Rectification optimization process for improving quality of acetic anhydride

Abstract

The invention relates to the technical field of chemical engineering, and discloses a rectification optimization process for improving the quality of acetic anhydride, the process realizes efficient pre-removal of high boiling impurities and colored substances in crude acetic anhydride by optimizing the structure of an evaporation tower, adopting combination of falling film evaporation and filler reinforcement, and accurately controlling the operation parameters of vacuum degree, temperature and feeding amount, so that the quality of the acetic anhydride is improved, and the quality of the acetic anhydride is improved. A traditional tower plate of the acetic acid rectifying tower is replaced with the efficient float valve tower plate, the internal part structure of the efficient float valve tower plate is optimized, the gas-liquid mass transfer efficiency in the rectifying tower is improved, and efficient and stable separation of acetic acid and acetic anhydride is achieved through precise linkage control over temperature and reflux ratio rectifying parameters. The main content of the product is improved; the impurity content is reduced; through whole-process online monitoring and regular optimization iteration in combination with a process parameter-product quality database, dynamic regulation and control and continuous improvement of the production process are realized, and finally the beneficial effects that the main content of acetic anhydride in the product is greater than or equal to 99.65% and the chromaticity is less than or equal to 6 are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of chemical engineering, in particular to a rectification optimization process for improving the quality of acetic anhydride. BACKGROUND

[0002] As an important acetylating agent and dehydrating agent, acetic anhydride plays an irreplaceable role in the industrial fields of cellulose acetate, medicine, spices, dyes, etc. With the continuous improvement of product quality requirements in the pharmaceutical industry, electronic chemicals and high-end cellulose acetate industries, the market demand for high-purity acetic anhydride is increasingly urgent. The traditional acetic anhydride rectification process mostly uses conventional evaporation and sieve plate tower rectification technology, which has problems such as low evaporation separation efficiency, easy occurrence of flow deviation in the rectification process, limited removal effect of colored and high-boiling point impurities, etc., resulting in that the purity of the final product is difficult to stably reach the requirements of high-end applications, and the quality fluctuates greatly during production. Therefore, it is urgent to develop an efficient, stable and continuously optimized rectification optimization process to realize the accurate control of the color and key impurity content of acetic anhydride and meet the stringent standards of high-end market for the quality of acetic anhydride. SUMMARY

[0003] (I) Technical problems solved

[0004] In view of the deficiencies of the prior art, the present application provides a rectification optimization process for improving the quality of acetic anhydride, which has the advantages of high evaporation efficiency, good separation effect, stable product quality, and intelligent control and continuous optimization, and solves the problems of low product purity, large colority and obvious quality fluctuation caused by low evaporation and rectification efficiency, easy occurrence of flow deviation and insufficient impurity removal capacity in the traditional process.

[0005] (II) Technical solutions

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solutions: a rectification optimization process for improving the quality of acetic anhydride, comprising the following steps:

[0007] Step 1, raw material preparation: selecting crude acetic anhydride prepared by acetic acid thermal cracking method as raw material and performing main content detection;

[0008] Step 2, raw material pretreatment: pretreating the crude acetic anhydride;

[0009] Step 3, equipment improvement: improving the evaporation mode of the evaporation tower and the sieve plate tray of the acetic acid rectification tower;

[0010] Step 4, setting of rectification process parameters: setting the evaporation tower parameters, acetic acid rectification tower parameters and acetic anhydride rectification tower parameters based on the structural characteristics of the improved equipment;

[0011] Step five, evaporation-rectification treatment: the crude acetic anhydride prepared in step two is sent to the improved evaporation tower at a set feeding amount, and evaporation operation is carried out under the evaporation parameters prepared in step four, and the generated crude acetic anhydride vapor enters the improved acetic acid rectification tower, and continuous rectification is carried out under the set rectification parameters; when the acetic anhydride content of the tower bottom discharge reaches the standard, it enters the subsequent refining process;

