Page 261 - Process Modelling and Simulation With Finite Element Methods

248 Process Modelling and Simulation with Finite Element Methods

One possible way of obtaining such data is to measure the spatial electrical
permittivity distribution of a flowing gas-liquid mixture using Electrical
Capacitance Tomography (ECT). This will give information regarding the phase
distribution about the pipe cross-section.
Tomographic instrumentation can provide images, non-invasively, of the
distribution of components within a process vessel or pipeline. Electrical
Capacitance Tomography (ECT) provides 2-D images of the dielectric
distribution of the components within a process pipe. Non-invasive
measurements of capacitance by electrodes - excited by a charge-discharge
principle [l 13 - are used in a mathematical reconstruction algorithm to create
images of materials having different pennittivities. This procedure allows
different phases to be determined. To date, process engineering studies
involving ECT have been sparse, but some areas of application include fluidised
beds and pneumatic conveying. McKee et al. [12] reported the use of
capacitance tomography for imaging pneumatic conveying processes in two
industrial pilot scale rigs. This work pioneered the application of ECT to dense-
phase pneumatic conveying and demonstrated the potential of capacitance
tomography as an aid to on-line process control. A good review of this area can
be found in [ 1 31.
The tomographic imaging device involves three main sub-units: an array of
sensors (typically 12 electrodes; 66 independent measurements), a data
acquisition system and an image reconstruction system. Measurements of
capacitance are obtained for all possible combinations of electrodes. For each
electrode pair the following charge-discharge procedure is adopted: the active
electrode is charged to a given voltage (15 volts) while the detecting electrode is
earthed; the active electrode then discharges to earth while the detecting
electrode connects to the input of a current detector. This detector then averages
the resultant oscillating current from the detecting electrode, creating a voltage
directly proportional to the unknown capacitance value.
The basic capacitance data acquisition system is based on the charge
transfer principle. The discharging current flows out of the current detector
producing a positive voltage output. The typical chargeldischarge cycle repeats
at a frequency of 1 MHZ, and the successive charging and discharging current
pulses are averaged in the two current detectors, producing two DC output
voltages.
Calibration of the instrument is performed before use of the electrode
arrangement and involves the sensor device being filled with the material of
lower permittivity. This procedure provides a reference value of permittivity. A
change in the measurement sensitivity of the circuit then occurs when the pipe is
filled with the material having the higher permittivity. A calibration procedure is
needed for each type of material studied.