KLA MEASUREMENT MECHANICALLY AGITATED

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Measurement of Volumetric Mass
Transfer Coefficient and Gas hold-
up in Mechanically Agitated Airlift
Bioreactor
Done By
Taher Issa Al-Dhahouri, Sara Khalfan Al-Gadidi, Osama Abdullatif Al-Balushi,
Khawla Ahmed Al-Mamari, Emtethal khamis Al-Alawi.
A Final Year Report
Submitted to Sohar University
in Partial Fulfillment of the
Requirements for the Bachelor Degree in
Chemical Engineering
Sohar, Sultanate of Oman
June 2018
Supervised by: Dr. Ahmed Jawad Ali Al-Dallal

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Declaration
We hereby declare that this report is based on our original work. We also
declare that it has not been previously and concurrently submitted for any
other degree or award.
Signatures
We further permit Sohar University to reproduce this thesis by repetition or
by other means, in total or in part, at the request of other institutions or
individuals for the purpose of scholarly research.
Signatures

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Acknowledgment
The project group would like to thank their supervisor Dr. Ahmed Al-Dallal
for his assistance, valuable advice, and support along the project.
Furthermore, the group members would also like to express their gratitude to
their loving parent and friends who had helped and encouraged them.
Without the mentioned parties, it was impossible to complete this final year
project.

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Executive summary
Agitated airlift bioreactor was widely spread in the modern bioprocess
technology as they are used in so many applications like animal and plant cell culture,
three phase reactions with solid particles and sewage treatment. The hydrodynamics
of the agitated airlift bioreactor are the main scope for research and development for
this type of bioreactors. Volumetric mass transfer coefficient and gas-holdup
measurements gives an indication on the performance of agitated airlift bioreactor.
The study was carried out in airlift bioreactor consists of a rectangular vessel of
height, width and length of 68.4, 17.9 and 22.49 cm respectively. With the addition of
agitation with stirrer speed between 0-400 rpm and aeration flow rate between 10 – 20
L/min. Three different impellers configurations were used including axial (45o
Pitched
blade turbine) and radial (Rushton disk Turbine and Concave disc (CD-6)) impellers.
Configuration-1 is 45o
pitched blade turbine with Rushton disk Turbine,
configuration-2 is two Rushton disk Turbine and configuration-3 is 45o
pitched blade
turbine with Concave disc (CD-6). For measurement of the volumetric mass transfer
coefficient a dynamic method is used that depend on the response of dissolved oxygen
concentration change in the reactor using sodium sulfite.
Volumetric mass transfer coefficient (KLa) and gas holdup (ε) were measured
in mechanically agitated airlift reactor. KLa was found to increase with increasing of
riser gas velocity, speed of impeller and at different impellers configuration. Each
configuration gives an indication of KLa performance. Radial types of impellers (45o
piched blade turbine) gives the highest KLa values at mixer speed > 100 rpm. This
increase is mainly caused by breakage of bubbles going through the blades of the
impeller. Additionally, it was found that increasing gas velocity and impeller speed
lead to an increase in hold up. An increase in gas hold-up was more pronounced when
using both 45o
Pitched blade turbine and Rushton disk Turbine impellers and less
pronounced when using CD-6 compared with non-mechanical agitated system.

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Table of Contents
CHAPTER ONE: Introduction.................................................................................................. 1
1.1 Background ..................................................................................................................... 1
1.2 Objectives of thesis.......................................................................................................... 2
CHAPTER TWO: Literature Survey ........................................................................................ 3
2.1 Mechanical Agitated airlift Bioreactors .......................................................................... 3
2.1.1 Impellers applied ...................................................................................................... 4
2.1.2 Advantage of agitated airlift bioreactor.................................................................... 6
2.1.3 Application of agitated airlift bioreactor .................................................................. 6
2.2 Hydrodynamics of agitated airlift bioreactor................................................................... 7
2.2.1 Gas hold up............................................................................................................... 7
2.2.2 Volumetric mass transfer coefficient kLa ................................................................. 8
2.3 Published work on hydrodynamics of mechanically agitated airlift bioreactors:............ 8
CHAPTER THREE: Experimental Work ............................................................................... 24
3.1 Equipment Setup ........................................................................................................... 24
3.2 impeller Setup................................................................................................................ 25
3.3 Design of mechanical agitated airlift bioreactor............................................................ 26
3.3.1 Motor/stirrer ........................................................................................................... 26
3.3.2 Dissolved Oxygen measurements........................................................................... 26
3.4: KLa measurements:...................................................................................................... 27
3.6 Gas hold-up measurements............................................................................................ 28
CHAPTER FOUR: Results and Discussion............................................................................ 29
4.1 Volumetric mass transfer coefficient KLa ..................................................................... 29
4.1.1 Effect of gas velocity on KLa...................................................................................... 33
4.1.2 Effect of impeller speed on KLa ................................................................................. 33
4.1.3 Effect of impeller configuration on KLa..................................................................... 33
4.2 Gas Holdup.................................................................................................................... 34
4.2.1 Effect of gas velocity on gas holdup .......................................................................... 36
4.2.2 Effect of impeller speed on gas holdup ...................................................................... 36
4.2.3 Effect of impeller configuration on gas holdup.......................................................... 37
CHAPTER FIVE: Conclusion and Recommendations for Future Work................................ 38
5.1 Conclusion..................................................................................................................... 38
5.2 Recommendations for future work................................................................................ 39
REFERENCES........................................................................................................................ 40

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Nomenclature
Volumetric oxygen transfer coefficient
E Fractional approach to equilibrium
C* Saturation dissolved oxygen concentration
C0 Initial dissolved oxygen concentration
t0 Initial time when a hydrodynamic steady-state has been reestablished
(≤ 60 s) upon the beginning of aeration
C dissolved oxygen concentration at any time t
The total gas holdup
hD height of gas-liquid dispersion
hL height of gas free liquid
c, d Constants depend on the operating scale (dimensions of the system, and the
physical properties of the gas-liquid mixture)
UG superficial gas velocity
Vgas volume of the gas phase
Vl volume of the liquid phase
CL bulk concentration of dissolved oxygen
saturation concentration of dissolved oxygen
Cs concentration of (SF) solid
conductivity of the liquid phase
measured conductivity (mS/cm)
CD-6 6-bladed compact disk
constants in Eq. (2.10) for the conventional stirred tank fermenter
OTR oxygen transfer rate
m mass of the working media
̇ mass flow rate of pure H2O2

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List of figures
Figure 2.1: Mechanical Agitated Bioreactors
Figure 2.2: Airlift bioreactor
Figure 2.3: Axial and radial flow impeller
Figure 2.4: 6-blade radial impeller
Figure 2.5: 4-blade axial impeller
Figure 3.1: Back few of the system
Figure 3.2: Font few of the system
Figure 3.3: Side few of the system
Figure 3.4: Arrangement of axial (45o
Piched blade turbine) and radial (Rushton disk
Turbine) impeller
Figure 3.5: Arrangement of axial (45o
Piched blade turbine) and axial (45o
Piched
blade turbine) impeller.
Figure 3.6: Arrangement of radial (Rushton disk Turbine) and radial Concave disc
(CD-6) impeller.
Figure 3.7: Impellers used
Figure 3.8: Motor used
Figure 3.9: Dissolved oxygen measurements
Figure 3.10: Set up used to avoid the bubbles that accumulate on the prop
Figure 3.11: KLa measurements
Figure 4.1: Ug vs. KLa at different impeller speed
Figure 4.2: Ug vs. KLa at different impeller speed
Figure 4.3: Ug vs. KLa at different impeller speed
Figure 4.4: KLa vs Ug at 100 rpm with three arrangements

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Figure 4.5: KLa vs Ug at 200 rpm with three arrangements
Figure 4.6: KLa vs Ug at 300 rpm with three arrangements
Figure 4.7: KLa vs Ug at 400 rpm with three arrangements
Figure 4.8: Ug vs. gas holdup at different impeller speed
Figure 4.9: Ug vs. gas holdup at different impeller speed
Figure 4.10: Ug vs. gas hold up at different impeller speed
Figure 4.11: Gas holdup % vs. impeller Speed (rpm) at different impellers
configuration and at fixed Ug.
Figure 4.12: Gas holdup % vs. Ug at different impellers configuration and at fixed 400
rpm.

