Recovering Attached Crude Oil from Hydrodesulfurization Spent Catalysts

Abstract

As environmental awareness grows, hydrodesulfurization (HDS) catalysts have become crucial in petroleum refining, yet their use results in oil-laden waste, poses environmental risks, and complicates subsequent treatment. Efficient oil removal is thus critical for processing spent catalysts. This study systematically compares three de-oiling methods, extraction, chemical thermal washing, and pyrolysis, to identify the optimal de-oiling method. In the experiments, extraction achieves a 94.12% oil removal rate at a liquid-to-solid ratio of 10 mL/g, a temperature of 45 °C, and a time of 60 min, maintaining around 90% efficiency after five cycles of solvent recovery. Chemical thermal washing achieves an oil removal rate of 96.26% after 4 h at 90 °C, with 0.15 wt.% SDS, 3.0 wt.% NaOH, and a liquid-to-solid ratio of 10 mL/g. The heavy oil emulsion is then decomposed with 4% CuO and 5% H₂O₂. The pyrolysis method removes 96.19% of oil at 600 °C in 60 min. While the extraction and chemical thermal washing methods are effective, they produce wastewater, raising environmental concerns. In contrast, the pyrolysis method is more environmentally friendly. SEM, EDS, and FT-IR analyses show that after oil removal, the metal structures on the alumina support of the spent HDS catalyst are clearly exposed, facilitating the subsequent recovery of valuable metals.

1. Introduction

With the continuous rise in global environmental awareness, automotive fuel specifications and atmospheric emission standards are becoming increasingly stringent. To meet these growing demands, the importance of hydrodesulfurization (HDS) catalysts in the petroleum refining process is steadily increasing. These catalysts are primarily used to remove impurities such as sulfur, nitrogen, and metals from crude oil [1], thereby producing cleaner energy. However, after multiple cycles of use, the catalysts gradually lose their catalytic activity due to sintering, heavy metal poisoning, coke accumulation, sulfide and residual oil deposition, or the wear and loss of active components [2], ultimately becoming spent catalysts. It is estimated that the global petrochemical industry generates over 170,000 tons of spent HDS catalysts annually [3]. The surfaces of these spent catalysts are typically covered with large amounts of crude oil components and heavy metals, which not only pose a threat to the environment but also may harm human health and have been classified as hazardous solid waste by several countries, including China and the United States [4]. Therefore, finding safe and efficient methods for handling oil-contaminated spent HDS catalysts has become an urgent issue.

Additionally, spent catalysts contain more valuable metals than natural ores, such as aluminum (Al), vanadium (V), molybdenum (Mo), and nickel (Ni) [3]. In particular, with the rapid development of the new energy industry in recent years, the secondary resource recovery of nickel has become a popular topic. The treatment of spent catalysts and the recovery of valuable metals not only contribute to environmental protection but also hold significant economic value. Traditional methods for handling spent catalysts primarily include sending them to metal recovery plants or commercial landfills or performing direct incineration [5]. However, regardless of the method used, the oil removal process remains an essential step. This is for two main reasons: first, the residual oil in spent catalysts contains toxic substances, such as aromatic hydrocarbons, resins, asphaltenes, and trace metals [6], and oil removal helps reduce pollution to air, water, and soil during landfilling; second, the crude oil components adhered to the surface of spent catalysts can interfere with the contact between the solid surface and the liquid phase, thus reducing the efficiency of chemical leaching to recover precious metals. Therefore, the oil removal treatment of spent catalysts is a crucial step to ensure both resource recovery and environmental safety.

Currently, the treatment technologies for oil-containing solid waste can be broadly categorized into two main types: harmless treatment and resource recovery. Harmless treatment methods include solidification [7], redox processes [8], biodegradation [9], and incineration [10], while resource recovery encompasses physical separation [11], pyrolysis [12], chemical thermal washing [1], ultrasonic treatment [13], and extraction [6]. Among these, incineration is the most widely used method for treating oil-containing solid waste in industrial settings; however, its environmental impact cannot be overlooked. During incineration, toxic and carcinogenic chemicals may be produced, and the presence of oil can also pose explosion risks. In contrast, pyrolysis has the advantage of recovering pyrolysis oil without introducing new substances into the system, thus minimizing environmental pollution. Chang et al. investigated the distribution characteristics of solid residues, liquid oil, and non-condensable gases by performing pyrolysis on oily sludge in a nitrogen atmosphere using dynamic thermogravimetric analysis (TGA) [12]. The surfactant method, due to its simplicity and low cost, is widely used for the removal of oil contaminants from oily solid wastes. Yang et al. effectively removed crude oil from spent oil-containing catalysts by adding sodium hydroxide and surfactants and significantly improved the oil removal efficiency by combining ultrasonic-assisted hydrothermal treatment [13]. Extraction methods are increasingly applied in industrial settings because they allow for the recovery of both oil resources and solvents during the cleaning process, enabling resource recycling, reducing costs, and minimizing environmental pollution. Gao et al. employed solvents such as o-xylene, acetone, dichloromethane, and n-hexane to extract oil from spent catalysts and separated the oil and solvent using a rotary evaporator, achieving the recovery and reuse of both the oil and solvent [6]. However, most of these studies focus on individual methods and lack systematic comparative analyses of different treatment approaches. Additionally, many studies have not delved deeply into the subsequent treatment of oil-containing cleaning liquids or emulsions, particularly in terms of oil recovery and the recycling of cleaning solutions. Given the increasing depletion of resources and the growing environmental pressures, transforming these oil-containing solid wastes into reusable energy is an urgent issue that needs to be addressed.

