J. Zahradnı́k

Duality of the gas-liquid flow regimes in bubble column reactors

The purpose of the present work was to examine the effect of design and working parameters of bubble column reactors on the formation and stability of the homogeneous and the heterogeneous bubbling regimes and to demonstrate the effect of bubbling regime transition on the hydrodynamic and mass transfer characteristics of gas-liquid beds in bubble column reactors. The factors determining the mode of gas-liquid contacting in bubble column reactors included the type and geometry of distributing plates and column geometry, as well as liquid-phase properties. Experimental evidence demonstrated extreme sensitivity of gas and liquid-phase mixing data to the transition from the homogeneous to the heterogeneous bubbling regime and, at the same time, indicated existence of direct links between the variations of RTD characteristics of both phases and values of bubble-bed voidage and kL aL induced by the changes of bubbling mode. The effect of design parameters on bubble-bed behaviour has been illustrated for air-water system. In contrast with the heterogeneous bubble beds, the range of the homogeneous regime stability and corresponding values of gas holdup have been found to be significantly influenced by the column diameter and aspect ration, with obvious implications for column design and scale-up. Experimental data obtained in aqueous solutions of electrolytes and aliphatic alcohols demonstrated significant synergistic effect of solute concentration and gas dispersion mode on the character of a bubble bed and on the values of its voidage. The influence of solutes on gas holdup has been considerably more pronounced in the homogeneous and transition bubbling regimes than in the heterogeneous bubbling regime while, at the same time, our data demonstrated significantly enhanced stability of the homogeneous bubbling regime in the presence of surface active solutes. Increased viscosity of the liquid phase suppresed formation of homogeneous bubble beds already at μL ⩾ 8mPa s. Gas holdup data corresponding to the homogeneous bubbling regime were adequately described on the basis of the slip velocity concept while the Zuber and Findlay drift-flux model fitted well the data obtained under the heterogeneous bubbling conditions both in a bubble column and in an external-loop reactor.

Homogeneous–heterogeneous regime transition in bubble columns

A simple physical model for homogeneous–heterogeneous regime transition in bubble columns is developed. The model is based on hydrodynamic coupling between gas and liquid phases. For the homogeneous regime, the coupling is made via bubble drift concept (Darwin, Proc. Camb. Phil. Soc. 49 (1953) 342). As a byproduct, a novel non-empirical formula for bubble slip velocity results, u/w=1−ae/(1−e). For the heterogeneous regime, the coupling is obtained in a simple formal way, recovering the classical result of Zuber and Findlay. The regime transition is considered as a smooth and gradual process characterized by a transition function. The model has five parameters: two terminal bubble velocities, bubble drift coefficient, Zuber–Findlay constant, and intermittency factor. All have clear physical meaning and are extractable from experimental data. The model gives formulas for the voidage-gas flow rate dependence separately for the homogeneous regime, heterogeneous regime and transition regime. The model gives a kinematic stability condition for the homogeneous regime and predicts the critical gas flow rate where the transition begins. It also predicts the maximum possible gas holdup in bubble columns. The model is verified by experiments with four different air–water bubble columns. Good agreement is found. Our results are compared with results of other authors. The model agrees with the drift-flux concept developed by Wallis (One-dimensional Two-phase, Flow, McGraw-Hill, New York, 1969) and with the stability theory of Shnip et al. (Int. J. Multiphase Flow 18 (1992) 705).

Effect of the ejector configuration on the gas suction rate and gas hold-up in ejector loop reactors

Abstract An experimental study has been aimed at examining the effect of ejector configuration on the rate and energy effectiveness of gas suction and on the values of gas hold-up in ejector loop reactors. Experimental data showed that insertion of a swirl body into the ejector nozzle increased the suction rate and dispersion efficiency of the ejector distributor and significantly improved its operating flexibility. In the absence of swirl elements, the gas suction rate increased slightly with the mixing tube length up to the length-to-diameter ratio equal to six. In the presence of swirls, however, the mixing tube length exhibited negative effect on the ejector performance and the highest values of gas suction rate and dispersion efficiency were observed for the configurations without the mixing tube. The ejector suction efficiency increased sharply with increasing swirl number Swb (related to the swirl body parameters) in the region of its values 0.06–0.12 while the increase of Swb above 0.2 resulted in a decrease of the gas suction rate. For Swb = 0.12–0.20, the gas suction rate was significantly higher than that achieved in the absence of swirls even at the optimum mixing bube length. Comparison of the energy effectiveness of gas suction indicated superiority of the single-orifice nozzles with swirl inserts over the other tested variants. Good agreement of gas holdup data from all experimental runs with values calculated from the relation u s = u oG ge G − u oL (1−ϵ G ) , for the slip velocity value us = 0.224 m s−1, proved adequacy of the slip velocity concept for description of gas-liquid flow in ejector loop reactors.

