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     

Multiplicity patterns in atmospheric fluidized bed coal combustors

A new mathematical model has been developed for the analysis of the steady state characteristics of Atmospheric Fluidized Bed Coal Combustors. A physical understanding of the different processes occuring within the bed is incorporated in the development of detailed mass and energy balances for the different reactor phases (bubble and emulsion phase, sorbent and coal particles). Care is placed on the modelling of the homogeneous and heterogeneous oxidation reactions, the growth of bubbles within the bed by coalescence and the heat transfer between the solids and the immersed steam generating tubes. Furthermore a population balance for the coal particles which accounts for variable size feed, fines elutriation and solids overflow enables the calculation of two very crucial bed operating variables: the carbon loading and the combustion efficiency. The model also predicts the temperature and the O₂ and CO concentrations within the combustor.The numerical solution of the dimensionless model equations is based on their reduction to a system of three nonlinear algebraic equations. Three steady states are obtained in the industrially important region of design and operational characteristics. They correspond to a high, intermediate and low reactor temperature and combustion efficiency. At the same time the respective values of carbon loading and O₂ concentrations are low, intermediate and very high. The results indicate that there exists a critical value of the coal feed rate, or of the superficial gas velocity, below which the bed is extinguished.     

A discrimination between some fluidized bed reactor models for ammoxidation of propylene to acrylonitrile

The study consists in making a discrimination between four fluidized bed reactor models: Davidson and Harrison, Partridge and Rowe, Kunii and Levenspiel, Kato and Wen models. Their predictions are compared with data obtained by carrying out experiments on the catalytic ammoxidation of propylene to acrylonitrile in a 165 mm dia. reactor. These experiments were designed to test the models as selectively as possible.On statistical ground, none of them is acceptable as representing the behaviour of the reactor over the whole range of operating conditions. However, the models of Partridge and Rowe, Kunii and Levenspiel and Kato and Wen lead to rather good predictions on limited ranges of gas flow rate or mass of catalyst. Among them, the bubble assemblage model of Kato and Wen provides the best overall predictions if it is slightly modified by including the wakes of the bubbles with their clouds or by reducing the bubble formation diameter, for instance.     

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].     

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.     

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.     

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     

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.     

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 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.     

Analysis of nitrogenous compounds in the effluent streams from a fluidized bed coal gasification reactor

A Texas lignite and a New Mexico subbituminous coal were gasified with steam and oxygen in a pilot-scale fluidized bed reactor at pressures from 770 kPa to 830 kPa, and temperatures form 795°C to 980°C. The make gas passed through a cyclone separator, and then a venturi scrubber in which condensable and water-soluble compounds were removed. The gasifier effluents (spent char, cyclone fines, tar, wastewater, and dry make gas) were analyzed for nitrogenous compounds.

For both coals, 6–12% of the nitrogen in the feed was retained in the spent char, with greater quantities being retained in the subbituminous coal char. Of the nitrogen volatilized from both coals, roughly 5% appeared in the tar, less than 0.2% appeared in the dry make gas as ammonia and NO_x, and the balance appeared in the wastewater as ammonia (60%), hydrolyzable nitrogenous compounds and possibly cyanate (10–15%), thiocyanate (1%), cyanide (0.5%), and other compounds (3–10%). The average concentration of NO_x in the dry gas was 7 ppm for lignite. No NO_x data for subbituminous coal were obtained. Reactor conditions (temperature, pressure, steam-to-carbon feed ratio) had no measurable effect on the production rates of nitrogenous compounds over the range of conditions investigated.     

Modeling of fixed bed catalytic reactors

The simulation of a fixed-bed catalytic reactor requires the selection of a model, which is a set of balance equations that describes the reactor, as well as correlations for the model parameters involved. In this work general criteria, leading to a better choice of a model that fulfills the objectives of the simulation, are established. Different ways in which the parameters can be obtained are analyzed, and the numerical methods for solving the model equations are discussed.     

Modelling of fluidized bed reactors—VI(b): The nonisothermal bed with stochastic bubbles

Two stochastic nonisothermal fluidized bed reactor models are developed to investigate the significance of the fluctuating nature of fluidized beds on reactor performance. Fluctuating bubble size distributions within the bed are simulated by stochastic mass and heat transfer coefficients. Results of hybrid computer simulations indicate that randomness can enhance or inhibit reactor performance depending on the operating parameters of the nonisothermal model. Bubble and dense phase concentration statistics are fairly similar to those of corresponding isothermal models because dense phase temperatures are relatively insensitive to transfer coefficient fluctuations due to the high dense phase beat capacity. However, the corresponding stochastic isothermal models predict decreases in conversion with increasing variance in the transfer coefficients for all operating conditions. Results indicate that a deterministic system with two stable steady states may have fewer stable random stationary solutions. The existence of the stationary states is dependent on fluctuation frequency and variance of the transfer coefficients.     

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.     

Fixed bed catalytic reactor modelling: The heat transfer problem

The issue of radial heat transport in fixed beds is discussed and discrepancies between observed and computed hot spots are attributed to neglect of axial dispersion of heat, which neglect also accounts for observed length-dependent radial conductivities.The Nusselt number for heat transport at the wall is inferred to be Nu_w = 5.73($\text{D}{}_{\mathrm{t}}\text{d}{}_{\mathrm{p}}$)^{$\text{1}\text{2}$}$\text{P}\text{r}$(0.11Re_p + 20.64)/Re_p^0.262 which is in accord with recently secured Sherwood numbers for mass transport at a packed wall.The recent success of unconventional reactor models is discussed.     

Statistical study of the grid zone behavior in a shallow gas—solid fluidized bed using a mini-capacitance probe

The behavior of the grid zone in a shallow-air-sand fluidized bed was studied by means of a mini-capacitance probe coupled with the on-line correlation technique. Experiments were carried out in a bed with a diameter of 203 mm. The bed had a height of 50 mm and was operated at different air flow rates. A perforated “plexiglas” plate with a hole diameter of 1.59 mm and a thickness of 9.53 mm was used as the distributor. The technique has been found to be effective in determining locally or microscopically the jet penetration depth and the dead zone height. The phenomenon of jetting in a shallow gas—solid fluidized bed is described semi-quantitatively.     

Measurements of bubble properties in continuously operated fluidized bed reactors at elevated temperatures

In a fluidized bed reactor the thermal decomposition of sodium hydrogen carbonate into sodium carbonate, carbon dioxide and water was carried out in continuous operation. The fluid dynamical structure of the bed was characterized by measurements with capacitance probes. Signal processing was carried out by a process computer.Data for bubble volume fraction, bubble rising velocity and bubble diameter have been obtained in the presence and absence of a chemical reaction. The influence of the reaction on the fluid-dynamical structure is significant.     

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.