It can be shown mathematically that an efficient design must have a high gas induction rate, a small diameter induced gas bubble, and relatively large mixing zone. The design of the nozzle or rotor, and of the internal baffles, is thus critical to the unit’s efficiency.
As measured in actual field tests, these units operate on a constant percent removal basis. Within normal ranges their oil removal efficiency is independent of inlet concentration or oil droplet diameter.
The nozzles, rotors, and baffles for these units are patented designs. Field experiments indicate that these designs can be expected to have removal efficiencies of about 50% per cell. Each cell is designed for approximately one minute retention time to allow the gas bubbles to break free of the liquid and form the froth at the surface. Each manufacturer gives the dimensions of this standard units and the maximum flow rate based on his criteria.
From Equation 7-16, a three-cell unit can be expected to have an overall efficiency of 87% and a four-cell unit an efficiency of 94%. An efficiency of 90% is usually used for design. The unit’s actual efficiency will depend on many factors that cannot be controlled or predicted in laboratory or field tests.
There are many different proprietary designs of dispersed gas units. All require a means to introduce gas into the flow stream, a mixing region where the gas can contact the oil droplets, a separation (flotation) region that allows the gas bubbles to rise to the surface, and a means to skim the froth. To operate efficiently, the unit must generate a large number of small gas bubbles. Tests indicate that bubble size decreases with increasing salinity. At salinities above 3%, bubble size appears to remain constant, but oil recovery often continues to improve.
Gas bubble/oil droplet attachment can be enhanced with the use of polyelectrolyte chemicals. These flotation aid chemicals can also be used to cause bubble/solid attachments, and thus flotation units can be used to remove solids as well.
Oil removal is dependent to some extent on oil droplet size. Flotation has very little effect on oil droplets that are smaller in diameter than 2 to 5 microns. Thus, it is important to avoid subjecting the influent to large shear forces (e.g., level control valves) immediately upstream of the unit. It is best to separate control devices from the unit by long lengths of piping (at least 300 diameters) to allow pipe coalescence to increase droplet diameter before flotation is attempted. Above 10 to 20 microns, the size of the oil droplet does not appear to affect oil recovery efficiency, and thus elaborate inlet coalescing devices are not needed.
Skimmed oily water volumes are typically 2 to 5% of the machine’s rated capacity and can be as high as 10% when there is a surge of water flow into the unit. Since skimmed fluid volume is a function of weir length exposure over time, operation of the unit at less than design capacity increases the water residence time but does not decrease the skimmed fluid volumes.