1. Introduction - Firenze University Press

extraction process at 400 °C (~ 623 K) corresponds to an exergy equal to Ex(Q) = (1-T/T0)·Q = 9.1
– 3.9 GJ/t-CO2. (T/T0)·Q is the exergy destruction, ED.
Table 7. Energy (Q), exergy destruction (ED) and requirement (EQ) of the process in GJ/t-CO2
246
Q ED EQ
Mg(OH)2 production
Kiln 9.09 3.89 5.20
DISS 0.48 0.46 0.02
PREP1 1.10 1.06 0.04
PREP 2 -10.5 -9.70 -0.84
MVR Compressor 1.18
Sep-4 -0.89 -0.65 -0.24
Heat exchangers -2.40 -1.73 -0.67
Total -1.99 -6.67 4.68 #
Mg(OH)2 Carbonation
Turbine -0.24
-0.24
Heat exchangers -1.78 -1.22 -0.55
Total -2.02 -1.22 -0.79
Net
3.88 #
#
These values are lowered by ~0.45 GJ/t-CO2 if the Fe form in mineral is assumed to be FeO
instead of the Fe2O3 used here.
The exergy destruction of a system, which is the measure of the amount by which the value of the
resource is consumed or degraded, is shown as (8) while the exergy flow is presented in (9);
(8)
where (S-S0) is the entropy change, T0 is the ambient temperature and (H-H0) the enthalpy change.
The results obtained here are compared with the results of a previous model [17] where no pinch
analysis was done.
The application of pinch analysis and the heat exchanger network as implemented in the Aspen Plus
model (Fig.5) resulted in a ~ 0.5 Gt/t-CO2 (~ 10% points) reduction in the exergy requirement of
producing Mg(OH)2. Mg(OH)2 carbonation unit provides ~ 17% points energy offset to the process.
When the Mg(OH)2 production and carbonation units are integrated, the process requirements of the
process becomes 3.88 Gt/t-CO2. This value becomes 3.4 GJ/t-CO2 (reducing by another ~0.5 GJ/t-
CO2) if the compound form of Fe in mineral is assumed to be FeO instead of the Fe2O3 used here.
5. Conclusions
This paper investigated the influence that reaction parameters has on the production of Mg(OH)2 by
analyzing the effects of Mg/Fe ratio, S/AS ratio, T and t on Mg extraction. Once produced Mg(OH)2
could be used to sequester carbon by direct reaction with flue gases or CO2 derived from power or
chemical plants. Notable among the results presented in this paper is the fact that olivine rocks are
5x less as reactive as their serpentinite counterparts. It was also obvious that serpentinite rocks with
Mg/Fe < 2.16 were less (>2x) reactive than others. This validates previous results which showed
that an increase in Mg/Fe ratio increases Mg extraction. Reaction time has a significant effect on
magnesium extraction as an increase in t above 25 minutes results in a 15 percent points’ increase in
Mg extraction, but this effect tends to diminish after 60 min. On the other hand, Mg extraction
shows a negative dependence on reaction temperature; T > 440 o C do not favor Mg extraction. This
1.18
(9)