Osvaldo M. Martínez

Effect of the liquid axial dispersion on the behavior of fixed bed three phase reactors

Abstract The role of the liquid phase axial dispersion in fixed bed three-phase reactors behavior is examined by the formulation of a mathematical model including the major hydrodynamic and mass transport phenomena occurring in these reactors. Furthermore, liquid axial dispersion is evaluated from experiments carried out in mock-ups for cocurrent downflow and upflow. The effect of the liquid axial dispersion is analyzed by considering either liquid reactant of gaseous reactant limited reactions. A general criterion valid in both cases is developed in order to evaluate when it is necessary to take into account the axial dispersion effect.

Evaluation of heat transfer parameters in packed beds with cocurrent downflow of liquid and gas

The results of an experimental investigation on heat transfer from a packed bed with cocurrent gas–liquid downflow to the wall are presented and analyzed in this contribution. The measurements cover the range of operating variables corresponding to the so-called trickle regime in beds presenting aspect ratios (tube to particle diameter ratio) from 4.67 to 34.26. Water and air were employed as model fluids. The heat transfer process was first analyzed by means of a two-dimensional pseudohomogeneous plug-flow model with two parameters, the effective radial thermal conductivity (ker) and the wall heat transfer coefficient (hw). ker is well correlated with liquid and gas Reynolds numbers and particle diameter, except for the lowest experimental aspect ratio (4.67). Instead, a meaningful correlation of hw stands only for aspect ratios larger than 15. These results are analyzed and the evidence points out to sustain the hypothesis that the model fails at low aspect ratios because an apparent contact resistance (1/hw) can no longer accommodate the effects of significant fluid bypassing and finite size of the near-wall region. The experimental set of data were also used to develop a correlation for the overall heat transfer coefficient (hT), which can be employed satisfactorily to predict heat transfer rates in the whole range of variables here investigated.

Effect of the catalyst wettability on the performance of a trickle-bed reactor for ethanol oxidation as a case study

Abstract A comparative study on the performance of a trickle-bed reactor packed either with hydrophilic, hydrophobic catalyst particles or mixtures of hydrophobic catalyst-hydrophilic inert support in different mass proportions is carried out. Ethanol oxidation at moderate temperature and atmospheric pressure is selected as a model reaction. Experimental runs employing pure oxygen, relatively high ethanol concentrations and different liquid and gas flow rates are performed for each type of bed. A remarkable improvement in the reactor performance is found when the hydrophobic catalyst is employed, whether the bed is completely hydrophobic or diluted with hydrophilic support. An increase in ethanol conversion of even 100% is achieved for certain conditions. Experimental results are examined through a comprehensive model that takes into account the influence of particles’ wettability on the reactor behavior. It allows us to understand the role of particles’ wettability and, consequently, to explain the large improvement in the performance of the trickle-bed reactor when hydrophobic catalyst is used. The model properly describes the effect of bed wetting characteristics on the reactor performance in a wide range of operating conditions.

A One-Dimensional Equivalent Model to Evaluate Overall Reaction Rates in Catalytic Pellets

The problem of finding a one-dimensional (ID) model to approximate the behaviour of actual three-dimensional (3D) catalyst pellets is undertaken. It is shown that the ID model proposed by Burghardt and Kubaczka (Chem Eng Proc 35: 65–74, 1996), called here the generalized cylinder (GC) model, is most suitable for this purpose, provided that its main parameter (the shape power σ) is fitted to the behaviour of the actual pellet at low reaction rates. Calculations from the GC model are expected to be precise at around 1% for most geometrical cases of practical interest. The evaluation of σ for a given pellet geometry involves the solution of a Poisson equation. An approximate method that greatly simplifies this task for finite cylinders of any cross-section shape is developed. The procedure assumes knowledge of σ just for the cross-section (at most, a 2D problem). This is readily available for some practical cases, but if not, a suitable numerical procedure based on the boundary element method (BEM) is proposed. BEM is also suitable for the general 3D case.

