Carbonization of petroleum feedstocks I: Relationships between chemical constitution of the feedstocks and mesophase development

Carbon Vol. 27. No. 6. pp. 877-887. 1989 KIOR-62231119 $3.110 + .OtI Printed in Great Britam. C 1989 Pergamon Press plc

CARBONIZATION OF PETROLEUM FEEDSTOCKS I: RELATIONSHIPS BETWEEN CHEMICAL

CONSTITUTION OF THE FEEDSTOCKS AND MESOPHASE DEVELOPMENT

SEMIH ESER and ROBERT G. JENKINS~

Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University. University Park, PA 16802, U.S.A.

(Received 16 September 1988; accepted in revised form 28 February 1989)

Abstract-The relationships between the chemical constitution of the petroleum feedstocks and the mesophase development were investigated. A range of coker feedstocks that were characterized by solvent fractionation, elemental analysis, and FT-IR were carbonized in conventional tubing bomb reactors. The solid carbonization products were examined by polarized-light microscopy. In general, a direct relationship was observed between the degree of aromaticity of the feedstocks and the extent of mesophase development. The separate carbonization of the asphaltene and maltene fractions of the feedstocks indicated that the asphaltenes dominated in determining the optical textures of the semi- cokes produced from the petroleum feedstocks.

Key Words--Carbonization, mesophase, delayed coking, petroleum feedstocks.

1. INTRODUCTION

In view of the new applications of petroleum-derived

carbons[l], a firm control over the structure and properties of solid carbons has become increasingly important. Delayedcoking[2], which employs liquid- phase carbonization of petroleum heavy residua, is the most widely used industrial process for the production of petroleum cokes. To a great extent, the properties of petroleum cokes are determined by their crystalline structure, which principally depends on the nature of the carbonaceous mesophase formed during the liquid-phase carbonization of the precursors[3]. Extensive work has been done, in the recent past, to investigate the mesophase development during carbonization of single organic compounds and their mixtures[4-81. A great deal of attention has also been given to the characterization and carbonization of petroleum feedstocks(5,9-111. However, to a large extent, these studies were con- ducted separately and independently by employing a variety of feedstocks, characterization methods, and carbonization conditions. The resultant diversity in the reported results precludes their integration for correlating the characteristics of the coker feedstocks and the mesophase development in relation to the chemistry of carbonization.

This study, as the first part of a systematic research on carbonization of selected petroleum feedstocks[l2], was aimed at investigating the relationships between the chemical nature of the feedstocks and the mesophase development. The first two pa-

tcurrent address: College of Engineering, University of Cincinnati, Cincinnati, OH 45244, U.S.A.

pers investigate the relationships between the mesophase development and the chemical constitution of the whole petroleum feedstocks and their asphaltene fractions. The studies on the chemistry and kinetics of carbonization in relation to the mesophase development will be presented in the third paper.

2. EXPERIMENTAL

To cover a range of feedstocks used in delayed cokers, seven petroleum residua produced by various processes were selected as samples. They in- clude three vacuum distillation residua (VDR) of different origin, one solvent-separated asphalt (%A), two catalytic cracker bottoms (CCB) and one ethylene tar (ETR). In addition, a sample of Ash- land 240 (A-240) petroleum pitch was also used to provide a basis for comparison.

The feedstocks were characterized by elemental analysis, solvent fractionation, and Fourier Trans- form Infrared Spectroscopy (FT-IR). Solvent fractionation was carried out using a sequential extraction procedure, similar to that suggested by Mima el a1.[13], to separate the feedstocks into toluene insolubles (applicable only for A-240 pitch), asphaltenes (toluene solubleipentane insolubles), and maltenes (toluene and pentane solubles). Ele- mental analysis (C, H, 0, S, and N) of the feedstocks and their solvent fractions were determined by Gal- braith Laboratories, Inc. (Knoxville, Tennessee, U.S.A.).

FT-IR spectra were obtained on a Digilab 15 B FTS system (Digilab Inc., Cambridge, Massachu- setts, U.S.A.) by co-adding 200 scans (interfero- grams) at a resolution of 2 cm-‘(141. Solid samples were prepared as standard KBr pellets. The mate-

877

rials that are gummy solids or viscous liquids at room temperature were first dissolved in chloroform and then smeared uniformly on preweighed KBr windows (Optovac, Inc., North Brookfield, Massachu- setts, U.S.A.). Chloroform was expelled by placing the KBr windows in a vacuum oven at 325 K for 2 h. Using the band assignments established for complex organic molecules[l4,15], the FT-IR spectra were used for a qualitative comparison of the molecular nature of the feedstocks.

‘H NMR spectra of CCB and ETR maltenes were obtained on a Bruker WP-200 pulse FT NMR spec- trometer operating at 200 MHz. Tetramethylsilane (TMS) was added as an internal standard to a lOO- mg sample of maltenes that was dissolved in 1 cc CDC&. The distribution of the hydrogen atoms between aromatic and aliphatic groupings was determined by digital integration of the aromatic (9.30- 6.30 ppm) and aliphatic (4.50-0.50 ppm) band intensities normalized over the complete spectrum.