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CHAPTER ONE
Introduction
1.1 Background
Bioreactors are devices that are widely used in production of pharmaceutical
product, milk and waste treatment processes. There are many commercial types of
bioreactors are used in the world; mostly widespread are airlift and agitated
bioreactors. Such types of bioreactors are used in bioprocessing; one of the most
widespread bioreactor is stirred tank reactor involve by different design of propellers
(Kura et al 1993). The second one is airlift bioreactors that are used much in bio-
industries which are characterized by the absence of mechanical agitation which are
suitable with relatively less viscous fluids (Chisti and Jauregui-Haza, 2002). The
relatively new promise type of bioreactors is the mechanically agitated airlift
bioreactors.
Studies with mechanically agitated airlift bioreactors shows a noticeable
increase in gas holdup and overall volumetric mass transfer coefficient, when
compared with typical airlift bioreactors. Where experiment is done thus, obtaining
data and conclusions allowing for scale up (de-Jesus et al. 2013). Many studies have
been conducted on the effects of impeller speed and air flow rate on overall
volumetric mass transfer coefficient and gas holdup where other studies changed
impeller type to see its effect on overall volumetric mass transfer coefficient and gas
holdup. The main advantage of mechanically agitated airlift bioreactors is;
employment of different types of impellers and different flow pattern will lead to
higher gas-liquid contact which cause higher volumetric mass transfer coefficient.
However, it consumed high amount of energy (Oscar et al. 2007). Mechanically
agitated airlift bioreactor is introduced for highly viscus fluid that contain
biodegradable solid, where air mainly oxygen is needed for bacteria to consume the
biodegradable solid with help of impellers which break air bubble into small bubble to
increase volumetric mas transfer coefficient. Also, it is commonly used for large-
scale productions. microbial, animal and plant cell culture etc.

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1.2 Objectives of thesis
In this project, by using a mechanically agitated airlift bioreactor, it is planned
to study the effect of three different parameters namely impeller speed (rpm), dual
impeller configuration and gas flow rate on gas hold up and volumetric mass transfer
coefficient. To recognize that, many experiments have been planned at different gas
flow rates ranges (10-20 L/min) will be applied. Also different arrangement of
impellers (axial, radial and CD-6) with different range of impeller speed (0-400 rpm)
will be applied.

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CHAPTER TWO
Literature Survey
2.1 Mechanical Agitated airlift Bioreactors
Airlift bioreactors as shown in Figure 2.1 are popularly utilized for
bioprocessing. Airlift bioreactors are helpful with generally less thick fluids and when
there is a need for gentle fomentation and low-cost oxygen transfer (Chisti and
Jauregui-Haza, 2002).
Figure 2.1: Airlift bioreactor
Airlift bioreactors are devices described by the nonattendance of mechanical
fomentation, by the height to diameter ratio, which provide to their less cost of both
structures and operation, furthermore to presenting less issues in connection to the
blend of air bubbles. In addition, these reactors present a main disadvantage in the low
gas-liquid mass transfer rates, when use non-Newtonian highly viscous medium. (De-
Jesus et al., 2014).
Agitated airlift reactors have become much used for application in
biochemical and chemical industries for its simple geometry, uniform mixing and low

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power consumption (Xiaoping et al., 2000). Agitated airlift reactor is containing a
riser and a downcomer as in Figure 2.2. They are interconnected near the top and
bottom of the reactor as shown in Figure 2.2.
Figure 2.2 Agitated airlift reactor
The riser is usually aerated with a gas whereas the downcomer is not directly
sparged. This difference in aeration causes higher gas hold-up in the riser than the
downcomer. As well it made a difference in the fluid bulk densities in these two zones
induces circulation by liquid up-flow in the riser and down-flow in the downcomer.
Because of this configuration of the top and the bottom zones, and also of the gas
velocity, the gas coming out of the riser may typically recirculate into the downcomer,
or it may all disengage so that only gas-free liquid returns to the downcomer. The
reactor design of these regions affects the difference in gas hold-up between the riser
and the downcomer therefore it affects the driving force for liquid circulation and all
hydrodynamic and mass transfer characteristics of airlift reactors, including gas-liquid
mass transfer and the homogenization of solids (Bang et al., 1998).
2.1.1 Impellers applied
Impellers are rotating devices designed to alter the flow. Impellers consist
of various vanes often blade shaped that arranged around a short central shaft.
Impellers become very important device in many manufactures such as agitation

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tanks, pumps and so many other devise that required fluids or gases to move in a
specific direction. There are mainly two types of impellers based on the configuration
of flow which are axial and radial flow impeller Figure 2.3.
In airlift bioreactors, impellers are used to enhance its performance and this
enhancement cause by impeller geometrical parameters, including impeller type,
number of impeller blades, blade pitch angle and blade thickness. Mostly in airlift
bioreactors, impellers are located along with the gas sparger in the region comprised
by the riser (Wu et al., 2014).
2.1.1.1 Radial impeller:
The gas holdup increases when using radial impellers. Furthermore, by using
radial impeller the volumetric mass transfer coefficient in hybrid airlift bioreactor.
however, the volumetric mass transfer coefficient is function of gas velocity. while
using radial impeller the bubble diameter increases with the spherical gas velocity
(De-Jesus et al., 2014). Figure 2.4 show a schematic of 6-blade radial impeller.
Figure 2.4: 6-blade radial impeller.
Figure 2.3: Radial and xial flow impellers

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2.1.1.2 Axial impeller
Axial impellers as shone in Figure 2.5 are the best chose for mixing
applications that require stratification or solid suspension. Axial impellers are found
to create effective top to bottom motion in the tank.
There are some common types of axial flow impellers include: marine
impellers, pitched blade impellers, and hydrofoils. Hydrofoil impellers are also known
as high efficiency impellers. They are a popular choice for applications that require a
range from general blending to storage tanks.
Figure 2.5: 4-blade axial impeller.
2.1.2 Advantage of agitated airlift bioreactor
The geometry of agitated airlift bioreactor offers many benefits such as easy
structure and operation, low power necessities and continuous spread flow (Xiaoping
et al., 2000). Mechanical agitation uses normally to improve mixing behavior and the
oxygen transfer comparative to when mechanical agitation was not use. The
volumetric mass transfer coefficient can be improving up to four times relative to a
classical internal airlift because breakage of bubbles going during the blades of the
impeller, involving higher gas hold-up, furthermore the oxygen transfer efficiency
was decrease by mechanical agitation (Bang et al., 1998).
2.1.3 Application of agitated airlift bioreactor
Air-lift bioreactors are a relatively new kind fermenter, offering several
advantages for large scale bioprocesses, for animal and plant cell culture in particular.

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Some of these advantages as well as of the limitations, all of which are determined by
the fluid dynamics and mass transfer characteristics of air-lift reactors (D. Jesus,
2015). Also, airlift bioreactor can supply an attractive alternative to stirred tank,
particularly for bioprocesses with gaseous reactants or products. The microbubbles
increase the mixing efficiency in airlift bioreactor, dispersal of gas phase throughout
the ALR occurs with decreasing the bubble size and phase slip velocity decreases with
smaller bubble size as gas rise rate decrease (Lopez, 1997). The majority of
wastewater produced by different activities (industries and municipal) can be
successfully treated in airlift reactor, with suitable control and analysis of the
environment. Both physic, chemical and biological process can assist the processes
occurring in airlift. Also, the application of airlift reactors for wastewater treatment
addresses usually the conventional biological treatment. Other applications like
microbial, plant, insect, animal, and human cells, and for both adherent & suspension
cell cultures (Peinado et al. 2006).
2.2 Hydrodynamics of agitated airlift bioreactor
2.2.1 Gas hold up
The quantity of gas retained in the column at a given time called gas hold-up,
that is the one of the key variables that determine how intense the contact between gas
and liquid is. It can reach a stationary value or can vary periodically. Together with
size and form factors, the actual throughputs, feed distribution, and whether there is a
packing or additional mixing, hold-up values determine the extent of the interfacial
area, as well as the flow regimes; these in turn determine transport efficiency heat and
mass (Saravanan et al., 1994).
Most industrial cultivation processes are operated as aerobic submerged
cultures, by supplied oxygen into the liquid medium. The volume fraction of the
dispersed gas phase is referred to as the gas hold-up. It is of considerable practical
importance as it determines the maximal volume of culture broth that can be operated
in a given bioreactor. Moreover, it influences the mass transfer of oxygen (O2) and
carbon dioxide (CO2) to determine the cross-sectional area for their transport.
Additionally, the mixing performance of the usually employed stirred tank reactor is
affected since the power introduced into the culture at a fixed stirrer speed usually

