Fatigue Analysis of a Pressure Swing Adsorption Vessel

Abstract Pressure Swing Adsorption technology is used to separate any gas from a mixture of gases under pressure according to the gas molecular characteristics and its affinity for an adsorbent material. The process operates at near-ambient temperatures and specific adsorptive materials (e.g., activated carbon, molecular sieves, etc.) are used to preferentially adsorb the target gas at high pressure. The process then swings to low pressure to desorb the adsorbed material. Adsorption and desorption normally take place alternately at equal time intervals which sets the pressure cycle in a pressure vessel. The cycles are usually produced by internal pressure or vacuum caused by the swing operation. This paper focuses on an investigation to determine the design fatigue life of an adsorption vessel subjected to over 10 million cycles of design internal pressure and vacuum. The paper also highlights the effect of reaching full vacuum conditions on the fatigue life of the vessel during the pressure cycle. The fatigue evaluation is performed considering the normal operation of the equipment during the entire life cycle of the plant. The adsorber vessel under investigation replaces an existing vessel due to the improved process performance requirements of the plant and hence provided with four leg supports welded directly to the vessel head to avoid modifications to existing base supports in the plant. A 3D model of the full vessel is considered, meshed and the fatigue evaluation is done as per ASME Sec VIII Div. 2, Part 5, Annexure 3-F.1. FEA software ANSYS® is used to perform the stress analysis. Vessel fatigue life is estimated according to the alternating stresses at the highest stress point i.e., at the leg to head junction.