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4.1.3 Axial Dispersion Coefficient Estimation No consensus exists in literature for describing axial dispersion effects in columns packed with small particles as estimates of the axial dispersion coefficient (DL) vary depending on the correlation used. Some studies have continued to use a correlation common for large particles.34, 45-47 Others have incorporated an alternative correlation that increases axial dispersion effects for small particles.30, 38 It is currently unknown how the estimate of DL affects simulation accuracy, which further contributes to a lack of understanding of its importance. As discussed in section 3.3.2, equation 3.15 is typically used to estimate the axial dispersion coefficient, DL, with γ1 and γ2 assumed as 0.7 and 0.5 respectively. While these estimates of γ1 and γ2 are suitable for columns of large particles, their applicability to columns of small particles is questionable. As early as 1968, Edwards and Richardson48 demonstrated the Pé∞ for small particles (< 2 mm) was much lower than 2 (Pé∞ = 1/ γ2), which requires γ2 to be much higher than 0.5. Suzuki and Smith31 further confirmed this using small glass beads to determine the effect of particle size on dispersion. They found a clear change in Pé∞ as particle diameter decreased for small particles. Langer et al.26 also found a similar relationship, illustrated in Figure 4.1. Although the reason Pé∞ is a function of particle size for small particles is not fully understood,24 Moulijn and Van Swaaij49 reason the difference is due to channeling within the column (rather than just near the wall). They postulate that small particles tend to 53PDF Image | LIMITS OF SMALL SCALE PRESSURE SWING ADSORPTION
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