Originally developed more than 35 years ago, the Lightnin "Line
Blender" or "In-Line Mixer", may be the most overlooked and under
appreciated fluid mixing device. Often confused with "In-line Static
Mixers", the Line Blender is a mechanically driven open bladed mixing
assembly that is mounted perpendicular to the direction of the pipe flow in a
specially configured mixing vessel or "mixing tee". Since it was
originally introduced into industry, there has been only minor improvements or
developments to the operation or design. Axial flow impellers have replaced the
radial impellers and the separating plate (Hayward Gordon refers to it as the
"Z") has been removed for flash mixing. We have only found two basic
designs with slight variations on these designs by two mixing vendors.
Historically the Line Blender has been used as an add-on component to an
existing process as a means of enhancement or overcoming process short falls.
Some of the more common applications included simple chemical dispersion and
shear addition for the breakup of agglomerated material.
The scope of this paper, documents the results of how CFD is used to better
understand the performance and operation of the various Line Blender designs and
to illustrate the capabilities of an enhanced design, for use as a "Process
Intensifier" with significant application potential. The use of CFD
provides us with the tools to show the shortcomings of current designs and means
to evaluate the performance of new configurations that can be targeted for
specific processes.
The selection and implementation of a line blender follows many of the
standard criteria used in specifying the selection of the appropriate agitator
for a CSTR application. Understanding the nature of the mixing criteria, i.e.
miscible liquids, liquid-liquid,
liquid-solids,
liquid-gas or any
multi-phase
variation, as well as the fluid behavior and desired results is key to a
successful design. Since the advent of new and more application specific
impeller technologies there is the ability to tailor a Process Intensifier to a
specific process result. The selection of an appropriate impeller or impeller
combinations can be used to customize micromixing and the shear-flow
relationship.
To date, Line Blenders have used shear and the resultant turbulence to
provide sufficient mixing at the expense of a high P/V, but relatively low
overall power. Improvements to the design of the Line Blender can expand the
range of applications to include the dispersion of gas into viscous fluids,
condition particulate in slurry flows, enhance gas-liquid mass transfer, control
droplet formation and size in liquid-liquid mass transfer applications,
micromixing, as well as improvement to processes that have used the currently
available Line Blender designs in the from of less back mixing, lower P/V,
reduced short circuiting, increased or reduced shear as required to name only a
few.
Some examples of applications, which are well suited for the inclusion of a
Process Intensifier, are as follows:
High Speed Dispersion of Bentonite
Bentonite is an aluminum silicate clay that occurs naturally, which can take
up several times its own weight of water to form a highly thixotropic colloidal
suspension or gel. Conventional agitators do not produce sufficient shear at
acceptable power levels to reduce particle size and activate the gelling effect.
Ex-situ Bioremediation of Organics
Process Intensifiers can be ideal for the formation and maintenance of
oil/water emulsions in conditioning systems of the bioreactor feed. When air or
oxygen is introduced into the emulsion, it helps to initiate bacterial growth
and oxidation of the organic compounds.
Rapid Mixing of Water Treatment Polymers
The introduction of polymers are used in both water and wastewater treatment
flocculation of fine suspended particulate. Rapid mixing is the high intensity,
short retention blend of the polymer into the primary stream.
Preparation of Coatings
The addition pigment used in the preparation of coatings can be supplied as a
slurry containing up to 60% solids, in solids free liquid or a dry powder. In
applications where the primary stream has a viscosity in excess of 2000 cP the
uniform blending of added liquids, slurries or dry solids can be a challenge.
Dry solids and slurry addition can result in agglomerates and impurities in the
coating mixture, which must be broken down or filtered to prevent defects in the
coatings surface. The use of a Process Intensifier subjects the slurry to
intense mixing; ensuring that the product fed to the coating plant is
agglomerate-free and does not require filtration.
Beverage Industry
The use of pipeline mixing has significant potential for applications in the
production of beverages. Quite often these types of applications require the
uniform blending of syrups and colorings with water at very exact ratios in
order to meet stringent quality control guidelines and frequently systems will
process more than one beverage formulation. The properties of the syrups and
colorings can vary dramatically, some will have an extremely high sugar content
while other may have an oily additive and subsequently their behavior when added
to water will be equally varied and this can even be further complicated by
things such as temperature and method of addition. Poorly agitated systems can
suffer form stratification, resulting from variation in fluid density and this
contradicts the process criteria, in an industry that relies heavily on
repeatability.
