Presentación, interpretación y análisis de los datos recogidos

Various solutions have been proposed to limit the influence of degradation on the operation of a CO2 capture plant. Usual solutions in commercial plants are referred to as reclaiming processes.

They aim at purifying the amine solvent that contains degradation products. However, it is also possible to act preventively in order to avoid solvent degradation, which is the objective of degradation inhibitors.

2.6.1 Reclaiming methods

The most commonly used reclaiming methods are described below based on the work of Cummings et al. (2007).

Solvent purge and make-up: as mentioned by Cummings et al. (2007), “this method might not be opposed by your amine supplier”. It is a simple and widely used solution in commercial CO2

capture processes but it implies a large consumption of fresh amine. Moreover, the purged solvent must be properly disposed of, inducing additional waste treatment costs and a low environmental efficiency.

Thermal distillation: the amine solvent is vaporized in a distillation column, and the non-volatile degradation products are recovered in the sump of the column. This is an interesting technique if the concentration of degradation products is high. There is less waste generated in comparison to the first method but the energy requirement of distillation is an important drawback in this method, especially for less degraded solutions. Thermal reclaiming may also induce additional thermal degradation with CO2 since the temperature in the bottom of the reclaimer may reach 150°C. Thermal distillation may be performed under vacuum for low volatile amine. It is not the case for MEA.

Neutralization: a strong base, usually NaOH is added to the solution. The undesired acid contaminant that has formed a salt with MEA associates with the strong base and releases the amine. This method limits the effect of degradation products, but does not remove them, so that solvent properties may remain affected. An interesting alternative to NaOH has been proposed by Xu and Rochelle (2009). KOH is used as the strong base, leading to the precipitation of

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K2SO4 crystals easily removable from the solvent. However, this method mainly addresses heat stable salts so that non-ionic degradation products are unaffected.

Ion exchange: the same principle as for neutralization is used. The undesired acid contaminant that has formed a salt with MEA is exchanged with a friendly anion brought into the system by a resin. This is an interesting method from the economic point of view, but it only addresses ionic degradation products.

Electrodialysis: Ionic degradation products migrate through ion-selective membranes placed in an electric field. The energy requirement of this technique is advantageous but it produces more waste than ion exchange.

Independently of their efficiency, these methods are only used with already degraded amine solvents, so that they do not prevent degradation. As a consequence, many negative effects induced by degradation are not addressed by such techniques.

2.6.2 Degradation inhibitors

The use of degradation inhibitors may be an interesting alternative to reclaiming techniques.

Indeed, some chemicals show the ability of inhibiting amine degradation, especially oxidative degradation. This attractive approach prevents the formation of degradation products, so that the fresh amine consumption is reduced, as well as the waste volume. However, degradation inhibitors may modify the solvent properties, and more research is needed to assess this effect.

Indeed, the biodegradability of amine solvents should be preserved in order to limit the environmental penalty of solvent emissions (either due to solvent volatility or to accidental release) so that a trade-off is necessary between solvent stability and environmental safety.

The role of a degradation inhibitor is to prevent or minimize the solvent degradation during the CO2 capture process. However, no degradation inhibitor has been proposed so far to prevent MEA thermal decomposition, thermal degradation with CO2 or NOx degradation, and only few studies have considered SO2 degradation. The reasons are the following: Experimental study of amine solvent degradation. Thermal decomposition of MEA occurs at temperatures higher than 200°C (Epp and Bathen, 2011), so that there is no need to consider it in usual CO2 capture processes.

Degradation due to CO2 results from the CO2 absorption in MEA, a mechanism that is desired, so that inhibiting this mechanism does not make much sense. Degradation with NOx and SOx

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has rarely been studied because of the low SO2 and NOx content achievable in power plant flue gas.

As a consequence, most studies about degradation inhibitors address oxidative degradation.

Oxidative degradation inhibitors may be separated into three main categories based on the oxidative degradation mechanisms (Bedell, 2009).

Chelating agents: they form a complex with dissolved metals, inhibiting their catalytic activity and limiting the initiation/propagation steps of the chain reaction.

Radical and O2 scavengers: dissolved O2 forms peroxides in water. Radical scavengers react with the peroxides to form stable products and stop the chain reaction. They are also called O2

scavengers since they stoichiometrically react with dissolved O2. Disadvantage of many radical scavengers is that they are consumed during the reaction and must be renewed.

Stable salts like KCl, KBr or KCOOH increase the ionic strength of water, so that the solubility of gases in the solvent decreases (Goff and Rochelle, 2003). However, these salts appeared to be poor inhibitors, decreasing the NH3 production by only 15% in the best case.

2.6.3 Degradation of MEA Solvent

One of the concerns with using MEA as a solvent is that it is prone to degradation at high temperatures by a number of mechanisms as outlined in this section.

a. Carbamate Polymerization

Carbamate polymerization is the most common mechanism of amine degradation. It occurs in the presence of CO2 and high temperature. The rate of degradation is a strong function of CO2 partial pressure and temperature. Carbamate polymerization is initiated by the formation of an oxazolidone. This forms as a five-member ring by the internal reaction of an alcohol and a carbamate. The parent amine then reacts with the oxazolidone to produce a substituted ethylenediamine. The final step in the degradation is the condensation of the substituted ethylenediamine to a substituted piperazine. Sterically hindered amines and tertiary amines do not have a strong tendency to form carbamate and hence, are not subjected to his form of degradation.

Degradation by carbamate polymerization is insignificant at temperatures lower than 100⁰C and hence, will be important only around the stripper and the reboiler sump. Since, the degradation reactions are favored at high CO2 loading, the degradation is more probable at the rich end of the stripper. In addition, the rate of polymerization has a high dependence on amine concentration

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and hence, solvents that use a lower amine concentration will have a lower rate of degradation.

Carbamate polymerization has been studied by a number of authors, however there is not much literature on the kinetics of the carbamate polymerization react.

b. Oxidative degradation

Oxidative degradation occurs due to the presence of oxygen in the flue. Neither carbon dioxide nor high temperature is required for oxidative degradation to occur. The products of oxidative degradation include various aldehydes, organic acids such as acetate, formate, glycolate, acetate and oxalate amines, NH3 and nitrosoamines. These products can have significant environmental impacts if released into the environment. Nitrosoamines are known to be carcinogens. Oxidative degradation also results in the formation of heat stable salts and loss of the solvent. The degraded solvent has to be replaces with make-up and this can be a significant cost in the process. In addition, the degradation reactions can significantly enhance the corrosion of the column and its internals. In industrial applications, Fe and Cu are likely to be catalysts that promote the degradation of the amine.

Oxidative degradation will most likely occur at short times and low temperature with contact in the absorber and at longer times and high temperature in the stripper. A number of authors have studied the oxidative degradation of MEA. A study at the University of Texas has identified that the oxidative degradation of MEA under industrial conditions is controlled by O2 mass transfer and that the degradation rate is likely to be 0.29 - 0.73kg MEA/m ton of CO2. In general, inhibitors are added to the system to prevent the oxidative degradation of MEA.