Tobacco extract cascade separation method based on cascade nanofiltration membrane combination process and application

Abstract

The invention discloses a tobacco extract cascade separation method based on a cascade nanofiltration membrane combination process and application, and belongs to the technical field of membrane separation, and the tobacco extract cascade separation method comprises the following steps: filtering a tobacco extracting solution by using multistage nanofiltration membranes in a combined manner to realize cascade separation of a tobacco extract; in the filtering process, penetrating fluid separated by the previous-stage nanofiltration membrane is used as feed liquid of the next-stage nanofiltration membrane; the nanofiltration membrane is a polyamide nanofiltration membrane, and the tobacco extract comprises at least two of an acidic substance, an alkaline substance or a neutral substance; before each stage of filtration, the charge state of the surface of a nanofiltration membrane and protonation and deprotonation behaviors of tobacco extract molecules are regulated and controlled by adjusting the pH value of a solution to be filtered, the tobacco extract with the same charge as the surface of the nanofiltration membrane is intercepted under the corresponding pH value, and the rest of the tobacco extract permeates through the nanofiltration membrane, so that the tobacco extract with the same charge as the surface of the nanofiltration membrane is obtained. And adjusting the pH value of the filtered penetrating fluid again, and filtering the penetrating fluid by a next-stage nanofiltration membrane until the target separation degree is reached.

Description

Technical Field

[0001] The invention relates to the technical field of membrane separation, and in particular to a tobacco extract stepwise separation method and application based on a cascade nanofiltration membrane combination process. Background Art

[0002] Tobacco extract is an important product of the tobacco industry, which contains rich bioactive ingredients such as alkaloids, organic acids and polyphenols. Tobacco extract not only gives tobacco a unique flavor and taste, but also shows extensive and far-reaching application potential in fine chemicals, biomedicine, agricultural environmental protection and other aspects.

[0003] At present, the preparation method of tobacco extract includes extracting active ingredients from tobacco leaves with an extracting solution, and further separating and purifying these extracts by extraction, distillation, chromatography, adsorption and other methods. The Chinese patent document with publication number CN101570717A discloses a preparation method of tobacco extract and its application, which uses enzymatic hydrolysis technology, ultrasonic extraction and combined with chromatography separation to obtain tobacco extracts whose main components are damascone, dihydrodamascone, solanone, geranyl acetone, acacia acetone, megastigmatrienone, neophytadiene, etc. The Chinese patent document with publication number CN113508919A discloses a tobacco extract and its preparation method and application, which extracts nicotine and flavor substances by combining extraction, reaction of tobacco extract with alcohol solvent, organic solvent extraction, distillation and purification. However, due to the complex composition of tobacco extract and the wide distribution of molecular weight (mostly between 100 and 500 Da), these traditional methods show the problems of high energy consumption and low separation efficiency, and it is difficult to achieve high-value utilization of tobacco crops.

[0004] Compared with separation technologies such as extraction and distillation, nanofiltration has the advantages of low process energy consumption, high separation accuracy, and no solvent addition. Most nanofiltration membranes are thin-layer composite membranes prepared by interfacial polymerization of water-phase monomers and oil-phase monomers on a porous support layer. The separation layer formed by interfacial polymerization has nanoscale pore size and charge characteristics, which helps to achieve selective separation of molecules. In addition, the nanofiltration process will not undergo phase changes in traditional separation methods, and there will be no quality loss or purity reduction caused by physical changes, which can ensure the integrity and stability of the bioactive components in tobacco extracts.

[0005] However, multiple components in tobacco extracts have similar properties, such as similar molecular weights of acidic, neutral, and alkaline active ingredients, which limits the application of nanofiltration technology in tobacco extract separation. This is due to the fast reaction rate of the interfacial polymerization process and the uneven distribution of aqueous monomers on the interface, resulting in the formed polyamide nanofiltration membrane having a wide pore size distribution. Therefore, it is difficult for existing nanofiltration membranes to directly and accurately screen molecules of similar sizes, especially when the compound size is smaller than the pore size, it is difficult to achieve separation.

