Modeling of hydrodynamics in microchannel reactor for Fischer–Tropsch synthesis

Hydrodynamics of liquid and gaseous products in microchannel reactor for Fischer–Tropsch synthesis is considered. It is supposed, that liquid and gaseous products of the synthesis move downward in annular flow regime. A microchannel with irregular internal walls is investigated in cylindrical symmetry. In proposed numerical technique the peculiarities of coating the microchannel walls by cobalt-based catalytic particles are taken into account. System of equations of two-phase hydrodynamics is based on the generalized equations for mass flowrate and momentum of liquid film and gaseous phase. Stable numerical algorithm for calculation the thermodynamic equilibrium in gas–liquid mixtures of synthesis products is proposed. Calculation results illustrate thermophysical properties of liquid and gaseous products. In the hydrodynamics model variations along a microchannel mass fractions and thermophysical properties of liquid and gaseous products were taking into account. Principal hydrodynamical difference between a smooth microchannel and a microchannel with random roughness is explained. Hydrodynamical parameters and gradient of pressure are investigated as functions of pressure, temperature, averaged diameter of a microchannel and chain growth probability.     

Temperature oscillation in a catalytic particle of Fischer–Tropsch synthesis

Variational method for investigation oscillation of temperature and synthesis gas concentration inside spherical catalytic particle is developed. Approximate distributions of temperature and concentrations in a particle with internal heat release and synthesis gas consumption are obtained. Stationary parameters are found on the base of equations for square of norms functions which present temperature and concentrations distributions. In the frame of small disturbances of temperature and concentrations thermal stability is investigated. Variational principle is used for estimation first eigenvalues of system of partial differential equations with essentially variable coefficients. It is revealed, that diffusion resistance of synthesis gas inside a porous particle can lead to occurrence oscillation regime. Approximated results are compared with direct numerical experiments for modeling nonstationary heat and mass transfer in a catalytic porous particle. Two approaches for describing heat and mass stability of catalytic particle with internal heat source and synthesis gas loss are discussed.     

Enhancing the production of hydrogen via water–gas shift reaction using Pd-based membrane reactors

In this work, it is described an experimental study regarding the performance of a Pd–Ag membrane reactor recently proposed and suitable for the production of ultra-pure hydrogen. A dense metallic permeator tube was assembled by an innovative annealing and diffusion welding technique from a commercial flat sheet membrane of Pd–Ag. A “finger-like” configuration of the self-supported membrane has been designed and used as a packed-bed membrane reactor (MR) for producing ultra-pure hydrogen via water–gas shift reaction (WGS).     

Membrane electrolysis of Bunsen reaction in the iodine–sulphur process for hydrogen production

In traditional IS process for production of hydrogen by water decomposition, the Bunsen reaction (SO₂ + I₂ + 2H₂O → H₂SO₄ + 2HI) was carried out by direct contact of SO₂ with aqueous solution of I₂ where a large excess of I₂ (8 mol) and H₂O (16 mol) were required. Excess amounts of these chemicals severely affected the overall thermal efficiency of the process and new ways including membrane electrolysis was reported in literature for carrying out Bunsen reaction where the amount of excess chemicals can be greatly reduced. We have carried out Bunsen reaction in a two-compartment membrane electrolysis cell containing graphite electrodes and Nafion 117 membrane as a separator between the two-compartments. Electrolysis was carried out at room temperature with continuous recirculation of anolyte and catholyte. Electrolysis was done in constant-current mode with current density in the range of 1.6 A/dm² to 4.8 A/dm². Initial concentrations of H₂SO₄ and HI were about 10 and 5 N, respectively and I₂/HI molar ratio in the catholyte was varied in the range of 0.25–1.5. Current efficiency was found to be close to 100% indicating absence of any side reaction at the electrodes. Cell voltage was found to vary linearly with current densities up to 80 A/dm² and for I₂/HI molar ratio in the range of 0.25–1.5 the cell voltage was found to be lowest for the value of 0.5.     