Therefore, based on the characteristics of spent HDS catalysts and market demands, this study systematically investigates three common oil removal methods: extraction, chemical hot washing, and pyrolysis. The extraction method uses an appropriate solvent to extract oil from the spent catalyst, followed by solvent recovery for recycling. The chemical hot washing method combines an alkaline solution with surfactants to effectively remove oil adhered to the catalyst surface through a thermal washing process. The pyrolysis method involves heating the catalyst in an inert atmosphere using a tubular furnace, in which high-temperature pyrolysis reactions generate pyrolysis oil and pyrolysis gas. This study aims to compare and analyze the oil removal performance of these three methods to identify the most efficient oil removal technique in order to achieve the effective recovery of crude oil from the catalyst surface and facilitate the resource utilization of the catalyst. Finally, the treated and untreated spent catalysts are characterized using SEM, EDS, and FT-IR to assess whether the oil removal process can provide favorable conditions for the subsequent recovery of valuable metals.

2. Results and Discussion

2.1. Optimization of Extraction Process Conditions

2.1.1. Selection of Extraction Agent

This section comprehensively considers the volatility, toxicity, and cost of the solvents and selects six commonly used pure solvents with similar polarity and good solubility to residual oil as oil extraction agents. These solvents are acetone, dichloromethane, toluene, p-xylene, isobutyl acetate, and petroleum ether. The oil extraction experiments are conducted with these solvents under the following conditions: 50 °C, 60 min, a solid-to-liquid ratio of 10:1, a stirring speed of 500 rpm/min, and an average particle size of 47.419 μm. The results are shown in Figure 1.

In Figure 1, the cleaning efficiency of the solvent in descending order is: toluene > para-xylene > methylene chloride > isobutyl acetate > acetone > petroleum ether. Generally, the cleaning efficiency of a solvent increases as its polarity decreases (

Table 1. Solvent polarity and boiling point [14].

Solvent	Polarity	Boiling Point (°C)
Petroleum ether	0.01	30–60
Toluene	2.4	110.6
P-Xylene	2.5	138.4
Methylene chloride	3.4	39.8
Acetone	5.4	58.08
Isobutyl acetate	—	116

) [6] because the oil in the spent HDS catalyst mainly consists of non-polar components. According to the principle of “like dissolves like”, solvents with lower polarity are more effective at dissolving non-polar oils, thereby improving cleaning efficiency. An exception is petroleum ether, which has lower polarity but performs worse than the other five solvents in extracting crude oil from spent HDS catalysts. This is because petroleum ether has a higher solubility for lighter oil fractions, while the oil attached to the spent HDS catalyst contains fewer light components, resulting in poorer cleaning efficiency. In addition, the selection of the solvent not only needs to consider its polarity but also its aromaticity. The analysis of the oil recovered via gas chromatography shows that the crude oil contains a large proportion of aromatic hydrocarbons. Toluene is more similar in composition to the aromatic hydrocarbons and asphaltenes in the crude oil than to the other solvents, giving it a better affinity for crude oil and a higher cleaning efficiency. Therefore, Toluene is selected as the cleaning solvent after careful consideration.

2.1.2. Optimization of Oil Removal Efficiency

In order to further improve the oil removal efficiency of toluene in spent HDS catalysts, this study investigates the impact of several factors, including the liquid-to-solid ratio, temperature, duration, stirring speed, and particle size of the spent catalyst, on the oil removal efficiency of the spent HDS catalyst.

During the extraction process, the amount of solvent used has a decisive impact on the extraction efficiency. Under the experimental conditions of an extraction temperature of 35 °C, an extraction time of 30 min, a particle size of 47.419 µm, and a stirring speed of 200 rpm/min, the effect of the liquid-to-solid ratio (L/S) on the oil removal efficiency was investigated. The results, which are shown in Figure 2a, indicate that as the liquid-to-solid ratio (L/S) increases, the oil removal efficiency of the spent catalyst significantly improves. This is because a higher L/S ratio increases the solubility of the oil in the solvent, thereby facilitating the recovery of more oil [15]. When the L/S ratio increased from 4:1 to 8:1, the oil removal rate exhibited a linear growth trend. However, when the L/S ratio exceeded 8:1, the rate of improvement in oil removal efficiency gradually slowed down. Therefore, considering both extraction efficiency and extraction cost, an L/S ratio of 10:1 is considered optimal.

Figure 2b illustrates the effect of temperature on the oil removal efficiency under the following conditions: a liquid-to-solid ratio of 10:1, an extraction time of 30 min, a particle size of 47.419 µm, and a stirring speed of 200 rpm/min. Within the temperature range of 25 °C to 45 °C, the oil removal efficiency of the spent HDS catalyst increases significantly with rising temperature, with a noticeable improvement. This phenomenon can be explained by the following factors: (1) the increase in temperature reduces the viscosity of the oil, thereby enhancing the fluidity of the crude oil components [16]; (2) the rise in temperature accelerates the diffusion of crude oil components and the Brownian motion of solvent molecules, which in turn increases the mass transfer rate of crude oil molecules from the solid phase to the liquid phase [16]; and (3) the adhesion force between the catalyst particles and crude oil molecules weakens with an increasing temperature, allowing more hydrocarbons to dissolve in the solvent [16]. However, as the temperature continues to rise, a distinct decline in oil removal efficiency is observed, along with a reduction in the amount of toluene recovered via filtration. This suggests that the decrease in efficiency at higher temperatures may be due to the volatilization of toluene, which prevents the sample from fully contacting the solvent. Considering the safety, equipment, and energy consumption issues involved in the oil removal process, 45 °C was selected as the optimal extraction temperature.

