S.P. Bressa

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.

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.

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.