Comparative study of the optimal operation of methane reforming process in cylindrical and spherical reactors using multi-objective optimization

We present a multi-objective optimization (MOO) based study of the optimal operation of methane reformer for spherical reactor and compare the results with the ones for the cylindrical reactor. We considered three objective functions for this comparative study, namely maximization of hydrogen production, minimization of carbon dioxide emission, and minimization of power loss due to pressure drop in the reactor. We solve four MOO problems, which include three 2-objective problems with each pair of the aforementioned three objectives. In addition, we also solve a three objective problem considering all the three objectives. The optimization variables considered for the MOO study correspond to the feed conditions. Specifically, the three variables include the inlet temperature and the molar feed ratios of oxygen to methane & steam to methane.     

Tri-reformer with O2 side-stream distribution for syngas production

Methane tri-reforming combines steam reforming, dry reforming and partial oxidation of methane in a single reactor. The heat generated by the exothermic partial oxidation of methane can be used to supply the energy for the other two endothermic reactions (dry and steam reforming of methane). The thermoneutral condition allows the use of a tri-reformer with a simpler reactor structure since no external heat supply is necessary. Thermodynamic analysis of the thermoneutral reactor was performed using Gibbs free energy minimization approach. Conventional tri-reformers have heat and mass management problems. We developed a novel tri-reformer concept that utilizes proper distribution of O₂ gas to the reactor to address the problems. The optimization of the proposed reactor was performed with the objective function of minimizing total annual cost. Maintaining the peak temperatures by adjusting the O₂ flow rate at the distribution point along the reactor was shown to provide good load flexibility for the change in methane flow rate.     

Multi-objective optimization of molten carbonate fuel cell system for reducing CO2 emission from exhaust gases

The aim of this paper is to investigate the implementation of a molten carbonate fuel cell (MCFC) as a CO₂ separator. By applying multi-objective optimization (MOO) using the genetic algorithm, the optimal values of operating load and the corresponding values of objective functions are obtained. Objective functions are minimization of the cost of electricity (COE) and minimization of CO₂ emission rate. CO₂ tax that is accounted as the pollution-related cost, transforming the environmental objective to the cost function. The results show that the MCFC stack which is fed by the syngas and gas turbine exhaust, not only reduces CO₂ emission rate, but also produces electricity and reduces environmental cost of the system.     

Performance assessment of thermochemical CO2/H2O splitting in moving bed and fluidized bed reactors

In this paper, we present the assessment of moving bed reactors and fluidized bed reactors operating in different fluidizing regimes for solar thermochemical redox cycles (STRC) for syngas production. The reduction reactor with a moving bed (MB_RED) while the oxidation reactor (OXI) is either a moving bed reactor (MB_OXI) or bubbling bed (BB_OXI) yields higher performance. It was observed that only water splitting is suitable at 1400 °C and 10⁻³ bar reduction conditions. The higher reduction temperature and pressure improved the efficiency of the CO₂/H₂O splitting unit. The requirement of the H₂/CO ratio drives the gas feed (CO₂/H₂O) into OXI. To achieve an H₂/CO ratio of 1, MB_OXI and BB_OXI require an equimolar mixture of CO₂ and H₂O at 1600 °C. However, to achieve a similar H₂/CO ratio at a lower temperature of 1500 °C, the gas feed of the CO₂/H₂O ratio required is 3. A similar H₂/CO ratio is achieved for OXI operating in a turbulent and fast fluidizing, but the selectivity is lower due to lower reaction rates. OXI as a transport bed is least suited based on solid conversion (X_OXI), H₂/CO, or efficiency. The results are useful in designing the redox reactors for syngas.     

