Heat of absorption using water lean solvents

https://doi.org/10.1016/j.seppur.2020.118193

“Most investigators who claim that water-lean solvents might be able to deliver lower regeneration heat duties employ the following equation to sustain their argument.Q≈Cp·ΔTΔq+LDMCO2·pDpCO2+ΔHCO2absMCO2

On the right-hand side of the equation above, the first term refers to sensible heat, the second term refers to latent heat and the last term refers to absorption (or desorption) heat. In literature, one will find investigators claiming that each one of these terms can be reduced by shifting from aqueous to water-lean solvents. Therefore, we shall look at each one separately, starting from the right and coming back to the left.”

“Authors such as Leites [31] have reported water-lean solvents absorbing CO₂ above their theoretical stoichiometric limit (i.e. after the depletion of amines for chemical reaction) and releasing less heat than regular aqueous solvents. The reason is that, for example, aqueous MEA absorbs CO₂ above loadings of 0.5 mol CO₂∙mol MEA⁻¹ by means of reactions forming bicarbonate, whereas nonaqueous MEA absorbs CO₂ above the same loadings by pure physical solvation, a process less exothermical than any chemical reaction. This can be visualized on Fig. 14 in data adapted from our previous work [59]. Authors such as Shamiri et al. [26] seem to think that this behavior in itself might bring a reduction in regeneration costs.”

“Fig. 14. Heat of absorption of aqueous 30 %wt. MEA compared to nonaqueous MEA measured at 40 °C. Adapted from Wanderley et al. [59].”

“However, Leites [31] also registers the heat of absorption before the stoichiometric limit of 0.5 mol CO₂∙mol MEA⁻¹. According to that report, the heat of absorption in aqueous MEA is of about 81 kJ∙mol CO₂⁻¹, whereas nonaqueous solvents with ethylene glycol, NMP and THFA all present the same heat of absorption of 92 kJ∙mol CO₂⁻¹ (values remarkably similar to those obtained experimentally by Wanderley et al. [59]). In other words, though reductions in heat of absorption after maximum loading are indeed attainable, this is hardly advantageous for water-lean solvents unless one is planning to operate the capture plant at high pressures. This is the case for pre-combustion CO₂ capture, with the treatment of CO₂ partial pressures above 700 kPa as seen on the Fundamentals of Natural Gas Processing [19].

A number of researchers have applied the Gibbs-Helmholtz correlation on their vapor–liquid equilibrium data to calculate the heat of absorption in water-lean solvents. For example, Murrieta-Guevara et al. [83] estimate heats of 89.1 kJ∙mol CO₂⁻¹ for MEA in water-free NMP, while Tan et al. [86] estimate 87.6 kJ∙mol CO₂⁻¹ for MEA in water-free TEG solutions. Heat of absorption calculations with the Gibbs-Helmholtz equation might be ridden with huge uncertainties, evaluated at ± 20% by Murrieta-Guevara et al. [83], but it is still remarkable that these authors do not deduce heats of absorptions below those of aqueous amines.dlnpCO2d1Tα=-ΔHCO2absR

One exception we should point out is that of Zheng et al. [106], who estimate a reduction of heat of absorption in DEG-AMP blends when comparing to aqueous AMP. All of these calculations assume a heat of absorption that is independent of temperature, since integration of the Gibbs-Helmholtz expression below removes temperature from the equation. At least for aqueous amines, however, heat of absorption has been seen to increase slightly with increasing temperatures (Kim et al., 2014; Kim and Svendsen, 2007).

If the VLE in water-lean solvents is indeed more sensitive to changes in temperature, as proposed by Rivas and Prausnitz [34] and verified by a number of researchers, then it makes sense that the heat of absorption calculated with the expression above will be consistently higher for water-lean solvents as well.

We should refer to other examples of studies that have measured the heat of absorption in water-lean solvents experimentally. Most of them did so in biphasic systems [32], [108], [174], wherein the exothermicity of CO₂ absorption is greatly enhanced by the spontaneous phase separation phenomenon. Mobley et al. [116] have measured the heat of absorption of a very interesting hydrophobic mixture of 2-fluorophenethylamine and octafluoropentanol that, though having many properties in common with other hybrid solvents, show an abrupt reduction of heat of absorption with temperature, from around 85 down to 50 kJ∙mol CO₂⁻¹ between 40 and 120 °C. This is unlike most amine solvents. However, this is one exceptional blend formulated with complex molecules specifically to attain this target behavior.”