Catalyst Wetting Research Articles

Hydrogenation of Nitrobenzene in Trickle Bed Reactor over Ni/Sio2 Catalyst

Trickle bed reactor was used to study the hydrogenation of nitrobenzene over Ni/SiO2 catalyst. The catalyst was prepared using the Highly Dispersed Catalyst (HDC) technique. Porous silica particles (capped cylinders, 6x5.5 mm) were used as catalyst support. The catalyst was characterized by TPR, BET surface area and pore volume, X-ray diffraction, and Raman Spectra. The trickle bed reactor was packed with catalyst and diluted with fine glass beads in order to decrease the external effects such as mass transfer, heat transfer and wall effect. The catalyst bed dilution was found to double the liquid holdup, which increased the catalyst wetting and hence, the gas-liquid mass transfer rate. The main product of the hydrogenation reaction of nitrobenzene was aniline. Reaction operating conditions, i.e., temperature, liquid flow rate, and initial feed concentration were investigated to find their influences on the conversion and rate of nitrobenzene hydrogenation. Under normal conditions without bed dilution, the system was mass transfer controlled. In the diluted reactor, on the other hand, the resistance of mass transfer was nearly absent and the system became under surface kinetic control. The catalyst showed significant deactivation during the reaction period due to the adsorption of intermediate amine products on the surface of the catalyst. The kinetic study revealed that the reaction is zero order with respect to nitrobenzene concentration for the range of concentration between 0.58 to 1.17 mol/L while it was of positive order for the initial concentration less than 0.58 mol/L

Hidden parameters for electrochemical carbon dioxide reduction in zero-gap electrolyzers

The electrochemical reduction of CO 2 from flue gas or direct air capture to relevant syngas mixtures is a promising route toward mitigation of environmental pollution and production of bulk chemicals and fuels. Among the different cell types, gas-fed zero-gap electrolyzers are promising as energy-efficient and scalable devices. We herein investigate the influence of operational parameters related to cathode water management, such as cell orientation, gas humidification, and cathode compression onto CO 2 R. By a stepwise optimization of our not yet fully optimized electrolyzer, stability could be improved by a factor of 3 up to 10 h at 3 V and 300 mA cm −2 . Faradic efficiency for CO after 2 h of electrolysis was increased from 14% to over 60%. Controlling the water management is a key parameter as high water input leads to flooding of the electrodes, whereas lower values decrease the performance of the anion exchange membrane and reduce catalyst wetting. • Water management is a key parameter for efficient CO 2 R • Operational parameters improve the performance of the electrolyzer significantly • Longtime stability can be improved by a factor of 3 Water management in gas-fed electrolyzers is a key parameter toward a stable and efficient conversion of CO 2 to syngas. Hoof et al. herein investigate less commonly studied but crucial operational parameters related to the water management in a CO 2 electrolyzer, including electrolyzer orientation, humidification of the gas stream, and cathode compression.

Deactivation of Fe-N-C catalysts during catalyst ink preparation process

The membrane electrode assembly (MEA) is a core component of low-temperature fuel cells. The first step of MEA manufacturing is the preparation of a catalyst ink suspension in which the catalyst powder is homogeneously dispersed in a liquid solvent through mechanical or sonic agitation. In this work, we have studied the effects of catalyst dispersion in water or alcohol solutions and subsequent drying processes on the physicochemical properties of Fe-N-C catalysts and their electrocatalytic oxygen reduction activities. We find that dispersing the model Fe-N-C catalyst comprising only FeNxCy moieties in water and subsequent drying treatment change neither its bulk structure nor surface composition, as indicated by various spectroscopic measurements before and after treatment. However, zeta potential measurements, which are very sensitive to the chemistry of functionalities present on the carbon surface, reveal that the Fe-N-C catalyst becomes slightly more acidic, and that the change in their acido-basicity is magnified with a) increasing treatment temperature and b) repetitions of a same wetting/drying treatment. This small change in the surface acido-basicity of the Fe-N-C catalyst results in a measurable and reproducible decrease in its electrocatalytic activity, which shows a positive correlation with the zeta potential changes measured at pH = 1. Observed on the Fe-N-C catalyst but not on Pt/C, it is surmised that the electrocatalytic activities of the oxygen-reducing FeNxCy moieties are influenced by the surface chemistry of the carbonaceous support. Since catalyst wetting and drying processes are essential for MEA fabrication for fuel cells, these results suggest that careful attention should be paid to the conditions employed to prepare and dry catalytic inks for the family of Fe-N-C catalysts in order to obtain their highest possible ORR activity.

