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Pyrolysis of petroleum residues: I. Yields and product analyses
PERGAMON
Carbon 37 (1999) 1567–1582
Pyrolysis of petroleum residues: I. Yields and product analyses a a a a, *, ´ ´ M. Martınez-Escandell , P. Torregrosa , H. Marsh , F. Rodrıguez-Reinoso b c c ´ ´ ´ ´ R. Santamarıa-Ramırez , C. Gomez-De-Salazar , E. Romero-Palazon a
´ ´ Departamento de Quımica Inorganica , Universidad de Alicante, E-03080 Alicante, Spain b Instituto Nacional del Carbon, Oviedo, Spain c ´ ´ de Cartagena, Cartagena, Spain REPSOL-PETROLEO , Centro de Investigacion Received 17 April 1998; accepted 22 January 1999
Abstract This study examines the pyrolysis of three petroleum pitch residues of different aromaticities, (R1), (R2) and (R3), varying the experimental parameters of pressure, temperature and soak time. The overall objective is to provide further detailed information of factors which influence formation of anisotropy or mesophase in resultant semicokes. Pressure was varied during the progress of a pyrolysis. Yields of gases, liquids and semicokes were obtained. Gases were analysed by gas chromatography, the liquids by simulated distillation and 1 H-NMR, and the semicokes by elemental analysis and FTIR. For the semicokes from R1, yields are dominantly a function of pressure, with little influence of temperature and soak time. For semicokes from R2, yields are dominantly a function of pressure and temperature, with little influence of soak time. For semicokes from R3, yields are dominantly a function of temperature and soak time, with little influence of pressure. The use of simulated distillation and pressure release, at reaction temperatures, provides additional information about mechanisms of the pyrolysis reactions. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Mesophase, Petroleum pitch, Pyrolysis, Semicoke; C. Infrared spectroscopy, Nuclear magnetic resonance (NMR)
1. Introduction Carbon anodes for the aluminium industry and graphitic electrodes for the steel industry, as well as many smaller applications, use delayed cokes (sponge- and needle-coke) as major constituents in their formulations [1]. In turn, delayed cokes are made by the industrial process of delayed coking, where selected petroleum residues or feedstocks are heated in vertical cylindrical drums, the inlet temperatures of the feedstock being |5008C. The feedstock is converted to a coke (of various qualities) via a mechanism involving the formation of mesophase [2,3]. Greinke [4] reviewed the early stages of the chemistry of petroleum pitch carbonisation (kinetics and mechanisms), and referred to the importance of mesophase formation. Mochida et al. [5] reviewed the chemistry and industrial operations in the production and utilisation of delayed cokes. Ultimately, it is the chemistry of the pyrolysis system which controls the structure and prop*Corresponding author. Fax: 134-6-590-34-54. ´ E-mail address: [email protected] (F. Rodrıguez-Reinoso) 0008-6223 / 99 / $ – see front matter PII: S0008-6223( 99 )00028-7
erties of resultant semicokes / cokes / carbons. But, the chemistry, in turn, is a function not only of the chemical properties of the original feedstocks, but also of the physical parameters incorporated into the system, such as carbonisation capacity (size of reactor), heating rate, soak time, final reaction temperature and pressure within the system. Dependent upon how the pyrolysis system is constructed, e.g. whether or not volatile material is lost from or retained within the pyrolysis system, these physical parameters control the chemical composition of the feedstock as it is pyrolysed through to a semicoke [6,7]. ´ et al. [8], Rodrıguez-Reinoso ´ Romero-Palazon et al. [9] and Adams [10] showed how multiphase systems within bulk pyrolysing systems influence coke quality and how gas evolution (amounts and temperatures of evolution) also influence coke quality, controlling how and when mesoph´ ´ ase formation occurs. Santamarıa-Ramırez et al. [6,7] surveyed those effects, chemical and physical, which influence mesophase growth and coalescence of mesophase within semicokes. The chemistry of carbonisation of petroleum residues and coal–tar pitches is complicated. The chemical re-
1999 Elsevier Science Ltd. All rights reserved.
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actions involve the thousands of molecular species contained within these materials. It is necessary to establish mechanisms of growth in molecular size, at the molecular level, in order to understand the patterns of molecular behaviour during pyrolyses and carbonisations, leading to mesophase and semicoke. Such molecular behaviour, as well as being a function of the chemical composition of the feedstock, is also a function of the rate of heating (Rc), of the final reaction temperature (T ), the time at temperature (soak time (tc)), pressure (P), and size and geometry of the reaction vessel containing the feedstock which is maintained in the system. These factors together control the overall behaviour of coking and the properties of the final product (semicoke or coke) [6]. The physico-chemical behaviour of coking, which also includes flow movement or turbulence within the liquid (fluid) carbonisation system, caused by thermal flow and gas evolution from within the system, also affects semicoke properties [8]. The objective of this study is to have an overall understanding of these processes and how they relate to the properties of the semicoke and any materials derived from these semicokes. More specifically, this study, part of a more comprehensive study of mechanism and kinetics of petroleum residue pyrolysis, ascertains the influence of the different chemistry of different feedstocks, as well as different operating parameters (Rc, T, tc, P) on (i) yields of semicoke, liquids and gases (to obtain a mass balance of the pyrolysis process) and on (ii) the composition of these products of pyrolysis. The results are relevant to the industrial process of delayed coking and to the formulation of new carbon materials which may be produced from the semicokes ultimately derived from petroleum residues.
2. Experimental
6008C. Heating rates were constant at |158C / min. The reactor tube was made of Pyrex glass, 65 mm diameter, 210 mm height and containing 350 g of feedstock. The tube was placed within the steel block which was within the sand bath (to ensure a maximum uniformity of temperature). All pyrolyses were carried out under an atmosphere of nitrogen, (99.999% pure), the nitrogen also being used to establish pressures .0.1 MPa in the system. In order to have a wide range of pyrolysis conditions (at constant heating rate), variations were introduced into the maximum pyrolysis temperature (T ) 420–4808C, the pressure (P) 0.1–1.0 MPa, and soak time 0–12 h, in various combinations. R2 is a feedstock which develops mesophase more slowly than R1 and R3, and for a level of development equal to that of the other two feedstocks, the reaction temperature had to be increased by 208C. When operating at 1.0 MPa, the system was depressurised at reaction temperatures at the end of each experiment and then cooled to room temperature. In these pyrolysis experiments, where pressure is reduced, on reduction of the pressure, or depressurisation, there occurs a second distillation. Note: there is no need to depressurise the system when operating at 0.1 MPa pressure.
2.2.2. Pyrolysis yields Mass of liquids was obtained during the experiment by opening the valve of the condensers. No loss of gases was detected while taking out the liquids. The volume of gases produced during the pyrolysis is obtained using a gas meter. Mass of gases is calculated using an average molecular weight obtained by gas chromatography. Mass of semicoke was obtained at the end of the experiment. During the experiment mass of semicoke can be calculated by difference. Gas, liquids and semicoke yields are expressed in terms of the original feedstock. Mass balances were obtained with an accuracy of |1 wt %.
2.1. Materials used 2.3. Gas analysis Three types of feedstocks are used as starting materials for the study:
(R1), an aromatic feedstock. (R2), a feedstock of relatively low aromaticity. (R3), an almost totally aliphatic feedstock.
2.2. Experimental methods 2.2.1. Pyrolyses of the feedstocks The pyrolysis of feedstocks was carried out in a laboratory pilot plant already described [7,9]. The special features of this system include a maximum sample capacity of 1000 cm 3 and an ability to work between the pressures of 0.1 and 4.5 MPa. Heating of the reaction tube occurred in a preheated fluidised sand-bath furnace, with a temperature control of 618C at 508C and of 628C at
The product gases were analysed using gas chromatography, model HP-5890 series II.
2.4. Liquid analysis During the experiments, liquids were removed from the system by opening the valve to the condensers. Hence, curves of the variation of amounts of distillates with time were obtained. The aromaticity of the liquids was determined by 1 H-NMR using DCCl 3 as the solvent and tetramethylsilane (TMS) as the internal standard. The integration intervals were: Hydrogen attached to aromatic carbon (H ar ) of 9–6 ppm, hydrogen of methyl groups attached to aromatic rings (H a-2 ), 4.5–3.5 ppm, hydrogen attached to a-carbon atoms (H a ) of 3.5–2 ppm, hydrogen attached to b-carbon atoms (H b ) of 2–1 ppm, and hydrogen attached to g-carbon atoms (H g ) of 1–0.5 ppm.
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In addition, the liquids were studied by simulated distillation, using chromatography, model HP-5890 series II, following the accepted ASTM D28-87 standard procedure.
2.5. Semicoke analysis The resultant semicokes were analysed for element content using a Carlo Erba Eager 200 CHNSO elemental analyser. Semicokes were examined for aromaticity by diffuse reflectance infrared spectroscopy, using a Nicolet 510P instrument. Using the computer programme Omnic v1.0, areas of absorption corresponding to C–H ar vibrations (2990–3150 cm 21 ) and C–H al (2800–2990 cm 21 ) were obtained. The aromaticity parameter ‘n’ was calculated as the ratio of the two areas for each semicoke. All samples were mounted in a resin block, and optically polished surfaces were examined by reflected polarised light microscopy. The percentage contents of isotropic and anisotropic (A) constituents were measured by analysing 25 fields of the semicoke; a total of 2500 points were counted for each sample. The nomenclature used to describe carbonisation experiments is: R-T-P/t, where R5petroleum residue, T5pyrolysis temperature (8C), P5pressure (0.1 or 1.0 MPa), and t5soak time (h).
3. Results
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Table 1 Compositional analyses of petroleum residue feedstocks R1 Elemental analyses
Solubility
1
H-NMR
Simulated distillation
R2
R3
C (wt %) H (wt %) N (wt %) S (wt %) HI (wt %) TI (wt %)
90.2 7.0 0.1 0.2 13 0
87.1 9.7 0.1 1.1 0 0
85.2 10.4 0.2 3.8 0 0
H ar (%) H a-2 (%) H a (%) H b (%) H g (%)
51 5 27 15 2
19 1 19 44 17
8 0 10 59 23
25–1008C 100–2008C 200–3608C 360–4208C 420–4408C 440–4608C 460–4808C .4808C
0 5 60 12 3 4 4 12
0 2 15 21 14 14 10 24
0 0 0 0 0 0 0 100
C 1 H 0.9
C 1 H 1.3
C 1 H 1.5
Reduced empirical formulae
between 420–4608C; therefore, large differences in yields due to increasing temperature are not expected.
