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• Different approaches to approximate the overall effectiveness factor in reactor scale model. • Sensitivity of the overall effectiveness factor to the reactor scale model. • Local overall effectiveness factor from pellet scale model integrated in reactor scale model. A steady-state diffusion model comprising of diffusion and reaction in a catalyst pellet was used to study and understand the effect of the approximation of the catalyst effectiveness factor in the modeling of trickle bed reactors. The hydrogenation of alpha-methylstyrene was chosen as the case study reaction from literature for liquid-limited reaction conditions. The effectiveness factor values in this study were sensitive to reactor scale equations during the prediction of reactor performance. The approximation of accounting for the overall catalyst effectiveness factor in the reactor scale model equations from different models and solving the reactor scale model equations by different modeling approaches was assessed. It was observed that evaluating the overall effectiveness factor from pellet scale model equations and integrating the parameter in the reactor scale axial dispersion model at every local axial collocation point to simulate the reactor performance showed better agreement to the experimental data. This approach evaluates the effectiveness factor locally with the variation of the reactant concentration at different axial points across the reactor rather than using one value for the entire reactor. The approach of using a single effectiveness factor for the whole bed and using the effectiveness factor approximated as a fitted polynomial to the reactant concentrations did not properly predict the reactor performance. [ABSTRACT FROM AUTHOR]

While treating the industrially produced wastes through various processes such as physical, chemical, biological and radiation processes have been implemented on various scales and is the focus of various studies and development, the trend of minimizing and/or eliminating the pollutions at the source via developing and selecting proper catalytic multiphase reactors is worth to be given proper attention and consideration which is the focus of this manuscript beside outlining the processes used for treating the wastes. In order to achieve such goal in minimizing the wastes, these multiphase reactors must be well understood, studied, scaled up and designed. This can be only achieved by developing and implementing advanced measurement and computing techniques which have been developed in our research laboratory and are briefly outlined here with selected results. [ABSTRACT FROM AUTHOR]

The knowledge and proper analyses of axial dispersion and the mixing phenomena of the coolant gas flow in the dynamic core of pebble-bed nuclear reactors are useful for safe design and efficient operation of these reactors. These processes can be characterized in terms of the residence time distribution and quantified in terms of the axial dispersion coefficient. Therefore, in this work, the axial dispersion coefficients of the gas phase and their residence time distributions were measured experimentally in a separate effect pilot-plant scale and cold-flow experimental setup of 0.3 m in diameter, using a sophisticated and advanced gaseous tracer technique. The non-ideal flow of the gas phase in the pebble bed was described using the axial dispersion model (ADM). The effect of the gas velocity on the axial dispersion was investigated using a range of velocities from 0.01 to 2 m/s, covering both the laminar and turbulent flow regimes. The effect of the bed structure (pebble size) on the axial dispersion coefficient was investigated, and the results indicate that the pebble size strongly affects axial dispersion and mixing in the packed pebble-bed reactor. The results show that the flow pattern of the gas phase does not deviate much from the idealized plug-flow condition at high flow rate, which depends on the gas flow rate and the bed structure of the pebble-bed. The present work provides insight on the extent of mixing and dispersion in the gas phase in the studied bed using an advanced gas dynamics technique and methodology that properly accounts for the external dispersion. [ABSTRACT FROM AUTHOR]

Abstract: Airlift reactors have been recognized as one of the promising photobioreactors for biomass/bio-energy production, where mixing has significant impact on the reactor performance. In recent years, using CFD simulations to track microorganism cells and to generate their trajectories in the reactor for reactor performance evaluations becomes more common. However, there is a lack of systematic and rigorous verifications and validations of the reliability of CFD models in particle tracking against experimental measurements in the open literature, which is vital for the faithful application of CFD in reactor design and scale-ups. In this work, we attempt to evaluate the reliability of using CFD simulations to generate trajectories of microorganisms in a draft tube column photobioreactor. A computationally promising CFD simulation model based on CFX5.7 was validated against a benchmark experimental database reported in . This model was then used to generate typical trajectories of microorganisms in the studied airlift column, which was further validated against experimentally measured tracer trajectories. The results indicated that the CFD model reasonably predicted the recirculation of the microorganism around the draft tube, however over-estimated the cells'' residence time in the wall regions. Proper treatment for the wall region such as griding and wall function is needed to better capture the movement of microorganism cells in such bioreactors. [Copyright &y& Elsevier]

Abstract: Airlift column bioreactors are gas–liquid contact devices characterized by a rising channel and a down flow channel due to gas holdup differences in these two channels. Local gas holdup distribution strongly affects the overall gas–liquid flow dynamics in airlift columns. In this work, local gas holdup distributions in a draft tube airlift column covering both bubbly flow and churn–turbulent flow regimes have been studied using computed tomography (CT) technique as well as conventional techniques. The radial and axial evolutions of the gas holdup distribution will be discussed, together with the effects of superficial gas velocity and geometry parameters. The obtained gas holdup results will also be used to verify various empirical and semi-empirical correlations in the literature. Moreover, the obtained gas holdup information, combined with liquid flow dynamic information reported in , forms a benchmark database for the design and scale-up of airlift column bioreactors and for computational fluid dynamic (CFD) modeling validations. [Copyright &y& Elsevier]

