See Tiam You, Abdul Aziz Abdul Raman*, Raja Shazrin Shah ...repository.um.edu.my/95701/1/Multiple-impeller stirred vessel... · ler, asymmetric deep hollow blade (BT-6), and Prochem

DOI 10.1515/revce-2013-0028 Rev Chem Eng 2014; 30(3): 323–336

See Tiam You, Abdul Aziz Abdul Raman*, Raja Shazrin Shah Raja Ehsan Shah and Mohamad Iskandr Mohamad Nor

Multiple-impeller stirred vessel studies

Abstract: Multi-impeller stirred vessels are widely used for industrial applications. Based on the numerous studies that reported the motivation and importance of studies on multi-impeller systems, a systematic study was conducted to identify the focus and objectives of research and types of experiments conducted using multi-impeller systems. Researchers mainly focused on the effects of impeller spacing, off-bottom clearance, and type of impeller com-binations. Most experiments were conducted on power number, power consumption, gas hold-up, and gas-liquid mass transfer. Research works have not exhausted all impeller-type combinations and there are still opportuni-ties for future work. Computational fluid dynamics stud-ies involving multi-impeller systems are also still lacking owing to flow complexities. This work can serve as a road-map for future study themes.

Keywords: flow pattern; gas hold-up; mass transfer coef-ficient; multi-impeller; power number.

*Corresponding author: Abdul Aziz Abdul Raman, Faculty of Engineering, Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia, e-mail: [email protected] Tiam You, Raja Shazrin Shah Raja Ehsan Shah and Mohamad Iskandr Mohamad Nor: Faculty of Engineering, Department of Chemical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

1 IntroductionMulti-impeller stirred vessels utilize two, three, or more impellers in a single shaft configuration (Figure 1). The number of studies reporting work on dual- and triple-impeller configurations are relatively few even though they are common in the industry, and fewer still focus on systems with more than three impellers (Armenante and Chang 1998, Armenante et al. 1999, Fajner et al. 2008) because increasing the number of impellers resulted in increased flow complexity (Zadghaffari et al. 2009). Addi-tionally, the tank height required for systems with more than three impellers is not practical for industrial applica-tions (Davis 2010).

Stirred vessels using multiple impellers are widely used in chemical, biotechnology, pharmaceutical, food processing, and many other industries for mixing pro-cesses. Researchers have focused on multiple-phase systems such as gas-liquid, gas-liquid-solid, and solid-liquid systems in view of the wide industrial applica-tions of multiple-impeller stirred vessels (Khopkar and Tanguy 2008, Min et al. 2008, Bao et al. 2012). Detailed knowledge of the mixing process in stirred vessels is of paramount importance for increasing process productiv-ity and reducing losses (Vrabel et al. 1999). Insufficient knowledge of dynamic behavior in stirred vessels has caused an estimated several billion dollar production loss annually in several industries (Smith 1990, Fishwick et al. 2005).

The present paper aims at giving a clear overview of research reported on multiple-impeller systems. A detailed review of relevant work on single- and multi-phase systems involving dual or triple impellers is pre-sented. The focus and motivation of reported works will provide an insight into current problems/issues being addressed.

2 Application of multiple-impeller systems for stirred vessels

Recent literature has indicated the advantageous char-acteristics of multiple-impeller systems (Pan et al. 2008, Wang et al. 2009, Li et al. 2012) desirable for specific industrial applications compared to single-impeller systems. Some researchers (Lehn et al. 1999, Vrabel et al. 2000, Shewale and Pandit 2006) reported the use of mul-tiple-impeller systems to solve industrial mixing problems despite their being more complex than systems having only one impeller. These systems also have a height of liquid-to-tank diameter ratio (H/T) exceeding 1.0, a value that would be considered as irregular in a single-impeller configuration. Research involving the tallest tank shows that it is a stirred vessel with H/T ratio of 4.0 and fitted with four impellers (Moucha et al. 1995). The practical number of impellers employed should agree with the fol-lowing expression:

Brought to you by | University of Malaya LibraryAuthenticated | [email protected] author's copy

Download Date | 7/17/14 11:32 AM

mailto:[email protected]

324 S. Tiam You et al.: Multi-impeller stirred vessel studies

- -22

H D H DnD D

> >

(1)

where H is the height of liquid in the vessel, n is the number of impellers used, and D is the diameter of impel-lers (Davis 2010). This is to ensure that the impellers are adequately spaced apart because if the impellers were placed too close together, the power imparted would be low; and if placed too far apart, there would not be ade-quate mixing (Babalona et al. 2005).

Examples of industrial gas-liquid reaction-based applications with multiple-impeller systems are fermenta-tions, hydrogenation dissolution, polymerization, crystal-lization, and wastewater treatment (Fujasová et al. 2004, Puthli et al. 2005, Ochieng and Lewis 2006, Shewale and Pandit 2006, Bao et al. 2007, Tamburini et al. 2009, Jafari

C4

C3

C2

C1

D

T

H

A B

Figure 1 Stirred tanks. (A) Flat bottom and (B) dish bottom.

et al. 2012, Li et al. 2012), which are found in industries such as chemical, mining, pharmaceutical, and biotechno-logical industries (Fishwick et al. 2005, Fajner et al. 2008, Khopkar and Tanguy 2008, Min et al. 2008, Zadghaffari et al. 2009, Montante et al. 2010, Taghavi et al. 2011). Mul-tiple-impeller systems in gas-liquid-solid applications are also employed in chemical industries, mineral processing industries, wastewater treatment plants, and biochemical industries to cater to specific mixing and contact require-ments between the phases (Dohi et al. 2004, Murthy et al. 2007, Bao et al. 2008, Panneerselvam et al. 2008).

Motivations of selection especially in gas-liquid and gas-liquid-solid systems are described in Gogate et al. (2000) and Shewale and Pandit (2006). Comparative advantages of multiple- over single-impeller systems are summarized in Table 1. In the presence of gas, multiple-impeller systems are reported to have better performance compared to single-impeller systems. Numerous research-ers have reported that the multiple-impeller system is more feasible and flexible than the single impeller system when dealing with a large amount of fluid (Gogate et al. 2000, Wang et al. 2009). Moreover, multiple-impeller con-figurations consume less power per volume (Figures 2–4), which results in significant power saving for the mixing process (Bouaifi and Roustan 2001, Alliet-Gaubert et al. 2006). Thus, there is a need for more research on indus-trial applications of impeller systems.

3 Design variationThere are many types of impellers used in multiple-impeller systems (Gogate et al. 2000, Min et al. 2008). The impeller designs used in industrial applications are the

Table 1 Advantages of multiple-impeller systems compare to single-impeller system.

