role of sparger design in mechanically agitated gas-liquid reac

386 Chem. Eng. Technol. 14 (1991) 386 - 393

Role of Sparger Design in Mechanically agitated Gas-Liquid Reactors Part 11: Liquid Phase Mixing

Vilas B. Rewatkar and Jyeshtharaj B. Joshi*

Liquid phase mixing time was measured in 0.57, 1 .O and 1 .5 m i.d. mechanically agitated gas- liquid reactors. Transient conductivity technique was used for the mixing time measurement. Pit- ched blade downflow turbine was employed. The design details of PTD impellers such as diameter (0.22 T to 0.5 T, and blade width (0.25 D to 0.35 D ) were studied. The influence of sparger types and their design on mixing time has been investigated. For this purpose, pipe, ring, conical, and concentric ring spargers were employed. The design details of the ring sparger, i.e. ring diameter, number of holes and hole size were also studied in depth. Sparger location with respect to the impeller was found to be the most important variable and, therefore, it was varied for practically all the spargers studied in this work. It was found that the liquid phase mixing time depends on the impeller design, sparger design, sparger location, impeller speed and superficial gas velocity. Correlations have been developed for the dimensionless mixing time.

1 Introduction

The knowledge of liquid phase mixing is important for the design of mechanically agitated multiphase (liquid-liquid, gas- liquid, solid-liquid and gas-liquid-solid) contactors. For these contactors, a considerable body of knowledge exists on liquid phase mixing, in the absence of other phases. However, very little work has been reported on mixing in the presence of gas (gas-liquid) or solids (solid-liquid) or both (gas-liquid-solid). Paca et al. [l], Einsele and Finn [ 2 ] , Joshi et al. [3] and Pandit and Joshi [4] measured liquid phase mixing time in the presence of gas. In general, it was observed that the presence of gas as well as solid particles [5 - 71 extends the liquid phase mixing time. This increase depends on impeller design, gas flow rate and impeller speed. Raghava Rao and Joshi [ S ] studied these aspects for different impeller designs and they recommended the pitched blade downflow turbine (PTD) for gas-liquid systems, on the basis of minimum power consumption per unit mass. However, in this investigation, sparger's design and its distance from the impeller were kept constant. The effect of sparger design on liquid phase mixing has not been investigated in the past. In the present paper, emphasis has been laid on the design data of the sparger and its location with respect to the impeller. The combined effect of sparger and impeller (PTD) designs on the liquid phase mixing was also studied. It was considered desirable to investigate these effects in a 1.5 m i.d. vessel.

2 Experimental

Experiments were carried out in 0.57, 1.0 and 1.5 m i.d. mechanically agitated contactors (MAC), fitted with four baf- fles, with widths amounting to 10% of the tank diameter. Pitch- ed blade downflow turbines with six blades (45" blade inclina-

* V.B. Rewatkar and Prof. J.B. Joshi, Department of Chemical Techno- logy, University of Bombay, Matunga, Bombay-400 019, India. Cor- respondence should be addressed to Prof. J.B. Joshi.

tion) were used for this study. Details pertaining to MAC and impellers are listed in Tables 1 and 2 (Part I), respectively. The impeller speed was varied in the range between 0.4 and 10.5 rls. Air and tap water were used as gas and liquid phases. The superficial gas velocity was varied from 0 to 15 mmls. Dif- ferent designs of sparger, such as pipe, ring, concentric ring, and conical spargers were used. The construction details of these spargers are presented in Fig. 1B and Table 3 (Part I). Im- peller geometries are the same as those in Part I.

The liquid phase mixing time was measured by using transient conductivity technique. Measurement details of mixing time have been reported by various investigators [4, 5 , 8, 91. An at- tempt was made to clarify the mechanism of gas dispersion on the basis of variation of liquid phase mixing time with impeller speed.

3 Mechanism of Gas Dispersion

The liquid phase mixing time (in the absence of gas) decreases continuously with increasing impeller speed. The value of" NOmi, remains practically constant for a given impeller design, location, diameter and tank size. This aspect has been reviewed by Rewatkar and Joshi [9].

