Acrylamide preparation process capable of improving purity

Abstract

The invention relates to the field of fine chemical engineering, and discloses an acrylamide preparation process capable of improving purity, which takes an acrylamide initial aqueous solution prepared by a biological method as a raw material, and comprises the following steps in sequence: firstly, removing most ion impurities through ion exchange resin; then carrying out low-temperature and deoxidation pretreatment on the feed liquid to inhibit spontaneous polymerization of acrylamide monomers; then, macromolecular impurities such as residual protein are efficiently removed by adjusting the pH value, adding a cation trapping agent and combining the synergistic effect of microbubble flotation; then, the flotation clarified liquid is subjected to nanofiltration membrane concentration and reverse osmosis membrane refining in sequence; and finally, carrying out deep polishing treatment through mixed bed resin to obtain the high-purity acrylamide aqueous solution. According to the method disclosed by the invention, low-temperature deoxidation and a pH regulation-flotation technology are combined, so that the technical problems that acrylamide is easy to polymerize and biological impurities are difficult to remove are effectively solved, the yield and the purity of the product are improved, and an electronic-grade product with extremely low conductivity can be obtained.

Description

Technical Field

[0001] This invention relates to the field of fine chemicals, and in particular to a process for preparing acrylamide with improved purity. Background Technology

[0002] Acrylamide, as an important fine chemical raw material, is widely used in water treatment, papermaking, mineral processing, and the electronics industry, especially as a core monomer in the production of polyacrylamide. Currently, the biological method for preparing acrylamide by catalyzing the hydration of acrylonitrile using bio-enzymes has become the mainstream production technology due to its advantages such as mild reaction conditions, high selectivity, and low energy consumption.

[0003] However, the purification process of the initial aqueous solution of acrylamide prepared by biological methods still faces challenges. This feed system is quite complex, containing not only the target product acrylamide but also inevitably residual macromolecular impurities such as proteins and enzymes acting as biocatalysts, as well as various inorganic salt ions. Furthermore, acrylamide monomers themselves are highly reactive and readily undergo spontaneous polymerization during purification processes (such as heating and concentration), leading to decreased product yield and the introduction of polymer impurities. This directly affects the quality and application range of the final product.

[0004] Existing technologies have limitations in handling such complex liquid solutions. Conventional purification methods struggle to simultaneously and efficiently address both the stability issues of acrylamide and the deep removal of biomacromolecules. Traditional heat-sensitive methods, such as heating and concentration, exacerbate the polymerization side reactions of acrylamide, while conventional filtration or sedimentation methods are ineffective at removing trace, dispersed protein impurities. To obtain high-purity acrylamide products with extremely low conductivity suitable for high-end applications such as the electronics industry, the removal of ionic impurities from the liquid solution must be extremely thorough. This poses a significant technical challenge to the systematic nature and depth of existing purification processes.

[0005] Therefore, this invention proposes a process for preparing acrylamide with improved purity to address the shortcomings of existing technologies. Summary of the Invention

[0006] The purpose of this invention is to provide a process for preparing acrylamide with improved purity, which solves the problems of easy polymerization leading to decreased yield and difficulty in deep removal of biomolecules and ionic impurities in the biological production of acrylamide.

[0007] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a process for preparing acrylamide with improved purity, the process comprising the following steps: S1. The acrylamide aqueous solution obtained by bio-enzyme catalytic hydration and ultrafiltration separation is subjected to ion exchange treatment to obtain the ion-exchanged feed solution. S2. Cool the ion-exchange solution to 5-10°C and deoxygenate it at this temperature until the dissolved oxygen concentration is below 0.5 mg / L to obtain a low-temperature, low-oxygen solution. S3. The pH of the low-temperature and low-oxygen feed solution is adjusted to cause protein impurities to flocculate, and then inert gas microbubbles are introduced into it for flotation separation to obtain a flotation clarified solution. S4. The flotation clarified liquid is concentrated by passing it through a nanofiltration membrane and purified by a reverse osmosis membrane to obtain a high-purity acrylamide aqueous solution.

[0008] In this technical solution, step S1, which involves treating the acrylamide aqueous solution obtained by bio-enzyme catalytic hydration and ultrafiltration with ion exchange to obtain the ion-exchanged solution, includes: passing the acrylamide aqueous solution sequentially through a cation exchange column packed with a strong acid cation exchange resin and an anion exchange column packed with a weak base anion exchange resin to remove cation and anion impurities from the aqueous solution and control the conductivity of the ion-exchanged solution to be below 20 μS / cm.

