Separation Using Coalescers

A coalescer achieves separation of an oily phase from water on the basis of density differences between the two fluids. These systems obviously work best with non-emulsified oils. Applications historically have been in the oil and gas industry, and hence the most famous oil/water separator is the API separator (API being the abbreviation for the American Petroleum Institute).

Modern-day designs are more sophisticated than the early, simple separators of a few decades ago that were introduced by the petroleum industry. Commercial systems are comprised of cylindrical vessels, rectangular vessels, above and underground installations. Figure 29 shows an underground system advertised by Tank Direct.

In this diagram the key features are: A - Diffusion baffle: this serves four roles. First to dissipate the velocity head, thereby improving the overall hydraulic characteristics of the separator. Next, to direct incoming flow downward and outward maximizing the use of the separator volume. Third, to reduce flow turbulence and to distribute the flow evenly over the separator's cross-sectional area. Finally, to isolate inlet turbulence from the rest of the separator. B- Internal chambers: In the sediment chamber, heavy solids settle out, and concentrated slugs of oil rise to the surface. As the oily water passes through the parallel corrugated plate coalescer (an inclined arrangement of parallel corrugated plates) the oil rises and coalesces into sheets on the underside of the plate. The oil creeps up the plate surface, and breaks loose at the top in the form of large globules. The globules then rise rapidly to the surface of the separation chamber where the separated oil accumulates. The effluent flows downward to the outlet downcomer, where it is discharged by gravity displacement from the lower region of the separator. C -PetroScreen™: This part of the design focuses on improved separation efficiency. It is essentially a polypropylene coalescer (a bundle of oleophilic (i.e., oil extracting) fibers). The oleophilic fibers are layered from coarse to fine, and are encapsulated within a solid framework. These are used to intercept droplets of oil that are too small to be removed by the parallel corrugated plate coalescer section. D- Monitoring systems: Various monitoring and control instruments are included in a system for level sensing and pumpout control purposes.

There are variations of coalescer designs, each achieving different degrees of separation depending upon the application and properties of the influent. One fairly popular design used primarily in the oil and gas industry is a so-called deoiler cyclone vessel and cyclone system. These systems take advantage of the hydroclone (or cyclone) principle of separationMore correctly stated, the cyclonic action results in an increase in the oil droplet size, enabling an more efficient separation of the phases. Deoiling cylcones have steep droplet-cut size curves, like the one illustrated in Figure 30. The typical performance curve for a high efficiency cyclone separator (or hydroclone) shows that a small increase in the droplet size from 5 to 10 microns typically increases the separation from 15 % to over 90 %. The inlet chamber of a conventional deoiling hydroclone is usually the largest chamber in the coalescer vessel, and has a residence time up to about 20 seconds. Some commercial units employ specially designed low-intensity pre-coalescing internals and inlet vanes that take advantage of this residence time by optimizing the flow distribution. This enhances the coalescence of droplets and enables a pre-coalescing stage.

Underground Oil Water Separator Design"/>
Figure 29. Underground installation of an oil/water separator. Source: Tanks Direct from their website:

Figure 30. Cut size curve for a deoiling hydroclone.

An example of a pre-coalescer unit is illustrated in Figure 31. In general, the technology largely relies on many years of experience and empiricism. Before investing in a system, it is wise to run batch tests and perhaps pilot tests using vendor facilities. Units may range in size, complexity of internals, configuration (e.g., rectangular, slant rib designs that borrow concepts from the classical Lamella separator, to cylindrical (the classical design case). There are varying degrees of claims for removal efficiency. Cost can vary greatly with coalescers, depending not only on throughput requirements and add-on controls and monitors, but with construction as well. Because of strict environmental regulations tied to underground and above ground tanks, double wall vessel construction is often needed. Double wall vessels are normally constructed in several ways.

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