Abstract: Airlift column reactors have been widely used in bioprocesses. The design, scale-up, and performance evaluation of such reactors all require extensive and accurate information about the gas–liquid flow dynamics, particularly as computational fluid dynamics (CFD) has become more popular in the last decade. However, due to the limitation of most conventional techniques for gas–liquid flow dynamics measurement, only global hydrodynamic parameters (e.g., cross-sectionally averaged liquid circulation velocity, overall gas holdup, and overall mass transfer rate) have been extensively studied. The local flow characteristics (e.g., the macro-mixing and the turbulence intensity) remain unclear. In this study, we use the computer automated radioactive particle tracking (CARPT) technique to investigate the details of the multiphase flow dynamics in a draft tube airlift bioreactor, such as the liquid velocity field, turbulent kinetic energy field, distributions of shear stresses, etc. The flow structures in the whole reactor, as well as the structure in individual regions, i.e., the top, the bottom, the riser, and the downcorner are also characterized. We found significantly large turbulent kinetic energy in the top and the bottom regions, with spots of very high shear stress, which were also found in the vicinity of the sparger. The results also suggest that the top and bottom clearances have significant effects on the flow structures, which may have substantial effects on the bioreactor performance. [Copyright &y& Elsevier]

Abstract: Airlift reactors are pneumatically agitated reactors that have been widely used in industries, particularly in bioprocesses. Extensive studies about the flow dynamics in airlift column reactors exist; however, most of these studies have focused on global hydrodynamic parameters using conventional techniques. The local flow characteristics, such as the macro-mixing and the turbulence intensity, are crucial for reliable design and scale-up, and they remain unclear. This work focuses on studying the macro-mixing in a draft-tube airlift bioreactor utilizing an advanced flow dynamic measurement technique, computer automated radioactive particle tracking (CARPT). True residence time distribution analyses for the overall column as well as individual regions, i.e., the riser, the downcomer, the top, and the bottom regions, are conducted for the first time based on CARPT measured particle trajectories. The effects of the superficial gas velocity and the top/bottom clearances on the macro-mixing are also discussed. The results suggest that although the flow structures in the overall draft-tube column reactor, as well as in the riser and in the downcomer, are close to plug flows, bypassing and stagnancy exist in the top and the bottom regions. [Copyright &y& Elsevier]

Abstract: Time-averaged local heat transfer coefficients were studied in a 0.16m inner diameter high-pressure air–water bubble column. The effects of the superficial gas velocity (up to 0.30m/s), pressure (up to 10 bar), probe position, and probe orientation were investigated. The heat transfer coefficients increased with superficial gas velocity, and the values in the center of the column were 9–16% greater than those near the wall region under atmospheric pressure (1bar). With the increase in pressure, the heat transfer coefficients decreased due to combined effects of bubble size decreasing, gas holdup increasing, and bubble number increasing. The heat transfer coefficients at 10bar were about 6–17% lower than those at 1bar for the studied conditions, where the radial profile of the heat transfer coefficients became flat with increasing pressure. The orientation of the probe qualitatively indicated the flow direction in different radial positions of the column. [Copyright &y& Elsevier]

Abstract: The gas–liquid mass transfer in a 0.162m high pressure stainless steel bubble column was investigated using three different gas sparger designs. An oxygen-enriched-air dynamic method and an optical oxygen probe technique were implemented to measure values in the bubble column reactor. Using the interfacial area values measured by a four-point optical probe technique at similar conditions (), the values were estimated. Axial dispersion model (ADM) and continuous stirred tank reactor (CSTR) model were used to calculate as a fitted parameter with the measured data. The ADM gave better fits to the experimental data than the CSTR model, especially at high axial locations for the bubble column used with a large ratio. The sparger design was found to have a noticeable effect on in the low gas velocity range but only a slight effect in the high gas velocity range . The sparger design showed almost no effect on the liquid side mass transfer coefficient, , at high gas velocity , where no significant variations of the bubble size distribution and hydrodynamics were obtained using different sparger designs. Although the values increased with the operating pressure, the pressure change from 0.1 to 0.4MPa yielded lower values, as a result of the reduced bubble size. However, as the pressure further increased to 1.0MPa, the and values increased, while the values negligibly decreased. In addition to the pressure and sparger design effects, the superficial gas velocity had effect of increasing the values, while such effect became small and flattened out at high superficial gas velocities. [Copyright &y& Elsevier]

Abstract: In this study, solids flow dynamics has been investigated in a 6 in. “cold-flow” circulating fluidized bed riser using non-invasive flow techniques. Gamma ray Computed Tomography (CT) has been used to measure the time-averaged cross-sectional solids holdup distribution. The time-averaged mean and fluctuating solids velocity fields have been quantified using the Computer Automated Radioactive Particle Tracking (CARPT) technique. Details of the experimental techniques, protocol of implementation and data analysis are discussed. The experimental studies examine operating conditions in fast fluidization and dilute phase transport regimes. Comparative and symbiotic analyses of the results obtained from CARPT and CT have been used to develop a coherent picture of the solids flow field. In addition, this work also demonstrates the power of CARPT and CT as flow mapping techniques in studying highly turbulent and opaque multiphase systems. [Copyright &y& Elsevier]

Abstract: The compartmentalization of conventional bubble columns by perforated trays constitutes a very effective method to reduce the liquid backmixing. The effect of tray design and operating conditions on the overall liquid mixing was studied in a bench-scale trayed bubble column. The extent of liquid backmixing in the column was investigated in light of liquid-phase tracer response experiments. In average, a three fold reduction in the liquid backmixing was achieved in the trayed column as compared to the column without the trays. Moreover, the tray open area and the superficial liquid velocity were found to have the strongest effects on the liquid backmixing. The N-CSTR with Backmixing Model was found to match the experimental tracer response curves better than the Axial Dispersion Model. [Copyright &y& Elsevier]

