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Delayed coker coke characterization: correlation between process conditions, coke composition and morphology Naydu P. Zambrano, Liseth J. Duarte, Juan Carlos Poveda-Jaramillo, Hector J. Picón, Fernando Martinez O., and Martha Eugenia Niño-Gómez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02788 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Delayed coker coke characterization: correlation between process conditions, coke composition and morphology Naydu P. Zambranoa, Liseth J. Duarteb, Juan Carlos Poveda-Jaramillod, Hector J. Picónc, Fernando Martínez O.a and Martha Eugenia Niño-Gómeza* a

Centro de Investigaciones en Catálisis, Universidad Industrial de Santander, Sede Guatiguará

Km. 2 vía El Refugio, 681011 Piedecuesta, Santander, Colombia b

Laboratorio de Superficies – SurfLab, Universidad Industrial de Santander, Sede Guatiguará

Km. 2 vía El Refugio, 681011 Piedecuesta, Santander, Colombia c

ECOPETROL S.A - Instituto Colombiano del Petróleo, 681011 Piedecuesta, Santander,

Colombia d

Laboratorio de Resonancia Magnética Nuclear, Universidad Industrial de Santander, Sede

Guatiguará Km. 2 vía El Refugio, 681011 Piedecuesta, Santander, Colombia

ABSTRACT Delayed coking is the technology most used to upgrade vacuum residue into high value products, but in this process, secondary reactions produce coke. It is already known that the chemical and physical properties and composition of the feedstock and processing conditions affect coke morphology. Recently, a new type of morphology called transition coke has been described, but this morphology should be avoided because it induces operational and safety risks to delayed

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coking units. Several studies have attempted to understand the complex structure of this type of coke and the factors that control the different final morphologies. Therefore, the purpose of this study is to shed additional insight on the correlation between surface chemical composition and the morphology of delayed coker coke with variables of the process such as temperature, pressure and elemental composition of the feedstock. All the samples had a surface area between 1-4 m2/g showing that the coke lacks porosity. Morphological classification by Scanning Electron Microscopy (SEM) showed transition/associated shot type and sponge type coke. By Xray Photoelectron Spectroscopy (XPS), it was possible to identify heteroatoms as N, S, Si and O on the surface of the samples. Metals were not found on the surface of the solids, despite the fact that Atomic Absorption (AA) and Total Reflection X-ray Florescence (TXRF) showed elements such as Ni and V in the bulk. Solid State Nuclear Magnetic Resonance (ssNMR) was used to identify the aromatic and aliphatic regions and to calculate the aromaticity factor (fA) and the relationship of peri- to cata-carbons, Cperi/Ccata, in the aromatic structure of the samples, giving us an idea of the condensation degree, φ, of the aromatic moieties. The results of this work allow us to verify that low metal amount in the samples are related to sponge and transition coke. Thus, it is possible that this fact, in combination with the amounts of nitrogen and sulfur on the surface, could influence the formation of transition coke and that variables such as temperature direct a change in morphology from transition to sponge coke.

1. INTRODUCTION Delayed coking is the technology most used to upgrade vacuum residue into high value products, but in this process, secondary reactions produce coke1,2. The carbon content in the petroleum coke represents 80% of its weight3, the volatile matter in coke ranges from 8 to 18%, the S ranges from 0.2 to 4.2% and the ash runs from 0.05 to 1.6% or higher, depending on the ash in the crude oil used and whether lime or other inorganic substances are added during processing4. Although the resulting coke is mainly used as primary fuel or in a coal/coke blend, variations in its properties and morphology may have some value as electrodes for aluminum manufacturing, graphite products, wear-resistant carborundum coatings and so on. The classification of the types of coke according to use and morphology differ. The morphological classifications can be divided into shot, sponge, transition and needle type coke. The uses can be divided into fuel coke and anode precursor coke, which are the most known, having defined parameters based on the


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quantities of volatile matter, ash, sulfur, metals such as vanadium and nickel and their coefficient of thermal expansion (CTE)5,6. It is already known that chemical and physical properties and composition of the feedstock and processing conditions affect the coke morphology3,5,7,8. Very aromatic feedstocks with a low asphaltenes (4 at 700 cm-1 19,20. Other functional groups identified in the samples were pyrrol (stretching band N-H), water (stretching band N-H), carboxylic acids (stretching band O-H), carbonyl and carboxyl groups (v C=O) and ether (stretching band CO)21,22. Comparisons of spectra for the same raw sample (1-1-1-1), after the first pretreatment with toluene (1-1-1-2) and the final sample with n-heptane (1-1-1-3) showed a decrease in the intensity of the bands related to aliphatic and aromatic groups, because of the removal of asphaltenes and resins fractions, as shown in Figure 1. However, the final structure of the delayer


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coker coke had these types of bonds, and some bands were not completely removed. The intensity bands of nitrogen and sulfur groups were also depleted. Nitrogen groups are well known to be related to the presence of pyrrol and pyridine groups in coke. No major differences were observed in the spectra of samples for different vacuum bottoms3.

