(530c) Investigation and Modeling of Polydisperse Drop Sedimentation in Liquid-Liquid Phase Separation

In the chemical industry, liquid-liquid extraction is a common separation technology. It is preferably applied when separation via distillation is inefficient or even impossible. Isolation of many bio-based chemicals, characterized by low volatility due to high oxygen content, may demand liquid-liquid extraction. The performance of extraction processes may suffer from poor phase separation properties.

In this work the phase separation has been studied in order to improve the design of settlers. Established design principles are based on separation experiments, carried out in laboratory scale settling cells. These experiments require optical monitoring of the separation process. Unlike academic test systems, industrial systems are often opaque, making optical monitoring impossible. Therefore, a new technique was used. With an ultrasonic scanning system the phase separation rate of opaque dispersions can be quantified. The speed of sound depends on the composition of the dispersion.

After phase separation, residual turbidity may limit the quality of the settler outflow. This is in particular true for high-viscosity and low interphase tension systems. As small droplets of turbid dispersions take much longer time to sediment and to coalesce they are a problem in industrial processes when they accumulate. With the ultrasonic scanning technique it is also possible to quantify residual turbidity of either phase and the rate of secondary sedimentation.

Current sedimentation models in the settler design are based on the assumption of monodisperse droplets. Application does poorly reflect hindered settling. Improvement is achieved by implementing a population balance. For a given drop size distribution the improved model can determine the change of the hold-up with height and time with high accuracy, since it takes into account the sedimentation rate based on each drop size, the hindered sedimentation through swarm effects and the countercurrent flow of the continuous phase. By comparing the error between model and experiment, the drop size distribution parameters can be fitted. The proposed scanning technique in combination with improved data interpretation and modeling will help to improve the design of settlers.