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.     

Catalyst pore plugging effects on hydrocracking reactions in an Ebullated bed reactor operation

This paper analyzes the deactivation effects of NiMo/Al₂O₃ catalyst during the operation in an Ebullated bed reactor for Heavy residue hydrocracking. The spent catalysts were characterized by chemical analysis, ¹³C NMR, ESM, DRX, and by using thermal programmed oxidation and diffusion studies in a shallow bed micro-reactor. The deactivations were performed in a 5 l continuously stirred tank reactor, while the spent catalysts were tested in a 0.05 l micro-reactor. The study focused on determining the properties of the external layer of the catalyst and on evaluating the internal coke and metal deposition. The results indicated that initial deactivation is mainly due to coke depositions, while its impact on mass transfer reaction control depends on temperature. In long-term deactivation, the metal deposition plays a more important role in blocking the internal micro- and meso-structures and in building up the external layer of the pellets.     

Optimization of the active component distribution through the catalyst bed for the case of adiabatic reactor

This article is the next one in the previous series of publications concerning the optimal active component distribution. The aim of the present work is to extend the optimization problem taking into account the energy balance. The variational problem on searching for the optimal active component distribution profile along the catalyst bed was formulated for the case of an adiabatic reactor at an arbitrary form of reaction rate expression. The Euler differential equation was derived and the existence conditions for the solution of variational problem were obtained. A numerical algorithm was suggested and the optimal profiles of the active component distribution were calculated for the first-order reaction. Under isothermal conditions with linear dependence of the reaction rate on reactant concentration at a constant mass transfer coefficient, the uniform distribution is optimal. As opposed to the case of an isothermal reactor with a first-order catalytic reaction, in an adiabatic reactor considerable economy in the active component loading might be achieved due to optimization of its distribution. It was shown that the optimal active component distribution profiles were axially decreasing for the first-order exothermic reactions and increasing for endothermic reactions. The optimal active component distribution profile was calculated for the case of methane combustion on catalytic monoliths.     

Effect of recycling the unconverted residue on a hydrocracking catalyst operating in an ebullated bed reactor

The effect of recycling the unconverted bottom on catalyst deactivation as a way to improve the hydrocracking conversion of heavy oil was analyzed using the experimental information obtained in a steady-state ebullated bed reactor. The recycle contained different amounts of partially converted (aged) material. Four sets of experiments were performed to demonstrate that after five passes through the reactor, the reactivity of the unconverted material decreased by 15% and its impact on catalyst deactivation increased by 30%. The results indicated that the higher the conversion, the lower is the reactivity and the higher is the catalyst deactivation. The production of an insoluble and refractory to convert material imposes a limit on the recycling benefit.     

固定床渣油加氢催化剂表面积炭及抑制研究进展

固定床渣油加氢技术是重质油轻质化的重要手段,积炭是造成催化剂失活、缩短渣油加氢装置运行周期的重要原因之一。本文介绍了固定床渣油加氢反应时催化剂积炭的来源、积炭类型及形成机理、影响积炭形成的因素、抑制催化剂积炭的方法。积炭分为软炭和硬炭,主要由渣油中的沥青质等稠合芳香环化合物吸附于催化剂表面脱氢缩合形成;渣油性质、催化剂物化性能和工艺条件共同影响积炭形成,低黏度渣油、大孔径催化剂及较高氢分压可以减少催化剂表面积炭量;反应过程中掺杂低黏度高芳香性馏分油可以较好地抑制积炭形成。文章指出通过对固定床渣油加氢催化剂积炭问题的分析,可以达到有效抑制催化剂表面积炭和延长催化剂运转周期的目的。     

Modelling of hydrodesulfurization catalysts: II. Effects of catalyst pore structures on deactivation by metal deposits

A mathematical model, previously developed to describe deactivation by metal deposition, has been applied to catalyst with macro-, micro- and random pore structures. Simulation results showed that effectiveness factor reduces progressively with the increasing age of the catalyst. For the macropore system, catalyst lifetime was found to be over 1 year and it is the highest in comparison with both micropore and random pore models. It was revealed that a pore structure in which the pore diameter is enlarged increases hydrodemetallization activity and will be very effective for improving metal resistance because pore-mouth plugging can be curtailed. Predicted catalyst lifetimes matched plant data very closely. The need to develop catalysts of optimum pore structure to achieve desirable goals has been validated.     

