Captured CO
CO recycle
BFW
Knock - out drum
Green H
Flash 1
Syn oil
Green H
H1
C1
FT
HP steam
HP steam
RWGS
Water
Water
FG
FG
PSA CO /FG
Heavy syn oil
Flash 7
Flash 3
E1
Flash 4
Flash 5
Water
FG
Flash 2
Lt syn oil
Flash 6
Water
Water
Figure 3 Simulation model flowsheet mass
Finally, two smaller streams are recycled back and mixed with the main input of CO 2 and hydrogen. The recovered synthetic oil, 139.4 kg/h in total, is drawn from three streams, as shown in the model flowsheet in Figure 3 . Its composition is shown in Table 1 . Alternative configurations and improvement prospects It is important to note that alternative configurations can be developed, where CO2 capture is conducted with a chemical absorption amine unit and its recycle to the feed starts after the water knock-out drum of the RWGS section. In any case, the amount of remaining CO2 depends largely on the conversion achieved in the RWGS step and, to a lesser extent, on the effect of the WGS side reaction in the FT step. The final water level is similar to conventional crude oil cargoes. The energy efficiency of the process can be improved with careful thermal integration, steam generation, and the use of elevated temperature electrolysis (solid oxide electrolysis, SOEC) for green hydrogen production. Also, depending on the capacity of the prospective plant, there is an option to use an
electrical heater in the RWGS section (such as Topsoe‘s eREACT) and thus increase the efficiency further while reducing or even eliminating the fuel gas requirements of a fired heater. In that case, the system feed ratio of H 2 :CO 2 also improves. Balance and energy flows A quick look at the mass balance and energy flows in the main equipment reveals the key challenges and obstacles that would need to be overcome for a meaningful deployment of this route. Even for this 30 bpd plant, the production of 89 kg/h of green hydrogen requires a disproportionately large investment in RES and electrolysis. With the current state of technology, approximately a 5 MW electrolysis unit in constant operation would be required, assuming an electrolysis efficiency of 71% (~56.0 kWhel/kg H 2 ). For the amount of power that would be needed for the operation of the electrolysis unit, a sufficient capacity of RES installations would have to be developed. It does not matter if these would be developed near the plant and thus they would have a direct connection to it or if the green origin of power would be secured through the use of PPAs.
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