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Reactors for Reactive Solids
Published in Salmi Tapio, Mikkola Jyri-Pekka, Wärnå Johan, Chemical Reaction Engineering and Reactor Technology, 2019
Salmi Tapio, Mikkola Jyri-Pekka, Wärnå Johan
where z = cj /c0j for the shrinking particle model and z = 1 for the product layer model. The index (i) refers to the fluid-phase component. As the reaction progresses, the mass transfer coefficients increase for the shrinking particle model, and a kinetic control is attained at the complete conversion of the solid component. Taking into account that a'=2 in Equation 8.202, it can be rewritten to kLi=Did0z1/32+b″z2/9
Enolate Anions and Condensation Reactions
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
In (a), under thermodynamic conditions, cyclohexanone is expected to react with itself, (self-condensation) to give the aldol product 2-(1-hydroxycyclohexyl)cyclohexanone (after hydrolysis). In (b), conversion to the kinetic enolate anion of 4-methylhexan-3-one under kinetic control conditions. No self-condensation can occur since all of the ketone was converted to the enolate and the equilibrium reaction is suppressed under kinetic control conditions. In order for the condensation to occur, 4-methylhexan-3-one is added to the enolate solution, and condensation gives the aldol product, 6-ethyl-6-hydroxy-3,5,7-trimethyloctan-4-one (after mild hydrolysis).
Silver Nanoparticles Colloidal Dispersions
Published in Girma Biresaw, K.L. Mittal, Surfactants in Tribology, 2017
Ali A. Abd-Elaal, Nabel A. Negm, Girma Biresaw, K.L. Mittal
A simple and effective method, UV-initiated photoreduction, has been reported for synthesis of silver nanoparticles in the presence of citrate, poly(vinyl pyrrolidone), poly(acrylic acid), and collagen. Huang and Yang [47] produced silver nanoparticles via the photoreduction of silver nitrate in layered inorganic laponite clay suspension as a stabilizing agent. The properties of the produced nanoparticles were studied as a function of UV irradiation time. Bimodal size distribution and relatively large silver nanoparticles were obtained when irradiated under UV for 3 h. Further irradiation disintegrated the silver nanoparticles into smaller sizes with a single distribution mode until a relatively stable size and size distribution were obtained. Silver nanoparticles (nanospheres, nanowires, and dendrites) have been prepared by ultraviolet irradiation photoreduction technique at room temperature using poly(vinyl alcohol) as a protecting and stabilizing agent. The concentrations of both poly(vinyl alcohol) and silver nitrate played a significant role in the growth of nanorods and dendrites [48]. Sonoelectrochemistry technique utilizes ultrasonic power primarily to manipulate the material mechanically. The sonoelectrochemical synthesis method involves alternating sonic and electric pulses, and electrolyte composition plays a crucial role in particle shape formation. It was reported [49] that silver nanospheres could be prepared by sonoelectrochemical reduction using a complexing agent, nitrilotriacetate, to avoid aggregation. Nanosized silver particles with an average size of 8 nm were prepared by photoinduced reduction using poly(styrene sulfonate)/poly(allylamine hydrochloride) polyelectrolyte capsules as microreactors [50]. Moreover, it was demonstrated [51] that the photoinduced method could be used for converting silver nanospheres into triangular silver nanocrystals with desired edge lengths in the range of 30–120 nm. The particle growth process was controlled using dual-beam illumination of nanoparticles. Citrate and poly(styrene sulfonate) were used as stabilizing agents. In the study by Malval et al. [52], silver nanoparticles were prepared through a very fast reduction of Ag+ by α-aminoalkyl radicals generated from hydrogen abstraction to an aliphatic amine by the excited triplet state of two-substituted thioxanthone series (TX−O−CH2−COO− and TX−S−CH2−COO−) (TX: thioxanthone moiety). The quantum yield of this prior reaction was tuned by a substituent effect on the thioxanthones and led to a kinetic control of the conversion of Ag+ to Ag0.
Shedding light on the factors controlling the mechanism, selectivity and reactivity of the Diels–Alder reactions between substituted pyridinones and ethylenes: a MEDT study
Published in Molecular Physics, 2021
Brahim Bayoud, Leila Barama, Abdelmalek Khorief Nacereddine, Abdelhafid Djerourou
An analysis of the activation energies in the gas phase indicates that the ortho-exo approach (TS2ox) is more favoured one by 6.74 kcal.mol−1 than the second more favoured approach (TS2on). Additionally, the ortho-exo approach is more stable than the most favoured meta approach TS2mx (ΔE#=27.01 kcal.mol−1) by 12.99 kcal.mol−1. On the other hand, we notice that all cycloadducts are stable, which account for the irreversibility for this DA reaction. Therefore, this DA reaction between pyridinone 2 and benzyloxyethylene 4 is only under kinetic control and favour the formation of a single isomer P2ox, which is generated from ortho-exo approach, as observed experimentally.
Kinetics of malachite leaching in alkaline glycine solutions
Published in Mineral Processing and Extractive Metallurgy, 2021
B. C. Tanda, E. A. Oraby, J. J. Eksteen
where x is reaction conversion, c is ligand concentration, dp is the mean particle size, ρ is slurry density, n is the rotational velocity, T is the absolute temperature and t is the reaction time. According to (Bingöl et al. 2005), the controlling steps during the leaching of copper oxides containing malachite in ammonia/ammonium carbonate are the interface transfer and diffusion across the product layer with an activation energy of 15 kJ/mol. Copper dissolution from a malachite ore in ammonium nitrate solutions was reported by Ekmekyapar et al. (2012) to be a mixed kinetic model, including both surface chemical control (30–50°C) and diffusion through a porous product layer (50–70°C). It was noted that the sequential stages had activation energies of 95.1 and 29.5 kJ/mol respectively. A kinetic study performed by Künkül et al. (2013) on the dissolution of malachite in ammonium acetate reported the leaching process follows a mixed kinetic control model with a calculated activation energy of 59.6 kJ/mol.
Investigation on the oxidation behavior and multi-step reaction mechanism of nuclear graphite SNG742
Published in Journal of Nuclear Science and Technology, 2020
Wei Lu, Xiaowei Li, Xinxin Wu, Libin Sun, Zhengcao Li
According to the relative resistance of chemical reaction kinetics and diffusion, the graphite oxidation process can be divided into three regimes, namely the chemical kinetics control regime, inner diffusion control regime (transition regime), and outer diffusion control regime. As shown in Figure 4(a), the oxidation rate is essentially stable when the temperature is higher than 700°C for gas volume flow rate of 0.2 SLPM and when the temperature is higher than 750°C for gas volume flow rate of 0.5 SLPM. The oxidation rate for 550–700°C is linearly proportional to 1000/T. Therefore, the temperature range of the chemical kinetic control regime is 550–700°C, and the apparent activation energy can be obtained by fitting experimental data in this region. This provides a way to slow down the corrosion of graphite in accidents of HTGR. In the reaction kinetic control regime with temperature lower than 700°C, lowering the temperature could effectively reduce the oxidation rate, while in the diffusion dominating regime with higher temperature, decreasing the flow rate in pebble bed core is the priority.