Sweep distribution effect

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. 

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 16.12 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...

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 this beam-sweeping scheme the effective spatial distribution of the ions sampled is defined by the characteristics of the sweeping action and the detector slit parameters [20]. Maintaining the fast rise time of the deflection pulse is critical in maintaining spatially small ion packets at the detector surface, and thus adequate resolution. The overall resolution for the differential impulse-sweeping mode in Fig. 12.3 can be estimated with the following equation developed by Bakker [20] ... 

Perez, J, E.R, Gonzalez, and E.A. Ticianelli, Oxygen electrocatalysis on thin porous coating rotating platinum electrodes. Electrochimica Acta, 1998, 44 pp. 1329-1339 Song, H,-K, H,-Y, Hwang, K.-H. Lee, and L.H, Dao, The effect of pore size distribution on the frequency dispersion of porous electrodes. Electrochimica Acta, 2000. 45 pp. 2241-2257 Srikumar, A, T.G. Stanford, and J.W, Weidner, Linear sweep voltammetry in flooded porous electrodes at low sweep rates. Journal of Electroanalytical Chemistry, 1998. 458 pp. 161-173... 

A MC module contains thousands of microporous hollow fibres, which are knitted into a fabric that is wound around a distribution tube with a central baffle as shown in Figure 1.15. The baffle ensures the water is distributed across the fibres, and also results in reduced pressure drop across the contactor. The hollow fibres are packed densely in a membrane module with a surfrce area of up to 4000 n / m. The liquid flows outside (shell side) the membrane, while vacuum is appHed on the inside of the fibre (tube side) forming a film across the pores of the membrane. Mass transfer takes place through this film and the pores due to the difference in the gas partial pressure between the shell side and tube side. Since the membranes are hydrophobic, they are not wetted by water, thereby, efiectively blocking the flow of water through the membrane pores. The membrane provides no selectivity. Rather its purpose is to keep the gas phase and the Hquid phase separated. In effect, the membrane acts as an inert support that allows intimate contact between gas and liquid phases without dispersion. Vacuum on the tube side of the membrane increases the mass transfer rate as in a vacuum tower. The efficiency of the process is enhanced with the aid of nitrogen sweep gas flowing on the permeate side of the membrane. 

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.

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