High-purity BHET low-temperature continuous production process, matched catalyst and catalyst preparation method

Abstract

The invention discloses a high-purity BHET low-temperature continuous production process which comprises the following steps: (1) uniformly mixing waste PET powder and a catalyst, adding a solvent, stirring and mixing to form a muddy mixture, pumping the muddy mixture and ethylene glycol into a depolymerization kettle, and carrying out a depolymerization reaction to generate a BHET crude product; (2) the BHET crude product is continuously introduced into a hydrolysis kettle, water is continuously introduced, the BHET crude product is fully dissolved, the temperature in the hydrolysis kettle is 60-100 DEG C, and the pressure in the hydrolysis kettle is normal pressure; (3) filtering the fully dissolved BHET crude product, conveying a BHET liquid-phase product obtained by filtering into a purification tower, and carrying out purification treatment, so as to obtain a purified BHET liquid-phase product; and (4) cooling and crystallizing the purified BHET liquid-phase product, filtering and drying to obtain the high-purity BHET. According to the method, the alcoholysis reaction of PET and ethylene glycol is catalyzed by using the high-activity catalyst, the alcoholysis reaction temperature is reduced, the continuous production mode is more met, the produced BHET is purified, metal residues are reduced, and the purity of the BHET is further improved.

Description

Technical Field

[0001] The invention relates to the technical field of polymer material degradation and recycling, and in particular to a high-purity BHET low-temperature continuous production process and a matching catalyst and a catalyst preparation method. Background Art

[0002] PET plastics are an indispensable part of modern society and are widely produced and used. However, this is accompanied by a large amount of waste and recycling problems. The production of plastics relies on a large amount of fossil energy, so recycling waste plastics is not only an effective way to reduce environmental pollution, but also a way to reuse fossil energy. Compared with traditional physical recycling methods, upgrading and recycling waste plastics and converting them into high-value chemicals can not only help alleviate plastic pollution problems, but also achieve efficient utilization of plastic resources.

[0003] As one of the products of PET alcoholysis, bis(2-hydroxyethyl) terephthalate (BHET) is a multifunctional monomer material widely used in industries such as resins, coatings, foams, and tissue scaffolds. However, the current process of preparing BHET by PET alcoholysis generally uses intermittent reactors for production, which cannot achieve continuous operation; in addition, due to the use of homogeneous catalysts or conventional oxide catalysts, the temperature required for the depolymerization process is high and the energy consumption required is high; in addition, the catalyst added during the reaction often leaves trace amounts of residual metals (mainly iron, cobalt, nickel, zinc, and vanadium) in the final product. However, since there are currently no relevant national standards or industry standards, most technical solutions do not demetallize the produced BHET monomers, which greatly limits the scope of application of BHET.

[0004] At present, the processes for preparing BHET by depolymerization of PET mainly include:

[0005] Chinese patent application CN116554025A discloses a method for chemically preparing BHET powder from polyester bottle flakes, which involves brittle waste polyester bottle flakes into powder and then depolymerizing to prepare BHET powder. This method requires operation in an intermittent reactor, which makes it impossible to achieve continuous production, and the homogeneous catalyst used is difficult to recover.

[0006] Chinese patent application CN116983690A discloses a system and process for preparing BHET monomer from waste PET materials by chemical method. Although continuous production equipment is used, the depolymerization temperature is high due to the use of conventional oxides or solid acid catalysts, which is 200°C to 260°C, and the metal in the catalyst is not removed.

[0007] Chinese patent application CN114656684A discloses a method for preparing high-purity recycled PET polyester using waste PET polyester, wherein the depolymerization reaction temperature is 190°C to 230°C, which is relatively high, and the metal in the catalyst is not treated.

