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Abstract: Fluidized Bed Membrane Reactors gain worldwide increasing interest for various applications. Nevertheless, fundamental research on the hydrodynamics of these reactors is required in order to improve the predictive capabilities of numerical models and to improve reactor performance. This study comprises an experimental investigation in a pseudo-2D gas–solid fluidized bed containing horizontal submerged membrane tubes, through which gas can be added to – or extracted from – the fluidized suspension. Employing Particle Image Velocimetry (PIV) combined with a novel Digital Image Analysis (DIA) technique, the effect of the presence of, and permeation of gas through the membranes, on both the solids flux profiles as well as the bubble properties has been investigated in great detail for different membrane configurations. It has been shown that merely the presence of the membrane tubes decreases the solid fluxes by about a factor three, but does not significantly change the overall circulation patterns in the bed. The permeation ratio and the membrane tube arrangement (in-line vs. staggered, size and number of membranes) hardly influence the time-averaged solid flux profiles. In addition, it has been found that, compared to a fluidized bed without inserts, the average bubble size with horizontally submerged membrane tubes decreases by a factor of approximately three, but is hardly dependent on the permeation ratio. The average total superficial gas velocity is more important than the permeation ratio. With respect to bubble size and number of bubbles, the staggered membrane arrangement with 9.6mm tubes outperforms the in-line arrangement, the arrangement with smaller tubes and the arrangement with fewer membrane tubes, which is of great benefit to reduce bubble-to-emulsion phase mass transfer limitations in fluidized bed membrane reactors under reactive conditions. [Copyright &y& Elsevier]

Abstract: Among many research studies on fluidized bed membrane reactors, only very few focus on the details of the effects of gas permeation through the membranes on the hydrodynamic properties of the membrane-assisted fluidized bed. For this reason, in the first part of this paper, a novel hybrid Immersed Boundary Method (IBM) Discrete Particle Model (DPM) has been developed. In the second part of the paper, this model is employed to investigate fluidized bed membrane reactors in more detail. Simulations without membrane inserts (having vertical membranes at the side walls instead), with membrane tubes in an in-line arrangement and membranes in a staggered arrangement are compared with each other. The time-averaged particle fraction maps display a lower bed height for the simulations with inserts. A very important aspect of the simulation with gas extraction/addition via the side walls is the solids circulation; an inversion of the solids circulation is observed when gas is added via the side walls with altered bubble properties. Because of this phenomenon, the bubble size counter-intuitively decreases when gas is added, and increases slightly when gas is extracted. The latter is a result of the existence of compacted zones near the membranes. For the simulation cases with membrane inserts, the solids circulation and bubble properties are different; in these cases they mainly depend on the local fluidization velocity, and not so much on the extent of permeation itself. Although the arrangement of the membrane tubes plays only a minor role in this matter, their presence has a pronounced influence on the bubble size. The system''s energy and energy dissipation are both smaller for the simulations with inserts compared to those with permeation via the side walls. Also in this respect, the local fluidization velocity is found to be more important than the extent of permeation. [Copyright &y& Elsevier]

Abstract: Many novel reactor concepts based on the integration of membranes in fluidized beds have been proposed. For the detailed numerical investigation of such systems, a method is required describing the motion of the particles while accounting for (i) the gas motion and (ii) the encounters between particles and confining walls/membranes. In this paper a novel hybrid computational method is presented, combining the Discrete Particle Model (DPM) and the Immersed Boundary Method (IBM). The first part focuses on the implementation and validation of these models. The IBM component of the model has been verified by comparing the computed drag coefficients for Simple Cubic (SC) arrays of cylinders with data of Sangani and Acrivos (1982) at creeping flow conditions. For higher Reynolds numbers the code has been compared to Ferziger and Peric (2002) for a single off-center cylinder, after which simulations with addition and extraction of gas through the membrane surface have been conducted. In the second part of this work we will focus on actual fluidized bed membrane systems, and will report results of simulations for several membrane geometries as well as permeation ratios. [Copyright &y& Elsevier]

Abstract: Particle Image Velocimetry (PIV) is a widely applied non-intrusive, optical experimental technique to measure characteristics of the solids motion in pseudo 2D fluidized beds. However, the majority of the research that concerns the use of PIV on granular systems focused thus far on either particle velocities or on particle fluxes with a very rudimentary implementation for the correction for the presence of bubbles. A reliable correlation between the 2D image intensity and the true 3D solids fraction in fluidized beds is required to obtain more accurate information on the solid particle flux profiles in granular systems. Using only experimental data, it is difficult – if not impossible – to obtain such a correlation, but by using data from Discrete Particle Model (DPM) simulations to create artificial 2D-images, we reconstructed this 2D–3D correlation for the solids fraction correction. In this paper the influence of particle size, fluidization velocity, bed depth and intensity distribution function on the 2D–3D correlation has been investigated, in particular for the application in gas-solid fluidized beds in the bubbling regime. It was found that the particle size and fluidization velocity have no influence on the correlation and the intensity distribution function only slightly affects the results for dilute regions, but the bed depth shows the most prominent effect. A detailed comparison between the DPM velocity field as a reference and the conventional and newly introduced Digital Image Analysis (cDIA/ nDIA) techniques shows that the error in the solids flux decreases from 18.7% to 12.3%. Also, we used PIV software to reconstruct the solids velocity field, based on a pair of artificial images from DPM simulations (as done for experimentally acquired data based on a pair of real images), and showed that the largest error is introduced in the determination (cross-correlation) of the particle velocity field. [Copyright &y& Elsevier]

Abstract: Fluidized bed membrane reactors gain worldwide increasing interest for various applications. Nevertheless, fundamental understanding of the hydrodynamics of these reactors is required in order to improve the future design of these reactors. This study focuses on a pseudo-2D fluidized bed containing flat vertical membranes, through which gas can be added to – or extracted from – the fluidized suspension. By employing Digital Image Analysis (DIA) in combination with Particle Image Velocimetry (PIV), both solids motion as well as bubble properties were investigated in detail. In addition the experimental results were compared to computational results obtained from a Discrete Particle Model (DPM) and a Two-Fluid Model (TFM). Comparison between results obtained from a 4cm wide bed and a (previously used) 30cm bed revealed that the effect of gas addition is qualitatively the same (solids motion inverts and average bubble size decreases). During gas extraction, bubbles (and solids) are forced towards the center, enhancing bubble coalescence compared to the reference condition. Both the DPM and the TFM qualitatively capture these phenomena well. However, these models are not able to predict the experimentally observed solids motion and gas bubbles behavior quantitatively. DPM and TFM simulations for a wider bed reveal that grid dependency and maximum solids fraction play an important role with respect to solids motion. Furthermore, the porosity distribution shows significant differences between the DPM and TFM models. [Copyright &y& Elsevier]

Abstract: Recently, many novel reactor concepts based on membrane fluidized bed reactors have been proposed. In this work, the effects of gas permeation through flat membranes on the hydrodynamics in a pseudo-2D membrane-assisted gas–solid fluidized bed have been investigated experimentally. A combination of the non-invasive techniques (Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA)) was employed to simultaneously investigate solids phase and bubble phase properties in great detail. Counter-intuitively, addition of secondary gas via the membranes, that constituted the confining walls of a gas–solid suspension at conditions close to incipient fluidization, did not result in a larger, but in a smaller equivalent bubble diameter, while gas extraction on the other hand, resulted in a larger equivalent bubble diameter, although in this case the effect was less pronounced. This could be explained by changes in the larger scale particle circulation patterns due to gas extraction and addition via the membranes. [Copyright &y& Elsevier]