阅读说明：本技术 一种丙交酯的分离提纯方法 (Method for separating and purifying lactide ) 是由 何岩 田博 杨颖� 范天熙 刘英俊 张红涛 朱小瑞 马朋成 张永振 乔小飞 于 2020-11-28 设计创作，主要内容包括：本发明公开了一种丙交酯分离提纯方法。该方法将反应工序的粗丙交酯先经熔融结晶,再经精馏步骤进行提纯。本工艺可高效脱除对下游聚乳酸聚合过程有显著影响的酸分以及异构体,显著改善丙交酯产品质量,减少杂质,并显著降低整个丙交酯分离提纯工序的能耗及生产成本。(The invention discloses a lactide separation and purification method. The method comprises the steps of melting and crystallizing crude lactide in a reaction process, and then purifying the crude lactide through a rectification step. The process can efficiently remove acid components and isomers which have obvious influence on the downstream polylactic acid polymerization process, obviously improve the quality of lactide products, reduce impurities, and obviously reduce the energy consumption and the production cost of the whole lactide separation and purification process.)

1. A method for separating and purifying lactide is characterized by comprising the following steps:

1) melting and crystallizing crude lactide from a reaction process, and separating the crude lactide into a stream a rich in L-lactide or D-lactide and a stream b rich in m-lactide, lactic acid and 2-5 polylactic acid oligomers;

2) rectifying and separating the stream a in the step 1) to obtain a purified stream D rich in L-lactide or D-lactide, a light component stream a-1 rich in water, lactic acid and m-lactide and a heavy component stream a-2 rich in lactic acid oligomers;

3) rectifying and separating the stream b in the step 1) to obtain a stream c rich in m-lactide, a stream b-1 rich in water and lactic acid as light components, and a stream b-2 rich in L-lactide or D-lactide and lactic acid oligomers;

4) melting and crystallizing the stream D obtained in the step 2) again to obtain an L-lactide or D-lactide product with high optical purity;

wherein, a crystallization device M is adopted in the step 1), the crystallization device M is one or two of a falling film crystallizer, a suspension crystallizer and a static crystallizer, and the suspension crystallizer is preferably adopted;

the crude lactide stream in the crystallizer flows through the crystallizer in a liquid form vertical direction, and heat exchange devices with cooling areas and heating areas alternately spaced are arranged in the flowing direction; the crystals are dispersed in a liquid phase in a suspension manner in the crystallizer, and the crystals and the liquid integrally pass through the heat exchange device in a countercurrent direction respectively.

2. The method as claimed in claim 1, wherein the a-1 in the step 2) is recycled to the reaction process for further utilization;

and/or b-2 in the step 3) is recycled to the reaction process for continuous utilization;

and/or, the stream D in the step 4) is in a gaseous form, condensed into a liquid phase, enters a crystallization device N, and is subjected to melt crystallization to obtain an L-lactide or D-lactide product with high optical purity and a stream relatively rich in m-lactide, wherein the stream relatively rich in m-lactide is recycled to the rectifying tower in the step 2).

3. The method as claimed in claim 1 or 2, wherein the temperature of the heating zone of the heat exchange device of the crystallizer in the step 1) is 90-120 ℃ and the temperature of the cooling zone is 50-90 ℃.

4. The method according to any one of claims 1 to 3, wherein the reaction process in step 1) comprises a process of pre-polymerizing lactic acid into lactic acid oligomer, and depolymerizing and cyclizing the lactic acid oligomer into lactide under the action of a catalyst.

5. The process according to any one of claims 1 to 4, characterized in that the crystallization device N of step 4) is a falling film crystallizer and/or a static crystallizer, preferably a combination of a falling film crystallizer and a static crystallizer.

6. A lactide product obtained by the lactide separation and purification method according to any one of claims 1 to 5.

Technical Field

The invention belongs to the field of separation and purification of synthetic products, and particularly relates to a method for separating and purifying lactide.

