Influence of Polyethylene Glycol on the Deep Desulfurization of

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The influence of Polyethylene glycol on the deep desulfurization of catalytic cracking gasoline by polyurethane membranes via pervaporation Yingfei Hou, Haiping Li, Yang Xu, Qingshan Niu, and Wenlei Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03654 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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The influence of polyethylene glycol on the deep desulfurization of catalytic cracking gasoline by polyurethane membranes via pervaporation Yingfei Hou*, Haiping Lia , Yang Xua, Qingshan Niua, Wenlei Wu b a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum

(East China),Qingdao. b

CHAMBROAD

CHEMICAL INDUSTRY RESEARCH

INSTITUTE

CO,LTD,Bingzhou.

Abstract Polyethylene glycol (PEG) is an important additive that can effectively improve the desulfurization performance of polyurethane membranes for fluid catalytic cracking (FCC) gasoline via pervaporation (PV). Polyurethane membrane was characterized using Fourier transform infrared spectroscopy (FT-IR) analysis and X-ray diffraction techniques. Permeation vaporization experiments were carried out for sulfur-containing compounds in FCC gasoline using homemade Polyvinylidene fluorid (PVDF) bottom membranes. Compared with the traditional osmogasification membrane, PEG/Polyurethane (PU) hybrid membrane shows higher sulfur enrichment factor (6.00) and permeation flux (2.20 kg/(m2.h)), which confirms the superior performances of proposed PU membrane. Through the analysis of meteorological S1

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chromatography, it can be clearly observed that the PEG/PU membrane can effectively remove the thiophene and its derivatives in FCC gasoline.

Key words: Pervaporation, Gasoline desulfurization, Polyurethane, Polyethylene glycol.

1.

Introduction

Sulfur species in the exhaust is the main source for air pollution. Concerns about the global environment have stimulated academic and industry attention to reducing vehicle emissions1-4. The sulfur-containing compounds in FCC gasoline are mostly in the form of thiophene and its derivatives, which account for more than 80% of sulfur content5.

There

are

several

ways

to

reduce

sulfur

content,

including

hydrodesulfurization (HDS)6-8, oxidative desulfurization(ODS)9-12, extractive or azeotropic distillation, etc13. In this context, pervaporation(PV) is considered to be one of the most promising alternative technologies with the economical and environmental advantages. Separation technology using PV membrane that is with high efficiency and low energy consumption, environmentally friendly, easy to operate, easy to integrate and control advantages14, 15, which will offers a number of potential advantages over conventional sulfur compounds removal process in the FCC gasoline desulfurization 16. Selection of suitable polymer membrane materials and the preparation approaches are key to pervaporation technology. In recent years, the development and S2


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application of membranes has injected new vitality into petrochemical industry, especially

the

development

of

various

synthetic

polymer

separation

membrane.[references] In 2002, Germany Grace Division developed the S-Brane membrane separation process for gasoline desulfurization.[references] Later effort has been paid to gasoline desulfurization using membrane.. Exxon Mobil Corporation Translonics and Marathon Oil,

and China University of Petroleum (East China),

Tsinghua University and Tianjin University and other institutions are committed to the development of the selection membrane material the optimization of process parameters, and the achievement of industrialization. Most of the research focused on process development, technology scale-up and optimization of operating parameters. Current research work is focused on the development of new membrane materials and modification of membrane materials of the present membranes in order to improve sulfur enrichment and flux. Lin et al.17 improved the performance of the PEG membranes significantly using cross-linking modification. Similarly, Qu et al.18 investigated the effects of different crosslinking degrees on the desulfurization performance of hydroxyethyl cellulose (HEC) membranes. Although the flux is found to decrease, the sulfur enrichment factor of the membrane is also enhanced with the increasing degree of crosslinking. When the crosslink density is 0.2, the sulfur enrichment factor is 3.47, and the corresponding permeation flux is 0.78 kg / (m2.h). Qi et al.19 modified the structure of polydimethylsiloxane (PDMS) polymer by cross-linking method to improve comprehensive performance. This above mentioned high polymer and modified methods are effective means to provide some helpful S3


