Hydrocarbon Processing - August 2022 - 20

Refining Technology
drop on MAN recovery is directly proportional. The column
operational trendline clearly specified that a low column pressure
drop provides improved MAN recovery.
important when designing distillation columns and are one of
the key reasons that many distillation experts use the process
simulation softwarec
. In this case, a rigorous distillation calcuMost
column revamps necessitate the evaluation
and modification of ancillary equipment. However,
associated equipment such as condensers,
receivers, pumps, valves and instrumentation
may remain as they are.
TROUBLESHOOTING APPROACH
In any unit operation, future modifications without a basic
engineering study are a risky adventure. Simulation provided
by an industry leader was used due to the company's widespread
use of 40-yr databases of tuned models with specific
vapor-liquid equilibrium (VLE) packages. These databases are
TABLE 1. Hydraulic performance comparison of old packing
vs. proprietary structured packinga
Proprietary
Key factors
Pressure drop at
plant match load
% capacity
At 120% of plant
match load
Existing packing
2.6 mbar
79.4
Packing is fl ooding
structured packinga
< 1.5 mbar
62.7
< 6 mbar
lation was performed on the existing column
using the actual temperature, pressures, compositions,
etc., of the column to accurately
match the original heat and material balance
and tune the steady-state model.
Rating the existing column. With the
two tuned models, the column was rated for
the new conditions. The rating determined
that the existing equipment could be used
in the new hydraulics operation. The hydraulic calculations
were then evaluated, and the column capacity was reviewed
using an analysis of simulated internal vapor and liquid flows.
With the original heat and material balances, the VLE data and
packed column efficiencies were checked. The existing vent
gas column was rated for the correct number of transit units
(NTS), reflux-to-stages ratio and packing hydraulics.
A plant match case simulation was performed with the
objective of determining whether lower surface area packing
should be used to improve column run length. However, the
simulation showed significant loss of MAN from the top due
to reduced NTS. Therefore, it was decided to not use the lower
surface area packing.
Furthermore, from the author's company's hydraulics calculation
of equivalent existing packing, it was observed that
the existing packing bottom section was operating near to
flood (79%) in a clean condition. This means that as soon
as fouling starts in the packing, the surface area and overall
cross-sectional area of the packing are reduced to the extent of
the fouling, resulting in the packed bed operating under flood
condition. As soon as the packing flooded, the pressure drop
across the packing was increased exponentially.
Application of structured packinga
. To resolve the structure
packing capacity limitation and to reduce the overall
pressure drop across the bed, the author's company checked
the hydraulics of its latest generation structured packinga
. The
FIG. 3. The flow pattern of the proposed proprietary structured packinga
.
pressure drop across the packing is much lower and the maximum
capacity can be extended up to 20%-30%. The new style
of packing incorporates a patented modification to the lower
and upper end of each packing element.
The new packing's internals were verified
with various operational variables/
changes to meet the desired targets of
the highest degree of MAN recovery
by 6.5 wt%. During simulation, various
options (pressure changes, temperature
reflux rate changes, optimizing the feed,
etc.) were studied.
For the revamp, different options were
FIG. 4. Pressure drop and MAN recovery post revamp.
20 AUGUST 2022 | HydrocarbonProcessing.com
also studied, including random packings.
The pros and cons of each option were
identified and compared; in this service,
however, random packing seems inapplicable
due to the increased settlement of
fouling materials and eventual high pres
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Hydrocarbon Processing - August 2022

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