Editing Mass transfer

{{short description|Net movement of mass from one location, phase, etc. to another}}
{{distinguish|Weight transfer}}
{{More citations needed|date=December 2009}}
{{chemical engineering}}

'''Mass transfer''' is the net movement of mass from one location (usually meaning  stream, [[Phase (matter)|phase]], fraction, or component) to another. Mass transfer occurs in many processes, such as [[Absorption (chemistry)|absorption]], [[evaporation]], [[drying]], [[precipitation (chemistry)|precipitation]], [[Membrane technology|membrane filtration]], and [[distillation]]. 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 diffusion|diffusive]] and [[convection|convective]] transport of [[chemical species]] within [[system|physical systems]].

Some common examples of mass transfer processes are the [[evaporation]] of [[water]] from a pond to the [[Earth's atmosphere|atmosphere]], the purification of blood in the [[kidney]]s and [[liver]], and the distillation of alcohol. In industrial processes, mass transfer operations include separation of chemical components in distillation columns, absorbers such as scrubbers or stripping, adsorbers such as activated carbon beds, and [[liquid-liquid extraction]]. Mass transfer is often coupled to additional [[transport phenomena|transport processes]], for instance in industrial [[cooling tower]]s. These towers couple heat transfer to mass transfer by allowing hot water to flow in contact with air. The water is cooled by expelling some of its content in the form of water vapour.

==Astrophysics==

In [[astrophysics]], mass transfer is the process by which [[matter]] [[gravitation]]ally 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 star|binary systems]], and may play an important role in some types of [[supernova]]e and [[pulsar]]s.

==Chemical engineering==

Mass transfer finds extensive application in [[chemical engineering]] problems. It is used in reaction engineering, separations engineering, heat transfer engineering, and many other sub-disciplines of chemical engineering like electrochemical engineering.<ref>Electrochimica Acta 100 (2013) 78-84. https://doi.org/10.1016/j.electacta.2013.03.134</ref>

The driving force for mass transfer is usually a difference in [[chemical potential]], when it can be defined, though other [[thermodynamics|thermodynamic gradients]] may couple to the flow of mass and drive it as well. A chemical species moves from areas of high chemical potential to areas of low chemical potential. Thus, the maximum theoretical extent of a given mass transfer is typically determined by the point at which the chemical potential is uniform. For single phase-systems, this usually translates to uniform concentration throughout the phase, while for multiphase systems chemical species will often prefer one phase over the others and reach a uniform chemical potential only when most of the chemical species has been absorbed into the preferred phase, as in [[liquid-liquid extraction]].

While thermodynamic equilibrium determines the theoretical extent of a given mass transfer operation, the actual rate of mass transfer will depend on additional factors including the flow patterns within the system and the [[mass diffusivity|diffusivities]] of the species in each phase. This rate can be quantified through the calculation and application of [[mass transfer coefficient]]s for an overall process. These mass transfer coefficients are typically published in terms of [[dimensionless quantities|dimensionless numbers]], often including [[Péclet number]]s, [[Reynolds number]]s, [[Sherwood number]]s, and [[Schmidt number]]s, among others.<ref name="basictext">{{cite book
|title=Fundamentals of momentum, heat, and mass transfer
|edition=2
|first1=James R.
|last1=Welty
|first2=Charles E.
|last2=Wicks
|first3=Robert Elliott
|last3=Wilson
|publisher=Wiley
|year=1976
|isbn=9780471022497
|url=https://books.google.com/books?id=hZxRAAAAMAAJ}}
</ref><ref name="BSL">{{cite book|title=[[Transport Phenomena (book)|Transport Phenomena]]|edition=2|first1=R.B.|last1=Bird|first2=W.E.|last2=Stewart|first3=E.N.|last3=Lightfoot|publisher=Wiley|year=2007}}</ref><ref name="TaylorKrishna">{{cite book|title=Multicomponent Mass Transfer|first1=R.|last1=Taylor|first2=R.|last2=Krishna|publisher=Wiley|year=1993}}</ref>

==Analogies between heat, mass, and momentum transfer==
{{main|Transport phenomena (engineering & physics)|l1=Transport phenomena}}
There are notable similarities in the commonly used approximate differential equations for momentum, heat, and mass transfer.<ref name="basictext"/> The molecular transfer equations of [[Newtonian fluid|Newton's law]] for fluid momentum at low [[Reynolds number]] ([[Stokes flow]]), [[Heat conduction|Fourier's law]] for heat, and [[Fick's laws of diffusion|Fick's law]] for mass are very similar, since they are all [[linear approximation]]s to transport of conserved quantities in a flow field. 
At higher Reynolds number, the analogy between mass and heat transfer and momentum transfer becomes less useful due to the [[nonlinearity]] of the [[Navier–Stokes equation]] (or more fundamentally, the [[Momentum#Relating to force – general equations of motion|general momentum conservation equation]]), but the analogy between heat and mass transfer remains good. A great deal of effort has been devoted to developing analogies among these three transport processes so as to allow prediction of one from any of the others.

==References==
{{Reflist}}

==See also==
* [[Crystal growth]]
* [[Heat transfer]]
* [[Fick's laws of diffusion]]
* [[Fractionating column]]
* [[McCabe–Thiele method]]
* [[Vapor–liquid equilibrium]]
* [[Liquid–liquid extraction]]
* [[Separation process]]
* [[Binary star]]
* [[Type Ia supernova]]
* [[Thermodiffusion]]
* [[Accretion (astrophysics)]]

{{Chemical engg}}
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{{DEFAULTSORT:Mass Transfer}}
[[Category:Transport phenomena]]
[[Category:Mechanical engineering]]
[[Category:Heating, ventilation, and air conditioning]]