Process system and process method for continuously preparing lactide through direct cyclization of lactic acid

Abstract

The invention provides a process system and a process method for continuously preparing lactide by direct cyclization of lactic acid, a fluidized bed reactor is provided with an upper reaction chamber and a lower reaction chamber, the middle is an expanding transition section, a lactic acid raw material, inert gas and a catalyst enter the reactor for countercurrent contact reaction, and the catalyst settles downwards to obtain a lactic acid product; extracting from a solid outlet at the bottom of the fluidized bed reactor and circulating back to the upper part of the reactor; inert gas with water, a small amount of lactic acid and a small-particle catalyst moves upwards and enters a cyclone separator at the top of the fluidized bed reactor, a separated gas phase is continuously separated through a separation tower and a condenser, a separated solid phase is returned into the fluidized bed reactor through an extension pipe, and a liquid product is discharged through an overflow port of an expanding transition section. According to the invention, lactide is prepared from lactic acid through a one-step liquid phase method in a self-circulation reaction mode, mass transfer and heat transfer in the reaction process are effectively ensured, continuous and stable proceeding of the whole reaction is ensured through catalyst circulation, continuity is realized, and lactide yield is high.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer materials, and in particular relates to a method for continuously preparing lactide by direct cyclization of lactic acid. Background Art

[0002] Polylactic acid, also known as polylactide (PLA), is made from renewable biomass such as corn, cassava, and straw. It is fermented to produce lactic acid monomers and then polymerized through a series of reactions. It has good mechanical properties, processability, and biodegradability. Its products can be composted and degraded into CO2 and water after use, realizing a cycle in nature.

[0003] At present, there are two main technical routes for the synthesis of polylactic acid, namely the indirect method (two-step method) and the direct method (one-step method). The direct method is to prepare polylactic acid by direct polycondensation of lactic acid as raw material, but the polylactic acid prepared by this method has problems such as low molecular weight and deviation of product performance indicators. The indirect method for synthesizing polylactic acid is to use lactic acid as raw material, firstly generate lactide through dehydration cyclization, and then obtain polylactic acid through ring-opening polymerization of lactide, as shown in the following formula.

[0004]

[0005] Since the ring-opening polymerization process of lactide is intramolecular ring-opening and intermolecular bond connection, no light components will be generated. By controlling the amount of catalyst added, the reaction process operating conditions, etc., the molecular weight of polylactic acid can reach more than 100,000 and can be adjusted. Therefore, it is suitable for large-scale industrial production and is also the main process for polylactic acid production.

[0006] Lactide is the key core of the entire polylactic acid industry chain. There are currently two main methods for synthesizing lactide. One is the "two-step method", which is the current industrialized production process. Specifically, lactic acid is polymerized to generate lactic acid oligomers, and then the oligomers are depolymerized to generate lactide. This process has good lactide selectivity, relatively high product single-pass yield, good product quality, and the technical route has been verified by industrialization. However, there are high energy consumption, long and complex reaction process, high requirements for reaction equipment, and harsh reaction conditions. A part of lactic acid polymers will be produced, affecting the overall yield. Another method is the "one-step method", that is, lactic acid is synthesized into lactide in the next step under the action of a catalyst. This process has low energy consumption, no negative pressure operation is required, and the process route is relatively simple, but there are also relatively low yields, uncontrollable processes, and products in the aqueous reaction system are easily hydrolyzed, and continuous industrialization is difficult. Among them, the development of efficient catalysts, reaction processes and equipment is its key technical difficulty.

[0007] There are two systems for the one-step method. One is a liquid phase one-step reaction process, with relatively low reaction temperature, high product yield and selectivity, but currently it is mostly carried out in batch operation, which makes it difficult to scale up. The other is a gas phase reaction, with relatively high reaction temperature, which is mostly carried out in fixed bed, fluidized bed and other reactors. The product yield is low and racemization is relatively serious, but it can be operated continuously.

[0008] DuPont disclosed a one-step gas phase continuous synthesis process for cyclic esters in patent US5043458. The process uses lactic acid as raw material, gas phase feed passes through a fixed bed or fluidized bed, uses Sn or Sb oxide or carboxylate as catalyst, reacts at 190-235°C, and uses N 2 As a carrier, the obtained lactide yield is 55%. The lactide is extracted by acetone and the product purity can reach 97.24% after preliminary purification. However, under the high temperature of the system, the presence of water or its untimely removal can easily cause the hydrolysis of the lactide generated by the reaction, resulting in a low product yield.

