Dissolved gas flotation (DGF) systems are used for a variety of applications throughout the world. The process floats solids, oils and other contaminants to the surface of liquids. Once on the surface these contaminants are skimmed off and removed from the liquids. Oil and gas production facilities have used flotation systems to remove oil and solids from their produced and processed water (

wastewater Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...

) for many years. The relative density of candle wax is 0.93, hence objects made of wax float on water.

Process description

The keys to good separation are both gravity and the creation of millions of very small bubbles. Based on

Stokes' law In fluid dynamics, Stokes' law gives the frictional force – also called drag force – exerted on spherical objects moving at very small Reynolds numbers in a viscous fluid. It was derived by George Gabriel Stokes in 1851 by solving the S ...

, the size of the oil droplet and density of the droplet will affect the rate of rise to the surface. The larger and lighter the droplet, the faster it will rise to the surface. By attaching a small gas bubble to an oil droplet, the density of the droplet decreases, which increases the rate at which it will rise to the surface. Therefore, the smaller the gas bubbles created the smaller the oil droplet floated to the surface. Efficient flotation systems need to create as many small bubbles as possible. The method in which the bubbles are introduced into the water stream and retention time are also important factors. The average retention time for a vertical unit is typically 4 to 5 minutes and 5 to 6 minutes for a horizontal unit.

DGF pump

The

impeller An impeller, or impellor, is a driven rotor used to increase the pressure and flow of a fluid. It is the opposite of a turbine, which extracts energy from, and reduces the pressure of, a flowing fluid. Strictly speaking, propellers are a sub-clas ...

in a DGF pump is designed with dual sides. One side is designed to drive the liquid like a normal

centrifugal pump Centrifugal pumps are used to transport fluids by the Energy transformation, conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are ...

and the other side is designed to draw

vapor In physics, a vapor (American English) or vapour (Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R ...

into the pump and mix it with the liquid. In addition to the new impeller, a special seal was invented to extend the life of the pump. With these innovations the pump creates a sub-atmospheric pressure region within the pump's seal chamber. As the impeller draws in the vapor it is mixed with the liquid being pumped and compressed into micro-fine bubbles. Because of the close tolerance between the backvanes of the impeller and the backplate of the pump the vapor is sheared into fine bubbles and then they are compressed in the sub-atmospheric pressure region of the pump. These fine bubbles become dissolved into the liquid within the

volute A volute is a spiral, scroll-like ornament that forms the basis of the Ionic order, found in the capital of the Ionic column. It was later incorporated into Corinthian order and Composite column capitals. Four are normally to be found on an ...

of the pump. The result of this process provides similar size bubbles to a

dissolved air flotation 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 an ...

system. The backpressure

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

on the discharge piping can regulate the bubble size in a DGF pump. The bubble size ranges from 50 down to 1

micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * Micrometre The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights ...

or less.

References

Design and operation of Dissolved-Gas Flotation Equipment for the Treatment Of Oilfield Produced Brines
M.C. Sport, Shell Oil Company, ''Journal of Petroleum Technology'', Volume 22, Number 8, August 1970. * {{Reflist Oil refining Flotation processes Waste treatment technology

Process description

DGF pump

See also

References