Xylene Isomerization in the Liquid Phase Using Large‐Pore Zeolites

Three large‐pore zeolites, Beta with Si/Al ratios of 25 and 35 and Mordenite with an Si/Al ratio of 30, were studied in the conversion of o‐xylene at 493 K. Maximum conversion was achieved by the catalyst with the highest Si/Al ratio due to faster diffusion of the isomer inside the zeolite channels because of the lower acidity of the solid even with larger crystal size. A kinetic study was then carried out over this catalyst between 473 and 513 K in a batch reactor in the liquid phase. The activation energies obtained do not indicate the presence of diffusional constraints towards any isomer. Finally, the kinetic model obtained was simulated in a fixed‐bed reactor and compared to ZSM‐5 in the temperature range from 493 to 533 K. An increment in p‐xylene production of 20 % on average was obtained.     

Pulse experiments over catalyst beds: A window of measurable reaction rate coefficients

The elimination of concentration profiles in porous catalyst pellets is not straightforward for transient kinetic experiments since profiles may arise from limited diffusion rates, but also from accumulation in the pellets. For TAP pulse experiments, dominated by Knudsen diffusion on the reactor scale, intrapellet diffusion is typically instantaneous for industrial catalysts. The Knudsen diffusivity for beds of porous pellets can be calculated from its value for non-porous pellets by accounting for the intrapellet porosity. The window of measurable rate coefficients is governed by a Damköhler number of the first type. A criterion is derived for experimental conditions, that allow intrinsic kinetic measurements with porous catalysts. The size of the admitted pulse should be sufficiently large to avoid pulse depletion by adsorption.     

Selective Hydrogenation of 1‐Butene Rich Cuts: the Impact of the Intraparticle Diffusion Limitations on the Selectivity

The impact of intraparticle diffusion limitations on the selectivity of an industrial reactor for selective hydrogenation of 1‐butyne and 1,3‐butadiene contained in 1‐butene rich cuts was evaluated. To this end, a simple model of a trickle‐bed reactor was employed and actual process operating conditions were chosen. A kinetic model was chosen whose parameters correspond to a commercial catalyst. These parameters were calculated from experiments conducted under industrial operating conditions. The complex diffusion and reaction phenomena occurring inside catalyst pellets placed at different depths of the reactor are comprehensively described. 1‐Butene losses in the range 20–30 %, which are usual in commercial plants, were predicted. It was concluded that the operating pressure is crucial for enhancing process selectivity.     

Heterogeneous Catalytic Reactor Design with Non‐Uniform Catalyst Considering Shell‐Progressive Poisoning Behavior

A novel methodology has been developed to design an optimum heterogeneous catalytic reactor, by considering non‐uniform catalyst pellet under shell‐progressive catalyst deactivation. Various types of non‐uniform catalyst pellets are modelled in combination with reactor design. For example, typical non‐uniform catalyst pellets such as egg‐yolk, egg‐shell and middle‐peak distribution are developed as well as step‐type distribution. A progressive poisoning behavior is included to the model to produce correct effectiveness factor from non‐uniform catalyst pellet. As opposed to numerical experiment with limited type of kinetic application to the model in the past, this paper shows a new methodology to include any types of kinetic reactions for the modeling of the reactor with non‐uniform catalyst pellet and shell‐progressive poisoning. For an optimum reactor design, reactor and catalyst variables are considered at the same time. For example, active layer thickness and location inside pellet are optimised together with reactor temperature for the maximisation of the reactor performance. Furthermore, the temperature control strategy over the reactor operation period is added to the optimization, which extends the model to three dimensions. A computational burden has been a major concern for the optimization, and innovative methodology is adopted. Application of profile based synthesis with the combination of SA (Simulated Annealing) and SQP (Successive Quadratic Programming) allows more efficient computation not only at steady state but also in dynamic status over the catalyst lifetime. A Benzene hydrogenation reaction in an industry scale fixed‐bed reactor is used as a case study for illustration.     

