Experimental Study on Vacuum Distillation Separation Characteristics of Tar Containing Solid Particles: Distributions of Light Components

Abstract

The impacts of the composition and properties of tar products on their utilization are of great importance, while the consequences of varying tar separation conditions on distillation fractions remain underexplored. Solid impurities in special tar products (e.g., subsurface in situ pyrolysis-derived tar-like substances) can contribute to the separation as well. In the present study, low-temperature coal tar (LTCT) was used as an analogue to pyrolysis product, mixed with semi-coke and coal dust, representing pyrolytic byproducts and nonpyrolytic substances, respectively. The LTCT mixtures were tested with vacuum distillation at various pressures and temperatures. The results revealed the role of pressure in fraction distribution across temperatures, with higher pressure concentrating fractions at lower temperatures. The impact of solid impurities on distillation primarily stemmed from adsorption. Minimal concentrations of solid impurities carried coal dust/semi-coke into the distillation, but higher levels retained them in the residue. The adsorption of coal dust was quite high at lower temperatures and waned as temperature increased, unlike semi-coke, which had consistent adsorption throughout the distillation. The present study can advance the understanding of vacuum distillation for tar products in the presence of solid impurities, offering a framework for the effective distillation/utilization of coal tar. By probing separation conditions, tar properties, and solid impurity effects, the present research will refine strategies for optimizing coal tar use, crucial for enhancing energy security and sustainable progress in China.

1. Introduction

Coal and oil have held positions of significance as essential primary energy sources on a global scale.¹ Situated as a nation of substantial repute in terms of energy acquisition and consumption, China’s energy landscape epitomizes a state abundant in coal yet deficient in oil, with a limited presence of natural gas. The total consumption of petroleum resources in China accounts for 19.41% of the aggregate primary energy consumption, which is an important primary energy source.^2,3 However, the average daily consumption of liquid fuel in China during 2021 amounted to 15 522 kilo barrels, while the corresponding production only reached 3994 kilo barrels per day, resulting in a conspicuous dependence on oil imports.² This issue is inextricably linked to energy security. Coal consumption in China accounts for more than 50% of primary energy, which is expected to be maintained for a long time.^4,5 Consequently, China’s recourse to mitigating the import pressure of oil and gas resources involved a reliance upon substantial coal resource production, particularly through coal tar production by pyrolysis. Coal tar production through pyrolysis refers to a mixed product containing a diverse array of intricate organic constituents obtained by heating coal to elevated temperature in an inert atmosphere and cooling the output pyrolysis gas.⁶⁻⁹ A diverse array of valuable products, including coal tar, crude benzene, and semi-coke, could commonly be derived.¹⁰ Simultaneously, the hierarchical and qualitative utilization of tar holds the potential to facilitate the more efficient utilization of this resource.

Therefore, a multitude of scholars have directed their focus toward the utilization of coal tar, delving into the realms of its hierarchical exploitation. Ma et al.¹¹ summarized in detail the acquisition of value-added chemicals and carbon materials from coal tar. They focused on the sources of coal tar, the types of compounds in coal tar, the characterization of coal tar, and the evaluation of the properties of coal tar pitch carbon-based materials. The structure and properties of coal tar through molecular characterization for more effective utilization of coal tar were discussed. Zhang et al.¹² investigated the mechanism of action of indole extraction from low-temperature coal tar (LTCT) using tetramethylguanidine-based ionic liquids (ILS) and found that hydrogen bonding between ILS and indole is crucial in this process. This finding contributed to the design of ILS for indole extraction, thereby reducing nitrogen-containing aromatic compound contamination in the LTCT utilization process. Smagulova et al.¹³ proposed a method to produce coke from a coal tar distillate fraction with a separation temperature above 280 °C. The properties of this coke were investigated in particular, and the effect of heat treatment of the fraction on the coke produced was studied. Yang et al.¹⁴ systematically summarized the research progress, technical characteristics, and industrial applications of slurry bed and boiling bed technologies suitable for treating coal tar heavy fraction or full fraction and discussed the development direction of coal tar heavy fraction hydrogenation. Previous research has highlighted the widespread consensus within the industry regarding the hierarchical utilization of coal tar. The commonly employed technique involves dividing the tar into different temperature-based fractions through distillation and then utilizing them separately. Distillation stands as the predominant and energy intensive separation method within the chemical industry.¹⁵ One potential avenue for enhancing its efficiency and reducing energy consumption lies in the application of vacuum distillation. However, there has been limited attention given to how the conditions under which the vacuum distillation is carried out affect the properties of the tar and its fractions, which significantly influence their utilization method. Therefore, a practical method for separating coal tar was chosen for the present study. The aim was to examine how the composition and properties of coal tar fractions change when subjected to different vacuum distillation conditions.

