*4.4. Hydrocracker*

Figure 5 displays the modeled hydrocracker. The long-chain hydrocarbons were brought together with several re-circulations in which the required hydrogen was also added. The hydrogen was produced using water electrolysis and the total amount adjusted so that the mass fraction of hydrogen after mixing was around 8% (mass). This value was chosen as a compromise between the suppression of both soot formation and catalyst deactivation (see Section 3.3) and minimization of the required additional electrolysis performance.

**Figure 5.** Excerpt from the process flow diagram: hydrocracker.

The mixed material flow was preheated and fed into the hydrocracker. The hydrocracker was modeled as a two-stage isothermal fixed bed reactor, whereby the two fixed beds were each modeled by a RYield block (i.e., a yield reactor). To ensure ideal hydrocracking, an operating pressure of 50 bar and a temperature of 350 ◦C were selected and a conversion per bed of 60% assumed. The yields of the respective cracking products were calculated in calculator blocks connected to Excel files, based on the uniform distribution of the cracking products of the ideal hydrocracking, which is described in more detail in Section 3.3 The heat required for the cracking process was provided by the exothermic reformer (see Section 4.6). The product stream was then separated into gas and liquid phases, and the long-chain hydrocarbons that had not been cracked were returned to increase the conversion of the hydrocracker. The gas stream was cooled in several stages for better energy utilization and the middle distillates were separated off, expanded to 30 bar, and passed to the carrier steam distillation. The gas stream separated in B-5 consisted mainly of hydrogen and *C*7−-hydrocarbons, H2O and CO2, and was partly returned to the hydrocracker in order to reduce the need for fresh hydrogen and thus the required performance of the water electrolysis system. A purge flow of 10% was expanded to 30 bar and fed to the reformer to prevent CO2 and short-chain hydrocarbons from accumulating in the hydrocracker.

#### *4.5. Carrier Steam Distillation*

The feed streams of the carrier steam distillation, which can be seen in Figure 6 were the product stream of the hydrocracker unit and the separated middle distillates from the product separation following the Fischer–Tropsch synthesis (see Figure 4). As the NRTL– RK material data model was to be used in the distillation column, which is not suitable for calculating mixtures with pseudo components, the remaining pseudo components (approximately 0.6% by weight) were assigned to the real component C29H60. A RYield block (RY-3) was used for this. In the next step, the feed was directed into a so-called pre-flash drum (B-6), in which part of the material flow was evaporated by releasing it to 1 bar. The use of a pre-flash drum is recommended in the literature [48] for the separation of some of the gases and thus simplify product separation in the column. In the simulation, almost all of the remaining CO2, hydrogen, and methane as well as most of the *C*2- to *C*5-hydrocarbons were separated by the flash evaporation. The liquid phase was preheated by heat integration and fed to the vaporizer (furnace) of the column, which was modeled with a Petrofrac block (K-1). The energy required to evaporate the feed was provided by the exothermic reformer (see Section 4.6). Steam was fed to the column via the sump, which reduced the partial pressure of the hydrocarbons and thus lowered the boiling temperatures. The column had two strippers with four stages and their own steam supplies, each for kerosene and diesel. There was no need for a side exhaust for gasoline, which is common in industrial applications. The number of stages in the column as well as the position of the feed and the side draws were determined using a sensitivity analysis in such a way that as much kerosene and diesel as possible was produced, but the requirements of the standards (see Table 2 were still met. The requirements of the standards were taken into account in two design specs through which the specified T50 temperature of the respective product flow was set by automatically varying the amount of kerosene or diesel withdrawn. At this point, it should also be noted that the use of pumparounds was dispensed with. Pump-arounds, through the recirculation of material flows within the column, represent a possibility for recovering thermal energy and reducing the column's energy requirement [48]. In the simulation, however, the use of pump-arounds led to considerable convergence problems. The bottom product, consisting of long-chain hydrocarbons, was returned to the hydrocracker, and the top product, together with the gas flow from the pre-flash drum, was compressed to 30 bar and sent to the reformer. A detailed analysis of the fuels produced is presented in Section 5.1.

**Figure 6.** Excerpt from the process flow diagram: carrier steam distillation.

**Table 2.** Characteristic values of the synthetic kerosene and diesel produced.


<sup>1</sup> Probably overrated; for an explanation see text.

#### *4.6. Reformer*

The reformer was used to convert the unusable gases into synthesis gas and thus to recycle them. The corresponding section of the simulation flow diagram is shown in Figure 7. The gas streams available at 30 bar from the distillation column, the hydrocracker and the product separation were combined with the addition of water. The amount of water added was automatically set using a design spec so that there was an H2O/C ratio of 0.6 after mixing. The feed stream was then heated to 900 ◦C with the waste heat from the product stream and passed into the reformer. The reformer was modeled with an RGibbs block (i.e., an equilibrium reactor). Due to the high operating temperature, the short residence time and gas phase reaction, the kinetics can be neglected [54] and the product composition can be determined by minimizing the Gibbs energy. The addition of oxygen was automatically controlled by a design spec so that a product temperature of 950 ◦C results. The reformer was exothermically-operated in order to provide the high-temperature heat required for the hydrocracker and the distillation column through the waste heat. The additional heat to be

produced was taken into account by a cooling capacity imposed on the reformer from the outside. The reformed synthesis gas was cooled and then mixed with the synthesis gas from the co-electrolysis unit and fed into the Fischer–Tropsch reactor.

**Figure 7.** Excerpt from the process flow diagram: reformer.

#### **5. Results from Process Analysis**

In this section, the simulation results of the power-to-fuel process presented in Sections 3 and 4 are presented. First, it was determined whether the diesel and kerosene fuels produced met the requirements of the relevant standard. The composition of the fuels was considered, the boiling curves were calculated and compared with the real boiling curves of kerosene and diesel, and finally the fuel property data were checked. Then, the process balance was considered and the PtL efficiency of the process established. The next step was to examine the extent to which heat extraction for the provision of thermal energy for CO2 separation is possible and useful. Finally, the developed process was compared with other power-to-fuel or PtL processes.
