Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Research on a Dual Solvent To Separate Olefin/Aromatic−Sulfide from Heavy Fluid Catalytic Cracking Naphtha Yuhao Zhang,†,‡ Yongtao Wang,†,‡ Feng Chen,‡ Suxin Liu,‡ Liang Zhao,*,‡ Jinsen Gao,‡ Tianzhen Hao,§ and Chunming Xu‡ ‡
State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, People’s Republic of China Hebei Jingzhi Technology Company, Limited, Cangzhou, Hebei 061000, People’s Republic of China
§
ABSTRACT: The separation of olefin from heavy fluid catalytic cracking (FCC) naphtha is beneficial to avoid the loss of octane number during the desulfurization process. Extractive distillation is a promising alternative to separate olefin and enrich sulfide at the same time. The vapor−liquid equilibrium of heavy FCC naphtha with different single solvents was studied first. Sulfolane showed the best ability to separate olefin with a selectivity coefficient θ of 2.81; however, sulfide selectivity coefficient β was only 1.78. N-Methylpyrrolidone showed the best ability to concentrate sulfide, with β up to 2.07 but lower θ of 2.74. The optimized dual solvent with a volume ratio of sulfolane/N-methylpyrrolidone at 85:15 (vol %) resulted in a selectivity coefficient θ of up to 4.01, while β was 1.97.
1. INTRODUCTION Sulfur compounds in vehicle gasoline has been well-focused as a result of the destruction of the environment by the exhaust gas. With the development of techniques, these sulfides could be successfully removed to meet the urgent need by many methods, such as hydrodesulfurizaiton,1−4 adsorptive desulfurization,5,6 oxidative desulfurization,7,8 and extractive desulfurization.9,10 To further improve the quality of vehicle gasoline, the content of olefin was also limited because olefin could bring many environmental problems and evaporation loss. 11 Especially, olefin becomes of critical importance if fluid catalytic cracking (FCC) naphtha is the main source of the gasoline pool. The most simple and direct way to reduce the content of olefin is hydrodesulfurization, which causes serious loss of the research octane number (RON). Obviously, it is the best way if olefin could be oriented to transform into high RON compounds, by which the content of olefin could be reduced, while the RON of gasoline will not be lost. As Sholl and Lively claimed, “the improvement of separation in refinery industry will reap great global benefits”.12 The separation of olefin from FCC naphtha is very important, while extraction desulfurization has great potential to solve this problem. The essence of extraction desulfurization is the separation process, which could complete the separation of olefin and enrichment of sulfide in naphtha at the same time using a highselectivity solvent.13 To be specific, the solvent with high polarity can dissolve the aromatics and thiophene compounds (called A and S, respectively) to its maximum ability, while other components, such as paraffin, olefin, and naphthene (called PON components), are less dissolved in this solvent. By this way, naphtha could be separated into raffinate with ultralow sulfur and extracted with concentrated sulfur. Commonly, the extraction is divided into solvent extraction and extractive distillation depending upon different mechanisms. The theory of solvent extraction is based on the difference of solubility. The solvent prefers to dissolve aromatics and sulfide (thiophene) other than paraffin, olefin, © XXXX American Chemical Society
and naphthene. Much work thus far has focused on the separation of aromatics and non-aromatics, such as the aromatic extraction process.14,15 For example, Ferreira et al.16 reported that the mutual solubility between the two co-existing phases was increased with the increase of the amount of aromatics by a liquid−liquid equilibria of ternary systems composed of ionic liquid and aromatic and aliphatic hydrocarbons. Therefore, solvent extraction has a limited ability to separate the feedstock containing excessive aromatics.17 Another process, extractive distillation, could fill up this deficiency. The extractive distillation process enhances the relative volatility of the mixture to finish the separation by a high-boiling-point solvent.18 The essence of this process is special distillation,19 which is suitable for treating high-aromatic feedstock. The solvent properties, such as solubility and selectivity, are two of the kernels of the extractive distillation process.20 In recent years, the research of screening solvents has just been focused on the desulfurization.21−24 Shen et al.25 reported that the dimethylformamide solvent has the preferable ability to concentrate sulfide and good blends for producing high-quality Europe IV gasoline. Also, N-formylmorpholine (NFM) was studied as a solvent for extractive distillation desulfurization. The result showed that the sulfur content of raffinate was reduced from 189.52 to 26.03 μg/g.26 Besides, other solvents, such as dimethyl sulfoxide, N-methylpyrrolidone, diethylene glycol, and tetraethylene glycol, were reported to study the ability of desulfurization.27,28 As mentioned above, most of the screening of solvents in FCC naphtha extractive distillation focused on removing sulfide with neglected olefin separation. It could be attributed to three factors. First of all, even if the solubility is small, a part of olefin will still dissolve into solvent and cannot be separated from the solvent during the subsequent distillation. Second, the existence Received: January 3, 2018 Revised: January 26, 2018 Published: January 29, 2018 A
