A Cryogenic Test Rig for Startup Procedures of Main Heat Exchangers in Flexible Air Separation Units

An increasing share of renewable energy sources in the power supply is leading to a higher volatility of the German energy market. One possible way to stabilize the power grid is the flexible operation of energy-intensive processes such as cryogenic air separation. Hence, the Kopernikus project “SynErgie”, which is funded by the German Federal Ministry of Education and Research (BMBF), investigates the enhancement of the load flexibility of air separation units (ASUs)^[1].

The main heat exchanger was identified as one of the key components for dynamic plant operation because the higher number of startups, shutdowns and load changes can lead to thermal fatigue and decrease the lifetime of this vital plant component. Aluminum plate-fin heat exchangers (PFHEs) are used in these plants because of their high process integration, low production cost and compact design.

Deeper understanding and experimental data from dynamic operation of representative PFHEs are of major interest. Thus, a large cryogenic test rig for dynamic operation of PFHEs was designed, constructed, and put into operation ^[2,3]. In a cyclic operation scheme, two PFHEs are alternately cooled down to cryogenic temperatures and heated up to simulate startup procedures in ASUs and induce thermal fatigue eventually. Extensive measurement technology, like distributed temperature sensing using optical fibers and detailed strain evaluation, offer interesting insights into the behavior of PFHEs under dynamic operation ^[4]. The collected data from the test rig is used for validation of a three-dimensional model for thermo-fluid simulation ^[5] and computational lifetime estimation tools.

Results show severe 3D temperature maldistribution, which results in high thermal stress for this harsh test scenario. Two promising operational measures for lowering thermal stress in dynamic operation of PFHEs were examined at the test rig. Results will provide important information for design and operation of main heat exchangers in flexible ASUs.

Literature:

[1] Klein, H.; Fritsch, P.; Haider, P.; Kender, R.; Rößler, F.; Rehfeldt, S.; Freko, P; Hoffmann, R.; Thomas, I.; Wunderlich, B.: Flexible Operation of Air Separation Units. Chemie Ingenieur Technik 92 (12), 2020, pp. 1921 – 1940

[2] Haider, P.; Freko, P.; Lochner, S.; Reiter, T.; Rehfeldt, S.; Klein, H.: Design of a test rig for the simulation of startup procedures in main heat exchangers of air separation plants. Chemical Engineering Research and Design 147, 2019, pp. 90-97

[3] Fritsch, P., Hoffmann, R., Flüggen, R., Haider, P., Rehfeldt, S. & Klein, H.: A Cryogenic Test Rig for Dynamically Operated Plate‐Fin Heat Exchangers. Chemie Ingenieur Technik, 93(8), 2021, 1230–1237.

[4] Fritsch, P., Hoffmann, R., Flüggen, R., Woitalka, A., Haider, P., Rehfeldt, S. & Klein, H.: Distributed Temperature and Strain Measurements at a Cryogenic Plate‐Fin Heat Exchanger Test Rig: in production. Chemie Ingenieur Technik, 2021, Pubblicazione anticipata on-line. https://doi.org/10.1002/cite.202100070

[5] Haider, P., Freko, P., Acher, T., Rehfeldt, S. u. Klein, H.: A transient three-dimensional model for thermo-fluid simulation of cryogenic plate-fin heat exchangers. Applied Thermal Engineering 180, 2020, p. 115791