Final Project 2013

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 AlDallal 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 was used to determine the value of kLa.

Table of Contents Declaration......................................................................................................................2 Acknowledgment..................................................................................................................3 Executive Summary..............................................................................................................4 Nomenclature.......................................................................................................................6 Chapter 1 Introduction.........................................................................................................8 Chapter 2: Literature Survey............................................................................................10 2.1 Methods for measuring kLa:.....................................................................................11 2.2 Factors that affect kLa value:................................................................................13 2.2.1 Effect of salt addition and viscosity:.................................................................13 2.2.2 Effect of probe position:...................................................................................18 2.2.3 Effect of aeration rate:......................................................................................19 2.2.4 Effect of Temperature:......................................................................................22 2.2.5 Effect of medium depth:...................................................................................23 2.2.6 Effect of mixing:...............................................................................................24 2.2.6.1 Effect of impeller Type:.............................................................................24 2.2.6.2 The effect of impeller speed:.....................................................................28 2.2.6.3 Effect of impellers position:......................................................................28 Chapter 3: Theory.............................................................................................................30 3.1 Determination of kLa from experiments:.................................................................30 3.2 Determination of kLa from empirical correlations:..................................................31 REFERENCES..................................................................................................................35

Nomenclature a

Interfacial area (m2/m3)

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)

k La

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)

ρ l

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 (k La) 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

A sparger and hence a compressor have to

parameter can be very high.

be installed, leading to more investment and to the process more complicated.

Large changes in the volumetric mass

More power consumption due to the use of

transfer parameter could be achieved by

a compressor.

changing the gassing rate and power input.

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 k La 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

k La

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 k La value decreases to about 60 to 70% of the value of k La 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 k La. 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 k La 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 Liquid volume

Used Value 0.0015 m3

Temperature

0.002 m3 20ᴼC

if one impeller if two impellers

Rotation speed Gas flow rate

5 to 15 r.p.s Varied between 2.5, 3.75 and 5 ×

Experiment time

10-5 m3/s 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 k La value due to its non-coalescing properties.

Although water is a

coalescing system and showed low values of k La 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 k La 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 k La 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

Viscosity of

Air flow rate

Value of kLa

CMC

solution

range 10

(s-1)

%w/v 0.25

mPa 4.33

-5

(m3/s) 0.9

0.0082

0.5

5.99

0.75

11

3.4 0.9 3.4 0.9 3.4

0.0161 0.0058 0.0095 0.0021 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 k La 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:

Agitatio

Air flow

Range of composition

Viscosity

kLa value

n speed

rate

of glycerol solution

(cpoise)

(min-1)

(rpm) 300

(L/min) 0.3

(% w/w) 10 50

0.935 6.948

2.65 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 k La, 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 Temperature Pressure

conditions Room temperature Atmospheric pressure gas 0.3 m/s

Superficial velocity Gas hold up Bubble diameter

0.009 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 k La 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 k La 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 1.81m 3. The following table and graph summarizes the results obtained [&]: Type of impeller

Speed of the impeller

Air flow rate range

kLa value

Triple impellers

(rpm) 300

applied (cc/s) 8.92 33.81 8.92 33.81 8.92 33.81 8.92 33.81

(s-1) 0.0026 0.0035 0.0045 0.0081 0.0062 0.0098 0.0082 0.0161

(disc turbinepitched blade downflow turbinepitched blade down-flow turbine

400 500 600

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 k La of the system. The operating condition used and the results obtained are all summarized in the coming tables [ ]: Volume

Type of

Speed of

Air flow rate

kLa value

(L)

impeller

impeller

Range

(s-1)

5

Six blades

(rpm) 500

(L/min) 0.5 6

0.02 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 k La. The operation conditions used in the experiments performed and the results obtained are as follows: Volume

Temperature

(L)

(ᴼC)

pH

Agitation rate

Range of

kLa value

(rpm)

flow rate of

(min-1)

air 0.6

37

7

300

(L/min) 0.15 0.9

1.728 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 k La 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 k La 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

o

C

to

29.0

o

C.

