A study of the effect of process conditions on the fluidization behaviour of cohesive industrial powders linked with rheological studies

The role that fluidized bed reactors and other unit operations play for a wide range of industries is well recognized. Although fluidized bed systems offer several advantages such as high heat transfer rate, rapid solids mixing, large surface contact, high heat and mass transfer rates between gas and particles, a complete understanding of the phenomena occurring in these reactors is still a challenge, with reference to the role of the process conditions, such as pressure, temperature and humidity. Generally, the temperature affects both the properties of the material and the fluid, such as density and fluidizing gas viscosity. These changes can influence significantly the design and efficiency of the reactor. For these reasons, the effect of the temperature on fluidization became the center of a significant academic effort aimed at providing a theoretical framework to underpin the major physical phenomena involved and, in particular, to develop correlations for the scale-up of fluidized bed reactors. Several works have demonstrated that process conditions can influence the role of the interparticle forces (IPFs) in the fluidization behaviour of powders. Given the complexity of the phenomena involved, a direct quantification of the particle-particle interactions in fluidized beds and of their changes at process conditions is very difficult. Within this framework, powder rheology represents an appealing tool to evaluate indirectly the effects of the interparticles forces on fluidization. The main objective of the present work is to provide a basis for understanding the factor responsible for changes in fluidization behaviour of industrial particles under realistic process conditions. In order to address the problem of assessing the fluidization behavior of powders at high temperature, a multidisciplinary approach linking micro and macro properties of the particulate system is adopted in this project. On the one hand, the investigation of the fluidization behavior at process conditions is carried out by means of standard fluidization tests; on the other hand, the characterization of the flow properties of the same powders is performed by means of powder rheology tests. To this end, the 5 experimental campaign was performed using a 140x1000 mm heated gas fluidized bed and a modified Schulze annular shear cell. Both experimental apparatuses allowed a safe operation of the system up to 600 °C. Five cuts of the same mother particles covering Group B, A and C of Geldart’s classification were investigated over a range of temperatures from ambient to 500 °C. Furthermore, two reacted samples of the same mother particles but, containing different levels of impurities were tested. Shear test experiments show changes of the flow properties at high temperatures. The powder cohesion is the parameter which appears to be mostly affected by temperature while the angle of internal friction shows a weaker dependence on temperature and consolidation level. A model combining the continuum approach and the particle–particle interaction description was used to correlate the powder tensile strength with the interparticle forces. In the presence of only van der Waals forces, the model with the assumption of plastic deformation at contact points and a reasonable value of the mean curvature radius is able to predict the correct order of magnitude of the tensile strength. Furthermore, the significant increase of the cohesion of the reacted material with increasing temperature can be only justified by considering an active role of capillary bridges between the particle asperities. These findings, together with the nature of the impurities characterized by means of EDX analysis applied to SEM imaging, strongly suggest that the observed changes for the reacted material are due to the occurrence of capillary bridges between particles, even if thermal analyses are not able to detect any significant phase changes. This work assessed also the validity of some classical concepts and equations commonly used for describing the fluidization behaviour at low and high temperature. The minimum fluidization conditions were well predicted by the Ergun equation when accounting for the experimental values of the bed voidage. The bed collapse test was used to quantify changes in the aeratability of the powders between low and high temperature and to identify the minimum bubbling conditions. For systems dominated by IPFs the analysis of the voidage of the dense phase and the overall bed expansion as a function of the flow rate allowed reconstructing the sequence of phenomena through which a stable flow of bubbles across the solid mass were achieved. 6 The role of hydrodynamic forces and of interparticle forces on the fluidization behaviour of the particulate systems studied was investigated by looking at the applicability of the Foscolo and Gibilaro stability criterion [Chem Eng Sci. 1984, 39 (12): 1667-1675]. In particular the analysis followed the approach indicated by Valverde et al. [Europhys Lett. 2007, 54: 329-334.], which makes use of the initial settling velocity of cohesive particles and of the Bond number derived from rheometry results. The results of the analysis show the capability of predicting the final structure of the bed with temperature when considering an aggregative fluidization behaviour caused by interparticle adhesive forces. These results also suggest the potential use of the powder rheometry carried out at high temperature as a sensitive method to detect phase changes in particulate systems that are limited to the particle surface that can significantly affect the working conditions of fluidized bed reactors. More in general, the results indicate that shear testing results at ambient and high temperatures allow to correctly estimate the intensity of interparticle forces in particulate systems

