Simple Model for Predicting the Cutting Temperature between Light

Nov 21, 2014 - State Key Laboratory of Heavy Oil Processing, and. ‡. Key Laboratory of Catalysis, China National Petroleum Corporation, China. Unive...
0 downloads 0 Views 834KB Size
Download PDF
Article pubs.acs.org/EF

Simple Model for Predicting the Cutting Temperature between Light and Heavy Fractions in Fluid Catalytic Cracking Naphtha Selective Hydrodesulfurization Processes Lei Wang,† Gang Shi,‡ Jian Xu,† Yu Fan,† and Xiaojun Bao*,‡ †

State Key Laboratory of Heavy Oil Processing, and ‡Key Laboratory of Catalysis, China National Petroleum Corporation, China University of Petroleum, No. 18 Fuxue Road, Changping, Beijing 102249, People’s Republic of China S Supporting Information *

ABSTRACT: In selective hydrodesulfurization processes for hydro-upgrading fluid catalytic cracking (FCC) naphtha with high olefin and sulfur contents, it is a common practice to split the feeding full-range FCC naphtha into a light fraction and a heavy fraction. This operation can effectively alleviate olefin saturation and thereby octane loss. Thus, the determination of a suitable cutting temperature plays a vital role in guaranteeing the success of the operation. Starting by fractionating two FCC naphthas into nine narrow cuts, this paper shows that, despite the great differences in the properties of the two FCC naphthas, both hydrocarbons and sulfides have almost the same distributions in the nine cuts. More importantly, it was observed that the distribution of sulfides in the narrow cuts is irrelevant to their true boiling points because of the formation of azeotropes between sulfides and hydrocarbons. On the basis of these findings, a simple model for estimating thiophene content in light FCC naphtha and, thereby, determining the cutting temperature was deduced and its applicability was verified using three other FCC naphthas sampled from different refineries. The salient feature of the model lies in that it only uses the total thiophene content of the feeding FCC naphtha to perform the estimation without the necessity to carry out time-consuming and cost-expensive prehydrogenation and fractionation tests. Thus, it can provide in-prior estimation for the design and operation optimization of FCC naphtha hydro-upgrading processes.

1. INTRODUCTION Increasingly strict environmental regulations on vehicle exhaust emissions over the world have led to the continuing reduction of the allowable level of sulfur in motor petrol. In the petrol pools of Asian countries, especially China, fluid catalytic cracking (FCC) naphtha is the major blending component, which contributes about 90% of sulfur to product petrol. Thus, deep and ultra-deep desulfurization of this stream becomes the main focus of clean petrol production.1 To achieve deep and ultra-deep desulfurization of FCC naphtha without causing serious olefin saturation and, thereby, large octane number loss,2 all of the existing hydrodesulfurization (HDS) processes for upgrading FCC naphtha, such as Prime-G+ from Axens,3 CDHydro/CDHDS from CDTECH,4 and GARDES from the China University of Petroleum, Beijing, China,5 employ a similar process configuration consisting of pre-hydrogenation, distillation, and hydrotreating steps, as illustrated in Figure 1. In these processes, the full-range FCC naphtha (FRFN) is first pre-hydrogenated in the pre-hydrogenation reactor, where small-molecular mercaptans react with dienes and olefins to form heavy sulfur compounds and dienes are simultaneously saturated into olefins. The pre-hydrogenated FRFN then goes into a distillation column, from which a light FCC naphtha (LCN) with very low sulfur content but high olefin content and a heavy FCC naphtha (HCN) with lower olefin content but higher sulfur content are obtained. The LCN can be used as a clean blending component, but the HCN with high sulfur content must be further treated in the followed HDS reactor(s). Obviously, the cutting temperature for LCN and HCN plays an important role in controlling the sulfur content © 2014 American Chemical Society

Figure 1. Schematic of a typical selective HDS process.

of LCN and the olefin content of HCN. Thus, a prior estimation of the cutting temperature is necessary for the design and operation of FCC naphtha hydro-upgrading units.6 Usually, after pre-hydrogenation, the vast majority of mercaptans originally existing in LCN have been transformed into heavier disulfides that can go into HCN after splitting, with thiophene being the predominant sulfide in the resultant LCN. Therefore, it is the thiophene content of LCN that determines the cutting temperature between LCN and HCN. To produce clean petrol that meets the Euro IV/Euro V standards, the Received: September 4, 2014 Revised: November 1, 2014 Published: November 21, 2014 7411

