Oxygen vacuum swing adsorption (VSA) has emerged as an important unit operation in many chemical engineering processes such as iron and aluminium smelting, making oxygen the third largest man-made chemical commodity in the world. Although a mature technology (with the first patents published in the 1970s), oxygen VSA processes are still not well understood due to their complicated batch-like operation, inherent non-linearities and inverse responses associated with the operating conditions. Step perturbations of manipulated variables together with the process response provide valuable information for the study of system dynamics, the extent of interaction and control loop pairings. The first part of this study presents data from input perturbations gathered from a pilot-scale experimental oxygen VSA process. The interesting time-variant temperature profiles, and bed and system pressures, flows and purity are the main focus of the discussion. Furthermore, the possible applications of this knowledge for heuristic-based control are discussed.

Pressure- and vacuum-swing adsorption processes are challenging to model and difficult to solve rapidly because the system response is a nonlinear function of both the axial and temporal domains with periodic boundary conditions. Extensive computing power is required to solve the conservation equations that describe the temperature, composition, and pressure profiles during operation of an adsorption process. Furthermore, the physics of the source/sink terms in the conservation equations of mass and energy, which relates to the ad/desorption of absorbable species, is not always easily described. These rigorous numerical models are useful for furthering our understanding of these complex processes and are the only methods available for the design of industrial units. However, such complicated numerical simulators for model-based control schemes are not feasible at the current level of computing resources. Simplification of the conservation equations is required to derive a practical mechanistic model for predictive control purposes. In a previous study, Beh and Webley (Adsorpt. Sci. Technol. 2003, in press) have demonstrated that much of the complexity of these processes can be captured through the use of a simple model consisting of a series of coupled tanks which approximates the bulk flows and pressures to a satisfactory degree. In this paper, an extension is made to this model to incorporate the time-varying composition variable. The limitations of this method will be discussed in relation to field operation.

The application of vacuum swing adsorption (VSA) as a method for the production of low-purity oxygen is now a mature and widely used technology. However, unlike other unit operations, VSA processes remain difficult to understand and, coupled by the lack of theoretical developments in batch process control, make them difficult to control. The objective of the second part of this study was to characterise the step response data gained from Part 1 using empirical modelling techniques. This information was then used to analyse the sensitivity of the input variables and hence permit determination of the extent of closed-loop interaction and identification of suitable controller parings.

A simplified energy-balance equation has been proposed as an aid to online measurement of the adsorbed-phase nitrogen loading in industrial-scale pressure and vacuum swing adsorption systems consisting of one preferentially adsorbed component. Implementation of this technique to current and future plants requires the addition of thermocouples (which are relatively inexpensive) located axially through the bed and pressure transmitters at the bottom and top of the bed. This methodology has the advantage that the composition front may be inferred accurately from direct measurements of the local pressure and temperature, and used as the basis for process monitoring and diagnostics of plant purity, recovery and production rate.

This paper investigates the thermal profiles that arise in oxygen VSA, which is a prominent example of a PSA bulk gas separation process. Experimentally, it is demonstrated that the severe axial thermal profile or "cold spot" that frequently characterizes oxygen VSA can only arise if there are multilayered adsorption beds or if there are readily adsorbed trace components (such as water) that create a de facto multilayered bed. A qualitative explanation is offered to explain how this cold spot is formed. This paper also details a technique for predicting the penetration of a water-loaded zone into an oxygen VSA adsorption bed based on the method of characteristics. The results of this technique compare well with experimental and numerically simulated results. Finally, this paper demonstrates that a water-loaded zone and an inert zone of activated alumina result in very similar cyclic steady-state thermal profiles, even though the transient behaviors are markedly different.