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drops with the gas holdup. And the relationships between the various influence
variables that can be used to estimate the gas hold-up are described (Vasconcelos et
al. 1995).
2.2.2 Volumetric mass transfer coefficient KLa
Volumetric mass transfer coefficient with unit of is term represent one
media absorbed in other media mostly for gas-liquid contact. There are many dynamic
methods for measuring the volumetric mass transfer coefficient. One of them is
absorbed gas (air) throw liquid using agitated airlift bioreactors. The oxygen transfer
rate can be measured by chemical or physical techniques (Tribe et al., 1995).
Volumetric mass transfer coefficient can be calculated by:
( ) (2.1)
2.3 Published work on hydrodynamics of mechanically agitated airlift
bioreactors:
The most significant characteristics of gas/liquid dispersion in agitated airlift
bioreactors are bubble size and coalescence. Also Gas holdup and volumetric mass
transfer coefficient (KLa) are one of the key parameters affecting formation of the
product in agitated airlift bioreactors.
The summaries below of many different studies show a critical review on the
parameters been measured for mechanically agitated airlift.
According to de-Jesus et al. (2013), the volumetric oxygen transfer
coefficient ( ) and gas hold up by using a stirred airlift bioreactor with agitated
with a Rusthon impeller and six blades, using water as the fluid model, was
investigated by computational fluid dynamics modeling. For each test, the fluid was
purged by bubbling nitrogen until reaching a dissolved oxygen concentration. Then,
the nitrogen flow was suspended, the outflow of its bubbles was allowed, and the
airflow was established to the required condition. The dissolved oxygen concentration
increase with time until the fluid became nearly saturated with oxygen. The was
calculated as the slope of the linear equation:
( ) ( ) (2.2)
where E can be estimated by:

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(2.3)
Gas Holdup The total gas holdup (εG) was measured by the volume expansion method
as follows:
(2.4)
For calculating the a by CFD, assume the diameter of the gas bubble in the
reactor to be constant and of the same diameter as the gas sparger, and the is no
effects of breakage and coalescence of bubbles in the models used for simulation, the
simulated data showed good fit with the experimental one Shah et al when the effects
of breakage and coalescence of the bubbles are considered. A correlation where the
superficial gas velocity (UGR) is the only variable for estimating the in bubble
column bioreactors. This correlation is given by:
= c (2.5)
The correlations between the experimental and computational data as a function
of UGR are given by:
(R2
= 0.973) – experimental
(2.6)
(R2
= 0.971) – computational (2.7)
The experiment has shown that model with constant bubble diameter and the
result is higher value of . Also, by increasing gas velocity causing the gas to
liquid recirculation will increase and that increase unlike without recirculation.
The impeller effect is prevalent at low gas flow rates and it propels the liquid against
the wells of vessel, the values obtained by the simulation were close to the
experimentally. In both cases, there was an increased gas holdup, as the air velocity in
the riser increases. The CFD simulation results were approximately 10% higher than
the experimental value.
As reported by de Jesus et al. (2014), the experiment Stirred Hybrid Airlift
Bioreactor used to determine and gas holdup, three-bladed marine and Lightnin

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A310 (axial flow impellers) t, and six-bladed Rusthon turbine and six-bladed Smith
turbine (radial flow impellers) are used. The distilled water at 250
C and a constant
rotation velocity of 800 rpm, without agitation (airlift mode); the superficial gas
velocity varied from 0.0157 to 0.0262 ms-1
was applied. Air was sparged in the
internal zone through a 0.05 m porous plate, with 90 holes of 0.001 m in diameter,
and 0.003 m equidistant, located at the bottom of the bioreactor and concentric to the
region comprised by the riser. The volumetric oxygen transfer coefficient ( ) was
measured using the dynamic gassing-in method. The fluid was purged by bubbling
nitrogen until reaching a dissolved oxygen concentration with less than 5% of air
saturation. After that, the nitrogen flow was suspended, allowing the outflow of its
bubbles, and establishing the airflow to the required condition. The dissolved oxygen
concentration was increase with time, the was calculated by equation (2.1) and
gas holdup by equation (2.4). The experiment shows the evolution of the volumetric
mass transfer coefficient in function of the superficial gas velocity for all the impellers
used at a constant rotation of 800 rpm. The kLa increase Also, the mechanical power
input required while using radial impellers is 730-1400% higher than with axial
impellers. In addition, by increasing the superficial gas velocity, the are very
close values of two types of radial impellers used. The best results being achieved
with the A310 model. When the agitation was performed with radial impellers, there
was a big increase in gas holdup and kLa; similar values were obtained for both the
impellers studied. The gas holdup increase a very small by using axial impellers,
compared to experiments performed in the absence of agitation, especially when using
a BM impeller, the RT impellers had higher gas holdup that the A31 for Studies
performed with mechanically stirred tank reactors.
In agreement with Pollard et al (1997), the oxygen transfer performance of a
conventionally operated multi configurable pilot scale concentric airlift bioreactor
containing baker’s yeast were big improved by using a marine propeller to bring
liquid down the draft tube and support recirculation at the base of the vessel. The
severe DOT heterogeneity of the reactor by propeller operation reduced and gave
DOT values below 1% air saturation in the riser because of producing DOTs above
40% around the vessel at maximum energy dissipation rate. The fermentation
conditions as follow: Packed baker’s yeast (Distillers Company Ltd, Surrey) was
suspended under non-sterile conditions in a medium containing (g/l): glucose 10,

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yeast extract 10, (NH4)2SO4 5, KH2PO4 2.5, polypropylene glycol 0.25 ml/l. A large
inoculum giving 10 g/l dry cell weight of packed yeast was used in the reactor unless
stated otherwise. hydrodynamic and oxygen transfer measurements performed during
the six hours fermentation the air flow rate and propeller speed were varied. The
overall rate of oxygen transfer was estimated by a gas balance (steady state) using a
mass spectrometer to monitor inlet and exit gas compositions. The rate of oxygen
transfer was used with the local dissolved oxygen concentration to estimate the
volumetric oxygen mass transfer coefficient, kLa. The volume expansion method as
the difference between the volume of the dispersion and that of the liquid use to
measure overall gas holdup. Riser gas holdup was measured using the pressure
differential between two Druck pressure transducers positioned at 0.53 and 2.11 m
heights in the riser. The experiment show by using a marine propeller situated in the
lower downcomer, the gas holdup and liquid circulation were increased and that led
to the increase of oxygen transfer rate. By using yeast, the annulus spared and
propeller operated configuration get better the gas holdup and kLa of the
conventionally aerated reactor with yeast unlike result with water.
Conforming to Kura et al. (1993), oxygen transfer studied in a stirred loop
fermenter and dual impeller system with dilute polymer solution with water in semi-
batch system. Polyethylene in water is used as polymer drag-reducing additive. The
strong liquid circulation produces by the flow from the pitch blade turbine installed
near the free surface is predominantly axial. The six -blade disk turbine was used for a
lower impeller and that lead to produces a radial discharge liquid stream to bubble
break up. To promotes axial flow use the concentric draft tube and that causes on
obstruction to the radial flow of liquid. the dynamic method uses to determine
volumetric mass transfer coefficients. The experiment correlation for bubble columns
proposed by Shah et al is:
(2.8)
It is applying for N rpm in the aeration -controlling region. Also use the
correlation of Van’t Riet the following correlation from extensive literature data for
aerated conventional stirred tank reactor with water.

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( ) ( ) (2.9)
In the agitation-controlling region (P/V ).
( ) ( ) ( ) (2.10)
Where . The exponent for (P/V) ranges between 0.4-
0.42 for water and 0.52-0.5 for electrolyte and superficial gas velocity 0.35 – 0.5 for
water and 0.26 – 0.62 for electrolyte. The experiment shows, in aeration controlling
region (n=0 rps) the addition of small amount of water soluble polymer was
mischievous in gas-liquid mass transfer rate and decreased by the polymer
additive. On other hand, by using water the increased with gas flow rate. A
dependence on the gas flow rate decreased with increasing impeller speed. At high
impeller speed is dependent of the impeller speed but not the gas flow rate.
Also, It was observed that the in the stirred loop fermentor with dilute polymer
solutions is higher compared with that for water.
In consonance with Lueske et al. (2015), the experimental used a stirred-tank
reactor, tank with draft tube configuration and the agitation system consisting of axial
flow impellers inside the draft tube and a radial gas dispersion impeller below the
draft tube and above the gas ring sparger. The draft tube is equipped with four flat
baffles. by using a fully baffled two-tier down-pumping hydrofoil (A320) agitation
system. Due to the radial impeller (Rushton turbine), the sparged air is scattered to
small bubbles to the liquid. Then, the two phases rise upwards in the riser or annular
area between the tank wall and the draft tube. The dissolved oxygen (DO)
concentration was measured by an YSI model 600XLD.O. and a temperature probe
with a standard membrane (Yellow Springs Instruments, Yellow Springs, OH, USA).
The two DO probes were mounted on a rod and positioned at two different location.
volumetric mass transfer coefficient kLa, which can be subdivided into unsteady-state
and steady-state measuring methods, depending on whether the concentration cL of
the dissolved gas in the liquid changes with time during the measuring process or
remains constant. The steady-state method was applied in this work, and for the first-
time ferric chloride was used as the catalyst for hydrogen peroxide decomposition.
The experimental matrix reflects a superficial gas velocity (vsg) at the riser in the
range of 0.04–0.12ms–1 and a specific power (e) range of 1.0–3.0Wkg–1
(5–15
hp/1000 gallons). With the assumptions of ideal gas behavior, a well-mixed liquid

