dissip ation rate
The effect of total energy dissipation rate (Pv t) on the hydrodynamic and oxygen transfer perform ance between the conventionally aerated and combined aerated and propeller operated reactor was studied for two gas velocities (figure 3.33). The total energy dissipation rate (section 2.8) consisted of the energy dissipation rate from aeration and propeller operation at the corresponding speed. Hence, the first data point for all curves in figure 3.33 referred to the energy dissipation rate from conventional aeration
(Pv a) and subsequent data were for total energy dissipation rate from combined aeration and propeller operation (Pv t) at increasing propeller speed.
For both gas velocities a small increase in P v t (corresponding to propeller speed increases to 600 rpm) produced a more pronounced influence on the overall and riser gas holdup (figure 3.33a). For the low gas velocity (0.036 ms'O the highest Pv t 590 Wm'^
0.16 0.14 0.14 0.12 ■ 0.10 0.10 0.08 0.08 -3 0.06 0.06 0.04 u O 0.02 0.02 0.00 0.00 y S o U 0.14 0.12 0.10 ▲ 0.08 CL - 0.06 2 00 I 0.04 0.02 0.00 0 200 400 600 800 1000 1200
Total energy dissipation rate ( W m'^ )
Figure 3.33 The effect of total energy dissipation rate on the hydrodynamic and oxygen transfer
performance of the combined aerated and propeller operated airlift reactor (H ^j of 2.77 m, 10 gL'^ dry cell weight baker’s yeast suspension) with superficial gas velocities(ms'^) 0.036 ( — ), 0.136( — )
corresponding to aeration and propeller operation at 1000 rpm produced an increase of the overall gas holdup by 57% when compared to conventional aeration at the Pvt of 198 Wm-3. This com pared to a 30% increase in riser gas holdup over the same energy dissipation rate increase. The increase of gas holdup from F y j increases with the gas velocity o f 0.136 ms'* (7 1 0 - I I 20 Wm*^) were not as significant as those observed at the gas velocity of 0.036 ms‘*.
An increase of Pvt from 198 - 290 Wm*^ produced a rapid reduction of the liquid circulation time from 28 - 17.4 seconds with the low gas velocity (figure 3.33b). For energy dissipation rates above 290 Wm ^ the circulation time decreased at a slower rate reaching a value of 14 seconds at the Pvt of 590 Wm-3. At the gas velocity of 0.136 ms- \ the Pvt increase from 7 1 0 -1 1 2 0 Wm’^ only produced a small reduction in circulation time from 13 to 11.9 seconds.
For the gas velocity of 0.036 ms'^ the mixing time decreased from 58 to 53 seconds with the PvT increase from 198 to 590 Wm*3 (figure 3.33b). However, at the higher gas velocity o f 0.136 ms'^ increases of Pvt from 710 to 810 Wm*^ produced no beneficial improvement o f the mixing performance of the vessel. This demonstrated that the Pvt from aeration dominated the mixing performance of the airlift reactor as shown in section 3.3.2.
For the k^a (low er riser) Pv t from 198 to 590 W m'^ produced a three fold increase in k^a from 0.008 to 0.03 S'^ with the low gas velocity. W hereas, a two fold increase in k^a from 0.055 to 0.11 s'^ occurred with the Pv tincrease from 710 to 1120 Wm*3 with the high gas velocity (figure 3.33c). Also, the rate of increase of k^a with increasing energy dissipation rates with the 0.136 ms*^ gas velocity was greater than that observed with the low gas velocity of 0.036 ms’f
As observed for gas holdup the OUR increased at the fastest rate with increases of energy dissipation rate corresponding to propeller speeds up to 600 rpm for both gas velocities (figure 3.33c). For the low gas velocity (0.036 m s ‘ 0 Pv trange from 198 to 590 W m'^ produced a 300% increase in the OUR compared to the high gas velocity where the power input range of 710 to 1120 Wm'^ produced a 24% increase in OUR.
