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Reactive adsorption desulfurization of FCC gasoline over a Ca-Doped Ni-ZnO/Al2O3-SiO2 adsorbent Feng Ju, Changjun Liu, Kai Li, Chun Meng, Shuai Gao, and Hao Ling Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01117 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 21, 2016

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Reactive adsorption desulfurization of FCC gasoline over a Ca-Doped Ni-ZnO/Al2O3-SiO2 adsorbent Feng Ju1 Changjun Liu1 Kai Li2

Chun Meng2 Shuai Gao2

Hao Ling2*

(1. Key Laboratory of Pressurized System and Safety, Ministry of Education 2. State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China)

Abstract: A Ca-doped Ni/ZnO-Al2O3-SiO2 adsorbent was prepared for reactive adsorption desulfurization (RADS) of fluidized catalytic cracked (FCC) gasoline. Various characterizations such as H2-Temperature Programmed Reduction (H2-TPR), the H2/O2 pulse titration (HOPT), NH3-Temperature Programmed desorption (NH3-TPD), N2 physisorption, powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive spectrometry (EDS) are used to evaluate the sorbent. NH3-TPD results showed that Ca loading could reduce the mild and strong acidities of the sorbent surface. HOPT results indicated that Ca doping could promote the dispersity and specific surface area of nickle component. The adsorbent doped with 1wt% Ca obtained a higher efficiency of desulfurization and regeneration performance than the undoped one. The breakthrough sulfur capacities of the fresh and the regenerated Ca-doped adsorbents could reach 54.07mg/g and 46.23mg/g, respectively, at a breakthrough sulfur level of 10 ppmw with a loss of 0.23 gasoline octane number. The introduction of Ca contributes to the reduction of surface acidity of adsorbent and also reduces the carbon deposition in the process of RADS, improving the desulfurization ability and the regeneration performance of the adsorbent.

ACS Paragon Plus Environment

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1. Introduction The deep desulfurization of fuel oil has attracted a great many investigations of new techniques due to tougher environmental legislations in many countries. The conventional hydrodesulfurization (HDS) process is the main process to remove sulfur from hydrocarbon fuels. However, it is difficult to remove aromatic heterocyclic sulfur compounds to the required levels such as 10 ppmw in gasoline

[1]

. HDS of

gasoline also results in the saturation of olefins, and leads to an inevitable octane loss of about 10 numbers[2,3]. One way to avoid this is to use adsorption desulfurization technology. Compared with HDS, RADS employs new reactive adsorbents and operates under moderate conditions, which could also save the consumption of hydrogen and reduce the drop of octane number [4,5]. The commercial S-Zorb process [6-10] developed by Conoco Philips Petroleum Co. is an effective RADS method for the production of ultralow sulfur gasoline and diesel at a low H2 pressure (0.7-2.1 MPa). In the RADS process, the active component Ni plays an important role in adsorbing the sulfur atoms. Sulfur atoms [11] of sulfur-containing molecules adsorb onto the adsorbents first, and then react with the adsorbents. The hydrocarbon portion of the molecule is released back into the product stream. Ni element, as the main active component in both HDS and RADS processes, has attracted more attention in recent studies. To enhance the efficiency of hydrodesulfurization and alkylation desulfurization, nickel based catalysts have been improved by many researches [12-19]. The mechanism and operating conditions of RADS over Ni/ZnO adsorbent is also an important issue

[20-24]

. Zhang et al.

[25]

reported the effect of ZnO particle size on the

adsorptive desulfurization performance for Ni/ZnO adsorbent. The desulfurization activity and sulfur capacity of Ni/ZnO adsorbent with small ZnO particle sizes are much higher than that with large sizes. To enhance the desulfurization and regeneration ability of the RADS sorbents, several studies focused on the doping technology. Zhang et al.

[26]

investigated the effect of Mn doping on the adsorptive

desulfurization performance of Ni/ZnO adsorbents. They found that sulfur removal efficiency by a


