Note: Descriptions are shown in the official language in which they were submitted.
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'-GAS-LIQUID CONTACTING TRAY WITH
SIDE DISCHARGING TRIANGULAR DOWNCOMERS"
FIELD
The invention relates to an improved high capacity
gas-liquid contacting apparatus used primarily as
fractionation trays for the separation of volatile chemical
compounds in a fractional distillation column.
BACKGROUND
US-A-3410540 illustrates a fractionation tray
design comprising alternating flat decking sections and
downcomers. This tray design employs a conventional
rectangular cross-section downcomer similar to that employed
in one embodiment of the subject invention.. Figure 11 shows
the use -of a contacting deck which has a saw-tooth shape
rather than the normal flat deck.
GB-A-2232365 illustrates a contacting column
having layers of packing located under perforated contacting
trays. The use of perforations in downcomer sidewalls to
allow the descent of liquid onto the packing beds is
mentioned.
US-A-2767967 :illustrates a type of dual-flow tray
referred to in the art as a ripple-tray. In this tray the
rising vapor and descending liquid both pass through
openings in the surface of the tray deck. The deck may have
many topologies ranging from the ainosoidal curve of Figures
3 and 4 to the more planar shape of Figures 5 and 6. The
variations in the elevation allow for less liquid depth on
higher portions of tray which in turn allows for upward
vapor passage, while liquid descends through the tray at
points which allow for a greater liquid depth.
US-A=4499035 j.a believed pertinent for its showing
of an arrangement on the surface of cross-flow vapor-liquid
contacting trays which employs both circular perforations
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and the vapor-directing slots on the decking panels of the
tray.
The use of "anti-jump~~ baffles located above the
inlet to an outlet downcomer is known in the art.
SUMMARY
The subject invention is a high capacity
fractionation tray which employs downcomers having a
triangular cross-section. The present invention provides a
highcapacity gas-liquid contacting apparatus possessing
good efficiency with features reminiscent of ripple trays
and Multiple Downcomer trays. The triangular cross-section
downcomers extend downward only a portion of the distance to
the next tray, with liquid exiting near the bottom of the
imperforate downcomer aidewalls.
One embodiment of the invention may be
characterized as a gas-liquid contactingtray having upper
and lower surfaces and comprising a perforated horizontal
tray deck comprising a plurality of substantially planar -
elongated parallel vapor-liquid contacting areas; at least
two downcomers, each downcomer bordering two of the planar-
vapor-liquid contacting areas and having an open rectangular
inlet located on the upper surface of the tray and with at
least one of said downcomers comprising a pair of planar
sidewalls which are attached to and inclined at
substantially equal angles from the tray deck such that the
portions of sidewalls located furthest away from the lower
surface of the tray deck join together to form a sealed V-
shaped bottom portion of the downcomer; and, means to allow
downward liquid flow from the tray comprising a row of
openings in the downcomer sidewalls, with all of the
openings being located. in the one-third of the sidewall
which is located furthest from the tray deck.
In some embodiments of the present invention a bed
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of structured or random (dumped) packing material is
suspended below the horizontal decking of the tray, with the
packing being wetted by liquid exiting the downcomers.
DRAWINGS
Figure 1 is a diagrammatic, sectional side view of
one embodiment ofthe subject gas-liquid contacting
apparatus employed as a part of a fractionation column 1.
Figure 2 is an enlarged, sectional isometric side -
view of a portion of a tray similar to the lowermost tray
shown in Figure 1 but with the packing 13 extending to a
higher elevation.
Figure 3 is a sectional view of a triangular
cross-section downcomer showing details of two optional tray
variations including vapor-directing slots 10.
Figure 4 is a diagrammatic, sectional side view of
a column 1 containing three fractionation trays arranged
according to another embodiment of the invention.
Figure 5 is a sectional side view of a portion of
a column employing both triangular and rectangular cross
section downcomers plus packing beds 13 below the tray
decks.
Figure 6 is a sectional side view of a
fractionation tray formed from adjoining V-shaped channels.
Figure 7 is an enlarged view of a section of the
tray shown in Figure 6.
