Note: Descriptions are shown in the official language in which they were submitted.
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BACKG~OUND OE7 TIIE INYENTION
In distillation, absorption ancl other chemical-separating processes and
' techniques, various types of vapor-liquid contacting trays, such as sieve trays, jet
~ trays, valve trays and the like, are employed, in order to provide for intimate contact
,, between an upwardly flowing vapor and a downwardly flowing liquid in a vapor liquid
contacting column. A wide variety of tray designs and concepts has been proposed,
generally directed around improving the tray efficiency, improving the tray capacity,
or improving the tray mechanical structure. However, experience has shown that
I, these design concepts often work against each other; for example, a tray design for
11 high efficiency typically sacrifices tray capacity and also complicates the tray
mechanical structure. Furthermore, claimed tray designs of improved tray efficiency
Il or capacity are not easily and convincingly verified in pilot-plant tests or in commercial
¦ I tray towers.
It is desirable to minimize the overhead of a tray design, wherein a
1¦ typical operating tray generally comprises a highly active heat mass-transfer region;
¦ I that is, the froth region across the horizontal surface of the tray, a poor heat
¦I mass-transfer region; that is, the droplet disengagement region above the froth region
and underneath the next tray, and the nonactive downcomer regions, where little
I l or no vapor-liquid contact occurs. Trays designed for high liquid-loading chemical
ill applications generally have to bear a heavy downcomer overhead.
A multiple downcomer tray has been developed which employs a
! sieve-type tray with multiple downcomers, with the downcomer inlet width between,
for example, I to 4 inches, and with the downcomer outlet terminating at an elevation
Il above the downcomer inlet of the next tray below, with a liquid seal of the downcomer
¦I provided by using orifices, spouts or screen materials (see, for example, ~I.S. Patents
3,410,540, 4,159,291, 4,278,621, and 4,297,329). By terminating the downcomer above
the froth on the tray below, the tray area occupied by the downcomer may be reduced
¦ by 50%. This additional tray deck can be used as an active bubbling area; for example,
¦ by putting additional valves on the tray deck below the downcomer, and thereby1 increAsing the crpacity of the tray. In addition downcomer bAcl~-up flooding, A mAj
Il -2-
¦!
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tray-capacity limiting factor in high liquid-loading applications, also can bc avoided
or reduced. The multiple downcomer tray, thus, provides for a high percentage ofavailable tray area and a lower tray spacing in the column, and, for high liquid-loading
applications, is equivalent to a higher tray capacity and higher efficiency per unit
tower height; for example, tray spacing of 10 to 12 inches, as compared to 20 to 2
inches for a typical valve tray.
However, such sieve-type multiple-downcomer trays, for example, having
a sieve-type tray deck and slotted downcomers, have an inherently poor turndown
ratio. The normal turndown ratio for the valve distillation trays can be greater than
6, while for sieve trays it is usually around 3, while with multiple-downcomer sieve
trays, the turndown ratio is often 2 or less. Trays have been developed employing
a flexible downcomer design (see U.S. Patent 3,784,175). This patent attempts toimprove the poor turndown ratio of the multiple-downcomer sieve tray, while
maintaining a positive downcomer seal, by employing a movable member. However,
because of the questionable reliability of the movable member, such trays have not
been widely accepted in the marketplace.
Some trays are composed of a plurality of plates made from a highly
porous, open-pored, sintered material, particularly for use in the absorption and/or
desorption of gases by liquids. Such plates are usually composed of a sintered
magnesium or aluminum silicate. The plates often have a plurality of generally parallel
passage openings of predetermined cross-sections extending therethrough, and include
a plurality of run-off means comprised of tubes which have, at the lower end thereof,
a gas shut-off device, to block the upward flow of gas in the downcomer (see, for
example, ~.S. Patent 3,958,964). These sintered materials have very small openings,
are subject io plugging, and are restricted to low liquid flow rates; therefore, the
sintered materials are much different from the packing materials. Furthermore,
the objective of using the sintered materials in the downcomer is to prevent gas flow
up the downcomer, rather than to promote good heat and mass transfer between gasand liquid in the downcomer.