[0008] In view of this, the present invention develops a new process for low-temperature continuous production of BHET. Summary of the invention

[0009] In order to solve the above technical problems, the present invention provides a high-purity BHET low-temperature continuous production process and a supporting catalyst, which utilizes a highly active catalyst to catalyze the alcoholysis reaction of PET and ethylene glycol, thereby reducing the alcoholysis reaction temperature to make it more suitable for a continuous production mode, and purifies the produced BHET to reduce metal residues, thereby significantly improving the purity of the BHET monomer.

[0010] The technical solution adopted by the present invention is:

[0011] In a first aspect, the present invention provides a low-temperature continuous production process of high-purity BHET, comprising the steps of:

[0012] (1) mixing waste PET powder and catalyst evenly, adding solvent and stirring to form a sludge mixture, pumping the sludge mixture and ethylene glycol into a depolymerization kettle for depolymerization reaction to generate a BHET crude product, wherein the reaction temperature in the depolymerization kettle is 120-180° C. and the pressure is normal pressure;

[0013] (2) continuously introducing the crude BHET product into a hydrolysis kettle, and continuously introducing water to fully dissolve the crude BHET product, wherein the temperature in the hydrolysis kettle is 60 to 100° C. and the pressure is normal pressure;

[0014] (3) filtering the fully dissolved BHET crude product, and conveying the filtered BHET liquid product to a purification tower for purification treatment to obtain a purified BHET liquid product;

[0015] (4) The purified BHET liquid product is cooled and crystallized, and then filtered and dried to obtain high-purity BHET.

[0016] Preferably, the catalyst in step (1) is a Fe-Mg / ZnO catalyst.

[0017] Preferably, in step (1), the mass ratio of the waste PET powder to the catalyst is 10 to 50:1; more preferably, the mass ratio of the waste PET powder to the catalyst is 10 to 20:1.

[0018] Preferably, the solvent added when mixing the waste PET powder and the catalyst in step (1) is ethylene glycol, and the mass ratio of the waste PET powder to the total amount of ethylene glycol is 1:2 to 10; more preferably, the mass ratio of the waste PET powder to the total amount of ethylene glycol is 1:5.

[0019] Preferably, in step (1), the reaction time in the depolymerization reactor is controlled to be 1 to 3 hours.

[0020] Preferably, in step (3), the purification tower is filled with activated carbon and decolorizing resin, the mass ratio of the adsorbent activated carbon to the decolorizing resin is 1:1-5, and the mass ratio of the adsorbent activated carbon to the catalyst is 1:40-60.

[0021] Preferably, in step (4), the cooling crystallization temperature is lower than 10°C and the drying temperature is 50-100°C.

[0022] In a second aspect, the present invention also provides a catalyst for realizing a low-temperature continuous production process of high-purity BHET, wherein the catalyst is a Fe-Mg / ZnO catalyst, comprising a ZnO carrier and an active component Fe and an auxiliary component Mg loaded on the ZnO carrier.

[0023] Preferably, relative to the ZnO carrier, the loading amount of the active component Fe is 1 to 10 wt %, and the loading amount of the auxiliary component Mg is 0.5 to 5 wt %.

[0024] In a third aspect, the present invention also provides a method for preparing a Fe-Mg / ZnO catalyst, comprising the steps of: dissolving a precursor Zn salt in water, and dripping a mixed aqueous solution of sodium carbonate and sodium hydroxide until pH=10 to form a turbid liquid; then adding a precursor Fe salt and a Mg salt to the turbid liquid, and continuing to drip a mixed aqueous solution of sodium carbonate and sodium hydroxide until pH=12 to form a precipitate, and filtering, washing, drying, and calcining to obtain a Fe-Mg / ZnO catalyst.

[0025] Preferably, the calcination temperature is 500-600° C., and the calcination time is 3-8 hours.

