Research On Crystallization Process Of L-lactide

L-lactide is an important intermediate for the synthesis of polylactic acid whichcan be used to produce biodegradable materials.The purity of L-lactide is the keyfactors to synthetize polylactic acid. At present, the researches on L-lactide are focuson the methods for synthetizing L-lactide, with fewer reports about solutioncrystallization and crystal morphology of L-lactide. Therefore, investigation oncrystallization process is important for the use and process of L-lactide and the studyof crystallization theory. Based on the research before, this dissertation has somefurther research on crystallization process of L-lactide.The crystal structure and morphology is primary to understanding and researchon the crystallization process. Single crystal of Form A and Form B of L-lactide wasprepared by solvent evaporation, and Form B is a new crystal form. The crystals wereanalyzed by single crystal XRD, and the cell parameters and space group data wereacquired. The crystal habit of Form A was simulated and predicted using BFDHmodel and AE model by Materials Studio software. In comparison with theexperimental result, AE model has the better adaption. Considering crystallizationmethod and different solvents for the habit variation of L-lactide, some experimentshave been done. The result showed that crystallization method and different solventscan affect the particle size of L-lactide, but have little influence on the crystal habit ofthe products.Basic thermodynamic data of crystallization process can conduct thedevelopment and optimization of crystallization process. The solubility of L-lactide indifferent solvents was determined by dynamic method, and was correlated by usingdifferent equations, such as ABC model, Wilson model, NRTL model, UNIQUACmodel. The dissolution enthalpy, entropy of L-lactide and the free Gibbs energychange of L-lactide in these solvents were obtained by using the van’t Hoff equation.It’s found that the solubility of L-lactide increases with increasing temperature andcan be combined to give accurate solubility data correlation using the NRTL modeland the modified Apelblat model. It’s also proved that the dissolution process ofL-lactide in these solvents is endothermic, entropically driven and not spontaneous.The main object of this thesis is to acquire the optimal operation of the cooling crystallization process of L-lactide. Based on the crystal structure, morphology andsolubility data mentioned before, the mixture of solvent A and B were used to purifycrude L-lactide, and the effects of the ratio of the solvent, the initial concentration, thefinal crystallization temperature, the cooling rate, the addition and the amount of seed,the crystal growing time, and the stirring velocity on purity and yield of L-lactidewere investigated using off-line analysis methods(DSC, XRD) and online analyticaltechnologies(FBRM, ATR-FTIR, PVM) to optimize the crystallization process ofL-lactide. Then, the optimal technological conditions were determined.