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Mn-doped NiO/ZnO adsorbent was higher than that by a NiO/ZnO adsorbent without Mn doping. Tang et al. [27] compared the performance of a Ni/MnOx adsorbent and a Ni/ZnO adsorbent. The breakthrough time of Ni/MnOx is 1.5 times longer than that of Ni/ZnO. The acidity of catalysts also affects the desulfurization and regeneration ability. Laurenti et al. [28] studied the effect of catalyst acidity. They found that the acidity of the support had a detrimental influence over hydrodesulfurization activity of Re sulfide. In reactive adsorption, carbon deposition affects the adsorptive capacity for removing sulfur compounds due to the fact that metal active sites will be blocked gradually. The speciﬁc bond formation reaction between sulfur atom and metal will be depressed. The acidity of the adsorbent impacts the carbon deposition on the surface of the adsorbent, which gives a negative effect of the desulfurization performance. Using alkaline metal could lead to the decline in acidity of the adsorbent, which would further alleviate the carbon deposition thus to improve the desulfurization performance of the RADS adsorbent. However, there are very limited reports investigating the alkaline doping effect on the RADS adsorbent so far. With the view of studying the effects of Ca doping, this work developed a series of adsorbents with different Ca loadings under different calcination temperatures. The regeneration performance and the reason for deactivation of the regenerated adsorbent were also discussed. 2. Experimental section 2.1. Adsorbent Preparation and Feedstock Properties All chemicals used for preparation of the adsorbent were of analytical grade. The feedstock employed in the work was supplied by Petrochina Jilin Petrochemical Company, and its properties are listed in Table 1. Table 1. Properties of the FCC gasoline °

Density(20 C) g/cm3

Sulfur content µg/g

Nitrogen content µg/g

0.723

243.48

28.22

The supporter ZnO-Al2O3-SiO2 of adsorbent was prepared by coprecipitation method, and then Ni and


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Ca components were loaded on the supporter by impregnation method, shown in Figure 1. A mixed aqueous solution of Al(NO3)3 and Zn(NO3)2 was dropwise added into a mixed solution of Na2CO3(0.2mol/L) and Na2SiO3(0.2mol/L) at a rate of 15mL/min at a precipitation temperature of 20°C, followed by aging at the same temperature for 2h. Then, the precipitation was filtrated out and washed with amount of deionized water to remove the residue sodium until the pH of the suspension is below 6.5[29-33]. After that, the filter cake was dried at 120°C in air for 12h, and then calcinated at different temperatures in a muffle furnace in dry air for 4h. In this way, the supporter ZnO-Al2O3-SiO2 was obtained. Solutions with different concentration of Ni (NO3)2 and Ca (NO3)2 were mixed with the above supporter and stirred for 2h. After that, a solution of Na2CO3 (0.2mol/L) was added dropwise into the mixed solution. The precipitation is the precursor of the sorbent. Amount of deionized water was used to wash away the residue sodium until the pH of the solution is below 6.5. The obtained filter cake was dried at 120°C in a vacuum oven for 12 h, and then calcinated at different temperatures in a muffle furnace for 2 h. Finally, the adsorbent was screened to 120 mesh and kept in a sealed bag before usage.


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Ca/Ni-ZnO/Al2O3-SiO2 Na2SiO3 and Na2CO3

Calcinating

solution Drying

Al(NO3)3 and Zn(NO3)2 solution

Precipitate Precipitate Na2CO3 solution Aging and Filtering Ni(NO3)2·6H2O and Ca(NO3)2·4H2O solution

Drying

ZnO/Al2O3-SiO2

Calcinating

Figure 1. Preparation process of the Ca-doped Ni/ZnO-Al2O3-SiO2 adsorbent 2.2. Desulfurization Experiments The desulfurization experiments were conducted in a continuous micro fixed-bed reactor [34]. A total of 3 grams of adsorbents in the oxidized form of adsorbents was loaded into the reactor per run. Before the reaction, the oxidized form sorbent needs to be reduced by hydrogen at a flow rate of 140mL/min under 2.0MPa and 440°C for 2h. Then the FCC gasoline was pumped into the reactor. The desulfurization products were collected periodically in a beaker for use of analyzing sulfur content. After desulfurization, an oxide gas containing 2% O2 diluted by 98% N2 was used to regenerate adsorbents and the tail gas was collected by a 0.1mol/L NaOH solution each hour. The experimental conditions are listed in Table 2.

Table 2. Experimental conditions of RADS process Temp. /°C Reduction

Hydrogen pressure/MPa Reduction time/h

Adsorption Desulfurization

Temp. /°C Hydrogen pressure/ MPa


440 2.0 2 419 2.9

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Weight hourly space velocity (MHSV)/h-1

10.84 0.3 90

Mole ratio (H2/oil) H2 volume/ Gasoline weight (mL/g) Temp. /°C Purge

H2 flow (mL/min)

360 200

Purge time /min

20

°

Temp. / C Regeneration time/min Regeneration

360 30

Total pressure/MPa Oxygen pressure/KPa Flow (mL/min)

480 60

0.15 3.0 200

The sulfur removal efficiency of adsorbent is defined according to the following equation:

Rs (%) =

C0 − Ct ×100 C0

(1)

where Rs is the sulfur removal efficiency of the sulfur compounds in the fuel (%), C0 is the sulfur content of the gasoline feedstock (µg/g), Ct is the sulfur concentration of the outlet product at any time (µg/g). The breakthrough sulfur capacity is determined as follows [35].

