Performance comparison of tire pyrolysis oils in
hydrotreating toward high-quality fuel

Meryem Badlaoui, Ngoc Yen Phuong Cao, Warumporn Pejpichestakul, Benoit Celse, Bertrand Guichard, Minh Tuan Nguyen, Nadège Charon, Joris W Thybaut

► To cite this version:

Meryem Badlaoui, Ngoc Yen Phuong Cao, Warumporn Pejpichestakul, Benoit Celse, Bertrand Guichard, et al.. Performance comparison of tire pyrolysis oils in hydrotreating toward high-quality fuel. Chemical Engineering Journal, 2025, 520, pp.165115. 10.1016/j.cej.2025.165115. hal-05215256

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# Performance comparison of tire pyrolysis oils in hydrotreating toward high-quality fuel

Meryem Badlaoui a,D,*, Ngoc Yen Phuong Cao a,D, Warumporn Pejpichestakul a,D,
Benoit Celse a,D, Bertrand Guichard a, Minh Tuan Nguyen a,D, Nadège Charon a,D, Joris
W. Thybaut b,D

## ARTICLE INFO

Keywords:
Waste tire
Pyrolysis oil
Vacuum gas oil
Hydrotreating
Renewable fuels

## ABSTRACT

Hydrotreating is a process that uses existing refining facilities to upgrade waste tire pyrolysis oils, containing high amounts of bio-carbon, by reducing heteroatoms (e.g., nitrogen, sulfur and oxygen) and aromatics content. A Ni-Mo/ γ-Al₂O₃ catalyst was used to assess hydrotreating reactivity differences between three tire pyrolysis oils and a petroleum straight run vacuum gas oil in a semibatch reactor at 140 bar for four hours at three different temperatures ranging from 352 °C to 397 °C. Hydrodenitrogenation performance for tire pyrolysis oils is higher than their vacuum gas oil counterpart despite their larger content of nitrogen in the feed. Hydrodesulfurization conversion is similar for all the studied tire pyrolysis oils with temperature having a stronger effect on vacuum gas oil. For hydrodeoxygenation, residual oxygen was low (<0.1 wt%) for all tire pyrolysis oils. The conversion of aromatic carbon content for tire pyrolysis oils (10%-25%) is significantly reduced compared to vacuum gas oil (55 %). On a feedstock basis, vacuum gas oil, containing less light hydrocarbons (370 °C –), yields higher amounts of bottom residues (370 °C +). Meanwhile, tire pyrolysis oils produce more naphtha and kerosene products as temperatures increase. Hydrogen consumption calculation shows that TPOs require increased quantities of hydrogen compared to VGO.

## 1. Introduction

With a global market size reaching US$ 164.9 Billion in 2023 and an expected growth rate of 4.9% through 2032 [1], the tire market shows signs of increasing demand, thus production, for the foreseen future. This increased demand is mainly driven by rising sales of automobiles, growing need for replacement tires and increasing penetration of electric and hybrid vehicles [2]. Tires are typically made of rubber (14%-30%), carbon black (20%-28%), steel (13%-25%) and other materials like textile, fillers, etc (10%-17%), with ratio between the different materials making up the tire varying depending on tire type and local regulations [3,4]. Tires composition as well as their cross-linked rubber structures make them highly resistant to biodegradation, photochemical decomposition, chemical reagents and high temperatures which makes their recycling a difficult challenge [3]. Waste tires are defined as tires which can no longer be used to perform their original function and are typically named end-of-life tires [5]. Waste tires are processed in several possible ways, for instance, in the EU during 2015, 46% were used for material recovery, 12% for reuse and

export, 8% were stored in illegal landfills or have had an unknown processing, 6% were retreaded while the remaining 28% were used for energy recovery (e.g., pyrolysis, gasification, and combustion) [5,6].

Waste tire pyrolysis is the thermochemical conversion of complex hydrocarbons in the tires at temperatures between 400 and 900 °C in the absence of oxygen or presence of inert gas [7] in order to obtain gas (10–30 wt%), carbon black which is also referred to as char (35–50 wt%), and oil (30–55 wt%) which is an important fraction due to its energetic and economic value [8]. Reactor design improvements and use of catalyst therefore aim to increase the yield of the pyrolysis oil [9]. Tires source, pyrolysis reactor type, conditions of operation (i.e., temperature, pressure, reaction time, and heating rate), catalyst and size of tire particles all have an effect on the final composition and yields of pyrolysis products [7,9,10]. Table 1 shows characteristics of some commercial tire pyrolysis oils (TPOs), along with vacuum gas oils (VGOs) and straight run light cycle oils (LCOs).

TPOs show a larger density range compared to VGOs and LCOs. Their hydrogen content reaches values as low as 7 wt%, due to the

<footnote>a IFP Energies nouvelles, Rond-point de l'échangeur de Solaize, BP 3, Solaize, 69360, France</footnote>

<footnote>b Laboratory for Chemical Technology, Ghent University, Technologiepark 125, Ghent, 9052, Belgium</footnote>