No. Multiple-impeller systems References

1 Higher gas hold-up value with same power consumption per volume (as shown in Figures 2 and 3)

Gogate et al. (2000), Li et al. (2012)

2 Better performance in gas utilization, momentum, heat and mass transfer per unit volume as well as gas distribution compared to single-impeller system (as shown in Figure 4)

Pinelli and Magelli (2000), Cabaret et al. (2008), Li et al. (2012)

Higher mass transfer coefficient (kLa) values although gas flow rates change 3 Gives sufficient mixing performance with lower shear strength compared to single-

impeller system (for bioreactor) Gogate et al. (2000), Shewale and Pandit

(2006)4 Each impeller can have own flow pattern and function. For example, incoming gas will

be dispersed by lowest impeller, and upper impeller ensures good top-to-bottom liquid mixing

Gao et al. (2001), Wang et al. (2009)

Without regularities of single impeller and more intricate 5 Lower power required for each impeller for solid suspension (as little as 48% and as high

as 84%) compared with single-impeller system Armenante et al. (1992, 1999), Armenante

and Uehara Nagamine (1997)



S. Tiam You et al.: Multi-impeller stirred vessel studies 325

turbine and paddle impeller designs. Turbine-type impel-lers can be further classified as axial flow or radial flow impellers. Previous researchers have reported four new impeller designs, namely, Rayneri-Sevin, hybrid impel-ler, asymmetric deep hollow blade (BT-6), and Prochem Maxflo (MF) hydrofoil (Foucault et al. 2006). Different combinations or different types of impellers such as axial and radial flow impellers will have different effects on mixing performance such as power number, mixing time, and gas hold-up (Wang et al. 2009). Table 2 summarizes the impeller types and combinations in recent research. Table 2 also shows that the Rushton turbine (RT) has been the most studied, followed by the pitched blade turbine (PBT) and combination of RT and PBT. In addition, a pairing system using the same impeller (RT) was used by

0.01

0.1

A

B

ε [-

]ε

[-]

TXU

RT

NS

TXD

PBU

PBD

0.01

0.1

50050

50050

P/VL (Wm-3)

P/VL (Wm-3)

3 NS

3 PBU

3 RT

3 TXU

3 TXD

3 PBD

Figure 2 Behavior of gas hold-up for single- and triple-impeller system (Pinelli 1994, Moucha et al. 2003, Fujasová et al. 2004).

0

2

4

6

8

10

12

0 1 2 3 4

Gas

Hol

d up

(%

)

P/V (kW/m3)

Triple impeller [a]

Dual impeller [b]

Single impelller [c]

Single impeller [d]

Figure 3 Comparison of gas hold-up for single-impeller and multiple- impeller system. (A) Gogate and Pandit (1999), (B) Arjunwadkar et al. (1998b), (C) Greaves and Barigou (1988), and (D) Abradi (1988).

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 1E-05 2E-05 3E-05 4E-05

Kla

(1/

s)

Gas flow rates (m3/s)

Triple impeller [PTD]

Dual impeller [PTD]

Single impeller [RT]

Figure 4 Comparison of mass transfer coefficient, kLa for single-impeller and multiple-impeller system (Puthli et al. 2005).

most of the researchers to avoid influence between impel-lers in terms of flow pattern.

A number of researchers have reported that an impel-ler-to-tank diameter ratio (D/T) of 0.3–0.5 leads to better fluid movement and power efficiency. More power is required for larger D, whereas small D causes weak fluid movement (Davis 2010). Impeller clearance and spacing usually vary between 0.5D and 2D, whereas a different flow pattern is created as impeller spacing changes (Baba-lona et al. 2005, Pan et al. 2008). However, a turbulent flow is created by adding baffle to the system to improve solid suspension. The standard margin for baffles is 0.08–0.10T with 90°C toward center (Martín et al. 2008). Unfor-tunately, no detailed discussion on the baffle size and shape and number of baffles is found in recent researches. However, some research work on the mixing performance has been done without using baffles (Cabaret et al. 2008).

4 Effect of multiple impellers on various parameters

4.1 Flow pattern

Flow patterns are a visual representation of fluid flow in a stirred vessel. The studies done by Wu and Patterson (1989) on mean velocities, turbulent intensities, one-dimensional energy spectra, and turbulence macro and micro scales for RT stirred tanks showed that 60% of the energy input dissipated in the immediate impeller and jet flow regions from impellers. Hence, power transfer from impeller to the fluid can be improved by understanding the underlying flow patterns (Babalona et al. 2005, Davis 2010). Based on the study done by Kuboi and Nienow (1982) on interaction between impellers, it was discov-ered that large impeller clearance and spacing have only little or no influence on flows produced by each impeller.




However, a flow pattern study using photographic method conducted by Weng (1983) showed that the flow pattern is significantly affected by the spacing between impellers for the dual RT system. Similarly, the flow pattern of an RT is also found to be very different from that of a single impel-ler when the spacing between impellers is rather small (Mishra and Joshi 1994). Rutherford et al. (1996) then sum-marized that flow patterns created by multiple-impeller system are strongly affected by the following parameters:1. Off-bottom clearance for the lower impeller from the

tank bottom, C1.2. Spacing between two impellers, S.3. Submergence of the upper impeller below the fluid

surface.

In addition, they also stated that the flow pattern basically could be divided into three types (Figure 5):1. Parallel flow2. Merging flow (a flow pattern with two main

circulations in a multiple-impeller stirred tank)3. Diverging flow (a flow pattern with three main

circulations in a multiple-impeller stirred tank)

Table 2 Summary of impellers used in multiple-impeller systems.

References Types of impellers used

Armenante and Chang (1998), Jaworski et al. (2000), Cabaret et al. (2008), Liu et al. (2008), Pan et al. (2008), Zadghaffari et al. (2009), Taghavi et al. (2011), Li et al. (2012)

Rushton turbine (RT)

Arjunwadkar et al. (1998a, 1999) Pitched blade turbine (PBT)Arjunwadkar et al. (1998a), Shukla et al. (2001), Shewale and Pandit (2006), Jahoda et al. (2007)

Combination:PBTRT

Li et al. (2009) RTHalf elliptical blade disk turbine (HEDT) Hydrofoil impeller up flow (TXU)

Wang et al. (2010) Combination: RTAlternate blade disc turbine (ABDT) PBT upflow (PBTU) PBT turbine downflow (PBTD)

Dohi et al. (2001) Top = PBTMiddle = PBTBottom = Pfaudler-type impeller

Foucault et al. (2006) RT (top) Rayneri-Sevin impellerHybrid impellerAnchor impeller

Min et al. (2008) Prochem Maxflo hydrofoil impeller (top and middle) Asymmetric deep hollow blade (BT-6) (bottom)

Montante et al. (2010) PBT (top) Radial concave blade turbine (bottom)

Bao et al. (2007, 2008) Up-pumping wide-blade hydrofoils (WHU) (top and middle) HEDT (bottom)

Pinelli and Magelli (2000) Hydrofoil impeller

1.40

A B C

1.40

0.88

0.40

0.15

0.55

1.40

0.40

0.715

0 0.5

Parallel flow

(C1=D=0.40T, C2=0.48T) (C1=D=0.40T, C2=0.315T) (C1=D=0.40T, C2=0.48T)

Merging flow Diverging flow

r/T0 0.5

r/T0 0.5

r/T

z/Tz/T z/T

Figure 5 Basic stable flow patterns. Reprinted from Pan et al. (2008) with permission from Elsevier.

