# Toward Sustainability of the Aqueous Phase Reforming of Wastewater: Heat Recovery and Integration
## 2 Materials and Methods  
Previous experimental works [14; 24] showed that the concentration of organic matter in the wastewater subjected to APR must be maintained below 5% in order to maximize substrate conversion and hydrogen production. On the other hand, low-concentration feedstocks showed the disadvantage of increased heat demand in the reactor, mainly due to preheating needs and higher water evaporation. In addition, organic matter concentration below 5% could be considered representative of wastewater streams generated in the food and beverage industries. Therefore, the case study considered for the simulated APR process was the treatment of a 180 m\({}^{3}\)/h stream containing 1-2%w of carbohydrate-like organic matter. The concentration of the wastewater was considered as a process variable. Sorbitol was selected in Aspen HYSYS databases as a substrate because it has been widely studied as an organic matter model compound in several works on APR [14; 24; 27], facilitating the comparison of results. The reactions involved in APR are mainly dehydrogenation and the breakage of C-C bonds of organic compounds to produce hydrogen and carbon monoxide, water-gas shift (WGS), methanation of CO/CO\({}_{2}\) with H\({}_{2}\) and other minor reactions such as Fischer-Tropsch, dehydration and hydrogenation [24; 28; 29]. Table 1 shows the main reactions involved in the APR process. Attending to both reaction enthalpy and the contribution to the global reaction scheme, two reactions were taken into consideration for the purpose of this work: reforming and methanation, shown in Equations (1) and (2), respectively, for the case of sorbitol. In addition, these two reactions represent a route for the transformation of substrate into gas products. \[\mathrm{C_{6}H_{14}O_{6}+6\cdot H_{2}O\leftrightarrow 6\cdot CO_{2}+13\cdot H _{2}\qquad\Delta H^{\circ}=229.9\ kJmol^{-1}} \tag{1}\]  
\[\mathrm{CO_{2}+4\cdot H_{2}\leftrightarrow CH_{4}+6\cdot H_{2}O\qquad\Delta H^{ \circ}=-163.0\ kJmol^{-1}} \tag{2}\]  
As expressed in Equation (1), the so-called reforming reaction includes both steps of the whole reaction: decomposition of sorbitol to H\({}_{2}\) and CO and further oxidation of CO to CO\({}_{2}\) through water-gas shift. For that reason, WGS was not considered as an independent reaction in the study. Aspen HYSYS V12 was used as a calculation tool. For thermodynamic property estimation, the Peng-Robinson-Strylek-Vera (PRSV) model was selected to reflect the non-ideal behavior of the vapor/gas phase. This equation of state has previously shown good results in the property estimation of the compounds involved in APR [20; 23]. Figure 1 shows a block flow diagram of the proposed APR process, in which heat consumption and possible heat production/recovery steps are indicated. This process scheme allowed a low-limit analysis (i.e., most limiting situation) for the case where the gas stream was valorized through combustion, with no other value recovery. \begin{table}
\begin{tabular}{l l} \hline
**Reforming** & \(\mathrm{C_{n}H_{2n+2}O_{n}+nH_{2}O\Leftrightarrow nCO+(2n+1)H_{2}}\) \\ \hline
**Water–Gas Shift** & \(\mathrm{CO+H_{2}O\Leftrightarrow CO_{2}+H_{2}}\) \\ \hline
**Methanation** & \(\mathrm{CO_{2}+4H_{2}\Leftrightarrow CH_{4}+2H_{2}O}\) \\  & \(\mathrm{CO+3H_{2}\Leftrightarrow CH_{4}+H_{2}O}\) \\ \hline
**Fischer–Tropsch Hydrogenation Dehydration** & \(\mathrm{(2n+1)H_{2}+nCO\rightarrow C_{n}H_{2n+2}+nH_{2}O}\) \\  & \(\mathrm{2 nH_{2}+nCO\rightarrow C_{n}H_{2n}+nH_{2}O}\) \\  & \(\mathrm{C_{n}H_{2n+2}O_{n}+nH_{2}\Leftrightarrow C_{n}H_{2n}+H_{2}O}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Main reactions involved in the APR process [23; 27; 28]. Figure 1: APR process block diagram. The simulated process comprised the following steps:  
* Feed conditioning: temperature and pressure of wastewater was increased to usual operating conditions in APR: 220 \({}^{\circ}\)C and 30 bar in the current work [3,4,23,30].