Studies on modeling and control of spouted bed reactors—II: Control

Control and operability characteristics of an adiabatic spouted bed reactor with a first order reaction are investigated. The reactor transient response to changing operating conditions is analyzed and the generic control problems are formulated. Reduced-order state space models suitable for process control studies are constructed by utilizing the methods of orthogonal collocation and singular perturbation.Based on modern control and estimation techniques a hierarchy of different control systems is systematically generated. The assessment of alternative d     

Solids motion at horizontal tube surfaces in a large gas-solid fluidized bed

The motion of fluidized particles at the surface of 38 mm diameter horizontal tubes, immersed in a 1.2 m square bed of silica sand (U_mf = 0.3 m/s), fluidized at 0.9 m/s has been observed using photographic techniques (200 fps). It is shown that the solids motion is different for centrally located tubes and those adjacent to a side wall. Data on particle velocity and surface contact are presented and the degree of particle contact at various zones around the tube circumference is shown to vary in a similar manner to published localized heat transfer rates.     

Hydrodynamics of constrained inverse fludization and semifluidization in a gas-liquid-solid system

Experiments were conducted to study the hydrodynamic behavior of the constrained inverse gas-liquid-solid fluidized bed and the semifluidized bed where the liquid is the continuous phase. Also included in the experiments was a study of the hydrodynamic behavior of countercurrent flow of gas and liquid in a packed bed. This part of the study simulates the packed section of a constrained inverse semifluidized bed. A mathematical model is developed to account for the friction factor between the liquid and solid in the packed bed. The gas hold-up and friction factor of the packed bed are analyzed and empirically correlated.Wallis' drift flux model is modified to describe the gas hold-up in a constrained inverse fluidized bed. For the constrained inverse semifluidized bed,     

The effect of liquid flow distribution on the behaviour of a trickle bed reactor

A mathematical model has been formulated of the effect of flow distribution of the liquid phase carrying a dissolved reactant on the progress of an nth order, irreversible, catalytic reaction with heat effects in an adiabatic trickle bed reactor. The model has been stated in terms of the density of irrigation, temperature and concentration of the reactant in the liquid, all treated as spatially distributed variables. Provisions have been made to account for the existence of the flow down the surface of the wall, which has no catalytic effect.Local concentration and temperature have been proven to be coupled by the invariant T + Uγc = γU. The same invariant governs also local concentration and temperature of the wall flow. Mathematically, the model is represented by a coupled set of nonlinear parabolic partial differential equations enabling concentration and temperature fields to be obtained for an arbitrary type of liquid distribution and intensity of the wall flow.Numerical solutions have been obtained by the finite-difference method simulating reactors irrigated by liquid distributors as central discs of different radii, or a central annulus, and strongly exothermic reactions with the reaction order ranging between 0.1 and 2. Numerical results have shown the effect of liquid distribution on the overall reaction conversion to be very complex. Optimum initial distribution varies depending on the reaction order as well as the required degree of conversion. In general, however, the entrance region flow pattern may play a significant role in affecting especially reactions exhibiting kinetics close to zero order (hydrogenations). The effect of the wall flow has been found unambigously adverse to reaching high conversions and of increasing importance for low order reactions.     

Studies on modeling and control of spouted bed reactors—I: Reactor modeling

This paper presents for the first time a theoretical nonisothermal dynamic model for spouted bed catalytic reactors. A pseudo-homogeneous on-dimensional model is developed for an adiabatic spouted-bed with a first order reaction. The effects of various design parameters on the steady-state reactor performance are established.     

