Catalyst regulation and control device for preparing lactide through continuous depolymerization and process system and method for preparing lactide through continuous depolymerization

Abstract

The invention provides a catalyst regulation and control device for preparing lactide through continuous depolymerization, a regulating valve is arranged in a tank body and can regulate the protruding degree of a protruding part protruding into the tank body, and when the protruding part is in the maximum protruding degree, the bottom of the protruding part is lower than an inlet and an outlet of the tank body; a depolymerization reactor, a circulating tank and a catalyst regulation and control device which are connected in sequence are adopted to realize continuous depolymerization to prepare lactide, the fluid state in the tank body is regulated and controlled according to the protruding degree of a protruding part, the amount of a catalyst entering a circulating material is controlled, regulation and control of the amount of the catalyst in the depolymerization reactor are realized, and long-period stable operation of production is realized. According to the present invention, with the catalyst regulation and control device, the redundant catalyst can be captured, the lactide racemization degree and the substrate coking carbonization probability are reduced, the slag discharge interval is effectively prolonged, and the oligomer conversion rate and the product quality are improved.

Description

Technical Field

[0001] The invention relates to the technical field of lactide preparation, in particular to a catalyst control device capable of realizing continuous reaction, and a corresponding process system. Background Art

[0002] Controlling the use of disposable non-degradable plastic products has become a global consensus, and degradable materials have ushered in major development opportunities. Currently, commercial biodegradable plastics include polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene succinate (PBS), polybutylene succinate-adipate (PBSA), polybutylene terephthalate-adipate (PBAT), etc. Among them, PLA is currently the most widely used and has broad development prospects in the future. It not only has the basic properties of general polymer materials, but also has better processing properties, physical and mechanical properties and biodegradability. It can be widely used in the packaging industry, textile industry, agricultural industry, consumer goods market, etc., and is considered to be the most likely biodegradable material to replace petroleum-based polyester.

[0003] Industrialized polylactic acid synthesis is mainly obtained through the ring-opening polymerization of lactide. Lactide is the key to the entire synthesis process. The production process barriers are relatively high. Usually, the lactic acid raw material needs to be prepared through polycondensation and depolymerization processes under a catalyst, high temperature, and high vacuum system. The continuous production process of lactide is an important basis for the industrial production of lactide. In the continuous production of lactide, in order to improve the utilization rate of raw materials, a cyclic depolymerization process is usually adopted, that is, the heavy components produced by depolymerization are mixed with fresh materials and then transported to the depolymerization reactor again. The more prominent problem in this process is that it will lead to the accumulation of high molecular weight heavy components and catalysts during the synthesis process, which is easy to cause coking and carbonization of the substrate on the reactor surface.

[0004] CN111153886A discloses a method and device for synthesizing lactide quickly and with high yield. After the light components are removed, the oligomeric lactic acid is added with a catalyst and then passes through a mixer, and then enters a depolymerization reactor for depolymerization into lactide. The heavy components are refluxed and then enter the depolymerization reactor again. The light components are purified and recovered to obtain a lactide product. The device can be used to efficiently synthesize lactide, and crude lactide with a yield of 94%-98% can be obtained within a short residence time of 0.5-5 minutes. After the light components are passed through a simple purification system, the lactide product has a L-lactide, D-lactide or DL-lactide content of 94%-98%, and an m-lactide content of 0.5%-5.5%. However, the heavy components produced by the depolymerization reaction of the present invention directly enter the depolymerization feed mixer and are mixed with the reaction raw materials to form a stream of materials, which then re-enter the depolymerization reactor for reaction. In the long-term operation, this not only affects the stable operation of the depolymerization reactor, but also increases the probability of coking and carbonization of the reaction substrate on the reactor surface as the molecular weight of the heavy components increases and the catalyst accumulates, thereby increasing the degree of racemization of lactide and affecting the continuous and stable operation of the reaction and the product quality.

