Experimental Evaluation of Pyrolysis Processes for Kazakhstan Oil Sludge

Abstract

The utilization of oil sludge for the creation of value-added petroleum products represents an important research direction, as certain processing routes do not incur the additional costs that are associated with more complex refining operations. The selection of the most appropriate treatment method is therefore critical for achieving cost-effective processing outcomes. The economic feasibility of a particular technology is largely determined by the physical–chemical properties and potential toxicity of oil sludge, and thus, it is essential to comprehensively characterize and assess the toxicity of this substance. In this study, the physical–chemical composition and principal characteristics of oil sludge obtained from a Kazakhstan oil company were examined. To clean the oil sludge, an alkaline solution was used as a surfactant with a solid–liquid ratio of 1:3. The solid content in the sludge was reduced from 23% to 0.76%. The results revealed that the hydrocarbon fraction of the oil sludge was predominantly composed of heavy fractions. In addition, the effects of thermal parameters on treatment efficiency were found to contribute to the secondary products present in high oil fractions. Treatment with inert gases improved processing efficiency rates by over 57%. The most efficient results included the pyrolysis of cleaned oil sludge with minimum solid residues (5.8% under CO₂) and maximum gas products (37.8% under N₂).

1. Introduction

As environmental requirements become more stringent, the inclusion of hydrocarbon components in the processing of oil-containing wastes is becoming particularly urgent, as it will help to solve the problems associated with increasing the resource base of the oil complex and will thus improve the state of the natural environment. The oil industry is characterized by the generation of a significant number of oil-bearing wastes of various origins. The formation of oily sludges during exploitation of oil fields occurs in different ways, including in discharges during oil preparation or during cleaning of reservoirs, in oil-containing washing fluids used in drilling operations, and during testing and overhaul of wells. Oil-bearing waste can also be generated by accidental spills, the transport of crude oil and its derivatives, as well as in the treatment facilities of petrochemical plants and refineries.

Thermal methods for processing oil sludge are currently the most effective, and based on considerations of economic social benefits and processing difficulties, pyrolysis has been recognized as the most suitable technique [1]. Pyrolysis is characterized by advantages such as the absence of harmful emissions into the atmosphere; the possibility of obtaining useful products; and the elimination of the need to sort the raw materials before loading them into the reactor. However, the problems associated with reducing capital costs and energy supply for the process have not been fully resolved [2].

Compared to other processing methods, pyrolysis/gasification of the oil sludge produces pyrolysis resin, gas, and syngas. The physical and chemical characteristics of oil sludges are presented in [3]. The principle, kinetic characteristics, influencing factors, pyrolysis/gasification reactors and pyrolysis/gasification product characteristics of oil sludges are discussed. To improve the efficiency of thermal conversion, catalytic pyrolysis and co-gasification are proposed. The process of pyrolysis of oil sludge has been developed from basic research to industrial scale before progressing to industrial applications. In industrial applications, the oil recovery rate exceeds 80%. Synthesized gas comprises 30–40 mass. % of gasification products.

Temperature is one of the most important working parameters in the complex pyrolysis process, and has a great influence on the products of pyrolysis, especially with regard to output and quality [4]. When processing different raw materials, the optimal temperature often differs slightly. The selection of the optimum temperature allows us not only to reduce energy consumption, but also to achieve the optimal use of the product. For sewage sludge, the temperature of pyrolysis is usually below 600 °C and the temperature of the oil sludge is similar to that of sewage sludge, which ranges between 500 °C and 700 °C.

In [5], low-temperature pyrolysis at 500–550 °C was studied, producing combustible gases and a solid residue. The study found that pyrolysis is suitable for recycling solid oil sludge with low water content (less than 3%). The choice of temperature depends on the composition of the feedstock, which can yield additional products.

Pyrolysis temperature is known to have the greatest impact on oil and gas yield. Pilot studies in the temperature range of 350–530 °C show that the main gaseous products are methane, ethane, ethylene, propane, and propylene. It should be noted that the yield of light hydrocarbons increases with increasing temperature, while aromatic compounds decrease [6]. This approach to decomposing the solid fraction indicates that the pyrolysis temperature is insufficient, resulting in low reactivity of the feedstock.

