Particle size of the reactants

Reaction depends on collisions. The more surface area on which collisions can occur, the faster the reaction. You can hold a burning match to a large chunk of coal and nothing will happen. But if you take that same piece of coal, grind it up very, very fine, throw it up into the air, and strike a match, you ll get an explosion because of the increased surface area of the coal. 

Increasing the number of collisions speeds up the reaction rate. The more reactant molecules there are colliding, the faster the reaction will be. For example, a wood splint bums okay in air (20 percent oxygen), but it burns much faster in pure oxygen. 

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Mampel extended the treatment to include due allowance for three-dimensional growth of product into the particles by considering the latter to consist of a series of concentric thin spherical shells. The fractional reaction within each such shell was calculated and the total reaction found by integration to include all such shells. This analysis, which includes the effects of overlap, ingestion and also particle size of the reactant, is not amenable to general solution, but the following special cases are of interest. 

The (en) compound developed nuclei which advanced rapidly across all surfaces of the reactant crystals and thereafter penetrated the bulk more slowly. Kinetic data fitted the contracting volume equation [eqn. (7), n = 3] and values of E (67—84 kJ mole"1) varied somewhat with the particle size of the reactant and the prevailing atmosphere. Nucleus formation in the (pn) compound was largely confined to the (100) surfaces of reactant crystallites and interface advance proceeded as a contracting area process [eqn. (7), n = 2], It was concluded that layers of packed propene groups within the structure were not penetrated by water molecules and the overall reaction rate was controlled by the diffusion of H20 to (100) surfaces. 

High-density spinel refractory brick is made by calcining (1200-1300°C for about 3 h) compacted powders and then sintering at 1700°C. The extent of spinel formation increases by calcination however, the powder mixture is not completely converted to spinel. Typically 10-15% of a-Al203 and 5-10% of MgO are observed by XRD, depending on the conditions of temperature, time and particle size of the reactants. 

The rate of the low temperature reaction is influenced by the particle size of the reactant [4,29], ageing [29] and the presence of volatile decomposition products [29]. Measurements have been made of the surface temperature of NH4QO4 during deflagration and other aspects of the combustion reactions [32,33]. 

A small particle size of the reactant powders provides a high contact surface area for initiation of the solid state reaction diffusion paths are shorter, leading to more efficient completion of the reaction. Porosity is easily eliminated if the initial pores are very small. A narrow size... 

Silica, in any of its forms, when healed with orthophosphoric acid dissolves to an extent dependent upon the temperature, concentration and particle size of the reactants. Various colloidal, amorphous and crystalline materials can be separated, some of which are orthophosphates. Although not all of these have been well defined, Si3(P04)4 and Si50(P04)6 are among the products which can be isolated as definite chemical individuals. 

Nature of the reactants Particle size of the reactants Concentration of the reactants Pressure of gaseous reactants Temperature Catalysts... 

In practical systems, solid state reaction in powder systems depends on several parameters. They include the chemical nature of the reactants and the product the size, size distribution, and shape of the particles the relative sizes of the reactant particles in the mixture the uniformity of the mixing, the reaction atmosphere the temperature and the time. The reaction rate will decrease with an increase in particle size of the reactants because, on average, the diffusion distances will increase. For coherent reaction layers and nearly spherical particles, the dependence of the reaction kinetics on particle size is given by Eq. (2.16) or Eq. (2.17). The reaction rate will increase with temperature according to the Arrhenius relation. Commonly, the homogeneity of mixing is one of the most... 

The rates at which reactions take place are generally dependent on the particle size of the reactants, i. e., on the reactive surface areas. Hence the raw meal should be of such fineness that in the burning process even its coarsest particles will react as completely as possible. As a rule, this condition is satisfied by cement raw meal with a residue of not more than 5-20% (by weight) retained on the 90 micron sieve, the actual maximum acceptable percentage being dependent on the composition of the meal and the type of kiln system. 

The effects of ball-milling time on the formation of forsterite have affirmed that the particle size of the reactants... 

Studies had been done in SHS to observe the effect of various parameters on wave-front velocity and product morphology. Among the parameters studied were the green (unreacted) density and particle sizes of the reactants. All the systems so far considered use liquid monomers or solid monomers in solution. Relatively little work has been done with solid monomers. Pojman et al, demonstrated frontal acrylamide polymerization with a variety of free-radical initiators (72). Savostyanov et al. studied transition metal complexes of acrylamide without initiator (73). No studies were performed on the effect of particle size and/or green density. We therefore investigated those two factors to compare to work done in intermetallic SHS systems. 

The average pore size and the pore size distribution should be such that physical limitations are not placed on the conversion of reactants to products. The particle size of the carrier must also be suitable for the purpose intended (i.e., small for fluidized bed reactors and significantly larger for fixed bed applications). 

Figures 1 shows the catalytic performance of the Fe-BEA catalysts in the temperature range of 250-550 °C. It is clear from the figure that propylene yield depends on particle size of the parent BEA zeolite. Effect of the N20 concentration has been analyzed under reaction regimes RS-1 and RS-2. Increase in N20 concentration resulted in the same propene yields but increased the N20 conversion and decreased the selectivity toward propylene. At higher temperature has been obtained increases in the formation of the molecular oxygen which further accelerates production of the undesired carbon oxides. Thus, at lower feed concentration of N20, i.e. at 1 1 feed ratio of reactants (RS-1), formation of carbon oxides is suppressed and the selectivity of ODHP reaction is...

As in the case of normal supported catalysts, we tried with this inverse supported catalyst system to switch over from the thin-layer catalyst structure to the more conventional powder mixture with a grain size smaller than the boundary layer thickness. The reactant in these studies (27) was methanol and the reaction its decomposition or oxidation the catalyst was zinc oxide and the support silver. The particle size of the catalyst was 3 x 10-3 cm hence, not the entire particle in contact with silver can be considered as part of the boundary layer. However, a part of the catalyst particle surface will be close to the zone of contact with the metal. Table VI gives the activation energies and the start temperatures for both methanol reactions, irrespective of the exact composition of the products. 

Reactants and catalyst must be contacted thus, a high external surface area of the catalyst (smaller particle size) is desirable to maximize reaction rate. The particle size of the catalyst must be optimized to permit filterability for ease of recycle while maintaining the high external surface area needed for maximum reactant-catalyst contacting. 

In both the electrothermographic and foil assembly methods, the rapid heating rates associated with combustion synthesis are reproduced. However, the powder reactant contact found in a compacted green mixture of particulate reactants is not adequately simulated. One way to overcome this is to investigate interactions of particles of one reactant placed on the surface of the coreactant in the form of a thin foil. The physical simulation corresponds to the reaction of a powder mixture where the particle size of one reactant is small while that of the coreactant is relatively large. Two methods have been used to initiate the interaction. 

The dependence on Pd particle size of the hydrogenations of alkenes, alkadienes, and alkynes has been studied in the gas and liquid phases and for pure reactants or simulated real feedstocks. This is illustrated by some typical examples later. 

Hence the effectiveness factor for a given pellet can be obtained by measuring the rate for the pellet and for a small particle size of the same catalyst at the same concentration of reactant. 

Internal diffusion limitations of reactant molecules can affect the activity properties and this depends on the pore size distribution as well as on the particle size of the catalyst. Figure 3 shows the variation of the carbon productivity as a function of the particle size of the catalyst. 

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