551433 Light Alkene Oligomerization for Shale Gas Upgrading: A Microkinetic Model

Monday, June 3, 2019: 4:48 PM
Texas Ballroom A (Grand Hyatt San Antonio)
Sergio Vernuccio1, Elsa Koninckx2, Elizabeth E. Bickel3, Han-Ting Tseng4, Rajamani Gounder3, Fabio Ribeiro5 and Linda J. Broadbelt1, (1)Chemical and Biological Engineering, Northwestern University, Evanston, IL, (2)Department of Chemical Engineering, Northwestern University, Evanston, IL, (3)Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN, (4)Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN, (5)Purdue University, W LAFAYETTE, IN

Introduction

In the last decade, large scale upgrading of shale gas has been considered a major issue in the US energy industry and has, for this reason, become a very popular research area. Unlike natural gas, shale gas contains higher concentrations of hydrocarbons other than methane, such as ethane and propane. Conversion of these light gases to liquid products would open up new opportunities for their use as valuable chemicals or fuels (Ka et al., 2009). Dehydrogenation of these hydrocarbons is a common process leading to light olefins that can be, in a second process, oligomerized over proton-exchanged zeolites.

The present work aims at developing and validating a microkinetic model to simulate the oligomerization process of light olefins (ethene and propene) over a proton-exchanged zeolite H-ZSM-5. Despite the industrial relevance of these processes, a detailed characterization of the kinetic parameters governing the process has not been proposed in the literature. All of the preceding results are related to “pathways-level models”, consisting of the lumping of several reactions in a single one describing the conversion of a reagent into a product and disregarding any reaction intermediates. Thus, due to the rough lumping by carbon number, these models are not detailed enough to predict the product distribution and have not been demonstrated to predict the evolution of the system under a wide range of operating conditions.

Methods

The oligomerization of light alkenes over acidic zeolites is characterized by an extremely complex product distribution due to the large diversity of ionic species formed as reaction intermediates during the process. However, this large reaction network can be condensed into a limited number of reactions occurring with similar chemistry (reaction families). The acid-catalysed oligomerization of an olefin can be fully described using five main reaction families (protonation, deprotonation, oligomerization, β-scission and hydride transfer) and several isomerization steps. The species present in the reacting system were represented using bond and electron (BE) matrices based on graph theory. A unique mathematical operator was specified for each reaction family (Broadbelt et al., 1994) and the reaction mechanism was automatically created by applying these operators to the reactants and their progeny.

The kinetic constants governing the process were assumed to obey an Arrhenius temperature dependence. The frequency factors were estimated directly from transition state theory knowing the entropy changes between the reactants and the corresponding transition states. These parameters were calculated for the ionic species characterized in the literature as alkoxides or carbenium ions. The activation energies were related to the heats of reaction of each elementary step through the Evans-Polanyí relationship. Group additivity approaches were used to express the heats of reactions based on the heats of formation of the reacting species (Bjorkman et al., 2014).

Results

The reaction network of olefin oligomerization is initiated via formation of an ionic species by protonation of the double bond of the reactant. The reverse reaction, deprotonation, is a termination step that desorbs the alkenes from the surface of the catalyst returning a proton to the zeolite surface. Oligomerization proceeds through addition of an alkene to an ionic species and consequent formation of a new sigma bond. β-scission breaks the bond between the carbon atom in the β-position with respect to the carbocationic center, representing the reverse step of oligomerization. The application of these reaction families was restricted to a limited number of reactants to generate molecular species with relatively low carbon number (≤ 12) to control the growth of the oligomerization mechanism (Susnow et al., 1997).

The resulting kinetic networks were coupled with the design equations of a PFR to build a continuum kinetic model. The proposed model was solved according to proper initial conditions to simulate reaction kinetics, product yields and selectivity. The proposed mathematical model, reliably validated, is able to predict the kinetic behaviour of the oligomerization system given its temperature, reactant pressure and reactant space velocity.

Conclusions

This paper presents a microkinetic analysis to describe the kinetics of light olefin oligomerization over acidic zeolites. The developed mathematical model represents a powerful tool to reproduce and predict conversion, selectivity and product distributions obtained from experimental operation under conditions relevant to industrial practice.

References

Ka, B., Pe K. (2009) Oil Gas J. 107, 50–55.

Labinger, J. A., Leitch, D. C., Bercawl, J. E., Deimund, M. A., Davis, M. E. (2015) Top. Catal. 58, 494–501.

Broadbelt, L. J., Stark, S. M., Klein, M. T. (1994) Ind. Eng. Chem. Res. 33, 790–799.

Bjorkman, K. R., Sung, C.-Y., Mondor, E., Cheng, J., C., Jan, D.-Y., Broadbelt, L. J., (2014) Ind. Eng. Chem. Res. 53, 19446-19452.

Susnow, R. G., Dean, A. M., Green, W. H., Peczak, P., Broadbelt, L. J., J. (1997) J. Phys. Chem. A 101, 3731–3740.


Extended Abstract: File Not Uploaded
See more of this Session: Light Hydrocarbon Reactions
See more of this Group/Topical: General Submissions