Corrosion rate determination of vessel walls agitated by double impeller and gas sparging
-
und
Abstract
Recently, multiple impeller gas sparged vessels have found wide application in many industries, such as food, pharmaceuticals, and biofuels. In this study, the rate of diffusion-controlled corrosion of the wall of nitrogen gas sparged-double impeller agitated vessel was studied by the dissolution of copper wall in acidified dichromate solution technique. The variables studied were the impeller rotation speed, the superficial gas velocity, and the clearance between the two impellers. The results were reported in terms of dimensionless number depicting the process conditions, Re, Sc, and the impeller clearance. For the agitated vessel, the corrosion rate correlation was
1 Introduction
Mixing is widely used in industrial operations to bring homogeneity and reduce concentration and temperature differences in the system. That is why agitated vessels are widely used in a variety of mixing processes in chemical industries, such as food, pharmaceutical, biofuel, and metallurgical industries (Ahmed et al., 2010; Arlov et al., 2008; Bouaifi & Roustan, 2001; Vasconcelos et al., 1995). They are characterized by low capital and operating costs compared to other mixing processes (Jakobsen, 2014; Kadic & Heindel, 2014; Kumaresan & Joshi, 2006; Ochieng et al., 2008; Zadghaffari et al., 2009). Also, they are very popular in liquid-gas processes such as wastewater aeration and fermentation (Cai & Dai, 2010; Shewale & Pandit, 2006; Vrábel et al., 2000; Wang et al., 2014). In these processes, the gas is fed to the bottom of the tank through a sparger (Cai & Dai, 2010; Stanbury et al., 1995a,b). Generally, the Rushton turbine is the most commonly used impeller in gas mixing (Cai & Dai, 2010; Hudcova et al., 1989; Sardeing et al., 2004), as they are recognized by good gas dispersion because of their high solidity ratio (Doran, 2013; Vrábel et al., 2000).
Many gas-liquid processes in chemical and biochemical industries are carried out in multiple-impeller stirred tanks. Multiple-impeller designs are very popular in practice and are implemented due to the shortcomings of the single-impeller system (Bouaifi & Roustan, 2001; Cabaret et al., 2008; Shewale & Pandit, 2006). They are characterized by a high height-to-diameter ratio, which render them of higher capacity than single-impeller vessels (Ahmed et al., 2010; Bombac & Zun, 2006; Bouaifi & Roustan, 2001). Moreover, this leads to better liquid recirculation and higher residence time for gas bubbles and consequently better mass transfer between the two phases (Cabaret et al., 2008; Kadic & Heindel, 2014; You et al., 2014).
Moreover, double impellers produce lower and homogenous shear force than single impeller and hence avoid microorganism degradation in bioreactors (Gogate et al., 2000). Finally, they are reported to be more energy efficient (Armenante et al., 1992, 1999).
The design of the multiple impellers allows producing different flow regimes. In general, there are three flow patterns produced: parallel, merging, and diverging (Kadic & Heindel, 2014; Rutherford et al., 1996; Saravanan et al., 2009; Shewale & Pandit, 2006; Vrábel et al., 2000). In the parallel pattern, the two impellers operate essentially independently from one another; each impeller produces its own pattern leading to the formation of ring vortices toward the wall. In the merging pattern, the two impeller streams merge and form two large ring vortices: the impeller streams followed an almost straight-line orientation toward one another and merge at an elevation approximately midway between the two impellers to form two large ring vortices. Finally, the diverging flow is for smaller bottom clearance. The streams produced from the upper impeller follow a path toward the wall, whereas the lower impeller stream is directed toward the vessel bottom: the flow generated by the upper impeller is unaffected by the presence of the lower impeller. Table 1 shows the previous studies regarding the different flow pattern produced by the double impellers.
