C.G. Xie

Design of capacitance electrodes for concentration measurement of two-phase flow

Capacitance electrode systems for the measurement of the volumetric concentration of two-phase flows have inherently non-uniform sensitivity distributions over the pipe cross section and therefore have different responses to different flow regimes (e.g. core, annular and stratified flows). In processes with varying flow regimes, this can lead to significant measurement errors. An electrode system which has a uniform sensitivity distribution and consequently a response independent of flow regime is therefore desirable. The system performance parameters of surface electrode systems used with stray-immune measurement transducers have been characterised quantitatively using a two-dimensional finite-element model. The electrode system design parameters have also been identified and their effects on system performance parameters have been investigated. As a result, optimum electrode designs are proposed, with responses to different flow regimes having a minimum deviation from linearity.

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.

Electrical tomography techniques for process engineering applications

Abstract This paper summarizes the characteristics of electrical tomography techniques, highlights their current applications and gives an indication of their future applications in the chemical process engineering environment.

Experimental techniques for monitoring sedimentation in optically opaque suspensions

Abstract A review of established and developing techniques for monitoring solid flux, and in some cases particle size, is presented for sedimenting multi-phase systems that are opaque and therefore unsuited to interrogation using conventional optical methods. The techniques are classified according to the physical principle employed for the measurement, relative cost of the equipment, precision, versatility, ease of operation, state of development and their ability to function in non-invasive or invasive modes. Requirements and limitations of these techniques are discussed with respect to their application to colloidal dispersions, emulsions and to mixed colloidal/non-colloidal suspensions. Practical examples of phase-flux determinations are given for devices involving (i) ultrasonics, (ii) capacitance, (iii) inductance and (iv) direct measurement by physical interruption and sampling of a sedimentation process.

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.

Capacitance-based instrumentation for multi-interface level measurement

An electrical capacitance method is described for the measurement of multi-interface levels of gas/liquid/solid, including the foam layer. It uses a novel multi-electrode capacitance sensing element. A capacitance transducer based on the charge/discharge principle is used, which is stray immune and capable of operating at high frequencies to reduce the effects of the liquid conductivity on the measurement. The presence of foam is detected by processing the fluctuating component of the capacitance signal, which reflects the inherently unstable feature of the foam, whereas the multi-interface levels are reconstructed by processing the steady-state component of the measured capacitances.

A novel sedimentation analyser

The authors describe a novel instrumentation system capable of measuring the particle volume fraction and solids flux of optically opaque suspensions undergoing sedimentation. Two independent but complementary measurement techniques-capacitance and pressure-are integrated into the system.

Sedimentation behaviour of complex polydisperse suspensions

Abstract Experimental data were acquired using a novel sedimentation analyser equipped with pressure and capacitance transducers, with the objective of developing a process-orientated model to enable prediction of the sedimentation behavoiur of concentrated polydisperse particulate mixtures within the size range 1 – 150 μm. Parallel experiments were performed in a batch vessel in which zones of the sedimentation column could be sectioned and removed for analysis. The sensitivity of the solid concentration profiles, solid flux and sediment characteristics to changes in the suspension properties (volume fraction, size distribution and particle zeta potential) is demonstrated and quantified using an empirical model. The experimental results are compared with a simulation of the sedimentation process using established hydrodynamics-based correlations. Hence, the regimes under which the use of hydrodynamic considerations alone fail in describing the actual sedimentation behaviour are identified. The physical mechanisms, significance and inter-relationships between the effects of size polydispersity, particle charge and solids concentration are discussed.

Tomographic Systems for Measurement and Control of Minerals Processing Plants

Abstract Improvements in the control of minerals processing plant may be possible by using tomographic techniques to measure the quantity and spatial distribution of material at certain points in the process. Electrical tomography can provide real-time (100 frames/sec) distribution maps with a resolution of 1 in 20 lines (400 pixels over cross-section) which can be used for i) model verification, ii) measuring material concentrations at critical locations within process vessels as part of an overall control scheme. Case studies are presented on i) electrical capacitance tomography for non-conducting process fluids (e.g. for use in pneumatic conveyors), ii) electrical resistance tomography for electrically conducting fluids (e.g. for use in hydrocyclones).