Hydrodechlorination of tetrachloroethylene over sulfided catalysts: kinetic study

In this work, a commercial sulfided hydrotreatment catalyst (2.8% NiO, 13.5% MoO₃, supported on γ-alumina, supplied by Shell) is compared with different iron sulfide based catalysts. These catalysts were prepared from a by-product (called red mud (RM)) of the bauxite leaching in the Bayer process. Two different activation procedures were tested, both based in dissolving the RM in an acid solution (HCl or HCl+H₃PO₄) followed by a precipitation with ammonia at pH=8 and calcining at 500 °C. All the catalysts were sulfided at 400 °C.

The commercial catalyst was more active than the iron sulfide catalysts in all the range of space times tested. However, considering the low prize of the RM based catalysts, they could be an interesting alternative to the hydrotreatment catalysts. The selectivity for ethane was near 100% for all the catalysts tested.

Kinetics results were successfully modeled with a Langmuir–Hinshelwood model, assuming that the chemisorption of hydrogen (considered as associative) and TTCE occurs over analogous active sites.     

A new method for enhancing the performance of red mud as a hydrogenation catalyst

A new method is presented for improving the performance of red mud as a hydrogenation catalyst (a residue from the production of alumina by the Bayer process that contains iron oxides), based on the method developed by K.C. Pratt and V. Christoverson, Fuel 61 (1982) 460. The activation method consists essentially in dissolving red mud in a mixture of aqueous hydrochloric and phosphoric acids, boiling the resulting solution, adding aqueous ammonia until pH=8, and filtering, washing, drying and calcining the resulting precipitate. The catalyst thus obtained is characterised, and after sulphidation, tested (activity and life) for the hydrogenation of a light fraction of an anthracene oil. The catalytic performance is compared with that of sulphided untreated red mud and sulphided red mud activated by the method of Pratt and Christoverson. This activation method has proved to be more effective in improving the performance of red mud as a hydrogenation catalyst than the method of Pratt and Christoverson, since the activated catalyst presents a slightly higher level of activity and a markedly extended active life.     

Removal of hydrogen sulfide using red mud at ambient conditions

Red mud (RM) is a caustic waste product of alumina industry. A laboratory study was conducted to investigate the removal of hydrogen sulfide (H₂S) gas using RM at ambient conditions. Red mud was characterized before reaction and after reaction by using XRD, BET, SEM/EDX, TG-DSC, FT-IR, and CHNS. The XRD and EDX data confirmed that the H₂S was removed in the form of FeS₂, FeS, CaSO₄·2H₂O, sulfur, sulfide, and bisulfide of Na. During the sulfidation reaction, the color of some RM changes from red to black, indicating that iron oxide was converted to iron sulfide. The black color in this case was an indicator of mineral sulfide compounds which indicated that some H₂S sorption and removal occurred. The removal capacity of sulfidic filtrate was higher as compared to the sulfidic red mud as revealed by thermogravimetric and EDX analysis.     

Low-temperature catalytic combustion of trichloroethylene over cerium oxide and catalyst deactivation

Cerium oxide catalysts prepared by a thermal decomposition method using the salt nitrate as precursor were tested for the catalytic combustion of trichloroethlyene (TCE), as a model of chlorinated volatile organic compounds (CVOCs). CeO₂ catalysts calcined at different temperature were found to possess high catalytic activity for catalytic combustion of TCE, and CeO₂ calcined at 550 °C was the most active catalyst and the complete combustion temperature (T_90%) of TCE was 205 °C. Effects of systematic variation of reaction conditions, including space velocity, inlet TCE concentration and water concentration on TCE catalytic combustion were investigated. Additionally, the stability and deactivation of CeO₂ catalysts were studied by various characterization methods (such as TG/DTA, EDS, XRD, Raman and XPS) and other assistant experiments.     

Inhibition effects of organosulphur compounds on the hydrodechlorination of tetrachloroethylene over Pd/Al2O3 catalysts

The hydrodechlorination of tetrachloroethylene in presence of thiophene over a commercial Pd/Al₂O₃ catalyst was studied in a continuous packed-bed reactor at different temperatures (200–300 °C) and thiophene concentrations (0–5 wt.%). Results indicate that thiophene reversibly decreases tetrachloroethylene conversion and increases the selectivity for trichloroethylene formation. The kinetics of the hydrodechlorination of tetrachloroethylene in the presence of thiophene to form trichloroethylene and ethane can be represented by a Langmuir–Hinshelwood model.     

