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8 Summary and final conclusions 82 8 Summary and final conclusions This thesis presents an experimentally-verified PSA mathematical model for high-purity applications with the aim to propose appropriate process optimisation schemes. 8.1 Overview of findings A mathematical model for the dynamic simulation of high-purity twin-bed N2-PSA was implemented in Aspen AdsorptionTM and validated at multiple process conditions, cycle organisation strategies, and plant design parameters. Key-features of the model are a Sips-based IAST-approximation and a quadratic driving force approach. The influence of operating temperature, adsorption pressure, half-cycle time, purge flow rate, cutting time, flow resistances in the piping system, and volume of N2-receiver tank on productivity and air demand was predicted qualitatively and mostly quantitatively correctly at two different product purity levels. Relative errors of simulated performance indicators were presented and their possible origin was discussed. It was clearly shown that a precise forecast of the PSA operation depends on the accurate representation of many factors, e.g. adsorption isotherms, mass transfer kinetics, pressure and temperature profiles along the adsorber, flow resistances, etc. When fitted to high- purity CMS materials, simulation errors at high-purity levels are expected to decline. A further improvement of the model is possible by introducing an additional gaseous key component combining the effects of moisture and carbon dioxide adsorption for a better simulation of thermal phenomena. The discussion of the adsorber column dynamics was conducted based on the validated mathematical model. The model is particularly capable to propose tailor-made process optimisation strategies. Recommendations for performance improvement, with a particular focus on the reduction of energy consumption, were given based on the simulation outcome at three product purity levels. 8.2 Critical discussion of the work The mathematical model developed in this work can be applied to evaluate the performance of any twin-bed PSA unit intended for high-purity nitrogen generation; as long as the kinetic parameters, as listed in Tab. 3.5-1, are adjusted according to the utilised CMS material. However, the credibility of model predictions is highly dependent on correct representation of pressure profiles; thus, on proper estimation of flow resistances in the piping system. It should be clearly stated that the assembly of different armature within the PSA piping system, including different types of automatic valves, will certainly result in discrepancies between experimental and numerical data; which constitutes the biggest disadvantage of this type of model. 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 classical approach suggests an experimental investigation of breakthrough curves and its detailed analysis followed by the subsequent suggestion of the suitable model. By means of this strategy, an assessment of the origin of mass transfer resistances can be performed. However, in this work, the mass transfer kinetics were found by examination of different available models in order to fit the experimental PSA performance indicators to simulation results. The empirical evaluation of the breakthrough curve was not performed, mostly due to the fact that thePDF Image | Modelling and Simulation of Twin-Bed Pressure Swing Adsorption Plants
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