Nitrogen and argon are widely used as carrier gases. Inert gas is important but does not participate in the reaction. The types of protective gases do not significantly affect the pyrolysis process, but the flow determines the residence time of the products, especially gaseous ones, thereby indirectly affecting the pyrolysis process. Studies on the catalytic pyrolysis of oil sludge [7] at 500 °C used inert nitrogen to increase the catalytic activity of the catalysts. The use of inert gas can lead to dehydrogenation and deoxygenation reactions, as evidenced by increased H₂ and CO₂ formation.

The pyrolysis of oil sludge is attributed to mild operating conditions, energy recovery, and smaller quantities of pollutants. In reference [8], the effect of pyrolysis temperature (350–550 °C) on products in a stationary layer reactor and external heating was studied, resulting in 7.4–11.65% gaseous products, 25.6–32.35% liquid products, and 56–67% solid residues. The calorific values were 23.9–48.2 MJ/Nm³ for gaseous products, 44.4–46.6 MJ/kg for liquid products, and 13.8–34.4 MJ/kg for solid residues. Moreover, the main compounds of the liquid products were alkanes and alkenes (C₅–C₂₉), while the gaseous fractions were predominantly represented by hydrocarbons and H₂.

Reference [9] found that up to 70–84% of liquid products are produced in a pseudo-cooled reactor at 460–650 °C. It was also found that the release of hydrocarbons increases at temperatures of 600–700 °C. A high temperature leads to an increase in the yield of light fractions and a decrease in solid residue.

It was also found that oil sludge contains many sulfur and nitrogen compounds. These compounds undergo transformation and migration during the pyrolysis process and are eventually distributed in the pyrolysis products, resulting in reduced product quality. Analysis of the initial oil-containing sludge sample and pyrolysis products identified the release of nitrogen compounds in various forms. It was found that a temperature of 400 °C is suitable for the pyrolysis of oilfield sludge, reducing emissions of nitrogen-containing pollutants and promoting environmentally friendly production methods [10]. However, this temperature does not lead to the desired increase in distillate fractions.

In [11], the combined pyrolysis of oily sludge and rice husks at 600 °C in various ratios was studied. Due to the synergistic effect, the content of saturated and aromatic compounds in the petroleum product increased by 55–86%, while the content of resins and asphaltenes decreased by 11–68%. The presence of rice husks leads to the formation of ash and alkali metals in the system, where co-pyrolysis can be classified as catalytic cracking, which promotes secondary reactions of liquid products. At the same time, higher pyrolysis temperatures lead to increased levels of H₂, CO, CO₂, methane, and ethane hydrocarbons in the gas product [12].

In different regions of Kazakhstan, the problem of elimination of oil sludge storage is particularly prevalent because it depends on the infrastructure of fuel and energy complex, the level of urbanization, and natural-climatic factors. The presence of open bunds with a large amount of accumulated liquid and paste oil sludge leads to constant pollution of the environment, such as the soil, surface water and groundwater, as well as atmospheric air with hydrocarbons, hydrogen sulfide, and other emissions from the evaporation of light fractions [13].

In a harsh continental climate, such as Kazakhstan, oil pollution under external environmental factors increases in size, evaporates, is absorbed by living organisms, and also undergoes transformation. Oil sludge is characterized by a highly stable suspension emulsion containing high quantities of polycyclic aromatic hydrocarbons, and is considered a hazardous industrial waste. The formation of oil sludge is mainly influenced by two main factors: inorganic residues (clay, sand, salt, dust, etc.) and storage tanks for oil. According to reference [14], approximately 10,000 m³ of oil sludge is produced annually in refineries with a daily capacity of up to 500 barrels. Another study showed that a refinery with a capacity of 105,000 barrels per year generates 50 tons of oil sludge annually; that is, between 0.3 and 0.5% of oil waste is produced from one ton of crude oil reprocessing [15]. Reference [16] showed that the resulting oil sludge represented 0.5% of the world’s total annual crude oil production. In addition, ref. [17] showed that oil sludge formation ranged from 0.1 to 1.5% and averaged 228.29 million tons per year worldwide. Thus, oil slurries pose a danger to the environment on a global scale.

