Vapor liquid contacting

A vapor liquid contacting apparatus having improved column throughput and mass transfer efficiency featuring, in one aspect, an overlay for dissipating vapor hole momentum and reducing vertical oscillations in the liquid vapor dispersion on the tray, and, in another aspect, an improved inlet weir.

BACKGROUND OF THE INVENTION 
This invention relates to vapor-liquid contacting in apparatus of the 
cross-flow type, e.g. in a fractional distillation column. 
In such systems, mass transfer is accomplished by liquid flow across 
horizontal orificed trays through which vapor is forced. Column throughput 
and efficient mass transfer are limited by a number of factors including 
priming, entrainment of the liquid droplets in the vapor flow, downcomer 
flooding, hydraulic gradient, weeping, transition to spray regime 
operation, non-plug flow of liquid across the trays, and liquid 
back-mixing. 
Webster, U.S. Pat. No. 3,467,365, shows a layer of expanded metal attached 
to a sieve tray to give gas entering the vapor/liquid dispersion a 
component of velocity in the direction of the liquid flow path. 
Mix and Erickson, U.S. Pat. No. 3,887,665, shows layers of wire mesh in 
contact with the tray which serve to limit the oscillations in the 
vapor/liquid dispersion at the tray surface. A similar function is 
performed by vertical sheets above the tray in Mix ('665) and Mix, U.S. 
Pat. No. 4,105,723. 
In both aforementioned Mix patents, zigzag woven wire mesh is placed above 
the vapor/liquid dispersion to recover entrained liquid droplets. In Mix 
('732), this zigzag deentrainment mesh is placed on top of an expanded 
metal support. 
Bruckert and Wang, U.S. Pat. No. 3,282,576, shows an inlet weir having a 
perforated and inwardly and downwardly sloping surface to initiate froth 
formation in the entering liquid. 
Uitti and Carson, U.S. Pat. No. 3,700,216 shows an inwardly and upwardly 
sloping inlet weir, forming the top of a vertical vapor inlet slot 
positioned above the tray, to initiate froth formation in the entering 
liquid. 
SUMMARY OF THE INVENTION 
In one aspect, the invention features, in general, a plurality of surfaces, 
positioned above an orificed tray of a vapor liquid contacting apparatus 
to contact vapor predominantly flowing along a first axis, and liquid 
flowing along a second axis transverse to the first axis, the surfaces are 
spaced along and oriented obliquely to the first axis, and spaced along 
the second axis in a staggered relationship to the first axis; the 
surfaces have a hydraulic mean diameter greater than one-half the diameter 
of the tray orifices, and the projection of the surfaces on the tray has 
no open areas whose greatest dimension is more than three times the 
hydraulic mean diameter, so that the surfaces deflect vapor and thereby 
dissipate vapor hole momentum, and reduce vertical oscillations in the 
vapor-liquid dispersion on the tray. 
In preferred embodiments, the surfaces are provided by a plurality of 
layers of unflattened expanded metal parallel to the tray and to the 
second axis, and perpendicular to the first axis; various layers are 
oriented to deflect vapor in respective directions having primary 
components normal to the second axis, in the positive direction of the 
second axis, and in the negative direction of the second axis; a first 
group of layers closest to the tray are closely spaced, being no more than 
one layer thickness from adjacent layers, and the bottom layer of the 
group is no further above the tray than the diameter of the tray orifices; 
additional layers are spaced above the first group, each of which is 
spaced apart from other layers, and the highest of which is no further 
from the tray than 1/2 the distance to the next higher stage. 
In another aspect, the invention features, generally, an improved inlet 
weir with a liquid inlet providing momentum to the liquid in the direction 
of the liquid flow path, and a vapor inlet including means for mixing gas 
with the liquid, and giving the vapor a predominant flow component in the 
direction of the inlet liquid momentum. 
In preferred embodiments, the inlet weir has a first surface that is joined 
at one end to the tray and extends upwardly from the tray and inwardly 
toward the liquid outlet to end at a juncture with a second surface that 
extends downwardly toward the outlet and the tray, and ends over an 
orificed portion of the tray, so as to define a vapor inlet between the 
surfaces and the tray; a vapor inlet slot is formed between the second 
surface and a third surface joined to the tray and positioned parallel to 
and below the second surface; the inlet weir extends at least as high as 
the downcomer apron; and a deflection plate joined to either the second or 
third surface is parallel to the tray and positioned between, above, or 
below layers of expanded metal. 
