Note: Descriptions are shown in the official language in which they were submitted.
W,('_92/13620 21 O ~ 2 7 6 PCT/CA92/00049
AN ACTIVE LI~IJID DISTRIB1JTOR
CQNTAINING PACR D COl.lJI~I
This invention relates to an active liquid
distributor containing, packed column. The present
invention is particu~arly useful for mass transfer
operations, such as, for example, distillation,
absorption stripping and liquid extraction.
It has already been proposed in, for example, United
States Patent No. 2,639,130, dated May l9, 1953
"Apparatus For Equalizing Distribution ln packed Columns"
P.N. Heere, to provide successive, radially extending
vane distributors ~n a pac~ed column to intercept liquid
flowing toward the column wall and to return it to
definite predetermined points across the column. This
compensates for the action of the upflowing phase, the
resistance of the column wall, surface tension and the
action of the packing itself, causing the liquid to have
a tendency to flow outwardly to the column wall and then
down the wall, and thus escaping direct contact with the
upwardly flowing gaseous phase.
-Whi~e the-packed c~lumns of Heere are useful
~l) the distributors contribute little to
increasing the gas/liquid contact and mass transfer,
(2) the distributors do not evenly distribute the
downward flow of liquid through the packed bed,
(3) the distributors may become flooded and cause
maldistribution of the liquid and lower the gas/liquid
contacting efficiency of the column.
It has already been proposed in, for example, United
States Patent No. 4,504,426, dated March 12, 1985.
"Gas-Li~uid Contacting Apparatus'l, K.T. Chuang et al, to
provide perforated, liquid distributing and frothing
trays at different levels in a gas-liquid contacting
column. Perforated trays generally of this type have
been used in upwardly extending, liquid distributing
spaces which separate packed beds in a packed column. It
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W092/13620 PCT/CA92/000
has always been considered essential for the perforated
trays to be placed in these spaces, which separate packed
beds and contribute nothing to the mass transfer between
the gas and the liquid, in order to provide unobstructed
flow of liquid across the perforated trays to ensure
uniform liquid distribution across the perforated trays
which is essential for maintaining uniform wetting of the
packing.
In a recent paper entitled "Troubleshooting a Packed
}0 Vacuum Column'i, the authors, McMullan et al. presented
the results of tests on a packed vacuum column at AlChE
Annual Meeting in Chicago on November 11-16, 1990. The
authors concluded from the results that the development
of uneven liquid distribution in a packed column is
inevitable and, for best performance, liquid should be
remixed in packing free portions of the column at
dif~erent levels between the packed beds therein.
It has also been proposed in "Better sieve tray?
Add packing" A-B.S.~. Salem et al, Hydrocarbon
Processing, May 1988, p. 76-G , and "Performance of
Combined Mesh Packing and Sieve Tray in Distillation" to
add a s~allow bed of packing material on top of sieve
trays to increase the efficiency of a sieve tray tower.
This development was also investigated by me in Chen et
al, "Performance of Combined Mesh Packing and Sieve Tray
in Distillation", Cdn. J. of Chem. Eng., Vol 68, June
1990. The systems investigated in these articles did not
provide for distribution of liquid onto the underlying
packing because the development relied on the use of
sieve trays. As is appreciated this type of tray
provides solely for transverse flow of liquid across the
tray towards its downcomer and thereby provides a
transverse distribution of liquid in the packing. As the
liquid flows across the tray, vapour bubbles upwardly
through the transversely flowing liquid to develop a
froth. It is difficult to maintain a level of froth in
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W~2/13620 2 ~ 7 ~ PCT/CA92/00049
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the packing but for the purposes described in these
articles an improvement in performance was noted.
There is a need for a packed column wherein lnstead
of standard perforated trays for supporting packing, or
other types of perforated trays in spaces between packed
beds, or inactive distributors, i.e. the distributors are
inactive by insignificantly contributing to the
gas/liquid contact, active distributors are provided,
which significantly contribute to the gas/liquid contact,
at the same time provide direct support for the packing,
and provide good liquid remixing.
