(271f) Formation of Carbonaceous Deposits During Thermal Cracking of Light Hydrocarbons

Formation
of carbonaceous deposits during thermal cracking of light hydrocarbons

A.Y. Ramírez¹, A.Molina ¹,
M.Kraft², J.C. Vivas³

¹ Universidad Nacional de
Colombia ? Sede Medellín, Facultad de Minas, Bioprocesos y Flujos reactivos,
Medellín, Colombia

² University of Cambridge, Department of
Chemical Engineering, Computational Modelling Group, Cambridge, England

³Instituto Colombiano del Petróleo,
Piedecuesta, Colombia

A
model that predicts coke deposition during steam cracking of light hydrocarbons
was developed. Steam cracking of light hydrocarbons, such as ethane and propane,
is the main route for the production of ethylene and propylene. Associated with
cracking, normally conducted at high temperatures in a tubular reactor located
in a furnace that supplies heat, there is undesirable coke deposition on
interior walls of the reactor. This deposit builds up to a point in which
pressure drop and heat transfer reduction are so significant that maintenance
stops are mandatory ^[1].

Simulation,
by a reliable model, of coke deposition in the thermal cracking furnace for
different inlet conditions is necessary if one wants to understand the effects
that changes in process conditions and raw materials have on process
performance. A typical model for coke deposition includes two independent
submodels: one that considers cracking of steam/hydrocarbons mixtures in
gaseous phase and a second one that predicts coke deposition. Both models have
to be integrated in order to simulate olefins production and the reduction in
the diameter during thermal cracking of light hydrocarbons ^[1,2].

At
last year AIChE Annual Meeting we presented a gas-phase mechanism that predicts
the production of olefins and coke precursor's ^[3,4] during the
steam-cracking of light hydrocarbons.  This year we propose a model that
predicts coke deposition.

The
available models in the literature for the prediction of coke deposition during
thermal cracking of ethane or ethane/propane mixtures are either based on
empirical theories which have limited application to the experimental condition
in which they were developed ^[1,2] or do not present any kinetic
information ^[5]. However, some of them give information about the
process of coke formation. Ranzi et al.^[5] and Albright et al.^[6],
present a very good description of the mechanism of coke deposition. After
analyses of various coke deposits, these authors proposed two main mechanisms of coke formation: initial
catalytic growth prompted by metals present on the coil surface and growth
after the first polymeric layer is formed by radical contributions from the
homogeneous phase. The second mechanism starts when simple aromatic compounds
are deposited over the polymeric layer. Despite this interesting model insight
from Ranzi's and Allbright's groups, they do not provide specific kinetics for
modeling coke deposition. Without the kinetic information, it is
not possible to predict the rate of coke deposition in the walls of the reactor

Based
on the aforementioned understanding of coke formation during ethane pyrolysis
and carrying out an analogy from other mechanisms, such as the HACA^[7]
mechanism, that consider the formation of a solid substance, such a soot, from
a gas phase hydrocarbon, we propose a two-reaction mechanism that represents
coke formation. The first reaction considers the formation of an active A^*
aromatic intermediate by reaction of A_1, a dummy species that
represents the total aromatic concentration in the gaseous mixture, with
hydrogen. This active intermediate builds, by reaction with acetylene, the coke
layer. 

Although
it would be desirable that the model includes the phase of catalytic growth
prompted by metals in the coil surface, the period of catalytic coke formation
is very short when compared to the typical thermal cracking furnace operation
cycle. Therefore, for modeling purposes the catalytic growth phase could be
neglected without incurring in a significant error.

The coke
formation mechanism was calibrated based on data obtained from a pilot plant
which consist in a tubular reactor over a range extending from 400°C to 900°C
and with data from the literature about the coke deposition in pilot and
industrial plants.

To evaluate the
ability of the model to predict coke deposition, an industrial steam-cracker of
ethane and ethane/propane mixtures was simulated. The solid phase model
predicts, in agreement with industrial data, that a 700-hours period before
shutdown is required due to coke deposition during thermal cracking of ethane.
Figure 1 shows the reduction in the diameter because of coke layer formation
for 50, 200, 400 and 700 hours.

Figure 1. Coil diameter reduction
because of coke deposition during steam cracking of ethane

The model was
used to address the effect that changes in the C/H ratio, residence time and
dilution factor had on the rete of coke deposition. To increase the C/H ratio,
the proportion of propane in the mixture was increased from 0.2 to 0.8 kg
propane/kg hydrocarbons. The maximum reduction in coil diameter because of coke
production increased from 22% (without propane) to 34% (0.2 kg propane/kg
hydrocarbons) and 58% (0.8 kg propane/kg hydrocarbons) after 500 hours of
furnace operation. In the case of the residence time and dilution factor, the
model predicts typical trends observed for coke formation during thermal
cracking such as the increase in coke production as both parameters increase.

REFERENCES

[1] Sundaram KM, Froment GF.
Kinetics of coke deposition in the thermal cracking of propane. Chemical
Engineering Science. 1979;34(5):635-44.

[2] Sundaram KM, Froment
GF, Van Damme PS. Coke deposition in the thermal cracking of ethane. AIChE
Journal 1981;27(6):946-51.

[3] Ramirez
A, Almanza L, Molina A, Kraft M, Vivas J., "Comparison of gas phase
mechanisms for the prediction of coke deposition during thermal cracking of
light hydrocarbons". Oral presentation. AIChE annual meeting 2012.

[4] Ramirez
A, Almanza L, Molina A, Kraft M., A
model for the prediction of olefin production and coke deposition during
thermal cracking of light hydrocarbons. 2013:1-12"

[5] Ranzi E, Dente M, Pierucci S, Bussani G. New
improvements in modeling kinetic schemes for hydrocarbons pyrolysis reactors. Chemical
Engineering Science. 1992;47(9-11):2692-34.

[6] Albright LF, Crynes RL,
Corcoran WH. Pyrolysis of Ethane and Propane. In: Press A, editor. Pyrolysis:
Theory and Industrial Practice; 1983, p. 25-87

[7] Frenklach M.
On surface growth mechanism of soot particles. Symposium (International) on
combustion. 1996;26(2):2285-93