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Dynamic Response/Characteristics of an Oxygen Swing Adsorption Process to Step Perturbations. Part 1 339 account to ensure that dynamic information is not lost, as this will affect the field performance of control schemes which are usually implemented and tuned offline. Also, design engineers should ensure that the effects of non-idealities are minimised (by appropriate piping layout and design of baffling or mixing points), especially in large oxygen production facilities where product vessels can assume capacities of several thousand litres. If such large unit operations were operated under less than ideal flow configurations, then dead time would dominate the response causing unneces- sary difficulties in the design and tuning of various control algorithms (both model-based and PID). The presence of time delay in a process causes a reduction from the theoretical maximum controller gain (i.e. a system with no dead time) and hence may result in sluggish controller performance. In the case of the oxygen VSA pilot plant, flow into the 60-l product tank was fed by 1/20 tubing running perpendicular to the vessel wall located near the top of the tank, while the exit stream was located near the bottom of the vessel and also ran perpendicularly via a 1/20 tube. No baffling or internal mixing was used in the tank. On average for the set of experiments conducted, ca. 20 l product gas entered and exited the tank per cycle at CSS. If the conditions were dominated by pure plug flow, then a step change in the exit product valve position would result in a stepwise change in flow after a delayed period (dead time of ca. three cycles in this case volume of product vessel/volume of gas exiting the vessel per cycle). On the other hand, if conditions within the product vessel were well-mixed, then the measured exit composition would follow an asymptotic relationship with the time constant of the response being a function of the capacity of the tank. However as the results show, a condition somewhere in between plug flow and well-mixed existed. This condition was not constant but was a function of the flow dynamics and depended on the magnitude and type of perturbations as characterised by the different measured time delays (refer to Table 2). For completeness, the full response time (time necessary to attain 100% value) of the paramagnetic analyser and connecting tubing was measured and found to be ca. 25–30 s for a step change in composition at constant flow. As the response of the analyser was much less than the period of the cycle studied (total cycle time = 60 s), this further demonstrated that the observed delay could be attributed to poor mixing in the tank. The performance of the FODT model, which was calibrated to a single experimental set for each of the four disturbances and then used to predict the effects of similar disturbances of differ- ing magnitudes, is summarised in Table 2. The integral absolute error, IAE, was used as a mea- sure of performance of the first-order model in comparison with plant data and is described by equation (9): (9) Failure of the FODT model to capture the responses that are clearly measured after the initia- tion of the step change is highlighted in Figures 16–19 (the FODT model shows sharp changes in the process variable initially after the time delay whereas the plant exhibits smoother transitions). A better representation may be achieved using higher-order approximations such as Nth order models or a series of interacting first-order lags (each with differing time constants). However, the satisfactorily low IAE and the observed approximation of the FODT model are sufficient to jus- tify its use over more computationally expensive higher-order methods. Furthermore, due to process/model mismatch, additional fine-tuning of the controller parameters is required once implemented in the field. The main use of the model should be to enable the determination of approximate gains that seek to minimise the IAE. Lastly, a PID controller with fixed gains or a model-based controller compensating on a constant time delay may fail if the dead time were to T IAE ya ym dt 0PDF Image | Dynamic Response and Characteristics of an Oxygen Vacuum Swing Adsorption
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