Performance of gas-phase photocatalytic reactors on hydrogen production

Three types of high-performance photocatalytic reactors were developed for gas-phase photocatalytic hydrogen (H₂) production from hydrogen sulphide (H₂S) and effective photocatalytic decomposition of gaseous H₂S at a very low concentration is investigated. In this paper, three lab-scale photocatalytic reactors viz., packed bed photocatalytic reactor, catalyst coated fixed bed photocatalytic reactor and catalyst dispersed photocatalytic reactors were developed to study the performance of reactors on hydrogen production. The novel photocatalyst (CdS + ZnS)/Fe₂O₃ and the optimized catalyst dosage, H₂S gas flow rate, pollutant concentration, light irradiations were used. The experimental result indicates that packed bed photocatalytic reactor can effectively splits the H₂S into hydrogen (i.e. 98%) and rapidly decompose H₂S toward zero concentration than the other two reactors. Hence the bench-scale photocatalytic reactor was fabricated in packed bed reactor and the maximum hydrogen conversion achieved from hydrogen sulphide was found to be 98%.     

Hydrogen enrichment and separation from synthesis gas by the use of a membrane reactor

One of the objectives of the CHRISGAS project was to study innovative gas separation and gas upgrading systems that have not been developed sufficiently yet to be tested at a demonstration scale within the time frame of the project, but which show some attractive merits and features for further development. In this framework CIEMAT studied, at bench scale, hydrogen enrichment and separation from syngas by the use of membranes and membrane catalytic reactors.In this paper results about hydrogen separation from synthesis gas by means of selective membranes are presented. Studies dealt with the evaluation of permeation and selectivity to hydrogen of prepared and pre-commercial Pd-based membranes. Whereas prepared membranes turned out to be non-selective, due to discontinuities of the palladium layer, studies conducted with the pre-commercial membrane showed that by means of a membrane reactor it is possible to completely separate hydrogen from the other gas components and produce pure hydrogen as a permeate stream, even in the case of complex reaction system (H₂/CO/CO₂/H₂O) under WGS conditions gas mixtures.The advantages of using a water-gas shift membrane reactor (MR) over a traditional fixed bed reactor (TR) have also been studied. The experimental device included the pre-commercial Pd-based membrane and a commercial high temperature Fe–Cr-based, WGS catalyst, which was packed in the annulus between the membrane and the reactor outer shell. Results show that in the MR concept, removal of H₂ from the reaction side has a positive effect on WGS reaction, reaching higher CO conversion than in a traditional packed bed reactor at a given temperature. On increasing pressure on the reaction side permeation is enhanced and hence carbon monoxide conversion increases.     

Reduced superstructure solution of MINLP problem in refinery hydrogen management

Minimizing Hydrogen waste into fuel gas within the H₂ network in a refinery is the objective function of an optimization problem in this paper. The superstructure obtained for a refinery wide concept, is first solved and validated for literature cases, then is reduced by heuristic rules, based on engineering judgment. The reduced superstructure contains all simulation procedures of pseudo-components definitions, fine tunings of all unit operations to reach actual operating conditions, reactions characterization, linear and nonlinear equalities and inequalities as system constraints. The set of governing equations are solved with Genetic Algorithm. Based on this optimization, in an Iranian refinery 22.6% reduction of H₂ production and a saving of 1.19 million $/year could be achieved.     

Optimization of tri-reformer reactor to produce synthesis gas for methanol production using differential evolution (DE) method

This paper presents a study on optimization of a fixed bed tri-reformer reactor (TR). This reactor has been used instead of conventional steam reformer (CSR) and auto thermal reformer (CAR). A theoretical investigation has been performed in order to evaluate the optimal operating conditions and enhancement of methane conversion, hydrogen production and desired H₂/CO ratio as a synthesis gas for methanol production. A mathematical heterogeneous model has been used to simulate the reactor. The process performance under steady state conditions was analyzed with respect to key operational parameters (inlet temperature, O₂/CH₄, CO₂/CH₄ and steam/CH₄ ratios). The influence of these parameters on gas temperature, methane conversion, hydrogen production and H₂/CO ratio was investigated. Model validation was carried out by comparison of the reforming model results with industrial data of CSR. Differential evolution (DE) method was applied as a powerful method for optimization. Optimum feed temperature and reactant ratios (CH₄/CO₂/H₂O/O₂) are 1100 K and 1/1.3/2.46/0.47 respectively. The optimized TR has enhanced methane conversion by 3.8% relative to industrial reformers in a single reactor. Methane conversion, hydrogen yield and H₂/CO ratio in optimized TR are 97.9%, 1.84 and 1.7 respectively. The optimization results of tri-reformer were compared with the corresponding predictions from process simulation software operated at the same feed conditions.     

