Method and apparatus for purifying lactic acid

Abstract

The present invention relates to a process for purifying an aqueous crude lactic acid composition, comprising the steps of i) heating and pressurizing the aqueous crude lactic acid composition to obtain a liquid heated and pressurized aqueous crude lactic acid composition, ii) flashing the liquid heated and pressurized aqueous crude lactic acid composition obtained from step i) to obtain a crude lactic acid vapor, iii) gradually condensing by directing the crude lactic acid vapor obtained from step ii) through at least three consecutive condensers to obtain at least three condensed streams, one of which is a purified lactic acid stream.

Description

[0001] The present invention relates to a method and apparatus for purifying lactic acid.

[0002] Lactic acid is a monomer and thus a necessary component of polylactic acid homopolymers and copolymers. Lactic acid polymers are of particular interest because they can be obtained from renewable resources and most are compostable and / or biodegradable. In addition, the technical and physicochemical properties of these polymers are very close to those of polymers derived from fossil-based resources, which also explains why these polymers are regarded as very promising alternatives to the latter. Moreover, lactic acid polymers have a wide range of applications. For example, polylactic acid is used in surgical implants, membranes (such as for packaging), fibers (such as for clothing), hygiene products, carpets, and disposable plastic products (such as disposable tableware or containers) in the biomedical field. In addition, polylactic acid also has a wide range of applications in composite materials, such as in fiber-reinforced plastics.

[0003] Generally, two alternative main methods for synthesizing polylactic acid are known. The first main method is the direct polycondensation of lactic acid into polylactic acid. The second known main method for synthesizing polylactic acid is the ring-opening polymerization of lactide (the cyclic diester of lactic acid). Lactide can be formed by the condensation of two lactic acid molecules. Alternatively, lactide can also be formed by first pre-polymerizing lactic acid and then subjecting the oligomers or prepolymers to a depolymerization reaction, respectively. For any of the above polylactic acid production methods, lactic acid is the starting material. Therefore, there is a need for pure lactic acid.

[0004] Lactic acid is mainly prepared by the fermentation of biomass such as carbohydrates in starch, sugar, or corn, resulting in a crude lactic acid composition that is not pure enough for use in synthesizing polylactic acid. Therefore, the crude lactic acid composition thus obtained must be purified before use.

[0005] A known method for purifying lactic acid is based on precipitating lactic acid in the form of a metal lactate and then subjecting it to a neutralization reaction with a strong acid such as sulfuric acid. However, this method is characterized by a relatively low yield and a relatively poor quality of the obtained lactic acid. Another known method for purifying lactic acid is based on the esterification of lactic acid with an alcohol, followed by distillation of the ester and finally hydrolysis of the distilled ester to lactic acid. Other known methods are based on electrodialysis, extraction, and / or ion exchange. However, all of these methods are relatively costly. An alternative method that is relatively cheaper than these methods is the distillation of lactic acid. However, purifying lactic acid by conventional distillation is not feasible because lactic acid is both an alcohol and an acid, and once water is removed, it undergoes self-catalyzed transesterification with itself. Thereby, lactic acid oligomers are formed and almost non-volatile ester chains are thus formed, which are not suitable for the conventional distillation method. However, in order to use distillation to purify lactic acid, US 6,489,508 B1 proposes first purifying lactic acid by anion and cation exchange, then concentrating lactic acid, and then distilling the concentrated anhydrous lactic acid. However, this process is complex and also faces relatively high capital investment costs (CAPEX) and operating costs (OPEX).

[0006] In view of this, the object of the present invention is to provide a method for purifying lactic acid that requires relatively low capital investment costs and relatively low operating costs and still results in pure lactic acid with a low content of impurities such as salts, proteins, sugars, yeasts, glycerol, etc., if any.

[0007] According to the present invention, this object is achieved by providing a method for purifying a crude lactic acid aqueous composition, the method comprising the following steps: i) heating and pressurizing the crude lactic acid aqueous composition to obtain a liquid heated and pressurized crude lactic acid aqueous composition, ii) flash distilling the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i) to obtain crude lactic acid vapor, iii) gradually condensing the crude lactic acid vapor obtained from step ii) by passing it through at least three consecutive condensers to obtain at least three condensate streams, one of which is a purified lactic acid stream.

[0008] This technical solution is based on the following discovery: By flash-vaporizing a liquid, heated and pressurized, aqueous crude lactic acid composition, most of the aqueous crude lactic acid composition will immediately evaporate, i.e., evaporate within a very short time period and so fast enough to reliably avoid dimerization and oligomerization of lactic acid. This is because the formation of low-polymer chains is relatively fast but not an immediate reaction, such that dimerization and oligomerization do not occur or at least do not occur measurably during the flash-vaporization process. Further, after flash-vaporization, since the corresponding reaction rate in the gas phase is extremely slow, dimerization and oligomerization do not occur or at least do not occur measurably. Before flash-vaporization, due to the water content (preferably 15 wt% or higher) and low temperature, oligomerization does not occur or only occurs to a small extent. After condensation of lactic acid, a certain degree of dimerization and oligomerization will occur. The purified lactic acid fraction thus obtained can then be further purified, as described later, for example by a stripping step in which other low-boiling acids, such as formic acid, acetic acid, and water, are stripped off. Thus, there is no need for further laborious and costly purification steps, such as ion-exchange steps, extraction steps, or electrodialysis steps. In summary, the method according to the invention requires relatively low equipment investment costs and relatively low operating costs and still results in pure lactic acid with a low content of impurities (such as salts, proteins, sugars, yeasts, glycerol, etc.), if any.

