1. Introduction to Mass
Transfer
Mass transfer is a fundamental concept in bioprocess
engineering, involving the movement of substances
within a system. It plays a crucial role in various
bioprocesses, impacting the efficiency and
effectiveness of these processes.
ErikaY.OrtizMontoya
eortiz@icesi.edu.co
FacultaddeIngeniería,DiseñoyCienciasaplicadas
Febrero2024
2. Mass transfer
“Mass transfer occurs in mixtures containing concentration variations. For example,
when dye is dropped into a pail of water, mass transfer processes are responsible for the
movement of dye molecules through the water until equilibrium is established and the
concentration is uniform. Mass is transferred from one location to another under the
influence of a concentration difference or concentration gradient in the system.
There are many situations in bioprocessing where the concentrations of compounds are
not uniform; we rely on mechanisms of mass transfer to transport material from regions
of high concentration to regions where the concentration is initially low.”
3. 1 2
Mass transfer
Oxygen supply in fermenters for aerobic
culture. The oxygen concentration on the
bubble's surface is higher than that of the
liquid. This concentration gradient promotes
oxygen transfer from the bubbles to the
medium.
Penicillin is extracted from fermentation broth
using organic solvents. When the solvent is added
to the medium, the high concentration of
penicillin in the aqueous phase and the low
concentration in the organic phase cause the
mass transfer of penicillin to the solvent.
4. MOLECULAR DIFFUSION
If the gradient is maintained by constantly supplying material to the region of high
concentration and removing it from the region of low concentration, diffusion will
be continuous. This situation is often exploited in mass transfer operations and
reaction systems.
Molecular diffusion is the movement of
component molecules in a mixture under the
influence of a concentration difference in the
system.
Diffusion of molecules occurs in the direction required to destroy
the concentration gradient, that is, from regions of high
concentration to regions of low concentration
5. Diffusion Theory
Concentration gradient of component A inducing mass
transfer across area a.
Consider a closed system containing
molecular components A and B. Initially,
the concentration of A in the system is not
uniform; as indicated in the figure,
concentration CA varies from CA1 to CA2
as a function of distance y.
In response to this concentration
gradient, molecules of A will diffuse away
from the region of high concentration until
eventually the whole system acquires
uniform composition.
If there is no large-scale fluid motion in
the system (e.g., due to stirring), mixing
occurs solely by molecular movement.
6. Assume that mass transfer of A occurs across area a perpendicular to the direction
of
diffusion. In single-phase systems, the rate of mass transfer due to molecular
diffusion is given by Fick’s law of diffusion, which states that the mass flux is
proportional to the concentration gradient:
JA is the mass flux of component A, NA is the rate of
mass transfer of component A, a is the area across
which mass transfer occurs, DAB is the binary
diffusion coefficient or diffusivity of component A in a
mixture of A and B, CA is the concentration of
component A, and y is distance. dCA/dy is the
concentration gradient, or change in concentration of
A with distance.
Diffusion Theory
7. Fick's Law of Diffusion
Principle Basis
Fick's law describes the
diffusion process
quantitatively, relating the flux
of a substance to the
concentration gradient and
diffusivity.
Application in Bioprocesses
This law provides a
fundamental framework for
understanding and predicting
mass transfer phenomena,
guiding the design and
optimization of bioprocess
operations.
8. MASS
TRANSFER G-
L
An important example of mass transfer in
bioprocessing is the supply of oxygen in
fermenters for aerobic culture. The
concentration of oxygen at the surface of air
bubbles is high compared with that in the bulk
of the liquid; this concentration gradient
promotes oxygen transfer from the bubbles into
the medium.
9. Definition of Molecular Diffusion
1 Intrinsic Process
Molecular diffusion
refers to the
spontaneous movement
of molecules from an
area of higher
concentration to an area
of lower concentration,
driven by the random
motion of molecules.
2 Rate of Diffusion
The rate of molecular
diffusion is influenced by
factors such as
concentration gradient,
temperature, and the
diffusing species'
properties.
3 Role in Bioprocesses
Molecular diffusion is
crucial for the transport
of nutrients, gases, and
metabolites in
bioprocesses, affecting
overall productivity and
product quality.
10. Importance of Molecular Diffusion in
Bioprocesses
Cellular Nutrient
Uptake
Molecular diffusion enables
the supply of essential
nutrients to cells,
supporting their growth and
metabolic activities within
bioprocess environments.
Waste Product
Removal
Efficient molecular diffusion
facilitates the removal of
waste products generated
during cellular activities,
maintaining optimal
conditions for
bioprocesses.
Gas Exchange
Biological systems rely on
molecular diffusion for the
exchange of gases, such
as oxygen and carbon
dioxide, necessary for
respiration and metabolic
processes.
11. Role of diffusion in bioprocessing
¿why is diffusive transport
important?
1 Scale of mixing
Mixing on a molecular scale
therefore relies on diffusion as
the final step in the mixing
process.
2 Solid-phase reaction.
Mass transfer within solid
particles is usually unassisted
by bulk fluid convection;
therefore, the only mechanism
for intraparticle mass transfer is
molecular diffusion.
3 Mass transfer across phase boundaries
When different phases come into contact, the fluid velocity near the phase
interface is decreased significantly and diffusion becomes crucial for mass
transfer (Oxygen transfer from gas bubbles to fermentation broth, penicillin
recovery from aqueous to organic liquid, and glucose uptake from liquid
medium into mould pellets)
12. Film theory: Mass transfer between pha
Mass transfer of solute from one phase to
another involves transport from the bulk of
one phase to the phase boundary or
interface, then movement from the interface
into the bulk of the second phase.
The film theory is based on the idea that a
fluid film or mass transfer boundary layer
forms wherever there is contact between two
phases.
13. Film theory: Mass transfer between pha
According to the film theory, turbulence in each
fluid dies out at the phase boundary. A thin film of
relatively stagnant fluid exists on either side of the
interface; mass transfer through this film is effected
solely by molecular diffusion.
Most of the resistance to mass transfer resides in
the liquid films rather than in the bulk liquid. For
practical purposes it is generally assumed that
there is negligible resistance to transport at the
interface itself; this is equivalent to assuming that
the phases are in equilibrium at the plane of
contact.
14. Applications of Diffusion Theory in
Bioprocesses
Biological Separations
Diffusion theory drives the
development of precise
separation techniques for
isolating biomolecules and
cells, essential in bioprocess
purification and analysis.
Drug Delivery Systems
Understanding diffusion
mechanisms is critical for
designing efficient drug
delivery systems, ensuring
targeted and controlled
distribution of
pharmaceutical agents.
Bioremediation
Strategies
Diffusion principles underpin
bioremediation approaches,
facilitating the removal of
pollutants and contaminants
from environmental and
industrial settings.
15. Conclusion and Key Takeaways
Process Efficiency
Optimizing mass
transfer processes is
essential for enhancing
bioprocess efficiency,
influencing product
yield and quality.
Product Quality
Maintaining precise
control over molecular
diffusion contributes to
consistent product
quality and purity in
bioprocess
applications.
Continuous
Improvement
Constant research and
innovation in mass
transfer phenomena
are crucial for
advancing bioprocess
technologies and
capabilities.
