**1. Introduction**

778 Mass Transfer - Advanced Aspects

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> The purification or generally separation processes play a major role in a chemical, petrochemical, oil and gas industry. After producing a synthetic material usually a purifying section should be designed. The purification processes contain broad units of absorption/ desorption, distillation, liquid-liquid extraction, leaching, humidification/dehumidification, drying, adsorption/desorption, evaporation, membrane and crystallization. Processing design of these units is one of duties of chemical engineers and for this reason literatures related to purification are taught to chemical engineers as different courses on unit operations and separation processes.

> The basic rules of mass transfer are as skeleton of different separation or purification processes. Concentration gradient of one or more constituents in a media can force the molecules to transfer in different directions based on Fick's first law of diffusion. Creating a convection form in flow of molecules using different ways in industrial units helps the molecules transportation and in other word the purification rate. Of course to this scenario addition of other phases also should be considered. Addition of a phase or an excess media to the first media produces the problem of passing molecules from the interface between phases and answer to the question of how much molecules are able to pass the interface dam. Therefore in this stage phase equilibrium coming from thermodynamics can help us to answer the question. Using different equilibrium data in handbooks helps us to construct the equilibrium curves.

> On the other hand, different contact processes of crosscurrent, countercurrent and cocurrent of different phases create operating line or operating curve that can show the conditions of flows pass next to each other in mass exchanger equipment. Stagewise and differential types of contact are two usual forms of phase contact in mass exchanger equipments. The equipments used these forms of contacts are tray and packed towers and number of theoretical trays and the height of packed beds are two processing parameters that need to be calculated in separation processes.

> Many good literatures have been published on separation processes. Different units have been studied in a valuable book under the title "Mass Transfer Operations" (Treybal, 1955) and added others as well, (McCabe et al., 1956), (Pavlov et al., 1979), (Henley et al., 1981), (Hines & Maddox, 1985), (Khoury, 1995) and (Geankoplis, 2003). The analysis of different separation processes was carried out from the same point of view. The processes were

Mass Transfer - The Skeleton of Purification Processes 781

Fig. 2. Processes used in purification and variation of transferring component concentration in absorption-like and desorption-like processes; (a1, a3) crosscurrent flow, absorption-like process; (a2, a4) crosscurrent flow, desorption-like process; (b1, b2) cocurrent flow, absorptionlike process; (b1, b3) cocurrent flow, desorption-like process; (c1, c2) countercurrent flow,

Usually on diffusion and transfer of a constituent through a phase to the interface produced in contact with other phases two cases can be considered. In the first case the constituent molecules move through a less dense phase to the interface and from interface to other phase contain denser material due to the first phase. This case is called as 'absorption-like' process and can contain different processes of absorption, dehumidification and adsorption. But sometimes the molecules move from a dense phase to less dense other phase and it is called a 'desorption-like' processes. Desorption, humidification, leaching and drying processes

absorption-like process; (c1, c3) countercurrent flow, desorption-like process

studied in details in mentioned references and usually the classification of separation processes was carried out based on phases concerned in units, for example in transfer between gas and liquid phases, humidification, gas absorption and distillation processes were presented. But it seems classifying the units into two broad branches of 'absorptionlike' and 'desorption-like' processes can be helpful on discussion of the main aspects of mass transfer. Therefore in the present chapter, separation units taking place in towers (tray or packed-bed type) are discussed based on a common view point and set into two broad branches. The transfer of molecules presented in a phase to the other phase is investigated from dense point of view of phases. Different separation processes are divided to 'absorption-like' processes and 'desorption-like' processes and a similar analysis procedure for units contain in each category is explained. This method of analysis of separation processes maybe helpful for undergraduate students of chemical engineering and also graduate students to make a common sense between different purification techniques.

In this chapter after dividing the different units into 'absorption-like' and 'desorption-like' processes in the second section, staged processes philosophy is brought in section three and followed with a brief discussion on thermodynamics of processes in section four. Operating line or curve related to processes will be discussed in the fifth section and it follows with staircase construction in sixth section. Different equilibrium diagrams and the strategy of using difference points in process calculations will be discussed in seventh section and section eight contains the calculation of packed bed height. The chapter will be ended through a conclusion section at last.
