Sohar University Study on Determination of Volumetric Mass Transfer Coefficient in Stirred Vessels Done By Laila Moosa Al-Blushi, Maryam Said Al-Oasimy Sahar Payam Allah Talebi, Shamsa Ahmed Al-Haddabi, and Sumaya Hamood Al- Maawali A Final Year Report Submitted to Sohar University in partial fulfilment of the requirements for the degree of Bachelor of Engineering in Chemical Engineering
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Sohar University
Study on Determination of
Volumetric Mass Transfer Coefficient
in Stirred Vessels
Done By
Laila Moosa Al-Blushi, Maryam Said Al-Oasimy Sahar Payam Allah Talebi, Shamsa Ahmed Al-Haddabi, and Sumaya Hamood Al- Maawali
A Final Year Report
Submitted to Sohar University
in partial fulfilment of the
requirements for the degree of
Bachelor of Engineering
in
Chemical Engineering
Sohar, Sultanate of Oman, June 2013
Supervised by: Dr. Ahmed Jawad Ali Al-Dallal
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
Acknowledgment
The project group would like to thank their supervisor Dr. Ahmed Al-Dallal for his
assistance, valuable advice, and support along the project. They also appreciate Mr.
Ibrahim Al-Ajmai and Mr. Rashid for their never end support, and their continuous help
during the design, manufacturing and experimenting the aeration device build.
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.
Executive Summary
In this thesis we will report the design and fabrication of an aeration experimental unit
used to calculate the oxygen gas-liquid volumetric mass transfer coefficient (kLa). The
total cost of manufacturing the unit was almost 500 O.R and was totally designed and
arranged by the students. The mixing tank designed to have a total volume of …… with
dimensions ……………. Distilled water at atmospheric pressure and room temperature
C Dissolved concentration of gas in liquid phase (mol/m3)
C* Saturation concentration of the gas in liquid (mol/m3)
D Diameter of impeller (m)
g Standard gravity (9.81 m/s2)
H Depth of liquid in the tank (m)
kL Mass transfer coefficient (m/s)
kLa The volumetric mass transfer coefficient (1/s)
m Mass of liquid in STR (stirred tank reactor) (kg)
N Impeller speed (rev/s)
NP Power number of the impeller
P Power (W)
Qg Gas flow rate (m3/s)
T Diameter of the tank or vessel (m)
to Initial time
v Kinematic Viscosity (m2/s)
VG Superficial gas velocity (m/s)
VL Volume of liquid inside the reactor (m3)
w Weight
μ Viscosity of liquid phase (Pa. s)
ρl Density of the liquid phase (Kg/m3)
σ Interfacial tension (N/m)
Chapter 1 Introduction
In all aerobic chemical and biological process, effective oxygen supply and absorption is
one the most required and important principal. Some examples on operations that utilizes
between oxygen gas and a liquid phase are leaching of metal concentrates and
microbiological fermentation. Aeration is the process in which oxygen is added and
dissolved in water by utilizing the principles of mass transfer. It is one of the most
important processes used in public health engineering as it eliminates both smell and taste
from water. Transfer of oxygen into a mass of water can either occur naturally for
example, surface aeration of polluted surfaces, or under imposed conditions such as, the
activated sludge process. The reverse of aeration is also possible for example, losses of
oxygen to the atmosphere from supersaturated water due to photosynthesis.
There are different devices that are used to bring oxygen and liquid phase to contact such
as, packed bed, bubble column, tray tower, and plate columns. However, in this thesis a
mechanically agitated device will be used in which the liquid phase is the continuous
phase, compressed air is the dispersed phase and agitation is insured by a rotating
impeller [sulpite]. Mechanically agitated vessels are very widely used in reactions that
involve gas and liquid phase, because they can provide high heat and mass transfer
coefficient and mixing [O].
Oxygen is a non-polar gas and it is more soluble in water compared to nitrogen gas.
When water is in equilibrium with air, it will contain one molecule of dissolved oxygen
for every two molecules of nitrogen. The solubility of oxygen in water depends on the
temperature at which this process occurs. It is observed that at 25ᴼC and 1atm of air,
fresh water will contain approximately 6.04 ml of oxygen per litre. On the other hand,
seawater contains almost 4.95 ml per litre. This is due to presence of salt and other
impurities in seawater that will reduce the solubility of oxygen in water.
