This document discusses different types of reactor operations including plug flow reactors. Plug flow reactors operate with liquid flowing at a constant velocity and have concentration gradients of substrates and products developing along the direction of flow. Mass balance equations are developed to model an ideal plug flow reactor and equations are presented to calculate the reactor length and residence time required to achieve a desired conversion. Challenges with practical implementation of plug flow reactors are also discussed. A comparison of plug flow, continuous stirred-tank, and batch reactors is provided in terms of substrate conversion and product concentration.

1. Reactor Engineering

2. Reactor Engineering

3. Ideal reactor operation

4. Sterilization
-Batch heat sterilization of liquid
-Continuous heat sterilization of liquid
-Filter sterilization of liquid
-Sterilization of air

5. Continuous operation of a plug flow reactor
Plug flow operation is…..
• Alternative to mixed operation for continuous reactor.
• No mixing occurs in ideal plug-flow reactor
• This is achieve at high flow rate which minimize
backmixing and variation in liquid velocity.
• Plug-flow is most readily achieve in column or tubular
reactor.

6. Plug-flow reactor
• Operated in upflow or downflow mode or, in some case,
horizontally.
• Plug-flow tubular reactor are known
by the abbreviation PFTR.
• Liquid in PFTR flows at constant velocity.
• Reaction in the vessel proceeds, concentration gradient
of substrate and product develop in direction of flow.

7. • This exit concentration can be relate to the inlet
condition and reactor residence time.

8. Enzyme reaction
• To develop equation for plug flow enzyme reactor
• Consider a small section of reactor of length ∆z
as indicated in the figure.
• Steady state balance on substrate around the
section using mass balance equation

9. Enzyme reaction /mass-balance equation is :
( at steady state, left-hand side
(1) of equation is 0. )
•F = volumetric flow rate through the reactor
•Fs│z = mass flow rate of substrate entering the system
• s│z = Substrate concentration at z
•Fs│z+ ∆z = mass flow rate of substrate leaving the section
• vmax = maximum rate of enzyme reaction
• Km = Michaelis constant
•S = Substrate concentration
•A∆z = section volume where A is the cross-sectional area of the reactor

10. Enzyme reaction
• The volumetric flow rate (F ) divided by the section
volume (A∆z) is equal to the vmax (maximum rate of
enzyme reaction) divvied by Km (Michaelis constant):
(2)
• The volumetric flow rate (F )divided by the reactor
cross-sectional area (A) is equal to the superficial
velocity through the column ( u ) :
(3)

11. Enzyme reaction
• For F and A constant, u is also constant.
• Eq.(3) is valid for any section in the reactor of
thickness ∆z. For it to be valid at any point in the
reactor, take the limit as
∆z 0:
(4)
• apply the definition of differential from equation :
(differential equation for the
(5) substrate concentration gradient
through the length of plug-flow
reactor)

12. Enzyme reaction
• Assuming u and the kinetic parameter are constant,
• Eq.(5) is ready for integration.
• Separating variable and integrating with boundary
condition s = si at z = 0
• Gives an expression for the reactor length (L) require
to achieve an outlet concentration of sf
(6)

13. Enzyme reaction
• Residence time (t) for plug-flow reactor
parameter L and u :
(7)
• Therefore Eq 6. can written as
(6) (8)
• Eqs (6) and (8) allow to calculate the reactor length
and residence time require to achieve conversion
of substrate from concentration si to sf at flow
rate u .

14. Enzyme reaction
• Plug- flow operation is generally impractical for
enzyme conversions unless the enzyme is immobilised
and retained inside the vessel.
• For immobilised enzyme reactions affected by
diffusion, Eq.(5) must be modified to account
for mass- transfer effect :
(5) (9)
nT is the total effectiveness factor representing internal and external
mass transfer limitation, s is bulk substrate concentration,
and vmax and Km are intrinsic kinetic parameter.

15. Enzyme reaction
• Plug-flow operation with immobilized enzyme is most
likely to be approached in packed-bed reactor.
• Packing in column can cause substantial backmixing
and axial dispersion of liquid, thus interfering with
ideal plug flow.