3.1. Feedstock R1 3.1.1. Effect of experimental parameters on yields Table 1 contains the analyses of the three feedstocks used. The three feedstocks are different chemically, one from the other. The values presented in Table 1 are considered as examples of a wide range of the feedstocks used industrially. However, the observed constituent differences are sufficiently distinct that behavioural differences are to be expected. R1 is relatively rich in carbon with a low content of heteroatoms (0.3 wt %). It is an aromatic feedstock, ((H ar )551%). There is a majority of hydrogen, as methyl hydrogen, attached to the aromatic rings. It has the highest insolubility in heptane (13 wt %). When studying the simulated distillation data, it is seen that 65 wt % of the compounds have boiling points lower than 3608C. This material distills during heating before reaching reaction temperatures (higher than 3608C). It also has a heavy fraction (12 wt %) with boiling point higher than 4808C. This amount coincides with the percentage of material insoluble in heptane, indicating that in this feedstock there exists a heavy fraction, highly condensed, which needs limited transformations to become mesogens. This heavy fraction does not distill and can be considered as the nearly minimum yield that could be obtained from this feedstock. There is little material with boiling points
3.1.1.1. Gases. Figs. 1(a–b) describe the variations of yields of gases, liquids and solids, with soak time (h), at 0.1 and 1.0 MPa pressure, for pyrolysis of R1. The yields of gases are only a small fraction of the original material. The feedstock R1 has the lowest quantity of gas evolution (,5 wt %) of the three feedstocks. Pressure exerts a major influence on gas yields, causing an increase from 2.2 to 5.5 wt % (4608C, 3 h) when the pressure is raised from 0.1–1.0 MPa, i.e. more than a doubling in the quantity of gas evolved. 3.1.1.2. Liquids. Yields of liquids from R1, Figs. 1(a–b), vary little with residence time (5 wt %), increasing slowly with increasing temperature, 420–4608C, (5 wt %) but more noticeably with increasing pressure, 0.1–1.0 MPa, (20 wt %), being between 52 and 75 wt %. 3.1.1.3. Solids. The variations in yields of semicoke (solids), Figs. 1(a–b) are similar to those described for the liquids because the amounts of gases are relatively small. The yields from R1 are almost constant with residence time, and vary little with temperature, but vary signifi-
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Fig. 1. Variation of yield (wt %) of gases (– –), liquids (---) and solids (—) with soak time (h), temperature (4208C (j), 4408C (m), 4608C (d), and 4808C (♦)) and pressure (MPa) for the feedstocks R1, R2 and R3.
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cantly with pressure of pyrolysis. Dependent on the pressure used, yields vary between 30–45 wt %.
3.1.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 3.1.2.1. Gas analysis. Gases produced during pyrolysis of R1 (R1-440-1.0 / 3) were collected to be compared with gases from R2 (R2-440-1.0 / 3) and R3 (R3-440-1.0 / 3), for the same pyrolysis conditions. Gases produced from R1 are from short-chain substituents, mainly methane and ethane together with a significant quantity of hydrogen (Table 2). Contents of compounds with three carbon atoms are quite low (,2 vol % of all gases). Similarly, the production of H 2 S is low (,1 vol % of all gases). 3.1.2.2. Liquid analysis. The simulated distillation data for R1 and its pyrolysis liquids (distillates) coming from different conditions are in Fig. 2(a), where the first histogram describes the feedstock and the rest of the histograms describe the liquids obtained during pyrolysis. These data refer to weights of original feedstock. The histograms of the feedstock were compared with those of the liquids obtained during pyrolysis to determine if these liquids come from distillation of the original feedstock or from cracking reactions. For R1, 5 wt % of the original feedstocks boils between 100–2008C, 60 wt % boils between 200–3608C, and 30 wt % boils .3608C. When operating at 0.1 MPa, 4208C, zero soak time, distillates of boiling point ,3608C are a little lower (2 wt %) than in the feedstock. The heaviest fraction of the feedstock, boiling point .3608C, decreases in the distillates, stays within the reactor and eventually forms mesogens and mesophase. With increasing soak time to 12 h, distillates of boiling point ,3608C increases, fraction 100–2008C being slightly higher than in the original feedstock (1 wt %). At 4608C, fraction ,3608C are slightly higher than in feedTable 2 Analyses of gases pyrolysis gases (vol %) Gas
Feedstock R1
R2
R3
CH 4 Hydrocarbon 2-C Hydrocarbon 3-C Hydrocarbon 4-C Hydrocarbon 5-C Hydrocarbon 6-C or higher H2 H2S
62.8 19.4 6.3 1.4 0.3 0.5
57.3 16.9 10.0 4.1 1.2 1.5
40.6 19.8 13.3 7.3 1.0 3.1
8.3 0.9
5.4 3.8
6.3 8.3
Mean molecular weight (amu)
20.5
24.5
28.0
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stock (3 wt %), and fraction .3608C increases when comparing to that of 4208C. Operating at 1.0 MPa, 4608C, comparing with 0.1 MPa, there is a decrease of compounds of boiling point .2008C in the distillates, but compounds of boiling point ,2008C increase (3 wt %). The variation of H ar with soak time, for liquids from R1 (4608C, 0.1–1.0 MPa) are shown in Fig. 3. The aromaticity of the distillates from R1 is clearly the highest of all three feedstocks, being similar to that of the original feedstock. 1 H-NMR of the distillates of R1 shows that the aromaticity at 0.1 MPa is independent of soak time, but at 1.0 MPa the aromaticity increases with increasing soak time. The effect is more pronounced with increasing temperature and residence time.
3.1.2.3. Solid analysis. For feedstock R1, an increase in soak time causes an increase in the anisotropy content, i.e. operating at 4208C, 0.1 MPa, anisotropy increases from 0 to 80% in 12 h soak time (Figs. 4(a–b)). An increase in temperature accelerates mesophase development, i.e. operating at 4608C, 0.1 MPa, anisotropy increases from 0 to 80% in 3 h soak time. The effect of increasing the pressure from 0.1 to 1.0 MPa is to slow down the process of formation of mesophase. When analysing the evolution of aromaticity of the semicokes by FTIR (Figs. 5(a–b)), aromaticity follows similar trends as observed for mesophase. It increases with soak time and temperature, i.e. operating at 4208C, 0.1 MPa, it increases from 0.35 to 1 in 12 h, operating at 0.1 MPa and 3 h soak time. With increasing temperature from 420 to 4608C, aromaticity increases from 0.4 to 1.5. The effect of pressure is rather different from mesophase development. Operating at 420 and 4408C, aromaticity increases slightly more at 0.1 MPa than at 1.0 MPa (i.e. 1.0 and 0.75, 420-0.1 / 12, 420-1.0 / 12, respectively). Operating at 4608C and 3 h soak time, the aromaticity is higher at 1.0 MPa (1.5–2.3 and 0.1–1.0 MPa, respectively). R1 initially develops mesophase in the form of spheres (.30 mm) which leads to domains or flow domains (Fig. 6(a)). The chemical analyses of the semicokes, Table 3, indicates that the ratio H / C decreases with temperature and soak time, H / C varying from 0.66 to 0.55 with increasing soak time from 0.5 to 6 h (4408C, 0.1 MPa). In R1 the heteroatom content is almost zero (N: 0.1 wt % and S: 0.2 wt %). 3.2. Feedstock R2 The feedstock R2 is intermediate in properties between R1 and R3. The simulated distillation data indicate that R2 molecules have a more continuous boiling points distribution than R1 (Table 1). Only 17 wt % of the feedstock compounds have boiling points below 3608C. Most of the feedstock is in the reactor when reaching reaction temperatures. Almost 40 wt % of the feedstock has boiling points between 420 and 4808C; therefore, large variations in
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Fig. 2. Simulated distillation distributions of liquid distillates both from within the feedstock (fd) and from pyrolysis reactions (pd) of R1, R2 and R3.
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wt % at 0.1 MPa, to 40–50 wt % at 1.0 MPa. Solid yields from R2 are influenced more than solid yields from R1, by temperature, i.e. varying from 36 to 14 wt % when increasing temperature from 440 to 4808C (0.1 MPa, 3 h). Pressure also exerts an influence, yields increasing more than 20 wt % with increasing pressure. Soak time also affect yields, mainly in the first 2 h (i.e. operating at 4408C, 0.1 MPa, yields decreasing from 55 to 35 wt %) (Figs. 1(c–d)).
3.2.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 3.2.2.1. Gas analysis. R2, when compared with R1, produces smaller quantities of methane and ethane but larger amounts of compounds with three carbon atoms or more (17 vol % of all gases). Similarly, the production of H 2 S is also higher (4 vol % of all gases) (Table 2). Fig. 3. Variation of content of H ar of all liquid distillates with soak time and pressure, at pyrolysis temperatures of 4608C (R1, R3) and 4808C (R2).
yields with increasing temperature are expected. It has 24 wt % with boiling points higher than 4808C, but it contains no material insoluble in heptane, indicating that this heavy fraction is highly aliphatic, in contrast to the heavy fraction of R1. It is a feedstock much less viscous than R1 and R3 at room temperature. Aromaticity is intermediate between R1 and R3 (Table 1).
