Solvent-cycle-assisted rice bran wax membrane refining process and its nanofiltration membrane preparation method

Abstract

This invention relates to a solvent-circulating assisted rice bran wax membrane refining process and its nanofiltration membrane preparation method. The process includes: heating and melting crude rice bran wax; adding a first portion of ethyl acetate to dissolve and filter to remove impurities; adding a second portion of ethyl acetate for dilution and then passing the solution into a first membrane separation unit for cross-flow decolorization; the first unit is filled with a first solvent-resistant composite nanofiltration membrane composed of an adsorption functional layer, a reduction functional layer, and a complex modification layer, with the adsorption functional layer selectively adsorbing pigments; the decolorized liquid is then passed into a second membrane separation unit for concentration; the second unit is filled with a second solvent-resistant composite nanofiltration membrane with the active layer facing opposite directions; the concentrate is cooled and filtered to obtain a filter cake; the membrane is evaporated to remove solvent and obtain refined rice bran wax melt; granulation is performed, and ethyl acetate is recovered and recycled in each step. The invention also includes a method for preparing the solvent-resistant composite nanofiltration membrane, employing a phase inversion method to prepare the adsorption functional layer, a solution impregnation-drying method to load the reducing agent, and an interfacial polymerization method combined with a coordination modification method to prepare the complex modification layer.

Description

Technical Field

[0001] This application relates to the fields of natural product refining and membrane separation technology, and in particular to a solvent-circulating assisted rice bran wax membrane refining process and its nanofiltration membrane preparation method. Background Technology

[0002] Rice bran wax, an important byproduct of rice bran oil refining, is a natural plant wax formed by the esterification of long-chain fatty acids and long-chain fatty alcohols. Due to its unique physicochemical properties, it is widely used in food coatings, cosmetic additives, pharmaceutical sustained-release agents, and chemical auxiliaries. As consumers increasingly demand higher quality from natural products, the market has raised the bar for the color purity of rice bran wax products. However, crude rice bran wax often contains fat-soluble pigments such as carotenoids, chlorophyll, and their derivatives. These pigment molecules have complex structures and are tightly bound to the wax, resulting in a dark brown or dark green color in the crude wax, severely limiting its application value in high-end products. Therefore, efficient and environmentally friendly decolorization and refining technology has become a key link in the upgrading of the rice bran wax deep processing industry.

[0003] Currently, there are two main industrial processes for decolorizing rice bran wax. One of them is the adsorption decolorization method. This typically involves dissolving crude rice bran wax in an organic solvent, then adding adsorbents such as activated clay and activated carbon for stirring and adsorption. Finally, the wax is filtered and the solvent is recovered to obtain refined wax. While this technology is mature and relatively simple to operate, it has significant technical drawbacks: First, the amount of adsorbent required is enormous, typically needing to reach 15% or even higher of the wax weight to achieve the desired decolorization effect. This not only increases raw material costs but also generates a large amount of saturated waste clay. This type of solid waste contains residual oils and organic solvents and is classified as hazardous waste, making it difficult and costly to treat. Second, while adsorbing pigments, the adsorbent also non-selectively adsorbs some wax and bioactive functional components, resulting in a 3% to 5% reduction in product yield and resource waste. Furthermore, this process is highly dependent on the organic solvent system, the subsequent solvent recovery process is extremely energy-intensive, and the production workshop poses flammable and explosive safety hazards, making it difficult to meet the requirements of green chemical development.

[0004] Another commonly used method is solvent extraction, which separates pigments and waxes based on their differences in solubility in different solvents. Through multi-stage countercurrent extraction, it can reduce product color to some extent. However, solvent extraction also faces bottlenecks in practical applications: on the one hand, to achieve effective phase separation and mass transfer efficiency, this process typically requires maintaining a high solvent-to-material ratio (often as high as 6:1 to 10:1), resulting in a huge solvent circulation load, and the energy consumption of the distillation recovery unit accounts for the majority of the total production cost; on the other hand, simple physical extraction is insufficient to remove dark-colored impurities with molecular structures similar to wax esters, making it difficult for the final product's color index to meet the standards of high-end cosmetic raw materials, and the longer process flow increases the risk of material oxidation, affecting product quality stability.

[0005] In recent years, membrane separation technology has been gradually introduced into the field of oil refining due to its advantages such as room temperature operation, no phase change, energy saving, and environmental protection. Nanofiltration membranes can retain organic matter with molecular weights between 200 and 1000 Daltons, and theoretically can achieve high-efficiency retention of pigment molecules. However, existing nanofiltration membrane technology still faces severe challenges when applied to rice bran wax decolorization. Conventional polyamide composite nanofiltration membranes are prone to swelling or dissolution in organic solvent systems such as ethyl acetate and n-hexane, leading to membrane structure collapse and loss of separation performance. While solvent-resistant materials (such as polytetrafluoroethylene) have good stability, they often have low flux and are difficult to functionalize, failing to meet the high-efficiency requirements of industrial production. In addition, rice bran wax solutions have high viscosity, which easily forms concentration polarization and gel layers on the membrane surface, leading to rapid decline in membrane flux and frequent cleaning, severely restricting the continuous and stable operation of membrane technology.

[0006] Therefore, we propose a solvent-circulating assisted rice bran wax membrane refining process and its nanofiltration membrane preparation method. Summary of the Invention

[0007] The purpose of this application is to provide a solvent-circulating assisted rice bran wax membrane refining process and its nanofiltration membrane preparation method, aiming to solve the problems of existing rice bran wax decolorization processes, such as large adsorbent consumption, serious solid waste pollution, high solvent consumption, high energy consumption for recovery, and the easy swelling and failure of conventional nanofiltration membranes in organic solvent systems, as well as the low decolorization efficiency of single separation mechanisms.

[0008] To achieve the above objectives, this application provides a solvent-circulating assisted rice bran wax film refining process, comprising the following steps;

[0009] S1. The crude rice bran wax is heated to 80-90℃ in a melting tank using a steam coil to melt it, thus obtaining molten rice bran wax;

[0010] S2. The molten rice bran wax is transported to a mixing tank, and the first part of ethyl acetate is added while maintaining the temperature at 80~90℃. The mass ratio of the first part of ethyl acetate to the molten rice bran wax is 0.8:1~1.2:1. The mixture is stirred until the rice bran wax is completely dissolved and the liquid phase is clear and transparent. The resulting mixture is filtered through a bag filter to remove large particulate impurities.

[0011] S3. The mixture filtered in S2 is sent to a decolorization tank, and a second portion of ethyl acetate is added to make the mass ratio of the total amount of ethyl acetate in the system to the initial rice bran wax 2.5:1~3.5:1. After dilution, the mixture enters the first membrane separation unit for cross-flow decolorization treatment, with an operating pressure of 0.5~2.0 MPa and a temperature of 70~90℃. The first membrane separation unit is filled with a first solvent-resistant composite nanofiltration membrane, which is composed of an adsorption functional layer, a reduction functional layer and a complex modification layer in sequence. The adsorption functional layer is used as the feed contact surface to selectively adsorb pigment molecules and allow rice bran wax and ethyl acetate to pass through.

[0012] S4. The decolorized clear liquid obtained in S3 is transferred to a concentration tank, heated to 50~60℃, and then enters the second membrane separation unit for cross-flow concentration treatment. The operating pressure is 1.5~4.0 MPa and the temperature is 50~60℃. The second membrane separation unit is filled with a second solvent-resistant composite nanofiltration membrane. Both the second solvent-resistant composite nanofiltration membrane and the first solvent-resistant composite nanofiltration membrane are composed of an adsorption functional layer, a reduction functional layer and a complex modification layer. The two membranes are arranged in mirror images with the active layers facing completely opposite directions in their respective membrane modules.

