Membrane separation processes

Membrane separation processes, in-situ approach by SAXS and micro PIV: concentration-velocity gradient-stress relationships, within the concentration polarization layer.

Tangential flow ultrafiltration of colloidal suspensions, although used in many industrial fields, is limited by the accumulation of particles on the membrane surface. This leads to two phenomena: concentration polarization and membrane fouling, which reduce process performance.

The objectives of this work are to understand the mechanisms governing concentration polarization phenomena in relation to the flow and structural properties of colloid suspensions in the concentrated layers near the membrane surface during tangential flow ultrafiltration. Various types of colloids of fundamental interest or with industrial applications are studied: clays, cellulose nanocrystals, carbon nanotubes, casein micelles, and jackfruit juice. 

A breakthrough in understanding the mechanisms involved in the formation of this accumulation of matter was achieved through the implementation of novel methodologies using instrumented filtration cells combined with small-angle X-ray and neutron scattering techniques (Figs. 1, 2). These novel filtration cells and the measurement techniques developed around them made it possible to obtain information at the nanometer scale within concentration polarization layers, at thicknesses ranging from 20 µm to several hundred micrometers, during cross-flow filtration under controlled pressure and flow conditions. 

These measurements allow the establishment of concentration profiles as a function of distance from the membrane surface and their temporal evolution. Furthermore, they also provide access to the spatial organization of the filtered colloids and the levels of inter-particle interactions reached in the concentrated layers. 

More recent developments have enabled the characterization of the velocity field as a function of the distance z from the membrane surface through in-situ local micro-PIV measurements (Fig. 3). By calculating the velocity gradient and understanding the rheological behavior of the filtered medium (stress-shear gradient-concentration relationship), it has been possible to deduce the local stresses within the polarization layer. 

These structural and velocity field data have made it possible to link concentration polarization phenomena to the onset of the sol-gel transition or changes in the rheological behavior of the systems during filtration. An example using a cellulose nanocrystal suspension system is discussed below.

Structure, rheological behavior, and flow fields of cellulose nanocrystal suspensions during cross-flow ultrafiltration process.

A result obtained on aqueous suspensions of unsonicated cellulose nanocrystals (parallelepiped-shaped rods 150–200 nm long and 4 x 60 nm in cross-section) is detailed here. SAXS measurements allowed for the local characterization of the particle concentration profile as a function of the distance z from the membrane surface, as well as the structural organization of the concentrated colloids, particularly their orientation (Fig. 4a).

Local micro-PIV measurements allowed for a detailed characterization of the velocity field as a function of the distance z from the membrane surface (Fig. 4b). In particular, a flow arrested zone was identified, along with a transition zone where the velocity gradually increases with distance from the membrane surface, corresponding to the concentration polarization layer.

The rheological behavior of the suspensions was characterized, and constitutive laws relating stresses to velocity gradients as a function of concentration were modeled using a power-law equation (Fig. 5).

A Matlab code was developed to derive velocity gradients from the micro-PIV velocity measurements V(z). By combining concentration data as a function of distance to the membrane surface C(z) with the rheological constitutive laws of the suspensions (shear stresses (velocity gradient)) for different concentrations, we were able to deduce, for the first time, the evolution of local stresses within the concentration polarization layers, and thus explain the phenomena governing the transition from the concentration polarization layer to deposit formation for these suspensions.

Fig. 6 highlights three distinct flow zones: a zone A corresponding to the formation of a concentrated deposit without flow (fooling layer).

In zone B, which corresponds to the concentration polarization layer, the shear gradient increases until it reaches a maximum. Over this distance, the velocity gradually increases from a near-zero value to a maximum. The velocity gradient increases steadily to values on the order of 40 s⁻¹, thus indicating the flow of the suspension over this zone B. The stress also reaches its maximum value of 0.07 Pa, while the suspension concentration returns to its initial value (0.7 wt%).

In a third zone C, a decrease in both the stress and the velocity gradient is then observed. In this zone, the suspension is circulated in the feed channel without being influenced by the accumulated particles, with a decreasing velocity gradient until it reaches the maximum velocity value between 2000 and 2500 µm above the membrane surface.

Figure 7 shows the relationship between concentration profiles, structural organization, and the velocity and stress levels reached within Zones A, B, and C during the accumulation of CNC particles near the membrane surface.

The results obtained on these cellulose nanocrystal suspensions revealed that the change in rheological behavior from Newtonian to shear-thinning (over dilute concentration ranges of 0.7 to 5 wt%) is responsible for particle arrest at the transition between the polarization layer (Zone B) and the formation of concentrated deposits (Zone A).

The innovative calculation of stresses within the polarization layer, derived from concentration and velocity field measurements, along with the spatio-temporal evolution of these stresses as a function of filtration conditions (flow rate and pressure), provides a better understanding of these concentration polarization phenomena and their progression toward fooling.

On an industrial level, it should make it possible to improve methods to reduce fooling or decrease of filtration performance over time, in particular by applying localized shear or stress through ultrasonic waves.

Fig. 4: a) concentration profiles of cellulose nanocrystal particles as a function of distance to the membrane surface, and associated in-situ SAXS scattering spectra; b) velocity fields corresponding to the vicinity of the membrane surface by in-situ PIV measurements. CCNC = 0.7 wt%, Q = 0.06 L.min-1, T = 25°C, TMP  = 1.1 x 105 Pa. 

Fig. 5: Flow curve and consistency parameter K and shear-thinning parameter n of the power-law rheological model. Identification of the transition from Newtonian to shear-thinning behavior in the vicinity of 5 wt%.

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