Measurement of fluidised bed dryer by different frequency and different normalisation methods with electrical capacitance tomography

To investigate the effect of moisture content, excitation frequency and normalisation model on image reconstruction with a fluidised bed dryer, an electrical capacitance tomography (ECT) sensor was mounted near the bottom of the drying chamber. An ECT system based on an HP4128 impedance analyser was used to measure capacitance and loss conductance between the electrode pairs in the sensor. It has been found that the capacitance depends on not only the particle moisture but also the excitation frequency. With a low moisture content, the relationship between capacitance and frequency is simple and linear. With a high moisture content, however, the relationship becomes more complex and non-linear. For image reconstruction, different normalisation models have been used: series, parallel, Maxwell and Böttcher models. The results show that with a low moisture content, these models give nearly the same image errors. With the increase in moisture content, the difference between these models becomes more and more obvious. With different gas-solids flow patterns, the four models also give slightly different images. In the end of this paper, solids distribution and averaged solids concentration profile for dynamic test with different excitation frequencies in a fluidised bed dryer are given.     

Modelling of batch fluidised bed drying of pharmaceutical granules

This paper presents a mathematical model based on a three-phase theory, which is used to describe the mass and heat transfer between the gas and solids phases in a batch fluidised bed dryer. In the model, it is assumed that the dilute phase (i.e., bubble) is plug flow while the interstitial gas and the solid particles are considered as being perfectly mixed. The thermal conductivity of wet particles is modelled using a serial and parallel circuit. The moisture diffusion in wet particles was simulated using a numerical finite volume method. Applying a simplified lumped model to a single solid particle, the heat and mass transfer between the interstitial gas and solid phase is taken into account during the whole drying process as three drying rate periods: warming-up, constant rate and falling-rate. The effects of the process parameters, such as particle size, gas velocity, inlet gas temperature and relative humidity, on the moisture content of solids in the bed have been studied by numerical computation using this model. The results are in good agreement with experimental data of heat and mass transfer in fluidised bed dryers. The model will be employed for online simulation of a fluidised bed dryer and for online control.     

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.     

Discrete element modelling of fluidised bed spray granulation

A novel discrete element spray granulation model capturing the key features of fluidised bed hydrodynamics, liquid-solid contacting and agglomeration is presented. The model computes the motion of every individual particle and droplet in the system, considering the gas phase as a continuum. Microscale processes such as particle-particle collisions, droplet-particle coalescence and agglomeration are directly taken into account by simple closure models. Simulations of the hydrodynamic behaviour of a batch granulation process are presented to demonstrate the potential of the model for creating insight into the influence of several key process conditions such as fluidisation velocity, spray rate and spray pattern on powder product characteristics.     

Co-combustion of coal and olive oil industry residues in fluidised bed

Recently new environmental regulations of fossil fuels have further increased interest in the use of waste and biomass for energy generation. Co-combustion is generally viewed as the most cost-effective approach to biomass and wastes utilisation by the electric utility industry.The aim of this paper is to assess the feasibility of co-firing coal and a very specific biomass waste from the olive oil industry: foot cake, in a fluidised bed. This waste is quite difficult material to be used in combustion process, due to its high moisture content and alkaline content in ashes.Two different Spanish coals were selected for this study: a lignite and an anthracite. The combustion tests were carried out in the CIEMAT bubbling fluidised bed pilot plant. In order to study the effect of different parameters on the emissions and combustion efficiency, the tests were done using different operating conditions: furnace temperature, share of foot cake in the mixtures and coal type.The pilot plant tests show that the combustion of foot cake/lignite or anthracite mixtures in bubbling fluidised bed is one way to utilise this biomass residue in energy generation. The presence of foot cake in the mixtures has not any significant effect on the combustion efficiency. SO₂ and NO_x emissions decrease when the amount of foot cake in the mixtures increases, while N₂O emission increases.     

An experimental study on gasification of Colombian coal in fluidised bed

The main results of an experimental work on gasification of Colombian coal in a fluidised bed are reported in this paper. Experiments were carried out at different steam/coal (F_s/F_c) and air/coal (F_a/F_c) ratios and temperatures of gasifying agent. In addition, the influence of bed temperature on coal conversion was analysed. Results show a maximum value in the curve of high heating value versus F_a/F_c. From the environmental standpoint, low concentrations of sulphur compounds were obtained but more work should be done in order to decrease particulate matter.     