dx.doi.org/10.1021/ef501978t | Energy Fuels 2014, 28, 7411−7417

Energy & Fuels

Article

cutting temperature is determined by the LCN sulfur content; namely, it is set to allow for as much olefin as possible to enter the LCN while guaranteeing the sulfur limitation of LCN as a clean petrol blending component.7 For different kinds of FCC naphthas with different sulfur contents, however, the cutting temperature needs to be determined by first pre-hydrogenating FRFNs, then splitting them at different temperatures via distillation, and finally analyzing the sulfur contents of the resulting LCNs by an online analyzer, which is time-consuming and cost-expensive. Obviously, if the cutting temperature could be predicted by some easily measurable properties of a FRFN naphtha (e.g., the total sulfur content and thiophene content), it will provide great help for the design and operation of the FCC naphtha hydro-upgrading processes, which is especially true for those plants with unstable feedstocks. Theoretically, the cutting temperature can be presumed by referring to the normal boiling points and distribution of sulfides. However, the true boiling points of sulfides obtained in true boiling point distillation experiments are usually much lower than the normal boiling points of some sulfides, because sulfides and aliphatic and aromatic hydrocarbons in FCC naphtha can form azeotropes, whose boiling points are significantly lower than the corresponding sulfides.8−10 Therefore, determining the cutting temperature according to the normal boiling points of sulfides is not reliable for real FCC naphtha, leading to the difficulty in designing and operating FCC naphtha hydrotreating processes. To the best of our knowledge, it seems that the only result regarding the prediction of the cutting temperature between LCN and HCN is that due to Liang et al.11 Observing that the distributions of olefins and sulfides in the narrow cuts of FCC naphthas are similar, they indicated that an optimum cutting temperature for producing Euro IV standard petrol should be in the range of 72−80 °C, at which the majority of C7-olefins could enter into the resultant LCN. Herein, we report a mathematic model to predict the optimal cutting temperatures between LCN and HCN for producing Euro IV/Euro V standard petrol. First, two FCC naphthas, with one having a high sulfur content and one having a lower sulfur content, sampled from two Chinese refineries were cut into various narrow cuts at different cutting temperatures, and the hydrocarbons and sulfides in the resultant narrow cuts were determined by the true paraffin, isoparaffin, olefin, naphthene, and aromatics (PIONA) method and a gas chromatography− sulfur chemiluminescence detector (GC−SCD), respectively. Second, when the distributions of hydrocarbons and sulfides in the narrow cuts and the azeotropes formed between sulfides and the corresponding hydrocarbons were analyzed, a mathematic model that only uses the thiophene content of FRFN for predicting the thiophene content in the LCN fraction was proposed to determine the optimal cutting temperature. Finally, three other naphthas were chosen to verify the applicability of the model, and their optimal cutting temperatures were determined by the model.

Table 1. Basic Properties of the Dushanzi and Dagang FRFNs property density at 20 °C (g/cm3) sulfur (μg/g) research octane number (RON) motor octane number (MON) hydrocarbon composition (wt %) n-paraffins isoparaffins olefins naphthenes aromatics distillation (ASTM D86) (°C) IBP 10% 50% 90% FBP

