Hydrocarbon Processing - June 2021 - 29

Special Focus Process Optimization
B. BLUM, Bazan Group, Haifa, Israel; H. KISTER, Fluor, Aliso Viejo,
California; and R. TSANG, Koch-Glitsch, Stoke-on-Trent, UK
Good distributor design for high-velocity
feed debottlenecks a crude preflash tower
Bazan Group's crude unit No. 3-not
designed by any of the authors' companies-was
bottlenecked by the preflash
tower. The tower experienced frequent
episodes-especially with light crudes
(high naphtha percent)-in which the
naphtha stream leaving the tower turned
dark, accompanied by a high tower ∆P.
Most likely, these episodes originated
from crude entrainment and restricted
the throughput of the entire crude unit.
From the lowest downcomer, a 4-in.
pipe drains the liquid into the top shed
deck. During two recent outages, the
4-in. pipe was found broken and falling,
with a hole in the seal pan from which
it originated. As the pipe was in front of
a high-velocity feed entry, it was likely
that feed impingement at high velocities
generated vibration and caused the 4-in.
pipe to break. It was believed that, due to
the pipe breakage, the high feed vapor velocities
picked up the descending liquid
or rose up the bottom downcomer, causing
the naphtha product to become dark.
The initial solution sought was reorientation
of the 4-in. pipe away from the feed.
Suspecting that there may be other
issues, Bazan requested Fluor to assist
with the problem analysis. Koch-Glitsch
was later added to the team to consult
on hardware changes. The analysis determined
that the breakage of the 4-in.
pipe was insufficient to explain the entrainment
and dark naphtha. It was determined
that the root cause was excessive
feed velocities, with feed entry too close
to the bottom tray.
To solve the problem, it was necessary
to break the incoming momentum
and to divert the feed to a lower point in
the tower. This was achieved by a unique
feed entry design for the very-high-velocity
feed, by removing two segments of
the top shed deck to permit feed entry at
a lower point, and by adding a specially
designed impingement baffle to break
the incoming momentum.
When the tower returned to service,
the dark naphtha episodes completely disappeared.
The naphtha product rate could
be raised significantly (by 20% and above)
without any adverse effects on operation.
No limit has been reached. The entrainment
problem has been eliminated.
This article details the problem, analysis
and solution.
Process and problem description.
The preflash tower (FIG. 1) is designed to
receive crude feed at 160°C, which contains
4.8% vapor by weight. The tower operates
at a pressure of 1.6 barg. Overhead
vapor from the tower is condensed, and
some is returned to the tower as reflux.
The rest is naphtha overhead product that
is sent to the main stabilizer tower. Bottom
product is preflashed crude that goes
to the heat exchangers, the crude heater
and, finally, to the atmospheric tower.
The 2.1-m-diameter preflash tower
contained 12 single-pass rectangular
moving valve trays above the crude feed
inlet and six shed decks with 34% open
windows beneath the feed inlet. A connection
permits stripping steam to be
added below the stripping section, but
this steam has not been used.
Since the 1990s, the tower experienced
dark naphtha (about 2-3 episodes/yr)
while distilling crudes with
high naphtha content. Following a turnaround
in 2018, the unit started running
on September 9, 2018. Beginning
November 11, 2018, the preflash tower
experienced episodes of dark naphtha
product. The dark naphtha episodes
happened about 15 times/yr-with each
dark naphtha event lasting approximately
12 hr, during which the naphtha was
continuously dark. The dark naphtha
was accompanied with a high pressure
drop, which remained high throughout
the episode. There was no problem of
instability of the bottom level during the
dark naphtha episodes; however, during
these episodes, the bottom level dropped
by approximately 20%.
The rest of the time, the tower was
working as intended because the refinery
limited the tower inlet temperature and
the total amount of naphtha in the top.
To get out of the event, the refinery needed
to cut crude charge rates or the feed
temperature. These episodes became the
prime crude unit capacity bottleneck.
FIG. 1. Bazan's preflash tower.
Hydrocarbon Processing | JUNE 2021 29
Hydrocarbon Processing - June 2021

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