CN113385037A - Pervaporation membrane for separating biodiesel and methanol and preparation method thereof - Google Patents
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Abstract
The invention discloses a pervaporation membrane for separating biodiesel and methanol, which is a polyvinyl alcohol membrane doped with modified chitosan, wherein the modified chitosan is glyoxal modified chitosan, phosphoric acid modified chitosan or N-phthaloyl chitosan. The invention also discloses a preparation method of the pervaporation membrane. The pervaporation membrane can effectively improve the swelling property and the mechanical property of the polyvinyl alcohol membrane in polar solution, and can also increase the membrane flux of the pervaporation membrane when separating the mixed solution of biodiesel and methanol.
Description
Technical Field
The invention relates to a pervaporation membrane for separating biodiesel and methanol and a preparation method of the pervaporation membrane for separating biodiesel and methanol.
Background
The biodiesel is an environment-friendly alternative fuel, can partially replace diesel oil when in use, and has positive effects on adjusting an energy structure, relieving energy crisis, protecting ecological environment, adjusting an agricultural structure and promoting social sustainable development. The production and popularization of the biodiesel have good prospect: the biodiesel raw material is easy to obtain and low in price, is beneficial to soil optimization, and the byproduct has good economic value, obvious environmental protection benefit and the like. When the biodiesel is prepared to obtain a product, the crude biodiesel contains impurities such as methanol, and the impurities can reduce the quality of the biodiesel, influence the storage of the biodiesel and even influence the performance of a used internal combustion engine. Therefore, the crude biodiesel needs to be further purified to meet the requirements of relevant standards.
If the biodiesel product cannot be effectively separated and purified, carbon deposition and coking can be caused when the biodiesel product is used as engine fuel, and further the problems of filter and nozzle blockage, oil ring adhesion, engine knocking and the like are caused. At present, the common methods for separating the biodiesel comprise a wet separation method, a dry cleaning method and a vacuum continuous distillation method (Hu xue Ling, Lin nationality friend, separation and purification of the biodiesel from waste oil of kitchens and kitchens [ J ] popular science and technology, 2020,22(04): 24-27.). Wherein the highest ester content of the biodiesel product obtained by wet separation is still lower than the minimum value (96.5%) required by the relevant standard. Generally, nearly 10L of wastewater can be generated when 1L of biodiesel is produced, the time consumption is long, the cost is high, the water consumption is large, and the problems that the washing wastewater is difficult to treat and recycle are greatly troubled for production enterprises; the dry cleaning method is mainly used for removing impurities in the product by using a plurality of solid adsorbents, but the solid adsorption method has common effect on the biodiesel synthesized by liquid alkali; the vacuum continuous distillation method has high distillation temperature, which not only causes high production energy consumption, but also carries glycerin molecules to influence the quality of the biodiesel.
Pervaporation (PV) membrane separation technology is used as a novel separation means with high efficiency and low energy consumption, and is widely applied to separation of near-boiling or azeotropic mixed liquor, separation of organic matter mixed liquor, removal of trace water in an organic solvent and recovery of high-value components in an aqueous solution, wherein polyvinyl alcohol (PVA) can be used for other various organic matter systems containing methanol; however, the linear polymer structure of polyvinyl alcohol (PVA) and a large number of hydroxyl groups on the side chain are easy to swell and even dissolve in methanol, so the pure polyvinyl alcohol film has a short service time, and cannot meet the requirement of the PVA film on the separation of biodiesel and methanol in practical application.
Disclosure of Invention
The purpose of the invention is as follows: the invention provides a polyvinyl alcohol pervaporation membrane doped with modified chitosan and a preparation method of the pervaporation membrane, aiming at the problems of low permeation flux, low separation coefficient and poor swelling resistance of the polyvinyl alcohol membrane when the polyvinyl alcohol membrane is used for separating an organic matter system containing methanol in the prior art.
The technical scheme is as follows: the pervaporation membrane for separating the biodiesel and the methanol is a polyvinyl alcohol membrane doped with modified chitosan.
Preferably, the doping amount of the modified chitosan is 10-50% of the mass of the polyvinyl alcohol; the modified chitosan is glyoxal modified chitosan, phosphoric acid modified chitosan or N-phthaloyl chitosan.