[0013] S5. The concentrate from S4 is transferred to a plate filter tank, cooled to 25~35℃, and then separated by a plate filter. The resulting filtrate is ethyl acetate, which is transferred to an ethyl acetate circulation tank. The resulting filter cake is a mixture of ethyl acetate and rice bran wax, wherein the mass ratio of ethyl acetate to rice bran wax is 0.8:1~1.2:1.

[0014] S6. Heat the ethyl acetate-rice bran wax mixture from S5 to 100-110°C in a thin-film evaporator to completely vaporize the ethyl acetate and condense it back to the ethyl acetate circulation tank to obtain refined rice bran wax melt.

[0015] S7. The refined rice bran wax melt is granulated to obtain rice bran wax particles, wherein the first part of ethyl acetate, the second part of ethyl acetate, and the ethyl acetate recovered in S4, S5, and S6 are all sent to the ethyl acetate circulation tank.

[0016] Preferably, the mass ratio of the first part of ethyl acetate to the molten rice bran wax is 0.9:1, and the mass ratio of the total amount of ethyl acetate in the system to the initial mass of rice bran wax after adding the second part of ethyl acetate is preferably 2.8:1.

[0017] Preferably, the first membrane separation unit and the second membrane separation unit adopt a cross-flow filtration method, specifically: in the first membrane separation unit, the adsorption functional layer is used as the feed contact surface, the cross-flow velocity is 0.8~1.5 m / s, and the transmembrane pressure difference is 0.8~1.2 MPa; in the second membrane separation unit, the complex modified layer is used as the feed contact surface, and the cross-flow velocity is set to 2.0~3.5 m / s, and the transmembrane pressure difference is set to 2.5~3.5 MPa.

[0018] Furthermore, the ratio of the transmembrane pressure difference between the first membrane separator and the second membrane separator is negatively correlated with the ratio of their molecular weight cutoffs, i.e., the ratio of the transmembrane pressure difference is 1.5~4.0:0.5~2.0, while the ratio of the molecular weight cutoffs is 200~500:500~1000.

[0019] Preferably, the ratio of the transmembrane pressure difference between the first membrane separator and the second membrane separator is 2.8:1 to 3.2:1, and the ratio of the molecular weight cutoff is 270 to 420 to 580 to 800.

[0020] Preferably, the specific process of the cross-flow decolorization treatment is as follows: After adding a second part of ethyl acetate to the mixture filtered by S2, it is driven into the first membrane separation unit by a circulating pump. The feed side pressure is 0.5~2.0 MPa, and the permeate side is at atmospheric pressure. The permeate is the decolorized clear liquid and is continuously discharged and transported to the concentration tank of S4. The concentrate is returned to the decolorization tank for recycling. The recycling decolorization time is controlled at 30~60 min until the Gardner color number of the permeate is ≤3, at which point the circulation is stopped. The remaining concentrate is discharged and returned to the S2 mixing tank for redissolution or treated as waste residue.

[0021] Preferably, the specific process of the cross-flow concentration treatment is as follows: the decolorized clear liquid obtained in S3 is pumped into the concentration tank and heated to 50~60℃, and then driven by the circulation pump into the second membrane separation unit. The feed side pressure is 1.5~4.0 MPa, and the permeate side is at atmospheric pressure. The concentrate is returned to the concentration tank, and the permeate (ethyl acetate clear liquid) is continuously discharged and transported to the ethyl acetate circulation tank. When the solid content of the liquid in the concentration tank reaches 30~40 wt%, the concentration is stopped, and the concentrate is discharged into the S5 plate filter tank.

[0022] To achieve the above objectives, this application also provides a method for preparing a solvent-resistant composite nanofiltration membrane for use in any of the above methods, wherein the nanofiltration membrane is sequentially stacked with an adsorption functional layer prepared by a phase inversion method, a reduction functional layer prepared by a solution impregnation-drying method, and a complex modification layer prepared by an interfacial polymerization method combined with a coordination modification method, characterized in that it further includes the following steps;

[0023] S1. The adsorbent, polymer binder and pore-forming agent are mixed to prepare a casting solution, which is then immersed in a coagulation bath for curing after molding.

[0024] S2. The polymer carrier is immersed in the reducing agent solution, and after the solution has fully penetrated into the internal pores of the carrier, it is dried so that the reducing agent is loaded in the form of microcrystals on the inner wall and surface of the polymer carrier pores.

[0025] S3. A polyamide active separation layer is formed on the surface of a polysulfone ultrafiltration membrane by an interfacial polycondensation reaction between piperazine and pyromellitic trimethylol chloride.

[0026] S4. The polyamide active separation layer is treated sequentially with tannic acid solution and ferric chloride solution, so that tannic acid molecules are adsorbed on the surface of polyamide and further undergo coordination reaction with iron ions to form a tannic acid-iron ion complex modified layer, and the molecular weight cutoff of the tannic acid-iron ion complex modified layer is 200-1000 Da.

[0027] S5. The adsorption functional layer faces the feed side, the complex modified layer faces the permeation side, and the reduction functional layer is sandwiched between the adsorption functional layer and the complex modified layer, and is compounded by hot pressing, coating or co-extrusion.

[0028] Preferably, in step S1, the mass percentage of each component in the casting solution is: 5%~20% adsorbent, 10%~25% polymer binder, 1%~8% pore-forming agent, and the balance is a polar solvent, namely N,N-dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidone.

[0029] Preferably, in step S1, the coagulation bath is deionized water or an ethanol-water mixture, the coagulation bath temperature is 10~40℃, and the phase transformation curing time is 1~10 minutes.

[0030] Preferably, the adsorbent is at least one and any combination of activated clay, powdered activated carbon, diatomaceous earth, or zeolite molecular sieve; the polymer binder is at least one and any combination of polyethersulfone, polyvinylidene fluoride, or polyacrylonitrile; and the pore-forming agent is at least one and any combination of polyvinylpyrrolidone, polyethylene glycol, or lithium chloride.

[0031] The mass ratio of the activated clay to the activated carbon is 3:1 to 4:1, preferably 3.5:1; the mass ratio of the polyethersulfone to the polyvinylidene fluoride is 1:1 to 3:1, preferably 2:1; the mass ratio of the polyethylene glycol to the lithium chloride is 2:1 to 8:1, preferably 3.8:1.

[0032] Preferably, in step S2, the polymer carrier is at least one and / or any combination of polypropylene nonwoven fabric, polyester web or polyvinyl alcohol sponge.

[0033] Preferably, in step S2, the reducing agent solution is a reducing inorganic solvent and / or a reducing organic solvent; the reducing inorganic solvent is at least one of sodium borohydride, sodium bisulfite, or sodium thiosulfate, and the reducing organic solvent is at least one of ascorbic acid, glucose, or gallic acid.

[0034] Preferably, the mass concentration of the reducing agent solution is 1% to 15%, the immersion time is 30 to 180 minutes, the drying temperature is 40 to 80°C, the drying time is 2 to 12 hours, and the loading of the reducing agent microcrystals is 5% to 25% of the mass of the polymer carrier.

[0035] Preferably, in step S3, the mass concentration of the piperazine aqueous solution is 0.1%~2.0%, the mass concentration of the trimesoyl chloride n-hexane solution is 0.05%~0.5%, the interfacial polycondensation reaction time is 30~180 seconds, and the reaction temperature is 15~35℃.

