Biomedical Engineering Reference
In-Depth Information
Resistances associated to either of these transfer steps caused that substrate concentration
in the site of biochemical reaction is lower than in the gas phase or in the bulk liquid and,
therefore, the overall rate of biological treatment decreased (Figure 3). The relative
importance of these resistances in providing a process rate limitation is influenced by
numerous factors, including the size and shape of the biomass aggregates, fluid and substrate
properties, intrinsic reaction kinetics of the microorganisms and the hydrodynamics of the
bulk phase. In this aspect, agitation and aeration are generally used to improve mass transfer
rate in reactors due to the hydrodynamic turbulence caused by them. They also can be used to
improve the performance of the reactors by enhancing the fluid mixing and controlling the
thickness of biofilms (Ebrahimi et al., 2006). However, the energy required to induce and
maintain the physical mass transfer pathways in the systems should be also considered.
A
(g)
+ B
(l)
→
R
(g)
A
(g)
+ B
(l)
→
R
(g)
C
Ai
C
Ai
C
B
C
B
P
A
P
A
C
A
C
A
C
Bs
C
Bs
C
As
C
As
P
Ai
P
Ai
Gas
phase
Gas
phase
Gas
phase
Gas
phase
Liquid
phase
Liquid
phase
Liquid
phase
Liquid
phase
Biomass
Biomass
Figure 3. Substrates profiles caused by the mass transfer resistances (P
A
: Partial pressure of A in the gas
phase; P
Ai
: Partial pressure of A in the interphase gas-liquid; C
Ai
: Concentration of A in the interphase
gas-liquid (in equilibrium with P
Ai
); C
A
, C
B
: Concentration of A and B in the bulk liquid; C
As
, C
Bs
:
Concentration of A and B inside the biomass aggregate).
Generally, agitation and aeration rates are the most critical parameters used for scale-up
biological treatment process and play significant roles in determining the efficiency of the
process. For this reason, great efforts were carried out in order to obtain engineering
correlations to the scaling-up of biochemical reactors which allows calculating shear stress
and mass transfer coefficients as a function of operational parameters such as superficial gas
and liquid velocities, stirring velocity or energy input.
