Fouling of aluminum oxide microfiltration membranes by semi-synthetic metal working fluids

Microfiltration is a membrane-based technology that has been shown to reduce contaminants in semi-synthetic metalworking fluid (MWF) streams and extend the useful life of the fluid. The greatest challenge to more extensive use of microfiltration in the machining industry is the loss of productivity through the membrane becoming clogged, that is, membrane fouling. This research seeks to identify and model the mechanisms causing membrane fouling in aluminum oxide microfiltration membranes. It also seeks to use the knowledge gained about the mechanisms to mitigate fouling through the reformulation of a semi-synthetic MWF while maintaining its usefulness, i.e. cooling and lubrication.;Through experimental microfiltration tests using a commercial semi-synthetic MWF it was determined that membrane fouling was caused by MWF particles aggregating and then depositing within the membrane pores. When the aggregates are smaller than the pores this process is called partial pore blocking and when the aggregates are larger than the pores it is referred to as complete pore blocking. The aggregates were found to be created in the MWF feed stream through the action of microemulsion particle impaction.;Modeling efforts on two scales were developed to investigate these fouling mechanisms. A probabilistic model was developed to show membrane level flux decline as a function of individual pores being partially blocked, blocked, and unblocked. The model includes a time-dependent particle size distribution to take into account particle aggregation within the feed stream. The influence of partial blocking is shown in the ability of particles smaller than membrane pores to still cause flux decline. The second model was developed at the individual pore level to show the progression of partial blocking from an unblocked pore to a completely blocked pore. Computational fluid dynamic simulations were conducted with microemulsion aggregates passing through and becoming stuck in a modeled tortuous pore structure with multiple inlets and outlets, as seen in the aluminum oxide membranes used in industry. The simulations show that as aggregates become lodged in the pore the flux declines until the pore is eventually sealed off through the collective work of multiple aggregates.;The modeled membrane fouling mechanisms were validated through the reformulation of a semi-synthetic MWF designed to have greater microemulsion stability. It was experimentally verified that the new fluid did not see significant aggregation. Accordingly this fluid did not show evidence of significant flux decline. Functionality tests showed no difference in cooling and lubrication between the reformulated fluid and a commercial semi-synthetic MWF.