Comparison between parallel, merging, and diverging flow in previous studies.
| Literature studies | Parallel flow | Merging flow | Diverging flow |
|---|---|---|---|
|
Khopkar and Tanguy (2008)
H =1.4T, H=0.65, T=0.45 m |
C
1=T/4, C2=T/2, C3=T/4 C1=0.15, C2=0.2, C3=0.3 |
C
1=T/3, C2=T/3, C3=T/3 C1=0.2, C2=0.15, C3=0.3 |
C
1=0.15T, C2=T/2, C3=0.35T C1=0.067, C2=0.28, C3=0.3 |
|
Pan et al. (2008)
H =1.4T, T=0.48 m |
C 1=0.4T, C2=0.48T, C3=0.12T | C 1=0.4T, C2=0.32T, C3=0.28T | C 1=0.15T, C2=0.4T, C3=0.45T |
|
Liu et al. (2008)
H =1.4T, T=0.48, H=0.67 m |
C 1=0.4T, C2=0.4T, C3=0.6T | C 1=0.4T, C2=0.31T, C3=0.69T | C 1=0.15T, C2=0.4T, C3=0.85T |
Corrosion is a problem for agitated vessels; tank perforation and leakage is a common accident in operations (Davis, 2000; Fontana, 2005; Revie, 2008, 2011). Moreover, corrosion product contamination can be a serious problem to some industries such as food, pharmaceutical, and fermentation industries (Bourgeois et al., 2001). Therefore, it is essential to determine the amount of corrosion in agitated sparged vessel to predict the lifetime of the tank and the amount of contamination (Cullen, 2009; Ende, 2011; Stanbury et al., 1995a,b).
In these industries, usually, steel is the predominant metal of construction, and the corrosion occurs by the formation of galvanic cells, where the oxidation and dissolution of metal occurs at anodic sites and the reduction of the depolarizer occurs at the cathodic sites. Accordingly,
At cathodic sites, in alkaline and neutral solutions, the reduction of the oxidizer (dissolved oxygen in this case) takes place:
Moreover, the two reactions products combine to form ferrous hydroxide:
which, upon further oxidation, forms ferric hydroxide that is deposited on the surface:
The deposited Fe(OH)3 film is easily removed by the moving fluid shear force, exposing the fresh metal surface to the solution for further corrosion. For the oxidation of steel in alkaline or neutral industrial solution, the oxygen transfer through the diffusion boundary layer attached to the wall is the slowest step and controls the rate of the corrosion process (Fontana, 2005; Poulson, 1993; Slaimana & Hasan, 2010). Under these conditions, the rate of reaction is diffusion controlled and the dissolved oxygen flux NO2 is given by
where CbO2 and CwO2 are the concentrations of oxygen in the bulk and at the wall, respectively, and k is the mass transfer coefficient and CbO2 is taken as 0.259×10−3 kmol/m3 (Perry & Green, 2008). Under these conditions, CwO2 is set to zero, as all the oxygen molecules reaching the wall are consumed by the reaction instantaneously. Accordingly, the equation for the reaction of Fe is
where the factor of 2 is to account that every molecule of oxygen reacts with 2 molecules of iron. The rate of corrosion of steel, CR (in m/s), can be obtained from the mass transfer coefficient k as the following equation (Davis & Frawley, 2009):
or by directly substituting the dimensionless number Sherwood instead of the mass transfer coefficient in this form of equation:
2 Materials and methods
2.1 Methodology
According to Gregory and Riddiford (1960), the rate of diffusion-controlled reaction can be studied by following the rate of reaction between the copper metal and an acidic solution of hexavalent chromium ion according to the equation:
Cu (anode) dissolves in the solution in the form of ions, and the rate is controlled by the transport of Cr+6 ions to the cathodic sites through the diffusion layer when they are consumed by the reaction.
2.2 Apparatus
The apparatus used in the present study is shown in Figure 1. It consists of a cylindrical vessel made of plexiglass of 0.12 m diameter and 0.25 m height lined with a pure copper sheet to form the inner wall of the vessel. The outer side of the copper sheet is completely isolated with an epoxy resin and glued to the plastic wall to make sure that the active area is only the inner side and no solution gets trapped between the walls. The vessel was equipped with four plexiglass rectangular baffles fixed to the vessel wall. Each baffle had a width of 0.01 m. The agitated vessel was fitted with two-disc turbine impellers of six blades with a diameter of 0.04 m mounted centrally on a steel rotating shaft of 0.005 m diameter connected to an automatic controlled variable speed motor. The two impellers and the shaft were isolated from the solution with an epoxy coating.