3.2.1. Effect of experimental parameters on yields 3.2.1.1. Gases. R2 has a higher yield of gas than R1, reaching a maximum of 16 wt % (R2-440-1.0 / 12). There is also a significant decrease in gas evolution at the lowest pressure of 0.1 MPa, 16–.7 wt %, (the gas yield increasing with increasing temperature). At 4808C, 3 h, the yields decrease, 13–.3.0 wt %, 1.0–.0.1 MPa (Figs. 1(c–d)). 3.2.1.2. Liquids. R2 is a feedstock which generates mesophase more slowly than the other two feedstocks. To obtain equal developments with the semicokes, the pyrolyses of R2 are 208C higher than for R1 and R3. During pyrolyses, R1 and R2 behave differently, in particular with respect to liquid yields, between 31 and 85 wt % for the latter. For R2, experimental conditions significantly affect yields: increasing pressure decreasing liquid yields by 40 wt %: increasing temperature increasing yields by 30 wt %: increasing soak time increasing yields by 10 wt % (Figs. 1(c–d)). 3.2.1.3. Solids. For R2, yields depend more on temperature and pressure, less on soak time. Yields of semicoke vary more than yields of liquids, changing from 12–25
3.2.2.2. Liquid analysis. Simulated distillation curves for R2 and its pyrolysis liquids (distillates) coming from different conditions are in Fig. 2(b). Analysis by simulated distillation of residue R2 reveals that its molecular constituents can be divided into two groups, 15 wt % of the feedstock is distilled between 200–3608C, and 82 wt % .3608C, indicating the homogeneity of molecular weight composition of R2. When analysing the distillates produced during carbonisation by the simulated distillation histograms, for the system at 0.1 MPa, there is an enrichment with compounds of boiling point 200–3608C, compared with the original feedstock, indicating that about 10–15% of the liquids, (pd), come from the cracking of the higher molecular weight molecules. At 1.0 MPa, this percentage is higher (20–22 wt %), as it includes also a fraction in the range 100–2008C. The evolution of aromaticity of liquids with soak time at 4808C and 0.1–1.0 MPa are shown in Fig. 3. At 0.1 MPa, aromaticity of liquids at zero soak time is very similar to the feedstock (H ar 520%), decreasing slowly with soak time. At 1.0 MPa the behaviour differs somehow, at zero soak time aromaticity is lower than that of the feedstock (H ar 515%), being almost constant with soak time. 3.2.2.3. Solid analysis. The development of mesophase (anisotropy) of R2 is shown in Fig. 4(c–d). Anisotropy increases with increasing soak time and temperature. When increasing pressure mesophase development seems to be more slow. When compared with pyrolysis of R1, R2 needs higher temperatures to give similar mesophase contents. The evolution of the aromaticity of the semicoke by FTIR (‘n’ parameter) with experimental conditions is shown in Fig. 5(c–d). As occurred in pyrolysis of R2, ‘n’ increases with soak time and temperature, varying from 0.35 and 2.75 depending on experimental conditions. The
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Fig. 4. Variation of mesophase content measured by optical microscopy (A) of the semicokes with soak time (h), pressure (0.1–1.0 MPa), for pyrolyses of R1, R2 and R3.
effect of pressure is smaller than with pyrolysis of R1, obtaining semicokes of slightly higher aromaticity at 0.1 MPa than at 1.0 MPa. When compared with R1 semicokes operating at the same experimental conditions (Fig. 5(a–
b)), R2 semicokes are less aromatic than R1, differences decreasing with increasing temperature. R2 give mesophase in the form of fluid domains (Fig. 6(b)).
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Fig. 5. Variation of aromaticity of the semicokes (‘n’, C–H ar / C–H al ) with soak time (h), pressure (0.1–1.0 MPa), for pyrolyses of R1, R2 and R3.
As occurred with aromaticity, R2 semicokes have higher H / C than R1 semicokes. For instance, R1-440-0.1 / 1.5 have an H / C50.63 whereas R2-460-0.1 / 1.5 have an H / C50.67, indicating the lower reactivity of R2 (Table 3).
3.3. Feedstock R3 The feedstock R3 contains much less carbon and more in the way of heteroatoms (4.0 wt %) together with more
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Fig. 6. Optical micrographs of semicokes from (a) R1 (R1-440-1.0 / 6); (b) R2 (R2-460-1.0 / 6); (c) R3 (R3-440-1 / 3).
aliphatic hydrogen associated with long-chain paraffinic groups. The content of (H b ) is not only indicative of paraffinic hydrogen attached to the aromatic ring systems but also to totally aliphatic constituent molecules, such as waxes and paraffins of high molecular weight. The high content of (H g ) indicates the presence of substituents of
long molecular chain-length. With R3 there is no volatile loss below 3608C. All the material has boiling points higher than 4808C (Table 1). No distillation during heating, and limited differences with pressure are to be expected. It is a viscous feedstock at room temperature (almost a solid).
Table 3 Elemental analyses of semicokes from pyrolyses of R1, R2 and R3 Experimental conditions
C (wt %)
H (wt %)
N (wt %)
S (wt %)
H/C
R1-440-0.1 / 0.5 R1-440-0.1 / 1.5 R1-440-0.1 / 6 R1-440-1.0 / 0.6 R1-460-0.1 / 3 R2-460-0.1 / 0.5 R2-460-0.1 / 0.5 R2-460-0.1 / 3 R2-480-0.1 / 3 R3-440-0.1 / 0 R3-440-0.1 / 1.5 R3-440-0.1 / 3 R3-440-1.0 / 3 R3-460-0.1 / 3
93.73 94.96 95.58 95.26 95.79 92.33 92.82 93.38 94.60 84.78 85.88 86.73 87.19 87.64
5.20 4.95 4.37 4.74 4.21 5.63 5.15 4.63 3.70 9.55 5.49 4.55 4.56 4.52
0.03 0.06 0.05 0.00 0.00 0.15 0.15 0.22 0.22 1.18 2.05 2.13 1.96 2.28
0.04 0.03 0.00 0.00 0.00 1.89 1.88 1.77 1.57 4.49 6.58 6.58 6.28 5.76
0.66 0.63 0.55 0.60 0.53 0.73 0.67 0.59 0.47 1.35 0.77 0.63 0.63 0.62
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3.3.1. Effect of experimental parameters on yields 3.3.1.1. Gases. Yields of gases vary between 1–13 wt % (of the original feedstock) depending on conditions. Operating at R3-420-0.1 / 3, the yield of gases is 6.5 wt %; at R3-460-0.1 / 3 the yield is 10.1 wt %. Soak time is important. At R3-420-0.1 / 6 yields increases by 7 wt %. Pressure has little influence on gas yields (Fig. 1(e–f)). 3.3.1.2. Liquids. R3, Fig. 1(e–f), differs more from R1 and is closer to R2. Yields of liquids are significantly influenced by temperature and residence time, the effect of pressure being least noticeable. It is the feedstocks with the lowest yield of distillates (5–59 wt %); increasing pressure changes the yields of distillates by 5–10 wt %.
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are no differences in aromaticity with pressure. R3 semicokes are less aromatic than R1 and R2 at equal conditions. R3 develops mesophase in the form of very small spheres in the early stages, leading to a mesophase in the form of coarse mosaics (Fig. 6(c)). R3 semicokes have a higher H / C than those of R1 and R2, the explanation being that R3 is more reactive than R2 and its composition is less aromatic (Table 3). R3 semicokes contain most of the heteroatom content of the original feedstock; similar results have been reported by other authors [11–13]. Heteroatoms are known to cause puffing in cokes, and are environmentally unacceptable.
4. Discussion
3.3.1.3. Solids. Yields for R3 are between 30–40 wt % at 0.1 MPa and 40–45 wt % at 1.0 MPa. Cracking reactions control yields, and these are sensitive to temperature and soak time (Fig. 1(e–f)). 3.3.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 3.3.2.1. Gas analysis. For R3, the percentage of methane in the gases is the lowest for the three feedstocks, but with yields of compounds of three carbon atoms and beyond being the highest (.24 vol % of all gases). R3 also contains the highest percentage of H 2 S, but with most of the sulphur being retained within the semicoke (Table 2). 3.3.2.2. Liquid analysis. Simulated distillation histograms for R3 and its distillates obtained in different conditions are in Fig. 2(c) where it can be seen that 100 % of the liquids come from cracking reactions, i.e. they are all pyrolysis distillates (pd). When analysing the distillates, at 0.1 MPa, the liquid has a wide range of boiling points, mainly 200–3608C. Increasing soak time enhances the liquids with boiling point in the range 100– 3608C. At a pressure of 1.0 MPa, there is an increase of material of boiling point ,2008C. The aromaticity of the distillates of R3 is the lowest of all (being similar to that of the original feedstock). It does not change with pressure, temperature and soak time. In general, it possesses hydrocarbon side-chains, varying in length and with branching. 3.3.2.3. Solid analysis. R3 is the most reactive feedstock (Fig. 4(e–f)). At 4208C, 0.1 MPa develops anisotropy much slower than R1, but when operating at 4608C, R3 develops mesophase faster than R1. As expected, the effect of increasing pressure from 0.1 to 1.0 MPa is negligible. Variations of aromaticity, as ‘n’, are in Fig. 5(a–f) for R1, R2 and R3, at 0.1 and 1.0 MPa, 420–4808C, 0–12 h soak time. The aromaticity of the semicokes varies from 0.25 to 0.8 depending on experimental conditions. There
4.1. Feedstock R1 4.1.1. Effect of experimental parameters on yields 4.1.1.1. Gases. The low yields of gases from R1 result from the aromaticity of this feedstock in association with little in the way of aliphatic side-chains attached to the aromaticity. When increasing pressure from 0.1 to 1.0 MPa the percentage of gases is almost doubled. The reason for this behaviour is that there is material that distills at 0.1 MPa and does not at 1.0 MPa. This additional material that does not distill at 1.0 MPa also undergoes thermal cracking reactions, so increasing the volume of released gases. There is no evidence to suggest that pressure alone promotes thermal cracking reactions. 4.1.1.2. Liquids. A simple model of the pyrolysis process includes the formation of two fractions, a volatile fraction (liquids and gases) and the semi-solid fraction of the semicoke. A volatile fraction (feedstock distillates (fd)), containing molecules of the low boiling point fraction of the feedstock, leaves the reactor without contributing to the pyrolysis reactions. A similar nonreacting fraction (pyrolysis distillates (pd)) results from the cracking of constituent molecules of much higher molecular weight. Dependent on their molecular weights, they are collected as either liquids or gases. Therefore, the measured liquid yields are a summation of volatile material distilled from the feedstock during a pyrolysis, (distinguished as (fd)) plus volatile material created during a pyrolysis (distinguished as (pd)), e.g. by cracking reactions [6]. These two contributions ((fd) and (pd)) can be separated making use of simulated distillation curves using gas chromatography. The limited variation in yields with soak time observed in R1 pyrolysis, once distillation has occurred (after heating time), is the result of limited production of pyrolysis liquids (pd) from cracking reactions. These data indicate that the molecular constituents of R1 essentially
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undergo very limited molecular transformations. Only the elimination of side-chains and condensation reaction seems to occur. Variations of yields with temperature are small because: (i) most of the compounds of the feedstock have boiling points ,3608C, and there is only a small fraction with boiling points between 420–4608C (7 wt %), and (ii) there is a low production of liquids coming from cracking reactions (pd). Fig. 7(a) shows the variations in liquid distillates (wt %) for the experiments R1-420-1.0 / 1.0, R1-440-1.0 / 1.0, and R1-460-1.0 / 1.0. This diagram shows that, when heating at 1.0 MPa, 15–40 wt % of the original material is removed. Very little material is lost between the heating and the depressurisation stages (maximum of 5 wt %) implying that little in the way of liquids comes from cracking reactions, the remainder being obtained during depressurisation (20–30 wt %). As the temperature increases from 420 to 4608C more material is removed during heating and less is removed during depressurisation. When operating at 0.1 MPa, most of the liquid originates during the heating stage, as there is no depressurisation, meaning that during soak time about 25–30 wt % of the initial feedstock remains in the reactor. At 1.0 MPa, after the heating period and before depressurisation, there exists about 60–70 wt % of the original feedstock in the reactor. Accordingly, the chemis-
try of the pyrolyses, at 0.1 and 1.0 MPa pressures, differs from each other.
