Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
NENHANCED CAPACITY MULTIPLE-DOWNCOMER FRACTIONATION TRAY"
FIELD OF THE INVENTION
The invention relates to the design and construction of high performance
vapor-liquid contacting apparatus; for example, apparatus used within
fractional
distillation columns to perform separations of volatile chemical compounds and
commonly referred to as fractional distillation or contacting trays.
BACKGROUND
Fractional distillation trays are widely employed in the hydrocarbon
processing, chemical, and petrochemical industries. Accordingly, a large
amount
of research, development, and creative thinking has been devoted to providing
improved fractional distillation trays.
Most trays have circular perforations evenly distributed across the
contacting surtace (decking) of the tray. These allow the rising vapor to flow
straight upward from the tray's surtace. A small subset of fractional
distillation trays
utilize mechanical means for directing the vapor in a specified direction as
it passes
upward through the contacting area of the tray. One example of this provided
in
US-A-3,045,989. This reference shows perforations, which can be considered to
be slot-like in nature, as a means to use rising vapor to direct the
horizontal liquid
flow in various directions depending upon specific embodiments. In Figure 4A
the
slots are oriented in diametrically opposite directions in order to promote
the
convergence of the liquid flow at an outlet downcomer 118'.
US-A-3,550,916 shows slot-like openings on the active surface area of a
fractionation tray deck being oriented to direct rising vapor flow in the
direction of
the outlet downcomer associated with the tray. US-A-4,065,52 shows another
arrangement for fractionation trays wherein slots are provided in the decking
of the
tray to direct the direction of gases emerging from the slots and thereby
direct
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liquid flow in desired patterns over the tray. In this instance the liquid
flows in
alternate centrifugal and centripetal patterns. An objective of the invention
is to
insure a uniform distribution of liquid across the contact plate.
US-A-3,282,576 teaches, as exemplified in column 5, that slots may be
placed across the surface of a cross-flow sieve tray to promote the flow of
liquid
across the tray without the aid of liquid gradients. The thrust directing
openings of
the slots are arranged in parallel rows and generally face the liquid outlet
of the
tray. US-A-3,417,975 teaches another variation in fractionation tray design in
which
the decking portion of the tray comprises both circular perforations and also
flow
directing slots. The slots shown in Figure 1 of this reference and in Figure 3
of the
above-cited '576 reference are similar in nature to those which may be
employed
in the subject invention. In the apparatus of the '975 reference, the flow-
directing
slots are spaced unevenly across the decking to provide a higher concentration
of
the slots near the periphery of the tray.
US-A-3,759,498 teaches another fractionation tray deck arrangement in
which both circular perforations and vapor-directing slots are employed. The
arrangement of the slots is intended to increase liquid velocity in the
peripheral
portions of the tray which do not lie on a direct flow path between the inlet
and
outlet downcomers. The orientation of the slots directs liquid into and
removes
liquid from this otherwise stagnant area therefore promoting the overall
efficiency
of the tray. It is to be noted that this tray like the trays cited above does
not
comprise a multiple downcomer tray having relatively closely spaced downcomers
which are not troubled by uneven flow patterns.
US-A-4,101,610 shows a cross-flow fractionation tray having decking which
comprises both circular perforations and flow-directing slots. The flow-
directing
slots are arranged to direct the liquid across the tray towards the outlet
downcomer. The intent is to decrease stagnant areas upon the surface of the
tray
and to reduce the liquid gradient across the tray.
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2Z~2~o5
US-A-4,499,035 shows another arrangement on the surface of cross-flow
vapor-liquid contacting trays which employ both circular perforations and the
vapor-
directing slots on the decking panels of the tray. This reference is specific
to the
provision of froth initiation or bubble-promoting means at the liquid entrance
to the
tray decking area from the inlet downcomer.
The function of the vapor directing slots in these references is different
than
in the multiple downcomer tray of the present invention. In the references one
basic use of the slots is to reduce liquid gradients across the tray which can
result
from long liquid flow paths. Another basic function of vapor directing slots
in the
prior art was to push liquid into and out of deck areas which were out of the
immediate flow path between the inlet and outlet downcomers. The distinctive
structure of multiple downcomer trays eliminates these two reasons to employ
vapor directing slots in the tray decking.
CA-A-785,739 is believed pertinent for the tray decking structure shown in
Figures 3a and 3b. The tray surface depicted in these figures has vapor-
directing
slots positioned in various directions including slots facing diametrically
opposite
directions. The tray employs slots to promote liquid flow across the tray and
to
prevent or lessen liquid height gradients from being established upon the
surface
of the tray.
The use of "anti-jump" baffles located above the inlet to an outlet
downcomer is known in the art as illustrated by Figure 10 in the ballast tray
design
manual issued by Glitsch Incorporated (Bulletin No. 4900- fourth edition,
copyright
1974).
US-A-3,410,540 is believed pertinent for its showing of the structure of a
prior art multiple downcomer tray employing the highly distinctive downcomer
design similar to that employed in the tray of the present invention.