Figure 1. ATR-FTIR spectra of delaying coker coke sample 1-1-1-3 (a); comparison before and after two pre-treatments with solvents (b).

3.2 Elemental characterization: Table 2 shows the elemental analysis of delayed coker coke from five different vacuum bottoms at different temperatures and pressures. Under the same temperature and pressure conditions, the contents of N, C, H and O did not show a significant difference between the different vacuum bottoms. However, a comparison of the same sample


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under different physical conditions demonstrates how the original species react in diverse ways; while the sulfur content remained the same in the most samples with increasing the pressure or temperature, samples 3-1-2-3 and 3-2-1-3 showed a jump up in the content. It is widely known that sulfur content under 3% leads to the formation of sponge coke, while values between 3-6% define the fuel coke, because the sulfur content tends to cause a puffing during the graphitization heat treatment, resulting in lower density and lower strength, making it impossible to manufacture graphite electrodes with this kind of coke 6. In this case, a variation in the processing conditions can lead to a change in the final morphology, as the sample from vacuum bottom 3 is a potential candidate for a change from sponge to fuel coke, while the samples from vacuum bottom 5 are the only one that present a fuel coke characteristic even when the lowest temperature and pressure were used in the process. Morphological characterizations by SEM imaging give a clearer result and confirm this appreciation. There is no a clear tendency on how the changes in the physical conditions were reflected in the bulk of every sample for the other four elements; however, the effects on the surface could be lead to a new conclusion.


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Table 2. Elemental analysis of delayed coker coke from five different vacuum bottoms at different temperatures and pressure. Sample

H [wt%]

C [wt%]

N [wt%]

O [wt%]

S [wt%]

1-1-1-3

3.81

91.31

1.84

1.97

1.07

2-1-1-3

3.29

91.03

1.31

1.81

2.56

3-1-1-3

4.01

91.76

1.51

1.59

1.13

4-1-1-3

3.54

90.43

1.37

2.27

2.39

5-1-1-3

3.49

89.96

1.43

1.85

3.28

1-1-2-3

3.35

90.87

1.40

2.80

1.58

2-1-2-3

3.52

90.80

1.15

1.86

2.67

3-1-2-3

3.51

88.28

1.59

2.55

4.08

4-1-2-3

3.43

90.46

1.41

2.21

2.48

5-1-2-3

3.64

89.87

1.47

1.68

3.34

1-2-1-3

3.71

90.99

1.72

2.45

1.14

2-2-1-3

4.74

90.66

1.03

0.83

2.73

3-2-1-3

3.36

88.24

1.68

2.69

4.03

4-2-1-3

3.31

90.17

1.61

2.36

2.56

5-2-1-3

3.26

89.67

1.51

2.24

3.33

The aromaticity factor was calculated following the procedure developed by Jin Xiao et. al3, and the aliphatic (2800-3000 cm-1) to aromatic (700-915 cm-1) ratio, Hal/Har, was obtained from the IR spectra using an OMNIC 8.0 (Thermo Fisher Scientific, Waltham, MA) to determine the peak area values Ia and Iar that reflect Hal and Har, respectively. The total values of C and H in Table 1 were converted to the atomic percent to use them in the following equations:

=

(1)


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=

= 1 −

(2)

(3)

Table 3 shows the results of aromaticity for the fifteen samples after the two pretreatments with solvents. Samples 2-2-1-3, 3-1-2-3 and 5-1-2-3 presented the lowest values, even though in all samples, the high aromaticity factor and low degree of metamorphism3 were observed. Samples 5-2-1-3 and 2-1-1-3 had the highest aromaticity; however, their behavior was different; while increases in the temperature or pressure of the process caused the sample from vacuum bottom 2 to decrease its aromaticity, the opposite was observed for the samples from vacuum bottom 5, where sample 5-1-1-3 showed a similar aromaticity value to the sample at higher temperature but lost aromaticity as the pressure increased. Initial components in the vacuum bottoms (2 and 5) may differ since the samples did not show the same behavior against changes in the process conditions.


