The flotation process for removing suspended materials from produced waters is widely used in the petroleum industry to improve the water quality. Flotation involves the introduction of gas or air into the water to create a foam or froth which will scrub the solids from the liquid and carry them out with the foam. The primary difference in flotation cells is the two ways of introducing the gas. Gas introduced into the bottom of a cell and dispersed into a water in the form of very fine bubbles will rise to the top of that cell and carry with it suspended solids and entrained oil. Additional frothing can be created by injecting surfactants or other water clarification compounds into the water to assist in removing the undesirable solids and oil from the water.
Mechanically-induced flotation cells (Wemco, Quadricell, Petrolite, US Filter) use mechanical rotors to put the gas into each cell. The hydraulically induced flotation cells (MultiSep, VeriSep, MonoSep) use water that is pumped into each cell. Flotation cells can be either single or multi-cell units. They can utilize dissolved gases entrained in the water or they can rely on field gas which will be induced into the water. The units are equipped with paddles for skimming the solids laden foam from the vessels.
In most units the water level is controlled inside the vessel and the wiper paddles are not used. The oil lockers and the wiper paddles should be washed down once a week to prevent polymer build up and to prevent reintroducing oil back into the unit.
The Wemco Unit was designed to remove impurities from mined ores, and over the years it has been adapted for use in the petroleum industry. The unit is designed to utilize field gas and any gas dissolved in the water to remove entrained oil from the produced water. The unit consists of a series of four separate compartments, each equipped with an external motor for driving the star rotor used to disperse the incoming gas into the water. The star rotor revolves at a high rate of speed and breaks the incoming gas into very fine bubbles which contacts the oil entrained in the water and causes it to foam. The foam, containing oil and solids, being lighter than water, rises to the top of the cell where it is removed by mechanical means.
The water settles to the bottom of the compartment, flows into the next compartment and the process is repeated. As the water flows from compartment to compartment the oil content of the water becomes less and less, and by the time the water exits the final compartment it should be free of oil. Each compartment provides additional retention time and opportunity for removal of the oil.
Foaming agents or surfactants can be used to enhance the foaming action and provide for increased oil removal.
Single cell flotation cells operate in much the same manner as the Wemco Unit with the exception of the use of the rotor for dispersing the gas into the water. The gas is jetted into the bottom of the cell where it disperses in the water.
The use of flotation cells for removal of oil wet iron sulfide from waters has been tried for many years, but the approach has not produced the desired results.