Vacuum cooling crystallization process of DMT in process of recovering PET by chemical method

Abstract

The invention provides a vacuum cooling crystallization process of DMT (Dimethyl Terephthalate) in a process of recovering PET (Polyethylene Terephthalate) by a chemical method, which comprises the following steps: conveying a reaction liquid of an ester exchange reaction kettle to a DMT crystallizing tank, carrying out vacuum cooling on the DMT crystallizing tank after the reaction liquid is conveyed to separate out DMT crystals, and enabling evaporated gas-phase methanol to enter a condenser group; nitrogen is introduced into the DMT crystallizing tank, normal pressure in the tank is recovered, then the DMT slurry in the tank is discharged into a DMT slurry storage tank, and a discharging pipeline of the DMT crystallizing tank is cleaned. According to the vacuum cooling crystallization process for DMT in the process of recovering PET by the chemical method, the quality of a DMT product is improved, DMT crystals with the particle size distribution being close to 150-200 [mu] m are obtained, and the yield of the DMT product is up to 90-93%.

Description

Technical Field

[0001] The invention belongs to the field of chemical industry, and in particular relates to a vacuum cooling crystallization process of DMT in a chemical method for recovering PET. Background Art

[0002] The vacuum cooling crystallization process is mainly used in the crystallization process of dimethyl terephthalate (DMT) in the chemical recycling of waste polyethylene terephthalate (PET). The target product DMT is generated by the transesterification reaction of BHET (ethylene terephthalate) and excess methanol. After the reaction, the reaction liquid contains unreacted methanol and by-products DMI (dimethyl isophthalate), ethylene glycol and other substances in addition to DMT. In order to separate DMT from it, after the transesterification reaction is completed, the reaction liquid is sent to the DMT crystallization tank, and the methanol in the reaction liquid is evaporated by vacuum suction. The DMT is cooled and crystallized by using the latent heat of evaporation of methanol, and high-purity DMT is obtained by centrifugal separation and remelting.

[0003] In the process of chemical recycling of PET, the DMT crystallization process affects the filtration and recovery of DMT in the subsequent centrifugal separation section, and the particle size distribution of DMT crystals obtained by the existing crystallization process is unqualified, resulting in more DMT loss in centrifugal separation. This is mainly because the cooling rate is the key to the distribution of crystal size. Too fast a cooling rate will result in a smaller particle size of DMT crystals, with the particle size concentrated in 50-100μm. A large amount of DMT fine crystals are dissolved in the liquid phase and are not centrifuged into the solid phase, resulting in a decrease in the DMT yield. If the cooling rate is too slow, the crystal particles obtained will be larger, but the crystallization time will be prolonged, affecting the overall process operation. Secondly, the liquid phase in the conventional mechanically stirred DMT crystallizer is not fully mixed, methanol evaporates in large quantities near the phase interface and the stirring paddle, DMT preferentially precipitates crystals near the phase interface and the stirring paddle, and the DMT crystallization rate in the crystallizer is inconsistent, resulting in uneven distribution of DMT crystal particle size. Summary of the invention

[0004] In view of this, the present invention aims to overcome the defects in the prior art and proposes a vacuum cooling crystallization process for DMT in the process of chemical recovery of PET.

[0005] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0006] The present invention provides a vacuum cooling crystallization process for DMT in a chemical method for recovering PET, comprising the following steps:

[0007] The reaction liquid of the transesterification reactor is transported to the DMT crystallization tank. After the transport is completed, the DMT crystallization tank is vacuum cooled to make DMT crystallize and evaporated gaseous methanol enters the condenser group;

[0008] Nitrogen is introduced into the DMT crystallization tank to restore the normal pressure in the tank, and then the DMT slurry in the tank is discharged into the DMT slurry storage tank, and the discharge pipeline of the DMT crystallization tank is cleaned.

[0009] Furthermore, the vacuum cooling step in step (1) is specifically as follows: closing the inlet pipeline switch valve, starting the vacuum pump, and adjusting the opening of the second pressure regulating valve.

[0010] Further, after the vacuum cooling step in step (1) is completed, the pressure in the DMT crystallization tank is 25-27 kPaA, the temperature of the reaction liquid is 40-43° C., and the time of the vacuum cooling step is 100-110 minutes.

