Yassir T. Makkawi

Fluidization regimes in a conventional fluidized bed characterized by means of electrical capacitance tomography

Abstract The purpose of the present study is to verify different analysis approaches and provide a quantitative and qualitative classification of fluidization regimes. The experimental study has been carried out in a cold conventional fluidized bed using electrical capacitance tomography (ECT). The system compromises a twin-plane ECT in a 15 cm diameter acrylic column. The experiments were carried out at ambient conditions and under different fluidization velocities, ranging from 0.2 to 2.0 m / s using air as the fluidizing gas. A mixture of spherical glass ballotini ranging from 150 to 1000 μm diameter and average density of 2600 kg / m 3 were used as the fluidizing particles. Measurement of solid volume fraction was recorded over a 20 s interval at a 100 Hz sample rate. Four different fluidization regimes were identified based on a distinct transition velocity: single bubble, slugging bed, turbulent flow and fast fluidization regime. Different analysis methods employed with the solid fraction fluctuations have shown good agreement. The transition velocities determined by standard deviation, amplitude and solid fraction distribution analysis almost coincide, while results obtained with the peak and cycle frequencies analysis only shows the transition to slugging bed and fast fluidization regime. Bubble rise velocity analysis shows a maximum at the onset of turbulent fluidization, but no further conclusions could be made. Analyses based on power spectra and probability distribution of amplitude are also discussed. The regime classification shows no variation with respect to height within the bottom level of the bed, however, regime transitions are strong functions of the radial measuring position. Conclusions are drawn about the adequacy of each analysis method applied in this study, and a brief description on the characteristics of each flow regime is presented. Several available correlations from the literature for U mf , U c and U k are tested and compared with the experimental findings.

The voidage function and effective drag force for fluidized beds

Abstract Here, an experimental investigation on the effective drag force in a conventional fluidized bed is presented. Two beds of different particle size distribution belonging to group B and group B/D powders were fluidized in air in a 13.8 cm diameter column. The drag force on a particle has been calculated based on the measurement of particle velocity and concentration during pulse gas tests, using twin-plane electrical capacitance tomography. The validity of the voidage function “correction function”, (1−es)n, for the reliable estimation of the effective drag force has been investigated. The parameter n shows substantial dependence on the relative particle Reynolds number (Re p ∗ ) , and the spatial variation of the effective static and hydrodynamic forces. It is also illustrated that, a simple correlation for the effective drag coefficient as function of the particle Reynolds number (Rep), expressed implicitly in terms of the interstitial gas velocity, can serve in estimating the effective drag force in a real fluidization process. Analysis shows that, the calculated drag force is comparable to the particle weight, which enables a better understanding of the particle dynamics, and the degree of spatial segregation in a multi-sized particle bed mixture. The analogy presented in this paper could be extended to obtain a generalized correlation for the effective drag coefficient in a fluidized bed in terms of Rep and the particle physical properties.

Optimization of experiment span and data acquisition rate for reliable electrical capacitance tomography measurement in fluidization studies—a case study

In a conventional fluidized bed, the particle behaviour is highly unpredictable and therefore the data collected during a specific experimental scenario is often highly representative of that isolated result, rather than being reproducible. To quantify the degree of representation/reproducibility, different scenarios of sampling span and measurement frequency were tested for studying the bed hydrodynamics. The experiments were carried out at ambient conditions in a non-reacting fluidized bed. A twin plane electrical capacitance tomography system was used to measure the solid fraction distribution in a 150 mm diameter acrylic column and at a selected gas velocity of 0.9 m s−1. The images produced from capacitance measurements are relatively low-resolution images. Thus an iterative method based on a LPB algorithm has been used and the recommended number of iterations required for enhanced images is presented. In measuring the hydrodynamic parameters, it is demonstrated that increasing the recording span considerably increases the measurement accuracy. It is also observed that the visualization of the axi-symmetrical nature of the bed hydrodynamics is not easy to achieve with a short recording span of 20 s, due to the high bed non-uniformity. For solid fraction measurements, the minimum recommended experimental sample is 4000 data points. When measuring dynamic parameters such as frequencies or standard deviation of solid fraction fluctuations, high data capture rates are of vital importance to properly characterize the hydrodynamic fluctuations. Local measurements were found to only represent the specific measuring location. In order to have a global estimate of a dynamic parameter such as the bubble frequency the analyses should be based on the average bed signal. When measuring the bubble rise velocity from the average bed signal it has been found that, within the operating conditions considered in this study, a combination of 60 Hz and 80 s (4800 points) or 100 Hz and 60 s (6000 points) produces an acceptable level of accuracy. Low data capture rates ≤40 Hz may dramatically underestimate or completely fail to provide an estimate of bubble rise velocity.