4.1.1.3. Solids. The short variation of solid yields with soak time is caused by the limited distillation of liquids (pd) generated during pyrolysis, as well as gases. Mainly, almost all liquids are produced during heating time (fd), and the amount of gases produced during pyrolysis is lower than 5 wt %, even at the highest temperature. So, pressure, by reason of its effect on feedstock distillates (fd), controls yields. The low influence of temperature is due to: (i) the feedstock has little material which distill between 420–4608C (Table 1); and (ii) there is little production of cracking products (gases and liquids). 4.1.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 4.1.2.1. Gas analysis. The gases come principally from the breaking of bonds, a to the aromatic ring: later, during the pyrolysis process, gases come from cleavage of aliphatic compounds with long branched side-chains, of higher molecular weight. The hydrogen comes from the stabilisation of an alkyl radical with formation of a double bond and from breakage of aromatic compounds with the
Fig. 7. Variation of weight of liquid (wt %) with soak time (h) and pyrolysis temperature (8C) for five pyrolysis experiments: R1-420-1.0 / 1; R1-440-1.0 / 1 and R1-460-1.0 / 1. (b) R2-440-1.0 / 1 and R2-480-1.0 / 1.
´ M. Martınez-Escandell et al. / Carbon 37 (1999) 1567 – 1582 Table 4 H 2 evolved as gases per H 2 in the reactor for one pyrolysis experiment (460-1 / 1) Feedstock
H 2 gases / H 2 reactor
R1
R2
R3
1.8
2.9
3.1
formation of a cyclic aromatic radical [14], or aromatisation of naphthenic compounds. For R1, the majority of the aliphatic hydrogen is from H a or H b , from the side-chains, producing gases of low molecular weight. Although the volume percentage of hydrogen is the highest of all the gases, the amount of hydrogen evolved, per total amount of hydrogen in the reactor added as petroleum residue, is the smallest (Table 4). This means that the process of increasing aromaticity is the least extensive of the three feedstocks, mainly because R1 is the most aromatic of the original feedstocks.
4.1.2.2. Liquid analysis. The data indicate that operating at 0.1 MPa, liquids are mainly produced by distillation of original R1 (fd), recovering all liquids of boiling point ,3608C and the light compounds of the heavy fraction (Fig. 2(b)). With increasing temperature there is distillation of heavier compounds. At 1.0 MPa a fraction of the compounds of boiling point 200–3608C does not distill and remains in the reactor. There is also a slight increase of compounds with boiling point 100–2008C compared with the feedstock, indicating the cracking of larger compounds. Yet, the effect of pressure is to suppress liquid yields, increasing slightly the cracking of larger compounds. The additional material retained by pressure within the system, of lower molecular weight, acts as a diluent to the system and retards the formation of mesophase [15–17], giving a small percentage of cracking reactions. The development of the aromaticity of distillates is dependent on pyrolysis conditions. The different behaviours found in the evolution of aromaticity of distillates with increasing soak time at 0.1 and 1.0 MPa are due to the depressurisation. When operating at 0.1 MPa, once distillation has occurred, there is little production of distillates (due to cracking reactions) and their aromaticity is similar or even lower than dis-
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tillates produced during heating. On the other hand, when operating at 1.0 MPa, a fraction of the distillates that distills at 0.1 MPa during heating is now retained in the reaction. This material has more time to eliminate side chains and increases its aromaticity. On depressurisation, part of this light material distills, its aromaticity being higher than that distilled before depressurisation and also higher than operating at 0.1 MPa. The aromaticity of the distillates obtained with depressurisations increases with soak time. Table 5 shows the differences in aromaticity and the percentage of the distillates obtained before depressurisation and after depressurisation. The aromaticity of the whole amount of distillates is also shown. It is seen that aromaticity of distillates obtained with depressurisation is much higher than that obtained before depressurisation. The effect of temperature is to accelerate this phenomena.
4.1.2.3. Solid analysis. Mesophase content increases with soak time or temperature. There is a rapid increase in aromaticity and a decrease of H / C relation as expected with this residue. With increasing pressure light material which does not distill reduces average molecular weights and further reaction time is then needed to produce mesogens which lead to mesophase, slowing down the process of mesophase formation and aromatisation. This residue produces highly ordered mesophase as it was expected from a highly aromatic feedstock. 4.2. Feedstock R2 4.2.1. Effect of experimental parameters on yields 4.2.1.1. Gases. The effect of experimental parameters on gas yields is similar. An increase of yield of gases from R2 with respect to R1 is due to its higher content of aliphatic side-chains. The behaviours of R1 and R2 are similar in that there is an increase in gas yield with increasing pressure, caused by more material being in the reactor due to pressure. 4.2.1.2. Liquids. The effect of experimental parameters on liquid yields differs from R1. In R1 yields are mainly affected by pressure, effects of temperature and soak time
Table 5 Aromaticity of liquids from R1 and R2 before and after depressurisation Experimental conditions
Yield of distillates before depressurisation (wt %)
Yield of distillates after depressurisation (wt %)
H ar (%) of distillates before depressurisation
H ar (%) of distillates after depressurisation
H ar (%) of total distillates
R1-460-1 / 1.5 R2-440-1 / 6
75 61
25 39
50 16
62 5
52 33
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being of less importance. In R2, the effect of temperature is more pronounced, the use being helpful of simulated distillation histograms of the feedstock (Table 1, Fig. 3(b)). At 3608C, only 17 wt % is distilled from the feedstock; between 440–4808C as much as 52–76 wt % is produced. This is typical of compounds with a homogeneous molecular weight distribution and with a narrow band of boiling points. In this way, temperature (more so at 0.1 MPa) and pressure exert a major influence on production of distillates. Cracking reactions also have an important role; thus, yields vary with soak time. Fig. 7(b) shows the variation of weight of distillates with soak for two temperatures (a) at R2-440-1.0 / 1, and (b) at R2-480-1.0 / 1. The distillations of R2 differ from R1. Operating at 4408C and 1.0 MPa there is no distillation during heating and about 15 wt % of liquids is lost during the soak period as products of cracking reactions. The remainder (|30 wt %) is produced during depressurisation. When operating at 4808C and 1.0 MPa, 25 wt % of the original feedstock is evolved as distillates (fd), about 15 wt % during the soak time, as pyrolysis distillates (pd), the remainder being evolved during depressurisation, |10 wt %. Amounts of distillates from cracking reactions (pd) increase compared with R1. As occurred with R1, amounts of liquids obtained during depressurisation decrease as the temperature increases. With R2, the fraction obtained during the second distillation is less than for R1, but higher yields of solids are obtained when operating at 1.0 MPa, Fig. 7(a–b).
4.2.1.3. Solid. Solid yields of pyrolysis of R2 vary with pressure, temperature and soak time. Variation of solid yield are a consequence of what occurred with liquids and gases. Pressure exerts each effect as it controls distillation, and there is an important fraction that can distill or not depending on the pressure, thus affecting solid yields. Temperature affects in two ways: (i) when increasing temperature there is an increase in liquids distilled during heating; (ii) there is also an increase of distillates due to cracking reactions. Soak time affects more, because there is more production of distillates and gases due to cracking reactions (mainly at 1.0 MPa.) 4.2.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 4.2.2.1. Gas analysis. For R2, it is aliphatic hydrogen which is responsible for the gas composition, and the length of chains of molecules within R2. Because of this, there is an increase in the average molecular weight of the gases with respect to R1. R2 produces larger amounts of hydrogen, methane and hydrocarbons of higher molecular weight than R1, indicating more extensive cracking reactions leading to smaller molecules. R2 evolves a higher percentage of overall hydrogen in the reactor than R1 indicating that it has had to undergo further molecular transformations to give products similar to R1 (Table 4).