The volumetric mass transfer coefficient (kLa) is a lamped parameter as it depends only
on time not on space. It indicates the rate of oxygen that has transferred from the gas
phase to the liquid phase and has the unit of [1/time]. kLa is a very important parameter in
designing, scaling up from laboratory scale to pilot scale or production scale of
bioreactors especially. There are several factors that affect the transfer of oxygen to liquid
phase such as water and they are:
Depth of water used.
Water composition, if there are any salt or impurities present.
Design and arrangement of diffusers.
Flow rate of air supplied.
Speed of agitator used.
Water temperature.
Type of impeller.
Air pressure.
In this thesis, the affect of different impeller types and speed, gas flow rate, liquid
viscosity, and addition of salts on kLa will be studied and observed. At the end, an
empirical equation will be determined for possible causes of study.
Chapter 2: Literature Survey
Aeration is the process of making air and water in direct close contact for the purpose of
removing dissolved gases or to oxidize dissolved metals. This process can also be used to
remove volatile organic chemicals (VOC) in the water [*]. There are several industrial
applications that use’s the concept of aeration, such as: (1) oxidation of soluble metals
(iron and manganese); (2) removal of carbon dioxides during cold lime softening; (3)
reducing the concentration of VOC in water by air stripping process. There are two main
methods of pond aeration systems there are used, diffused aerators and surface aerators.
Diffusion aeration uses a tube that is connected to an air compressor to force the air in the
tube and forms bubbles which are then released in the water. The tubing system is usually
placed in the bottom of the pond. This method is also known as the lake-bed aeration or
bottom aeration. Diffused aerators has higher efficiency in deep ponds, depths more than
8 feet, because of the longer contact time and it's safer due to the ability of keeping the
electricity out of water by placing the compressor away and avoud direct contact with
water.
Surface aeration uses a pump which is placed just above the pond surface to circulate
water from the pond surface into the air or it can be just right at the surface. Surface
aerators are preferred in shallow ponds but it doesn't suits deep lakes.
Figure 2.1: An example on diffusion aeration and surface aeration [].
Advantages Disadvantages
The value of volumetric mass transfer
parameter can be very high.
A sparger and hence a compressor have to
be installed, leading to more investment
and to the process more complicated.
Large changes in the volumetric mass
transfer parameter could be achieved by
changing the gassing rate and power
input.
More power consumption due to the use of
a compressor.
Table 2.1: The advantages and disadvantages of diffusion aeration [].
2.1 Methods for measuring kLa:kLa is a common measure of mass transfer characteristic between a gas and a liquid phase
and a very important parameter in designing and scaling up bioreactors. It is calculated
using the film model of mass transfer between the two phases by assuming that the
resistance at the gas phase is negligible. There are several methods that are used for
measuring kLa and are described shortly below:
Dynamic method. This method applies changes in the dissolved oxygen
concentration either by a chemical reaction of by changing the pressure of the
system and consists of the following sub methods:
- Dynamic pressure method. Changes in concentration are achieved by
changing the pressure in the system and this will cause changes in the partial
pressure of oxygen. Pinelli et al. (2010) reported that if there is any sudden
change in the system pressure will cause a noticeable change on the partial
pressure of oxygen in all bubbles.
- Gassing in and gassing out method. The change in oxygen concentration of
the entering gas is done by either a step change from lower concentration to
higher and is known as the gassing in method or a step change from higher
concentration to lower and it is known as gassing out method. This is done
usually by using pure nitrogen gas. Although this method is expensive and
requires very pure nitrogen gas (99%), but it is fast, low time requirement and
the results obtained compromises the high cost.
The kLa value is calculated by fitting a model that had been chosen to represent
the system to the recorded transient curve of dissolve oxygen. There are several
factors that might affect the dynamic model such as, fluid dynamic model used for
the gas that is introduced to the system and the response time of oxygen probe.
