Germán D. Mazza

Plasma-enhanced chemical vapor deposition of nitrides on fluidized particles

Abstract A new plasma jet fluidized bed reactor working at atmospheric pressure is presented. The plasma arc is created inside the bed of particles in order to improve the reactivity between excited gaseous species and the particles to be coated. Metal chlorides, nitrogen and hydrogen were used as precursors of deposited layers. TiN, Si3N4 and SiOx coatings were deposited on silica and corundum particles by this plasma-assisted CVD process. Optical emission spectroscopy and electron microanalysis characterized the plasma and the layer composition, respectively.

Approximation of the effectiveness factor in catalytic pellets

The 1D model proposed by Burghardt and Kubaczka [Chem. Eng. Proc. 35 (1996) 65] to approximate the behavior of 3D catalytic pellets has been recently found able to provide accurate results for evaluating effective reaction rates when its parameter σ is suitable adjusted [Chem. Eng. Res. Des., submitted for publication]. This parameter represents the contraction of the cross-section available for diffusion. A formulation coupling a first-order Galerkin approximation with a truncated asymptotic expansion is proposed here to evaluate the effectiveness factor of single reactions in the range of interest −1/5 <σ< 5 [Chem. Eng. Res. Des., submitted for publication]. The formulation provides a 3% level of precision for essentially all normal kinetics of practical interest and a large range of abnormal kinetics. In particular, this conclusion includes reaction rates approaching a zero-order reaction, for which large deviations arise from the use of previous approximations proposed in the literature. On the other hand, the extent of abnormal kinetics being accurately approximated is significantly enlarged.

On the validity of the addition of independent contributions for evaluating heat transfer rates in gas fluidized beds

Abstract Different approaches for the approximate prediction of heat transfer rates between gas fluidized beds and immersed surfaces are analyzed in this work. They concern the interstitial gas and radiant contributions that have been frequently accounted for by adding approximate terms to the particle convective component which is usually the principal component. A generalized heterogeneous model considering simultaneously the three mechanisms and their interactions is presented and used to assess the validity of some previous approximations. This analysis is carried out by scanning practical ranges of the main decision variables: operating pressure, temperature and particle size.

The gas contribution to heat transfer between fluidized beds of large particles and immersed surfaces

Abstract The gas contribution to heat transfer between large particle fluidized beds and immersed surfaces is analysed through a heterogeneous model for the dense phase. An approximate solution of the governing balances provides a conceptually appropriate and precise enough expression for the gas heat transfer coefficient, h g . For most practical conditions h g amounts to 75—85% of the limiting heat transfer coefficient at the surface wall, h wg . The results are compared with previous correlations for experimental conditions at which they were formulated. The analogous mass transfer process is included in this analysis as a particular case of the proposed theory.

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.

A BEM FORMULATION FOR EVALUATING CONDUCTIVE HEAT TRANSFER RATES IN PARTICULATE SYSTEMS

A formulation based on the boundary-element method (BEM) to solve the conductive heat transfer problem between a particle and an adjacent heat exchange surface or between two particles is presented. This simple geometric configuration corresponds to a unit cell representing the thermal behavior near the wall or in the bulk of fixed and fluidized beds. The BEM is formulated by employing a variable number of nodes defined by the same quadrature points on which the integral coefficients of the influence matrix are evaluated. It is shown that this is a very efficient choice as compared to standard BEM formulations.

EVALUATION OF OVERALL HEAT TRANSFER RATES BETWEEN BUBBLING FLUIDIZED BEDS AND IMMERSED SURFACES

A mechanistic model to evaluate heat transfer rates between the dense phase of gas fluidized beds and immersed surfaces has been recently presented by the authors. This model, denoted Generalized Heterogeneous Model (GHM), is formulated in terms of effective thermal properties for particles and interstitial gas. It has been conceived with the purpose of achieving a generalized formulation accounting simultaneously for conductive, gas convective and radiant effects. The model was previously tested as regards its capability to predict radiative heat transfer rates in beds at high temperature and gas convective contribution in beds of large particles and high operating pressures. It is the principal object of this contribution to evaluate the performance of the GHM for a wide range of particle sizes, covering from The purely conductive regime to the gas convection dominant regime. Also, the main assumptions incorporated in the model are revised and some modifications are introduced, mainly on the basis of th...

EVALUATION OF CONDUCTIVE HEAT TRANSFER MECHANISMS BETWEEN AN IMMERSED SURFACE AND THE ADJACENT LAYER OF PARTICLES IN BUBBLING FLUIDIZED BEDS

The evaluation of the heat transfer coefficient hwp between a heat exchanging surface immersed in a gas fluidized bed and the adjacent layer of dense phase particles is analyzed in this contribution. Gas convective and radiant effects are not included in the present analysis. The inclusion of hwp, or an equivalent formation, in mechanistic models describing heat transfer has been necessary because the sudden voidage variation close to the immersed wall restrains significantly the heat transfer rate. However, there is not at present a widely accepted expression to evaluate hwp. A precise formulation for hwp accounting for transient conduction inside spherical particles, the Smoluchowski effect, the concentration of particles in the adjacent layer (Np) and an effective separation gap (l0) is developed here. Although Np can be estimated, in principle, from experimental evidence in packed beds, and it is reasonably expected that l0 = 0, the analysis of experimental heat transfer rates in moving beds, packed b...

On the use of incremental quotients in Michelsen's method for stiff ordinary differential equations

Abstract A systematic procedure for the use of incremental quotients instead of analytical derivatives in the Jacobians of Michelsens semi-implicit method (STIFF3) for stiff differential equations is presented. Runge—Kutta constants and step length adjustment criterion are modified to this end, and a mechanism for control of increment sizes is proposed. The procedure does not increase the number of function evaluations and its performance for some typical stiff systems is fully comparable to that with analytical derivatives.