Kinetic Model for Formation of Aromatics in the High Temperature Chlorination of Methane1

A preliminary account of this work was presented at the symposium on Natural Gas Upgrading 11, Division of petrolium Chemistry, American Chemical Society, San Francisco Meeting, April 5-10, 1992.

ABSTRACT

A detailed thermochemical kinetic model consisting of 165 reversible reactions has been developed to describe catalyzed polymerization of methane under adiabatic conditions. The model, an extension of that previously proposed (Weissman and Benson, 1984, 1989), was tested against available experimental data for CH₃C1 pyrolysis in the absence and presence of CH₄ in the temperature range 1260-1310 K (Weissman and Benson, 1984). Predictions of major product yields are reasonable although predictive inadequacies are discussed. Product distributions in the CH₄/CI2 reaction were studied as a function of inlet/mixing temperature (T_o = 750-900 K), reactor pressure (P_R = 1.0 —5.0 atmospheres) and CH₄:C1, mole ratio (1.0-2.0). Product distribution is shown to be a strong function of inlet temperature and reactanl mole ratio. Choice of these parameters define the “window” of accessible reaction conditions experimentally available for useful product formation. Commercially attractive C₂ yields (C₂ H ₂, C₂H₄), with minimal soot formation (as identified by the yields of precursor molecules, styrene and naphthalene), are predicted under "ideal" conditions. On the basis of the proposed model, the simulations indicate in situ mixing of preheated CH₄and CI₂ feeds is required. The latter must be achieved on millisecond time-scales in order to provide a homogeneous mixture at the onset of reaction. Only limited homogeneous formation of high molecular weight products is predicted at the residence times and temperatures considered, suggesting that the significant yields of soot observed experimentally may be a consequence of heterogeneous and/or mixing effect contributions. The effects of the latter experimental “perturbations” on homogeneous product formation, neglected in the simulations which were conducted under “ideal” conditions, are considered in detail.

Key Words:

• Methane
• chlorination
• soot formation
• kinetic model
• aromatics formation
• production of C₂ hydrocarbons
• high temperatures

Notes

A preliminary account of this work was presented at the symposium on Natural Gas Upgrading 11, Division of petrolium Chemistry, American Chemical Society, San Francisco Meeting, April 5-10, 1992.