16. Cell culture
• Analysis of plug-flow reactor for cell culture follows
the same procedure as for enzyme reaction.
• If the cell specific growth rate is constant ,
equal to µ max throughout the reactor
and cell death can be neglected.
• Equation for reactor residence time are analogous
to those derived in section cell culture for batch
fermentation,
- t is the reactor residence time
- x i is the biomass concentration at
(10) inlet
- x f is the biomass concentration
at outlet.

17. Cell culture
• Plug-flow operation is not suitable for cultivation of
suspended cells unless the biomass is recycle or there
is continuous inoculation of the vessel.
• Plug flow operation with cell recycle is used for large
scale wastewater treatment; however application are
limited.
• Even so, operating problems such as those mentioned
in Section packed bed mean that PFTRs are rarely
employed for industrial fermentation.

18. Comparison between major model of
reactor operation
• The relative performance of batch, CSTR and PFTR
reactors can be consider from a theoretical point of
view in term of the substrate conversion and product
concentration obtained from vessel of the same size.
• Because the total reactor volume is not fully utilized at all
times during fed-batch operation,
• It is difficult to include this mode of operation in a
general comparison.

19. Comparison between major model of
reactor operation
• Kinetic characteristics of PFTRs are the same as
batch reactor; the residence time required for
conversion in plug-flow reactor is the same as in a
mixed vessel operated in batch.
• The number of stages in a CSTR cascade increases,
the conversion characteristics of the entire
system approach those of an ideal plug-flow or
mixed batch reactor.

20. • Concentration change in PFTR, single CSTR and
multiple CSTR vessel.
• smooth dashed curve represent
the progressive decrease in substrate
concentration with time spent in a
PFTR or batch reactor;
concentration is reduce from si at
the inlet to sf at the outlet.
• single well-mixed CSTR operated with
the same inlet and outlet concentration,
because condition in vessel are uniform
In cascade of CSTRs, the concentration in uniform in each
reactor but there is a step-wise drop in concentration between
each stage.

21. Comparison between major model of
reactor operation
• The benefits associated with particular reactor
design or modes of operation depend on the
kinetic characteristics of the reaction.
• For zero-order reaction there is no difference
between single batch, CSTR and PFTR reactor
in term of overall conversion rate.
• For most reaction including first-order and
Michaelis-Menten conversions,
rate of reaction decrease as the concentration
of substrate decrease.

22. Comparison between major model of
reactor operation
• Reaction rate is therefore high at the start of batch culture
or at the enhance to plug-flow reactor because the
substrate level is greatest.
• Subsequently, the reaction velocity falls gradually as
substrate is consumed.
• In contrast, substrate entering CSTR is immediately diluted
to the final or outlet steady-state concentration so that the
rate of reaction is comparatively low for the entire reactor.
• For first-order and Michaelis-Menten reaction, CSTRs
achieve lower substrate conversion and lower product
concentration than batch reactor or PFTRs of the same
volume.

23. Comparison between major model of
reactor operation
• The comparison between reactors yields a different
result if the reaction is autocatalytic.
• Catalyst is produce by reaction in fermentation
processes; therefore, the volumetric rate of
reaction increase as the conversion proceeds
because the amount of catalyst builds up.
• Volumetric reaction rate continues to increase until
the substrate concentration becomes low, then it
declines due to substrate depletion.

24. Comparison between major model of
reactor operation
• Rate of conversion in chemostats operated close to
the optimum dilution rate for biomass productivity
are greater than in PFTR or batch reactors.
• For most fermentations, CSTRs offer significant
theoretical advantage over other modes of reactor
operation.
• Despite productivity benefits associated with CSTRs,
an overwhelming majority of commercial fermentations
are conducted in batch. The reasons with the
advantages associated with batch culture

25. Comparison between major model of
reactor operation
• Batch processes have a lower risk of contamination
than continuous-flow reactor; equipment and
control failures during long term continuous
operation are also potential problem.
• Continuous fermentation is feasible only when the
cells are genetically stable
• In contrast freshly-produced inocula are used in
batch fermentation giving closer control over the
genetic characteristics of the culture.

