(44a) Electrical Capacitance Tomography – the State of the Art

Electrical tomography is well suited for pattern identification of multi-phase flows and velocity profile measurements in opaque pipes or conduits. It can generate a series of cross-sectional images and reveal the flow pattern from the images at some defined location in a pipe or process vessel. The permittivity distribution of dielectric materials might represent desired properties, such as density or chemical composition. By obtaining two series of images of a flow at two cross-sections and cross-correlating the elements in the two series of images, the velocity profile may be obtained. This opens a new way for on-line measurement of the volumetric flow rate. The data acquisition rates of an ECT system are fast (up to 200 images per second). An ECT sensor can be constructed to be sufficiently robust to cope with the harsh industrial environments. In addition, it is quite simple to operate an ECT system. A drawback of electrical tomography is its relatively low spatial resolution ? typically 3?10% of a pipe diameter. However, the resolution should be viewed in the context of the practical industrial applications. With electrical tomography both qualitative and quantitative data needed for modelling a multi_-phase system can be obtained. For instance, different flow patterns are characterised, in a qualitative way by using both time and space scales. The qualitative model may be verified by comparison between calculated fields of concentration or velocity and measurement results. As tomographic data provides, in a non-invasive way, cross-sectional profiles of the distribution of materials and/or velocities in a process vessel or pipeline, the results obtained from tomographic measurements can then be applied for process design and process control. In certain cases, electrical tomography is one of the most attractive methods for real-time imaging of industrial processes, because of its inherent simplicity and high speed. This paper will report the latest developments in the ECT systems including data obtained from a fluidized bed, a discharge process of solids from silos, and solids concentration and velocity profile measurements in a pneumatic conveyor. It will also report an approach to flow-pattern recognition without image reconstruction by finding the geometrical properties hidden in a frame corresponding to a set of capacitance measurments. Omitting an explicit image reconstruction phase, which is time-consuming, is particularly important for online control. In this aspect, the latest developments in fluidised bed dryier control using dual-modality tomography will also be reported.