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Mass transfer is the net movement of mass from one location to another. Mass transfer is used by different scientific disciplines for different processes and mechanisms. The phrase is commonly used in engineering for physical processes that involve molecular and convective transport of atoms and molecules within physical systems.
Some common examples of mass transfer processes are the evaporation of water from a pond to the atmosphere; the diffusion of chemical impurities in lakes, rivers, and oceans from natural or artificial point sources; separation of chemical components in distillation columns. In Cooling towers, hot water flows down over the fill material as air flows up and contact between water and air evaporates some of the water. Evaporation requires heat; the heat is removed from the remaining water lowering its temperature.
In astrophysics, mass transfer is the process by which matter gravitationally bound to a body, usually a star, fills its Roche lobe and becomes gravitationally bound to a second body, usually a compact object (white dwarf, neutron star or black hole), and is eventually accreted onto it. It is a common phenomenon in binary systems, and may play an important role in some types of supernovae and pulsars.
Mass transfer finds extensive application in chemical engineering problems. Often, chemical species transfer between two phases through an interface or diffusion through a phase. The driving force for mass transfer is a difference in concentration; the random motion of molecules causes a net transfer of mass from an area of high concentration to an area of low concentration. For separation processes, thermodynamics determines the extent of separation, while mass transfer determines the rate at which the separation will occur. The amount of mass transfer rate can be quantified through the calculation and application of mass transfer coefficients.
Analogies between heat, mass, and momentum transfer
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