[0009] Step S2 has a specific technical function. Cooling the ion-exchange solution to a low-temperature environment of 5-10°C and controlling its dissolved oxygen concentration to below 0.5 mg / L can inhibit the spontaneous thermal polymerization reaction of acrylamide monomers in subsequent treatment processes, thereby reducing the generation of macromolecular impurities such as polyacrylamide and maintaining the stability of the solution.

[0010] The combined operation in step S3 is the core of achieving efficient protein removal. First, the pH of the low-temperature, low-oxygen feed solution is adjusted to a specific range, such as 4.0–5.0. This pH range is close to the isoelectric point of protein impurities remaining from the biocatalytic process. Under these conditions, the net surface charge of protein molecules decreases, intermolecular repulsion weakens, and solubility decreases, leading to aggregation and the formation of micro-flocs. Subsequently, inert gases (such as nitrogen or argon) are introduced into the feed solution to generate microbubbles with a diameter of 10–50 μm. These microbubbles have a large specific surface area and can act as carriers to effectively capture and adsorb the protein flocs already formed in the water. Under the action of buoyancy, the microbubbles carry the protein impurities to the surface, forming a scum layer. By separating the scum layer, a flotation clarified solution free of large-molecule protein impurities can be obtained.

[0011] In a specific technical solution, to further enhance the flocculation effect of protein impurities in step S3, a cationic scavenger is added to the low-temperature, low-oxygen feed solution before introducing the inert gas microbubbles. The cationic scavenger, for example, is chitosan, added in such a way that its final concentration in the low-temperature, low-oxygen feed solution is 1–10 mg / L. Chitosan with a degree of deacetylation of 85%–95% is selected, as the amino groups on its molecular chain are protonated and become positively charged under acidic conditions. This allows it to neutralize charges and bridging negatively charged protein flocs into larger, denser flocs through charge neutralization and adsorption, thereby significantly improving the removal efficiency of microbubble flotation.

[0012] Step S4 employs a two-stage membrane separation system for further treatment of the flotation clarified solution. First, concentration is achieved using a nanofiltration membrane, for example, at a cross-flow rate of 2.0–4.0 m / s and an operating pressure of 0.5–1.5 MPa. The nanofiltration membrane can retain acrylamide molecules while allowing most water molecules and some monovalent salt ions to pass through, achieving effective concentration of the acrylamide solution at room temperature and avoiding polymerization problems that may occur with heating concentration. Subsequently, the nanofiltration concentrate is purified using a reverse osmosis membrane, for example, at a cross-flow rate of 0.5–1.5 m / s and an operating pressure of 2.0–4.0 MPa. The reverse osmosis membrane has a higher rejection rate, further removing water and residual trace small-molecule impurities, thereby obtaining a high-concentration acrylamide aqueous solution.

[0013] To obtain a product with extremely high purity, the preparation process may further include step S5: polishing the high-purity acrylamide aqueous solution through a mixed-bed resin. The mixed-bed resin is composed of a strongly acidic cation exchange resin and a strongly basic anion exchange resin, which can deeply remove residual trace ions from the solution, resulting in an extremely low conductivity of the final product.

[0014] In the above scheme, the ion exchange treatment sequentially passes through a strongly acidic cation exchange resin and a weakly basic anion exchange resin to remove most of the cations and anions in the feed solution. The nanofiltration membrane is a polyamide membrane composite nanofiltration membrane with a molecular weight cutoff of 100–200 Da, which is used to retain acrylamide molecules in the concentration operation of step S4. The reverse osmosis membrane is a polyamide membrane composite reverse osmosis membrane with a sodium chloride removal rate greater than 99%, which is used to remove residual dissolved ionic impurities in the purification operation of step S4 to ensure treatment efficiency and the chemical stability of the membrane element.

[0015] A second aspect of the present invention provides a high-purity acrylamide aqueous solution, which is prepared by any of the aforementioned preparation processes. This high-purity acrylamide aqueous solution comprises the following components in parts by weight: Acrylamide: 45-55 parts; Water: 45-55 parts; Acrylic acid and other ionizable impurities: 0 to 0.01 parts; Residual protein and macromolecular impurities: 0 to 0.0001 parts.

