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

14-72 EQUIPMENT FOR DISTILLATION, GAS ABSORPTION, PHASE DISPERSION, AND PHASE SEPARATION
volumetric distribution improves along the bed, the concentration
profile could have already been damaged, and pinching occurs
(Bonilla, Chem. Eng. Prog., p. 47, March 1993).
8. Liquid maldistribution lowers packing turndown. The 2-in Pall
rings curve in Fig. 14-59 shows HETP rise upon reaching the distributor
turndown limit.
9. The major source of gas maldistribution is undersized gas inlet
and reboiler return nozzles, leading to the entry of high-velocity gas
jets into the tower. These jets persist through low-pressure-drop
devices such as packings. Installing gas distributors and improving gas
distributor designs, even inlet baffles, have alleviated many of these
problems. Vapor distribution is most troublesome in large-diameter
columns. Strigle (Packed Tower Design and Applications, 2d ed., Gulf
Publishing, Houston, Tex., 1994) recommends considering a gas distributing
device whenever the gas nozzle F-factor (F N = u Nρ G 0.5 )
exceeds 27 m/s (kg/m 3 ) 0.5 , or the kinetic energy of the inlet gas exceeds
8 times the pressure drop through the first foot of packing, or the
pressure drop through the bed is less than 0.65 mbar/m. Gas maldistribution
is best tackled at the source by paying attention to the gas
inlet arrangements.
10. A poor initial liquid maldistribution may cause gas maldistribution
in the loading region, i.e., at high gas rates [Stoter, Olujic, and de
Graauw, IChemE Symp. Ser. 128, A201 (1992); Kouri and Sohlo,
IChemE Symp. Ser. 104, B193 (1987)]. At worst, initial liquid maldistribution
may induce local flooding, which would channel the gas. The
segregation tends to persist down the bed. Outside the loading region,
the influence of the liquid flow on gas maldistribution is small or negligible.
Similarly, in high-gas-velocity situations, the liquid distribution
pattern in the bottom structured packing layers is significantly influenced
by a strongly maldistributed inlet gas flow [Olujic et al., Chem.
Eng. and Processing, 43, 465 (2004)]. Duss [IChemE Symp. Ser. 152,
418 (2006)] suggests that high liquid loads such as those experienced
in high-pressure distillation also increase the susceptibility to gas
maldistribution.
11. The effect of gas maldistribution on packing performance is
riddled with unexplained mysteries. FRI’s (Cai, Paper presented at
the AIChE Annual Meeting, Reno, Nev., 2001) commercial-scale
tests show little effect of gas maldistribution on both random and
structured packing efficiencies. Cai et al. (Trans IChemE 81, Part A,
p. 85, 2003) distillation tests in a 1.2-m-diameter tower showed that
blocking the central 50 percent or the chordal 30 percent of the tower
cross-sectional area beneath a 1.7-m-tall bed of 250 m 2 /m 3 structured
packing had no effect on packing efficiency, pressure drop, or capacity.
The blocking did not permit gas passage but allowed collection of
the descending liquid. Simulator tests with similar blocking with packing
heights ranging from 0.8 to 2.4 m (Olujic et al., Chemical Engineering
and Processing, 43, p. 465, 2004; Distillation 2003: Topical
Conference Proceedings, AIChE Spring National Meeting, New
Orleans, La., AIChE, p. 567, 2003) differed, showing that a 50 percent
chordal blank raised pressure drop, gave a poorer gas pattern, and
prematurely loaded the packing. They explain the difference by the
ability of liquid to drain undisturbed from the gas in the blocked segment
in the FRI tests. Olujic et al. found that while gas maldistribution
generated by collectors and by central blockage of 50 percent of
the cross-sectional areas was smoothed after two to three layers of
structured packing, a chordal blockage of 30 to 50 percent of crosssectional
area generated maldistribution that penetrated deeply into
the bed.
12. Computational fluid dynamics (CFD) has been demonstrated
effective for analyzing the effects of gas inlet geometry on gas maldistribution
in packed beds. Using CFD, Wehrli et al. (Trans. IChemE
81, Part A, p. 116, January 2003) found that a very simple device such
as the V-baffle (Fig. 14-70) gives much better distribution than a bare
nozzle, while a more sophisticated vane device such as a Schoepentoeter
(Fig. 14-71c) is even better. Implications of the gas inlet geometry
to gas distribution in refinery vacuum towers was studied by
Vaidyanathan et al. (Distillation 2001, Topical Conference Proceedings,
AIChE Spring National Meeting, Houston, Tex., p. 287, April
22–26, 2001); Paladino et al. (Distillation 2003: Topical Conference
Proceedings, AIChE Spring National Meeting, New Orleans, La.,
p. 241, 2003); Torres et al. (ibid., p. 284); Waintraub et al. (Distillation
2005: Topical Conference Proceedings, AIChE Spring National
Meeting, Atlanta, Ga., p. 79, 2005); and Wehrli et al. (IChemE Symp.