In yet another embodiment, the invention features, in general, a method for 
retrofitting a vapor liquid-contacting apparatus by increasing the height 
of the outlet weir and providing means disposed above the tray to limit 
the increase in froth height to less than that increase which would 
otherwise accompany the increase in height of the outlet weir. In 
preferred embodiments, the outlet weir is at least 4 inches high; and the 
froth height limiting means is a plurality of layers of expanded metal. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
We turn now to a description of the preferred embodiments of the invention 
after briefly describing the drawings.

STRUCTURE 
Referring to the drawings, cylindrical housing 10 of liquid-vapor 
contacting and separating column 12 has a central vapor inlet below its 
bottom stage. A series of contact stages is vertically arranged inside 
housing 10. Considering an intermediate stage shown as typical, a plate 22 
extends across housing 10. The downcomer 32 with apron 35 carries liquid 
from the tray above into seal pan 33 with base 34. Inlet weir 23 
communicates between pan 33 and the active section of plate 22. 
The active section of plate 22 between inlet weir 23 and outlet weir 29 is 
perforated with holes 25, 1/2 inch in diameter on 11/2 inch triangular 
centers. The area of the holes thus amounts to 10% of the total area of 
the active section of the plate, but in low pressure drop applications 
that area can be higher, e.g., between 15 and 40 percent. 
Seven layers 14 (numbered 1-7 sequentially from bottom to top in the Figs.) 
of expanded metal (of which selected surfaces 27 are shown in FIGS. 4 and 
5) are positioned above the active portion of plate 22. The expanded metal 
has parallelogram shaped openings (short diagonal between 1/2 and 3/4 
inch) and is made from stainless steel sheet stock of between 12 and 18 
gauge (e.g., Exmet SS (304) 561/2). Vapor rising through the plate strikes 
surfaces 27 obliquely, imparting a net deflection in the direction of the 
short axis A of the parallelogram opening, as depicted in FIGS. 4. 
The bottom four layers (1-4) form a matrix no more than the diameter of the 
tray holes above, and preferably in contact with, the upper surface of the 
active section of the plate. As depicted in FIG. 3, layers 1 and 3 are 
oriented with axis A normal to the predominant direction L of liquid flow, 
deflecting the vapor flow to the right in that figure. Layers 2 and 4 are 
oriented 180.degree. to layers 1 and 3, deflecting vapor to the left in 
the figure. The layers are closely spaced, each layer being no further 
than one layer thickness from, and preferably in contact with, adjacent 
layers. 
Layers 5, 6, and 7 are spaced apart, parallel to, and spaced above layers 
1-4. Layer 5 is oriented in the same direction as layers 1 and 3, and 
layer 6 is oriented in the same direction as layers 2 and 4; layer 7 is 
oriented to deflect vapor in the direction L of liquid flow across the 
tray (FIG. 3). Layer 5 is spaced above the tray to be located at the top 
of outlet weir 29; layers 6 and 7 are spaced above layer 5 to create a gap 
between each of the three layers of between 0.2 and 5 times the height of 
the outlet weir (e.g., 1.5 inches). The highest layer is spaced above the 
tray to be less than 1/2 the distance to the tray of the next highest 
stage. Layers 6 and 7 extend into the area 51 above downcomer 52. 
Inlet weir 23 has surface 26 which meets base 34 at an obtuse angle on the 
inlet side and is joined to downwardly and inwardly sloping surface 28 
which ends above the tray. Surface 41 is parallel to and below surface 28, 
joining the tray at an acute angle on the inlet side. Inlet 42 formed 
between surfaces 41 and 28 communicates with the vapor space 45 beneath 
the tray through slot 24 between surfaces 28 and 41. Deflector 44 joins 
surface 41 parallel to plate 22 and is positioned between the tray and the 
uppermost expanded metal layer adjacent the tray. 
Alternatively, the inlet weir can be constructed, as in FIG. 6, without 
surface 41 and with deflector plate 44' attached to surface 28'. Deflector 
plate 44' can be over some or all of layers 14'. That embodiment shows six 
layers in contact with each other. 
FIG. 7 depicts yet another alternate embodiment in which deflector plate 
44" is attached to surface 41" and is in contact with the tray. 
Outlet weir 29 extends at least four and typically six inches or more above 
tray 22. Existing apparatus can be retrofitted by increasing the outlet 
weir height and adding the expanded metal overlay to limit the froth 
height increase to less than would otherwise accompany the increased weir 
height. 