qbreakthrough =

v ⋅ C0 t Rdt 1000 ⋅ m ∫0

(2)

where qbreakthrough is the breakthrough sulfur capacity of the adsorbent(mg/g), v is the feed volumetric flow rate (mL/min), C0 is the initial sulfur content in the fuel (mg/L), m is the weight of the adsorbent (g). The breakthrough time is defined as the RADS time when the sulfur concentration of effluent desulfurized gasoline exceeded 10µg/g. Research octane number (RON) is calculated by following equation: RON=∑αiwi where αi is the effective octane number of i component, wi is the weight fraction of i component. Motor octane number (MON) is calculated by following equation: MON=10+0.8RON The octane number of FCC gasoline (AKI) is calculated by following equation:


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AKI=(RON+MON)/2 2.3. Characterization of adsorbents The sulfur concentrations of various samples are analyzed by an Antek 9000 total sulfur analyzer. The surface area, pore volume, average pore diameter, and pore size distribution of the adsorbents were determined by N2 physisorption at -196°C using a Micromeritics ASAP 2010 instrument. TGA-DSC of sorbent is analyzed by SDT-Q600 simultaneous thermal analyzer. The acid strength distributions of sorbents were analyzed by Micromeritics AutochemII 2920 instrument and the signal was collected by TCD detector. The crystalline structures of the adsorbents were characterized through X-ray diffraction (XRD) by using a Bruker D8 Advance X-ray diffractometer with a Cu Kα=0.154 nm monochromatized radiation source, operating at 40 kV and 100 mA. The data was recorded over a 2θ range of 10-80° in a step-scan mode of 0.02°/s. The elements distributions of the adsorbents were characterized by SEM/EDS with an EDXA energy dispersive spectrometer (EDS). The crystal lattice of the adsorbents was surveyed by JEM-2100 transmission electron microscope (TEM). 3. Results and discussion 3.1.1 XRD results A series of adsorbents with three different Ca weight loading (0.5wt%, 1wt% and 2wt%) were tested . The XRD patterns of 1wt% Ca-doped and undoped adsorbents are compared in Figure 2. The XRD pattern shows distinguishable characteristic diffraction peaks. These peaks are attributed to the crystalline phases of ZnO (2θ=28.6°, 47.6°, 56.5°, 61.6°) and NiO (2θ=43.3°, 62.3°, 75°). For patterns of undoped adsorbent, the characteristic diffraction peaks of ZnO (2θ=43.3°) and NiO (2θ=43.3°) are sharp and strong. However, the Ca doping sample shows that the height of peaks decreases and peak width broadens, which mean the degree of crystallinity of the active phases NiO and ZnO is decreased, and its dispersity is promoted. Small grain size of the active phase NiO is advantageous to the desulfurization reaction, and small ZnO grain size


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is beneficial to hence the breakthrough sulfur capacit. From the XRD patterns, almost all the intensity of peaks in undoped adsorbent is stronger than those of Ca-doped adsorbent. It can be assumed that since Ca makes the active phases Ni and Zn form Ni-Ca-Zn phase to reduce the crystal degree of phase NiO and ZnO, thus Ca element plays a positive role in the reduction of grain size and improves the dispersion of active phases. ●-ZnO □-NiO

●

Ca-doped undoped

□ ● ● Intensity(a.u.)

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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● ●

●

● □

10

20

30

40

50

60

70

80

2-Theta (degree)

Figure 2. XRD patterns of Ca-doped adsorbent and undoped adsorbent 3.1.2. NH3-TPD results Figure 3 shows the different profiles of NH3-TPD of Ca doped and undoped adsorbents. The undoped adsorbent has three broad peaks located at 101°C, 442°C and 651°C, corresponding to weak acidity (50~250°C), mild acidity (250~500°C) and strong acidity (500~700°C), respectively. Modification of the doped sorbent with 1.0 wt% Ca results in a considerable decrease in the amount of NH3 desorbed, indicating a decrease in the mild acidity and the strong acidity. The decrease of the mild acidity and strong acidity on the sorbent surface could reduce the carbon deposition in the RADS process, contributing to desulfurization ability. The alkaline Ca loading reduces the number of mild acidity and strong acidity


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centers.

Ca-undoped Ca-doped

TCD Signal( a.u.)