Syngas and H2 production from natural gas using CaFe2O4 – Looping: Experimental and thermodynamic integrated process assessment

Experimental methods and thermodynamic simulations were employed in this study to assess the H₂ production potential of a CaFe₂O₄ based blue H₂ production process from natural gas (NG, >90 vol%CH₄). Fixed bed reactor testing was used to verify the product outcomes. Syngas production from methane using CaFe₂O₄ was demonstrated. Stable H₂ production with high steam conversion was demonstrated with the CaFe₂O₄ when reduced with methane. The thermodynamic integrated process simulation enabled simulation of the process in two and three reactor configurations to understand the feasibility of a heat integrated system. The 2-reactor process used the generation of syngas as the prevailing mode for H₂ generation while the 3-reactor system utilized steam water splitting in a dedicated reactor as the prevailing mode to generate H₂. Simulation of the 2-reactor process's FR showed syngas generation similar to the products from fixed bed demonstrations, establishing a connection between thermodynamic simulation and experimental data. The H₂ yield potentials of the various configurations were determined and compared to steam methane reforming with capture (SMR-CCS) and a Fe₂O₃ based system from the literature. The 2-reactor process has the potential to generate 1.6–2.1 mol of H₂/mole of NG fed to the system. Three reactor configurations showed the highest potential for H₂ yield with a range of 2.2–2.56 mol of H₂/mole NG but with the need for additional CCS at the highest yield. A thermal management approach was introduced that combined the chemistries of CaFe₂O₄ and CuFeAlO₄ which enabled increasing the potential yield to 2.66 mol H₂/mol NG and enabling a system without the need for addition carbon capture to meet 90% threshold targets. The three reactor cases showed the most competitive performance in comparison to SMR-CCS with up to a 14.6% improvement in H₂ yield.     

Optimal design and operation of ammonia decomposition reactors

The design and steady-state operation of a packed bed reactor with tubular geometry is optimized. Direct optimal control methods are used. Two objective functions are considered: (i) minimization of the ammonia mass fraction at reactor outlet and (ii) minimization of the heat flux necessary to reach a predefined value of the ammonia mass fraction at reactor outlet. The optimization process is performed by using different controls, that is, the space distributions of (1) tube wall temperature T_w , (2) circular tube diameter D_tube , and (3) diameter d_p of the catalyst spherical particles. Results for the first objective function are as follows. The optimal distribution of T_w along the reactor consists of a constant temperature or a U-shaped space temperature distribution, respectively, depending on the allowed range of variation of T_w . The optimal space distribution of D_tube (or, in other words, the shape of the reactor tube) depends of T_w . For smaller values of T_w the tube is narrower at inlet and larger at outlet while the reverse situation happens for larger values of T_w . For lower T_w values, particles with smaller diameter d_p are placed at reactor inlet while when higher values of T_w are considered, particles with larger d_p are placed at reactor inlet. When both D_tube and d_p are used as controls, the optimization results are generally different from the results obtained from one-control optimization. Results for the second objective function are as follows. The optimal space distribution of T_w starts with high values at reactor inlet. Next, the temperature decreases abruptly towards a minimum (which is lower for longer tubes). Finally, the temperature increases smoothly towards a maximum near the reactor outlet. The required heat flux slightly decreases by increasing the tube length. The optimal D_tube ranges between its maximum allowed value (at reactor inlet) and its minimum allowed value (at reactor outlet). The best performance is obtained for catalyst particles of the smallest allowed diameter.     

