(75b) Design of a Test Rig for the Simulation of Startup Procedures in Main Heat Exchangers of Air Separation Plant

The Kopernikus project “SynErgie” investigates an enhancement of the load flexibility of air separation units. Aluminium plate fin heat exchangers (PFHE) are used in these plants because of their high process integration, low production cost and compact design. The main heat exchanger was identified as one of the key components for a dynamic plant operation because startup, shutdown and load change procedures can lead to thermal stress and an increased lifetime consumption.

The transient behavior of temperature profiles within a PFHE can be calculated with satisfying accuracy. Furthermore, a FEM-based model is available to investigate the thermal stress induced by dynamic temperature changes in PFHE. Hence, the reduction of lifetime due to 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. In order 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. The location of maximum thermal stress in the FEM model matches typical cracks observed in 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.