Dynamic optimization of membrane dual-type methanol reactor in the presence of catalyst deactivation using genetic algorithm

This paper presents a study on optimization of a membrane dual-type methanol reactor in the presence of catalyst deactivation. A theoretical investigation has been performed in order to evaluate the optimal operating conditions and enhancement of methanol production in a membrane dual-type methanol reactor. A mathematical heterogeneous model has been used to simulate and compare the membrane dual-type methanol reactor with conventional methanol reactor. An auto-thermal dual-type methanol reactor is a shell and tube heat exchanger reactor which the first reactor is cooled with cooling water and the second one is cooled with synthesis gas. In a membrane dual-type reactor the wall of the tubes in the gas-cooled reactor is covered with a pd–Ag membrane, which is only hydrogen-permselective. The simulation results have been shown that there are optimum values of reacting gas and coolants temperatures to maximize the overall methanol production. Here, genetic algorithms have been used as powerful methods for optimization of complex problems. In this study, the optimization of the reactor has been investigated in two approaches. In the first approach, the optimal temperature profile along the reactor has been studied and then a stepwise approach has been followed to determine the optimal profiles for saturated water and gas temperatures in three steps during the time of operations to maximize the methanol production rate. The optimization methods have enhanced 5.14% and 5.95% additional yield throughout 4 years of catalyst lifetime for first and second optimization approaches, respectively.     

A comparison of auto-thermal and conventional methanol synthesis reactor in the presence of catalyst deactivation

This study proposed a one-dimensional dynamic plug flow model to analyze and compare the performance of an auto-thermal and a conversional methanol synthesis reactor in the presence of catalyst deactivation. An auto-thermal two-stage industrial methanol reactor type is a system with two catalyst beds instead of one single catalyst bed. In the first catalyst bed, the synthesis gas is partly converted to methanol in a water-cooled reactor. In the second bed which is a gas-cooled reactor, the reaction heat is used to preheat the feed gas to the first bed. To analyze the effect of important control variables on the rector performance, steady state and dynamic simulations are utilized to investigate effect of operating parameters on the performance of reactors. The simulation results show that there is a favorable profile of temperature along the two-stage auto-thermal reactor type in comparison with conventional single stage reactor type. In this way the catalysts are exposed to less extreme temperatures and, catalyst deactivation via sintering is reduced. Overall, this study resulted in beneficial information about the performance of the reactor over catalyst life-time.     

Application of hydrogen-permselective Pd-based membrane in an industrial single-type methanol reactor in the presence of catalyst deactivation

This work presents application of palladium-based membranes in a conventional single-type methanol reactor. A novel reactor configuration with hydrogen-permselective Pd and Pd–Ag membrane are proposed. In this configuration the reacting synthesis gas is fed to the shell side of reactor while the high pressure product is routed from recycle stream through tubes of the reactor in a co-current mode with reacting gas. The reacting gas is cooled simultaneously with recycle gas in tube and saturated water in outer shell. The permselective palladium layer on inner tube allows hydrogen to penetrate from the tube side to the reaction side. In this work, the results of two types of novel membrane reactors are compared with a conventional methanol synthesis reactor at identical process conditions. Also the effect of key parameters such as membrane thickness, reaction and tube side pressure, ratio of tube side flow rate to reaction side flow rate on performance of reactor are investigated. The steady-state and quasi-steady-state simulations results show that there are favorable profiles of temperature and methanol mole fraction along the reactor in proposed reactor relative to conventional reactor system. Therefore using this novel configuration in industrial single-type methanol reactor improves methanol production rate.     

