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Heating oil emulsions
Using heat to treat crude oil emulsions has four basic benefits; It reduces viscosity, increases droplets, dissolves paraffin crystals, and increases density between oil and water.
Heat reduces the viscosity of the oil
Which allows the water droplets to collide with greater force and to settle more rapidly. The chart in Fig. 1 can be used to estimate crude-oil viscosity/temperature relationships. Crude-oil viscosities vary widely, and the curves on this chart should be used only in the absence of specific data. If a crude oil’s viscosity is known at two temperatures, it can be approximated at other temperatures by drawing a straight line along those temperature/viscosity points on the chart. Viscosity that is known at one temperature can be approximated at other temperatures by drawing a straight line parallel to the curves already on the chart. If the viscosity is unknown at any temperature, the chart’s curves may be used. API Spec. 12L recommends that crude oil be heated so that its viscosity is 50 cSt for dehydration. Viscosity should be < 7 cSt for desalting.
Heat increases the droplets
Molecular movement, which helps coalescence by causing the dispersed-phase droplets to collide more frequently.
Heat might deactivate the emulsifier
e.g., dissolve paraffin crystals), or might enhance the action of treating chemicals, causing the chemical to work faster and more thoroughly to break the film around the droplets of the dispersed phase of the emulsion.
Heat also might increase the density difference between the oil and the water
Thus accelerating settling. In general, at temperatures below 180°F, adding heat will increase the density difference. Because most light oils are treated below 180°F, the effect of heat on gravity is beneficial. For heavy crudes ( < 20°API), which normally are treated above 180°F, heat might have a negative effect on the density difference. In special cases, increased heat might cause the density of water to be less than that of oil. This effect is shown in Fig. 2.
Fig. 2—Relationship of density with temperature for three crude oils.
Expense of heating well fluids
Heating well fluids is expensive. Adding heat can cause a significant loss of the lower-boiling-point hydrocarbons (light ends). This causes “shrinkage” of the oil, or loss of volume. Because the light ends are boiled off, the remaining liquid has a lower American Petroleum Institute (API) gravity and thus might have less value. Figs. 3 and 4 illustrate typical gravity and volume losses, respectively, vs. temperature for 33°API crude. The vapor leaving the oil phase can be vented to a vapor recovery system or compressed and sold with the gas. Either way, there probably will be a net income loss.
Fig. 3—API-gravity loss vs. temperature for 33o API crude oil.
Fig. 4—Percent loss by volume vs. temperature for 33o API crude oil.
The gas that is liberated when crude oil is treated also might create a problem in the treating equipment if the equipment is improperly designed. In vertical emulsion treaters and gun barrels, some liberated gas could rise through the coalescing section, creating enough turbulence and disturbance to inhibit coalescence. Perhaps more importantly, the small gas bubbles are attracted to surface-active material and, hence, to the water droplets; thus, they tend to keep the water droplets from settling and might even cause them to be discharged with the oil.
Fuel is required to provide heat, and so the cost of fuel must be considered. If the oil is above inlet-fluid temperature when it is discharged from the treating unit, it can be flowed through a heat exchanger with the incoming well fluid to transfer the heat to the cooler incoming well fluid. This will minimize evaporation losses and reduce fuel cost; however, it also will increase the vapor pressure of the crude, which might be limited by contract.
If properly done, heating an emulsion can greatly benefit water separation. Using less heat and a little more chemical, agitation, and/or settling space can obtain the most economical emulsion treatment.
In some geographic areas, emulsion-heating requirements vary in accordance with daily and/or seasonal atmospheric temperatures. Emulsions usually are more difficult to treat when the air is cooler—e.g., at night, during a rain, or in winter months. On the other hand, treatment, especially heating, might not be required in the warmer summer months. When the treating problem is seasonal, some emulsions can be resolved successfully by adding more chemical demulsifiers during winter months. The proper economic balance of heat and chemicals requires evaluation.
- API Spec. 12L, Specification for Vertical and Horizontal Emulsion Treaters, fourth edition, reaffirmed 2000. 1994. Washington, DC: API.
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