Electrical capacitance tomography (ECT) has been developed since the late 1980s for visualization and measurement of a permittivity distribution in a cross section using a multi-electrode capacitance sensor. While the hardware and image reconstruction algorithms for ECT have been published extensively and the topics have been reviewed, few papers have been published to discuss ECT sensors and the design issues, which are crucial for a specific application. This paper will briefly discuss the principles of ECT sensors, but mostly will address key issues for ECT sensor design, with reference to some existing ECT sensors as a good understanding of the key issues would help optimization of the design of ECT sensors. The key issues to be discussed include the number and length of electrodes, the use of external and internal electrodes, implications of wall thickness, earthed screens (including the outer screen, axial end screens and radial screens), driven guard electrodes, dealing with high temperature and high pressure, twin planes for velocity measurement by cross correlation and limitations in sensor diameter. While conventional ECT sensors are circular with the electrodes in a single plane or in twin planes, some non-conventional ECT sensors, such as square, conical and 3D sensors, will also be discussed. As a practical guidance, the procedure to fabricate an ECT sensor will be given. In the end are summary and discussion on future challenges, including re-engineering of ECT sensors.

https://iopscience.iop.org/article/10.1088/0957-0233/21/4/042001/pdf

Design of electrical capacitance tomography sensors

Advanced computational modelling for drying processes – A review

Mathematical modeling, computational fluid dynamics (CFD) analysis and electrical capacitance tomography (ECT) is used to develop an understanding of batch fluidized-bed drying, which is a complex air-solids, multiphase process. The mathematical model is based on a three-phase theory that describes the mass-and heat-transfer between the air and solids phases. The CFD model is based on a two-fluid model (TFM) approach, in which both phases are considered to be continuous and fully interpenetrating. The hydrodynamic parameters used to model the heat-and mass-transfer between the two phases are determined by a correlation approach, and the estimated parameters are incorporated into a user defined function (UDF) in FLUENT. The moisture diffusion in the air and solids phases is simulated using the user defined scalar (UDS) transport equation in the software package. ECT is used for the online solids concentration and moisture measurement, based on dynamic calibration. Comprehensive comparisons between results from mathematical modeling, CFD simulation and ECT measurement are presented, and are validated using experiment results. Online measurement of moisture content is compared with a static calibration. The mathematical model, CFD and ECT presented are being integrated into an online process control system for a batch fluidized-bed drying application in the pharmaceutical industry. © 2007 American Institute of Chemical Engineers AIChE J, 2008

Investigation of batch fluidized-bed drying by mathematical modeling, CFD simulation and ECT measurement

Online measurement and control of solids moisture in fluidised bed dryers

Application of electrical capacitance tomography in particulate process measurement – A review

Modelling of batch fluidised bed drying of pharmaceutical granules

In this study, operating conditions for a batch fluidised bed dryer are found that achieve the specifications on the final moisture level, while minimising the energy consumption. The operating conditions are obtained by solving an optimisation problem, where the specification for the final moisture content of the bed is included as a constraint. The optimisation is based on a lumped mechanistic model that describes the heat and mass transfer between solid, gas and bubble phases in the bed. Experimental validation of this model shows that it can be used to predict the particle moisture content and temperature profiles during the drying process and it is shown that the wall temperature has a major effect on the predictions. Compared to the standard approach of maintaining the inlet air temperature and flow rate at constant values, the optimal operating conditions for a lab-scale bed dryer reduce the energy consumption by 26% and shorten the duration of a batch by 20 min.

R.S. Raghavan

Papers

Investigation of batch fluidized-bed drying by mathematical modeling, CFD simulation and ECT measurement

Modelling of batch fluidised bed drying of pharmaceutical granules

Optimal operating conditions for a batch fluidised bed dryer

Modelling, measurement and analysis of fluidised bed dryer using ECT sensor

Investigation of fluidised bed drying by combination of CFD and ECT