1. Introduction - Firenze University Press

The base case for the CO2 purification and compression option is also shown in Figure 1: The
retentate stream after the membranes is expanded and catalytically combusted in an oxy-fired
combustor. Chemical equilibrium predicts that complete combustion of the remaining combustible
species is feasible with almost no oxygen surplus [13] but this is not practically easily achievable.
The required oxygen depends on how the upsteam units (ATR and WGS performance) are operated:
for example assuming TATR=1050°C and S/CATR=1.5 for the base case, the required amount of
oxygen for the post combustor increases the total oxygen production in the ASU by 17.5%. The hot
flue gases deliver heat in a secondary Heat Recovery Steam Generator where the feedwater is
transformed to high-pressure superheated steam. Next, the water content of the flue gases is
removed in a flash separator, and the almost pure CO2 is then compressed and pumped. The
operating parameters of the ATR and WGSMR play an important role in the amount of heat present
in the retentate stream. The present study is focused in the CO2 stream purification and
compression block based on two separation methods, (a) flash separator and (b) distillation column
(see Fig. 2). Apart from CO2, the retentate stream mainly consists of H2O, H2, CH4, N2 and Ar (see
Table 1).
Table 1. Retentate stream T, P and composition
This gas stream is firstly expanded and then is cooled down to around 220°C. Some of the heat is
recovered for generating superheated intermediate pressure steam at 315°C. Expanding the gas has
a triple positive effect: firstly, the mixture is separated more easily at lower pressures. Secondly, the
manufacturing cost of equipment such as the evaporator is significantly lower if they operate at
lower pressures. Thirdly, as the content of water in the retentate stream is about 25% w/w, the CO2
rich mass flow rate that is compressed is less than the corresponding stream that is expanded,
contributing positively to the total power balance.
After that, cooling water is used to bring the stream to water condensation conditions at 28°C. The
next part of the Purification and Compression Units differs for the two proposed schemes:
2.1 Scheme 1: Double flash separation – internal cooling
This system is auto-refrigerated with no additional cooling system required (Figure 2). Flash
separations are performed at two different temperatures, and at the same pressure level. Before each
flash, there is a Heat Exchanger that cools the inlet stream. The required cooling loads are taken
from the final steams as can be seen in Figure 2a. The rich-CO2 liquid steams are throttled
adiabatically and their temperature is reduced (Joule–Thomson effect). The level of throttling has
been set so as to permit heat transfer at the two Heat Exchangers, without temperature crossovers,
assuming a minimum temperature approach =3°C. The final streams come out of the
Purification Unit at the temperature of 18°C and the rich-CO2 stream enters the Compression Unit,
where it is compressed in a three stage inter-cooled compressor up to 80 bar to supercritical
conditions. Then, it is cooled, liquefied and pumped up to 28°C/110bar and is transported for
storage.
Since the temperature at the 2 nd Flash is -54.5°C, (near the triple point of CO2) the parameters that
determine the system’s efficiency are the expander outlet pressure and the temperature of the 1st
flash separation.
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