Background

Polylactic acid (PLA), also known as polylactide, is a typical biodegradable polymer material prepared by chemical synthesis from renewable plant resources. At present, the global market for PLA is rapidly increasing, demand will increase at a rate of 2-3 ten thousand tons per year, the premium can reach 20% -30%, and it is expected that short supply will be in place for a long time. Global plastic inhibition and waste classification have brought a huge potential market for degradable plastics, particularly disposable plastic articles.

At present, the industrial production of PLA generally adopts a lactide ring-opening polymerization method, lactide is taken as a key intermediate for synthesizing PLA, and the synthesis process and key equipment are the key directions of the research in the industry. At present, the lactide synthesized industrially is generally prepared by cracking and cyclization reaction of polylactic acid with a certain molecular weight and a certain degree of polymerization in the presence of a catalyst to obtain crude lactide, and the crude lactide is purified to obtain a qualified lactide product.

In the preparation process of PLA, in order to obtain higher molecular weight and product performance, the requirements on impurities such as isomers, moisture, acid content and the like of the raw material lactide are very strict. For example, the lactide product of Dadal-Cobyne generally has a controlled moisture content of less than 100ppm, an acid content of less than 7meq/kg, and an optical purity of > 99.5%. However, removal of acid components is very difficult due to physical properties. Rectification schemes are disclosed, for example, in EP0893462A2, US6277951B1, WO95/09879, CN100586921C, CN101821315A, CN107531663, etc. However, the removal of the above impurities, particularly acid, by rectification is very difficult, expensive, and requires large equipment investment and energy consumption, and the by-products reduce the purification yield, which results in high production cost. Other separation and purification techniques, such as the melt crystallization process disclosed in US5264592, US6310218, CN208426686, the rectification recrystallization technique disclosed in CN105324164, CN105814110A, CN10528994A, etc., the solvent crystallization disclosed in CN109400574A, the water washing disclosed in CN1112559A, and the combination of the solvent crystallization techniques, have been developed, but the above methods have limited effect on removing acid components.

Lactide can be divided into three types, L-lactide, D-lactide, m-lactide (meso-lactide). The melting crystallization technology and the solvent crystallization technology appear as the supplement of the rectification technology, are mainly used for purifying L (or D) -lactide with medium optical purity to L (or D) -lactide with high optical purity, and still have the problems of unreasonable separation flow, low efficiency, large equipment investment, high energy consumption, high production cost and the like. In addition, the existing melt crystallization process, particularly a crystallizer, has the problems of complicated operation, low processing capacity, complex equipment, high manufacturing cost and the like, and the intermittent operation is usually adopted in the industry, so that the processing capacity is limited, and the production cost is high.

In summary, the separation and purification technology of lactide disclosed in the prior art solves the problems of lactide impurity separation and quality control to a certain extent, but how to remove the acid component in the lactide with high efficiency and low cost to obtain a lactide product with lower acid component still needs to be solved urgently.

Disclosure of Invention

The invention aims to provide a method for removing acid components and isomers in lactide more efficiently at low cost, which can remove impurities such as acid components and isomers which are difficult to remove in the traditional separation process more efficiently, simplify production equipment and reduce production cost.

In order to achieve the above purpose, the technical scheme of the invention is as follows:

a method for separating and purifying lactide, comprising the following steps:

1) melting and crystallizing crude lactide from a reaction process, and separating the crude lactide into a stream a rich in L-lactide or D-lactide and a stream b rich in m-lactide, lactic acid and 2-5 polylactic acid oligomers;

wherein, a crystallization device M is adopted in the step 1), the crystallization device M is one or two of a falling film crystallizer, a suspension crystallizer and a static crystallizer, and the suspension crystallizer is preferably adopted;

the crude lactide stream in the crystallizer flows through the crystallizer in a liquid form vertical direction, and heat exchange devices with cooling areas and heating areas alternately spaced are arranged in the flowing direction; the crystals are dispersed in a liquid phase in a suspension manner in the crystallizer, and the crystals and the liquid integrally pass through the heat exchange device in a countercurrent direction respectively.