Thermodynamic optimization of biomass gasification for decentralized power generation and Fischer–Tropsch synthesis

In recent years, biomass gasification has emerged as a viable option for decentralized power generation, especially in developing countries. Another potential use of producer gas from biomass gasification is in terms of feedstock for Fischer–Tropsch (FT) synthesis – a process for manufacture of synthetic gasoline and diesel. This paper reports optimization of biomass gasification process for these two applications. Using the non–stoichometric equilibrium model (SOLGASMIX), we have assessed the outcome of gasification process for different combinations of operating conditions. Four key parameters have been used for optimization, viz. biomass type (saw dust, rice husk, bamboo dust), air or equivalence ratio (AR = 0, 0.2, 0.4, 0.6, 0.8 and 1), temperature of gasification (T = 400, 500, 600, 700, 800, 900 and 1000 °C), and gasification medium (air, air–steam 10% mole/mole mixture, air–steam 30%mole/mole mixture). Performance of the gasification process has been assessed with four measures, viz. molar content of H₂ and CO in the producer gas, H₂/CO molar ratio, LHV of producer gas and overall efficiency of gasifier. The optimum sets of operating conditions for gasifier for FT synthesis are: AR = 0.2–0.4, Temp = 800–1000 °C, and gasification medium as air. The optimum sets of operating conditions for decentralized power generation are: AR = 0.3–0.4, Temp = 700–800 °C with gasification medium being air. The thermodynamic model and methodology presented in this work also presents a general framework, which could be extended for optimization of biomass gasification for any other application.     

Optimization of liquid–liquid phase separation characteristics in the Bunsen section of the sulfur–iodine hydrogen production process

In order to optimize the sulfur–iodine thermochemical cycle, a series of experiments were conducted to investigate the separation characteristics of the liquid–liquid phase in the H₂SO₄/HI/I₂/H₂O quaternary solution produced by Bunsen reaction. The effects of the solution composition in the feed and the operating temperature on the separation characteristics were analyzed to determine the preferable operating conditions in the Bunsen section. The increases in both the iodine content and the operating temperature improved the separation characteristics of the liquid–liquid phase when the occurrence of secondary reactions was neglected. The amount of impurities in both phases obviously decreased as the iodine content increased. The effect of the iodine content was more significant relative to that of the operating temperature. The optimal operating conditions were proposed to achieve the concentrations of HI in the HIx phase in excess of the azeotropic composition.     

Continuous hydrogen production via the steam–iron reaction by chemical looping in a circulating fluidized-bed reactor

The steam–iron reaction was examined in a two-compartment fluidized-bed reactor at 800–900 °C and atmospheric pressure. In the fuel reactor compartment, freeze-granulated oxygen carrier particles consisting of Fe₃O₄ supported on inert MgAl₂O₄ were reduced to FeO with carbon monoxide or synthesis gas. The reduced particles were transferred to a steam reactor compartment, where they were oxidized back to Fe₃O₄ by steam, while at the same time producing H₂. The process was operated continuously and the particles were transferred between the reactor compartments in a cyclic manner. In total, 12 h of experiments were conducted of which 9 h involved H₂ generation. The reactivity of the oxygen carrier particles with carbon monoxide and synthesis gas was high, providing gas concentrations reasonably close to thermodynamic equilibrium, especially at lower fuel flows. The amount of H₂ produced in the steam reactor was found to correspond well with the amount of fuel oxidized in the fuel reactor, which suggests that all FeO that was formed were also re-oxidized. Despite reduction of the oxygen carrier to FeO, defluidization or stops in the solid circulation were not experienced. Used oxygen carrier particles exhibited decreased BET specific surface area, increased bulk density and decreased particle size compared to fresh. This indicates that the particles were subject to densification during operation, likely due to thermal sintering. However, stable operation, low attrition and absence of defluidization were still achieved, which suggest that the overall behaviour of the oxygen carrier particles were satisfactory.     

Study of hydrogen production and storage based on aluminum–water reaction

The rate and yield of hydrogen production from the reaction between activated aluminum and water has been investigated. The effect of different parameters such as water–aluminum ratio, water temperature and aluminum particle size and shape was studied experimentally. The aluminum activation method developed in-house involves 1%–2.5% of lithium-based activator which is diffused into the aluminum particles, enabling sustained reaction with tap water or sea water at room temperature. Hydrogen production rates in the range of 200–600 ml/min/g Al, at a yield of about 90%, depending on operating parameters, were demonstrated. The work further studied the application in proton exchange membrane (PEM) fuel cells in order to generate green electric energy, demonstrating theoretical specific electric energy storage that can exceed batteries by 10–20 folds.     