[0012] Step six, refining rectification and decolorization: the tower bottom discharge of the acetic acid rectification tower after step five reaches the standard and is sent to the acetic anhydride rectification tower, and the rectification operation is started according to the parameters set in step four, and at the same time, the fraction containing trace colored substances is collected through the middle side line, and when the colority of the tower bottom discharge does not reach the standard, the reflux ratio or the tower bottom temperature is adjusted, and finally refined acetic anhydride is obtained;

[0013] Step seven, post-treatment: the refined acetic anhydride obtained from the bottom of the acetic anhydride rectification tower in step six is sent to a cooler, and after cooling, it is finally filtered through a precision filtering device;

[0014] Step eight, quality detection: the filtered acetic anhydride is detected for main content and colority, and when the target acetic anhydride content is ≥99.65% and the colority is ≤6, high-purity acetic anhydride product is obtained;

[0015] Step nine, process parameter optimization iteration: during the operation of the rectification optimization process, the operating parameters of each tower and the discharge quality detection data are recorded in real time, and a process parameter-product quality correlation database is established for experience data analysis, and when the product quality fluctuates, the process parameters are adjusted in combination with the equipment operating state to realize continuous optimization iteration of the process.

[0016] Preferably, in step one, the raw material preparation: acetic acid is cracked into ethylene ketone by acetic acid thermal cracking, and then absorbed by acetic acid to prepare crude acetic anhydride, which is used as the raw material for rectification process, and the main content of the crude acetic anhydride raw material is controlled to be 85%-95%.

[0017] Preferably, in step two, the raw material pretreatment: the crude acetic anhydride selected in step one is pretreated by a filtering device with a filtering precision of 5-10 μm, and the crude acetic anhydride material is obtained after pretreatment.

[0018] Preferably, the evaporation tower in step three is improved: the original evaporation method of the crude acetic anhydride evaporation tower is improved, and a combination evaporation structure of falling film evaporation and packing is adopted, corrugated packing is added in the evaporation tower, the packing is selected from 316L stainless steel material θ ring packing or metal wire mesh corrugated packing, the θ ring packing is filled in the lower liquid phase evaporation section of the evaporation tower, and the filling height is 1 / 3-1 / 2 of the height of the tower section; the metal wire mesh corrugated packing is filled in the upper gas-liquid mass transfer section of the evaporation tower, and the filling height is 1 / 2-2 / 3 of the height of the tower section; meanwhile, the temperature distribution range of the evaporation tower is optimized, the temperature in the gas phase zone of the tower is controlled to be 138-142℃, the temperature in the liquid phase zone is controlled to be 135-137℃, and an online colority monitor is added at the top of the evaporation tower.

[0019] Preferably, the acetic acid rectification tower in step three is improved: the traditional sieve plate tower plate in the original acetic acid rectification tower is replaced with a new type of high-efficiency float valve tower plate, which is selected from F1 type float valve tower plate or V-4 type strip-shaped float valve tower plate, when the F1 type float valve tower plate is used, the spacing is adjusted to 380-400mm; when the V-4 type strip-shaped float valve tower plate is used, a 120° radial arrangement mode is adopted, the tower plate opening rate is controlled to be 13%-15%, a mist prevention baffle is added on the tower plate, the height of the mist prevention baffle is 70-80mm, meanwhile, the distributor structure in the tower is optimized, the original single-inlet pipe distributor is replaced with a double-inlet tank liquid distributor, a liquid distribution hole with a diameter of 9-10mm is arranged at the bottom of the distributor tank body, a guide plate with an inclination angle of 30° is arranged below the tank body, the length of the guide plate is 1 / 6-1 / 5 of the tower diameter, so that the feeding liquid is uniformly distributed along the tower section; meanwhile, an arc-shaped guide vane is added at the connection between the tower wall and the distributor, and a two-stage condenser is added at the top of the acetic acid rectification tower.

[0020] Preferably, the parameters of the evaporation tower in step four are set: the feeding amount is controlled to be 18-22m³ / h, the vacuum degree is-0.09~-0.095MPa, and the heat source is 1-1.2MPa saturated steam.

