Dolomite decomposition in a high temperature fluidised bed reactor

The decomposition of dolomite was investigated in a bench-scale fluidised bed reactor (30 cm diameter) at temperatures between 600°C and 1000°C in batch and continuous operations. The composition of the solids was determined by X-ray analysis, the gas composition by infrared (i.r.) spectroscopic analysis. The reaction kinetics were investigated in a small fluidised bed (2.6 cm diameter) as well as in the bench-scale equipment and compared with kinetic data evaluated from differential thermal gravimetric (DTG) curves. The measurements were carried out by adding small amounts of dolomite into the isothermal fluidised bed. The resulting CO₂ concentration within the gas could then be very low, thus MgCO₃ and CaCO₃ decompose as parallel reactions (singlestage reaction). At somewhat higher CO₂ gas concentrations prevailing in technical operational conditions the decomposition mechanism changes into a two-stage reaction where MgCO₃ decomposes first. The bed temperature, pressure drop, superficial gas velocity, solid conversion, and specific interfacial area of solids were determined as functions of time and/or reactor wall temperature with a constant temperature increase rate for batch runs. The distributions of the residence time of solids indicate that in the continuously operated fluidised bed well-mixed conditions prevail. The concentrations of dolomite, CaCO₃, MgO, CaO, and solid conversion as well as the specific surface area of particles were determined as functions of the mean residence time in the continuous reactor. By means of the CO₂ concentration in the gas phase and the mean residence time the conversions of the consecutive reactions can be controlled.     

The influence of COD,hydraulic, temperature and pH shocks on the stability of an unheated fluidised bed reactor

An anaerobic fluidised bed reactor, operating at room temperature, was used to treat a synthetic meat extract wastewater with an influent COD of 1250 mg 1^–1. It was subjected to large increases in influent COD and flow rate for 4-and 8-h periods. The reactor tolerated these transient changes satisfactorily and with no long term detrimental effects. Normal operation was regained within 9h. Variations in operating temperature and influent pH were also examined. Four hour duration, 10°C temperature increases and decreases had little effect on effluent quality, but long term operation at a low influent pH was considered inadvisable. A simulated working week experiment was carried out with a 48-h 90% influent flowrate reduction, followed by a return to normal operation. The reactor tolerated these conditions well, with full recovery made within a few hours. The resistance of the reactor to unfavourable process conditions is discussed. The extrapolation of laboratory data to practical operating conditions is also considered.     

Beneficiation of ilmenite to rutile by selective chlorination in a fluidised bed

The paper deals with studies on selective chlorination of iron oxides in Kerala Ilmenite. Optimum conditions were established with a view to developing an autothermal process.     

Characteristics of fluidisation behaviour in a pressurised bubbling fluidised bed

The experiments were carried out in a bench‐scale fluidised bed of 90 mm in diameter to determine the influence of pressure on fluidisation characteristics of Geldart A and B particles over the range of pressure 0.1–4.5 MPa. For Geldart B particles, the results indicate that minimum fluidisation velocity (u_mf) was found to decrease with pressure whilst bed voidage at u_mf was unaffected, and the bed expansion height increase with pressure at fixed value of gas velocity was observed for both Geldart B and A particles. For Geldart A particles, minimum bubbling velocity (u_mb) bed voidage at u_mb and dense phase voidage were found to increase obviously with pressure, but a slight influence of pressure on u_mf was observed. The prediction values of high‐pressure fluidisation characteristics from the references' correlations developed at pressure were in agreement with the experimental data. © 2012 Canadian Society for Chemical Engineering     

Bubble dynamics and its effect on the performance of a jet fluidised bed gasifier simulated using CFD

A computational fluid dynamic modelling study of a jet fluidised bed gasifier has been carried out. The modelling was based on the Eulerian-Eulerian models for gas and solid flows, which take into account the hydrodynamics, mass and heat transfer, and heterogeneous and homogeneous reactions. The bubble dynamics was simulated in detail, enabling its effect on temperature distributions, gasification reactions and gas compositions in the bed to be examined. The results revealed that jet growth, bubble rise, and the associated convective flow play a significant role in the heat exchange and mass transfer, and in turn, affect the gasification reactions.     

