(252623346) Understanding Bubble Hydrodynamics in Bubble Columns

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Experimental Thermal and Fluid Science 45 (2013) 6374

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Experimental Thermal and Fluid Science

journal h omepage: www.else vier.com/locate/etfs

Understanding bubble hydrodynamics in bubble columnsAmir Sheikhi 1, Rahmat Sotudeh-Gharebagh , Reza Zarghami, Navid Mostou, Mehrdad Al

Multiphase Systems Research Lab., Oil and Gas Processing Centre of Excellence, School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box 11155/4563, Tehran, Iran

a r t i c l e i n f o

Article history:Received 31 March 2012Received in revised form 19 July 2012Accepted 9 October 2012Available online 30 October 2012

Keywords: Gasliquid column Hydrodynamics Vibration inspection Pressure uctuations Frequency analysis Wavelet transform

a b s t r a c t

Compared to conventional gasliquid bubble columns, gasliquid columns including both gas and liquid ows have been investigated less due to their complex hydrodynamics and operational difculties. In this study, simultaneous non-intrusive methods of column shell vibration and pressure uctuation measure- ments were coupled with direct photography and image analysis for bubble characterization. Various sta- tistical and frequency analyses were conducted on the acceleration and pressure uctuations signals to determine their capability of interpreting bubble behavior inside the column. The standard deviation of vibration signals showed less sensitivity to bubble behavior compared to that of pressure uctuations. The skewness of vibration and pressure uctuations could detect bubble regime transition points at all studied gas and liquid velocities while vibration and pressure uctuations kurtosis could only detect the main transition point of the column at a moderate liquid velocity. It was found that besides regular statistical methods, the energy of pressure signals could predict bubble regime transition points success- fully. While vibration-based inspection showed more sensitivity to bubble size distribution (similar to the standard deviation of pressure signals), frequency analysis on pressure signals proved to be a strong representative of bubble Sauter mean diameter alteration by liquid ow variation at constant gas veloc- ities. Moreover, by means of energy-based discrete wavelet transformation, we dened the exact path- way of various sub-signal gradual evolutions throughout a wide range of operating conditions of gas and liquid velocities and captured regime transition points accordingly. The proposed methods in this work can be used for non-intrusive hydrodynamic characterization in industrial bubble columns. 2012 Elsevier Inc. All rights reserved.

1. Introduction

Gasliquid contactors are widely used in various industries such as chemical [14], biochemical, biotechnology [57], biomed- ical [8], petrochemical and rening [9,10], environmental, separa- tion and purication [1118], nanotechnology [19,20] and gas processing industries [2123]. Among such contactors, bubble col- umns, with or without liquid ow, are of a considerable impor- tance in numerous process units. Industrial plants which are dealing with physical [24] and/or chemical interactions between gas and liquid phases as well as gasliquidsolid [25] contactors prot from the ease of construction [26], high interfacial area and consequently high mass [27] and heat [28] transfer rates, sta- ble temperature [29], the ease of energy providence and the high liquid hold up [30] of bubble columns [31].Successful design, operation, scales up and optimization of bubble columns highly depends on the hydrodynamics of such

Corresponding author. Tel.: +98 21 6697 6863; fax: +98 21 6646 1024.E-mail addresses: amir.sheikhi@mail.mcgill.ca (A. Sheikhi), sotudeh@ut.ac.ir(R. Sotudeh-Gharebagh).1 Present address: Chemical Engineering Department, McGill University, Montreal, Quebec H3A 0C5, Canada.