DOI: 10.1021/acs.energyfuels.8b00026 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Properties of Extraction Solvents
initial boiling point (IBP)−80, 80−100, 100−130, 130−150, and 150− 183 °C fractions using the TD10L-06A-B true boiling point distillation unit. It was designed in accordance with the ASTM D2887 method, which has 17 theoretical plates and a reflux controller with magnetic force. All of the operating conditions were managed by a computer and operated at an ordinary pressure. The reflux ratio was 5:1, and the distilling rate was controlled in the range of 1.5−2.0 distillations/s. 2.2.2. VLE Experiment. The VLE data were measured using a VLE still at atmospheric pressure, and all of the VLE experiments were tested at solvent/feed (S/F) = 1. The detailed procedures were (1) ensuring the gas tightness of the whole system, (2) mixing the solvent and FCC naphtha with an equal amount of 25 mL adequately and then added to the equilibrium still, (3) keeping the vapor temperature stable and the vapor and liquid circulated completely while keeping the temperature at an equilibrium value for 1 h by adjusting the heating, (4) sampling the vapor and liquid components in the chromatogram vial, and (5) removing the residual solvent from samples by deionized water, conducting the paraffin, olefin, naphthene, and aromatic (PONA) and sulfur chemiluminescence detector (SCD) analyses by a gas chromatograph. 2.2.3. Batch Extractive Distillation. Batch extractive distillation equipment was set up in the laboratory, which consists of magnetic stirring heating pot, 500 mL three-mouth flask, rectifying column filled with ϕ 3 × 3 mm Dixon padding, condenser column, vacuum connecting tube, volumetric cylinder, etc. The height of the rectifying column is 125 cm, which is about 31 pieces of theory plate. The diagrammatic sketch is shown in Figure 1. The batch extractive distillation was operated at S/F = 3. The detailed procedure was (1) adding the solvent and FCC naphtha into the flask and sufficient stirring for 1 h with the temperature holding at 75 °C, (2) controlling the heating power slowly until the fraction was distilled off and recording the initial boiling point, (3) keeping the distilling rate in the range of 1.5−2.0 distillations/s, (4) setting the bottom temperature below 180 °C, (5) collecting the light fraction (called raffinate) and the mixture including oil and solvent (called extract) to the conical flask, and (6) removing the residual solvent and analyzing PONA and SCD data by a gas chromatograph. 2.3. Analysis and Calculation Method. The compositions of olefin, aromatics, and sulfide were analyzed by an Agilent 7890B gas chromatograph equipped with a capillary column (50 m × 0.2 mm × 0.2 μm), a flame ionization detector (FID), and a SCD. The chromatographic column temperature was programmed for an initial temperature of 35 °C maintained for 15 min and a final temperature of 180 °C with a heating rate of 2 °C/min maintained for 10 min. Besides, the initial and final temperatures of the FID were set 220 and
of dissolved olefin will lead to the decrease of the dissolution of aromatics or sulfide in the solvent, inhibiting the removal of them by extractive distillation. In previous studies, sufficient attention has been paid to the desulfurization of this extraction process; however, the content of olefin dissolved into the solvent has not been fully concerned. Third, olefin has a strong interaction with aromatics/sulfide in FCC naphtha from the perspective of desulfurization. The solvent has to break this interaction before it extracts sulfide from naphtha, which exactly leads to the separation difficulty. Thus, there were few reported works about separating olefin and enriching sulfide at the same time. It is very important to find out a proper solvent to deliver the above task. Usually, single-solvent method can hardly approach this objective with effective sulfide removal and high-selectivity olefin separation, but it could be accomplished by developing a dual solvent. In this work, a heavy fraction of FCC naphtha with a high aromatic content was selected as the feedstock to test the separation performance of solvents. To study the unique properties of each solvent, vapor−liquid equilibrium (VLE) experiments were applied involving the heavy FCC naphtha and six kinds of single solvents. A series of dual-solvent groups were prepared, evaluated by VLE and batch extractive distillation experiments. The related abilities of separation of olefin and enrichment of sulfide from heavy FCC naphtha were analyzed and summarized.