Dushanzi

Dagang

0.737 872 94.34 85.33

0.725 345 93.75 84.87

4.06 26.95 42.43 7.15 19.41

5.04 20.45 39.43 10.43 24.65

30.4 50.3 91..6 179.6 199.6

29.8 49.7 90.5 173.7 195.9

meets the ASTM D2892 standard.12 As shown in Figure S1 of the Supporting Information, the distillation unit includes the following parts: a 10 L distillation flask with a side arm as a thermowell, an electric heating mantle, a fractionating column with an efficiency equivalent to 15 theoretical plates at total reflux and operated in batch mode under atmospheric pressure, a condenser using a coolant temperature of −20 °C, a fraction collector, and a moderate temperature measurement and control system. The calibration procedure of the related components can refer to the ASTM D2892 standard.12 In addition, it is emphasized that, on the top of the fractionation column, different narrow cuts, i.e., LCNs within different boiling ranges, can be obtained at the reflux ratio of 5:1. When the overhead temperature is controlled, nine narrow-boiling cuts were obtained and denoted as NCIBP−55 [from initial boiling point (IBP) to 55 °C], NC55−60 (from 55 to 60 °C), NC60−65 (from 60 to 65 °C), NC65−70 (from 65 to 70 °C), NC70−75 (from 70 to 75 °C), NC75−80 (from 75 to 80 °C), NC80−85 (from 80 to 85 °C), NC85−90 (from 85 to 90 °C), and NC90−FBP [from 90 °C to final boiling point (FBP)]. 2.3. Analysis Methods. The PIONA analysis of the FCC naphthas and their narrow cuts were conducted on a SP 3420A GC (Beijing Beifen Ruili Analytical Instruments Co., Ltd.) equipped with a BFB-93105 column and a flame ionization detector (FID). The carrier gas was nitrogen, flowing at 40 mL/min. The temperatures of the injector and detector were 280 and 180 °C, respectively. The oven temperature program was as follows: started at 35 °C and maintained at this temperature for 10 min, then increased to 60 °C at a 0.5 °C/ min temperature ramp, and finally increased to 180 °C at a ramp rate of 2 °C/min and kept there for 10 min. The qualitative and quantitative determination of the sulfur compounds in the various narrow cuts were made on a HP 7890 GC (Agilent Technologies, Wilmington, DE) installed with a model 355 SCD (Siever Instruments, Inc., Boulder, CO) according to the modified ASTM D5623-94 standard.13 The qualitative determination of the target compounds was based on the standard substances (methanthiol, ethanethiol, propanethiol, thiophene, butanethiol, and methylthiophene). The quantitative determination of major sulfur compounds and total sulfur content was carried out based on the linear response of the SCD, using diphenyl sulfide as an internal substance. The instrument was equipped with a HP 7683 autosampler and a Supelco SPB-1 column (30 m × 0.32 mm, 4 μm film thickness). The oven temperature program was as follows: started at 35 °C for the first 4.5 min, then increased to 230 °C at a ramp rate of 10 °C/min, and finally maintained there for 6 min. The HP ChemStation software was used for data acquisition and instrument control.

2. EXPERIMENTAL SECTION 2.1. Materials. The two FRFNs used in the present investigation were kindly provided by Dushanzi Petrochemical Company, PetroChina Company, Ltd. (Dushanzi, Xinjiang, China) and Dagang Petrochemical Company, PetroChina Company, Ltd. (Tianjin, China), and their basic properties are summarized in Table 1. 2.2. Experimental Apparatus and Process. The fractionation of the two FRFNs was conducted on a homemade distillation unit that 7412

dx.doi.org/10.1021/ef501978t | Energy Fuels 2014, 28, 7411−7417

Energy & Fuels

Article

Table 2. Hydrocarbon Distributions in the Narrow Cuts Obtained from Dushanzi FCC Naphtha narrow cut

(C4 + C5) olefins (wt %)

(C4 + C5) saturated hydrocarbons (wt %)

C6 olefins (wt %)

C6 saturated hydrocarbons (wt %)

C7 olefins (wt %)

C7 saturated hydrocarbons (wt %)

C7 aromatics (wt %)

(C8 + C8+) hydrocarbons (wt %)

total (wt %)

NCIBP−55 NC55−60 NC60−65 NC65−70 NC70−75 NC75−80 NC80−85 NC85−90 NC90−FBP total

11.56 0.16 0.02 0.00 0.00 0.00 0.00 0.00 0.00 11.77

5.78 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5.78

1.24 2.66 2.39 1.28 2.29 0.27 0.19 0.07 0.00 10.39

2.48 2.37 1.24 0.52 0.75 0.12 0.03 0.00 0.00 7.53

0.00 0.06 0.08 0.10 1.03 0.82 1.37 1.95 1.24 6.65

0.00 0.07 0.08 0.09 0.93 0.78 1.38 1.87 4.21 9.41

0.00 0.00 0.00 0.00 0.00 0.01 0.03 0.11 1.73 1.88

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.13 46.49 46.62

21.06 5.35 3.83 1.99 4.99 2.00 2.99 4.13 53.67 100

Table 3. Hydrocarbon Distributions in the Narrow Cuts Obtained from Dagang FCC Naphtha narrow cut

(C4 + C5) olefins (wt %)

(C4 + C5) saturated hydrocarbons (wt %)

C6 olefins (wt %)

C6 saturated hydrocarbons (wt %)

C7 olefins (wt %)

C7 saturated hydrocarbons (wt %)

C7 aromatics (wt %)

(C8 + C8+) hydrocarbons (wt %)

total (wt %)

