Thermodynamic and kinetic determinants of citric‑acid crystallization: upstream influences, process integration and industrial design implications

Abstract

Citric acid is produced industrially through Aspergillus niger fermentation, but the efficiency and predictability of its downstream crystallization are strongly governed by upstream decisions that determine the composition and thermodynamic character of the mother liquor. This report develops an integrated framework linking substrate choice, fermentation conditions, in‑situ product‑removal strategies and recovery pathways to the crystallization behavior of citric acid. Each upstream operation establishes a distinct ionic and organic load that shifts activity coefficients, alters solubility and defines realistic supersaturation trajectory. Fundamental thermodynamic principles (chemical potential, supersaturation, solubility, and hydration–dehydration equilibria) are combined with kinetic concepts such as Classical Nucleation Theory (CNT), Ostwald’s Rule of Stages, the terrace–step–kink mechanism and interfacial poisoning to explain observed solid‑form outcomes and sensitivity to feed variability. The citric‑acid-water T-RH stability diagram provides the basis for controlling monohydrate-anhydrate transitions during crystallization, drying and storage. The analysis shows that crystallizer performance, to include form selection, metastable‑zone width (MSZW), nucleation suppression and particle‑size control, depends on how well upstream composition has been stabilized. High‑ionic‑strength or impurity‑rich feeds require slower supersaturation generation and higher seed masses, whereas polished or membrane‑conditioned feeds support tighter control and more reproducible growth. This integrated perspective connects process decisions across the entire manufacturing chain and provides a comprehensive basis for improving solid‑form control and consistency between batches in industrial citric‑acid production.