2. EXPERIMENTAL SECTION 2.1. Materials. A suitable solvent for separating olefin from aromatics and sulfide should have several characteristics: (1) high boiling point to avoid vaporization in the process of extractive distillation, (2) low melting point for easy mass transferring, and (3) high density for well mixing and contacting with components. According to these requirements, six kinds of solvents, including propylene carbonate (PC), N-methylpyrrolidone (NMP), sulfolane (SUL), diethylene glycol (DEG), tetraethylene glycol (TEG), and dimethyl sulfoxide (DMSO), were selected for separating olefins from aromatics and sulfide by VLE experiments. The properties of these solvents were listed in Table 1. 2.2. Apparatus and Experimental Procedure. 2.2.1. FCC Naphtha Distillation. The full range FCC naphtha was cut into B
DOI: 10.1021/acs.energyfuels.8b00026 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
where Ki or K0 is the phase equilibrium constant, xi is the sulfur content of liquid with solvent (ng/μL), x0 is the sulfur content of the liquid phase without solvent (ng/μL), yi is the sulfur content of vapor without solvent (ng/μL), and y0 is the sulfur content of liquid without solvent (ng/μL). The efficiency of the sulfide concentration can be shown as the enrichment coefficient τ, which is calculated by
τ=
xsL xsV
(4)
xLs
where is the different types of the sulfur content of the liquid phase with solvent (ng/μL) and xVs is the different types of the sulfur content of the vapor phase with solvent (ng/μL). Also, desulfurization efficiency could show the ability to concentrate the sulfide, which is calculated by
desulfurization efficiency =
Figure 1. Equipment of the batch extractive distillation: (1) kettle, (2) flask with three necks, (3) thermometer, (4) rectifying column, (5) Dixon padding, (6) condenser column, (7) tube with two joints, (8) condenser column, (9) vacuum connecting tube, and (10) volumetric cylinder.
