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5 Validation of the mathematical model 47 5 Validation of the mathematical model Product purity, productivity, and air demand are the three main performance indicators used for the evaluation of the N2-PSA effectiveness. When operating the adsorber, the product purity at the outlet of the system is the consequence of the gas flow rate through the column. This would suggest that a representation of process performance data should consider productivity as an independent variable, while the product purity, as well as the air demand, are dependent variables. This approach is consistent with the conventional demonstration of PSA performance results in the literature. However, in commercial applications, values for productivity and air demand are listed for a certain purity class separately, since the nitrogen quality, not quantity, indicates go/no-go decisions for a specific gas application. In this work, experimental results of PSA performance indicators, i.e. productivity and air demand, are compared with the outcome of the mathematical model at fixed product purity levels, which is consistent with commercial conventions. The difference between simulated and empirical results is expressed as the relative error of the simulation Φ according to Eq. 5-1. Thus, the performance indicator value is underestimated by the mathematical model when Φ is positive; reversely, it is overestimated when Φ is negative. =100% PIEXP − PISIM (Eq. 5-1) PI EXP Selected operating conditions, cycle organisation strategies, and plant design parameters are in agreement with the vast majority of relevant industrial applications. The effects presented here on PSA performance indicators are studied individually. Therefore, one parameter is investigated at a time, while all others follow the conditions of the reference process (as described in Chapter 4.3). The process performance data were collected upon reaching the cycle-steady-state (CSS); namely, when process variables do not fluctuate over subsequent PSA cycles. In order to assure cycle-steady-state conditions in the system, performance indicators are determined after at least ten hours of uninterrupted operation of the PSA unit. 5.1 Effect of operating conditions 5.1.1 Operating temperature Since nitrogen generators are commonly exploited in both cold and warm environments, the influence of operating temperature on the PSA performance indicators is investigated in the range of 5 – 45 °C. Experimental and simulated results are presented in Tab. 5.1.1-1–2 and Fig. 5.1.1-1. Independent from the purity level, Fig. 5.1.1-1 shows that productivity and air demand, simulated with the model premises listed in Chapter 3, fit well to experimental data in the temperature range between 12 °C and 36 °C. At investigated conditions, the productivity reaches its maximum at 28 °C and 20 °C at a purity level of 1000 and 10 ppm O2, respectively. The effect is evident when taking into consideration a competition of both adsorption thermodynamic and kinetic factors, which is especially relevant for kinetically-controlled separations. The equilibrium loading in the adsorbent isPDF Image | Modelling and Simulation of Twin-Bed Pressure Swing Adsorption Plants
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