Method and system for preparing p-hydroxyacetophenone through continuous high-temperature alkali fusion demethylation

Abstract

The invention belongs to the technical field of organic synthesis, and relates to a method and system for preparing p-hydroxyacetophenone through continuous high-temperature alkali fusion demethylation. A three-temperature-zone tubular reactor is matched with a falling film quench cooler, a spiral plate heat exchanger and other devices which are connected in series, p-methoxyacetophenone and a 30% sodium hydroxide solution with the molar ratio being 1: 2-1: 3 serve as raw materials, the raw materials are premixed through a static mixer and then enter the three-temperature-zone tubular reactor, a reaction is conducted for 15-30 min at a certain temperature and pressure, the temperature of a main reaction zone is 245-255 DEG C, and the reaction time is 30-50 min. And carrying out acidification crystallization on the reaction product after millisecond-level quenching. According to the method disclosed by the invention, the problems of incomplete demethylation, acetyl hydrolysis and the like in a traditional intermittent process are solved through processes such as short high-temperature retention time and a spiral turbulence enhanced mass transfer technology, the yield reaches 96.7%, and the purity is greater than 99%; meanwhile, an aluminum trichloride catalyst which is high in price and easy to cause environmental pollution is avoided, so that the method is green and environment-friendly, and the production cost is reduced by about 32%.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of organic synthesis, and particularly relates to a method and system for preparing p-hydroxyacetophenone through continuous high-temperature alkali melting demethylation. BACKGROUND

[0002] As a fine chemical intermediate, p-hydroxyacetophenone can be used as a pharmaceutical intermediate, such as for the synthesis of paracetamol and racemic levo-dopa, anti-premature birth drugs, atenolol, etc. In addition, p-hydroxyacetophenone is also a cosmetic raw material, which is included in the 2014 Catalogue of Cosmetic Raw Materials Used in China by the China Food and Drug Administration. P-hydroxyacetophenone has certain antiseptic ability when used in cosmetics, has strong black mold killing ability, and has certain inhibitory effect on pseudomonas aeruginosa. Chinese patent CN104490716A discloses the compounding of p-hydroxyacetophenone with 1,2-hexanediol and traditional Chinese medicine compositions, and the compounded composition has good antiseptic effect and no harm to human skin, overcoming the defect that current antiseptics are harmful to human skin.

[0003] At present, there are two traditional preparation methods for p-hydroxyacetophenone, one is a two-step method, and the other is a one-step method. Among them, the preparation steps of the two-step method are: first, phenol is esterified to form phenyl acetate, and then Fries rearrangement reaction occurs under the action of anhydrous aluminum chloride catalyst to obtain p-hydroxyacetophenone. The domestic main manufacturers all adopt this route, and the disadvantage of this route is that by-product o-hydroxyacetophenone is produced, and the yield of p-hydroxyacetophenone is between 30-60%, and the corresponding by-product o-hydroxyacetophenone is between 60-30%. And the two-step method is complex, and the reverse reaction will occur in the Fries rearrangement process to produce phenol, which will inevitably enter the by-product aluminum chloride hexahydrate, limiting the application of the by-product aluminum chloride hexahydrate as a water purifying agent. The preparation steps of the one-step method are: phenol and acetyl chloride are used to obtain p-hydroxyacetophenone and o-hydroxyacetophenone, respectively, under the action of anhydrous aluminum chloride catalyst. Compared with the two-step method, this method can limitedly improve the content of the p-position, but there is no essential difference, and the production of o-hydroxyacetophenone cannot be completely eliminated. The yield of p-hydroxyacetophenone prepared by the one-step method is 76%, and the yield of the by-product o-hydroxyacetophenone is 21%.

[0004] In summary, the product prepared by the traditional method has more or less a considerable amount of o-position isomers, which affects the yield of p-hydroxyacetophenone product, and a large amount of AlCl3 is used in the reaction process, resulting in complex post-treatment and serious environmental pollution. SUMMARY

[0005] The present application aims at solving the problems of excessive by-products, serious environmental pollution and low reaction efficiency in the prior art, and provides a method and system for preparing p-hydroxyacetophenone through continuous high-temperature alkali fusion demethylation, which is a collaborative innovation of deep integration of 'chemical process' and 'engineering equipment'.