Dynamics of Stocks and Flows in a Regenerative Economy

This seminar will present a “trailer” for material in a book – “Living well on a Finite Planet: a systems approach to sustainability, society and economy” - by Roland Clift, George Martin and Simon Mair, to be published by Springer in 2023. The book will develop an approach to socio-economic restructuring that looks beyond the Circular Economy to envisage a repurposed economy addressing the three components of sustainability: economy, environment, and society. Rather than the usual economic concern with flows, the analysis takes an industrial ecology approach: it starts from demand for the services provided by the stock of products and materials in use and works out from there, through analysis of remanufacturing and recycling, to the associated material flows which are treated as responses rather than drivers. An earlier analysis, developed by Stahel and Clift, has been extended to stocks that change over time, to generate simple metrics accounting for the effect of stock growth on material demand allowing for product life. Applying the analysis to selected scarce metals shows how it can help to understand the development of “closed loop” systems. It also reveals why setting targets in terms of “circularity” can have perverse consequences

Hydrodynamics of compartmented fluidized beds for concentrated solar power applications

Application of fluidized beds to collection and thermal storage of solar radiation is beneficial in Concentrated Solar Power (CSP) systems thanks to their well-known inherently good thermal performances. Non-conventional design and operation of fluidized beds based on uneven or unsteady (pulsed) fluidization (1), may further enhance their thermal performances improving the potential for applications in the very demanding CSP systems. Dense gas-fluidized beds have the potential to effectively accomplish three complementary tasks: the collection of incident solar radiation; the heat transfer of the incident power to immersed tube bundles of high-efficiency steam and/or organic Rankine cycles (ORC) and the thermal energy storage equalizing the inherent time-variability of the incident radiation for stationary CHP generation. A novel concept of solar receiver for CHP (combined heat and power) generation consisting of a compartmented dense gas fluidized bed has been proposed (2). The present study addresses the hydrodynamics of a dense gas-fluidized bed operated at ambient conditions and equipped with a compartmented windbox. Figure 1 outlines the experimental apparatus which consists of a nearly-2D fluidization column (2850x1860x200mm) equipped with two sparger-type gas distributors extending along 20 and 80% of the fluidized bed width. The regions of the bed above the two spargers were marked as compartments A and NA, respectively. The bed material was fine silica sand with a mean Sauter diameter of 145 µm. A pressure measurement system was used to monitor pressures and pressure gradients at different locations inside the fluidized bed. A pressure gradient exceeding a threshold of 0.11mbar/mm was assumed to mark the onset of local fluidization. A procedure was developed to draw the separation boundary (dashed line) between fluidized and non-fluidized regions for different bed heights (0.55, 0.95, 1.39 and 1.85m) as the gas superficial velocity was varied in either regions of the bed (UA and UNA=0-4Umf). Selected fluidization maps are shown in figure 2 where separation boundaries at different values of UA and UNA for a static bed height of 1.85 are reported. Figure 3 reports the fractional extension of the fluidized region at a level of 400mm for different values of UA e UNA as the static bed height was varied. Results indicate that a perfectly compartmented fluidized bed cannot be obtained simply using a compartmented windbox, but a proper choice of the operating conditions enables good control of the local fluidization conditions and of the gas cross-flow between the compartments. Please click Additional Files below to see the full abstract

The relevance of surface impurities on the effect of temperature on powder flow behavior

Cohesive interparticle forces may have a relevant role in several industrial process operations involving particulate materials, such as fluidization, granulation and drying, as well as storage and solids handling units. Several of these operations require process conditions which involve high temperatures which, in turn, may affect the intensity of interparticle forces such as van der Waals, capillary and electrostatic forces. The mean by which the system temperature can affect all these forces is the change of particle hardness, the generation of liquid phases, which determines the formation of liquid bridges, and the modification of the particle dielectric properties. A direct measure of interparticle forces is possible but can be affected by large fluctuations and require a great number of repetitions. Interparticle forces, instead, play in averaged ensembles in bulk properties such as powder cohesion. It is of interest, therefore, to have the possibility to measure powder cohesion at the process temperature and to observe possible changes due to temperature variations to infer possible changes at the particle level. Powder shear testing is one of the available methods able to measure powder cohesion and it has the great advantage of measuring well established physical properties and of being able to produce highly repeatable results. It has to be remarked, however that to date no established procedure exists to relate powder cohesion measured at the bulk level to powder fluidization properties. In this paper a systematic study on the effect of the process conditions on the fluidization quality of ceramic powders is presented. Tests were carried out on powders of industrial interests, characterized by different particle size distributions and by different amounts of surface impurities, ranging from easy-flowing to cohesive flow behaviour.Two different experimental facilities were used: a modified ring shear tester available at the University of Salerno and aX-ray high temperature fluidization facility available at University College of London. The first apparatus was used to characterize powder cohesion at different temperatures between ambient and 500°C. Experimental results have been interpreted in terms of possible changes in interparticle forces as a function of temperature. The powder samples without impurities show an increase of cohesion with temperature as a result of an increase of interparticle van der Waals forces. A larger increase of cohesion was observed in the case of the powder samples with chemical impurities. The behaviour can be explained only by considering a cooperative effect of both van der Waals and capillary forces. It is noteworthy that the amount of surface impurities that is able to determine significant changes of powder flow properties is still so small that no evidence of phase transition could be detected by means of sample thermal analysis.The same powders have been characterized in terms of fluidization quality by using the x-ray fluidization facility available at University College of London under the same temperature range. Moreover the changes by temperature on the flow properties of the bulk solid evaluated with the shear cell and the behaviour of particles under fluidization conditions are analysed and discussed. Though a direct quantification of the particle-particle interactions in fluidized beds and of their changes under process conditions is very difficult, this paper suggests a method by which powder rheology can be used to indirectly evaluate the effects of the interparticle forces on fluidization

