Coke deposition in the thermal cracking of ethane

Abstract

A novel experimental setup allowed both the kinetics of the thermal cracking of ethane and of the coke formation to be studied over a temperature range extending from 750 to 870°C. The overall kinetics of ethane disappearance are in excellent agreement with previously reported pilot results. Coking rates are initially high but rapidly decrease to reach an asymptotic value. The initial product distribution also differs from the asymptotic product distribution. A kinetic model for the coking is derived from the experiments and used in conjunction with a set of conservation equations for the simulation of industrial ethane cracking. The predicted run length, thickness of the coke layer and evolution of the tubeskin temperatures are in agreement with industrial Observations. SCOPEThermal cracking of hydrocarbons around 800°C is the most important process for the production of feedstocks for the petrochemical industry. Side reactions always lead to carcycle has to be interrupted tor decoking by burning off the coke. Little is known about the rate of formation of coke. Yet, this is required for a better planning of the production and for the coking during ethane cracking in a novel experimental set up and makes use of this information to industrial operation and predict the evolution of the process parameters and the coke layer with time.bonaceous material that deposits on the wall ofthe cracking coil more optimal operation. This Paper Presents a kinetic Study of thus reducing heat transfer, requiring critical tubeskin temPeratures, increasing the Pressure drop so that after 20-60 days, depending upon the severity of operation, the production CONCLUSIONS AND SIGNIFICANCESpecially designed equipment consisting of a completely mixed reactor in which a hollow cylinder is suspended at the arm of an electrobalance allows the kinetics of the main reaction(s) and of the coking to be determined. The reactor, with a volume of some 5 mL yields kinetic parameters for the main reactions in agreement with those obtained in a pilot plant. Accurate rates of coking a r e obtained which are used in the simulation of an existing industrial cracking unit. In this way, it becomes possible to predict the evolution of conversion, product distribution, pressures, and coke layer in the cracking coil. The predicted results are in line with industrial observations.

A DFT‐Based Investigation of Hydrogen Abstraction Reactions from Methylated Polycyclic Aromatic Hydrocarbons

“…Initially, semiempirical models were used to predict coke formation during thermal cracking for a limited range of feedstocks. [14,15] A model based on elementary reactions was, however, proposed by Wauters and Marin and this should have a much wider applicability. [2,3] It is in any case essential to have reliable estimates of the reaction rates in the mechanism, as pointed out by Zador et al in a recent sensitivity analysis study.…”

Modelling coke formation in an industrial ethane‐cracking furnace for ethylene production

“…Coke formation is a complex phenomenon. The major mechanisms for coke formation are catalytic (also referred to as heterogeneous catalytic), pyrolytic (also referred to as heterogeneous non‐catalytic), and condensation (also referred to as homogeneous non‐catalytic). Catalytic coke is formed by a series of reactions catalyzed by nickel and iron present on the surface of the coils .…”

“…It is clear that the number of possible reactions that contribute to coke formation is extremely large . Since the number of species and reactions is very large, taking into account all of the possible coking reaction paths would lead to an unrealistically complex model (with an intractable number of kinetic parameters to estimate) for routine use in simulation of industrial cracking furnaces …”

“…Sundaram et al proposed a simple kinetic model for pyrolytic coke formation during ethane cracking. In this model, ethylene is identified as the main precursor molecule leading to the simplified reaction: …”

“…This reaction leads to the corresponding rate expression in Equation (9) in Table . Sundaram's estimates of the pre‐exponential factor and activation energy for k Pyro , obtained from laboratory experiments, are provided in Table . Several researchers have developed empirical rate expressions for pyrolytic coke formation; however, the proposed coking mechanisms in the literature are diverse and somewhat reactor‐dependent due to limitations of the corresponding laboratory‐scale reactors .…”