Samples of the feedstocks (5 g) and their solvent fractions (3 g) were carbonized in tubing bomb reactors under autogeneous pressure at 773 K for 2 h. Prior to carbonization, the reactors were evacuated and pressurized with nitrogen to 0.2 MPa. The relatively small volume (15 cc) of these rectors enabled isothermal operation by rapid heating to the desired temperature in a fluidized-bed sand bath and by rapid quenching in a water bath upon termination of the experiments. During carbonization, the reactors, attached to a rocking mechanism, were agi- tated by vertical oscillation with a frequency of 200 cycles/min and an amplitude of 2.5 cm.

The solid carbonization products were examined by optical microscopy[ 161 for assessing the shape and size of the anisotropic constituents, which are determined by the degree of mesophase development during carbonization. That is, the larger the size of the anisotropic structures, the higher the extent of the related mesophase development was. The final optical texture of the carbonaceous solids obtained from a particular feedstock was designated as that which did not contain any isotropic material con- vertible to anisotropic semi-coke. The degree of mesophase development was measured as the largest dimension of the isochromatic regions having the most frequently observed size range on the surface of the semi-coke specimens. This qualitative mea- sure was defined as the principal optical texture size. It was determined visually by scanning the entire surface of the polished specimens using the optical microscopy and allowing a range of variation -tlO urn for large flow domains (>lOO pm) and *5 pm for domains (15-60 pm). The overall uniformity of the size and shape of the constituent anisotropic structures enabled the determination of a characteristic size that is representative of the optical texture of the semi-coke samples. In all the cases, the anisotropic structures having the characteristic size within the allowed range of variation constituted a large fraction (> approximately 80%) of the area on

878 S. ESER and R. G. JENKINS

the surface of the semi-coke specimens. The polished specimens of the carbonaceous solids were prepared using the conventional techniques[l7]. A Zeiss microscope (Model GFL, Carl Zeiss Inc., West Ger- many) equipped with polarizing filters and a phase- sensitive plate (red quartz, l/Cwave) was used for viewing the surface of the polished sections with both air and oil-immersion objectives (10x and 40x magnification) in reflected light. A Nikon camera (C 34, 47 60 70) attached to the microscope was employed for taking the photographs of selected specimens.

3. RESULTS AND DISCUSSlON

3.1 Solvent fractionation and elemental analysis of the feedstocks

The distribution of the asphaltenes and maltenes as weight percentages of the original feedstocks is given in Table 1. Among the feedstocks, only the A-240 pitch contains toluene insoluble material (6.9% wt), most of which (86.9% wt) is also insoluble in pyridine. Soxhlet extraction of the A-240 pitch with pyridine yielded 6.0% wt insolubles. An- other distinction between the A-240 pitch and the other materials is the markedly higher asphaltene content of the pitch (77% wt), which can be explained by the additional thermal and chemical processes it has undergone[l8]. It is interesting to note that the ethylene tar has a markedly higher asphaltene content (41% wt) than the catalytic cracker bottoms (23% wt). The samples of vacuum distillation residua (VDR 1, 2, and 3) show a wide range of distribution of asphaltenes (20%-43% wt). The solvent-separated asphalt has a higher asphaltene content (50% wt) than the vacuum distillation residua. These findings indicate that the chemical constitution of the petroleum heavy residua (e.g. asphaltenes/maltenes ratio) depends upon the nature of the starting materials (i.e. the original crude- oil or its distilled fraction) and the processes by which they are derived.

Table 2 gives the elemental composition of the feedstocks. Some variations are observed in the carbon contents of the feedstocks, but the most pro- nounced differences are seen in the hydrogen and sulfur contents. Comparatively low sulfur and nitro-

Table 1. Distribution of asphaltenes and maltenes in the feedstocks

% Weight

Feedstocks Asphaltenes Maltenes

Ashland 240 pitch 77.1 16.0 Catalytic cracker bottoms 23.1 76.9 Ethylene tar 41.0 59.0 Solvent-separated asphalt 49.9 50.1 Vacuum distillation residue 1 20.0 80.0 Vacuum distillation residue 2 43.0 57.0 Vacuum distillation residue 3 33.0 67.0

