Mass polymerisation section for preparing polylactic acid

Abstract

The present invention relates to the field of polymerisation of lactic acid to form polylactic acid (PLA). The present invention in particular relates to a process for mass polymerisation in a PLA production process. The present invention also relates to an apparatus for performing the process of the invention, and to the use of the present process or apparatus in the production of a polylactide polymer.

Description

[0001] MASS POLYMERISATION SECTION FOR PREPARING POLYLACTIC ACID

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of polymerisation of lactic acid to form polylactic acid (PLA). The present invention in particular relates to a process for mass polymerisation in a PLA production process. The present invention also relates to an apparatus for performing the process of the invention, and to the use of the present process or apparatus in the production of a polylactide polymer.

[0004] BACKGROUND OF THE INVENTION

[0005] The demand for biodegradable polymers with excellent material properties is rapidly growing. Biodegradable polymers can be used in various applications, from biomedicine, additive technologies, film, fibres, packaging, automotive to agriculture, etc. Polylactide, which is also referred to as polylactic acid and abbreviated as PLA, has been receiving increased attention in recent years for use in these applications, because of its excellent performances in renewability, mechanical properties, biocompatibility, and biodegradability.

[0006] PLA is an aliphatic polyester, which can be manufactured from renewable resources. Such manufacture may involve the fermentation of starch, sugar, or other renewable organic substrates into lactic acid. Polylactide can be produced by direct polycondensation of lactic acid, i.e. lactate monomers. However, this has the drawback that a high molecular weight is not easily reached. Therefore, PLA is usually prepared by ring-opening polymerisation (ROP) of lactide, the cyclic dimer of lactic acid, which in turn is usually manufactured by polycondensation of lactic acid into PLA oligomers, followed by de-polymerisation of these oligomers by a so-called ‘backbiting’ mechanism in the presence of a suitable catalyst. After purification, the produced lactide can be converted into PLA of controlled molecular weight by means of a ring-opening polymerisation reaction (ROP) in the presence of a polymerisation catalyst and initiator. Ring-opening polymerisation allows to control the polymerisation process and thereby the structure of the produced PLA. This method can be used to manufacture PLA of high molecular weight. The molecular weights of the polymer fabricated by the ring opening polymerisation can be controlled by residence time, catalyst and initiator concentration, impurities concentration and temperature. The sequence and ratio of L- and D-lactic acid units in the final polymer can also be controlled.

[0007] A mass polymerisation section for polylactic acid (PLA) production from lactide typically consists of a series of reactors arranged in series. The reactors are designed to efficiently and effectively convert lactide monomers into high molecular weight PLA polymer. The reactors can be of different configurations, such as continuous stirred tank reactors (CSTR), plug flow reactors (PFR), or a combination of both. In a CSTR configuration, the lactide monomers are continuously fed into the reactor and mixed with an initiator to start the polymerisation reaction. The reaction mixture is continuously stirred to ensure even reaction conditions throughout the reactor. In a PFR configuration, the lactide monomers are fed into the reactor at one end, and are gradually polymerised as they move through the reactor due to the flow of the reaction mixture.

[0008] However, having several reactors in series means higher capital and operational expenses (CAPEX and OPEX). The need for multiple reactors increases the required investment in equipment and increases the ongoing operational costs such as maintenance and energy consumption. Additionally, the need to transfer the reaction mixture between reactors can also lead to some losses of the product and increase the complexity of the process. To mitigate these disadvantages, it is important to optimise the design of the reactors and the overall process to minimise costs while still achieving high quality and consistent PLA production.

[0009] Mass polymerisation of lactide to produce polylactic acid (PLA) is an exothermic reaction that releases heat. This heat can cause temperature fluctuations along the reaction train, which can affect the reaction kinetics and lead to an uneven temperature profile. The increasing viscosity of the reaction mixture as the polymerisation progresses also exacerbates the temperature control issue, as the heat generated by the reaction becomes trapped in the increasingly viscous material, leading to localised hot spots and further temperature fluctuations. These difficulties in controlling the temperature profile can result in decreased product quality, lower polymer yields, and increased energy consumption. As a result, existing reactor configurations for mass polymerisation of lactide face challenges in maintaining a consistent temperature profile, which can affect the efficiency and economics of the PLA production process.

[0010] It is therefore an object of the present invention to provide an improved process and apparatus for mass polymerisation in a PLA production process, which overcomes at least some of the above-mentioned drawbacks.

[0011] It is an object of the present invention to provide a process and apparatus for mass polymerisation in a PLA production process that lowers CAPEX and / or OPEX. It is an object of the present invention to provide a process and apparatus for mass polymerisation in a PLA production process reduces investment costs, such as caused by complex equipment. It is an object of the present invention to provide a process and apparatus for mass polymerisation in a PLA production process reduces operational costs, such as maintenance and energy consumption. It is also an object of the present invention to provide a process and apparatus for mass polymerisation in a PLA production process that only requires simple operations, while maintaining optimal polymer quality.

[0012] It is also an object of the present invention to provide a process and apparatus for mass polymerisation in a PLA production process that allows for a more controlled temperature profile. It is an object of the present invention to provide a process and apparatus for mass polymerisation in a PLA production process that takes into account the increased viscosity.

[0013] It is also an object of the present invention to provide a process and apparatus for mass polymerisation in a PLA production process that results in improved product quality. It is also an object of the present invention to provide a process and apparatus for mass polymerisation in a PLA production process that results in reduced energy consumption.

[0014] SUMMARY OF THE INVENTION

[0015] It has now surprisingly been found that some or all of the above demands and objectives can be attained either individually or in any combination by a process and an apparatus for preparing polylactic acid (PLA) as described herein.

[0016] The present invention relates to a process and to an apparatus for preparing PLA.

[0017] In a first aspect, the present invention relates to a process for preparing a polylactic acid (PLA) polymer from lactide. The process preferably comprises the steps of: feeding lactide or a partially polymerized lactide / PLA mixture into an apparatus, the apparatus comprising a plug flow reactor (PFR); and, polymerizing the lactide or the partially polymerized lactide / PLA mixture to polylactic acid in the vertical plug flow reactor (PFR).

[0018] The present invention also relates to a process for preparing a polylactic acid (PLA) polymer from lactide, comprising the steps of: feeding lactide or a partially polymerized lactide / PLA mixture into an apparatus, the apparatus comprising a plug flow reactor (PFR); and, polymerizing the lactide or the partially polymerized lactide / PLA mixture to polylactic acid in the plug flow reactor (PFR); characterized in that the plug flow reactor (PFR) is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0019] The present invention also relates to a process for preparing a polylactic acid (PLA) polymer from lactide, comprising the steps of: feeding lactide or a partially polymerized lactide / PLA mixture into an apparatus, the apparatus comprising a plug flow reactor (PFR); and, polymerizing the lactide or the partially polymerized lactide / PLA mixture to polylactic acid in the plug flow reactor (PFR); characterized in that the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor.

[0020] In some most preferred embodiments, the plug flow reactor (PFR) is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils. In some other most preferred embodiments, the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor. In some embodiments, the apparatus comprises both types of PFRs.

[0021] In some preferred embodiments, the apparatus comprises a shell-and-tube type plug flow reactor.

[0022] In some preferred embodiments, the apparatus further comprises a shell-and-tube type plug flow reactor.

[0023] In some preferred embodiments, the apparatus comprises a plug flow reactor (PFR) that is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0024] In some preferred embodiments, the internal coils are spiral coils.

[0025] In some preferred embodiments, the internal coils are placed in one or more coil boxes.

[0026] In some preferred embodiments, the apparatus further comprises a plug flow reactor (PFR) that is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0027] In some preferred embodiments, the lactide or the partially polymerized lactide / PLA mixture is polymerised to polylactic acid in only one plug flow reactor (PFR).

[0028] In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series, wherein the last-placed plug flow reactor is a shell-and-tube type plug flow reactor or a static mixer reactor. Preferably, all plug flow reactors are shell-and-tube type plug flow reactors.

[0029] In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series, preferably wherein the last-placed plug flow reactor is a static mixer reactor.

[0030] In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in a continuous stirred-tank reactor.

[0031] In some preferred embodiments, the apparatus further comprises a continuous stirred-tank reactor, preferably wherein lactide or a partially polymerized lactide / PLA mixture is partially polymerised to polylactic acid in said continuous stirred-tank reactor.

[0032] In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in a plug flow reactor (PFR) that is placed in series after a continuous stirred-tank reactor (CSTR).

[0033] In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is subsequently polymerised to polylactic acid in a continuous stirred-tank reactor, followed by a shell-and-tube type plug flow reactor, followed by a third reactor selected from: an additional shell-and-tube type plug flow reactor or a static mixer reactor (SMR).

[0034] In some preferred embodiments, the process comprises the step of controlling the temperature in the continuous stirred-tank reactor, either by adjusting the feed preheating temperature or by a head condenser for temperature control with the reactor being operated at P / T equilibrium.

[0035] In some preferred embodiments, the process comprises the step of feeding catalyst and / or initiator at multiple stages along the reactor(s).

[0036] In some preferred embodiments, lactide is fed to the plug flow reactor (PFR) through multiple lactide feed inlets, preferably at the top section of the vertical plug flow reactor.

