Heterogeneous systems rate-controlling steps

While in homogeneous systems the reaction is occurring throughout the entire volume of the reaction vessel and the partial pressures (concentrations) of the species participating in the rate-controlling step are often directly observable, the same is not true for heterogeneous systems. Here, reaction is confined to a monomolecular layer at the surface, around 10 "6 of the total volume of the reaction system, and the concentrations of... 

Modeling of Time Dependence. Simple Topological Description of the Overall Reaction. A better understanding of the reaction can be achieved by plotting the conversion of a specific structural unit. The conversion is proportional to the normalized peak area, and is plotted vs the normalized time necessary for the complete reaction [289, 301-305]. Similar plots are used in heterogeneous catalysis to study the rate-controlling step of a process [321]. In Fig. 70 top, a plot for the aromatic rings (3060 cm1) at temperatures of 783 K (V), 812 K (+) and 841 K ( ) is presented. In Fig. 70 bottom, the imide system (725 cm1) is plotted at the same temperatures (783 K A 812 K X and 841 K A). 

On heating, many hydrides dissociate reversibly into the metal and Hj gas. The rate of gas evolution is a function of both temperature and /KH2) but will proceed to completion if the volatile product is removed continuously [1], which is experimentally difficult in many systems. The combination of hydrogen atoms at the metal surface to yield Hj may be slow [2] and is comparable with many heterogeneous catalytic reactions. While much is known about the mobility of H within many metallic hydride phases, the gas evolution step is influenced by additional rate controlling factors. Depending on surface conditions, the surface-to-volume ratio and the impurities present, the rate of Hj release may be determined by either the rate at which hydrogen arrives at the solid-gas inteifece (diffusion control), or by the rate of desorption. 

In the case of a heterogeneous reaction the overall process rate must consider the kinetics of both chemical and physical steps. If a very slow step can be identified, this controls in generally the global reaction rate. More often, the analysis should consider several limiting steps, both of physical and chemical nature. Therefore, the computation of the reaction rate in heterogeneous systems requires information that is hardly available at the conceptual stage. 

Batch emulsion polymerization of styrene in the presence of NIPAM monomer shows rapid consumption of NIPAM monomer [22]. In fact, around 80% of NIPAM monomer is reacted before the styrene monomer starts to polymerize. Then, the formed poly(NIPAM) chains bearing surface active properties acts as a surfactant during the styrene polymerization. Consequently, the initial amount of NIPAM controls the polymerization rate, the particle size, the size distribution, and also the thickness layer of the formed shell. Thus, polymerization should be considered as emulsion polymerization with in situ surfactant production. Examination of the particle size of latexes as a function of polymer conversion by transmission electron microscopy shows that the polymerization of a heterogeneous system such as this occurs in numerous steps that will not be depicted in this chapter, but can be consulted in Refs. [22,24]. 

The rate at which each of these steps occurs ultimately determines the distribution of the participating species (reactants and products) in the system in addition, it plays a major role in determining the over-all rate of heterogeneous catalytic reactions. The factors and effects influencing each of these rates is considered in the next four sections. The final section is concerned with analyzing and determining the controlling step or steps for the reaction in question. This topic will be reviewed qualitatively since much of this material is both difficult and complex a qualitative presentation is beyond the scope of this text. 

These problems are avoided if a continuous process is employed for the precipitation however, this makes higher demands on the process control. In a continuous process all parameters as temperature, concentrations, pH, and residence times of the precipitate can be kept constant or altered at will. Continuous operation is, for instance, used for the precipitation of aluminum hydroxide in the Bayer process. Bayer aluminum hydroxide is the main source for the production of cata-lytically active aluminas. The precipitation step of the Bayer process is carried out continuously. An aluminum solution supersaturated with respect to Al(OH)3, but not supersaturated enough for homogeneous nu-cleation, enters the precipitation vessel which already contains precipitate so that heterogeneous precipitation is possible. The nucleation rate has to be controlled very carefully to maintain constant conditions. This is usually done by controlling the temperature of the system to within 2-3 degrees [7]. 

The second-phase reaction is heterogeneous and occurs at the surface of the particle. The reaction causes the reacting surface to shrink and to leave an ash layer as the particle moves through the reactor. Unlike the first-phase reaction, which is only slightly affected by temperature, the second-phase reaction is quite sensitive to variations in temperature for tests conducted in a semiflow system (10). Since a high gas flow rate was maintained in semiflow tests, gas diffusion probably does not affect the rate. At temperatures below 1700°F., the first-phase reaction rate is an order or two larger than the second-phase reaction rate, but as the temperature approaches 2000°F., the two rates become comparable. This is, of course, true only when the reaction is controlled by the chemical step. 

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