Variables affecting rate reaction

Miscellaneous Methods At the beginning of this section we noted that kinetic methods are susceptible to significant errors when experimental variables affecting the reaction s rate are difficult to control. Many variables, such as temperature, can be controlled with proper instrumentation. Other variables, such as interferents in the sample matrix, are more difficult to control and may lead to significant errors. Although not discussed in this text, direct-computation and curve-fitting methods have been developed that compensate for these sources of error. ... 

Table 12-3. Variables Affecting Observed Reaction Rate...

Structural control in sol-gel processes is complicated because many and diverse variables affect concurrent reactions differently. Inductive and steric factors contribute to the reaction rates. pH is probably the single most important variable in these reactions. It accounts for differences when DCCAs are used and for the rapid gel times in Si(OAc)4 sols, as well as differences in the two predominant growth mechanisms nucleation and growth and cluster-cluster aggregation. 

The foregoing objectives do not require reference to all those studies that simply show how the rate varies with some variable under a single set of experimental conditions, where the variable may for example be the addition of an inactive element or one of lesser activity, the particle size or dispersion, the addition of promoters, or an aspect of the preparation method. Such limited measurements rarely provide useful information concerning the mechanism, and many of the results and the derived conclusions have recently been reviewed elsewhere. We look rather to the determination of kinetics and product distributions to show how the variable affects the reaction mechanism. 

Reaction Kinetics and Thennoffynamics The influence of extensive variables (pressure, tenperature, concentration) on reactor performance is defined by reaction kinetics and reaction equilibrium. These extensive variables affect the reaction rate and determine the extent to which reactants can be converted into products in a given reactor or the size of reactor needed to achieve a given conversiom Additionally, catalysts are used to increase the rate of reaction. Thermodynamics sets a theoretical limit on the extent to which reactants can be converted into products and cannot be changed by catalysts. 

The extent to which each of the above reactions occur is strongly influenced by feed quality and the levels selected for the major process variables pressure, temperature, recycle rate, and frequency of regeneration. From a process viewpoint, these variables affect catalyst requirement, gasoline yield, and coke make. 

The primary process variables affecting the economics of sulfuric acid alkylation are the reaction temperature, isobutane recycle rate, reactor space velocity, and spent acid strength. To control fresh acid makeup, spent acid could be monitored by continuously measuring its density, the flow rate, and its temperature. This can reduce the acid usage in alkyla-tion units. 

Now that we have a good picture of how SN2 reactions occur, we need to see how they can be used and what variables affect them. Some SN-2 reactions are fast, and some are slow some take place in high yield and others, in low yield. Understanding the factors involved can be of tremendous value. Let s begin by recalling a few things about reaction rates In general. 

Moreover, the current-potential curves are affected by the disproportionation reaction therefore, other variables (the rate constant for the disproportionation reaction) must be taken into account. Since experimental results for many interesting systems show clear evidence of slow kinetics, ad hoc simulation procedures have typically been used for the analysis of the resulting current-potential curves [31, 38, 41, 48]. As an example, in reference [38], it is reported that a clear compropor-tionation influence is observed for an EE mechanism with normal ordering of potentials and an irreversible second charge transfer step. In this case, the second wave is clearly asymmetric, showing a sharp rise near its base. This result was observed experimentally for the reduction of 7,7,8,8-tetracyanoquinodimethane in acetonitrile at platinum electrodes (see Fig. 3.20). In order to fit the experimental results, a comproportionation rate constant comp = 108 M-1 s-1 should be introduced. 

The analysis presented in this chapter was based on three crucial assumptions. Namely, mass transfer and reaction kinetics were neglected. Further, constant flow rates were assumed. Although these assumptions are valid in many applications, an extension to finite mass transfer and reaction kinetics as well as variable flow rates seems challenging for future research in this field. As indicated in the last section, finite mass transfer and reaction kinetics may affect the feasible products of integrated reaction separation processes quite significantly. Strong impact can therefore also be expected for the dynamic behavior. The same applies to variable convective flow rates due to nonequimolar reactions. While this effect is not too important... 

We can draw a very useful general conclusion from this simple binary system that is applicable to more complex processes changes in production rate can be achieved only by changing conditions in the reactor. This means something that affects reaction rate in the reactor must vary holdup in liquid-phase reactors, pressure in gas-phase reactors, temperature, concentrations of reactants (and products in reversible reactions), and catalyst activity or initiator addition rate. Some of these variables affect the conditions in the reactor more than others. Variables with a large effect are called dominant. By controlling the dominant variables in a process, we achieve what is called partial control. The term partial control arises because we typically have fewer available manipulators than variables we would like to control. The setpoints of the partial control loops are then manipulated to hold the important economic objectives in the desired ranges. 

The ways in which reaction parameters affect a two phase batch reaction are similar to those considered above for the three phase systems. Since there is no gas phase, agitation only serves to keep the catalyst suspended making it more accessible to the dissolved reactants so it only has a secondary effect on mass transfer processes. Substrate concentration and catalyst quantity are the two most important reaction variables in such reactions since both have an influence on the rate of migration of the reactants through the liquid/solid interface. Also of significant importance are the factors involved in minimizing pore diffusion factors the size of the catalyst particles and their pore structure. 

Data obtained in both bench and pilot-plant equipment are valuable, and it is common practice to carry out investigations with both before building the commercial-scale reactor. The first yields a better rate equation and more knowledge about the kinetics of the reaction i.e., it tells the engineer more accurately just what variables affect the rate of the chemical step and how they influence the course of the reaction. This information is particularly valuable in case it is necessary to predict how the commercial-scale plant will be affected by a change of operating conditions not specifically considered in the pilot-plant work. 

The data will be analyzed graphically by plotting the volume of hydrogen gas produced vs. time for each set of conditions. By comparing the slopes of each graph, we will be able to determine how changing the variables affect the rate of the chemical reaction. 

The relative sizes and locations of anodic and cathodic areas are important variables affecting corrosion rates. As stated previously, these areas may vary from atomic dimensions to macroscopically large areas. In Fig. 1.6, areas have been depicted over which the anodic and cathodic reactions occur, designated as Aa and Ac. If the current is uniformly distributed over these areas, then the current densities, ia = Ia/Aa and ic = Ic/Ac, may be calculated. The current density is fundamentally more... 

Equation 2.45 is frequently used to close the material balance for components that do not affect the reaction rate. A second use of Equation 2.45 is to eliminate some of the composition variables from rate expressions. Divide Equation 2.45 by V... 

---