<footnote>* Corresponding author.
E-mail address: meryem.badlaoui@ifpen.fr (M. Badlaoui).</footnote>
<table><caption>**Table 1**
Characteristics of commercial TPOs, VGOs and LCOs.</caption><thead><tr><th></th><th></th><th>TPOs</th><th>VGOs</th><th>LCOs</th></tr></thead><tbody><tr><td>Density</td><td>kg m<sup>-3</sup></td><td>727-1020</td><td>881-983</td><td>899-973</td></tr><tr><td>Carbon</td><td>wt%</td><td>68-92</td><td>84-89</td><td>87-91</td></tr><tr><td>BioCarbon</td><td>wt%</td><td>2-68</td><td>-</td><td>-</td></tr><tr><td>Hydrogen</td><td>wt%</td><td>7-13</td><td>10-14</td><td>9-11</td></tr><tr><td>Nitrogen</td><td>mg kg<sup>-1</sup></td><td>1020-13200</td><td>1010-4250</td><td>70-1300</td></tr><tr><td>Sulfur</td><td>wt%</td><td>0.10-1.54</td><td>0.12-3.32</td><td>0.04-2.55</td></tr><tr><td>Oxygen</td><td>wt%</td><td>0.4-4</td><td>0.13-0.51</td><td>-</td></tr><tr><td>Silicon</td><td>mg kg<sup>-1</sup></td><td>0-353</td><td>-</td><td>-</td></tr><tr><td>Chlorine</td><td>mg kg<sup>-1</sup></td><td>1-866</td><td>0-1</td><td>-</td></tr><tr><td>Total aromatics</td><td>wt%</td><td>38-77</td><td>33-48</td><td>33-80</td></tr><tr><td>Mono-aromatics</td><td>wt%</td><td>28-50</td><td>10-16</td><td>7-33</td></tr><tr><td>Di-aromatics</td><td>wt%</td><td>4-18</td><td>7-15</td><td>17-53</td></tr><tr><td>Tri-aromatics</td><td>wt%</td><td>0.7-19</td><td>5-9</td><td>0.9-14</td></tr><tr><td>SimDis</td><td></td><td></td><td></td><td></td></tr><tr><td>5 wt%</td><td>°C</td><td>39-408</td><td>265-409</td><td>163-248</td></tr><tr><td>50 wt%</td><td>°C</td><td>79-446</td><td>399-488</td><td>250-335</td></tr><tr><td>95 wt%</td><td>°C</td><td>342-720</td><td>508-581</td><td>304-435</td></tr></tbody></table>

Data presented in this table is collected from internal IFPEN analysis database.

presence of high amount of unsaturated carbon (aromatics and olefins). The advantage of TPOs is the presence of bio-carbon in high amounts, reaching values up to 68 wt%, which could make them an excellent source to produce renewable fuels (e.g., sustainable aviation fuel SAF).

Nitrogen content in TPOs is extremely high compared to VGOs and LCOs, with TPOs reaching values up to 13200 mg kg⁻¹. Mirmiran et al. [11] investigated nitrogen compounds in TPO naphtha fraction (30–204 °C) and found many nitrogenous elements (e.g., indoles, quinolines, anilines, benzothiazoles) with pyridine and their alkyl derivatives being the most abundant. The origin of these compounds is linked to the thermal degradation of the accelerants (e.g., N,N-diisopropyl-2-benzothiazole-sulfenamide, 2-mercaptobenzothiazole) present in tires [12]. On the other hand, sulfur, used mainly as a cross-linking agent in vulcanization for more rigid and durable tires [13], is lower in content compared to VGOs and LCOs.

Other impurities such as oxygen, silicon, and chlorine (used as fillers for example) are also present in TPOs but found in negligible quantities in VGOs and LCOs. Their presence has a negative effect due to (i) catalyst poisoning by water vapor formation through oxygen and hydrogen reaction [14] (ii) catalyst poisoning by silicon builds-up leading to decreased access to active sites therefore catalyst life cycle reduction [15] (iii) catalyst poisoning by chlorine reaction product (i.e., HCl) leading to Ni particles gradual agglomeration and sintering decreasing metallic dispersion therefore catalyst activity [16], in addition to its corrosive effect on reactors [17].

Another particularity of TPOs is their wide distribution of boiling points, ranging from mono-aromatics like toluene, xylene and limonene to heavy elements with boiling point exceeding 600 °C linked to the presence of high molecular weight compounds like polycyclic aromatic hydrocarbons (PAH) and polycyclic aromatic sulfur hydrocarbon (PASH) [18]. TPO upgrading, through hydrotreating (HDT) and/or hydrocracking (HCK), is therefore necessary to produce drop-in fuels since it contains high amount of heteroatom impurities, in addition to aromatics and heavy molecules with a boiling point above 350 °C.

Several studies on TPO upgrading have been conducted in literature, for instance Straka et al. [19] studied the production of alternative transportation fuels via TPO hydrotreating in a temperature range of 210–360 °C and pressures of 60 and 100 bar in a lab scale fixed-bed catalytic unit over a commercial Ni–Mo/γ–Al₂O₃ catalyst. Complete hydrogenation of olefins, aromatics content reduction, desulfurization and denitrogenation were achieved. In addition, properties of the hydrotreated fuel fractions were improved compared to the feedstock. However, only one TPO was used to conduct this study, which raises the question of the effect of feedstock on HDT performance and products quality. Palos, et al. [20] also carried out hydrotreating tests of TPO/LCO blend (TPO 20 v%) in a fixed bed reactor at 400 °C and

80 bar using three different commercial catalysts, i.e., Co–Mo/Al₂O₃, Ni–Mo/SiO₂–Al₂O₃ and Ni–W/USY zeolite. These catalysts showed similar hydrotreating performance by producing quality fuels with low amounts of sulfur, nitrogen and oxygen. It would have been also interesting to study the effect of TPO ratio in the blend on the studied catalysts, higher TPO content in the blend may create inhibiting effect on hydrotreating reactions, thus decreasing HDT performance.

To the authors best knowledge of the literature, there is no extensive comparison between different TPOs and a fossil feed (commercial crude petroleum vacuum gas oil VGO) hydrotreating in a semibatch reactor. In this work, TPOs and VGO were hydrotreated under industrial conditions for high conversions, in order to compare their hydrotreating performance. Detailed feed and hydrotreated product analysis was performed in order to compare hydrodenitrogenation (HDN), hydrodesulfurization (HDS), hydrodearomatization(HDCa) and hydrodeoxygenation (HDO) performances in terms of residual impurities of three TPOs, each from a different supplier, in addition to a commercial VGO. The hydrotreating was performed using a commercial Ni–Mo/γ–Al₂O₃ hydrotreating catalyst in a 500 mL semibatch reactor. The hydrotreating reactions were modeled using a power law equations model and model parameters were estimated using a least square estimator.

## 2. Materials and methods

### 2.1. Analytical methods

Carbon and hydrogen contents in liquid phase were measured using elemental analyzer based on thermal conductivity following American Society for Testing and Materials (ASTM) D5291 standard.

