Types of packing

Provide a large surface area a high interfacial area between the gas and liquid. 

Promote uniform liquid distribution on the packing surface. 

Promote uniform vapour gas flow across the column cross-section. 

Many diverse types and shapes of packing have been developed to satisfy these requirements. They can be divided into two broad classes  

Packings with a regular geometry such as stacked rings, grids and proprietary structured packings. 

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Molecular mechanics (MM) energy minimiza tion indicates that the two modes lead to monolayers exhibiting different types of packing arrangements, but comparable in their ground state energies. (The monolayer resulting from the mode is more stable by 2.5 kJ/mol (0.6 kcal/mol))... 

The performance coefficients usually attained have been about 5.2 for water loadings in excess of 750 lb/(h-ft ), though new types of packing are improving... 

Selection of Equipment Packed columns usually are chosen for very corrosive materials, for liquids that foam badly, for either small-or large-diameter towers involving veiy low allowable pressure drops, and for small-scale operations requiring diameters of less than 0.6 m (2 ft). The type of packing is selected on the basis of resistance to corrosion, mechanical strength, capacity for handling the required flows, mass-transfer efficiency, and cost. Economic factors are discussed later in this sec tion. 

Figure 14-12 illustrates the influence of system composition and degree of reaetant eonversion upon the numerical values of for the absorption of CO9 into sodium hydroxide solutions at constant conditions of temperature, pressure, and type of packing. An excellent experimental study of the influence of operating variables upon overall values is that of Field et al. (Pilot-Plant Studie.s of the Hot Carbonate Proce.s.s for Removing Carbon Dioxide and Hydrogen Sulfide, U.S. Bureau of Mines Bulletin 597, 1962). 

This subsection will help you select the type of packing to use for your studies and gives typical HETP and HTU values for ballpark estimates when time is short. 

The height and/or type of packing can be changed to improve mass transfer without purchasing new equipment ... 

Numerous types of packing have historically been employed ranging from lumps of coke or quartz to complicated manufactured ceramic and plastic shapes. The arrangement of a typical packed tower is illustrated in Figure 1. 

Liquid distribution in a packed bed is a function of the internal vapoi/liquid traffic, the type of packing employed, and the quality of the liquid distributors mounted above the packed bed. Vapor distribution is controlled by the internal vapor/liquid traffic, by the type of packing employed, and by the quality of the vapor distributors located below the packed beds. 

Figure 8-15. Vorious types of packing. (Courtesy McGraw-Hill Book Company.l...

Eor a long time there have been discussions about which type of sorbent is the best for SEC separations in various mobile phases. In principle, organic (copolymer) and inorganic packings can be used. Each type of packing has its benefits and drawbacks. Table 9.3 summarizes major sorbent properties and reveals some interesting aspects of SEC separations and its requirements on packings. 

There are two basic types of packed-bed reactors those in which the solid is a reactant and those in which the solid is a catalyst. Many e.xaniples of the first type can be found in the extractive metallurgical industries. In the chemical process industries, the designer normally meets the second type, catalytic reactors. Industrial packed-bed catalylic reactors range in size from units with small tubes (a few centimeters in diameter) to large-diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat transfer rates in large-diameter packed beds are poor and where high heat transfer rates are required, Jluidized beds should be considered. ... 

The process of absorption conventionally refers to the intimate contacting of a mixture of gases with a liquid so that part of one or more of the constituents of the gas will dissolve in the liquid. The contact usually takes place in some type of packed column. 

This fitted the data well up to volume fractions of 0.55 and was so successful that theoretical considerations were tested against it. However, as the volume fraction increased further, particle-particle contacts increased until the suspension became immobile, giving three-dimensional contact throughout the system flow became impossible and the viscosity tended to infinity (Fig. 2). The point at which this occurs is the maximum packing fraction, w, which varies according to the shear rate and the different types of packings. An empirical equation that takes the above situation into account is given by [23] ... 

Structured packing requires specially designed distributors recommended by the respective manufacturers to ensure the same important uniform liquid distribution across any bed of this type of packing. 

For applications and design details refer to the manufacturer concerning these types of packings. 

Internal uniform distribution Properly designed devices to distribute and redistribute liquid entering the column is critical to obtain best performance of these types of packings. 

Material of construction The materials of fabrication for this type of packing are more critical to long life due to pad size, wall thickness of metal or plastic components, and actual selection based on the system corrosion, hydrogen attack, and oxygen attack in the column environment. 

Due to the unique HETP characteristics, it is important to consult the manufacturer on the specific system involved and operating conditions. Care must be exercised in selecting these or any other type of packing since plugging with suspended solids, polymer formation on surfaces, and similar mechanical problems can influence performance and life of the packing system. 

The pressure drop for a given number and type of packing deck is expressed [38] ... 

Water quality is important, not only from an environmental point of view but also in relation to the type of packing to be specified. Analysis of the circulating water is simple to obtain, but it is very seldom offered to the cooling tower designer. The quality, or lack of it, will determine the type of pack to be used, the selection of structural materials and whether the tower should be induced or forced draft, counterflow or crossflow. Water treatment, in the shape of chemicals to control pH and to act as counter-corrosion agents or as biocides, all has a bearing on tower selection. 

It is well known in crystallography that, when spheres of equal radius are packed together, the closest type of packing is one in which each sphere has 12 other spheres in contact with it. In Sec. 24 it was mentioned that in water at room temperature each molecule has, on the average, only 4.4 other molecules in contact with it. If we wanted to place one or two additional H20 molecules in contact with any H20 molecule, there would be plenty of room to do this without seriously disturbing the neighbors that are already in contact with this molecule. Similarly, if this molecule is replaced by a solute particle of the same size, the same remark could be made about placing molecules in contact with the solute particle. 

The packings used in modern HPLC consist of small, rigid particles having a narrow particle-size distribution. The types of packing may conveniently be divided into the following three general categories. 

The absorption of reactants (or desorption of products) in trickle-bed operation is a process step identical to that occurring in a packed-bed absorption process unaccompanied by chemical reaction in the liquid phase. The information on mass-transfer rates in such systems that is available in standard texts (N2, S6) is applicable to calculations regarding trickle beds. This information will not be reviewed in this paper, but it should be noted that it has been obtained almost exclusively for the more efficient types of packing material usually employed in absorption columns, such as rings, saddles, and spirals, and that there is an apparent lack of similar information for the particles of the shapes normally used in gas-liquid-particle operations, such as spheres and cylinders. 

Lowe and Christie 17 used a 1.3 m square experimental column fitted with a number of different types of packing and measured heat and mass transfer coefficients and pressure... 

The index n was found to vary from about 0.4 to 0.8 according to the type of packing. It will be noted that when n 0.75, there is close agreement with the results given by equation 13.57. 

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