Defying the Force of Gravity
Ultra-High-Capacity
ConSep Trays
DANIEL SUMMERS
SULZER CHEMTECH
4160
In recent years, the petrochemical and refining
industries have placed an emphasis on
maximizing the capacity of existing plants. To
exploit the maximum capacity of the complete
plant, it is necessary to identify and to
debottleneck specifically those units and column
internals that limit the overall volume. In alliance
with Shell, Sulzer Chemtech offers distillation
trays that can operate at a capacity beyond
anything stationary trays were able to achieve until
now. The ConSep® tray has demonstrated its
potential not only in research facilities but in
numerous commercial applications as well.
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The Shell ConSep tray was the
first device tested at the independent research consortium,
Fractionation Research Inc. (FRI),
to break through the limitation
faced by all distillation devices,
namely gravity (Fig. 1). Without
this limitation, process engineers
can now design columns with
a distillation capacity 40–50% higher than that of devices limited by
gravity. The ConSep tray is a product of the alliance between Shell
Global Solutions and Sulzer Chemtech (see STR 2/2005, p. 11).
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1 Sulzer Chemtech and Shell Global Solutions have partnered to provide
column internals for ultra-high-capacity applications. The combination of a
conventional sieve tray with high-capacity swirl tubes enables the capacity
of distillation trays to overcome the limits posed by gravity.
Combining Proven Technologies
A traditional distillation tray purifies the valuable products by
forcing vapor bubbles upward
through a pool of liquid and securing suitable interface contacting. The degree of mixing and the
time it takes for the vapor to contact with the liquid determine how
well a distillation tray performs.
The upward-flowing vapor must
“shed” its liquid before it travels
up to the next distillation tray. If
the vapor drags extra liquid with
it—as it would when the system is
operating close to maximum tray
capacity—this liquid is carried upward into the next tray, and will
eventually fill the tray above. This
phenomenon is called flooding
System Limit
The system limit of a column internal is defined as the point when the upward shear
force of the vapor will just equal the gravitational pull on the liquid droplets in the
tower. This behavior sometimes is erroneously referred to as “Stokes’ Law,” which
only applies to the laminar vapor flow
regime. A distillation tray, however, oper-
30
Liquid rate m3/h-m2
SULZER TECHNICAL REVIEW 3+4/2005
20
40
60
80
100
120
140
160
0.50
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0
0.40
0.30
0.20
ConSep tray
operation
0.10
0
0
10
20
30
40
50
Liquid rate, gpm/ft2
60
70
Vapor capacity
factor Cs, m/s
0
Vapor capacity
factor Cs, ft/s
Physical constrictions limit the capacity
of gravity-driven distillation trays. The
ConSep tray overcomes these restrictions and can be operated at a capacity
well above that of a conventional highcapacity tray. Capacity factor—a measure of vapor velocity—and liquid rate—
the fluid discharge—are based on tower
cross-sectional area.
ates very far away from laminar flow. Vapor velocities above the system limit are
not possible for normal distillation trays
that are limited by gravity. The ConSep tray
uses centrifugal force instead of gravity as
the mechanism to disengage the liquid
from the vapor before the vapor passes to
the next tray above.
and limits both the capacity and
the separation effect of the distillation tower.
For many years, Shell has used
swirl tubes in knockout drums and
vapor-liquid separators to isolate
entrained liquid from vapor
streams. In 1995, the application
of the swirl-tube technology to
normal high-capacity distillation
trays led to the development of the
ConSep tray.
Because it has separate contacting
and separation tray decks on one
single tray, the ConSep tray can
defy the force of gravity. The swirl
tubes enable the tray to operate
fully flooded; only vapor reaches
the tray above (Fig. 2). FRI tested
performance and capacity of these
trays at commercial boundary conditions and found the capacity increase to be “substantial,” and a
FRI topical report from July 2004
noted that “this device offers exciting new possibilities in debottlenecking distillation columns.”
The first commercial application
was a debutanizer in a Shell natural gas liquids (NGL) facility. Natural gas liquids—ethane, propane,
butane, isobutene, and natural
gasoline—are usually removed
from natural gas, which is used as
heating and cooking fuel. After the
NGLs are removed, they are reprocessed in the fractionator to
separate them for individual sale.
The debutanizer’s previous performance indicated that it was the
main limiting factor for a further
capacity increase of the NGL plant.
Since this column was already
equipped with conventional highcapacity trays, the use of a novel
ultra-high-capacity ConSep tray
design was considered to be the
only feasible option to increase the
capacity while maintaining the existing column vessel.
At the time of this project, the
ConSep tray had just been developed, but had not yet been installed at any plant. Nevertheless,
confidence in the ConSep technology for this revamp was very high
for a number of reasons:
The ConSep tray is, in principle,
just a normal tray for which the
basic design rules are well established.
The additional separator technology used for ConSep is based
on a long tradition of separator
technology within Shell. Furthermore, key design criteria
such as separator pressure drop
and separation efficiency are
well-known, because they have
been tested in the laboratory under realistic conditions.
During research, the ConSep
tray was tested from low to high
pressure under realistic hydrocarbon test conditions.
2 In a ConSep tray, guide
vanes cause a rotation of the
upward flow inside the swirl
tubes. The centrifugal force
directs the liquid toward the
walls of the swirlers. The
vapor, which has less mass
and momentum than the
liquid, travels through the
center of the swirlers nearly
free of all liquid.
Clear vapor
Guide vanes
Liquid entrainment
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The hydraulic loads required to
achieve the revamp targets
were too high for a conventional high-capacity tray, but still
about 30% below the proven
capacity of the ConSep tray.
creased by more than 20% after the
revamp. In 2005, the column is in
its tenth year of operation with
consistent good performance and
trouble-free operation.
Ten Years of
Trouble-free Operation
To further validate the design,
additional instrumentation was installed on this first commercial
application. Some trays were
equipped with instrumentation to
measure the downcomer backup
and liquid levels on the trays after
the revamp. The results from these
measurements confirmed the FRI
findings and showed that the trays
were still operating well below
their flood point (Fig. 3).
For the debutanizer, a one-for-one
tray replacement with ConSep
trays was selected. For this particular revamp, a conservative tray
efficiency was assumed, which increased the reflux requirement by
about 10%. The hydraulic capacity
of the trays was expected to increase by about 50%; therefore, the
overall capacity of the debutanizer
was not in jeopardy. The capacity
of the NGL train ultimately was in-
140
120
Tray efficiency (%)
Successful Plant Application
100
80
60
VGPlus tray
40
ConSep tray
40% over
VGPlus
20
0
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Column load factor (m/s)
3 FRI measured the efficiency of several high-capacity
trays. The measurement results show a remarkable
increase in capacity for the ConSep tray with only a
slight loss in tray efficiency.
Contact
Dan Summers
Sulzer Chemtech USA Inc.
4019 South Jackson Avenue
Tulsa, OK 74107
USA
Phone +1 (1)918 447 76 54
Fax
+1 (1)918 446 53 21
dan.summers@sulzer.com
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