For liquid flow, superficial velocity profiles inside a packed bed were obtained for monodisperse packings of spheres, deformed spheres, cylinders and Raschig-ring by averaging over several thousand local measurements of the axial flow components within a cross-section. At fully developed flow, the profiles are constant along the packing except for short inlet and outlet zones of about three particles layer length with either a buildup of the profile at the inlet or a degeneration at the exit of the packed bed. Besides velocity profiles, porosity distribution functions were also evaluated experimentally. These were introduced in the extended Brinkman equation which simulates the fluctuating profiles well if the average porosity of the packing is replaced by the radial porosity distribution. The effective viscosity is adjusted to obtain best agreement between measurements and solutions of the Brinkman equation. This effective property, however, influences the flow profile in the very near wall range only, up to approximately a distance of about half the particle diameter. The effective viscosity is found to depend on the porosity data close to the wall, on particle shape, on the Reynolds number, and on the pressure loss relation.

Measured and modeled superficial flow profiles in packed beds with liquid flow

' analysis and design of multiphase reactors is one of the most I intensely studied areas of chemical reaction engineering. The ^r types of industrial fluid/solid reactors can be broadly divided into two categories: one where the solids are fixed and the other where the solids are in a suspended state. Solids suspension may be achieved by stirring (stirred slurry reactor), by gas-induced liquid motion (slurry bubble column or air-lift) or by forced fluid flow (three-phase fluidised bed). These reactors are sketched in Figure 1. For a particular reaction, it may not always be clear which reactor type is most advantageous. Different types have to be considered or are actually used for the same conversion (e.g. FischerTropsch Synthesis). More recently Krishna and Sie (1994) recommended a three level strategy for the selection of type of multiphase reactor for a given process. Their selection criteria is conveniently split into three, separate, sequential decisions on the following aspects:

Trickle bed reactors

Meticulous inspection of the literature has unveiled the weakness of several empirical methods for predicting the macroscopic mass- and heat-transfer characteristics relevant to gas−liquid cocurrent downflow and upflow packed-bed reactors. In response, using a wide experimental database consisting of 5279 measurements for trickle beds (downflow) and 1974 measurements for packed bubble columns (upflow), a set of reliable correlations has been recommended for the prediction of the gas−liquid interfacial area (agl), the volumetric liquid- (kla) and gas-side (kga) mass-transfer coefficients, the wall (ηeklw) and bed (ηekls) liquid−solid mass-transfer coefficients, the wall heat-transfer coefficient (hw), the bed effective radial thermal conductivity (λe), and the particle-to-fluid heat-transfer coefficient (hp). Some of these correlations are from the literature, and others have been developed by combining artificial neural networks and dimensional analysis. The accuracy of the proposed correlations surpasses...

http://fs.teledos.gr:2206/%3ERESEARCH%20PUBLICATIONS/CHEMICAL%20ENGINEERING/Mass%20Transfer/Heat%20and%20Mass%20Transfer%20in%20Cocurrent%20Gas-Liquid%20Packed%20Beds.%20Analysis,%20Recommendations,%20and%20New%20Correlati.pdf

Heat and Mass Transfer in Cocurrent Gas−Liquid Packed Beds. Analysis, Recommendations, and New Correlations

Generalized equations for predicting minimum fluidization velocities in two-phase L-S and three-phase G-L-S inverse fluidized bed are developed covering a wide range of variables using the experimental data obtained from the present work and that available in the literature. It is found that the classical Wen and Yu equation for predicting minimum fluidization velocity can satisfactorily be used for two-phase inverse fluidized bed. A model based on empiricism and another based on the gas perturbed liquid model, which reduce to limiting conditions such as liquid solid and three-phase batch liquid systems are proposed.