Biocarbon was measured based on ASTM D6866 (method B) standard. The method relies on radiocarbon ¹⁴C measurement using accelerator mass spectroscopy (AMS).

Nitrogen content was measured based on oxidative combustion and chemiluminescence detection following Norme Française (NF) M07-058/ASTM D4629 standards.

Sulfur content was measured by a method based on wavelength-dispersive X-ray fluorescence spectrometry using a 4 kW analyzer equipped with a Cr anode. 6 mL of the solution was introduced within a cup with a Mylar 6 µm film, after irradiation of the sample with primary X-rays, intensity of secondary X-rays specific to sulfur atoms was measured. Sulfur content of the sample was determined using an external calibration based on standards and reference materials. The C/H and S matrix effect were compensated using the Cr Compton line according to an internal IFPEN method.

Oxygen contents were measured by a two-step method, pyrolysis of the sample under an inert atmosphere followed by the transformation of the resulting oxygenated compounds to CO which was detected by infrared analysis. Quantification of the oxygen content was determined using an external calibration based on the analysis of a mixture of model compounds.

Chlorine and silicon total contents were determined using two internal IFPEN methods. A 4 kW Axios (Panalytical, Almelo, Netherlands) equipped with a Cr anode was used to perform the wavelength dispersive X-ray fluorescence analysis (WDXRF). A minimum of 4 mL of solution was introduced in a cup equipped respectively with a mylar (for Cl) and a polypropylene (for Si) 6 mm film.

Density was determined using an oscillating U-tube densimeter based on Organisation Internationale de Normalisation (ISO) 12185:2024 standard.

Conradson carbon residue was determined following ASTM D189 standard.

Bromine number (BN) was determined by electrometric titration according to an internal method adapted from the ASTM D1159. Samples were first diluted in dichloromethane then measured in a titration flask. The flask was filled to the mark with the titration solvent (a mixture of
<table><caption>**Table 2**
Physico-chemical properties of VGO and TPOs.</caption><thead><tr><th></th><th></th><th>VGO</th><th>TPO A</th><th>TPO B</th><th>TPO C</th></tr></thead><tbody><tr><td>Density</td><td>kg m<sup>-3</sup></td><td>930.4</td><td>1019.2</td><td>949.0</td><td>952.0</td></tr><tr><td>Conradson carbon residue</td><td>wt%</td><td>0.25</td><td>14.14</td><td>1.19</td><td>1.01</td></tr><tr><td>Bromine number</td><td>g Br<sub>2</sub>/ 100 g</td><td>-</td><td>50</td><td>56</td><td>46</td></tr><tr><td>Carbon</td><td>wt%</td><td>85.7</td><td>87.8</td><td>87.1</td><td>88.1</td></tr><tr><td>BioCarbon</td><td>wt%</td><td>-</td><td>43</td><td>50</td><td>47</td></tr><tr><td>Hydrogen</td><td>wt%</td><td>12.14</td><td>8.80</td><td>10.81</td><td>10.64</td></tr><tr><td>C/H</td><td>au/au</td><td>0.59</td><td>0.83</td><td>0.67</td><td>0.69</td></tr><tr><td>Nitrogen</td><td>mg kg<sup>-1</sup></td><td>1680</td><td>7630</td><td>5655</td><td>4180</td></tr><tr><td>Sulfur</td><td>wt%</td><td>1.84</td><td>1.00</td><td>0.90</td><td>0.77</td></tr><tr><td>Oxygen</td><td>wt%</td><td>0.20</td><td>1.49</td><td>0.90</td><td>0.59</td></tr><tr><td>Silicon</td><td>mg kg<sup>-1</sup></td><td>-</td><td>11.2</td><td>&lt; 10</td><td>&lt; 10</td></tr><tr><td>Chlorine</td><td>mg kg<sup>-1</sup></td><td>-</td><td>251</td><td>18</td><td>22</td></tr><tr><td>Aromatics</td><td>wt%</td><td>49.31</td><td>*</td><td>62.96</td><td>58.12</td></tr><tr><td>Mono-aromatics</td><td>wt%</td><td>25.53</td><td>*</td><td>45.34</td><td>37.51</td></tr><tr><td>Di-aromatics</td><td>wt%</td><td>11.36</td><td>*</td><td>9.27</td><td>11.03</td></tr><tr><td>Tri-aromatics</td><td>wt%</td><td>12.42</td><td>*</td><td>8.35</td><td>9.58</td></tr></tbody></table>

glacial acetic acid, dichloromethane, methanol and sulfuric acid) then cooled to a temperature between 0 and 5 °C, the titration was then performed using a bromide/bromate solution while maintaining that temperature range.

Simulated distillation was performed by gas chromatography with a flame ionization detector (FID) according to a method adapted from ASTM D2887 enabling the measurement boiling range distribution of petroleum fractions. An on-column injector, a non-polar (100% dimethylpolysiloxane) capillary column (10 m × 0.53 mm × 0.88 µm) and ChromDis software were used for this analysis. A calibration mixture of n-alkanes diluted in carbon disulfide was used to determine the relationship between retention time and the boiling point of n-alkanes.

Mono-, di- and poly-aromatic contents were measured by UV absorption spectrometry using an analyzer which covers the range 190–350 nm. The sample was first diluted in cyclohexane prior to measurement of the absorbance of the solution at the maxima of several bands, i.e. 197, 230 and 260 nm. Aromatics contents were determined using the Beer–Lambert Law and based on external calibration curves resulting from the analysis of model compounds.

Unsaturated carbon content (UC) was determined by liquid state quantitative ¹³C NMR analysis that was performed on an analyzer 600 MHz equipped with a 5 mm Quattro Nucleus Probe using TopSpin software. All spectra were obtained after 512 scans and referenced to deuterated chloroform (76.9 ppm). Prior to analysis, the samples were diluted in deuterated chloroform with relaxant. Quantification of the atomic proportion of unsaturated carbons was determined as a ratio between the signals area specific to unsaturated carbons (160–100 ppm) and the total area of unsaturated carbons and saturated carbons (70–0 ppm).