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 []. Vessel diameter Table 2.9: and operating used by

Depth to

The six blade

diameter ratio

in this study

H/T

shapes. shows

(m)

Superficial gas the

velocity (m/s)

of kLa value

Gas flow rate

of

the

0.392

(vvm)

it is noticed depend on the type of impeller.

2

Geometries condition Vasconcelos. turbine had

0.013

used

several

Figure

2.8

dependence 0.125

on the shape impeller

and

that does not

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), double impeller consisting of disc turbine-pitched blade downflow turbine (PTD), triple impeller consisting of disc turbine-pitched blade downflow turbine- pitched blade

downflow turbine. The reactor used in this study was concentrated of glass with baffles and it has the following dimensions:      

Height to diameter ratio = 1.692 Internal diameter = 13 cm Number of baffles = 4 Baffle height = 19 cm Baffle width = 0.7 cm Volume of the tank = 1.81 m3

Moreover, the impeller was located 4.5 cm from the bottom of the reactor. The ratio of the diameter of the impeller into the diameter of the tank was 0.333 in the three cases. Impeller

blades

Impeller

Impeller

Impeller blade Power

configuration

diameter

blade

width (cm)

number

Single

(cm) 4.33

length (cm) 1.4

1.2

(Np) 4.8

Six blade DT 4.33 450 PTD (4 4.33

1.4 1.8

1.2 1.4

6.3

blades) Six blade DT 4.33 0 45 PTD (4 4.33

1.4 1.8

1.2 1.4

7.8

blades) 450 PTD

1.8

1.4

Six blade DT

impeller Double impellers Triple impellers

(4 4.33

blades) Table 2.10: Impellers configuration used by Puthli et al []. During the study it was noticed that using single, double and triple impellers had an effect on the kLa vakues at different impeller speed and gas flow rates. Distilled water was used in this experiment at a temperature of 35 C0. With an impeller speed range of 300 to 600 rpm and the rang of the gas flow rate was 8.92-33.8 cc/s the results of kLa value at 600rpm and 33.8 cc/s are shown in the following table: Type of impeller Single impeller

Impeller speed

Gas flow rate

kLa value

(rpm)

(cc/s)

(s-1) 0.0073

Double impeller Triple impeller

600

33.18

0.0098 0.0161

Table 2.11: kLa value of single, double, and triple impeller obtained by Puthli et al. [].

Figure 2.12: Effect of impeller speed on kLa with respect to impeller configuration (triple impeller (♦) kLa (8.92 cc/s), (□) kLa (19.5 cc/s), (○) kLa (29.4 cc/s), (∆) kLa (33.81 cc/s) by Puthli et al. []. As a conclusion to this study, kLa values in triple impeller was higher compared to single and double impeller this results from the sudden brakeage in gas bubbles as the impeller speed increased in triple impeller therefore the size of bubbles will be smaller which will lead to higher interfacial area so the value of the mass transfer coefficient will increase.

2.2.6.2 The effect of impeller speed: The effect of the impeller speed on the k La value was studied by Ozbek and Gayik (2001) using speed range of 100 to 500 rpm and at gas flow rate of 0.31 L/min. The temperature was 37 C 0 and the fluid used was distilled water. It was reported that the value of kLa increased as the impeller speed increased from 100 to 500 rpm and the values were 0.31 and 5.274 min-1 respectively. However, it was noticed that the relation between kLa value and the impeller speed, as it is shown in

figure 2.10, consists of two regions first one, when the impeller speed was lower than 300 rpm the slope was very small compared with second region at impeller speed above 300 rpm where the k La value increased noticeably.

Figure 2.13: the effect of impeller speed on the kLa value obtained by Ozbek and Gayik [].

