2. 2 Several important chemical engineering concepts in
Bioprocess Engineering are transport phenomena (fluid flow, mixing,
heat and mass transfer), unit operations, reaction engineering, and
bioreactor engineering. Fluid flow, mixing, and reactor engineering
are skipped in this class. They are available more detail in
several chemical engineering books. We start with the heat transfer
in bioreactors
3. In situ batch sterilization of liquid medium. In this
process, the fermenter vessel containing medium is heated using
steam and held at the sterilization temperature for a period of
time; cooling water is then used to bring the temperature back to
normal operating conditions Temperature control during reactor
operation. Metabolic activity of cells generates heat. Some
microorganisms need extreme temperature conditions (e.g.
psycrophilic, thermophilic microorganisms) Heat transfer
configurations for bioreactors: jacketed vessel, external coil,
internal helical coil, internal baffle-type coil, and external heat
exchanger. 3
4. External jacket and coil give low heat transfer area. Thus,
they are rarely used for industrial scale. Internal coils are
frequently used in production vessel; the coils can be operated
with liquid velocity and give relatively large heat transfer area.
But the coil interfere with the mixing in the vessel and make
cleaning of the reactor difficult. Another problem is film growth
of cells on the heat transfer surface. External heat exchanger unit
is independent of the reactor, easy to scale up, and provide best
heat transfer capability. However, conditions of sterility must be
met, the cells must be able to withstand the shear forces imposed
during pumping, and in aerobic fermentation, the residence time in
the heat exchanger must be small enough to ensure the medium does
not become depleted of oxygen. 4
5. Double-pipe heat exchanger Shell and tube heat exchanger
Plate heat exchanger Spiral heat exchanger In bioprocess, the
temperature difference is relatively small. Thus, plate heat
exchanger is almost never being used The concepts and calculation
for heat exchangers and their configurations are available in the
text book ( Pauline Doran, Bioprocess Eng Principle, chapter 8)
5
6. Mass Transfer in Fermentation
7. dy dC DJ A ABA = 7 The Ficks law of diffusion Role of
diffusion in Bioprocess Scale of mixing Mixing on a molecular scale
relies on diffusion as the final step in mixing process because of
the smallest eddy size Solid-phase reaction The only mechanism for
intra particle mass transfer is molecular diffusion Mass transfer
across a phase boundary Oxygen transfer to gas bubble to
fermentation broth, penicillin recovery from aqueous to organic
liquid, glucose transfer liquid medium into mould pellets are
typical example.
8. 8 The two film theory is a useful model for mass transfer
between phase. Mass transfer of solute from one phase to another
involves transport from bulk of one phase to the interface, and
then from the interface to the bulk of the second phase. This
theory is based on idea that a fluid film or mass transfer boundary
layer forms whenever there is contact between two phases. According
to film theory, mass transfer through the film is solely by
molecular diffusion and is the major resistance. CA1i CA1 Bulk
fluid 1 Bulk fluid 2 CA2i CA2 Film 2 Film 1
9. ( ) ( )AGiAGGAG ALALiLAL CCakN CCakN = = ( )AiAoA CCkaN = 9
It refers to mass transfer occurring in the presence of bulk fluid
motion k: mass transfer coefficient [m/s] a: area available for
mass transfer [m2 /m3 ] CAo: concentration of A at bulk fluid CAi:
concentration of A at interface For gas-liquid system, A from gas
to liquid:
10. ( )ALALLA LGG CCaKN ak m akaK = += * '11 10 Refers to the
book Geankoplis (2003), Transport Processes and Separation Process
Principles, 4th ed, chapter 10.4. Oxygen transport to fermentation
broth can be modeled as diffusion of A through stagnant or
non-diffusing B. If A is poorly soluble in the liquid, e.g. oxygen
in aqueous solution, the liquid-phase mass transfer resistance
dominates and kGa is much larger than kLa. Hence, KLa kLa.
11. Eight steps involved: i. Transfer from the interior of the
bubble to the gas-liquid interface ii. Movement across the
gas-liquid interface iii. Diffusion through the relatively stagnant
liquid film surrounding the bubble iv. Transport through the bulk
liquid v. Diffusion through the relatively stagnant liquid film
surrounding the cells vi. Movement across the liquid-cell interface
vii. If the cells are in floc, clump or solid particle, diffusion
through the solid of the individual cell viii. Transport through
the cytoplasm to the site of reaction. 11
12. i. Transfer through the bulk phase in the bubble is
relatively fast ii. The gas-liquid interface itself contributes
negligible resistance iii. The liquid film around the bubble is a
major resistance to oxygen transfer iv. In a well mixed fermenter,
concentration gradients in the bulk liquid are minimized and mass
transfer resistance in this region is small, except for viscous
liquid. v. The size of single cell