OPERATION 
The downcomer 32 from the tray above carries liquid from the tray above 
into seal pan 33 with base 34. The liquid flows under downcomer apron 35 
onto the active portion of tray 22 between the base of the inlet downcomer 
apron 33 and vertical outlet weir 29. Vapor is forced through holes 25 
into contact with the liquid. In traversing the holes, the vapor acquires 
additional momentum (called vapor hole momentum) due to the restricted 
area of the hole passage. As the vapor obliquely contacts angled surfaces 
27 of the expanded metal which are wetted with liquid, the additional 
momentum is transferred to the liquid and in turn dissipated in drag on 
the expanded metal surfaces. These deflections of the vapor by the wetted 
angled surfaces therefore dissipate the hole momentum and generate more 
efficient mass transfer than would occur if the hole momentum were 
dissipated in other ways--e.g., in jetting and spray formation, which 
would lead to vertical oscillation of the gas-liquid dispersion and 
earlier onset of droplet entrainment and priming, and generate high froth 
heights and low froth densities. The projection of the surfaces on the 
tray has no open areas whose greatest dimension is more than three times 
the hydraulic mean diameter. 
In the vicinity of the plate there is in general an upward flow of vapor 
and liquid in the region over the plate holes, and a return of liquid to 
the plate in the region between the holes. The return liquid flow is 
driven by gravity and surface tension forces. The resistance to liquid 
return flow is a function of the Fanning friction factor which is in turn 
a function of the Reynolds number. The Reynolds number is proportional to 
the hydraulic mean diameter of the channels in the plate overlay. Sherwood 
and Pigford in the second edition of their book Absorption and Extraction, 
on page 239, present the following equation for calculating the hydraulic 
diameter: 
EQU D.sub.e /4=.epsilon./a.sub.v 
wherein 
.epsilon.=fraction of voids in the overlay 
a.sub.v =surface area per unit volume of the overlay, and 
D.sub.e =hydraulic mean diameter. 
The larger the hydraulic mean diameter, the larger will be the Reynolds 
number and the lower will be the Fanning friction factor. For 1/2 inch 
number 18 expanded metal, the surface area per unit volume of staggered 
layers in close contact can be calculated to be 56.5 ft.sup.2 /ft.sup.3, 
while the void fraction is 0.866. The mean hydraulic diameter for this 
overlay is therefore about 0.061 ft. 
The large hydraulic diameter of the expanded metal overlay contributes to 
the efficiency of the gas-liquid contacting. The low resistance to liquid 
return flow generated by the passages in the metal increases liquid in the 
region directly over the plate holes to ensure efficient contacting in 
this critical region of high vapor velocities where a significant fraction 
of the mass transfer is likely to occur. 
An additional advantage of the expanded metal overlay is the possibility 
with the expanded metal of increasing the fluid mixing normal to the 
direction of flow across the tray so as to approach more closely plug flow 
and the full plate to point effect on plate efficiency. This can be 
achieved with the expanded metal overlay by orienting the layers so as to 
have the vapor deflected normal to the direction of liquid flow across the 
tray. 
A spaced pair of deflecting layers, oriented so as to deflect vapor normal 
to the liquid flow path, will result in little or no net deflection of the 
flow if the deflections of the two layers are opposite to each other, but 
will result in increased liquid mixing normal to the liquid flow path. The 
scale of mixing generated normal to the liquid flow between spaced layers 
will increase with spacing between them, while the mixing along the liquid 
flow path axis will be relatively unaffected. Thus the stagnant flow 
regions which are apt to result from chordal flows can be made to mix with 
the regions of active flow resulting in a much closer approach to plug 
flow and increased plate efficiency due to the cross-flow effect. If the 
space between layers is too large, the flow deflection can interfere with 
the uniformity of contacting by generating a region relatively free of 
liquid at one side wall and a region with excess liquid at the opposite 
side wall. 
Since the orientation of the deflecting layers can be varied locally, it is 
also possible to direct the flow of liquid across the tray, so that it is 
made to diverge as the flow path diverges, without flow separation and 
without the formation of these stagnant regions. The ability of the 
overlays to direct the gas-liquid flow thus makes possible the obtaining 
of a close approximation to plug flow and zero hydraulic gradient without 
the lowering in liquid residence time and decrease in mass transfer. 
