Oil and Water Separators

Oil and Water Separators

Conventional Gravity Separators (API)

Conventional gravity separators use the force of gravity to separate oil and other contaminants from water. This works because oil is less dense or lighter than water. Therefore, when oily water is put into a separator, the oil rises to the top, solids and sediment sink to the bottom and the water is left in the middle. The oil is then skimmed off the top and disposed of, while water is free to flow through an outlet on the side of the tank for further processing or to be released into the environment. Solids and the sediment layer are scraped off the bottom into a sludge pump for disposal.

The API separator is a gravity separation device designed using Stokes' law principles that define the rise velocity of oil droplets based on their density, size and water properties.

The design of the separator is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. Based on that design criterion, most of the suspended solids will settle to the bottom of the separator as a sediment layer, the oil will rise to top of the separator, and the wastewater will be the middle layer between the oil on top and the solids on the bottom. The API Design Standards, when correctly applied, make adjustments to the geometry, design and size of the separator beyond simple Stokes Law principles. This includes allowances for water flow entrance and exit turbulence losses as well as other factors. API Specification 421 requires a minimum length to width ratio of 5:1 and minimum depth-to-width ratio of 0.3:0.5.

Typically in operation of API separators the oil layer, which may contain entrained water and attached suspended solids, is continually skimmed off. This removed oily layer may be re-processing to recover valuable products, or disposed of. The heavier bottom sediment layer is removed by a chain and flight scraper (or similar device) and a sludge pump.

Parallel Plate Gravity Separators

In this type of water oil separator system, corrugated plates made of an oleophilic material are placed parallel to one another in a tank with a space of about 6-12 millimeters between them. Water and oil are then allowed to flow through the tank and in between these plates. The oil in the water is attracted to and gets stuck on the plates.

As more and more oil droplets attach, they form larger droplets and eventually rise to the surface where they are collected and discharged into a used oil tank. The water, now free of oil, is allowed to continue flowing through the tank and into a clean water outlet, where it can then be released for other uses.

Plate separators or Coalescing Plate Separators are similar to API separators, in that they are based on Stokes Law principles, but include inclined plate assemblies (also known as parallel packs). The underside of each parallel plate provides more surfaces for suspended oil droplets to coalesce into larger globules. Coalescing plate separators may not be effective in situation where water chemicals or suspended solids restrict or prevent oil droplets coalesce. In operation it is intended that sediment will slide down the topside of each parallel plate, however in many practical situations the sediment can adhere to the plates requiring periodic removal and cleaning.

Such separators still depend upon the specific gravity between the suspended oil and the water. However, the parallel plates can enhance the degree of oil-water separation for oil droplets above 50 micron in size. Alternatively parallel plate separators are added to the design of API Separators and require less space than a conventional API separator to achieve a similar degree of separation.

Hydrocyclone Water Oil Separator System

A hydrocyclone (often referred to by the shortened form cyclone) is a device to classify, separate or sort particles in a liquid suspension based on the ratio of their centripetal force to fluid resistance. This ratio is high for dense (where separation by density is required) and coarse (where separation by size is required) particles, and low for light and fine particles. Hydrocyclones also find application in the separation of liquids of different densities.

A different description: A hydrocyclone is a mechanical device designed to reduce or increase the concentration of a dispersed phase, solid, liquid or gas of different density, by means of centripetal forces or centrifugal forces within a vortex.

The mixture is injected into the hydrocyclone in such a way as to create the vortex and, depending upon the relative densities of the two phases, the centrifugal acceleration will cause the dispersed phase to move away from or towards the central core of the vortex.

A hydrocyclone will normally have a cylindrical section at the top where liquid is being fed tangentially, and a conical base. The angle, and hence length of the conical section, plays a role in determining operating characteristics.

A hydrocyclone water oil separator system uses an active vortex created when oily water is injected at an angle into the tank. The centrifugal force accelerates as it creates a cyclone or spiral shape. Since water is denser than oil, it is pushed to the outer edges and allowed to fall towards an outlet that leads to further processing or into the environment.

The lighter oil is forced towards the centre and upwards through the top into a used oil tank. This type of separator is appropriate for highly efficient oil removal from water, such as uses in factories and highly contaminated waters.

Flotation Separators

The last of the water oil separator systems is commonly referred to as DAF (dissolved air flotation), uses dissolved air to increase the rate at which the oil rises. In the separator oily water is mixed with water saturated with dissolved air, creating small air bubbles. The oil in the water attaches to these bubbles and are brought to the surface of the separator where the oil, sludge, and solids are skimmed off and held in another tank. Any solids or sediment to heavy to rise sinks to the bottom where it is collected in a drain. The DAF wastewater then passes through an outlet, ready to be used again or filtered more.

Dissolved air flotation (DAF) is a water treatment process that clarifies wastewaters (or other waters) by the removal of suspended matter such as oil or solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank basin. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it may then be removed by a skimming device.

The feed water to the DAF float tank is often (but not always) dosed with a coagulant (such as ferric chloride or aluminium sulphate) to coagulate the colloidal particles and/or a flocculant to conglomerate the particles into bigger clusters.

A portion of the clarified effluent water leaving the DAF tank is pumped into a small pressure vessel (called the air drum) into which compressed air is also introduced. This results in saturating the pressurized effluent water with air. The air-saturated water stream is recycled to the front of the float tank and flows through a pressure reduction valve just as it enters the front of the float tank, which results in the air being released in the form of tiny bubbles. Bubbles form at nucleation sites on the surface of the suspended particles, adhering to the particles. As more bubbles form, the lift from the bubbles eventually overcomes the force of gravity. This causes the suspended matter to float to the surface where it forms a froth layer which is then removed by a skimmer. The froth-free water exits the float tank as the clarified effluent from the DAF unit.

Some DAF unit designs utilize parallel plate packing material (e.g. lamellas) to provide more separation surface and therefore to enhance the separation efficiency of the unit. DAF systems can be categorized as circular (more efficient) and rectangular (more residence time). The former type requires just 3 minutes. A particular circular DAF system is called "Zero speed", allowing quite water status then highest performances; a typical example is an Easyfloat 2K DAF system. The rectangular type requires 20 to 30 minutes. One of the bigger advantages of the circular type is its spiral scoop.