[0011] Further, the vacuum cooling step in step (1) is specifically as follows: in the first 20-25 minutes of the vacuum cooling step, the opening of the second pressure regulating valve is 30-40%, the pumping rate is 2-5m3 / min, the pressure of the DMT crystallization tank drops from normal pressure -105kPaA to 65-70kPaA, and the temperature drops from 70-73°C to 65-68°C; within 25-80min of the vacuum cooling step, the opening of the second pressure regulating valve is 60-70%, the pumping rate is 1-4m3 / min, the pressure of the DMT crystallization tank drops from 65-70kPaA to 27-30kPaA, and the temperature drops from 65-68°C to 42-45°C; within 80-100min of the vacuum cooling step, the opening of the second pressure regulating valve is 50-60%, the pumping rate is 1-3m 3 / min, the pressure of the DMT crystallization tank dropped from 27-30kPaA to 25-27kPaA, and the temperature dropped from 42-45℃ to 40-43℃.

[0012] Furthermore, the pumping rate is calculated by the following formula:

[0013] Se=2.3×a(V+V1) / Δt×lg(p1 / p2)

[0014] Where a is the amplification factor, 1.5≤a≤1.8, V is the volume of the vacuum system, V1 is the amount of non-condensable gas generated by the vacuum system, Δt is the pumping time, p1 is the initial pressure of the system, and p2 is the final pressure of the system pumping.

[0015] Furthermore, the temperature of the cooling medium of the condenser group in step (1) is 30-35°C.

[0016] Further, after the reaction liquid in step (1) is transported, the liquid phase at the bottom of the DMT crystallization tank is extracted and circulated back to the DMT crystallization tank through a circulation pump; the circulation volume of the circulation step is 1100-1150m 3 / h.

[0017] The present invention also provides a vacuum cooling crystallization system for DMT in a chemical method for recovering PET, comprising a DMT crystallization tank, a condenser group, and a vacuum pump. The upper portion of the DMT crystallization tank is provided with an eccentric feed port, a gas phase outlet, a nitrogen inlet, and a condensate reflux port, and the lower portion is provided with a discharge port. The gas phase outlet is connected to the condenser group through a pipeline, and the condenser group is respectively connected to the condensate reflux port and the vacuum pump through pipelines.

[0018] Further, the angle between the center line of the eccentric feed port and the center line of the DMT crystallization tank is 30-60 degrees; preferably, the angle between the center line of the eccentric feed port and the center line of the DMT crystallization tank is 45-55 degrees.

[0019] Furthermore, the system is also provided with a circulation pump, a circulation feed port is provided in the middle of the DMT crystallization tank, a circulation discharge port is provided at the bottom, the circulation liquid discharge port is connected to the circulation pump through a pipeline, and the circulation pump is connected to the circulation feed port through a pipeline.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] The vacuum cooling crystallization process of DMT in the chemical method for recovering PET described in the present invention resets the vacuum cooling program according to the formula and actual operation data, sets different cooling rates at different crystallization stages, ensures that the supersaturation level in the crystallization tank is controlled within a reasonable range during the entire cooling process, and achieves optimal operation from a time dimension; adopts forced circulation outside the tank to simulate the flow field in the tank, obtains the optimal circulation volume and eccentric feeding angle, fully mixes the materials in the radial and axial directions in the entire crystallization tank, ensures that the supersaturation of the entire crystallization system is uniform, and achieves optimal operation from a spatial dimension; and accurately controls the supersaturation of the crystallization process in time and space.

[0022] The vacuum cooling crystallization process of DMT in the chemical method for recovering PET described in the present invention not only improves the quality of DMT products, but also obtains DMT crystals with a particle size distribution concentrated around 150-200 μm, and the DMT product yield is as high as 90-93%. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of a vacuum cooling crystallization system for DMT in a chemical method for recovering PET according to an embodiment of the present invention;

[0024] Figure 2 It is a schematic diagram of the DMT crystallization tank described in the embodiment of the present invention;

[0025] Figure 3A bottom view of the DMT crystallization tank according to an embodiment of the present invention;

[0026] Figure 4 It is a cooling curve diagram of the existing DMT crystallization process described in the embodiment of the present invention;

