**3.1.3.3 V2O5 chlorination enthalpy**

For the implementation of an industry process based on chemical reaction is fundamental to know the amount of heat generated or absorbed from that. Exothermic processes (heat is released) reach higher temperatures, and frequently demand engineering solutions for protecting the oven structure against the tremendous heat generated by the chemical phenomena. In this context, endothermic processes (heat is absorbed) are easier controlled, but the energy necessary to stimulate the reaction must be continuously supplied, making the energy investment larger.

The variation of the total enthalpy of the system for the chlorination process in question was calculated as a function of *P*(Cl2) (Figure 30). The partial pressure of O2 was forced to be equal to 1.93.10-22atm and four temperature levels were studied, 1000 K, 1100 K, 1300 K and 1700 K. It can be seen that the total enthalpy for the process conducted at 1000 K reduces with the advent of the chlorination reactions, indicating that the chlorination process is exothermic. However, the molar enthalpy magnitude is progressively lower up to a certain temperature where it is zero. Above that, the molar reaction enthalpy becomes positive, and Figure (30) illustrates its value for 1700 K.

 This is perfectly consistent with the results presented on Table (3). As temperature gets higher, the mol fractions of VCl4 and VOCl3 reduce and that for VCl3 and VCl2 experience a significant elevation. For some temperature between 1300 K and 1373 K the mol fractions of VCl4 and VCl3 assume equal values. This point is related to the condition where the chlorination enthalpy is zero. For higher temperatures, where x(VCl4) < x(VCl3) the process

The variations depicted on Figures (27), (28) and (29) are consistent with the occurrence of reactions represented by Eq. (51). As *P*(O2) achieves higher values, it reacts with VCl3, VCl2 and or VCl4 resulting in VOCl3. Such phenomena could explain the significant reduction of VCl3, VCl4 and VCl2 concentrations, and the concomitant elevation of the VOCl3 mol

> 32 3 22 4 3 42 3 2

The participation of VCl4 in the second reaction is supported by the fact that its equilibrium concentration lowering is more sensible to LnP(O2) than observed for VCl3 (Figure 27). The consumption of VCl4 by the second reaction is also consistent with the maximum observed in the curve obtained for VOCl3 concentration (Figure 29). As less VCl4 is available, less

The occurrence of reactions represented by Eq. (51) is supported by classical thermodynamics, as the equilibrium constant (*K*) computed at 1373 K for all chemical

For the implementation of an industry process based on chemical reaction is fundamental to know the amount of heat generated or absorbed from that. Exothermic processes (heat is released) reach higher temperatures, and frequently demand engineering solutions for protecting the oven structure against the tremendous heat generated by the chemical phenomena. In this context, endothermic processes (heat is absorbed) are easier controlled, but the energy necessary to stimulate the reaction must be continuously supplied, making

The variation of the total enthalpy of the system for the chlorination process in question was calculated as a function of *P*(Cl2) (Figure 30). The partial pressure of O2 was forced to be equal to 1.93.10-22atm and four temperature levels were studied, 1000 K, 1100 K, 1300 K and 1700 K. It can be seen that the total enthalpy for the process conducted at 1000 K reduces with the advent of the chlorination reactions, indicating that the chlorination process is exothermic. However, the molar enthalpy magnitude is progressively lower up to a certain temperature where it is zero. Above that, the molar reaction enthalpy

 This is perfectly consistent with the results presented on Table (3). As temperature gets higher, the mol fractions of VCl4 and VOCl3 reduce and that for VCl3 and VCl2 experience a significant elevation. For some temperature between 1300 K and 1373 K the mol fractions of VCl4 and VCl3 assume equal values. This point is related to the condition where the chlorination enthalpy is zero. For higher temperatures, where x(VCl4) < x(VCl3) the process

(51)

VCl 0.5O VOCl VCl O VCl 2VOCl VCl 0.5O VOCl 0.5Cl

 

Table 6. Equilibrium constants at 1373 K for reactions represented by Eq. (51)

reactions above assume values appreciably greater than unity (see Table 6).

becomes positive, and Figure (30) illustrates its value for 1700 K.

fraction.

VOCl3 can be produced.

K Chemical reaction 8.01.106 <sup>3</sup> <sup>2</sup> VOCl3 0.5OVCl 1.89.1013 <sup>22</sup> <sup>4</sup> VOCl2VClOVCl <sup>3</sup> 1.01.105 <sup>4</sup> <sup>2</sup> <sup>3</sup> <sup>2</sup> VCl 0.5O VOCl 0.5Cl

**3.1.3.3 V2O5 chlorination enthalpy** 

the energy investment larger.

becomes progressively more endothermic. It is interesting to see that he explained behavior is consistent with the fact that the global formation reactions of VCl3 and VCl2 are associated with positive molar reaction enthalpies and that of VOCl3 and VCl4 with negative molar reactions enthalpies (Figure 31).

Fig. 30. Total as a function of *P*(Cl2)

Fig. 31. Molar reaction enthalpy for the formation of gaseous VCl3, VCl2, VCl4 and VOCl3 as a function of temperature
