Capacitance tomography

Many studies on the flow distribution in random packed beds have been reported in the literature. Mercandelli et al. [8] published a short review of the flow distribution work in random packed trickle bed, which includes the list of various techniques used to determine and quantify the flow distribution. Conventional methods include, for example, collecting liquid at the bottom of the column from different zones while advanced methods include tomographic techniques. Mercandelli et al. [8] used several techniques to quantify liquid distribution in columns of diameters up to 30 cm with three different distributor designs. They used global pressure drop measurements, global residence time distribution (RTD) of the liquid, local heat transfer probes, capacitance tomography and a collector at the bottom of the column. 

Examples of the use of tomographic techniques to determine gas/liquid distribution in stmctured packing is limited in the open literature. Mewes et al. [9] studied flow distribution in monoliths using capacitance tomography. A spatial... 

W. Warsito, L.S. Fan, Measurement of real-time flow structures in gas—liquid and gas—liquid-solid flow systems using electrical capacitance tomography (ECT), Chem. Eng. Sci. 56 (2001) 6455-6462. 

T. Loser, M. Geweke, D. Mewes, Measuring the Phase Distribution in a Transient Gas—Solid Flow Using Capacitance Tomography. 3rd Israeli Conference for Conveying and Handling of Particulate Solids, Dead Sea, Israel, 2000, pp. 15.11-15.15. 

The fluidized bed systems have been utilized extensively in many physical, chemical, petrochemical, electrochemical, and biochemical processes. Successful applications of the fluidization systems lie in a comprehensive understanding of hydrodynamics, heat and mass transfer properties, and mixing. Various non-intrusive measurement techniques, such as electric capacitance tomography and radioactive particle tracking technique, are available to advance the fundamental understanding of the microscopic and macroscopic phenomena of fluidization. Till date, the... 

The capacitance tomography scheme consists of sampling at low voltages (15 V) and high frequencies (1.25 MHz) all combinations of successive pairs of electrodes around the bed (Figure 3-46, left). The displacement current is sensed for each electrode pair, which is assumed proportional to the permittivity of the material between the electrodes. The wave length of the electric field is much greater than the diameter of the bed so that any queried... 

Beck, M. S., T. Dyakowski, and, Wang, S. J., Measurement of Fluidization Dynamics in a Fluidized Bed Using Capacitance Tomography, International Fine Particle Research Institute Annual Report, Nov. 30, 1995. 

Kuhn, F.T., J. C.Schouten, C. M. Van den Bleek, and B. Scarlett, Electrical Capacitance Tomography Applied to Fluidization Dynamics for Chaos Analysis, Second Internl. Particle Tech. Forum, 5th World Congress of Chemical Engineering, VI, AIChE, San Diego, CA., July 14-18, 1996, pp. 216-219. 

Du, B., Warsito, W., and Fan, L.S. (2006), Imaging the choking transition in gas-solid risers using electrical capacitance tomography, Industrial Engineering Chemistry Research, 45(15) 5384-5395. 

Gamio, J.C., Castro, J., Rivera, L., Alamilla, J., Garcia-Nocetti, R, and Aguilar, L. (2005), Visualization of gas-oil two-phase flows in pressurized pipes using electrical capacitance tomography, Flow Measurement and Instrumentation, 16(2-3) 129-134. 

Makkawi, Y.T., and Wright, P.C. (2002), Fluidization regimes in a conventional fluidized bed characterized by means of electrical capacitance tomography, Chemical Engineering Science, 57 2411-2437. 

Pugsley, T., Tanfara, H., Malcus, S., Cui, H., Chaouki, 1., and Winters, C. (2003), Verification of fluidized bed electrical capacitance tomography measurements with a fiber optic probe, Chemical Engineering Science, 58 3923-3934. 

Advanced measuring techniques With the advent of sophisticated imaging and velocimetry techniques, better understanding of the flow behavior and consequently a better prediction of regime transition is possible. The techniques used in the literature are listed below with the corresponding references. Readers interested in a specific technique may refer to these references Electrical capacitance tomography (ECT) (Bennett et al. 1999 Tapp et al. 2003) Electrical impedance tomography (EIT) (Tapp et al. 2003)... 

Ismail I, Shafquet A, Karsiti MN. (2011) Application of electrical capacitance tomography and differential pressure measurement in an air-water bubble column for online analysis of... 

Tomographic techniques allow to reconstruct the instantaneous particle holdup (porosity) of a 3D fluidized bed, thus allows to extract information especially on the bubble phase. Well-known techniques include electrical capacitance tomography. X-ray tomography, and magnetic resonance imaging (MRI). 

Malcus S, Pugsley TS. Investigation of the axial variation of the CFB riser hydrodynamics by means of electrical capacitance tomography (ETC). In Kwauk M, Li J, Yang WC, eds. Fluidization X. New York Engineering Foundation, 2001, pp 763-770. 

In capacitance tomography, the measurement setup consists of an array of electrodes installed around the external surface of the monitored system, and measurements of capacitance between various pairs of electrodes are performed, so that data is received on the capacitance in various directions within the system. Then, from the algorithm described above, the volume fraction of solids or gas within the system can be calculated. Examples of various sources for fluidized bed CTs are presented in Table 14. An example setup and results are shown in Fig. 43. 

Dyakowski T, Edwards R, Xie C, Williams R. Application of capacitance tomography to gas-solid flows. Chem Eng Sci 52(13), 1997. 

Rhodes M, Wang X. Stud5nng fast fluidization using electrical capacitance tomography. Werther J, ed. Proc 6 Conf CFB. Wurzburg, Germany, 1999, pp 891 896. 

Arko A., Waterfall R.C., Beck M.S., Dyakowski T., Sutcliffe P. and Byars M., Development of Electrical Capacitance Tomography for Solids Mass Flow Measurement and Control of Pneumatic Conveying Systems, Proceedings of the C World Congress on Industrial Process Tomography, 140 - 146, 1999. 

The industrial process tomography systems can be based on a number of measurement principles, including nucleonic, optical, acoustic, microwave, NMR and electrical methods [21] - each of them usually having a number of distinctive variants. From the practical point of view only a few are routinely used for investigating fluidised beds. This chapter will concentrate therefore on nucleonic transmission tomography (x-ray and y-ray tomography) and electrical capacitance tomography (ECT). 

Electrical capacitance tomography (ECT) belongs to a larger family of electrical impedance methods. Here, unlike in nucleonic tomography where, the photon beam attenuation is measured, the measured quantity is electneal impedance (i.e. conductance and/or capacitance) between all possible pairs of electrodes placed around the vessel or pipe periphery. 

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