Flow management in nanofiltration spiral wound modules with ladder-type spacers

Abstract

The incompressible laminar and two-dimensional flow in narrow rectangular channels filled with ladder-type spacers that have the transverse filaments adjacent to a semi-permeable wall, was investigated in order to have insight on the feed flow in a nanofiltration (NF)/reverse osmosis (RO) spiral wound module. The study was based on numerical simulations and on experiments of flow visualisation with tracer injection and pressure drop measurements. The numerical simulations were performed assuming fully developed laminar flow quasi-periodically repeating in successive inter-filament regions. Ladder-type spacers with inter-filament distances/channel height ratios ( L f) of 1.9, 3.8 and 5.7 were investigated. The ratio ( P f) between the transverse filaments height and channel height was investigated between 0.25 and 0.75 in the numerical study and was fixed to 0.5 in the experiments. Flow visualisation with tracer injection and friction factor measurements were made for the water flow in a spacer-filled rectangular channel with 2 mm height, 30 mm wide and 200 mm long, in the range of Reynolds numbers (based on the channel height and on the average inlet velocity) between 50 and 1000. These results show that for increasing inter-filament distances the transition critical Reynolds number decreases from 300 to 150. Below this critical value, the flow is observed to be laminar and two-dimensional. The numerical simulations show that the flow structures are associated to the occurrence of recirculation zones downstream each transverse filament. For low values of the Reynolds number and P f and high values of L f, the recirculation region does not reach the second filament, while for high values of the Reynolds number and P f and low values of L f the recirculation region extends from the first to the second filament. Additionally, for P f=0.75, a secondary recirculation region can be formed inside the main recirculation region. Each of these situations is associated to different patterns of concentration boundary layers growth and subsequently different solute concentration distributions at the membrane surface.