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

When Z is large or Γ/ρB F is so small that liquid penetration is complete,
and
k� = 11.800 D�/BF
H � = 0.95 ΓB F/D �
OTHER TOPICS FOR DISTILLATION AND GAS ABSORPTION EQUIPMENT 14-85
(14-174)
(14-175)
A comparison of experimental data for carbon dioxide absorption
obtained by Hatta and Katori (op. cit.), Grimley [Trans. Inst. Chem.
Eng., 23, 228 (1945)], and Vyazov [Zh. Tekh. Fiz. (U.S.S.R.), 10,
1519 (1940)] and for absorption of oxygen and hydrogen by Hodgson
(S.M. thesis, Massachusetts Institute of Technology, 1949), Henley
(B.S. thesis, University of Delaware, 1949), Miller (B.S. thesis, University
of Delaware, 1949), and Richards (B.S. thesis, University of
Delaware, 1950) was made by Sherwood and Pigford (Absorption
and Extraction, McGraw-Hill, New York, 1952) and is indicated in
Fig. 14-79.
In general, the observed mass-transfer rates are greater than those
predicted by theory and may be related to the development of surface
rippling, a phenomenon which increases in intensity with increasing
liquid path.
Vivian and Peaceman [Am. Inst. Chem. Eng. J., 2, 437 (1956)]
investigated the characteristics of the CO 2-H 2O and Cl 2-HCl, H 2O
system in a wetted-wall column and found that gas rate had no effect
on the liquid-phase coefficient at Reynolds numbers below 2200.
Beyond this rate, the effect of the resulting rippling was to increase
significantly the liquid-phase transfer rate. The authors proposed a
behavior relationship based on a dimensional analysis but suggested
caution in its application concomitant with the use of this type of relationship.
Cognizance was taken by the authors of the effects of column
length, one to induce rippling and increase of rate of transfer,
one to increase time of exposure which via the penetration theory
decreases the average rate of mass transfer in the liquid phase. The
equation is
k�h
� D�
= 0.433� � 1/2
µ�
�
ρ�D� � � 1/6
� � 0.4
2 3 ρ� gh 4Γ
�
µ� 2
where D � = diffusion coefficient of solute in liquid, ft 2 /h
g = gravity-acceleration constant, 4.17 � 10 8 ft/h 2
h = length of wetted wall, ft
k � = mass-transfer coefficient, liquid phase, ft/h
� µ�
(14-176)
TABLE 14-17 Relative Fabricated Cost for Metals
Used in Tray-Tower Construction*
Relative cost
per ft2 of tray
area (based on
Materials of construction carbon steel = 1)
Sheet-metal trays
Steel 1
4–6% chrome—a moly alloy steel 2.1
11–13% chrome type 410 alloy steel 2.6
Red brass 3
Stainless steel type 304 4.2
Stainless steel type 347 5.1
Monel 7.0
Stainless steel type 316 5.5
Inconel 8.2
Cast-iron trays 2.8
*Peters and Timmerhaus, Plant Design and Economics for Chemical Engineers,
4th ed., McGraw-Hill, New York, 1991. To convert cost per square foot to
cost per square meter, multiply by 10.76.
Γ=mass rate of flow of liquid, lb/(h)(ft of periphery)
µ � = viscosity of liquid, lb/(ft)(h)
ρ� = density of liquid, lb/ft 3
The equation is dimensionless.
The effect of chemical reaction in reducing the effect of variation
of the liquid rate on the rate of absorption in the laminar-flow
regime was illustrated by the evaluation of the rate of absorption of
chlorine in ferrous chloride solutions in a wetted-wall column by
Gilliland, Baddour, and White [Am. Inst. Chem. Eng. J., 4, 323
(1958)].
Flooding in Wetted-Wall Columns When gas and liquid are in
counterflow in wetted-wall columns, flooding can occur at high gas
rates. Methods for calculating this flood are given in “Upper Limit
Flooding in Vertical Tubes.” In the author’s experience, Eq. (14-204)
has had an excellent track record for calculating flooding in these
columns.
COLUMN COSTS
Estimation of column costs for preliminary process evaluations
requires consideration not only of the basic type of internals but also of
their effect on overall system cost. For a distillation system, for example,
the overall system can include the vessel (column), attendant
structures, supports, and foundations; auxiliaries such as reboiler, condenser,
feed heater, and control instruments; and connecting piping.
The choice of internals influences all these costs, but other factors
influence them as well. A complete optimization of the system requires
a full-process simulation model that can cover all pertinent variables
influencing economics.
Cost of Internals Installed costs of trays may be estimated from
Fig. 14-80, with corrections for tray material taken from Table 14-17.
For two-pass trays the cost is 15 to 20 percent higher. Approximate
costs of random packing materials may be obtained from Table 14-18,
but it should be recognized that, because of competition, there can be
significant variations in these costs from vendor to vendor. Also, packings
sold in very large quantities carry discounts. In 1995, costs of
structured packings, made from sheet metal, averaged $90 to $110 per
cubic foot, but the need for special distributors and redistributors can
double the cost of structured-packings on a volumetric basis. Note
TABLE 14-18
January 1990
Costs of Random Packings, Uninstalled,
Prices in dollars per ft3 , 100 ft3 orders, f.o.b. manufacturing plant
Size, in, $/ft3 1 1a2 3
Raschig rings
Chemical porcelain 12.8 10.3 9.4 7.8
Carbon steel 36.5 23.9 20.5 16.8
Stainless steel 155 117 87.8 —
Carbon
Intalox saddles
52 46.2 33.9 31.0
Chemical stoneware 17.6 13.0 11.8 10.7
Chemical porcelain 18.8 14.1 12.9 11.8
Polypropylene
Berl saddles
21.2 — 13.1 7.0
Chemical stoneware 27.0 21.0 — —
Chemical porcelain
Pall rings
33.5 21.5 15.6 —
Carbon steel 29.3 19.9 18.2 —
Stainless steel 131 99.0 86.2 —
Polypropylene 21.2 14.4 13.1
Peters and Timmerhaus, Plant Design and Economics for Chemical Engineers,
4th ed., McGraw-Hill, New York, 1991. To convert cubic feet to cubic
meters, multiply by 0.0283; to convert inches to millimeters, multiply by 25.4;
and to convert dollars per cubic foot to dollars per cubic meter, multiply by 35.3.