[0005] It can be seen that how to achieve rapid adjustment of the circulating reaction materials in the continuous production process of lactide and adjust the catalyst therein to the appropriate feed amount is one of the key factors in achieving high-quality production of lactide. Summary of the invention

[0006] To address the above shortcomings, the present invention provides a catalyst control device for continuous depolymerization to prepare lactide and a process system for continuous depolymerization to prepare lactide. Through the catalyst control device, the catalyst in the circulating material can be adjusted in time according to the accumulation of the catalyst in the polymerization reactor, so as to flexibly realize the continuous production of lactide.

[0007] To achieve the above technical objectives, the first aspect of the present invention provides a catalyst control device for preparing lactide by continuous depolymerization, which includes a tank body with a sealed upper portion, an inlet and an outlet arranged opposite to each other on both sides of the tank body, and at least one protruding component is arranged on the top of the tank body, and is controlled as a whole by at least one regulating valve or separately by multiple regulating valves. When the protruding component forms a protruding state, it bulges into the interior of the tank body and occupies a certain space in the tank body, and when the protruding component is at the maximum protrusion degree, its bottom is lower than the inlet and outlet of the tank body.

[0008] Furthermore, when the protruding component is at its maximum protrusion, the distance between the bottom of the protruding component and the bottom of the tank body is not less than the inlet diameter of the catalyst regulating device, and not more than 2 times the inlet diameter, preferably 1-1.5 times.

[0009] Furthermore, when the protruding part is at its maximum protrusion, the height of the protruding part accounts for 2 / 3-9 / 10 of the height of the tank body, preferably 2 / 3-5 / 6.

[0010] Furthermore, the position of the inlet is lower than the position of the outlet, and the inlet is arranged at a position of 1 / 4 to 3 / 4 from bottom to top of the tank body.

[0011] Furthermore, when the protruding component protrudes downward, the plane formed by it is a smooth surface, preferably an arc-shaped surface.

[0012] Furthermore, the number of the protruding parts is 1-3, preferably 1 or 2, and most preferably 1.

[0013] Furthermore, the tank body is cylindrical, and its bottom is flat or U-shaped, preferably U-shaped.

[0014] In the catalyst control device of the present invention, the material enters the device from the inlet and leaves the device from the outlet. In this process, the protrusion degree of the protruding component is adjusted by the regulating valve to adjust the flow state of the fluid in the tank. Specifically, in the initial stage of the reaction, the amount of catalyst in the depolymerization reactor is small, and it is expected that the material returned to the depolymerization reactor contains more catalyst. At this time, the degree of downward protrusion of the protruding component is increased, so that the fluid in the tank of the catalyst control device forms turbulence, the catalyst is stirred, and no or less catalyst is controlled in the tank. When in the later stage of the reaction, more catalyst accumulation is generated in the depolymerization reactor, and it is expected that the material returned to the depolymerization reactor contains less catalyst, at this time, the degree of downward protrusion of the protruding component is reduced, so that the fluid forms a smaller turbulence or tends to a flat flow state. At this time, the catalyst and the heavy components in the material will form a deposit and remain in the tank, and the material leaving the device to be returned to the depolymerization reactor contains less catalyst and heavy components.

[0015] The technical purpose of the second aspect of the present invention is to provide a process system for preparing lactide by continuous depolymerization, comprising a depolymerization reactor, a circulation tank and a catalyst control device connected in sequence, the outlet of the catalyst control device is connected to the inlet of the depolymerization reactor, the bottom outlet of the depolymerization reactor is connected to the circulation tank, and a lactide outlet is arranged on the top.

[0016] Furthermore, at least two catalyst control devices are arranged in parallel and connected to corresponding switching pipelines to prevent excessive catalyst and heavy components from being deposited in one catalyst control device, which affects the subsequent control process, and to switch to another catalyst control device in time to ensure continuous reaction.

[0017] Furthermore, the volume of the catalyst control device does not exceed 1 / 3 of the volume of the circulation tank.

[0018] Furthermore, a pump is provided between the depolymerization reactor and the circulation tank for conveying materials from high vacuum to low vacuum or normal pressure, and at the same time plays a role in isolating high and low vacuum.

