Non-isothermal effects in a single pellet diffusion reactor

A theoretical analysis of an adiabatic non-isothermal single-pellet diffusion reactor shows that non-isothermal effects can alter the value of the centerplane concentration. Deviation from isothermal behavior is significant under exothermic conditions where multiplicity of solutions can lead to reactor instabilities or falsification of reaction parameters. Conversely, it is shown that the internal isothermal model is a good approximation to detect non-isothermal pellet behavior from centerplane concentration measurements.     

Finite element collocation solution of tubular and packed-bed reactor two point boundary value problems

Nonlinear two point boundary value problems (TPBVP's) are frequently encountered in chemical engineering. TPBVP's describing tubular and packed-bed reactors are important and have received much attention in the literature. For certain ranges of parameter values of the reactors, the TPBVP's become stiff and pose problems in numerical solution. These problems have been efficiently solved by the finite element collocation method using Hermite and B-spline functions in conjunction with quasilinearization. Numerical results have been presented and discussed. The problems have been solved for a wide range of parameter values without encountering any numerical difficulty.     

Photocatalytic ammonia synthesis in a fluidised bed reactor

Photocatalytic ammonia synthesis was successfully performed in a fluidised reactor of parallel-wall configuration by irradiating iron-doped titanium dioxide with near ultraviolet light. The catalyst was prepared in such a way that good quality of fluidisation could be obtained. Mixing the catalyst with γ-alumina was found to affect the fluidisation behaviour. Ammonia production was increased when the catalyst was suitably fluidised because of enhanced utilisation of light energy. Quantum efficiencies of more than 10% have been reached. The parallel-wall reactor configuration was found to give higher energy conversion than a cylindrical one [1a].     

Mixing of chemically reactive fluids by swirling in a tubular reactor

To achieve a varying degree of radial mixing in a single phase fluid, a stable laminar swirl is created by two fluid streams tangentially entering a tubular reactor. The details of the resultant fluid motion is described using the material stretching coordinate technique. The combination of fluid motion, molecular diffusion, and intrinsic chemical kinetics provides the basis for investigating mixing with chemical reactions for nonpremixed feeds.Computations on an infinitely fast second-order reaction for the radial Peclet number ranging from 10² to 10⁶ reveal the following features for     

Kinetics of catalytic hydrodesulfurization of a petroleum residue in a batch-recycle trickle bed reactor

The kinetics of catalytic hydrodesulfurization of an atmospheric residuum were investigated in a batch-recycle trickle bed reactor with a commercial catalyst. The global rate equation determined was R = Catalyst effectiveness factors η, remaining activity ε, effective pore diffusivities D_e and activation energies E have been determined quantitatively. The data have also been interpreted in terms of the two parallel, first-order reactions model and the respective parameters have been determined. Solid—liquid chromatography fractions of three hydrocarbon and sulfur compound groups have been determined in addition to the asphaltenes content of the feedstock and the hydrotreated product. These data are discussed and conclusions are drawn with respect to their behaviour during HDS and a reaction network is proposed which explains the complicated interrelations involved.     

Solid particle mixing in a continously operated fluidized bed reactor

In a bench-scale fluidized bed reactor, 20cm in dia., residence time distribution of solid particles (d_p, = 137μm) were measured by the radionuclide technique (²⁴Na₂C0₃) in the absence and in the presence of the chemical reaction 2NaHCO₃→Na₂C0₃+C0₂+H₂O. The residence time distributions were evaluated by a backflow cascade model by nonlinear optimization. The radial and logitudinal concentration profile of NaHCO₃ in the emulsion phase of the reactor were measured during steady-state operation. The solid is well-mixed.The connection of the measured bubble properties with the longitudinal solid dispersion coefficient and the use of the Haines-King-Woodburn model allow     

The electrochemical recovery of iron in a two-compartment batch electrochemical reactor

