CHEMICAL ENGINEERING

SECTION 7

Liquid Mixing

PROBLEM 7.1

A reaction is to be carried out in an agitated vessel. Pilot plant experiments were performed
under fully turbulent conditions in a tank 0.6 m in diameter, fitted with baffles and
provided with a flat-bladed turbine. It was found that satisfactory mixing was obtained
at a rotor speed of 4 Hz, when the power consumption was 0.15 kW and the Reynolds
number was 160,000. What should be the rotor speed in order to retain the same mixing
performance if the linear scale of the equipment is increased 6 times? What will be the
power consumption and the Reynolds number?

Solution

See Volume 1, Example 7.3.

PROBLEM 7.2

A three-bladed propeller is used to mix a fluid in the laminar region. The stirrer is 0.3 m
in diameter and is rotated at 1.5 Hz. Due to corrosion, the propeller has to be replaced
by a flat two-bladed paddle, 0.75 m in diameter. If the same motor is used, at what speed
should the paddle rotate?

Solution

For mixing in the laminar region, the power requirement is:

PDk^0 N^2 D^3 (equation 7.17)

wherek^0 D1964 for a propeller and 1748 for a flat paddle.
Thus, fora propeller0.3 m in diameter rotating at 1.5 Hz:

PD 1964 ð 1. 52 ð 0. 32 D 119 .3W

and fora paddle, 0.75 m in diameter using the same motor:

119. 3 D 1748 N^2 ð 0. 753 andND 0 .403 Hz(24 rpm)

103