4.2.2.2. Liquid analysis. As occurred with R1 distillates, with R2 distillates there is an enrichment in low boiling point compounds (,3608C). This enrichment indicates that distillates are produced by cracking of larger molecules (higher boiling point). This phenomenon is enhanced when operating at 1.0 MPa because at this pressure more material is available for cracking reactions, because of retention of higher amounts of the .3608C fraction. The percentage of distillates coming from cracking reactions is higher in R2 than in R1 (10–25 versus 3%, respectively). This indicates that the stability of the heavy fraction of R1 is higher than R2 (they are more aromatic). Although the aromaticity of the distillates from R2 is much less than that of R1, the changes with respect to increasing temperature and soak time are similar. The aromaticity of distillates from R1 and R2, at 1.0 MPa, increase with soak time. The reason is that, at 1.0 MPa, liquids from the second distillation are more aromatic than the rest, because they have been in the reactor longer, and so have had more time to aromatise and eliminate sidechains (Table 5). Aromaticity of the distillates increases significantly after depressurisation. Extents of aromaticity before depressurisation are close to those in a pyrolysis at 0.1 MPa pressure. 4.2.2.3. Solid analysis. The behaviour of R2 is different (Fig. 5(c and d)). The anisotropy content increases with soak time and temperature, but higher temperatures (208C) are needed to achieve the same anisotropy contents as with R1. According to the low aromaticity of the feedstock a higher reactivity was expected. Whereas it is generally understood that reactivity in terms of mesophase formation increases with increasing aromaticity of the feedstock, the behaviour of R2 indicates that there can be exceptions to this general rule. It is difficult in such a complex system to separate out the competing influences of radical formation, radical capping via hydrogen transfer reactions, dehydrogenation and dealkylation reactions as well as cyclisation processes. In this way R2 gives typical products for a low reactive system in which low viscosity conditions have occurred during pyrolysis. The influence of pressure is much less than is in R1. It was expected that there exists wider differences in extents of anisotropy when pressure is increased from 0.1 to 1.0 MPa, as occurred in R1 (in R2 yields vary considerably with pressure). The composition does not change much (is more homogeneous than in R1) with pressure and does not affect as much the development of anisotropy. 4.3. Feedstock R3 4.3.1. Effect of experimental parameters on yields 4.3.1.1. Gases. R3 has percentages of gases very similar to R2 (operating at lower temperatures than R2) and more than R1 because it contains a greater proportion of material that cracks into smaller molecules. NMR spectra of the
´ M. Martınez-Escandell et al. / Carbon 37 (1999) 1567 – 1582
feedstocks indicate that the percentage of chains in R2 and R3 is much greater than in R1, the most aromatic of the feedstocks, Table 1. Pressure does not affect gas yields mainly because there is little change in amounts of material in the reactor with pressure (Fig. 1(e–f)).
4.3.1.2. Liquids. In R3 Pyrolysis, no material (fd) is distilled from the feedstock during the heating-up stage, all liquids coming from cracking reactions of the high molecular weight constituent material. Extended soak times and higher reaction temperatures promote cracking reactions. Effects of pressure are not as important as with R1 and R2 because of the lack of sensitivity of cracking reactions to pressure, unlike the distillations of materials from R1 and R2, during heating-up (Fig. 1(e–f)). 4.3.1.3. Solids. Solid yields decrease with temperature and soak time because of the evolution of products of cracking (liquids and gas). Temperature and soak time promote cracking reactions but pressure has a negligible effect (Fig. 1(e–f)). 4.3.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 4.3.2.1. Gas analysis. Gases of R3 have a higher molecular weight than for R1 and R2. Generally, the average molecular weight of gases evolved in a pyrolysis increases as the aliphatic nature of the feedstock increases. Amounts of hydrogen evolved per hydrogen in the reactor is the highest of the three feedstocks, indicating that R3 is undergoing the largest molecular transformations of the three feedstocks during formation of semicokes (Table 4). 4.3.2.2. Liquid analysis. The simulated distillation diagrams of R3 liquids (Fig. 2(c)) indicate that all liquids come from cracking reactions. With increasing soak time, low boiling point compounds are produced. With increasing temperature, molecules of higher stability are cracked, resulting in a distillate of higher molecular weight. When increasing pressure, there is more material in the reactor, therefore, cracking reactions are ‘promoted’, increasing amounts of material of low boiling point (,2008C) (simply as a consequence of having increased amounts of material in the reactor). As expected, aromaticity of distillates is almost constant during pyrolysis. In contrast with R1 and R2, the distillates from R3 hardly change in aromaticity with soak time at 1.0 MPa, noting that extents of material obtained in the second distillation are minimal. 4.3.2.3. Solid analysis. Mesophase develops slowly at high temperatures, the rate increasing with temperature. This is the typical behaviour of a reactive system. This behaviour is expected as it is an aliphatic feedstock (Fig. 4(e–f)). Clearly, the amount of aromaticity for the same tem-
1581
perature in the semicoke from R1, Fig. 5(a–b) is the highest, followed by R2, Fig. 5(c–d) with low values of aromaticity of semicokes from R3, Fig. 5(e–f). The lower aromaticities of the semicokes of R3 may be associated with its higher molecular reactivity, enabling mesogens to be formed early on and these may not have enough time to lose completely some of their aliphatic side-chains. Differences in aromaticity with pressure do not exist as there is no variation in amount of material in the reactor and highly aromatic compounds are not diluted. Mesophase formed from different petroleum residues differ not only in structure but also in H / C content and aromaticity (‘n’ parameter). R1 and R2 develop mesophase in the form of domains or flow domains, which have a low H / C ratio and high ‘n’ parameter, while R3 develops mesophase in form of mosaics of higher H / C ratio and lower aromaticity (‘n’ parameter), Table 3.
5. Conclusions
1. A laboratory pilot plant has been designed to provide a deeper insight into the chemistry of pyrolysis reactions of petroleum residues by following, simultaneously, the formations of solids, liquids and gases during the entire process under a wide range of pyrolysis conditions. 2. Effects of carbonisation parameters (pressure, temperature and soak time) depend on the chemical composition of original feedstock. For semicokes from R1 (the most aromatic), yields are dominantly a function of pressure, with little influence of temperature and soak time. For semicokes from R2 (intermediate aromaticity), yields are dominantly a function of pressure and temperature, with little influence of soak time. For semicokes from R3 (least aromatic), yields are dominantly a function of temperature and soak time, with little influence of pressure. Increasing pressure increases solid yields, thus reducing liquid yields. Increases are larger for R1 and R2 which contains larger amounts of light material which distills during heating. Increases are smaller for R3 which contains larger amounts of material which undergo cracking reactions. 3. The use of pressure release at reaction temperatures causes a second distillation of volatile materials, amounts of material decreasing with increasing temperature and soak time. In the pyrolysis of R1, yields are dominantly controlled by distillation from the system. In the pyrolysis of R2, yields are dominantly controlled by both distillation from the system and by cracking reactions of molecular constituents (elimination of alkyl side-chains). In the pyrolysis of R3 yields are dominantly controlled by cracking reactions of molecular constituents (elimination of alkyl sidechains). 4. Simulated distillation procedures enable a distinction to be made between liquids coming from distillation from
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the original feedstock or from cracking reactions. For example, only 5% of the liquids of R1 come from cracking reactions, 22% from R2, while in R3 all liquids derive from cracking reactions. 5. R1 (most aromatic feedstock) produces semicokes with a high aromaticity and flow domain textures while R3 (most aliphatic) produces semicokes with the lowest aromaticity and an optical texture of coarse mosaics. R2 (intermediate aromaticity) reacts more slowly than expected to give highly ordered mesophase. Differences in aromaticity are not enough to explain differences in reactivity and the resultant optical textures of mesophase. Whereas it is generally understood that reactivity in terms of mesophase formation increases with increasing aromaticity of the feedstock, the behaviour of R2 indicates that there can be exceptions to this general rule. It is difficult in such complex systems to separate out the competing influences of radical formation, radical capping via hydrogen transfer reactions, dehydrogenation and dealkylation reactions as well as cyclisation processes.
Acknowledgements Support from BRITE-EURAM Program, Contract BRE.2CT.0901 is acknowledged. H.M. thanks the Spanish DGICYT (SAB95-0086).
References [1] Heintz EA. Carbon 1996;34:699.
[2] Marsh H, Diez M-A. In: Shibaev VP, Lam L, editors, Liquid crystalline and mesomorphic polymers, New York: SpringerVerlag, 1994, pp. 231–57, Chapter 7. [3] Marsh H, Latham CS. The chemistry of mesophase formation in: Bacha JD, Newman JW, White JL, editors, Petroleum derived carbons, American Chemical Society, Washington DC, USA, 1986, pp. 1–28, Symposium series 303. [4] Greinke RA. In: Thrower PA, editor, Chemistry and physics of carbon, Vol. 24, New York: Marcel Dekker, 1994, p. 1. [5] Mochida I, Fujimoto K, Oyama T. In: Thrower PA, editor, Chemistry and physics of carbon, Vol. 24, New York: Marcel Dekker, 1994, p. 111. ´ ´ [6] Santamarıa-Ramırez, R. Ph.D. Thesis. Spain: University of Alicante 1992. ´ ´ ´ E., Salazar, C.G., [7] Santamarıa-Ramırez, R., Romero-Palazon, ´ ´ ´ Rodrıguez-Reinoso, F., Martınez-Saez, S., Martınez-Escandell, M., Marsh, H. Carbon, in press, 1999. ´ [8] Romero-Palazon, E. Ph.D. Thesis. Spain: University of Alicante, 1990. ´ ´ E, Diez [9] Rodrıguez-Reinoso F, Santana P, Romero-Palazon M-A, Marsh H. Carbon 1998;36:105. [10] Adams HA. In: Marsh H, Heintz EA, Rodriguez Reinoso F, editors, Introduction to carbon technologies, Universidad de Alicante: Secretariado de Publicaciones, 1997, p. 71. [11] Jakob RR. Hydrocarbon Processing 1971;50:132. [12] Debiase R, Elliott JD, Harnett TE. In: Bacha JD, Newman JW, White JL, editors, Petroleum derived carbons, Washington DC, USA: American Chemical Society, 1986, p. 155, Symposium series 303. [13] Lieberman HP. Oil Gas J 1980;78:71. [14] Greinke RA. Carbon 1992;30:407. [15] Makabe M, Itoh H, Ouchi K. Carbon 1976;14:365. [16] Park YD, Mochida I. Carbon 1989;27:925. [17] Rhee B, Chung DH, In SJ, Edie DD. Carbon 1991;29:343.