Aromatic carbon (AC) and olefinic carbon (O), in wt%, in TPOs were calculated using the Eqs. (1) and (2), adapted from ASTM D1159 standard, based on UC and BN analysis.

$$AC = UC - O \qquad (1)$$

$$O = \frac{BN}{160} \times 24 \qquad (2)$$

## 2.2. Physico-chemical properties of feeds

The TPOs and VGO used were sourced from commercial providers, their physico-chemical properties and simulated distillation curves are summarized in Table 2 and Fig. 1 respectively.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Simulated distillation curves by GC-FID of VGO and TPOs.</figcaption></figure>

## 2.3. Catalyst sulfiding

A mass $m_{cat} = 41.63$ g of a commercial VGO hydrotreating Ni–Mo/γ–Al₂O₃ catalyst (Axens, Rueil-Malmaison, France) was loaded in a glass cell which was then placed in a lab scale cylindrical oven. The catalyst was inerted using argon flow for 10 min. The argon was then exchanged for a ratio 3/17 H₂S to H₂ gas mixture at a 70 NL h⁻¹ flow for the total duration of the sulfiding. A temperature ramp of +5 °C min⁻¹ was applied to reach a temperature of 400 °C in which the catalyst was sulfided for 2 h. The gas flow was then switched to argon while the catalyst was cooling to ambient temperature and once that is achieved the cell inlet was closed while the outlet was connected to a vacuum pump for 5 min to remove the remaining gas. The cell was finally sealed and stored for further use.

## 2.4. Reaction setup

The TPOs and VGO were hydrotreated using type 1967 1000 semi-batch autoclave reactor of volume $V_{rea} = 500$ mL from Top Industrie (Vaux-le-Pénil, France), a simplified diagram of the reactor is presented in Fig. 2. A mass $m_{feed} = 200$ g of feed was loaded in the reactor. The catalyst cell was opened and emptied in a metal basket of volume $V_{bas} = 46.25$ mL placed in the reactor.

After a valid sealing and tightness test, the reactor was depressurized then heated using a temperature ramp of 20 °C min⁻¹. Reaction time count started when the targeted reactor temperature and pressure was achieved under continuous H₂ injection. In parallel with the heating, the self-priming Rushton turbine used for stirring, therefore increasing the liquid-gas mass transfer coefficients, was manually raised to 1000 rpm using a ramp of 200 rpm, Fig. 3 shows liquid and gas flow inside the reactor.

After the reaction time $t = 4$ h had elapsed, H₂ pressure regulation valve was closed, stirring stopped and reactor's temperature regulation disabled. The liquid phase was then cooled down by an air vortex system using a temperature ramp of -20 °C min⁻¹.

A pre-vacuumed 5 l gas bottle was connected to the reactor in order to evacuate the gas phase to be analyzed. When ambient pressure is reached in the reactor, it was opened and liquid phase was sampled, stripped off residual gas contents using N₂, weighted, then stored for analysis. Table 3 summarizes the experiment operating conditions. These operating conditions were selected to achieve different levels of hydrotreating and reduce residual nitrogen content to 10 mg kg⁻¹.

<footnote>* TPO A samples out of scope for aromatics analysis.</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Semibatch unit simplified diagram (not drawn to scale). 1-N₂ intake, 2-H₂ intake, 3-H₂ pressure control valve, 4-Cartridge heaters, 5-Rushton stirring turbine, 6-Electrical resistance heating, 7-Reactor, 8-Autoclave, 9-Air vortex cooling system, 10-Catalyst Basket, 11-Liquid phase with temperature control, 12-Gas phase with temperature control, 13-gas bottle, 14-gas purging outlet.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Semibatch reactor (a) liquid flow (b) gas flow [21].</figcaption></figure>

<table><caption>**Table 3**
Reaction parameters.</caption><thead><tr><td>Parameter</td><td>Notation</td><td>Value/Range</td></tr></thead><tbody><tr><td>Reactor volume</td><td>V<sub>rea</sub></td><td>500 mL</td></tr><tr><td>Basket volume</td><td>V<sub>bas</sub></td><td>46.25 mL</td></tr><tr><td>Feed mass</td><td>m<sub>feed</sub></td><td>200 g</td></tr><tr><td>Catalyst</td><td>m<sub>cat</sub></td><td>41.63 g</td></tr><tr><td>Operating pressure</td><td>P</td><td>140 bar</td></tr><tr><td>Operating temperature</td><td>T</td><td>[342 °C, 397 °C]</td></tr><tr><td>Reaction time</td><td>t</td><td>4 h</td></tr></tbody></table>

Consequently, temperatures were adjusted based on the initial nitrogen content of each feed.

Additional experiments were performed in order to ensure the reaction setup is free of thermodynamic and mass transfer limitations, results are presented in Supplementary data S1 and S2 respectively.

## 2.5. Kinetics modeling

Modeling chemical reactions involved in HDT is complex due to the extremely large number of reactions and challenges in measuring feedstock and product compositions [22]. In the present work, reactants considered are N, S, O heteroatoms compounds, and aromatics. A simplified kinetics model based on pseudo first-order kinetics with reaction rate constant from Arrhenius equation [23] was used and the following assumptions were made: the semibatch reactor was considered well mixed with a constant volume throughout the whole reaction, no liquid evaporation or vapor condensation took place, no mass or heat transfer limitations, wetting of catalyst was complete, the reaction occurred in

isothermal and isobaric conditions, resulting in negligible variations in the partial pressure of H₂. Therefore, it is reasonable to simplify the rate of reaction by omitting the H₂ concentration.