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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100

200

300

400

500

600

700

800

Temperature (℃ )

Figure 3. NH3-TPD profiles of Ca doped and undoped adsorbents 3.1.3. Dispersity of Ni component To identify the impact of Ca loadings on Ni dispersity, the hydrogen and oxygen pulse titration method is used. Table 3 shows the bulk metal dispersity (D), specific surface area (SANi) and metal crystal size (MCS) of Ni component in undoped and Ca doped sorbents. It can be seen that Ni component in adsorbent is difficult to be reduced after hydrogen and oxygen pulse titration, and the bulk metal dispersity is only about 0.05%. The introduction of Ca, to some extent, could raise the bulk metal dispersity. After Ca doping, the bulk metal dispersity increases from 0.0236% to 0.0522%. The higher dispersity of Ni component provides more active centers to react with sulfur compounds, promoting the efficiency. The specific surface area of Ca doped sorbent is twice of undoped sorbent, and the metal crystal size is decreased by 52%. It indicates that Ni component distributes more evenly, while the Ca doped sorbents have more active sites. The Ca loading prevents the aggregation of Ni component and decreases the thickness of Ni layer, which promotes the transformation of NiS and the formation of ZnS. Also, it can be seen that Ca component influences the distribution of other components during the calcination, thus, the effect of calcination temperature needs to


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be researched. Table 3. Ni dispersion of adsorbents Sample Adsorbent (undoped) Adsorbent (Ca doped)

D (%)

SANi (m2/g)

MCS (nm)

0.0236 0.0522

0.1568 0.3309

3582.4852 1697.5938

3.1.4. TPR results The reduction ability of adsorbent directly impacts the desulfurization performance, so reduction abilities of adsorbents with different Ca loading are compared by TPR method, as showed in Figure 4 and Table 4. In Figure 4, each of three adsorbents show only one hydrogen consumption peak. Three adsorbents perform the characteristic peaks located at 628°C, 598°C and 635°C, and peak areas are 46.5, 38.5 and 42.7, respectively. Among them, the temperature and area of characteristic peak of adsorbent with 1wt% Ca loading are the lowest and smallest. It indicates that the 1wt% Ca adsorbent could be more easily reduced at a lower temperature by hydrogen than other adsorbents. Less peak area means less cost of hydrogen for reduction. Indirectly, it illustrates that the active component Ni in adsorbent with 1wt% Ca loading disperses well. Also, it can be predicted that the 1wt% Ca adsorbent would have a good desulfurization performance.

0.5% CaCO3 1% CaCO3 2% CaCO3

TCD signal (a.u.)

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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0

100

200

300

400

500

600

700

800

Temperature (℃)

Figure 4. TPR of Ni/ZnO-Al2O3-SiO2 adsorbents with different Ca loading


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Table 4. TPR results of adsorbents with different Ca loading Sample

characteristic peak temperature

characteristic peak area

0.5wt%Ca adsorbent 1wt%Ca adsorbent 2wt%Ca adsorbent

628 °C 598 °C 635 °C

46.5 38.5 42.7

3.1.5. TPO results Temperature-programmed oxidation (TPO) was conducted for the spent and regenerated adsorbents, shown in Figure 5. Normally, coke deposits could be classified as reactive soft coke and refractory coke. Reactive soft coke could be removed under regeneration conditions, while refractory coke is adsorbed at the catalyst support and difficult to be removed in the process of regeneration. Figure 5(a) shows TPO results of the spent adsorbents. It could be found that two large peaks locate around 550 °C and 700 °C. The peak at 550 °C associated with the reactive soft coke. This result agrees well with the TPO results reported by Dasgupta[36] and Thompson[37]. The intensity of the peak of the Ca-doped adsorbent is not as strong as that of the undoped adsorbent, which means that Ca doping alleviates the soft coke deposits happened in RADS. Another peak at 700 °C related to the refractory coke. As the regeneration condition cannot reach such high temperature, the refractory coke will stay on the adsorbent surface and leads to a drop of adsorbent activity. The TPO results of regenerated adsorbents also showed that the refractory coke cannot be removed by regeneration temperature, shown in Figure 5(b). However, peaks of reactive soft coke are almost disappeared in that the soft coke is removed by oxidative regeneration. It can also found that the peak intensity at 700 °C of the regenerated Ca doping adsorbent is not as strong as that of the regenerated undoped adsorbent. This indicates that the Ca doping method could alleviate the forming of the refractory coke on the surface of the adsorbent.


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TCD Signal(a.u.)

undoped adsorbent after RADS Ca-doped adsorbent after RADS

100

200

300

400

500

600

700

Temperature(℃)

(a)

undoped adsorbent after regeneration Ca-doped adsorbent after regeneration

TCD Signal(a.u.)

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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100

200

300

400

500

600

700

Temperature(℃)

(b) Figure 5. TPO results of used and regenerated adsorbents (a. spent adsorbents b. adsorbents after regeneration) 3.1.6. Effect of Ca loading on desulfurization Figure 6 shows the different desulfurization and regeneration performance between the 2wt% Ca-doped sorbent and undoped sorbent. It can be seen that the desulfurization capacity of Ca-doped sorbent is much stronger than that of the undoped sorbent. This indicates that the Ca component promotes the activity of desulfurization of the sorbent and prolongs the service life. The regeneration ability is also enhanced by the Ca doping. The Ca doping increases the dispersion of active components and improves the ability of


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resistance to sintering since the main reason of the deactivation is the carbon deposit on the surface of the sorbent[34].

