**2. 'Absorption-like' and 'Desorption-like' purification processes**

There are different processes that can be applied in purification of products obtained in a chemical industry. Fig. 1 shows different separation processes that can be taking place in tray, packed-bed towers or both. Classification can be carried out based on phases presented in the processes but these processes can be classified into two big branches of 'absorptionlike' and 'desorption-like' processes as well. But before explaining these two branches it needs to describe the mass transfer basis of the purification processes.

Fig. 1. Classification of purification processes that takes place in tray, packed-bed towers or both

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.

both

**2. 'Absorption-like' and 'Desorption-like' purification processes** 

needs to describe the mass transfer basis of the purification processes.

There are different processes that can be applied in purification of products obtained in a chemical industry. Fig. 1 shows different separation processes that can be taking place in tray, packed-bed towers or both. Classification can be carried out based on phases presented in the processes but these processes can be classified into two big branches of 'absorptionlike' and 'desorption-like' processes as well. But before explaining these two branches it

Fig. 1. Classification of purification processes that takes place in tray, packed-bed towers or

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

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

Mass Transfer - The Skeleton of Purification Processes 783

d. Hydraulic driving force- Imposing the pressure gradient using decreasing the total pressure in one phase forces the molecules that move more and faster to the interface and be separated. Drying of a moist solid in a vacuum gas media is a good example in this case that as we expect the moisture molecules are separated faster and more in

On the other hand, for mass transfer to carry out better some conditions can be used to increase mass flux from increasing the mass transfer coefficient viewpoint. Reaching this condition is available from using higher turbulence conditions and consequently increasing the Reynolds number. But in the present section the focus is done on increasing driving forces. The processes that are used for purification in chemical industries need at least two phases to contact each other and this contact should be performed in a contactor or mass exchanger equipment. What is important in contacting phases is that during the course of separation of components transferring molecules causes the concentration gradient to be decreased along the time and the system of finite volume from two phases gets closer to the equilibrium condition just at temperature and pressure of the system. The question is that 'is the equilibrium available really what we need?'. It maybe true sometimes but not often. What needs to do is contacting phases more and more or in other words stage by stage. Each stage can be considered as a control volume that mass transfer taking place on it. Imposing different operating conditions of temperature and pressure in each stage can impose different equilibrium conditions that can be accessible and the flows leaved the stage can be reached them theoretically. Therefore each stage is called as a 'theoretical stage' and the

Fig. 3 shows a stage as a control volume in separation processes generally. As it is shown two streams from two phases enter the stage with known flow rates, compositions and enthalpies (noted as L, y and HL in light phase and H, x, HH in heavy phase, respectively) and after getting the equilibrium condition at temperature and pressure of the stage, two streams made from two phases leave the stage. To get the desire conditions for the flows leave the stage, a set of stages is often used. Therefore the streams enter the stage 'n' coming

What happened on the stage, as mentioned, are the contact and consequently the component transfer between phases. Transport of mass between phases contains the phenomena of mass transfer inside a phase from bulk to the interface, passing the interface,

from stage 'n-1' and stage numbers are brought as subscripts of flow characteristics.

comparison with the cases that vacuum condition not be used.

separation processes sometimes are called 'staged processes'.

Fig. 3. Schematic of a theoretical stage n

can fall in this group. Distillation process that contains rectifying and stripping sections together in a tower is set in 'desorption-like' processes group. Usually Liquid-liquid extraction also falls in 'desorption-like' processes group.

Transferring mass of a component is occurred when there is a difference in component chemical potential between phases. This is a thermodynamics fact of mass transfer. The assumptions simplify this fact into measurable parameter of concentration of component in different phases. The concentration of a component i can be translated as mole or mass concentration (as mole or mass of i to volume which is shown as Ci or ρi, respectively), mole or mass fractions (as mole to total moles that is shown as xi or yi and as mass to total masses that is shown as ωi) and partial pressure for gaseous components (as pressure of i that is shown as pi).

From processing point of view, the differences between purification processes can be categorized in two ways of 'phases contact form' and 'component transferring form'. The contact between phases depends on direction of streams to and from the separator and can be divided into crosscurrent, cocurrent and countercurrent flows. The component transferring form also specifies the way of mass transfer and as mentioned it divides to absorption and desorption-like processes. The operation of different processes is determined from form of varying the component concentrations in streams passing the equipment due to the equilibrium condition from component transferring point of view. The variation of concentrations can be presented as an operating line and the conditions at equilibrium between phases are referred as equilibrium curve. Fig. 2 shows different contact form of phases in purification processes and the form of locating operating line and equilibrium curve in each case generally. All purification processes often fall into different cases shown in Fig. 2. The parameters X and Y show mole fraction of transferring component to mole fraction of non-transferring component and sometimes called as mole ratios. [X=x/(1-x) and Y=y/(1-y)]
