N. Midoux

Influence of coalescence behaviour of the liquid and of gas sparging on hydrodynamics and bubble characteristics in a bubble column

Abstract This experimental study is aimed at investigating the effect of liquid phase properties and gas distribution on bubble and hydrodynamic characteristics in bubble columns. With the various measuring techniques used, systematic measurements of bubble size, velocity and frequency and gas hold-up are possible. Bubble size distribution and shape factors which are rarely found in literature, are also available. Water–alcohol solutions are used to simulate the behaviour of industrial non-coalescing organic mixtures. The experimental results obtained with three different spargers in the coalescence inhibiting solutions are compared with data on standard coalescing air–water system. Evolutions of bubble characteristics and gas hold-up have been interpreted successfully by considering the simultaneous influence of the hydrodynamic regime of the gas–liquid flow and of the operating regime of the distributor. It has also been put into evidence that bubble frequency measurements are good tools to evaluate distributor efficiency. The influence of the distributor has been shown to be enhanced in non-coalescing media. Bubble shape and bubble size distributions are dramatically modified by addition of minute quantities of alcohol in water. Bimodal distributions can be observed even in the homogeneous regime with orifice nozzle spargers.

Experimental study of a trickle-bed reactor operating at high pressure: two-phase pressure drop and liquid saturation

Abstract The effect of pressure on the hydrodynamics of trickle-bed reactors is investigated. The two-phase pressure drop and the liquid hold-up (liquid RTD determination) were measured for pressures up to 8.1 MPa. The influence of pressure, gas and liquid flow rates, viscosity, the coalescence behaviour of the liquid, and the particle size was examined. The experimental results were compared to correlations from the literature and two new correlations for the pressure drop and the liquid hold-up for non-foaming liquids are proposed; they are based on 1500 experimental results. Consideration of systems exhibiting non-foaming behaviour shows that the two-phase pressure drop is correctly described by the introduction of the modified Lockhart and Martinelli parameter. The liquid saturation data analysis shows that this hydrodynamic parameter is pressure-independent for very low gas superficial velocities allowing for an acceptable estimation at atmospheric pressure.

Numerical Simulation of Multiphase Flow in Bubble Column Reactors. Influence of Bubble Coalescence and Break-Up

Population balance equations have been combined to a classical hydrodynamic Euler/Euler simulation to investigate the operation of a cylindrical bubble column. The MUSIG (mutiple-size-group) model implemented in the CFX 4.3 commercial software has been used. Hydrodynamic experimental variables, i.e. local axial liquid velocity and local gas hold-up, have been compared to the corresponding calculated values, showing a quite good agreement, except for the gas hold-up when the column is no more operating in the homogeneous regime. Bubble sizes have been investigated, showing that two domains of superficial gas velocities can be distinguished. In the first domain, coalescence occurs predominantly, Sauter diameter increases with the superficial gas velocity, bubble size distribution is narrow and Sauter diameter is continuously evolving along the column axis. In the second domain, break-up becomes more intensive and compensates coalescence, bubble size distribution becomes wider, since more small bubbles are formed, an equilibrium Sauter diameter appears when the superficial gas velocity increases. Furthermore an equilibrium Sauter diameter appears along the column axis, and it can be noticed that this phenomenon appears lower in the column when the gas flow rate is increased. In these two domains the characteristics of the bubbles are typical of those of the homogeneous and transition regimes.

Study of hydrodynamic behaviour in bubble columns and external loop airlift reactors through analysis of pressure fluctuations

Recent methods of regime identification based on pressure fluctuations analysis have been applied both in a bubble column and an external loop airlift reactor with several spargers. Their ability to determine regime transition and to extract regime features is compared. A new method based on the auto-correlation function is proposed. This method is shown to be simple and efficient. It also provides quantitative information about the characteristic time and the axial dimension of the flow structure in the prevailing regime.