US-A-4,174,363 discloses a design for small metal enclosures which are
bolted to the upper surface of trays to encourage the flow of stagnant liquid
away
from the wall of the column toward the active area of the tray. This reference
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2132~0~
shows these devices being used on cross-flow and on Multiple Downcomer trays
such as employed in the subject invention. The reference also shows the usage
of flow-directing slots on the active deck surface of a cross-flow tray.
In contrast to the teachings of these references, the slots of the subject
invention
apparatus are employed to increase capacity while maintaining equal
efficiency.
SUMMARY OF THE INVENTION
The invention is an improved multiple downcomer tray, as opposed to the
more widely used cross-flow trays, which provides an increase in vapor
capacity.
The invention appears to function by reducing froth height and lowering liquid
entrainment in the vapor rising toward higher trays. In the invention vapor-
directing
slots are placed on the relatively narrow decking areas located between
adjacent
downcomers to direct vapor towards the nearest downcomer. Preferably the slots
will be pointed toward the closest downcomer. The trays also comprise an anti-
jump baffle located above the downcomer inlet to prevent droplets of liquid
from
being tossed over the downcomer to a different portion of decking. This baffle
extends upward between the descending downcomers of the next higher tray.
One embodiment of the invention may accordingly be characterized as an
apparatus for performing fractional distillation which comprises vertically
aligned
upper first and a lower second vapor-liquid contacting trays, each tray having
a
generally circular circumference, and comprising: (i) at least two narrow,
trough-
shaped downcomers which are parallel to each other and equidistantly spaced
across the tray, each downcomer being formed by two opposing side walls and
two end walls which are shorter than the side walls, the side walls and end
walls
oriented perpendicular to the plane of the tray, each downcomer having a
liquid
sealable outlet means; and, (ii) a plurality of elongated vapor-liquid
contacting
decks, with a vapor-liquid contacting deck being located adjacent each
downcomer
side wall such that the tray has at least one more vapor-liquid contacting
deck than
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. 2.~32~05
downcomer means; with perforations being substantially evenly distributed
across
the entire surface provided by the vapor-liquid contacting decks and with at
least
a portion of said perforations being in the form of vapor flow directional
slots having
openings facing a downcomer side wall; with the apparatus being turtner
characterized in that the downcomers of the second tray are perpendicular to
those
on the first tray and in that each tray comprises a plurality of anti-jump
baffles
comprising a vertical plate centrally mounted over the downcomers of the tray
and
parallel to the downcomer sidewalls, with the anti-jump baffles of the lower
second
tray having notches which surround the bottom portion of downcomers of the
upper first tray.
DESCRIPTION OF THE DRAWING
Figure 1 is the view seen looking downward toward one embodiment of the
subject fractionation tray. The parallel downcomers 2 are arranged between the
vapor-liquid contacting decks 3.
Figure 2 is a cross-sectional view taken on a vertical plane through a
fractional distillation column 1 employing a slightly different embodiment of
the trays
of the subject invention.
Figure 3 is a vertical cross-sectional view of an upper portion of a
downcomer 2 having an anti-jump baffle plate 7 held in a central location
above its
upper opening by the legs 8.
Figure 4 is a vertical cross-sectional view showing an anti-jump baffle
extending downward into a downcomer and a liquid sealable outlet means at the
bottom end of the downcomer comprising a perforated bottom plate 14.
Figure 5 is a cross section of a lower portion of a downcomer 2 illustrating
an alternative construction of the liquid sealable outlet at the lower end of
the
downcomer comprising a seal trough 16 attached to the open bottom end of the
downcomer by legs 11.
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213240
Figure 6 is a cross section through a section of a deck 3 illustrating a side
view of a vapor-directing slot 4.
Figure 7 is a cross-sectional view of a portion of deck 3 illustrating the
view
along section line 7 as seen facing into the outlet of a vapor-directing slot
4.
Figure 8 is a representation of a small section of decking drawn to actual
scale to illustrate the relative size and placement of suitable circular
perforations 9
and vapor-directing slots 4 upon a piece of decking material.
Figure 9 is a cross-sectional view of a fractional distillation column 1
looking
downward upon a multiple downcomer tray having an anti-jump baffle 7 and
showing alternative types of perforation 10.
Figure 10 illustrates an embodiment of the invention in which the anti-jump
baffle extends upward above the bottom of the downcomer of the next-above
tray.
Figure 11 is an overhead view of a section of a downcomer showing details
of the bracing bar 15 and tapered brace 18.
DETAILED DESCRIPTION
Vapor-liquid contacting devices are used in a wide variety of applications for
bringing into contact a liquid, which flows in a generally downward direction
in the
overall device, with a rising vapor stream. For instance, the device is widely
used
to contact a gas stream with a treating liquid which selectively removes a
product
compound or an impurity from the gas stream. The subject apparatus can
therefore be used in an acid gas absorber or stripper or in an ethylene oxide
absorber. Another application of vapor-liquid contacting apparatus is in the
separation of chemical compounds via fractional distillation. The subsequent
discussion herein is focused on use in a process for separation by fractional
distillation, but this is not intended to in any way to restrict the apparatus
invention
to that mode of operation.
The subject apparatus can be used in the separation of essentially any
chemical compound amenable to separation or purification by fractional
distillation.