Heterogeneous Modeling Approach for Gas‐Limited Reactions in an Inclined Rotating Fixed Bed Reactor

AbstractA heterogeneous modeling approach for an inclined rotating fixed bed reactor with concentric internal tube is introduced. The novel reactor is designed to intensify the mass transfer of gas‐limited heterogeneous catalyzed reactions by intermittent catalyst wetting, which is enabled exposing the packed bed to rotation and inclination. A simulation study for the hydrogenation of α‐methylstyrene is presented. In particular, the influence of period length and different wetting‐draining cycles on the space‐time yield of the reactor is analyzed.

Effect of reactor configuration on performance of vacuum gas oil (VGO) hydrotreater: Modelling studies

While kinetics is independent of either scale or configuration of hydrotreating reactor, hydrodynamics of reactor depend on both. The hydrodynamics of reactor which comprise of phase mass-transfer, catalyst wetting and pressure drop affect its performance significantly and should be addressed adequately while deciding on configuration or scale up issues. This study evaluates and compares the performance of different configurations of commercial VGO hydrotreater by employing mathematical model encompassing kinetics and hydrodynamics. Two configurations have been studied:•Conventional trickle-bed reactor, subdivided into (a) beds in parallel and (b) beds in series.•Pre-saturated one liquid flow (POLF) reactor, subdivided into (a) reactor with single pre-saturator (POLF-SP) and (b) multiple reactors in series with intermittent pre-saturators (POLF-MP).The performance of conventional reactor in series/parallel is found to be superior to POLF configurations. The inferior performance of POLF configurations is attributed to mixed flow behaviour due to recycle as against plug flow behaviour in conventional trickle-bed reactors.

Hydrodynamics of co-current two-phase flow in an inclined rotating tubular fixed bed reactor — Wetting intermittency via periodic catalyst immersion

The hydrodynamics of an inclined rotating tubular fixed bed reactor operated with gas–liquid co-current downflow are studied. Reactor inclination is applied to force phase segregation, while the superimposed rotation of the reactor results in a wetting intermittency via periodic catalyst immersion. The fixed bed is clamped to avoid abrasion of the catalyst. The inclined rotating reactor is presented as a new reactor concept for process intensification of heterogeneous catalytic reactions requiring enhanced mass transfer of the gaseous phase and partial catalyst wetting. Four different flow regimes with stratified, sickle, annular and dispersed flow patterns are determined experimentally by applying a compact gamma-ray computed tomography system. The effects of (i) gas and liquid superficial velocities, (ii) inclination angle and rotational velocity of the reactor and (iii) physico-chemical properties of the liquid phase on the occurrence of the flow regimes are investigated. The results of these investigations are illustrated with flow maps. In addition, pressure drop and liquid saturation depending on the operating conditions are shown.

Successful Scale-up of an Industrial Trickle Bed Hydrogenation Using Laboratory Reactor Data

This work validates the appropriate application of chemical reaction engineering principles in the successful design of a full-scale industrial trickle bed reactor for a proprietary hydrogenation reaction over a palladium catalyst. After identifying an effective catalyst formulation in a continuous laboratory scale trickle bed reactor, the project team used the same small-scale reactor to generate kinetic data for scale-up. The scale factor from the laboratory to the final design was about 3 × 106. The development effort identified and resolved three important problems: (1) incomplete catalyst wetting of the small catalyst bed, even though the catalyst was diluted with inert fines; (2) lower than economically attractive catalyst productivity; and (3) catalyst deactivation.

Performance of Phosphonated Hydrocarbon Ionomer in the Fuel Cell Cathode Catalyst Layer

Inexpensive and environmentally friendly electrolyte polymers that can be operated at higher temperatures and drier conditions are highly interesting for PEM fuel cells for automotive, portable power and stationary electricity generation applications. In this study an ionomer based on polysulfone grafted with poly(vinylphosphonic acid) (PSUgPVPA) in the cathode Pt/C catalyst layer was electrochemically characterized and compared to Nafion. The performance at different levels of humidity at 80°C was evaluated by polarization measurements, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The results show that the performance of the PSUgPVPA-based cathode catalyst layer is comparable to that of Nafion at 100% relative humidity (RH) but with some instabilities. However, at drier conditions significant losses of performance for the PSUgPVPA-based cathode was observed. This could be an effect of catalyst poisoning by the ionomer interfering with ORR. However, the concomitant decrease of the electrochemical surface area, double layer capacitance and increased imaginary impedance, indicate that the poorer performance at low humidity is mainly an effect of reduced catalyst wetting by the ionomer in combination with the decreased proton conduction in the ionomeric phase.

