The roles of contact time and contact pressure on the coalescence of water droplets suspended in concentrated bitumen solutions

The coalescence of water drops in diluted bitumen solutions has been previously studied via micropipette experiments in which water drops are brought to contact, and the onset of drop coalescence is evaluated as a function of bitumen dilution, type of solvent, etc. Due to the opacity of concentrated bitumen solutions, most of these studies have been conducted in highly diluted systems. Under those conditions (typically solvent/bitumen or S/B mass ratios greater than 4), asphaltenes tend to form skins at the oil/water interface that prevent coalescence altogether. In 'naphthenic' froth treatment (NFT) process, however, S/B ratios of 0.5-2 are often used. To study S/B ratios relevant to NFT operations, we developed a Hele-Shaw version of the micropipette aspiration experiment, in which drops held on micropipettes are sandwiched to disks by two hydrophobically-treated plates spaced less than a few hundred microns apart. The resulting thin layer of bitumen is optically transparent in brightfield microscopy, allowing us to execute micropipette contacting experiments at industrially relevant bitumen dilutions. Another departure from almost all the literature in the past is that we have measured the time, tc, required to observe coalescence from the moment two disk-shaped drops are contacted under an imposed contact pressure, P. tc was observed to decrease when P, pH or bitumen dilution was increased. Deasphalting bitumen also led to a significant decrease in tc, while increasing the aging time of the interface from 15 to 60 min dramatically increased tc. The coalescence time was found to obey an exponential relationship with P, and was explained to be a result of an activated process related to the formation of a bridge of water across the thin film of bitumen in the contact zone between the drops. The tc-P correlations measured in this work provide the foundation for designing coalescers that coarsen the drop size distribution in water-in-bitumen emulsions under given conditions of bitumen concentration and water pH.