Estimation of mid‐distillates production rate in a low temperature Fischer–Tropsch process

BACKGROUND: The Fischer–Tropsch process is the most important path for converting natural gas to high quality liquid hydrocarbons. Low temperature Fischer–Tropsch synthesis in slurry bubble column reactors with cobalt‐based catalysts is used for mid‐distillates production. RESULTS: In this work the slurry bubble column reactor was simulated by applying the two‐bubble class mathematical model. In addition, the effect of operating parameters on synthesis gas conversion was studied. The distribution of products was also predicted from the simulation framework. CONCLUSIONS: The effect of synthesis gas inlet velocity on mid‐distillates production rate was studied in the present work. A maximum production rate for mid‐distillates of about 23 kg s^?1 was predicted from the simulation program. Copyright © 2011 Society of Chemical Industry     

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.     

Optimization of Fischer‐Tropsch Synthesis Using Neural Networks

Fischer‐Tropsch synthesis is an important chemical process for the production of liquid fuels and olefins. Optimization of hydrocarbon products such as diesel and gasoline produced by Fischer‐Tropsch synthesis usually requires the knowledge of the complex polymerization mechanism and the kinetic parameters associated with it in order to optimize production. The Fischer‐Tropsch reaction mechanism is still not fully understood, making optimization a hard task. In this work, a neural network was used in substitution to the reaction mechanism to optimize diesel and gasoline production based on few experimental data for the reaction. The neural network has yielded satisfactory predictions of the product distribution (with prediction errors lower than 5 %) and the optimum operating conditions for gasoline and diesel production were found for a commercial iron based catalyst.     

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.     

Optimal Design of a Gas‐to‐Liquids Process with a Staged Fischer‐Tropsch Reactor

The optimal design of a natural gas‐to‐liquid hydrocarbons (GTL) process with a multistage cobalt‐based Fischer‐Tropsch reactor and interstage product separation is considered. The objective function is to maximize the wax (C₂₁₊) production rate at the end of the reactor path. Sectioning of the Fischer‐Tropsch reactor increases the chain growth probability inside the reactor which results in a higher production of wax. The carbon efficiency of the two‐stage reactor is distinctly higher than that of the single‐stage reactor.     

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.     

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.     

Polymerization Kinetics of Fischer‐Tropsch Reaction on Iron Based Catalysts and Product Grade Optimization

The polymerization kinetics of Fischer‐Tropsch reactions on a K‐promoted Fe catalyst was studied. To represent the product distribution, a kinetic model was developed based on alkyl and alkenyl mechanisms for hydrocarbon chain propagation, which were assumed to occur simultaneously in the Fischer‐Tropsch synthesis. The conclusion was drawn that superimposed Anderson‐Schulz‐Flory (ASF) distributions with different chain growth probabilities, on iron catalysts, can be the result of different chain growth mechanisms. The polymerization mechanism was used to obtain the product distribution for several conditions, and the optimum conditions for the production of transportation fuels were found.     

Rate equations for the fischer‐tropsch reaction on a promoted iron catalyst

Intrinsic rates for the Fischer‐Tropsch synthesis reaction over a promoted iron catalyst fabricated at the Research Institute of the Petroleum Industry (RIPI) have been obtained in the temperature range of 290°C to 310°C, pressure range of 1500 to 2300 kPa, molar hydrogen to carbon monoxide ratio of 0.76 to 1.82, and a space velocity of 3300 h^?1 under conditions of constant catalyst activity. To this end, the initial reaction rates have been measured at constant temperature (±1°C) in the absence of diffusion limitations, and power‐law equations have been fitted in terms of the hydrogen and carbon monoxide partial pressures for the reaction rates.     

Multidimensional modeling of a microfibrous entrapped cobalt catalyst Fischer‐Tropsch reactor bed

Thermal management of highly exothermic Fischer‐Tropsch synthesis (FTS) has been a challenging bottleneck limiting the radial dimension of the packed‐bed (PB) reactor tube to 1.5 in. ID. A computational demonstration of a novel microfibrous entrapped cobalt catalyst (MFECC) in mitigating hot spot formation has been evaluated. Specifically, a two‐dimensional (2‐D) model was developed in COMSOL^®, validated with experimental data and subsequently employed to demonstrate scale‐up of the FTS bed from 0.59 to 4 in. ID. Significant hot spot of 102.39 K in PB was reduced to 9.4 K in MFECC bed under gas phase at 528.15 K and 2 MPa. Improvement in heat transfer within the MFECC bed facilitates higher productivities at low space velocities (≥1000 h^?1) corresponding to high CO conversion (≥90%). Additionally, the MFECC reactor provides an eightfold increase in the reactor ID at hot spots ≤ 30 K with CO% conversions ≥ 90%. This model was developed for a typical FTS cobalt‐based catalyst where CO₂ production is negligible. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1723–1731, 2018     