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2.~3~~~~
Fractionation trays are widely used in the separation of specific hydrocarbons
such
as propane and propylene or benzene and toluene or in the separation of
various
hydrocarbon fractions such as LPG (liquified petroleum gas), naphtha or
kerosene.
The chemical compounds separated with the subject apparatus are not limited to
hydrocarbons but may include any compound having sufficient volatility and
temperature stability to be separated by fractional distillation. Examples of
these
materials are acetic acid, water, acetone, acetylene, styrene acrylonitrile,
butadiene,
cresol, xylene, chlorobenzenes, ethylene, ethane, propane, propylene,
xylenols,
vinyl acetate, phenol, iso and normal butane, butylenes, pentanes, heptanes,
hexanes, halogenated hydrocarbons, aldehydes, ethers such as MTBE and TAME,
and alcohols including tertiary butyl alcohol and isopropyl alcohol..
Two determinants of the quality of a contacting tray are its efficiency for
performing a separation and its capacity in terms of liquid or vapor traffic.
It is an
objective of the subject invention to increase the capacity of multiple-
downcomer
trays.
Often the capacity of a fractionation tray is limited by its ability to handle
increased rates of upward vapor flow through the tray. This limitation is
normally
associated with the tendency of the liquid "on" the tray to be entrained in
rising
vapor and to rise upward towards the next tray. The vapor capacity of a tray
is
therefore often reached when the height of the "froth" upon an upper surface
of the
tray reaches the bottom surface of the next above tray. An excessive froth
height
on a fractionation tray, or any type of contacting tray, can cause liquid to
pass
upward through the decking of the above tray. Liquid is then passed upward. A
decrease in the froth height may accordingly be desirable to increase tray
vapor
capacity. It is therefore another objective of the subject invention to reduce
the
froth height present on a multiple downcomer tray.
Before proceeding further with a description of the invention, it is useful to
define and characterize the type of tray referred to herein as a "multiple
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2132~~5
downcomer-type" tray. This term is used herein to distinguish the subject
invention
from other types of fractionation trays.
A multiple downcomer tray is distinguished from the conventional crossflow
tray by several structural characteristics. First, a multiple downcomer tray
does not
have a "receiving pan". This is the normally imperforated section located
below an
inlet downcomer opening. Reference is made to previously cited US-A-3,550,916
which illustrates a receiving pan 1 in Figure 1. This is the imperforate space
upon
which the liquid descending through the downcomer impacts before passing onto
the decking of the tray. In multiple downcomer fractionation trays, the
horizontal
surface area of the tray is basically divided into downcomer means and vapor-
liquid
contacting area normally referred to as decking. There are no impertorate
areas
allocated to receiving descending liquid from the tray located immediately
above.
Another distinguishing feature of multiple downcomer fractionation trays is
the provision of a relatively large number of trough-like downcomer means
across
the tray. These downcomer means are spaced relatively close together compared
to the customary crossflow fractionation trays. The distance between adjacent
downcomers {measured between their sidewalls or weirs) of the same tray is
normally between .3 and 1.0 m and will often be less than 0.5 m. This results
in the
multiple downcomer tray having a unique design when viewed from above
consisting of the alternating decking areas and downcomer means evenly spaced
across the upper surface of the fractionation tray--see Figure 1.
The actual downcomer means of a multiple downcomer tray 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 distance located between the two trays.
The
downcomer descending from the tray above therefore stops well above the top or
inlet to the downcomers of the tray below. The top or inlet to the downcomer
of
a tray often functions as the outlet weir of the tray, and it is therefore
seen that the
bottom of the downcomer of a multiple downcomer tray above is well above the
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2Z32j~05
outlet weir of the tray located below.
Downcomers on a multiple downcomer tray are normally oriented at 90
degrees from the trays located immediately above and below. The downcomers
on vertically adjacent multiple downcomer trays are therefore perpendicular
rather
than parallel.
Yet another distinguishing feature of multiple downcomer fractionation trays
is the provision of a liquid sealable means in the bottom or outlet of the
downcomer means. The bottom of the downcomer means is therefore partially
closed off by a plate having various perforations or by some other means
intended
to retard the direct downward flow of liquid out of the downcomer means--see
Figures 4 and 5. This liquid sealable outlet is located well above the deck of
the
tray located immediately below and is at a point 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
these openings or other mechanical arrangement.
There is no inlet weir at the bottom of the downcomer of the subject multiple
downcomer trays as in a cross-flow tray. The liquid sealable outlet performs
this
function--see Figure 2.
Multiple downcomer trays are characterized by a very short liquid flow path
between the point at which the liquid first falls on the tray and the point at
which the
liquid exits the tray via the downcomer means. This is due primarily to the
close
spacing of the downcomers as described above. Except for between the column
wall and the end downcomers, liquid descending from the tray above will fall
to the
surface of a tray at a point midway between two adjacent downcomers. The
distance from the point of liquid reaching the tray to the downcomer inlet is
therefore always less than one-half of the distance between downcomers for
most
of the liquid. This short distance the liquid must travel coupled with the
agitation
attendant with the passage of vapor upward through the decking results in
multiple
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downcomer trays having essentially no liquid level gradient from the liquid
inlet to
exit points.