Packing size and shape effects on forced convection in large rectangular packed ducts with asymmetric heating

An experimental and numerical investigation of fully developed forced convection in large rectangular packed ducts is presented. The horizontally oriented ducts have the length-to-separation distance ratio of L/H=16 with two aspect ratios of W/H=8 and 4. A constant heat flux is supplied to the top wall, while the bottom and side walls are insulated. Packing of hard polyvinyl chloride Raschig rings with outside diameters of 48 and 34 mm, and expanded polystyrene spheres with diameters of 48, 38 and 29 mm are used in the air flow passage. The experiments are carried out for 200 < (Rep=udp/nf) < 1450, and 4.5 < de/dp < 9.0. The pressure drop, the heat flux, axial and transverse temperature profiles of air flow inside the ducts are measured at the steady state. Similar experiments have also been carried out with empty ducts. Numerical predictions of two-dimensional quasi-homogeneous model are found to be in agreement with the experimental results of the packed ducts. The correlation equations for the Nusselt number are obtained. It is found that the introduction of packing into the air flow passage yields about a three times increase in the wall-to-air heat-transfer rate compared with that of the empty duct. # 1999 Elsevier

Numerical analysis of convection heat transfer in a rectangular packed duct with asymmetric heating

Fully developed, forced convection heat transfer in a rectangular packed duct is analyzed numerically based on a two-dimensional model incorporating the effects of Raschig ring packing on the Ergun equation. The boundary conditions are based on asymmetric heating with only the top wall supplied with a constant heat flux, while the other walls are adiabatic. The numerical predictions are compared with measured temperature profiles and local Nusselt numbers obtained from a large scale packed duct with L/H = 16 and W/H = 8, using Raschig rings of 4.8 cm in size. The prediction represents the experimental data satisfactorily. The volumetric entropy generation of the duct is also evaluated and displayed graphically that shows the distribution of entropy generation due to heat transfer and pressure drop in the duct.

Mass transfer in fluidized bed drying of moist particulate

Bubbling fluidized bed technology is one of the most effective mean for interaction between solid and gas flow, mainly due to its good mixing and high heat and mass transfer rate. It has been widely used at a commercial scale for drying of grains such as in pharmaceutical, fertilizers and food industries. When applied to drying of non-pours moist solid particles, the water is drawn-off driven by the difference in water concentration between the solid phase and the fluidizing gas. In most cases, the fluidizing gas or drying agent is air. Despite of the simplicity of its operation, the design of a bubbling fluidized bed dryer requires an understanding of the combined complexity in hydrodynamics and the mass transfer mechanism. On the other hand, reliable mass transfer coefficient equations are also required to satisfy the growing interest in mathematical modelling and simulation, for accurate prediction of the process kinetics. This chapter presents an overview of the various mechanisms contributing to particulate drying in a bubbling fluidized bed and the mass transfer coefficient corresponding to each mechanism. In addition, a case study on measuring the overall mass transfer coefficient is discussed. These measurements are then used for the validation of mass transfer coefficient correlations and for assessing the various assumptions used in developing these correlations.

