M. Kulkarni, BASF Chemicals India Pvt. Ltd., Mumbai, India
In oil refineries and chemical plants, chemical reactions and separations take place in the distillation column. The introduction of heat exchangers improves the efficiency of the system with recirculation of the column fluid. These heat exchangers vaporize a fraction of the bottom product of the column fluid. Reboilers are heat exchangers typically used to provide heat to the bottom of the industrial distillation column, boiling liquid and generating vapors that are returned to the column for distillate separation.
Reboiler steam provides the heat needed for vaporization. Vaporizers heat and vaporize a processing fluid by adding latent heat to the processing fluid. When the fluid flow takes place with a natural temperature gradient and pressure difference, the system is called a thermosyphon, which imposes limits on pipe length, pipe bends and fittings that increase the pressure drop in the flow and affect the phenomenon. Thermosyphon reboiler systems can be of either the recirculating type (shown in FIG. 1) or the once-through type.
Flexibility study of the system. During the design of the piping system around the column and the reboiler, equipment layout preparation occurs as per process requirements. Special care must be given to achieve the thermosyphon effect, considering its limitations. A general practice throughout industry is to verify the flexibility of the system. A flexibility analysis can be carried out using a customized tool based on a finite element analysis (FEA) approach using 1D elements (commercial applications for the specific use include CAESAR-II, AutoPIPE, RHO2, etc.).
The column-reboiler system design is compact and must be analyzed for flexibility due to thermal growth of the equipment. One major challenge in the system design is to limit the excessive resulting forces on equipment nozzles.1 A null-point concept and spring supports are generally used in the system design to bring the nozzle loads within specified limits. 1D elements consider global stiffness for the flexibility analysis of the system. Here, however, the integrated system is so compact that a global stiffness approach provides very high reactions at the interconnection nodes (which will place piping loads on the nozzles). To analyze this system, a general purpose FEA tool using shell or solid 3D elements is required.
Null-point concept. When the reboiler rests on an external supporting structure during piping flexibility analysis (FIG. 2), the supporting point is kept so the differential thermal growth is nullified.
Spring supports. Another way to manage thermal growth due to the differential expansion of the column and reboiler is by adding spring supports between the reboiler lug and the supporting structure (FIG. 3). Locking and unlocking the springs during startup, shutdown and during maintenance is challenging, and springs are susceptible to corrosion in moist atmospheres. Due to these factors, the use of spring supports is becoming increasingly rare.
Column-supported reboiler. External support structures require space and pose challenges in a compact plant layout. Generally, the reboiler is attached to a structural frame fixed on the column, as shown in FIG. 4. As the system is compact and piping flexibility is limited by the addition of fittings, the calculated nozzle loads in the piping system analysis are generally very high. Therefore, in this case, springs are introduced to reduce nozzle loads.
Load cases. The following load cases are considered:
The piping system was analyzed for the following critical load cases:
Piping flexibility analysis model. Conventionally, piping systems are analyzed using customized programs like CAESAR II, AutoPIPE, etc. In terms of FEA, these tools use a line model with 1D beam elements with section properties. The model used for the piping flexibility analysis is shown in FIG. 5.
Detailed FEA model. The same column reboiler arrangement is modeled in an FEA. When a detailed FEA model is prepared, the column, reboiler and supporting members are modeled in detail, as per geometry, using 3D shell or solid elements (FIG. 6).
Discussion. Piping flexibility analysis tools use beam or pipe elements that are 1D line elements and suitable for macro analysis. A local stress analysis is not correctly captured by these elements. Additionally, nozzle and vessel connections are point connections, which also adds to the conservativeness of the approach.
It is worth noting that the piping system analysis is based on the maximum shear stress theory, and the calculated stress intensity is evaluated with allowable stresses.
However, when the same system is considered as an integrated model with vessels and interconnecting pipes and analyzed using detailed FEA by modeling the equipment with all details, the connection between the nozzle and vessel is either a common line or a common area. The same connection in the piping model will be by a single point.
According to ASME Section VIII, Div. 2, Part 5, Table-1, “Secondary loading by considering thermal load is at operating temperature only and induced stresses are evaluated using the Von-Mises theory (i.e., equivalent stresses are evaluated against allowable stress value).” 2,3
Therefore, if the 1D element model of the piping's flexibility analysis is compared with the 3D detailed FEA model, the piping model is very conservative and difficult to qualify the nozzle loads.
It is recommended to apply a detailed 3D FEA model, which will result in optimal design and may avoid spring supports, which are commonly used to compensate for the thermal expansion of 1D pipe elements.
A detailed comparison of 1D elements and 3D elements for the column reboiler system analysis will be the future scope of Part 2 of this article.
Takeaways. Column reboiler process requirements and various supporting arrangements have been examined here. Commercial tools used for piping flexibility analysis have been compared with general purpose FEA tools, as well as the advantages and technical overview of 1D and 3D elements. A detailed FEA can simulate the realistic analysis of a column-reboiler circuit. HP
LITERATURE CITED
Mahesh Kulkarni works as a Manager and Team Lead for the mechanical and machinery team in technical expertise and discipline engineering at BASF, Mumbai. He is a Mechanical Engineering graduate and completed an MS degree in Mechanical-Heat Power Engineering. He has obtained Charted Engineer, Fellow and Professional Engineer titles from the Institution of Engineers, India, and received Professional Engineer title from the Engineering Council of India. Kulkarni has more than 23 yr of professional experience and specializes in FEA in the process industries, including extensive hands-on experience on all types of static equipment in chemical and petrochemical projects. He has also worked as an FEA expert, project engineer, root cause analysis and troubleshooting expert in refinery projects. The author can be reached at Mahesh.Kulkarni@basf.com.