Eq. (4) is amine protonation and Eq. (5) represents carbamate
formation. For monoamines protonation of the carbamate species
may occur and the pKa is around 7 for MEA (Fernandes et al.,
2012a). This reaction was included for MEA as it may contribute
to the chemistry at high CO2 loadings. The pKa for the AMP carbamate
is not known but it can be considered irrelevant as the
AMP-carbamate species forms to only a minor extent. For diamines
like PZ the formation of the dicarbamate needs to be included, Eq.
(7). Protonation of the mono-carbamate species occurs typically
with a higher pKa than monoamines (Fernandes et al., 2012a) and
this reaction needs to be included as Eq. (6). Dicarbamate formation
involves a number of reactions with a number of forms of mono and
dicarbamate species existing in equilibrium (Conway et al., 2013).
Eq. (7) represents the dicarbamate formation equilibrium that is
relevant at PCC conditions. The pKa for protonation of the dicarbamate
species has been determined to be 7.99 (Conway et al., 2013)
so its inclusion is also necessary (Eq. (8))