RU2575709C2 - Method for obtaining polyhydroxycarboxylic acid - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to method for obtaining polylactic acid. Method for obtaining polylactic acid includes the following stages: (i) realisation of polymerisation with cycle opening, with application of catalyst, and either catalyst deactivator, or additive, blocking terminal groups, to obtain non-purified polylactic acid with molecular weight higher than 10000 g/mol, (ii) purification of polylactic acid by removal and separation of low-boiling compounds, including lactide and admixtures, from non-purified polylactic acid by removal of volatile low-boiling compounds in form of gas-phase flow, (iii) purification of lactide at stage of removal of volatile components and removal of admixtures from gas-phase flow of evaporated low-boiling compounds by means of crystallisation by desublimation from gas-phase, in which lactide is purified, and removed admixtures include catalyst residue and compound, containing at least one hydroxyl group, with thereof purified lactide being further polymerised by supplying it back into polymerisation with cycle opening. Device for method realisation is also claimed.

EFFECT: simplification of technology.

25 cl, 10 dwg, 2 tbl, 2 ex

Description

The present invention relates to a method for producing polyhydroxycarboxylic acid, in particular polylactic acid, in which the yield is increased relative to the final product due to the reuse of the lactide from the side fraction resulting from the purification of the crude polylactic acid, and due to the reuse of the lactide obtained as a result of this cleaning up. In addition, the present invention relates to a device for performing this method of producing polylactic acid. The present invention also relates to a method for melt crystallization of a vaporous biodegradable intermolecular cyclic alpha-hydroxycarboxylic acid diester.

Polylactic acid, which will also be referred to as PMA in the following text, is a biodegradable polymer that is synthesized from lactic acid. A particular advantage of such polymers is their biocompatibility. The term "biocompatibility" means that they have only a very limited negative effect on any living creatures in the environment. A further advantage is that polylactide polymers are obtained from completely renewable raw materials, such as, for example, starch and other saccharides derived from sugar cane, sugar beet and the like.

Polylactide polymers are increasingly turning into a source of income since the middle of the 20th century. However, mainly due to the limited availability of monomer and high manufacturing costs, their original use was mainly carried out in the medical field, such as surgical implants or surgical threads, for example, pins, screws, suturing material or reinforcing material for bone cracks. An interesting property of PMC is the decomposition of polylactide polymers in the body, eliminating the need for secondary surgical treatment to remove any implant. In addition, MVP can be used in sustained release capsules for controlled dosage of drugs.

In recent decades, due to a significant increase in crude oil prices and the recognition of the importance of environmental problems, along with improved production methods, the production of polylactide polymers has become more attractive for packaging, in particular, food, both rigid packaging and flexible foils, such as uniaxial and biaxially stretched films. Other uses are fibers, for example, for fabrics used in clothing, in furniture upholstery or in carpets. In addition, extruded products similar to disposable cutlery or containers are produced for office supplies or hygiene products. Polylactide polymers can also be combined with other materials to form composite materials.

Currently, there are two known methods for the production of PMC.

The first of these production methods involves direct polycondensation of lactic acid to form polylactic acid, as described in JP 733861 or JP 5996123. A solvent is used in addition to lactic acid to carry out the polycondensation reaction. In addition, it is necessary to continuously remove water throughout the entire polycondensation process in order to enable the formation of high molecular weight polylactide polymers. For all these reasons, this method is not used in mass production.

The method, which was created for the mass production of PMC, uses the lactide intermediate to begin the subsequent polymerization with the opening of a ring leading from lactide to polylactic acid. A number of variants of these methods are disclosed, for example, in US 5142023, US 4057537, US 5521278, EP 261572, JP 564688 B, JP 2822906, EP 0324245, WO 2009121830. The methods described in these documents collectively have the following main stages. In the first stage, raw materials, for example, starch, or other saccharides extracted, for example, from sugar cane or sugar beets, corn, wheat; in a second step, fermentation is carried out using suitable bacteria to produce lactic acid; in a third step, a solvent — usually water — is removed from the mixture to prevent the negative effects of the solvents in subsequent steps. In a fourth step, lactic acid is subjected to catalytic dimerization to form crude lactide. An optional intermediate step is usually carried out, which involves pre-polymerizing lactic acid to low molecular weight polylactic acid and subsequent depolymerization to form crude lactide. The fifth stage involves the purification of lactide to remove foreign substances that can have a negative effect on the polymerization and contribute to the staining, as well as to the smell of the final product. Separation can be performed either by distillation or crystallization. In a sixth step, ring-opening polymerization is obtained to obtain a high molecular weight crude polylactic acid. The molar mass is from about 20,000 to 500,000 g / mol in accordance with US 6187901.

Optionally, copolymerization compounds may be added during ring opening polymerization. In a seventh step, the crude polylactic acid is purified to obtain purified polylactic acid. In this step, low boiling compounds are removed that would reduce the stability of the polymer and adversely affect the parameters of subsequent plastics production, such as viscosity or rheological properties of the molten polymer, and which would contribute to staining and undesirable odors of the final product. According to US Pat. No. 5,880,254, the crude polylactic acid can be solidified to form a granulate which is brought into contact with a moderate inert gas stream, for example in a fluidized bed. The lowest boiling compounds from crude polylactic acid are carried away by an inert gas. Another method is described in US 6187901. In accordance with this method, liquid crude polylactic acid is sprayed with a plurality of nozzles to form a plurality of liquid filaments. Inert gas flows around the liquid filaments and the lactide evaporates into the hot inert gas stream. The low boiling point stream typically contains up to 5% by weight of dilactide.

Lactic acid has two enantiomers, L-lactic acid and D-lactic acid. Chemically synthesized lactic acid contains L-lactide and D-lactide in a racemic mixture consisting of 50% of each of the enantiomers. However, the fermentation process is made more selective by using appropriate microbial cultures to selectively produce L- or D-lactic acid.

Lactide molecules that are obtained by dimerization of lactic acid exist in three different forms: LL-lactide, also called L-lactide, DD-lactide, also called D-lactide, and L, D-lactide (or D, L-lactide ), which is called mesolactide. L and D-lactides are optically active, while mesolactide is not. The purification steps for purifying the crude lactide typically include separating a stream rich in L-lactide and a stream rich in D-lactide, and then a stream rich in mesolactides, each of which may be separately purified. By mixing at least two of the three forms of lactides, the mechanical properties and the melting point of the polymers formed by polylactic acid can be affected. For example, by mixing a suitable amount of one enantiomer with another, the polymer crystallizes at a lower rate, which, in turn, allows foaming of the produced plastic without interference caused by curing too fast.

Attempts have been made to increase the yield of the polylactic acid process and reduce the manufacturing costs of polylactic acid.

US 5142023 reports that a gaseous stream of low boiling components from the stage of purification of the crude lactide is fed, at least partially, back to the lactide reactor. A heavy residue forms in the lactide reactor, which can be partially withdrawn back to the reactor itself or fed back to a separation device to separate the solvent from lactic acid after fermentation.

US 7488783 reports that the crude lactide crystallizes to form purified lactide. A second crystallization step is performed on the remainder of the first crystallization step in order to isolate the lactide from there. This lactide is fed back to the first stage of crystallization or to one of the previous stages of the process in accordance with this method.

US 5521278 reports that the crude lactide is crystallized. The residue stream is evaporated, selectively condensed and returned back to one of the previous process steps in accordance with this method.

JP 2822906 discloses the solidification of a gaseous crude lactide stream into pure lactide. The residue, which does not solidify, is returned back to the lactide rector.

JP 10101777 describes that a gaseous crude lactide stream partially hardens by cooling with an inert gas stream to form pure lactide. The residue is fed back to the lactide reactor. This crude lactide stream originates from a direct polycondensation reaction. This crude lactide stream is gaseous. By cooling the crude lactide stream created by the indicated polycondensation reaction to a temperature at which the lactide crystallizes, it crystallizes in a countercurrent crystallization device with a self-cleaning function. The countercurrent crystallization device has rotational drive means for rotating two screws located in the cylinder, whereby the rotating screws are arranged with engaged gears. The cylinder is cooled by a cooling medium circulating in a cooling jacket located in the cylinder wall to a temperature at which part of the lactide compound with a low molecular weight and lactic acid crystallizes and is transferred to the outlet with two screws and flows back from the outlet to the periodic condensation reactor actions. Crystallization is carried out using a solvent. Such a solvent, such as water, is used to lower the viscosity of the melt, which is believed to improve mass transfer. Therefore, compounds with a low melting point are more fully separated from compounds with a high melting point, which form a crystalline fraction on the crystallization surface of the crystallization device. Thus, it is believed that the fouling of the crystals should decrease if the melt viscosity is reduced. The purpose of this invention, as disclosed in JP 10101777, is the removal of solvent.