[0037] In some preferred embodiments, the shell-and-tube type plug flow reactor is heated / cooled by thermal fluid on the shell side and has the polymer flowing through the tubes. In some preferred embodiments, the shell-and-tube type plug flow reactor comprises a polymer distribution plate in the exchanger head.

[0038] In a second aspect, the present invention relates to an apparatus configured for preparing polylactic acid (PLA) from lactide. The apparatus preferably comprises: a plug flow reactor (PFR) characterized in that the plug flow reactor (PFR) is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; or in that the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor.

[0039] The process according to the first aspect, and (preferred) embodiments thereof, is preferably performed in the apparatus according to the second aspect, and (preferred) embodiments thereof.

[0040] The apparatus according to the second aspect, and (preferred) embodiments thereof, is preferably configured to perform the process according to the first aspect, and (preferred) embodiments thereof.

[0041] In a third aspect, the present invention relates to the use of an apparatus according to the second aspect, and (preferred) embodiments thereof, for preparing polylactic acid, preferably for performing the process according to the first aspect, and (preferred) embodiments thereof. In a fourth aspect, the present invention relates to polylactic acid formed in the apparatus according to the second aspect, and (preferred) embodiments thereof. In a fifth aspect, the present invention relates to polylactic acid formed by the process according to the first aspect, and (preferred) embodiments thereof.

[0042] (Preferred) embodiments of the first, second, third, fourth, or fifth aspect of the invention are also (preferred) embodiments of the other aspects of the invention.

[0043] The processes and apparatuses according to the invention provide a simpler solution compared to existing reactor configurations, thereby reducing investment costs and CAPEX. The processes and apparatuses according to the invention require less operational costs, such as maintenance and energy consumption, thereby reducing OPEX. The present invention uses reactors that are cheaper and easier to maintain compared to loop reactors. Vertical plug flow reactors are also cheaper and easier to maintain compares do static mixer actors.

[0044] The present invention also provides a process and apparatus for mass polymerisation in a PLA production process that allows for a more controlled temperature profile. The present invention also provides a process and apparatus for mass polymerisation in a PLA production process that takes into account the increased viscosity.

[0045] The present invention also provides a process and apparatus for mass polymerisation in a PLA production process that results in improved product quality. The present invention also provides a process and apparatus for mass polymerisation in a PLA production process that results in higher polymer yields. The present invention also provides a process and apparatus for mass polymerisation in a PLA production process that results in reduced energy consumption.

[0046] The present invention also relates to the use of a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils, for producing a polylactic acid (PLA), wherein said PLA polymer has a yellowness index of at most 35, as determined by ASTM D1925-70.

[0047] The present invention also relates to the use of a shell-and-tube type plug flow reactor for producing a polylactic acid (PLA) polymer, wherein said PLA polymer has an amount of black specks with a size of 0.1 -0.5 mm of at most 10.0 / 100g of PLA polymer.

[0048] The independent and dependent statements set out particular and preferred features of the invention. Features from the dependent statements may be combined with features of the independent or other dependent statements as appropriate.

[0049] The present invention will now be further described. In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

[0050] DETAILED DESCRIPTION OF THE FIGURES

[0051] FIG. 1 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a single vertical plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0052] FIG. 2 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises two plug flow reactors (110), placed in series, the two vertical plug flow reactors (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0053] FIG. 3 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises two plug flow reactors, placed in series, the first plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; and the second plug flow reactor comprising a static mixer reactor or SMR (130).

[0054] FIG. 4 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by a plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0055] FIG. 5 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises two plug flow reactors, placed in series, the first plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; and the second plug flow reactor comprising a shell-and-tube type plug flow reactor (120).

[0056] FIG. 6 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by two shell-and-tube type plug flow reactors (120).

[0057] FIG. 7 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by two vertical plug flow reactors (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0058] FIG. 8 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by a vertical plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils, followed by a plug flow reactor comprising a static mixer reactor or SMR (130).

[0059] FIG. 9 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by a shell-and-tube type plug flow reactor (120), followed by a plug flow reactor comprising a static mixer reactor or SMR (130).

[0060] FIG. 10 provides a zoomed in schematic illustration of a vertical plug flow reactor (110) suitable for an embodiment of the invention, the vertical plug flow reactor (110) comprising a reactor jacket (112), motorised agitation through agitator blade tips (114), and spiral coils (116) placed between the agitator blade tips (114). A heat transfer medium is circulated within the internal coils (116).

[0061] The following (non-limiting) reference numbering will be adhered to in the figures:

[0062] 100 - apparatus; 102 - feed line; 104 - devolatilization preheater; 110 - vertical PFR with motorised agitation and coils; 112 - reactor jacker; 114 - agitator blade tips; 116 - internal coils; 120 - shell-and-tube PFR; 130 - SMR; 140 - CSTR; 142 - reflux condenser.

[0063] DETAILED DESCRIPTION OF THE INVENTION

[0064] When describing the invention, the terms used are to be construed in accordance with the following definitions, unless a context dictates otherwise.

[0065] Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

[0066] In the following passages, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. As used herein, the singular forms "a”, "an", and "the" include both singular and plural referents unless the context clearly dictates otherwise. By way of example, "a polymer" means one polymer or more than one polymer.

[0067] The terms "comprising", "comprises" and "comprised of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements, or method steps. It will be appreciated that the terms "comprising", "comprises" and "comprised of" as used herein comprise the terms "consisting of", "consists" and "consists of".

[0068] The terms “apparatus”, “device” may be used herein as synonyms.

[0069] The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g., 1 to 5 can include 1 , 2, 3, 4 when referring to, for example, a number of elements, and can also include 1 .5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g., from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

[0070] The term "about" as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of + / -10% or less, preferably + / -5% or less, more preferably + / -1% or less, of and from the specified value, insofar such variations are appropriate to perform in the disclosed invention. It is to be understood that the value to which the modifier "about" refers is itself also specifically, and preferably, disclosed.

[0071] The terms “wt%,” “vol%”, or “mol%” refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, which includes the component.

[0072] All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

[0073] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some, but not other, features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

[0074] The terms described above, and others used in the specification are well understood to those skilled in the art.

[0075] Preferred statements (features) and embodiments, resins and uses of this invention are set herein below. Each statement and embodiment of the invention so defined may be combined with any other statement and / or embodiment unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features or statements indicated as being preferred or advantageous. Hereto, the present invention is, in particular, captured by any one or any combination of one or more of the below numbered aspects and embodiments, with any other statement and / or embodiment.

[0076] 1. Process for preparing a polylactic acid (PLA) polymer from lactide, the process comprising the steps of: feeding lactide or a partially polymerized lactide / PLA mixture into an apparatus, the apparatus comprising a plug flow reactor (PFR); and, polymerizing the lactide or the partially polymerized lactide / PLA mixture to polylactic acid in the vertical plug flow reactor (PFR); characterized in that the plug flow reactor (PFR) is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; or in that the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor.

[0077] 2. Process according to any one of the previous statements or according to some embodiments, wherein the apparatus comprises a shell-and-tube type plug flow reactor.

[0078] 3. Process according to any one of the previous statements or according to some embodiments, wherein the apparatus comprises a plug flow reactor (PFR) that is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0079] 4. Process according to any one of the previous statements or according to some embodiments, wherein the lactide or the partially polymerized lactide / PLA mixture is polymerised to polylactic acid in only one plug flow reactor (PFR).

[0080] 5. Process according to any one of the previous statements or according to some embodiments, wherein lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series.

[0081] 6. Process according to any one of the previous statements or according to some embodiments, wherein the last-placed plug flow reactor is a shell-and-tube type plug flow reactor or a static mixer reactor.

[0082] 7. Process according to any one of the previous statements or according to some embodiments, wherein all plug flow reactors are shell-and-tube type plug flow reactors.

[0083] 8. Process according to any one of the previous statements or according to some embodiments, wherein lactide is polymerised to polylactic acid in a plug flow reactor (PFR) that is a static mixer reactor (SMR).

[0084] 9. Process according to any one of the previous statements or according to some embodiments, wherein lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series wherein the last-placed plug flow reactor is a static mixer reactor (SMR).

[0085] 10. Process according to any one of the previous statements or according to some embodiments, wherein lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series wherein the plug flow reactors (PFR) are shell-and-tube type reactors, except wherein the last-placed plug flow reactor is a static mixer reactor (SMR). 11. Process according to any one of the previous statements or according to some embodiments, wherein lactide is fed to the plug flow reactor (PFR) through multiple lactide feed inlets, preferably at the top section of the plug vertical flow reactor.

[0086] 12. Process according to any one of the previous statements or according to some embodiments, wherein polylactic acid is discharged from the plug flow reactor (PFR) through a discharge gear pump, and is transferred to a next reactor and / or to a devolatilisation section.

[0087] 13. Process according to any one of the previous statements or according to some embodiments, wherein lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in a continuous stirred-tank reactor (CSTR).

[0088] 14. Process according to any one of the previous statements or according to some embodiments, wherein the continuous stirred-tank reactor (CSTR) is the first-placed reactor.

[0089] 15. Process according to any one of the previous statements or according to some embodiments, wherein lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in a plug flow reactor (PFR) that is placed in series after a continuous stirred-tank reactor.

[0090] 16. Process according to any one of the previous statements or according to some embodiments, wherein lactide or a partially polymerized lactide / PLA mixture is subsequently polymerised to polylactic acid in a continuous stirred-tank reactor, followed by a shell-and-tube type plug flow reactor, followed by a third reactor selected from: an additional shell-and-tube type plug flow reactor or a static mixer reactor.