Figure 2c demonstrates the effect of extraction time on the oil removal efficiency under the following conditions: a liquid-to-solid ratio of 10:1, an extraction temperature of 45 °C, a particle size of 47.419 µm, and a stirring speed of 200 rpm/min. In the range of 30 min to 60 min, as the time increases, the oil removal efficiency of the spent HDS catalyst continuously improves, reaching its maximum oil removal rate at 60 min. The reasons may be as follows: when the contact time is short, the reaction process is incomplete, leading to a low oil removal rate; as the time increases, the spent catalyst and toluene have sufficient contact; and the oil removal rate continues to improve with good separation efficiency. However, if the reaction time is extended further, the oil removal rate decreases. This is because, by 60 min, the solvent has essentially reached a saturated state, and the extraction process reaches equilibrium. As time increases, the oil removed from the catalyst surface is re-adsorbed, which leads to a reduction in oil removal efficiency. Therefore, the optimal extraction time is selected as 60 min.

Figure 2d depicts the effect of stirring speed on the oil removal efficiency under the following conditions: a liquid-to-solid ratio of 10:1, an extraction temperature of 45 °C, a particle size of 47.419 µm, and an oil removal time of 60 min. Stirring speed plays a crucial role in determining the contact efficiency between the catalyst and the solvent. At low stirring speeds, the interaction between the spent catalyst and the solvent is inadequate, leading to limited solvent involvement in the cleaning process. As the stirring speed increases, the frequency of contact between the crude oil and solvent molecules rises, enhancing the mass transfer rate of the spent catalyst to the solvent. However, if the stirring speed becomes too high, the contact time between the crude oil and solvent molecules shortens, which hinders effective extraction. Experimental results show that the solvent extraction efficiency peaks at 300 rpm/min. Therefore, 300 rpm/min was selected as the optimal stirring speed.

Figure 2e presents the effect of the spent catalyst particle size on the oil removal efficiency under the following conditions: a liquid-to-solid ratio of 10:1, an extraction temperature of 45 °C, an extraction time of 60 min, and a stirring speed of 300 rpm/min. By comparing the oil removal efficiencies of spent HDS catalysts with different particle sizes, it can be concluded that grinding the spent HDS catalyst significantly improves its oil removal efficiency. This is because reducing the particle size increases the exposed surface area and pore volume of the catalyst, allowing more oil to be released, thereby enhancing the oil removal performance. However, when the particle size of the spent catalyst is smaller than D₅₀ = 47.419 µm, the oil removal efficiency does not continue to increase and even slightly decreases. This is because the smaller particles expose fewer oil-laden or unpolluted regions, causing the oil to potentially re-adsorb onto the catalyst surface [17]. Additionally, studies have shown that smaller particles tend to form stronger bonds with the colloids in the oil. Therefore, D₅₀ = 47.419 µm is selected as the optimal particle size for the spent catalyst.

2.1.3. Solvent Recycling

The significant advantage of solvent-cleaning spent HDS catalysts lies in the ability to recover both oil resources and solvents during the cleaning process, thereby achieving resource recycling, reducing costs, and minimizing environmental pollution. After extraction, the spent catalyst and the extracted liquid undergo solid–liquid separation through vacuum filtration. The solid phase, after drying, can be further utilized in the subsequent metal extraction stage, enabling further resource recovery. The extracted liquid is then separated into solvent and crude oil using a distillation unit (Figure 3a). The recovered oil appears brownish-yellow, and its color intensity is closely related to the content of double bonds in the crude oil. It is speculated that the recovered oil contains relatively high levels of aromatic hydrocarbons and asphaltenes.

The recovered solvent was reused in the experimental study for recycling washing oil from spent catalysts, while any lost solvent was replenished to maintain a constant L/S ratio of 10:1 throughout the washing process. The cyclic experiment was conducted at 45 °C, 60 min, and 300 rpm/min, with the D₅₀ of the spent catalyst being 47.419 µm. The experimental results, as shown in Figure 6b, indicate that after five cycles, the toluene extraction efficiency for crude oil from the spent HDS catalyst remains approximately 90%. This demonstrates that solvent recycling for oil recovery from spent catalysts is an economically viable and feasible approach. The recovered solvent volume is shown in red in Figure 3b, with a recovery rate ranging from 88 to 94%. The solvent may be lost during mixing, filtration, and transferring liquids [18]. Therefore, to reduce the loss of solvent, it is possible to optimize the sealing of equipment, simplify the process flow in practical operations, and reduce the frequency of liquid transfer. By doing so, the cleaning cost can be further controlled.

2.2. Optimization of Chemical Thermal Washing Process Conditions

2.2.1. Optimization of Oil Removal Efficiency

To further enhance the efficiency of chemical thermal washing in the oil removal process of spent HDS catalysts, the effects of factors such as SDS concentration, NaOH concentration, liquid-to-solid ratio, temperature, time, stirring speed, and catalyst particle size on the oil removal efficiency were investigated. The data can be found in Figure 4.

