Please
select an application from the list below to learn more.
Liquid
Blending
Liquid blending is defined as the mixing
of two or more miscible liquids to a measurable level of uniformity.
It is often quantitatively characterized by a “blend
time.” This is the time required from the addition or
contact of the ingredients to the point at which any sample
taken from the vessel can be expected to show a ratio of mixed
components within five percent of a sample taken from any
other point. “Crude” blending is achieved when
the composition is within ten percent. “Precise”
blending is achieved when the composition is within two percent.
As a rule of thumb, crude blending takes about half the time
of normal blending, precise blending takes about twice as
long.
Vessel Design
Considerations
When the viscosities are water-like we expect the flow to
be turbulent. Almost any vessel configuration is allowed.
It can be short and squatty or tall and skinny. It can be
round, square, rectangular, horizontal cylindrical or even
irregularly shaped. However, the vessel geometry will restrict
where the mixer(s) must be placed for effective blending.
For example, you can’t expect to be able to place one
mixer close to the end of a long horizontal cylindrical tank.
Odd shaped vessels often require more than one mixer as do
very shallow pits and sumps.
When the liquid exhibits significant
viscosity, the flow will be laminar. In this flow regime,
your choice of vessel geometry is restricted. Relatively big
impellers will be required and these will often run close
to the sides of the vessel. Round vessels are the best choice,
but square vessels with rounded corners and filleted bottoms
may sometimes be used. Tall skinny vessels can be mixed, but
as the aspect ratio increases the blend time becomes much
longer at the equivalent level of power and cost of the agitator.
Solids suspension is defined as
maintaining a slurry of solid particles in a liquid. It usually
pertains to systems in which solids would settle to the bottom
of the vessel if no agitation were provided. It can also refer
to maintaining a slurry of solids that would otherwise float.
Suspension or “complete suspension” normally means
that no particle will rest more than one second on the floor
of the vessel. “Uniform” suspension means that
the concentration of solids measured in sample taken at any
point in the vessel will be within five percent of the concentration
of a sample taken at any other point.
Vessel Design Considerations
The optimum vessel for solids suspension is round and has
an aspect ratio of about 1:1. It must either be baffled with
the mixer on center or the mixer can be angle-offset mounted.
A center mounted mixer in an unbaffled tank will not effectively
suspend solids. Offset mixers in unbaffled vessels will leave
a significant heal of solids in one sector of the vessel floor.
Angle offset mixers are generally restricted to relatively
small volumes although we have used them successfully in 25,000
gallon vessels.
It is important to recognize that solids
can easily be suspended to a height of about 85% of the vessel
diameter with a single impeller. However, even when multiple
impellers are used, it is generally impractical to suspend
solids higher than about 1.4 times the vessel diameter and
it is impossible to suspend them more than 2 times the vessel
diameter in a conventional vessel. If these constraints are
unacceptable, a draft tube must be installed in the vessel.
When the system contains very small solid
particles, their tendency to settle out is exclusively controlled
by the viscosity of the liquid. This is called Stokes settling.
Most of these systems can be treated as liquid blending applications,
and many of the restrictions above do not apply. However,
if there is a power failure and the solids are allowed to
settle, they may be difficult or impossible to re-suspend
if the restrictions above were ignored in the design of the
vessel.
Dispersion (liquid-liquid) is defined as
the production of a suspension of droplets of one liquid in
another immiscible liquid. A dispersion is usually produced
in order to enhance mass transfer of a component from one
of the liquids to the other. Dispersions are produced by high
shear mixing devices that impart a high level of turbulent
energy to the system. This energy appears as turbulence which
produces droplets of one of the liquids which is “dispersed”
into the other. However, the energy level must be controlled
so as not to produce a stable emulsion (see Emulsion). When
the mass transfer is complete, the liquids will be expected
to separate into two distinct phases which can be drawn off
for further processing.
Vessel Design Considerations
These applications almost always require a vertical round
vessel. Exceptions may be acceptable when the viscosity is
relatively high, but these are unusual. The vessel should
be selected so that the static height of the greater component
is no higher than the vessel diameter. Angle offset mounting
can be used, but we normally expect to see baffles installed
in the vessel.
Dissolution is defined as the solution
of solid (particles) in a liquid to form a homogeneous phase.
It is generally a sub-category of solids suspension. Note
that complete suspension is a sufficient specification for
most dissolution operations. When all particles are suspended,
mass transfer from the particle surface to the liquid controls
the rate, and additional mixing is not cost effective. It
is seldom practical to design a mixing system to mechanically
break solid particles as an aid to dissolution. Agitated slurries
often show evidence of some particle attrition, but this usually
has only a small effect on dissolution rate. Specialized attrition
devices are available if the application demands it.
Vessel Design Considerations
The optimum vessel for solids dissolution is round and has
an aspect ratio of about 1:1. It must either be baffled with
the mixer on center or the mixer can be angle-offset mounted.
A center mounted mixer in an unbaffled tank will not effectively
suspend solids. Offset mixers in unbaffled vessels will leave
a significant heal of solids in one sector of the vessel floor.
Angle offset mixers are generally restricted to relatively
small volumes although we have used them successfully in 25,000
gallon vessels.
