Page 265 - Applied Process Design For Chemical And Petrochemical Plants Volume II

254 Applied Process Design for Chemical and Petrochemical Plants

Table 9-16 square centers for small towers less than 36 in. in diame-
Packing Service Application ter, and should number (D/6)* streams for 36in. and larg-
.
.
. ,. . - . . . . . .. .. er, where D is the tower inside diameter in inches [22].
General Service When the liquid stream spacings exceed 6-in. square
Packing Material Application Remarks pitch, consideration should lean to this figure. Most man-
... .
Glazed and un- Neutral and acid con- Unglazed usual ufacturers make some type of distributor giving one
glazed, Porcelain ditions except hydro- type specified ex- stream every 6 in.* of tower area.
or Chemical fluoric, solvents. Not cept special re- The number of irrigation or “drip-points” or entrance
Stoneware good in hot caustic quirement of low points per square foot of flat surface of the tower should
(above 70°F) adsorption on sur- be uniform for orifice, weir-type gravity, or pressure dis-
face. Special ceram-
ics available for tributors, and need not exceed 10 points/ft2 [82]. This
mild caustic. Por- uniformity must not be disturbed by support rings for sup-
celain stronger and porting the distributor itself. The distribution must
more resistant than include the area adjacent to the wall, and the design must
stoneware. not “force” more liquid at the wall where it contacts the
Carbon Hot alkali, all acids ex- Stand thermal packing. Uniformity of points of distribution to the pack-
cept nitric, no oxidiz- shock, low cubic ing surface is extremely important. The volume flow per
ing atmospheres. weight point must be carefully calculated.
Plastic Alkali, salts, aqueous Light weight Bonilla [131] presents an excellent examination of liq-
and acids depending
on resin uid distributors in packed towers. Packed towers with ran-
Steel and other Hot alkali for steel, May be heavier dom packing or structured packing are more sensitive to
light gauge other service to suit than ceramic, poor or non-uniform distribution of liquid than tray tow-
metals metals more expensive ers. This requires that liquid and vapor enter the packing
evenly distributed. Often, only the liquid distributor at the
top of the packing is considered, but vapor distribution at
the bottom or intermediate in the tower is quite important.
(text continued porn page 246)
The ultimate performance of a packing depends signifi-
Poor distribution reduces the effective wetted packing cantly on the initial distribution [131], with non-uniform
area and promotes liquid channeling. distribution resulting in reduced packing efficiency, which
The final selection of the mechanism of distributing the can be expressed as a higher HETP, height equivalent to a
liquid across the packing depends upon the size of the theoretical plate/stage/tray. A higher surface area struc-
tower, type of packing (exposed surface, configuration), tured packing is more sensitive to the initial liquid non-uni-
tendency of packing to divert liquid to tower walls, and form distribution than a lower surface area packing.
materials of construction for distribution. Figures 9-8A- It is important to recognize that each packing has a nat-
9-8L illustrates a few distribution types. Spray nozzles are ural liquid distribution [ 1311 that will develop if sufficient
used, but care must be taken in evaluating the percent of bed depth is available. If the distribution is poorer than
the total liquid that hits the walls and never enters the the natural distribution, the system will end up with con-
packing. Full cone nozzles with spray angles which will centration gradients and higher HETP values. When/if an
keep most of the liquid on the center portion of the pack- improvement from a poor to natural distribution occurs
ing for initial contact will perform quite well. slowly over many feet of packing, Bonilla [131] states that
Spray nozzle manufacturers have spray angle data for adding extra packing to a bed to compensate for the ini-
various pressures at the nozzle inlet (pipe), see Figure tial maldistribution does not work because the return to
9-8L. This should be considered carefully in the distribu- the natural distribution is not fast enough to compensate
tor design, and the volume discharge per square foot of for the concentration gradients that have already formed.
flat tower cross-section must be as uniform as possible. Most liquid distributors feed onto the packing by gravi-
Careful layouts of nozzle arrangements are usually ty, rather than being pressurized. For any given tower
required, Figure 9-8K. Maximum rate variation of 2 5 to design the distributor design and installation is an impor-
6% of average flow is necessary in design [82]. tant component for assisting in aiding the packing to do
There are many other types and variations in addition its job. For best performance for the average distillation
to those listed, although they are usually special-purpose the distributor should be installed level to a tolerance of
trays and not necessarily generally adaptable. *?4 to H6 in. and should be able to be leveled once it is in
Good design generally considers that the streams of liq- the tower. Table 9-18 [131] presents a comparison of
uid should enter onto the top of the packing on 3 to 6-in. many of the factors necessary to the selection and design