THE ROLE OF GAS DISTRIBUTION IN FLUIDIZED BED CHEMICAL REACTOR DESIGN†

Abstract

The design of fluid bed gas distributors may have a marked influence on the performance of a fluid bed reactor. The primary physical reason for this influence is that the distributor design influences the hydrodynamics and thus the gas/solid contacting pattern in the fluidized bed.

In the paper presented here the influence of distributor design on mass transfer and chemical reaction has been investigated systematically in fluid bed reactors with diameters of 0.2 and 1.0 meter. Coefficients of mass transfer between the bubble phase and the suspension phase were determined from chemical conversion and tracer gas residence time distribution measurements. In the experimental program the height of the fluidized bed was varied between 0.3 m and 0.9 m with superficial gas velocities in the range of 0.06 m/s to 0.30 m/s.

The comparison of the experimental results with a suitably modified and extended two-phase model yields quantitative relationships which allow to account for the influence of the gas distributor in the design of fluid bed chemical reactors.     

THE ROLE OF GAS DISTRIBUTION IN FLUIDIZED BED CHEMICAL REACTOR DESIGN†

The design of fluid bed gas distributors may have a marked influence on the performance of a fluid bed reactor. The primary physical reason for this influence is that the distributor design influences the hydrodynamics and thus the gas/solid contacting pattern in the fluidized bed.

In the paper presented here the influence of distributor design on mass transfer and chemical reaction has been investigated systematically in fluid bed reactors with diameters of 0.2 and 1.0 meter. Coefficients of mass transfer between the bubble phase and the suspension phase were determined from chemical conversion and tracer gas residence time distribution measurements. In the experimental program the height of the fluidized bed was varied between 0.3 m and 0.9 m with superficial gas velocities in the range of 0.06 m/s to 0.30 m/s.

The comparison of the experimental results with a suitably modified and extended two-phase model yields quantitative relationships which allow to account for the influence of the gas distributor in the design of fluid bed chemical reactors.     

A correlation to the solution of the two-phase dispersion model

A correlation to the solution of the two-phase dispersion model has been developed for gas-solid fluidized bed reactors operating in the bubbling regime. An analytical solution was obtained for fractional gas conversion by using an exponential function to characterize the dense phase gas concentration profile. The coefficient of the exponential function was found to depend on gas axial dispersion and, in order to determine this parameter, a Peclet number correlation was developed. Model predicted gas conversions were in excellent agreement with experimental conversions for a variety of fluidized bed reaction data over a conversion range from 2.5 to 99%.     

Grundlagen der Wirbelschichttechnik

Fundamentals of fluidized bed technology . Although gas/solids fluidized beds have long been successfully employed in a variety of industrial processes, the design and optimization of fluidized bed reactors still presents considerable difficulties, owing to the complex nature of gas/solids flow in the fluidized bed. The present review collects various flow-mechanical principles for the design of fluidized bed equipment. The central role of flow mechanics in the operating properties, and in particular for scale-up of fluidized bed reactors, is illustrated for the model of a simple heterogeneously catalyzed gas-phase reaction.     

A transport-oriented approach to residence time distributions and two-phase chemical reactors

This paper focuses on two-phase (e.g., fluid-solid catalyst) chemical reactors where one phase participates in the feed and effluent, first order chemical reactions proceed in the other phase and linear, species-dependent, interphase transport connects the phases. A class of reactors exists in which the Laplace transform of the dynamic reaction/interphase transport equations reduces to essentially that of an effective set of first order reactions amongst effluent phase species only, in an imaginary single phase reactor. This result bears on scale-up: Since the imaginary reactor simply scales with the usual residence time distribution, so too do reactors of this class scale with its analog. This easily measurable analog turns out to be just the non-adsorbing tracer experiment. Significantly, certain reactors outside of this class do not scale likewise, even with first order chemistry; an example illustrates. The class in question includes, but is not limited to, fixed bed and Berty (CSTR) reactors. The analysis allows design inferences for two-phase reactors, including fluidized beds.     

