Phenomenological approach to interpret the effect of liquid flow modulation in trickle bed reactors at the particle scale

This work analyzes the influence of liquid flow modulation on the behavior of a reaction occurring in a spherical porous particle within a trickle bed reactor. A single first-order reaction between a gaseous reactant and a non-volatile liquid reactant is considered. Non-steady-state mass balances for gas and liquid reactants are formulated and solved under isothermal conditions in order to focus the analysis on the mass transport effects. Dynamic reactant profiles inside the catalytic particle are obtained for different cycling and system conditions. The enhancement factor (ε) due to periodic operation is defined to evaluate the impact of induced liquid flow modulation on reaction rate. Influence of cycling and system parameters on the enhancement factor is also reported for a wide range of conditions. Experimental trends observed by several authors can be explained with this approach.     

Effect of partial wetting on liquid/solid mass transfer in trickle bed reactors

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.     

A sequential approach to modeling catalytic reactions in packed-bed reactors

A sequential modeling approach is proposed to simulate catalytic reactions in packed-bed reactors. The hydrogenation of alpha-methylstyrene and wet oxidation of phenol are selected as studied cases. The modeling scheme combines a reactor scale axial dispersion model with a pellet scale model. Without involving any fitting parameters, such an approach accounts for the non-linear reaction kinetics expression and different types of pellet-liquid wetting contact. To validate the developed modeling scheme and the parallel approach reported in the literature, the experimental observations for hydrogenation of alpha-methylstyrene to cumene have been employed. The predicted results by both approaches agree reasonably with the experimental data for both gas- and liquid-limited reaction. The proposed sequential approach was also used to simulate the dynamic performance of the reactor and pellets for the catalytic wet oxidation of aqueous phenol over a newly developed but rapidly deactivated catalyst (MnO₂/CeO₂). The simulation results for the catalytic wet oxidation process by both approaches were compared. The simulation describes the time evolution of the catalyst stability at different pellet points along the reactor axis. The performance of trickle beds and packed bubble columns over a range of operating conditions were also investigated, and packed bubble columns were found to achieve higher phenol conversion at the cost of more rapid catalyst deactivation.     

Experimental investigation of fast-mode liquid modulation in a trickle-bed reactor

Unsteady-state operation of trickle-bed reactors (TBRs) is a promising technique to improve reactor performances especially when mass transfer phenomena are rate controlling. Among the different techniques, fast-mode modulation of the liquid flow rate seems to be one of the most successful. In fact cycling the liquid flow rate at very low frequencies can induce the reactor to work at the high-interaction regime where mass and heat transfer phenomena are strongly enhanced. Fast-mode periodic operation, then, can be considered an extension of the natural high-interaction regime at a mean range of gas and liquid flow rate normally associated with trickling regime in steady-state conditions.Experimental tests have been performed in a TBR employing α-methyl styrene hydrogenation on Pd/C catalyst in unsteady-state conditions by “on-off” fast-mode liquid modulation. Results have been compared with the steady-state experiments at the corresponding average liquid flow rate, revealing a conversion rate improvement up to 60%. All experiments have been performed in isothermal conditions, so conversion improvement can be ascribed only to mass transfer increase and not to thermal effects. The variation of gas and liquid flow rates and liquid cycle parameters presented several important implications about the optimal working conditions.     

Liquid flow texture analysis in trickle bed reactors using high-resolution gamma ray tomography

Trickle bed reactor performance and safety may suffer from radial and axial liquid maldistribution and thus from non-uniform utilization of the catalyst packing. Therefore, experimental analysis and fluid dynamic simulation of liquid–gas flow in trickle bed reactors is an important topic in chemical engineering. In the present study for the first time a truly high-resolution gamma ray tomography technique was applied to the quantitative analysis of the liquid flow texture in a laboratory cold flow trickle bed reactor of 90 mm diameter. The objective of this study was to present the comparative analysis of the liquid flow dynamics for two different initial liquid distributions and two different types of reactor configurations. Thus, the hydrodynamic behavior of a glass bead packing was compared to a porous Al₂O₃ catalyst particle packing using inlet flow from a commercial spray nozzle (uniform initial liquid distribution) and inlet flow from a central point source (strongly non-uniform initial liquid distribution), respectively. The column was operated in downflow mode at a gas flow rate of 180 L h⁻¹ and at liquid flow rates of 15 and 25 L h⁻¹.     

