Mixed oxide supported hydrodesulfurization catalysts—a review

Support effects form important aspect of hydrodesulfurization (HDS) studies and mixed oxide supports received maximum attention in the last two decades. This review will focus attention on studies on mixed oxide supported Mo and W catalysts. For convenience of discussion, these are divided into Al₂O₃ containing mixed oxide supports, TiO₂ containing mixed oxide supports, ZrO₂ containing mixed oxide supports and other mixed oxide supports containing all the rest. TiO₂ containing mixed oxides received maximum attention, especially TiO₂–Al₂O₃ supported catalysts. A brief discussion about their prospects for application to ultradeep desulfurization is also included. An overview of the available literature with emphasis on research carried out in our laboratory form the contents of this publication.     

Niobium-containing catalysts—the state of the art

This review article is devoted to the materials containing niobium, which have been discovered or developed in the past few years and exhibit the potential application in heterogeneous catalysis. Niobium oxides and mixed oxides as well as sulfides, nitrides (oxynitrides), carbides (oxycarbides), and phosphates are considered. Among the catalytic processes in which Nb-containing materials were tested, liquid and gas phase oxidation is described in details, and the role of niobium in the prevention of the catalyst from SO₂ poisoning is mentioned.     

Importance of proper hydrodynamics modelling in fixed‐bed reactors: Fischer‐Tropsch synthesis study case

The importance of hydrodynamics, particularly gas density, superficial gas velocity, and total pressure in axial and radial directions, was analyzed for the modelling of a catalytic reactor using a non‐isothermal pseudo‐homogeneous approach. The modelling of a fixed‐bed reactor in one and two stages for CO conversion by Fischer‐Tropsch synthesis was taken as a study case. For the validation of the proposed model, the results of the simulations for the CO conversion and temperature profiles were compared with experimental data reported in the literature. Simulations for CO conversion and reactor temperature profiles confirmed the model's ability to predict the selectivity of the liquid products in the Fischer‐Tropsch synthesis reactor in one and two stages. The proposed model predicts more suitable profiles of CO conversion and temperature along the reactor, which makes it a more robust and efficient tool for design, optimization, and control purposes.     

Oxidation of ethanol over supported manganese catalysts—effect of the carrier

MnO_x catalysts supported on alumina, titania or yttria-stabilised zirconia were studied in ethanol oxidation. Catalysts were characterized by determining their XRD, TPR, TPD-O₂ and TPD-NH₃ properties and light-off behavior. The effect of kind of carrier on activity in the ethanol oxidation and on selectivity to acetaldehyde (ACA) was determined. Relation between the TPR properties of the catalysts and their activity in ethanol conversion is suggested. The maximum of selectivity to ACA appears in the same sequence of temperatures as the first peak of oxygen desorption from supported MnO_x catalysts.     

Recycling of spent magnesite and ZAS bricks for the production of new basic refractories

Changing environmental awareness and regulations, the cost of waste disposal, and concerns about future liabilities have caused increased interest in recycling of spent refractories. The densification parameters of spent magnesite containing up to 10.0 wt.% spent ZAS (zirconium aluminium silicate) sintered at 1450–1550 °C were investigated. X-ray, microstructure and microchemistry analysis were used to establish the present phases. The technological parameters in terms of RUL (refractoriness under load) and TSR (thermal shock resistance) of the prepared refractories attaining their maximum densification were evaluated. The results revealed that, the addition of spent ZAS up to 5.0 wt.% to spent magnesite enhanced the physico-mechanical, refractory and thermal properties due to the development of highly refractory phases MA spinel and MgO·ZrO₂ solid solution. Samples containing more than 5.0 wt.% spent ZAS exhibited lower refractory and thermal properties due to the formation of M₂S ferroan beside the high flux CMS phase.     

A comparison of co-current and counter-current modes of operation for a novel hydrogen-permselective membrane dual-type FTS reactor in GTL technology

In this work, a comparison of co-current and counter-current modes of operation for a novel hydrogen-permselective membrane reactor for Fischer-Tropsch Synthesis (FTS) has been carried out. In both modes of operations, a system with two-catalyst bed instead of one single catalyst bed is developed for FTS reactions. In the first catalytic reactor, the synthesis gas is partly converted to products in a conventional water-cooled fixed-bed reactor, while in the second reactor which is a membrane fixed-bed reactor, the FTS reactions are completed and heat of reaction is used to preheat the feed synthesis gas to the first reactor. In the co-current mode, feed gas is entered into the tubes of the second reactor in the same direction with the reacting gas stream in shell side while in the counter-current mode the gas streams are in the opposite direction. Simulation results for both co-current and counter-current modes have been compared in terms of temperature, gasoline and CO₂ yields, H₂ and CO conversion, selectivity of components as well as permeation rate of hydrogen through the membrane. The results showed that the reactor in the co-current configuration operates with lower conversion and lower permeation rate of hydrogen, but it has more favorable profile of temperature. The counter-current mode of operation decreases undesired products such as CO₂ and CH₄ and also produces more gasoline.     