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DETAILED DESCRIPTION
Gas-liquid contacting trays find utility in the
separation of a wide variety of chemicals including
hydrocarbons in petroleum refineries, petrochemical and
chemical manufacturing plants. As with any useful
industrial equipment, there are constant demands to improve
the performance and lower the coat of the equipment.
Several different types of trays, each having its
own advantages and disadvantages have been developed. One
of the biggest disadvantages of some trays is the cost of
manufacturing the tray, which is greatly influenced by the
complexity of its design. The more pieces required to
assemble a tray, the more it costs to fabricate and then
assemble the pieces into the finished tray. Therefore a
very simple tray, such as a dual flow tray, should have the
advantage of low manufacturing costs. A dual flow tray
typically comprises a flat deck with uniform perforations
sized large enough to allow both liquid to descend and vapor
to rise through the same openings. Dual flow trays
therefore do not require downcomers or other accessories and
are low in cost. However, dual flow trays tend not to work
very well at tray diameters larger than four feet. Cross-
flow trays use down,comers to achieve higher tray
efficiencies but have the disadvantage of being more costly
to fabricate and install. Ripple trays are similar to dual
flow trays but have variations in the height of the tray
deck as shown in US-A-2767967. These variations provide
depressions which allow liquid to collect and drain to the
next tray much like a downcomer. Dual flow and ripple trays
are also very sensitive to departures from the optimum
(design) fluid flow rates.
It is an objective of this invention to provide a -
low cost high-capacity fractionation tray. It is a further
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objective of the invention to provide increased separation
efficiency in high capacity fractionation trays.
The subject trays have large flat tray deck areas
reminiscent of dual flow trays but also have depressed areas
similar to ripple trays. The depressed areas however are
partially blanked-off sections or are formed from
substantially imperforate sidewalls with openings for liquid
flow being provided only in selected regions of the sidewall
including near the bottom portion of these depressed areas.
The subject trays also differ from some other trays in that
they can have a high percentage of . open area -- the area
provided by perforations in the tray deck -- on the order of
40%, like dual flow trays. This open area can range from 35
- 45%. In contrast a normal cross-flow sieve tray or a
Multiple Downcomer tray will usually have an open area less
than about 20%. Another characteristic of the subject
invention is the use of relatively large perforations onthe
order of 0.95 - 1.3 cm (3/8 - 1/2 inch) diameter in the case
of round openings as compared to the typical sieve tray
opening of about 0.5 cm (3/16 inch) diameter.
As the invention relates to the design of a
downcomer which is only one part of a tray, there are a wide
variety of different overall tray constructions~which can
employ the invention.
The accompanying drawings -illustrate by way of
examplA some embodiments of the present invention.
Referring now to Figure 1, there is shown a portion of a
gas-liquid contacting apparatus comprising a cyliwdrical
outer vessel, 1, employed as a fractionation column. A feed
stream containing a mixture of chemical compounds having
different relative volatilities enters the column via means
not shown and falls onto a fractionation tray 2. The column
contains a plurality, e.g., 10-300 of-these fractionation or
frothing trays located at uniform vertical intervals within
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the interval volume of the outer vessel. In the embodiment
of the invention shown in Figure 1 each tray 2 comprises
four trough-like downcomers having a triangular cross
section spaced across the tray, with sections of planar tray
deck 8 being adjacent each downcomer sidewall 3. Vapor
rises upward through perforations 7 in the deck sections of
the tray. Liquid descends through the column and is
collected on each tray in the downcomers, with a portion of
the liquid jetting out of perforations b in the downcomer
sidewall.
Figure 2 illustrates the use of an optional bed 13
of packing located beneath a lowermost tray deck. That is,
the tray may be modified by the addition ofpacking between
and-below the downcomers as desired. Theperforations in
this instance are employed as a means to spread liquid onto
the packing bed(s). The packing 13 may consist of
individual beds which fill the volumes between adjacent
downcomers and also extend a short distance below the bottom
of the downcomers.
The great majority of each downcomer is located
below the horizontal plane of the tray with only a smaller
portion located-above the tray. Optional vertical walls 9
may extend above the tray and terminate with a straight
upper edge 5which functions as the inlet weir to the
downcomer.