Experimental set-up: (1) DC motor, (2) speed controller, (3) plexiglass, (4) baffles, (5) Rushton turbine impeller, and (6) shaft.
Three different spacings between the two impellers (i.e. 0.04, 0.08, and 0.12 m) were used in the present work. The clearance between the bottom of the vessel and the lower impeller was maintained constant at 0.04 m. The vessel bottom was made of a perforated plexiglass disc covered with a synthetic cloth, which acted as a gas distributor. Nitrogen gas was introduced from the bottom of the vessel through the distributor, and the flow rate was measured and controlled by gas flow meter.
Three sets of experiments were carried out. The first one, the agitation process, was carried out only by mechanical agitation where the rotational speed was varied from 1.7 to 11.7 rps (100–700 rpm) at the different spacings between the two impellers.
In the second set of experiments, the agitation process was carried only by gas sparging and the superficial gas velocity ranged from 0.0029 to 0.0147 m s−1.
In the third set, both nitrogen gas sparging and mechanical agitation by impellers were applied using the same range of superficial gas velocities, rotational speeds, and impellers spacing.
2.3 Procedure
Before each run, the copper surface was degreased with trichloroethylene, etched with diluted HCl to remove any formed surface oxides, and then washed multiple times with distilled water. Then, 2.4 liters of fresh acidified dichromate solution were used in each run. The solution composition was 0.003 m K2Cr2O7 and 0.5 m H2SO4.
The change in dichromate concentration with time during each run was monitored by withdrawing samples every 5 min and titrating against standard ferrous ammonium sulfate using diphenylamine barium salt as indicator (Vogel & Mendham, 2000). The experiments were carried out at a temperature range from 25°C to 27°C.
All solutions were prepared using A.R.-grade chemicals and distilled water. The solution viscosity and density were determined using an Ostwald viscometer and density bottle (Findlay & Kitchener, 1965). The diffusivity of dichromate was taken from the literature (Gregory & Riddiford, 1956, 1960).
3 Results and discussion
3.1 Mass transfer calculation
For the diffusion-controlled reaction, the rate of reaction can be given by
which upon integration gives
Figure 2 shows that the present experimental data fits equation (6). The mass transfer coefficient (k) was calculated from the slopes of
Typical ln C0/C versus t plot at different impeller rotational speeds Vg=0 m/s and C2=0.04 m.
3.2 Effect of superficial gas velocity and impeller rotation speed on the mass transfer coefficient
The sparging and agitation effects on the mass transfer rate were calculated according to the following equations:
where N and Vg ranged from 1.7 to 11.7 rps and 0.0029 to 0.0147 m/s, respectively. Sparging and agitation effects can be illustrated by plotting kSp. vs. Vg at different N values (Figure 3) and kAg. vs. N at different Vg values (Figure 4). In Figure 3, mass transfer increased with increasing the sparging velocity at different impeller speeds. Also, sparging has a significant effect on mass transfer at lower impeller speeds and decreases at higher impeller speeds. For example, mass transfer increased by more than 500% at 1.7 rps in comparison to 100% at an 11.7 rps when Vg was 0.0147 m/s. The observed increase in the mass transfer rate with the increase of the gas velocity can be attributed to the following (Katkout et al., 1988; Konsowa et al., 2004; Noseir et al., 1997; Nosier, 1997; Nosier et al., 2006; Soltan et al., 2003; Taha et al., 2003; Zaki et al., 1997, 2001; Zarraa et al., 1991, 1994):
Bubbles rising upward close to the wall generate turbulence that disturbs and decreases the diffusion boundary layer thickness.
The collision of bubbles with the wall penetrates the diffusion layer and brings fresh solution to the wall.
The swarm of rising bubbles imparts radial momentum transfer, which brings a fresh supply of dichromate solution to the active wall surface.
Sparging effect on the mass transfer coefficient, C2=0.04 m.