However, Mao (1998) stated that besides three stable flow patterns there are another four unstable flow pat-terns observed with different clearances. He also further stated that for liquid height of H = 2T, the impellers oper-ated independently of one another when the impeller




spacing, C2, was not less than the tank diameter, T, with each impeller being able to produce its own flow char-acteristic as if in a single-impeller system. This phenom-enon is termed as parallel flow. When C2 is no more than 1/3T, flow is almost a straight line orientation toward one another behind impeller streams. The flow was found to merge at an elevation midway between the impellers and forms two large vortices. This is named as merging flow. Liu et al. (2008) and Pan et al. (2008) studied the flow pattern using advanced technologies to investigate the relation between impeller spacing and flow pattern (Table 3). Most of the researches were done on a dual RT system as it is widely used for a variety of mixing pro-cesses. The flow structures developed using the RT are complex (Liu et al. 2008).

4.2 Power number

Power number is a dimensionless parameter used for esti-mating the power consumed by mixing impellers. Power number is a function of fluid density, impeller dimen-sions, and rotational speed of impeller (Taghavi et al. 2011). In addition, Wang et al. (2009) reported that power number is constant and dependent on impeller design and geometric parameters of the vessel and internals in the absence of gas.

Bittins and Zehner (1994) proposed the following equation to calculate the power number:

P 3 5

PNN Dρ

=

(2)

where ρ is the density of the mixing fluid used, N is the impeller rotational speed, P is the power consumption of the mixing process, and D is the diameter of the impeller used.

Previous researchers have investigated whether the total power number was equal to the summation or number of impellers used for multiple-impeller systems especially RT and PB impellers (Armenante and Chang 1998, Armenante et al. 1999). Moreover, it was discovered that power number is dependent on the design of the impeller such as impeller pacing and position. Hudcova et al. (1989) reported that power number increased and reached the individual additive power with increas-ing impeller spacing for an unaerated dual-disc turbine impeller system. Findings on this issue are summarized in Table 4. Most of the findings agree that total power number is equal to the summation of power for each impeller under certain conditions.

Table 3 Relation between flow pattern and impeller spacing (Pan et al. 2008).

Rutherford et al. (1996)

Mao (1998)

Pan et al. (2008)

Flow pattern

H = 2T, C1 = 0.33TD = 0.33T

H = TD = 0.33T

D = 0.33T D = 0.40 D = 0.50T

Parallel flow

C1 > 0.20TC2 > 0.385TC3 < 0.415T

C2 ≥ 0.50T C1 = DC2 ≥ 0.40T

C1 = DC2 ≥ 0.38T

C1 = DC2 ≥ 0.32T

Merging flow

C1 > 0.17TC2 > 0.385T

C2 ≤ 0.33T C1 = DC2 ≥ 0.38T

C1 = DC2 ≥ 0.36T

C1 = DC2 ≥ 0.27T

Diverging flow

C1 > 0.15TC2 > 0.385T

C1 ≤ 0.15T C1 ≤ 0.15T C1 ≤ 0.15T

pT p1 p2 p , for 1nSN N N ND

≈ + + + >�

(3)

pT p1 p2 p , for 1nSN N N ND

< + + + <�

(4)

In addition, power number is related to flow pattern (parallel, diverging, and merging). Parallel flow pattern dominates when impeller spacing is large, indicating that each impeller does not cause obstruction to mixing (Mahmoudi and Yianneskis 1992, Armenante and Chang 1998). Moreover, Pan et al. (2008) observed that power number decreases slightly in the diverging-flow case (Np 2-RT = 9.5) and decreases significantly in the merging-flow case (8.4).

The wealth of experimental data available for mul-tiple-impeller systems opens avenues for numerical pre-dictions. Taghavi et al. (2011) discovered that the largest relative standard deviation among the replicate determi-nations of 200 short measurements during 340 s of meas-urement time was < 1% for single-phase systems. This shows that the reproducibility of measurement is quite reliable. The deviation of experimental power number values from computed values is shown in Figure 6. The difference of power number from the numerical method is only 2% higher than the experimental result. The average power number reported is approximately 8.9. Contrary to other researchers, Jaworski et al. (2000) calculated the power number for dual RT stirred tanks with ΔC = T by the standard κ-ε and RNG κ-ε model, which was 8.32 and 8.11, respectively. At the same time, the power numbers reported by Kasat et al. (2008), Khopkar and Tanguy (2008), and Pan et al. (2008) for parallel-flow regime with dual RT were 10, 7.6 and 7.14, respectively. This shows that power number prediction




by computational methods is reliable and comparable to those from experiments.

4.3 Power consumption

Power consumption per unit mass is one of the key parameters for designing a stirred vessel, especially in scaling up. It is a function of the number of impellers, types of impellers, agitation speed, physical properties of the fluid, the phases to be dispersed, and geometrical design of the system (all dimensions and positions of

impellers are within the tanks) (Armenante et al. 1999). Similar to power number, there are concerns on how the geometrical design of stirred tanks affects power consumption, especially impeller design and position. Table 5 summarizes the effects of geometrical design of impeller on power consumption. It shows the same results as power number. Armenante and Uehara Nag-amine (1997) reported that when the lower impeller was close to the tank bottom (1/48 < C1/T < 1/8), power con-sumption for both dual-disc turbine system and PBT system was less than twice of that consumed by a single-impeller system.

Table 4 Summary of power number studies (Armenante and Chang 1998, Armenante et al. 1999).

References Conditions Finding Remark

Nienow and Lilly (1979)

Dual RTS/D = 2S/D ≤ 1

Np1 = 4.9NpT = 10.2NpTn = n × Np

=pT

p1

2.082NN

Kuboi and Nienow (1982)

Dual RTH/T = 1

Np1 = 3.6Np2 = 3.9NpT = 7.5

NpT = Np1+Np2

Roustan (1985) Dual RTTriple RT

NpT = 10.4NpT = 14.2

pT

p1

2.122NN

=

pT

p1

2.898NN

=

Machon and Vlcek (1985)

RT and PBTS/D < 1S/D = 1

NpT < ∑Np

NpT < ∑Np

Nocentini et al. (1988)

Single impellerFour impeller

Np = 4.6Np = 18.5

pT

p1

4.022NN

=

Np not affected by liquid heightLu and Yao (1991)

Triple impeller

pT

p1

3NN

≈ Each impeller has a different Np (5.05, 5.5, and 3.75

for top, middle and bottom, respectively)

Puthli et al. (2005)

Single impeller RTRT+PBTRT+2PBT

Np = 4.8Np = 6.3Np = 7.8

Np for RT = 4.8 and Np for PBT = 1.5. Total Np equal to summation Np for each impeller.