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Table 3. Aromaticity of fifteen delayed coker coke samples Sample

Ial

Iar

Hal/H

H/C

Hal/Cal

Fa

1-1-1-3

142.27

318.44

0.30

0.50

1.80

0.91

2-1-1-3

23.05

83.60

0.21

0.43

1.80

0.95

3-1-1-3

69.57

169.45

0.29

0.52

1.80

0.91

4-1-1-3

90.10

139.43

0.39

0,47

1.80

0.90

5-1-1-3

87.42

357.34

0.19

0.46

1.80

0.95

1-1-2-3

68.21

195.02

0.25

0.44

1.80

0.94

2-1-2-3

78.30

239.31

0.24

0.47

1.80

0.94

3-1-2-3

35.64

44.71

0.44

0.48

1.80

0.88

4-1-2-3

58.01

166.05

0.25

0.46

1.80

0.93

5-1-2-3

66.95

99.99

0.40

0.49

1.80

0.89

1-2-1-3

198.78

584.29

0.25

0.49

1.80

0.93

2-2-1-3

183.65

299.03

0.38

0.63

1.80

0.87

3-2-1-3

45.88

74.04

0.38

0.46

1.80

0.90

4-2-1-3

36.01

91.82

0.28

0.44

1.80

0.93

5-2-1-3

70.35

407.97

0.14

0.44

1.80

0.96

3.3 Total Reflection X-ray Fluorescence (TXRF): TXRF results identified trace metals and elements not identified by any other techniques. Table 4 (Figure S2) shows results for the 15 coke samples. All samples present quantities of Mo and Ar associated with the anode and gas injected in the equipment; therefore, they are not shown in the table. Ca, Ti, V, Mn, Fe, Ni, Cu and Zn metals were observed in all samples. Fe and Ni were detected in all samples despite the changes in operational conditions (T and P). Mn is not observed in most samples; however, samples 3-1-2-3 and 1-2-1-3 have a higher content of Mn when the process variables changed, and exceptions are shown in the samples coming from vacuum bottom 5 that have a large enough quantity of Mn to not be affected by the variations in temperature and pressure. Nickel,


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vanadium and iron are well known to be related with pyrrole and pyridinic structures after the decomposition of metallo-porphyrins from the vacuum bottom after the delayed coking6. Samples 3-1-1-3 and 4-1-2-3 contrasted the rest and contained Al and Ba, respectively; this is an indication of the different compositions of the vacuum bottom; however, these two metals were not characterized in the feedstock. It is important to notice the absence of some metals in the same sample when the process conditions were changed. This absence does not mean that these metals were completely eliminated but instead indicates that the quantity was depleted enough that it did not overcome the instrument detection limit.

Table 4. Trace metals observed on 15 samples of delayer coker coke bulk [ppm] by TXRF Sample