2) Rectifying and separating the stream a in the step 1) to obtain a purified stream D rich in L-lactide or D-lactide, a light component stream a-1 rich in water, lactic acid and m-lactide and a heavy component stream a-2 rich in lactic acid oligomers;

3) rectifying and separating the stream b in the step 1) to obtain a stream c rich in m-lactide, a stream b-1 rich in water and lactic acid as light components, and a stream b-2 rich in L-lactide or D-lactide and lactic acid oligomers;

4) and melting and crystallizing the stream D obtained in the step 2) again to obtain the L-lactide or D-lactide product with high optical purity.

In the lactide purification technology provided by the invention, rectification is generally adopted as a main separation means for removing light components such as lactic acid, water and the like and separating heavy components such as m-lactide isomer, lactic acid oligomer and the like. However, due to the nature of the system, the removal of lactic acid and water during the rectification process is difficult, on one hand, because the relative volatility of the lactic acid and lactide system is low, the number of rectification theoretical plates and huge energy consumption are required, on the other hand, because the lactide system is unstable, and during the relatively high-temperature operation of rectification, significant side reactions can occur, such as the hydrolysis of lactide to produce lactic acid, the oligomerization of lactic acid to produce water, the transesterification of lactic acid oligomer, and the hydrolysis to produce lactic acid. The rectification and purification process has been widely affected in the industrial production of lactide, but for the above reasons, the current rectification process needs to be improved.

Aiming at the problem of poor effect of removing key impurities such as lactic acid, water and the like by adopting a rectification method in the traditional separation and purification process, in the known lactide purification technology, a crystallization technology is mainly used as a purification method of high-purity lactide (such as optical purity of more than 99.5 percent), and is generally used after a rectification process and used as a supplement of further purification of the rectification process. Although there are reports in the literature on a process for purifying lactide by crystallization without rectification, the actually adopted crystallization equipment has a complex structure and low processing capacity, and the equipment investment and production cost are high, and the adoption of the traditional crystallization process as the only purification means of lactide is not economically feasible. The technology disclosed by the invention is mainly characterized in that melt crystallization is adopted as a main means for removing lactic acid and moisture, and most of lactic acid and moisture are removed under mild conditions such as low temperature and the like; and the innovative crystallization equipment is adopted, the problems of low treatment capacity and large equipment investment of the traditional crystallization equipment are solved, the production cost is obviously reduced, and the difficulty and the production cost of the lactide purification are finally obviously reduced.

The invention provides a novel crystallizer, and aims to simplify the crystallization process, improve the processing capacity of a device and reduce the production cost of a crystallization device. As shown in fig. 6, heating and cooling areas are arranged at intervals, after raw material liquid crude lactide enters a crystallizer, lactide in a cooling liquid phase is partially crystallized and separated out, crystals move downwards under the action of gravity, but the crystals are suspended in the liquid phase under the action of stirring and are in a slow settling state as a whole; the uncrystallized product still remains in the liquid phase to obtain a liquid phase stream with lower lactide content, the whole body moves upwards, and more lactide product is separated out through continuous condensation and crystallization in the moving process. Then the crystals settle and enter a heating area, and partial crystals are melted to obtain solid lactide crystals with higher purity and a liquid phase with lower lactide content; the liquid phase and the crystal integrally run upwards in a countercurrent state, and the lactide enters a cooling area to be partially crystallized and separated out again. Through the multi-stage partial melting and crystallization processes, the lactide content in the crystal is increased step by step, and theoretically, if the number of the steps is enough, the crystal purity can be obtained to be high enough. In addition, the crystallization operation is carried out in a continuous state, so that the equipment structure is obviously simplified, and the production efficiency is improved.

The other key technology of the invention is an innovative continuous operation high-flux crystallizer which is arranged in front of a rectifying tower and is used as a main means for removing lactic acid in crude lactide. The melting crystallization device of the step 1) and the step 4) adopts a falling film crystallizer, a suspension crystallizer, a static crystallizer or a combination of the falling film crystallizer, the suspension crystallizer and the static crystallizer. Preferably, a static crystallizer or a suspension crystallizer is used in step 1), and most preferably, the novel suspension crystallizer disclosed in the present invention is used. Step 4) employs a falling film crystallizer and/or a static crystallizer, more preferably step 4) employs a combination of a falling film crystallizer and a static crystallizer.