A model has been developed for a two-compartment batch electrochemical reactor in which simultaneous iron deposition and hydrogen evolution occur at the cathode, the anodic reaction being the production of hydrogen ions. The model incorporates ionic diffusivities through the diaphragm and a composite kinetic parameter for the cathodic reactions. In order to apply the model to data from a small batch reactor, the diffusivities have been obtained under conditions in which hydrogen alone is evolved at the cathode. The value of the kinetic parameter has been chosen to provide the best agreement between the model predictions and the experimental results. Satisfactory predictions of the current efficiency for iron deposition and the final concentrations in the reactor has been obtained, the largest discrepancies between predicted and actual concentrations being at high anolyte hydrogen ion concentrations. Reasons for these discrepancies are discussed.     

Variable measurement structures for the control of a tubular reactor

The feasibility of the variable measurements structure is investigated within the framework of controlling a tubular chemical reactor. An on-line sequential analysis of the temperature measurements along the length of the reactor, allows a fast and time-adaptive selection of the best measurements to be used in a low order, simple and robust control scheme. The approach has been developed for both static and dynamic cases. It is based on a recursive computation of the state estimate errors. Its theoretical and computational simplicity makes it attractive for on-line implementation. The procedure has been applied for the adaptive tubular reactor under pseudo-steady state and dynamic conditions with very encouraging results.     

Residence time distribution of plug flow in a finite packed-bed chemical reactor

The residence time distributions in a finite packed-bed chemical reactor under plug flow conditions have been studied. Analysis was performed upon the well-known axial dispersion model in one dimension. Of paramount importance it was to construct a high order approximate solution to the corresponding initial-boundary value problem which appeared to be extremely convenient for fast numerical calculations. To this purpose singular perturbation techniq were applied using the reciprocal of the Péclet number as a small parameter. An error analysis was subsequently established for the special case of a pulse-function tracer input. The so-called “tailing phenomenon” of the response curve was simulated by an appropriate parameter-depending boundary condition of diffusion type.     

Drying of porous granular materials

Drying characteristics have been measured for two types of granular compound fertilizer and for granular mono calcium phosphate. Air was forced through a fixed bed, and the humidity of the air leaving the bed was analysed by means of an infrared photometer.All results confirm the assumption of vapour diffusion as the rate determining mechanism. Experiments with varying temperature and air humidity show thA shrinking core model was successfully used for predicting the drying behaviour of two of the materials. For the third material the drying rate fell r     

Modeling a low pressure steam-oxygen fluidized bed coal gasifying reactor

A comprehensive model accounting for the jetting region and homogeneous dilute phase reactions is developed for the adiabatic and continuous gasification of coal particles in a fluidized bed. The division of flows in the bed is determined by means of a modified two-phase theory which considers inlet gas jets, bubbles, free of particles, which develop at the top of the jets and grow in size as they rise, and an emulsion phase consisting of particles and the surrounding interstitial gas. The model describes the gasification of coal particles by pyrolytic devolatilization and three heterogeneous chemical reactions: oxidation by oxygen and steam, and reduction of carbon dioxide. Carbon monoxide and hydrogen produced by the heterogeneous reactions can be oxidized to carbon dioxide and steam by incoming oxygen within the dilute phase jets and bubbles. Furthermore, the water-gas shift reaction can occur in the dilute phase and interstitial gas. Simulations both with and without homogeneous reactions occurring in the jets and bubbles indicate that dilute phase homogeneous reactions have considerable influence on carbon conversion, bed temperature, and product gas composition. It has also been found that the jetting-emulsion mass and beat interchange has a substantial effect on overall bed performance and the temperature of the bed close to the inlet gas distributor. Results indicate that water-gas shift equilibrium is established rapidly and significant quantities of hydrogen and carbon monoxide and a nonuniform steam concentration are present within the combustion zone.     

