Brief review of solvent degradation

https://doi.org/10.1016/j.ijggc.2022.103597

“Since amine degradation appears to be unavoidable in long-term operations of PCC, test campaigns of several hundred or thousand hours are typically performed to ascertain amine stability along with qualitative and quantitative analysis of degradation compounds (Cousins et al., 2015; Morken et al., 2019; Moser et al., 2021, 2020; Reynolds et al., 2015). In some of these campaigns, CO₂ capture performance indices such as the regeneration energy were reported over the operation period. Interestingly, the regeneration energy often remained stable or decreased despite the accumulation of degradation compounds (Cousins et al., 2015; Moser et al., 2021). For example, Moser et al. (2021) performed a long-term operation of pilot scale PCC using AMP-PZ aqueous solution from combustion gasses of coal-fired power plant at the constant CO₂ capture rate of 90%; the regeneration energy after 1000 hours and 4000 hours operation rather decreased compared to that after 420 hours despite the accumulation of degradation compounds. Unfortunately, an explanation for these unexpected results was not provided in detail. The accumulation of degradation compounds will alter solution properties (e.g., vapor liquid equilibrium: VLE, specific heat capacity, viscosity) and changes in solution properties will necessarily affect the process performance. Chronological changes in solution properties were rarely reported in these long-term campaigns, and the link between accumulation of degradation compounds and the process performance remains unknown. Several studies have reported the solution properties of degraded solutions in real world CO₂ capture plants (Aronu et al., 2014; Ju et al., 2018), but the corresponding process performance were not reported.

In contrast to the lack of information connecting solution properties to process performance, there are many qualitative and quantitative reports on the degradation compounds generated in long-term operations (Morken et al., 2017; Reynolds et al., 2015). The main amine degradation compounds are weak acids (e.g., formic acid, acetic acid, glycolic acid, nitric acid, and oxalic acid) that form a salt (Heat Stable Salts: HSS) that remains intact even at high temperature in the stripper (Morken et al., 2017; Reynolds et al., 2015). All PCC campaigns of sufficient length have observed non-negligible amounts of weak acids that act as progenitor of HSS. In general, HSS are recognized as undesired components in the PCC system because they increase the viscosity (Nielsen, 2018) and decrease the concentration of free amine which can react with CO₂ or act as a proton acceptor (Weiland and Sivasubramanian, 2004).

From a review of the bulk of the literature, the question of how HSS affect solution properties, and subsequently the regeneration energy, arises naturally. Some research has focused on HSS’s impact on CO₂ absorption and desorption. Freguia and Rochelle (2003) simulated PCC using an aqueous monoethanolamine (MEA) solution containing HSS. They showed that the addition of HSS reduces the regeneration energy by 5%, but they did not provide a mechanism linking solution properties and the regeneration energy. Weiland and Sivasubramanian (2004) explained how the presence of HSS reduces the CO₂ loading at a given CO₂ partial pressure, noting that HSS provide benefits to stripper operation and negative impact on absorber operation. They simulated H₂S removal using N-Methyldiethanolamine (MDEA) aqueous solutions which are partially neutralized (0∼10%) by phosphoric acids, and results showed that the regeneration energy was minimized when ∼2% of MDEA were neutralized by phosphoric acids. However, their explanation only accounted for the trade-off between loss of free amine and improvement in amine regeneration. Ling et al. (2019) examined the change in CO₂ absorption and desorption in MEA aqueous solution due to HSS via batch type experiments. The results indicated that HSS reduce the initial solution pH, equilibrium CO₂ solubility, CO₂ absorption rate, and cyclic CO₂ solubility, but HSS also increase the CO₂ desorption rate.

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