Stoyan Nedeltchev

A New Statistical Parameter for Identifying the Main Transition Velocities in Bubble Columns

The identification of the main flow regime boundaries in bubble columns is essential since the degrees of mixing and mass and heat transfer vary with the flow regime. In this work, a new statistical parameter was extracted from the time series of the cross-sectional averaged gas holdup. The measurements were performed in bubble columns by means of conductivity wire-mesh sensors at very high sampling frequency. The columns were operated with an air/deionized water system under ambient conditions. As a flow regime indicator, a new dimensionless statistical parameter called “relative maximum number of visits in a region” was introduced. This new parameter is a function of the difference between the maximum numbers of visits in a region, calculated from two different division schemes of the signal range.

New Approaches for Theoretical Estimation of Mass Transfer Parameters in Both Gas-Liquid and Slurry Bubble Columns

Bubble columns (with and without suspended solids) have been used widely as chemical reactors, bioreactors and equipment for waste water treatment. The key design parameters in bubble columns are: • gas holdup; • gas-liquid interfacial area; • volumetric liquid-phase mass transfer coefficient; • gas and liquid axial dispersion coefficients; Despite the large amount of studies devoted to hydrodynamics and mass transfer in bubble columns, these topics are still far from being exhausted. One of the essential reasons for hitherto unsuccessful modeling of hydrodynamics and mass transfer in bubble columns is the unfeasibility of a unified approach to different types of liquids. A diverse approach is thus advisable to different groups of gas-liquid systems according to the nature of liquid phase used (pure liquids, aqueous or non-aqueous solutions of organic or inorganic substances, non-Newtonian fluids and their solutions) and according to the extent of bubble coalescence in the respective classes of liquids. It is also necessary to distinguish consistently between the individual regimes of bubbling pertinent to a given gas-liquid system and to conditions of the reactor performance. The mechanism of mass transfer is quite complicated. Except for the standard air-water system, no hydrodynamic or mass transfer characteristics of bubble beds can be reliably predicted or correlated at the present time. Both the interfacial area a and the volumetric liquid-phase mass transfer coefficient kLa are considered the most important design parameters and bubble columns exhibit improved values of these parameters (Wilkinson et al., 1992). For the design of a bubble column as a reactor, accurate data about bubble size distribution and hydrodynamics in bubble columns, mechanism of bubble coalescence and breakup as well as mass transfer from individual bubbles are necessary. Due to the complex nature of gas-liquid dispersion systems, the relations between the phenomena of bubble coalescence and breakup in bubble swarms and pertinent fundamental hydrodynamic parameters of bubble beds are still not thoroughly understood. The amount of gas transferred from bubbles into the liquid phase is determined by the magnitude of kLa. This coefficient is an important parameter and its knowledge is essential