Meso-Lactide Solvent Crystallization for Free Acid Removal
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Solution Overview
Problem
Existing methods for purifying meso-lactide are inefficient in removing contaminants, particularly free acids, leading to undesirable side reactions and reduced yield, and do not meet the stringent requirements for commercial-scale production of high-purity meso-lactide.
Innovation Solution
A solvent crystallization process using specific ketones and ethers at controlled temperatures to purify meso-lactide, reducing free acid content from at least 30 meq/kg to at most 20 meq/kg in multiple sequential steps without intermediate crystallization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional distillation methods are used to purify meso-lactide, then separation of meso-lactide from L- and D-lactide is achieved, but free acid contaminants remain at high levels (at least 30 meq/kg) causing side reactions and reduced yield
Solution Approach 1:
The invention changes the physical state parameter of meso-lactide from liquid to solid through controlled crystallization. By adjusting temperature and solvent composition parameters, the process selectively precipitates solid meso-lactide crystals that exclude free acid contaminants, achieving purification from at least 30 meq/kg to at most 20 meq/kg free acid content
Solution Approach 2:
The invention utilizes the phase transition of meso-lactide from dissolved state to solid crystalline state. During this phase change, the crystal lattice formation selectively incorporates meso-lactide molecules while excluding free acid impurities, effectively separating them. The solid crystals are then filtered and dried to achieve the desired purity level
2Manufacturing precision
If multiple purification steps are implemented to reduce free acid content, then purity increases, but process complexity and production time increase
Solution Approach 1:
The invention performs preliminary separation of meso-lactide from L- and D-lactide through distillation before the crystallization step. This preliminary action concentrates the meso-lactide in the feed stream, allowing the subsequent crystallization step to focus solely on removing free acid contaminants rather than dealing with complex mixtures, thereby simplifying the overall process
Solution Approach 2:
The invention extracts free acid contaminants from the meso-lactide system during the crystallization process. By forming solid meso-lactide crystals and separating them through filtration, the free acids remain in the mother liquor and are effectively removed. This extraction approach achieves purification in a single crystallization step rather than requiring multiple sequential purification operations
3Manufacturing precision
If high-purity meso-lactide is produced through extensive purification, then contaminant levels decrease, but yield and production efficiency decrease due to losses
Solution Approach 1:
The invention optimizes crystallization parameters including temperature, solvent composition, and cooling rate to maximize both purity and yield. By carefully controlling these parameters, the process achieves high purity (at most 20 meq/kg free acid) while minimizing meso-lactide loss in the mother liquor, maintaining high productivity
Solution Approach 2:
The invention applies partial crystallization where not all meso-lactide is converted to crystals in a single step. By controlling the extent of crystallization, the process achieves sufficient purification while leaving some meso-lactide in solution that can be recovered in subsequent steps, thereby optimizing the balance between purity and yield
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process achieves high-purity meso-lactide with minimal formation of lactic acid and oligomers, ensuring efficient yield and meeting commercial-grade purity standards.
Implementation Method 1
subjecting a feed comprising at least 75 wt.% meso-lactide to solvent crystallisation followed by recovery of product meso-lactide, wherein the solvent crystallisation step is carried out in a solvent comprising a total of at least 80 wt.% of at least one compound selected from the group of ketones
Data Source
Figure 1
AI summary
The invention pertains to a process for purifying meso-lactide which has a free acid content of at least 30 meq/kg comprising at least one step of subjecting a feed comprising at least 75 wt.% meso-lactide to solvent crystallisation followed by recovery of product meso-lactide, wherein the solvent crystallisation step is carried in a solvent comprising a total of at least 70 wt.% of at least one compound selected from the group of ketones of the formula R1-C(=O)-R2, wherein R1 and R2 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl and ethers of the formula R3-O-R4, wherein R3 and R4 are independently selected from methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, t-butyl, and isobutyl, the solvent being present in an amount of 5-50 wt.% calculated on the total of feed and solvent, the solvent crystallisation conditions being selected such that at least 15% of the meso-lactide in the feed crystallises in solid form, wherein the solid meso lactide recovered from a final step of solvent crystallisation followed by recovery of product meso-lactide has a free acid content of at most 20 meq/kg. The process according to the invention yields meso-lactide in high purity and high yield in an efficient manner, with minimum formation of lactic acid and linear lactic acid oligomers. The invention also pertains to a process for producing purified meso-lactide and a purified L- lactide and/or D-lactide stream, and to a process for producing polylactide which comprises a step of purifying meso-lactide.