Electrical Process Tomography
Electrical process tomography including electrical capacitance tomography (ECT), electrical impedance tomography (EIT) and electromagnetic tomography (EMT), is based on the specific properties of materials principally sensed by each technique.
Electrical resistance tomography (ERT) is a particular case of electrical impedance tomography when the real component of electrical impedance is the dominant property of materials in an EIT process application. ECT senses the permittivity distribution of dispersed materials in a fluid process with a non-conductive continuous phase; EIT is specified for a process that has a conductive continuous phase; EMT is mainly applied for high conductive fluids, which can induce measurable current under a magnetic field. In the case of EIT, the sensor is made from multiple electrodes arranged around the periphery of the internal wall of the process vessel or pipeline, in contact with the process medium but not intrusive to the medium. An alternating current is applied to some electrodes and voltages are measured from the remaining electrodes, according to a predefined sensing strategy. Then these voltage measurements are used to reconstruct the impedance distribution inside the vessel with a specific inverse algorithm.
The basis of electrical tomography or electrical impedance tomography (EIT) is the detection and exploitation of differing electrical properties (capacitance, resistance, induction) of the material or materials under investigation within the imaging volume. Based on these properties, electrical measurements from numerous points around a vessel or pipeline are utilised. An array of equally spaced electrodes, often arranged in a transverse plane, are fixed to the boundary wall of a vessel to map the changing spatial distribution of permittivity and/or resistivity (or its inverse, conductivity) of the material contained within. The resulting signals are interpreted by a computer and the resulting boundary datasets, inverted by means of an image reconstruction technique to provide a distribution map of the internal resistivity/permittivity, are used to create a series of ‘live’ 2D or 3D images of the process occurring within. Interpolation over several planes of sensor arrays enables the rendering of 3D images for imaging different stages of a process.
Aided by fast data acquisition and image reconstruction speeds it is possible to examine concentration profiles, phases and chemicals within a wide range of industrial environments. The resulting data can be used to monitor process reactions, improve product quality and yields, monitor flow regimes and concentrations and provide data for on-line process control. Typical industrial applications of EIT include monitoring changing concentration profiles within mixing and separation vessels (single phase and multiphase), imaging distribution and the determination and control of flow characteristics. EIT is therefore capable of contributing to the on-line, real-time control, monitoring and understanding of process operations, providing instant feedback on transport processes, reaction progression and process efficiency, hence, it can help reduce waste and improve overall plant energy efficiency, resulting in time, productivity gains for the operator. It is also adaptable to a wide range of process length scales (mm to m) and requires a considerably lower financial outlay than other tomographic techniques such as computerised x-ray tomography (XMT) and magnetic resonance imaging (MRI). The major drawback is that the images produced have a relatively low spatial resolution (typically 3-10% of a pipe diameter) when compared to imaging techniques such as XMT (spatial resolution in the region of 1%). However, this is often sufficient for many industrial applications.