[0009] In the article "Shape-selective zeolite catalysis for bioplastics production", the M. Dusselier team of the University of Leuven in Belgium disclosed a process using 50% L-lactic acid as raw material, H-beta molecular sieve as catalyst, toluene or o-xylene as solvent, and reacting at 140°C (toluene as solvent) or 170°C (o-xylene as solvent). The conversion rate of lactic acid in this process can reach more than 95%, and the yield of lactide can reach 83%. Although the reaction process temperature is low, the degree of product racemization is small, and the yield is high compared with the gas phase method, the process uses organic solvents and is limited to laboratory-scale intermittent operation.

[0010] Jilin University and the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences also used L-lactic acid as raw material in the article "Breaking the Si / Al limit of nanosized β Zeolites: promoting catalytic production of lactide". By preparing nanoscale Beta molecular sieve catalysts with Si / Al ratios of 20-100, lactide was synthesized in one step in the solvent liquid phase. This article first proposed the concept that lactic acid trimers can be cyclized in the catalyst pores to form lactic acid dimers. The lactic acid conversion rate can reach 99.0%, and the lactide yield can reach 74%. Starting from the perspective of catalyst preparation, this article improves catalyst performance and improves catalytic efficiency, but it still does not solve how to achieve continuous operation.

[0011] Compared with the two-step method for preparing lactide, by comparing the existing literature, it is found that the process of direct cyclization of lactic acid in liquid phase into lactide in one step does not have the problem of racemization, and the product yield is relatively high. It is an application method with great application prospects. The main reasons limiting its large-scale application are: first, the development of continuous reaction process and reactor form; second, from the perspective of catalytic reaction principle, the direct cyclization of lactic acid dimer / trimer to lactide is mainly due to the "shape-selective catalysis" effect in the catalyst pores. Therefore, the effective catalysts are mostly molecular sieve catalysts, and the pores of this type of catalyst are relatively small. During long-term operation, as the number of lactic acid units on the lactic acid polymer increases, it is very easy to cause the pores of the molecular sieve catalyst to be blocked, causing it to be inactivated. In addition, the molecular sieve is in the hot water / lactic acid system for a long time, and the stability of the catalyst becomes a problem. Summary of the invention

[0012] In view of the deficiencies in the prior art, the present invention provides a process system and a process method for continuously preparing lactide by direct cyclization of lactic acid. The present invention adopts a fluidized bed self-circulating reaction device to ensure the uniformity of the reaction temperature and the stability of the catalyst activity in the whole reaction process. Through the continuous circulation or regeneration of the catalyst, the catalyst deactivation caused by the blocking of the molecular sieve catalyst pores by lactic acid polymers or carbonization is avoided, thereby affecting the stability and continuity of the whole reaction process. And the water removal reaction system produced by the fluidized bed gas phase entrainment reaction can effectively avoid the use of organic solvents in the current liquid phase one-step method for preparing lactide, reduce the probability of lactide re-hydrolysis when it encounters water, effectively improve the efficiency of the reaction, the temperature of the whole reaction process is low, the degree of racemization is low, and continuous and stable production can be achieved.

[0013] In order to achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0014] The technical purpose of the first aspect of the present invention is to provide a process system for continuously preparing lactide by direct cyclization of lactic acid, comprising a fluidized bed reactor, wherein the fluidized bed reactor is provided with an upper reaction chamber and a lower reaction chamber, the diameter of the upper reaction chamber is larger than the diameter of the lower reaction chamber, and the middle is a diameter expansion transition section, a feed inlet is provided at the bottom of the fluidized bed reactor, and a solid inlet is provided at the upper part of the lower reaction chamber, which is communicated with the reaction chamber of the fluidized bed reactor; an overflow port is provided on the side wall of the diameter expansion transition section, the overflow port is connected to a liquid product outlet, and the liquid product outlet is provided with branches, which are respectively a crude lactide collection pipeline and a return line. A circulating material pipeline at the bottom of the recirculating fluidized bed reactor; a cyclone separator is arranged on the upper part of the upper reaction chamber of the fluidized bed reactor, and the inlet of the cyclone separator is communicated with the upper reaction chamber of the fluidized bed reactor; the gas outlet of the cyclone separator is connected to a separation tower, and the separation tower is connected to a condenser; the solid outlet of the cyclone separator extends to the bottom of the reaction chamber of the fluidized bed reactor through an extension pipe at the bottom of the cyclone separator; a solid outlet is arranged at the bottom of the fluidized bed reactor, and the solid outlet is provided with branches, which are respectively a solid material extraction pipeline and a solid material circulation pipeline connected to the solid inlet at the upper part of the fluidized bed reactor.

