Measuring the Gas-Solids Distribution in Fluidized Beds - A Review

This paper reviews techniques for measuring the voidage distribution in gas-solid fluidized beds, with a focus on the developments during the last ten years. Most attention is given to recent progress in tomography and pressure measurements, but visual observations, capacitance probes and optical probes are also covered

Effect of sieving and isopropanol on the fluidization behavior of TiO2 nanoparticles

The fluidization of ABF nanoparticles has gained the attention of many researchers due to its interesting applications but difficult fluidization. Typically, these particles are sieved to remove the large agglomerates that are formed during storage. Otherwise, the larger agglomerates stay near the distributor plate, hindering the proper fluidization of the nanoparticles due to the formation of channels throughout the bed. To solve that, several papers propose to improve the fluidization conditions using external assistance methods. Such methods impose an external force that can break up the agglomerates; examples are magnetic or electric fields, vibration or centrifugal beds [1]. A different approach is to change the surface properties of the nanoparticles, decreasing the cohesive forces. Tahmasebpooret al. [2] analysed the influence of the hydrogen bonds during the fluidization of nanoparticles. They showed that the use of isopropanol vapour (ISP) in the fluidizing gas can reduce the cohesive forces between nanoparticles increasing the bed aspect ratio. Sieving of nanoparticles and the use of ISP in the gas stream have been commonly used to improve the fluidization quality during the last years. However, the influence of both processes on the fluidization behaviour has not been studied in detail. For the former, the effect of the sieving size on the bed dynamics is still unknown. Regarding the ISP, its influence for long fluidization times has not been addressed yet. For instance, whether the ISP should be continuously on the gas stream or working with gas pulses to improve the fluidization has not been clarified. Therefore, the objective of this experimental work is to further understand the influence of the sieving size and the effect of ISP during the fluidization of TiO2 nanoparticles. The experiments are carried out in a 5 cm inner diameter column with a porous distributor. Nitrogen is used as fluidizing gas. The experiments are analysed using a 2D tomography setup. The attenuation of the X-rays are measured when they go through the fluidized by a plate detector, with a size of 30 cm x 30 cm and 1524x1548 pixels. The fluidization of TiO2 nanoparticles sieved with a 350 µm mesh shows higher bed expansion than the powder sieved with a 850 µm mesh. Considering the effect of time, the use of ISP initially increases the bed expansion, but after that the bed height decreases faster than for the situation without ISP. Please click Additional Files below to see the full abstract

Pressure and X-ray tomography characterization of the fluidization behavior of TiO2 nanoparticles

During recent years the fluidization of nanoparticles has been attracting the interest of the scientific community as the number of industrial applications has increased [1]. In these systems, the powder tends to agglomerate during the fluidization showing either bubble-less and smooth behavior for agglomerate particulate fluidization (APF), and bubbling behavior with little bed expansion for agglomerate bubbling fluidization (ABF) [2]. Regarding the ABF regime, relatively high gas velocities are required to fluidize these nanoparticles, which causes a large powder elutriation and the reduction of the fluidization quality. In this sense, it would be interesting to detect the changes in the fluidization behavior using an easy and reliable measurement technique. However, the literature commonly uses of the bed expansion ratio or the bed pressure drop [1], which are not able to detect maldistributions inside the fluidized bed. Therefore, in this work is proposed the use of the pressure fluctuation signals as a tool to describe the state of a fluidized bed of nanoparticles. The differential pressure signals will be analyzed in the time and frequency domain following earlier work for micron-sized particles [3, 4]. TiO2 nanoparticles (dp= 21nm) were fluidized in a Perpex column of 5 cm inner diameter at different gas velocities. The particles were sieved with a 350μm mesh to remove the large agglomerates and dry nitrogen was used during all experiments. To validate the pressure results, an X-ray tomography system is employed. This technique measures the attenuation of the X-rays through the fluidized bed, which is placed between the X-ray source and the detector. A square detector of 30 cm x 30 cm with a pixel resolution of 1524x1548 is employed. In this way, it is possible to obtain 2D pictures of the fluidization regime at a frame rate of 22 Hz (see Fig. 1). The results show that the main frequencies of the power spectrum are moved towards to higher frequencies as the gas velocity is increased (see Fig. 2). We will demonstrate that the pressure fluctuation data can indicate whether or not the nanoparticle bed is properly fluidized . Please click Additional Files below to see the full abstract

A fast reconstruction algorithm for time-resolved X-ray tomography in bubbling fluidized beds

