Planes de Clase - Ismael

Plate-pack separators (PPS) were first used in the oil industry in the 1950’s. Introduced by Shell, they were the first separation device designed to give API separator capacity and performance in a compact vessel. Plate- pack separators come in many different configurations, but all are derivations of the same basic design.

This basic design used a series of parallel flat plates installed at a 45° angle to the flow direction. The plates themselves sit in a rectangular tank that has inlet distri- bution weirs to ensure adequate flow distribution to the total plate system. All designs have a weir arrangement for oil removal and a sump to collect separated solids. The plates act as a coalescing medium for oil droplets that attain a critical mass and then progress along the underside of the plates to the top of the vessel, where oil is skimmed off. Any solids in the system settle in the laminar quiescent region between the plates and fall to the base of the vessel.

As Fig. 5 shows, the flow of the oil droplets along the plate surface is countercurrent to the water flow. Hence, to maintain the coalescing action of the plates, laminar flow must be maintained to avoid stripping of the oil film. Solids that fall to the bottom of the channel can build up and block the laminar flow region. This reduces the surface area available for coalescence and hence oil removal efficiencies. The balance between having a large number of plates in a vessel to maximize coalescence and having a wide enough plate spacing to avoid solids blocking is a key consideration in the design of a PPS for solids-laden waters.

The plates themselves have been the focus of many design derivatives. Corrugated plates are popular be- cause they provide more surface for coalescence than flat plates; they also provide a concave channel for the oil to flow upward. This design has become very popular recently as improvements in the oil collection weirs and solids-handling sump accompanied the development of the corrugated plate concept.

Corrugated Plate

Interceptors (CPI) are the most common and

effective plate pack separators.

Significant derivatives of the traditional plate-pack sepa- rators are the SP-Pack Coalescer and the Vertical Tube Coalescer. The SP-Pack, a serpentine-path coalescer, uses free-flow turbulent coalescence to increase oil droplet size and thereby enhance separation efficiency. The device itself is essentially a collection of pipes joined by a series of helical swirl bends that cause the water to flow in a pattern conducive to coalescence without generating enough turbulence to shear oil droplets. The primary role of the bends is to generate this flow pattern, but evidence shows that they also provide a surface to assist coalescence.12 _{This device does not operate as a separator} but merely as a coalescer. Thus, it must be used with other equipment. This device can be used as a pretreat- ment device for flotation cells and hydrocyclones. But it probably is best used in settling tanks where it can act as a coalescer and inlet flow spreader (Fig. 6).

The Vertical Tube Coalescer operates like a plate pack, but the plates are replaced with vertical tubes. The vertical tubes offer a five-fold increase in coalescing area over traditional plate packs. In addition, because the flow is across the tubes, oil rises more easily to the separation weir (as opposed to the PPI, where the oil flow is coun- tercurrent to water flow).13 _{The tubes are made from} polypropylene and look like a loosely knit cartridge filter (Fig. 7). Because the material is oleophillic, it easily attracts small and large oil droplets. The inlet flow passes through the tubes, where oil coalesces and rises to the surface to be skimmed off. Solids and sludge fall through the tubes and are collected in a sump at the bottom of the vessel. As in the PPI, the vessel itself consists of an inlet tank arrangement, flow distribution baffles, the tube tank, and an oil collection system.

FLOTATION

Flotation has become the industry standard for de-oiling produced water because of its excellent separation capa- bilities. Conceptually, it is a simple process. Gas bubbles unite with oil droplets to reduce their apparent specific gravity, thus accelerating their rise velocity and separa- tion. Practically, flotation occurs in a vessel that provides Flotation has been the

industry standard for oil removal for many years.

bubble generation, a contact zone for the gas and oil, retention time for the separation process, and an “oily froth” removal device.

The three basic types of flotation systems are designated by their mode of bubble generation: dissolved flotation, rotor- induced flotation, and eductor-induced flotation. In the dissolved gas system, oily water and gas are com- bined under pressure, usually less than 50 psig. This mixture then flows across a pressure control valve into the atmospheric flotation tank. The pressure relief causes tiny bubbles to be generated. These bubbles then can unite with the oil droplets to initiate flotation (Fig. 8). In a rotor-induced gas system, a fine dispersion of gas bubbles is generated in the water stream by drawing gas from the surface of the flotation cell and mixing it with the water with a rotating impeller (Fig. 9). In these sys- tems, the flotation device usually operates at 6 to 12 in. of water gauge pressure, which is usually enough to pre- vent air ingress. (Air ingress is undesirable in terms of explosive mixture/safety and corrosion mitigation.) The eductor flotation cell is similar to the rotor-induced system, except that gas and water are contacted in the eductor and then fed to the flotation cell (Fig. 10).

In all three variants, the separation theory is the same. It consists of five basic steps.

1. Small gas bubbles are generated in oily water. 2. A gas bubble and an oil droplet suspended in the

water come in contact.

3. The gas bubble attaches to the oil droplet.

4. The gas bubble/oil droplet rise to the liquid surface for removal.

5. The oil-rich froth is removed.