Any of the methods described relates to the reuse of a partial stream from purification of the crude lactide. Any of these methods serves to increase the yield of this method, but does not disclose whether it is possible to reuse lactide, which is still in crude polylactic acid in a percentage concentration up to 5%.

No. 6,187,901 relates to a method for removing lactide from polylactide and for recovering lactide from a lactide-containing gas. The crude polylactic acid is sprayed into the space containing the hot inert gas using spray nozzles. By means of which thin threads are formed. These threads fall under the action of gravity and in a laminar flow. Due to this, the polymer melt flows faster in the inner parts of the thread than in the surface. By means of which the polymer melt flowing in the inner part of a sufficiently thin filament forms a transition surface of a new material for the evaporation of lactide in the process of its downward movement. Lactide partially evaporates and collects in an inert gas, from which it crystallizes in a crystallization chamber due to rapid cooling. The resulting crystals are separated in a cyclone or on a filter device and reused in the polymerization reactor. The amount of lactides in polylactic acid can be reduced by this stage of the process up to 1%. However, reuse of lactide requires an inert gas stream, which must be purified before it is discharged as a waste stream.

No. 5,880,254 discloses a method for producing polylactic acid. The crude polylactic acid is crystallized in the form of a granulate. The granulate is exposed to a stream of hot inert gas passing through the granulate, forming a fluidized bed. The lactide contained in the granulate evaporates and is carried away with a stream of inert gas and fed back to the polymerization reactor. The purified polylactic acid still contains about 1% dilactide.

Each of the methods of US 6187901 or US 5880254 requires the supply of an inert gas, which must be processed for reuse, which, in turn, requires additional equipment and leads to increased costs for the purification of polylactic acid.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide an improved method for producing polylactic acid without the disadvantages of the previously described methods, and a further goal is to reduce the equipment necessary for the treatment of inert gas and increase the yield compared to methods in accordance with US 6187901 and US 5880254.

In accordance with the present invention, the first objective is achieved by a method for producing polylactic acid, comprising the steps of ring-opening polymerization using a catalyst, and either a catalyst deactivator compound or an end-blocking additive to produce crude polylactic acid with a molecular weight of more than 10,000 g / mol, purification of the crude polylactic acid by removing and separating low boiling compounds, including lactide and impurities, from the crude polylactic acid by y deletion of the volatile components of low-boiling compounds in the form of a gas-phase stream, purification of lactide from the stage of removal of volatile components and removal of impurities from the gas-phase stream of evaporated low-boiling compounds by crystallization by desublimation from the gas phase in which the lactide is purified, and the removed impurities include a catalyst residue and a compound containing at least one hydroxyl group, so that the purified lactide is then polymerized, feeding it back to the ring opening polymerization.

A further objective is achieved by a device for implementing this method, including a ring-opening polymerization reactor to obtain crude polylactic acid, a volatile component removal device for separating low boiling compounds including lactide and impurities from crude polylactic acid, and a crystallization device for purifying lactide and removing impurities by desublimation and crystallization in the same crystallization device.

In a preferred embodiment of this method, desublimation occurs on a chilled surface. In another preferred embodiment of this method, the crystallization device and the volatile component removal device operate under substantially the same vacuum conditions. In another preferred embodiment, desublimation and crystallization occur in the same device. In yet another preferred embodiment, the crystallization apparatus does not have an inert gas stream. In another preferred embodiment, the vaporized gas phase stream from the volatile component removal step contains at least 30% lactide, preferably at least 60%, most preferably at least 90%. In yet another preferred embodiment, the lactide is first subjected to a sweating step followed by a melting step before being fed back to the ring opening polymerization. In yet another preferred embodiment, the removal of volatile components operates at a partial lactide pressure of less than 20 mbar (2 kPa), preferably less than 10 mbar (1 kPa), particularly preferably less than 5 mbar (0.5 kPa). In yet another preferred embodiment, the removed impurities include either an organometallic compound or a carboxylic acid. In yet another preferred embodiment, at least a portion of the crystallization purge stream is reused in the purification step of the crude lactide, the prepolymerization and dimerization step, or the solvent removal step in the production of purified lactide. In another preferred embodiment, the liquid from the sweating step is collected and recrystallized to recover the lactide.

In a preferred embodiment of the device of the present invention, no throttling means or vacuum pumps are located between the crystallization device and the volatile component removal device. In another preferred embodiment, a heat exchanger is disposed between the volatile component removal device and the crystallization device. In yet another preferred embodiment, the crystallization device has heat exchange surfaces for solidifying the gaseous stream.

Detailed description of the present invention

The aim of the present invention is a method comprising purifying polymerizable monomers or oligomers similar to lactide, by crystallization, in which, in the first step, ring-opening polymerization is carried out to obtain crude polylactic acid with a high molecular weight of more than 10,000 g / mol;

in the second stage, the crude polylactic acid is purified to obtain purified polylactic acid, whereby in the second stage the low boiling compounds are removed and the low boiling compounds are separated from the polylactic acid by removal of volatile components, and in the third stage the lactide is reused and the impurities are removed from the vaporized gas phase stream of the second stage by crystallization or solidification from the gas phase. In the third stage process, impurities are removed so that the purified lactide can be added back to the ring opening polymerization of the second stage. Such impurities may include coloring or odor forming compounds or any additional by-products such as water, catalyst residues, for example organometallic compounds, reaction by-products, compounds comprising at least one hydroxyl group (OH), acidic compounds, such like carboxylic acids, catalyst deactivator compounds or end-blocking additives.

As used herein, by “polymerization is carried out” is meant a process in which the lactide monomer is contacted with a catalyst and optionally an initiator (a substance containing hydroxyl functional groups, for example, water, alcohols), at a temperature and for a period of time which are sufficient to providing polymerization of lactide molecules, i.e. to provide a chemical reaction that “opens” and chemically binds lactide molecules in several long chains to form polymer lactic acid.

The polymerization of lactide into polylactic acid is generally described and reviewed in the literature, e.g. A. Sodergard and M. Stolt in "Industrial Production of High Molecular Weight Poly (Lactic Acid) as Chapter 3 of Poly (lactic acid): Synthesis, Structures, Properties, Processing, and Applications, edited by R. Auras, L.-T. Urn, SEM Selke, and H. Tsuji, John Wiley, Hoboken, New Jersey, 2010 (ISBN 978-0-470-29366-9).

A particularly preferred continuous lactide-opening continuous polymerization process is described in WO 2010/012770 A1.

Suitable catalysts for use within the scope of the invention are known in the art. In one embodiment of the present invention, the lactide is polymerized using a catalyst of the formula

(M) (X1, X2 ... Xm) n,

where M is selected from metals of groups 1, 2, 4, 8, 9, 10, 12, 13, 14 and 15 of the Periodic table of elements; X1, X2 ... Xm are independently selected from the group of alkyls, aryls, oxides, carboxylates, halides, alkoxides, alkyl esters; m is an integer from 1 to 6; and n is an integer from 1 to 6, where the type values depend on the oxidative state of the metal ion.

In group 1, the use of Na is preferred. In group 2, Ca is preferred. In group 4, Ti is preferred. In group 8, the use of Fe is preferred. In group 12, the use of Zn is preferred. In group 13, the use of Al, Ga, In and Tl may be mentioned. In group 14, the use of Sn and Pb is preferred. In group 15, the use of Sb and Bi is preferred. In general, the use of metals of groups 4, 14 and 15 is preferred. It is preferred that M be selected from Sn, Pb, Sb, Bi and Ti. The use of a Sn-based catalyst may be particularly preferred for the lactide monomer.

In the case of halides, tin halides such as SnCl ₂ , SnBr ₂ , SnCl ₄ and SnBr ₄ may be mentioned. In the case of oxides, SnO and PbO may be mentioned. In the alkyl ester group, octanoates (= 2-ethylhexanoates), stearates and acetates, for example, in the form of Sn octanoate (also known as Sn (II) bis-2-ethylhexanoate Sn (II)), Sn stearate, dibutyl diacetate, butyl tris (2 ethyl hexanoate), (2-ethylhexanoate) Sb, (2-ethylhexanoate) Bi, Sb triacetate, (2-ethylhexanoate) Na, Ca stearate, Mg stearate and Zn stearate.

