American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
Shell has used statistically designed bottle tests for a number of years to optimize crude oil dehydration systems. A brief review of important parimeters in crude oil dehydration is reviewed plus the statistical design method and the method of data evaluation is discussed.
Oil production in the U.S. usually produces normal emulsions (water-in-oil) with some reverse emulsions (oil-in-water) found associate with thermal and chemical floods. In the strict thermodynamic sense, the coalescence of droplets in a stable system is accompanied by an increase in Gibbs free energy. Coalescence of a stable system will not occur spontaneously as only a decrease in free energy will occur naturally. Conversely, an unstable emulsion would separate spontaneously if given sufficient time. However this time might be weeks, so in a practical sense it will be considered as being stable. Since coalescence causes a decrease in the interfacial area and a small increase in interfacial tension which decreases the free energy, emulsions are probably never truly stable in the thermodynamic sense but east as a pseudoequilibrium. pseudoequilibrium. Droplet size of emulsions is very important in oil-water separators as stated in Stokes Law. They are also important in differentiating between an emulsion and a dispersion. Usually emulsion droplets are in the .5 to 50 micron range with an average in the 20 micron range. An arbitrary division would be to class everything above the 100-150 micron range as dispersions.
Emulsions have an emulsifying agent or surface-active agent absorbed at the oil-water interface usually oriented so a monomolecular film is formed. It is these agents which interfere in dehydration. These agents are usually indigenous to the crude oil system. The emulsifying agents include asphaltenes, resins, organic acids, heavy paraffins, various metallic salts, silts, clays, scale and bacteria wastes. Whether a normal or reverse emulsion is formed depends upon whether the agent is more water compatible (hydrophilic) or oil compatible (lipophilic or hydrophobic). According to Bancroft's rule' of thumb, the continuous phase is the one in which the emulsifier is the most soluble.
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