282168 Comparison of Gas Phase Mechanisms for the Prediction of Coke Deposition During Thermal Cracking of Light Hydrocarbons
Comparison of gas phase mechanisms for the prediction of coke deposition during thermal cracking of light hydrocarbons
Facultad de Minas, Universidad Nacional de Colombia - Sede Medellín
2 Instituto Colombiano del Petróleo, ICP
2 Chemical Engineering and Biotecnology departemt. Univeristy of Cambridge
Thermal cracking of light hydrocarbons is the main route for the production of important raw materials for the chemical industry, such as ethylene and propylene. The current and most used technology for olefin production involves injection of a mixture of hydrocarbons, preferably ethane, into a long tubular coil (around 80m long) located in a furnace with multiple burners that provide the required energy for the highly endothermical cracking reactions. Steam is added to the hydrocarbon mixture at a ratio (known as dilution factor) in order to control the reactive flow temperature and reduce the partial pressure of hydrocarbons, increasing the forward rate of reaction preferential to light olefins[1, 2] .
An undesirable effect during thermal cracking is coke deposition on the walls of the tubular reactor. Coke deposits build with reactor operation time and increase up to a point in which the reduction in heat transfer across the reactor's wall is so high that external skin coil temperature needs to be significantly increased to maintain a constant heat flux to the reactor. Clearly this decreases the thermal efficiency. This coke layer leads to a higher pressure drop over the reactor which is a very undesirable situation because it affects product selectivity. When the pressure drop along the reactor and the reduction on heat transfer across the reactor's wall are too high, the furnace operation is interrupted and a decoking operation is conducted in which the coke is burned off with a controlled air/steam mixture [3–5]. The operational time before decoking is of the order of 20 to 90 days, depending on process conditions and load.
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, 6]. This paper deals with the selection of a proper gaseous phase model that renders the information required to properly construct a coke deposition model.
In the refereed literature, the seminal work of Sundaram and Froment is recognized as one of the first studies on this area. These authors proposed using a global mechanism with 5 and 10 reactions to model the thermal cracking of ethane and ethane/propane mixtures respectively , . After this first approximation, they proposed a radical reaction scheme which has 110 reactions and describes the pyrolysis of ethane, propane, isobutene, ethylene, propylene and n-butane as well as their mixtures  .
Subsequent to Sundaram and Froment's work, Ranzi and collaborators developed a model which takes into account elemental reactions for the thermal cracking of light hydrocarbons such as ethane, propane and propylene as well as heavier feedstocks such as naphtha[8, 9]. The mechanism for light hydrocarbons takes into account 85 species and 1351 reactions and includes hydrocarbon fuels up to 3 C atoms. The work by Ranzi and collaborators finally lead to the SPYRO code, currently used in the hydrocarbon industry to predict thermal cracking. This code is currently licensed by Pyrotec, a divison of Technip .
Among the numerous kinetic mechanisms developed to describe the combustion and pyrolysis of hydrocarbons, it is important to analyze those which are relevant to pyrolysis of light hydrocarbons, particularly those developed by and Frenklach et al [11, 12] and Wang et al. [13, 14].
The kinetic mechanism of Frenklach et al. (ABF mechanism) consists of 99 chemical species and 531reactions [11, 12] . It includes the pyrolysis and oxidation of C1 and C2 species. One of the most important things of this mechanism is that it goes until de formation of pyrene as the higher-weight aromatic compound. This is a good characteristic because pyrene, as benzene, has been widely used to start the mechanism to describe solid phase formation of soot and coke.
The kinetic mechanism described by Wang et al. (USC mechanism) has been changed during the last decade in more than four occasions. The last published mechanism takes into account the combustion of H2/CO/C1-C4 and the description of pyrolysis and combustion at high temperature of normal alkenes up to n-dodecane. It has as the most complex aromatic compound benzene [13, 14] .
This paper addresses the differences between all these mechanisms under the light of developing a model to predict coke deposition as final goal. The predictions with the mechanisms by Sundaram and Froment (global [3, 4] and detailed  ), Ranzi et al. [8, 10], Frenklach et al. [11, 12]and Wang et al. [13, 14] were compared with measurements from an ethane cracker operator.
While the global and detailed mechanisms by Sundaram and Froment give good prediction of the high-concentration species, they yield a poor prediction of the low-concentration species and do not consider key species important to model coke deposition. The ABF mechanism correctly predicts species concentrations but the highest-molecular weight species it considers is ethane. In the case of Ranzi's and USC, both mechanisms consider propane pyrolysis and correctly predict major and minor species. However, predictions with the USC mechanism are in better agreement with industrial data and give more insight into some precursors for coke formation. Table 1 shows the comparison in the prediction of the gaseous species as a ratio between model predictions and industrial data.
The results suggest that from the mechanisms available in the open literature, the USC is the one that will be more suitable to be coupled with a solid-phase model for the prediction of coke deposition during the pyrolysis of light hydrocarbons.
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