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Hydrodynamic data obtained from laboratory‐scale trickle‐beds often fail to accurately represent industrial‐scale systems with high packing aspect ratios and column‐to‐particle diameter ratios. In this study, pressure drop, liquid holdup, and flow regime transition were investigated in a pilot‐scale trickle‐bed column of 33 cm ID and 2.45 m bed height packed with 1.6 mm × 8.4 ± 1.4 mm cylindrical extrudates for air‐water mass superficial velocities of 0.0023 – 0.094 kg/m2s and 4.5 – 45 kg/m2s, respectively, at atmospheric pressure. Significant deviation was observed from pressure drop and liquid holdup correlations at low liquid flows rates, corresponding to gravity‐driven flow limit. Likewise, liquid saturation is overestimated by correlations at high liquid flow rates, owing to significantly reduced wall effects. Lastly, trickle‐to‐dispersed bubble flow and trickle‐to‐pulsing flow regime transitions are reported using a combination of visual observations and analysis of the magnitude of local pressure fluctuations within the column. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2560–2569, 2018 [ABSTRACT FROM AUTHOR]

In this contribution we report on spatially resolved analysis of multiphase hydrodynamics in solid foam packed trickle bed reactors. For the investigation we used ultrafast X-ray computed tomography and fast response pressure transducers. The SiSiC foams’ pore density, the liquid distribution system as well as gas and liquid flow rates were varied. The transient behavior of the liquid holdup at trickle and pulse flow as well as after drainage were examined and correlations for static and dynamic holdups were derived. The correlations are based on Eötvös, Reynolds and Galileo number, using porosity and specific area for the definition of the hydraulic diameter. The correlations are applicable to a wide range of foam morphologies, pore densities and operating conditions reported in the literature. The axial pressure gradients in the solid foams showed significantly lower pressure drop compared to particle packings of similar specific surface area. The evolution of liquid spreading was analyzed qualitatively and quantitatively with various irrigation patterns. In addition, an approach for the determination of radial liquid dispersion coefficients in solid foams is presented. [ABSTRACT FROM AUTHOR]

Flow regime effect and reactor modeling studies in a gas-liquid downward flow fixed bed reactor were conducted for acetophenone hydrogenation on 1% Rh/Al 2 O 3 catalyst, a relatively complex reaction scheme typical of pharmaceutical applications, using a 7.1 mm ID stainless steel reactor with 0.5 mm catalyst spheres at elevated pressures. A flow regime transition map for trickle and bubbly flows was determined visually in a transparent reactor surrogate and confirmed by monitoring pressure drop fluctuations for different gas/liquid systems, tube/particle materials, and operating pressure and temperature. The flow regime at each operating condition for the opaque stainless steel reactor was identified solely using pressure drop fluctuations. The beneficial effect of bubbly flow on reaction rate was confirmed experimentally at gas and liquid superficial velocity ranges of 0.02–0.19 m/s and 2.5–12 mm/s, respectively, under 80–100 °C, 11–26 bar and 0.04–0.6 M initial substrate concentration. Gas flow rate, temperature, and pressure variations were used to study the effects on reaction performance of partial wetting of the catalyst and of liquid-solid diffusion limited reaction. A reactor model including external/internal mass transfer and flow regime effects was developed using an adjustable parameter to account for partial wetting and flow regime effects. The parameter was fitted using a subset of the experiments, and the model provided good prediction (R 2 > 95%) of all remaining experiments. [ABSTRACT FROM AUTHOR]

Two-phase flow dynamics and hydrodesulfurization (HDS) performance were numerically analyzed for vertical, inclined, and oscillating trickle beds with a prospect of assessing the potential of implementing HDS operations on marine floating platforms. An unsteady-state three-dimensional model based on the macroscopic volume-averaged mass, momentum, energy and species balance equations coupled with simultaneous diffusion and chemical reaction within sulfide CoMo/alumina catalyst was developed for the purpose of comparative analyses using H 2 /dibenzothiophene/ n -hexadecane model system. Angular uniform and sinusoidal oscillatory motion of the reactor between two angled symmetrical positions and between vertical and an inclined position was considered. As in the case of cold-flow passive conditions, in chemical reaction environment two-phase downflow deviates considerably from axial symmetry at higher reactor inclinations with noticeable liquid accumulation in the bottommost reactor cross-sectional area on the tilting side. Also, externally-induced reactor periodic oscillation generates complex reverse secondary flow in radial and circumferential directions and oscillatory patterns of flow field with the amplitude and propagation frequency affected by the type (uniform or sinusoidal), amplitude and period of angular motion of reactor. Inclined, symmetric and asymmetric oscillating trickle-bed reactors underperform the vertical configuration and the decline of symmetric oscillating trickle-bed reactor hydrodesulfurization performance is lower under uniform oscillatory motion conditions. Oscillating trickle-bed reactors generate an oscillatory hydrodesulfurization performance which is affected by the parameters of angular motion of reactor. The inhibiting effect induced by H 2 S on the hydrogenolysis is considerable at higher packed bed inclinations and in symmetric oscillating trickle-bed reactors. [ABSTRACT FROM AUTHOR]

The trickle-bed reactor model proposed by Mostoufi et al. was rebuilt for use in commercial software, Aspen Plus ® . Coupling with thirty hydrogenation rate equations, the process model could simulate a pyrolysis gasoline hydrogenation unit. With an optimization procedure, the thirty rate constants were obtained after minimizing the relative-concentration-based objective function. In this work, the hydrodynamic parameters including physical properties were simultaneously calculated from the software platform according to the reactor conditions, such as temperature, concentration, etc. Therefore, the effect of parameter variation can be avoided not only along the catalyst bed, but also at the start of run and end of run. This semi-empirical method can be applied to highly exothermic hydrogenation reactor models, and then used it to estimate residue catalyst activity much more theoretically than the weighted average bed temperature method and interpret plant data more in practice, for example, bed temperature profile and its deviation, the relationship between the throughput and pressure drop. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) used to investigate pressure drop and liquid holdup in high- and low-pressure trickle-bed reactors (TBRs). CFD results compared with experimental data from literature and found that CFD is a useful tool for hydrodynamics investigation of high-pressure TBRs. It is shown that by increasing pressure in TBRs, pressure drop increased and liquid holdup decreased. Finally, it is observed that pressure drop is more sensitive to velocity changes than to the reactor pressure changes. This result is the same as several findings by experiments in the literature. [ABSTRACT FROM PUBLISHER]

