A dryer vessel is designed for the purpose of removing water from ‘wet’ propylene liquid, where ‘wet’ denotes the presence of a small quantity of water mixed with the propylene. The water is adsorbed using a zeolite bed within the dryer, which over time becomes saturated. To regenerate the zeolite bed and remove adsorbed water, hot propylene gas is passed through the bed, a process known as ‘regeneration’.
The vessel operates under two distinct conditions: a cool drying phase and a high-temperature regeneration phase. This cyclic operation causes the vessel’s temperature to fluctuate multiple times daily. Variations in heat transfer rates across different parts of the vessel result in some areas heating up or cooling down more rapidly than others. These temperature differentials can lead to thermal bending stresses within the vessel.
Analysis
The analysis of the dryer vessel utilized Finite Element Analysis (FEA) software, specifically FE/Pipe, to create a 2-D axisymmetric model. Heat transfer into the vessel wall depended on the heat transfer coefficient and the local temperature difference between the fluid and the vessel material. A 1-D model predicted the temperature profile within the zeolite bed based on heat transfer between the bed and the passing fluid. Heat transfer coefficients throughout the vessel were estimated using standard fluid dynamics solutions.
These estimated temperature profiles and heat transfer coefficients served as boundary conditions for the 2-D axisymmetric model. Given the significant temporal variation in process temperatures, transient FEA simulations were conducted to model stress variations over time.
Results
The analysis identified areas of high bending stress, particularly at the intersections of the vessel heads (both top and bottom) with their connecting nozzles. These locations experienced rapid temperature changes due to higher heat transfer coefficients from increased fluid flow velocities within the nozzles. To ensure compliance with pressure vessel design codes, recommendations were made to round the nozzle/head intersections and refine certain welds.
By addressing these thermal stresses through simulation and design modifications, the dryer vessel can effectively manage the cyclic thermal conditions encountered during its operation, ensuring reliability and safety in industrial applications.