Dynamic modeling and operability analysis of a dual-membrane fixed bed reactor to produce methanol considering catalyst deactivation

The goal of this research is dynamic operability analysis of dual-membrane reactor considering catalyst deactivation to produce methanol. A dynamic heterogeneous one-dimensional model is developed to predict the performance of this configuration. In this configuration, a conventional reactor has been supported by a Pd/Ag membrane tube for hydrogen permeation and alumina–silica composite membrane tube to remove water vapor from the reaction zone. To verify the accuracy of the considered model, the results of conventional reactor are compared with the plant data. The main advantages of the dual-membrane reactor are: higher catalyst activity and lifetime, higher CO₂ conversion and methanol production.     

Flow distribution and velocity measurement in a radial flow fixed bed reactor using electrical resistance tomography

Electrical resistance tomography is a relatively simple and inexpensive technique for imaging electrically conducting systems. It has been applied to visualise the flow pattern and distribution inside a radial flow packed bed of novel design for improving reactor performance with lower pressure drop. The density of information yielded by electrical tomography is suitable for validation of Computational fluid dynamics. Sets of tomographic images representing slices through a packed bed have been obtained for a 8-plane × 16-electrode sensor configuration which produces of the order 10³ conductivity measurements in three-dimensions. Pulse injections of high conductivity tracer, both uniformly in the feed and localised, can be imaged as multiple tomographic images or 3D solid-body images, revealing the internal flow pattern. Differentiation of the motion of the tracer peak conductivity within pixels in the sensing planes and between the planes allows the local flow velocities and directions to be determined. This quantifies the flow pattern for uniformity and radial distributive properties.     

Dynamic modelling of catalytic liquid-phase reactions in fixed beds—Kinetics and catalyst deactivation in the recovery of anthraquinones

Dynamic modelling of catalytic fixed-bed reactors with liquid-phase feed is of crucial importance, since catalyst deactivation often plays a central role in reaction engineering. General dynamic modelling of liquid-phase fixed beds was considered, including complex reaction kinetics and catalyst deactivation. The modelling concept was applied on a catalytic liquid-phase conversion reaction. The model was tested with pilot-plant data and showed a good predictivibility. The model can be used to optimize the production life cycles of fixed beds with catalyst deactivation.     

Improved one-dimensional model of a tubular packed bed reactor

The major drawback of one-dimensional models of tubular fixed bed reactors, which are often used if the computational effort should be small, is the fact that the reaction rate is calculated using the average temperature over the cross-section of the reactor. The difference between this reaction rate and the average reaction rate over the cross-section becomes increasingly significant with increasing temperature difference over the radius of the reactor and with increasing activation energy of the reaction(s). Improved versions of the one-dimensional model, such as that of Hagan et al. [1988. A simple approach to highly sensitive tubular reactors. SIAM Journal of Applied Mathematics 48, 1083-1091] are available, which use an analytical approximation of the radial temperature profile to improve the prediction of the average reaction rate. However, application of these models involves solving of implicit equations. Here, a new model is proposed as an alternative to the existing one-dimensional models. It has the same form as the conventional one-dimensional model and contains only explicit functions. It is demonstrated that, at conditions not too close to runaway, the new model performs better than the well-known α-model.     

数学模型在固定床反应器中的应用

化学反应器是整个化工生产过程的核心装置，其中固定床反应器是应用较为广泛的反应设备，建立能准确描述其特性的数学模型，不但可以给反应器设计和最优化操作提供理论依据，更减少了工作量。实现其优化操作，具有重要意义。主要介绍了固定床反应器中各种吸附过程数学模型的建立及其相应解法。     

Methanol synthesis in a novel axial-flow, spherical packed bed reactor in the presence of catalyst deactivation