Vapor–Liquid Equilibrium (VLE)-Based Modeling for the Prediction of Operating Regimes in a Heavy Gas Oil Hydrotreater

In a pilot-plant trickle-bed hydrotreating reactor, ideal operating regimes (plug flow, full catalyst wetting, and absence of reactor wall effects) are desirable to generate reliable, reproducible, and representative data for kinetics studies and commercial scale-up. In this study, the operating regimes of a heavy gas oil pilot-plant hydrotreating reactor were investigated under operating conditions of commercial interest using real petroleum feedstocks. Vapor–liquid equilibrium experiments and calculations were performed to accurately predict the fluid flows and the physical/transport properties of the vapor and liquid phases. These properties were subsequently used to predict the operating regimes under various operating conditions and to generate a map of operating conditions within which the desired operating regimes can be maintained.

Hydrodynamics characteristics of HDS trickle bed test reactor

Abstract Laboratory-scale trickle bed reactors are often used in testing of catalyst life time and activity. One of the important processes is represented by hydrodesulphurization (HDS) of motor fuels and petrochemical fractions. The study of stability of catalysts activity can be influenced both reactions kinetics, hydrodynamics and mass transfer phenomena. In case of ideal behaviour, the liquid could report piston flow. Non-ideal liquid flow can lead to incomplete catalyst particle wetting, which results in poor utilization of the catalytic surface of the particles. It may also expected to occur stagnant zones and imperfect heat transfer and overheating of the catalytic particles. For limitation of this negative phenomena should be diluted catalytic bed by small inert particle provide more contact area. The aim of this study was to determine degree of the effect of hydrodynamic conditions in model trickle bed reactors by residence time distribution method. Objective was to evaluate liquid holdup, axial dispersion and pressure drop of reactor bed consisting of catalyst particles diluted by inert fines to obtain data for process description by PD mathematical model. Hydrodynamic experiments were carried out in model glasses reactor with I.D 30.4 mm. Catalyst –trilobe extrudes of 1.3 mm O.D. was diluted by fine grain SiC 0.1-0.15 mm to obtained complete catalyst wetting and uniform liquid distribution. In presumed range of liquid and gas flow rates, air and water mass flow corresponded to values of hydrogen and hydrocarbons mixture in high-pressure reactor for the study of HDS catalysts. Gas flow was measured by MFC and liquid flow rate was checked by weighing, pressure drop was measured by pressure probe. Along the bed were located three electrodes to measure conductivity signal of responses to impulse of KCl solution. Data acquisition was made by four channel chromatography Clarity SW/HW, with subsequent evaluation by program in Matlab. Simultaneously, the reproducibility of three bed sections of trickle bed catalyst reactor formation of bed diluted by fines was tested by two methods. Effective mean residence time, liquid holdup and axial dispersion were evaluated for each. The range of experimental operation conditions (gas and liquid flow rates) negligible influenced by hydrodynamics was evaluated both experimentally and by hydrodynamics PD model.

Modelling and simulation of trickle‐bed reactors using computational fluid dynamics: A state‐of‐the‐art review

AbstractTrickle‐bed reactors (TBRs), which accommodate the flow of gas and liquid phases through packed beds of catalysts, host a variety of gas–liquid–solid catalytic reactions, particularly in the petroleum/petrochemical industry. The multiphase flow hydrodynamics in TBRs are complex and directly affect the overall reactor performance in terms of reactant conversion and product yield and selectivity. Non‐ideal flow behaviours, such as flow maldistribution, channelling or partial catalyst wetting may significantly reduce the effectiveness of the reactor. However, conventional TBR modelling approaches cannot properly account for these non‐ideal behaviours owing to the complex coupling between fluid dynamics and chemical kinetics. Recent advances in the application of computational fluid dynamics (CFD) to three‐phase TBR systems have shown promise of achieving a deeper understanding of the interactions between multiphase fluid dynamics and chemical reactions. This study is intended to give a state‐of‐the‐art overview of the progress achieved in the field of CFD simulation of TBRs over the past two decades. The fundamental modelling framework of multiphase flow in TBRs, advances in important constitutive models, and the application of CFD models are discussed in detail. Directions for future research are suggested.

