Crystallization in multiphase systems is one of the major problems that is encountered during oil processing and oil transporting processes. In the petroleum industry oil and water are produced and transported together and this causes mixing of oil and water which may result in different flow behaviour. This study is concerned with the crystallization of calcium carbonate in oil-in-water emulsions.
The crystallization process is studied by following the kinetics in which different CaCO3 polymorphic phases are developed on a surface and in the bulk solution using a combination of X-Ray Diffraction and Scanning Electron Microscopy. Kinetic measurements combined with microscopic observations have been monitored with time and found as a useful way for characterizing CaCO3 scale formation on a surface and in the bulk solution. This new approach enables a better understanding of the different stages of CaCO3 scale formation in single and multiphase systems. This includes the formation of different polymorphs, the kinetics of growth of each polymorph, the order of polymorphic appearance, the mechanisms of transformation of the metastable phase to the stable one and the prediction of the time required for a complete transformation to a stable phase.
The results suggest the importance of the early period of the crystallization process. This period of time represents the nucleation and the growth of the metastable phase which exhibits simultaneously with the transformation to the stable phase. The solution supersaturation ratio is found to reduce by about 96% during this period.
It is found that the formation of the more stable phase mainly depends on the formation and the transformation of the less stable phase. The results show the existence of an inhibition effect when adding an oil phase to the surface crystallization through retarding both the nucleation and the dissolution processes of the metastable phase. The oil phase impacts the dissolution process by changing the mechanism of transformation from surface-controlled to diffusion-controlled. This affects the kinetics of formation of the stable phase and results in a reduction in the overall surface deposition.
The inhibition effect has also been shown to take place in the bulk solution where less nucleation has been detected for homogeneous type nucleation. At the same time the oil provides an extra surface where heterogeneous nucleation takes place.
The presence of oil was shown to have more impact on surface crystallization kinetics than the bulk precipitation kinetics which showed similar bulk behaviour between systems with and without oil at 30o C and 60o C. The results suggest the existence of two simultaneous processes for the precipitation kinetics; the first is the rapid dissolution of the metastable phase followed by the re-crystallization to the stable phase according to a surface-controlled mechanism. This process takes place and ceases rapidly. The second process is the slow dissolution of the metastable phase and this is followed by the re-crystallization of the stable phase according to a dissolution-controlled mechanism. This process is very slow and proceeds for a much longer time.
The existence of two processes in the bulk solution compared to only one process for the crystallization on a surface results in much faster kinetics on the surface than in the bulk solution. The results also suggest the similarity in the mechanism of formation and transformation of CaCO3 scale between surface and bulk solution