A cellular automata model for liquid distribution in trickle bed reactors

A cellular automata model for liquid distribution studies in trickle bed reactors is presented. It is a potential tool for describing non-uniform distribution of gas and liquid in a trickle bed. This non-uniformity may arise from a wide range of potential sources, such as improper distribution of the feed, random or radial porosity variation, wall effect, partial wetting of the catalyst, and gas-liquid surface tension related effects. Axial and radial dispersion of the liquid flow are inherently included in the model, since the fundamental model probability parameters are directly related to the dispersion coefficients. The present model is extremely fast due to simple single-event modeling, and it is well suited for parallelization. Three examples of the model performance are shown. In the first a liquid jet spreading from a point source is followed, and in the second, effect of radial porosity profile to wall flow is examined. The third example illustrates the potential of the model to predict pulsing flow regime.     

Hydrodynamics of trickle bed reactors at high pressure: Two-phase flow model for pressure drop and liquid holdup, formulation and experimental validation

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.     

Modulation of liquid holdup along a trickle bed reactor with periodic operation

Temporal variations of the liquid holdup in a mini-pilot scale trickle bed reactor cold-mockup, induced by an ON-OFF liquid flow modulation strategy of operation, are explored at different axial positions. The reactor is packed with porous beads of γ-Al₂O₃ and the liquid holdup is approximately estimated with a conductimetric technique, using probes that mimic the packing. The effects of the liquid and gas superficial velocities, the bed depth and the cycling parameters, cycle period and split, on the liquid holdup modulation are examined for a wide range of conditions. For slow and intermediate cycle periods, the liquid holdup time dependence observed during the dry period is represented by an exponential function. The characteristic value of the decay is correlated to the examined variables. The correlation allows reconstruction of the liquid holdup time dependence along the column.     

CFD modeling of multiphase flow distribution in catalytic packed bed reactors: scale down issues

Flow maldistribution in either a bench-scale or commercial scale packed bed is often responsible for the failure of the scale down unit to mimic the performance of the large reactor. The modeling of multiphase flow in a bench-scale unit is needed for proper interpretation of reaction rate data obtained in such units. Understanding the mechanism of flow maldistribution is the first step to avoiding it. In order to achieve this objective, computational fluid dynamic (CFD) simulations of multiphase flow under steady state and unsteady state conditions in bench-scale cylindrical and rectangular packed beds are presented for the first time. The porosity distribution in packed beds is implemented into CFD simulation by pseudo-randomly assigned cell porosity values within certain constraints. The flow simulation results provide valuable information on velocity, pressure, and phase holdup distribution.     

Characteristics of liquid and tracer dispersion in trickle-bed reactors: Effect on CFD modeling and experimental analyses

The characteristics of mechanical dispersion of tracer and liquid are analyzed using CFD modeling and experimental results from the literature. The most significant differences are underlined and their impact is discussed further. When compared to uniform liquid distribution, the more complicated flow conditions in liquid source measurements are considered to have a significant effect on result analysis and should be paid more attention to. Modeling of mechanical dispersion of liquid using CFD is discussed. Finally, liquid source dispersion cases are simulated and the results are compared to the experimental liquid as well as tracer dispersion results.     

CFD simulation of dilute phase gas-solid riser reactors: Part I—a new solution method and flow model validation

A three-dimensional simulation of a dilute phase riser reactor (solid mass flux: ) is performed using a novel density based solution algorithm. The model equations consisting of continuity, momentum, energy and species balances for both phases, are formulated following the Eulerian-Eulerian approach. The kinetic theory of granular flow is applied. The gas phase turbulence is accounted for via a k-ε model. An extra transport equation describes the correlation between the gas and solid phase fluctuating motion. The solution algorithm allows a simultaneous integration of all the model equations in contrast to the sequential multi-loop solution in the conventional pressure based algorithms, used so far in riser simulations. The simulations show an unsteady behaviour of the flow, but a core-annulus flow pattern emerges on a time-averaged basis. The abrupt nature of the T type outlets causes a significant recirculation of gas and solid from the top of the riser. The flow near the outlets is highly non-symmetric and has a three-dimensional character. A significant decrease of the gas phase turbulence and particle granular temperature across the riser length is attributed to the presence of small particles, which is qualitatively consistent with the experimental data from literature.     