Other suitable compounds include tetraphenyltin, Tris (ethylene glyoxide) Sb, aluminum alkoxides and zinc alkoxides.

The concentration of the catalyst, as a rule, is at least 5 ppm. based on the weight of the metal, more specifically at least 10 ppm When amounts in excess of conventional are used, the catalyst concentration is at least 30 ppm, more specifically at least 50 ppm. The concentration of the catalyst, as a rule, is not more than 200 ppm, in particular, not more than 150 ppm

The use of Sn (II) bis (2-ethylhexanoate), also referred to as tin octanoate, may be preferred since this material is commercially available, liquid at room temperature and reaction temperature and soluble in liquid lactide. In addition, the compound has received FDA approval for many everyday end use applications.

If desired, a cocatalyst may be added to the lactide and catalyst, i.e. a compound that further increases the rate of polymerization. Suitable cocatalysts are known in the art. For example, see US 6166169.

It is known that polymerization with the opening of the lactide cycle in polylactic acid proceeds by the mechanism of coordination incorporation, through which the lactide molecules are sequentially attached to the ends of the polymer chain (elongation). This reaction is reversible and can lead to degradation of the polymer through double-chain interaction reactions also occurring at the end of the polymer chain that release lactide molecules (depolymerization).

Catalyst deactivator compounds are any substances that react, chemically or through the formation of stable complexes, with the particular catalyst used for the polymerization reaction, inhibiting its activity of accelerating the polymerization reaction. Thus, the catalyst deactivator compounds increase the resistance of the crude polylactic acid to depolymerization, discoloration, and degradation in general. Examples of suitable catalyst deactivator compounds are organic peroxides, antioxidants, phosphorus compounds, multifunctional carboxylic acids, screened phenolic compounds, deactivation catalysts, such as screened alkyl, aryl and phenolic hydrazides, amides of aliphatic and aromatic mono- and dicarboxylic acids, cyclic amides, hydrazones and bishydrazones of aliphatic and aromatic aldehydes, hydrazides of aliphatic and aromatic mono- and dicarboxylic slot, bis-acylated hydrazine derivatives, and heterocyclic compounds. The polymer is treated with a catalyst deactivator compound by mixing it with the polymer, for example, at a temperature of the same order as the polymerization temperature. This can be done using a static mixer, an extruder, or any other conventional method of mixing materials, at least one of which is highly viscous.

The end-blocking additive is any substance that is capable of chemically reacting with the end elements of the polylactic acid chains and, thus, replacing the chemical functionality of the end of the chain. Examples of end-blocking additives include anhydrites such as acetic anhydride, succinic anhydride and phthalic anhydride.

Thus, both end-blocking additives and catalyst deactivator compounds are used to increase polymer stability.

According to the present invention, the “catalyst residue” is an unreacted catalyst or degradation product or a reaction by-product of the catalyst. Thus, it can be an organometallic compound. The organometallic complex can be the same as the catalyst, for example, Sn (II) bis (2-ethylhexanoate), or it can be a complex formed between metals and the catalyst deactivator compound, an end-blocking additive, a monomer, an oligomer , an impurity or a reaction by-product (as opposed to the desired polymerization reaction), such as a hydrolysis product, such as lactic acid, or a product of thermal degradation of a monomer, or compound, or additive. Also, the catalyst residue can be formed mainly by the organic part of the catalyst (such as 2-ethylhexanoic acid in the case of Sn (II) bis (2-ethylhexanoate) as a catalyst), which, as a rule, is a volatile organic compound.

In embodiments in which a catalyst deactivator compound is added, the “catalyst residue” is often a direct product of the chelation reaction between the catalyst deactivator compound and the catalyst or its thermal degradation product.

This feature is described as a residue since it is typically a derivative of an added catalyst (such as a chelating complex with a catalyst deactivator compound) or a product of thermal degradation of the added catalyst or its reaction product.

According to the present invention, the compound containing at least one hydroxyl group is an impurity present in the feed material, such as water, lactic acid, oligomers or low molecular weight alcohols. Alternatively, it may be a hydrolysis product of a lactide monomer, such as lactic acid, a volatile low molecular weight oligomer of polylactic acid, such as a dimer or trimer, etc., or a product of hydrolysis or thermal degradation, or a by-product of the reaction of the lactide monomer, catalyst, deactivator compound a catalyst or additive, such as a terminal blocking additive. Thus, such impurities containing hydroxyl groups can be derived from a monomer, catalyst, a catalyst deactivator compound or an end-blocking additive in a ring opening polymerization step.

Advantageously, the molecular weight of the crude polylactic acid is at least 10,000 g / mol, preferably at least 15,000 g / mol, particularly preferably at least 20,000 g / mol. Optionally, other polymerizable monomers or oligomers may be included, such as at least one of the group of glycolactide copolymer, polyglycolic acid or polyglycolic acid (PHA), block copolymer of styrene-butadiene-methacrylate (SBM), polystyrene copolymer, 1.4 polybutadiene, syndiotactic polymethyl methacrylate (PMMA), a triple block copolymer with a central block of polybutyl acrylate surrounded by two blocks of polymethyl methacrylate, polymethyl methacrylate (PMMA), polyetheretherketone (PEEK), polyethylene oxide (PEO), polyethylene glycol (PEG), polycaprolactam, polycaprolactone, polyhydroxybutyrate.

Typical comonomers for the copolymerization of lactic acid or lactide are glycolic acid or glycolide (HA), ethylene glycol (EG), ethylene oxide (EO), propylene oxide (PO), (R) -β-butyrolactone (BL), δ-valerolactone (VL), ε-caprolactone, 1,5-doxepan-2-one (DXO), trimethylene carbonate (TMK), N-isopropyl acrylamide (IPAA).

The crude polylactic acid may also contain other impurities. However, in this application there are no specific restrictions regarding the chemical nature of the catalyst included in the crude composition and in the condensed stream, and it certainly depends on the type of pre-treatment of the crude composition.

At the end of the polymerization, a temperature-dependent equilibrium between the monomer and the polymer is achieved, whereby the crude polylactic acid contains about 5% by weight of unreacted lactide. The monomer content should be reduced to less than 0.5 wt.% In order to obtain the required mechanical, chemical, rheological and thermal properties of the polymer for further processing.

The vaporized gas phase stream leaving the volatile component removal step can be condensed, whereby a condensed stream is obtained. The vaporized gas phase stream contains at least 30 wt.% Lactide. Impurities should be present only in small quantities, so water should be at most 10 ppm, preferably 5 ppm, particularly preferably less than 0.5 ppm. Any lactic acid in the vaporized gas phase stream should be less than 100 mmol / kg, preferably less than 50 mmol / kg, particularly preferably less than 10 mmol / kg. The condensed stream crystallizes from its liquid state, and crystallization is preferably carried out without solvent. This is a particular advantage that no further steps are required to remove any solvents.

The crystallization step is advantageously carried out in one of the melt layer crystallization devices or in a desublimation device, such as at least one of a falling film crystallization device or a stationary crystallization device, or crystallization from a suspension which is carried out in at least one suspension crystallization device. If a suspension crystallization device is used, the condensed stream is cooled so as to obtain lactide crystals floating freely in the liquid phase of the suspension crystallization device, thereby forming a partially crystallized liquid stream, which is subsequently fed to the washing device.

Alternatively, the vaporized gas phase stream may be desublimated, namely, cooled from the gas phase directly to the solid phase in the desublimation step.

The crystalline fraction obtained by crystallization in accordance with any of the alternatives set forth above contains purified lactide. Advantageously, the removal of volatile components works at a lactide partial pressure of less than 20 mbar (2 kPa), preferably less than 10 mbar (1 kPa), particularly preferably less than 5 mbar (0.5 kPa). The solidified fraction containing purified lactide can be melted in a subsequent heating step to be fed back to the ring opening polymerization. The sweating step may be carried out before the heating step for the solidified fraction present in crystalline form on the crystallization surfaces. The mother liquor may remain between the crystals and thereby form inclusions containing impurities. During the sweating step, these impurities are removed.

The vaporized gas phase stream from the volatile component removal step contains at least 30% lactide, preferably at least 60% lactide, most preferably at least 90% lactide. To increase the yield of lactide from the vaporized gas phase stream, the mother liquor and / or liquid from the sweating stage can be fed to the recrystallization step.

According to a preferred embodiment of the present invention, the crystallization device is connected directly to the device for removing volatile components by means of a gas pipe or optional heat exchanger located between the removal of volatile components and crystallization. The heat exchanger, in particular, has a gas cooler configuration. Such a heat exchanger has the particular advantage of reducing the desublimation surface of the crystallization device, since part of the contact heat can be removed from the steam stream already before entering the crystallization device.

