Industrial Fluidized Bed Reactors Research Articles

Evaluation of heat transfer conditions during polydisperse gas–solid flow in an industrial fluidised bed reactor

Knowledge of heat transfer conditions in large-scale circulating fluidised bed reactors plays an essential role in their proper design and optimization of theirs. This work presents an evaluation of the heat transfer conditions taking into account the coordinate height of the furnace chamber and the fluidised bed flow regime in the reactor. Experimental tests used method and measurement technique that provides reliable temperature data. The vertical and horizontal temperature profiles were obtained at different 966MWth fluidised bed reactor operating conditions. Four characteristic areas can be distinguished in the furnace chamber: the bottom region with a high-temperature gradient from 10.6 °C/m to 22.7 °C/m; the dense region with a medium temperature gradient from 4.7 °C/m to 7.3 °C/m; the dilute area with the lowest temperature gradient from 0.7 °C/m to 3.1 °C/m and the exit region with a temperature gradient (4.3 °C ≤ grad T ≤ 9.6 °C) similar to dense region. The horizontal temperature profiles confirmed a core-annular flow structure inside the furnace chamber. At the pressure drop below 5 kPa, the low variation of average temperature (from ±0.7 °C to ±4.2 °C) inside the furnace chamber of the CFB reactor was observed. As a result of fly ash recirculation of 3.86 kg/s, the lowest temperature gradient 0.2 °C/m was achieved in the bottom region of the furnace chamber. In the range of bed material circulation rates of 32 to 68 were registered temperature fluctuations 115 °C. The experimental results also showed the variation of the bed temperature was small and kept at a very low level of ±13 °C whereas the fluidised bed reactor was operated with a fluidisation velocity range of 1.8 to 2.4. Obtained findings provide unique data on heat transfer conditions in industrial facilities and thus fill the gap in literature data for CFB reactors. Based on main tests carried out in a large-scale CFB reactor, it can be concluded that detailed experimental data on thermal conditions provide important information to validate guidelines and for verifying empirical relationships, which are used in the design, optimization, and scaling of heat transfer surfaces in commercial fluidised bed reactors.

Distribution and movement of fine particles injected in a gas–solid fluidized bed with hydrocarbon liquid spray

In a gas–solid fluidized bed with hydrocarbon liquid spray, a liquid-affected zone (cloudy zone) and a gas–solid zone (non-cloudy zone) coexist. Fine particles with different diameters are injected and shuttle between the two zones to simulate the motion of growing catalyst particles in an industrial fluidized bed reactor. The movement and distribution patterns of the fine particles are investigated via high-speed image and infrared image approaches developed. The probability density based on the coordinates of fine particles reflects clearly the distribution characteristics of fine particles in the bed at different times. Fine particles with smaller diameters are more easily enriched locally during the injection and diffusion process. Moreover, the proportion of fine particles in the cloudy zone to the total fine particles increases with the diameter of fine particles.

Probing the differences in CO2 adsorption/desorption behaviors of solid amine sorbents in fixed and fluidized beds

Circulating fluidized bed temperature swing adsorption (CFB-TSA) CO2 capture process using solid amine sorbents has become a widely recognized solution to reduce stationary sources CO2 emissions. The successful design and operation of the adsorber and desorber requires much information on the CO2 adsorption and desorption behaviors of the sorbents in fluidized bed reactors. However, among the existing studies, little attention has been paid to desorption performance, which is equally important as adsorption performance. And almost all of them were conducted in TGAs or fixed beds, the lack of research in fluidized beds making it difficult to guide the design of industrial fluidized bed reactors. In addition, the systematic comparison of the above-mentioned adsorption/desorption behaviors and bed hydrodynamics (e.g. pressure drops) in fixed and fluidized beds can help provide theoretical support for the optimization of adsorbers, desorbers and coolers, which has not been reported yet. Therefore, we for the first time investigated the CO2 adsorption/desorption behaviors as well as hydrodynamics in both fluidized and fixed beds using a solid amine sorbent in a temperature swing adsorption (TSA) experimental rig. The results show that the sorbent exhibits faster adsorption and desorption kinetics in fluidized beds than those in fixed beds, as reflected by the fitted parameters of the Avrami kinetic model and the change of bed temperature during adsorption and desorption tests. And the bed pressure drops in fluidized beds are also much lower than that in fixed beds, especially at high superficial gas velocities and high sorbent loadings. By further discussion, a fluidized bed can also achieve lower equipment investment and less sorbent loading compared to multiple fixed beds in parallel. This study demonstrates the advantages of fluidized beds over fixed beds in large-scale TSA units in view of higher reactor efficiency, lower energy consumption and equipment investment.