6

7

8

9

10

11

12

150 200 250 300 350 400 450 500 550 600 650

Np

N (rpm)

Experimental data

Simulation data

Figure 6 Comparing experimental and simulation power number with increasing impeller rotational speed (Taghavi et al. 2011).




T 1 2 , for 1.5n

SP P P PD

≈ + + + >�

(5)

T 1 2 , for 1.5n

SP P P PD

< + + + <�

(6)

Additionally, Wang et al. (2009) reported that axial impellers consumed less energy than radial impellers in both single- or multiple-phase systems. Moreover, the axial impeller displayed insensitivity to aeration regard-less of gas flow rate. Kasundra et al. (2008) studied several types of impellers such as self-inducing impeller, pipe impeller, and disc impeller. They reported that the self-inducing turbine impeller has higher power consumption compared to those impellers chosen for experiments in multiple-impeller stirred vessels, whereas the hydrofoil-type impeller could create better flow at the same power consumption. However, the radial impeller used higher power consumption because more power was needed to disperse gas bubbles. Li et al. (2009) also found that the combination of RT+Techmix 335 hydrofoil upflow (TXU)+half-elliptical blade disc turbine (HEDT) is superior to RT+TXU+RT and RT+TXU+TXU based on mass transfer coefficient, kLa, when comparing power consumption.

The RT+TXU+TXU combination displayed the lowest per-formance, which demonstrates that axial-type impellers are unsuitable for multiphase mixing.

Nienow and Bujalski (2002) studied the formation of different cavity structures behind the RT. Growth in cavity size causes reduction in power consumption. Four types of cavity structures have been identified as cavity grows: ragged cavities, 3-3 structure, clinging cavities, and vortex cavities (Doran 1995). Bao et al. (2012) have recently studied the cavities behind the impeller blade and gas cir-culation. It was discovered that an increase in top impel-ler diameter caused an increase in cavity size behind the larger up-pumping wide-blade hydrofoils (WHUs), which decreased power consumption. Therefore, an increase in top impeller diameter creates more blockage, which causes recirculation of gas to the impeller region, result-ing in power consumption reduction.

4.4 Mixing time

Mixing time is defined as the time required to achieve the desired or specified degree of homogeneity. The desired degree of mixing used is normally fixed at 95%, which

Table 5 Effect of geometrical design of impeller on power consumption (Armenante and Chang 1998, Armenante et al. 1999).

References Conditions Findings Remarks

Bates et al. (1963)

S/D < 4, dual PBTS/D < 1, dual flat-blade turbines (FBT)

1

2tPP

<

1

1.25tPP

≈

Ho et al. (1987)

H/T = 3S/D = 2.0, dual RTS/D = 2.5, triple RT Pt = ∑P

For S/D < 1.5, P increases steeply as S/D increases. When S/D = 1.5, P total equal to 90% summation of each P

Abradi (1988)

S/D = 2 and C1/D = 1Dual impeller

1

2tPP

=

Hudcova et al. (1989)

D = 0.33T, H/T = 1 or 2C1 = DS/D = 0.20.5 < S/D < 1.5S/D > 2

1

1.29tPP

=

1

1.54 1.91tPP

< <

1

2tPP

=

Abrardi (1990)

Multiple-impeller system1.5 < S/D < 2 Pt = ∑P

Chiampo (1991)

C1 = DHigh speed ( > 450rpm), S/D ≥ 1.6Low speed, S/D ≥ 1.9 1

2tPP

= S/D < 0.6

P increases moderately0.6 < S/D < 1.3Slightly increasedS/D > 1.3If turbulence occurs power increases steeply




means that mixing time is the time when tracer concentra-tion reaches or remains constant within 5% range of the final concentration (Mavros 2001, Jahoda et al. 2007).

θ ∞

∞

=i

-( )

-tC C

tC C

(7)

where Ct, C∞, and Ci are the concentration of tracer at respective time t, equilibrium or final concentration, and initial concentration, respectively.

Mixing time for a multiple-impeller system is much higher than for a single-impeller system (Figure 7). Yi (2006) reported that mixing time increased significantly for dual and triple RTs (about 50% and 75%, respectively), whereas mixing time was almost the same for triple-, dual-, and single-impeller systems for other hydrofoil impellers.

The focus is more on the effect of impeller combi-nation in studies of mixing time in multiple-impeller systems. Kasat and Pandit (2004) studied mixing time on two combinations of triple impeller (PBT-PBT-PBT and PBT-PBT-RT) and reported that using RT as bottom impel-ler reduced mixing time by 10–15%, but power consump-tion doubled. Bao et al. (2005b) showed that mixing time was reduced by over 40% for radial impellers in single liquid phase. Wang et al. (2009) reported that radial impellers like RT or alternate blade disc turbine (ABDT) broke the injected electrolyte lump into smaller sizes by great shear force. Thus, there is better dispersion, which results in better mixing and shorter mixing time compared to that of axial impellers such as PBTD or PBTU. However, when gas is present, the axial liquid movement acceler-ates the diffusion of electrolyte solution, which shortens the mixing time compared to that of radial-type impellers. A number of studies have explored the potential of coaxial

0

5

10

15

20

25

30

35

40

45

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Mix

ing

time

(s)

1/N (s)

6 RT

6 RT-4 MFU

6 RT-4 MFD

6 RT -6 RT

Figure 7 Comparison of mixing time between single- and dual-impeller systems (Abradi 1988).

mixing systems. Schneider and Todtenhaupt (1990) were the first to investigate the performance of a coaxial mixer in counter-rotating mode. Reduction of mixing time was observed in counter-rotating mode compared to the single Viscopro impeller configuration. Tanguy et al. (1997) studied the mixing performance of dual-impeller stirred vessels, which were composed of a disc turbine and a helical ribbon impeller mounted in the same axis but rotating at different speeds. Improved performance in top-to-bottom pumping was observed compared to a standard helical ribbon impeller. Espinosa-Solares et al. (2002) studied the mixing time performance of a hybrid dual-impeller system (Rushton impeller+helical ribbon impeller) by keeping the rotational speed ratio constant. Improved performance was observed for this hybrid system compared to the single-impeller system. Foucault et al. (2004) compared the performance of coaxial mixer configurations consisting of a wall-scraping impeller and several dispersing impellers operated in counter- and co-rotation modes with Newtonian and non-Newtonian fluids. Co-rotating mode was found to have shorter mixing time for both types of fluids in transition regime. Foucault et al. (2006) studied the coaxial mixing system consisting of an anchor impeller and three different types of impel-lers (Rushton, Rayneri-Sevin impeller, and hybrid dis-persing impeller) using Newtonian and non-Newtonian (shear-thinning) fluids. The introduction of an anchor impeller reduced mixing time in co-rotating mode used for all three impellers; however, mixing time increased in counter-rotating mode.