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phase, and equilibrium oxygen concentration at the gas bubble interface, kLa is
determined from the oxygen transfer rate and the oxygen concentration in the liquid.
The volumetric mass transfer coefficient can be expressed by following equations:
( ) (2.1)
(2.2)
The gas volume fraction or gas holdup in the liquid is of interest for the correct
dimensioning of the height of the gas head space of a stirred tank. The gas holdup is
defined in equation follow:
= Vgas / (Vgas + Vl) (2.13)
The experiment shows the mass transfer increases with increasing power input. In the
bottom region of the tank, the mass transfer rate increases with increasing specific
power and gas flow rate. Also, the lowest kLa is achieved at 1.0VVM, and the highest
kLa value is realized at 1.4VVM. After reaching a specific power greater than
2.25Wkg–1
, kLa remains constant. This means that a power input higher than
2.25Wkg–1
will not increase or improve the mass transfer. In general, for low-
viscosity media and in coalescence-inhibited systems, with increasing gas holdup, the
volumetric mass transfer coefficient kLa increases. Furthermore, the gas holdup
influences the networking media volume.
de Jesus et al. (2017) studied the hydrodynamics, mass transfer and gas hold-
up using four bioreactors (bubble column, concentric tube airlift, concentric tube
stirred airlift, and mechanically stirred tank). To study this behaviors, a solution of
viscous a Newtonian fluid (glycerol 65%) and a non-Newtonian fluid (xanthan
0.25%) with deionized water were used. Stirred tank bioreactor with a dissolved
oxygen electrode (O2-sensor InPro6800/12/220 Mettler Toledo, Switzerland) and pH
probe (405-DPAS-SC-K8S/225 Mettler Toledo, Switzerland) were used. The stirring
action was done by a Rushton turbine impeller with six blades, 0.06 m in diameter.
The stirring speed ranged from 400 to 800 rpm. All experimental runs were carried
out at atmospheric pressure and at 25-2o
C. Gas hold-up (G) was estimated by visual
observation of the increase in volume level caused by aeration or aeration/agitation.
Gas holdup is given by equation (2.4).

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The volumetric oxygen transfer coefficient (kLa) was measured as follow:
The oxygen content in the fluid was removed by bubbling of gaseous nitrogen until
the dissolved oxygen concentration became less than 5% air saturation, and then the
reactor was aerated in accordance with the conditions of each experiment.
The dissolved oxygen concentration was collected every second through a computer
program. The increase in the dissolved oxygen concentration was followed over time
until the fluid became saturated with oxygen (>90%). For each air velocities were
collected approximately 40 points until oxygen saturation. kLa was calculated as the
slope of the linear equation (2.1). The experiments showed that the gas holdup have a
significant difference because of four reactors used. Bubble column and airlift
reactors shows lower values than other. Also increase of air flow rate will increase gas
hold-up and viscose fluid cause bubble coalescence, gas hold-up decrease. Adding
stirred will increase gas hold-up as well as kLa and kLa increased with air flow.
Based on the experiment done by Bang, et al. (1998), volumetric mass transfer
coefficient ( ) and gas hold-up had been measured by dynamic absorption of
oxygen using an airlift bioreactor. ALR used having area of 1.8 m2
. A marine impeller
is used with three blades having 50 mm diameter. Four baffles located at 25 mm
above the bottom draft tube to avoid the formation of thin deep vortex from the free
surface down to the stirrer. The agitation speed was from 0 to 2000 rpm. The system
used was Air/ Glucose solution (5-38 w.v%), Al2O3 50&100 m (0.01-4%). The
dissolved oxygen concentration was measured by using oxygen probe (YSI 57). The
concentration of oxygen was de-aerated from the liquid by bubbling nitrogen. To
calculate overall gas hold-up, liquid global height was measured for aerated and non-
aerated conditions. The rate of oxygen transfer is related to with material balance
on dissolved oxygen assumed a perfect mixed reactor is shown in equation (2.1).
The result was found the volumetric mass transfer coefficient and gas hold-up are
increased with superficial gas velocity and with glucose solution volumetric mass
transfer coefficient and gas hold-up are decreased because of coalescence of bubble in
the riser. For using stirrer speed, volumetric mass transfer coefficient increased with
stirrer speed increased and for solid loading, volumetric mass transfer coefficient
decreased.

23. Page | 15
Xiaoping et al. (1998) studied the volumetric mass transfer coefficient ( )
and gas hold-up using DO meter by means of dynamic gas dissolution method. An
airlift reactor with area of 0.92 m were used which is made of plexiglass. 4-Pitched
disk turbine impeller with diameter of 0.085 m was used. To avoid formation of
vortex, 4 baffles were used. Air/tap water and carboxymethyl (CMC) aqueous
solution used as the system of this experiment. Average gas hold-up was obtained by
the volume expansion method. To calculate volumetric mass transfer coefficient
( ), tow correlation was developed in term of , :
for case without static mixer with stander deviation of 0.8975:
( ) (2.14)
for case with static mixer with stander deviation of 0.9417:
( ) (2.3)
The result found to be as the viscosity of the liquid increase the volumetric mass
transfer coefficient ( ) decrease and spherical gas velocity and stirring speed
increased, volumetric mass transfer coefficient ( ) will increased too. volumetric
mass transfer coefficient ( ) is higher with static mixer than without. Furthermore,
with mechanical agitation, is larger than static mixer even the media is very
viscous. Gas hold-up increased with increased impeller speed and gas superficial
velocity and gas hold-up decreased with increased of fluid viscosity.
As stated in Chisti and Jaureui-Haza (2001), airlift bioreactors and
mechanically stirred tank are widely used in bioprocessing. Airlift bioreactors are
useful with relatively less viscous fluids and when there is a need for gentle agitation
and low-cost oxygen transfer. To improve the circulation of the fluid used installation
of an axial flow impeller. A 5-bladed impeller was used. The system used was
Air/Hard tap-water/A aqueous NaCl (0.15) solution/ 2-4% suspensions of Solka flow
cellulose fibers in aqueous sodium chloride (0.15). The agitation speed was from 0 to
260 rpm. The bioreactor vessel was 0.755m in diameter and its overall height was
3.21m. The vessel was sparger (96 holes of 0.02 m in diameter located on 2 rings of
0.013m tube diameter). The Ar/Ad ratio was 1.27. The working volume and the overall
volume of bioreactor were 1.10 and 1.46 m3. The bioreactor made of stainless steel.
The dissolve oxygen elect-ode was measured by using oxygen probe (YSI 5739). The

24. Page | 16
SF slurries behaved as non-Newtonian power law fluids and their consistency (K) and
flow (n)
K= (8.9 Cs2
- 80.7Cs - 70.4) *10-3
(2.4)
n = (996 - 44.7Cs + 0.6Cs
2
) *10-3
(2.5)
It was found that the fractional gas holdup increases with increase aeration and
agitation rates and the holdup declined with increasing concentration of the cellulose
fiber solid in slurry. Also, the maximum reduction in holdup was 60% relative to the
value in the solid-free-system. The average error in gas holdup measurement was less
than 3%.
In keeping with de Jesus et al. (2015), The influence of impeller type in a
mechanically stirred airlift bioreactor was analyzed in relation to the non-Newtonian
viscous fluids. The agitation was carried out through a marine impeller and paddle
impeller. The bioreactor was sparged with air under different velocity (0.036 – 0.06)
m/s. carboxy-methyl cellulose 1.94% and xanthan 1.80% were used as a fluid model.
The stirring speed ranged from 0 to 800 rpm. An axial 3-bladed marine impeller or a
radial 4-bladed paddle impeller with the same geometric similarities and 40 mm of
diameter. The Ar/Ad ratio was 1.8. The system used was air/CMC solution
1.94%/xanthan gum solution 1.80%. The spherical oxygen velocity in the riser UGR
ranged from 0.0157 to 0.0262 m/s.
It was found the mixing time decrease linearly with the spherical gas velocity. Also,
the agitation increased the gas fraction in the bioreactor, and this increase was greater
with a paddle impeller. The gas fraction was very low, when using xanthan gum as
model fluid. Furthermore, the increased viscosity is a critical factor that must be taken
into consideration when choosing the bioreactor. Non-Newtonian viscous fluids
generally have low gas holdup that is independently of the reactor type.
As it is mentioned by Xiaoping et al. (1998), KLa was determined by using an
internal loop airlift reactor equipped with a mechanical stirrer. The height of column
is 1.5 m with diameter of 0.15 m. The diameter of the riser is 0. 1 m. Air, water and
glass beads (2630 kg/ m3
) are used as a system of this experiment. Superficial gas
velocity varies from 0.5 to 19.0 cm/s, solid loading from 1% to 4%, and impeller
rotation speeds are 0-1200 rpm. 4 inclined paddle blade impeller was used.