3. RESULTS AND DISCUSSION 3.1. Property of the Feedstock. It is well-known that FCC naphtha mainly consists of paraffins (P), olefins (O), naphthas (N), aromatics (A), and some impurities, such as sulfides (S). In view of the complex compositions, the content and distribution of components greatly influence the separation efficiency, especially olefins, aromatics, and sulfides. Therefore, the full-range FCC naphtha was cut into several narrow fractions. The distribution of PONA in narrow fractions was shown in Table 2. Table 2 showed that the content of n-alkanes and isoalkanes decreased gradually in the fraction after the cutting point at 100 °C. Besides, the content of olefin decreased obviously when the cutting temperature was over 100 °C. However, a reverse phenomenon that the content of aromatics rose with the increase of the cutting point was found in the distribution of aromatics. When the cutting point was over 130 °C, the aromatic content jumped at 38.8 wt %. As was emphasized above, the feedstock rich in olefins and aromatics is badly in need of being separated; consequently, the naphtha fraction selected should involve plenty of olefins and aromatics simultaneously. It was easily to have the majority of sulfide concentrated in the fraction of over 100 °C. Considering the high olefin and aromatic/sulfide contents, the above 100 °C fraction was identified as the feedstock to investigate the separation properties between olefin and aromatic/sulfide in this research. The PONA component of the >100 °C fraction presented in Table 3 showed the rule of carbon number distribution. It indicated that the olefin content is the highest in all types of
where yA is the aromatic content of the vapor phase with solvent (ng/ μL) and xA is the aromatic content of the liquid phase with solvent (ng/μL). The separation efficiency between olefin and aromatics can be expressed by the separation coefficient θ, which is calculated as y x /x θ = A O = kA O yA /yO xO (2) where yA is the aromatic content of the vapor phase with solvent (ng/ μL), yO is the olefin content of the vapor phase with solvent (ng/μL), xA is the aromatic content of the liquid phase with solvent (ng/μL), and xO is the olefin content of the liquid phase with solvent (ng/μL). In addition, the concentration of sulfide is also an important part to be studied in this paper. The sulfide selectivity coefficient β shows the ability to concentrate sulfide using different solvents, which could be calculated using the following equation:
β=
=
xi/yi x0/y0
(5)
where Sf is the sulfur content of feedstock (ng/μL) and Sr is the sulfur content of raffinate (ng/μL).
280 °C, respectively, while the temperatures for SCD were 250 and 800 °C, respectively. Selectivity is the main factor to reflect the solvent property. Therefore, several parameters were defined to describe it in this paper. The ability to combine aromatics can be expressed by the aromatic distribution coefficient kA, which could be calculated by the following equation: x kA = A yA (1)
1 Ki 1 K0
Sf − Sr × 100% Sf
(3)
Table 2. Property and PONA Compositions of Narrow Fractions of Golmud FCC Naphtha composition (wt %) distillation fraction
nP
iP
O
N
A
RON
sulfur content (ng/μL)
yield (wt %)
IBP−80 °C 80−100 °C 100−130 °C 130−150 °C 150−183 °C >183 °C full range
7.80 6.52 6.73 4.36 4.77 3.89 6.04
34.74 33.56 32.10 15.15 12.67 19.16 30.38
54.03 37.71 29.46 27.75 22.51 15.57 40.81
3.43 18.03 13.24 6.77 11.18 11.02 8.28
0.00 4.18 18.47 45.97 48.87 50.36 14.50
94.6 86.6 109.6 122.8 95.7 95.9 93.5
138 615 1022 1305 1199 995 610
42.1 15.1 15.5 12.6 14.7 5.7
C
DOI: 10.1021/acs.energyfuels.8b00026 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
was obviously higher than that of other solvents at 60 °C, which may be beneficial to avoid the two liquid-phase region during the extractive distillation process. Whereafter, these solvents have been selected to investigate the separation performance of olefin from FCC naphtha by VLE experiments. The contents of olefins and aromatics in vapor and liquid phases were displayed in Table 4. The base experiment was tested with the >100 °C fraction without any solvents using an ordinary pressure VLE experiment, while six solvents were added to this feedstock for separation property tests under the same conditions. In comparison to the blank test, it was found that olefins were moved into the vapor phase and aromatics were moved into the liquid phase. Basically, the separation between olefins and aromatics is determined by the selectivity of the solvent. The key of separation between olefins and aromatics is suggested that the binding force of aromatics and solvent needs a fairly strong but huge gap to olefin. To present the separation of olefin without the influence of the yield, the separation coefficient θ was introduced. It is obviously that DMSO has the best ability to separate olefin, in which θ was 3.57. Also, it is well-established that SUL or NMP were suitable for separating aromatics and non-aromatics, such as GT-BTX,29 SED,30 and Distapex31 processes. In this paper, θ of SUL was 3.03, just lower than DMSO, showing great performance on the separation of olefin. From the above results, it can be deduced that the performance rank of solvent to separate olefin was DMSO > SUL > NMP in the VLE experiment. However, DMSO has a low boiling point (189 °C) and was hard to separate from the extract in real extractive distillation treating heavy FCC naphtha. The property of separating olefin between SUL and NMP can be demonstrated by the olefin and aromatic distribution in the vapor phase from the VLE experiment, as shown in Figure 3.
