1. INTRODUCTION

Removing sulfur compounds from hydrocarbon fuels, such as crude oil, has become an important focus of environmental and catalysis investigations [1-5]. The existence of sulfur compounds in fuels has several negative impacts on fuel derivative quality as well as on the environment and health [1,6,7]. Specifically, sulfur compounds in fossil fuels shorten the life of machines, equipment, and pipes due to their corrosive impact [8]. They are also poisonous to the catalysts used in a refinery [1,9]. Furthermore, sulfur compounds cause several environmental issues [10,11], such as contaminating the atmosphere with sulfur oxides produced during fuel combustion. Particularly, sulfur dioxide (SO2) has been reported as a serious cause of several respiratory diseases [12,13].

Catalytic hydrodesulfurization (HDS) is a well-known process for sulfur removal [14,15]. However, its main drawback is a decrease in the octane number of the fuel products [16] due to the saturation of olefins [17]. In addition, HDS requires intense reaction conditions, resulting in a high hydrogen consumption and a reduction of catalyst life that increases desulfurization penalty fees. Thus, several alternative approaches have been investigated for sulfur removal [18]. These approaches are classified into three main categories: pretreating, posttreating, and enhancing sulfur conversion to hydrogen sulfide (H₂S) during the fluid catalytic cracking (FCC) process of naphtha [19]. These methods use one or more processes such as a catalytic process, selective HDS, adsorption using solid adsorbents (with or without using H₂), membranes, and a biochemical process [20,21]. Oxidative desulfurization (ODS) has attracted much attention due to several advantages over traditional HDS. These advantages include mild reaction conditions (atmospheric pressure and low temperatures), no hydrogenation requirement, and the ability to remove sterically hindered sulfides [22]. Thus, investigations of different oxidizing agents such as NO₂, tert-butyl hydroperoxide (C₄H₁₀O₂), and hydrogen peroxide (H₂O₂) have been conducted on the ODS process. H₂O₂ is the most desirable oxidant agent because it is environmentally friendly [22].

On the other hand, adsorption has several advantages over other methods, such as minimal operational requirements and low penalty fees for sulfur removal [23,24]. One of the most important factors associated with the adsorption method is the choice of adsorbent, which eventually determines the economics, efficiency, and versatility of the process. Activated carbon is the material most often used in adsorption processes due to its high surface area and adsorbing ability. However, to enhance the cost-effectiveness of the adsorption process agriculture and food wastes have been investigated as adsorbent materials due to their high cost-effectiveness and adsorption ability [25]. Eggshell has been investigated as a potential adsorbent for certain heavy metals and organic compounds due to its high porosity, which makes it an attractive sorbent [26,27]. In addition, it was chosen due to its promising efficiency as a catalyst as reported in the literature [28]. Also, it is an abundant material, so there is no need to regenerate it after using. However, to the best of our knowledge, using eggshell as an adsorbent for desulfurization of heavy naphtha (one of the crude oil fractions) has not been evaluated yet.

In this work, catalytic desulfurization of heavy naphtha (HN) with sulfur content equal to 25 ppm was investigated using adsorption by granular activated carbon (GAC) and white eggshells (WES). In addition to their roles as adsorbents, GAC and WES acted as catalysts for the oxidation reaction of sulfur compounds [29]. We investigated the impact of several operating conditions on desulfurization efficiency, specifically, the H₂O₂ to HN volume ratio, solution pH, agitation speed, temperature, contact time, and solid weight at atmospheric pressure in a batch shaking unit. This work provides new insights into the HN desulfurization process with an adsorption method that can remove up to 86 and 65% of the initial sulfur content using GAS and WES, respectively.

2. MATERIALS AND METHODS

Commercial GAC (Unicarbon, an Italian firm) was used in the current work. The GAC was dried in an oven at 110 °C overnight to completely remove any moisture and stored in a desiccator prior to its use. White eggshell (WES) from chickens was used after washing with distilled water to remove any dirt or fine impurities. The WES was dried at sunlight for 24 hours and crushed manually. The surface area, porosity, and pore volume were determined using the 13-points Brunauer–Emmett–Teller (BET) adsorption isotherm method with N₂ (g) as the adsorbate, and in which solid catalysts were first degassed for two hours at 373 K. The physio-chemical properties and elemental composition of GAC and WES are presented in Tables 1 and 2, respectively. The HN was supplied from the Dura Refinery (Baghdad-Iraq). The properties of the HN fraction are listed in Table 3 below.

