A method for separating water-containing isopropyl alcohol in the production of high-purity pseudo-boehmite by alcohol aluminum method

Abstract

The present application relates to the technical field of organic alcohol recovery treatment in pseudo-boehmite production, and discloses an alcohol separation method for water-containing isopropyl alcohol in high-purity pseudo-boehmite production by an alcohol-aluminum method. The method collects water-containing isopropyl alcohol mixture generated in the process of preparing high-purity pseudo-boehmite by alcohol-aluminum hydrolysis, and sends the mixture into a composite filter prepared by coating, drying, calcining, water-containing isopropyl alcohol pre-wetting and flushing of alumina powder, boehmite sol, polyvinyl alcohol, polyacrylic acid and polyvinylpyrrolidone, to obtain pretreated water-containing isopropyl alcohol liquid. The pretreated water-containing isopropyl alcohol liquid is sent into a hydrophilic dehydration membrane separation unit after preheating, the permeate is condensed and collected on the permeation side, and low-water isopropyl alcohol liquid is collected on the non-permeation side, and the low-water isopropyl alcohol liquid is returned to the alcohol-aluminum synthesis, alcohol-aluminum dissolution or alcohol circulation process. The method is suitable for the separation and reuse of water-containing isopropyl alcohol in the process of high-purity pseudo-boehmite production by the alcohol-aluminum method.

Description

Technical Field

[0001] This invention relates to the field of organic alcohol recovery and treatment technology in the production of boehmite, and particularly to an alcohol separation method for hydrous isopropanol in the production of high-purity boehmite using the aluminum alkoxide method. Background Technology

[0002] Boehmite is an important type of hydrated alumina material, used in catalyst supports, adsorbents, and alumina precursors. For high-purity boehmite, aluminum alkoxide hydrolysis is one of the more commonly used preparation routes. This process typically uses aluminum alkoxide or aluminum alkoxide alcohol solutions as raw materials, and obtains the boehmite product through hydrolysis, aging, separation, and drying. During hydrolysis, water or an aqueous medium is usually added to ensure complete conversion of the aluminum alkoxide; simultaneously, aluminum alkoxide hydrolysis releases corresponding organic alcohols. Therefore, a certain amount of aqueous isopropanol mixture is generated during the hydrolysis alcohol separation, aging alcohol evaporation, mother liquor treatment, and drying tail gas condensation stages.

[0003] For the aforementioned aqueous isopropanol mixture, direct discharge or low-value treatment would result in isopropanol loss and a burden on subsequent wastewater treatment. If the isopropanol needs to be returned to aluminum alkoxide synthesis, aluminum alkoxide dissolution, or alcohol recycling processes, the water content needs to be reduced. Traditionally, aqueous isopropanol separation can be carried out using methods such as distillation. However, for isopropanol-water systems, distillation processes typically involve high energy consumption, heavy equipment load, and long operating cycles. To reduce the burden of distillation, hydrophilic dehydration membranes are also used to dehydrate aqueous isopropanol, allowing water to be removed through the permeate side of the membrane, resulting in a low-water isopropanol solution on the non-permeate side.

[0004] However, the aqueous isopropanol mixture produced during the aluminum alkoxide process for producing high-purity boehmite is not entirely the same as a conventionally prepared aqueous isopropanol solution. This type of aqueous isopropanol mixture may contain fine boehmite powder, colloidal aluminum-containing substances, soluble aluminum components, and small amounts of solid impurities carried over with the mother liquor. If this type of aqueous isopropanol mixture is directly introduced into the hydrophilic dehydration membrane separation unit without proper pre-membrane treatment, it can easily cause fluctuations in the feed state of the membrane separation unit and may increase the burden of membrane module cleaning and maintenance.