Binary mixtures of biomass and inert components in fluidized beds: experimental and neural network exploration

Considering the little understanding of the hydrodynamics of multicomponent particle beds involving biomass, a detailed investigation has been performed, which combines well-known experimental and theoretical approaches, relying, respectively, on conventional pressure drop methods and artificial neural network (ANN) techniques. Specific research tasks related to this research work include: i. to experimentally investigate by means of visual observation the mixing and segregation behavior of selected binary mixtures when varying the biomass size and shape as well as the properties (size and density) of the granular solids in cold flow experiments; ii. to carry out a systematic experimental investigation on the effect of the biomass weight and volume fractions on the characteristic velocities (e.g., complete fluidization velocity and minimum slugging velocity) of the investigated binary mixtures in order to select the critical weight fraction of biomass in the mixtures beyond which the fluidization properties deteriorate (e.g., channeling, segregation, slugging); iii. to analyze the results obtained in about 80 cold flow experiments by means of ANN techniques to scrutinize the key factors that influence the behavior and the characteristic properties of binary mixtures. Experimental results suggest that the bed components’ density difference prevails over the size difference in determining the mixing/segregation behavior of binary fluidized bed, whereas the velocities of minimum and complete fluidization increase with a growing biomass weight fraction in the bed. The training of ANNs demonstrated good performances for both outputs (Umf and Ucf); in particular, the best predictions have been obtained for Umf with a MAPE1 <4% (R2=0.98), while for Ucf the best ANN returned a MAPE of about 7% (R2=0.93). The analysis on the importance of each individual input on ANN predictions confirmed the importance of particle density of the bed components. Unexpectedly, results showed that morphological features of biomass have a limited importance on Ucf

Hydrodynamics of a Loop-seal Operated in a Circulating Fluidized Bed: Influence of The Operating Conditions on Gas and Solid Flow Patterns

Hydrodynamic features of a loop-seal operated as solids re-injection device in a labscale cold CFB apparatus are studied. Gas flow patterns are characterized by means of gas tracing experiments with continuous injection of CO2 in the loop-seal chambers. Solids flow patterns are characterized by impulsive injection of dyecoloured particles into the supply chamber, followed by particle tracking

HYDRODYNAMICS OF UNCONVENTIONAL FLUIDIZED BEDS: SOLIDS FLOW PATTERNS AND THEIR INFLUENCE ON MIXING/SEGREGATION OF A LARGE FLOTSAM PARTICLE IN A BED OF FINER SOLIDS

Gross solids circulation of solid phase and its influence on mixing/segregation of a large flotsam particle in beds of finer solids in unconventional fluidized beds has been investigated. A tapered two-dimensional fluidization column and a fluidization column equipped with a diverging cone as gas distributor have been adopted. The hydrodynamics of the gas-solid suspension in the two apparatus has been qualitatively assessed by visual observation and the trajectories of the centre-of-gravity of large flotsam particles have been evaluated to assess the extent of mixing/segregation

Thermal behaviour of fluidized beds directly irradiated by a concentrated solar radiation

Directly-irradiated fluidized bed reactors are very promising in the context of concentrated solar power applications as they can be operated at process temperatures high enough to perform thermochemical storage with high energy density. The present study aims at experimentally investigating the direct interaction between a concentrated simulated solar radiation and a fluidized bed by measuring the time-resolved bed surface temperature with an infrared camera under different fluidization gas velocities. The effect of a localized generation of bubbles was investigated too, by injecting a chain of bubbles through a nozzle located just at the centre of the concentrated solar beam. The obtained results encourage the localized generation of bubbles, just at the larger value of the impinging radiative heat flux, as a strategy to reduce the overheating of the bed surface and, as a consequence, the energy losses related to fluidizing gas and radiative re-emission

Influence of bubble bursting on heat transfer phenomena in directly irradiated fluidized beds

