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To jump
to a specific category, click the respective button in this table.
Sometimes
it isn't easy to determine which mixing category is important to consider for
some chemical processes. This page should help.
One thing in common with all of the mixing described here on the Post Mixing
website is that a liquid is the predominate phase. Solids mixing or
processing where a fluid, like water, makes up less than 5% of the total batch
is not described here. Also, this website does not discuss the mixing of
two or more gas phases, or a gas-solid mixture.
All of the phase categories in this table are self explanatory except for Fluid
Motion and perhaps Miscible Liquids. Definitions and examples
of all of the Phase Properties categories are listed below. To jump
to a specific category, click the respective button in this table.
Fluid Motion is when the
main topic of mixing is the description of a single phase flow generated by one
or more impellers inside a vessel and its characterization. The bulk of
mixing articles belong in this category, which really doesn't have anything to
do with mixing, but is the first step in describing mixing parameters.
Many of these articles are studies in water. Much of this information can
be transferred to other mixing topics of other phase properties with the proper
modifications.
 | Macro-level mixing
topics covered here are flow patterns, velocity profiles,
velocity gradients, shear rates and distribution, turbulence
spectrum, energy dissipation, pressure gradients, and impeller
power consumption. Methodologies to measure these are also
discussed. Many of these characteristics change with the flow regime,
so the discussion of the effects of laminar, transitional, and turbulent
flow regimes also belongs here. Obviously, the Reynolds-number, Re, is
a main parameter that describes these changes. Since Re is a function
of viscosity, the discussion of the viscous properties of the fluids and
slurries are also discussed here and how they affect mixing. Many
other dimensionless parameters are mentioned here, too, such as the power
number, Np, the flow number, Nq, and the head number, Nh. |
| Micro-level mixing
topics covered here are flow related "chemical"
processes. These are processes that we can't see with our eyes, but we
can measure. Examples are processes that happen in the boundary layers
at the vessels walls, such as heat transfer and electrochemical
mass transfer. |
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Miscible Liquids is the
mixing of two or more liquids which are of the same final phase.
| Macro-level
mixing topics covered here are mixing or blending times based
on several different methods. Acid-base neutralization with an
indicator makes mixing or blend time studies visible to the naked eye and
allows the investigator to see the location of the worst mixing.
Mixing paints or dyes can also be used to determine mixing times, but
because these are usually opaque, the determination of the mixing time is
restricted to what you can see at the tank wall, or during the draining of
the tanks contents. Other mixing studies are done with ionic
conductivity, radioactive tracers, density difference (stratified mixing
layers), or pH which are not visible. Since they require
instrumentation, they should belong in the micro-level mixing topics, but
because mixing time studied based on acid-base neutralization are so
colorful and visible, all mixing and blend time studies are grouped
here. Residence time distributions of continuous processes (including continuous
stirred tank reactor models, CSTR) also belong here as well as the resulting
characteristics such as back mixing, short circuiting and dead zones .
Typical reactor studies in chemical and mechanical engineering just talk
about perfect mixing and plug flow models. Studies describing
imperfect mixing, which probably describes 90% of all industrial mixing
applications, belong here. |
| Micro-level
mixing topics covered here includes the entire description of chemical
reactions and how they are affected by mixing. This includes
micromixing, selectivity, yields, product distribution, etc. Other
mixing application examples include bulk polymerization and solution
polymerization. |
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Liquid-Liquid
is the mixing of two or more liquids which result in two final phases.
Reactor types include extraction columns, mixer-settlers, emulsifiers, etc.
| Macro-level mixing topics
covered here describe how mixing affects the dispersion of the two phases,
which are most often aqueous and organic. Dispersion topics
here include drop size, drop size distribution, creation of fines,
entrainment, drop break-up mechanisms, phase continuity, flooding,
dispersion stability, and operating ranges for stabile dispersions. Emulsions
and the process of emulsification are described here, too. Both
stabile dispersions and emulsions can often be described as a single phase
and studies from Fluid Motion and Miscible
Liquids can apply here when using the appropriate physical parameter
averages. Reactors include the batch reactor and continuous flow
reactors such as column extractors and mixer-settlers. A common
application of liquid-liquid dispersion mixing is the purification of
pharmaceutical intermediates. Another application is hydrometallurgy.
In the mining industry, solvent extraction finds applications in the
production of 99.999% copper, nickel, zinc, uranium, vanadium, and rare
earths by dispersing the pregnant acidic leach solution into an organic
fluid largely consisting of kerosene . Often the limiting, controlling
step is just making a stabile dispersion, not the liquid-liquid mass
transfer or extraction of desired products. |
| Micro-level mixing topics
deal almost entirely with liquid-liquid mass transfer across a phase
boundary, in this case through the droplets. Another word for this is extraction.