Modular Simulation of Fluidized Bed Reactors

Simulation of chemical processes involving nonideal reactors is essential for process design, optimization, control and scale‐up. Various industrial process simulation programs are available for chemical process simulation. Most of these programs are being developed based on the sequential modular approach. They contain only standard ideal reactors but provide no module for nonideal reactors, e.g., fluidized bed reactors. In this study, a new model is developed for the simulation of fluidized bed reactors by sequential modular approach. In the proposed model the bed is divided into several serial sections and the flow of the gas is considered as plug flow through the bubbles and perfectly mixed through the emulsion phase. In order to simulate the performance of these reactors, the hydrodynamic and reaction submodels should be integrated together in the medium and facilities provided by industrial simulators to obtain a simulation model. The performance of the proposed simulation model is tested against the experimental data reported in the literature for various gas‐solid systems and a wide range of superficial gas velocities. It is shown that this model provides acceptable results in predicting the performance of the fluidized bed reactors. The results of this study can easily be used by industrial simulators to enhance their abilities to simulate the fluidized bed reactor properly.     

Distribution of the gas flow in fluidized beds with a slugging behaviour

Capacitance probe measurements of the visible bubble flow rate have been made in a pressurized fluidized bed burning coal. The bed, of 0.3 × 0.3 m cross-section, was operated at pressures between 1.0 and 2.0 MPa and at temperatures between 750 and 900°C. The fluidizing velocity was 0.95 m/s and the mean particle diameter was 0.9 mm. Based on the experimental results, a model of the gas distribution between the bubble phase and the particulate phase in fluidized beds with a slugging behaviour was developed. The model accounts for the lack of bubble flow obtained if the two-phase theory is employed. In order to verify the model, simultaneous measurements of the visible bubble flow rate and of the gas flow rate through the bubbles were carried out in a bed of similar geometry but operating at ambient conditions. In this bed the fludizing velocity was varied between 1.6 and 2.7 m/s and the mean particle diameter was 1.0 mm. The through-flow of gas was measured with the aid of pressure probes. Evaluation of the experimental results using the model showed that this gas through-flow in the bubble phase subsequently increases the superficial gas velocity in the particulate phase between the vertically aligned bubbles (slugs), and that this gas velocity in excess of the incipient fluidization velocity is responsible for the large deviation from the two-phase theory. The associated increase of the particulate phase voidage was calculated via the Ergun equation.     

Comparison of two-phase upflow and downflow fixed bed reactors performance: Catalytic SO2 oxidation

A time- and space-dependent model based on the piston-dispersion-exchange model for liquid flow was developed to analyze the performance of two-phase upflow and downflow fixed bed reactors and was applied to the catalytic SO₂ oxidation. The hydrodynamic parameters were determined from residence time distribution measurements, using an imperfect pulse method for time-domain analysis of nonideal pulse tracer response. A transient diffusion model of the tracer in the porous particle coupled with the PDE model was used to interpret the obtained RTD curves. Gas-liquid mass transfer parameters were determined by a stationary method based on the least square fit of the calculated concentration profiles in gas phase to the experimental values. It is shown that two-phase downflow fixed bed reactor performs better at low liquid flow rates, while two-phase downflow fixed bed reactor performs better at low liquid flow rates, while two-phase upflow performs better at high liquid flow rates.     

The hydrogenolysis of n-Butane on a nickel on silica catalyst: II Fluidized bed studies

This study was undertaken to obtain experience in developing steady-state mathematical models of full-scale chemical reactors through the use of a combination of bench-scale experiments and actual plant operating data. In this case, the hydrogenolysis of n-butane on a nickel on silica gel catalyst was studied in a %inch diameter pilot plant fluidized bed reactor. The kinetic parameters in a mechanistic reaction model were obtained in a bench-scale integral reactor and these rate equations were included in a simple two-phase fluidized bed model. The parameter relating to interchange of gas between bubbles and emulsion was determined from the fluidized bed data. A comparison of the predicted and observed selectivities suggested a weakness in the kinetic model. This shortcoming was then partially corrected by using the fluidized bed data. In this way, a reactor model was achieved which was satisfactory for overall plant simulations.     