Regulation of Catalytic Reaction via Vapor-Phase Condensation. 1. Experimental Study on the Reaction Rate Hysteresis Loops

To experimentally verify the existence of reaction rate hysteresis in the gas phase operation due to capillary condensation, benzene hydrogenation was carried out in a fixed bed under isothermal condition. Through changing the flow rate of benzene and cyclohexane mixture in a sequence from low to high and then vice versa, a group of distinctive conversion profiles was obtained. By plotting the reaction rate against the relative vapor pressure, hysteresis loops in reaction rate are confirmed to exist. It was observed the reaction rate kept nearly constant in the region of p/p0 0.8. In providing an intrinsic understanding on the influence of vapor condensation on the reaction rate of a partially wetted catalyst, the reaction rate and internal effectiveness factor are plotted against the catalyst wetting fraction, from which a new correlation between the overall reaction rate and catalyst wetting fraction was proposed.

Experiment based model development for the enzymatic hydrolysis in a packed-bed reactor with biphasic reactant flow

A systematic experiment based development of a kinetic model for the immobilized lipase (Novozym 435) catalyzed hydrolysis of a short-chain fatty acid methyl ester mixture in a biphasic organic–aqueous system was conducted. It included kinetic experiments to find the influence of the operating conditions on the reaction rate, the examination of the catalyst wetting and of the liquid–liquid phase behavior, as well as the formulation of an integral mathematical model for the used packed-bed reactor assembly. From experiments it was found that the initial reaction rate was independent of the total water content and directly proportional to the catalyst to organic phase mass ratio. The produced fatty acids and methanol reduced the reaction rate reversibly. During the experiments no negative impact on the enzyme activity was observed for temperatures <65 °C. The organic phase retained in the particle pores and around the particle prevented the aqueous phase from penetrating it. The influence of the aqueous phase pH-value on the reaction rate was negligible. A good description of the liquid–liquid phase behavior was obtained from a UNIFAC (Dortmund) and literature data based UNIQUAC model. For the reaction rate and the chemical equilibrium activity based expressions were chosen. The very good consistency of the resulting kinetic model with the experimental data confirmed the ability of the chosen approach to elucidate the relationships between the singular effects simultaneously occurring in the reaction system.

Hydrodynamics of cocurrent two‐phase flows in slanted porous media—Modulation of pulse flow via bed obliquity

AbstractA packed bed operated with descending gas–liquid cocurrent flows in slanted configuration to force trickle flow pattern to segregate due to gravity was studied, in addition to dependences to inclination angle of liquid saturation and pressure drop. Inception of pulse flow regime could take place regardless of inclination angle and the transition from segregated/trickle regime to pulse flow regime was experimentally determined. The Grosser et al. flow regime transition model was modified by considering only axial components of the gas and liquid superficial velocities to predict the slant‐dependent shifts in transition from segregated/trickle to pulse flow and was found to agree with experimental data. Pulse flow regime at different inclinations was characterized with respect to frequency, velocity, and shape of pulses. Bed obliquity was unveiled as a new artifice to pulse flow modulation with possible prospects for catalytic reactions requiring antagonistically high‐interaction regime mass transfer coefficients and partial catalyst wetting. © 2010 American Institute of Chemical Engineers AIChE J, 2010

Numerical Studies of Catalyst Wetting and Total Organic Carbon Reaction on Environmentally Based Trickle-Bed Reactors

This work is devoted to the computational fluid dynamics (CFD) simulation of trickle-bed reactors (TBRs) with environmentally based applications on advanced wastewater remediation technologies. Recently, TBRs have assumed a major role in industrial wastewater treatment plants being envisaged as a breakthrough paraphernalia to move forward superseded decontamination processes. According to the literature, the generous research in environmental reaction engineering has indicated that scale-up of TBRs is erroneous if one considers isothermal operation and uses either a pseudohomogeneous or a heterogeneous model with plug flow for gas and liquid phases. Even though an axial dispersion model may account for liquid distribution nonuniformity, the reaction parameters are strongly dependent on the reactor fluid dynamics. In our case-study, first we developed a volume-of-fluid CFD framework which accounts for the surface tension and wall adhesion effects. A three-dimensional computational domain was employed to in...