[0019] Furthermore, the discharge port of the circulation tank is arranged at the lowermost part of the circulation tank and is connected to the catalyst regulating device, and the position of the catalyst regulating device is preferably lower than the circulation tank.

[0020] Furthermore, the gas phase lactide outlet of the depolymerization reactor is connected to a crude lactide purification and refining system, including a process of purifying and refining the crude lactide.

[0021] The technical purpose of the third aspect of the present invention is to provide a process method for preparing lactide by continuous depolymerization using the above process system, wherein fresh lactic acid oligomer raw materials and a depolymerization catalyst are added to a depolymerization reactor, and gaseous lactide produced by depolymerization leaves the depolymerization reactor from the top, and materials and heavy components that have not yet reacted are discharged from the bottom of the depolymerization reactor, and enter the catalyst control device after passing through the circulation tank. In the initial stage of the reaction, the amount of catalyst in the depolymerization reactor is small, and it is expected that the material returned to the depolymerization reactor contains more catalyst. At this time, the degree of downward protrusion of the protruding part of the catalyst control device is increased, so that the fluid in the tank body forms turbulence, the catalyst is stirred, and will not or less be retained in the tank body, and enters the depolymerization reactor with the fluid; in the later stage of the reaction, more catalyst accumulation is generated in the depolymerization reactor, and it is expected that the material returned to the depolymerization reactor contains less catalyst. At this time, the degree of downward protrusion of the protruding part is reduced, so that the fluid forms a smaller turbulence or tends to a flat flow state, and the catalyst and the heavy components in the material will form a deposit and remain inside the catalyst control device, and the material returning to the depolymerization reactor from the device contains less catalyst and heavy components.

[0022] Furthermore, in the above process method, the method for judging whether a large amount of catalyst accumulation is generated in the depolymerization reactor is as follows: at the initial stage of the reaction, the regulating protrusion of the regulating valve of the catalyst control device is in the maximum protrusion position, and when it is detected that the content of m-lactide in the crude lactide sample at the discharge port of the circulation tank is between 2.5% and 3.0%, the regulating valve protrusion begins to be continuously retracted until it is retracted to the end point.

[0023] Furthermore, the molecular weight of the lactic acid oligomer is 800-4000, preferably 1200-3000, and is generally prepared from L-lactic acid or D-lactic acid by dehydration and polycondensation. The dehydration is to remove free water in the lactic acid, which can be done under normal pressure or reduced pressure; the reaction temperature of the polycondensation process is 140-170°C, the reaction time is 0.5-4.0h, and the absolute pressure is 1000-2000Pa.

[0024] Furthermore, in the above process, the depolymerization catalyst is a tin catalyst selected from at least one of stannous octoate, SnCl2 and SnO. The amount of the depolymerization catalyst is 0.4%-3.0% by weight of the lactic acid oligomers, preferably 0.8%-2.0%.

[0025] Furthermore, in the above process, the depolymerization reactor is a scraped film depolymerization reactor, and its main forms include any one of a thin film evaporator, a molecular distillation evaporator or other stirred film evaporator.

[0026] Furthermore, in the above process, a pump is provided between the depolymerization reactor and the circulation tank for conveying materials from high vacuum to low vacuum or normal pressure, and at the same time serves to isolate high and low vacuum.

[0027] Furthermore, in the above process, the reaction temperature of the depolymerization reactor is 180-220°C, preferably 190-220°C, and the vacuum degree is 200-1500Pa.

[0028] Furthermore, in the above process method, the feed mass ratio of the fresh material to the recycled material is 1:6-2:1.

[0029] Furthermore, in the above process method, the feed amount is increased, and the total feed amount of fresh material and recycled material is adjusted to 4-8 times the actual reaction amount, so as to reduce the residence time of lactic acid oligomers in the scraped film depolymerization reactor, inhibit the occurrence of the polymerization process, improve the overall yield, and ensure product quality.

[0030] Furthermore, in the above process, the liquid level in the circulation tank is maintained at 30%-70%, the pressure is maintained at 10kPa-normal pressure, and the temperature is maintained at 160-200°C to reduce the probability of continued intermolecular polymerization of lactic acid oligomers and reduce coking and carbonization.

