Oxygen Vacuum Swing Adsorption (O2VSA) is a relatively new process for the production of 90-95% O2 from air using adsorbents in a pressure swing cycle. Many adsorbents such as molecular sieve zeolites preferentially adsorb nitrogen leaving the un-adsorbed oxygen to be recovered as product from the exit of a packed bed. Removing the nitrogen is done by reducing the bed pressure to below atmospheric, purging with some oxygen product, and repressurising the bed with either product or feed or both. Thus a sequence of steps is used with multiple adsorbent beds to produce a "pseudo" continuous product stream of oxygen. During normal operation of a O2VSA, the product composition, flow rate, and pressure, all oscillate to a small extent. These oscillations are buffered by using an oxygen product tank so that the customer receives a relatively steady flow rate of constant purity oxygen at a relatively steady pressure. When such a condition occurs, the system is said to have achieved cyclic steady state.
Unfortunately, cyclic steady state is an idealisation which seldom occurs. A large variety of disturbances can and do affect the performance of an O2VSA such as changes in ambient temperature, changes in the customer demands (ie. attempting to take more or less oxygen), changes in valve coefficients and therefore flow rates as the valves wear, and changes in machinery performance. As a result of these disturbances, the product composition, flow rate and pressure often start to diverge from their set points and control must be taken to restore them to their required values. This can be an exceedingly difficult task: there are a large number of variables which can be manipulated (step times, valve positions) all or some of which may affect the product composition, flow rate, and pressure. It is the goal of this research project to understand the dynamic response of an O2VSA to disturbances in customer demand and develop control strategies to allow efficient return of the VSA to cyclic steady state at the desired setpoints.
To achieve this goal, both experimental and modelling strategies have been adopted. An experimental O2VSA plant has been built to duplicate the expected large scale behaviour. To achieve this, it is important that the unit approaches adiabatic operation since the presence of large temperature gradients in the bed could significantly affect the dynamic behaviour and required control. The VSA built is a 2 bed unit, each bed consisting of insulated PVC 10cm piping of height 1.8m. The beds contain two layers: a prelayer of inert alumina and a main bed of LiX zeolite. It is the presence of this prelayer which causes significant non-isothermal behaviour.
The VSA is fully instrumented and PLC controlled and a wide variety of possible cycles can be programmed. In our study, the feed valve, product
valve, and purge valve are the main three variables to be manipulated to control product purity, pressure, and flow rate. The response of the system to load disturbances and the ability of these manipulated valves to restore the system to its set point has been the focus of our study. A variety of control schemes have been developed and tested. For more information on this project see the publications or contact Paul Webley.