A pseudo-dynamic optimization of a dual-stage methanol synthesis reactor in the face of catalyst deactivation

This paper presents an optimization investigation on methanol synthesis reactor in the face of catalyst deactivation using multi-objective genetic algorithms. Catalyst deactivation is a challenging problem in the operation of methanol synthesis reactor and has an important role on productivity of the reactor. Therefore, determination of the optimal temperature profile along the reactor could be a very important effort in order to cope with catalyst deactivation. Our previous studies clarify the benefits of a two-stage reactor over a single stage reactor. In this study, an optimal temperature trajectory is obtained for each stage of the corresponding two-stage reactor. Here, steady state optimization is performed in six different activity levels by maximizing the yield and minimizing the temperature of the first stage of the reactor. Multi-objective genetic algorithms are used to solve this two-objective optimization. The set of optimal solutions obtained for six activity levels represents an optimal temperature trajectory for each stage, which has been extended and proposed as a dynamic optimization. This optimization resulted in an additional 3.6% yield, during the course of 4-year process.     

Factors affecting methanol photocatalytic oxidation and catalyst attrition in a fluidized-bed reactor

A resolution IV fractional factorial experimental design explored the effects of seven factors on both the methanol photocatalytic oxidation (PCO) rate and the catalyst particle size distribution using a fluidized-bed reactor. The seven factors were as follows: calcination temperature, calcination time, grinding order, particle size, vibration amplitude, carrier gas humidity, and fluidization velocity. Decreasing calcination temperature from 726 to 623 K increased the activity of TiO₂/Al₂O₃ catalysts for methanol PCO. Attrition during fluidization liberated small TiO₂ particles from the bulk catalyst and the rate of attrition increased with gas velocity. Attrition was the primary cause of catalyst elutriation and not the presence of fine particles initially present in the bed from catalyst preparation. Increasing humidity caused agglomeration of fine particles, which reduced the amount of catalyst carryover. Removal of fines from the catalyst bed prior to fluidization caused an increase in catalyst attrition until the amount of fines present in the bed was similar to that of a bed in which fines were not removed.     

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.     

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.     

Dynamic operation of butadiene dimerization reactor undergoing catalyst deactivation

Operation of fixed-bed catalytic reactors undergoing catalyst deactivation has been investigated as an optimal control problem to yield optimal temperature policies. An efficient numerical scheme using a control vector iteration method based on gradients in functional space is developed. The procedure is applied to develop optimal temperature profiles for a butadiene dimerization process. The temperature-time trajectories and dynamic activity profiles are strongly influenced by kinetics. A sensitivity analysis is done to study the effect of flow rates, conversion level and parameters that influence kinetic and deactivation processes. These results have been validated with experimentation on a lab scale reactor and a 9.14 m pilot-plant reactor.     

铜基甲醇合成催化剂失活原因的探讨

采用加压微型反应器和化学分析、原子吸收光谱法、X 射线荧光分析、X 射线衍射峰宽化法和其它分析方法对工业使用前后的铜基甲醇合成催化剂M K101 进行了分析和讨论。探讨了该催化剂的失活原因。     

Study on the deactivation phenomena of Cu-based catalyst for methanol synthesis in slurry phase

A commercial Cu-based catalyst for methanol synthesis was studied using a stirred autoclave reactor system in the present study. The synthesis reactions were conducted for different time under the same reaction conditions in order to get catalyst samples with different deactivation degrees. The composition and morphology of the catalyst samples before and after reaction were characterized by the means of temperature programmed reduction (TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), elemental analysis (EA) and nitrogen adsorption techniques. The experimental results indicated that Cu composition of the catalyst had not changed significantly during the reaction, and sintering of Cu particles of the catalysts was the main cause of the catalyst deactivation with time on stream.     

Contribution to emission reduction of CO2 by a fluidized-bed membrane dual-type reactor in methanol synthesis process

In this work, a fluidized-bed membrane dual-type reactor was evaluated for CO₂ removal in methanol synthesis process. The feed synthesis gas is preheated in the tubes of the gas-cooled reactor and flowing in a counter-current mode with reacting gas mixture in the shell side. Due to the hydrogen partial pressure driving force, hydrogen can penetrate from feed synthesis gas into the reaction side through the membrane. The outlet synthesis gas from this reactor is fed to tubes of the water-cooled packed-bed reactor and the chemical reaction is initiated by the catalyst. The methanol-containing gas leaving this reactor is directed into the shell of the gas-cooled reactor and the reactions are completed in this fluidized-bed side. A two-phase dynamic model in bubbling regime of fluidization was developed in the presence of long-term catalyst deactivation. This model is used to compare the removal of CO₂ in a FBMDMR with a conventional dual-type methanol synthesis reactor (CDMR) and a membrane dual-type methanol synthesis reactor (MDMR). The simulation results show a considerable enhancement in the CO₂ conversion due to have a favourable profile of temperature and activity along the fluidized-bed membrane dual-type reactor relative to membrane and conventional dual-type reactor systems.     

