Page 233 - Separation process principles 2

198 Chapter 6 Absorption and Stripping of Dilute Mixtures

maximum to minimum vapor capacity). At the limiting gas to flow mainly up the center of the column, thus greatly
vapor capacity, jlooding of the column occurs because of reducing the extent of vapor-liquid contact. In that case, a i
excessive entrainment of liquid droplets in the vapor causing liquid red~stributor should be installed. i
the liquid flow rate to exceed the capacity of the downcomer Commercial packing materials include random (dumped) ' '
and, thus, go back up the column. At low vapor rates, weep- packings, some of which are shown in Figure 6.7a, and
ing of liquid through the tray openings or vapor pulsation structured (also called arranged, ordered, or stacked pack-
becomes excessive. Because of their low relative cost, sieve ing~), some of which are shown in Figure 6.7b. Among the
trays are preferred unless flexibility is required, in which random packings, which are poured into the column, are the
case valve trays are best. Bubble-cap trays, which exist in old (1895-1950) ceramic Raschig rings and Berl saddles,
many pre-1950 installations, are rarely specified for new in- which are seldom specified for new installations. They have
stallations, but may be preferred when the amount of liquid been largely replaced by metal and plastic Pall rings, metal
holdup on a tray must be controlled to provide adequate res- Bialecki rings, and ceramic Intalox saddles, which provide
idence time for a chemical reaction or when weeping must more surface area for mass transfer, a higher flow capacity,
be prevented. and a lower pressure drop. More recently, through-flow
Apacked column, shown in detail in Figure 6.6, is a ver- paclungs of a lattice-work design have been developed.
tical, cylindrical pressure vessel containing one or more sec- These packings, which include metal Intalox IMTP; metal,
lions of a paclung material over whose surface the liquid plastic, and ceramic Cascade Mini-Rings; metal Levapak;
flows downward by gravity, as a film or as droplets between metal, plastic, and ceramic Hiflow rings; metal tri-packs;
packing elements. Vapor flows upward through the wetted and plastic Nor Pac rings, exhibit even lower pressure drop
packing, contacting the liquid. The sections of packing are per unit height of paclung and even higher mass-transfer
contained between a lower gas-injection support plate, rates per unit volume of packing. Accordingly, they are
which holds the paclung, and an upper grid or mesh hold- called "high-efficiency" random packings. Most random
down plate, which prevents packing movement. A liquid dis- paclungs are available in nominal diameters, ranging from
tributor, placed above the hold-down plate, ensures uniform 1 in. to 3.5 in. As packing size increases, mass-transfer
distribution of liquid over the cross-sectional area of the col- efficiency and pressure drop may decrease. Therefore, for a
umn as it enters the packed section. If the depth of packing is given column diameter an optimal packing size exists that
more than about 20 ft, liquid channeling may occur, causing represents a compromise between these two factors, since
the liquid to flow down the column mainly near the wall, and low pressure drop and high mass-transfer rates are both
desirable. However, to minimize channeling of liquid, the
nominal diameter of the paclung should be less than one-
Gas out eighth of the column diameter. Most recently, a "fourth
A generation" of random packings, including VSP rings,
Fleximax, and Raschig super-rings, has been developed,
which features a very open undulating geometry that pro-
motes even wetting, but with recurrent turbulence promo-
tion. The result is lower pressure drop, but sustained mass-
transfer efficiency that may not decrease noticeably with
increasing column diameter and may permit a larger depth
of packing before a liquid redistributor is necessary. Metal
paclungs are usually preferred because of their superior
strength and good wettability. Ceramic packings, which
have superior wettability but inferior strength, are used only
to reslst corrosion at elevated temperatures, where plastics
would fail. Plastic packings, usually of polypropylene, are
inexpensive and have sufficient strength, but may experi-
ence poor wettability, particularly at low liquid rates.
Representative structured packings include the older cor-
rugated sheets of metal gauze, such as Sulzer BX, Montz A,
Gempak 4BG, and Intalox High-Performance Wire Gauze
Packing. Newer and less-expensive structured packings,
which are fabricated from sheet metal and plastics and may
or may not be perforated, embossed, or surface roughened,
include metal and plastic Mellapak 250Y, metal Flexipac,
Liquid out metal and plastic Gempak 4A, metal Montz B1, and metal
Figure 6.6 Details of intemals used in a packed column. Intalox High-Performance Structured Paclung. Structured