[0005] Furthermore, the high-purity boehmite production system is highly sensitive to exogenous impurities. Treating the aqueous isopropanol with resins, silica filter aids, phosphate-based treatment agents, or other added chemical agents may introduce impurities or burden subsequent separation processes. Therefore, it is necessary to provide an alcohol separation method suitable for the aqueous isopropanol treatment in this system, taking into account the material characteristics of the aluminum alkoxide process for producing high-purity boehmite. This method would allow the aqueous isopropanol to undergo appropriate pre-membrane treatment before entering the hydrophilic dehydration membrane separation unit, achieving stable recovery and recycling of isopropanol without significantly altering the conventional membrane dehydration process. Summary of the Invention

[0006] In view of this, the present invention proposes an alcohol separation method for the production of hydrous isopropanol from high-purity boehmite using the aluminum alkoxide process. This method targets the hydrous isopropanol mixture generated during the hydrolysis of aluminum alkoxide to prepare high-purity boehmite. First, a composite filter element made from aluminum-based raw materials such as alumina powder and boehmite sol is used for membrane pretreatment. Then, the pretreated hydrous isopropanol solution is sent to a hydrophilic dehydration membrane separation unit for dehydration separation. The resulting low-water isopropanol solution is returned to the aluminum alkoxide synthesis, aluminum alkoxide dissolution, or alcohol recycling processes.

[0007] The technical solution of this invention is achieved as follows: This invention provides an alcohol separation method for producing hydrous isopropanol from high-purity boehmite using the aluminum alkoxide process, comprising the following steps: Collect the aqueous isopropanol mixture generated during the preparation of high-purity boehmite by aluminum alkoxide hydrolysis; The aqueous isopropanol mixture is fed into a composite filter for filtration to obtain a pretreated aqueous isopropanol solution. The pretreated aqueous isopropanol solution is preheated and then fed into a hydrophilic dehydration membrane separation unit. A negative pressure is formed on the permeate side of the hydrophilic dehydration membrane separation unit and the permeate is collected by condensation. Low-water isopropanol solution is collected on the non-permeate side. The low-water isopropanol solution is returned to the aluminum alkoxide synthesis process, the aluminum alkoxide dissolution process, or the alcohol recycling process; The composite filter element is obtained by applying a coating slurry containing alumina powder and boehmite sol to a porous alumina support, followed by drying, calcination, pre-wetting with aqueous isopropanol, and rinsing.

[0008] In some embodiments, the aqueous isopropanol mixture originates from one or more of the following: aluminum alkoxide hydrolysis alcohol fraction, aged alcohol evaporation condensate, filtration mother liquor de-alcoholization liquid, and drying tail gas condensate. The aqueous isopropanol from these different sources can be processed separately in a composite filter, or they can be collected and blended before being processed in the composite filter. By limiting the source of the aqueous isopropanol, the processing object of this invention corresponds to the actual process of producing high-purity boehmite using the aluminum alkoxide method, avoiding the mixing of ordinary isopropanol aqueous solution with the aqueous isopropanol mixture described in this invention.

[0009] In some embodiments, the aqueous isopropanol mixture has a water content of 20–45 wt%, preferably 30–40 wt%. This range covers the aqueous isopropanol states commonly encountered during hydrolysis, alcohol distillation, condensation, and mother liquor de-alcoholization, thus providing good process adaptability for subsequent membrane pretreatment and membrane separation steps.

[0010] In some embodiments, the composite filter element is prepared by the following method: polyvinyl alcohol is dissolved in water and / or aqueous isopropanol, and polyacrylic acid is added to obtain a dispersion; alumina powder, boehmite sol, and polyvinylpyrrolidone are added to the dispersion and mixed to obtain a coating slurry; the coating slurry is applied to a porous alumina support, dried, and then calcined; the calcined porous alumina support is pre-wetted with aqueous isopropanol, then rinsed with aqueous isopropanol, and the initial filtrate is discarded to obtain the composite filter element. The above preparation method uses aluminum-based inorganic materials as the main component and does not require the introduction of resin exchangers, silica filter aids, or phosphate treatment agents, making it suitable for pretreatment of aqueous isopropanol membranes in high-purity boehmite production systems.

[0011] In some embodiments, the alumina powder is α-Al₂O₃ powder, or a mixture of α-Al₂O₃ powder and γ-Al₂O₃ powder. The D50 particle size of the α-Al₂O₃ powder is 0.1–1.0 μm, preferably 0.3–0.6 μm; the D50 particle size of the γ-Al₂O₃ powder is 0.1–1.0 μm, preferably 0.3–0.6 μm. By combining alumina powder within the above particle size range with boehmite sol, a slurry system suitable for coating and calcination can be obtained, and the composite filter element can have filtration channels suitable for the passage of aqueous isopropanol.