The directional slots shown in the previously cited cross-flow tray references
are employed to promote liquid flow across the surface of the tray. This is
done
to eliminate stagnant areas, uneven residence times and liquid level
gradients.
Multiple downcomer trays suffer from none of these maladies and therefore do
not
require the use of directional slots as a remedy to these cross-flow tray
problems.
The slots are instead used to increase vapor capacity.
With the distance between downcomer walls (the width of a decking section)
being between 1 and 0.3 m, the average liquid flow path would be less than 0.5
to
0.15 m.
The following are measurement ranges of commercially employed multiple
downcomer trays which are presented for the dual purposes of providing
guidance
in the design and use of the subject apparatus and for distinguishing the
multiple
downcomer trays of the subject invention from the conventional cross-flow
fractionation tray. The spacing between vertically adjacent trays would
normally be
between 20 and 91 cm (8-36 in) and is preferably between 25-61 cm (10-24 in).
The total open area of the deck is generally in the range of 6 to 5 to 15
percent of
the deck area. This includes the open area provided by both circular openings
and
elongated slots of the present invention in the decking area of the tray. The
normal
hole diameter of the circular pertorations may range from 0.3 to 2.6 cm (1 /8 -
1.0
in). A hole size of 0.47 to 0.64 cm (3/16 - 1 /4 in) is normally preferred.
The open
area provided by slots is preferably from 0.25 to 5 percent of the area of the
deck.
A representative thickness of the decking is about 0.19 cm to 0.34 cm.
The inlet openings of the downcomers of a multiple downcomer tray are
normally 6 to 25 cm wide (2.5 - 10 in). The height of a downcomer as measured
from the horizontal top edge of the outlet weir to the bottom of the liquid
sealable
means is normally between 15.2 to 45.7 cm (6 - 18 in). This includes the
height
that the downcomer extends above the decking and below the decking. The anti-
CA 02132405 2004-10-04
jump baffle located above the downcomer would normally be at least 7.5 cm tall
and no more than 35.6 cm in height (3-14 in) and will normally be
approximately
equal in length to the associated downcomer means. For further information on
multiple downcomer trays, see US-A-3,410,540.'
The subject invention achieves the objective of increasing the vapor capacity
of a multiple downcomer tray through the provision of a number of vapor-
directing
slots in the decking section of the fractionation tray. The slots are oriented
such
that the gas rising upward through the deck through these slots imparts a
horizontal thrust or momentum to the liquid or froth on the tray in the
direction of
the nearest downcomer means. A multiple downcomer fractionation tray would
typically have at least two downcomer means, but smaller trays can have a
single
downcomer. Therefore, each multiple downcomer tray will normally have at least
one section of decking which has downcomer means along each lateral side. The
slots on the portions of deck having downcomers on two sides will therefore
have
slots oriented in diametrically opposite direction towards the nearest
downcomer
means.
This unique structural design has the novel function of directing the froth
towards and into the downcomers of the multiple downcomer tray. In comparison,
the prior art slots are employed to direct liquid flow across the relatively
lengthy flow
paths of cross-flow trays to reduce liquid gradients or to eliminate stagnant
areas.
The vapor rising upward through the slots leaves the slots at an angle to the
tray surface having a definite horizontal component and imparts some of the
horizontal momentum of the vapor to the liquid phase or suspended droplets
above
the deck surface. This results in a net force pushing the froth towards the
downcomer means. There is therefore achieved a more rapid passage of the froth
into the downcomer means and a decrease in the froth height on the tray.
The agitation which occurs upon the decking can cause erratic and
sometimes very powerful horizontal movement of suspended liquid droplets. It
is
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2132~p~
therefore a feature of the instant invention that "anti-jump" baffles be
placed over
the inlet of the downcomer means in order to prevent the passage of liquid
across
the inlet to the downcomer. It is the function of the anti-jump baffle to
intercept
liquid passing horizontally over the downcomer inlet and to direct this liquid
into the
downcomer. Basically the anti-jump baffle absorbs the horizontal momentum of
the
liquid particles which may otherwise carry it over the baffle and allows the
liquid to
fall by the action of gravity into the downcomer inlet. The provision of the
anti-jump
baffles has been found to be a positive enhancement to the performance and
structure of the tray and useful in achieving the objectives of the invention.
A more complete understanding of the subject invention may be obtained by
reference to the drawings. Figure 1 shows the view seen looking downward
toward
the upper surface of a multiple downcomer tray. The particular tray
represented
in this drawing has six downcomers and is surrounded by the cylindrical wall
of the
fractionation column 1. Each downcomer means 2 is comprised of two downcomer
end walls 5 and two parallel side walls 6. The downcomers are uniformly spaced
across the tray. Located between the downcomers is the perforated decking or
deck 3 portion of the tray. Decks also extend between the extrememost
downcomer means and the outer periphery of the tray. That is, the portion of
the
tray enclosed between the end downcomers and the perimeter of tray is also
filled
with perforated decking and has active vapor-liquid contacting means placed
thereon. The decks will comprise both the standard symmetrical (circular)
perforations 9 which are uniformly distributed across the decking surface and
the
vapor-directing slots 4. The slots and perforations are not drawn to scale in
order
to show detail.