Comparison of conventional and spherical reactor for the industrial auto-thermal reforming of methane to maximize synthesis gas and minimize CO2

Auto-thermal reforming (ATR), a combination of exothermic partial oxidation and endothermic steam reforming of methane, is an important process to produce syngas for petrochemical industries. In a commercial ATR unit, tubular fixed bed reactors are typically used. Pressure drop across the tube, high manufacturing costs, and low production capacity are some disadvantages of these reactors. The main propose of this study is to offer an optimized radial flow, spherical packed bed reactor as a promising alternative for overcoming the drawbacks of conventional tubular reactors. In the current research, a one dimensional pseudo-homogeneous model based on mass, energy, and momentum balances is applied to simulate the performance of packed-bed reactors for the production of syngas in both tubular and spherical reactors. In the optimization section, the proposed work explores optimal values of various decision variables that simultaneously maximize outlet molar flow rate of H₂, CO and minimize molar flow rate of CO₂ from novel spherical reactor. The multi-objective model is transformed to a single objective optimization problem by weighted sum method and the single optimum point is found by using genetic algorithm. The optimization results show that the pressure drop in the spherical reactor is negligible in comparison to that of the conventional tubular reactor. Therefore, it is inferred that the spherical reactor can operate with much higher feed flow rate, more catalyst loading, and smaller catalyst particles.     

CO2 gasification of coal cokes using internally circulating fluidized bed reactor by concentrated Xe-light irradiation for solar gasification

For the solar thermochemical gasification of coal coke to produce CO + H₂ synthetic gas using concentrated solar radiation, a windowed reactor prototype is tested and demonstrated at laboratory scale for CO₂ gasification of coal coke using concentrated Xe light from a 3-kW_th sun simulator. The reactor was designed to be combined with a solar reflective tower or beam-down optics. The results for gasification performance (CO production rate, carbon conversion, and light-to-chemical efficiency) are shown for various CO₂ flow rates and ratios. A kinetics analysis based on homogeneous and shrinking core models and the temperature distributions of the prototype particle bed are compared with those for a conventional fluidized bed reactor tested under the same Xe light irradiation and CO₂ flow-rate conditions. The effectiveness and potential impacts of internally circulating fluidized bed reactors for enhancing gasification performance levels and inducing consistently higher bed temperatures are discussed in this paper.     

Thermodynamic analysis of steam assisted conversions of bio-oil components to synthesis gas

The aim of this study is to investigate the thermodynamics of steam assisted, high-pressure conversions of model components of bio-oil – isopropyl alcohol, lactic acid and phenol – to synthesis gas (H₂ + CO) and to understand the effects of process variables such as temperature and inlet steam-to-fuel ratio on the product distribution. For this purpose, thermodynamic analyses are performed at a pressure of 30 bar and at ranges of temperature and steam-to-fuel ratio of 600–1200 K and 4–9, respectively. The number of moles of each component in the product stream and the product composition at equilibrium are calculated via Gibbs free energy minimization technique. The resulting optimization problems are solved by using the Sequential quadratic programming method. The results showed that all of the model fuels reached near-complete conversions to H₂, CO, CO₂ and CH₄ within the range of operating conditions. Temperature and steam-to-fuel ratio had positive effects in increasing hydrogen content of the product mixture at different magnitudes. Production of CO increased with temperature, but decreased at high steam-to-fuel ratios. Conversion of model fuels in excess of 1000 K favored molar H₂/CO ratios around 2, the synthesis gas composition required for Fischer–Tropsch and methanol syntheses. It was also possible to adjust the H₂/CO ratios and the amounts of CH₄ and CO₂ in synthesis gas by steam-to-fuel ratio, the value depending on temperature and the fuel type. Product distribution trends indicated the presence of water–gas shift and methanation equilibria as major side reactions running in parallel with the steam reforming of the model hydrocarbons.     

