Parallel reactions temperature effect

Example 18-2 illustrates the effect of two process variables (concentration and temperature) on the instantaneous fractional yield for two parallel reactions. 

Common features in the various theoretical explanations of compensation behavior referred to in Section II, A, 1-7 are the occurrence of parallel reactions that are characterized by different values of the kinetic parameters (A, E) and/or a systematic change in the effective concentrations of reactants across the temperature interval used in the measurements of the Arrhenius parameters. Both influences are based on reaction models for which the kinetic behavior cannot be represented as a single desorption step and, indeed, the overall surface interactions could be much more complicated. 

The chemical process gives the enthalpy of reaction, the flow rate, the reaction time, and the required reaction temperature. The first step in the sizing procedure is to calculate the required number of channels for the heat exchanger. Then the pass arrangement is selected in order to achieve the highest possible Reynolds number within an acceptable pressure drop. For example, if the total number of channels is fixed by the residence time channels in series will induce high velocities and high pressure drop channels in parallel will induce low velocities and low pressure drop. The second step is to estimate the heat transfer coefficient and to check that the heat flux can effectively be controlled by the secondary fluid (the lower heat transfer coefficient should be on the reaction side). 

It has been shown theoretically that for the case of two competing parallel reactions, two stable combustion modes can exist (Khaikin and Khudiaev, 1979), where the reaction route depends on the ignition conditions. This effect was first observed experimentally by Martirosyan et al. (1983) for the Zr-C-H2 system (Fig. 50). Using relatively low ignition temperatures (1300 K), the combustion... 

During ECM, electrochemical dissolution of anode and cathodic evolution of hydrogen proceeds on the electrodes (the WP and TE, respectively). Along with these basic reactions, parallel reactions proceed concurrently, for example, oxygen anodic evolution, cathodic reduction of nitrate ions, if NaNC>3 electrolyte is used. It is important to note that electrochemical reactions in a narrow IEG result in gas evolution. The temperature of the electrolyte in the IEG and the void fraction increase as the electrolyte flows along the gap. This leads to a variation in the electrolyte conductivity that has an effect on the distributions of current and metal dissolution rate over the WP surface. The electrode processes and the processes in... 

Further measurements need to be made on the temperature and potential dependence of the rates of simple ionic redox reactions at electrodes with proper corrections for double-layer effects at various temperatures, so that the temperature dependence of (3 for an elementary electron transfer reaction, without chemisorption and coupled atom transfer, would become better known. This is an essential requirement for progress in understanding the true significance of the temperature effects on electrode-kinetic behavior reliable experiments will not, however, be easy to accomplish and will require parallel double-layer studies over a range of temperatures. 

FIGURE 78 Arnett plot of polymers made with Cr/silica-titania catalysts. Raising the activation temperature from 700 to 982 °C resulted in greater deviation from the linear reference line, indicating increased LCB. However, raising the reaction temperature from 98 to 110 °C caused points to move parallel to the JC gridlines, indicating no effect on LCB. 

The effects of complex mechanisms with serial or parallel shifts in rate limiting steps must also be considered in the analysis of the temperature dependence of isotope effects [28]. tn addition, tedious attention to details of temperature effects on pH, acidity constants, reaction volumes, and substrate or catalyst stabilities may be needed in some cases to avoid problematic interpretations. For these reasons, temperature studies of isotope effects may not be as convincing as the observation of very large isotope effects in providing evidence for tunneling. 

Catalytic partial oxidation of hydrocarbons represents an important class among petrochemical reactions. Complete oxidation of hydrocarbons gives CO2, H2O. During the partial oxidation processes the conversion of a certain percentage of reactants and/or products to complete combustion products cannot be avoided. The main role of the catalyst in these reactions is to accelerate (at relatively lower temperatures) the reaction paths to the desired product without having the same effect on the paths to the complete combustion products. The partial oxidation reactions are usually consecutive or consecutive/parallel reactions with quite complex networks in many cases. 

Temperature. As regards dependence on temperature, there was an interesting parallel to the anomalous temperature effects (36) observed in many enzyme reactions (above 35°C, the rate increased bv 40%/10°C and below 35°C by 80-100%/10°C). 

The effect of the residence time on the IBA conversion and products selectivity at the temperature of 235C is drawn in Figure 1, for the Ko sample (ammoniacal salt) calcined at 320C. The proportionality between conversion and residence time allows to exclude diffusion as the rate-determining step. Moreover, it is shown that the selectivities to the various products were substantially independent on the conversion. This is in favour of a reaction network constituted of parallel reactions (probably sharing a common reaction intermediate, obtained by IBA activation) for the formation of methacryhc add, acetone plus CO2, propylene plus CO, and carbon oxides from combustion (1,4,7). 

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. 

---