The slots located on a decking section located between any adjacent pair
of downcomer means are divided into two groups oriented in diametrically
opposite
directions. Vapor rising from one grouping of slots travels in a horizontal
direction
180 degrees opposite from vapor rising through the second group of slots
located
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2~3240~
closer to the other downcomer associated with this portion of deck. The
particular
tray illustrated in Figure 1 does not employ an anti-jump baffle.
Figure 8 provides a pictorial representation of an actual portion of decking
material 3. This Figure illustrates representative commercial sizes and shapes
of
the circular perforation 9 and the vapor-directing slot 4. In this instance,
the
openings point toward the lower end of the Figure and would direct the rising
gases
in a downward direction away from Figure 1. The precise alignment or spacing
of
the circular perforations is not believed to be a controlling variable in the
subject
invention. Likewise, the placement of the flow-directing slots is not believed
to be
critical as long as the direction of thrust of the vapor rising through the
slots imparts
a horizontal momentum to the froth and lipuid upon the tray in the general
direction
of the nearest downcomer. The slots may be arranged in straight lines or in
zigzagged rows across the surface of the tray. For convenience of
presentation,
only two rows of the flow-directing slots 4 are illustrated on Figure 1. In
actuality
many more slots would be placed upon the decking area.
A representative maximum spacing between any two flow-directing slots is
on the order of from about 5 to 17.8 cm (2 to 7 in). The perforations are
preferably
spread in a relatively uniform manner across the entire deck area. To minimize
fabrication expense the deck material is normally constructed by first
perforating the
deck material to provide the desired number of circular openings. A second
pertoration step is then pertormed to impart the flow-directing slots. No
attempt is
made to align the slot openings with or to have the slot openings fall between
the
circular perforations. Therefore, as shown in Figure 8, some of the slot
openings
will actually fall in the same location as the circular openings and the
portion of the
deck material which forms the slot may also have a perforation.
Figure 2 is a cross-sectional view looking in a horizontal direction through
a fractionation column. The figure shows three multiple downcomer
fractionation
trays, each of which employs anti-jump baffles. This view illustrates the
preferred
and customary perpendicular arrangement of the downcomers on alternating
trays.
13
The uppermost tray shows the view when seen looking directly towards the side
wall 6 of the downcomers. This view also shows the anti-jump baffle 7 held
above
the downcomer by the braces or support legs 8. One feature of the anti-jump
baffle illustrated in this figure is that the bottom edge 17 of the baffle is
above the
upper edge 13 of the associated downcomer. This view also illustrates the
customary feature of multiple downcomer trays that the downcomer means is
situated with about 1 /5 - 1 /4 of its total height located above tray decking
to
provide outlet weirs with the remainder of the downcomer means extending below
the decking.
The depiction of the middle tray in Figure 2 shows the alignment of the anti-
jump baffle means parallel to the downcomer means 2. It also illustrates the
orientation of the vapor directing slots 4 towards the nearest downcomer
means.
It may therefore be seen that the vapor-directing slots located between two
adjacent downcomer means will point in opposite directions. That is, those
slots
located closest to a downcomer will be pointed towards that downcomer. The
direction of slot orientation is therefore divided along a line intermediate
between
the two adjacent downcomer means which separates the slots into the two
respective groupings pointed at the closest downcomer means. Figure 3 is
an enlargement of the upper portion of a downcomer 2 shown in Figure 2 and a
section of the associated decking material. The decking 3 is attached to the
side
wall 6 of the downcomer 2 by means not shown on the drawing. Typically a lower
piece of "angle" stock is welded to the downcomer wall to support the decking.
A
second piece of "angle" stock is bolted to the wall above the decking to form
a
groove. The circular perforations 9 are uniformly dispersed through the
decking
surface. The vapor-directing slots are located on each side of the downcomer
means with the opening of the slot facing the downcomer side wall 6. The
liquid
anti-jump baffle means 7 is supported by intermittent braces 8 which may be
attached as by bolting to both the baffle 7 and the side walls 6. The anti-
jump
baffle is preferably centrally located between the downcomer side walls and
14
2~.~~40~
essentially as long as the downcomer. The total height of the actual plate of
baffle
is at least one-third the height of a downcomer side wall. The baffle plate
may be
located entirely above the inlet of the downcomer but preferably extends into
the
downcomer as shown in Figure 4.
Figure 4 illustrates an alternative arrangement of a downcomer and an anti-
jump baffle 7. The side wall of the downcomer 2 is again attached to the
decking
3 as described in Figure 3, but the anti-jump baffle 7 is supported by a
bracing bar
bolted to the upper end of the downcomer side walls 6. The bar is
perpendicular to the anti-jump baffle 7. This arrangement is preferred as it
10 increases the rigidity of the downcomer means itself. Another feature of
the
arrangement shown in Figure 4 is that the lower edge 17 of the anti-jump
baffle is
located within the downcomer means. That is, the lower edge of the anti-jump
baffle is below the upper edge 13 of the downcomer inlet and a lower portion
of the
baffle is located within the downcomer. The baffle may extend downward to the
15 level of the decking. This figure also illustrates one of the several
alternative ways
in which the bottom surface of the downcomer means 2 may be constructed. In
this embodiment a bottom plate 14 connects the two side walls 6. A number of
relatively large openings or perforations 10 are provided in the bottom plate
14 for
the purpose of allowing the rapid exit of the liquid which accumulates within
the
downcomer. The purpose of the plate 14 is to retard the liquid flow
sufficiently that
the bottom of the downcomer means is sealed by liquid to the upward passage of
vapor.
