F.J. Dickin

Application of electrical resistance tomography to interrogate mixing processes at plant scale

This paper presents an application of electrical resistance tomography to the investigation of mixing processes at plant scale. An 8-plane 16-electrode ring sensor installed within a stirred tank with a 1.5 m inner diameter is described. Three-dimensional and non-stationary behaviour of mixing processes are illustrated by the images obtained simultaneously from eight axial levels along the tank height. The results illustrate air-core vortex detection, miscible fluid mixing and gas-liquid mixing processes.

ELECTRICAL RESISTANCE TOMOGRAPHIC SENSING SYSTEMS FOR INDUSTRIAL APPLICATIONS

Abstract The effects of electrode configuration and design are investigated using a grouped node finite element method (FEM). An estimated expression for an appropriate experimental/actual electrode-angle β is given for an electrode system modelled with grouped-node FEM or single-node FEM. Analyses and methods for reducing sensor noise are discussed in practical terms. Examples of ERT sensors for different applications are given, which comprise from one to eight sensing planes sized from 6 to 1500 mm in diameter. A novel image reconstruction method based on the sensitivity theorem is presented. The capabilities of ERT and its data interpretation and image processing are discussed for pipelines, hydrocyclones and mixing vessels.

Determination of composition and motion of multicomponent mixtures in process vessels using electrical impedance tomography-I. Principles and process engineering applications

Abstract A method of mapping composition profiles and phase boundaries within process reactors and pipelines using robust, low-cost and non-intrusive electrical sensors is described. The method involves utilising differences in electrical resistivity between phases in a disperse system to chart their momentary distribution in a cross-section of the vessel. Sensing electrodes are placed at known locations around the periphery of the vessel and the sensor signals are fed into a suitable algorithm to solve the boundary value problem. The algorithm solves Laplaces equation (∇·ϱ−1∇V=0) inversely: i.e. a computed image of the resistivity profile of the two-dimensional cross-section is formed using electrical measurements obtained from the sensing electrodes. The technique is analogous to that used in medical tomography, but the sensing system employed in this method is solid-state, of much lower cost and can be used in a process plant environment. Composition maps can be obtained in real time. The principles, scope and limitations of electrical impedance tomography and some practical details of its implementation on process-scale vessels are discussed and illustrated by two case studies.

Multi-phase flow measurements in powder processing

Abstract This paper describes recent advances in the design and application of a number of promising techniques that employ non-intrusive transducers to enable the interrogation of multi-phase particulate systems. Most of these techniques are based on electrical sensing methods which have the particular advantages of being non-hazardous, high speed, low cost, and utilise reliable solid-state devices that can operate independently of the opacity of the particulate dispersion. Consequently, it is possible to acquire data on phase concentration, velocity, mass flow and spatial distribution in fast-moving process pipelines or vessels by relating the data obtained from several transducers simultaneously. The principle of measurement techniques based on capacitance, impedance, ultrasonics, electrodynamics and pressure transducers are reviewed and then illustrated for specific powder processing applications involving (i) analysis of solid/liquid sedimentation processes, (ii) pneumatic conveying, (iii) slurry transport in pipelines and (iv) imaging of solid/gas, solid/liquid and liquid emulsions in real time. The limitations and future uses of such instrumentation for powder and materials handling operations are discussed.

Development of mixing models using electrical resistance tomography

Abstract The network-of-zones concept provides a simplified basis for constructing mixing models appropriate to tomography. For miscible single-phase mixing in three-dimensions, of the order of 10 4 volume elements (voxels) are needed to interpret mixing results from 16 element electrical resistance tomographic (ERT) sensing rings stacked axially. Experimental mixing and dispersion in passive scalar pulse tests can be acquired at video frame rates by ERT and simulated by integration of sets of first-order ordinary differential equations. Non-uniformities in gas hold-up and solids suspension can also be detected and quantified in three-dimensions by resistance tomography. Modelling the approximately axisymmetric mixing of these two-phase examples can be achieved by a much smaller number of algebraic equations. Tomographic modelling provides new insights into the fundamentals of mixing in a stirred vessel. The reduced computational requirements of networks-of-zones make real-time model-based control more feasible.

On Detecting Mixing Pathologies Inside a Stirred Vessel Using Electrical Resistance Tomography

Electrical resistance tomography (ERT) has the capability to resolve the 3-D conductivity field inside a stirred mixing vessel using multiple planes of axially spaced sensors. This capability has already been exploited to qualitatively image vortex formation and geometry, pseudo-stationary gas-liquid mixing and unsteady dynamic brine tracer mixing inside a plant scale (1.5 m) stirred vessel (Mann et al , 1997 1 ). This feature of qualitative imaging, arising from simplified reconstruction by back-projection, nevertheless can visualize key features of mixing characteristics without the need to resort to extensive iterations to converge on quantitative images. This approach is therefore useful in detecting and identifying pathological behaviour caused by equipment malfunction. Examples are presented for (i) misplaced gas sparger, (ii) inadvertent solids accumulation and (iii) displaced feed point behind a baffle.

Smoothness-constrained inversion for two-dimensional electrical resistance tomography

Two-dimensional image reconstruction in electrical resistance tomography (ERT) involves determination of the electrical conductivities throughout the cross section of the region being imaged. Quantitative image reconstruction in ERT is an ill-conditioned problem and therefore some sort of regularization is required in order to provide meaningful images. Typically, this is achieved using the Marquardt - Levenberg regularization. In this work, a new data acquisition system developed at UMIST, the Mark 2a, is briefly described and a smoothness-constrained regularization is presented, which is shown to be superior to the widely used Marquardt - Levenberg regularization in the presence of Gaussian noise in the collected data.

Air core imaging in cyclonic separators: implications for separator design and modelling

Abstract The control and stability of the air core and factors affecting the flow split to the underflow or ‘heavy’ product are two of the least understood aspects in the operation of hydrocyclone and dense medium separators. Existing methods for assessing these factors using conventional and tomographic measurement instrumentation are described. New results obtained using electrical resistance tomography (ERT) are described for a pilot-scale large diameter coal dense medium separator (LARCODEMS). Such data have important implications for the development of (a) empirical correlations to describe air-core characteristics as a function of operational variables (feed rate, feed slurry viscosity, feed density etc) and their effect on separation efficiency, and (b) computational fluid dynamics models of liquid and slurry flow in centrifugal separators.

Further development of a tomographic imaging system using optical fibres for pneumatic conveyors

This paper describes the further development of optical sensor hardware for a process tomography system in which emitters and detectors are used to exploit the optical characteristics of multiphase flow regimes. The optical arrangement is described and the importance of fibre beam position discussed. The proportion of the measurement volume interrogated by the beams is derived. The response of a single fibre is shown followed by a reconstructed concentration profile.

Using electrical impedance tomography for controlling hydrocyclone underflow discharge

Abstract This paper describes the results of a series of experiments using Electrical Impedance Tomography (EIT) to investigate the distribution of solids inside a hydrocyclone. A case study is presented for a 44 mm diameter hydrocyclone. Amongst the characteristics examined are: (i) identification of particle distribution inside the separator, (ii) air core formation as a function of the feed rate and solids concentration, and (iii) the manner in which the air core behaviour can be related to the type of underflow discharge (spray, rope).