The introduction of high SG or even low SG additives with Process
Intensifiers can result in dramatically shorter blend times in mix tanks, reduce
the mixing criteria in these tanks or even eliminate the process variation
between different beverage formulations being processed by systems originally
designed for one specific application. The potential for the elimination of mix
tanks in certain applications is a viable option provided the fluid properties
and behavior are clearly understood and the subsequent Process Intensifier size
and configuration, including wetted parts, have been implemented.
There are other applications in the beverage industry that could benefit form
the inclusion of Process Intensifiers. Processes which are characterized by the
addition of gas, in some cases CO2 and in others O2, which require specific
bubble sizes and uniform gas dispersion in closed loop pressurized systems, can
benefit from this technology. The use of a pipe mixer for the dispersion of gas
can be far more cost effective than the implementation of a much larger sealed
tank agitator and significantly more reliable and repeatable than using bubblers
or gas jet nozzles in tanks with a larger capacity. As is the case with most
gas-liquid applications regardless of the industry, as the volume of the liquid
is increased in the tankage, the ability to control the behavior of the gas
becomes increasingly more challenging. For those that believe good gas
dispersion can be achieved by using an injection port upstream of a static mixer
that may or may not meet the necessary coefficient of convergence, maybe
crossing their fingers and hoping that flow variations will not effect the
performance of their system and ultimately the final product, which could be
compensated for by the flexibility and the applied power of an inline mixer.
Flotation
Flotation is a process used to separate a desired mineral (i.e., copper,
nickel, zinc) from an undesired material commonly known as gangue. Flotation is
used extensively throughout minerals processing including after the ore has been
reduced to a very small particle size by crushing and milling, the cleaning of
hydrometallurgical liquors and the preparation of final concentrate.
After milling, the finely ground ore, which has the consistency of sand, is
then transported in the form of a slurry with a relatively high percentage of
solids. As part of this process, the use of a specific chemical called a
collector is introduced, which enables a specific mineral to attach to a bubble
and rise to or near to the surface. This process takes place in flotation cells
designed for maximum particle bubble contact and resultant separation of the
desired minerals from unwanted particulate. Either injecting air or entraining
air through slurry flow into the flotation cell causes the bubbles. The float
material is then overflowed or skimmed off while the bottom material is sent to
another separation process or to waste (tailings). Flotation can be used to
separate the desired mineral from the waste (gangue) or from another mineral.
Each mineral has certain conditions, which affect its ability to float. One
of the important variables of these conditions is pH. pH as a variable in
flotation affects: 1) the form of many ionic species present, 2) the charge
state of the mineral surfaces, and 3) the form of many collectors. While it is
clear that the pH of the slurry is an important issue and is usually controlled
by the addition of chemicals, the addition of reagents as collectors is as
crucial. The effectiveness of collectors relies heavily on the uniform blending
of these reagents with the slurry and more importantly the exposed surfaces of
ore to be recovered.
Process Intensifiers have the ability to provide high intensity mixing of
these collectors with the slurry, prior to the flotation cell. Particulate that
may have oxidized or partially oxidized as a result of entrained air in
turbulent pipe flow can benefit form the high energy, high shear inter-particle
kinetics that a Process Intensifier can provide. Often this work is done through
the inclusion of equipment that can prove a level attrition in a tank; there are
numerous existing processes where the installation of this technology may not be
an option. As an intense chemical blending device or as a potential particulate
conditioner the Process Intensifier offers a new and extremely flexible dynamic.
Other Applications
Other potential applications would include chemical extraction,
series-parallel reactions, oxidation processes, emulsification applications, dry
material wetting, and chemical neutralization. Because of the potentially high
shear in these devices, mixing of high viscosity shear thinning fluids benefit
because the apparent viscosity is decreased a great deal. These are only a few
of the potential applications, which would benefit from the use of a mixing
assembly designed to be a Process Intensifier.
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