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5 Validation of the mathematical model 68 Fig. 5.3.1-7 PSA performance at asymmetric pressure equalisation at reference process conditions at product purity level of 10 ppm O2 5.3.2 Volume of the N2-receiver tank The N2-receiver tank is implemented in the PSA unit in order to operate the cyclic process with a constant flow rate of a high-pressure nitrogen stream that is requested in the majority of industrial applications. Moreover, a mixing space for the product gas is provided which is generated during the production step at inconstant composition. In the considered pilot plant, the purge and the backflow streams, directed counter-currently to the adsorber columns, are served also from the N2-receiver tank. The selection of the smallest possible buffer vessel is of essential importance while aiming for a reduction of the unit footprint. However, an insufficient volume of the tank would give rise to enlarged pressure differences in the system and could be a reason for reduced unit performance. Thus, the influence of the N2-receiver volume on the PSA performance indicators is investigated in the range of 5 – 12 L by replacing the originally assembled tank by smaller tanks. The results are presented in Tab. 5.3.2-1–2 and Fig. 5.3.2-1. During the PSA operation, the pressure maintained in the N2-receiver directly affects the gas flow rate through the packed bed which consequently influences the mass transfer conditions. The experimental pressure profiles of adsorbers and product buffer tank at different volume ratios of the N2-receiver to the adsorber column are presented in Fig. 5.3.2-2. As a consequence of the co-current pressurisation of the packed bed with a simultaneous counter-current backflow of product, the pressure in the N2-receiver tank declines at the beginning of the PSA cycle. That pressure downfall becomes more extensive as the volume of the product buffer tank is reduced. An increased pressure difference between the adsorber column and the N2-receiver is created which causes an enlarged gas velocity in the adsorbent fixed-bed and, therefore, a decreased contact time of gas and solid phases. Furthermore, the final production pressure is achieved in the adsorber column with a significant delay, diminishing the efficiency of the adsorption process. For that reason, at every investigated process condition, the reduction of the volume ratio of the N2-receiver tank to the adsorber column results in lower nitrogen productivity and higher air demand.PDF Image | Modelling and Simulation of Twin-Bed Pressure Swing Adsorption Plants
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