Modelling of the pyrolysis of tert-butylbenzene in supercritical water

The thermal decomposition of tert-butylbenzene (TBB) in supercritical water (SCW) was investigated. From experimental data of the overall rate and for mation of 30 products (more than 50 experiments; T=500-540 degrees C, p=5-2 5 MPa, five different environments including SCW) and assuming a free radic al mechanism, a high pressure reaction model was developed on the basis of a chemical mechanism. The model, which ultimately consists of 171 elementar y reactions (ER), and the chemical mechanism, which identifies the reaction paths for the formation of the main products, were evaluated in an interac tive process. For the calculation of the entire set of 342 coefficients of the model, an optimisation method was applied to solve the 'inverse problem ', using: (1) as initial values all available kinetic data at normal pressu re; (2) best estimates for the remaining coefficients, considering limits f or the different types of ER; (3) 'punishment functions', which force the p arameters to vary only within certain limits. Model simulations show reasonable agreement with experimental data. The mai n difference of the reaction in SCW and at low pressure in an inert environ ment is the strong inhibition of the overall reaction by a factor of 1000. Simulations indicate that mainly radical decomposition reactions are respon sible for this effect. It is assumed that a cage effect of water molecules reduces the reactivity of these species. Also, the pressure dependence of t he different types of ER is discussed. The product spectra of the reaction in SCW has a greater variety than at 0.1 MPa and below. Simulations show th at this is caused by a promotion of substitution reactions and the suppress ion of decomposition reactions at SCW conditions. Thus, the difference of t he decomposition of TBB in SCW regarding the product spectra to the reactio n at normal conditions is mainly a pressure effect. These results seem to b e of general significance for free radical reactions. They are in agreement with previous studies of the pyrolysis of ethyl-benzene in SCW as well as in inert conditions at atmospheric pressure. (C) 1999 Elsevier Science B.V. All rights reserved.