[0012] In some embodiments, the boehmite sol has a solid content of 20–30 wt%, preferably 25 wt%. In the coating slurry, based on the total solid mass, the alumina powder content is 55–75 wt%, the boehmite sol content (based on AlOOH) is 20–35 wt%, the polyvinyl alcohol content is 3–6 wt%, and the polyvinylpyrrolidone content is 2–6 wt%. Alumina powder serves as the main inorganic framework component; the boehmite sol participates in the coating layer formation and together with the alumina powder forms a filter layer; polyvinyl alcohol is used to improve the film-forming properties and coating stability of the slurry; and polyvinylpyrrolidone is used to adjust the passing performance after calcination.

[0013] In some embodiments, the porous alumina support is a tubular alumina support. The average pore size of the porous alumina support is 5–15 μm, and the porosity is 35–45%. The coating slurry is applied to the porous alumina support by dip-coating. This support type facilitates its arrangement in a continuous filter housing and its connection with the subsequent hydrophilic dehydration membrane separation unit.

[0014] In some embodiments, the drying temperature is 80–110°C, the calcination temperature is 400–550°C, and the calcination time is 2–4 h. The drying step is used to remove some volatile components from the coating layer and maintain the coating layer in an adherent state; the calcination step is used to fix the coating layer onto the porous alumina support. The aqueous isopropanol pre-wetting involves circulating an aqueous isopropanol solution with a water content of 20–30 wt% for 60–90 min. After pre-wetting, the filter element is rinsed with an aqueous isopropanol solution with a water content of 20–30 wt% for 10–30 min, and the initial filtrate is discarded. The composite filter element is then used for filtration of the aqueous isopropanol mixture.

[0015] In some embodiments, the aqueous isopropanol mixture is filtered through a pre-filter with a pore size of 20 μm before entering the composite filter element; the composite filter element has a filtration accuracy of 0.1–2 μm, preferably 0.5 μm, a filtration temperature of 45–60°C, and a filtration pressure difference of 0.08–0.15 MPa. The filtration accuracy of the composite filter element can be determined using the standard particle retention method. By combining the pre-filter with the composite filter element, the risk of instantaneous clogging of the composite filter element can be reduced, and the composite filter element can be primarily used to treat fine solids and aluminum-containing micro-components in aqueous isopropanol.

[0016] In some embodiments, after continuous filtration for 24 hours, the pressure difference between the inlet and outlet of the composite filter element is no higher than 1.5 times the initial pressure difference. By monitoring the operating pressure difference of the composite filter element, its operating status can be determined; when the inlet and outlet pressure difference rises to a set value, the composite filter element can be replaced or routine cleaning can be performed.

[0017] In some embodiments, the pretreated aqueous isopropanol solution is preheated before entering a hydrophilic dehydration membrane separation unit. This hydrophilic dehydration membrane separation unit employs a NaA molecular sieve membrane pervaporation dehydration unit or a PVA composite membrane pervaporation dehydration unit. The temperature of the pretreated aqueous isopropanol solution entering the hydrophilic dehydration membrane separation unit is 80–85°C, and the absolute pressure on the permeate side of the unit is 5–8 kPa. This step can be implemented using existing hydrophilic dehydration membrane components and a matching negative pressure condensation system, without requiring changes to the basic dehydration method of the membrane separation unit.

[0018] In some embodiments, the water content of the low-water isopropanol solution collected on the non-permeable side of the hydrophilic dehydration membrane separation unit is no higher than 0.5 wt%, preferably no higher than 0.3 wt%. The low-water isopropanol solution can be returned to the aluminum alkoxide synthesis process, the aluminum alkoxide dissolution process, or the alcohol recycling process. The alcohol recycling process includes alcohol recovery and storage, alcohol preparation for aluminum alkoxide synthesis, alcohol preparation for aluminum alkoxide dissolution, or alcohol reuse. The permeate collected by condensation on the permeable side of the hydrophilic dehydration membrane separation unit can be returned to the aluminum alkoxide hydrolysis solution preparation process, the aging replenishment process, or the tail gas absorbent preparation process. Thus, the low-water isopropanol solution and the water-rich permeate can respectively enter subsequent processes adapted to their compositions, reducing the outflow of materials during the treatment of aqueous isopropanol.

