A numerical performance study of a fixed-bed reactor for methanol synthesis by CO2 hydrogenation

Synthetic fuels are needed to replace their fossil counterparts for clean transport. Presently, their production is still inefficient and costly. To enhance the process of methanol production from CO₂ and H₂ and reduce its cost, a particle-resolved numerical simulation tool is presented. A global surface reaction model based on the Langmuir-Hinshelwood-Hougen-Watson kinetics is utilized. The approach is first validated against standard benchmark problems for non-reacting and reacting cases. Next, the method is applied to study the performance of methanol production in a 2D fixed-bed reactor under a range of parameters. It is found that methanol yield enhances with pressure, catalyst loading, reactant ratio, and packing density. The yield diminishes with temperature at adiabatic conditions, while it shows non-monotonic change for the studied isothermal cases. Overall, the staggered and the random catalyst configurations are found to outperform the in-line system.     

Cu+-ZrO2 interfacial sites with highly dispersed copper nanoparticles derived from Cu@UiO-67 hybrid for efficient CO2 hydrogenation to methanol

A series of Cu@ZrO₂-U framework catalysts derived from Cu@UiO-67 precursors with adjustable copper loadings were constructed by the deposition-precipitation method, and the catalytic activities of methanol synthesis via CO₂ hydrogenation were investigated. The optimized 20-Cu@ZrO₂-U catalyst showed the best catalytic activity. At 3 MPa and 260 °C, the space-time yield of CH₃OH (STY_CH3OH) reached 2.28 mmol_CH3OH/(g_cat·h), which was 3.5 times higher than that of 20-Cu/ZrO₂. The catalyst of 20-Cu@ZrO₂-U also showed good stability during the 100-h time on stream test. The catalysts were further characterized by XRD, N₂ sorption, TEM, XPS, H₂-TPR, CO₂/H₂-TPD and in situ DRIFTS. The characterization results showed that the stable ZrO₂ framework derived from UiO-67 is propitious to the confine of copper nanoparticles and formation of Cu⁺-ZrO₂ interfacial sites, which should be responsible for the excellent performance of methanol synthesis. Moreover, in situ DRIFTS was used to probe that the methanol synthesis via CO₂ hydrogenation over 20-Cu@ZrO₂-U follows a HCOO1-intermediated reaction pathway.     

Techno-economic analysis of hydrogen production from dehydrogenation and steam reforming of ethanol for carbon dioxide conversion to methanol

Decreasing carbon dioxide (CO₂) emission by converting to higher-valued product has become of interest. Hydrogen (H₂) is an important feedstock required in thermochemical conversion of CO₂ to chemicals such as methanol. The cost and availability of H₂ affect the cost of CO₂ conversion. This study is focused on the process simulation of H₂ production from ethanol feedstock. Steam reforming of ethanol is compared with dehydrogenation of ethanol to H₂ with valued products including ethyl acetate and acetaldehyde. Form this study, steam reforming of ethanol presents the lowest cost of H₂ production at 1.58 USD/kg H₂ while dehydrogenation of ethanol presents the cost at 3.24 and 1.97 USD/kg H₂, respectively. Although presenting the lowest cost, steam reforming of ethanol provides a net positive CO₂ emission in the overall CO₂ conversion to methanol process. In contrast, ethanol dehydrogenation to H₂ and byproducts, ethyl acetate and acetaldehyde, promotes a net negative CO₂ emission of −819.20 kg/ton methanol and −5.42 kg/ton methanol, respectively. The results present a decreasing CO₂ emission with an increasing cost of H₂ production.     

