Optimizing the Operation of a Direct-Flow Filtration Device

 

Direct-flow filtration is an ultrafiltration membrane process in which hydrostatic pressure forces a liquid through the semipermeable walls of a hollow fibre module in the absence of a cross flow; the far end of the feed channel is capped and therefore all of the feed fluid is forward to pass the membrane walls. This filtration mode offers an efficient combination of the benefits of classical dead-end and crossflow filtration providing there is periodic back-flush. In this study, mathematical models for the flow in a 2D filtration channel and in a 3D hollow fibre have been developed to investigate the flow behaviour within such direct-flow filtration devices. These models were originally developed by applying lubrication equations to systems with walls assumed to be zero wall thickness.  This constraint was lifted in the second part of the work. The space taken by the walls of hollow fibres is not negligible and the influence of wall thickness was important.

The pressure profile in the permeate channel influences the pressure profile in the feed channels. We study the influence of permeate spacing and other parameters, and found that there was a strong axial dependence of the filtration performance. Figure 1 shows the transmembrane pressure difference (TMP) profiles versus axial distance at different module spacing parameter values in (a) a 2D channel and (b) a 3D module. The module productivity was evaluated for various input and operating conditions in order to find the operating regimes of the device that maximize the spatial uniformity in the TMP. Figure 2 shows the cumulative fluid volume transported through the fibre walls as function of axial position for (a) a 2D channel and (b) a 3D module for various values of module spacing parameters. Ideally one should optimize the use of the entire membrane area. The membrane area is not evenly used especially when the module are tightly packed. Behaviour of 2D channels is a rough guide to the behaviour of 3D modules. Additional studies, which will be part of the presentation, indicate there exists an optimum packing range, below which reduction in packing density will have very little effect on process performance, however productivity will be reduced as packing density is reduced.

Figure 1. The transmembrane pressure difference (TMP) profiles versus axial distance, at different module spacing parameter values in (a) a 2D channel and (b) a 3D module.

Figure 2. The cumulative fluid volume transported through the fibre walls as function of axial position for (a) a 2D channel and (b) a 3D module for various values of module spacing parameters.