[0036] Preferably, in step S4, the mass concentration of the tannic acid solution is 0.5%~5%, the mass concentration of the ferric chloride solution is 0.2%~2%, the treatment time of the tannic acid solution is 5~30 minutes, the treatment time of the ferric chloride solution is 5~20 minutes, and the treatment temperature is 20~40℃.

[0037] To achieve the above objectives, this application also provides a rice bran wax decolorization system based on nanofiltration membrane method, wherein the nanofiltration membrane prepared by any of the above preparation methods comprises:

[0038] The beneficial effects of the technical solution of this invention are as follows:

[0039] An ethyl acetate closed-loop circulation system is employed, organically coupling four units: decolorization, concentration, crystallization, and evaporation, resulting in a solvent recovery rate of ≥98%. Compared to traditional solvent extraction methods in existing technologies, this invention significantly reduces the amount of fresh solvent replenishment and waste solvent discharge, substantially saving production costs and fundamentally solving the safety hazards and environmental pollution problems caused by solvent evaporation. A stepwise dissolution and dilution process is also adopted, first dissolving in a 1:1 ratio, then diluting in a 3:1 ratio, effectively optimizing the viscosity of the feed solution before it enters the membrane module. Compared to existing technologies that involve adding a large amount of solvent at once, leading to uneven mixing or improper viscosity control, this invention effectively solves the hydrodynamic challenges of high-viscosity materials during pipeline transportation and membrane filtration, reduces pumping energy consumption, effectively suppresses concentration polarization on the membrane surface, and extends membrane lifespan.

[0040] A gradient control strategy coupled with temperature and pressure was adopted. High-temperature operation (70~90℃) was used in the primary membrane decolorization stage to reduce feed viscosity and improve mass transfer efficiency; low-temperature operation (50~60℃) was used in the secondary membrane concentration stage to prevent oxidation of heat-sensitive components. This approach ensured both processing efficiency and product quality, achieving a deep fit between process parameters and material characteristics.

[0041] The adsorption functional layer is prepared using a phase inversion method, with activated clay and powdered activated carbon as adsorbents, polyvinylidene fluoride as a binder, and polyvinylpyrrolidone as a pore-forming agent to form a porous membrane structure. When used on the feed side, activated clay (3.5:1 mass ratio with activated carbon) exhibits preferential adsorption capacity for polar pigment molecules (such as chlorophyll derivatives and free fatty acid oxidation products), while activated carbon efficiently captures non-polar pigment molecules (such as carotenoids and sterol ester pigments). Furthermore, through the synergistic adsorption of activated clay and powdered activated carbon, the initial color is reduced when molten rice bran wax (temperature 60–80°C) passes through the membrane layer.

[0042] The reducing functional layer uses a polypropylene microporous membrane as a carrier, and loads sodium borohydride microcrystals (loading 5%–25%) via an impregnation-drying method, sandwiched between the adsorption functional layer and the complex modified layer. When the molten wax passes through the adsorption functional layer, the residual conjugated double-bond type stubborn pigments (such as some oxidative degradation products) that are difficult to remove by physical adsorption come into contact with the reducing agent microcrystals, and a solid-liquid interface reduction reaction occurs at the melting temperature. The unsaturated chromophores in the pigment molecules are hydrogenated or reduced to colorless derivatives, further reducing the total color. The use of a hydrophobic polypropylene carrier ensures good compatibility with the molten wax, guaranteeing smooth passage of the wax liquid. The reducing agent is stably loaded in microcrystalline form, avoiding the problem of decreased reaction efficiency caused by uneven dissolution and dispersion in a solvent-free system.

[0043] The complex-modified layer uses a polyamide thin-film composite membrane as its matrix. A dense TA-FeⅢ complex network is constructed via tannic acid adsorption and Fe³⁺ coordination reaction, facing the permeate side. This effectively retains residual pigment molecules, oxidation byproducts, and trace amounts of colloidal components with a molecular weight greater than 600 Da, further reducing the color of the permeate to the Klett value. Simultaneously, the TA-FeⅢ complex layer is rich in phenolic hydroxyl groups and hydrophilic groups, endowing the membrane surface with excellent hydrophilicity and exhibiting good antifouling ability and chemical stability in high-temperature molten wax environments. The coordination crosslinking structure significantly enhances the pressure resistance and oxidation resistance of the polyamide separation layer. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the overall process of the rice bran wax film refining process assisted by solvent circulation in one embodiment of this application;

[0045] Figure 2This is a process flow diagram of a method for preparing a solvent-resistant composite nanofiltration membrane in one embodiment of this application.

[0046] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0047] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0048] Furthermore, descriptions using terms such as "first" and "second" in this application are for descriptive purposes only (e.g., to distinguish identical or similar elements) and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, features defined with "first" or "second" may explicitly or implicitly include at least one feature. Additionally, technical solutions from different embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. If a combination of technical solutions is contradictory or impossible to implement, such a combination should be considered nonexistent and not within the scope of protection claimed in this application.

[0049] With the rapid development of the oil and fat processing industry and the increasing demands of consumers for the quality of natural products, rice bran wax, as an important byproduct in the refining process of rice bran oil, is receiving growing attention for its high-value utilization. Rice bran wax is rich in high-value-added components such as natural higher fatty alcohols and sterols, and is widely used in cosmetics, food, pharmaceuticals, and fine chemicals. However, crude rice bran wax contains a large amount of pigments such as carotenoids, chlorophyll, and oxidation products, resulting in a dark brown to dark green color, which severely restricts its application in high-quality products.

[0050] Traditional methods for decolorizing rice bran wax primarily rely on physical adsorption by adsorbents such as activated clay and activated carbon, or a combination of chemical oxidation-reduction treatment. However, existing decolorization processes have significant shortcomings in practical applications: Firstly, adsorbent decolorization typically requires an organic solvent system, followed by multiple steps such as plate and frame filtration and solvent recovery, resulting in a lengthy process, high solvent consumption, the risk of solvent residue, and a lack of alignment with green manufacturing development trends. Secondly, chemical decolorization methods easily introduce external chemical reagents, potentially leading to the degradation of natural active ingredients in rice bran wax and affecting product quality. Furthermore, traditional methods exhibit poor selectivity in removing pigment molecules, making it difficult to retain the effective components of rice bran wax while achieving efficient decolorization. The contradiction between decolorization effectiveness and component retention has long remained unresolved.

[0051] Currently, membrane separation technology has been gradually applied to the field of oil decolorization. However, existing nanofiltration membranes are mostly designed for organic solvent systems and have poor adaptability to solvent-free molten liquids. In addition, conventional nanofiltration membranes have a single function and are difficult to achieve multi-level synergistic removal of pigment molecules.

[0052] See Figure 1 This invention proposes a solvent-circulating assisted rice bran wax film refining process, which includes the following steps;

[0053] S1. The crude rice bran wax is heated to 80-90℃ in a melting tank using a steam coil to melt it, thus obtaining molten rice bran wax;

[0054] S2. The molten rice bran wax is transported to a mixing tank, and the first part of ethyl acetate is added while maintaining the temperature at 80~90℃. The mass ratio of the first part of ethyl acetate to the molten rice bran wax is 0.8:1~1.2:1. The mixture is stirred until the rice bran wax is completely dissolved and the liquid phase is clear and transparent. The resulting mixture is filtered through a bag filter to remove large particulate impurities.