Al

Ca

Ti

V

Mn

Fe

Ni

Cu

Zn

Ba

1-1-1-3

0.0

0.0

15.4

0.0

0.0

48.9

56.9

0.0

3.6

0.0

2-1-1-3

0.0

0.0

0.0

0.0

0.0

22.7

14.8

23.5

0.0

0.0

3-1-1-3

80.0

3.5

3.0

33.6

0.0

20.1

105.3

8.6

6.2

0.0

4-1-1-3

0.0

12.8

10.3

22.6

0.0

55.8

14.9

17.1

22.6

0.0

5-1-1-3

0.0

0.0

7.1

115.0

2.2

55.6

35.4

0.0

0.9

0.0

1-1-2-3

0.0

0.0

36.0

0.0

0.0

45.8

23.1

0.0

0.0

0.0

2-1-2-3

0.0

7.8

7.5

131.8

0.0

73.8

47.2

0.0

0.0

0.0

3-1-2-3

0.0

0.0

0.0

143.0

2.3

69.4

37.8

5.0

11.1

0.0

4-1-2-3

0.0

4.4

0.0

50.5

0.0

18.5

25.4

2.6

8.3

15.0

5-1-2-3

0.0

5.8

21.1

101.0

2.0

44.4

29.9

0.0

3.7

0.0

1-2-1-3

0.0

5.8

21.8

34.8

3.2

128.4

84.3

0.0

5.3

0.0

2-2-1-3

0.0

0.0

14.2

122.9

0.0

0.0

40.0

0.0

0.0

0.0

3-2-1-3

0.0

5.4

3.8

184.0

0.0

90.0

52.4

8.5

3.2

0.0

4-2-1-3

0.0

8.6

5.2

73.7

0.0

43.4

36.4

1.4

3.4

0.0

5-2-1-3

0.0

0.0

21.6

144.0

4.4

123.6

66.0

0.0

9.3

0.0


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All metals found in the samples are under the established limits: 300-600 ppm for nickel and vanadium and lower than 250 ppm for iron, which leaves open the possibility for the use of this materials in the manufacturing of anodes 6.

3.4

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C solid state NMR Spectroscopy: Three samples of coke (1-1-1-3, 1-1-2-3 and 1-2-1-3)

were analyzed using ssNMR. The 13C NMR spectra were acquired using two different experiments. In the first experiment, CP-TOSS (Cross Polarization Total Sidebands Suppression) from proton to carbon was possible. In this experiment, the signals of carbons directly attached or spatially close to hydrogen (1H) increased the signal as a consequence of the energy transfer during the spin lock. The efficiency of this energy transfer depends of the proximity between the nucleus and the number of 1H nuclei near the 13C, and this type of interaction allows for an increase in the 13C signal. For this reason, in the CP-TOSS spectra of the coke samples, signals from protonated carbons ( and ) intensified regarding quaternary carbons ( , ,

). In the CP-TOSS 13C-NMR spectra, we can attribute different spectral regions to different carbons depending on the local chemical environment. We can identify the main regions as follows: aliphatic region from 5 to 60 ppm and aromatic region from 85 to 160 ppm. The second region has two sub regions, from 80 to 129.2 ppm where the resonances of protonated C and the pericondensed aromatic carbons C appear and from 129.2 to 160 ppm where the substituted C and pericondensed carbons C

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appear, as seen in Figure 2a.

Another type of experiment performed was zgig (Figure 2b), where polarization transfer was not possible because there is no spin-locking and decoupling in the proton channel is activate during the 13C FID acquisition time; thus, the 13C is not increased by CP, and the final spectrum is quantitative, allowing us to determine the relative content of different types of carbon atoms in the coke structure and calculate some of the structural parameters.


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Figure 2. 13C-Solid State MAS NMR spectra of coke sample 1-1-1-3. a) CP-TOSS spectrum and b) zgig spectra (proton decoupled one pulse).

From Figure 2a, we can observe that a small amount of aliphatic carbons could be detected using the polarization transfer technique, and this result is in agreement with the signals detected in the 1440-1370 cm-1 region in the infrared spectra, as shown in Figure 1. From the proton decoupled 13C one pulse NMR spectra, shown in Figure 2b, it was possible to calculate the aromatic factor ! as C ⁄C"# ; our results showed that less than three percent of the carbons correspond to saturated carbons with an ! higher than 0.97 (Table 5). Similar results were obtained for samples 1-1-2-3 and 1-2-1-3, as shown in Figure 3, 4 and Table 5.


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From the relative carbon content calculated from the spectra, it can be noted that soft delayed coking conditions such as 763.15 K and 10 psi do not produce as strong of coking (cokes with a is up to 8% in weight. A major degree of large amount of aromatic condensation), as the

coking was reached using 763.15 K and 20 psi as the operational conditions (Sample 1-1-2-3).

Table 5. Quantittive 13C-ssNMR data Sample

%

%

Fa

1-1-1-3

45.69

46.12

8.19

0.92

1-1-2-3

48.71

49.54

1.74

0.98

1-2-1-3

48.76

48.18

3.06

0.97


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A detailed analysis of the aromatic carbons in the CP-TOSS experiment reveals an increment in the assigned signal for protonated aromatic carbons and peri-condensate, and these results indicated that a major number of protonated aromatic carbons are present in sample 1-1-1-3 when compared to samples 1-1-2-3 and 1-2-1-3. More aromatic condensation is present in sample 1-2-1-3, accompanied by a minor number of aliphatic carbons bound to the aromatic nucleus and a consequence of the higher degree of cata/peri condensation as seen in Table 6. By % ' )/ ' % ' we can observe that a larger comparing the samples using the ratio ('

enhancement was observed in sample 1-1-2-3 because of the higher number of protonated aromatic carbons, which is consistent with the idea of a minor amount of aromatic nuclei.