Table 1. Physio-chemical properties of GAS and WES

Table 2. Elemental composition (weight %) of GAS and WES

Table 3. Properties of used heavy naphtha fractions

Desulfurization of HN (sulfur content 25 ppm) involved using solid adsorbent (GAC and WES) in an orbital water bath shaker(Innova 4080, New Brunswick Scientific Company) at atmospheric pressure and different operating conditions. The experimental procedure was started by adding 100 ml of HN and H₂O₂ (27% w/w aqueous solution, Alfa Aesar) solution at the required ratio in 250 ml Erlenmeyer flask. Formic acid ( CH₂O₂, 97% Alfa Aesar) and deionized water were added together to the flask. Formic acid was used to adjust the pH of the aqueous solution and its interaction with H₂O₂ forms performic acid ( CH₂O₃), which oxidizes the sulfur compounds. The use of 50 ml deionized water was to increase the volume of the polar phase and thereby enhance the phase transfer of the corresponding sulfonic species formed by oxidation of sulfur compounds. These sulfonic components, which are produced from the oxidation of sulfur components, are adsorbed by solid GAC or WES. The aqueous mixture was heated and shaken at the appropriate temperature and agitation speed for the desired period. Then the sample was withdrawn and filtered using Whatman paper No.3 twice. The desulfurized concentration was determined by Antek 9000N/S analyzer according to the American Society for Testing Materials (ASTM) D5453. The operating conditions for these desulfurization processes involved varying the volume ratio of H₂O₂ to HN (0.01~0.05), agitation speed (Uspeed) of the water bath shaker (100~500 ± 1 rpm), pH of sulfur solution (1~5), amount of adsorbent (0.1~2.5 g), desulfurization temperature (25~85 ± 1 °C) and contact time (10~180 minutes). Here we only report the results of each parameter at the optimum values of other parameters. The procedure was triplicated, and good reproducibility was observed. The desulfurization efficiency was calculated by Equation (1)

\(\mathrm{R} \%=100 * \frac{\mathrm{C}_{i n}-\mathrm{C}_{o u t}}{\mathrm{C}_{i n}}\)             (1)

where C_in: sulfur intial concentration in the petroleum fraction before desulfurization

C_out: sulfur residual concentration of the petroleum fraction after desulfurization

3. Results and Discussion

The impact of changing the operational parameters on the efficiency of catalytic and adsorption desulfurization processes via batch mode at various parameters was investigated and the results are presented as follows.

The impact of increasing (H₂O₂/HN) volume ratio from 0.01 to 0.05 was investigated for all cases. The results showed that desulfurization efficiency increased when the ratio increased when all other variables were constant at the optimum values (Uspeed = 400, pH = 1, catalyst weight = 2 g, temperature = 80 o C, time = 150 min) to reach maximum values at ratio 0.035. Increasing the volume ratio above 0.035 led to a sharp decrease in the desulfurization efficiency of GAC. On the other hand, WES showed a steady increase in desulfurization efficiency by increasing the ratio from 0.035 to 0.05. Specifically, increasing the volume ratio from 0.01 to 0.035 increased the removal percentage of GAC and WES from 40 and 33 to 86 and 65, respectively. A further increase in the ratio to 0.05 resulted in decreasing the efficiency of GAC to 39, while no change was observed with WES as shown in Figure 1.

Fig. 1. The effect of volume percentage ratio of H₂O₂ to HN on the desulfurization efficiency.

This result demonstrates the significant role of the (H₂O₂/HN) ratio on the production of hydroxyl (OH^- ) ions and performic acid (CH₂O₃), which increases when the volume of H₂O₂ is increased. This increase enhances the oxidation process of sulfur compounds in the HN fraction forming SO₄^2- which is then absorbed by the solid adsorbent. This was verified by adding BaCl₂ into the resulting aqueous phase, resulting in a white precipitate with SO₄^2- ions. We attributed the better performance of GAC in the volume ratio range between 0.01 to 0.035 to its better catalytic and adsorption activity, which enhanced the oxidation of the sulfur components as well as its adsorption on the GAC high surface area [22,30]. Specifically, H₂O₂ affected the oxygenated functional groups on the activated carbon surface, which in turn enhanced the catalytic oxidation of sulfur components in naphtha and adsorbing them [31]. More details about GAC and WES roles: they catalyze the decomposing of hydrogen peroxide into a hydroxyl group (HO^-) and a highly reactive hydroxyl radical (HO●) according to Equation (1). The hydroxyl radical in turn attacks sulfur compounds forming sulfones as shown in Equation (2) [32]. The consistency of the desulfurization efficiency without any change after the ratio of 3.5 using WES suggests that increasing the volume ratio beyond 0.035 inhibited the catalytic activity of GAC as well as its adsorption ability, while it had no effect on WES. Specifically, Haw et al. reported that the decomposing of hydrogen peroxide to water and gaseous oxygen, Equation (3), competes with the previous decomposing pathway that forms hydroxyl radicals [32]. Inhibition of hydroxyl radical formation leads to inhibiting the desulfurization process, resulting in the poor performance of GAC beyond the ratio of 0.035. Unlike GAC, WES is rich in oxygen molecules which work as a stabilizer for H₂O₂ that prevents the decomposition of H₂O₂ to H₂O and O [33]. Thus, after the ratio of 0.035% only GAC shows a decrease in the activity. However, we do not rule out completely the role of diffusion in decreasing the desulfurization activity above the ratio of H₂O₂ to HN of 0.035.

