**3. Results**

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 TiO2 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.

**Table 1.** Chemical composition of lemongrass dry basis.


#### *3.1. Processes Simulation Results*

3.1.1. Simulation of Large-Scale Production of Chitosan Microbeads Modified with TiO2 Nanoparticles via Green Chemistry

Process simulation flowsheet of large-scale production of chitosan microbeads modified with TiO2 nanoparticles is shown in Figure 4. And the pressure, temperature, composition and flows of main process streams are shown in Table 2. As is described in Section 2.2, the system is composed of three sections: (1) Lemongrass oil extraction; (2) TiO2 nanoparticles synthesis, and (3) CMTiO2 production. For subprocess (1), stream 1 represents the inlet flow of the main feedstock (lemongrass). This stream is sent to washing and drying stages for cellulosic material removal and moisture reduction, respectively. After feedstock cleaning, the material is sent to a crusher unit for size-reduction. The above is performed with the aim of increasing the surface contact between the lemongrass and the extraction media (water). The liquid-solid extraction (infusion stage) is developed by a heating-sedimentation stage where the oil extract is separated in the upper site of the unit while the residual solids remain at the bottom. The oil extract is obtained with high moisture content (99% w/w) so the stream passes

through an evaporation unit to reduce the water content close to 3% w/w. Finally, the oil extract is cooled until 28 ◦C, and it is stored. Subprocess (2) starts with the preparation of TTIP solution and the hydrolysis reaction where the TiO2 nanoparticles are synthesized (see stream 39). In the case of subprocess (3), this stage starts with the production of the chitosan gel considering inlet flow according to the composition of stream 43. Stream 39 is added to the mixing tank where the CMTiO2 are formed maintaining the required proportion of 1:1 for Chitosan and TiO2, respectively. Finally, the product obtained with a flow of 232.03 kg/h according to stream 56.

```
(a)
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(**b**)

**Figure 4.** *Cont*.

**Figure 4.** Process flowsheet of large-scale production of chitosan microbeads modified with TiO2 nanoparticles via a green chemistry method: (**a**) Lemongrass oil extraction (**b**) TiO2 nanoparticles synthesis; (**c**) chitosan microbeads production.


**Table 2.** Main process streams for Chitosan microbeads modified with TiO2 nanoparticles.

3.1.2. Simulation of Large-Scale Production of Chitosan Microbeads Modified with TiO2 and Magnetite Nanoparticles

The simulation for the second processing route of bio-adsorbents production was performed based on general mass/energy balances, and operational conditions obtained from literature and lab experiments. Figure 5 shows the simulated process flowsheet of large-scale production of chitosan microbeads modified with TiO2 nanoparticles and magnetite. Table 3 summarizes the main streams and operational conditions of this process. As it was described, this designed system is constituted by two main subprocesses: (1) Magnetite nanoparticles synthesis, and (2) chitosan microbeads production (with nanoparticles modification). For magnetite nanoparticles synthesis, stream 1 and 2 represent the inlet flow of main feedstocks: FeCl2·4H2O and FeCL3·6H2O, respectively. The reaction between these iron chlorides and NaOH generates magnetite (desired product) along with hematite, water and sodium chloride. For simulation purpose, it was assumed that all produced hematite is magnetite. After reaction stage, the stream is sent to separations processes where all impurities are removed from

the product. The magnetic properties of these nanoparticles allow separating them using a magnetic field or a magne<sup>t</sup> [19]. Finally, the dried and purified magnetite nanoparticles are sent to the second subprocess, the chitosan microbeads modified with the nanoparticles (TiO2 and magnetite) are sent to purification stage where the desired product is obtained dried and with high purity with a mass flow rate of 307.86 kg/h.

**Figure 5.** Process flowsheet of Large-scale production of chitosan microbeads modified with TiO2 and magnetite nanoparticles: (**a**) Magnetite nanoparticles synthesis; (**b**) chitosan microbeads with modifications subprocess.


**Table 3.** Main process streams for chitosan microbeads modified with TiO2 and magnetite nanoparticles.

3.1.3. Simulation of Large-Scale Production of Chitosan Microbeads Modified with Thiourea

The simulation of the third processing route was developed according to the subprocess stage for chitosan microbeads production described for CMTiO2 and CMTiO2-Mag processes. In this case, the modification of the microbeads does not require the synthesis of any nanoparticles because the thiourea is introduced to the process as an available raw material. After this stage, the stream is sent to microbeads production and purification stages. Table 4 reports the main mass flows and operational conditions of this process. Figure 6 shows the simulated process flowsheet of large-scale production of chitosan microbeads modified thiourea. Finally, in this process alternative are produced 155.23 kg/h of chitosan microbeads modified with thiourea according to stream 13.

**Table 4.** Main process streams for chitosan microbeads modified with thiourea.


#### *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.


**Table 5.** Chemical exergy of main components for bio-adsorbent production processes.

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.

**Table 6.** Results for exergy performance of 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 CMTiO2-Mag with an exergy flow of 182,698.34 MJ/h, followed by CMTiO2 route with destroyed exergy of 144,445.08 MJ/h. The performance of this parameter is congruen<sup>t</sup> respect to exergy efficiencies obtained for all processes. For CMTiO2 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 CMTiO2-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.

**Figure 7.** Comparison of exergy destruction for each processing route.

On the other hand, the exergy of residues was higher for CMTiO2-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 CMTiO2 and CMTiO2-Mag were obtained exergy flows of 144,405.08 MJ/h and 182,698.34 MJ/h, respectively.

**Figure 8.** Comparison of exergy of residues for each processing route.

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 CMTiO2, 88,030.46 MJ/h for CMTiO2-Mag, and 105,964.41 MJ/h for CMThio. For the case of CMTiO2-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 CMTiO2 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.

**Figure 9.** Comparison of exergy utilities for each processing route.

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.
