Gabriel Wild

Measuring techniques in gas-liquid and gas-liquid-solid reactors

This article offers an overview of the instrumentation techniques developed for multiphase flow analysis either in gas/liquid or in gas/liquid/solid reactors. To characterise properly such reactors, experimental data have to be acquired at different space scale or time frequency. The existing multiphase flow metering techniques described give information concerning reactor hydrodynamics such as pressure, phases holdups, phases velocities, flow regime, size and shape of dispersed inclusions, axial diffusion coefficients. The measuring techniques are presented in two groups: the non-intrusive techniques that deliver global, cross-section-averaged or local data, and the intrusive probes that are dedicated to local measurements. Eventually some examples of multiphase instrumentation development are reported (trickle-bed and slurry bubble column at semi-industrial scale) in the refinery or petrochemical area.

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.

Gas and solid behavior in cracking circulating fluidized beds

Abstract Gas and solid hydrodynamics have been studied in dilute circulating fluidized beds under conditions occurring in catalytic cracking risers. Gas radial velocity profiles and dispersions were established by a tracer technique in a cold set-up. The gas axial dispersion was determined in an industrial riser. The local concentrations of the solid phase were measured by a tomographic technique. This has allowed an assessment of the core—annulus structure of the bed and an estimate of the solid radial and axial dispersions. The axial solid concentration profiles were determined in pilot and industrial scale beds. These show an important accumulation upstream of the abrupt exit. The overall conclusion is that the gas flow can be considered to be plug flow with a radial velocity profile and a radial dispersion; the solid flow is slightly more dispersed due to the core—annulus structure and a high radial mixing.

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.

Gas}liquid interfacial mass transfer in trickle-bed reactors: state-of-the-art correlations

The state-of-the-art of the gas-liquid mass transfer characteristics in trickle-bed reactors was summarized and its quantification methods were reevaluated based on a wide-ranging data base of some 3200 measurements. A set of three unified whole-flow-regime dimensionless correlations for volumetric liquid- and gas-side mass transfer coefficients, and gas–liquid interfacial area, each of which spanned four-order-of-magnitude intervals, were derived. The correlations involved combination of artificial neural networks and dimensional analysis. The dimensionless interfacial area, ShL and ShG were expressed as a function of the most pertinent dimensionless groups: ReL, ReG, WeL, WeG, ScL, ScG, StL, XG, MoL, FrL, Eom, Sb.

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.