100 95 Sulfur removal(%)

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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90 85 80 fresh Ca-doped sorbent regenerated Ca-doped sorbent fresh undoped sorbent regenerated undoped sorbent

75 70 0

5

10

15

20

25

30

35

40

45

Volume of effluent gasoline/Weight of adsorbent(mL-G/g-A)

Figure 6. Effect of CaCO3 on desulfurization and regeneration performance of the Ni-ZnO/Al2O3-SiO2 adsorbents 3.1.6. Effect of Ca weight The desulfurization and regeneration performance of different Ca weight sorbents are shown in Figure 7. In the first three cycles, all of the sorbents have a good performance, while, in the fourth cycle, the sorbent with 2wt% CaCO3 cannot keep its sulfur removal efficiency upon the breakthrough curve, showing a worse desulfurization capacity than the others. In the fifth cycle, the sorbent with 0.5wt% CaCO3 shows unstability of desulfurization and has a trend of breakthrough. The sorbent with 1wt% CaCO3 keeps high efficiency of sulfur removal from cycle 1 to cycle 5. It may result from the fact that a moderate amount of CaCO3 could enhance the anti-sintering ability of sorbent by preventing the interaction between active component and supporter, which is beneficial to the regeneration. A low (lower than 1wt%) Ca loading cannot provide enough anti-sintering components for the adsorbent. In comparison, a high (higher than 1wt%) Ca loading leads to the drop of active component and the supporter, also, changes the acidity


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strength on the sorbent surface, resulting in increasing carbon deposit and reducing service life of sorbent. From the above analysis, the proper CaCO3 loading is 1wt%. 100

0.5%CaCO3

95 90 85 80 75

95 90

1%CaCO3

Sulfur removal (%)

70 100

85 80 75

70 100 95

2%CaCO3

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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Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5

90 85 80 75 70

0

5

10

15

20

25

30

35

40

45

50

55

Volume of effluent gasoline/Weight of adsorbent(mL-D/g-A)

Figure 7. Effect of the loading of CaCO3 on desulfurization and regeneration performance of the Ni-ZnO/Al2O3-SiO2 adsorbent 3.2. Effect of calcination temperature 3.2.1. Stability of adsorbents precursors

The thermo gravimetric analysis (TGA) combined with differential scanning calorimetry (DSC) curves of support precursor and adsorbent precursor are shown in Figure 8. It can be seen that after 400°C the curves become stable, which means carbonate and subcarbonate have already been decomposed while the supporter and adsorbent precursors are being prepared. On one hand, under high calcination temperature, it would result in a certain degree of sintering of adsorbent. On the other, an appropriate degree of sintering leads to reduce the specific surface area. It can also improve the strength and structure stability of the adsorbents. Therefore, to enhance the desulfurization performance and regeneration ability of the adsorbents, it is essential to optimize the calcination temperature through experiments.


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100 Weight (%) Heat Flow (W/g) Deriv. Weight (%/°C)

98 96 94 92 100

Support precursor Weight (%) Heat Flow (W/g) Deriv. Weight (%/°C)

95 90 85 80 75 70

Adsorbent precursor

100

200

300

400

500

600

700

800

Temperature (°C)

Figure 8. TGA-DSC profiles of support and adsorbent precursors 3.2.2. Effect of calcination temperature on desulfurization and regeneration Based on the analysis of TGA and DCS, the desulfurization performance of adsorbents under calcination temperature of 400°C, 500°C and 600°C are investigated. Figure 9 illustrates the desulfurization and regeneration performance under different calcination temperatures. From the Figure 8, the fresh adsorbents under 400°C and 500°C calcination temperature show higher desulfurization activity than the adsorbents calcinated at 600°C. The adsorbents under 500°C calcination temperature have the best regeneration ability, whose breakthrough sulfur capacity is above 47mg/g. After regeneration, the desulfurization activity of the adsorbents calcinated at 400°C reduced significantly, which indicates that the structure stability is too poor to keep the desulfurization capacity. It can be explained that under high regeneration temperature the crystal structure has been changed, leading to decreased desulfurization ability. The fresh and refreshed adsorbents under 600°C calcination temperature perform worse desulfurization ability than other two adsorbents. From Figure 8, the TGA curves show that ranging from 500°C to 800°C, the adsorbent keeps adsorbing heat, which may lead to a serious sintering of the grain, making the grain size become bigger and the active component on the support surface disperses nonuniform. The desulfurization ability reduces