Low volatile content and low bound carbon content give the oil sludge some properties such as hydrophobicity, a complex chemical structure, and persistence [18]. The oil sludge contains chemicals that cause cancer and are immunotoxic [19,20]. Various metals and poisonous organic compounds, such as toluene, benzene, phenolic acids, and xylol, and a small number of non-acid compounds, such as esters, ketones, and amides, make up most of the oil sludge. Among all the chemical components present in different types of sludges, iron (III) oxide is the most common compound (51%). The high content of iron oxide in oil sludges provides inhibitory and low water absorption properties, which can be used as additives for road surfaces and for synthesis gas production.

The process of refining oil sludge results in valuable products but also causes many problems. Most disposal methods inadvertently result in toxic, mutagenic, and carcinogenic chemicals being released into the environment. These include bacteria, sulfur compounds, nitrogen compounds, paraffins, asphaltenes, heavy metals, and organic pollutants in the form of polyaromatic hydrocarbons [21]. Under the influence of sunlight, oil sludge undergoes several aging and weathering processes that result in an increase in average molecular weight.

If the concentration of oil in water exceeds the ecological limit of rivers or lakes, it will reduce aquatic biodiversity. High-molecular-weight hydrocarbons clog the pores on the surface, thereby reducing the amount of dissolved oxygen and making it difficult to transfer water and energy to them, which can lead to rotting and plant root death. Oilfields affect the growth of seeds and the length of shoots and roots, and may also cause a sharp drop in the length, energy index, and weight of the probed seedlings. They may also cause oxidative stress in plants. It was shown that the high content of oil sludge caused a change in the osmotic relationship between seeds and water, thus reducing the amount of water absorbed. This proves that the nutrients and oil concentrations of the oil sludge were toxic to plants [22].

Some oil sludge contains radioactive compounds, complicating its processing. A reclamation procedure is proposed in [23] as one possible method for reducing the radioactivity of oil sludge, making it suitable for use as a building material. This is due to the potential for soil and groundwater contamination. Thermal processing of oil sludge converts radioactive compounds into a solid residue, making them easier to dispose of.

The authors of [24] present the results of a study on the thermal processing of oil sludge at temperatures up to 800 °C. It is shown that high temperatures lead to the release of gaseous pollutants such as nitrogen and sulfur.

In [25], the authors conducted a study on the earthworm to determine the toxicity of oil sludge using mortality, growth, cocooning, juvenile production, and avoidance behavior as test endpoints. The toxic effects have been found to depend on the concentration of oil in the oil sludge. In [26], a rain worm was used as a test organism to assess the ecotoxicity of soils containing oil sludges. The results of the analysis showed that low concentrations of oil sludge have a detrimental effect on biomass and the reproduction of artemia.

In [27], the authors conducted studies on plant growth to observe the growth performance and element content of three different kinds of plants grown in soil containing oil sludge. It was found that different species of plants reacted differently to oil sludges: the ryegrass plant proved more resistant to toxicity than oats and barley. Absorption of heavy metals by the studied plants was not significant.

In [28], a field experiment was conducted to assess the risk of oil sludge to human health on plots of land under arid conditions. The evaporation of hydrocarbons has been found to result in a significantly higher concentration of volatile organic compounds in the air, which can pose a serious health hazard.

Reference [29] assessed the potential environmental risk of heavy metals in oil sludge and its pyrolysis using environmental risk index values. The risk assessment code is used to assess the environmental risk of heavy metals in the ash of an oil sludge. The potential environmental hazard coefficient is used to estimate heavy metals contained in pyrolysis residues. In all studies, it was found that with the increase in pyrolysis temperature, the complex environmental hazard index of heavy metals in residues gradually decreases.

Our literature review shows that for most types of oil sludge, there are no common technologies that can be used to process oil sludge in order to produce recycled raw materials. That is, existing technologies for refining oil sludge include the production of kiln fuel, fuel oil, and oil based on these products, but the use of such methods for slurry and bituminous containing heavy hydrocarbon fractions (asphaltenes and resins) and a large number of mineral components is uneconomical. In this regard, the aim of this work is to test the thermal technology for the disposal of oil sludge containing an increased amount of iron oxide in order to reduce the technogenic impact on the environment and to demonstrate the possibility of obtaining secondary raw materials.

