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MIXING / AGITATION














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-MIXING AND AGITATION

1                 Mild agitation is obtained by circulating the liquid with an impeller at superficial velocities of 0.1-0.2 ft/sec, and intense agitation at 0.7-1.0ft/sec.

                    2. Intensities of agitation with impellers in baffled tanks are measured by power input, HP/1000 gal, and impeller tip speeds: 0peration  HP /1000 gal Tip speed

                    (ft/min) Blending 0.2-0.5 Homogeneous reaction 0.5-1.5 7.5-10 Reaction with heat transfer  1.5-5.0 10-15 Liquid-liquid mixtures  5 15-20 Liquid-gas mixtures  5-10 15-20 Slurries 10

2                 Proportions of a stirred tank relative to the diameter D: liquid level = D; turbine impeller diameter = D/3; impeller level above bottom = D/3; impeller blade width = D/15; four vertical baffles with width = D/10.

3                 Propellers are made a maximum of 18 in., turbine impellers to 9 ft.

4                 Gas bubbles sparged at the bottom of the vessel will result in mild agitation at a superficial gas velocity of 1 ft/min, severe agitation at 4 ft/min.

5                 Suspension of solids with a settling velocity of 0.03 ft/sec is accomplished with either turbine or propeller impellers, but when the settling velocity is above 0.15 ft/sec intense agitation with a propeller is needed.

6                 Power to drive a mixture of a gas and a liquid can be 25-50% less than the power to drive the liquid alone.

7                 In-line blenders are adequate when a second or two contact time is sufficient, with power inputs of 0.1-0.2 HP/gal.
















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