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significantly. 100 95 90

Sulfur Removal(%)

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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85 Tc=400°C,Fresh

80

Tc=400°C,Sulfurized

75

Tc=500°C,Fresh Tc=500°C,Sulfurized

70

Tc=600°C,Fresh Tc=600°C,Sulfurized

65 60 0

30

60

90

120

150

180

210

240

270

300

330


Figure 9. Effect of calcination temperature on desulfurization and regeneration performance of adsorbents

Table 5 shows the sulfur breakthrough capacity of adsorbents prepared under different calcination temperatures. It can be concluded that the fresh and refreshed adsorbents under 500°C perform better desulfurization capacity and regeneration ability than the adsorbents under 600°C. The breakthrough sulfur capacity of refreshed adsorbent is much higher than the adsorbents under 400°C. Thereby, it is safe to say that the proper calcination temperature is 500°C based on the data acquired from experiments.

Table 5. Sulfur breakthrough capacity of adsorbents prepared under different calcination temperature Calcination temperature 400°C

Breakthrough Volume of gasoline/Weight of adsorbent(mL/g)

Breakthrough sulfur capacity (mg/g)

Fresh adsorbent

Regenerated adsorbent

Fresh adsorbent

Regenerated adsorbent

330

186

57.72

32.3

°

300

270

57.12

47.33

°

220

170

38.49

29.46

500 C 600 C

3.2.3. Effect of calcination temperature on acid strength distribution According to the theory of carbonium ion, the formation of carbon deposition is associated with the acidity of the sorbents. In the acid center of the sorbent, hydrogen transfer reaction will happen to the


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hydrocarbon, including the reaction among olefin molecules, naphthene and aromatic hydrocarbon. It results in that some olefin reacts into alkanes and the hydrocarbon which provides hydrogen atoms may condense to coke. The acidity of the sorbents plays an important role in desulfurization performance. In the high temperature RADS, the strong acidity of the adsorbent is not expected because it could enhance the degree of carbon deposition, which will reduce the service life of the adsorbents. The strength and distribution of the acidity of the adsorbents can be changed by the calcination temperature. Figure 10 is the NH3-TPD photos of the fresh sorbents under different calcination temperatures.

Tc=600°C Tc=500°C Tc=400°C

TCD Signal(a.u.)

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

100

200

300

400

500

600

700

800

Temperature (°C)

Figure 10. NH3-TPD profiles of fresh adsorbents prepared under different calcination temperature Figure 10 shows that the acid strength distribution changes slightly with the calcination temperature increasing. The area of mild acidity reduces regularly and the area of strong acidity increases with the calcination temperature increasing. It indicates that with the strong acidity rising, there could be an increase in deactivation by coking formation. The adsorbents under 500°C calcination temperature show a moderate strength of mild acidity and strong acidity. However, the desulfurization ability of sorbents not only depends on acid strength, but also affected by specific surface and pore size. 3.2.4. BET results


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According to the principle of the adsorbents, the adsorption capacity depends on its specific surface area and pore size. The bigger specific surface is, the better adsorption ability could be. The bigger pore size is, the higher efficiency factor of internal diffusion could be, which contributes to the mass transfer. The calcination process can directly affect the specific surface area and pore size of the sorbents. Figure 11 is the N2 adsorption isotherms of fresh adsorbents prepared under different calcination temperatures. The adsorption amounts of N2 on the adsorbent under 400°C and 500°C calcination temperature are much higher than that of the adsorbent under 600°C. It displays that under 600°C, both of specific surface area and pore volumes descend, attributing to the sintering of the adsorbent grain.

Tc=400°C

200

Quantity Adsorbed(cm3/g,STP)

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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Tc=500°C Tc=600°C

150

100

50

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure(p/p0)

Figure 11. N2 adsorption isotherms of fresh adsorbents prepared under different calcination temperature


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0.016 0.014

Tc=400°C Tc=500°C

0.012

Pore volume(cm3/g,STP)

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

Tc=600°C

0.010 0.008 0.006 0.004 0.002 0.000 2

4

6

8

10

12

14

Pore Diameter (nm)

Figure 12. Pore size distributions of fresh adsorbents prepared under different temperatures Figure 12 shows the pore size distributions of fresh adsorbents prepared under different temperatures. Table 6 shows the specific surface areas, pore volumes and pore diameters of the adsorbents under different temperatures. It can be seen that under 400°C and 500°C, the pore size distributions are almost the same, while, under 600°C, the pore size distribution shifts toward large sizes, with a reduction in pore volumes at a diameter range of 2.5 to 5.5 nm. These large pores are formed because of the sintering of relatively small pores under high reaction temperatures, and the reduction of small pores is mainly due to the carbon blockage and site coverage besides the sintering effect [38]. Table 6. Textural properties of adsorbents prepared under different calcination temperature Sample

SBET(m2/g)

Vtotal(cm3/g)

DA(nm)

Tc=400°C

147.32

0.31

5.42

Tc=500°C

149.01

0.33

5.90

Tc=600°C

118.93

0.32

7.42

From Table 6, under 400°C and 500°C calcination temperature, the microstructure parameters remain nearly unchanged. However, under 600°C, the specific area is reduced by 20% and the pore diameters are promoted by 26%, which indicates that a high degree of sintering of the grain occurs.