[0091] 17. Process according to any one of the previous statements or according to some embodiments, comprising the step of controlling the temperature in the continuous stirred-tank reactor, either by adjusting the feed preheating temperature or by a head condenser for temperature control with the reactor being operated at P / T equilibrium.

[0092] 18. Process according to any one of the previous statements or according to some embodiments, comprising the step of agitating the lactide and prepared polylactic acid in the continuous stirred-tank reactor (CSTR) by an axial type agitation system.

[0093] 19. Process according to any one of the previous statements or according to some embodiments, comprising the step of controlling the temperature in the continuous stirred-tank reactor (CSTR) by adjusting the feed preheating temperature.

[0094] 20. Process according to any one of the previous statements or according to some embodiments, comprising the step of controlling the temperature in the continuous stirred-tank reactor (CSTR) by a head condenser for temperature control. Process according to any one of the previous statements or according to some embodiments, comprising the step of controlling the temperature in the continuous stirred-tank reactor (CSTR) with the reactor being operated at P / T equilibrium. Process according to any one of the previous statements or according to some embodiments, comprising the step of controlling the temperature in the continuous stirred-tank reactor (CSTR) by flowing a heat transfer medium in a jacket and / or a set of internal coils. Process according to any one of the previous statements or according to some embodiments, comprising the step of feeding catalyst and / or initiator before the inlet of the first-placed reactor. Process according to any one of the previous statements or according to some embodiments, comprising the step of feeding catalyst and / or initiator at an intermediate stage of the first-placed reactor. Process according to any one of the previous statements or according to some embodiments, comprising the step of feeding catalyst and / or initiator after the first- placed reactor. Process according to any one of the previous statements or according to some embodiments, comprising the step of feeding catalyst and / or initiator at multiple stages along the reactor(s). Process according to any one of the previous statements or according to some embodiments, wherein the shell-and-tube type plug flow reactor is heated / cooled by thermal fluid on the shell side and has the polymer flowing through the tubes. Process according to any one of the previous statements or according to some embodiments, wherein the shell-and-tube type plug flow reactor comprises a polymer distribution plate in the exchanger head. Apparatus configured for preparing polylactic acid (PLA) from lactide, said apparatus comprising a plug flow reactor (PFR) characterized in that the plug flow reactor (PFR) is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; or in that the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor. Apparatus according to any one of the previous statements or according to some embodiments, wherein the plug flow reactor (PFR) is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; or in that the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor. Apparatus according to any one of the previous statements or according to some embodiments, wherein the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor. Apparatus according to any one of the previous statements or according to some embodiments, comprising only one plug flow reactor (PFR). Apparatus according to any one of the previous statements or according to some embodiments, comprising two or more plug flow reactors (PFR) placed in series. Apparatus according to any one of the previous statements or according to some embodiments, wherein the last-placed plug flow reactor is a shell-and-tube type plug flow reactor or a static mixer reactor. Apparatus according to any one of the previous statements or according to some embodiments, wherein all plug flow reactors are shell-and-tube type plug flow reactors. Apparatus according to any one of the previous statements or according to some embodiments, comprising a plug flow reactor (PFR) that is a static mixer reactor (SMR). Apparatus according to any one of the previous statements or according to some embodiments, comprising two or more plug flow reactors (PFR) placed in series wherein the last-placed plug flow reactor is a static mixer reactor (SMR). Apparatus according to any one of the previous statements or according to some embodiments, comprising two or more plug flow reactors (PFR) placed in series wherein the plug flow reactors (PFR) are shell-and-tube type reactors, except wherein the last-placed plug flow reactor is a static mixer reactor (SMR). Apparatus according to any one of the previous statements or according to some embodiments, comprising multiple lactide feed inlets, preferably at the top section of the plug vertical flow reactor. Apparatus according to any one of the previous statements or according to some embodiments, comprising a discharge gear pump. Apparatus according to any one of the previous statements or according to some embodiments, comprising a transfer line to a next reactor and / or to a devolatilisation section. Apparatus according to any one of the previous statements or according to some embodiments, comprising a continuous stirred-tank reactor (CSTR). Apparatus according to any one of the previous statements or according to some embodiments, wherein the continuous stirred-tank reactor (CSTR) is the first-placed reactor. 44. Apparatus according to any one of the previous statements or according to some embodiments, comprising a plug flow reactor (PFR) that is placed in series after a continuous stirred-tank reactor.

[0095] 45. Apparatus according to any one of the previous statements or according to some embodiments, comprising a continuous stirred-tank reactor, followed by a shell-and- tube type plug flow reactor, followed by a third reactor selected from: an additional shell-and-tube type plug flow reactor or a static mixer reactor.

[0096] 46. Apparatus according to any one of the previous statements or according to some embodiments, comprising means for adjusting the feed preheating temperature.

[0097] 47. Apparatus according to any one of the previous statements or according to some embodiments, comprising a head condenser for temperature control.

[0098] 48. Apparatus according to any one of the previous statements or according to some embodiments, comprising a continuous stirred-tank reactor (CSTR) with an axial type agitation system.

[0099] 49. Apparatus according to any one of the previous statements or according to some embodiments, comprising a continuous stirred-tank reactor (CSTR) comprising a heat transfer medium in a jacket and / or a set of internal coils.

[0100] 50. Apparatus according to any one of the previous statements or according to some embodiments, comprising a catalyst and / or initiator feed before the inlet of the first- placed reactor.

[0101] 51. Apparatus according to any one of the previous statements or according to some embodiments, comprising a catalyst and / or initiator feed at an intermediate stage of the first-placed reactor.

[0102] 52. Apparatus according to any one of the previous statements or according to some embodiments, comprising a catalyst and / or initiator feed after the first-placed reactor.

[0103] 53. Apparatus according to any one of the previous statements or according to some embodiments, comprising a catalyst and / or initiator feed at multiple stages along the reactor(s).

[0104] 54. Apparatus according to any one of the previous statements or according to some embodiments, comprising a shell-and-tube type plug flow reactor configured to be heated / cooled by thermal fluid on the shell side and has the polymer flowing through the tubes.

[0105] 55. Apparatus according to any one of the previous statements or according to some embodiments, wherein the shell-and-tube type plug flow reactor comprises a polymer distribution plate in the exchanger head.

[0106] 56. Apparatus according to any one of the previous statements or according to some embodiments, wherein the heat transfer medium comprises heat transfer oil. 57. Apparatus according to any one of the previous statements or according to some embodiments, wherein the coils are horizontal spiral coils.

[0107] 58. Apparatus according to any one of the previous statements or according to some embodiments, wherein multiple coils, for example 2, are placed in a coil box.

[0108] 59. Apparatus according to any one of the previous statements or according to some embodiments, comprising multiple coil boxes, for example 30.

[0109] 60. Apparatus according to any one of the previous statements or according to some embodiments, comprising a header configured to supply the coil boxes.

[0110] 61. Apparatus according to any one of the previous statements or according to some embodiments, comprising an agitator between each of the coil boxes.

[0111] 62. Apparatus according to any one of the previous statements or according to some embodiments, configured for performing the process according to any one of the previous statements or according to some embodiments.

[0112] 64. Process according to any one of the previous statements or according to some embodiments, performed in the apparatus according to any one of the previous statements or according to some embodiments.

[0113] 49. Use of the apparatus according to any one of the previous statements or according to some embodiments, for performing the process according to any one of the previous statements or according to some embodiments.

[0114] 50. Polylactic acid formed in the apparatus according to any one of the previous statements or according to some embodiments.

[0115] 51 . Polylactic acid formed by the process according to any one of the previous statements or according to some embodiments.

[0116] In a first aspect, the present invention relates to a process for preparing a polylactic acid (PLA) polymer from lactide. The process preferably comprises the steps of: feeding lactide or a partially polymerized lactide / PLA mixture into an apparatus, the apparatus comprising a plug flow reactor (PFR); and, polymerizing the lactide or the partially polymerized lactide / PLA mixture to polylactic acid in the vertical plug flow reactor (PFR).

[0117] In some most preferred embodiments, the plug flow reactor (PFR) is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils. In some other most preferred embodiments, the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor. In some embodiments, the apparatus comprises both types of PFRs.

[0118] In some preferred embodiments, the apparatus comprises a shell-and-tube type plug flow reactor. In some preferred embodiments, the apparatus comprises a plug flow reactor (PFR) that is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0119] In some preferred embodiments, the lactide or the partially polymerized lactide / PLA mixture is polymerised to polylactic acid in only one plug flow reactor (PFR).

[0120] The terms “PLA”, “polylactic polymer”, and “polylactic acid” are used herein as synonyms.

[0121] In some preferred embodiments, the PLA comprises a PLA homopolymer, a PLA copolymer, or a combination thereof.

[0122] The process and apparatus of the present invention are typically suitable for the preparation of PLA polymer. In some embodiments, the PLA polymer is PLA homopolymer. In some embodiments, the PLA polymer is a PLA copolymer. In some embodiments, the PLA polymer is a combination of a PLA homopolymer and a PLA copolymer.