Prediction of pulsation frequency of pulsing flow in trickle-bed reactors using artificial neural network

Based on an extensive experimental database (946 measurements) set up from the literature published over past 30 years, a new correlation relying on artificial neural network (ANN) was proposed to predict the basic pulsation frequency of pulsing flow in the trickle-bed reactors. Seven dimensionless groups employed in the proposed correlation were liquid and gas Reynolds (Re_L,Re_G), liquid Weber (We_L), gas Froude (Fr_G), gas Stokes (St_G) and liquid Eötvös numbers and a bed correction factor (S_b). The performance comparisons of literature and present correlations showed that ANN correlation is significantly an improvement in predicting pulsation frequency with an AARE of 10% and a standard deviation less than 18%. The effects of the variables including the properties of fluid and bed, and flow rate of liquid and gas on pulsing frequency were investigated by ANN parametric simulations and the trends were compared with exiting experimental results that confirmed the coherence of the proposed method with the previous experiments.     

Cyclic variation of the liquid flow residence time in periodically operated trickle-bed reactors

The cyclic variation of the mean residence time of the liquid phase is investigated in a trickle-bed reactor operated with a liquid feed rate modulated in a periodic square wave pattern. Experiments made using a salt tracer method are compared to a residence time model, based on the concept of continuity shock waves. The model predicts accurately the mean residence times and their cyclic variation in case of a non-zero base liquid flow rate. A particular application of the model is the adjustment of the feed cycle parameters in order to obtain a constant residence time of the liquid, no matter the moment at which it enters the bed. This particular cycle duration depends, among others, on the feed rates, but also on the bed length.     

Flow through packed bed reactors: 2. Two-phase concurrent downflow

A model for the prediction of pressure drop and liquid holdup for trickling flow in packed bed reactors has been developed, based on the relative permeability concept. The relative permeabilities for gas and liquid as functions of corresponding phase saturations have been studied with 1300 newly measured data pairs of pressure drop and liquid holdup obtained for a wide range of commercially relevant operating conditions (including pressures up to 50 bar) as well as types of packing (both in terms of size and shape). The relative permeabilities are found to be solely the functions of corresponding phase saturations and it is shown that the functional form of the correlations developed, which are otherwise purely empirical by nature, has its roots in the physics of flow at the microscale level. The proposed model requires no prior experimental knowledge about the packed bed and is able to predict liquid holdup and pressure drop to within 5% and 20%, respectively, regardless of the type of packing or operating range investigated.     

Effect of gas feed flow and gas composition modulation on activated carbon performance in phenol wet air oxidation

Modulation of gas feed composition (air/N₂ cycling) and gas feed flow (on-off air cycling) was investigated in the catalytic wet air oxidation of phenol over activated carbon (AC). Fifty hours lasting experiments were conducted in a laboratory trickle bed reactor at 140-160 °C, 2 bar of oxygen partial pressure and different splits and periods to determine the set of cycling parameters that optimise the periodic reactor operation. To follow the dynamic behaviour of the phenol oxidation, temperature and conversion were continuously monitored by means of computerised data acquisition and automatic liquid sampling. Several long term tests over 144 h were also run using both periodic operating strategies to compare the activity and stability of AC with those obtained in a steady state operation at otherwise same conditions. The results show that, depending on the selection of split and period, modulation of the gas phase significantly improves the stability of AC compared to steady state operation, thereby performing a superior long term phenol conversion.     

Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.     

Comparison of fluidized bed flow regimes for steam methane reforming in membrane reactors: A simulation study

An important decision in the design of fluidized bed reactors is which of several flow regimes to choose. Almost all fluidized bed reactor models are restricted to a single flow regime, making comparison difficult, especially near the regime boundaries. This paper examines the performance of fluidized bed methane reformers with three models—a simple equilibrium model and two kinetic distributed models, based on different assumptions of varying sophistication. Membranes are incorporated to improve reactor performance. Eighteen cases are simulated for different flow regimes and membrane configurations. Predictions for the fast fluidization and turbulent flow regimes show that the rate-controlling step is permeation through the membranes. Bubbling regime simulations predict somewhat less hydrogen production than for turbulent and fast fluidization, due to the effects of interphase crossflow and mass transfer. Overall reactor performance is predicted to be best under turbulent fluidization operation. Practical considerations also affect the advantages, shortcomings and ultimate choice of flow regime.     