In the sphere of coal tar production, some scholars have advanced the notion of employing underground in situ pyrolysis technology for coal tar generation. This method entails the direct pyrolysis of coal within its native seam through the application of thermal energy, leading to the expulsion of pyrolysis gas to the surface.¹⁶ This pyrolysis product is similar to LTCT, characterized by pyrolysis temperatures spanning the range of 500–700 °C.¹⁷ Kelly et al.¹⁸ evaluated the life cycle energy and greenhouse gas impacts of all process stages of coal heat treatment. The results indicated that underground coal thermal treatment could produce high-quality liquid products and gas mixtures with greenhouse gas emissions within reasonable limits. The product yield at low temperatures, heating rate, number of heaters, and moisture content of the coal are key factors in determining the feasibility of this process. Feng et al.¹⁹ investigated the gas production performance and reaction zone evolution law of underground coal gasification with different directions and speeds of continuous moving injection. Optimization methods of the moving gas injection scheme were proposed for low ash and low volatile coal seams, respectively. Diverging from established coal-to-tar methodologies, the in situ underground pyrolysis technique obviates the necessity for traditional coal excavation and the establishment of external pyrolysis infrastructure, resulting in an efficient and environmentally friendly approach.^20,21 Additionally, the pyrolytic process creates a porous framework within the coal seam, affording a reservoir for CO₂ sequestration.^22,23 Nevertheless, it is noteworthy that the outputs derived from subterranean in situ pyrolysis exhibit a certain degree of instability,^24,25 and the production process unavoidably introduces solid impurities. Consequently, when considering the separation of pyrolysis products for subsequent applications, it is necessary to take into account the presence of particulate matter within them because this factor could have an impact on the properties of the separated products.

Henceforth, a comprehensive set of experiments was designed to explore the interplay between separation conditions and the intrinsic attributes of tar fractions during the vacuum distillation process. Moreover, this study delved into the influence of the presence of solid impurities within the tar separation. To simulate tar-like products potentially blended with solid impurities, samples were made by amalgamating solid particles with different characteristics with LTCT. Specifically, these particles included coal dust and semi-coke. The former represents substances without undergoing pyrolysis, while the latter constitutes byproducts from comprehensive pyrolysis. A dedicated vacuum distillation setup was constructed for LTCT, and LTCT blended with varying ratios of coal or semi-coke powders was adopted as the focal point of inquiry to study the influences of distillation pressure and temperature as well as the character and quantity of solid particles on the separation process. The samples were partitioned into discrete distillation fractions under diverse conditions. The chemical constitution of the distillation fractions was subject to examination via gas chromatography–mass spectrometry (GC-MS), affording an analysis of the impacts of these factors on the organic composition within the distillation fractions. This investigation augments the comprehension of the vacuum distillation mechanism pertaining to such distinct tar products containing solid particles, concurrently providing theoretical guidance for the distillation and effective utilization of these products.

2. Experimental Section

2.1. Materials

In the present experiments, low-temperature coal tar was used, and experimental samples were prepared by blending various fractions of solid particles (including coal dust particles and semi-coke particles) into the low-temperature coal tar. Semi-coke represented the byproduct of the coal pyrolysis process, while coal dust represented unreacted solid particles from the underground pyrolysis process. These samples were used to simulate tars containing solid particles, such as the products of underground in situ pyrolysis of tar-rich coal. Hence, the samples were employed without dehydrated treatment. LTCT was prepared using an endothermic low-temperature dry distillation furnace. The proximate analyses of the coal dust and raw coal of LTCT used for the present experiments is shown in Table 1. The methods used in the proximate analyses were under the national standard GB/T 212-2008. The moisture (M), ash (A), and volatile matter (V) in the coals were measured directly from the experiments, while the fixed carbon (FC) content was calculated by the difference subtraction method. The volatile matter fraction of dry ash-free basis (V_daf) of the raw coal used for tar production was 35.6%, and that of the tar-rich coal was 35.4%, both of which are high-quality bituminous coals with high levels of volatile matter. Semi-coke is a byproduct of LTCT production, and its process parameters and surface properties are shown in Table 2. The process parameters were determined by reference to the proximate analysis of the coals, but moisture was not included in the analysis because semi-coke is a pyrolysis product of the coals. Surface property data were obtained using a BET (Brunauer–Emmett–Teller) tester model TB440A from JWGB Instruments. The testing method employed was the static volumetric method. During the testing process, cooling was facilitated by liquid nitrogen with the adsorption temperature maintained at 77.35 K. Nitrogen (N₂) served as the adsorbent for the measurements.

Table 1. Proximate Analyses of Raw Coal of LTCT and Experimental Coal Dust^a.

sample	Mad (%)	Aad (%)	Vad (%)	FCad (%)
raw coal of LTCT	12.0	5.1	29.5	53.4
coal dust particle	5.7	7.6	31.5	55.2

^aM is the content of moisture; A is the ash content; V is the content of volatile matter; and FC is the content of fixed carbon. The subscript “ad” represents air-dry basis.

Table 2. Production Parameters and Surface Properties of Semi-Coke^a.

sample	tjp (°C)	Ad (%)	Vd (%)	FCd (%)	SBET (m2/g)	VBJH (cm3/g)
semi-coke	60–80	7.8	3.5	88.7	2.416	0.008

^at_jp is the temperature of semi-coke leaving the distillation furnace; A is the ash content; V is the fraction of volatile matter; and FC is the content of fixed carbon. The subscript “d” means dry basis. S_BET is the BET surface area, and V_BJH is the BJH (Barrett–Joyner–Halenda) pore volume.