Carbon 37 (1999) 1567–1582
Pyrolysis of petroleum residues: I. Yields and product analyses a a a a, *, ´ ´ M. Martınez-Escandell , P. Torregrosa , H. Marsh , F. Rodrıguez-Reinoso b c c ´ ´ ´ ´ R. Santamarıa-Ramırez , C. Gomez-De-Salazar , E. Romero-Palazon a
´ ´ Departamento de Quımica Inorganica , Universidad de Alicante, E-03080 Alicante, Spain b Instituto Nacional del Carbon, Oviedo, Spain c ´ ´ de Cartagena, Cartagena, Spain REPSOL-PETROLEO , Centro de Investigacion Received 17 April 1998; accepted 22 January 1999
Abstract This study examines the pyrolysis of three petroleum pitch residues of different aromaticities, (R1), (R2) and (R3), varying the experimental parameters of pressure, temperature and soak time. The overall objective is to provide further detailed information of factors which influence formation of anisotropy or mesophase in resultant semicokes. Pressure was varied during the progress of a pyrolysis. Yields of gases, liquids and semicokes were obtained. Gases were analysed by gas chromatography, the liquids by simulated distillation and 1 H-NMR, and the semicokes by elemental analysis and FTIR. For the semicokes from R1, yields are dominantly a function of pressure, with little influence of temperature and soak time. For semicokes from R2, yields are dominantly a function of pressure and temperature, with little influence of soak time. For semicokes from R3, yields are dominantly a function of temperature and soak time, with little influence of pressure. The use of simulated distillation and pressure release, at reaction temperatures, provides additional information about mechanisms of the pyrolysis reactions. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Mesophase, Petroleum pitch, Pyrolysis, Semicoke; C. Infrared spectroscopy, Nuclear magnetic resonance (NMR)
1. Introduction Carbon anodes for the aluminium industry and graphitic electrodes for the steel industry, as well as many smaller applications, use delayed cokes (sponge- and needle-coke) as major constituents in their formulations [1]. In turn, delayed cokes are made by the industrial process of delayed coking, where selected petroleum residues or feedstocks are heated in vertical cylindrical drums, the inlet temperatures of the feedstock being |5008C. The feedstock is converted to a coke (of various qualities) via a mechanism involving the formation of mesophase [2,3]. Greinke [4] reviewed the early stages of the chemistry of petroleum pitch carbonisation (kinetics and mechanisms), and referred to the importance of mesophase formation. Mochida et al. [5] reviewed the chemistry and industrial operations in the production and utilisation of delayed cokes. Ultimately, it is the chemistry of the pyrolysis system which controls the structure and prop*Corresponding author. Fax: 134-6-590-34-54. ´ E-mail address: [email protected] (F. Rodrıguez-Reinoso) 0008-6223 / 99 / $ – see front matter PII: S0008-6223( 99 )00028-7
erties of resultant semicokes / cokes / carbons. But, the chemistry, in turn, is a function not only of the chemical properties of the original feedstocks, but also of the physical parameters incorporated into the system, such as carbonisation capacity (size of reactor), heating rate, soak time, final reaction temperature and pressure within the system. Dependent upon how the pyrolysis system is constructed, e.g. whether or not volatile material is lost from or retained within the pyrolysis system, these physical parameters control the chemical composition of the feedstock as it is pyrolysed through to a semicoke [6,7]. ´ et al. [8], Rodrıguez-Reinoso ´ Romero-Palazon et al. [9] and Adams [10] showed how multiphase systems within bulk pyrolysing systems influence coke quality and how gas evolution (amounts and temperatures of evolution) also influence coke quality, controlling how and when mesoph´ ´ ase formation occurs. Santamarıa-Ramırez et al. [6,7] surveyed those effects, chemical and physical, which influence mesophase growth and coalescence of mesophase within semicokes. The chemistry of carbonisation of petroleum residues and coal–tar pitches is complicated. The chemical re-
1999 Elsevier Science Ltd. All rights reserved.
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actions involve the thousands of molecular species contained within these materials. It is necessary to establish mechanisms of growth in molecular size, at the molecular level, in order to understand the patterns of molecular behaviour during pyrolyses and carbonisations, leading to mesophase and semicoke. Such molecular behaviour, as well as being a function of the chemical composition of the feedstock, is also a function of the rate of heating (Rc), of the final reaction temperature (T ), the time at temperature (soak time (tc)), pressure (P), and size and geometry of the reaction vessel containing the feedstock which is maintained in the system. These factors together control the overall behaviour of coking and the properties of the final product (semicoke or coke) [6]. The physico-chemical behaviour of coking, which also includes flow movement or turbulence within the liquid (fluid) carbonisation system, caused by thermal flow and gas evolution from within the system, also affects semicoke properties [8]. The objective of this study is to have an overall understanding of these processes and how they relate to the properties of the semicoke and any materials derived from these semicokes. More specifically, this study, part of a more comprehensive study of mechanism and kinetics of petroleum residue pyrolysis, ascertains the influence of the different chemistry of different feedstocks, as well as different operating parameters (Rc, T, tc, P) on (i) yields of semicoke, liquids and gases (to obtain a mass balance of the pyrolysis process) and on (ii) the composition of these products of pyrolysis. The results are relevant to the industrial process of delayed coking and to the formulation of new carbon materials which may be produced from the semicokes ultimately derived from petroleum residues.
2. Experimental
6008C. Heating rates were constant at |158C / min. The reactor tube was made of Pyrex glass, 65 mm diameter, 210 mm height and containing 350 g of feedstock. The tube was placed within the steel block which was within the sand bath (to ensure a maximum uniformity of temperature). All pyrolyses were carried out under an atmosphere of nitrogen, (99.999% pure), the nitrogen also being used to establish pressures .0.1 MPa in the system. In order to have a wide range of pyrolysis conditions (at constant heating rate), variations were introduced into the maximum pyrolysis temperature (T ) 420–4808C, the pressure (P) 0.1–1.0 MPa, and soak time 0–12 h, in various combinations. R2 is a feedstock which develops mesophase more slowly than R1 and R3, and for a level of development equal to that of the other two feedstocks, the reaction temperature had to be increased by 208C. When operating at 1.0 MPa, the system was depressurised at reaction temperatures at the end of each experiment and then cooled to room temperature. In these pyrolysis experiments, where pressure is reduced, on reduction of the pressure, or depressurisation, there occurs a second distillation. Note: there is no need to depressurise the system when operating at 0.1 MPa pressure.
2.2.2. Pyrolysis yields Mass of liquids was obtained during the experiment by opening the valve of the condensers. No loss of gases was detected while taking out the liquids. The volume of gases produced during the pyrolysis is obtained using a gas meter. Mass of gases is calculated using an average molecular weight obtained by gas chromatography. Mass of semicoke was obtained at the end of the experiment. During the experiment mass of semicoke can be calculated by difference. Gas, liquids and semicoke yields are expressed in terms of the original feedstock. Mass balances were obtained with an accuracy of |1 wt %.
2.1. Materials used 2.3. Gas analysis Three types of feedstocks are used as starting materials for the study:
(R1), an aromatic feedstock. (R2), a feedstock of relatively low aromaticity. (R3), an almost totally aliphatic feedstock.
2.2. Experimental methods 2.2.1. Pyrolyses of the feedstocks The pyrolysis of feedstocks was carried out in a laboratory pilot plant already described [7,9]. The special features of this system include a maximum sample capacity of 1000 cm 3 and an ability to work between the pressures of 0.1 and 4.5 MPa. Heating of the reaction tube occurred in a preheated fluidised sand-bath furnace, with a temperature control of 618C at 508C and of 628C at
The product gases were analysed using gas chromatography, model HP-5890 series II.
2.4. Liquid analysis During the experiments, liquids were removed from the system by opening the valve to the condensers. Hence, curves of the variation of amounts of distillates with time were obtained. The aromaticity of the liquids was determined by 1 H-NMR using DCCl 3 as the solvent and tetramethylsilane (TMS) as the internal standard. The integration intervals were: Hydrogen attached to aromatic carbon (H ar ) of 9–6 ppm, hydrogen of methyl groups attached to aromatic rings (H a-2 ), 4.5–3.5 ppm, hydrogen attached to a-carbon atoms (H a ) of 3.5–2 ppm, hydrogen attached to b-carbon atoms (H b ) of 2–1 ppm, and hydrogen attached to g-carbon atoms (H g ) of 1–0.5 ppm.
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In addition, the liquids were studied by simulated distillation, using chromatography, model HP-5890 series II, following the accepted ASTM D28-87 standard procedure.
2.5. Semicoke analysis The resultant semicokes were analysed for element content using a Carlo Erba Eager 200 CHNSO elemental analyser. Semicokes were examined for aromaticity by diffuse reflectance infrared spectroscopy, using a Nicolet 510P instrument. Using the computer programme Omnic v1.0, areas of absorption corresponding to C–H ar vibrations (2990–3150 cm 21 ) and C–H al (2800–2990 cm 21 ) were obtained. The aromaticity parameter ‘n’ was calculated as the ratio of the two areas for each semicoke. All samples were mounted in a resin block, and optically polished surfaces were examined by reflected polarised light microscopy. The percentage contents of isotropic and anisotropic (A) constituents were measured by analysing 25 fields of the semicoke; a total of 2500 points were counted for each sample. The nomenclature used to describe carbonisation experiments is: R-T-P/t, where R5petroleum residue, T5pyrolysis temperature (8C), P5pressure (0.1 or 1.0 MPa), and t5soak time (h).
3. Results
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Table 1 Compositional analyses of petroleum residue feedstocks R1 Elemental analyses
Solubility
1
H-NMR
Simulated distillation
R2
R3
C (wt %) H (wt %) N (wt %) S (wt %) HI (wt %) TI (wt %)
90.2 7.0 0.1 0.2 13 0
87.1 9.7 0.1 1.1 0 0
85.2 10.4 0.2 3.8 0 0
H ar (%) H a-2 (%) H a (%) H b (%) H g (%)
51 5 27 15 2
19 1 19 44 17
8 0 10 59 23
25–1008C 100–2008C 200–3608C 360–4208C 420–4408C 440–4608C 460–4808C .4808C
0 5 60 12 3 4 4 12
0 2 15 21 14 14 10 24
0 0 0 0 0 0 0 100
C 1 H 0.9
C 1 H 1.3
C 1 H 1.5
Reduced empirical formulae
between 420–4608C; therefore, large differences in yields due to increasing temperature are not expected.