Steady state methods. In these methods a condition is created to keep the
concentration of dissolved oxygen constant at the same time that oxygen is
introduced into the system as an inlet gas and is continually consumed by an
oxygen sink such as, microbiological organisms. The kLa is calculated either from
the stoichiometric oxygen consumption and time required for total consumption
or from the mass balance on oxygen. The first method requires data generated for
dissolved oxygen and time and the second method requires the flow rate and the
oxygen concentration of the inlet and outlet gas. It is very important that the
reaction rate and the mass transfer rate are in right proportional to each other. If
the reaction rate is low compared to the mass transfer rate than the process is
controlled by the kinetics of the reaction, but if the reaction rate is very high than
the mass transfer will be higher and better [ اطروحه]. Sodium sulfite oxidation method. The sodium sulfite method is used in chemical
reactions to measure kLa, where it is used to calculate the oxygen uptake rate. The
sulfite is oxidized to sulfate which is consuming oxygen in the process [+]. by the
following reaction:
2Na2SO3 + O2 2Na2SO4
The amount of sodium should be around 10 to 15% of the total liquid volume and
has a concentration range from 0.04 to 1N [klz correlatin]. A catalyst of cobalt
should be introduced to the tank before the sodium sulphite. It is loaded only once
at the beginning of the process [_]. The reaction rate is faster than the oxygen
transfer rate, so the rate of the oxidation reaction will be controlled by the rate of
the mass transfer. If the overall rate is measured than the mass transfer rate could
be calculated. The concentration of the catalyst added affects the reaction rate
and bubble coalescence. The kLa value was estimated using steady state sulphite
method by Puskeiler and Weuster-Botz (2005). It was found that kLa value
increased as the power input increases in the steady state sulphite method on log-
log plot. Using dynamic sulphite method a maximum value of kLa can be reached
at certain power input after that as the power input value increases value the mass
transfer coefficient will decrease as it is shown in figure below.
2.2 Factors that affect kLa value:
2.2.1 Effect of salt addition and viscosity:One of factor that affects the transfer rate of oxygen and the value of kLa is the water
compositions and the presence of solid particles in water. This factor is very important in
soil treatment in slurry bioreactors. The efficiency of those types of reactors is described
as the amount of soil treated per unit volume of reactor, but there is a limit to the amount
of soil treated due to limitation in oxygen supplied. Researches and experiments have
shown that kLa values are significantly dependent on particle size of the soil and the clay
content, but are not affected noticeably with the concentration of soil organic matter. As
the soil content increases above 40% the kLa value decreases to about 60 to 70% of the
value of kLa in pure water. It was also noticed that the oxygen supply is limited, diameter
and the number of bubbles also decreases if the clay content is very high [ ].
Robinson et al. (1973) and Linek et al. (1987) have studied the effect of medium
properties on the value of kLa. They have found that adding small amounts of solute to
distilled water will cause reduction in bubble size even if other properties were kept
constant. They also reported in their research paper on kLa values in electrolytic mediums
that for electrolytic solutions the decrease in bubble size is due to the electrical effect of
the concentration gradient of the ionic species between the gas and liquid interface. This
makes the medium capable of preventing coalescence of bubbles so an increase in the
value of kLa is noticed [ two refrences ].
Juarez and Orejas (2001) reported the effect of medium properties on the value of kLa by
considering two different liquids which were distilled water and 0.5M Na2SO4 solution.
The distilled water is known to act as a coalescing system and the aqueous solution acts
as a non-coalescing system. The operating conditions used could be summarized in the
below table:
Operating variable Used Value
Liquid volume 0.0015 m3 if one impeller
0.002 m3 if two impellers
Temperature 20ᴼC
Rotation speed 5 to 15 r.p.s
Gas flow rate Varied between 2.5, 3.75 and 5×10-5 m3/s
Experiment time 200 to 700 s
Table 2.2: The operating condition used by Juarez and Orejas [ ].
The difference in the two mediums was absorbed from the results obtained and it was
noticed that sulphate solution has a higher kLa value due to its non-coalescing properties.
Although water is a coalescing system and showed low values of kLa when using only
one impeller, but increasing the number of impellers to two will help to break down the
bubble size and cause an increases in kLa. On the other hand, in sulphate solution the
value of kLa obtained is not affected if one or two impellers are used because the solution
prevents collection of bubbles.