2. Materials and Methods

The oily sludge used in this study came from the oil company “PetroKazakhstan Oil Products”. Four sludge samples were collected from different locations of the sludge-settling pond. The physical–chemical properties of hydrocarbon fraction of oil sludge were determined according to standard procedures: water content [30], mechanical impurities [31], kinematic viscosity, and calculated dynamic viscosity [32], density [33], and pour point [34].

The concentration of crude oil in oil sludge was determined by surfactant extraction using a solution of sodium hydroxide (2 wt. %). Oil sludge samples of 30 g were mixed with 90 mL of sodium hydroxide with pH 13 in a 200 mL flask at 50 °C. The resulting substance was mixed for 30 min and oil products were extracted by filtering. After that, the extracted oil products were placed in a centrifugal tube and centrifuged for approximately 10 min at a speed of 5000 rpm. The sodium hydroxide was filtered. Each oil sludge sample was extracted four times, and the filtrate was combined. The extracted petroleum hydrocarbon content was then isolated and quantified by gravimetric methods. Finally, the hydrocarbon group composition was determined according to existing procedures.

The SARA composition of pyrolysis products was determined using a conventional procedure. The asphaltene content in the sample was measured via precipitation. The concentration of resins in the obtained maltenes was then determined by adsorption: the maltenes were applied onto activated silica gel (ASC), placed into a Soxhlet extractor, and sequentially washed with n-hexane to separate the oil fraction, followed by ethanol–benzene mixture to extract the resins.

2.1. Analysis of Gaseous Products

The gaseous products of pyrolysis were analyzed by gas chromatography using a Khromos GX-1000 chromatograph (Chromos Engineering company, Dzerzhinsk, Russia). Hydrocarbon gases from C₁ to C₅ were separated by gas–liquid chromatography on a column packed with triethylene glycol dibutyrate on spheronchrom. The column was 7 m in length and 3 mm in diameter, and was operated in an isothermal mode at 40 °C. Helium (He) was used as the carrier gas at a flow rate of 150 mL/min.

2.2. Description of the Laboratory Pyrolysis Setup

Pyrolysis of oil sludge was performed in a stationary porcelain boat placed inside a horizontal steel tubular reactor (Figure 1), installed in an automated, temperature-controlled pyrolysis furnace. Both the reactor inlet and outlet were equipped with coiled condensers to maintain the required temperature. At the reactor inlet, a gas cylinder (1) and a flow-control unit (2) were connected to regulate the pyrolysis atmosphere. At the reactor outlet, an adapter was connected to a condensation system consisting of a reflux condenser, a receiver, and two sequentially connected glass traps immersed in ice (4), as well as a gas meter for collecting pyrolysis gases (5). The collected gas subsequently underwent chromatographic analysis (6).

Preliminarily ground oil sludge samples with a defined degree of dispersion, in an amount of 14 g, were placed into a porcelain boat with a length of 19.5 cm and a width of 1.7 cm. Pyrolysis experiments were conducted at 700 °C under nitrogen and carbon dioxide atmospheres at atmospheric pressure. The flow rates of nitrogen and carbon dioxide supplied to the reactor were controlled using a rheometric tube and varied within the range of 8–9 mL/min. The oil sludge heating rate was 10 °C per minute. The average reaction process duration was 40 min.

2.3. Hydrocarbon Composition of Liquid Oils

Separation was performed using a DB-35MS capillary column (30 m length, 0.25 mm inner diameter, 0.25 μm film thickness) with helium as the carrier gas at a constant flow rate of 1 mL/min. The chromatographic temperature program ranged from 40 °C to 300 °C at a heating rate of 5 °C/min. Detection was carried out in SCAN mode over the range m/z 10–850. System control, data acquisition, and processing were performed using Agilent MSD ChemStation software (version 1701EA). Mass spectral interpretation was based on Wiley 7th edition and NIST’02 libraries.