Toward the practical application of direct CO2 hydrogenation technology for methanol production

Methanol production via direct CO₂ hydrogenation is one of the most promising means of utilizing greenhouse gases owing to the significant market for methanol and the potential to simultaneously reduce CO₂ emissions. However, the practical applications of this process still suffer from high production costs owing to the expensive raw materials required and the severe operating conditions. Herein, we propose an economically attractive methanol production process that also works to sequester CO₂, developed through technoeconomic optimization. This economically optimized process design and the associated operating conditions were simultaneously obtained from among thousands of possible configurations using a superstructure optimization. A modified machine learning-based optimization algorithm was also employed to efficiently achieve this complex superstructure optimization. The optimum process design involves a multistage reactor together with an interstage product recovery system and substantially improves the CO₂ conversion to greater than 52%. Consequently, the revenue obtained from methanol production changes from a $4.3 deficit to a $2.5 profit per ton. In addition, the proposed process is capable of generating the same amount of methanol with only half the CO₂ emissions associated with conventional methanol production methods. A comprehensive sensitivity analysis is also provided along with the optimum process design to identify the influence of various technoeconomic parameters.     

Low-temperature alcohol-assisted methanol synthesis from CO2 and H2: The effect of alcohol type

Carbon dioxide (CO₂) conversion to higher-value products is a promising pathway to mitigate CO₂ emissions. Methanol is a high-value-chain chemical in industries that can be produced through CO₂ hydrogenation, which is an exothermic reaction. Due to thermodynamic limitations, a typical synthesis temperature between 250 °C and 300 °C results in a low conversion of CO₂ at equilibrium. To enhance the CO₂ conversion, high pressures of 50–100 bar are required, which inevitably causes the process to be energy-intensive. In this study, an alternative method called alcohol-assisted methanol synthesis is investigated. In this method, alcohol is used as a catalytic solvent and helps decrease the reaction temperature and pressure (150 °C and 50 bar) and significantly increases methanol yield. Ethanol is used as the alcohol due to its reactivity, providing a high methanol yield (47.80%) with 63.93% CO₂ conversion and 67.54% methanol selectivity. However, due to unwanted side reactions, ethanol generates ethyl acetate as a byproduct that forms an azeotrope with methanol, leading to difficulty in product purification. The effects of alcohol type (molecular weight and structure), including ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, iso-butanol, tert-butanol and 1-pentanol, on CO₂ conversion, methanol yield and byproducts are investigated. It is found that smaller-molecule alcohols provide a higher methanol yield. Moreover, n-alcohols provide a higher methanol yield than branched alcohols, and the byproducts of the reaction with n-alcohols do not form an azeotrope with methanol. Therefore, 1-propanol is compared with ethanol providing 26.55% methanol yield, 69.02% CO₂ conversion and 70.82% methanol selectivity.     

Techno-economic assessment of a synthetic fuel production facility by hydrogenation of CO2 captured from biogas

In this study, a thermodynamic and economic analysis of a synthetic fuel production facility by utilizing the hydrogenation of CO₂ captured from biogas is carried out. It is aimed to produce methanol, a synthetic fuel by hydrogenation of carbon dioxide. A PEM electrolyzer driven by grid-tie solar PV modules is used to supply the hydrogen need of methanol. The CO₂ is captured from biogas produced in an actual wastewater treatment plant by a water washing unit which is a method of biogas purification. The required power which is generated by PV panels, in order to produce methanol, is found to be 2923 kW. Herein, the electricity consumption of 2875 kW, which is the main part of the total electricity generation, belongs to the PEM system. As a result of the study, the daily methanol production is found to be as 1674 kg. The electricity, hydrogen and methanol production costs are found to be $ 0.043 kWh^?1, $ 3.156 kg^?1, and $ 0.693 kg^?1, respectively. Solar availability, methanol yield from the reactor, and PEM overpotentials are significant factors effecting the product cost. The results of the study presents feasible methanol production costs with reasonable investment requirements. Moreover, the efficiency of the cogeneration plant could be increased via enriching the biogas while emissions are reduced.     