Puthli et al. (2004) experimented the effect of liquid viscosity on kLa values using three
different concentrations of carboxymethyl cellulose (CMC) which were 0.25, 0.5 and
0.75% w/v. The actual volume used was 1.81m3. The effect of changing the viscosity was
estimated at an impeller speed of 600 rpm and gas flow rate range from (0.9-3.4)*10-5
m3/s. The following table and graph shows the results obtained [&]:
Concentration of CMC Viscosity of solution Air flow rate range Value of kLa
%w/v mPa 10 -5(m3/s)
(s-1)
0.25 4.33 0.9 0.0082
3.4 0.0161
0.5 5.99 0.9 0.0058
3.4 0.0095
0.75 11 0.9 0.0021
3.4 0.0037
Table 2.3: The effect of medium velocity on the kLa value obtained by Puthli et al [].
Figure 2.2: Effect of viscosity at 600 rpm at varying gas flow rate (triple impeller (♦) water, (□) 0.25% CMC (w/v), (∆) 0.375% CMC (w/v), (○) 0.5%CMC (w/v)) obtained by Puthli et al [].
From the results obtained the writes concluded that increasing the viscosity of the
medium will reduce the kLa values regardless the speed of impeller or the gas flow rate
used. This behaviour is probably because of the decreases in the surface area of the
bubbles due to the viscous forces generated in the fluid.
Ozbek and Gayik (2000) have studied the effect of adding glycerol to distilled water with
the following composition 10,20,30,40 and 50% glycerol solution w/w. A volume of 0.6
L at a temperature of 37ᴼC and a pH of 7 was used .The operation condition used in the
experiments and the values obtained is described and summarized as following:
Agitation
speed
(rpm)
Air flow
rate
(L/min)
Range of composition of
glycerol solution
(% w/w)
Viscosity
(cpoise)
kLa value
(min-1)
300 0.3 10 0.935 2.65
50 6.948 1.4
Table 2.4: The effect of changing the viscosity of water by Ozbek and Gayik [].
Figure 2.3: The kLa values versus the viscosity of the medium by Ozbek and Gayik [].
The graph proves the truth that any increase in the medium viscosity will decrease the
value of kLa. The authors than investigated what happens if medium viscosity is changed
under the same operating condition, but also the agitation speed and the flow rate of air is
varied. The following graphs were obtained when changing both impeller speed and
aeration rate:
Figure 2.4: The effect of stirrer speed on kLa in glycerol solution by Ozbek and Gayik [O].
Figure 2.5: The kLa value verses air flow rates in glycerol solution by Ozbek and Gayik [O].
It is noticed that although increasing the viscosity will decreases the value of kLa, but
increasing the agitation rate or the gas flow rate could reduce this effect as it is noticed
from the graphs. It is also noticed from the figure that after 300 rpm the value of kLa
increased linearly with increasing the agitation rate.
2.2.2 Effect of probe position: Hashsham, S.A. in his research article under the title of “Design of an Experimental Unit
for Determination of Oxygen Gas-Liquid Volumetric Mass Transfer Coefficient using
Dynamic Re-Oxygenation Method” has experimented and determined the volumetric
mass transfer coefficient in a bubble column reactor.
The writer’s goal was to design and build a bubble column reactor for measuring kLa. He
installed several polarographic dissolved oxygen sensors in various axial positions along
the column to measure the dissolved oxygen. The author assumed that the bubble column
follows the model of a CSTR and the liquid used was distilled water at room temperature.
The equipments used could be described as following:
1- Bubble column. A hexagonally shaped tower with a height of 1.27m and a diameter of 0.318m was used. Three holes were drilled carefully at three different positions along the length of the column to hold the sensor seal. Water was filled from the top using a plastic tube and removed through the bottom hole made for the sensor. The conditions of the reactor under which the experiments were performed are summarized in the below table:
Parameter conditionsTemperature Room temperaturePressure Atmospheric pressureSuperficial gas velocity 0.3 m/sGas hold up 0.009Bubble diameter 7.5 mm
Table 2.5: Operating conditions used by Hashsham, S.A.