NCIBP−55 NC55−60 NC60−65 NC65−70 NC70−75 NC75−80 NC80−85 NC85−90 NC90−FBP total

10.24 0.18 0.03 0.00 0.00 0.00 0.00 0.00 0.00 10.45

8.86 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 8.86

1.57 2.15 2.02 1.21 0.50 0.39 0.30 0.05 0.00 8.19

3.82 2.46 1.86 0.54 0.36 0.12 0.04 0.00 0.00 9.2

0.00 0.07 0.21 0.21 0.18 0.56 1.27 1.68 2.42 6.6

0.00 0.04 0.15 0.28 0.25 0.55 1.77 2.55 3.21 8.8

0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.26 4.87 5.19

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 42.73 42.73

24.47 4.89 4.28 2.25 1.29 1.62 3.44 4.54 53.23 100

the two FRFNs are almost the same. The first narrow cut NCIBP−55 contains exclusively all methanthiol and ethanethiol, with a small part of 1-propanethiol and thiophene in it,17 while the others mainly contain thiophenes. Specifically, the content of 1-propanethiol is the highest in NC55−60 and decreases in the heavier narrow cuts; while thiophene can be found in all of the narrow cuts, it is mainly and uniformly distributed in NC60−65, NC65−70, NC70−75, and NC75− 80; methylthiophene emerges in NC65−70, and its content in other heavier cuts increases gradually; and most methylthiophene and all C2-thiophene, C3-thiophene, C4-thiophene, benzothiophene, and methylbenzothiophene are distributed in the heaviest cut NC90−FBP (see Figure S8 of the Supporting Information). The above results indicate that the distribution behavior of the sulfur compounds in these narrow cuts are distinctively different from that predicted from the boiling points of the individual sulfur compounds, especially for 1propanethiol and thiophene. As known, 1-propanethiol has a boiling point of 67.8 °C, and thiophene has a boiling point of 84.2 °C; therefore, these two sulfides should have their maximum contents in NC65−70 and NC80−85 rather than singularly in NC55−60 and NC65−70, respectively. This phenomenon was also reported by Pang and Zhang.18 The reason for this is that some sulfides and hydrocarbons in FCC naphtha can form azeotropes, whose boiling temperatures are lower than those of the corresponding individual sulfides. By digging out the data reported in the literature,8,9,19−24 we found that typical sulfides that can form azeotropes with hydrocarbons existing in FCC naphtha are 1-propanethiol, thiophene, 2methylthiophene, and 3-methylthiophene, as shown in Table 4. Intuitively, it is conceived that, once a hydrocarbon and sulfide form an azeotrope, their distributions in the different

3. RESULTS AND DISCUSSION 3.1. Hydrocarbon Group Compositions. The PIONA compositions of the narrow cuts obtained from the two FRFNs are given in Tables 2 and 3. It can be seen that the major hydrocarbon type in the narrow cuts with their FBPs < 90 °C is olefin.11,14 However, the olefins of different carbon numbers have their highest contents in the different narrow cuts: C4 and C5 olefins are concentrated in NCIBP−55, C6 olefins are distributed in NC55−60, NC60−65, NC65−70, and NC70− 75, and C7 olefins are present in NC75−80, NC80−85, and NC85−90. Although the contents of the different individual hydrocarbons in the two FRFNs are quite different, their distributions in the different narrow cuts are almost the same: most C4 and C5 hydrocarbons are concentrated in the first narrow cut NCIBP−55; C6 hydrocarbons are distributed in NC55−60, NC60−65, NC65−70, and NC70−75, with the last cut simultaneously having a large amount of C6 hydrocarbons and C7 hydrocarbons; C7 hydrocarbons are mainly in NC75− 80, NC80−85 and NC85−90, and in NC75−80, aromatic hydrocarbons emerge and their content gradually increases with the increasing cutting temperature; 16 and C 8 and C 8 + hydrocarbons are distributed in NC90−FBP. 3.2. Sulfur Compounds. The sulfur compounds in the various narrow cuts obtained from the two FRFNs were identified by GC−SCD, and the results in the cuts, except NC90−FBP, are shown in Figure 2. The typical sulfides include methanthiol, ethanethiol, propanethiol, thiophene, butanethiol, methylthiophene, dimethylthiophene, trimethylthiophene, tetramethylthiophene, benzothiophene, and methylbenzothiophene.15,16 It can be seen that, despite the large difference in the total sulfur contents of the two FRFNs, the distributions of the different sulfur compounds in the different narrow cuts of 7413

dx.doi.org/10.1021/ef501978t | Energy Fuels 2014, 28, 7411−7417

Energy & Fuels

Article

Figure 3. Comparison of contents of thiophene and the corresponding hydrocarbons: (I) NCIBP−55, (II) NC55−60, (III) NC60−65, (IV) NC65−70, (V) NC70−75, (VI) NC75−80, (VII) NC80−85, and (VIII) NC85−90. The hydrocarbons that can form azeotropes with thiophene are listed in Table 4.

