**1. Introduction**

Nowadays, there is an increasing interest in developing sustainable and green-chemistry-based ways to synthesize novel materials for applications in emerging industries. Industrial wastewater treatment is a major global problem, mainly due to the restricted quantities of water that can be re-used along with the high cost of purification. In recent years, appropriate technologies for the treatment of wastewater and other effluents have raised grea<sup>t</sup> interest due to more strict laws and regulations [1]. The literature reports a variety of processes to water treatment and there are a lot of technologies used for this purpose. Among these methods, adsorption stands out as an effective, efficient and low-cost technology [2]. The adsorbents may be of mineral, organic or biological origin and are selected according to their applications. Polymeric adsorbents have been widely used to remove organic pollutants from industrial wastewater, but in recent years, the use of adsorbents produced from biomasses, organic residues and biopolymers have attracted interest due to its high availability and environmental issues related to disposal of residual biomasses and wastes [3]. In this sense, chitosan-based adsorbents have been evaluated showing high removal yield for several substances such as heavy metals, polycyclic aromatic hydrocarbons, among others [4].

Green chemistry is related to a new idea which has developed in the industry and research contexts as a natural evolution of pollution-prevention strategies [5]. The use of these types of processes brings the concept of developing chemical plants that can reduce waste generation and the resource demands. Recently, the green chemistry concept is also related to those processes that use smaller amounts of energy maintaining process yield/efficiency while providing reasonably market-valuable products [6]. On the other hand, nanotechnology is an innovative science/field associated with the manipulation of components and materials at the atomic/molecular scale and explains the behavior of these substances when used on the nanoscale. Recently, metal oxide nanoparticles have attracted interested by their potential application in different fields [7].

Modification of bio-adsorbents produced from natural polymers as chitosan microbeads with chemical species as thiourea or magnetite nanoparticles gives to these materials a higher adsorption capacity and selectivity to several heavy metals and liquid hydrocarbons, and higher efficiency in recovery processes via magnetic field application, which was studied at the lab-scale [8], however, the behavior at large scale of these emerging technologies is unknown, so the computer-aided simulation of these technologies at large-scale is relevant for further industrial applications [9].

Biomass-based processing technologies (and many chemical processes) commonly require huge amounts of energy and water, therefore, there are some published studies addressing the application of exergy analysis to measure the process performance from an energy viewpoint. Ojeda et al. [10] applied the exergy analysis to assess process alternatives for producing ethanol from lignocellulosic biomass. Peralta-Ruiz et al. [11] used exergy analysis as a tool for screening process alternatives for microalgae oil extraction. Aghbashlo et al. [12] applied an exergy analysis of a lignocellulosic-based biorefinery along with a sugarcane mill for simultaneous lactic acid and electricity production. Meramo-Hurtado et al. [13] developed an exergy analysis of bioethanol production from rice residues, the results for this study showing that pretreatment stage was the unit with the lowest exergy efficiency. They pointed out that this subprocess can be potentially improved to obtain a better energy performance. Restrepo-Serna et al. [14] developed a study related to the energy efficiency of various biorefinery schemes using sugarcane bagasse as raw material. They performed the assessment based on process simulation and exergy analysis, so it is shown in these studies that exergy analysis can be considered a useful evaluation instrument for the diagnosis of novel technologies involving sustainability goals.

In this work, three production processes for chitosan microbeads modified with nanoparticles were evaluated using exergy analysis as an instrument for screening design alternatives and as a decision-making tool for evaluation of novel technologies from energy viewpoint. Indicators as exergy efficiency, total irreversibilities, exergy utilities, and exergy destruction were determined for each processing alternative evaluated.

#### **2. Materials and Methods**

In this study, chitosan obtained from shrimp exoskeletons is used as feedstock to synthesize bio-adsorbents which are further modified with nanoparticles. Chitosan is a polymer present in many organic structures in Nature. It is commonly obtained from crustacean wastes through chitin deacetylation techniques [15]. The design and modeling of the processing routes for bio-adsorbents production is developed using a computer-aided process engineering software. This tool requires process information as mass and energy balances, operational conditions, reaction yields, among others. It is important to point out that the information required for the simulations were obtained from literature and experimental results previously published by the authors [16]. Another issue that must be considered is the input of an adequate thermodynamic model which allows estimating most accurate physical-chemical properties of the substances. The simulations give the extended energy and mass balances considering the operational conditions and processing routes of each design. The above parameters are important due to their uses to perform the exergy analysis for each alternative. Finally, exergy indicators are evaluated to compare each chitosan microbeads processing technology with the aim to identify improvement opportunities and screen the most suitable design from an exergy basis.

