Chemical demulsifiers sold under various trade names, such as Tretolite ™, Visco™, and Breaxit™, are highly useful in resolving emulsions. Demulsifiers act to neutralize the effect of emulsifying agents. Typically, they are surface active agents and thus their excessive use can decrease the surface tension of water droplets and actually create more stable emulsions. There are four important actions required of a demulsifier:
1. Strong attraction to the oil-water interface.
4. Solid wetting.
When these actions are present, they promote the separation of oil and water. The demulsifier must have the ability to migrate rapidly through the oil phase to the droplet interface, where it must compete with the more concentrated emulsifying agent. The demulsifier must also have an attraction for droplets with a similar condition. In this way large clusters of droplets gather which, under a microscope, appear like bunches of fish eggs. The oil will take on a bright appearance since small droplets are no longer present to scatter the light rays. At this point the emulsifier film is still continuous. If the emulsifier is weak, the flocculation force may be enough to cause coalescence. This is not true in most cases and the demulsifier must therefore neutralize the emulsifier and promote a rupture of the droplet interface film. This is the opener that causes coalescence. With the emulsion in a flocculated condition the film rupture results in rapid growth of water-drop size.
The manner in which the demulsifier neutralizes the emulsifier depends upon the type of emulsifiers. Iron sulfides, clays, and drilling muds can be water wet causing them to leave the interface and be diffused into the water droplet. Paraffins and asphaltenes could be dissolved or altered to make their films less viscous so they will flow out of the way on collision or could be made oil wet so they will be dispersed in the oil. It would be unusual if one chemical structure could produce all four desirable actions. A blend of compounds is therefore used to achieve the right balance of activity.
The demulsifier selection should be made with the process system in mind. If the treating process is a settling tank, a relatively slow-acting compound can be applied with good results. On the other hand, if the system is a chemelectric process where some of the flocculation and coalescing action is accomplished by an electric field, there is need for a quick-acting compound, but not one that must complete the droplet building action.
Emulsion-breaking chemicals are most commonly tested with bottle tests, which involve mixing various chemicals with samples of the emulsion and observing the results. Such tests are effective in eliminating some chemicals and selecting those that appear to be more efficient. Bottle tests also provide an estimate of the amount of chemical required. Bottle tests should be performed on a representative sample as soon as the sample is obtained because of the possible detrimental effects of aging. These tests should also be performed at conditions that are as close to field treating conditions as possible. Synthetic water should not be used in place of produced water in bottle tests because the produced water may have very different properties, and it may contain impurities that are not present in the synthetic water.
While candidate chemicals and approximate dosages can be determined in bottle tests, the dynamic nature of the actual flowing system requires that several candidates be field-tested. In actual conditions, the emulsion undergoes shearing through control valves, coalescence in flow through pipes, and changes to the emulsion that occur inside the treating vessel as a result of inlet diverters, water wash sections, etc. Static bottle tests cannot model these dynamic conditions. As field conditions change, the chemical requirements can change.
If the process is modified, e.g., very low rates on electrostatic units, the chemical requirement can change. Seasonal changes bring paraffininduced emulsion problems. Workovers contribute to solids content, which alters emulsion stability. So, no matter how satisfactory a demulsifier is at one point in time, it cannot be assumed that it will always be satisfactory over the life of the field.
Categories: Emulsion Treating Teory | Tags: Coalescence, demulsifier, emulsion, settling tank, workover | Leave a comment