[0021] Preferably, the parameters of the acetic acid rectification tower in step four are set: the crude acetic anhydride steam generated by the evaporation tower is received as the feed, the tower bottom temperature is controlled to be 145-148℃, the tower top temperature is controlled to be 115-120℃, and the reflux ratio is set to be 3-5:1.

[0022] Preferably, the parameters of the acetic anhydride rectification tower in step four are set: the bottom discharge of the acetic acid rectification tower is received, the tower bottom temperature is controlled to be 149-152℃, the tower top temperature is controlled to be 139-141℃, the reflux ratio is set to be 4-6:1, the light component impurities are collected at the top of the tower, and the preliminary refined acetic anhydride is obtained at the bottom of the tower, and a side line outlet is arranged at 2 / 3 of the height of the tower.

[0023] Preferably, the step five evaporation-primary rectification process is monitored in real time for the acetic acid distillation column overhead acetic acid production and the acetic anhydride content of the bottom discharge, when the acetic anhydride content of the bottom discharge is ≥98.5%, it can enter the subsequent refining process, when the content is <98.5%, then adjust the reflux ratio or the feed amount for parameter correction;

[0024] The step six refined distillation and decolorization process uses an online color monitor to monitor the color of the bottom discharge in real time, when the color is ≥8, the reflux ratio is increased or the bottom temperature is reduced.

[0025] Preferably, the step seven bottom discharge post-processing cooling medium is circulating water, the temperature after cooling is controlled to be between 40-50°C, and the filtering precision of the precision filtration device is 0.22μm.

[0026] Compared with the prior art, the present application provides a rectification optimization process for improving the quality of acetic anhydride, which has the following beneficial effects:

[0027] 1、The present application realizes the efficient pre-removal of high-boiling impurities and colored substances in crude acetic anhydride by optimizing the structure of the evaporation tower, using falling film evaporation and packing reinforcement combination, and accurately controlling the operating parameters of vacuum degree, temperature and feed amount, providing purer feed for subsequent rectification, and finally achieving the beneficial effects of product acetic anhydride main content ≥99.65% and colority ≤6.

[0028] 2、The present application realizes the efficient and stable separation of acetic acid and acetic anhydride by replacing the traditional tray of the acetic acid distillation column with a high-efficiency float valve tray and optimizing the structure of its internals, such as using a double-inlet slot distributor, to enhance the gas-liquid mass transfer efficiency in the rectification tower, and through precise linkage control of temperature and reflux ratio rectification parameters, and finally achieving the beneficial effects of improving the main content of the product and reducing the impurity content.

[0029] 3、The present application realizes dynamic regulation and continuous improvement of the production process by constructing an online monitoring and automatic feedback control system (such as online colorimeter, content analysis and DCS central control) throughout the whole process, and combining with the process parameter-product quality database for regular optimization iteration, thereby achieving the beneficial effects of long-term stable production of high-quality acetic anhydride products and reducing quality fluctuations. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The process flow chart of the present application. DETAILED DESCRIPTION

[0031] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0032] Please refer to Figure 1 An improved rectification optimization process for improving the quality of acetic anhydride, comprising the following steps:

[0033] Step one, raw material preparation: selecting crude acetic anhydride prepared by acetic acid thermal cracking method as raw material, and performing main content detection to meet the basic requirements of subsequent rectification treatment;

[0034] Step two, raw material pretreatment: performing pre-filtering treatment on the crude acetic anhydride to remove solid particle impurities;

[0035] Step three, equipment improvement: improving the evaporation mode of the evaporation tower, and improving the sieve plate tower plate of the acetic acid rectification tower to ensure the synergistic improvement of evaporation efficiency and impurity removal effect, and to avoid the occurrence of gas-liquid partial flow phenomenon;

[0036] Step four, rectification process parameter setting: setting the evaporation tower parameters, acetic acid rectification tower parameters and acetic anhydride rectification tower parameters based on the structural characteristics of the improved equipment;

