Mass Transfer Controlled Corrosion of the Wall of Air Sparged Agitated Vessel With Ring Sparger

8/13/2019 Mass Transfer Controlled Corrosion of the Wall of Air Sparged Agitated Vessel With Ring Sparger

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KEYWORDINDEXAUTHORINDEXTITLEINDEX8 C B

( & ), A, , 20

D, 2010

Mass Transfer Controlled Corrosion of the Wall of Air Sparged Agitated

Vessel with Ring Sparger

Yehia M.S. El-Shazly

Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria

21544; Egypt.

Abstract

Air sparged mechanical agitated vessel is an important gas liquid contactor that is used in many

processes: in gas stripping, in bioreactors, in fermentation, in pulp bleaching, and in wastewater treatment

to reduce the oxygen demand in waste streams. In this study, the mass transfer coefficient from the wall

of an air sparged mechanically agitated vessel with Rushton turbine is studied as a function of the air

superficial velocity (3, 9, 15 and 21 L/min) and the speed of rotation of the impeller (100, 200, 400 and

600 rpm). The dissolution of copper in acidified dichromate solution technique was adopted in this study.

It was found that the rate of mass transfer increases with the increase of the speed of agitation and to alower extent with the increase in the air flow rate. Moreover, at low speed of agitation (100 rpm), the

mass transfer is controlled by the air flow rate, while the effect of mechanical agitation is dominating at

the higher speeds. The data were correlated with the mass transfer equation:

It was found that the value of the mass transfer coefficient from this study is much higher than expected

from the correlation of Calderbank and Moo-Young.

Keyword:Mass transfer; Sparged vessel; diffusion; corrosion; Power consumption

1. IntroductionAir sparged agitated vessel is an important equipment that is widely used in many industrial applicationswhere a high interfacial area between liquid and gas is required for mass and heat transfer [1]. They are

used in fermenters, wastewater aeration, gas absorbers and strippers, catalytic and hydrogenation reactors

The basic principle is the introduction of a gas into the liquid with the agitation by an impeller to increase

the dispersion of the gas and uniformity of the mix. They are very effective in transferring mass between

the two phases, subject to the constraints of physical and chemical equilibrium. Usually the gas is injected

from the bottom of the liquid tank or near the impeller, as the effectiveness of the process depends

primarily on the impeller speed and the gas injection rate as well as the physical properties of the system[1-2].

Different flow regimes that may occur in this process; with axial flow impellers, the upward flowing gasbubbles are allowed to rise near the shaft where the shear forces are small, so that the flow pattern and

pumping effect is disrupted. Therefore, axial flow impellers are not very useful for gas dispersion. On the

other hand, radial impellers are very useful for gas dispersion where the gas is introduced just below the

impeller at its axis and drawn up to the blades and chopped into fine bubbles increasing the contact area

[3].
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Table 1 List of symbols used in the current research

A Wall surface area; cm2

D Diffusivity; cm2/sC Potassium Dichromate concentration at timet; mol/L

C0 Initial Potassium Dichromate concentration; mol/L

Oxygen concentration; mol/L

d Tank diameter; cm

di Impeller diameter; cm

g Gravity acceleration;

H Height of liquid; cm

K Mass transfer coefficient; for the depolarizer to the metal wall surface; cm/s

n Impeller rotation speed; rpm

N Dissolved oxygen flux; mol/m

2

sNp Power Number

Pm Agitation power consumption under aeration; erg/s

Pm0 Agitation power consumption under no aeration; erg/s

Psp Sparging power consumption; erg/s

PT Total power consumption; erg/s

Q Air flow rate; cm3/s

t Time; s

Re Reynolds Numberfor impeller mixing

Reg Reynolds Numberfor air sparging

Sc Schmidt Number

Sh Sheerwod Number

v Superficial air velocity; cm/s

Vs Liquid volume; cm3

Liquid viscosity; g/cm s

Liquid density; g/cm3

Previous studies on the performance of the different combinations of ring sparger and Rushton radialimpeller reported that the best results in terms of gas handling and mass transfer are obtained with the

sparger located below the impeller and with a diameter at least equal to the impeller diameter [4-5].

Mass transfer from the wall of the vessel can be an important consideration in the design of the reactor;

most corrosion processes in industrial applications are diffusion controlled [6-8]. Corrosion of metals as a

material of construction involves the formation of macroscopic or microscopic galvanic cells where twosimultaneous reactions are taking place; the anodic dissolution of the metal: MM

2++2e; and cathodic

reduction of the oxidizing agent such as dissolved oxygen: O2+2H2O+4e4OH-. Side reactions between

the cathodic product and the anodic product may lead to the formation of porous solid film of corrosion

product on the corroding metal surface; Fe(OH)2 or the oxidized form Fe(OH)3, through which oxygen

must diffuse for the corrosion process to continue. Under these conditions, the rate of corrosion is [7, 9]:


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(1)

Moreover, the analogy between mass and heat transfer is a well known phenomenon since the work ofColburn [10], whereas the similarity of the mathematical formulations of both the heat flux and mass flux

enable the estimation of one by knowing the second, which has been used to study different systems of

transfer [11-16].