[0015] Furthermore, a perforated sieve plate is provided at the overflow port of the fluidized bed reactor, and the hole diameter of the sieve plate is not larger than the particle size of the catalyst used, so as to prevent larger particles of solid from flowing out with the liquid phase product.

[0016] Furthermore, a distributor is arranged above the bottom feed port of the fluidized bed reactor to disperse the feed.

[0017] Furthermore, the ratio of the diameter of the upper reaction chamber to the diameter of the lower reaction chamber of the fluidized bed reactor is 3:1-1.2:1, preferably 3:1-1.5:1.

[0018] Furthermore, the bottom plate of the expansion transition section is a perforated sieve plate, the hole diameter of the sieve plate is not larger than the maximum particle size of the catalyst used, so as to allow gas and liquid to pass through but prevent larger particles of catalyst from entering the overflow port.

[0019] The fluidized bed reactor of the present invention is configured to be larger at the top and smaller at the bottom, so that the velocity of the gas flow in the expansion transition section is instantly reduced, which can reduce the entrainment of the catalyst by the gas to a certain extent and allow more catalyst to fall back into the lower reaction chamber; and preferably, the bottom plate of the expansion transition section is configured to be a sieve plate with holes, which can further reduce the outflow of the catalyst.

[0020] Furthermore, an internal component is also provided in the fluidized bed reactor, and the internal component is a baffle, specifically, a porous plate, a disc-shaped baffle, etc. The baffles are arranged in multiple layers in a staggered order longitudinally on the wall of the lower reaction chamber to suppress and break up bubbles, improve the residence time distribution of the gas in the bed, reduce the drag-out of catalyst particles, increase the contact time between the reaction raw materials and the catalyst, strengthen the heat and mass transfer between the reaction systems, increase the reaction conversion rate, and improve the selectivity and yield.

[0021] The technical purpose of the second aspect of the present invention is to provide a process method for continuously preparing lactide by direct cyclization of lactic acid using the above-mentioned process system, comprising: liquid lactic acid raw material and inert gas enter the fluidized bed reactor from the feed port at the bottom of the fluidized bed reactor, and the catalyst enters the lower reaction chamber of the reactor from the solid inlet of the fluidized bed reactor, and the two are contacted and reacted in countercurrent, and most of the catalyst settles downward by its own gravity, and after a period of reaction, it is extracted from the solid outlet at the bottom of the fluidized bed reactor; water generated by the reaction, a small amount of lactic acid entrained, and smaller particles of catalyst are entrained upward by the inert gas and enter the top of the fluidized bed reactor. The separated gas enters the separation tower to separate lactic acid, and the rest passes through the condenser to obtain inert gas and water; the solid phase separated by the cyclone separator is returned to the fluidized bed reactor through the extension pipe; the liquid product in the fluidized bed reactor is discharged from the liquid phase product outlet through the overflow port, part of which is returned to the bottom of the fluidized bed reactor through the pipeline as a circulating material to continue the reaction, and part of it is extracted as a crude lactide product; the catalyst settled at the bottom of the reactor is discharged through the solid outlet and enters the solid inlet at the top through the pipeline to form a catalyst cycle. After the catalyst activity is reduced or deactivated, all or part of it is extracted and fresh catalyst is supplemented.

[0022] It should be understood by those skilled in the art that in the method of the present invention, in the fluidized bed reactor, the lactic acid raw material is mainly carried by the inert gas to flow from bottom to top and contact the catalyst in reverse. After the feed liquid enters the reactor from the inlet, the inert gas velocity decreases due to the expansion of the diameter, and the catalyst with larger particles is suspended by its own gravity and then gradually sinks to the bottom of the reactor. The catalyst with smaller particles moves upward along with the inert gas to entrain the water generated by the reaction. After the lactic acid material reacts, the generated lactide, lactic acid dimer, trimer or polymer and unreacted lactic acid components are withdrawn or circulated by gravity. In this way, the heat and mass transfer between the reactants and the catalyst is improved, and the reaction is accelerated.

