Method for preparing 2-phenylethanol with high purity by coupling membrane separation and molecular distillation

Abstract

This invention relates to the field of chemical separation and purification technology, and discloses a method for preparing high-purity 2-phenylethanol using membrane separation coupled with molecular distillation. The method includes clarifying yeast fermentation broth using a cross-flow microfiltration system to obtain a clarified microfiltration permeate; placing the permeate in a conditioning tank and adding an aqueous solution of sodium hexametaphosphate or trisodium citrate, stirring, and then adding an aqueous solution of sodium carbonate to adjust the pH value to obtain a pre-conditioned solution; pumping this solution into a nanofiltration system to obtain a concentrated 2-phenylethanol solution; adding an aqueous solution of citric acid to a neutralization tank to adjust the pH value to obtain a neutralized concentrated 2-phenylethanol solution; and finally pumping this solution into a short-path molecular distillation apparatus to obtain high-purity 2-phenylethanol. This invention prevents inorganic salt scaling and slows down the deposition of organic matter on the nanofiltration membrane surface through two-step chemical conditioning operations, maintaining a stable permeate flux in the nanofiltration system; and avoids thermal degradation of the material during distillation, ensuring the total recovery rate and product purity of 2-phenylethanol.

Description

Technical Field

[0001] This invention relates to the field of chemical separation and purification technology, specifically a method for preparing high-purity 2-phenylethanol using membrane separation coupled with molecular distillation. Background Technology

[0002] Yeast fermentation broth is the raw material for extracting 2-phenylethanol. It contains calcium ions and organic matter. When the yeast fermentation broth is concentrated and purified using a nanofiltration system, calcium ions react to form inorganic salt scale, clogging the nanofiltration system pores. Organic matter is adsorbed and deposited on the nanofiltration membrane surface. Both inorganic scale and organic deposits cause a decrease in the permeate flux of the nanofiltration system, reducing the flux recovery rate of the membrane module. The concentrated feed solution, lacking chemical preconditioning, has a complex composition when entering the short-path molecular distillation unit. The presence of impurities necessitates setting a higher evaporator heating temperature for the short-path molecular distillation unit.

[0003] The material undergoes degradation and polymerization losses under heating conditions. The slightly alkaline feed solution, without neutralization and rebalancing, directly enters the short-path molecular distillation unit, triggering side reactions of heat-sensitive substances and generating impurities. The lack of coordinated chemical control between the separation and concentration unit and the distillation and purification unit leads to reduced equipment operational stability, resulting in a decrease in the overall recovery rate and product purity of 2-phenylethanol.

[0004] Therefore, this invention proposes a method for preparing high-purity 2-phenylethanol through membrane separation coupled with molecular distillation to overcome the shortcomings of existing technologies. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing high-purity 2-phenylethanol using membrane separation coupled with molecular distillation. This method solves the problems of reduced permeate flux caused by nanofiltration membrane fouling and organic contamination during the separation and extraction of yeast fermentation broth, as well as the decrease in total 2-phenylethanol recovery rate and product purity due to material degradation and side reactions of unconditioned materials during high-temperature molecular distillation.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing high-purity 2-phenylethanol via membrane separation coupled with molecular distillation, comprising the following steps: The yeast fermentation broth was clarified using a cross-flow microfiltration system to obtain a clarified microfiltration permeate. The clarified microfiltration permeate was placed in a conditioning tank, and sodium hexametaphosphate aqueous solution or trisodium citrate aqueous solution was added and stirred. Sodium carbonate aqueous solution was added dropwise to adjust the pH value of the clarified microfiltration permeate, thus obtaining a feed solution that has been pre-conditioned by composite chemicals. The feed solution, which had undergone compound chemical preconditioning, was pumped into a nanofiltration system to obtain a concentrated 2-phenylethanol solution. The 2-phenylethanol concentrate was transferred to a neutralization tank, and citric acid aqueous solution was added dropwise to adjust the pH value of the 2-phenylethanol concentrate, thus obtaining the neutralized 2-phenylethanol concentrate. The neutralized 2-phenylethanol concentrate was pumped into a short-path molecular distillation apparatus to obtain high-purity 2-phenylethanol.

[0007] The microfiltration membrane module of the cross-flow microfiltration system is a polyvinylidene fluoride (PVDF) hollow fiber membrane with a nominal pore size of 0.2 μm.

[0008] When clarifying yeast fermentation broth using a cross-flow microfiltration system, the operating temperature is controlled at 25℃, the transmembrane pressure difference is maintained at 0.1MPa, and the cross-flow velocity on the membrane surface is 2.0m / s.

[0009] When sodium hexametaphosphate aqueous solution is added, the sodium hexametaphosphate aqueous solution is prepared by dissolving commercially available industrial-grade sodium hexametaphosphate in pure water to a concentration of 10% (w / w); when trisodium citrate aqueous solution is added, the trisodium citrate aqueous solution is prepared by dissolving commercially available industrial-grade trisodium citrate in pure water to a concentration of 10% (w / w).

