**3.1. Pilot plant and experimental procedure**

**Name of the AOP**

**Types** UV/Fe2+

Fe(OH)2+ + UV

Fe2+ + H

Photo catalysts

Photo catalysts

TiO2

*e*

− + *h*

− → heat

(*cb*)

H<sup>2</sup>

•OH + dyes

> **Table 2.**

Mechanisms of different advanced oxidation processes.

→

colourless

O<sup>2</sup>

(*νb*)

+ *h*

•OH + H+

> − →

(*νb*)

⎯

⎯⟶*e*

− + *h*

−

(*cb*)

(*νb*)

Oxidation [22]


*λ*<400nm

O2

→2 Fe(OH)2+ + •OH

→ Fe2+ + •OH

**Mechanism reaction**

**Highlights** - Reaction molecules reaches their excited state

absorbing UV and promotes further reaction



UV radiation has been noted


Chen and Chou (1994)

328 Phenolic Compounds - Natural Sources, Importance and Applications

degradation [21]

**References**

Currently, a non-conventional AOP called Direct Contact Thermal Treatment (DiCTT) process has been investigated, whose main attraction are the use of natural gas as the energy source, the demonstrated ability to oxidize phenolic compounds at low temperatures and atmospheric pressure, and the generation of free radicals (•OH, •H, •CH3 and •CHO) resulting from combustion of natural gas (methane). The installation of the DiCTT technology presents a compact reactor configuration that involves maintaining the reactor in a vertical position, favouring the immediate application of this technology in off-shore oil platforms, where natural gas is available and space is limited [4, 27].

The experimental unit used is mainly composed of a vertical stainless steel reactor, being 1.359 mm high and 203 mm internal diameter, a gas-liquid separator, a first tank (Tank 1) for preparation of phenol synthetic solutions, 400 L volume, a second tank (Tank 2) for feeding in polluted waters, also of 400 L volume, and a natural gas burner with 50 kW maximum power. The combustion air is provided by an axial fan with 0.3 HP power. The output pressure of the natural gas supply is 2 × 105 Pa, followed by a reduction to 1 × 105 Pa for combustion in the burner.

In this process, the effluent is tangentially injected into the reactor to produce a helical flow in its inner walls. The helical flow allows an intimate contact between the effluent and free radicals produced in the combustion flame of the natural gas, resulting in a thermochemical oxidation in the liquid phase, while avoiding their incineration. The high flame temperature contributes to the oxidation performance of the effluent in the presence of free radicals and works for the oxidation process to be carried out in completely liquid phase, by transferring some of the excess oxygen present in the flame.

The DiCTT process is a thermochemical oxidation method in aqueous medium, and generating free radicals resulting from the combustion of natural gas (methane) according to the reaction mechanism described by the whole of Eq. (1) following [27]:

$$\begin{aligned} \bullet \text{CH}\_4 &\rightarrow \bullet \text{CH}\_3 + \bullet \text{H} \\ \bullet \text{CH}\_3 + \text{O}\_2 &\rightarrow \text{CH}\_2\text{O} + \bullet \text{OH} \\ \bullet \text{OH} + \text{CH}\_4 &\rightarrow \bullet \text{CH}\_3 \\ \bullet \text{OH} + \text{CH}\_4 &\rightarrow \bullet \text{CH} + 2 \bullet \text{OH} \end{aligned} \tag{1}$$

This technique presents operational and capital costs 2.5 times lower than those of wet air oxidation (WAO) and 4.1 times lower than those of electric plasma oxidation (EPO) [27].

**Figure 1** show a schematic representation of the pilot plant used in the experiments that was composed of a vertical, stainless-steel reactor and a gas-liquid separator.

The phenol solution was prepared in Tank 1. The operation of the system was stabilized by heating the water to almost 70°C for an hour and a half; phenol was subsequently added to Tank 1, and the synthetic effluent was transferred from Tank 1 to Tank 2. The reactor had an internal helical groove with a rectangular shape, in the axial direction, through which the liquid effluent flowed. Wastewater polluted with phenol was injected into the reactor

**Figure 1.** Pilot plant using the DiCTT process.

tangentially to produce a liquid helical stream on its inner walls. The combustion gases were vented to the atmosphere through a chimney; a fraction of recycled combustion gases (of the total flow rate *Q*RG) was immediately injected into Tank 2 by adjusting an open valve to heat the solution in the recirculation tank (Tank 2) more rapidly and to dissolve a fraction of the residual oxygen from combustion into the reaction liquid, thereby inducing the thermochemical oxidation of the phenolic compounds. For the experiments, 250L of effluent was prepared. For each experiment 250mL samples of effluent in black plastic bottles were collected at previously chosen points and put to cool in a refrigerator. For the analyses, 250mL of treated drinking water was employed as a reference. To initiate the oxidation reaction, some millilitres of phenol/hydrogen peroxide with a mole ratio, RPH was introduced into Tank 2. Liquid samples were withdrawn for analysis through a collector located at the entrance of the tubes connecting the feed tank (Tank 2) to the reactor inlet (**Figure 1**).
