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approach a maximum Pé number of two at high Reynolds numbers, common in large PSA applications.26 3.3.3 Mass Transfer Zone In an adsorption process, a feed mixture is introduced at one end of the column and product exits at the other end. As the adsorbate moves from the fluid phase into the adsorbed phase, a concentration wave forms in the column. The concentration wave, or MTZ, travels through the column and eventually reaches the opposite end, illustrated in Figure 3.3. As it exits the column, the adsorbate outlet concentration increases and eventually reaches the inlet concentration. The shape of the exit (“breakthrough”) curve is heavily dependent on the adsorption isotherm shape and whether equilibrium is favorable or unfavorable. Figure 3.4 illustrates how to determine adsorption isotherm favorability. If favorable, the concentration wave is compressive or self-sharpening (Figure 3.5), since the high adsorbate concentration front travels faster than the low adsorbate concentration front because of a material balance in the MTZ. The limit of this wave is a shock wave or a step discontinuity, which cannot occur in reality because adsorption is not instantaneous. A sharp wave front produces a small MTZ and indicates a high adsorption rate and separation efficiency. An unfavorable isotherm produces a dispersive wavefront, the opposite of a compressive front.9 This occurs during the desorption step of a PSA process and is illustrated in Figure 3.5. In realistic adsorption processes, mass and heat transfer limitations will spread the MTZ. Mass transfer limitations include diffusional and axial dispersion, as previously discussed. Since adsorption is exothermic and finite heat transfer resistances exist in any packed column, 44PDF Image | LIMITS OF SMALL SCALE PRESSURE SWING ADSORPTION
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