[0016] The composition of this product clearly reflects the technical effectiveness of the process of this invention. The upper limit of the content of acrylic acid and other ionizable impurities (0.01 parts) corresponds to extremely low conductivity, indicating that the ion exchange and membrane separation system thoroughly removes ionic impurities. The upper limit of the content of residual proteins and macromolecular impurities (0.0001 parts) directly demonstrates the effectiveness of the combined process of low-temperature deoxygenation pretreatment and pH adjustment-collection-microbubble flotation in removing residual biocatalysts.

[0017] In summary, the present invention has at least one of the following beneficial technical effects: 1. This invention effectively suppresses the spontaneous polymerization reaction initiated by the high reactivity of acrylamide monomers by pretreating the feed solution at low temperature and with deoxygenation before the core purification step. This measure reduces the generation of polymer impurities at the source, ensures the stability of the feed solution throughout the entire process, and removes a key obstacle to obtaining high-purity products.

[0018] 2. This invention employs a two-stage treatment strategy combining pH adjustment and microbubble flotation to achieve highly efficient and targeted removal of macromolecular impurities such as residual proteins from biocatalysis processes. By first adjusting the pH value to alter the surface charge of protein molecules to promote flocculation, and then using inert gas microbubbles for capture and flotation separation, this synergistic effect significantly enhances the purification depth of biomacromolecular impurities.

[0019] 3. This invention constructs a multi-stage, end-to-end deep desalination system, from initial ion exchange and membrane purification to final mixed-bed resin polishing. This system can systematically remove various cations, anions, and ionizable impurities from aqueous solutions, ultimately resulting in a product with extremely low conductivity and achieving the high electrical purity required for electronic applications.

[0020] 4. This invention further enhances the efficiency of the microbubble flotation step by introducing a cationic chelating agent to bridge and trap flocculated proteins. The application of this chelating agent can aggregate tiny protein flocs into larger flocs that are more easily carried by bubbles, significantly improving solid-liquid separation and ensuring more thorough removal of protein impurities.

[0021] 5. This invention utilizes a two-stage membrane separation system of nanofiltration and reverse osmosis to concentrate and purify acrylamide aqueous solution under ambient temperature conditions. This non-thermal purification pathway not only completely avoids the quality degradation and impurity generation problems that may be caused by traditional heating concentration, but also significantly reduces energy consumption in the production process, making the process more economical and environmentally friendly. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the preparation process of the present invention. Detailed Implementation

[0023] The following is in conjunction with the appendix Figure 1 The present invention will be further described in detail below.

[0024] Experimental materials: The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0025] The initial aqueous solution of acrylamide was obtained by enzymatic hydration of acrylonitrile and ultrafiltration separation, wherein the mass concentration of acrylamide was 50% and the residual protein concentration was approximately 100 mg / kg.

[0026] Strongly acidic cation exchange resin (hydrogen form): styrene-divinylbenzene copolymer matrix, sulfonic acid functional groups, CAS No.: 69011-20-7.

[0027] Weakly basic anion exchange resin (free basic type): styrene-divinylbenzene copolymer matrix, tertiary amine functional groups.

[0028] Strongly basic anion exchange resin (hydrogen-oxygen type): styrene-divinylbenzene copolymer matrix, quaternary ammonium functional groups, CAS No.: 69011-18-3.

[0029] Chitosan: Deacetylation degree is 85%-95%, CAS No.: 9012-76-4.

[0030] High-purity nitrogen (Nitrogen-Gas): Purity ≥ 99.999%, CAS No.: 7727-37-9.

[0031] Sulfuric acid: analytical grade, mass concentration 98%, CAS number: 7664-93-9.

[0032] Sodium hydroxide: analytical grade, CAS No.: 1310-73-2.

[0033] Acrylamide Standard: Chromatographic grade, purity ≥99.9%, CAS No.: 79-06-1.

[0034] Bovine serum albumin (BSA): biochemical reagent grade, used for protein content determination standard curve, CAS No.: 9048-46-8.

[0035] Coomassie Brilliant Blue G-250: Biochemical reagent grade, CAS No.: 6104-58-1.

[0036] Methanol: chromatographic grade, CAS No.: 67-56-1.

