Types of Mass Transfer Equipment

A gas component A in air is absorbed into water at latm and 20 °C. The Henry s law constant Hm of A for this system is 1.67 x 103 Pa m3 kmol-1. The liquid film mass-transfer coefficient kL and gas-film coefficient kc are 2.50 x 10-6ms-1 and 3.00 x 10 3 m s-1, respectively, (i) Determine the overall coefficient of gas-liquid mass transfer K, (ms-1), (ii) When the bulk concentrations of A in the gas phase and liquid phase are 1.013 x 104 Pa and 2.00 kmol m-3, respectively, calculate the molar flux of A. 

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Numerous types of equipment are available for gas-liquid, liquid-liquid, and solid-liquid mass transfer operations. However, at this point only few representative types are described, on a conceptual basis. Some schematic illustrations of three types of mass transfer equipment are shown in Figure 6.2. 

Figure 6.2 Types of mass transfer equipment (a) packed column, (b) packings, (c) bubble column, and (d) packed bed.

Another type of mass transfer equipment, shown in Figure 6.2d, is normally referred to as the packed- (fixed-) bed. Unlike the packed column for gas-liquid mass transfer, the packed-bed column is used for mass transfer between the surface of packed solid particles (e.g., catalyst particles or immobilized enzyme particles) and a single-phase liquid or gas. This type of equipment, which is widely used as reactors, adsorption columns, chromatography columns, and so on, is discussed in greater detail in Chapters 7 and 11. 

In most types of mass-transfer equipment, the interfacial area, a, that is effective for mass transfer cannot be determined accurately. For this reason, it is customary to report experimentally observed rates of transfer in terms of mass-transfer coefficients based on a unit volume of the apparatus, rather than on a unit of interfacial area. Calculation of the overall coefficients from the individual volumetric coefficients is made practically, for example, by means of the equations ... 

The imerfadal aree A is often unknown in many types of mass transfer equipment. In such cases mass transfer rates frequently are based on unit volume or the equipment, instead of unit inlerfacial area. The rate equations (7.1-37) and (7.1-39) are then modified to the following form ... 

To illustrate the principle of an equilibrium-stage cascade, two typical countercurrent multistage devices, one for distillation and one for leaching, are described here. Other types of mass-transfer equipment are discussed in later chapters. 

Thus far, we have considered mass-transfer correlations developed from analogies with heat transfer. In this section, we present a few of the correlations developed directly from experimental mass-transfer data in the literature. Others more appropriate to particular types of mass-transfer equipment will be introduced as needed. 

The overall mass transfer coefficients Kg and Ky have units of (moIes)/(time-interfacial area-unit mole fraction driving force). In the case of a wetted-wall column, the interfacial area is known. However, for most types of mass transfer equipment the interfacial area cannot be determined. It is necessary therefore to define a quantity a that is the interfacial area per unit of active equipment volume. Although separate compilations of a can be found in handbooks and vendor literature, this parameter is usually combined with the mass transfer coefficients to define capacity coefficients (k a) and (K a) for the liquid phase and (K,a) or (k,a) for the vapor phase, which then have the dimensions of moles per unit time per unit driving force per unit of active equipment volume. The application of these composite coefficients to the design of packed towers is now demonstrated. 

Other types of mass-transfer equipment, such as the packed bed, do not have the discrete stages defined by plates, and the concept of the transfer unit seems more appropriate. When a given component is present at different ffigacities in two phases in contact, mass transfer occurs. The flux is expressed in terms of a mass-transfer coefficient, K, and a fugacity driving force ... 

For case 1, a screening unit of high flexibility and availability is needed. The extraction volume can be small. For case 2 the amount of extract should be apt to determine the course of the extraction with time. It has proved that about 100 to 500 g of solid substrate is appropriate, meaning that the extraction vessel volume should be about 1000 cm If a gas cycle is added, a parameter study can be carried out on extraction and precipitation. For case 3 the equipment should be able to carry out the total process or to simulate all the process steps in sequence in the same manner as intended in a production process. This means that for purposes of screening and a parameter study the precipitation of caffeine in a decaffeination process can be carried out by adsorption on active charcoal or by absorption in water. But for demonstration of process principles, the individual steps have to be carried out by the same unit operation and with the same type of mass transfer equipment as intended for use in a large scale process. Otherwise, no indication of the joint operation can be obtained. For information on scale up, the extractor volume must be at least one order of magnitude larger than in the laboratory type of equipment. 

The concept of overall mass-transfer coefficients is in many ways similar to that of overall heat-transfer coefficients in heat-exchanger design. And, as is the practice in heat transfer, the overall coefficients of mass transfer, are frequently synthesized through the relationships developed above from the individual coefficients for the separate phases. These can be taken, for example, from the correlations of Chap. 3 or from those developed in later chapters for specific types of mass-transfer equipment. It is important to recognize the limitations inherent in this procedure [6]. 

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