Modelling and optimization of Fischer–Tropsch products through iron catalyst in fixed-bed reactor

The most important issue for alternative renewable energy source instead of fossil fuels is environmental concern. Human energy supply from carbon waste has an important role in environmental health. Fischer–Tropsch synthesis is a reaction whereby carbon waste such as woods, foodstuffs, sewage, and any material that contains carbon in their structure, produces all oil derivatives including gasoline, diesel, etc. In this paper, the products model and the effective parameters of the model and also, the interaction between the parameters were investigated. $P_{{\text{H}}_2}$ and $T1P_{{\text{H}}_2}$ are effective parameters and interaction in the model, respectively. Finally, the optimization of products was discussed. The best operating conditions for minimizing CO₂ and CH₄ and maximizing C₂H₄ and C₂H₆ were investigated. This method can be suitable for environmental purposes.     

Solid particle decomposition and hydrolysis reaction kinetics in Cu–Cl thermochemical hydrogen production

This paper examines cupric chloride solid conversion during hydrolysis in the thermochemical copper-chlorine (Cu–Cl) cycle of hydrogen production. The hydrolysis reaction is a challenging step, due to the excess steam requirement and decomposition of cupric chloride (CuCl₂) into cuprous chloride (CuCl) and chlorine (Cl₂). In this paper, the hydrolysis and decomposition reactions are analyzed with respect to chemical equilibrium conversion and the reaction kinetics. The effects of operating parameters are examined, including the temperature, pressure and excess steam, on equilibrium conversion. It is shown that the reaction kinetics expression that represents a reversible reaction reflects the equilibrium limitation on the solid conversion, rather than first-order kinetics.     

Process integration of hydrolysis and electrolysis processes in the Cu–Cl cycle of hydrogen production

Process integration of the hydrolysis and electrolysis processes is one of the most important engineering challenges associated with the Cu–Cl cycle of hydrogen production. The kinetics of the hydrolysis reaction indicates the reversibility of this process which requires H₂O in excess of the stoichiometric quantity, which significantly decreases the overall thermal efficiency of the Cu–Cl cycle. Moreover, the HCl concentration in the produced gas mixture of H₂O and HCl in the hydrolysis reaction is in much lower concentration of the electrolysis reaction requirement for an effective electrolytic cell performance. This paper simulates an integrated process model of the hydrolysis and electrolysis processes by introducing intermediate heat recovery steam generator (HRSG) and HCl–H₂O separation process consisting of rectification and absorption columns. In the separation processes, the influence of operating parameters including reflux ratio, mole fraction of HCl in the feed stream, solvent flow rate and temperature, and column configuration variables such as the location of feed stage and number of stages on the heat duty requirements and the composition of products are investigated and analysed. It is shown that the amount of steam generated in the HRSG unit satisfies the extra steam requirement of the hydrolysis reaction up to 14 times more than its stoichiometric value and the separation process effectively provides HCl acid up to the concentration of 22 mol% for the electrolysis reaction.     

The mechanism and kinetics study of Fischer‒Tropsch reaction over iron-nickel-cerium nano-structure catalyst

The kinetic of Fischer‒Tropsch reaction was investigated using the iron‒nickel‒cerium nano-structure catalyst synthesized by the hydrothermal method in the fixed-bed micro reactor for the first time. The kinetic tests carried out under operating conditions including the pressure of 2–10 bar, temperature of 230–250 °C, molar ratio H₂/CO of 1, and the gas space velocity of 3000 h⁻¹. Twenty-two set of reaction mechanisms were proposed on the basis of the adsorption nature of carbon monoxide, and hydrogen using the Langmuir‒Hinshelwood‒Hougen‒Watson, and Eley Rideal adsorption theories for the FT reaction. The rate equations of CO consumption were obtained based on the proposed reaction mechanisms. The best kinetic model was chosen by non-linear regression analysis and its kinetic parameters including activation energy, adsorption enthalpies of H₂, and CO were estimated 60.4, −3.24, and −65.7 kJ/mol respectively. The nanocatalyst was characterized by various techniques such as XRD, FESEM, and Brunauer–Emmett–Teller surface area measurements.     