The suspension crystallizer preferably adopted in the step 1) has the following characteristics: the crude lactide stream flows horizontally or vertically in liquid form through the crystallizer, and cooling and heating heat exchange devices are arranged in the flow direction. The crude lactide stream meets a cooling area in the flowing direction, at least part of products are crystallized and separated out after cooling and exist in a suspension state in the liquid phase of the crude lactide stream, the liquid phase is relatively rich in L or D lactide with higher concentration, and the liquid phase is relatively rich in m-lactide, lactic acid oligomer and the like with higher concentration. The stream then enters a heated zone where the suspended crystals at least partially melt into the liquid phase under the action of the heat exchange means, but still retain at least a portion of the suspended crystalline particles, during which the retained crystals contain a further increased concentration of L or D lactide, while the melted-off liquid phase contains a higher concentration of m-lactide, lactic acid and lactic acid oligomers. After multi-stage partial crystallization and melting, the obtained suspended crystals reach the required L or D lactide content, and most of lactic acid and part of m-lactide, lactic acid oligomer and the like enter a liquid phase stream. The crystallizer temperature setting is crucial for the separation effect, the temperature range needs to span the crystallization and melting region of the crude lactide stream, typically 50-120 ℃, wherein the heating region is 90-120 ℃, the cooling region is 50-90 ℃, and the heating and cooling regions are alternately and alternately arranged. In order to achieve better separation effect, different heating areas can be the same or different, and preferably, the temperature of the heating areas is gradually reduced along the flow direction of the flow, and the temperature of the cooling area is gradually increased. The flow rate of the raw crude lactide is also a key factor, and the preferred flow rate in the present invention can ensure that the crystals produce good suspension in the liquid phase. In a vertical suspension crystallizer as shown in fig. 6, a higher flow velocity is preferred to ensure good crystal suspension and the residence time is ensured by the length and number of crystallizers to obtain the desired crystal purity.

By using the novel continuously operated high-throughput crystallizer disclosed by the invention, crude lactide is separated into a stream a rich in L (or D) -lactide and a stream b rich in m-lactide, lactic acid and lactic acid oligomers of 2-5 poly; through the step, most of lactic acid, part of m-lactide and 2-5 poly lactic acid oligomer are removed to the stream b. And the stream a rich in L or D lactide is further rectified, separated and purified to obtain the lactide product with qualified optical purity.

Reasonable process flow and circulating flow are beneficial to improving the yield of the lactide purification process and further reducing the production cost. In order to achieve the purpose, the b-1 is recycled to the reaction process for further utilization.

In the invention, the a-2 in the step 3) is recycled to the reaction process for continuous utilization.

In the present invention, the stream D in step 4) is in a gaseous form, condensed into a liquid phase, enters a crystallization device N, and is subjected to melt crystallization to obtain an L-lactide or D-lactide product with high optical purity, and a stream relatively rich in m-lactide, wherein the stream relatively rich in m-lactide is recycled to the rectifying column in step 2).

Wherein the streams c and d in the steps 2) and 3) are located at the side position of the rectifying tower. The rectifying tower in the steps 2) and 3) is a rectifying tower with a side line extraction, as shown in the attached drawing 1, and more preferably, the rectifying tower in the steps 2) and 3) is a clapboard rectifying tower with a middle clapboard, as shown in the attached drawing 2. The rectifying device with the side line can simplify the flow, reduce the equipment investment and the production cost, and is a relatively mature technology of the metric system in the industry. The partition plate rectifying tower technology with the partition plates is low in light and heavy component content, and can further reduce investment and cost and achieve remarkable benefits in the rectifying operation with m, L and D lactide as main components.

In order to further simplify the flow of the rectification process, it is preferred in the present invention to withdraw streams c and D rich in m-lactide and L or D lactide, respectively, at side-cut positions of the same rectification column. Namely, the stream a rich in L (or D) -lactide and the stream b rich in m-lactide, lactic acid and 2-5 poly lactic acid oligomers enter a rectifying tower from the lower part and the upper part of the rectifying tower respectively, and the rectifying tower is separated from the upper side to the lower side to obtain qualified streams c and D rich in m-lactide and L (or D) -lactide, as shown in the attached figure 3.

