Sugar crystallization is a tightly controlled unit operation in which variations in supersaturation, temperature, and impurity load can directly affect Brix, purity, color, and crystal size. However, data-driven soft sensors often face limited labeled data, measurement noise, missing observations, and poor generalization under changing operating conditions. This study presents a synthetic digital-twin benchmark for vacuum pan crystallization and a physics-guided machine-learning framework based on gradient-boosting decision trees for simultaneous prediction of multiple quality attributes. The benchmark generates batch trajectories using simplified mass–energy balances and crystallization-rate proxies, while incorporating industrially relevant sensor noise, drift, missingness, actuator lag, feed-quality disturbances, and operating-regime shifts. Windowed process histories are transformed into process-informed descriptors and used to train gradient-boosting models for Brix, purity, color, and crystal-size prediction. Physical plausibility was assessed through process-informed features, monotonicity checks, and residual-based scoring, while robustness was evaluated using in-distribution (Test-ID) and out-of-distribution (Test-OOD) scenarios and conformal prediction intervals. The proposed models achieved strong nominal performance for Brix, purity, and crystal size, with Test-ID RMSE values of 0.440, 0.810, and 18.502, respectively. Under operating shifts, Brix and purity remained comparatively stable, whereas color and crystal size showed greater performance degradation, indicating higher sensitivity to regime changes. Conformal analysis provided informative uncertainty estimates under nominal conditions but revealed reduced coverage under Test-OOD conditions. Overall, the results demonstrate the potential of gradient-boosting soft sensors for crystallization monitoring while highlighting the need for improved uncertainty quantification and distribution-shift resilience for reliable industrial deployment.


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