[0037] Phosphoric acid: analytical grade, mass concentration 85%, CAS No.: 7664-38-2.

[0038] Example 1: A process for preparing acrylamide with improved purity, the specific steps of which are as follows: S1. Take 1000 kg of the initial aqueous solution of acrylamide obtained by bio-enzyme catalytic hydration and ultrafiltration separation, and pass it sequentially through a pretreated hydrogen-form strong acid cation exchange resin column and a free base-form weak base anion exchange resin column, controlling the flow rate of the feed liquid to be 2 times the bed volume / hour, and collect the treated ion exchange feed liquid.

[0039] S2. The ion-exchange solution collected in step S1 is transferred to a jacketed, cooled, sealed storage tank to lower the solution temperature to 8°C and maintain it. High-purity nitrogen is then introduced into the solution for bubbling deoxygenation until the online dissolved oxygen detector reading stabilizes at 0.3 mg / L, resulting in a low-temperature, low-oxygen solution.

[0040] S3. Slowly add dilute sulfuric acid dropwise to the low-temperature, low-oxygen feed solution from step S2 while stirring, adjusting the pH to 4.5. Then, add chitosan solution to achieve a final chitosan concentration of 5 mg / L. Pump the adjusted feed solution into a microbubble flotation column, introducing high-purity nitrogen microbubbles with an average diameter of 30 μm at a flow rate of 0.5 L / min. Collect the clarified flotation liquid flowing from the bottom of the flotation column and scrape off the top scum.

[0041] S4. The flotation clarified solution obtained in step S3 is fed into a membrane separation system. First, it is concentrated using a polyamide membrane composite nanofiltration membrane, with operating parameters set to a cross-flow velocity of 3.0 m / s and an operating pressure of 1.0 MPa. Then, the concentrated solution from the nanofiltration membrane is fed into a polyamide membrane composite reverse osmosis membrane for purification, with operating parameters set to a cross-flow velocity of 1.0 m / s and an operating pressure of 3.0 MPa.

[0042] S5. The high-purity acrylamide aqueous solution obtained after purification by reverse osmosis membrane in step S4 is polished by passing it through a mixed bed resin column composed of hydrogen-form strong acid cation exchange resin and hydroxide-form strong base anion exchange resin at a flow rate of 3 times the bed volume / hour to obtain the final product P1.

[0043] Example 2: A process for preparing acrylamide with improved purity, the specific steps of which are as follows: S1. The ion exchange treatment steps are the same as in Example 1.

[0044] S2. The temperature of the ion-exchange feed solution collected in step S1 is lowered to 5°C and maintained. High-purity nitrogen gas is introduced into the feed solution for deoxygenation until the dissolved oxygen concentration drops to 0.4 mg / L, resulting in a low-temperature, low-oxygen feed solution.

[0045] S3. Adjust the pH of the low-temperature, low-oxygen feed solution from step S2 to 4.0. Then, add chitosan to achieve a final concentration of 1 mg / L. Introduce high-purity nitrogen microbubbles with an average diameter of 10 μm for flotation separation, and collect the clarified flotation solution.

[0046] S4. The flotation clarified solution obtained in step S3 is fed into the membrane separation system. The operating parameters for nanofiltration concentration are set to a cross-flow velocity of 2.0 m / s and an operating pressure of 0.5 MPa. The operating parameters for reverse osmosis purification are set to a cross-flow velocity of 0.5 m / s and an operating pressure of 2.0 MPa.

[0047] S5. The polishing process is the same as in Example 1, resulting in the final product P2.

[0048] Example 3: A process for preparing acrylamide with improved purity, the specific steps of which are as follows: S1. The ion exchange treatment steps are the same as in Example 1.

[0049] S2. The temperature of the ion-exchange feed solution collected in step S1 is lowered to 10°C and maintained. High-purity nitrogen gas is introduced into the feed solution for deoxygenation until the dissolved oxygen concentration drops to 0.2 mg / L, resulting in a low-temperature, low-oxygen feed solution.

[0050] S3. Adjust the pH of the low-temperature, low-oxygen feed solution from step S2 to 5.0. Then, add chitosan to bring the final concentration to 10 mg / L. Introduce high-purity nitrogen microbubbles with an average diameter of 50 μm for flotation separation, and collect the clarified flotation solution.