[0055] S3. The mixture filtered in S2 is sent to a decolorization tank, and a second portion of ethyl acetate is added to make the mass ratio of the total amount of ethyl acetate in the system to the initial rice bran wax 2.5:1~3.5:1. After dilution, the mixture enters the first membrane separation unit for cross-flow decolorization treatment, with an operating pressure of 0.5~2.0 MPa and a temperature of 70~90℃. The first membrane separation unit is filled with a first solvent-resistant composite nanofiltration membrane, which is composed of an adsorption functional layer, a reduction functional layer and a complex modification layer in sequence. The adsorption functional layer is used as the feed contact surface to selectively adsorb pigment molecules and allow rice bran wax and ethyl acetate to pass through.

[0056] In one embodiment, the mass ratio of the first ethyl acetate to the molten rice bran wax is preferably 0.9:1, and the mass ratio of the total amount of ethyl acetate to the initial rice bran wax in the system after adding the second ethyl acetate is preferably 2.8:1.

[0057] S4. The decolorized clear liquid obtained in S3 is transferred to a concentration tank, heated to 50~60℃, and then enters the second membrane separation unit for cross-flow concentration treatment. The operating pressure is 1.5~4.0 MPa and the temperature is 50~60℃. The second membrane separation unit is filled with a second solvent-resistant composite nanofiltration membrane. Both the second solvent-resistant composite nanofiltration membrane and the first solvent-resistant composite nanofiltration membrane are composed of an adsorption functional layer, a reduction functional layer and a complex modification layer. The two membranes are arranged in mirror images with the active layers facing completely opposite directions in their respective membrane modules.

[0058] Specifically, the second solvent-resistant composite nanofiltration membrane uses its reduction functional layer as the feed contact surface, while the first solvent-resistant composite nanofiltration membrane uses its adsorption functional layer as the feed contact surface. During the cross-flow concentration process, the second membrane separation unit effectively retains rice bran wax on the retention side, while allowing ethyl acetate and residual trace small molecule impurities to permeate to the permeate side, thereby achieving the concentration of rice bran wax and solvent separation. After the concentration treatment is completed, the collected retentate is the rice bran wax concentrate, with a solid content of 40~60wt%. The permeate is the recovered ethyl acetate, which is temporarily stored in the solvent recovery tank after condensation.

[0059] Furthermore, the specific process of the cross-flow decolorization treatment is as follows: after adding a second part of ethyl acetate to the mixture filtered by S2, it is driven into the first membrane separation unit by a circulating pump. The feed side pressure is 0.5~2.0 MPa, and the permeate side is at atmospheric pressure. The permeate is the decolorized clear liquid and is continuously discharged and transported to the concentration tank of S4. The concentrate is returned to the decolorization tank for recycling. The recycling decolorization time is controlled at 30~60 min until the Gardner color number of the permeate is ≤3 and the circulation is stopped. The remaining concentrate is discharged and returned to the S2 mixing tank for redissolution or treated as waste residue.

[0060] In one embodiment, the first membrane separator and the second membrane separator adopt a cross-flow filtration method, specifically: in the first membrane separator, the adsorption functional layer is used as the feed contact surface, the cross-flow velocity is 0.8~1.5 m / s, and the transmembrane pressure difference is 0.8~1.2 MPa; in the second membrane separator, the complex modified layer is used as the feed contact surface, and the cross-flow velocity is set to 2.0~3.5 m / s, and the transmembrane pressure difference is set to 2.5~3.5 MPa.

[0061] Furthermore, the ratio of the transmembrane pressure difference between the first membrane separator and the second membrane separator is negatively correlated with the ratio of their molecular weight cutoffs, i.e., the ratio of the transmembrane pressure difference is 1.5~4.0:0.5~2.0, while the ratio of the molecular weight cutoffs is 200~500:500~1000.

[0062] In one preferred embodiment, the ratio of the transmembrane pressure difference between the first membrane separator and the second membrane separator is preferably 2.8:1 to 3.2:1, and the ratio of the molecular weight cutoff is preferably 270 to 420:580 to 800.

[0063] Further, the specific process of the cross-flow concentration treatment is as follows: the decolorized clear liquid obtained in S3 is pumped into the concentration tank and heated to 50~60℃, and then driven by the circulation pump into the second membrane separation unit. The feed side pressure is 1.5~4.0 MPa, and the permeate side is at atmospheric pressure. The concentrate is returned to the concentration tank, and the permeate (ethyl acetate clear liquid) is continuously discharged and transported to the ethyl acetate circulation tank. When the solid content of the liquid in the concentration tank reaches 30~40 wt%, the concentration is stopped, and the concentrate is discharged into the S5 plate filter tank.

[0064] S5. The concentrate from S4 is transferred to a plate filter tank, cooled to 25~35℃, and then separated by a plate filter. The resulting filtrate is ethyl acetate, which is transferred to an ethyl acetate circulation tank. The resulting filter cake is a mixture of ethyl acetate and rice bran wax, wherein the mass ratio of ethyl acetate to rice bran wax is 0.8:1~1.2:1.

[0065] S6. Heat the ethyl acetate-rice bran wax mixture from S5 to 100-110°C in a thin-film evaporator to completely vaporize the ethyl acetate and condense it back to the ethyl acetate circulation tank to obtain refined rice bran wax melt.

[0066] S7. The refined rice bran wax melt is granulated to obtain rice bran wax particles, wherein the first part of ethyl acetate, the second part of ethyl acetate, and the ethyl acetate recovered in S4, S5, and S6 are all sent to the ethyl acetate circulation tank.

[0067] See Figure 2 The present invention also proposes a method for preparing a solvent-resistant composite nanofiltration membrane, wherein the nanofiltration membrane is sequentially stacked with an adsorption functional layer prepared by phase inversion method, a reduction functional layer prepared by solution impregnation-drying method, and a complex modification layer prepared by interfacial polymerization method combined with coordination modification method, and further includes the following steps;

[0068] S1. The adsorbent is mixed with polymer binder and pore-forming agent to prepare casting solution, which is then immersed in coagulation bath for curing after molding to obtain adsorption functional layer.

[0069] In one embodiment, the mass percentage of each component in the casting solution is: 5%~20% adsorbent, 10%~25% polymer binder, 1%~8% pore-forming agent, and the balance is a polar solvent, namely N,N-dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidone.

[0070] Further, the adsorbent is at least one and any combination of activated clay, powdered activated carbon, diatomaceous earth, or zeolite molecular sieve; the polymer binder is at least one and any combination of polyethersulfone, polyvinylidene fluoride, or polyacrylonitrile; and the pore-forming agent is at least one and any combination of polyvinylpyrrolidone, polyethylene glycol, or lithium chloride. The mass ratio of activated clay to activated carbon is 3:1 to 4:1, the mass ratio of polyethersulfone to polyvinylidene fluoride is 1:1 to 3:1, and the mass ratio of polyethylene glycol to lithium chloride is 2:1 to 8:1.

[0071] In a preferred embodiment, the mass ratio of activated clay to activated carbon is 3.5:1. Activated clay exhibits excellent adsorption capacity for polar pigment molecules (such as chlorophyll derivatives), while activated carbon shows better adsorption performance for non-polar pigment molecules (such as carotenoids). Furthermore, the 3.5:1 mass ratio ensures balanced adsorption of various pigments while avoiding an excessively high proportion of activated carbon that would make the film too hydrophobic and affect the permeability of the molten wax. The polymer binder is polyvinylidene fluoride (PVDF), with a concentration of 20 wt% of the solid content of the casting solution.

[0072] For example, 200-mesh food-grade activated clay and 300-mesh food-grade powdered activated carbon are mixed in a dry state in a mixer at a speed of 60 rpm for 2 hours to ensure uniform mixing.