Experimental study on gas-solids flows in a circulating fluidised bed using electrical capacitance tomography

It is difficult to measure the gas-solids flow in a circulating fluidised bed (CFB) because of the complicated and rapid transient process. Electrical capacitance tomography (ECT), a cross-sectional imaging technique, has been used to measure the dilute flow in a large square CFB. A sensor has been specifically designed for the measurement and a new algorithm has been developed for image reconstruction. Flow conditioning parts (internals) are designed and placed inside the CFB, aiming to enhance the contact between gas and particles in the dilute gas-solids flow. The dynamic characteristics and detailed information were obtained on two sections in the bed at different height. The performance of the internals is related to their size, combination, height in the bed and the superficial gas velocity. It has been confirmed that a particular combination of internals can increase the solids concentration in the central area of a cross-section, and can improve the probability density distribution (PDD) with a moderate gas velocity. Using a combination of a large internal at an upper location and a small one at a lower location can optimise the flow in the CFB.     

Optimal design of a thermally coupled fluidised bed heat exchanger reactor for hydrogen production and octane improvement in the catalytic naphtha reformers

The present study combines simultaneously the definition of fluidisation and process intensification (thermally coupled heat exchanger reactor) concept and determines the optimum operational conditions in both sides of the reactor, using Differential Evolution (DE) optimisation approach. The exothermic hydrogenation of nitrobenzene to aniline takes place in a set of tubular reactors which is placed inside the naphtha reactors and thermally handle the endothermic reaction of reforming. A single objective function consists of four terms including aromatic mole fraction of the reformate and hydrogen production from each reactor in the endothermic side as well as the total molar flow rate of aniline and nitrobenzene conversion in the exothermic side is defined. Seven decision variables such as inlet temperature of exothermic and endothermic sides, exothermic molar flow rates for the first and the second reactors and the number of tubes are considered during the optimisation procedure. Temperature constraints have been considered in both sides during the optimisation in order to reduce the possibility of rapid catalyst deactivation by sintering. Results show approximately 464.4 and 598.9 kg/h increase in aromatic and aniline production rates in optimised thermally coupled fluidised bed naphtha reactor (OTCFBNR) compared with non‐optimised case (TCFBNR), respectively. Such a theoretical study is necessary prior to designing new pilot plants and revamping industrial units. © 2011 Canadian Society for Chemical Engineering     

A comparative study of a fluidised bed reactor and a gas lift loop reactor for the IBE process: Part I. Reactor design and scale down approach

For the continuous production of isopropanol-butanol mixtures by immobilised Clostridium spp. (the IBE process) two reactor types were studied: a fluidised bed reactor with liquid recycling (FBR) and an external loop gas lift reactor (GLR). A large scale design (50–65 m³) was made for both reactors. A regime analysis, by evaluating the time constants for e.g. mixing and conversion, identified the ruling regime. Via the scale down approach two representative model reactors were developed: a 10 dm³ FBR reactor (H/D=25, D=0.08 m) and a 15 dm³ external loop GLR (H/D = 12.5, D_r=D_d=0.08 m). For both reactors the hydrodynamical behaviour and the total reactor performance were studied and are described in parts II and III of this study.     

A comparative study of a fluidised bed reactor and a gas lift loop reactor for the ibe process: Part III. Reactor performances and scale Up

Continuous fermentations using Clostridium spp. DSM 2152 immobilised in calcium alginate beads to produce butanol and isopropanol from glucose were carried out in a fluidised bed reactor with liquid recycle (FBR, 10.9 dm³ working volume, 41 % solids) and in a gas lift loop reactor (GLR, 11.4 dm³ working volume, 32% solids). In the FBR in-situ produced non-coalescing gas bubbles had a negligible influence on the fluidisation pattern and the steady state results of the fermentation were in accordance with those predicted by a reactor model. The FBR was operated reliably for 5 weeks without decrease of activity. The GLR behaved as a three phase reactor because of the recycled fermentation gas. The steady state fermentation results were as predicted by the GLR model. Scale up to a 50 m³ FBR and a 65 m³ GLR led to development of a plug flow with recycle model for the FBR and a stirred tank model for the GLR. On the basis of overall reactor performance and ease of integration with a simultaneous product recovery the FBR is preferred to the GLR for application in a large scale butanol/isopropanol process using immobilised Clostridia spp.     