[0027] Figure 5 It is an optimal cooling curve diagram of the DMT crystallization process described in the embodiment of the present invention;

[0028] Figure 6 It is a flow field simulation diagram in the crystallization tank under different circulation rates according to the embodiment of the present invention;

[0029] Figure 7 This is a curve diagram showing the change of concentration with different circulation amounts within 200 seconds according to an embodiment of the present invention;

[0030] Figure 8 This is the concentration distribution diagram of the plane 1.75m high according to the embodiment of the present invention;

[0031] Fig. 9 This is a concentration distribution diagram of a plane 3.7 m high according to an embodiment of the present invention;

[0032] Fig.10 This is a concentration distribution diagram of a plane 4.75 m high according to an embodiment of the present invention;

[0033] Fig.11 It is a flow field simulation diagram in the crystallization tank at different feeding angles according to the embodiment of the present invention;

[0034] Fig.12 A comparison diagram of the actual on-site cooling curve and the optimal cooling curve according to the embodiment of the present invention;

[0035] Fig.13 It is a normal distribution curve diagram of DMT crystals according to an embodiment of the present invention: wherein, Figure A is a cooling curve of the existing process, and Figure B is an optimal cooling curve;

[0036] Fig.14 It is a comparison curve diagram of the normal distribution curve of DMT crystal with different cooling curves described in the embodiment of the present invention;

[0037] Fig.15 It is a normal distribution curve diagram of DMT crystals described in an embodiment of the present invention: wherein, Figure A is mechanical stirring, and Figure B is circulation outside the tank;

[0038] Fig.16 This is a comparison diagram of normal distribution curves of DMT crystals in different stirring modes according to the embodiments of the present invention;

[0039] Fig.17It is a normal distribution curve diagram of DMT crystals described in an embodiment of the present invention: wherein, Figure A is central feeding, and Figure B is eccentric feeding;

[0040] Fig.18 This is a comparison diagram of normal distribution curves of DMT crystals at different circulating feed ports according to an embodiment of the present invention;

[0041] Fig.19 It is a comparison diagram of the normal distribution curve of DMT crystals described in the embodiment of the present invention.

[0042] Description of reference numerals:

[0043] 1. DMT crystallization tank; 2. Circulation pump; 3. Primary condenser; 4. Secondary condenser; 5. Vacuum pump; 6. Inlet pipeline switch valve; 7. Outlet pipeline switch valve; 8. Exhaust line switch valve; 9. First pressure regulating valve; 10. Second pressure regulating valve, 11. Sampling port; N1, eccentric feed port; N2, circulation feed port; N3, discharge port; N4, gas phase outlet; N5, circulation discharge port; N6, condensate reflux port; N7, nitrogen inlet. DETAILED DESCRIPTION

[0044] Unless otherwise defined, the technical terms used in the following examples have the same meanings as those generally understood by those skilled in the art to which the present invention belongs. The test reagents used in the following examples, unless otherwise specified, are all conventional biochemical reagents; the experimental methods, unless otherwise specified, are all conventional methods.

[0045] The present invention will be described in detail below with reference to the embodiments.

[0046] Example 1

[0047] In the existing DMT crystallization process, the vacuum pump has a pumping rate of 8-12m / s. 3 / min, using a cooling curve such as Figure 4 As shown: In the early stage of crystallization, the pressure regulating valve opening is 60-80%, and the pumping rate is 4-10m 3 / min, the vacuum degree drops rapidly. After 20-25min, the pressure of the crystallizer drops from normal pressure -105kPaA to 40-45kPaA, and the temperature of the crystallizer drops from 70-73℃ to 53-56℃. The supersaturation rises sharply, and the crystal nucleus generation rate is too fast, which is much greater than the crystal nucleus growth rate. A large number of DMT fine crystals with a particle size of 50-100μm are generated, resulting in a wider range of DMT crystal size distribution and reduced product quality. In the middle and late stages of crystallization, the vacuum pump's exhaust rate decreases due to the increase in vacuum degree. Therefore, when the pressure regulating valve opening is 60-80%, the exhaust rate is 1-3m 3 / min, the vacuum degree decreases slightly. During the cooling time of 25-80min, the vacuum degree decreases from 40-45kPaA to 27-30kPaA, and the crystallizer temperature decreases from 53-56°C to 42-45°C. During the cooling time of 80-100min, the vacuum degree decreases from 27-30kPaA to 25-27kPaA, and the crystallizer temperature decreases from 42-45°C to 40-43°C. The supersaturation level is low, which is not conducive to the crystallization of residual DMT in the mother liquor. The degree of supersaturation changes greatly during the entire cooling process, and the obtained DMT crystals are finely broken, with a particle size distribution concentrated near 75-125μm, and the product yield is only 65-68%.

