(566j) Comprehensive 3D Modeling of Steam Cracking Furnaces: Influence of Flue Gas Radiative Properties, Burner Geometry and Shadow Effects

Yu Zhang, Carl M. Schietekat, Kevin M. Van Geem, Feng Qian, Guy B. Marin.

Laboratory for Chemical Technology, Department of Chemical Engineering, Ghent University, Technologiepark 918, B-9052 Zwijnaarde, Belgium

Large fuel-fired furnaces have many applications in refining and (petro-)chemistry. In petrochemistry, one of the most important applications is in the hot section of steam cracking units for the production of olefins and aromatics. Multiple tubular reactors are suspended in these furnaces. The process is very energy-intensive as most reactions are highly endothermic, consuming about 8% of the total chemical industries’ energy use ¹. In order to improve the process’ energy efficiency and the olefin selectivity, a lot of efforts have been made towards the rigorous simulation of steam cracking units. To this end detailed reaction networks describing the gas-phase radical chemistry ² and coking models ³ have been implemented in one-dimensional, two-dimensional and three-dimensional reactor models^4-7. To obtain accurate run length predictions with these reactor simulations a well-defined heat flux profile to the reactors is required. The latter can only be obtained by detailed furnace simulations with models describing combustion and radiative heat transfer.

In recent years, significant progress has been made in the modeling of steam cracking furnaces using computational fluid dynamics (CFD). However, furnace modeling can still be improved in three aspects. First, the flue gas radiative properties have been treated as gray in most of the works, which can lead to temperature underpredictions of more than 100 K ⁸. Second, the detailed geometry of the burners is often omitted in industrial furnace simulations, although it is accounted for during burner design. Hence, the effect of the burner geometry details on the heat flux to the reactors has never been assessed. Last, the so-called “shadow effect” ⁹arising from the projected shadows between adjacent reactors always leads to significant circumferential heat flux non-uniformities. This phenomenon is important but is rarely considered in furnace simulations as often only a single reactor is simulated.

This work solves all three aforementioned issues in the simulation. As demonstration case an industrial naphtha cracking furnace equipped with long-flame floor burners has been modeled. A “nine-band model” is developed from the exponential wide band model (EWBM) ¹⁰ and is validated against Leckner’s correlation ¹¹for the simulation of a small test furnace. It is then used as a non-gray gas radiative property model and compared to the gray gas implementation of the Weighted Sum of Gray Gas Model (WSGGM) in the commercial software FLUENT 14.0. Two furnace configurations were built to investigate the effect of modeling the burner details on the furnace and reactor simulations. In the first configuration, all the geometrical details such as stages and tips are modeled, while in the second configuration the burners are modeled as a rectangular inlet in the furnace floor. Three cases are studied to study the gas properties and the burner geometry; the gray detailed, non-gray detailed and non-gray simplified case. Shadow effects are evaluated in all three cases as all the reactors in the furnace are simulated separately.

Results show that the gray gas simulation overpredicts the flue gas temperature by about 70 K compared to the non-gray gas simulation, resulting in a 3.6% higher thermal efficiency and 44 K higher average Coil Outlet Temperature (COT). The thermal efficiency obtained in the non-gray gas simulation matches closely the industrially reported efficiency of 45.5%.  For the simulation with simplified burners, longer flames are simulated and more uniform heat flux profiles to the reactors are obtained. Hence, the tube metal temperature is underpredicted by about 20 K compared to the detailed burner simulation. Finally, shadow effects in the furnace cause strong differences in the total heat flux per reactor. The maximum COT difference between two reactors is about 30 K. This information can be utilized to distribute the flow rate of the naphtha feedstock to the reactors or the fuel to the burners to reduce these non-uniformities.

Acknowledgment

The computational work was carried out using the STEVIN Supercomputer Infrastructure at Ghent University, funded by Ghent University, the Flemish Supercomputer Center (VSC), the Hercules Foundation and the Flemish Government – department EWI. The authors also acknowledge the financial support from the China Scholarship Council (CSC).

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