Electrical capacitance tomography for gas–solids flow measurement for circulating fluidized beds

Gas–solids flows in the risers of circulating fluidised beds (CFBs) and cyclones exhibit complex physical behaviour, such as local backflow and recirculation. The difficulties in accurate measurement of gas–solids flows stem from various flow regimes, which exist in multi-phase flows in pipelines and vessels. It is necessary to investigate the solids’ fraction profile, flow regime identification, image reconstruction, flow acceleration and flow velocity. Electrical capacitance tomography (ECT) is regarded to be a successful technology for imaging industrial processes containing dielectric materials. ECT would help understanding of gas–particle interaction, particle–boundary interaction and the influence of gas on the solids’ flow turbulence.The first part of this paper covers some new developments in ECT, i.e., algorithms for 3D image presentation and on-line iterative image reconstruction. The second part presents a novel non-intrusive technique for measuring axial and angular velocities. Theoretical and experimental studies, carried out using cross-correlation techniques in a cyclone separator dipleg, confirm the feasibility of on-line velocity measurement. Experimental results from various gas–solids flow facilities, CFB and cyclone, are presented.     

Imaging conductive materials with high frequency electrical capacitance tomography

Electrical capacitance tomography (ECT) is known as an imaging technique for dielectric permittivity imaging. A novel ECT sensor model at a high excitation frequency is proposed to examine the capability of the ECT system to image both conductivity and permittivity contrasts. The proposed model uses a complex impedance forward model for the ECT system. This new model indicates that in higher excitation frequency both conductive and dielectric imaging may be feasible. Normally, capacitance tomography is designed for the measurements of imaginary part and resistance tomography is used to take the measurements of real part. The drawback of a typical capacitance tomography at a low excitation frequency, such as 200 kHz is that it cannot be used to measure the conductive phase of a conductive/dielectric mixed fluid, e.g. the gas/water flow. By increasing the excitation frequency, the capacitive impedance of the conductive material decreases and dielectric phenomena of the conductive fluid dominates so that it is possible to use capacitance tomography to characterise the dielectric/conductive flows. This paper presents a development of capacitance tomography with a high excitation frequency in measuring the gas/liquid mixture i.e. gas/water and gas/oil multiphase distributions. Both theoretical and experimental results are presented to verify this feasibility study.     

Theoretical study of vertical slug flow measurement by data fusion from electromagnetic flowmeter and electrical resistance tomography

Electrical resistance tomography (ERT) can be used to obtain the conductivity distribution or the phase distribution of gas/liquid flows (e.g. slug flow). Using proper parameter models and flow regime identification models, the measurement of phase size, void fraction, and pattern recognition can be realized. Electromagnetic flowmeters have been used to measure conductive single-phase liquid flows. However, neither ERT nor electromagnetic flowmeters (EMF) can provide accurate measurement of gas/liquid two-phase flows. This paper presents an approach to fuse the information from ERT and an electromagnetic flowmeter. A model for the measurement signal from the electromagnetic flowmeter has been developed based on the flow pattern and the phase distributions, which are obtained from the reconstructed images of ERT, aiming to reduce the measurement error of the electromagnetic flowmeter and enhance the measurement accuracy. Through the simulation research of virtual current density distribution, the feasibility of fusion of electromagnetic flowmeter and ERT to measure gas/liquid two-phase vertical slug flow is verified. By theoretical analysis, the relationship between the output of electromagnetic flowmeter and flow parameters is established. The electrical potential difference of the electromagnetic flowmeter, average velocity, volume flow rate and gas void fraction between the bubble size and location are also investigated. The fusion approach can be used to measure vertical slug flows.     