[0048] According to the existing DMT crystallization process, the process data of the DMT crystallization tank in the crystallization process was obtained through experiments, and the results are shown in Table 1.

[0049] Table 1 Process data of crystallization tank in existing DMT crystallization process

[0050]

[0051] Where T is the measured value of the thermometer, P is the measured value of the pressure gauge, and Se is the measured value of the volume flow meter. According to the calculation formula of the intermittent vacuum system pumping rate and combined with Table 1, the following formula is derived:

[0052] Se=2.3×a(V+V1) / Δt×lg(p1 / p2) (1)

[0053] Se—evacuation rate of vacuum system, m 3 / min

[0054] a—Amplification factor

[0055] V—Volume of vacuum system, m 3

[0056] V1—the amount of non-condensable gas generated by the vacuum system, m 3

[0057] Δt—Evacuation time, min

[0058] p1—initial system pressure, kPaA

[0059] p2—System vacuuming end pressure (after Δt time), kPaA

[0060] The volume of the crystallization system is 90m 3, and according to the process data in Table 1, the amplification factor is fitted to be 1.5-1.8. The amount of non-condensable gas V1 generated by the vacuum system is obtained based on theoretical calculations, and the calculation process is as follows: First, the reaction liquid in the DMT crystallizer is simplified to a ternary mixture of methanol (1)-ethylene glycol (2)-DMT (3). According to the material balance, a batch of material feed is 15,000 kg and the molar fraction of each component in the ternary mixture is obtained. Since DMT continuously crystallizes from the liquid phase to the solid phase during the cooling process and deviates from the vapor-liquid equilibrium, in order to simplify the calculation, it is assumed that the molar fraction of DMT remains unchanged and the liquid phase is treated as an ideal solution. The composition of each component in the mixture is as follows: z1=0.65, z2=0.30, z3=0.05. The crystallizer is a low-pressure system, so the gas phase is regarded as an ideal gas mixture, so the vapor-liquid equilibrium constant can be simplified to

[0061] K i =y i / x i =p s i / p2 (2)

[0062] Through simulation, the saturated vapor pressure of each substance at different temperatures in the pure component state is obtained, and the Antoine equation constant of each substance is fitted based on this. The specific formula is as follows:

[0063] Methanol: lnp s 1=9.223-3258.87 / (T-54.1) (3)

[0064] Ethylene glycol: LNP s 2=9.1188-4520.4 / (T-72.759) (4)

[0065] DMT: lnp s 3=7.983-4795.0 / (T-91.3) (5)

[0066] Therefore K i It is a function of T and p2. According to the material balance equation

[0067] z i =x i (1-η)+y i η (6)

[0068] η—gas phase fraction

[0069] Combined with the vapor-liquid equilibrium relationship, we get

[0070] y i =z i K i / (1+η(K i-1)) (i=1,2,3) (7)

[0071] And the normalized equation of gas phase composition gives η with respect to K i The equation

[0072] ∑z i K i / (1+η(K i -1))=1 (8)

[0073] According to the temperature and pressure data in Table 1, the gas phase fraction η is calculated by trial and error method, and the vaporization amount in the crystallization tank at different temperatures and pressures is obtained therefrom, and V1 is 2-5% of the vaporization amount.

[0074] At the same time, according to the T and P data in Table 1, a TP relationship is fitted as follows:

[0075] P=0.00013×T 4 -0.0264×T 3 +2.02×T 2 -67.496×T+851.517 (9)

[0076] In order to obtain a more ideal DMT particle size distribution and a higher product yield, the above formula is combined to calculate the required pumping rate under the ideal cooling rate. The theoretical calculation results are shown in Table 2, and the optimal cooling curve is drawn according to Table 2. Figure 5 shown.