A direct connection between the crystallization device and the device for removing volatile components gives the result in that both devices operate in almost the same vacuum conditions. This means that no throttling or vacuum pumps are located between the crystallization device and the volatile component removal device.

The inventors have found that, oddly enough, the viscosity of the condensed lactide fraction in the crystallization stage from the melt makes possible sufficient mass transfer and, in turn, sufficient purification of the crystalline fraction. Crystallization from melt should be understood as crystallization that does not contain a solvent. The melt viscosity can be up to 100 mPa · s, whereby the viscosity is preferably less than 10 mPa · s, particularly preferably less than 5 mPa · s.

According to a preferred embodiment, the method comprises a first step in which the feed is processed to extract fermentable polysaccharides. Raw materials can come from grains, sugar crops, reeds, potatoes, or other fermentable polysaccharides. In a second step, fermentation is carried out using suitable bacteria to produce crude lactic acid. In a third step, the solvent is removed from the mixture. According to a preferred method, the solvent can be removed by evaporation. The solvent may in particular be water. In a fourth step, lactic acid is subjected to catalytic dimerization to form crude lactide. An optional intermediate step may be carried out which involves pre-polymerizing lactic acid to low molecular weight polylactic acid and subsequent depolymerization to form crude lactide. Lactic acid, which has not reacted to form crude lactide, can be separated and reused in the device for the third stage. Heavy residues from a lastidium reactor can be reused in a second or third stage reactor. Part of the heavy residues can also be added to the next sixth stage, which includes the polymerization of purified lactide to polylactic acid or can be reused in the device for the third stage.

In the fifth stage, the lactide is purified to remove foreign substances that can adversely affect the polymerization and contribute to the coloring and also to the smell of the final product. The separation can be carried out either by distillation or crystallization. Undesirable compounds, such as unreacted lactic acid, other carboxylic acids, are contained in the vapor phase using evaporation. These undesirable compounds are present in the non-crystallizable residue. The stream of undesired compounds can be reused in any device of the third or fourth stage.

In a sixth step, ring opening polymerization takes place to obtain a high molecular weight crude polylactic acid. During the polymerization, a temperature-dependent equilibrium is reached between the monomer and the polymer. The crude polylactic acid contains about 4-6 wt.% Unreacted lactide. The monomer content should be reduced to less than 0.5% in order to obtain the required mechanical properties of the polymer for its further processing. Therefore, crude polylactic acid must be purified.

In a seventh step, the crude polylactic acid is purified to obtain purified polylactic acid. In this step, low boiling compounds are removed, which usually contribute to the coloring and undesirable odors of the final product or may contain additives that could have an undesirable effect on the ring opening polymerization process when reused. Separation of low boiling compounds from crude polylactic acid is achieved by the removal of volatile components, for example, flash evaporation under vacuum. The vaporized stream contains at least 30% lactide which has not reacted to polylactic acid in the ring opening polymerization process according to the sixth stage. Moreover, the vaporized gas phase stream may contain other low boiling compounds that contribute to the coloring or to the smell of the final product, both of which are the most undesirable properties, reaction by-products or additives that have an undesirable effect on ring opening polymerization upon repeated use.

Cleaning in accordance with the seventh stage can be carried out in one or more subsequent stages of removal of volatile components. The main part of the lactide contained in the crude polylactic acid stream is stored in the first stage of removal of volatile components, which is a total amount of 5% relative to the main part.

In the eighth stage, the lactide is purified and reused from the vaporized gas phase stream of the seventh stage by crystallization, which may include desublimation, and hence crystallization from the gas phase. In the process of this step, stain and odor forming compounds or unwanted additives are removed, so that the purified lactide can be added back to the sixth stage ring opening polymerization, thereby preventing any accumulation of stain and odor forming compounds or that would destruct the process in sixth stage of the process.

The lactide content in the purified PMA, leaving the stage of removal of volatile components, as a product stream is less than 2%. Preferably, the lactide content in the purified PMC is less than 0.5 wt.%.

The lactide content in the vaporized gas phase stream is at least 30 wt.%, Preferably at least 60%, most preferably at least 90%.

In accordance with one embodiment of this method of the present invention, the vaporized stream leaving the stage of removing the volatile components is condensed and crystallized from its liquid state. Such crystallization can be carried out without solvent as layer-by-layer crystallization in a crystallizing device with a falling film or in a stationary crystallization device. Alternatively, crystallization can be carried out in a suspension crystallization device in which the condensed mixture is cooled so as to form lactide crystals freely floating in the liquid, whereby forming a partially crystallized liquid stream. This partially crystallized liquid stream is fed to a washing device in which solids are separated from the liquid residue.

The crystalline fraction obtained using any of the above crystallization devices contains purified lactide and melts in the last crystallization stage to be fed back to ring opening polymerization in accordance with the sixth stage. The non-crystallized mother liquor must be discharged from the process in the form of a waste stream or can be at least partially reused in any of the above process steps located upstream, for example, 3, 4, 5, as shown in FIG. 2.

According to one embodiment of the method of the present invention, a crystallization device in which lactide crystals are formed must be directly connected to a device for removing volatile components. The removal of volatile components works at a partial pressure of lactide of less than 20 mbar (2 kPa), preferably less than 10 mbar (1 kPa), particularly preferably less than 5 mbar (0.5 kPa). The lactide from the vaporized gas-phase stream solidifies on cooled crystallization surfaces provided with a crystallization device forming crystallization layers. The solidified fraction containing purified lactide is melted in a subsequent heating step to be fed back to ring opening polymerization in accordance with a sixth step. The liquid fraction that has not been precipitated as crystals on the crystallization surfaces should be removed from the process as a waste stream.

The heating step for melting the crystals on the crystallization surfaces may be preceded by a sweating step. During the sweating stage, crystals are partially melted. Any residual unwanted compounds located between the crystals of the polycrystalline layers or on their surfaces can be separated and removed from the lactide crystals. A polycrystalline layer is understood to mean a layer that contains many crystals. Between crystals of such a polycrystalline layer, impurities can accumulate. These impurities can be removed by a sweating step. The liquid fraction formed during the sweating stage must be removed from the process as a waste stream.

In layer-by-layer crystallization, polycrystalline layers are formed on heat transfer surfaces provided by a crystallization device. According to a preferred embodiment, the heat exchange surfaces are plates or tubes through which a cooling medium circulates. A crystallization device having plates as heat exchange surfaces is also known as a stationary crystallization device. A crystallization device having tubes as heat exchange surfaces is also known as a falling film crystallization device.

To increase the purity of the lactides formed from the vaporized gas-phase stream from the stage of removal of volatile components, layer-by-layer crystallization can be carried out in many stages. The molten crystals resulting from the crystallization of the liquefied vaporized gas phase stream can crystallize in a further crystallization step, whereby the purity of the crystallized fraction resulting from this second crystallization step is recrystallized, thereby increasing the purity of the crystals of the second step. The liquid residue from the second stage of crystallization can be fed back together with any liquid fraction from the stage of sweating in the feed pipe of the first stage of crystallization.

More than two crystallization stages can be foreseen in advance, by means of which the liquid residue from the last crystallization stage can be fed back together with any liquid fraction from the stage of sweating to the feed pipe of any of the previous crystallization stages. The optimal number of crystallization stages depends on the desired purity of the lactide.

Moreover, crystals formed by solidification from the gas phase can be melted and then recrystallized to increase the purity of the lactide.

According to a further embodiment, in order to increase the yield of lactide from the vaporized gas stream, the mother liquor and / or liquid from the sweating stage can be collected and recrystallized to recover the lactide still contained in these two fractions.

The mother liquor from the first stage of crystallization, and hence the liquefied stream of vaporized gas, is crystallized to obtain a lactide in the form of a crystallized fraction. The lactide content in the mother liquor and / or liquid from the sweating stage of this recrystallization stage is lower than in the corresponding fraction from crystallization of the liquefied vaporized gas stream. The crystallization product from such a recrystallization step can also be fed to a sweating step and subsequently melted to be added to the liquefied volatile component fraction. Further recrystallization steps can be used, whereby the lactide content in the liquid residue and / or liquid from the sweating step of the subsequent recrystallization step is reduced compared to each previous recrystallization step. Thus, the mother liquor and / or liquid from the sweating step of the subsequent recrystallization step is supplied to the previous recrystallization step, and the molten crystallization product is fed to the subsequent recrystallization step. The number of recrystallization stages is determined by optimizing the costs of the entire process.