Figure 2. Distributions of sulfur compounds in the narrow cuts of the (a) Dushanzi and (b) Dagang FRFNs: (1) methanthiol, (2) ethanethiol, (3) 1-propanethiol, (4) thiophene, and (5) methylthiophene.

Table 4. Azeotropes Formed between Sulfides and Hydrocarbons in FCC Naphtha boiling point of the component (°C)

component 1-propanethiol8,21 2,3-dimethylbutane 2-methylpentane 3-methylpentane n-hexane thiophene9,19−24 n-hexane methylcyclopentane 2,4-dimethylpentane cyclohexane 2-methylthiophene9,24 n-heptane 2,2-dimethylhexane 2,5-dimethylhexane 3-methylthiophene9,24 ethylcyclopentane 2,5-dimethylhexane

67.80 58.10 60.40 63.35 68.75 84.20 68.75 71.85 80.55 80.85 113.00 98.40 106.85 109.15 114.00 103.45 109.15

contents of thiophene and some hydrocarbons (n-hexane, methylcyclopentane, 2,4-dimethylpentane, and cyclohexane) listed in Table 4 do have the same distribution in the different narrow cuts, exactly in agreement with our inference. It is because of the formation of these azeotropes that the distribution patterns of these sulfides in the narrow cuts become almost irrelevant to their boiling points but dependent upon the distribution patterns of the corresponding hydrocarbons. 3.3. Simple Model To Predict the Thiophene Content in LCN. The above discussion suggests that the formation of azeotropes may lead to an unclear division of thiophene in LCN and HCN. A prior prediction of the optimal cutting temperature that can simultaneously guarantee the control of the total sulfur content of LCN and ensure the maximum quantity of olefins in LCN is necessary for the design and operation of FCC naphtha hydro-upgrading units. It is known that, after mercaptan conversion via pre-hydrogenation, nearly all mercaptans originally existing in a FRFN can be converted into heavy sulfur compounds (see Figure S9 of the Supporting Information) and the predominant sulfide remaining in the LCN after fractionation is thiophene; therfore, a correlation for predicting the thiophene content in LCN becomes the main focus. The thiophene content C(LCN,T)(t) in a LCN in terms of μg/ g, which is a function of the cutting temperature t, can be simply expressed by

boiling point of the azeotrope (°C) 57.54 59.20 61.26 64.35 68.46 71.47 76.58 77.90 97.77 104.62 106.12 102.82 107.12

cuts should be the same. To confirm this, the distributions of the typical sulfides that can form azeotropes with the hydrocarbons in the narrow cuts listed in Table 4 were investigated and the results are shown in Figure 3 and Figures S2−S7 of the Supporting Information. It can be seen that the 7414

dx.doi.org/10.1021/ef501978t | Energy Fuels 2014, 28, 7411−7417

Energy & Fuels C(LCN,T)(t ) =

Article

F(T)(t ) = 419.191 − 24.0746t + 0.41832t 2 − 0.00211t 3

M(LCN,T)(t ) (1)

(5)

where M(LCN,T)(t) is the mass of thiophene in the LCN obtained at cutting temperature t and M(LCN,H)(t) is the total mass of the LCN. M(LCN,T)(t) and M(LCN,H)(t) can also be expressed by

f(H) (t ) = −297.816 + 12.6881t − 0.16598t 2 + 0.00075t 3

M(LCN,H)(t )

M(LCN,T)(t ) = M(FRFN,T)F(T)(t )

(6)

By substituting eqs 5 and 6 into eq 4, we have C(LCN,T)(t ) = C(FRFN,T)((419.191 − 24.0746t + 0.41832t 2

(2)

− 0.00211t 3)/( −297.816 + 12.6881t

M(LCN,H)(t ) = M(FRFN,H)f(H) (t )

(3)

− 0.16598t 2 + 0.00075t 3))

In eq 2, M(FRFN,T) is the total thiophene mass in a FRFN and F(T)(t) is the ratio of the thiophene mass in the LCN to the total thiophene mass in the FRFN. In eq 3, M(FRFN,H) is the total mass of the FRFN and f(H)(t) is the ratio of the LCN mass to the total FRFN mass. Therefore, eq 1 can be transformed into the following equation: C(LCN,T)(t ) =

M(FRFN,T)F(T)(t ) M(FRFN,H)f(H) (t )

= C(FRFN,T)

