Page 353 - Applied Process Design For Chemical And Petrochemical Plants Volume II

342 Applied Process Design for Chemical and Petrochemical Plants

(text continuedhtn page 339) system in question by the packing manufacturer, referenc-
ing to a specific packing size and style. Several competitive
vapor capacity factor that in the vacuum systems is less designs are often helpful for a final design selection.
than 60%-70% of the corresponding flood point (the
actual decrease depending on the mixture). The aver- The test studies of several different structured packings
age optimum load factor is given [123]: of Dean et al. [ 1171 indicated that structured packing per-
forms well in water removed by triethylene glycol (TEG)
(9 - 78) from natural gas from field wells, giving a design point of
F, = 3 (where F, = V, I&, V, = superficial velocity, pg = gas
2. Specific efficiency of packing: density) for sizing. A high efficiency drip-point distributor

n,/H = 5, theoretical trays/meter is recommended, because it was far superior to a notched
trough or spray nozzle distributor. Excellent turndown to
3. Average pressure drop per theoretical tray: 12:l was shown, still meeting outlet gas low water vapor
(Ap/nJOpt = 5 mm water column specifications. HETP values ranged from 44 in. to 102 in.,
in this application. The number of theoretical trays was 2.8
where H = height of packing, m to 4.6, varying with liquid and gas rates and packing
n, = number of theoretical trays
Ap = pressure drop, mm water column style/size. Flooding occurred at 2.28 to 3.98 F, value at 0.7
u,, = vapor velocity per unit of free column cross- gpm/ft2 at 650 psig.
section, m/sec A commercial 4ft diameter refinery depropanizer
pv = density of vapor, kg/m3 unit’s performance after replacement of the lower half of
a trayed tower with Intalox-2T (R) structured packing is
Guidelines for Structured Packings
described in Reference 118.
1. Pressure drop: Range usually 0.3-0.5 in. water/ft, or, Nutter [141] Montz B-1 structured packing, licensed
30-40 mm water/m of packed height [ 1161. from Julius Montz GmbH of Hilden, Germany, Figures
2. Lower holdup of liquid: Less than other packings, 9-6LL and 9-6MM, is of uniform sinusoidal corrugations,
may require special attention to operational controls. which avoids sharp corners. The embossing on both sides
3. More difficult to purge: Operating vapors more diffi- of the sheet metal is closely spaced projections in a dot-
cult to purge from the system than random packings matrix pattern, which spreads the liquid in a uniform film
and trays. This poses potential fire, explosion, and [141], without holes or slots. The design allows for an
toxic hazard. effective liquid-vapor seal at the tower wall by means of a
4. Lower pressure drop for vacuum systems: Allows for metal wiper band. A representative performance is shown
better low pressure and vacuum system operation and for pressure drop, Figure 9-66, and HETP, Figure 9-67.
lower bottoms temperature, with less degradation of This packing is effectively used in low pressure (or vacu-
bottoms product. um) columns where large theoretical stages and low pres
5. Internal uniform distribution: Properly designed sure drop are required, producing the lowest possible
devices to distribute and redistribute liquid entering pressure drop per stage. Style El is embossed sheet metal;
the column is critical to obtain best performance of Style BSH, is expanded textured sheet metal, high effi-
these types of packings. ciency with maximum surface area; and Style A-3 is wire
6. Material of construction: The materials of fabrication gauze construction.
For this type of packing are more critical to long life
due to pad size, wall thickness of metal or plastic com-
ponents, and actual selection based on the system Structured Packing Scale-up
corrosion, hydrogen attack, and oxygen attack in the
column environment. Applications of structured packing into ethylene plant’s
7. HETP values: Small packing with narrow corruga- various column systems [ 1191 have been successfully
tions gives low HETP values, but usually higher pres- achieved, but the individual manufacturers must be con-
sure drops. Plastic and some metal packing sheets, sulted to use their most directly applicable pilot and com-
wires or corrugations may require special surface mercial data, which are generally not published. The use
treatment to ensure good wettability. of published general correlations should only be used for
8. Preliminary designs: These can usually be made from a “first” or approximation design, while the delicate or
the generalized literature references; however, they important final design must be performed in cooperation
are not a substitute for direct design of the specific with the manufacturer.