Highlights • The presence of internals significantly impacts on the bubble properties. • Internals administrates the profiles and distributions of the bubble properties. • Low aspect ratio enhances bubbles breakup and effects on the fully developed region. • New benchmark data for CFD code validation were provided. Abstract This work investigates, for the first time, the effects of the presence of internals and low dynamic liquid levels on the bubble dynamics in industrial-size pilot plant bubble columns. Experimental work that conducted in a bubble column of 0.6 m inner diameter and 3.9 m height with an air-water system was utilizing our advanced four-point fiber optical probe technique to measure the radial profiles of the bubble properties. The superficial gas velocity varied from 0.2 to 0.45 m/s to cover the churn turbulent flow regime. PVC pipe of 0.06 m diameter used to represent the heat exchanging internals, occupying 24% of the column cross-section area, and three different aspect ratios (H/D = 3, 4, and 5) were used. Data obtained show that the presence of internals slightly increases the overall gas holdup and significantly affects the radial profiles and distribution of the bubble properties, particularly in the fully developed flow region. However, the presence of internals increases the local gas holdup, the bubble pass frequency, and the interfacial bubble area, especially in the wall region, while decreases the bubble chord length and the bubble rise velocity. The variation in the aspect ratio (H/D) and the presence of internals exhibited a slight impact on the bubble dynamics in the sparger region, whereas the effect of internal on the local gas holdup was concentrated in the wall region. Meanwhile, the axial location (Z), where the fully developed flow region occurs, appears a high sensitivity toward the internals existence and the variations in the aspect ratio and the superficial gas velocity. However, the presence of internals and the increase in aspect ratio both show that the fully developed flow region begins at lower axial locations, while an increase of the superficial gas velocity delays the transition to fully developed flow to a higher axial location. [ABSTRACT FROM AUTHOR]

Highlights • Local gas holdup and bubble properties in the bubble columns were measured via a 4-point optical probe. • Influence of vertical internals configurations on the gas holdup and bubble properties was assessed. • The magnitude of the interfacial area was decreased in the bubble column with the circular configuration. • Distinct asymmetrical radial profiles of gas holdup and the specific interfacial area were observed in the bubble column with hexagonal configuration. Abstract The effect of the vertical heat-exchanging tube bundle configurations on local gas holdup and bubble dynamics, including specific gas-liquid interfacial area, bubble chord length, bubble rise velocity, and bubble passage frequency have been studied using the 4-point optical fiber probe technique. Two different tube bundle configurations were investigated, circular tube bundle and hexagonal tube bundle. 30 internal tubes, each with a diameter of 0.5 inches (0.013 m), were used in each configuration occupying 25% of the column cross-section area to represent the heat exchanging tubes utilized in the Fischer–Tropsch process. The experimental work was performed in a 0.14 m inner diameter Plexiglas bubble column using an air-water system. The applied superficial gas velocities were based on the free cross-sectional area of the column available for fluid flow and were in the range of 0.02 to 0.45 m/s covering bubbly, transition, and churn turbulent flow regimes. Although the size and the number of the tubes in both configurations were similar, their effects on the hydrodynamics were found to be different. When compared to bubble column without internals, the circular tube bundle showed a significant increase in the local gas holdup in the core region and a decrease in the wall regions. Simultaneously, a substantial increase in the bubble chord length and bubble velocity was seen. Another important observation was the decrease in the interfacial area while using circular tube bundle. A distinct asymmetrical effect on the radial profiles of gas holdup and the specific interfacial area was observed when the hexagonal configuration was used. The gas holdup and interfacial area significantly increased on one side of the column and decreased on the other side. The bubble chord length and bubble rise velocity decreased, exhibiting a narrower distribution with smaller values, in comparison to the bubble column without internals. [ABSTRACT FROM AUTHOR]

Highlights • Non-invasive radioactive particle tracking technique was utilized. • Visualization of the hydrodynamics in airlift reactor was performed. • 3D Liquid velocity and turbulence kinetic energy were quantified. • Scenedesmus green microalgae was cultured. • Benchmark data for CFD validation was obtained. Abstract In this study, an advanced radioactive particle tracking (RPT) technique was used to investigate for the first time the details of the cells’ movements (trajectory) and multiphase flow hydrodynamics during microalgae culturing in a cylindrical split airlift photobioreactor. The cells’ trajectory, liquid velocity field, distributions of shear stresses, and the turbulent kinetic energy field were studied under superficial gas velocity of 1 and 3 cm/s. The structures of the flow in the whole reactor, the riser, the downcomer, as well as the structure above and below the split plate were characterized. The effects of the cells’ concentration and different aeration rate at different axial levels on the studied parameters were discussed. It has been found that the cells’ fluctuations reduced and its movement frequency between the light (wall) and dark zone decreased during the culturing particularly when the cells concentrations becomes large after 30 days of culturing. Distinguishing behaviors were observed for all the parameters, with a higher magnitude at the superficial gas velocity 3 cm/sec than at 1 cm/sec. This effect positively enhanced the liquid circulation and the movement between the reactor sides, the riser, and the downcomer. This circulation and good mixing phenomena had a large positive impact on the culture’s continuity. The obtained results are reliable as benchmark data to validate computational fluid dynamics (CFD) simulation and other models that can be later used to be integrated with dynamic growth and light intensity models for optimized. [ABSTRACT FROM AUTHOR]