3. Results and Discussion

Table 1. Properties of the oil sludge sample.

Parameters	Unit	Actual Value
Hydrocarbon content	wt. %	61.86–71.22
Water	wt. %	0.33–5.20
Mineral content	wt. %	23.58–37.81
Density	kg/m3	892–901
pH		6–9
Mechanical impurities’ chemical content:
SiO2	wt. %	35.6
Fe2O3	wt. %	11.5
CaSO4	wt. %	5.3
CaO	wt. %	3.3
MgO	wt. %	2.6
Al2O3	wt. %	1.2
Mass loss on ignition	wt. %	40.5

listed the characteristic composition of oil sludge samples collected from different locations. The water content in the investigated sludges varied between 0.33 and 5.20%. The major hydrocarbon fraction of the sludge accounted for 61.86–71.22%, and was characterized by a relatively high density of 901 kg/m³. The chemical composition of the mechanical impurities was mainly represented by quartz, calcium sulfate, and iron oxide; the latter formed as a result of the corrosion of transportation pipelines and oil storage tanks.

The physical–chemical properties of the hydrocarbon fraction of oil sludge are presented in

Table 2. Chemical properties of hydrocarbon fractions of the oil sludge.

Parameters	Unit	Actual Value
Density at 20 °C	kg/m3	989.0
Mechanical impurities	wt. %	0.76
Viscosity conditional at 80 °C	°VC	2.84
Boiling point	°C	75
Flash point in open range	°C	124
Viscosity is dynamic at 20 °C	Pa·s	0.002–0.006
Viscosity is dynamic at 50 °C	Pa·s	0.004–0.009
Viscosity is dynamic at 80 °C	Pa·s	0.0011–0.0015
Pour point	°C	−2
Heating value	MJ/kg	23–26
Coking value	wt. %	12.5
Asphaltenes	wt. %	1.3
Resins	wt. %	11.3
Oils	wt. %	87.4
Elemental composition:	wt. %
C (carbon)		84.9
H (hydrogen)		12.7
S (sulfur)		0.3
N + O (nitrogen + oxygen)		1.1

. In terms of component distribution, the asphaltene fraction accounted for 1.3%, while resins comprised 11.3%, with the oily compounds predominantly represented by paraffins. The elemental composition revealed a low sulfur content, which is considered advantageous for the subsequent processing of oil sludge.

Oil sludge with hydrocarbon content (over 40–50%) is a good feedstock for the pyrolysis process. It provides a high yield of liquid products and gases. A heavy organic content (including resins and asphaltenes) requires high-temperature pyrolysis (higher than 600 °C). The presence of some mineral compounds (clay, sand, carbonates, metal oxides) leads to a reduction in the calorific value and the overall yield of target oil products. However, some components (Fe₂O₃, CaO, Al₂O₃) can act as cracking and dehydrogenation catalysts. A high amount of light hydrocarbons produces fuel gas and diesel fractions. An elemental composition with high carbon and hydrogen content indicates good potential for producing liquid and gaseous fuels. A nitrogen quantity of more than 2% promotes NO_x formation during gas combustion and is important for assessing environmental risks.

Pyrolysis of Oil Sludge in an Inert Atmosphere

As noted, the inert atmosphere indirectly affects the pyrolysis process and reduces the formation of gaseous products. The effect of feedstock types, with and without mechanical impurities caused by oil sludge, on the yield of pyrolysis products at 700 °C in N₂ and CO₂ atmospheres was investigated. The material balance of pyrolysis for contaminated and uncontaminated oil sludge under nitrogen and carbon dioxide atmospheres is presented in Figure 2.

