Kinetics, catalyst types

The shape of the kinetic curves depends on the catalyst type and polymerization conditions (ethylene pressure, temperature, concentration of inhibitors in reaction medium) (89, 97, 98). The types of the kinetic curves obtained. at ethylene polymerization under various conditions are presented in Fig. 1. 

S. Y Wang, and J. P. Chen et al., Selective Debenzylation in the Presence of Aromatic Chlorine on Pd/C Catalysts Effects of Catalyst Types and Reaction Kinetics , paper presented at 20th Organic Reactions Catalysis Society Meeting, March 21-25, 2004, Hilton Head Island, SC, USA. 

Selective Debenzylation in the Presence of Aromatic Chlorine on Pd/C Catalysts Effects of Catalyst Types and Reaction Kinetics... 

In another article, however, [59] it was shown that in the most case this reaction gave mixtures of two heterocycles 53 and 54. To develop procedures allowing regioselective synthesis of both heterocyclic systems, the authors of [59] studied an influence of temperature regime and catalyst type on the direction of this MCR. With application of ultrasonication and microwave irradiation it was estabhshed that the reaction studied can pass under kinetic and thermodynamic control. 

Heats of reactions were estimated from heats of formations and chemical compositions of feed and product using standard procedures. For REY catalysts, we estimated approximately 130 Btu/lb heat of reaction. The heat of reaction was close to 200 Btu/lb for USY catalysts. These values are in close agreement with reported data (21)- The activation energies for different catalyst types were estimated from our extensive pilot plant data base, and found to be a weak function of catalyst type. The adsorption constants and other kinetic parameters used in these simulations were fitted to a large in-house data base. Typical parameter values are reported in Tables III and V. The kinetic parameters (k-, and Aj) are a strong function of catalyst used, whereas the adsorption parameters were found to be relatively insensitive. One could estimate these parameters even from a limited data base as illustrated below for Catalyst D. 

G. -M. Schwab History of Concepts in Catalysis. -J. Haber Crystallography of Catalyst Types. -G.Froment, L. Hasten Catalytic Kinetics Modelling. -A J. Lecloux Texture of Catalysts. - K Tanabe Solid Acid and Base Catalysts. 

The units of space velocity are the reciprocal of time. Usually, the hourly volumetric feed-gas flow rate is calculated at 60 °F (15.6 C) and 1.0 atm (1.01 bar). The volumetric liquid-feed flow rate is calculated at 60 F (15.6 °C). Space velocity depends on the design of the reactor, reactor inlet conditions, catalyst type and diameter, and fractional conversion. Walas [7] has tabulated space velocities for 102 reactions. For exanple, for the homogeneous conversion of benzene to toluene in the gas phase, the hoiuly-volumetric space velocity is 815 h . This means that 815 reactor volumes of benzene at standard conditions will be converted in one hoiu. Although space velocity has limited usefulness, it allows estimating the reaction volume rapidly at specified conditions. Other conditions require additional space velocities. A kinetic model is more useful than space velocities, allowing the calculation of the reaction volume at different operating conditions, but a model requires more time to develop, and frequently time is not available. 

Pure component studies indicate the rate of mercaptan formation is sufficiently rapid at hydrotreating conditions compared to the saturation step which lead to alkane [8]. The exothermic reversible reaction, which shifts to the left at higher hydrogen sulfide partial pressure, is also dependent on temperature, feedstock type, total sulfiir, partial pressure of hydrogen and alkenes, space velocity and catalyst type. Furthermore the size of the reactor affect the balance between the kinetic sulfur removal and alkene saturation [9]. 

Apart from taking into account these possible steric limitations, the chronic weakness of kinetic modelling in heterogeneous catalysis lies in the absence of a direct relation between the catalyst type (composition, structure, morphology, etc.) and its reactivity. In practice, the nature and structure of the catalyst are only involved through the values of kinetic constants. These values vary from one catalyst to another and, in principle, must be re-estimated whenever the catalyst is changed. 

The ratio of parallel to consecutive reaction rates is a function of the hydrocarbon structure, the catalyst type, and the reaction temperature. In studying the kinetics of catalytic benzene oxidation to maleic anhydride Hammar (107) came to the conclusion that benzene oxidizes by two independent routes the formation of maleic anhydride, and the complete combustion to carbon dioxide and water via unidentified intermediates. He suggested the following scheme for benzene oxidation over a vanadium catalyst ... 

