Scale up of bubble columns : a regime transition approach

Bubble column reactors are multiphase contactors based on the dispersion of gas phase in form of bubbles inside a vessel where the liquid phase circulates (Leonard, Ferrasse et al. 2015). Generally, it could be two-phase reactors, or three-phase one, depending on the presence or absence of solid load. In this work, only two-phase systems are considered. Despite the fact that bubble columns are widely established within the process industry, common research focus are on the description of bubble hydrodynamic parameters under atmospheric conditions. Industrial production, on the other hand, is usually conducted at pressure above atmospheric and temperature above ambient (Deckwer and Field 1992, Wilkinson, Spek et al. 1992, Krishna and Ellenberger 1996, Leonard, Ferrasse et al. 2015, Rollbusch, Bothe et al. 2015). Therefore, this work has the aim to give an overview based on a wide range of literature survey covering different value of pressure and temperature. In particular, the focus is on the gas holdup, the most important hydrodynamic parameter on which the design and the performance of reactors is based on (Lemoine, Behkish et al. 2008). Starting from a survey of more or less 60 studies of most relevant authors from 70ens to more recent period of nowadays, a systematic classification of reactor geometry, operating variables and hydrodynamic parameters is done with high accuracy. The idea is to extract a very general correlation that can link physical, geometrical variables to the hydrodynamic parameters that govern the system. Wilkinson (Wilkinson, Spek et al. 1992) in his paper has already proposed a correlation to predict the gas holdup, starting from physical properties of the working fluids in an industrial relevant range of operation. The methodology that Wilkinson proposed in his work is very revolutionary and efficiently described the gas holdup behaviour. The negative aspect of this equation, on the other hand, is a lack of generality, since it does not take in account the geometry of the reactor. Therefore, in this work a generalization is made in order to obtain a correlation that also takes in account the geometry of the system. Validation of the proposed relation is then performed, based on the literature survey data and an error up to 20% is considered. Bubble reactors are widely used in the field of wastewater treatment for wet air oxidation (WAO) processes application (Debellefontaine, Chakchouk et al. 1996, García-Molina, Kallas et al. 2007, Lemoine, Behkish et al. 2008). Those processes operate at high pressure and temperature in the most of the cases. A design procedure is then performed, by maximizing the mass transfer properties, in particular, the liquid-gas interfacial area (a). In fact to achieve a high conversion and good efficiency, a is a crucial parameter that should be considered. On the other hand a is strictly dependent on fluid properties and gas holdup (Sideman, Hortaçsu et al. 1966). Wilkinson (Wilkinson, Haringa et al. 1994), in the following years, in his work of 1994 proposed also an empirical correlation for the liquid-gas interfacial area that has the advantage of taking account the influence of pressure of the system. Although, previously many equations have been proposed, none of them directly considered the pressure influence, which is a critical parameter in mass transfer properties. In the final section of this work, the LOPROX process is considered. This is a low pressure WAO patented by Bayer and widely spread all around the world (Lemoine, Behkish et al. 2008). First of all a design of geometrical parameter is performed, by keeping constant fluid physical-chemical properties and operating conditions, then, once column diameter, column height and sparger nozzle diameter are established, a discussion is made about the influence of presence of organic impurities (which decrease the liquid surface tension) and of operating pressure on the liquid-gas interfacial area.