Consecutive reactions temperature effect

Consecutive reactions. Marcu and Segal [96] considered two consecutive reactions of the reaction order (RO) type. The a,T curves were particularly influenced by the values (which determine the onset temperatures of the reactions). Differences of greater than 10 Id mol between , values of the two processes led to their total separation. Different values for the pre-exponential factor, A, could compensate for this effect. The value of the reaction order also influenced separation. Thus, at low a the kinetic parameters are approximately those of the first reaction step. 

The decomposition of titanium hydride in vacuum between 523 and 773 K was slower than the rate predicted by diffusion calculations and the controlling step was identified [12] as the surface combination of hydrogen atoms. The rate of reaction was sensitive to traces of gaseous Hj, but not to Oj. The inhibiting effect exerted by the presence of helium was ascribed to opposition to the diffusive dispersal of product from the vicinity of the desorption interface. The rates of decomposition of the hydrides of four related metals [13] (TiHj, ZrHj, NbH and TaH) studied between 343 and 973 K pass through a temperature maximum. This was explained by the occurence of two consecutive reactions first-order decomposition of the hydride, followed by second-order combination of the hydrogen atoms before desorption. 

In order to study the effect the reaction temperature has, the reaction was carried out with the same method but in the 180-80-C temperature range (the difference in temperature between consecutive steps was 20 °C). The resultant conversion and TOE show an increase in the values when the reaction took place at higher temperatures. 

The data in Fig. 1 show a drastic loss of selectivity with increasing reaction temperature and conversion, indicating a great sensitivity of benzaldehyde towards consecutive transformations, but do not allow discrimination between the effect of temperature and conversion of toluene and oxygen. Therefore, experiments were carried out at isotemperature and isoflow rate with different amounts of the catalyst or the 02/toluene ratio in the feed. The results are summarized in Figures 4A and 4B, respectively. 

Figure 3 shows the catalytic behavior of the K3 sample. The dependence of conversion and selectivities on the temperature is similar to that exhibited by the Ko sample the only difference consists in a decrease of the selectivity to MAA (and of acetone, too) approaching total IBA conversion, with an increase in combustion products. This effect is likely related to the higher surface area of the K3 catalyst, favouring consecutive reactions of MAA combustion. 

The effect of residence time on isobutane conversion and on selectivity to the various products at the temperature of 320°C, under isobutane-rich conditions, is illustrated in Figure 3. The data indicate that methacrolein, methacrylic acid, and carbon dioxide are all formed through direct, parallel reactions acetic acid and possibly carbon monoxide are instead formed through consecutive reactions. Methacrolein undergoes consecutive reactions of transformation to acetic acid, to carbon oxides and possibly in part also to methaciylic acid. Indeed the selectivity to the latter product seems to increase slightly with increasing isobutane conversion. 

The first requirement is mainly important for the assessment of chemical reactions. In the overwhelming majority of chemical processes, not only the chemical conversion into the single desired product takes place. Instead, the desired reaction is accompanied by numerous parallel and consecutive reactions. Under the defined operating conditions resulting from the optimization work, the effect of these simultaneous reactions on yield and selectivity has been minimized by the choice of mode of operation (continuous, batch or semibatch) and of process parameters, such as pressure, temperature, concentration, pH-value, mass flow rates etc. A performance of the safety tests under conditions deviating fi-om those chosen for the plant process would inadvertently favour those secondary reactions in a different manner. Values for the gross value of heat output and reaction rate obtained this way would not be suitable for any process safety evaluation. Modem reaction calorimeters, like those commercially available today, enable the conduction of experiments with sufficient similarity to actual plant conditions. 

Wang and Wu [70] analyzed the extraction equilibrium of the effects of catalyst, solvent, NaOH/organic substrate ratio, and temperature on the consecutive reaction between 2,2,2-trifluoroethanol with hexachlorocyclotriphosphazene in the presence of aqueous NaOH. Wu and Meng [69] reported the reaction between phenol with hexachlorocyclotriphosphazene. They first obtained the intrinsic reaction-rate constant and overall mass transfer coefficient simultaneously, and reported that the mass transfer resistance of QX from the organic to aqueous phase is larger than that of QY from the aqueous to organic phase. The intrinsic reaction-rate constant and overall mass transfer coefficients were obtained in three ways. 

For most unsaturated hydrocarbons, addition of OH is the first and rate-limiting step of the photochemical reaction chain. In the case of aromatics, which are emitted from automobiles, forest fires and fuel wood burning [11], the addition reaction is reversible at atmospheric temperatures. The effective rate constant for removal of the aromatic depends on consecutive reactions of the adduct. Prior to LACTOZ, consecutive reactions with O2 had not been detected for benzene -OH... 

The last question that will be examined on the subject of chain transfer is the effect of coupling between chain reactions in series on the selectivity of the system. The example chosen is the gas-phase oxidation of methane at controlled temperatures. The two consecutive reactions that are important in the early stages of the reaction are ... 

The same applies to the oxyethylation of FAME. Thus, the reaction depends on the rate of EO transport to the liquid phase. The delivered EO is immediately consumed in successive parallel reactions giving products with various numbers of oxyethylene groups. The rate constants of the first, second, third, and nth reaction steps are denoted by Kq, k-y, and respectively. However, the contribution of the reaction depends on the process temperature and degree of oxyethylation. An increase in temperature enhances the role of diffusion, whereas an increase in oxyethylation degree has an opposite effect caused by an increased concentration of EO dissolved in the liquid phase. All this means that the computed constants cannot be considered as typical kinetic constants but only as relative estimates of consecutive reaction steps. 

In general, temperature effects may significantly affect the performance of adsorptive processes. For this reason we have further extended previous work on the use of PSR reactors for consecutive irreversible reactions to exothermic reactions and non-isothermal conditions. In the present case we will deal with external cooling, by using a multi-tubular... 

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