One area of variation in the structure of multiple downcomer tray is the
arrangement of the openings provided in the bottom of the downcomer and which
form a part of the sealable outlet means necessary at the bottom of the
downcomer. The openings may be circular, square or elongated in either
direction,
that is, along the width or length of the downcomer means. Circular openings
and
elongated grooves extending between the side walls 6, sometimes referred to as
louvers, as shown in Figure 9 are preferred.
~~3.~~p~'
Figure 5 illustrates an alternative configuration which is suitable as the
liquid
sealable means necessary at the bottom of the downcomer means. In this
embodiment of the invention, a seal trough 16 is attached to the lower portion
of
the downcomer means 2 by the provision of short vertical legs 11 which may be
bolted to the bottom end of the side walls 6 and to the sides of shallow
trough 16.
The space between the bottom edge 12 of the downcomer sidewall and the
interior
surface of the trough is left open for the passage of liquid. A minor seal
difference
provided by the elevation of the top of the trough above the lower edge of the
downcomer together with the momentum of the descending liquid is sufficient to
prevent the entrance of gas into the downcomer means. The entrance of gas into
the downcomer means is always undesirable as it would allow gas to bypass the
contacting area of the tray.
Figures 6 and 7 illustrate details of one embodiment of the vapor-directing
slots 4. Figure 6 shows the view looking across the directional slot with the
circular
perforations 9 being distributed across the decking material 3 while the slot
4 is
formed in a section of the deck. The slot is formed by cutting and stretching
the
metal such that the slot raises at an angle a above the surface of the deck.
The
deck is normally mounted in an absolutely horizontal position in the
fractionation
tray when in use. The angle "a" is preferably between 5 and 45 degrees. Figure
7 is a cross-section of the same small piece of decking material as Figure 6
and
shows the view looking into the opening of the slot 4. In this particular
instance,
it is seen that the slot is formed by a relatively flat upper surface which is
connected
to the tray by the sloping side surfaces.
The slots could be produced having alternative configurations. For
instance, the overall shape of the top surface of the slot could be circular
or
elliptical when viewed looking into the opening as in Figure 7. Although it is
preferred that the top of the slot is connected to the decking by the sloping
side
surfaces, there is no requirement for effective vapor-directing slots , to be
so
constructed. Therefore, the slots could be formed with the metal being cut
along
16
CA 02132405 2004-10-04
the sides of the slot in addition to being cut at the open front portion of
the slot.
Siots constructed in this manner are preferably relatively long such that only
a
minimal amount of the total gas volume passing upward through the slot may
pass
outward through the sides of the slot in a direction parallel to the downcomer
means. It is preferred that the gases are allowed to pass upward through the
tray
in a very large number of flow-directing slots. A typical slot density will
exceed 250
slots per m2 of deck area (24 slots per ft~ of decking area).
The louvers, openings, or holes provided in the bottom of the downcomer
means should be located such that the liquid exiting the openings will fall
upon
decking material rather than into the open upper end of the downcomer means
located on the next lower tray. Allowing liquid to fall directly into the next
lower
downcomer is undesirable.
Figure 9 is a more detailed view as seen looking downward into a portion
of a fractionation column 1 employing a multiple downcomer tray of the subject
invention. The tray itself extends outward to the inner surface of the vessel
shell
or outer wall of the column. A deck 3 extends from the inner surface of the
vessel
wall to the first downcomer 2. This portion of the tray surface contains vapor-
directing slots which are oriented only towards this specific downcomer means.
The portion of the tray surface located between this first downcomer means and
the next inward downcomer contains vapor-directing slots oriented in opposite
directions. Again, the slots are not drawn to scale in order to allow easier
representation. This figure illustrates the view seen looking downward into a
downcomer means 2 having an anti-jump baffle 7 aligned with the downcomer
means and equidistant between the side walls 6. The anti-jump baffle is held
in
place and supported by the bracing bars 15 located along the length of the
downcomer. This particular figure illustrates two alternative opening
configurations
in the bottom plate of the downcomer. The circular openings 10 are shown in
one
portion of the innermost downcomer while elongated louvers 10' are shown in
another portion of this same downcomer and in the next inward downcomer. The
17
openings are arranged in groupings corresponding to the presence of decking on
the next tray downward in the column.
Figure 10 is a sectional view looking horizontally across a column 1
showing two vertically aligned fractionation trays each having four downcomers
2.
The dimensions of this figure are approximately proportional to those of the
second
test apparatus described below. This figure illustrates the embodiment in
which the
baffle of the lower tray extends upward above the downcomer such that the
upper
edge 19 of the anti-jump baffle is above the bottom edge 12 of the downcomer
sidewall, and hence above the bottom of the downcomer itself. In this
embodiment
the upper edge of the baffle of the lower tray, and those below it, are
slightly
notched to accommodate the downcomers. The upper edge of the anti-jump baffle
of the uppermost tray is flat and preferably bent to provide a slight lip
which
increases the rigidity of the baffle. The upper edge of individual portions of
the
baffles on lower trays may also be tipped at the top. The lower edge of all of
the
anti-jump baffles is preferably straight and located at about the level of the
tray
decking.