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suggestions to the involved research field. PU is composed of block copolymer with special structure of soft and hard segment alternately in the material. PU is known as versatile material with excellently high elasticity, wearing resistance and corrosion resistance. Due to its unique molecular structure and physicochemical properties, PU has attracted much attention in the field of PV membrane separation20. Based on our previous work, this paper mainly studies the hybird of PEG to improve the flux in order to increase the low flux of real gasoline PU membrane. In this work, membrane characterization was conducted by FT-IR analysis and XRD studies. Gas chromatography provides a detailed analysis of the feed and permeate side material, which provides a reference for a better understanding of the PU mechanism. All of these surveys will provide useful advice for emerging membrane desulphurization technologies. 2. Experimental 2.1. Materials Elastollan® PU1180A10 (manufactured by BASF） was selected as the base polymer to produce the membrane. PEG (molecular weight of 200) was purchased from West Long Chemical Co., Ltd (Guangdong, China). Tetrahydrofuran (THF) was purchased from China National Pharmaceutical Group Corporation (Shanghai, China). Gasoline feed was obtained from Shandong Chambroad Holding Co., Ltd (Shandong, China). PVDF bottom membranes, (homemade), the thickness of 110~120µm. 2.2. The preparation of membrane S4


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Five membrane casting solutions were prepared at different PEG concentrations. The weight of PU polymer was 2.00 g in all the solutions, while the weight of PEG in the casting solutions was varied from 0.00 to 7.00 wt%. Desired amount of PU polymer and PEG as pore-forming agent was dissolved in THF (12.00 g) under vigorous stirring at room temperature to from a solution. Then, the solution was cast onto the porous PVDF support using a casting knife. After evaporating THF at 40 °C for 20 min, the obtained membrane was stored in dust-free and dry atmosphere for further characterization. The membranes were designated as M0, M1, M2, M3, M4 and M5 with respect to the weight of PEG. The thickness of the active layer was about 10.00-20.00µm 2.3. Characterization of membrane 2.3.1. Surface structure analysis The surface morphology of the PU membranes and the longitudinal direction of the cross-section were observed and recorded by a JEOL JSM-6300F scanning electron microscope (SEM). The surface roughness of the PU membranes was characterized by atomic force microscopy (AFM). 2.3.2. Fourier transform infrared spectroscopy The Fourier transform infrared (FT-IR) spectra of the membranes were recorded using a Nicolet Avatar 370RCT basic FTRI spectrometer (USA) within the range of 4000-500cm-1. 2.3.3. X-ray diffraction (XRD) studies X-ray diffraction patterns of the PU membranes were performed at the application S5


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laboratory of PaNalytical in Almelo with an X’Pert PRO MPD diffractometer (Netherlands). The X-ray was generated by a Cu Kα source. The angle of diffraction was varied from 5° to 75°. 2.4. Pervaporation experiment The pervaporation experiments were carried on an equipment manufactured by Shi Yi Science and Technology Industrial Development Crop. The capacity of the feed cell was about 500 mL and the effective surface area of the membrane in contact with the gasoline feed was 1.245×10-3m2. The permeate vapor was collected in a glass trap suspended inside the liquid nitrogen jar. The experiments were conducted with sulfide content of 200 ppm at room temperature with the permeate pressure of 20 mmHg to separate FCC gasoline with a temperature gradient of 65-85 °C. From the pervaporation data, separation performance of the membrane was estimate in terms of total flux (J) and sulfur enrichment factor (α). J can be calculated from the weight of permeate collect after pervaporation run using eq17 (1):

J=

×

(1)

Where Q is the total weight of permeate (Kg) collected in time t (h) and α is the effective membrane area (m2). α is defined as eq. (2):

α=

(2)

Where CF and CP are the total sulfur contents of the feed and permeate samples, respectively. The sulfur content was analyzed by a fluorescence sulfur analyzer. Composition analysis of gasoline was carried out by Varian GC-3800 gas chromatography (GC). S6


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3. Results and discussion 3.1. Membrane characterization 3.1.1. FTIR analysis The IR spectra of PU and PEG / TPU composite membranes were as shown in Fig.1 and 2 respectively. The perk at the wavenumber 3333 cm-1 corresponds to the characteristic hydroxyl group and in the region of 2958 cm-1 to the C-H group. Generally, the peak at 1733 cm-1and 1533 cm-1are representative of the carbamate and the amide group in PU. The remaining isocyanate groups in PU can react with the hydroxyl groups in PEG and produce carbamate groups confirmed by the disappearance of the isocyanate peak at 2286 cm-1 and the decreased intensity of hydroxyl group (characteristic of PEG) at 3333 cm-1 It can be observed from the FTIR a low reaction yield21.