[0077] Table 2 Theoretical calculation of pumping rate

[0078]

[0079] Under normal pressure, the vacuum pump has a pumping rate of 6-10m 3 / min, with the increase of vacuum degree, the vacuum pump's exhaust rate decreases. Combined with the theoretically calculated exhaust rate, the valve opening at each crystallization stage can be determined, and the vacuum cooling program can be reset as follows: in the early stage of crystallization, 20-25 minutes before cooling, the second pressure regulating valve opening is 30-40%, and the exhaust rate is 2-5m 3 / min, the crystallization tank pressure drops from normal pressure -105kPaA to 65-70kPaA, the temperature drops from 70-73℃ to 65-68℃, the temperature is slowly lowered to control the supersaturation level to prevent the crystal nucleation rate from being too fast; within the cooling time of 25-80min, the second pressure regulating valve opening is 60-70%, and the pumping rate is 1-4m 3 / min, the crystallization tank pressure drops from 65-70kPaA to 27-30kPaA, and the temperature drops from 65-68℃ to 42-45℃, maintaining a high supersaturation during the crystal nucleus growth period to obtain a high precipitation amount and DMT crystals with appropriately large particle size and concentrated distribution; within the cooling time of 80-100min, the second pressure regulating valve opening is 50-60%, and the pumping rate is 1-3m 3 / min, the crystallization tank pressure dropped from 27-30kPaA to 25-27kPaA, and the temperature dropped from 42-45°C to 40-43°C to ensure that the residual DMT in the solution was precipitated as much as possible.

[0080] Conventional crystallization processes mostly use mechanical stirring to mix the reaction liquid. However, the liquid phase at the phase interface and near the stirring paddle has a large gas-liquid contact area or a large degree of turbulence, resulting in a high degree of supersaturation. There are dead zones in other parts of the DMT crystallizer, and the liquid flow effect is poor, resulting in different crystallization rates in different parts of the crystallizer.

[0081] Therefore, if Figure 1-4 As shown, on the basis of resetting the vacuum cooling program, forced circulation outside the tank is adopted. The solution in the mixing tank is mixed by circulating at a large flow rate to promote mass transfer and heat transfer between the solute and the solvent during the crystallization process. A new circulation pipeline is added outside the DMT crystallizer. After the crystallizer receives the reaction liquid, the circulation pump is turned on to extract the liquid phase at the bottom of the crystallizer and pump it back into the crystallizer through the circulation pump to increase the turbulence of the liquid phase in the crystallizer. The greater the flow rate of forced circulation outside the tank, the greater the shear force and turbulence between the materials in the tank, which helps to speed up the mixing speed and improve the heat transfer efficiency. However, too high a circulation flow rate will also increase the energy consumption of the pump and increase the wear of the pump. Therefore, it is necessary to optimize the circulation flow rate according to the specific process requirements to achieve the best mixing effect and economic benefits. The volume of a batch of materials in the crystallizer is 45-55m 3 The vacuum cooling process takes about 100-110 minutes, and the circulating pump flow rate is initially selected to be 700m 3 / h or more, and select the circulating feed port as the central feed and the circulating flow rate is 700, 800, 900, 1000, 1100, 1200m 3 / h, and simulate the flow field of the crystallizer. Assuming that the initial material concentration in the tank is 0, the material concentration at the circulating feed port is 1, different circulating amounts of material are introduced into the crystallizer, and the changes in the material concentration in the crystallizer over time are recorded. The flow field simulation in the crystallizer under different circulation amounts is as follows: Figure 6 Take three points on the axis of the crystallizer, 1.75m, 3.7m, and 4.75m from the bottom of the crystallizer, and record the changes in concentration at these three points with different circulation volumes within 200s, as shown in Figure 1. Figure 7 shown.

[0082] from Figure 6 It can be seen intuitively that within 200s, as the circulation volume increases, the mixing effect of the liquid in the crystallizer becomes better. Figure 6 The concentrations at the three points in the crystallization tank all increase with the increase of the circulation volume. When the circulation volume is 1100m 3 / h, the increase of the three-point concentration increases sharply, and when the circulation volume is 1200m 3 / h, the increase of the three-point concentration tends to be gentle. Obviously, when the circulation volume is 1100m 3 / h or more, the influence of the increase in circulation volume on the mixing effect in the crystallization tank gradually decreases. Considering the energy consumption and wear of the pump, the present invention selects a flow rate of 1100m 3 / h circulation pump for forced circulation.