The vertical distance "d" between the top edge of the anti-jump baffle and
bottom surtace of tray decking (between the downcomers) can be varied to a
considerable extent. The upper edge of the anti-jump baffle preferably extends
upward between the downcomers such that the distance "d" is substantially
equal
to the equivalent distance provided in the anti-jump baffle notch which
surrounds
the bottom of the downcomers. This maximum upward extension is shown on the
figure by the hashed line 20 which follows the shape of the lower surface of
the
upper tray. The separation between the tower surface of the upper tray and the
top
edge of the anti-jump baffle would then be substantially uniform along the
entire
length of the anti-jump baffle. Alternatively, the maximum upward extension of
the
baffle may occur only along those longer portions of the baffle which are
between
downcomers, with the terminal portions being level with the portion under the
downcomers. The anti-jump baffle may be attached to the downcomer to further
18
2~.~~~05
stiffen the baffle.
Figure 10 also shows an outline of a stiffening brace 15 which extends
across the downcomer 2 and is notched to receive the lower edge of the anti-
jump
baffle 7. The bottom of the brace is therefore below the baffle and extends
into the
downcomer. The extension of the baffle 7 and braces 15 into the downcomer
helps
reduce turbulence in the downcomer and improve the separation of vapor and
liquid.
Figure 11 shows the details of the structural elements which are preferably
employed to support the anti-jump baffle above the downcomer. These elements
comprise the bracing bars 15 and the tapered brace 18. These elements
alternate
along the length of the baffle. The bracing bars 15 are preferably but not
necessarily located entirely within the trough of the downcomer. The bracing
bar
configuration of this figure has a central notch which receives and surrounds
a
lower portion of the anti-jump baffle 7 thus preventing sideways motion and
also
supporting the baffle. The braces extend across the width of the downcomer and
are attached to both downcomer sidewalls 6. The tapered braces 18 are used in
pairs, with one brace being attached to each side of the anti-jump baffle. In
the
view provided by Figure 10, the triangular profile of the top portion of these
braces
can be seen. These braces are intended to provide increased rigidity to the
upper
portions of the anti-jump baffle. Figure 11 also shows a group of openings 10
in
the bottom plate 14 of the downcomer.
One embodiment of the subject invention may be characterized as a tray
useful in the fractional distillation of chemical compounds, the tray having a
generally circular circumference and comprising at least three narrow, trough-
like
rectangular downcomers which are parallel to each other and equidistantly
spaced
across the tray, each downcomer being formed by two opposing side walls and
two opposing end walls which are shorter than the side walls, the side walls
and
end walls extending perpendicular to the plane of the tray in both directions
and
having upper edges located on one side of the plane of the tray, each
downcomer
19
having a substantially open inlet and a liquid sealable outlet located on a
second
side of (below) the plane of the tray; a plurality of elongated vapor-liquid
contacting
decks, with a vapor-liquid contacting deck being located adjacent each
downcomer
means side wall such that the tray has one more vapor-liquid contacting deck
than
downcomer means, with uncovered symmetrical, preferably circular, perforations
being evenly distributed across the vapor-liquid contacting decks and with the
side
walls of the downcomer means forming parallel liquid outlet weirs on opposite
sides
of the contacting decks; directional slots located on the vapor-liquid
contacting
decks and having openings facing the closest downcomer means, with the vapor-
liquid contacting decks located between adjacent downcomer means having a
first
grouping of the slots being oriented in a diametrically opposite direction
from a
second grouping of slots; and, a liquid anti-jump means comprising at least
one
vertical plate extending vertically outward and above the inlet of the
downcomer
means and aligned parallel with the side walls of the downcomer means. This
baffle may have one of the several forms described herein. The tray, in
differing
embodiments, can have one, two, three, four, five or more individual
downcomers.
A more inclusive embodiment of the invention is a fractionation column
comprising an enclosed cylindrical outer vessel having a plurality of the
subject
trays mounted therein, with the vertically adjacent trays (as defined by
downcomer
orientation) being perpendicular to each other. The overall apparatus would
include
the customary accessories for feeding the liquid and vapor streams to be
contacted
such as a reboiler and reflux system.
EXAMPLES
The subject trays, containing flow-directing slots and anti-jump baffles were
tested in a number of experiments in two different apparatuses. The object of
the
experiment was to compare the subject trays to conventional multiple downcomer
trays. The conventional trays contain only circular perforations with an open
area
roughly equal to the subject trays. For each tray type, the froth height was
measured at different liquid rates and F-factors. The F-factor is defined as
vs
multiplied by the square root of d9/(d,-dg) where v$ = air velocity based on
bubbling
area, d9 = air density and d, = density of liquid (water). Thus, the froth
heights
could be compared at different F-factors to evaluate the froth height
reduction
capabilities inherent in the subject trays. These runs were also to evaluate
and
compare the vapor capacity of each tray type. The maximum vapor capacity is
determined as the condition where the froth height reaches the tray above.