The variation of mixing time with impeller speed in the presence of gas (at constant V,) is rather complex (Fig. I) . This variation was found to depend on the dispersion patterns (flooding, cavity formation, complete dispersion of gas, recirculation of gas-liquid mixture) in the vessel which, in turn, depend on the flow generated by the impeller. The different dispersion patterns exert a strong influence on the impeller's pumping capacity and, hence, corresponding changes are observed in the liquid phase mixing time. The variations of this mixing time with impeller speed for different sparger designs and locations are presented in Fig. 1.

1) List of symbols at the end of the paper.

0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 199 I 0930-75 16/91/06 12-0386 $03 S O + ,2510

Chem. Eng. Technol. 14 (1991) 386-393

55

50-

45- - - 4 D -

0:

f - 35- r E

W

u

30- VI a

0 a a 1 5 -

20-

387

1 = 1.5m ,D: C:T /3 , W I D = 0.3 vg = 9.4~16 m l r -

S R l g ISR400-168-3-390 1 S R R ~ I S R R 4 1 0 - 1 B 9 - 2 - ( - 4 5 1 1

SRR2.JSRR1000-168-3 -370 I

101 I I I 1 0 1 2 3 I

IMPELLER SPEED,N I r l r )

Fig. 1. Variation of liquid phase mixing time with impeller speed in mechanically agitated gas-liquid contactors.

3.1 Sparger Below and Close to Impeller

Curve 1 in Fig. 1 corresponds to the variation of mixing time in the case of a ring sparger located 100 mm below the impeller. At very low impeller speeds, the impeller generates negligible liquid flow, compared to that produced by the sparged gas and the mixing process is practically controlled by the gas flow rate (VG). Under these conditions, the mixing time decreases with increasing impeller speed along the curve L , M , . With increasing impeller speed in the range M , N , , the impellers role and its interaction with the gas phase become more and more significant. This causes a substantial deviation of the mixing time behaviour from that of pure liquid phase (curve Lo).

On a further increase of the impeller speed beyond N , , the liquid flow generated by the impeller increases and, at a par- ticular speed (NCD), the downward liquid flow becomes suffi- cient to entrain the gas. In this zone ( N , O , ) , the liquid flow generated by the impeller counteracts that produced by the sparged gas. This reduces the overall liquid circulation velocity in the vessel and, as a result, the mixing time increases continuously. At point 0,, the liquid circulation generated by the impeller is strong enough to disperse the gas down to the bottom of the vessel. The relevant speed is referred to as N, , (Part I). Beyond this point, a sudden reduction of mixing time is observed within the narrow range of impeller speeds ( O , P , ) . This is attributed to the instant elimination of resistance to liquid flow by gas sparging. At point P,, the impeller action is predominant and completely controls the flow pattern. Beyond P,, the pumping capacity continuously increases and, as a result, the mixing time decreases continuously. The mixing times above N c , are

always longer than those in the absence of gas. The difference increases with increasing superficial gas velocity.

3.2 Sparger Below and Distant from Impeller

For a sparger below and distant from the impeller, a typical variation of mixing time with impeller speed (curve 2) resembles case one (curve I ) in Fig. 1. However, the entire curve is displaced to the left of curve I (sparger below and close to impeller).

The mixing times before N,, (L,N,) were found to be the shortest for this sparger. Since the gas is sparged at a considerable distance from the impeller, this allows bulk of the gas to bypass the impeller zone. For this reason, the impellers pumping capacity is slightly reduced. Gas flow also promotes liquid circulation. Therefore, shorter mixing times are observed. The values of NPG at such impeller speeds are also higher for this type of sparger than in case I (sparger near impeller) which supports the above arguments (see Fig. 3 in Part I).

The mixing time starts increasing in the zone of cavity formation (M2N2) . Comparing the values of Ornix at N , and N,, the value at N , is seen to be lower, indicating a smaller cavity. The power numbers at these points (Fig. 3 in Part I) are also in agreement with these observations.

At NcD, the mixing times are more or less the same for both spargers. However, above NCD, the mixing times are shorter for this sparger than in case one (sparger close to impeller). This observation can be explained on the basis of differences in the fractional gas hold-ups. Since, in case one, the gas is sparged near the impeller, the bubbles generated in the vessel are smaller than in case 2, which results in higher fractional gas hold-ups. As a result, the impellers pumping capacity decreases and the mixing time lengthens for case one, in rela- tion to case two.

