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Sizing of Packed Towers in Acid Plants

PHYSICAL FACTORS

Void Fraction

Since gas in an acid plant packed tower flows up and the liquid flows down to achieve the required gas-liquid contact, the packing functions as a liquid surface generation device.  While good mixing action by the packing is a desired feature in that it promotes mass transfer, packing must not cause the tower to fill with liquid and prevent gas counterflow.  The void fraction of packing, which is the space available for the gas and liquid to flow, depends on many factors.  These factors include the shape of the packing, the diameter of the tower relative to the size of the packing, the degree of packing chips or sulfate accumulation and the liquid flow rate.  Figure 2 illustrates the variation of packing density with the ratio of the tower diameter to packing characteristic dimension.  Ceramic packings typically have void fractions around 0.75, although a number of references from small sized towers have declared 0.80 void fraction for the same packing.  This small difference is significant in that a 0.05 decrease in void fraction will add over fifty percent to the pressure drop.  Fouling with sulfate or chips can easily double the pressure drop as well and it may be desirable at the initial design stage to make an allowance for such fouling.

Effective Surface

While the actual surface of the packing pieces can be deduced from geometry, tests with Raschig rings have demonstrated that a significant portion of the surface is not wetted by the circulating liquid and is, therefore, ineffective.  The same conclusion is not necessarily true for other packings.  The HP^TM saddle and Pall Ring both have less surface than their predecessors but their surface is much more effective.  With an identical shape, the surface required for satisfactory mass transfer is similar for small or large packing.  The smaller packings, however, typically have a much larger surface area per unit packing volume and, therefore, will need less packing height.  The penalty is that smaller packing has a larger pressure drop so that the tower diameter must be increased to handle a given gas and liquid flow rate.  Note that liquid distribution also becomes more difficult as the diameter of a tower increases.  In addition, tower costs are mostly a function of diameter and not of height.  It is therefore, not economical to use small packing.

Gas Passage Size

For a given packing height, the number of time the gas must detour around a packing piece depends on the size of the packing, while the mass transfer depends to a significant degree on the extent to which the gas stream is split and contacts the liquid.   The larger the packing, the higher is the height required for the mass transfer duty and the smaller the required tower diameter.  The pressure drop depends largely on the number of gas detours as the gas passes through the packing.

Random Orientation

Packing can be structured or random.  Structured packing is a relatively new development.  It still remains expensive and difficult to install.   Its capacity is, however, marginally higher than the standard 3" saddle.   A serious shortcoming of structured packing is that good initial liquid distribution is absolutely necessary to achieve the required mass transfer efficiency.   In other applications involving structured packing, ten irrigation points per square foot are commonly specified.  In acid tower applications, a layer of saddle packing is often used on top of the structured packing to achieve adequate liquid distribution.  This requirement voids the potentially higher capacity of structured packing.  Most packings used in the sulfuric acid industry are random and the standard 3" saddle is a good example.  The Berl Saddle, which preceded it, can stack in a tower and is now rarely used for that reason.  The Raschig ring and derivatives are also good random shapes.  For effective use, a packing dimension should be small compared to the tower in which it is installed and preferably packing should not have a long and thin shape.  Such a geometry can lead to gas bypassing at the wall of the tower.

Mechanical Strength

Ceramic packing can break easily if it is not properly fabricated.   The two techniques by which such packing is made are by extrusion and by slip casting.  Extrusion is the most common technique as it is relatively inexpensive.   Slip casting produces a saddle of greater density, which has much lower rates of absorption of acid and water and which is much harder to break.  In addition, the packing shape must be designed to provide sufficiently thick cross sections to minimize breakage.  The CECEBE HP^TM saddle benefits from both thick sections and the slip casting technique to give a packing that is exceptionally strong, nearly three times stronger than conventional ceramic 3" saddles.   Structured packing uses thin sheets which are fragile.

A further factor affecting strength in general is the clay that is used in the manufacture.  Most packing is made from mined clay with little further preparation and the clay composition can very depending on where in the pit it comes from.  Clay used for porcelain or domestic fixtures is normally formulated to a specified grain size distribution and chemical composition and results in a stronger and more homogenous saddle.  The HP^TM saddle, for example, uses the same clay that is used in the fabrication of toilets and sinks.  It is of a much higher quality than the clay mined directly from the pit.  This is important to an owner because packing can be broken during installation or during operation, resulting in chip generation which will then lead to excess pump wear, plugging of acid coolers and distributors, higher pressure drop, and expensive downtime for cleaning.

Acid Absorption

Where ceramics have been produced by extrusion, voids in the packing must be expected.  This results in significant absorption of acid into the packing and extended acid weeping on shutdown, making access and maintenance difficult.  Voids in slip cast ceramics are much less common.  This difference reveals itself in the specific gravity of packing which can range from 2.3 in the extruded product to 2.7 in the slip cast porcelain.

Pressure Drop Prediction

By: Gordon M. Cameron (CECEBE Technologies) and Ian Chang (NORAM Engineering)