https://doi.org/10.1016/j.petlm.2016.11.002
“Amine stability is one of the key parameters that signify the potential of an amine solution (single or blended) for CO2 capture application. Stable amine solutions translate to lower degradation. As amine solvents degrade during CO2 capture, a slipstream of the CO2 lean amine solution is sent to the reclaiming unit to recover fresh amine solvents. Slipstream for thermal reclaiming is withdrawn from the CO2 lean amine process line upstream of the lean/rich heat exchanger while that required for ion exchange/electrodialysis reclaiming is withdrawn from the CO2 lean amine process line entering the absorber [182]. Recent results [182] showed that thermal reclaiming still offer better performance for recovering amine solvent and separating degradation products (Table 1). The removal of metals and non–ionic degradation products (neutral degradation products) also make thermal reclaiming preferable. Thermal reclaiming is also cost competitive (Table 2) [182].
Table 1. Amine recovery and degradation products removal of different reclaiming technologies [182].
| Reclaiming technology | Amine Recovery (wt.%) | HSS Removal (wt.%) | Metals/Non–ionic Product Removal (wt.%) |
|---|---|---|---|
| Thermal reclaiming | 95 | 100 | 100 |
| Electrodialysis | 97 | 91.5 | 0 |
| Ion exchange | 99 | 90 | 0 |
Table 2. Normalized capital cost estimates of different reclaiming technologies [182].
| Reclaiming technology | US$ per kg/hr |
|---|---|
| Thermal reclaiming | 76,000 |
| Electrodialysis | 70,000 |
| Ion exchange | 101,000 |
Though researchers [84], [85] identify heat of regeneration as the major culprit in high operating cost of CO2 capture process plant, it is important to note that high degradation rates can also be the major problem. This is because high degradation rate of an amine solution will lead to higher corrosion rate, increased amine make–up (high solvent loss), significant increase in viscosity (increased pumping duty and mass transfer limitation), higher foaming [183], [184], [185], [186], [187], [188], emissions of potentially toxic substances in the capture plant off–gas. In addition, high degradation rate also lead to frequent solvent reclaiming and waste disposal and increased energy requirement both in the water wash section of the absorber and in the desorber (regenerator or stripper) overhead condenser (Fig. 19).
Fig. 19. Effect of amine degradation (amine instability) towards plant operating costs.
It can be seen from Fig. 19 that a combination of the key variables affected by amine degradation can surpass regeneration energy as the major contributor to the operating costs. Therefore, it is very important to understand the chemistry and pathways of amine degradation in order to reduce the accompanying risks, costs, and environmental concerns. Amine degradation (irreversible chemical reactions) can occur due to high temperature in the desorber typically operated between 110 and 130 °C [189], [190], [191]. This temperature range will speed up thermal degradation, the undesirable amine break–down process. Chemical degradation is also triggered by oxygen (oxidative degradation) present in the flue gas which could be as high as 11%vol [177]. Other impurities including NOx, SOx and fly ash in the flue gas stream can also significantly contribute to the degradation of amine solvents [12], [131], [177], [183], [192], [193], [194], [195], [196], [197], [198], [199], [200], [201], [202], [203], [204]. Though there are several degradation products reported in various studies, however, nitrosamines and nitramines seem to be the most concern as they pose more environmental and health risk compared to others [205], [206], [207], [208], [209]. Nitrosamine is the major degradation product generated from reaction of NOx in the flue gas and it is mainly produced by secondary amines followed by tertiary amines and then primary amines [209], [210]. This is because secondary amines form stable nitrosamines and nitramines while the nitrosamines formed by primary amines degrade rapidly to release N2 and a carbocation [211]. Smith and Loeppky stated that nitrosation of tertiary amines forms an unstable cationic intermediate that dealkylates to yield a nitrostable secondary amine [212]. Yu et al. further revealed that tertiary alkanolamines that contain two 2-hydroxyethyl groups showed higher nitrosamine formation potential compared with other tertiary amines with one or three 2-hydroxyethyl groups [210]. Their study also indicated that when tertiary amines contain same number of 2-hydroxyethyl groups, the amine with shorter alkyl chains enhanced formation of nitrosamine.