[0009] According to the invention, a liquid, heated and pressurized, aqueous crude lactic acid composition means any composition containing lactic acid, where the composition is in liquid form, has a temperature above ambient temperature (i.e., 23 °C) and has a pressure above atmospheric pressure (i.e., 101325 Pa).

[0010] According to the invention, a lactic acid oligomer means a lactic acid condensation product containing 2 to 100 lactic acid units, while according to the invention, a lactic acid condensation product containing more than 100 lactic acid units is regarded as polylactic acid. Thus, lactide, i.e., the cyclic dimer of lactic acid, is also regarded as a lactic acid oligomer.

[0011] According to the invention, guiding the crude lactic acid vapor obtained from step ii) through at least three consecutive condensers means guiding the crude lactic acid vapor through the first of at least three consecutive condensers and effecting partial condensation therein. While removing the condensed portion, guiding the remaining vapor through the second of at least three consecutive condensers and effecting partial condensation therein. While removing the condensed portion, guiding the remaining vapor through the third of at least three consecutive condensers and effecting partial or complete condensation therein. While removing the condensed portion, removing any remaining vapor, or, if there are more than three consecutive condensers, guiding it through the next of the more than three consecutive condensers.

[0012] The present invention has no particular limitation on the composition of the crude lactic acid aqueous composition as long as it contains water and lactic acid. For example, the crude lactic acid aqueous composition is produced by the fermentation of carbohydrates in biomass (such as starch, sugar or corn). Preferably, before subjecting it to step i), the crude lactic acid aqueous composition, such as the crude lactic acid aqueous composition obtained in the form of a fermentation broth, is filtered through a (micro) filter having an average pore size of 0.05 to 1.0 µm, more preferably 0.1 to 0.5 µm, and most preferably 0.1 to 0.3 µm to remove microbial cells. Further preferably, as an alternative to the above (micro) filtration or after the above (micro) filter, the crude lactic acid aqueous composition is filtered through a (nano) filter having an average pore size of 1 to 100 nm and more preferably 10 to 50 nm to (also) remove at least most of the unreacted sugars, proteins and ions.

[0013] In addition, it is preferred to adjust the water content of the microfiltered and / or nanofiltrated crude lactic acid aqueous composition, for example by evaporating part of the water, so that the starting water concentration reaches 10 to 50% by weight, preferably 15 to 30% by weight, and more preferably 15 to 20% by weight. For example, the fermentation broth that has been microfiltered and / or nanofiltrated and evaporated contains, based on 100% by weight of the crude lactic acid aqueous composition: - 80% to 85% by weight of lactic acid and - 15 to 20% by weight of water.

[0014] The crude lactic acid aqueous composition refers to the composition that is heated and pressurized to obtain a liquid heated and pressurized crude lactic acid aqueous composition, and then subjected to flash evaporation. This does not exclude mixing the lactic acid aqueous composition produced by the fermentation of carbohydrates in biomass with additional water and / or a recycle stream of heavy substances and / or one or more other components, and then reheating and pressurizing it into a liquid heated and pressurized crude lactic acid aqueous composition. Also in this case, the crude lactic acid aqueous composition refers to the composition that is heated and pressurized, that is, the composition obtained after adding additional water and / or a recycle stream and / or one or more other components.

[0015] According to the present invention, the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i) is flash-vaporized to obtain crude lactic acid vapor. According to the present invention, flash-vaporization means evaporating the liquid crude lactic acid composition by reducing the pressure of the heated and pressurized crude lactic acid aqueous composition. The pressure reduction can be achieved by any suitable means, as further described hereinafter. Thus, flash-vaporization is at least largely affected by the potential energy of the liquid heated and pressurized crude lactic acid aqueous composition (i.e., the thermal energy and pressure energy of the heated and pressurized crude lactic acid aqueous composition). In view of this, it is preferred that at least 70% of the energy required for flash-vaporization of the liquid heated and pressurized crude lactic acid aqueous composition in step ii) is obtained from the potential energy stored in the form of thermal energy and pressure energy in the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i). Good results are obtained especially when at least 80%, more preferably at least 90%, still more preferably at least 95%, even more preferably at least 99%, and most preferably all of the energy required for flash-vaporization of the liquid heated and pressurized crude lactic acid aqueous composition in step ii) is obtained from the potential energy stored in the form of thermal energy and pressure energy in the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i). This enables the liquid heated and pressurized crude lactic acid aqueous composition to evaporate immediately, i.e., within an extremely short period of time, and thus is faster and more energy-efficient than the case of guiding an unheated and unpressurized liquid onto a heating surface. In other words, just before and / or during flash-vaporization, preferably at most 30%, more preferably at most 20%, still more preferably at most 10%, even more preferably at most 5%, even more preferably at most 1%, and most preferably no energy is applied externally to the liquid heated and pressurized crude lactic acid aqueous composition, for example in the form of a heating surface.

[0016] Good results are obtained especially when heating the crude lactic acid aqueous composition in step i) to obtain a liquid heated and pressurized crude lactic acid aqueous composition having a temperature of 140 to 220 °C, preferably 180 to 200 °C, such as about 190 °C.

[0017] In a further development of the inventive concept, it is proposed to pressurize the crude lactic acid aqueous composition in step i) to obtain a liquid heated and pressurized crude lactic acid aqueous composition having a pressure of 0.2 to 5.0 MPa, preferably 0.7 to 2.5 MPa, and more preferably 1.0 to 1.5, such as about 1.2 MPa.