[0023] Furthermore, the weight ratio of the recycled material at the liquid product outlet to the fresh lactic acid raw material feed is 1:1-4:1, and the output of the crude lactide product at the liquid product outlet is equivalent to the volume of the fresh lactic acid raw material feed.

[0024] Those skilled in the art should also understand that the inert gas carries moisture and smaller particles of catalyst into the separation section, which mainly includes two parts. The first part is to separate the solid catalyst from the gas phase components through a cyclone separation device; the second part is to separate the moisture from the entrained lactic acid through a separation tower, and the moisture and nitrogen are further separated through a condenser.

[0025] Furthermore, in the fluidized bed reactor, the reaction temperature is 120-150° C., the pressure is 0.1-0.3 MPa, and the average residence time of the catalyst particles in the bed is 20-40 min per pass.

[0026] Furthermore, the lactic acid raw material is a 30wt%-60wt% lactic acid aqueous solution, and the feed temperature is 80-100°C.

[0027] Furthermore, the inert gas refers to a gas that does not react with the reaction system and has relatively stable properties in the reaction system, preferably nitrogen or argon.

[0028] Furthermore, the feed temperature of the inert gas is 100-120°C, the pressure is 0.1-0.3Mpa, and the gas flow rate satisfies u / u mf =3-8,u mf The critical fluidization velocity refers to the minimum gas velocity in the empty bed cross section that can fluidize solid particles. Its value is related to the viscosity and density of the fluid, and the size and density of the solid particles. For all catalyst particles in the present invention, it is calculated according to the empirical formula as follows:

[0029]

[0030] Among them, d p is the average diameter of catalyst particles, ρ p is the catalyst density, ρ g is the density of the fluid, and μ is the viscosity of the fluid.

[0031] Furthermore, the volume of the lactic acid raw material in the feed accounts for 0.5%-5% of the volume of the inert gas.

[0032] Further, the catalyst is a solid catalyst capable of realizing the production of lactide by liquid phase reaction of lactic acid, which is well known to those skilled in the art. As one of the specific embodiments, the catalyst is a catalyst of molecular sieve loaded with Sn and / or Ti, and one of the more preferred embodiments is a catalyst of Beta molecular sieve loaded with Sn and Ti. The preparation method of the catalyst is prepared by conventional impregnation loading method in the art, which is a method well known to those skilled in the art.

[0033] Further, the catalyst is a shaped catalyst of any shape with a cross-sectional diameter of 0.5-3.0 mm, preferably 1.0-2.5 mm, and a length of 0.5-3.0 mm, preferably 1.0-2.5 mm, and is obtained by pressing a powder through an extrusion molding process. For a catalyst of a regular shape, its cross-sectional diameter is easy to determine, and for a catalyst of an irregular shape, its cross-sectional diameter refers to the diameter of the circumscribed circle of its cross section. The catalyst is preferably spherical or cylindrical.

[0034] Furthermore, the catalyst with reduced activity or deactivated can be selectively regenerated and recycled. Catalyst regeneration is to heat the catalyst to 450-550°C at a heating rate of 1-5°C / min and then maintain the temperature for 4.0-10.0h to remove lactic acid polymers or carbonized coke in the catalyst pores, clear the catalyst pores, and restore catalyst performance.

[0035] Through the above method, after the reaction is stabilized, the composition of the crude lactide extracted is L-lactide 74.0%-84.0%, m-lactide ≯2.0%, L-lactic acid 3.0%-10.0%, lactic acid dimer 3.7%-7.2%, trimer 2.3%-5.6%, lactic acid polymer 2.0%-8.0%. The total lactide yield can reach 80% or more of the theoretical yield.

