Fluid Motion
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.
Miscible Liquids
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.
Liquid-Liquid
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.
Liquid-Solid
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.
Liquid-Gas
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 topicscovered 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.
Liquid-Solid-Gas
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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