Mechanistic Model of Coaxial Microfiltration for Semi-Synthetic Metalworking Fluid Microemulsions

This paper discusses the development of a mechanistic model that describes the rate of flow reduction (i.e., flux decline) for a semi-synthetic metalworking fluid (MWF) during the application of microfiltration for extended MWF reuse and recycling. For the transport of unused semi-synthetic MWF through microfiltration membranes ranging in pore size from 0.2 to 5.0 micrometers, Environmental Scanning Electron Microscopy (ESEM) and Confocal Scanning Laser Microscopy (CSLM) are used to identify three interdependent and sequential mechanisms of flux decline: pore constriction, pore blockage, and surface film retardation. These mechanisms are modeled together mathematically as a four-parameter model that quantitatively describes flux decline versus time for semi-synthetic MWF as a function of membrane pore size and transmembrane pressure. The four parameters of the model are the rate constants for pore constriction and pore blocking, the steady-state effective internal pore constriction, and the specific surface film resistance. Independent experimental observations confirmed both the existence of the three stages of flux decline, and the physical interpretation of the model parameters across the pore size range of polycarbonate membranes investigated. It was also found that the mechanistic model fit experimental flux data over time with low error and that the magnitudes and trends of the model parameters closely fit direct microscopic observations and expected behavior of fouled membrane surfaces. Consequently, the mechanistic model enables quantitative modeling of microscopic phenomena at the membrane surface using only macroscale flux observations. This will enable a better understanding of the relationship between MWF formulation and membrane transport in novel MWF recycling applications.