[0006] The electrostatic interaction between molecules and membrane surface charges provides another selectivity mechanism. The charge of a compound usually comes from a dissociable or protonable group, such as a carboxyl group, an amine group, a pyridyl group, etc. The charge on the membrane surface comes from unreacted residual amine groups or acyl chloride groups. When a charged molecule approaches a membrane surface with the same charge, the transmembrane energy barrier of the molecule increases significantly, and is thus repelled by the membrane. By manipulating this interaction, the selectivity of the membrane for charged molecules can be significantly improved, and it is even possible to achieve selective separation of molecules in a mixture when the molecular size is smaller than the membrane pore size. Based on this, the present invention provides a stepwise separation method for tobacco extracts based on a cascade nanofiltration membrane combination process. Summary of the invention

[0007] In order to address the deficiencies in the prior art, the present invention provides a method for the step-by-step separation of tobacco extracts based on a cascade nanofiltration membrane combination process. This method can achieve the step-by-step separation of all electrically charged molecules in the tobacco extract by regulating the charge state of the nanofiltration membrane surface and the protonation and deprotonation behavior of the tobacco extract molecules, thereby helping to promote the application of nanofiltration technology in the separation of tobacco extracts.

[0008] The specific technical solutions adopted are as follows:

[0009] A stepwise separation method for tobacco extract based on a cascade nanofiltration membrane combination process, comprising: using multiple nanofiltration membranes to filter tobacco extract to achieve stepwise separation of tobacco extract; during the filtration process, the permeate separated by the previous nanofiltration membrane is used as the feed liquid of the next nanofiltration membrane;

[0010] The nanofiltration membrane is a polyamide nanofiltration membrane, and the tobacco extract includes at least two of acidic substances, alkaline substances or neutral substances;

[0011] Before each stage of filtration, the charge state of the nanofiltration membrane surface and the protonation and deprotonation behavior of the tobacco extract molecules are regulated by adjusting the pH value of the solution to be filtered. At the corresponding pH value, the tobacco extract with the same charge as the nanofiltration membrane surface is retained, and the remaining tobacco extract permeates through the nanofiltration membrane. The pH value of the permeate obtained by filtration is re-adjusted and the next nanofiltration membrane filtration step is performed until the target separation degree is achieved.

[0012] The present invention is based on the relationship between the pH value of the solution to be filtered and the ionization state of the groups on the surface of the nanofiltration membrane: when the pH value of the solution to be filtered is higher than the isoelectric point of the nanofiltration membrane, the surface of the nanofiltration membrane is negatively charged, and when the pH value of the solution to be filtered is lower than the isoelectric point of the nanofiltration membrane, the surface of the nanofiltration membrane is positively charged; and based on the relationship between the pH value of the solution to be filtered and the molecular dissociation constant pKa of the tobacco extract: for acidic tobacco extract molecules, when the pH value of the solution to be filtered is higher than its molecular dissociation constant pKa, the molecules will be deprotonated and negatively charged, and for alkaline tobacco extract molecules, when the pH value of the solution to be filtered is lower than its molecular dissociation constant pKa, the molecules will be protonated and positively charged; by changing the pH value of the solution to be filtered, the electrical properties of the charges carried by the nanofiltration membrane surface and the tobacco extract molecules are regulated at the same time, under electrostatic interaction, the tobacco extract and the nanofiltration membrane surface are retained when they carry the same charge, and the rest of the tobacco extract is permeated, and the desired separation effect is achieved by combining multiple nanofiltration membranes.

[0013] Preferably, the nanofiltration membrane is a polyamide nanofiltration membrane prepared by interfacial polymerization, with an average effective pore size of 0.2-1.0 nm and containing unreacted amino groups or acyl chloride groups on the surface.

[0014] Preferably, the molecular weight of the tobacco extract is 100-500 Da.

[0015] Specifically, the acidic substance includes phthalic acid, benzoic acid, malic acid, proline or ethyl vanillin, the alkaline substance includes nicotine, and the neutral substance includes megastigmatrienone, 2-phenylethanol or neophytadiene.

[0016] Specifically, when the tobacco extract in the tobacco extract includes acidic substances, alkaline substances and neutral substances, two-stage nanofiltration membranes are used in combination to achieve the separation of acidic substances, alkaline substances and neutral substances in the tobacco extract.

[0017] Specifically, when the tobacco extract in the tobacco extract liquid includes x acidic substances, y alkaline substances and z neutral substances, x+y level nanofiltration membranes are used in combination to achieve separate separation and extraction of each acidic substance and each alkaline substance.