An experimental reactor for kinetic studies: continuously-fed batch reactor

The use of a continuously-fed batch reacter (CFBR) as an experimental tool for the kinetic studies is presented for liquid- and gas-phase reactions, and noncatalytic gas-solid reactions. An experimental CFBR run is demonstrated using the reaction involving sucrose conversion to substantiate the theoretical results. The concept used for the CFBR is the fact that at an extremum of concentration the rate of concentration change is zero. For the liquid- and gas-phase reactions, the CFBR can be used as an alternative to the CSTR. For slow reactions, the rate determined from a CFBR run is more accurate than the rate determined from a CSTR run. Determination of the rates over the entire conversion range is made easy for fast reactions with the use of the CFBR. Results are extended to noncatalytic gas-solid reactions. It is shown that the fractional change of reacting surface area of the solid reactant can be obtained using the CFBR under the limiting condition of negligible effect of mass transfer.     

A revised model for predicting the performance of a fluidised-bed catalytic reactor

In a gas fluidised-bed, bubble coalescence gives rise to a very rapid mass exchange rate with the dense phase. The extent of this exchange rate is calculated from the observed changes in bubble size with height in the bed. It is calculated that with a catalytic reaction in progress, the difference in composition between the dense phase and bubble phase rapidly falls to a negligible value for all cases where the reaction rate and throughput are su that a significant height of bed is required for nearly complete conversion. A corresponding model of the fluidised-bed catalytic reactor is analysed f which it transpires that its behaviour is very close to that of a CSTR, as is apparent when published data on reactor performance is examined. The stat mixing of the dense phase, if it were to depart from the completely mixed condition assumed, would influence this conclusion.     

Effectiveness factor of a catalyst pellet in a trickle bed reactor: Limiting reactant in the gas phase

The influence of both internal and external wettings on the effectiveness factor of a partially wetted catalyst pellet in a trickle-bed reactor when the limiting reactant is in the gas phase is analyzed.A new parameter, not considered up to now, measuring the gas-liquid interfacial area inside and on the pellet surface, is introduced to study different forms of wetting.The reaction in the dry zone is also taken into account and its influence on the effectiveness factor for both the pores mouth and pores end dry is studied.An approximate analytical expression for the effectiveness factor, giving close results to the numerical solution, is proposed.     

Deactivation of an adiabatic fixed bed reactor by a Langmuir-Hinshelwood fouling mechanism

Fouling of an adiabatic fixed bed reactor has been simulated for a process where both the main reaction and the fouling reaction occur by a Langmuir-Hi mechanism. Series and parallel fouling mechanisms were investigated assuming a pseudo -steady state in the simulation and results were obtained in the where only unique solutions were possible. Comparison with first order fouling showed that Langmuir-Hinshelwood fouling is generally more severe and in case fouling by this means gives results contrary to previous predictions. An analysis of this abnormal behaviour was made by varying the activation en of the fouling reaction, the Thiele modulus and the adsorption constants.     

Kinetics of the catalytic dehydrogenation of cyclohexanol to cyclohexanone on a zinc oxide catalyst in a gradientless reactor

The catalytic dehydrogenation of cyclohexanol to cyclohexanone on a ZnO catalyst has been studied in a gradientless reactor (GLR) in the temperature range of 550–650 K. The data were fitted to several kinetic models using a non-linear least squares fitting computer programme. From model discrimination procedures, the most suitable model was found to be a Langmuir-Hinshelwood model, involving associative adsorption of cyclohexanol, cyclohexanone and hydrogen on the same kind of sites. No effects of film diffusion or intraparticular temperature gradients could be detected, while a certain influence of pore diffusion has to be taken into account at temperatures above 620 K. The mixing characteristics of the GLR have been investigated by measuring the mean residence time distribution and the recirculation ratio. Furthermore, the possibility of temperature gradients along the catalyst bed are discussed. The limitations of application of the GLR are shown and some suggestions to overcome the shortcomings are presented.