Contact Equipment

Height Equivalent to an Equilibrium Stage (Theoretical Plate) 

Consider a packed tower of unit cross section, as in Fig. 8.1-7. The total effective interfacial surface for mass transfer, as a result of spreading of the liquid in a film over the packing, is S per unit tower cross section. This is conveniently described as the product of a specific interfacial surface, surface per volume of 

The quantity of solute A in the gas passing the differential section of the tower under consideration is Gy mol/(area) (time), and the rate of mass transfer is therefore d Gy) mol A/(differential volume) (time). Since N = Q and N / 

Both G andy vary from one end of the tower to the other, but the solvent gas which is essentially insoluble, does not. Therefore, 

The value of y, can be found by the methods of Chap. 5, using Eq. 5 21) with 

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Process Solvent Solvent additives and reflux conditions Operating tempera-ture, °C Contacting equipment Comments... 

The material on solids drying is divided into two subsections, Solids-Drying Fundamentals, and Sohds-Drying Equipment. In this introductory part some elementary definitions are given. In solids-gas contacting equipment, the solids bed can exist in any of the following four conditions. 

Note that the product of the mass-transfer coefficient and the interfacial area is a volumetric coefficient and obviates the need for a value of the interfacial area. While areas for mass transfer on plates have been measured, the experimental contacting equipment cuffered significantly from that used for commercial distillation or gas absorption, and the reported areas are considered unreliable for design purposes. 

Liquid-hquid contacting equipment may be generally classified into two categories stagewise and continuous (oifferential) contact. 

Three regions are identified with different requirements of and n, and for which particular lands of contacting equipment may be best ... 

Inventory Reduce inventory of chemicals Continuous operation may be preferable to batch Low residence time contacting equipment may be better than cheaper alternatives etc. Monitor temperature, pressure flow, composition, freedom from contamination and other appropriate properties of all streams where relevant. Consider automatic control... 

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

Packed towers are used as contacting equipment for gas-liquid and liquid-liquid systems. Figures -1A and B present a cross-section of a typical unit. The shell is usually q lindrical, although square wooden, light metal, or reinforced plastic towers are used. The basic unit consists of ... 

Later publications have been concerned with mass transfer in systems containing no suspended solids. Calderbank measured and correlated gas-liquid interfacial areas (Cl), and evaluated the gas and liquid mass-transfer coefficients for gas-liquid contacting equipment with and without mechanical agitation (C2). It was found that gas film resistance was negligible compared to liquid film resistance, and that the latter was largely independent of bubble size and bubble velocity. He concluded that the effect of mechanical agitation on absorber performance is due to an increase of interfacial gas-liquid area corresponding to a decrease of bubble size. 

Continuous operation may be preferable to batch Low residence time contacting equipment may be better than cheaper alternatives etc. 

Many of these treatments can be straightforwardly scaled-up to industrial quantities by adding steam to calcination furnace atmospheres and carrying out various forms of solution ion exchange or chemical treatment and/or acid extractions in suitable powder and granule contacting equipment. The products vary in stability and catalytic properties because the treated materials are no longer freshly crystal-... 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

Because of the diversity of contacting equipment available, it is unlikely that all these contactors will be available in any one laboratory or pilot plant. Consequently, unless test work is carried out on similar contactors, the system may not be optimized. Since mixer-settlers are the easiest to construct, are simple to operate, and require little room and low-flow rates, these contactors are, in many cases, the only ones used to investigate a continuous solvent extraction process. This is by no means ideal and may result in abandonment of a process that, using another type of contactor, could be found to be entirely satisfactory. 

Labor and maintenance costs can usually be kept to the order of a few cents per pound of metal produced. However, where a circuit has been improperly designed and several manual operations are required, then labor and maintenance costs can be high. Excessive solvent entrainment losses, mentioned earlier, are added to labor costs. Any crud that forms and has to be removed can result in additional costs from labor requirements. For example, if incorrectly designed contacting equipment is chosen, which results in periodic shutdowns necessary for cleaning because of formation of cruds and precipitates, a major maintenance problem could arise. 

Two-phase systems are often exposed to turbulent flow conditions in order to maximize the interfacial area of the fluids being contacted. In addition, turbulence is often present in wind tunnels and other laboratory equipment, as well as in nature where it can influence breakup processes (F5). Prediction of drop or bubble sizes in turbulent contacting equipment for any geometry and operating conditions is a formidable problem, primarily because of the inherent theoretical and experimental diflBculties in treating turbulent flows. To these difficulties, which exist in single phase systems, must be added the complexity of interaction of dispersed particles with turbulent flow fields. 

A variety of gas-liquid contacting equipment with mechanical moving elements (e.g., stirred (agitated) tanks with gas sparging) are discussed in Chapters 7 and 12, including rotating-disk gas-liquid contactors and others. 

Almost every kind of holding or contacting equipment has been used as a chemical reactor at some time, from mixing nozzles and centrifugal pumps to the most elaborate towers and tube assemblies. This section is devoted to the general characteristics of the main kinds of reactors, and also provides a gallery of selected examples of working reactors. 

A combination of indirect- and direct-contact equipment is commonly used. Trays containing the material to he dried may be placed on... 

Cooling towers fall under the general category of direct-contact equipment. In direct-contact equipment, fouling resistances are automatically eliminated because a surface is no longer available on which they can form. This is a unique feature because it allows direct-contact equipment to operate indefinitely without interruption in thermal performance. 

Definitions Gas absorption is a unit operation in which soluble components of a gas mixture are dissolved in a liquid. The inverse operation, called stripping or desorption, is employed when it is desired to transfer volatile components from a liquid mixture into a gas. Both absorption and stripping, in common with distillation (Sec. 13), make use oF special equipment for bringing gas and liquid phases into intimate contact. This section is concerned with the design of gas-liquid contacting equipment, as well as with the design of absorption and stripping processes. 

As discussed in Sec. 7, the factor E represents an enhancement of the rate of transfer of A caused by the reaction compared with physical absorption, i.e., Kq is replaced by EKq. The theoretical variation of E with Hatta number for a first- and second-order reaction in a liquid film is shown in Fig. 19-25. The uppermost line on the upper right represents the pseudo first-order reaction, for which E = Ha coth (Ha). Three regions are identified with different requirements of liquid holdup 8 and interfacial area a, and for which particular kinds of contacting equipment may be best ... 

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