Theoretical Investigation of a Pd‐membrane Reactor for Methanol Synthesis

A numerical study is performed in order to evaluate the performance and optimal operating conditions of a palladium membrane reactor for methanol synthesis. A novel reactor configuration with a Pd wall, which is perm‐selective to hydrogen, has been proposed. In this configuration the reactants are added to the tube side while pure hydrogen is added to the shell side, as a result, the hydrogen diffuses across the membrane from the shell side to the tube side. In this membrane reactor, hydrogen penetrates to the reaction side in order to maintain a suitable hydrogen level in the whole length of the reactor and shift the equilibrium reaction. The effects of different parameters on the methanol output mole fraction were investigated in the co‐current mode. These parameters were membrane thickness, reaction side flow rate, reaction side pressure, shell side pressure and H₂/CO₂ ratio in the feed.     

A comparison of conventional and optimized thermally coupled reactors for Fischer–Tropsch synthesis in GTL technology

In this research, the conditions at which a thermally coupled reactor – containing the Fischer–Tropsch synthesis reactions and the dehydrogenation of cyclohexane – operates have been optimized using differential evolution (DE) method. The proposed reactor is a heat exchanger reactor consists of two fixed bed of catalysts separated by the tube wall with the ability to transfer the produced heat from the exothermic side to the endothermic side. This system can perform the exothermic Fischer–Tropsch (F–T) reactions and the endothermic reaction of cyclohexane dehydrogenation to benzene simultaneously which can save energy and improve the reactions' thermal efficiency. The objective of the research is to optimize the operating conditions to maximize the gasoline (C₅⁺) production yield in the reactor's outlet as a desired product. The temperature distribution limit along the reactor to prevent the quick deactivation of the catalysts by sintering at both sides has been considered in the optimization process. The optimization results show a desirable progress compared with the conventional single stage reactor. Optimal inlet molar flow rate and inlet temperature of exothermic and endothermic sides and pressure of exothermic side have been calculated within the practicable range of temperature and pressure for both sides.     

Hydrocracking of Fischer‐Tropsch Wax with Tungstovanadophosphoric Salts as Catalysts

The synthesis of liquid hydrocarbons from CO₂ and H₂, based on renewable energy and H₂O electrolysis, respectively, in a power‐to‐liquid process is a promising concept for the substitution of fossil fuels. Such a process is based on Fischer‐Tropsch synthesis followed by hydrocracking to convert waxy products into transportation fuels such as gasoline and diesel oil. Heteropolyacid cesium salts as catalysts show appropriate activity for hydrocracking, and the selectivity in cracking model hydrocarbons and Fischer‐Tropsch wax can be tuned by the vanadium content of the catalyst. Thermal stability and surface properties were investigated, and the catalysts are compared with a classical H‐Y‐type zeolite used for hydrocracking.     

Hydrocracking of Fischer‐Tropsch waxes: Kinetic modeling via LHHW approach

A lumped kinetic model to describe the hydrocracking of complex mixtures of paraffins, such as Fischer‐Tropsch waxes, has been developed. A Langmuir‐Hinshelwood‐Hougen‐Watson approach has been followed, accounting for physisorption by means of the Langmuir isotherm. Finally, a complete form of the rate expression is used, thus introducing the equilibrium constants for dehydrogenation and protonation elementary steps. To minimize the number of model parameters, the kinetic and thermodynamic constants are defined as functions of the chain length. Vapor–liquid equilibrium is calculated along the reactor, and the hydrocarbons concentrations are described by means of fugacity. The model provides quite a good fitting of experimental results and is able to predict the effects of the operating conditions (temperature, pressure, H₂/wax ratio, and WHSV). Outstandingly, the estimated values and trends of the kinetic and thermodynamic constants (activation energies, Langmuir adsorption constants, etc.) are in line with their physical meaning. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Notes on the issues of equilibrium in the Fischer–Tropsch synthesis

The product distribution for the Fischer–Tropsch synthesis is normally described using the kinetically derived (Anderson–Schultz–Flory) ASF model. Variations of the kinetic model have been proposed to explain deviations from the ASF distribution. The Fischer–Tropsch system can be equally well described using a pseudo‐element (CH₂, H₂, O) equilibrium approach. A one‐parameter equilibrium model is derived for the product distributions for alkenes, alkanes and alcohols. The Fischer–Tropsch system should be considered as three separate partial equilibria systems: the product homologous series; the water gas shift system, and the redox behaviour of the catalyst with the H₂/O ratio of the gas. This approach correctly predicts the impacts of changes in a variety of parameters (temperature pressure, feed composition) on the ASF product distribution. In addition, the catalyst phase changes with gas composition and pressure, indicative of an equilibrium response. Equilibrium is of much greater importance to the Fischer–Tropsch system than previously thought, and the decision to use a complex kinetics‐based model rather than a simpler equilibrium based model should be taken with care.     