Preferably, the preparation method of the glyoxal modified chitosan comprises the following steps: adding chitosan into an acetic acid solution with the mass fraction of 2% to obtain a chitosan solution, dropwise adding a sodium hydroxide solution into the chitosan solution until the chitosan solution is weakly acidic, adding an glyoxal solution with the volume of 5-15% of that of the chitosan solution, stirring for 12-24 h after adding, standing overnight, dropwise adding a sodium hydroxide solution with the concentration of 1-2 mol/L into the mixed solution until a dark brown suspension is generated, then, the mixed solution is neutral, filtering the solution, and drying the filtered substance at room temperature for 12-24 h to obtain glyoxal modified chitosan.
Wherein, the addition amount of the chitosan is 10mg in each 1mL of acetic acid solution.
Preferably, the preparation method of the phosphoric acid modified chitosan comprises the following steps: adding chitosan, urea, diammonium phosphate and phosphoric acid in a mass ratio of 1: 3-7: 0.5-1.5 into N, N-dimethylformamide, heating and stirring the mixed material at 100-200 ℃ for 1-2 hours, and freeze-drying to obtain the phosphoric acid modified chitosan.
Preferably, the preparation method of the N-phthaloyl chitosan comprises the following steps: adding chitosan and phthalic anhydride in a mass ratio of 1: 3-7 into N, N-dimethylformamide, reacting for 4-8 h at 100-200 ℃, and drying in vacuum to obtain the N-phthaloyl chitosan.
The preparation method of the pervaporation membrane for separating the biodiesel and the methanol specifically comprises the following steps:
(1) dissolving modified chitosan in an acetic acid solution to obtain a modified chitosan solution;
(2) mixing the modified chitosan solution with the polyvinyl alcohol aqueous solution, heating to obtain a casting solution, and casting to form a film.
Preferably, in the step (1), the mass fraction of the acetic acid solution is 2-8%, and the mass fraction of the modified chitosan solution is 3-5%.
Preferably, in the step (2), the mass fraction of the polyvinyl alcohol aqueous solution is 3-11%; the mass of the modified chitosan solution is 10-50% of that of the polyvinyl alcohol aqueous solution.
Preferably, in the step (2), the heating condition is 80-90 ℃ for treatment for 2-6 hours, and the curing temperature of the casting solution is 20-30 ℃.
According to the invention, the modified chitosan is doped into the polyacrylic acid film by a blending method, so that the hydrophilicity of the film is effectively improved, the film and methanol molecules are more easily combined, the adsorption content of methanol is improved, the preferential selectivity of the film to the methanol molecules is realized, and the swelling resistance of the polyvinyl alcohol film can be enhanced by the doped modified chitosan (a large amount of hydroxyl groups on a polyvinyl alcohol side chain are reacted); finally, the modified chitosan is doped in the membrane, a more complex network structure can be formed (the modified chitosan and the polyvinyl alcohol are subjected to cross-linking polymerization reaction), the structure is changed from a one-dimensional structure to a three-dimensional network structure, and the mechanical strength of the membrane can be effectively increased; the polyvinyl alcohol membrane doped with the modified chitosan has smaller swelling degree, greatly increases the service time, increases the adsorption sites of the membrane, and greatly increases the permeation flux of methanol.
Has the advantages that: the membrane pervaporation membrane effectively solves the problems of low permeation flux, low separation coefficient and poor swelling resistance of the polyvinyl alcohol membrane in the process of separating an organic matter system containing methanol in the prior art, and effectively improves the mechanical strength of the composite membrane by doping the modified chitosan in the polyvinyl alcohol membrane, so that the swelling resistance of the membrane is improved, and compared with the membrane before doping, the swelling degree is reduced by 24-116%, and can be as low as 7.3%; the tensile strength is increased by 0.7-2.5 times and is higher than 0.067 MPa; the membrane flux of the pervaporation membrane is increased by 4-6 times compared with that before doping when the pervaporation membrane is used for separating the mixed liquid of the biodiesel and the methanol and is higher than 544.6 g.m-2·h-1(ii) a The separation factor is increased by 2-4 times and is higher than 19.5.
Detailed Description
The technical solution of the present invention is further described with reference to the following specific embodiments.