[0006] The technical scheme of the present application is as follows:

[0007] The present application provides a method for preparing p-hydroxyacetophenone through continuous high-temperature alkali fusion demethylation, which comprises the following steps:

[0008] (1) The p-methoxyacetophenone and the sodium hydroxide solution are fully mixed at a molar ratio of 1:2-1:3, and the mixing time is controlled within 1.0 s to form a uniform slurry.

[0009] The molar ratio of the raw material p-methoxyacetophenone to the 30% NaOH solution is controlled within 1:2.0-1:3.0, and the control basis is that the excess alkali ensures complete demethylation, and ≤3 can avoid product condensation.

[0010] (2) The slurry is sent into a tubular reactor, which is divided into three temperature zones, wherein the preheating zone temperature is 220-230 DEG C, the main reaction zone temperature is 245-255 DEG C, and the cooling zone temperature is 240-245 DEG C, the pressure is set to 8-12 MPa, and the reaction time is 15-30 min.

[0011] The tubular reactor is temperature-controlled, and the three temperature zones are independently PID temperature-controlled with an error of ±1 DEG C.

[0012] (3) The material after the reaction is rapidly cooled, the temperature of the material is first rapidly reduced from the reaction temperature to below 80 DEG C within 0.1 s, and then further cooled to below 40 DEG C, so as to terminate the reaction.

[0013] (4) An acidic solution is slowly added to the reaction liquid after the reaction, the pH is adjusted to 5.5-6.5, and p-hydroxyacetophenone crystals are precipitated; then centrifugal separation, washing and drying are carried out to obtain p-hydroxyacetophenone crystal product.

[0014] Further, in step (1), the liquid p-methoxyacetophenone and the 30% sodium hydroxide solution with a molar ratio of 1:2.5 are respectively delivered to a static mixer at a constant flow rate, and fully mixed within 0.5 s to form a uniform reaction slurry.

[0015] Further, in the step (2), three temperature zones are arranged along the length of the tubular reactor, wherein the preheating zone accounts for 20% of the total length of the tubular reactor, the main reaction zone accounts for 60% of the total length of the tubular reactor, and the cooling zone accounts for 20% of the total length of the tubular reactor.

[0016] Further, in the step (2), the reaction pressure is 10 MPa, the temperature of the main reaction zone is 250±2℃, and the total residence time of the material in the tubular reactor is 22 min.

[0017] Further, the tubular reactor is made of Hastelloy C276 material, and the length-diameter ratio is 100:1.

[0018] The tubular reactor is internally provided with a spiral guide vane, the lead angle of which is 25-35°, and the Reynolds number Re is 8000-10000. The lead angle (preferably 30°) can generate strong turbulent flow, greatly enhancing the mass transfer and heat transfer efficiency, avoiding local overheating, and being the key to realizing high selectivity.

[0019] Further, in the step (3), the quenching rate is >100℃ / s, and the temperature is reduced to 80℃ within 0.05-0.1 s.

[0020] Further, in the step (4), 20% hydrochloric acid aqueous solution is slowly added to the reaction liquid after the reaction is terminated under stirring, the pH is adjusted to 6.0, at which time p-hydroxyacetophenone is crystallized and precipitated, and the obtained crystals are separated, washed, and dried to obtain the p-hydroxyacetophenone crystal product.

[0021] The application also provides a continuous high-temperature alkali fusion demethylation system for preparing p-hydroxyacetophenone, comprising:

[0022] A mixing unit is used for pre-mixing the reaction raw materials; the mixing unit comprises a storage tank and a static mixer;

[0023] A reaction unit is used for receiving the pre-mixed reaction raw materials for reaction; the reaction unit comprises a three-zone tubular reactor;

[0024] A quenching unit is used for instantaneously reducing the material to low temperature after the reaction is terminated, so as to terminate the reaction process; the quenching unit comprises a two-stage quenching heat exchange device;

[0025] A crystallization unit is used for acidifying and crystallizing the reaction product; the crystallization unit comprises an online pH control crystallization device.

[0026] Further, the static mixer is provided with an electric heating jacket; the three-temperature-zone tubular reactor is internally provided with three-head helical guide vanes; the two-stage quenching heat exchange device comprises a falling film quencher and a spiral plate heat exchanger connected in series, and the total heat exchange coefficient is greater than or equal to 2000 W / (m2*K); and the on-line pH regulation crystallization device is an acidification crystallization kettle provided with stirring and on-line pH monitoring.