Carbonization of petroleum feedstocks I 879

Table 2. Elemental composition of the feedstocks

% Weight

Material C H S N 0 (C/H)At

A-240 91.2 5.4 2.0 0.1 0.6 1.41 CCB 88.3 6.9 4.6 0.1 0.7 1.07 CCB A 90.4 7.4 1.3 0.2 0.5 0.97 ETR 91.0 7.4 - 0.1 - 1.02 SSA 87.2 8.5 4.6 0.1 1.3 0.85 VDR 1 86.1 10.3 2.4 0.6 0.2 0.70 VDR 2 82.9 9.6 5.6 0.7 1.1 0.72 VDR 3 83.8 9.8 4.8 0.5 0.7 0.71

gen contents of the A-240 pitch and CCB A can be attributed to the low heteroatom contents of their precursors and the extensive cracking processes they had undergone during their production. Among the vacuum distillation residua, VDR 1 has the lowest, while VDR 2 has the highest sulfur content. The atomic C/H ratios of the feedstocks are also given in Table 2. It can be seen that the A-240 pitch has the highest atomic C/H ratio, followed by CCB, ETR, SSA, and VDR samples. The low C/H ratios of the VDR and SSA are consistent with their being highly paraffinic in nature, while the high C/H ratios of the A-240 pitch, CCB and ETR can be explained by their high degree of aromaticity[lQ]. It should be noted that the C/H ratios of the different VDRs are very similar and slightly lower than that of SSA. The oxygen contents of the feedstocks do not seem to show any meaningful trend.

Table 3 gives the elemental composition of the solvent fractions of the selected feedstocks. The comparison of Table 2 with Table 3 shows that the carbon contents of the whole materials and their asphaltene fractions are rather similar. There are, however, significant variations in the hydrogen contents. As expected, the maltene fractions of the feedstocks were invariably found to contain more hydrogen than the whole materials, while the asphaltene fractions, obviously, less. The heteroatoms (S, 0, N) tend to concentrate in the asphaltene fractions. It is interesting to note that even the hydrogen- rich (maltene) fraction of the A-240 pitch has a

higher C/H ratio than the hydrogen-poor (asphaltene) fractions of the vacuum distillation residua.

The elemental analyses of the feedstocks and their solvent fractions indicate salient differences among the chemical composition of both whole materials and the solvent fractions of each material. Addi- tional information on the molecular nature of these materials was sought by using FT-IR.

3.2 FT-IR analysis of the feedstocks and their solvent fractions

Figure 1 shows the FT-IR absorbance spectra of the three solvent fractions of the A-240 pitch, all normalized to a sample weight of 1 mg. Thus, they are directly comparable. The strong broad band near 3400 cm-’ in the spectrum of the toluene insolubles (obtained on a pellet prepared by grinding the sample with KBr) is very weak in the spectra of the asphaltenes and maltenes (obtained on the samples smeared on KBr windows), strongly suggesting that this band is due to the absorption by water adsorbed during pellet making. As neither the blank KBr pellet, nor the asphaltenes that were smeared on the KBr window showed any significant bands near 3400 cm-‘, the cogrinding of KBr and the asphaltenes appears to have a synergistic effect on the adsorption of water during pellet making. This is a commonly encountered problem associated with the pellet preparation[20.21].

The relative intensities of aromatic C-H (3050 cm-‘) and aliphatic C-H (2860-2960 cm-‘) bands in the spectra in Fig. 1 indicate that the toluene insolubles have the highest degree of aromaticity followed by the asphaltenes and maltenes, respectively. This is confirmed by the respective intensities of the band at 1600 cm-’ for aromatic C-C stretch. The strong absorptions at 1580 and 1490 cm-’ indicate that the toluene insolubles contain highly conjugated polycyclic aromatic ring systems[ 151. These bands appear only as shoulders in the spectra of the other two fractions. The band for methylene and methyl bending vibrations (1450 cm -I), which is better resolved in the case of asphaltenes and maltenes. suggests the presence of alkyl groups at high con- centrations in all the fractions. Another distinctive

Table 3. Elemental composition of the solvent fractions

% Weight

Material C H S N 0 (CIHIAt

A-240 tol. insol. A-240 asphaltenes A-240 maltenes CCB asphaltenes ETR asphaltenes VDR 1 asphaltenes VDR 1 maltenes VDR 2 asphaltenes VDR 3 asphaltenes VDR 3 maltenes

90.8 4.0 1.4 0.2 1.9 1.89 91.7 5.2 2.0 0.2 1.0 1.47 88.8 6.5 1.9 0.0 0.7 1.14 89.5 5.7 - 0.2 - 1.31 92.0 6.5 - 0.2 - 1.18 86.0 8.3 3.5 1.3 1.0 0.86 82.9 11.0 2.1 0.4 0.4 0.63 82.5 8.0 7.0 1.2 1.5 0.86 83.5 7.9 6.5 1.0 1.0 0.88 84.2 10.8 3.6 0.4 0.7 0.65


TOLUENE INSOLUBLE

II 1 ' II 1 I,11 1 ,I 11 II 10 II 11 ,I 11 ,I 1 I 3600 3400 3000 2600 2200 1600 1400 1000 600

WAVENUMBERS, cm”

Fig. 1. FT-IR absorbance spectra on the solvent fractions of A-240 pitch in the range 3800-600 cm-‘.

feature of the toluene insolubles is the prominence

of the band at 1375 cm-‘, indicating that most of the alkyl groups are in the form of methyl substituents on the aromatic ring systems. The relatively low intensities of the bands at 700-900 cm-’ (aromatic C- H out-of-plane bending modes) also indicate highly substituted and conjugated aromatic rings. The bands at 1690 and near 1300 cm-’ assigned to the C=O stretch can be attributed to the presence of aryl ketones which may be partially responsible for the observed molecular conjugation.