Preparation and characterization of H4SiW12O40@MIL-100 (Fe) and its catalytic performance for synthesis of 4,4'-MDA

The catalytic performance of commonly used heteropoly acids(H_3 PW_(12)O_(40). H_4 SiW_(12)O_(40) and H_3 PMo_(12)O_(40)) for the synthesis of 4,4'-methylenedianiline(4.4'-MDA) from aniline and formaldehyde was evaluated and the result showed that H_4 SiW_(12)O_(40) with moderate acid strength exhibited the best catalytic performance. Then H_4 SiW_(12)O_(40)@MIL-100(Fe) was prepared by encapsulating H_4 SiW_(12)O_(40) within the pores of MIL-100(Fe) to facilitate its recovery and reuse. The prepared H_4 SiW_(12)O_(40)@MIL-100(Fe) was characterized by means of FT-IR, N_2 adsorption-desorption, XRD, TG and then the catalytic performance was evaluated. The result showed that H_4 SiW_(12)O_(40) was highly dispersed in the pores of MIL-100(Fe), and both the Keggin structure of H_4 SiW_(12)O_(40)and the crystal skeleton structure of MIL-100(Fe) could be effectively preserved. Furthermore. H_4 SiW_(12)O_(40)@MIL-100(Fe) showed excellent catalytic performance under the following reaction conditions: a molar ratio of aniline to formaldehyde = 5, a mass ratio of catalyst to formaldehyde = 1.2, a reaction temperature of 120 ℃and a reaction time of 6 h. Under the above reaction conditions, the conversion of aniline was 41.1%, and the yield and selectivity of 4.4'-MDA were 81.6% and 79.2%, respectively. Unfortunately, an appreciable loss in the catalytic activity of the recovered H_4 SiW_(12)O_(40)@MIL-100(Fe) was observed because of the blocking of the pores and the change of the acidity resulted from the adsorption of alkaline organics such as aniline and 4,4'-MDA.The adsorbed alkaline organics could be cleaned up when the recovered catalyst was washed by methanol and DMF. Then the catalyst was effectively reused up to three cycles without much loss in its activity.     

Numerical assessment of diffusion-convection-reaction model for the catalytic abatement of phenolic wastewaters in packed-bed reactors under trickling flow conditions

Computational fluid dynamics (CFD) modeling of trickle-bed reactors with detailed interstitial flow solvers has remained elusive mostly due to the extreme CPU and memory intensive constraints. Here, we developed a comprehensible and scalable CFD model based on the conservative unstructured finite volume methodology to bring new insights from the perspective of catalytic reactor engineering to gas-liquid-solid catalytic wet oxidation. First, the heterogeneous flow constitutive equations of the trickle bed system have been derived by means of diffusion-convection-reaction model coupled within a Volume-of-Fluid framework. The multiphase model was investigated to gain further evidence on how the effect of process variables such as liquid velocity, surface tension and wetting phenomena affect the overall performance of high-pressure trickle-bed reactor. Second, as long as the application of under-relaxation parameters, mesh density, and time stepping strategy play a major role on the final corroboration, several computational runs on the detoxification of liquid pollutants were validated accordingly and evaluated in terms of convergence and stability criteria. Finally, the analysis of spatial mappings for the reaction properties enables us to identify the existence of relevant dry zones and unveil the channeling phenomena within in the trickle-bed reactor.     

Simulation of activity loss of fixed bed catalytic reactor of MTO conversion using percolation theory

In this investigation, a reactor model for prediction of the deactivation behavior of MTO's porous catalyst in a fixed bed reactor is developed. Effect of coking on molecular transport in the porous structure of SAPO-34 has been simulated using the percolation theory. Thermal effects of the reaction were considered in the model and the temperature profile of the gas stream in the reactor was predicted. The predicted loss in catalyst activity with time-on-stream was in very good agreement with the experimental data. The resulting coke deposition and gas temperature profiles along the length of reactor suggested a reaction front moving toward the outlet of the fixed bed reactor at the operating experimental conditions of 1 h⁻¹ and 723 K for methanol space velocity and inlet temperature, respectively. Effects of space time, coordination of Bethe network, and effective diffusivity of component in reaction mixture on the reactor performance are presented.