Operation conditions affecting scale formation in membrane - distillation An in situ scale study based on optical coherence tomography

The formation of an inorganic deposit layer (scaling) is one of the most critical challenges affecting the long-term stability of membrane distillation processes applied in desalination applications. The consequences are a reduced permeate flux and a lowered permeate quality e.g., throughout wetting. Conventionally, scale formation is assessed based on the detected flux reduction without online proof. Optical coherence tomography (OCT) allows for a non-invasive, in situ visualization and quantification of scale layers in a fully operated MD system. Therefore, OCT was used in this work for the in situ scale study. Scale parameters were derived as membrane-based process key parameters, facilitating the quantification and objective morphological assessment of the scale layer. Variation of the temperature (76 degrees C and 60 degrees C) showed a significant influence on the formation of calcite scale layers. A high temperature on the feed side lowered the ion solubility, which caused the massive and fast formation of big crystals on the membrane surface. However, the variation of Air Gap and Direct Contact configuration showed no difference in the formation of scaling. The application of different water matrixes caused the formation of different chemical and morphological scale layers. Both revealed the same flux reduction although the degree of coverage and the morphology of the crystals was different. The results emphasize the insufficient information supply based on single flux detection. It only allows a limited interpretation of scale formation, whereas the calculation of scale parameters determined the differences clearly and enables the calculation of scale formation rate and crystal morphology. Additionally, wetting phenomena were noninvasively visualized by means of OCT.

Automated summary

The use of optical coherence tomography (OCT) for in situ scale study allowed for visualization and quantification of scale layers in a membrane distillation system. Temperature variation had a significant influence on the formation of calcite scale layers, while different water matrices resulted in the formation of different chemical and morphological scale layers. However, the configuration of air gap and direct contact showed no difference in scaling formation.