Measuring the Gas-Solids Distribution in Fluidized Beds -- A Review

Abstract

In gas-solid fluidized beds, the distribution of the particles typically varies both in time and space. Since the gas-solids distribution and its variation have a strong influence on the performance of a fluidized bed for a given process, it is very important to accurately measure the gas-solids or voidage distribution. This paper reviews techniques for measuring the voidage distribution in gas-solid fluidized beds, with a focus on the developments during the last ten years. We will treat subsequently direct visualisation, tomography, optical probes, capacitance probes, and pressure measurements.Dense gas-solids flows are typically opaque to visible light. This makes optical techniques only of limited use in dense gas-solids flow. However, direct visualization can be useful for very dilute systems, pseudo 2-D beds, and the outer layer of dense, 3-D systems. Tomography is frequently used to obtain the voidage distribution in a horizontal cross-section of the bed. Electric capacitance tomography is fast, but its spatial resolution is limited and image reconstruction is still troublesome. Although some steps forward have been made, research is continuing at this point. For nuclear (X-ray and gamma-ray) tomography, the image reconstruction is much easier and the spatial resolution better, but its temporal resolution is typically much lower. Therefore, research efforts for nuclear tomography are mainly aimed at increasing the measurement frequency. Optical probes determine the voidage as a function of time in a small measurement volume, either by the degree of reflection or by the degree of transmission of a light bundle. Capacitance probes determine the voidage as a function of time in a small measurement volume by measuring the dielectric permittivity of the gas-solids suspension in the measurement volume. Both optical and capacitance probe techniques are reasonably well-developed; the current research effort spent at improving them seems * We would like to thank M.O. Coppens, J. Nijenhuis, R.W.S Bohlken, and M.A. Wormsbecker for useful suggestions during preparation of this manuscript. limited, especially for capacitance probes. Time-averaged pressure measurements are commonly used to determine the average bed density and bed height. By sampling the local pressure at a sufficiently high frequency (typically in the order of 200 Hz), much more information can be obtained about the fluidized bed hydrodynamics. However, obtaining quantitative voidage data from pressure fluctuations measurement remains a difficult task; in-bed pressure fluctuation (and acoustic) measurements are mostly used to determine changes in the voidage dynamics and distribution.

Use of α -shapes for the measurement of 3D bubbles in fluidized beds from two-fluid model simulations

“…Such techniques discriminate gas bubbles from the emulsion phase by means of pixel intensity in the solids volume fraction distribution maps and have been widely reported in literature [2][3][4][5][6]. Applied to 3D simulations, the DIA algorithm detected bubble contours in 2D void fraction maps traced through the 3D bed center (Figure 4.a).…”

“…The main drawback of these techniques is the need of visual access. Since dense gas-solid flows are typically opaque to visible light, this limits the use of the technique to 2D systems although most of the fluidized beds are cylindrically shaped [5]. To circumvent this limitation, some techniques were developed to characterize experimental 3D beds by means of non-intrusive tomography (electric capacitance tomography [6] or nuclear tomography [7]), particle tracking techniques [8][9][10], optical and capacitance probes [11,12] or pressure transducers [13].…”

Acoustic chemometrics for on-line monitoring and end-point determination of fluidised bed drying

“…However, monitoring methods based on pressure measurements still remain as the conventional and most used monitoring techniques [8]. One of the drawbacks with pressure sensors is the intrusive deployment.…”

“…One of the drawbacks with pressure sensors is the intrusive deployment. Intrusive sensors are highly prone to blockage and thus it is possible to acquire pressure signals that are nonrepresentative of the hydrodynamics in the bed being monitored [8]. Furthermore, among the PAT approaches adapted for fluidised bed monitoring, on-line techniques are preferred because of the non-invasive sensor probes.…”

“…However, adaptation of this technique and other acoustic methods for fluidised bed characterisation is still very low. This fact can be seen in a critical review on the various experimental measurement principles applied in fluidisation technology as presented by Omen and Mudde [8]. The experimental techniques according to the review included direct visual inspection during fluidisation, tomography, optical probe, capacitance probe, pressure measurements and acoustic measurements.…”