3.1. Feedstock R1 3.1.1. Effect of experimental parameters on yields Table 1 contains the analyses of the three feedstocks used. The three feedstocks are different chemically, one from the other. The values presented in Table 1 are considered as examples of a wide range of the feedstocks used industrially. However, the observed constituent differences are sufficiently distinct that behavioural differences are to be expected. R1 is relatively rich in carbon with a low content of heteroatoms (0.3 wt %). It is an aromatic feedstock, ((H ar )551%). There is a majority of hydrogen, as methyl hydrogen, attached to the aromatic rings. It has the highest insolubility in heptane (13 wt %). When studying the simulated distillation data, it is seen that 65 wt % of the compounds have boiling points lower than 3608C. This material distills during heating before reaching reaction temperatures (higher than 3608C). It also has a heavy fraction (12 wt %) with boiling point higher than 4808C. This amount coincides with the percentage of material insoluble in heptane, indicating that in this feedstock there exists a heavy fraction, highly condensed, which needs limited transformations to become mesogens. This heavy fraction does not distill and can be considered as the nearly minimum yield that could be obtained from this feedstock. There is little material with boiling points
3.1.1.1. Gases. Figs. 1(a–b) describe the variations of yields of gases, liquids and solids, with soak time (h), at 0.1 and 1.0 MPa pressure, for pyrolysis of R1. The yields of gases are only a small fraction of the original material. The feedstock R1 has the lowest quantity of gas evolution (,5 wt %) of the three feedstocks. Pressure exerts a major influence on gas yields, causing an increase from 2.2 to 5.5 wt % (4608C, 3 h) when the pressure is raised from 0.1–1.0 MPa, i.e. more than a doubling in the quantity of gas evolved. 3.1.1.2. Liquids. Yields of liquids from R1, Figs. 1(a–b), vary little with residence time (5 wt %), increasing slowly with increasing temperature, 420–4608C, (5 wt %) but more noticeably with increasing pressure, 0.1–1.0 MPa, (20 wt %), being between 52 and 75 wt %. 3.1.1.3. Solids. The variations in yields of semicoke (solids), Figs. 1(a–b) are similar to those described for the liquids because the amounts of gases are relatively small. The yields from R1 are almost constant with residence time, and vary little with temperature, but vary signifi-
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Fig. 1. Variation of yield (wt %) of gases (– –), liquids (---) and solids (—) with soak time (h), temperature (4208C (j), 4408C (m), 4608C (d), and 4808C (♦)) and pressure (MPa) for the feedstocks R1, R2 and R3.
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cantly with pressure of pyrolysis. Dependent on the pressure used, yields vary between 30–45 wt %.
3.1.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 3.1.2.1. Gas analysis. Gases produced during pyrolysis of R1 (R1-440-1.0 / 3) were collected to be compared with gases from R2 (R2-440-1.0 / 3) and R3 (R3-440-1.0 / 3), for the same pyrolysis conditions. Gases produced from R1 are from short-chain substituents, mainly methane and ethane together with a significant quantity of hydrogen (Table 2). Contents of compounds with three carbon atoms are quite low (,2 vol % of all gases). Similarly, the production of H 2 S is low (,1 vol % of all gases). 3.1.2.2. Liquid analysis. The simulated distillation data for R1 and its pyrolysis liquids (distillates) coming from different conditions are in Fig. 2(a), where the first histogram describes the feedstock and the rest of the histograms describe the liquids obtained during pyrolysis. These data refer to weights of original feedstock. The histograms of the feedstock were compared with those of the liquids obtained during pyrolysis to determine if these liquids come from distillation of the original feedstock or from cracking reactions. For R1, 5 wt % of the original feedstocks boils between 100–2008C, 60 wt % boils between 200–3608C, and 30 wt % boils .3608C. When operating at 0.1 MPa, 4208C, zero soak time, distillates of boiling point ,3608C are a little lower (2 wt %) than in the feedstock. The heaviest fraction of the feedstock, boiling point .3608C, decreases in the distillates, stays within the reactor and eventually forms mesogens and mesophase. With increasing soak time to 12 h, distillates of boiling point ,3608C increases, fraction 100–2008C being slightly higher than in the original feedstock (1 wt %). At 4608C, fraction ,3608C are slightly higher than in feedTable 2 Analyses of gases pyrolysis gases (vol %) Gas
Feedstock R1
R2
R3
CH 4 Hydrocarbon 2-C Hydrocarbon 3-C Hydrocarbon 4-C Hydrocarbon 5-C Hydrocarbon 6-C or higher H2 H2S
62.8 19.4 6.3 1.4 0.3 0.5
57.3 16.9 10.0 4.1 1.2 1.5
40.6 19.8 13.3 7.3 1.0 3.1
8.3 0.9
5.4 3.8
6.3 8.3
Mean molecular weight (amu)
20.5
24.5
28.0
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stock (3 wt %), and fraction .3608C increases when comparing to that of 4208C. Operating at 1.0 MPa, 4608C, comparing with 0.1 MPa, there is a decrease of compounds of boiling point .2008C in the distillates, but compounds of boiling point ,2008C increase (3 wt %). The variation of H ar with soak time, for liquids from R1 (4608C, 0.1–1.0 MPa) are shown in Fig. 3. The aromaticity of the distillates from R1 is clearly the highest of all three feedstocks, being similar to that of the original feedstock. 1 H-NMR of the distillates of R1 shows that the aromaticity at 0.1 MPa is independent of soak time, but at 1.0 MPa the aromaticity increases with increasing soak time. The effect is more pronounced with increasing temperature and residence time.
3.1.2.3. Solid analysis. For feedstock R1, an increase in soak time causes an increase in the anisotropy content, i.e. operating at 4208C, 0.1 MPa, anisotropy increases from 0 to 80% in 12 h soak time (Figs. 4(a–b)). An increase in temperature accelerates mesophase development, i.e. operating at 4608C, 0.1 MPa, anisotropy increases from 0 to 80% in 3 h soak time. The effect of increasing the pressure from 0.1 to 1.0 MPa is to slow down the process of formation of mesophase. When analysing the evolution of aromaticity of the semicokes by FTIR (Figs. 5(a–b)), aromaticity follows similar trends as observed for mesophase. It increases with soak time and temperature, i.e. operating at 4208C, 0.1 MPa, it increases from 0.35 to 1 in 12 h, operating at 0.1 MPa and 3 h soak time. With increasing temperature from 420 to 4608C, aromaticity increases from 0.4 to 1.5. The effect of pressure is rather different from mesophase development. Operating at 420 and 4408C, aromaticity increases slightly more at 0.1 MPa than at 1.0 MPa (i.e. 1.0 and 0.75, 420-0.1 / 12, 420-1.0 / 12, respectively). Operating at 4608C and 3 h soak time, the aromaticity is higher at 1.0 MPa (1.5–2.3 and 0.1–1.0 MPa, respectively). R1 initially develops mesophase in the form of spheres (.30 mm) which leads to domains or flow domains (Fig. 6(a)). The chemical analyses of the semicokes, Table 3, indicates that the ratio H / C decreases with temperature and soak time, H / C varying from 0.66 to 0.55 with increasing soak time from 0.5 to 6 h (4408C, 0.1 MPa). In R1 the heteroatom content is almost zero (N: 0.1 wt % and S: 0.2 wt %). 3.2. Feedstock R2 The feedstock R2 is intermediate in properties between R1 and R3. The simulated distillation data indicate that R2 molecules have a more continuous boiling points distribution than R1 (Table 1). Only 17 wt % of the feedstock compounds have boiling points below 3608C. Most of the feedstock is in the reactor when reaching reaction temperatures. Almost 40 wt % of the feedstock has boiling points between 420 and 4808C; therefore, large variations in
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Fig. 2. Simulated distillation distributions of liquid distillates both from within the feedstock (fd) and from pyrolysis reactions (pd) of R1, R2 and R3.
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wt % at 0.1 MPa, to 40–50 wt % at 1.0 MPa. Solid yields from R2 are influenced more than solid yields from R1, by temperature, i.e. varying from 36 to 14 wt % when increasing temperature from 440 to 4808C (0.1 MPa, 3 h). Pressure also exerts an influence, yields increasing more than 20 wt % with increasing pressure. Soak time also affect yields, mainly in the first 2 h (i.e. operating at 4408C, 0.1 MPa, yields decreasing from 55 to 35 wt %) (Figs. 1(c–d)).
3.2.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 3.2.2.1. Gas analysis. R2, when compared with R1, produces smaller quantities of methane and ethane but larger amounts of compounds with three carbon atoms or more (17 vol % of all gases). Similarly, the production of H 2 S is also higher (4 vol % of all gases) (Table 2). Fig. 3. Variation of content of H ar of all liquid distillates with soak time and pressure, at pyrolysis temperatures of 4608C (R1, R3) and 4808C (R2).
yields with increasing temperature are expected. It has 24 wt % with boiling points higher than 4808C, but it contains no material insoluble in heptane, indicating that this heavy fraction is highly aliphatic, in contrast to the heavy fraction of R1. It is a feedstock much less viscous than R1 and R3 at room temperature. Aromaticity is intermediate between R1 and R3 (Table 1).
3.2.1. Effect of experimental parameters on yields 3.2.1.1. Gases. R2 has a higher yield of gas than R1, reaching a maximum of 16 wt % (R2-440-1.0 / 12). There is also a significant decrease in gas evolution at the lowest pressure of 0.1 MPa, 16–.7 wt %, (the gas yield increasing with increasing temperature). At 4808C, 3 h, the yields decrease, 13–.3.0 wt %, 1.0–.0.1 MPa (Figs. 1(c–d)). 3.2.1.2. Liquids. R2 is a feedstock which generates mesophase more slowly than the other two feedstocks. To obtain equal developments with the semicokes, the pyrolyses of R2 are 208C higher than for R1 and R3. During pyrolyses, R1 and R2 behave differently, in particular with respect to liquid yields, between 31 and 85 wt % for the latter. For R2, experimental conditions significantly affect yields: increasing pressure decreasing liquid yields by 40 wt %: increasing temperature increasing yields by 30 wt %: increasing soak time increasing yields by 10 wt % (Figs. 1(c–d)). 3.2.1.3. Solids. For R2, yields depend more on temperature and pressure, less on soak time. Yields of semicoke vary more than yields of liquids, changing from 12–25
3.2.2.2. Liquid analysis. Simulated distillation curves for R2 and its pyrolysis liquids (distillates) coming from different conditions are in Fig. 2(b). Analysis by simulated distillation of residue R2 reveals that its molecular constituents can be divided into two groups, 15 wt % of the feedstock is distilled between 200–3608C, and 82 wt % .3608C, indicating the homogeneity of molecular weight composition of R2. When analysing the distillates produced during carbonisation by the simulated distillation histograms, for the system at 0.1 MPa, there is an enrichment with compounds of boiling point 200–3608C, compared with the original feedstock, indicating that about 10–15% of the liquids, (pd), come from the cracking of the higher molecular weight molecules. At 1.0 MPa, this percentage is higher (20–22 wt %), as it includes also a fraction in the range 100–2008C. The evolution of aromaticity of liquids with soak time at 4808C and 0.1–1.0 MPa are shown in Fig. 3. At 0.1 MPa, aromaticity of liquids at zero soak time is very similar to the feedstock (H ar 520%), decreasing slowly with soak time. At 1.0 MPa the behaviour differs somehow, at zero soak time aromaticity is lower than that of the feedstock (H ar 515%), being almost constant with soak time. 3.2.2.3. Solid analysis. The development of mesophase (anisotropy) of R2 is shown in Fig. 4(c–d). Anisotropy increases with increasing soak time and temperature. When increasing pressure mesophase development seems to be more slow. When compared with pyrolysis of R1, R2 needs higher temperatures to give similar mesophase contents. The evolution of the aromaticity of the semicoke by FTIR (‘n’ parameter) with experimental conditions is shown in Fig. 5(c–d). As occurred in pyrolysis of R2, ‘n’ increases with soak time and temperature, varying from 0.35 and 2.75 depending on experimental conditions. The
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Fig. 4. Variation of mesophase content measured by optical microscopy (A) of the semicokes with soak time (h), pressure (0.1–1.0 MPa), for pyrolyses of R1, R2 and R3.