25. Page | 17
The experiment shows the volume mass transfer coefficient increases with solid
loading and the maximum increase occurs at approximately 2%. but it decreased with
more addition of solid and volumetric mass transfer coefficient increased with
superficial gas velocity.
In line with de Jesus et al. (2014), the experiment stirred hybrid airlift
Bioreactor used, The comparative study was performed using three-bladed marine and
Lightnin A310 (axial flow impellers) t, and six-bladed Rushton turbine and six-bladed
Smith turbine (radial flow impellers) to determine gas hold up and KLa . The
experiments were conducted using distilled water at 250C and a constant rotation
velocity of 800 rpm, as well as in the absence of agitation (airlift mode); the
superficial gas velocity varied from 0.0157 to 0.0262 ms-1
. The gas holdup and
oxygen transfer coefficient was higher with the use of radial impellers; however, the
mechanical power input required while using radial impellers is 730-1400% higher
than with axial impellers. Air was sparged in the internal zone through a 0.05 m
porous plate, with 90 holes of 0.001 m in diameter, and 0.003 m equidistant, located
at the bottom of the bioreactor and concentric to the region comprised by the riser.
The volumetric oxygen transfer coefficient (KLa) was measured using the dynamic
gassing-in method. For each test, the fluid was purged by bubbling nitrogen until
reaching a dissolved oxygen concentration with less than 5% of air saturation. Later,
the nitrogen flow was suspended, allowing the outflow of its bubbles, and establishing
the airflow to the required condition. The increase in dissolved oxygen concentration
was followed with time, until the fluid became nearly saturated with oxygen (>90%).
The was calculated as the slope of the linear equation (2.13). The total gas holdup
( ) was measured by the volume expansion method eq (2.3). The experiment shows
the evolution of the volumetric mass transfer coefficient in function of the superficial
gas velocity for all the impellers used at a constant rotation of 800 rpm, and in the
absence of agitation. The presence of agitation in the bioreactor contributed, in all
cases, to the kLa increase; however, when the radial impellers were used, there was an
increase of 50% over the airlift mode and, 30-40% in relation to the axial impellers. It
was also noticed that by increasing the superficial gas velocity, very close values of
kLa were obtained for the two types of radial impellers used. Comparative studies with
RT and A315 impellers in cultures of Streptomyces in a stirred bioreactor revealed
that the kLa value increases by 50% with the use of RT impellers. The axial impellers

26. Page | 18
were not big contributors to the increases of gas holdup and kLa, with the best results
being achieved with the A310 model. In contrast, when the agitation was performed
with radial impellers, there was a significant increase in gas holdup and kLa; similar
values were obtained for both the impellers studied. The presence of agitation favored
the increase of gas holdup, especially when using radial impellers. The experiments
performed with radial impellers provided very similar results, in which there was an
increase in function of the superficial gas velocity. However, when the experiments
were performed with axial impellers, there was a very small increase in gas holdup,
compared to experiments performed in the absence of agitation, especially when using
a BM impeller. Studies performed with mechanically stirred tank reactors, showed
that the RT impellers had higher gas holdup that the A31.
Pollard et al (1997) found that, oxygen transfer performance of a
conventionally operated multi-configurable pilot scale (0.25 m3
) concentric airlift
bioreactor containing baker’s yeast were significantly improved by operating a marine
propeller to draw liquid down the draft tube and aid recirculation at the base of the
vessel. Propeller operation reduced the severe DOT heterogeneity of the reactor,
which gave DOT values below 1% air saturation in the riser, by producing DOTs
above 40% around the vessel at maximum energy dissipation rate. The fermentation
conditions as follow: Packed baker’s yeast (Distillers Company Ltd, Surrey) was
suspended under non-sterile conditions in a medium containing (g/l): glucose 10,
yeast extract 10, (NH4)2SO4 5, KH2PO4 2.5, polypropylene glycol 0.25 ml/l. A large
inoculum giving 10 g/l dry cell weight of packed yeast was used in the reactor unless
stated otherwise. The vessel contents were maintained at pH 7 and 26°C. During the
six hours fermentation the air flow rate and propeller speed were varied to perform
hydrodynamic and oxygen transfer measurements. Samples (10 ml) were removed
before and after the fermentation to confirm that the dry cell weight remained
relatively constant throughout the fermentation. The overall rate of oxygen transfer
was estimated by a gas balance (steady state). Using a mass spectrometer to monitor
inlet and exit gas compositions. The rate of oxygen transfer was used with the local
dissolved oxygen concentration to estimate the volumetric oxygen mass transfer
coefficient, kLa. Overall gas holdup was measured by the volume expansion method
as the difference between the volume of the dispersion and that of the liquid.
Measurements of aerated and unaerated liquid heights were made with a graduated

27. Page | 19
rod suspended from the reactor top plate. Riser gas holdup was measured using the
pressure differential between two Druck pressure transducers positioned at 0.53 and
2.11 m heights in the riser. The result shows the gas holdup and liquid circulation
performance of the annulus air spared airlift reactor were increased by the operation
of a marine propeller situated in the lower downcomer. This led to the increase of
oxygen transfer rate. Also, the agitator disrupted the large spherical capped bubbles
increasing the gas holdup. No improvement of gas holdup was observed with water,
whereas in this study with yeast, the annulus spared and propeller operated
configuration improved the gas holdup and kLa of the conventionally aerated reactor
with yeast. The decreasing impact of the propeller on gas holdup and liquid
circulation with increasing gas velocities may be due to the increase in turbulence in
the riser. This is similar to the decrease in the rate of improvement of liquid
circulation with increasing superficial gas velocity for aerated only airlift reactors due
to increased energy loss from turbulence.
Pursuant to de Jesus et al. (2015), investigate the influence of impeller type in
a mechanically stirred airlift bioreactor for non-Newtonian viscous fluids. The
agitation was carried out through a marine impeller and paddle impeller. The
bioreactor was sparged with air under different velocity (0.036 – 0.06) m/s.
carboxymethylcellulose 1.94% and xanthan 1.80% were used as a fluid model. The
stirring speed ranged from 0 to 800 rpm. An axial 3-bladed marine impeller or a radial
4-bladed paddle impeller with the same geometric similarities and 40 mm of diameter.
The Ar/Ad ratio was 1.8. The system used was air/CMC solution 1.94%/xanthan gum
solution 1.80%. The spherical oxygen velocity in the riser UGR ranged from 0.0157 to
0.0262 m/s. The result was found the mixing time decrease linearly with the spherical
gas velocity. Also, the agitation increased the gas fraction in the bioreactor, and this
increase was greater with a paddle impeller. The gas fraction was very low, when
using xanthan gum as model fluid. Furthermore, the increased viscosity is a critical
factor that must be taken into consideration when choosing the bioreactor. Non-
Newtonian viscous fluids generally have low gas holdup that is independently of the
reactor type.
Oscar, et al. (2007) measured the oxygen transfer, mixing time and gas
holdup characterization in a hybrid bioreactor. Global oxygen transfer coefficient
(KLa), gas holdup (ε) and mixing time were characterized in mechanically agitated