The oil removal efficiency from spent catalysts is closely related to the interfacial tension, which is influenced by the concentrations of SDS and NaOH, liquid-to-solid ratio, temperature, time, stirring speed, and particle size of the spent catalyst. The effect of SDS concentration on oil removal efficiency was investigated under the following conditions: 2 wt.% NaOH, an L/S of 6:1, a hot washing temperature of 60 °C, a hot washing time of 30 min, a particle size of 47.419 µm, and a stirring speed of 200 rpm/min. The results are shown in Figure 4a. The oil removal efficiency first increases and then decreases, reaching a maximum at an SDS concentration of 0.15 wt.%. This is because when the surfactant concentration reaches the critical micelle concentration, micelles are formed in the solution, which solubilizes the oil components, making previously insoluble oil soluble. As the concentration increases, the solubilizing effect becomes more pronounced. The surfactant also helps to lift the oil from the solid surface [16,19]. However, when the concentration becomes too high, the solubilizing effect of the surfactant reaches saturation. At this point, interactions occur between the hydrophobic groups, forming bilayer adsorption on the surface of the spent catalyst [16,20]. The mixed system transitions into emulsification, which results in a decrease in de-oiling efficiency. Moreover, excessively high concentrations not only reduce oil removal efficiency but also lead to the waste of chemicals, increasing costs. Therefore, 0.15 wt.% is selected as the optimal SDS concentration.

Figure 4b displays the effect of NaOH concentration on oil removal efficiency under the following conditions: 0.15 wt.% SDS, a liquid-to-solid ratio of 6:1, a hot washing temperature of 60 °C, a hot washing time of 30 min, a particle size of 47.419 µm, and a stirring speed of 200 rpm/min. The effect of NaOH concentration has a similar trend to that of SDS. This is likely because adding an appropriate amount of NaOH can enhance the activity of the surfactant, reduce the oil–water interfacial tension, and neutralize the acidic substances in the spent HDS catalyst, which helps separate the oil, gum, and coke from the catalyst, thus improving the oil removal rate [21]. However, an excessive amount of NaOH can cause the contact angle to shift from water-wet to oil-wet, blocking the water channels between the estimated surface and the oil phase, which reduces the oil removal efficiency. An excessive NaOH concentration not only reduces the oil removal efficiency but also leads to reagent waste and increased costs. Thus, the appropriate ratio of alkali to surfactant is key. Considering these factors, 3 wt.% is chosen as the optimal NaOH concentration.

The effect of NaOH concentration on oil removal efficiency was investigated under the following conditions: 0.15 wt.% SDS, 3 wt.% NaOH, a hot washing temperature of 60 °C, a hot washing time of 30 min, a particle size of 47.419 µm, and a stirring speed of 200 rpm/min. The results are shown in Figure 4c. The oil removal efficiency of the spent HDS catalyst increases with the L/S ratio. A higher L/S ratio increases the solubility of oil in the solvent, thereby increasing the amount of oil recovered. When the L/S ratio increases from 4:1 to 8:1, the oil removal rate increases linearly. However, when the L/S ratio exceeds 8:1, the growth rate of oil removal efficiency gradually slows down. Therefore, considering both washing efficiency and cost, an L/S ratio of 10:1 is selected.

Figure 4d illustrates the effect of temperature on oil removal efficiency under the following conditions: 0.15 wt.% SDS, 3 wt.% NaOH, a liquid-to-solid ratio of 10:1, a hot washing time of 30 min, a particle size of 47.419 µm, and a stirring speed of 200 rpm/min. Within the temperature range of 25 °C to 90 °C, the oil removal efficiency of the spent HDS catalyst increases with temperature, with a noticeable increase. This is because increasing the temperature enhances the molecular motion, increases the activity of the washing agent, reduces hydration, and facilitates micelle formation. It also accelerates the reaction rate and decreases the viscosity of the crude oil, increasing the flowability of the crude oil components, reducing the adhesion between the crude oil and catalyst, and improving the contact between the washing agent and catalyst, thereby enhancing the oil removal efficiency [16]. When the temperature is between 90 °C and 95 °C, the oil removal efficiency increases but at a slower rate, and higher temperatures result in greater energy consumption. Therefore, considering these factors, the optimal washing temperature is selected as 90 °C.

Figure 4e shows the effect of hot washing time on oil removal efficiency under the following conditions: 0.15 wt.% SDS, 3 wt.% NaOH, a liquid-to-solid ratio of 10:1, a hot washing temperature of 90 °C, a particle size of 47.419 µm, and a stirring speed of 200 rpm/min. As the hot washing time increases, the oil removal efficiency of the hot washing process continuously improves. This is because the viscosity of the residual oil attached to the spent HDS catalyst is high. If the cleaning time is too short, the catalyst cannot be fully dispersed in the cleaning solution, and the residual oil does not come into complete contact with the washing agent, resulting in poor oil removal. As the washing time increases from 0.5 h to 4 h, the spent HDS catalyst is more effectively stirred and dispersed, allowing for better contact between the cleaning agent and the spent catalyst, significantly improving oil removal efficiency. However, as the washing time increases further, the dispersion of the spent catalyst in the cleaning solution reaches saturation, and the improvement in oil removal efficiency becomes less noticeable. Extended washing times not only fail to significantly increase oil removal but also increase energy consumption. Therefore, considering the optimal cleaning time, 4 h is selected.

The effect of stirring speed on oil removal efficiency is demonstrated in Figure 4f, which was obtained under the following conditions: 0.15 wt.% SDS, 3 wt.% NaOH, a liquid-to-solid ratio of 10:1, a hot washing temperature of 90 °C, a hot washing time of 4 h, and a particle size of 47.419 µm. As the stirring speed increases, the oil removal efficiency improves. Increasing the stirring speed helps promote the full contact between the spent HDS catalyst and the washing agent, facilitating the action of the surfactant to lift and peel off the oil, thus improving the oil removal rate. However, very high stirring speeds can not only increase energy consumption but also potentially cause the formation of water-in-oil emulsions, which can interfere with the oil–water separation process [22]. Considering both oil removal efficiency and cost, an optimal stirring speed of 800 rpm/min is selected.