The impact of dense vertical internal tubes and their configurations on the gas holdup distributions and their diametrical profiles in pilot-scale bubble column is visualized and quantified for the first time ever using an advanced gamma-ray computed tomography (CT) technique. Two arrangements of vertical internals (circular and hexagonal configurations) occupying the same cross-sectional area (CSA) of the column (about 25% of the total cross-sectional area to represent the heat exchanging tubes that are used in the Fischer-Tropsch synthesis), were examined in addition to the measurement in the bubble column without vertical internals. Moreover, the gas holdup distribution results of the 18-inch (0.46 m in outer diameter, O.D.) bubble column are compared with an available data of 6-inch (0.15 m in O.D.) bubble columns with and without vertical internals. CT scans have been conducted for 18-inch bubble columns with and without vertical internals for the air-water system under a wide range of superficial gas velocity (0.05–0.45 m/s). The experimental results indicate that an improvement in the gas holdup distribution over the column's cross-sectional area is obtained when the vertical internal tubes (arranged in either a circular or a hexagonal configuration) were used. However, better cross-sectional gas holdup distribution was achieved in the bubble column with vertical internals arranged in a hexagonal configuration as compared to the bubble column without and with vertical internals arranged in a circular arrangement. Additionally, the averages of the cross-sectional gas holdup and their profiles for bubble column with and without vertical internals are close to each other when the bubble column with vertical internals is operating at a high superficial gas velocity, which is calculated based on the free cross-sectional area for the flow. Furthermore, the gas holdup distributions are further improved when the larger bubble column with vertical internals was used as compared to the 6-inch bubble columns with and without vertical internals. [ABSTRACT FROM AUTHOR]

In this study, a cylindrical stainless-steel column with dimensions of 250 cm in height and 16.2 cm in diameter was employed to investigate how fluid properties influence phase distribution within slurry bubble columns. The research focused on various superficial gas velocities and operating pressures. Data collection was carried out using single-source Computed Tomography and overall gas holdup. The gas holdup was found to be directly proportional to both the superficial gas velocity and the operating pressure. Results revealed that both liquid properties and operating pressure have a considerable impact on flow regime transition. Interestingly, when comparing water to Therminol LT, higher gas holdup measurements were observed when water was used, displaying a relative difference of approximately 29% in the bubbly flow regime under ambient conditions. The effect of liquid properties on holdup profiles became more pronounced at high pressures. For example, under higher pressure conditions, Therminol LT exhibited a higher gas holdup rate due to its lower viscosity and surface tension, leading to an increased rate of bubble breakup. While the influence of physical properties on the solids' holdup profile was relatively less significant, it remained noteworthy when compared to its impact on the gas holdup profile. • Hydrodynamics of a slurry bubble column reactor operating at mimicked Fischer-Tropsch (FT) reaction conditions. • Non-invasive gamma ray computed tomography measurement technique. • Effect of liquid phases on phases distribution was studied at different superficial gas velocities, operating pressures. • Findings can be utilized as a benchmarking tool for CFD and phenomenological modeling. [ABSTRACT FROM AUTHOR]

An advanced gamma-ray computed tomography (CT) technique was used for the first time to visualize and quantify the impacts of the presence of heat-exchanging tubes and their configurations on the gas-liquid distributions and their profiles in a 6-inch (0.1524 m O.D.) Plexiglas® bubble column in an air-water reactor. Two superficial gas velocities (i.e., 0.2 and 0.45 m/s) were employed to simulate the churn turbulent flow regime. To investigate the impact of vertical internals configurations, three arrangements (i.e., hexagonal, circular without a central internal, and circular with a central internal) were employed in addition to the column with no internals. Using the same sized vertical internals and the same occluded cross-sectional area (CSA), it was found that the configuration of the vertical internals significantly impacted the gas holdup distribution over the CSA of the column. All studied superficial gas velocities resulted in symmetrical gas holdup distributions over the CSA of the bubble columns without vertical internals; however, the columns equipped densely with vertical internals did not have symmetrical gas holdup distributions. The presence of an extra central tube in the circular configuration played a key role in the gas-liquid distribution over the CSA of the bubble column. The hexagonal configuration had the advantage of providing the best spread of the gas phase over the entire CSA of the column. Gas holdup values at the wall region of the bubble column increased with the addition of vertical tubes in all investigated configurations. However, a remarkable increase in the gas holdup values was obtained with the hexagonal configuration. The experimental data (i.e., gas holdup distributions and their diametrical profiles) can help to evaluate and validate three-dimensional (3-D) computational fluid dynamics (CFD) simulations to better predict the hydrodynamic parameters involved in these types of reactors. [ABSTRACT FROM AUTHOR]

The effects of the presence of the vertical internals of different sizes at a wide range of superficial gas velocity on the overall, local gas holdup distributions and their profiles have been studied and quantified in a 6-in. (0.14 m) Plexiglas® bubble column with air-water system using a non-invasive advanced gamma-ray computed tomography (CT) technique. In this study, two sizes of Plexiglas® vertical internals, having the same occupying area (∼25%) of the column's cross-sectional area (CSA) that represents those used in Fischer-Tropsch synthesis, have been used within a range of superficial gas velocities that cover bubbly and churn turbulent flow regimes (0.05–0.45 m/s). The reconstructed CT scan images revealed that the bubble columns equipped with or without internals displayed a uniform cross-sectional gas holdup distribution (symmetric) for all studied superficial gas velocities. However, the bubble column equipped with 1-in. vertical internals exhibited more uniform gas holdup distribution than the column with 0.5-in. internals. Also, the visualization of the gas-liquid distributions for bubble columns with and without internals reveal that the well-known phenomenon of the core-annular liquid circulation pattern that observed in the bubble column without internals still exists in bubble column packed densely with vertical internals. Moreover, a remarkable increase in the gas holdup values at the wall region was achieved in the churn turbulent flow regime based on the insertion of the vertical internals inside the column as compared with using a bubble column without obstacles. Furthermore, the values of the gas holdup in the core region of the bubble column with vertical internals are similar to those of the bubble column without vertical internals when they are operated at high superficial gas velocity (churn turbulent flow regime), based on the free cross-sectional area (CSA) for the flow. In general, the magnitude of the gas holdup increased significantly with increasing superficial gas velocity for the bubble columns with and without internals. However, the gas holdup profile was shaped like a wavy line in the bubble column with vertical internals, whereas it exhibited a parabolic gas holdup profile in the bubble column without obstacles. [ABSTRACT FROM AUTHOR]