28. Page | 20
airlift. For the measurements, water and culture medium were used under different
agitation and aeration conditions. The gas-liquid oxygen transfer, mixing, gas holdup
and liquid circulation in a draft-tube airlift bioreactor (diameter and height were
0.755 and 3.21 m, respectively). Characterization was carried out using water and
cellulose fibre slurries (2-4%) in sodium chloride (0.15 M); achieving an
improvement in mixing and oxygen transfer capacity due to mechanical agitation. the
liquid circulation time, gas holdup and oxygen transfer performance in a concentric
airlift reactor with 0.25 m3 as total volume (internal diameter of 0.371 m and the
ratio of down comer – riser cross sectional area Ad / Ar was 0.83). The area-volume
ratio was 0.036 m2
. Agitation was made with two Rushton turbines, the 6-bladed
turbines, 0.075 m in diameter (di) were placed at the centerline of the reactor vessel.
The stirring rates 0, 50, 100, 200, 300 and 450 rpm and superficial gas velocities
referred to the riser area (UGr) 0, 0.003- 0.012 m s-1
. A diminution of mixing time is
observed with the increase of superficial gas velocity, however significant
improvement are not appreciated at UGr ≥ 0.01 m s-1
, showing an agitation rate
independence. at 50 r.p.m. there is an increase in the mixing time, for impeller speeds
until 300 r.p.m. it decreases, and further speeds do not make any significant
improvement. Mixing time for water and culture medium at the same aeration and
agitation conditions has not significant difference (< 5s). A residence time increase
for impeller speeds ≤ 300 r.p.m. while the superficial gas velocity is kept constant.
For all cases, increasing the superficial gas velocity is observed a fast gas holdup rise
until the coalesced bubble flow regime is reached, where is generated a reduction in
the holdup increment.
Kawalec and Cisiak (2001) studied the interrelation among liquid velocity,
impeller speed and gas flow rate in stirred airlift reactor. In these experiments a
reactor volume of 10.6 m3
was used. The diameter of the outer “column1” is
120/110mm and the height 1550 mm while the inner “column 2” has a diameter of
80/72mm and a height of 1200 mm. The lower edge of the inner column is at the
height of 22 mm above the reactor bottom. The ring (50 mm in diameter) was made
of a perforated tube with diameter 6/5.5mm is applied as an air sparger with 13 holes,
0.5 mm in diameter, are arranged along the tube every 10 mm. The two-blade
propeller stirrer 3 is placed in the inner column150 mm above the reactor bottom.
The diameter of the stirrer is 60 mm. The blade angle is 30o
. The agitation speed was

29. Page | 21
from 0 to 2400 rpm. The system used was Air/Water/Aqueous saccharose solutions.
The cross-sectional area was used 0.8. The results for this experiment the riser gas
hold-up in the reactor without the gas feeding increases with the stirrer speed and
reaches the value of 0.038at the stirrer speed of 40s±1.The gas hold-up increases also
with an increase of the impeller speed for low gas velocities. The gas hold-up seems
to be almost independent on the stirrer speed at the gas velocity of 0.03 ms-1
. The
riser gas hold-up diminishes with an increase of the stirrer speed for gas velocities
higher than the mentioned value of 0.03 ms-1
. The gas hold-up difference increases
monotonically with increasing gas flow rates. The average square error of the above
equation is found to be11%. It can be noticed that the value of the exponent of the
gas velocity is consistent with that obtained by several authors (Merchuk et al, 1981,
Siegel et al. 1986 and Kawalec-Pietrenko et al. 1992). What can also be observed, is
that the liquid circulation velocity at low values of the stirrer speed i.e. 5 ms-1
and 8.3
ms-1
increases monotonically with an increase of the gas velocity.
S. Kura et al. (1993) investigate the oxygen transfer in a stirred loop
fermenter with dilute polymer solutions. The air was sparged into the fermentor
through the ring sparger after passing through a calibrated rotameter. The air flow
rate was varied between 1.67 x 10-4
and 8.33 x 10-4
m3
/s. The revolutions of the
impellers were varied between 0 and 8 rps. Volumetric mass transfer coefficients
were determined by the dynamic method. A galvanic oxygen probe was used to
measure transient response of the liquid phase oxygen con- centration. The polymer
drag-reducing additive used in this experiment is polyethylene oxide (PEO, Wako
Co., 163-13815) in concentrations of 100, 500 and 1,000 ppm, in water. Vessel:
working volume 0.040 m 3 diameter 0.35 m height 0.60 m, Impeller Pitched blade
turbine (45 ~ ) diameter 0.133 m height of blade 0.0302 m width of blade 0.0302 m
location 0.38 m above the bottom, Six-blade disk turbine diameter 0.119 m disk
diameter 0.0796 m height of blade 0.02485 m width of blade 0.03085 m location
0.08 m above the bottom, Baffles: number 3 width 0.0319 m, Sparger (ring sparger):
number of orifices 24 orifice diameter 0.002 m ring diameter 0.t 3275 m location
0.035 m above the bottom Draft tube: height 0.3 m diameter 0.175 m. The results for
this experiment. The presence of gas phase has been known to reduce the power
consumption. This is mainly due to a decrease in the density of the liquid around the
impeller because of the presence of air bubbles. The reduction in power consumption

30. Page | 22
of about 50% due to the introduction of gas was found. In the range of Ug > 1 x 10-3
m/s, the power consumption was almost independent of the gas flow rate. For
reference, power consumption data for water are also plotted. The reduction in power
consumption due to aeration in dilute polymer solutions seems to be slightly smaller
than that in pure water. An increase of the KLa coefficient with gas flow rate was
found. A dependence on the gas flow rate decreased with increasing impeller speed.
In aerated stirred tank reactors, there are two limiting mixing regions: Agitation -
contr011ing region at high impeller speeds and aeration-controlling region at low
impeller speeds. At high impeller speeds, KLa is dependent of the impeller speed but
not the gas flow rate.
Rostami et al. (2005) measured the volumetric mass transfer coefficient ( )
and gas hold-up in agitated airlift reactor. The reactor was mechanically agitated by
two sets of 5-bladed Prochem impellers. The impeller speed was varied from 0 to 5
revolutions per second. The dimensions of the airlift reactor used were 3.22 m height,
0.755 m inner diameter, with draft tube of 2.055 m height and 0.32 m diameter. The
reactor was loaded with the coveted fluid (water, 1, 2, and 3% SF slurries), which
formed the continuous phase. The slurry was removed by bubbling nitrogen gas into it
and agitating it. The airflow rates were measured with a precalibrated rotameter. In
order to study the effect of riser superficial gas velocity on and KLa, air was
sparged to the pool of liquid (or slurry) at precalibrated rotameter readings. As well
effect of impeller velocity was studied on and KLa. Dissolved oxygen
concentration in the liquid was measured as a function of time. The transient gassing-
in technique was used to determine the dissolved oxygen concentration in liquid. A
polaro- graphic probe No. 5739 with standard membrane was connected to YSI model
57 dissolved oxygen-meter, and the output response was connected to a Pantos U228
recorder. The membrane of the oxygen probe was changed after every set of
experiments to ensure proper maintenance of the probe sensitivity. KLa is calculated
by . Gas holdup is an important parameter in bioreactor design
because liquid circulation velocity, residence time of the gas in the liquid, total
dispersion volume, and the gas liquid interfacial area, a, for mass transfer all depend
on gas holdup. It calculated by question 2.3. The results were, increasing riser
superficial dispersed phase gas velocities (UGR), and KLa increased. If viscous
forces increased, the bubble terminal rise velocities increased and therefore the riser

31. Page | 23
fractional dispersed phase gas holdup ( ) decreased. Further, since decreased a,
consequently KLa decreased. however, it can be seen that as the impeller speed was
increased the and KLa values increased.
Tervasmäki et al. (2016) measured the overall mass transfer coefficient by
dynamic gassing method and measured the local gas hold-up by electrical impedance
tomography (EIT) using two types of reactors, the OKTOPs9000 reactor which is
agitated with a single impeller located just below the draft tube and STR (stirred tank
reactor) with three Rushton turbines. Both reactors have the same geometries. Overall
gas–liquid mass transfer coefficient (KLa) was measured by absorbing gas into liquid.
The liquid was sparged with nitrogen to displace all dissolved oxygen. When DO had
reached 0%, agitation and nitrogen flow were stopped. Dissolved oxygen
concentration was measured from two liquid levels, and KLa was calculated from the
saturation phase. Values for KLa were estimated using
(̅̅̅ ) (2.17)
Local values for gas hold-up were measured by electrical impedance tomography
(EIT). The measurement method is based on electrical conductivity of the dispersion
and has previously been used for gas hold-up measurements in a flotation cell as well
as for the measurement of solids distribution in a stirred tank. It is calculated by
( )
(2.18)
The results were OKTOPs9000 reactor was found to have higher kLa values than the
STR with similar agitation power and gas flow rate. The overall gas hold-up was
similar in both geometries at same power inputs and gas flow rates.