To estimate the kinetic parameters for HDT reactions, the time variation of the reactant concentration $dC/dt$ was modeled using the differential equation (3) $k_0$ is the pre-exponential factor, $E_a$ is the apparent activation energy in J mol⁻¹, $R = 8,314$ J K⁻¹ mol⁻¹ is the ideal gas constant, and $T$ is temperature in K. In fact:

$$ \frac{dC_T(t)}{dt} = -k_0 \exp(-\frac{E_a}{RT}).C_T(t)^n \quad (3) $$

Integrating Eq. (3) to solve for the time evolution of the concentration (content) $C_T(t)$ with $n = 1$ as the assumed reaction order yields Eq. (4), where $C_0$ is the initial reactant concentration.

$$ C_T(t) = C_0 \exp\left[-k_0 t \exp\left(-\frac{E_a}{RT}\right)\right] \quad (4) $$

A fixed time $t^* = 4h$ was selected and $C_T^* = C_T(t^*)$ was used to estimate the model parameters $k_0^* = \ln(k_0)$ and $E_a$. The set of the l tested temperature values is denoted by $T_s$. The model parameters are estimated using the least-squares estimator. Using Lmfit package [24,25], the sum of squared residuals between the expected model concentration $\hat{C}_T^*$ and the measured experimental value $C_T^*$ was minimized. Hence, the estimated values $\hat{k}_0^*$ and $\hat{E}_a$ are given by Eq. (5). Plots of the objective function with respect to model parameters are presented in Supplementary data S3.

$$ (\hat{k}_0^*, \hat{E}_a) = \arg \min_{(\hat{k}_0^*, \hat{E}_a) \in T_s} \sum_{T \in T_s} (\hat{C}_T^* - C_T^*)^2 \quad (5) $$

To assess the accuracy of $(\hat{k}_0^*, \hat{E}_a)$ estimates, the Lmfit package calculates the standard deviations $\hat{\sigma}_{\hat{k}_0^*}$ and $\hat{\sigma}_{\hat{E}_a}$ of $\hat{k}_0^*$ and $\hat{E}_a$ respectively.

To compare the HDT reactivity estimated in the various experiments, the conversion %HDXT for a given hydrotreating reaction (X being either N, S, O, or Ca) and temperature $T \in T_s$ is calculated according to formula (6) where $C_{0,X}$ is the initial concentration of element X, $m_{feed}$ is defined in Table 3 and $m_{product}$ is the liquid hydrotreated product mass.

$$ \%HDX_T = \left(1 - \frac{C_T^*}{C_{0,X} \frac{m_{product}}{m_{feed}}}\right) \times 100 \quad (6) $$

## 2.6. Hydrogen consumption

The hydrogen consumption $Hydrogen_{consumption}$, in g/100 g of feed, is calculated based on $Hydrogen_{tot}$ the total mass of hydrogen in the products (in g) and $Hydrogen_{feed}$ the mass of hydrogen in the feed (in g), as presented in Eq. (7).

$$ \text{Hydrogen}_{\text{consumption}} = \frac{\text{Hydrogen}_{\text{tot}} - \text{Hydrogen}_{\text{feed}}}{m_{\text{feed}}} \times 100 \quad (7) $$

$Hydrogen_{tot}$ is calculated based on Equation (8). It represents the total mass of hydrogen necessary for the production of light hydrocarbon gases, H₂S, NH₃, H₂O, and hydrogenation reactions. Light hydrocarbon gases produced were measured using GC-FID analysis (Agilent, California, United States of America), their hydrogen consumption is calculated based on Eq. (9). Mass of hydrogen for H₂S, NH₃, H₂O, and hydrogenation reactions calculations are presented in Eqs. (10), (11), (12), and (13) respectively, where $M_X$ is the molecular weight of element X and %Hydrogen_{product} is the percentage of hydrogen in the liquid product from elemental analysis.

$$ \begin{aligned} \text{Hydrogen}_{\text{tot}} = \text{Hydrogen}_{\text{H}_2\text{S}} + \text{Hydrogen}_{\text{NH}_3} + \\ \text{Hydrogen}_{\text{H}_2\text{O}} + \text{Hydrogen}_{\text{HC}_{\text{gas}}} + \text{Hydrogen}_{\text{product}} \end{aligned} \quad (8) $$

$$ \text{Hydrogen}_{\text{HC}_{\text{gas}}} = \frac{\% \text{Hydrogen}_{\text{HC}_{\text{gas}}}}{100} m_{\text{HC}_{\text{gas}}} \quad (9) $$

$$ \text{Hydrogen}_{\text{H}_2\text{S}} = C_{0,S} \frac{\%\text{HDS}_T}{100} \frac{M_{\text{H}_2\text{S}} - M_S}{M_S} m_{\text{feed}} \quad (10) $$
$$Hydrogen_{NH_3} = C_{0,N} \frac{\%HDN_T}{100} \frac{M_{NH_3} - M_N}{M_N} m_{feed} \quad (11)$$

$$Hydrogen_{H_2O} = C_{0,O} \frac{\%HDO_T}{100} \frac{M_{H_2O} - M_O}{M_O} m_{feed} \quad (12)$$

$$Hydrogen_{product} = \frac{\%Hydrogen_{product}}{100} m_{product} \quad (13)$$

$Hydrogen_{feed}$ is calculated based on %Hydrogen_{feed} the initial percentage of hydrogen in the feed from elemental analysis results following Equation (14).

$$Hydrogen_{feed} = \frac{\%Hydrogen_{feed}}{100} m_{feed} \quad (14)$$

# 3. Results and discussion

## 3.1. Feed analysis

Physico-chemical properties of TPOs vary depending on their origin as shown in Table 2 The nitrogen content in TPO A is extremely high reaching 7630 mg kg⁻¹ compared to TPO B and TPO C with 5655 mg kg⁻¹ and 4180 mg kg⁻¹ respectively, while VGO has the lowest nitrogen content of 1680 mg kg⁻¹. The sulfur contents of TPOs A, B and C are lower than VGO.

Other impurities, namely oxygen, silicon, and chlorine are highly present in TPO A, with values reaching 1.49 wt% for oxygen, 11.2 mg kg⁻¹ for silicon, and 251 mg kg⁻¹ for chlorine. TPO B and TPO C also contain these impurities but at lower concentrations. The presence of these heteroatoms have negative effects on HDT due their poisoning effect on catalysts.

