, driving force - concentration gradient -
component of a mixture -
/ mechanism - , , ,
, molecular diffusion -
thermal diffusion - to arise from - to result from - - pressure diffusion - by virtue of - forced diffusion - /
interface - ,
, .-. to confront - humidification - cutting - welding - ablation - heat shield - deaeration - feed water - steam boiler - heat treatment -
waste treatment -
eddy current - ,
non-equilibrium -
mode - , , , ,
, , to dominate - ,
moisture laden air -
subsequent precipitation - , / to be concerned with -
I. ,
1. What definition of mass transfer can you give?
2. What mechanisms of mass transfer are mentioned in the text?
3. Can you explain the difference between the words "mechanism" and "mode"?
4. What examples of mass transfer does the author give?
5. Can you add any other examples?
II. :
Forced convection mass transfer Interphase mass transfer Molecular mass transfer Convective mass transfer Moisture laden air
TEXT A
1. In this chapter another driving force, concentration gradient, is introduced. This driving force causes the transport of a component of a mixture from a region of high concentration to a region of low concentration. The transport process is known as mass transfer. The mechanisms of mass transfer are varied. They can be classified into eight types: 1. Molecular (ordinary) diffusion, resulting form a concentration gradient. 2. Thermal diffusion, arising from a temperature gradient. 3. Pressure diffusion, which occurs by virtue of a pressure gradient. 4. Forced diffusion, resulting from external forces other than gravity. 5. Forced- convection mass transfer. 6. Natural-convection mass transfer. 7. Turbulent mass transfer resulting from eddy currents in a fluid. 8. Interphase mass transfer occurring by virtue of non-equilibrium at an interface.
2. These types divide naturally into two distinct modes of transport. The first four are molecular mass transfer; the last four are convective mass transfer. Although the two modes often occur simultaneously, one mode usually dominates and we can understand the mechanisms better by considering them separately.
3. Examples of mass transfer in everyday life are legion: the diffusion of sugar in a cup of coffee; vaporization of water in a tea-kettle; the movement of moisture- laden air over the ocean with its subsequent precipitation on dry land; combustion and air-conditioning process, cloud formation; clothes drying. The chemical engineer is concerned with gas absorption, separation, crystallization and extraction, the mechanical engineer confronts the mass-transfer process in humidification, drying, cutting and welding metals, ablation of heat shields in high-speed flight, deaeration of feed water in steam boilers, and the production and heat treatment of metals; and civil engineers make use of mass transfer in waste treatment.
