Chap 1(a)-Molecular Diffusion in Gases.ppt

8/19/2019 Chap 1(a)-Molecular Diffusion in Gases.ppt

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BKF 2432: MASS TRANSFERFKKSA, UMP

Principles of Mass Transfer

CHAPTER 1CHAPTER 1

Molecular Difusion inMolecular Difusion in

GasesGases

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Topic Outcomes

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It is expected that student will be able to:

Apply the diffusivity coefficient of molecular

diffusion in gases.

olve mathematical solution of molecular diffusion

in gases.


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CONTENTS

Mass Transfer

Molecular Diffusion Convective Mass Transfer

Gases Liquid Solid

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!"uimolar #ounter diffussionin $ases

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Molecular %iffusion in $ases

Equimolar Counter diffussion in Gases

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For a binary gas mixture of A and B, the diffusivity coefficient D AB= D BA

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Problem &.'(' )pg *+,- Diffusion of Methane

Through Helium

A gas of #* and e is contained in a tube at '/'.0, 1Pa pressure and

,23 4. At one point the partial pressure of methane is p A1 = &/.52 1Pa6

and at a point /./, m distance away6 p A2 7 ,/.,& 1Pa. If the total pressure

is constant throughout the tube6 calculate the flux of #* )methane- atsteady state for e"uimolar counter diffusion.

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!xample &.,(' )pg *'+- Equimolar Counterdiffusion

o Ammonia gas )A- is diffusing through a uniform tube /.'/ m long

containing 8, gas )9- at './'0, x '/+ Pa pressure and ,23 4. At point

'6 p A1 = './'0 x '/* Pa 6 and at point ,6 p A2 7 /.+/5 x '/* Pa. The

diffusivity %A9 7 /.,0/ x '/(* m,s.

'. '. #alculate the flux J* A at steady state

,. ,. ;epeat for J* B

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8umerical

value


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BKF 2432: MASS TRANSFER FKKSA, UMPProblem &.,(' )pg *+,- Equimolar Counterdiffusion of a Binar !as

Mi"ture

• elium and nitrogen gas are contained in a conduit + mm in diameter

and /.' m long at ,23 4 and a uniform constant pressure of './ atm

abs. The partial pressure of e at one end of the tube is /./&/ atm and

the other end is /./,/ atm. #alculate the following for steady(state

e"uimolar counterdiffusion:

'. >lux of e in 1g mols.m,

'. ,. >lux of 8,

,. 0. Partial pressure of e at a point /./+ m from either end.

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%iffusion of $ases A and 9 Plus#onvection )$eneral #ase- )pg *'&-

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Convection is the concerted, collective

movement of ensembles of molecules

within fluids (e.g., liquids, gases)


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?A@ %iffusing Throughtagnant6 8ondiffusing ?9@

)pecial #ase-

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!xample &.,(, )pg *'2- Diffusion of #aterThrough $tagnant% &ondiffusing Air

ater in the bottom of a narrow metal tube is held at a constant

temperature of ,/o#. The total pressure of air )assumed dry- is'./ atm

and the temperature is ,/o#. ater evaporates and diffuses through

the air in the tube6 and the diffusion path B, C B' is /.'+,* m )/.+ ft-

long. The diagram is similar to >ig &.,(,a. #alculate the rate ofevaporation at steady state in lb molhr.ft, and 1g mols.m,. The

diffusivity of water vapor at ,/o# and ' atm pressure is /.,+/x'/(*

m,s. Assume that the system is isothermal.

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Problem &.,(0 )pg *,0- Diffusion of A Through

$tagnant B and Effe't of Tpe of Boundar on (lu"

Ammonia gas is diffusing through 8, under steady state conditions

with 8, nondiffusing since it is insoluble in one boundary. The total

pressure is './'0 x '/+ Pa and the temperature is ,23 4. The partial

pressure of 80 at one point is '.000 x '/* Pa6 and at the other point

,/ mm away it is &.&&& x '/0 Pa. The D AB for the mixture at './'0 D '/+

Pa and ,23 4 is ,.0/ x '/(+ m,s.

a- calculate the flux of 80 in 1g mols.m,

b- do the same as )a- but assume that 8, also diffuses6 both

boundaries are permeable to both gases and the flux is e"uimolar

counterdiffusion. In which case is the flux greaterE

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%iffusion Through #rossectional Area )phere-

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!xample &.,(* )pg *,'- E)aporation of &aphthalene $phere

A sphere of naphthalene having a radius of ,./ mm is suspended in a

large volume of still air at 0'3 4 and './'0,+ x '/+ Pa. The surface

temperature of the naphthalene can be assumed to be at 0'3 4 and its

vapor pressure at 0'3 4 is /.+++ mm g. The D AB of naphthalene in airat 0'3 4 is &.2, x '/(& m,s. #alculate the rate of evaporation of

naphthalene from the surface.

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Problem &.,(+ )pg *+0- Mass Transfer from a &aphthalene $phere to Air

Mass transfer is occurring from a sphere of naphthalene having radius

of '/ mm. The sphere is in large volume of still air at +,.&F# and ' atm

abs pressure. The vapor pressure of naphthalene at +,.&F# is './

mmg. The diffusitivity of naphthalene in air at /F# is +.'& x '/ (& m,s.

#alculate the rate of evaporation of naphthalene from the surface in1g mols.m,.

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%iffusion #oefficientfor $ases

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!xample &.,(+ )pg *,5- Estimation of Diffusi)it of a !as Mi"ture

• ormal butanol !A" is diffusing through air !B" at # atm abs.

$sing the Fuller et al. method, estimate the diffusivity D AB for

the follo%ing tem&eratures and com&are %ith the ex&erimental

data. 'Given M A !butanol" = ().# *g !mass"+*g mol, M B !air" =

- *g !mass"+*g mol

#. For /oC.

. For 0.-oC

1. For /o

C and ./ atm abs

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8umericalvalue


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21st (EB-.A-/ 2+1,

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Chap 1(a)-Molecular Diffusion in Gases.ppt

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