Table 3. PONA Component and Sulfur Content of >100 °C Fraction composition (wt %) carbon number
nP
iP
O
N
A
sulfur content (ng/μL)
5 6 7 8 9 10 11 total
0 0.3 1.3 1.5 0.9 0.5 0.1 4.6
0.1 1.3 5.3 7.9 6.2 4.1 1.4 26.3
0.1 3.7 11.3 9.3 6.7 2.1 0.1 33.3
0 0.6 2.5 2.9 2.1 0.4 0 8.5
0 0.3 4.4 11.7 9.5 1.2 0 27.3
953.7
hydrocarbons, reaching up to 33.3 wt % with a carbon number distribution from C6 to C10. The olefin content of C7−C9 is much higher than that of others, accounting for 82.0 wt % in total olefin in this selected fraction. The aromatic content is the second highest in the hydrocarbons, presenting 27.3 wt %. The carbon number distribution of them appears C6−C10, where C7−C9 contributed to 94.0 wt % of total aromatics. On basis of these data, it can be concluded that C7−C9 hydrocarbons are the target components to be separated in this research. 3.2. Separation Performance of a Single Solvent. As we know, the intermiscible properties of various solvents and FCC gasoline are a key factor in the extractive distillation process. Therefore, six kinds of solvents have been selected to test the solubility. As seen from Figure 2, the solubility of NMP
Figure 3. Olefin and aromatic distribution in the vapor phase. Figure 2. Solubility of FCC heavy naphtha in six kinds of solvents.
It indicated that the content of olefin in the vapor phase using NMP was less than that using SUL. The later one had a Table 4. Olefin/Aromatic Contents of VLE Experiments (by a Single Solvent) solvent
olefin content in vapor (yO, wt %)
olefin content in liquid (xO, wt %)
aromatic content in vapor (yA, wt %)
aromatic content in liquid (xA, wt %)
DMSO SUL NMP PC DEG TEG blank
42.08 41.67 30.21 38.55 42.49 39.72 31.93
31.98 31.68 32.06 28.48 31.22 31.73 31.57
10.68 12.03 9.78 14.75 14.41 14.30 16.60
28.97 27.74 28.48 29.54 27.95 26.48 27.53
D
kA
θ
2.71 2.31 2.91 2.00 1.94 1.85
3.57 3.03 2.74 2.71 2.64 2.32
DOI: 10.1021/acs.energyfuels.8b00026 Energy Fuels XXXX, XXX, XXX−XXX
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respectively, both showing good performance to concentrate sulfide. Therefore, the concentration efficiency of sulfide had an order of NMP > DMSO > SUL. However, DMSO was eliminated as a result of a low boiling point. The abilities to concentrate different types of sulfide by SUL and NMP were analyzed using SCD tests for comparison. The VLE data were shown in Table 6.
better separation ability of C6 and C7 olefins for its generic solubility. The low carbon olefins undissolved in the SUL solvent were easy to be distillated into the vapor phase. The solubility and selectivity of NMP were in contradiction with each other. It preferred to separate C7−C9 olefins, while SUL tended to separate C6−C8 olefins. It can explain that the NMP solvent preferred to dissolve small molecules in the same group composition. The limited dissolution vacancy was first occupied by small molecules, such as C6 and C7 olefins, while C8 and C9 olefins lacked the opportunity to dissolve into the solvent. As a result, the boiling point of C6 and C7 olefins with solvent was increased sharply and not distilled into the vapor phase. NMP also showed a better performance on concentrating C7 aromatics than SUL, while C8 aromatics in the vapor phase using either SUL or NMP solvent showed a significant content. In comparison to those reported work, it is generally accepted that a low carbon number of olefin usually shows a higher value of the octane number.32 SUL was particularly attractive as an alternative to separate olefin as a result of its ability in the separation of small-molecule olefins. Meanwhile, the performance of the sulfide concentration of the selected solvents has been well-studied. Table 5 showed the sulfur content data using six single solvents in comparison to the blank group.