The next task was to model the reformer itself to understand design issues and be able to predict performance of various reactor/catalyst types and transient behavior. However, upon trying to obtain kinetic rate expressions for the reforming reactions, it was found that very little information existed in the public domain. This led to the decision/need to develop reaction kinetics for catalytic partial oxidation and steam reforming at National Energy Technology Laboratory s (NETL s) onsite research facility. 

A quantitative kinetic model, denominated TC4, for the catalytic conversion of n-butane is proposed. The model considers 56 elementary reactions, six of them were chosen to occur in heterogeneous phase. The TC4 model can be used to predict the product distribution and the heterogeneous rate constants for a wide range of conditions and on different catalyst types. The model can fit also the experimental data from the isobutane dehydrogenation reaction. A plot, that we have denominated "the graphic s performance of a catalyst", is proposed for the evaluation of the maximum yield of a catalyst with a minimum of experimental data. 

Most reactions that have been investigated using PTC in supercritical fluids have been solid-SCF systems, not liquid-SCF. The first published example of PTC in an SCF is the displacement reaction of benzyl chloride 1 with potassium bromide in supercritical carbon dioxide (SCCO2) with 5 mol % acetone, in the presence of tetraheptylammonium bromide (THAB) [19-20] (Scheme 4.10-1) to yield benzyl bromide 2. The effects on reaction rate of traditional PTC parameters, such as agitation, catalyst type, temperature, pressure, and catalyst concentration were investigated. The experimental technique is described below. PTC appeared to occur between an SCF phase and a solid salt phase, and in the absence of a catalyst the reaction did not occur. With an excess of inorganic salt, the reaction was shown to follow pseudo-first order kinetics. 

This chapter describes the coordination polymerization of acyclic and cyclic vinylic monomers, conjugated dienes, and polar vinylic monomers with the most important catalytic systems known in this area. A chronological classitication for the development of the main coordination catalyst types is outlined, as well as polymerization kinetics and mechanisms and applications of polymers obtained through different metallic complexes. 

Kinetic Rate Constants The kinetic rate constants usually depend on temperature, as well as catalyst type and concentration [48]. In literature [49, 50], it has been proposed that the esterification and polycondensation reactions are acid catalyzed and that the corresponding rate constants can be expressed as... 

Simulation Model Results Initially, the assumption was tested that succinic acid can act as its own catalyst in the esterification reaction. In Figure 4.6, the experimental results on the esterification of PPSu are compared to the theoretical model predictions using kinetic rate constant that are either acid catalyzed (dashed and dotted lines) or not (solid line). As can be seen, the simulation of the experimental data by the theoretical model is very good when the kinetic rate constants used are not acid catalyzed. However, when the kinetic rate constants are assumed to be acid catalyzed, using Equations 4.30 and 4.31, the experimental data are not predicted equally well. Using values to accurately predict the initial rate data, the final data are underestimated. In contrast, when such values are used to predict the final experimental data, the initial data are overestimated. Thus, it was concluded that in the synthesis of the poly(alkylene succinates) studied here, the presence of the metal catalyst tetrabutoxy titanium (TBT) leads to a poor activity of self-catalyzed acid. This was also observed for PBSu by Park et al. [42]. Therefore, Equations 4.30 and 4.31 were not used and only parameters and Arg need to be estimated. The values of these parameters were calculated for every different system studied from fitting to the experimental data. The final values are reported in Table 4.2. Notice that these values are correct only for the specific catalyst type. 

Marafi A, Fukase S, Al-Marri M, Stanislaus A. A comparative study of the effect of catalyst type on hydrotreating kinetics of Kuwaiti atmospheric residue. Energy Fuels 2003 17 661-668. 

The reduction of Oj to HjO (Equation 3.5) is the desired process in a Hj/Oj PEMFC and produces twice as many electrons as the reduction to hydrogen peroxide (Equation 3.6). The H2O2 can be further reduced to HjO depending on the catalyst type and its kinetics, as shown in Equation 3.7. 

---