The relationship describing the shock wave velocity was formulated for the trickle-bed reactor operating at periodi-cally changed feeding the bed with liquid phase. The values of shock wave velocity calculated from derived equations were compared with experimental values obtained for both fast and slow mode of base-pulse periodic liquid feed-ing and using liquids differing in physicochemical properties. A good agreement between these two sets of values of shock wave velocity was obtained. It has to be emphasized that the relationship (Eq. (26)) derived in this study enables to estimate the values of the shock wave velocity when only mean values of variables of a process are known. [ABSTRACT FROM AUTHOR]

An experimental investigation of the hydrodynamic parameters in a trickle bed reactor is presented. The operating conditions are selected for the operating system to be at trickle flow regime. The effects of the two-phase flow rates, reactor pressure and temperature on the pressure drop, external liquid holdup and liquid axial dispersion are discussed. Pressure drop was measured using differential pressure transducer, while liquid holdup and axial dispersion were estimated using RTD technique with a reactive dye as a tracer. The results confirmed that pressure drop is proportional to flow rate of fluids and operating pressure while it is inversely proportional to temperature. Liquid flow rate has a proportional effect on liquid holdup and axial dispersion while gas flow rate and temperature presented a different image. A comparison between the results of present work and that of literature is presented and discussed. Empirical correlations for pressure drop, liquid holdup and axial dispersion with operating conditions are developed with correlation coefficient of 98.4 to 99.7 %. [ABSTRACT FROM AUTHOR]

Abstract: Familiarized with the steady state behavior and advantages of the trickle bed reactors (TBR), for the past two decades, researchers and process engineers are continuously exploring the unsteady state hydrodynamics of periodically (or cyclic) operated trickle bed reactors to extract even more from its efficiency and performance. Despite its complicated nonlinear behavior that attributes to the process control safety at the commercial scale, cyclic operation of TBR is a promising process intensification technology that has immense potential for implementation on industrial TBR. In this paper, we have summarized and reviewed the research and progresses made in recent years on cyclic operation of TBR and potential applications of various mode of its operation. Several issues associated with its hydrodynamics scale-up and designs have been discussed with an emphasis on shock wave attenuation that arises during flow modulation. [Copyright &y& Elsevier]

Characterization of the hydrodynamicsenables operation of tricklebed reactors within the desired flow regime and under conditions foruniform distribution of gas and liquid, resulting in an essentiallyplug flow contacting pattern. Studies reported in the literature aretypically restricted to systems of beds packed with catalyst supportsof a uniform size. This work addresses the impact of supports withparticle size distributions on reactor hydrodynamics. An experimentaldatabase of pressure drop and liquid holdup was developed and, bycareful definition of the particle diameter, literature models wereadapted to account for the particle size distribution. The resultingmodels give improved predictions for packing media with a particlesize distribution while maintaining applicability to uniform systems. [ABSTRACT FROM AUTHOR]

Local liquid velocity measurements in Trickle Bed Reactors (TBRs) are one of the essential components in its hydrodynamic studies. These measurements are used to effectively determine a reactor's operating condition. This study was conducted to validate a newly developed technique that combines Digital Industrial Radiography (DIR) with Particle Tracking Velocimetry (PTV) to measure the Local Liquid Velocity (VLL) inside TBRs. Three millimeter-sized Expanded Polystyrene (EPS) beads were used as packing material. Three validation procedures were designed to test the newly developed technique. All procedures and statistical approaches provided strong evidence that the technique can be used to measure the VLL within TBRs. [ABSTRACT FROM AUTHOR]

Highlights: [•] Validation of the CFD code on the basis of a experimental database. [•] Selection of the optimum correlation for the radial and axial porosity profile in the bed. [•] Estimation of Ergun parameters in the closure equation for the momentum exchange between phases. [•] Influence of the ratio D/dp on values of hydrodynamic parameters computed has been investigated. [ABSTRACT FROM AUTHOR]

Highlights: [•] Sub-particle scale bifurcation of area specific liquid–solid mass transfer. [•] Extend of stagnancy highly dependent on prewetting procedure. [•] No relation between static and residual holdup found. [•] Static LS mass transfer liquid velocity dependent and higher than prior reports. [ABSTRACT FROM AUTHOR]

Trickle bed reactors are commonly used in chemical industries. Scale-up of trickle bed reactors is difficult and simple scaling rules often lead to poor design. Fluid dynamics of the trickle bed reactors is complex and sensitive to the scale of the reactors. Flow mal-distribution, channeling, wetting of catalyst and local temperature variation are some of the important parameters which controls the overall performance of the trickle bed reactors. Two-phase pressure drop and liquid holdup are two important hydrodynamic parameters which should be considered while analysing and designing a trickle bed reactor. By using computational fluid dynamics, we can understand fluid dynamics and its interactions with chemical reactions. In this study a computational fluid dynamic model based on Eulerian multiphase 2D model has been developed for simulating two phase flow in a trickle bed laboratory scale reactor. The porous media concept was used to model solid packings in the reactor. Model predictions were evaluated by comparing them with published experimental data. Pressure drop and liquid hold up along the bench scale reactor were obtained and were in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

The myth of coexistence of flow regimes in trickle bed reactor were experimentally explored using 40 ppm and 60 ppm highly foaming aqueous surfactant-air system. Since the last publication, none of literature present adequate experimental investigation of these flow irregularities arise near regime transitional boundary. The influence of physiochemical as well as hydrodynamic variables has been investigated to verify the possibilities of coexistence of flow regimes at corresponding liquid and gas flow velocities. The increment in the gas-liquid throughput resulted in an uneven disturbance velocity leads to formation of coexistence of flow regimes. The implementation of modified liquid-gas distributor slightly improve the flow maldistribution over packed bed but limited to low gas-liquid superficial velocities and temperatures. A relatively new technique, two gas-liquid distributors has been attempted first time to nullify the these kind of flow irregularities arise, trends evident an outstanding improvement of regime flow patterns compared with earlier cases. [ABSTRACT FROM AUTHOR]