[0010] Add an aqueous solution of sodium hexametaphosphate to achieve a final concentration of sodium hexametaphosphate in the clarified microfiltration permeate of 200 ppm to 500 ppm; or add an aqueous solution of trisodium citrate to achieve a final concentration of trisodium citrate in the clarified microfiltration permeate of 50 ppm.

[0011] The conditioning tank is equipped with a stirrer and online pH monitoring. When adding sodium carbonate aqueous solution, the 1.0M sodium carbonate aqueous solution is added slowly to precisely control and stabilize the pH value of the clarified microfiltration permeate at 8.5 to 9.5.

[0012] The nanofiltration membrane in the nanofiltration system is a polyamide composite spiral wound membrane with a molecular weight cutoff of 200 Da.

[0013] When the pre-treated feed solution is pumped into the nanofiltration system, the operating temperature is maintained between 20°C and 40°C, and the operating pressure is set between 1.0 MPa and 2.5 MPa.

[0014] When adding citric acid aqueous solution, add 20% (w / w) citric acid aqueous solution to adjust the pH of the 2-phenylethanol concentrate back to 7.0.

[0015] When the neutralized 2-phenylethanol concentrate is pumped into the short-path molecular distillation apparatus, the evaporator heating temperature of the short-path molecular distillation apparatus is set to 60°C to 85°C, the system vacuum is maintained at 5Pa, and the built-in condenser temperature is 15°C.

[0016] This invention uses an aqueous solution of sodium hexametaphosphate or trisodium citrate to form a stable soluble complex with calcium ions in the yeast fermentation broth, thus preventing the formation of inorganic salt scale when sodium carbonate aqueous solution is added subsequently to raise the pH value.

[0017] At the same time, increasing the pH value changes the charge characteristics of organic matter in the feed solution and the membrane surface, and uses electrostatic repulsion to slow down the adsorption and deposition of organic matter on the membrane surface.

[0018] Two-step chemical conditioning eliminates the risk of inorganic scaling, inhibits organic contamination, maintains stable permeation flux in the nanofiltration system, prevents hard scale formation, and ensures flux recovery rate. The feed solution, after composite chemical preconditioning, provides the short-path molecular distillation unit with a simple composition and controllable impurities, avoiding material degradation or polymerization losses due to heat, reducing impurity generation, and ensuring the purity and total recovery rate of 2-phenylethanol.

[0019] This invention provides a method for preparing high-purity 2-phenylethanol via membrane separation coupled with molecular distillation. It offers the following advantages: 1. This invention adjusts the pH of the clarified microfiltration permeate by adding an aqueous solution of sodium hexametaphosphate or trisodium citrate to the conditioning tank and dropwise adding an aqueous solution of sodium carbonate. This promotes the formation of a stable, soluble complex between the chelating agent and calcium ions, preventing the formation of inorganic salt scale when the pH value increases. Increasing the pH value alters the charge characteristics of organic matter in the feed solution and the nanofiltration membrane surface, utilizing electrostatic repulsion to slow down the adsorption and deposition of organic matter on the nanofiltration membrane surface. This composite chemical preconditioning maintains a stable permeate flux in the nanofiltration system, prevents hard scale formation, and ensures flux recovery rate.

[0020] 2. This invention provides a concentrated 2-phenylethanol solution by pumping a pre-treated feed solution with composite chemicals into a nanofiltration system, offering a feed with simple composition and controllable impurities for short-path molecular distillation. The pure feed allows the short-path molecular distillation unit to set a lower evaporator heating temperature, avoiding material degradation or polymerization losses due to heat, suppressing side reactions and reducing impurity formation, thereby improving the overall recovery rate and product purity of 2-phenylethanol.

[0021] 3. This invention completes the extraction process by coupling a cross-flow microfiltration system for clarification, a nanofiltration system for concentration, and a short-path molecular distillation unit for purification, combined with chemical conditioning operations in conditioning and neutralization tanks. The addition of citric acid aqueous solution in the neutralization tank adjusts the pH of the 2-phenylethanol concentrate, eliminating the risk of side reactions caused by alkaline feed entering the short-path molecular distillation unit. The synergistic cooperation between the separation unit and the chemical conditioning unit improves the operational stability of the preparation process and the extraction yield of the target product. Attached Figure Description

[0022] Figure 1This is a gas chromatogram comparison of the final products prepared in Example 1 and Comparative Example 1 of the present invention; Figure 2 This is a comparison chart of the turbidity of the feed solution after the composite chemical preconditioning step in Example 1 and Comparative Examples 1-3 of the present invention. Figure 3 This is a comparison graph showing the change of normalized flux over time in Example 1 of the present invention and Comparative Examples 1-3 during nanofiltration operation; Figure 4 This is a comparison chart of flux recovery rates of Example 1 and Comparative Examples 1-3 after chemical cleaning following nanofiltration membrane fouling. Figure 5 This is a comparison chart of the total recovery rate of 2-phenylethanol in the embodiments and comparative examples of the present invention. Detailed Implementation