Layer crystallization in an embodiment of crystallization from melt or solidification from the gas phase, i.e. desublimation is a batch process. Advantageously, these steps are carried out in one or more crystallization devices, such as a melt crystallization device or a desublimation device. The operating procedure of these devices is organized mainly so as to crystallize or desublimate in one of the devices, while sweating or melting in any other device. Thus, the continuous release of the vaporized gas phase stream for crystallization is guaranteed without the need for intermediate storage.

A significant advantage of reusing lactide from an vaporized gas phase stream from a volatile component removal device is the use of less sophisticated equipment and simpler processes than the prior art, such as the processes described in US 6187901 and US 5880254. The crystallization device is a simple mechanical design. Moreover, inert gas flows are not required, so any processing steps for such an additional inert gas stream are not necessary, which leads to a significant economic effect in favor of the lactide regeneration process in accordance with the present invention.

A further object of the present invention is to improve the purification of the vaporous biodegradable intermolecular cyclic alpha-hydroxycarboxylic acid diester and to keep the amount of waste to a minimum and to reduce the equipment for purification.

This goal is achieved by the method of layer-by-layer crystallization from a melt of a vaporous biodegradable intermolecular cyclic complex diester of alpha-hydroxycarboxylic acid of the formula I

where R is selected from hydrogen or one of linear or branched aliphatic radicals having from one to six carbon atoms, from a melt stream containing a diester of formula I.

In particular, the temperature of the melt stream at the inlet of the melt layer crystallization apparatus for performing melt layer crystallization is set between 0 ° C and 130 ° C, preferably between 10 ° C and 110 ° C, to crystallize the diester of formula I when the partial the pressure of the diester in the vaporized gas phase stream is not more than 20 mbar (2 kPa), preferably not more than 10 mbar (1 kPa), particularly preferably not more than 5 mbar (0.5 kPa). The concentration of the diester of the formula I in the melt stream is preferably controlled to a minimum value of 30% by weight, preferably to a minimum value of 40% by weight, particularly preferably to a minimum value of 60% by weight, in particular to a minimum a value of 70 wt.%. According to a preferred embodiment, the melt stream has a water content of less than 10%, in particular less than 5%, most preferably less than 1%. This method is particularly suitable for the purification of a complex diester of formula I, which is 3,6-dimethyl-1,4-dioxane-2,5-dione (dilactide), in particular L, L-dilactide.

According to an advantageous embodiment, at least a portion of the diester of formula I is formed from a purification device located upstream, which can, in particular, start in at least one of the stages of the method for producing polylactide, polycondensation of lactic acid thermal depolymerization of oligomers of lactic acid with an average molecular weight of between 500 g / mol and 5000 g / mol, rectification of dilactide, polymerization of the reaction mixture containing dilactide, ring opening, vacuum de monomerization of polylactide or its copolymers. The upstream cleaning may include two or more process steps of the above processes and / or several of the above processes simultaneously.

In particular, alpha-hydroxycarboxylic acid of formula I from alpha-hydroxycarboxylic acid of formula II

can be used to obtain a biodegradable intermolecular cyclic complex diester in which R is selected from hydrogen or one of linear or branched aliphatic radicals having from one to six carbon atoms. According to a particularly preferred embodiment, the alpha-hydroxycarboxylic acid of formula II is lactic acid.

The concentration of alpha-hydroxycarboxylic acid of formula II in the melt stream is preferably controlled to a maximum value of 20 wt.%, Preferably to a maximum value of 5 wt.%, Particularly preferably to a maximum value of 1 wt.%. If the concentration of alpha hydroxycarboxylic acid in the melt stream can be limited to less than 10 wt.%, the lactide obtained by the melt crystallization device can be of higher purity, and this lactide can be fed back previous purification step to increase the purity of the final product, namely lactic acid. In this way, polylactic acid having high purity and high molecular weight can be obtained.

If the concentration of alpha-hydroxycarboxylic acid in the biodegradable intermolecular cyclic diester can be kept low, the polymerization can also be controlled and the physical and chemical properties of the biodegradable intermolecular cyclic diester can be controlled in accordance with formula I.

In particular, polylactic acid (PMA), especially L- or D-polylactic acid (PLLA or PDLA), having a molecular weight of at least 10,000, is achievable. Preferably, the molecular weight of the PMC is at least 20,000, and the molecular weight of at least 50,000 is particularly preferred.

The lactide recovered and reused in accordance with the method of the present invention is of sufficient purity to be reused in the polymerization process leading to PMC with the above desired parameters.

A layered crystallization device in accordance with the present invention includes a vessel receiving a melt stream containing a biodegradable intermolecular cyclic alpha-hydroxycarboxylic acid diester of formula I,

where R is selected from hydrogen or one of linear or branched aliphatic radicals having from one to six carbon atoms. The layer-by-layer crystallization device further includes a heat exchanger having a heat transfer surface, a heat transfer medium for cooling the heat transfer surface, and a crystallization surface provided on the heat transfer surface for growing crystals of the diester of formula I.

The polymerization apparatus for the polymerization of the diester of the formula I includes a layer crystallization device in accordance with the present invention. The polymerization unit may further include at least a device for purifying the biodegradable intermolecular cyclic diester in accordance with formula I, and / or at least one depolymerization reactor located upstream of the layer crystallization device.

These and other objectives and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a flow diagram of a method of the present invention.

FIG. 2 shows the regeneration of lactide from a vaporized gas phase stream of a volatile component removal step by crystallization.

FIG. 3 shows the regeneration of lactide from a vaporized gas phase stream of a volatile component removal step by desublimation.

FIG. 4 shows a phase diagram of a lactide.

FIG. 5 shows the regeneration of lactide from a vaporized gas phase stream of a volatile component removal step by the crystallization step of the crude lactide.

FIG. 6 shows an embodiment of a crystallization apparatus.

FIG. 7 shows an embodiment of a slurry crystallization apparatus.

FIG. 8 shows an embodiment of a desublimation apparatus.

FIG. 9 shows a first embodiment of a layer crystallization device.

FIG. 10 shows a second embodiment of a layer crystallization device.

FIG. 1 shows a process for preparing PMC from lactide and ring opening polymerization. The steps of FIG. 1 include a cooking step 26 followed by a fermentation step 27 carried out in the fermentation apparatus. During the preparation step 26, the supplied biomass material 80 is converted to a feed stream 28. After the preparation step 26, a feed stream 28 containing polysaccharides and / or polysaccharides is supplied to the fermentation device for the fermentation step 27. The fermentation device may be a reaction vessel containing a liquid reaction mixture. If necessary, a stirring element may be provided for homogenizing the reaction mixture during the course of the fermentation reaction. Fermentation can be carried out as a batch process or as a continuous process. The product of the fermentation step exiting the fermentation apparatus is crude lactic acid in solution 29.

As a next step, the solvent must be removed from the crude lactic acid in the solvent removal step 30 to obtain purified lactic acid 35. The solvent can be recycled and reused at least partially to add during the fermentation step 20. The purified lactic acid is subjected prepolymerization steps and dimerization steps 40 to obtain crude lactide 45.

As a next step, the crude lactide 45 should be purified in the purification step of the crude lactide 50. The product of the crude lactide purification step is pure lactide 55. The pure lactide stream contains at least 85% by weight of lactide. Any lactic acid present in the pure lactide stream is less than 0.2%, and any water is present in an amount of less than 1%, preferably less than 0.1%. Pure lactide 55 is processed to the crude PMC 65 in the ring opening 60 polymerization step. The crude PMC 65 can be further purified in the cleaning step of the crude PMC 70 to obtain pure PMC 75. Any impurities are removed from the purification device as purge 77.

FIG. 2 shows the regeneration of lactide from a vaporized gas phase stream 135 by crystallization. FIG. 2 relates in particular to purge treatment 77 of FIG. 1. In FIG. 2 stages that have already been discussed in connection with FIG. 1 are not explained again. These stages contain the same reference numbers and are not further explained in detail. The crude PMC 65 from the ring opening 60 polymerization step is cleaned in a purification step 170. This purification step 170 is carried out as the removal of volatile components in a volatiles removal section. By this purification step, purified PMC 175 is obtained. In the volatiles removal section, low boiling fractions from crude PMC 65 containing lactide are vaporized under vacuum. In this way, the vaporized gas phase stream 135 is obtained. The vaporized gas phase stream 135 is cooled and condensed in the condensation step 140. Condensate 145 is fed to the crystallization step 100. During the crystallization step, a pure lactide stream 110 is obtained which can be fed into the polymerization step with ring opening 60 together with a stream of pure lactide 55. The purge 120 from the crystallization step 100 is a waste stream, however it is possible to reuse at least a portion of it in the purification step of the crude lactide 50, in the step complete polymerization and dimerization 40 or in the solvent removal step 30.