[0019] The present invention has the following advantages over the prior art: This invention addresses the aqueous isopropanol mixture generated during the production of high-purity boehmite using the aluminum alkoxide process. A composite filter made from aluminum-based raw materials such as alumina powder and boehmite sol is installed before the hydrophilic dehydration membrane separation unit. This allows the aqueous isopropanol to undergo pre-filtration, which is compatible with the high-purity aluminum system, before entering the hydrophilic dehydration membrane separation unit for dehydration. Compared to directly feeding the aqueous isopropanol into the membrane separation unit, this method is more suitable for handling aqueous isopropanol streams containing fine boehmite powder, colloidal aluminum-containing substances, or other fine solids. Compared to relying solely on traditional distillation separation, this method reduces the burden of full-volume distillation and allows the resulting low-aqueous isopropanol solution to be returned to the aluminum alkoxide synthesis, aluminum alkoxide dissolution, or alcohol recycling processes. The membrane pretreatment material of this method is mainly prepared from aluminum-based inorganic raw materials, which reduces the risk of introducing exogenous impurities into the high-purity boehmite production system. The overall process flow has good compatibility with existing aluminum alkoxide hydrolysis, hydrous isopropanol recovery and hydrophilic membrane dehydration equipment, and is suitable for application in high-purity boehmite production lines. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and should not be construed as limiting the scope of protection of the present invention. Unless otherwise stated, all raw materials used below can be commercially available materials conventional in the art; all detection methods used can be conventional detection methods in the art.

[0021] In the following embodiments, the aqueous alcohol mixture is an aqueous isopropanol stream generated during the hydrolysis of aluminum alkoxide to prepare high-purity boehmite. The aqueous isopropanol mixture can be one of the following: hydrolysis alcohol fraction, aged alcohol condensate, filtrate mother liquor de-alcoholized liquid, or drying tail gas condensate; or it can be a mixture of two or more of the above streams.

[0022] Detection methods Moisture content was determined using the Karl Fischer method. For aqueous isopropanol mixtures with high moisture content, the volumetric Karl Fischer method was used for moisture determination; for isopropanol solutions with low moisture content, the volumetric or coulometric Karl Fischer method was used for moisture determination.

[0023] Isopropanol content was determined by gas chromatography. During testing, the sample was taken, appropriately diluted, and injected. The isopropanol content was calculated using either the external standard method or the area normalization method. For production control, existing gas chromatography detection methods established by the company can also be used.

[0024] Turbidity was measured using a turbidimeter, and the results are expressed in NTU.

[0025] The suspended solids content was determined by the filtration weighing method. A certain mass or volume of sample was taken and filtered through a filter membrane that had been pre-dried to constant weight. After filtration, the filter membrane was dried to constant weight, and the suspended solids content was calculated based on the difference in mass of the filter membrane before and after filtration.

[0026] Soluble aluminum content was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). During the test, the sample was first filtered through a 0.45 μm filter membrane, and the filtrate was acidified and diluted before determining the aluminum content. For samples with high alcohol content, matrix dilution or a matrix-matched standard curve was used before testing to reduce the influence of the isopropanol matrix on the measurement results. The soluble aluminum content mentioned in this paper refers to the aluminum content measured in the filtrate after filtration through a 0.45 μm filter membrane using the above method.

[0027] Sodium, iron, and silicon impurities in low-aqueous isopropanol solutions were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES) or inductively coupled plasma mass spectrometry (ICP-MS). Appropriate dilution and digestion were performed based on the isopropanol content of the sample during the detection process.

[0028] The particle size of alumina powder was measured using a laser particle size analyzer and is expressed as D50. The filtration accuracy of the composite filter element was determined using the standard particle retention method. Specifically, a suspension containing standard particles with a known particle size distribution was passed through the composite filter element, and the change in the concentration of standard particles before and after filtration was detected. The filtration accuracy of the composite filter element was characterized by the particle size of the standard particles that could be stably retained.

[0029] The membrane flux of a hydrophilic dehydration membrane is calculated using the following formula: J=m / (A×t) In the formula, J is the membrane flux, with units of kg / (m²). 2 •h); m is the mass of permeate collected by condensation on the permeate side, in kg; A is the effective membrane area, in m². 2 t represents the running time, in hours (h).

[0030] Membrane flux retention is calculated using the following formula: R = Jt / J0 × 100% In the formula, R is the membrane flux retention rate after t hours of operation, J0 is the initial membrane flux, and Jt is the membrane flux after t hours of operation.