26. Comparison between major model of
reactor operation
• Continuous culture is not suitable for production
of metabolites normally formed near stationary
phase when the culture growth rate is low,
but productivity in a batch reactor is likely to
be greater under these conditions.
• Continuous fermentation must be operated for
lengthy periods to reap the full benefits of their
high productivity.

27. Evaluation of kinetic and yield
parameters in chemostat culture
• In steady-state chemostat with sterile feed and
negligible cell death, the specific growth rate (µ) is
equal to the dilution rate (D) .
• This relationship is useful for determining kinetic
and yield parameters in cell culture. If growth can
be modeling using Monod kinetics, for chemostat
culture,
µmax = maximum specific growth rate
Ks = substrate constant
• (11) s = the steady-state substrate
concentration in reactor

28. Evaluation of kinetic and yield
parameters in chemostat culture
• Eq. (11) gives the following linearised equation
which can be used for Lineweaver-Burk,
Edie-Hofstee and Langmir plots, respectively:
• (12)
(13)
•
• (14)

29. Evaluation of kinetic and yield
parameters in chemostat culture
• Chemostat operation is convenient for determining
true yields and maintanace coefficient for cell
culture.
• In chemostat culture with µ = D .
(11) (15)
Y xs = observed biomass yield from substrate
Y xs= true biomass yield from substrate
ms = maintenance coefficient

30. Graphical determination of maintenance coefficient
ms and Yxs using data from chemostat culture .
• Plot of 1/ Y xs Vs. 1/D gives
a straight line with slope (ms )
and intercept 1/Y xs
• In chemostat with sterile
feed, the observed biomass
yield from substrate Y xs is
obtained as follws ;
x = steady –state cell
(16) s = substrate concentrations
si = inlet substrate concentrations

31. Sterilization
• The methods available for sterilization including ;
o chemical treatment,
o exposure to ultraviolet,
o gamma and X -ray radiation,
o sonication,
o filtration and heating .
• Aspect of fermentor design and construction
for aseptic operation were considered in part
section (aseptic operation and fermentation
inoculation and sampling ).
• In this section consider design of sterilization
system for liquid and gasses.

32. Sterilization
Batch heat sterilization of liquids
• Liquid medium is most commonly sterilized in batch
in the vessel.
• Liquid is heated to sterilization temperature by
introducing steam into the coils or jacket of the
vessel or steam is bubbled directly into the medium,
or the vessel is heated electrically.
• If direct steam injection is used, allowance must be
made for dilution of the medium by condensate
which typically adds 10-20% to the liquid volume.

33. Sterilization
Batch heat sterilization of liquids
• Typical temperate-time profile for batch sterilization
is shown in below figure
(Variation of temperature with time for batch sterilization of liquid medium.)
• Depending on the rate heat
transfer from the steam or
electrical element ,
• The holding or sterilization
temperature is reached,
temperature is held constant
for a period of time t hd .
Cooling water in the coils or jacket of the fermentor is then
used or reduce the medium temperature to required value.

34. Sterilization
Batch heat sterilization of liquids
• Operation of batch sterilization systems, we must be able to
estimate the holding time required to achieve the desired level
of the cell destruction.
• Destroying contaminant organisms, heat sterilization also
destroys nutrients in the medium. To minimize this loss,
holding time at the highest sterilization temperature
should be kept as short.
• Cell death occur at all times during batch sterilization,
including the heating-up and cooling-down periods.
The holding time t hd can be minimized by taking into
account cell destruction during these periods.

35. Sterilization
Batch heat sterilization of liquids
(Reduction in number of viable cells during batch sterilization)
The number of contaminants
present in the raw medium No
>>During heating period No is
reduced to N1 .
>>The end of the holding period,
the cell number is N2 ; final number
after cooling = Nf ,
>>Ideally Nf = 0; at the end of
sterilization cycle we want
to have no contaminants present.
Normally, the target level of contamination is expressed as a
fraction of a cell , which is related to possibility of contamination

36. Sterilization
Batch heat sterilization of liquids
• Rate of heat sterilization is governed by the
equation for thermal death outline .
• In batch vessel where cell death is the
only process affecting the number of viable cells :
N = number of viable cells
(17)
t =time and
kd = specific death constant .
• Eq (17) applied to each stage of the batch
sterilization cycle: heating, holding and cooling.

