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4 Experimental program 42 (Eq. 4.4-3) (Eq.4.4-4) (Eq.4.4-5) (Eq. 4.4-6) Qproduct = VCMS 1 (Qfeed feed −Qtail−gastail−gas ) VCMS product Qproduct = 1 (Q feed feed x O2 feed −Q tail−gas tail−gas x O2tail−gas ) VCMS Qfeed Q product Q feed Qproduct VCMS product xO2 product = product + Qtail−gastail−gas feed Q product feed = product xO2 product + Qtail − gas tail − gas xO2tail − gas feed xO2 feed Qproduct feed xO2 feed A perfectly determined performance indicator attains exactly the same value whether calculated from the two material balances (according to the implicit method) or directly read from the flow meters (according to the explicit method). In fact, it is necessary to accept a certain tolerance of incompatibilities between values due to inevitable measuring errors. Any significant differences in values of performance indicators are the evidence of a faulty operation of the PSA system, e.g. leakages, incorrect calibration of measuring devices, errors in transmitting electrical signals to the equipment, errors in the software code, accumulation of pollutants in valves, diffusion intake due to porous or inadequate piping or seals, or exhausted filters. In this work, a performance indicator value is considered to be determined correctly when the difference between the results obtained from the different established calculation paths does not exceed a tolerance level of 1 %, determined as a relative error δexp. If δexp between differently calculated performance indicators is lower than 1 %, obtained values are considered as correct. If δexp is larger than 1 %, obtained values are discarded. Admittedly, this 1 %-tolerance level of δexp seems to be set arbitrarily, however justified by laboratory experience gained in decades of quality control. Additionally, separate performance indicators of each single adsorber can also be calculated, which allows the disclosure of potential imbalances of operation between two packed beds. Maintaining such a strict tolerance becomes more challenging the higher the product purity is addressed. Here the accuracy of measuring devices comes to the fore since a limited measurement precision mathematically prohibits solving equations with an arbitrary low tolerance. 4.5 Measurement accuracy In order to solve the balance equations (Eq. 4.4-1 and 4.4-2), precise data of the feed, the product, and the tail gas flow rates are required. However, only one of three flows is constant in a PSA system and the others fluctuate along the cycle. In this study, the product flow rate is set constant. Thus, the measurement of feed and tail gas flow rates during the rapid system pressurisation/depressurisation could be burdened by a comparably high experimental error mainly due to the limitation in scan-speed of the measuring devices as presented in Fig. 4.5-1.PDF Image | Modelling and Simulation of Twin-Bed Pressure Swing Adsorption Plants
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