Many techniques have been used for the study of the mass transfer, among them the dissolution of solidinto liquid [17-23], the electrochemical technique [24-27], adsorption [28-29] and CFD [30-33].

2. Effect of power on mass transferThe rate of mass transfer is related to the power consumed in causing the turbulence responsible for this

process: according to Calderbank and Moo-Young [34], the power consumed in driving the process

affects the rate of mass transfer. They presented their equation to estimate the mass transfer coefficient

between a fixed phase and its surrounding fluid which is in turbulent conditions as:

(2)

3.

Power consumption in sparged agitated vessels

The power requirement for the mixing and agitation is an important consideration in the design and scale-

up of reactor and mixing vessels [2]. For the current case, two equipment are consuming power: the motor

responsible for driving the shaft of the turbine, and the compressor used for injecting the air in the liquid.

The power used for mechanical agitation can estimated from the following equation [35-36]:

(3)

and the value ofNpreported for the six blade Rushton turbine is 5.75.

For the case of presence of air sparging along the mechanical agitation, the power used for mixing is

decreased and can be estimated according to the equation [37]:

(4)

For the case of air sparging, the power required to drive the air against the liquid pressure can becalculated as [38]:

(5)


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The total power is the sum of its two components: mechanical mixing and air sparging:

(6)

4. Materials and Methodology

4.1.Experimental techniqueTo study the rate of mass transfer from the wall of the vessel under mechanical agitation and air sparging,

the dissolution of copper in acidified dichromate method is adopted[39]. The arrival of the oxidizing

agent, the Cr6+, to the surface of the copper wall, determines the rate of the reaction:

(7)

This technique has been used widely in the study the role of the surface geometry and hydrodynamics in

the rate of diffusion controlled corrosion in view of simplicity and accuracy [7, 40-41].

To report the rate of mass transfer from the bulk of the liquid to the surface of the wall, the method of

dimensional analysis is adopted, and a correlations in terms of Sh,J,Re,Fr, and Scnumbers are reporteddepending on the conditions.

For example, for the case of agitated vessels, a correlation in the form Shnumber as a function ofReandScis usually reported [7-8, 42]

(8)

While for the case of gas sparging, a correlation in the form of

(9)

is usually reported [40, 43-44].

While in the case of the presence of mechanical agitation and air sparging, the correlation is reported is

the form [45]:

(10)

where,x1,x2, x3, , , , and are constant to be determined experimentally depending on the geometryand conditions. The value of the power of Schmidt number is usually fixed at 0.33[9].

4.2.Experimental set-upFigure 1shows the set up of the experiment: A tank made of plexiglass with it internal wall covered witha pure copper sheet to form the inner wall of the tank. The outer side of the sheet is completely isolated to

insure that the exposed area is only the inner one. Four vertical plexiglass baffles were attached to the

copper wall to prevent swirl flow of the solution. The Rushton turbine and its shaft are completely

isolated with an epoxy coating. The shaft is connected to a variable speed motor with a digital controller.


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The air is introduced through a ring sparger of 7.5 cm in diameter and is fixed at 2 cm below the turbine.

It is connected through a plastic tube that is attached to the wall of the tank to a flowmeter and then to the

air compressor. Table 2summarize the dimensions used for the agitated vessel.

The speed of rotation tested was 100, 200, 400 and 600 rpm and the air flow rate was 3, 9, 15, and 21

liter/min. A solution of 1 Molar sulphuric acid and 0.003 Molar potassium dichromate was used. The

volume of the solution was 2.7 liter in each run, and a 5 mL sample was drawn every 5 minutes for

analysis. The dichromate concentration was determined by titration against standard ferrous ammonium

sulphate using diphenylamine indicator [46]. The density and viscosity of the solution were determined

using a density bottle and an Ostwald viscometer respectively to be 1.061 g/cm3and 0.0107 g/cm s with

6% reproducibility [47]. The diffusivity of the dichromate was taken from literature [48]. ANALAR grade

chemicals and distilled water were used in the experiments.

1. DC Motor2. Speed controller3. Flowmeter4. Baffles5. Rushton turbine6. Ring sparger

Table 2 Dimensions used

in the experimental setup

Figure 1 Schematic diagram of the experimental setup.