[0031] Preparation of composite filter elements Preparation Example 1 Weigh 4 parts of polyvinyl alcohol and add them to deionized water. Stir and dissolve at 85°C to obtain a polyvinyl alcohol solution. Add 1 part of polyacrylic acid to the polyvinyl alcohol solution and stir until homogeneous to obtain a dispersion.

[0032] 70 parts of α-Al₂O₃ powder, 96 parts of boehmite sol, and 2 parts of polyvinylpyrrolidone were added to the dispersion, and stirring was continued to obtain a coating slurry. The D50 particle size of the α-Al₂O₃ powder was 0.5 μm; the solid content of the boehmite sol was 25 wt%, corresponding to 24 parts of AlOOH solids. Therefore, based on the total solid mass, the coating slurry contained 70 wt% α-Al₂O₃ powder, 24 wt% AlOOH, 4 wt% polyvinyl alcohol, and 2 wt% polyvinylpyrrolidone.

[0033] A tubular porous alumina support was used. The average pore size of the tubular porous alumina support was 10 μm, the porosity was 40%, the outer diameter was 12 mm, the inner diameter was 8 mm, and the effective length was 250 mm. The coating slurry was applied to the outer surface of the tubular porous alumina support using an dip-coating method. After coating, it was dried at 90℃ for 4 h, and then calcined at 450℃ for 3 h. After calcination, the resulting material was placed in a circulating wetting device and circulated with an isopropanol aqueous solution with a water content of 20 wt% for 60 min, followed by rinsing with the same solution for 20 min. The initial filtrate was discarded, yielding composite filter A. The filtration accuracy of composite filter A was measured to be 0.5 μm using the standard particle retention method.

[0034] Preparation Example 2 This preparation example is basically the same as Preparation Example 1, except that the amount of α-Al2O3 powder is 60 parts, the amount of boehmite sol is 120 parts, the amount of polyvinyl alcohol is 5 parts, and the amount of polyvinylpyrrolidone is 5 parts. The D50 particle size of the α-Al2O3 powder is 0.5 μm; the solid content of the boehmite sol is 25 wt%, corresponding to 30 parts of AlOOH solids. Therefore, in the coating slurry, based on the total solid mass, the amount of α-Al2O3 powder is 60 wt%, AlOOH is 30 wt%, polyvinyl alcohol is 5 wt%, and polyvinylpyrrolidone is 5 wt%.

[0035] The coating, drying, calcining, pre-wetting, and rinsing conditions were the same as in Preparation Example 1, resulting in composite filter element B. The filtration accuracy of composite filter element B was determined to be 0.5 μm using the standard particle retention method.

[0036] Preparation Example 3 This preparation example is basically the same as Preparation Example 1, except that the α-Al2O3 powder is replaced with a mixture of α-Al2O3 powder and γ-Al2O3 powder, wherein the mass ratio of α-Al2O3 powder to γ-Al2O3 powder is 3:1, and the D50 particle size of both α-Al2O3 powder and γ-Al2O3 powder is 0.5 μm; the drying temperature is 100℃, the calcination temperature is 500℃, and the calcination time is 2 h; pre-wetting is performed by circulating a 30 wt% isopropanol aqueous solution for 90 min, followed by rinsing with a 30 wt% isopropanol aqueous solution for 20 min, and the initial filtrate is discarded to obtain composite filter C. The filtration accuracy of composite filter C is measured to be 0.5 μm by the standard particle retention method.

[0037] Obtaining the aqueous isopropanol mixture The hydrolyzed alcohol fraction and the aged alcohol condensate produced during the preparation of high-purity boehmite by aluminum alkoxide hydrolysis were mixed at a mass ratio of 1:1 to obtain aqueous isopropanol mixture A. The basic composition of aqueous isopropanol mixture A was determined and recorded in Table 1.

[0038] Table 1. Basic composition of aqueous isopropanol mixture A

[0039] Table 1 shows that, in addition to isopropanol and water, aqueous isopropanol mixture A also contains a certain amount of suspended solids, soluble aluminum, and trace impurities such as sodium, iron, and silicon. Turbidity and suspended solids content can reflect the load of fine powder, colloids, or micro-solids entrained in the aqueous alcohol; the soluble aluminum content is calculated based on the filtrate measured after filtration through a 0.45 μm filter membrane, and can be used to reflect aluminum-containing components in the sample that are difficult to remove by conventional coarse filtration.

