Ind. Eng. Chem. Res. 1995,34, 2499-2514
2499
Gas-Inducing-TypeMechanically Agitated Contactors: Hydrodynamic Characteristics of Multiple Impellers Kaliannagounder Saravanan and Jyeshthar4 B. Joshi* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
The rate of gas induction was measured in gas-inducing-type mechanically agitated contactors (GIMAC)provided with two impellers. Three vessels of 0.57, 1.0,and 1.5m i.d. were used. Tap water was used as the liquid phase, and air was used as the gas phase. Six different impeller designs (pitched-blade downflow and upflow turbines straight-blade turbine, disk turbine, and upflow and downflow propellers) were employed. From these designs, six different impeller combinations were made and a n optimum combination has been proposed. The impeller speed was varied in the range of 0.30 to 15.45 reds. The ratio of impeller diameter to tank diameter (DIT) and the submergence (S) of upper impeller from the top were varied. The effects of clearance of lower impeller from the tank bottom ((21) and the impeller spacing (C3,distance between the two impellers) were also varied over a wide range. The design of the lower impeller was optimized in terms of diameter (D), blade width (W), blade angle (&), number of blades (nb), and the blade thickness ( t b ) . Rational correlations have been developed for the critical impeller speed for gas induction and the rate of gas induction.
1.0. Introduction In many gas-liquid reacting systems, the recirculation of gas from the head space back into liquid is desired when the conversion per pass is low. This can be achieved by using different reactor designs such as sparged loop reactors (SLR), surface aerators (SA), and gas-inducing-typemechanically agitated contactors (GIMAC). The comparison of GIMAC with other reactors has been reported by Saravanan et aZ. (1994). These reactors are useful for unit processes such as alkylation, ozonolysis, and hydrochlorination where it is desirable to have complete utilization of the solute gas. It is particularly useful for situations where the gas is available at relatively low pressure and the gas is introduced in the contactor by generating a low-pressure region in the vicinity of impeller. Many designs of gasinducing impellers have been reported in the published literature (Zlokamik and Judat, 1967;Joshi and Sharma, 1977;Zundelevich, 1979;Joshi, 1980;Raidoo et d., 1987,Mundale and Joshi, 1995). These designs include the pipe impeller consisting of a hollow shaft and hollow impeller, the flattened cylindrical impeller, the turboaerator, the shrouded-disWcurved-bladeturbine, and the pitched-blade downflow turbine. Mundale and Joshi (1995)have examined the different impeller designs and have shown the pitched-blade downflow turbine (PBTD) t o be the most energy efficient. However, these reactors having only one impeller need further improvements. The rate of gas induction decreases with an increase in the impeller submergence from the top. Therefore, the gas-inducing impeller needs to be as close to the surface as possible. On the other hand, the dispersion ability of the impeller decreases as the impeller is brought closer t o the liquid surface. In addition, it is also known that the suspension ability of the gas-inducing impeller is very poor. These limitations of the single-impeller system have impeded the commercializationof the gas-inducing-type mechanically agitated reactor.
* Author to whom correspondence should be addressed. Q888-5885/95/2634-2499$09.QQIQ
The limitation of the single-impeller system can be overcome by using a double-impeller system. The top impeller acts as a gas-inducing impeller, and the second impeller distributes the gas bubbles throughout the vessel. Thus the functions of gas introduction and gas dispersion are assigned t o two different impellers. There is yet another advantage of the second impeller. The gas-induction ability of the gas-inducing impeller depends upon the supply of liquid to that impeller. If the second impeller is designed in such a way that it pumps liquid to the gas-inducing impeller, then the ability of the inducing impeller enhances. Further, the second impeller is closer to the bottom and it is more effective for the purpose of solid suspension. Thus, the induction, the dispersion, and the suspension ability of the two-impeller system is far superior to those of the single-impeller system. Though the gas-inducing impellers have been investigated for the past 30 years, there is practically no information in the published literature on the multiple impeller system. Therefore, it was thought desirable to undertake a systematic investigation t o optimize the impeller combination. The scale-up of the multiple impeller system was also studied. 2.0. Experimental Section
Investigations were carried out in 0.57, 1.0, and 1.5 m i.d. gas-inducing type of mechanically agitated reactors (GIMAC). The schematic diagrams of the GIMAC and the experimental setup are shown in Figure 1. The design details of vessels are given in Table 1. The prime mover was a dc motor, driven by a thyristor-controlled power source. The speed of rotation of the shaft was controlled by regulating the voltage applied to the armature of the motor by a ten-turn potentiometer. Fixed pulleys and twin C section were used t o connect the agitator shaft to the motor. The entire device was able to control the speed and maintain it within f0.1% accuracy. The top impeller together with stator formed the gasinducing assembly. Raidoo e t al. (1987)and Mundale
0 1995 American Chemical Society
2500 Ind. Eng. Chem. Res., Vol. 34,No. 7, 1995 THYRISTOR
PULSATION DAM PE NER
FLOW METER
LEVEL INDICATOR
Figure 1. Schematic diagram of the experimental setup. I
I
thick 15" vo n es 12 no
j
30. 9+ m
W I
DI r
Dit
I
(SBT) were employed. The impeller speed was varied in the range of 0.30-15.45 revls. The ratio of impeller diameter to vessel diameter (DIT)was varied from 0.25 to 0.33 in the 0.57 m i.d. vessel, 0.19 to 0.33 in the 1.0 m i.d. vessel, and 0.13 t o 0.33 in the 1.5 m i.d. vessel. The ratio of blade width to impeller diameter (WID)was varied in the range of 0.20-0.40. Blade angle (B&was varied from 30" to go", keeping the blade width constant. The lower impeller clearance from the tank bottom (C1)was also varied over a wide range (C1= TI2 to T110) in 1.0 m i.d. vessel, T11.5 t o TI6 in 0.57 m i.d. vessel and TI3 t o TI8 in 1.5 m i.d. vessel. The effect of impeller blade thickness (tb) and number of blades (nb)for the lower impeller was also studied. The effect of top impeller submergence (8)was studied in the range of 0.15-0.70 m. All these details are summarized in Table 1 and Table 2. Saravanan et al. (1994) have given additional details of the experimental procedure. 3.0. Results and Discussion
4 >
Figure 2. Various components of the stator rotor system for gasinducing impeller.