Continuous toluene vapour photocatalytic deduction in a multi-stage fluidised bed

Continuous deduction of 1000 ppm toluene vapour in a multi-stage gas fluidised bed photoreactor was studied using TiO₂ coated activated carbon (AC/TiO₂) particles as the fluidising media. Due to the enrichment of the local toluene concentration, the efficiency of the toluene removal from the gas stream by the mass transfer-limited photocatalytic reaction was significantly improved using AC/TiO₂ particles as the fluidising media. In the stage addition studies, the results showed that the toluene removal efficiency at steady state increased approximate 3-4% for every stage addition. The three-stage design effectively utilised the freeboard region and improved the toluene removal efficiency for approximately 37% when comparing to the single-stage system. Using SiO₂ powders as fluidising media could enhance the light utilisation by light scattering and promotes the toluene deduction during the initial to steady toluene deduction period.     

Gas quenching probe and field measurements of gaseous cadmium in a 12-MWth circulating fluidised bed combustion boiler

A probe-based technique for measurement of gas phase metal species in flue gas at high temperatures has been developed. This paper deals with work carried out to test and evaluate the technique through field measurements of cadmium species during co-combustion of sewage sludge and other solid fuels in a circulating fluidised bed combustion boiler. During the measurements of vaporous cadmium forms, solid fly ash samples were also collected at the measurement positions. Chemical composition data from these fly ash samples were used to discuss the influence that the fly ash filter cake built up on the probe tip can have on the measurements. Additionally, the solid samples were examined by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques to have a better insight into the problem. Using the data from the field measurements and/or from the integrated measurements and weighing devices of the boiler the mass balance of cadmium from the combustion process was calculated by different methods. The calculation results were fairly consistent. In one method, data from the field measurements was used. This method gave the mass balance for cadmium in the process Cd_out/Cd_in=113.6%, which was a very good result considering that it involved the analysis of trace amount of metals in full-scale measurements.     

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).     

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.     

Agglomeration in fluidised bed gasification of biomass

Three promising biomass fuels for southern Mediterranean regions were tested for their agglomeration tendency in an atmospheric lab-scale fluidised bed (FB) gasifier using quartz and olivine as bed materials. The defluidisation temperatures of the energy crops Giant Reed (Arundo donax L.) and Sweet Sorghum bagasse were respectively approx. 790 °C and 810 °C, in both bed materials, while the agro industrial residue olive bagasse caused defluidisation of the quartz bed at 830 °C and olivine bed at > 850 °C. Agglomerates from these tests were analysed with SEM/EDS. Coatings and necks between bed particles were formed due to ash derived potassium silicate melt. For the first two fuels cluster-type agglomerates around remains of char particles were observed. Thermodynamic equilibrium simulations of each chemical system were performed to cross examine the predicted ash melting temperatures and chemistry with experimental findings. Predictions of potassium liquid compounds, like K₂O·SiO_2(l) were verified by EDS analyses on the particle coatings. FB gasification of olive bagasse resisted defluidisation up to higher temperatures because of its lower potassium and higher calcium content, especially in the case of olivine bed. The latter experimental finding coincided with thermodynamic predictions.     

Continuous on-line measurement of solid moisture content during fluidized bed drying using triboelectric probes

Wet granulation and drying of solids in fluidized beds are widely used in the pharmaceutical, food and fertilizers industries. Although the moisture content of fluidized solids is the key parameter for on-line process monitoring, reliable, accurate and economical moisture sensors are lacking. The aim of this work was, therefore, to develop a new technique using triboelectric probes for real-time measurement of moisture content in fluidized beds, and to validate its applicability to fluid bed drying of glass beads (Sauter-mean diameter of 171 μm) and silica sand particles (Sauter-mean diameter of 190 μm) wetted by water. Several triboelectric probes, installed at different locations throughout the bed, monitored the bed moisture content during both the liquid spraying and the following drying process. The measuring technique developed in this study makes use of inexpensive probes that do not require any maintenance. The triboelectric signals were continuously recorded by a data acquisition system and, at selected times, samples of bed solids were taken and analyzed for their moisture content using Karl Fischer titration. The triboelectric signals were correlated with the moisture contents determined by titration to calibrate the technique, which was found to be sensitive to moisture contents below 100 ppm. For most of the experiments the bed was operated in the bubbling regime with a superficial gas velocity of 0.4 m/s. The relationship between triboelectric signal and moisture content was unaffected by changes in the fluidization gas velocity of up to 25%, and could be easily automated for direct control of industrial granulation and drying processes.     

