Counterflow module

In the crossflow module illustrated in Figure 8.5(a), the pooled permeate stream has a water concentration of 1.88%. The counterflow module illustrated in Figure 8.5(b) performs substantially better, providing a pooled permeate stream with a concentration of 3.49%. Not only does the counterflow module perform the separation twice as well, it also requires only about half the membrane area. This improvement is achieved because the gas permeating the membrane at the residue end of the module contains much less water than the gas permeating the membrane at the feed end of the module. Permeate counterflow dilutes the permeate gas at the feed end of the module with low-concentration permeate gas from the residue end of the module. This increases the water concentration driving force across the membrane and so increases the water flux. 

Figure 8.6(a) shows the concentration of water vapor on the feed and permeate sides of the membrane module in the case of a simple counterflow module. On the high-pressure side of the module, the water-vapor concentration in the feed gas drops from 5000 ppm to about 1500 ppm halfway through the module and to 500 ppm at the residue end. The graph directly below the module drawing shows the theoretical maximum concentration of water vapor on the permeate side of the membrane. The actual calculated permeate-side concentration is also shown. The difference between these two lines is a measure of the driving force for water-vapor transport across the membrane. At the feed end of the module, this difference is about 15 000 ppm, but at the permeate end the difference is only about 500 ppm. 

Figure 8.6(b) shows an equivalent figure for a counterflow module in which 10% of the residue gas containing 500 ppm water vapor at lObar is expanded to lbar and introduced as a sweep gas. The water-vapor concentration in the permeate gas at the end of the membrane then falls from 4500 to 500 ppm, producing a dramatic increase in water-vapor permeation through the membrane at the residue end of the module. The result is a two-thirds reduction in the size of the module required for the separation. 

Counterflow modules are always more efficient than crossflow modules, but the advantage is most noticeable when the membrane selectivity is much higher than the pressure ratio across the membrane and a significant fraction of the most permeable component is being removed from the feed gas. This is the case for air-dehydration membrane modules, so counterflow capillary modules are almost always used. With most other gas-separation applications, the advantage offered by counterflow designs does not offset the extra cost of making the counterflow type of module, so they are not widely used. 

Fig. 4.6.1 (a) Di agram ofthe counterflow principle of membrane and dialysate flow in a hemodialyzer module, (b) Schematic depiction ofthe module cross section, (c-d) Photographs ofthe cross sections ofthe mini-hemodialyzer modules used in this study. 

Fig. 4.6.2 ID velocity profiles of counterflow through the mini-hemodialyzer modules of type SMC (a) and SPAN (b), where velocity along z is plotted versus the x position axis (total width 9 mm), with z representing the flow direction. Positive velocities correspond to membrane-side flow (M) and negative velo-...

Fig. 4.6.5 2D VEXSY experiments performed on axial counterflow of water in mini-hemo-dialyzer modules of type SMC and SPAN. Two different mixing times tm for two typess of modules are displayed. The applied flow rate for the SMC module is 2.1 mL min"1 (M) and 1.9 mL min"1 (D). The flow rate for the SPAN...

We would like to thank Peter Blunder and Simone Laukemper-Ostendorf who initially established the collaboration between the company Membrana and the RWTH. They started NMR flow imaging studies to characterize filtration in hemodialyzer modules, and Volker Gobbels measured the first 2D VEXSY data of counterflow in such applications. All experimental work has been accomplished in the Magnetic Resonance Center (MARC) directed by Bernhard Bliimich, whose support and leadership is greatly acknowledged. 

In air-drying applications, it is important to operate the modules in a counterflow mode, usually with a small sweep flow from the residue gas. Some calculations illustrating the importance of counterflow and counterflow/sweep operation are shown in Figure 8.5. 

Figure 8.5 Comparison of (a) crossflow, (b) counterflow and (c) counterflow sweep module performance for the separation of water vapor from air. Membrane water/air selectivity = 100, water permeance = 1000 gpu.

In the case of the counterflow/sweep membrane module illustrated in Figure 8.5 (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... 

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