2- Air diffuser. The air diffuser used was bought from a pet store and had a diameter of 7.6 cm. However the writes suggested using special designed air diffusers for further experiments.
3- Dissolved oxygen sensors. Three sensors were used in the experiments, but they were not calibrated. The author recommended that for better and accurate results the sensors should be calibrated. A Vernier dissolved sensor was used that contains a platinum cathode and Ag/AgCl reference anode dipped in KCl electrolyte and are separated from the surrounding sample solution by an oxygen permeable membrane.
A fixed voltage is applied to the platinum electrode and the following reactions take place:
0.5 O₂ + H₂O + 2e⁻ 2OH⁻ reduction reaction
Ag + Cl⁻ AgCl + e oxidation reaction
As a result of the reactions, an electrical current flow is generated and measures the
concentration. The current is converted to voltage, amplified, and recorded.
4- Other equipments such as, sensor seal, air compressor, nitrogen cylinders, and personal computer were all used.
To determine the volumetric mass transfer coefficient first of all, any amounts of
dissolved oxygen in the water was removed by supplying nitrogen gas until the
concentration of oxygen felled below 1%. This step usually took 30 min. After that,
oxygen was introduced to the column as compressed air through the diffuser, and the
dissolved oxygen was measured every second until water became saturated. This process
usually took less than 20 min. The experiments were performed three times and the re-
oxygenation profile for each experiment was analyzed individually.
The author noticed that the re-oxygenation plots were almost linear and the estimated
mass transfer coefficient for the three sensors was almost 0.31 ± 0.01 min⁻1. This
observation proves that kLa is a lumped parameter as it didn’t depend on the position of
the sensors.
2.2.3 Effect of aeration rate:Another important factor that affects the kLa values is the gas flow rate which is also
referred to as the aeration rate. Any changes in the flow rate of air entering the system
will cause changes in fractional gas hold up and the interfacial area between gas and
liquid phases, so changes in kLa values are expected to take place.
Puthli et al. (2004) have experimented the effect of changing the air flow rate in a
bioreactor. They have observed that any increase in the gas flow rate will cause increase
in the interfacial area between the two phase due to increase in the surface area of the
bubbles, thus causing an increases in kLa value especially when very high air flow rate
were applied to the system. The total capacity of the bioreactor used was 2.01m3 and the
actual volume was 1.81m3. The following table and graph summarizes the results
obtained [&]:
Type of impeller Speed of the impeller
(rpm)
Air flow rate range
applied (cc/s)
kLa value
(s-1)
Triple impellers
(disc turbine-
pitched blade
downflow turbine-
pitched blade
down-flow turbine
300 8.92 0.0026
33.81 0.0035
400 8.92 0.0045
33.81 0.0081
500 8.92 0.0062
33.81 0.0098
600 8.92 0.0082
33.81 0.0161
Table 2.6: The experimental values of kLa obtained by Puthli et al. using different speeds of impeller and air flow rate [].
Figure 2.6: Effect of aeration rates on kLa values at different impeller speeds (triple impeller (♦) kLa (300 rpm), (□) kLa (400 rpm), (○) kLa (500 rpm), (∆) kLa (600 rpm)) by Puthli et al [].
Eric Jackson studied the effect of aeration rate of kLa values and he also noticed that any
increase in the gas flow rate will increase the kLa of the system. The operating condition
used and the results obtained are all summarized in the coming tables [ ]:
Volume
(L)
Type of
impeller
Speed of
impeller
(rpm)
Air flow rate
Range
(L/min)
kLa value
(s-1)
5 Six blades 500 0.5 0.02
6 0.12
Table 2.7: The operating conditions and the effect of air flow rate of kLa used by Jacjson.
Figure 2.7: kLa versus Aeration Rate in distilled water by Eric Jackson [O].