[0031] Furthermore, in the above process method, regular slag discharge is performed with reference to the cumulative amount of catalyst circulation in the catalyst control device. In order to optimize the continuous operation of the device, slag discharge is performed when it is detected that the cumulative amount of catalyst circulation in the catalyst control device is greater than 15% of the circulating material; preferably, slag discharge is performed when it is greater than 15% but not greater than 20%, and most preferably, slag discharge is performed when it is greater than 15% but not greater than 18%. When discharging slag, switch to the parallel catalyst control device, and the lactic acid polymer discharged by slag discharge can be hydrolyzed and reused as lactic acid under certain conditions. The cumulative amount of catalyst circulation in the catalyst control device is monitored in real time by sampling and analyzing the catalyst content from the outlet at the rear end of the catalyst control device every 2h-4h.

[0032] Furthermore, in the above process, the depolymerization reactor is provided with a gas phase lactide outlet, which is directly transported to a separation and purification section. The separation and purification section can be any combination of single-stage or multi-stage distillation, solvent extraction and melt crystallization.

[0033] Furthermore, in the above process, the crude lactide product obtained has an L-lactide content of 85%-90%, an m-lactide content of 0.5%-3.5%, an L-lactic acid content of 2.0%-6.0%, and a dimer and trimer content of 2.0%-5.0%.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] (1) During the lactide production process, the accumulation of catalysts can easily lead to coking and carbonization of the reaction substrate on the reactor surface, and long-term stable operation cannot be guaranteed. By setting up a catalyst control device, excess catalyst can be captured, reducing the degree of lactide racemization and the probability of substrate coking and carbonization, effectively extending the slag discharge interval, and improving the oligomer conversion rate and product quality. In the obtained crude lactide, the m-lactide content is not more than 3.5%, and the lactic acid oligomer conversion rate can reach more than 95%. Compared with the traditional cyclic depolymerization process, the degree of lactide racemization is reduced by more than 50%, and the lactic acid oligomer conversion rate is increased by more than 10%.

[0036] (2) On the basis of adopting the cyclic depolymerization process, setting up more than two catalyst control tank pipelines can effectively ensure the continuous and stable operation of the device. Slag discharge can be achieved by simply cutting out the catalyst control tank, which greatly improves the operability of the process.

[0037] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the catalyst control device of the present invention;

[0039] Figure 2 The figure is a schematic diagram of the process system for preparing crude lactide according to the present invention. DETAILED DESCRIPTION

[0040] The following non-limiting embodiments may enable a person skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0041] The lactic acid used in the embodiment of the present invention is heat-resistant grade L-lactic acid with a lactic acid content of 88% or more, and an optical purity of not less than 99.0%.

[0042] The present invention adopts Malvern Viscotek OMNISEC GPC / SEC gel chromatograph to analyze the molecular weight of lactic acid oligomers. The conventional calibration method is adopted, polystyrene (PS) is used as the internal standard, the chromatographic column model is T3000, the size is 300mmL×8.0mm, the column temperature is 40°C, the flow rate is 1.0mL / min, the sample concentration is 2-5mg / mL, and the single injection volume is 500μL.

[0043] The present invention adopts Agilent high performance liquid chromatograph to analyze the chemical purity of lactide, L-lactic acid and dimer and trimer content, ultraviolet detector, phosphoric acid and acetonitrile as mobile phase, chromatographic column model is ZORBAX SB-Aq, column length is 250mm, column inner diameter is 4.6mm, internal filler particle size is 5μm, detection wavelength is 200nm, column temperature is 40℃, flow rate is 1mL / min, injection volume is 5μL.

[0044] The present invention adopts Agilent gas chromatograph to analyze the content of different optical isomers of lactide, selects CYCLOSIL-B model chromatographic column, vaporization chamber temperature of 250°C, detector temperature of 280°C, hydrogen flame ionization detector, column temperature program heating initial temperature of 100°C, maintained for 5 minutes, heated to 140°C at a rate of 4°C / min, maintained for 7 minutes, heated to 200°C at a rate of 8°C / min, maintained for 20 minutes, carrier gas N2 flow rate of 1.4mL / min, hydrogen flow rate of 30mL / min, air flow rate of 400mL / min, injection volume of 0.5μL.

