(207g) Accurate Molecular Reconstruction of Cracking Feeds Improves the Predictions of Ethylene Yields

Accurate
molecular reconstruction of cracking feeds improves the predictions of ethylene
yields

Eliseo
Ranzi^a, Sauro Pierucci^a, Mario Dente^a,

^aCMIC Department Politecnico di
Milano, Italy

Eliseo.Ranzi@polimi.it,
Sauro.Pierucci@polimi.it, Mario.Dente@polimi.it

Marco
van Goethem^b, Dirk van Meeuwen^b

^b Technip Stone & Webster Process Technology, Zoetermeer,
The Netherlands.

mvangoethem@technip.com, dvanmeeuwen@technip.com

Eric Wagner^c

^cTechnip Stone & Webster Process Technology, Inc., Claremont,
California, USA

ewagner@technip.com

Accurate characterization of
petrochemical feeds plays a crucial role in describing the different pyrolysis
pathways and reactivity of the various isomer species. The isomer distribution
is proved to have a critical role in characterizing product yields. Isomers
with almost identical physical properties, such as the isomers of branched
alkanes and cyclo-alkanes, can exhibit very different cracking behaviors, and
hence, a poor estimate of their internal distribution can significantly affect
the ethylene yield predictions. Thus, the horizontal lumping, i.e. the grouping
of species with the same molecular weight, becomes as critical as the vertical
one. Despite recent advancements of analytical techniques, a
satisfactory molecular description cannot be obtained simply from these methods
and the molecular reconstruction of the feedstocks needs to rely also on other bases.
This paper emphasizes the role of the probability of methylation and
alkylation inside the families of homologous hydrocarbons. For the first time,
the importance of the internal distribution in the molecular feedstock
reconstruction is highlighted. For these purposes, SPYRO^® kinetic
model has been revised and extended to more than 600 species, covering a large
detail of feed components, and it is now able to properly show and quantify the
effect of isomer distributions on cracking yields.

Since its
introduction in the 1990s, comprehensive two-dimensional gas chromatography has
demonstrated very promising perspectives for the analysis of complex hydrocarbon
mixtures. The main reason lies in the higher peak capacity obtained with the
combination of two chromatographic columns that develop complementary
selectivities so that the entire sample is analysed with two orthogonal
separations. Both reverse and normal column combination are useful to
characterize the hydrocarbon composition of liquid fuels. In general, six
hydrocarbon classes (n-paraffins, iso-paraffins, cyclo-paraffins, mono-, di-
and poly-aromatics) can be identified and quantified using flame ionization
detectors. While the boiling curve and GCxGC analysis give important
information on molecular weight distribution, the quantification of the
individual isomers remains more difficult. In the meantime, it is possible to
observe a relative regularity in the internal distribution of the isomers
inside the different fractions of virgin feedstocks. To give an example, the description
of the whole fraction of isomers of branched-alkanes with eight C-atoms needs
only a few isomers: ethyl-hexane, three mono-methyl-heptanes, and four
dimethyl-hexanes with a tertiary C structure. In spite of the different origins
of these feeds, there is clear regularity with regard to their composition. In
fact, methyl-heptanes prevail over dimethyl-hexanes and ethyl-hexanes.
Trimethyl-pentanes are less abundant and quaternary C atoms are of very limited
importance. On this basis, it was possible to empirically derive the internal
distribution of the isomer mixture, simply based on the probability of
methylation, and or different alkyl substitutions along the carbon chain. GC
analysis of heavy naphtha, kerosene and light gasoils indicates the prevailing
presence of poly-isoprane structures characterized by an average probability of
methyl substitution of about 0.20-0.30. Of course, these internal distributions
remain a rough approximation, they are a peculiarity of the virgin fractions and
can drastically change after thermal or catalytic refinery processes. Moreover,
it seems quite important to extrapolate and validate these assumptions and
approach to higher fractions.

Unfortunately, accurate
and detailed analysis of different isomers for branched alkanes and
cyclo-alkanes with more than 10 C-atoms are very scarce in the literature. A
carefully executed investigation to the selection of surrogates for jet fuels states
the average distribution of substituents in iso-paraffins, cyclo-paraffins and
aromatics, based on literature data complemented by NMR analysis. That investigation
indicates a higher methylation probability for the iso-paraffin compounds, as
well as for the aromatic fractions. From the NMR data of the investigation
constraints can be observed on the number of side chain substitution on
aromatic carbons, as well as on the length and branching of the substituents.
The suggested surrogate composition of aromatics with 10 C-atoms, all
substituents but one being methyl, was about 20% ethyl-toluene, 5%
propyl-toluene, 55% ethyl-xylene, and 20% propyl-xylene.

Panel a) of
Figure 1 shows the estimated internal distribution of branched alkanes C10 in
terms of the relative amounts of methyl-nonanes, di-methyl-octanes,
ethyl-octanes, and tri-methyl-heptanes as a function of the methylation degree.
There is a clear increase of poly-methylated compounds, with a decrease of the
mono substituted species. Note that these distributions refer to a fixed
probability of ethyl substitution of ~10%. Panel b) of the same figure shows the predicted cracking
yields, as obtained in conventional pyrolysis coils, at standard operating
conditions. As expected, the increase of the methylation degrees corresponds to
a net decrease of about 3% of ethylene yields with a parallel increase of
methane, propylene, and C5+ components. These significant differences in
ethylene yields justify the efforts of a better investigation of the effect of
the internal distribution of different isomers, not only inside the branched
paraffins, but also with respect to the cyclo-paraffins, where the internal
ratio between 5- and 6-membered rings constitutes a further parameter for the
correct internal isomer distribution.

Figure
1: Panel a) internal distribution of branched alkanes C10 vs the methylation
degree. Panel b) Effect of the internal distributions of branched alkanes C10
on the cracking yields at constant coil outlet temperature 

This study
confirms the importance of highlighting a limited number of intrinsic
parameters useful for the proper characterization of the internal distribution
of isomers, inside the different hydrocarbon classes. The probability of
methylation and different alkylation on the carbon chain, the average length of
side substitutions on the rings as well as the relative presence of
cyclo-hexane and and cyclo-pentane structures (as well as the one of tetraline
and indane), constitute a limited number of freedom degrees, mainly dependent
on the origin of the feed.

Due to the
current computer facilities, it is now convenient to enlarge the kinetic scheme
by including several individual isomers, still maintaining the empirical
distribution rules, when analytical and detailed data are not available. This
approach allows enlarging the range of applications of the kinetic model. For
instance, the decomposition of a particular structural isomer can be
individually investigated. Furthermore, the lumping rules inside the individual
isomeric fractions can be tailored in a very effective way, based on a limited
number of intrinsic characterization factors depending on the actual liquid
feed. Finally, further advancements in revised and combined GCxGC analysis are
expected to reveal in the near future more detailed chemical structures, which
could assist in a more comprehensive identification of these freedom degrees.
They are not simply adaptive or empirical factors, but intrinsic characterization
parameters, a molecular finger print of the different petroleum fractions.