Equation 7 states that, for a given FRFN with a total thiophene content C(FRFN,T), the thiophene content C(LCN,T)(t) in LCN depend upon only the cutting temperature t. To verify the applicability of eq 7 to different FRFNs, fractionation experiments were conducted at the cutting temperatures of 55, 60, 65, 70, 75, 80, 85, and 90 °C using three other FRFNs sampled from Dalian PetroChemical Company, PetroChina Company, Ltd. (C(FRFN,T) = 33 μg/g), Golmud Refinery, PetroChina Company, Ltd. (C(FRFN,T) = 10 μg/g), and Ningxia PetroChemical Company, PetroChina Company, Ltd. (C(FRFN,T) = 51 μg/g), and the results are shown in Table 5. It can be seen that the experimental results obtained from the three FRFNs are in exact agreement with those calculated by eq 7. The comparison of the predicted and experimental thiophene contents in the LCNs of the three RFRNs shows that the equation works satisfactorily and, thus, can be used to predict the cutting temperature for splitting LCN and HCN. 3.4. Determination of the Cutting Temperature between LCN and HCN. As stated above, the very reason for splitting a feeding FRFN into a LCN stream and a HCN stream in all of the existing FCC naphtha hydro-upgrading processes is to maximize the content of olefins in LCN and, thus, avoid the saturation olefins; at the same time, the choice of the cutting temperature should also meet the requirement of product blending for the sulfur content of LCN. Therefore, an optimal cutting temperature should be set to allow for olefins to enter LCN as much as possible while guaranteeing the sulfur limitation for LCN. As shown in Figure 5, the temperature is determined by controlling the sulfur content of the LCN to be equal to the allowable level of the final blending product, e.g., 50 μg/g for Euro IV petrol and 10 μg/g for Euro V petrol. Meanwhile, the accurate determination of the cutting temperature is very vital because its subtle change has a great influence on the olefin content in the resultant LCN, owing to the fact that large portions of C5 and C6 olefins have their boiling points in the range of 40−70 °C. After the mercaptan conversion via pre-hydrogenation, nearly all small-molecular mercaptans originally existing in a FRFN have been converted into heavy sulfur compounds; thus, the predominant sulfide existing in the LCN after fractionation is thiophene, whose content in the LCN determines the cutting temperature between LCN and HCN. For the above three FRFNs after pre-hydrogenation, as shown in Table 6, the optimal cutting temperatures estimated through eq 7 are 52.9 °C (for the Ningxia FRFN), 53.9 °C (for the Dalian FRFN), and 61.2 °C (for the Golmud FRFN) for producing Euro V petrol and 61.0 °C (for the Ningxia FRFN) and 67.1 °C (for the Dalian FRFN) for producing Euro IV petrol. The fractionation experiments were carried out at the corresponding optimal cutting temperatures, and the sulfur contents of the

F(T)(t ) f(H) (t )

(7)

(4)

where C(FRFN,T) is the thiophene content in the FRFN. Equation 4 shows that C(LCN,T)(t) is related to the total thiophene content C(FRFN,T) in the FRFN that is independent upon the cutting temperature and the two functions F(T)(t) and f(H)(t) that are dependent upon the cutting temperature. According to the results described in Figure 3, F(T)(t) should be approximately equal to the ratio of the mass of the four hydrocarbons (n-hexane, methylcyclopentane, 2,4-dimethylpentane, and cyclohexane) in the LCN to their total mass in the FRFN (see Table S1 of the Supporting Information). Meanwhile, owing to the fact that the compositions of hydrocarbons in FCC naphthas from various sources are quite similar and their distributions in the different narrow cuts are almost the same, as reported by Liang et al.,11 the functions F(T)(t) and f(H)(t) of the two different FCC naphthas should have a similar change trend. Figure 4 shows the changing trends

Figure 4. F(T)(t) and f(H)(t) versus cutting temperature t.

of F(T)(t) and f(H)(t) with the increasing cutting temperature for the Dagang and Dushanzi FRFNs. Interestingly, we can see that both F(T)(t) and f(H)(t) are the unary functions of cutting temperature t, regardless of the properties of the FRRNs involved, exactly in agreement with our above inference. For the ease of computation, the explicit expressions of F(T)(t) and f(H)(t) are obtained by fitting the results in Figure 4 using polynomial regression, as shown in eqs 5 and 6. 7415

dx.doi.org/10.1021/ef501978t | Energy Fuels 2014, 28, 7411−7417

Energy & Fuels

Article

Table 5. Comparison of the Predicted and Experimental Thiophene Contents in LCN Fractions cutting temperatures (°C) LCN LCN LCN LCN LCN LCN

thiophene thiophene thiophene thiophene thiophene thiophene

content content content content content content

of of of of of of

Dalian naphtha (experimental) (μg/g) Dalian naphtha (predicted) (μg/g) Golmud naphtha (experimental) (μg/g) Golmud naphtha (predicted) (μg/g) Ningxia naphtha (experimental) (μg/g) Ningxia naphtha (predicted) (μg/g)