32. Page | 24
CHAPTER THREE
Experimental Work
3.1 Equipment Setup
In this work, agitated airlift bioreactor was equipped with 4 porous ceramic air
distributers. The experimental unit used in this work is shown in Figures 3.1- 3.3.
The Equipment used are:
1- Flow meter input and Output
2- Compressor
3- Airlift bioreactor
4- Stirrer (CAT R100S-D)
5- Dissolved oxygen meter (AZ 8403)
6- Computer: used to monitor the dissolved oxygen by handheld (V3) program,
7- Water supply pipe
8- Drainage
9- Flow meter
10- Air distributer
11- Impellers (three types used).

33. Page | 25
3.2 impeller Setup
Three types of impeller were used in three arrangements as shown in Figures 3.4- 3.6.
Figure 3.7 shows the impellers used in this work where:
impeller A: Concave disc (CD-6)
impeller B: 45o
Piched blade turbine
impeller C: Rushton disk Turbine
Figure 3.5:
Configuration-2.
Figure 3.4:
Configuration-1.
Figure 3.6:
Configuration-3.
A B C
Figure 3.7: Impellers used.

34. Page | 26
3.3 Design of mechanical agitated airlift bioreactor
The airlift bioreactor consists of a rectangular vessel of height, width and length of
68.4, 17.9 and 22.49 cm respectively. A riser and two down comers are made with
two vertical plates with dimension of 33.15×17.5 cm. The liquid level in the
bioreactor is 58 cm, so the working volume is 23.35 L the area of the riser and the
down comers are 205.41 cm2
and 182.401 cm2
respectively. Four porous ½” ceramic
spargers were used for distributing the air. The height of internal gas distributor was
6 cm from bottom. By using a rotameter, The air flow was controlled where the
superficial gas velocity in the riser was varied between 0.0081 - 0.021 m/s and the
stirrer speed was varied between 0 - 400 rpm. Different impellers configurations of
bioreactor were used to find hydrodynamic parameters such as gas holdup and
volumetric mass transfer coefficient.
3.3.1 Motor/stirrer
The brand and type of stirrer used for Agitating the water in the Agitated airlift
bioreactor is shown in Figure 3.8. CAT R100S-D is a stirring motor with power of
100 watt and with speed range of 40-2000 rpm.
3.3.2 Dissolved Oxygen measurements
The instrument used in this work was AZ 8403 dissolved oxygen meter as shown in
Figure 3.9. The data are imported to the computer using handheld program.
Figure 3.8: Stirrer Used.

35. Page | 27
Figure 3.9 shows also the set up used to avoid the bubbles that accumulate on the
probe by dipping it at the bottom
Figure 3.9: Dissolved oxygen apparatus.
3.4: KLa measurements:
Calibration is done whenever a session of experiment is done, and procedure
for the calibration is in the Operation manual where it says four steps:
1- In normal mode, hold the probe in the air, wait for few minutes until the
reading on LCD is stable enough. Press CAL/Esc button on the meter to
calibrate for 100% saturation calibration, CAL icon will be flashing on LCD.
2- Wait a couple of seconds, when the reading is stable, press ENTER button on
meter to finish the calibration, the meter will automatically calibrate to 100%
air saturation and return to normal mode.
3- You can stop the calibration by pressing CAL/Esc.
4- Whenever an error occurs during the calibration, the ERR indicator will
appear.
For measurement of the volumetric mass transfer coefficient a dynamic method is
used that depend on the response of dissolved oxygen concentration change in the
reactor. In this method the concentration of dissolved oxygen was reduced to
maximum of 0.5 ppm by using sodium sulfite. Then the air is supplied at the same

36. Page | 28
time the concentration of dissolved oxygen measured with time until saturation point
is reached. Equation (2.1) used to find the mass transfer coefficient.
Separating the variables and integrating from C0 to C and t0 to t as in equation (2.2).
The plot between ( ) and t will result in a line with a slope of - KLa as shown
in the following graph:
Figure 3.11 shows the procedure for measuring of volumetric mass transfer
coefficient according to equation (2.2)
Figure 3.11 Calculation procedure for KLa
3.6 Gas hold-up measurements
The general method to evaluate the gas hold-up is to measure the distance between the
un-gassed and gassed liquid level. The overall gas holdup was calculated using
equation (2.4). The steps are as follows:
- The level of water was measured at rest.
- The compressor was switched on to supply air with flow rates (10- 20 L/min).
- The increase in the level of water was measured by ruler.

37. Page | 29
CHAPTER FOUR
Results and Discussion
Mass transfer coefficient KLa and Gas holdup were measured at constant
conditions with different gas velocity (Ug), different arrangement of various impellers
and impeller speed. three arrangements of impellers were used are shown in Figure
3.4, 3.5 and 3.6 to study the effects of impeller speed and riser gas velocity (Ug) on
volumetric mass transfer coefficient and gas holdup in mechanically agitated airlift
bioreactor. The results were obtained at five different ranges of air flow rate at the
same conditions.
4.1 Volumetric mass transfer coefficient KLa
Figure 4.1 shows the effect of different Ug on KLa at different impeller speed
for the arrangement of impeller (Configuration-1) as shown in Figure 3.4 (45o
Pitched
blade turbine and Rushton disk Turbine).
Figure 4.1: Ug vs. KLa at different impeller speed (Configuration-1).
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016 0.017
KLa(1/S)
Ug (m/s)
0 rpm 100 rpm 200 rpm 300 rpm 400 rpm

38. Page | 30
Figure 4.2 shows the effect of different Ug on KLa at different impeller speed
for the arrangement of impeller (Configuration-2) as shown in Figure 3.5 (dual
Rushton disk Turbine).
Figure 4.2: Ug vs. KLa at different impeller speed (Configuration-2).
Figure 4.3 shows the effect of different Ug on KLa at different impeller speed
for the arrangement of impeller (Configuration-3) as shown in Figure 3.6 (45o
piched
blade turbine and concave disc (CD-6)).
Figure 4.3: Ug vs. KLa at different impeller speed.
0
0.005
0.01
0.015
0.02
0.025
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016 0.017
KLa(1/S)
Ug (m/s)
0 rpm 100 rpm 200 rpm 300 rpm 400 rpm
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016 0.017
KLa(1/S)
Ug (m/s)
CHART TITLE
0
0.005
0.01
0.015
0.02
0.025
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016 0.017
KLa(1/S)
Ug (m/s)
0 rpm 100 rpm 200 rpm 300 rpm 400 rpm

39. Page | 31
Figure 4.4 shows the values of KLa vs Ug at 100 rpm with three arrangements
A1(Configuration-1), A2(Configuration-2) and A3(Configuration-3).
Figure 4.4: KLa vs Ug at 100 rpm with three arrangements
Figure 4.5 shows the values of KLa vs Ug at 200 rpm with three arrangements
A1(Configuration-1), A2(Configuration-2) and A3(Configuration-3).
Figure 4.5: KLa vs Ug at 200 rpm with three arrangements.
0.004
0.005
0.006
0.007
0.008
0.009
0.01
0.011
0.012
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016
KLa(1/S)
Ug (m/s)
A1 A2 A3
0.004
0.005
0.006
0.007
0.008
0.009
0.01
0.011
0.012
0.013
0.014
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016
KLa(1/S)
Ug(m/s)
A1 A2 A3

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Figure 4.6 shows the values of KLa vs Ug at 300 rpm with three arrangements
A1(Configuration-1), A2(Configuration-2) and A3(Configuration-3).
Figure 4.6: KLa vs Ug at 300 rpm with three arrangements.
Figure 4.7 shows the values of KLa vs Ug at 400 rpm with three arrangements
A1(Configuration-1), A2(Configuration-2) and A3(Configuration-3).
Figure 4.7: KLa vs Ug at 400 rpm with three arrangements.
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016
KLa(1/S)
Ug (m/s)
A1 A2 A3
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0.02
0.022
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016
KLa(1/S)
Ug (m/s)
A1 A2 A3