37. Sterilization
Batch heat sterilization of liquids
• kd is a strong function of temperature,
direct integration of Eq.(17) is valid only when the
temperature is constant, i.e. during the holding period. The
result is:
(18) (19)
or
thd = holding time
N1 = number of viable cells at the start of holding
N2 = number of viable cells at the end of holding
• kd is evaluated as a function of temperature (20)
using the Arrhenius equation:
A = Arrhenius constant or frequency factor,
Ed = activation energy for the thermal cell death,
R = ideal gas constant
T= absolute temperature

38. Sterilization
Batch heat sterilization of liquids
• To use Eq. (19) we must know N1 and N2.
• These numbers are determined by
considering the extent of cell death during the heating
and cooling periods when the temperature is not constant.
• Combining Eq. (17) and (20) gives: >> (21)
• Integration of Eq. 21 gives
for heating period: >>> (22)
and for cooling period : >>> (23)
t1 = time at the end of heating / t2 = time at the end of holding
and tf = time at the end of cooling

39. Generalized temperature-
time profile for the heating
and cooling stages of batch
sterilization
General equations for
temperature as a
function of time during
heating and cooling
periods of batch
sterilization

40. Sterilization
Batch heat sterilization of liquids
• Applying an appropriate expression for T in Eq.(22)
(22)
• From table allows to evaluate the cell number N1
at the start of the holding period.
• Similarly , substituting for T in Eq.(23) for cooling gives N2
at the end of the holding period.
(23)
• Use of the resulting values for N1 and N2 in Eq.(19)
completes the holding-time calculation.
(19)

41. Sterilization
Batch heat sterilization of liquids
• The design procedures outlined in this section
apply to batch sterilization of medium when the
temperature is uniform throughout the vessel.
• However, the liquid contains contaminant particle
in the form of flocs or pellets, temperature
gradient may develop.
• Cell death inside the particles is not as effective as
in the liquid.
• Longer holding times are require to treat solid-
phase substrate and media containing particles.

42. Sterilization
Batch heat sterilization of liquids
• Heat sterilization is scale up to larger volumes,
• Scale-up also affects the temperature-time profile for heating
and cooling.
• Heat-transfer characteristics depend on the equipment used;
heating and cooling of large volumes usually take more time.
• Sustained elevated temperature during heating and cooling
are damaging to vitamins, proteins and sugar in nutrient
solutions and reduce the quality of the medium.
• Because it is necessary to hold large volume of medium for
longer periods of time, this problem is exacerbated with
scale-up.

43. Sterilization
Continuous heat sterilization of liquids
• Continuous sterilization, particularly a high-
temperature, short-exposure-time process, can
reduce damage to medium ingredients while
achieving high level of cell destruction.
• Improved steam economy and more reliable scale up.
• Time require is significantly reduced because
heating and cooling are virtually instantaneous.

44. Sterilization
Continuous heat sterilization of liquids
• Typical equipment configurations for continuous sterilization
are shown in below figure

45. Sterilization
Continuous heat sterilization of liquids
• Heat-exchange systems are more expensive to construct than
injection devices; fouling of the internal surfaces also
reduces the efficiency of heat transfer between cleaning.
• On the other hand, a disadvantage associated with steam
injection is dilution of the medium by condensate; foaming
from direct stream injection can also cause problem with
operation of the flash cooler.
• Important variable affecting performance of continuous
sterilizers is the nature of fluid flow in the system.

46. Sterilization
Continuous heat sterilization of liquids
• The type of flow in pipes where there is neither
mixing nor variation in fluid velocity is
called plug flow

47. Sterilization
Continuous heat sterilization of liquids
• Deviation from plug flow behavior is characterized
by the degree of axial dispersion in the system.
• Axial dispersion is critical factor affecting design
of continuous sterilizers.
• The relative importance of axial dispersion and
bulk flow in transfer of material through the pipe is
represented by a dimensionless variable called the
Peclet number.

48. Sterilization
Continuous heat sterilization of liquids
Pe = Peclet number,
(24) u = average linear fluid velocity,
L = pipe length
Dz = axial- dispersion coefficient.
• For perfect plug flow, Dz = 0,
•Pe is infinitely ; in practice, Paclet number between
3 and 600 are typical.
•The value of Dz for a particular system depend on the
Reynolds number and pipe geometry.

