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Dr. Sang Phil Han, LG Chemicals Ltd., Daejeon, Korea
The Lightnin A320 and other internals near the base of the vessel
The dispersion of gases in liquids is a process that is used in the chemical,
petrochemical, and pharmaceutical industries for fermentation and oxidation
reactions, synthesis, and the manufacture of fine chemicals, for example.
Stirred tanks, equipped with a gas delivering sparger near the base, are
typically used for this purpose. If the gas flow rate is high, the behavior
of the gas-liquid mixture differs considerably from that of the liquid
alone. The power requirements are different as well. While the power required
to drive a single or multiple impeller system is lowered in the presence
of the gas, there is an additional power demand to operate the sparger.
For optimal gas-liquid mixing, this device should deliver a uniform flow
of gas through each of the many holes that cover its surface.
One of the sparger systems used at LG Chemicals is a continuous stirred
tank reactor, driven by two Lightnin agitators: an A310 near the top of
the shaft and an A320 near the base. The reactor has four baffles, a ring-type
gas sparger positioned below the A320 with numerous side and bottom holes,
side circulation inlets, and an outlet at the bottom with a vortex breaker
and a degassing ring. Gas phase reactants are supplied through the sparger
holes, and liquid phase products are extracted through the outlet. A portion
of the product stream is recycled to the reactor through the side inlet.
The sparger assembly
Path lines illustrate some of the bubble trajectories
The gas flow in the sparger
For a recent project, several simulations of the reactor were performed
in an attempt to reduce the pressure difference through the sparger holes
that had caused an overload problem on some of the compressors. In order
to accomplish the goal without any loss in productivity, a decision was
made to enlarge the sparger hole sizes. Changing the sparger hole sizes
had to be carefully studied, however, because new problems might be introduced
in the process. Using FLUENT, several aspects of the planned changes that
would be critical to successfully achieving the goal were checked. First,
the flow in the sparger itself was precisely investigated for various
hole sizes. The results were used to assess the distribution of the gas
flow rate per hole, and to test whether the pressure difference for the
gas exiting through the holes was properly adjusted. Next, the liquid
flow pattern in the reactor was calculated. These results were used to
check for possible problems in the mixing patterns in the vessel. As
a result of this effort, it was found that by modifying the agitator system,
a better mixing pattern could be achieved. The revised liquid solution
was then used as the basis for the gas sparging calculation, which was
performed using the discrete phase model (DPM). This calculation was used
to ensure that the hole size proposed in the first phase of the project
would not lead to any unforeseen problems when the sparger was activated.
During this phase of the project, the underlying assumptions for the
DPM were validated, and the fundamental concepts for bubble formation
by a gas emitted from a sparger hole in a liquid were investigated.
As a result of the project work, the most appropriate hole sizes for
the spargers was chosen that would satisfy the process goals while introducing
no unexpected problems in reactor operation. The results also helped identify
ways to modify other aspects of the agitating system so that better gas
dispersion could be obtained. All of the ideas have since been applied
in the field, and the reactor is now operating successfully.
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