Apparatus for linear organic alcohol separation and method of separating linear organic alcohols

Abstract

The application relates to the technical field of product separation and purification, and discloses a device for linear organic alcohol separation and a method for separating linear organic alcohol, which comprises at least one membrane separation unit, the membrane separation unit comprises a membrane assembly and an electrostatic field generator, the membrane assembly is arranged in an electric field generated by the electrostatic field generator, and the membrane assembly is a charged nanofiltration separation membrane. The device and the method provided by the application have low production cost and high selectivity for linear organic alcohol.

Description

Technical Field

[0001] This invention relates to the field of product separation and purification technology, and specifically to an apparatus and method for separating linear organic alcohols. Background Technology

[0002] Linear organic alcohols are an important class of organic chemical raw materials with a wide range of applications, including but not limited to ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol. They can be used to manufacture polyester fibers, polyester resins, hygroscopic agents, plasticizers, surfactants, synthetic fibers, cosmetics, and explosives, and as solvents for dyes, inks, etc.

[0003] Heterocyclic impurities, including cyclic ketones and nitrogen-containing heterocyclic impurities, are commonly found in linear organic alcohol products. The presence of these impurities significantly affects product quality and purity, such as ultraviolet transmittance and thermochromic properties. Currently, industrial plants typically employ mature and effective physical adsorption methods for ethylene glycol purification. However, physical adsorption methods face various challenges, including high adsorbent costs, short lifespans, and inconsistent product quality during adsorbent replacement periods.

[0004] Therefore, there is an urgent need to research and develop a highly selective linear organic alcohol separation and purification process to overcome the aforementioned shortcomings of traditional purification methods. Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention provides an apparatus and a method for separating linear organic alcohols. The apparatus and method provided by this invention not only have low production costs but also exhibit high selectivity for linear organic alcohols.

[0006] The first aspect of the present invention provides an apparatus for the linear separation of organic alcohols, the apparatus comprising: at least one membrane separation unit, the membrane separation unit comprising a membrane module and an electrostatic field generator, wherein the membrane module is placed in an electric field generated by the electrostatic field generator, and the membrane module is a charged nanofiltration membrane.

[0007] A second aspect of the present invention provides a method for separating linear organic alcohols, the method comprising:

[0008] Under the influence of an electrostatic field, the linear organic alcohol to be separated is subjected to at least one stage of membrane separation, and the membrane separation adopts a charged nanofiltration membrane.

[0009] The technical solution provided by this invention separates linear organic alcohols through a charged nanofiltration membrane under the action of an electrostatic field, and has the following beneficial effects:

[0010] (1) It has high selectivity for linear organic alcohols and heterocyclic impurities with similar molecular weights, that is, it selectively passes through linear organic alcohols and intercepts heterocyclic impurities.

[0011] (2) Significantly reduce the operating cost of the linear organic alcohol purification and separation process.

[0012] (3) Through this separation method, the recovery rate of linear organic alcohols is greater than or equal to 95%, and the removal rate of heterocyclic impurities is greater than or equal to 95%. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of an apparatus for the separation of linear organic alcohols according to a specific embodiment of the present invention.

[0014] Explanation of reference numerals in the attached figures

[0015] 1. Raw material supply unit; 2. Additive supply unit; 3. First booster pump

[0016] 4. Second booster pump; 5. Electrostatic field negative electrode; 6. Plate and frame nanofiltration membrane module.

[0017] 7. Positive electrode of electrostatic field 8. Product tank 9. Wastewater tank Detailed Implementation

[0018] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0019] The first aspect of this invention provides an apparatus for the linear separation of organic alcohols, such as... Figure 1 As shown, the device includes: at least one membrane separation unit, the membrane separation unit including a membrane module and an electrostatic field generator, and the membrane module is placed in the electric field generated by the electrostatic field generator, the membrane module being a charged nanofiltration membrane.