[0083] The circulating discharge port is 500-600mm above the crystallizer discharge port, and the liquid at the bottom of the crystallizer is extracted to prevent large-size DMT crystals from accumulating and clogging at the discharge pipe port after deposition. The circulating feed port is in the middle of the crystallizer cylinder, 2400-2600mm away from the tangent line of the lower head. The conventional feed port is a central feed, and the center line of the pipe port coincides with the center line of the crystallizer, which has little effect on the flow field in the tank. Therefore, the circulating feed port is changed from central feed to eccentric feed, and the center line of the pipe port and the center line of the crystallizer form a certain angle, so that the material flows in a spiral manner in the tank body after passing through the circulating feed port, which greatly affects the flow field in the tank. At the same time, the angle of the eccentric feed port will also affect the flow field distribution in the tank. As the feed angle increases, the speed and kinetic energy of the fluid entering the tank increase, which may lead to stronger vortices and larger mixing areas, thereby enhancing the turbulence of the fluid in the tank and making the fluid in the tank more evenly mixed. However, too high a feed angle may lead to excessive mixing, thereby increasing energy consumption and the instability of the tank structure. Therefore, four groups of crystallization tanks with eccentric feed angles of 0°, 35°, 45°, and 55° were selected to simulate the flow field in the tank, and the results are as follows: Fig.11 Assume that the bottom nozzle of the crystallizer is the reference horizontal plane, take three planes with heights of 1.75, 3.7, and 4.75 m in the crystallizer, and record the concentration distribution of each point on the horizontal straight line at the center of these three planes under different incident angles within 200 s, as shown in Figure 8-10 Show.

[0084] The flow field simulation in the crystallization tank at different feeding angles is as follows Fig.11As shown, it can be seen that the eccentric feed changes the flow field distribution in the crystallizer. The concentration of the eccentric feed on the horizontal straight line at the center of the three planes is higher than that of the central feed. After passing through the circulating feed port, the material flows in a spiral manner in the tank body, which enhances the turbulence of the fluid in the tank and improves the mixing efficiency. The concentration distributions of the eccentric angles of 45° and 55° are similar. Considering the stability of the tank structure, the optimal eccentric feed angle is selected as 45°, which not only improves the mass and heat transfer efficiency of the fluid, reduces the nucleation and scaling problems of crystallization to a minimum, but also ensures the stability of the crystallizer and extends the service life of the crystallizer.

[0085] Through field experiments, the vacuum degree and temperature of crystallization at different stages under the vacuum cooling program were recorded. The results are shown in Table 3. The three groups of experimental data and the optimal cooling curve are compared. Fig.12 As shown;

[0086] Table 3 Process data of the crystallization tank after resetting the vacuum cooling program

[0087]

[0088] from Fig.12 It can be seen that the cooling curve during actual operation on site is consistent with the optimal cooling curve, so it can be shown that the vacuum cooling program reset according to the above formula and the relationship between the vacuum pump's suction volume and the valve opening is accurate and repeatable, verifying the correctness of the above formula. By changing the valve opening of the second pressure regulating valve on the vacuum suction pipeline, the pressure drop in the crystallization system can be controlled, thereby controlling the cooling rate at different crystallization stages, and achieving gradient cooling to achieve optimal operation.

[0089] Five crystallization tests were conducted under the existing process cooling curve and the optimal cooling curve, and 1 kg of the DMT slurry obtained after crystallization was filtered, dried and weighed to obtain DMT crystals (m1) and filtrate (m2). The filtrate was analyzed by liquid chromatography to obtain the content of DMT component in the filtrate (x1), and the total mass M and yield y of DMT were calculated according to formulas (10) and (11):

[0090] M=m1+m2x1 (10)

[0091] y=m1 / M (11)

[0092] The total mass M and yield of DMT were calculated; DMT crystal particles of different particle sizes were screened using sieves of different meshes, and the weight percentage (wt%) of the material under the sieves of different sieves was obtained to obtain the DMT particle size distribution data, and the intermediate particle size MS was calculated. The intermediate particle size MS can represent the volume average particle size of the DMT crystals. The results are shown in Table 4. The average value of the five groups of data was taken to draw a normal distribution curve as shown in Table 4. Fig.13As shown, and compare the normal distribution curves of the two Fig.14 shown.

