Article pubs.acs.org/EF
Research on the Production of Aromatic Hydrocarbon via Hydroreforming a Light Fraction in Direct Coal Liquefaction Oil Peng Huang,*,†,‡ Xiao-Jing Zhang,†,‡ and Xue-Feng Mao†,‡ †
Beijing Coal Chemistry Research Institute, China Coal Research Institute, Beijing 100013, People’s Republic of China State Key Laboratory of Coal Mining and Clean Utilization, Beijing 100013, People’s Republic of China
‡
ABSTRACT: Direct coal liquefaction (DCL) oil is known for the production of aromatic hydrocarbon because of the large amount of cyclic compounds with a high aromatic content. In this work, naphtha from the DCL oil was used as raw material and the hydroreforming experiments were carried out in a small continuous fixed-bed reactor for the production of aromatics. The results show that the n-alkane isomerization reaction occurred and naphthenics were converted to aromatics via dehydrogenation and aromatization during the process of hydroreforming. The yield of C1−C4 hydrocarbon gas was 6.03%, and the yield of hydrogen was 3.6%. The aromatic content reached 83.2% with 61.3% of C6−C8 after reforming, which is a better raw material to extract benzene, toluene, ethylbenzene, and xylenes. It is found that the naphtha distillation range also affected the composition and yield of aromatic hydrocarbons and the appropriate temperature range was 60−160 °C. According to the experimental results, the technical route to produce aromatic hydrocarbon from DCL oil naphtha was also proposed.
1. INTRODUCTION Aromatics are a very important basic organic chemical raw material, including benzene, toluene, ethylbenzene, and xylenes (BTEX), and play a very important role in many aspects of the national economy. With the rapid development of industry, China’s demand for aromatics is improving. There a larger gap between supply and demand, which caused the need to import large quantities each year. According to statistical analysis, in 2012, China’s net imports reached 6.09 million tons of aromatics, with foreign dependence at 44%. Most aromatics came from petrochemical production in the past. Naphthenicbased crude oil is a good raw material for extraction of aromatics, but its reserves account for only 2−3% of the total reserves of crude oil. With the scarcity of oil resources and oil prices continuing to rise, exploring new sources of aromatics to meet the supply and demand gap has become imperative. China is rich in coal resources. The coal chemical industry has developed rapidly, which has large-scale coal-based oil output. The main mechanism of the reforming reaction is that the number of carbon atoms does not change before and after the reaction. From the cyclization reaction, C6 can be transformed into C6 aromatics (benzene), C7 can be transformed into C7 aromatics (toluene), and C7 and above were also transformed in the same way. Through the disproportionation and transalkylation reactions in the disproportionation unit, the C7 aromatic, C9 aromatic, and C10 aromatic can be transformed into high-value-added aromatic C8. The schematic diagram of the disproportionation and transalkylation is shown in Scheme 1. Therefore, the potential aromatic content of naphtha directly determines the yield of C8 aromatics, which determines the economic benefits of the device.1−4 Calculation method of the potential aromatic content (Ar%) as follows: Ar% =
aC6N %
+
bC 7N %
+
cC8N %
+
o
∑ Ar %
© 2014 American Chemical Society
where A is the total aromatic content of the feedstock and a, b, and c are conversion coefficients between CN6 , CN7 , and CN8 and corresponding generated benzene (T), toluene (B), and xylenes (X), respectively, with their values as follows: a = MB/MC6N = 78/84 = 0.93
b = M T /MC7N = 92/93 = 0.94 c = MX /MC8N = 102/112 = 0.95
wherein MB, MT, and MX are B, T, and X molar masses, respectively, and MCN6 , MCN7 , and MCN8 are the molar masses of C6 cycloalkane, C7 cycloalkane, and C8 cycloalkane, respectively. Direct coal liquefaction (hereinafter referred to as DCL) oil is a kind of coal-based oil. There was a lot of research about the group composition of the coal-based oil (coal liquefaction oil and coal tar). The group compostion of the coal liquefaction was researched in refs 5−11 according to liquid chromatography analysis. The result shows that coal liquefaction oil contained a lot of macromolecules of polycyclic aromatic hydrocarbons (PAHs). Zhang et al.12 used gas chromatography−mass spectrometry (GC−MS) and liquid chromatography−mass spectrometry (LC−MS) to analyze the liquid products of the coal−oil co-processing. The results showed that aromatic compounds included mainly single aromatic rings, followed also by bicyclic aromatic hydrocarbons, in addition to the aromatic compound, containing some naphthenes. The middle- and low-temperature coal tar from northern Shaanxi was researched by Chen et al. The data show light oil containing a large amount of aromatic hydrocarbons according to GC−MS analysis.13 Received: September 23, 2014 Revised: December 10, 2014 Published: December 11, 2014
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dx.doi.org/10.1021/ef502146a | Energy Fuels 2015, 29, 86−90
Energy & Fuels
Article
Scheme 1. Schematic Diagram of Disproportionation and Transalkylation
According to the research of the light fraction in coal-based oil (it mainly included coal tar or hydrogenated product naphtha fraction and coal liquefaction oil naphtha fraction), it could be found that the naphtha fraction has a higher potential aromatic content and more cyclic compounds and, after hydroretreating to remove impurities and heterocyclic compounds, naphtha could be taken as the ideal feedstock of the catalytic reforming. With the coal-based naphtha as raw material for the production of aromatics, there is a high theoretical aromatic yield. In addition, it can reduce the severity of the reforming conditions, which is because the coal-based naphtha reforming reaction requires only cycloalkane dehydrogenation. The petroleum naphtha needs not only cycloalkane dehydrogenation but also cyclization of paraffins to improve the yield of the aromatics. The cyclization reaction conditions are very harsh.14−20 Studies by Bonnifay and Barbier showed that, under catalytic reforming conditions, C6 paraffin dehydrocyclization completed only 0−5% and C7 and C8 paraffins were difficult over 50%.21 The extraction of the aromatic from coalbased oil by hydroreforming has a theoretical advantage: there is no relevant work carried out abroad. In this experiment, the naphtha fraction from the product of coal liquefaction oil was used as the raw material. The experiment studied the group composition change and the main aromatic compound yield.