A CFD study on H2-permeable membrane reactor for methane CO2 reforming: Effect of catalyst bed volume

A 2D axisymmetric model is developed for a H₂-permeable membrane reactor for methane CO₂ reforming. The effect of catalyst bed volume on CH₄ conversion and H₂ permeation rate is investigated. The simulation results indicate that catalyst bed volume with a shell radius of 9 mm is optimal for a tubular Vycor glass membrane with a diameter of 10 mm and H₂ permeance of 2x10⁻⁶ mol/m²/Pa/s. The concentration polarization at the retentate side and the accumulation of H₂ at permeate side make it hard to extract the H₂ production at the zone far from the membrane surface. Though increasing pressure at the retentate side enhances H₂ permeation, CH₄ conversion is even decreased due to unfavorable thermodynamics. And increasing sweep gas flow rate at permeate side benefits to both CH₄ conversion and H₂ permeation. This work highlights the importance of determining the optimal catalyst bed volume to match the membrane in the design of membrane reactors.     

An oxy-fuel mass-recirculating process for H2 production with CO2 capture by autothermal catalytic oxyforming of methane

A new oxy-fuel H₂ generation process with CO₂ avoidance is provided. The process utilizes mass recirculation of CO and H₂O to the oxyforming reactor. A comparison between non-recirculating and mass-recirculating oxyforming reactor operation is given. Main benefits of mass recirculation are emphasized. The oxyforming reactor is integrated with the H₂ and CO₂ separators, fuel cell and O₂ generator. In the process C/O is equal to 0.5 while C/H determines the temperature level in the reactor. The reaction system includes combustion, steam reforming and water–gas shift reactions. The oxyforming process is found to be mass transport controlled with O₂ as the limiting reactant. It is emphasized that under MR conditions the decomposition of H₂/CO₂ by water–gas shift reaction is suppressed by means of CO/H₂O-enrichment and hence MR conditions allow for higher temperatures beneficial to endothermic steam reforming reaction. Under MR conditions the thermodynamic equilibrium limits are overcome and all reactions are forced to proceed to the completion which enables 100% selectivities to H₂ and CO₂. The effects of operation parameters such as temperature, flow rate, pressure and composition are examined. The derived S-terms enable for the concise interpretation of the effect of pressure on the concentration gradients transverse to the flow. The consistent control algorithm of the oxyforming reactor is provided.     

A CFD approach on simulation of hydrogen production from steam reforming of glycerol in a fluidized bed reactor

Hydrogen production from steam reforming of glycerol in a fluidized bed reactor has been simulated using a CFD method by an additional transport equation with a kinetic term. The Eulerian–Eulerian two-fluid approach was adopted to simulate hydrodynamics of fluidization, and chemical reactions were modelled by laminar finite-rate model. The bed expansion and pressure drop were predicted for different inlet gas velocities. The results showed that the flow system exhibited a more heterogeneous structure, and the core-annulus structure of gas–solid flow led to back-mixing and internal circulation behaviour, and thus gave a poor velocity distribution. This suggests the bed should be agitated to maintain satisfactory fluidizing conditions. Glycerol conversion and H₂ production were decreased with increasing inlet gas velocity. The increase in the value of steam to carbon molar ratio increases the conversion of glycerol and H₂ selectivity. H₂ concentrations in the bed were uneven and increased downstream and high concentrations of H₂ production were also found on walls. The model demonstrated a relationship between hydrodynamics and hydrogen production, implying that the residence time and steam to carbon molar ratio are important parameters. The CFD simulation will provide helpful data to design and operate a bench scale catalytic fluidized bed reactor.     

