Hydrocarbon Processing - June 2022 - 32

Process Optimization
of the transmitter changes, then the gas static head in the piping
would also change, according to Eq. 3.
Due to a very different temperature in
its connecting piping, the measured
pressure drop by a PDT in an ASU cannot
be used to judge packing hydraulics
permanence directly.
Inferences. Despite being a theoretical exercise, the calculations
in this paper are adequate to conclude that the deviation
between the predicted dynamic pressure drop and the PDT
reading is caused by the difference between the gas density in
the HP column and the gas density in the pressure piping due
to the great temperature difference. The larger the difference in
gas density, the greater the deviation.
This can also explain why only a 10%-30% deviation has
been observed with the LP and CAR columns. First, the gas
density difference is still the primary reason for the deviation.
Due to much lower operating pressure, the gas density in the
LP and CAR columns are in the range of 5 kg/m3
-8 kg/m3
;
therefore, the gas static head becomes a smaller portion of the
total pressure drop (the curves of static and total pressure drop
would shift downwards in FIG. 1 in such a case). Using gaseous
N2
, FIG. 5 illustrates the impact of column operating pressure
on the gas density at different temperatures.
Takeaway. Due to the temperature gradient along the piping
of the differential pressure transmitter and its varying position
in the cold box, it would be challenging-if not impossible-
to deduce from a PDT reading either the dynamic pressure
drop of packings or the total pressure drop of a column. Hence,
unlike in other applications, PDT readings in an ASU cannot
be used to judge how far the columns are from flooding, nor
can they provide any input to process simulation and upstream
equipment sizing, significantly limiting typical uses. To obtain
FENGJIE XU is a Senior Engineer at China National Air
Separation Engineering Co. Ltd., where he is responsible for
R&D of ASU processes and ASU energy-saving technologies.
He has been engaged in the field of air separation for 15 yr, and
holds a BE degree in energy and environmental engineering.
FENGHUA ZHOU is a Senior Engineer at China National
Air Separation Engineering Co. Ltd. She specializes in the
automation design and R&D of control technology for ASUs.
She has more than 19 yr of experience in the field of air
separation automatic control and holds an MA degree in
control engineering.
ZHIMING CAO is a Senior Project Manager at China National
Air Separation Engineering Co. Ltd. with responsibility over
equipment integration, construction, erection and project
management. His 21 yr of process project management
experience ranges from engineering design, equipment
installation to commissioning, etc., for ASU projects.
QUAN YANG is a Sulzer Senior Technical Expert, specializing
in mass transfer components. He is in charge of air separation
knowledge management within Sulzer Chemtech. Recently,
his field expanded to process optimization for DWC.
Dr. Yang holds a PhD in environmental engineering from
the National University of Singapore.
LAURENT ZUBER heads Process Innovation and Technology
Management at Sulzer Chemtech Ltd. in Winterthur,
Switzerland. He joined Sulzer in 1995 as a Process Engineer
for chemical and petrochemical separation units, and has
held different positions in application management. He has
a special interest in advanced distillation solutions and difficult
product separation. Dr. Zuber holds a PhD in chemical
engineering from the Swiss Institute of Technology of Zurich.
FIG. 5. Density of gaseous N2
at different temperature and pressure.
32 JUNE 2022 | HydrocarbonProcessing.com
JIAO-JIAO DONG is a Principle APT Engineer at Sulzer
Chemtech China in Shanghai, China. In her 13 yr with Sulzer,
her main responsibilities have included driving ASU business
in the China market and advocating the best designs and
practices of column internals optimized for ASUs. She holds
an MS degree in chemical engineering from the East China
University of Science and Technology.
the correct pressure drop for ASU columns, zero calibration5
can be considered based on gas density. However, this requires
the gas temperature in the pressure piping.
To investigate the difference between the PDT reading
and the predicted pressure drop, the operation data
of HP columns in the ASU were collected and processed.
It was determined the deviation is caused by significant
temperature difference between the HP column and the
differential pressure transmitter piping. The PDT reading
corresponds neither to the dynamic pressure drop of
packings nor the total pressure drop of the column.
NOTES
a Sulzer's Sulcol™
b Sulzer MellapakPlus™ series
c YOKOGAWA EJA110E
LITERATURE CITED
1
2
Cai , T. J. and M. R. Resetarits, " Pressure drop measurements on distillation columns, "
Chinese Journal of Chemical Engineering, 2011.
Kehrer, F., L. Spiegel, E. Kolesnikov and P. Choo, " Experimental investigation and
modelling of Sulzer I-ring hydraulics, " Chemical Engineering Research and Design,
Vol. 84, Iss. 11, November 2006.
3
Yang, Q., J. J. Dong, F. J. Feng and X. D. Zhang, " Revamping an air separation unit, "
PTQ, 2019.
4 Kister, H. Z., " Distillation operation, " McGraw-Hill Inc., New York, 1990.
5
American Institute of Chemical Engineers (AIChE), Tray distillation columns: A
guide to performance evaluation, 2nd Ed., Wiley, New York, New York, 1987.
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