[0051] S4. The flotation clarified solution obtained in step S3 is fed into the membrane separation system. The operating parameters for nanofiltration concentration are set to a cross-flow velocity of 4.0 m / s and an operating pressure of 1.5 MPa. The operating parameters for reverse osmosis purification are set to a cross-flow velocity of 1.5 m / s and an operating pressure of 4.0 MPa.

[0052] S5. The polishing process is the same as in Example 1, resulting in the final product P3.

[0053] The following are examples of comparisons: Comparative Example 1: Compared with Example 1, the difference is that after the ion exchange treatment in step S1, the feed solution was not subjected to the cooling and deoxygenation treatment in step S2, but was directly used in the operation of step S3. The rest are the same. Comparative product DP1 was obtained.

[0054] Comparative Example 2: Compared with Example 1, the difference is that in step S3, after adjusting the pH value and adding chitosan, microbubbles are not introduced for flotation separation. Instead, the feed solution is allowed to stand in a settling tank for 2 hours, and the supernatant is taken for subsequent steps. All other steps are the same. Comparative product DP2 is obtained.

[0055] Comparative Example 3: The difference from Example 1 is that chitosan solution was not added in step S3, while all other steps were the same. Comparative product DP3 was obtained.

[0056] Comparative Example 4: Compared with Example 1, the difference is that the mixed bed resin polishing treatment in step S5 is not performed, and the product after reverse osmosis purification in step S4 is used as the final product; all other aspects are the same. Comparative product DP4 is obtained.

[0057] The test example is as follows: Test Example 1: Determination of Acrylamide Concentration and Overall Process Yield: To evaluate the acrylamide concentration and overall process yield of the products obtained in the above examples and comparative examples, high performance liquid chromatography (HPLC) was used to determine the initial raw materials and final products (P1, P2, P3, DP1, DP2, DP3, DP4).

[0058] Chromatographic conditions: Instruments: High performance liquid chromatograph; Column: C18 reversed-phase column (4.6 mm × 250 mm, 5 μm); Mobile phase: methanol: water = 10:90 (volume ratio); Flow rate: 1.0 mL / min; Column temperature: 30 ℃; Detection wavelength: 210 nm; Injection volume: 10 μL.

[0059] The experimental steps are as follows: Preparation of standard curve: Accurately weigh acrylamide standard and prepare a 1000 mg / L stock solution with ultrapure water. Dilute the stock solution to prepare standard working solutions with concentrations of 1, 5, 10, 50, and 100 mg / L. Perform HPLC analysis on the above standard working solutions respectively. Plot a standard curve with peak area on the ordinate and concentration on the abscissa to obtain the linear regression equation.

[0060] Sample preparation and determination: Accurately weigh a certain mass of initial aqueous solution of acrylamide and each final product, quantitatively dilute them with ultrapure water until the concentration is within the linear range of the standard curve, filter through a 0.22 μm microporous membrane, and then inject for analysis. Record the peak area of ​​the sample.

[0061] Calculation: Based on the peak area of ​​the sample and the regression equation of the standard curve, the mass concentration of acrylamide in the sample dilution is calculated. Based on the dilution factor and initial sample mass, the mass concentration (%) of acrylamide in each final product is calculated. The overall process yield (%) is calculated using the following formula: Overall process yield = (total mass of final product × mass concentration of acrylamide in final product / total mass of initial aqueous solution × mass concentration of acrylamide in initial aqueous solution) × 100%; The test results are as follows: The final products obtained from each embodiment and comparative example were measured, and the results are summarized in Table 1.

[0062] Table 1: Test results of acrylamide concentration and overall process yield for each example and comparative example Sample No. Mass concentration of acrylamide in final product (%) Total process yield (%) P1 51.2 97.6 P2 50.8 96.8 P3 51.5 97.1 DP1 48.3 85.3 DP2 49.5 92.5 DP3 50.1 94.2 DP4 51.1 97.5 Experiment Summary: The results of Comparative Example 1 show that the overall process yield decreased significantly without the low-temperature and deoxygenation pretreatment steps. This is because acrylamide monomers are chemically active in normal temperatures and oxygen-containing environments, easily undergoing spontaneous polymerization to form undesirable polymers. The low-temperature and deoxygenation steps in this technical solution effectively suppress this side reaction by lowering the activation energy of the reaction environment and removing the inducing factor (oxygen) of the polymerization reaction, thereby maximizing the preservation of the target product molecules and ensuring a high overall process yield.