Under the conditions of 0.15 wt.% SDS, 3 wt.% NaOH, a liquid-to-solid ratio of 10:1, a thermal washing temperature of 90 °C, and a stirring speed of 800 rpm/min, with a thermal washing duration of 4 h, the effect of spent catalyst particle size on oil removal efficiency was investigated. The related results are displayed in Figure 4g. As seen in Figure 4g, grinding the spent HDS catalyst significantly improved the oil removal efficiency. This is because the reduction in particle size exposes more oil in the catalyst pores, thereby enhancing the oil removal effect. However, when the particle size of the spent catalyst is smaller than D₅₀ = 47.419 µm, the oil removal efficiency does not increase further and even slightly decreases. This is due to the fact that excessively small particles may expose areas with lower oil content or unpolluted regions, causing the oil to re-adsorb onto the catalyst surface. It has also been reported that smaller particles have a stronger interaction with the colloidal substances in the oil. Therefore, D₅₀ = 47.419 µm is chosen as the optimal particle size for the spent catalyst.

2.2.2. Wastewater Treatment

The de-oiling alkali solution is an emulsion of heavy oil containing a large number of alkanes that cannot be directly discharged; thus, the emulsion is decomposed via the catalytic degradation method using CuO. The specific operation is as follows: add 4% CuO and 5% H₂O₂ to the de-oiling alkali solution, heat it to 50 °C, and react for 1 h. The infrared spectrum of the emulsion is shown in Figure 5. The infrared spectrum analysis of the emulsion is as follows [23]: The broad absorption peak at 3447 cm⁻¹ is the stretching vibration of -OH, -NH in alcohol, phenol, and water; the absorption peak at 1396 cm⁻¹ is the asymmetric bending vibration of -CH₃; the absorption peak at 1110 cm⁻¹ is the bending vibration of alkane. The sample after the emulsion is degraded with NaOH is consistent with the peak of pure NaOH solution, and the characteristic peak of crude oil is basically eliminated, indicating that the catalytic degradation can effectively treat the emulsion.

2.3. Optimization of Pyrolysis Process Conditions

In the pyrolysis experiments, the heating rate was initially set to 10 °C/min with a pyrolysis time of 60 min. Pyrolysis was conducted at different temperatures (300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C) to determine the optimal pyrolysis temperature. Once the optimal temperature was identified, the experiment was continued in a nitrogen atmosphere, maintaining the heating rate at 10 °C/min and setting various residence times (30 min, 60 min, 90 min, and 120 min) to optimize the pyrolysis duration. The oil products obtained after pyrolysis were subsequently recovered. The results of the experiment are presented in Figure 6.

Figure 6a shows the oil removal efficiency at different pyrolysis temperatures, and Figure 6a shows the oil removal efficiency at different pyrolysis temperatures. The results indicate that as the pyrolysis temperature increases, the oil removal efficiency rises rapidly, but the rate of increase slows down after 500 °C. When the pyrolysis temperature reaches 600 °C, the yield of pyrolysis oil decreases, likely because, above this temperature, liquid products further crack into small-molecule gases, leading to a decrease in yield and an increase in pyrolysis gas. Therefore, 600 °C is determined as the optimal pyrolysis temperature. Figure 6b shows the effect of different pyrolysis times on oil removal efficiency at 600 °C. It can be seen that as the pyrolysis time increases, the oil removal efficiency first increases and then stabilizes. The extended pyrolysis time promotes the generation of pyrolysis gas, which results in a decrease in the yield of pyrolysis oil; thus, 60 min is chosen as the optimal pyrolysis time.

Oil removal via pyrolysis demonstrates high efficiency due to its ability not only to remove residual oil from the surface of spent catalysts but also to effectively clean the oil trapped within the catalyst’s pores. Additionally, the pyrolysis process generates pyrolysis oil, enabling the recovery of residual oil and laying the foundation for the secondary high-value utilization of waste oil. The characterization of the oil-removed spent catalysts allows for the evaluation of both the oil removal efficiency and changes in pore structure, thereby creating favorable conditions for the subsequent efficient recovery of strategic metals from the spent catalysts.

2.4. Characterization

2.4.1. SEM and EDS

The morphology of spent HDS catalysts before and after treatment via three different oil removal methods was analyzed using scanning electron microscopy (SEM) (Figure 7). The results show that the dispersion of the spent HDS catalyst before oil removal is poor (Figure 7(a₁)), and its surface is completely covered by an oil layer (Figure 7(a₂,a₃)). As a result, the particle size of the catalyst before oil removal (Figure 7(a₁)) is significantly larger than that of the catalysts treated with the three oil removal methods (Figure 7(b₁,c₁,d₁)). After treatment via the three methods, the crude oil on the surface of the spent HDS catalyst and the oil in the pores are effectively removed (Figure 7(b₂,b₃,c₂,c₃,d₂,d₃)), and the dispersion of the spent catalyst is significantly improved (Figure 7(b₁,c₁,d₁)). Additionally, it was observed that the catalyst treated via pyrolysis exhibited better dispersion compared to those treated via the extraction and chemical thermal washing methods. Further analysis of Figure 7(b₂,b₃,c₂,c₃,d₂,d₃) shows that the amount of residual oil on the surface of the spent catalyst after pyrolysis treatment is significantly lower than that after the other two methods, providing more favorable conditions for the subsequent leaching of valuable metals. Therefore, the pyrolysis method demonstrates superior performance in comparison to extraction and chemical thermal washing in the oil removal process of spent HDS catalysts. Moreover, all three oil removal methods effectively expose the metal structures loaded on the alumina in the spent catalyst (Figure 7(b₃,c₃,d₃)), offering better conditions for the subsequent chemical recovery of metals.

