**3.3. Parameters and calculated data**

Phenol oxidation is a common reaction stoichiometry described in Eq. (2):

$$\rm C\_6H\_5OH + 14H\_2O\_2 \to 6CO\_2 + 17H\_2O \tag{2}$$

From Eq. (2) others relations were calculated.

Molar ratios other than 100% were calculated proportionally using the reaction stoichiometry in Eq. (2).

In this work, natural gas (89.24% of methane) supplied by COPERGAS (Pernambuco, Brazil) was employed [29]. Stoichiometrically 9.881 mol of air reacts with one mol of methane. So, the excess air (E) in the combustion and the equivalent ratio (Φ) may be evaluated employing Eqs. (3) and (4) [30, 31].

$$E = \frac{1}{9.881} \left(\frac{Q\_{s\alpha}}{Q\_{c\alpha}}\right) - 1\,\tag{3}$$

$$
\varphi = 9.881 \left( \frac{Q\_{\text{cN}}}{Q\_{\text{sM}}} \right) \tag{4}
$$

where *Q*AR denotes the volumetric flow rate of air, and *Q*GN denotes the volumetric flow rate of natural gas.

The power dissipated by the burner (P) was calculated using Eq. (5):

$$P = Q\_{\rm GN} \cdot \text{PCM},\tag{5}$$

where PCM denotes the average heat of combustion of natural gas, which has a value of 34,740 kJ m−3 [29].

The percent degradation of phenol (*XF* ) was calculated using Eq. (6):

The percent degradation of phenol \(X\_{\sharp}\) was calculated using Eq. (6):

$$X\_{\circ} = \left(\frac{Q\_{\iota} \cdot C\_{\text{Ph}\_{i}} - Q\_{\iota} \cdot C\_{\text{Ph}} - F\_{\iota} \cdot C\_{\text{Ph}\*}}{Q\_{\iota} \cdot C\_{\text{Ph}\_{i}}}\right) \times 100\,,\tag{6}$$

where *QL* represents the volumetric flow rate, *C*ph0, the initial phenol concentration, *C*ph, the phenol concentration at time *t, FG* the dry air mass flow rate, *C*phv, the phenol concentration in the condensate at time *t*. TOC conversion was evaluated via Eq. (7).

The TOC conversion (*XT*) was evaluated via Eq. (7):

The TOC conversion \(X\_p\) was evaluated via Eq. (7):

$$X\_r = \left(\frac{Q\_\text{L} \cdot \text{TOC}\_0 - Q\_\text{L} \cdot \text{TOC} - F\_\text{C} \cdot \text{TOC}}{Q\_\text{L} \cdot \left(\text{TOC}\_0 - \text{TOC}\_\text{s}\right)}\right) 100\,,\tag{7}$$

where TOC0 denotes the initial total organic carbon concentration, TOC and TOCV denote the total organic carbon and the total organic carbon in the condensate, respectively, at a time point *t* of the process and TOCB denotes the total organic carbon in the blank.

### **3.4. Effect of the liquid phase flow rate**

This work was to evaluate the influence of the liquid phase flow rate on the level of phenol oxidation, settling other process variables. These operational parameters are listed in **Table 3**.


**Table 3.** Operational parameters for the study of the influence of the liquid phase flow rate.

**Figure 2a** and **b** show, respectively, the evolution of the temperature and pH of the liquid effluent present in the feed tank (Tank 2) during the process, varying the volumetric flow of the same.

*E* = \_\_\_\_\_ <sup>1</sup>

332 Phenolic Compounds - Natural Sources, Importance and Applications

*ϕ* = 9.881(

The power dissipated by the burner (P) was calculated using Eq. (5):

*QL* . *C*Ph<sup>0</sup>

the condensate at time *t*. TOC conversion was evaluated via Eq. (7).

The TOC conversion (*XT*) was evaluated via Eq. (7):

of natural gas.

34,740 kJ m−3 [29].

where *QL*

where TOC0

**Table 3**.

The percent degradation of phenol (*XF*

*XF* <sup>=</sup> (

*XT* <sup>=</sup> (

**3.4. Effect of the liquid phase flow rate**

**Tests QL(L h−1) QGN(m<sup>3</sup>**

9.881(

where *Q*AR denotes the volumetric flow rate of air, and *Q*GN denotes the volumetric flow rate

 *P* = *QGN* ⋅ *PCM*, (5) where PCM denotes the average heat of combustion of natural gas, which has a value of

<sup>−</sup> *QL* . *<sup>C</sup>*Ph <sup>−</sup> *FG* . *<sup>C</sup>*Phν \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *QL* . *<sup>C</sup>*Ph<sup>0</sup>

phenol concentration at time *t, FG* the dry air mass flow rate, *C*phv, the phenol concentration in

*QL* . TOC0 − *QL* . TOC − *FG* . TOC*ν*

total organic carbon and the total organic carbon in the condensate, respectively, at a time

This work was to evaluate the influence of the liquid phase flow rate on the level of phenol oxidation, settling other process variables. These operational parameters are listed in

point *t* of the process and TOCB denotes the total organic carbon in the blank.

