HOLLOW FIBER AIR DRYING

Compressed air can easily be dried with hollow fiber modules made with water permeable membranes. The selectivity of these membranes for water over air is high, over 4000: 1; the water permeability per membrane thickness is also high, over 5 cm/sec (6 X 10(7) barrers per cm). As a result, the mass transfer is influenced not only by the membrane resistance, but also by the resistances in the feed and the permeate streams in the module. This mass transfer can often be enhanced by recycling some of the retentate as a sweep stream on the permeate side. These mass transfer resistances and the sweep stream interact in subtle ways which can be quantitatively predicted using correlations and models in the literature. For example, both experiments and predictions show that under some conditions, the module with the smallest membrane area for a given task may have a less selective membrane.