55

60

65

70

75

80

85

90

12.74 13.71 4.05 4.15 21.93 21.19

27.67 29.37 8.45 8.90 46.51 45.39

44.03 44.07 13.54 13.35 68.95 68.10

53.25 57.62 16.21 17.46 84.35 89.05

67.97 68.89 20.54 20.87 103.22 106.46

76.07 76.17 23.14 23.08 111.54 117.71

79.27 77.77 23.44 23.57 116.03 120.19

74.26 72.76 22.14 22.05 105.83 112.45

of great value for the design and operation optimization of FCC naphtha selective hydrodesulfurization processes, because it allows for quick estimation of the optimal cutting temperature by only using the thiophene content of a full-range FCC naphtha, without the necessity to carry out time-consuming and cost-expensive pre-hydrogenation and fractionation tests.

■

ASSOCIATED CONTENT

S Supporting Information *

Comparison of F(T)(t) and ratio of the mass of the corresponding hydrocarbons (Table S1), image of the homemade fractionation unit (Figure S1), comparison of contents of sulfides and hydrocarbons that can form azeotropes with the corresponding sulfides (Figures S2−S7), distribution of sulfur compounds in NC90−FBP (Figure S8), and distributions of sulfur compounds before and after pre-hydrogenation (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. Determination of the optimal cutting temperature between LCN and HCN.

■

resultant LCNs are compared to those predicted values: 49.34 versus 50.00 μg/g (for the Dalian FRFN) and 51.27 versus 50.00 μg/g (for the Ningxia FRFN) for producing Euro IV petrol and 9.61 versus 10.00 μg/g (for the Dalian FRFN), 10.25 versus 10.00 μg/g (for the Ningxia FRFN), and 9.88 versus 10.00 μg/g (for the Golmud FRFN) for producing Euro V petrol. We can see that the predicted and experimental values are in good agreement for the three RFRNs, demonstrating the outstanding predictability of eq 7.

AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-10-89734979. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China through the National Basic Research Program of China (Grant 2010CB226905) and the National Natural Science Foundation of China (Grant 21276270).

4. CONCLUSION In summary, we have successfully demonstrated that, for different FCC naphthas, the distributions of hydrocarbons and sulfides in different narrow cuts follow almost the same patterns and, most importantly, the distribution of sulfides is not directly related to their boiling points but depends upon the distribution patterns of the corresponding hydrocarbons that can form azeotropes with them. On the basis of these findings, a simple model for predicting the thiophene content in LCN is proposed to estimate the optimal cutting temperature between LCN and HCN. The verification experiments showed that the model predictions are in good agreement with the experimental results. From the practical application perspective, the model is

■

REFERENCES

(1) Brunet, S.; Mey, D.; Pérot, G.; Bouchy, C.; Diehl, F. On the hydrodesulfurization of FCC gasoline: A review. Appl. Catal., A 2005, 278, 143−172. (2) Ito, E.; van Veen, J. A. R. On novel processes for removing sulphur from refinery streams. Catal. Today 2006, 116, 446−460. (3) Cosyns, J.; Debuisschert, Q.; Didillon, B. The domino interaction of refinery processes for gasoline quality attainment. Proceedings of the

Table 6. Comparison of the Predicted and Experimental Sulfur Contents in LCNs at Optimal Temperatures Euro IV FCC naphthas Dalian naphtha Golmud naphtha Ningxia naphtha

predicted experimental predicted experimental predicted experimental

Euro V

optimal cutting temperature (°C)

sulfur content of LCN (μg/g)

optimal cutting temperature (°C)

sulfur content of LCN (μg/g)