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Examples 1-3: Example 1: This embodiment provides a method for preparing high-purity 2-phenylethanol via membrane separation coupled with molecular distillation, including the following steps: Yeast fermentation broth with an initial concentration of 3.5 g / L of 2-phenylethanol was used to clarify the broth using a cross-flow microfiltration system. The microfiltration membrane module of the cross-flow microfiltration system was a polyvinylidene fluoride (PVDF) hollow fiber membrane with a nominal pore size of 0.2 μm. The operating temperature was controlled at 25 °C, the transmembrane pressure difference was maintained at 0.1 MPa, and the cross-flow velocity on the membrane surface was 2.0 m / s. A clarified microfiltration permeate was obtained. The clarified microfiltration permeate was placed in a conditioning tank equipped with a stirrer and online pH monitoring. A pre-prepared sodium hexametaphosphate aqueous solution was added (the sodium hexametaphosphate aqueous solution was prepared by dissolving commercially available industrial-grade sodium hexametaphosphate in pure water to a concentration of 10% (w / w), so that the final effective concentration of sodium hexametaphosphate in the clarified microfiltration permeate was 200 ppm. The mixture was stirred for 15 min, and then 1.0 M sodium carbonate aqueous solution was slowly added dropwise to precisely control and stabilize the pH value of the clarified microfiltration permeate at 9.0, thus obtaining a feed solution that had undergone composite chemical preconditioning. The feed solution, which has undergone composite chemical preconditioning, is pumped into a nanofiltration system. The nanofiltration membrane of the system is a polyamide composite spiral wound membrane with a molecular weight cutoff of 200 Da. The operating temperature is maintained at 30°C and the operating pressure is set at 1.8 MPa, resulting in a 2-phenylethanol concentrate. The 2-phenylethanol concentrate was transferred to a neutralization tank, and 20% (w / w) citric acid aqueous solution was added dropwise to adjust the pH of the 2-phenylethanol concentrate back to 7.0, thus obtaining the neutralized 2-phenylethanol concentrate. The neutralized 2-phenylethanol concentrate was pumped into a short-path molecular distillation apparatus. The evaporator of the short-path molecular distillation apparatus was set to a heating temperature of 75°C, the system vacuum was maintained at 5 Pa, and the built-in condenser temperature was 15°C, thus obtaining high-purity 2-phenylethanol.

[0025] Example 2: This embodiment provides a method for preparing high-purity 2-phenylethanol via membrane separation coupled with molecular distillation, including the following steps: Yeast fermentation broth with an initial concentration of 3.5 g / L of 2-phenylethanol was used to clarify the broth using a cross-flow microfiltration system. The microfiltration membrane module of the cross-flow microfiltration system was a polyvinylidene fluoride (PVDF) hollow fiber membrane with a nominal pore size of 0.2 μm. The operating temperature was controlled at 25 °C, the transmembrane pressure difference was maintained at 0.1 MPa, and the cross-flow velocity on the membrane surface was 2.0 m / s. A clarified microfiltration permeate was obtained. The clarified microfiltration permeate was placed in a conditioning tank equipped with a stirrer and online pH monitoring. A pre-prepared sodium hexametaphosphate aqueous solution was added (the sodium hexametaphosphate aqueous solution was prepared by dissolving commercially available industrial-grade sodium hexametaphosphate in pure water to a concentration of 10% (w / w), so that the final effective concentration of sodium hexametaphosphate in the clarified microfiltration permeate was 500 ppm. The mixture was stirred for 15 min, and then 1.0 M sodium carbonate aqueous solution was slowly added dropwise to precisely control and stabilize the pH value of the clarified microfiltration permeate at 9.5, thus obtaining a feed solution that had undergone composite chemical preconditioning. The feed solution, which has undergone composite chemical preconditioning, is pumped into a nanofiltration system. The nanofiltration membrane of the system is a polyamide composite spiral wound membrane with a molecular weight cutoff of 200 Da. The operating temperature is maintained at 40°C and the operating pressure is set at 2.5 MPa, resulting in a 2-phenylethanol concentrate. The 2-phenylethanol concentrate was transferred to a neutralization tank, and 20% (w / w) citric acid aqueous solution was added dropwise to adjust the pH of the 2-phenylethanol concentrate back to 7.0, thus obtaining the neutralized 2-phenylethanol concentrate. The neutralized 2-phenylethanol concentrate was pumped into a short-path molecular distillation apparatus. The evaporator of the short-path molecular distillation apparatus was set to a heating temperature of 85°C, the system vacuum was maintained at 5 Pa, and the built-in condenser temperature was 15°C, thus obtaining high-purity 2-phenylethanol.

