(57) In order to dissolve a gas such as oxygen in a liquid, typically aqueous, a stream
of liquid is taken from tank 2 by a pump 10 and pressurised thereby. Oxygen is introduced
into the stream via a conduit 18 upstream of a venturi 20. Turbulence created as the
stream passes through the venturi 20 helps to disperse the oxygen as fine bubbles
in the stream. The dispersion is then conducted to a sparge pipe 14 and introduced
into the main volume 4 of liquid in the tank 2. The bubbles are thereby sheared into
very small bubbles that readily dissolve in or are consumed by the volume 4 of liquid.
Some of the gas in the dispersion is separated therefrom at a relatively downstream
position and drawn along a pipe 26 back into the stream at a relatively upstream position
by the action of the stream passing through the venturi 20. A valve 28 is operable
to regulate the rate at which gas is so recycled.
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