#### *2.1. Processes Simulation*

Process simulation mainly implies selecting chemical components used in the process, choosing an appropriate thermodynamic model, setting processing capacity, using suitable operating units and setting up input conditions such as mass flow rates, temperature, pressure, among others [17]. For the simulation of large-scale production processes of chitosan microbeads was used the industrial process software Aspen Plus, the chemical species required for the simulations were taken from the software database. For those compounds which are not available in the software database, molecules were created using the Aspen Property Estimator based on the properties reported for these species in the literature [18]. It was necessary to create components such as titanium isopropoxide and chitosan during the development of this work. The thermodynamic model Non-Random Two Liquids (NRTL) was selected for process simulations taking into account its good performance in the prediction of thermodynamic properties for polar/non-polar mixtures.

2.1.1. Process 1: Production of Chitosan Microbeads Modified with TiO2 Nanoparticles via Green Chemistry

The production process of chitosan microbeads modified with TiO2 nanoparticles (CMTiO2) is developed through three specific subprocesses. The first subprocess is the lemongrass oil extraction where organic compounds such as myrcene, undecyne, neral, among others are obtained based on a green chemistry synthesis. These substances are used as surfactants to guarantee the nanosize of the particles. Lemongrass is first pretreated for cellulosic material removal, then it is sent to drying and crushing to reduce the particle size. Thus, the lemongrass oil is obtained in a liquid-solid extract. The second subprocess is the synthesis of TiO2 nanoparticles. This stage uses titanium tetraisopropoxide (TTIP) as a precursor for TiO2 nanoparticles through hydrolysis reactions. Propanol is formed as a by-product of the reaction. Hydrolysis reaction stoichiometry is described as follows:

$$\text{Ti(OC}\_3\text{H}\_7)\_4 + 2\text{H}\_2\text{O} \rightarrow \text{TiO}\_2 + 4\text{C}\_3\text{H}\_7\text{OH}^-$$

As mentioned earlier, this stage involves the formation of TiO2 nanoparticles via hydrolysis of TTIP. This substance is first diluted to a concentration of 100 mM under controlled conditions and sent to the hydrolysis reactor system. On the other hand, the oil extract (obtained from the first subprocess) is added and mixed with the main stream in the reactor [9]. Figure 1 shows the process diagram for the large-scale production of chitosan microbeads modified with TiO2 nanoparticles. After TiO2 synthesis the main stream is sent to a separation train composed of three washing units where the pH is stabilized. In the second unit, ethanol is used for washing, while for the first- and third-units water is used. In this separation stage, moisture content of the product is reduced. Finally, the product is sent to a drying unit to obtain the bio-adsorbents with water content equal to zero. The third subprocess begins after TiO2 nanoparticles formation. Chitosan microbeads are prepared from chitosan (main raw material). In the first tank, a chitosan dissolution is prepared at a concentration of 2 w/v %. The solution is mixed in a second vessel with acetic acid (2 w/v %) obtaining a chitosan gel solution. This gel mixture is sent to another mixing stage where TiO2 nanoparticles are added. The mass ratio between chitosan and TiO2 nanoparticles is 1:2. In this stage, chitosan microbeads modified with TiO2 nanoparticles are formed. For product formation is necessary to set alkaline conditions, hence, NaOH (at a concentration of 2.5 M) is used to reach the pH required [16]. Finally, the microbeads are physically mixed through an ultrasound-assisted agitation system. It is important to point out that the above procedure is developed at a temperature of 28 ◦C. The resulting microbeads have high moisture content so it is necessary to perform a separation stage where a train of washing and drying units are used to purify the bio-adsorbents. For all process units the operational pressure is 1 atm.