[0026] The beneficial effects of the present invention are:

[0027] The present invention provides a low-temperature continuous production process of high-purity BHET and a supporting catalyst. The supporting Fe-Mg / ZnO catalyst of the present invention has high catalytic activity for PET and ethylene glycol alcoholysis, reduces the depolymerization temperature, and can react under normal pressure. In addition, the PET depolymerization efficiency is high and the BHET selectivity is high, so that it is more suitable for a continuous production mode and realizes the low-temperature continuous production of BHET. In addition, the present invention also performs purification treatment on BHET, which not only realizes decolorization, but also reduces metal residues, and significantly improves the purity and quality of the BHET monomer. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 It is a process flow chart of the continuous production of BHET of the present invention. DETAILED DESCRIPTION

[0030] The present invention provides a high-purity BHET low-temperature continuous production process and a supporting catalyst and a catalyst preparation method. In order to make the purpose, technical scheme and effect of the present invention clearer and more specific, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0031] On the one hand, the present invention provides a catalyst for the alcoholysis reaction of PET and ethylene glycol, wherein the catalyst is an Fe-Mg / ZnO catalyst, comprising a ZnO carrier and an active component Fe and an auxiliary component Mg loaded on the ZnO carrier, and relative to the ZnO carrier, the loading amount of the active component Fe is 1 to 10 wt%, and the loading amount of the auxiliary component Mg is 0.5 to 5 wt%. Preferably, the loading amount of the active component Fe is 5 to 10 wt%, and the loading amount of the auxiliary component Mg is 0.5 to 1 wt%.

[0032] The preparation method of the Fe-Mg / ZnO catalyst comprises the steps of: dissolving a precursor Zn salt in water, adding a mixed aqueous solution of sodium carbonate and sodium hydroxide until the pH value is 10 to form a turbid liquid; then adding a precursor Fe salt and a Mg salt to the turbid liquid, continuing to add a mixed aqueous solution of sodium carbonate and sodium hydroxide until the pH value is 12 to form a precipitate, filtering, washing, drying and calcining to obtain the Fe-Mg / ZnO catalyst.

[0033] In a preferred embodiment of the present invention, the precursors Zn salt, Fe salt, Mg salt are soluble salts such as nitrate, chloride, sulfate, acetate or carbonate, and preferably, nitrate is used.

[0034] The above-mentioned Fe-Mg / ZnO catalyst has high catalytic activity for the alcoholysis reaction of PET and ethylene glycol, can reduce the reaction temperature of the alcoholysis reaction of PET and ethylene glycol, and make the alcoholysis reaction of PET and ethylene glycol proceed at a relatively low temperature and room temperature, which is more useful for realizing continuous production.

[0035] On the other hand, the present invention uses the above-mentioned Fe-Mg / ZnO catalyst to realize the preparation of high-purity BHET by continuous reaction of PET and ethylene glycol at low temperature and normal pressure. The continuous production process includes the following steps:

[0036] (1) waste PET powder and Fe-Mg / ZnO catalyst are mixed evenly, and a small amount of ethylene glycol is added and stirred to form a muddy mixture, and then the muddy mixture and ethylene glycol are pumped into a depolymerization kettle for depolymerization reaction to generate a BHET crude product, the reaction temperature in the depolymerization kettle is 120-180° C., the pressure is normal pressure, and the reaction time in the depolymerization kettle is controlled to be 1-3 hours;

[0037] (2) continuously introducing the crude BHET product into a hydrolysis kettle, and continuously introducing water to fully dissolve the crude BHET product, wherein the temperature in the hydrolysis kettle is 60 to 100° C. and the pressure is normal pressure;

[0038] (3) filtering the fully dissolved BHET crude product, and conveying the filtered BHET liquid product to a purification tower for purification treatment to obtain a purified BHET liquid product;

[0039] (4) The purified BHET liquid product is cooled and crystallized, and then filtered and dried to obtain high-purity BHET.

[0040] In a preferred embodiment of the present invention, the mass ratio of the waste PET powder to the catalyst in the above step (1) is 10 to 50:1; preferably, the mass ratio of the waste PET powder to the catalyst is 10 to 20:1.