Alternatively, the step of removing volatile components may be carried out in more than one step. During each such additional step, an vaporized gas phase stream may be generated.

One such additional condensation step 150 is shown in FIG. 2 for a vaporized gas phase stream 155 from such an additional step of removing volatile components. The condensate 156 from this additional condensation step 150 is fed either to the condensate stream 145 or directly to the crystallization device to carry out the crystallization step 100.

FIG. 3 shows a variant of the method, as shown in FIG. 2. The condensation step 140, 150 and the crystallization step 100 are replaced by a desublimation step 200. Thus, condensation and crystallization take place in the same device because the vaporized gas phase stream is cured directly from the gas phase stream.

Alternatively, a plurality of desublimation steps can be envisaged, in particular if a plurality of steps for removing volatile components are provided. An additional step of desublimation 210 is shown in FIG. 3 with a dashed line as an optional alternative. Purge stream 215 is a waste stream, however, it can be recycled, at least in part, to the purification step of the crude lactide 50, to the prepolymerization and dimerization step 40, or to the solvent removal step 30.

Such desublimation is possible in the low pressure region. In the lactide phase diagram, a phase transition from the gas phase to the solid phase is possible along curve 220. Curve 220 extends from the y axis, which corresponds to a temperature of 60 ° C, to a triple point 230. When the lactide is cooled at a pressure or partial pressure of less than 2 mbar ( 0.2 kPa), a direct transition from the gas phase to the solid phase occurs.

FIG. 5 shows a further embodiment of the method in accordance with FIG. 2. FIG. 5 shows the regeneration of lactide from a vaporized gas phase stream 135 by crystallization. In FIG. 5 stages that have already been discussed in connection with FIG. 1 or FIG. 2, repeatedly not explained. The stages by which the same tasks are performed as in FIG. 1 or FIG. 2 contain the same item numbers and are not explained in further detail below. The crude PMC 65 from the ring opening 60 polymerization step is cleaned in the purification step 170. This purification step 170 is performed as removing volatile components in a volatile component removal section. The vaporized gas phase stream 135, containing the low boiling fraction from the volatile component removal step, is cooled and condensed in the condensation step 140. Condensate 145 is fed to the equipment for the purification step of lactide 50, which may include a crystallization step. A purge stream containing impurities that should not be present in the PMC can be discharged from the lactide purification device to carry out the lactide 50 purification step.

Volatile component removal can be carried out in more than one volatile component removal section. The condensation 150 of the vaporized gas phase stream 155 can be carried out separately from the condensation 140 of the first stage of removal of volatile components.

Example 1

Ring opening polymerization in the absence of solvent to obtain crude polylactic acid was carried out in two different tests. The following conditions were applied to the first and second tests of Example 1: the crude polylactic acid was sent to a purification device to remove volatile components. The product of the removal of volatile components is purified polylactic acid and a vaporized gas phase stream containing boiling compounds such as lactide. The vaporized gas phase stream from the stage of removal of volatile components has a lactide content of about 98.5%, and is liquefied in the refrigerator and fed to the vessel of the layer crystallization device for solidification to form a solidified mass. Hardening occurs due to the crystallization of lactide on the heat exchange surfaces of the layered crystallization device. Then, the hardened mass is melted after delivery to the layer-by-layer crystallization device by heating the vessel to form a molten mass. Then the molten mass is fed back to the process, that is, to the ring opening polymerization device.

The crystallization step for this test was performed twice, as shown in table 1a. During the first stage of crystallization, the molten mass crystallized, the liquid residue was discharged. Then the hardened mass was sweated. The process of sweating was carried out in two stages. At the end of each step, a solidification point was measured. The solidification point of the mixture is related to the purity of the main component in the mixture, in accordance with the phase diagram of the lactide, according to FIG. 4, and therefore, allows to evaluate the course of cleaning. The lactide purity achieved after the first stage of sweating of the first stage of crystallization was 99.5%. A purity of 99.6% was achieved after the second stage of sweating of the second stage of crystallization.

For the second test, analysis for individual impurities, that is, Sn ions and free acids, was carried out for all fractions, that is, for the feed, residue, sweating fraction and hardened mass forming the crystallization product. The results of the second test are shown in table 1b. In this test, the sweating stage was carried out only once.

In the third test, the crystallization product of the second test was again melted and crystallized. In this test, only the residue was discharged and no sweating step was carried out. The results of this crystallization are summarized in table 2.

Sn ions arise from the catalyst. Free acids are understood to mean any acids that would act as a chain terminating agent in the polymerization process. Moreover, the color and smell of the raw material and crystallization product obtained from each of the sweating stages are compared with each other in table 1a and table 1b.

Example 2

Desublimation

In this test, the effect of desublimation separation on lactide purity was tested.

The vaporized gas phase ring-opening polymerization stream, which was also used for the tests of Example 1, was fed into a pipe having an inner diameter of 50 mm and a length of 3 m, in which the lactide was desublimated, thus curing directly from the gas phase, to form a crystallization product on the heat exchange surfaces of the layered crystallization apparatus used for testing in accordance with Example 1. The residue was fed back to the main process stream, thus to the subsequent removal stage eniya volatile components.

A solid layer of a thickness of 10 to 15 mm was obtained and deposited on the inner surface of the pipe. Upon completion of desublimation, part of the deposited solid layer was discharged from the pipe and melted to form a molten mass. The solidification point of this molten mass was determined. The solidification point was measured to be 96, 97 ° C. The hardened molten mass was almost colorless and had only a faint odor.

Each of the test results of the first and second examples shows that the purification of lactide from the vaporized gas-phase stream is quite close to the melting point of pure lactide. The purity of the lactide obtained by desublimation in accordance with this example was approximately 99.5%. For L-lactide, the melting point is 97.7 ° C.

These tests were carried out in a stationary crystallizer for laboratory tests with the following structural details. A stationary crystallizer is a special embodiment of a layer crystallization device in which the melt does not undergo any forced convection during the crystallization process. The stationary mold for testing consists of a vertically arranged, jacketed pipe with a diameter of 80 mm, a length of 1200 mm and having a nominal capacity of 6 liters. The pipe has a tight-fitting cap on top, which allows the pipe to be loaded with incoming melt and to close the pipe tightly during crystallization. In the lower part, the pipe diameter is reduced to 20 mm, and there is an outlet valve located directly below the passage of reduced diameter. This valve allows you to drain liquid fractions from the pipe under the influence of gravity. A heat transfer medium circulates in the jacket of this pipe, which supplies cooling or heating energy for crystallization and subsequent stages of sweating and melting. The heat transfer medium is either heated or cooled in a commercial thermostatic device with time-programmed temperature profiles.

After loading the crystallization tube with the incoming molten mass, the loading opening is closed. The temperature of the heat transfer medium then drops to the value of the onset of crystallization, and then it is reduced in accordance with the programmed temperature / time profile to the final crystallization value. During this cooling, crystals form nuclei and begin to grow on the inner wall of the crystallization pipe. After crystallization is complete, the non-crystallized precipitate is poured into the receiving container, opening the outlet valve at the bottom of the pipe. The sweating fraction is collected in various containers, if required in several shoulder straps. After the sweating is completed, the outlet valve is closed, and the remaining crystals are melted and drained from the crystallization tube into an appropriate container by re-opening the outlet valve.

During operation, the first two stages were carried out under the following operating conditions. The crystallization tube was pre-cooled to 95 ° C to begin crystallization. The temperature of the heat transfer medium was gradually reduced to a final value of 90 ° C. over six hours, respectively. During this period, lactide was crystallized on the heat transfer surfaces. The melt was held in a vessel of a crystallization device to allow crystals to grow. When crystallization was completed, the outlet valve was opened to discharge the liquid residue, thus the mother liquor.

After opening the exhaust valve to drain the residue, the temperature of the heat transfer medium was gradually increased to 98 ° C to carry out the sweating step. The sweating stage lasted for five hours. After completion of the sweating step, the liquid residue was again discharged, opening the outlet valve.

Then the crystallization product must be removed from the heat exchange surfaces of the layer crystallization device. Melting was carried out at a temperature of 120 ° C. During melting, the outlet valve was kept closed and opened only after the completion of the melting stage to discharge the melt from the crystallization vessel.

