Page 355 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mixing of Liquids 323

be certain that the proper fluid mixture ts sampled or etc., will be the same in the larger system. Once it can be
removed. determined whether the process result is controlled,
Side entering mixers (usually propellers) as shown in more or less, by flow or turbulence, the scale-up can be
Figure 5-23G are placed 18-24 inches above a flat tank more intelligently analyzed with the use of the relations
floor with the shaft horizontal, and at a 10° horizontal previously presented; noting that head, H, is the turbu-
angle with a vertical plane through the tank centerline. lence indicator. Generally speaking, for fixed power input
This equipment is used for fluids up to 500 centipoise the relative proportions of flow and turbulence in a given
[21). For fluids from 500-5000 c.p., the mixer is usually system vary as indicated in Table 5-5.
top entering.
Viewing from behind the mixer, for a clockwise rota- Table 5-5
tion of the impeller, the mixer must be angled to the left Expected Proportions of Flow and Turbulence in a
at the 7°-10° angle to the vertical plane noted above. Mixing System
Side-entering mixers usually run at 280 to 420 rpm, com-
pared to 30 to 100 rpm for top entering units. The side- Relative Percent Percent Relative
entering units are primarily used for large blending tanks Impeller Diameter Flow Turbulence Speed
of gasoline, oils, chemicals (watch the shaft seal through Large ............. ... High, > 50 Low, <50 Low
Medium .............. About 50
the tank here), paper stock blending, and similar large Small. .. .......... ... Low,< ,50 About 50 Medium
systems, but they do not necessarily have to be limited to High, > 50 High
large systems.
The proper placement of the impeller for specific
applications is necessary for good mixing performance; Operations such as blending, solids-suspension, dis-
therefore, a thorough discussion with a mixing company solving, heat transfer and liquid-liquid extraction are typ-
specialist is useful. ical of systems requiring high flow relative to turbulence,
In gas dispersion systems the gas inlet should normally while gas-liquid reactions and some liquid-liquid contact-
be directly below the impeller inlet, or on a circular pat- ing require high turbulence relative to flow. The case of
tern at the periphery of the impeller. (1) 100% of suspension-requires head to keep particles
In order to achieve uniform solid suspension or pick- suspended and (2) 100% uniformity of distribution of
up of solid particles off the bottom, the upward velocities particles-requires head for suspension plus flow for dis-
of the fluid streams in all portions of the vessel must tribution.
exceed the terminal settling velocity of the particular par- In the case of heat transfer using coils in the tank, it is
ticles. This can be determined by small scale tests. generally necessary to increase the horsepower per unit
For solids which float, or which are added from the volume from small scale to full size equipment.
top, a vortex action helps t.o draw the material down into For some scale-up situations, particularly when the
the impeller. Often a draft tube is used to serve as a sue- change in size of the system is not great and the fluid
t.ion entrance for the impeller. Uniform suspensions are properties remain unchanged, the use of horsepower per
difficult to maintain when the tank liquid height is much unit of liquid volume is an acceptable scale-up tool.
greater than the tank diameter. The impeller is normally The horsepower per unit volume is fairly constant with
placed )i of liquid depth off the bottom [21). increasing tank volume, actually falling off slightly at large
volumes. Therefore, a usually safe scale-up is to maintain
Process Results a constant HP /volume.
Bates [8] describes the handling of process results for
The effect on the process of a change in operation of reaction completion, gas absorption, phase distribution
the mixer system (impeller, baffles, etc.) is the final mea- when related to power, as shown in the log-log plot of Fig-
surement of performance. Thus, operations such as ure 5-35.
biending, uniform particle suspension, reaction, gas The slope of the line is significant and serves as a guide
absorption, etc., may be acceptable under one physical to the type of mixing mechanism required. The irregular
system and not so to the same degree under a slightly line to the left of point U indicates non-uniformity of tank
modified one. The ratio per unit volume on scale-up must contents. At U, homogeneity is accomplished. The System
be determined experimentally. C line indicates no change is required in horsepower in
Generally as system size increases the impeller flow per order to achieve better or different results. This would be
fixed power input will increase faster than will the turbu- typical of a blending operation. For System B or any sys-
lence of the system. Even the same degree of turbulence tem between C and B where the slope of the line is
is no insurance that the rates of mixing, mass transfer, between O and 0.1, the act.ion is only slightly influenced by