This is a continuation of the
Process Intensifier - Optimization with CFD: Part 1 paper.
This report shows the versatility of using CFD to model and understand a complex mixing device such as the Process Intensifier. In the past, the use of CFD often meant very long computing time and it was often quicker to do the experiment. Not any more. ACUSOLVE was successfully able to determine the power number of the impellers within 1% of reported values without the use of fudge factors on a repeatable basis. The output information from this extremely powerful tool has conclusively proven its capability to assist in the optimization of the design and implementation of sophisticated process equipment such as the Process Intensifier. This demonstrates that the ACUSOLVE CFD code formulation and its adherence to fundamental physics are extensible to handle the arbitrary geometric structures and flow conditions of inline mixers. ACUSOLVE Turbulence was evaluated and validated in the context of standard configuration tank models, where an abundance of experimental data and generally accepted correlations exist.
A process and procedure to assess the effects of turbulent flow characteristics around the blades with attached surface flow criteria adequately categorize and calibrate the class of discretization and nature of solution to be executed.
ACUSOVLE computational results were consistent with the present state of understanding of the global behavior. Further, detailed study and review provided substantial insight to the fluid mechanics and mixing characteristics relative to potential process improvements. The concepts used in this study have clearly illustrated the extensive potential to impact many new and even conventional mixing applications. Horizontal pressurized mixing vessels such as autoclaves used in the mining industry, have very similar flow patterns and could benefit greatly. There are also many existing process that have employed this type of technology as an enhancement or even as a substitute for tankage with conventional mixers that may or may not be fully aware of how effective or in-effective their installations may be.
We studied four basic Process Intensifiers and have already shown several
ideas for improving upon the mixing by studying the velocity profiles, flow
patterns, tracer pathways, pressure drops, eddy viscosity, and residence time
distributions. Placement of the injection ports for mixing of additive
components into the bulk flow is very crucial. The assumption that a single high
velocity nozzle attached to the pipe wall (or even multiple low pressure
diffusers) located upstream, will be sufficient to result in uniform blending
can be a mistake. It is also important to effectively disturb as much of the
pipe cross-section as possible to prevent short-circuiting. Impeller size and
selection is key. Be too small and they are highly ineffective; too big, and the
level of applied power may be significantly more than what is required and for
some processes overmixing can be as bad as undermixing. The mixer must be able
to create as much flow as possible or in some cases sufficient flow combined
with shear. The T-section of the Process Intensifier can become a recirculation
zone as well as the chamber areas defined by a Z-plate; however there are design
modifications that can exploit these to benefit or eliminate them as a
determent. Swirl is present in all designs and is wasting energy. Methodology
for the elimination or at least the reduction of this condition is a topic for
future discussion (Part 2).
For low viscosity high intensity mixing, the LTR appeared to do the best job.
It could effectively retain the tracers and they left the pipe with a very
narrow time distribution. The HGR does better in high viscosity, since the
Z-plate ends are matched with the intensity of the impellers. These units
required an order of magnitude more power and also suffered from a relatively
high-pressure drop. LTA was not particularly effective in the configuration that
we tested, basically requiring larger impellers and more of them. The HGA
assembly did a very good job of mixing with little power and pressure drop.
However the mixing in the region of the T-section of the vessel needs to be
addressed in some fashion.
For all designs, the impeller's flow pumping capacity is no match for typical
flow rates through pipes even if the P/V can be as much as 2000 Hp/1000 Gallons
(400 kW/m3). Impeller blades need to be wider than standard blades used for
vertical cylindrical tanks. The spacing between the impellers should also be
closer so that there is a good flow pattern from one impeller to the next,
otherwise the cross flow will cause short-circuiting (as with LTA). As a means
of improvement it may be best to install a Process Intensifier into a vessel,
which is one pipe size larger in diameter than the balance of the pipeline. This
can be easily done using oversized flanges on the connection piping. The Process
Intensifier could also benefit from the use of pipe reducers and expanders at
the outlet and inlet of the mixing vessel. This will slow down the cross flow
enough to allow the impellers to really mix. Should a process require high
intensity mixing for a longer period of time, the pipeline can easily have
several of these mixers in series. The overall power cost is quite low for the
amount of fluid being treated.
Whereas Part 1 focused on four existing styles of
inline mixers and the effects their designs have on mixing, Part 2 will show how
the flaws of these designs can be eliminated and how the Process Intensifier can
be optimized for very specific applications and process results. Designs will
vary by application, scale and process criteria.
Although there are many Line Blenders / Pipeline Mixers currently operating
in applications that range from the very simple to significantly more
challenging, it is apparent that existing conventional designs maybe only
marginally effective or even over designed to compensate and guarantee process
results. Employing the latest in CFD technology, we have been able to
successfully remove the "black box" that has limited the application
potential for this type of technology and assisted in the development of design
modifications including impeller sizing and selection for specific results. It
has also given us the means whereby we can accurately us CFD to evaluate the
performance of existing configurations in the case of a process audit.
With the advent of so many innovative and even new commercially available
impeller designs, which could be incorporated as part of the Process
Intensifier, the real secret will be making the right choice of impeller or
impeller combinations and fully understanding how these will perform to provide
the required results. For some, this may be an introduction to new process
technology for others this may be an explanation of what is really taking place
when a conventional Line Blender is introduced into a process stream or this
just might be a simple case of "what once was old, is new again".
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