Table 6. Different Type of Sulfides in Vapor and Liquid Phases Using SUL and NMP content of sulfur (ng/μL) vapor
a
solvent
liquid sulfur content (xi, ng/μL)
NMP DMSO SUL PC TEG DEG blank
372 398 427 451 575 587 732
1023 1012 1008 997 990 982 971
β 2.07 1.92 1.78 1.67 1.30 1.26
enrichment coefficient τ
sulfide
SUL
NMP
SUL
NMP
SUL
NMP
dimethylthiophene thiophene 2-methylthiophene 2-pentanethiol methyl propyl sulfide
71.5 49.1 85.0 65.8 24.3
64.6 50.1 68.0 51.1 20.2
202.7 79.8 112.5 72.3 24.5
250.8 85.9 132.9 88.6 25.1
2.83 1.63 1.32 1.10 1.01
3.88 1.71 1.95 1.73 1.24
By comparison of the distribution of the sulfur compound in the vapor phase, it indicated that the content of different types of sulfide were similar and can be in the order of 2methylthiophene > dimethylthiophene > 2-pentanethiol > thiophene > methyl propyl sulfide. Besides, trace amounts of ethyl isopropyl sulfide and C4 thiophene were detected only in the SUL-treated vapor phase but not in the one of the NMP solvent. The reason was thought that the SUL molecule contains a sulfur atom, which may has weak repulsion to other sulfides, while the structure of NMP is a symmetrical chain compound, which has the property to attract circular thiophene as well as chain sulfide. The distribution of sulfur type in the liquid phase was similar to the vapor phase, except C3 thiophene. The enrichment ratio τ of main sulfide was shown in Table 6. Wherein the enrichment coefficient τ of dimethylthiophene for SUL was up to 2.83, other thiophenes were also above 1. However, thioether and mercaptan were found more difficult to be separated. It was attributed to the polarity of thiophenic sulfur, which was much bigger than those of thioether and mercaptan. The enrichment coefficient τ of NMP showed a similar rule of concentrating sulfide. However, the value of τ was holistically higher than that of SUL. Another interesting finding was that the enrichment coefficient τ of 2-pentanethiol was greater than that of SUL. Therefore, NMP showed better ability to concentrate sulfide with respect to more kinds as well as amount. In this work, the separation of olefin was aiming at avoiding the loss of the ctane number during the olefin saturation process via hydrodesulfurization. It should be pointed out that the best solvent to separate olefin of heavy naphtha was SUL in previous research, but the dissolving property (affecting the stability of distillation) and the ability to concentrate sulfide still needed to be improved. Zhang et al. have discovered that the NMP solvent has a good dissolving property of the FCC model compound.28 NMP was proven to possess the best performance in the sulfide concentration based on our research. Therefore, a series of dual solvents in which SUL was the main solvent and NMP was the subordinate solvent were prepared to optimize the property of olefin separation, sulfide concentration, and component dissolution for heavy FCC naphtha.
Table 5. VLE Data of the Sulfur Content (Single Solvents)a vapor sulfur content (yi, ng/μL)
liquid
desulfurization efficiency (%) 60.3 57.6 54.6 52.0 38.7 37.5 22.1
Total sulfur content of feed = 939 ng/μL.