41. Page | 33
4.1.1 Effect of gas velocity on KLa
Figures 4.1-4.3 shows KLa versus the gas velocity where it appears that the
value of KLa increases as the air velocity increases. This is due to the fact that In the
heterogeneous regime ratio KLa/εG tends to have constant values and since the hold-up
is increased with increasing Ug, KLa will also increase (Vandu and Krishna, 2004).
This behavior will be in agreement of the work of Bang et al. (1998), Xiaoping et al.
(1998), de Jesus et al. (2014) and Rostami et al. (2005). This increase will be inhibited
when we reached flooding.
4.1.2 Effect of impeller speed on KLa
Figure 4.1-4.7 illustrate the values of KLa with respect to different impeller
speed (0-400 rpm). KLa value increased as the impeller speed increased. As the speed
of impeller increase the bubble size decrease, this will cause higher gas-liquid area of
contact and this will increase KLa. Rostami and Moo-Young (2005), Lukić et al.
(2017), Xiaoping et al. (2000), De Jesus et al. (2014) and Chisti and Jauregui-Haza
(2002) found similar trend where using mechanically agitated airlift bioreactor affects
gas-liquid mass transfer behavior. KLa value was enhanced by increasing aeration and
agitation rates; bubbles with a smaller diameter and lower rise velocities were
observed.
4.1.3 Effect of impeller configuration on KLa
According to Figures 4.4-4.7 the arrangement congfiguration-1 and
congfiguration-3 have a behavior where both gives the same response, where axial
and radial type were used. Wu et al. (2014) stated that axial impellers are circulating
the fluid inside the airlift bioreactor while radial impellers are breaking the bubbles
into small bubbles. At configuration 3 concave disc impeller (CD-6) were used and it
is very efficient in power consumption where its uses less power for the same mixing
speed. Whereas the arrangement congfiguration-2 was getting better in KLa values as
the impeller speed increased. De Jesus et al. (2014) indicate that when the radial
impellers were used, there was an increase of 50% over the airlift mode and, 30-40%
in relation to the axial impellers. It was also noticed that by increasing the superficial
gas velocity, very close values of KLa were obtained for the two types of radial
impellers used. Comparative studies with RT and axial impellers (A315) in cultures of
Streptomyces in a stirred bioreactor De Jesus et al. (2014) revealed that the kLa value

42. Page | 34
increases by 50% with the use of RT impellers. with the best results being achieved
with the A310 model.
4.2 Gas Holdup
Figure 4.8 shows the effect of different Ug on gas holdup at different impeller
speed (45o
Pitched blade turbine and Rushton disk Turbine).
Figure 4.8: Ug vs. gas holdup at different impeller speeds at A1 (configuration-1).
Figure 4.9 shows the effect of different Ug on gas holdup at different impeller
speeds for dual impeller (dual Rushton disk Turbine).
Figure 4.9: Ug vs. gas holdup at different impeller speeds A2 (configuration-2).
3
4
5
6
7
8
9
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016
GasHoldup%
Ug (m/s)
0 rpm 100 rpm 200 rpm 300 rpm 400 rpm
4
4.5
5
5.5
6
6.5
7
7.5
8
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016
GasHoldup%
Ug (m/s)
0 rpm 100 rpm 200 rpm 300 rpm 400 rpm

43. Page | 35
Figure 4.10 shows the effect of different Ug on gas holdup at different
impeller speed for dual impeller (45o
Piched blade turbine and Concave disc (CD-6)).
Figure 4.10: Ug vs. gas holdup at different impeller speed A3 (configuration-3).
Figure 4.11 shows the percentage of gas hold up at different impeller speed
at 0.0162 ms-1
for the three configurations.
Figure 4.11: Gas holdup % vs. impeller Speed (rpm) at different impellers
configuration at Ug of 0.0162 m/s.
6
6.2
6.4
6.6
6.8
7
7.2
7.4
7.6
7.8
8
0 50 100 150 200 250 300 350 400
GasHoldup%
Impeller Speed (rpm)
A1 A2 A3
3.5
4
4.5
5
5.5
6
6.5
7
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016
GasHoldup%
Ug (m/s)
0 rpm 100 rpm 200 rpm 300 rpm 400 rpm

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Figure 4.12 shows the effect of different Ug on gas holdup at different
impeller configuration for 400 rpm.
Figure 4.12: Gas holdup % vs. Ug at 400 rpm.
4.2.1 Effect of gas velocity on gas holdup
As can be observed from Figures 4.8 - 4.10, the gas hold-up is increased with
increasing the gas velocity (Ug), because the bubbles are relatively small, increasing
the gas velocity will result in increasing the circulation of the air through the
downcomer thus increasing the gas holdup. Chisti and Jaureui-Haza (2001) and de-
Jesus et al. (2013) found that fractional gas holdup increases with increase aeration
rates.
4.2.2 Effect of impeller speed on gas holdup
As shown in Figures 4.8 – 4.10, as the impeller speed increased, the gas
holdup increased. And the reason for that, is that the larger bubble rise faster and
make the gas holdup reduced, so having agitation will break the bubbles reducing
their size thus increasing the gas holdup. Which corresponds with [de Jesus et al.
(2017)] who says Adding stirred will increase gas hold-up as well as KLa and KLa
increased with air flow. Also, De Jesus et al. (2014) claimed that the presence of
agitation favored the increase of gas holdup.
4
4.5
5
5.5
6
6.5
7
7.5
8
0.008 0.009 0.01 0.011 0.012 0.013 0.014 0.015 0.016 0.017
GasHoldup%
Ug (m/s)
A1 A2 A3

45. Page | 37
4.2.3 Effect of impeller configuration on gas holdup
As shown in Figures 4.8 - 4.10, the gas holdup in Configuration-1 was very
high where it reached maximum of 8%, Configuration-2 and Configuration-3 gives
good response in gas holdup. Also, impeller speed has relatively close response where
increasing the impeller speed increases the holdup. De Jesus et al. (2014) stated that
when the agitation was performed with radial impellers, there was a significant
increase in gas holdup relative to the axial impellers that were not big contributors to
the increases of gas holdup. The presence of agitation favored the increase of gas
holdup, especially when using radial impellers. The experiments performed with
radial impellers provided very similar results, in which there was an increase in
function of the superficial gas velocity. However, when the experiments were
performed with axial impellers, there was a very small increase in gas holdup,
compared to experiments performed in the absence of agitation, especially when using
a BM impeller.

46. Page | 38
CHAPTER FIVE
Conclusion and Recommendations for Future Work
5.1 Conclusion
In this project, many experiments have been done to study the effect of three different
parameters namely impeller speed (rpm), impeller type and gas flow rate on gas hold
up and volumetric mass transfer coefficient in mechanically agitated airlift
bioreactor. Three impellers configurations are selected using 45o
Pitched blade
turbine, Rushton disk Turbine and Concave disc (CD-6). Different gas flow rates (10-
20 L/min) and impeller speeds (0 – 400 rpm) were applied.
The actual experiments data evaluated in several steps to find the KLa. First step is to
draw the result in linear equation form (equation (2.2)) by using excel file from lap
computer. And the slop for linear equation is kLa and has an error (R2
), the kLa that
are found have different values in all parameters and the error (R2
) is how close the
data are from the fitted regression line. It was between 0.85-0.99, have average 0.96
for all data. The experiments are repeated three times to find a good accuracy in the
result and readings. 225 experiments were done to get the result that are in this study.
Finally, KLa values were selected at different gas flow, impeller speed and type. And
KLa vs Ug was drawn with different speed to compare between them. Likewise, the
gas holdup values were obtained by measuring the bubbles increase with ruler and
then equation (2.4) was used to find the gas holdup .
The following conclusions can withdraw from this study:
- KLa value increased as the impeller speed increased, when the speed of
impeller increases the bubble size decrease, the transfer area will increase thus
the diffusivity rate will increase, this will cause higher gas-liquid contact.
- The gas hold-up is increased with increasing the gas velocity (Ug) and impeller
speed. Increasing the gas velocity will result in increasing the circulation of
the air through the downcomer thus increasing the gas holdup and reducing the
bubble size by the agitation will also increase the gas holdup.

47. Page | 39
- The value of KLa increases as the gas velocity increases. As the gas holdup is
increased because of increasing the gas velocity, results in having more air
holds up in the tank thus increasing the KLa.
- The gas holdup in Configuration-1 was very high where it reached maximum
of 8%, Configuration-2 and Configuration-3 gives relatively low gas holdup.
Having the highest holdup for the first arrangement of impeller is maybe due
to the axial impeller (45o
Pitched blade turbine) the contribute greatly in
pushing the water up and low hold up in the third arrangement is because the
impeller CD-6 doesn’t involve much in increasing the gas holdup.
This result just for three different type impellers but the world doesn’t stop, it has
so many types of impellers and for each impeller they have Special properties and
technology for it.
5.2 Recommendations for Future Work
In order to have better understanding of the hydrodynamics of this type of
contactors the following points should be addressed in more details in future:
- Measuring the liquid circulation velocity at different operating
conditions.
- Study the effect of liquid viscosity on mixing time and overall mass
transfer coefficient at different operating conditions.
- Use of more impeller design and types to see their effect on KLa.

48. Page | 40
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