Exothermic Synthesis Reaction Research Articles

Mathematical modelling of thermal microexplosion in a catalyst granule with point sources of heat release

A mathematical model of heat and mass transfer in a spherical catalyst granule is proposed. Exothermic synthesis reactions are carried out on point active centres located inside a porous ceramic granule. From the surface of the granule the heat of catalytic reactions is removed into liquid synthesis products. The rate of a chemical reaction is modelled by a modified Arrhenius law. In contrast to the homogeneous model of a catalytic granule methods for calculating heat transfer processes in a system of point, active centres do not develop. An iterative procedure is suggested to calculate the unknown temperature and concentration of the reagent at the active centre. It is shown that the temperature of the active centres is significantly higher than in the volume of the granule. The results of modelling a thermal explosion with increasing granule size and reactor temperature are presented.

In-Situ Manufacturing of NiAl-TiC Composites with Three Dimensional (3D) Discrete Particular Network and Bi-Continuous Microstructures

NiAl-TiC intermetallic-ceramic composite system is a candidate for structural applications. Thus, in this research, near fully dense NiAl–xTiC composites [x = 0, 28 and 48 vol fraction percent (vol.%)] were produced through in-situ manufacturing process using exothermic synthesis reactions of Al, Ni, Ti and C precursors and their consolidation via hot press apparatus. NiAl-28 vol.% TiC composite showed a simple three dimensional (3D) discrete networks of TiC particular fillers in the continuous polycrystalline monolithic NiAl intermetallic matrix as a thermal management strategy for elevated temperature applications while NiAl-48 vol.% TiC composite indicated interconnectivity of TiC particles in certain regions of NiAl matrix known as bi-continuous microstructure. Reasons for the higher mechanical properties of NiAl-28 vol.% TiC in comparison monolithic NiAl were also discussed.

Concept, design, and energy analysis of an integrated power-to-methanol process utilizing a tubular proton-conducting solid oxide electrolysis cell

An alternative power-to-methanol process based on an integration of a tubular proton-conducting solid oxide electrolysis cell into a methanol synthesis unit is explained, energetically evaluated and technically presented. Being currently developed in the joint research project DELTA, the novel process has the potential for a significant increase in system efficiency, if the heat from the exothermic synthesis reaction can be utilized and/or kinetic advantages can be achieved. For the experimental proof of the concept and a comprehensive characterization of the process, a test platform is currently under construction. The design of the flexible test facility with the complex technical integration of both processes (electrolysis and synthesis) is described briefly. The chemical reactor, where electrolysis and synthesis are taking place, allows for an operation at temperatures (for electrolysis) up to 700 °C, pressures of 10 MPa and a current (across the electrolysis cell) of up to 100 A. Moreover, a precise pressure balancing system between both gas volumes, an axial temperature measurement and the possibility of regulating both processes inside the pressure vessel are pivotal properties of the test facility.

Heat and mass transfer in Fischer–Tropsch catalytic granule with localized cobalt microparticles

The productivity of Fischer-Tropsch reactors is determined by the efficiency of heat and mass transfer processes inside the catalyst granules. To reduce the diffusion resistance the granules base is made from ceramic highly porous material. The porous structure of the granules causes a discrete arrangement of cobalt metallic microparticles whose size can reach tens of microns. Cobalt particles are the active centres on which the synthesis reactions are realized. The distance between these active centres significantly exceeds their characteristic size and the homogeneous model for heat and mass transfer is incorrect. In our paper a mathematical model of heat and mass transfer processes in a porous spherical granule with localised active centres is proposed. The heat of the exothermic synthesis reaction is removed from the surface of the granule by heat transfer into the synthesis gas stream washing the granule. The components of the synthesis gas enter to the granule surface as a result of mass transfer. On the basis of the mean-field approach the values of the temperature and concentration of the synthesis gas components at the active centres inside the granule were determined. In the reactor tube, where the catalyst granules are placed, there is a critical temperature. The excess of the critical temperature leads to a thermal explosion, i.e. a substantial overheating of the active centres. In this case, the surface of the catalyst granule is superheated slightly. The principal difference between the homogeneous and heterogeneous models in catalytic reactions is discussed. We analysed influence of the temperature inside the reactor tube, size of the granule, and coefficient of thermal conductivity on the thermal stability of the granule.

Mathematical model of Fischer-Tropsch catalyst pellet with pointed centers of synthesis

The productivity of Fischer-Tropsch reactors is determined by the efficiency of heat and mass transfer processes inside the catalyst pellets. To reduce the diffusion resistance, the pellet base is made porous. The porous structure of the granules causes a discrete arrangement of cobalt metallic microparticles whose size can reach tens of microns. The distance between these active centres significantly exceeds their characteristic size and the homogeneous catalyst model is incorrect. A mathematical model of heat and mass transfer processes inside a porous spherical pellet with localised active centres is proposed. The heat of the exothermic synthesis reaction is removed from the surface of the granule to the synthesis gas stream washing the catalyst pellet by heat transfer. The components of the synthesis gas enter the granule surface as a result of mass transfer. On the basis of the self-consistent field method, the values of the temperature and concentration of the synthesis gas components at the active centres were determined. It is shown that there is a critical temperature of the synthesis gas washing the granule, exceeding critical temperature leads to a substantial overheating of the active centres. In this case, the surface of the catalyst pellet is superheated slightly. The principal difference between the homogeneous and heterogeneous models in catalytic reactions is discussed.