contactors. Although various theoretical efforts to model two- phase gasliquid contactors have been undertaken [3238], new experimental approaches for hydrodynamic inspections are of great interests in industrial and R&D communities. Wall pressure uctuations were used to study the effect of various sparger geom- etries on bubble ow regimes in bubble columns [39]. Flow pattern and structure were investigated by means of pressure uctua- tions combined with particle image velocimetry [40]. Chaotic behavior of bubbles were studied using pressure signals and laser-phototransistor [41]. Also, bubbling-to-jetting regime transi- tion was investigated by plenum pressure uctuations monitoring [42]. Turbulence in the heterogeneous bubble regime was charac- terized by chaos analysis on pressure uctuations [43].Simonnet et al. studied the drag coefcient on the gas bubble swarm using laser Doppler velocimetry [44]. Harteveld et al. [45] and Olmos et al. [46] used laser Doppler anemometry for the accu- rate estimation of turbulence power spectra and ow regime tran- sition identication, respectively. Magnetic resonance imaging was utilized to characterize hydrodynamics of opaque multiphase sys- tems such as slurry bubble columns [47]. Also, particle image velocimetry was found to be able to dene bubble velocity and ow regimes inside a two-phase gasliquid column [4850]. Similar advanced, non-intrusive but expensive methods, such as

0894-1777/$ - see front matter 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermusci.2012.10.008

Nomenclature

A. Sheikhi et al. / Experimental Thermal and Fluid Science 45 (2013) 637465

ak approximation sub-signalAn amplitude in a time series at a certain frequency of fn(kPa or m/s2)CWT continuous wavelet transform

N data point number in a sampleL segment (window) number in a time seriesp indicating pressure uctuation experimentsPxx average power spectrum (kPa2/Hz or m2/s4 Hz)n 2 2 4de equivalent bubble diameter (m)

Pxx

power spectrum for each segment (kPa /Hz or m /s

Hz)dm,j minor length (smallest Feret diameter) of bubbles (m)dM,j major length (largest Feret diameter) of bubbles (m)dS Sauter mean diameter of bubbles (m)Dk detail sub-signalX(f) discrete Fourier transformDWT discrete wavelet transformE energy of PSDF (kPa2 or m2/s4)Ea energy of approximation sub-signal coefcient (kPa2 or m2/s4)ED energy of detail sub-signal coefcient (kPa2 or m2/s4)f desired frequency (Hz)i imaginary unitk sub-signal numberK kurtosis or the forth momentn counternj bubble number with equivalent diameter of de,j

PSDF power spectral density function (kPa2/Hz or m2/s4 Hz)q time-lag coefcientS skewness or the third momentt time (s)Ug gas velocity (m/s)Ul liquid velocity (m/s)v indicating vibration (acceleration) experimentsxn time series data (kPa or m/s2)

Greek symbolsd scale factorr standard deviations shift factorw mother wavelet function

computer-automated radioactive particle tracking [51], computed tomography [52], electrical resistance [53] and capacitance [54] tomography have also been used by a variety of researchers.Recently, Abbasi et al. suggested the non-intrusive measure- ment and analysis of vibrations in both time [55] and frequency [56] domains to characterize the hydrodynamics of gassolid uid- ized beds. They were able to dene main transition points inside the bed as well as bubble behavior using regular signal processing methods. Sheikhi et al. [57] have shown that vibration inspection can also be used as a reliable method for the hydrodynamic char- acterization of liquidsolid uidized beds. They predicted mini- mum liquid-uidization and solid-regime transition conditions. Yet, no effort has been made to characterize gasliquid contactors by means of simultaneous vibration and pressure uctuations analyses. The aim of this work is a critical comparative study on the applicability of vibration and pressure uctuation signal pro- cessing for the hydrodynamic characterization of bubble behavior inside bubble columns. Electrical signals obtained from vibration inspection and pressure uctuations were processed in time and frequency domains and the extracted information were used to determine the hydrodynamic state of a bubble column at a wide range of industrial gasliquid two-phase reactor operating conditions.

2. Materials and methods

2.1. Experimental set-up

The bubble column used in this study was made of a 2 m height Plexiglas column with an inner diameter of 0.09 m, presented in Fig. 1. Air at ambient temperature, produced by a compressor, was introduced into the 0.1 m high gasliquid engagement section from the bottom of the bed using a cylindrical porous ceramic air sparger with a 0.03 m diameter and 0.085 m length consisting of0.0001 m pores. Tap water, as the continuous phase, was pumped into the engagement section. Engagement section was lled with0.01 m glass beads for better mixing of air and water. The mixture of gas and liquid was then sent into the bed thr