[0123] A “PLA polymer” as used herein refers to a polymer of lactide (monomers). Lactide, also referred to as 3, 6-dimethyl-1 ,4-dioxane-2, 5-dione, can exist in three different geometric structures, which have a stereomeric relationship. The term “lactide” (or “lactide monomer”) as used herein may therefore be L-lactide (derived from two L-lactic acid molecules), D-lactide (derived from two D-lactic acid molecules), meso-lactide (derived from a L-lactic acid molecule and a D-lactic acid molecule), or a mixture of two or more of the above. A 50 / 50 mixture of L- lactide and D-lactide with a melting point of about 126°C is often referred to in the literature as D, L-lactide or racemic lactide (and is also denoted as “rac-Lactide” or “racemic lactide” or “rac- lactide” herein). A PLA polymer as defined herein may thus be a polymer of lactide (monomer) selected from the group comprising L-lactide, D-lactide, meso-lactide, racemic lactide and any mixture of two or more thereof.

[0124] In certain embodiments, a PLA polymer as defined herein is a polymer of lactide (monomer) as defined herein only, / .e., such polymer does not comprise any other monomer which is not a lactide. In certain embodiments, a PLA polymer which does not comprise any monomer which is not a lactide, is also denoted herein as a “PLA homopolymer”. Such PLA homopolymer may thus consist of lactide, e.g., lactide which is selected from the group comprising L-lactide, D-lactide, meso-lactide, racemic lactide and any mixture of two or more thereof.

[0125] In certain embodiments, the PLA polymer is selected from the group comprising, and preferably consisting of, poly(L-lactic acid) (PLLA), poly(D-lactic acid) (PDLA), and poly(L-,D- lactic acid) (PLDLA), and any mixture thereof. Stereo-complexes of PLLA and PDLA, as described for example in WO 2010 / 097463, can also be used as PLA polymer.

[0126] In certain embodiments, the PLA polymer may comprise (limited) amounts of a comonomer which is not a lactide as defined herein. More in particular, the PLA polymer may include a PLA copolymer, i.e., a copolymer of a lactide and a non-lactide comonomer. The term “PLA copolymer” as used herein intends to refer to a polymer of lactide (monomer) (as defined herein) and a comonomer which is not lactide ( / .e., a non-lactide comonomer).

[0127] In a certain embodiment, a non-lactide comonomer is selected from the group comprising urethanes, carbonates, lactones. For instance, copolymers of lactide and trimethylene carbonate may be used. For instance, copolymers of lactide and urethanes may be used. For instance, copolymers of lactide and lactones may be used. In a preferred embodiment, said comonomer is a lactone. Preferably said lactone is selected from the group comprising caprolactone, valerolactone, and butyrolactone. For instance, copolymers of lactide and caprolactone may be used in the polymer composition.

[0128] In some embodiments, the introduction of comonomers to PLA increases the ductility ( / .e., decreases the brittleness) of the PLA. Additionally, it is appreciated that if polymer composition comprises a PLA copolymer, as defined herein such PLA copolymer comprises a non-lactide comonomer content in a specific range. Preferably, the amount of a non-lactide comonomer in a PLA copolymer, for use in the present invention, is at most 30 % by weight, based on the total weight of the PLA copolymer, and preferably comprised between 1 and 20 % by weight or between 1 and 10 % by weight, or between 2 and 7 % by weight, or between 2 and 5 % by weight, based on the total weight of the PLA copolymer. A PLA copolymer as applied herein can be understood to mean any type of copolymer, including but not limited to a random copolymer, a block copolymer, a gradient copolymer, and a statistical copolymer.

[0129] Generally, two alternative methods for synthesizing polylactic acid are known. The first method is the direct polycondensation of lactic acid to polylactic acid, which leads to low molecular weight polymer only. The second method is the ring-opening polymerisation of lactide. In preferred embodiments of the present process, the PLA is obtained via a ring-opening polymerisation of lactide. In such method, lactide monomer is polymerised in the presence of catalyst, and optionally initiator, in a reactor to form a reaction mixture which comprises the resulting polylactide in a molten phase and unreacted lactide.

[0130] The term “partially polymerized lactide / PLA mixture” as used herein may therefore be a mixture comprising polylactide and (unreacted) lactide as defined herein. The fraction of (unreacted) lactide in said mixture is determined by the extent of polymerization or conversion. The conversion of a polymerization reaction is typically expressed as a percentage calculated based on the initial and final concentrations of monomers (e.g., lactide). In some embodiments, the partially polymerized lactide / PLA mixture may have a conversion of at least 5.0%, or at least 10.0%, or at least 20.0%, or at least 30.0%. In some embodiments, the partially polymerized lactide / PLA mixture may have a conversion of at most 90.0%, or at most 80.0%, or at most 70.0%. It is known to those skilled in the art of producing PLA which analytical techniques can be used to determine the initial and final concentrations of monomers (e.g., lactide). In some preferred embodiments, the first polymerisation reactor section comprises one or more CSTR type reactors. In some preferred embodiments, the first polymerisation reactor section comprises only CSTR type reactors. In some preferred embodiments, the first polymerisation reactor section comprises one or more CSTR type reactors and one or more PFR type reactors.

[0131] CSTRs and PFRs are differentiated by the amount of required liquid renewal before a composition can be changed and are often expressed in number of average residence time. A reactor or reactor system is usually considered a CSTR when one needs to wait 3 residence time and it is considered to be a PFR when one has only to wait for less than 1.5 residence time (see literature). From an engineering point of view, CSTRs are the cheapest type of reactor and allow significant CAPEX reduction.

[0132] The behaviour of a CSTR is often approximated or modelled by that of a Continuous Ideally Stirred-Tank Reactor (CISTR). All calculations performed with CISTRs assume perfect mixing. In a perfectly mixed reactor, the output composition is identical to composition of the material inside the reactor, which is a function of residence time and rate of reaction. If the residence time is 5-10 times the mixing time, this approximation is valid for engineering purposes.

[0133] As used herein, the term “CSTR type reactor” refers to CSTR reactors as well as to CSTR equivalents in the case of a loop reactor made of tubular reactors.

[0134] An ideal plug flow reactor has a fixed residence time which is the flowrate divided by the useful volume of the reactor. A real plug flow reactor has a narrow residence time distribution that is centred around the mean residence time also called the average residence time. Usually the behaviour is considered to be a good PFR when 2 sigma of the residence times are covered within + / -0.25 average residence time

[0135] As used herein, the term “PFR type reactor” refers to PFR reactors as well as to PFR equivalents in the case of tubular reactors.

[0136] As illustrated herein, the PFR type reactors are represented vertically. However, the present invention equally comprises the use of horizontal PFR type reactors.

[0137] CSTR type reactors are typically cheaper than other types of reactors. However, the transition time is longer. Therefore, the benefits of the present application are more outspoken when CSTR type reactors are used, particularly in the first section. This combines a reduction in cost while still keeping the transition time short.

[0138] PFR type reactors are typically better for PLA in terms of precision of temperature adjustment. PFR type reactors also have the advantage of being able to handle higher viscosities. In some preferred embodiments, the second polymerisation reactor section comprises one or more PFR type reactors. In some preferred embodiments, the second polymerisation reactor section comprises only PFR type reactors. In some preferred embodiments, lactide is polymerised to polylactic acid in only one plug flow reactor (PFR). This has the advantage that the entire process is performed in just one reactor, which reduces CAPEX and OPEX. This also has the advantage that operations are simpler, from both production and maintenance points of view.

[0139] In some preferred embodiments, lactide is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series. This allows for more flexibility. This may also allow multiple lactide injection points and multiple initiator injection points, to obtain more specific PLA product. In most cases, it also allows going to a higher lactide conversion.

[0140] In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series, wherein the last-placed plug flow reactor is a shell-and-tube type plug flow reactor or a static mixer reactor. Preferably, all plug flow reactors are shell-and-tube type plug flow reactors.

[0141] A plug flow reactor (PFR) is a type of continuous flow reactor in which reactants are introduced at one end of the reactor and progress through the reactor in a "plug-like" manner, with little or no back mixing. In the production of polylactic acid (PLA) from lactide, a plug flow reactor can be utilised to promote efficient polymerisation and improve product quality. The plug flow design of the reactor provides a more consistent residence time for the reactants, promoting a more uniform and controlled reaction and resulting in improved product quality. The plug flow design allows for a higher space-time yield compared to other reactor designs, leading to higher production volumes. The well-defined flow profile of a plug flow reactor allows for precise control of reaction conditions, making it easier to optimise the process and achieve desired product properties. The plug flow design minimises the risk of reactor runaway reactions, making it a safer option compared to other reactor designs. A 95% conversion target requires to complete the reaction through a PFR due to the relatively high viscosity.

[0142] In some preferred embodiments, lactide is polymerised to polylactic acid in a plug flow reactor (PFR) that is a shell-and-tube type reactor. This type of PFR is cheaper than a static mixer reactor (SMR) thanks to its relatively simple mechanical design. This type of PFR is very robust and reliable. This type of PFR has been found to be ideal in terms of thermal dilatation, and avoids issues of differential thermal expansion, reducing stress on the equipment, subsequently reducing the risks on crack and / or leakage.

[0143] A shell-and-tube reactor is a type of heat exchanger in which a fluid flows through tubes within a cylindrical shell, and heat is exchanged between the fluid and the walls of the tubes. Reference is made to the TEMA definition of a shell-and-tube reactor, such as in Perry's Chemical Engineers' Handbook, 9th Edition, Chapter 11 , Section 17, hereby incorporated by reference.