[0027] The beneficial effects of the present application are as follows:

[0028] (1) The present application innovates the synthesis route, completely discards the traditional catalytic system, and solves the problems of pollution and separation difficulty:

[0029] The traditional route needs to use anhydrous aluminum chloride as a catalyst or rearrangement reagent, resulting in a large amount of phenol / aluminum-containing waste residue, complex post-treatment, serious environmental pollution, and the isomer of the by-product o-hydroxyacetophenone produced by the traditional route is difficult to separate, which restricts the product yield and purity. The present application adopts a direct path of high-temperature alkali demethylation, using p-methoxyacetophenone as a raw material, without any metal catalyst, and the reaction system is only NaOH and the product, which completely eliminates the generation of aluminum-containing waste residue from the source, and the post-treatment only needs simple acid precipitation, greatly reducing the environmental pressure and the cost of three-waste treatment.

[0030] (2) The continuous process of the present application can realize the leap of yield and productivity, and realize the innovation of yield and efficiency:

[0031] The method and system of the present application can realize a substantial increase in yield, with a yield of up to 96.7%, which is much higher than that of the traditional one-step method (76%) and two-step method (30-60%); this is mainly due to the plug flow characteristics in the continuous reactor, which ensures that all materials have uniform and accurately controlled reaction time, avoiding the common local over-reaction or insufficient reaction in batch tanks. At the same time, the reaction time of the present application is shortened from several hours (6 hours) of the traditional batch method to less than half an hour, realizing the leap from batch to continuous, and the production efficiency is improved by one order of magnitude, meeting the requirements of modern chemical industry for process intensification and cost reduction and efficiency improvement.

[0032] (3) The method of the present application realizes the control of impurities, and ensures the ultra-high purity of the product through kinetic control and engineering means:

[0033] The present application precisely controls the high temperature (optimum 250 DEG C) and short residence time (optimum 22 min), utilizes the huge difference (k1 / k2 = 35.75) between the demethylation (k1) and acetyl hydrolysis (k2) reaction rates, and greatly inhibits the occurrence of the acetyl hydrolysis side reaction in thermodynamics.

[0034] The application blocks the side reaction by engineering means, adopts millisecond quenching technology (temperature reduction within 0.05s), and lowers the material to low temperature at the moment after the reaction, so as to completely freeze the reaction process from the engineering aspect, and avoid the degradation and condensation of the product at high temperature.

[0035] The impurity level of p-hydroxyacetophenone prepared by the system and method is extremely low; the total amount of main by-products in the final product is less than 0.3%, and no raw material (methoxy) residue is detected, and the purity is as high as 99.32%; this shows that the process not only has complete conversion, but also has extremely high selectivity, and the product purity meets the requirements of high-end applications of medicines and cosmetics. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 The system structure schematic diagram for preparing p-hydroxyacetophenone by continuous high-temperature alkali demethylation provided by the application is shown in the figure.

[0037] Figure 2 The high-performance liquid chromatogram provided for example 1 of the application is shown in the figure. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0039] In order to further understand the application, the application will be further described in combination with the drawings and embodiments.

[0040] In the following embodiments, if the specific technology or condition is not specified, the technology or condition described in the literature in the art or according to the product manual is used. If the manufacturer of the reagent or instrument is not specified, it is a conventional product that can be purchased through a regular channel.

[0041] The application provides a method for preparing p-hydroxyacetophenone by continuous high-temperature alkali demethylation, which comprises the following steps:

[0042] (1) Raw material preparation and premixing:

[0043] The solid p-methoxyacetophenone is heated to its melting point (above 38℃) to completely melt into a liquid, and is delivered by a metering pump at a constant flow rate; at the same time, a 30% sodium hydroxide solution is delivered at a constant flow rate according to a molar ratio of 2:1-3:1 to p-methoxyacetophenone; the two materials are jointly introduced into a static mixer to realize efficient and sufficient mixing within 0.5-1.0s, and form a uniform reaction slurry.

[0044] (2) Continuous reaction:

[0045] The mixed reaction slurry obtained in step (1) is fed into a Hastelloy C276-made tubular reactor with built-in spiral guide vanes (length-diameter ratio of 100:1), the operating pressure of which is controlled at 8-12 MPa, and the reaction temperature along the length of the tubular reactor is divided into three precisely controlled temperature zones: a preheating zone (220-230℃, about 20% of the total length of the tubular reactor), a main reaction zone (245-255℃, about 60% of the total length of the tubular reactor), and a cooling-down zone (240-245℃, about 20% of the total length of the reactor), and the total residence time of the material in the tubular reactor is controlled at 15-30 min; the reaction is ensured to be carried out at 250±2℃ in the core section of the main reaction zone.