[0026] Example 3: This embodiment provides a method for preparing high-purity 2-phenylethanol via membrane separation coupled with molecular distillation, including the following steps: Yeast fermentation broth with an initial concentration of 3.5 g / L of 2-phenylethanol was used to clarify the broth using a cross-flow microfiltration system. The microfiltration membrane module of the cross-flow microfiltration system was a polyvinylidene fluoride (PVDF) hollow fiber membrane with a nominal pore size of 0.2 μm. The operating temperature was controlled at 25 °C, the transmembrane pressure difference was maintained at 0.1 MPa, and the cross-flow velocity on the membrane surface was 2.0 m / s. A clarified microfiltration permeate was obtained. The clarified microfiltration permeate was placed in a conditioning tank equipped with a stirrer and online pH monitoring. A pre-prepared trisodium citrate aqueous solution was added (the trisodium citrate aqueous solution was prepared by dissolving commercially available industrial-grade trisodium citrate in pure water to a concentration of 10% (w / w), so that the final effective concentration of trisodium citrate in the clarified microfiltration permeate was 50 ppm. The mixture was stirred for 15 min, and then 1.0 M sodium carbonate aqueous solution was slowly added dropwise to precisely control and stabilize the pH value of the clarified microfiltration permeate at 8.5, thus obtaining a feed solution that had undergone composite chemical preconditioning. The feed solution, which has undergone composite chemical preconditioning, is pumped into a nanofiltration system. The nanofiltration membrane of the system is a polyamide composite spiral wound membrane with a molecular weight cutoff of 200 Da. The operating temperature is maintained at 20°C and the operating pressure is set at 1.0 MPa, resulting in a 2-phenylethanol concentrate. The 2-phenylethanol concentrate was transferred to a neutralization tank, and 20% (w / w) citric acid aqueous solution was added dropwise to adjust the pH of the 2-phenylethanol concentrate back to 7.0, thus obtaining the neutralized 2-phenylethanol concentrate. The neutralized 2-phenylethanol concentrate was pumped into a short-path molecular distillation apparatus. The evaporator of the short-path molecular distillation apparatus was set to a heating temperature of 60°C, the system vacuum was maintained at 5 Pa, and the built-in condenser temperature was 15°C, thus obtaining high-purity 2-phenylethanol.

[0027] Comparative Examples 1-3: Comparative Example 1: The difference from Example 1 is that no chemical preconditioning treatment is performed on the clarified microfiltration permeate before nanofiltration preconcentration; otherwise, they are the same.

[0028] Comparative Example 2: Compared with Example 1, the difference is that in the composite chemical preconditioning step, only the pH of the clarified microfiltration permeate is adjusted to 9.0, without adding an aqueous solution of sodium hexametaphosphate; all other steps are the same.

[0029] Comparative Example 3: Compared with Example 1, the difference is that in the composite chemical preconditioning step, sodium hexametaphosphate aqueous solution is only added to the clarified microfiltration permeate until the final effective concentration is 200 ppm, without pH adjustment treatment, and the rest are the same.

[0030] Test Examples 1-5: Test Example 1: The experimental steps are as follows: The final products obtained in Example 1 and Comparative Example 1 were taken respectively, and each sample was diluted 100 times with methanol as solvent to prepare test solutions for gas chromatography-mass spectrometry (GC-MS) and gas chromatography (GC) analysis.

[0031] Qualitative analysis of each sample was performed by GC-MS. The instrument used was a gas chromatograph-mass spectrometer, and the chromatographic column was an HP-5MS capillary column (30m × 0.25mm × 0.25μm). The injection port temperature was set to 250℃, the carrier gas was high-purity helium, and the flow rate was 1.0mL / min.

[0032] The column temperature program was set as follows: initial temperature 50℃, hold for 2 min, then increase to 250℃ at a rate of 10℃ / min, hold for 5 min. The mass spectrometer used an electron impact (EI) ion source with an ionization energy of 70 eV and a mass scan range of 30-350 amu. The retention times of the main chromatographic peaks of each sample were compared with those of the 2-phenylethanol standard, and their mass spectra were matched with the National Institute of Standards and Technology (NIST) to identify the constituent substances.

[0033] GC quantitative analysis was performed on each sample. A gas chromatograph equipped with a flame ionization detector (FID) was used, and the chromatographic conditions were the same as those in step 2. The peak area percentage of each component was calculated using the area normalization method to determine the mass percentage of the main component, 2-phenylethanol.

[0034] The experimental data are as follows: Table 1. Comparative analysis results of product composition and purity between Example 1 and Comparative Example 1 Figure 1 The horizontal axis represents retention time in minutes; the vertical axis represents detector signal intensity. The solid line represents the chromatogram of the product from Example 1, and the dashed line represents the chromatogram of the product from Comparative Example 1. The main peak at a retention time of 10.42 min is the target product 2-phenylethanol.

