Modeling the dynamics of filtration processes under variable flow conditions

To rate the performance of filtering devices, several test procedures have been proposed in the past. The standard ISO 16889 is a well-established and widely accepted method to evaluate the lifetime, dust holding capacity and the fractional efficiencies of a filter element. However, the continuous ingression of test dust at relatively high concentrations and the constant flow rate have been subject to some criticism. A major argument for this is that under real operating conditions, both the concentration of contaminants and the flow rate will not be constant in time and therefore, a static multi-pass test with high ingression rates can underestimate relevant effects such as the resuspension of captured particles into the liquid. Several alternative multi-pass test procedures were proposed to resolve this issue by prescribing conditions such as a variable flow rate for (at least part of) the test duration, a reduced base upstream gravimetric level and so-called stabilization phases during which no test dust is injected at all. In this study, a macroscopic model for static flow is extended to dynamic flow. Therefore, additional effects caused by the rapid change of filtration velocity are taken into account. This might result in different ratings of filter elements. For instance, it was observed that filter media (or elements) with almost the same performance in a static test can display a quite different efficiency behavior when tested under variable flow conditions. It is shown that the incorporation of dynamic flow conditions results in different performance (lifetime, efficiency) of the filtering medium when compared to static conditions. In the synthetic test case at hand, the difference in the fractional efficiencies of the small particles results in a lower pressure drop. The simulations show that one important ingredient, also for the real test conditions, is the choice of the particle counting intervals.