A CFD approach on the effect of particle size on char entrainment in bubbling fluidised bed reactors

The fluid – particle interaction inside a 41.7 mg s-1 fluidised bed reactor is modelled. Three char particles of sizes 500 µm, 250 µm, and 100 µm are injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. Due to the fluidising conditions and reactor design the char particles will either be entrained from the reactor or remain inside the bubbling bed. The particle size is the factor that differentiates the particle motion inside the reactor and their efficient entrainment out of it. A 3-Dimensional simulation has been performed with a completele revised momentum transport model for bubble three-phase flow according to the literature as an extension to the commercial finite volume code FLUENT 6.2.

Studies in three phase gas-liquid-solid fluidised systems

The work is a logical continuation of research started at Aston some years ago when studies were conducted on fermentations in bubble columns. The present work highlights typical design and operating problems that could arise in such systems as waste water, chemical, biochemical and petroleum operations involving three-phase, gas-liquid-solid fluidisation; such systems are in increasing use. It is believed that this is one of few studies concerned with `true' three-phase, gas-liquid-solid fluidised systems, and that this work will contribute significantly to closing some of the gaps in knowledge in this area. The research work was mainly experimentally based and involved studies of the hydrodynamic parameters, phase holdups (gas and solid), particle mixing and segregation, and phase flow dynamics (flow regime and circulation patterns). The studies have focused particularly on the solid behaviour and the influence of properties of solids present on the above parameters in three-phase, gas-liquid-solid fluidised systems containing single particle components and those containing binary and ternary mixtures of particles. All particles were near spherical in shape and two particle sizes and total concentration levels were used. Experiments were carried out in two- and three-dimensional bubble columns. Quantitative results are presented in graphical form and are supported by qualitative results from visual studies which are also shown as schematic diagrams and in photographic form. Gas and solid holdup results are compared for air-water containing single, binary and ternary component particle mixtures. It should be noted that the criteria for selection of the materials used are very important if true three-phase fluidisation is to be achieved: this is very evident when comparing the results with those in the literature. The fluid flow and circulation patterns observed were assessed for validation of the generally accepted patterns, and the author believes that the present work provides more accurate insight into the modelling of liquid circulation in bubble columns. The characteristic bubbly flow at low gas velocity in a two-phase system is suppressed in the three-phase system. The degree of mixing within the system is found to be dependent on flow regime, liquid circulation and the ratio of solid phase physical properties. Evidence of strong `trade-off' of properties is shown; the overall solid holdup is believed to be a major parameter influencing the gas holdup structure.

Gasification of rubberwood in a downdraft gasifier

The objectives of this research were to investigate the perforamnce of a rubberwood gasifier and engine with electricity generation and to identify opportunities for the implementation of such a system in Malaysia. The experimental work included the design, fabrication and commissioning of a throated downdraft gasifier in Malaysia. The gasifier was subsequently used to investigate the effect of moisture content, dry wood capacity and particle size of rubberwood on gasifier performance. Additional experiments were also conducted to investigate the influence of two different nozzle numbers and two different throat diameters on tar cracking. A total of 101 runs were completed during the duration of the research. From the experimental data, the average mass balance was found to be 92.65%. The average energy balance over the gasifier to hot raw gas was 98.7%, to cold clean gas was 102.4% and over the complete system was 101.9%. The heat loss from the gasifier was estimated to range from 10-26% of the chemical energy of the feedstock. From the downstream operation, the heat loss was estimated to range from 17-37% of the chemical energy of rubberwood feedstock. The maximum throughput for stable operation was found to be 60-70% of the maximum dry wood capacity. The gasifier was found to have a maximum turndown ratio of 5:1. It is also postulated that the phenomenon of turndown of the gasifier is due to a `bubble theory' occurring at the gasification zone, and this hypothesis is explained. For stable power output, the working range of the engine was found to be 5-33.5 kWe. The thermal efficiency and diesel displacement of the engine was found to be 17-18% and 65-70% respectively. The research also showed that rubberwood gasification in Malaysia is feasible if the price of diesel is above MR35/l and the price of wood is below MR120/tonne.

