Page 354 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

322 Applied Process Design for Chemical and Petrochemical Plants

In this case foam can be re-entered into the mixture by mixed region develops between the impellers. Mul-
this action, and solids or liquids added will enter the tiple axial-How impellers have less tendency in this
impeller rather rapidly. The deeper the liquid above the regard than do multiple radial flow impellers. As vis-
baffle the greater the rotating action. Bottom swirling cosity of one fluid increases, the flow pattern
action allows the segregation of heavy solid particles [21]. becomes more radial, thus the tendency for zone
In general, some sidewall baffles are desirable in most mixing increases. When axial flow impellers are too
mixing operations. Baffles allow the system to absorb rel- close together on one shaft, they tend to behave as
atively large amounts of power which is needed for devel- a single larger impeller, with decrease in power draw
opment of mixing turbulence, and still avoid vortex and but with a decrease in pumping capacity also.
swirling action, that is, the tank fluid stays under control. E. For radial flow turbine, locate 1.50 to 2.00 apart,
This is indicated typically by Figure 5-15 for the flat blade with liquid coverage over the top impeller of mini-
impeller in area CD (or the similar region on other fig- mum l'2 to 3.00, depending on surface motion
ures). Use 4 vertical baffles, Figures 5-23B, C and 5-34. desired.
A large number of mixing problems operate in this F. Polyethylene polymer autoclave type reactors usually
region and can be easily interpreted. Here large amounts contain 8 to 120 impellers of the same or different
of power can be added to the system for greater volume circulation designs on a single shaft to ensure rapid
and/ or shear forces by simply increasing the speed. This total homogeneous mixing in the reactor, which con-
is one reason for variable speed drives. However, in the tains a gas at about 30,000 psi and, hence, the fluid
portion of Figure 5-15 lettered EF as well as AB, BC, and is neither a gas or a liquid because the densities are
BE, the power changes exponentially with the speed. The about the same.
advantage of using baffles is that the flow pattern is fixed
to follow the portion CD of the curves. Reference [30] suggests the depth distances in a tank
When fluid swirling is prominent, it is difficult or over which the turbines are effective mixers:
impossible to reproduce the same mass transfer effect in
any other size vessel, and hence cannot be reproduced by Liquid Viscosity, Gp Turbine Diameters (Vertical Spacing)
geometric similarity [21]. <5,000 3.0-4.0
Mixing operations are not limited to a flat bottom tank, 5,000 2.5
but in general, for each system there is a tank configura- 15,000 2.0
tion which is optimum. The difference between them is 25,000 1.7
small in many instances. 50,000 1.4
A tank bottom of dished or spherical shapes is usually
better than a flat bottom as it requires less horsepower for The turbine should be positioned to give 35% of the liq-
the system. uid below and 65% above the turbine. For depths exceed-
ing these values, multiple impellers should be used. For
Impeller Location and Spacing: Top Center Entering total liquid depths less than the effective height of a tur-
bine impeller, the turbine should be located 0.35 Z from
For base tank dimensions, Z/T, (height/tank diame- the tank bottom, unless there is an overriding restraint.
ter) equal to 1.0 [29]: The top entering mixer units are better for producing
flow at constant power than the side entering units [29].
A. For blending and solid suspension, use ZIT for min- The radial flow impellers require more horsepower when
imum power at about 0.6 to 0.7, but recognize that compared to the marine impellers.
this may not be the most economical. lf a propeller is located quite close to the bottom of a
B. One or two impellers can operate on one shaft at liq- tank, the flow becomes radial like that of the flat blade tur-
uid coverages (Z-IC) over the impeller ofat least 3D, bine. In a properly baffled system the propeller flow is
see Figure 5-34. axial. When dynamic similarity is accomplished, the sys-
C. Draw-off of the mixed liquids influences the loca- tems are similar [21]. For a first approximation, placing
tion of the impeller. For blending, the preferred the impeller at Ji of liquid height off the bottom is good.
impeller location in a Z/T = 1.0 Vessel is at the mid- When possible it is best to withdraw the fluid from a
point of liquid depth for a continuous flow process, propeller-mixed system directly below the propeller. This
but may not be for a batch system. allows removal of all solids and mixed liquids.
D. It is important to avoid zone mixing, when each For a turbine the preferred location for withdrawal of
impeller on a shaft (of more than one impeller) mixed fluid is at the side opposite the turbine impeller. A
mixes its own zone, and then a stagnant or less- study of the flow pattern of the system should be made to