Since in the foreseeable future liquid hydrocarbon fuels will play a significant role in the transportation sector, methanol might be used potentially as a cleaner and more reliable fuel than the petrochemical-based fuels in the future. Consequently, enhancement of methanol production technology attracts increasing attention and, therefore, several studies for developing new methanol synthesis reactors have been conducted worldwide. The purpose of this research is to reduce the pressure drop and recompression costs through the conventional single-stage methanol reactor. To reach this goal, a novel axial-flow spherical packed bed reactor (AF-SPBR) for methanol synthesis in the presence of catalyst deactivation is developed. In this configuration, the reactor is loaded with the same amount of catalyst in the conventional single-stage methanol reactor. The reactants are flowing axially through the reactor. The dynamic simulation of the spherical reactors has been studied in the presence of long-term catalyst deactivation for four reactor configurations and the results are compared with the achieved results of the conventional tubular packed bed reactor (CR). The results show that the three and four stages reactor setups can improve the methanol production rate by 4.4% and 7.7% for steady state condition. By utilizing the spherical reactors, some drawbacks of the conventional methanol synthesis reactors such as high pressure drop, would be solved. This research shows how this new configuration can be useful and beneficial in the methanol synthesis process.     

Study of the catalyst deactivation in an industrial gasoil HDS reactor using a mini-scale laboratory reactor

The activity of a hydrodesulphurization catalyst loaded in an industrial hydrotreater is studied at start up and end of run. Catalyst initial and final activity was determined by performing HDS experiments at industrial conditions in a laboratory mini-scale hydrotreater. The results show that the deactivation of the catalyst samples collected from three different places of the industrial reactor do not vary significantly, the maximum difference among the catalyst samples, being less than ±4%. The experimentally determined deactivation level of the catalyst samples is compared with the deactivation estimated for the same industrial reactor and the same load using a hybrid neural network model trained with operational data of the industrial and the results are in close agreement. Catalyst deactivation appears to be faster for hydrogen consumption reactions than for hydrodesulphurization reactions indicating a decreasing hydrogen consumption trend with time in operation for specific sulphur content in the product.     

Correlation of reactor performance with catalyst structural changes during coke formation in FCC processes

In the present study an attempt is made to relate the structural properties of Fluid Catalytic Cracking (FCC) catalysts with the process performance. Stochastic network models are employed to represent the porous structure of two catalysts with similar catalytic activity but different pore structure based on N₂ porosimetry experiments. Accordingly, reaction and deactivation are studied by means of a transient model based on percolation theory. Extension at the reactor level is achieved by employing the mixing cell in series model. The results show that at early times the catalyst with the larger surface area and pore volume shows higher conversion and yield while, at the same time, has a lower fraction of poisoned sites in the porous network. At later stages the catalyst with the higher degree of connectivity exhibits stronger resistance to structural phase transition due to pore plugging. Comparison between experimental and simulation N₂ isotherms during catalyst deactivation shows excellent agreement between model and experiment confirming the mechanism of coke formation. Further comparison with experimental results at the reactor level shows very good agreement in terms of both conversion and yield differences between the two catalysts. The close agreement between model and experiments in both catalyst structure and reactor conversion for two entirely different structures of FCC catalysts opens up the possibility of architectural design of these catalysts for optimum process performance achievement.     

Simulation of a spouted bed reactor for solid catalyst alkylation

A process simulator was developed to afford a better understanding of the performance of a butene-isobutane alkylation plant that uses a three-phase spouted bed reactor and a super acid solid catalyst. The mass and energy balances were numerically solved using lump of reactions and kinetic expressions previously developed. The results indicate the strong influence of the gas linear velocity and temperature in the riser and downcomer behavior that affects the activity, selectivity, and stability of the catalytic systems that cannot be predicted otherwise. The optimal operating conditions were determined for a particular catalyst and set of costs. The impacts of the recycling alkylate and isobutane to the reactor as well as the presence of hydrogen to control the coke formation are analyzed.     

An integrated dynamic model for reaction kinetics and catalyst deactivation in fixed bed reactors: skeletal isomerization of 1-pentene over ferrierite

Catalyst deactivation in fixed beds is a critical issue in many industrial processes. A general dynamic model for fixed bed reactors involving catalyst deactivation was presented and simplified. The simplifications were based on the pseudo-steady state hypothesis for the fluid phase. Catalyst deactivation was treated by a linear model with respect to surface intermediates. A semi-analytical solution was obtained for the surface intermediates. The approach was applied to skeletal isomerization of 1-pentene over ferrierite. The comparison of experimental results and model predictions revealed that the model is able to describe the essential features of catalyst deactivation in skeletal isomerization and it can be useful for process scale-up.     