[0037] Step five, evaporation-primary rectification treatment: conveying the crude acetic anhydride pretreated in step two to the improved evaporation tower according to the set feed amount, performing evaporation operation under the evaporation parameters set in step four, and the generated crude acetic anhydride vapor enters the improved acetic acid rectification tower to perform continuous rectification under the set rectification parameters; when the acetic anhydride content of the tower bottom discharge reaches the standard, it enters the subsequent refining process;

[0038] Step six, refining rectification and decolorization: conveying the acetic acid rectification tower bottom discharge after step five to the acetic anhydride rectification tower, starting the rectification operation according to the parameters set in step four, and through the tower top, the light components are taken out, the high-boiling impurities are removed by heating at the tower bottom, and at the same time, the fraction containing trace colored substances is taken out through the middle side line (the side line output is 3% to 5% of the feed amount), when the colority of the tower bottom discharge does not reach the standard, the reflux ratio or the tower bottom temperature is adjusted, and finally the refined acetic anhydride is obtained;

[0039] Step seven, post-treatment: conveying the refined acetic anhydride obtained from the bottom of the acetic anhydride rectification tower in step six to a cooler, and after cooling, performing terminal filtering through a precision filtering device to remove possible trace impurities;

[0040] Step eight, quality detection: the filtered acetic anhydride is detected for main content (using gas chromatography, detection accuracy ± 0.05%) and colority (using platinum-cobalt colorimetry, detection accuracy ± 1 Hazen unit), and when the target acetic anhydride content is ≥99.65% and the colority is ≤6, high-purity acetic anhydride product is obtained;

[0041] Step nine, process parameter optimization iteration: during the operation of the rectification optimization process, the operating parameters of each tower and the quality detection data of the discharge are recorded in real time, and a process parameter-product quality correlation database is established; the database data is analyzed regularly (every 48-72 hours), and when the product quality fluctuates (such as a decrease in content ≥0.1% or an increase in colority ≥1), the process parameters (such as optimizing the heat source temperature and cleaning the packing) are adjusted in combination with the equipment operating state (such as the fouling condition of the evaporator packing and the tray efficiency of the rectification tower) to realize continuous optimization iteration of the process and ensure stable production of high-purity acetic anhydride products.

[0042] Specifically, in step one, the raw material preparation: acetic acid is cracked into ethylene ketone by a hot cracking method, and then absorbed by acetic acid to prepare crude acetic anhydride. The crude acetic anhydride is used as the raw material for the rectification process, and the main content of the crude acetic anhydride is controlled to be 85%-95%. The crude acetic anhydride still contains 5%-15% of acetic acid and a small amount of high-boiling impurities and colored impurities, and there is no obvious mechanical impurity and caking phenomenon in the crude acetic anhydride raw material.

[0043] Specifically, in step two, the raw material pretreatment: the crude acetic anhydride selected in step one is pretreated by a filtering device with a filtering precision of 5-10 μm to remove solid particle impurities (such as trace carbon particles and catalyst residues) that may exist in the raw material, so as to avoid equipment blockage or affect the separation efficiency caused by solid impurities entering the subsequent evaporation and rectification system. The pretreated crude acetic anhydride material is clear.

[0044] Specifically, the evaporation tower in step three is improved: the evaporation mode of the original crude acetic anhydride evaporation tower is optimized, a combination evaporation structure of falling film evaporation and packing enhancement is adopted, corrugated packing is added in the evaporation tower, the packing is selected from 316L stainless steel θ ring packing or metal wire mesh corrugated packing, the θ ring packing is filled in the lower liquid phase evaporation section of the evaporation tower, the filling height is 1 / 3-1 / 2 of the tower section height, the large specific surface area is used to strengthen the dispersion and evaporation efficiency of the liquid phase material; the metal wire mesh corrugated packing is filled in the upper gas-liquid mass transfer section of the evaporation tower, the filling height is 1 / 2-2 / 3 of the tower section height, the fine mesh structure is used to enhance the capture and mass transfer of the trace liquid droplets in the gas phase, further remove the colored substances and high-boiling impurities carried in the steam, thereby increase the contact area and mass transfer efficiency of crude acetic anhydride and heat source in the evaporation process, improve the pre-removal capacity of the evaporation tower for color substances and high-boiling impurities in crude acetic anhydride; at the same time, the temperature distribution range of the evaporation tower is optimized, the gas phase zone temperature in the tower is controlled to be 138-142℃, the liquid phase zone temperature is 135-137℃, so as to ensure the synergistic improvement of evaporation efficiency and impurity removal effect, and an online colority monitor is added at the top of the evaporation tower.