Agitation effect on the mass transfer coefficient, C2=0.08 m.
On the contrary, the agitation effect can be explained with the help of Figure 4. Increasing impeller rotation speed leads to the increase of the mass transfer rate. This can be ascribed to the increase in the intensity of turbulence generated by the movement of the impellers. These eddies are directed radially toward the vessel wall, disturbing the diffusion layer and reducing its thickness with a consequent increase in the mass transfer rate.
This behavior is in agreement with the previous studies that have been conducted on the solid-liquid mass transfer in agitated vessels (Abdel-Aziz, 2013; El Shazly, 2011; El-Shazly et al., 2004; Fouad et al., 2013; Sedahmed et al., 1998).
3.3 Power consumption effect on mass transfer
Calderbank and Moo-Young (Calderbank, 1995) developed an equation for measuring the mass transfer coefficient as a function of the specific power consumption in the process:
The power used for mechanical agitation can be estimated from the following equation (McCabe et al., 2005; Sinnott, 2009):
The power number, Np, was taken to be 8.4, 10, and 10 for the case of dual Rushton impeller with clearance of 0.04, 0.08, and 0.12 m, respectively (Pan et al., 2008).
For the case of gas sparged agitated vessel, the power used for mixing can be estimated from the equation (Michel & Miller, 1962):
where, for the case of sparging system, the power consumed can be calculated as (Joshi & Doraiswamy, 2008)
Figure 5 shows the ratio of PAg.–PSp. used in this study.
Ratio of power consumed in agitation in the presence of sparging (PAg.) to the power consumed in sparging (Psp.), related to the current study for C2=0.08 m.
Furthermore, as the two processes occur in parallel, the total power PT will equal the sum:
Figure 6 shows a comparison of the mass transfer coefficient obtained from the present study in three different systems (agitation, sparging, and combination of agitation and sparging) according to the Calderbank and Moo-Young correlation.
Comparison of the mass transfer values for agitation, sparging, and combination of agitation and sparging system according to the Calderbank and Moo-Young correlation.
The slope increased in the following order: agitation system, gas sparging system, and, finally, the combination of agitation and gas sparging. These results show that, at the same specific power, the corrosion rate is higher in the case of sparging than in the case of mixing. Moreover, a higher rate of corrosion is obtained if the total power is distributed between the two processes.
3.4 Corrosion rate correlations
3.4.1 Corrosion rate in the mechanically agitated vessel
Figure 7 shows that the corrosion rate increases with increasing impeller rotational speed. The data fit the relation:
Effect of the impeller rotational speed on the corrosion rate at different distances between the two impellers with no gas sparging.
Figure 8 shows that the corrosion rate increases with increasing impeller spacing. The data fit the relation:
Effect of the dimensionless factor (C2/H) on the corrosion rate at different impeller rotational speeds with no gas sparging.
The increase in the corrosion rate with the increase in impeller spacing is due to the change in the flow pattern with the increase of spacing: at impeller spacing of 0.04 m, the flow follows the merging pattern where the double impellers behave as a single impeller. The flow leaving the two impellers collides midway, decreasing the turbulence intensity and forming only two ring vortices. However, as the clearance between the two impellers increases, each impeller starts to develop his own flow and the pattern follows the parallel flow: the double impeller acts as two separate impellers, forming a total of four stable ring vortices reaching the vessel wall. This results in higher eddies and turbulence reaching the wall and a larger wall affected area. This explains the gradual increase of the corrosion rate with the increase of the double impeller spacing.
3.4.2 Corrosion rate in the gas sparging vessel
Figure 9 shows that the corrosion rate increases with increasing superficial gas velocity. The data fit the relation:
Effect of the superficial gas velocity on the corrosion rate and N=0.
3.4.3 Corrosion rate in the mechanically agitated sparged vessel
Figure 10 shows that the corrosion rate increases slightly with rising impeller rotational speed at the different superficial gas velocity. The data fit the relation:
Effect of the impeller rotational speed on the corrosion rate at different superficial gas velocities, C2=0.04 m.