Computational methods are also used to predict mixing time for multiple-impeller systems. Jaworski et al. (2000) reported computational dynamics (CFD) simulation results for the dual RT using ANSYS Fluent (ANSYS, Inc., PA, USA)and found that mixing time was two to three times higher than the experimental data. Jahoda et al. (2007) showed that large eddy simulation approach described the real flow in a stirred tank better and reflected more realistic courses of the liquid homogenization. Zadghaffari et al. (2009) compared the CFD and experimental results using Particle Image Velocimetry (PIV) technique. The results showed that there was only 5.5% overprediction to experimental data. Hence, available computational methods can com-plement results of experimental studies.

Finally, researchers have studied the effects of off-center shaft operation for multiple impeller system. Karcz et al. (2005) showed that mixing time decreased with increased shaft eccentricity for both axial and radial impellers but with undesirable increase of power con-sumption. Hall et al. (2005) showed that eccentrically agitated vessels needed slightly shorter mixing time than




baffled configurations at the same power consumption per unit volume. Cabaret et al. (2007) studied the dual shaft mixer with off-center shaft operating in counter-rotating mode, which prevented flow compartmentalization and reduced mixing time.

4.5 Gas hold-up

Gas hold-up is defined as the ratio of the gas phase volume to the total volume.

G o

G

-H HH

ε=

(8)

A number of studies on the effects of multiple-impeller systems on gas hold-up (Pinelli 1994, Moucha et al. 2003), using identical triple impellers on a single shaft, reported that axial flow pattern produced greater gas hold-up. However, Arjunwadkar et al. (1998a) used RT and axial flow PB impellers and discovered that the com-bination of PB pumping down in the upper stages and RT in the bottom stage is the most effective configuration. Moucha et al. (2003) showed that down-pumping impel-ler configuration increased liquid circulation, which impedes rising bubbles and increases gas hold-up. As a result, gas hold-up is dependent on the impeller type used. Fujasová et al. (2004) reported that gas hold-up is consistent for all types of impellers investigated (Figure 2). Bao et al. (2006) recommended using concave-blade disk turbine (HEDT) or radial impeller as the lowest impeller. It was reported that the radial bottom impel-ler efficiently disperses gas while operating at higher relative power demand and provides better gas hold-up. Bao et al. (2012) studied the influence of top impeller

diameter on gas hold-up. The gas hold-up increases with increasing top impeller diameter. However, this effect decreases, while gas flow rate increases. Therefore, an increase in top impeller diameter increases gas recircula-tion in the tank, which increases bubble residence time and gas hold-up. However, most studies on gas hold-up have focused on mixing medium, solid content and gas properties such as superficial gas velocity and density instead of the effects of multiple impellers (Pinelli and Magelli 2000, Shukla et al. 2001, Bao et al. 2005a,b, 2008).

4.6 Mass transfer coefficient

Gas-liquid mass transfer coefficient (kLa) is one of the global parameters that is dependent on the impeller design, tank geometry, power consumption, and proper-ties of the gas-liquid system. It determines the efficiency of gas transfer within the stirred vessels. The rate of oxygen transfer in nitrogen-purged medium or dissolved oxygen probe was commonly used to calculate gas-liquid mass transfer coefficient. As shown in Eq. (8), kLa will be deter-

mined from the plotted graph of final

in

*-ln

*-C CC C

against t,

where kLa is the slope of the graph.

finalL

in

*-1a ln*-

C Ck

t C C

=

(9)

The average value of kLa in a stirred tank can be calculated through Eq. (10) (Cabaret et al. 2008):

L Top L BottomL mean

a aa

2k k

k+

=

(10)

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

Exp

erim

enta

l kLa

(s-1

)

Predicted kLa (s-1)

Centered shaft without baffles

Off-centered shaft

Dual shaft Co-rotating

Dual shaft Counter-rotating

Centered shaft with baffles

Figure 8 Comparison between experimental kLa and predicted kLa (Cabaret et al. 2008).




The study done by Lu (2004) showed that kLa for each impeller was the same at any position, but the mean or average value for kLa in a stirred tank for a multiple-impeller system was not equal to the mean value. Eq. (11) describes the mass transfer coefficient in a multiple impeller system. Shewale and Pandit (2006) and Cabaret et al. (2008) compared the correlation using Eq. (12) with experimental data, which is shown in Figure 8. The exper-imental data in the multiple impeller system fitted well into both correlations.

L 3a 0.134 0.0039Qk

ND

= + (11)

a b

Lak cN Q=

(12)

In addition, Arjunwadkar et al. (1998b) and Shukla et al. (2001) studied bioreactors using dual-impeller system and showed that kLa in a fermenter is a strong function of mode of energy dissipation and physical-chemical prop-erties of the liquid media. Moucha et al. (2003) studied the effects of impeller configurations on kLa at constant power consumption and reported that the kLa values are approximately independent of impeller configuration at higher power consumption (800 W/m3); the impeller con-figuration with high power number provides significantly high kLa at low power consumption (300W/m3), Suhaili et al. (2010) reported that the combination of two concave blade disc turbines enhances mass transfer coefficient by about 5–50% and 18–65% compared to dual RT in New-tonian and non-Newtonian systems, respectively. Karimi et al. (2013) reported that mass transfer coefficient behav-ior for single and dual impellers was almost the same, with the dual impeller having insignificantly higher value compared to the single impeller. Additionally, RT shows higher performance ( > 50%) compared to PBT in single- and dual-impeller systems.

5 ConclusionRecent studies on multiple-impeller systems focused on the effects of multiple impeller combinations and off-bottom clearance and space between impellers on power

number, power consumption, flow pattern, gas hold-up, mass transfer coefficient, and mixing time. An overview of their results is also reviewed in this work. This review signals that there is room for future study on multiple impeller systems, particularly in the aspects below:1. Studies on different combinations other than RT and

PB impeller.2. CFD studies on multiple-impeller systems with data

focused on flow pattern, power consumption, and gas hold-up.

3. Studies on solid suspensions using multiple-impeller systems, particularly on systems involving gas.

NomenclatureH Height of liquid in stirred tank, mHG Height of liquid in the presence of gas, mHo Height of liquid in absence gas, mn Number of impellers usedD Diameter of impeller used, mT Diameter of stirred tank, mC1 Off-bottom clearance of lower impeller, mC2 Spacing between lower and middle impellers, mC3 Spacing between middle and top impellers, mS Impeller spacing, mNp Power numberP Power consumption, kg m2/s3

ρ Density of fluid, kg/m3

N Agitation speed, rpmθ(t) Mixing time, sC Concentration of tracerε Gas hold-upRT Rushton turbinePBT Pitched blade turbineHEDT Half-elliptical blade disk turbineTXU Hydrofoil impeller up flowWHU Up-pumping wide-blade hydrofoilABDT Alternate blade disc turbineCFD Computational fluid dynamics

Acknowledgments: This research is supported by a Post-graduate Research Fund (PPP) with project number PG115-2012B from the University of Malaya.