Concentrating Solar Power (CSP) is a fast-growing technology in which several hundreds/thousands of optical mirrors concentrate the solar energy onto a receiver. The received energy can be used to produce electricity through thermodynamic cycles or to drive an endothermic chemical reaction for chemicals production (e.g. solar fuels) or for chemical storage. The receiver is a crucial part of the whole system, as it owns the severe task of collecting and transferring the concentrated solar energy minimizing the heat losses. Dense fluidized beds have been proposed as CSP receivers thanks to their large heat-transfer and thermal diffusivity coefficients, and their use is currently under investigation (1-2). Directly-irradiated fluidized bed receivers are very promising in the context of solar chemistry and CSP applications, but they can undergo to extensive bed surface overheating. Tailoring bed hydrodynamics close to the region where the incident power is concentrated may disclose effective measures to improve the interaction between the incident radiative flux and the bed in order to maximize the receiver efficiency and mitigate the bed surface overheating. The present work addresses the study of the interaction between a concentrated solar radiation and bed surface. The experimental apparatus schematically reported in figure 1a mainly consists of: i) a fluidized bed reactor (square 0.78 x 0.78 m bed column, 0.6 m tall); ii) a simulated solar radiation source, consisting of a short‑arc Xe lamp coupled with an elliptical reflector, whose spatial flux distribution map is shown in Figure 1b; iii) a Bubble Generation System (BGS), able to produce bubbles with a minimum diameter of 0.045 m at a maximum frequency of 2 Hz, connected to a submerged nozzle aligned with the focal point of the simulated solar beam. SiC particles (127 mm) were used as solid bed material. The main diagnostic tool is represented by an infrared camera used to map the bed surface temperature. Tests were performed at incipient fluidization condition injecting through the nozzle bubbles with different diameters and at different frequencies keeping constant the gas flow rate. Two snapshot sequences of the bubble eruption phenomena are reported in Figure 2. It can be observed that increasing bubble diameter the lateral dispersion heat at bed surface is more efficient, as the hot particles are shifted towards a larger annular region. Nevertheless, the long delay between two successive bubble eruption events brings to a larger bed surface overheating, which could result into a fluidized particles degradation or into higher heat losses due to re-irradiation. A trade-off between these two-fold results has to be found to optimize bed hydrodynamics in CSP applications. Please click Additional Files below to see the full abstract

Combustion of lignin-rich residues with coal in a pilot-scale bubbling fluidized bed reactor

The deployment and the exploitation of bioethanol as automotive fuel became more and more relevant to reduce the emissions of greenhouse gases and to limit the dependence on countries supplying fossil fuels. However, the production of second-generation bioethanol, i.e. using lignocellulosic biomass or scraps of agricultural crops as feedstock, generates a waste stream consisting of lignin-rich residues whose fate has to be found (1-2). This work aims at investigating the combustion of lignin-rich residues (in the following simply called lignin), coming from a second-generation bioethanol production plant, with coal in a pilot-scale bubbling fluidized bed combustor (FBC). The pilot-scale 200kWth FBC schematically shown in Fig. 1 basically consist of a AISI 310 stainless steel fluidization circular column (370 mm ID for 5.05 m and 700 mm ID for 1.85m in the upper part of freeboard), a continuous over-bed feeding system, two cyclones for flue gas de-dusting, a propane premixed burner for the start-up and different heat removal devices located along the fluidization column. On-line gas analyzers (ABB AO2020) measured flue gas composition sampled at the exhaust. Fuels were the lignin-rich residue, a bituminous coal and wood chips. Silica sand (0.8-1.2mm) was used as bed material. An experimental campaign was carried out to study gaseous and particulate emissions and thermal regimes during the co-combustion of different mixtures of coal-lignin varying the percentage of lignin fed with coal, the bed temperature, the excess air and the fluidization velocity. Figure 2 reports the main results in terms of normalized emissions of NO, SO2, particulate and carbon in particulate as a function of the O2 concentration measured at the exhaust obtained during the steady state operation of the pilot-scale FBC. A large part of the investigated experimental conditions regarded the operation using a mixture lignin-coal at 30%w in lignin. Experiments with coal, with a mixture at 40%w in lignin and with a mixture coal-wood chips at 20%w in wood chips were carried out for comparison. The analysis of the experimental results mainly highlights that: 1) the gaseous emissions do not significantly change with respect to coal or to reference biomass-coal mixture at least until the mixture content of lignin is 30-40%w; 2) the particulate emissions increase with the percentage of residues content, but, at the same, the carbon content is significantly reduced. Bottom bed particles were analyzed at the end of each experiments highlighting the absence of agglomerates but a significant enrichment of metals like Fe, Mg, Na, Ca and K coming from lignin ash when the FBC was operated for long time and at high temperature. Please click Additional Files below to see the full abstract