[0040] Example 1 The aqueous isopropanol mixture A was first filtered through a 20 μm pre-filter, and then fed into the composite filter element A obtained in Preparation Example 1 for filtration at 50°C and a filtration pressure difference of 0.10 MPa to obtain a pretreated aqueous isopropanol solution. After continuous filtration for 24 h, the pressure difference between the inlet and outlet of the composite filter element A was 1.25 times the initial pressure difference.

[0041] The pretreated aqueous isopropanol solution is fed into a preheater and preheated to 80°C, then fed into a NaA molecular sieve membrane pervaporation dehydration unit. The effective membrane area of ​​the NaA molecular sieve membrane pervaporation dehydration unit is 0.5 m². 2The absolute pressure on the permeate side is controlled at 5 kPa, and the operating time is 24 hours. During operation, the permeate is collected by condensation on the permeate side, and low-water isopropanol solution is collected on the non-permeate side. The resulting low-water isopropanol solution is returned to the aluminum alkoxide synthesis process or the alcohol recycling process.

[0042] The detection results before and after filtration and the membrane separation operation results of Example 1 are shown in Table 2.

[0043] Table 2 Detection results of Example 1

[0044] Table 2 shows that after treatment with composite filter A, the turbidity, suspended solids content, and soluble aluminum content of the aqueous isopropanol mixture A were significantly reduced. After subsequent dehydration via NaA molecular sieve membrane pervaporation, the water content of the low-aqueous isopropanol solution obtained on the non-permeable side was 0.22 wt%, and the aluminum content in the low-aqueous isopropanol was below the detection limit. The membrane flux retention rate of the hydrophilic dehydration membrane was 93.5% after 24 h of operation and 89.2% after 48 h of operation.

[0045] Example 2 This embodiment is basically the same as Example 1, except that the composite filter element B obtained in Preparation Example 2 is used for pre-membrane filtration; the filtration temperature is 60℃, and the filtration pressure difference is 0.15 MPa; the pretreated aqueous isopropanol solution is preheated to 85℃ and then enters the PVA composite membrane pervaporation dehydration unit, with the absolute pressure on the permeate side controlled at 8 kPa and the operating time being 24 h. After continuous filtration for 24 h, the pressure difference between the inlet and outlet of the composite filter element B is 1.32 times the initial pressure difference.

[0046] The test results of Example 2 are shown in Table 3.

[0047] Table 3 Detection results of Example 2

[0048] As shown in Table 3, composite filter element B can also reduce the turbidity, suspended solids content, and soluble aluminum content in aqueous isopropanol mixture A. After pervaporation dehydration through the PVA composite membrane, the resulting low-water isopropanol solution has a water content of 0.28 wt%, the aluminum content in the low-water isopropanol is below the detection limit, and the membrane flux retention rate is 91.8% after 24 h.

[0049] Example 3 This embodiment is basically the same as Example 1, except that the composite filter element C obtained in Preparation Example 3 is used for pre-membrane filtration; the aqueous isopropanol mixture A is not pre-filtered at 20 μm before entering the composite filter element, but directly enters the composite filter element C at 45°C and 0.08 MPa filtration pressure difference; the pretreated aqueous isopropanol solution is preheated to 80°C and then enters the NaA molecular sieve membrane pervaporation dehydration unit, with the absolute pressure on the permeate side controlled at 5 kPa and the running time at 24 h. After continuous filtration for 24 h, the pressure difference between the inlet and outlet of the composite filter element C is 1.38 times the initial pressure difference.

[0050] Example 3 illustrates the pre-treatment of the aqueous isopropanol mixture using composite filter C without a pre-filter, under the conditions of aqueous isopropanol mixture A. The test results of Example 3 are shown in Table 4.

[0051] Table 4. Detection results of Example 3

[0052] As shown in Table 4, under the feeding conditions used in this embodiment, the composite filter C can perform pre-membrane treatment on the aqueous isopropanol mixture A, making the treated aqueous isopropanol solution suitable for entering the subsequent NaA molecular sieve membrane pervaporation dehydration unit. For conditions with higher solid loads or longer continuous operation times, a pre-filter can be installed based on pressure differential changes.

[0053] Comparative Example 1 Take the same aqueous isopropanol mixture A as in Example 1, filter it only through a 20 μm pre-filter without filtering through the composite filter, and then perform membrane separation under the same preheating conditions and NaA molecular sieve membrane pervaporation dehydration conditions as in Example 1.