According to the present invention there is provided
a packed column for use in gas/liquid mass transfer
operations, which includes:
a) an upwardly extending casing having,
i) an upper, liquid receiving, casing
interior portion,
ii) an intermediate, casing interior portion
forming a flow path for the flow o~ active
liquid downwardly therethrough and the
flow of gas upwardly therethrough, and
iii~ a lower, ~iquid col~ecting, casi~g
interior portion.
b) a gas outlet from the casing above the liquid
receiving, casing interior portion,
c) means for distributing liquid across the liquid
receiving, casing interior portion,
d) a liquid outlet from the liquid collecting,
casing interior portion,
e) a gas inlet to the liquid collecting, casing
interior portion,
f) a series of spaced apart porous trays
partitioning transversely the whole of the flow path in
the intermediate, interior portion at different levels,
each tray being perforated across the partitioning area
thereof, and for directly supporting a packed bed
thereon,
W092/13620 ~ 6 PCT/CA92/0~0
g) a plurality of discrete, gas/liquid contacting,
packing elements supported on each tray,
h) the improvement comprising:
i) the packing elements forming packed beds,
with each packed bed filling the whole of
the partitioned part of the inter~ediate,
casing interior portion above the tray on
which that packed bed is supported, and
ii) the trays are active liquid frothing and
distributing trays, to provide in
operation a desired degree of controlled
liquid level on each tray to promote
frothing above the tray and within the
packing whereby each tray, together with
substantially the whole of each packed
bed, and thus substantially all of the
intermediate casing interior portion,
contributing signi~icantly to gas/liquid
contact in the casing.
In the accompanying drawings which illustrate, by
way of example, embodiments of the present invention,
F~3u~e 1 - ~5 a -diagrammatic side-view-of a-liquid-
distributing, packed column.
Figure 2 is a plan view along II-II, Figure l,
Figure 3 is a similar view to Figure 1 but of an
upwardly extending portion of a different packed column,
Figure 4 is a view along IV-IV, Figure 3 with the
packing,
Figure 5 is a graph showing effectiveness of
different kypes of packing, and
Figure 6 is a graph showing mass transfer
performance of various tower designs.
In Figures 1 and 2 there is shown a preferred
embodiment of an active, liquid distributor containing,
packed column, which includes:
a) an upwardly extending casing 1, having,
,
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W ~ 2/13620 21 O 1 2 7 6 PCT/CA92/00049
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- i~ an upper, liquid receiving, casing interior portion 2,
ii) an intermediate, casing interior portion 4
forming a flow path for the flow of active
liquid downwardly therethrough and the
flow of gas upwardly therethrough, and
iii) a lower, liquid collecting, casing
interior portion 6,
b) a gas outlet 8 from the casing 1 above the
liquid receiving, casing interior portion 2,
c) means lo for distributing liquid asross the
liquid receiving, casing interior portion 2,
d) a liquid outlet 12 from the liquid collecting,
casing interior portion 6 which in operation, is sealed
against the escape of gas therethrough,
e) a gas inlet 13 to the liquid collecting casing
interior portion,
f) a series of porous trays, 14 to 16, partitioning
transversely the whole of the flow path in the
intermediate, casing interior portion 4 at different
levels, each tray 14 to 16 being perforated across the
whole partitioning area thereof, and for directly
supporting a packed bed thereon and, in operation,
together with the packed bed supported directly
thereabove, distributing active liquid across any packed
bed therebelow, and
g) a plurality of discrete, gas/liquid contacting,
packing elements }8 supported on each try 14 to 16, and
wherein the improvement comprises,
i) the packing elements 18 form packed beds
20 to 22, with each packed bed 20 to 22
filling the whole of the partitioned part
of the intermediate, casing interior
portion 4 above the tray 14 to 16 on which
that packed bed 20 to 22 is supported,
ii) the trays are active liquid frothing and
distributing trays, to provide in
W092/13620 2 10 1 2 '1 ~ PCT/C~92/000~ ~
operation, a desired degree of controlled
liquid level on each tray to provide
frothing above the tray and within the
packing whereby each tray 14 to 16,
together with substantially the whole of
each packed bed 20 to 22, and thus
substantially all of the intermediate,
casing interior portion 4, contributing
significantly to gas/liq~lid contact in the
casing 1.
The device 10 for dis~ributing liquid across the
liquid receiving, casing interior portion 2 comprises a
pipe 24 with space apart liquid spray nozzles 26 to
distribute liquid across the top of the packing.
The liquid outlet 12 comprises a pipe 27 with a
U-shape trap 28 forming a liquid seal at the tower base
to facilitate liquid removal and prevent escape of vapour
through the pipe 27.
~n operation, liquid is sprayed on to the uppermost
packed bed 22 while gas is fed through the inlet 13 and
upwardly through the lowermost packed bed 20.