E1 170 4 40 500 50 50 E2 100 4 40 500 50 50

**Table 3.** Operational parameters for the study of the influence of the liquid phase flow rate.

) was calculated using Eq. (6):

represents the volumetric flow rate, *C*ph0, the initial phenol concentration, *C*ph, the

denotes the initial total organic carbon concentration, TOC and TOCV denote the

 **h−1) E(%) Cph0(mgL−1) QRG(%) RP/H(%)**

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *QL* . (TOC0 <sup>−</sup> TOC*B*) ).100 , (7)

\_ *Q*AR

> \_ *Q*GN

*<sup>Q</sup>*GN) <sup>−</sup> <sup>1</sup>, (3)

*<sup>Q</sup>*AR ), (4)

) <sup>×</sup> <sup>100</sup> , (6)

**Figure 2.** (a) Evolution of temperature of the liquid effluent as a function of the operating time. (b) Evolution of pH as a function of the operating time. *E* = 40%, *Q*GN = 4 m3 h−1, *C*Ph0 = 500 mg L−1, *R*P/H = 50% and *Q*RG = 50%.

**Figure 2a** indicates that the elevation of effluent flow affects the heating curve of liquid effluent, getting it faster to obtain steady-state temperature in Tank 2. **Figure 2a** also show that for the larger effluent flow (170 L h−1), the same reaches a maximum temperature of 348 K, quite greater than the temperature of the effluent flow of the 100 L h−1 (344 K), featuring a small effect, since the difference in these temperatures reaches an order of magnitude lower than the measurement uncertainty by thermocouples.

The oxidation process indicates that the heating profile is characterized by two distinct steps, a first, approximately 110 min, characterized by a rapid temperature rise and a second, after 110 min of operation, showing a temporal increase rate of low temperature. **Figure 2b** show that the evolution of pH also identifies these two steps, the first of about 110 min, characterized by a decreased less than hydrogen potential as a function of time, and the second-fastest where the temporal decay profile of pH is more significant. It is shown a low influence in liquid flow in the dynamics of acid formation [24].

**Figure 3a** and **b** show, respectively, the profile of phenol degradation and residual fraction of the TOC as a function of time for the two flows of liquid effluent studied, 100 and 170 L h−1.

**Figure 3.** (a) Evolution of phenol degradation as a function of the operating time. (b) Evolution of TOC conversion as a function of the operating time. *E* = 40%, *Q*GN = 4 m3 h−1, *C*Ph0 = 500 mg L−1, *R*P/H = 50% and *Q*RG = 50%.

**Figure 3a** indicates that, in the range studied, the variation of the effluent flow does not affect the profile of phenol degradation and total degradation of the same is almost completely achieved in 180 min of operation, reaching values of 99.5 and 97.4%, respectively, at effluent flow of 100 and 170 L h−1. **Figure 3b** indicates that the increase of the effluent flow of 100 to 170 L h−1 allows a higher speed of phenol mineralization, due the acceleration of the lowest value of the pH, but not interfering in the maximum value TOC conversion, around 28% with an operating time of 210 min.

**Figure 4a** and **b** indicates, respectively, the time profiles of the hydroquinone and catechol formed by thermochemical phenol oxidation, for the two flows of liquid effluent, 100 and 170 L h−1. Analysing the same figures, it can be seen that the rate of formation of these species becomes appreciable after the induction period, approximately 110 min, previously observed by the curves of the evolution of pH, phenol degradation and TOC conversion.

The evolution of the hydroquinone and catechol concentrations happened quickly because of the thermochemical oxidation reaction of phenol with high speed, regardless of the flows of liquid effluent studied. It has been observed that hydroquinone and catechol concentrations are reached when phenol consumption rate is maximum, which is identified in the process time between 140 and 150 min. After reaching the maximum hydroquinone and catechol formation, an immediate reduction in the concentration of these two species is observed, indicating that to achieve the maximum consumption of phenol, the oxidation rate of these two organic compounds becomes greater than its rate of formation, enabling to be degraded, thus favouring the formation of other organic compounds that are not acids, because the pH remained almost constant at 2.5−2.8, after

**Figure 4.** (a) Evolution of hydroquinone formation as a function of the operating time. (b) Evolution of catechol formation as a function of the operating time. *E* = 40%, *Q*GN = 4 m3 h−1, *C*Ph0 = 500 mg L−1, *R*P/H = 50% and *Q*RG = 50%.

140−150 min of operation. The products resulting from the oxidation of hydroquinone and catechol are probably aldehydes (Glyoxal, for example, in the case of hydroquinone and catechol) and alkenes (1,4-dioxo-2-butene, for example, in the case of hydroquinone) [24].

**Figure 4a** and presents the results obtained in the quantification of concentrations of hydroquinone and catechol, respectively. It also show a speed of formation and consumption of these species not significantly affected by variation of the flows of liquid effluent. It is also observed a rate of production and disappearance slightly larger (especially in the case of catechol) with use of effluent flow rate 170 L h−1, similarly what was evidenced in the evolution of pH, being less for the flow rate of 170 L h−1, allowing more oxidation of phenol to hydroquinone and catechol. However, regardless of the flow of the liquid studied, catechol concentrations were approximately two times higher compared to those obtained in relation to the hydroquinone.
