Sweep module performance

Figure 4.18 Comparison of (a) cross-flow, (b) counter-flow and (c) counter-flow sweep module performance for the separation of water vapor from natural gas. Pressure-normalized methane flux 5 x 10 6cm3(STP)/cm2 s cmHg membrane selectivity, water/methane 200...

The Effect of Sweep Uniformity on Gas Dehydration Module Performance... 

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. 

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 ...

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.

Figures 16.7 and 16.8 illustrate the effect of sweep variation on module performance. An average sweep fraction of 0.1 was used for the calculations.

Variability in the sweep flow around individual fibres has little effect on module performance. Only slight decreases in dry gas recovery and flow rate occur as the variability in sweep flow increases. The dry gas flow rate drops by less than 10% over the range of dew points considered as the variability in sweep flow rate increases to 20%. The dry gas recovery changes by less than 1%. 

Figures 16.9 and 16.10 compare the effects of inner diameter and sweep variation on module performance. Like sweep variability, inner diameter variability is detrimental to performance - dry gas recovery and flow rate deaease as the variability increases. However, the effect of inner diameter variability is significantly larger. The dry gas flow rate decreases by a factor of two with 20% inner diameter variation for the lower dry gas dew points. Changes in dry gas recovery are smaller and do not exceed 5% at the lowest dew points considered.

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...

The effect of sweep variability in air dehydration is comparable to that observed previously for fibre variability in nitrogen production from air [30,31]. Significant changes in module performance are found only for inner diameter variability. 

Figures 16.12 and 16.13 illustrate the performance predictions for the different configurations. The locations of the inlet and outlet regions for the sweep have little effect on performance. Dry gas flow rate and recovery decrease by less than 5% over the dew point range considered. The change in performance increases as dew point increases. Such results are consistent with the results obtained assuming a Gaussian distribution of the sweep around each fibre - variations in sweep flow rate do not significantly affect module performance.

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. 

In the case of the counter-flow/sweep membrane module illustrated in Figure 4.18(c) a portion of the dried residue gas stream is expanded across a valve and used as the permeate-side sweep gas. The separation obtained depends on how much gas is used as a sweep. In the calculation illustrated, 5 % of the residue gas is used as a sweep even so the result is dramatic. The concentration of water vapor in the permeate gas is 13 000 ppm, almost the same as the perfect counter-flow module shown in Figure 4.18(b), but the membrane area required to perform the separation is one-third of the counter-flow case. Mixing separated residue gas with the permeate gas improves the separation The cause of this paradoxical result is illustrated in Figure 4.19 and discussed in a number of papers by Cussler et al. [16]. 

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