Optimization of flame stabilization methods in the premixed microcombustion of hydrogen–air mixture

The premixed combustion of a lean hydrogen–air mixture is analyzed in this study to examine various properties and flame stabilization. A two-dimensional (2D) analysis of a microscale combustor is performed with various shapes of bluff bodies (e.g., circular and triangular). Nine bluff bodies are placed at the entrance of the microscale combustor and solved with 2D governing equations. The analysis is performed with the three velocities of 10, 20, and 30 m/s, but the equivalence ratio is fixed in all cases. The various characteristics of the microscale combustor are studied such as the temperature of the wall, difference in peak temperature, the mean velocity at the outlet, and temperature of the exhaust gases. Flame stabilization depends on various factors such as bluff body shape and size, and the velocity of the fuel–air mixture at the inlet and recirculation zone. In comparison to all bluff body cases, we observe that the wall blade bluff body is the most efficient (low exhaust gas temperature, large recirculation zone, low mean velocity at the outlet of the microcombustor, and high wall temperature) compared with all eight other bluff body cases. Combustion efficiency is directly proportional to the wall temperature, meaning that the microcombustor with wall blade bluff bodies is more efficient with a stabilized flame. The simulation results are compared with published data on an L/D ratio of 15.     

Optimization of hydrogen production via toluene steam reforming over Ni–Co supported modified-activated carbon using ANN coupled GA and RSM

Hydrogen (H₂) is a clean fuel that can be produced from various resources including biomass. Optimization of H₂ production from catalytic steam reforming of toluene using response surface methodology (RSM) and artificial neural network coupled genetic algorithm (ANN-GA) models has been investigated. In RSM model, the central composite design (CCD) is employed in the experimental design. The CCD conditions are temperature (500–900 °C), feed flow rate (0.006–0.034 ml/min), catalyst weight (0.1–0.5 g) and steam-to-carbon molar ratio (1–9). ANN model employs a three-layered feed-forward backpropagation neural network in conjugation with the tangent sigmoid (tansig) and linear (purelin) as the transfer functions and Levenberg-Marquardt training algorithm. Best network structure of 4-14-1 is developed and utilized in the GA optimization for determining the optimum conditions. An optimum H₂ yield of 92.6% and 81.4% with 1.19% and 6.02% prediction error are obtained from ANN-GA and RSM models, respectively. The predictive capabilities of the two models are evaluated by statistical parameters, including the coefficient of determination (R²) and root mean square error (RMSE). Higher R² and lower RSME values are reported for ANN-GA model (R² = 0.95, RMSE = 4.09) demonstrating the superiority of ANN-GA in determining the nonlinear behavior compared to RSM model (R² = 0.87, RMSE = 6.92). These results infer that ANN-GA is a more reliable and robust predictive steam reforming modelling tool for H₂ production optimization compared to RSM model.     

Numerical analysis of the electrochemical Bunsen reaction for hydrogen production from sulfur–iodine cycle

The sulfur–iodine (S-I) water-splitting cycle is one of the most promising hydrogen production methods. The Bunsen reaction in the cycle affects the flowsheet complexity and thermal efficiency, but an electrochemical technique has recently been applied to make the S-I cycle more simplified and energy efficient. However, the performance of the electrochemical Bunsen reaction, especially the electrode reactions inside the electrolytic cell (EC) are not clear at present. In this work, a two-dimensional numerical model of EC was developed. The detailed reaction process was numerically calculated with considering the coupling of mass transfer and electrochemical reactions, and was verified using experimental data. The effects of various operating parameters on the reactions were investigated. The results showed that the increase of current density significantly improves the conversion rates of reactants. A higher temperature is unfavorable for concentrating H₂SO₄ and HI. Increase in the inlet flow rate reduces the conversion rates of reactants, but the impact declines with further rising flow rate. An optimal operating condition is also proposed. The theoretical simulation study will provide guidance for the improvement of experimental work.     

Apparent kinetics of the Bunsen reaction in I2/HI solution for the iodine–sulfur hydrogen production process

Massive hydrogen production featuring high efficiency, CO₂ free, and cost effectiveness is a crucial challenge for the hydrogen economy. Nuclear hydrogen production through thermochemical iodine–sulfur (IS) process is a potential candidate for this purpose. Chemical reaction kinetics data are indispensable for developing a high-performance reactor as well as the scaling up of the process. The apparent kinetics of the reaction under simulated recycling conditions of IS closed cycle operation was studied by initial rate method. The effects of key parameters, including agitation speed, SO₂ partial pressure, I₂ concentration, and reaction temperature, on reaction rate, were systematically investigated by measuring the variation in SO₂ pressure with reaction time. Initial rate analysis method indicated that the Bunsen reaction rates were 0.23 ± 0.01 and 0.77 ± 0.01 order with respect to SO₂ pressure and I₂ concentration. The apparent activation energy was 5.86 ± 0.21 kJ/mol. Based on these results, an exponential rate expression of the Bunsen reaction was established. In addition, a simplified method for calculation of kinetics parameters was proposed and compared with conventional techniques. Experimental results provide theoretical basis for design and development of Bunsen reactors and for elucidating the reaction process.     