Therefore, there exists a need for an improved vapor-liguid contact
tray of simple mechanical structure, but which tray can achieve high tray efficiency
and high tray capacity and a wide turndown ratio, through a reduction in tray overhead.
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SlJMMAl~Y OF THE INVENTION
The present invention is directed to a vapor-liquid contact tray and
to vapor-liquid contacting towers containing such trays, and to a method of operating
and manufacturing such trays. ln particular, the invention concerns an improved
valve vapor-liquid contact distillation tray for use in high liquid-loading applications,
and to vapor-liquid columns containing a plurality of such trays.
The present invention concerns an improved gas liquid contacting tray,
and more particularly a valve-type tray useful for high liquid-loading applications
in which the downcomer is terminated above the froth level of the tray below, and
the effects of vapor by-passing through the downcomer are obviated or minimized.The contact trays of the invention employ heat and~or mass-transfer packing material,
and particularly a structured packing material, in one or more, and preferably all,
the downcomers. Thereore, instead of sealing the downcomer area by the conventional
methods, as used in the multiple-downcomer trays, or by other techniques, the
high-surface-area packing material in the downcomer of the improved gas liquid contact
tray of the invention is utilized to provide intimate countercurrent vapor-liquid
contacting, thereby providing a tray design which can achieve high tray efficiency,
high tray capacity and a wide turndown ratio. Generally, both heat and mass transfer
occur during distillation and other gas liquid processes, based on temperature
differences for heat transfer and concentration differences for mass transfer; however~
for the purposes of this invention, the term mass transfer, as it relates to packing
materials used in the downcomer, shall be employed to indicate that one or both
transfers are covered or occur.
The vapor-liquid contact tray, thus, provides mass-transfer means,
by the use of a structured or nonstructured packing, or a combination thereof~ in
the downcomer for the upwardly flowing vapor in the downcomer passageway, so
that these vapors in the downcomer passageway can achieve or approach an equivalent
level of mass transfer as the vapor which normally flows through the horizontal tray
deck. For example, in one embodiment, the mass-transfer packing elements employed
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in the downcomer may comprise packing elements made of a plurality of metsl sheets,
such as corrugated metal sheets, which also may be provided with a plurality of
apertures, as well as with fine fluting on the surface of the sheets. The flutings may
be disposed on one or both sides of the sheets, and typically are in the form of grooves
which extend at right angles to the axis of the column. Such packing element is
I commercially sold for exchange columns and is known as Flexipac (a registered
¦ trademark of Koch Engineering Company, Inc.), as described more particularly, for
¦ example, in U.S. Patent 4,296,050 of October 20, 1981 and other pPtents,
The improvement in the downcomer performance of the vapor-liquid
contacting tray of the present invention over the multiple-downcomer trays is dramatic,
in that, in the multiple-downcomer trays, the potential efficiency drop caused by
vapor by-passing through the downcomers is a major design concern. Therefore, insuch multiple-downcomer trays, in order to establish a proper liquid seal on each
downcomer, the downcomer exit openings are quite restrictive; for example, only
a few slots are provided for liquid delivery. Since the liquid-discharge rate through
a slotted-plate-type device is proportional to the sq~are root of the liquid head above
the plate, snd since the allowable liquid head for each downcomer of a
multiple-downcomer tray is usually less than 60~6 of the tray spacing, the downcomer
turndown ratio for this type of multiple-downcomer tray is usually very small. For
example, for a downcomer with an 8-inch maximum allowable liquid head and a 2-inch
minimum required liquid seal, the downcomer turndown ratio is only 2.0; that is, ~7~
Thus, the turndown ratio for multiple-downcomer trays is usually limited to a value
of approximately 2.
In the vapor-liquid contact tray of the invention, a packed bed of
structured or random-type packing, or a combination, is employed in the downcomer,
with a height which may vary as desired. The mass-transfer packing material employed
in the downcomer may not entirely stop the upward vapor flow through the downcomer,
but provides a mass-transfer means for the upward flowing vapor in the downcomer,
so that ~his vapor can achieve a high or equivalent level of heat and/or mass transfer
as the vapor normally flowing through the tray deck. This design also permits the
:l
utilization of a previously nonactive trav-deck area as an active heat and/or
mass-transfer area.