Role of mesoscale structure in gas–solid fluidization: Comparison between continuum and discrete approaches

The coarse-grained discrete particle model (DPM) is fast growing into a powerful tool and a useful counterpart of the widely used two-fluid model (TFM) in simulation of large-scale reactors. This work aims to study the role of mesoscale modeling in both TFM and DPM approaches to understand the advantage and disadvantage of each approach for further development. Both simulation approaches with and without considering mesoscale structures in drag modeling are systematically investigated through simulations of an industrial diameter-transformed fluidized bed reactor with complex reactions. It is found that considering mesoscale drag can obviously improve the prediction in solid concentration for both approaches, and the effect of mesoscale drag for TFM modeling is more significant than for DPM approach. Besides, the DPM approach can reveal local heterogeneous structures without using mesoscale drag because it can distinguish different parcels in each fluid cell, but it overestimates the accumulation of solid particles below the distributor, as the large coarse-grain ratio may over-enhance the particle collision. For reaction, the coke content can be better predicted by both approaches with mesoscale drag, and the DPM simulation can capture more heterogeneous distribution of coke content than TFM modeling. The predicted temperature and product distribution still have obvious deviation from industrial data, suggesting a need of mesoscale heat and mass transfer modeling. The underlying mechanisms are further analyzed together with proposing future work.

Partitioning of a wide bubbling fluidized bed with vertical internals to improve local mixing and bed material circulation

Industrial scale fluidized bed reactors are characterized by limited mixing rates, either local or global, especially when using low-pressure drop gas distributors to reduce operational costs. In this work, partitioning of wide beds using vertical internals is proposed as an effective technique to improve local mixing in large reactors, i.e., mixing in specific zones of the bed. The effect of the vertical internals height on local solids mixing within partitions was experimentally evaluated in a pseudo-2D bed by analyzing the velocity and flow structure of the solids and the circulation time within individual partitions. In the presence of internals, global mixing, i.e., mixing between neighboring partitions and across the entire reactor, may be reduced as vertical internals compartmentalize the bed. Thus, the effect of the internals height on global mixing was also quantified while using bed materials with the same properties, but differing in color, in the different partitions, and analyzing the time evolution of the concentration of solids. Furthermore, the effect of internals on bubbles was also evaluated for different internal heights. It was found that internals with a height between the gulf-stream height and the fixed bed height promote the appearance of vortex pair structures in each partition of the wide bed. These structures substantially improve local mixing within each partition, while global mixing between partitions is practically unaffected by the presence of these short internals. • Pseudo-2D bed was used to characterize the effect of vertical partitions on mixing. • Solids with different colors were used to evaluate local and global mixing. • A model is developed to determine the exchange efficiency between partitions. • Vortex pair structures appear with internals above the gulf-stream height. • Vortex structures improve local mixing, global mixing remaining nearly unaffected.