[0046] (3) Reaction termination and quenching:

[0047] The material after the reaction is immediately quenched, first setting the quenching rate >100℃ / s, and then rapidly reducing the temperature of the material from the reaction temperature to below 80℃ within 0.1 s, and then further cooling to below 40℃, so as to instantaneously terminate the reaction and prevent the hydrolysis or degradation of the product;

[0048] (4) Product separation and purification:

[0049] To the quenched reaction liquid, 20% hydrochloric acid aqueous solution is slowly added under stirring, and the pH of the system is accurately adjusted to 5.5-6.5 to promote the crystallization of p-hydroxyacetophenone; the obtained slurry is centrifuged, the filter cake is washed with deionized water, and then dried by a drying device to obtain high-purity p-hydroxyacetophenone crystal product.

[0050] As shown in Figure 1 The application also provides a continuous high-temperature alkali demethylation system for preparing p-hydroxyacetophenone, comprising:

[0051] A mixing unit comprising a p-methoxyacetophenone storage tank, a 30% NaOH storage tank, and an electrically heated static mixer, wherein the raw materials are transported from the storage tanks to the static mixer by metering pumps or high-pressure plunger pumps, and the efficient premixing of the reaction raw materials is realized in the static mixer;

[0052] A reaction unit comprising a three-zone tubular reactor, which is connected to the static mixer and receives the premixed reaction raw materials, and the three-zone tubular reactor is provided with three-head spiral guide vanes with a lead angle of 25-35° and a Reynolds number Re of 8000-10000, which greatly enhances the mass and heat transfer efficiency and avoids local overheating, so as to ensure that the uniformly mixed raw materials can be fully reacted in the reactor and reduce the generation of by-products;

[0053] The quenching unit comprises a falling film quencher and a spiral plate heat exchanger connected in series; the two-stage quenching heat exchange device is connected to the three-temperature-zone tubular reactor, which can instantly reduce the material to low temperature after the reaction is completed, thereby terminating the reaction process;

[0054] The crystallization unit comprises an acidification crystallization kettle with stirring and on-line pH monitoring and a continuous centrifuge; the reaction product of the quenching unit enters the acidification crystallization kettle, and the crystals are precipitated through acidification crystallization, and then the product crystals are separated through the continuous centrifuge.

[0055] Example 1

[0056] The embodiment provides a continuous high-temperature alkali fusion demethylation method for preparing p-hydroxyacetophenone, and comprises the following steps:

[0057] (1) Raw material preparation and premixing:

[0058] After the solid p-methoxyacetophenone is heated to 38 DEG C and completely melted into a liquid, the p-methoxyacetophenone is conveyed at a rate of 20 kg / h; meanwhile, the 30% sodium hydroxide solution is conveyed at a rate of 260 L / h (molar ratio 1:2.8); the two materials are jointly introduced into a static mixer, and the dispersion and efficient mixing are completed within 0.5 s, so that the uniform reaction slurry is formed;

[0059] (2) Continuous reaction:

[0060] The mixed reaction slurry obtained in step (1) is sent into a tubular reactor (length-diameter ratio is 100:1) made of Hastelloy C276 with an internal spiral guide vane, the operating pressure of the reactor is controlled at 10 MPa, and the reaction temperature is divided into three precisely controlled temperature zones along the length of the tubular reactor: the preheating zone is 225 DEG C, the residence time of the reaction slurry in the preheating zone is controlled at 7 min, the main reaction zone is 250 DEG C, the residence time of the reaction slurry in the main reaction zone is controlled at 12 min, and the cooling zone is 245 DEG C, the residence time of the reaction slurry in the cooling zone is controlled at 3 min;

[0061] (3) Reaction termination and quenching:

[0062] The material after the reaction is immediately quenched, the reaction liquid is reduced to 80 DEG C within 0.05 s through a falling film quencher, and then cooled to 40 DEG C through a spiral plate heat exchanger, and the reaction is terminated;

[0063] (4) Product separation and purification:

[0064] 20% hydrochloric acid aqueous solution is slowly added into the quenched reaction liquid under stirring, the pH of the system is accurately adjusted to 6.0, and p-hydroxyacetophenone is precipitated; the obtained slurry is separated through centrifugation, the filter cake is washed with deionized water, and then dried through a drying device, so that the high-purity p-hydroxyacetophenone crystal product is obtained.