Received August 13, 2013; accepted January 26, 2014; previously published online March 27, 2014




ReferencesAbradi V, Rovero G, Sicardi S, Baldi G, Conti R. Sparged vessels

agitated by multiple impellers. Proc Eur Conf Mixing 1988; 6: 329–336.

Abradi V, Rovero G, Baldi G, Sicardi S, Conti R. Hydrodynamics of a gas-liquid reactor stirred with a multiple-impeller system. Chem. Eng. Res. Des 1990. 68: 516–522.

Alliet-Gaubert M, Sardeing R, Xuereb C, Hobbes P, Letellier B, Swaels P. CFD analysis of industrial multi-staged stirred vessels. Chem Eng Process Process Intensif 2006; 45: 415–427.

Arjunwadkar SJ, Saravanan K, Pandit AB, Kulkarni PR. Optimizing the impeller combination for maximum hold-up with minimum power consumption. Biochem Eng J 1998a; 1: 25–30.

Arjunwadkar SJ, Sarvanan K, Kulkarni PR, Pandit AB. Gas-liquid mass transfer in dual impeller bioreactor. Biochem Eng J 1998b; 99–106.

Armenante PM, Chang G-M. Power consumption in agitated vessels provided with multiple-disk turbines. Ind Eng Chem Res 1998; 37: 284–291.

Armenante PM, Uehara Nagamine E. Solid suspension and power dissipation in stirred tanks agitated by one or two impellers at low off-bottom impeller clearances. In: Proceedings of the 9th European Conference on Mixing. Nancy, France, 1997.

Armenante PM, Huang Y-T, Li T. Determination of the minimum agitation speed to attain the just dispersed state in solid-liquid and liquid-liquid reactors provided with multiple impellers. Chem Eng Sci 1992; 47: 2865–2870.

Armenante PM, Mazzarotta B, Chang G-M. Power consumption in stirred tanks provided with multiple pitched-blade turbines. Ind Eng Chem Res 1999; 38: 2809–2816.

Babalona E, Bahouma D, Tagia S, Pantouflas E, Markopoulos J. Power consumption in dual impeller gas liquid contactors: impeller spacing, gas flow rate, and viscosity effects. Chem Eng Technol 2005; 28: 802–807.

Bao Y, Gao Z, Hao Z, Long J, Shi L, Smith JM, Kirkby NF. Effects of equipment and process variables on the suspension of buoyant particles in gas-sparged vessels. Ind Eng Chem Res 2005a; 44: 7899–7906.

Bao Y, Hao Z, Gao Z, Shi L, Smith JM. Suspension of buoyant particles in a three phase stirred tank. Chem Eng Sci 2005b; 60: 2283–2292.

Bao Y, Hao Z, Gao Z, Shi L, Smith JM, Thorpe RB. Gas dispersion and solid suspension in a three-phase stirred tank with multiple impellers. Chem Eng Commun 2006; 193: 801–825.

Bao Y, Gao Z, Huang X, Shi L, Smith JM, Thorpe RB. Gas-liquid dispersion with buoyant particles in a hot-sparged stirred tank. Ind Eng Chem Res 2007; 46: 6605–6611.

Bao Y, Zhang X, Gao Z, Chen L, Chen J, Smith JM, Kirkby NF. Gas dispersion and solid suspension in a hot sparged multi-impeller stirred tank. Ind Eng Chem Res 2008; 47: 2049–2055.

Bao Y, Yang J, Chen L, Gao Z. Influence of the top impeller diameter on the gas dispersion in a sparged multi-impeller stirred tank. Ind Eng Chem Res 2012; 51: 12411–12420.

Bates RL, Fondy PL, Corpstein RR. An examination of some geometric parameters of impeller power. Ind Eng Chem Process Des Dev 1963; 2: 310–314.

Bittins K, Zehner P. Power and discharge numbers of radial-flow impellers. Fluid-dynamic interactions between impeller and baffles. Chem Eng Process Process Intensif 1994; 33: 295–301.

Bouaifi M, Roustan M. Power consumption, mixing time and homogenisation energy in dual-impeller agitated gas-liquid reactors. Chem Eng Process Process Intensif 2001; 40: 87–95.

Cabaret F, Rivera C, Fradette L, Heniche M, Tanguy PA. Hydrodynamics performance of a dual shaft mixer with viscous Newtonian liquids. Chem Eng Res Des 2007; 85: 583–590.

Cabaret F, Fradette L, Tanguy PA. Gas-liquid mass transfer in unbaffled dual-impeller mixers. Chem Eng Sci 2008; 63: 1636–1647.

Chiampo F. Gas-liquid mixing in a multiple impeller stirred vessel. In: Proceedings of the 7th European Conference on Mixing. Bruge, Belgium, 1991.

Davis RZ. Design and scale-up of production scale stirred tank fermentors. Unpublished Master of Science dissertation. Utah State University, Logan, UT, USA, 2010.

Dohi N, Matsuda Y, Itano N, Minekawa K, Takahashi T, Kawase Y. Suspension of solid paticles in multi-impeller three phase stirred tank reactors. Can J Chem Eng 2001; 79: 107–111.

Dohi N, Takahashi T, Minekawa K, Kawase Y. Power consumption and solid suspension performance of large-scale impellers in gas-liquid-solid three-phase stirred tank reactors. Chem Eng J 2004; 97: 103–114.

Doran PM (1995). Bioprocess Engineering Principles 2nd: Mixing. UK, Elsevier Ltd. 8: 274–275.

Espinosa-Solares T, Brito-de la Fuente E, Tecante A, Medina-Torres L, Tanguy PA. Mixing time in rheologically evolving model fluids by hybrid dual mixing systems. Chem Eng Res Des 2002; 80: 817–823.

Fajner D, Pinelli D, Ghadge RS, Montante G, Paglianti A, Magelli F. Solids distribution and rising velocity of buoyant solid particles in a vessel stirred with multiple impellers. Chem Eng Sci 2008; 63: 5876–5882.

Fishwick RP, Winterbottom JM, Parker DJ, Fan X, Stitt EH. Hydrodynamic measurements of up-and down-pumping pitched-blade turbines in gassed, agitated vessels, using positron emission particle tracking. Ind Eng Chem Res 2005; 44: 6371–6380.

Foucault S, Ascanio G, Tanguy PA. Coaxial mixer hydrodynamics with Newtonian and non-Newtonian fluids. Chem Eng Technol 2004; 27: 324–329.

Foucault S, Ascanio G, Tanguy PA. Mixing times in coaxial mixers with Newtonian and non-Newtonian fluids. Ind Eng Chem Res 2006; 45: 352–359.

