EFFECTS OF BAFFLE DESIGN ON THE LIQUID-MIXING IN AN AERATED STIRRED-TANK WITH STANDARD RUSHTON TURBINE IMPELLERS

The effects of width and number of baffles in mechanically agitated ve ssels with standard Rushton turbine impellers are examined for systems with and without aeration. The insertion of the appropriate number of baffles clearly improves the extent of liquid mixing. However, excess ive baffling (i.e. n(b) >8 or B/T >0.2) and sparging gas through the i mpeller would interrupt liquid mixing and lengthen the mixing time. Th is study found that sparging gas through the impeller leads to an incr ease in the mixing time of more than 20% because it reduces the liquid pumping capacity of the impeller. A numerical technique was applied t o examine the same effect on several extreme baffle conditions and the cases of higher gas flow rates and rotational speeds. To generalize o ur results obtained, the numerical technique was applied to simulate f or a system with triple impellers. The trends of the mixing time were found to be very similar to the single impeller system. By correlating the mixing time with n(b), B/T, Q(g) and N, the following correlation is obtained for the system with single Rushton turbine impeller under non-gassed and aerated systems. Nt(M) = 55.7(n(b))(-0.30)(B/T)(-0.153 5)(Q(g)/ND3)(0.0296) and the similar correlation for the triple impell er system can be given as Nt(M) = 46.5(n(b))(-0.295)(B/T)(-0.327)(Q(g) /ND3)(0.010). (C) 1997 Elsevier Science Ltd.