OPTIMIZING THE FEED CONDITIONS IN A DIMETHYL ETHER PRODUCTION PROCESS TO MAXIMIZE METHANOL CONVERSION USING A HYBRID FIRST PRINCIPLE NEURAL NETWORK APPROACH

Modeling is a fundamental step in plant optimization and simulation. In this work, a new technique for modeling a gas-solid heterogeneous fixed-bed reactor is developed. Gas diffusion into the solid catalyst pellets requires solving the mass balance equations inside the catalyst. The computational load needed can be quite time-consuming due to system complexities and nonlinearities. This bottleneck prevents on-line optimization of the process. In this work, a trained three-layer neural network model is used to replace major parts of these computations. The model is then incorporated within the overall model of an adiabatic fixed-bed reactor to produce dimethyl ether (DME) from methanol dehydration over solid acidic catalysts. The performance of the reactor simulated using this procedure indicated good agreement with its experimental operation. Then an optimizer is employed to determine the best feed conditions. The proposed strategy can be applied to any heterogeneous fixed-bed reactor.     

Effective kinetics of 3-hydroxypropanal hydrogenation over Ni/SiO2/Al2O3 catalyst

Concentration-time curves at 45-80°C and 2.60-5.15 MPa were measured, in a spinning basket reactor, to model the hydrogenation of 3-hydroxypropanal (HPA) to 1,3-Propanediol (PD) over Ni/SiO₂/Al₂O₃ catalyst pellets. A mathematical model whose parameters are effective diffusion coefficients and intrinsic kinetic parameters is proposed to describe this process and to avoid the dependence of the model parameters on the catalyst particle size. This model fits the experimental data reasonably and allows a reliable scale up of this process in comparison to other empirical models.     

Diffusion of asphaltene molecules through the pore structure of hydroconversion catalysts

When hydrotreatment of heavy cuts by heterogeneous catalysis is carried out in liquid phase, the molecules’ state of containment in the porous network is very high. Moreover, at that state of containment, the size of asphaltenes and resins, from various origins, can be the cause for the different hydrotreatment yields. Consequently, volume constraints are added to the kinetic and thermodynamic ones (adsorption equilibrium): a given species can penetrate in the solid only if the necessary volume is available within the network.Hindered diffusion and adsorption of asphaltene molecules inside hydrotreatment catalysts’ carriers were studied. The system's kinetics was investigated by visible absorption spectroscopy. Asphaltenes were prepared by n-heptane separation and solubilized in toluene at a known concentration and put in contact with a given amount of catalyst support. The evolution of the concentration in the asphaltene's solution was followed, as a function of time, by measuring the absorbance of a monochromatic visible radiation (750 nm) through the asphaltene suspension.A model based on the “Stefan–Maxwell” equations, that takes into account the volume constraints by the Fornasiero's formulation, which supposes that the molecules collide only by equivalent volume, was developed. The parameters estimation has been performed and discussed. The results show that the diffusional limitations are important in the catalyst used for heavy oil hydrotreatment and the asphaltene adsorption is very strong in this type of material.     

OPTIMUM REACTOR DESIGN IN METHANATION PROCESSES WITH NONUNIFORM CATALYSTS

A generic methodology is developed to design a heterogeneous catalytic reactor for methanation processes. For the optimization of a heterogeneous catalytic reactor, nonuniform catalyst pellets such as a layered catalyst are considered with respect to reaction type, reactor performance, and component distribution inside the catalyst. Heterogeneous uniform and nonuniform catalyst models were developed to analyze the effect of mass and heat transfer between both bulk phase and catalyst surface and inside a catalyst pellet. Then, concentration profiles of hydrogen and carbon monoxide in the catalyst pellet and along the reactor axis were obtained by analyzing simulation results. It was shown that the application of different types of nonuniform catalyst pellets at a certain number of separate zones within a reactor could produce higher catalyst performance than a reactor with uniform catalyst. Furthermore, it proved a significant decrease of catalyst deactivation behavior such as coking and sintering. Layered catalysts were optimized to maximize an overall reactor performance over the catalyst lifetime, achieving capital cost reduction characterized by reactor size, catalyst amount, and degree of catalyst deactivation. Last, temperature control throughout the reactor operating periods was strategically planned for a reactor operation with distribution of nonuniform catalyst pellets. This methodology can also be usefully applied to the design of heterogeneous catalytic reactors for other processes such as hydro-treating process and cracking process.     