The liquid trickles downwardly through the packed
beds 20 to 22 in the direction of arrow 7 while the gas
passes upwardly through them in the direction of arrows 9
to demonstrate the counterflow aspect of the vapour and
liquid flows.
At each of the trays 14 to 16, the gas passing
upwardly through the perforations in the trays causes
liquid immediately above them to be frothed to a level
generally indicated at 23. It has been discovered that
significant bene~it can be derived from developing a
significant froth height in the packing above the
respective tray. According to this invention, the
feature of frothing can only be accomplished by using an
active liquid frothing and distributing tray. The term
active is intended to distinguish the subject tray from
tray designs which do not promote or sustain a frothing
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W~92/13620 ~ 2 7 ~ P~T/CA92/00049
action. Active trays would naturally exclude the use of
perforated trays which do not provide for a controlled
hold-up of liquid on the tray because upwardly flowing
vapour can not develop a froth in the packing above the
tray.- Such trays which do not normally provide for
liquid hold-up are the common packing support tray which
has relatively large perforations which allow the liquid
to flow freely therethrough yet retain the packing pieces
above the tray. Furthermore, trays which preclude or
inhibit frothing by other means are excluded. Such trays
would include those which only provide for an upflow of
vapour ~hrough the tray with little or no downflow of
liquid through the tray, for example, sieve trays ancl
bubble cap trays which involve transverse flow of lic~id
across the entire tray toward a downcomer to the
corresponding side of the next tray are excluded.
Acceptable active trays are therefore of a category
which can develop the desired degree of frothing in the
packing while allowing liquid to ~low downwardly through
the tray onto packing immediately beneath the tray. The
tray has a sufficient number of perforations or apertures
to provide for adequate re-distribution of downflowing
liquid over the immediately underlying packing. It is
appreciated that a variety of tray configurations with
aperture designs can be provided to accomplish this
feature of an active type of tray. As a guideline the
tray configuration can be best characterized according to
a preferred embodiment of the invention where the
perforated area of the tray can be in the range of 10% to
40% of the cross-sectional area of the tower which is
partitioned by the tray. Exemplary types of trays are
shown in Figures 2 and 4. The aperture sizing is
preferably in the range of 0.125 to l.0 in. (3.2 to 25.4
mm). The tray of Figure 2 is similar to the types of
support trays commonly used at the base of a packed
tower, only the modified extent of perforations and size
of perforations are formed in a manner so as to be within
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WO92/13620 ~l ~i ~ 7 6 PCT/CA92/00049~
8 \ 'L
the above ranges. The tray of Figure 4 is simiIar to the
tray of Figure 2, only the section is modified across its
surface to provide parallel rows of downcomer troughs.
Most of liquid flows downwardly through the downcomers
and is distributed over the underlying bed, while vapour
flows upwardly through the perforated plate portions
between the downcomers. The downcomers are provided with
weirs to hold back on the tray sufficient liquid to
develop the desired froth height above the tray and
within the packing. With reference to the tower of
Figure l, the packed beds 20 to 22 fill the whole of the
partitioned spaces of the intermediary casing interior
portion 4, and the amount of perforated area of, and the
cross sectional areas in the, perforated trays 14 to 16
are chosen to provide a desired degree of controlled
liquid level on each tray to promote frothing above the
tray and within the packing so that each active tray 14
to 16, together with substantially the whole of each
packed bed 20 to 22, contributes significantly to
gas/liquid contact within the casing l.
For best results, each tray 14 to 16 has a
perforated area within the aforementioned range, that is
the preferred more limited range 20% to 40% of the
cross-sectional area partitioned by that tray, and
perforations 17 as shown in Fig. 2 having a diameter in
the range 0.125 to l.0 ins ~3.2 to 25.4 mm). With this
type of active plate structure, the desired froth heights
23 are achieved and maintained during operation as well
as the desired distribution of down~lowing liquid.
Also ~or best results, the trays 14 to 16 partition
the flow path in the casing l at heights in the range of
lO to 80 ins. (250 to 2000 mm).
The preferred packings include random type packings,
e.g. Glitsch~, Mini-rings~, Norton~, metal Intalox
saddles~j and ordered-bed type packings, e.g. Koch~,
FlexpacX, York mesh~, Glitsch~, GoodlocD, and Sulzer Bx~
packings.