The slugs which transport vapor through the froth, in the froth regime, 
generate troughs and crests in a wavelike action at the froth surface as 
they erupt through the froth and disengage. Only minimal mass transfer 
occurs between vapor and liquid in this disengagement region. This region 
can represent a substantial fraction (equaling 50 percent or greater) of 
the froth height. Spaced horizontal layers of deflecting surfaces located 
in this region can damp these wave oscillations, and increase the height 
of froth over which vapor liquid mass transfer occurs, by increasing the 
height above the tray of the troughs. At the same time, by reducing the 
surface oscillations, these spaced layers also reduce liquid back mixing, 
and hence eddy diffusivity, and thereby increase realizable plate-to-point 
efficiency gain. Thus, the individual horizontal layers of expanded metal 
spaced vertically above the plate further restrict oscillations in the 
froth, and increase the density of the upper regions of the froth to 
improve the gas-liquid contacting in these regions. The layers further 
increase throughput by reducing entrainment, by increasing the velocity at 
which priming will be produced, and by increasing the density of the froth 
entering the outlet downcomer and thereby increasing the density of the 
downcomer fluid. 
The spaced individual layers are of small enough hydraulic diameter to damp 
the wave oscillations effectively, but are sufficiently open not to 
flood--i.e., interfere with the return drainage to the tray of liquid that 
has passed up through the layer. To avoid flooding, the hydraulic mean 
diameter should be such as to make j.sub.g * less than C.sup.2 according 
to the following equations (compare equations 11.84-11.87 of Wallace, 
One-Dimensional, Two-Phase Flow, McGraw-Hill (1969)). 
EQU j.sub.g *&lt;C.sup.2 
where 
EQU j.sub.g *=j.sub.g.rho..sub.g.sup.1/2 [gD(.rho..sub.f 
-.rho..sub.g)].sup.-1/2 
and where 
j.sub.g is the superficial gas velocity 
D is the hydraulic mean diameter 
g is the gravitational acceleration 
.rho..sub.f is the liquid density 
.rho..sub.g is the gas density 
and 
C is a constant between 0.7 and 1.0. 
The overlays are sufficiently open (i.e., of large enough hydraulic 
diameter), and flow through them is sufficiently active throughout, to 
resist deposition and fouling. 
The design of the inlet weir 24 further improves mass transfer and permits 
operation without weeping at lower vapor velocities, and with trays having 
higher free areas, by promoting froth formation in the entering liquid. 
Incoming liquid flows under downcomer apron 35, over inlet weir surface 
26, down inlet surface 28 and onto horizontal deflection surface 44. The 
liquid thus enters as a thin stream with velocity and momentum in the 
direction of liquid flow across the tray. The vapor from the chamber below 
the tray enters through slot 25 with a predominant velocity and momentum 
in the direction of liquid flow. This inlet vapor momentum is partially 
transferred to the liquid, accelerating and thinning it and preventing the 
development of a high head of unaerated liquid in this region. The vapor 
enters under the liquid and flows concurrent with it, promoting froth 
formation, and ensuring reasonable contact time and hence efficient mass 
transfer despite the thin liquid layers in this region. 
Because the expanded metal overlays dissipate hole momentum efficiently 
without jetting and spray formation, they make feasible operation at lower 
rates of liquid flow without blowing (i.e., vapor flow through the plate 
sweeping liquid off the tray) and without excessive entrainment. 
OTHER EMBODIMENTS 
Other embodiments are within the following claims. For example, variations 
in the number and orientation of the layers of metal can be arranged to 
affect the liquid flow in different ways. Other trays can be used, for 
example with valve or bubble cap gas openings, or 40 percent free area 
trays having 1/8 inch diameter holes on 3/16 inch triangular centers. 
Other thickness sheets of expanded metal can be used. The bottom layer of 
metal can be suspended above the tray. 
The invention is applicable to high-pressure systems as well as for 
low-pressure systems. It is also applicable over wide ranges of liquid 
rates. For low pressure-drop applications, such as vacuum fractionations, 
the combination of the expanded metal overlays and the inlet 
weir/froth-initiator make possible stable operation with high free area 
trays as to reduce pressure drop across the tray. 
The improved inlet weir can be used with or without the expanded metal 
layers, and with or without the baffles described in my U.S. Pat. No. 
4,105,723, which is hereby incorporated by reference. Retrofitting can be 
accomplished by using those baffles to limit the froth height increase 
accompanying increased outlet weir height. 
The invention is of course generally applicable to types of vapor-liquid 
contacting other than the fractionated distillation disclosed above, such 
as acid gas removal and particulate scrubbing, and the like.