[0018] Specifically, the pH value of the solution to be filtered is adjusted in the range of 3-12.

[0019] Preferably, before each filtration, the adjustment range of the pH value of the solution to be filtered is jointly determined by the isoelectric point of the nanofiltration membrane and the pKa of the target retained tobacco extract in the solution to be filtered; when the target retained tobacco extract is an acidic substance, the pH value of the solution to be filtered is adjusted to be greater than the isoelectric point of the nanofiltration membrane and the pH value of the solution to be filtered is greater than the pKa of the target retained tobacco extract, and the target acidic substance is retained; when the target retained tobacco extract is an alkaline substance, the pH value of the solution to be filtered is adjusted to be less than the isoelectric point of the nanofiltration membrane and the pH value of the solution to be filtered is less than the pKa of the target retained tobacco extract, and the target alkaline substance is retained.

[0020] When the tobacco extract contains phthalic acid, benzoic acid, ethyl vanillin, nicotine and 2-phenylethanol, the pH value of the solution to be filtered is first adjusted to 3.0-4.0, and the first-stage nanofiltration membrane filtration separation is performed to obtain nicotine and a permeate solution 1; the pH value of the permeate solution 1 is then adjusted to 5.0-6.0, and the second-stage nanofiltration membrane filtration separation is performed to obtain phthalic acid and a permeate solution 2; the pH value of the permeate solution 2 is then adjusted to 7.0-9.0, and the third-stage nanofiltration membrane filtration separation is performed to obtain benzoic acid and a permeate solution 3; the pH value of the permeate solution 3 is then adjusted to 10.0-12.0, and the fourth-stage nanofiltration membrane filtration separation is performed to obtain ethyl vanillin and 2-phenylethanol.

[0021] In the cascade nanofiltration membrane combination process of the present invention, each stage of the nanofiltration membrane undertakes a specific separation task. By finely regulating the pH value of the solution to be filtered at each stage, the separation of specific tobacco extracts under specific pH conditions is achieved in turn. Through the step-by-step nanofiltration steps, the method of the present invention can achieve effective step-by-step separation of all acidic tobacco extracts or alkaline tobacco extracts. This separation method not only improves the separation efficiency, but also improves the accuracy and purity of the separation.

[0022] The present invention also provides the application of the tobacco extract step separation method based on the cascade nanofiltration membrane combination process in the fields of electronic cigarettes, high-value utilization of waste tobacco, etc.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The process of the present invention is simple and highly controllable. By adjusting the pH value of the solution to be filtered, the charge properties of the nanofiltration membrane and the tobacco extract molecules can be flexibly changed to achieve step-by-step separation of the tobacco extract. The process is easy to implement, has low equipment requirements, is highly universal, and has many advantages such as energy saving, environmental protection, and cost-effectiveness.

[0025] (2) The present invention utilizes the relationship between the pH value of the solution to be filtered and the isoelectric point of the nanofiltration membrane, and between the pH value of the solution to be filtered and the pKa of the tobacco extract molecules, to regulate the surface charge of the nanofiltration membrane and the protonation and deprotonation behavior of the tobacco extract molecules under different pH conditions, and achieves the step-by-step separation of molecules with different charges through electrostatic interactions, making the separation more effective and the purity of the separated tobacco extract higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the separation principle of the stepwise separation method of tobacco extract based on the cascade nanofiltration membrane combination process of the present invention;

[0027] Figure 2 This is a process flow chart for the stepwise separation of tobacco extracts using the cascade nanofiltration membrane combination process used in Example 1 of the present invention. DETAILED DESCRIPTION

[0028] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the following is a detailed description through specific embodiments. In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0029] The operating methods in the following examples without specifying specific conditions are usually carried out under conventional conditions or according to the conditions recommended by the manufacturer. The contents not described in detail in this specification belong to the prior art known to professionals in the field. The experimental materials used in the following examples, unless otherwise specified, can be purchased from conventional biochemical reagent companies.

