Hydrocarbon Processing - April 2022 - GP-19

PROCESS OPTIMIZATION
Investigations of high pressure drop
observed in ASU columns
F.-J. XU, F.-H. ZHOU and Z.-M. CAO, China National Air Separation Engineering Co.; Q. YANG and L. ZUBER,
Sulzer Chemtech Ltd., Winterthur, Switzerland; and J.-J. DONG, Sulzer Chemtech China, Shanghai
The measured pressure drop of columns in air separation
units (ASU) is often reported as being higher than the
predicted pressure drop. Particularly for high-pressure (HP)
columns, the difference can be up to four times higher, depending
on the operating pressure of the column. To investigate
the difference, the operation data of recently revamped
HP columns were collected; then, vapor and liquid loads as
well as their physical properties were generated using a process
simulation tool. Thereafter, a proprietary hydraulic rating
toola
was utilized to predict the dynamic pressure drop caused
by the column internals. The use of the differential pressure
transmitter and its connecting piping inside the cold box was
also scrutinized. It was deduced from calculations that the
average temperature of gas in the differential pressure transmitter
(PDT) piping located inside the cold box was more
than four times higher than the temperature of gas inside the
column. The resultant different gas static head at the two locations
leads to the deviation between the measured and the
predicted pressure drop.
Since the 1980s, structured packings have progressively replaced
trays, first beginning with the crude argon (CAR) column
and low-pressure (LP) columns. Around 2011, in China,
structured packings eventually became a default option for
HP columns.
It was determined that the pressure drop of columns in the
ASUs was higher than what was predicted by the hydraulic
rating toola
. The extent of the reported deviations was inconsistent-the
typical difference was ~20%, which was initially
believed to be within the accurate parameters of pressure drop
correlations. When the packings-equipped HP columns began
to operate nationwide, the observed deviation jumped.
For a 550-kPa HP column, the measured pressure drop was
three times higher, and for an HP column operated at 900 kPa,
the measured pressure drop was four times higher.
Pressure drop in a column is caused by two factors: dynamic
pressure drop and static pressure drop (commonly called
gas static head). Dynamic pressure drop is the resistance of
column internals, such as packings, to gas flow.1,2
The pressure
drop decreases with declining gas throughput, and reaches
zero if gas flow stops. For a packed column, the dynamic
pressure drop is closely linked to packings capacity-e.g., for
a proprietary structured packing seriesb
, 5.5 mbar/m and 12
mbar/m correspond to 90% and 100% capacity, respectively.
Therefore, the dynamic pressure drop measured from a plant
is typically used to monitor packings hydraulic performance.
In contrast, the static pressure drop is caused by the pressure
exerted by gas weight over a certain height.
The gas density in ASU columns can reach 40 kg/m3
, depending
on the operating pressure of the columns, and structured
packings generate much less dynamic pressure drop than
trays. Therefore, the gas static head can contribute a significant
portion to the total pressure drop for ASU packed columns.
The proprietary hydraulic rating tool only calculates dynamic
pressure drop, so the gas static head must be manually added to
the dynamic one to obtain the total pressure drop of a column.
However, the measured pressure drop in ASU plants matches
neither the proprietary hydraulic rating tool'sa
predicted dynamic
pressure drop nor the total manually calculated pressure
drop. Rather, it lies between.
Although the deviation does not cause any practical challenges
to column design and operation, a joint investigation
between the authors' companies began in early 2020 with the
objective of benefitting the ASU industry.
Data collection and analysis. Among the various columns in
an ASU, the HP column is the best option for investigation: the
simplest HP column consists of only one feed (e.g., the compressed
air feed) and two products (i.e., liquid nitrogen at the
top and oxygen-enriched liquid at the bottom). The high pressure
drop observed in HP columns will not be masked by the
inaccuracy of any pressure drop correlations.
In the last 2 yr, the authors' companies have successfully revamped
HP columns from tray to structured packings, and the
details of the first project were published elsewhere.3
The operation data of two identical revamped HP columns
in a plant were collected in June 2021. The vapor and liquid
loads and their physical properties were generated using a process
simulation tool, and then the dynamic pressure drop of
packings was calculated using the proprietary hydraulic rating
toola
. The gas static head inside the HP column was calculated
manually using Eq. 1:
∆Pstatic pressure drop in HP
where,
ρg
= ρg × g × h
(1)
is the average gas density in the HP column,
g is the gravitational acceleration and h refers to
the total vertical distance between the pressure taps.
The relevant data are listed in TABLE 1. The measured pressured
drop is about three times the dynamic pressure drop preGas
Processing & LNG | MARCH/APRIL 2022 19

Hydrocarbon Processing - April 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - April 2022

Contents
Hydrocarbon Processing - April 2022 - Cover1
Hydrocarbon Processing - April 2022 - Cover2
Hydrocarbon Processing - April 2022 - Contents
Hydrocarbon Processing - April 2022 - 4
Hydrocarbon Processing - April 2022 - 5
Hydrocarbon Processing - April 2022 - 6
Hydrocarbon Processing - April 2022 - 7
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Hydrocarbon Processing - April 2022 - 88A
Hydrocarbon Processing - April 2022 - 88B
Hydrocarbon Processing - April 2022 - 89
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Hydrocarbon Processing - April 2022 - Cover3
Hydrocarbon Processing - April 2022 - Cover4
Hydrocarbon Processing - April 2022 - GP-1
Hydrocarbon Processing - April 2022 - GP-2
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Hydrocarbon Processing - April 2022 - GP-19
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Hydrocarbon Processing - April 2022 - GP-21
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Hydrocarbon Processing - April 2022 - GP-23
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