[0020] The apparatus provided by this invention separates linear organic alcohols using a charged nanofiltration membrane. The linear organic alcohols pass through the pores of the nanofiltration membrane into the filtrate side, while annular impurities contained in the linear organic alcohols to be separated remain in the mother liquor, ultimately forming the concentrate side. The filtrate side then flows through a pipeline into a product tank, and the concentrate side flows through a pipeline into a wastewater tank.

[0021] The apparatus provided by this invention can employ a single-stage membrane separation unit. In order to obtain a higher yield of linear organic alcohol products and a higher impurity removal rate, a multi-stage membrane separation unit can be used for separation and purification.

[0022] Preferably, the device includes two or more membrane separation units, such as 2-5 stages, preferably connected in series. In this case, the concentrate from the upper membrane separation unit is used as the feed for the lower membrane separation unit. The filtrates from each stage of the membrane separation unit are combined and enter the product tank, while the concentrate from the final membrane separation unit enters the wastewater tank.

[0023] According to a preferred embodiment of the present invention, when the device includes two or more membrane separation units, all the multi-stage membrane modules are in the electric field generated by the electrostatic field generator.

[0024] According to a preferred embodiment of the present invention, the membrane module is a positively charged nanofiltration membrane or a negatively charged nanofiltration membrane.

[0025] Preferably, the membrane module is a plate-and-frame nanofiltration membrane.

[0026] The membrane module described in this invention can be commercially available, and this invention does not have any particular limitation on it.

[0027] According to a preferred embodiment of the present invention, the electrostatic field generator includes a positive electrostatic field electrode and a negative electrostatic field electrode. The present invention does not particularly limit the arrangement of the electrostatic field generator, as long as the linear organic alcohol to be separated is separated by membrane separation under the action of the electrostatic field. Preferably, the positive and negative electrostatic field electrodes are arranged parallel to the direction in which the linear organic alcohol to be separated passes through the membrane module. This preferred embodiment is more conducive to improving the throughput of linear organic alcohols in the membrane module, thereby improving the recovery rate.

[0028] According to one specific embodiment of the present invention, the apparatus further includes a raw material supply unit in communication with the membrane module; the raw material supply unit is used to provide the linear organic alcohol to be separated.

[0029] The apparatus provided by this invention is suitable for the separation of linear organic alcohols. It can select from a wide range of specific types of linear organic alcohols, including branched-chain and straight-chain linear organic alcohols. Preferably, the linear organic alcohols are straight-chain linear organic alcohols. During their research, the inventors discovered that the apparatus provided by this invention has higher selectivity for straight-chain linear organic alcohols.

[0030] More preferably, the linear organic alcohol is selected from at least one of ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol.

[0031] Preferably, the device further includes a first booster pump 3 disposed between the membrane module and the raw material supply unit.

[0032] In this invention, the need for an auxiliary agent, i.e., whether to set up an auxiliary agent supply unit, can be considered based on the molecular weight of the linear organic alcohol to be separated. During the research process, the inventors of this invention discovered that when the molecular weight of the linear organic alcohol is below 150, an auxiliary agent may not be used. When the molecular weight of the linear organic alcohol is above 150, it is preferable to mix the linear organic alcohol to be separated with an auxiliary agent before performing membrane separation.

[0033] In a preferred embodiment, the device further includes an additive supply unit connected to the membrane module; the additive supply unit is used to provide additives selected from at least one of inorganic bases corresponding to alkali metal elements and water-soluble inorganic salts.

[0034] According to the present invention, preferably, the auxiliary agent supply unit is connected to the raw material supply unit such that the auxiliary agent and the linear organic alcohol to be separated are mixed in the raw material supply unit or in the connecting pipeline. The auxiliary agent and the linear organic alcohol to be separated can be mixed in the raw material supply unit or in the connecting pipeline, as long as the auxiliary agent and the linear organic alcohol to be separated pass through the membrane assembly together.

[0035] According to a preferred embodiment of the present invention, the device further includes a second booster pump 4 disposed between the additive supply unit and the membrane assembly.

