Mass Transfer - The Skeleton of Purification Processes

Abstract

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