As also shown in Figure 1-, the downcomers on a
particular tray are oriented perpendicular to those on the
vertically adjacent trays. Thus in the view shown in the
Figure one is looking endwise down the length of the
downcomers on the top and bottom trays but across the
downcomer toward a sidewall on themiddle tray. This is a
normally optimal design configuration but can be departed
from if desired.
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The trays of the subject invention have some
structural features simi7.ar to Multiple Downcomer trays of
US-A-3410540. For instance, the subject tray does not have
a "receiving pan". This is the normally imperforate section
located below an inlet downcomer opening. A-receiving pan
is the imperforate area of a tray upon which the liquid
descending through the downcomer impacts before passing
horizontally onto the decking of the tray. Often the
receiving pan is separated from the active decking area of
the tray by an inlet weir. Receiving pans are therefore
normally located directly below the downcomer leading from
the next above conventional fractionation tray. The
horizontal surface area of the present fractionation tray is
divided into depressed areas functioning as downcomer means
and flat vapor-liquid contacting area-normally referred to
as decking. There are no imperforate areas allocated to
receiving descending liquid from the tray located
immediately above.
Another distinguishing feature of the subject
fractionation trays is the provision of a relatively large
number- of the triangular cross-section downcomer means
across the tray. The traye can employ from one to fifteen
or-more downcomers. These downcomer means are spaced
relatively close together compared- to the customary
crossflow fractionation trays as they are spread across the
surface of the tray rather than being at the periphery of
the tray. The distance between -adjacent downcomers
(measured between their aidewalls) of the same tray will be
between 0.2 and 1.0 meters and preferably less than 0.5
meter. This results in a tray having a unique design
consisting of the alternating decking areas and downcomer
means evenly spaced across the 'upper surface of the
fractionation tray, as shown in Figures 1 and 4.
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The downcomer means of the subject trays are also
unique compared to the downcomers employed upon normal
cross-flow fractionation trays. The downcomer means do not
extend downward to the next fractionation tray. Rather they
stop at an intermediate level located between the two trays. '
The downcomer descending from the tray above therefore stops
well above the deck surface of the lower tray and the inlet
to the downcomera of the tray below. The top or inlet to
the downcomer of the tray functions as the outlet weir of
the tray, and the bottom of the downcomer of a multiple
downcomer tray above is therefore well above the outlet weir
of the tray located below. Thus there-is no inlet weir at
the bottom of the downcomer of the tray as there is on a
cross-flow tray.
Another - distinguishing- feature of - Multiple
Downcomer type fractionation trays is the provision of a
liquid sealable means near the bottom of the downcomer
means. The bottom of the downcomer means is closed off to
retard- the direct downward flow of -liquid out of the
downcomer means. This liquid sealable outlet is located
well above the deck of -the tray located immediately below
and is at a level above the inlet of the downcomers
associated with this next lower tray. The descending liquid
is collected in the lower portion of the downcomer means and
spills forth upon the next lower tray through openings in
the side of-the downcomer side walls.
The major embodiments of subject trays are similar
to Multiple Dovmcomer trays in that they use a plurality of
narrow, open-ended parallel downcomers to divide the tray
deck into strips of planar decking. The downcomers do not
extend to the tray be7.ow but instead terminate at an
intermediate point. The subject trays are also similar to
ripple trays in that a depressed portion of the tray deck is
used for liquid transfer to the next-lower tray. In the
a
subject trays the depth of these depressions, that is, the
depth of the triangular downcomers, is only from 5-10
centimeters. Another difference from ripple trays is that
these depressions are partially imperforate while ripple -
trays have very uniform perforations.
Figure 2 provides a better illustration of -the
preferred triangular cross-section of the downcomers which
are formed from two opposing inclined sidewal k 3. The
single row of spaced apart openings 6 in the bottom portion
of the inclined sidewalk allows liquid to flow out of the
downcomer toward the next lower tray. Several rows of holes
can be provided if desired. Vapor flows upward through
perforations 7 in the decking 8.
Figure 3-presents a cross-sectional sideview of a
small portion of a tray with a single triangular downcomer.