Solids mixing and segregation in a gas spout-fluid bed-effect of the distributor design

This work is concerned with the assessment of a newer version of the spout-fluid bed where the gas is supplied from a common plenum and the distributor controls the operational phenomenon. Thus the main body of the work deals with the effect of the distributor design on the mixing and segregation of solids in a spout-filled bed. The effect of distributor design in the conventional fluidised bed and of variation of the gas inlet diameter in a spouted bed were also briefly investigated for purpose of comparison. Large particles were selected for study because they are becoming increasingly important in industrial fluidised beds but have not been thoroughly investigated. The mean particle diameters of the fraction ranged from 550 to 2400 mm, and their specific gravity from 0.97 to 2.45. Only work carried out with binary systems is reported here. The effect of air velocity, particle properties, bed height, the relative amount of jetsam and flotsam and initial conditions on the steady-state concentration profiles were assessed with selected distributors. The work is divided into three sections. Sections I and II deal with the fluidised bed and spouted bed systems. Section III covers the development of the spout-filled bed and its behaviour with reference to distributor design and it is shown how benefits of both spouting and fluidising phenomena can be exploited. In the fluidisation zone, better mixing is achieved by distributors which produce a large initial bubble diameter. Some common features exist between the behaviour of unidensity jetsam-rich systems and different density flotsam-rich systems. The shape factor does not seem to have an affect as long as it is only restricted to the minor component. However, in the case of the major component, particle shape significantly affects the final results. Studies of aspect ratio showed that there is a maximum (1.5) above which slugging occurs and the effect of the distributor design is nullified. A mixing number was developed for unidensity spherical rich systems, which proved to be extremely useful in quantifying the variation in mixing and segregation with changes in distributor design.

Flowback of solids through distribution plates of gas fluidized beds and associated phenomena

This work is concerned with a study of certain phenomena related to the performance and design of distributors in gas fluidized beds with particular regard to flowback of solid particles. The work to be described is divided into two parts. I. In Part one, a review of published material pertaining to distribution plates, including details from the patent specifications, has been prepared. After a chapter on the determination of the incipient fluidizing velocity, the following aspects of multi-orifice distributor plates in gas fluidized beds have been studied: (i) The effect of the distributor on bubble formation related to the way in which even distribution of bubbles on the top surface of the fluidized bed is obtained, e.g. the desirable pressure drop ratio ?PD/?PB for the even distribution of gas across the bed. Ratios of distributor pressure drop ?PD to bed pressure drop at which stable fluidization occurs show reasonable agreement with industrial practice. There is evidence that larger diameter beds tend to be less stable than smaller diameter beds when these are operated with shallow beds. Experiments show that in the presence of the bed the distributor pressure drop is reduced relative to the pressure drop without the bed, and this pressure drop in the former condition is regarded as the appropriate parameter for the design of the distributor. (ii) Experimental measurements of bubble distribution at the surface has been used to indicate maldistribution within the bed. Maldistribution is more likely at low gas flow rates and with distributors having large fractional free area characteristics (i.e. with distributors having low pressure drops). Bubble sizes obtained from this study, as well as those of others, have been successfully correlated. The correlation produced implies the existence of a bubble at the surface of an orifice and its growth by the addition of excess gas from the fluidized bed. (iii) For a given solid system, the amount of defluidized particles stagnating on the distributor plate is influenced by the orifice spacing, bed diameter and gas flow rate, but independent of the initial bed height and the way the orifices are arranged on the distributor plate. II. In Part two, solids flowback through single and multi-orifice distributors in two-dimensional and cylindrical beds of solids fluidized with air has been investigated. Distributors equipped with long cylindrical nozzles have also been included in the study. An equation for the prediction of free flowback of solids through multi-orifice distributors has been derived. Under fluidized conditions two regimes of flowback have been differentiated, namely Jumping and weeping. Data in the weeping regime have been successfully correlated. The limiting gas velocity through the distributor orifices at which flowback is completely excluded is found to be indepnndent of bed height, but a function of distributor design and physical properties of gas and solid used. A criterion for the prediction of this velocity has been established. The decisive advantage of increasing the distributor thickness or using nozzles to minimize solids flowback in fluidized beds has been observed and the opportunity taken to explore this poorly studied subject area. It has been noted, probably for the first time, that with long nozzles, there exists a critical nozzle length above which uncontrollable downflow of solids occurs. A theoretical model for predicting the critical length of a bundle of nozzles in terms of gas velocity through the nozzles has been set up. Theoretical calculations compared favourably with experiments.

