(560eb) Isobutane Alkylation Kinetics with Mixed C4 Olefins Catalyzed By Sulfuric Acid

Ling Zhao, Weizhong Zheng, Piao Cao, and Weizhen Sun

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

The alkylate, has the advantages of high-octane number, low vapor pressure, and zero content of olefins and aromatics that allow it to become a desirable blending component for high-quality gasoline. It is well accepted that the isobutane alkylation reaction proceeds following the classic carbonium ion mechanism. Considerable efforts have been made to fully understanding the mechanism, providing the fundamental insight of the isobutane alkylation. However, due to simultaneously considerable reactions occurring in the alkylation system and the exist of more than three dozen isoparaffins which are hard to detect owing to the lack of the effective analysis method, few literature was devoted to the isobutane alkylation kinetics with sulfuric acid as catalyst to explain the formation pathway of several key components in alkylate, such as trimethylpentanes (TMPs), dimethylhexanes (DMHs), and heavy ends (HEs).

In this work the alkylation kinetics of isobutane with mixed C4 olefins using sulfuric acid as catalyst were investigated under conditions of industrial interest. The kinetic model was established on the basis of the carbonium ion mechanism with three key components in alkylate being measured, including TMPs, DMHs, and HEs. Meanwhile, the reaction pathways for polymerization of C4 olefins to produce HEs were modified and the isomerization reaction between butenes was fully considered in this model. Furthermore, the transition state and activation energy of the isomerization reaction between three C4 olefins were calculated using DFT method and further compared with the predicted results by the kinetic model. Using MD simulation, the diffusion coefficients of three C4 olefins were calculated to demonstrate their fast diffusion and self-assembly at interface before alkylation reaction.

According to the result of alkylation experiments carried out under the temperature range of 276.2-285.2 K, the concentration changes of three key components in alkylate, i.e. TMPs, DMHs, and HEs, were well predicted by the kinetic model with satisfactory agreement between experiments and model calculations. The rate constants of transformation rate of 1-butene into 2-butene and 2-butene into isobutene are two and one order of magnitudes larger than that of the corresponding reversible reaction, respectively. The reliability of the kinetic model was verified by the isobutane alkylation experiments with mixed C4 olefins with different compositions, confirming the good transferability of the model regardless of the species of butenes in the feed. Furthermore, MD simulations show that butenes can diffuse more easily than isobutane in the H₂SO₄, facilitating the fast polymerization reaction and the growth of heavy ends (HEs).