An inverted microscopy technique was implemented to scrutinizethe wall-region hydrodynamics of gas–liquid cocurrent flowsin micropacked beds. Digital image analysis enabled characterizationof two contiguous flow regimes, hysteresis, and transition thereof.Low- and high-interaction regimes featuring, respectively, slow andrapid displacements of gas–liquid boundary were identified.The onset of regime changeover was delineated by distinguishing thefluctuating behavior in time of characteristic lengths extracted fromthe areas occupied by gas and liquid in the field of view. A Charpentierand Favier flow regime map demarcating low and high interaction regimesin conventional macroscale trickle beds was elaborated for the sakeof comparison of micropacked bed transitions for three different gas–liquidsystems (air–water, argon–water, and argon–sucrosesolution). The flow regime map suggests that micropacked bed transitionoccurs at considerably lower L/Gvalues. Manifestation of hysteresis in micropacked beds was apprehended viapressure drop measurements and wetting fraction determinationboth in imbibition and drainage modes. In agreement with macroscalepacked bed observations, the drainage branch revealed a larger pressuredrop and wetting fraction compared with the imbibition branch forthe same set of bed and fluid flow rates. [ABSTRACT FROM AUTHOR]

Abstract: Aiming to accomplish stringent regulations for the secure disposal of hazardous compounds, trickle beds have been envisaged from a process intensification point of view to adhere to environmental guidelines. In this work, several experimental and computational runs were performed under reacting flow conditions and cyclic (flow rate) modulations for the mineralization of liquid pollutants through electrical capacitance tomography (ECT) and computational fluid dynamics (CFD) techniques, respectively. First, we have investigated the hydrodynamics of propagating liquid pulsations generated via ON–OFF liquid, ON–OFF gas, and gas/liquid alternating cyclic operations. Here, the liquid pulsation intensity generated in ON–OFF gas mode indicated a negligible dependence with respect to cycle frequency apart from being qualitatively analogous with respect to ON–OFF liquid mode. Second, the effect of oxidation temperature and the gas and liquid flow rates unveiled the gas/liquid alternating mode as being able to generate sizable liquid payloads interrupted by gas and liquid streams. Correspondingly, we found higher detoxification rates due to the periodic pulsations sustained down to the reactor outlet by conferring improved catalyst wetting efficiencies. Finally, two-dimensional mappings of different cross-sections allowed inferring by means of ECT and CFD how the liquid payloads were morphologically in disposition to preserve or dissipate their original structure. [Copyright &y& Elsevier]

A radiotracer study was carried out in a trickle bed reactor (TBR) independently filled with two different types of packing i.e., hydrophobic and hydrophilic. The study was aimed at to estimate liquid holdup and investigate the dispersion characteristics of liquid phase with both types of packing at different operating conditions. Water and H gas were used as aqueous and gas phase, respectively. The liquid and gas flow rates used ranged from 0.83 × 10-16.67 × 10 m/s and 0-3.33 × 10 m (std)/s, respectively. Residence time distribution (RTD) of liquid phase was measured using Br as radiotracer and about 10 MBq activity was used in each run. Mean residence time (MRT) and holdup of liquid phase were estimated from the measured RTD data. An axial dispersion with exchange model was used to simulate the measured RTD curves and model parameters (Peclet number and MRT) were obtained. At higher liquid flow rates, the TBR behaves as a plug flow reactor, whereas at lower liquid flow rates, the flow was found to be highly dispersed. The results of investigation indicated that the dispersion of liquid phase is higher in case of hydrophobic packing, whereas holdup is higher in case of hydrophilic packing. [ABSTRACT FROM AUTHOR]

Abstract: Numerical simulations of the two-phase flow distribution in a trickle-bed reactor used for fuel hydrodesulfurization are reported. As a first step, the heat and mass transfer, as well as the chemical reactions, are not considered. The reactor has four packed-beds and a distribution tray above the catalytic beds equipped with cylindrical chimneys. The flow distribution at the outlet of the circular chimney predicted by the simulations is not axisymmetric because of the spatial distributions of the liquid and gas inlets in the chimney. This causes that the liquid entering the packed beds is distributed in three main streams. For the simulation of the two-phase flow in the packed beds, an Eulerian three-phase model that considers the particles of catalyst as a granular static phase has been used following the Holub single slit model for particle–fluid interaction to compute the liquid–solid and gas–solid drag coefficients. Numerical simulations of the dispersion of water–air flow in a column of glass beads using this model were initially carried out and results were found to be in reasonable agreement with numerical and experimental data available in the literature. The simulations consider the flow dispersion in the central region of the reactor bed as well as in the region close to the cylindrical lateral wall of the reactor. In both cases most of the liquid spreading takes place in the top part of the bed. The distributions of the liquid volume fraction do not change significatively as the depth of the bed is increased except in the third bed and at the interface of two beds with different porosity. [Copyright &y& Elsevier]

The selective hydrogenation of 1-heptyne over a 2 wt % Pd/Al2O3catalyst was studied in a trickle bed reactor operating in both batch recycle and continuous modes. The reaction was studied in a range of different solvents, including isopropanol, hexane, mixtures of these two solvents, and also isopropanol with small quantities of water and base (NaOH) added. It was found that the rate of reaction was fastest in hexane, owing to the higher hydrogen solubility in this solvent. However, the selectivity toward 1-heptene was higher in isopropanol, with over 95% selectivity being maintained for 120 min of the total 240 min reaction time. The addition of water or base led to an increase in reaction rate, possibly through modification of the adsorption equilibria at the catalyst surface or direct involvement in the reaction. The hydrodynamics of trickle flow upon the reaction were investigated, showing that increasing liquid flow rate led to enhancement of reaction rate, although a plateau was eventually reached at the higher flows. The higher flows led to improved catalyst wetting, liquid hold up, and mass transfer rates, thus explaining the enhanced reaction rate. The concentration profiles were fitted according to a Langmuir–Hinshelwood kinetic expression. Operation in continuous flow was demonstrated, although a long residence time was required for high conversion, leading to lower 1-heptene selectivity compared with batch recycle operation. [ABSTRACT FROM AUTHOR]