During the second stage, the crystallization tube was pre-cooled to 96 ° C to begin crystallization. The temperature of the heat transfer medium was then gradually reduced to a final value of 92 ° C over six hours. After opening the exhaust valve to drain the residue, the temperature of the heat transfer medium was gradually increased to 98 ° C at the end of sweating. Sweating lasted five hours. Melting was carried out at a temperature of 120 ° C.

Melt crystallization in the absence of solvent is used on an industrial scale. A crystallization device including falling film crystallizers, as described, for example, in US Pat. No. 3,621,664, is manufactured industrially by Sulzer Chemtech Ltd., Switzerland.

Alternatively, the crystallization device may include stationary crystallizers, as described, for example, in EP 0728508 (A1), EP 1092459 (B1), EP 0891798 (B1), and manufactured industrially by, for example, Litwin, France, and Sulzer Chemtech Ltd., Switzerland. A stationary mold essentially consists of a reservoir that is filled with crystallized melt, and of cooling surfaces that are cooled / heated from the inside by a heat transfer medium. The heat transfer medium circulates in a vertical beam of plates, as shown in FIG. 9, or in a tube bundle, as shown in FIG. 10. Crystals grow on the outer walls of these heat transfer surfaces.

Alternatively, the crystallization device may include a suspension crystallization device, as described, for example, in US 6719954 B2, EP 1245951 A1, US 6241954 B1, US 6467305 B1, US 7179435 B2, US 2010099893 (A1), and manufactured industrially by GEA Messo PT , Germany, and Sulzer Chemtech Ltd., Switzerland. In such a crystallization apparatus, small crystals are formed from the suspension, which grow in suspension in the growth chamber. The growth chamber and the crystallization device from the suspension can be connected together in one unit. The suspension is then transferred to a washing column, where the crystals are washed countercurrently, the fraction of molten crystals is partially returned, and the washing liquid saturated with undesirable components is discarded as a residue. The residue from the first crystallization device from the suspension can be collected, recrystallized and washed again in the second crystallization device from a suspension of a similar design in order to remove the lactide from the residue of the first device.

In FIG. 6 shows a melt layer crystallization device including a stationary crystallizer 1 containing a plate bundle. The design of such a crystallizer has the same or corresponding elements as in the crystallization device shown in FIG. 9. The crystallizer 1 is loaded with a portion of the molten mass, which must be crystallized, through line 2 using pump 3 from the supply vessel 4 with lactide. The feed enters the feed vessel through feed line 5. The feed may be either a gaseous stream or a melt stream. In particular, the feed may be a vaporized gas phase stream from a volatile component removal device (70, 170), as shown in FIG. 1, 2, 3, 5.

The residue from the crystallizer 1, as well as the sweated fraction and the molten crystal fraction, are poured into the corresponding vessels 6, 4 and 7, respectively, through the exhaust pipe 8 and the exhaust valve 9. The distribution manifold 10 with the necessary valves allows the individual fractions to be sent to the corresponding vessels. The collector has the function of a liquid distributor. The residue is collected in vessel 6. The fraction of molten crystals that contains purified lactide is poured into vessel 7. The residue and purified lactide can be transported to their destination by transfer pumps 11 and 12. The sweated fraction can be collected in vessel 6 and discharged in the same way. as the remainder, or it can be collected in a vessel 4 for reuse in the crystallizer 1 through line 2. A bunch of platinum, as shown in FIG. 9, it is cooled and heated by a heat transfer medium entering through the pipe 21 and leaving the beam through the pipe 22. The circulation pump 23 allows the heat transfer medium to continuously circulate in the energy system. The cooling and heating energies are supplied by means of heat exchangers 24 and 25. The heat exchangers shown here represent only one, simple possibility of supplying energy to the crystallization system. There are other possible solutions, such as systems with energy buffer tanks and other energy supply systems, known to those skilled in the art from industrial practice.

In the embodiment of FIG. 7, the liquefied lactide from the volatile component removal step is continuously supplied through a conduit 301 to the crystallization compartment of the crystallization device from the molten suspension. A device for crystallization from a molten suspension includes a crystallizer and / or scraper 302 and a vessel 303 for crystal growth. The transfer pipe 305 leads from the mold 302 to the vessel 303, the transfer pipe 306 - from the vessel 303 to the mold 302. A circulation pump 304 can be installed in the transfer pipe 306, which allows the suspension to circulate between the mold 302 and the vessel 303. The mold and / or scraper has a cooling jacket 321 for cooling the walls of the crystallization device. The crystal nuclei on the inner wall are formed on the surfaces of the inner wall of the crystallizer 302. The crystal nuclei are then continuously scraped off from the inner wall surfaces by a scraper element 322. The crystal nuclei are allowed to grow while they are suspended in the melt, which is a lactide melt according to the preferred embodiment implementation.

In an alternative version, both devices, the mold 302 and the vessel 303, can be combined into a common unit. The lactide can also be supplied to the crystallizer 302 or either to the circulation pipe 305 or to the transfer pipe 306, instead of the vessel 303. Structural details of mass-produced crystallization devices from molten slurry are known to those skilled in the art.

A part of the suspension stream is separated from the circulation pipe to the pipe 307 to the washing column 308. The flow rate of this part of the stream is controlled by the valve 309. The flow rate is practically the same as the flow rate of the raw materials in the pipe 301. The crystals contained in the washing column 308 contain slurry, forced to move towards the top of the wash column, and the remainder of the melt moves toward the opposite end. The crystals are moved by a mechanical element 310, like a screw conveyor or a piston with a head in the form of a sieve, which repeatedly causes the crystals to move in one direction, allowing the melt to pass in the opposite direction. In another type of commercially available wash column 308, the required types of crystal flow and melt flow are set by the corresponding design of the interior of the vessel so that no moving parts are needed.

The crystal suspension is guided by a mechanical element 310 to the end of the column, in this example, to the lower end or sump, and then discharged by a circulation circuit 311. The forced circulation of the crystal suspension is maintained by a circulation pump 312. The crystal suspension then flows through a melting device 313, in which the crystals are melted with the formation of molten mass. One part of such a molten mass is continuously discharged through a discharge line 314 and a control valve 315. This part is, in a preferred embodiment, in a polymerization apparatus for the production of polylactic acid, purified lactide, which is then returned to the polymerization reactor or to the stage of removal of volatile components. The remainder flows through the return line 316 back to the flushing column. This part is used to maintain the countercurrent flow of crystals and melt inside the wash column. At the other end of the wash column, namely at the top of the column, the residual melt is discharged from the column through line 317 and valve 318. This residual melt is a purge stream.

In the embodiment of FIG. 8, lactide vapor enters through a supply line 401 from the step of removing volatile components through an open valve 402 and a branch of the pipe 403 to a solidification device 404, where it solidifies on cooled surfaces 405. The solidification device may be, for example, at least one of a sublimation device or mold. Uncured residual vapor may flow through line 406 back to the main process stream, for example, to the second stage of removal of volatile components, or discharged. The heat exchange system is similar to the system disclosed in FIG. 6, and is not further described here. Reference is made to the description of FIG. 6.

After a part of the gaseous flow has solidified on the heat exchange surfaces of the solidification device 404, the valve 402 is closed and the valve 407 is opened to direct steam to the second solidification device 408, in which the solidification of the steam takes place. The second solidification device operates in essentially the same way as the solidification device 404.

The solidification device 404 is pressurized by allowing an inert gas, such as nitrogen, to flow through valve 409, increasing the working pressure to melt the solidified mass. The hardened mass contains, in accordance with a preferred embodiment of the purification of lactides, a fraction of the lactide and is a crystallization product. The heat exchange surfaces are currently heated by a heat transfer medium to melt the hardened mass to obtain a molten mass. The molten mass, in particular the molten lactide, is discharged through a valve 410 to a collection vessel 411, from which it can be transported by a pump 412 to the polymerization stage or the stage of removal of volatile components.

After the solidified mass is melted, the outlet valve 410 closes and the solidification device 404 is drained by means of the valve 413 and pipe 406 before the subsequent solidification begins.

At least two solidification devices are necessary to ensure continuous reception of lactide vapor, however, the number of such devices may be larger, and it is not limited.

