Muthanna H. Al-Dahhan

Multiple-Zone Model for Partially Wetted Trickle Flow Hydrodynamics

This paper describes an implicit 1-D two-fluid hydrodynamic trickle-bed model designed to approximate the actual two-phase flow and bed void using four inclined slits consisting of a dry, a gas-(static)liquid, a (dynamic)liquid-(static)liquid and a gas-(dynamic)liquid slits. Based on the Holub et al. mono-slit phenomenological approach, the model can be used to predict the frictional two-phase pressure drop, the external, dynamic and static liquid hold-ups, the pellet-scale external, dynamic and static wetting efficiency in trickle-bed reactors operated under trickle flow regime and partially wetted pellet conditions. In this context, a collection of data relative to the wetting efficiency, liquid hold-up and pressure drop measured under high pressure/high temperature has been compiled from the literature concerning the trickle flow regime in undiluted and diluted beds of catalysts and fines. This useful data provided sufficient and pertinent information to practically implement and successfully validate the model.

Local time-averaged gas holdup in fluidized bed reactor using gamma ray computed tomography technique (CT)

Many invasive and non-invasive techniques have been used to analyze the hydrodynamics of fluidized beds. In this study, the effect of superficial gas velocity and bed particle density on the hydrodynamics of gas–solid fluidized beds was investigated by using a cylindrical plexiglas fluidized bed column, 14xa0cm in diameter. Air at room temperature was used as the fluidizing gas and two different Geldart type-B particles were used: glass beads and copper particles with material densities of 2.5 and 5.3xa0g/cm3, respectively, with the same size particle, 210xa0µm. To measure the time-averaged cross-sectional gas and solid holdup distribution, gamma ray computed tomography was used for the first time as a non-invasive technique instead of using X-rays (due to the height attenuation of the copper particles). The results show that gas holdup increases by increasing the superficial gas velocity, and decreasing the particle density increases the gas holdup in the bed.

Study of Local Gas Holdup and Specific Interfacial Area in a Split-Column Airlift Bioreactor Using Sphosticated 4-Point Optical Probe for Culturing Microlgae/Cyanobacteria

Local gas holdup (ϵ) and interfacial area (a) at different axial locations of the riser and downcomer of a split-column airlift bioreactor were investigated using a sophisticated four-point optical probe. Such a type of a reactor has been found to outperform both bubble-column and draft-tube airlift bioreactors for culturing microalgae. The effect of superficial gas velocity (0.3–2.8 cm/s) on both gas holdup and interfacial area was studied using air–water system. It was found that both gas holdup and interfacial area significantly decrease from the top to the bottom of the downcomer for all superficial gas velocities, while their variation from the bottom to the top for the riser was found to be much less than that of the downcomer at the same superficial gas velocities. It was found that the interfacial area of the riser tends to increase by 35% from the bottom to the upper middle point of the column (6.15 Z/D from the bottom), then declines by 10% at the top location (7.7 Z/D from the bottom). Empirical correlations were obtained relating the gas holdup and specific interfacial area to superficial gas velocity of the riser and the downcomer of the bioreactor. It was found that the riser has to be represented as upper and lower halves to be best correlated, while the only upper half of the downcomer was successfully correlated. Having obtained variable interfacial area (a) at different locations of both the riser and the downcomer of the bioreactor, the local KLa consequently changes as a function of the location of the bioreactor and hence needs to be investigated locally as opposed to the current studies that have only measured and correlated the overall KLa.

Effect of Distributor Design on Gas-Liquid Distribution in Monolithic Bed at High Gas/Liquid Ratios

Abstract Experiments were carried out to investigate the liquid flow distribution at high gas/liquid ratios in a cold model monolith bed of a 0.048 m diameter with 62 cells per cm 2 . Three types of distributor for the liquid distribution were used to evaluate their distribution performance. Local liquid saturation in individual channels was measured using 16 single-point optical fiber probes mounted inside the channels. The results indicate that 1) The optical fiber probe technique can measure phase distribution in the monolith bed; 2) Liquid saturation distribution along the radial direction of the monolith bed is not uniform and the extent of non-uniformity depends on the distributor design and phase velocities; and 3) The tube array distributor provides superior liquid distribution performance over the showerhead and nozzle distributors.

