Efficiency and safety are paramount in the operation of ethylene-cracking furnaces. These critical processes produce ethylene through thermal cracking of hydrocarbon feedstocks, such as ethane and propane. However, managing heat flux profiles in these furnaces is crucial to prevent issues like coking, tube overheating, and equipment damage. In this blog post, we will explore the significance of heat flux correlations and how they can be applied to various ethylene furnace configurations.
Ethylene Furnace Configurations
Ethylene furnaces come in three major configurations:
- Floor-Fired Only
- Wall-Fired Only
- Floor Plus Wall-Fired
These configurations vary in terms of burner placement and heat release mechanisms. To optimize their performance, engineers need to understand how heat flux correlates with operating conditions and design parameters.
Heat Flux Profile
The heat flux profile, which represents the distribution of radiant heat incident to a tube at different elevations, plays a crucial role in the operation of ethylene-cracking units. It is a critical factor for managing the thermal cracking process efficiently and safely.
The thermal cracking process is highly endothermic, requiring intense heat. This heat is supplied by specialized burners operating at extremely high temperatures. However, if heat is applied non-uniformly, side reactions can lead to the deposition of carbonaceous polymers on the tube walls, known as coke. This can result in local overheating and, if left unaddressed, tube rupture.
To achieve the optimal balance between conversion efficiency, run length, and equipment life, it’s essential to establish a suitable heat flux profile. This profile is a function of various parameters, including the elevation at which the maximum heat flux occurs (zmax) and the heat flux at the floor (y0).
Correlation Equations
To predict and control heat flux, equations and correlations have been developed for all major ethylene-cracking furnace configurations. Let’s delve into some key aspects:
- Floor-Fired Furnaces: Heat flux from floor burners, as it varies with elevation, is described by a differential equation. The analogy of jet theory helps establish this equation, which simplifies the correlation between heat flux and elevation.
- Wall-Fired Furnaces: Wall-fired furnaces are predominantly used with premixed burners, making heat flux calculations relatively straightforward. These calculations are based on the point source model and the inverse aspect ratio of the furnace.
- Floor Plus Wall-Fired Furnaces: In this case, the floor serves as the reference elevation for heat release. The correlation equation is adjusted accordingly.
Predicting Heat Flux
Knowing how to predict heat flux as a function of operating, burner and furnace factors is crucial. This predictive capability allows engineers to influence heat flux profiles early in the design process and make informed decisions about burner configurations and operating conditions.
We have correlated key parameters like heat flux at the floor (y0) and elevation of peak het flux (zmax) with operating, furnace and burner factors, resulting in semi-empirical models that predict their influence. This means that if you know the elevation of the maximum heat flux (zmax) and the initial heat flux (heat flux at the floor, y0) you may determine the entire curve. These models have been incorporated into our proprietary program which calculates not only heat flux but also emissions like NOx and CO, among other critical design and performance outputs.
Conclusion
In summary, understanding and predicting heat flux correlations in ethylene-cracking furnaces are essential for optimizing their performance. Equations and correlations have been developed for different furnace configurations, and these predictive models offer a valuable tool for furnace design, operation, and safety. By working with burner design engineers and utilizing these models, ethylene producers can ensure efficient and reliable operations while maintaining the highest safety standards.
