Wang, Ziming; (2025) Advanced CFD modelling for predicting liquid-particle mass transfer coefficients in agitated vessels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Estimating liquid-particle mass transfer coefficients in agitated vessels is crucial for optimizing mass-transfer-limited processes in several industrial applications, in particular pharmaceutical ones. Existing experimental and theoretical approaches often suffer from various limitations, such as theoretical inadequacy, case-specific applicability and failure when the process is scaled up. To overcome these challenges, this research proposes a novel method. Mass transfer strongly depends on fluid dynamics. In agitated vessels, multiphase turbulence is often present. To account for it, various turbulent multifluid models exist, but their applicability to the flow of liquid-particle suspensions in agitated vessels remains unexamined. I critically analyse these models; in particular, the Fox model, derived rigorously via Reynolds-averaging the Eulerian-Eulerian balance equations for mass, linear momentum and energy, and other models that instead lack theoretical justification but are widely adopted. My analysis validates the applicability of the Fox model for the system of interest but shows that the discrepancies between this model and the less rigorous ones impact the results minimally, explaining why these models yield reasonably accurate predictions. Additionally, industrial applications generally involve many particles. Therefore, an accurate correlation for local liquid-particle mass transfer coefficients in dense suspensions is required. Assuming local homogeneity, I develop a correlation using scaling and order-of-magnitude analysis. Unlike many available correlations constrained to specific Reynolds number ranges, my formulation is valid across multiple orders of magnitude and achieves a relative error below 30% against experimental data. Finally, I propose a novel method to estimate the globally averaged mass transfer coefficient in an agitated vessel by averaging (over the vessel) the local mass transfer coefficient profile obtained from the validated turbulent multifluid model coupled with my new local mass transfer coefficient correlation. Compared with conventional approaches, this method improves the agreement with experimental data significantly.

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