Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 81:951–957 (2006) DOI: 10.1002/jctb.1475 Gas hold-up estimation in bubble columns using passive acoustic waveforms with neural networks Waheed A Al-Masry 1∗ and Adel Abdennour 2 1 Department of Chemical Engineering, King Saud University, Riyadh, Saudi Arabia 2 Department of Electrical Engineering, King Saud University, Riyadh, Saudi Arabia Abstract: Passive acoustic waveforms produced experimentally from a bench-scale two-phase bubble column were recorded using a miniature hydrophone at three axial positions. The generated acoustic waveforms were processed and trained using artificial intelligence against global gas hold- up measurements. Two neural network architectures, the radial basis function (RBF) neural network and the recurrent Elman neural network, were employed. Both neural network techniques achieved accurate gas hold-up estimation, characterised by low mean square errors of 2.70 and 1.68% for the RBF and recurrent Elman networks respectively. The designed and trained neural networks were found to be a powerful tool for learning and replicating complex two-phase patterns. Passive acoustic waveforms were found to be a useful measuring technique for gas hold-up estimation in bubble columns under moderate operating conditions.  2006 Society of Chemical Industry Keywords: bubble column; acoustic; hydrophone; neural network; gas hold-up INTRODUCTION Bubble columns are frequently used as gas–liquid reactors in the process industries, biotechnology and wastewater treatment. They are increasingly favoured because of their good mixing, heat transfer and mass transfer properties, while their relatively low shear stresses render them attractive for shear-sensitive biosystems. Mixing in bubble columns is achieved by the sparging action of the gas alone. Thus a major advantage of bubble columns over other reactor types is their design simplicity, which, owing to the absence of any moving mechanical parts, provides ease of operation and maintenance. A typical bubble column consists of a cylindrical tank with an aspect ratio between 2 and 10. At the bottom a gas sparger generates bubbles, which rise in the continuous liquid phase. The density difference between the gas and liquid leads to circulating flow in the liquid phase. The property most frequently measured is the gas hold-up, which is the fraction of the two-phase mixture volume occupied by the gas phase. Gas hold-up is an important property for bubble column design because of its direct influence on column size. In addition, it is indirectly related to the gas–liquid surface area and hence to mass transfer. 1,2 Gas hold-up can be measured directly by bed expansion or indirectly by manometric, photographic, light transmission and probe techniques. The bed expansion method requires a transparent vessel and an accurate liquid level determination, particularly at high gas flow rates or when foaming systems are used. Manometric methods measure pressure difference at specific pressure taps across the column and can be subject to fouling. One promising technique for measuring global gas hold-up in bubble columns is based on the use of acoustic probes. Acoustic measurements can be either active or passive. Active acoustics involves the measurement of the effect of a process on a transmitted acoustic wave. Passive acoustics, on the other hand, deals with the measurement of the acoustic emission created by the process itself. Such acoustic emissions are caused by physical and chemical events occurring within processes and could therefore provide valuable information. The difficulty in extracting information from acoustic signals is the main reason why process acoustic emission monitoring is not widespread. 3 An acoustic wave is characterised by its speed, frequency and wavelength. The amplitude of an acoustic wave varies with time and depends on the distance from the sound source at which it is measured. As an acoustic wave travels through a medium, its amplitude decreases owing to attenuation caused by adsorption and scattering. There has been much work in the literature relating the physics of the bubble formation sound with bubble size and bubble population. The ∗ Correspondence to: Waheed A Al-Masry, Department of Chemical Engineering, King Saud University, Riyadh, Saudi Arabia E-mail: walmasry@ksu.edu.sa (Received 17 May 2005; revised version received 17 August 2005; accepted 31 October 2005) Published online 8 March 2006  2006 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2006/$30.00 951