Measuring solid cuttings transport in Newtonian fluid across horizontal annulus using electrical resistance tomography (ERT)

The in-situ visualization of flow in the opaque pipes and annulus is a major cause of concern for studying hole cleaning issues and multiphase flow patterns in the oil and gas industry. Electrical resistance tomography is one of the latest non-intrusive technologies, which can provide real-time cross-sectional images of multiphase flow patterns in opaque pipes and annulus. Most studies conducted using the ERT system in the area of multi-phase flow is limited to opaque pipes and two-phase systems. Therefore, in this work is the suitability of the ERT technology has been tested to examine the three-phase flow (air-liquid-solid) and solids cuttings transport in a horizontal flow loop annulus system at wide operating conditions. The effect of different eccentricities (0–50%), inner pipe rotation speed (0–120 RPM), liquid flow rates (164–373 kg/min) and air input pressure (0–0.8 bar) on solids concentration in the specified ERT zone was examined using ERT concentration tomograms.     

双层电容层析成像传感器及应用

电容层析成像在气力输送过程中具有十分重要的应用地位，可进行固相浓度分布测量，流型判断。利用双层电容层析成像传感器不仅可以进行浓度测量还可以进行相关测速。基于这种思想，设计了一种双层电容传感器，与电容测量系统结合，用于气力输送过程固相浓度和速度的测量。     

Independent component analysis of interface fluctuation of gas/liquid two-phase flows — experimental study

It is important to understand the behaviour of two-phase flows in industry. This paper presents a study of the interface fluctuation between gas/liquid two-phase flows in a horizontal pipeline. Having obtained the data of a gas/liquid flow by electrical resistance tomography (ERT), an independent component analysis (ICA) method can be applied not only to extract the flow regime information but also the interface fluctuation of the flow. The efficiency of ICA with the ERT data has been assessed by experiment. The independent components have been interpreted by comparing the obtained independent components with the reconstructed images by ERT, showing that ICA is not only effective in extracting flow regime information, but also provides the fluctuation of each individual phase and the interface between the two phases. Without modelling the forward problem, this method can be applied to other electrical tomography modalities.     

Velocity measurement for two-phase flows based on ultrafast X-ray tomography

The ultrafast electron beam X-ray tomography scanner ROFEX is used for the investigation of multiphase flows. Its functional principle allows us to obtain sequences of cross-sectional flow images, which shows local attenuation properties of the flow. Hence, the X-ray CT images mainly reveal the shape and interfaces of flow constituents, such as gas, liquid and solids via their X-ray contrast. It is, however, more difficult to obtain velocity information from multiphase flows. In this article we discuss different methods to extract information on the velocities of particles or interfaces as well as for continuous phase. For disperse phase velocity measurement, e.g. in gas–liquid or gas–solids flows, we employ cross-correlation based techniques using two imaging planes. Apart from the standard cross-correlation technique we developed a method and algorithm, which is capable to identify identical bubbles in the two planes giving us a unique Lagrangian particle-related velocity information. Eventually we give an example of velocity measurement in the continuous liquid phase using an X-ray contrast agent.     

Tomography for multi-phase flow measurement in the oil industry

Electrical capacitance tomography (ECT) is regarded as a successful method for visualising cross-sectional distribution and measuring multi-phase flows (MPFs). Because of the “soft-field” nature of ECT and the non-linear relationship between electrical measurements and the permittivity of the measured material, image reconstruction for ECT is complicated. However, ECT offers some advantages over other tomography modalities, such as no radiation, rapid response, low cost, being non-intrusive and non-invasive, and the ability to withstand high temperature and high pressure. In principle, ECT can deal with the complexity of MPF measurement by explicitly deriving the component distributions at two adjacent planes along a pipeline. Images of the component distributions can be cross-correlated to obtain the velocity profile of the flow. Multiplying the component concentration and velocity profiles yields a measure of volumetric flow rate for each phase accurately. This paper covers the development of ECT for MPF metering and oil separator in the oil industry. The principal strategies and technologies that may be used to measure three-phase flows will be discussed, and the status of currently available tomography solutions will be reviewed.     

