Wall-cooled Fixed-bed Reactor Research Articles

Dynamic Carbon Dioxide Methanation in a Wall-Cooled Fixed Bed Reactor: Comparative Evaluation of Reactor Models

A comprehensive model comparison is presented for the description of the steady state and dynamic behavior of a wall-cooled fixed bed methanation reactor. First, the maximal conversion predicted by different steady state models is compared and evaluated. Second, the dynamic reactor behavior is investigated based on positive and negative step changes of the inlet flow rate. The examined model complexity ranges from a simple one-dimensional pseudohomogeneous reactor model to a two-dimensional heterogeneous model. The results show that similar maximal conversions are predicted by each model, but large discrepancies appear for the prediction of the dynamic reactor behavior. Our data clarify the impact of the considered heat and mass transfer resistances on the model prediction and allow us to give specific recommendations for further steady state and dynamic modeling works.

Influence of Particle Size and Single‐Tube Diameter on Thermal Behavior of Fischer‐Tropsch Reactors. Part II. Eggshell Catalysts and Optimal Reactor Performance

AbstractThe optimal combination of particle and tube size for simulation of a single tube of a wall‐cooled multitubular Fischer‐Tropsch (FT) reactor with cobalt as catalyst was determined. The maximum size of the tubes, realized without temperature runaway, enhances with increasing particle size until an optimal value is reached, thereby improving the production rate of liquid fuels per tube. Reasons for this are that heat transfer to the cooled tube wall for a given tube size is considerably enhanced by increasing the particle size and that the influence of pore diffusion on the effective rate of FT synthesis gets stronger with rising particle size, which reduces the temperature sensitivity of the reactor and decreases the danger of a temperature runaway. The simulations indicate that the use of FT eggshell catalysts is not an option for fixed‐bed reactors. The temperature sensitivity of the reactor is strongly enhanced, which decreases the maximum tube size and with that the productivity per tube. All these effects are valid in general for wall‐cooled fixed‐bed reactors. Respective criteria are presented.

Oxidative dehydrogenation of ethane to ethylene in a membrane reactor: A theoretical study

This contribution presents a theoretical study of a multitubular packed-bed membrane reactor for the ethane to ethylene oxidative dehydrogenation reaction over a highly active and selective Ni–Nb–O mixed oxide catalyst. This theoretical study takes into account the radial composition and temperature profiles using a two-dimensional pseudo-homogeneous model on the reaction side. The feasibility and convenience of using this novel design, as well as the influence of the main operating variables on the reactor performance, are analyzed. The introduction of the membrane leads to lower oxygen partial pressures inside the catalyst tubes, which results in an improved selectivity to ethylene (lower heat generation rates) and high effective heat transfer area per unit volume. The multitubular membrane reactor enables significant ethylene productions per tube and milder temperature profiles than a conventional wall-cooled fixed-bed reactor. Operating conditions have to be carefully adjusted to avoid undesired oxygen accumulation inside the tubes. The presence of small amounts of oxygen at the reactor inlet significantly improves the ethylene production rates.

Optimizing control of a wall-cooled fixed-bed reactor

A novel on-line optimizing control strategy for wall-cooled fixed-bed reactors is presented. It is essentially a nonlinear model predictive algorithm in which the profit is uniquely expressed as the objective function, instead of a square sum of the deviations between model predicted outputs and a desired output variable trajectory. Excessive bed temperature is avoided by setting a constraint on the maximum reactor temperature. Excellent performance including fast tracking to the optimum operating condition and small overshooting of the reactor temperature has been demonstrated by taking advantage of a dynamic KL-NN reactor model.

A hybrid neural network-first principles model for fixed-bed reactor

In the present work, we combine first principles, in the form of mass and energy balance equations, with artificial neural networks (ANNs) as estimators for some of the important process parameters in modeling a wallcooled fixed-bed reactor. Experiments were carried out in a pilot wall-cooled fixed-bed reactor with benzene oxidization to maleic anhydride as a working reaction to show the performance of the proposed hybrid models. Compared with the two-dimensional pseudo-homogeneous models, the hybrid models predict equally well, but are simpler in structure and therefore easier to meet the real-time requirements of control and on-line optimization.

Initial estimation of heat transfer and kinetic parameters of a wall-cooled fixed-bed reactor

A numerical method is developed for the initial guesses of heat transfer and kinetic parameters in a wall-cooled fixed-bed reactor accompanied with side-stream analysis in both the axial and radial directions. The differential method is found to be the only way to decouple the heat and mass transport equations, and thus enables the estimation of the heat transfer and kinetic parameters by separate steps. Compared with simultaneous estimation by the integral methods, the differential method possesses a unique advantage arising from the separate estimation of the parameters, because the parametric correlation can be reduced since the parameter number in the separate estimating process is less than that in the integral process. Numerical analysis and experimental studies reveal that the most appropriate method for presenting the axial derivative is by the Pad≐ approximation, and that for the radial derivatives is by the orthogonal collocation with one point.

Simultaneous estimation of kinetic and heat transfer parameters of a wall-cooled fixed-bed reactor

Heat transfer and kinetic parameters of wall-cooled fixed-bed reactors are traditionally obtained through separate heat transfer and kinetic experiments. In spite of its simplicity, the uncertainties resulting from the low parametric sensitivity and high correlationship among the parameters inevitably make the estimation unsuitable for safe extension to actual reacting conditions. In this paper, an effort was made to estimate simultaneously both heat transfer and kinetic parameters under reacting conditions in a single tube wall-cooled fixed-bed reactor, and a two-stage parameter estimation procedure was developed. The correlation and confidence region analysis performed in this paper verifies both theoretical and experimental feasibility of simultaneous estimation of heat transfer and kinetic parameters under real reacting conditions.