Interaction of kinetics and internal diffusion in complex catalytic three-phase reactions: Activity and selectivity in citral hydrogenation

Kinetics and mass transfer effects were studied for the complex catalytic liquid-phase hydrogenation in a semi-batch reactor, where finely dispersed and larger catalyst particles were used. Citral was used as a model molecule. The intrinsic kinetics was determined with crushed catalyst particles at and at pressures ranging from 10 to 40 bar. A kinetic model was proposed and successfully fitted to the experimental data. In order to elucidate the influence of internal diffusion on the selectivity and activity in complex reactions, citral hydrogenation was performed with larger catalyst pellets, in a pressurized autoclave equipped with a catalyst basket. As expected, the activity decreased with increasing catalyst particle size. The product distribution was shifted from the primary hydrogenation product (citronellal) to the fully hydrogenated end product (3,7-dimethyloctanol) as the catalyst particle size was increased. The concentrations of the secondary hydrogenation products were minor throughout the experiment. A complete reaction-diffusion model was developed for large pellets and complex reactions systems.     

Heterogeneous modeling of an autothermal membrane reactor coupling dehydrogenation of ethylbenzene to styrene with hydrogenation of nitrobenzene to aniline: Fickian diffusion model

Coupling of reactions in catalytic membrane reactors provides a route to process intensification. Dehydrogenation of ethylbenzene and hydrogenation of nitrobenzene form a promising pair of processes to be coupled in a membrane reactor. The heat released from the hydrogenation side is utilized to break the endothermality on the dehydrogenation side, while hydrogen produced on the dehydrogenation side permeates through the hydrogen-selective membranes, enhances the equilibrium conversion of ethylbenzene and reacts with nitrobenzene on the permeate side to produce aniline. Mathematical reactor models are excellent tools to evaluate the extent of improvement before experiments are set up. However, a careful selection of phenomena considered by the reactor model is needed in order to obtain accurate model predictions.To investigate the effect of the intraparticle resistances on the performance of the cocurrent configuration of the coupling reactor, a heterogeneous fixed bed reactor model is developed with Fickian diffusion inside the catalyst pellets. For the condition of interest, the styrene yield is found to be 82% by the homogenous model, 73% by the heterogeneous model for isothermal pellets, and 69% by the heterogeneous model with non-isothermal pellets. Hence, the homogeneous model overestimates the yield by 5–15% of their actual values.     

Modelling of a vacuum residue hydrocracking in an industrial slurry phase reactor

The upgradation of the bottom of the barrel has gained much interest across the refineries due to severe environmental rules, limitations of conventional oil reserves, and its flexibility to produce light end products which benefit end users. Slurry phase hydrocracking is one of the growing technologies to fulfil the increasing demand for light cut. Modelling of an industrial slurry phase reactor (SPR) for vacuum residue hydrocracking using different kinetic models is proposed. The axial dispersion model (ADM) is used for modelling an industrial SPR. The mathematical model of the reactors is incorporated for the three different lump kinetic models. This study deals with the continuous stirred tank reactor (CSTR) and SPR modelling, followed by industrial SPR modelling. The small lab-scale reactor models are validated with the experimental data reported in the literature. The study's objective was to investigate the one-dimensional and two-dimensional concentration dynamics of each lump along the axial and radial positions of industrial SPR. The vacuum residue conversion into the light fractions was obtained by more than 73% in industrial SPR. Also, the yield of vacuum gas oil and resins were evaluated as 49% and 63%, respectively. The sensitivity analysis was performed to explain the effect of process variables. The optimum range was found as a length of 15 m, liquid hourly space velocity (LHSV) of 0.2 h⁻¹, 1% catalyst concentration, and 420°C reaction temperature to enhance the throughput of the reactor.     

Modelling of catalytic reactors containing core–shell pellets with various morphologies for series reactions

Modelling of series reactions was performed for core–shell catalysts. Mathematical solutions of concentrations inside the pellets were derived from reaction–diffusion equations considering inert-core thickness (ξ_c) for first-order kinetics. Transient behaviours of catalytic reactors containing core–shell pellets were predicted, assuming pseudo-steady state approximation. In a batch reactor, the removal rate of reactants increased with increasing Thiele modulus and decreasing ξ_c in the order of sphere > cylinder > slab. The transient concentration of the intermediate product was maximum and affected by the distribution coefficient, diffusivity ratio, particle shape, and ξ_c. In a continuously stirred tank reactor, the concentration was affected by feed rate and catalyst loading, and conversion could be enhanced by a cascade connection. In a fixed-bed reactor, the concentration increased with increasing ξ_c due to an insufficient catalyst volume. Péclet number and particle shape also affected the concentration, implying that axial dispersion and interfacial area are important design parameters.     