[0036] Furthermore, the above method also includes the process of obtaining a refined lactide product after pre-treatment and purification of the crude lactide. Among them, the pre-treatment, purification and refining of lactide are well known to those skilled in the art. As one of the specific embodiments, the pre-treatment and purification is to wash the crude lactide with a mixture of water and ethanol to remove the unreacted lactic acid and polymers therein, and the elution temperature is 0-10°C to reduce the hydrolysis rate of water to lactide and the solubility of ethanol to lactide, thereby improving the product yield; as a second specific embodiment, the pre-treatment is in the form of eluting with a dilute alkaline solution and then eluting with cold water to remove the unreacted lactic acid and lactic acid polymers therein, and the elution temperature is 0-10°C. In the above two specific embodiments, after the crude lactide is eluted, it is further separated by filtration and centrifugation and then sent to a cyclone dryer or a rotary vacuum dryer for drying. During the pre-treatment of the crude lactide, the lactide yield can reach more than 95.0%. The purity of L-lactide can reach more than 92%, meeting the requirements of subsequent refining and purification. The refining and purification is to further treat the pre-treated lactide through melt crystallization or distillation process to obtain a lactide product that meets the polymerization requirements.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] (1) The present invention adopts a fluidized bed reactor, and the whole process realizes the one-step liquid phase method of preparing lactide from lactic acid in a self-circulating reaction mode, replacing the kettle-type one-pot reaction process, effectively ensuring the mass transfer and heat transfer of the reaction process, and through the catalyst circulation, ensuring the continuous and stable progress of the whole reaction, avoiding the blockage of the molecular sieve catalyst pores caused by lactic acid polymers or coking carbonization. The reaction temperature of the whole process is uniform, the catalyst activity is stable, and the reaction efficiency is high.

[0039] (2) The present invention utilizes catalyst circulation to ensure that the catalyst is stable during the reaction and is easy to remove and regenerate. The catalyst regeneration method is simple and the performance after regeneration is consistent with that of a fresh catalyst, thereby ensuring that the reaction is economical and efficient.

[0040] (3) The process system and process method of the present invention can effectively avoid the use of organic solvents in the traditional one-step liquid phase reaction process. The present invention can effectively ensure that the water generated in the reaction process escapes from the reaction system through the entrainment of inert gas, effectively ensuring the efficiency of the reaction, and effectively ensuring the complete reaction of the components through the recycling of materials. The lactide synthesis yield of the whole process can reach more than 80.0% of the theoretical yield.

[0041] (4) Under the reaction mode of the present invention, the process of re-separation and activation of the catalyst and the reaction system in the traditional kettle reactor can be effectively avoided, which effectively increases the service life of the catalyst and improves the catalyst utilization rate.

[0042] (5) In the reaction method of the present invention, the chemical purity of the crude lactide reaches 78%, and can reach up to 86%, and the m-lactide content can be controlled within 1.0%.

[0043] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 . Schematic diagram of the circulating fluidized bed reaction device used in the method of the present invention;

[0045] 100. fluidized bed reactor, 1001. upper reaction chamber, 1002. lower reaction chamber, 1003. expansion transition section, 101. feed inlet, 102. distributor, 103. overflow port, 104. liquid product outlet, 105. crude lactide collection pipeline, 106. liquid material circulation pipeline, 107. solid inlet, 108. solid outlet, 109. circulation pump, 110. solid material circulation pipeline, 111. solid material extraction pipeline, 112. internal component, 113. bottom plate of expansion transition section, 200. cyclone separator, 201. cyclone separator inlet, 202. extension pipe, 300. separation tower, 400. condenser. DETAILED DESCRIPTION

[0046] The following examples further illustrate the method for preparing lactide by direct cyclization of lactic acid and its effects. The examples are implemented based on the technical solution of the present invention, and detailed implementation methods and specific operation processes are given, but the protection scope of the present invention is not limited to the following examples.

[0047] The experimental methods in the following examples, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, can all be purchased from biochemical reagent stores.

[0048] The lactic acid used in the embodiments of the present invention is heat-resistant grade L-lactic acid, and its optical purity is not less than 99.0%.

[0049] The present invention adopts Agilent high performance liquid chromatograph to analyze the chemical purity of lactide, 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 5um, detection wavelength is 200nm, column temperature is 40℃, flow rate is 1mL / min, injection volume is 5μL.

[0050] 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 250°C, detector temperature 280°C, hydrogen flame ionization detector, column temperature program heating initial temperature 100°C, maintain 5min, heat to 140°C at a rate of 4°C / min, maintain 7min, heat to 200°C at a rate of 8°C / min, maintain 20min. Carrier gas N 2 The flow rate was 1.4 ml / min, the hydrogen flow was 30 mL / min, the air flow was 400 mL / min, and the injection volume was 0.5 μL.