2.2.6.3 Effect of impellers position: Gas-liquid mass transfer in reactors with two Rushton Disk turbines has been studied by Arcella et al. (1997). This paper shows the suitable placement of two disk turbines for gas liquid mass transfer. A flatPlexiglas bottomed cylindrical was used in the experiment. The reactor was equipped with two six-blade Disk turbine, four baffles, and a ring sparger with eight orifices. Temperature Diameter of the reactor The orifice of Liquid used

250C 0.202m 1mm Distilled water

Dispersed gas phase used

air

Table 2.12: The operating conditions used by Arcella et al. []. In this work the correct position of the two disk turbine (the upper and lower) has been obtained experimentally. After all the investigation in the experiment, it was found that the upper disk turbine should be moved upward to apposition near the static liquid surface. This is to increase the contribution of gas entrainment from free surface to gasliquid mass transfer. On the other hand, the lower disk turbine should be moved near the sparger to re-disperse the aerated gas immediately. So the basic role obtained for the upper disk turbine is for both redispersing the aerated gas and sucking in the gas from the top space of the reactor, while the lower disk turbine is for re-dispersing the aerated gas. It should be mentioned that the increase in gas entrainment from free space as a result from the upward movement of the upper disk turbine, increases the volumetric mass transfer parameter. Sometimes, gas entrainment from free space is unwanted, especially when back-mixing of gas phase is unwanted. In such cases, increasing the immersion depth of the upper Disk turbine is a must. (A study of gas-liquid mass transfer in reactors with two Disk turbines).

Chapter 3: Theory

3.1 Determination of kLa from experiments: The Dynamic method that depends on the response of an oxygen probe to changes in the concentration of dissolved oxygen in the liquid medium during the absorption or desorption of oxygen was used in this thesis. In absorption dynamic technique, the concentration of dissolved oxygen is eliminated and reached to zero at the beginning either by bubbling nitrogen gas or using sodium sulfite. Then air is supplied to the liquid and the increase in the concentration of oxygen is measured with time until saturation point is reached. In desorption dynamic method, air is supplied at the beginning of the process until the saturation level of oxygen is reached. After that, nitrogen gas is supplied and the decrease in the oxygen concentration is recorded as a function of time. The mass transfer resistance in the gas phase is usually neglected, because it is much smaller compared to the resistance in the liquid phase [=]. For a perfectly mixed liquid, the mass balance on the dissolved oxygen could be established as following: dC dt

= OTR – OUR

(3.1) OTR is the oxygen transfer rate (mol O2/m3.s) and is:

Where

OTR = kLa (C* - C)

(3.2)

OUR is the oxygen uptake rate (mol O2/m3.s). Because the system used in this thesis does not use any microorganisms than OUR is equal to zero and the equation becomes as: dC dt

= kLa (C* - C)

(3.3) Separating the variables and integrating from C0 to C and t0 to t:

C∗−C ln ( C∗−Co ) = - kLa (t – t0)

(3.4) C∗−C The plot between ln ( C∗−Co ) and t will result in a line with a slope of - kLa as in the following example graph:

Figure 3.1: An example on how to use equation (3.4) [=].

3.2 Determination of kLa from empirical correlations: Over the past decades different equations have been developed for estimating k La as a function of different variables. However, very large deviations are found in values of kLa between the experimental and estimated values, especially when k La for systems containing microorganisms are estimated for equations developed for aqueous solutions. Empirical correlations for estimating kLa values depend on many geometrical parameters, but there is no specific rules followed in literatures. For example, Van’t Riet (1979) proposed a correlation showed in table (3.2) that depends only on (P/V) and (V G). He has not considered the effect of stirrer geometry and number of impellers.

Many developed correlations apply dimensionless quantities such as, Reynolds (Re). The impeller Reynolds number represents the ratio of the inertial forces created by the rotating impeller and the viscous forces of the liquid:

ReN =

ρl . N . D2 μ

(3.5) When the impeller Reynolds number is less than 10 4 then the liquid is in the laminar region; while in the turbulent region, the impeller Reynolds number is more than 10 4. Usually the mechanical stirred vessels operate in turbulent region, where the velocity fluctuates in both time and three dimensions in space. The table below shows other dimensionless groups used in kLa correlations: Dimensionless quantities Froude Number (Fr) gas flow rate (FlG) Sherwood (Sh) Stanton (St) Schmidt (Sc) Weber (We) Aeration (A)