Trickle-Bed Laboratory Reactors for Kinetic Studies

In this paper, different phenomena affecting laboratory trickle-bed reactors are carefully reviewed. Before focusing on laboratory scale, some general considerations about trickle-bed reactors are discussed: regimes, industrial uses, etc. The main factors a laboratory trickle bed reactors are presented and detailed. The greatest six factors include: homogeneity of the catalyst bed, catalyst wetting, axial dispersion, channeling, isothermicity and mass transfer. These factors are reviewed and different criteria, suggested in the literature, are given in order to minimize the influence of these six factors; for each criterion, a simple application is presented. Finally, the application of all the criteria given in the paper is based on a hypothetical example (hydrodesulfurization).

Measurements and Modeling of Wetting Efficiency in Trickle-Bed Reactors: Liquid Viscosity and Bed Packing Effects

An experimental parametric study on wetting efficiency is reported which evaluates the influence of liquid viscosity, as well as the effect of particle size(/shape) and bed porosity (εB), separately. A 10-fold increase of liquid viscosity improves slightly catalyst wetting (by about 10%), while an increase of either bed porosity or particle size has the opposite effect. Wetting efficiency is reduced by about 0.1 for an increase of εB from 0.38 and 0.40 due to a change of particle size from 1.8 to 7 × 10−3 m while the decrease is only 0.05 for a similar variation of εB (0.38−0.41) with the same particles. The effect of particle shape (cylindrical/trilobe extrudates or spheres) appears very small in the investigated conditions. A new correlation for wetting efficiency is proposed, using a bounded function and only three dimensionless groups (liquid Froude and Morton numbers and bed porosity). This correlation is able to predict wetting efficiency with a very good precision on a large database, provided wetting liquids are used. Adding fines in the fixed bed is also examined, and its positive effect can be correlated with the size ratio between catalyst particles and fines.

Design, scale up and safe piloting of thymol hydrogenation and menthol racemisation

The hydrogenation of thymol and the racemisation of a menthol mixture were investigated as part of the development of novel continuous process for the production of l-menthol. The key innovation in this l-menthol process is enzymatic enantioselective resolution of l-menthol from 8 menthol isomers mixture. This continuous biocatalytic step has significant impact on process economy. The productivity of the enzyme (mass of l-mentylacetate produced/mass enzyme) is a key performance measurement of this process. The demonstration of the required enzyme productivity was therefore a key objective of the process piloting. The effect of possible side products and impurities on the process also needed to be tested and therefore five cycles of piloting were performed using approximately 500 kg of feed into resolution in each cycle. The required quantities of the racemic menthol mixture were obtained by thymol hydrogenation in pilot plant trickle-bed reactor. Both hydrogenation and racemisation were developed as batch processes at laboratory scale and tested at bench scale in a continuous trickle-bed reactor system and scaled up to a pilot scale trickle-bed reactor. A bench scale trickle-bed reactor was designed, constructed and extensively used in the optimisation of the hydrogenation of thymol to racemic menthol. The process was successfully operated for more than 660 h in a bench scale reactor and subsequently scaled up to a pilot scale trickle-bed reactor. The pilot scale experiments were used to verify process scale-up and generate material for menthol resolution and downstream processing. Both the bench and pilot scale reactors were operated as adiabatic systems and the operation was found to be safe and reliable. Safety problems leading to formation of undesirable hot spots were experienced as consequence of insufficient catalyst wetting, high thymol concentration in the feed (above 40 wt.%) and because of possible equipment, operation and power failure.

Vapor−Liquid Equilibrium Study in Trickle-Bed Reactors

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.

Pressure drop hysteresis in trickle bed reactors

An understanding of the hydrodynamics of trickle bed reactors (TBR) is essential for their design and prediction of their performance. Flow variables, packing characteristics, physical properties of fluids and operation modes influence the behavior of the TBR. The existence of multiple hydrodynamic states or hysteresis (pressure drop, liquid holdup, catalyst wetting, gas–liquid mass transfer) is due to the different flow structures in the packed bed and can be attained by a set of different operating procedures. Experiments were performed to study the effect of liquid and gas velocity, liquid surface tension, liquid viscosity and the particle diameter of the packing on two-phase pressure drop hysteresis. The parallel zone model for pressure drop hysteresis in the trickling flow was used for analysis of experimental data and flow structure. Theoretically predicted pressure drop hysteresis loop is in satisfactory agreement with experimental data.