Tantalum Diboride Obtained by Reactive Sintering–Properties and Wear Resistance

The paper present mechanical properties and wear behavior of tantalum diboride at temperatures in the range from room temperature up to 800°C. The materials for studies were obtained using Spark Plasma Reactive Sintering method from tantalum and boron powders. The mixture was prepared according to the reaction Ta+2B→TaB2. Reactive sintering processes were carried out at temperature of 2100°C during 30 minutes and pressure of 48MPa. During the heating, exothermic synthesis reaction, with visible change of sample height and temperature increase of about 160°C, takes place. The obtained materials have an apparent density of 11.22g/cm3, the Young's modulus of 500 GPa and Vickers hardness of 19.5 GPa. The tribological investigations were carried out using ball-on-disc method in dry friction conditions, at temperatures in the range from room temperature up to 800°C. The influence of temperature on the friction coefficient, specific wear rate and Vickers hardness of sintered materials was determined. In order to study the mechanisms of the surface oxide layer formation, the microstructural analyzes of cross-section were carried out. The values of friction coefficient up to 600°C were stable. The increase of temperature up to 700°C resulted in significant decrease of the friction coefficient to the value of 0.22. The microstructure analysis of the materials indicates that the oxidation of the surface takes place above the temperature of 500°C. The negative effect of the oxides surface layer formation is a gradual decrease of the material surface hardness.

Real-time monitoring of room-temperature ionic liquid purity through optical diode-based sensing

Room-temperature ionic liquids (RTILs) are promising for use in many industries due to their unique properties, including wide electrochemical windows, low vapor pressures, high ionic conductivities, and chemical and thermal stability. All of these properties require high RTIL purity, and achieving this high purity is a major driver of RTIL manufacturing costs. Continuous flow processes to synthesize highly pure RTILs at a reduced cost have been developed, but due to exothermic synthesis reactions and temperature dependent reaction rates, these processes require real-time control. An ultraviolet LED based optical sensor has been designed to measure RTIL purity at millisecond sampling rates using a liquid flow cell. The sensor is demonstrated by measuring 1-methylimidazole (MIM) concentration in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). Sensor results are compared against spectroscopic measurements with good agreement.

Synthesis of Al<sub>2</sub>O<sub>3</sub> Ceramics Matrix Composites by Thermal Explosion under Pressure and Hot Pressing

Three kinds of Al2O3 ceramics matrix composites, such as Al2O3-TiB2, Al2O3-SiC and Al2O3-TiC, were prepared by two steps, which were synthesis of the composites by thermal explosion under a pressure of 20MPa and hot pressing of the composites at 1700°C under a pressure of 32MPa respectively. The preparation process, the composition and the microstructure of the composites were investigated in detail. The Gibbs free energy and the adiabatic temperature (Tad) of three exothermic synthesis reactions were calculated. Disc-shaped and elongated TiB2 grains were synthesized in the Al2O3-TiB2 composite. Some whiskers and platelet grains were synthesized in the Al2O3-SiC composite. Nano-particles were synthesized in the Al2O3-TiC composite and distributed both within the Al2O3 matrix grains and along the grain boundaries.

Padding, Welding and Freeform Fabrication of Nickel Aluminide Intermetallic Compound by Reactive Rapid Prototyping Process

A novel method for padding, welding and freeform fabrication of intermetallic alloys based on an exothermic synthesis reaction between powder and droplets is proposed, and its feasibility is examined using nickel monoaluminide, NiAl, as a demonstration material. In an experiment for NiAl padding on steel, a small amount of nickel powder was fed onto a steel surface, followed by supplying an aluminum droplet onto the powder. The nickel and aluminum exothermically reacted and produced a NiAl bead on the steel surface, bringing about strong bonding between the NiAl bead and the steel. In an experiment for welding of NiAl, when an aluminum droplet was dropped onto nickel powder fed into a root gap between two NiAl base metals, they exothermically reacted and produced a molten NiAl bead. The heat from the reaction melted the base metals near the interface with the molten NiAl bead. After solidification of the molten NiAl bead and the melted parts of the base metals, welding of the base metals was completed. In an experiment for freeform fabrication of NiAl, when an aluminum droplet was dropped onto a nickel powder bed, the two metals reacted and produced a small NiAl bead. When a next droplet was fallen to a position very close to the NiAl bead, a new NiAl bead was similarly produced and was bonded to the former one. By continuous dropping of aluminum droplets a two-dimensional structure of the NiAl beads was configurated. After the two-dimensional structure was finished, nickel powder was added until it wholly covered the structure on the former plane, and then a new structure was similarly configured in the added nickel powder bed. The NiAl beads were bonded to each other in both horizontal and vertical directions. Finally, a three-dimensional structure was finished after repeating the addition of the powder and the supply of the droplet.