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Energy & Fuels

60 50

2286

40 T (%)

30

2873

776

20

︸

3333 O-H groups

10

︸

2958 C-H groups

1079

︸

0

︸

︸ 1533 1173-1219 1736 urethane-C=O vibration

4000

3500

3000 2500 2000 1500 wavenumber(cm-1)

1000

500

Fig 1. FTIR spectra of PU membrane.

60 50 40 T (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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509

30 771

︸

20

3333 ︸ O-H groups 2957 C-H groups

︸

10

︸ 1733

0

1530 amide

carbamate

4000

3500

3000

2500 2000 1500 wavenumber (cm-1)

1000

500

Fig 2. FTIR spectra of PEG/PU membrane. 3.1.2. X-ray diffraction studies To investigate the influence of PEG on the arrangement of polymer chains, the crystalline structure of PU is analyzed by XRD. Fig.3.respresents the XRD spectra of S8


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pure PU membrane and PEG/PU Hybrid membrane. Pure PU membrane in Fig 3a reveals two characteristic peaks at 2θ=17.33 and 22.53, which indicate the presence of semicrystalline structure of the sample. The polymer molecular layer spacing and the chain spacing obtained by Bragg equation is shown in Table 1. Form the Table 1, when the low molecular weight PEG is doping in the casting fluid, the spacing of the molecular layer becomes smaller and the chain spacing increases, reducing the mass transfer resistance, which makes the penetration flux significantly increase.

(b)

intensity

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Energy & Fuels

(a)

0

10

20

30

40

50

60

70

80

90

100 110 120

2θ

Fig 3. XRD patterns of the PU and PEG/PU. Table1 Distance between molecular layer and between molecular chain of PU membrane and PEG/PU membrane.

S9


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Material

The first

Distance

The

The distance between

type

dispersion

between

dispersion

the molecular chains

peak2θ（°）

molecular layer

peak2θ（°）

17.33

5.11

22.57

3.94

19.06

4.93

21.35

4.13

PU PEG/PU 3.1.3. SEM

S 10


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Fig.4. SEM images of the PU and PEG/PU. surface morphologies of the pure PU, (b)surface image of 5%PEG/PU, (c) cross-section morphologies of pure PU, (d) cross-section morphologies of 5%PEG/PU composite membranes.(e) and cross-section of the active layer of pure PU,(f) and cross-section of the active layer of 5%PEG/PU. In the hybrid membranes, the addition of PEG significantly affected the desulfurization of gasoline during the pervaporization process, characterized by SEM in Fig .4. By contrast the PU and PEG / PU hybrid membranes, it was observed that the surface morphologies of PEG / PU hybrid membrane was rough compared with PU and cross-section morphologies of the pore size of PEG / PU hybrid membrane was larger mainly due to the fact that the membrane is placed in a water coagulation bath during the membrane process to dissolve part of the PEG in water22.

3.1.4 AFM It assume that the membrane surface consists of a dense array of roughness elements to predict the change in surface area of a membrane. Each roughness element is approximated as a cone with an aspect ratio characterized by height of the cone (h) versus the radius at the base of the cone (R). The enhancement in surface area by surface roughness compared to a perfectly smooth one is related to the aspect ratio (h/R).

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Energy & Fuels

= ^2 1

roughness h=100nm h=10nm h=1nm

100

S u rfa c e a re a e n h a n c e m e n t

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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membrane

80

60

h

40

20

1

10

100

1000

R

R (nm)

Fig 5. Surface area enhancement versus footprint radius of roughness element for 3 different roughness heights. The following chart summarizes the effects of the aspect ratio in surface area. The results show that the increase of surface area is the most important factor of roughness with the highest aspect ratio. We believe that this is a highly idealized scenario because real membranes would not have such well-defined roughness elements with such a high packing density, which would imply that the surface area enhancement is less than the power-law scaling shown here.