In this study, the effects of the dense vertical internals on the liquid velocity field and turbulence parameters (Reynolds stresses, turbulent kinetic energy, and turbulent eddy diffusivities) are experimentally investigated for the first time by using advanced Radioactive Particle Tracking (RPT) technique. The experimental work was carried out in a Plexiglas bubble column with 5.5 in. (0.14 m) and a height of 72 in. (1.83 m) for the air-water system. In this work, thirty vertical Plexiglass internals of 0.5 in. (0.0127 m) outer diameter were used which covered ∼25% (typical for Fischer–Tropsch processes) of the total cross-sectional area of the column where they arranged as the triangular pitch of 0.84 in. (0.0214 m). The superficial gas velocities based on both total cross-sectional area and free cross-sectional area available for the flow were utilized (0.08, 0.2, and 0.45 m/s), which covered the transition and churn-turbulent flow regime to meet the industrial applications of FT synthesis. The experimental data show that the presence of the internals at a given superficial gas velocity causes an increase in the axial centerline liquid velocity and a sharp decrease in turbulence parameters while the increase in superficial gas velocity in the presence of internals causes an increase in axial centerline liquid velocity and turbulent parameters. The obtained data are reliable as a benchmark data for validation computational fluid dynamics (CFD) simulation, and models. [ABSTRACT FROM AUTHOR]

Moving bed reactors have a wide range of applications in the chemical, petrochemical, and alternative energy industries in a special situation where there is a need to either continuously recirculate or replace solid particles catalysts. They are also under consideration as one of the 4th generation nuclear reactors known as pebble bed modular reactor (PBMR). In this work a continuous cold flow pebble recirculation experimental set-up, mimicking the flow of pebbles in a PBMR, is designed, developed, and tested at Missouri S&T. A unique experimental work has been performed on assessment of the possibility of using pebble bed modular reactor (PBMR) as static packed bed approximation for the needed experimental investigation. For this purpose, a radioactive particle tracking (RPT) technique is implemented around the continuous pebble recirculation experimental set-up, to compare the packing characteristics of static and moving pebble beds. The photopeak counts during RPT calibration were collected by placing radioactive particle (tracer) at different positions in the test reactor under different operating conditions of moving and static conditions. The photopeak counts between moving and static conditions show that attenuation characteristics of the medium in between tracer and detectors are not changing significantly due to movement of pebbles. This confirms that PBMR could be well approximated by static packed beds for experimental investigation which simplify the needed experimentation to advance PBMR technology and commercialization. Obtained results are serving as a unique benchmark data for an evaluation of the contact force model that used in the discrete element method (DEM) based simulations. [ABSTRACT FROM AUTHOR]

The granular flow of pebbles in a pebble bed reactor (PBR) under the influence of gravity is a dense granular flow with long-lasting frictional contacts. The basic governing physics is not fully understood and hence the dynamic core of a PBR and non-idealities associated with pebbles flow inside the reactor core are of non-trivial significance from the point of view of safety analyses, licensing, and thermal hydraulics. In the current study, overall and zonal pebbles residence time investigation is carried out by implementing noninvasive radioisotope-based flow visualization measurement techniques such as residence time distribution (RTD) and radioactive particle tracking (RPT). The characteristics of overall pebble residence time/transient number, zonal residence time, and the z-component of average zonal velocities at different initial seeding positions of a tracer particle have been summarized. It is found that the overall pebbles residence time/transient number increases (the z-component of average zonal velocities decreases) from the center towards the reactor wall. Also, pebbles’ zonal residence time results (the whole core is divided into three zones) which provide more insight and understanding about PBR core dynamics have been reported. The benchmark data provided could be used for assessment of commercial/in-house computational methodologies related to granular flow investigations. [ABSTRACT FROM AUTHOR]

The effect of heat exchanging internals that represent Fischer–Tropsch (FT) synthesis in bubble column has been investigated for the first time using gamma ray Computed Tomography (CT) in a 5.5″ (0.14 m) inner diameter Plexiglas bubble column operated at atmospheric condition with air water system. Thirty vertical Plexiglas rods of 0.5″ outer diameter were used which covered ∼25% of the total cross-sectional area and were arranged in a triangular pitch of 0.84″ (2.14 cm). The superficial gas velocities applied were based on both total cross-sectional area as well as free cross-sectional area available for the flow and were in the range of 5–45 cm/s covering the bubbly through churn-turbulent flow regimes. New knowledge and findings have been obtained which have not been reported in previous studies. In churn turbulent flow regime, the overall gas holdup and the profiles of gas holdup obtained in bubble columns without internals can be extrapolated to those with internals in the central region of the column if the superficial gas velocity is based on the free cross-sectional area (CSA) available for the flow of the phases provide that symmetric time averaged gas holdup cross-sectional distributions are achieved. The results show that a significant increment in overall and local gas holdup are obtained upon inserted of the internals when the velocity measured based on the total cross sectional area of the column. The gas holdup distribution over the cross-sectional area (CSA) of the column in the presence of internals has a symmetrical shape at low gas velocities and asymmetry at higher ones for the honeycomb configuration and its installation used in this study. At a high superficial gas velocity that is based on free CSA for the flow, the influence of dense internal becomes insignificant at the central region of the column. However, the profiles of the gas holdup in the column with internals become less steeper compared to those that are like parabolic shape at high gas velocity in the column without internals due to higher gas holdup was obtained in the region near the wall with internals. [ABSTRACT FROM AUTHOR]