The CO₂ atmosphere was associated with a slightly higher yield of liquid products: 55.5% compared to the 52.25% yield observed in the N₂ atmosphere. A higher degree of gas formation was also recorded. This effect may be attributed to the heterogeneity of initial oil sludge composition and the presence of mineral components. Thermal treatment accelerates the coking process of hydrocarbons; however, heat transfer in the presence of an inert gas ensures direct contact of oil sludge in pyrolysis reactor, which collectively slows down coke formation and increases the yield of liquid products. At elevated temperatures, cleavage of high-molecular-weight aromatic hydrocarbons occurs, leading to the formation of lower-molecular-weight hydrocarbons. The mineral fraction of oil sludge likely acts as a heat-retaining component due to its high natural heat capacity, whilst also acting as a natural catalyst [35,36]. Comparative analysis of the pyrolysis products of cleaned oil sludge under the examined gaseous atmospheres indicates that the inert environment has no significant effect on product yields. This is presumably due to the homogeneous composition of the oil sludge emulsion and the absence of impurities capable of influencing thermal transformation. Pyrolysis gases might be collected and used as fuel gas [37].

More detailed information was obtained from the study on the SARA composition of pyrolysis products of oil sludge.

Table 3. SARA composition of liquid products from the pyrolysis of uncleaned oil sludge.

Process Type	SARA Composition, wt. %
	Insoluble Substances	Oils	Resins
Oil sludge	26.60	64.99	8.41
Under nitrogen atmosphere	11.34	73.92	14.74
Under carbon dioxide atmosphere	6.03	74.39	19.58

and

Table 4. SARA composition of liquid products from the pyrolysis of cleaned oil sludge.

Process Type	SARA Composition, wt. %
	Insoluble Substances	Oils	Resins
Oil sludge	1.3	87.4	11.3
Under nitrogen atmosphere	1.4	81.47	17.13
Under carbon dioxide atmosphere	1.57	75.25	23.18

present the analytical data on the component composition of uncleaned and cleaned oil sludge, respectively.

Table 3 shows the results of the SARA composition of liquid products obtained from the pyrolysis of uncleaned oil sludge at 700 °C. The data indicate that under a CO₂ atmosphere, fewer insoluble substances are formed, while the oil content in the liquid products remains nearly identical. Moreover, it can be observed that the CO₂ atmosphere promotes the formation of larger amounts of resinous oily components.

The original oil sludge, as an industrial waste, represents a complex system containing a significant number of insoluble substances such as clay and inorganic salts, some of which can influence hydrocarbon transformation processes and act as catalysts. This, in turn, affects both the composition and the yield of pyrolysis products. Table 4 summarizes the results of the determination of the component composition of liquid products derived from the pyrolysis of cleaned oil sludge at 700 °C under different atmospheres.

Resin and oil fractions were mutually alkylated via the Friedel–Crafts reaction. Thus, oil fractions were converted into resins in the presence of various metal ions originally present in the initial oil sludge samples. During the conversion process, the aromaticity degree of molecules increases, the cycloalkane and aliphatic carbon content decreases, and the content of methyl groups increases. All these reactions lead to the formation of new products (gaseous, liquid, and solid residues) that are not present in the initial samples. In our case, the target product was an increase in the oil fraction; the main reactions were cleavage reactions, leading to the formation of aromatic hydrocarbons [38,39].

The hydrocarbon phase obtained after thermal treatment of cleaned oil sludge exhibits significant differences in chemical characteristics compared with the uncleaned oil sludge. Following thermal conversion, the liquid product is characterized by increase in oil components in a nitrogen atmosphere and reduced resin content. Conversely, under a CO₂ atmosphere, the oil fraction decreases by more than 6%, while the resinous components increase by approximately 6%.

Table 5. Hydrocarbon composition of oils.

Hydrocarbon Composition	Content, wt. %
	Cleaned Oil Sludge	Under Nitrogen Atmosphere	Under Carbon Dioxide Atmosphere
Paraffins	19.28	19.36	16.61
Uncondensed cycloparaffins	15.18	14.05	11.73
Condensed cycloparaffins with 2 rings	15.2	14.99	10.81
Condensed all cycloparaffins	13.57	12.44	13.11
Benzenes	8.59	8.17	8.97
Naphthenobenzenes	6.23	6.65	8.52
Dinaphthenobenzenes	3.97	5.06	6
Naphthalene compounds	7.29	9.2	9.63
Acenaphthenes	3.63	2.41	4.71
Fluorenes	3.91	3.36	4.49

presents the hydrocarbon composition of the oils, which are mainly represented by paraffinic hydrocarbons. The pyrolysis oils of oil sludge obtained in an inert nitrogen atmosphere contain a high amount of cyclic paraffinic hydrocarbons, including bicyclic cycloparaffins. In addition, condensed tricyclic cycloparaffins decrease with a concentration of 12.44%. The presence of polycyclic aromatic hydrocarbons such as naphthenobenzenes, dinaphthenobenzenes, naphthalenes, acenaphthenes, and fluorenes in the oil composition is associated with the fact that the feedstock formed from heavy crude oil and was stored in open barns.