[0093] Table 4 DMT crystal size distribution and yield obtained from different cooling curves

[0094]

[0095] By resetting the vacuum cooling program, the optimal cooling curve is obtained, the temperature is slowly cooled during the nucleation period, the cooling rate is fast during the crystal growth period, and the residence time is guaranteed as long as possible in the late crystallization period, so that the supersaturation at different crystallization periods is maintained at a reasonable level, the nucleation and scaling problems during the crystallization process are greatly reduced, and the generation of DMT fine crystals is effectively reduced. As shown in Table 4, after changing the vacuum cooling program, the concentrated distribution of DMT crystal size increases from 75-125μm to around 100-150μm, the intermediate particle size MS increases from 104.5μm to 126.7μm, and the DMT product yield increases from 65-68% to 75-78%.

[0096] Conventional cooling crystallization usually uses mechanical stirring to achieve the purpose of mixing the liquid phase. The present invention uses forced circulation outside the tank on the basis of resetting the vacuum cooling procedure, and selects a flow rate of 1100m 3 / h circulation pump for external circulation of the tank, and sampling and comparison of the two stirring methods, the sampling and calculation methods are as described above, four groups of experiments are repeated to obtain DMT yield and particle size distribution data, draw a normal distribution curve and calculate the intermediate particle size MS, the results are shown in Table 5, take the average value of the five groups of data to draw a normal distribution curve as shown in Fig.15 As shown, and compare the normal distribution curves of the two Fig.16 shown.

[0097] Table 5 DMT crystal size distribution obtained by different stirring methods

[0098]

[0099]

[0100] The data of DMT crystal size distribution in Table 5 can fully illustrate that the external circulation of the tank intensifies the turbulence of the liquid phase in the DMT crystallization tank, fully mixes the materials in the radial and axial directions, improves the mass transfer and heat transfer efficiency of the fluid in the tank, eliminates local excessive supersaturation, ensures uniform supersaturation in the entire crystallization tank, avoids local nucleation explosion, keeps the DMT crystallization speed consistent, and obtains DMT crystals with appropriately large particle size and concentrated particle size distribution. Fig.16It can be seen that after changing the stirring mode, the particle size distribution of DMT crystals is more concentrated, the concentrated particle size distribution area increases from 100-150μm to 150-200μm, the DMT fine crystals with a particle size less than 100μm are significantly reduced, and the intermediate particle size MS increases from 126.7μm to 166.3μm, which not only improves the product quality, but also increases the DMT product yield from 75-78% to 85-88%.

[0101] On this basis, the circulating feed port was changed from central feed to eccentric feed, and a crystallization tank with an eccentric feed angle of 45° was selected. The same sampling comparison was performed to obtain the DMT particle size distribution data, draw a normal distribution curve, and calculate the intermediate particle size MS. The results are shown in Table 6. The average value of the five groups of data was taken to draw a normal distribution curve as shown in Fig.17 As shown, and compare the normal distribution curves of the two Fig.18 shown.

[0102] Table 6 DMT crystal size distribution obtained from different circulating feed ports

[0103]

[0104] from Fig.18 It can be seen intuitively that the DMT crystal particle size distribution obtained by eccentric feeding is narrower and the peak value is higher. From the data in Table 6, it can be seen that the DMT intermediate particle size MS increases from 166.3μm to 175.6μm. Therefore, eccentric feeding can enhance the turbulence of the fluid in the crystallizer, make the material mixing more complete, and further improve the mass and heat transfer efficiency between the solute and the solvent in the DMT crystallizer. The yield of DMT product is increased from 85-88% to 90-93%.