,1
The range of downcomer liquid capacity can be rated quite conservatively
at 170 gpm per square foot or more of the downcomer area. Data taken with a
5 I commercial-scale tower have shown that the same downcomer can be operated
satisfactorily at liquid flow rates as low as 30 gpm per square foot and as high as
400 gpm per square foot. This gives a turndown ratio greater than 10. Typically,
' I at high liquid capacity, the downcomer will be sealed by the high downward liquid
I¦ flux, while, at lower liquid loadings, vapor can by-pass through the downcomer;
10 1 however, this by-passing vapor stream will undergo efficient heat and mass transfer
with the high-flow liquid stream, for example, over 30 gpm per square foot, in the
packed section of the downcomer.
A further advantage of the improved vapor-liquid contact tray design
l is in the downcomer liquid delivery. By employing restrictive slots in the lower portion
15 ¦¦ of the downcomer, the liquid streams leaving each downcomer of a multiple-downcomer
tray possess a higher velocity and momentum which may disturb the performance
of the next lower tray deck. Typically, to divert this liquid momentum over a broader
tray-deck area, multiple downcomers are used, with the width of each downcomer
l less than about 4 inches. This restriction will not be as severe for the improved
20 1 vapor-liquid contact tray of the invention, since the down-flowing liquid will be spread
more evenly over a greater area of the succeeding tray by the insert within the
downcomer, thus reducing any disturbance of the liquid on the lower tray deck.
An additional advantage of the improved vapor-liquid contact tray design
l is in the design of the tray deck itself. The multiple-downcomer trays use sieve-type
tray decks which have an inherently poor turndown ratio. One preferred embodiment
of the present vapor-liquid contact tray design will use the field-proven, valve-type
tray deck, for example, the Type-A or Type-T Flexitray (a registered trademark of
Koch Engineering Company, Inc.). Therefore, the efficiency and the turndown ratio
of the tray deck also can be improved, along with the turndown ratio of the downcomer.
The downcomer of the tray may comprise one or more downcomers
on each tray in the column, and, for example, the tray may have a single-side
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I downcomer, with the side downcomer alternatin~ from one to the other side of the
I column with each tray. The tray may comprise a plurality of downcomers, typically,
¦ but not limited to, uniformly spaced across the tray deck.
¦ The mass-transfer devices may comprise structured-type gas liquid
¦I devices, xuch as, but not limited to, generally parallel sheets in a close, spaced-apart
I relationship with flutes, apertures or corrugations, or combinations thereof, or comprise
¦ fine-opening, knitted plastic or wire mesh, or discrete particulate random-type gas
¦ liquid packing, such as plastic, ceramic or metal, pall rings, saddles and the like,
¦ or combinations, in each passageway, or on different trays of such gas liquid devices.
¦ Typically and preferred, structured sheet or mesh-type packing is used.
¦ The mass-transfer device is usually positioned in the lower portio
n of
the downcomer passageway, and preferably is generally flush with the bottom
of
the lower outlet or extends below the lower outlet; for example, 0 to 12 in
ches, such
as 0 to 6 inches. The extension; that is, the exposed height, of the mass-t
ransfer
device permits the liquid to be spread more evenly over a larger area to th
e tray
below, to avoid disturbance of the lower tray liquid level, and providing a
more uniform
liquid layer on the lower tray deck. The distance from the bottom of the ma
ss-transfer
insert to the lower tray-deck may also vary as desired, but generally range
s from
about 0 to 18 inches; for example, 0 to 12 inches. The height of the mass-t
ransfer
device or insert in the downcomer may vary, but generally ranges between ab
out
1 inch and 90%; for example, 1 inch to 70%, of the height of the tray spaci
ng.
In the operation of a gas liquid column, such as in a distillation
or
separation process, the downwardly flowing liquid from the horizontal tray
deck,
passing through the downcomer passageway, is placed in intimate contact wit
h the
upwardly flowing gas in the passageway by means of the mass-transfer materi
al in
the passageway, to provide an active gas liguid contact area. The scrambled
liquid
in the passageway is then directed over a large area above the froth region
and into
the liquid level on the lower tray deck. The invention is particularly usef
ul in valve
trays employed in high liquid-loading operations, but is usefully adapted f
or other
operations with other types of trays.
