Figure 1: A Typical Sulfuric Acid Plant Layout (Print Version)
Packed towers are key components in sulfuric acid plants.
Drying of the sulfur furnace air is necessary to avoid acid condensation and
corrosion in downstream equipment and to minimize mist formation. Absorption of SO3
in the Interpass and Final Towers recovers the product sulfuric acid. Great
attention to detail is required in the design of packed towers to achieve the necessary
absorption efficiency. In many ways, the sulfuric acid industry is unique in that
packed towers of exceptionally large diameters with relatively small packing height are
common. In addition, the use of large size ceramic packing has become the industry
standard. A result of the unusual features of packed towers employed in acid plant
service is that truly applicable design data are not readily available and that
discrepancies reveal themselves when the designs of different technology suppliers are
compared. For the engineer facing the task of sizing a packed tower or selecting a
supplier, it is tempting to assume that the design techniques for acid plant towers are
well proven and that differences in supplier's offers simply reflect differences is design
conservatism. It is suggested, however, to take into consideration the following
points:
A standard three inch saddle is available from a number
of suppliers at relatively low costs. This saddle has been used for over thirty
years in the sulfuric acid industry. Only modest profit margins in making and
supplying this type of packing can be expected, not sufficient to commission
significant development work on packing performance in large towers, especially when
competing suppliers would gain the benefit of the development at no cost.
Contractors are in a similar position, leaving the owners as the only party likely to gain
from development work. Where new packing is proposed, there is a need to compare it
with existing packing to see if there is an improvement which justifies the development
expense or makes the changeout of packing attractive to the owners. This economic
reality has limited the introduction of new packing over the past twenty years.
Engineers involved in sizing towers have at their
disposal a number of different techniques for tower sizing, ranging from rules of thumb
based on gas velocity and irrigation rate, to dated theoretical work in the handbooks, to
software programs from packing vendors, and finally to proprietary in-house design
techniques. The resulting tower sizes vary significantly, as this paper will show.
In addition, there is a need for design approaches which can be used with new
packing for which there is little data. Most of the experimental work on packing
pressure drop was carried out over forty years ago, almost exclusively in small pilot
towers. Norton, for example, did much of their early work using thirty inch diameter
towers while Koch used a thirty-six inch diameter tower. When the packing sizes were
relatively small, the effect of the tower diameter on the packing density was minimal, but
when larger packings were used in these pilot towers, there were significant edge effects
and the void fraction in the test column was much larger than that found in large towers
typical in acid plants with the same packing. The result was very optimistic
predictions of pressure drop. Figure 2, reproduced from a brochure published by VFF
Industries, shows the relative number of pieces of packing per unit volume, the packing
density, as a function of the ratio of the tower diameter to the characteristic dimension
of the packing. The curve uses a reference packing density in a tower with a
diameter twenty times the nominal size of the packing. For a three inch nominal size
saddle, this would give a tower diameter of six feet. For a three foot tower, the
packing density would be ninety seven percent of that of the reference tower, while for a
twenty foot tower, the actual packing density would approach one hundred and ten percent
of the reference case. The packing void fraction will vary accordingly with high
packing densities resulting in low void fractions. The packed tower pressure drop
and flooding limits are very sensitive to the void fraction, as will be shown later in
this paper. It has been found that the pressure drop for a given large size packing
in a plant scale tower can exceed twice the pressure drop measured in a pilot tower under
identical process conditions.
Figure 2: Packing Density Correction Factors (Print Version)
The Generalized Pressure Drop Correlation (GPDC) is the
classic sizing method for packed towers and is used in many industries. It is,
however, based mostly on the small pilot tower data. As long as the correlation is
applied to small packing, it appears to give reasonable results, but when the performance
of large packing in large towers is assessed, then the results appear overly optimistic.
A second rule of thumb method is to size the tower based on a packing exit gas
velocity of 8 ft/s with an acid irrigation rate of 10 USGPM/ft2. Most
often the velocity used in sizing the tower is that of the gas leaving the packing.
Different techniques should be applied to the tower bottom depending on the temperature of
the inlet gas and the relative acid flow. A third sizing approach is to use the
published pressure drop curves for the air and water system which are publicized in the
suppliers' literature. Figure 3 shows one such plot published by U.S.
Stoneware. This approach is more practical but, as previously discussed, the data
were again mostly developed in small pilot towers, even when large packing was
studied. On the basis of proprietary design approaches, one from a packing supplier,
and the second from a chemical company, a design approach was developed by CECEBE based on
ideas originally proposed by Dr. Max Leva. Field data were carefully taken in full
sized towers to refine the correlations and design methods. The method of Dr. Leva
starts with dry bed pressure drop, which is then corrected for the liquid flow through the
packing and also for loading. With moderate gas flows and pressure losses, the
incremental increase in pressure drop due to liquid irrigation depends on the liquid flow
but does not change with gas velocity. Greater liquid flow causes greater liquid
hold-up and higher interstitial gas velocities. These two effects combine to result
in higher pressure drop. In this range of moderate gas and liquid flow rates,
pressure drop curves for irrigated packing run parallel to the dry gas pressure drop
curve. Further increases in gas and liquid flow, beyond a critical limit called
"loading point", result in a rapid increase in the liquid hold-up due to spray
re-entrainment and causes increased pressure loss. Eventually, the liquid hold-up
fills a significant part of the packing void and it becomes difficult to force gas flow
through the packing. The tower ultimately floods.
Figure 3: Pressure drop vs. gas rate (1 1/2-in. Intalox
saddles--ceramic) (Print
Version)
The various design approaches described earlier have been
used in this paper in a standard absorber to predict the tower size on the basis of
operating with standard packing. In addition, tower sizes were developed for both
the HPTM Saddle Packing developed
by CECEBE Technologies and the Flexeramic® Structured Packing developed by Koch.