[0030] In the following embodiments, the separation principle diagram of the stepwise separation method of tobacco extract based on the cascade nanofiltration membrane combination process is as follows: Figure 1 As shown; the tobacco extract is provided by Yunnan Tobacco Research Institute, and the tobacco extract contains acidic substances (phthalic acid, benzoic acid and ethyl vanillin), alkaline substances (nicotine) and neutral substances (2-phenylethanol); the nanofiltration membrane can be prepared by conventional interfacial polymerization method, and the preparation method can be selected from:

[0031] (1) pouring the prepared piperazine aqueous solution (piperazine mass concentration of 0.05-0.5wt%, additive Na3PO4 mass concentration of 0.2-0.8wt%) onto the surface of the fixed polysulfone porous support membrane, soaking for 1-10 minutes, pouring out the aqueous solution and removing the residual solution on the surface of the porous support membrane;

[0032] (2) pouring the prepared trimesoyl chloride oil phase solution (the mass concentration of trimesoyl chloride is 0.05-0.5wt%) onto the surface of the porous support membrane obtained in step (1) to carry out interfacial polymerization reaction for 1-10 minutes, then pouring out the oil phase solution and immediately placing it in an oven for heat treatment at a temperature of 50-100°C for a holding time of 1-15 minutes to obtain a polyamide nanofiltration membrane; the corresponding nanofiltration membrane has an average effective pore size of 0.2-1.0 nm, and the surface contains unreacted amino groups or acyl chloride groups.

[0033] Example 1

[0034] The tobacco extract contains acidic substances (phthalic acid, benzoic acid and ethyl vanillin), alkaline substances (nicotine) and neutral substances (2-phenylethanol). The process flow chart of the cascade nanofiltration membrane combination process for the stepwise separation of tobacco extracts used in this embodiment is as follows: Figure 2 shown.

[0035] The pH value of the tobacco extract is adjusted to 3.2, so that the corresponding pH value is less than the isoelectric point of the nanofiltration membrane and the corresponding pH value is less than the pKa of nicotine, and after filtering and separating through the first-stage nanofiltration membrane, a retentate solution 1 (nicotine) and a permeate solution 1 (phthalic acid, benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; the purity of nicotine in the retentate solution 1 is 80%;

[0036] The pH value of the permeate 1 is adjusted to 5.6, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of phthalic acid. After filtering and separating through the second-stage nanofiltration membrane, the retentate 2 (phthalic acid) and the permeate 2 (benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; the purity of phthalic acid in the retentate 2 is 78%;

[0037] The pH value of the permeate 2 is adjusted to 8.3, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of benzoic acid. After filtering and separating through the third-stage nanofiltration membrane, the retentate 3 (benzoic acid) and the permeate 3 (ethyl vanillin, 2-phenylethanol) are obtained; the purity of benzoic acid in the retentate 3 is 72%;

[0038] The pH value of the permeate three is adjusted to 11.5, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of ethyl vanillin. After filtration and separation through the fourth-stage nanofiltration membrane, retentate four (ethyl vanillin) and permeate four (2-phenylethanol) are obtained; the purity of ethyl vanillin in the retentate four is 75%, and the purity of 2-phenylethanol in the permeate four is 90%.

[0039] Example 2

[0040] Tobacco extract contains acidic substances (phthalic acid, benzoic acid and ethyl vanillin), alkaline substances (nicotine) and neutral substances (2-phenylethanol);

[0041] The pH value of the tobacco extract is adjusted to 11.5, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of the acidic substance, and after filtering and separating through the first-stage nanofiltration membrane, a retentate I (phthalic acid, benzoic acid, ethyl vanillin) and a permeate I (nicotine, 2-phenylethanol) are obtained; in the retentate I, the purity of phthalic acid is 35%, the purity of benzoic acid is 32%, and the purity of ethyl vanillin is 33%;

[0042] The pH value of the tobacco extract is adjusted to 3.3, so that the corresponding pH value is less than the isoelectric point of the nanofiltration membrane and the corresponding pH value is less than the pKa of nicotine. After filtering and separating through the second-stage nanofiltration membrane, a retentate second (nicotine) and a permeate second (2-phenylethanol) are obtained; the purity of nicotine in the retentate second is 79%, and the purity of 2-phenylethanol in the permeate second is 88%.

