Mass transfer apparatus

Eliminating the mole fraction yAm from (1.215) by yAm = fcHxAeq and the mole fraction yAl by (1.217), gives, with (1.216), a equivalent relationship equivalent to eq. (1.220) 

The resistance to mass transfer according to (1.221) and (1.223) is made up of the individual resistances of the gas and liquid phases. Both equations show how the resistance is distributed among the phases. This can be used to decide whether one of the resistances in comparison to the others can be neglected, so that it is only necessary to investigate mass transfer in one of the phases. Overall mass transfer coefficients can only be developed from the mass transfer coefficients if the phase equilibrium can be described by a linear function of the type shown in eq. (1.217). This is normally only relevant to processes of absorption of gases by liquids, because the solubility of gases in liquids is generally low and can be described by Henry s law (1.217). So called ideal liquid mixtures can also be described by the linear expression, known as Raoult s law. However these seldom appear in practice. As a result of all this, the calculation of overall mass transfer coefficients in mass transfer play a far smaller role than their equivalent overall heat transfer coefficients in the study of heat transfer. 

In mass transfer apparatus one of two processes can take place. Multicomponent mixtures can either be separated into their individual substances or in reverse can be produced from these individual components. This happens in mass transfer apparatus by bringing the components into contact with each other and using the different solubilities of the individual components in the phases to separate or bind them together. An example, which we have already discussed, was the transfer of a component from a liquid mixture into a gas by evaporation. In the following section we will limit ourselves to mass transfer devices in which physical processes take place. Apparatus where a chemical reaction also influences the mass transfer will be discussed in section 2.5. Mass will be transferred between two phases which are in direct contact with each other and are not separated by a membrane which is only permeable for certain components. The individual phases will mostly flow countercurrent to each other, in order to get the best mass transfer. The separation processes most frequently implemented are absorption, extraction and rectification. 

In absorption one or more components from a gaseous mixture are absorbed by either a liquid or a solid. In extraction individual components of a mixture 

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The transfer interface produced by most of the mass transfer apparatus considered in this book is in the form of bubbles. Measuring the surface area of swarms of irregular bubbles is very difficult. This difficulty in determining the interfacial area is overcome by not measuring it separately, but rather lumping it together with the mass transfer coefficient and measuring kLa as one parameter. 

T. Dobre, Highly Efficient Mass Transfer Apparatus- Mobile Packed Bed Column Thesis, Polytechnic Institute of Bucharest, 1985. 

The film and boundary layer theories presuppose steady transport, and can therefore not be used in situations where material collects in a volume element, thus leading to a change in the concentration with time. In many mass transfer apparatus fluids come into contact with each other or with a solid material for such a short period of time that a steady state cannot be reached. When air bubbles, for example, rise in water, the water will only evaporate into the bubbles where it is contact with them. The contact time with water which surrounds the bubble is roughly the same as that required for the bubble to move one diameter further. Therefore at a certain position mass is transferred momentarily. The penetration theory was developed by Higbie in 1935 [1.31] for the scenario described here of momentary mass transfer. He showed that the mass transfer coefficient is inversely proportional to the square root of the contact (residence) time and is given by... 

A material balance is always required to determine the size of mass transfer apparatus, irrespective of the design. So as the energy balance links the temperatures of the fluid flows in a heat exchanger, the mass balance delivers the concentrations of the fluids. 

Heat and mass transfer apparatus normally consist of channels, frequently tubes, in which a fluid is heated, cooled or changes its composition. While the boundary layers in flow over bodies, for example over a flat plate, can develop freely without influence from neighbouring restrictions, in channels it is completely enclosed and so the boundary layer cannot develop freely. In the following the flow, and then the heat and mass transfer in tubes will be discussed. After this we will study flow through packed and fluidised beds. 

Packed beds serve as regenerators in heat transfer. As so-called packed columns they are frequently implemented as mass transfer apparatus. This normally involves the introduction of a liquid mixture at the top of a column with a gas of different composition flowing in the opposite direction, as illustrated in Fig. 3.37. Through mass transfer one or more components of the gas are transferred into the... 

Many heat and mass transfer problems can be solved using the balance equations and the heat and mass transfer coefficients, without requiring too deep a knowledge of the theory of heat and mass transfer. Such problems are dealt with in the first chapter, which contains the basic concepts and fundamental laws of heat and mass transfer. The student obtains an overview of the different modes of heat and mass transfer, and learns at an early stage how to solve practical problems and to design heat and mass transfer apparatus. This increases the motivation to study the theory more closely, which is the object of the subsequent chapters. 

V. M. Olevskii, V. R. Ruchinskii Rotary-Membranous Heat- and Mass-Transfer Apparatus, Khimiya, Moscow (1977) (in Russian). 

As will be shown, in practical mass-transfer apparatus the bulk concentrations in the contacted phases normally vary considerably from place to place, so that a point such as P in Figs. 5.3 and 5.4 is just one of an infinite number which forms a curve on these figures. Thus, in the countercurrent wetted-wall gas... 

Mallinson, R.H. and C. Ramshaw, Mass transfer apparatus and process," European Patent 0053881 (1982). 

This accessible time is the time through which the liquid phase stays in the mass transfer apparatus while the crmsidered reactions take place in the liquid phase. 

N. Kolev, Liquid phase distribulcff fi>r heat and mass transfer apparatuses. Bulprian patent... 

The way applied in the whirlwind mass-transfer apparatus with a screw torch of an irrigation, as presented inFigure 1.1 (Aliev, 1988), can be one of the ways of maintenance of an optimum regime of wet clearing of gas emissions. 

Bunkin, V. A. Working Out and Research of the Mass-Transfer Apparatus with Direct-Flow Rotary Cormections. Ref Diss. Cand. Tech. Sci. Kazan KCTU 1970. Samsonov, V. T In the Collector Ventilation and an Air Conditioning at the Polygraphic Factories. Works Scientific Research Institute a Polygraph, M. Book. 1974. 

Bunkin, V. A. Working out and research of the mass-transfer apparatus with direct-flow rotary cormections. Ref Diss. Cand. Tech. Sci. Kazan KCTU 1970. 

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