[0035] The test results are as follows: Based on the data in Table 1 and Figure 1 The presented gas chromatographic comparison chromatograms can be used to thoroughly evaluate the effectiveness of the method of the present invention.

[0036] Figure 1 The compositional differences between the final products of Example 1 and Comparative Example 1 are visually presented. The spectrum of Example 1 (solid blue line) shows a sharp and symmetrical main peak at 10.42 min, corresponding to 2-phenylethanol. Simultaneously, the response values ​​of other impurity peaks in the spectrum are very low, indicating high product purity. In contrast, the spectrum of Comparative Example 1 (dashed red line) not only shows a decrease in the absolute response intensity of the 2-phenylethanol main peak, but also an increase in the area and number of impurity peaks at positions such as 9.71 min, 11.05 min, and the newly appearing 11.8 min.

[0037] This difference is directly attributed to the omission of the complex chemical preconditioning step in Comparative Example 1. Without preconditioning, membrane fouling and concentration polarization problems are exacerbated in the nanofiltration process, which not only reduces treatment efficiency but may also lead to prolonged residence time of the feed solution on the membrane surface or excessively high local concentrations. This unstable operating environment creates conditions for the degradation of heat-sensitive substances or side reactions during subsequent high-temperature molecular distillation, resulting in more impurities and affecting the final yield of the target product.

[0038] therefore, Figure 1 The data not only confirms that the method of this invention can successfully prepare the target product, but also, through direct comparison with comparative examples, demonstrates the crucial role of the core technical step of composite chemical preconditioning in suppressing impurity formation and ensuring the purity of the final product. The preconditioning scheme proposed in this invention, by stabilizing the front-end membrane separation process, provides a purer feed for the back-end distillation, which is a necessary prerequisite for achieving the preparation of high-purity 2-phenylethanol. Test Example 2:

[0039] The experimental steps are as follows: Take 50 mL of the feed solution sample from each of Examples 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 after completing step (2) (or the corresponding step) and before entering the nanofiltration system. Among them, the sample of Comparative Example 1 is a clear microfiltration permeate that has not undergone any chemical conditioning.

[0040] After thoroughly mixing each liquid sample, take a sample immediately and measure the turbidity value of each liquid using a calibrated turbidimeter. The unit is NTU. Each sample is measured three times, and the average value is taken.

[0041] Centrifuge the remaining liquid samples at 4000 rpm for 10 min and carefully aspirate the supernatant.

[0042] Soluble calcium ions (Ca) in each supernatant were analyzed using atomic absorption spectrometry (AAS). 2+ Quantitative analysis of calcium concentration was performed. A standard curve was plotted using calcium standard solution, and the calcium content in each sample was calculated based on the standard curve. 2+concentration.

[0043] The experimental data are as follows: Table 2. Effects of each preconditioning regimen on turbidity and soluble calcium ion concentration of the feed solution. Figure 2 The horizontal axis represents different sample sources, and the vertical axis represents turbidity, in NTU. The stems and markers in the figure represent the turbidity values ​​measured for each sample.

[0044] The test results are as follows: Based on the data in Table 2 and Figure 2 The results show that the core mechanism of the composite chemical preconditioning step of this invention has been verified.

[0045] In Comparative Example 2, simply raising the pH of the clarified microfiltration permeate to 9.0 caused the turbidity of the feed solution to increase from 1.85 NTU at the baseline (Comparative Example 1) to 189.4 NTU, while the soluble Ca in the supernatant also increased. 2+ The concentration decreased from 90.2 mg / L to 6.4 mg / L. This phenomenon is evidence that divalent metal ions such as calcium and magnesium, which are commonly present in fermentation broth, react with anions such as phosphate and carbonate under alkaline conditions to form insoluble salt precipitates.

[0046] As a control, Comparative Example 3, with the addition of a chelating agent alone without pH adjustment, exhibited turbidity similar to that of Ca. 2+ The concentrations were essentially the same as in the initial state (Comparative Example 1), indicating that the chelating agent itself does not cause any change in the solution properties. However, in Example 1, by adding the chelating agent first and then adjusting the pH, although the final pH was the same as in Comparative Example 2, the solution remained clear with a turbidity of only 2.13 NTU, and the soluble Ca in the solution was maintained at a high level of 85.7 mg / L. 2+ Concentration. This indicates that the pre-added sodium hexametaphosphate successfully reacted with Ca. 2+ A stable soluble complex was formed, preventing precipitation from occurring upon subsequent pH increases.

[0047] Therefore, this series of comparative experiments not only reproduced the technical difficulties encountered when the method of this invention is lacking (i.e., scaling under alkaline conditions), but also confirmed that the pre-protection of metal ions through chelation is the solution to this problem. This pre-conditioning step removes the most significant obstacle to the long-term stable operation of the subsequent nanofiltration membrane and ensures the smooth implementation of the entire process.