PCDD/PCDF emissions from co-combustion of coal and PVC in a bubbling fluidised bed boiler

This paper reports on the results obtained in the study of the co-combustion of PVC with hard coal from South Africa in a 0,5 MW_th Bubbling Fluidised Bed Boiler. The research has included the study of the effect of combustion temperature, fluidisation velocity and PVC content. The addition of urea to the raw fuel, as a dioxin-preventing compound has also been evaluated. Results have been analysed in terms of combustion efficiency, major pollutants emission (NO_x, CO), and PCDD/Fs formation in the flue gas and in the fly ash. Under the experimental conditions tested, co-combustion of coal and PVC has proved to be feasible from the combustion efficiency and emission of PCDD/Fs points of view, whose levels remained below limits set by existing legislation on persistent organic pollutants. The addition of solid urea to the fuel blend reduces the amount of chlorinated compounds emitted. However, it has a negative impact on nitrogen pollutants formation     

The influence of the riser exit on the particle residence time distribution in a circulating fluidised bed riser

This paper reports measurements of the influence of riser exit geometry upon the particle residence time distribution in the riser of a square cross section, cold model, circulating fluidised bed. The bed is operated within the fast fluidisation regime. The fast response particle RTD technique developed by Harris et al. (Chem. Eng. J. 89 (2002) 127-142) was used to measure the residence time distribution.The geometry of the riser exit is shown to have a modest but consistent influence upon the particle RTD; the influence of operating conditions, i.e. superficial gas velocity and solids flux is more significant.Increasing the refluxing effect of the riser exit increases the mean, variance and breakthrough time and decreases the coefficient of variation of the residence time distribution. Changes in reflux do not have a systematic effect upon the skewness of the RTD.     

The influence of geometrical factors and feedstock on gasification in a high temperature fluidised bed

Results are presented for gasification of coal and char by means of air or air-steam mixtures in fluidised bed reactors of three different volumes. Two sizes of coal feedstock particles, 0.5-1.0 mm and 1.0-1.5 mm, and one size of char particles, 0.5-1.5 mm, were used. The calorific value of generated gas and the carbon conversion are presented as a function of particle residence time. For coal gasification higher carbon conversion has been obtained at the same particle residence time than for char gasification. For the steam gasification, a lower gas heating value of about 4 MJ/m³ (S.T.P.) was obtained.     

Combustion of oil palm stone in a pilot-scale fluidised bed reactor

Biomass is being generated in vast amounts from oil palm plantations particularly in developing countries such as Malaysia, Thailand and Indonesia. Oil palm stone (OPS) is currently considered a waste material and has not previously been considered for energy purposes. The main objective of this study was to investigate the thermochemical conversion of OPS in a pilot-scale fluidised bed combustor. The net heating value of OPS was 24.93 MJ/kg. The effect of primary air flowrate and initial bed temperature were the main parameters investigated. The bed and bed's surface temperature were found to decrease as the primary air flowrate increased. In all tests CO emissions were less than 0.2%. The emissions of SO₂ and HCl ranged from 0.02 ppm to 0.05 ppm, significantly below the permitted levels set by legislation. Stable combustion was observed at a bed temperature of 950 °C. The most abundant elements found in the ash were Al, Ca, Fe, K, Mg, Mn, P, S and Si. However, due to the temperature regime used in the study fouling would not be an issue.     

Numerical study of cluster and particle rebound effects in a circulating fluidised bed

Gas-particle flows in a vertical two-dimensional configuration appropriate for circulating fluidised bed applications were investigated numerically. In the computational study presented herein the motion of particles was calculated based on a Lagrangian approach and particles were assumed to interact through binary, instantaneous, non-frontal, inelastic collisions including friction. The model for the interstitial gas phase is based on the Navier-Stokes equations for two-phase flows. The numerical study of cluster structures has been validated with experimental results from literature in a previous investigation. Numerical experiments were performed in order to study the effects of different cluster and particle rebound characteristics on the gas-particle flow behaviour.Firstly, we investigated the hard sphere collision model and its effect on gas-particle flow behaviour. The coefficient of restitution in an impact depends not only on the material properties of the colliding objects, but also on their relative impact velocity. We compared the effect of a variable restitution coefficient, dependent on the relative impact velocity, with the classical approach, which supposes the coefficient of restitution to be constant and independent of the relative impact velocity.Secondly, we studied the effects of different cluster properties on the gas-particle flow behaviour. Opposing clustering effects have been observed for different particle concentrations: within a range of low concentrations, groups of particles fall faster than individual particles due to cluster formation, and within a well-defined higher concentration range, return flow predominates and hindered settling characterises the suspension. We propose herein a drag law, which takes into account both opposing effects and have compared the resulting flow behaviour with that predicted by a classical drag law, which takes into account only the hindered settling effect.     

Effect of external mass transfer in a packed bed reactor system for a reversible enzyme reaction

A mathematical model was developed to describe the effect of external mass transfer for a packed-bed enzyme reactor in which a reversible, one-substrate, two-intermediate enzyme reaction took place. The model equation was applied to the analysis of an immobilized glucose isomerase reactor system. A Colburn-type mass transfer correlation was obtained from the Colburn j-factor versus Reynolds number plot: i.e., j_D = 0.045N_Re^−0.48. The values of mass transfer coefficient for the system under study ranged from 0.01 to 0.1 cm h⁻¹ depending on the substrate flow rate. Very good agreements were observed between the computer simulation using a plug flow reactor model with the derived mass transfer correlation and the experimental results obtained from the packed-bed reactor operation.     