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Energy & Fuels

(a)

(b)

Fig 6. AFM images of (a) neat PU membrane, (b) PEG / PU hybrid membranes. In order to detect the effect of the smoothness of the membranes on the desulfurization performance of the gasoline, the roughness of the surface of the membranes was measured using atomic force microscope (AFM). Fig.6 demonstrates the three dimensional surface AFM images of the neat and PEG modified PU membrane. In these images, the brightest area presents the highest point of the membrane surface and the dark regions indicate valley or membrane pores. The surface roughness parameters of the membrane were calculated based on the scan size of 5μm × 5μm. The mean roughness (Ra) of the neat PU-membrane and modified membrane (neat PU,PEG/PU) are 3.821 nm, 4.324 nm respectively. Since increasing membrane surface enhances permeation flux, we can conclude from Figure 5 that the surface area of PEG/PU hybrid membrane is greater than that of pure PU membrane, which is very good for explaining the S 13


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reason of the increase of permeability flux. 3.2 Pervaporation characteristics A non-porous polymeric membrane usually serves the separating barrier for the PV process. The selected polymer membrane generally has a high affinity or a faster diffusion coefficient in contact with the sulfide in the gasoline so that the sulfur compounds is preferentially removed from the gasoline. In PV process, the macromolecule polymer membranes preferentially pass through sulfur compounds, evaporates into the gas phase, after which sulfur compounds is compressed, depending on the difference between the hydrocarbons and sulfur the degree of affinity. In order to allow the pervaporation process to proceed continuously, PV contacts directly with the liquid feed on one side of the membrane and the other side of the cold trap to ensure a very low absolute pressure to provide power. The temperature of the liquid feed and the PEG content will be discussed as two important parameters in the following text. At the same time, the PV process of the relevant parameters of the study can help to determine a reliable new sulfur removal and expand the design of the underlying technology. 3.2.1. Effect of PEG Content on Membrane Properties S 14


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The effect of different PEG contents on PEG / PU hybrid membranes is shown in Fig.5 In this experiment, only the content of PEG from 0% to 7% was investigated because when the PEG content is greater than 7%, however, when the content of the added polyethylene glycol is more than 7%, the flux of the membrane is too large and the sulfur enrichment factor is almost 1. It can be seen from the figure that when the content of PEG reaches the maximum, the permeation flux is the largest and the sulfur enrichment factor is the smallest. The appropriate ratio can be determined according to the performance of the obtained properties of the membrane 23. It can be seen in Fig. 7 that when the PEG content increases, the flux increases and the enrichment factor decreases. This phenomenon is possibly induced by the addition of small molecular weight PEG disrupts the density of the polymer membrane to a certain extent, so the flux increases and the enrichment factor decreases. Secondly, in consequence of the proximity of solubility parameter of PEG to thiophenic compounds, affinity of PEG to thiophenic compounds is higher than that of PU. Therefore, the addition of PEG weakens the degree of the proximity between PU and thiophene in the point of solubility parameters, which leads to the reduction of the enrichment factor, so the flux increased. S 15


Energy & Fuels

5

9 PU PU+1%PEG PU+3%PEG PU+5%PEG PU+7%PEG

4

8

PU PU+1%PEG PU+3%PEG PU+5%PEG PU+7%PEG

7 3

6

E

J (Kg/（（.h） )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5

2 4 1

3 2

0

65

70

75

80

85

65

90

Temperature/℃

70

75

80

85

Temperature/℃

(a)

(b)

Fig 7. Effect of different PEG content on desulfurization performance of PEG / PU hybrid membrane, (a) effects of different PEG contents on permeation flux (J), (b) effects of different PEG contents on sulfur enrichment factors. 3.2.2. Effect of feed temperature Temperature is one of the most important factors affecting the performance of permeable vaporization. The effect of liquid temperature on the pervaporation performance of 5% PEG / PU hybrid membrane is shown in Fig.6. As mentioned in the experimental section, PV process liquid feed (FCC gasoline) contains sulfur content of 200 ppm, peristaltic pump speed 90 r / min, operating pressure 20 mmHg. Increasing the temperature of the feed increases the sulfur flux while reducing the enrichment factor As expected, it is further observed from Fig 8 that the permeation flux (J) increased with the increasing feed temperature. However, sulfur enrichment factor (E) decreased

S 16


Page 17 of 23

with the increasing temperature24. It is highly possible that the temperature not only affects the transport of compound, but also affected the membrane. Increasing temperature providing a greater impetus for the transport of the feed liquid. Furthermore, the free volume of the polymer matrix is enhanced by the movement of the elevated temperature25.