Compared to the toluene insolubles, the asphaltenes, which constitute the bulk of the A-240 pitch, appear to have a lower degree of conjugation (or ring condensation) and less substitution on the aromatic rings in view of the higher hydrogen aromaticity as inferred from the strong aromatic C-H out-of-plane bending modes. In a clear contrast to the toluene insolubles, most of the aliphatic C-H in the asphaltenes appear to be in the form of methylene (CH,) functionalities which may exist as hydroaromatic structures of alkyl bridges connecting the aromatic ring systems. The whole A-240 pitch gave a spectrum that is very similar to that of the asphaltenes[22].

The maltenes, on the other hand, are the most paraffinic fraction of the A-240 pitch, as can be seen from the comparison of the intensities of the respective aliphatic and aromatic absorptions with those of the other fractions. The band at 1,735 cm-’ ((C=O) stretch) associated with the maltenes, but much weaker in the other two spectra, suggests the presence of saturated esters in appreciable concen- trations.

Figure 2 shows the FT-IR absorbance spectra of the ETR, CCB, SSA, and VDR 3. The spectra were plotted using the scale expansion facility of the data processing unit, which sets the intensity of the strongest peak (2920 cm-’ band in this case) to a preassigned maximum value and adjusts the intensities of the other bands proportionally. This is an alternative procedure to the scaling of the spectra,

which is useful in comparing the band intensities of the spectra taken on samples with different weights. As expected, both CCB and ETR are considerably aromatic in nature as deduced from the strong absorptions at 3050, 1600, and 700-900 cm-‘, ETR being comparatively more aromatic. The distribution of the aliphatic C-H among methyl and methylene functionalities also shows some variations between CCB and ETR, as revealed by the differences in the relative intensities of the bending modes. For the same intensity of the band at 1375 cm-’ in both spectral profiles, the higher intensity of the composite band at 1465 cm-’ in the spectrum of ETR suggests that, compared to CCB, ETR has a higher proportion of methylene groups, which can be explained by the existence of longer alkyl chains (or bridges) in its molecular structure.

It can be seen in Fig. 2 that the 3050, 1600, and 700-900 cm-’ bands in the spectra of the SSA and VDR 3 are much weaker than those in the spectra of CCB and ETR, indicating the highly paraffinic nature of SSA and VDR 3. In fact, VDR 3 appears to be the most paraffinic material among the feedstocks used in this study. The other two samples of vacuum distillation residua (VDR 1 and VDR 2) gave FT-IR spectra more similar to that of SSA rather than VDR 3. The different nature of VDR 3 among the distillation residua should be due to the crude slate when it was produced, since the distillation conditions employed during the production of the three residua are reportedly similar.

The FT-IR spectra of the asphaltene fractions of CCB, ETR, SSA, and VDR 3 are shown in Fig. 3.

En?

CCB

SSA

VDR 3

'"00 3400

WAVENUMBERS, cm”

Fig. 2. FT-IR absorbance spectra of the feedstocks in the range 3400-2600 and 1800-600 cm-‘.


-00 lSLT----- 3400 1400 1000 600

WAVENUMBERS, cm”

Fig. 3. FT-IR absorbance spectra of the asphaltene fractions of the feedstocks in the range 3400-2600 and lSOO-

600 cm-l.

A qualitative comparison of the aliphatic and aro-

matic C-H absorption modes suggests that CCB asphaltenes are more aromatic than ETR asphaltenes, which is, interestingly, the reversal of the relation-

ship between the whole materials. As expected, the asphaltene fractions of both materials are more ar-

omatic than the respective parent feedstocks. In parallel to that observed with the parent materials, CCB asphaltenes appear to contain more methyl groups than the ETR asphaltenes. Although the asphaltene fractions of both SSA and VDR 3 appear to be more aromatic than the respective parent residua (Fig. 2), they are still much less aromatic than the asphaltenes of CCB and ETR. VDR 3 asphaltenes, in parallel to that observed with the whole feedstocks, are the most paraffinic material among the asphaltenes obtained in this study.

In all the cases, the maltene fractions were found to be more paraffinic than the respective parent materials and the asphaltenes. There are differences, however, among the different maltenes concerning the ratio of the aromaticlaliphatic C-H, which can be related back to the chemical constitution of the parent materials. The A-240 maltenes, for example, are considerably more aromatic than the VDR maltenes.

