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1 Introduction 8 The crucial aspect regarding modelling of kinetically-controlled air separation is the correct representation of the mass transfer kinetics in highly microporous CMS adsorbent. The commonly known linear driving force (LDF) model is frequently applied due to its simplicity and physical consistency [37]. Details of the LDF approach are presented in Chapter 2.2.1. Interestingly, the N2-PSA process simulations involving LDF model present results almost exclusively at low product purity levels (< 99 vol.-% N2) [38–43]. Shirley and Lemcoff were the sole authors of predicted and experimental data in the range of higher nitrogen purity (> 99.9 vol.-% N2), describing the mass transfer kinetics through the Langmuir adsorption rate expression as an approximation of the slit-potential-rate model introduced by LaCava [44–46]. The model was deduced from knowledge of the structure of carbon molecular sieves and basic physical principles. Since the diffusing molecules experience a net repulsive interaction upon entering very narrow pores of molecular dimensions, they must pass an energy barrier to gain admittance to the adsorption volume. The difference of magnitude of these energy barriers for various adsorptives gives rise to different diffusion rates and therefore causes separation [47]. Thus, the adsorption process consists of two steps: (1) crossing the activation energy barrier at the entrance to the slit, and (2) adsorption on the cavity surface, which, according to the authors, is following the classical Langmuir adsorption rate [45]. Based on those considerations, the mass transfer rate is quantified as shown in Eq. 1.2-1. k =slitskKc1−− w (t) k w ( ) (Eq. 1.2-1) tK(1−)kk kk kk where: w(t) is the adsorbed phase concentration, ws is monolayer capacity, K is the Langmuir equilibrium constant, kslit is the slit potential rate constant, c is gas phase concentration, and θ=w/ws is the fractional surface coverage. Fig. 1.2-1 Comparison between the predicted and experimental product rate for two cycle times [44]PDF Image | Modelling and Simulation of Twin-Bed Pressure Swing Adsorption Plants
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