Unit 1_ Introduction (II).pdf

Topic 1 Introduction to Mass Transfer and Separation Processes AECH 4210 Mass Transfer & Separation Processes  Modes of Mass Transfer - Mass transfer by diffusion (molecular or eddy): ① Mass transfer is due to concentration difference. ② Microscopic...

Topic 1 Introduction to Mass Transfer and Separation Processes AECH 4210 Mass Transfer & Separation Processes  Modes of Mass Transfer - Mass transfer by diffusion (molecular or eddy): ① Mass transfer is due to concentration difference. ② Microscopic level, diffusion velocity. ③ Diffusion through a layer of stagnant fluid – molecular diffusion. ④ When one of the diffusing fluids in turbulent motion – eddy diffusion. ⑤ Mass transfer: eddy diffusion > molecular diffusion. ⑥ Example of molecular diffusion – a purple dye diffusing in a test tube. ⑦ Example of eddy diffusion – dissipation of smoke from a smoke stack. Turbulence causes mixing and transfer of smoke to the ambient air. 2  Modes of Mass Transfer - Mass transfer by convection: ① Macroscopic level, bulk movement of liquid in large scale. ② Depends on transport properties and on dynamic (laminar or turbulent characteristics). ③ Free convective mass transfer: caused by buoyancy forces due to density differences in the fluid. Example: evaporation of ether. ④ Forced convective mass transfer, induced by external means such as fans or pumps to increase fluid flow. Example: when air moves very fast on the surface of the lake due to high speed of wind, it takes water particle with it. 4 Analogy Between Mass and Heat Transfer 1. Example of diffusion mass transfer: Mass Convective Transfer: Drying the Paint 2. Example of convection mass transfer: - It is well-known that sweet-scented osmanthus is very fragrant, but only under the role of wind the scent of osmanthus can Drying Process spread very far. 6 Comparison of diffusion and convection mass transfer processes ◼ The rate of diffusion mass transfer is very slow, and therefore greatly lowers the scale of mass transfer process. ◼ The rate of convection mass transfer can be very fast. So, it can realize large scale mass transfer process. ◼ Convection mass transfer process inevitably involves of diffusion process. This process is very like a high-speed train carrying a crawling turtle. Diffusion mass transfer Convection mass transfer 7  Interphase Mass Transfer Interface mass transfer depends on molecules diffusing from one distinct phase to another and is based upon differences in the physico-chemical properties of the molecules, such as vapour pressure or solubility. There is a concentration gradient between bulk and interface, however under steady state, at interface equilibrium is assumed. 8  Interphase Mass Transfer Understanding by examples: Absorption of ammonia from Ammonia-Air Mixture by liquid water Absorption of CO2 from flue gas by amine solvents Removal of H2S from natural gas In above examples gas is absorbed at the interface of two phases and gets transported to the bulk of liquid 9  Two film theory In many cases with gas-liquid transfer we have transfer considerations from both sides of the interface. Therefore, we use the Lewis-Whitman (1923) two-film model as described below. It is a physical model that says that when a gas comes into contact with the liquid, the existence of a gas-liquid interface can be seen, forming a gaseous and a liquid two layers, through which the gas is transferred by molecular diffusion. A typical example: Your native language is Arabic, but my native language is Chinese. We communicate with each other in English. Because of the two layers of language resistance, it is very difficult for us to communicate with each other. So we have to slow down the speed of language communication. 10  Terminology Concentrations Liquid Phase Gas/Vapour Phase Partial pressure* Mass fraction Mass fraction Mole fraction Mole fraction Mass concentration Mass concentration Mole concentration Mole concentration 11  Terminology Concentrations 12 Terminology 13 Terminology 14  Terminology Relation Between Partial Pressure and Concentration 15  Terminology Ideal Gas Mixtures: 16 Terminology 17 www.udst.edu.qa