[0045] Specifically, the acetic acid rectification tower in step three is improved: the traditional sieve plate tray in the original acetic acid rectification tower is replaced with a new type of high-efficiency float valve tray, which is selected from F1 type float valve tray or V-4 type strip-shaped float valve tray, when F1 type float valve tray is used, the spacing is adjusted to 380-400mm; when V-4 type strip-shaped float valve tray is used, a 120° radial arrangement method is adopted, the tray opening rate is controlled to be 13%-15%, and a mist prevention baffle is added on the tray, the height is 70-80mm, the flexible opening and closing of the float valve is used to adapt to the gas-liquid flow change under different loads, so as to enhance the mass transfer uniformity of the gas-liquid two-phase in the tower, improve the separation efficiency of low-boiling-point acetic acid impurities in crude acetic anhydride, optimize the distributor structure in the tower, replace the original single-inlet pipe distributor with a double-inlet tank liquid distributor, the liquid distribution holes with a diameter of 9-10mm are opened at the bottom of the distributor tank body (the hole spacing is 15-20mm), and a 30° inclined guide plate is arranged below the tank body, the length is 1 / 6-1 / 5 of the tower diameter, so that the feed liquid is uniformly distributed along the tower section; at the same time, an arc-shaped guide vane (the arc matches the tower wall) is added at the connection between the tower wall and the distributor, which eliminates the liquid retention area caused by the tower wall effect, so as to avoid the gas-liquid deflection phenomenon, and a two-stage condenser is added at the top of the acetic acid rectification tower to improve the relative volatility of acetic acid-acetic anhydride.

[0046] Specifically, in step four, the parameters of the evaporation tower are set: the feed quantity is controlled at 18-22 m³ / h, the vacuum degree is -0.09 to -0.095 MPa, and the heat source is 1-1.2 MPa saturated steam, so as to ensure that the evaporation rate of crude acetic anhydride is ≥96%, and the content of high-boiling impurities in the gas phase after evaporation is ≤0.3% and the removal rate of colored substances is ≥65%. By maintaining a high vacuum environment, the boiling point of the material is significantly reduced, avoiding thermal decomposition or condensation reaction of acetic anhydride and impurities at high temperatures. By precisely controlling the liquid film thickness, temperature distribution and gas phase residence time in the evaporator, efficient liquid-phase enrichment and separation of high-boiling impurities and colored substances are achieved.

[0047] Specifically, in step four, the parameters of the acetic acid rectification tower are set: crude acetic anhydride vapor generated by the evaporation tower is received as feed, the tower bottom temperature is controlled at 145-148°C, the tower top temperature is controlled at 115-120°C, and the reflux ratio is set at 3-5:1. Acetic acid impurities (acetic acid purity ≥98.5%, which can be recycled for use in the ethylene ketone absorption process) are collected at the tower top, and the crude acetic anhydride content at the tower bottom is increased to 98.5%-99.0%. The tower bottom temperature is precisely controlled by the tower bottom reboiler, so as to ensure that the low-boiling acetic acid is fully vaporized and rises to the tower top. At the same time, the reflux liquid flow is adjusted by the tower top reflux pump to match the set reflux ratio, so as to strengthen the mass transfer and separation of the gas-liquid two-phase on the float valve tray, thereby achieving efficient separation of acetic acid and crude acetic anhydride.