Optimization of Fischer‐Tropsch Process in a Fixed‐Bed Reactor Using Non‐uniform Catalysts

Optimization of Fischer‐Tropsch (FT) process in a fixed‐bed reactor is carried out using non‐uniform catalysts. The C₅⁺ yield of the reactions is maximized utilizing a combination of non‐uniform catalysts across the bed. A 1D heterogeneous model is developed to simulate the bed containing uniform and non‐uniform catalysts. It is found that the egg‐shell and surface‐layered catalysts result in higher C₅⁺ yield. Moreover, effects of cooling temperature are studied. Genetic Algorithm (GA) and Successive Quadratic Programming (SQP) methods are applied. Feed and cooling temperature are selected as decision variables together with distribution of non‐uniform catalysts along the bed. The optimization result shows 14.47 % increase in the C₅⁺ yield with respect to the base condition.     

Microstructured Fischer‐Tropsch Reactor Scale‐up and Opportunities for Decentralized Application

Current projects focusing on the energy transition in traffic will rely on a high‐level technology mix for their commissioning. One of those technologies is the Fischer‐Tropsch synthesis (FTS) that converts synthesis gas into hydrocarbons of different chain lengths. A microstructured packed‐bed reactor for low‐temperature FTS is tested towards its versatility for biomass‐based syngas with a high inert gas dilution. Investigations include overall productivity, conversion, and product selectivity. A 60‐times larger pilot‐scale reactor is further tested. Evaporation cooling is introduced which allows to increase the available energy extraction from the system. From that scale on, an autothermal operation at elevated conversion levels is applicable.     

Synthesis Gas Production Configurations for Gas‐to‐Liquid Applications

Different syngas configurations in a gas‐to‐liquid plant are studied including autothermal reformer (ATR), combined reformer, and series arrangement of gas‐heated reformer and ATR. The Fischer‐Tropsch (FT) reactor is based on a cobalt catalyst and the degrees of freedom are steam‐to‐carbon ratio, purge ratio of light ends, amount of tail gas recycled to synthesis gas (syngas) and FT synthesis units, and reactor volume. The production rate of liquid hydrocarbons is maximized for each syngas configuration. Installing a steam methane reformer in front of an ATR will reduce the total oxygen consumption per barrel of product by 40 % compared to the process with only an ATR. The production rate of liquid hydrocarbons is increased by 25.3 % since the flow rate of the purge stream for the ATR is the highest one compared to other configurations and contains mainly CO₂.     

Influence of Particle Size and Single‐Tube Diameter on Thermal Behavior of Fischer‐Tropsch Reactors. Part II. Eggshell Catalysts and Optimal Reactor Performance

The optimal combination of particle and tube size for simulation of a single tube of a wall‐cooled multitubular Fischer‐Tropsch (FT) reactor with cobalt as catalyst was determined. The maximum size of the tubes, realized without temperature runaway, enhances with increasing particle size until an optimal value is reached, thereby improving the production rate of liquid fuels per tube. Reasons for this are that heat transfer to the cooled tube wall for a given tube size is considerably enhanced by increasing the particle size and that the influence of pore diffusion on the effective rate of FT synthesis gets stronger with rising particle size, which reduces the temperature sensitivity of the reactor and decreases the danger of a temperature runaway. The simulations indicate that the use of FT eggshell catalysts is not an option for fixed‐bed reactors. The temperature sensitivity of the reactor is strongly enhanced, which decreases the maximum tube size and with that the productivity per tube. All these effects are valid in general for wall‐cooled fixed‐bed reactors. Respective criteria are presented.     

Modeling of Multi‐Tubular Reactors for Fischer‐Tropsch Synthesis

The results of the simulation of multi‐tubular Fischer‐Tropsch reactors based on a two‐dimensional pseudo‐homogeneous model are presented. The model takes into account the intrinsic kinetics of two commercial iron and cobalt catalysts, intraparticle mass transfer limitations, and the radial heat transfer within the fixed bed and to the cooling medium (boiling water). The effective rate with Co is slightly higher than with Fe. Hence, a temperature level can be used for Co that is 20 °C lower compared to Fe. The conversion and product selectivies are then almost the same and the reactor can be operated safely without a temperature runaway. The results of the simulations are consistent with literature data and show that there is still room for improvement of fixed bed FT reactors, e.g., by an enhanced heat transfer.