Multiscale CFD Simulation of an Industrial Diameter-Transformed Fluidized Bed Reactor with Artificial Neural Network Analysis of EMMS Drag Markers

Modeling of gas–solid, heterogeneously catalytic, diameter-transformed fluidized bed (DTFB) reactors is intrinsically complex and requires considering the variation of material properties and operating conditions, because of reactions and/or diameter transformation. The EMMS-matrix drag model, which correlates both operating conditions and local parameters, has been applied in computational fluid dynamics (CFD) simulation of such complex reactors by simplifying the macroscale operating conditions with one set of constant parameters. However, a complete scheme has not been reported that covers a wide range of datasets for a DTFB reactor with complex reactions. To this end, the artificial neural network (ANN), which enables exploring a multivariate relation with the contribution of a set of different parameters, is chosen to couple with EMMS drag modeling. A complete scheme of EMMS-ANN drag for hot, reactive simulation of DTFB is thereby established, with comprehensive evaluation of the contribution of drag markers successively considering the variation of gas properties and operating parameters. Both a priori tests and CFD simulations show that the voidage and slip velocity are the dominant factors in modeling of drag correction, and the effects of dynamic variation of gas properties and operating hydrodynamics are marginal; even the heterogeneous reactions and the change in bed diameter give rise to a remarkable variation in gas properties and operating parameters. The underlying mechanism is then analyzed to provide important clues for drag modeling of gas–solid, heterogeneous catalytic fluidized-bed reactors.

The hydrodynamics of low-density particles and optimization of the EMMS calculation process

Different reaction technology determines different catalytic materials and solid density affects the hydrodynamics behavior in a fluidized bed. An efficient hydrodynamics study is, therefore, highly desirable, even for evaluating the assumptions through pilot-scale investigation. To develop new fluidized bed technology that requires low-density particles (e.g., the vapor-phase Beckman rearrangement to produce ε-caprolactam), the authors thoroughly studied the hydrodynamics of low-density particles (true density 750 kg/m3, packing density 450 kg/m3) in a turbulent fluidized bed. Pilot-scale cold experiments were conducted in an ∅800 mm column with optical fiber probes. The TFM was adopted, where the Wen&Yu model, EMMS model, and six filtered models were coupled, respectively. The solution of the EMMS model is often challenging because of the iterative solution of nonlinear equations. In this work, the calculation process of the EMMS model was optimized, saving one global search for every possible state solution, and a set of nonlinear equations can be linearized. Comparisons between experiment and simulation results revealed that the EMMS models gave the best prediction. This work can provide a research model and guidance for the design of industrial fluidized bed reactors using low-density particles.

Study on pressurized isothermal pyrolysis characteristics of low-rank coal in a pressurized micro-fluidized bed reaction analyzer

In order to investigate the pressurized isothermal pyrolysis characteristics of coal, the effect of pressure on gas release characteristics and the kinetics of pressurized isothermal pyrolysis are explored for the first time in a pressurized micro-fluidized bed reaction analyzer (P-MFBRA). This work finds that the yields of CO2, CO, CH4, and H2 increases with temperature and pressure. The difference in the order of gas-releasing reduces as temperature and pressure rises, but that of gas-ending first decreases and then increases with pressure. The most probable mechanism functions of CO2, CO and CH4 change from shrinking core model to homogeneous model at 1 MPa, 0.8 MPa and 0.5 MPa, respectively, showing that reaction is controlled by chemistry under low pressure but affected by diffusion effect with elevating pressure. The rate constant and activation energy (Ea) of each gas appear an increasing-decreasing tendency and the difference between Ea of each gas reduces with pressure. Compared with non-isothermal experiments, the Ea (20.8–475 kJ mol−1) and pre-exponential factor in P-MFBRA are less than those (70–150 kJ mol−1) in the pressurized thermogravimetric analyzer (P-TGA), indicating P-MFBRA can effectively reduce the diffusion inhibition, and the kinetics obtained is close to the reaction in industrial fluidized bed reactor.