Rohstoffliches Recycling von glasfaserverstärktem Polyamid‐6

It is well known that polyamide 6 (PA‐6) can be depolymerised to regenerate caprolactam monomer by treatment with phosphoric acid, and can thus be subjected to tertiary recycling (=according ASTM D 5033‐90: the process technologies of producing chemicals from scrap or waste plastics). However, broad general application of this process — widely used in industrial practice especially for PA‐6 fibre waste — to fibreglass‐reinforced or filled PA‐6 is precluded by the fact that phosphoric acid reacts with the fillers. This causes a high phosphoric acid consumption; moreover, the method gives an unsatisfactory yield of caprolactam (around 56%) and also requires costly disposal of phosphorus‐containing waste. Therefore, processes have been developed for tertiary recycling of fibreglass‐reinforced or filled polyamide 6, which plays an important role as an engineering plastic. Two different processes have been investigated, viz. hydrolytic depolymerisation and catalysed depolymerisation in vacuum.     

Fischer‐Tropsch Synthesis Over Co‐Ru/γ‐Al2O3 Catalyst in Supercritical Media

The Fischer‐Tropsch synthesis (FTS) in gaseous and supercritical phases was examined in a continuous, high‐pressure fixed‐bed reactor by employing a cobalt catalyst (Co‐Ru/γ‐Al₂O₃). The kinetic modeling of the FTS was investigated in the reactor over a 60–80 mesh cobalt catalyst. The Langmuir‐Hinshelwood kinetic equation was used for both the Fisher‐Tropsch (FT) and water gas shift (WGS) reactions. The kinetic model was applied for simulation of the reactor with 16–20 mesh cobalt catalyst. The simulation results showed a good agreement with the experimental data. The experimental data showed that higher CO conversion and lower CH₄ and CO₂ selectivities were achieved in supercritical media compared to the gaseous phase. The BET surface area and pore volume enhancement results provided evidence of the higher in situ extraction and greater solubility of heavy hydrocarbons in supercritical media than in gaseous phases. Furthermore, the effects of supercritical solvent such as n‐pentane, n‐hexane, n‐heptane and their mixtures were studied. Moreover, the influence of reaction temperature, H₂/CO ratio, W/F_(CO+H2) and pressure tuning in the supercritical media FT synthesis were investigated, as well as the effect of the supercritical fluid on the heat transfer within the reactor. The product carbon distribution had a similar shape for all types of solvents and shifted to lighter molar mass compounds with increasing temperature, H₂/CO ratio, and W/F_(CO+H2). Finally, the product distribution shifted to higher molar mass hydrocarbons with increasing pressure. As a result, one may conclude that a mixture of hydrocarbon products of the FTS can be used as a solvent for supercritical media in Fischer‐Tropsch synthesis.     

Staging of the Fischer‐Tropsch Reactor with a Cobalt‐Based Catalyst

A method for systematic reactor design, described by Hillestad [1], is applied to the Fischer‐Tropsch synthesis. The reactor path is sectioned into stages and design functions are optimized to maximize an objective function. Two different objective functions are considered: the yield of wax and a measure of the profitability. With the chosen kinetic model [2] and the path temperature constrained by 240 °C, staging of the Fischer‐Tropsch synthesis based on the first criteria will increase the yield of wax. By introducing the cost of heat transfer area in the objective function, the total heat transfer area requirement of a two‐stage reactor is significantly less than of a single‐stage reactor.     

Influence of Reaction Conditions on the Conversion of Methane‐Rich Gases to Fischer‐Tropsch Products

Evidence is provided that stable operation of a microstructured reactor for steam‐assisted catalytic partial oxidation (sCPOX) and its subsequent coupling with a Fischer‐Tropsch synthesis (FTS) reactor is possible at pressures up to 25 bar. The product composition of the sCPOX was determined and subsequently used as feed composition for a downstream FTS reactor to prove the possibility of coupling with syngas generation. After stable operation was proven in both setups, they were coupled and operated together, feeding the product gas stream of the sCPOX to the FTS. In addition, the negative influence of sulfur in the sCPOX‐gas feed was evaluated.     

Unpromoted and Mn‐Promoted Cobalt Catalyst Supported on Carbon Nanotubes for Fischer‐Tropsch Synthesis

Langmuir‐Hinselwood (LH) and power rate equations were applied to describe the kinetics of the Fischer‐Tropsch reaction on cobalt catalysts and manganese‐doped cobalt catalysts supported on carbon nanotubes (CNTs). LH‐based kinetics characterize the activity behavior of the unpromoted Co/CNT system satisfactorily, but fail with respect to the manganese‐promoted Co/CNT catalyst. An alternative LH equation is able to fit the experimental data, but the fitting parameters are out of the range of usual values and underrate the activity at ambient pressure regardless of manganese promotion. Application of power law rate expressions results in satisfying characterization of the kinetics in the whole CO pressure range in the promoted case and within a defined range of CO pressure in the unpromoted case.     

Simulation and Analysis of a Tubular Fixed‐Bed Fischer‐Tropsch Synthesis Reactor with Co‐Based Catalyst

A two‐dimensional pseudohomogeneous reactor model is proposed to simulate the performance of fixed‐bed Fischer‐Tropsch synthesis (FTS) reactors by lumped thought. A CO consumption kinetics equation and a carbon chain growth probability model were incorporated into the reactor model. The model equations discretized by a two‐dimensional orthogonal collocation method were solved by the Broyden method. Concentration and temperature profiles were obtained. The validity of the reactor model against the pilot plant test data was investigated. Satisfactory agreements between model prediction values and experiment results were obtained. Further simulations were carried out to investigate the effect of operating conditions on the reaction behavior of the fixed‐bed FTS reactor.