(81b) Drawdown of Liquid-Liquid Systems: Comparing Performance of Conventional Stirred Tank and Confined Impeller Stirred Tank

Conventional stirred tanks (ST) are widely used to produce liquid-liquid dispersions and emulsions. The flow field and energy dissipation rate throughout these tanks are highly inhomogeneous meaning that the rate of change in the drop size distribution (DSD) varies significantly with the location in the tank early in the liquid dispersion process. In the impeller region characterized by high energy dissipation rate, smaller drops are produced. At the same time, even with the Reynolds number of 20 000 the top third of the tank might not be in a fully-developed turbulent regime that leads to low levels of energy dissipate rate and, thus, very little break-up. Having homogeneous flow in the tank is crucial for industrial process, such as emulsion polymerization, where a uniform DSD is required.

A comparison study of liquid drawdown in a conventional stirred tank and a confined impeller stirred tank (CIST) was performed to demonstrate the advantages of using the latter tank for dispersion testing. The tanks are shown in the figure below. Canola oil and tap water are used as working fluids with the oil forming the dispersed phase. The dispersed phase volume fraction varies from φ=10% to 30% by volume. Sets of PBT, Intermig, A310 and Rushton turbines are used. Several parameters are determined for each experiment: the point of entrainment is defined as the impeller rotational speed when the interface between the liquids is disrupted; the fully dispersed speed gives the impeller rotational speed where complete dispersion is achieved; the time required to reach fully dispersed is also recorded; and the total mixing energy required to fully disperse the oil is the product of dissipation and mixing time. It is demonstrated that the CIST requires less energy than the stirred tank to form a complete dispersion of the immiscible liquids. For instance, for φ=10 and the PBT impeller the energy requirement for CIST is 2.5 times less than for the stirred tank.

Figure. Liquid drawdown in (a) conventional stirred tank (T=0.24 m and H=T) (b) CIST (T=0.076 m and H=3T). The diameter of the Rushton turbine is equal to T/2 for both tanks. The dispersed phase volume fraction is 30%.