[0041] In a preferred embodiment of the present invention, the molar ratio of the waste PET powder to the total amount of ethylene glycol in the above step (1) is 1:10-50; more preferably, the mass ratio of the waste PET powder to the total amount of ethylene glycol is 1:2-10, preferably 1:2-5, and the above total amount of ethylene glycol refers to the total content of the small amount of ethylene glycol added when the waste PET powder and the catalyst are mixed and the ethylene glycol added to the depolymerization reactor.

[0042] In a preferred embodiment of the present invention, the reaction temperature in the depolymerization reactor in the above step (1) is 130-150°C. In some specific embodiments, the reaction temperature can be set to 130°C, 140°C, or 150°C.

[0043] In a preferred embodiment of the present invention, the mass ratio of water added to the hydrolysis kettle in the above step (2) to ethylene glycol added in the step (1) is 1 to 5:1, so as to reduce the viscosity of the system, remove water-insoluble impurities, and dissolve water-soluble impurities, thereby facilitating filtration and purification in subsequent steps.

[0044] In a preferred embodiment of the present invention, since waste PET is usually colored, the prepared BHET product is also colored accordingly and needs to be decolorized. At the same time, the reaction product also contains a catalyst, etc. Removing the metal oxide catalyst can further improve the purity of BHET. In step (3) of the present invention, the crude BHET product is purified by filling a purification tower with activated carbon and a decolorizing resin. The mass ratio of the adsorbed activated carbon to the decolorizing resin is 1:1 to 5, which can achieve decolorization and removal of the metal oxide catalyst. When the adsorbed activated carbon uses the catalyst of the present invention, the mass ratio of the catalyst to the adsorbed activated carbon is 40 to 60:1.

[0045] In a preferred embodiment of the present invention, the cooling crystallization temperature in the above step (4) is lower than 10°C, preferably 0°C, and the drying temperature is 50-100°C.

[0046] Example 1

[0047] In this embodiment, the Fe-Mg / ZnO catalyst is prepared, and the specific steps are as follows:

[0048] First, 500g of zinc nitrate was dissolved in 5L of deionized water, and after stirring, a 1mol / L mixed aqueous solution of sodium carbonate and sodium hydroxide (molar ratio of 1:1) was added dropwise, stirring until pH = 10, and then stirring was continued for 2h to form a turbid solution; then 92g of ferric nitrate and 13g of magnesium nitrate were added to the turbid solution, and after stirring, a 1mol / L mixed aqueous solution of sodium carbonate and sodium hydroxide was added dropwise, stirring until pH = 12, and stirring was continued for 6h before filtering. The obtained solid precipitate was washed 3 times with deionized water, and then dried at 150°C for 12h, and then placed in a calcination furnace, and heated to 550°C at a heating rate of 2°C / min, and calcined in an air atmosphere at 550°C for 5h. After the calcination, a Fe-Mg-Zn mixed oxide catalyst was obtained, which was recorded as Fe-Mg / ZnO catalyst.

[0049] In the catalyst prepared in this example, based on the mass of ZnO, the loading amount of Fe is 10 wt %, and the loading amount of Mg is 1 wt %.

[0050] Example 2

[0051] The difference between this embodiment and embodiment 1 is that in the Fe-Mg / ZnO catalyst prepared in this embodiment, the loading amount of Fe is 5 wt % and the loading amount of Mg is 0.5 wt % based on the mass of ZnO.

[0052] Example 3

[0053] Reference Figure 1In this embodiment, the Fe-Mg / ZnO catalyst prepared in the above embodiment 1 is used to catalyze the alcoholysis of waste PET and ethylene glycol to prepare BHET. The specific continuous production process of BHET is as follows:

