Calcination reactions

Fig. 4. Elfect of calcination temperature on conversion and hydrogenolytic behaviors catalyst 10 wt % M0O3-AI2O3 reduced at 450°C for 2 hr after calcination reaction temperature, 350°C WjF, 7.9 g catal hr mole , H2/(2,5,3 TrMeDPM -(- benzene) molar ratio, 2.0 total pressure, 1 atm.

Calcination of limestone has been chosen as a model reaction and pore size distributions of the limestone particles are determined at different extents of calcination at different temperatures. Although the calcination reactions have been investigated for ages there are still questions about the actual mechanism of such reactions. The literature does not involve the structural variations. 

In this work it is shown from the variations of pore structure that a zone reaction model similar to the one suggested by Ishida and Wen (9) can explain the mechanism of calcination reaction studied. 

Experimental findings show that calcination reaction starts at every interior point of the reactant and after a certain time the reactant solid near the external surface is completely exhausted forming a bidisperse inert product layer. The period of reaction prior to the formation of the ash layer is designated as the first stage and the period following the formation of the ash layer as the second stage. 

During the calcination, the gaseous product CC>2 diffuses out through the pores. The rate of CO2 evolution depends upon whether diffusion or surface reaction is the controlling mechanism. Since calcination reaction is reversible concentration profile of CO2 within the pores would strongly effect the apparent rate of... 

A different DTA/DSC endotherm shape would be expected for calcination reactions such as CaCC, ) = CaO(s) + C02(9) (Figure 3.13). Given that the partial pressure of CO2 in dry air is... 

A classic example of a solid—fluid ceramic powder synthesis reaction is that of calcination and dehydration of natural or synthetic raw materials. Calcination reactions are common for the production of many oxides from carbonates, hydrates, sulfates, nitrates, acetates, oxalates, citrates, and so forth. In general, the reactions produce an oxide and a volatile gaseous reaction product, such as CO2, SOg, or HgO. The most extensively studied reactions of this type are the decompositions of magnesium hydroxide, magnesium carbonate, and calcium carbonate. Depending on the particular conditions of time, temperature, ambient pressure of CO2, relative humidity, particle size, and so on, the process may be controlled by a surface reaction, gas diffusion to the reacting... 

TGA and XRD studies showed that dehydration occurred between 200-300 C, without loss of the basal spacing. Further heating was accompanied by partial oxidation and decarboxylation, the latter being complete by about 600 C. This heating yielded a mixed oxide with a spinel structure (XRD). The expected stoichiometry of the calcination reaction for the Co/Al = 3 system should result in a... 

If the converted particle is the main reaction product (e.g. for calcination reactions), the recycle ratio N and the hei t of the reactor determine the average conversion of the discharged particles Mp (and hence the product quality) as illustrated by eqn. (3). Smolders et al. [8] illustrated these principles for a CFB limestone calciner. 

Fig. 2. Thermogravimetric examination (TGA) of potassium cyantrimethylsilylamide. The mass loss at T= 300 °C of 62% is identical with the theoretically expected value for the depicted reaction. The volatile bis(trimethylsilyl)carbodiimide (BTSC) was detected by mass spectrometry. Bulk calcination experiments proved the quantitative formation of pure BTSC (NMR) and potassium cyanamide (powder XRD). This reaction is analogous to calcination reactions of hydroxides forming oxides.

D. L. Hankey, Calcination Reaction Mechanisms and Kinetics in Lead Zirconate Titanate Powder Compacts, Ph.D. thesis, Pennsylvania State University, 1980. 

The calcining reaction occurs rapidly and is always complete in less than 10 min. if the driving force exceeds 0.3 atm. The driving force provided in the feasibility study—i.e.y 2.4 atm.—appears adequately conservative. 

A peculiar memory effect is apparent in the data of Figure 11 in that the calcining reaction is significantly retarded by a high C02 partial pressure in the acceptor pores before initiating the calcining regime. This points to a diffusion-controlled mechanism of some kind. The... 

Figure 3. GIXD spectra of Pt/AhOs - A after A calcination - B calcination + reaction (s I) - C aging (see Table 2 for conditions)...

Here, the forward and reverse reactions are typically referred to as the carbonation and calcination reaction, respectively. The optimal operation temperature for the reactors in which the carbonation and calcination reactions proceed can be easily determined by plotting the equilibrium partial pressure of CO2 as a function of temperature using, e.g. the correlation of Barin and Platzki [6], as shown in Fig. 6.1. 

Although the calcination reaction has been extensively studied, there is still no consensus as to its rate-limiting step. As a typical non-catalytic, gas-solid reaction, one or more of the following steps may limit calcination [9] ... 

Effect of the Partial Pressure of CO2 on the Kinetics of the Calcination Reaction... 

The last aspect of the calcination reaction to be considered here is the effect of the total pressure of the system on the rate of the calcination reaction. In fact, so far no conclusive explanation has been drawn on why the total pressure of the system affects the rate of reaction, even in the absence of C02- For example, Dennis and Hayhurst [50] observed that increasing the pressure results in a decreasing reaction rate even in the absence of CO2 they suggested an empirical expression for Qc (rate of reaction of one particle) of the form ... 

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