Most existing studies in foaming trickle bed reactors aim at the improvement of efficiency and operational parameters leading to high economic advantages. Conventionally, most of the industries rely on frequently used gas continuous flow (GCF) where operational output is satisfactory but yields are not as efficient as in pulsing flow (PF) and foaming pulsing flow (FPF). Hydrodynamic characteristics like regime transitions are significantly influenced by the foaming nature of liquid as well as gas and liquid flow rates. The aim of this study was to experimentally demonstrate the effects of liquid flow rate, gas flow rates and liquid surface tension on regime transition. These parameters were analyzed for the air-aqueous sodium lauryl sulphate and air-water systems. More than 240 experiments were done to obtain the transition boundary for trickle flow (GCF) to foaming pulsing flow (PF/FPF) by use of excessive foaming 15-60 ppm surfactant compositions. The trickle to pulse flow transition appeared at lower gas and liquid flow rates with decrease in liquid surface tension. All experimental data were collected and drawn in the form of four different transitional plots which are compared and presented by using flow coordinates proposed by different researchers. A prominent decrease in dynamic liquid saturation was observed, especially during the regime transitional change. The reactor two phase pressure had an evident sharp rise, verifying the regime transition shift from GCF to PF/FPF. The study revealed that the regime transition boundary is significantly influenced by any change in hydrodynamic as well as physiochemical properties including surface tension. [ABSTRACT FROM AUTHOR]

Abstract: A numerical model for liquid mal-distribution in randomly trickle bed reactors has been investigated and the results are compared with the experimental data. A CFD model based on the three-phase Eulerian approach is developed and a two-fluid model is utilized to perform the inter-phase momentum exchanges. Furthermore, radial distribution of the bed porosity is considered adjacent to the reactor wall. Two different types of liquid inlet distributors have been used in order to study the accuracy of the CFD model. To validate the CFD model, the simulation results are compared with the experimental data and the results from the porous media concept in which the permeability model has been applied to implement the inter-phase momentum exchange. Experimental results have been obtained on an industrial trilobe catalyst under a trickling flow regime in a pilot scale reactor setup. Co-current liquid and gas streams have entered to the reactor through a mono or multi orifice distributor. Results of the developed CFD model have found more accurate than that of the porous media concept when compared with the experimental data. [Copyright &y& Elsevier]

The hydrodynamics of countercurrent gas and liquid flows through a trickle bed reactor with unstructured (8 mm Raschig rings) and structured (Bale) packing has been investigated using the residence time distribution (RTD) approach. The operation was carried out with gas and liquid Reynolds number over the range 0 < ReG< 150 and 0 < ReL< 1500, respectively. Gas phase Peclet number, PeG, was generally higher in the randomly packed Raschig rings than in the structured Bale packing but increased with ReG. However, gas phase mixing decreased with increasing liquid flow rate. Specifically, the dependency on ReGand ReLwas given by, PeG= AGReGbGexp(−dReL). The gas phase hold-up, H0Galso increased with gas flow rate but decreased with the liquid flow rate. Owing to the statistical nature of the gas flow through beds, a Snedecor F-distribution was used to describe the gas phase RTD data and hence, the dynamics of the macroscopic flow pattern within the bed was determined from the intensity function, I(t). This indicated initial gas recirculation in stagnant zones within the bed with the flow pattern subsequently evolving into predominantly a channelling and bypassing type. Interestingly, the liquid phase mixing behavior appeared to be independent of ReGand thus, PeL,Raschig= ALReLbLfor the unstructured bed, but flow through the more open Bale packing revealed two types of mixing regimes, with a crossover from the region of decreasing PeLto increasing PeLwith an increased liquid flow rate at ReLof about 750 and accordingly, PeL,Bale= A0͔帬එ碽. Liquid hold-up, H0L, could be decoupled into a static component, HSL, and dynamic contribution, HDL, with the latter being proportional to the liquid flow rate. The Bale packing has a higher static hold-up (0.225) than the randomly packed Raschig rings (0.100) due to its more open structure. On the whole, it seemed that liquid phase back-mixing was generally lower in the unstructured packed bed than in the structured Bale packing. The associated higher static hold-up and reduced sensitivity with increasing liquid flow rate makes the latter the preferred choice for catalytic distillation for operations bounded by the loading and flooding envelopes. [ABSTRACT FROM AUTHOR]

The sensitivity of catalytic reactions to concurrent filtration and fines deposition in trickle-bed reactors (TBRs) was assessed by means of the catalytic hydrogenation of α-methylstyrene from kaolin–kerosene suspension flows. A negative correlation between catalytic conversion and the specific deposit of the bed resulted from the extra mass-transfer step that built up on the collectors in the course of filtration. The severity of the evolving extra mass-transfer step was sensitive to deposit compaction resulting from higher gas superficial velocities. Furthermore, irreversible catalyst activity losses were observed after catalyst cleanup and were ascribed to irretrievable active sites by catalyst intraparticle fines trapping. Fines accumulation in the catalyst bed was notably reduced by means of high-porosity solid-foam modules used as guard filters and placed upstream of, and adjacent to, the trickle bed. Hydrodynamic studies were carried out with alumina and silicon carbide solid foams to assess their role on the distribution of suspension and the reduction of specific deposit and pressure drop in trickle beds. It was found that the foam capture efficiency critically depended on the open connectivity of foam cells and the physicochemical nature of foam materials. The numbering up of foam modules led to reduced overall specific deposits of the beds and pressure drops, foreseeing a prospect for prolonging the hydrotreatment TBR lifetime, using solid foam as guard beds. [ABSTRACT FROM AUTHOR]