TPOs have a higher content of total aromatics compared to VGO, specifically mono-aromatics. Therefore it is important to assess the effect of this on hydodearomatization reactions. Studied TPOs also have wide distribution of boiling points compared to VGO, as shown in Fig. 1, which affects yields of the different products, favoring lighter fractions production.

## 3.2. Heteroatoms removal

### 3.2.1. Hydrodenitrogenation (HDN)

Despite very high contents of nitrogen in studied TPOs compared to VGO, high HDN performances were obtained for TPO B and TPO C as shown in Fig. 4. Thus, there is a high proportion of nitrogen species in those TPOs that are very reactive and easily removed under moderate severity conditions. TPO A, with higher N content, has lower reactivity than VGO and the other two TPOs, which can either be related to (i) presence of more refractory nitrogen compounds (ii) auto-inhibition of nitrogen compounds (iii) inhibition effect of chlorine compounds [26] since their content (Cl = 251 mg kg⁻¹) is particularly high in this feedstock.

Parameters estimates are shown in Table 4 and based on this data, the reactivity curves along with the experimental values are presented in Fig. 4. It is important to note that hydrotreating was performed under industrial conditions in order to achieve high conversions. Therefore, it is expected to obtain low apparent activation energies overall. TPOs show a significantly lower apparent activation energies and $k_0$ compared to VGO, which is expected due to their high reactivity. TPO A has the lowest apparent activation energy among the TPOs, which confirms the presence of other factors limiting the N conversion in this feed, namely the presence of impurities leading to catalyst poisoning or mass transfer limitations. Despite a total 35 °C difference between the tests, residual organic nitrogen did not reach values lower than 200 mg kg⁻¹. For TPO C and VGO, a 10 mg kg⁻¹ residual organic nitrogen is reached at temperatures around 370 °C even though TPO C nitrogen content in feed is at least 2.4 times higher. The same residual organic nitrogen is reached by TPO B at temperatures around 397 °C, but with an initial N content 3.3 times higher.

<table><caption>**Table 4**
Kinetics parameters for HDN.</caption><thead><tr><th></th><th></th><th>VGO</th><th>TPO A</th><th>TPO B</th><th>TPO C</th></tr></thead><tbody><tr><td>N in feed</td><td>mg kg<sup>-1</sup></td><td>1680</td><td>7630</td><td>5655</td><td>4180</td></tr><tr><td>k̂<sup>*</sup><sub>0</sub> ± σ<sub>k<sup>*</sup><sub>0</sub></sub></td><td></td><td>21.42 ± 0.22</td><td>5.20 ± 0.13</td><td>7.65 ± 0.41</td><td>13.14 ± 0.91</td></tr><tr><td>E<sub>a</sub> ± σ<sub>E<sub>a</sub></sub></td><td>kcal mol<sup>-1</sup></td><td>113.75 ± 1.17</td><td>30.02 ± 0.71</td><td>40.11 ± 2.22</td><td>68.04 ± 4.77</td></tr></tbody></table>

<table><caption>**Table 5**
Kinetics parameters for HDS.</caption><thead><tr><th></th><th></th><th>VGO</th><th>TPO A</th><th>TPO B</th><th>TPO C</th></tr></thead><tbody><tr><td>S in feed</td><td>wt%</td><td>1.84</td><td>1.00</td><td>0.90</td><td>0.77</td></tr><tr><td>k̂<sup>*</sup><sub>0</sub> ± σ<sub>k<sup>*</sup><sub>0</sub></sub></td><td></td><td>15.63 ± 3.32</td><td>5.41 ± 0.55</td><td>5.69 ± 2.82</td><td>6.84 ± 0.89</td></tr><tr><td>E<sub>a</sub> ± σ<sub>E<sub>a</sub></sub></td><td>kJ mol<sup>-1</sup></td><td>85.70 ± 17.71</td><td>31.74 ± 2.97</td><td>34.08 ± 15.20</td><td>38.85 ± 4.65</td></tr></tbody></table>

<table><caption>**Table 6**
Kinetics parameters for HDO.</caption><thead><tr><th></th><th></th><th>TPO A</th><th>TPO B</th><th>TPO C</th></tr></thead><tbody><tr><td>O in feed</td><td>wt%</td><td>1.49</td><td>0.90</td><td>0.59</td></tr><tr><td>k̂<sup>*</sup><sub>0</sub> ± σ<sub>k<sup>*</sup><sub>0</sub></sub></td><td></td><td>4.01 ± 0.59</td><td>23.97 ± 2.46</td><td>8.16 ± 0.76</td></tr><tr><td>E<sub>a</sub> ± σ<sub>E<sub>a</sub></sub></td><td>kJ mol<sup>-1</sup></td><td>24.07 ± 3.18</td><td>131.63 ± 13.98</td><td>43.83 ± 4.02</td></tr></tbody></table>

### 3.2.2. Hydrodesulfurization (HDS)

TPOs show high HDS reactivity compared to VGO, as shown in Fig. 5, which may be explained by presence of lower content of refractory sulfur compounds since sulfur presence in TPO feed is lower than its VGO counterpart. At 362 °C, TPO C with the lowest initial sulfur content has the highest conversion. It was also observed that temperature variation has its biggest effect on VGO hydrodesulfurization.

Regarding performance, the lowest HDS achieved was 83% for VGO at 352 °C while the highest was 96% for TPO B at 392 °C. Ultra-low sulfur content (< 10 mg kg⁻¹) in hydrotreated products was not obtained and the lowest sulfur content reached was 426 mg kg⁻¹ by TPO B.

HDS model parameters and experimental results are presented in Table 5 and Fig. 5 respectively. Considering the high conversions obtained, low apparent activation energies were estimated. Apparent activation energies and $k_0^*$ of TPOs are lower than their VGO counterpart, with TPO A having the lowest apparent activation energy while TPO B and TPO C have similar values.

### 3.2.3. Hydrodeoxygenation (HDO)

Since VGO feed contains negligible amount of oxygen compared to TPOs, this section will mainly focus on reactivity of TPOs. Experimental results, reactivity curves and kinetic parameters estimated are presented in Fig. 6 and Table 6 respectively.