It was found that the sulfur contents in the vapor phase from all solvent treatments were lower than that of the blank group. The lowest was 372 ng/μL sulfide in the vapor phase for NMP solvent. Next were the sulfone solvents. Apparently, the sulfur contents in the liquid phase after solvent treatment were higher than that of the blank. The relative volatility of sulfide was enhanced obviously as a result of its combination with highboiling-point solvents. However, the sulfur content of vapor and liquid phases could not fully embody the properties of the solvents for the enrichment of sulfide. Therefore, the sulfide selectivity coefficient β and desulfurization efficiency (%) were introduced to describe the ability of the solvents to concentrate sulfide. NMP was the only solvent with β greater than 2, which showed a great ability to enhance the boiling point of sulfide. The desulfurization efficiency indicated the performance on the removal of sulfide from the vapor phase. It was found that NMP solvent also had the highest data, at up to 60.3%. It was suggested that the double bond between the oxygen atom and nitrogen of NMP appeared to have a great polarity property, more likely to combine with thiophene. On the basis of the above results, the type of sulfide in >100 °C FCC naphtha was mostly thiophene,33 which would prefer to dissolved into the NMP solvent. The desulfurization efficiencies of DMSO and SUL were 57.6 and 54.6%, E
DOI: 10.1021/acs.energyfuels.8b00026 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 7. VLE Data of Olefin/Aromatic Contents (Dual Solvents) group
SUL/NMP (vol %)
vapor olefin content (yO, wt %)
liquid olefin content (xO, wt %)
vapor aromatic content (yA, wt %)
liquid aromatic content (xA, wt %)
kA
θ
1 2 3 4 contrast
95:5 90:10 85:15 80:20 SUL
33.54 41.52 44.70 38.13 41.67
31.22 30.43 31.76 28.97 31.68
5.87 8.42 10.25 14.55 12.03
22.54 24.81 29.54 31.32 25.74
3.84 2.95 2.88 2.15 2.31
4.13 4.02 4.06 2.83 3.03
Table 8. VLE Data of the Sulfur Content (Dual Solvents)a
a
group
SUL/NMP (vol %)
vapor sulfur content (yi, ng/μL)
liquid sulfur content (xi, ng/μL)
β
desulfurization efficiency (%)
1 2 3 4 contrast
95:5 90:10 85:15 80:20 SUL
423 423 400 441 427
993 996 1046 1006 1008
1.77 1.78 1.97 1.72 1.78
55.0 55.0 57.4 53.0 54.6
Total sulfur content of feed = 939 ng/μL.
Table 9. Comparison of Raffinate and Contrast Groups composition (wt %) group
nP
iP
O
N
A
sulfur content (ng/μL)
yield of raffinate (vol %)
dual solvent contrast feedstock
6.57 8.83 5.70
31.84 30.86 22.69
47.20 37.33 31.75
11.70 14.26 12.42
2.69 8.72 27.44
245.6 990.5 953.7
20.0 20.0
3.3. Dual-Solvent Performance. In this section, the dual solvent volume ratios of SUL and NMP were 95:5, 90:10, 85:15, and 80:20 (vol %). The VLE data of olefin and aromatic contents using dual solvents were shown in Table 7. It can be seen that the dual solvent showed great selectivity of aromatics. For the aromatic distribution coefficient kA, the dual solvent was up to 3.84 with the volume ratio of 95 (SUL):5 (NMP). With the addition of more NMP, kA gradually decreased. The separation coefficient θ showed a similar trend. It was noted that θ of group 3 was 4.06, between groups 1 and 2. It was thought that adding NMP improved the solubility of aromatics and led to a rise of the aromatic content in the liquid phase. However, the dissolution of olefin was weakly enhanced simultaneously and obviously presented between groups 1, 2, and 3 and the contrast group (using single SUL as the solvent). The ability to separate olefin was reduced by adding NMP to SUL, especially at the volume ratio over 85:15. It was suggested to find an optimal ratio of adding NMP in this research. The ability of the sulfide concentration was also studied. The VLE data of the sulfur content in vapor and liquid phases using mixed solvents were shown in Table 8. It indicated that dual solvents had great performance in sulfide separation. The vapor sulfur content was stable at about 400 ng/μL. For the sulfide selectivity coefficient β, group 3 with the volume ratio of 85 (SUL):15 (NMP) was the highest among four experimental groups and the contrast group, reaching up to 1.97. The desulfurization efficiency of group 3 was 57.4%, about 3% greater than the one from the single SUL solvent. It was interesting to see that the dual solvent with a volume ratio of SUL/NMP at 85:15 showed significant performance in both the sulfide concentration and olefin separation, presenting selectivity coefficients θ and β up to 4.01 and 1.97, respectively. It should be noted that the content of sulfide in real FCC naphtha is far less than that in olefins or aromatics. The