[0073] Weigh 100g of this mixed adsorbent and gradually add it to a mixture containing 200g of PVDF in DMF solution (concentration of 5wt%) and 5g of polyvinylpyrrolidone (PVPK30, as a porogen).

[0074] Stir at 500 rpm for 4 hours using a mechanical stirrer, maintaining a water bath temperature of 50°C during the process to reduce the viscosity of the solution, until a uniform, viscous casting solution is formed.

[0075] The resulting casting solution was poured onto a clean, flat glass plate, and a doctor blade was used to control the wet film thickness to 300 μm. The glass plate was immediately immersed in a 25°C deionized water coagulation bath for 24 hours, during which the PVDF solidified and precipitated a pore-forming agent, forming a porous structure with micron-level interconnected pores.

[0076] The nascent adsorption functional layer was removed and rinsed with plenty of deionized water for 48 hours to thoroughly remove residual DMF solvent and PVP porogen. It was then stored in deionized water for later use. This yielded an adsorption functional layer with a dry thickness of approximately 180 μm, a porosity of approximately 65%, and an average pore size of approximately 1.5 μm.

[0077] In a preferred embodiment, the coagulation bath is deionized water or an ethanol-water mixture, the coagulation bath temperature is 10~40℃, and the phase transformation curing time is 1~10 minutes.

[0078] In a preferred embodiment, the mass ratio of polyethersulfone to polyvinylidene fluoride is 2:1.

[0079] In a preferred embodiment, the mass ratio of polyethylene glycol to lithium chloride is 3.8:1.

[0080] S2. The polymer carrier is immersed in the reducing agent solution, and after the solution has fully penetrated into the internal pores of the carrier, it is dried so that the reducing agent is loaded in the form of microcrystals on the inner wall and surface of the polymer carrier pores.

[0081] In one embodiment, the polymer carrier is at least one and / or any combination of polypropylene nonwoven fabric, polyester web or polyvinyl alcohol sponge.

[0082] In a preferred embodiment, the reducing agent solution is a reducing inorganic solvent and / or a reducing organic solvent.

[0083] The reducing inorganic substance is at least one of sodium borohydride, sodium bisulfite, or sodium thiosulfate, and the reducing organic substance is at least one of ascorbic acid, glucose, or gallic acid.

[0084] For example, sodium borohydride (NaBH4) is used to chemically reduce pigment molecules that are difficult to be physically adsorbed after passing through the adsorption functional layer (such as some stubborn pigments with conjugated double bond structures) into colorless or light-colored derivatives.

[0085] The polymer carrier is a polypropylene (PP) microporous membrane with an average pore size of 0.2 μm, a thickness of 50 μm, and a porosity of 45%. Polypropylene material possesses excellent chemical stability and hydrophobicity; its microporous structure can effectively adsorb and contain NaBH4 solution through capillary action, and after drying, NaBH4 crystals are stably deposited on the pores and surface of the membrane. Furthermore, the sodium borohydride is loaded onto the polypropylene microporous membrane using a solution immersion-drying method.

[0086] Specifically, a 0.5 mol / L NaBH4 methanol / water mixed solution (volume ratio 1:1) was first prepared. The addition of methanol helps improve the wettability of the hydrophobic PP membrane and promotes solution permeation. The PP microporous membrane was completely immersed in the above NaBH4 solution and allowed to stand at 25°C for 12 hours to ensure that the solution fully permeates all pores. After loading, the membrane was removed, and residual droplets on the surface were gently blotted dry with filter paper. Then, it was immediately transferred to a vacuum oven at 40°C (vacuum degree -0.095 MPa) for 2 hours to dry completely, allowing the solvent to evaporate and the NaBH4 crystals to be stably fixed in the pores and surface of the PP membrane in the form of microcrystals.

[0087] In a preferred embodiment, the mass concentration of the reducing agent solution is 1% to 15%, the immersion time is 30 to 180 minutes, the drying temperature is 40 to 80°C, the drying time is 2 to 12 hours, and the loading of the reducing agent microcrystals is 5% to 25% of the mass of the polymer carrier.

[0088] S3. A polyamide active separation layer is formed on the surface of a polysulfone ultrafiltration membrane through an interfacial polycondensation reaction between piperazine and trimesoyl chloride.

[0089] In one embodiment, the mass concentration of the piperazine aqueous solution is 0.1% to 2.0%, the mass concentration of the trimesoyl chloride n-hexane solution is 0.05% to 0.5%, the interfacial polycondensation reaction time is 30 to 180 seconds, and the reaction temperature is 15 to 35°C.

[0090] In a preferred embodiment, the mass concentration of the piperazine aqueous solution is 1.2%, the mass concentration of the trimesoyl chloride n-hexane solution is 0.25%, the interfacial polycondensation reaction time is 120 seconds, and the reaction temperature is 30°C.

[0091] S4. The polyamide active separation layer is treated sequentially with tannic acid solution and ferric chloride solution, so that tannic acid molecules are adsorbed on the polyamide surface and further undergo coordination reaction with iron ions to form a tannic acid-iron ion complex modified layer, and the molecular weight cutoff of the tannic acid-iron ion complex modified layer is 200-1000 Da.

[0092] Furthermore, if the tannic acid concentration is too low, the surface adsorption is insufficient, making it difficult to form a continuous and complete modified layer, which affects the subsequent coordination and cross-linking effect of iron ions. If the concentration is too high, it may lead to multilayer adsorption or aggregation, increasing additional mass transfer resistance. The ferric chloride concentration needs to be matched with the tannic acid adsorption to ensure that iron ions fully participate in the coordination reaction and form a stable three-dimensional network structure. If the concentration is too low, the coordination and cross-linking will be insufficient, and the modified layer structure will be loose. If the concentration is too high, excessive free iron ions or iron precipitates may be generated, affecting the uniformity of the film surface.

[0093] In one embodiment, the mass concentration of the tannic acid solution is 0.5%~5%, the mass concentration of the ferric chloride solution is 0.2%~2%, the treatment time of the tannic acid solution is 5~30 minutes, the treatment time of the ferric chloride solution is 5~20 minutes, and the treatment temperature is 20~40℃.

[0094] In a preferred embodiment, the mass concentration of the tannic acid solution is 2.5%, the mass concentration of the ferric chloride solution is 0.5%, the treatment time for the tannic acid solution is 20 minutes, the treatment time for the ferric chloride solution is 10 minutes, and the treatment temperature for both is 35°C. At this time, tannic acid molecules can form a uniform and stable adsorption layer on the surface of the polyamide separation layer. Its abundant catechol groups undergo a highly efficient coordination reaction with the iron ions in the ferric chloride, constructing a dense and hydrophilic tannic acid-iron ion complex modified layer.

[0095] For example, prepare an aqueous solution containing 2.0 wt% piperazine (PIP) and 0.5 wt% sodium dodecyl sulfate (SDS, as a surfactant to improve the spreadability of the aqueous phase on the base film), and a hexane solution containing 2.0 wt% trimesoyl chloride (TMC).

[0096] Immerse the PSF ultrafiltration membrane in the PIP aqueous solution for 2 minutes to ensure the membrane surface is fully wetted. After removal, gently scrape off excess droplets with a rubber roller at a speed of 0.5 m / min to form a uniform thin liquid film. Immediately immerse the membrane with the PIP aqueous solution in a TMC hexane solution for 60 seconds. During this time, the PIP in the aqueous phase and the TMC in the organic phase undergo a condensation reaction at the oil-water interface, forming a cross-linked polyamide (PA) active separation layer. The reaction equation can be expressed as:

[0097]

[0098] Remove the membrane and heat-treat it in a 60°C forced-air oven for 5 minutes to complete the polymerization and remove residual solvent. Then rinse with n-hexane for 30 seconds to remove unreacted monomers, and obtain the original polyamide thin-layer composite nanofiltration membrane (PA-TFC).

