas in the absorption bottle was extracted, and the contents of CO2 and HCl in the gas were measured respectively by gas phase molecular absorption spectrometer to obtain the selectivity of the ionic liquid for HCl / CO2.

[0041] It should be noted that the selectivity of ionic liquids for HCl / CO2 is calculated as: Selectivity = HCl absorption / CO2 absorption.

[0042] The results are shown in Table 3, which illustrate that the method for separating HCl from an acidic gas mixture using an ionic liquid provided by the present invention achieves a high absorption capacity of HCl while achieving an HCl / CO2 selectivity of greater than 5 for the ionic liquid.

[0043] Table 3 Selectivity test of ionic liquids for HCl / CO2 The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0044] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for separating HCl from an acidic gas mixture using an ionic liquid, wherein the acidic gas mixture also contains CO2, characterized in that The method comprises the following steps: an absorption step of contacting the ionic liquid with the acidic gas mixture, wherein the absorption temperature is 10-50° C. and the absorption pressure is 0.1 MPa-4.0 MPa; A desorption process in which the ionic liquid that has absorbed the acidic gas discharges the acidic gas, wherein the desorption temperature is 60-120°C and the desorption pressure is 0.1 kPa-1.0 MPa; The ionic liquid includes at least one of the compounds shown in the following structural formulas (1) to (6): Structural formula (1); Structural formula (2); Structural formula (3); Structural formula (4); Structural formula (5); Structural formula (6).

2. The method according to claim 1, characterized in that The volume ratio concentration of HCl in the acidic gas mixture is 0.5%VOL-80%VOL.

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