Ozbek and Gayik (2000) have also studied the effect of aeration rate on kLa values in a
system containing distilled water. They have also proved that increasing the aeration rate
will cause an increase in the value of kLa. The operation conditions used in the
experiments performed and the results obtained are as follows:
Volume
(L)
Temperature
(ᴼC)
pH Agitation rate
(rpm)
Range of
flow rate of
air
(L/min)
kLa value
(min-1)
0.6 37 7 300 0.15 1.728
0.9 5.35
Table 2.8: The experimental values of kLa obtained using different air flow rates for a distilled water system by Ozbek and Gayik [O].
Figure 2.8: The kLa values versus air flow rate in distilled water system obtained by Jackson [O].
In conclusion it is noticed that the kLa value is a strong function of the aeration rate with
a direct proportional relationship between them. Any increase in the gas flow rate will
cause on increases in the number of bubbles and the interfacial area, so kLa value
increases.
2.2.4 Effect of Temperature: Another important factor that affects the kLa value is the temperature at which the
operation takes place. Zhen et al. (2003) have studied this factor and its effects, by
measuring the temperature using a thermometer or by the dissolved oxygen meter. They
assured that the difference in temperature had an accuracy of ± 0.50C at the beginning and
the end of each test. Figure 2.7 shows the standard oxygen transfer coefficient in the
range of tested water temperatures between 27.60 oC to 29.0 oC.
The tests were done in 0.30 m depth of water [].
Figure 2.9: The effect of water temperature on kLa [].
2.2.5 Effect of medium depth:Zhen et al. (2003) have studied the effect of different water depths on the standard
oxygen transfer efficiency (SOTE). Figure 3 shows that SOTE increases as water depth
increases. This is because of the increase on oxygen partial pressure and because of the
increase in time contact between the bubble and water [].
Figure 2.10: The effect of Water Depth on SOTE
2.2.6 Effect of mixing: Mixing is one of the most important parameters that must be studied is biochemical
reaction. Several types of impellers are used in gas- liquid system some examples are: (1)
disc turbines impellers with radial flow are capable of eliminating uniformity of the flow
caused by open turbines. Moreover, they can collect the gas moistures and force it to stay
at high shear zone near the blades where the bubbles are form; (2) the classical Rushton
Disk turbine which is one of the most commonly used mixers for gas-liquid mass transfer
especially in the cases of low and intermediate gassing rate, because it creates high local
shear that is good for dispersion processes (Myers et al., 1994).
2.2.6.1 Effect of impeller Type:Vasconcelos et al. (2000) studied the performance of six blade turbine in an agitated tank
to find the effect of using blade turbine on many factors such as power consumption,
mixing, gas hold up and mass transfer characteristics. These factors were studied under
turbulent agitation. Six blade turbines are the most important impellers used in industries.
However, there are some limitations of six blade turbine an example is that when the gas
is introduced to the system it causes a sudden decrease by about 50% of total power
demand. This limitation will lead to a reduction in both potential of mass and heat
transfer. The liquid used in the experiments was tap water at 37 0C. A compressor used to
supply air to the vessel through 0.66 mm tube. In addition, the diameter of the standard
Rushton turbine D =T/3 [].
Table 2.9: Geometries and operating condition used by Vasconcelos.
Vessel diameter (m) 0.392
Depth to diameter
ratio H/T
2
Superficial gas
velocity (m/s)
0.013
Gas flow rate (vvm) 0.125
The six blade turbine used in this study had several shapes. Figure 2.8 shows the
dependence of kLa value on the shape of the impeller and it is noticed that does not
depend on the type of impeller.
Figure 2.11: kLa results for the extreme values of constant air flow rate QG, by Vasconcelos et al. Full lines represent the overall correlation. Dashed lines stand for 95% confidence interval [].
Nelson et al. (1998) reported that Rushton turbine is used in most industries but
according to the limitations discussed previously which leads to power drop due to the
gas cavitations near the blades. In this experiment, hollow blade impellers which are
concave in shape, a tank of 0.4m diameter made of clear Perspex to notice the gas
dispersion, and baffles were used. It was noticed that hollow blade impellers can
eliminate the reduction in the power demand, provide more handling capacity and gas
hold up capacity which makes them better than Rushton turbine.
The effect of using single, double and triple impellers was studied by Puthli et al. (2005).
The impellers used in this study were single impeller contisting of a disc turbine (DT),