2.2. Experimental Procedure and Conditions

A specialized vacuum distillation experimental bench was built to study the separation characteristics of LTCT containing solid particles by vacuum distillation, and the schematic diagram of the experimental system is depicted in Figure 1. The system consisted of a separation part, a collection part, and a safety part, and the low-pressure environment was constructed by a vacuum pump. The experimental samples were heated by an electric heating unit, and a small amount of air was passed into the liquid by a capillary tube to stabilize the distillation process and prevent burst boiling. The distillation residue was left in the two-neck flask, and the light components mixed with air entered the condenser tube and turned into liquid droplets in the collection bottle. Air was passed through an activated carbon wash bottle to filter out any light components that were incompletely captured. A safety bottle with a valve and a gas washing device were connected to the rear for equalizing the pressure and treating the exhaust gas, respectively. The vacuum environment was first constructed, regulated, and maintained by the vacuum pump and the valve. The heating process was started to carry out the separation process after the pressure was stabilized. The light components separated from the collection bottle were prepared in ppm concentration using dichloromethane as the solvent, whose components were analyzed by GC-MS. The carrier gas used for GC-MS analysis was helium flowing at 1 mL/min. The inlet temperature was maintained at 250 °C, and a sample volume of 10 μL was injected at once. The column temperature was initially set to 50 °C for 5 min and then ramped up to 280 °C at a rate of 5 °C/min, followed by a hold time of 8 min. A solvent delay of 4.5 min was applied. The results were compared with the standard maps of organic compounds in the National Institute of Standards and Technology (NIST) spectral library to semiquantitatively analyze the organic components in the fractions.

Figure 1.

Schematic diagram of vacuum distillation test bench in the present study.

The experiments were designed according to the pressure, temperature, and mass percentage of solids in distillation to study the influencing factors in LTCT vacuum distillation separation. In industrial production, the different fractions of tar separated by distillation are classified as shown in Table 3, and their corresponding boiling points at 13.3, 30, and 45 kPa are provided. Considering that the water in LTCT is difficult to be completely separated and that its dew point is too low to be collected under vacuum, the light oil and water were separated by distillation at atmospheric pressure as the same distillation fraction. The remaining components were classified into four fractions: 170–230 °C, 230–300 °C, 300–360 °C, and the distillation residue, with reference to the classification used in the industry and simplified. The separation mechanism of the LTCT containing solids in vacuum distillation was investigated by controlling the nature and content of the solids. There were two types of solids blended in the LTCT: coal dust represented the solid particles that were directly brought out without participating in pyrolysis, and semi-coke represented the material after complete pyrolysis. They were blended at 5–20%, respectively, during sample preparation. The experimental conditions are shown in Table 4. Vacuum distillation at 30 kPa and the LTCT sample without any solids were taken as the baseline experimental conditions.

Table 3. Distillation Fractions of LTCT Classified in Industrial Production and Their Corresponding Boiling Points under Vacuum Conditions.

	temperature range (°C)
fraction of distillation	at atmospheric pressure	at 13.3 kPa	at 30 kPa	at 45 kPa
light oil	<170	-	-	-
phenol oil	170–210	∼144	∼168	∼181
naphthalene oil	210–230	144–163	168–187	181–201
wash oil	230–300	163–226	187–253	201–268
anthracene oil	300–360	226–282	253–311	268–326
distillation residue	>360	>282	>311	>326

Table 4. Experimental Conditions for Atmospheric Pressure Vacuum Distillation of LTCT Samples Containing Solid Particles.

condition no.	solids	solid mass ratio (%)	distillation pressure (kPa)	final distillation temperature of each fraction (°C)
P13.3	-	-	13.3	170 at atmospheric pressure, 163, 226, 282 in vacuum
P30	-	-	30	170 at atmospheric pressure, 187, 253, 311 in vacuum
P45	-	-	45	170 at atmospheric pressure, 201, 268, 326 in vacuum
P30-C	coal dust	5, 10, 15, 20	30	170 at atmospheric pressure, 187, 253, 311 in vacuum
P30-T	semi-coke	5, 10, 20	30	170 at atmospheric pressure, 187, 253, 311 in vacuum

3. Results and Discussion

3.1. Influences of Distillation Slicing Temperature

Figure 2 and Table 5 present the distributions of mass percentages and properties among the fractions acquired through varied temperatures during distillation, conducted under a pressure of 30 kPa. The separation of moisture and light oil via distillation presents challenges due to their inherent intermingling. Meanwhile, since moisture is difficult to collect due to its low boiling point under vacuum, the moisture and light oil fractions were separated by distillation at atmospheric pressure, while the remaining fractions were obtained through vacuum distillation at 30 kPa. During the experiment, 51.55 g of tar was used, resulting in the distillation of 25.59 g of product, with a successful collection of 25.01 g of this product. This corresponded to a yield of approximately 47.86%, with a negligible loss of merely 1.11%. Within the distillation fraction occurring below 170 °C under atmospheric pressure, a distinct layering of water and oil was observed, with the latter constituting a minor proportion. The fractions subjected to distillation at 187 and 253 °C under 30 kPa pressure exhibited more pronounced oil content, existing uniformly as liquid phases. Conversely, the fraction acquired at 311 °C showed a denser and less lustrous liquid composition, accompanied by the presence of a solid phase. As the distillation temperature increased, a gradual reduction in the mass proportion of each fraction was evident.

Figure 2.

Properties of the distillation fractions obtained at 30 kPa (the superscript * indicates that the temperature is for distillation at atmospheric pressure).

Table 5. Yield of the Distillation Fractions Obtained at 30 kPa^a.

element	units	value
temperature range of distillation	°C	<170*	<187	187–253	253–311
yield of distillation fraction	%	29.74	6.66	6.61	4.86

^aThe superscript * indicates that the temperature is for distillation at atmospheric pressure.

Table 6 presents the outcomes of a semiquantitative analysis conducted via GC-MS to discern the composition of substances within each distillation fraction. The initial fraction was obtained via distillation under atmospheric pressure. Notably, as the distillation temperature increased, the organic constituents tended to concentrate toward species characterized by longer retention times, presenting with heightened structural complexity. Within the initial fraction, phenols and phenol substituents dominated the organic composition, with a limited presence of naphthalene. In the subsequent fractions, there was a reduction in phenolic content, coupled with an increase in naphthalene content. As the distillation temperature increased to over 253 °C in a vacuum atmosphere, a pronounced concentration of organic matter toward species with longer retention times was evident. Simultaneously, the organic structure became more intricate, with the emergence of tricyclic organic compounds such as fluorene. In the final fraction, naphthalene became the most abundant constituent, existing in more intricate forms, and there was a notable rise in the proportion of tricyclic organic matter.

