Page 276 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Packed Towers 265

= [ (vapor velocity) (vapor density) / (liquid - vapor The use of V-notches in a trough wall for overflow is
densities))O.>] more sensitive to leveling problems than the other
DQ = distributor quality designs, and for the same *%L to %in. level tolerance pro-
a = surface area of an orifice hole, in.2 duces a more severe non-uniform flow distribution. The
d = diameter of hole, in. quality of distribution from a V-notch is poor compared to
h = liquid height over orifice, in. the other types of trough distributor, but does have advan-
hd = vapor pressure drop across distributor, in. (Calculate hd tages in slurry systems [ 1311. It should not be used for crit-
by the vapor flow equation through distributor open ical distillation applications, but is good for heat transfer
area.) and where solids are in the system.
n = number of holes
Q = liquid volumetric flow rate, gpm Pipe orifice Headers (POH)
K = flow coefficient
V = horizontal liquid velocity, ft/sec (in distributor) These distributors are fabricated of pipe lengths tied to
a central distribution header (usually) with orifice holes
The use of narrow trough PAN distributors is a better drilled in the bottom of the various pipe laterals off the
choice to prevent leakage from towers above 3-9 ft diam- header. This style of distributor can be fed by pressure or
eter and larger [131]. gravity for clean fluids. The gravity feed is considered bet-
Turndown on a PAN type tray should be limited to 2:l ter for critical distillation application when uniformity of
(ratio of high to low flow rates), which results in a reasonable the flow of the drip points (or flow points) through out
design. Bonilla [131] points out that it is costly to design for the cross-section of the tower is extremely important, and
short-term high turndown rates such as start-up, shutdown, is excellent for low flow requirements such as below 10
or for other short term periods, because it is better to gpm/ft2 [131].
increase reflux ratio to increase internal loads for such peri- This design is restricted to a 2:l turndown and is not
ods rather than design the distributor for large turndown. practical for large liquid rates compared to the NTD or
RTD styles [ 1311.
Trough Distributors (NTD)
Spray Nozzle Headers (SNH)
These consist of multiple troughs 3-4 in. wide [131],
These are similar in design to the pipe orifice distributor
and are fed by feed or parting boxes mounted above, or by using small angles (<go”) spray nozzles instead of orifices.
multiple pipes mounted 90” to the direction of the Because the sprays can be selected to cover varying cross-
troughs. The parting boxes distribute the liquid to troughs sections per spray, the total number can be small compared
through calibrated orifices in the bottom or wall. The feed to the orifice holes that would be required. The spray from
points out of the troughs to the packing are usually in the the nozzle should be “full cone” and not “hollow cone” to
bottom or at the wall of the troughs. Locating the holes in provide a uniform liquid circle that should overlap to avoid
the trough sidewalls allows for collection of rust and any dry-spots. The spray nozzles operate under pressure, with
other sediment in the trough bottoms and avoids plugging the manufacturer providing flow rates and patterns for
the orifice holes. This style has a capability of handling variations in system pressure. A careful layout is required to
large vapor loads and the distributor design allows for evaluate how much liquid flows through each square foot
good liquid distribution. These troughs can be readily lev- of tower and therefore the packing. This style of distribu-
eled across a tower, and can still handle low to medium liq- tor is good for heat transfer and vapor washing, with little
uid rates, and can handle turndown of up to 5:l by special or no fractionation, although they have been used success-
and careful design [ 1311. This should not be used for slur- fully for distillation operations.
ry systems; rather, a Vnotch version is better suited, but Turndown is usually limited to 0.5:1, and liquid distrib-
may not be as accurate for liquid distribution. ution can be poor if the sprays are not carefully laid out
The NTD’s are quite often used with structured packing and the system flow tested for uniformity. Another prob-
where it may be desired to have an overflow sheet of liquid lem is misting of the liquid from the sprays and the result-
ofito the packing and (rather than through holes) orient- ing entrainment out of the tower or up to overhead mist
ed at 90” to the top structured layer [131]. eliminators.
With a trough distributor it is often more difficult to
obtain uniform distribution next to the tower wall than Number of Flow wfip Points Required [131]
with a PAN or orifice pipe distributor. Bonilla [131] rec-
ommends the rule of thumb: Any 10% of the outer sur- 1. For low purity hydrocarbon fraction, the number of
face area of packing in the tower should not receive more drip or separate flow points should be 6-10 drip
or less liquid than the average 10% of the surface area. points/ft2 of tower cross-section surface area.