[0036] In the preferred embodiment described above, the linear organic alcohol to be separated and the additives are mixed evenly in the pipeline by their respective booster pumps, and then enter the membrane separation unit.

[0037] According to a preferred embodiment of the present invention, the apparatus further includes a security filter disposed between the membrane module and the feed supply unit. The security filter is used to filter out small particles in the material entering the membrane separation unit, thereby protecting the nanofiltration membrane in the membrane module. There is no particular limitation on the type of security filter, as long as it can achieve the above-mentioned objective.

[0038] A second aspect of the present invention provides a method for separating linear organic alcohols, the method comprising:

[0039] Under the influence of an electrostatic field, the linear organic alcohol to be separated is subjected to at least one stage of membrane separation, and the membrane separation adopts a charged nanofiltration membrane.

[0040] The range of linear organic alcohols to be selected can be the same as the range of linear organic alcohols to be selected as described in the first aspect above, and will not be repeated here.

[0041] The method provided by this invention is applicable to the separation of linear organic alcohols containing impurities. There is no particular limitation on the type of impurities; preferably, the linear organic alcohol to be separated is a byproduct of the ethylene oxide hydration process.

[0042] Preferably, the linear organic alcohol to be separated also contains impurities, which are small cyclic impurities generated by oxidation side reactions in the ethylene oxide process, with a molecular weight between 100 and 150. More preferably, they are selected from at least one of cyclic ketone impurities and nitrogen-containing heterocyclic impurities, and more preferably, cyclic ketone impurities. The present invention does not particularly limit the specific types of cyclic ketone impurities, such as at least one of 3-methyl-1,2-cyclopentanedione, 1,3-dimethyl-1,2-cyclopentanedione, and 3,4-dimethyl-1,2-cyclopentanedione.

[0043] According to the present invention, preferably, the linear organic alcohol content in the linear organic alcohol to be separated is 5-50% by weight, preferably 10-40% by weight.

[0044] According to the present invention, preferably, the content of impurities in the linear organic alcohol to be separated is 0.1-1% by weight, more preferably 0.2-0.5% by weight.

[0045] According to the present invention, there are no particular limitations on other components in the linear organic alcohol to be separated; for example, the balance may be water.

[0046] According to a preferred embodiment of the present invention, the linear organic alcohol has a molecular weight of 150 or higher. The method further includes mixing the linear organic alcohol to be separated with an auxiliary agent, and then performing membrane separation. The auxiliary agent is selected from at least one of an inorganic base corresponding to an alkali metal element and a water-soluble inorganic salt. As mentioned above, preferably, the use of the auxiliary agent depends on the molecular weight of the linear organic alcohol to be separated. When the molecular weight of the linear organic alcohol is <150, the material to be separated may not require the use of an auxiliary agent. When the molecular weight of the linear organic alcohol is ≥150, the linear organic alcohol to be separated is mixed with the auxiliary agent, and then membrane separation is performed. This preferred embodiment is more conducive to improving the throughput of linear organic alcohols in the membrane module, thereby improving the recovery rate.

[0047] According to a preferred embodiment of the present invention, the auxiliary agent is selected from at least one of NaOH, KOH, NaCl and KCl.

[0048] According to a preferred embodiment of the invention, the adjuvant is introduced in the form of a solution, such as an aqueous solution. The concentration of the adjuvant aqueous solution is selected from a wide range, and preferably, the concentration of the adjuvant aqueous solution is 5-40% by weight.

[0049] According to the present invention, preferably, the mass concentration of the auxiliary agent used is in the range of 500-3000 ppm, more preferably 600-1800 ppm, based on the linear organic alcohol to be separated. This preferred embodiment is more advantageous in improving the permeation rate of linear organic alcohols in the membrane module, thereby increasing the recovery rate.

[0050] According to the present invention, preferably, the membrane separation employs a positively charged nanofiltration membrane or a negatively charged nanofiltration membrane.