Kinetic modelling and reactor simulation for methanol synthesis from hydrogen and carbon monoxide on a copper-base catalyst

Some hypothesized reaction mechanisms were tested out against the supplied experimental data on methanol synthesis from syn gas on copper-based catalyst at different temperatures. Each reaction mechanism consists of three to five intermediate steps, one of which being inevitably the rate determining step for overall reaction CO + 2H₂ = CH₃OH. It was concluded from these tests that the reaction between the adsorbed reactants on the catalyst seemed to be the rate determining step when diffusion resistance was assumed to be negligible. Once the rate determining step is known, the rate equation can be derived, and the modelling of an actual reactor is relatively a routine procedure including the setting up of mass, momentum, and energy balance equations assuming necessary conditions. As the result of simulation, plots of component concentration, reaction rate, temperature, and pressure profiles against reactor length and a table of summary were presented.     

Activity and deactivation of a catalyst for vinyl acetate synthesis in an industrial fluidized reactor

The synthesis of vinyl acetate from acetylene and acetic acid has been carried out using two industrial fluidized reactors. Both reactors have an inverted conical shape with a conical angle of 3°20′, and the diameter of the base is 3.28 m. Both have a production capacity of 50 ton/day. An activated carbon supported zinc acetate catalyst of 0.4 mm in average diameter is used.In order to investigate the optimum fluidizing and reaction conditions with the above industrial reactors, a series of operational tests have been carried out over a period of two years.As a result of such operation tests, it has been confirmed that the subject catalyst is free from hysteresis, because, in the operational tests using the catalysts with the same activity level, it shows the same reaction activity under the same reaction conditions, irrespective of the procedure for setting such reaction conditions. Further, it is considered that the rule of additivity of catalyst activity can be applied to this reaction system, because the subject catalyst shows a reaction activity corresponding to its mixing ratio, when using mixtures of catalysts with different activity levels.In these operational tests, we have introduced a method to calculate space time yield (STY) values at a certain past time point, under such conditions that STY changes on standing by a deterioration of catalyst activity. We measured the STY at a certain time point under certain reaction conditions and continued the operation under the same conditions. Several days later, we measured the STY under the same reaction conditions and, then, proceeded with the operational tests under different reaction conditions and measured the STY again. The STY under the latter reaction conditions at the first time point can be estimated by multiplying the ratio of STY actually measured, just before and after changing reaction conditions, and the STY value measured at the first time point.     

Dynamic modeling of the methanol synthesis fixed-bed reactor

A dynamic model for a fixed-bed reactor for methanol synthesis is presented. The model is compared with its steady state version. The analysis points out that the numerical stability of the dynamic model is improved by opportunely increasing the level of detail. It is appropriate to introduce the diffusion terms, to work with mass fractions, to select good discretization methods for each term of the model equations. Since these aspects are usually neglected in steady state analysis, this paper investigates step-by-step their implementation, emphasizing their importance (I) in the transformation of an original hyperbolic PDE system into a parabolic PDE system; (II) in removing non-physical oscillations generated by first-order systems that may lead to relevant model prediction errors; and (III) in the approximation of the convection terms using the forward formulation, which is more stable and provides more realistic solutions.     

