Chemical Engineering November 2013 - 34

Feature Report
Variables
Table 1. Comparison of HigH-liquid-raTe TesT wiTH operaTion
operation
Feed flowrate, m3/h
Reflux flowrate, m3/h
Steam flowrate, ton/h
Feed temperature, °C
Bottom temperature, °C
H2S in bottom, ppm
Calculated Variables
Internal liquid load, m3/h
Internal vapor load, m3/s
Percent jet flood, %
Downcomer backup, mm
Downcomer inlet velocity, m/s
All downcomer clearances were
checked and recorded. Without exception,
all the clearances were within
the manufacturer's tolerance.
It was observed that the seal from
the Tray 19 downcomer had been lost
as the side section of tray 18 (underneath
the tray 19 downcomer) was
also bent downwards. This loss of seal
offered an explanation to the observed
flood. It was postulated that the steam
flow from tray 17 in the top section was
routed preferentially through the tray
19 downcomer rather than through the
fixed valves of tray 19. This explained
why the top section of the column
quickly flooded (evident from a sudden
increase in tray differential pressure)
and flooded the reflux drum.
The observation that the flood was
vapor-sensitive and liquid-insensitive
is very consistent with a sealing-loss
flood theory. When the vapor rate is
high enough, it will break through the
downcomer's broken seal and severely
interfere with liquid drainage, inducing
flood on the tray above. In this
References
1. Lieberman, N. P., " Process Engineering for a
Small Planet " , Wiley, N.J., 2010.
2. Beychok, M. R., " Aqueous Wastes " , John
Wiley and Sons, London, 1967.
3. Kister, H.Z., " Distillation Troubleshooting " ,
Wiley-Interscience, N.J., 2006.
4. Hauser, R., and R. T. Kirkey, Refinery Tests
Demonstrate Fixed Valve Trays Improve
Performance in Sour Water Stripper, in " Distillation
2003: on the Path to High Capacity,
Efficient Splits " , Topical Conference Proceedings,
p. 163, AIChE Spring National Meeting,
New Orleans, La., March 31-April 3, 2003.
20
1.5
2.8
90-95
128
<10
27
0.54
37
170
0.075
3.1
70
115
100-200
41
0.60
48
210
0.115
case, the small hole area (7.3% of the
active area) will aggravate the sealing
problem by resisting flow through the
tray holes and acting to divert vapor
to the downcomer. Sealing loss leads
to a vapor flood, just as observed in
the high liquid test.
The damage was repaired and the
column returned to service. During a
performance test, it has had no problem
handling the feed flowrate of 35
m3/h at the corresponding steam flowrate
with the reboiler online and is
producing a water bottom stream of
<5 ppmw, as per design.
Lessons learned
The key for avoiding damage in water
strippers is to keep the feed hot. This
lesson was also advocated in two other
cases of column internals damage in
water strippers reported in Ref. 3.
Awareness of and understanding the
mechanism of damage when cold feed
enters a column containing a steamwater
system may lead to many additional
means of minimizing its inci5.
The FRI Design Practices Committee, Reboiler
Circuits for Trayed Columns, Chem.
Eng., p. 26, January 2011.
6. Kister, H. Z., P. M. Mathias, D. E. Steinmeyer,
W. R. Penney, B. B. Crocker, and J. R. Fair,
Equipment for Distillation, Gas Absorption,
Phase Dispersion and Phase Separation, in
D. W. Green and R. H. Perry " Perry's Chemical
Engineers' Handbook " 8th Ed., Section
14, McGraw Hill, New York, 2008.
7. Lieberman, N. P., " Troubleshooting Process
Operations " , 4rd Ed., PennWell Books, Tulsa,
Okla., 2009.
8. Kister, H. Z. " Distillation Operation " ,
McGraw-Hill, N.Y., 1990.
34 ChemiCal engineering www.Che.Com november 2013
High-liquid Test
35
dence, as advocated in one of the two
case studies reported in Ref. 3.
The case confirms our experiences
that sour water strippers generally do
not foam. As stated earlier, our design
case evaluation concluded that the
trays/downcomers were not adequately
designed for foaming conditions. The
downcomer velocities are far too high
and the system factor application in
the design is questionable. Yet, now
that the damage has been fixed, the
tower has been operating well above
design rates, suggesting the absence of
significant foaming in this column. ■
Edited by Gerald Ondrey
Authors
Jeremy Ponting has been
working as a process engineer
in the oil-and-gas industry for
19 years. The last eight years
have been with Fluor Ltd.
(140 Pinehurst Road, Farnborough,
GU14 7BF, U.K.;
Phone: +44 1252-291000;
Fax: +44-1252-292222; Email:
jeremy.ponting@fluor.com) as
a lead process engineer and
he is currently on assignment
in Manila, the Philippines (Phone: +63-2-8764292),
acting as a " process coach. " He has been
involved with two successful plant startups and
was directly involved with the troubleshooting
and startup of the sour-water stripper unit that
this paper was based on. Ponting graduated from
Loughborough University of Technology with a
B.Eng. (Honors) and M.Sc. in chemical engineering.
He is a Fellow of the Institute of Chemical
Engineers (IChemE).
Henry Z. Kister is a senior
fellow and director of fractionation
technology at Fluor
Corp. (3 Polaris Way, Aliso
Viejo, CA 92625; Phone +1949-349-4679;
Email: henry.
kister@fluor.com). He has
over 30 years experience
in design, troubleshooting,
revamping, field consulting,
control and startup of
fractionation processes and
equipment. He is the author of three books, the
distillation equipment chapter in Perry's Handbook,
and about 100 articles, and has taught the
IChemE-sponsored " Practical Distillation Technology "
course more than 400 times. A recipient
of several awards, Kister obtained his B.E. and
M.E. degrees from the University of New South
Wales in Australia. He is a Fellow of IChemE
and AIChE, member of the NAE, and serves on
the FRI Technical Advisory and Design Practices
Committees.
Richard (Dick) B. Nielsen
has over 40 years of experience
in the design, revamp,
troubleshooting, and startup
of process plants. Formerly a
vice president of Process Technology
for Fluor Corp. (same
address as Kister; Phone: +1949-349-4217,
Email: dick.
nielsen@fluor.com) specializing
in gas processing and
treating, he now consults for
Fluor on various projects. Together with Art Kohl,
he is a coauthor of the 5th ed. of " Gas Purification, "
Gulf Professional Publishing, 1997. Nielsen
has an M.S. degree from the University of Southern
California, an M.B.A. from the University
of California, Irvine, and a B.S. degree from the
Massachusetts Institute of Technology (MIT).
http://www.Che.Com
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