Absorption (Lec # 2)
Absorption
(Lec
#
2)
DMT bJ dPE
Process Mass & Momentum Transfer
D
r.
M
.
T
ayye
b
J
ave
d
,
PE
Room C-217
Ext. 3243
Email: [email protected]
Ref.:
Chemical En
g
ineerin
g
b
y
Richardson and Coulson Vol. 2
ggy
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Diffusion-Absorption-Lecture Slides, Slides of Technical Writing
This lecture was delivered by Kiran Ramesh at Allahabad University for Absorption course. It includes: Absorption, Diffusion, Concentration, Gradient, Equalize, Continuous, Convection, Eddy, Turbulent
Typology: Slides
2011/2012
1 / 15
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Absorption (Lec # 2)Absorption^ (Lec # 2)^ D^ M T^
Process Mass & Momentum Transfer b J^ d PE^ Dr. M. Tayyeb Javed, PE^ Room C-217Ext. 3243 Email: [email protected] Ref.:Chemical Engineering by Richardson and Coulson Vol. 2g^ g^ y
Diffusion ^ Diffusion^ is^ the^
movement,^ under^
the influence^ of^ a^ physical
stimulus^ of^ an influence^ of^ a^ physical
stimulus,^ of^ an individual component through a mixture A^ concentration^ gradient
tends^ to^ move^ the A^ concentration^ gradient
tends^ to^ move^ the component in such a direction as to equalizeconcentrations and destroy the gradient Drop^ of^ ink^ in^ water,
ammonia^ in^ air, fragnance of roses, smell of petrol Can be continuous diffusion Can be continuous diffusion Molecular^ Diffusion^
and^ convection^ (eddy diffusion))
Diffusion… •^ Molecular diffusion does not have asignificant role to play in mass transfer in asignificant role to play in mass transfer in aturbulent medium except in a region veryclose to the phase boundary •^ Molecular diffusion is governed by Fick’sLaw
Fick’s Law of Diffusion ^ Molar^ flux^ of^
a^ species^ moving
is proportional to the concentration gradient ofproportional to the concentration gradient ofthe species If A diffuses in a binary mixture of A & B, the
y^ , flux of A is D is^ ‘diffusion^ AB^
coefficient’^ or^ the
‘diffusitivity’ of A in a mixture of A and B‘diffusitivity’ of A in a mixture of A and B dC^ Aj^ aAAd
j^ dz dCAj^ D^^ = - A^ ABdz^ dz^5 docsity.com
Stefan’s Law •^ The concept of a stationary component may beenvisaged by considering the effect of movingenvisaged by considering the effect of movingthe box, in the opposite direction to that inwhich B is diffusing, at a velocity equal to its^ diffusion velocity, so that to external observer Bappears to be stationary.
Since the total mass transfer rate of B is zero, there must bea ‘bulk flow’ of the systemtowards the liquid surfaceexactly to counterbalance theexactly to counterbalance thediffusional flux
Stefan’s Law^ Thus the bulk flow enhances the massThus^ the bulk flow enhances the masstransfer rate by a factor C
/C^ , known asTB drift factor.
Equilibrium b/t Liquid and Gas… In an ammonia-air mixture containing 13.1 percent of ammonia,the^ partial^ pressure^ of^ the
(^2) ammonia is 13.3 kN/m and the maximum concentration of the ammonia in the water at 303 K is
10 maximum^ concentration^ of^ the
ammonia^ in^ the^ water^ at^303
K^ is 93 kg per 1000 kg of water
Equilibrium b/t Liquid and Gas… ^ Equilibrium distribution of a solute betweentwo phasestwo phases ^ Extent to which the solute can dissolve in theliquidq ^ Equilibrium gas solubility varies with T & P,nature of solute & liquidHi^ h^ th^ l^ t^
t^ ti^ i^ th ^ Higher the solute concentration in the gasphase,^ higher^ is^ the
solubility^ of^ the^ gas solute Solubility of a gas in a liquid decreases withtemp. almost as a ruleS^ b^ ti^ i^ d^
t l^ t^ t ^ So absorption is done at lower temperature
Henry’s Law ^ Equilibrium data for many non-ideal gas-liquidsystems at low concentrations can be expressed bysystems at low concentrations can be expressed byHenry’s Law^ * p^ Hx = p^ A^ A^ A^ A ^ H^ is Henry’s Law constant ^ Constant strongly increases with temperature ^ Other forms of Henry’s Law const^ *^ A^
y^ mx = A
*^ A^ Henry's law is used to quantify the solubility of gasesin solvents
¢ p H C = A
13 in^ solvents.