Vapor/liquid contact apparatus

An apparatus and method for sealing vapor/liquid contacting trays on startup of a downcomer-equipped vapor/liquid contactor comprising a series of gas/liquid contacting trays, at least some of said trays having a weir to maintain a liquid level thereon, a downcomer whereby liquid may flow down to the next lower tray and a seal area positioned underneath said downcomer for providing a liquid seal between successive trays in said series which comprises, prior to full liquid loading of a tray, collecting a portion of the first liquid on said tray, and passing it directly to said seal area positioned underneath said downcomer.

BACKGROUND OF THE INVENTION 
This invention relates to improvements in vapor/liquid contact apparatus 
and more particularly to an improved sealing vapor/liquid contact tray 
downcomer construction for absorption and similar towers or columns. 
A major concern in commencing operation of a tray-containing vapor/liquid 
contacting apparatus such as an absorption or a distillation column is the 
sometimes difficult problem of establishing a liquid seal in the 
downcomers. 
In conventional operation of a countercurrent vapor/liquid contacting tray 
(also referred to herein as a deck) vapor flows upwardly through tray 
perforations and liquid flows through the downcomer. The liquid seal at 
the bottom of the downcomer keeps vapor from entering the downcomer and 
the velocity of the vapor keeps most of the liquid from weeping through 
the tray perforations. 
During startup of the apparatus, a reverse situation occurs. Until a 
downcomer seal is established, vapor flows upwardly through the downcomer 
as well as the tray perforations, and at low vapor velocity the liquid 
pours through the perforations rather than flow across the tray and outlet 
weirs to reach the downcomers. As vapor and liquid rates are increased the 
situation may occur that, before the liquid rate is high enough to seal 
the downcomer, the vapor flow up the downcomer is sufficient to prevent 
liquid downflow. In the current art, the start-up procedure must be 
changed or the trays/downcomers modified to avoid this problem. 
When designing a vapor/liquid contactor, the traditional way to widen the 
operating conditions over which downcomer sealing and successful column 
startup can occur is to use smaller clearance area under the downcomers; 
however, the need to accommodate high flow rates in operation may make it 
impossible to reduce the clearance area enough to avoid a sealing problem 
on startup. Further, low downcomer clearance in a seal pan (typically on 
the bottom tray downcomer) runs the risk of plugging caused by deposition 
of solids, which solids may be present in or formed during processing of 
the contact fluids. 
Another traditional way of promoting successful contactor startup is to 
incorporate recessed inlet pans; however, this is a relatively costly 
solution. Moreover, like the low downcomer clearance there remains a 
possibility of plugging. Further, inlet pans have the additional 
disadvantage of a lower separation efficiency, especially in small 
columns, since the flow through the requisite drain holes bypasses two 
column trays. 
The present invention provides a novel method and apparatus to seal 
vapor/liquid contacting trays on startup. 
SUMMARY OF THE INVENTION 
The invention provides apparatus for sealing vapor/liquid contacting trays 
on startup in a downcomer-equipped vapor/liquid contactor, said apparatus 
comprising: 
a downcomer-equipped upright vapor/liquid contacting column, comprising a 
series of gas/liquid contacting trays, at least some of said trays having 
associated therewith a weir to maintain a liquid level thereon, a 
downcomer whereby liquid may flow down to the next lower tray and into a 
liquid seal area formed by an inlet weir or seal pan, said seal area being 
positioned underneath said downcomer for providing a liquid seal between 
successive trays in said series; and 
collecting means, provided on at least one said contacting tray having said 
weir, said downcomer and said seal area associated therewith, and said 
collecting means having an upper part disposed at an elevation between 
that of said tray and below said weir and having a lower part disposed to 
provide liquid to said seal area, said collecting means being dimensioned 
for collecting liquid from said tray so that no appreciable level has to 
build up on said tray before it flows into and through said lower part 
into said seal area which is disposed underneath said downcomer. 
The invention further provides a method for startup of an upright 
downcomer-equipped vapor/liquid contactor comprising a series of 
gas/liquid contacting trays, at least one of said trays having associated 
therewith a weir to maintain a liquid level thereon during normal 
operation, a downcomer whereby liquid may flow down to the next lower 
tray, and a seal area positioned underneath said downcomer for providing a 
liquid seal between successive trays in said series, which method 
comprises: 
starting said apparatus with vapor and liquid at operating conditions 
substantially less than conditions during normal operation; 
collecting, prior to flow of liquid on said tray over said weir and through 
said downcomer, a portion of the first liquid on said tray; and 
passing it directly to said seal area underneath said downcomer.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The apparatus of this invention may be applied to any of the well known 
columns for countercurrent contacting of a vapor and a liquid, such as 
fractionating columns, absorbing columns, rectifying columns, scrubbing 
columns and the like. Further, the apparatus of the invention may be 
suitably applied with all types of trays, e.g., sieve, bubble cap, valve 
tray, etc., and all types of downcomers. 
Referring now to the drawings, FIG. 1 is a schematic partial-sectional view 
of an upright, downcomer equipped vapor/liquid contactor 10 showing a 
portion of contactor wall 12 and two of the internal vapor/liquid contact 
trays 14 and 16, respectively. A seal area 18 formed by inlet weir 19 is 
shown under the downcomer area 20 of each tray. The bottom tray of a 
series of trays will typically have a sealing area formed by a seal pan 
(not shown) rather than an inlet weir. Said trays 14 and 16 in a preferred 
embodiment are perforated plates, having a plurality of perforations 22 to 
facilitate vapor/liquid contact during normal operation. Said trays 14 and 
16 each have a weir 24, also referred to herein as a baffle, to maintain a 
liquid level on said trays during normal operation. In accordance with the 
invention, the trays further contain collecting means 26 with an upper 
part 28 having an opening 30 at an elevation substantially that of the 
tray deck it depends from, and having a substantially narrower lower part 
32 disposed to deliver liquid flowing downwardly therethrough into said 
seal area 18. Preferably said collecting means 26 is disposed on said 
trays 14 and 16 proximate to said weir 24, and most preferably as close to 
said weir as possible to avoid bypassing of the active area of the tray 
during normal operation. 
It is preferred that the top of collecting means 26 have an elevation even 
with the tray deck so during startup no appreciable liquid level has to 
buildup on the tray before liquid flows into said collecting means. Thus 
tray pressure drop and downcomer gas flow are low when sealing liquid 
becomes available and is provided by collecting means 26 to seal area 18. 
FIG. 2 is a top plan view showing a preferred embodiment wherein 
collecting means 26 is disposed as close as possible to weir 24. FIG. 3 is 
a fragmentary vertical section along line 3--3 of FIG. 1 illustrating an 
embodiment of collection means 26, having opening 30 in upper part 28 of 
larger area than the discharge opening in the lower part 32. The 
cross-sectional area of opening 30 of collecting means 26 needs to be 
large enough so that during startup vapor flowing up the lower part 32 at 
the time liquid reaches said opening 30, will not blow liquid out of the 
upper part of collecting means 28. The depth of said upper part 28 should 
be sufficient to make it possible to build up enough head to stop vapor 
flow and to allow the liquid to flow down the tube. Moreover, the area of 
the lower part should not be so large as to appreciably reduce froth 
height on the tray during normal operation since it is, in effect, a weir 
bypass. The upper part 28 of collecting means 26 preferably should have a 
cross-sectional area no less than about twice the cross-sectional area of 
the lower part 32 of collecting means 26. Further, the vertical height of 
upper part 28 preferably should be equal to or greater than the tray 
liquid height equivalent to the dry tray pressure drop at normal vapor 
rates.