Effect of size and shape of catalyst microparticles on pellet pore structure and effectiveness

Catalyst pellets with bidisperse pore structure were simulated by packed bed of microporous ion exchange resin (sulfonated styrene-divinylbenzene copolymer) and inert glass particles. Inert particles of different size and shape were used to modify the shape and volume of transport macropores (voids between the particles).Tortuosities of pellets were obtained from the experimentally measured effect of internal diffusion on the rate of catalytic methanol dehydration.Analysis of results shows that in the region of strong internal diffusion influence the highest pellet effectiveness is achieved for equal-size spherical microparticles. To ensure optimum macropore structure even in the transition region between the kinetic region and the region of strong diffusion influence, volume of transport macropores has to be reduced by combination of appropriately-sized spherical catalyst microparticles.     

Dynamic operation of a three-phase upflow reactor for the hydrogenation of phenylacetylene

The selective catalytic hydrogenation of phenylacetylene to styrene presents an attractive and interesting process both from scientific and industrial viewpoints. The process mechanism consists of two consecutive reaction steps in which a high selectivity towards the intermediate product is desired.

There have been many papers published so far, that report an increase in the selectivity of adsorption-desorption-based gas-phase processes performed on solid catalysts, due to a periodic (pulsing) operation of the reactor system. Pulsing the gas feed is believed to create more preferential adsorption conditions for the required reactant (transient conditions).

This paper presents the results of a theoretical and experimental study concerning the influence of forced periodic oscillations of the gas-phase component on the performance of a three-phase reactor. The theoretical results are based on a heterogeneous mathematical reactor model considering both the transient mass balance over the reactor length as well as accumulation inside the catalyst pellets.

Numerical dynamic reactor simulations with a simplified model neglecting accumulation of components within the catalyst pellet showed little improvement in conversion and selectivity when compared to results of the steady-state model. These results were supported by dynamic experiments in a bench scale reactor. It is shown that due to mass transfer limitations, the original square wave shape of the oscillation in the gas-phase concentration was almost completely faded into a smooth and rather flat oscillation at the catalyst surface. Simulations with a model in which accumulation of the relevant components inside the particles was considered showed no further improvement in performance at transient conditions, but demonstrated the impact of accumulation capacity of the pellets on the shape of oscillation present at the catalyst surface.

The study is a result of direct cooperation between Politecnico di Torino, where the modelling and calculations were performed, and DSM Research. Its aim was to offer an attractive alternative to the conventional approach.     

Evaluation of published kinetic models for tert-amyl ethyl ether synthesis

The published kinetic models for liquid phase synthesis of tert-amyl ethyl ether (TAEE) by addition of ethanol to isoamylenes on acidic ion exchange resins were evaluated by comparison with own experimental data. Fixed bed and batch reactor experiments were carried out in liquid phase on Amberlyst-35 ion exchange resin as catalyst. Among the published kinetic models, our experimental data fits the best with the model published by [J.A. Linnekoski, A.O. Krause, L.K. Rihko, Kinetics of the heterogeneously catalyzed formation of tert-amyl ethyl ether, Ind. Eng. Chem. Res. 36 (1997) 310–316.]. In the simulation study for the fixed bed reactor experiments, the influences of axial mixing, liquid–solid mass transfer and internal diffusion steps on the overall process kinetics were theoretically evaluated. The results evidenced that on the working temperature domain, significant kinetic limitations by internal diffusion can appear for catalyst pellets size over 1 mm. The external mass transfer step has a weak influence on the process kinetics and can be important only at lower limit of the flow rates domain. Our computations evidenced also a negligible influence of the axial mixing on the reactants conversion in the experimental fixed bed reactor, on the working domain investigated.     