[0045] The calculation formula for the conversion rate of lactic acid oligomers in the crude lactide synthesis process is as follows:

[0046] Lactic acid oligomer conversion rate = m / M × 100%

[0047] Wherein, m is the total mass of crude lactide, and M is the mass of initial lactic acid oligomers.

[0048] Example 1

[0049] A catalyst control device for continuous depolymerization to prepare lactide, such as Figure 1 As shown, it includes a tank body 30 with an upper sealing, the tank body 30 is U-shaped, and an inlet 33 and an outlet 34 are arranged on both sides of the upper part thereof, the inlet 33 is located lower than the outlet 34, and the inlet 33 is arranged at a position of 1 / 4 of the tank body from bottom to top. A regulating valve 31 is connected to the top of the tank body, which can adjust the protrusion degree of the protruding part 32 that bulges into the inside of the tank body. When the protruding part forms a protruding state, a smooth arc surface is formed, which occupies a certain space in the tank body 30, and when the protruding part 32 is at the maximum protrusion degree, its bottom is lower than the inlet and outlet of the tank body, and the distance between the bottom of the protruding part and the bottom of the tank body is 1.5 times the diameter of the inlet pipe of the catalyst control device, and when the protruding part is at the maximum protrusion degree, the height of the protruding part occupies 2 / 3 of the height of the tank body.

[0050] Example 2

[0051] A process system for preparing lactide by continuous depolymerization, such as Figure 2As shown, it includes a depolymerization reactor 1, a circulation tank 2 and two catalyst control devices 300 and 301 of Example 1 connected in parallel, the outlets of the catalyst control devices 300 and 301 are connected to the inlet of the depolymerization reactor 1, the bottom outlet of the depolymerization reactor 1 is connected to the circulation tank 2, and a lactide outlet is arranged on the top of the depolymerization reactor 1, which is connected to the crude lactide purification system 4.

[0052] The method for producing lactide using the above process system is as follows: fresh lactic acid oligomer raw material 5 to which a depolymerization catalyst has been added enters the depolymerization reactor 1, gaseous lactide 8 produced by depolymerization leaves the depolymerization reactor 1 from the top, and the material and heavy components 6 that have not yet reacted are discharged from the bottom of the depolymerization reactor 1, and enter the catalyst control device 300 after passing through the circulation tank. The amount of catalyst in the circulating material is adjusted by adjusting the protrusion degree of the protruding component 32 of the catalyst control device, and the circulating material 7 is obtained and returned to the depolymerization reactor 1. When the slag discharge condition is met, another catalyst control device is turned on, and after normal operation, the valves before and after the first catalyst control device are closed, and the catalyst control device is removed to clean and discharge slag, and then it is returned to the original position after cleaning for the next switch.

[0053] The specific production process and results of lactide using the above system and method are as follows:

[0054] Example 3

[0055] Preparation of lactic acid oligomers: (1) Dehydration of lactic acid: Take 10 kg of L-lactic acid (lactic acid content of about 88.0%) and add it into a four-necked flask with a stirring system. Use a vacuum circulating water pump to maintain the pressure of the system at about 50 kPa. Start heating under vacuum and gradually heat to 110-120°C. Dehydrate for 2 hours. At this time, the free water in the reaction system is slowly evaporated out of the reaction system. (2) Preparation of lactic acid oligomers: After the free water in the system is almost completely removed, increase the vacuum degree of the system and slowly reduce the pressure of the system to about 1.0 kPa. The temperature of the feed liquid is gradually raised to 160°C and reacted for 3 hours. At this time, a polycondensation reaction occurs between lactic acid molecules. The water generated by the reaction in the system is evaporated out of the system to obtain a lactic acid oligomer with a molecular weight of 2200.