[0018] The heating and pressurization of the crude lactic acid aqueous composition can be carried out in a single step in step i) or in two or more subsequent steps in any order. Thus, the heating can be carried out before, after or during the pressurization of the crude lactic acid aqueous composition. The heating of the crude lactic acid aqueous composition is preferably carried out using one or more heat exchangers, while the pressurization of the crude lactic acid aqueous composition can be carried out using one or more pumps.

[0019] Preferably, the flash evaporation in step ii) is carried out such that as much of the liquid, heated and pressurized, crude lactic acid aqueous composition as possible evaporates as quickly as possible. In particular, when the flash evaporation of the liquid, heated and pressurized, crude lactic acid aqueous composition in step ii) is carried out such that at least 80% of the liquid, heated and pressurized, crude lactic acid aqueous composition evaporates within at most 1 second, more preferably within less than 0.75 seconds, even more preferably within at most 0.5 seconds, and most preferably within 0.1 to 0.5 seconds, good results are obtained.

[0020] In step ii), the faster the liquid, heated and pressurized, crude lactic acid aqueous composition evaporates, the lower the chance of dimerization and oligomerization of lactic acid.

[0021] In a further development of the inventive concept, it is proposed that the flash evaporation in step ii) is carried out in a vessel or column, the interior of which is adjusted to atmospheric pressure or more preferably sub-atmospheric pressure. Thereby, the pressure difference between the liquid, heated and pressurized, crude lactic acid aqueous composition and its flash evaporation location is maximized. Thus, it is particularly preferred to maintain or adjust the pressure inside the vessel or column to be less than 100 kPa, more preferably less than 10 kPa, even more preferably less than 5 kPa, and most preferably less than 2 kPa, for example about 1 kPa. In principle, there is no lower limit for the pressure inside the vessel or column to be maintained or adjusted respectively. However, for economic reasons, the pressure inside the vessel or column is preferably maintained or adjusted to be at least 0.1 kPa respectively.

[0022] In order to obtain efficient flash evaporation during step ii), in step ii), it is further preferred to guide the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) through a flash evaporation device which comprises one or more openings or nozzles. The one or more openings or nozzles act as throttle valves, thereby achieving the pressure drop required to effect the flash evaporation. In particular, good results are obtained when the pressure loss of the liquid, heated and pressurized, crude lactic acid aqueous composition within the one or more openings or nozzles of the flash evaporation device is at least 0.1 MPa, more preferably at least 5.0 MPa, even more preferably at least 7.5 MPa, and most preferably at least 10.0 MPa.

[0023] According to a particularly preferred embodiment of the invention, in step ii), the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) is guided through a flash evaporation device in the form of an annular tube, which flash evaporation device comprises an inlet for the liquid, heated and pressurized, crude lactic acid aqueous composition and a plurality of openings or nozzles in order to flash evaporate the liquid, heated and pressurized, crude lactic acid aqueous composition.

[0024] According to an alternative and particularly preferred embodiment of the present invention, in step ii), the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) is directed through a flash evaporation device in the form of a T-tube, which flash evaporation device comprises an inlet for the liquid, heated and pressurized, crude lactic acid aqueous composition and two openings or nozzles in order to flash evaporate the liquid, heated and pressurized, crude lactic acid aqueous composition. As further described below, steps ii) and iii) are preferably carried out in a vessel or a column, for example in a vertically arranged column having a substantially circular cross-section, wherein the vessel or the column is respectively bounded by a wall. Preferably, the two openings or nozzles are respectively oriented to direct the liquid towards the vessel wall or the column wall respectively. The method can be implemented such that partial flash evaporation has occurred within the T-tube, such that the mixture of liquid and vapour is respectively directed through the two openings or nozzles towards the vessel wall or the column wall respectively. In order to protect the walls of the vessel or the column respectively, the ends of the T-tube (each comprising an opening or a nozzle) are designed to redirect the crude lactic acid vapour and the liquid that may remain respectively after leaving the opening or the nozzle towards the periphery of the vessel or the column. For this purpose, V-shaped metal plates can be respectively arranged exactly downstream of each opening or nozzle in order to redirect the outflowing liquid and vapour and to prevent them from respectively directly impinging on the walls of the vessel or the column.

[0025] According to a further alternative and particularly preferred embodiment of the present invention, in step ii), the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) is directed through a flash evaporation device in the form of a tube, which flash evaporation device comprises a plurality of arms, for example from 4 to 20, and preferably from 8 to 15 arms, each arm being provided with one or more openings or nozzles in order to flash evaporate the liquid, heated and pressurized, crude lactic acid aqueous composition. As further described below, steps ii) and iii) are preferably carried out in a vessel or a column, for example in a vertically arranged column having a substantially circular cross-section, wherein the vessel or the column is respectively bounded by a wall. Preferably, the flash evaporation device comprises one or more nozzles, wherein one or more nozzles effect pneumatic atomization or solid cone spraying of the crude lactic acid vapour in order to flash evaporate the liquid, heated and pressurized, crude lactic acid aqueous composition. Particularly preferred are solid cone nozzles with a spray angle of 60 to 90°.

