Page 341 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mixing of Liquids 309

used, or request the mixer manufacturer to recommend B. Decrease Design Problems
the type best suited to the service. 1. Reduce required shaft diameter and length, while
maintaining complete mixing effectiveness.
Flow Patterns 2. Limit or eliminate the need for submerged or
internal guide bearings.
The pattern of the fluid motion is a function of the
fluid system, impeller, vessel configuration, and location Entrainment
of the impeller in the fluid system relative to the vessel
walls and/ or bottom. The patterns illustrated in Figures 5- Entrainment is an important element in the m1x111g
23A-5-23K indicate that almost any pattern can be estab- operation and involves incorporation of low velocity fluid
lished. provided the particular impeller type is located in into the mass of the fluid stream or jet issuing from a
the proper position. This is easier to accomplish in some source such as a mixing impeller. The axial flow from a
systems than others. propeller under proper physical conditions serves as a cir-
The use of vertical side wall baffles usually destroys the cular cross-section jet to produce mixing by turbulence
rotary and swirling motion in vertical tanks. This also can and entrainment. The flat-blade turbine issues a jet for
be accomplished to a degree by setting the mixer off cen- entrainment at the top and bottom areas of the ring [2].
ter. These baffles should be Yio to Yi2 of the width or diam- It is significant to estimate the relative amount of liquid
eter of the tank. Six baffles generally give slightly better involved due to entrainment, as this helps to describe the
performance than four; although four is the usual num- effectiveness of the operation.
ber, with three not being as good for most situations. From a propeller, the entrainment by circular jet is [9]:

Draft Tubes
(5- 42)
The application of draft tubes as related to various mix-
ing operations is shown in Figures 5-231 and 5-24A-5-241.
The draft tubes are basically a tube or shell around the where Q., = volume entrained, cu ft/sec
shaft of the mixer including the usual axial impeller, X = distance from impeller source, not to exceed 100
which allows a special or top-to-bottom fixed flow pattern jet diameters, ft
0
to be set up in the fluid system. The size and location of D = diameter of jet at origin, ft
the tube are related to both the mechanical and mixing
performance characteristics as well as peculiar problems This relation is sufficiently accurate for large scale
of the system. Usually they are used to ensure a mixing design.
1
flow pattern that cannot or will not develop in the system. The maximum Q/P i 3 for a circular jet is at X = 17.1
Weber gives the following points for draft tubes [23]: D 0 (Refs. [9, 21]) or in other words the optimum jet ori-
With a draft tube inserted in a tank, no sidewall baffles gin diameter is 1/ 17.1 of the distance desired for effective
are required, and, the flow into the axial impeller mount- entrainment. Since the entrainment efficiency does not
ed inside the tube is flooded to give a uniform and high fall off too rapidly, it is not necessary to use only the ratio
flow pattern into the inlet to the impeller. The upflow in given, but rather to stay in close proximity, say ± 25-35
the annulus around the tube has sufficient velocity to percent. Large diameter jet streams are more effective for
keep particles in suspension, if necessary. the same power than small streams [ 17]. Data on flat
blade turbines has not been fully evaluated.
A. Increase mixing efficiency
Batch or Continuous Mixing
l. Prevent short circuiting of fluid, define a specific
path. Often pilot plant or research data for developing a
2. Improve heat transfer coefficient by forcing flow process are obtained on a batch operation. Later, a con-
past coil surfaces. tinuous process will usually prove that smaller equipment
3. Provide more complete reaction in a gas-liquid can be used and that the operation will be more econom-
system by recirculation of unreacted gases. ical. Normally batch mixing requires 10%-25% more
4. Minimize areas of inadequate turbulence in vessel. power than continuous [29] for stable conditions; howev-
5. Accentuate the direct mechanical shearing action er, the reaction time for continuous flow is always longer
of the mixing impeller upon the fluid. than the react.ion time for batch flow, but the practical
6. Amplify mixing action by effectively increasing result may show batch time cycle is increased by filling,
the ratio of mixer to container diameter. (lal continued 011 page 312)