Typical Applications and Uses
For both air pollution control and recovery of process gases, packed towers are one of the most common mass transfer devices in current use.
They are used for control of soluble gases such as halide acids (such as HF and HC1) and to remove soluble organic compounds such as alcohols and aldehydes. When the scrubbing solution is charged with an oxidant such as sodium hypochlorite, they are used to control sulfide odors from wastewater treatment facilities and rendering plants. They are used to absorb and concentrate acids for recovery. When gases and aerosols are both present, the packed tower is frequently used ahead of aerosol collectors such as fiberbeds and wet electrostatic precipitators.
Packed towers are also used as gas coolers and condensers (Please see Chapter 22 on energy recovery). They sometimes are used after a hot gas quencher to act as a gas cooler. Some are fitted with ceramic packing that can resist temperature extremes. When fitted ahead of a Venturi scrubber to function as a water vapor condenser/absorber, the packed tower becomes a critical part of a flux force condensation system for particulate control. The tower in this case acts as both an acid gas absorber and a direct contact vapor condenser.
They are also used after Venturi scrubbers on medical waste incinerators to control acid gases such as HC1.
To control the combined vent gases from semiconductor manufacturing, large, packed towers are used. Called house scrubbers, they clean the small concentrations of acid gases usually using pH control and neutralization with caustic. In contrast, the same industry uses small, packed towers at specific tools in a point-of-use configuration. The point-of-use scrubbers are designed to treat the specific emission source and often vent into a combined ventilation system, eventually leading to a house scrubber. The emissions are effectively double scrubbed before the carrying gas is released to atmosphere.
Pulp and paper mills often use packed towers for bleach plant applications to control chlorine and chlorine dioxide where fibers or chemical scaling is minimal. Fluidized bed-type scrubbers are used in cases where fibers or scaling are known challenges.
As mentioned in Chapter 1, absorbers function by extending the surface of a solvent (usually water) so that the mass transfer of a gas into that solvent is enhanced. The mass transfer of a gas into the liquid is limited by the gas/ liquid interface conditions. Only a certain mass of gas can move into the liquid per unit area. Once into the liquid, only a certain amount of dissolved gas can remain, per unit volume. Therefore, to effectively remove the gas, one must have sufficient liquid surface area and an adequate volume of liquid.
The packing (or medium) in a packed tower provides the liquid-extending function to increase its area. The liquid inlet system provides the adequate volume. By selecting the proper type and quantity of media, the conditions can be created for optimum mass transfer. The result is a tower containing the design amount of media (or an excess) irrigated by the design amount of liquid (or an excess). If the gas flows vertically, the tower may contain just a few feet of this medium, or over 50 ft of medium, depending upon the absorption characteristics of the contaminant and the neutralizing capability of the liquid. Towers may also be required in series to reach the desired gas outlet conditions.
Packed towers are essentially probability machines. The individual contaminant gas molecule is in contact with the descending liquid for only a fraction of a second. By increasing the number of chances of such random contacts through increasing the height of the packed bed, the chances that the molecule will be absorbed are increased. If you do not absorb it now, you might absorb it later. Also, it takes time for the gas to diffuse to the liquid surface. If one gives such diffusion more time by letting the gas move slowly through a long-contact bed, one increases the chance of successful absorption.
The standard vertical (counterflow) packed tower has the components shown in Figure 12.1. The vessel contains a grid that supports the packing medium. The medium is irrigated from above by a liquid distribution device (usually a spray header or headers). The liquid hits the medium and
Vertical counterflow packed tower components (Bionomic Industries, Inc.).
high-surface area liquid films and/or drip points are formed as the liquid flows over and through the medium. The gas, flowing in the opposite direction as the liquid, is caused to take a tortuous path through the medium, thus bringing the gas close to the absorbing liquid. The gas contacts the liquid surface and, if the liquid is not saturated with the contaminant, is absorbed. If some contaminant is already present in the liquid, not all the contaminant gas will be absorbed. Therefore, a large volume of packing is often used so that, particularly at the top of the packed tower, the scrubbing liquid can absorb and retain the gas. If not, the removal efficiency of the packed tower will be reduced.
A crossflow arrangement (Figure 12.2) is similar except that some of the gas and liquid move concurrently and that the liquid is rejected downward along the entire vessel path length. For gases that are absorbed and react with dissolved compounds, the crossflow and counterflow towers behave similarly. If the gas does not react with chemicals in the liquid, the crossflow tower can demonstrate a reduced efficiency since the liquid is carried, with its dissolved gas cargo, toward the gas discharge point, creating a vapor pressure condition that favors the gas. This means that the liquid may not have
Crossflow packed tower components (Bionomic Industries, Inc.).
Dumped-type packing media (Bionomic Industries, Inc.).
the same absorption capacity in the crossflow design as in the counterflow design when no liquid phase reaction occurs.
There are hundreds of types of packed tower packing material that forms the packed bed. Figure 12.3 shows a variety of basic types of dumped-type packing media. These media may be made from thermoplastic material such as polypropylene, metals such as stainless steel or corrosion resistant alloys, or even in the form of cast ceramics. Figure 12.4 shows media offered by RVT Process Equipment, Inc., and Figure 12.5 shows media designed and supplied by Lantec Products, two of the leading domestic suppliers of this type of media.
You can see by the designs that certain configurations produce large surface films and others have small holes or openings that form numerous drip points. In general, where scaling can occur, packing with large openings that produce drip passages rather than film surfaces are used because scaling is a surface phenomenon. The various vendors seek to combine a balance between mass transfer enhancement and plugging and scaling resistance. The resulting packing must be structurally sound as well because the material rests on and is supported by the medium beneath it. In a more subtle manner, the packing must resist side-to-side motion under the influence of gas or liquid flow. If the packing moves around easily, valleys or mounds of packing can form in the tower, upsetting its performance.
RVT packed tower hiflow media (RVT Process Equipment, Inc.).
Lantec packed tower media (Lantec Products, Inc.).
Structured ceramic packing (Lantec Products, Inc.).
Media can also be in the form of shaped and/or perforated panels. This is called structured packing because the media are structurally self-supporting. Figure 12.6 shows a type of structural packing. The plastic versions are cousins to cooling tower fill, and many look like corrugated plastic panels. Other fill material is made of woven mesh, much like the mesh used in a mesh pad droplet eliminator. This type of medium is used in distillation columns and applications, in general, where no solids are present. If solids are present, the medium can act as a liquid filter and plug.
Primary Mechanisms Used
Gas absorption in a counterflow (vertical gas flow) packed tower is dictated by the equilibrium conditions between the contaminant gas and the absorbing liquid. The overall controlling mechanisms are ruled by the solubility of the gas in the liquid and by any reactions that may be caused to occur in the liquid with a reacting chemical. If the gas reacts with a chemical forming a lower vapor pressure compound, the equilibrium shift favors further absorption. If the absorbed gas builds up in the liquid, the equilibrium shifts to inhibit subsequent absorption.
Diffusion is used to move the gas to the liquid surface. At or near the liquid surface, phoretic forces such as thermophoresis or diffusiophoresis may be in play.
In essence, however, packed towers are equilibrium and probability machines. The overall gas/liquid equilibrium controls the design of the tower. Because the gas is absorbed at the liquid surface, the more liquid-to-gas interactions that can be caused to occur, the greater the probability of absorption. The more difficult the absorption, therefore, the greater the medium depth. This increases the number of contact possibilities, thus increasing the likelihood that a contact will be successful, and the gas will be absorbed.