3.3 Sparger Above Impeller

Curve 3 corresponds to the variation of mixing time for a sparger located above the impeller. This variation is similar to case one. As discussed in Part I, cavity formation is gradual and cavity size appears to be the largest for this sparger location. For this reason, the pumping capacity increases gradually with increasing impeller speed and thus a continuous reduction in the mixing time was observed (L3N3) .

The region N , 0 3 (break-up of cavity) is wider than for the other spargers. A similar observation was made on the basis of power number variation with impeller speed. Before complete dispersion of gas, the range of impeller speeds (over which the dispersion is unstable) is larger at this sparger location. With the gas being easily available to the impeller zone, the value of N,, is also relatively high under such conditions. Beyond the point of complete dispersion, a continuous reduction in the mixing time was observed. However, the actual values of Ornix are higher than those for any other case.

388

I

E a-16-

Chem. Eng. Technol. 14 (1991) 386-393

SYSTEM 2 PURE LlOUlD P H A S E T = 0 . 5 7 r n , D = C = T I J , W \ D = 0 . 3

3.4 Large Ring Sparger (20) Below Impeller

In the case of a large ring sparger, the mixing time decreases continuously along the curve L, Q,. As discussed in Part I, the flow pattern of this sparger is characteristically different from those generated by the other three spargers. These differences in the flow pattern reflect typical variations of the power number (Fig. 3 in Part I) and mixing time (curve 4 in Fig. 1). The regions of cavity formation, growth and break-up are in this case practically nonexistent. Therefore, the mixing time decreases continuously with increasing impeller speed. The mixing times after complete gas dispersion are practically the same as those for sparger 2.

4 Results and Discussion

4.1 Effect of Sparger Type

It was observed that, even in the absence of gas, the sparger type exerts a strong influence on the dimensionless mixing time, (NOmix). The results are shown in Fig. 2 .

The variation of mixing time with impeller speed (in gas-liquid system) was also found to depend on the sparger type. In order to study this effect, different spargers, namely, ring, pipe, conical and concentric ring spargers were used. The typical variations for different sparger types are shown in Figs 3 and 4. The different shapes of N - Omix curves are due to different power consumptions and different impeller pumping capacities. This aspect has already been discussed in Part I .

4.2 Effect of Super-cial Gas Velocity (V,)

The effect of superficial gas velocity on liquid phase mixing time was studied for all sparger and impeller designs. The range of V, was 1.5 to 15 mm/s. At very low impeller speeds, the mixing time decreases with increasing superficial gas velocity. At such impeller speeds, the liquid circulation caused by the impeller action is weak and that generated by gas sparging

I M P E L L E R S P E E D , N I r / s l

Fig. 2. Effect of spargers on mixing time.

SYSTEM : GAS-LIOUID T - 0 . 5 7 m , D : C -713 ,WID : 0.1 SRRl I S R R Z O O - 2 8 - 2 - 1 5 2 1

- 1 8

I I I I 1 I I I I 2 3 L S 6 7 8 9

IMPELLER S P E E D , N I t l r I

Fig. 3. Effect of sparger design on emi, - N relationship (at V, = 9.4 x lo- m/s).

is predominant. As a result, the mixing time in the vessel is determined by the gas flow rate and, therefore, Ornix decreases with increasing V, in this range.

With increasing impeller speed, the impeller flow increasingly predominates. Therefore, the mixing time continuously decreases in this zone (MN). However, the values are higher than those in the absence of gas. The increase in VG in this region increases the cavity size. As a result, the power number decreases with increasing V,. The value of Ornix in this zone increases with increasing V,.

In the range of impeller speeds at cavity break-up (region NO), the mixing time increases continuously and reaches a maximum at NCD. Liquid circulation caused by the sparged gas is counteracted by impeller action, resulting in a decrease of the overall liquid circulation and an increase in the liquid phase tur- bulence. Therefore, the mixing time increases and reaches a maximum at around NCD. The liquid circulation velocity (generated by gas sparging) increases with increasing V, which enhances the resistance to impeller action. As a result, the increase in Omix increases with increasing V,. For lower VG and sparger distant from the impeller (in the 1.5 m vessel), this in-

- E ID - L o -

5

E 35- r! I

30- U I G

5 25- 0 - _I

20 - 1 I I I I 0 1 2 3 4

IMPELLER SPEED , N l r l s l

Fig. 4. Effect of sparger design on Ornix - N relationship.