[0051] The yield Y of the one-step cyclization of lactic acid to prepare L-lactide is the ratio of the actual amount of L-lactide produced to the theoretical amount of L-lactide produced, and the calculation formula is as follows:

[0052]

[0053] Among them, m 0 is the lactic acid feed rate, x 0 is the content of pure lactic acid in the feed, M 丙 is the molecular weight of L-lactide, M 乳 is the molecular weight of lactic acid, m is the generation rate of L-lactide, and is the rate of L-lactide product produced from the outlet after the reaction is stabilized (specifically calculated from the production rate of crude lactide and the content of L-lactide therein).

[0054] Example 1

[0055] This embodiment provides a process system for continuously preparing lactide by direct cyclization of lactic acid, such as Figure 1 As shown, a fluidized bed reactor 100 is provided, wherein an upper reaction chamber 1002 and a lower reaction chamber 1001 are provided in the fluidized bed reactor, wherein the ratio of the diameter of the upper reaction chamber to the diameter of the lower reaction chamber is 2:1, and an expansion transition section 1003 is provided in the middle, wherein a feed inlet 101 is provided at the bottom of the fluidized bed reactor 100, and a solid inlet 107 is provided at the upper part of the lower reaction chamber 1001, which is communicated with the reaction chamber of the fluidized bed reactor; an overflow port 103 is provided on the side wall of the expansion transition section 1003, and the overflow port is connected to a liquid product outlet 104, and the liquid product outlet 104 is provided with branches, which are respectively a crude lactide collection pipeline 105 and a circulating material pipe returning to the bottom of the fluidized bed reactor. 106; a cyclone separator 200 is arranged at the upper part of the upper reaction chamber 1002, and the inlet 201 of the cyclone separator is connected to the upper reaction chamber 1002 of the fluidized bed reactor; the gas outlet separated by the cyclone separator is connected to the separation tower 300, and the separation tower 300 is connected to the condenser 400; the solid outlet obtained by separation is extended to the bottom of the reaction chamber of the fluidized bed reactor 100 through the extension pipe 202; a solid outlet 108 is arranged at the bottom of the fluidized bed reactor 100, and the solid outlet 108 is provided with branches, which are respectively a solid material extraction pipeline 111 and a solid material circulation pipeline 110 connected to the solid inlet of the upper part of the fluidized bed reactor through a circulation pump 109. At the same time, a sieve plate with holes is arranged at the overflow port 103 to isolate larger particles of solid from flowing out with the liquid phase product, and a distributor 102 is also arranged above the bottom feed port 101 of the fluidized bed reactor 100 to disperse the feed. The bottom plate 113 of the expansion transition section is a sieve plate with holes to prevent most of the catalyst from overflowing.

[0056] Example 2

[0057] The process for continuously preparing lactide by direct cyclization of lactic acid using the process system shown in Example 1 is as follows:

[0058] The liquid lactic acid raw material and the inert gas enter the fluidized bed reactor 100 from the feed port 101 at the bottom of the fluidized bed reactor, and the catalyst enters the reactor from the solid inlet 107 of the fluidized bed reactor. The two react in countercurrent contact in the lower reaction chamber 1001 of the fluidized bed reactor 100. Most of the catalyst settles downward by its own gravity. After a period of reaction, it is extracted from the solid outlet 108 at the bottom of the fluidized bed reactor; the water generated by the reaction, a small amount of lactic acid entrained, and smaller particles of catalyst particles are entrained upward by the inert gas and enter the cyclone separator 200 at the top of the fluidized bed reactor. The separated gas enters the separation tower 300 to separate the lactic acid, which can be optionally refluxed to the fluidized bed reactor. The rest passes through the condensation tower 400 to obtain the inert gas and water; the solid phase separated by the cyclone separator 200 is returned to the fluidized bed reactor 100 through the extension pipe 202; the liquid product in the fluidized bed reactor 100 is discharged from the liquid product outlet 104 through the overflow port 103, part of which is returned to the bottom of the fluidized bed reactor through the pipeline 106 as a circulating material, and continues to participate in the reaction after being mixed with the fresh lactic acid raw material, and part of which enters the crude lactide collection pipeline 105 and is produced as a crude lactide product; the catalyst settled at the bottom of the reactor passes through the solid outlet 108, the circulation pump 109, and the solid material circulation pipeline 110 into the upper solid inlet, forming a catalyst circulation. After the catalyst activity is reduced or deactivated, it is fully or partially produced through the solid material production pipeline 111, and fresh catalyst is supplemented.