Equation 2 N D Fr = g

FlG

Qg = N . D3

Sh =

k La D² T

St =

k LaV L Qg

Sc =

μ ρT

We = A=

ρN ² D³ σ ND VG

Table 3.1: Dimensionless groups used in kLa correlations []. Another important parameter used in calculating kLa is the power consumption of an impeller which depends on the power number that is strongly dependent on the impellers geometry, vessel geometry if baffles are used and how many, the impellers clearance

from the bottom of the vessel, and the spacing between impellers if multi impellers are used. The power could be calculated by, where NP is found from graph 3.2:

P = NP. ρl. N3. D5

(3.3)

Figure 3.1: Power number estimation for different impellers Another method for determining the power consumed by the impeller is using a laboratory dynamometer. Before performing experiments, the torque of the impeller could be measured at different impeller speeds and in air. This value represents the system friction. During the experiments, the torque value is measured again in water. Subtracting the two values than substituting in the below equation will result the amount of power consumed by the impeller in the liquid:

P=K×N×S

(3.4)

Where

K is the dynamometer constant S is the net value of torque (N.m)

In the table below different correlations from literatures are reported. Some depends on the gassing rate and the mixing power of the impellers, others are based on dimensionless groups: Researchers Calderbank (1958) van 't Riet (1979) Hickman (1988)

Linek et al. (1987) Linek et al. (1991) Moucha et al. (1995) Robinson and Wilke (1973) Smith (1991) Smith et al. (1977) Smith et al.(1977) Stenberg and Andersson (1987) Van’t Riet (1979) Whitton and Nienow (1993) Zhu et al. (2001) Juarez and Orejas (2001) Wu (1995) Perez and Sandall (1974) Yagi and Yoshida (1975) Nishikawa et al. (1981) Albal et al. (1983) Schluter and Deckwer (1992)

Correlation Proposed kla =0.026(P/VL)0.4(VG)0.5 For T = 0.60 m kLa = 0.043 (P/VL)0.4(VG)0.57 For T = 2 m kLa = 0.027(P/VL)0.54(VG)0.68 kLa = 4.95 × 10-3 (P/VL)0.593(VG)0.4 kLa =3.84 × 10-3 (P/ VL)0.654(VG)0.4 kLa =14.6 × 10-3 (P/ VL)0.611(VG)0.554 kLa = (P/ VL)0.4(VG)0.35 kLa =1.25 ×10-4 (D/T)2.8(Fr)0.6(ReN)0.7 (FlG)0.45(D/g)-0.5 kla = 0.01(P/VL)0.475(VG)0.4 kLa =10.4 × 10-3 (P/ VL)0.475(VG)0.4 kLa = 5.3 × 10-3 (P/ VL)0.55(VG)0.32 kLa =26 × 10-3 (P/ VL)0.4(VG)0.5 kLa =0.57 (P/m)0.4(VG)0.55 kLa =0.031 (P/ VL)0.4(VG)0.5 kLa =1.11 × 10-3 (P/ VL)0.9504(VG)0.6282 kLa =10.36 × 10-3 (P/ VL)0.67(VG)0.56 kLa=21.2 (Re)1.11(Sc)0.5 (Vg.T/σ)0.45(µG/µa) (DL/T2) kLa=0.06 (Sc)0.5(Re)1.5 (Vg.µa/σ)0.6 (Fr)0.19 (A) (DL/T2) kLa=0.368 (Re)1.38 (Sc)0.5 (Vg.µa/σ) (Fr)0.367 (A)0.167(T/D)0.25 ((P/V)/ρN3T5)0.75 (DL/D2) kLa=1.41× 10-3 (Re)0.67 (Sc)0.5 (We)1.29 (DL/T2) kLa= C. [(P/VL)/ρ (v.g4)1/3] [(Q/VL). (v/g2)1/3]0.23 (g2/ v)

Table 3.2: Correlations proposed by different researchers to estimate the value of kLa in stirred tank reactor [three ref].

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