A study of the ageing and deactivation phenomena occurring during operation of an iron molybdate catalyst in formaldehyde production

Catalyst from a Perstorp Formox formaldehyde plant, operating with high inlet concentration of methanol (10.2 vol.%), was discharged from single tubes of a multi-tube reactor after half the expected lifetime of the catalyst and again after termination of the load. Each tube was filled with two different layers of catalyst. From the inlet of the reactor the first layer was a catalyst mixed with inert rings, which was followed by a second layer of pure catalyst extending from the middle to the outlet of the reactor. Catalyst fractions from the two layers were characterized with various techniques including BET, Fourier transform Raman (FT-Raman) spectroscopy, X-ray diffraction (XRD), elemental analysis with atomic absorption spectroscopy (AAS) and activity measurements. It was found that the surface area of the catalyst in the mixed layer increases during operation while a small decrease is noticeable for the catalyst below in the pure layer. Elemental analysis, XRD and FT-Raman show that during operation of the catalyst there is migration of Mo species from the upper part of the reactor towards the outlet. Activity measurements reveal severe deactivation of the catalyst in the mixed layer. It is concluded that the deactivation primarily is due to formation of volatile species formed by the MoO₃ surface reacting methanol, causing a decrease of the MoO₃/Fe₂(MoO₄)₃ mole ratio in the catalyst. Concerning the catalyst in the pure layer, the condensation of needle-like crystals of MoO₃ mainly occurs on the external surface of the catalyst ring.     

Hydrodechlorination of dichloromethane, trichloroethane, trichloroethylene and tetrachloroethylene over a sulfided Ni/Mo–γ-alumina catalyst

The hydrodechlorination reactions of dichloromethane, 1,1,1-trichloroethane, trichloroethylene and tetrachloroethylene were examined over a commercial Ni/Mo–γ-alumina catalyst in a packed-bed reactor. This preliminary study was focused in the influence of the catalyst pre-treatment (sulfidation), temperature, pressure and nature of the solvent over the reaction yield. The evolution of the catalytic activity was also examined. As an overall, results indicate that catalytic hydrodechlorination might be a suitable method for the destruction of the above mentioned chlorinated compounds, since conversions to non-chlorinated organics were found to be close to 100%, operating at 100 bar and 350°C over a sulfided Ni/Mo–γ-alumina catalyst. However, the catalyst resistance to deactivation must be enhanced.     

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.     

Catalytic applications of red mud, an aluminium industry waste: A review

Red mud is a by-product of bauxite processing through Bayer process. The amount of red mud generated depends largely on the type of ore used and the processing. Use of red mud as a catalyst can be a good alternative to the existing commercial catalysts. Its properties such as iron content in form of ferric oxide (Fe₂O₃), high surface area, sintering resistance_, resistance to poisoning and low cost make it an attractive potential catalyst for many reactions. Besides red mud, ferric ion sludge from wastewater treatment plant has also been studied for its catalytic properties, mainly due to its ferric oxide constituent. This paper reviews the studies on red mud as a catalyst. The catalyst characteristics, reaction mechanisms involved and performance are examined and compared with iron oxide catalyst and commercial catalysts.     

Thermocatalytic decomposition of methane using palm shell based activated carbon: Kinetic and deactivation studies

Catalytic characteristics of activated carbon manufactured from palm shell (ACPS) for methane decomposition was studied using a thermobalance by measuring thee mass gain with time. A reaction order of 0.5 is obtained for methane decomposition over the activated carbon and the activation energy is 210 KJ mol^− 1. The activity of the activated carbon had decreased in almost a linear relationship with the amount of carbon deposited at 800 °C, which indicates that the carbon deposition occurred uniformly on to the carbon surface, while diffusion effect appeared to occur significantly at the last stage of the process at reaction temperatures up to 950 °C. The study of mass gain using different particle sizes indicates the existence of mass transfer effect and the pore mouth blocking which seem to take place particularly in large particles. Comparison between ACPS and commercial based activated carbon (AC) shows almost similar values with regard to change of mass gain with time, maximum amount of carbon deposited before deactivation and the deactivation time.     

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.     

The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane

Pt supported on γ-Al₂O₃, TiO₂ and ZrO₂ are active catalysts for the CO₂ reforming of methane to synthesis gas. The stability of the catalysts increased in the order Pt/γ-A1₂O₃ < Pt/TiO₂ < Pt/ZrO₂. For all catalysts, the decrease in activity with time on stream is caused by carbon formation, which blocks the active metal sites for reaction. With Pt/TiO₂ and Pt/ZrO₂, deactivation started immediately after the start of the reaction, while the Pt/γ-A1₂O₃ catalyst showed an induction period during which carbon was accumulated without affecting the catalytic activity.     