[0065] The p-hydroxyacetophenone crystal product prepared in Example 1 was detected to obtain the spectrum as shown in Figure Figure 2 The analysis results are shown in Table 1 below:

[0066] Table 1 High performance liquid chromatography (HPLC) detection results

[0067] Indicator Results Detection method Product yield 96.7% HPLC area normalization p-Hydroxyacetophenone purity 99.32% HPLC area normalization Main by-products <0.3% HPLC area normalization Methoxy residue Not detected HPLC area normalization

[0068] Example 2

[0069] The present embodiment provides a method for preparing p-hydroxyacetophenone by continuous high-temperature alkali demethylation, comprising the following steps:

[0070] (1) Raw material preparation and premixing:

[0071] After the solid p-methoxyacetophenone is heated to 38°C to completely melt into a liquid, it is fed at a rate of 20 kg / h; at the same time, a 30% sodium hydroxide solution is fed at a rate of 185.6 L / h (molar ratio 1:2); the two streams are jointly fed into a static mixer, and the dispersion is completed within 0.7 s to achieve efficient and thorough mixing, forming a uniform reaction slurry;

[0072] (2) Continuous reaction:

[0073] The mixed reaction slurry obtained in step (1) is fed into a tubular reactor made of Hastelloy C276 with an internal spiral guide vane (length-diameter ratio of 100:1), and the operating pressure of the reactor is controlled at 8 MPa. The reaction temperature along the length of the tubular reactor is divided into three precisely controlled temperature zones: a preheating zone at 225°C, the residence time of the reaction slurry in this zone is controlled at 3 min, a main reaction zone at 248°C, the residence time of the reaction slurry in this zone is controlled at 9 min, and a cooling zone at 245°C, the residence time of the reaction slurry in this zone is controlled at 3 min;

[0074] (3) Reaction termination and quenching:

[0075] The material after the reaction is immediately quenched, and the reaction liquid is cooled to 80°C within 0.05 s by a falling film quencher, and then cooled to 40°C by a spiral plate heat exchanger, and the reaction is terminated;

[0076] (4) Product separation and purification:

[0077] To the quenched reaction liquid, 20% hydrochloric acid aqueous solution is slowly added under stirring, and the pH of the system is accurately adjusted to 6.0 to promote the crystallization of p-hydroxyacetophenone. The obtained slurry is separated by centrifugation, the filter cake is washed with deionized water, and then dried by a drying device to obtain high-purity p-hydroxyacetophenone crystal product.

[0078] Example 3

[0079] The embodiment provides a method for preparing p-hydroxyacetophenone by continuous high-temperature alkali fusion demethylation, and comprises the following steps:

[0080] (1) raw material preparation and premixing:

[0081] After the solid p-methoxyacetophenone is heated to 38 DEG C and completely melted into a liquid, the p-methoxyacetophenone is fed at a rate of 20 kg / h; meanwhile, a 30% sodium hydroxide solution is fed at a rate of 232 L / h (molar ratio 1:2.5); the two streams of materials are jointly fed into a static mixer, and the dispersion is completed within 1.0 s, so that efficient and sufficient mixing is realized, and a uniform reaction slurry is formed;

[0082] (2) continuous reaction:

[0083] The mixed reaction slurry obtained in step (1) is fed into a tubular reactor made of Hastelloy C276 and provided with a built-in spiral guide vane (length-diameter ratio 100:1), the operating pressure of the reactor is controlled at 12 MPa, and the reaction temperature is divided into three precisely controlled temperature zones along the length of the tubular reactor: a preheating zone at 225 DEG C, the residence time of the reaction slurry in the zone being controlled at 6 min, a main reaction zone at 252 DEG C, the residence time of the reaction slurry in the zone being controlled at 18 min, and a cooling zone at 245 DEG C, the residence time of the reaction slurry in the zone being controlled at 6 min;

[0084] (3) reaction termination and quenching:

[0085] The material after the reaction is immediately quenched, the reaction liquid is lowered to 80 DEG C within 0.05 s through a falling-film quencher, and then cooled to 40 DEG C through a spiral plate heat exchanger, and the reaction is terminated;

[0086] (4) product separation and purification:

[0087] 20% hydrochloric acid aqueous solution is slowly added to the quenched reaction liquid under stirring, the pH of the system is accurately adjusted to 6.0, and p-hydroxyacetophenone is caused to crystallize and precipitate; the obtained slurry is centrifuged, the filter cake is washed with deionized water, and then dried by a drying device, so that a high-purity p-hydroxyacetophenone crystal product is obtained.