Advances in catalytic synthesis and utilization of higher alcohols

Alcohols can be used directly as fuels and fuel additives or as intermediates to form high octane or high cetane ethers. Catalysts have been improved to synthesize the higher alcohols, especially isobutanol, from coal- or natural gas-derived H 2/CO synthesis gas, and reaction engineering approaches are being investigated to further increase the activities and selectivities of the reactions forming the higher alcohols. These approaches include (a) using a dual catalyst bed reactor for gas phase conversions and (b) employing a slurry phase reaction system for increasing the reaction rate of the exothermic synthesis reactions. These developments have led to higher alcohol productivities. For example, with double bed Cs/Cu/ZnO/Cr 2O 3 catalysts, productivities of isobutanol and total alcohols as high as 202 and 947 g/kg catalyst/h, respectively, have been achieved. However, declining petroleum prices over the last three years have pushed the economic break-even point to even greater productivity levels of isobutanol and of a mixture of the higher alcohols. It is shown that alcohols and certain ethers have desirable properties as octane enhancers, while other ethers could be used as additives to enhance the cetane number of diesel fuel.

In-situ Joining of Nickel Monoaluminide to Iron by Reactive Sintering.

A cylindrical block of nickel monoaluminide, NiAI, is produced from a mixture of nickel and aluminum powders by sintering a powder compact under a pseudo-isostatic pressure, and is simultaneously joined to an iron block with the same shape. When the joining couple of the powder compact and the iron block is heated to approximately 900 K, a violent exothermic synthesis reaction, Ni+Al→NiAl, suddenly starts, and the temperature of the compact quickly rises owing to the heat of reaction and exceeds the melting point of NiAI, 1911 K. Because the molten NiAI wets the contacting surface of the iron block, an iron-rich NiAl-base alloy and an iron-base ternary solid solution are produced on each side of the joining interface. No cracks or cavities are formed in the NiAI even in the vicinity of the joining interface. Hardness continuously changes across the joining interface from approximately 330 in NiAI to approximately 55 in iron. All of five specimens of a four-point bending test fractured in NiAI, the fracture surface being 1 to 2 mm away from the joining interface.

Assessment of hazards linked to accumulation in semi-batch reactors

Abstract In the assessment of hazards linked to chemical reactions, two aspects must be considered: the behavior of the reactor under normal operating conditions, i.e. the controllability of the reaction rate and the behavior in case of failure. The maximum temperature which can be attained in the case of a cooling failure by an exothermic synthesis reaction (MTSR) is a parameter of great significance for the assessment of the thermal safety of semi-batch reactions. This temperature depends on the accumulation of non-converted reactants at the instant of the failure. In most cases, the determination of MTSR is possible after one experiment performed in a reaction calorimeter following standard operating procedures. Such an experiment allows the evaluation of the heat of accumulation as a function of time during addition of reactants. However in some cases, with multiple reactions, another experimental approach, including simulation of the failure and coupling of spectroscopic methods, has to be used. Examples will illustrate the determination of the MTSR in such more difficult cases.

The combustion synthesis of advanced materials

This article discusses the versatility of utilizing exothermic combustion synthesis reactions to produce ceramic, intermetallic, and composite materials in a one-step process. The emphasis is placed on controlling the reaction parameters of these exothermic synthesis reactions as a means of controlling the microstructure and properties of the synthesized materials and composites. The application of this technology to the synthesis and control of a selection of advanced materials is demonstrated (i.e., dense composite materials, uniformly porous or “foamed” ceramics, and functionally graded materials). In addition, the control of product morphology (e.g., submicrometer particles or whiskers by gas-solid reactions that use vapor phase species deliberately generated at the reaction front) is also demonstrated, as is the effect of microgravity processing on some of these reactions.

Combustion Synthesis of Silicon Carbide in Nitrogen Atmosphere

Fine SiC powders were synthesized by burning the mixed reactionts Si and C in a nitrogen atmosphere of 3 to 10 MPa. The exothermic synthesis reaction propagated spontaneously after igniting the reactants at room temperature. The SiC powders obtained had a uniform size distribution of about 0.2 μm. The combustion velocity was 0.8 to 1.5 mm / s. The maximum temperature measured at the reaction was 2500 K, which was higher than the adiabatic combustion temperature of SiC, but slightly lower than the decomposition temperature of Si3N4 under nitrogen pressure.

Cold metal crucible system for synthesis, zone refining, and Czochralski crystal growth of refractory metals and semiconductors

A cold metal crucible apparatus with induction heating has been used for synthesis, purification and growth of single crystals of refractory semiconductors and metals. The system is generally applicable without having to meet conditions of chemical compatibility between melt and crucible and it can be used with an ambient at a pressure of a few atmospheres. The apparatus enables one to control highly exothermic synthesis reactions and to conduct simple phase diagram studies.