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

� � = 0.62� � 0.5
� � 0.33
� � 0.29 gρL 2 dB
�
µL 2
gρLd B 2
kLa dB
µ L
� DL
U G
� (ρLDL)
ρG �
ρL
� σ
× � � 0.68
� � 0.04
�
(gdB) 0.5
(14-221)
where k La = overall mass-transfer coefficient, d B = bubble diameter =
0.003 m, D L = diffusivity of gas in liquid, ρ =density, µ =viscosity, σ =
interfacial tension, g = gravitational acceleration.
As mentioned earlier, surfactants and ionic solutions significantly
affect mass transfer. Normally, surface affects act to retard coalescence
and thus increase the mass transfer. For example, Hikata et al. [Chem.
Eng. J., 22, 61–69 (1981)] have studied the effect of KCl on mass transfer
in water. As KCI concentration increased, the mass transfer
increased up to about 35 percent at an ionic strength of 6 gm/l. Other
investigators have found similar increases for liquid mixtures.
Axial Dispersion Backmixing in bubble columns has been
extensively studied. Wiemann and Mewes [Ind. Eng. Chem. Res., 44,
4959 (2005)] and Wild et al. [Int. J. Chemical Reactor Eng., 1, R7
(2003)] give a long list of references pertaining to backmixing in
bubble columns. An excellent review article by Shah et al. [AIChE J.,
24, 369 (1978)] has summarized the literature prior to 1978. Works by
Konig et al. [Ger. Chem. Eng., 1, 199 (1978)], Lucke et al. [Trans. Inst.
Chem. Eng., 58, 228 (1980)], Riquarts [Ger. Chem. Eng., 4, 18
(1981)], Mersmann (op. cit.), Deckwer (op. cit.), Yang et al. [Chem.
Eng. Sci., 47(9–11), 2859 (1992)], and Garcia-Calvo and Leton
[Chem. Eng. Sci., 49(21), 3643 (1994)] are particularly useful references.
Axial dispersion occurs in both the liquid and the gas phases. The
degree of axial dispersion is affected by vessel diameter, vessel internals,
gas superficial velocity, and surface-active agents that retard coalescence.
For systems with coalescence-retarding surfactants the
initial bubble size produced by the gas sparger is also significant.
The gas and liquid physical properties have only a slight effect on the
degree of axial dispersion, except that liquid viscosity becomes important
as the flow regime becomes laminar. With pure liquids, in the
absence of coalescence-inhibiting, surface-active agents, the nature of
the sparger has little effect on the axial dispersion, and experimental
results are reasonably well correlated by the dispersion model. For the
liquid phase the correlation recommended by Deckwer et al. (op.
cit.), after the original work by Baird and Rice [Chem. Eng. J., 9,
171(1975)] is as follows:
= 0.35� � 1/3 gD
�
UG 2
EL
�
(DUG)
(14-222)
Gases and liquids may be intentionally contacted as in absorption and
distillation, or a mixture of phases may occur unintentionally as in
vapor condensation from inadvertent cooling or liquid entrainment
from a film. Regardless of the origin, it is usually desirable or necessary
ultimately to separate gas-liquid dispersions. While separation
will usually occur naturally, the rate is often economically intolerable
and separation processes are employed to accelerate the step.
GAS-PHASE CONTINUOUS SYSTEMS
Practical separation techniques for liquid particles in gases are discussed.
Since gas-borne particulates include both liquid and solid particles,
many devices used for dry-dust collection (discussed in Sec. 17
under “Gas-Solids Separation”) can be adapted to liquid-particle separation.
Also, the basic subject of particle mechanics is covered in Sec.
6. Separation of liquid particulates is frequently desirable in chemical
processes such as in countercurrent-stage contacting because liquid
entrainment with the gas partially reduces true countercurrency. Sep-
PHASE SEPARATION
PHASE SEPARATION 14-111
where EL = liquid-phase axial dispersion coefficient, UG = superficial
velocity of the gas phase, D = vessel diameter, and g = gravitational
acceleration.
The recommended correlation for the gas-phase axial-dispersion
coefficient is given by Field and Davidson (loc. cit.):
EG = 56.4 D1.33� � 3.56
UG
(14-223)
where EG = gas-phase axial-dispersion coefficient, m2 �
ε
/s; D = vessel
diameter, m; UG = superficial gas velocity, m/s; and ε =fractional gas
holdup, volume fraction.
The correlations given in the preceding paragraphs are applicable
to vertical cylindrical vessels with pure liquids without coalescence
inhibitors. For other vessel geometries such as columns of rectangular
cross section, packed columns, and coiled tubes, the work of Shah
et al. (loc. cit.) should be consulted. For systems containing coalescence-inhibiting
surfactants, axial dispersion can be vastly different
from that in systems in which coalescence is negligible. Konig et al.
(loc. cit.) have well demonstrated the effects of surfactants and
sparger type by conducting tests with weak alcohol solutions using
three different porous spargers. With pure water, the sparger—and,
consequently, initial bubble size—had little effect on back mixing
because coalescence produced a dynamic-equilibrium bubble size not
far above the sparger. With surfactants, the average bubble size was
smaller than the dynamic-equilibrium bubble size. Small bubbles produced
minimal back mixing up to ε ≈ 0.40; however, above ε ≈ 0.40
backmixing increased very rapidly as UG increased The rapid increase
in back mixing as ε exceeds 0.40 was postulated to occur indirectly
because a bubble carries upward with it a volume of liquid equal to
about 70 percent of the bubble volume, and, for ε≈0.40, the bubbles
carry so much liquid upward that steady, uniform bubble rise can no
longer be maintained and an oscillating, slugging flow develops, which
produces fluctuating pressure at the gas distributor and the formation
of large eddies. The large eddies greatly increase backmixing. For the
air alcohol-water system, the minimum bubble size to prevent
unsteady conditions was about 1, 1.5, and 2 mm for UG = 1, 3, and 5
cm/s, respectively. Any smaller bubble size produced increased backmixing.
The results of Konig et al. (loc. cit.) clearly indicate that the
interaction of surfactants and sparger can be very complex; thus, one
should proceed very cautiously in designing systems for which surfactants
significantly retard coalescence. Caution is particularly important
because surfactants can produce either much more or much less
backmixing than surfactant-free systems, depending on the bubble
size, which, in turn, depends on the sparger utilized.
aration before entering another process step may be needed to prevent
corrosion, to prevent yield loss, or to prevent equipment damage
or malfunction. Separation before the atmospheric release of gases
may be necessary to prevent environmental problems and for regulatory
compliance.
GENERAL REFERENCES
G-1. Buonicore and Davis, eds., Air Pollution Engineering Manual, Van Nostrand
Reinhold, New York, 1992.
G-2. Calvert and Englund, eds., Handbook of Air Pollution Technology, Wiley,
New York, 1984.
G-3. Cheremisinoff, ed., Encyclopedia of Environmental Control Technology,
vol. 2, Gulf Pub., Houston, 1989.
G-4. McKetta, Unit Operations Handbook, vol. 1–2, Dekker, New York, 1992.
G-5. Wark and Warner, Air Pollution: Its Origin and Control, 2d ed., Harper &
Row, New York, 1981.
G-6. Hesketh, Air Pollution Control, 1979; Fine Particles in Gaseous Media,
Ann Arbor Science Pubs., Ann Arbor, MI, 1977.
G-7. Stern, Air Pollution, 3d ed., vols. 3–5, Academic, Orlando, FL, 1976–77.