Chem. Eng. Techno]. 14 (1991) 386-393

10

389

I I 1

I I I

crease in Ornix was insignificant and a continuous decrease in the mixing time was observed.

At impeller speeds beyond N,,, Omix decreases continuously but it still exceeds those for pure liquid phase. The extension of mixing time, as compared to the case of sole liquid phase, increases with increasing V,. This can be explained on the basis of an increase in the gas hold-up with V,. The presence of gas reduces the impeller power consumption and liquid circulation velocity. This causes an increase in the value of Ornix. This extension of mixing time was found to depend on the sparger and impeller designs since the gas hold-up is directly related to these parameters.

4.3 Effect of Sparger Location with Respect to Impeller

Sparger location influences the mechanism of gas dispersion as well as the power consumption and thus the liquid phase mixing time in a gas-liquid system. The effect of location of ring (SR,, SR,,, SR,,) and concentric ring (SRR,, SRR,) spargers on liquid phase mixing time was studied in the 0.57 m and 1.5 m i.d. vessels. The variations of mixing time with impeller speed at constant V, for different sparger designs and locations are shown in Figs 5, 6 and 7. It is seen from Fig. 5 that, for all sparger designs, as the sparger approaches the impeller from the bottom of the vessel, the hydrodynamic sequence of flooding to recirculation is displaced towards higher impeller speeds. This displacement reaches a maximum when the gas is sparged above the impeller.

Comparison of the Ornix values before complete dispersion of the gas (curve LN in Fig. 1) shows that the concentric ring and ring spargers distant from the impeller produce shorter mixing times than at other locations. The increase in Ornix in the vicinity of cavity break-up 'was found to be at maximum for the sparger located above the impeller (SRR,). Similar observations were made in the case of ring sparger (Fig. 6).

Beyond N,,, the mixing time decreases continuously with impeller speed for all sparger locations and the values depend

SYMBOL 1-1 SPARGER 0 SRR, l S R R L 1 0 - ~ 1 8 9 - 2 - 3 9 0 1

0 SRR, I S R R 4 1 0 - 1 8 9 - 2 - 1 - 4 5 1 1

A S R ? ' l S R L 0 0 - 1 8 9 - 2 - 1 0 0 1

A S R I ~ I S R 4 0 0 - 1 8 9 - 2 - 3 9 0 1

151 I I I I I I 0 1 2 3 1 5

IMPELLER SPEED,N l r f r l

Fig. 5. Effect of sparger location on Ornix - N relationship.

1 : 0 . 5 7 ~ . 0 z T I ) . C = T I 3 . W / O : 0.3 SYMBOL Ye x l O ' i m / s I

L. 8

9. L 15

I I I I I 3 4 5 6 - 7 8 9 1 0

I M P E L L E R S P E E D , N i r / s l

Fig. 6. Variation of mixing time with impeller speed for sparger SR,.

strongly on the distance between the sparger and the impeller. The mixing time was longest in the case of sparger (SRR,), compared to spargers below the impeller. Among all the spargers located below the impeller, the mixing time was longest for the ring sparger (SR,,) located 100 mm below the impeller.

Comparing the concentric ring and ring spargers, distant from the impeller, Ornix values of the ring sparger (SR,,) were found to be larger than those of the concentric ring sparger (SRR,). These differences in mixing times are attributable to different fractional gas hold-ups. The maximum increase in Ornix during cavity break-up was observed for a sparger located above the impeller (SR,). The effect of V, in this zone was found to be more pronounced for this sparger.