An experimental investigation was carried out to determine the effects of gas and liquid flow velocities and surface tension on the two-phase phase pressure drop a in a downflow trickle bed reactor. Water and non- Newtonian foaming solutions were employed as liquid phase. More than 240 experimental points for the trickle flow (GCF) and foaming pulsing flow (PF/FPF) regime were obtained for present study. Hydrodynamic characteristics involving two-phase pressure drop significantly influenced by gas and liquid flow rates. For 15 and 30 ppm air-aqueous surfactant solutions, two-phase pressure drop increases with higher liquid and gas flow velocities in trickle flow and foaming/pulsing flow regimes. With decrease in surface tension i.e. for 45 and 60 ppm air-aqueous surfactant systems, two-phase pressure drop increases very sharply during change in regime transition at significantly low liquid and gas velocities. Copyright © 2011 BCREC UNDIP. All rights reserved. [ABSTRACT FROM AUTHOR]

Modulation of electrical conductivity in kaolin/kerosene suspensions was examined as a means for the attenuation of fines retention in trickle-bed reactors. The suspension stability was remarkably enhanced through ON-OFF concentration modulation of an electrolyte-based kerosene conductivity improver and resulted in an efficient bed-cleaning strategy under operating conditions. Periodic additions of the conductivity improver enabled fines and deposits to gain momentarily large and similar electrical charges undoing, or impeding, multilayer deposition. The time evolution of the two-phase pressure drop and specific deposit with/without conductivity improver was monitored, as well as the corresponding local deposition structure via electrical capacitance tomography (ECT). Electrical conductivity modulation was found to reduce the bed-specific deposit by an order of magnitude and the bed pressure drop increment by a factor 14. ECT imaging evaluated the efficacy of this modulation strategy in mitigating deposition and in preventing filtration-induced flow maldistribution. © 2010 American Institute of Chemical Engineers AIChE J, 2011 [ABSTRACT FROM AUTHOR]

Abstract: Hydrodynamics and transport phenomena inside Trickle Bed Reactors are strongly modified when superficial liquid velocity is not sufficient to insure a perfect wetting of catalyst particles. For this reason, the impact of partial wetting on catalyst effectiveness has been widely studied in past, in case of isothermal reactions. Heat transfer limitations, inside or outside catalyst particles, have also been investigated, but only in case of total wetting. In the present study, the effect of partial wetting is quantified numerically for various kinetics, mass and heat transfer limitations. When possible, an analogy is proposed between 1D and 2D models of particles. Robust rules are proposed to take into account for partial wetting in 1D-radial models of particles. The case of partial evaporation inside catalyst pores is also studied. Evaporation phenomena is approximated following a simple approach. Performed calculations show that the effect of partial wetting can affect very strongly catalyst effectiveness factors, especially in case of evaporation in the pores. The use of 1D approximations gives a good prediction of the global catalyst efficiency, as far as boundary conditions are symmetric between heat and mass transfer. Otherwise, 1D models are not relevant anymore. [Copyright &y& Elsevier]

In the present study, a computational modeling approach capable of simultaneously tracking the multiphase attributes of trickle flow is accomplished at high-pressure by means of computational fluid dynamics (CFD) codes. The morphological features of the gas−liquid structure accounted for by the CFD framework were embedded with a volume-of-fluid (VOF)-based interface capturing methodology for monitoring the gas−liquid interface dynamics supplemented with the droplets and rivulets formation triggered at unsteady-state conditions. Our research effort on transient operated trickle-bed reactors (TBRs) relied on assured benefits of their performance enhancement that ultimately investigated the application of the VOF model with the purpose to quantify the relationship between bed structure and downstream flow distribution in cocurrent packed beds. To accomplish this concept, first we evaluated the influence of model tuning parameters. Second, the effect of process liquid flow rates and operating pressures was examined thoroughly on liquid holdup and two-phase frictional pressure drop. Finally, we have performed a three-dimensional analysis of interstitial flow hydrodynamics that enabled us to identify the inhomogeneous nature of the multiphase flow environment on the catalytic packing scale. [ABSTRACT FROM AUTHOR]

The hydrodynamic behavior of a monolith reactor experiencing deposition from fines-contaminated flows was studied to evaluate its aptitude to be an alternative for trickle bed reactors under filtration conditions. Bed was subjected to single-pass kaolin-kerosene suspension and air flows to study the influence of different operating conditions. To reduce the extent of deposition, feed flow rate modulation strategies for cleaning of deposits were tested. The dynamics of liquid pulsations generated via cyclic operation strategies was monitored using electrical capacitance tomography imaging. The X-type deposition in small opening channels can keep all the channels in operation to get the maximum benefit of catalyst. Instabilities induced by the modulation strategies favored detachment and removal of deposition for the large-opening channel monoliths. In small-channel opening monoliths, detachment followed by re-deposition downstream in the bed increased the specific deposit and pressure buildup in flow modulation. Consequently, small-opening channels can be recommended for isoflow filtration of suspensions whereas large-diameter channels are preferable for cyclic operations. [ABSTRACT FROM AUTHOR]

Abstract: The influence of periodic operation on trickle-bed reactor (TBR) hydrodynamics and gas–liquid mass transfer was investigated. Two-phase pressure drop, dynamic liquid hold-up and gas–liquid mass transfer coefficient (k L a) were determined at various liquid flow rates and for different modes of liquid flow variation (increasing and decreasing liquid flow rate). The results reveal the considerable influence of type of liquid flow rate modulation on k L a values (deviations of up to 80% in k L a). Simulation studies on gas-limited reaction in a periodically operated TBR indicate that an enhancement in conversion of about 14% can be expected from an appropriate selection of the operating mode, thus clearly demonstrating the quantitative process intensification feasible through increased gas–liquid mass transfer. [ABSTRACT FROM AUTHOR]

