Design of a test rig for the simulation of startup procedures in main heat exchangers of air separation plants

Abstract

Within the Kopernikus project “SynErgie”, the sub-project “FlexASU” investigates an enhancement of the load flexibility of air separation units with the goal of synchronizing their power consumption with the availability of renewable energy sources. The main heat exchanger of these plants was identified as a key component for a dynamic plant operation, because startup, shutdown and load-change procedures can lead to thermal stress and an increased lifetime consumption of the apparatus. Aluminum-brazed plate-fin heat exchangers (PFHE) are used in these plants because of their high process integration, low production cost and compact design.The main focus of this work, which is an extended version of Haider et al. (2018a), is the transient behavior of PFHE. The temperature profiles within a PFHE can be calculated, and a FEM-based model is available for investigation of the thermal stress induced by dynamic temperature changes in the apparatus. Hence, the reduction in lifetime due to certain dynamic plant operating scenarios can be estimated, and critical operating modes can be identified.For the further development of the model, a PFHE test rig is designed which is representative of extreme operating conditions in air separation units. A test scenario matching the conditions of plant startup after extended downtime in a cold state has been designed. The scenario leads to thermal stress that is expected to damage the PFHE within a few weeks of testing. The location of maximum stress in the FEM-model matches typical cracks observed in PFHE which have been exposed to severe thermal stress.