Downcomers for vapor-liquid contact trays

Gas and liquid contact apparatus – Contact devices – Wet baffle

Reexamination Certificate

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Details

C261S114500

Reexamination Certificate

active

06224043

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND OF THE INVENTION
This invention relates in general to mass transfer and exchange columns and, more particularly, to downcomers used in association with vapor-liquid contact trays employed in such columns.
Vapor-liquid contact trays are used in mass transfer or heat exchange columns to facilitate interaction and mass transfer between descending liquid streams and ascending vapor streams. The trays are generally horizontally disposed and vertically spaced apart within an open interior region of the column. Each tray typically includes a flat deck portion that includes a plurality of vapor flow apertures that allow ascending vapor to pass through the tray deck and interact with liquid flowing across the upper surface of the tray deck. A downcomer is positioned at an opening at an outlet end of the tray deck to remove liquid from the deck and direct it downwardly to a liquid receiving area at the inlet end of an underlying tray. The liquid then flows across the tray deck of the underlying tray, interacts with vapor passing through the tray deck, and then flows downwardly through the associated outlet downcomer to the next underlying tray. This flow pattern is then repeated for each successively lower tray.
In conventional columns where high liquid flow rates are encountered, it has been suggested in U.S. Pat. No. 5,213,719 that a second downcomer can be used on each tray to increase the liquid handling capacity of the tray and thereby reduce the opportunity for flooding of the downcomer to occur. The second downcomer, referred to as the upstream downcomer, is positioned adjacent to the downstream downcomer and is shorter than the downstream downcomer in vertical length.
FIG. 1
, taken from U.S. Pat. No. 5,213,719, illustrates this downcomer construction with the upstream downcomer
10
and downstream downcomer
12
positioned at the outlet end of the tray deck
14
.
It has also been suggested, in U.S. Pat. No. 5,453,222, that the normally planar downcomer inlet wall can be shaped in a semi-conical fashion to form a vapor tunnel along the undersurface of the semi-conical wall. The vapor tunnel imparts a horizontal flow vector to the vapor stream and facilitates disengagement of liquid from the vapor stream.
FIGS. 2 and 3
are taken from U.S. Pat. No. 5,453,222 and illustrate a tray
16
with a downcomer
18
having a semi-conical inlet wall
20
. Venting chambers
22
positioned in the liquid receiving trough
24
on the underlying tray
26
allow vapor to flow through the chambers
22
for upward passage through the overlying vapor tunnel
28
formed by the semi-conical downcomer inlet wall
20
.
It would be desirable to combine the advantages afforded by the double downcomer disclosed in the above-mentioned U.S. Pat. No. 5,213,719 with those provided by a downcomer with a semi-conical inlet wall as taught by U.S. Pat. No. 5,453,222 discussed above. Several problems, however, would result from such a combination because the upstream downcomer would need to be of a relatively short vertical dimension so that it does not protrude downwardly into the vapor tunnel and interfere with the desired flow of vapor through the vapor tunnel. If a relatively short upstream downcomer is used, liquid issuing from the bottom of the upstream downcomer would be discharged directly into the vapor stream flowing along the vapor tunnel. The momentum of the vapor stream would cause the discharged liquid to be blown away from the downcomer and across the tray. The vapor-liquid contact and energy and mass exchange occurring in such blowing liquid as it moves through the vapor is not as good as is to be desired. In addition, the blown liquid would bypass portions of the tray deck and would not experience the vapor-liquid interaction that would otherwise occur if the liquid flowed completely across the tray deck. Therefore, it is desirable to minimize or eliminate this effect.
Another untoward effect which may occur as a consequence of utilizing an upstream downcomer is it may “starve” liquid flow from the downstream or primary downcomer under low flow conditions. A further consequence of this effect is that the downstream downcomer may have too little liquid flowing through it and it may lose the liquid seal at the bottom region of the downcomer that blocks undesired entry of vapor into the downcomer. Loss of the liquid seal will allow vapor to flow upwardly through the downcomer and bypass interaction with liquid on the overlying tray. The possibility that such an effect will occur decreases the operating flexibility of the column taken as a whole.
A still further undesirable result from the use of an upstream downcomer of small vertical extent is that liquid issuing from the bottom of the upstream downcomer falls in free-fall vertically downward to the tray deck below. The large momentum of the falling liquid is transmuted into pressure when the liquid hits the tray below and locally depresses the vapor flow in the impact area and, in consequence, allows the liquid to weep through the vapor apertures at that point in the tray deck.
While the foregoing undesirable effects of utilizing an upstream downcomer of short vertical extent have been described in connection with a downcomer system utilizing a vapor tunnel structure, those skilled in the art will appreciate that these undesirable effects can also be encountered when the upstream downcomer is of slight vertical extent, even if there is no vapor tunnel. It would thus be desirable to overcome these disadvantages in a double downcomer system.
BRIEF SUMMARY OF THE INVENTION
It is an object of this invention to provide a vapor-liquid contact tray with a double downcomer constructed in a manner that does not completely block the desired vapor flow pattern in the area of the downcomer, but is able to at least partially shield the liquid discharged from the upstream portion of the downcomer so that the vapor flow does not carry the discharged liquid away from the downcomer and thereby interfere with the desired vapor-liquid interaction in the vicinity of the downcomer.
It is also an object of this invention to provide a double downcomer that does not completely block the desired vapor flow pattern and in which the upstream portion of the downcomer chokes the flow of liquid so that it can accumulate within the upstream portion of the downcomer and flow into the downstream portion of the downcomer even under low liquid flow conditions, thereby creating the liquid seal necessary to resist upward vapor flow through the downstream portion of the downcomer.
It is a further object of this invention to provide a double downcomer as described that does not completely block the desired vapor flow pattern but is able to discharge liquid near the surface of the underlying tray deck in a manner that disrupts the downward momentum of the liquid to reduce the incidence of liquid weeping through the vapor flow apertures on the tray deck as a result of such downward momentum.
To accomplish these and other related objects of the invention, a vapor-liquid contact tray is provided, comprising a tray deck having an opening for removing liquid from an upper surface of the tray deck and a plurality of apertures for allowing vapor to flow upwardly through the tray deck to interact with said liquid on the upper surface. An upstream downcomer is provided and extends downwardly at the opening in the tray deck and has an inlet at an upper end to receive at least a portion of the liquid entering the opening from the tray deck and a lower discharge outlet through which at least part of the portion of the liquid exits the upstream downcomer. A downstream downcomer is also provided and extends downwardly at the opening in the tray deck and has a lower discharge outlet through which a second portion of liquid exits the downstream downcomer. A partition wall separates the downstream downcomer from the upstream downcomer along at least a portion of their lengths and an inlet wall which defines a portion of the ups

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