This
condition is termed the flood point of the tray.
The first test apparatus used was a square column with the dimensions
being roughly 0.61 m by 0.61 m (2 ft. by 2 ft.). The apparatus uses air and
water
test fluids to test new ideas since its size allows changes to be made
readily. The
high vapor rates available in this column were used to evaluate and compare
the
froth heights and vapor capacity of conventional multiple downcomer trays and
the
subject multiple downcomer trays. The test column consisted of three trays
with
tray spacings of 38.1 cm (15 in). Each tray contained a single 12.7 cm (5 in)
wide
downcomer with a total of 0.29 m2 (3.1 ftz) of bubbling area. The total
downcomer
height was 16.5 cm (6.5 in) of which 3.8 cm (1.5 in) extended above the tray
deck
as an outlet weir. The decks were made of 0.19 cm (.076 in) thick stainless
steel
and contained perforations with diameters of 0.476 cm (0.1875 in). For the
slotted
trays, the open area of each "C" slot was 24.4 mm2 (0.038 in2) and each tray
contained a single 16.5 cm (6.5 in) high anti-jump baffle. The circular
perforations
of the subject trays were 14.2 percent of the tray deck active (bubbling)
area, and
21
the slots were 2.0 percent of the active area. The unslotted (conventional)
trays
had a 16.3 percent open area (circular perforations only).
The results of these tests revealed the slotted multiple downcomer trays
significantly reduced the froth heights when compared to the unslotted
multiple
downcomer trays. At high vapor rates, the slotted trays reduced the observed
froth
height by about 8.4 cm (3.3 in). The decrease in froth height allows the
subject
trays to be operated at much higher vapor rates (i.e. F-factors). Thus, these
experiments determined the subject trays with slots and baffles have a greater
vapor capacity than the unslotted and unbaffled multiple downcomer trays.
The second apparatus enabled the subject trays to be evaluated on
commercial scale equipment. This column is roughly eight feet in diameter and
contains three trays at 30.5 cm (12 in) tray spacings. The trays contained
four 12.7
cm (5 in) wide downcomers at a total downcomer height of 16.5 cm (6.5 in).
These
downcomers extended above the tray decks by 2.54 cm (1.0 in). The slotted
trays
had 12.5 percent open area due to the circular perforations and a 1.8 percent
open
area due to the slots. Also, the trays contained 20.3 cm (8.0 in) high anti-
jump
baffles. The conventional tray open area was 14.4 percent (circular
perforations
only).
The eight-foot test column operates at atmospheric pressure and uses air
and water as the test fluids. Test runs on the trays indicated the subject
trays
reduced the froth height considerably when compared to the conventional trays.
A test at a liquid weir loading of 6.05 1 /m sec (.065 CFS/ft of weir), the
slotted
trays reduced the observed froth height by about 7.3 cm at high vapor rates.
The
results of these tests confirmed the earlier findings in the smaller test
column. The
subject slotted and baffled multiple downcomer trays have a higher vapor
capacity
than the conventional multiple downcomer trays due to the ability of the
subject
trays to reduce froth height. Flood points from the eight-foot column at
various
liquid flow rates are given in TABLE 1. The liquid flow rates are given in
cubic feet
per sec/ft of weir and in liters per sec/meter of weir. The F-factors of TABLE
1
22
.~.~~~~~5
represent the vapor rates where the froths on the trays completely filled the
tray
spacings. For liquid weir loadings up to 6.05 1 /m-sec (0.065 CFS/ft), the
subject
trays were found to have a vapor capacity increase of 15 to 22 percent over
the
conventional multiple downcomer trays.
TABLE 1
Liquid flow F Factor
rate
CFS/ft Is/m Conventional Improved
tray tray
0.040 3.72 0.350 0.405
0.052 4.84 0.333 0.388
0.065 6.05 0.300 0.367
While not wishing to be bound to any particular theory, it is believed that
the
subject invention achieves its improved performance by imparting a net
horizontal
movement to the liquid droplets suspended above the tray deck. This movement
is the result of momentum transfer from gases rising through the flow-
directing slots
to the suspended liquid droplets. A larger number of the droplets will
therefore flow
into the area above the open upper end of the downcomer while suspended and
fall into the downcomer. The froth is in this manner more quickly removed from
the
tray surface.
It is also possible that the slots tend to cause the droplets to have a
flatter
(more horizontal) average trajectory and thereby impact with vertically rising
droplets sprayed upward from the circular perforations. This could reduce the
average height reached by the droplets having an initial totally vertical
trajectory.
The increased performance of the subject trays has been verified in two
additional tests performing actual separations. In a test performed at a
university
test facility using a 60 cm diameter column to separate methanol and water it
was
23
determined that the efficiency of the subject trays did not appear to be
degraded
as compared to prior art multiple downcomer trays.
A more convincing test result was obtained when the conventional multiple
downcomer trays installed in a commercial fractionation column used to remove
ethane from ethylene were replaced with the trays of the subject invention.
This
change allowed the column to be operated at 115 percent of its former
capacity.
Separation efficiency was not decreased. It is believed an even greater
increase
in capacity could have been achieved, but there was no additional column feed
available.
24