Katalytische Suspensions-Reaktoren

Catalytic suspension reactors . Reactions occurring in slurries are frequently encountered in industrial chemistry. The field of application of suspension reactors and their advantages and disadvantages compared with other catalytic reactors are underlined. It should be borne in mind when calculating space-time yields that several drag components (gas/liquid and liquid/solid mass transfer, as well as pore diffusion inhibition) precede the actual reaction. They are best accounted for by the concept of series switching of partial resistances. In the case of highly active and finely divided catalysts, however, absorption acceleration may take place. Methods are presented for determining reaction and transport parameters and correlations of mass and heat transfer are considered. Particular attention is devoted to the question whether the characteristic quantities of heat and mass transfer in a three-phase system can be estimated from the experimental data for two-phase systems (gas/liquid and liquid/solid). This is shown to be feasible in many cases with an accuracy adequate for practical purposes. Reactor and catalyst performance show opposing trends in suspension reactors. Guidelines for optimal operation can be derived on the basis of simplified model concepts.     

(Gas‐)liquid‐solid circulating fluidized beds and their potential applications to bioreactor engineering

Compared with conventional fluidized beds, circulating fluidized beds have many advantages including better interfacial contacting and reduced backmixing (Lim et al., 1995). While there are many reports on the gas—solid circulating fluidized systems, liquid—solid and gas—liquid—solid circulating fluidized bed systems have been scantily studied. However, extending current knowledge obtained in gas—solid systems to liquid—solids and gas—liquid—solid three‐phase systems is shown to open new horizons for applications of circulating fluidized bed technology and expected to lead to the development of highly efficient liquid—solid and gas—liquid—solid reactors, especially for the ever growing field of biotechnology. In order to fully appreciate the potential of those two types of liquid phase circulating fluidized beds, recent progress is reviewed in this article. Their potential applications to biochemical processes are also discussed.     

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.     

Mass transfer at the confining wall of helically coiled circular tubes with gas–liquid flow and fluidized beds

Experiments were conducted to investigate the effect of various dynamic and geometric parameters on mass transfer coefficients in two-phase helically coiled flow systems. Computation of mass transfer coefficients was facilitated by the measurement of limiting current at the electrodes fixed flush with the inner surface of the tube wall. Two flow systems were chosen: a two-phase liquid solid fluidized bed and a two-phase gas–liquid up flow. An equimolar potassium ferrocyanide and potassium ferricyanide solution in the presence of sodium hydroxide was used as the liquid phase. In the fluidized bed, glass spheres and sand of different sizes were employed as fluidizing solids. In two-phase flow system nitrogen was employed as inert gas. The pressure drop in the presence of fluidizing solids in helical coils was found to increase with increase in the pitch of the coil and was maximum for straight tube. The mass transfer coefficients were found to increase with increase in liquid velocity. The mass transfer coefficients in case of gas–liquid flow were found to be independent of liquid velocity and the pitch of the coil, and were largely influenced by gas velocity only. The data were correlated using j_D factor, Helical number, Froude number and Stanton number.     

Fluidized bed catalytic reactor modeling across the flow regimes

Fluidized bed reactor models are generally specific to a single flow regime resulting in ambiguities and discontinuities at the regime boundaries. In practice, only the bubbling, turbulent and fast fluidization regimes are of industrial significance for catalytic reactions. The turbulent fluidization regime is especially advantageous because of improved interphase mass transfer, resulting in improved selectivities and conversions. It is shown that some of the difficulties in modeling can be resolved by means of the probabilistic-averaging model, recently published by Thompson et al. (1999). This model interpolates between the Grace (1984) two-phase bubbling bed model at low velocities and single phase axially dispersed flow for fully established turbulent fluidization conditions, leading to improved predictions of conversion and selectivity for catalytic fluidized bed reactors operated at flow rates covering the full range between bubbling and fully turbulent fluidization. An analogous approach should be useful for beds operated at higher gas velocities as fast fluidization conditions are approached.     

Two-phase modeling of a gas phase polyethylene fluidized bed reactor

A two-phase model is proposed for describing the behavior of a fluidized bed reactor used for polyethylene production. In the proposed model, the bed is divided into several sequential sections where flow of the gas is considered to be plug flow through the bubbles and perfectly mixed through the emulsion phase. Polymerization reactions occur not only in the emulsion phase but also in the bubble phase. Voidages of the emulsion and bubble phases are estimated from the dynamic two phase structure hydrodynamic model. The kinetic model employed in this study is based on the moment equations. The hydrodynamic and kinetic models are combined in order to develop a comprehensive model for gas-phase polyethylene reactor. The results of the model are compared with the experimental data in terms of molecular weight distribution and polydispersity of the produced polymer. A good agreement is observed between the model predictions and actual plant data. It has been shown that about 20% of the polymer is produced inside the bubble phase and as such cannot be neglected in modeling such reactors.     