Pressure Drop and Fluid Flow Characteristics in a Packed Pebble Bed Reactor

Abstract In the dynamic core of nuclear pebble bed reactors, the prediction of the fluid flow within the packing determines the heat transfer characteristics and, hence, the performance of these reactors. The fluid flow of the gas phase can be characterized and quantified in terms of the pressure drop coefficient. Therefore, in this work, the pressure drop in a packed pebble bed having different aspect ratios (ratio of the diameter of the bed to the diameter of the pebbles) has been measured experimentally in a separate-effects pilot-plant scale and cold-flow experimental setup of 0.3 m in diameter using a differential pressure transducer technique. The effects of superficial gas velocity have been investigated using a range of velocities from 0.01 to 2 m/s covering both the laminar and turbulent flow regimes. In addition, the effect of bed structure (aspect ratio) on the pressure drop coefficient has been investigated for the studied packed pebble bed. The results show the strong dependence of the pressure drop on both the aspect ratio and, hence, the porosity of the bed and the coolant gas velocity. The obtained experimental results have been used to evaluate the predictions of the correlations recommended for pressure drop estimation in packed pebble bed nuclear reactors. The present work provides insight on the pressure drop and fluid flow of the gas phase in the studied bed using an advanced technique and methodology.

Local Liquid Velocity Measurement in Trickle Bed Reactors (TBRs) Using the X-ray Digital Industrial Radiography (DIR) Technique

This work describes the development of a new technique to measure local liquid velocity (VLL) for multiphase flows in trickle bed reactors (TBRs) (gas–liquid–solid system). In the studied TBR, the liquid phase is represented by water, gas by air and 3xa0mm expanded polystyrene beads as the solid packing. Three different superficial liquid velocities (VSL) and a constant superficial gas velocity (VSG) were used in the packed bed with an internal diameter of 4.25xa0cm. While the liquid is moving into the packed bed, tracking particles of 106–125 μm diameter (16.45% difference) are injected. The movement of the tracking particles is monitored and digitally recorded by a complementary metal–oxide–semiconductor detector. In this experiment, x-rays were used as the radiation source. Four replications were made with fresh packing. Comparable observations can be found from other published techniques (i.e. magnetic resonance imaging). Results from this study indicate that, at VSL = 0.13 cm s−1, the measured VLLxa0can reach up to 51xa0times that of its VSL, while for VSL = 0.27 cm s−1, the measured VLLxa0reached up to 35xa0times higher than the VSL and for VSL = 0.39 cm s−1, the VLLxa0reached up to 39.8xa0times higher than its VSL. Through statistical analysis, the implementation of such a method is found to be reproducible throughout the experiments. The mean per cent difference in the measured VLL was 10% and 5% for lower implemented VSLxa0 of 0.13 and 0.27xa0cm s−1, respectively. At higher VSLxa0(0.39xa0cm s−1), the particle tracer was greatly distributed and carried away by the high liquid flow rate. The variance or the range of the measured VLLxa0does not vary for all replications in every VSL, which confirms the reproducibility of the experimental measurements, regardless of the VSL.

Discrete Element Method–Based Investigations of Granular Flow in a Pebble Bed Reactor

Abstract In a pebble bed reactor (PBR) core, nuclear fuel in the form of pebbles moves slowly under the influence of gravity. Due to the dynamic nature of the core, a thorough understanding about slow and dense granular flow of pebbles is required from both a reactor safety point of view and a performance evaluation point of view. In the current study, validation of discrete element method (DEM)–based simulation for the pebble flow in a PBR was carried out. Validation of DEM-based simulations necessitates validation of the employed numerical method of simulating packed structure. Hence, a parametric sensitivity study of packing interaction properties was initially conducted and also validation of the numerical method simulating packed structure at first. The parametric sensitivity analysis suggests that static friction characteristics play an important role from a packed/pebble bed structural characterization point of view. In addition, the simulated packed structure approach has shown a good agreement with the available benchmark data. Afterward, the effect of two different half-cone angles of 30 deg and 60 deg on pebble flow field in a PBR was studied by EDEMTM-based simulations. Results of streamlines, velocity radial profiles, and direct observation of discharge indicated a plug-type flow in the upper cylindrical region, whereas results indicated converging-type flow near the bottom conical region. EDEMTM results of granular flow were validated against experimental benchmark data and show a fair agreement in terms of Lagrangian trajectories and velocity profile. Therefore, this validated EDEMTM-based simulation can be used to obtain reliable results of pebble dynamics in a PBR and to enhance understanding of this phenomenon in a PBR. However, additional experimental investigations are recommended to be carried out for different sizes of test reactors, different bottom cone angles, and different sizes of pebbles to further assess DEM simulation results before using them for full-scale reactor simulations.