The effect of a membrane reactor upon catalyst deactivation during hydrodechlorination of dichloroethane

The hydrodechlorination of 1,2-dichloroethane was studied over a Rh/SiO₂ catalyst. The catalyst deactivated with use; only part of the deactivation was reversible. The reversible deactivation could be quantitatively accounted for by assuming quasi-equilibrium between surface chlorine and gas phase HCl. An empirical power-law rate expression was found that adequately described the kinetics when used in combination with this assumption of quasi-equilibrium. Experimental results as well as simulation showed that a membrane reactor can reduce the degree of catalyst deactivation by selective removal of HCl. The membrane reactor can also lessen deactivation through dilution of the reaction zone. In this study, using a porous membrane, the predominant effect is one of dilution, not selective HCl removal.     

Sulfur tolerance of methanol synthesis catalysts: Modelling of catalyst deactivation

A series of supported Pd and commercial Cu-based catalysts was tested for methanol synthesis activity and activity maintenance in the presence of 2 ppm H₂S in a differential fixed-bed reactor operated at 523 K and 1.5 MPa. Sulfur breakthrough curves were obtained by monitoring the H₂S concentration in both the feed and exit streams using a gas chromatograph with a flame photometric detector. A comparison of the resistance to sulfur poisoning of the various catalysts is given using a qualitative examination of activity maintenance plots as well as a mathematical model for catalyst deactivation. The rates of deactivation of the Cu-based catalysts in the absence or presence of H₂S were very similar, with 70–90% of the initial activity being lost during 300 h on stream. No H₂S breakthrough was observed with these catalysts whereas the Pd catalysts exhibited breakthroughs after short times (10–30 h) on stream. The Pd catalysts retained a much higher residual activity in the presence of H₂S than those reported for Fe, Co, Ni and Ru at much lower H₂S pressures, and Pd/SiO₂ possessed a stable, lined-out activity in 2 ppm H₂S that was near half its original activity. Problems that arise in catalyst evaluation when a single deactivation model does not apply in all cases are discussed. The Szepe-Levenspiel approach, with a modification recently proposed by Fuentes for poison-tolerant catalysts, was found to give meaningful quantification of sulfur resistance of Pd-based catalysts.     

流化床透氧膜反应器用于天然气制氢的工艺模拟

氢气是一种重要的化工原料,流化床透氧膜反应器是一种新型的天然气制氢装置。建立了该新型装置的AspenPlus模型,并模拟了反应压力、氧碳比(OC)、水碳比(SC)对反应温度、合成气成分的影响,并与普通的流化床自热反应器进行了比较。结果表明,流化床透氧膜反应器由于分离了空气中的N2,反应可在高的氧气摩尔分数下进行,合成气中的H2摩尔分数大大提高,甲烷转化率较大,氢气产量也提高。     

Modeling, simulation and control of a methanol synthesis fixed-bed reactor

In this paper, the dynamic behavior and control of the low pressure methanol synthesis fixed bed reactor have been investigated. For simulation purpose, a heterogeneous one-dimensional model has been developed. First, the reactor simulation is carried out under steady-state condition and the effects of several parameters such as shell temperature, feed composition (especially CO₂ concentration) and recycle ratio on the methanol productivity and reactor temperature profile are investigated. Using the steady state model and a trained feedforward neural network that calculates the effectiveness factor, an optimizer which maximizes the reactor yield has been developed. Through the dynamic simulation, the system open loop response has been obtained and the process dynamic is approximated by a simple model. This model is used for the PID controller tuning and the performances of fixed and adaptive PID controllers are compared for load rejection and set point tracking. Finally the proposed optimizer is coupled with a controller for online optimization and hot spot temperature protection.     

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.     

Experimental determination of reactor activity factors for structure-sensitive catalyst deactivation

An experimental basis is presented for the determination of reactor activity factors for the multiple reactions affected by structure-sensitive catalyst deactivation, which causes a different deactivation effect on different reaction paths. The reactor activity factor is the local catalytic activity specific to a given reaction path integrated over the reactor length. The soundness of the experimentally determined reactor activity factor is demonstrated using ethylene oxidation over a silver catalyst. The reactor activity factor is useful not only for the control of a reactor undergoing deactivation but more importantly for following the progression of catalytic activity loss when there is no detailed knowledge of catalyst deactivation as it affects different reaction paths differently.