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3.3. Desulfurization and regeneration performance Considering the effect of Ca loadings and calcination temperature, the optimized sorbents are prepared. In order to simulate industrial continuous desulfurization process and investigate the regeneration performance, the recycling desulfurization experiments have been operated. The experimental conditions are listed in Table 2. The multi-cycle desulfurization and regeneration performance are shown in Figure 13. In the five cycles, the sulfur content in the product fuel has always been below 10ppm and the efficiency of sulfur removal keeps upon 96%. The desulfurization activity is quite stable. It means that the sorbents perform a high efficiency on desulfurization and regeneration. The breakthrough sulfur capacities of fresh and regenerated adsorbents are 54.07mg/g and 46.23mg/g. 100

95

Sulfur Removal(%)

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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Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5

90

85

80 0

50

100

150

200

250

300

350

400

450


Figure 13. Multi-cycle desulfurization and regeneration curves 3.4. PONA analysis and octane number Table 7 shows the paraffins, olefins, naphthenes and aromatics (PONA) analysis of the FCC gasoline before and after desulfurization process. From Table 7, olefins content reduces 8.42% and alkane content rises 8.03%. It indicates that hydrogenation reaction occurs. FCC gasoline from different crude oil contains different constituents. The side effect of the reaction should be restrained as much as possible in order not to affect the desulfurization reaction and not to increase the cost of hydrogen.


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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

Table 7. PONA analysis of FCC gasoline before and after desulfurization PONA sample FCC gasoline Desulfurization product change

paraffins wt%

isoparaffin wt%

olefins wt%

naphthenes wt%

aromatics wt%

total wt%

4.53

23.60

55.35

7.67

8.70

99.85

7.40

28.76

46.93

7.16

9.37

99.62

2.87↑

5.16↑

8.42↓

0.51↓

0.67↑

-

The change of octane number after desulfurization is displayed in Table 8. The octane number is calculated based on the equation in section 2.2. RON is calculated by the result of PONA analysis. After desulfurization, the AKI number is only decreased by 0.23. Compared with HDS process, the cost of hydrogen and the loss of octane number are negligible. The Ca-doped Ni/ZnO-Al2O3-SiO2 adsorbent performs an effective desulfurization ability and a high economical efficiency. Table 8. Change of octane number after desulfurization

FCC gasoline Desulfurization product

MON

RON

AKI

82.92

91.21

87.07

82.71

90.97

86.84

3.5 EDS results The EDS spectra of the fresh, used and regenerated adsorbents are depicted in Figure 14. The element distribution of the fresh, used and regenerated adsorbent are almost the same, primarily containing nickel, zinc, aluminum, silicon and oxygen. This is in accordance with the expectation of adsorbent compositions which have been calculated before preparation. After desulfurization and regeneration, no loss of the main component is found. The peak of Ca loading is not displayed in the spectra due to the fact that the compositions of the sorbent disperse nonuniform and the Ca2+ to Ni2+ ratio is too low, which results in that Ca component is covered by Ni and cannot be detected. Compared with fresh sorbent, there appears a characteristic peak of C element in the used sorbent spectra. The content of C element reaches a high degree of 12.87wt%. Under high reaction temperature, the carbon


Energy & Fuels

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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deposition is unavoidable because of the polymerization reaction in hydrocarbon compounds on the surface of sorbent. The peak of S element is also detected. The sulfur in the FCC gasoline is partly retained by the active component of adsorbent and then forms ZnS. The carbon deposit and the formation of ZnS are main factors for the deactivation of the RADS absorbents [34]. It blocks the sorbent pores and hinders the reaction between the active components and sulfur compounds. From the spectra of regenerated sorbents, the peak of carbon disappears. During the regeneration process, the carbon is burned into COx under oxygen atmosphere. However, the content of sulfur decreased to 1.52wt%, which means the sulfide is not burned out completely. This is the main reason that adsorbents cannot be regenerated entirely. Zn

