Coalescing plate separators

Coalescing plate separators allow water ballast to be discharged in-shore and on the high seas without risk to the environment and without detectable oil slicks. In standard form they reduce the oil content of discharges to better than 15 ppm, and all oil droplets discharged are smaller than 20 pm and are thus quickly degraded by seawater. The recovered oil normally has a water content of less than 3% and can be used as furnace fuel without further treatment, or can be reprocessed. 

Parallel and corrugated plate separators are improved types of oil-water separators with tilted plates installed at an angle of 45°. This increases the collection area many times while decreasing the overall size of the unit accordingly. As the water flows through the separator, the oil droplets coalesce on the underside of the plates and travel upward to where the oil is collected. 

Because of the coalescing action, these separators can separate oil droplets as small as 0.006 mm (6 microns) in diameter and produce effluent-free oil concentrations as low as 10 mg/L [27], There is a broad range of applications for tilted-plate separators. As little space is required, they can be installed to polish the effluent from existing API separators that are either overloaded or improperly designed, or they can be installed parallel with existing separators, reducing the hydraulic load and enhancing the oil removal capacity of the system. 

Vessel Internals. Most vessel internals were discussed in last month s auricle on two-phase separation. Two common internals not previously discussed are coalescing plates and methods for removing solids or sand. 

Paraffin. Separator operation can be adversely affected by paraffin accumulation. Coalescing plates in the liquid section and mesh pad mist extractors in the gas section are particularly prone to plugging by paraffin. If paraffin is an actual or potential problem, use of plate type or centrifugal mist extractors should be considered. Manways, handholes and nozzles should be provided to allow steam, solvent or other types of cleaning of separator internals. 

Parallel-plate interceptor. The first form of plate coalescer was the parallel-plate interceptor (PPI). This involved installing a series of plates parallel to the longitudinal axis of an API separator (a horizontal, rectangular cross-section skimmer). When viewed perpendicular to the axis of flow, the plates form a "V so that the oil sheet migrates up the underside of the coalescing plate and to the sides Sediments migrate toward the middle and down to the bottom of the separator where they are removed. 

The coalescent principle becomes the significant principle for small-oil-droplet removal. When the droplets coalesce, they do not form floes as the solid particles can, but coalesce into larger droplets. Interfacial or surface tension of the liquid tends to make the droplets from spherical shapes, which follows the assumption of Stokes s law mentioned previously. With coalescent principle, the Stokes s law can be applied. The coalescent technique widely used in oil droplet removal is the plate separator. 

The plate separator is a major configuration of gravity separators. This equipment uses differences in densities between oils and water as a principle separation technique. The module can be a single plate or three plates or may contain a nest of parallel plates, which is typically 10-20 mm wide. The operational principles of plate separators are to allow oil droplets rise to a plate surface where coalescence can occur and to form larger oil droplets, which are easier to separate. The performance of the plate separator can be enhanced by selecting appropriate plate designs or by the use of a coalescer. 

Separators are often made with baffles or other interior devices that increase the residence time and thus the degree of separation. The parallel plate separator is a special model of gravity separator. Many parallel plates are placed perpendicular to the flow, creating areas of low water turbulence where drops of oil can re-coalesce from the water and rise to the surface. 

Fig. 7.6 Collisions of a water and an ethanol drop (a) head-on collision with coalescence and separation of one satellite (We =20,X = 0) (b) reflexive separation with formation of a small satellite due to Marangoni forces We = 38.5, X = 0.02) (c) stretching separation with formation of three satellite droplets (We = 82.3, X = 0.82). Droplets move from right to left the water droplet coming from above is marked with w [45] (With kind permission from Springer Science+Busi-ness Media Experiments in Fluids [45], Plates 3, 5 6, Copyright Springer-Verlag 2005)...

Lamella separators or plate separators where the oil is collected directly by the lower surface of oblique plates and then brought up to the surface. The plates have a dual function. They define very short routes for die droplets and they have a coalescence effect. Both of these functions are due to the close interlamella spacing. 

The emulsions in produced water will become oil in the form of dispersed droplets after the emulsion film is broken. The droplets will coalesce to yield an oily film that can be separated from the produced water using gravity settling devices such as skim vessels, coalescers, and plate separators. However, small droplets require excessive gravity settling time, so flotation cells or acceleration enhanced methods such as hydrocyclones and centrifuges are used. Equipment selection is based on the inlet oil s droplet diameter and concentration. 

The oil/water/sediment coalescing separator exhibits the same advantages and disadvantages as plate separators. The one additional... 

An existing low-pressure separator, skimmer, or plate separator can be retrofitted with a coalescing section. 

Another type of gravity separator used for small amounts of oily water, the oil interceptor, is widely used both offshore and onshore. These devices work by encouraging oil particles to coalesce on the surface of plates. Once bigger oil droplets are formed they tend to float to the surface of the water faster and can be skimmed off. A corrugated plate interceptor (CPI) is shown below and demonstrates the principle involved. However there are many varieties available. Plate interceptors can typically reduce oil content to 50-150 ppm. 

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. 

Cross Flow Interceptors employ sloping corrugated plates to provide some coalescing effect and to reduce the effective separtion path. They olso provide effective separation of solids which accumulate at the bottom of the plate pock and can be removed by periodic blow-down. They con ochieve very high separation efficiencies but only when designed for known droplet sizes. 

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