Though amine blends offer huge benefits in lowering regeneration energy they might engage in undesired secondary reactions. These can involve individual amine reaction with degradation products of the other amine (in the blend) and/or the reactions between the degradation products of the individual amines in the blend. Degradation of amine blends (MEA–MDEA, MDEA–PZ, MDEA–DEA, MEA–PZ, MEA–AMP) have been studied [84], [131], [196], [197], [213], [214]) and it was reported that more degradation products were produced in the blends compared to their individual amines. A study done by Idem et al. on MDEA–MEA degradation in boundary dam pilot plant revealed that MEA degraded faster in the blend (i.e. 2.3 mol %/day) than in single MEA (i.e. 0.5 mol %/day) [85]. Lawal et al. discovered that MEA degraded by O2 at a slower rate when blended with MDEA than in a single MEA solvent [213]. This was because MDEA was preferentially degraded in the blend thereby reducing the degradation of MEA. Experimental results of Wang and Jens showed that PZ oxidatively degraded slower as a single solvent than when it was mixed with AMP [204]. Similar trend was reported in the thermal degradation studies of MDEA–PZ and AMP–PZ blends [215], [216]. Based on Li et al. and Closmann et al. studies, oxazolidone degradation product formed in AMP and MDEA mixture could be blamed for such a high degradation of PZ in the mixed solvent [131], [217]. Rochelle reported that PZ–AEPD (PZ–2-Amino-2-ethyl-1,3-propanediol) thermally degraded faster than PZ–AMP due to the additional OH group (hydroxyl group) of AEPD, which made it more prone to form oxazolidone than AMP [215]. This has been also confirmed by Du et al. that an additional OH group decreases the thermal stability of the amine making it more likely to form oxazolidone [201]. Namjoshi showed that the rate of thermal degradation of PZ–DMAEE (piperazine–dimethylaminoethoxyethanol) was lower than that of PZ–MDEA, because DMAEE cannot form oxazolidone [218].
Namjoshi et al. further stated that diamines (without OH group)–PZ blends showed higher thermal stability than both tertiary amine–PZ and hindered amine–PZ blends [216]. Rochelle previously stated that ether amines are expected to exhibit higher stability than their corresponding alkanolamines [215]. This is because ether amines are less likely to degrade by carbamate polymerization like the alkanolamines. Table 3 shows some studied ether amines. Du et al. recently confirmed that ether amines are more stable than their alkanolamines counterparts because ether amines do not easily form oxazolidone like alkanolamines [201]. This was evidenced in their experimental results as PZ–BMEA and PZ–MOPA blends degraded much more slowly compared to most PZ–acyclic alkanolamines blends. This means that the type of functional group and its position in an amine can also affect the stability of the amine when it is blended with other amine solvents.
Previous investigations have now shown that degradation products of an amine solvent (in a blend) can catalyze the degradation of the other amine solvent in the blend. Hence, in order to optimize an amine blend to withstand degradation, proper understanding of the degradation products of the individual amines in the blend, the type of functional group and its position in the amine solvent is very important. It is also true that once adequate understanding of the amine degradation chemistry with respect to functional group type and position (in an amine) is well studied, characterising a novel polyamine with all the desired properties (e.g. type of functional group and position) will offer a lasting and reliable solution. This will lead to the deployment of the desired single amine (not blended amine) which will be a polyamine for CO2 capture application.
From Arrhenius relationship in Eq. (28) a 10 °C increase in temperature doubles reaction rate, and this is also the case for amine degradation rate. Therefore, low temperature regeneration (temperatures below 100 °C) will greatly minimize the rate of amine degradation. Nwaoha et al. also suggested additional benefits of low temperature regeneration which also covers less environmental implication, reduced operating and capital costs, lower amine make–up, minimized waste treatment and disposal [97].(28)k=Ae−EaRT
where; k is the rate constant, A is the frequency factor, Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 J/K.mol) while T is the temperature (K).
In addition to the effect of temperature towards amine degradation rate (rdeg, kg/hr), Eq. (28) can be expanded to include concentration effects of O2, SOx, NOx, fly ash (Cash), metals concentration (Cmetals) and water concentration (CH2O). Additionally, when various amine concentration is investigated (single or blended amine solvents), the amine concentration (Camine, kmol/m3) variable should be included. This will lead to the development of a model that is analogous to a power law kinetic model as shown in Eq. (29).(29)rdeg=Ae−EaRT[CO2]a[O2]b[SOx]c[NOx]d[Cash]e[Cmetals]f[CH2O]g[Camine]hwhere; a, b, c, d, e, f, g and h coefficients are the order of the reaction and can predict the contribution and influence of each component towards amine degradation. These coefficients can be determined by regression. The rdeg parameter in Eq. (29) can also be replaced with either the mass flow rate of the CO2 lean amine solution that is reclaimed (Fm, kg/hr) or its volumetric flow rate (Fv, L/hr). The choice of parameter to use (on the left hand side) will depend on the available data.
If nitric acid (HNO3) and sulphurous acid (H2SO3) were used to represent NOx and SOx respectively during the experiment then NOx and SOx should be replaced with HNO3 and H2SO3 in Eq. (29).
Fig. 20 depicts the summary of possible options for minimizing degradation of amine solvent. The combination of the proposed options will help towards enhancing CO2 capture efficiency, lower regeneration energy and reduce amine degradation rate.
Fig. 20. Several options of reducing amine solvent degradation during CO2 capture.
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