Table 6. Relative enhancement by polarization transfer 13C{1H} of aromatic carbons in the CPTOSS and one pulse proton decoupled experiments )*+, %

%

1-1-1-3

32.58

1-1-2-3 1-2-1-3

Sample

Ratio CP-TOSS

One Pulse

55.41

1.70

1.01

31.20

46.78

1.50

1.02

36.01

52.03

1.44

0.99

Additional efforts are needed to understand the possible correlation between the vacuum residues that are used as starting material in the reactor and the final coke product.

3.5 Surface characterization: XPS results showed the presence of five elements on the surface of every sample: C, O, N, S and Si. Figure 5 displays the high-resolution spectra of cokes from the same vacuum bottom but carried out using different process variables in the delayed coking unit. Despite the changes in temperature and pressure, the same species were identified in the samples. High-resolution N 1s XPS spectra (394-404 eV) was used to measure two important signals related to the pyrrole (400.4 eV) and pyridine (398.7 eV) groups, and these groups were also observed in the ATR-FTIR spectra (see Figure 1). Five and six-membered nitrogen


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heterocyclic carbons in the coke are related to vacuum bottom porphyrins, which suggest that the temperature of the process caused a bond cleavage of the macrocyclic-structure of the porphyrins in its primary components; these functional groups could also be related to resin polymerization but only in minor quantities 3,15,24. In the high-resolution S 2p XPS spectra, two signals for S 2p3/2 (164.0 eV) and 2p½ (165.2 eV) with a 2:1 area relation were observed. According to its binding energy, this sulfur species correspond to thiophene3,15,25,26. There was no evidence of oxidized sulfur species (as sulfoxide) or any other compounds different from thiophene, in contrast with the results reported by Siskin et al.15. Increasing the temperature and pressure enhanced the amount of the S in the spectrum because thiophene is not easy to oxidize, and for these samples, all the sulfur content is related to this species 26. Finally, high-resolution Si 2p XPS spectra showed the presence of one or two different silicon species. Normally, silicon oxide (102.6 eV) is reported as the predominant species

3,5,15,27

.

Particularly, in some of the samples (1-1-1-3 and 1-2-1-3) another Si species was detected at 101.4 eV, which could be associated with the Si-C bond. However, its amount is very low, and its presence was not confirmed in the C1s spectra. Table S3 and Figure 6 summarize the XPS results for all 15 samples, including the calculated mass percentages. High-resolution C 1s and O 1s XPS spectra are shown in the supporting information (Figure S3).


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Figure 5. High-resolution XPS spectra of petroleum cokes from vacuum bottom one at different values of temperature and pressure: (a), (b) and (c) N 1s, S 2p and Si 2p of 1-1-1-3; (d), (e) and (f) N 1s, S 2p and Si 2p of 1-1-2-3; (g), (h) and (i) N 1s, S 2p and Si 2p of 1-2-1-3.

From vacuum bottom 2, samples 2-1-1-3 and 2-1-2-3 presented the highest quantities of sulfur on the surface (see Table S2), and similar quantities were evidenced in the elemental analysis (Table 1), which leads to the conclusion that the behavior of the sample in surface and bulk are similar in term of the sulfur content and that the sulfur species seen are not greatly affected by the changes in the operational conditions. On the other hand, oxygen behaves in an analogous way when the process is carried out under the same conditions; for samples with temperature 1


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and pressure 1, the oxygen content was similar despite the differences in the origins vacuum bottom. When the pressure or temperature increased, the surface oxygen was enhanced, leaving evidence in the oxidative nature of all samples (Figure 3); a deeper characterization of this phenomenon was made by TGA (Figure S4) in oxidizing atmosphere and can be seen in the supporting information, which is an advantage for the use of fuel coke. The nitrogen content had the clearest tendency among all the surface elements. Apart from sample 1-1-1-3, all remaining samples showed similar quantities as observed in the gray line of Figure 4, and an increase of temperature and pressure did not affect the nitrogen-related surface species. For silicon, a less obvious tendency is presented, and Si-O-Si or/and Si-C quantities tend to increase with increasing pressure despite the originating vacuum bottom. The carbon quantities tend to prevent what makes the enhancement of elements other than oxygen and silicon possible; a similar phenomenon was reported by Shamsi

28

by following the carbon on a

TPO analysis and showing how increasing the temperature removed the carbon on the surface.

