Catalysis Today 79–80 (2003) 181–188 Intensification of bubble columns by vibration excitement J. Ellenberger, J.M. van Baten, R. Krishna ∗ Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Abstract In this paper we show that application of low-frequency vibrations, in the 40–120 Hz range, to the liquid phase of an air–water bubble column causes significantly smaller bubbles to be generated at the nozzle. In experiments with a single capillary nozzle, the bubble size is reduced by about 40–50%, depending on the vibration frequency and amplitude. CFD simulations show that the vibrations tend to lead to earlier detachment of the bubbles from the nozzles, leading to smaller bubble sizes. Using a 12-capillary nozzle arrangement, the gas holdup, ε, was measured for a range of superficial gas velocities. Ap- plication of vibrations to the liquid phase leads to a significant increase in the gas holdup. The increase in the gas holdup is attributed mainly to a significant reduction in the rise velocity of the bubble swarm due to the generation of standing waves in the column. Furthermore, application of vibrations to the liquid phase serves to stabilize the homogenous bubbly flow regime and delay the onset of the churn-turbulent flow regime. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Bubble column; Vibration excitement; Process intensification; Gas holdup; Standing waves 1. Introduction A bubble column reactor is commonly used in the process industries for carrying out a variety of liquid phase reactions [1]. There are two regimes of operation for a bubble column. At low superfi- cial gas velocities, U, we have homogeneous bubbly flow in which the dispersion consists of bubbles that are roughly uniform in size. Homogeneous bubbly flow is sustainable up to a superficial gas velocity U = U trans , called the transition gas velocity. When U is increased to values beyond U trans , we enter the heterogeneous or churn-turbulent flow regime. In this churn-turbulent flow regime, we have a wide dis- tribution of bubble sizes, ranging from 3 to 50 mm depending on the system properties [2]. The wide ∗ Corresponding author. Tel.: +31-20-525-7007; fax: +31-20-525-5604. E-mail address: krishna@science.uva.nl (R. Krishna). distribution of bubble sizes causes a wide gas phase residence time distribution; this is often detrimental to reactor conversion and selectivity. Uniformity of bubble sizes in gas–liquid reactors can be achieved by means of mechanical agita- tion or stirring. However, the stirring action of the agitator causes intense backmixing of both liquid and gas phases. In some biotechnological applica- tions, the high shear, experienced near the agitator, is undesirable. The major objective of the present communication is to demonstrate the advantages of a shaken or vibrated bubble column reactor in which the liquid phase is subjected to low-frequency vibrations in the 40–120 Hz range. A special vi- bration excitement device is used for this purpose. The advantage of using low-frequency vibrations over mechanical agitation is that the plug flow char- acter of the bubbles is maintained and no addi- tional large-scale backmixing of the liquid phase is induced. 0920-5861/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-5861(03)00003-8