[0054] (1) After cleaning, drying and preheating, the PET waste is crushed into 100 mesh particles in a grinder to obtain waste PET powder. The waste PET powder and Fe-Mg / ZnO catalyst are mixed uniformly at a mass ratio of 20:1, and a small amount of ethylene glycol is added and stirred to form a uniform muddy mixture. The muddy mixture and ethylene glycol are then pumped into a depolymerization kettle for depolymerization reaction to generate a BHET crude product. The pumping rate of the muddy mixture is 4.5 kg / h, of which the delivery rate of PET powder is about 4 kg / h, the delivery rate of catalyst is about 0.2 kg / h, and the rest is a small amount of ethylene glycol added when preparing the muddy mixture. The pumping rate of ethylene glycol is 18 L / h. The reaction temperature in the depolymerization kettle is adjusted to 150° C. The pressure valve is opened to keep the pressure in the reactor at normal pressure. When the pressure in the depolymerization kettle rises to the set value, the flow pump is started to start feeding, and it is ensured that the PET and ethylene glycol in the depolymerization kettle are fully reacted. The reaction time of the reaction materials in the depolymerization kettle is controlled to be 2 hours;

[0055] (2) The crude BHET product from the depolymerization reactor is continuously pumped into the hydrolysis reactor using a pump body, and the pumping rate is kept the same as the feed rate of the depolymerization reactor to maintain a stable liquid level in the depolymerization reactor; 40 L / h of water is continuously pumped into the hydrolysis reactor using a flow pump, the temperature of the hydrolysis reactor is adjusted to 90° C., and the pressure valve is opened to keep the pressure of the reactor at normal pressure, so that the crude BHET product is fully dissolved;

[0056] (3) continuously extracting the raw materials in the hydrolysis kettle through a pump body, and the extraction rate is kept the same as the feed rate of the hydrolysis kettle to maintain a stable liquid level in the hydrolysis kettle; after filtering the extracted reaction liquid with heat, a BHET liquid phase product and a catalyst mixed phase product are obtained, and the catalyst mixed phase product mainly consists of a small amount of unreacted PET, a small amount of water-insoluble dimer / polymer and catalyst, which can be further used for the depolymerization reaction in the above step (1) after drying and recovery, and the filtered BHET liquid phase product is pumped into a purification tower through a pump body for purification treatment, and the tower is filled with adsorption activated carbon and decolorization resin in a mass ratio of 1:1, and the filling amount of adsorption activated carbon and decolorization resin is 2% of the total mass of the added catalyst, to obtain a purified BHET liquid phase product;

[0057] (4) The purified BHET liquid product flows into a cooling tank, is cooled at 0°C for 2 hours, is filtered after crystallization, and the crystals are collected and dried at 50°C for 12 hours to obtain a high-purity BHET monomer.

[0058] Example 4

[0059] In this embodiment, the Fe-Mg / ZnO catalyst prepared in the above embodiment 1 is used to catalyze the alcoholysis of waste PET and ethylene glycol to prepare BHET. The specific continuous production process of BHET is as follows:

[0060] (1) After cleaning, drying and preheating, the PET waste is crushed into 100 mesh particles in a grinder to obtain waste PET powder. The waste PET powder and Fe-Mg / ZnO catalyst are mixed uniformly in a mass ratio of 10:1, and a small amount of ethylene glycol is added and stirred to form a uniform muddy mixture. The muddy mixture and ethylene glycol are then pumped into a depolymerization kettle for depolymerization reaction to generate a BHET crude product. The pumping rate of the muddy mixture is 4.65 kg / h, of which the delivery rate of PET powder is about 4 kg / h, the delivery rate of catalyst is about 0.4 kg / h, and the rest is a small amount of ethylene glycol added when preparing the muddy mixture. The pumping rate of ethylene glycol is 18 L / h. The reaction temperature in the depolymerization kettle is adjusted to 150 ° C, and the pressure valve is opened to keep the pressure of the reactor at normal pressure. When the pressure in the depolymerization kettle rises to the set value, the flow pump is started to start feeding, and it is ensured that the PET and ethylene glycol in the depolymerization kettle are fully reacted. The reaction time is controlled to ();