The impact of three oil removal methods on the elemental composition of spent HDS catalyst surfaces was analyzed using EDS (Figure 8). It has been reported that HDS catalysts consist of active metals, such as vanadium (V), molybdenum (Mo), and nickel (Ni), supported on an alumina (Al₂O₃) carrier [24]. Therefore, a clean catalyst should primarily contain elements like Al, V, Mo, and Ni. However, as shown in Figure 8a, the spent HDS catalyst with residual oil mainly contains C, O, Al, S, Fe, and Ca. Among these, C and part of O originate from hydrocarbons in the crude oil, while S, Fe, and Ca are impurity elements present in the crude oil. Due to the presence of crude oil, the contents of expected elements, such as Al, O, V, Mo, and Ni, are relatively low in the spent catalyst. After treatment with the three oil removal methods, the contents of elements such as C, Ca, and S significantly decreased or even disappeared entirely, indicating that these methods are effective in removing the crude oil from the spent catalyst surface.

Table 2. Atomic contents of the main elements on the surface of the spent catalyst after degreasing using three different methods.

Oil Removal Method		Untreated Spent HDS Catalyst	Spent HDS Catalyst Treated with the Extraction Method	Spent HDS Catalyst Treated with the Chemical Thermal Washing Method	Spent HDS Catalyst Treated with the Pyrolysis Method
Atomic content of major elements (%)	C	51.24	41.78	35.44	36.02
	O	34.55	10.93	25.74	31.90
	Al	11.11	28.45	24.74	23.08
	V	0.49	0.28	4.89	2.67
	Mo	—	0.46	—	1.53
	N	—	4.72	0.29	3.09

shows that after the removal of crude oil, the Al₂O₃ support and active metals of the catalyst were exposed, with a marked increase in the contents of surface elements, such as Al, O, Ni, and V. Specifically, the data in Table 2 show that after treatment with the extraction method, the C content decreased, indicating that part of the oil was removed, exposing the Al, Mo, and Ni elements of the catalyst. After treatment with the chemical thermal washing method, the contents of C and O significantly decreased, indicating that most of the oil was removed, which in turn increased the contents of the Al, V, and Ni elements. In contrast, after treatment with the pyrolysis method, although the C and O contents slightly increased, the levels of Al, V, Mo, and Ni significantly increased, suggesting that pyrolysis is effective in removing oil from the spent catalyst. The slight increase in the C and O contents may be due to the generation of a small amount of coke during the pyrolysis of the oil.

These results indicate that while the extraction and chemical thermal washing methods are more effective in removing C, the total contents of valuable surface elements in the spent catalyst treated with these methods are lower than those of the catalyst treated via pyrolysis. This suggests that pyrolysis is superior to the other two methods in terms of exposing valuable elements. This could be related to the fact that pyrolysis, under high-temperature conditions, facilitates a more thorough thermal decomposition of crude oil. Therefore, pyrolysis offers significant advantages in providing favorable conditions for subsequent metal recovery.

2.4.2. FT-IR

Figure 9 shows the FT-IR spectra of three different methods of oil removal before and after the treatment of spent HDS catalysts. The red line represents the infrared spectrum of the oil-containing spent HDS catalyst before treatment, while the blue, purple, and green lines represent the infrared spectra of the spent HDS catalyst after the extraction, chemical thermal washing, and pyrolysis methods, respectively. There is a broad absorption band in the range of 3500~3200 cm⁻¹, which may be the stretching vibration absorption peak of -OH [25,26]. The peaks at 2922 cm⁻¹ and 2852 cm⁻¹ correspond to the asymmetric and symmetric stretching vibrations of -CH₂ [26,27]. The absorption peak at 1626 cm⁻¹ is the symmetric stretching of the -COO group, indicating that there is more asphaltene in the oil-containing spent HDS catalyst [28,29]. The absorption peaks at 1458 cm⁻¹ and 1378 cm⁻¹ are due to the symmetric and asymmetric bending vibrations of -CH₃ [27,28]. The absorption peak at 1104 cm⁻¹ is the stretching vibration of Al-O-Al, which is a structure that makes it easy to form hydrogen bonds with oil molecules [30]. There are also two absorption bands in the range of 900~600 cm⁻¹, which may be the deformation vibration of C-H in aromatic rings, further proving the presence of aromatic compounds [31,32].

After toluene extraction, the intensity of the asymmetric (C-H, 2922 cm⁻¹) and symmetric (C-H, 2852 cm⁻¹) stretching vibrations of the catalyst is significantly reduced, indicating that a large amount of alkane substances is removed. The intensity of the symmetric stretching vibration of the -COO group (C=O, 1626 cm⁻¹) is weakened, indicating that there was still a large amount of asphalt pitch in the treated oil-containing spent HDS catalyst. Meanwhile, the peaks at 1458 cm⁻¹ and 1378 cm⁻¹ almost disappeared.

After being treated via chemical thermal washing, the stretching vibration absorption peak of -OH (3500~3200 cm⁻¹) became narrower, indicating that part of the hydrocarbons and water were removed. The intensity of the symmetric stretching vibration of the -COO group (C=O, 1626 cm⁻¹) is weakened, indicating that there is still a part of asphalt pitch in the treated oil-containing spent HDS catalyst. Meanwhile, the absorption peak near 1104 cm⁻¹ is also significantly weakened. Furthermore, the two absorption bands in the range of 900~650 cm⁻¹ almost disappeared.