Three-dimensional electrochemical Bunsen reaction characteristics in the sulfur–iodine water splitting cycle for hydrogen production

Hydrogen production from the sulfur–iodine water splitting cycle integrated with solar or nuclear energy has been proposed as a promising technique. Bunsen reaction is one of the three main steps in the cycle and electrochemical method has been applied to this reaction. In present work, a three-dimensional numerical study of the electrochemical Bunsen reaction was conducted. A three-dimensional, steady state, laminar and isothermal mathematical model of electrolytic cell was developed and verified by experiments. The spatial maldistribution of species concentration was found between electrodes and proton exchange membrane (PEM). The electric power drives most H₂SO₄ and I₂ to the anode and cathode surface, respectively, while the proton attraction contributes to HI enrichment on the surface of PEM. At the high inlet H₂SO₄ concentration of 50 wt%, the transformation of flow channel from single serpentine to single entry & double serpentine with the same inlet flow rate cannot solve the insufficient problem of SO₂. But the increase of the overall inlet flow rate in the double entry & double serpentine flow channel make SO₂ sufficient for anode reaction. Further decreasing the inlet H₂SO₄ concentration to 40 wt% and 30 wt% make the initial SO₂ sufficient for overall reactions. The single serpentine channel gives the highest SO₂ conversion rate, followed by the single entry & double serpentine and double entry & double serpentine flow channels. The single serpentine flow channel at the H₂SO₄ inlet concentration of 40 wt% is found optimal for achieving a high electrochemical Bunsen reaction performance.     

Effect of mass transfer on the deactivation model and GPLE parameters of Co/γ-Al2O3 catalysts in the Fischer Tropsch synthesis

The activity of catalysts with various sizes was compared in a fixed-bed Fischer–Tropsch reactor under similar operating conditions by determining the deactivation model. Catalyst size had no impact on the type of deactivation model. The smaller catalyst showed a smaller deactivation constant of catalyst (k_d) and a lower deactivation rate in the initial stage. The decline in the activities of the catalyst with a mesh size of 40 was lower than the other catalysts, suggesting its higher long-term stability (a_ss). Larger catalyst sizes led to the fouling of carbon and heavy hydrocarbons, decreasing the specific surface of the catalyst, thus increasing the pore diffusion resistance and further decrementing the catalyst activities.     

Activated carbon catalysts for the production of hydrogen via the sulfur–iodine thermochemical water splitting cycle

Seven activated carbon catalysts obtained from a variety of raw material sources and preparation methods were examined for their catalytic activity to decompose hydrogen iodide (HI) to produce hydrogen, a key reaction in the sulfur–iodine (S–I) thermochemical water splitting cycle. Activity was examined under a temperature ramp from 473 to 773 K. Within the group of lignocellulosic steam-activated carbon catalysts, activity increased with surface area. However, both a mineral-based steam-activated carbon and a lignocellulosic chemically activated carbon displayed activities lower than expected based on their higher surface areas. In general, ash content was detrimental to catalytic activity while total acid sites, as determined by Boehm's titrations, seemed to favor higher catalytic activity within the group of steam-activated carbons. These results suggest that activated carbon raw materials and preparation methods may have played a significant role in the development of surface characteristics that eventually dictated catalyst activity and stability as well.     

Influence of the structural and surface characteristics of activated carbon on the catalytic decomposition of hydrogen iodide in the sulfur–iodine cycle for hydrogen production

Coal-based activated carbon (AC-COAL) catalysts subjected to acid treatment were tested to evaluate their performance on hydrogen-iodide (HI) decomposition for hydrogen production in sulfur-iodine (SI or IS) cycle. The effects of acid treatment on catalysts and the relations between sample properties and catalytic activities were discussed. The AC-COAL obtained by non-oxidative acid treatments had the best catalytic activity. However, the catalytic activity of AC-COAL decreased after the treatment of nitric acid. Higher surface area, higher carbon contents, lower ash contents and fewer surface oxidation groups contributed to the catalytic activity of ACs. HI decomposition on the AC surface itself may be due to high densities of unpaired electrons associated with structural defects and edge plane sites with similar structural ordering. Moreover, the oxygen-containing groups reduced the electron transfer capability associated with the basal plane sites.