Degradation of solvent and inhibitors – brief and general discussion

https://doi.org/10.1016/j.cesx.2021.100096

“Degradation is as much an operational as an economic problem, bringing forth operational interruptions, incurring in solvent and plant replacement costs, and increasing emissions, which impacts the environmental footprint of the process. The benchmark solvent for amine-based flue gas scrubbing, monoethanolamine (MEA), is known for its tendency to undergo rapid and uncontrollable oxidative degradation. Experience from pilot campaigns with MEA proves that degradation is often the reason for ending a campaign (Dhingra et al., 2017).

Mechanistically, oxidative degradation is not as well understood as the thermal degradation pathways within the process, and different routes for the formation of a multitude of identifiable degradation compounds have been suggested (Lepaumier et al., 2009a, Lepaumier et al., 2009b, Vevelstad et al., 2016, Vevelstad et al., 2011). It is commonly accepted that the initial step of the oxidative degradation mechanism is a radical reaction, where the amine reacts with dissolved oxygen originating from the flue gas. Such radical reactions are assumed to be catalyzed by dissolved metals such as iron and copper, which have proven to increase degradation rates in laboratory scale studies (Blachly and Ravner, 1963, Goff, 2005, Sexton and Rochelle, 2009). In the initial reaction step, volatile ammonia and organic acids such as formic, acetic and oxalic acid are formed. These acids can react further with one another or with other amine molecules to create further degradation compounds, and are also known to give rise to corrosion of the construction material (Rooney and DuPart, 2000).

As a means of tackling the issues of degradation and corrosion in CO₂ capture plants, a plethora of degradation inhibitors have been suggested. There are typically three categories of oxidative degradation inhibitors for amine solutions:

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Oxygen or peroxide scavengers;

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Chelating agents;

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Stable salts.

The first approach to finding degradation inhibitors was published in 1964, when Blachly and Ravner tested ethylenediaminetetraacetic acid (EDTA) as a chelating agent for inhibiting the reaction between amine and metals by forming chelate complexes with metal ions. They also successfully proposed bicine as an efficient peroxide scavenger, which proved to be an excellent degradation inhibitor in metal-free solutions (Blachly and Ravner, 1964). Their findings have since been employed for aiding CO₂ separation in nuclear submarines (Blachly and Ravner, 1966, Blachly and Ravner, 1965), and EDTA has remained an attractive inhibitor, being thoroughly tested by many researchers (Chi and Rochelle, 2002, Goff and Rochelle, 2006, Lee et al., 2012, Sexton and Rochelle, 2009, Supap et al., 2011). More recently, a number of other chelating agents and oxygen/peroxide scavengers have been assessed in Fytianos et al. (2016).

One big issue with these two categories of inhibitors is that these materials lose effect with time and require replenishing throughout the process. On the other hand, Goff and Rochelle (2006) tested a range of heat stable salts as degradation inhibitors including potassium chloride (KCl), potassium bromide (KBr) and potassium formate (CHKO₂) in concentrations between 10 and 1000 mM, observing a small decrease of ammonia formation rate with both KBr and CHKO₂. The great advantage of salt addition, compared to scavenging additives, is that these salts simply change the properties of the solvent without getting exhausted with time. As such, their need for replenishing is minimal. Another significant advantage of employing stable salts for reduction of amine degradability is their toxicity when compared to many suggested degradation inhibitors, such as reactive vanadium or copper salts. Halide containing stable salts were already pointed out as inhibitors for the oxidative degradation of organic acids by Lee and Rochelle (1987). Their work proved that iodide (I⁻) worked as a powerful scavenger for sulfate (SO₄²⁻), inhibiting its oxidation of the organic acid (Cl⁻ < Br⁻ < I⁻). A recently published patent also identifies iodide as an effective oxidative degradation inhibitor in the context of organic acids (Sjostrom et al., 2020).”