(530d) Turbidity Control in Liquid-Liquid Extraction

Liquid-liquid extraction will play a major role in the biobased industries. Liquid-liquid extraction of carboxylic acid from pulp effluents with trialkylphosphine oxides, e.g. trioctylphosphine oxide dissolved in dodecane, is a state of the art process. Depending on the effluent matrix and the operation temperature the quality of the raffinate phase and the extract phase may suffer from turbidity due to fines, either picked up during extraction or caused by formation due to low interphase tension. Temperature change of the raffinate phase may also lead to inacceptable turbidity because of solvent supersaturation with spontaneous phase separation. While extract phase contamination may limit the product quality, raffinate phase load with stable solvent fines affects wastewater quality and economic feasibility of the process because of solvent loss.

Several technologies have been applied in turbidity control, representatively mist eliminators, installed in the settler, shall be mentioned. Resizing of settling equipment is probably not a good choice because turbidity control oriented design will increase the settling area by orders of magnitude. Chemical turbidity control with flocculants may help, but does need a second settling step and does have a negative impact on solvent quality. Turbidity control of the solvent phase by AC or DC electro coalescence is a highly efficient and appropriate measure in upgrading of the solvent phase. Turbidity control in the aqueous phase excludes direct contact of effluents with either power source. The working electrode has to be separated from the effluent with a dielectric barrier, limiting power source to AC power supply.

In this project turbidity control of aqueous effluents by electro coalescence has been investigated. Experiments were performed in a glass tube coalescer. For process modelling dodecane spiked aqueous feed was prepared in a high shear mixer. After phase separation the “turbidity laden” aqueous phase was poured into the coalescer and coalescence was started. The same procedure was applied for turbidity control experiments of the water spiked solvent phase. The current-voltage characteristic of the setup was frequently controlled to avoid data misinterpretation because of changing chemical composition of the test system.