[0063] Comparing the data from Example 1 with Comparative Examples 2 and 3, it is evident that both static sedimentation and flotation lacking a cationic precipitant lead to a decrease in process yield. This is because protein flocs formed solely through pH adjustment are small and dispersed, making complete separation difficult through static sedimentation, resulting in some acrylamide being removed along with the waste sludge. The introduction of a cationic precipitant, however, increases the size and density of the flocs through bridging, and combined with the efficient solid-liquid separation of microbubble flotation, impurities can be separated more thoroughly from the liquid phase, reducing the loss of the target product during the separation process.

[0064] Based on the results of Examples 1, 2, and 3, this technical solution organically combines low-temperature deoxygenation pretreatment, pH adjustment-collector-microbubble flotation, and subsequent membrane separation technology to construct a complete process system designed to maximize the retention of acrylamide monomers and efficiently remove various impurities. The steps within this system work together to improve the yield efficiency of the final product, and the results are verified by test data.

[0065] Test Example 2: Determination of Residual Protein Content: To evaluate the residual protein content in the products obtained in the above examples and comparative examples, the Coomassie Brilliant Blue G-250 method (Bradford method) was used for determination.

[0066] Instruments and reagents: Instruments: UV-Vis spectrophotometer; Reagents: Coomassie Brilliant Blue G-250 solution, bovine serum albumin (BSA) standard; The experimental steps are as follows: Preparation of Standard Curve: Accurately weigh bovine serum albumin standard and prepare a 100 μg / mL stock solution with ultrapure water. Accurately pipette this stock solution and dilute it with ultrapure water to prepare a series of working standard solutions with concentrations of 0, 2, 5, 10, 15, and 20 μg / mL. Take 1 mL of each working standard solution and add 5 mL of Coomassie Brilliant Blue G-250 solution, mix thoroughly, and incubate at room temperature for 5 minutes. Measure the absorbance at 595 nm. Plot a standard curve with absorbance on the ordinate and protein concentration on the abscissa to obtain the linear regression equation.

[0067] Sample determination: Accurately measure 1 mL of each of the final products (P1, P2, P3, DP1, DP2, DP3, DP4), add 5 mL of Coomassie Brilliant Blue G-250 solution, mix well, and incubate at room temperature for 5 minutes. Measure the absorbance at a wavelength of 595 nm. Each sample was measured three times, and the average value was taken.

[0068] Calculation: Substitute the average absorbance of the sample into the regression equation of the standard curve to calculate the protein concentration (mg / L) in the sample. The final result is converted into mass content (mg / kg) based on the approximate density of the liquid solution of 1 kg / L.

[0069] The test results are as follows: The final products obtained from each embodiment and comparative example were measured, and the results are summarized in Table 2.

[0070] Table 2: Test results of residual protein content in each example and comparative example Sample No. Average absorbance (Abs @ 595 nm) Residual protein content (mg / kg) P1 0.058 0.65 P2 0.076 0.88 P3 0.064 0.73 DP1 0.061 0.69 DP2 0.625 15.7 DP3 0.243 5.4 DP4 0.06 0.68 Experiment Summary: The data in Table 2 show that the processes described in Examples 1, 2, and 3 can control the residual protein content in the final product to an extremely low level. The technical mechanism behind this effect lies in the combined application of pH adjustment and microbubble flotation. By adjusting the pH of the feed solution to near the isoelectric point of the protein, the originally dissolved protein molecules undergo charge neutralization and aggregate into tiny flocs. Subsequently, inert gas microbubbles are introduced as hydrophobic carriers, efficiently adsorbing and capturing these flocs, separating them from the bulk solution through buoyancy, thereby achieving deep removal of large molecular weight protein impurities.

[0071] The comparison between Comparative Example 2 and Example 1 clearly reveals the role of microbubble flotation in separation efficiency. Comparative Example 2, employing the traditional static sedimentation method, showed significantly lower protein removal efficiency compared to Example 1. This is because gravity sedimentation alone is inefficient and time-consuming for separating small protein flocs with densities close to water. Microbubble flotation, on the other hand, provides an active and efficient solid-liquid separation pathway, significantly enhancing the driving force and thoroughness of separation, making it a key step in achieving efficient purification of protein impurities.

