(28e) Modeling Catalyst Deactivation In Dehydrogenation of Ethylbenzene to Styrene | AIChE

(28e) Modeling Catalyst Deactivation In Dehydrogenation of Ethylbenzene to Styrene

Authors 

Baghalha, M. - Presenter, Sharif University of Technology
Babaei, J. - Presenter, Sharif University of Technology


Industrial dehydrogenation of ethylbenzene to styrene monomer is carried out using potassium-promoted iron oxide catalyst. Active sites in this catalyst are KFeO2. Many efforts have been made to identify the deactivation mechanisms of these active sites that influence the activity, selectivity, stability and lifetime of the catalyst. Coke deposition on the active catalyst sites is one major deactivation mechanism. The other deactivation mechanisms include migration, loss and redistribution of the potassium promoter, changes in the oxidation state of iron, and physical degradation of the catalyst structure. All of these deactivation mechanisms are related to each other and occur simultaneously. In this work, the data from a continuous styrene plant operation over 30 months is implemented to investigate the catalyst long-term deactivation. The plant data is related to an industrial unit with three adiabatic radial flow reactors in series. The 1st reactor only contains the dehydrogenation catalyst. In the 2nd and 3rd reactors, there are catalytic oxidation sections to facilitate reaction of hydrogen (that is produced in the previous reactor) with oxygen (that is intentionally added to the feed). Through this technique, hydrogen is consumed and pushes the dehydrogenation reaction forward to higher styrene yield. Substantial amount of heat is also generated to increase the stream temperature back to 600 °C levels. The exit streams from these oxidation sections enter the dehydrogenation sections to increase the ethylbenzene conversion.

A short-term catalyst deactivation during the unit start up occurs due to rapid coke formation. However, after a monolayer of coke forms, rate of coke formation decreases. In fact, the interaction between coke formation and gasification leads to a dynamic value of equilibrium coke content on the catalyst surface. Coke deposition is a reversible short-term deactivation process that reaches a pseudo steady state condition after the first few hours of operation. However, during the 30 months of plant operation, other deactivation mechanisms, such as potassium loss and ferric reduction to ferrous, further reduce the catalyst activity with time-on-stream. In practice, to compensate for the lower catalyst activity with time and to maintain a high ethylbenzene conversion level during the long-term plant operation, the inlet reactors temperatures are gradually increased. Higher temperatures, in turn, accelerate the catalyst degradation process.

For modeling the catalyst deactivation, two deactivation functions (one due to coke formation and the other due to potassium loss) were introduced in the reaction kinetics that consists of six chemical reactions. Using the plant data for 30 months of operation, 4 unknown parameters of the two deactivation functions were fitted. The obtained results show that deactivation due to coke formation decreases the catalyst activity to 50% of the fresh catalyst activity in the 1st month and to 40% in the 30th month of the plant operation. The effect of potassium loss on the catalyst activity was found to be much more pronounced. While the catalyst activity for the fresh catalyst with fully charged potassium was 100%, it reduced to less than 10% of the initial value at the end of operation (month 30). In conclusion, the long-term catalyst deactivation due to potassium loss was identified to be the dominant degradation mechanism that shortens the catalyst life. Hence, for design of new styrene catalysts with longer life, it is inevitable to find more stable potassium compounds with good activities for inclusion in the catalyst that can maintain high levels of activities at the reaction temperatures of 600 °C for longer periods of operation.

Checkout

This paper has an Extended Abstract file available; you must purchase the conference proceedings to access it.

Checkout

Do you already own this?

Pricing

Individuals

2011 Spring Meeting & 7th Global Congress on Process Safety
AIChE Pro Members $150.00
AIChE Graduate Student Members Free
AIChE Undergraduate Student Members Free
AIChE Explorer Members $225.00
Non-Members $225.00
Fuels and Petrochemicals Division only
AIChE Pro Members $100.00
Fuels and Petrochemicals Division Members Free
AIChE Graduate Student Members Free
AIChE Undergraduate Student Members Free
AIChE Explorer Members $150.00
Non-Members $150.00