[0144] For a shell-and-tube type reactor, the oil is typically in the shell, while for a coil PFR or an SMR, the oil is in the tubes. This makes a shell-and-tube type reactor more robust. In the production of polylactic acid (PLA) from lactide, it has been found that a shell-and-tube reactor can be utilised as a heat exchanger to control the temperature of the reaction mixture and regulate the polymerisation process. The shell-and-tube design allows for efficient heat transfer, enabling precise control of the temperature of the reaction mixture, which is critical for successful polymerisation. Another advantage of the shell-and-tube design is that thermal risks in the polymerization process may be reduced, which avoids fast and local heat buildup. The efficient heat transfer provided by the shell-and-tube design allows for reduced energy consumption compared to other heat exchanger designs, leading to cost savings and improved energy efficiency. The shell-and-tube design can be easily scaled up or down to accommodate different production volumes, making it an adaptable option for the polymerisation of lactide to produce PLA. The shell-and-tube design is relatively simple and robust, making it a reliable option for temperature control in the production of PLA from lactide in the tubes side.

[0145] In some embodiments, lactide is polymerised to polylactic acid in a plug flow reactor (PFR) that is a static mixer reactor (SMR). Preferably, the last-placed PFR is an SMR. An SMR has the advantage that it allows to maintain relatively low pressure drop.

[0146] A static mixer reactor (SMR) is a type of reactor in which a mixture of reactants is passed through a series of fixed mixing elements to promote chemical reactions. In the production of polylactic acid (PLA) from lactide, a static mixer reactor is used to create homogeneous reaction conditions and promote efficient and consistent polymerisation. The design of the static mixer elements ensures complete mixing of the reactants, leading to a more uniform and consistent reaction. The high shear forces generated by the mixer elements can lead to faster reaction kinetics, resulting in increased production rates. The compact design of a static mixer reactor allows for a smaller overall process equipment footprint compared to other reactor designs. The absence of moving parts and simple design of the static mixer elements make the reactor easy to operate and maintain.

[0147] In some preferred embodiments, lactide is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series wherein the last-placed plug flow reactor is a static mixer reactor (SMR) or a shell-and-tube PFR. Such configurations allow for improved heat profile and reduces issues with viscosity. Such a configuration increases conversion and reduces pressure drop. Therefore, there is less required design pressure of reactors and / or reduced power consumption of melt pumps.

[0148] In some embodiments, lactide is fed to the plug flow reactor (PFR) through multiple lactide feed inlets, preferably at the top section of the plug vertical flow reactor. This was found to provide the best compromise between CAPEX and energy.

[0149] In some embodiments, polylactic acid is discharged from the plug flow reactor (PFR) through discharge gear pump, and is transferred to a next reactor and / or to a devolatilisation section. Providing a discharge pump allows to minimize the design pressure of the reactor as the discharge pump will get the necessary pressure to go through the downstream equipment.

[0150] In some preferred embodiments, the shell-and-tube type plug flow reactor is heated / cooled by thermal fluid on the shell side and has the polymer flowing through the tubes. In this case, the exchanger heads and nozzles may be jacketed with thermal fluid. In some preferred embodiments, the shell-and-tube type plug flow reactor comprises a polymer distribution plate in the exchanger head. This distributor is preferably a perforated plate which ensures equal pressure drops are obtained across all tubes. This plate is preferably positioned such that no stagnant zones are present in the exchanger head.

[0151] In some embodiments, the temperature in the PFR is at least 150°C and at most 200°C, for example about 170°C. In some embodiments, the pressure in the PFR is at least 10 barg and at most 200 barg, for example about 50 barg. In some embodiments, the residence time in the PFR is at least 20 minutes and at most 2 hours, for example about 45 minutes.

[0152] In some embodiments, the process of the present invention may comprise one or more plug flow reactors (PFRs).

[0153] In some embodiments, the temperature in each PFR is at least 150°C and at most 200°C, for example about 170°C. In some embodiments, the pressure in each PFR is at least 10 barg and at most 200 barg, for example about 50 barg. In some embodiments, the residence time in each PFR is at least 20 minutes and at most 2 hours, for example about 45 minutes.

[0154] In some embodiments, the temperature in at least one PFR is at least 150°C and at most 200°C, for example about 170°C. In some embodiments, the pressure in at least one PFR is at least 10 barg and at most 200 barg, for example about 50 barg. In some embodiments, the residence time in at least one PFR is at least 20 minutes and at most 2 hours, for example about 45 minutes.

[0155] In some preferred embodiments, lactide is also polymerised to polylactic acid in a continuous stirred-tank reactor (CSTR). In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in a continuous stirred-tank reactor.

[0156] In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in a plug flow reactor (PFR) that is placed in series after a continuous stirred-tank reactor (CSTR). A CSTR is typically cheaper than a PFR, but cannot handle high viscosities.

[0157] In some preferred embodiments, lactide or a partially polymerized lactide / PLA mixture is subsequently polymerised to polylactic acid in a continuous stirred-tank reactor, followed by a shell-and-tube type plug flow reactor, followed by a third reactor selected from: an additional shell-and-tube type plug flow reactor or a static mixer reactor (SMR).

[0158] Maintaining a maximum viscosity in CSTR, particularly if it is placed as a first-placed reactor, enables an easy temperature control. The CSTR may be equipped with standard equipment (reactors; agitator, condenser). Use of a CSTR allows for improved flexibility in operation. Use of a CSTR allows for reduced CAPEX, easier maintenance, and a reduction in overall costs. A continuous stirred tank reactor is a type of reactor in which reactants are continuously fed into a tank and mixed by a stirring device to promote chemical reactions. In the production of polylactic acid (PLA) from lactide, a CSTR can be utilised to create homogeneous reaction conditions and promote efficient polymerisation. The continuous stirring provided by the CSTR design ensures complete mixing of the reactants, leading to a more uniform and consistent reaction. The CSTR design allows for easy adjustment of the reaction conditions, making it a flexible option for the polymerisation of lactide to produce PLA. The well-mixed reaction mixture in a CSTR allows for precise control of reaction conditions, making it easier to optimise the process and achieve desired product properties. The relatively simple design of a CSTR compared to other reactor designs makes it a cost-effective option for the polymerisation of lactide to produce PLA.

[0159] In some embodiments, lactide is polymerised to polylactic acid in one continuous stirred-tank reactor (CSTR) followed by one plug flow reactor (PFR).

[0160] In some embodiments, the continuous stirred-tank reactor (CSTR) wherein lactide is polymerised to polylactic acid is the first-placed reactor.

[0161] In some preferred embodiments, lactide is polymerised to polylactic acid in a plug flow reactor (PFR), preferably two or more plug flow reactors (PFR), that are placed in series after a continuous stirred-tank reactor (CSTR). A CSTR is a cheaper type of reactor (few times cheaper than a loop reactor, for example). After the CSTR, preferably PFRs are used for viscosity reasons. Such a configuration is a very flexible option in terms of operations, as well as cheap configuration, since it allows to use standard reactors rather than specific plug flow reactors. There is no limitation in line capacity, since the CSTR will take care of polymerisation at a relatively low viscosity while the plug flow reaction section will complete the polymerisation until the equilibrium point is reached between monomer and polymer.

[0162] In some embodiments, the process comprises the step of agitating the lactide and prepared polylactic acid in the continuous stirred-tank reactor (CSTR) by an axial type agitation system. This type of agitator is easy to take care of in maintenance.

[0163] In some embodiments, the process comprises the step of controlling the temperature in the continuous stirred-tank reactor (CSTR) by feed temperature adjustment.

[0164] In some preferred embodiments, the process comprises the step of controlling the temperature in the continuous stirred-tank reactor (CSTR) by operating at P / T equilibrium. In some preferred embodiments, the process comprises the step of controlling the temperature in the continuous stirred-tank reactor (CSTR) at P / T equilibrium, preferably by using a reflux head condenser for temperature control. A pressure set point allows to control the reactor bulk temperature. In this later case, it is preferred to pre-heat the feed sufficiently high so as to generate an excess heat to be removed by a head condenser. In another embodiments, CSTR bulk temperature is simply adjusted through the set point of the feed-preheating. If for example the conversion inside the CSTR brings a heat corresponding to a delta T of 45°C (exothermic reaction) while operating at 170°C then the feed preheater would preferably target a temperature at least of 125°C.

[0165] In some preferred embodiments, the process comprises the step of controlling the temperature in the continuous stirred-tank reactor, either by adjusting the feed preheating temperature or by a head condenser for temperature control with the reactor being operated at P / T equilibrium.

[0166] In some embodiments, the temperature in the CSTR is at least 100°C and at most 200°C, for example about 170°C. In some embodiments, the pressure in the CSTR is at least 0.01 bar abs and at most 20 bar abs, for example about 2 bar abs. In some embodiments, the residence time in the CSTR is at least 20 minutes and at most 2 hours, for example about 45 minutes. In some embodiments, the process comprises the step of feeding catalyst. This improves the productivity.

