Parameter Screening and Optimization for DL-Methionine Cooling Crystallization Using Response Surface Methodology

Cooling crystallization of DL-methionine requires the joint control of crystallization yield and bulk density because operating conditions that favor one response may impair the other. Here, single-factor experiments, Plackett–Burman screening, and a three-factor Box–Behnken response-surface design were combined to identify a local operating window. Stirring speed, crystallization time, and solution pH were retained for response-surface modeling. The quadratic models for crystallization yield and bulk density were significant, with R2 values of 0.9910 and 0.9892, respectively, and nonsignificant lack-of-fit terms. Multi-response optimization selected a stirring speed of approximately 332 r/min, a crystallization time of 1.47 h, and a pH of 5.55. Three validation experiments produced yields of 48.98–49.57% and bulk densities of 0.2925–0.3035 g/mL, with relative errors below 5% compared with the model predictions. X-ray diffraction showed no detectable change in the principal DL-methionine crystal phase across representative products. X-ray photoelectron spectroscopy further showed closely matched near-surface C 1s, N 1s, O 1s, and S 2p features between the raw material and the product obtained under the optimized conditions. The sodium nitroprusside assay gave total methionine contents of 99.64–99.79% for the raw material and five representative products; for the model-selected product, the colorimetric result (99.79%) agreed with the amino acid analyzer result (99.93%) to within 0.14%. The combined PB–BBD/RSM workflow therefore supports local parameter selection within the tested design space while maintaining the principal crystal phase, near-surface chemical-state profile, and total methionine content of the recovered product.