However, different technical variants can be selected according to different situations, for example, as shown in fig. 4, the composition of m-lactide in the crude lactide stream of the reaction process is controlled to be low, which does not have significant negative effect on the quality of lactide and downstream PLA, and the c stream rich in m-lactide can be eliminated, so as to simplify the process.

The stream rich in L (or D) -lactide is withdrawn from the side in gaseous form in step 2), condensed into liquid phase, and enters the crystallizer in step 4).

The reaction process comprises the steps of pre-polymerizing lactic acid into a lactic acid oligomer with a certain molecular weight, and depolymerizing and cyclizing the lactic acid oligomer into lactide under the action of a catalyst.

It is also an object of the present invention to provide a lactide product.

A lactide product, which is obtained by the lactide separation and purification method.

The invention has the advantages that:

crude lactide from a reaction process is subjected to crude separation through melt crystallization, and the crude lactide is separated into a stream rich in L (or D) -lactide and a stream rich in m-lactide, lactic acid and lactic acid oligomer of 2-5 poly. Particularly, through the melt crystallization of the step, the lactic acid and lactic acid oligomers in the stream rich in L (or D) -lactide are remarkably reduced, and the impurities are difficult to separate in the traditional rectification technology, so that the product quality is reduced, or the equipment investment and energy consumption cost are extremely high, so that the production cost is remarkably increased. The invention firstly carries out melt crystallization and crude separation, purifies the lactide to meet the requirement of medium optical purity through rectification crystallization, and further adopts the most efficient separation means in different process stages in the process of obtaining the lactide with high optical purity through melt crystallization.

Drawings

FIG. 1 is a schematic flow chart of a lactide separation and purification process;

FIG. 2 is a schematic diagram of a separation and purification process of a rectifying tower with a partition plate;

FIG. 3 is a schematic diagram of a separation and purification flow of an integrated rectification column;

FIG. 4 is a schematic view of a separation and purification scheme without separating m-lactide;

FIG. 5 is a schematic diagram of a falling film crystallizer, wherein HO is temperature-controlled heat transfer oil inlet oil, and HOR is temperature-controlled heat transfer oil return oil;

FIG. 6 is a schematic view of a novel vertical large-flux continuous suspension bed crystallizer, in which CW is a cooling medium inlet and CWR is a cooling medium return; HO is heating heat conducting oil inlet, and HOR is heating heat conducting oil return; the cold and hot ports are shown in thin and thick lines, respectively.

Detailed Description

Embodiments of the present invention are further illustrated by the following figures and examples. The invention is not limited to the embodiments listed but also comprises any other known variations within the scope of the invention as claimed.

Raw material information: the crude lactide adopted in the embodiment of the invention is self-made, the lactic acid product of Jiangxi Wuzang wild company is adopted as the raw material, the chemical purity of L lactic acid is 88 percent, and the optical purity is 99.7 percent;

the characterization equipment comprises the following steps: in the examples the quantitative detection of substances was performed by GC,

chromatographic column and chromatographic conditions

Drawing a standard curve of each component, and carrying out quantitative analysis on each component according to the following formula:

and substituting the peak area of the sample into a standard curve formula to calculate the concentration of the sample. The results of the analysis are reported in mg/L.

Calculating the content of lactide isomers by adopting an external standard curve method:

wherein the content of the first and second substances,

ω_L-the concentration of L-lactide in the sample%

A_LArea of L-lactide peak

m-weight sample mass, g

Wherein, ω is_D-the concentration of D-lactide in the sample%

A_DArea of-D-lactide Peak

m-weight sample mass, g

Wherein the content of the first and second substances,

ω_m-m-lactide concentration in the sample%

A_mArea of m-lactide peak

m-weighing sample mass, g.

Example 1

Adopts the separation and purification flow shown in figure 1 and takes L-lactide as a main product.