[0054] The test results for Comparative Example 1 are shown in Table 5.

[0055] Table 5. Detection results of Comparative Example 1

[0056] Table 5 shows that, without the use of composite filters for pre-treatment, although a low-aqueous isopropanol solution with a water content of 0.35 wt% can be obtained after pervaporation and dehydration through the NaA molecular sieve membrane, the aluminum content in the low-aqueous isopropanol is 8.5 mg / L, and the 24-hour membrane flux retention rate is 76.3%. This indicates that when only a 20 μm pre-filter is used, some aluminum-containing components in the aqueous isopropanol mixture A will still enter the recycled alcohol product along with the non-permeate side alcohol solution, affecting the operational stability of subsequent membrane separation units.

[0057] Comparative Example 2 Aqueous isopropanol mixture A, identical to that in Example 1, was first filtered through a 20 μm pre-filter, and then through a standard polypropylene filter cartridge with a filtration precision of 0.5 μm. The filtered aqueous isopropanol solution was then subjected to membrane separation under the same preheating conditions and NaA molecular sieve membrane pervaporation dehydration conditions as in Example 1.

[0058] The test results for Comparative Example 2 are shown in Table 6.

[0059] Table 6. Detection results of Comparative Example 2

[0060] As shown in Table 6, under the condition of the same filtration accuracy of 0.5 μm, ordinary polypropylene filter cartridges can reduce the turbidity and suspended solids content of aqueous isopropanol mixture A, but the reduction in soluble aluminum content is limited. After the treated liquid enters the NaA molecular sieve membrane pervaporation dehydration unit, the membrane flux retention rate is 83.6% after 24 h, and the aluminum content in the low-aqueous isopropanol is 5.2 mg / L, which is lower than that of Comparative Example 1 but still higher than that of the other examples.

[0061] Comparative Example 3 Aqueous isopropanol mixture A, identical to that in Example 1, was first filtered through a 20 μm pre-filter, and then through a common alumina ceramic filter element with a filtration precision of 0.5 μm. The filtered aqueous isopropanol solution was then subjected to membrane separation under the same preheating conditions and NaA molecular sieve membrane pervaporation dehydration conditions as in Example 1.

[0062] The test results for Comparative Example 3 are shown in Table 7.

[0063] Table 7 Detection results of Comparative Example 3

[0064] As shown in Table 7, under the condition of a filtration accuracy of 0.5 μm, the ordinary alumina ceramic filter element has a good effect on reducing the turbidity and suspended solids content in the aqueous isopropanol mixture A, but its reduction in soluble aluminum content is still lower than that of the composite filter element used in Examples 1-3. After the feed liquid treated by the ordinary alumina ceramic filter element enters the NaA molecular sieve membrane pervaporation dehydration unit, the membrane flux retention rate is 87.2% after 24 h, and the aluminum content in the low-aqueous isopropanol is 3.8 mg / L.

[0065] Results Comparison The main results of Examples 1-3 and Comparative Examples 1-3 are summarized in Table 8.

[0066] Table 8 Summary of Results from Examples and Comparative Examples

[0067] As shown in Table 8, the composite filter elements in Examples 1-3 all exhibited good reduction effects on turbidity, suspended solids content, and soluble aluminum content. Specifically, the soluble aluminum removal rate in Examples 1-3 was 73.4%-77.8%, higher than that of ordinary PP filter elements and ordinary alumina ceramic filter elements with the same filtration precision. Compared with Comparative Examples 1-3, Examples 1-3 showed higher 24-hour membrane flux retention during subsequent hydrophilic dehydration membrane separation, and also lower aluminum content in low-water isopropanol.

[0068] The above results demonstrate that the composite filter element made from alumina powder, boehmite sol, polyvinyl alcohol, polyacrylic acid, and polyvinylpyrrolidone, used in this invention, can reduce the load of fine solids and aluminum-containing components entering the hydrophilic dehydration membrane separation unit after pre-treatment of the aqueous isopropanol mixture generated during the aluminum alkoxide process for producing high-purity pseudoboehmite. This improves the operational stability of the subsequent dehydration membrane separation and yields a low-aqueous isopropanol solution suitable for return to aluminum alkoxide synthesis, aluminum alkoxide dissolution, or alcohol recycling processes.

