Investigations on the behaviour of ceramic micro- and mesoporous membranes at hydrothermal conditions

• Successful design of a membrane apparatus dealing with hydrothermal conditions.
• Investigations of tubular oxidic ceramic membranes up to 160 °C and 375 bar.
• Determination of pure water permeance in parameter studies (p, T, TMP).
• Filtration tests with two organic model substances to determine retention behaviour.
• Approach to increase hydrothermal membrane stability by doping of separation layer.

Hydrothermal processes can be used for extraction, reaction or analytical purposes. In combination with membrane technologies, energy consumption of such processes can be reduced considerably in terms of a nearly isobaric and isothermal regeneration of the hot and pressurized fluid. However, investigations on the performance of this hybrid process concept require experimental data on the use of membrane processes under hydrothermal conditions.Consequently, the main objective of this work is the determination of characteristics such as permeation and filtration behaviour for several ceramic, asymmetric micro- and mesoporous membranes (TiO2/ZrO2) with average pore diameters from 0.9 to 3.0 nm at hydrothermal conditions. Performed studies are comprised of experimental investigation of pure water permeance and organic model substance retention in dependence on feed pressure (200–375 bar), feed temperature (40–140 °C) and transmembrane pressure (5–25 bar). All investigations were carried out in a newly developed stainless steel membrane separation apparatus.Due to seriously corrosive hydrothermal conditions, a degradation of the glass coated membrane end seals and the membrane separation layer at temperatures above 140 °C has been observed. Subsequently, an inorganic and more stable end seal has been designed. Moreover, this work gives a first promising approach for stabilizing the membrane separation layer by Y2O3 modification. As a result, pure water permeance has been found to be independent of feed and transmembrane pressure, but showed an almost linear dependence on feed temperature.In a following step, the filtration characteristic towards organic model substances such as Inulin and Neohesperidin dihydrochalcone (NHDC) has been studied. Experimental results showed retention up to 88% for Inulin and 75% for NHDC.

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