\(\mathrm{H}_{2} \mathrm{O}_{2} \stackrel{\text { catalyst }}{\longrightarrow} \mathrm{OH} \bullet+\mathrm{HO}^{-}\)              (1)

\(\text{(R)}_2\text{S}+\text{OH}\bullet\rightarrow\underset{\text{sulfoxide}}{\text{(R)}_2\text{SO}}\overset{+\text{OH}\bullet}{\longrightarrow}\underset{\text{sulfone}}{\text{(R)}_2\text{SO}_2}\)              (2)

\(\mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{H}_{2} \mathrm{O}+1 / 2 \mathrm{O}_{2}\)              (3)

Figure 2 shows the impact of increasing the agitation speed (U_speed) from 100 to 500 rpm on the desulfurization efficiency at optimum conditions (H₂O₂/HN = 0.035, pH = 1, catalyst weight = 2 g, temperature = 80 ^oC, time = 150 min). As can be seen from Figure 2, the desulfurization efficiency was enhanced when the U_speed was increased to 400 rpm, while no change was observed for a further increase to 500 rpm. Increasing the desulfurization efficiency by increasing the U_speed is attributed to an increase in the turbulent flow, resulting in better interaction between the sulfur compounds and the solid adsorbent, which enhances the adsorbent rate. In addition, increasing the U_speed from 100 to 400 rpm resulted in a huge difference between the efficiencies of GAC and WES. Specifically, at U_speed 100 rpm the efficiency of GAC and WAE were 40 and 33%, respectively, which are very close to each other, while at U_speed equal to 400 rpm the efficiency was 86 and 65, respectively. This big difference implies that the increasing U_speed enhanced both the catalytic and adsorption activities, which resulted in better removal of GAC. The consistency of the desulfurization efficiency by increasing the U_speed above 400rpm indicates that the U_speed reached an ideal value that maximizes contact between the adsorbent and absorbent.

Fig. 2. The effect of agitation speed, U_speed, on the desulfurization efficiency.

Figure 3 shows that increasing the pH value decreases the desulfurization efficiency at the optimum conditions (H₂O₂/HN = 0.035, U_speed = 400, catalyst weight = 2 g, temperature = 80 ^oC, time = 150 min). This decrease is expected because the low pH value resulting from formic acid is favorable for sulfur oxidation. In addition, the low pH value provides a suitable environment for the reaction between formic acid and H₂O₂ which produces performic acid and promotes the sulfur oxidation process [29]. The more efficient sulfur removal of GAC at lower pH is attributed to three characteristics [29]: First, adsorption of sulfur components on the GAC surface may increase the collision probability of these components and the active oxygen species, which would speed the oxidation reaction. Second, OH^- radicals generated from H₂O₂ can be resonance-stabilized on the carbon surface, which could also promote oxidation of sulfur components. Finally, GAC has a strong affinity with HN and can be dispersed easily in it. Thus, GAC acts as a phase transfer agent for performic acid which is formed in situ and adsorbed on the GAC surface. WES has some similarity with GAC but less efficiency.

Fig. 3. The effect of pH on the desulfurization efficiency.

Figure 4 shows that increasing the catalyst amount enhanced the desulfurization efficiency at the optimum conditions (H₂O₂/HN = 0.035, U_speed = 400, pH = 1, temperature = 80 ^oC, time = 150 min). This enhancement was attributed to an increase in the available surface area for adsorption and catalytic activities. Increasing the solid catalyst surface area increases the number of active sites on the surface, which increases the availability of binding sites for oxidation and consequently increases the efficiency of the oxidation of sulfur compounds in both catalyst types. In addition, a large surface area increases the adsorption sites, which enhances the adsorption ability.

Fig. 4. The effect of catalyst amount on the desulfurization efficiency.