Table 1. Properties of the Raw Material and Reformed Oil sample properties
naphtha
refined naphtha
reformed oil
density (kg cm−3) H (%) C (%) S (mg/kg) N (mg/kg) Pb (μg/kg) As (μg/kg) H/C distillation range (°C) IBP 10% 20% 30% 50% 70% 90% FBP
0.792 12.3 85.81 650 1880 180
pore volume (mL/g)
mechanical strength (N/cm)
0.45−0.55
>100
Table 4. Material Balance of the Refining Experiments input (wt %)
output (wt %)
naphtha H2 total refined naphtha H2O gas C1−C4 H2S NH3 total
output/input (%)
Table 6. Group Composition Analysis of Refined Naphtha and Reformed Oil
100.00 2.63 102.63 100.28 0.96 0.63 0.39 0.07 0.21 102.54 99.9
balance is shown in Table 5. The distillation range were presented in Table 1. Table 5. Material Balance of the Reforming Experiments input (wt %) output (wt %)
output/input (%)
refined naphtha total reformed oil H2 C1−C4 total
100.00 100.00 90.11 3.60 6.03 99.7 99.7
As seen from the results of Table 5, under this condition, the refined oil had a good reforming feature. The yield of the reformed oil was about 90%. The yield of the C1−C 4 hydrocarbon was 6.03%. The results showed that there were some side reactions (hydrogenolysis and hydrocraking reactions, which could be seen in Scheme 2) during the hydroreforming process, which could cause high-value raw
components (%)
refined naphtha
reformed oil
n-alkanes 1 C6 2 C7 3 C8 4 C9+ isoparaffins 1 C4−C5 2 C6 3 C7 4 C8 5 C9+ naphthenics 1 C6 2 C7 3 C8 4 C9+ aromatics 1 C6 2 C7 3 C8 4 C9−C10 5 C11+
8.77 1.13 1.52 3.75 2.37 2.57 0.21 1.36 0.64 0.36
2.48 1.01 0.52 0.34 0.6 4.77 1.68 1.11 1.73 0.25
37.55 12.07 9.26 7.74 8.48 54.22 5.22 13.33 14.64 11.72 9.31
8.89 2.86 2.15 2.03 1.85 83.2 11.97 22.15 26.91 10.97 11.2
Table 6 shows the group composition of refined naphtha and reformed oil. The results show that naphthenics in refined naphtha were 37.55% (C6, C7, and C8 cycloalkane contents were 12.07, 9.26, and 7.74%), the aromatic content in refined naphtha was 50.25%, and the aromatic potential content of refined naphtha was 77.56%, calculated using eq 1. C8 components were the majority in n-alkanes (aromatics can be generated by the cyclization reaction of paraffins). Overall, refined naphtha is a suitable catalytic reforming feedstock. After the catalytic reforming process, the aromatic content was 83.2% in the reformed oil (the C6−C8 aromatic content
Scheme 2. Hydrogenolysis and Hydrocraking Reactions
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dx.doi.org/10.1021/ef502146a | Energy Fuels 2015, 29, 86−90
Energy & Fuels
Article
was 61.03%), the naphthene content was 8.89%, and the paraffins had less content (isoparaffin, 4.77%; n-paraffins, 2.48%). Analysis of the data in Table 6 showed that, during the reforming process, n-alkanes decreased by 3.27% and isoparaffins increased by 2.2%, when the n-alkane isomerization reaction occurred (Scheme 3): Scheme 3. Isomerization Reaction
Naphthenics decreased by 28.66%, and aromatics increased by 32.95%, which showed that naphthenics converted to aromatics by dehydrogenation and aromatization (Scheme 4). Figure 3. Main benzene compound content of refined naphtha and reformed oil.
Scheme 4. Dehydrogenation and Aromatization Reactions
naphtha hydrorefining unit, catalytic reforming unit, and aromatic extraction unit, the aromatic hydrocarbons were separated. The procedure was shown in Figure 4. Naphtha in DCL oil contains more heteroatoms. The hydrorefining process should be carried out in harsh conditions, which adapted to select a high temperature, high pressure, and lower space velocity. Moreover, to obtain a higher yield of the aromatics and reduce the catalyst deactivation rate, the final boiling point of naphtha should be chosen reasonably. According to the results of this experiment, the end point of refined naphtha can be controlled at
