Aqueous Phase Reforming of Dairy Wastewater for Hydrogen Production: An Experimental and Energetic Assessment
3.2. Hydrogenation + APR
As reported in the literature regarding the hydrothermal treatment of sugary solution, and confirmed in the previous section, the solid formation was ascribed to the presence of high glucose concentration [32,33]. We demonstrated in a previous work with glucose, xylose, and hydrolysate from ethanol production that the pre-hydrogenation of a sugar solution may be a solution to mitigate this undesired pathway, because it would hinder the production of 5-HMF, being a precursor for solid formation [22]. Despite apparent nonsense, this approach is worthy of consideration if a net hydrogen production is obtained. Alvear et al. and Aho et al. used formic acid as a hydrogen donor for the APR of xylose and birch/pine hemicellulose hydrolysates, respectively, without obtaining a significant advantage [34,35]. Oliveira et al. used a similar technological coupling for the APR of maltose for simulating brewery wastewater, observing higher H2 selectivity [29]. To our knowledge, this approach has never been tested before for lactose, and in the following, the results of our investigation are provided.
The typical reaction temperature range for the hydrogenation step is lower than the APR one. This is mainly due to the more favorable kinetics of the aldehyde group for sugars with respect to C-C bond breaking, necessary in the APR mechanism. In accordance with previous works of the authors and similar to the industrial practice, herein it was varied between 180 and 220 °C [22,36]. The consequences of this pretreatment on the APR performance are depicted in Figure 5. Please note that the values reported in the x-axis refer to the temperature used in the pre-hydrogenation step, while the subsequent APR step was still performed at 270 °C. Figure 5A shows the carbon distribution among the different phases. It is clearly observed that the hydrogenation step drastically modified the carbon fate with respect to the APR-alone configuration (Figure 2). Carbon ended up prominently in the gaseous phase, followed by the liquid phase, while the solid residue accounted for approximately 10% of the initial carbon content. This result confirms the effectiveness of the hydrogenation pretreatment to stabilize the feed and hinder undesired pathways. Looking at the gas phase composition (Figure 5B), it was also different with respect to the APR-alone case. There was a gradual increase in the hydrogen concentration with the increase in the hydrogenation temperature: therefore, a gas phase with a higher calorific value was gradually produced. Even other small hydrocarbons were present, such as propane and ethane, which were not detected previously. Finally, carbon monoxide concentration was significantly lower. The improvement of the performance was also reflected by the strong increase in the APR H2 selectivity, which reached 70% (while it was ca. 8% in the one-pot configuration).
In the liquid phase, lactose was not detected, as well as lactitol, which is the direct hydrogenation product of lactose. Galactose was not identified, while a small percentage of glucose was still detected. This is due to the further hydrolysis of lactitol, which can be hydrolyzed into glucose and sorbitol. The formation of the former may be the cause of the slight solid formation. It can be expected that glucose may undergo two possible pathways: on one side, its catalytic (heterogeneous) hydrogenation into sorbitol, which will subsequently lead to hydrogen production via APR; on the other side, its non-catalytic (homogeneous) decomposition into humins.
Focusing on the catalyst textural properties, it was observed that the specific surface area after APR thanks to the pre-hydrogenation conducted at 220 °C was equal to 347 m2/g, which was significantly higher with respect to the stand-alone APR, even if it was still ca. 62% lower than the fresh one. Even the pore volume increased up to 0.528 cm3/g, and the average pore size was more similar to the fresh one, i.e., 5.9 nm.
It is finally important to look at the net hydrogen consumption. Analyzing the results for 2.5 wt.% of lactose for the direct APR, the hydrogen production was equal to 3.4 mol H2/mol lactose, and hence with a 14% H2 yield (normalized with respect to the theoretical stoichiometry which should give 24 moles of hydrogen per mole of lactose). If the pre-hydrogenation step is considered, the hydrogen yield strongly increased up to 87%, but there was not a net hydrogen production, since the hydrogen consumed during the pretreatment was higher than the amount obtained during the APR itself.
These results highlight not only the importance of a pretreatment like the pre-hydrogenation to reduce the solid formation (and hence increase the catalyst lifetime) and improve the hydrogen selectivity, but point out at the same time the need for further investigation, especially in the hydrogenation step, which should be optimized to avoid unselective reactions.
3.3. Energetic Assessment
Given the experimental results reported above, we were finally interested in evaluating the possible coupling of the cheese whey APR with the cheese-making industry itself. Chinese et al. pointed out that a significant amount of energy is used in these processes due to the milk pasteurization step, as well as for heating and refrigeration [37].
For this reason, an energetic assessment was carried out taking into consideration an average energetic consumption and the energy produced from the APR step to see if this coupling is potentially feasible.
