44526-AC9
Drop Size Distributions of Oil-in-Water Emulsions During High Pressure Production and Storage
Oil-in-water emulsions are ubiquitous in petroleum-based consumer products, including cosmetics, creams, lotions and agricultural products. Emulsions are also encountered throughout the petroleum industry with applications at many stages of petroleum recovery, transportation, and processing. In petroleum recovery applications, emulsion formation at the well-head is undesirable while emulsions are critical for enhanced oil recovery as drilling muds and fluids. Heavy crude oils typically have viscosities in the range of a few hundred to several thousand centipoise, and therefore they cannot be transported economically in conventional pipelines without reducing their viscosity. Emulsifying heavy oils with an aqueous solution significantly reduces the viscosity, allowing for lower pumping power requirements and more economical transportation. The emulsion drop size distribution has a significant effect on emulsion viscosity, and hence pipeline frictional losses and pumping power, as well as emulsion stability. For petroleum applications, emulsions with small drops are desirable to promote stability and minimize viscosity. Coarse emulsions are typically formed by low-shear mixing in an agitated vessel, followed by a high-shear process to create smaller drops. The prepared
emulsions are then stored under static conditions before introduction into the pipeline. Drop breakage and coalescence during high-shear preparation have a substantial impacts on the drop size distribution. While drop breakage under laminar conditions has been extensively studied, predictive models for drop breakage and coalescence under turbulent conditions are lacking. Due the lack of suitable models, emulsified products are currently developed by combining a broad knowledge of previous product formulations with empirical scientific experimentation to create new formulations with desired properties.
We developed a population balance equation (PBE) model for pure drop breakage processes from high pressure homogenization experiments and investigated model extensibility over a range of emulsion formulation and homogenizer operating variables. To test the assumption of negligible drop coalescence, we performed a recoalescence test in which an emulsion prepared with multiple homogenizer passes at a high pressure was subjected to homogenization at a lower pressure. The drop volume distribution was not significantly changed by low pressure homogenization where the breakage rate was expected to decrease from that at the higher pressure, suggesting that the coalescence rate was neglibibly small. Adjustable parameters in the mechanistic breakage functions were estimated from measured drop volume distributions by constrained nonlinear least-squares optimization. Satisfactory prediction of measured bimodal distributions was achieved by the incorporation of two different breakage functions that accounted for large drop breakage due to turbulent shear and for small drop breakage due to collisions between drops and turbulent eddies. We used computational fluid dynamics (CFD) to better understand the homogenizer flow regimes and the relevant drop breakage mechanisms. Our CFD results showed the presence of both high and low shear regions, providing theoretical support for multiple breakage mechanisms.
Model extensibility to different emulsion compositions and homogenizer pressures was investigated by comparing model predictions generated with the base case parameters to drop volume distributions measured under the different conditions. The PBE model satisfactorily accounted for changes in the dispersed phase volume fraction and the interfacial tension with the base case parameters. By contrast, improved predictions for the continuous phase viscosity and multiple formulation variables were obtained through re-estimation of the model parameters using multiple data sets in which the associated variables were systematically varied. The model was not able to satisfactorily predict drop volume distributions resulting from homogenizer pressure changes. The CFD results suggest that the inability of the model to account for pressure changes may be atttributable to strong pressure heterogeneities across the homogenizer domain, a result in direct contradiction to the model assumption of a well-mixed, homogeneous system. Despite this limitation, we concluded that PBE models of drop breakage can be used to predict the effects of emulsion formulation variables on drop volume distributions and have the potential for guiding experimental efforts aimed at the analysis and design of pipeline emulsions.
We believe that our PBE modeling approach represents a necessary first step towards addressing the design problem for oil-in-water emulsions prepared with high-shear processes. By utilizing drop breakage functions that depend explicitly on formulation and processing variables, the PBE model has the capability to reproduce trends in the drop volume distribution that result when these variable are changed. Consequently, the model can be used to predict the effects of different formulation variables on emulsion properties before performing the associated experiments. This capability will not only allow novel emulsions to be identified but also will enable unacceptable solutions to be eliminated more rapidly and/or with less experimental effort.