Finally it can be observed from figure 3.33 that with the low gas velocity and propeller speed at 1000 rpm the P v t was 590 Wm*^ which was similar to the P v t of the high gas velocity with no propeller operation of 710 Wm"^. Hence, it was interesting to note that these similar energy dissipation rates resulted in similar values of gas holdup and liquid circulation time although the engineering environment and the operating conditions were quite different. However, these similar values o f Pv t did not produce sim ilar mixing performance. The mixing time with the 0.036 ms"^ gas velocity and 1000 rpm propeller operation was 52 seconds compared to 30 seconds for the 0.136 ms'^ gas velocity. This provided further evidence that the mixing performance o f the vessel was dominated by aeration as shown in section 3.3.2.
3 .3 .8 D escrip tio n o f the flow regim e w ith p ro p eller and aera tio n o p era tio n
W ater was used to visualise the effect of the propeller on the flow regime of aerated reactor using the individual side vessel sight glasses as described in section 3.1.5. As the marine propeller was used in conjunction with the annulus sparger then the side vessel sight glasses showed the bubble regime in the riser.
The propeller only operation (no aeration) with water was found to produce a vortex down from the surface of the liquid, as viewed from the top plate sight glass. This was due to the action of the propeller drawing the liquid down the draft tube. As the propeller speed was increased the vortex increased in length down the centre o f the top section. At 1000 rpm the vortex reached a depth of 0.4 m from the liquid surface.
At low gas velocities (0.018 to 0.036 ms**) and propeller speeds below 600 rpm the bubbles passed the lower and middle sight glasses at a greater speed than with normal aeration. The bubbles moved in a direct upward movement compared to the more swirl like movement with aeration only operation as described in section 3.1.5. The bubble sizes observed in the lower and middle sight glasses were similar to those observed with normal aeration. Hence, a large population of spherical 2 mm diameter bubbles were observed am ongst some larger 5 mm slug shaped bubbles. For this aeration and propeller operation no vortex occurred in the liquid surface as observed from the top plate sight glass. However, the liquid surface with propeller operation at 300 rpm was considerably more turbulent than that observed with aeration only operation. The turbulent liquid surface with the gas velocity of 0.036 ms'^ and propeller speed at 600 rpm, was similar to the liquid turbulence observed with aeration only operation with a gas velocity above 0.072 m s'k At propeller speeds above 600 rpm the bubbles moved past the sight glass at high speeds and the bubble population changed to a more varied population o f sizes. Middle sized bubbles o f 5 - 7 mm in diameter were most abundant but some large 7 - 9 mm diameter slug shaped bubbles and 0.5 - 2 mm diameter bubbles were observed passing the lower and middle sight glasses. These bubbles passed the lower sight glass in a swirling upward motion. The direction of the swirling was in the direction o f propeller rotation from the lower downcomer. The swirling motion was less distinct at the middle sight glass where bubbles rose in a more upward direction. At the higher speeds (900 rpm) the liquid surface was more turbulent than at propeller speeds below 600 rpm.
At high gas velocities between 0.091- 0.136 ms'^ and propeller speeds below 600 rpm a large number o f bubbles observed at the lower sight glass were between 5 - 8 mm in diameter, with some smaller 0.5 - 2 mm bubbles. This was similar to normal aeration operation. A small population o f large slug shaped bubbles of 8 - 12 mm in diam eter were also observed. However, these bubbles were smaller than the slug shaped bubbles o f 10 - 20 mm in diameter observed during normal aeration. As the propeller speed was increased above 600 rpm the large slug shaped bubbles seemed to decrease in size and the
bubbles moved with the side ways swirly upward motion from the direction o f propeller rotation. However, the bubbles travelled passed the sight glass at high speed which made it difficult to clarify the actual bubble size changes. This also made it impossible to determine whether more gas was present in the riser, due to the increase in gas holdup observed above 600 rpm. The liquid surface with the gas velocity of 0.136 ms-' and propeller speed at 1000 rpm was more turbulent than with conventional aeration only operation.
3.3.9 The effect o f top section size on the com bined aerated and propeller