[0026] According to yet another alternative and particularly preferred embodiment of the present invention, in step ii), the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) is guided through a flash evaporation device in the form of a cyclone separator. The cyclone separator may comprise a first cylindrical container having a first diameter and a second cylindrical container having a second diameter greater than the first diameter, wherein the second cylindrical container concentrically surrounds the first cylindrical container, and wherein, when viewed in the vertical direction, the first cylindrical container is arranged offset with respect to the second cylindrical container. Preferably, the first cylindrical container is arranged such that, when viewed in the vertical direction, its lower end is arranged at a position corresponding to 30% to 70%, preferably 40% to 60%, for example approximately 50% of the vertical length of the second cylindrical container, such that the lower half of the second cylindrical container does not accommodate the first cylindrical container, but only the upper half of the second cylindrical container. The upper end of the first cylindrical container is preferably arranged above the upper end of the second cylindrical container. The liquid, heated and pressurized, crude lactic acid aqueous composition is introduced into the interior of the second cylindrical container in order to direct the liquid, heated and pressurized, crude lactic acid aqueous composition against the inner wall of the second cylindrical container, so that the crude lactic acid aqueous composition evaporates completely. The vapors of the crude lactic acid aqueous composition thus obtained are removed from the flash evaporation device through the interior of the first cylindrical container.

[0027] According to yet another alternative and particularly preferred embodiment of the present invention, in step ii), the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) is guided through a flash evaporation device in the form of a Schoepentoeter container. Such a container has a trapeze shape in its plan view, wherein the width of the container gradually decreases in the direction from the inlet to the outlet. Both sides of the container are edged with deflector plates to enable the passage of vapors.

[0028] According to yet another alternative and particularly preferred embodiment of the present invention, in step ii), the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) is guided through a flash evaporation device in the form of a vapor horn, which is a cylindrical container comprising an outer wall and an inner wall concentric therewith, which delimit an annular channel therebetween, through which the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) is guided. The inner wall is formed by deflector plates to enable the passage of vapors.

[0029] According to the present invention, during step iii), the crude lactic acid vapor obtained from step ii) is gradually condensed by being guided through at least three consecutive condensers, thereby obtaining at least three condensate streams, one of which is a purified lactic acid stream. To this end, in step iii), in each of at least three consecutive condensers (except for the last condenser), a portion of the vapor is condensed to obtain a condensed portion and a remaining vapor portion, wherein the condensed portion obtained in each of the at least three condensers is taken out in liquid form, and the remaining vapor portion in each of the at least three condensers (except for the last condenser) is guided to the next one of the at least three condensers. In the last condenser, a portion of the vapor may also be condensed to obtain a condensed portion and a remaining vapor portion; alternatively, all of the vapor may be condensed to obtain only a condensate stream. If only a portion of the vapor is condensed in the last condenser, the remaining vapor portion obtained after the last condenser is taken out.

[0030] Good results are obtained especially when steps ii) and iii) are carried out in a vessel or a column, for example in a vertically arranged column having a substantially circular cross-section, wherein at least three subsequent condensers are vertically arranged one above the other. Thus, these two steps can be carried out in a relatively small and compact device.

[0031] Preferably, the column further comprises a flash device for flashing the liquid, heated and pressurized, crude lactic acid aqueous composition, wherein all at least three subsequent condensers are arranged above the flash device. Thereby, in step iii), the crude lactic acid vapor can be efficiently condensed by being guided through at least three consecutive condensers to obtain three condensate streams.

[0032] According to the present invention, in step iii), the crude lactic acid vapor obtained from step ii) is gradually condensed by being guided through at least three consecutive condensers, thereby obtaining at least three condensate streams, one of which is a purified lactic acid stream. In practice, in step iii), it is sufficient for the crude lactic acid vapor to be gradually condensed by being guided through exactly three consecutive condensers. The lowermost condenser condenses the heavy substances, i.e., the components in the crude lactic acid vapor having a boiling point higher than that of lactic acid; and the second condenser (i.e., the condenser arranged downstream of the first condenser, or, if the condensers are all vertically arranged one above the other, the condenser arranged above the first condenser) condenses a mixture of lactic acid and water and, if any, only a small amount of impurities; and the third condenser (and any additional condensers, if present) condenses water, a small amount of lactic acid, and light substances, i.e., the components in the crude lactic acid vapor having a boiling point lower than that of lactic acid. The remaining light substances and water (if any) in the form of vapor remaining after the last condenser are removed as an overhead stream.

[0033] Preferably, a condensate stream is withdrawn from the first of at least three or exactly three consecutive condensers, the condensate stream comprising lactic acid oligomers, lactide, proteins, metals and one or more organic dicarboxylic acids.

[0034] Furthermore, preferably a purified lactic acid condensate stream is withdrawn from the second of at least three or exactly three consecutive condensers, the stream comprising a mixture of lactic acid with water and, if at all, only minor impurities (such as lower carboxylic acids, such as formic acid and acetic acid).

[0035] In order to obtain a particularly pure lactic acid product composition, in a further development of the inventive concept it is proposed to further purify the purified lactic acid condensate stream obtained in the second condenser in the condenser to separate water and other possible impurities therefrom. For example, good results are obtained when the purified lactic acid condensate stream obtained in the second condenser in the condenser is passed through a stripping column. The stripping column preferably comprises a heat exchanger (or reboiler, respectively) at its bottom, which is used to partially evaporate the purified lactic acid condensate stream, thereby stripping water and light organic molecules, preferably formic acid and acetic acid, from the purified lactic acid condensate stream. Instead of or in addition to the heat exchanger or reboiler, respectively, steam is injected into the stripping column to partially evaporate the purified lactic acid condensate stream, thereby achieving a stripping effect. As an alternative to stripping, the purified lactic acid condensate stream can be passed through a further distillation column comprising one or more stages.