49. Sterilization
Continuous heat sterilization of liquids
The extent of cell destruction in
sterilizer can be related to the
specific death constant kd
N1 is the number of viable cells
entering the system,
N2 is the number of cells leaving ,
Pe is the Peclet number as defined
by Eq.(24) and
Da is another dimensionless
number called the Damkohler number
kd = specific death constant,
(25) L = the length of the holding pipe
u i= average linear liquid velocity.
The lower the value of N2/N1
the greateris the level of cell destruction

50. Sterilization
Continuous heat sterilization of liquids
• Heating and cooling in continuous sterilization are
so rapid that in design calculation they are
considered instantaneous.
• While reducing nutrient deterioration, this feature
of the process can cause problems if there are
solids present in the medium.
• It is important therefore that raw medium be
clarified as much as possible before it enters a
continuous sterilizer.

51. Sterilization
Filtration sterilization of liquids
• Sometimes, fermentation media or selected
ingredients are sterilized by filtration rather than
heat.
• For example, media containing heat-labile
components such as enzymes and serum are easily
destroyed by heat and must be sterilized by other
mean.
• Membrane used for filter sterilization are made of
cellulose esters or other polymers and have pores
between 0.2 and 0.45 µm in diameter.

52. Sterilization
Filtration sterilization of liquids
• Bacteria and other particles with dimensions greater than the
pore size are screened out and collect on the surface of the
membrane.
• To achieve high flow rates, large surface areas are required.
• Liquid filtration is generally not as effective as heat sterilization.
Viruses and mycoplasma are able to pass through membrane
filters; care must also be taken to prevent holes or tears in the
membrane.
• Usually, filter-sterilized medium is incubated for a period of time
before use to test its sterility.

53. Sterilization
Sterilization of air
• The number of microbial cells in air is of the order
103 - 104 m-3.
• Filtration is the most common method for sterilizing
air in large scale bioprocesses.
• Depth filters consisting of compacted beds or pads
of fibrous material such as glass wool have been
used widely in the fermentation industry.
• Depth of the filter medium required to produce air
of sufficient quality depends on the operating flow
rate and the incoming level of contamination.

54. Sterilization
Sterilization of air
• Cells are collected in depth filters by a combination of
impaction, interception, electrostatic effects.
• Depth filters do not perform well if there are large
fluctuations is flow rate or if the air is wet; liquid
condensing in the filter increase the pressure drop,
cause channeling of the gas flow.
• Cartridge filters, these filters
use steam-sterilizable polymeric
membrane which act as surface filter
trapping contaminants as on a sieve.

55. Sterilization
Sterilization of air
• Containment is particularly important when
organisms used in fermentation are potentially
harmful to plant personnel or the environment;
companies operating fermentations with pathogenic
or recombinant strains are require by regulatory
authorities to prevent escape of the cells.

56. Summary reactor Engineering
This chapter contains a variety of qualitative and
quantitative information about design and operation
of bioreactors. After studying this chapter,
you should
1 be able to assess in general terms the effect of
reaction engineering on total production costs in
bioprocessing
2 be familiar with a range of bioreactor configurations
in addition to the standard stirred tank including
bubble column, airlift, packed-bed, fluidized- bed
and trickle-bed designs;

57. 3 understand the practical aspects of bioreactor
construction, particularly those aimed at
maintaining aseptic condition;
4 be familiar with measurements used in fermentation
monitoring and the problems associated with lack
of online methods for important fermentation
parameter;
5 be familiar with established and modern approaches
to fermentation control;
6 be able to predict batch reaction time for enzyme
and cells reaction;

58. 7 be able to predict the performance of fed-batch
reactors operated under quasi-steady-state
conditions;
8 be able to predict and compare the performance of
continuous stirred-tank reactor and continuous
plug flow reactor;
9 know how to use steady-state chemostat data to
determine kinetic and yield parameter for cell
culture and
10 know how batch and continuous system are
designed for heat sterilization of liquid medium
and methods for filter sterilization of fermentation
gases.

59. Reactor Engineering

60. END
Reactor Engineering
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