[0099] Prepare an aqueous solution of 2.0 mg / mL tannic acid (TA) and an aqueous solution of 1.0 mg / mL ferric chloride (FeCl3). Immerse the PA-TFC membrane prepared above in the TA aqueous solution for 10 minutes to allow TA molecules to be adsorbed and bound to the polyamide surface through multiple hydrogen bonds and hydrophobic interactions.

[0100] Remove and gently rinse with deionized water for 10 seconds to remove loosely adsorbed TA molecules. Then immerse in FeCl3 aqueous solution for 10 minutes. Fe³⁺ ions will undergo a rapid and efficient coordination reaction with the catechol groups on the TA molecules, forming a stable TA-FeⅢ complex network, which deposits and covers the surface of the PA layer. The complexation reaction occurs almost instantaneously, forming a stable modified layer. The coordination process can be simplified as follows:

[0101] TA + Fe³⁺ → [TA−Fe]³⁺ (complex)

[0102] Rinse thoroughly with deionized water for 60 seconds to remove unfixed substances, and obtain TA-Fe modified polyamide nanofiltration membrane (TA-Fe / PA-TFC).

[0103] S5. The adsorption functional layer faces the feed side, the complex modified layer faces the permeation side, and the reduction functional layer is sandwiched between the adsorption functional layer and the complex modified layer, and is compounded by hot pressing, coating or co-extrusion.

[0104] In this embodiment, the adsorption functional layer on the feed side first physically adsorbs and retains most of the large pigment particles and some colloidal impurities. The intermediate reduction functional layer uses loaded reducing agent microcrystals to chemically reduce the pigment molecules (especially carotenoids with conjugated double bond structures) that permeate through the adsorption layer, destroying their chromophores and causing them to fade or be converted into smaller molecules that are easily retained. The complex modification layer on the permeation side further retains residual pigment molecules and reduction products through precise pore size sieving and electrostatic repulsion. Through this layer-by-layer decolorization mechanism, nanofiltration is performed directly on molten rice bran wax without the need for any organic solvents. This not only avoids the high energy consumption and solvent residue risks associated with solvent recovery, but also features a short process flow, simple operation, and conforms to the development trend of green chemistry.

[0105] On the other hand, the adsorption functional layer on the feed side acts as a "sacrificial layer," preferentially intercepting most of the pollutants that may cause membrane pore blockage, thus protecting the internal reduction functional layer and complex modification layer. At the same time, the tannic acid-iron ion complex on the surface of the complex modification layer has hydrophilic and antifouling properties, effectively reducing membrane concentration polarization and the formation of gel layers, and significantly extending the membrane's service life.

[0106] Furthermore, by modifying the polyamide active separation layer through the coordination reaction of tannic acid and iron ions, the effective pore size (200-1000 Da) was precisely controlled, which ensured that the main components of rice bran wax (such as triglycerides) could pass through smoothly while efficiently retaining pigment molecules, thus achieving the best balance between product yield and decolorization effect.

[0107] Comparative Example 1

[0108] To investigate the role of the adsorption functional layer in the entire composite membrane, this comparative example omits the adsorption functional layer described in S1, retaining only the reduction functional layer and the complex modification layer, and directly composites the two. The specific configuration is as follows:

[0109] A polypropylene (PP) microporous membrane with an average pore size of 0.2 μm, a thickness of 50 μm, and a porosity of 45% was used as a carrier. It was immersed in a 0.5 mol / L NaBH4 methanol / water mixed solution (volume ratio 1:1) for 12 hours and then vacuum dried at 40 °C for 2 hours to obtain a reducing functional layer with a reducing agent loading of approximately 18.2%.

[0110] Prepare an aqueous solution containing 2.0 wt% piperazine (PIP) and 0.5 wt% sodium dodecyl sulfate (SDS, as a surfactant to improve the spreadability of the aqueous phase on the base film), and a hexane solution containing 2.0 wt% trimesoyl chloride (TMC).

[0111] Immerse the PSF ultrafiltration membrane in the PIP aqueous solution for 2 minutes to ensure the membrane surface is fully wetted. After removal, gently scrape off excess droplets with a rubber roller at a speed of 0.5 m / min to form a uniform thin liquid film. Immediately immerse the membrane with the PIP aqueous solution in a TMC hexane solution for 60 seconds. During this time, the PIP in the aqueous phase and the TMC in the organic phase undergo a condensation reaction at the oil-water interface, forming a cross-linked polyamide (PA) active separation layer. The reaction equation can be expressed as:

[0112]

[0113] Remove the membrane and heat-treat it in a 60°C forced-air oven for 5 minutes to complete the polymerization and remove residual solvent. Then rinse with n-hexane for 30 seconds to remove unreacted monomers, and obtain the original polyamide thin-layer composite nanofiltration membrane (PA-TFC).

[0114] Prepare an aqueous solution of 2.0 mg / mL tannic acid (TA) and an aqueous solution of 1.0 mg / mL ferric chloride (FeCl3). Immerse the PA-TFC membrane prepared above in the TA aqueous solution for 10 minutes to allow TA molecules to be adsorbed and bound to the polyamide surface through multiple hydrogen bonds and hydrophobic interactions.

[0115] Remove and gently rinse with deionized water for 10 seconds to remove loosely adsorbed TA molecules. Then immerse in FeCl3 aqueous solution for 10 minutes. Fe³⁺ ions will undergo a rapid and efficient coordination reaction with the catechol groups on the TA molecules, forming a stable TA-FeⅢ complex network, which deposits and covers the surface of the PA layer. The complexation reaction occurs almost instantaneously, forming a stable modified layer. The coordination process can be simplified as follows:

[0116] TA + Fe³⁺ → [TA−Fe]³⁺ (complex)

[0117] Rinse thoroughly with deionized water for 60 seconds to remove unfixed substances, and obtain TA-Fe modified polyamide nanofiltration membrane (TA-Fe / PA-TFC).

[0118] The reducing functional layer and the complex modified layer were hot-pressed together at 120℃ and 0.5MPa for 30 seconds to obtain a bilayer composite film.

[0119] Furthermore, the test conditions were limited to molten wax at 70°C, operating pressure of 1.2 MPa, and continuous operation for 24 hours, as shown in Table 1 below.

[0120]

[0121] Table 1

[0122] As shown in Table 1 above, omitting the adsorption functional layer significantly reduces the separation performance of the composite membrane. The removal rate of polar pigments decreases from over 92% to 62.5%, and the removal rate of non-polar pigments decreases from over 88% to 58.3%. Furthermore, the stable permeation flux decreases from 108 L·m⁻²·h⁻¹ to 69.3 L·m⁻²·h⁻¹, while the molecular weight cutoff increases from 200~1000 Da to 450±30 Da, resulting in a significant decrease in separation accuracy.

[0123] Comparative Example 2

[0124] To investigate the effect of the ratio of activated clay to activated carbon in the adsorption functional layer on the performance of the composite membrane, this comparative example, while keeping the total solids content of the casting solution and other process conditions constant, adjusted the mass ratio of the two materials to set two sets of deviation ratios. For example, Comparative Example 2a: activated clay: activated carbon = 1:1 (the proportion of activated carbon is too high); Comparative Example 2b: activated clay: activated carbon = 6:1 (the proportion of activated clay is too high).