Table 6. GC-MS Compositional Analysis of Distillation Fractions of LTCT at 30 kPa^a.

retention time	organic components	molecular formula	relative content
distillation temperature range (°C)			<170*	<187	187–253	253–311
10.785	phenol	C6H6O	4.10	-	-	-
13.445	phenol, 2-methyl-	C7H8O	16.45	-	-	-
14.224	p-cresol	C7H8O	37.62	30.33	8.27	-
16.687	phenol, 3,4-dimethyl-	C8H10O	16.46	33.38	6.36	3.46
16.750	phenol, 2,3-dimethyl-	C8H10O	8.57	-	11.67	-
17.305	phenol, 4-ethyl-	C8H10O	-	7.66	-	-
17.413	phenol, 3-ethyl-	C8H10O	-	-	4.74	4.94
17.905	naphthalene	C10H8	7.42	6.81	4.25	-
18.173	1,2,2,3-tetramethylcyclopent-3-enol	C9H16O	3.26	-	-	-
18.901	benzaldehyde, 2,4-dimethyl-	C9H10O	-	8.08	2.83	-
19.517	benzene, 1-ethyl-4-methoxy-	C9H12O	1.81	-	2.86	-
20.209	phenol, 2-ethyl-4-methyl-	C9H12O	-	5.92	-	2.20
20.213	phenol, 3-ethyl-5-methyl-	C9H12O	-	-	7.44	-
21.373	naphthalene, 1-methyl-	C11H10	4.29	-	13.06	2.63
21.376	naphthalene, 2-methyl-	C11H10	-	7.82	-	-
21.926	benzaldehyde, 4-ethyl-	C9H10O	-	-	4.81	-
23.972	benzene, 1-ethyl-3-(1-methylethyl)-	C11H16	-	-	-	3.82
23.976	ethanone, 1-(3,4-dimethylphenyl)-	C10H12O	-	-	3.82	-
24.899	naphthalene, 2,3-dimethyl-	C12H12	-	-	5.94	2.54
25.025	naphthalene, 1,7-dimethyl-	C12H12	-	-	-	4.76
25.606	benzene, 1-ethyl-3-(1-methylethyl)-	C11H16	-	-	-	3.63
25.610	benzene, 2,4-diethyl-1-methyl-	C11H16	-	-	3.76	-
26.638	acenaphthene	C12H10	-	-	-	6.24
27.453	1,1,4,5,6-pentamethyl-2,3-dihydro-1H-indene	C14H20	-	-	6.49	-
27.547	1-naphthalenol	C10H8O	-	-	-	8.09
27.795	naphthalene, 2,3,6-trimethyl-	C13H14	-	-	2.48	6.26
28.227	naphthalene, 1,6,7-trimethyl-	C13H14	-	-	4.33	-
28.667	3-(2-methyl-propenyl)-1H-indene	C13H14	-	-	2.58	-
29.201	fluorene	C13H10	-	-	4.32	6.93
29.933	1-naphthalenol, 2-methyl-	C11H10O	-	-	-	9.98
30.125	9H-fluoren-9-ol	C13H10O	-	-	-	9.14
31.392	naphthalene, 1,6-dimethyl-4-(1-methylethyl)-	C15H18	-	-	-	3.81
32.405	9H-fluorene, 9-methyl-	C14H12	-	-	-	5.31
32.770	chamazulene	C14H16	-	-	-	11.53
33.917	anthracene	C14H10	-	-	-

^aThe superscript * indicates that the temperature is for distillation at atmospheric pressure.

3.2. Effects of Vacuum Distillation Pressure

Figure 3 depicts the distribution of mass fractions across various pressure conditions, with the horizontal axis showing the distillation temperature for each fraction within distinct vacuum environments. The fractions falling within the temperature range of <100 °C and 100–170 °C are categorized as a single fraction. Table 7 offers the employed experimental conditions and the associated mass losses during distillation. The observed losses remain within an acceptable range. The yields of distillation residues across diverse conditions exhibit a high degree of resemblance, and the yields of the initial fractions are similar. Since all of these fractions were distilled under atmospheric pressure, it is evident that samples from distinct conditions can be considered as comparable. The subsequent three fractions underwent distillation under vacuum conditions. It is worth noting that substances distilled at 13.3 kPa predominantly accumulated in the second fraction within vacuum environments, whereas at 30 kPa, they concentrated toward the lower-temperature fraction, and at 45 kPa, they favored the higher-temperature fraction. This observed phenomenon could be attributed to the interplay of two factors. First, the variation in distillation temperatures corresponding to different pressures exerts an influence on the type and quantity of organic matter distilled. Second, differing distillation temperatures lead to alterations in factors such as liquid tension and further influence the separation of fractions. Therefore, the composition of each fraction was studied via a semiquantitative analysis by GC-MS to elucidate the impact of these factors on the distillation.

Figure 3.

Fraction yields at different pressures (the superscript * indicates that the temperature is for distillation at atmospheric pressure).