Hydrotreated TPO products contained oxygen lower than 0.1 wt%, it is strongly dependent on the TPO type, with temperature having the greatest effect on TPO B. Conversion wise, each TPO exhibited the highest conversion rate at the most severe conditions. TPO A achieved a conversion of 95.5 wt%, while TPO B reached 98.6 wt%. TPO C, on the other hand, reached the highest conversion with a value of 97.9 wt%, as a result, low apparent activation energies were estimated. These apparent activation energies as well as pre-exponential factors are dependent on the type of feedstock, TPO A feed, containing the most oxygen with 1.49 wt%, has the lowest apparent activation energy and pre-exponential factor, followed by TPO C then TPO B.

## 3.3. Hydrodearomatization (HDCa)

Hydrodearomatization of feedstocks is studied through aromatic carbon content in hydrotreated products, reactivity and experimental values are shown in Fig. 7 while model kinetic parameters are presented in Table 7. Olefins in the TPO C were measured at the lowest hydrotreating temperature (T = 342 °C) using a bromine test and a
<figure><img src="image_7.png" /><figcaption>Fig. 4. Hydrodenitrogenation (HDN) (a) reactivity experimental data (markers) and model data (lines) (b) %HDN<sub>362</sub> is the HDN conversion at T = 362 °C.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. Hydrodesulfurization (HDS) (a) reactivity experimental data (markers) and model data (lines) (b) %HDS<sub>362</sub> is the HDS conversion at T = 362 °C.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Hydrodeoxygenation (HDO) (a) reactivity experimental data (markers) and model data (lines) (b) %HDO<sub>362</sub> is the HDO conversion at T = 362 °C.</figcaption></figure>

<table><caption>**Table 7**
Kinetics parameters for HDCa.</caption><thead><tr><th></th><th></th><th>VGO</th><th>TPO A</th><th>TPO B</th><th>TPO C</th></tr></thead><tbody><tr><td>AC in feed</td><td>wt%</td><td>19.50</td><td>50.50</td><td>23.60</td><td>28.60</td></tr><tr><td>k'<sub>0</sub> ± σ<sub>k'<sub>0</sub></sub></td><td></td><td>7.05 ± 2.12</td><td>-2.74 ± 1.14</td><td>-3.11 ± 4.55</td><td>4.17 ± 2.09</td></tr><tr><td>E&#x0302;<sub>a</sub> ± σ<sub>E<sub>a</sub></sub></td><td>kJ mol<sup>-1</sup></td><td>46.85 ± 11.22</td><td>&lt; 1</td><td>&lt; 1</td><td>35.67 ± 11.05</td></tr></tbody></table>

value of 0.21 wt% olefinic carbon was found (corresponding to 1.4 g Br<sub>2</sub>/100 g). Therefore, complete olefin hydrogenation is considered for higher temperatures.

A reactivity comparison at 362 °C shows that the aromatic carbon conversion of TPOs is significantly lower than that of VGO, particularly

for TPO B, while TPO A and TPO C exhibit similar levels of hydrogenation. Also, temperature has insignificant effect on TPO aromatic carbon content. This trend was also observed by Palos et al. [20] and was linked to hydrogenation reaction of aromatics that favors dehydrogenation or condensation reactions at high temperatures due to their highly exothermic reversible nature. This low conversion may also be due to (i) presence of refractory aromatic compounds (ii) presence of contaminants (i.e., Si and Cl) in TPOs (iii) thermodynamic limitations [27], further investigation is required in order to confirm these hypotheses. Concerning kinetic parameters of TPOs, they are much lower than VGO, TPO A and TPO B apparent activation energies are almost null and their pre-exponential factors are extremely low.

Density of hydrotreated product, presented in Fig. 8, shows that temperature has more effect on VGO than TPOs, given the correlation
<figure><img src="image_10.png" /><figcaption>Fig. 7. Hydrodearomatization (HDCa) (a) reactivity experimental data (markers) and model data (lines) (b) %HDCa₃₆₂ is the HDCa conversion atT = 362 °C, AC aromatic carbon.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Density of hydrotreated products.</figcaption></figure>

between density and aromatics content, it confirms that VGO aromatics content is significantly decreased with increased temperature.

Aromatic distributions in hydrotreated product are also measured and presented in Fig. 9. Total aromatics content of TPOs decreases with temperature but at a much lower rate than its VGO counterpart, which is in line with previous AC and density results. Mono-aromatics content in TPOs decreases with temperature while di- and tri-aromatics content is more or less stable, which indicates that the total aromatics decrease stems majorly from hydrogenation of single aromatics compounds. TPO B and TPO C, with lower di-and tri-aromatics content compared to VGO, were hydrogenated at a much lower rate. In fact, the presence of polycyclic aromatic hydrocarbons (PAH) makes the HDCa of TPOs challenging due to the stability of their aromatic rings.

### 3.4. Product yields

The main goal of upgrading TPO through hydrotreating is to produce quality drop-in fuels or to prepare for further treatment (hydrocracking) therefore it is necessary to assess distillation fraction yields. Table 8 presents the yields of four different product fractions from VGO and different TPOs: naphtha (Initial Boiling Point IBP-180 °C), kerosene (180 °C-250 °C), diesel fuel (250 °C-370 °C), bottom residues (370 °C +) and Fig. 10 shows hydrotreated products yields of the same fractions estimated from simulated distillation curves by GC-FID.