Hydrodynamic studies of trickle bed reactors (TBRs) are essential for the design and prediction of their performance. The hydrodynamic characteristics involving pressure drop and dynamic liquid saturation are greatly affected by the physical properties of the liquids. In the present study experiments have been carried out in a concurrent downflow air - liquid trickle bed reactor to investigate the dynamic liquid saturation and pressure drop for the water (non-foaming) and 3% polyethylene glycol and 4% polyethylene glycol foaming liquids in the gas continuous regime (GCF) and foaming pulsing regime (FP). In the GCF regime the dynamic liquid saturation was found to increase with increase in liquid flow rate for non-foaming and foaming liquids. While for 3% and 4% polyethylene glycol solutions the severe foaming was observed in the high interaction regime and the regime is referred to as foaming pulsing (FP) regime. The decrease in dynamic liquid saturation followed by a sharp rise in the pressure drop was observed during transition from gas GCF to FP regime. However in the FP regime, a dip in the dynamic liquid saturation was observed. The pressure drop for foaming liquids is observed to be manifolds higher compared to non-foaming liquid in the GCF regime. [ABSTRACT FROM AUTHOR]

Abstract: Electrical capacitance tomography was used to follow the behavior of long and brief solitary liquid-rich waves in a trickle-bed reactor. A quantitative description of wave propagation and attenuation was attempted using a simplified two-phase Eulerian computational fluid dynamics modeling by tracking long and brief pulses as encountered in slow- and fast-mode cyclic operation strategies. The results assert the efficacy of slow mode operation since the wave attempts to preserve its identity while propagating in the bed. On the contrary, the decay time period in case of fast mode cyclic operation is prolonged and continues to attenuate down the reactor length. The predictive computational model applied in this work, in spite of many simplifications, conforms arguably well with the experimental observations. [Copyright &y& Elsevier]

Abstract: Computational fluid dynamics is used to describe both wetting efficiency in simple configurations of stacked solid particles and catalyst efficiency inside such partially wetted catalyst particles. The volume of fluid (VOF) model used to describe hydrodynamics leads to realistic and promising results for surface wetted ratio. Catalyst efficiency simulations have been performed for different shapes of particles and different Thiele modulus values for a first order reaction rate. Results show that for all the shapes studied, catalyst efficiencies of partially wetted particles can be calculated by simply using a modified Thiele modulus defined as ratio of the actual Thiele modulus to the wetting efficiency f. [Copyright &y& Elsevier]

Abstract: The response of an isothermal TBR to a liquid flow modulation ON–OFF strategy is examined through a model aimed at the reactor scale. The reaction is gas-limited and first order with respect to both reactants. Liquid hydrodynamic behavior is considered by means of two different approaches: the Liquid Draining Approach, based on experimental results and the ideal, Square Wave Approach. Model allows the evaluation of, among other variables, the liquid holdup, the liquid velocity and the liquid reactant conversion time variations at different axial positions within the reactor. An enhancement factor due to periodic operation is defined by computing a temporal average of the liquid reactant conversion during an invariant cycling state, referred to its corresponding steady-state conversion. For all the conditions investigated, attainable enhancements are lower when the actual draining model is assumed. [Copyright &y& Elsevier]

Abstract: Multiphase flow in trickle-bed reactors (TBR) is known to be extremely complex and depends on a multitude of effects including the physico-chemical properties of both gas, liquid and solid phases, the ratio of column diameter to particle diameter and most importantly the gas and liquid superficial velocities. Despite several works devoted to the experimental investigation of liquid distribution, there is yet no universal agreement on the influence of interstitial phenomena on overall TBR hydrodynamics. Consequently, a Eulerian multiphase model was developed to predict the liquid holdup and pressure drop in the trickling flow regime with a 3D computational grid. The multiphase model was optimized in terms of mesh density and time step for the successful hydrodynamic validation activities. The model predictions correctly handled the effect of different numerical solution parameters. Afterwards, particular attention is paid to the consequences on flow development and hydrodynamic parameters of imposing liquid maldistribution at the bed top with three types of liquid distributors. Several computational runs were carried out querying the effect of gas and liquid flow rate on overall hydrodynamics. Computational fluid dynamics (CFD) predictions demonstrated that liquid flow rate had a prominent effect on radial pressure drop profiles at the higher values whereas the gas flow rates had it major outcome at lower regimes. Regarding the liquid holdup predictions, several time averaged for radial and axial profiles illustrated that a five times increase on liquid flow rate cannot be matched by an equivalent change on gas flow rate. The increase in both flow rates was found to smooth the oscillatory behaviour of local phenomena, but the gas flow rate had an outstanding consequence on both hydrodynamic parameters. Finally, CFD simulations at atmospheric conditions were compared with the pressurized ones. Liquid holdup fluctuations of about 25% between the liquid-rich and the gas-rich zone can be smoothened as long as the operating pressure is increased until 30bar. [Copyright &y& Elsevier]

Abstract: Trickle bed reactors are encountered throughout the process industry. Considerable attention has been given to the study of the hydrodynamics of this type reactor. It has been identified that, in the trickle flow regime, the hydrodynamic parameters (e.g. pressure drop and liquid holdup) are not unique functions of the operating and system conditions, but depend on the flow history. This study reviews the experimental trends identified in literature on the basis of a limiting cases framework and then evaluates the three-dimensional pore-scale liquid distribution using computed tomography (CT) data. This leads to the identification of 20 phenomenological trends that characterize hydrodynamic multiplicity, including hydrodynamic flow hysteresis as well as the effects of pre-wetting. The CT study yields additional experimental insight into the role of capillary pressure and ultimately leads to the proposal of a capillary gate mechanism based on contact angle hysteresis as the root cause of multiplicity. The mechanism is incorporated into a simple pore-network model. It is shown that the qualitative performance of the model corresponds closely to the majority of phenomenological trends and is capable of explaining the observed experimental behaviour. [Copyright &y& Elsevier]