CO2 hydrogenation to methanol over CuOZnOTiO2ZrO2 catalyst prepared by a facile solid-state route: The significant influence of assistant complexing agents

A series of CuOZnOTiO₂ZrO₂ catalysts were prepared by a facile solid-state route, and the effects of adding citric acid or oxalic acid on the physicochemical properties and catalytic performance for CO₂ hydrogenation to methanol were investigated. The catalysts were characterized by several techniques, including X-ray diffraction (XRD), N₂ adsorption, transmission electron microscopy (TEM), reactive N₂O adsorption, X-ray photoelectron spectroscopy (XPS), atomic absorption spectroscopy (AAS), temperature-programmed reduction with H₂ (H₂-TPR), and adsorption of CO₂ followed by temperature programmed desorption (CO₂-TPD). The results reveal that the addition of assistant complexing agents can improve the CuO dispersion in the catalysts and increase the metallic Cu surface area, and consequently greatly increase the CO₂ conversion and methanol yield, with oxalic acid being a more effective assistant complexing agent than citric acid.     

Insights into the role of Zn and Ga in the hydrogenation of CO2 to methanol over Pd

The hydrogenation of CO₂ to methanol is a viable alternative for reducing greenhouse gases net emissions as well as a route for hydrogen storage and transportation. In this context, the synthesis of active and selective catalysts is a relevant objective. In this work, we study the promotion of Pd with Ga and Zn in the hydrogenation of CO₂ to methanol at 800 kPa and 220–280 °C. Mono and intermetallic catalysts (Pd/SiO₂, PdGa/SiO₂ and PdZn/SiO₂) were synthesized by incipient wetness impregnation with the aid of triethanolamine as an organic additive, obtaining similar average metal particle sizes (between 9 and 12 nm). Kinetic analysis reveals that the addition of Ga and Zn increases the turnover frequency for methanol formation by an order of magnitude without significant changes in the reaction rate of the reverse water-gas shift (r-WGS) which is a parallel undesired reaction. The selectivity to methanol (at 220 °C) thus increases from 3% for Pd/SiO₂ to 12% for PdGa/SiO₂ and 30% for PdZn/SiO₂. XPS studies, Infrared analysis of CO adsorption, and XRD analyses show the presence of intermetallic phases Pd₂Ga and PdZn on the surface. The results suggest that Ga and Zn promote Pd, increasing its activity towards the synthesis of methanol, by creating more active sites for this reaction. These sites are likely formed by intermetallic compounds such as Pd₂Ga and PdZn.     

Analysis of a 2-D model of a packed bed reactor for methanol production by means of CO2 hydrogenation

In recent years, methanol received the attention of many researchers as a building block of the circular economy, because of its diversified applications in different areas. Generally, methanol is produced by syngas, however recent studies are dealing with its production via carbon dioxide hydrogenation. With the aim to predict conversions, efficiencies as well as concentration, pressure and temperature profiles inside the packed bed methanol reactor, mathematical models are developed in one- (1-D) and two- (2-D) dimensions. However, a deep study about a 2-D mathematical model and conditions where its use is advisable to get reliable predictions is missing in the literature. In this research, a two dimensional model for methanol reactor via carbon dioxide hydrogenation is suggested, comparing a structured catalytic packing with a more common packed bed of catalyst pellets, which differ mainly for the respective thermal conductivity. The system of partial differential equations is solved in MATLAB® and the same operating conditions set in a previous work about a one dimensional model are considered. Results show that the 2-D model is useful for both reactor typologies under the examined operating conditions, although definitely more important for the non-structured reactor, where higher temperature and concentration differences on tube cross sections are calculated because of a stronger resistance to radial heat transfer. In addition, a higher efficiency is predicted for a structured reactor in terms of carbon dioxide selectivity to methanol and methanol yield, then a lower recycle flow rate is required in this case. A sensitivity analysis is also developed for the two reactor typologies, changing feed inlet temperature, wall heat transfer coefficient and tube diameter. Conditions are investigated, for which 2-D model results tend to corresponding outputs of a 1-D model.     