and Joshi (1995)have investigated comprehensively the effect of impeller design on gas induction. They have shown that the pitched-blade downflow turbine (PBTD) is the most energy efficient impeller for gas induction. Therefore, in the present work, the top impeller was PBTD in all the experiments. For the bottom impeller, six different impeller designs, namely, pitched-blade downflow turbine (PBTD),pitched-blade upflow turbine (PBTU), disk turbine (DT), propeller upflow (PU), propeller downflow (PD), and straight-blade turbine
3.1. Comparison of Impeller Combinations. In the present investigation, a double-impeller system has been used where the top impeller acted as the gasinducing impeller as shown in Figure 1. Raidoo et al. (1987) and Mundale and Joshi (1995) have optimized the design of gas-inducing impeller, and they have recommended the use of a pitched-blade downflow turbine for the purpose of gas induction. In the present investigation, the performance of the bottom impeller on the performance of gas induction has been investigated. For this purpose, six different impeller designs, namely, pitched-bladed o d o w turbine (PBTD),pitchedblade upflow turbine (PBTU),disk turbine (DT), straightblade turbine (SBT), upflow propeller (PU), and downflow propeller (PD)were employed. Using these designs, the following six combinations were tested in the 1.0 m
Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995 2601
C1-c Distance between upper impeller(centre) to the bottom
CZ-Lower
impeller clearance from bottom
C p Distance between two impellers
D +Diameter
of impeller
T +Diameter
of Tank
Ds-LDiameter of stater
I
hs+Heiqht
of stater
H +Height
of liquid level
S +Submergence
of upper impeller
c
Figure 3. Various components of GIMAC for multiple-impeller system. Table 1. Design Details of Gas-Inducing Mechanically Agitated Contactor diameter of vessel 0.57m, 1.0m, 1.5m impeller diameter 0.19m, 0.225m, 0.33m, 0.38m, 0.42m, 0.50m 10% of vessel diameter bame width number of baffles 4 construction material transparent acrylic for vessel geometry cylindrical with flat bottom lower impeller clearance TI10, Tl8,Tl6,Tl4,Tl3,T12, from the bottom T11.5 submergence of upper impeller 0.15m, 0.30m, 0.40 m, 0.50m, from the top liquid surface 0.60m, 0.70 m, 0.80m, 1.0m 0.175m, 0.25m, 0.35m, 0.45m impeller spacing
i.d. vessel: (1)PBTD-PBTD(45); (2) PBTD-PBTU(45); (3) PBTD-DT (4) PBTD-PD (5) PBTD-PU (6) PBTDSBT. In all these cases, the DIT ratio was 0.225, the clearance from the bottom for the lower impeller was T/3, the interimpeller clearance was 0.550, and the impeller submergence (of the upper impeller) was 0.35 m. The overall clear liquid height was equal to the vessel diameter. Rate of gas induction (QG) and power consumption (PI were measured for all these combinations. Figure 5 shows the rate of gas induction against the power consumption for the six combinations. The effect of impeller design can be explained on the basis of the mechanism of gas induction. Saravanan et al. (1994) have proposed that a vortex in the impeller zone plays a key role in the gas-induction process, and they have explained the sequence of events leading t o the gas induction. These are briefly summarized here: 1. When the impeller is static, the liquid level within the standpipe corresponds to the liquid level in the vessel. As the impeller imparts tangential velocity to
Table 2. List of Impellers Studied in This Work no. of blade proj blade blade impeller impeller blades width width angle diam tswe (nb) (W)(m) (Wd(m) ( B d ( d e d (D)(m) PBTD 6 0.057 0.048 45 0.190 45 0.225 6 0.068 0.056 0.099 45 0.330 6 0.080 0.120 45 0.400 6 0.100 0.150 45 0.500 6 0.125 0.068 30 0.225 6 0.056 0.068 60 0.225 6 0.056 0.057 45 0.190 0.048 PBTU 6 0.068 45 0.225 0.056 6 0.099 45 0.330 0.080 6 0.068 30 0.225 0.056 6 0.068 60 0.225 0.056 6 0.068 90 0.225 0.056 6 45 0.120 0.400 0.100 6 0.150 45 0.500 0.125 6 0.045 45 0.225 0.056 6 0.068 45 0.225 0.056 6 0.090 45 0.225 0.056 6 0.225 0.056a 0.045 DT 6 0.190 PU 3 0.225 3 0.330 3 0.380 3 0.420 3 0.500 3 0.225 PD 3 a
Disk diameter.
the fluid, the level begins t o fall and simultaneously a forced vortex takes a shape. A paraboloid of free surface appears (Figure 4A). The vortex approaches the impeller as the speed is increased. When the vertex of this paraboloid meets the impeller, gas induction can begin. Meeting of the vortex with the impeller is a necessary condition for the critical speed of gas induction which
2502 Ind. Eng. Chem. Res., Vol. 34,No. 7, 1995
IMPELLER SPEED N
