Lactide Process
An industrially demonstrated flowsheet integrating reactive distillation and divided-wall distillation to suppress racemization, deliver high-purity L-lactide, and enable high-molecular-weight PLA.
Process Overview
Step 1 — Polycondensation & First Reactive Distillation
Lactic acid is polycondensed under tightly controlled temperature and residence time. A forced-circulation reactive distillation system (expandable to two stages) enhances heat/mass transfer, drives the forward reaction, and continuously removes water. Oligomers with number-average molecular weight of 450–600 are obtained. Keeping residence time < 8 hours suppresses racemization, minimizing D-lactic acid and meso-lactide.
Step 2 — Depolymerization & Second Reactive Distillation
Oligomer depolymerization is integrated with crude lactide pre-purification in a second reactive distillation column designed as a divided-wall column. This lowers acid value, reduces impurities (lactic acid, lactyl lactate), and cuts energy use versus conventional multi-step separations.
Step 3 — Melt Crystallization Refining
Intermediate-purity lactide is refined by melt crystallization. Plate crystallizers (economic choice) achieve L-lactide ≥ 99.5% via two stages; suspension or falling-film crystallizers can push to ≥ 99.8% at higher CAPEX.
Step 4 — Fractionation & By-product Handling
Mother liquors go to a third distillation tower: top product enriches M-lactide (recovered); side/heart cuts recycle as purified L-lactide to the crystallization loop; D-lactide-rich streams are purged to protect optical purity.
Step 5 — PLA Polymerization & Devolatilization
A multi-stage static-mixer reactor train enables precise temperature staging for consistent Mw build. A high-vacuum static-extrusion devolatilizer strips unreacted lactide and light oligomers, boosting purity and recycling valuable monomer.