390

20

4.4 Effects of Ring Sparger Design

T:O.Slm,D- C : T / 3 , W I D : 0.3 -

The effects of ring sparger design (ring diameter, hole area and hole size) on mixing time were studied in detail. The ring diameter was varied from 0.5 to twice the impeller diameter (SR,, SR,,, SR,, SR,,, SR,,) and results, obtained at constant superficial gas velocity (9.4 mm/s) are shown in Figs 8 and 9. These diagrams show that the Ornix - N relationship is similar for all spargers up to the ring diameter of the sparger being equal to the impeller diameter. The increase in Ornix in the vicinity of NcD was largest for the 0.5D sparger and it decreas- ed with increasing ring diameter. The increase is practically eliminated for the 2 0 sparger. This elimination of mixing time extension was observed at all superficial gas velocities (Fig. 10). This is explained by the changes in pumping capacity. For a small size ring sparger, the gas passes directly into the impeller region and develops large gas cavities in the vicinity of NcD which reduces the pumping capacity.

18- - I

2 1 6 -

w r t- 14- " z x - z 12- a w Y)

2 10-

For a large ring sparger (2D), the mechanism of gas dispersion is different. In this case, the regions of cavity formation, growth and break-up are practically non-existent. For this reason, the changes in pumping capacity are gradual. Therefore, the pumping capacity increases continuously with increasing impeller speed and a corresponding decrease is observed in the mixing time. This behaviour was observed at all superficial gas velocities (Fig. 10).

4.4.1 Number of Holes and Hole Size

The effects of the number of holes and hole size were studied at two locations (100 mm and 390 mm from impeller). A ring sparger 0.8 D in diameter was used in the I .5 m i.d. vessel. The results are shown in Figs 11 and 12.

In general, the variation of mixing time with impeller speed was similar for all spargers. Furthermore, the effect of the number of holes and hole size on mixing time was found to be negligible when the sparger was nearer to the impeller (Figs

I S Y M B O L I SPARGER I S R , I S R 9 5 - 6 - 3 - 1 0 0 1 1 l l SR68 I , S R l 5 2 ~ 1 2 - 3 - ~ 0 0 1 ~ , I I

1 2 1 4 5 6 7 0 9

S R , ISR190-6-3-1001

6

I M P E L L E R S P E E D , N ( r / r l

Fig. 8. Effect of ring diameter on Ornix - N relationship.

Chem. Eng. Techno]. 14 (1991) 386-393

S Y S T E M : G A S - L I a U I D 1 : l . S r n , 0 : C z T I 3 , W I O : 0.3

1s vo: 9.4X16'rnfS

z 40-

I w

t- 35-

* z

2 30- - x

w Y) a 2 2 5 - 0

: 20- 0 SR,, [ S A 1000-168 -3-370 I

Fig. 9. Effect of ring diameter on Ornix - N relationship.

l l a and 12a). However, the effects became significant when the sparger was distant from the impeller (Figs 1 Ib and 12b). For a smaller number of holes (SR,,) and a larger hole size (SR,,), the sparger generates larger bubbles and more gas reaches the impeller zone before NCD. For this reason, the impeller pumping capacity is more reduced. Therefore, the mixing times are longer for these spargers before the gas dispersion is complete. (Figs I l b and 12b). The increase of Ornix in the vicinity of NCD was also greater for these spargers. However, very little change in mixing time is observed after complete gas dispersion and the effects of hole size and number become negligible.

4.5 Effects of PTD Impeller Design

The effects of design details of the PTD impeller such as diameter and blade width on the liquid phase mixing time were investigated in the 1.0 and 1.5 m i.d. vessels.

T = 1 6 m , D = C = T I 3 , W / O = O . 3

Z 30- T w

-

= 0

n

5 20-

15 I I I 1 2 3 L

I M P E L L E R S P E E D , N I r f s l I 10

Fig. 10. Variation of mixing time with impeller speed for ring sparger of diameter 2 0 (SR,,).

Chem. Eng. Technol. 14 (1991) 386-393

5 0 -

- 1 5 - - i 1 0 -

m.

; 15- W T

0 z

x - $ 30-

W w a

D : 20-

I 2 5 -

3

2

15

39 1

I I I I 1 2 3 1

I M P E L L E R S P E E D , N l r l r l

~ _ _ _

S R q 2 I S R l 0 0 - O L - 1 - 1 0 0 1

SR,, I S R l 0 0 - 1 6 8 - 3 - 1 0 0 I

50

- - 4 5

W T

E 10-

c 35- m z x -

30-

W VI a

n

0 -8 20-

2 5 -

- 3

15. 0

m z - x 3 5 - z W VI

2 30- a n

2 2 5 - - 3

A

-

SR,, I S R l 0 0 - 0 1 - 3 -3901

I I I I I I 1 2 3 1 5

201 I I I 1 I 1 2 3 1 5

I M P E L L E R S P E E 0 , N I r / s l

Fig. l l a . Effect of number of holes on Ornix - N relationship for sparger close to impeller.