[0056] Take 5000g of the above-mentioned lactic acid oligomer, add 50g of stannous octoate catalyst, mix well and transport to the scraped film depolymerization reactor at a flow rate of 150g / h, control the depolymerization reaction conditions of the lower reaction zone as follows: vacuum degree 300Pa, reaction temperature 200℃, single-pass residence time 2min, the material that has not reacted in time and the heavy components after the reaction are discharged into the circulation tank, the material of the circulation tank is discharged from the bottom, and then after passing through a catalyst control device, the material returns to the depolymerization reactor for cyclic depolymerization. In the initial stage of cyclic depolymerization, the protruding part 32 of the catalyst control device regulating valve 31 is in the maximum protruding position. When the content of m-lactide in the crude lactide sample is detected to reach 2.5%, the regulating valve 31 will continuously retract the protruding part 32 until it reaches the end point. After that, the catalyst content is analyzed by sampling from the rear end of the catalyst control device to monitor the material at the bottom of the catalyst control device in real time to detect the cumulative amount of catalyst circulation. When the cumulative amount of catalyst circulation reaches 15% of the circulating material, another catalyst control device is turned on. After normal operation, the valves before and after the first catalyst control device are closed, and the catalyst control device is removed for cleaning and slag removal. After cleaning, it is returned to its original position for the next switch. The catalyst control devices work alternately to achieve continuous and stable operation of the entire process. During the reaction, the mass ratio of fresh material to circulating material is adjusted to 1:2, and the total feed amount is adjusted to 4 times the actual reaction amount. As the reaction proceeds, the liquid level of the circulation tank is controlled to be maintained at 50%, the pressure is maintained at 50kPa, and the temperature is maintained at 180°C.

[0057] Analysis of the crude lactide product showed that the L-lactide content was 87.3%, the m-lactide content was 2.4%, the L-lactic acid content was 3.4%, the dimer and trimer content was 3.6%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process reached 97.2%.

[0058] Example 4

[0059] The preparation process of lactic acid oligomer is the same as that of Example 3, except that the final reaction temperature is 170° C., the reaction time is 4.0 h, and the molecular weight of the obtained lactic acid oligomer is 4000.

[0060] Take 5000g of the above-mentioned lactic acid oligomer, add 150g of stannous octoate catalyst, mix well and transport to the scraped film depolymerization reactor at a flow rate of 150g / h, control the depolymerization reaction conditions in the lower reaction zone as follows: vacuum degree 200Pa, reaction temperature 220°C, one-way residence time 2min, the material that has not reacted in time and the heavy component after the reaction are discharged into the circulation tank, the circulation tank material is discharged from the bottom, and then the material is returned to the depolymerization reactor for cyclic depolymerization after passing through a catalyst control device. The operation of the catalyst control device is the same as in Example 3. During the reaction, the mass ratio of fresh material to circulating material is controlled to be 2:1, and the total feed amount is regulated to be 4 times the actual reaction amount. As the reaction proceeds, the circulation tank liquid level is controlled to be maintained at 60%, the pressure is maintained at normal pressure, and the temperature is maintained at 200°C.

[0061] Analysis of the crude lactide product showed that the L-lactide content was 86.8%, the m-lactide content was 3.1%, the L-lactic acid content was 2.6%, the dimer and trimer content was 3.8%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process reached 96.1%.

[0062] Example 5

[0063] The preparation process of lactic acid oligomer is the same as that of Example 3, except that the reaction time is 2.0 h, and the molecular weight of the prepared lactic acid oligomer is 800.

[0064] Take 5000g of the above-mentioned lactic acid oligomer, add 20g of stannous octoate catalyst, mix well and transport to the thin film depolymerization reactor at a flow rate of 150g / h, control the depolymerization reaction conditions of the lower reaction zone as follows: vacuum degree 1500Pa, reaction temperature 180°C, one-way residence time 2min, the material that has not reacted in time and the heavy component after the reaction are discharged into the circulation tank, the circulation tank material is discharged from the bottom, and then the material is returned to the depolymerization reactor for cyclic depolymerization after passing through a catalyst control device. The operation of the catalyst control device is the same as in Example 3. During the reaction, the mass ratio of fresh material to circulating material is regulated to 1:6, and the total feed amount is regulated to 8 times the actual reaction amount. As the reaction proceeds, the liquid level of the circulation tank is controlled to be maintained at 70%, the pressure is maintained at 10kPa, and the temperature is maintained at 160°C.