Combination of X-ray Digital Industrial Radiography (DIR) and Particle Tracking Velocimetry (PTV) techniques for local liquid velocity measurement ( V LL ) has been newly developed and successfully applied for trickle bed reactor (TBR). The technique was validated against newly developed fiber optical probe technique. This work attempts to highlight the applicability of this newly developed technique on a liquid–solid packed bed reactor. In this work, liquid was represented by water and solids were represented by EPS beads. The EPS beads were chosen because of its low density property. Three superficial liquid velocities ( V SL ) were applied to the system. The experiment was replicated four times. The digital industrial radiography (DIR) consists of a complementary metal oxide semiconductor (CMOS) digital detector and X-ray source. Results of this work suggest that the technique has been successfully applied and comparable with previous work that has been done in the literature. It also suggests that there will be a maximum measurable interstitial liquid velocity when it travel inside the packed bed. The measured V LL can have a maximum range that is between 4 and 4.7 times that of its V SL . For V SL =0.42±±2%, the V LL-Max is in between 1.7 cm/s and 1.9 cm/s, V SL =0.84±±2%, the V LL-Max is in between 3.6 cm/s and 4.0 cm/s, and for V SL =1.11±±2%, the V LL-Max is in between 4.3 cm/s and 4.8 cm/s. [ABSTRACT FROM AUTHOR]

• Different techniques for the measurement of dispersed phase holdup. • E-LTT limitation through the nature of the incident light wave. • E-LTT setup and its different components. • Continuous measurements of the dispersed phase holdup. Expanded Laser Transmission Technique (E-LTT), where a laser beam is used in conjunction with a beam expander large enough to cover the average drop size up to a few millimeters in the path of the laser in a liquid–liquid dispersion mixture, has been applied for online continuous measurement and investigation in a non-invasive manner of the drop phase volume fraction in an agitated flow mixer of a pilot plant scale. The limitations of beam scattering by the drops through the dispersion path were overcome by having liquids of matching refractive indices enabled by the temperature control system. This study reports for the first time the continuous measurements of the line averaged dispersed phase holdup for a pilot plant scale liquid–liquid mixer equipped with a commercial design mixer, where the measurements have no limitations to the geometrical aspects. Experimental results from a cubic mixing tank with a dispersion depth of 30 c m were discussed. Online measurements were carried out in the presence of a revolving impeller and transmission of the expanded laser beam. The net volume of the mixer was 20.42 l i t e r , and the dispersed phase holdup that was successfully measured ranged between 0.15 and 0.75. In this work, arrangement of the laser setup was made to scan and measure continuously the line average dispersed phase holdup along the height of the pilot plant scale of the flow agitated liquid–liquid mixer. The E-LTT measurements were validated by comparing their results with those obtained from the mixer by shut-down procedure. [ABSTRACT FROM AUTHOR]

• A comprehensive database of critical heat flux containing more than 15,000 experimental data points is generated. • Multiple machine learning (ML) algorithms are used to predict critical heat flux trained and tested on the generated database. • Artificial neural network (ANN) and extreme gradient boosting (XGBoost) algorithms outperform other counterpart techniques. • Critical heat flux for vertical flow systems is found to be highly dependent upon Weber number and boiling number. The critical heat flux (CHF) associated with the departure from nucleate boiling (DNB) determines the design and safety aspects of two-phase flow boiling systems. Despite the availability of several predictive tools, within the thermal engineering community, the pursuit of an accurate and robust CHF model remains a significant challenge. In this unique study, we extracted a substantial database from literature to develop machine-learning (ML) models to predict CHF for vertical flows commonly employed in the process industry. The extensive database encompasses 15,006 experimental data points gathered from diverse sources covering wide range of geometric and operational ranges of D, L, P, G, h, and x. This database is then employed to develop five ML – based models, namely Artificial Neural Networks (ANN), K-Nearest Neighbors (KNN), Adaptive Boosting (AdaBoost), Extreme Gradient Boosting (XGBoost), and Random Forests (RF). The selection of optimal input features is conducted by exploring various combinations of both dimensional and dimensionless parameters to identify the most influencing inputs for CHF prediction. The models incorporating features D, L, P, G, h, x, L/D, ρ r , We, and BO achieve CHF predictions with Mean Absolute Errors (MAEs) of 8.85 % for the ANN model and 10.39 % for the XGBoost model. The optimized ML models outperformed even the highly reliable CHF correlations and lookup tables. The feature importance analysis of input parameters highlights strong dependence of CHF on dimensionless numbers such as Weber number and Boiling number. [ABSTRACT FROM AUTHOR]