50

- -: 4 5 -

E m W

+ m

5 1 0 -

z 5 3 5 - T

w m

10- a

0 2 2 5 - -8

- r I SR,' I S R L O O - 1 8 9 - 2 - 1 0 0 1 S R l l [ SR 100 - 01-3 - 1 0 0 I SRq8 ISR LOO- 2 1 - 6 - 1 0 0 I u

S Y S T E M : G A S - L I O U I O T : 1 . 5 r n I D r C : T l 3 , W I D r O . 1

0 1 2 3 L I M P E L L E R S P E E D , N i r l r l

Fig. 12a. Effect of hole size on Ornix - N relationship for sparger close to impeller.

4.5.1 Impeller Diameter

The effect of impeller diameter was studied in the 1.5 m i.d. vessel. Impellers 0.33, 0.5 and 0.75 m in diameter, of blade width 0.30, were used for this purpose. The effect of impeller diameter was studied for ring spargers 0 .80 in diameter, at two locations (SR,, SR,, SR,,, SR19, SR,,, SR,,). The variation of mixing time with impeller speed for different impeller diameters, at constant V,, is shown in Fig. 13.

It can be seen from Fig. 13 that the variation of mixing time with impeller speed is similar for all impeller diameters and spargers except SR,, ( 0 = 0.75 m). For sparger SR,, (D = 0.75 m) the mixing time decreases continuously with increasing impeller speed. No increase in Ornix was observed during the process of cavity break-up. This behaviour of mixing time was observed at all superficial gas velocities. The different behaviour of sparger SR,, can be explained on the basis of impeller pumping capacity. As discussed in Part I , the tendency to cavity formation decreases with increasing impeller diameter

0 1 2 3 L 5 6 7 0 IMPELLER SPEED, N I r l s I

Fig. 13. Effect of impeller diameter on Ornix - N relationship.

392 Chem. Eng. Technol. 14 (1991) 386-393

-

E m

20

1 6 -

" 1 6 -

5

5 1 L -

; 12-

- r W

I a

0 5 10- 2

8

and cavities are practically non-existent in the case of 0.75 m diameter impeller. Therefore, the influence of gas on the pumping capacity is smaller in this case. As a result, the extension of mixing time near N,, is smaller for the 0.75 m diameter impeller. Furthermore, the increase is eliminated if the sparger is distant from the impeller.

T : l - O m , 0 : 0.33, c z 1 / 3

-

I I I I I I I 2 3 L 5 6 7 6

I M P E L L E R SPEED, ti ( r / s )

In general, after complete gas dispersion, the values of NOmix were found to decrease with increasing impeller diameter and the following relationships were formulated:

vessel diameter. When all three vessels were of the same diameter, NO,,, was found to vary as T2. This is attributed to the fact that the volumetric flow rate, generated by a given impeller at a given speed, remains constant in all three vessels. However, the average liquid circulation velocity varies as T- because of the increase in the cross-section of flow. The reduction of circulation velocity with increasing Texplains the variation of \\ 1111 \essel diameter. Similar observation was made in the absence of gas [lo].

- For a sparger close to the impeller 5 Correlations

- For a sparger distant from the impeller

4.5.2 Blade Width

The ratio WID was varied between 0.25 and 0.35 in the 1 .O m i.d. vessel. The impeller diameter was kept constant at T13. Pipe sparger (SP,) was used in the investigation of the effect of blade width.

The variation of mixing time with impeller speed at constant superficial gas velocity is shown in Fig. 14. The value of N,, was found to decrease with increasing WID ratio. Furthermore, when gas dispersion was complete, the value of NOmix was found to decrease with increasing WID ratio in the range from 0.25 to 0.35.