[0051] According to the present invention, preferably, the membrane separation adopts a plate and frame nanofiltration membrane.

[0052] According to a preferred embodiment of the present invention, the membrane separation conditions include: a pressure of 1-4 MPa, an electrostatic voltage of 220-380 V, and a membrane separation yield of 90-99%.

[0053] The method provided by this invention can employ single-stage membrane separation. In order to obtain a higher yield of linear organic alcohol products and a higher impurity removal rate, multi-stage membrane separation can be used for separation and purification.

[0054] Preferably, the membrane separation is a series of two or more stages, for example, two to five stages. In this case, the concentrate from the upper stage membrane separation is used as the feed for the lower stage membrane separation. The filtrates from each stage of membrane separation are combined and enter the product tank, while the concentrate from the final stage membrane separation unit enters the wastewater tank.

[0055] Preferably, the yield of each stage of membrane separation gradually increases according to the material flow direction. This preferred configuration is more conducive to improving the retention rate of impurities in the membrane.

[0056] Unless otherwise specified, the content of each component in the materials to be separated in the following examples and comparative examples is by mass percentage.

[0057] Example 1

[0058] The material to be separated is a mixture of ethylene glycol and 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione. Ethylene glycol accounts for 40%, 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione contain 1000 ppm and 1500 ppm respectively, and the remaining component is water. Figure 1As shown, the raw material supply unit 1 feeds the material into the plate-and-frame nanofiltration membrane module 6 (the nanofiltration membrane is a negatively charged membrane element, purchased from GE Nanofiltration Membrane MUNI-NF-365-FF) at a flow rate of 50 L / min via the first booster pump 3. The plate-and-frame nanofiltration membrane module 6 is placed in the electric field generated by an electrostatic field generator, which includes an electrostatic field negative electrode 5 and an electrostatic field positive electrode 7. The membrane box operating pressure is 2 MPa, the electrostatic field generator voltage is 220 V, and the membrane box yield is controlled at 97%. The liquid filtered out from the nanofiltration membrane box enters the product tank 8, and the liquid concentrated from the nanofiltration membrane box enters the wastewater tank 9. Finally, the content of linear organic alcohols and cyclopentanedione in the liquid filtered out from the nanofiltration membrane box is analyzed to determine the recovery rate of linear organic alcohols and the rejection rate of cyclopentanedione. The results are shown in Table 1.

[0059]

[0060] In the formula, α is the recovery rate of the linear organic alcohol; V is the liquid volume in mL / min; and w1 is the mass fraction of the linear organic alcohol in mg / mL.

[0061]

[0062] In the formula, γ is the rejection rate of cyclopentanedione; V is the liquid volume in mL / min; and w2 is the mass fraction of cyclopentanedione in mg / mL.

[0063] The content of linear organic alcohols was determined by gas chromatography (GC), and the content of cyclopentanedione was determined by high performance liquid chromatography (HPLC).

[0064] Example 2

[0065] The method of Example 1 was followed, except that the materials to be separated were a mixture of diethylene glycol and 3-methyl-1,2-cyclopentanedione and 3,4-dimethyl-1,2-cyclopentanedione. The diethylene glycol content was 40%, the cyclopentanedione content was 1000 ppm and 1500 ppm respectively, and the remaining component was water. The membrane chamber operating pressure was 2.5 MPa, and the membrane chamber yield was controlled at 98%. The results are shown in Table 1.

