Decarbonisation Technology - February 2023

Temperature

0 K

50 K

100 K

150 K

200 K

250 K

300 K

350 K 400 K

450 K

500 K

550 K

600 K

650 K 700 K

1 kbar

100 MPa

Super- critical water

Water critical point 647 K (374˚C) 22.064 MPa

Supercritical CO

Liquid CO

CO triple point 216.55 K (-56.6˚C) 518 kPa

100 bar

10 MPa

CO critical point 304.25K (31.1˚C) 7.39 MPa

10 bar

1 MPa

Water freezing point at 1 atm 273.15 K(0˚C) 101.325 kPa

Solid CO (dry ice) Gaseous CO

1 bar

100 kPa

Water boiling point at 1 atm 373.15 K (100˚C) 101.325 kPa

CO sublimation point at 1 atm 194.65 K (-78.5˚C) 101.325 kPa

Liquid water

10 kPa

100 mbar

Gaseous water (steam)

Solid water (ice)

10 mbar

1 kPa

Water triple point 273.16 K (0.01˚ ° C) 611.657 Pa

1 mbar

100 Pa

-200 ˚C -250˚C

-150 ˚C

-100 ˚C

-50 ˚C

0 ˚C

50 ˚C

400 ˚C 350 ˚C 300 ˚C 250 ˚C 200 ˚C 100 ˚C 150 ˚C

Figure 2 Phase diagrams of pure CO 2 and H 2 O

CO with water (i.e. CO + H 2 O ↔ CO 2 + H 2 ) is considered inevitable in the FT reactor, but with limited effect. After the main endothermic RWGS reaction, a water knock-out drum is operated to remove the water that was formed and thus separate the liquid phase from the CO/CO 2 / H 2 gas mixture. The conversion of CO 2 to CO improves with increased temperature and stoichiometric surplus of hydrogen in the feed. With a temperature of 908°C and an H 2 :CO 2 ratio of ~3:1, a CO 2 conversion of 77.9% is accomplished. The feed to the FT synthesis section is enriched with a supplementary stream of 4 kg/h green hydrogen to maintain an H 2 :CO ratio of 3:1. Since the target products of the plant are liquid hydrocarbons in the range of C 5 to C 20, no hydrocracker unit is being used. This reduces the need for additional hydrogen requirements but implies that the chain length in FT synthesis can be controlled. Currently, such a synthesis technology has a TRL of 6, compared to a TRL of 9 for the traditional wax FT synthesis. Separation takes place after the FT reactor. A seven-step flash separation section is incorporated to represent the separation of liquid products from the gaseous byproducts, reaction water, and unreacted gases. An expander turbine is used to achieve a pressure drop from 25 bar to 15

bar, alleviating the need for electricity for compressors and pumps. A CO 2 /fuel gas separation section retrieves fuel gas (C 2¯: 95.3% mol) for internal use, and a CO 2 -rich stream is recycled back to the mixer for the RWGS feed. This section uses a feed stream rich in H 2 /CO 2 , coming out of the third flash, well above the temperature of -76°C (the sublimation point of CO 2 at 1 atm). There are three options for this sub-section. The first involves a second expander to 1 atm, which allows a very high retrieval of CO 2 from the stream, but the subsequent cooling would result in dry ice formation, making this option the most difficult to handle. The second involves moderate cooling of the stream to -53°C, then compressing it to as much as 110 bar so that a significant part of the CO2 is retrieved in liquid form without any dry ice formation. The temperature-pressure phase diagrams of pure CO 2 and H 2 O are shown in Figure 2 . The third option uses the classical pressure swing absorption (PSA) technology.

H/Cs

% mol

C 1 -C 5 C 6 -C 11 C 12 -C 19 C 20 -C 30

13.9 48.7 29.3

6.9 1.3

Others/oxygenates

Table 1 Synthetic oil H/Cs distribution