Figure 6. Summary of XPS results from 15 delayed coker coke by elements C (left) and O, N, S, Si (right).


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3.6 Morphological characterization: Raw samples (without solvent pretreatment) were analyzed under a scanning electron microscope. The micrographs shown below correspond to magnifications from 200X to 2000X. A highly anisotropic intern mosaic was observed, with a very smooth surface that is characteristic of transition morphology formations. However, there was space for the initial formation of pores (Figure 7) at 600X, and a zoomed view of the region allows us to determine the length and size of the pore (≈50 µm) giving an indication of a mix of morphologies, which are shot and sponge. This type of mix is also known as shot-associated or transition coke 9,13,29.

Figure 7. Micrographs at different magnifications of sample 1-1-1-1. 200X (a). 600X (b) and 2000X (c).

The previous results (TXRF, Elemental Analysis), including the BET surface areas between 2-4 m2/g that showed lack of porosity in the samples (Figure S1), support the hypothesis of transition coke morphology formation. Figure 8 shows the edge corresponding to sample 1-1-1-1; the left side of the micrograph (I) shows a fine mosaic domain while the right side (II) is notorious for its smooth surface with the beginning of stretch marks. In the center of both regions (III) appears to be a pore formed in the center dividing the two diverse sides of the same sample, a typical morphological featured of a transition coke type (referred as shot-associated) as reported by Picón-Hernandez 13.


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Figure 8. SEM 200X micrograph of the edge of sample 1-1-1-1 showing the anisotropic morphology inside.

Coke sample 1-1-2-1 is presented in Figure 9a and shows a similar morphology as that described in sample 1-1-1-1, with a larger pore size and large diversity of shapes. When the process pressure is increased, the vaporization of low molecular weight hydrocarbons is inhibited. Thus, the small molecules are enriched in liquid to lower the viscosity of the matrix, which favors the movement and alignment of mesogens, and the condensation rate is lower, as reported in Guo et al7. Here, again, there were two distinct regions, a smooth surface (I) and uncompacted matter (granular texture) accumulated in the walls of the coke (II). Sample 1-2-1-1 (Figure 9b) shows a larger fine mosaic domain with an agglomeration of volatile matter on the walls of the small pores (I). Free radicals formed from the cleavage of bonds in the vacuum bottom molecules react drastically at high temperature and will not be stabilized by hydrogen transfer reactions, which leads to fast condensation and faster release of vapors; the resultant high velocity of the vapors helps to disorganize the system and inhibit the intermediate solidifying of the asphaltenic coke7. The same pathway is followed in the formation of shot type coke; however, the characteristics of the vacuum bottom, such as the temperature and pressure of the delayed coking, were not enough to complete the formation of the small spheres. Instead, a transition morphology was reached.


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Figure 9. SEM 200X micrograph of samples 1-1-2-1 (a) and 1-2-1-1 (b)

Micrographs of the samples 2-1-1-1, 2-1-2-1 and 2-2-1-1 are shown in Figure 10. Figure 10 (a) shows a micrograph of a sample that corresponds to the normal process conditions at minimum temperature and pressure, with a tendency to form pores but with a smooth surface resulting in a mix of characteristics from shot and sponge coke; this mix can be characterized as transition coke. Figure 10 (b) displays a sample formed when the pressure of the process is increased by 10 psi. Bigger pores are formed with the internal formation of small pores, the smooth region decreases while a region formed of uncompact matter was observed. Finally, Figure 10 (c) is a sample formed when the temperature has been increased by 20 °C. The pore size is greatly different from before, and the magnification used in all previous micrographs is focusing inside a big pore instead of an entire region. Again, the change in this process variable led to a change in the final morphology, changing the sample from transition to sponge coke type.


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Figure 10. Micrographs of samples from vacuum bottom 2 at different process conditions: a) 21-1-1, b) 2-1-2-1 and c) 2-2-1-1.

A bigger magnification of the sample 2-2-1-1 is shown in Figure 11. Region (I) represents the zone where smooth and granular surfaces coexist, and the pore size is bigger than the previous samples from vacuum bottom 1 and 2. In micrograph 10b, there are stretch marks forming on the surface, as non-compact matter accumulates. All the characteristics previously described classify this sample as a sponge type coke. A justification for this morphological change can be found on the elemental analysis of the sample 2-2-1-1. The %H and %O were higher for this sample than for the 14 remaining samples (Table 2), and at the same time, the Ca and Na content in vacuum bottom 2 were 30.87 ppm, the highest amount between the five initial feedstocks. There is a synergistic effect between the chemical composition of the coke, the chemical composition of the vacuum bottom and the increase in temperature that results in the production of sponge coke 7,10,13,29

.