Insight into the intraparticle diffusion of residue oil components in catalysts during hydrodesulfurization reaction

Well‐defined and uniform pore structure catalysts were used to study the intraparticle diffusion of fractionated Saudi vacuum residue under hydrodesulfurization (HDS) reaction conditions. HDS rates of residue oil cuts with different molecular weights are determined as functions of pore size, temperature, and pressure in a trickle‐bed reactor. Credible intrinsic and bulk diffusivities of organosulfur compounds in residue oil were obtained for the first time, from the apparent and intrinsic reaction kinetic constants. Intrinsic diffusivities ranged from 2 × 10^?7 to 8 × 10^?7 cm²/s for the residual oil molecules; diffusivity decreases with increasing molecular weight of the residual oil. The intrinsic diffusivity for molecular weights ～1000 Daltons increases with pore size for pores <70 nm, but is nearly independent of pore size for pores >70 nm. The diffusivity dependences on pore size and molecular weight suggest that the onset of restricted diffusion occurs for ratios of molecular diameter to pore diameter of ～0.04. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3267–3275, 2014     

加压高温内循环无梯度催化反应器的设计与试验

无梯度催化反应器的发展,促进了催化反应动力学的研究工作。尤其是内循环无梯度催化反应器,不仅可以较准确地测算催化动力学方程,而且可以测定工业粒度催化剂的内表面利用率,模拟工业反应器内各点反应速率,沟通了理论研究与工业应用。 本文简要介绍四川大学研制的一种加压高温内循环无梯度催化反应器,并将该反应器用于天然气水蒸汽转化动力学的研究。     

Hydroconversion kinetics of Marlim vacuum residue

A kinetic model was obtained for the Marlim crude vacuum residue (VR) hydroconversion. Marlim VR mixed with FCC decant oil in an 80%/20% weight basis was put in contact with a commercial NiMo on γ-alumina catalyst in a stirred batch reactor. Several temperatures and oil/catalyst ratios were used over different times (0–240 min), at a 110 bar pressure and constant hydrogen flow. The analysis of the collected product showed residua conversions of up to 70%. Hydroconversion kinetics involving thermal and catalytic cracking contributions was proposed to represent the obtained data. The resulting system of differential equations of the kinetic model was solved within reaction time, taking into account the experimental temperature profile. The chi-square objective function was minimized to adjust model parameters. A proposed effective hybrid minimization method was used, by applying a Newton-type method between certain simulated annealing minimization steps. The proposed kinetic model allowed the representation of thermal and catalytic cracking effects, in order to take into account different catalyst concentrations. Therefore it is possible to consider distinct reactor hydrodynamics, such as expanded or bubble column reactors.     

MONOLITHIC CATALYSTS: DIFFUSIONAL MASS TRANSFER OF CO THROUGH Y-ALUMINA SUBSTRATES UNDER REACTING CONDITIONS

Experimental kinetic data have been obtained for the CO oxidation reaction over laboratory made platina-rhodium monolithic catalyst. Inert layers of y-yalumina were deposited over the active layer in order to elucidate the diffusional effects on the monolithic catalyst. A synthetic gas mixture consisting of CO₁, O₂ and N₂ was the feed of an integral tubular reactor between 180 and 370°C. A first order kinetic rate expression with respect to CO concentration, which includes an inhibition term of second order was found to fit the experimental data via a model which also accounts for diffusional resistances. The obtained kinetic and diffusional data can be used in the design of a catalytic converter with improved efficiency. The results can also be used to interpret the diffusion mechanism inside the catalyst itself and it was concluded that Knudsen diffusion mechanism prevails.     

Vacuum residue coking process simulation using molecular-level kinetic model coupled with vapor-liquid phase separation

In this work,a molecular-level kinetic model was built to simulate the vacuum residue (VR) coking pro-cess in a semi-batch laboratory-scale reaction kettle.A series of reaction rules for heavy oil coking were summarized and formulated based on the free radical reaction mechanism.Then,a large-scale molecular-level reaction network was automatically generated by applying the reaction rules on the vacuum residue molecules.In order to accurately describe the physical change of each molecule in the reactor,we cou-pled the molecular-level kinetic model with a vapor-liquid phase separation model.The vapor-liquid phase separation model adopted the Peng-Robinson equation of state to calculate vapor-liquid equilib-rium.A separation efficiency coefficient was introduced to represent the mass transfer during the phase separation.We used six sets of experimental data under various reaction conditions to regress the model parameters.The tuned model showed that there was an excellent agreement between the calculated val-ues and experimental data.Moreover,we investigated the effect of reaction temperature and reaction time on the product yields.After a comprehensive evaluation of the reaction temperature and reaction time,the optimal reaction condition for the vacuum residue coking was also obtained.