EDS of fresh adsorbent

fresh

Element O Al

Ni O Si Al Ni

Atomic % 40.32

4.91

7.14

Si

4.78

6.67

Ni

22.73

15.19

Zn

51.13

30.68

Total

100

100

Zn

EDS of spent adsorbent

Zn

spent Ni O

Si Al

Ni

S

C

Zn

Zn

Element C

Weight % 12.87

Atomic % 34.28

O

11.11

22.21

Al

3.26

Si

3.04

3.47

S Ni

1.99 19.38

1.98 10.56

Zn

48.34

23.65

Total

100

100

3.86

EDS of regenerated adsorbent

regenerated

Ni O Si Al

0

Weight % 16.45

Ni

S

200

400

600

Zn

800

Element O Al Si S Ni Zn Total

Weight % 13.01 3.86 3.54 1.52 27.26 50.8 100

1000

Atomic % 34.3 6.03 5.32 2.01 19.58 32.77 100

1200

Figure 14. EDS spectra of the 1%CaCO3 doped Ni-ZnO/Al2O3-SiO2 adsorbents 3.6 XRD results Figure 15 shows the XRD patterns of the Ca-doped Ni-ZnO/Al2O3-SiO2 adsorbents. For the fresh adsorbent, most peaks diffuse well. Several peaks of ZnO (located at 2θ=31.8°, 36.3°, 56.6°, 62.3°, 68.3°) are relatively strong. But the peak of Ca element has not been detected, which is in accordance of EDS results. It means active component Ni and supporter ZnO, SiO2 and Al2O3 have a good dispersity. In the used XRD


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curves, the peaks of carbon element (located at 2θ=28.6°, 44.7°, 51.3°, 75.3°) are detected. And the crystalline phases of ZnS (2θ=47.8°) are formed. It indicates that the carbon deposit and Zn sulfide are formed on the sorbent surface. After the regeneration, the peaks of carbon disappear, while the peaks of ZnS remains, which means ZnS is not completely combusted. The results are also in accordance of EDS results. In the regenerated adsorbent, under high reaction temperature, the crystalline phases of Ca2SiO4 are formed, but no calcium compound is shown in the fresh sorbent XRD results. On one hand, calcium elements diffuse uniformly in the fresh adsorbent. On the other hand, the formation of calcium compound states that slight sintering occurred in the internal structure after regeneration. The appearance of more ZnO and NiO peaks after regeneration also support the slight sintering.

▼

▼

C ZnS

NiO

ZnO

Ca 2SiO 4

Intensity (a.u.)

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

regenerated

spent

fresh 10

20

30

40

50

60

70

80

2-Theta (degree)

Figure 15. XRD patterns of the 1%CaCO3/Ni-ZnO/Al2O3-SiO2 adsorbents 3.7 TEM results Figure 16 illustrates the TEM photos of fresh adsorbent and spent adsorbent. For the fresh sorbent, the spots in diameters close to 5 nm are nickel components, which are evenly dispersed on the supporter surface. From the figure 16b, after desulfurization process, the diameters of the black spots increase to 10nm, which indicates that the nickle components are agglomerated to some extent. In figure 16c and 16d, there are some regular stripes, while, no stripe is found in TEM photos of fresh sorbent. These stripes may


Energy & Fuels

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

be formed by the crystalline phases of SiO2, ZnO and NiO. Compared with fresh sorbent, the phases have been crystallized again. No regular stripe of graphite carbon is found. The sorbent shows good sulfur compounds adsorption and regeneration performances.

Figure 16. TEM images of adsorbent before and after desulfurization (a) fresh sorbent×100000 (b) spent sorbent×50000 (c) spent sorbent×500000 (d) spent sorbent×500000

4. Conclusions The Ca-doped Ni-ZnO/Al2O3-SiO2 adsorbent is an effective adsorbent for RADS of FCC gasoline. Ca loading decreases the mild and strong acidities of the sorbent surface, promotes the dispersity of active component Ni and enhances desulfurization and regeneration ability. The calcination temperature affects the acid strength and pore size distributions and regeneration. After the optimization of Ca loading and calcination temperature, the sorbent calcinated at 500°C with 1wt% Ca loading has a good breakthrough sulfur capacity of 54.07mg/g. After regeneration, the breakthrough sulfur capacity drops to 46.23mg/g. The reason of deactivation of the regenerated adsorbent is also discussed. ZnS cannot be combusted completely,


Page 24 of 28

Page 25 of 28

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

which leads to the decline of sulfur capacity of regenerated adsorbent. The formation of crystalline phase CaSiO4 results in the slight sintering. The sulfur capacity of regenerated adsorbent cannot reach that of fresh adsorbent. In the long-term desulfurization test, the Ca-doped Ni-ZnO/Al2O3-SiO2 adsorbent shows an excellent stability, and the octane number of FCC gasoline only decreases by 0.23.

Acknowledgements

The support from the Fundamental Research Funds for the Central Universities of China is gratefully acknowledged.


Energy & Fuels

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