[0125] Furthermore, the preferred ratio of 3.5:1 in the above embodiments was used as a baseline control. The casting solution formulation was: 15wt% total adsorbent (mixed according to the ratio), 20wt% PVDF, 5wt% PVP, and the balance DMF. The remaining preparation steps were the same as those in Comparative Example 1 above, as shown in Table 2 below.

[0126]

[0127] Table 2

[0128] According to Table 2 above, when using Comparative Example 2a (activated clay: activated carbon = 1:1, a higher proportion of activated carbon), the increased proportion of activated carbon led to enhanced hydrophobicity of the membrane, increased surface contact angle, and increased resistance to molten wax permeation. The initial flux decreased by approximately 21% compared to the example. Furthermore, due to the insufficient proportion of activated clay, the adsorption capacity for polar pigments was further weakened, and the removal rate of polar pigments decreased to 78.5%. During operation, the hydrophobic membrane surface more readily adsorbed hydrophobic components from the wax, and both the flux decay rate and the decolorization rate decay rate were higher than those of the example. When using Comparative Example 2b (activated clay: activated carbon = 6:1, a higher proportion of activated clay), the excessively high proportion of activated clay resulted in insufficient adsorption capacity for non-polar pigments, and the removal rate of non-polar pigments decreased to 72.5%. The removal rate of polar pigments was basically the same as that of the example, but the porosity and pore size increased slightly.

[0129] In summary, the effect of adsorbent ratio on membrane performance exhibits a significant non-linear characteristic. When the proportion of activated carbon is high (e.g., Comparative Example 2a, ratio 1:1), the hydrophobicity of the membrane layer is enhanced, and the surface contact angle increases. On the one hand, this increases the permeation resistance of molten wax, leading to a significant decrease in the initial permeate flux. On the other hand, the hydrophobic membrane surface easily adsorbs hydrophobic components from the wax, accelerating membrane fouling, resulting in higher flux decay rates and decolorization rate decay rates than in the examples. Simultaneously, insufficient activated clay ratio weakens the adsorption capacity for polar pigments, leading to a decrease in the removal rate of polar pigments. When the proportion of activated clay is high (e.g., Comparative Example 2b, ratio 6:1), although the removal effect on polar pigments is basically equivalent to that in the examples, and the porosity and pore size are slightly increased, the insufficient activated carbon content significantly reduces the adsorption capacity for non-polar pigments, resulting in a substantial decrease in the removal rate of non-polar pigments, making it difficult to achieve simultaneous and efficient removal of complex pigment components.

[0130] Compared with the preferred ratio (3.5:1) used in the example, the optimal match was achieved in terms of hydrophilicity-hydrophobicity balance, pore size structure and adsorption synergy, which not only ensured high-throughput operation, but also achieved efficient synergistic removal of polar and non-polar pigments.

[0131] Comparative Example 3

[0132] This comparative example prepared reducing functional layers with different loadings by changing the combination of NaBH4 solution concentration and impregnation time. For example, Comparative Example 3a (low loading): 0.1 mol / L NaBH4 solution, impregnation for 2 h, loading of approximately 4.5%; Comparative Example 3b (medium loading): 0.3 mol / L NaBH4 solution, impregnation for 6 h, loading of approximately 12.8%.

[0133] Furthermore, the preferred loading of 0.5 mol / L NaBH4 solution from the above embodiments was used for immersion for 12 hours, with a loading of approximately 18.5% serving as a baseline control. All external conditions were kept consistent, such as using a polypropylene (PP) microporous membrane with an average pore size of 0.2 μm, a thickness of 50 μm, and a porosity of 45% as the carrier, and drying conditions of 40°C for 2 hours, as shown in Table 3 below.

[0134]

[0135] Table 3

[0136] According to Table 3 above, in Comparative Example 3a, the initial stubborn pigment removal rate was only 67.3%, which did not reach an effective decolorization level. Furthermore, after 24 hours of operation, the reducing agent loss rate was as high as 62.3%, and the decolorization rate decreased to less than 60%. In Comparative Example 3b, the initial stubborn pigment removal rate increased to 81.5%, meeting the basic decolorization requirements. However, the reducing agent loss rate after 24 hours was still 38.5%, and the decolorization rate decreased to approximately 73%. In contrast, the preferred loading in the examples resulted in an initial stubborn pigment removal rate ≥85%, and the loss rate decreased to 24.6% after 24 hours of operation.

[0137] In summary, the loading of the reducing functional layer has a decisive impact on the overall decolorization performance and operational stability of the microporous membrane. When the loading is too low (e.g., Comparative Example 3a), although the initial permeation flux is high, there are insufficient effective reduction sites, making it difficult to achieve efficient removal of stubborn pigments. Furthermore, the reducing agent is rapidly lost during operation, leading to a rapid decline in decolorization performance. When the loading is increased to a moderate level (e.g., Comparative Example 3b), the decolorization effect is improved, but due to the still unsatisfactory binding strength between the reducing agent and the carrier, it continues to be lost during operation, limiting long-term stability.

[0138] Compared to the preferred loading amount used in the example (immersion in 0.5 mol / L NaBH4 solution for 12 h, loading amount of about 18.5%), while ensuring a high initial decolorization rate, it significantly enhances the binding stability between the reducing functional layer and the carrier, effectively inhibits the loss of reducing agent, and enables the membrane to maintain excellent decolorization performance and flux stability after continuous operation for 24 h.

[0139] In one embodiment, the heating and melting temperature is 70-95°C, and the melting time is 30-120 minutes.

[0140] In a preferred embodiment, the decolorization process employs a cross-flow filtration method, with a cross-flow velocity of 0.5-2.0 m / s, a pressure of 0.5-1.5 MPa, and an operating temperature of 70-90°C.

[0141] In a preferred embodiment, the pigment molecules include at least one of carotenoids, flavonoids, and chlorophyll residues.

[0142] Furthermore, the adsorption functional layer has a physical adsorption retention rate of 40%-70% for pigment molecules, the reduction functional layer has a chemical reduction decolorization rate of 20%-40% for pigment molecules, and the complex modified layer has a molecular sieving retention rate of 60%-85% for pigment molecules.

[0143] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a rice bran wax film refining process that includes a series of elements, such as a process, apparatus, article, or solvent-circulating auxiliary process, includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, apparatus, article, or solvent-circulating auxiliary process. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of additional identical elements in the rice bran wax film refining process that includes the element.