**Figure 1.** Process diagram of large-scale production of chitosan microbeads modified with TiO2 nanoparticles.

#### 2.1.2. Process 2: Production of Chitosan Microbeads Modified with TiO2 and Magnetite Nanoparticles

Production of chitosan microbeads modified with TiO2 and magnetite nanoparticles (CMTiO2-Mag) is divided into two main subprocesses. Subprocess (1) is related to the synthesis of magnetite nanoparticles while subprocess (2) to the microbeads formation. For magnetite nanoparticles synthesis, the first stage implies the preparation of iron oxides solutions (FeCl3·H2O and FeCl2·4H2O) at a temperature of 301.15 K [19]. Subsequently, these solutions are mixed and sent to a heat exchanger unit to reach a temperature of 353.15 K. Thus, the resulting stream is fed into the iron oxides reactor where magnetite is produced along with NaCl and H2O. In order to form the nanoparticles, NaOH is added at a concentration of 2% v/v. The outlet stream of the reactor is cooled to 301.15 K, and sent to a separation stage. In this stage, the nanoparticles are separated using magnetic fields. The above allows removing non-ferrous material. To reach higher purity, the magnetite nanoparticles are fed into a separation train composed of three consecutive washing units (an analog system used for CMTiO2 process). Finally, the main stream is sent to a furnace unit where the microbeads are completely dried. For the final product, it is desired to obtain a mass ratio for chitosan, TiO2 and magnetite of 2:1:1, respectively. On the other hand, second subprocess CMTiO2-Mag alternative is similar to subprocess (3) of CMTiO2 design described in Section 2.1.1 but the modification with magnetite is incorporated after TiO2 nanoparticles modification. Thus, it is important to mention that these modifications are related to physical mixing processes. Figure 2 shows the process diagram for the large-scale production of chitosan microbeads modified with TiO2 and magnetite nanoparticles.

**Figure 2.** Process diagram of large-scale production of chitosan microbeads modified with TiO2 and magnetite nanoparticles.

#### 2.1.3. Process 3: Production of Chitosan Microbeads Modified with Thiourea

The third process evaluated consists in the large-scale production of chitosan microbeads modified with thiourea. In this process, thiourea is mixed with the main stream after chitosan gel formation. The process follows to the microbeads formation using diluted NaOH. Finally, the microbeads are purified through washing and drying stages. The mass ratio for chitosan and thiourea is 1:1. Figure 3 shows the process diagram for the large-scale production of chitosan microbeads modified with thiourea

**Figure 3.** Process diagram of large-scale production of chitosan microbeads modified with thiourea.

#### *2.2. Exergy Analysis*

Thermodynamics-based tools such as energy analysis, exergy analysis, emergy analysis, among others have been applied for evaluation of industrial systems and thermal energy storage processes [20]. The first and second law of thermodynamics serve as a theoretical basis for energy analysis, along with the calculation of energy efficiencies for the studied processes. However, an energy balance not necessary provides information associated with the energy losses or quantifies the quality of the mass and energy streams of the evaluated routes. Exergy analysis is presented as an alternative tool which allows surpass the limitations of the laws of thermodynamics for a desired process [21]. Exergy analysis also shows the source of energy degradation in a process and can help to optimize an operation, a technology or a processing unit [22]. In addition, exergy analysis allows evaluating several process technologies to improve the design of a process. The above means that the exergy analysis is an appropriate tool to assess novel technologies for any chemical process. The exergy is defined as the maximum theoretical useful work that could be obtained from a system that interacts only with the environment. Considering that the exergy of a system depends on the selected state of reference, it is usual to choose standard environmental conditions as the reference state. Based on steady-state conditions, four balance equations have to be addressed to determine work/heat interactions. The first equation is the mass/matter conservation principle given by Equation (1), the second equation refers to the first law of thermodynamics given by Equation (2). The third equation refers to the second law of thermodynamics given by Equation (3):

$$\sum\_{\mathbf{i}} (\mathbf{m}\_{\mathbf{l}})\_{\mathbf{in}} = \sum\_{\mathbf{i}} (\mathbf{m}\_{\mathbf{l}})\_{out} \tag{1}$$

