(768g) Effect of Coke Solubility on Product Yield on the Ethylene Oligomerization Under Supercritical Conditions

Supercritical fluids are substances above the critical temperature and pressure, which possess unique transport properties compared with liquids and gases. Previous publications [1]–[3] from the 50’s to the 80’s showed that supercritical conditions reduces the dielectric constant of substances, making them excellent solvents in systems involving high molecular weight hydrocarbons. For example, Diepen et al.[1] compared the effects of subcritical and supercritical ethylene on the solubility of naphthalene and showed an increase in naphthalene solubility under supercritical conditions relative to near-critical conditions. Furthermore, Prausnitz et al.[3] studied the solubility of long alkanes in gases and showed that methane is an excellent solvent at supercritical conditions. However, these solubility studies did not focus on the application of supercritical fluids as reactants and solvents in chemical reactions. Therefore, this talk will address the effect of supercritical fluid conditions and changes in solubility on chemical reactions. In a previous publication from our research group[4], we used a cell mounted with ZnSe windows to study the effects of supercritical fluids in reactive systems. We showed strong evidence that ethylene can dissolve coke molecules produced during the ethylene oligomerization over heterogeneous catalysts under supercritical conditions. We characterized the coke formed under supercritical conditions as a macromolecule formed by cyclic species connected via sp³ hybridized linear hydrocarbons. However, we still lack understanding of the mechanism behind the solubilization of coke molecules by supercritical ethylene and the effects of coke solubility on the product distribution. To address this issue, we developed a solubility and a kinetic study under subcritical and supercritical conditions. For the solubility study, we propose the use of model compounds to mimic the coke formed during the ethylene oligomerization over the Ni-H-Beta catalyst on the same reaction system used for the kinetic study under subcritical and supercritical conditions.

In this talk, we will report results with n-decane, n-tetradecane, ethylcyclohexane, dimethylcyclohexane, and cyclooctane as model compounds to simulate the coke formed during the ethylene oligomerization over the Ni-H-Beta catalyst under supercritical conditions. Additionally, we will report the use of naphthalene to mimic the coke formed over heterogeneous catalysts in other reaction systems, such as pyrolysis and petrochemical processes. We propose a flow system to study the solubility of these compounds under ethylene and nitrogen. To ensure that the system is under equilibrium we successfully replicated the solubility data produced by Diepen and Prausnitz.[1]–[3] We also show that the flow rates do not affect the equilibrium under the conditions used for the solubility experiment. The core of this work consists on the development of a thermodynamic model coupled with equations of state (EOS), such as Virial, Peng-Robinson, and Soave-Redlich-Kwong, which will be validated using the experimental results. Our results are in agreement with the literature and indicate that ethylene is able to dissolve more heavy hydrocarbons than nitrogen, especially under supercritical conditions. We used the experimental solubility data to calculate the binary interaction parameters for the gas-liquid mixtures studied in the present work. These results help to explain the effects of the solubility of coke by the supercritical ethylene on the kinetics for the ethylene oligomerization under supercritical conditions.

References

[1] G. A. M. Diepen and F. E. C. Scheffer, “The Solubility of Naphthalene in Supercritical Ethylene,” J. Am. Chem. Soc., vol. 70, no. 12, pp. 4085–4089, 1948.

[2] J. M. Prausnitz and P. R. Benson, “Solubility of liquids in compressed hydrogen, nitrogen, and carbon dioxide,” AIChE J., vol. 5, no. 2, 1959.

[3] S. G. D. Avila, B. K. Kaul, and J. M. Prausnitz, “Solubilities of Heavy Hydrocarbons in Compressed Methane and Nitrogen,” vol. 21, no. 4, pp. 488–491, 1976.

[4] G. V. S. Seufitelli and F. L. P. Resende, “Study of the catalytic reactions of ethylene oligomerization in subcritical and supercritical media over a NiBEA catalyst,” Appl. Catal. A Gen., vol. 576, pp. 96–107, 2019.