Modelling of steam gasification of char in a circulating fluidised bed

In the last few years, asphaltenes have been of immense interest for exploration techniques, since it was reported that they possess structural features of the related source rock kerogens. This is because the use of asphaltenes from crude oils may help to overcome the lack of source rock samples in basin analysis when reliable predictions for the generation of hydrocarbons are required. Potential source rocks are described in terms of quantity, quality and level of thermal maturity of organic matter, but pertinent source rock information is frequently absent because exploratory drilling does not reach deeply buried source facies. Even if the source is reached, samples are often inappropriate for reliable oilsource rock correlation due to low maturity or organic facies variation.H is often considered a very capable future energy vector. It can be produced from renewable wind or solar power via water electrolysis and has a wide range of potential applications in all important fields of energy supply. The gravimetric storage density of hydrogen is excellent. One kilogram H2 carries 33.3 kWh (LHV) of energy. However, being the chemical element with the lowest density, the volumetric storage density of hydrogen is only 3 Wh/liter at ambient pressure. In existing technical applications hydrogen is, therefore, either stored as gas under very high pressures (up to 700 bar, called “Compressed Gaseous Hydrogen” or CGH2) or in its liquid state at 253°C (called “Liquid Hydrogen” or LH2). A very attractive way to store and release hydrogen is in form of “Liquid Organic Hydrogen Carriers” (LOHC) systems. Aromatic molecules, such as, e.g., N-ethylcarbazole (NEC) or dibenzyltoluenes can be reversibly hydrogenated and dehydrogenated in order to store and transport hydrogen in form of diesel-like liquids. The presentation introduces shortly the LOHC concept for energy storage and future hydrogen logistics. Afterwards, it concentrates on material and process aspects of LOHC hydrogenation and dehydrogenation catalysis covering the full range from studies on the molecular level (XPS, IR studies) to reactor design and demonstration units. Challenges and optimization potentials will be discussed; novel options (LOHC transfer hydrogenation, hydrogen purification through LOHC hydrogenation/dehydrogenation) will be presented.H is necessary to establish a sustainable and environmentally-friendly society. However, hydrogen could degrade materials strength. Therefore, one of the key issues to deploy high-pressure hydrogen containment systems is how to optimize the cost, performance and safety of those systems. For this issue, many studies on hydrogen-affected fracture are under way in order to identify fundamental mechanisms, develop predictive performance models, develop next generation materials, reduce regulations, develop design methods, identify appropriate material testing standards in high-pressure hydrogen environment, and so on. Fretting fatigue is a kind of fatigue at the contact part between mechanical components. As can be expected from the fact that fretting is sometimes termed as fretting corrosion, it involves some chemical reactions, which might have a great impact on fatigue properties. Since hydrogen could influence both fatigue and the phenomena occurring at the contact surface such as friction, wear, oxidation, etc., the effects of hydrogen on fretting fatigue are very complicated. In fact, fretting fatigue strength of austenitic stainless steels is significantly lower in hydrogen than in air. As a result, the industries related to hydrogen-containment systems are deeply concerned about fretting fatigue in hydrogen. When considering service conditions of hydrogen-containment systems, some amount of impurities in hydrogen should be accepted. For example, the purity of hydrogen for PEM fuel cell is designated by ISO standard as 99.99%. On the other hand, positive use of impurities is expected based on the report in which the addition of small amounts of oxygen to hydrogen inhibited hydrogen-affected fracture. The objective of this study is to clarify the effect of oxygen and water vapor added to hydrogen on fretting fatigue strength of an austenitic stainless steel. For the fretting fatigue test, a controlled method for the addition of ppmlevel oxygen to hydrogen environment was established. Fretting fatigue tests in hydrogen containing 0.088, 5, 35 and 100 volumeppm oxygen were carried out using the test apparatus. The fretting fatigue strength in the oxygen-hydrogen mixture was different depending on the oxygen level. In the fretting fatigue test in hydrogen with humidification, it was found that the humidification of hydrogen significantly reduced the fretting fatigue strength. Based on the XPS (X-ray photoelectron spectroscopy) analysis of the fretted surface, it was found that the fretting removed the original protection layer of the stainless steel, however, the addition of water vapor or ppm-level of oxygen produced an oxide layer on the fretted surface during the fretting that surpassed the removal effect of the initial oxide layer by fretting. In fact, a strong adhesion between the contacting surfaces occurred and no fretting wear particles were observed in the high-purity hydrogen. On the other hand, oxidized fretting wear particles were found in the oxygen-hydrogen mixture. In addition, the reasons for the change in the fretting fatigue strength in hydrogen due to the addition of impurities were examined from the view point of the change in mechanical stress conditions.