[0167] In some embodiments, the polylactic acid is obtained by polymerizing lactide, in the presence of a suitable catalyst and preferably in the presence of a compound of formula (VI), acting as a co-initiator and transfer agent of the polymerisation, R3-OH (VI) wherein R3is selected from the group consisting of Ci-2oalkyl, Ce-soaryl, and C6-3oarylCi-2oalkyl, each group being optionally substituted by one or more substituents selected from the group consisting of halogen, hydroxyl, and Ci.6alkyl. Preferably, R3is a group selected from C3- walkyl, Ce- aryl, and Ce- arylCs-^alkyl, each group being optionally substituted by one or more substituents each independently selected from the group consisting of halogen, hydroxyl, and Ci-ealkyl; preferably, R3is a group selected from Cs-walkyl, Ce- aryl, and Ce- arylCs-walkyl, each group being optionally substituted by one or more substituents each independently selected from the group consisting of halogen, hydroxyl and Ci-4alkyl. The alcohol can be a polyol such as diol, triol or higher functionality polyhydric alcohol. The alcohol may be derived from biomass such as for instance glycerol or propanediol or any other sugar- based alcohol such as for example erythritol. The alcohol can be used alone or in combination with another alcohol.

[0168] In some embodiments, non-limiting examples of initiators include 1-octanol, decanol, isopropanol, propanediol, trimethylolpropane, 2-butanol, 3-buten-2-ol, 1 ,3-butanediol, 1 ,4- butanediol, 1 ,6-hexanediol, 1 ,7-heptanediol, benzyl alcohol, 4-bromophenol,1 ,4- benzenedimethanol, and (4-trifluoromethyl)benzyl alcohol; preferably, the compound of formula (VI) is selected from decanol, 1-octanol, isopropanol, and 1 ,4-butanediol. The catalyst employed for this process may have general formula M(Y1,Y2, ... Yp)q, in which M is a metal selected from the group comprising the elements of columns 3 to 12 of the periodic table of the elements, as well as the elements Al, Ga, In, Tl, Ge, Sn, Pb, Sb, Ca, Mg and Bi; whereas Y1, Y2, ... Ypare each substituents selected from the group comprising alkyl with 1 to 20 carbon atoms, aryl having from 6 to 30 carbon atoms, alkoxy having from 1 to 20 carbon atoms, aryloxy having from 6 to 30 carbon atoms, and other oxide, carboxylate, and halide groups as well as elements of group 15 and / or 16 of the periodic table; p and q are integers of from 1 to 6. As examples of suitable catalysts, we may notably mention the catalysts of Sn, Ti, Zr, Zn, and Bi; preferably an alkoxide or a carboxylate and more preferably Sn(Oct)2, Ti(OiPr)4, Ti(2-ethylhexanoate)4, Ti(2-ethylhexyloxide)4, Zr(OiPr)4, Bi(neodecanoate)3, (2,4-di- tert-butyl-6-(((2-(dimethylamino)ethyl)(methyl)amino)methyl)phenoxy)(ethoxy)zinc, or Zn(lactate)2. Preferably, the catalyst comprises Sn(Oct)2.

[0169] In some embodiments, the process comprises the step of feeding initiator. This improves the quality.

[0170] In some preferred embodiments, the process comprises the step of feeding catalyst and / or initiator at multiple stages along the reactor(s).

[0171] In some embodiments, the process comprises the step of feeding catalyst and / or initiator before the inlet of the first-placed reactor. A catalyst is a pre-requisite for PLA production, so there has to be an injection point either in the feed line or in the CSTR itself. Injecting in the feed line allows to premix with the feed which is beneficial for CSTR homogeneity.

[0172] In some embodiments, the process comprises the step of feeding catalyst and / or initiator at an intermediate stage of one or more of the reactors, for example at an intermediate stage of the first-placed reactor. Adding a second injection point coupled at an intermediate stage allows to fine tune the productivity while keeping the first reactor at a relatively lower conversion rate. This is beneficial notably when the first reactor is a CSTR.

[0173] In some embodiments, the process comprises the step of feeding catalyst and / or initiator after the first-placed reactor. Having a second initiator injection point downstream of the first reactor allows to fine-tune the molecular weight distribution of the PLA.

[0174] In some preferred embodiments, the process comprises the step of feeding catalyst and / or initiator at multiple stages along the reactor(s). This allows for more precise control of the polymerisation rate along the reactor(s). This also allows to produce broader Mw distribution PLA grades, resulting in new or more flexible product development.

[0175] In some embodiments, the process comprises the step of injecting meso-lactide, preferably side-injecting meso-lactide.

[0176] The present invention has the advantage that there is improved control of the temperature inside the reactor thanks to the combination of coils and agitation. The reactants are typically very fluid at top, yet very viscous at the bottom. As lactide polymerization is an exothermic reaction, there is preferably provided a system to remove the excess heat. The system also allows to heat up the fluid during start-up of the line. A preferred way to add or remove heat allow a reactor is to have internal coils where a heat transfer oil is circulated. While it is possible in the case of a SMR to have the internal coils making the mixing as well, this complexifies the design and brings up the CAPEX. Associating a coil with an agitator enable a simple and cheap design.

[0177] Preferably the coils are spiral coils.

[0178] In a second aspect, the present invention relates to an apparatus configured for preparing polylactic acid (PLA) from lactide. The apparatus preferably comprises: a plug flow reactor (PFR) characterized in that the plug flow reactor (PFR) is a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; or in that the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor.

[0179] The specific geometry and agitator design allows to maintain homogeneity of the monomer / polymer solution, thus avoiding channelling. The internal coils manage the heat balance inside the reactor (they add and / or remove heat). Therefore, the internal coils provide a good control of the temperature profile.

[0180] The present invention results in less equipment and simpler operation, thereby reducing both CAPEX and OPEX. The present invention results in simpler process design and operation.

[0181] Advantages of the apparatus according to the present invention, and (preferred) embodiments thereof, are similar to the advantages of the process according to the present invention, and (preferred) embodiments thereof.

[0182] In some embodiments, the apparatus comprises multiple reactors that are fluidly connected in series. As used herein, the term “fluidly connected” or “in fluid connection” are used herein as synonyms and intend to refer to a connection between two components (or units or devices or the like) of the apparatus as given herein, so that through said connection a fluid can flow from one component (e.g. unit, device, reactor, etc) to the other. Preferably two fluidly connected components are connected so that an outlet of the first component is connected by a pipe, a conduit or a series of pipes or conduits, to an inlet of the second component. Other components, such condensers, and the like, may be fluidly connected between two fluidly connected components.

[0183] The term “means for transferring” or “transfer system” are also used herein as synonyms and refer to suitable systems allowing the transfer (transport) of a stream or reaction product or reactant, e.g. from one unit or device to another unit or device, and includes for instance pipes, conduits, recovery lines, outlet lines, connecting lines, and the like.

[0184] The term “means for recovering” or “recovery system” are used herein as synonyms and refer to suitable systems allowing the recovery of a stream or reaction product or reactant, and includes for instance pipes, recovery lines, outlet lines, connecting lines, and the like. “Means for recovering” or “recovery system” may comprise separation means.

[0185] The terms “means for supplying” or “supply system” are used herein as synonyms and refer to suitable systems allowing the supply of a stream, or reaction product, or reactant, and includes for instance pipes, conduits, supply lines, inlet lines, connecting lines, and the like.

[0186] In some preferred embodiments, the apparatus comprises only one plug flow reactor (PFR), as described for the process above. In some embodiments, the apparatus comprises two or more plug flow reactors (PFR) placed in series, as described for the process above.

[0187] In some embodiments, at least one plug flow reactor (PFR) is a static mixer reactor (SMR). In some embodiments, the apparatus comprises two or more plug flow reactors (PFR) placed in series wherein the last-placed plug flow reactor is a static mixer reactor (SMR). In some embodiments, the apparatus comprises two or more plug flow reactors (PFR) placed in series wherein the plug flow reactors (PFR) are shell-and-tube type reactors, except wherein the last- placed plug flow reactor is a static mixer reactor (SMR).

[0188] In some embodiments, the plug flow reactor (PFR) comprises multiple lactide feed inlets, preferably at the top section of the plug vertical flow reactor.

[0189] In some embodiments, the plug flow reactor (PFR) comprises a discharge gear pump to transfer the PLA to a next reactor, and / or to a devolatilisation section.

[0190] In some preferred embodiments, the apparatus further comprises a continuous stirred-tank reactor (CSTR); preferably wherein the plug flow reactor (PFR), preferably the two or more plug flow reactors (PFR), are placed in series after the continuous stirred-tank reactor (CSTR). In some embodiments, the apparatus comprises a continuous stirred-tank reactor (CSTR). In some embodiments, the apparatus comprises one continuous stirred-tank reactor (CSTR) followed by one plug flow reactor (PFR).

[0191] In some embodiments, the continuous stirred-tank reactor (CSTR) is the first-placed reactor. In some embodiments, the plug flow reactor (PFR), preferably the two or more plug flow reactors (PFR), are placed in series after the continuous stirred-tank reactor (CSTR).

[0192] In some embodiments, the continuous stirred-tank reactor (CSTR) comprises an axial type agitation system.

[0193] In some embodiments, the temperature in the continuous stirred-tank reactor (CSTR) is controlled by feed temperature adjustment. In some embodiments, the temperature in the continuous stirred-tank reactor (CSTR) is controlled by a head condenser for temperature control.

