Chemical Engineering Science 59 (2004) 623–632 www.elsevier.com/locate/ces Time-dependent gas holdup variation in an air–water bubble column Chengzhi Tang, Theodore J. Heindel * Department of Mechanical Engineering, 2025 Black Engineering Building, Iowa State University, Ames, IA 50011-2161, USA Received 7 August 2003; received in revised form 17 October 2003; accepted 22 October 2003 Abstract Time-dependent gas holdup variation in a two-phase bubble column is reported with air and tap water as the working uids. The results indicate that time-dependent gas holdup is closely related to the water, whose quality is unsteady and changes, not only during the two-phase ow, but also during idle periods. The signicance and characteristics of the time-dependent gas holdup variation are inuenced by the bubble column operation mode (cocurrent or semi-batch), the sparger orientation, the supercial gas velocity, and the supercial liquid velocity. It is proposed that a volatile substance (VS), which exists in the water in very small concentrations and inhibits bubble coalescence, evaporates during column operation and results in a time-dependent gas holdup. The inuence of bubble column operation mode, sparger orientation, supercial gas velocity, and supercial liquid velocity on the time-dependent gas holdup variation are explained based on their eects on bubble size, bubble contacting frequency and mixing intensity. This work reveals that regular tap water may cause signicant reproducibility problems in experimental studies of air–water two-phase ows. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Bubble column; Coalescence; Hydrodynamics; Multiphase ow; Transient response; Voidage 1. Introduction Gas–liquid two-phase bubble columns are the subject of much research, and water and air are used in a majority of these studies. It is usually assumed that the properties of air and water are steady and the results obtained in these air–water systems are reproducible. However, Anderson and Quinn (1970) compared gas holdup in a 21 cm ID semi-batch bubble column using distilled water, deionized water, and tap water; they found that at the same supercial gas velocity, the gas holdup in tap water was up to 50% higher than in distilled water, while the gas holdup in deion- ized water was intermediate to the former values. They also found that solutions made from mixing varying quantities of tap and distilled water also gave dierent gas holdup re- sults at the same supercial gas velocities. In addition, they observed that heating tap water could change the gas holdup and regime transition in the bubble column. They inferred that there were some bubble coalescence-inhibiting impuri- ties in the water and the concentrations of these impurities diered among water types. They concluded the gas holdup * Corresponding author. Tel.: +1-515-294-0057; fax: +1-515-294-3261. E-mail address: theindel@iastate.edu (T.J. Heindel). variations were the result of the coalescence-inhibiting im- purities and ordinary purity precautions did not necessarily result in reproducible results. Using tap water and air from a compressor, Maruyama et al. (1981) noticed that dierent gas holdups were ob- tained in each of the 3 experiments repeated in the same 2-D semi-batch bubble column without ltering the air or changing the water. They attributed the dierence to the ac- cumulation of trace impurities, such as compressor oil, in the water. In other experiments reported in the same pa- per, they found that impurities of dierent concentrations could change the ow transition in bubble columns. Ueyama et al. (1989) revealed that types of water (tap water or ion-exchange water) and gas (compressed air or N 2 ) could change the gas holdup and ow behavior in a semi-batch bubble column. They hypothesized that there were some coalescence-inhibiting impurities in the water and the con- centrations were higher in tap water than in ion-exchange water. They also inferred that the presence of a mist in the air, believed to be oil droplets, hindered bubble coales- cence as the mist accumulated in the water. They believed that the coalescence-hindering impurities in the tap water, together with the coalescence-weakening mist in the com- pressed air, caused a rather complex hysteresis behavior ob- served in the bubble column. 0009-2509/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2003.10.016