A simple method for regime identification and flow characterisation in bubble columns and airlift reactors

A new diagnosis method for regime identification in bubble columns and airlift reactors based on a theoretical analysis of the auto-correlation function (ACF) of wall pressure fluctuations is proposed. It yields quantitative information, such as a characteristic time and a characteristic frequency of the two-phase flow, which are closely related to the flow structure in the prevailing regime. This method is shown to be simple, low-cost, reliable and efficient and has been applied successfully to a bubble column and an external loop airlift reactor. Experimental data on both reactors are shown to be in good agreement with theoretically predicted values. The order of magnitude of the characteristic time can be used for regime identification. Combined with an analysis of the cross-correlation function (CCF) of two signals recorded simultaneously, the method is also able to yield an estimate of the axial dimension of the flow structures. This analysis is, therefore, promising for regime identification and flow structure characterisation in industrial equipment.

Chemical kinetics of the reaction of carbon dioxide with amines in pseudo m-nth order conditions in aqueous and organic Solutions

The specific rate of absorption for the gas-liquid system CO2 with cyclohexylamine (CHA) in a toluene solution was measured in a laboratory wetted-wall contactor. This reaction was found to be first order with respect to CO2 and second order with respect to amine. The values of the specific rate of absorption are such that this reaction may be used to determine interfacial areas in reactors working with organic liquids.

A new, improved liquid hold-up correlation for trickle-bed reactors

Abstract Presently available liquid hold-up correlations for trickle-bed reactors are not applicable over wide ranges of system conditions. The new, improved correlation for the liquid hold-up in a trickle-bed reactor presented here, derived from fundamental considerations and a wide-ranging data base of some 5000 hydrodynamic experimental results, is applicable to industrial trickle-bed reactors since it is based on wide variations of all the important variables, including measurements at high pressures. No other previously derived correlations are applicable to high pressure operations. A comparison with recent data on liquid hold-up in co-current upflow of gas and liquid shows that the correlation proposed for high interaction downflow gives a fair estimate of the upflow data, as long as the liquid is not foaming.

A new, improved pressure drop correlation for trickle-bed reactors

Abstract Presently-available pressure drop correlations for trickle-bed reactors are not applicable over wide ranges of system conditions. The new, improved correlation for the pressure drop in a trickle-bed reactor presented here - derived from fundamental considerations and a wide-ranging data base of some 4600 hydrodynamic experimental results - is applicable to industrial trickle-bed reactors since it is based on wide variations of all the important variables, including measurements at high pressures. No other previously derived correlations are applicable to high pressure operations.

The use of micro-electrodes in the study of the flow regimes in a packed bed reactor with single phase liquid flow

Abstract The utilization of micro-electrodes in a non-conducting wall with subsequent signal analysis allowed the determination and the study of the flow regimes in a packed bed reactor with single-phase liquid flow. In effect, the analysis of the rate of fluctuation of the liquid—solid mass transfer coefficient (velocity gradient) as a function of liquid flow rate clearly showed the transition between inertial flow regime (nonlinear steady-state laminar) and turbulent flow regime. This transition, located between Reynolds number values of 110 and 370, corresponds to an unsteady-state laminar flow regime. Examination of the power spectral densities of these fluctuations led to a better understanding of liquid flow behaviour during this transition.

Experimental and theoretical analysis of the hydrodynamics in the riser of an external loop airlift reactor

A detailed description of the local hydrodynamic parameters of both liquid and gas phases is obtained in the riser of an external loop airlift reactor (EL-ALR) in order to achieve a better understanding of the local phenomena and to obtain experimental data for CFD validation. The radial and the axial evolutions of these parameters are studied. A valve placed in the downcomer is used to increase the pressure drop due to friction effects, in this section, to study the influence of downcomer and junction geometry on the local flow. A simple analytical model is proposed to estimate the radial evolution of the local shear stress in the riser. Bubble-induced turbulence is shown to prevail, which leads to a strong anisotropy of the Reynolds stress tensor. Local hydrodynamic parameters are shown to be influenced both by the overall liquid circulation, which depends essentially on reactor geometry, and by the bubble-scale phenomena which exhibit little dependence on reactor geometry. Experimental data and results from the analytical model are finally compared with simulations obtained using the CFD code FLUENT® 4.51. An acceptable agreement is achieved only in the homogeneous regime, and the inaccuracy of the simulations at high gas flow rate is shown to be due to a poor estimation of the influence of bubble-induced turbulence.