Simulation of catalyst loss from an industrial fluidized bed reactor on the basis of labscale attrition tests

Attrition of catalyst represents a significant economic penalty due to the necessity to make-up the losses on a continual basis. Designing fluidized bed systems to minimize attrition may sacrifice gas-solid contact efficiency that requires pressure to distribute gases in process vessels uniformly. In this paper, the attrition performance of DuPont's vanadium pyrophosphate oxide catalyst (VPP) used in their commercial Circulating Fluidized Bed (CFB) process to convert n-butane to maleic anhydride has been examined. The analysis is based on simulating the commercial conditions in the individual vessels combined with the physical characteristics of the catalyst determined in the laboratory. The comparison of the catalyst loss rate measured in the industrial plant and the simulation results indicates that the measured values are below the values predicted by the simulation. However, the relative change in the catalyst loss rate with operating time is well described. The simulation allows the quantification of the attrition sources in the process. The simulation results revealed that in the present case over 60% of the catalyst loss originates from attrition in the fluidized beds, whereas only 16% originates from attrition in the cyclones. The simulation is an appropriate tool to investigate the influence of parameter changes. Using the simulation the design parameters and operating conditions can be optimized to minimize the catalyst loss rate in the industrial plant.     

Production of hydrogen and carbon nanotubes from methane decomposition in a two-stage fluidized bed reactor

Methane decomposition over a Ni/Cu/Al₂O₃ catalyst is studied in a two-stage fluidized bed reactor. Low temperature is adopted in the lower stage and high temperature in the upper stage. This allows the fluidized catalysts to decompose methane with high activity in the high temperature condition; then the carbon produced will diffuse effectively to form carbon nanotubes (CNTs) in both low and high temperature regions. Thus the catalytic cycle of carbon production and carbon diffusion in micro scale can be tailored by a macroscopic method, which permits the catalyst to have high activity and high thermal stability even at 1123 K for hydrogen production for long times. Such controlled temperature condition also provides an increased thermal driving force for the nucleation of CNTs and hence favors the graphitization of CNTs, characterized by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and XRD. Multistage operation with different temperatures in a fluidized bed reactor is an effective way to meet the both requirements of hydrogen production and preparation of CNTs with relatively perfect microstructures.     

Pore plugging effects on the performance of ZSM-5 catalyst in MTP reaction using a discrete model

Coke is an important medium for connecting reaction and regeneration of the methanol to propylene pro-cess on the ZSM-5 catalyst. Coke grows in the meso and macro pores, it gradually worsens the diffusion inside the catalyst particle. Furthermore, pore plugging is inevitable which causes the deactivation of ZSM-5 catalyst. However, current continuum model cannot reflect the changes in pore structure with clear physical concepts. A discrete model that is verified by the carbon deposition experiments is intro-duced to indicate the behavior of pore plugging effects. Results show that the pore plugging has a signif-icant effect on the performance of the catalyst. The time varying profile of effectiveness factor is obtained, indicating a regular reduction with the increase of the pore plugging effect. Spatial distributions of pore size that would significantly enhance the plugging effect are also identified.     

Study of gas cavity size hysteresis in a packed bed using DEM

When a high velocity gas jet is introduced into a packed bed a cavity is formed. The size of the cavity shows hysteresis on increasing and decreasing gas flow rates. This hysteresis leads to different cavity sizes at same gas flow rate depending on the bed history. The size of cavity affects the gas flow profiles in the packed bed. In this study the cavity size hysteresis phenomenon has been modeled using discrete element method along with turbulent gas flow. A reasonable agreement has been found between computed and experimental results on cavity size hysteresis. The effect of various parameters, such as nozzle height from the bed bottom and packing height, on the cavity size hysteresis has been studied. It is found that inter-particle interaction forces along with gas drag and bed porosity play an important role in describing the cavity size hysteresis. The injection of gas flow allows the particles to go to an unconstrained state than they were previously in, and their ability to remain in that state, even under decreased gas drag force, leads to the phenomenon of cavity size hysteresis.