3.3 Optical textures of the semi-cokes produced from the feedstocks and their solvent fractions

The principal optical textures of the semi-cokes produced by the carbonization of the feedstocks at

Table 4. Principal optical textures of the semi-cokes produced from the feedstocks

Material

A-240 CCB CCB A ETR SSA VDR 1 VDR 2 VDR 3

Principal optical texture of semi-cokes

Elongated flow domains, 500 km Elongated flow domains, 200 Frn Elongated flow domains, 200 pm Elongated flow domains, 200 pm Flow domains, 60 pm Domains, 50 km Domains, 25 km Mosaics, 5 pm

773 K are described in Table 4. The micrographs of the surface of selected semi-cokes are shown in Fig. 4. Table 4 and Fig. 4 show a wide range of variation in the optical texture size of the semi-cokes produced. A-240 pitch produced the largest optical texture (500 pm), followed by CCB, CCB A, and ETR, which gave rise to the formation of large flow domains (200 km). Compared to the A-240 pitch and heavy aromatic oils (CCB and ETR), SSA and VDRs produced semi-cokes with smaller optical textures, indicating a lower degree of mesophase development. Among the VDR samples, VDR 1 showed the most extensive mesophase development producing large domains (50 pm) that are comparable in size to those formed during the carbonization of SSA (60 pm). On the other hand. VDR 3 produced the smallest texture (mosaics, 5 urn) observed in the semi-cokes. The semi-cokes obtained from VDR 2 exhibited domains of 25 pm in size, which were intermediate between those of VDR 1 and VDR 3 semi-cokes.

Table 5 describes the principal optical textures of the semi-cokes produced by the carbonization of the solvent fractions separated from the selected feedstocks. The carbonization experiments were carried out at 773 K for 2 h. The optical micrographs of the

Table 5. Principal optical textures of the semi-cokes produced from the solvent fractions of the feedstocks

Material Principal optical texture of semi-cokes

A-240 toluene insol. Fine mosaics, 1 ym A-240 asphaltenes Flow domains, 500 wrn A-240 maltenes Domains, 100 km CCB asphaltenes Flow domains, 200 pm CCB maltenes Flow domains, 100 pm ETR asphaltenes Flow domains, 120 km ETR maltenesf Flow domains, 200 pm SSA asphaltenes Domains, 60 pm SSA maltenes Domains, 30 km VDR 1 asphaltenes Domains, 50 pm VDR 1 maltenes Domains, 60 pm VDR 2 asphaltenes Domains, 15 km VDR 2 maltenes Domains, 30 pm VDR 3 asphaltenes Mosaics, 2 km VDR 3 maltenes Domains, 30 pm

tcarbonized at 823 K for 1 h.

S. ESER and R. G. JENKINS

A-240 CCB

ETR SSA

VDR 2 VDR 3

Fig. 4. Optical micrographs of the semi i-cokes produced from the feedstocks.

selected semi-cokes produced from the asphaltenes are shown in Fig. 5. The asphaltenes obtained from the A-240 pitch and CCB produced well-developed flow domains (500 and 200 p,rn in size, respectively) similar to those in the semi-cokes produced from the parent feedstocks. The semi-cokes obtained from the ETR asphaltenes displayed smaller flow domains (120 pm). The SSA, VDR 1, and VDR 2 asphaltenes led to the semi-cokes with still smaller optical textures (60, 50, and 15 km, respectively), while the

VDR 3 asphaltenes produced the smallest optical texture (mosaics, 2 km) among the asphaltenes.

Table 5 further shows that, with the exception of the solvent fractions of ETR, the principal differences among the optical textures are seen in those produced from the asphaltenes. These differences are, in fact, closely related to those observed in the semi-cokes obtained from the parent feedstocks (Ta- ble 4). The maltenes of the paraffinic and the aromatic feedstocks, on the other hand, produced

similar textures, consisting predominantly of domains (30, 60 urn), and flow domains (100 km), respectively. The maltenes of the VDRs produced larger textures than the respective asphaltenes, while the reverse was observed for the SSA and the aromatic feedstocks with the exception of ETR. The optical texture of the semi-cokes produced from the maltene fractions of the A-240 pitch, VDR 3, and VDR 1 is shown in Fig. 6.


in the optical texture of the semi-cokes obtained from the maltenes. The fine mosaics (<l p,m in di- ameter), commonly observed as separate bands sur- rounding the semi-coke particles, are believed to have formed via the gas phase cracking of the light (i.e. volatile) materials present in the maltenes. In all the cases, the fine mosaic regions constituted only a small fraction (~5% area) of the surface of the polished semi-coke specimens.

It can be noted in the micrographs of Fig. 6 that there exist some fine mosaic and isotropic structures

As distinct from all the other maltenes, the ETR maltenes did not produce any solids upon reaction

A-240 ASPHALTENES CCB ASPHALTENES

SSA ASPHALTENES VDR 1 ASPHALTENES

VDR 2 ASPHALTENES VDR 3 ASPHALTENES

Fig. 5. Optical micrographs of the semi-cokes produced from the asphaltene fractions of the feedstocks.