The invention will be described for the purpose of illustration on
ly in
¦ connection with certain embodiments; however, changes, additions, modific~tions
and improvements in the illustrated embodiments may be made by those persons skilled
in the art, all falling within the spirit and scope of the invention.
BRIEF DESCRlPTlON OF T~IE DRAWINGS
Fig. 1 is ~ schematic illustration of a gas liquid contact column containing
the trays of the invention with side downcomers;
Fig. 2 is a schematic, fragmentary, perspective, illustrative view of
a portion of a tray of Fig. l;
Fig. 3 is a schematic illustration of a gas liquid cont~ct column containing
10 the trays of the invention with multiple downcomers; and
Fig. 4 is a schematic, fragmentary, perspective, illustrative view of
a portion of a tray of Fig. 3.
DESCRIPTION OF THE EMBODIMENTS
Fig. 1 illustrates a gas li~uid contact system 10, having a column 12 with
15 a plurality of spaced-apart, horizontal valve trays 14, 16 and 18 having valves 20, 22
and 24, with side downcomers 26, 28 and 30 and with mass-transfer packing inserts
32, 34 and 36 in the downcomer passages. The inserts 32, 34 and 36 comprise
spaced-apart corrugated plates with apertures and grooves therein, as described,for example, in U.S. Patent 4,296,050. The inserts extend slightly below the outlet
20 of the downcomers 26, 28 and 30. In the drawing, hl is the height of the inserts 32,
34 and 36, h2 is the height of the exposed extension of the inserts 32, 34 and 36 beyond
the end of the outlet of the downcomers 26, 28 and 30, h3 is the height of the bottom
of the extended inserts from the next lower tray, h4 is the height of the weir of the
downcomer extending above the tray decks 14, 16 and 18, and hs is the tray spacing.
25 With the structured packing illustrated, hl is between 1 inch and 90% of hs, h2 is
between 0 and 4 inches and h3 is between 0 and 20 inches grester th~n h4. Fig. 2is a perspective, illustrative view of a portion of one tray (16 of Fig. I) also illustrating
the wide distribution of the liquid from the bottom of the insert 34 over the liquid
in the tray below.
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~or conventional valve trays, the downcomers extend very close to
the tray deck of the next lower tray. For example, in Fig. I for a conventional tray,
the downcomer would occupy an area of the tray 18, and no valves, caps, holes orother devices for achieving intimate gas liquid contact could be used in this area.
By terminating the downcomer at h3, this area 71 can be used to expand the bubbling
area of the tray and increase its capacity, such as by the use of the same or different
type valves in this area, such as valves with bubble cap-type heads, to prevent weeping
in this area and to promote the transverse movement of the exiting liquid from the
downcomer from this area and across the tray.
Fig. 3 illustrates a system 50 with a large-diameter gas liquid column
52; for example, over 12 feet, having valve trays 54 and 56 with valves 58 and 60,
with a plurality of horizontal parallel and uniformly spaced downcomers 62 and 64
extending across the surface of the trays 54 and 56, with mass-trsnsfer packing inserts
66 and 68 in each downcomer, such as structured corrugated sheet packing or knitted
wire mesh.
Fig. 4 is a perspective, illustrative view of a portion of valve tray 54
with downcomer 62 and insert 66, and showing the distribution of liquid from thebottom and extended or exposed sides of the insert 66 to the tray below.
Thus, the advantage of this gas liquid contact tray over a conventional
valve tray is that the downcomers, with the packing material for mass transfer,
terminate above the tray deck, allowing additional tray-deck area to be used forefficient heat and mass transfer. Also, the advantage of this gas liquid contact tray
over a multiple-downcomer tray is its greater turndown ratio. In the preferred
embodiment, the greater turndown ratio is the result of using a valve tray deck, rather
than a sieve tray deck, and using packing in the downcomers, to achieve good heàt
and mass lransfer between the liquid and any by-passing vapor.
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