Chemical kinetic control

Prior to conducting the DOE (design of experiments) described in Table 3, it was established that no reaction took place in the absence of a catalyst and that the reactions were conducted in the region where chemical kinetics controlled the reaction rate. The results indicated that operating the reactor at 1000 rpm was sufficient to minimize the external mass-transfer limitations. Pore diffusion limitations were expected to be minimal as the median catalyst particle size is <25 pm. Further, experiments conducted under identical conditions to ensure repeatability and reproducibility in the two reactors yielded results that were within 5%. 

Price, S.G. Helditch, D.J. Streat, M. Diffusion or chemical kinetics control in a chelating ion exchange resin system. In Ion Exchange for Industry Streat, M., Ed. Ellis Horwood Ltd. Chichester, U.K., 1988. 

This observation is very important because kinetic studies should de carried out preferably in upflow system in order to avoid mass transfer limitations (interphase). Of course, additional experiments have to be performed to study intraphase gradients consists of determining the conversion for particles of different size at constant space velocity [4]. If conversion is constant indicates that the system is imder chemical kinetic control. 

The solubility of terephthalic acid in the above-mentioned solvents is very low, which means that the acid must diffuse continuously from the solid particules to the solution where the reaction takes place. In such a case, the first question which arises is does the diffusion control the kinetics of the overall process In all cases, the authors claimed that the reaction rate is never affected by the amount of undissolved terephthalic acid and that the reaction proceeds through a chemical kinetic control. Under the experimental conditions used by Bhatia et al. the diffusion rate of terephthalic acid from the solid particles to the solution is 9.5x 10 mol cm" s at 100 °C and that of ethylene oxide from the gas phase to the liquid is 19.4 x 10" mol cm" s" . These values are far above the rate of formation of the diester(bishydroxy-ethylterephthalate), as this is only 5.84 x 10" mol cm" s" . Moreover, the independence of the reaction rate on the mass transfer effects was shown by varying the values of some parameters (e.g., ethylene oxide flow-rate, stirrer-speed, particule size, terephthalic acid charge) in a large range. 

I. Leenzon. Chemical Kinetics Control Questions and Tasks, http //www.chemjnsu.su/rus/ teachingAeenson/zadaniya/welcome.htm, (in Russian)... 

At conversions well above o)p 0.8, the value of kp corresponds to that found in the absence of chemical kinetics control The reaction diffusion constant [44] is denoted by kp.rd and may be estimated from the Smoluchowski expression [45]... 

As noted above, this situation occurs when the rate of diffusion through the interstices among the grains presents a negligible resistance to the progress of reaction and thus chemical kinetics controls the process. 

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