Study on effect of gas distributor in fluidized bed reactors by hydrodynamics-reaction-coupled simulations

Deep understanding of the complex relationship between the complex hydrodynamics and reactor performance in a reactive gas-fluidized bed is crucial to optimization of engineering design and industrial operation. In this work, multiphase particle-in-cell (MP-PIC) simulations coupled with reaction kinetics models are conducted to investigate the effect of gas distribution on reactor performance. A batch fluidized bed reactor with different distributors for regenerating spent FCC catalyst is simulated under different superficial gas velocities. In all simulation cases, the air flowrate and the spent catalyst inventory are kept constant. The results show an indiscernible effect of superficial gas velocity, but the induced-maldistribution conditions generally deteriorate the reactor performance. In addition to wall-slug formation, solids holdup, and slip velocities also decrease significantly in the plugged-gas distribution cases. Strong linear relationship between hydrodynamics and reactor performance are established. Present study sheds light on the importance of uniform gas distribution in industrial fluidized bed reactors.

Comparison of the properties of segregated layers in a bidispersed fluidized bed to those of a monodispersed fluidized bed

Since industrial fluidized-bed reactors typically operate with polydispersed particles, approximating such reactors as the superposition of corresponding monodispersed fluidized beds would greatly simplify their design and operation. To examine the validity of this superposition, we evaluate the effects of bidispersity by comparing three-dimensional liquid-solid monodispersed and segregated bidipsersed fluidized beds. This work demonstrates that, despite the clear segregation into layers that behave like monodispersed beds, the transition region is governed by complex bidispersed mechanisms that cannot be explained in terms of the particle behavior in the segregated layers.

Coupling Euler-Euler and Microkinetic Modeling for the Simulation of Fluidized Bed Reactors: an Application to the Oxidative Coupling of Methane.

We propose a numerical methodology to combine detailed microkinetic modeling and Eulerian–Eulerian methods for the simulation of industrial fluidized bed reactors. An operator splitting-based approach has been applied to solve the detailed kinetics coupled with the solution of multiphase gas–solid flows. Lab and industrial reactor configurations are simulated to assess the capability and the accuracy of the method by using the oxidative coupling of methane as a showcase. A good agreement with lab-scale experimental data (deviations below 10%) is obtained. Moreover, in this specific case, the proposed framework provides a 4-fold reduction of the computational cost required to reach the steady-state when compared to the approach of linearizing the chemical source term. As a whole, the work paves the way to the incorporation of detailed kinetics in the simulation of industrial fluidized reactors.

Study on Pressurized Isothermal Pyrolysis Characteristics of Low-Rank Coal in a Pressurized Micro-Fluidized Bed Reaction Analyzer

The development of clean coal technology has prompted the study of pressurized pyrolysis for coal. Most of the existing studies are non-isothermal kinetic experiments conducted in P-TGA, which cannot reflect the particle movement in actual furnace, so there is a lack of relevant data on pressurized isothermal pyrolysis characteristics. In this work, the effects of pressure on the characteristics of gas release and pressurized isothermal pyrolysis kinetics are explored in P-MBFRA. This work finds that the yields of CO2, CO, CH4, and H2 increases with temperature and pressure. The most probable mechanism function of CO2, CO and CH4 change from shrinking core model to homogeneous model at 1MPa, 0.8MPa and 0.5MPa, respectively, showing that reaction is controlled by chemistry under low pressure but affected by diffusion effect with elevating pressure. The rate constant and activation energy of each gas appear an increasing-decreasing tendency and the difference between activation energy of each gas reduces with pressure. Compared with non-isothermal experiments, the activation energy and pre-exponential factor in P-MBFRA are less than those in P-TGA, indicating P-MBFRA can effectively reduce the diffusion inhibition, and the kinetics obtained is close to the reaction in industrial fluidized bed reactor.

Two-stage modeling strategy for industrial fluidized bed reactors in gas-phase ethylene polymerization processes

Fluidized bed reactor is one of the main units in gas-phase ethylene polymerization processes. Due to the complexity in the polymerization kinetics, hydrodynamic behavior and heat transfer, the modeling and simulation have played an important role in the real operations of industrial polymerization reactors. In this paper, a two-stage modeling strategy for industrial fluidized bed reactors in gas-phase ethylene polymerization processes is proposed. The kinetics of polymerization reactions is computed in the first stage based on a comprehensive kinetics model while the hydrodynamic behavior and heat transfer in the industrial fluidized bed reactors are computed in the second stage based on a set of partial differential equations (PDEs). The polymerization kinetics, hydrodynamic behavior and heat transfer are integrated into the modeling and simulation of industrial fluidized bed reactors in gas-phase ethylene polymerization processes.