Ser 152, London, 2006). Vaidyanathan et al. and Torres et al. examined
the effect of the geometry of a chimney tray (e.g., Fig. 14-72)
above the inlet on gas distribution and liquid entrainment. Paladino
et al. demonstrated that the presence of liquid in the feed affects the
gas velocity profile, and must be accounted for in modeling. Paladino
et al. and Waintraub et al. used their two-fluid model to study the
velocity distributions and entrainment generated by different designs
of vapor horns (e.g., Fig. 14-71). Wehrli et al. produced pilot-scale
data simulating a vacuum tower inlet, which can be used in CFD
model validation. Ali et al. (Trans. IChemE, vol. 81, Part A, p. 108,
January 2003) found that the gas velocity profile obtained using a
commercial CFD package compared well to those measured in a 1.4m
simulator equipped with structured packing together with commercial
distributors and collectors. Their CFD model effectively
pointed them to a collector design that minimizes gas maldistribution.
PACKED-TOWER SCALE-UP
Diameter For random packings there are many reports [Billet,
Distillation Engineering, Chem Publishing Co., New York, 1979;
Chen, Chem. Eng., p. 40, March 5, 1984; Zuiderweg, Hoek, and
Lahm, IChemE. Symp. Ser. 104, A217 (1987)] of an increase in
HETP with column diameter. Billet and Mackowiak’s (Billet, Packed
Column Analysis and Design, Ruhr University, Bochum, Germany,
1989) scale-up chart for Pall ® rings implies that efficiency decreases as
column diameter increases.
Practically all sources explain the increase of HETP with column
diameter in terms of enhanced maldistribution or issues with the
scale-up procedure. Lab-scale and pilot columns seldom operate at
column-to-packing diameter ratios (DT/Dp) larger than 20; under
these conditions, lateral mixing effectively offsets loss of efficiency
due to maldistribution pinch. In contrast, industrial-scale columns
usually operate at DT/Dp ratios of 30 to 100; under these conditions,
lateral mixing is far less effective for offsetting maldistribution pinch.
To increase DT/Dp, it may appear attractive to perform the bench-scale
tests using a smaller packing size than will be used in the prototype.
Deibele, Goedecke, and Schoenmaker [IChemE Symp. Ser. 142, 1021
(1997)], Goedecke and Alig (Paper presented at the AIChE Spring
National Meeting, Atlanta, Ga., April 1994), and Gann et al. [Chem. Ing.
Tech., 64(1), 6 (1992)] studied the feasibility of scaling up from 50- to 75mm-diameter
packed columns directly to industrial columns. Deibele
et al. and Gann et al. provide an extensive list of factors that can affect
this scale-up, including test mixture, packing pretreatment, column
structure, packing installation, snug fit at the wall, column insulation and
heat losses, vacuum tightness, measurement and control, liquid distribution,
reflux subcooling, prewetting, sampling, analysis, adjusting the
number of stages to avoid pinches and analysis issues, evaluation procedure,
and more. Data from laboratory columns can be particularly sensitive
to some of these factors. Goedecke and Alig show that for wire-mesh
structured packing, bench-scale efficiency tends to be better than largecolumn
efficiency, while for corrugated-sheets structured packing, the
converse occurs, possibly due to excessive wall flow. For some packings,
variation of efficiency with loads at bench scale completely differs from
its variation in larger columns. For one structured packing, Kuhni Rombopak
9M, there was little load effect and there was good consistency
between data obtained from different sources—at least for one test mixture.
Deibele et al. present an excellent set of practical guidelines to
improve scale-up reliability. So, it appears that great caution is required
for packing data scale-up from bench-scale columns.
Height Experimental data for random packings show that HETP
slightly increases with bed depth [Billet, Distillation Engineering,
Chemical Publishing Co., New York, 1979; “Packed Tower Analysis
and Design,” Ruhr University, Bochum, Germany, 1989; Eckert and
Walter, Hydrocarbon Processing, 43(2), 107 (1964)].
For structured packing, some tests with Mellapak 250Y [Meier,
Hunkeler, and Stöcker, IChemE Symp. Ser. 56, p. 3, 3/1 (1979)]
showed no effect of bed height on packing efficiency, while others
(Cai et al., Trans IChemE, vol. 81, Part A, p. 89, January 2003) did
show a significant effect.