[0048] Specifically, in step four, the parameters of the acetic anhydride rectification tower are set: the tower bottom discharge from the acetic acid rectification tower is received, the tower bottom temperature is controlled at 149-152°C, the tower top temperature is controlled at 139-141°C, and the reflux ratio is set at 4-6:1. Light component impurities are collected at the tower top, and preliminary refined acetic anhydride is obtained at the tower bottom. A side line discharge outlet is arranged at 2 / 3 of the tower height for further removal of trace colored substances. The tower bottom temperature sensor and the reboiler heating power are linked to control the temperature to avoid decomposition of acetic anhydride caused by excessively high tower bottom temperature. The discharge amount is controlled by the side line discharge pump to timely discharge the intermediate fraction enriched with colored substances. The contact time of gas and liquid in the tower is prolonged by means of high reflux ratio to improve the removal effect of high-boiling impurities.

[0049] Specifically, in step five, the acetic acid discharge amount at the top of the acetic acid rectification tower and the acetic anhydride content of the tower bottom discharge are monitored in real time. When the acetic anhydride content of the tower bottom discharge is ≥98.5%, the subsequent refining process can be entered. When the content is <98.5%, the reflux ratio (±0.5) or the feed quantity (±1 m³ / h) is adjusted for parameter correction. The acetic anhydride content is analyzed in real time. When the content is not up to standard, the central control system automatically sends instructions to the reflux adjusting valve to adjust the reflux ratio or to the feed pump to adjust the feed quantity, so as to realize automatic control of parameter correction and avoid product quality fluctuations caused by manual adjustment lag.

[0050] Specifically, in step six, the online color monitor (detection accuracy ≤1 Hazen unit) is used to monitor the colority of the tower bottom discharge in real time during the refining rectification and decolorization process. When the colority is ≥8, the reflux ratio is increased (+0.5-1) or the tower bottom temperature is decreased (-1-2℃) to ensure that the main content of the tower bottom discharge is ≥99.65% and the colority is ≤6. Through the data linkage of the online color monitor and the central control system, the real-time colority value is converted into a control signal. When the colority exceeds the standard, the central control system preferentially triggers the reflux adjustment valve opening to increase, prolonging the contact time of gas and liquid in the tower and strengthening the separation of colored substances and acetic anhydride. When the colority still does not meet the standard after adjusting the reflux ratio, the heating power of the tower bottom reboiler is automatically reduced to control the tower bottom temperature to decrease slightly, avoiding the decomposition of impurities at high temperature to produce new colored substances. At the same time, combined with the real-time flow monitoring of the side line discharge outlet, the side line discharge amount is ensured to be stable, and the intermediate fraction enriched in colored substances is discharged synchronously, forming a closed loop control of monitoring-adjusting-discharging impurities, thereby ensuring that the colority and main content of the tower bottom discharge meet the standards.

[0051] Specifically, in step seven, the cooling medium in the post-treatment of the tower bottom discharge is circulating water, and the temperature after cooling is controlled between 40-50℃. The filtering precision of the precision filter device is 0.22μm. The countercurrent heat exchange of refined acetic anhydride and circulating water is realized through the shell-and-tube cooler. A temperature sensor is arranged at the outlet of the cooler to feedback temperature data in real time. When the temperature exceeds the range of 40-50℃, the central control system automatically adjusts the opening degree of the circulating water valve to adjust the cooling efficiency. The precision filter device with a filter core made of polytetrafluoroethylene material is used to intercept possible small solid particles (such as filler debris falling off in the rectification tower), thereby ensuring that the final product is free of mechanical impurities.

[0052] The refined acetic anhydride is distilled by the rectification optimization process of the present application, and the performance of the product is detected to obtain the following data:

[0053] Table 1

[0054]

[0055] Seven groups of acetic anhydride samples prepared by the present application (Examples 1 to 7) and seven products prepared by the traditional process (Comparative Examples 1 to 7) are randomly selected for comparison of test data, and the data are as shown in Table 2:

[0056] Table 2

[0057]

[0058] Examples 1 to 7 of the present application are superior to Comparative Examples 1 to 7 of the traditional process in terms of colority, acetic anhydride content, evaporation residue, iron content and potassium permanganate reduction index, and completely meet the strict requirements of high-end acetic anhydride users for colority ≤6 and content ≥99.65%.