Fujasová M, Linek V, Moucha T, Prokopová E. Energy demands of different impeller types in gas-liquid dispersions. Sep Purif Technol 2004; 39: 123–131.

Gao Z, Smith JM, Müller-Steinhagen H. Gas dispersion in sparged and boiling reactors. Chem Eng Res Des 2001; 79: 973–978.

Gogate PR, Pandit M. Scale effect on gas hold-up characteristics in multiple-impeller fermenter. In: AIChE Spring Meeting, Houston, TX, 1999.

Gogate PR, Beenackers AACM, Pandit AB. Multiple-impeller systems with a special emphasis on bioreactors: a critical review. Biochem Eng J 2000; 6: 109–144.

Greaves M, Barigou M. The internal structure of gas-liquid dispersions in a stirred reactor. In: Proceedings of the 6th European Conference on Mixing, Cranfield, UK, 1988: 313.




Hall J, Barigou M, Simmons MJH, Stitt EH. Comparative study of different mixing strategies in small high throughput experimentation reactors. Chem Eng Sci 2005; 60: 2355–2368.

Ho CS, Oldshue JY, editors. Biotechnology processes: scale-up and mixing. New York: American Institute of Chemical Engineers, 1987: 107–115.

Hudcova V, Machon V, Nienow AW. Gas-liquid dispersion with dual Rushton impellers. Biotechnol Bioeng 1989; 34: 617–628.

Jafari R, Tanguy PA, Chaouki J. Experimental investigation on solid dispersion, power consumption and scale-up in moderate to dense solid-liquid suspensions. Chem Eng Res Des 2012; 90: 201–212.

Jahoda M, Moštek M, Kukuková A, Machon V. CFD modelling of liquid homogenization in stirred tanks with one and two impellers using large eddy simulation. Chem Eng Res Des 2007; 85: 616–625.

Jaworski Z, Bujalski W, Otomo N, Nienow AW. CFD study of homogenization with dual Rushton turbines – comparison with experimental results: Part I: Initial studies. Chem Eng Res Des 2000; 78: 327–333.

Karcz J, Cudak M, Szoplik J. Stirring of a liquid in a stirred tank with an eccentrically located impeller. Chem Eng Sci 2005; 60: 2369–2380.

Karimi A, Golbabaei F, Mehrnia MR, Neghab M, Mohammad K, Nikpey A, Pourmand MR. Oxygen mass transfer in a stirred tank bioreactor using different impeller configurations for environmental purposes. Iran J Environ Health Sci Eng 2013; 10: 1–9.

Kasat GR, Pandit AB. Mixing time studies in multiple impeller agitated reactors. Can J Chem Eng 2004; 82: 892–904.

Kasat GR, Pandit AB, Ranade VV. CFD Simulation of Gas-Liquid Flows in a Reactor Stirred by Dual Rushton Turbines. Int J Chem React Eng 2008; 6: 1628.

Kasundra RB, Kulkarni AV, Joshi JB. Hydrodynamic and mass transfer characteristics of single and multiple impeller hollow self-inducing reactors. Ind Eng Chem Res 2008; 47: 2829–2841.

Khopkar AR, Tanguy PA. CFD simulation of gas-liquid flows in stirred vessel equipped with dual Rushton turbines: influence of parallel, merging and diverging flow configurations. Chem Eng Sci 2008; 63: 3810–3820.

Kuboi R, Nienow AW. The power drawn by dual impeller systems under gassed and ungassed conditions. In: Proceedings of the 4th European Conference on Mixing, Noordwijkerhout, the Netherlands, 1982: 247.

Lehn MC, Myers KJ, Barker A. Agitator design for solids suspension under gassed conditions. Can J Chem Eng 1999; 77: 1065–1071.

Li X, Yu G, Yang C, Mao Z-S. Experimental study on surface aerators stirred by triple impellers. Ind Eng Chem Res 2009; 48: 8752–8756.

Li Z, Hu M, Bao Y, Gao Z. Particle image velocimetry experiments and large eddy simulations of merging flow characteristics in dual Rushton turbine stirred tanks. Ind Eng Chem Res 2012; 51: 2438–2450.

Liu X, Bao Y, Li Z, Gao Z, Smith JM. Particle image velocimetry study of turbulence characteristics in a vessel agitated by a dual Rushton impeller. Chin J Chem Eng 2008; 16: 700–708.

Lu WM. Gas-liquid mass transfer in gassed mechanically agitated vessel. In: Lu WM, editor. Multiple impeller gas-liquid contactors. Taiwan: RESI Corporation, 2004; 8: 153–190.

Lu WM, Yao CL. Gas dispersion in a multi-stage impeller stirred tank. In: Proceedings of the 7th European Conference on Mixing, Brugge, Belgium, 1991.

Machon V, Vlcek J. Dual impeller systems for aeration of liquids: an experimental study. In: Proceedings of the 5th European Conference on Mixing. Wurzburg, Germany, 1985: 155–169.

Mahmoudi S, Yianneskis M. The variation of flow pattern and mixing time with impeller spacing in stirred vessels with two Rushton impellers. In fluid mechanics of mixing. Springer Netherlands, 1992: 11–18.

Mao DM. Study on mixing and flow in vessels stirred by multideck impellers. Unpublished doctoral dissertation. Zhejiang University, Hangzhou, China, 1998.

Martín M, Montes FJ, Galán MA. On the contribution of the scales of mixing to the oxygen transfer in stirred tanks. Chem Eng J 2008; 145: 232–241.

Mavros P. Flow visualization in stirred vessels: A review of experimental techniques. Chem Eng Res Des 2001; 79: 113–127.

Min J, Bao Y, Chen L, Gao Z, Smith JM. Numerical simulation of gas dispersion in an aerated stirred reactor with multiple impellers. Ind Eng Chem Res 2008; 47: 7112–7117.

Mishra V, Joshi J. Flow generated by a disc turbine. IV: Multiple impellers. Chem Eng Res Des 1994; 72: 657–668.

Montante G, Laurenzi F, Paglianti A, Magelli F. Two-phase flow and bubble size distribution in air sparged and surface-aerated vessels stirred by a dual impeller. Ind Eng Chem Res 2010; 49: 2613–2623.

Moucha T, Linek V, Prokopová E. Gas hold-up, mixing time and gas-liquid volumetric mass transfer coefficient of various multiple-impeller configurations: Rushton turbine, pitched blade and Techmix impeller and their combinations. Chem Eng Sci 2003; 58: 1839–1846.

Moucha T, Linek V, Sinkule J. Measurement of kLa in multiple-impeller vessel with significant axial dispersion in both phases. Chem Eng Res Des 1995; 73: 286–290.

Murthy B, Ghadge RS, Joshi JB. CFD simulations of gas-liquid-solid stirred reactor: prediction of critical impeller speed for solid suspension. Chem Eng Sci 2007; 62: 7184–7195.

