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

Using the GPDC method, the capacity parameter [by Eq. (14-140)] =
U t[ρG/(ρL −ρG)] 0.5 Fp 0.5 ν 0.05 , which is roughly equivalent to
Fs
� ρL 0.5
F p 0.5ν0.05 = 270.5 1.53
� (1.0)
0.5 62.4
= 1.01
Referring to Fig. 14-55, the intersection of the capacity parameter and the flow
parameter lines gives a pressure drop of 0.38 inches H2O/ft packing.
Using the Robbins method, Gf = 986Fs(Fpd/20) 0.5 = 986(1.53)(24/20) 0.5 = 1653.
Lf = L (62.4/ρL)(Fpd/20) 0.5 µ 0.1 = 9000 (1.0)(1.095)(1.0) = 9859. Lf /Gf = 5.96.
From Fig. 14-58, pressure drop = 0.40 in. H2O/ft packing.
PACKING EFFICIENCY
HETP vs. Fundamental Mass Transfer The two-film model
gives the following transfer unit relationship:
H OG = HG +λHL
where HOG = height of an overall transfer unit, gas concentration
basis, m
HG = height of a gas-phase transfer unit, m
HL = height of a liquid-phase transfer unit, m
λ=m/(LM/GM) = slope of equilibrium line/slope of
operating line
EQUIPMENT FOR DISTILLATION AND GAS ABSORPTION: PACKED COLUMNS 14-63
(14-152)
In design practice, a less rigorous parameter, HETP, is used to
express packing efficiency. The HETP is the height of packed bed
required to achieve a theoretical stage. The terms HOG and HETP may
be related under certain conditions:
ln λ
�
(λ−1)
HETP = HOG� � (14-153)
and since Z p = (H OG)(N OG) = (HETP)(N t) (14-154)
N OG = Nt[ln λ/(λ−1)] (14-155)
Equations (14-153) and (14-155) have been developed for binary mixture
separations and hold for cases where the operating line and equilibrium
line are straight. Thus, when there is curvature, the equations
should be used for sections of the column where linearity can be
assumed. When the equilibrium line and operating line have the same
slope, HETP = H OG and N OG = N t (theoretical stages).
An alternative parameter popular in Europe is the NTSM (number
of theoretical stages per meter) which is simply the reciprocal of the
HETP.
Factors Affecting HETP: An Overview Generally, packing
efficiency increases (HETP decreases) when the following occur.
• Packing surface area per unit volume increases. Efficiency increases
as the particle size decreases (random packing, Fig. 14-59) or as the
channel size narrows (structured packing, Fig. 14-60).
• The packing surface is better distributed around a random packing
element.
• Y structured packings (45° inclination) give better efficiencies than
X structured packings (60° inclination to the horizontal) of the same
surface areas (Fig. 14-60).
• For constant L/V operation in the preloading regime, generally
liquid and vapor loads have little effect on random and most corrugated
sheet structured packings HETP (Figs. 14-59 and 14-60).
HETP increases with loadings in some wire-mesh structured
packing.
• Liquid and vapor are well distributed. Both liquid and vapor maldistribution
have a major detrimental effect on packing efficiency.
• Other. These include L/V ratio (lambda), pressure, and physical
properties. These come into play in some systems and situations, as
discussed below.
HETP Prediction HETP can be predicted from mass-transfer
models, rules of thumb, and data interpolation.
Mass-Transfer Models Development of a reliable mass-transfer
model for packing HETP prediction has been inhibited by a lack of
understanding of the complex two-phase flow that prevails in packings,
by the shortage of commercial-scale efficiency data for the newer
packings, and by difficulty in quantifying the surface generation in
modern packings. Bennett and Ludwig (Chem. Eng. Prog., p. 72,
April 1994) point out that the abundant air-water data cannot be reliably
used for assessing real system mass-transfer resistance due to
variations in turbulence, transport properties, and interfacial areas.
More important, the success and reliability of rules of thumb for predicting
packing efficiency made it difficult for mass-transfer models to
compete.
For random packings, the Bravo and Fair correlation [Ind. Eng.
Chem. Proc. Des. Dev. 21, 162 (1982)] has been one of the most
popular theoretical correlations. It was shown (e.g., McDougall,
Chem SA, p. 255, October 1985) to be better than other theoretical
correlations, yet produced large discrepancies when compared to
test data [Shariat and Kunesh, Ind. Eng. Chem. Res. 34(4), 1273
(1995)]. For structured packings, the Bravo, Fair, and Rocha correlation
[Chem. Eng. Progr. 86(1), 19 (1990); Ind. Eng. Chem. Res.
35, 1660 (1996)] is one of the most popular theoretical correlations.
This correlation is based on the two-film theory. Interfacial areas
are calculated from the packing geometry and an empirical wetting
parameter.
Alternate popular theoretical correlations for random packings,
structured packings, or both (e.g., Billet and Schultes, “Beitrage zur
Verfahrens-und Umwelttechnik,” p. 88, Ruhr Universitat, Bochum,
Germany, 1991) are also available.
Rules of Thumb Since in most circumstances packing HETP is
sensitive to only few variables, and due to the unreliability of even the
best mass-transfer model, it has been the author’s experience that
rules of thumb for HETP are more accurate and more reliable than
mass-transfer models. A similar conclusion was reached by Porter
and Jenkins (IChemE Symp. Ser. 56, Summary paper, London,
1979).
The majority of published random packing rules of thumb closely
agree with one another. They are based on second- and third-generation
random packings and should not be applied to the obsolete
first-generation packings. Porter and Jenkins’s (loc. cit.), Frank’s
(Chem. Eng., p. 40, March 14, 1977), Harrison and France’s (Chem.
Eng., p. 121, April 1989), Chen’s (Chem. Eng., p. 40, March 5, 1984),
and Walas’ (Chem. Eng., p. 75, March 16, 1987) general rules of
thumb are practically the same, have been successfully tested against
an extensive data bank, and are slightly conservative, and therefore
suitable for design.
For small-diameter columns, the rules of thumb presented by
Frank (loc. cit.), Ludwig (Applied Process Design for Chemical and
Petrochemical Plants, vol. 2, 2d ed., Gulf Publishing, Houston, Tex.,
1979), and Vital et al. [Hydrocarbon Processing, 63(12), 75 (1984)]
are identical. The author believes that for small columns, the more
conservative value predicted from either the Porter and Jenkins or
the Frank-Ludwig-Vital rule should be selected for design. Summarizing:
HETP = 18D P
(14-156a)
HETP > DT for DT < 0.67 m (14-156b)
where DP and D T are the packing and tower diameters, m, respectively,
and the HETP is in meters. In high-vacuum columns (