A novel tomographic sensing system for high electrically conductive multiphase flow measurement

Electrical resistance tomography (ERT) has been widely applied in order to extract flow information from various multiphase flows, e.g. the concentration and velocity distributions of the gas phase in gas–water two phase flows. However, the quality of measurement may become very poor from a multiphase flow whose continuous phase has a considerably high electrical conductivity, e.g. seawater (5.0 S/m), using a conventional current-injected ERT system. It is known that a large current excitation is necessary in order to enhance the measurement sensitivity. In practice, it will be very challenging to build a current source with a large amplitude (more than 75 mA) and a high output impedance at a high excitation frequency. This paper presents an implementation of an ERT system with a voltage source and current sensing to overcome the limits of the current source. The amplitude of the current output can reach more than 300 mA. A logarithmic amplifier is used to compress the signal’s dynamic ranges from 18.32 dB to 1.66 dB. The structure and features of this system are presented in this paper and the performances of key circuits are reported. Finally the experimental results from a highly conductive flow (1.06 S/m) are analysed and compared with the measurements obtained from a low conductive flow.     

Visualisation of gas–oil two-phase flows in pressurised pipes using electrical capacitance tomography

Electrical capacitance tomography (ECT) was used to image various two-phase gas–oil horizontal flows on a 7.62 cm (3 in.) pressurised test loop. ECT is a novel non-invasive technique for imaging mixtures of electrically non-conducting substances. One of its most promising applications is the visualisation of gas–oil flows. This work presents a series of imaging experiments using a pressure-resistant ECT sensor. By varying the oil and gas flow rates, different flow regimes were established in the test loop. ECT images were obtained for each case and compared with (a) the flow observed through a transparent section in the loop and (b) the prediction of the Taitel–Duckler flow map. The results confirm the suitability of ECT for imaging gas–oil flows.     

Dual-plane ultrafast limited-angle electron beam x-ray tomography

Electron beam x-ray tomography is an imaging technique, which can provide cross-sectional images of an object of interest with about 1 mm spatial resolution at frame rates of up to 10,000 frames per second. As a non-intrusive method it is especially suited for studying multiphase flows. For this purpose we devised an experimental limited-angle scan setup which utilizes linear beam deflection to generate radiographic projections. This setup was employed in the study of gas–liquid flow in an experimental flow loop operated at different liquid and gas flow rates. Electron-beam tomography images were compared with image data of a wire-mesh sensor. The latter is a fast but intrusive imaging device which is commonly used in gas–liquid flow imaging and achieves comparable frame rates but at lower spatial resolution. As a novelty we implemented a dual-plane limited-angle electron beam x-ray tomography which allows us to gain information about the phase velocities using cross-correlation data analysis.     

Measurement of the solids volume fraction and velocity distributions in solids–liquid flows using dual-plane electrical resistance tomography

This paper describes a dual-plane Electrical Resistance Tomography (ERT) system for measuring the local solids volume fraction distribution and the local solids axial velocity distribution in solids–liquid flows. The paper also describes a local, intrusive conductivity probe system for providing reference measurements of the distributions obtained using the dual-plane ERT system. Experiments were performed using both the ERT system and the local probe system in vertically upward and inclined solids–water flows. Good agreement between the two techniques was observed. The local solids volume fraction distribution and the local solids axial velocity distribution obtained using the ERT system enable good estimates to be made of the mean solids volume fraction, the mean solids velocity and the solids volumetric flow rate.     

Gamma-ray tomography applied to hydro-carbon multi-phase sampling and slip measurements

Gamma-ray tomography is a technique well suited to visualize gas void fraction distribution in two-phase flows. The liquid phase considered in this paper is a homogeneous mixture of oil and water. Gamma-ray tomography will be used to qualitatively visualize the distribution of gas in the flow, and also to provide more quantitative average void fraction measurements. The subject treatment is practical and experimental with a primary focus on multiphase sampling. Experimental results for total average void fraction are compared to the drift–flux model for two-phase flow by comparing measurements with the calculated slip.     