A novel induction heating fluidized bed reactor: Its design and applications in high temperature screening tests with solid feedstocks and prediction of defluidization state

A novel mini induction heating fluidized bed reactor (IHFBR) is introduced which was developed to carry out screening tests of high temperature reactions up to 1500°C particularly for solid feedstocks. Despite conventional mini reactors, this reactor mimics real scenario of solid feeding in industrial reactors: cold feedstock is injected within 1 s from a lift tube, then particles reach reaction temperature in less than 5 s in a reaction zone. The lift tube (9.5 cm diameter) is also the gas distributor of the fluidized bed (2.5 cm diameter) so that the bed is completely fluidized with uniform gas distribution. Beside facilities to perform tests in a fluidized bed, another important feature of this reactor is prediction of the defluidization state in the bed. Not only reproducible data are generated, but also many tests can be conveniently carried out, that is, one test per hour. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1507–1523, 2015     

Dynamic modeling of gas phase propylene homopolymerization in fluidized bed reactors

A new model with comprehensive kinetics for propylene homopolymerization in fluidized bed reactors was developed to investigate the effect of mixing, operating conditions, kinetic and hydrodynamic parameters on the reactor performance as well as polymer properties. Presence of the particles in the bubbles and the excess gas in the emulsion phase was considered to improve the two-phase model, thus, considering the polymerization reaction to take place in both the bubble and emulsion phases. It was shown that in the practical range of superficial gas velocity and catalyst feed rate, the ratio of produced polymer in the bubble phase to the total production rate is roughly between 10% and 13%, which is a substantial amount and cannot be ignored. Simulation studies were carried out to compare the results of the improved two-phase, conventional well-mixed and constant bubble size models. The improved two-phase and well mixed models predicted a narrower and safer window at the same running conditions compared with the constant bubble size model. The improved two-phase model showed close dynamic behavior to the conventional models at the beginning of polymerization, but starts to diverge with the evolution of time.     

High temperature fluidized bed reactor: measurements,hydrodynamics and simulation

This paper was made possible through the development of a novel high temperature optical fiber probe to study the hydrodynamics of a high temperature fluidized bed reactor. The experimental results show that the hydrodynamic parameters considerably change with bed temperature when fluidizing FCC particles. For a given superficial gas velocity, the average local particle concentration, the dense phase fraction and the particle concentration in the dense phase decrease with increasing bed temperature. As a result of an increase in temperature, the fluidized behavior of the FCC particles progressively shifts from typical Geldart A towards B. Consequently, a modified two-phase model, based on the simple two-phase model, integrating the effects of temperature and superficial gas velocity on the hydrodynamics, is proposed. Simulation of a reactive catalytic system using a conventional simple two-phase model and the modified model is achieved. The predicted reactor performances strongly differ for each model. In the present case, the simple two-phase model underestimates the reactor performance by inadequately accounting for the solid fractions in the bubble and dense phases and their dependence on temperature and superficial gas velocity. This suggests that the hydrodynamic models should take into account the effects of temperature and superficial gas velocity when simulating the performance of a high temperature fluidized bed reactor.     

Two-Phase and Three-Phase Modeling of an Industrial Fluidized Bed for Maximizing Iso-Paraffins

The maximizing iso-paraffins (MIP) process is a new fluid catalytic cracking route to produce cleaner gasoline. Its major innovation is the diameter-transformed fluidized bed reactor which can be flexibly regulated to multiple distinct reaction zones. This study aims to accurately reveal complex behaviors in MIP reactor by two-phase modeling and three-phase modeling to extend its application. Both simulations of an industrial MIP reactor are compared in terms of solids and liquid concentration, temperature, coke content, gas velocity, and product yield. It is found the two-phase case is enough for predicting the region far away from the oil feedstock injection, and the other is the better choice especially in the first zone if the heat transfer model can be reasonably built.