[0066] Example 3

[0067] The material to be separated is a mixture of triethylene glycol and 3-methyl-1,2-cyclopentanedione and 3,4-dimethyl-1,2-cyclopentanedione. The triethylene glycol content is 30%, the 3-methyl-1,2-cyclopentanedione and 3,4-dimethyl-1,2-cyclopentanedione contents are 1000 ppm and 1000 ppm respectively, and the remaining component is water. Figure 1As shown, the raw material supply unit 1 mixes the material with the auxiliary agent via the first booster pump 3 at a flow rate of 50 L / min, and then feeds it in streams into the plate and frame nanofiltration membrane module 6 (the nanofiltration membrane is a negatively charged membrane element). The auxiliary agent is supplied via the auxiliary agent supply unit 2 using the second booster pump 4. The auxiliary agent used is a 10% by weight NaOH aqueous solution at a flow rate of 300 mL / min. The plate and frame nanofiltration membrane module 6 is placed in the electric field generated by an electrostatic field generator, which includes a positive electrode 7 and a negative electrode 5. The membrane box operating pressure is 3 MPa, the electrostatic field generator voltage is 220 V, and the membrane box yield is controlled at 99%. The liquid filtered from the nanofiltration membrane box enters the product tank 8, and the liquid concentrated from the nanofiltration membrane box enters the wastewater tank 9. Finally, the content of linear organic alcohols and cyclopentanedione in the liquid filtered from the nanofiltration membrane box is analyzed to determine the recovery rate of linear organic alcohols and the rejection rate of cyclopentanedione. The results are shown in Table 1.

[0068] Example 4

[0069] The method of Example 3 was followed, except that the auxiliary agent used was a 30% by weight NaOH aqueous solution, and the results are shown in Table 1.

[0070] Example 5

[0071] The method of Example 3 was followed, except that the material to be separated was a mixture of tetraethylene glycol and 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione. The tetraethylene glycol content was 10%, and the contents of tetraethylene glycol, 3-methyl-1,2-cyclopentanedione, and 1,3-dimethyl-1,2-cyclopentanedione were 1000 ppm and 1000 ppm, respectively. The remaining component was water. The results are shown in Table 1.

[0072] Example 6

[0073] The method of Example 3 was followed, except that the material to be separated was a mixture of tetraethylene glycol and 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione. The tetraethylene glycol content was 10%, the 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione contents were 1000 ppm and 1500 ppm respectively, and the remaining component was water. The auxiliary agent used was a 20% by weight NaOH aqueous solution.

[0074] Example 7

[0075] The method of Example 3 was followed, except that the material to be separated was a mixture of tetraethylene glycol and 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione. The tetraethylene glycol content was 10%, the contents of 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione were 1000 ppm and 1500 ppm respectively, and the remaining component was water. The auxiliary agent used was a 30% by weight NaOH aqueous solution.

[0076] Example 8

[0077] The method of Example 3 was followed, except that the material to be separated was a mixture of tetraethylene glycol and 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione. The tetraethylene glycol content was 10%, the 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione contents were 1000 ppm and 1500 ppm respectively, and the remaining component was water. No additives were used in the separation process.

[0078] Example 9

[0079] The method of Example 3 was followed, except that the materials to be separated were a mixture of tetraethylene glycol and 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione. The tetraethylene glycol content was 10%, and the contents of 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione were 1000 ppm and 1500 ppm, respectively. The remaining components were water, and the auxiliary agent used was a 30% by weight NaOH aqueous solution. The membrane module was arranged in a two-stage configuration and both stages were placed in an electrostatic field. The operating pressure of the first-stage membrane chamber was 2 MPa, and the operating pressure of the second-stage membrane chamber was 3 MPa. The voltage of the electrostatic field generator was 220 V, and the yield of the first-stage membrane chamber was controlled at 98%, while the yield of the second-stage membrane chamber was 99%.

[0080] Comparative Example 1

[0081] The method of Example 3 was followed, except that the material to be separated was a mixture of tetraethylene glycol and 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione. The tetraethylene glycol content was 10%, the 3-methyl-1,2-cyclopentanedione and 1,3-dimethyl-1,2-cyclopentanedione contents were 1000 ppm and 1500 ppm respectively, the remaining component was water, no additives were used in the separation process, and the plate-and-frame nanofiltration membrane module 6 was not placed in an electrostatic field.