[0048] Test Example 3: The experimental steps are as follows: The feed solutions from Example 1 and Comparative Examples 1-3 were pumped into the same nanofiltration system, which had undergone standard chemical cleaning and had its initial pure water flux measured, under the nanofiltration operating conditions of their respective examples / comparative examples.

[0049] During the continuous operation of the nanofiltration system for 480 minutes, the volume of permeate flowing out per unit time, as well as the feed pressure and temperature at that time, were recorded every 60 minutes.

[0050] Based on the recorded data, the membrane permeation flux at each time point is calculated and recorded.

[0051] The membrane permeate flux at each time point is compared with the initial flux at the start of the run to calculate the normalized flux and assess the degradation of membrane flux. After each test, the membrane module is thoroughly chemically cleaned until the pure water flux recovers to more than 98% of its initial value before proceeding to the next set of experiments.

[0052] The experimental data are as follows: Table 3. Variation of normalized flux in nanofiltration for each scheme Figure 3 The horizontal axis represents the running time in minutes; the vertical axis represents the normalized flux. Different line types and markers distinguish the data groups in the figure: solid lines with dots represent Example 1, dashed lines with squares represent Comparative Example 1, dotted lines with triangles represent Comparative Example 2, and dotted lines with diamonds represent Comparative Example 3.

[0053] The test results are as follows: Based on the data in Table 3 and Figure 3 As can be seen from the curve, the preconditioning method used in this invention plays a decisive role in maintaining the stability of the nanofiltration system.

[0054] Figure 3 The curves in the figure illustrate the evolution of membrane performance over time under different treatment methods. The curve of Example 1 showed high stability during a continuous run of up to 480 minutes, with its normalized flux consistently remaining above 0.94 and exhibiting very small attenuation. This indicates that the composite chemical preconditioning method used in this invention, namely the synergistic effect of chelating agents and pH regulation, inhibited the occurrence of membrane fouling.

[0055] In contrast, the curve for Comparative Example 2 showed a rapid decline in flux shortly after the start of operation, dropping to below 30% of the initial value after 180 minutes and quickly reaching a very low value. This result confirms that directly increasing the pH value without chelating divalent ions leads to severe inorganic salt scaling, rapidly clogging the membrane pores and making the separation process unsustainable.

[0056] The performance of Comparative Example 1 (no treatment) and Comparative Example 3 (with chelating agent only) falls between the two. Comparative Example 3, having solved the inorganic scaling problem, exhibits better initial flux stability than Comparative Example 1, but still shows a certain flux decline over time. This is due to the adsorption and deposition of organic matter such as proteins and polysaccharides in the fermentation broth on the membrane surface. Comparative Example 1, on the other hand, faces the dual effects of organic fouling and potential inorganic scaling, resulting in a more severe flux decline.

[0057] Comprehensive analysis, Figure 3 The results not only demonstrated that the lack of preconditioning (Comparative Example 1) or improper preconditioning (Comparative Example 2) leads to rapid fouling of the nanofiltration membrane, but also revealed that the chelation and pH adjustment steps in this invention are indispensable. This composite conditioning fundamentally eliminates the risk of inorganic fouling and, by adjusting the pH, alters the charge state of the membrane surface and pollutant molecules, mitigating organic fouling through electrostatic repulsion, ultimately ensuring the nanofiltration unit can achieve stable, long-term, and highly efficient operation.

[0058] Test Example 4: The experimental steps are as follows: In Test Example 3, the nanofiltration membrane modules that had been continuously run for 480 minutes using the feed solutions from Example 1 and Comparative Examples 1-3 were removed from the system.

[0059] A standard chemical cleaning procedure was performed on each contaminated membrane module. First, a 0.2% (w / w) citric acid aqueous solution was circulated at 35°C for 30 min; then rinsed with pure water until neutral. Next, a 0.1% (w / w) sodium hydroxide aqueous solution was circulated at 35°C for 30 min; finally, the system was rinsed with pure water until neutral.

[0060] The cleaned membrane module was reinstalled into the nanofiltration system, and its pure water permeation flux was measured under the same operating conditions as the initial test.

[0061] The pure water flux measured after cleaning is compared with the initial pure water flux of the membrane module before fouling. The flux recovery rate is calculated and recorded as follows: Flux recovery rate = (pure water flux after cleaning / initial pure water flux) × 100.

[0062] The experimental data are as follows: Table 4. Comparison of flux recovery rates after nanofiltration membrane fouling for each scheme Figure 4 The horizontal axis represents different sample sources, and the vertical axis represents flux recovery rate, in percentage (%). The vertical lines and top markers in the figure together indicate the performance recovery level of the membrane module corresponding to each sample after cleaning.

[0063] The test results are as follows: Based on the data in Table 4 and Figure 4 As a result, the method of the present invention is of great value in ensuring the renewability and long service life of nanofiltration membranes.

