M. Cooke

The Transition from Homogeneous to Heterogeneous Flow in a Gassed, Stirred Vessel

Much ’old’ work has been undertaken studying gas-liquid systems in stirred reactors (Nienow1). However, this ‘old’ work has generally been limited to specific ungassed energy dissipation rates, ( ɛ T ) 0 , up to ∼2 Wkg −1 and gas flow rates of up to ∼2 vvm (or superficial gas velocity, ν s , of −1 ). Recent developments have led to the need to operate modern reactors at much higher values of ɛ T and ν s . This preliminary study reports work under such conditions and also builds on studies at very high gas velocities in bubble columns. Under such conditions, a transition from homogeneous to heterogeneous flow occurs. The results show that under well dispersed conditions, the transition occurs at lower gas velocities for the coalescence-inhibited solutions in comparison to water. Possible reasons for these findings and for the formation of large bubbles at this transition are discussed. A simple model on the basis of a gas volume balance is proposed which matches the trend. The impact of the intense operating conditions on a range of other mixing parameters is also reported.

Mass transfer and hold-up characteristics in a gassed, stirred vessel at intensified operating conditions

The steady state mass transfer coefficient, k L a, and the hold-up, e, have been measured in a vessel 0.29 m in diameter, sparged with air using 6RT (single and dual) or 6SRGT (single) impellers at unaerated energy dissipation rates of up to ∼100 kW/m 3 and superficial gas velocities of up to 0.13 m/s in deionized water (coalescing) and an aqueous solution of 0.2 M -1 sodium sulphate (non-coalescing). Under these intense conditions, the flow structure changed from homogeneous to heterogeneous, the hold-up could be as high as ∼65% and the gassed to ungassed power ratio for the RT and 6SRGT as low as 0.2 and 0.6, respectively, k L a values up to ∼2.0 s were found and the results could be correlated well by k L a=k(P/V L ) a v S β with a single parameter set for each fluid, based on a well-mixed model and a χ 2 -method of data fitting.

Spinning Cones as Pumps, Degassers and Level Controllers in Mechanically Stirred Tanks

The literature on spinning cones as pumps and degassers is reviewed. Experiments on a spinning cone rig designed to measure spinning cone-pumping rates are described. A large number of experiments were carried out, measuring pumping rates as a function of cone angle and immersion depth. Cone half angles were from 15 to 60°. Most of the tests were done with water but a number of runs were carried out with 75% by volume glycerol solution. This changed the viscosity and fluid density. An equation is proposed to predict the volumetric pumping rate (Q) of a cone in terms of its geometry and the physical properties of the fluids. Experiments with gassed fluids indicate that the liquid pumping rate of a spinning cone is independent of any gas present. It was also found that both outside and inside surfaces of the cone contribute equally to the fluid pumping process. The effectiveness of spinning cones as a degasser and/or level controller have been tested in mechanically agitated baffled vessels using cones mounted on the agitator shaft at the liquid surface. Under gassed conditions and with surfactants added to the liquid, the spinning cone was shown to be effective in controlling the level and reducing the gas voidage over a wide range of operating conditions. The effectiveness of the cone as a defoamer appears to scale at equal tip speed, suggesting that shear rate is the dominant mechanism. Tests were done on 2 ft (0.61 m) and 3 ft (0.914 m) diameter stirred tanks.

Investigating Jet Mixing Using Electrical Resistance Tomography

Coaxial jet and side entry mixers are used in a wide range of industries for a variety of processes including precipitation, polymerization and neutralization duties. Jet mixers are characterized by short contact times between the fluids and can be operated continuously or semi‐batch. Coaxial and side entry jets can be designed in order to deliver rapid turbulent mixing using short sections of pipeline. As the energy required for mixing is provided by the addition stream, the process‐side pressure drop required for homogeneity is very low. A key design parameter for jet mixers is the mixing length, the length of pipe downstream of the injection point required to achieve a given degree of homogeneity. The mixing length can be affected by the addition geometry (for example coaxial or side entry), orifice size and shape, operating conditions and material properties. This paper presents the use of Electrical Resistance Tomography (ERT) to monitor jet mixing via the addition of a conductivity tracer through co...