China 
Africa 
Explore chapters and articles related to this topic
Residue Upgrading by Hydrovisbreaking and Hydrotreating
Published in Michael C. Oballa, Stuart S. Shih, Catalytic Hydroprocessing of Petroleum and Distillates, 2020
The HVB pretreatment step did not improve subsequent catalytic performance in terms of desulfurization (Figure 5), demetalation (Figure 6), CCR reduction (Figure 7), and asphaltene conversion (Figure 8) for either small-pore or large-pore catalysts. Most data even suggest that die hydrovisbroken residue in die HVB/HDT combination is more refractory than die virgin residue in the HDT only case. This is not surprising since hydrovisbreaking, similar to thermal visbreaking, can produce and concentrate refractory asphaltenes concentrated with metals and polar functional groups [3,10]. However, hydrovisbreaking appears to improve the hydrogenation activity of the large-pore catalyst as shown in Figure 2. The reasons for this improvement are not clear. It is possible that the hydrovisbreaking reduces size of the residue molecules and lowers the diffusion resistance. However, this improvement was not observed for the demetalation which is a diffusion-controlled reaction.
Generation of Particles by Reactions
Published in Ko Higashitani, Hisao Makino, Shuji Matsusaka, Powder Technology Handbook, 2019
Kakeru Fujiwara, Sotiris E. Pratsinis, Hisao Suzuki
Preparation of powders from liquid phase is usually controlled by the two reaction processes, namely the diffusion-controlled reaction and the rate-limiting reaction. Crystallization by the diffusion-controlled reaction will be described in the next chapter. Therefore, preparation by the rate-limiting reaction is focused in this chapter. Formation or precipitation of particles and/or powders through the liquid phase occurs according to the following three steps: (1) increase in the concentration of the required cations in the solution over the saturation concentration; (2) formation of the new nuclei (embryo); (3) growth of the nuclei over the critical size to stabilize the precipitated particles. Therefore, we can classify the precipitation of powders or particles into two categories depending on the method to increase the concentration of the cations over the saturation concentrations, namely chemical reaction controlled method and physical phenomena controlled method as shown in Figure 4.2.3. As the concentration reaches a certain degree of supersaturation value, nucleation is induced and then the nuclei with a size over the critical size ultimately grow into the particles. Critical size is calculated by the following equation: ΔG=4πr3ΔGV/3+4πr2γ
Transport equations
Published in Yu. K. Tovbin, The Molecular Theory of Adsorption in Porous Solids, 2017
Here it is assumed that the surface area does not change during the reaction and the concentration of particles may be characterized as ‘the degree of filling’ the surface θi $ {{\theta }}_{i} $ . Equations (28.1)–(28.3) suggest that the equilibrium distribution of the molecules forms in the reaction system, and that this limits the stage of chemical transformation. It is also assumed: 1) the absence of diffusion transport at the macroscopic level (uniform distribution in the macrovolume), 2) the absence of the effect of external fields, 3) the absence of a diffusion controlled reaction regime at the molecular level, 4) the lack of influence of intermolecular interactions, and 5) the proportion of particles, reacting in unit time, is so small that it does not distort the equilibrium distribution of molecules on the surface.
Curie-supported accelerated curing by means of inductive heating – Part I: Model building
Published in The Journal of Adhesion, 2022
Morten Voß, Marvin Kaufmann, Till Vallée
Netzsch Kinetics Neo® offers the possibility to describe each reaction step as a diffusion-controlled reaction. In contrast to cold curing at RT, monomer mobility during inductive heating is not restricted since the adhesive-CP mixture takes on temperatures above Tg (entropy-elastic range) most of the time. An exclusion of the aforementioned point represents the late cooling phase after the induction device has been switched off and the GiR specimens slowly start to cool down to RT. However, for the kinetic modelling it was assumed that curing of the two adhesives was already well advanced up to this point, thus, eliminating the need to implement a diffusion-controlled polymerisation as diffusion control comes into play once curing temperatures fall below Tg.
Accelerated curing of G-FRP rods glued into timber by means of inductive heating – Influences of curing kinetics
Published in The Journal of Adhesion, 2022
Netzsch Kinetics Neo® offers the possibility to describe each reaction step as a diffusion-controlled reaction. In the context of the present study, this was omitted, since curing with the presented induction process takes place at elevated temperatures. In contrast to cold curing, the mobility of the monomers is not restricted with increasing curing degree α, since the adhesive temperature during inductive heating, with the exception of the late cooling phase, takes on values above Tg (entropy-elastic range). For the kinetic modelling (see section 3.3), it is assumed that curing for all adhesives is already well advanced at the end of the inductive heating. After the induction device has been switched off, the adhesives are still very hot (<100°C) and cooling progresses relatively slowly. Therefore, modelling of a diffusion-controlled polymerisation is not necessary, since diffusion control comes into play once the adhesive temperature falls below Tg.
Chicken feathers derived materials for the removal of chromium from aqueous solutions: kinetics, isotherms, thermodynamics and regeneration studies
Published in Journal of Dispersion Science and Technology, 2022
Rupa Chakraborty, Anupama Asthana, Ajaya Kumar Singh, Renu Verma, Sreevidya Sankarasubramanian, Sushma Yadav, Sónia A. C. Carabineiro, Md. Abu Bin Hasan Susan
Weber and Morris[68] developed the intra-particle diffusion model which can be presented as: where qt (mg/g) is the amount of chromium(VI) adsorbed at time t (minutes) per unit of weight of CFs, kid is the intra-particle diffusion rate constant (mg/g min1/2), and C is the intercept. The linear plot of qt versus t1/2 is used to test a diffusion-controlled reaction. The increasing values of C indicate that there is an increase in boundary layer thickness of molecule which plays a major role of the surface adsorption in the rate-limiting step.[69] In Figure 7c, the linear fitting plot does not pass through the origin, which shows that the intra-particle diffusion is involved in the adsorption process but it is not the rate governing step for adsorption of Cr(VI) ions onto CFs.