Table 7. Experimental Conditions and Losses of LTCT Sample Distillation at Different Pressures.

condition no.	distillation pressure (kPa)	LTCT mass (g)	mass of substance evaporated (g)	mass of substance received (g)	loss (%)	yield of distillation residue (%)
P13.3	13.3	52.26	25.59	25.01	1.11	51.03
P30	30	53.48	26.52	25.24	2.39	50.41
P45	45	55.70	26.94	24.45	4.47	51.64

Table 8 provides a comprehensive overview of the compositional analyses via GC-MS conducted on the <170 °C fraction under atmospheric pressure. Notably, these analyses were carried out under identical experimental conditions across various samples. The organic matter in the fractions was mainly dominated by phenol and its substituents. The fractions also contained about 7% of naphthalene and a small amount of naphthalene substituents. The resemblance in composition among the different experimental groups serves as a strong indicator of the stability of the experimental outcomes and the homogeneity of the experimental materials.

Table 8. GC-MS Analysis of Fractions Distilled at Atmospheric Pressure at Temperatures Lower Than 170 °C.

retention time	organic components	molecular formula	relative content
experimental number			P13.3	P30	P45
10.785	phenol	C6H6O	5.07	4.10	11.85
13.445	phenol, 2-methyl-	C7H8O	13.40	16.45	15.56
14.224	p-cresol	C7H8O	32.56	37.62	33.18
16.332	phenol, 2-ethyl-	C8H10O	1.33	-	1.46
16.687	phenol, 3,4-dimethyl-	C8H10O	9.74	16.46	15.78
16.750	phenol, 2,3-dimethyl-	C8H10O	2.51	8.57	2.81
17.404	phenol, 2,4-dimethyl-	C8H10O	8.60	-	-
17.905	naphthalene	C10H8	7.44	7.42	7.36
18.173	1,2,2,3-tetramethylcyclopent-3-enol	C9H16O	-	3.26	2.64
19.218	phenol, 3-(1-methylethyl)-	C9H12O	1.80	-	1.44
19.390	phenol, 2,4,6-trimethyl-	C9H12O	1.68	-	-
19.410	phenol, 2,3,6-trimethyl-	C9H12O	-	-	1.39
19.517	benzene, 1-ethyl-4-methoxy-	C9H12O	2.22	1.81	-
19.551	phenol, 2-(1-methylethyl)-	C9H12O	-	-	1.69
19.702	phenol, 2-ethyl-4-methyl-	C9H12O	0.90	-	-
19.722	phenol, 2-ethyl-5-methyl-	C9H12O	3.65	-	2.43
20.609	phenol, 2,4,6-trimethyl-	C9H12O	0.91	-	-
21.373	naphthalene, 1-methyl-	C11H10	5.48	4.29	1.58
24.899	naphthalene, 1,5-dimethyl-	C12H12	1.69	-	-
24.916	naphthalene, 2,3-dimethyl-	C12H12	-	-	0.84
25.019	naphthalene, 1,7-dimethyl-	C12H12	1.02	-	-

Table 9 provides a detailed account of the GC-MS findings pertaining to the first vacuum distillation fraction across distinct experimental conditions. These analyses reveal notable similarities in the organic composition of fractions subject to different distillation pressures. The organic fraction was dominated by phenolic compounds, while the content of naphthalene increased compared to the previous fraction. Dimethylphenol emerged as the predominant constituent, constituting approximately 60% of the total composition. It was noteworthy that elevated distillation temperatures and pressures led to a heightened diversity of organic substances within the fractions. This expansion in diversity could be attributed to the increased thermal degradation effect experienced by organic substances under elevated temperatures. The most notable manifestation was the presence of phenol in the distillate fractions at the experimental conditions of 45 kPa and 201 °C, whereas it was undetected in the rest of the fractions.

Table 9. GC-MS Compositional Analysis of the First Vacuum Distillation Fraction (Corresponding to 230 °C at Atmospheric Pressure).

retention time	organic components	molecular formula	relative content
distillation pressure (kPa)			13.3	30	45
distillation temperature (°C)			162	187	201
10.884	phenol	C6H6O	-	-	2.27
13.477	phenol, 2-methyl-	C7H8O	9.99	-	8.39
14.251	p-cresol	C7H8O	34.87	30.33	28.74
16.705	phenol, 3,4-dimethyl-	C8H10O	28.69	33.38	30.78
17.305	phenol, 4-ethyl-	C8H10O	-	7.66	-
17.912	naphthalene	C10H8	5.28	6.81	5.72
18.177	phenol, 3-ethyl-	C8H10O	-	-	3.52
18.901	benzaldehyde, 2,4-dimethyl-	C9H10O	3.11	8.08	-
19.211	phenol, 2-ethyl-5-methyl-	C9H12O	2.51	-	-
19.233	phenol, 2-(1-methylethyl)-	C9H12O	2.64	-	5.44
20.216	phenol, 2-ethyl-4-methyl-	C9H12O	5.00	5.92	4.88
20.48	phenol, 2,3,6-trimethyl-	C9H12O	-	-	1.26
21.376	naphthalene, 2-methyl-	C11H10	5.67	7.82	7.27
24.895	naphthalene, 2,3-dimethyl-	C12H12	2.23	-	1.73

Table 10 provides a detailed account of the GC-MS findings pertaining to the second fraction obtained under vacuum conditions. An overarching similarity emerged in the composition of fractions across varying distillation pressure–temperature settings, with phenols and naphthalene constituting the predominant constituents. As both distillation temperature and pressure increased, there was an observable broadening of the spectrum of organic components within the fractions, which tended toward species characterized by shorter retention times. This phenomenon was most conspicuous in the fraction obtained under a pressure of 45 kPa, where the component with the shortest retention time was detected at approximately 7 min, while the last component was identified before the 28 min mark. This is due to the phenomenon of organic matter degradation caused by the high temperatures during distillation separation at elevated pressures.