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5. Results and DiscussionThe rate of the mass transfer diffusion of the wall of the vessel is expressed in terms of the mass transfer

coefficient which can be obtained from the dichromate concentration data. For these experiments, the rate

of reaction is given by:

(11)

Integrating this equation gives:

(12)

Figure 2 shows a typical ln(C0/C)vs. time

plot for the effect of the degree of agitation onthe rate of dichromate deletion, whileFigure 3

shows the effect of air sparging on dichromate

depletion. It is seen that the increase in either the

speed of rotation or the air flow rate result in

increasing the rate of dichromate arrival to the

wall. Figure 4 shows the combined effect ofthe mechanical agitation and air sparging on the calculated value of the mass transfer coefficient, K. As

expected, the mass transfer coefficient increases with the increase of the rotation speed and the air flow

rate. This is attributed to the increase of turbulence resulting from the eddies generated by the movement

of the impeller or from the bubbles coalescence and breakdown. This turbulence results in the decrease ofthe thickness of the hydrodynamic boundary layer and the faster transport of the oxidizer to the wall of

the vessel. It is also noted a higher increase in mass transfer coefficient upon increasing the rate ofmechanical agitation than on the increase of the air flow rate for the tested range.

Diameter of the vessel (d) 15 cm

Diameter of the Impeller (di) 5 cm

Diameter of the sparger (dS) 7.5 cm

Height of solution in the vessel (H) 15 cm

Impeller clearance(H2) 5 cm

Sparger clearance(C) 2 cm

Width of Baffles (Bw) 1 cm


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Figure 2 ln(C0/C) versus time curves for the different mechanical agitation speed.

Figure 3 ln(C0/C) versus time curves for the different air flow rate.


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Figure 4 Combined effect of mechanical agitation and air sparging on the rate of mass transfer from the

wall of the vessel.

For the effect of air sparging,Figure 5shows that the mass transfer coefficient is related to the superficialair velocity by the equation:

(13)

It is seen that the exponent of the velocity is lower than reported in literature [40, 49]. This might be

attributed to the fact that the diameter of the sparger is smaller than the diameter of the tank, with the

result that the air bubbles flowing out of the sparger are more concentrated at the centre of the tank than at

the wall, thus resulting in higher turbulence in the bulk zone away from the wall. Moreover, due to the

fact that the air bubbles move to the upward direction, a dead zone where no turbulence occurs is formedbelow the sparger, makes the actual affected area by the turbulence lower than the wall area and the lower

area of the tank not affected by gas bubbles.

shows that for the case of sparging alone, the mass transfer correlation is:

(14)

Figure 7 shows that for the case of mechanical agitation in the absence of air sparging, the non

dimensional mass transfer correlation can be represented as:

(15)


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Figure 5 Effect of superficial air velocity on the rate of mass transfer for the case of air sparging alone.

6 .

Figure 7 Mass transfer correlation for the case of mechanical agitation.


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The mass transfer correlation for the combined mechanical agitation and gas sparging is ():

(16)

with a correlation coefficient equal to 0.92. It is seen from this correlation that the mass transfer is

affected by the combined effect of the mechanical agitation and air sparging. The higher exponent of the

Recompared to that ofRegshows that under the present range of conditions mechanical stirring is more

influential than gas sparging in enhancing the rate of mass transfer.

Figure 8 Overall mass transfer correlation for the case of the combined effect of mechanical agitationand air sparging.

Figure 9 shows a comparison between the values ofKobtained from the current study with the valuesobtained from Calderbank and Moo-Young correlation (equation 2) [34]. For the power calculations,

equations 3 through 6 were used. At low mechanical agitation speed, a small difference between the

experimental and the calculated values of K is found, though upon increasing the air flow rate, the

difference increases in favor for the experimental results. On increasing the rotation speed, the difference

increases largely. This discrepancy may be attributed to that the value of the rate of mass transfer in this

case is not only a result for the decrease in the thickness of the hydrodynamic layer at the wall from the

agitation turbulence, but also the radial flow resulting from the impeller, directs the air bubbles outwardswhere they impinge on the wall, exposing fresh surface for mass transfer and enhancing the turbulence

near the wall. This is different from the isotropic turbulence for which Calderbank equation applies.

6. ConclusionsIn this study, the rate of mass transfer of the wall of a mechanical agitated sparged vessel is examined. A

mass transfer correlation relating the mass transfer to the turbulence and physical properties of the

solution is reported. It is found that the rate of mass transfer increases with the increase in speed of

rotation, and to a lower extent, with the increase in the air flow rate. The experimental results showed ahigher mass transfer coefficient than expected from Calderbank and Moo-Young correlation.


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Mass Transfer Controlled Corrosion of the Wall of Air Sparged Agitated Vessel With Ring Sparger