Co-gasification of meat and bone meal with coal in a fluidised bed reactor

After the Bovine Spongiform Encephalopathy illness appeared, the meat and bone meat (MBM) produced from animal residues became an important waste. In spite of being a possible fuel due to its heating value (around 21.4 MJ/kg), an important fraction of the meat and bone meal is being sent to landfills. The aim of this work is to evaluate the co-gasification of low percentages of meat and bone meal with coal in a fluidised bed reactor as a potential waste management alternative. The effect of the bed temperature (800-900 °C), the equivalence ratio (0.25-0.35) and the percentage of MBM in the solid fed (0-1 wt.%) on the co-gasification product yields and properties is evaluated. The results show the addition of 1 wt.% of MBM in a coal gasification process increases the gas and the liquid yield and decreases the solid yield at 900 °C and 0.35 of temperature and equivalence ratio operational conditions. At operational conditions of 900 °C and equivalence ratio of 0.35, the specific yield to gas (y_gas) increases from 3.18 m³(STP)/kg to 4.47 m³(STP)/kg. The gas energy yield decreased 24.1% and the lower heating value of the gas decreases from 3.36 MJ/m³(STP) to 2.16 MJ/m³(STP). The concentration of the main gas components (H₂, CO and CO₂) hardly varies with the addition of MBM, however the light hydrocarbon concentrations decrease and the H₂S concentration increases at the higher temperature (900 °C).     

Continuous biotransformation of pyrogallol to purpurogallin using cross‐linked enzyme crystals of laccase as catalyst in a packed‐bed reactor

Cross‐linked enzyme crystals (CLEC) of laccase were prepared by crystallizing laccase with 75% (NH₄)₂SO₄ and cross‐linking using 1.5% glutaraldehyde. The cross‐linked enzyme crystals were further coated with 1 mmol L^?1 β‐cyclodextrin by lyophilization. The lyophilized enzyme crystals were used as such for the biotransformation of pyrogallol to purpurogallin in a packed‐bed reactor. The maximum conversion (76.28%) was obtained with 3 mmol L^?1 pyrogallol at a residence time of 7.1 s. The maximum productivity (269.03 g L^?1 h^?1) of purpurogallin was obtained with 5 mmol L^?1 pyrogallol at a residence time of 3.5 s. The productivity was found to be 261.14 g L^?1 h^?1 and 251.1 g L^?1 h^?1 when concentrations of 3 mmol L^?1 and 7 mmol L^?1 respectively were used. The reaction rate of purpurogallin synthesis was maximum (2241.94 mg purpurogallin mg^?1 CLEC h^?1) at a residence time of 3.5 s, when 5 mmol L^?1 pyrogallol was used as the substrate. The catalyst to product ratio calculated for the present biotransformation was 1:2241. The CLEC laccase had very high stability in reuse and even after 650 h of continuous use, the enzyme did not lose its activity. Copyright © 2006 Society of Chemical Industry     

Hydrolysis of milk lactose in a packed bed reactor system using immobilized yeast cells

BACKGROUND: Hydrolysis of lactose with β‐D‐galactosidase is one of the most promising biotechnological applications in the food industry because of its use in the production of low lactose milk products and whey hydrolysis. To overcome the problem of enzyme extraction from cells due to the intracellular nature of β‐D‐galactosidase and the poor permeability of the cell membrane to lactose, permeabilization of yeast cells was investigated. Permeabilized whole cells have been claimed to have an advantage over more pure enzyme preparations. In view of the advantages of immobilized cell systems over free cell systems, permeabilized cells were immobilized by an entrapment method in calcium alginate gel. A packed bed reactor together with this immobilized cell system has been used for hydrolysis of milk lactose in a continuous system. RESULTS: Different process parameters (temperature, substrate feed rate, biomass load and time‐course) were optimized to maximize lactose hydrolysis. The immobilized yeast cells (300 mg dry wt) resulted in 87.2% hydrolysis of milk lactose at 30 °C and flow rate 7 mL h^?1 in a packed bed reactor system. CONCLUSION: This convenient and relatively inexpensive method of immobilization, resulting in high hydrolysis potential in a continuous system, indicates that permeabilized yeast cells have the potential for the production of low lactose milk and milk products. Copyright © 2010 Society of Chemical Industry