[0082] Table 1

[0083]

[0084]

[0085] As can be seen from the results in Table 1, in the preferred embodiment of the method and apparatus of the present invention, the recovery rate of linear organic alcohols is greater than 95%, and the removal rate of heterocyclic impurities is greater than 95%.

[0086] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. An apparatus for separating linear organic alcohols and cyclic impurities, the apparatus comprising: At least one membrane separation unit, the membrane separation unit includes a membrane module and an electrostatic field generator, and the membrane module is placed in the electric field generated by the electrostatic field generator. The electrostatic field generator includes an electrostatic field positive electrode and an electrostatic field negative electrode. The arrangement direction of the electrostatic field positive electrode and the electrostatic field negative electrode is parallel to the direction of the linear organic alcohol to be separated through the membrane module, so that the linear organic alcohol to be separated is separated by the membrane under the action of the electrostatic field. The membrane module is a charged nanofiltration separation membrane. The device further includes a raw material supply unit and an auxiliary agent supply unit connected to the membrane module; the raw material supply unit is used to provide the linear organic alcohol to be separated, and the auxiliary agent supply unit is used to provide an auxiliary agent selected from at least one of an inorganic base corresponding to an alkali metal element and a water-soluble inorganic salt; the auxiliary agent supply unit is connected to the raw material supply unit such that the auxiliary agent and the linear organic alcohol to be separated are mixed in the raw material supply unit or in a connecting pipeline; When the molecular weight of the linear organic alcohol is ≥150, the linear organic alcohol to be separated is mixed with an auxiliary agent, and then the membrane separation is performed. Alternatively, if the molecular weight of a linear organic alcohol is <150, no auxiliaries are used.

2. The apparatus according to claim 1, wherein, The membrane module is a positively charged nanofiltration membrane or a negatively charged nanofiltration membrane.

3. The apparatus according to claim 1 or 2, wherein, The membrane module is a plate-and-frame nanofiltration membrane.

4. The apparatus according to claim 1, wherein, The linear organic alcohol is a straight-chain linear organic alcohol.

5. The apparatus according to claim 4, wherein, The linear organic alcohol is selected from at least one of ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol.

6. The apparatus according to claim 1 or 2, wherein, The device also includes a booster pump and / or a security filter disposed between the membrane module and the feed supply unit.

7. A method for separating linear organic alcohols and cyclic impurities, the method being carried out in the apparatus of any one of claims 1-6, comprising: Under the influence of an electrostatic field, the linear organic alcohol to be separated is subjected to at least one stage of membrane separation, and the membrane separation adopts a charged nanofiltration membrane.

8. The method according to claim 7, wherein, The linear organic alcohol to be separated has a content of 5-50% by weight.

9. The method according to claim 8, wherein, The cyclic impurities in the linear organic alcohol to be separated are selected from at least one of cyclic ketone impurities and nitrogen heterocyclic impurities.

10. The method according to claim 9, wherein, The cyclic impurities are cyclic ketone impurities.

11. The method according to claim 9, wherein, The linear organic alcohol to be separated contains 0.1-1% by weight of cyclic impurities.

12. The method according to any one of claims 7-11, wherein, The additive is selected from at least one of NaOH, KOH, NaCl, and KCl.

13. The method according to any one of claims 7-11, wherein, Based on the linear organic alcohol to be separated, the mass concentration of the auxiliary agent used ranges from 500 to 3000 ppm.

14. The method according to claim 13, wherein, Based on the linear organic alcohol to be separated, the mass concentration of the auxiliary agent used ranges from 600 to 1800 ppm.

15. The method according to any one of claims 7-11, wherein, The membrane separation conditions include: pressure of 1-4 MPa, electrostatic voltage of 220-380 V, and membrane separation yield of 90-99%.

16. The method according to any one of claims 7-11, wherein, The membrane separation is a series of two or more membrane separation stages.

17. The method according to claim 16, wherein, The yield of membrane separation gradually increases according to the direction of material flow.

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