Hydrogen production from photo-driven electrolysis of biomass-derived oxygenates: A case study on methanol using Pt-modified WO3 thin film electrodes

In this paper, we demonstrate the feasibility of H₂ production from biomass-derived oxygenates with photoelectrochemical cells (PECs) based on the tandem cell hybrid photoelectrode configuration. As a proof of concept, we have studied the simplest oxygenate, methanol, which is photoelectrochemically oxidized at thin film tungsten oxide (WO₃) photoelectrodes. When the methanol oxidation reaction (MOR) is coupled with the hydrogen evolution reaction (HER), this process is known as methanol electrolysis. We demonstrate that catalytic modification of the WO₃ surface by the electrodeposition of Pt particles can greatly increase MOR activity at the photoanode, resulting in a significant increase in H₂ production rates from methanol electrolysis. This improvement is greatest at low overpotentials and high Pt loadings, with the demonstrated MOR current density of Pt-WO₃ being nearly four times that of the oxygen evolution reaction (OER) on WO₃ at a potential of 0.8 V vs. the Reversible Hydrogen Electrode. We also illustrate how the increase in WO₃ photocurrent and the decrease in the oxidation onset potential, compared to the OER, make it possible to use WO₃-based photoelectrodes in a simple tandem cell configuration whereby a common PV component such as a-Si can provide the remaining voltage to achieve unassisted methanol electrolysis. Results from methanol electrolysis reveal the potential to utilize a similar approach for larger biomass-derived oxygenates, which could be a promising pathway to H₂ production from renewable feedstock using photo-driven electrolysis.     

Methanol synthesis from CO2 hydrogenation over CuO-ZnO-ZrO2-MxOy catalysts (M=Cr,Mo and W)

Three CuO-ZnO-ZrO₂-M_xO_y (CZZM, M = Cr, Mo and W) mixed oxides were prepared by a co-precipitation method and tested as catalysts for methanol synthesis from CO₂ hydrogenation. The catalysts were characterized by XRD, N₂ adsorption/desorption, XPS, reactive N₂O adsorption, H₂-TPR, and CO₂-TPD techniques. The results indicated that the methanol selectivity and yield of the CuO-ZnO-ZrO₂ catalyst noticeably increased by the additions of MoO₃ and WO₃, but slightly decreased by Cr₂O₃ doping. Combining with the characterization results, the difference in methanol yield over the three catalysts can be attributed to the differences in their BET specific surface areas (S_BET) and adsorption capacities for CO₂, while the methanol selectivity is closely correlated to the ratio of surface contents of Zn to Cu, as well as the fraction of strong basic sites in the total basic sites of the investigated catalysts.     

Effect 3A and 5A molecular sieve on alcohol-assisted methanol synthesis from CO2 and H2 over Cu/ZnO catalyst

Low-temperature methanol synthesis from CO₂ and H₂ was carried out using ethanol as a catalytic solvent. The alcohol-assisted method reduced synthesis temperature and enhanced methanol yield (33.80%) at 150 °C (5.0 MPa, Cu/ZnO catalyst). However, ethyl acetate and water were generated as byproducts from the reaction. The byproducts formed azeotrope mixture with methanol and led to a complex product purification. Therefore, in this study, molecular sieves (MS) were introduced to adsorb the byproducts. The effect of different MS (3A and 5A) was studied. It was found that MS helped enhancing methanol yield. The highest methanol yield (42.8%) was obtained when adding MS_3A to adsorb water. The MS_5A could separate methanol and ethyl acetate, providing high methanol purity. The effect of operating conditions was also investigated. When reducing temperature to 130 °C, methanol yield decreased but methanol selectivity (>98%) significantly increased. Controlling temperature and using MS could help enhance the yield and selectivity of methanol.     

