Behavior of oil droplets at the membrane surface during crossflow microfiltration of oil–water emulsions

A fundamental study of microfiltration membrane fouling by emulsified oil was conducted using a combination of real-time visualization, force balance on a droplet, and permeate flux analysis. The model 0.1% v/v hexadecane-in-water emulsions contained sodium dodecyl sulfate (0.1mM, 0.4mM, or 0.8mM) to regulate interfacial tension. Direct Observation Through the Membrane tests with Anopore (dpore=0.2µm) and track-etch (dpore=5µm) membranes revealed three characteristic stages of membrane fouling: (1) droplet attachment and clustering, (2) droplet deformation, and (3) droplet coalescence. In qualitative agreement with visualization results, the force balance predicted that droplets ≲36–40µm would remain pinned at dpore=5µm pores while larger droplets would be swept off the surface by the crossflow drag. In a separate set of constant pressure crossflow filtration tests with track-etch membranes, the average oil rejection was ≥98% while the permeate flux decreased to a pseudo-steady-state ~10% of the initial value. The results indicate that membrane fouling by emulsified oil is controlled by droplet coalescence and crossflow shear: the transport of oil to the membrane surface by the permeate flow is balanced by the shear-induced removal of the droplets that coalesce to exceed a critical size. [Display omitted] •DOTM method is used to visualize oil droplets at the membrane surface.•Fouling stages include droplet clustering, deformation, and coalescence.•Droplets that coalesce to exceed a critical size are removed by shear.•Force balance on a droplet predicts the critical droplet size.•Droplet coalescence and crossflow shear control membrane fouling by oil droplets.