Prediction of Minimum Fluidization Velocity in Two and Three Phase Inverse Fluidized Beds

Heat transfer in a packed bed reactor with cocurrent downflow of a gas and a liquid

Hydrodynamics and heat transfer in packed bed with cocurrent upflow

The authors investigate the radial heat transfer in packed columns with liquid flow and compare their results with those of studies carried out with gas flow. The heat transfer experiments are performed in a packed bed heated electrically through the wall with upward flow of water and aqueous solutions of ethylene glycol. The particle diameter has been varied from 1 to 6 mm, and the reactor inner diameter is either 50 or 100 mm. Radial and axial temperature profiles are measured inside the packed bed. The heat transfer parameters of the packed bed are obtained by fitting the parameters of two-dimensional, homogeneous model to the measured profiles: the radial effective thermal conductivity of the packed bed (Λr), and the wall heat transfer coefficient. The conductivity of the packed bed incresaes linearly with the fluid velocity but when the ratio of Λr to the thermal conductivity of the fluid is plotted against the fluid Peclet number (Ped = Red Pr), the slope is found to be lower for liquid than predicted for gas. This different behaviour is explained by differences in the flow regimes. The experiments in gas flow are usually carried out in turbulent flow, whereas the experiments in liquid flow are in an intermediate regime (between laminar and turbulent). A new correlation is proposed for this regime, depending on both Prandtl and Reynolds number of the fluid. Results on heat transfer coefficient at the wall and on wall channelling are also presented.

Hydrodynamics and heat transfer in packed beds with liquid upflow

We investigate radial heat transfer in packed columns with cocurrent upflow of gas and liquid. The gas is saturated nitrogen and the liquids are aqueous solutions of either ethylene glycol (viscous) or pentanol (foaming), or pure water. The influence of liquid properties on hydrodynamics and heat transfer parameters is shown. The operating parameters have been varied in the range of 5.4≤Pr≤22.6 for the liquid Prandtl number, 2≤Re l ≤50 (up to 125 for water in single phase flow) for the liquid Reynolds number and 0≤Re g ≤30 for the gas Reynolds number. The particle diameter is 2 mm and the reactor inner diameter is 100 mm. The reactor is heated by a constant heat flux through its wall. Radial temperature profiles are measured inside the packed bed. The heat transfer parameters of the packed bed reactor are obtained by fitting the parameters of a two-dimensional, homogeneous model to the measured profiles (the radial effective thermal conductivity of the packed bed, Λ r , and the wall heat transfer coefficient α w ). The liquid holdup is measured by conductimetric probes, using a salt tracer technique. In single phase liquid flow, the packed bed conductivity increases first linearly with the fluid flow rate, but tends to increase stronger than linearly at higher liquid Reynolds number (Re l >80). The packed bed conductivity decreases with increasing liquid viscosity. In two-phase flow, the heat transfer is influenced by the flow regimes. Flow regimes and liquid holdup measurements are first presented and then heat transfer parameters are discussed. Results are compared to existing correlations and a new correlation is proposed. The packed bed conductivity also decreases with liquid viscosity but increases with foaming in two-phase flow

Influence of viscosity and foaming properties on heat transfer in packed bed with cocurrent upflow of gas and liquid

Mechanical properties of membrane-electrode assemblies (MEAs) for proton exchange membrane fuel cells (PEMFCs) have been studied with the small-punch technique. MEAs were operated according to a non-accelerated aging protocol, with changes in humidification and cell temperature. After operation, degradation of the MEA is reflected by a decrease in power density, increase in the internal resistance, and decrease in open circuit voltage. Small disks (8 mm diam.) were taken from different locations of the degraded MEA to test the mechanical properties by small-punch. The results show an increase in plastic deflection and a decrease in maximum load of degraded MEAs, that result in lower apparent Young modulus. The degradation is larger in the area close to oxygen outlet. SEM microscopy shows that degradation affects majorly to the fibers of the carbon cloth gas diffusion layer (GDL). Combined small-punch and SEM correlate the change in mechanical properties on aged MEA with carbon fibers degradation. Such GDL degradation must also be principal responsible for the increase in the internal resistance.

M.T. Colli Serrano

Papers

Hydrodynamics and heat transfer in packed bed with cocurrent upflow

Hydrodynamics and heat transfer in packed beds with liquid upflow

Influence of viscosity and foaming properties on heat transfer in packed bed with cocurrent upflow of gas and liquid

Study of Mechanical Properties of Membrane-Electrode Assemblies for Proton Exchange Membrane Fuel Cells By the Small-Punch Technique