The perforations in the tray deck 8 are provided by vapor
directing slots 10 formed by a mechanical pressing and
cutting operation. A detailed view of a flow directing slot
is presented in Figure 5 of US-A-4101610 and in Figure 6 of
US-A-4499035. These slots are employed in one embodiment of
the invention to cause the froth which forms on the surface
of tray deck to move toward the downcomer inlet. This is
intended to increase the capacity of trays. In addition a
vertical substantially imperforate wall or baffle 11
supported by members 12 may be centrally placed over the
downcomer inlet to intercept horizontally flowing froth,
thus increasing the capacity of the tray.
The perforations 6 in the downcomer sidewalls are
preferably arranged in a single row running along the major
axis of the downcomer of a tray having flat decking areas.
It is preferred that the holes are located in the aidewalls
rather than along the bottom of the V-shaped downcomer.
This helps impart horizontal velocity to the egressing
liquid causing it to travel away from the downcomer. This
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is beneficial in spreading the liquid over any packing 13
which-is located below the tray hence ensuring. more uniform
wetting of the packing and is also useful in directing the
liquid onto the decking areas of the tray below depending on
the orientation and location of the lower tray decks.
The perforations in the downcomer sidewalls are
preferably circular but could have other shapes including
horizontal or diagonal slots. The use of a smaller number
of larger perforations is preferred although the
perforations should be located more or less uniformly along
the length of both sidewalls of the downcomer in a single
row. Circular openings of 0.5-2.5 centimeters diameter are
suitable. Another important factor in the placement of the
downcomer perforations is the provision of an adequate
distance between the upper surface of the tray, which is
normally coextensive with downcomer inlet, and the
perforations to allow the entering froth to separate into
clear liquid and vapor: This is important to good tray
efficiency and performance in general. This distance should
also provide sufficient liquid head to prevent the upward -
paseage of vapor through the downcomer perforations. This
desirable placement of --the downcomer perforations can be
characterized as being in the lower third of the downcomer
sidewall.
The deck portions between the downcomera is
preferably substantially planar; that is, flat and
horizontal. These decking portions are preferably provided
with uniformly distributed openings of adequate total cross-
sectional area to allow the expected vapor flow to pass
upward through the tray at a reasonable velocity. Uniform
circular openings of a standard sieve tray are preferred but
can be supplemented by the vapor flow directing slots of
Figure 3. The open area provided by deck perforations may
be as high as 30-45% of the tray deck, as compared to a
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lower area of up to 20% normally used on ripple trays. The
circular perforations may be up to 1.27 cm (1/2 inch) in
diameter.
Figures 6 and 7 illustrate a unique embodiment
which has no significant horizontal surface on the vapor -
liquid contacting area of the tray. The tray of this
embodiment, as ahownbest in Figure 6, is composed of a
number of adjacent V-shaped structures which are attached to _
one another. Figure 7 is an enlargement of a portion of the
tray of Figure 6, showing the added upper row of
perforations not needed in the embodiment of Figure 1. The
inclined sidewalls 3 join another sidewall at both the upper
and lower edges of the eidewall, which are horizontal when
the tray is in use. Each sidewall will have at least an
upper and a lower row of openings 6. The upper row allows-
vapor to-pass upward through the tray. The lower row of
openings functions as the openings in the bottom of the
downcomer through which liquid drains downward to the next
tray.
The openings 6 are grouped into two spaced apart
rows in the sidewall to leave an imperforate central region
which runs the length of the sidewall (across the column)
from one:face of 'the inner surface of the column to the
opposing face. Three or more rows of openings can be placed
at different heights from the lower edge but preferably the
openings are staggered such that they do not lay above one
another when viewing the broad surface of the sidewall. The
distance "a" between the center lines of the upper and lower
openings 6 should-be 1/3 to-2/3 of the total height (a + b
+ c) of the sidewall. A. sidewall with a greater height is
not inclined as much from the horizontal as equal inclines
would lead to greater liquid accumulations on the tray. The
vertical distance between the downcomer inlet and the liquid
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outlet perforations near the bottom of the downcomer should
be in the general range of 5-13 cm.