Table 10. GC-MS Compositional Analysis of the Second Vacuum Distillation Fraction (Corresponding to 300 °C at Atmospheric Pressure).

retention time	organic components	molecular formula	relative content
distillation pressure (kPa)			13.3	30	45
distillation temperature (°C)			226	253	268
7.485	o-xylene	C8H10	-	-	1.17
13.488	phenol, 2-methyl-	C7H8O	-	-	2.84
14.3	p-cresol	C7H8O	19.79	8.27	16.38
16.725	phenol, 3,4-dimethyl-	C8H10O	14.74	6.36	-
16.765	phenol, 2-ethyl-	C8H10O	-	-	6.94
17.31	phenol, 4-ethyl-	C8H10O	5.18	-	-
17.412	phenol, 3,5-dimethyl-	C8H10O	-	-	12.13
17.42	phenol, 2,3-dimethyl-	C8H10O	-	11.67	-
17.919	naphthalene	C10H8	-	4.25	4.92
18.181	phenol, 3-ethyl-	C8H10O	-	4.74	4.60
18.914	benzaldehyde, 2,4-dimethyl-	C9H10O	-	2.83	-
19.233	phenol, 3-(l-methylethyl)-	C9H12O	3.55	-	-
19.537	benzene, 1-ethyl-4-methoxy-	C9H12O	-	2.86	-
19.541	phenol, 2-(l-methylethyl)-	C9H12O	-	-	3.69
20.213	phenol, 3-ethyl-5-methyl-	C9H12O	11.34	7.44	10.48
20.479	phenol, 2,3,5-trimethyl-	C9H12O	-	-	3.29
21.256	thymol	C10H14O	-	-	2.98
21.382	naphthalene, 1-methyl-	C11H10	15.12	13.06	6.16
21.926	benzaldehyde, 4-ethyl-	C9H10O	-	4.81	-
22.628	1H-inden-5-ol, 2,3-dihydro-	C9H10O	4.89	-	3.56
23.145	phenol, 4-(2-methylpropyl)-	C9H12O	1.52	-	-
23.976	ethanone, 1-(3,4-dimethylphenyl)-	C10H12O	-	3.82	-
23.989	phenol, 2-(2-methyl-2-propenyl)-	C10H12O	-	-	3.55
24.234	naphthalene, 2-ethyl-	C12H12	1.58	-	-
24.899	naphthalene, 2,3-dimethyl-	C12H12	-	5.94	6.10
25.036	naphthalene, 1,7-dimethyl-	C12H12	7.19	-	-
25.61	benzene, 2,4-diethyl-1-methyl-	C11H16	-	3.76	2.79
26.65	acenaphthene	C12H10	-	-	3.31
27.453	1,1,4,5,6-pentamethyl-2,3-dihydro-1H-indene	C14H20	3.37	6.49	3.63
27.795	naphthalene, 2,3,6-trimethyl-	C13H14	-	2.48	1.50
28.227	naphthalene, 1,6,7-trimethyl-	C13H14	9.11	4.33	-
28.667	3-(2-methyl-propenyl)-1H-indene	C13H14	2.64	2.58	-
29.201	fluorene	C13H10	-	4.32	-

Table 11 provides detailed insights into the compositional analysis of the third vacuum distillation fraction. Similar to the previous observations, fractions obtained under different experimental conditions exhibited some commonalities. Primarily, within the current temperature ranges, it is noteworthy that phenolic compounds were undetected, with naphthalene dominating the fractions distilled at 13.3 kPa. Conversely, fractions obtained under lower pressures contained a greater abundance of complex organic compounds. Second, the phenomenon of organic components shifting toward shorter retention times became increasingly pronounced with an increase in distillation temperature. For instance, within the fractions obtained at 13.3 kPa, organic matter with retention times exceeding 30 min constituted 58.67% of the composition. In contrast, this proportion declined to 34.55% and 10.12% within the fractions obtained at 30 and 45 kPa, respectively. This observation underscores the substantial influence of distillation temperature on the compositional dynamics of these fractions.

Table 11. GC-MS Compositional Analysis of the Third Vacuum Distillation Fraction (Corresponding to 360 °C at Atmospheric Pressure).