It is noticed that the exponent of the agitation rate has dropped to one-fifth of its value in the mechanical agitation. This shows that the effect of the mixing rate on the corrosion rate is more pronounced when agitation is carried out without any gas sparging. This is explained by the contradicting effects of the gas flow on the hydrodynamics of the system. On the lower part of the vessel, the intensity of liquid turbulence that is created by the impellers is weakened by the rising of the gas and that the flow pattern has changed from radial to axial (Vasconcelos et al., 1995).
Figure 11 shows that the impeller spacing effect on the corrosion rate has vanished. The exponent of the dimensionless factor (C2/H) has dropped to almost equal to zero:
Effect of the dimensionless factor (C2/H) on the corrosion rate at different superficial gas velocities, N=3.3 rps.
Figure 12 shows that the corrosion rate increases with increasing superficial gas velocity at the different impeller rotational speeds. The result follows the relation:
Effect of the superficial gas velocity on the corrosion rate at different impeller rotational speeds and C2=0.04 m.
Furthermore, for the current experiments, the mixing power is much lower than the sparging power, indicating that the flow pattern is controlled by the gas flow. Referring to the ratio of the power used in agitation to sparging (Figure 5), we find that these results confirm the pervious results of Jakobsen (2008) who reported:
If PAg./PSp.≤1, the gas sweep upward, passing unhindered by the impeller or the shaft, resulting in a non-uniform distribution in the liquid.
If PAg./PSp.=2–3, the flow pattern becomes controlled by the mixer, and the gas bubbles get pushed toward the wall.
If PAg./PSp.=5, the mixer drives the bubbles down to the bottom of the tank, and the distribution of the gas in the liquid becomes completely uniform.
Overall corrosion rate correlations were developed for the agitated vessels and for the gas sparged agitated vessels.
Figure 13 shows that, for the conditions: Sc=960, 2800<ReAg.<19,600 and 0.19<C2/H<0.58, the experimental data for the sparged vessel corrosion rate (CR; m/s) fits the equation:
Overall corrosion rate correlation for the agitation system.
with and average deviation of ±2.9%.
Figure 14 shows that, for the conditions: Sc=960, 2800<ReAg.<19,600, 370<ReSp.<1855, and 0.19<C2/H<0.58, the experimental data for the corrosion rate (m/s) for the mechanically agitated sparged vessel fits the following equation:
Overall corrosion rate correlation for the mechanically agitated with gas sparging system.
with an average deviation of ±6.7%.
3.5 Corrosion rate of sparged vessels at different conditions
Figure 15 shows a comparison between the values of corrosion rate at different parts of mechanically agitated sparged vessels. The result shows that the corrosion rate increased in the following order: sparged vessel (current study), sparged column (Nosier, 1997), and mechanically agitated sparged vessel (current study). The corrosion rate in the sparged column was higher than the sparged vessel because of the effect of the high length-to-diameter ratio of the column. Moreover, the superficial gas velocities used in the current study ranged from 0.0029 to 0.0147 m/s, which is lower than the velocities used in the previous study (0.0105–0.059 m/s).
Comparison between the current studies with the Nosier (1997) study.
4 Conclusions
In this study, the effect of mechanical agitation and gas sparging using the dissolution of copper in acidified dichromate technique on corrosion rate is measured. The results have shown that:
In case of the agitation system, the corrosion rate increases with the increase of the distance between the impellers and with the increase of the agitation rate.
In case of the sparging system, the corrosion rate increased with the increase of the superficial gas velocity.
In case of the combination of agitation and sparging system, the superficial gas velocity has the more effect on the corrosion process. On the contrary, the clearance between the impellers does not have any influence on the corrosion rate of the wall.
Regarding the power consumption, corrosion is higher for the combination of mixing and sparging followed by sparging and then by mixing.
Correlations related to the rate of corrosion have been developed in terms of the rate of agitation, the rate of sparging, the physical properties of the solution, and the distance between the two impellers.