TPOs contain higher fraction of light hydrocarbons (370 °C -) with a maximum value of 51.2 wt% for compounds with boiling point over

<table><caption>Table 8
Yields in wt% of distilled fractions of VGO and TPOs (by GC-FID analysis).</caption><thead><tr><th>Fraction</th><th>VGO</th><th>TPO A</th><th>TPO B</th><th>TPO C</th></tr></thead><tbody><tr><td>IBP-180 °C</td><td>1.6</td><td>4.6</td><td>5.1</td><td>3.2</td></tr><tr><td>180 °C-250 °C</td><td>1.6</td><td>16.1</td><td>12.6</td><td>12.0</td></tr><tr><td>250 °C-370 °C</td><td>5.9</td><td>36.8</td><td>31.1</td><td>45.1</td></tr><tr><td>370 °C +</td><td>90.9</td><td>42.5</td><td>51.2</td><td>39.7</td></tr></tbody></table>

370 °C compared to VGO with at least 90 wt% of same fraction. Therefore, after hydrotreating, TPOs produce a greater amount of light fractions than VGO. A general trend for TPOs is observed: the naphtha and kerosene fractions increase with rising temperatures, while the diesel fraction remains relatively stable. A rapid increase was noted when hydrotreating temperature reached values over 390 °C (TPO A and TPO B) which is generally linked to cracking. VGO, on the other hand, exhibits a similar trend for the three fractions (i.e., naphtha, kerosene, and diesel). While the heavy fraction decreases with increasing temperature, its values remain high (at least 82.5 wt%) compared to those of TPOs. Another factor to higher naphtha and kerosene yields in TPOs is the increased HDN and HDS observed. In fact, the greater removal of heteroatoms, either due to larger amounts present in TPOs or to higher conversion rates compared VGO, leads to the production of lighter molecules.
<figure><img src="image_12.png" /><figcaption>Fig. 9. Aromatics distribution in hydrotreated products (a) mono-aromatics (b) di-aromatics (c) tri-aromatics (d) total aromatics.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10. Experimental (markers) and trend (lines) yields in wt% of distilled fractions of hydrotreated VGO and TPOs (a) naphtha, IP - 180 °C (b) kerosene, 180 °C-250 °C (c) diesel, 250 °C-370 °C (d) bottom residues, 370 °C +. The linear trends are provided as a visual guide and should not be interpreted as indicative of a definitive trend.</figcaption></figure>

### 3.5. Hydrogen consumption

Important quantities of hydrogen are needed for hydrotreating processes, which makes its consumption calculation calculation necessary, Fig. 11 represents the H₂ consumption of each experimental point with respect to the reaction temperature. The total hydrogen consumption is estimated from the hydrogen mass in feed and products as detailed in Section 2.6.

Overall, hydrogen consumption increases with the reaction temperature, as expected, due to the higher severities. This clearly shows that TPOs have higher consumption than VGO. However, increasing HDT temperatures leads to increased hydrogen consumption for VGO, which is linked to the increased aromatics hydrogenation. TPOs, with lower aromatics hydrogenation, see their consumption variation less affected by temperature. It is also important to note the difference between the TPOs consumption, with TPO B and TPO C having a close values while TPO A consumption is the highest among the hydrotreated
<figure><img src="image_14.png" /><figcaption>Fig. 11. Hydrogen consumption in wt% of HDT reactions.</figcaption></figure>

feeds. In fact, TPO A has the highest amount of nitrogen and aromatics in its feed, which requires an increased quantity of hydrogen despite having the lowest nitrogen conversion among hydrotreated feedstock and reduced hydrogenation. TPO A has the highest AC of 50 wt%. After HDT, the AC content is around 40 wt%. The aromatic ring saturation of TPO A is large enough to contribute to the highest H₂ consumption among the feeds.

## 4. Conclusion

HDT performance of three different TPOs was studied and compared to that of a commercial VGO. Using a Ni-Mo/γ-Al₂O₃ catalyst in a semibatch reactor, the different feeds were processed and the product impurities concentration was measured and used to estimate the HDT reactions kinetic parameters according to a simplified first order Arrhenius power law.

In HDN, TPOs exhibited higher reactivity than VGO, as shown by their lower apparent activation energy and pre-exponential factor despite higher initial N content. It was hypothesized that the higher reactivity is due to the presence of less refractory nitrogen compounds. In HDS, the same conclusion holds true, with TPOs still showing better performance. VGO HDS exhibited a more pronounced temperature dependence compared to TPOs. In HDO, the oxygen impurities in TPOs were successfully reduced while VGO was not studied due to its negligible oxygen content. Finally, in HDCA, it was observed that aromatic carbon conversion of TPOs, contrary to VGO, is experimentally independent of temperature variation. Several hypotheses have been proposed to explain this independence with further investigation is still required.

Products yields analysis showed that TPOs, with high amounts of light compounds in the feed, primarily produces more economically interesting lighter fractions (naphtha, kerosene and diesel) compared to VGO, which tends to produce heavier fractions.

Hydrogen consumption during hydrotreating is higher for TPOs compared to VGO. However, VGO shows an increased temperature dependence due to its higher aromatics hydrogenation. TPOs exhibit less temperature sensitivity, with TPO A consuming the most hydrogen due to its high nitrogen and aromatics content, despite lower nitrogen conversion.

Results acquired from these experiments show that TPOs can be a good feed alternative for fuel production when upgraded using a

commercial hydrotreating catalyst. These findings suggest that TPO hydrotreating can address challenges related to waste tires management, while contributing to a more sustainable and circular economy model. However, further hydroprocessing is needed (hydrocracking) in order to comply with fuel quality directives and regulations.

## CRediT authorship contribution statement

**Meryem Badlaoui:** Writing – original draft, Visualization, Methodology, Data curation, Conceptualization. **Ngoc Yen Phuong Cao:** Writing – review & editing, Visualization, Validation. **Warumporn Pejpichestakul:** Writing – review & editing, Visualization, Validation. **Benoit Celse:** Writing – review & editing, Visualization, Validation, Supervision, Methodology. **Bertrand Guichard:** Writing – review & editing, Visualization, Validation. **Minh Tuan Nguyen:** Writing – review & editing, Visualization, Validation. **Nadège Charon:** Writing – review & editing, Visualization, Validation, Supervision, Methodology. **Joris W. Thybaut:** Writing – review & editing, Visualization, Validation.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Appendix A. Supplementary data

Supplementary material related to this article can be found online at https://doi.org/10.1016/j.cej.2025.165115.

## Data availability

Data will be made available on request.

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