# Toward Sustainability of the Aqueous Phase Reforming of Wastewater: Heat Recovery and Integration
## 3 Results and Discussion  
### Process Heat Balance at Different APR Operating Conditions  
Figure 4 shows the flow and composition (dry basis) of the gas stream produced for different values of reforming and methanation reaction conversion for wastewater containing 1% of organic matter. At any reforming conversion value, the lowest H\({}_{2}\) production was obtained for higher extension of methanation reaction due to hydrogen consumption in this reaction. On the contrary, the increase in methanation conversion caused an increase in CO\({}_{2}\) concentration in the gas stream: in this case, much slighter than that observed for H\({}_{2}\) concentration. The differences in the stoichiometry of reforming and methanation reactions resulted in lower gas flow at low reforming conversion and high methanation conversion, even though the stream composition was not altered. As explained previously, the evaporation of water in the reactor played an important role in overall heat balance. The molar flow of evaporated water in the reactor vs. reforming and methanation conversion is plotted in Figure 5. Due to stoichiometry of the reactions involved, at constant pressure, the gas-stream molar flow was higher for high reforming conversion. Higher total gas flow resulted in higher water flow because the water molar fraction in this stream was conditioned by equilibrium. Figure 4: Gas production and composition (dry basis) at the APR reactor. Figure 3: Aspen HYSYS process flow diagram used for heat integration. As the evaporation of water is the most important contribution to energy demand in the reactor, the influence of operating pressure in the flow of evaporated water is relevant. Evaporated water vs. reactor operating pressure values for different reforming reaction conversion are plotted in Figure 6. At every pressure and reforming conversion, a mean value of evaporated water with methanation conversion in the interval between 0 and 55% was considered. A decrease in water molar flow could be observed when pressure was increased up to 50 bar. A moderate increase of pressure from 30 to 35 bar led to a 50% reduction in water flow. From 45 bar, a minor effect of pressure could be observed. However, an operating pressure of 50 bar was selected to minimize energy demand from the evaporation of water, since the compression of wastewater was a minor contribution to overall energy demand. These results are in concordance with those reported by [PERSON] et al. [23]; the evaluation of APR of sorbitol by simulation with Aspen Plus in the 30 to 50 bar range suggested that the reactor operating pressure must be maintained over 40 bar to minimize the heat reactor duty. Figure 7 shows HC for the APR process and its dependence on the conversion of reforming and methanation reactions. The negative values obtained for the HC parameter (\(-6.7\) to \(-13.5\%\)) indicated a net heat excess within the whole range considered for reforming and methanation reaction conversion. A higher excess was obtained at a higher extension of reforming reaction due to a larger production of gas. Net heat excess was also higher for high methanation conversion due to the higher heating value of methane in  
Figure 5: Molar flow of water evaporated in APR reactor vs. reforming and methanation conversion. Figure 6: Molar flow of water evaporated in APR reactor vs. operating pressure. comparison to hydrogen. The steepness of the contour map in the y-axes indicated that methanation has a higher impact in heat excess than reforming. These results indicated that the orientation of the APR process, i.e., towards hydrogen or towards alkanes, has to take into consideration whether the produced gas will be sold as a commodity or valorized in the APR plant. In the latter case, higher HC is achieved by promoting alkane production. Finally, the influence of wastewater concentration in the heat balance was studied using different reactor operating pressures in the 30-50 bar interval (every 5 bar).