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Figure 11. Micrographs of sample 2-2-1-1 at a) 200X and b) 600X

Figures S5 presents the micrographs of samples from vacuum bottom 3, where a similar morphology is observed for 3-1-1-1 and 3-1-2-3. However, when increasing the pressure, less pores and a smoother surface is seen. Figure S6 presents the results for vacuum bottom 4, where deeper pores are formed in the sample when the temperature has a value of 490 °C at two different pressure values. However, increasing the temperature to 510 °C caused the already formed pores to expand their domain, making the sample hollow, and all micrographs presented smooth surface regions. Finally, samples from vacuum bottom 5 (Figure S7) presented extended smooth surface regions with the formation of pores in the inter-domain area. As in the samples from vacuum bottom 4, increasing the temperature tends to enhance the size of the pores. These described characteristics are typical of the transition/shot-associated coke type13. Table 7 presents the morphological classification given to the fifteen delayed coker cokes according to SEM analysis. The results show that increasing the temperature and pressure of the process caused the number of pores to be enhanced while at the same time the smooth surface region decreased in sample 2. Sample 2-1-1-1 presented a higher amount of sulfur (2.64%wt) on the surface when the delayed coking was carried out in normal conditions and with a clear tendency to increase its content as the physical variables changed (Table 1 and Table S2). There is likely a close relation between the sulfur species and the final morphology.


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Table 7. Morphological classification of coke samples according to SEM observation Sample

Morphology

1-1-1-1

Transition

2-1-1-1

Transition

3-1-1-1

Transition

4-1-1-1

Transition

5-1-1-1

Transition

1-1-1-1

Transition

2-1-2-1

Transition

3-1-2-1

Transition

4-1-2-1

Transition

5-1-2-1

Transition

1-2-1-1

Transition

2-2-1-1

Sponge

3-2-1-1

Transition

4-2-1-1

Transition

5-2-1-1

Transition

4. CONCLUSION The current study aimed to present a correlation among the delaying coke process conditions, coke and feedstock composition and final morphology of delayed coker coke using several characterization techniques such as FT-IR, XPS, TXRF, ssNMR, Elemental Analysis and SEM. The main conclusions of this paper are condensed as follows: 1) Asphaltenes and resin fractions are removed by pretreatment using toluene and n-heptane respectively; however, these solvents also remove pyrrole compounds. A pretreatment is necessary to discriminate between the carbon from the sample and carbon one from the contaminants. 2) There were no vanadium and nickel species in the surface of the samples despite the presence of these metals in bulk; C, N, O, S and Si were the only elements found by XPS. Nitrogen was in the form of pyrrole and pyridine,


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silicon was in the form of silicon oxide and silicon carbide, and sulfur was surprisingly found in just one form, thiophene. 3) A majority of the samples showed a transition/shot associated type of morphology, which is in agreement with the results of the characterization, as follows: a low metal content, a high aromaticity, and a low surface area. However, one sample presented sponge morphology as a result of its surface sulfur content, which in combination with the increase in temperature led to a change in the initial morphology. 4) In general, increasing the process conditions, such as temperature and pressure, tends to decrease the amount of carbon in the surface of the samples and enhance the amount of oxygen and silicon. It also increases the number and irregularity of pore shapes and the amount of non-compact volatile matter. These coke samples presented a great potential for future use in graphite anodes and environmental remediation, which could be done by making modifications on the coke surface and by anchoring diverse functional groups or subsequent thermic treatments30–32.

AUTHOR INFORMATION Corresponding Author Email: [email protected]

ACKNOWLEDGMENT The authors gratefully acknowledge the support from the Instituto Colombiano del Petróleo (ICP) and Universidad Industrial de Santander project 9445. In addition, the authors are thankful to the Laboratorio de Análisis Instrumental, Laboratorio de Microscopía, Grupo de Investigación en Minerales, Biohidrometalurgia y Ambiente and Grupo de Investigación en Polímeros of the Universidad Industrial de Santander.


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For Table of Contents Only

Keywords: delayed coking, delayed coker coke, XPS, SEM


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Delayed coker coke characterization: correlation ... - ACS Publications

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