Page 214 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 203

These trays are somewhat sensitive to rapid changes in
tower conditions. Towers over 40 trays must be controlled
within fine limits.
The perforated plate, punched plate, or Dual-Flow plate
are terms used to refer to a tray operating without down-
comers, with tapor and liquid passing countercurrent
through perforations in the tray. The Dual-Flow term has The orifice coefficient can be read from Figure 8-128.
been coined by Fractionation Research, Inc., and its Sutherland used C, = 0.85 and 0.73 for %in. and %in.
design know-how is restricted to contributing members, holes respectively, in %-in. plate.
and cannot be presented in this book.
Effective Head, h,
Diameter
Although Sutherland did not obtain an equation for
There is essentially no published work on specific tests total tray pressure drop, correlation at this time indicates
with these trays as relates to entrainment, etc. However, that it follows the effective head concept of Hughmark.
the very close similarity between a perforated plate with- This is a limited evaluation because the data available did
out downcomers and one with downcomers is sufficient to not indicate any clear liquid heights over about 0.75 in.
just+ using some data for one in the design of the second. When “head of liquid” is considered “clear liquid on the
This is the case with diameter determination. The rela- tray,” Figure 8-130 may be used to read the effective head,
tion of Equation 8-250 for the perforated tray or sieve tray he. Values of h,l beyond 1 in. have not been checked for
with downcomers can be used for the plate without down- lack of data, but do agree generally with the plotted results
comers. Generally, the liquid level and foam-froth height of Sutherland [69].
will be higher on this tray, hence the value of h,, clear liq-
uid on the tray, may range from 1-in. to 6in. depending Total Wet Tray Pressure Drop
on the servic.e.
For the data checked,
Capacity
ht = hdt + h,, (also see Equation 8-268) (8-306)
In general, the vapor capacity for a given tray diameter is
10-35% greater than bubble cap trays and somewhat These results cannot be expected to correlate for a tray
greater than sieve trays with downcomers. The flexibility or just becoming active (very IOW liquid on tray, 0.1 in. k), but
range is limited because reasonable efficiencies fall-off near have been satisfactory at 0.2-in. for clear liquid height, h,l.
the dump point for most systems. Usual designs limit the To determine a tray operation with respect to pressure
lower operating point to 60-70 of the flood point, unless drop, the value of h,l must be assumed at a reasonable
particular data is available to safely allow reduction in lower value,-the larger the better the contact, and higher the
limits without the accompanying loss in tray efficiency. pressure drop. Values of h,l should be limited to about 4
in., following sieve tray practice.
Pressure Drop
Hole Size, Spacing, Percent Open Area
The pressure drop of these trays is usually quite low.
They can be operated at an effective bubbling condition Hole size is as important in perforated plates without
with acceptable efficiencies and low pressure drops. For downcomers as far the sieve tray. Published data limits a
more efficient operation the clear liquid height on the full analysis of the relationships; however, the smaller
tray appears to be similar to the sieve tray, i.e., 1.5-2-in. holes, %-in., %-in., %-in. appear to give slightly higher effi-
minimum. This is peculiar to each system, and some oper- ciencies for the same tray spacing [47]. Unfortunately the
ate at 1 in. with as good an efficiency as when a 2-in. is data [69] for the larger %-in. holes was not evaluated for
used. IA%en data is not available, 2 in. is recommended as efficiencies. Experience has indicated efficiencies equal to
a median design point. or only slightly, 10-15%, less for %-in. holes when com-
pared to %pin. holes for some systems. Holes as small as
Dry Tray Pressure Drop !4rrin., %Pin. and %An. were considered unsatisfactory for
high surface tension materials such as water [471.
A$ should be expected, the relation of Hughmark [31] Sutherland reports frothy type contact for %in. holes
correlated the data of Sutherland [69] quite well. and jetting spray bubbly action for %-in. holes.