A-240 MALTENES

VDR 3 MALTENES

VDR 1 MALTENES

Fig. 6. Optical micrographs of the semi-cokes produced from the maltene fractions of the feedstocks.

at 773 K for 2 h. The reaction product obtained at 773 K was a mixture of liquids and a pitch-like material. Therefore, a higher temperature (823 K) was employed to carbonize the ETR maltenes, as indicated in Table 5. The resultant semi-coke displayed flow domains that were 200 km in size, i.e. larger than those produced by the ETR asphaltenes.

Interestingly, the toluene insoluble fraction of the A-240 pitch produced a semi-coke that had the smallest optical texture (fine mosaics, 1 pm) observed in this study.

3.4 Relationships between chemical constitution of feedstocks and optical texture of resultant semi-cokes

The elemental composition of the feedstocks (Ta- ble 2) indicated that there were significant variations especially in the C/H ratios of the various types of feedstocks. The A-240 pitch and the samples of heavy aromatic oils (CCB, CCB, and ETR)have markedly higher C/H ratios than the vacuum distillation residua (VDR 1, 2, and 3). The C/H ratio of the SSA, on the other hand, has an intermediate


value between those of the CCB and VDR. This trend correlates, in general, with the variation in the optical textures of the semi-cokes produced. That is, the A-240 pitch and CCB with the higher C/H ratios produced much larger textures in the semi-cokes than VDRs, while SSA maintained its intermediate position with respect to both parameters. This general correlation does not, however, hold for the low C/H ratios. In other words, the VDR samples with similar C/H ratios produced significantly different optical textures in the semi-cokes.

In the most general terms, C/H ratios can be related to the degree of aromaticity of the feedstocks. However, at low C/H ratios (i.e for highly paraffinic materials) the relationship between the C/H ratios and the degree of aromaticity may be obscured by other factors concerning the molecular configuration of the main constituent structures. For example, the differences in the length of the constituent aliphatic chains and in the degree of condensation of the aromatic ring systems can be important in determining the C/H ratios of the paraffinic materials, such as VDRs. The qualitative comparison of the FT-IR spectra (Fig. 2) has, in fact, shown that VDR 3 was the most paraffinic material among the VDR samples, while VDR 1 and VDR 2 were more similar in nature to SSA. The FT-IR analysis has also indicated that the A-240 pitch is more aromatic than CCB, which is consistent with the higher C/H ratio of the A-240 pitch compared to CCB. The comparison of the FT-IR spectra has further shown that

ETR has a slightly higher degree of aromaticity than CCB. This difference was not, however, reflected in the optical texture of the semi-cokes produced from CCB and ETR. As noted above, CCB and ETR produced semi-cokes with very similar optical textures to each other, despite the higher degree of aromaticity of ETR (Fig. 2). It was also noted that, in contrast to the relationship between the whole CCB and ETR, the asphaltene fraction of the CCB is more aromatic than that of ETR (Fig. 3). This discrepancy between the whole materials and their asphaltene fractions as well as the unique nature of the ETR maltenes can explain the insensitivity of the size of the optical texture to the differences in the degree of aromaticity of the whole feedstocks.

To compare the CCB and ETR maltenes, they were analyzed by using IH NMR. The ‘H NMR spectra of the two maltenes are shown in Fig. 7. The calculated hydrogen aromaticities of CCB and ETR mahenes are 31% and 34%, respectively. Another difference between the maltenes can be seen in the comparatively stronger absorptions in the ranges 4.6-2.5 and 2.0-1.4 ppm in the spectrum of ETR maltenes, which are assigned to hydrogen atoms in alpha and beta CH, of hydroaromatic rings, respectively. It can, therefore, be concluded that ETR maltenes have a higher hydrogen aromaticity and contain hydroaromatic structures at higher concen- trations than CCB maltenes. In a relevant study, Seshadri et a/.[231 noted that the hydraaromatic structures were concentrated in the hexane soluble

ETR MALTENES

I I I I I I I I I I I IO 9 8 7 6 5 4 3 2 1 0

5, PPm

Fig. 7. ‘H NMR spectra of the ETR and CCB maltenes in the range 10-O ppm.


fraction of a sample of ethylene tar, whereas the hydroaromatic compounds were retained in the hexane insoluble fraction of a decant-oil sample. These observations, which are consistent with the findings of this study, can explain the slower carbonization of ETR maltenes and the resulting higher degree of mesophase development[24].