retention time	organic components	molecular formula	relative content
distillation pressure (kPa)			13.3	30	45
distillation temperature (°C)			281	310	316
16.713	phenol, 3,4-dimethyl-	C8H10O	-	3.46	-
17.411	phenol, 2,5-dimethyl-	C8H10O	-	-	3.25
17.413	phenol, 3-ethyl-	C8H10O	-	4.94	-
18.176	phenol, 2-ethyl-	C8H10O	-	-	1.65
20.209	phenol, 2-ethyl-4-methyl-	C9H12O	-	2.20	6.70
21.400	naphthalene, 1-methyl-	C11H10	-	2.63	5.90
22.621	1H-inden-5-ol, 2,3-dihydro-	C9H10O	-	-	5.83
23.972	benzene, 1-ethyl-3-(1-methylethyl)-	C11H16	2.45	7.45	6.83
23.980	ethanone, 1-(2,4-dimethylphenyl)-	C10H12O	-	-	5.28
25.004	naphthalene, 2,6-dimethyl-	C12H12	3.00	-	-
25.025	naphthalene, 1,7-dimethyl-	C12H12	-	4.76	9.90
25.470	naphthalene, 2,3-dimethyl-	C12H12	-	2.54	-
26.638	acenaphthene	C12H10	3.42	6.24	10.59
27.178	4-(2,2-dimethyl-6-methylenecyclohexylidene)-3-methylbutan-2-one	C14H22O	-	-	5.17
27.471	1,1,4,5,6-pentamethyl-2,3-dihydro-1H-indene	C14H20	-	-	6.55
27.547	1-naphthalenol	C10H8O	11.40	8.09	-
27.685	naphthalene, 1,6,7-trimethyl-	C13H14	2.18	-	-
27.888	naphthalene, 2,3,6-trimethyl-	C13H14	4.56	6.26	11.40
28.748	naphthalene, 1-methyl-7-(1-methylethyl)-	C14H16	-	-	3.85
29.223	fluorene	C13H10	6.04	6.93	6.98
29.933	1-naphthalenol, 2-methyl-	C11H10O	8.26	9.98	-
30.125	9H-fluoren-9-ol	C13H10O	9.10	9.14	-
30.467	9H-xanthene	C13H10O	3.90	-	8.49
31.392	naphthalene, 1,6-dimethyl-4-(1-methylethyl)-	C15H18	8.51	3.81	-
32.389	1-naphthol, 5,7-dimethyl-	C12H12O	9.24	-	-
32.405	9H-fluorene, 9-methyl-	C14H12	-	5.31	-
32.770	chamazulene	C14H16	14.76	11.53	-
33.046	1-naphthol, 6,7-dimethyl-	C12H12O	3.57	-	-
33.263	naphtho[2,1-b]furan, 1,2-dimethyl-	C14H12O	4.01	-	-
33.917	anthracene	C14H10	5.58	4.74	1.63

Combining the GC-MS results of the distillation fractions from the four temperature ranges, a discernible pattern emerges. The elevated pressure and temperature in vacuum distillation appears to enhance the high-temperature degradation effect significantly. This effect, which is consistent with observations made in the study by Feng et al.,²⁶ manifests as an increase in phenolic content and a decrease in the content of oxygenated nonphenolic (ONP) compounds within the organic fractions. These alterations are particularly prominent in the latter fractions. Interestingly, despite the changes in the distillation conditions, there is little variation in the yield of distillation residue. This suggests that changes in distillation pressure exert only a slight influence on the overall yield of light fractions. However, a higher distillation temperature makes the fraction more prone to organic matter with simpler structures and fewer substituent groups. In the case of the initial vacuum distillation fraction labeled P45, it can be inferred that the degradation effect may have caused the dew point temperatures of the distilled organic compounds to decrease to such an extent that they became challenging to collect in the subsequent processes. Consequently, this phenomenon could serve as the primary source of experimental losses.

3.3. Effects of Solid Particles on Vacuum Distillation

3.3.1. Separation Performance of Samples Containing Semi-Coke Particles

Figure 4 provides an overview of the properties exhibited by the fractions obtained during the experimental procedures. The fractions generated within each experimental group showed similarities with the fractions obtained from samples without solid components. However, some fractions exhibited darker hues and minor solid sediments. The final fraction exhibited higher viscosity and essentially appeared as a solid substance. The experimental working conditions and losses of LTCT samples containing semi-coke are presented in Table 12. The samples contained approximately 5%, 10%, and 20% semi-coke, with negligible loss during the experiment being minimal. Notably, the yield of distillation residue decreased by 3.74% with the addition of 5% semi-coke but subsequently increased by 4.34% upon further elevating the semi-coke content by an additional 5%. When the proportion of semi-coke reached 20%, the distillate residue yield registered a modest increase of 2.53%. These observations indicated that in samples containing semi-coke, the semi-coke is easily carried by the vapor into the collection bottle when the percentage of semi-coke is relatively low. After the content of semi-coke is elevated, it can be adsorbed with LTCT and agglomerated together to prevent it from being easily detached from the distillation residue.

Figure 4.

Properties of distilled fractions of LTCT samples containing semi-coke.

Table 12. Experimental Conditions and Results of LTCT Samples Containing Semi-Coke.

condition no.	LTCT (g)	solids (g)	evaporated fraction (g)	received fraction (g)	losses (%)	yield of distillation residue (%)
P30	53.48	0	26.52	25.24	2.39	50.41
P30-T-5	47.57	2.53	26.70	25.17	3.05	46.67
P30-T-10	45.28	5.09	24.67	22.45	4.41	51.01
P30-T-20	40.39	10.01	23.33	20.84	4.94	53.54

Figure 5a offers a comparative view of the yields of fractions from samples with different levels of semi-coke during the distillation process. To facilitate this analysis, it is assumed that the solid components in the samples are non-interactive with LTCT and ultimately remain within the distillation residue. Consequently, under these conditions, the ideal yield for each fraction can be calculated using the individual fraction yields observed in the solids-free LTCT samples. Figure 5b shows the difference between the actual yield and the ideal yield (termed as D_y). D_y is influenced by two counteracting processes: the entrainment of solids into the fraction by vapor and the adsorption of solids onto LTCT components, causing them to be retained together in the residue. D_y is positive when the former prevails and negative when the latter prevails. It becomes evident that as the proportion of semi-coke in the sample progressively increased, D_y gradually increased for fractions below 170 °C at atmospheric pressure. In contrast, for the other fractions, the opposite trend was observed. This implies that during the initial low-temperature phase of distillation, semi-coke is more prone to being carried away, resulting in a significant increase in D_y as the semi-coke content increases. Subsequently, semi-coke appears to preferentially remain in the distillation residue and continues to adsorb to LTCT. Consequently, D_y exhibits a monotonic increase with the augmentation of semi-coke content.