The mass transfer is a function of the intensity of mixing and the phase
loading and is described here. A mixing application example is an emulsion
polymerization. |
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Liquid-Solid
is the mixing of solid particles in a liquid phase. Reactor types include
chests, circulators, crystallizers and precipitators.
| Macro-level mixing topics
covered here describe how the flow pattern generated by the impeller or
impellers affects the suspension or incorporation of and the distribution of
the particles within the vessel. Most often people think of solids
suspension from the bottom of the vessel because solids are usually
heavier than the displaced liquid, but the incorporation of floating
solids is also described here. Parameters that affect Liquid-Solid
mixing are the shape of solids, solid size distribution, solid
concentration, solid density, and liquid density and viscosity. Solid
dispersions (slurries) and pastes are described here. The quality of
the solids distribution is discussed, which includes the description of
fillets, on bottom motion, off bottom motion, and uniform solids
suspension. Reactors include batch reactors and continuous flow
reactors (CSTR). Some mixing application examples are agitated
leaching in the mining industry, rubber crumb, crystallization,
precipitations, etc. Abrasion and impeller wear are important factors
to consider in solid-liquid mixing. |
| Micro-level mixing
topics deal almost entirely with liquid-solid (solid-liquid) mass
transfer across a phase boundary, in this case through the solids. This
category deals more with the rate of the mass transfer, and less with the
physical distribution of the solids. The solids can be porous catalysts
for catalytic reactions, active agents for adsorption, polymers and
co-polymers for suspension polymerization, or particles that
need to be dissolved or coated. In the case of crystallization and
precipitation, the solids are the final product. In the special case
of dissolving, the solids are generally added to the surface.
Although the solid density is greater than the liquid density, the solids
may initially float and gel with each other. The gel creates a
"protective layer" around the solids, making it nearly impossible
for the solids inside the gel to be wetted out. This phenomena is particularly
interesting in the polymer industry. |
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Liquid-Gas
is the mixing of a continuous gas stream in a liquid phase that may be batch or continuous.
Reactor types include bubble columns, sparged columns, fermenters, hydrogenators,
surface aerators, draft-tube aerators and submerged aerators.
| Macro-level mixing topics
covered here describe how the flow pattern generated by the impeller or
impellers affects the dispersion of gas bubbles within the
vessel. The discussion of the quality of the gas distribution,
which includes flooding, minimum, intermediate, and uniform gas
distribution, belongs here. The description of the sparging or gas
introduction device and how it affects gas handling is also a topic
here. Spargers include sparge pipes, lances, sparge rings, sparge
plates and caps, and shaft induction spargers. Other parameters that
affect the gas dispersion are impeller power, isothermal gas expansion
power, superficial gas velocity, specific volumetric gas flow rate (vvm),
pressure, and temperature. The gas hold-up is also a major
topic in this mixing category. Boiling is quite different from
the dispersion of a continuously forced gas stream, but also is discussed
here. |
| Micro-level mixing topics
deal almost entirely with liquid-gas (gas-liquid) mass transfer
across a phase boundary, in this case through the gas bubbles. This
category deals more with the rate of the mass transfer, and less with the
physical distribution of the bubbles. Absorption of gas by a
liquid is a subset of gas-liquid mass transfer. Factors that affect
gas-liquid mass transfer are the solubility of the gas in the liquid
(saturation), impeller power per unit volume, superficial gas velocity,
liquid properties affecting the alpha- and beta-factors, temperature, and
the viscosity. Sometimes it is also considered to be an affect of the
gas hold-up. Some mixing applications include fermentations, hydrogenations,
waste water aeration, oxidations, chlorinations, brominations,
fluoridations, sour gas neutralization, gassing with
methane, etc. Some of these may also have a solid phase, but the
solids are not the limiting case and are therefore neglected here.
Where the solids have an effect, see the next Phase Properties
Category. |
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Liquid-Solid-Gas is the
mixing of a continuous gas stream in a liquid phase that may be batch or continuous
that also has a solid phase which can be dissolving, or forming, or taking part
in a reaction. This is obviously a combination of all of the other Phase
Properties Categories. Reactor types include Pachucas, autoclaves,
draft-tube circulators and aerators, flotation cells, etc.
| Macro-level
mixing topics covered here describe how the flow pattern generated
by the impeller or impellers affects the
 | dispersion of gas bubbles within the vessel as a function of
the solid concentration and particle size distribution or |
| suspension of the solid particles within the vessel as a
function of the gas flow rate or boiling rate. |
Most often these studies are done in water, air, and with an inert solid
(sand, quartz, ore sample, polymer beads, rubber crumbs, titanium dioxide,
gypsum, etc.). |
| Micro-level
mixing topics deal almost entirely with the phase controlling
mass transfer of the particular process, where the mixing of all three
or more phases have an effect on the process results. Some application
examples are supercritical solvent extraction, flue-gas desulfurization,
high pressure gold autoclaving, copper ammoniacal leaching, gold cyanide
leaching, carbon-in-pulp (CIP) leaching, uranium leaching, terephthalic acid
oxidizers, hydrogenations, precipitations, flotations, phosphoric acid
attach tanks, etc. |
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