[0061] (2) The crude BHET product from the depolymerization reactor is continuously pumped into the hydrolysis reactor using a pump body, and the pumping rate is kept the same as the feed rate of the depolymerization reactor to maintain a stable liquid level in the depolymerization reactor; 40 L / h of water is continuously pumped into the hydrolysis reactor using a flow pump, the temperature of the hydrolysis reactor is adjusted to 90° C., and the pressure valve is opened to keep the pressure of the reactor at normal pressure, so that the crude BHET product is fully dissolved;

[0062] (3) continuously extracting the raw materials in the hydrolysis kettle through a pump body, and the extraction rate is kept the same as the feed rate of the hydrolysis kettle to maintain a stable liquid level in the hydrolysis kettle; after filtering the extracted reaction liquid with heat, a BHET liquid phase product and a catalyst mixed phase product are obtained, and the catalyst mixed phase product mainly consists of a small amount of unreacted PET, a small amount of water-insoluble dimer / polymer and catalyst, which can be further used for the depolymerization reaction in the above step (1) after drying and recovery, and the filtered BHET liquid phase product is pumped into a purification tower through a pump body for purification treatment, and the tower is filled with adsorption activated carbon and decolorization resin in a mass ratio of 1:1, and the filling amount of adsorption activated carbon and decolorization resin is 2% of the total mass of the added catalyst, to obtain a purified BHET liquid phase product;

[0063] (4) The purified BHET liquid product flows into a cooling tank, is cooled at 0°C for 2 hours, is filtered after crystallization, and the crystals are collected and dried at 50°C for 12 hours to obtain a high-purity BHET monomer.

[0064] Example 5

[0065] In this embodiment, the Fe-Mg / ZnO catalyst prepared in the above embodiment 1 is used to catalyze the alcoholysis of waste PET and ethylene glycol to prepare BHET. The specific continuous production process of BHET is as follows:

[0066] (1) After cleaning, drying and preheating, the PET waste is crushed into 100 mesh particles in a grinder to obtain waste PET powder. The waste PET powder and Fe-Mg / ZnO catalyst are mixed uniformly at a mass ratio of 20:1, and a small amount of ethylene glycol is added and stirred to form a uniform muddy mixture. The muddy mixture and ethylene glycol are then pumped into a depolymerization kettle for depolymerization reaction to generate a BHET crude product. The pumping rate of the muddy mixture is 4.65 kg / h, of which the delivery rate of PET powder is about 4 kg / h, the delivery rate of catalyst is about 0.2 kg / h, and the rest is a small amount of ethylene glycol added when preparing the muddy mixture. The pumping rate of ethylene glycol is 9 L / h. The reaction temperature in the depolymerization kettle is adjusted to 150° C. The pressure valve is opened to keep the pressure in the reactor at normal pressure. When the pressure in the depolymerization kettle rises to the set value, the flow pump is started to start feeding, and it is ensured that the PET and ethylene glycol in the depolymerization kettle are fully reacted. The reaction time is controlled to (2 h);

[0067] (2) The crude BHET product from the depolymerization reactor is continuously pumped into the hydrolysis reactor by a pump body, and the pumping rate is kept the same as the feeding rate of the depolymerization reactor to maintain a stable liquid level in the depolymerization reactor; 20 L / h of water is continuously pumped into the hydrolysis reactor by a flow pump, the temperature of the hydrolysis reactor is adjusted to 90° C., and the pressure valve is opened to keep the pressure of the reactor at normal pressure, so that the crude BHET product is fully dissolved;

[0068] (3) continuously extracting the raw materials in the hydrolysis kettle through a pump body, and the extraction rate is kept the same as the feed rate of the hydrolysis kettle to maintain a stable liquid level in the hydrolysis kettle; after filtering the extracted reaction liquid with heat, a BHET liquid phase product and a catalyst mixed phase product are obtained, and the catalyst mixed phase product mainly consists of a small amount of unreacted PET, a small amount of water-insoluble dimer / polymer and catalyst, which can be further used for the depolymerization reaction in the above step (1) after drying and recovery, and the filtered BHET liquid phase product is pumped into a purification tower through a pump body for purification treatment, and the tower is filled with adsorption activated carbon and decolorization resin in a mass ratio of 1:1, and the filling amount of adsorption activated carbon and decolorization resin is 2% of the total mass of the added catalyst, to obtain a purified BHET liquid phase product;

[0069] (4) The purified BHET liquid product flows into a cooling tank, is cooled at 0°C for 2 hours, is filtered after crystallization, and the crystals are collected and dried at 50°C for 12 hours to obtain a high-purity BHET monomer.