Example 1
The invention relates to a preparation method of a pervaporation membrane for separating biodiesel and methanol, which comprises the following steps:
(1) preparing glyoxal modified chitosan: weighing 0.3g of chitosan (the deacetylation degree of the chitosan is more than or equal to 95 percent, and the viscosity is 100-200 mpa · s), dissolving the chitosan in 30mL of acetic acid solution with the mass fraction of 2 percent, stirring for 6 hours to obtain a chitosan solution, dropwise adding NaOH solution with the concentration of 0.01mol/L into the chitosan solution until the pH value of the chitosan solution reaches 5.6, adding 3.0mL of glyoxal solution with the mass fraction of 50 percent into the chitosan solution, violently stirring for 12 hours, and standing overnight; dropwise adding a sodium hydroxide solution with the concentration of 2mol/L into the mixed solution until a dark brown suspension is generated, filtering insoluble crosslinked chitosan-glyoxal particles until the pH of the mixed solution reaches 7, washing the insoluble crosslinked chitosan-glyoxal particles with deionized water and acetone, crushing the product, standing the product, and drying the product in the air at room temperature for 24 hours to obtain glyoxal modified chitosan;
(2) preparing a polyvinyl alcohol aqueous solution: according to the mass ratio of 3: 97 mixing polyvinyl alcohol (the polymerization degree of the polyvinyl alcohol is 1700, and the alcoholysis degree is 98% -99%) with water to obtain 100g of a mixture, stirring the mixture at 80 ℃ for 6h, and cooling to room temperature after the polyvinyl alcohol is fully dissolved to obtain a polyvinyl alcohol aqueous solution;
(3) weighing 0.3g of glyoxal modified chitosan, dissolving in 10mL of 2% acetic acid solution by mass fraction, and stirring for 6h to obtain a modified chitosan solution; mixing the modified chitosan solution with a polyvinyl alcohol aqueous solution, heating at 80 ℃ and stirring for 6 hours to obtain a membrane casting solution; placing the membrane casting solution in a container, and curing for 48 hours at 20 ℃ to obtain a polyvinyl alcohol membrane doped with modified chitosan; in the polyvinyl alcohol film, the doping amount of the modified chitosan is 10 percent of the mass of the polyvinyl alcohol.
Example 2
The invention relates to a preparation method of a pervaporation membrane for separating biodiesel and methanol, which comprises the following steps:
(1) weighing 2.1g of chitosan (the deacetylation degree of the chitosan is more than or equal to 95 percent, and the viscosity is 100-200 mpa · s), 10.5g of urea, 2.1g of diammonium phosphate and 2.1g of phosphoric acid, adding the mixture into 210mLN, N-dimethylformamide, heating and stirring the mixture at 150 ℃ for 1h, cooling the obtained suspension to room temperature, centrifuging, thoroughly washing the precipitate with deionized water, and freeze-drying in a freeze dryer to obtain phosphoric acid modified chitosan;
(2) according to the mass ratio of 7: 93 mixing polyvinyl alcohol and water to obtain 100g of a mixture, stirring the mixture at 85 ℃ for 4 hours, and cooling to room temperature after polyvinyl alcohol is fully dissolved to obtain a polyvinyl alcohol aqueous solution;
(3) weighing 2.1g of phosphoric acid modified chitosan, dissolving in 52.5mL of acetic acid solution with the mass fraction of 5%, and stirring for 6 hours to obtain a modified chitosan solution; mixing the modified chitosan solution with a polyvinyl alcohol aqueous solution, heating at 85 ℃ and stirring for 4 hours to obtain a membrane casting solution; placing the membrane casting solution in a container, and curing for 36h at 25 ℃ to obtain a polyvinyl alcohol membrane doped with modified chitosan; in the polyvinyl alcohol film, the doping amount of the modified chitosan is 30% of the mass of the polyvinyl alcohol.
Example 3
The invention relates to a preparation method of a pervaporation membrane for separating biodiesel and methanol, which comprises the following steps:
(1) weighing 5.5g of chitosan (the deacetylation degree of the chitosan is more than or equal to 95 percent, the viscosity is 100-200 mpa · s) and 27.5g of phthalic anhydride, adding the chitosan and the phthalic anhydride into 100mL of N, N-dimethylformamide containing 5 percent (mass fraction) of water, reacting for 6 hours at 130 ℃, cooling, filtering out unreacted substances, washing with ethanol for several times, and drying in vacuum to obtain N-phthaloylated chitosan;
(2) according to the mass ratio of 11: 89 mixing polyvinyl alcohol and water to obtain 100g of a mixture, stirring the mixture at 90 ℃ for 2 hours, and cooling to room temperature after the polyvinyl alcohol is fully dissolved to obtain a polyvinyl alcohol aqueous solution;
(3) weighing 5.5g of N-phthaloyl chitosan, dissolving in 110mL of acetic acid solution with the mass fraction of 8%, and stirring for 6h to obtain a modified chitosan solution; mixing the modified chitosan solution with a polyvinyl alcohol aqueous solution, heating at 90 ℃ and stirring for 2 hours to obtain a membrane casting solution; placing the membrane casting solution in a container, and curing for 24h at 30 ℃ to obtain a polyvinyl alcohol membrane doped with modified chitosan; in the polyvinyl alcohol film, the doping amount of the modified chitosan is 50% of the mass of the polyvinyl alcohol.
