In‐situcharacterization of deposits in ceramic hollow fiber membranes by compressed sensing RARE‐MRI

Abstract

Ultrafiltration with ceramic hollow fiber membranes was investigated by compressed sensing rapid acquisition relaxation enhancement (CS-RARE) magnetic resonance imaging (MRI) to characterize filtration mechanisms. Sodium alginate was used as a model substance for extracellular polymeric substances. Dependent on the concentration of divalent ions like Ca 21 in an aqueous alginate solution, the characteristics of the filtration change from concentration polarization to a gel layer. The fouling inside the membrane lumen could be measured by MRI with a CS-RARE pulse sequence. Contrast agents have been used to get an appropriate contrast between deposit and feed. The lumen was analyzed quantitatively by exploring the membrane's radial symmetry, and the resulting intensity could be modeled. Thus, different fouling mechanisms could be distinguished. CS-RARE-MRI was proven to be an appropriate in situ tool to quantitatively characterize the deposit formation during in-out filtration processes. The results were underlined by flux interruption experiments and length dependent studies, which make it possible to differentiate between gel layer or cake filtration and concentration polarization filtration processes.

In situ measurement of deposit layer formation during skim milk filtration by MRI

“…It describes the convective transport of the retained species to the membrane surface and back‐diffusive transport in the opposite direction. Therefore, an accumulation of the retained substances results in a mono‐exponential behavior …”

“…During dead‐end filtrations at higher pressures, a deposit layer develops, which is not reversed by back‐diffusion (Figure ). As a result, the filtration mechanism cannot be consistently described by a loose concentration polarization that can easily be reversed by back diffusion, when pressure is released, as has been shown in alginate filtration experiments in the work of Schuhmann et al To further detach the remaining fouling layer and fully recover the filtration performance of the hollow fiber membrane, additional cleaning of the membrane layer must be performed. For example, back‐washing or even chemical cleaning of the hollow fiber are needed to detach the adhesive or agglomerated part of the fouling layer …”

“…The experiments were performed in dead‐end filtration mode. A complete description of the filtration setup and its configuration options can be found in previous studies …”

“…In order to obtain detailed insight into the phenomena and mechanisms of deposit layer in an opaque hollow fiber setup during the fractionation of micellar caseins, time‐resolved and spatially resolved measurements are essential. MRI was used for an in situ analysis of many membrane processes . MRI allows not only a measurement in an optically transparent setup but is also a non‐invasive analysis method in opaque ceramic hollow fibers that are used in this publication …”

“…MRI was used for an in situ analysis of many membrane processes. [17,[26][27][28][29] MRI allows not only a measurement in an optically transparent setup but is also a non-invasive analysis method in opaque ceramic hollow fibers that are used in this publication. [17,[30][31][32][33][34] 2 | EXPERIMENTAL…”