[0059] While embodiments of the application have been shown and described, it is to be understood that the application is not limited to the details of the embodiments described, since numerous changes, modifications, substitutions and variations can be made thereto without departing from the spirit and scope of the application as defined by the appended claims and their equivalents.

Claims

1. A rectification optimization process for improving the quality of acetic anhydride, characterized in that, The method comprises the following steps: Step one, raw material preparation: select crude acetic anhydride prepared by acetic acid thermal cracking method as raw material, and perform main content detection; Step two, raw material pretreatment: pretreatment of crude acetic anhydride by filtering; Step three, equipment improvement: improve the evaporation mode of the evaporation tower, and improve the sieve plate of the acetic acid rectification tower; Step four, setting of rectification process parameters: based on the structural characteristics of the improved equipment, set the evaporation tower parameters, the acetic acid rectification tower parameters and the acetic anhydride rectification tower parameters; Step five, evaporation-primary rectification treatment: the pretreated crude acetic anhydride in step two is transported to the improved evaporation tower according to the set feeding amount, and the evaporation operation is performed under the evaporation parameters prepared in step four, the generated crude acetic anhydride vapor enters the improved acetic acid rectification tower, and the continuous rectification is performed under the set rectification parameters; when the acetic anhydride content of the tower bottom discharge reaches the standard, it enters the subsequent refining process; Step six, refining rectification and decolorization: the tower bottom discharge of the acetic acid rectification tower in step five is transported to the acetic anhydride rectification tower, the rectification operation is started according to the parameters set in step four, and at the same time, the fraction containing a small amount of colored substances is collected through the middle side line, when the colority of the tower bottom discharge does not reach the standard, the reflux ratio or the tower bottom temperature is adjusted, and finally the refined acetic anhydride is obtained; Step seven, post-treatment: the refined acetic anhydride obtained from the tower bottom of the acetic anhydride rectification tower in step six is transported to a cooler, and after cooling, it is finally filtered through a precision filtering device; Step eight, quality detection: the filtered acetic anhydride is detected for main content and colority, and when the target acetic anhydride content is greater than or equal to 99.65% and the colority is less than or equal to 6, the high-purity acetic anhydride product is obtained; Step nine, process parameter optimization iteration: during the operation of the rectification optimization process, the operating parameters of each tower and the discharge quality detection data are recorded in real time, and a process parameter-product quality correlation database is established for experience data analysis, when the product quality fluctuates, the process parameters are adjusted combined with the equipment operating state, and the continuous optimization iteration of the process is realized.

2. The process for rectification optimization for improving the quality of acetic anhydride as claimed in claim 1 wherein: In the step one, the raw material preparation: acetic acid is cracked into ethenone by acetic acid thermal cracking method, and then absorbed by acetic acid to prepare crude acetic anhydride; the crude acetic anhydride is used as the raw material of the rectification process, and the main content of the crude acetic anhydride raw material is controlled to be 85%-95%.

3. The process as claimed in claim 1, wherein the process for optimization of rectification for improving the quality of acetic anhydride is characterized by: In the step two, the raw material pretreatment: the crude acetic anhydride selected in step one is pretreated by filtering through a filtering device with a filtering precision of 5-10 μm, and the crude acetic anhydride material is obtained after pretreatment.

