Design of a Test Rig for the Simulation of Startup Procedures in Main Heat Exchangers of Air Separation Plants

Abstract

The Kopernikus project “FlexASU” investigates an enhancement of the load flexibility of air separation units with the goal to synchronize 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. Aluminium brazed plate fin heat exchangers (PFHE) are used in these plants because of their high process integration, low production cost and compact design. Finding the optimum operating scheme for the rectification columns depends on detailed knowledge of their effect on the lifetime of the PFHE.The transient behavior of temperature profiles within a PFHE can be calculated and a FEM-based model is available to investigate the thermal stress induced by dynamic temperature changes in the apparatus. Hence, the reduction of lifetime due to certain dynamic plant operating scenarios can be estimated and critical operating modes can be identified. For further development of the model, a PFHE test rig is designed which is representative for extreme operating conditions in air separation units. A test scenario matching the conditions of plant startup after extended downtime in cold condition has been designed. To reach a level of thermal stress comparable to industrial sized plants, the PFHE weighing around 1.5 tons is heated up to 50 °C before rapidly being cooled down by a stream of gaseous nitrogen at -173 °C in a cyclic operation scheme. This test scenario leads to thermal stress that is expected to damage the PFHE within a few weeks of testing and the location of maximum thermal stress in the FEM model matches typical cracks observed in PFHE of air separation units. The test rig is designed to gain a clearer understanding of the underlying damage mechanism and to evaluate measures for lowering thermal stress in further test series. The knowledge gained from the experimental investigations is crucial for maximizing the operation flexibility of an ASU plant.