Novel reactors for multiphase processes

Process intensification tools, such as the capillary reactor, offer several benefits to the chemical process industries due to the well-defined high specific interfacial area available for heat and mass transfer, which increases the transfer rates, and due to low inventories, they also enhance the safety of the process. This has provided motivation to investigate three such tools, namely the capillary microreactor, spinning disc and rotating tube reactors, in this study.The gas-liquid slug flow capillary microreactor intensifies reactor performance through internal circulation caused by the shear between the continuous phase/wall surface and the slug axis, which enhances the diffusivity and consequently increases the reaction rates. However, integrating the complex hydrodynamics of this reactor with its chemical kinetics is a mathematically challenging task. Therefore, in this study, a simple-to-complex approach, using a set of state-of-the-art computational fluid dynamic tools, has been used. Firstly, simulations were performed without any chemical reaction to ascertain the extent of slug flow regime. The model also clearly captured the slug flow generation mechanism which can be used to structurally optimize the angle of entry in these reactors. Finally, the hydrodynamic model was also capable of estimating the pressure drop and slug lengths. After successfully simulating the hydrodynamics of the system, a reaction model was incorporated to study the chemical reaction kinetics. The results were compared with the published experimental work and were found to be in good agreement.The spinning disc reactor utilizes the centrifugal and shear forces to generate thin liquid films characterized with intense interfering waves. This enables a very high heat transfer coefficients to be realized between the disc and liquid, as well as very high mass transfer between the liquid and the bulk gas phase. The waves formed also produce an intense local mixing with very little back mixing. This makes a spinning disc reactor an ideal contactor for multiphase processes. The focus of this study has been to elucidate the hydrodynamic behaviour of the liquid film flow over the horizontal spinning disc. Investigations were also performed to elaborate the local and overall hydrodynamic characteristics of a fully developed spinning disc reactor. Simulation results showed a continuous linear liquid film on the horizontal spinning disc and intense mixing performance in the annulus of the reactor around the disc surface. Finally, the film thickness data from the simulations were compared with the limited amount of data available for this novel process.Rotating tube reactor also uses centrifugal forces to generate the liquid film and a high degree of mixing along with an improved control over the reactant retention times. In this work we have conducted a CFD analysis to understand the hydrodynamics of this new technology for future developments

Grain boundary engineering: An emerging pathway toward efficient electrocatalysis

Electrochemical transformation processes involving carbon, hydrogen, oxygen, nitrogen, and small-molecule chemistries represent a promising means to store renewable energy sources in the form of chemical energy. However, their widespread deployment is hindered by a lack of efficient, selective, durable, and affordable electrocatalysts. Recently, grain boundary (GB) engineering as one category of defect engineering, has emerged as a viable and powerful pathway to achieve improved electrocatalytic performances. This review presents a timely and comprehensive overview of recent advances in GB engineering for efficient electrocatalysis. The beneficial effects of introducing GBs into electrocatalysts are discussed, followed by an overview of the synthesis and characterization of GB-enriched electrocatalysts. Importantly, the latest developments in leveraging GB engineering for enhanced electrocatalysis are thoroughly examined, focusing on the electrochemical utilization cycles of carbon, hydrogen, oxygen, and nitrogen. Future research directions are proposed to further advance the understanding and application of GB engineering for improved electrocatalysis. (Figure presented.)

Low-carbon methanol production using solar thermal energy: A techno-economic assessment

This work presents a techno-economic and greenhouse gas emissions assessment of a proposed low-carbon methanol production process. The process takes a novel low-carbon methanol production method and reduces its emissions further by integrating a solar thermal energy (STE) system to provide for its electricity requirements. The authors investigate three different scenarios for the STE system, analysing the effects of solar irradiance design point, and the impact of thermal energy storage on system performance. Case-1 (15.57 MW) was designed for daytime operation and sized for the maximum annual solar irradiance, with Case-2 (24.91 MW) incorporating thermal energy storage to enable 24-hour operation. Case-3 (48.66 MW) also utilised thermal energy storage to enable 24-hour operation, but was sized for the minimum annual solar irradiance. The methanol process and the STE system were modelled in Aspen Plus and Aspen HYSYS, with the simulation data being used with mass and energy balances to conduct a techno-economic and greenhouse gas emissions assessment. The standalone methanol plant was estimated to emit 0.37 t CO2/t CH3OH, with the integration of the STE system reducing this to 0.29 (Case-1), 0.23 (Case-2), and 0.10 (Case-3), resulting in one of the lowest emitting traditional methanol production methods available. The cost of abatement of these remaining emissions through carbon capture and storage was also considered. The economic analysis found that the integrated processes in Case-1 and 2 were financially viable, resulting in a net present value of $247 million and$72 million respectively. The net present value was reduced to $185 and$21 million when considering the cost of carbon capture and storage. Conversely, the net present value for Case-3 was negative, requiring an increase in the price of methanol to ∼$600/t, and a reduction in the cost of capital to ∼8%, to become financially viable

Development of a kinetic model for Fischer-Tropsch synthesis over a Ru promoted Co/Al2O3 catalyst in a slurry reactor

A detailed kinetic model for hydrocarbon formation rate over a wide range of operating conditions by using a stirred tank slurry reactor (STSR), which closely mimics the kinetics of the commercialized slurry process, is put forward. Kinetics of the Fischer-Tropsch (F-T) synthesis over 0.27% Ru 25% Co/Al 2 O 3 catalyst was studied using the STSR. Hydrogen-assisted dissociative adsorption of CO followed by hydrogenation of dissociatively adsorbed CO was the likely path for formation of the monomer (methylene) and was the likely rate controlling step in F-T synthesis. Rates obtained from the best kinetic model provided a satisfactory fit to the experimental data. This is an abstract of a paper presented at the 2011 AIChE Spring Meeting & 7th Global Congress on Process Safety (Chicago, IL 3/13-17/2011)

Kinetics of the fischer-tropsch reaction over a Ru-promoted Co/Al<inf>2</inf>o<inf>3</inf>catalyst

The kinetics of the Fischer Tropsch (FT) synthesis reaction over 0.27 % Ru 25 % Co/Al 2 o 3 catalyst was studied in a 1L stirred tank slurry reactor (STSR). Experiments were conducted at reactor pressures of 1.4 MPa and 2.4 MPa, temperatures of 205°C and 220°C, H 2 /CO feed ratios of 1.4 and 2.1 and gas space velocities ranging from 2 to 15 NL/g-cath. Langmuir-Hinshelwood-Hougen-Watson (LHHW) type rate equations were derived on the basis of a set of reactions originating from carbide and enolic pathways for hydrocarbon formation. Derived rate equations were fitted to the corrected experimental rate using Levenberg-Marquardt method to obtain model parameters. Physical and statistical tests were used to discriminate between rival models. It was found that a model based on hydrogen-assisted dissociative adsorption of CO followed by hydrogenation of dissociatively adsorbed CO provides the best fit to the data. Copyright © 2011, AIDIC Servizi S.r.l