[0059] Reaction conditions: The raw material lactic acid is 50% lactic acid aqueous solution, which is heated to 95°C in a raw material preheater and then fed to the bottom feed port of the fluidized bed reactor at a feed rate of 3.0 L / h. 2 After being heated to 140°C in a preheater, the pressure is 0.3Mpa, and the gas enters the bottom of the fluidized bed at an inlet flow rate of 300L / h. After being mixed and dispersed with the reaction liquid, it enters the fluidized bed reactor through a distributor. The catalyst is a Sn / Ti co-loaded Beta molecular sieve catalyst with a catalyst particle size of 0.1mm spherical and a catalyst space velocity of 2.0h -1 After being fed from the upper part of the fluidized bed reactor, it is in countercurrent contact with the reaction materials. The reaction temperature in the fluidized bed reactor is 130°C, the pressure is 0.2Mpa, the circulating material flow rate is 7.0L / h, and the crude lactide production rate is 3.0L / h.

[0060] Under the above reaction process conditions, the crude lactide obtained has an L-lactide content of 80.2%, an m-lactide content of 0.9%, an L-lactic acid content of 6.3%, a lactic acid dimer content of 4.6%, a trimer content of 4.9%, and a polymer content of 2.8%. Through measurement and calculation, it is obtained that the total yield of L-lactide can reach 85.2%.

[0061] Example 3

[0062] The reaction process is the same as in Example 2.

[0063] Reaction conditions: The raw material lactic acid is 35% lactic acid aqueous solution, which is heated to 90°C in a raw material preheater and then fed to the bottom feed port of the fluidized bed reactor at a feed rate of 5.0 L / h. 2 After being heated to 120°C in a preheater, the pressure is 0.2Mpa, and the gas enters the bottom of the fluidized bed at an inlet flow rate of 200L / h. After being mixed and dispersed with the reaction liquid, it enters the fluidized bed reactor through a distributor. The catalyst is a Sn / Ti co-loaded Beta molecular sieve catalyst with a particle size of 0.2mm and a length of 0.6mm to obtain a cylindrical shape. The catalyst space velocity is 1.5h -1 After feeding from the upper part of the fluidized bed reactor, it is in countercurrent contact with the reaction materials. The reaction temperature in the fluidized bed reactor is 140°C, the pressure is 0.25Mpa, the circulating material flow rate is 10.0L / h, and the crude lactide production rate is 5.0L / h.

[0064] Under the above reaction process conditions, the crude lactide obtained has an L-lactide content of 78.8%, an m-lactide content of 1.6%, an L-lactic acid content of 7.3%, a lactic acid dimer content of 5.7%, a trimer content of 5.6%, and a polymer content of 1.0%. Through measurement and calculation, it is obtained that the total yield of L-lactide can reach 82.0%.

Claims

1. A process system for continuous preparation of lactide by direct cyclization of lactic acid, It is characterized in that The invention comprises a fluidized bed reactor, wherein the fluidized bed reactor is provided with an upper reaction chamber and a lower reaction chamber, wherein the diameter of the upper reaction chamber is larger than that of the lower reaction chamber, and an expansion transition section is provided in the middle, a feed port is provided at the bottom of the fluidized bed reactor, and a solid inlet is provided at the upper part of the lower reaction chamber, which is communicated with the reaction chamber of the fluidized bed reactor; an overflow port is provided on the side wall of the expansion transition section, and the overflow port is connected to a liquid product outlet, and branches are provided at the liquid product outlet, which are respectively a crude lactide collection pipeline and a circulating material pipeline returning to the bottom of the fluidized bed reactor; A cyclone separator is arranged on the upper part of the upper reaction chamber of the fluidized bed reactor, and the inlet of the cyclone separator is communicated with the upper reaction chamber of the fluidized bed reactor; the gas outlet of the cyclone separator is connected to a separation tower, and the separation tower is connected to a condenser; The solid outlet of the cyclone separator extends to the bottom of the reaction chamber of the fluidized bed reactor through an extension pipe at the bottom of the cyclone separator; a solid outlet is arranged at the bottom of the fluidized bed reactor, and branches are arranged at the solid outlet, which are a solid material extraction pipeline and a solid material circulation pipeline connected to the solid inlet at the top of the fluidized bed reactor.

