Dispersed Gas Units

In dispersed gas units gas bubbles are dispersed in the total stream either by the use of an inductor device or by a vortex set up by mechanical rotors. Figure 7-14 shows a schematic cross section of a unit that employs a hydraulic eductor. Clean water from the effluent is pumped to a recirculation header (E) that feeds a series of venturo eductors (B). Water flowing through the eductor sucks gas from the vapor space (A) that is released at the nozzle (G) as a jet of small bubbles. The bubbles rise causing flotation in the chamber (C) forming a froth (D) that is skimmed with a mechanical device at (F).

Dispersed gas flotation unit with eduction.

Hydraulic eductor units are available with one, three, or four cells. These devices use less power and less gas than mechanical rotor units. Gas/water ratios are typically less than 10 ft2/bbl at design throughput. The volume of gas dispersed in the water is not adjustable, so throughputs less than design result in higher gas/water ratios.

Figure 7-15 shows a cross section of a dispersed gas flotation cell that utilizes a mechanical rotor. The rotor creates a vortex and vacuum within  the vortex tube. Shrouds assure that the gas in the vortex mixes with and  is entrained in the water. The rotor and draft inducer causes the water to  flow as indicated by the arrows in this plane while also creating a  swirling motion. A baffle at the top directs the froth to a skimming tray as  a result of this swirling motion.

Dispersed gas flotation unit with rotor (courtesy ofPetrolite Corp.).

Most dispersed gas units contain three or four cells. Bulk water moves in series from one cell to the other by underflow baffles. Field tests have indicated that the high intensity of mixing in each cell creates the effect of plug flow of the bulk water from one cell to the next. That is, there is virtually no short circuiting or breakthrough of a part of the inlet flow to the outlet weir box.

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