Sensitivity study of a full-scale industrial spray-injected fluidized bed reactor

The industrial fluidized bed reactor (FBR) described here is designed to convert an aqueous solid laden stream into a consistent granular product. The FBR is heated to 650 °C and chemically reducing conditions are achieved by using steam as the fluidizing gas and coal as a source of carbon. The objective of this study is to use the MFiX (Multiphase Flow with Interphase eXchanges) two-fluid model as a screening tool to investigate the effects of off-nominal conditions to establish performance guidelines. This information will be helpful in assessing whether the identified boundaries of the system are steep or gradual. Initially, 13 independent operating parameters were sampled in a one-at-a-time manner using high, low, and base values and ranked against a set of performance criteria indicative of defluidization. Five operating parameters were found to have the largest effect (bed particle size, bed particle density, coal particle size, spray feed flow rate, and fluidizing gas flow rate) on three quantities of interest—bed differential temperature, low solids velocity, and bed voidage. Latin-hypercube sampling was used to generate a minimal number of random values for various combinations of input parameters. The spray feed flow rate was the most significant parameter affecting the temperature differences, followed by the coal particle and bed particle size. The bed particle size had the largest effect on the low velocity of the bed particles, with the coal particle size as a contributing second effect. The high solid packing shows the coal particle size with the largest effect, and significant secondary contributions from the feed flow rate and fluidizing gas flow rate. The fits to the five-dimensional Gaussian Process models were 0.7797, 0.8664, and 0.9440 for the temperature, velocity, and solids packing, respectively.

Experimental study of non-uniform bubble growth in deep fluidized beds

Non-uniform bubble growth occurs in deep fluidized beds, resulting in generation of large bubbles or even slugs. Gas tracer method, in combination with analysis of pressure fluctuations, is employed to quantify the time-averaged and transient characteristics of bubble growth with respect to the bed depth in a 300-mm ID cold flow unit. The bubble distribution, growth and its residence time in the transition from uniform to non-uniform bubble growth and finally to slugging are investigated. In addition, appreciably better conversion and selectivity in a 1200-mm ID industrial fluidized bed reactor with ammoxidation of p-chlorotoluene (PCT) is obtained by eliminating non-uniform bubble growth.

Quantifying the uncertainty introduced by discretization and time‐averaging in two‐fluid model predictions

The two‐fluid model (TFM) has become a tool for the design and troubleshooting of industrial fluidized bed reactors. To use TFM for scale up with confidence, the uncertainty in its predictions must be quantified. Here, we study two sources of uncertainty: discretization and time‐averaging. First, we show that successive grid refinement may not yield grid‐independent transient quantities, including cross‐section–averaged quantities. Successive grid refinement would yield grid‐independent time‐averaged quantities on sufficiently fine grids. Then a Richardson extrapolation can be used to estimate the discretization error, and the grid convergence index gives an estimate of the uncertainty. Richardson extrapolation may not work for industrial‐scale simulations that use coarse grids. We present an alternative method for coarse grids and assess its ability to estimate the discretization error. Second, we assess two methods (autocorrelation and binning) and find that the autocorrelation method is more reliable for estimating the uncertainty introduced by time‐averaging TFM data. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5343–5360, 2017

Optimal conditions in direct dimethyl ether synthesis from syngas utilizing a dual-type fluidized bed reactor