[0036] Furthermore, preferably a condensate stream is withdrawn from the third of at least three or exactly three consecutive condensers, the condensate stream comprising water, a small amount of lactic acid and light components (i.e., components in the crude lactic acid vapor with a boiling point lower than that of lactic acid). The lactic acid content in this condensate stream is typically less than 5% by weight and even does not exceed about 2% by weight, so that the stream contains a small amount of lactic acid but is not at all a purified lactic acid composition.

[0037] The residual light substances and water in the form of vapor remaining after the last condenser are removed as an overhead stream.

[0038] According to another aspect, the present invention relates to an apparatus for purifying a crude aqueous lactic acid composition, comprising: - one or more means for heating and pressurizing the crude aqueous lactic acid composition into a liquid heated and pressurized crude aqueous lactic acid composition, - a flash evaporation device for flash evaporating the liquid heated and pressurized crude aqueous lactic acid composition obtained in the means for heating and pressurizing the crude aqueous lactic acid composition, - at least three consecutive condensers for gradually condensing the crude lactic acid vapor obtained in the flash evaporation device.

[0039] The device may comprise a single device that serves as a heating device and as a device for pressurizing the crude lactic acid aqueous composition into a liquid heated and pressurized crude lactic acid aqueous composition. Alternatively, the device may comprise a device that serves as a device for heating the crude lactic acid aqueous composition and a device that serves as a device for pressurizing the crude lactic acid aqueous composition, but the two devices may be arranged in any order to produce a liquid heated and pressurized crude lactic acid aqueous composition. The device for heating the crude lactic acid aqueous composition preferably comprises one or more heat exchangers, and the device for pressurizing the crude lactic acid aqueous composition preferably comprises one or more pumps.

[0040] According to a particularly preferred embodiment of the invention, the device comprises a column in which a flash device and at least three successive condensers are arranged. Preferably, the column is a vertically arranged column having a substantially circular cross-section, in which at least three successive condensers are arranged vertically one above the other. Thereby, two steps can be carried out in a relatively compact and small device.

[0041] As described above, the flash device preferably comprises one or more openings or nozzles. Preferably, the flash device comprises one or more nozzles, wherein the one or more nozzles preferably perform pneumatic atomization or solid cone spraying of the crude lactic acid vapour in order to flash the liquid heated and pressurized crude lactic acid aqueous composition.

[0042] In a further development of the inventive concept, it is proposed that the flash device is an annular tube comprising a plurality of openings or nozzles.

[0043] According to an alternative particularly preferred embodiment of the invention, the flash device is in the form of a T-shaped tube and comprises an inlet for the liquid heated and pressurized crude lactic acid aqueous composition and two (outlet) openings or nozzles. More specifically, the flash device preferably comprises an inlet pipe section for the liquid heated and pressurized crude lactic acid aqueous composition and two outlet pipe sections, each outlet pipe section comprising one or more openings or nozzles in order to flash the liquid heated and pressurized crude lactic acid aqueous composition. Preferably, the flash device is arranged in a container or column, for example in a vertically arranged column having a substantially circular cross-section, wherein the container or column is bounded by walls respectively. Preferably, the two openings or nozzles are respectively oriented to direct the liquid or the mixture of liquid and vapour towards the container wall or the column wall respectively. In order to protect the container or column wall, it is further preferably provided that the ends of the T-shaped tube (respectively comprising the openings or nozzles) are designed to redirect the crude lactic acid vapour and the possibly remaining liquid that leave the openings or nozzles respectively towards the periphery of the container or column. For this purpose, V-shaped metal plates are preferably arranged directly downstream of each opening or nozzle in order to redirect the outflowing liquid and vapour and to prevent them from directly impinging on the wall of the container or column respectively.

[0044] According to yet another alternative and particularly preferred embodiment of the present invention, the flash evaporation device is in the form of a pipe including a plurality of branch pipes, for example, 4 to 20 branch pipes, and preferably 8 to 15 branch pipes. Each branch pipe is provided with one or more openings or nozzles for flash evaporating the liquid, heated and pressurized, crude lactic acid aqueous composition. More specifically, the flash evaporation device preferably includes an inlet pipe section for the liquid, heated and pressurized, crude lactic acid aqueous composition, distribution branch pipes, and a plurality of outlet pipe sections. Each outlet pipe section includes one or more openings or nozzles for flash evaporating the liquid, heated and pressurized, crude lactic acid aqueous composition. The function of the distribution branch pipes is to evenly distribute the liquid to the plurality of outlet branch pipes. The distribution branch pipes may be omitted, such that the flash evaporation device only includes an inlet pipe section for the liquid, heated and pressurized, crude lactic acid aqueous composition and a plurality of outlet pipe sections. Each outlet pipe section includes one or more openings or nozzles. Additionally, preferably, the evaporation device is arranged in a container or a tower, for example, arranged in a vertically arranged tower having a substantially circular cross-section, wherein the container or the tower is bounded by walls respectively.