Claims

1. A method for separating hydrous isopropanol from high-purity boehmite produced by the aluminum alkoxide process, characterized in that, Includes the following steps: Collect the aqueous isopropanol mixture generated during the preparation of high-purity boehmite by aluminum alkoxide hydrolysis; The aqueous isopropanol mixture is fed into a composite filter for filtration to obtain a pretreated aqueous isopropanol solution. The pretreated aqueous isopropanol solution is preheated and then fed into a hydrophilic dehydration membrane separation unit. A negative pressure is formed on the permeate side of the hydrophilic dehydration membrane separation unit and the permeate is collected by condensation. Low-water isopropanol solution is collected on the non-permeate side. The low-water isopropanol solution is returned to the aluminum alkoxide synthesis process, the aluminum alkoxide dissolution process, or the alcohol recycling process; The composite filter element is manufactured according to the following method: Polyvinyl alcohol is dissolved in water and / or aqueous isopropanol, and polyacrylic acid is added to obtain a dispersion. Alumina powder, boehmite sol, and polyvinylpyrrolidone were added to the dispersion and mixed to obtain a coating slurry; The coating slurry is applied to a porous alumina support, dried, and then calcined. The calcined porous alumina support was pre-wetted with aqueous isopropanol and then rinsed with aqueous isopropanol. The initial filtrate was discarded to obtain the composite filter.

2. The alcohol separation method according to claim 1, characterized in that, The aqueous isopropanol mixture is one or more of the following: hydrolysis alcohol fraction, aged alcohol condensate, mother liquor de-alcoholized liquid, and dried tail gas condensate.

3. The alcohol separation method according to claim 1, characterized in that, The aqueous isopropanol mixture has a water content of 20–45 wt%, preferably 30–40 wt%.

4. The alcohol separation method according to claim 1, characterized in that, The alumina powder is α-Al2O3 powder, or a mixture of α-Al2O3 powder and γ-Al2O3 powder; the D50 particle size of the α-Al2O3 powder is 0.1–1.0 μm, preferably 0.3–0.6 μm; the D50 particle size of the γ-Al2O3 powder is 0.1–1.0 μm, preferably 0.3–0.6 μm.

5. The alcohol separation method according to claim 1, characterized in that, The boehmite sol has a solid content of 20-30 wt%; in the coating slurry, based on the total solid mass, the alumina powder content is 55-75 wt%, the boehmite sol content (calculated as AlOOH) is 20-35 wt%, the polyvinyl alcohol content is 3-6 wt%, and the polyvinylpyrrolidone content is 2-6 wt%.

6. The alcohol separation method according to claim 1, characterized in that, The porous alumina support is a tubular alumina support; the average pore size of the porous alumina support is 5-15 μm, and the porosity is 35-45%; the coating slurry is applied to the porous alumina support by dip-coating.

7. The alcohol separation method according to claim 1, characterized in that, The drying temperature is 80-110℃, the calcination temperature is 400-550℃, and the calcination time is 2-4 h; the pre-wetting is carried out by circulating a 20-30 wt% isopropanol aqueous solution for 60-90 min, and the rinsing is carried out by rinsing a 20-30 wt% isopropanol aqueous solution for 10-30 min.

8. The alcohol separation method according to claim 1, characterized in that, Before entering the composite filter element, the aqueous isopropanol mixture is filtered by a pre-filter with a pore size of 20 μm; the composite filter element has a filtration accuracy of 0.1 to 2 μm, preferably 0.5 μm, a filtration temperature of 45 to 60°C, and a filtration pressure difference of 0.08 to 0.15 MPa.

9. The alcohol separation method according to claim 1, characterized in that, The hydrophilic dehydration membrane separation unit adopts a NaA molecular sieve membrane pervaporation dehydration unit or a PVA composite membrane pervaporation dehydration unit; the temperature of the pretreated aqueous isopropanol solution entering the hydrophilic dehydration membrane separation unit is 80-85℃, and the absolute pressure on the permeation side is 5-8 kPa.

10. The alcohol separation method according to claim 1, characterized in that, The water content of the low-water isopropanol solution collected on the non-permeable side is not higher than 0.5 wt%; the permeate collected by condensation on the permeable side is returned to the aluminum alkoxide hydrolysis solution preparation process, the aging replenishment process, or the tail gas absorption liquid preparation process.

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