Mass transfer coefficient for drying of moist particulate in a bubbling fluidized bed

Experiments on drying of moist particles by ambient air were carried out to measure the mass transfer coefficient in a bubbling fluidized bed. Fine glass beads of mean diameter 125?µm were used as the bed material. Throughout the drying process, the dynamic material distribution was recorded by electrical capacitance tomography (ECT) and the exit air condition was recorded by a temperature/humidity probe. The ECT data were used to obtain qualitative and quantitative information on the bubble characteristics. The exit air moisture content was used to determine the water content in the bed. The measured overall mass transfer coefficient was in the range of 0.0145–0.021?m/s. A simple model based on the available correlations for bubble-cloud and cloud-dense interchange (two-region model) was used to predict the overall mass transfer coefficient. Comparison between the measured and predicted mass transfer coefficient have shown reasonable agreement. The results were also used to determine the relative importance of the two transfer regions.

Protein deposition at polymer surfaces:the bulk and surface structure-property effects of hydrogels

The primary objective of this research has been to investigate the interfacial phenomenon of protein adsorption in relation to the bulk and surface structure-property effect s of hydrogel polymers. In order to achieve this it was first necessary to characterise the bulk and surface properties of the hydrogels, with regard to the structural chemistry of their component monomers. The bulk properties of the hydrogels were established using equilibrium water content measurements, together with water-binding studies by differential scanning calorimetry (D.S.C.). Hamilton and captive air bubble-contact angle techniques were employed to characterise the hydrogel-water interface and from which by a mathematical derivation, the interfacial free energy (ðsw) and the surface free energy components (ð psv, ðdsv, ðsv) were obtained. From the adsorption studies using the radio labelled iodinated (125I) proteins of human serum albumin (H.S.A.) and human fibrinogen (H.Fb.), it was Found that multi-layered adsorption was occurring and that the rate and type of this adsorption was dependent on the physico-chemical behaviour of the adsorbing protein (and its bulk concentration in solution), together with the surface energetics of the adsorbent polymer. A potential method for the invitro evaluation of a material's 'biocompatibility' was also investigated, based on an empirically observed relationship between the adsorption of albumin and fibrinogen and the 'biocompatibility' of polymeric materials. Furthermore, some consideration was also given to the biocompatibility problem of proteinaceous deposit formation on hydrophilic soft' contact lenses and in addition a number of potential continual wear contact lens formulations now undergoing clinical trials,were characterised by the above techniques.

Constant shear along a boundary of a flume

'I'he accurate rreasurement of bed shear stress has been extremely difficult due to its changing values until white propunded a theory which would give constant shear along the bed of a flume. In this investigation a flume has been designed according to White's theory and by two separate methods proven to give constant shearing force along the bed. The first method applied the Hydrogen Bubble Technique to obtain accurate values of velocity thus allowing the velocity profile to be plotted and the momentum at the various test sections to be calculated. The use of a 16 mm Beaulieu movie camera allowed the exact velocity profiles created by the hydrogen bubbles to be recorded whilst an analysing projector gave the means of calculating the exact velocities at the various test sections. Simultaneously Preston's technique of measuring skin friction using Pitot tubes was applied. Twc banks of open ended water manometer were used for recording the static and velocity head pressure drop along the flume. This tvpe of manometer eliminated air locks in the tubes and was found to be sufficiently accurate. Readings of pressure and velocity were taken for various types and diameters of bed material both natural sands and glass spheres and the results tabulated. Graphs of particle Reynolds Number against bed shear stress were plotted and gave a linear relationship which dropped off at high values of Reynolds number. It was found that bed movement occurred instantaneously along the bed of the flume once critical velocity had been reached. On completion of this test a roof curve inappropriate to the bed material was used and then the test repeated. The bed shearing stress was now no longer constant and yet bed movement started instantaneously along the bed of the flume, showing that there are more parameters than critical shear stress to bed movement. It is concluded from the two separate methods applied that the bed shear stress is constant along the bed of the flume.

Size dependent enhancement of helium ion irradiation tolerance in sputtered Cu/V nanolaminates

We have investigated the evolution of radiation damage and changes in hardness of sputter-deposited Cu/V nanolaminates upon room temperature helium ion irradiation. As the individual layer thickness decreases from 200 to 5 nm, helium bubble density and radiation hardening both decrease. The magnitude of radiation hardening becomes negligible for individual layer thickness of 2.5 nm or less. These observations indicate that nearly immiscible Cu/V interface can effectively absorb radiation-induced point defects and reduce their concentrations.