[0072] The comparison between Comparative Example 3 and Example 1 verifies the function of the cationic scavenger in this technical solution. Without the addition of a scavenger, the protein removal rate significantly decreased. The mechanism lies in the fact that the molecular chains of the cationic scavenger (such as chitosan) connect the initially formed fine protein flocs into larger, more structurally stable flocs through adsorption bridging and charge neutralization. This increased size makes the flocs easier for microbubbles to capture and carry, thereby significantly improving the overall removal efficiency of the flotation process and ensuring the high purity of the final product.

[0073] Test Example 3: Conductivity Measurement To evaluate the ionic purity of the products obtained in the above embodiments and comparative examples, the conductivity of the final products was measured using a conductivity meter.

[0074] Instruments and conditions: Instrument: High-precision conductivity meter (equipped with temperature compensation function); Measurement temperature: 25.0±0.1℃; The experimental procedure is as follows: Instrument calibration: The conductivity meter and its electrodes are calibrated using certified standard conductivity solutions (e.g., 1.3 μS / cm, 12.88 μS / cm) until the reading error is within the allowable range.

[0075] Sample preparation: Place the final products (P1, P2, P3, DP1, DP2, DP3, DP4) in a constant temperature water bath and allow their temperature to stabilize at 25.0℃.

[0076] Sample Measurement: Rinse the conductivity cell three times with the sample to be tested. Immerse the conductivity cell in the sample to be tested, ensuring the electrode is completely submerged and free of air bubbles. After the instrument reading stabilizes, record the conductivity value. Each sample should be measured independently three times, and all readings should be recorded.

[0077] Calculation: Calculate the average of the three measurements for each sample, which is taken as the final conductivity of the sample.

[0078] The test results are as follows: The final products obtained from each embodiment and comparative example were measured, and the results are summarized in Table 3.

[0079] Table 3: Conductivity test results of each embodiment and comparative example Experiment Summary: Table 3 shows that the final products obtained in Examples 1, 2, and 3 all exhibit extremely low electrical conductivity, indicating that their ionic impurity content is controlled at an extremely low level. In contrast, Comparative Example 4, after omitting the final polishing step, shows a significantly higher electrical conductivity than the products from the Examples. This data comparison directly demonstrates that the final polishing step plays a decisive role in achieving high electrical purity in the product.

[0080] The multi-stage desalination system constructed in this technical solution is the technological basis for obtaining products with low conductivity. The initial ion exchange step is responsible for removing most of the strong electrolyte ions in the feed solution. The subsequent reverse osmosis membrane purification step, utilizing the membrane's high solute rejection characteristics, further removes the remaining ions and small molecule impurities. However, trace amounts of ions and dissolved weak electrolytes (such as carbon dioxide) may still remain from the previous steps, thus limiting the final purity that the product can achieve.

[0081] The final mixed-bed resin polishing treatment is crucial for achieving deep desalination. This mixed-bed resin, composed of a strongly acidic cation exchange resin and a strongly basic anion exchange resin, captures and exchanges any trace cations and anions remaining in the solution, converting them into water molecules. This process reduces the ion concentration in the solution to a theoretically minimum level, resulting in an extremely low conductivity in the final product. Comparative data validates the effectiveness of this deep purification mechanism.

[0082] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for preparing acrylamide with improved purity, characterized in that, The preparation process includes the following steps: S1. The acrylamide aqueous solution obtained by bio-enzyme catalytic hydration and ultrafiltration separation is subjected to ion exchange treatment to obtain the ion-exchanged feed solution. S2. Cool the ion-exchange solution to 5-10°C and deoxygenate it at this temperature until the dissolved oxygen concentration is below 0.5 mg / L to obtain a low-temperature, low-oxygen solution. S3. The pH of the low-temperature and low-oxygen feed solution is adjusted to cause protein impurities to flocculate, and then inert gas microbubbles are introduced into it for flotation separation to obtain a flotation clarified solution. S4. The flotation clarified liquid is concentrated by passing it through a nanofiltration membrane and purified by a reverse osmosis membrane to obtain a high-purity acrylamide aqueous solution.

