Lec9_Principles of Diffusion and Mass Transfer.ppt

Principles of Diffusion and Mass transfer

1. PRINCIPLES OF DIFFUSION
AND MASS TRANSFER
BETWEEN PHASES
Lecture No: 9

2. DIFFUSION
Diffusionisthemovement,undertheinfluenceofa
physicalstimulus,ofanindividualcomponentthrougha
mixture.
Definition:
Common Cause:
Concentrationgradient
What happened?
Aconcentrationgradienttendstomovethecomponentin
suchadirectionastoequalizeconcentrationsanddestroy
thegradient.
Whenthegradientismaintainedbyconstantlysupplying
thediffusingcomponenttothehigh-concentrationendof
thegradientandremovingitatthelow-concentrationend,
thereisasteady-statefluxofthediffusingcomponent.

3. DIFFUSION
High Concentration End Low Concentration End
Direction of diffusion
A -Substance
B -Substance
This is characteristic
of many mass-transfer
operations

4. DIFFUSION
Example of Mass Transfer with Diffusion:
Gas mixture with ammonia Water
Interface
Removing of Ammonia from gas mixture by absorption in water
•Steady-state diffusion
•Unsteady-state diffusion

5. DIFFUSION
What Other causes of diffusion?
Usual cause:Concentration Gradient
Activity Gradient (as in Reverse Osmosis)
Pressure Gradient
Temperature Gradient
External Force field (as in centrifuge)
Other Causes:
Molecular diffusion induced by temperatureisThermal Diffusion
Molecular diffusion induced by external fieldisForced Diffusion
Diffusionalsotakesplacewhenfluidsofdifferentcompositionsaremixed.
Thefirststepisoftenmasstransfercausedbytheeddymotion
characteristicofturbulentflow.ThisiscalledEddyDiffusion

6. ROLE OF DIFFUSION IN
MASS TRANSFER
Inallthemass-transferoperations,diffusionoccursinatleastonephaseand
ofteninbothphases.
InDistillation:thelowboilerdiffusesthroughtheliquidphasetotheinterfaceand
awayfromtheinterfaceintothevapor.Thehighboilerdiffusesinthereverse
directionandpassesthroughthevaporintotheliquid.
InLeaching:diffusionofsolutethroughthesolidphaseisfollowedbydiffusion
intotheliquid.
InLiquidExtraction:thesolutediffusesthroughtheraffinatephasetothe
interfaceandthenintotheextractphase.
InCrystallization:solutediffusesthroughthemotherliquortothecrystalsand
depositsonthesolidsurfaces.
InHumidificationorDehumidification:thereisnodiffusionthroughtheliquid
phasebecausetheliquidphaseispureandnoconcentrationgradientthroughit
canexist;butthevapordiffusestoorfromtheliquid-gasinterfaceintooroutof
thegasphase.
InMembraneSeparations:diffusionoccursinallthreephases:inthefluidson
eithersideofthemembraneandinthemembraneitself.

7. THIS IS NOT THE END,
BUT JUST BEGENING
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