Comparative example 1
Adding a polyvinyl alcohol film of unmodified chitosan: according to the mass ratio of 7: 93 mixing polyvinyl alcohol and water, weighing 100g of the mixture, stirring at 85 ℃ for 4h, and cooling to room temperature after the polyvinyl alcohol is fully dissolved to obtain a polyvinyl alcohol aqueous solution (casting solution); weighing 1.4g of chitosan, dissolving the chitosan in 140mL of 5% (mass fraction) acetic acid solution, and stirring for 6 hours to obtain a chitosan solution; mixing the chitosan solution and the polyvinyl alcohol solution, heating at 85 ℃ and stirring for 4 hours to obtain a membrane casting solution; and (3) placing the casting solution in a container, and curing for 36h at 25 ℃.
Comparative example 2
Weighing 0.15g of chitosan (the deacetylation degree of the chitosan is more than or equal to 95 percent, and the viscosity is 100-200 mpa · s), dissolving the chitosan in 15mL of acetic acid solution with the mass fraction of 2 percent, stirring for 6 hours to obtain a chitosan solution, dropwise adding NaOH solution with the concentration of 0.01mol/L into the chitosan solution until the pH value of the chitosan solution reaches 5.6, adding 1.5mL of glyoxal solution with the mass fraction of 50 percent into the chitosan solution, violently stirring for 12 hours, and standing overnight; dropwise adding a sodium hydroxide solution with the concentration of 2mol/L into the mixed solution until a dark brown suspension is generated, filtering insoluble crosslinked chitosan-glyoxal particles until the pH of the mixed solution reaches 7, washing the insoluble crosslinked chitosan-glyoxal particles with deionized water and acetone, crushing the product, standing the product, and drying the product in the air at room temperature for 24 hours to obtain glyoxal modified chitosan; according to the mass ratio of 3: 97 mixing polyvinyl alcohol (the polymerization degree of the polyvinyl alcohol is 1700, and the alcoholysis degree is 98% -99%) with water to obtain 100g of a mixture, stirring the mixture at 80 ℃ for 6h, and cooling to room temperature after the polyvinyl alcohol is fully dissolved to obtain a polyvinyl alcohol aqueous solution; weighing 0.15g of glyoxal modified chitosan, dissolving in 5mL of 2% acetic acid solution by mass fraction, and stirring for 6h to obtain a modified chitosan solution; mixing the modified chitosan solution with a polyvinyl alcohol aqueous solution, heating at 80 ℃ and stirring for 6 hours to obtain a membrane casting solution; placing the membrane casting solution in a container, and curing for 48 hours at 20 ℃ to obtain a polyvinyl alcohol membrane doped with modified chitosan; in the polyvinyl alcohol film, the doping amount of the modified chitosan is 5% of the mass of the polyvinyl alcohol.
Comparative example 3
Weighing 7.7g of chitosan (the deacetylation degree of the chitosan is more than or equal to 95 percent, and the viscosity is 100-200 mpa · s) and 38.5g of phthalic anhydride, adding the chitosan and the phthalic anhydride into 140mL of N, N-dimethylformamide containing 5 percent of water (mass fraction), reacting for 6h at 130 ℃, cooling, filtering out unreacted substances, washing with ethanol for several times, and drying in vacuum to obtain N-phthaloylated chitosan; according to the mass ratio of 11: 89 mixing polyvinyl alcohol and water to obtain 100g of a mixture, stirring the mixture at 90 ℃ for 2 hours, and cooling to room temperature after the polyvinyl alcohol is fully dissolved to obtain a polyvinyl alcohol aqueous solution; weighing 7.7g of N-phthaloyl chitosan, dissolving in 154mL of acetic acid solution with the mass fraction of 8%, and stirring for 6 hours to obtain a modified chitosan solution; mixing the modified chitosan solution with a polyvinyl alcohol aqueous solution, heating at 90 ℃ and stirring for 2 hours to obtain a membrane casting solution; placing the membrane casting solution in a container, and curing for 24h at 30 ℃ to obtain a polyvinyl alcohol membrane doped with modified chitosan; in the polyvinyl alcohol film, the doping amount of the modified chitosan is 70 percent of the mass of the polyvinyl alcohol.