• Local convective heat transfer coefficients (HTC) are higher in near-wall region. • Local HTC variation in the PBR is due to the variation of local gas velocities. • Local HTC are lower at center due to low void fractions and high flow resistance. • Variation of Local HTC at different diameter lines is higher at low Re number. The aim of this study was to investigate the local heat transfer coefficients between the pebbles and the coolant gas in a PBR using an advanced measurement technique that consists of a heated pebble probe, a micro-foil heat flux sensor mounted flush on the surface of the heated pebble probe, and a thermocouple in the center of the bed void in front the sensor. In this work, the local heat transfer coefficients were derived at various axial levels, radial and angular locations and at different superficial inlet gas velocities that cover both transitional and turbulent flow conditions. The local heat transfer coefficients were found to be 22 - 32 % , 34 - 39 % , and 18 - 20 % higher near the wall at superficial inlet gas velocities of 0.3 , 1.2 , and 2 m / s , respectively. This is due to higher volumetric flow at the wall where larger void in the pebble bed exists compared to the center region of the bed where the flow of the gas in the packed bed follows the least resistance path. Large deviations were obtained between the experimental overall heat transfer coefficients in the bed and the predictions of seven correlations that were selected from the literature. Furthermore, these correlations cannot predict the local heat transfer coefficients inside the PBR. This necessitates the development of new correlations for the prediction of the local heat transfer coefficients using the data obtained in this work. A pseudo-3D correlation was developed and was found to provide predictions that are in good agreement with the experimental values for the conditions used, with an averaged absolute relative error (AARE) of 3.33 % at high Reynolds numbers for our operating and design conditions. [ABSTRACT FROM AUTHOR]

Abstract: One-dimensional (1D) and two-dimensional (2D) cell network models were developed to simulate the steady-state behavior of trickle-bed reactors employed for the highly exothermic hydrotreating of benzene. The multiphase mass transfer-reaction model and novel solution method are discussed in this report. The 1D model was shown to satisfactorily simulate the axial temperature field observed experimentally for multiphase flow with exothermic reactions. The 2D reactor modeling provided valuable information about local hot spot behavior within the multiphase reactor, identifying situations in which hot spots may form. The model took into consideration the heterogeneous nature of liquid distribution, including radial liquid maldistribution and partial external wetting. This approach was proven to be stable and efficient in dealing with the complex interaction of phase vaporization and temperature rise. Through analysis and discussion, this report established the cell network model as a valid representation of the flow environment produced in a trickle bed with exothermic reactions. [Copyright &y& Elsevier]

Computational fluid dynamics simulations were used to study the effect of bottom configuration and a hanging baffle on the mixing inside a gas-lift digester filled with non-Newtonian sludge. The Navier–Stokes and continuity equations were solved numerically using commercially available finite element method-based solver. The results from this simulation were found to be in good agreement with previously reported experimental findings. At a gas recirculation rate of 84.96l/h, the poorly mixed zones inside a flat bottom digester were about 33.6% of the digester volume, while in the case of digesters with 25° and 45° conical bottoms poorly mixed zones were about 31.9% and 29.6%, respectively. The power law viscosity index, n, did not have a significant effect on the mixing pattern under the conditions studied. Introduction of a hanging baffle in combination with a 45° hopper bottom resulted in reduction of the poorly mixed zone by a factor of three compared to a flat bottom without baffle configuration. Although the introduction of a hanging baffle was able to significantly reduce the size of the poorly mixed zones inside a gas-lift digester, further optimization of the digester geometry may lead to additional improvements. [Copyright &y& Elsevier]

Abstract: The trajectories of a solid tracer with the same characteristics as the circulating solids in the risers of two pilot-scale circulating fluidized beds (CFBs), obtained by the computer automated radioactive particle tracking (CARPT) technique, are examined in detail to get further insights into the complex solids dynamics of these systems. The analysis uses tools from the theory of non-linear dynamics and symbolic dynamics. Distinct features of the solids dynamics within the fast fluidization and the dilute phase transport flow regimes are highlighted and related to the operating conditions. The possibility of downflow cluster existence within the central region of the riser is particularly examined. [Copyright &y& Elsevier]

Abstract: Successful design and scale-up of Slurry Bubble Column Reactors (SBCRs) require proper understanding of how operating conditions affect their flow behavior. Presently, there is little information on the flow dynamics of solids (e.g., distribution of velocities and turbulent parameters) in slurry systems that are operated at industrially relevant conditions of high pressure, high superficial gas velocities, and high solids loading. Computer Automated Radio Particle Tracking (CARPT) is widely recognized as one of a few techniques that can be reliably used even in highly turbulent and opaque slurry flows. This work utilizes an improved CARPT technique to investigate the effect of reactor pressure (0.1–1MPa) and superficial gas velocity (0.08–0.45m/s) on solids phase velocity and shear stress in a pilot scale 0.16m diameter stainless steel column using an air–water–glass beads () system. The solids axial velocity and shear stress were found to increase noticeably with pressure and superficial gas velocity in the churn turbulent flow regime. [Copyright &y& Elsevier]

The building of a sustainable society will require reduction of dependency on fossil fuels and lowering of the amount of pollution that is generated. Wastewater treatment is an area in which these two goals can be addressed simultaneously. As a result, there has been a paradigm shift recently, from disposing of waste to using it. There are several biological processing strategies that produce bioenergy or biochemicals while treating industrial and agricultural wastewater, including methanogenic anaerobic digestion, biological hydrogen production, microbial fuel cells and fermentation for production of valuable products. However, there are also scientific and technical barriers to the implementation of these strategies. [Copyright &y& Elsevier]