Among the identified compounds in the oils obtained under a carbon dioxide atmosphere, paraffinic components were found at relatively low concentrations (approximately 52%), compared with the oils derived from initial oil sludge and those produced in a nitrogen atmosphere. A slight increase in the naphthenobenzene and acenaphthene content was also observed.

Table 6. Gaseous composition of oil sludge pyrolysis in different atmospheres.

Gaseous Composition	Uncleaned Oil Sludge, wt. %	Cleaned Oil Sludge, wt. %
Methane (CH4)	3.50	28.53
Ethylene (C2H4)	0.57	15.51
Ethane (C2H6)	0.29	7.42
Propylene (C3H6)	0.14	5.65
Propane (C3H8)	0.05	1.06
Butin (C4H6)	0.03	0.59
n-butane (C4H10)	0.08	1.11
Iso-pentane (i-C5H12)	–	0.008
n-pentane (C5H12)	–	0.003
Carbon monoxide (CO)	–	27.81
Oxygen (O2)	–	10.12
Other gases (N2, SOx, He)	95.34	2.20

presents the gaseous composition of the pyrolysis products of uncleaned and cleaned oil sludge in a nitrogen atmosphere. The main gaseous product of pyrolysis is methane, with its concentration exceeding 28% during the pyrolysis of cleaned oil sludge, compared with only 3.5% for initial oil sludge. The gaseous pyrolysis products also contain significant amounts of ethane, ethylene, propylene, propane, carbon monoxide, and oxygen. The presence of considerable amounts of other gases, such as nitrogen, in the pyrolysis products is attributed to the inert nature of the atmosphere itself. In the case of cleaned oil sludge, the pyrolysis products also include iso- and n-pentanes in concentrations no greater than 1 wt. %. Methane, ethylene, ethane, and carbon monoxide are used as fuel gases and as raw materials for chemical synthesis and play important roles in increasing the energy efficiency of the process and reducing the environmental impact. The pyrolysis process provides a closed energy cycle, creating the preconditions for waste-free recycling when integrated with CO₂ capture and wastewater treatment systems [40,41].

4. Conclusions

In this study, to remove the hydrocarbon phase from the oil sludge, we used a highly efficient cleaning method that resulted in a successful extraction rate of 98%. The oil sludge contained significant amounts of heavy petroleum residues and a considerable number of mechanical impurities, as silica and iron oxide had formed as a result of the corrosion of storage tanks.

The optimum condition of the cleaning process is as follows: The oil sludge sample should be washed four times with an alkaline solution (sodium hydroxide (2%)) for 30 min at a temperature 50 °C, a stirring rate of 5000 rpm, and a solid–liquid ratio of 1:3. Under these conditions, the solid content in the sludge can be reduced from 23% to 0.76%.

The material balance and SARA characterization of the pyrolysis process samples of uncleaned oil sludge showed higher yields of gaseous products (25.78% under N₂) and solid residues (24.17% under a CO₂). On the other hand, pyrolysis of cleaned oil sludge led to a decrease in the formation of solid residues (5.8% under a CO₂ atmosphere) and an increase in gas products (37.8% under N₂).

In addition, our research results can improve the applied experimental knowledge of oil sludge processing, enabling targeted production of additional hydrocarbon feedstocks from petroleum industry waste. Moreover, the new data on the chromatographic characteristics of the pyrolysis products, which include a detailed determination of their hydrocarbon composition, contribute to the advancement of this scientific field. Future studies should focus on determining the light fraction and hydrocarbon fraction of oil sludge and refining the catalysis and development of the practical methodology based on key principles.