In view of the observations presented above, it can be concluded that the size of the optical texture of the semi-cokes is, in general, directly proportional to the degree of aromaticity of the feedstocks. This general relationship is in good agreement with the delayed coking practice in which the highly aromatic feedstocks (i.e. catalytic cracker bottoms) produce needle coke (with large flow domains texture), while vacuum distillation residua produce sponge coke (domains and flow domains) and, occasionally, shot coke (mosaics)[24,25]. It should be noted, however, that there are exceptions to the general relationship between the degree of aromaticity of the feedstocks and the optical texture size of the resultant semi- cokes. A notable example to that effect is that the toluene insoluble fraction of the A-240 pitch which is the most aromatic material used in this study produced the smallest optical texture observed in the semi-cokes. The reasons for this apparent discrepancy are discussed elsewhere in relation to the chemistry of mesophase formation[22].

The elemental composition of the feedstocks (Ta- ble 3) has also shown some differences in the heteroatom (0, S, N) contents of the feedstocks. Especially the sulfur contents, which were appreci- ably higher than the oxygen and nitrogen contents in all the feedstocks, showed important differences. None of these differences, however, could be directly related to the differences observed in the optical texture of the semi-cokes produced from the feedstocks. Although the A-240 pitch and the heavy aromatic oils have, in general, lower heteroatom contents than the SSA and VDR, the larger semi- coke textures produced by A-240 pitch, CCB and ETR cannot solely be attributed to their lower heteroatom contents due to the major structural differences between these feedstocks. Further, the large differences in the heteroatom contents of the two samples of the catalytic cracker bottoms (CCB and CCB A (Table 2)) did not cause any noticeable variation in the optical texture of the resultant semi- cokes. Among the VDRs, VDR 1 with the lowest heteroatom content produced, in fact, the largest optical texture. On the other hand, VDR 2, which has the highest heteroatom content, gave rise to an intermediate-size optical texture between those of VDR 1 and VDR 3. The apparent lack of a consistent correlation between the heteroatom content and the size of the optical texture should not rule out the possible effects of the heteroatoms on the mesophase development. It has been reported elsewhere[26] that the heteroatoms can severely retard the mesophase development. These effects critically depend on the concentration and the functionality

of the constituent heteroatoms[27] as well as on the nature of the feedstocks[28]. In this respect, the results obtained within the scope of this study are not conclusive in terms of the effects of heteroatoms on the mesophase development.

The comparison of the asphaltene contents of the feedstocks (Table 1) with the size of the principal optical texture of the resulting semi-cokes shows that there is no correlation between these two parameters. The A-240 pitch and the heavy aromatic oils with very different asphaltene contents (ranging from 23% to 77% wt) produced semi-cokes with similar optical textures. On the other hand, VDR 2, with an asphaltene content (43% wt) close to that of ETR (41% wt) produced much smaller optical textures than ETR. Further, VDR 3, which has an intermediate asphaltene content (33% wt) among the VDRs, led to the smallest optical texture in the semi-cokes produced from the feedstocks.

The general parallelism observed between the optical textures of the semi-cokes produced from the feedstocks (Table 4) and their asphaltene fractions (which are, in general, the higher molecular weight and more complex fractions of the feedstocks) (Ta- ble 5) indicates a dominating role of the asphaltenes in determining the optical texture of the semi-cokes produced from the parent feedstocks. It should be noted that, except for the A-240 pitch, the concentration of the asphaltenes is lower than that of the maltenes in the feedstocks used (Table 1). However, the optical texture of the semi-cokes obtained from these feedstocks as well as SSA (which contains 50% wt asphaltenes) was essentially the same as that produced by their asphaltene fractions. The dominant effect of the asphaltenes on mesophase development appears to be dependent on the chemical nature of the different asphaltenes, rather than on their con- centrations in the feedstocks. Similar observations, called “the dominant partner effect,” have been reported in the literature(291 concerning the carbonization of mixtures of the different materials. Con- sidering the feedstocks as an intimate mixture of the two solvent fractions, the asphaltenes act as the dominant partners during the carbonization of the feedstocks mentioned above. A notable exception to this general behavior was displayed by ETR, the maltene fraction of which appeared to have a strong influence on the development of the texture during the carbonization of ETR. The generally observed dominant effect of the asphaltenes on the formation of the optical texture can be attributed to their higher tendency (compared to the maltenes) to carbonize and to initiate the mesophase formation without any significant involvement of the maltenes.

In view of the dominating role of the asphaltene fractions in determining the optical texture of the semi-cokes produced from the petroleum feedstocks, the relationships between the chemical constitution of the feedstock asphaltenes and mesophase development were further investigated by a more extensive characterization of the asphaltenes[30].


4. CONCLUSIONS

The mesophase development during the carbonization of the petroleum heavy residua critically depends upon the chemical constitution of the feedstocks. A general relationship was observed between the degree of aromaticity of the feedstocks and the principal optical texture size of the resultant semi-cokes. Although the optical texture size of the semi-cokes tends to increase with the increasing degree of aromaticity of the feedstocks, there are notable exceptions to this general trend. The asphaltene and heteroatom contents of the feedstocks did not appear to be critically significant in determining the degree of mesophase development. The asphaltene fractions of the feedstocks. however, have a dominating influence in determining the degree of mesophase development during the carbonization of the feedstocks.