An amount of 1 kg/h of cheese production was assumed as the basis. According to Lappa et al., it can be assumed that approximately 9 kg/h of cheese whey are produced [38]. The amount of lactose can vary in such effluent: in our assessment, a sensitivity study considering a range between 2.5 and 10 wt.% was performed. The gross energy production was estimated considering three scenarios: 100%, 50%, and 14% hydrogen yield, where the latter was the one obtained in the APR-alone configuration. As observed in Figure 6, the energy production increased with the initial lactose concentration and the hydrogen yield, as can be expected. This energy was obtained from the lower heating value of the produced gas stream.
In order to estimate the net energy production, it was necessary to estimate the energy duty of the process. With regards to the pressurization step, it varied between 0.05 and 0.09 MJ/h, depending on the final pressure, modified between 60 and 100 bar, respectively. With regards to the energy necessary for heating the feed up to the desired 270 °C, it accounted for 0.45 MJ/h.
However, the main energetic expense regarded the APR reactor. This was due to two main aspects, i.e., the heat of water vaporization and the enthalpy of reaction. Figure 7 reports the results of this evaluation in a contour plot. The reactor duty increased with the lactose concentration because of the endothermicity of the reaction. Furthermore, with a fixed lactose concentration, it decreased with the decrease in the conversion. These outcomes are quite trivial. Far more interesting is the effect of the pressure. It can be observed that increasing the pressure of the system led to a drastic decrease in the energy duty. Let us take as an example the case with 2.5 wt.% lactose, at 60 and 100 bar (Figure 7A). The energy duty moved from 12.1 to 1.8 MJ/h spanning from the lowest to the highest pressure level. This dramatic drop was not caused by the extent of the reaction, but rather by the vaporization of water. In fact, when the gas phase was produced inside the reactor, water evaporated because of its tendency to saturate the just-produced gas bubble. The extent of such evaporation decreased with the increase in the system pressure, since it is directly proportional to the partial pressure of water: consequently, with the total pressure being higher, the molar fraction will be lower, and hence the endothermicity associated with the lower evaporation. The importance of this phenomenon increased also with the extent of the gas yield, since the lower the production, the lower the amount of water necessary for the saturation. Moving towards lower H2 yields (50% in Figure 7B and 14% in Figure 7C), the same trend in the energy requirement was observed: it increased with the lactose concentration, while it decreased with the pressure. The duty was significantly lower because of two reasons. On one side, lower hydrogen yield means lower lactose reactivity, and hence there is a lower influence attributed to the endothermicity of the reaction. At the same time, a lower hydrogen yield means a lower gas production, which is associated with a minor amount of water necessary to saturate the gas phase. Since the evaporation leads to a loss of energy, a lower duty will be required.
Overall, it appeared that increasing the pressure of the system can be advantageous since the small expense for the pressurization was rewarded by the amount saved due to the vaporization of water.
Figure 8 depicts the net energy of the system, calculated as the difference between the gross energy production in the APR step and all the energy duties of the process, i.e., the pressurization, the trim heater requirement, and the reactor duties. As can be observed from the complex geometry of the contour plots, the net result was due to a tradeoff between the higher production at the higher lactose concentration, but also the higher expense because of the water vaporization associated with it. For this reason, by increasing the conversion level, the span of the net energy increased strongly (from ca. 1 MJ/h at 14% H2 yield to ca. 16 MJ/h at 100% H2 yield).
Independently from the H2 yield level, a minimum pressure of 70 bar appeared necessary to have a net energy higher than zero (the only exception was at a high lactose concentration and 100% H2 yield).
Finally, it is necessary to compare these values with the cheese-making industry duties. According to Chinese et al., the range is wide, moving from 0.2 to 3 kWh/kg for electricity and 0.1 to 8 MJ/kg for thermal energy [37]. Considering the scenario with the highest net energy production (10% lactose and 100% H2 yield), APR was able to produce 8 MJ/kg of thermal energy. According to the numbers cited above, it means that it may be sufficient to satisfy entirely the thermal requirement of the most demanding scenario. In the case where the electrical energy is the target, it can be supposed that hydrogen may be fed to a fuel cell. Therefore, assuming 50% efficiency, ca. 1 kWh of electricity may be obtained. Looking at the range of electricity requirement cited above, it means that APR may be able to satisfy up to 33% of the most demanding scenario, which was supposed to be equal to 3 kWh/kg.
Additionally, one further consideration regards the lowest energetic requirement, i.e., 0.2 kWh/kg and 0.1 MJ/kg. In order to satisfy this need, APR should produce approximately 0.8 MJ/h of energy: this value may be reached also with the actual yields (i.e., 14%), if working at a high pressure and lactose concentration (P > 90 bar and lactose > 7.5 wt.%). However, one final remark here is related to the low catalyst stability, which would not allow a stable performance.