67.1 67.1

50.00 49.34

61.0 61.0

50.00 51.27

53.9 53.9 61.2 61.2 52.9 52.9

10.00 9.61 10.00 9.88 10.00 10.25

7416

dx.doi.org/10.1021/ef501978t | Energy Fuels 2014, 28, 7411−7417

Energy & Fuels

Article

National Petrochemical and Refiners Association (NPRA) Annual Meeting; San Antonio, TX, March 26−28, 2000; Paper AM-00-61. (4) Song, C. S. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211−263. (5) Fan, Y.; Shi, G.; Bao, X. J. A process for producing ultraclean gasoline by coupling efficient hydrodesulfurization and directional olefin conversion. AIChE J. 2013, 59, 571−581. (6) Largeteau, D.; Ross, J.; Laborde, M.; Wisdom Axens, L. Challenges and opportunities of 10 ppm sulphur gasoline: Part 2. Pet. Technol. Q. 2012, 17, 25. (7) Towler, G. P.; Schultz, M. A. Fractionation for full boiling range gasoline desulfurization. U.S. Patent 6,540,907, 2004. (8) Denyer, R. L.; Fidler, F. A.; Lowry, R. A. Azeotrope formation between thiols and hydrocarbons. Ind. Eng. Chem. 1949, 41, 2727− 2737. (9) Desty, D. H.; Fidler, F. A. Azeotrope formation between sulfur compounds and hydrocarbons. Ind. Eng. Chem. 1951, 43, 905−910. (10) Miracca, I.; Judzis, A.; Sahay, N.; Sanfilippo, D. Catalytic distillation. In Handbook of Heterogeneous Catalysis; Ertl, G., Knözinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 2008; pp 2188−2198. (11) Liang, Y.; Shi, Q.; Liu, X.; He, J. Study on the cut point of light and heavy fractions for selective hydrodesulfurization of FCC gasoline. Pet. Process. Petrochem. 2011, 42, 40−44. (12) ASTM International. ASTM D2892-03, Distillation of Crude Petroleum (15-Theoretical Plate Column); ASTM International: West Conshohocken, PA, 2003. (13) ASTM International. ASTM D5623-94, Sulfur Compounds in Light Petroleum Liquids by Gas Chromatography and Sulfur Selective Detection; ASTM International: West Conshohocken, PA, 1994. (14) Yin, C.; Xia, D. Distribution of sulfur compounds in the fullrange FCC and RFCC gasoline. J. Fuel Chem. Technol. 2001, 29, 256− 258. (15) Yin, C.; Zhu, G.; Xia, D. A study of the distribution of sulfur compounds in gasoline fraction produced in China: Part 2. The distribution of sulfur compounds in full-range FCC and RFCC naphthas. Fuel Process. Technol. 2002, 79, 135−140. (16) Yin, C.; Xia, D. A study of the distribution of sulfur compounds in gasoline produced in China. Part 3. Identification of individual sulfides and thiophenes. Fuel 2004, 83, 433−441. (17) Yin, C.; Xia, D. A study of the distribution of sulfur compounds in gasoline produced in China. Part 1. A method for the determination of the distribution of sulfur compounds in light petroleum fractions and gasoline. Fuel 2001, 80, 607−610. (18) Stumpf, Á .; Tolvaj, K.; Juhász, M. Detailed analysis of sulfur compounds in gasoline range petroleum products with high-resolution gas chromatography−atomic emission detection using group-selective chemical treatment. J. Chromatogr. A 1998, 819, 67−74. (19) Sapei, E.; Zaytseva, A.; Uusi-Kyyny, P.; Keskinen, K. I.; Aittamaa, J. Vapor-liquid equilibrium for binary system of thiophene + 2,2,4-trimethylpentane at 343.15 and 353.15 °C and thiophene + 2ethoxy-2-methylpropane at 333.15 and 343.15 °C. Fluid Phase Equilib. 2007, 261, 115−121. (20) Bai, J.; Guo, B.; Xia, S.; Li, Y.; Ma, P. Isobaric vapor−liquid equilibrium for four binary systems of thiophene. Fluid Phase Equilib. 2012, 315, 84−90. (21) Erlin, S.; Zaytseva, A.; Uusi-Kyyny, P.; Younghun, K.; Keskinen, K. I.; Aittamaa, J. Vapor-liquid equilibrium for binary system of 1propanethiol, thiophene, and diethyl sulfide with toluene at 90.03 kPa. J. Chem. Eng. Data 2006, 51, 1372−1376. (22) Sapei, E.; Zaytseva, A.; Uusi-Kyyny, P.; Keskinen, K. I.; Aittamaa, J. Vapor−liquid equilibrium for binary system of thiophene + n-hexane at (338.15 and 323.15) K and thiophene + 1-hexene at (333.15 and 323.15) K. J. Chem. Eng. Data 2006, 51, 2203−2208. (23) Giles, N. F.; Wilson, L. C.; Wilson, G. M.; Wilding, W. V. Phase equilibria on eight binary mixtures. J. Chem. Eng. Data 1997, 42, 1067−1074.

(24) Guo, B.; Bai, J.; Li, Y.; Xia, S.; Ma, P. Isobaric vapor−liquid equilibrium for systems containing sulfur compounds. Fluid Phase Equilib. 2013, 353, 87−92.

7417

dx.doi.org/10.1021/ef501978t | Energy Fuels 2014, 28, 7411−7417