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Hydrocracking of long chain linear paraffins
Full text linkThe hydrocracking reactivity of two model compounds, namely n-C(16)H(34) (n-C16) and n-C(28)H(58) (n-C28), was investigated on a Pt/SiO(2)-Al(2)O(3) catalyst. Conversion and products distribution have been determined under a wide range of operating conditions (i.e. pressure: 20-80 bar; temperature: 270-330 degrees C; weight hourly space velocity: 0.33-1.0 h(-1): H(2)/n-paraffin feeding ratio 0.05-0.15 wt/wt). The latter were changed according to a central composite design. The present paper summarises the results obtained on both the model paraffins, depending on the reaction conditions. A first, simple kinetic elaboration is also presented, based on an ideal PFR model and a first order kinetics. The reaction confirmed to be first order with respect to the n-paraffin. Experimental data showed that for both n-C16 and n-C28 conversion was affected by H(2)/n-paraffin ratio. The change of conversion was explained in terms of vapour liquid equilibrium (VLE), which in turn is affected by the H(2)/n-paraffin ratio, so leading to a different vaporisation degree of reactant. In agreement with the VLE data, the effect of H(2)/n-paraffin on conversion was lower for n-C28. VLE calculations have been carried out to estimate the H(2) partial pressure and degree of vaporisation of the normal paraffin. The reaction order for hydrogen was -1 and -0.5 for n-C16 an n-C28, respectively. However, in the case of n-C16 the data obtained at the lower bound of the pressure range examined displayed an increase of the reaction order. The apparent activation energy was calculated after correction of the contact time taking into account the liquid-vapour equilibrium: similar values have been estimated for n-C16 and n-C28, ca. 32 and 31 kcal/mol, respectively
Introduction of a Breakage Probability Function in the Hydrocracking Reactor Model
No full textThis paper shows how a breakage probability function for the C-C bonds, elaborated from experimental evidence reported in literature, is introduced in the reactor model for the hydrocracking of Fischer-Tropsch waxes. The results demonstrate a better response to the variation of the operating conditions (especially as concerns temperature) and show product distributions closer to the experimental ones than those predicted by the previous model [Pellegrini, L. A. et al. Chem. Eng. Sci. 2008, 63, 4285]. The agreement with the experimental data has also been enhanced introducing a dependence on temperature (in addition to the dependence on the number of carbon atoms) in the expressions for the Langmuir constants and giving the equilibrium constants for isomerization reactions a new function derived from a thermodynamic study
Modelling of hydrocracking with vapour-liquid equilibrium
No full textHydrocracking Fischer-Tropsch waxes is a catalytic process that occurs both in vapour phase and in liquid phase. In a previous model, worked out by the authors, only the presence of the vapour phase was considered. In this paper it is shown how to account for the vapour-liquid equilibrium (VLE). In particular, the method used to calculate the critical properties of heavy hydrocarbons and the computing procedure that allows one to introduce the VLE calculation into the reactor model are explained. The results show that by accounting for the VLE a better agreement between experimental data and model outputs can be achieved. This results from the improved ability of the model to take into account the effect of the H2/waxes ratio both on the conversion and on the product distribution
Thermodynamic characterization of heavy paraffin mixtures to employ the Soave-Readlich-Kwong equation of state
No full textVapour liquid equilibrium in the reactor for the hydroconversion of Fischer-Tropsch products
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Liquid fuels from Fischer-Tropsch wax hydrocracking: Isomer distribution
No full textConsidering the current need of low emission fuels for the automotive market and the need of renewable
fuels that will emerge in the very next future, Fischer-Tropsch (FT) based technologies should be
considered a valid option to accomplish both low emission and renewable fuel production targets. A
hydrocracking step is necessary for obtaining high quality fuels from FT wax. Isomerisation is an
important reaction that takes place during the hydroconversion process. The amount and the type of the
isomers in the produced fuels heavily influence both cold flow properties and cetane number. In this
paper the results of a detailed method of analysis which allows the distinction between mono-branched
and multi-branched isomers in fuels obtained from an FT wax hydrocracking process, are presented and
discussed. In particular the influence of the operating conditions and the wax conversion on the isomer
distribution is pointed out
Equilibrium Constants for Isomerization of n-Paraffins
No full textA correlation between lumped isomerization equilibrium constants (Keq) and the number of carbon atoms in
paraffinic chains has been developed. The aim of the work is to check the meaningfulness from a chemicophysical
point of view of previously estimated values of Keq in order to improve the performances of a
simulation model for the isomerization/hydrocracking of Fischer-Tropsch (F-T) waxes, a mixture made up
of normal paraffins covering a wide range of molecular weights. Owing to the lack of experimental data for
paraffins with more than ten carbon atoms, a procedure has been developed to determine equilibrium constants
extrapolating the thermodynamical data of low carbon number paraffins. Since in our case the hydrocracking
simulation model considers lumped classes of isomers (i.e., monobranched and multibranched), the equilibrium
data do not take into account single isomerization reactions but those from a n-paraffin to the lump of its
monobranched isomers and from the lump of monobranched isomers to the lump of multibranched ones. The
coherence of the estimated constants has been verified by comparison with the little data available in the
literature on lumped equilibrium constants. The analysis of equilibrium constants at different temperatures
has shown that isomerization becomes endothermic from a number of carbon atoms equal to about 13-14
onward; this result is also supported by enthalpy data
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