## 2 Development of a Process Model for Two-Stage Bio-Oil Hydrogenation (HDO)
### Process Setup  
As discussed above, the whole HDO process was divided into two main stages which included a mild hydrogenation stage and a subsequent deep hydrogenation stage. The system flow diagram for the bio-oil hydrogenation process is shown in Figures 1 and 2. The whole HDO process consists of seven unit operations: (1) hydrogen preheating; (2) bio-oil preheating; (3) mild hydrogenation reactor; (4) mild hydrogenation product solid-liquid separation; (5) mild hydrogenation product liquid-gas separation; (6) deep hydrogenation reaction; and (7) product cooling and pressure separation.  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Properties** & **Bio-Oil** & **Mild-Stage Production** & **Deep-Stage Production** \\ \hline Density (kg L\({}^{-1}\)) & 1.1–1.2 & 1.2 & 0.84 \\ Viscosity (c St at 25 \({}^{\circ}\)C) & 19–25 & 7.4 & 2.8 \\ Water content (wt \%) & 20–30 & 11.4 & 0.02 \\ Higher heating value (MJ kg\({}^{-1}\)) & 16–18 & 29.5 & 45.6 \\ pH & 2.7 & 4.2 & – \\ Elemental analysis (wt \%, a. f) & & & \\ C & 44–46 & 78.3 & 88 \\ H & 6–7 & 9.6 & 11.5 \\ O & 46–48 & 11.4 & 0.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Properties of the feedstock and products of the mild and deep stages [9; 11; 12].  
Figure 1: Simulation procedure of the bio-oil hydrogenation (HDO) process.  
Figure 2: Bio-oil hydrogenation reaction process.  
The organic compounds of raw bio-oil (BIO-OIL), hydrogen (H2-1), and the catalyst (CATALYST) are treated as input parameters under normal conditions that are first preheated by the stream (MIXTURE3) and then continuously fed into the first stage HDO, or the mild hydrogenation reactor (RCSTR). According to the reported research and experimental data, operating temperature for the mild hydrogenation reactor of raw bio-oil is set at 250 \({}^{\circ}\)C, 280 \({}^{\circ}\)C, and 300 \({}^{\circ}\)C, respectively. The kinetic reactions added to this RCSTR reactor include the main bio-oil hydrogenation reaction plus side reactions of the catalyst coking [12].  
The product stream (MIXTURE1) from mild hydrogenation reactor first enters a solid-fluidized bed separator (SEP-1) to remove the used catalysts (with soluble and insoluble coke deposits), and the liquid product then flows into a three-phase separator (SEP-2) to be separated into: (1) water and aqueous phase organic compounds (AQO) formed during the hydrogenation reaction; (2) the unreacted bio-oil with an oil-phase organic compound (ODF); and (3) a mixture of gaseous products and unreacted hydrogen (GAS-1). The organic compound (ODF) and unreacted hydrogen (H2-2) are separated from stream (GAS-1) and then fed into the secondary hydrogenation reactor (RSTOIC) for the deep hydrogenation. The product stream (MIXTURE3) is separated after the heat exchanger. Excessive hydrogen (H2-C) from the deep hydrogenation is continuously recycled and fed to the mild reactor. The heat recovery of the process is achieved by a series of heat exchangers between process streams and feedstock, and the only unit operation that requires external heat supply within the entire process is the secondary hydrogenation reactor.