[0070] Example 6

[0071] In this embodiment, the Fe-Mg / ZnO catalyst prepared in the above embodiment 2 is used to catalyze the alcoholysis of waste PET and ethylene glycol to prepare BHET. The specific continuous production process of BHET is as follows:

[0072] (1) After cleaning, drying and preheating, the PET waste is crushed into 100 mesh particles in a grinder to obtain waste PET powder. The waste PET powder and Fe-Mg / ZnO catalyst are mixed uniformly at a mass ratio of 20:1, and a small amount of ethylene glycol is added and stirred to form a uniform muddy mixture. The muddy mixture and ethylene glycol are then pumped into a depolymerization kettle for depolymerization reaction to generate a BHET crude product. The pumping rate of the muddy mixture is 4.5 kg / h, of which the delivery rate of PET powder is about 4 kg / h, the delivery rate of catalyst is about 0.2 kg / h, and the rest is a small amount of ethylene glycol added when preparing the muddy mixture. The pumping rate of ethylene glycol is 18 L / h. The reaction temperature in the depolymerization kettle is adjusted to 130 ° C, and the pressure valve is opened to keep the pressure of the reactor at normal pressure. When the pressure in the depolymerization kettle rises to the set value, the flow pump is started to start feeding, and it is ensured that the PET and ethylene glycol in the depolymerization kettle are fully reacted. The reaction time is controlled to ();

[0073] (2) The crude BHET product from the depolymerization reactor is continuously pumped into the hydrolysis reactor using a pump body, and the pumping rate is kept the same as the feed rate of the depolymerization reactor to maintain a stable liquid level in the depolymerization reactor; 20 L / h of water is continuously pumped into the hydrolysis reactor using a flow pump, the temperature of the hydrolysis reactor is adjusted to 70° C. The pressure valve is opened to keep the pressure of the reactor at normal pressure, so that the crude BHET product is fully dissolved;

[0074] (3) continuously extracting the raw materials in the hydrolysis kettle through a pump body, and the extraction rate is kept the same as the feed rate of the hydrolysis kettle to maintain a stable liquid level in the hydrolysis kettle; after filtering the extracted reaction liquid with heat, a BHET liquid phase product and a catalyst mixed phase product are obtained, and the catalyst mixed phase product mainly consists of a small amount of unreacted PET, a small amount of water-insoluble dimer / polymer and catalyst, which can be further used for the depolymerization reaction in the above step (1) after drying and recovery, and the filtered BHET liquid phase product is pumped into a purification tower through a pump body for purification treatment, and the tower is filled with adsorption activated carbon and decolorization resin in a mass ratio of 1:1, and the filling amount of adsorption activated carbon and decolorization resin is 2% of the total mass of the added catalyst, to obtain a purified BHET liquid phase product;

[0075] (4) The purified BHET liquid product flows into a cooling tank, is cooled at 5°C for 4 hours, is filtered after crystallization, and the crystals are collected and dried at 80°C for 10 hours to obtain a high-purity BHET monomer.

[0076] Example 7

[0077] The difference between this embodiment and embodiment 1 is that in this embodiment, only decolorizing resin is loaded into the purification tower during purification treatment.

[0078] The BHET monomer samples prepared in the above Examples 3-7 were sampled and dissolved, and liquid chromatography analysis and elemental analysis were performed to test the BHET purity and the Fe-Mg-Zn metal content in the samples, as shown in Table 1 below.