After being treated via pyrolysis, the stretching vibration absorption peaks of -OH (3500~3200 cm⁻¹) in the catalyst were significantly narrower, indicating that large amounts of hydrocarbons and water were removed. Meanwhile, the asymmetric (at 2922 cm⁻¹) and symmetric (at 82,852 cm⁻¹) stretching vibration peaks of C-H almost disappeared, indicating that alkanes are removed in large quantities. The symmetric stretching intensity of C=O (1626 cm⁻¹) in the -COO group is significantly weakened, indicating that the pyrolysis method also has an effect on asphalt. The absorption peak around 1104 cm⁻¹ is also significantly weakened. In addition, the peaks at 1458 cm⁻¹ and 1378 cm⁻¹ and the two absorption bands in the range of 900 to 650 cm⁻¹ were almost absent.

These changes indicate that the three different treatment methods can remove the crude oil components in the spent HDS catalyst, thereby exposing the metal structure loaded on the alumina more clearly, which facilitates the subsequent recovery of valuable metals. In particular, the efficiency of the pyrolysis method in oil removal is prominent.

2.5. Experimental Results and Analysis

2.5.1. Experimental Results of the Three Methods

Through the aforementioned experimental studies, the optimal process parameters for the extraction, chemical thermal washing, and pyrolysis methods were determined. A comprehensive analysis and summary of the optimization conditions for each method were conducted. The specific results are summarized in

Table 3. Optimal process parameters and oil removal efficiency of the three methods [14].

Types of Methods		Extraction Method	Chemical Hot Washing Method	Pyrolysis Method
Process Conditions	Extractant	Toluene		—
	Concentration of Surfactant	—	0.15 wt.% SDS	—
	Concentration of Alkali	—	3.0 wt.% NaOH	—
	L/S (mL/g)	10:1	10:1	—
	Temperature (°C)	45	90	600
	Time (min)	60	240	60
	Stirring Speed (rpm/min)	300	800	—
	Particle size of spent catalyst (µm)	47.419	47.419	Not ground
	Gas atmosphere	Air	Air	Inert gas (N2)
Oil removal efficiency (%)		94.12	96.26	96.19
Waste liquid volume (mL)		45 (One-time generated amount)	47 (One-time generated amount)	250 (Amount generated after multiple uses)

, which provides a detailed listing of the oil removal efficiency for each method under optimal operating conditions.

According to the data presented in Table 3, all three methods are effective in removing the crude oil adhered to the surface of the spent catalysts, with oil removal efficiencies exceeding 90%. In a comprehensive comparison, pyrolysis exhibits slightly higher oil removal efficiency than the extraction and chemical hot washing methods. Although the extraction method offers the advantage of recoverable extractants, solvent loss occurs during the recovery process, and the solvents, being highly volatile, may pose secondary environmental pollution risks. The chemical hot washing method, on the other hand, involves a longer process duration, leading to higher energy consumption, and requires the treatment of heavy oil emulsions, further increasing operational costs. When handling the same quantity of raw material, both the extraction and chemical hot washing methods generate waste liquid roughly equivalent to the volume of solvent used. Although these waste liquids can be disposed of through subsequent treatment, this adds complexity and cost to the overall process. In contrast, pyrolysis offers a clear advantage in terms of waste liquid generation, producing the least amount of waste and having a relatively lower environmental impact. Since pyrolysis generates almost no waste liquid during the process, the primary source of waste liquid comes from the exhaust gas absorption stage. The absorption liquid can be recycled multiple times, with replacement occurring only when the solution becomes saturated, thus effectively reducing waste liquid generation and simplifying waste treatment.

2.5.2. Environmental and Economic Impacts of Pyrolysis Method

By comparing the comprehensive performance of three treatment methods, pyrolysis demonstrates superior overall effectiveness, showcasing its significant potential for resource recovery and waste management applications. However, despite its high oil removal efficiency when processing oil-containing spent catalysts, a holistic evaluation is still required, particularly regarding potential environmental impacts and economic feasibility.

From an environmental perspective, the primary challenges of pyrolysis include effectively controlling emissions and addressing the carbon emissions associated with energy consumption during the process. Economically, the key factors for feasibility analysis include equipment investment, energy demand, and operational costs. Nevertheless, with advancements in technology and process optimization, the pressure on both environmental impact and economic cost is expected to be alleviated, thereby enhancing the sustainability and market competitiveness of the pyrolysis method.

In practical applications, it is essential to fully consider the physicochemical properties of the spent catalysts, the scale of treatment, and the recovery value, as well as to develop scientifically sound process flows and investment decisions based on these factors. To effectively address these challenges, continuous technological innovation and improvement are required across various aspects, including the pyrolysis technology itself, exhaust gas treatment systems, equipment design and manufacturing, and operational optimization. By continuously advancing improvements in these areas, we expect to ensure that pyrolysis meets both environmental and economic sustainability requirements while also enhancing its industry applicability and practical benefits.

3. Materials and Methods

3.1. Materials and Chemicals

The raw material used in this study is a spent hydrodesulfurization catalyst (V-Mo-Ni/Al₂O₃, HDS), which has a cylindrical structure and is black due to the presence of crude oil and coke on its surface. Its oil content is approximately 26%. The acetone (>99.5%), dichloromethane (AR), toluene (AR), p-xylene (AR), ethyl acetate (>99%), petroleum ether (AR), sodium hydroxide (NaOH), sodium dodecyl sulfate (SDS) are all of analytical reagent grade and are purchased from the Aladdin Chemical Reagent Co., Ltd. (Riverside, CA, USA). All designated concentrations of solutions are prepared using deionized water.