Adaptive filtering techniques for velocity estimation of tomographic electric capacitance signals of two-phase gas/oil flows

The development of adaptive real-time flow velocity estimation algorithms for two-phase flows can contribute to monitoring the pipelines of various complex processes, such as energy, chemical, petroleum and nuclear industries. Among the different non-invasive tomography techniques, electrical capacitance tomography (ECT) is gaining increasing attention for its potential use in real-time imaging and characterization of multiphase flow systems. The nature of ECT signals for two-phase flows can significantly degrade the velocity estimation process with cross-correlation approaches. We address the unique challenges of such signals and propose a preprocessing technique to improve the performance and robustness of the velocity estimation algorithm. Two adaptive filters are used to estimate the velocity of a two-phase type flow. A least mean square (LMS) and a fast block LMS (FBLMS) are used to model the time delay between the two signals captured by the twin sensor (ECT). Performance of the proposed technique is assessed by applying it to ECT data obtained from an experimental flow rig. The computed estimates are then compared with the calculated velocity from tracking motion of bubbles captured by a high speed camera monitoring the two phase flow in the pipe. Results show that the proposed technique provides consistent results across various flow patterns, and is advantageous compared to cross-correlation based techniques, specially for chaotic flow conditions. Furthermore, the proposed estimation algorithms can be applied to other electric based tomographic techniques.     

Effect of IR Transceiver orientation on gas/liquid two-phase flow regimes

Multiphase flows play a vital role in many industrial and naturally occurring processes. Recent trend of miniaturization in mini/micro fluid reactors, compact heat exchangers and micro thrusters requires a thorough knowledge on multiphase flow phenomena in mini/micro channels. The present work is focused on the effect irradiation behavior of infrared rays (IR) during gas liquid two phase flow consisting of thin liquid films inside a mini channel. The influence of size and shape of the slug regime and liquid film thickness on IR rays is analyzed with COMSOL Multi physics package. Experiments are carried out in a 2.5 mm diameter borosilicate glass tube with wall thickness of 0.3 mm. The refraction and transmittance behavior of IR rays on slug and bubbly flow is studied by analyzing the Current-time output of an IR photodiode kept at different angles with the test section. The results are found to be in good agreement with experimental image processing technique and COMSOL results. The results obtained will be useful for designing of IR sensor arrays sensitive to multiphase flows. It can also be used for measurement of liquid film thickness with proper calibration.     

Data fusion in dual-mode tomography for imaging oil–gas two-phase flow

Process tomography (PT) techniques have been developed rapidly for visualizing the internal behavior of industrial processes, e.g. multi-phase flow measurement. Most of tomography systems employ a single measurement technique, such as computerized tomography (CT), optical tomography (OT), electrical resistance tomography (ERT) or electrical capacitance tomography (ECT). It is now possible to fit two or more tomographic systems to an industrial process. Detailed information from different modalities can be gained by inspection of separate tomographs, and the advantage of the strongest features provided by each unit can be taken. A combined tomogram can be produced of superior quality to any of the separate tomograms. To maximize the information available from the combined tomographic system, data fusion is the better option. In this paper, a dual-mode tomography system based on capacitance sensor and gamma sensor was developed to capture oil–gas two-phase flow. The two modalities can work at the same time. Two fusion methods, namely image fusion method and data fusion method, are proposed. Both simulation and static experiments for oil–gas two-phase flow were conducted. The reconstruction results of different fusion methods and modalities were compared and discussed.     

A reference conductivity fitting method for ERT system to achieve void fraction of gas/liquid flow

The void fraction is one of the key parameters in the measurement of gas/liquid two-phase flow. It can be derived from the absolute conductivity distribution based on Maxwell׳s theory. With Electrical Resistance Tomography (ERT) technology, the absolute conductivity distribution is obtained by multiplying the relative conductivity image with the reference conductivity which is conventionally the liquid conductivity of a gas/liquid flow. Unfortunately the liquid conductivity is not always available. Therefore, a conductivity fitting method is proposed in this paper, to find an optimal reference conductivity, which will be used in substituting the liquid conductivity to reconstruct the quasi-absolute conductivity image. The optimal reference conductivity fitting method is proposed and validated by simulation and experiments under certain flow regimes, e.g. slug flow, annular flow and bubbly flow. The simulation and experimental results show that, independent from prior-knowledge, the fitted quasi-homogenous conductivity is close to the average conductivity of the sensing field. It also leads to a much more accurate estimation of void fraction than the conventional method using liquid conductivity as the reference. With the proposed method, the ERT technique can play a more significant role in the measurement of multiphase flow (MPF).     

