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Energy Transition
Figure 6: Comparison kettle type vs. K ° FLEX reboiler.
The common heat exchanger type for reboilers in industry is the shell & tube( S & THE). And in amine systems the kettle type S & THE is often seen. It is reliable, well known and can work at almost 0 % turndown ratio. However, it is also heavy and big. When developing large-scale CCS projects, project developers face several challenges. First of all, the number of reboilers that need to be put around the column gets too big to be practical. When more than four units are needed, the space around the column gets congested and the vapour return piping gets very complex. Secondly, more and more project developers are seeking modularisation of their systems. This requires more compact and lower weight heat exchangers. Welded plate heat exchangers can offer great benefits in use as amine reboilers. Kelvion’ s K ° FLEX is such a welded plate heat exchanger. In one study case, four heavy kettle type reboilers with a weight of 89 tonnes and a footprint of 6,900 x 11,400 mm could be replaced by two K ° Flex thermosyphon reboilers with a weight of 58 tonnes and a footprint of 3,870 x 7,060 mm. A welded plate heat exchanger will also result in less solvent degradation and hence less system fouling and corrosion. Also, a welded plate heat exchanger works at lower wall temperatures on the solvent side and will have a lower residence time.
Carbon capture by desublimation – cryogenic Desublimation is the direct phase change of a gaseous substance into the solid, crystalline phase. This physical phenomenon is known from nature: in winter air-bound moisture is deposited as white frost. In the chemical industry, desublimation is used to separate chemical components from a gas stream. Kelvion has been supplying switch condensers for the production of phthalic anhydride( PA) since the 1950s. It is called switch condenser because of the batch operation nature, where in a first step you cool desublimator and collect crystals. When the unit is completely loaded with crystals, you‘ switch’ to the recovery mode, where you heat up the unit end melt off the collected component. Several parallel units are used for a semi-continuous operation. Desublimation has been used in many different processes such as cleaning tank venting from naphthalene and the protection of vacuum systems.
More than a decade ago, Kelvion started to investigate the removal of CO 2 by means of desublimation. The first challenge to overcome was the fact that there were no solid-gas equilibria available in the literature. Mathematical equilibrium models were developed in collaboration with the Ruhr University Bochum( RUB) at the Department of Fluid Process Engineering with Prof. Grünewald. There have been several master theses using this model predicting separation of CO 2 in solid form in binary and multicomponent mixtures with methane, pentane, butane, etc. Of course the question then is: how accurate is such a mathematical model? Verification of the models was done through experiments at CSIRO in its cryogenic lab in Melbourne, Australia. Important results were achieved in this experiment. First of all, it gave a visualisation of the formation of CO 2 crystals which proved the CO 2 separation concept and gave insight into the morphology of the crystals. Secondly, it was measured that a gas stream containing 20 % CO 2 at the inlet only contained 0.5 % CO 2 at the outlet. Thirdly, the mathematical model developed by
Figure 7: Full-scale desublimator for bio-LNG production.
Figure 8: CO 2 crystals captured in bio-LNG process.
26 Heat Exchanger World April 2025 www. heat-exchanger-world. com