3.2. Experimental Procedure

3.2.1. Extraction Method

A total of 5 g of spent HDS catalyst was added to a flask containing solvent and sealed with plastic wrap. The flask was then placed in a water bath and subjected to an oil removal experiment under magnetic stirring conditions. After the oil removal process, a vacuum pump with circulating water was used to achieve solid–liquid separation. The separated solid was then dried in an oven at 120 °C for 12 h. The extract was subsequently processed using a distillation apparatus (Figure 3a) to separate the solvent from the crude oil. The separated solvent was reused to clean the spent catalyst, and the recovered oil was saved for further use. The extraction process flow diagram is shown in Figure 10.

3.2.2. Chemical Hot Washing Method

A total of 5 g of spent HDS catalyst was added to a beaker containing an alkaline SDS solution and sealed with plastic wrap. The beaker was then placed in a water bath and subjected to the oil removal experiment under magnetic stirring conditions. After the oil removal process, a vacuum pump with circulating water was used to achieve solid–liquid separation. The separated solid was then dried in an oven at 120 °C for 12 h. Subsequently, the de-oiled alkaline emulsion was subjected to catalytic degradation by adding 4% CuO and 5% H₂O₂ [33], and the reaction was carried out at 50 °C for 1 h. The chemical thermal washing process flow diagram is shown in Figure 11.

3.2.3. Pyrolysis Method

Weigh 100 g of spent HDS catalyst and place it in a quartz boat, which is then positioned inside the steel tube of a tubular furnace. Connect the carrier gas delivery line, the steel tube, and the condensation purification unit, and continuously introduce N₂ at a flow rate of 200 mL/min. Once the air is purged from the system, adjust the N₂ flow rate to 100 mL/min. Quickly transfer the quartz boat into the constant-temperature reaction zone to initiate the pyrolysis reaction. The schematic diagram of the pyrolysis reaction system is shown in Figure 12.

3.3. Oil Removal Efficiency Determination

The dried spent HDS catalyst was weighed, and the oil removal efficiency was calculated using Equation (1) [1].

$$\mathsf{\alpha }={\displaystyle \frac{{\mathrm{c}}_0-{\mathrm{c}}_{\mathrm{i}}}{{26\%\times \mathrm{c}}_0}}\times 100\%$$

(1)

Here, $\mathsf{\alpha }$ is the oil removal efficiency of the spent HDS catalyst (%); ${\mathrm{c}}_0$ is the initial mass of the spent HDS catalyst (g); and ${\mathrm{c}}_{\mathrm{i}}$ is the mass of the residual substance of the spent HDS catalyst after oil removal (g).

3.4. Characterization

The morphology of the spent HDS catalyst before and after treatment is characterized using a scanning electron microscope (SEM, JEOL Field Emission Scanning Electron Microscope JSM-7800F, Shenyang, China) and energy-dispersive X-ray spectroscopy (EDS, JEOL Field Emission Scanning Electron Microscope JSM-7800F, Shenyang, China). The functional groups of the surface-attached crude oil on the spent HDS catalyst before and after treatment are characterized using a Fourier-transform infrared spectrometer (FT-IR, Nicolet iS50FTIR, Thermo Fisher Scientific, Shenyang, China).

4. Conclusions

In this study, based on the characteristics of spent HDS catalysts and market demand, three oil removal methods were selected: extraction, chemical thermal washing, and pyrolysis. The effectiveness of these methods in removing crude oil adhered to the surface of spent HDS catalysts was investigated. Experimental results show that all three methods can effectively remove the crude oil adhered to the surface of the spent catalyst. Specifically, the extraction method achieves an oil removal rate of 94.12% when using a liquid-to-solid ratio of 10 mL/g and extracting at 45 °C for 60 min. After five cycles of solvent recovery, the oil removal efficiency remains around 90%. The chemical thermal washing method, when conducted at 90 °C with 0.15 wt.% SDS, 3.0 wt.% NaOH, and a liquid-to-solid ratio of 10 mL/g for 4 h, achieves an oil removal rate of 96.26%. Subsequently, heavy oil emulsions are degraded using 4% CuO and 5% H₂O₂. The pyrolysis method achieves an oil removal efficiency of 96.19% under the conditions of 600 °C and 60 min.

The oil removal efficiency of the three methods is similar; however, the extraction method generates a large amount of oily liquid during the process, and the crude oil cannot be directly obtained after extraction, requiring an additional distillation step. The distillation process involves the heating and separation of the solvent, which consumes a significant amount of energy, increasing both economic costs and environmental pressure. The chemical thermal washing method generates a large volume of oily emulsion during oil removal, which complicates and raises the cost of subsequent treatment and may pose potential environmental hazards. In contrast, the pyrolysis method does not produce large quantities of waste liquids during oil removal and is more environmentally friendly. Additionally, pyrolysis can perform fractional cracking of the oil components at different temperature ranges, thereby improving resource recovery efficiency.

Furthermore, a comparison of the SEM, EDS, and FT-IR analysis results before and after the treatment of spent HDS catalysts shows that all three methods are effective in removing crude oil adhered to the surface of the spent HDS catalyst. Notably, the pyrolysis method demonstrates superior performance in crude oil recovery. During pyrolysis, the dispersibility of the spent catalyst significantly improves, and the metal structure loaded on alumina is clearly exposed, creating more favorable conditions for the subsequent chemical recovery of valuable metals, thus simplifying the recovery process.