Sweep distribution

This work reports simulations of sweep distribution within the shell and its effect on module performance. Two types of simulations are considered (1) simulations that assume the sweep flow around each fibre is distributed in a Gaussian manner and (2) simulations that explicitly predict flow fields within the shell based on how the sweep gas is introduced. 

Predictions based on explicit calculation of the shell flow are in good agreement with those based a Gaussian sweep distribution using a standard deviation in sweep flow equal... 

Results obtained for a Gaussian sweep distribution are compared to results obtained for a Gaussian fibre inner radius distribution. Previous work [30,31] has shown that variation in fibre inner radius has the largest impact on module performance when the variability in fibre properties is included in module performance simulations. 

Figure 16.2 Boundary conditions used in explicit simulations of sweep distribution to evaluate module performance (a) lumen-side boundary conditions and (b) shell-side boundary conditions. Note that the boundary conditions for the shell correspond to one of the configurations used to simulate shell flows. Similar boundary conditions apply for the others. The shell extensions allow establishment of uniform velocity and concentration fields as described previously 124 ...

The explicit sweep distribution simulations were performed with increasingly refined meshes until the results did not change by more than 5%. Typical meshes contained 5000 elements. 

Module Performance with Ideal Sweep Distribution... 

Figures 16.3 and 16.4 illustrate the effect of sweep fraction (i.e. fraction of the dry product returned as sweep to the shell) on module performance assuming uniform, ideal sweep distribution. For all sweep fractions, the product gas flow rate and recovery decrease as the dew point decreases since increased water removal is accompanied by increased loss of oxygen and nitrogen.

Figure 16.11 Three different sweep configurations used in explicit sweep distribution calculations to determine module performance. The arrows indicate the macroscopic flow direction. The thick solid lines indicate the location of the inlet and outlet...

Explicit sweep distribution calculations were performed for three different configurations as illustrated in Figure 16.11. The sweep is introduced either (1) on the periphery of the fibre bundle adjacent to the tube sheet, (2) on the periphery offset partially from the tube sheet, or (3) internally from within fibre bundle. 

Figure 16.12 Effect of sweep configuration on dry gas flow rate as a function of dew point diamond - internal, circle - shell, triangle - offset. The solid line corresponds to uniform sweep distribution. Note the diamond and circle symbols overlap...

Surprisingly, the large variations in concentration that exist do not impact overall module performance. The mixing cup average concentrations calculated from the concentration and velocity fields are nearly identical to the concentrations calculated assuming uniform sweep distribution and no radial variation in the concentration or velocity fields. This fortuitous result implies the inlet and outlet locations for the sweep are not a critical design variable. 

The effect of non-uniform sweep distribution is examined by (1) assuming a Gaussian variation in the sweep flow around each fibre and (2) explicitly calculating the sweep distribution within the bundle for specified sweep inlet and outlet locations. In both cases, non-uniform sweep flows has little effect on module performance. The explicit sweep distribution calculations indicate large radial concentration gradients are present in the module. Surprisingly, these concentration gradients are not detrimental to performance. 

We believe the analysis presented here will be useful in evaluating alternative methods for introducing sweep in membrane modules for both gas and liquid separations. Although uniform sweep distribution appears not to be critical for air dehydration, other applications may be more sensitive. 

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