Chemical Engineering and Processing 48 (2009) 120–125 Contents lists available at ScienceDirect Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep The Gaussian spectral pressure distribution applied to a fluidized bed M.R. Parise a,∗ , P.R.G. Kurka b,1 , O.P. Taranto a a School of Chemical Engineering, University of Campinas (UNICAMP), P.O. Box 6066, 13083-970 Campinas, SP, Brazil b Faculty of Mechanical Engineering, University of Campinas (UNICAMP), P.O. Box 6122, 13083-970 Campinas, SP, Brazil article info Article history: Received 31 August 2007 Received in revised form 21 February 2008 Accepted 22 February 2008 Available online 29 February 2008 Keywords: Gas–solid fluidized bed Defluidization Pressure fluctuations Fourier transform Gaussian distribution curve abstract The present work applies the methodology proposed by Parise et al. [M.R. Parise, O.P. Taranto, P.R.G. Kurka, L.B. Benetti, detection of the minimum gas velocity region using Gaussian spectral pressure distribution in a gas–solid fluidized bed, Powder Technol. 182 (2008) 453–458], as an alternative to the spectral analysis of pressure fluctuation measurements to find the region where the minimum velocity gas takes place in a gas–solid fluidized bed, that is, the zone where the bed is tending to defluidization. The technique is applied to analyze the effect of fixed bed height and particle density in defluidization conditions for particles of microcrystalline cellulose and sand. Tests are carried out for three fixed bed heights (0.15, 0.20 and 0.25 m) and two particle densities (980 and 2650 kg/m 3 ). Experiments show that the best conditions for identifying the defluidization zone are obtained with lower bed aspect ratios (H/D) and lower par- ticle density. The results indicate the high potential of the proposed method for industrial applications, especially for on-line control of gas–solid fluidized bed processes where the defluidization phenomenon needs to be detected and avoided. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Fluidization is an important technology employed in industrial processes, such as coal and biomass combustion, drying of solids, particles coating, material processing and biotechnology [1]. An adequate mixture of gas and particles is essential in fluidized bed processes. In practical terms, however, the mixing of particles may become inefficient during operation. The process of drying of solids or particles coating, for example, may suffer the undesired effect of agglomeration of particles due to the presence of mois- ture, subsequently followed by defluidization of the bed, which may cause critical situation leading to an inevitable process shut down. The superficial gas velocity is responsible for the formation and maintenance of the fluidized bed condition. If the velocity is not kept to a sufficiently high value, the defluidization of the bed may occur affecting the process. Such a phenomenon may happen so abruptly, that its detection by simple visualization is just not pos- sible. A more appropriate way to perform the identification of the minimum gas flow to avoid the defluidization is through the anal- ysis of pressure fluctuations, which are usually originated by the formation, rise and eruption of bubbles, gas turbulence, bed mass oscillation and bubble coalescence [2,3]. ∗ Corresponding author. Tel.: +55 19 3521 3895; fax: +55 19 3521 3910. E-mail address: parise@feq.unicamp.br (M.R. Parise). 1 Tel.: +55 19 3521 3175; fax: +55 19 3289 3722. Pressure fluctuations have been used to describe the fluidized beds characteristics, such as the quality of fluidization [3], bubble frequency [4], transition from bubbling to turbulent fluidization [5], differentiation of states of typical fluidization [6], the minimum fluidization gas velocity [7–10], and the minimum fluidization gas velocity region [11]. Normally, time series data originated from pressure fluctuations are treated by statistical analysis, by spectral analysis, and by chaos analysis [3]. Parise et al. [11] developed a methodology to identify the region where the bed is tending to defluidization, in order to be applied in gas–solid fluidized bed processes. This technique is based on Fourier transform and Gaussian distribution, using pressure fluc- tuation measurements. The objective of this work is to verify the influence of the fixed bed height and the solid density on the methodology proposed by Parise et al. [11], specifically for microcrystalline cellulose and sand particles. 2. Normal spectral distribution in the fluidization process The stages of fluidization carry different dynamic characteris- tics that can be observed in the spectral distribution of the plenum pressure. The dynamics of a fixed bed tends to be that of a bulk or “heavy” body, displaying lower frequencies in the pressure spectral range. The transition bed has a combination of the dynamics of bulk and light bodies, displaying a few frequencies that are higher in the spectral range, together with the characteristic low frequencies of the fixed bed behavior. 0255-2701/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2008.02.010