A new tomographic reconstruction algorithm is proposed for fast image reconstruction. The results are based on a high speed X-ray tomography system, consisting of 3 X-ray sources and 32 detectors for each source. The proposed algorithm combines void measurements of each X-ray beam into a triangular mesh, which is formed by the intersection points of all the beams. Simulations and real fluidized bed data are utilized to assess the quality of the proposed algorithm compared to the Simultaneous Algebraic Reconstruction Technique (SART). The influence of the number, position and diameter of the phantoms on the proposed reconstruction method is studied. The new method provides images with similar quality to SART reconstructions, although obtaining smaller bubble sizes. The low computing time needed to reconstruct each image with the new method, which is more than 5000 times faster than SART for a 40 × 40 mesh, encourages the use of the new method for the online image reconstruction of X-ray measurements

Sleep disturbance in patients with an implantable cardioverter defibrillator:Prevalence, predictors and impact on health status

Background:  Sleep disturbances are highly prevalent in patients with cardiac diseases and associated with poor health outcomes. However, little is known about sleep disturbance in patients with an implantable cardioverter defibrillator. Aims:  We examined the prevalence and predictors of sleep disturbance and the impact on perceived health status in a Dutch cohort of implantable cardioverter defibrillator patients. Methods:  Patients ( n=195) enrolled in the Web-based distress program for implantable cardioverter defibrillator patients (WEBCARE) trial completed questionnaires at the time of implantable cardioverter defibrillator implantation, three, six and 12 months afterwards. Sleep disturbance was assessed with the corresponding item #3 of the Patient Health Questionnaire 9. Results:  At baseline, 67% ( n=130) reported sleep disturbance (cut off ≥1). One year later, the prevalence was 57% ( n=112). Younger age (odds ratio=0.96, 95% confidence interval 0.92-0.99; p=0.012) and high negative affectivity/low social inhibition (odds ratio=4.47, 95% confidence interval 1.52-13.17; p=0.007) were associated with sleep disturbance at 12 months in adjusted analyses. Sleep disturbance was not associated with health status at 12 months. Charlson Comorbidity Index, anxiety, Type D personality and high negative affectivity/low social inhibition were associated with impaired health status at follow-up. Conclusions:  Sleep disturbance was highly prevalent in patients with an implantable cardioverter defibrillator. Younger age and high negative affectivity predicted sleep disturbance 12 months post-implantation independent of other demographic, clinical, intervention and psychological covariates. Sleep disturbance was not associated with impaired health status at the 12-month follow-up

Concomitant endocarditis and spondylodiscitis due to coagulase-negative Staphylococci and a review of the literature

Background: Coagulase-negative staphylococci (CoNS) are part of the normal skin flora. Although CoNS are generally considered as low pathogenic microorganisms, they can cause serious infections, particularly in the context of foreign body material. Case report: Here we present two cases of concomitant infectious endocarditis and spondylodiscitis; one caused by S. epidermidis, the other by S. haemolyticus. Additionally, we reviewed the literature for previously reported cases of concomitant endocarditis and spondylodiscitis due to CoNS. Conclusion: In patients with back pain and a cardiac device in situ, CoNS should be considered as causative pathogens for possible endocarditis and/or spondylodiscitis, and should not be regarded as contamination.</p

CFD-DEM simulation of nanoparticle agglomerates fluidization with a micro- jet

Nanoparticles can be fluidized as agglomerates, but for some materials this is cumbersome due to the cohesive nature. Micro-jets are shown to be effective for improving the fluidization in such cases (1). In this study, the mechanisms of micro-jet assistance are investigated by using an adhesive CFD-DEM (Computational Fluid Dynamics – Discrete Element Modelling) model. In previous studies, the complex agglomerates found in a fluidized bed are treated as the discrete elements (2). Here we use the simple agglomerates as the discrete elements, which are the building blocks of the larger complex agglomerates. The collision of the simple agglomerates are modeled by including collision mechanisms of elastic-plastic, cohesive and viscoelastic forces. Particles with =40 and =250 are used to represent the simple agglomerates. The cohesive force is expressed by the non-dimensional parameter , definded by the ratio of der Waals force over the particle gravity. A fluidized bed with dimension of 3 mm × 0.4 mm × 12 mm containing ~120,000 particles is simulated. At different cases, a micro-jet with horizontal cross-section size of 20 x 20 pointing downwards is turned ON or OFF (36 m/s) while the gas velocity to the bed is set as 2.8 cm/s or 4 cm/s, respectively. The schematic of the microjet in the bed is shown in Figure 1. In this way, like in our previous study, we keep the total amount of gas provided to the bed equal (2). Please click Additional Files below to see the full abstract