Packed Bed flooding.pdf - Youngstown State University's Personal ...

operation, for the resistance of a septum to the flow of gas increases
when it is wet. The air permeabilities for water-submerged porous
carbon of some of the grades listed in the table are shown in Fig. 14-96.
The data were determined with septa 0.625 in thick in water at 70°F.
Comparable wet-permeability data for 1-in Alundum plates of two
grades of fineness are given in Table 14-24.
The gas rate at which coalescence begins to reduce the effectiveness
of dispersion appears to depend not only on the pore size and
pore structure of the dispersing medium but also on the liquid properties,
liquid depth, agitation, and other features of the sparging environment;
coalescence is strongly dependent on the concentration of
surfactants capable of forming an electrical double layer and thus
produce ionic bubbles, long-chain alcohols in water being excellent
examples. For porous-carbon media, the manufacturer suggests that
the best dispersion performance will result if the broken-line regions
of Fig. 14-96 are avoided. For porous stainless-steel spargers, which
extend to a lower pore size than carbon, Micro Metallic Division, Pall
Corp., recommends (Release 120A, 1959) a working limit of 8 ft/min
(0.044 m/s) to avoid serious coalescence. This agrees with the data
reported by Konig et al. (loc. cit.), in which 0.08 m/s was used and
bubbles as small as 1 mm were produced from a 5-µm porous
sparger.
Slabs of porous material are installed by grouting or welding
together to form a diaphragm, usually horizontal. Tubes are prone to
produce coalesced gas at rates high enough to cause bubbling from
their lower faces, but they have the advantage of being demountable
for cleaning or replacement (U.S. Patent 2,328,655). Roe [Sewage
(a) (b)
PHASE DISPERSION 14-105
FIG. 14-95 Comparison of bubbles from a porous septum and from a perforated-pipe sparger. Air in water at 70°F. (a) Grade
25 porous-carbon diffuser operating under a pressure differential of 13.7 in of water. (b) Karbate pipe perforated with 1/16-in
holes on 1-in centers. To convert inches to centimeters, multiply by 2.54; °C = 5 ⁄9 (°F − 32). (National Carbon Co.)
Works J., 18, 878 (1945)] claimed that silicon carbide tubes are superior
to horizontal plates, principally because of the wiping action of
the liquid circulating past the tube. He reported respective maximum
capacities of 2.5 and 1.5 cm 2 /s of gas/cm 2 of sparger for a horizontal
tube and a horizontal plate of the same material (unspecified grade).
Mounting a flat-plate porous sparger vertically instead of horizontally
seriously reduces the effectiveness of the sparger for three reasons: (1)
The gas is distributed over a reduced cross section; (2) at normal rates,
the lower portion of the sparger may not operate because of difference
in hydrostatic head; and (3) there is a marked tendency for bubbles
to coalesce along the sparger surface. Bone (M.S. thesis in
chemical engineering, University of Kansas, 1948) found that the oxygen
sulfite solution coefficient for a 3.2- by 10-cm rectangular porous
carbon sparger was 26 to 41 percent lower for vertical than for horizontal
operation of the sparger, the greatest reduction occurring when
the long dimension was vertical.
Precipitation and Generation Methods For a thorough understanding
of the phenomena involved, bubble nucleation should be considered.
A discussion of nucleation phenomena is beyond the scope of
this handbook; however, a starting point with recent references is Deng,
Lee, and Cheng, J. Micromech. Microeng., 15, 564 (2005), and Jones,
Evans, and Galvin, Adv. Colloid and Interface Sci. 80, 27 (1999).
Gas Dispersion—Vessel Headspace Boerma and Lankester
have measured the surface aeration of a nine-bladed disk-type turbine
(NOTE: A well-designed pitched-blade turbine will give equal or better
performance). In a fully baffled vessel, the optimum depth to obtain
maximum gas dispersion was 15 to 50 percent of the impeller diameter.