[0045] According to yet another alternative and particularly preferred embodiment of the present invention, the flash evaporation device is in the form of a cyclone separator. The cyclone separator may include a first cylindrical container having a first diameter and a second cylindrical container having a second diameter greater than the first diameter, wherein the second cylindrical container concentrically surrounds the first cylindrical container, and wherein, when viewed in the vertical direction, the first cylindrical container is arranged offset from the second cylindrical container. Preferably, the first cylindrical container is arranged such that, when viewed in the vertical direction, its lower end is arranged at a position corresponding to 30% to 70%, preferably 40% to 60%, for example, approximately 50% of the vertical length of the second cylindrical container, such that the lower half of the second cylindrical container does not accommodate the first cylindrical container, but only the upper half of the second cylindrical container accommodates it. The upper end of the first cylindrical container is preferably arranged above the upper end of the second cylindrical container. During its operation, the liquid, heated and pressurized, crude lactic acid aqueous composition is introduced into the interior of the second cylindrical container to direct the liquid, heated and pressurized, crude lactic acid aqueous composition towards the inner wall of the second cylindrical container, thereby completely evaporating the crude lactic acid aqueous composition. The vapor of the crude lactic acid aqueous composition thus obtained is taken out of the flash evaporation device through the interior of the first cylindrical container.

[0046] According to yet another alternative and particularly preferred embodiment of the present invention, the flash evaporation device is in the form of a Schoepentoeter container. Such a container is trapezoidal in its plan view, wherein the width of the container gradually decreases in the direction from the inlet to the outlet. Both sides of the container are bounded by guide plates to enable the vapor to pass through.

[0047] According to yet another alternative and particularly preferred embodiment of the present invention, the flash device is in the form of a vapor horn, which is a cylindrical container comprising an outer wall and an inner wall concentric therewith, the inner wall and the outer wall defining an annular channel therebetween through which the liquid, heated and pressurized, crude lactic acid aqueous composition obtained from step i) passes. The inner wall is formed by a deflector plate to enable the vapor to pass through.

[0048] Preferably, at least three subsequent condensers are vertically arranged above the flash device in the column.

[0049] Subsequently, the present invention is described by way of example and not limitation with reference to the accompanying drawings, in which: Figure 1 A schematic view of a device for purifying lactic acid according to an embodiment of the present invention is shown.

[0050] Figure 2 A schematic view of a flash device in a device suitable for purifying lactic acid according to an embodiment of the present invention is shown.

[0051] Figure 3 A schematic view of a flash device in a device suitable for purifying lactic acid according to an embodiment of the present invention is shown.

[0052] Figure 4 A schematic view of a flash device in a device suitable for purifying lactic acid according to another embodiment of the present invention is shown.

[0053] Figure 5 A schematic view of a flash device in a device suitable for purifying lactic acid according to another embodiment of the present invention is shown.

[0054] Figure 1 The device 10 for purifying a crude lactic acid aqueous composition shown comprises: an inlet 12 for a lactic acid composition obtained by fermentation; and an inlet 14 for water, the two being interconnected to form a pipeline 16 for the crude lactic acid composition. The pipeline 16 for the crude lactic acid composition comprises: a static mixer 18 for mixing the components; a heat exchanger 20 for heating the crude lactic acid composition; and a pump 22 for pressurizing the crude lactic acid composition to form a liquid, heated and pressurized, crude lactic acid aqueous composition. A pipeline 24 for the liquid, heated and pressurized, crude lactic acid composition leads from the pump 22 to an evaporation device 26, which evaporation device 26 comprises a plurality of nozzles, wherein Figure 1Only one nozzle 27 is shown. The evaporation device 26 or the nozzle 27 is respectively arranged at the lower part of the tower 28, and the tower 28 further includes three condensers 30, 30', 30'', which are vertically arranged on top of each other and are respectively located above the evaporation device 26 or the nozzle 27. A bottom line 32 is arranged at the bottom of the tower 28, and this line is divided into a recirculation line 34 leading to the line 16 for the crude lactic acid composition and a removal line 36 for heavy substances. Below each condenser 30, 30', 30'', collectors 38, 38', 38'' are arranged, and removal lines 40, 40', 40'' lead out of the tower 28 from these collectors. The removal line 40 from the collector 38 of the lowermost condenser 30 leads into a container 42, and the removal line 36' for heavy substances leads out of the device 10 from this container 42. The removal line 40' from the collector 38' of the second condenser 30' leads into a stripping column 44, which respectively includes structured packing 46 and a heat exchanger or reboiler 48 at its lower part. The removal line 50 for purified lactic acid leads out of the device 10 from the stripping column 44, and the return line 51 for stripping gas leads back to the tower 28. The removal line 40'' from the collector 38'' of the uppermost condenser 30'' leads into a container 52, and the removal line 54 for light substances leads out of the device 10 from this container 52. Finally, from the top of the tower 28, the top removal line 56 leads to two containers 58, 58', each of which is provided with a freezing condenser 59, 59', and from each container, the removal lines 60, 60' lead to a container 62, and the removal line for light substances leads out of the device 10 from this container 62. Finally, the device 10 includes a vacuum unit 66, which controls the vacuum in the tower 28, that is, adjusts and maintains the vacuum in the tower 28 at a required value.