The effect of bubbling regime on gas and liquid phase mixing in bubble column reactors

An experimental study has been aimed at linking the extent of axial mixing in gas and liquid phases in tall bubble column reactors with the respective bubbling regimes (homogeneous, transition, heterogeneous) encountered in such reactors and with corresponding macro-scale flow patterns of both phases. The experimental programme, carried out in a bubble column reactor of 0.14 m in diameter and 4.1 m in height, included determination of gas and liquid phase residence time distribution (RTD) characteristics for different modes of primary gas dispersion and flow visualisation experiments aimed at identification of macro-scale flow patterns in the reactor. The experimental results proved an essential effect of gas dispersion mode (bubbling regime) on the extent of gas and liquid phase mixing in the reactor. The respective dependences of PeG and PeL on the superficial gas velocity, obtained with the distributing plate generating sequentially homogeneous, transition and heterogeneous bubbling regimes, indicated very sensitively the transition from the homogeneous to the heterogeneous bubbling mode (the onset of the transition bubbling regime) occurring at u0G = 0.04 m s−1 with the appearance of local macroeddies in the bed. The comparison of our mixing data with corresponding dependences of gas holdup and kLaL on the superficial gas velocity revealed the existence of direct links between the variations of RTD and mass transfer characteristics induced by the changes of gas dispersion (bubbling) mode. The results of the analysis of variances of gas phase RTD curves justified (in agreement with corresponding PeG data) the plug flow approximation for the gas phase in the homogeneous bubbling regime and indicated the convective mechanism of gas mixing within the whole region of heterogeneous bubbling conditions. Analogically, the data for the liquid phase yielded the convective mechanism of liquid mixing within the whole range of experimental conditions. The mixing data obtained for both phases in the heterogeneous bubbling regime were adequately described by the model of consecutive circulation cells with the backflow between adjacent cells, proposed on the basis of the flow visualisation experiments as a realistic simplified representation of the macro-scale flow structures in heterogeneous bubble beds.

Liquid-phase properties and sparger design effects in an external-loop airlift reactor

Abstract A synergistic effect of liquid-phase properties and sparger design on the performance of an external-loop airlift reactor was observed in our experiments. The absolute changes of gas holdup reflecting the variations of gas dispersion mode and system properties were, however, less pronounced than in the bubble column reactors due to the equalizing effect of induced liquid circulation on the riser hydrodynamics. The total increase of gas holdup observed in aqueous solutions of inorganic salts was independent of the type of salt and varied only with the gas dispersion mode. For all electrolytes studied, the respective limiting dependence e vs uG were obtained at concentrations close to the transition coalescence concentrations of individual electrolytes, while further increase of salt concentration had a negligible effect on gas holdup values. The Zuber and Findlay drift flux model fitted well the data of riser gas holdup for the heterogeneous bubbling conditions; it, however, failed to reflect the transition between the heterogeneous and homogeneous bubbling regimes. Data for the homogeneous bubbling regime were adequately described by the slip velocity concept when the functional relation between slip velocity and riser gas holdup was expressed by the modified equation of Davidson and Harrison.

Influence of viscosity and surface tension on performance of gas–liquid contactors with ejector type gas distributor

Abstract Effect of the liquid-phase viscosity (0.7– 25 mPa s ) and surface tension (33– 65 mN m −1 ) on hydraulic and mass-transfer characteristics in a bubble column with an up-flow ejector-type gas distributor was studied. The coalescent character of the aqueous batches was changed by addition of a polymeric thickener, sucrose and an alcoholic foam breaker. Mass transfer was studied using steady-state physical absorption of air. Better performance of the ejector with swirl body for all tested liquid batches was achieved. Gas hold-up in the bubble column increased with increasing viscosity and surface tension in all batches. Mass-transfer coefficient in the ejector decreased with increasing viscosity by 60% maximally in the maximum viscous coalescent batch. It was correlated within ±10% using the gas monodispersion model. Mass-transfer coefficient in the column decreased with increasing viscosity by 30% maximally in the same coalescent viscous batch. It was correlated with large relative error, ±27%.