- Nomenclature
- Symbol
-
Name
-
Units
- A
-
Area of active cylindrical vessel mass transfer
-
m2
- C 1
-
Lower impeller clearance with double impellers
-
m
- C 2
-
Impeller separation
-
m
- C 3
-
Upper impeller submergence
-
m
- C
-
Potassium dichromate concentration at time t
-
kmol/m3
- C 0
-
Initial concentration of dichromate solution
-
kmol/m3
- C bO2
-
Concentration of O2 in the bulk of solution
-
kmol/m3
- C wO2
-
Concentration of O2 at the wall
-
kmol/m3
- CR
-
Corrosion rate
-
m/s
- D
-
Diffusivity
-
m2/s
- d i
-
Impeller diameter
-
m
- g
-
Gravity acceleration
-
m/s2
- H
-
Liquid height
-
m
- k
-
Mass transfer coefficient
-
m/s
- M Fe
-
Molar mass for Fe, taken as 56
-
kg/kmol
- N
-
Impeller speed
-
rps
- N p
-
Power number
-
–
- N Fe
-
Iron flux
-
kmol/m2s
- N O2
-
Oxygen flux
-
kmol/m2s
- P Ag.
-
Agitation power consumption under no sparging
-
W
- P Ag.Sp.
-
Agitation power consumption with sparging
-
W
- P Sp.
-
Sparging power consumption
-
W
- P T
-
Total power consumption
-
W
- Q
-
Volumetric gas flow rate
-
m3/s
- Q L
-
Liquid volume
-
m3
- T
-
Tank diameter
-
m
- t
-
Time
-
s
- V g
-
Superficial gas velocity
-
m/s
- Greek symbols
- ρ
-
Liquid density
-
kg/m3
- ρ Fe
-
Steel density, taken as 7870
-
kg/m3
- μ
-
Liquid viscosity
-
kg/m s
- Dimensionless numbers
- ReAg.
-
Reynolds number for mechanical agitation
-
ρNdi2/μ
- ReSp.
-
Reynolds number for gas sparging
-
ρVgT/μ
- Sc
-
Schmidt number
-
μ/ρD
- Sh
-
Sherwood number
-
kT/D
- Abbreviations
- RT
-
Rushton-type impeller (radial flow impeller)
-
–
- STR
-
Stirred-tank reactor
-
–
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©2017 Walter de Gruyter GmbH, Berlin/Boston
Artikel in diesem Heft
- Frontmatter
- In this issue
- Obituary
- Roger Washburne Staehle (February 4, 1934– January 16, 2017)
- Original articles
- Microstructural analysis and corrosion behavior of Fe, B, and Fe-B-modified Cu-Zn-Al shape memory alloys
- Characterisation of localised corrosion behaviour of Ti-Ta-Mo alloy for the nuclear industry
- Investigating the influence of hydrogen on stress corrosion cracking of 2205 duplex stainless steel in sulfuric acid by electrochemical impedance spectroscopy
- Corrosion analysis of copper T2 exposed to polluted atmospheres and study on prediction model
- Influence of the electric double-layer capacitance at the rate of corrosion at the phase interface
- Corrosion rate determination of vessel walls agitated by double impeller and gas sparging
- Retraction
- Retraction of: Nanoporous copper: fabrication techniques and advanced electrochemical applications
Artikel in diesem Heft
- Frontmatter
- In this issue
- Obituary
- Roger Washburne Staehle (February 4, 1934– January 16, 2017)
- Original articles
- Microstructural analysis and corrosion behavior of Fe, B, and Fe-B-modified Cu-Zn-Al shape memory alloys
- Characterisation of localised corrosion behaviour of Ti-Ta-Mo alloy for the nuclear industry
- Investigating the influence of hydrogen on stress corrosion cracking of 2205 duplex stainless steel in sulfuric acid by electrochemical impedance spectroscopy
- Corrosion analysis of copper T2 exposed to polluted atmospheres and study on prediction model
- Influence of the electric double-layer capacitance at the rate of corrosion at the phase interface
- Corrosion rate determination of vessel walls agitated by double impeller and gas sparging
- Retraction
- Retraction of: Nanoporous copper: fabrication techniques and advanced electrochemical applications
