Biomedical Engineering Reference
In-Depth Information
Figure 6.1. Filtrate resistance to flow due to membrane and gel layer resistances.
membrane bringing nonpermeable solutes to the membrane surface where they accu-
mulate. This phenomenon is known as concentration polarization, and it results in the
formation of a thin solute layer at the membrane surface known as the “gel” layer [14].
The gel layer forms below the fluid boundary layer and creates an additional resistance to
flow that together with the membrane resistance influences the mass transfer rate and the
membrane selectivity (Fig. 6.1).
To counteract the adverse effect of concentration polarization, a high-velocity fluid
stream is applied tangentially to the membrane surface. In some cases, screens are
inserted in the membrane channels to promote turbulent flow, which reduces the fluid
boundary layer compared to that of laminar flow. The tangential flow provides turbu-
lence and shear that transport solutes from the membrane surface back to the bulk
fluid stream. However, as the filtration proceeds and the retained solute concentration
increases, the concentration polarization and the gel layer resistance increase.
This results in an increase over time of the mass transfer coefficient, k, which lowers
the filtrate flux rate (Fig. 6.2). This dynamic process means that TFF operations never
operate in steady state, and as a result a precise understanding of the operating parameters
Figure 6.2. Tangential flow between membrane sheets transports solutes from membrane
surface into bulk fluid stream. As solute concentration increase, gel layer resistance increases,
reducing filtrate flux.
 
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