It is clear from the foregoing discussion that the dependence of mixing time on N and V, is complex, when a wide range of impeller speeds is considered. However, in practice, the desirable impeller speed is above N,, in order to ensure effective gas- liquid contacting. Therefore, it was considered desirable to develop correlations for mixing time in the range of speeds above NcD.

Dimensionless mixing times (NOmix) were correlated by the following equations.

5.1 For Spargers Close (100 mm) to the Impeller ( I 74 Data Points)

T = 0.57, 1.0, 1.5 m, 0.22 I DIT I 0.5, WID = 0.3, CIT = 0.3, k = 0.0028 m, 1.5 I V, I 15 mmls.

(3) NOmix = 38.58 T'.93 D- ' .* Vk2"

S.D = 6% .

, 4.6 Effect of Vessel Size and Scale-up

In order to investigate the effect of the vessel diameter, experiments were performed in the 0.57, 1.0 and 1.5 m i.d. vessels. At constant DIT (113), CIT (113), and WID (0.3), the dimensionless mixing time was practically independent of the

5.2 For Ring Spargers Distant (390 mm) from the Impeller (130 Data Points)

Fig. 14. Effect of WID ratio on Ornix - N relationship.

T = 1.5 m, 0.22 I DIT 5 0.5, WID = 0.3, CIT = 0.3, k = 0.0028 m, 1.5 5 VG I 15 IS .

NOmix = 37.13 D-2.05 Vk1* , (4)

S.D = 7% .

5.3 For Spargers at 100 mm and 390 mm from the Impeller (304 Data Points)

T = 0.57, 1.0, 1.5 m, 0.22 5 DIT 5 0.5, WID = 0.3, CIT = 0.3, k = 0.0028 m, 1.5 5 V, 5 15 mmls .

Chem. Eng. Technol. I 4 (1991) 386-393 393

NOmi, = 38.58 T2 D-.94 VE226

S.D = 7.5% .

, ( 5 ) - Correlations have been put forward for dimensionless mixing time in the presence of gas.

Acknowledgement

54 For Oncentric Ring purger (sRR4) Above the Impeller We are grateful to the University Grants Commission of the Government of India for the award of a fellowship to V.B.R. The research was supported by a grant under the Indo-US Col- laborative Materials Science Program (CE-1).

T = 1.5 m, DIT = 0.3, WID = 0.3, CIT = 0.3, k = 0.0028 m, 1.5 I V, I 9.4 mmls .

NOmix = 1755 V,$58 , (6)

S.D = 2% . Received: Octuber 12, 1990 [CET 3291

Symbols used 6 Conclusions

- The variation of liquid phase mixing time with impeller speed (Omixvs N ) at a constant VG can be explained on the basis of corresponding visual observations of hydrodyna- mics around the impeller in vessel. Furthermore, the shapes of Ornix - N curves were similar to those of NPG - N curves described in Part I.

- The variation of liquid phase mixing time with impeller speed was found to depend on the superficial gas velocity, sparger design, sparger location and the design of the PTD impeller.

PG T v c

VG W

impeller clearance of tank bottom impeller diameter gravitational acceleration height of liquid in tank blade thickness distance between sparger and impeller impeller rotational speed critical impeller speed for gas dispersion power number in presence of gas PGl~,N3DS power consumption in presence of gas tank diameter liquid circulation velocity in bulk superficial gas velocity blade width

- The value of NcD was found to increase with decreasing distance between sparger and impeller and it reached a maximum when the gas was sparged above the impeller. In addi- tion, for this sparger location, the fractional gas hold-ups were largest and liquid phase mixing times longest, com- Q G [kdm31 gas density

pared to other sparger locations.

Greek symbols

e L [ w m 3 1 liquid density Is1 mixing time mix

- The effect of ring spargers outer diameter was predominant when it exceeded the impeller diameter and the sparger was below and distant from the impeller. The values of NcD were lowest when the ring diameter was double the impeller diameter.

- Hole size and number of holes in the ring sparger exert a negligible effect when the sparger is close to the impeller. However, these parameters become important when the sparger is distant from the impeller.

- With increasing blade width and blade thickness, the value of NO,,, decreases above NCD.

- The Ornix - N curves were found to depend on the absolute distance between sparger and impeller and, at the same distances, the effect of size was negligible.

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