The chemical composition of lemongrass oil used in process 1 is summarized in
Table 1. This raw material reports high content of moisture, and organic compounds as myrcene, undecyne, nerol, geranial, among other [
24]. These substances are the key compounds in the lemongrass extract and are used to guarantee the nanosize of the TiO
2 nanoparticles. The oil content in the lemongrass is about 1.10% of the total mass while the moisture and solid contents are approximately 71.23% and 27.67%, respectively. The above implies that the oil extraction method has to be highly efficient to reach acceptable/required yields. Most of the solid contents in lemongrass is cellulosic biomass, therefore it was assumed (for simulation purpose) that all solid content is cellulose.
3.2. Exergy Analysis of the Routes Simulated
These large-scale production processes can be generally divided into two main stages: nanoparticles preparation and chitosan microbeads formation. Considering the results of simulations and composition of each stream, chemical and physical exergies were estimated.
Table 5 shows the estimated and reported chemical exergies of main components for bio-adsorbent processing routes.
The higher chemical exergies correspond to those compounds with the longest carbon chains in its molecular structures (undecyne, myrcene and chitosan). Chemical exergy for common components as water, NaOH, or acetic acid was found in literature [
25].
Table 6 reports the results obtained for exergy performance indicators for each processing route.
CMTiO2 route presents the lowest exergy efficiency performance with a 0.04%. For CMTiO2-Mag and CMThi processes were obtained a corresponding exergy efficiency of 2.83% and 2.50%, respectively. From a general viewpoint, the performance of the exergy efficiency was significantly low for all bio-adsorbent processes. This result implies that these designs might require technological improvements to reach better exergy and energy performances. In this sense, CMTiO2 requires special attention due to its exergy efficiency shows that almost 100% of the inlet exergy is lost through the operation.
Figure 7 shows the exergy destruction of each bio-adsorbent processing route. The process with higher irreversibilities is CMTiO
2-Mag with an exergy flow of 182,698.34 MJ/h, followed by CMTiO
2 route with destroyed exergy of 144,445.08 MJ/h. The performance of this parameter is congruent respect to exergy efficiencies obtained for all processes. For CMTiO
2 route
, it was found that the stage with the highest irreversibilities was the separation train. This stage is composed of three consecutive centrifuges representing approximately 53.00% of total irreversibilities for this process. The use of other separation technologies may contribute to reduce exergy losses. For the case of CMTiO
2-Mag route, it was obtained that microbeads-drying unit was the stage with the highest irreversibilities with a contribution of 41.05%, followed by washing unit in the separation stage (see “Lav4” in
Figure 5b) with a 24.20%. In the case of CMThi route, drying unit was the stage with the highest exergy destruction representing 92.48% of total irreversibilities. The above results (for all cases) imply that these designs require better/improved separation technologies/stages to avoid several irreversibilities. This also could contribute to obtaining higher exergy efficiencies for each process.
On the other hand, the exergy of residues was higher for CMTiO
2-Mag route, which was expected due to this process presents higher mass inventory respect to the other processing routes.
Figure 8 shows the exergy of residues for each assessed process. CMThi process destroys less exergy due to residues with a flow of 33,654.48 MJ/h, while for CMTiO
2 and CMTiO
2-Mag were obtained exergy flows of 144,405.08 MJ/h and 182,698.34 MJ/h, respectively.
Finally, the exergy of utilities was estimated for each processing route.
Figure 9 shows the comparison of this parameter for each bio-adsorbent processing route. The results for exergy of utilities present a similar performance for the three alternatives, obtaining an exergy flow of 91,033.74 MJ/h for CMTiO
2, 88,030.46 MJ/h for CMTiO
2-Mag, and 105,964.41 MJ/h for CMThio. For the case of CMTiO
2-Mag route, drying unit was the most significant stage representing an around 99% of the total exergy by utilities for this process. The above result indicates that this unit probably has important energy requirements that implies a high demand of industrial utilities. In this sense, the application of process optimization techniques could contribute to decrease the energy requirements, or obtain a better energy distribution. For CMTiO
2 process was found that hydrolysis reactor is a critical stage due to the most of exergy utility is consumed in this unit with a contribution of 47.22% of the total. It is explained by studying the thermodynamics of this reaction (hydrolysis of TTIP) because it is highly exothermic, thus, a cooling system is needed to maintain the reaction temperature constant.
As described by CMTiO2-Mag route, for CMThio alternative, the drying stage was also the most critical unit for exergy of utilities parameter with an exergy flow of 105,942.41 MJ/h which represents almost a 100% of the total. This result confirms the described behavior for exergy destruction performance which was previously explained for this bio-absorbent route.