2. The process system according to claim 1, It is characterized in that A sieve plate with holes is arranged at the overflow port of the fluidized bed reactor, and the hole diameter of the sieve plate is not larger than the particle diameter of the catalyst used.

3. The process system according to claim 1, It is characterized in that A distributor is also arranged above the bottom feed port of the fluidized bed reactor.

4. The process system according to claim 1, It is characterized in that The ratio of the diameter of the upper reaction chamber to the diameter of the lower reaction chamber of the fluidized bed reactor is 3:1-1.2:

1.

5. The process system according to claim 1, It is characterized in that The bottom plate of the expansion transition section is a sieve plate with holes, and the hole diameter of the sieve plate is not larger than the particle diameter of the catalyst used.

6. The process system according to claim 1, It is characterized in that An internal component is also arranged in the fluidized bed reactor. The internal component is a baffle. The baffles are arranged in multiple layers in a staggered order longitudinally on the wall of the lower reaction chamber.

7. A process for continuously preparing lactide by direct cyclization of lactic acid using the process system of claim 1, include: Liquid lactic acid raw materials and inert gas enter the fluidized bed reactor from the feed port at the bottom of the fluidized bed reactor, and the catalyst enters the lower reaction chamber of the reactor from the solid inlet of the fluidized bed reactor. The two react in countercurrent contact, and most of the catalyst settles downward by its own gravity. After a period of reaction, it is taken out from the solid outlet at the bottom of the fluidized bed reactor; the water generated by the reaction, a small amount of lactic acid entrained, and smaller particles of catalyst are entrained upward by the inert gas and enter the cyclone separator at the top of the fluidized bed reactor. The separated gas enters the separation tower to separate the lactic acid, and the rest passes through the condenser to obtain the inert gas and water; The solid phase separated by the cyclone separator is returned to the fluidized bed reactor through the extension pipe; the liquid product in the fluidized bed reactor is discharged from the liquid phase product outlet through the overflow port, part of which is returned to the bottom of the fluidized bed reactor through the pipeline as a circulating material to continue the reaction, and part of which is taken out as a crude lactide product; the catalyst settled at the bottom of the reactor is discharged through the solid outlet and enters the solid inlet at the top through the pipeline, forming a catalyst circulation. After the catalyst activity is reduced or deactivated, all or part of it is taken out and fresh catalyst is supplemented.

8. The process according to claim 7, It is characterized in that The weight ratio of the recycled material at the liquid product outlet to the fresh lactic acid raw material feed is 1:1-4:1, and the output of the crude lactide product at the liquid product outlet is equivalent to the volume of the fresh lactic acid raw material feed.

9. The process according to claim 7, It is characterized in that In the fluidized bed reactor, the reaction temperature is 120-150° C., the pressure is 0.1-0.3 MPa, and the average residence time of the catalyst particles in the bed is 20-40 min.

10. The process according to claim 7, It is characterized in that The lactic acid raw material is a 30wt%-60wt% lactic acid aqueous solution, and the feed temperature is 80-100°C.

11. The process according to claim 7, It is characterized in that The inert gas feed temperature is 100-120°C, the pressure is 0.1-0.3Mpa, and the gas flow rate meets u / u mf =3-8, u mf is the critical fluidization velocity.

12. The process according to claim 7, It is characterized in that The volume of lactic acid raw material in the feed accounts for 0.5%-5% of the volume of the inert gas.

13. The process according to claim 7, It is characterized in that The catalyst is a catalyst in which a molecular sieve is loaded with Sn and / or Ti.

14. The process according to claim 7, It is characterized in that The cross-section of the catalyst has a diameter of 0.5-3.0 mm and a length of 0.5-3.0 mm.

15. The process according to claim 7, It is characterized in that Catalysts with reduced activity or deactivated can be selectively regenerated and recycled. Catalyst regeneration is to heat the catalyst to 450-550°C at a heating rate of 1-5°C / min and then keep the temperature constant for 4.0-10.0h.

16. The process according to claim 7, It is characterized in that The method also includes the process of pre-treating and purifying the crude lactide and refining and purifying the crude lactide to obtain a refined lactide product.

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