Rising velocity of oil droplets

The principle of gravity separation is obviously based on the difference in density between water and insoluble oil. Under the laminar flow conditions that characterize industrial separators and correspond to Reynolds numbers of less than 800, the rising velocity v of an 

Theoretical sizing of an oil remover therefore requires data on  

Any increase in water temperature that can be detected decreases the viscosity and increases the velocity (Figs. 19 and 20). A difference in density of at least 0.15 is desirable so that separators do not have to he sized too big. 

With seawater (Fig. 20), the favorable effect of a higher density is offset by a higher viscosity than for soft water. 

HC density poses a problem. It is well known during production which involves one or more well-defined crudes and is still more or less predictable during tanker transportation. However, it can no longer be assessed in oily water and accidentally oily water effluents where the nature of the HC, crude, intermediate cut or finished product varies according to malfunctions and routine operations. The choice of a maximum density for use in designing an oil separator facility would hardly be reasonable if the range of specific gravities is recalled  

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Oil and Water Viscosity. These data are used in computing vertical rising velocity of oil droplets in water. It has an important bearing in deciding the layout of coalescing media inside the equipment and on relative paths of oil and water. 

Dispersed oil can consist of oil droplets ranging in size from about 0.5 pm in diameter to greater than 200 pm in diameter. The oil droplet size distribution is one of the key parameters influencing the produced water treating performance. According to Stokes law, the rising velocity of an oil droplet is proportional to the square of the droplet diameter. For equipment that operates on the principle of Stokes law, the diameter of the oil droplet has a major effect on the separation and removal of the oil droplet from the water. 

In the preseparation chamber, the less dense oil droplets rise, collide, and fuse with adjacent droplets. According to Stoke s law, the larger the diameter of a particle, the faster is its rate of rise. Thus, as small droplets coalesce to form larger droplets, their upward vertical velocity increases. Coalescing tubes or plates are designed to enhance the separation of oil-water emulsions. The emulsion free water is directed away from the tubes or plates and enters the separation section. Some separators are built with an outlet zone for the discharge of clarified water. 

The larger an oil droplet, the larger the square of its diameter, and the greater its vertical velocity. That is, the bigger the droplet size, the less time required for it to rise to a collection surface and, thus, the easier it is to treat the water. 

From this equation it can be seen that if the diameter of the oil droplet is doubled, its rising velocity increases by a factor of 4. If the oil droplet can be quadrupled in size, the rising velocity increases 16-fold... 

The combined gas-oil droplet not only has a lower density, but it also has a larger diameter which further increases the rising velocity. In this case, both factors work towards decreasing either the separation time or size of the separating vessel. 

The rising velocity for a given size oil droplet can be calculated from a variation of Stokes Law. However, since small droplets rise slowly, the surface area required is large. This area can be greatly reduced by stacking a number of shallow basins on top of each other (Fig. 2)... 

Natural gravity settlers and separators that droplets of oil rise through at a given upward velocity defined by their specific gravity. 

Vo = rising vertical velocity of the oil droplet relative to the water continuous phase, ft/sec (m/sec),... 

Figure 3.11 shows that an oil droplet entering the space between the plates will rise in accordance with Stokes law. At the same time, the oil droplet will have a forward velocity equal to the bulk water velocity. By solving for the vertical velocity needed by a particle entering at the base of the flow to reach the coalescing plate at the top of the flow, the resulting droplet diameter can be determined. 

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