Figure 5 illustrates a portion of a fractionation
column comprising all of one tray 2 and parts of the next
higher and lower trays 2. The trays differ from those of
the other Figures in that downcomers having both triangular
and rectangular cross sections are employed. The aidewalls
3' of- the rectangular downcomer are vertical and therefore
perpendicular to the tray deck 8. Each rectangular
downcomer has a planar bottom plate 15 having openings for
downward liquid-flow. This Figure more clearly illustrates
the preferred- gap 14 between the lower surface of the
decking 8 and the upper surface of the bed of packing 13.
On the middle tray the packing rises above the
sidewall openings 6 which hinders the horizontal
distribution of the liquid. From this standpoint the
arrangement of Figure 1 with the packing being located below
the bottom of the downcomers is preferred. Depending on the
horizontal distance between the adjacent downcomers, it may
be desirable to promote downward liquid flow through the
perforations 7 normally limited to upward vapor flow. This
would aid in maintaining a uniformly wetted bed of packing.
A further feature of Figure 5 is the lack ofany inlet weir
at the upper end of the downcomers.
In Figure 4 there is shown a portion ofa casing
(outer vessel) 1 containing three structurally simple trays
2. Each tray comprises four or five triangular cross-
section downcomers which are aligned parallel to the
downcomera on the next higher and lower trays. The
downcomers are formed by sidewalla 3 inclined at equal
angles from the horizontal decking 8 to form an angle a of -
from 20 to 90 degrees. The downcomers of vertically
adjacent trays are offset .horizontally., This preferred
horizontal offset assures that the liquid descending from
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7
the openings 6 in the downcomer sidewall 3 will fall onto
decking 2 of the tray below rather than into the open-inlet
of the downcomers of the next tray. As the liquid exiting
the downcomer sidewalls will travel horizontally for several
inches, the downcomers could be mounted directly over each
other.
The packing-containing versions of the subject
apparatus can be used in a variety of gas-liquid contacting
operations including absorption, gas treating for impurity
removal or .other processes including fractional
distillation. In each of these processes, gas rises upward
through the column while liquid trickles downwardly through
the packing in a well-distributed manner and emerges
therefrom as droplets which fall onto the frothing trays.
The droplets falling onto the frothing trays form a liquid
level thereon some of which is frothed by the gas passing
upwardly through the column. Some of-the liquid on the tray
may be allowed to escape through the decking perforations,
or specifically provided openings, to form droplets which
are well distributed over the packing.
Tests have shown that the packing sections need
only be short, say 200 mm top to bottom, and so wall effects
are insignificant. It may be noted there is no packing
material on the surface of the frothing trays or in the
volume immediately above the trays. This allows
conventional frothing and liquid flow to occur.
The optional packing beds may contain random
packings; e.g., rings, spheres, saddles, structured or
ordered bed packings; e.g., corrugated, rolled, screens or
plates.
The apparatus according to the present invention
can be in the form of a new apparatus or a modified existing
apparatus. That is, an existing trayed column .may be
modified to employ the subject invention. Any known
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countercurrent tray arrangement; such as, for example, dual-
flow trays, cross-flow sieve trays and Multiple Downcomer
trays may be used.
One embodiment of the invention may be
characterized as a gas-liquid contacting apparatus
comprising a pair of fractionation trays comprising a upper w
and a lower tray, with each fractionation tray comprising a
perforated tray deck having upper and lower surfaces and a
plurality of dowhcomers spaced across the tray deck, each
downcomer having a triangular cross-section defined by two
planar sidewalls attached to and extending away from the
lower surface of the tray deck and joining together to form
a sealed bottom portion of the downcomer with the planar
sidewalls having a substantially imperforate central region
extending along the central (major) axis of the sidewall and
having a width equal to at least 50 percent of the surface
area of the sidewall; a bed of packing material located
between said pair of trays and above the upper surface of -
the lowermost tray; and, means to distribute liquid from the
upper tray over the bed of packing material comprising a
series of perforations located in the one third of each
downcomer sidewall located furthest away from the upper tray
deck.
The beds of packing material may be located in the
normally open spaces located between and/or below adjacent
downcomers. These beds may be retained by a single support
screen at the level of the bottom of the downcomers or by a
number of individual screens extending horizontally between
the downcomers. As a further alternative the packing
support screen may be located some distance below the bottom
of the downcomers with the packing between the downcomers
thus being part of a larger overall packing bed which
extends upward between the downcomers as chown in Figures 2
and 5. A further variation is that the packing may be in
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the form of a single bed of uniform thickness suspended
below the downcomera and above the next lower tray as shown
in Figure 1. In this embodiment the means to supply liquid
to the packing bed comprises the perforations in the
sidewalls.