[0064] Figure 4 The impact of different pretreatment methods on membrane cleaning performance was clearly quantified. The membrane module treated in Example 1 achieved a flux recovery rate of 98.7%, indicating that the fouling layer formed during operation is primarily reversible and can be effectively removed through standard chemical cleaning procedures. This high recovery rate directly reflects the process's good economic efficiency and sustainability.

[0065] In contrast, Comparative Example 2 reflects a serious problem, with a flux recovery rate of only 24.6%. This result is consistent with the rapid flux decay observed in Test Example 3, both pointing to the conclusion that the inorganic scale formed directly under high pH conditions is a chemically stable and firmly bonded hard scale to the membrane surface. This hard scale cannot be effectively removed by conventional acid and alkali cleaning, leading to a permanent loss of membrane flux, which is unacceptable in industrial applications.

[0066] The results of Comparative Examples 1 and 3 provide a more in-depth comparison. The flux recovery rate of Comparative Example 3 (with chelating agent only) (91.5%) was superior to that of Comparative Example 1 (untreated, 81.3%), indicating that eliminating inorganic fouling is key to improving membrane cleaning performance. However, even without inorganic fouling, the recovery rate of Comparative Example 3 was still lower than that of Example 1. This suggests that the pH adjustment step in Example 1, in addition to preventing fouling, may have reduced the irreversible binding of organic matter to the membrane by altering the charge characteristics of organic matter in the feed solution and the membrane surface, thereby making the organic fouling layer easier to clean.

[0067] Therefore, the conclusion of Test Example 4 is that the value of the composite chemical preconditioning scheme of the present invention lies not only in the suppression of membrane fouling during operation, but also in its prevention of the formation of irreversible fouling (especially inorganic scale) that is difficult to remove from the source, ensuring that the membrane element can be restored to its initial performance through routine maintenance, which is crucial for reducing replacement costs and extending the service life of the equipment.

[0068] Test Example 5: Experimental steps: For each complete process run of Examples 1-3 and Comparative Examples 1-3, the initial volume of the added yeast fermentation stock solution and the initial 2-phenylethanol concentration measured by GC analysis were accurately recorded.

[0069] After each process step is completed, all the obtained high-purity 2-phenylethanol products are collected and their final mass is accurately weighed.

[0070] The purity of the final product, i.e., the mass percentage of 2-phenylethanol, is determined by GC analysis.

[0071] The method for calculating the total recovery rate of 2-phenylethanol is as follows: multiply the final high-purity product mass by the purity of the product to obtain the mass of pure 2-phenylethanol; then divide this mass value by the product of the initial yeast fermentation liquid volume and the initial concentration of 2-phenylethanol in the liquid (i.e., the total mass of 2-phenylethanol contained in the raw material); finally, multiply the obtained ratio by 100.

[0072] Experimental data: Table 5. Comparison of total recovery rates of 2-phenylethanol for each scheme Figure 5 The horizontal axis represents the different sources of 2-phenylethanol, and the vertical axis represents the total recovery rate of 2-phenylethanol, in percentage. The solid dots in the figure represent the final calculated total recovery rate values ​​for each scheme.

[0073] The test results are as follows: Based on the data in Table 5 and Figure 5 As a result, the method of the present invention demonstrates a clear advantage in overall process efficiency.

[0074] The data reveals a clear grouping: the overall recovery rates of Examples 1-3 consistently remained above 90%, while the recovery rates of all comparative examples were below this level. This macroscopic efficiency difference is the ultimate manifestation of the cumulative and amplified effects of the front-end pretreatment throughout the entire process chain. Examples 1-3, through effective composite chemical preconditioning, ensured the stable and efficient operation of the nanofiltration unit. This not only meant minimal 2-phenylethanol retention loss in the concentration step, but more importantly, it provided a high-quality feedstock with simple components and controllable impurities for the subsequent molecular distillation unit. The pure feedstock allows molecular distillation to operate under milder conditions, avoiding the thermal degradation or polymerization loss of the target product caused by extending heating time or increasing evaporation temperature to handle complex feed solutions.

[0075] The results of the comparative examples, however, confirm this from the opposite perspective. The recovery rate of Comparative Example 2 was only 15.1%, which is close to the level of process failure. This corresponds to the severe membrane fouling and irreversible performance loss exhibited in Test Examples 3 and 4, indicating that this route is not feasible in practice.

[0076] While the recovery rates of Comparative Examples 1 and 3 (72.8% and 83.4%, respectively) were significantly higher than those of Comparative Example 2, they still lagged considerably behind those of the Example. This indicates that even without catastrophic inorganic fouling, progressive membrane fouling caused by organic matter (as in Comparative Example 3) or the coexistence of multiple fouling factors (as in Comparative Example 1) can still continuously erode the final product yield by reducing separation efficiency and potentially introducing side reactions.

