(398z) Modelling Direct-Flow Hollow Fibre Membrane Filtration at Fixed Pump Driving Pressure

Key words:
hollow fibre membrane, direct-flow filtration, packing density, membrane
fouling, fibre diameter, pumping pressure

Direct-flow
(DF) filtration operating in capped hollow fibre membrane (HFM) modules with a
regular intermittent backwash has become the dominant technology for potable
water treatment. Since the capped-end design ensures no outlet flow from the
module, the pressure drop along the feed channel is very small and smaller
diameter fibres can be used. Our previous analytical investigation [1,
2]
indicated that fibres with large inner diameter and small wall thickness were
to be preferred. However the reason for the clear industrial preference for
fibre diameters in the tight range 0.8mm to 0.9mm inner diameter (ID) and 1.3mm
to 1.4mm outer diameter (OD) was not clear. Therefore, the underlying trade-off
between the fibre diameters and other key design considerations, e.g.
the module packing density was explored. This work presents an MATLAB model for
simulating a constant flux, fixed pump-driving pressure DF-HFM filtration. The
model was validated against our previous mathematical model resulting from solution
of the Navier-Stokes equations. The new model provides a more practical method
for investigating the solution space and here we present an analysis of the
filtration process, including pressure drops, velocity profiles and the module
productivities. Specifically this presentation focuses on the analysis of the
effects of the fibre diameter and module packing densities on the overall
filtration outputs. The modelling results (Figure 1) show that at fixed pumping
pressure of 10 kPa (above outlet permeate pressure), 55% to 65% packing of
1.0mm to 1.2mm OD fibres generate higher outputs than other designs. However, a
slightly higher OD range would be preferred if one wants to avoid the sharp
decline of filtration throughputs which occur at higher packing densities. Given
that the packing density is non-uniform this can be an advantage. Additional
investigations that will be part of the presentation. There will include an
exploration of the effect of fibre lengths and the effect of imposed flux.  

Fig. 1. The influence of packing density, , and
fibre OD upon the overall module flowrate,  at fixed pumping
pressure of  above permeate outlet
pressure. The module had an inner diameter of 165mm
and a length of 1.6m.  

[1]          J. G. Herterich, Q. Xu, R. W. Field, D. Vella, and I. M. Griffiths,
"Optimizing the operation of a direct-flow filtration device," Journal
of Engineering Mathematics, pp. 1-17, 2016.

[2]          Q.
Xu, G. K. Pearce, and R. W. Field, "Pressure driven inside feed (PDI)
hollow fibre filtraitons: Optimizing the geometry and operating
parameters," Journal of Membrane Science [Submitted for publication], 2017.