[0055] During the operation of the device 10, the lactic acid composition obtained by fermentation is fed into the device 10 through pipeline 12, and water is fed into the device 10 through pipeline 14. The lactic acid composition, water, and heavy substances containing lactic acid dimers and oligomers removed from the tower 28 through pipeline 32 are introduced into the static mixer 18 through pipeline 16, where a homogeneous mixture is formed by the components. The thus formed crude lactic acid composition is heated in the heat exchanger 20 and pressurized in the pump 22, thereby forming a liquid heated and pressurized crude lactic acid aqueous composition, which is introduced into the flash device 26 or the nozzle 27 of the tower 28 through pipeline 24 respectively and flashes immediately after leaving the nozzle 27. The remaining liquid heavy substances are taken out from the bottom of the tower 28 through pipeline 32, most of which are taken out of the device 10 through pipeline 36, while a smaller part is recycled to pipeline 16 through pipeline 34. The flashed crude lactic acid vapor is successively introduced into the condensers 30, 30', 30''. In each condenser 30, 30', 30'', a part of the crude lactic acid vapor is condensed and collected in the corresponding collectors 38, 38', 38'', and removed from the tower 28 through pipelines 40, 40', 40''. The condensate taken out through pipeline 40 mainly contains heavy substances such as oligomeric lactic acid, metals, proteins, succinic acid, oxalic acid, etc., and this condensate is introduced into the container 42 and then removed from the container 42 through pipeline 36'. The condensate taken out through pipeline 40' from the collector 38' of the second condenser 30' is introduced into the stripping column 44, where water and light substances (such as formic acid and acetic acid) are removed from the lactic acid and sent back to the tower 28 through the return pipeline 51. The purified lactic acid is removed through pipeline 50. The condensate taken out through pipeline 40'' from the collector 38'' of the uppermost condenser 30'' is introduced into the container 52 and removed from the device 10 through pipeline 54. Finally, the top fraction of the tower is passed into the containers 58, 58' through pipeline 56, and the condensed part generated by the refrigeration condensers 59, 59' is passed into the container 62 through pipelines 60, 60' and led out of the device 10 through pipeline 64.

[0056] Figure 2 The figure shows a schematic diagram of the flash device 26 applicable to the device 10 for purifying lactic acid according to the present invention. The flash device 26 is an annular tube 68, which includes an inlet 24 for the liquid heated and pressurized crude lactic acid aqueous composition and a plurality of openings or nozzles 27 for flashing the liquid heated and pressurized crude lactic acid aqueous composition.

[0057] Figure 3The illustrated flash evaporation device 26 is in the form of a T-shaped tube and is arranged in the column 28. The T-shaped tube includes an inlet pipe section 74 for a liquid heated and pressurized crude lactic acid aqueous composition, and two outlet pipe sections 76, 76', each of which includes an opening or nozzle 27 for flash evaporating the liquid heated and pressurized crude lactic acid aqueous composition. Downstream of each opening or nozzle 27, a V-shaped metal plate 78 is arranged respectively.

[0058] Figure 4 The illustrated flash evaporation device 26 is in the form of a tube and includes an inlet pipe section 74 for a liquid heated and pressurized crude lactic acid aqueous composition, a distribution branch pipe 80, and twelve outlet pipe sections 76, 76', each of which includes one or more openings or nozzles 27 for flash evaporating the liquid heated and pressurized crude lactic acid aqueous composition. The role of the distribution branch pipe is to evenly distribute the distribution branch pipe to a plurality of outlet branch pipes.

[0059] Figure 5 The illustrated flash evaporation device 26 is a cyclone separator. The cyclone separator 26 includes a first cylindrical container 82 having a first diameter and a second cylindrical container 84 having a second diameter greater than the first diameter. The second cylindrical container 84 concentrically surrounds the first cylindrical container 82, wherein when viewed from the vertical direction, the first cylindrical container 82 is arranged offset from the second cylindrical container 84. More specifically, the first cylindrical container 82 is arranged such that, when viewed from the vertical direction, its lower end 86 is arranged at approximately 50% of the vertical length of the second cylindrical container 84, so that the lower half of the second cylindrical container 84 does not accommodate the first cylindrical container 82. The upper end 88 of the first cylindrical container 82 is arranged above the upper end 90 of the second cylindrical container 84. During its operation, the liquid heated and pressurized crude lactic acid aqueous composition is introduced into the interior of the second cylindrical container 84 to direct the liquid heated and pressurized crude lactic acid aqueous composition towards the inner wall of the second cylindrical container 84, thereby completely evaporating the crude lactic acid aqueous composition. The crude lactic acid aqueous composition vapor thus obtained is taken out of the flash evaporation device 26 through the upper end 88 of the first cylindrical container 82.

[0060] Reference numerals 10 Equipment 12 Inlet for lactic acid composition obtained by fermentation 14 Inlet for water 16 Pipeline for crude lactic acid composition 18 Static mixer 20 Heat exchanger 22 Pump 24 Pipeline for liquid heated and pressurized crude lactic acid composition 26 Evaporation device 27 (One or more) openings / nozzles 28 towers 30, 30', 30'' condensers 32 bottom pipeline of the tower 34 recirculation pipeline 36, 36' pipelines for removing heavy substances 38, 38', 38'' collectors 40, 40', 40'' removal pipelines 42 containers 44 stripping towers 46 structured packings 48 heat exchanger / reboiler 50 removal pipeline for purifying lactic acid 51 return pipeline for stripping gas 52 containers 54 removal pipeline for light substances 56 top removal pipeline of the tower 58, 58' containers 59, 59' freezing condensers 60, 60' removal pipelines 62 containers 64 removal pipeline 66 vacuum units 68 annular pipes 74 inlet pipe segments 76, 76' outlet pipe segments 78 V-shaped metal plates 80 distribution branch pipes 82 first cylindrical containers 84 second cylindrical containers 86 lower end of the first cylindrical container 88 upper end of the first cylindrical container 90 upper end of the second cylindrical container

Claims

1. A method for purifying a crude lactic acid aqueous composition, comprising the following steps: i) heating and pressurizing the crude lactic acid aqueous composition to obtain a liquid heated and pressurized crude lactic acid aqueous composition, ii) flash evaporating the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i) to obtain crude lactic acid vapor, iii) gradually condensing the crude lactic acid vapor obtained from step ii) by passing it through at least three consecutive condensers to obtain at least three condensate streams, wherein one of the condensate streams is a purified lactic acid stream.