[0077] Therefore, the high recovery rate of 2-phenylethanol is not merely the result of optimizing a single unit operation, but rather a comprehensive reflection of the systematic advantages of the overall process design of this invention. It demonstrates that the value of composite chemical preconditioning, as a core step, is consistent throughout the process. By ensuring the stability of the front-end separation, it lays the foundation for achieving high efficiency in the back-end distillation, ultimately forming the cornerstone of the economic and technical feasibility of the entire process route.

Claims

1. A method for preparing high-purity 2-phenylethanol via membrane separation coupled with molecular distillation, characterized in that, Includes the following steps: The yeast fermentation broth was clarified using a cross-flow microfiltration system to obtain a clarified microfiltration permeate. The clarified microfiltration permeate was placed in a conditioning tank, and an aqueous solution of sodium hexametaphosphate or trisodium citrate was added and stirred. An aqueous solution of sodium carbonate was added dropwise to adjust the pH value of the clarified microfiltration permeate, thus obtaining a feed solution that had undergone composite chemical preconditioning. The pre-treated feed solution was pumped into a nanofiltration system to obtain a concentrated 2-phenylethanol solution. The 2-phenylethanol concentrate was transferred to a neutralization tank, and citric acid aqueous solution was added dropwise to adjust the pH value of the 2-phenylethanol concentrate, thus obtaining a neutralized 2-phenylethanol concentrate. The neutralized 2-phenylethanol concentrate was pumped into a short-path molecular distillation apparatus to obtain high-purity 2-phenylethanol.

2. The method for preparing high-purity 2-phenylethanol by membrane separation coupled with molecular distillation according to claim 1, characterized in that, The microfiltration membrane module of the cross-flow microfiltration system is a polyvinylidene fluoride (PVDF) hollow fiber membrane, and the nominal pore size of the polyvinylidene fluoride (PVDF) hollow fiber membrane is 0.2 μm.

3. The method for preparing high-purity 2-phenylethanol by membrane separation coupled with molecular distillation according to claim 1, characterized in that, When clarifying the yeast fermentation broth using the cross-flow microfiltration system, the operating temperature is controlled at 25°C, the transmembrane pressure difference is maintained at 0.1 MPa, and the cross-flow velocity on the membrane surface is 2.0 m / s.

4. The method for preparing high-purity 2-phenylethanol by membrane separation coupled molecular distillation according to claim 1, characterized in that, When the sodium hexametaphosphate aqueous solution is added, the sodium hexametaphosphate aqueous solution is prepared by dissolving commercially available industrial-grade sodium hexametaphosphate in pure water to a concentration of 10% (w / w); When the trisodium citrate aqueous solution is added, the trisodium citrate aqueous solution is prepared by dissolving commercially available industrial-grade trisodium citrate in pure water to a concentration of 10% (w / w).

5. The method for preparing high-purity 2-phenylethanol by membrane separation coupled molecular distillation according to claim 1, characterized in that, Add the sodium hexametaphosphate aqueous solution so that the final effective concentration of sodium hexametaphosphate in the clarified microfiltration permeate is 200 ppm to 500 ppm. Alternatively, the aqueous solution of trisodium citrate may be added so that the final effective concentration of trisodium citrate in the clarified microfiltration permeate is 50 ppm.

6. The method for preparing high-purity 2-phenylethanol by membrane separation coupled with molecular distillation according to claim 1, characterized in that, The conditioning tank is equipped with a stirrer and online pH monitoring. When adding the sodium carbonate aqueous solution, 1.0M sodium carbonate aqueous solution is added slowly to precisely control and stabilize the pH value of the clarified microfiltration permeate at 8.5 to 9.

5.

7. The method for preparing high-purity 2-phenylethanol by membrane separation coupled with molecular distillation according to claim 1, characterized in that, The nanofiltration membrane of the nanofiltration system is a polyamide composite spiral wound membrane with a molecular weight cutoff of 200 Da.

8. The method for preparing high-purity 2-phenylethanol by membrane separation coupled molecular distillation according to claim 1, characterized in that, When the pre-treated feed solution is pumped into the nanofiltration system, the operating temperature is maintained between 20°C and 40°C, and the operating pressure is set between 1.0 MPa and 2.5 MPa.

9. The method for preparing high-purity 2-phenylethanol by membrane separation coupled with molecular distillation according to claim 1, characterized in that, When adding the citric acid aqueous solution, add 20% (w / w) of the citric acid aqueous solution to adjust the pH of the 2-phenylethanol concentrate back to 7.

0.

10. The method for preparing high-purity 2-phenylethanol by membrane separation coupled molecular distillation according to claim 1, characterized in that, When the neutralized 2-phenylethanol concentrate is pumped into the short-path molecular distillation apparatus, the evaporator heating temperature of the short-path molecular distillation apparatus is set to 60°C to 85°C, the system vacuum is maintained at 5Pa, and the built-in condenser temperature is 15°C.

Images