Gas phase solid-catalysed chemical reaction in spouted beds

Previously reported experimental work on the decomposition of ozone in a 0.15 m diameter spouted bed of angular iron oxide catalyst particles has been extended to spherical catalyst particles in both 0.15 m and 0.22 m dia. columns. Other variables, aside from column diameter, included catalyst particle size, bed depth, spouting velocity, and reaction rate constant (varied by adjusting the bed temperature). The use of a photometric analyser allowed analysis of concentrations at the inlet and exit from the bed, at different radial positions at the bed surface, and at several positions within the bed itself. Very little effect of particle size was found over the range studied (2.58 to 4.38 mm). Contrary to previous expectations, the conversion was found to decrease as the bed diameter was increased.The concentration gradients observed in the annulus indicate that the streamtube model is superior to the one-dimensional model, despite the fact that overall conversions predicted by these previously derived models are again similar. The observed gradients were not, however, as pronounced as those predicted by the streamtube model, and this suggests that radial dispersion in the annulus cannot be entirely neglected.     

Oxidation of sodium sulphide in a foam bed contactor

Oxidation of sodium sulphide to sodium thiosulphate has been experimentally investigated in a foam bed contactor using air as oxidizing medium. The var     

Modelling of fluidized bed reactors—VI(a): An isothermal bed with stochastic bubbles

A two-phase stochastic isothermal fluidized bed reactor model with first order reaction in the dense phase is developed to investigate the significance of the fluctuating nature of fluidized beds on reactor performance. Several stochastic processes are employed as the overall mass transfer coefficient between phases. Analytical moment solutions are obtained for white noise coefficients while hybrid computer simulation was used for correlated stochastic coefficients. Results indicate that a gamma distributed coefficient is preferred over white noise and Gaussian correlated coefficients. When compared with the deterministic model, randomness in the mass transfer coefficient is seen to lead to a decrease in reactor performance. Deviation from the deterministic model increases with increasing variance and decreasing fluctuation frequency of the correlated stochastic coefficients.     

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].     

Mass transfer from a slug in a fluidised bed at elevated pressures

The rate of mass transfer between bullet-shaped bubbles (slugs) and the dense phase of gas fluidised beds of fine particles (glass ballotini 64 μm and 101 μm average diameter) has been measured at pressures between 1 and 5 bar.The experimental values were compared with the theoretical predictions of Hovmand and Davidson[1] and Guedes de Carvalho and Davidson[1]; most values fall within 40% of the theoretical predictions, but the effect of gas diffusivity on mass transfer is not corroborated by experiment except for one set of values. Reduction of mass transfer by saturation of the wake is also considered and an approximate quantitative prediction of this effect is derived that is thought to be applicable to slugs in beds of fine particles.     

Flow regimes for condensation of a vapour inside a horizontal tube

Experimental data are reported for local heat transfer coefficients during condensation of a pure vapour inside a horizontal tube. Approximately 800 lo     

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.     

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.     

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     

Mass transfer with chemical reaction in a foam bed contactor

On the basis of dodecahedral structure of a foam bed, a model to predict conversion in a foam bed contactor with mass transfer with chemical reaction has been developed. To verify the proposed model, experiments have been carried out in a semi-batch apparatus for the absorption of lean CO₂ gas in a foam of sodium hydroxide solution. The proposed model predicts fairly well the experimentally found absorption values.     

Estimation of the particle diffusivity in a liquid-solids fluidized bed based on a stochastic model

A simple method was developed for estimating the particle diffusivity or mixing coefficient in a liquid-solids fluidized bed from the variance (dispersion) of the number of particles in a given section of the bed during relaxation in the bed expansion. This variance was estimated from the pressure drop history above the location of the pressure sensor recorded continuously after the stepwise disturbance in the velocity of the fluidizing medium. The method has been verified by comparing the particle diffusivities obtained by the present method with the available data obtained by other methods.     

Investigating the eruptions of fine−grain material from a nonuniform fluidized bed

A model is proposed for the entrainment of particles from the surface of a fluidized bed. It is shown that the deep voids produced by coalescence of twThe gas flux across the elongated ellipsoidal voids at the bed surface was investigated when the superficial gas velocity was equal to w_mf andThe analytical solution for the gas flow through the ellipsoidal void when the Reynolds number is small was derived and an empirical correlation is givAn empirical correlation for prediction of the maximum entrainment height was obtained. This includes a dependence on diameter and height of the bed, f