Step 1: and (5) a crystallization step. A suspension crystallizer as shown in figure 6 is adopted, the diameter is 270mm, the height is 3m, and 7-stage push-up propellers are arranged. And performing melt crystallization on the crude lactide prepared in the reaction process, wherein the total residence time is 4h calculated by the total flow of crude lactide streams, the temperature of hot oil in a heating area is 98 ℃, and the temperature of cooling medium in a cooling area is 70 ℃.

The results are shown in the following table, where L-LD stands for L-lactide, m-LD stands for m-lactide, D-LD stands for D-lactide, and LA stands for lactic acid.

Example 2

Adopts the separation and purification flow shown in figure 1 and takes L-lactide as a main product.

Step 1: the crystallization process adopts a suspension crystallizer as shown in figure 6, and crude lactide prepared by the reaction process is used for melt crystallization, and the total residence time is 2.5h calculated by the total flow of crude lactide streams, wherein the temperature of hot oil in a heating area is 120 ℃, and the temperature of cooling medium in a cooling area is 50 ℃.

The correlation results are shown in the following table.

Results analysis of examples 1 and 2: through crystallization and purification, m-lactide, lactic acid and oligomer in the crude lactide are all obviously reduced.

Example 3

Adopts the flow shown in the attached figure 1 and a separation flow taking L-lactide as a main product.

And 2, step 3: and (5) a rectification process. The rectifying tower is filled with 3 sections of Pro-Pak Bulletin-packing of POPE company in America, the height of the Pro-Pak Bulletin-packing is 50cm respectively, the feeding position is between the first section packing and the second section packing from top to bottom, the target product is extracted from the side line, and the extraction position is between the second section packing and the third section packing from top to bottom.

	L-LD/％	m-LD/％	D-LD/％	LA/％	Fraction of oligomers	Optical purity/%)
							Starting material	89.4	5.6	0.4	2.8	1.85	93.7

The rectifying tower operating conditions were as follows:

	oil bath/. deg.C	Temperature of the kettle/. degree.C	Top temperature/. degree.C	Reflux ratio	Pressure PaA
						Stream a rectifying column	135	120	104	4:1	500
Stream b rectifying column	165	150	120	4:1	500

The rectification separation results are as follows:

	LA	oligomer	Lactide	Optical purity of L-LD
					Product c	0.0689％	1.89％	98.02％	37.14％
Product d	0.0667％	3.58％	96.35％	98.79％

Example 4

Adopts the flow shown in the attached figure 1 and a separation flow taking L-lactide as a main product.

And 4, step 4: crystallizing and purifying the lactide with high optical purity. A single-tube falling-film crystallizer as shown in figure 5 is adopted, the diameter is 26mm, and the length is 300 mm.

The rectification product is used as the raw material, and the batch melting crystallization operation comprises the following steps: reducing the temperature from 98 ℃ to 85 ℃ (30 ℃/h) within 26min, crystallizing for 1 hour, and pouring out the mother liquor; the temperature is increased from 85 ℃ to 95 ℃ (10 ℃/h) for 60min, and sweating is carried out until no liquid is dropped.

The correlation results are as follows:

secondary crystallization: the product obtained by the secondary crystallization is used as a raw material, the temperature is reduced from 98 ℃ to 85 ℃ (30 ℃/h) within 26min, crystallization is carried out for 1 hour, and mother liquor is poured out; the temperature is increased from 85 ℃ to 95 ℃ (10 ℃/h) for 60min, and sweating is carried out until no liquid is dropped.

The correlation results are shown in the following table.

As can be seen from the above analysis, L-lactide with high optical purity can be obtained more efficiently by crystallization.

Comparative example 1

The comparative example is prior art, crude lactide is purified by rectification, and the rectification equipment is the same as in examples 2 and 3.

	L-LD/％	m-LD/％	D-LD/％	LA/％	Fraction of oligomers	Optical purity/%)
							Starting material	89.4	5.6	0.4	2.8	1.85	93.7

The operation conditions were the same as in example 3, and the rectification results were as follows:

the comparison shows that the acid value and the optical purity of the product are poorer than those of the process of the invention under the same conditions by adopting the traditional rectification process, and the lactide contains a large amount of lactic acid and has low optical purity, thereby influencing the application of downstream PLA.