W092/13620 2 ~ 012 7 6 PCT/CA92/00049
In Figures 3 and 4 as previously described, the
trays 14 to 16 are according to an alternative embodiment
of this invention, provided with downcomers such as the
downcomers 32 to 35 shown for tray 16. Each downcomer 32
to 35 has overflow weirs, such as overflow weirs 36 and
38, and perforated side plates and bottom plates, such as
side plates 40 and 42 and bottom plate 44. The
arrangement for the plate functions in the same manner as
the plate of Figure ~. The weirs 36 and 38 are of
sufficient height to hold-up liquid on the tray surface
above perforated portions 30 between respective downcomer
troughs so that upwardly flowing vapour through apertures
17 in the direction of arrows 45 develop the desired
froth height in the packing as generally designated at
level 23 of Figure 3. The number of downcomer troughs 32
through 35 are selected for the size of tray to provide
adequate distribution of downflowing liquid over packing
located immediately beneath the tray. As demonstrated by
arrows 46, the downcomer troughs direct the flow of
liquid in a dispersed manner so as to distribute the
downflowing liquid substantially evenly over the
underlying packing.
Tests to verify the present invention were made
using a 30-cm diameter methanol/water distillation column
which had 5 trays, adjacent trays being spaced apart by
46 cm. The tests showed that, at low methanol
concentrations for a multiple of 20% perforation
dual-flow trays of the type shown in Figure 2 combined
with 46 cm beds packed with 25 mm Mini-rings, the Height
Equivalent to a Theoretical plate ~HETP) was 12 in. as
compared with an HETP value of 30 in. for the same
packing but equipped with conventional packing support
plate. The results indicated that the present invention
can improve the packed column efficiency by a factor of
2.5 which is the ratio of HETP of the prior art to the
HEPT of this invention i.e: 30/12.
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WO92/13620 PCT/CA92/0004
Further tests were conducted to demonstrate the
improvements in columns efficiencies provided by this
invention. A column of the type shown in Figure 1 was
used to separate a methanol (MeOH) and water (H2O)
mixture. The effectiveness of various packings was
investigated to determine the best packing for the tower.
The results of Figure 5 indicate the best packing to be
the mini-rings based on the lo~est value for the height
equivalent to a theoretical plate (HETP) of the packing.
In Figure 5, the F-factor is an expression of gas kinetic
energy which is defined as Uv ~Pv where Uv is vapour
velocity in m/s and Pv is vapour density in kg/m2. The
results in operating the tower under various conditions
including with or without the preferred mini-rings is
shown in Figure 6. A comparison of the performance is
provided of the dual flow tray of Figure 2, a column of
mini-rings without any separating trays and the invention
which consisted of the dual flow trays and full packing
of the spaces between trays with the mini-rings.
In Figure 6, the NTP/m is equal to the number of
theoretical plates per metre which is the inverse of HETP
ie: 1/HETP. The plot of the data derived from operating
the tower under the above conditions is done for a value
of F = 1.6. From Figure 6, the increase in performance
is more than a factor of 2 in comparing the dual flow
tray to this invention of the tray with full mini-ring
packing. The invention also performs better than
standard packing of mini-rings by a factor of at least
1.3.
Although not indicated on the graph of Figure 6, the
perfo~mance of a sieve tray having a layer of packing
thereon as suggested by Chen et al (supra) is
approximately 1.1 times more effective than the operation
of the dual flow tray without packing. Hence the
arrangement of this invention provides an increase in
performance over Chen et al (supra) by a factor of 2 or
greater. This is an extremely significant development in
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W092/13620 21 O 12 7 ~ PCT/CA92/00049
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the field of mass transfer when one compares the 200
benefit versus a 10% increase in efficiency which is
normally considered quite significant.
According to this invention, the positioning of
active liquid frothing and distributing trays throughout
the column provides for a more fr~quent re-distribution
of downflowing liquid while enhancing the gas/liquid
contact throughout the height of the column. Unlike
normal systems which would include liquid distributors
every 6 to 8 m in a tower, the system of this invention
provides a distributor inherent in the active tray, every
0.3 to 0.5 m. As already noted, the active tray may be
of the type shown in Figures 2 and 4 where it is
understood that the downcomer tray of Figure 4 is more
useful in large diameter applications e.g. in excess of 1
m diameter or in large liquid downflow rates.
Although preferred embodiments of the invention are
described herein in detail, it will be understood by
those skilled in the art that variations may be made
thereto without departing from the spirit of the
invention or the scope of the appended claims.
