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4.1.1 Current Assumptions Large diameter zeolite particles (> 2 mm) are well known to exhibit significant mass transfer resistance in their macropores.2, 27 Since it represents the dominant resistance in columns packed with these particles, a mass transfer rate controlled by macropore diffusional resistance is typically assumed. Other resistances external to the particle such as axial dispersion and film transfer resistance are usually considered small or negligible. These assumptions generally provide a reasonable estimation of mass transfer limitations for simulations involving large zeolite particles. Current small scale PSA processes often operate with small particles (~ 0.3 mm to 0.7 mm in diameter) to permit rapid cycling. Smaller particles provide both a higher surface area for diffusional flux into the particle and a shorter diffusional path inside the particle. Decreasing macropore diffusional resistance through particle size reduction increases the influence of other factors affecting the mass transfer rate, particularly effects external to the particle. While this notion is generally accepted in literature, it remains unclear at what particle size these external factors become significant. Zhong et al.28 used a simulation study to determine the effect of LiLSX particle size on process performance. They found axial dispersion effects begin to dominate the mass transfer rate at a particle size < 1 mm for velocities often used in small scale PSA processes. However, the study was never confirmed experimentally. Alpay et al.29 experimentally studied the effect of 5A particle size on a single bed air separation process and found an optimal particle size to maximize product purity. They also found axial dispersion effects were more significant for small particles than predicted by common correlations. Rao and Farooq30 came to a similar conclusion about axial dispersion effects for their single 49PDF Image | LIMITS OF SMALL SCALE PRESSURE SWING ADSORPTION
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