Hydrodynamic model for three-phase internal- and external-loop airlift reactors

A mathematical model predicting the hydrodynamic behaviour of three-phase airlift reactors, working with low-density solids and with high solids loading, was developed. The model allows for the prediction of local gas holdup and liquid velocity in airlift bioreactors. Model was validated for an external-loop airlift reactor and an internal-loop airlift reactor with an enlarged degassing zone, being a good agreement obtained between calculated and experimental data.

A networks-of-zones analysis of mixing and mass transfer in three industrial bioreactors

Abstract The original version of the networks-of-zones (N-o-Z) model developed for the description of gas–liquid flow in stirred-vessel reactors (R. Mann, Gas–liquid stirred vessel mixers: towards a unified theory based on network of zones, Transactions of the Institution of Chemical Engineers 64 (1986) 23–34) has been extended and enhanced to cover distributed bubble sizes, gas–liquid mass transfer, bioreaction kinetics and multiple-impeller operation. In addition, a modified version of the N-o-Z model for bubble columns has been simply derived from the impeller version, assuming the existence of a two-loop axisymmetrical circulation pattern induced by the non-uniform distribution of gas holdup in bubble columns. The liquid circulation velocity has been expressed as a function of gas flow rate and the density difference between the gas and liquid phases, based on Zehners circulation model (P. Zehner, G. Schuh, A concept for the description of gas phase mixing in bubble columns, German Chemical Engineering 5 (1985) 282–289). These two variants of the N-o-Z model have been used for modelling three different industrial fermenters: 3 and 31 m 3 triple-impeller stirred reactors, and a 236 m 3 bubble column reactor. The performance of these three reactors, typical of the fine chemicals, bioprocessing and pharmaceutical process industries was evaluated and compared in terms of geometry/size, gas flows, power inputs, pressure, liquid mixing, oxygen mass transfer, reaction speed and spatial variability of behaviour. This provides potentially valuable insights into the relative factors influencing the selection of an appropriate reactor type.

Intermittent transition from bubbling to jetting regime in gas-liquid two phase flows

Abstract The transition from bubbling to jetting regime in nitrogen-water system was studied experimentally. The gas was introduced into a pool of stagnant liquid through a single orifice plate above a gas chamber. Two quantities were measured: pressure fluctuations in the gas chamber and velocity of liquid circulations near the orifice. Individual bubbles were formed at low gas flow rates (bubbling regime) while a continuous jet of gas was formed at high rates (jetting regime). The transition from bubbling to jetting regime (transition regime) displayed intermittent character. Jetting bursts of various length appeared at random in originally periodic pressure signal. The distribution of bubbling portion in the pressure signal was hyperbolical with exponent −1.33 indicating type III intermittency. Similar characteristic time scales were found in power spectra of both signals. 1/ f noise was revealed in the velocity spectrum. This kind of noise usually accompanies intermittent transitions. These results implied that liquid circulations with 1/ f noise induced by bubbles affected the bubble dynamics itself as a feed-back and caused the intermittent regime transition. The point of the regime transition was indicated by a sudden drop of Kolmogorov entropy, correlation dimension of the attractor, and Mann-Whitney statistic calculated from pressure signal. An explanation for this drop is suggested on the base of combination of properties of two attractors coexisting/competing within the intermittency range.

Dispersion efficiency of ejector-type gas distributors in different operating modes

The effect of ejector configuration on the energetic efficiency of gas-liquid contacting and on the efficiency of gas-phase utilization was examined in the two alternative operating modes of ejector-distributor reactors, i.e. under conditions of free suction or forced gas supply to the ejector. Data for the free suction regime proved the independence of the gas phase utilization from the ejector configuration. The dependence of gas holdup (eG) on the superficial gas velocity (u0G) was, within the whole range of our experimental conditions, well described by a single empirical equation, eG = 2.8u0G0.9. Good agreement of the experimental data with calculations based on the slip velocity concept confirmed the ‘homogeneous’ character of gas-liquid beds in the ejector distributor reactors. The energy effectiveness of bubble bed formation in the free suction regime varied with the ejector geometry. The optimal values of the mixing tube and diffuser length derived from our experimental data agreed well with those proposed previously by Henzler (1983). No significant differences were, however, observed between the energy effectiveness data for the two different ejector configurations, with and without the mixing tube, at constant values of the total ejector length. Experimental data obtained under conditions of the forced gas supply clearly demonstrated a decrease of the ejector dispersion efficiency with increasing gas feed rate as well as significant non-uniformity of gas holdup distribution at high gas flow rates. A critical ratio of the gas and liquid flow rates was determined, representing the upper limit of the ejectors working region in the forced-gas-supply regime.