Emulsification is defined as the production
of an apparently homogeneous liquid from two or more immiscible
liquids. In an emulsion one of the liquids is generally dispersed
in the other in the form of small droplets. Emulsions are
relatively stable. That is the liquids will not separate on
standing for a reasonable amount of time. What time is reasonable
is determined by the nature and purpose of the emulsified
product. Emulsions are often produced by high shear mixing
device that impart a high level of turbulent energy to the
system. The energy appears as turbulence which tears apart
suspended drops to produce droplets small enough to remain
entrained and to resist coalescence. However, emulsification
operations can be aided by “emulsifying agents,”
often surfactants, whose chemical properties enhance drop
stability.
Vessel Design
Considerations
These applications almost always require a vertical round
vessel. Exceptions may be acceptable when the viscosity is
relatively high, but these are unusual. The vessel should
be selected so that the static height of the greater component
is no higher than the vessel diameter. Angle offset mounting
can be used, but we normally expect to see baffles installed
in the vessel.
Many emulsions in particular are very
viscous. These applications often require scraped wall agitation
in conjunction with a second high speed disperser mixer in
the same vessel. Needless to say, these vessels are always
round.
Washing or leaching is defined at the
preferential solution of one or more components of a solid
(particle) leaving other components behind and undissolved.
From the mixer designer’s point of view it is very similar
to dissolution.
Vessel Design
Considerations
The optimum vessel for solids suspension is round and has
an aspect ratio of about 1:1. It must either be baffled with
the mixer on center or the mixer can be angle-offset mounted.
A center mounted mixer in an unbaffled tank will not effectively
suspend solids. Offset mixers in unbaffled vessels will leave
a significant heal of solids in one sector of the vessel floor.
Angle offset mixers are generally restricted to relatively
small volumes although we have used them successfully in 25,000
gallon vessels.
It is important to recognize that solids
cannot be suspended to a height of more than about 85% of
the vessel diameter with a single impeller. Even when multiple
impellers are used, it is generally impractical to suspend
solids higher than about 1.4 times the vessel diameter and
it is impossible to suspend them more than 2 times the vessel
diameter in a conventional vessel. If these constraints are
unacceptable, a draft tube must be installed in the vessel.
When the system contains very small
solid particles, their tendency to settle out is exclusively
controlled by the viscosity of the liquid. This is called
Stokes settling. Most of these systems can be treated as liquid
blending applications, and many of the restrictions above
do not apply. However, if there is a power failure and the
solids are allowed to settle, they may be difficult or impossible
to re-suspend if the restrictions above were ignored in the
design of the vessel.
Gas dispersion is defined as the production
of a suspension of bubbles of gas in an immiscible liquid.
A gas dispersion is usually produced in order to enhance mass
transfer of a component from the gas to the liquid, but occasionally
from the liquid to the gas. Dispersions are produced by high
shear mixing devices that impart a high level of turbulent
energy to the system. This energy appears as turbulence which
breaks up gas pockets into bubbles and disperses these into
the mass of liquid. This increases the surface area interface
between the gas and the liquid. Generally the gas is introduced
close to the bottom of the vessel. However, there are a variety
of specialized devices and vessel configurations used for
specific gas dispersion applications.
Vessel Design
Considerations
These applications almost always require a vertical round
vessel. Exceptions may be taken when the viscosity is relatively
high, but these are unusual. This is on of the few cases where
tall skinny vessels are preferred; some of these have aspect
ratios greater than 3.0.
Gas dispersion is almost always a
turbulent operation. Round baffled vessels are required. Offset
mounting is strongly discouraged because irregular unbalanced
hydraulic forces will be very large. This requires costly
“oversizing” of the mixer in order to prevent
destructive vibration.
Crystallization is opposite of dissolution.
It is the production of a suspension of solids from a solution
(often called a “mother liquor”) of the solid
component in a liquid. Mechanisms include evaporation, cooling
and chemical reaction. Crystallization is often a subcategory
of solids suspension. Uniform suspension is often required.
Minimizing particle attrition is often an important consideration.
There are a variety of specialized vessel and mixer configurations
used in crystallization operations on an application specific
basis.
Vessel Design Considerations
See Solids Suspension. However, there are many specialized
“crystallizers” that use unique vessel designs
for individual applications. These must be configured on a
case by case basis.
Heat transfer (as mixing application)
is defined as the transfer of heat between the contents of
the vessel and a heating or cooling surface provided for that
purpose. The surface may consist of coils, plates, the wall
of the vessel, etc. A heat transfer coefficient can predicted
for most systems of interest. This coefficient is called the
“inside” heat transfer coefficient. It is a measure
of the heat flow capacity at the surface in direct contact
with the vessel contents. The coefficient of the wall of the
surface and from the heating/cooling medium to the outside
wall of the surface must be determined separately, and combined
with the inside coefficient to determine an “overall”
coefficient.
Vessel Design Considerations
See Liquid Blending. Liquid blending and heat transfer have
much in common. The difference is that a heat source (or sink)
is installed in the vessel. These may be pipe coils, plates
or the vessel may be jacketed. There are a great variety of
coil arrangements used for heat transfer. Some coil assemblies
can serve as baffles, but when they cannot, baffles must be
installed in the vessel when required. Jacketed vessels are
almost always round and usually used when scraped wall agitation
is expected. However a jacketed vessel can also be used in
most other circumstances.