$$\sum\_{\mathbf{i}} (\mathbf{m}\_{\mathbf{l}} \mathbf{h}\_{\mathbf{i}})\_{\mathbf{in}} = \sum\_{\mathbf{i}} (\mathbf{m}\_{\mathbf{l}} \mathbf{h}\_{\mathbf{i}})\_{\text{out}} + \mathbf{Q} - \mathbf{W} = \mathbf{0} \tag{2}$$

$$\sum\_{\mathbf{i}} (\mathbf{m}\_{\mathbf{i}} \mathbf{S}\_{\mathbf{i}})\_{\mathbf{in}} = \sum\_{\mathbf{i}} (\mathbf{m}\_{\mathbf{i}} \mathbf{S}\_{\mathbf{i}})\_{\text{out}} + \sum\_{\mathbf{i}} \frac{\mathbf{Q}\_{\mathbf{i}}}{\mathbf{T}\_{\mathbf{i}}} + \mathbf{Q} - \mathbf{W} = \mathbf{0} \tag{3}$$

A fourth equation is addressed to formulate the exergy balance for around the system during a finite time interval given by Equation (4):

$$\text{Exergy input} - \text{Exergy output} - \text{Exergy consumption} = \text{Exergy accumulation} \tag{4}$$

Exergy consumed is the product between the entropy generated and the environmental temperature, as follows in Equation (5) [9]:

$$\text{Exergy consumed} = \text{Enviornmental temperature} \times \text{Entropy} \tag{5}$$

The Equation (4) can be expressed in terms of the mass exergy entering or leaving the system (Ex*mass*,*in* and Ex*mass*,*out*), exergy flow by heat (Ex*Heat*), exergy flow by work (Ex*work*), and exergy loss (Ex*loss*), in mathematical terms as follows:

$$\rm{Ex}\_{\rm{mass,in}} - \rm{Ex}\_{\rm{mass,out}} + \rm{Ex}\_{\rm{Heat}} - \rm{Ex}\_{\rm{work}} = \rm{Ex}\_{\rm{loss}} \tag{6}$$

Calculation of exergy flow by mass transfer is shown in Equation (6), it can be defined as a sum of physical exergy (Exphy), chemical exergy (Exche), potential exergy (Expot), and kinetic exergy (Exkin), according to Equation (7):

$$\mathbf{Ex\_{mass}} = \mathbf{Ex\_{phy}} + \mathbf{Ex\_{che}} + \mathbf{Ex\_{pot}} + \mathbf{Ex\_{kin}} \tag{7}$$

The chemical exergy is calculated using the chemical exergy per mole of each component. The estimation of the chemical exergy per mole is performed based on the free energy of formation (ΔGi) and the specific chemical exergy of each atom presented in the molecule (Exchem,j):

$$\text{Ex}\_{\text{chem},\text{i}} = \Delta \text{G}\_{\text{i}} + \sum\_{\text{j}} \text{n}\_{\text{ele}} \text{Ex}\_{\text{chem},\text{j}} \tag{8}$$

The exergy of a heat stream Q is determined by the Carnot factor as follows:

$$\text{Ex}\_{\text{heat}} = \text{Q} + \left(1 - \frac{T\_{\text{o}}}{\text{T}}\right) \tag{9}$$

where To is the environment (or reference) temperature and T is the temperature at which Q is available. Finally, the last term in the exergy balance (Exwork) is associated for losses by work. Equation (10) gives the relation between these parameters:

$$\text{Ex}\_{\text{work}} = \mathcal{W} \tag{10}$$

There is exergy destruction in operational processes due to its irreversibilities, the thermodynamic efficiency is associated with the amount of exergy loss. For measuring these losses, process exergy efficiency (Ψ) is formulated. This term is defined as an indicator to determine the degree of used exergy, given by Equation (11):

$$\Psi = 1 - \left(\frac{\text{Exergy loss}}{\text{Exergy input}}\right) \tag{11}$$

Sorin et al. [23] mentioned that it is possible to perform the exergy balance considering all incoming and out-going streams in a defined system. The total exergy input of any real process/system is ever higher than its output. The above is related to the fact that always an amount of exergy is irreversibly lost within the system.
