Emulsion layer growth in continuous oil–water separation

The relationship between emulsion coalescence and emulsion layer growth was investigated using batch, continuous, and decay experiments on model emulsion systems. Model oil-in-water emulsions were prepared at 45 °C from water containing NEO-10 surfactant and an organic phase consisting of 50 vol% toluene and 50 vol% n-heptane. Model water-in-oil emulsions were prepared at 60 °C from water containing AOT and Tween 80 and an organic phase (1.5 g/L asphaltenes in a solution of 50% toluene and 50% n-heptane). In a batch experiment, the emulsion was allowed to coalesce and the height of the free water and oil layers were measured over time. The bulk coalescence rate was determined from the change in height over time. Continuous experiments were performed where the emulsion was continuously fed to a separator until a steady state condition was reached. Decay experiments involved shutting of the feed and measuring the decrease in emulsion layer height over time.It was demonstrated that the bulk coalescence rate is proportional to the volume of the emulsion layer consistent with droplet–droplet coalescence. Using a model based on this assumption, the steady state height in continuous experiments was predicted from the initial coalescence rate determined from a decay experiment. Interestingly, for oil-in-water emulsions, the coalescence rate was found to decrease exponentially over time possibly due to surfactant accumulation in thinning continuous phase films. Hence, compaction of the emulsion layer can slow that rate of coalescence. For water-in-oil emulsions, the coalescence rate was constant for approximately 30 min and then decreased to zero, suggesting that the aging of asphaltene films may play a significant role in emulsion layer growth. The effect of flow rate of the emulsified feed, separator geometry, and wall effects were also studied.

Photograph and micrograph of accumulating emulsion layer in the separation of a model oil-in-water emulsion.Figure optionsDownload as PowerPoint slideHighlights
► Both oil-in-water and water-in-oil emulsions coalesced via droplet-droplet interaction; coalescence was proportional to the emulsion volume.
► Steady state emulsion layer height in steady state separation was predicted using batch data at the same dispersed phase volume fraction.
► The bulk coalescence rate in oil-in-water emulsions decreased exponentially as the emulsion layer is compacted possibly due to local surfactant accumulation.
► The coalescence rate in water-in-oil emulsions stabilized by asphaltenes was initially constant and then decreased to zero, possibly due to irreversible film formation.