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| Sulphuric Acid on the WebTM | Technology Manual | DKL Engineering, Inc. |
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Knowledge for
the Sulphuric Acid Industry Introduction
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Packing in an acid plant tower serves to bring into intimate contact, gas and acid to promote more efficient mass and heat transfer. To achieve this requirement, the following properties are desired:
Early packings were simply randomly shaped ceramic pieces dumped in a tower. Pressure drops were high and mass transfer efficiency relatively low. Early plants were small so these drawbacks were not critical. As plant size increased, the deficiencis of the early packings became more noticeable. The design and operation of towers was revolutionized by the introduction of the Raschig ring. The rings were uniform in shape so tower performance was predicatable and consistent.
Random Packing
Structured Packing
Installation of random packing is important in order to prevent gas or liquid channelling, high pressure drops and broken or chipped packing. To ensure a uniform density throughout the bed the packing should be installed evenly across the the entire cross-section of the tower. Large piles or mounds of packing should be avoided. If the density of the bed is different in one area, gas or liquid channelling may occur. Packing should be carefully laid or poured out from the box or bag onto the surface of the packing already in the tower. The packing should never be dumped from any great height. Small chips created from improper handling have been known to plug pump strainers, acid coolers and acid distributors. Installers should not stand directly on the packing but on plywood boards laid on top of the packing surface. The plywood serves to spread out the weight which will avoid further chipping and breakage of the packing. Ensure that all boxes and bags are removed from the tower as soon as they are empty. Ensure that no plywood, rope, or other construction material gets buried in the packing all debris is removed from the tower. The tower should always be packed up to the required level as shown on the drawings. Usually extra packing is order to allow for settling and breakage. The volume of the packaging will not equal the volume of the tower. The wall effects of the smaller packaging results in a different volume when the packing is installed in the tower. Standards ASTM C515 provides a standard for ceramic random packing in terms of the chemical composition, physical size of the packing and the number of pieces in a given volume which ensures a minimum quality for the packing. The standard does not cover properties that affect actual operational performance (i.e. pressure drop, mass transfer, etc.)
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Copyright© 2005,
2006, 2007, 2008 DKL
Engineering, Inc., All Rights Reserved |
The next advancement in tower packings was the introduction of the
saddle. The saddle offered further improvements in mass transfer efficiency and
lower pressure drop. Today, saddles are still the standard for acid plant towers.
The term 'Intalox' is oftened used to reference saddles in general but the term
'Intalox' is actually a tradename belonging to Norton. Other equivalent names are
Novalox and Flexisaddle (Koch). There are also generic type saddles available from
various manufacturers. Different saddles may appear similar but care should be taken
to ensure the product will perform as anticipated. The basic characteristics of the
packing (i.e. pieces/volume, specific surface area, bulk density, etc. should be examined
and check against the parameter assumed in the design of the tower. As well, the
mechanical properties and dimensions of the saddles should be checked against the
standard. Wall thicknesses may vary considerably resulting in a saddles that may
appear similar to another manufacturer's saddle but is not as robust and may be subject to
chipping nd breakage.
Small improvements and variations have been made to the basic saddle shape to
further enhance the packing performance. Norton developed Super Intalox saddles
which incorporated scallops or ridges along the curved edge of the saddle. As well,
small holes were introduced in the body of the saddle. These modifications were
intended to improve mass transfer efficiency and reduce pressure drop. The scallops
creates points at which liquid droplets can form and fall to next piece of packing,
exposing new liquid surface for mass transfer. The holes allow gas to more easily
flow through the packing resulting in less pressure drop. The packing has not been
widely used in the industry even though the performance data shows an improvement over
regular saddles. 
Another variation of the
basic saddle shape is the CECEBE HP™ packing. The packing is characterized by its
larger size, larger rectangular holes and scalloped edges. These features are
similar to the Super Intalox saddles but are magnified in terms of size.
A complete departure from
random packing is structured packing. The packing comes in 1 cubic foot block and is
stack in the tower so that each layer is orientated 90 degrees to the layer above or below
it. At the edge of the tower the packing can be shaped to the contour of the inside
curvature or the gap filled with smaller packing to prevent gas bypassing.
Structured packing exhibits extremely low pressure drops. Liquid and gas divided and
recombined by the packing thus providing the high mass transfer rates by continuously
renewing the liquid surface. The packing has not caught on for new tower
installations because of its high cost but has found a niche in the plant upgrade
market. Installing structured packing can allow a tower to handle a high gas flow
without the need to completely replace the tower. In these cases, the packing cost
is small compared to the cost of a new tower designed for a higher capacity.
Another relatively new
packing is WavePak™ available from Monsanto. The packing gets its name from the
unique shape of the packing which is a complete departure from the traditional saddle
shape.
The newest packing to hit the market is from Koch Knight LLC which already
market a line of packing under the tradename Flexisaddle™. The newest packing
is Flexisaddle™ LPD (low pressure drop) (patent applied for) which is basically a
saddle shaped random packing with holes in the surface of the saddle. A significant
difference is a rib in the middle of the saddle which appears to extend the surface area
available for each piece of packing. The added surface area combined with the
numerous holes appears to give the packing the low pressure drop and high performance
(i.e. high mass transfer rates) claimed by Koch Knight LLC.