Applicability of contact angle techniques used in the analysis of contact lenses, part 1:comparative methodologies

Objectives and Methods: Contact angle, as a representative measure of surface wettability, is often employed to interpret contact lens surface properties. The literature is often contradictory and can lead to confusion. This literature review is part of a series regarding the analysis of hydrogel contact lenses using contact angle techniques. Here we present an overview of contact angle terminology, methodology, and analysis. Having discussed this background material, subsequent parts of the series will discuss the analysis of contact lens contact angles and evaluate differences in published laboratory results. Results: The concepts of contact angle, wettability and wetting are presented as an introduction. Contact angle hysteresis is outlined and highlights the advantages in using dynamic analytical techniques over static methods. The surface free energy of a material illustrates how contact angle analysis is capable of providing supplementary surface characterization. Although single values are able to distinguish individual material differences, surface free energy and dynamic methods provide an improved understanding of material behavior. The frequently used sessile drop, captive bubble, and Wilhelmy plate techniques are discussed. Their use as both dynamic and static methods, along with the advantages and disadvantages of each technique, is explained. Conclusions: No single contact angle technique fully characterizes the wettability of a material surface, and the application of complimenting methods allows increased characterization. At present, there is not an ISO standard method designed for soft materials. It is important that each contact angle technique has a standard protocol, as small protocol differences between laboratories often contribute to a variety of published data that are not easily comparable. © 2013 Contact Lens Association of Ophthalmologists.

Agile or adaptable? Finding a paradigm for an uncertain world

This paper looks at the way in which, over recent years, paradigms for manufacturing management have evolved as a result of changing economic and environmental circumstances. The lean production concept, devised during the 1980s, proved robust only until the end of the bubble economy in Japan caused firms to re-examine the underlying principles of the lean production paradigm and redesign their production systems to suit the changing circumstances they were facing. Since that time a plethora of new concepts have emerged, most of which have been based on improving the way that firms are able to respond to the uncertainties of the new environment in which they have found themselves operating. The main question today is whether firms should be agile or adaptable. Both concepts imply a measure of responsiveness, but recent changes in the nature of the uncertainties have heightened the debate about what strategies should be adopted in the future.

Modelling a biochemical reaction with computational fluid dynamics

Earlier investigations (Cartland Glover et al., 2004) into the use of computational fluid dynamics (CFD) for the modelling of gas-liquid and gas-liquid-solid flow allowed a simple biochemical reaction model to be implemented. A single plane mesh was used to represent the transport and reaction of molasses, the mould Aspergillus niger and citric acid in a bubble column with a height to diameter aspect ratio of 20:1. Two specific growth rates were used to examine the impact that biomass growth had on the local solids concentration and the effect this had on the local hydrodynamics of the bubble column.

Predicting the effect of mixing on oxygen transfer and nutrient removal in activated sludge basins

