Simulating the behaviour of skim-milk during ultrafiltration
AffiliationChemical and Biomolecular Engineering
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2021-04-26. This item is currently available to University of Melbourne staff and students only, login required.
© 2019 Dr. Malavika Haribabu
Many commercial dairy products such as cheese and concentrated milk proteins are produced via ultrafiltration of skim-milk. During skim-milk ultrafiltration, a permeate stream comprising lactose, water and other minerals is removed from the system leaving behind the retentate, rich in whey proteins and casein micelles. The retained proteins result in a concentration boundary layer near the membrane. Solids build-up within this boundary layer results in resistance to the permeate flow in addition to the membrane resistance, causing a flux decline. Solids build-up also increases the contamination risk in the retained product. In order to overcome these problems, periodic cleaning of the membrane is performed which increases the overall operational cost. Numerical models that predict the behaviour of a filtration process have gained importance in the development of membrane modules and optimizing operating conditions. While there have been numerous studies based on the single-phase models, for the non-dilute volume fractions relevant to ultrafiltration applications, this framework lacks rigorous theoretical justification. A multiphase framework is better equipped to model solid-solid and solid-fluid interactions. In this thesis, a mixture model based on the multiphase framework is developed to predict the behaviour of dairy filtration. In this model, continuity and momentum equations are solved for the mixture, in addition to a solid phase continuity equation that employs a relative velocity between the phases, calculated from the differences in forces acting on each phase. This thesis details the implementation of the mixture model for typical filtration channels with large aspect ratios via geometric scaling. Mathematical similarity is achieved between the actual and the scaled filtration domains. The mixture model is then used to simulate the ultrafiltration of whey proteins in both dead-end and crossflow filtration modes. Simulations that employ hard-sphere models for the osmotic pressure and drag coefficient produce results that are consistent with relevant dead-end and crossflow filtration experiments, with the use of independently evaluated parameters such as voluminosity and high-density gel pore length scale. A deformation model is then proposed to capture the particle deformation occurring during ultrafiltration in the regions with high concentrations. This model, based on the Young's modulus of the material and initial particle size, is able to predict the filtration of deformable particle systems such as soft latex and casein micelles dispersions in the dead-end mode. The largest discrepancy between simulations and experiments occurs for skim milk during crossflow ultrafiltration - possible reasons for this discrepancy are discussed and presented as avenues for future work. The mixture model and the deformation model presented in this thesis are important in developing a comprehensive model to predict the behaviour of skim-milk ultrafiltration. This model is robust and valid across a wide range of operating conditions for filtration of proteins and latex dispersions. Though the present model is capable of modelling only one dispersed phase (either whey proteins or casein micelles), the model can be extended in the future for the study of two or more dispersed phases, relevant to the multicomponent nature of real milk.
Keywordscomputational fluid dynamics; ultrafiltration; multiphase modelling; skim milk; colloidal filtration
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