4. The process as claimed in claim 1, wherein the process for optimization of rectification for improving the quality of acetic anhydride is characterized by: In the step three, the evaporation tower improvement: the evaporation mode of the original crude acetic anhydride evaporation tower is improved, the combination evaporation structure of falling film evaporation and packing is adopted, the corrugated packing is added in the evaporation tower, the packing is selected from 316L stainless steel θ ring packing or metal wire mesh corrugated packing, the θ ring packing is filled in the lower liquid phase evaporation section of the evaporation tower, the filling height is 1 / 3-1 / 2 of the height of the tower section, the metal wire mesh corrugated packing is filled in the upper gas-liquid mass transfer section of the evaporation tower, the filling height is 1 / 2-2 / 3 of the height of the tower section, the temperature distribution range of the evaporation tower is optimized, the gas phase zone temperature in the tower is controlled to be 138-142 ℃, the liquid phase zone temperature is controlled to be 135-137 ℃, and an online colority monitor is added at the top of the evaporation tower.

5. The process as claimed in claim 1, wherein the process for optimization of rectification for improving the quality of acetic anhydride is characterized by: The step three acetic acid rectification column improvement: the original acetic acid rectification column traditional sieve plate is replaced with a new type of high-efficiency float valve plate, which is selected from F1 type float valve plate or V-4 type strip float valve plate, when F1 type float valve plate is used, the distance is adjusted to 380-400 mm; when V-4 type strip float valve plate is used, a 120° radial arrangement mode is adopted, the plate opening rate is controlled to be 13%-15%, and a mist prevention baffle is added to the plate, the height of which is 70-80 mm, at the same time, the structure of the distributor in the tower is optimized, the original single-inlet pipe distributor is replaced with a double-inlet tank liquid distributor, a liquid distribution hole with a diameter of 9-10 mm is arranged at the bottom of the tank body of the distributor, and a guide plate with an inclination angle of 30° is arranged below the tank body, the length of which is 1 / 6-1 / 5 of the tower diameter, so that the feed liquid is uniformly distributed along the tower cross section; at the same time, an arc-shaped guide vane is added at the connection between the tower wall and the distributor, and a two-stage condenser is added at the top of the acetic acid rectification column.

6. The process as claimed in claim 1, wherein the process for optimization of rectification for improving the quality of acetic anhydride is characterized by: The step four evaporation tower parameter setting: the feed quantity is controlled to be 18-22 m³ / h, the vacuum degree is-0.09~-0.095 MPa, and the heat source is 1-1.2 MPa saturated steam.

7. The process as claimed in claim 6, wherein the process is an optimized rectification process for improving the quality of acetic anhydride. The step four acetic acid rectification column parameter setting: the crude acetic anhydride vapor generated by the evaporation tower is received as the feed, the bottom temperature is controlled to be 145-148℃, the top temperature is controlled to be 115-120℃, and the reflux ratio is set to be 3-5:

1.

8. The process for rectification optimization for improving the quality of acetic anhydride as claimed in claim 7 wherein: The step four acetic anhydride rectification column parameter setting: the bottom discharge of the acetic acid rectification column is received, the bottom temperature is controlled to be 149-152℃, the top temperature is controlled to be 139-141℃, the reflux ratio is set to be 4-6:1, the light component impurities are collected at the top, and the preliminary refined acetic anhydride is obtained at the bottom, at the same time, a side line discharge outlet is arranged at 2 / 3 of the tower height.

9. The process as claimed in claim 1, wherein the process for optimization of rectification for improving the quality of acetic anhydride is characterized by: The step five evaporation-primarily rectification treatment: the acetic acid discharge amount at the top of the acetic acid rectification column and the acetic anhydride content of the bottom discharge are monitored in real time, when the acetic anhydride content of the bottom discharge is greater than or equal to 98.5%, the subsequent refining process can be entered, and when the content is less than 98.5%, the reflux ratio or the feed quantity is adjusted for parameter correction. The step six refined rectification and decolorization process: an online colority monitor is used to monitor the colority of the bottom discharge in real time, when the colority is greater than or equal to 8, the reflux ratio is increased or the bottom temperature is reduced.

10. The process as claimed in claim 1, wherein the process for optimization of rectification for improving the quality of acetic anhydride is characterized by: The step seven bottom discharge post-treatment: the cooling medium is circulating water, the temperature after cooling is controlled to be 40-50℃, and the filtering precision of the precision filter device is 0.22 μm.

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