Activated sludge basins (ASBs) are a key-step in wastewater treatment processes that are used to eliminate biodegradable pollution from the water discharged to the natural environment. Bacteria found in the activated sludge consume and assimilate nutrients such as carbon, nitrogen and phosphorous under specific environmental conditions. However, applying the appropriate agitation and aeration regimes to supply the environmental conditions to promote the growth of the bacteria is not easy. The agitation and aeration regimes that are applied to activated sludge basins have a strong influence on the efficacy of wastewater treatment processes. The major aims of agitation by submersible mixers are to improve the contact between biomass and wastewater and the prevention of biomass settling. They induce a horizontal flow in the oxidation ditch, which can be quantified by the mean horizontal velocity. Mean values of 0.3-0.35 m s-1 are recommended as a design criteria to ensure best conditions for mixing and aeration (Da Silva, 1994). To give circulation velocities of this order of magnitude, the positioning and types of mixers are chosen from the plant constructors' experience and the suppliers' data for the impellers. Some case studies of existing plants have shown that measured velocities were not in the range that was specified in the plant design. This illustrates that there is still a need for design and diagnosis approach to improve process reliability by eliminating or reducing the number of short circuits, dead zones, zones of inefficient mixing and poor aeration. The objective of the aeration is to facilitate the quick degradation of pollutants by bacterial growth. To achieve these objectives a wastewater treatment plant must be adequately aerated; thus resulting in 60-80% of all energetic consummation being dedicated to the aeration alone (Juspin and Vasel, 2000). An earlier study (Gillot et al., 1997) has illustrated the influence that hydrodynamics have on the aeration performance as measure by the oxygen transfer coefficient. Therefore, optimising the agitation and aeration systems can enhance the oxygen transfer coefficient and consequently reduce the operating costs of the wastewater treatment plant. It is critically important to correctly estimate the mass transfer coefficient as any errors could result in the simulations of biological activity not being physically representative. Therefore, the transfer process was rigorously examined in several different types of process equipment to determine the impact that different hydrodynamic regimes and liquid-side film transfer coefficients have on the gas phase and the mass transfer of oxygen. To model the biological activity occurring in ASBs, several generic biochemical reaction models have been developed to characterise different biochemical reaction processes that are known as Activated Sludge Models, ASM (Henze et al., 2000). The ASM1 protocol was selected to characterise the impact of aeration on the bacteria consuming and assimilating ammonia and nitrate in the wastewater. However, one drawback of ASM protocols is that the hydrodynamics are assumed to be uniform by the use of perfectly mixed, plug flow reactors or as a number of perfectly mixed reactors in series. This makes it very difficult to identify the influence of mixing and aeration on oxygen mass transfer and biological activity. Therefore, to account for the impact of local gas-liquid mixing regime on the biochemical activity Computational Fluid Dynamics (CFD) was used by applying the individual ASM1 reaction equations as the source terms to a number of scalar equations. Thus, the application of ASM1 to CFD (FLUENT) enabled the investigation of the oxygen transfer efficiency and the carbon & nitrogen biological removal in pilot (7.5 cubic metres) and plant scale (6000 cubic metres) ASBs. Both studies have been used to validate the effect that the hydrodynamic regime has on oxygen mass transfer (the circulation velocity and mass transfer coefficient) and the effect that this had on the biological activity on pollutants such as ammonia and nitrate (Cartland Glover et al., 2005). The work presented here is one part to of an overall approach for improving the understanding of ASBs and the impact that they have in terms of the hydraulic and biological performance on the overall wastewater treatment process. References CARTLAND GLOVER G., PRINTEMPS C., ESSEMIANI K., MEINHOLD J., (2005) Modelling of wastewater treatment plants ? How far shall we go with sophisticated modelling tools? 3rd IWA Leading-Edge Conference & Exhibition on Water and Wastewater Treatment Technologies, 6-8 June 2005, Sapporo, Japan DA SILVA G. (1994). Eléments d'optimisation du transfert d'oxygène par fines bulles et agitateur séparé en chenal d'oxydation. PhD Thesis. CEMAGREF Antony ? France. GILLOT S., DERONZIER G., HEDUIT A. (1997). Oxygen transfer under process conditions in an oxidation ditch equipped with fine bubble diffusers and slow speed mixers. WEFTEC, Chicago, USA. HENZE M., GUJER W., MINO T., van LOOSDRECHT M., (2000). Activated Sludge Models ASM1, ASM2, ASM2D and ASM3, Scientific and Technical Report No. 9. IWA Publishing, London, UK. JUSPIN H., VASEL J.-L. (2000). Influence of hydrodynamics on oxygen transfer in the activated sludge process. IWA, Paris - France.

MRI pressure and stress measurement in novel homogeneous soft solids

This paper presents MRI measurements of a novel semi solid MR contrast agent to pressure. The agent is comprised of potassium chloride cross linked carageenan gum at a concentration of 2% w/v, with micron size lipid coated bubbles of air at a concentration of 3% v/v. The choice for an optimum suspending medium, the methods of production and the preliminary MRI results are presented herein. The carageenan gum is shown to be ideally elastic for compressions relating to volume changes less than 15%, in contrast to the inelastic gellan gum also tested. Although slightly lower than that of gellan gum, carageenan has a water diffusion coefficient of 1.72×10-9 m2.s-1 indicating its suitability to this purpose. RARE imaging is performed whilst simultaneously compressing test and control samples and a maximum sensitivity of 1.6% MR signal change per % volume change is found which is shown to be independent of proton density variations due to the presence of microbubbles and compression. This contrast agent could prove useful for numerous applications, and particularly in chemical engineering. More generally the method allows the user to non-invasively image with MRI any process that causes, within the solid, local changes either in bubble size or bubble shape. © 2008 American Institute of Physics.

3D simulation of the effects of sphericity on char entrainment in fluidised beds

The paper presents a 3-dimensional simulation of the effect of particle shape on char entrainment in a bubbling fluidised bed reactor. Three char particles of 350 μm side length but of different shapes (cube, sphere, and tetrahedron) are injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. Due to the fluidising conditions, reactor design and particle shape the char particles will either be entrained from the reactor or remain inside the bubbling bed. The sphericity of the particles is the factor that differentiates the particle motion inside the reactor and their efficient entrainment out of it. The simulation has been performed with a completely revised momentum transport model for bubble three-phase flow, taking into account the sphericity factors, and has been applied as an extension to the commercial finite volume code FLUENT 6.3. © 2010 Elsevier B.V.All rights reserved.