Nienow A, Bujalski W. Recent studies on agitated three-phase (gas-solid-liquid) systems in the turbulent regime. Chem Eng Res Des 2002; 80: 832–838.

Nienow AW, Lilly MD. Power drawn by multiple impellers in sparged agitated vessels. Biotechnol Bioeng 1979; 21: 2341–2345.

Nocentini M, Magelli F, Pasquali G, Fajner D. A fluid-dynamic study of a gas-liquid, non-standard vessel stirred by multiple impellers. Chem Eng J 1988; 37: 53–59.

Ochieng A, Lewis AE. CFD simulation of solids off-bottom suspension and cloud height. Hydrometallurgy 2006; 82: 1–12.

Pan C, Min J, Liu X, Gao Z. Investigation of fluid flow in a dual rushton impeller stirred tank using particle image velocimetry. Chin J Chem Eng 2008; 16: 693–699.

Panneerselvam R, Savithri S, Surender GD. Computational fluid dynamics simulation of solid suspension in a gas-liquid-solid mechanically agitated contactor. Ind Eng Chem Res 2008; 48: 1608–1620.

Pinelli D, Magelli F. Analysis of the fluid dynamic behavior of the liquid and gas phases in reactors stirred with multiple hydrofoil impellers. Ind Eng Chem Res 2000; 39: 3202–3211.




Pinelli D. Hold-up in low viscosity gas-liquid systems stirred with multiple impellers. comparison of different agitator types and sets. Inst Chem Eng Symp Ser 1994; 136: 81–88.

Puthli MS, Rathod VK, Pandit AB. Gas-liquid mass transfer studies with triple impeller system on a laboratory scale bioreactor. Biochem Eng J 2005; 23: 25–30.

Roustan M. Power consumed by rushton turbines in non-standard vessels under gassed conditions. In: Proceedings of the 5th European Conference on Mixing, Wurzburg, Germany, 1985.

Rutherford K, Lee KC, Mahmoudi SMS, Yianneskis M. Hydrodynamic charateristics of dual Rushton impeller stirred vessels. AICHE J 1996; 42: 332–346.

Schneider T, Todtenhaupt E. Mixing times and heat transfer in coaxial stirrers. EKATO Ruehr-Mischtech. G.m.b.H., Schopfheim, Germany. Chem Ing Tech 1990; 63: 208–209.

Shewale SD, Pandit AB. Studies in multiple impeller agitated gas-liquid contactors. Chem Eng Sci 2006; 61: 489–504.

Shukla VB, Veera UP, Kulkarni PR, Pandit AB. Scale-up of biotransformation process in stirred tank reactor using dual impeller bioreactor. Biochem Eng J 2001; 8: 19–29.

Smith J. Industrial needs for mixing research. Chem Eng Res Des 1990; 68: 3–6.

Suhaili N, Mohamed MS, Mohamad R, Ariff AB. Gas-liquid mass transfer performance of dual impeller system employing Rushtons, concave-bladed disc (CD-6) turbines and their combination in stirred tank bioreactor. J Appl Sci Res 2010; 6: 234–244.

Taghavi M, Zadghaffari R, Moghaddas J, Moghaddas Y. Experimental and CFD investigation of power consumption in a dual Rushton turbine stirred tank. Chem Eng Res Des 2011; 89: 280–290.

Tamburini A, Cipollina A, Micale G, Ciofalo M, Brucato A. Dense solid-liquid off-bottom suspension dynamics: simulation and experiment. Chem Eng Res Des 2009; 87: 587–597.

Tanguy PA, Thibault F, Fuente EBDL, Espinosa-Solares T, Tecante A. Mixing performance induced by coaxial flat blade-helical ribbon impellers rotating at different speeds. Chem Eng Sci 1997; 52: 1733–1741.

Vrabel P, Van der Lans RGJM, Cui YQ, Luyben KCAM. Compartment model approach: mixing in large scale aerated reactors with multiple impellers. Chem Eng Res Des 1999; 77: 291–302.

Vrabel P, van der Lans RGJM, Luyben KCHAM, Boon L, Nienow AW. Mixing in large-scale vessels stirred with multiple radial or radial and axial up-pumping impellers: modelling and measurements. Chem Eng Sci 2000; 55: 5881–5896.

Wang T, Yu G, Yong Y, Yang C, Mao Z-S. Hydrodynamic characteristics of dual-impeller configurations in a multiple-phase stirred tank. Ind Eng Chem Res 2009; 49: 1001–1009.

Weng ZX. The effect of the distance between multiple impellers in the turbulent tank. Chem Eng 1983; 6: 1–6.

Wu H, Patterson G. Laser-Doppler measurements of turbulent-flow parameters in a stirred mixer. Chem Eng Sci 1989; 44: 2207–2221.

Yi M. Mixing in stirred tanks with multiple impellers. Journal of East China University of Science and Technology, Natural Science Edition 2006; 32: 357–360.

Zadghaffari R, Moghaddas JS, Revstedt J. A mixing study in a double-Rushton stirred tank. Comput Chem Eng 2009; 33: 1240–1246.

See Tiam You, a native Malaysian, graduated with a chemical engi-neering degree in 2012 and enrolled in a master’s degree program at the University of Malaya, Malaysia, in the same year. Encouraged by research interests in the field of mixing and with excellent labo-ratory skills, his current research focuses on performance studies in a multi-impeller stirred vessel for gas-liquid-solid system aiming at identifying key parameters that enhance performance in a multi-impeller stirred vessel.

Raja Shazrin Shah graduated with a chemical engineering degree in 2004 from the University of Malaya, Malaysia, and received his master’s degree in 2010 from the same university. He worked on applied research for resource recovery for waste streams coming from natural rubber and palm oil industries in Malaysia, with partic-ular emphasis on membrane technologies. He joined as a doctoral candidate in 2012 in the same university, working on hydrodynamic studies on multiphase systems in stirred reactors.




Abdul Aziz completed his PhD in the area of three-phase mixing. Currently, he is an associate professor and holds the position of deputy dean at the Faculty of Engineering, University of Malaya, Malaysia. His research interests are in mixing in stirred vessels and cleaner production technologies. He is also active in consultancy projects and supervised many PhD candidates. He is a member of a number of professional and learned societies such as the Institution of Chemical Engineers (IChemE, UK), the Institution of Engineers Malaysia (IEM), and the American Chemical Society (ACS).

Mohamad Iskandr Mohamad Nor is a senior lecturer in the Depart-ment of Chemical Engineering at University of Malaya, Malaysia. He received his BE and MSc in Chemical Engineering from Lake-head University and Queen’s University, Canada, respectively. His interests include applications of computational fluid dynamics (CFD) in Chemical Engineering, Linux OS and open source software in engineering.



Documents

See Tiam You, Abdul Aziz Abdul Raman*, Raja Shazrin Shah ...repository.um.edu.my/95701/1/Multiple-impeller stirred vessel... · ler, asymmetric deep hollow blade (BT-6), and Prochem