Adaptive calibration of a capacitance tomography system for imaging water droplet distribution

A highly sensitive electrical capacitance tomography (ECT) system based on an HP4284 impedance analyser has been developed and used to quantify low concentration multi-phase flows in wet gas separation processes. The system hardware provides high accuracy (0.05%) and high resolution (10⁻¹⁷ F). The sensor was calibrated in an environmental chamber with solid samples of known permittivity over ranges of temperature and humidity. Adaptive calibration and adjacent electrode pair correction techniques were applied to image very low concentration profiles. This paper describes the techniques used and presents the experimental results obtained from a test flow rig called Twister, which has been designed to separate liquid droplets from wet gas streams. The test results over a range of operating conditions (20–95% humidity) demonstrate that the ECT system is capable of reconstructing clear images of the droplet distribution inside Twister. Changes as small as 1 gWater/kgAir in the form of liquid droplets were detectable. It has also been shown that the concentration of the condensable phase can be estimated quantitatively within 20% in comparison with the reference measurements.     

Characterization of flow homogeneity downstream of a slotted orifice plate in a two-phase flow using electrical resistance tomography

Two-phase flows are complex and unpredictable in nature, commonly encountered in a majority of fluid transport systems. The accurate measurement of two-phase flow is critical for a wide range of applications from wet stream to multiphase flows. There are different methods to meter two-phase flow in various industries. One approach is to produce a flow meter that does not require the individual flow components to be separated and measured separately. This goal can be met if a homogenized mixture is produced which can be measured by a standard single phase flow meter. The slotted orifice plate was invented as a flow meter for single phase flows, it is independent upon upstream flow conditions. Slotted orifice plate flow meter's utilization in two-phase flow revealed that it is highly capable of working as a flow conditioner transforming most of the multiphase flow regimes into a fairly uniform mixture. This study measures how the relative homogeneity of an air/water mixture varies downstream of the slotted plate in a horizontal pipe for various upstream conditions including elongated bubble and slug flow regimes using electrical resistance tomography (ERT). According to this study, the optimal location with a maximum homogeneity was determined to be between 1.5 and 2.5 pipe diameters downstream of the slotted orifice plate. This indicates that placing a slotted orifice plate at the obtained distance upstream of another flow meter such as a venturi coupled with a density measuring device like a radiation based densitometer or an electrical impedance device will help in obtaining accurate multiphase flow rate measurement.     

Analysis of crude palm oil composition in a chemical process conveyor using Electrical Capacitance Tomography

In this work, we aim to analyse the capability of using a 16-segmented Electrical Capacitance Tomography (ECT) sensor system to monitor the internal composition of a chemical process conveyor that carries crude palm oil (CPO) multiphase flow. The source used to excite the electrodes is a differential potential, instead of the conventional single potential source, in order to obtain an improved sensitivity of the sensor, especially in the central area of the pipe. This system aims to recognise the phase concentration of the flow. The attained concentration profile that is received from the capacitance measurements is capable of providing an image of the liquid and liquid mixture in the pipeline, making the separation process (between oil and liquid waste) much easier and the CPO's quality can be dependably monitored. Experimental results and analysis are presented, and the new excitation technique is shown to provide better sensor sensitivity in the central pipe area. The visualisation results deliver information regarding the flow regime and concentration distribution in a two-phase flow-rate measurement system incorporating a liquid flow-measuring device. The information obtained will assist the design of process equipment, and the verification of existing computational modelling and simulation techniques.