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

Summary In the preloading regime, packing size, type, and distribution
affect HETP. With aqueous-organic systems, HETP may be
sensitive to underwetting and composition. A lambda value
(λ=mG M/L M) outside the range of 0.5 to 2.0 causes HETP to rise, and
so does a high hydrogen concentration. HETP of structured packings
may also be affected by pressure (at high pressure), and vapor and liquid
loads.
MALDISTRIBUTION AND ITS EFFECTS
ON PACKING EFFICIENCY
Modeling and Prediction Maldistribution may drastically
reduce packing efficiency. HETP may increase by a factor as high as 2
or 3 due to maldistribution. Shariat and Kunesh [Ind. Eng. Chem.
Res., 34(4), 1273 (1995)] provide a good demonstration.
Early models [Mullins, Ind. Chem. Mfr., 33, 408 (1957); Manning
and Cannon, Ind. Eng. Chem. 49(3), 347 (1957)] expressed the effect
of liquid maldistribution on packing efficiency in terms of a simple
channeling model. A portion of the liquid bypasses the bed, undergoing
negligible mass transfer, and then rejoins and contaminates the
rest of the liquid. Huber et al. [Chem. Ing. Tech. 39, 797 (1967);
Chem. Eng. Sci. 21, 819 (1966)] and Zuiderweg et al. [IChemE Symp.
Ser. 104, A217 (1987)] replaced the simple bypassing by variations in
the local L/V ratios. The overirrigated parts have a high L/V ratio, the
underirrigated parts a low L/V ratio. Regions with low L/V ratios experience
pinching, and, therefore, produce poor separation.
Huber et al. (loc. cit.) and Yuan and Spiegel [Chem. Ing. Tech. 54,
774 (1982)] added lateral mixing to the model. Lateral deflection of
liquid by the packing particles tends to homogenize the liquid, thus
counteracting the channeling and pinching effect.
A third factor is the nonuniformity of the flow profile through the
packing. This nonuniformity was observed as far back as 1935 [Baker,
Chilton, and Vernon, Trans. Instn. Chem. Engrs. 31, 296 (1935)] and
was first modeled by Cihla and Schmidt [Coll. Czech. Chem. Commun.,
22, 896 (1957)]. Hoek (Ph.D. Thesis, The University of Delft,
The Netherlands, 1983) combined all three factors into a single
model, leading to the zone-stage model below.
The Zone-Stage Model Zuiderweg et al. [IChemE Symp. Ser.
104, A217, A233 (1987)] extended Hoek’s work combining the effects
of local L/V ratio, lateral mixing, and flow profile into a model describing
the effect of liquid maldistribution on packing efficiency. This
work was performed at Fractionation Research Inc. (FRI) and at The
University of Delft in The Netherlands. The model postulates that, in
the absence of maldistribution, there is a “basic” (or “true” or “inherent”)
HETP which is a function of the packing and the system only.
This HETP can be inferred from data for small towers, in which lateral
mixing is strong enough to offset any pinching. For a given initial
liquid distribution, the model uses a diffusion-type equation to characterize
the splitting and recombining of liquid streams in the horizontal
and vertical directions. The mass transfer is then calculated by
integrating the liquid flow distribution at each elevation and the basic
HETP. Kunesh et al. successfully applied the model to predict measured
effects of maldistribution on packing efficiency. However, this
model is difficult to use and has not gained industrywide acceptance.
Empirical Prediction Moore and Rukovena [Chemical Plants
and Processing (European edition), p. 11, August 1987] proposed the
empirical correlation in Fig. 14-64 for efficiency loss due to liquid
maldistribution in packed towers containing Pall ® rings or Metal
Intalox ® packing. This correlation was shown to work well for several
case studies (Fig. 14-64), is simple to use, and is valuable, at least as a
preliminary guide.
To quantify the quality of liquid irrigation, the correlation uses the
distribution quality rating index. Typical indexes are 10 to 70 percent
for most standard commercial distributors, 75 to 90 percent for intermediate-quality
distributors, and over 90 percent for high-performance
distributors. Moore and Rukovena present a method for calculating a
distribution-quality rating index from distributor geometry. Their
method is described in detail in their paper as well as in Kister’s book
(Distillation Operation, McGraw-Hill, New York, 1990).
Maximum Liquid Maldistribution Fraction fmax. To characterize
the sensitivity of packed beds to maldistribution, Lockett and
EQUIPMENT FOR DISTILLATION AND GAS ABSORPTION: PACKED COLUMNS 14-69
FIG. 14-64 Effect of irrigation quality on packing efficiency. (a) Case histories
demonstrating efficiency enhancement with higher distribution quality rating. (b)
Correlation of the effect of irrigation quality on packing efficiency. (From F. Moore
and F. Rukovena, Chemical Plants and Processing, Europe edition, Aug. 1987;
reprinted courtesy of Chemical Plants and Processing.)
Billingham (Trans. IChemE. 80, Part A, p. 373, May 2002; Trans.
IChemE. 81, Part A, p. 134, January 2003) modeled maldistribution as
two parallel columns, one receiving more liquid (1 + f )L, the other
receiving less (1 − f )L. The vapor was assumed equally split (Fig. 14-65)
(a)
(b)