Distillation Column - Flooding
Tracerco’s TRU-SCAN® service provides reliable
information on distillation column performance
while the column is online and operating at
normal, test, or upset conditions.
The scan provides an “inside look” at the column’s process
from the top tangent down to the base liquid level. The
interpretation of a TRU-SCAN® can be used to diagnose
virtually any operating malfunction that can be identified by
an observable density difference. This includes most
mechanical, process, or rate related problems - such as tray
damage, foaming, weeping, and entrainment - to name a few.
FrothView™ Technology for Trayed Towers
Plant engineers have been asking the question “How much
more useful capacity do I have with the trays in a tower”. This is
where FrothView™ technology offers quantifiable and less
subjective information using TRU-SCAN® data. FrothView™,
has been developed and tested, where the total froth height on
a tray is measured using new detector technology and software
interpretation. Dividing the total froth height by the tray space
provides the percentage of tray space occupied by tray froth.
The “% tray space” has been shown to correlate very well with
% flood, allowing field measurements to be directly related to
specific tray design criteria to determine the useful capacity of
operating trays.
Tracerco TRU-SCAN® technology provides a new methodology,
FrothView™ that has been developed and tested whereby the total
froth height on a tray is measured from gamma scan data using
new detector technology and software interpretation.
Project Field Test
A column had been experiencing poor separation, higher-thannormal pressure drop, and unacceptable carryover. Because the
column had recently undergone a major retrofit, the customer
was convinced that the column had either sustained damage
during re-commissioning or was experiencing problems relating
to poor column design.
TRU-SCAN® services were performed two times on the column
under different operating conditions. (Figure 1) The first shown in
Figure 1 (green-dashed line) was at design conditions. At design
rates the column was delivering off-spec performance. After
reviewing initial scan results onsite, it was agreed to scan the
column a second time at reduced reflux rates. The second scan
is depicted by the blue (solid) line. The scan plot results focus on
the top seven trays of the column.
Project Analysis
Summary of results and the first scan at design rates
(green-dashed line):
Results of the scan at design conditions found that the mist
eliminator above Tray 1 was in place with liquid from Tray 1 being
entrained upwards, saturating it. The allowable liquid/vapor
disengagement space between the froth height and the tray above
was greatly reduced on Trays 2 - 4. In general, a single-point,
reduced vapor space is a telltale sign symptomatic
of moderate-to-severe entrainment. The total froth height on Tray 1
was 34-36”, nearly reaching the elevation of the mist eliminator.
Trays 2, 3 and 4 had total froth heights of essentially 100% or the
total tray space was filled with froth - the classic definition of
flooding. The trays below Tray 4 appeared to be operating
normally with good vapor/liquid disengagement.
Summary of results and the second scan at reduced reflux
rates (blue solid line):
Results from the second scan at reduced reflux rates showed all
trays were found to be in place with no evidence of damage. Tray 1
held 10-12” of total froth which is much more typical. Tray 2 held a
total froth height of 38 ±3% of tray spacing and the trays below Tray
4 had similar froth levels. These trays now had clear vapor spaces
between the trays rather than the flooding seen earlier. At the
reduced reflux rates the column appeared to be operating normally.
Customer Conclusion
Based upon the TRU-SCAN® information, the customer conducted
a thorough investigation that focused on the reflux rate entering the
column. The reflux rate set point should not have caused the trays
to flood. Confident that the tray design was correct and that the
trays had been installed correctly, the only other explanation was
a reflux rate greater than what the flow instruments indicated. The
investigation found that an orifice plate in the reflux measuring
system had not been installed properly, therefore the measured
reflux rates had been greatly understated. The poor column
performance had been a direct result of too much reflux flow,
and not tray damage or improper design.
Figure 1 - Scan results at design and reduced reflux rates
provided the customer the information they needed to eliminate
an unnecessary unit shutdown.
This study not only eliminated an unnecessary unit shutdown and
column inspection, but enabled the customer to achieve on-spec
production at higher unit feed rates. Tracerco has used this same
approach countless times with other customers to identify the
root cause of an operating problem. This is especially true with
entrainment concerns, or to give the customers additional insight
as to what is physically taking place inside their columns at
adjusted conditions.
TRU-SCAN® services have proven to be cost effective by reducing
off-spec production and equipment downtime. In today’s highly
competitive refining, petrochemical, and chemical processing
markets, more customers are using TRU-SCAN® information to
stop plant bottlenecks and to optimize performance.
For further information:
If you would like to obtain additional information on Tracerco’s Process Diagnostics® technology please contact a technical
advisor in your area to schedule an onsite presentation or visit our website at www.tracerco.com.
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