A trickle-bed reactor (TBR) was modeled by means of the volume of fluid (VOF) model to provide a hydrodynamic behavior analysis in trickling flow conditions. Fluid dynamics of the TBR is characterized by poor liquid distribution and inefficient catalyst utilization and conventional modeling techniques are unable to address these key design issues. Therefore, the VOF code was used to investigate the major hydrodynamic parameters in a three-dimensional packed bed providing a more rigorous physical description of the underlying flow process. Several numerical solution parameters including different mesh densities, time steps, and convergence criteria were optimized in order to provide computational independent results. During the parametric optimization it was found that the VOF model is more sensible to mesh density and time step than with respect to convergence criteria. The computational fluid dynamic model was thoroughly validated by comparing the model predictions with the published experimental data for liquid holdup and two-phase pressure drop. After the VOF optimization, selected values for the numerical solutions parameters were used to perform the assessment of different turbulent flow models at two nominal gas flow rates. Afterward, several computational runs were performed in the evaluation of the influence of either gas or liquid flow rate on TBR hydrodynamics. [ABSTRACT FROM AUTHOR]

Vapor−liquid equilibrium (VLE) in trickle-bed hydroprocessing reactors can significantly change the fluid hydrodynamics and the distribution of reacting species in both the vapor and liquid phases and, ultimately, change the reactor performance. VLE is especially important to pilot-plant studies in which ideal operating regimes (plug flow, full catalyst wetting, absence of reactor wall effects, etc.) are desired to generate reliable, reproducible, and representative data for commercial scale-up and kinetics studies. In this article, we report VLE flash experiments that were conducted in a continuous-flow unit with hydrogen and various petroleum middle distillates under typical hydrotreating conditions to study the relative distribution of the oil in the two phases. The experimental data were further used to evaluate the interaction coefficients, required to perform VLE flash calculations, between hydrogen and hydrocarbon boiling-point pseudocomponents. Furthermore, flow hydrodynamics were predicted in a pilot-plant trickle-bed reactor for hydrotreating two different middle distillate feeds to provide a mapping of operating conditions under which the desired operating regimes could be maintained. [ABSTRACT FROM AUTHOR]

Abstract: CFD simulations of trickle-bed reactors are presented with radial spreading of the liquid due to mechanical and capillary dispersion. Simulations are performed with various particle sizes and the significance of the dispersion mechanisms at the industrially relevant particle size range is analyzed. The effect of the bed porosity distribution and particle size to the simulation results is also discussed. The choice of the radial porosity profile is found to have a significant impact to the simulation results, especially when the column to particle diameter ratio, D/dp , is small, in which case the wall flow is important. The dependence of the standard deviation of porosity on the sample size is determined experimentally. Introducing just random variation of porosity to the model is found to describe inadequately the dispersive flow behavior. The presented hydrodynamic model with proper capillary and mechanical dispersion terms succeeds in capturing the features of the two independent physical phenomena. Separate models are presented for each dispersion mechanisms and it is shown that they both can have a significant contribution to the overall dispersion of liquid flowing through a packed bed. The hydrodynamic model is validated against the experimental dispersion profiles from Herskowitz and Smith [1978. Liquid distribution in trickle-bed reactors. A.I.Ch.E Journal 24, 739–454], Boyer et al. [2005. Study of liquid spreading from a point source in trickle-bed via gamma-ray tomography and CFD simulation. Chemical Engineering Science 60, 6279–6288] and Ravindra et al. [1997. Liquid flow texture in trickle-bed reactors: an experimental study. Industrial & Engineering Chemistry Research, 36, 5133–5145]. The extent of liquid dispersion predicted by the presented hydrodynamic model is in excellent agreement with the experiments. [Copyright &y& Elsevier]

Abstract: This study aims to investigate the hydrodynamic behaviour of a trickle-bed reactor (TBR) at high pressure (30bar) in terms of pressure drop and liquid holdup after the development of a multiphase model by means of computational fluid dynamics (CFD) codes. Taking into account transport phenomena expressed as interphase coupling terms in the momentum transfer between the gas, liquid and solid phases, an Euler–Euler model was developed resulting from the volume averaging of the continuity and momentum equations and solved for a 3D representation of the catalytic bed. The CFD calculations were validated with experimental data from the literature and different mesh sizes were evaluated for a grid-independent CFD solution of multiphase flow in the packed bed. During grid optimization, coarse and fine physical mesh domains were applied in the hydrodynamic prediction of trickle-bed reactor. After the grid adjustment in terms of number of cells, several spherical particle diameters were tested to study its effect on hydrodynamics and it was found that pressure drop is strongly influenced by the packing size. The Eulerian mutiphase model was then used in the computation of pressure drop and liquid holdup and over a wide range for the calculated flow regime as a function of gas and liquid flow rates, the CFD theoretical predictions were in good agreement for both hydrodynamic parameters. [Copyright &y& Elsevier]

Abstract: Hydrodynamic studies on trickle-bed reactors at non-ambient conditions overwhelmingly addressed coalescing systems despite numerous industrial applications concern the processing of foaming liquids for which engineering data are scantier. To fill this gap, the effects of temperature and moderate pressure (non-ambient conditions) are reported in this study on the shift of the transition from trickle-to-foaming-pulsing flow regimes, on the two-phase pressure drop, the liquid holdup, and the pulse frequency and velocity for Newtonian (air–cetyltrimethylammoniumbromide (CTAB)) foaming and non-Newtonian (air–0.25% CTAB–carboxymethylcellulose (CMC)) foaming systems. At constant superficial gas velocity, the trickle-to-foaming-pulsing flow transition boundary was observed at lower superficial liquid velocity in comparison to non-foaming systems. The transition boundary shifted towards higher gas and liquid superficial velocities with increasingly temperatures and pressures. The pulse frequency increased with temperature and/or pressure whereas the pulse velocity increased with temperature but it decreased with increasing pressure. Respective comparisons with the coalescing alter ego, namely, air–water and air–CMC/water systems, showed that Newtonian and non-Newtonian foaming systems behaved qualitatively similarly regarding the effects of temperature and pressure as the coalescing systems. [Copyright &y& Elsevier]

