The Optimisation of Pressure Measurements for the Control of Bubbling Fluidised Beds

A common form of chemical reactor is the bubbling fluidised bed where gas is introduced into a free bed of particles at a rate exceeding that necessary to support their weight. The behaviour of the reactor is dependent on the distribution and characteristics of the bubbles within the bed. Information about these can be inferred from pressure measurements within the bed, which can also be used to characterise the dynamics of a bed and therefore to control it and the reaction. It is necessary to understand what the pressure signals are measuring, how many of them are necessary, and where they should be placed for them to be used to characterise the state of the bed. Pressure measurements are made at single points within a bed, and it is not immediately clear whether the measurements are local (owing to some sort of wall effect, for example), characterise the region close to the sensor, or the whole bed. It is shown how the dynamics of the bed can be characterised from the pressure signals, and that these signals are sensitive to the state of the bed. It is also necessary to evaluate where the sensors should be placed and how many of them should be used. Important considerations here are the extent to which the measurements of pressure are axisymmetrical, and whether in some circumstances and positions the local dynamics mask the overall bed dynamics. Axisymmetry is shown to exist under many conditions and that a single transducer can be sufficient. Furthermore, the depth of the bed is also shown to have a significant effect on its dynamics. It is also shown how the characterised pressure measurements can be used to control the state of the fluidised bed and used to enhance the performance of a fluidised bed reactor.