Acknowledgement-This study was financially supported in part by a grant from the Aluminum Company of America (ALCOA).

1.

2.

3 4:

5. 6.

7.

REFERENCES

J. D. Bacha, J. W. Newman, and J. L. White (Eds.), In Petroleum Derived Carbons, ACS Symposium Se- ries, No. 303. American Chemical Society, Washing- ton, D.C. (1986). R. DeBiase, J. D. Elliott and J. E. Hartnett, In Pe- troleum Derived Carbons (Edited by J. D. Bacha, .I. W. Newman, and J. L. White), ACS Symposium Se- ries, No. 303, p. 155. American Chemical Society, Washington. D.C. (1986). H. Honda, Carbon 26. 139 (1988). E. Fitzer, K. Mueller, and W. Schaefer, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 7, p. 237. Marcel Dekker, New York (1971). I. C. Lewis, Cnrbon 20, 519 (1982). H. Marsh and P. L. Walker, Jr., In Chemistry and Physics of Carbon (Edited by P. L. Walker. Jr. and P. A. Thrower), Vol. 15, p. 230. Marcel Dekker, New York (1979). 1. Mochida and Y. Korai, In Petroleum Derived Car- bons (Edited by J. D. Bacha, J. W. Newman, and J. L. White), ACS Symposium Series, No. 303. p. 29. American Chemical Society, Washington. D.C. (1986).

8.

9.

10. 11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21. 22.

23.

24.

25. 26.

27.

28.

29.

30

H. Marsh and C. S. Latham, In Petroleum Derived Carbons (Edited by J. D. Bacha, J. W. Newman, and I. L. White), ACS Symposium Series, No. 303, p. I. American Chemical Society, Washington. D.C. (1986). M. P. Whittaker and L. I. Grindstaff, Carbon 10, 165 (1972). J. L. White and R. J. Price, Carbon l2, 321 (1974). I. Mochida, Y. Korai, H. Fujitsu, T. Oyama, and Y. Nesumi. Carbon 25. 259 (1987). S. Eserl Ph.D. thesis, The Pennsylvania State Uni- versity, University Park, Pennsylvania (1986). M. J. Mima, H. Schultz, and W. E. McKinstry, In Analytical Methods for Coaland Coal Products (Edited by C. Karr, Jr.), Vol. 1, p. 557. Academic Press, New York (1978). P. C. Painter, R. W. Snyder, M. Starsinic. M. M. Cole- man. D. W. Kuehn. and A. Davis. ADDI. Svectrosc. 81 ‘ 35. 475 (1981). L. J. Bellamy, In The Infrared Spectra of Complex Molecules. Methnen and Co. Ltd.,. London (1958). J. Smith and H. Marsh. In Analvtical Methods for Coal and Coal Products (Edited by C. Karr. Jr.), Vol. 2, p. 371. Academic Press, New York (1978). J. Dubois. C. Agache. and J. L. White, Metallography 3, 337 (1970). J. W. Newman, In Petroleum Derived Carbons (Edited by M. Deviney and T. O’Grady), Symposium Series No. 21. p. 52. American Chemical Society, Washing- ton, D.C. (1976). C. A. Stokes and V. J. Gurcio, Amer. Chem. Society, Div. Pet. Chern., Preprints, 29(2), 468 (1984). I. A. Breger and J. C. Chandler, Anal. Chem. 415, 506 (1969). M. Nakamizo and K. Tamari, Carbon 22, 193 (1984). S. Eser and R. G. Jenkins, Carbonization of Petroleum Feedstocks III, Carbon (1989). K. S. Seshadri, E. W. Albaugh, and J. D. Bacha, Fuel 61, 336 (1982). S. Eser. R. G. Jenkins, M. Malladi. and F. J. Der- byshire, Carbon 24, 77 (1986). K. E. Rose, Hydrocarb. Process. 51, 85 (1971). M. Monthioux, M. Oberlin, A Oberlin, X. Bourrat, and R. Boulet, Carbon 20, 167 (1982). E. Fitzer and H. Tillmans, In Extend. Abstr., 12th Biennial Conf. Carbon. Am. Carbon Sot.. 217 (1972). I. Mochida. K. Maeda,‘K. Takeshita. N. Kaji, Y. Suer- suyu, and T. Yoshida, Fuel Process. Technol. 1. 103 (197711978). H. Marsh. I. Macefield, and J. Smith, In Extend. Abst.. 13th Biennial Conf. Carbon, Am. Carbon Sot. 21 (1977). S. Eser and R. G. Jenkins. Carbonization of Petroleum Feedstocks If. Carbon 27(6) (1989).

Documents

Carbonization of petroleum feedstocks I: Relationships between chemical constitution of the feedstocks and mesophase development