Analysis of operating variables on the performance of a reactor for total hydrogenation of olefins in a C3C4 stream

Abstract The effect of process and operating variables in the catalytic hydrogenation of unsaturate traces in C 3 C 4 streams, intended for aerosol propellant use, has been analysed. The results from catalytic tests carried out on a commercial Pd/Al 2 O 3 catalyst have been used to estimate the kinetic parameters of rate expressions. The set of rate expressions is used in a mathematical model of a three-phase fixed-bed catalytic unit operated in up-flow mode. The mathematical model allowed studying the effect that variables such as temperature, pressure, hydrogen mass flow and feed composition will exert on the reactor performance. The volatility of the hydrocarbon mixture is found to be a paramount factor in the process, as H 2 becomes diluted in the vapour phase and, consequently, the amount of H 2 dissolved in the liquid stream and the hydrogenation rates decrease significantly. A temperature rise turned out to be detrimental for the reactor performance, as the increased hydrocarbon volatility overcomes the effect on the kinetic coefficients. This conclusion precludes the usual operating practice of rising temperature to compensate for catalytic activity decay. Instead, increasing the H 2 input and/or the operating pressure were shown to be effective alternatives for this purpose.

Liquid hold-up and backmixing in cocurrent upflow three-phase fixed-bed reactors

Liquid-phase hydrodynamics is studied in a three-phase fixed bed with cocurrent upflow of gas and liquid. Residence-time distributions are measured to determine liquid hold-up and backmixing using different liquids and glass beads of two sizes. Two types of correlations are tested for liquid saturation. Correlations based on the drift flux concept are found to account for experimental results. Liquid mixing is described in terms of the axial dispersion model. A close relation between the axial dispersion coefficient and the bubbles sizes estimated using the theory of fluid emulsions is found. Hence, a new type of correlation is proposed to estimate axial dispersion coefficients in this type of reactors. This correlation considers the influences on the Peclet number of the liquid Reynolds number and the two-phase flow dissipation power rate, which would determine bubbles sizes.

Evaluation of Structural Properties of Cylindrical Packed Beds Using Numerical Simulations and Tomographic Experiments

This contribution is focused on the analysis of the structure of packed beds of spherical particles at relatively low aspect ratios (i.e., particle to tube diameter ratio) as those arising in multitubular fixed bed reactors. On one hand, the computed tomography (CT) technique is employed to evaluate the position of each particle in the packing and from this information local properties such as particle center distribution and radial porosity profile were obtained. On the other hand, results from a previously developed algorithm to simulate packings were compared with those from our CT data and from literature sources. The agreement was very satisfactory.

Computing radial packing properties from the distribution of particle centers

The relationship between the distribution of particle centers in random beds of uniformly sized spheres and radial properties, in particular radial voidage profiles, is undertaken in this work. To this end, closed expressions for six geometrical quantities related to the intersection of spheres with cylindrical surfaces are presented. The relations to compute radial properties are then expressed in terms of those geometrical quantities and it is shown that for realistic types of particle center distribution, the calculations can be carried out without resorting to simplifying assumptions or numerical integration. The significance of radial profiles of particle surface area is also discussed.

Evaluation of an adsorption system to concentrate VOC in air streams prior to catalytic incineration

Catalytic combustion is a well-developed process for the removal of volatile organic compounds (VOCs). In order to reduce both the amount of catalyst needed for incineration and the surface area of recuperative heat exchangers, an evaluation of the use of thermal swing adsorption as a previous step for VOC concentration is made. An air stream containing ethyl acetate and ethanol (employed as solvents in printing processes) has been taken as a case study. Based on the characteristics of the adsorption/desorption system and the properties of the stream to be treated, a monolithic rotor concentrator with activated carbon as adsorbent material is adopted. Once the temperature of the inlet desorption stream TD is chosen, the minimum possible desorption flow rate, WD,min, and the amount of adsorbent material can be properly defined according to the extent of the Mass Transfer Zone (MTZ) at the end of the adsorption stage. An approximate procedure to speed up the calculations needed for sizing the bed and predicting the operating variables is also presented. In the case studied here, the concentration of the VOC stream can reach 6 times that of the primary effluent when TD = 200 °C is chosen.

An algorithm for evaluating reaction rates of catalytic reaction networks with strong diffusion limitations

Abstract An h -adaptive mesh procedure to solve the reaction–diffusion problem for multiple reactions in catalytic pellets presenting strong diffusion limitations is developed. The discretization approach selected for this purpose is based on an integral formulation of the conservation equations. The adaptive mesh procedure relies on estimating the error of local reaction rate evaluations. By adding or removing nodes the errors will eventually become bounded within pre-set limits. The algorithm is tried on some test cases derived from the liquid phase catalytic hydrogenation of butadiene and butyne in butene (1, 2- cis and 2- trans ) rich hydrocarbon mixtures.