Techno-economic and environmental assessment of methanol steam reforming for H2 production at various scales

According to global trend of transition to a hydrogen society, needs for alternative hydrogen (H₂) production methods have been on the rise. Among them, methanol steam reforming (MSR) in a membrane reactor (MR) has received a great attention due to its improved H₂ yield and compact design. In this study, 3 types of economic analysis – itemized cost estimation, sensitivity analysis, and uncertainty analysis – and integrative carbon footprint analysis (iCFA) were carried out to investigate economic and environmental feasibility. Unit H₂ production costs of MSR in a packed-bed reactor (PBR) and an MR for various H₂ production capacities of 30, 100, 300, and 700 m³ h⁻¹ and CO₂ emission rates for both a PBR and an MR in H₂ production capacity of 30 m³ h⁻¹ were estimated. Through itemized cost estimation, unit H₂ production costs of a PBR and an MR were obtained and scenario analysis was carried out to find a minimum H₂ production cost. Sensitivity analysis was employed to identify key economic factors. In addition, comprehensive uncertainty analysis reflecting unpredictable fluctuation of key economic factors of reactant, labor, and natural gas obtained from sensitivity analysis was also performed for a PBR and an MR by varying them both simultaneously and individually. Through iCFA, lowered CO₂ emission rates were obtained showing environmental benefit of MSR in an MR.     

Methanol production by CO2 hydrogenation: Analysis and simulation of reactor performance

Methanol is a very valuable chemical with a variety of uses, either as a fuel or as building block for the synthesis of other chemicals. In the last years, interest was growing in the production of methanol from CO₂, based on the so called “Power-to-Fuel” concept. In this research, an equilibrium analysis of a methanol reactor with pure CO₂ and H₂ in the feeding stream was developed. Three novel reactor configurations at equilibrium conditions were considered: once-through reactor, reactor with recycle of unconverted gases after separation of methanol and water by condensation; reactor equipped with membrane permeable to water. An additional important feature of this work was the development of a methodology that assists in comparison of different process schemes by simulation of two different methanol plants configurations in ChemCad®. An adiabatic kinetic reactor with recycle of unconverted gases was considered and simulated in Aspen Plus®, while the performance of a methanol reactor with heat exchange at the pipe wall was simulated in MATLAB. Results show that at equilibrium conditions a reactor with the recycle of unconverted gases ensures the highest CO₂ conversion: 69% at 473 K and 55 bar. In addition, the use of pure CO₂ and H₂ in the feeding stream allows an overall reaction enthalpy change lower than that obtained by the use of syngas in the feed. The kinetic simulation of the methanol reactor in MATLAB showed that axial dispersion phenomena are negligible and the effect of the global heat exchange coefficient on reactor performance is less important than the effect of isothermal heat exchange fluid temperature.     

Comparative performance analysis of a grid connected PV system for hydrogen production using PEM water,methanol and hybrid sulfur electrolysis

This paper presents comparative performance analysis of photovoltaic (PV) hydrogen production using water, methanol and hybrid sulfur (SO₂) electrolysis processes. Proton exchange membrane (PEM) electrolysers are powered by grid connected PV system. In this system design, electrical grid is considered as a virtual energy storage system (VESS) where the surplus of PV production can be injected and subsequently taken to support the electrolyser. Methanol (ME) and hybrid sulfur (HSE) electrolysis are compared to the conventional water electrolysis (WE) in term of operating cell voltage. Based on the experimental results reported in the literature, semi-empirical models describing the relationship between the hydrogen production rate and the electrolyser cell power input are proposed. Furthermore, power and hydrogen management strategy (PHMS) is developed. Case study is carried out to show the impact of each type of electrolysis on the system component sizes and evaluate the hydrogen production potentialities. Results show that the use of ME allows to produce 65% more hydrogen than with using WE. Moreover, the amount of hydrogen produced is almost double in the case of HSE. At Algiers city, based on a grid connected PV/Electrolyser system, it is possible to produce about 25 g/m² d and 29 g/m² d of hydrogen, respectively, through ME and HSE compared to 15 g/m² d of hydrogen when using WE.     