Increasing the temperature from 15 to 80 ^oC at the optimum conditions (H₂O₂/HN = 0.035, U_speed = 400, pH = 1, catalyst weight = 2 g, time = 150 min) enhanced desulfurization efficiency for both catalysts as shown in Figure 5. This was attributed to the production of a greater number of active oxygen species as well as the oxidation of sulfur compounds at higher temperatures. The formation of active oxygen species may accelerate the oxidation reaction of sulfur when the reaction temperature increases from 25 to 80 °C. Furthermore, increasing the temperature decreases viscosity [34], which might facilitate penetration by the heavy fluid into the solid pores, which enhances the catalytic and adsorption reactivities. However, a further increase of the temperature to 85 ^oC did not produce any further increase in the desulfurization efficiency, which makes 80 ^oC the best temperature for desulfurization

Fig. 5. The effect of temperature on the desulfurization efficiency.

Figure 6 shows the impact of process time on desulfurization efficiency at the optimum conditions (H₂O₂/HN = 0.035, U_speed = 400, pH = 1, catalyst weight = 2 g, temperature = 80 ^oC). The results show that increasing the process time increased the percentage of desulfurization efficiency. This is probably due to more contact time between liquid reactants, which increases the oxygenation percentage of sulfur compounds. Furthermore, increasing the process time increases the contact time between adsorbent, GAC or WES, and sulfur compounds, which increases the adsorption efficiency [25]. No change in the desulfurization efficiency was observed after 80 minutes. Thus, a steady state approximation was assumed and a quasi-equilibrium situation was accepted. This is probably due to the availability of a high number of vacant surface sites for adsorption during the first 80 minutes. After this period, no more adsorption occurred at those vacant surface sites due to saturation [35]. 

Fig. 6. The effect of time on the desulfurization efficiency.

Regeneration of the used catalysts is an essential step to reuse them again after their saturation with sulfide compounds. As mentioned, those materials are not expensive, especially WES, and their regeneration would not have an economic value for this process. Thus, the regeneration of those materials has not tested here. However, the recyclability of those materials in other applications, such as using them as additives to concrete as a replacement for cement or fine aggregate, is strongly suggested to enhance the economic advantages of this process [36,37].

5. Conclusions

Combining oxidation and adsorption processes to remove sulfur components from HN showed promising results. Catalytic oxidation of sulfur compounds using H₂O₂ combined with adsorption using GAC and WES successfully removed 86% and 65%, respectively, of the initial sulfur content at optimum operating conditions. Increasing the H₂O₂/HN ratio increased the removal efficiency to a maximum value at a volume ratio of 0.035 due to the formation of more hydroxyl (OH^- ) ions and performic acid (CH₂O₃). This increase in OH^- and CH₂O₃ enhanced the oxidation process of sulfur compounds in the HN fraction through the formation of the SO4 2- which was then absorbed by the solid adsorbent. Unlike GAS, WES showed stability in its activity after a ratio of 0.035 due to its enrichment of oxygen molecules, which prevents H₂O₂ decomposing to H₂O and O. In addition, the low surface area of WES is probably the reason behind its lower performance compared with GAC. Similarly, increasing Uspeed enhanced the desulfurization efficiency by increasing the turbulent flow, which resulted in better interaction between the sulfur compounds and the solid adsorbent which enhanced the adsorbent rate. In contrast, increasing the pH value decreased the removal efficiency because the low pH value resulting from formic acid is favorable for sulfur oxidation. In addition, a low pH value provides a suitable environment for a reaction between formic acid and H₂O₂ which produces and promotes the sulfur oxidation process. Increasing the amount of the solid catalyst-adsorbent enhances the desulfurization process due to the increase in the surface area as well as the active sites for both catalytic and adsorption processes.

Temperature plays an important role in the desulfurization process. Increasing the temperature to 80 ^oC enhanced the production rate of active oxygen species as well as the oxidation of sulfur compounds. The formation of active oxygen species may accelerate the oxidation reaction of sulfur. Furthermore, increasing the temperature decreases viscosity, which may increase the penetration of the heavy fluid inside the solid pores and enhance the catalytic and adsorption reactivities. Finally, increasing the contact time between the liquid reactants enhances the desulfurization process by increasing the oxygenation percentage of sulfur compounds. Furthermore, increasing the process time increases the contact time between adsorbent, GAC or WES, and sulfur compounds, which increases the adsorption efficiency.

Acknowledgments

The authors would like to acknowledge support from Mustansiriyah University, and AlKut Technical Institute - The Middle Technical University (MTU).