[0105] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A vacuum cooling crystallization process for DMT in a chemical recovery process of PET, characterized in that: The steps include: (1) The reaction liquid of the transesterification reactor is transported to the DMT crystallization tank. After the transport is completed, the DMT crystallization tank is vacuum cooled to allow DMT to crystallize and evaporate gaseous methanol to enter the condenser group; (2) nitrogen is introduced into the DMT crystallization tank to restore the normal pressure in the tank, and then the DMT slurry in the tank is discharged into the DMT slurry storage tank, and the discharge pipeline of the DMT crystallization tank is cleaned.

2. The vacuum cooling crystallization process for DMT in the chemical method for recovering PET according to claim 1, characterized in that: The vacuum cooling step in step (1) is specifically as follows: closing the inlet pipeline switch valve, starting the vacuum pump, and adjusting the opening of the second pressure regulating valve.

3. The vacuum cooling crystallization process for DMT in the chemical method for recovering PET according to claim 2, characterized in that: After the vacuum cooling step in step (1) is completed, the pressure in the DMT crystallization tank is 25-27 kPaA, the temperature of the reaction liquid is 40-43° C., and the time of the vacuum cooling step is 100-110 minutes.

4. The vacuum cooling crystallization process for DMT in the chemical method for recovering PET according to claim 3, characterized in that: The vacuum cooling step in step (1) is specifically as follows: in the first 20-25 minutes of the vacuum cooling step, the opening of the second pressure regulating valve is 30-40%, and the pumping rate is 2-5m 3 / min, the pressure of the DMT crystallization tank drops from normal pressure -105kPaA to 65-70kPaA, and the temperature drops from 70-73℃ to 65-68℃; within 25-80min of the vacuum cooling step, the opening of the second pressure regulating valve is 60-70%, and the pumping rate is 1-4m 3 / min, the pressure of the DMT crystallization tank drops from 65-70kPaA to 27-30kPaA, and the temperature drops from 65-68℃ to 42-45℃; within 80-100min of the vacuum cooling step, the opening of the second pressure regulating valve is 50-60%, and the pumping rate is 1-3m 3 / min, the pressure of the DMT crystallization tank dropped from 27-30kPaA to 25-27kPaA, and the temperature dropped from 42-45℃ to 40-43℃.

5. The vacuum cooling crystallization process for DMT in the chemical method for recovering PET according to claim 4, characterized in that: The pumping rate is calculated by the following formula: Se=2.3×a(V+V1) / Δt×lg(p1 / p2) Where a is the amplification factor, 1.5≤a≤1.8, V is the volume of the vacuum system, V1 is the amount of non-condensable gas generated by the vacuum system, Δt is the pumping time, p1 is the initial pressure of the system, and p2 is the final pressure of the system pumping.

6. The vacuum cooling crystallization process for DMT in the chemical method for recovering PET according to claim 1, characterized in that: The temperature of the cooling medium of the condenser group in step (1) is 30-35°C.

7. The vacuum cooling crystallization process for DMT in the chemical method for recovering PET according to claim 1, characterized in that: After the reaction liquid in step (1) is transported, the liquid phase at the bottom of the DMT crystallizer is extracted and circulated back to the DMT crystallizer through a circulation pump; the circulation volume of the circulation step is 1100-1150m 3 / h.

8. A vacuum cooling crystallization system for DMT in a chemical process for recovering PET, characterized in that: The invention comprises a DMT crystallizer, a condenser group and a vacuum pump. The upper part of the DMT crystallizer is provided with an eccentric feed port, a gas phase outlet, a nitrogen inlet and a condensate reflux port, and the lower part is provided with a discharge port. The gas phase outlet is connected to the condenser group through a pipeline, and the condenser group is connected to the condensate reflux port and the vacuum pump through pipelines respectively.

9. The vacuum cooling crystallization system for DMT in the chemical recovery of PET according to claim 8, characterized in that: The angle between the center line of the eccentric feed port and the center line of the DMT crystallization tank is 30-60 degrees; preferably, the angle between the center line of the eccentric feed port and the center line of the DMT crystallization tank is 45-55 degrees.

10. The vacuum cooling crystallization system for DMT in the chemical recovery of PET according to claim 8, characterized in that: The system is also provided with a circulation pump, a circulation feed port is provided in the middle of the DMT crystallization tank, a circulation discharge port is provided at the bottom, the circulation liquid discharge port is connected to the circulation pump through a pipeline, and the circulation pump is connected to the circulation feed port through a pipeline.

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