Techno-economic analysis of alternative processes for alcohol-assisted methanol synthesis from carbon dioxide and hydrogen

The novel methanol production from carbon dioxide (CO₂) and hydrogen (H₂) called alcohol-assisted process is simulated. Although the alcohol-assisted process allows the reduction in operating temperature and pressure, the subsequent product purification is complicated. Comparative studies between the conventional CO₂ hydrogenation and the alcohol-assisted processes are carried out (case I–V). The alcohol-assisted processes present the opportunity of increasing the CO₂ conversion per-pass and reducing 25% of the hydrogen consumption, the barriers in the conventional process. However, the product purifications remain challenging due to the azeotrope of methanol and by-products. Energy consumptions decrease in the feed and reaction sections of the alcohol-assisted processes but significant increase in the product purifications. The formation of by-products and the sequence of purification units affect process performance and economics. The obtained results indicate that the product purification and the catalyst development to increase methanol selectivity and produce an easy-separated by-product play key roles in the enhancement of the process feasibility.     

A novel configuration for hydrogen production from coupling of methanol and benzene synthesis in a hydrogen-permselective membrane reactor

Coupling energy intensive endothermic reaction systems with suitable exothermic reactions improve the thermal efficiency of processes and reduce the size of the reactors. One type of reactor suitable for such a type of coupling is the heat-exchanger reactor. In this work, a distributed mathematical model for thermally coupled membrane reactor that is composed of three sides is developed for methanol and benzene synthesis. Methanol synthesis takes place in the exothermic side and supplies the necessary heat for the endothermic dehydrogenation of cyclohexane reaction. Selective permeation of hydrogen through the Pd/Ag membrane is achieved by co-current flow of sweep gas through the permeation side. A steady-state heterogeneous model of the two fixed beds predicts the performance of this novel configuration. The co-current mode is investigated and the simulation results are compared with corresponding predictions for an industrial methanol fixed-bed reactor operated at the same feed conditions. The results show that although methanol productivity is the same as conventional methanol reactor, but benzene is also produced as an additional valuable product in a favorable manner, and auto-thermal conditions are achieved within the both reactors and also pure hydrogen is produced in permeation side. This novel configuration can increase the rate of methanol synthesis reaction and shift the thermodynamics equilibrium. The performance of the reactor is numerically investigated for various key operating variables such as inlet temperatures, molar flow rates of exothermic and endothermic streams, membrane thickness and sweep gas flow rate. The reactor performance is analyzed based on methanol yield, cyclohexane conversion and hydrogen recovery yield. The results suggest that coupling of these reactions in the presence of membrane could be feasible and beneficial. Experimental proof-of-concept is needed to establish the validity and safe operation of the novel reactor.     

Proposal of a natural gas-based polygeneration system for power and methanol production

A new kind of natural gas-based polygeneration system for methanol and power production is proposed in this paper. With the sequential connection between chemical production and power generation, the new system adopts innovative integration of partial-reforming and partial-recycle scheme in methanol synthesis subsystem. To reveal the characteristics of the new system, exegetic comparisons between the new system and a reference polygeneration system with full-reforming and once through methanol synthesis scheme have been carried out. Results indicate that the new system can save energy about 6 percentages versus single product systems. By the aid of graphical exergy analysis methodology, the specific information on internal phenomena of key processes was illustrated. The analysis shows that it is the synergetic combination of partial-reforming and partial-recycle schemes that makes the significant contribution to the performance improvement, and plays the most important role in system integration.     

NiFexP@NiCo-LDH nanoarray bifunctional electrocatalysts for coupling of methanol oxidation and hydrogen production

NiFe_xP@NiCo-LDH/CC nanosheet core-shell nanoarrays electrocatalysts are prepared by electrodeposition and phosphating methods for relatively low potential production of H₂ and value-added formate in methanol/water electrolysis systems. During the oxygen evolution reaction process, the over potential is 269 mA at the current density of 50 mA cm^?2 and the Tafel slope is 97 mV dec^?1. It also realizes the stable long-term electrocatalysis of methanol to formate under high current density and maintains a relatively high Faraday efficiency of 100%. Meanwhile, the energy saving is higher than 10% in the methanol oxidation and co-hydrogen production system that achieves great attention. The superior performance of NiFe_xP@NiCo-LDH/CC bifunctional electrocatalysts might be beneficial from the interaction to Ni and Fe bimetallic extranuclear electrons that exposes more active sites.