*3.1. Conventional Exergy Analysis*

The parameters of the exergetic analysis were calculated for each state throughout the entire studied system. Table 5 shows the flow rate ( .*m*), temperature (*T*), pressure (*P*), specific chemical exergy (*eCH*), specific physical exergy (*ePH*), specific kinetic exergy (*eKN*), and exergy rate ( .*E*) of each stream.


**Table 5.** Thermodynamic values of the streams.


**Table 5.** *Cont.*

The exergy rate of the fuel ( *EF*) and the product ( *EP*), the exergetic (*nex*) and energetic (*nen*) efficiencies, and the exergy destruction ratios (*y*<sup>∗</sup> *D*,*k* and *yD*,*<sup>k</sup>*) were calculated for each component in the system. The results are summarized in Table 6. The components with the highest exergy fuel rates were the B, the MHX, and the SD. The MHX is the component with the highest exergetic e fficiency (38.9%), followed by the boiler (37%). There is a big di fference between the exergetic and the energetic efficiencies of the majority of the components, and consequently the overall system also exhibited the same behavior. Therefore, despite the energy e fficiency of the system (the conservation of the quantity of energy) being 67.8%, the overall exergy e fficiency (the quality of that energy) was only 33.3%. Similar results were obtained in a study on the spray drying process in an industrial scale ceramic factory, in which the energetic e fficiency was found to be between 43% and 87% [34], and the exergetic e fficiency was between 12% and 64% [35]. However, in a pilot-scale study of spray drying of cherry puree the energetic and exergetic e fficiencies were only 3.2% and 0.7%, respectively [11]. This, along with laboratory-scale studies [10,12,36], demonstrates that pilot-scale and laboratory-scale studies do not accurately represent the energetic and exergetic performances of the industrial-scale spray drying process.

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Figure 2 shows the fuel and product exergy rate of the overall system, and the destroyed exergy rate of each component. The results show that the components that had electric energy as the main fuel exergy source such as the vibrating screen, belt, and fans had the lowest impact on the exergetic destruction. This occurs because the electric energy was used for mechanical operations, instead of being used as a heat source. The exergy destruction ratio (*yD*) was lower than 5% for these components. These results were similar to other studies that determined an exergy destruction ratio lower than 2% for the compressors and pumps in a CCHP system [37]. Furthermore, in a yogur<sup>t</sup> plant the devices that required electric energy accounted for less than 5% of the total exergy destruction [38].


**Table 6.** Results of the exergy analysis of all the components of the spray drying system.

**Figure 2.** Grassmann's diagram of the spray drying process.

Conversely, the boiler destroyed 39.4% of the overall fuel exergy rate. This percentage was similar to other plants where the boiler was used as an auxiliary supply of steam. For instance, in a factory, which produces ghee, the boiler has the highest exergy destruction ratio 39% [39]. This is because the main purpose of this component is to convert a high-quality energy (chemical energy of fuel oil) to a low-quality energy (heat).

The MHX also has a high exergy destruction rate, despite having one of the highest exergetic efficiencies. The air heater used in this process was a steam-heated type, which is one of the most used in food industry, it had an exergy e fficiency of 38.9% and a high specific exergy destruction of 287 kJ per kg of heated air, with a minimum temperature di fference of 12 ◦C. There are other types of air heaters that could reduce the exergy destruction rate and the minimum temperature di fference such as a system with a heat exchanger that uses geothermic fluid. A previous study showed that this kind of heat exchanger has an exergy e fficiency of 42% and specific destruction exergy of 57.5 kJ per kilogram of heated air with a minimum temperature di fference of 5 ◦C [40]. Another type of air heater was one that uses electric energy as the source of heat. A previous study on the spray drying of photochromic dyes determined that the exergy e fficiency of this kind of heater was 16.4% [12], this has the lowest exergy e fficiency because it is transforming high quality energy (electric energy) to low quality energy (heat).

The SD also a ffects the performance of the overall system, since it has one of the highest rates of exergy destruction at 595 kJ/kg of evaporated water. Previous studies by Bühler et al. [31] found that the spray dryer is a highly exergy-destructive component in a powdered milk factory. Similarly in a large dairy factory producing primarily milk powder, they obtained an exergy destruction rate of 1345 kJ/kg of evaporated water [14]. In a ceramic plant, the exergy destruction rate was 1111.4 kJ/kg of evaporated water [35].