If no subsequent stage of the removal of volatile components is provided, the residue is a waste stream, which subsequently needs to be processed in the waste processing. Optionally, a sweating step may be provided. The heat exchange surface may advantageously be in the form of a pipe, which is provided with a cooling jacket. If the solidification device is configured as a falling film crystallization device, it can be configured as shown in FIG. 10. Using a cooling jacket, the temperature created on the inner surface of the pipe is maintained below the solidification point for a given partial pressure of desublimated steam, in particular lactide.

FIG. 9 shows an embodiment of a layer crystallization device. The crystallization device 250 has a container 253 for receiving the melt, which contains lactide and impurities that must be removed from the lactide, that is, the product of the removal of volatile components, namely the vaporized gas-phase stream or the stream of its melt. In this container 253 there are many wall elements 255, while the wall elements are spaced from each other. Wall elements 255 contain closed channels 257 for circulating a liquid heat transfer medium. These wall elements are also called plate bundles. Each wall element 255 is a selectively heated or cooled circulation of a liquid heat transfer medium inside closed channels 257. The closed channels 257 open into the inlet radiator 259 and the output radiator 260, which serve to distribute the liquid heat transfer medium to the individual channels 257 or to receive the liquid heat transfer medium from separate channels.

The intermediate spaces 256 between the wall elements 255 during operation are filled with a melt, which contains the cleaned lactide. The melt is distributed over the entire set of wall elements through tributaries 261, which open into the distribution elements of the tributaries 262 so that the wall elements 255 are completely surrounded by the melt. After filling the crystallization device 250 with the melt through the channels 257, a liquid heat exchange medium is passed as a cooler, whereby the wall elements 255 are cooled. The melt crystallizes on the wall elements 255 to a crystallization layer, the thickness of which is continuously increasing. Due to the different melting points of the individual lactide and impurities in the melt, the crystallization product layer contains a higher part of the lactide having a high melting point. First, solid lactide is deposited on the crystallization surfaces of the wall elements 255, which means that it is therefore concentrated in the crystallization product layer. If the melt is cooled further, impurities with slightly lower melting points may also begin to crystallize.

Most of the impurities remain in the liquid phase and are discharged by effluent streams that are located in the region of the base 264 of the crystallization device 250. The liquid phase is also called the mother liquor. Impurities melting at a lower temperature, in comparison with lactide, are concentrated in the mother liquor. The mother liquor in this case contains a waste product.

Wall elements 255 are again heated in the second crystallization phase. During this second stage, in the so-called sweating stage, partial melting of the crystallization product layer may also take place. The lactide fraction, still containing some impurities present as a result of the incorporation of the mother liquor between the crystalline surfaces during crystal growth, can be selectively separated during the sweating phase. The crystallization product layer essentially remains bound to the wall elements in the sweating phase; only individual drops of melt are selected. Impurities with a low melting point, which were only released due to the partial melting of crystals, are concentrated in these first drops. Thus, a very selective separation of impurities is possible in the sweating phase. The surface temperature of the wall elements 255 is preferably continuously increased during the sweating step. In this case, a plurality of fractions can be taken during the sweating phase.

In the third phase, the crystallization product layer melts, that is, the crystallization product is removed from the wall elements 255. For this purpose, the channels 257 in the wall elements 255 are in contact with a liquid heat exchange medium, which is used as a liquid heating medium.

FIG. 10 shows a falling film crystallization device 270. A falling film crystallization device 270 includes a container 271 containing a plurality of tubes forming a bundle of tubes 272. The container receives lactide from the stage of removal of volatile components, and it is supplied to the container as a vaporized gas phase stream or melt stream . The feed stream enters the crystallization device through the inlet pipe 273. The tubes from the tube bundle 272 are hollow to form a passage for the heat exchange fluid. The heat exchange fluid enters the tube bundle through the inlet pipe 275 and leaves the tube bundle through the exhaust pipe 276. The inlet pipe opens into the fluid distribution element in fluid connection with the tube passages of the tube bundle. The passages of the tubes are received in the liquid collection element, which is in fluid connection with the exhaust pipe 276.

The heat exchange fluid may be a heating fluid or a cooling fluid, depending on the mode of operation of the crystallization device. In crystallization mode, the coolant circulates in the tubes, thereby lowering the temperature of the outer surfaces of the pipe relative to the temperature of the feed. The temperature drops to crystallize compounds having the highest melting points. A liquid fraction that does not crystallize, namely the mother liquor, leaves the container in the waste tank when the crystallization device is in crystallization mode. In crystallization mode, it is intended for the implementation of the crystallization stage. Both crystallization devices of FIG. 9 and FIG. 10 are designed for intermittent operation. This means that after the crystallization stage is carried out, a melting stage is performed to melt the crystalline fraction and discharge it into a waste tank, from which it is discharged by discharge pipe 274. During the crystallization mode, the crystalline fraction is deposited on the outer surfaces of the tube bundle tubes.

Crystallization with a falling film makes crystallization faster than a melt crystallization device using wall elements in the form of flat beams.

Claims (25)

1. A method of producing polylactic acid, comprising the steps of:
(i) ring-opening polymerization using a catalyst and either a catalyst deactivator compound or an end-blocking agent to produce a crude polylactic acid with a molecular weight of more than 10,000 g / mol,
(ii) purifying the crude polylactic acid by removing and separating low boiling compounds, including lactide and impurities, from the crude polylactic acid by removing volatile low boiling compounds as a gas phase stream,
(iii) purifying the lactide from the step of removing volatile components and removing impurities from the gas phase vaporized low boiling point stream by crystallization by desublimation from the gas phase,
wherein the lactide is purified and the impurities removed include a catalyst residue and a compound containing at least one hydroxyl group, the lactide thus purified then being polymerized, feeding it back to the ring opening polymerization. 2. The method of claim 1, wherein the desublimation occurs on a chilled surface. 3. The method according to p. 1 or 2, in which the crystallization device and the device for removing volatile components operates essentially under the same vacuum conditions. 4. The method according to p. 1 or 2, in which desublimation and crystallization occur in the same device. 5. The method according to p. 3, in which desublimation and crystallization occur in the same device. 6. The method of claim 3, wherein the crystallization apparatus does not have an inert gas stream. 7. The method of claim 4, wherein the crystallization apparatus does not have an inert gas stream. 8. The method according to p. 1 or 2, in which the vaporized gas phase stream from the stage of removal of volatile components contains at least 30% lactide, preferably at least 60%, most preferably at least 90%. 9. The method according to p. 1 or 2, in which the lactide first enters the stage of sweating, followed by the stage of melting, before it is fed back to the polymerization stage with the opening of the cycle. 10. The method according to p. 1 or 2, in which the removal of volatile components occurs at a partial pressure of lactide of less than 20 mbar (2 kPa), preferably less than 10 mbar (1 kPa), particularly preferably less than 5 mbar (0.5 kPa) . 11. The method according to p. 1 or 2, in which the removed impurities include either an organometallic compound or a carboxylic acid. 12. The method according to p. 3, in which the removed impurities include either an organometallic compound or a carboxylic acid. 13. The method of claim 4, wherein the removed impurities include either an organometallic compound or carboxylic acid. 14. The method of claim 5, wherein the removed impurities include either an organometallic compound or carboxylic acid. 15. The method according to claim 6, in which the removed impurities include either an organometallic compound or a carboxylic acid. 16. The method according to claim 7, in which the removed impurities include either an organometallic compound or a carboxylic acid. 17. The method of claim 8, wherein the removed impurities comprise either an organometallic compound or carboxylic acid. 18. The method of claim 9, wherein the removed impurities include either an organometallic compound or carboxylic acid. 19. The method according to claim 1 or 2, in which at least part of the purge stream from crystallization is returned back to the stage of purification of the crude lactide, the stage of prepolymerization and dimerization or to the stage of solvent removal in the production of purified lactide. 20. The method according to p. 9, in which the liquid from the stage of sweating is collected and recrystallized to extract lactide. 21. A device for implementing the method according to claim 1, including:
a loop opening polymerization reactor to obtain crude polylactic acid,
a device for removing volatile components for separating low-boiling compounds, including lactide and impurities, from crude polylactic acid,
and a crystallization device for purifying lactide and removing impurities by desublimation and crystallization in the same crystallization device. 22. The device according to p. 21, in which no means of throttling or vacuum pumps are located between the crystallization device and the device for removing volatile components. 23. The device according to any one of paragraphs. 21-22, in which the heat exchanger is located between the device for removing volatile components and the crystallization device. 24. The device according to any one of paragraphs. 21-22, wherein the crystallization device has heat exchange surfaces for solidifying the gaseous stream. 25. The device according to p. 23, in which the crystallization device has a heat exchange surface for solidification of the gaseous stream.

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