Six Langmuir-Hinshelwood-Hougen-Watson models have been derived for the kinetics of conversion of carbon monoxide to hydrocarbons in the Fischer-Tropsch synthesis. The models were fitted to experimental data obtained in an internal recycle reactor over a wide range of operating conditions. Two models, one based on the hydrogenation of surface carbon and the other on a hydrogen-assisted dissociation of carbon monoxide as rate limiting steps were both able to provide a satisfactory fit to the experimental rate data.A general model was also developed for the rate of methanation in the presence of higher hydrocarbons. The same two rate limiting assumptions as those used in formulating the rate of total CO conversion are used in these models. The two models were fitted to experimental data for methane formation.It was the model assuming CH formation as rate limiting that showed the best fit for both CO conversion for CH, formation.Six modkles Langmuir-Hinshelwood-Hougen-Watson ont CtC calculCs pour la cinetique de conversion de l'oxyde de carbone en hydrocarbures dans le procCdC Fischer-Tropsch. Ces modeles ont CtC adapt& 2 des donnCes exPCrimentales obtenues dans un rCacteur de recirculation interne sur une large gamme de conditions operatoires. Deux modkles, I'un base sur I'hydrogenation du carbone de surface, I'autre sur une dissociation par I'hydrogkne de I'oxyde de carbone servant pour la limitation des vitesses, ont permis de caler de manikre satisfaisante les donnCes de vitesse exptkimentales.Un mo&le gCnCral a Cgalement CtC dCveloppC pour la vitesse de mkthanation en prCsence d'hydrocarbures supkrieurs. On a utilisC dans ces deux modkles les m&mes hypotheses de limitation des vitesses qui ont servi dans la formulation de la vitesse de la conversion totale de CO. Ces deux modkles ont CtC adaptCs aux donnCes expirimentales pour la formation de mfthane.C'est le modkle supposant la formation de CH comme limitation des vitesses qui correspond le mieux B la conversion de CO et h la formation de CH,.
The catalytic cracking and skeletal isomerization of n‐hexenes on 60/80 mesh ZSM‐5 zeolite were studied in the temperature range 350–405°C. By applying the time‐on‐stream theory, the products of the reaction were identified as primary, secondary or both according to their optimum performance envelope (OPE) curves on product selectivity plots.
The products of cracking were found to be almost exclusively mono‐olefins and those in the range C3‐C5 were found to be stable primary, or primary plus secondary products. No C1 was found, and only traces of C2 as ethylene. The observed product distributions can be explained by a dimerization‐cracking mechanism with no product species having more than twelve carbon atoms. The probability of a fragment undergoing further cracking before desorption increased with temperature and the observed initial selectivities must be corrected to account for this process.
Methylpentenes, formed as unstable primary products, were the main isomers produced by skeletal rearrangement, with those derived from more stable carbenium ions predominating.
Paraffins, coke and aromatics were found in small amounts only.
The catalytic cracking of n‐alkenes on ZSM‐5 zeolite at 405°C can occur both by a monomolecular mechanism and a bimolecular process. In the latter, cracking is preceeded by dimerization. We show that pentenes are cracked exclusively by the bimolecular process. The dominant cracking mechanism for n‐hexenes also requires initial dimerization, although a small proportion (<19%) of the total cracking may proceed by a monomolecular process. Cracking of n‐heptenes is predominantly monomolecular, with only 13% of the total occurring via an initial dimer formation. The cracking of n‐octenes and n‐nonenes can be interpreted by assuming a monomolecular mechanism only. Thus it appears that at 405°C olefins smaller than C6 are stable with respect to direct cracking and must dimerize before a species is formed which is unstable enough to crack.
No molecular hydrogen was produced in any of the cracking reactions reported here in the range of conversions studied.
Two models describing the distribution of linear and monomethyl alkanes in Fischer‐Tropsch products from a cobalt catalyst are formulated. Distribution parameters for monomethyl isomers are derived by assuming either a post‐branching effect on linear chain growth or a decreasing reactivity toward branching with increasing chain length. Distribution parameters for each of the models have been extracted from an extensive experimental data set. We find that the experimental results are well represented by both the models.
A “break” in the distribution of linear paraffins has also been observed and is modelled as a sum of the yields of two Anderson‐Schulz‐Flory distributions. We postulate that the two chain growth processes which produce the distributions are governed by two kinds of termination.
A theoretical treatment of the decay of activity in aging catalysts is presented. In this treatment it is assumed that catalyst activity is a function of time‐on‐stream only and on the basis of this assumption the behavior of static bed catalytic reactors is studied. From these considerations it is deduced that there are three distinguishable types of behavior in decaying catalysts. These types are described in terms of their characteristic behavior and it is shown how the performance of certain rapidly decaying catalysts can be optimized in plant operation.
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