| Summary: | Sasol Limited experiences extremely high particulate fouling rates inside shell-and-tube heat
exchangers that utilize process cooling water. The water and foulants are obtained from
various natural and process sources and have irregular fluid properties. The fouling
eventually obstructs flow on the shell side of the heat exchanger to such an extent that the
tube bundles have to be replaced every nine months. Sasol requested that certain aspects
of this issue be addressed.
To better understand the problem, the effects of various tube and baffle configurations on
the sedimentation rate in a shell-and-tube heat exchanger were numerically investigated.
Single-segmental, double-segmental and disc-and-doughnut baffle configurations, in
combination with square and rotated triangular tube configurations, were simulated by using
the CFD software package, STAR-CCM+. In total, six configurations were investigated.
The solution methodology was divided into two parts.
Firstly, steady-state solutions of the six configurations were used to identify the best
performing model in terms of large areas with high velocity flow. The results identified both
single-segmental baffle configurations to have the best performance.
Secondly, transient multiphase simulations were conducted to investigate the sedimentation
characteristics of the two single-segmental baffle configurations. It was established that the
current state of available technology cannot adequately solve the detailed simulations in a
reasonable amount of time and results could only be obtained for a time period of a few
seconds.
By simulating the flow fields for various geometries in steady-state conditions, many of the
observations and findings of literature were verified. The single-segmental baffle
configurations have higher pressure drops than double-segmental and disc-and-doughnut
configurations. In similar fashion, the rotated triangular tube configuration has a higher
pressure drop than the square arrangement. The single-segmental configurations have on
average higher flow velocities and reduced cross-flow mass flow fractions. It was concluded
from this study that the single-segmental baffle with rotated triangular tube configuration had
the best steady-state performance.
Some results were extracted from the transient multiphase simulations. The transient
multiphase flow simulation of the single-segmental baffle configurations showed larger
concentrations of stagnant sediment for the rotated triangular tube configuration versus
larger concentrations of suspended/flowing sediment in the square tube configuration. This
result was offset by the observation that the downstream movement of sediment was quicker
for the rotated triangular tube configuration.
No definitive results could be obtained, but from the available results, it can be concluded
that the configuration currently implemented at Sasol is best suited to handle sedimentation.
This needs to be verified in future studies by using advanced computational resources and
experimental results. === Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013
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