[0043] Example 3

[0044] Tobacco extract contains acidic substances (phthalic acid, benzoic acid and ethyl vanillin), alkaline substances (nicotine) and neutral substances (2-phenylethanol);

[0045] The pH value of the tobacco extract is adjusted to 4.0, so that the corresponding pH value is less than the isoelectric point of the nanofiltration membrane and the corresponding pH value is less than the pKa of nicotine, and after filtering and separating through the first-stage nanofiltration membrane, a retentate solution 1 (nicotine) and a permeate solution 1 (phthalic acid, benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; the purity of nicotine in the retentate solution 1 is 70%;

[0046] The pH value of the permeate 1 is adjusted to 6.0, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of phthalic acid. After filtering and separating through the second-stage nanofiltration membrane, the retentate 2 (phthalic acid) and the permeate 2 (benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; in the retentate 2, the purity of phthalic acid is 72%;

[0047] The pH value of the permeate 2 is adjusted to 8.5, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of benzoic acid. After filtering and separating through the third-stage nanofiltration membrane, the retentate 3 (benzoic acid) and the permeate 3 (ethyl vanillin, 2-phenylethanol) are obtained; in the retentate 3, the purity of benzoic acid is 68%;

[0048] The pH value of the permeate three is adjusted to 10.5, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of ethyl vanillin. After filtration and separation through the fourth-stage nanofiltration membrane, retentate four (ethyl vanillin) and permeate four (2-phenylethanol) are obtained; the purity of ethyl vanillin in the retentate four is 73%, and the purity of 2-phenylethanol in the permeate four is 85%.

[0049] Example 4

[0050] Tobacco extract contains acidic substances (phthalic acid, benzoic acid and ethyl vanillin), alkaline substances (nicotine) and neutral substances (2-phenylethanol);

[0051] The pH value of the tobacco extract is adjusted to 3.0, so that the corresponding pH value is less than the isoelectric point of the nanofiltration membrane and the corresponding pH value is less than the pKa of nicotine, and after filtering and separating through the first-stage nanofiltration membrane, a retentate solution 1 (nicotine) and a permeate solution 1 (phthalic acid, benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; the purity of nicotine in the retentate solution 1 is 86%;

[0052] The pH value of the permeate 1 is adjusted to 11.6, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of the acidic substance. After filtering and separation through the second-stage nanofiltration membrane, a retentate 2 (phthalic acid, benzoic acid, ethyl vanillin) and a permeate 2 (2-phenylethanol) are obtained; in the retentate 2, the purity of phthalic acid is 36%, the purity of benzoic acid is 30%, the purity of ethyl vanillin is 35%, and in the permeate 2, the purity of 2-phenylethanol is 88%.

[0053] Example 5

[0054] Tobacco extract contains acidic substances (phthalic acid, benzoic acid and ethyl vanillin), alkaline substances (nicotine) and neutral substances (2-phenylethanol);

[0055] The pH value of the tobacco extract is adjusted to 3.3, so that the corresponding pH value is less than the isoelectric point of the nanofiltration membrane and the corresponding pH value is less than the pKa of nicotine, and after filtering and separating through the first-stage nanofiltration membrane, a retentate solution 1 (nicotine) and a permeate solution 1 (phthalic acid, benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; the purity of nicotine in the retentate solution 1 is 78%;

[0056] The pH value of the permeate 1 is adjusted to 4.0, and after the second-stage nanofiltration membrane filtration separation, the tobacco extract in the permeate 1 cannot be retained, and the permeate 2 (phthalic acid, benzoic acid, ethyl vanillin, 2-phenylethanol) is obtained;

[0057] The pH value of the permeate 2 is adjusted to 7.5, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of phthalic acid and benzoic acid, and after filtering and separating through the third-stage nanofiltration membrane, the retentate 3 (phthalic acid, benzoic acid) and the permeate 3 (ethyl vanillin, 2-phenylethanol) are obtained; in the retentate 3, the purity of phthalic acid is 58%, and the purity of benzoic acid is 42%;

[0058] The pH value of the permeate three is adjusted to 10.0, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of ethyl vanillin. After filtration and separation through the fourth-stage nanofiltration membrane, retentate four (ethyl vanillin) and permeate four (2-phenylethanol) are obtained; the purity of ethyl vanillin in the retentate four is 70%, and the purity of 2-phenylethanol in the permeate four is 84%.