The hydrodynamics of a trickle-bed reactor subjected to fines-contaminated feed flows was studied under cyclic operation to assess whether or not periodic flows are able to reduce fines deposition and hence to extend reactor operational life under filtration conditions. The bed was subjected to single-pass kaolinkerosene suspension and air flows to study liquid–, gas– and alternating liquid–gas cyclic operation policies. It was found that fast- and slow-mode liquid cyclic operation policies were not be able to decrease either the pressure drop or the specific deposit; however, with adjusting carefully the parameters of liquid cyclic operation it would be possible to prolong the trickle-bed cycle life. The alternating gas–liquid cyclic feed was also found useful to reduce the levels of pressure drop and bed specific deposit and exhibited the same efficiency for both short and tall beds. [Copyright &y& Elsevier]

An Euler–Euler computational fluid model was developed successfully for the hydrodynamic prediction of a trickle-bed reactor (TBR) designed for advanced wastewater treatment facilities. Catalytic wet air oxidation of phenolic acids was simulated in a TBR by means of computational fluid dynamic (CFD) in the temperature range and pressures . The hydrodynamic model validation was accomplished through the comparison of simulated pressure drop and liquid holdup with experimental data from the literature. In a broad range of gas and liquid flows studied ( and ) at different operation conditions, CFD demonstrated the considerable effect of operating pressure in pressure drop, whereas a minor influence was detected for the liquid holdup. CFD runs were then performed for the catalytic wet air oxidation of aqueous phenolic acids solution. The reactor behaviour was analysed by means of total organic carbon profiles which reflected the influence of temperature, pressure, gas–liquid flows and initial pollutant concentration. [Copyright &y& Elsevier]

The wetting efficiency of liquid trickle flow over a fixed bed reactor has been measured for a wide range of parameters including operating conditions, bed structure and physico-chemistry of liquid/solid phases. This data bank has been used to develop a new correlation for averaged wetting efficiency based on five different non-dimensional numbers. Finally liquid/solid mass transfer has been determined in partial wetting conditions to analyse what are the respective effects of wetting and liquid/gas flow turbulence. These effects appear to be separated: wetting being acting on liquid/solid interfacial area while the liquid/solid mass transfer coefficient is mainly connected to flow turbulence through the interstitial liquid velocity. A correlation has been proposed for liquid/solid mass transfer coefficient at very low liquid flow rate. [Copyright &y& Elsevier]

A large experimental database has been established at IFP on the same experimental setup to measure simultaneously pressure drop and liquid holdup in packed bed reactor operated in trickle for a large range of operating conditions. The varying parameters are liquid viscosity and density, gas density, bed particle shape and size. The range for gas density range is particularly large (from 1.3 to ), thanks to the use of dense gas to simulate very high pressure conditions. This data bank has been first used to compare the prediction accuracy of the different models from the literature. Finally, the mechanistic model proposed by Attou et al. [1999. Modelling of the hydrodynamics of the cocurrent gas–liquid trickle flow through a trickle-bed reactor. Chemical Engineering Science 54, 785–802] has been improved by adding a new formulation for liquid film tortuosity in two-phase flow conditions. This model has been validated over the whole data range and the accuracy has been checked with data external to the data bank. The prediction accuracy is significantly increased when compared with the best available models for pressure drop and liquid retention in trickle flow reactors. [Copyright &y& Elsevier]

Process intensification using periodic operation of trickle bed reactors (TBRs) is still a long way from replacing conventional steady-state operation in industrial use, despite the numerous benefits described in the literature. Complex interactions between hydrodynamics, mass transfer and reaction phenomena make the design of periodically operated TBRs an almost insurmountable challenge. The development of hydrodynamic models able to provide reliable quantitative predictions of flow behaviour and possessing a sound physical basis, is an essential prerequisite for obtaining the necessary insights into this complexity. In this work, the two-phase pressure drop and dynamic liquid hold-up during max/min and on/off periodical operation were predicted using a model based on the relative permeability concept. In order to demonstrate the utility of this approach, a systematic investigation of the quantitative influence of the liquid-phase physical properties was carried out. The results obtained show that the modelling of the hydrodynamics in periodically operated TBRs using the relative permeability concept is feasible. By selecting suitable permeability parameters, unsteady-state hydrodynamics for different periodic operating modes can be predicted successfully. [Copyright &y& Elsevier]

Abstract: The effect of temperature on the hydrodynamics of trickle-bed liquid cyclic operation was studied. The trickling-to-pulsing transition in cyclic operation was compared to spontaneous transition in constant-throughput flow, as well as the corresponding pulsing flow regimes. The effect of temperature on and , and on liquid holdup and shock wave plateau time, was assessed for (non-)Newtonian liquids. The effects of temperature and superficial gas velocity on pulse velocity and pulse frequency were evaluated for pulsing flow in cyclic and constant-throughput flows. With increased temperature, shifted to higher gas and liquid velocities; however, the modified Charpentier–Favier diagram still adequately captured both and evolutions with temperature. [Copyright &y& Elsevier]

Abstract: Evidence exists that, depending on the manner in which a bed is pre-wetted, some regions in a trickle-bed reactor may not be utilised due to the absence of liquid flow in these regions. However, liquid-limited trickle-bed reactors are usually modelled as if all of the particles are actively wetted. An average wetting efficiency is then used to represent the whole reactor. Previous work shows that, depending on the underlying hydrodynamic characteristics of the bed, an average wetting efficiency may not be representative for the whole population. By means of a simple reactor model, it is shown that for liquid-limited reactors the particle wetting distribution can accurately be represented by the average wetting efficiency and the fraction of completely dry particles alone. Data regarding the latter parameter is reported for different flow conditions and is shown to correlate well with related parameters in literature. [Copyright &y& Elsevier]

Abstract: The hydrodynamic behavior of trickle bed and monolith reactors experiencing deposition from fines-contaminated feed flows was studied to assess the potential aptitude of monolith to be an alternative for disability of trickle bed reactors under filtration condition. Beds with equal collection surface area were subjected to single-pass suspension and air flows to study the influence of filtration for different gas and liquid superficial velocities. It was found that for equivalent gas and liquid velocities, specific deposits in trickle bed are up to two times more than in monolith. [Copyright &y& Elsevier]