[0144] The above description is only a preferred embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural changes made based on the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A solvent circulation assisted rice bran wax membrane process refining process, characterized by, Includes the following steps; S1. The crude rice bran wax is heated to 80-90℃ in a melting tank using a steam coil to melt it, thus obtaining molten rice bran wax; S2. The molten rice bran wax is transported to a mixing tank, and the first part of ethyl acetate is added while maintaining the temperature at 80~90℃. The mass ratio of the first part of ethyl acetate to the molten rice bran wax is 0.8:1~1.2:

1. The mixture is stirred until the rice bran wax is completely dissolved and the liquid phase is clear and transparent. The resulting mixture is filtered through a bag filter to remove large particulate impurities. S3. The mixture filtered in S2 is sent to a decolorization tank, and a second portion of ethyl acetate is added to make the mass ratio of the total amount of ethyl acetate in the system to the initial rice bran wax 2.5:1~3.5:

1. After dilution, the mixture enters the first membrane separation unit for cross-flow decolorization treatment, with an operating pressure of 0.5~2.0 MPa and a temperature of 70~90℃. The first membrane separation unit is filled with a first solvent-resistant composite nanofiltration membrane, which is composed of an adsorption functional layer, a reduction functional layer and a complex modification layer in sequence. The adsorption functional layer is used as the feed contact surface to selectively adsorb pigment molecules and allow rice bran wax and ethyl acetate to pass through. S4. The decolorized clear liquid obtained in S3 is transferred to a concentration tank, heated to 50~60℃, and then enters the second membrane separation unit for cross-flow concentration treatment. The operating pressure is 1.5~4.0 MPa and the temperature is 50~60℃. The second membrane separation unit is filled with a second solvent-resistant composite nanofiltration membrane. Both the second solvent-resistant composite nanofiltration membrane and the first solvent-resistant composite nanofiltration membrane are composed of an adsorption functional layer, a reduction functional layer and a complex modification layer. The two membranes are arranged in mirror images with the active layers facing completely opposite directions in their respective membrane modules. S5. The concentrate from S4 is transferred to a plate filter tank, cooled to 25~35℃, and then separated by a plate filter. The resulting filtrate is ethyl acetate, which is transferred to an ethyl acetate circulation tank. The resulting filter cake is a mixture of ethyl acetate and rice bran wax, wherein the mass ratio of ethyl acetate to rice bran wax is 0.8:1~1.2:

1. S6. Heat the ethyl acetate-rice bran wax mixture from S5 to 100-110°C in a thin-film evaporator to completely vaporize the ethyl acetate and condense it back to the ethyl acetate circulation tank to obtain refined rice bran wax melt. S7. The refined rice bran wax melt is granulated to obtain rice bran wax particles, wherein the first part of ethyl acetate, the second part of ethyl acetate, and the ethyl acetate recovered in S4, S5, and S6 are all sent to the ethyl acetate circulation tank.

2. The solvent circulation-assisted rice bran wax membrane process according to claim 1, characterized by, The preferred mass ratio of the first ethyl acetate to the molten rice bran wax is 0.9:1, and the preferred mass ratio of the total amount of ethyl acetate to the initial rice bran wax in the system after adding the second ethyl acetate is 2.8:

1.

3. The solvent circulation-assisted rice bran wax membrane process refining process according to claim 1, characterized by, The first membrane separator and the second membrane separator adopt a cross-flow filtration method, specifically: in the first membrane separator, the adsorption functional layer is used as the feed contact surface, the cross-flow velocity is 0.8~1.5 m / s, and the transmembrane pressure difference is 0.8~1.2 MPa; in the second membrane separator, the complex modified layer is used as the feed contact surface, and the cross-flow velocity is set to 2.0~3.5 m / s and the transmembrane pressure difference is set to 2.5~3.5 MPa. Furthermore, the ratio of the transmembrane pressure difference between the first membrane separator and the second membrane separator is negatively correlated with the ratio of their molecular weight cutoffs, i.e., the ratio of the transmembrane pressure difference is 1.5~4.0:0.5~2.0, while the ratio of the molecular weight cutoffs is 200~500:500~1000.

4. The solvent circulation-assisted rice bran wax membrane process according to claim 3, characterized by, The ratio of the transmembrane pressure difference between the first membrane separator and the second membrane separator is preferably 2.8:1 to 3.2:1, and the ratio of the molecular weight cutoff is preferably 270 to 420 to 580 to 800.

5. The solvent circulation-assisted rice bran wax membrane process according to claim 1, characterized by, The specific process of the cross-flow decolorization treatment is as follows: After adding a second part of ethyl acetate to the mixture filtered by S2, it is driven into the first membrane separation unit by a circulating pump. The feed side pressure is 0.5~2.0 MPa, and the permeate side is at atmospheric pressure. The permeate is the decolorized clear liquid and is continuously discharged and transported to the concentration tank of S4. The concentrate is returned to the decolorization tank for recycling. The recycling decolorization time is controlled at 30~60 min until the Gardner color number of the permeate is ≤3 and then the circulation is stopped. The remaining concentrate is discharged and returned to the S2 mixing tank for redissolution or treated as waste residue.

6. The solvent circulation-assisted rice bran wax membrane process according to claim 5, characterized by, The specific process of the cross-flow concentration treatment is as follows: the decolorized clear liquid obtained from S3 is pumped into the concentration tank and heated to 50~60℃, and then driven by the circulation pump into the second membrane separation unit. The feed side pressure is 1.5~4.0 MPa, and the permeate side is at atmospheric pressure. The concentrate is returned to the concentration tank, and the permeate (ethyl acetate clear liquid) is continuously discharged and transported to the ethyl acetate circulation tank. When the solid content of the liquid in the concentration tank reaches 30~40wt%, the concentration is stopped, and the concentrate is discharged into the S5 plate filter tank.

7. A method for preparing a solvent-resistant composite nanofiltration membrane for use in any one of claims 1-6, wherein the nanofiltration membrane is sequentially stacked with an adsorption functional layer prepared by a phase inversion method, a reduction functional layer prepared by a solution impregnation-drying method, and a complex modification layer prepared by an interfacial polymerization method combined with a coordination modification method, characterized in that, It also includes the following steps; S1. The adsorbent, polymer binder and pore-forming agent are mixed to prepare a casting solution, which is then immersed in a coagulation bath for curing after molding. S2. The polymer carrier is immersed in the reducing agent solution, and after the solution has fully penetrated into the internal pores of the carrier, it is dried so that the reducing agent is loaded in the form of microcrystals on the inner wall and surface of the polymer carrier pores. S3. A polyamide active separation layer is formed on the surface of a polysulfone ultrafiltration membrane by an interfacial polycondensation reaction between piperazine and pyromellitic trimethylol chloride. S4. The polyamide active separation layer is treated sequentially with tannic acid solution and ferric chloride solution, so that tannic acid molecules are adsorbed on the surface of polyamide and further undergo coordination reaction with iron ions to form a tannic acid-iron ion complex modified layer, and the molecular weight cutoff of the tannic acid-iron ion complex modified layer is 200-1000 Da. S5. The adsorption functional layer faces the feed side, the complex modified layer faces the permeation side, and the reduction functional layer is sandwiched between the adsorption functional layer and the complex modified layer, and is compounded by hot pressing, coating or co-extrusion.

8. The method for preparing the solvent-resistant composite nanofiltration membrane according to claim 7, characterized in that, In step S1, the mass percentage of each component in the casting solution is as follows: adsorbent 5%~20%, polymer binder 10%~25%, pore-forming agent 1%~8%, and the balance is a polar solvent, namely N,N-dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidone.

9. The method of claim 7, wherein the solvent resistant composite nanofiltration membrane is prepared by the steps of: In step S1, the coagulation bath is deionized water or an ethanol-water mixture, the coagulation bath temperature is 10~40℃, and the phase transformation curing time is 1~10 minutes.

10. The method of claim 7, wherein the solvent resistant composite nanofiltration membrane is prepared by the steps of: The adsorbent is at least one or any combination of activated clay, powdered activated carbon, diatomaceous earth or zeolite molecular sieve; the polymer binder is at least one or any combination of polyethersulfone, polyvinylidene fluoride or polyacrylonitrile; the porogen is at least one or any combination of polyvinylpyrrolidone, polyethylene glycol or lithium chloride. The mass ratio of the activated clay to the activated carbon is 3:1 to 4:1, preferably 3.5:1; the mass ratio of the polyethersulfone to the polyvinylidene fluoride is 1:1 to 3:1, preferably 2:1; the mass ratio of the polyethylene glycol to the lithium chloride is 2:1 to 8:1, preferably 3.8:1.

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