2. The method according to claim 1, wherein the crude lactic acid aqueous composition is prepared by filtering a crude composition using a filter having an average pore size of 0.05 to 1.0 μm, more preferably 0.1 to 0.5 μm, and most preferably 0.1 to 0.3 μm and / or a filter having an average pore size of 1 to 100 nm, and more preferably 10 to 50 nm, and preferably then adjusting the water content of the microfiltered and / or nanofiltrated crude lactic acid aqueous composition to 10 to 50% by weight, preferably 15 to 30% by weight, and more preferably 15 to 20% by weight.

3. The method according to claim 1 or 2, wherein at least 70%, preferably at least 80%, more preferably at least 90%, still more preferably at least 95%, even more preferably at least 99%, and most preferably all of the energy required for flash evaporation of the liquid heated and pressurized crude lactic acid aqueous composition in step ii) is obtained from the potential energy stored in the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i).

4. The method according to any one of the preceding claims, wherein in step i) the crude lactic acid aqueous composition is heated to obtain a liquid heated and pressurized crude lactic acid aqueous composition having a temperature of 140 to 220 °C, and preferably 180 to 200 °C.

5. The method according to any one of the preceding claims, wherein in step i) the crude lactic acid aqueous composition is pressurized to obtain a liquid heated and pressurized crude lactic acid aqueous composition having a pressure of 0.2 to 5.0 MPa, preferably 0.7 to 2.5 MPa, and more preferably 1.0 to 1.5 MPa.

6. The method according to any one of the preceding claims, wherein in step ii) the flash evaporation of the liquid heated and pressurized crude lactic acid aqueous composition is carried out such that at least 80% of the liquid heated and pressurized crude lactic acid aqueous composition evaporates within at most 1 second.

7. The method according to any one of the preceding claims, wherein in step ii) the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i) is passed through a flash evaporation device comprising one or more openings or nozzles, wherein the pressure loss of the liquid heated and pressurized crude lactic acid aqueous composition within the one or more openings or nozzles is at least 0.1 MPa, preferably at least 5.0 MPa, more preferably at least 7.5 MPa, and most preferably at least 10.0 MPa in order to flash evaporate the liquid heated and pressurized crude lactic acid aqueous composition.

8. The method according to any one of the preceding claims, wherein in step ii), the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i) is directed through a flash evaporation device, which is a cyclone separator or an annular tube comprising a plurality of openings or nozzles, so as to flash evaporate the liquid heated and pressurized crude lactic acid aqueous composition, and / or wherein in step ii), the liquid heated and pressurized crude lactic acid aqueous composition obtained from step i) is directed through a flash evaporation device comprising one or more nozzles, wherein the one or more nozzles perform pneumatic atomization or solid cone spraying on the crude lactic acid vapor so as to flash evaporate the liquid heated and pressurized crude lactic acid aqueous composition.

9. The method according to any one of the preceding claims, wherein in step iii), in each of at least three consecutive condensers, a portion of the vapor is condensed to obtain a condensed portion and a remaining vapor portion, wherein the condensed portion obtained in each of the at least three condensers is taken out as a liquid, and except for the last condenser among the at least three condensers, the remaining vapor portion of each of the at least three condensers is directed to the next condenser among the at least three condensers.

10. The method according to any one of the preceding claims, wherein the flash evaporation of the liquid heated and pressurized crude lactic acid aqueous composition in step ii) and the gradual condensation of the crude lactic acid vapor in step iii) are carried out in a tower, wherein at least three subsequent condensers are vertically arranged one above the other, and wherein the tower further comprises a flash evaporation device for flash evaporating the liquid heated and pressurized crude lactic acid aqueous composition, and all at least three subsequent condensers are arranged above the flash evaporation device.

11. The method according to any one of the preceding claims, wherein in step iii), the crude lactic acid vapor is gradually condensed by directing it through three consecutive condensers, thereby obtaining three condensed streams, wherein a condensed stream is taken out from the first condenser among the three consecutive condensers, which contains lactic acid oligomers, lactide, proteins, metals, and one or more organic dicarboxylic acids.

12. The method according to claim 11, wherein a purified lactic acid condensed stream is taken out from the second condenser among the three consecutive condensers, and wherein the purified lactic acid condensed stream is further purified preferably by: directing the purified lactic acid condensed stream through a stripping column comprising a heat exchanger to partially evaporate the purified lactic acid condensed stream so as to strip water and light organic molecules, preferably formic acid and acetic acid, from the purified lactic acid condensed stream.

13. An apparatus for purifying a crude lactic acid aqueous composition, comprising: - one or more devices for heating and pressurizing the crude lactic acid aqueous composition into a liquid heated and pressurized crude lactic acid aqueous composition, - a flash evaporation device for flash evaporating the liquid heated and pressurized crude lactic acid aqueous composition obtained in the device for heating and pressurizing the crude lactic acid aqueous composition, - at least three consecutive condensers for gradually condensing the crude lactic acid vapor obtained in the flash evaporation device.

14. The apparatus according to claim 13, wherein the apparatus comprises a column in which the flash device and the at least three successive condensers are arranged.

15. The apparatus according to claim 13 or 14, wherein the flash device comprises one or more openings or nozzles, wherein preferably the flash device is an annular tube comprising a plurality of openings or nozzles, wherein preferably an annular collector is provided below the annular tube, and wherein the annular collector preferably has a U-shaped trough.

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