[0194] In some embodiments, the apparatus comprises a catalyst and / or initiator injection point before the inlet of the first-placed reactor. In some embodiments, the apparatus comprises an additional catalyst and / or initiator injection point at an intermediate stage of one or more of the 1 reactors, for example at an intermediate stage of the first-placed reactor. In some embodiments, the apparatus comprises an additional catalyst and initiator injection point after the first-placed reactor.

[0195] In some preferred embodiments, the apparatus comprises a multi-injection inlet system for a catalyst and / or an initiator along the reactor(s).

[0196] In some preferred embodiments, the plug flow reactor (PFR) comprises a radial agitation inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils. The internal coils can be used to recover heat generated during the polymerisation reaction, leading to increased energy efficiency, and reduced operating costs. The use of internal coils can be easily scaled up or down to accommodate different production volumes, making the PFR system adaptable for the polymerisation of lactide to produce PLA. In some embodiments, the heat transfer medium comprises heat transfer oil. Some suitable types of heat transfer oils for use in coils in PFRs include:

[0197] Mineral oils, which are typically derived from petroleum and are commonly used in industrial heating applications. They have a low viscosity, good thermal stability, and are relatively inexpensive.

[0198] Synthetic oils, which are typically artificially produced and can provide improved performance compared to mineral oils. Examples include poly-alpha-olefins (PAO) and esters, which have high thermal stability, low viscosity, and good heat transfer characteristics.

[0199] Bio-based oils, which are typically derived from renewable resources and can provide environmental benefits compared to mineral and synthetic oils. Examples include vegetable oils and biodiesel, which have good heat transfer characteristics and are biodegradable.

[0200] Fluorinated oils, which are typically synthesised using fluorine and offer superior thermal stability, low viscosity, and good heat transfer characteristics. They are commonly used in high-temperature applications.

[0201] For PLA, the best compromise between performance, cost and HSE, was found to be the use of mineral oil.

[0202] In some embodiments, multiple coils, for example 2, are placed in a coil box.

[0203] There are several options and configurations for coil boxes, including single-coil boxes, multicoil boxes, and dual-purpose coil boxes. The use of multiple coils in a coil box provides several advantages, including improved temperature control, increased energy efficiency, higher rigidity, and scalability. Single-coil boxes contain a single coil, which can be used to introduce heating or cooling media into the reaction mixture. Multi-coil boxes contain multiple coils, which can be used to introduce multiple heating or cooling media into the reaction mixture, providing increased control over the temperature profile. Dual-purpose coil boxes contain coils that can be used for both heating and cooling, providing additional flexibility in the temperature control of the reaction mixture.

[0204] In some embodiments, the PFR comprises multiple coil boxes, for example 30.

[0205] Multiple coils can provide increased control over the temperature profile, allowing for precise regulation of the reaction temperature, which is critical for efficient polymerisation. The use of multiple coils can reduce the temperature gradient within the reactor, leading to improved heat transfer and increased energy efficiency. The use of multiple coils can be easily scaled up or down to accommodate different production volumes, making the PFR system adaptable for the polymerisation of lactide to produce PLA.

[0206] In some embodiments, the PFR comprises a header configured to supply the coil boxes.

[0207] In some embodiments, the PFR comprises an agitator between each of the coil boxes. Preferably the coils are spiral coils.

[0208] In some preferred embodiments, the plug flow reactor (PFR) is a shell-and-tube type reactor. In some embodiments, the tubes are vertically disposed within the shell. In some embodiments, the tubes have a length of more than 0.5 m, more than 1.0 m, or more than 1.5 m. In some embodiments, the tubes have a length between 0.5 and 6.0 m, or a length between 1.0 and 4.0 m.

[0209] In some embodiments, the shell is cylindrical, and preferably has an outer diameter of at least 0.5 and at most 4.0 m.

[0210] In some embodiments, the shell-and-tube plug flow reactor comprises more than 500 tubes, or more than 1000 tubes, or more than 1500 tubes, or more than 2000 tubes, or more than 3000 tubes. Preferably, the tubes are made of stainless steel.

[0211] The shell-and-tube type reactor may have a top end and bottom end, wherein the tubes are disposed between the top end and the bottom end.

[0212] In some preferred embodiments, the shell-and-tube type plug flow reactor is heated / cooled by thermal fluid on the shell side and has the polymer flowing through the tubes. In this case, the exchanger heads and nozzles may be jacketed with thermal fluid. In some preferred embodiments, the shell-and-tube type plug flow reactor comprises a polymer distribution plate in the exchanger head. This distributor is preferably a perforated plate which ensures equal pressure drops are obtained across all tubes. This plate is preferably positioned such that no stagnant zones are present in the exchanger head.

[0213] Elements in the apparatus are preferably composed of a material capable of withstanding a differential pressure and vacuum as well as an elevated operating temperature. Without indenting to be limiting, an example of a suitable material may be steel. In case the polymer material, such as a polymer comprising PLA, may cause corrosion, appropriate steel alloys may be used, such as 316L or duplex steel grades. Elements of the apparatus may optionally comprise additional elements such as insulation or reinforcement plating. The process according to the first aspect, and (preferred) embodiments thereof, is preferably performed in the apparatus according to the second aspect, and (preferred) embodiments thereof.

[0214] The apparatus according to the second aspect, and (preferred) embodiments thereof, is preferably configured to perform the process according to the first aspect, and (preferred) embodiments thereof.

[0215] In a third aspect, the present invention relates to the use of an apparatus according to the second aspect, and (preferred) embodiments thereof, for preparing polylactic acid, preferably for performing the process according to the first aspect, and (preferred) embodiments thereof. In a fourth aspect, the present invention relates to polylactic acid formed in the apparatus according to the second aspect, and (preferred) embodiments thereof. In a fifth aspect, the present invention relates to polylactic acid formed by the process according to the first aspect, and (preferred) embodiments thereof.

[0216] (Preferred) embodiments of the first, second, third, fourth, or fifth aspect of the invention are also (preferred) embodiments of the other aspects of the invention.

[0217] As used herein, the terms “polylactic acid” or “poly-lactide” or “PLA” are used interchangeably and refer to poly(lactic acid) polymers comprising repeat units derived from lactic acid.

[0218] The number average molecular weight is measured by gel permeation chromatography using a solvent between 25°C to 30°C like for example chloroform.

[0219] In some embodiments, the polylactic acid has a weight average molecular weight (Mw) of at least 30 kDa to at most 500 kDa, preferably at least 50 kDa to at most 400 kDa, for example at least 60 kDa to at most 300 kDa, for example about 200 kDa. Measurement of the molecular masses may be performed at 25°C using a liquid chromatograph PolymerChar system.

[0220] In some embodiments, the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is generally from 1.0 to 4.0, preferably from 1.2 to 3.6, preferably from 1 .4 to 3.0, preferably from 1 .6 to 2.4, preferably from 1 .8 to 2.0.

[0221] In some embodiments, the polylactic acid has a density of at least 1.16 g / cm3to at most 1.40 g / cm3, for example at least 1.18 g / cm3to at most 1.35 g / cm3, preferably at least 1.20 g / cm3to at most 1.30 g / cm3, preferably at least 1.22 g / cm3to at most 1.28 g / cm3, preferably at least 1.23 g / cm3to at most 1 .26 g / cm3, for example about 1.24 g / cm3, as determined in accordance with ASTM D1505 at 23°C.

[0222] The polylactic acid may optionally include additives known in the art to improve processing and application of polymer films, e.g., anti-block additives, slip additives and viscosity enhancers. When used to enhance the production of polymer films, it should be noted that these additives are not essential for blowing the PLA films per se, but may be preferentially employed to enhance the processing, performance and look of the final product. In an embodiment, the polylactic acid comprises a slip agent, preferably erucamide. In an embodiment, the polylactic acid comprises an anti-block additive, preferably talc or other silica- based products.

[0223] In another aspect, the present invention relates to the use of a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils, for producing a polylactic acid (PLA), wherein said PLA polymer has a yellowness index of at most 35, or at most 34, or at most 33, or at most 32, or at most 31 , or at most 30 as determined by ASTM D1925-70.

[0224] “Yellowness” is an undesirable property, especially when a clear or light-coloured polymer (product) is desired. A reduced yellowness has the advantage that a high quality polymer (product) may be obtained with a reduced amount of impurities or by-products.

[0225] In another aspect, the present invention relates to the use of a shell-and-tube type plug flow reactor for producing a polylactic acid (PLA) polymer, wherein said PLA polymer has an amount of black specks with a size of 0.1 -0.5 mm of at most 10.0 / 100g, or at most 9.0 / 100g, or at most 8.0 / 100g, or at most 7.0 / 100g, or at most 6.0 / 100g, or at most 5.0 / 100g, of PLA polymer.

[0226] “Black specks” or “black dots” are material contaminations that may change both optical and mechanical properties of a polymer (product), which is undesirable for various applications. In addition, said contaminants may cause abrasion and wear on downstream polymer processing equipment such as moulds, dies, and extrusion tools. Reducing or avoiding the formation of black specks during polymer production therefore has the advantage that a polymer (product) may be obtained with improved aesthetic quality and performance. (Preferred) embodiments of the first, second, third, fourth, or fifth aspect of the invention are also (preferred) embodiments of the other aspects of the invention.

[0227] EXAMPLES

[0228] Reactor configurations

[0229] The Figures provide examples of reactor configurations suitable for an apparatus according to the invention. All pumps, for example illustrated at the bottom of the PFRs shown in the Figures are to be considered optional. Reflux condensers (145) as illustrated in the Figures are also to be considered optional.

