(177a) Influence of Electrolytes On Liquid-Liquid Separation

In liquid-liquid separation coalescence is one of the governing factors. An exact determination of the coalescence behaviour for any specific system is essential as it determines the apparatus geometry. But even nowadays, a precise prediction of the coalescence behaviour is a challenging task. Many investigations have proven the strong influence of impurities on the separation behaviour of dispersions. These impurities can alter the separation even at extremely low concentrations where accurate detection of relevant compounds is almost impossible. Consequently, coalescence experiments with the original system used in the process have to be performed. Modelling of the apparatus geometry can then be based on the experimental results.

Impurities which often occur in technical processes are e.g. surfactants, solids and electrolytes. A particular challenge when dealing with electrolytes is that the same electrolyte can inhibit as well as promote coalescence in different systems. Additionally, the concentration of the electrolyte determines in many cases how coalescence is influenced. Experimentally it can be found that an electrolyte inhibits coalescence at low concentration while it promotes coalescence at high concentrations. This behaviour can be understood in terms of the DLVO theory. Furthermore, impurities like solid particles cannot only hinder coalescence but suppress it completely. This behaviour has been reported in numerous cases for small solid particles in extraction systems. The appearance of a stable dispersion in liquid-liquid extraction processes, the so-called crud, can in some cases be attributed to particles. When solids are present, electrolytes can additionally promote the formation of crud and lead to a significant increase in crud volume. This leads to severe challenges in the operation of liquid-liquid separators. To be able to account for the undesired influence of impurities, the characterization of the separation behaviour is required. Because even a small amount of electrolytes has an influence on the phase separation, but not on relevant physical parameters like interfacial tension or viscosities of the liquid phases, the settling behaviour has to be characterized by batch-settling experiments. At AVT-TVT a method to design gravity settlers based on these lab-scale settling experiments has been developed. Combining the experimental results with a model for the settling behaviour allows the determination of a single parameter that characterizes the entire liquid-liquid separation.

The influence of electrolytes on the settling behaviour of organic-aqueous systems was investigated using our standardized batch-settling cell. In this presentation the results of these experiments will be discussed in detail and interpreted in terms of the DLVO theory. Furthermore, experimental results concerning interactions with other impurities like surfactants and solids will be shown. In order to systematically account for the aforementioned parameters when designing a liquid-liquid separation process, a heuristic tool which supports the overall process design was developed which will be briefly described as well.