Experimental comparison of biochar species on in-situ biomass tar H2O reforming over biochar

Experimental investigations of in-situ tar H₂O reforming over various biochar species were carried out in the two-stage fluidized bed/fixed bed reactor. The physicochemical structures of biochar were studied by SEM, mercury intrusion porosimetry and FTIR methods. The mechanism of tar H₂O reforming over biochar was studied through the results of tar yields and quantitative analysis of typical tars by GC/MS. According to the theory of organic mass spectrometry and current mechanisms of tar transformation, the reaction path of typical tar H₂O reforming over biochar was constructed. The results show that the tar reforming rate over sawdust biochar is the most significant among the three kinds of biochar samples (i.e., rice husk, sawdust and cornstalk). The metallic species contribute greatly to the weight loss of biochar in 15 vol% H₂O atmosphere at 800 °C, while they are not the only determinants of tar H₂O reforming. The selectivity of biochar on the in-situ tar H₂O reforming is determined by the coupling effects of its physical and chemical characteristics. The biochar, with the porous surface structures, a certain amount of metallic species and the carbon structure with low polymerization, would be effective on in-situ tar H₂O reforming.     

Enhanced activity of NiZrBEA catalyst for upgrading of biomass pyrolysis vapors to H2-rich gas

The main objective of these studies was development of competitive catalyst for the upgrading of biomass pyrolysis vapors to H₂-rich gas. The performed experiments were devoted to determination of the effect of incorporation of zirconium into the structure of BEA zeolite on the performance of NiBEA in the mentioned process. Moreover, the most important parameters responsible for the increased activity of NiZrBEA catalyst in comparison to nickel supported on parent zeolite have been identified. The activity of synthesized catalysts was tested in two step fixed bed quartz reactor. Firstly, cellulose or pine were heated to the 500 °C in order to decompose lignocellulosic feedstock. Then, formed pyrolysis vapors were directed through catalyst bed (700 °C) where their upgrading took place. The obtained results revealed that an introduction of zirconium in the structure of BEA zeolite allowed for the increase in the efficiency of Ni catalyst in the formation of H₂-rich gas. It was related to the increase in pore volume of the synthesized materials, formation of smaller nickel oxide crystallites and creation of the catalysts with moderate acidity.     

Supercritical water gasification of organic solid waste: H2 yield and cold gas efficiency optimization considering modeling uncertainties

Supercritical water gasification (SCWG) is one of the typical hydrothermal treatment technologies for organic solid waste. However, the current SCWG optimization methods perform deterministic optimization without considering the uncertainty of the model for calculating the objective function, which leads to low reliability of the optimization results. Therefore, an optimization framework that considers the prediction uncertainties of SCWG data-driven models is proposed to optimize the H₂ yield and cold gas efficiency of organic solid waste SCWG. An ensemble prediction model integrating random forest, gradient boosting regression, and K-nearest neighbor algorithms by the stacking learning method are built to predict SCWG gas yields. The cold gas efficiency prediction model is constructed based on the gas yield prediction models. The SCWG optimization models are constructed by combining the H₂ yield and cold gas efficiency prediction models. The uncertainties in the H₂ yield and cold gas efficiency prediction models are analyzed and integrated into the optimization models. The case studies were conducted to test the proposed framework. The optimization results were verified by the results of similar experimental conditions. It demonstrates that the proposed framework can obtain the robust results of the organic solid waste SCWG optimization, which can provide a reference for SCWG optimization.     

Biomass steam gasification in bubbling fluidized bed for higher-H2 syngas: CFD simulation with coarse grain model

A comprehensive coarse grain model (CGM) is applied to simulation of biomass steam gasification in bubbling fluidized bed reactor. The CGM was evaluated by comparing the hydrodynamic behavior and heat transfer prediction with the results predicted using the discrete element method (DEM) and experimental data in a lab-scale fluidized bed furnace. CGM shows good performance and the computational time is significantly shorter than the DEM approach. The CGM is used to study the effects of different operating temperature and steam/biomass (S/B) ratio on the gasification process and product gas composition. The results show that higher temperature enhances the production of CO, and higher S/B ratio improves the production of H₂, while it suppresses the production of CO. For the main product H₂, the minimum relative error of CGM in comparison with experiment is 1%, the maximum relative error is less than 4%. For the total gas yield and H₂ gas yield, the maximum relative errors are less than 7%. The predicted concentration of different product gases is in good agreement with experimental data. CGM is shown to provide reliable prediction of the gasification process in fluidized bed furnace with considerably reduced computational time.