[0230] FIG. 1 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a single vertical plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0231] FIG. 2 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises two plug flow reactors (110), placed in series, the two vertical plug flow reactors (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0232] FIG. 3 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises two plug flow reactors, placed in series, the first plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; and the second plug flow reactor comprising a static mixer reactor or SMR (130). Alternatively, the second plug flow reactor comprises a shell-and-tube type plug flow reactor (120). The shell- and-tube type plug flow reactor (120) is ideal to handle the increased viscosity. It is noted that an SMR is typically more expensive than a shell-and-tube type reactor.

[0233] FIG. 4 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by a plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0234] FIG. 5 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises two plug flow reactors, placed in series, the first plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils; and the second plug flow reactor comprising a shell-and-tube type plug flow reactor (120). The shell- and-tube type plug flow reactor (120) is ideal to handle the increased viscosity. It is noted that an SMR is typically more expensive than a shell-and-tube type reactor.

[0235] FIG. 6 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by two shell-and-tube type plug flow reactors (120).

[0236] FIG. 7 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by two vertical plug flow reactors (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

[0237] FIG. 8 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by a vertical plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils, followed by a plug flow reactor comprising a static mixer reactor or SMR (130). Alternatively, the second plug flow reactor comprises a shell-and-tube type plug flow reactor (120). FIG. 9 provides a schematic illustration of an embodiment of an apparatus (100) according to the invention, whereby the apparatus comprises a continuous stirred-tank reactor or CSTR (140), followed by a shell-and-tube type plug flow reactor (120), followed by a plug flow reactor comprising a static mixer reactor or SMR (130). Alternatively, the second plug flow reactor comprises a shell-and-tube type plug flow reactor (120).

[0238] FIG. 10 provides a zoomed in schematic illustration of a vertical plug flow reactor (110) suitable for an embodiment of the invention, the vertical plug flow reactor (110) comprising a reactor jacket (112), motorised agitation through agitator blade tips (114), and spiral coils (116) placed between the agitator blade tips (114). A heat transfer medium is circulated within the internal coils (116). In this particular example, between each agitator blade tip (114), there are 2 horizontal spiral coils (116). These spirals are placed in a coil box and welded. The exemplified configuration is built up from coil boxes alternating with blades. More or less (spiral) coils may be placed between the blades, resulting in a compromise between homogeneity and head exchange. The present configuration with 2 (spiral) coils between each blade was found to be optimal.

[0239] Reactor conditions

[0240] The below reactor conditions provide examples of conditions suitable for a process according to the invention. In the below examples, the lactide was fed to the reaction section at 50 kg / h; 120°C; 150ppm tin octoate; and 1500ppm decanol.

[0241] Example A

[0242] Reactor 1 : CSTR; 50 litres useful volume; 150°C; 50% conversion

[0243] Reactor 2: Shell-and-tube PFR; 30 litres; 150°C to 175°C; 85% conversion

[0244] Reactor 3: SMR; 30 litres; 175°C to 185°C; 95% conversion Example B

[0245] Reactor 1 : CSTR; 70 litres useful volume; 150°C; 50% conversion

[0246] Reactor 2: Shell-and-tube PFR; 60 litres; 150°C to 185°C; 95% conversion Example C

[0247] Reactor 1 : agitated vertical PFR; 120 litres useful volume; 115°C to 185°C; 90% conversion

[0248] In the below examples, the lactide was fed to the reaction section at 240 kg / h; 200°C; 50ppm tin octoate; and 2500ppm decanol.

[0249] Example D

[0250] Reactor 1 : CSTR; 84 litres useful volume; 200°C

[0251] Reactor 2: Shell-and-tube PFR; 18 litres; 200°C; 87% conversion; Yellowness index 38 Example E

[0252] Reactor 1 : agitated vertical PFR; 84 litres useful volume; 200°C Reactor 2: Shell-and-tube PFR; 18 litres; 200°C; 95% conversion; Yellowness index 32

[0253] When comparing examples D and E, it can be observed that the yellowness of the produced PLA polymer can be reduced when using a vertical plug flow reactor comprising a radial agitation system inserted in between a series of internal coils. Hence, resulting in a higher quality PLA polymer.

[0254] In the below examples, the lactide was fed to the reaction section at 45 kg / h; 200°C; 50ppm tin octoate; and 2500ppm decanol.

[0255] Example F

[0256] Reactor 1 : CSTR; 20 litres useful volume; 200°C; 79% conversion; 27 / 100g of black specs with a size between 0.1 -0.5mm

[0257] Example G

[0258] Reactor 1 : CSTR; 5 litres useful volume; 200°C

[0259] Reactor 2: Shell-and-tube PFR; 4 litres; 200°C; 79% conversion; 9 / 100g of black specs with a size between 0.1 -0.5mm

[0260] The below reactor conditions provide yet another example of conditions suitable for a process according to the invention. In the below examples, the lactide was fed to the reaction section at 80 kg / h; 200°C; 50ppm tin octoate; and 2500ppm decanol.

[0261] Reactor 1 : CSTR; 25 litres useful volume; 200°C

[0262] Reactor 2: Shell-and-tube PFR; 18 litres; 200°C; 94% conversion; 18 / 100g of black specs with a size between 0.1 -0.5mm

[0263] When comparing examples F, G, and H, it can be observed that the amount of black specks of the produced PLA polymer can be (significantly) reduced when using a shell-and-tube type reactor. Hence, this results in a higher quality PLA polymer.

[0264] Reactor 1 : Shell-and-tube PFR; 25 litres useful volume; 200°C

[0265] Reactor 2: Shell-and-tube PFR; 18 litres; 200°C; 95% conversion; 5 / 100g of black specs with a size between 0.1 -0.5mm When comparing examples F, G, H, and I, it can be observed that the amount of black specks of the produced PLA polymer can even be further reduced when using two shell-and-tube type plug flow reactors. Hence, this results in an even higher quality PLA polymer.

Claims

CLAIMS1. Process for preparing a polylactic acid (PLA) polymer from lactide, the process comprising the steps of: feeding lactide or a partially polymerized lactide / PLA mixture into an apparatus (100), the apparatus comprising a plug flow reactor (PFR); and, polymerizing the lactide or the partially polymerized lactide / PLA mixture to polylactic acid in the plug flow reactor (PFR); characterized in that the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor (120).

2. Process according to claim 1 , wherein the shell-and-tube type plug flow reactor (120) comprises a polymer distribution plate in the exchanger head.

3. Process according to any one of claims 1 or 2, wherein the apparatus (100) further comprises a plug flow reactor (PFR) that is a vertical plug flow reactor (110) comprising a radial agitation system inserted in between a series of internal coils, wherein a heat transfer medium is circulated within the internal coils.

4. Process according to any one of claims 1 to 3, wherein the lactide or the partially polymerized lactide / PLA mixture is polymerised to polylactic acid in only one plug flow reactor (PFR).

5. Process according to any one of claims 1 to 4, wherein lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in two or more plug flow reactors (PFR) placed in series, wherein the last-placed plug flow reactor is a shell-and-tube type plug flow reactor (120) or a static mixer reactor (130); preferably wherein all plug flow reactors are shell-and-tube type plug flow reactors (120).

6. Process according to any one of claims 1 to 5, wherein the apparatus (100) further comprises a continuous stirred-tank reactor (140), and wherein lactide or a partially polymerized lactide / PLA mixture is partially polymerised to polylactic acid in a continuous stirred-tank reactor (140).

7. Process according to claim 6, wherein lactide or a partially polymerized lactide / PLA mixture is polymerised to polylactic acid in a plug flow reactor (PFR) that is placed in series after a continuous stirred-tank reactor (140).

8. Process according to claim 7, wherein lactide or a partially polymerized lactide / PLA mixture is subsequently polymerised to polylactic acid in a continuous stirred-tank reactor (140), followed by a shell-and-tube type plug flow reactor (120), followed by a third reactor selected from: an additional shell-and-tube type plug flow reactor (120) or a static mixer reactor (130).

9. Process according to any one of claims 6 to 8, comprising the step of controlling the temperature in the continuous stirred-tank reactor (140), either by adjusting the feed preheating temperature or by a head condenser for temperature control with the reactor being operated at P / T equilibrium.

10. Process according to any one of claims 1 to 9, comprising the step of feeding catalyst and / or initiator at multiple stages along the reactor(s).

11. Process according to any one of claims 1 to 10, wherein the shell-and-tube type plug flow reactor (120) is heated / cooled by thermal fluid on the shell side and has the polymer flowing through the tubes.

12. Apparatus (100) configured for preparing polylactic acid (PLA) from lactide, said apparatus comprising a plug flow reactor (PFR) characterized in that the plug flow reactor (PFR) is a shell-and-tube type plug flow reactor (120).

13. Apparatus (100) according to claim 12, configured to perform the process according to any one of claims 1 to 11.

14. Process according to any one of claims 1 to 11 , performed in the apparatus (100) according to any one of claims 12 or 13.

15. Use of a shell-and-tube type plug flow reactor (120) for producing a polylactic acid (PLA) polymer, wherein said PLA polymer has an amount of black specks with a size of 0.1 -0.5 mm of at most 10.0 / 100g of PLA polymer.

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