[0088] The above description is only preferred embodiments of the present application, and is not a limitation on the present application, although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements to some technical features. Any modification, equivalent replacement, modification, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A process for the continuous production of p-hydroxyacetophenone by high temperature base demethylation, characterized in that, The method comprises the following steps: (1) uniformly mix p-methoxyacetophenone and sodium hydroxide solution at a molar ratio of 1:2-1:3 within 1.0 s to form a uniform slurry; (2) feed the slurry into a tubular reactor, which is divided into three temperature zones, wherein the preheating zone temperature is 220-230 DEG C, the main reaction zone temperature is 245-255 DEG C, and the temperature of the cooling zone is 240-245 DEG C, the pressure is set to 8-12 MPa, and the reaction time is 15-30 min; (3) after the reaction is completed, the material is rapidly cooled, the temperature of the material is first rapidly reduced from the reaction temperature to below 80 DEG C within 0.1 s, and then further cooled to below 40 DEG C, thereby terminating the reaction; (4) slowly add an acidic solution to the reaction liquid after the reaction is terminated, adjust the pH to 5.5-6.5, and precipitate p-hydroxyacetophenone crystals; then centrifugal separation, washing and drying are performed to obtain p-hydroxyacetophenone crystal product.

2. The method of claim 1, wherein, In step (1), liquid p-methoxyacetophenone and 30% sodium hydroxide solution at a molar ratio of 1:2.5 are respectively delivered to a static mixer at a constant flow rate, and fully mixed within 0.5 s to form a uniform reaction slurry.

3. The method of claim 1, wherein, In step (2), the reaction temperature is set along the length of the tubular reactor into three temperature zones, wherein the preheating zone accounts for 20% of the total length of the tubular reactor, the main reaction zone accounts for 60% of the total length of the tubular reactor, and the cooling zone accounts for 20% of the total length of the reactor.

4. The method of claim 1, wherein, In step (2), the reaction pressure is 10 MPa, the main reaction zone temperature is 250±2 DEG C, and the total residence time of the material in the tubular reactor is 22 min.

5. The method of claim 1, wherein, The tubular reactor is made of Hastelloy C276 material, and the length-diameter ratio is 100:1; The tubular reactor is internally provided with a helical guide vane, the lead angle of which is 25-35 DEG, and the Reynolds number Re is 8000-10000.

6. The method of claim 1, wherein, In step (3), the quenching rate is >100 DEG C / s, and the temperature is reduced to 80 DEG C within 0.05-0.1 s.

7. The method of claim 1, wherein, In step (4), 20% hydrochloric acid aqueous solution is slowly added to the reaction liquid after the reaction is terminated under stirring, the pH is adjusted to 6.0, at which point p-hydroxyacetophenone crystals are precipitated, and the obtained crystals are separated, washed and dried to obtain p-hydroxyacetophenone crystal product.

8. A system for continuous high temperature alkali demethylation for the production of p-hydroxyacetophenone, characterized in that, It comprises: a mixing unit for pre-mixing reaction raw materials; the mixing unit comprises a storage tank and a static mixer; a reaction unit for receiving the pre-mixed reaction raw materials for reaction; the reaction unit comprises a three-zone tubular reactor; a quenching unit for instantaneously reducing the material to low temperature after the reaction is completed, thereby terminating the reaction process; the quenching unit comprises a two-stage quenching heat exchange device. a crystallization unit for acidifying and crystallizing the reaction product; the crystallization unit comprises an online pH control crystallization device.

9. The system of claim 8, wherein, The static mixer is provided with an electric heating jacket; the three-zone tubular reactor is internally provided with a three-head helical guide vane; the two-stage quenching heat exchange device comprises a falling film quencher and a spiral plate heat exchanger connected in series; and the online pH control crystallization device is an acidification crystallization kettle provided with stirring and online pH monitoring.

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