煤基合成气甲烷化用镍基催化剂失活热力学和抗失活预测

利用热力学数据对煤基合成气甲烷化用镍基催化剂硫中毒以及积炭热力学进行了详细的计算。计算发现,活性金属Ni、Mo在甲烷化反应条件下与H₂S、COS发生反应是自发进行的过程。10^-10数量级分压的H₂S含量、10^-14数量级分压的COS含量即可使镍金属活性组分生成硫化镍而使催化剂失活;当Mo作为助剂添加到Ni基催化剂时,硫含量不能超过10^-6数量级。不同温度区间发生的积炭反应类型不同,当温度为633.15～898.15K时,积炭反应主要以CO歧化反应、CO还原反应为主;898.15～983.15K时以CH₄裂解反应为主。另外,在0.1MPa下,添加摩尔分数为11.11%及以上含量水蒸气可以避免积炭。     

On the dynamical instability of packed-bed reactors in the presence of catalyst deactivation

Due to the thermal instability of the packed-bed reactor running an exothermic reaction, unsteady-state operation (for example a fluctuating inflow temperature) can result in a variety of thermal responses. These include the amplification of input temperature perturbations and high-temperature pre-extinction waves. Catalyst deactivation adds further dynamical features to these scenarios. We explore them numerically, using a first-order exothermic reaction and a pseudo-homogeneous (single phase) model of the PBR together with a first-order deactivation model of the catalyst. At low deactivation rate, moving hot spots are found, as well as a non-uniform activity profile of the catalyst. At high deactivation rate, however, high-temperature waves (so-called pre-extinction waves) are followed by the complete extinction of the reactor. The amplification of input temperature perturbations is generally enhanced by the presence of catalyst deactivation. Finally, a power-law model is derived numerically that predicts the resonance frequency for amplification as a function of operating parameters.     

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.     

Evaluation of red mud as surface treatment for carbon steel prior painting

Red mud (RM) is the waste product of the Bayer process for obtaining alumina from bauxite. The alkaline nature of RM suspensions together with the presence of Fe³⁺ species point towards the possibility of using RM as a good corrosion inhibitor for carbon steel. Based on this idea, the possible use of RM suspensions as pre-treatment for carbon steel was studied by recording the electrode potential and the electrochemical impedance spectroscopy (EIS) evolution with immersion time. Different parameters regarding the steel surface finishing (grinding, pickling or degreasing) and RM suspension condition (stirred or steady, decanted or filtered) have been considered in the study; the passivation was obtained when ground samples were immersed in decanted RM suspensions and subjected to continuous stirring. The influence of chlorides and pH were analysed by potentiometric titration. The obtained results indicate that treated samples depassivate at lower Cl⁻/OH⁻ ratio than untreated ones. Regarding the pH parameter, treated samples remain passive at lower pH values than the untreated ones. Finally, some treated and untreated samples were painted and subjected to cathodic polarisation experiments, including an artificial defect in the coating. The comparative study was done using EIS technique; the impedance diagrams would indicate an effective passivation of the steel surface for treated samples.     

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.     

赤泥用作高性能水泥性能调节组分的研究

采用化学活化和热活化的方法,对赤泥进行预处理后再与硅酸盐水泥熟料配合,在实验室条件下就不同化学试剂处理、不同温度下加热处理和不同掺量的赤泥-硅酸盐水泥熟料体系的胶凝性能进行了系统的试验研究。结果表明,未经任何处理的赤泥掺入水泥后,尤其是赤泥掺量大于20%时,水泥砂浆后期强度显著降低,而经特定化学试剂处理和一定温度煅烧处理后的赤泥掺入水泥熟料后,水泥砂浆的后期强度降低幅度得到明显减缓。     

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.     

Modeling and analysis of rapid catalyst aging cycles

The primary mode of deactivation of automotive emission control catalysts is thermal aging, and it is well-known that high-temperature lean aging conditions are particularly detrimental. Since evaluating the long-term durability of automotive catalysts is costly and time-consuming, rapid catalyst aging cycles have been developed to mimic (in a reduced time) the catalyst deactivation under real-world driving conditions. One of the commonly used rapid catalyst aging tests is an exothermal aging cycle, which involves a combination of fuel-rich engine operation and supplemental air injection to generate high-temperature lean conditions within the catalyst bed. In this work, we use the previously developed transient three-way catalyst model to investigate the time evolution of the axial temperature profiles and exhaust air–fuel ratio (A/F) along the catalyst bed during the course of the exothermal rapid aging cycles. We find that the thermal front propagates downstream through the catalyst bed relatively slowly (compared to the concentration front) and this can limit the location within the catalyst bed and duration for high-temperature lean exposure. We also investigate how variations of some of the key system design and operating parameters can affect the extent and duration of high-temperature lean exposure. Finally, a simple analytical expression is developed which allows one to estimate the time it takes for the thermal front to travel through the catalyst bed. This time can be compared with the period of the lean A/F operation during the aging cycle to determine the location and duration of high-temperature lean exposure.