[0059] Example 6

[0060] Tobacco extract contains acidic substances (phthalic acid, benzoic acid and ethyl vanillin), alkaline substances (nicotine) and neutral substances (2-phenylethanol);

[0061] The pH value of the tobacco extract is adjusted to 3.5, so that the corresponding pH value is less than the isoelectric point of the nanofiltration membrane and the corresponding pH value is less than the pKa of nicotine, and after filtering and separating through the first-stage nanofiltration membrane, a retentate solution 1 (nicotine) and a permeate solution 1 (phthalic acid, benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; the purity of nicotine in the retentate solution 1 is 75%;

[0062] The pH value of the permeate 1 is adjusted to 8.0, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of phthalic acid and benzoic acid, and after filtering and separating through the second-stage nanofiltration membrane, a retentate 2 (phthalic acid, benzoic acid) and a permeate 2 (ethyl vanillin, 2-phenylethanol) are obtained; in the retentate 2, the purity of phthalic acid is 57%, and the purity of benzoic acid is 43%;

[0063] The pH value of the permeate II was adjusted to 10.0, so that the corresponding pH value was greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value was greater than the pKa of ethyl vanillin. After separation through the third-stage nanofiltration membrane, retentate III (ethyl vanillin) and permeate III (2-phenylethanol) were obtained. The purity of ethyl vanillin in the retentate III was 70%, and the purity of 2-phenylethanol in the permeate III was 78%.

[0064] Example 7

[0065] Tobacco extract contains acidic substances (phthalic acid, benzoic acid and ethyl vanillin), alkaline substances (nicotine) and neutral substances (2-phenylethanol);

[0066] The pH value of the tobacco extract is adjusted to 3.5, so that the corresponding pH value is less than the isoelectric point of the nanofiltration membrane and the corresponding pH value is less than the pKa of nicotine, and after filtering and separating through the first-stage nanofiltration membrane, a retentate solution 1 (nicotine) and a permeate solution 1 (phthalic acid, benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; the purity of nicotine in the retentate solution 1 is 78%;

[0067] The pH value of the permeate 1 is adjusted to 5.5, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of phthalic acid. After filtering and separating through the second-stage nanofiltration membrane, the retentate 2 (phthalic acid) and the permeate 2 (benzoic acid, ethyl vanillin, 2-phenylethanol) are obtained; in the retentate 2, the purity of phthalic acid is 80%;

[0068] The pH value of the permeate 2 is adjusted to 10.0, so that the corresponding pH value is greater than the isoelectric point of the nanofiltration membrane and the corresponding pH value is greater than the pKa of benzoic acid and ethyl vanillin. After filtering and separation through the third-stage nanofiltration membrane, retentate 3 (benzoic acid, ethyl vanillin) and permeate 3 (2-phenylethanol) are obtained; in the retentate 3, the purity of benzoic acid is 52%, the purity of ethyl vanillin is 48%, and in the permeate 3, the purity of 2-phenylethanol is 84%.

[0069] Comparative Example 1

[0070] Without adjusting the pH value of the solution, the tobacco extract is filtered and separated through a first-stage nanofiltration membrane to obtain a permeate 1 (phthalic acid, benzoic acid, ethyl vanillin, nicotine, and 2-phenylethanol); the permeate 1 is filtered and separated through a second-stage nanofiltration membrane to obtain a permeate 2 (phthalic acid, benzoic acid, ethyl vanillin, nicotine, and 2-phenylethanol); the permeate 2 is filtered and separated through a third-stage nanofiltration membrane to obtain a permeate 3 (phthalic acid, benzoic acid, ethyl vanillin, nicotine, and 2-phenylethanol);

[0071] The test shows that the method in this comparative example cannot achieve the separation of tobacco extract. Comparative Example 1 Compared with Example 1, the method in Comparative Example 1 does not control the pH value of the solution, and separates the tobacco extract based on the pore size screening mechanism, but due to the small molecular weight of the tobacco extract, the pore size of the nanofiltration membrane cannot meet the separation requirements.

[0072] The embodiments described above provide a detailed description of the technical solutions of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements or similar substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A stepwise separation method for tobacco extract based on a cascade nanofiltration membrane combination process, characterized in that: include: The tobacco extract is filtered by combining multiple nanofiltration membranes to achieve the stepwise separation of the tobacco extract. During the filtration process, the permeate separated by the previous nanofiltration membrane is used as the feed liquid for the next nanofiltration membrane; The nanofiltration membrane is a polyamide nanofiltration membrane, and the tobacco extract includes at least two of acidic substances, alkaline substances or neutral substances; Before each stage of filtration, the charge state of the nanofiltration membrane surface and the protonation and deprotonation behavior of the tobacco extract molecules are regulated by adjusting the pH value of the solution to be filtered. At the corresponding pH value, the tobacco extract with the same charge as the nanofiltration membrane surface is retained, and the remaining tobacco extract permeates through the nanofiltration membrane. The pH value of the permeate obtained by filtration is re-adjusted and the next nanofiltration membrane filtration step is performed until the target separation degree is achieved.

