T. Dyakowski

Applications of electrical tomography for gas–solids and liquid–solids flows — a review

Abstract This paper presents a review of electrical tomography methods for investigating, monitoring and controlling gas–solids and liquid–solids systems. The physical laws governing the electrical measurements and issues associated with image reconstruction are described in some detail. Experimental results, obtained for a number of case studies conducted in the pilot plant scale and industrial rigs, are presented. These include circulating fluidised bed, pneumatic and hydraulic conveyor, multiphase flow metering and hydrocyclone flow. Instantaneous images, captured with the speed up to 200 frames per second, illustrate how flow patterns vary, and reveal the dynamic behaviour of two-phase systems. Application of electrical tomography for control and fault diagnosis in industrial systems is addressed — the examples include dense pneumatic conveying and hydrocyclone performance.

Application of electrical capacitance tomography for measurement of gas-solids flow characteristics in a pneumatic conveying system

Transient three-dimensional multiphase flows are a characteristic feature of many industrial processes. The experimental observations and measurements of such flows are extremely difficult, and industrial process tomography has been developed over the last decade into a reliable method for investigating these complex phenomena. Gas-solids flows, such as those in pneumatic conveying systems, exhibit many interesting features and these can be successfully investigated by using electrical capacitance tomography. This paper discusses the current state of the art in this field, advantages and limitations of the technique and required future developments. Various levels of visualization and processing of tomographic data obtained in a pilot-plant-scale pneumatic conveying system are presented. A case study outlining the principles of measuring the mass flow rate of solids in a vertical channel is shown.

Application of capacitance tomography to gas-solid flows

The fundamental principles of electrical capacitance tomography (ECT) are presented. In particular, the recent developments in the use of a back-projection algorithm applied to solve an inverse problem are discussed. Experimental results obtained for monitoring a pneumatic conveying system and bubbling fluidization are reviewed and revised data are presented. The performance of the ECT is assessed in quantitative and qualitative terms. The use of ECT to provide novel on-line measurement information suitable for modelling or control purposes is illustrated.

Process tomography applied to multi-phase flow measurement

This paper presents the state of the art in measuring multi-phase flows by using tomographic techniques. The results presented show a wide range of industrial applications of process tomography from the nuclear and chemical to the food industry. This is illustrated by examples of the application of various tomographic sensors to the measurement of geometric or kinematic parameters of multi-phase flows. An application of process tomography for the validation of computational fluid dynamic models and the possibility of constructing a flowmeter for multi-phase flow are addressed.

Modelling turbulent flow within a small-diameter hydrocyclone

A revised approach to modelling turbulent flow in small-diameter (10–44 mm) hydrocyclones is presented and tested quantitatively against experimental measurements. Some of the limitations inherent in the existing approaches, such as the anisotropy of turbulent viscosity and the non-linear interaction between mean vorticity and mean strain rate have been overcome utilising a k—e model coupled with equations for calculating normal components of Reynolds stresses. The model predicts correctly the behaviour of a conventional water-fed hydrocyclone of various geometries, and has also been used to examine how the flow field can be modified by the insertion of a solid rod in place of the central air core in order to sharpen the classification performance of a hydrocyclone treating particulate suspensions.

Real time capacitance imaging of bubble formation at the distributor of a fluidized bed

Abstract This paper describes the use of a transputer-based 8-electrode capacitance tomography system for imaging gas bubbles in a fluidized bed in the vicinity of an air distributor plate. The quantitative results show how the solid concentration distribution varies as a function of time for three different flow regimes: bubbling, slugging and the transition to turbulent. Bubble shape, length and coalescence can be observed.

Industrial Monitoring of Hydrocyclone Operation using Electrical Resistance Tomography

Abstract The development and application of electrical resistance tomography for on-line auditing of an industrial hydrocyclone separation is reported. The work demonstrates the retrofitting of electrodes into a commercially available separator and their use in laboratory, pilot plant scale investigations of clay refining. A number of new and significant applications are described including: the development of methodologies to allow observation of the occurrence of faults in underflow discharge (spraying, roping, blockage); accurate measurement of the air core size for different operational conditions; direct calculation of solid concentration profiles based on parametric reconstruction of conductivity data in three dimensions.

Direct flow-pattern identification using electrical capacitance tomography

Abstract Non-invasive techniques such as process tomography are beginning to make promising contributions to control systems and are well fitted for flow pattern identification in opaque pipes or conduits. Even though process tomography and electrical capacitance tomography are usually associated with imaging, the image in itself can very seldom be directly used for control purpose. The path to be followed from data collection to flow-pattern identification often involves an image reconstruction phase, followed by some form of image processing and analysis. When online control is concerned, this approach may be too time consuming. This paper proposes a direct approach that discriminates between an annular and stratified flow pattern without the need for imaging. The model relies on capacitance data collected from an experimental rig, and extracts the information content from these raw-data measurements. Flow-pattern identifiers are kept simple and are designed to be rapidly evaluated for online observation purpose. Additional information and monitoring, such as the phase distribution or the detection of a slug building process, can also be obtained from the data by using specific electrode pair combinations.

Solids Flow Imaging and Attrition Studies in a Pneumatic Conveyor

Abstract This work describes the first application of a non-invasive capacitance tomographic technique to monitoring the behaviour of industrial-scale pneumatic conveyors. Dynamic images of the solids distribution within a pipe have captured the movement of slugs in dense-phase solids conveying, and the presence of saltation in dilute-phase conveying. It has also been possible to acquire images downstream of a pipe bend and to use this information to establish a suitable location for obtaining a representative sample of a material stream. The information provided by cross-sectional images facilitates the validation of theoretical models and ultimately will allow improved design as the sensitivity and resolution of the technique are enhanced. A study of particle (sea salt) attrition is reported using a conveying line 32 m in length. It is demonstrated that particle breakage can be described as a function of the conveying velocity and solids-gas loading factor. Particle breakage is described in terms of the mass-specific surface area, and is seen to increase with conveying velocity. Particle breakage rates are found to be inversely related to the solids loading factor.

On-line monitoring of dense phase flow using real time dielectric imaging

Abstract Two significant advances in the development of on-line tomographic imaging methods for visualization and quantification of powder flow are illustrated based on pilot-plant studies of pneumatic conveying and fluidization. The dynamic behaviour of a circulating fluidized bed is described, in which the thickness of the downflowing solids along the wall is measured. The data enables a more realistic (non axi-symmetric) model or heat transfer to the wall to be formulated. The application of electrical capacitance images to derive, in real time, visualization of moving stratified powder beds and slugs and subsequent analysis of image data for identification of the prevailing flow structure for control purposes is described.