Photosynthetic algal cultures are a potential source of many high-value products. In photobioreactors (PBR), the availability and the intensity of the light, which are affected by the cells’ movement, are the major factors controlling the biomass productivity. Hydrodynamics, hence, play a significant role in the reactor''s performance, as they determine not only the flow field, i.e. liquid flow and mixing, shear stresses, etc., but also the movements of the cells. In this work, computer-automated radioactive particle tracking (CARPT) technique was employed to evaluate its feasibility for characterizing PBRs. Liquid velocity profiles, cells’ movement, and the temporal irradiance patterns obtained by coupling the cells’ trajectories and the irradiance distribution model have been determined. The effects of the biomass concentration, reactor geometry, and the aeration rate on the irradiance patterns are discussed. The results demonstrate that the CARPT technique is promising for PBR analysis. It provides fundamental information needed to advance the cells’ growth prediction and modeling, and the design, scale-up and operation of PBRs. [Copyright &y& Elsevier]

• Hydrodynamics of a slurry bubble column reactor operating at mimicked Fischer-Tropsch (FT) reaction conditions. • Non-invasive gamma ray computed tomography measurement technique. • Variables studied are operating pressure, solids loading, and superficial gas velocity. • Findings can be utilized as a benchmarking tool for CFD and phenomenological modeling. This study aims to investigate the phases distribution and hydrodynamics in an air-Therminol LT-glass beads slurry bubble column reactor in bubbly and churn-turbulent flow regimes by implementing a gamma-ray computed tomography. It is found that gas holdup increases with increasing superficial gas velocity and operating pressure while increasing solids loading has an opposite effect on gas holdup measurements. It is noticed that the pressure effect is more significant at high solids loading and the solids loading is relatively more substantial at lower pressure. Solids loading has the most significant a stronger effect on the solids holdup profile than on the gas holdup profile. Current results were compared with the Sedimentation-Dispersion Model (SDM) predictions. The findings of this study advance the knowledge of the true behavior of phases distribution in FT mimicked slurry bubble columns which are pertinent in designing such reactors. [ABSTRACT FROM AUTHOR]

Abstract Temperature and velocity sophisticated measurement techniques have been integrated in a novel way for plenum-to-plenum natural circulation thermal hydraulics investigations in a separate-effects dual-channel facility designed with a representative geometry of prismatic modular reactor core. Time-and-frequency domain analysis has been conducted for the time series data obtained to determine the appropriate sampling time and frequency needed to collect data that statistically represents the time-averaged mean value. Preliminary tests have been conducted to examine the validity of implementing these techniques as well to investigate if flow fields inside prismatic core heated channels are affected by other different temperatures core channels. It is worth mentioning that the velocity measurements (i.e., obtained by using hot wire anemometry) were corrected to account for temperature differences between calibration and experimental conditions. Preliminary results reveal that the top section of the heated channel is significantly affected by cooled channel. This implies that naturally-driven flow has a delicate nature and flow fields inside core heated channels are influenced by temperature gradients exist within the prismatic reactor core after loss of flow accidents. [ABSTRACT FROM AUTHOR]

Reliable measurements of thermal and velocity fields were experimentally investigated under different natural circulation intensities in a dual channel facility designed and developed with a representative geometry of prismatic modular reactor core. Experiments were conducted under steady state conditions to obtain statistically stationary time series signals of temperature and velocity. Insignificant radial temperature variation was observed along the down-comer channel implying that the majority of supplied heat is removed through the upper plenum. Velocity results obtained emphasize the delicate nature of natural circulation phenomena in terms of flow destabilization, and recirculation penetration length. Observed air velocity distribution reveals that downward velocity is proportional to the extent of cooling applied to the setup. The calculated values of the dimensionless group Froude number ( Fr ) in the range of 2 to 10 indicate that air is dominated by momentum and is exiting the channel as a jet. Quantification of turbulent intensities implies that flow destabilization is noticeable along the down-comer channel particularly for the case of high cooling intensity. For 5 ° C cooling water temperature, turbulent intensity ( v rms / v ) peaks at the mid-channel (z/L = 0.5) reaching a value of 0.5. This value of turbulent intensity is much higher than the inlet (z/L = 0.96) and outlet (z/L = 0.04) turbulent intensities of 0.2 and 0.06 respectively. This destabilization is because of the flow reversal and heat conduction effects through stainless steel flanges connecting both channels. Current results provide detailed velocity and temperature variation which can be useful for validating computational fluid dynamics codes. [ABSTRACT FROM AUTHOR]

The effects of helium pressure on the convection heat transfer coefficient and temperature fields (helium and wall surface temperatures) in a unique scaled-down dual-channel natural circulation loop with upper and lower plena have been investigated in this study. Natural convection is one of the passive safety systems of the prismatic very high-temperature reactors (VHTRs) during the accident scenarios. The operating helium pressure was varied from 413.47 to 689.12 kPa in the temperature range from 6 to 196 ° C. Radial and axial measurements were carried out along the flow channels using a new sophisticated flush wall-mounted heat transfer coefficient probe in conjunction with the radial adjuster for T-thermocouple that are integrated in a novel way to characterize natural convection heat transfer in terms of heat transfer coefficient, Nusselt number, helium temperature, and wall surface temperature. The obtained experimental results along the flow channels showed the dependence of natural convection on the system's pressure in which the Rayleigh number is proportional to the square of the helium pressure (Ra α P 2 ). Also, it was found that upon increasing the helium pressure from 413.47 to 689.12 kPa, the heat transfer coefficient and Nusselt number are increased by 30% and 35%, respectively. Moreover, the wall surface temperature along the downcomer and riser channels are decreased by 12.7% and 18% with increasing the helium pressure from 413.47 to 689.12 kPa, respectively. [ABSTRACT FROM AUTHOR]