2. The stepwise separation method of tobacco extract based on cascade nanofiltration membrane combination process according to claim 1, characterized in that: The nanofiltration membrane is a polyamide nanofiltration membrane prepared by interfacial polymerization, with an average effective pore size of 0.2-1.0 nm and unreacted amino groups or acyl chloride groups on the surface.

3. The stepwise separation method of tobacco extract based on cascade nanofiltration membrane combination process according to claim 1, characterized in that: The molecular weight of the tobacco extract is 100-500Da.

4. The stepwise separation method of tobacco extract based on cascade nanofiltration membrane combination process according to claim 1, characterized in that: The acidic substance includes phthalic acid, benzoic acid, malic acid, proline or ethyl vanillin, the alkaline substance includes nicotine, and the neutral substance includes megastigmatrienone, 2-phenylethanol or neophytadiene.

5. The stepwise separation method of tobacco extract based on cascade nanofiltration membrane combination process according to claim 1, characterized in that: When the tobacco extract in the tobacco extract includes acidic substances, alkaline substances and neutral substances, two-stage nanofiltration membranes are used in combination to achieve the separation of the acidic substances, alkaline substances and neutral substances in the tobacco extract.

6. The stepwise separation method of tobacco extract based on cascade nanofiltration membrane combination process according to claim 1, characterized in that: When the tobacco extract in the tobacco extract liquid includes x acidic substances, y alkaline substances and z neutral substances, x+y level nanofiltration membranes are used in combination to achieve the individual separation and extraction of each acidic substance and each alkaline substance.

7. The stepwise separation method of tobacco extract based on cascade nanofiltration membrane combination process according to claim 1, characterized in that: The pH value of the solution to be filtered is adjusted in the range of 3-12.

8. The stepwise separation method of tobacco extract based on cascade nanofiltration membrane combination process according to claim 1, characterized in that: Before each filtration, the adjustment range of the pH value of the solution to be filtered is jointly determined by the isoelectric point of the nanofiltration membrane and the pKa of the target tobacco extract to be retained in the solution to be filtered; when the target tobacco extract to be retained is an acidic substance, the pH value of the solution to be filtered is adjusted to be greater than the isoelectric point of the nanofiltration membrane and the pH value of the solution to be filtered is greater than the pKa of the target tobacco extract to be retained, and the target acidic substance is retained; When the target tobacco extract to be retained is an alkaline substance, the pH value of the solution to be filtered is adjusted to be less than the isoelectric point of the nanofiltration membrane and the pH value of the solution to be filtered is less than the pKa of the target tobacco extract to retain the target alkaline substance.

9. The stepwise separation method of tobacco extract based on cascade nanofiltration membrane combination process according to claim 1, characterized in that: When the tobacco extract contains phthalic acid, benzoic acid, ethyl vanillin, nicotine and 2-phenylethanol, the pH value of the solution to be filtered is first adjusted to 3.0-4.0, and the first-stage nanofiltration membrane filtration separation is performed to obtain nicotine and a permeate solution 1; the pH value of the permeate solution 1 is then adjusted to 5.0-6.0, and the second-stage nanofiltration membrane filtration separation is performed to obtain phthalic acid and a permeate solution 2; the pH value of the permeate solution 2 is then adjusted to 7.0-9.0, and the third-stage nanofiltration membrane filtration separation is performed to obtain benzoic acid and a permeate solution 3; the pH value of the permeate solution 3 is then adjusted to 10.0-12.0, and the fourth-stage nanofiltration membrane filtration separation is performed to obtain ethyl vanillin and 2-phenylethanol.

10. Application of the tobacco extract stepwise separation method based on the cascade nanofiltration membrane combination process according to any one of claims 1 to 9 in the field of electronic cigarettes and high-value utilization of waste tobacco.

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