A complete and to the point guide to Pharmacy 4rth prof students regarding mass transfer and its implementation on industrial scale.

1. Mass transfer
SamiaGhani
M.PhilPharmaceutics

2. • Mass transfer is the net movement of mass from one location,
usually meaning stream, phase, fraction or component, to
another.
• Mass transfer occurs in many processes, such as
absorption, evaporation, drying, precipitation,
membrane filtration, and distillation.
• Mass transfer is used by different scientific disciplines for
different processes and mechanisms.

3.  Occurs through different mechanisms such as molecular diffusion, convection / bulk
flow & turbulent mixing
Extraction – transfer of solute
Humidification – transfer of water
molecule Evaporation
 Complex phenomenon occurs almost in all unit operations
Drying
Distillation
simultaneous heat & mass transfer

4. Mass Transfer
4
Movement of the molecule occurs due to concentration gradient known
as molecular diffusion.
Molecular Diffusion
In Gases In Liquid

5. Molecular diffusion in gases:
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partition
 GasAmoves towards chamber B and gasAtowards chamberA.
 Concentration of A with distance towards chamber B &B towards A, variation
in concentration of component with distance in the system called concentration
gradient.
 Movement of molecule A or B occurs due toconcentration gradient known as
molecular diffusion.
Gas A Gas B
dx
CA decreasing
CB decreasing

6. DIFFUSION
• It is defined as a process of mass transfer of individual
molecules of a substance brought about by random
molecular motion and associated with a driving force
such as a concentration gradient.
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7. 4

9. Mass TransferMechanisms
1. Convective Mass
Transfer
2.
Diffusion
http://www.timedomaincvd.com/CVD_Fundamentals/xprt/xprt_conv_diff.h

10. Convective transport
• Convective transport occurs when a constituent of the fluid
(mass, energy, a component in a mixture) is carried along
with the fluid.
• Convection is mass transfer due to the bulk motion of a fluid.
For example, the flow of liquid water transports molecules or
ions that are dissolved in the water. Similarly, the flow of air
transports molecules present in air, including both
concentrated species (e.g., oxygen and nitrogen) and dilute
species (e.g., carbon dioxide)

11. Mass TransferMechanisms
Convective and Diffusion

12. Mass transfer in turbulent & laminar flow:
Mrs. Poonam Purkar, GES'S 1/8/2018
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Explained by boundary layer or film theory
when fluid flows adjacent to the surface forms the boundary layer
Considers two regions
 boundary layer
 bulk
• If bulk flows in laminar fashion – rate of mass transfer
depends given by molecular diffusion equation
• If fluid bulk is turbulent – mass transfer depends upon transfer rate across the
boundary layer.

13. Boundary layer consist of 3 sub layers
 Laminar sub layer adjacent to surface
 Buffer / transient sub layer
 Turbulent region towards the bulk of fluid.
Turbulent layer : eddies move under inertial forces causing mass transfer. The
rate of mass transfer is high and conc. gradient is low
Buffer layer : combination of eddy and molecular diffusion responsible for mass
transfer
Laminar sub layer : molecular diffusion is the only mechanism of mass
transfer. Concentration gradient is high and rate of mass transfer is low.
The rate of mass transfer can be estimated by considering a film which offers the
resistance equivalent to boundary layer.
Mrs. Poonam Purkar, GES'S 1/8/2018
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14. Let,
PAi = partial pressure of A atsurface
PAl = partial pressure of Aat laminar sub layer
According to Fick’s law for diffusion,
We know, therefore
where, CAi & CAb concentration of A on either side of the film.
X'
of thickness X
PAb= partial pressure of A at the edgeof boundary
layer.
conc.grade
int 
PAi – PAb
.
Mrs. Poonam Purkar, GES'S 1/8/2018
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RT X'
D PAi – PAb
NA 
X’ is not known , hence kg constant known as mass transfer coefficient isintroduced.
NA  kg.CAi CAb
RT
CA 
PA
NA  kg.CAi CAb

15. Interphase Mass Transfer:
Mrs. Poonam Purkar, GES'S 1/8/2018
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Involves two phase mass transfer
e.g. distillation, liquid-liquid extraction.
Different theories involved :
 Two film theory
 Penetration theory
 Surface Renewal theory

16. Two film theory
Mrs. Poonam Purkar, GES'S 1/8/2018
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Theory has been developed by Nernst, Lewis and Whitman.
Postulates that two non-turbulent fictitious films are present on either side of the
interface between thw film
Mass transfer across these films purely occurs molecular diffusion.
Total resistance for mass transfer is summation of resistance of two films

17. Penetration theory:
Mrs. Poonam Purkar, GES'S 1/8/2018
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This theory proposed by Higbie
considers unsteady state at interface
Fluid eddies travel from bulk to interface by convection & remain remain there
for equal but limited period of time
When eddies comes at interface, solute moves into it by molecular diffusion &
get penetrated into bulk when eddies moves to bulk.
According to this theory, rate of mass transfer directly proportional to square root
of molecular diffusion and inversely proportional to exopsure time of eddies at
interface.

18. Surface renewal theory:
Mrs. Poonam Purkar, GES'S 1/8/2018
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This theory proposed by Dankwort
Each eddies gets equal exposure time at interface
Continuous renewal of interface by fresh eddies which have composition that of
bulk
Turbulent eddies remain at interface for time varying from 0 to ∞ and taken back
into bulk phase by convection current.
According to this theory, rate of mass transfer is directly proportional to
square root of molecular diffusivity.