2. The acrylamide preparation process for improving purity according to claim 1, characterized in that, In step S1, the acrylamide aqueous solution obtained through bio-enzymatic hydration and ultrafiltration is subjected to ion exchange treatment to obtain the ion-exchanged feed solution. The acrylamide aqueous solution is passed sequentially through a cation exchange column packed with a strong acidic cation exchange resin and an anion exchange column packed with a weak base anion exchange resin to remove cation and anion impurities from the aqueous solution and control the conductivity of the solution after ion exchange to be less than 20 µS / cm.

3. The acrylamide preparation process for improving purity according to claim 1, characterized in that, Step S3, which involves adjusting the pH of the low-temperature, low-oxygen feed solution to cause protein impurities to flocculate, followed by introducing inert gas microbubbles for flotation separation to obtain a clarified flotation solution, includes the following steps: After pH adjustment and before the introduction of inert gas microbubbles, a cationic scavenger is added to the low-temperature, low-oxygen feed solution. The cationic scavenger interacts with the protein flocs formed by pH adjustment to increase the floc size.

4. The acrylamide preparation process for improving purity according to claim 3, characterized in that, The cation scavenger is chitosan, which is added in the form of an aqueous solution, and the amount of chitosan added is such that the final concentration of chitosan in the low-temperature, low-oxygen solution is 1-10 mg / L; the degree of deacetylation of the chitosan is 85%-95%.

5. The acrylamide preparation process for improving purity according to claim 3, characterized in that, The pH adjustment involves adjusting the pH value of the low-temperature, low-oxygen feed solution to 4.0–5.0, within which protein impurities form flocs; the inert gas microbubbles have an average diameter of 10–50 μm, and the microbubbles adsorb the protein flocs when they float in the low-temperature, low-oxygen feed solution.

6. The acrylamide preparation process for improving purity according to claim 1, characterized in that, In step S4, the step of concentrating the flotation clarified solution by passing it through a nanofiltration membrane and purifying it by a reverse osmosis membrane to obtain a high-purity acrylamide aqueous solution includes: The flotation clarified solution is passed through the nanofiltration membrane at a cross-flow velocity of 2.0–4.0 m / s and an operating pressure of 0.5–1.5 MPa, allowing water molecules to pass through the nanofiltration membrane while acrylamide molecules are retained by the nanofiltration membrane, thereby concentrating the acrylamide in the flotation clarified solution.

7. The acrylamide preparation process for improving purity according to claim 1, characterized in that, In step S4, the step of concentrating the flotation clarified solution by passing it through a nanofiltration membrane and purifying it by a reverse osmosis membrane to obtain a high-purity acrylamide aqueous solution includes: The acrylamide aqueous solution, concentrated by the nanofiltration membrane, is passed through the reverse osmosis membrane at a cross-flow rate of 0.5–1.5 m / s and an operating pressure of 2.0–4.0 MPa to further remove residual ionic impurities in the acrylamide aqueous solution.

8. The acrylamide preparation process for improving purity according to claim 1, characterized in that, The preparation process further includes step S5: The high-purity acrylamide aqueous solution was polished by passing it through a mixed bed resin composed of a strong acid cation exchange resin and a strong base anion exchange resin. The polishing process is used to capture trace amounts of cations and anions remaining in the high-purity acrylamide aqueous solution. The cations are exchanged for hydrogen ions by the strongly acidic cation exchange resin, and the anions are exchanged for hydroxide ions by the strongly basic anion exchange resin. The hydrogen ions and hydroxide ions combine to form water.

9. The acrylamide preparation process for improving purity according to claim 1, characterized in that, The nanofiltration membrane is a polyamide membrane composite nanofiltration membrane with a molecular weight cutoff of 100-200 Da, which is used to retain acrylamide molecules in the concentration operation of step S4; the reverse osmosis membrane is a polyamide membrane composite reverse osmosis membrane with a sodium chloride removal rate of greater than 99%, which is used to remove residual dissolved ionic impurities in the purification operation of step S4.

10. The acrylamide preparation process for improving purity according to claim 1, characterized in that, The high-purity acrylamide aqueous solution comprises the following components in parts by weight: Acrylamide: 45-55 parts; Water: 45-55 parts; Acrylic acid and other ionizable impurities: 0 to 0.01 parts; Residual protein and macromolecular impurities: 0 to 0.0001 parts.

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