appropriate amount of NMP into SUL could improve the interaction between solvent and sulfide, while the separation of olefin would not be weakened. Moreover, the soluble sulfide may help aromatics combine with solvent, thus enhancing the property of olefin separation. 3.4. Batch Extractive Distillation Using Dual Solvents. It is well-known that the solvent dosage is an important factor affecting the performance of extraction. Many studies have indicated that the optimal volume ratio of solvent and feedstock (S/F) was 2−4.17 In this work, it was verified under S/F = 3 by a batch extractive distillation experiment. Table 9 showed the PONA/sulfur content data comparing to the contrast group data. The contrast group was distilled without solvent. The yield of distillate composition was the same as raffinate (20.0 vol %), in which the bottom temperature had reached up to 180 °C using a dual solvent. As seen from Table 9, the olefin content of optimal conditions was 47.20 vol %, 9.87% greater than the contrast group. The content of aromatics reduced to 2.69 vol %. In addition, the sulfur content also reduced from 953.7 to 245.6 ng/μL. Figure 4 showed the comparison of the carbon number distribution of olefin in raffinate at S/F = 3 using the optimal dual solvent and the olefin content of the blank group. As seen from Figure 4, the dual solvent was more likely to separate C6−C8 olefins, illustrating that the addition of NMP did not affect the selectivity of SUL to separate small-molecule olefins. This part of olefins has a high octane number and needs to be separated and avoided to send into the hydrodesulfurization unit, where hydrogenation saturation is occurring. The content of small olefins in raffinate was obviously greater than that of the contrast group. Olefins with a low octane number were concentrated into the extract. F
DOI: 10.1021/acs.energyfuels.8b00026 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 4. Olefin and aromatic distribution of batch extractive distillation at S/F = 3.
4. CONCLUSION The solvents of extractive distillation were screened for separating critical components by VLE experiments, wherein SUL was found as the optimal single solvent treating heavy FCC naphtha. The separation coefficient θ was 2.81, and the sulfide selectivity coefficient β was 1.78. PONA analysis showed that SUL preferred to separate small-carbon olefins of heavy FCC naphtha, such as C6−C8 olefins. SCD analysis showed that NMP preferred to enrich thiophene as well as 2pentanethiol. Then, a dual solvent with the volume ratio of SUL/NMP = 85:15 was prepared, in which θ reached up to 4.06 and β was 1.97. With S/F of 3, the content of olefins was increased from 37.33 to 47.20 wt % and the content of aromatics was reduced from 8.72 to 2.69 wt % compared to the contrast group. It is worth noting that there is a chance to directly separate the critical component of FCC naphtha by combining the normal distillation and extractive distillation. This kind of technology as a platform can realize the refining industry under a molecular level. We still have a lot of work to investigate more functions of the compound solvent to enhance the ability to separate specific carbon number olefins and improve the concentration of sulfides. The separated olefin is suitable to be the feedstock of many crafts, such as isomerization to make high-octane-number gasoline components.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 86-10-89739078. Fax: 86-10-69724721. E-mail:
[email protected]. ORCID
Liang Zhao: 0000-0003-3585-6020 Author Contributions †
Yuhao Zhang and Yongtao Wang contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China (21476260, 21336011, and 21236009) and the Science Foundation of China University of Petroleum, Beijing (2462015YQ0311). G
DOI: 10.1021/acs.energyfuels.8b00026 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.8b00026 Energy Fuels XXXX, XXX, XXX−XXX