[0065] Analysis of the crude lactide product showed that the L-lactide content was 85.4%, the m-lactide content was 2.1%, the L-lactic acid content was 5.6%, the dimer and trimer content was 4.8%, and the lactic acid oligomer conversion rate in the crude lactide synthesis process reached 95.8%.

[0066] Example 6

[0067] The preparation process of lactic acid oligomers and lactide is the same as that of Example 3, except that the tin catalyst used is SnCl2. The crude lactide product was analyzed to have an L-lactide content of 86.9%, an m-lactide content of 2.7%, an L-lactic acid content of 3.6%, a dimer and a trimer content of 3.9%, and a lactic acid oligomer conversion rate of 96.8% in the crude lactide synthesis process.

[0068] Example 7

[0069] The preparation process of lactic acid oligomers and lactide is the same as that of Example 3, except that the tin catalyst used is SnO, and the addition amount is 30g. After analysis, the crude lactide product has an L-lactide content of 86.2%, an m-lactide content of 2.3%, an L-lactic acid content of 3.7%, a dimer and a trimer content of 4.2%, and a lactic acid oligomer conversion rate of 96.3% in the crude lactide synthesis process.

[0070] Example 8

[0071] The preparation process of lactic acid oligomers and lactide is the same as that of Example 3, except that the depolymerization reactor used is a molecular distillation evaporator. After analysis, the crude lactide product has an L-lactide content of 87.4%, an m-lactide content of 2.4%, an L-lactic acid content of 3.6%, a dimer and a trimer content of 4.0%, and a lactic acid oligomer conversion rate of 96.9% in the crude lactide synthesis process.

[0072] Example 9

[0073] The preparation process of lactic acid oligomers and lactide is the same as that of Example 3, except that only one catalyst control device is provided, and the reaction needs to be terminated when slag is discharged. In the reaction process of the previous stage, this process can still operate more stably than the prior art, but in the long-term reaction process, frequent start-up and shutdown are required, and the operation is complicated, which affects the quality of the lactide product to a certain extent.

[0074] Comparative Example 1

[0075] The preparation process of lactic acid oligomers and lactide is the same as that of Example 3, except that the heavy components after depolymerization are mixed with fresh materials and directly fed into the depolymerization reactor without passing through the catalyst control device. In the long-term reaction process, the heavy components will accumulate more and more, which may easily cause the reaction substrate to coke and carbonize on the reactor surface. For more than 50 hours, the lactide content in the product is less than 80%.

[0076] Comparative Example 2

[0077] The preparation process of lactic acid oligomers and lactide is the same as in Example 3, except that when the cumulative amount of catalyst circulation in the catalyst control device reaches 25%, it is switched to another catalyst control device and the first one is slag treated. Slag is discharged when the cumulative amount of catalyst circulation is large, resulting in coking on the reactor surface and a large content of m-lactide in the lactide product. After analysis, the crude lactide product has an L-lactide content of 85.7%, an m-lactide content of 6.3%, an L-lactic acid content of 3.1%, a dimer and a trimer content of 3.5%, and a lactic acid oligomer conversion rate of 74.7% in the crude lactide synthesis process.

Claims

1. A catalyst control device for continuous depolymerization to prepare lactide, characterized in that: It includes a tank body with a sealed upper portion, an inlet and an outlet arranged opposite to each other on both sides of the tank body, at least one protruding component arranged on the top of the tank body, and controlled as a whole by at least one regulating valve or separately by multiple regulating valves. When the protruding component forms a protruding state, it bulges toward the inside of the tank body and occupies a certain space in the tank body. When the protruding component is at the maximum protrusion degree, its bottom is lower than the inlet and outlet of the tank body.

