M. KULKARNI and D. GOSWAMI, BASF, Mumbai, India
In chemical and process plants, reboilers play a crucial role in enhancing the efficiency of chemical reactions occurring within process columns by utilizing steam to heat the process fluid. This article presents an integrated study of column reboiler systems, focusing on the significance of minimizing pressure drops through optimized piping and fittings that facilitate thermosyphon effects. The structural configuration of reboilers, which are supported by frames connected to columns via steel structures, ensures that the entire weight of the exchanger is effectively transferred to the column.
This article builds upon previous work by illustrating a specific case study of a column reboiler system, contrasting traditional analysis methods with an advanced approach. The traditional methodology employs a piping flexibility analysis tool to assess local loads at the shell-nozzle junction and flange connections, utilizing an approach specified in WRC-107/537 for evaluation. In comparison, an integrated finite element model (FEM) using 3D shell elements, with a commercial general purpose finite element analysis (FEA) toola, has been developed to analyze the same system, enabling a thorough assessment of stresses and displacements. The results of the FEA are compared with those obtained from the traditional method using 1D beam (pipe) element using a customized commercial software toolb, leading to recommendations aimed at optimizing system design and reducing costs associated with springs, additional flexibility using bends and loops, and additional supports.
This article underscores the importance of integrated analytical approaches to ensure the structural integrity of the entire column reboiler system in chemical and processing industries. This article will be useful for future analysis and avoiding additional supports and accessories by considering the flexibility of the entire system using 3D elements.
Scope. In chemical and processing plants, chemical reactions taking place in the processing column and recirculation for efficiency improvement are done using vertically or horizontally placed heat exchangers (reboilers), where process fluid is heated using steam. For the thermosyphon effect, a minimal pressure drop is achieved by optimizing piping and fittings between the column and reboiler connection. Reboilers are placed on a supporting frame that is connected to the column by beams and bracings. Therefore, all the weight of the exchanger is transferred to the column through the frame.
In this article, a column reboiler system is studied with respect to a traditional approach—i.e., analyzing the system using a piping flexibility analysis tool. The same example is analyzed utilizing an integrated FEM with 3D-shell elements and stresses, and displacements are compared with the traditional approach. Finally, recommendations are made, along with the pathway for saving the pipe looping and supports structure.
ILLUSTRATIVE EXAMPLE
Input information. For this example, a reboiler is supported on the column with a steel frame. Dimensions are detailed in FIG. 1.
Conventional approach: 1D analysis. The 1D beam (pipe) element analysis was completed by using a customized commercial software toolb in accordance with general industrial practices, and its evaluation as per process piping standard ASME B31.3. This approach facilitates an assessment of the system's response to dead load and thermal expansion.
FIG. 1 illustrates the overall dimensions of the system and the temperature profile used for the analysis.
FIG. 2 shows the deformation plot for the thermal load case, while FIG. 3 provides node numbers. Stresses are due to thermal expansion. Calculations from the software toolb are tabulated in TABLE 1.
FEA using 3D elements: FEA modeling. A detailed 3D FEM was prepared using a general-purpose FEM toola with a structural shell element. The thickness of each element was provided by assigning section properties of the elements, along with material properties. Individual tubes are not modeled for simplification reasons, but the effect of the same is captured by adjusting the elastic modulus of the heat exchanger’s shell. The total weight of the exchanger is managed by adjusting the density of the material. Modeling of the interconnecting piping and equipment flanges was ignored for modeling simplicity. The column was modeled beyond a length equal to 5 (where R is the radius, and T is the thickness of the column) from the extreme end of the reinforcement pad of the top nozzle, so that the effect of discontinuity will not affect the boundary conditions.
A simple static elastic analysis was performed, with the following load cases considered:
Temperature difference (thermal expansion)
Internal pressure
Gravity load.
FIGS. 4 and 5 illustrate the FEA model for analysis, including dimensions.
Load and boundary conditions. All degrees of freedom (translational and rotational) at the base of the skirt are fixed in all directions. FIGS. 6 and 7 illustrate the temperature load and pressure (0.6 MPa) + gravity load, respectively.
Results. As this article illustrates pipe stresses, only surfaces are compared (primary + secondary stresses). All stresses in plots are in megapascals (MPa). FIGS. 8 and 9 illustrate stress due to temperature load. FIGS. 10 and 11 illustrate stress due to internal pressure and gravity load. FIGS. 12 and 13 illustrate stress due to combined load. FIG. 14 illustrates a displacement plot due to combined load.
TABLE 2 shows the stresses at the shell’s nozzle junctions at the inlet/outlet of the channel and column.
Discussion. The resulting stresses are compared in FIG. 15.
As 1D analysis is based on 1D elements and the stiffness of the vessel and nozzle are ignored, it is always recommended to use 3D FEA to compute stresses in the column reboiler system.
Takeaways. Stresses in the reboiler and supporting column were calculated using a conventional 1D tool and 3D FEA with the same load and boundary conditions. The resulting stresses in both approaches were compared. It was found that 3D analysis provides a minimum of two times less thermal stress, as it captures the stiffnesses of the system in all directions. Note: the stresses calculated for temperature load are very high when a straight pipe connection is included without a bend, as there is no flexibility in the piping. Therefore, it is recommended to use 3D stress analysis for column-reboiler integrated system analysis. The resulting nozzle local loads are very high and generally exceed allowable limits. By performing an integrated FEA, nozzle local load analysis calculation can be eliminated.
This analysis approach can also be used to study the stresses in other integrated systems, such as a fluid catalytic cracking unit’s reactor-regenerator system, where the equipment are connected to each other directly with pipes. HP
NOTES
ANSYS
CAESAR II
REFERENCES
Chowdhary, M. G., P. Darji and R. K. Sharma, "Improve vertical reboiler piping analysis," Hydrocarbon Processing, February 2007.
ASME, “ASME Section VIII, Division 2, Part-5: Design by analysis,” ASME 2025.
ASME, “ASME B31.3: Process Piping,” ASME, 2012.
Wichman, K. R., A. G. Hopper and J. L. Mershon, “Precision equations and enhanced diagrams for local stresses in spherical and cylindrical shells due to external loadings for implementation of WRC Bulletin 107,” Welding Research Council, 2022.
Kulkarni, M., “Integrated column-reboiler system stress and flexibility analysis approach,” Hydrocarbon Processing, May 2024.