1. Introduction - Firenze University Press

while it is compressed. The distillation column outlet streams can be used to assist the cooling of
the retentate stream after water removal (reducing energy consumption).
The refrigeration system comprises a two stage compressor in order to provide cooling loads at two
different temperatures: at -20°C for the retentate stream cooling and at around -65°C (depending on
recovery rate that determines the dew point of the outlet gas stream) to fulfil the condenser duties at
the distillation column. The most suitable cooling medium that is employed in the modelling is
R1150 (ethylene – C2H4). Assuming that the temperature approach of all heat exchangers is 3°C,
the pressure levels of the evaporation are 2.26 and 23.47 bar and that for condensing is 27 bar.
Finally, the compressor’s polytropic efficiency is assumed to be equal to 0.82. Since the present
cooling medium cannot reject its heat directly to the ambient while it is condensed, a secondary
auxiliary cooling cycle is required. The cooling medium in this refrigerant cycle is the commercial
R134a and the COP of this cycle is assumed 3.59.
The thermochemical properties of the PCU block are calculated according to the Peng- Robinson
equation of state [8]. The technical data of the system are presented at the Table 1.
Table 1. Process model specifications
3. Results and Discussion
3.1 Process parameters investigation
In order to come up with concluding remarks about the efficiency of the proposed systems, the
comparison of exergetic efficiencies of the schemes is performed. The exergy balance of the PCU
block is shown in Figure 5a. The exergy input comes from the retentate stream while any power or
heat duties are considered as part of the exergy outputs.
The exergy dissipation for the cases under investigation is depicted by Grassmann diagrams, which
are displayed in Figures 5b-d. Arrows that are at the upper side of the main exergy arrow
correspond to exergy that is not lost (power production or heat recovery at the steam cycle) whereas
the remaining arrows refer to exergy losses (irreversibilities). As far as the process of the oxycombustion
option is concerned, heat recovery and power generation are exergy that is not lost.
The retentate stream at the exit of the burner is at high temperature and a large fraction of the
exergy input is utilised for high enthalpy superheated steam generation.
The ASU consumption for extra oxygen production is not taken into account in the exergy balance
and its effect on total efficiency is investigated below. On the other hand, both separation options
can recover almost half of the total exergy by recovering the combustible content.
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