2. The catalyst control device according to claim 1, characterized in that: When the protruding component is at its maximum protrusion, the distance between the bottom of the protruding component and the bottom of the tank body is not less than the inlet pipe diameter of the catalyst regulating device, and is not greater than 2 times the inlet pipe diameter.

3. The catalyst control device according to claim 1, characterized in that: When the protruding component is at its maximum protrusion, the height of the protruding component accounts for 2 / 3-9 / 10 of the height of the tank body.

4. The catalyst control device according to claim 1, characterized in that: The inlet position is lower than the outlet position, and the inlet is arranged at the position of 1 / 4 to 3 / 4 from the bottom to the top of the tank body.

5. The catalyst control device according to claim 1, characterized in that: When the protruding part protrudes downward, the plane formed by it is a smooth surface; the tank body is cylindrical, and its bottom is flat or U-shaped.

6. The catalyst control device according to claim 5, characterized in that: The number of the protruding parts is 1 to 3.

7. A process system for preparing lactide by continuous depolymerization, characterized in that: The invention comprises a depolymerization reactor, a circulation tank and a catalyst regulating device which are connected in sequence, wherein the outlet of the catalyst regulating device is connected to the inlet of the depolymerization reactor, the bottom outlet of the depolymerization reactor is connected to the circulation tank, and a lactide outlet is arranged on the top.

8. The process system according to claim 6, characterized in that: At least two catalyst control devices are arranged in parallel and connected to corresponding switching pipelines.

9. The process system according to claim 6, characterized in that: The volume of the catalyst control device does not exceed 1 / 3 of the volume of the circulation tank.

10. The process system according to claim 6, characterized in that: Furthermore, the gas phase lactide outlet of the depolymerization reactor is connected to a crude lactide purification and refining system, including a process of purifying and refining the crude lactide.

11. A process for preparing lactide by continuous depolymerization using the process system according to claim 6, characterized in that: Fresh lactic acid oligomer raw materials and depolymerization catalysts are added to the depolymerization reactor, and the gaseous lactide produced by depolymerization leaves the depolymerization reactor from the top, and the materials and heavy components that have not yet reacted are discharged from the bottom of the depolymerization reactor, and enter the catalyst control device after passing through the circulation tank. In the early stage of the reaction, the amount of catalyst in the depolymerization reactor is small, and it is expected that the materials returned to the depolymerization reactor contain more catalysts. At this time, the degree of downward protrusion of the protruding parts of the catalyst control device is increased to make the fluid in the tank body form turbulence, and the catalyst is stirred, and will not or less be retained in the tank body, and enter the depolymerization reactor with the fluid; in the late stage of the reaction, more catalyst accumulation is generated in the depolymerization reactor, and it is expected that the materials returned to the depolymerization reactor contain less catalyst. At this time, the degree of downward protrusion of the protruding parts is reduced to make the fluid form a smaller turbulence or tend to a flat flow state. At this time, the catalyst and the heavy components in the material will form a deposit and remain in the catalyst control device, and the material returning to the depolymerization reactor leaving the device contains less catalyst and heavy components.

12. The process according to claim 11, characterized in that: The method for judging whether a large amount of catalyst accumulation is generated in the depolymerization reactor is as follows: at the initial stage of the reaction, the regulating protrusion of the regulating valve of the catalyst control device is in the maximum protrusion position, and when it is detected that the content of m-lactide in the crude lactide sample at the discharge port of the circulation tank is between 2.5% and 3.0%, the regulating valve protrusion begins to be continuously retracted until it is retracted to the end point.

13. The process according to claim 11, characterized in that: The feed mass ratio of fresh material to recycled material entering the depolymerization reactor is 1:6-2:

1.

14. The process according to claim 11, characterized in that: Regular slag discharge is carried out with reference to the cumulative amount of catalyst circulation in the catalyst control device. In order to achieve optimal continuous operation of the device, slag discharge is carried out when it is detected that the cumulative amount of catalyst circulation in the catalyst control device is greater than 15% of the circulating material.

Images