Chemical Engineering January 2019 - 35

* V: percent of the internal vapor flowrate
Case 1, all valve floats in place. This case
gave the expected performance of the intact
trays. As the geometry is symmetrical,
there is equal distribution of vapor and liquid
for all trays in the section. The percent
jet flood was 93% and 84%, respectively,
for the center-to-side and side-to-center
flow trays. The clear liquid backups in the
side and center downcomers were 56%
and 55%, respectively.
Case 2, valve floats missing per inspection.
In this case, 90% of valve floats were
missing on tray 4, 80% missing on trays 5
and 6, and 30% missing on tray 7. This situation
increases the open area of tray 4 from
10.1% to 13.1%. The open area of trays 5
and 6 are increased to 12.6% and 12.7%,
and the open area of tray 7 is increased to
11.0%. In the model, the geometry is symmetrical,
so there is still equal distribution of
vapor and liquid for all trays in the section.
The change in geometry from Case 1 to
Case 2 caused flooding in the actual tower,
as shown in Figure 6, but the hydraulic calculations
showed no flood. Percent jet flood
marginally rose, from 81 to 84%, on trays 4
and 6 (side-to-center flow) and from 93% to
94% on tray 5 (center-to-side flow). On tray
7, the jet flood decreased from 93% to 92%.
The percent flood stays relatively the same
despite the large increase in open area because
with the valve floats missing, the tray is
more similar to a sieve tray than a valve tray,
and the hydraulic diameters of the holes/slots
are much larger. The downcomer backup decreased
by 3-9% for trays 4-7. We conclude
that, based on conventional hydraulic calculations,
the loss of valves would not explain
the flooding observed on trays 4-6.
Cases 3 and 4, asymmetric valve loss. We
explored whether the maldistribution resulting
from losing valve floats asymmetrically
could result in the flood patterns observed on
trays 4-6. These cases also show the extent
to which maldistribution propagates through
the tray section. Here, we report the results
for two of these cases. More detailed analysis
and another case can be found in Ref.
16. In all modeled cases, asymmetric valve
losses generated flooding patterns vastly different
from that observed in the actual tower.
We therefore conclude that an asymmetric
valve loss is not the root cause of the flood
observed on trays 4-6.
Our
analysis
precluded
downcomer
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Chemical Engineering January 2019

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