Study on the structure and gasification characteristics of selected South African bituminous coals in fluidised bed gasification

The gasification characteristics of three South African bituminous coals were investigated in a bubbling fluidised bed reactor. The three coals are similar in rank, but two are inertinite-rich coals and the third has a high vitrinite content. The microstructural characteristics of the parent coals and their resultant chars were determined using XRD, FT-IR, Raman and petrographic analysis. The microstructural changes that occurred in the organic (maceral) and the inorganic (mineral) fractions of the selected coals were evaluated. The change in the carbon structure was correlated to the proportions of inertinite and vitrinite macerals in the coals. High vitrinite content resulted in an increase in the order of the disordered carbon structure after gasification and this leads to greater graphitised ordered carbon structures. While a high inertinite content resulted in low or no structural transformation of the chemical structure. The transformation of inorganic mineral constituents of the coal was correlated to the amount of inertinite present in the selected coals. Higher proportions of inertinite macerals and inertinitic chars resulted in higher proportions of melted minerals. Char samples with low proportions of organic matter resulted in higher proportion of melted minerals covering the char surface.     

The heat transfer coefficient between a particle and a bed (packed or fluidised) of much larger particles

The heat transfer coefficient has been measured for a heated phosphor-bronze sphere (diam. 2.0, 3.0 or 5.56 mm) added to a bed of larger particles, through which air at room temperature was passed. The bronze heat transfer sphere was attached to a very thin, flexible thermocouple and was heated in a flame to before being immersed in the bed. The cooling of the bronze sphere enabled the heat transfer coefficient, h, to be measured for a variety of U/U_mf, as well as diameters of both the particles in the bed and the heat transfer sphere. It was found that before the onset of fluidisation, h rose with U, but h reached a constant value for U?U_mf. These measurements indicate that in this situation (of a relatively small particle in a bed of larger particles) all the heat transfer is between the hot bronze sphere and the gas flowing over it. Consequently, a Nusselt number, based on the thermal conductivity of the gas, is easy to define and for U?U_mf (i.e. a packed bed), Nu is given by

     

Kinetics of fluidised bed melt granulation III: Tracer studies

Previous work (Proceedings of World Congress on Particle Technology, Sydney 2002, 629-636.) has shown that granule breakage occurs during fluidised bed melt granulation (FBMG) and should not be neglected when it comes to addressing granulation kinetics. In the current work, we have developed and verified tracer experiments in FBMG, in an attempt to decouple the influence of granule size and age on breakage kinetics. The tracer data during granulation shows that granule breakage is occurring at a much slower rate than aggregation, while the breakage-only tracer experiments reveal the breakage selection rate to be independent of size at an approximate value of . The observations allow us to deduce that the aggregation rate for smaller granules is actually faster than larger ones during granulation.     

Investigation into gas-solid heat transfer in a cryogenic vibrated fluidised bed

Experiments were carried out in a cryogenic vibrated fluidised bed to investigate the heat transfer between gas and rubber particles obtained from discarded tyres. The effects of parameters such as bed layer thickness and gas flow rate on the gas-solid heat transfer were investigated, and a heat transfer correlation obtained by regressing the experimental data. Theoretical analysis based on radial thermal conductivity indicated that higher heat transfer efficiency could be obtained by the use of a fluidised bed rather than a fixed bed or a moving bed, especially for rubber particles having low thermal conductivity under cryogenic conditions. A numerical modelling was developed, based on assumptions of the movement of the particles and the vibrating bed plate, using a unique method of regarding particles as the source term in the energy equation. Computational results from the modelling showed good agreement with the experimental data.     

Binder addition methods and binder distribution in high shear and fluidised bed granulation

Fluidised beds and high shear mixers are both important in industrial granulation. The binder addition method (pouring, melt-in, spraying) affects the growth and properties of granules and is therefore of vital importance to the fundamental understanding of this detailed process. Non-uniformity of binder distribution is well known in high shear melt granulation, however, there is limited literature surrounding binder distribution in fluidised bed granulation. It was therefore the aim of the paper to compare the binder distribution using alternative addition methods in both high shear mixer and fluidised bed.In this work two binder addition methods, ‘wet’ and ‘dry’, in a fluidised bed and high hear mixer were used to successfully produce granules with a typical pharmaceutical size, 150-300 μm. The granules were analysed for final binder distribution in different size classes using Patent V blue dye and ultra-violet spectrometry.All binder addition methods supported previous work showing non-uniformity of binder distribution throughout the size classes. High shear mixer results show great similarity in binder content whichever binder addition method was chosen. This is likely to be due to the same mechanisms occurring due to the impeller forces in the process, mean while the fluidised bed results show little similarity. The binder distribution by mass is also investigated and shows that although most studies show a relative higher binder content in the larger size classes that actually the majority of binder can instead be found around the mean size of the batch.