[0079] Table 1

[0080]

[0081] It can be seen from Table 1 that in Examples 3-7, Fe-Mg / ZnO catalysts were used to catalyze the alcoholysis of PET and ethylene glycol to prepare BHET. At a relatively low reaction temperature of 130-150°C, the Fe-Mg / ZnO catalyst already had a very high catalytic activity, and during the reaction, the PET conversion rate, BHET selectivity, and BHET purity were all very high. Moreover, after adding activated carbon during the purification process, the metal substances doped in BHET can be basically completely removed.

[0082] It should be noted that some parts not described in the present invention can be implemented by adopting or drawing on existing technologies.

[0083] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A low-temperature continuous production process of high-purity BHET, characterized in that: Includes steps: (1) mixing waste PET powder and catalyst evenly, adding solvent and stirring to form a sludge mixture, pumping the sludge mixture and ethylene glycol into a depolymerization kettle for depolymerization reaction to generate a BHET crude product, wherein the reaction temperature in the depolymerization kettle is 120-180° C. and the pressure is normal pressure; (2) continuously introducing the crude BHET product into a hydrolysis kettle, and continuously introducing water to fully dissolve the crude BHET product, wherein the temperature in the hydrolysis kettle is 60 to 100° C. and the pressure is normal pressure; (3) filtering the fully dissolved BHET crude product, and conveying the filtered BHET liquid product to a purification tower for purification treatment to obtain a purified BHET liquid product; (4) The purified BHET liquid product is cooled and crystallized, and then filtered and dried to obtain high-purity BHET.

2. A low-temperature continuous production process of high-purity BHET according to claim 1, characterized in that: The catalyst in step (1) is a Fe-Mg / ZnO catalyst.

3. A low-temperature continuous production process of high-purity BHET according to claim 1, characterized in that: In the step (1), the mass ratio of the waste PET powder to the catalyst is 10 to 50:1; In the step (1), the solvent added when the waste PET powder and the catalyst are mixed is ethylene glycol, and the mass ratio of the waste PET powder to the total amount of ethylene glycol is 1:2-10.

4. A low-temperature continuous production process of high-purity BHET according to claim 1, characterized in that: In the step (1), the reaction time in the depolymerization reactor is controlled to be 1 to 3 hours.

5. A low-temperature continuous production process for high-purity BHET according to claim 1, characterized in that: In the step (3), the purification tower is filled with activated carbon and decolorizing resin, the mass ratio of the adsorbed activated carbon to the decolorizing resin is 1:1-5, and the mass ratio of the adsorbed activated carbon to the catalyst is 1:40-60.

6. A low-temperature continuous production process of high-purity BHET according to claim 1, characterized in that: In the step (4), the cooling crystallization temperature is lower than 10°C and the drying temperature is 50-100°C.

7. A catalyst for realizing the low-temperature continuous production process of high-purity BHET according to any one of claims 1 to 6, characterized in that: The catalyst is a Fe-Mg / ZnO catalyst, which comprises a ZnO carrier and an active component Fe and an auxiliary component Mg loaded on the ZnO carrier.

8. The catalyst according to claim 7, characterized in that: Relative to the ZnO carrier, the loading amount of the active component Fe is 1-10 wt%, and the loading amount of the auxiliary component Mg is 0.5-5 wt%.

9. The method for preparing a catalyst according to claim 7, characterized in that: The method comprises the steps of: dissolving a precursor Zn salt in water, and dripping a mixed aqueous solution of sodium carbonate and sodium hydroxide until the pH value is 10 to form a turbid liquid; Subsequently, the precursors Fe salt and Mg salt are added to the turbid solution, and a mixed aqueous solution of sodium carbonate and sodium hydroxide is continuously added dropwise until the pH value is 12 to form a precipitate, which is then filtered, washed, dried, and calcined to obtain a Fe-Mg / ZnO catalyst.

10. The method for preparing a catalyst according to claim 9, characterized in that: The calcination temperature is 500-600°C and the calcination time is 3-8h.

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