Figure 5.

Comparison of distillation fraction yields of LTCT samples containing semi-coke (the superscript * indicates that the temperature is for distillation at atmospheric pressure).

3.3.2. Separation Performance of Samples Containing Coal Dust Particles

Figure 6 shows the properties of the fractions collected in the experiments. The fractions obtained in the various experimental groups were very similar to the fractions obtained from the samples containing semi-coke. The last fraction also appeared as solid, and the solid settlements were more pronounced in all fractions. The experimental conditions and results of the LTCT distillation experiments containing coal dust particles are presented in Table 13. The content of coal dust in the four samples was 5%, 10%, 15%, and 20%, respectively, and the experimental losses were less than 5%. Next, they were compared with the LTCT samples without solids. The difference in the yield of distillation residue was slight, and the distillation residue was only increased by 6.77% when the percentage of coal dust was increased from 5% to 20%. This indicates that the coal dust could be carried out by the vapor during the distillation process, or the coal dust was pyrolyzed during the distillation process, and it entered the product. When the content of coal dust increased, the distillation residue production first decreased and then gradually increased, which indicates that coal dust is also prone to be carried out by the vapor when its content is low. When the content of coal dust particles increases, the coal dust is more likely to be adhered to the tar and agglomerated together and, therefore, preserved in the distillation residue. A sudden increase in the residue yield was noted when the coal dust content was changed from 15% to 20%, whereas for the sample containing semi-coke, this phenomenon occurred when the content was increased from 5% to 10%. In addition, when the LTCT samples contained the same proportion of solid particles, the yield of residue containing semi-coke was always higher than the yield of residue containing coal dust. This indicates that coal dust is less adsorptive to LTCT components throughout the distillation process compared to semi-coke.

Figure 6.

Properties of distilled fractions of LTCT samples containing coal dust.

Table 13. Experimental Conditions and Results of LTCT Samples Containing Coal Dust.

condition no.	LTCT (g)	solids (g)	evaporated fraction (g)	received fraction (g)	losses (%)	yield of distillation residue (%)
P30	53.48	0	26.52	25.24	2.39	50.41
P30-C-5	47.54	2.50	27.85	26.97	1.76	44.34
P30-C-10	45.36	5.08	28.42	27.81	1.21	43.66
P30-C-15	43.28	7.51	27.87	26.20	3.29	45.13
P30-C-20	40.71	10.00	24.79	23.79	1.97	51.11

Figure 7 provides a comprehensive view of the yield of each fraction during distillation, along with the difference between the actual yield and ideal yield (D_y). In fractions below 170 °C, D_y initially increased and then decreased. This trend is probably due to the relatively loose nature of coal dust, making it easily entrained and carried away until the coal dust content becomes sufficiently high to agglomerate with LTCT and remain in the distillation residue. In the second fraction, D_y for distillation fractions under different conditions consistently displayed negative values, indicating that the adsorption of coal dust onto LTCT predominates at this stage, and the coal dust is strongly adsorbed to LTCT. This finding aligns with the observations of Yue et al.,²⁷ who noted that powdered coal possesses a large pore volume and specific surface area, leading to a high adsorption capacity. However, a notable shift occurred in subsequent fractions, where D_y abruptly became positive, indicating a significant amount of coal dust being carried away. This deviation stems from the fact that coal dust is a solid substance without pyrolysis, which is in contrast to semi-coke. As a result, coal dust undergoes pyrolysis at elevated temperatures, resulting in changes in adsorption behavior and its subsequent entrainment out of the distillation residue. Consequently, it can be inferred that coal dust exhibits weaker adsorption capabilities compared to semi-coke throughout the distillation process, despite it showing high adsorption capacity for LTCT at initial stages.

Figure 7.

Comparison of distillation fraction yields of LTCT samples containing coal dust (the superscript * indicates that the temperature is for distillation at atmospheric pressure).

4. Conclusion

To delve into the variables influencing the vacuum distillation separation of tar containing solid impurities, diverse LTCT samples with distinct solid ratios underwent vacuum distillation under varying conditions. The yield and composition of each distillation fraction were analyzed to investigate the separation mechanism. An impact assessment of the distillation temperature and pressure was achieved by examining the fraction yields of LTCT samples at differing pressures and temperatures, coupled with semiquantitative GC-MS analysis. Notably, alterations in distillation pressure had a slight effect on the overall yield of light fractions but significantly influenced the distribution of substances across distinct temperature ranges. Lower distillation pressures engendered higher fraction yields within the elevated temperature spectrum, while higher pressure concentrated fractions toward a lower temperature range. Concurrently, low pressure inclined fractions to include more intricate organic substances with greater carbon atom content. Conversely, appropriate pressure elevation led to fractions encompassing a broader array of simpler-structured organic components.

Examining LTCT samples comprising 5–20% semi-coke powder or coal dust disclosed that differences in solid particle nature had a significant effect on the distillation. Both solid particles exhibited a tendency to be carried into the distillation fraction at low contents, while heightened content prompted their retention in the distillation residue and exhibited adsorptive properties on the LTCT components. Coal dust demonstrated heightened adsorption capability at lower temperatures. However, it could decompose during the distillation process at elevated temperatures, simultaneously affecting its adsorption capacity, resulting in elevated high-temperature distillation fraction yields. In contrast, semi-coke, as a byproduct of coal pyrolysis, was poorly affected by a high-temperature environment throughout the distillation process, endowing it with stronger adsorption capacity for LTCT components.

Data Availability Statement

All the data that support the present research have been provided in the relevant paragraphs.

The authors declare no competing financial interest.