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

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

The Effect of IFN-γ, Alum and Complete Freund Adjuvant on TNP-KLH Induced Ig.G1, IgE and IgG2a Responses in Mice

Adjuvants are considered to play an important role in directing the isotype and amount of antibodies produced upon immunization by conducting the development of either Th-1 or Th-2 cells upon T-cell stimulation. This is based on the different cytokine production patterns that were observed after in vitro resttmulation of T cells isolated from mice immunized with antigen either adsorbed on alum or emulsified in complete Freund adjuvant (CFA). However, other studies suggest that primarily the type of antigen determines which isotypes are produced and to what extent. In these studies, however, IgE was not determined. Therefore, this study examined whether alum and CFA influenced the amount and/or ratio of IgG1, IgE and IgG2a produced after TNP-KLH immunization. Similar levels of IgG1, IgE and IgG2a antibodies were found upon immunization with TNP-KLH either adsorbed on alum or emulsified in CFA. Moreover, administration of IFN-γ in combination with TNP-KLH adsorbed on alum did not increase the amount of IgG2a produced. IFN-γ treatment resulted in an increased IL-6 and decreased IFN-γ production by spleen cells upon Con A stimulation, whereas it did not change the IL-4 production in similar conditions. The presented results suggest that upon immunization with TNP-KLH high IL-4 levels are produced, resulting in an antibody response that is dominated by IgG1, independent of the adjuvant employed. The IL-4 inducing property of TNP-KLH is substantiated by the finding that repeated immunization of mice with TNP-KI, without adjuvant, increases the serum total IgE level. The presented data suggest that the carrier part of TNP-KLH preferentially results in Th-2 cell activity after which the adjuvant merely enhances the antibody responses generated

Comparison of optical probes and X-ray tomography for bubble characterization in fluidized bed methanation reactors

The performance of many fluidized bed reactors strongly depends on the bubble behavior since they influence the mass transfer to the dense phase where the catalyst is present. An example is the methanation in a fluidized bed that allows for conversion of unsaturated hydrocarbons in the gasification gas without catalyst deactivation [1]. The BFB reactor is a very challenging step in the process chain to produce SNG out of biomass as feedstock since next to the bubble behavior a lot of other parameters like temperature, pressure, particle size, attrition of the catalyst, internals, bed height and reactor diameter etc. affect the overall performance. The focus of this research work lies on the determination of the bubble properties which are an important factor to model a bubbling fluidized methanation reactor in order to predict and optimize its performance and to support its scale-up [2]. Tomographic methods such as X-ray measurements are often used to characterize bubbles in a fluidized bed. Compared to intrusive measurement, e.g. optical probing, this method possesses the advantage of measuring bubbles throughout the entire cross section. However, X-ray measurements cannot be applied to all installation, especially not in large scale plants. For these purpose, we have developed optical probes that can be employed to investigate the fluidization state in a hot pilot scale reactor. A main drawback of the optical measurements lies in their locally limited detection of the hydrodynamic pattern since they are only able to measure at one point in the reactor. Therefore, conclusions on the bubble behavior of the whole cross section based on optical measurements are not easy to derive. To compare the influence of the measurement method on the measured bubble properties, in the scope of this study, an artificial optical signal is created out of the existing X-ray measurement data set for a cold flow model of the pilot scale methanation reactor. The obtained bubble properties of both methods (i.e. evaluation of the derived artificial optical probe signal and image reconstruction based on the original X-ray tomographic data) are compared with regard to the hold-up, bubble rise velocity and the bubble size (for the X-ray method) or chord length (for the optical evaluation method), respectively. The process to obtain an artificial optical signal is depicted in Figure 1. The comparison shows that for the evaluation of optical probe data, statistical effects have to be considered carefully. The detected mean chord length of the optical method does not represent the mean bubble size determined by the X-ray method. Moreover, also a difference in the bubble rise velocity was detected for some fluidization states. This knowledge may be the basis for the derivation of a statistically sound method to calculate different hydrodynamic properties in fluidized bed reactors based on optical probe measurements. Please click Additional Files below to see the full abstract

Fast X-ray tomography for the quantification of the bubbling-, turbulent- and fast fluidization-flow regimes and void structures

Multiple fluidization regimes were studied using X-ray tomography. Geldart B sand particles were used in a 14 cm (ID) column with a dual cyclone return system. Cross sectional solids concentration (/) was measured and the time averaged / ð /Þ decreased with velocity and axial height except in the turbulent regime where / remained constant. Radial profiles of / decreased to the centre, while all turbulent regime velocities resulted in similar radial / profiles. Results confirm the bubbling-turbulent transition velocity (Uc) determined from pressure fluctuations is a reliable quantification technique. The system exhibited slugging behaviour at higher bubbling regime velocities with voids taking on cylindrical shapes. Turbulent regime voids were characterised by elongated cylinders with diameters slightly less than the bubbling regime’s slugs or fast fluidization regime’s core annulus. Distribution curves of the / signal indicated a distinct dense phase in the bubbling and turbulent regime with a velocity independent solid concentration. Void velocity analysis suggested that the bubble linking algorithm was unable to detect fast rising voids at higher velocities.http://http://www.elsevier.com/locate/cejhb2013ai201

Intensifying the Fischer-Tropsch Synthesis by Reactor Structuring—A

Abstract This paper investigates the intensification of Fischer-Tropsch Synthesis in two types of threephase catalytic reactors: slurry bubble columns and multi-tubular fixed beds. A simple mathematical model is used to analyse the effect of structuring on the C 5+ productivity of these two types of reactors. The results of the model show that decreasing the backmixing with a factor 4 and increasing the gas residence time in a slurry bubble column considerably enhances the production of C 5+ . On the other hand in a fixed bed reactor a similar improvement is obtained when the heat transfer coefficient is improved with a factor 2.5 and the diffusion length in catalyst particles is decreased with a factor 2. Both reactors show a potential improvement in productivity per reactor volume; 20% in the slurry bubble column and 40% in the fixed bed reactor