1. Immobilized Cell System
Provide high cell concentration
Reuse cell
Elimination of washout problem at high dilution rate and cell recovery
May provide favorable microenvironmental conditions
May prove genetic stability
Protect against shear damage
Can perform multi-step biosynthesis reactions that are not practical purified immobilized enzyme preparation.
Disadvantages:
Diffusional limitation are important.
Growth and gas evalution may lead to significant mechanical disruption of the immobilizing matrix.

2. Immobilized Cell System
Immobilized methods:
- Active immobilization of cells
Entrapment and Binding
similar to immobilized enzyme except covalently binding (toxic agents, e.g. glutaraldehyde)
- Passive immobilization of cells: biological film.
Multi-layer growth of cells on solid surface.
e.g. common in biological waste water treatment and mold fermentation
thick biofilm: high concentration but diffusion problem
Bioreactor consideration:
- low hydrodynamic shear: packed-column, fluidized-bed or airlift reactors.

3. Bioreactor Operation Consideration
Mixing - Agitation and Aeration
To provide adequate mixing of its contents is one the most factors to consider in design a fermenter.
- to disperse the air bubbles
- to suspend the organism cells
- to enhance heat and mass transfer in the medium
Agitation: providing mixing
Aeration: providing oxygen and mixing in aerobic systems

4. Bioreactor Operation Consideration
Some basic bioreactor types for aerobic cultivation of suspended cells:
Reactors with Internal mechanical agitation.
high KLa, flexible, medium with high viscosity
Bubble column relying on gas sparging for agitation
must avoid plugging of spargers
Loop reactors: mixing and liquid circulation are induced by the motion of an injected gas, by a mechanical pump, or by a combination of the two.
similar to the above.

5. Scale-up stirred-tank reactor bubble column jet loop reactor
airlift loop reactor
propeller loop reactor

6. Bioreactor Operation Consideration
- Internal mechanical Agitation
Rushton impeller: impeller diameter 30-40% of the tank diameter, 1980s
Axial flow system hydrofoil impellers:
increasingly popular
Baffle: to augment mixing and gas dispersion

7. Bioreactor Operation Consideration
Volumetric oxygen mass transfer coefficient determination
Unsteady state method:
An examined reactor filled with pure water or a medium, no organism cells.

8. Bioreactor Operation Consideration
- Heat removal: internal coil and jacketed vessel
Foam: increases pressure drop, decreases gas flow and provide a pathway for contaminated cells’ entry
- mechanical foam breaker
- surfactant (toxic)
The working volume is 75% of the total fermenter volume.
Sterility: pressurized steam for in-place sterilization of reactors, seals, probes and valves.
limited openings.
- Clean: spray balls for clean-in-place

9. Scale-up/down Scale-up:
For the optimum design of a product-scale fermentation system (prototype), the data on a small scale (model) must be translated to the large scale.
The fundamental requirement for scale up is that the model and the prototype should be similar to each other.
Two kinds of conditions must be satisfied to ensure similarity between the model and the prototype.
Geometric similarity of the physical boundaries
- Dynamic similarity of the flow fields

10. Scale-up/down Scale-up:
Geometric similarity of the physical boundaries:
- same type of reactor
- all linear dimensions of the model must be related to the corresponding dimensions of the prototype by a constant scale-up factor.
i.e. Keep the ratio of the height H to diameter Dt same in the model and prototype.
Normally H/Dt is 2~3.

11. Scale-up
For an example, 2 litre vessel (Vm) is scaled up to 2000 litre (Vp) fermenter by geometrical similarity, H/Dt=2, impeller diameter Di,m =3.24 cm what are the dimensions of the model (Hm, Dt,m), and prototype (Hp, Dt,p, Di,p)?

12. Scale-up/down Scale-up: Dynamic similarity of the flow fields
To achieve dynamic similarity in a stirred-tank reactor, the dimensionless numbers for the prototype and model should be equal.
Power number: P0/ρN3Di5
Renolds number: ρNDi2/µ
Froude: N2Di/g
P0: energy input (W); N: impeller speed (rpm); ρ: density (kg/m3)
Di: impeller diameter (m),30-40% of the diameter of the tank (Dt)
µ: viscosity (kg/ms); g: acceleration due to gravity (m/s2)
However, it is difficult to satisfy the dynamic similarity when more than one dimensionless group is involved in a system.

13. Scale-up/down Scale-up:
Design criteria related to dynamic properties in a stirred-tank reactor:
constant power input: P0/V or
constant liquid circulation rate inside the reactor: Q/V
(pumping rate of impeller per unit volume)
constant impeller tip speed: NDi
constant Renolds number: ρNDi2/µ
V: working volume
In scale-up, generally any of the above criterion should be met with geometric similar boundaries.

14. Scale-up/down Scale-up:
Relating the above criteria to impeller diameter Di and
speed N:

15. Scale-up/down Scale-up:
In scale-up of a stirred-tank reactor, the design calculations are as follows:
Determine the scale-up factor Dp/Dm
Calculate the dimensions of the prototype (height H and diameter Dt of tanks, impeller diameter Di) by multiplying that of the model with the scale-up factor.
Select criterion related to dynamic properties and keep it constant in both the model and the prototype.
Determine the parameters such as impeller speed for the scale-up reactor.

16. Scale-up
For an example, 2 litre vessel (Vm) is scaled up to 2000 litre (Vp) fermenter, (H/Dt)model=2, impeller diameter Di,m is 3.24 cm, impeller speed Nm is 500 rpm, what is the impeller speed of the larger reactor for
- constant impeller tip speed
- constant Renolds number

17. Scale-up/down Scale-down:
To provide an experimental system at a smaller scale that duplicates the environment that exists at the larger scale.
Mimic the production facilities at a smaller scale
Parameters can be tested more quickly and inexpensively than at the production scale.
Design calculations used in scale-down are the same as that in scale-up.
Please read the example 10.3