Chemical Engineering June 2010 - 27

Distillation
Tray Design
Department Editor: Scott Jenkins
I
n a distillation column tray, vapor
passes upward through liquid that is
flowing across a horizontal perforated
plate. Vapor passing through the perforated
plate forms a two-phase mixture
with the liquid and enables mass transfer
contacting. This mixture is typically quite
turbulent. Tray design must allow the
turbulent liquid to fall away from the
rising vapor in the space above the tray,
while also enabling the vapor bubbles
to rise out of the falling liquid in the
downcomer. The downcomer is usually
a vertical plate that enables the already
contacted froth to travel down to the next
tray without remixing with the up-flowing
vapor from the tray below.
Downcomers
area. In that case, the downcomer is
sloped such that its bottom area is 60%
of its top area.
Active area
The active area of a distillation tower is
where the vapor contacts the liquid to
effect mass transfer. Above the active
area, where the liquid falls away from
the rising vapor, is the volume where the
vapor can expand. Typically, the active
area is calculated to be the tower crosssectional
area minus the downcomer top
and downcomer bottom area.
The minimum active area (ft2) for normal
valve trays can be determined from
the following relationship, which is a
modification of a commonly used correlation
[1] taken at 82% of jet flood:
Active area = V-Load / [TS0.5
0.00092(ρV2)) - 0.011WL
(0.0762 -
]
Vapor flow
Where,
V-Load = CFSV
(ρV / (ρL - ρV))0.5
Liquid flow
Side view of a simple tray arrangement
Generally, designing a column tray entails
determining the minimum downcomer
area that still allows vapor bubbles
to rise through the liquid, selecting the
number of downcomers, determining
the active area, and checking the flow
path length to see if a person can pass
through a tray manway. These factors
are the primary drivers for determining
overall tower size.
Downcomer area is determined by the
maximum recommended downcomer velocity.
Divide the volumetric flow of liquid
by the downcomer velocity to obtain the
downcomer top area. Typically a curve
of maximum downcomer velocity versus
the density difference between liquid and
vapor is consulted during this process.
Maximum downcomer velocity guideline
0.45
0.4
0.35
0.3
0.25
0.2
0.15
10 20 30 40 50 60
Delta-density (L-V), lb/ft3
A downcomer is generally straight
unless its area exceeds 8% of the tower
TS = Tray spacing, in.
ρV = Vapor density, lb/ft3
WL = Weir loading, gal/min per in.
CFSV
= Vapor volumetric flow, ft3/s
The required active area is dependant
on the vapor density and weir loading.
Note that the weir loading need not be
known at this point. Assume a weir loading
value of 5 gal/min per in. intially.
Typical tray spacings are 24 in.
Tower area and diameter
Based on the above areas, the overall
tower area and diameter can be determined
by the following:
AT
D = 2(AT/ π)0.5
Where,
AT
= ADtop + ADbottom + AA
= Tower area, ft2
ADtop = Downcomer area at top, ft2
ADbottom = Downcomer area at bottom, ft2
AA = Active Area, ft2
D = Tower inner dia., ft
Number of downcomers
Once the tower diameter is determined,
then the number of downcomers can be
chosen. As a starting point, an initial
design should use a single downcomer.
The resulting weir length is calculated
from a standard chord-length calculation,
which is iterative for a given
downcomer area.
BW = {[(πD2/360) cos-1
Where,
Z = [(D2
/4) - BW
2]0.5
BW = Weir length of one downcomer, ft
(2Z/D)] - 2AD
}/Z
D
Tray geometry parameters
BW
Z
AD
A good place to start the iterative
process is with a weir length 0.8 times
the tower diameter. If the resulting weir
loading is greater than 12 gal/min per
in., then increase the number of tray
passes to two. Recalculate the outlet weir
length for each of the side downcomers
of the column by using half the downcomer
area. Check the weir loading again
(for the tray with side downcomers). If
the weir loading continues to exceed 12
gal/min per in., increase the number of
tray passes to four. It is assumed that the
two-pass tray with side downcomers has
the shortest weir length.
The simplest approach to designing
4-pass trays is to assume equal bubbling
area and make the side downcomers onequarter
of the total downcomer area, and
make the center (and off-center) downcomers
one-half of the total downcomer area.
Maintaining the resulting downcomer
widths at 6 in. or more will allow a person
to reach into the downcomer for installation.
In addition, make sure the resulting
tray-flow path-length is 16 in. or greater
to enable a person to physically pass
through the trays. These minimum size
criteria may increase the column diameter
to above the previously calculated value.
Other considerations
Other criteria that need to be considered
are; downcomer backup, spray
fluidization, and entrainment. In addition,
minimum load conditions need to be
determined. The criteria for determining
the low-end vapor and liquid range are
weeping, tray stability and dry-tray pressure
drop.
Reference
1. Glitsch Inc. " Ballast Tray Design Manual; Bulletin
No. 4900. " 3rd Ed. Glitsch Inc., Dallas,
Tex.,1974.
Note: Material for the June " Facts at Your Fingertips "
was supplied by Dan Summers, tray technology
manager, Sulzer Chemtech USA Inc.
Max. downcomer
entrance velocity, ft/s
Chemical Engineering June 2010

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