2.4 E J

6.10

2.2 6.05

1.8

5.95 5.90

1.6

J (Kg/(（.h))

2.0

6.00

E

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

5.85 1.4 5.80 65

70

75

80

85

1.2

温温 (℃)

Fig 8. The effect of feed temperature on performance of 5%PEG/PU membrane. 3.2.3. Effect of GC-MS the hydrocarbon composition and sulfur species distribution analysis of feed GC-MS analyzes the type of hydrocarbon composition of the FCC gasoline and the main hydrocarbon compounds are found to be olefins and naphthenes as seen in Table 2. However, when the content of olefins in the material is high, the permeation flux and S 17


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sulfur enrichment factor will be affected. From Table 3, principal sulfur species in FCC gasoline are thiophene species, which contributes to the most of total sulfur content. Moreover, the sulfide through the membrane side is predominantly dimethylthiophene from FCC gasoline, which corresponds to the solubility parameter discussion in above sections26. Table 2 desulphurization performance for the hydrocarbon composition and typical sulfur species in FCC gasoline.

Peak No

class

dt

1

Alkane

19.96

2

olefin

41.936

3

naphthenic hydrocarbon

33.417

4

Aromatic hydrocarbons,

3.819

5

thiophene

0.394

6

dimethylthiophene

0.108

7

trimethylthiophene

0.043

8

Oxygenated compound

0.324

Total

100

Table 3

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GC typical sulfur speciesanalysis of feed and permeate sample. class

Feed sample

Permeate sample

Enrichment factor

thiophene

50,061.853

290,358.971

5.82

dimethylthiophene

13,734.490

85,153.838

6.21

trimethylthiophene

5470.333

32.821.998

6.00

Unknowns

3893.286

3687.212

0.95

4. Conclusions Desulfurization performance of PEG/PU hybrid membrane hybird membrane were investigated by various experiments including the content of PEG and operation temperature. Our work has shown that changing the content of PEG in polymer polyurethane is important to obtain a membrane with excellent performances, which can increase the penetration flux without too much loss of sulfur enrichment factor. The results presented in this paper a significantly improved desulfurization performance in comparison to the PU membrane for FCC gasoline. Sulfur enrichment factor can reach 6.00 while the flux is greater than 2.20 kg / (m2.h). It was also observed that the highest pervaporation separation index (PSI) was obtained at 85 ° C on a 5% PEG / PU hybrid membrane among all fabrication and performance conditions. This work provides useful S 19


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information for further development of the separation of sulfur compounds using membrane technologies. Acknowledgements The authors gratefully acknowledge the financial support of this work by the Fundamental Research Funds for the Central Uni-versities (15CX05014A); State Key Laboratory of Separation Mem-branes and Membrane Processes (Tianjin Polytechnic University) (M2-201503). References

1. Song, C.; Ma, X., Ultra-Clean Diesel Fuels by Deep Desulfurization and Deep Dearomatization of Middle Distillates. Journal of Biomechanics 2006, 43, (3), 579-582. 2. Ali, M. F.; Al-Malki, A.; El-Ali, B.; Martinie, G.; Siddiqui, M. N., Deep Desulfurization of Gasoline and Diesel Fuels Using Non-Hydrogen Consuming Techniques. Fuel 2006, 85, (10), 1354-1363. 3. Konietzny, R.; Koschine, T.; Rätzke, K.; Staudt, C., POSS-hybrid membranes for the removal of sulfur aromatics by pervaporation. Separation & Purification Technology 2014, 123, (123), 175-182. 4. Yahaya, G. O.; Hamad, F.; Bahamdan, A.; Tammana, V. V. R.; Hamad, E. Z., Supported ionic liquid membrane and liquid–liquid extraction using membrane for removal of sulfur compounds from diesel/crude oil. Fuel Processing Technology 2013, 113, (9), 123-129. 5. Liu, K.; Fang, C. J.; Li, Z. Q.; Young, M., Separation of thiophene/n-heptane mixtures using PEBAX/PVDF-composited membranes via pervaporation. Journal of Membrane Science 2014, 451, (2), 24-31. 6. Sawhill, S. J.; Layman, K. A.; Wyk, D. R. V.; Engelhard, M. H.; Wang, C.; Bussell, M. E., Thiophene hydrodesulfurization over nickel phosphide S 20


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gasoline components in polyethylene glycol membranes. Journal of Membrane Science 2008, 325, (1), 438-445.

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Influence of Polyethylene Glycol on the Deep Desulfurization of

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