Page 320 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Chapter

5

Mixing of Liquids

Mixing of fluids is necessary in many chemical process- Mixing applications often include one or more of the
es. It may include mixing of liquid with liquid, gas with liq- following [26]:
uid, or solids with liquid. Agitation of these fluid masses
does not necessarily imply any significant amount of actu- • bulk mixing
al intimate and homogeneous distribution of the fluids or • chemical reaction
particles, and for this reason mixing requires a definition • heat transfer
of degree and/or purpose to properly define the desired • mass transfer
state of the system. • phase interaction (suspending/dispersing)

In order for the mixing operation tu accomplish the
overall process requirement of this step in the system, it is Mixing is accomplished by the rotating action of an
necessary to establish which factors are significant for a impeller in the continuous fluid. This action shears the
mixing device that provides the required end result for fluid, setting up eddies which move through the body of
the industrial application. Because the "art" of mixing is the system. In general the fluid motion involves (a) the
still not an exact science, it is really not practical for the mass of the fluid over large distances and (b) the small
design engineer to expect to totally design a mixer, that is, scale eddy motion or turbulence which moves the fluid
over short distances [21, 15].
define its type, diameter, operating speed, and shape/
type of impeller. Rather it is reasonable for the engineer The size and shape of the vessel to be used for the mix-
to understand the mechanical and processing essentials ing operation is important in achieving the desired mix-
and anticipated performance when dealing technically ing results; therefore, this aspect of the design must
with a mixing equipment representative. For standard accompany the actual mechanical mixer design/size
nomenclature See references [ 4 7, 48]. The technical per- selection.
formance and economics of various designs often need to The performance of mixers involves high volume or
be examined in order to make a good, cost-effective selec- flow operations, or high head or shear operations. Many
tion of the device that will be the "heart" of this step in a mixing processes utilize a combination of these two,
process. In some situations, particularly chemical reaction although, surprisingly enough there are many which can
and/or mass transfer, it may be necessary to conduct test have only high volume or only high head. Some opera-
work to develop a sound basis for a larger scale industrial tions listed in decreasing order of high volume require-
unit. In other cases, the needed data mav be drawn from ments include: blending, heat transfer, solids suspension,
the public technical literature or a man�facturer's appli- solids dissolving, gas dispersion, liquid-liquid dispersion
cation files (see References [l, 4, 10, 11, 19, 20, 24, 25, 26, (immiscible), solid dispersion (high viscosity).
27, 28, 29, 31, 33, 42,43, 44, 45, 46, 47, 48]). Impeller types usually used with mixing and listed in
decreasing order of high volume ability (hence in increas-
Mixer performance is often related in terms of the ing order of high head ability or requirement) are: pad-
fluid velocity during agitation, total pumping capacity dle, turbine, propeller, sawtooth impeller or propeller,
(flow of the fluid in the system) generated by one cut-out impeller disc (no blades), colloid mill.
impeller, and the total flow in the tank ( or sometimes as Figures 5-1 and 5-2 are useful as guides in the general
blending time or a solids-suspension criterion) [25]. selection of mixing impellers and associated vessels. Note

288