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Dynamic Response/Characteristics of an Oxygen Swing Adsorption Process to Step Perturbations. Part 1 323 this step was omitted for purposes of simplicity. However, bed-to-bed coupling (important for studying system dynamics) was retained through the inclusion of the purge step. The following step changes were studied (perturbations about cyclic steady state): Case 1: An increase of 5% in the feed valve (CV1) position during the feed steps (steps 2 and 5). Case 2: An increase of 3% in the purge valve (CV3) position during the purge steps (steps 3 and 4). Case 3: A decrease of 10% in the product valve (CV4) position during the feed steps (steps 2 and 5). Case 4: Product load disturbance of 5 kPa via water level in a load tank. The magnitudes of the perturbations of the manipulated variables were chosen on the basis of previous experience in O2 VSA operation and are upper limits of what would typically be needed to control purity, flow and pressure. The responses measured were bed pressures, flow rates, tem- peratures and product composition as a function of absolute time and cycle time. Of particular interest were product composition, product flow rate and product pressure since these directly impact upon the customer and are usually the variables selected for control. Furthermore, it should be noted that the case studies listed above are by no means exhaustive of the varying conditions experienced by a VSA plant during operation, but they do encapsulate the important situations that cause fluctuations in performance. In addition, the valves listed are typically used for control purposes in the VSA process to ensure that purity, flow and pressure targets are met. MECHANISTIC MODEL DEVELOPMENT To help understand the observed responses, it was considered important to develop a computa- tionally simple yet physically representative model. The model should account for the direction of the response (increase or decrease) as well as the approximate magnitude (steady-state gain) and response time (number of cycles needed to reach 95% of the final value). The eventual use of this model will be in a model-predictive structure for future on-line model-based control. It is worth- while elaborating on the need for an appropriate model since there is already an abundance of adsorption models in the literature. The reader is therefore justified in questioning the need for an additional model. Explanation of the dynamic response of a cyclic adsorption system and devel- opment of a control model can be done on at least three different levels: (a) Detailed solution of the governing differential conservation equations with appropriate time- varying boundary conditions Many adsorption simulators have been developed over the past decade (Todd et al. 2001). This modelling approach is not appropriate in the current study for at least three reasons: 1. Unfortunately, the vast majority of adsorption simulators [including our own, MINSA (Todd et al. 2001)] are single-bed simulators. Multi-bed systems are modelled in one-bed simulators by storing information on beds providing gas and using this information when needed later for modelling beds receiving gas. This approach is entirely appropriate when only information at a cyclic steady state is needed, since eventual convergence on the correct profiles will be achieved for moderate computational demand. However, when bed coupling is present (as inPDF Image | Dynamic Response and Characteristics of an Oxygen Vacuum Swing Adsorption
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