The permeation flux, separation coefficient, swelling degree and tensile strength of the pervaporation membranes obtained in examples 1 to 3 and those obtained in comparative examples 1 to 3 were measured, and the results are shown in table 1.
TABLE 1
As can be seen from table 1, compared with the polyvinyl alcohol film added with unmodified chitosan, the polyvinyl alcohol film added with modified chitosan has greatly increased separation coefficient and permeation flux when separating biodiesel from methanol solution and reduced swelling degree to a certain extent under the same liquid inlet speed, concentration, reaction temperature and film back pressure, which indicates that the swelling resistance of the polyvinyl alcohol film doped with modified chitosan is improved to a certain extent. When the amount of the doped modified chitosan is less than 10%, the swelling degree and tensile strength of the polyvinyl alcohol film are improved little, and when the amount of the doped modified chitosan is more than 50%, the swelling degree and tensile strength of the polyvinyl alcohol film are reduced on the contrary.
Claims (10)
1. A pervaporation membrane for separating biodiesel and methanol, characterized in that: the pervaporation membrane is a polyvinyl alcohol membrane doped with modified chitosan.
2. The pervaporation membrane for separation of biodiesel and methanol according to claim 1, characterized in that: the doping amount of the modified chitosan is 10-50% of the mass of the polyvinyl alcohol.
3. The pervaporation membrane for separation of biodiesel and methanol according to claim 1, characterized in that: the modified chitosan is glyoxal modified chitosan, phosphoric acid modified chitosan or N-phthaloyl chitosan.
4. The pervaporation membrane for separating biodiesel and methanol according to claim 3, wherein the preparation method of the glyoxal-modified chitosan comprises: adding chitosan into an acetic acid solution with the mass fraction of 2% to obtain a chitosan solution, dropwise adding a sodium hydroxide solution into the chitosan solution until the chitosan solution is weakly acidic, adding an glyoxal solution with the volume of 5-15% of that of the chitosan solution, stirring for 12-24 h after adding, standing overnight, dropwise adding a sodium hydroxide solution with the concentration of 1-2 mol/L into the mixed solution until a dark brown suspension is generated, then, the mixed solution is neutral, filtering the solution, and drying the filtered substance at room temperature for 12-24 h to obtain glyoxal modified chitosan.
5. The pervaporation membrane for separation of biodiesel and methanol according to claim 4, characterized in that: the amount of chitosan added was 10mg per 1mL of acetic acid solution.
6. The pervaporation membrane for separating biodiesel and methanol according to claim 3, wherein the preparation method of the phosphoric acid modified chitosan comprises the following steps: adding chitosan, urea, diammonium phosphate and phosphoric acid in a mass ratio of 1: 3-7: 0.5-1.5 into N, N-dimethylformamide, heating and stirring the mixed material at 100-200 ℃ for 1-2 hours, and freeze-drying to obtain the phosphoric acid modified chitosan.
7. The pervaporation membrane for separating biodiesel and methanol according to claim 3, wherein the preparation method of said N-phthaloylated chitosan is: adding chitosan and phthalic anhydride in a mass ratio of 1: 3-7 into N, N-dimethylformamide, reacting for 4-8 h at 100-200 ℃, and drying in vacuum to obtain the N-phthaloyl chitosan.
8. A method for preparing a pervaporation membrane for separating biodiesel and methanol according to claim 1, which is characterized in that the method comprises the following steps: dissolving modified chitosan in an acetic acid solution to obtain a modified chitosan solution; mixing the modified chitosan solution with the polyvinyl alcohol aqueous solution, heating to obtain a casting solution, and finally casting to form a film.
9. The method of preparing a pervaporation membrane for separating biodiesel and methanol according to claim 8, characterized in that: the mass fraction of the acetic acid solution is 2-8%, and the mass fraction of the modified chitosan solution is 3-5%; the mass fraction of the polyvinyl alcohol aqueous solution is 3-11%; the mass of the modified chitosan solution is 10-50% of that of the polyvinyl alcohol aqueous solution.
10. The method of preparing a pervaporation membrane for separating biodiesel and methanol according to claim 8, characterized in that: the heating condition is 80-90 ℃, the treatment is carried out for 2-6 hours, and the curing temperature of the casting solution is 20-30 ℃.
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