Openings in the decking material may in addition
also be used for-this purpose. Some portion; e.g., 25-40
vol.% of the liquid flowing across the tray deck 8, may
therefore flow downward through openings 7 in the deck to
allow liquid to flow onto the packirig l3. Those skilled in
the art will recognize there are a number of ways to
distribute liquid from the tray to the packing. It is
preferred to avoid the use of any mechanically complex
system involving conduits, pipes and valves, inclined
troughs, etc. Liquid spread across the packing may be
derived from a downcomer, a separate liquid collection area -
or from the tray deck itself. For-most purposes it is
preferred to utilize some form of "dual flow" tray decking.
That is, the tray intentionally allows liquid to "weep"
downward. Valuing means known to the art may also be placed
on the tray to regulate liquid and vapor flow and to
accommodate variations in these flows due to changes in feed
or reflux rates in the column.
The packing beds may be suspended by a porous
woven wire screen. The screen itself may be held in place
in a number of ways. The screen may rest on a grid or a
plurality of support bars not shown which crisscross the
internal volume of the column in a plane parallel to the
tray. The screen or the individual packing elements may
alternatively be suspended (hung) from the tray above.
These mechanical details may be varied to suit individual
situations and are not deemed a limitation on the inventive
concept.
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The upper surface of the packing bed is not shown
being located adjacent (in contact with) the bottom surface
of the tray deck. Contact of packing with deck openings is
believed to aid weeping which could make such contact
useful. The top of the packing may alternatively be from 1
to 8 centimeters below the tray as shown in Figures 2 and 5,
with a gap of 2-3 centimeters being preferred. Inthis
instance, the packing will also be in contact with at least
a portion of the sidewalls of the downcomer and thus fill
the normally empty volume located -between adjacent
downcomers. With a dumped packing the upper surface of the
bed will be somewhat irregular. The packing material bed
may be in the form of preassembled units which are installed
under-preexisting trays, fabricated in situ under trays or
made as an integral part of the overall tray structure.
Structured packing may be added to the tray as bundles or -
subassemblies which together form the packing bed. A dumped
packing may be added to the bed through openings in the wall
of the outer vessel or through openings in the tray deck
above. This unstructured packing may also be preloaded in
porous retaining basket~a which are then installed under the
tray.
The amount of packing used with any one tray is
preferably equal in volume to less than 50 percent of the
open volume of the column between the upper and lower trays
of the pertinent tray pair. The packing bed of the subject
invention preferably extends downward to within S
centimeters of the intended upper surface of the froth on
the next lower tray. The bottom of the packing is therefore -
well above the top of the next tray. A minimum bed
thickness of 10 cm is desired, with beds up to 150 cm thick
beingcontemplated. Thickness is measured downward from the
lower surface of the upper tray. The subject packing bed
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therefore will not touch the next lower tray or interfere
with fluid flow across the tray.
The packing used in the invention may be
characterized as comprising a plurality of beds of packing -
material with each bed of said plurality being: (i) placed
between a pair of trays; (ii) located closer to an upper
tray than to a lower of said-pair of trays; (iii) being
equal-in volume to less than one-half of the available
volume in the outer vessel between the pair of trays; and,
optionally (iv) at least partially filling the volumes
located between the parallel downcomers.
The trays of the subject invention are
fractionation trays as compared to liquid distributors found -
in packed columns. Some characteristics of fractionation
types include: a closer vertical spacing for fractionation
trays; a design which causes intimate, vigorous contact of
liquid retained on the tray with vapor passing upward
through the tray and the formation of froth on the surface
of fractionation trays; and an abundance of closely spaced
perforations across a high percentage of the decking area of
the fractionation trays. Fractionation trays make no
attempt to collect and segregate vapor and/or liquid flow
before redistributing these phases. By its very nature, a
packed column liquid distributor would have no use for
downcomers which transfer liquid downward any appreciable
distance before releasing it to the column.
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