Concerns over environmental pollution and ever-increasing energy demand have urged the global community to tap clean-burning fuels among which dimethyl ether is a promising candidate for contribution in the transportation sector. Direct dimethyl ether synthesis from syngas, in which methanol production and dehydration take place simultaneously, is arguably the preferred route for large scale production. In this study, direct dimethyl ether synthesis is proposed in an industrial dual-type fluidized bed reactor. This configuration involves two fluidized bed reactors operating in different conditions. In the first catalytic reactor (water-cooled reactor), the synthesis gas is partly converted to methanol after being preheated by the reaction heat in the second reactor (gas-cooled reactor). A two-phase generalized comprehensive reactor model, comprised of the flow in three different regimes is applied and a smooth transition between flow regimes is provided based on the probabilistic averaging approach. The optimal operating conditions are sought by employing differential evolution algorithm as a robust optimization strategy. The dimethyl ether mole fraction is considered as the objective function during the optimization. The results show considerable dimethyl ether enhancement by 16% and 14% compared to the conventional direct dimethyl ether synthesis reactor and dual-type fixed bed dimethyl ether reactor arrangements, respectively.

Dynamic modeling and Molecular Weight Distribution of ethylene copolymerization in an industrial gas-phase Fluidized-Bed Reactor

A dynamic model for ethylene copolymerization in an industrial Fluidized-Bed Reactor (FBR) is developed to describe its behavior and calculate the properties of polyethylene. The presented model considers particle entrainment and polymerization reaction in two phases. Two-site kinetic and hydrodynamic models in combination, provide a comprehensive model for the gas phase fluidized-bed polyethylene production reactor. The governing moment and hydrodynamic differential equations were solved simultaneously and the results compared with a similar work, as well as industrial data. The dynamic model showed accurate results for predicting Polydispersity Index (PDI), Molecular Weight Distribution (MWD), reactor temperature and polymer production rate.

Hydrodynamic modeling of an industrial turbulent fluidized bed reactor with FCC particles

This study presents a CFD modeling of the hydrodynamics in an industrial turbulent fluidized bed reactor with FCC particles. Considering the mesoscale structure of particle clusters and the impact of reactor temperatures, a gas-solid drag model based on the equivalent diameter of particle clusters is proposed. A CFD model is developed, the model parameters are evaluated, and the gas-solid flow behaviors are explored. Compared with the conventional drag models, the proposed model gives a reasonable hydrodynamic prediction in the industrial turbulent fluidized bed reactor. The hydrodynamics show more sensitive to the drag models than to the flow types, the kinetics theories and the restitution coefficients. There exist two coexisting flow regions in the industrial turbulent fluidized bed: a bottom dense, bubbling region and an upper dilute, dispersed flow region. Moreover, the bed density in the dense phase is much lower than those in the cold experiments. In the dilute phase, particles ascend in the center and descend near the wall, showing a core-annulus flow. In the bottom dense phase, some vortex flows appear, leading to the non-uniform distribution of particle velocity and solid holdup. With increasing the superficial gas velocity, the bed surface rises; and the bed density decreases in the bottom dense phase and increases in the upper dilute phase.

The Influence of the System for Feedstock Feed in Fluidized Bed Reactors on the Reaction Efficiency with Isoparaffin Dehydrogenation in the Fluidized Aluminochromium Catalyst Bed as an Example

A mathematical model was developed for two units of industrial fluidized bed reactors equipped with different systems for feeding gaseous feedstock: three toroidal rings with nozzles in unit 1, and a false bottom with nozzles distributed through it in unit 2. Analysis of the efficiency (expressed as the yield of the target product – isobutylene) of the units during 4 month operation under industrial conditions demonstrated a higher efficiency of unit 2. The reasons for the differences in the product yields in two units were found using numerical solutions to obtain characteristic patterns of concentration fields of catalyst particles and temperature fields in the units; the results obtained demonstrated more uniform and dense catalyst distribution and more uniform reactorheating characteristics of unit 2. The constructed patterns of main circulative flows of the catalyst accounted for the considerable differences in the catalyst concentration and gas temperature fields. The numerical solutions were used for comparative analysis of the operation efficiencies of the units to show good agreement with the analytic results on industrial reactors. The proposed approach can be used for the design of new and optimization of operating units.