effect of pressure is smaller than with pyrolysis of R1, obtaining semicokes of slightly higher aromaticity at 0.1 MPa than at 1.0 MPa. When compared with R1 semicokes operating at the same experimental conditions (Fig. 5(a–
b)), R2 semicokes are less aromatic than R1, differences decreasing with increasing temperature. R2 give mesophase in the form of fluid domains (Fig. 6(b)).
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Fig. 5. Variation of aromaticity of the semicokes (‘n’, C–H ar / C–H al ) with soak time (h), pressure (0.1–1.0 MPa), for pyrolyses of R1, R2 and R3.
As occurred with aromaticity, R2 semicokes have higher H / C than R1 semicokes. For instance, R1-440-0.1 / 1.5 have an H / C50.63 whereas R2-460-0.1 / 1.5 have an H / C50.67, indicating the lower reactivity of R2 (Table 3).
3.3. Feedstock R3 The feedstock R3 contains much less carbon and more in the way of heteroatoms (4.0 wt %) together with more
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Fig. 6. Optical micrographs of semicokes from (a) R1 (R1-440-1.0 / 6); (b) R2 (R2-460-1.0 / 6); (c) R3 (R3-440-1 / 3).
aliphatic hydrogen associated with long-chain paraffinic groups. The content of (H b ) is not only indicative of paraffinic hydrogen attached to the aromatic ring systems but also to totally aliphatic constituent molecules, such as waxes and paraffins of high molecular weight. The high content of (H g ) indicates the presence of substituents of
long molecular chain-length. With R3 there is no volatile loss below 3608C. All the material has boiling points higher than 4808C (Table 1). No distillation during heating, and limited differences with pressure are to be expected. It is a viscous feedstock at room temperature (almost a solid).
Table 3 Elemental analyses of semicokes from pyrolyses of R1, R2 and R3 Experimental conditions
C (wt %)
H (wt %)
N (wt %)
S (wt %)
H/C
R1-440-0.1 / 0.5 R1-440-0.1 / 1.5 R1-440-0.1 / 6 R1-440-1.0 / 0.6 R1-460-0.1 / 3 R2-460-0.1 / 0.5 R2-460-0.1 / 0.5 R2-460-0.1 / 3 R2-480-0.1 / 3 R3-440-0.1 / 0 R3-440-0.1 / 1.5 R3-440-0.1 / 3 R3-440-1.0 / 3 R3-460-0.1 / 3
93.73 94.96 95.58 95.26 95.79 92.33 92.82 93.38 94.60 84.78 85.88 86.73 87.19 87.64
5.20 4.95 4.37 4.74 4.21 5.63 5.15 4.63 3.70 9.55 5.49 4.55 4.56 4.52
0.03 0.06 0.05 0.00 0.00 0.15 0.15 0.22 0.22 1.18 2.05 2.13 1.96 2.28
0.04 0.03 0.00 0.00 0.00 1.89 1.88 1.77 1.57 4.49 6.58 6.58 6.28 5.76
0.66 0.63 0.55 0.60 0.53 0.73 0.67 0.59 0.47 1.35 0.77 0.63 0.63 0.62
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3.3.1. Effect of experimental parameters on yields 3.3.1.1. Gases. Yields of gases vary between 1–13 wt % (of the original feedstock) depending on conditions. Operating at R3-420-0.1 / 3, the yield of gases is 6.5 wt %; at R3-460-0.1 / 3 the yield is 10.1 wt %. Soak time is important. At R3-420-0.1 / 6 yields increases by 7 wt %. Pressure has little influence on gas yields (Fig. 1(e–f)). 3.3.1.2. Liquids. R3, Fig. 1(e–f), differs more from R1 and is closer to R2. Yields of liquids are significantly influenced by temperature and residence time, the effect of pressure being least noticeable. It is the feedstocks with the lowest yield of distillates (5–59 wt %); increasing pressure changes the yields of distillates by 5–10 wt %.
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are no differences in aromaticity with pressure. R3 semicokes are less aromatic than R1 and R2 at equal conditions. R3 develops mesophase in the form of very small spheres in the early stages, leading to a mesophase in the form of coarse mosaics (Fig. 6(c)). R3 semicokes have a higher H / C than those of R1 and R2, the explanation being that R3 is more reactive than R2 and its composition is less aromatic (Table 3). R3 semicokes contain most of the heteroatom content of the original feedstock; similar results have been reported by other authors [11–13]. Heteroatoms are known to cause puffing in cokes, and are environmentally unacceptable.
4. Discussion
3.3.1.3. Solids. Yields for R3 are between 30–40 wt % at 0.1 MPa and 40–45 wt % at 1.0 MPa. Cracking reactions control yields, and these are sensitive to temperature and soak time (Fig. 1(e–f)). 3.3.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 3.3.2.1. Gas analysis. For R3, the percentage of methane in the gases is the lowest for the three feedstocks, but with yields of compounds of three carbon atoms and beyond being the highest (.24 vol % of all gases). R3 also contains the highest percentage of H 2 S, but with most of the sulphur being retained within the semicoke (Table 2). 3.3.2.2. Liquid analysis. Simulated distillation histograms for R3 and its distillates obtained in different conditions are in Fig. 2(c) where it can be seen that 100 % of the liquids come from cracking reactions, i.e. they are all pyrolysis distillates (pd). When analysing the distillates, at 0.1 MPa, the liquid has a wide range of boiling points, mainly 200–3608C. Increasing soak time enhances the liquids with boiling point in the range 100– 3608C. At a pressure of 1.0 MPa, there is an increase of material of boiling point ,2008C. The aromaticity of the distillates of R3 is the lowest of all (being similar to that of the original feedstock). It does not change with pressure, temperature and soak time. In general, it possesses hydrocarbon side-chains, varying in length and with branching. 3.3.2.3. Solid analysis. R3 is the most reactive feedstock (Fig. 4(e–f)). At 4208C, 0.1 MPa develops anisotropy much slower than R1, but when operating at 4608C, R3 develops mesophase faster than R1. As expected, the effect of increasing pressure from 0.1 to 1.0 MPa is negligible. Variations of aromaticity, as ‘n’, are in Fig. 5(a–f) for R1, R2 and R3, at 0.1 and 1.0 MPa, 420–4808C, 0–12 h soak time. The aromaticity of the semicokes varies from 0.25 to 0.8 depending on experimental conditions. There
4.1. Feedstock R1 4.1.1. Effect of experimental parameters on yields 4.1.1.1. Gases. The low yields of gases from R1 result from the aromaticity of this feedstock in association with little in the way of aliphatic side-chains attached to the aromaticity. When increasing pressure from 0.1 to 1.0 MPa the percentage of gases is almost doubled. The reason for this behaviour is that there is material that distills at 0.1 MPa and does not at 1.0 MPa. This additional material that does not distill at 1.0 MPa also undergoes thermal cracking reactions, so increasing the volume of released gases. There is no evidence to suggest that pressure alone promotes thermal cracking reactions. 4.1.1.2. Liquids. A simple model of the pyrolysis process includes the formation of two fractions, a volatile fraction (liquids and gases) and the semi-solid fraction of the semicoke. A volatile fraction (feedstock distillates (fd)), containing molecules of the low boiling point fraction of the feedstock, leaves the reactor without contributing to the pyrolysis reactions. A similar nonreacting fraction (pyrolysis distillates (pd)) results from the cracking of constituent molecules of much higher molecular weight. Dependent on their molecular weights, they are collected as either liquids or gases. Therefore, the measured liquid yields are a summation of volatile material distilled from the feedstock during a pyrolysis, (distinguished as (fd)) plus volatile material created during a pyrolysis (distinguished as (pd)), e.g. by cracking reactions [6]. These two contributions ((fd) and (pd)) can be separated making use of simulated distillation curves using gas chromatography. The limited variation in yields with soak time observed in R1 pyrolysis, once distillation has occurred (after heating time), is the result of limited production of pyrolysis liquids (pd) from cracking reactions. These data indicate that the molecular constituents of R1 essentially
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undergo very limited molecular transformations. Only the elimination of side-chains and condensation reaction seems to occur. Variations of yields with temperature are small because: (i) most of the compounds of the feedstock have boiling points ,3608C, and there is only a small fraction with boiling points between 420–4608C (7 wt %), and (ii) there is a low production of liquids coming from cracking reactions (pd). Fig. 7(a) shows the variations in liquid distillates (wt %) for the experiments R1-420-1.0 / 1.0, R1-440-1.0 / 1.0, and R1-460-1.0 / 1.0. This diagram shows that, when heating at 1.0 MPa, 15–40 wt % of the original material is removed. Very little material is lost between the heating and the depressurisation stages (maximum of 5 wt %) implying that little in the way of liquids comes from cracking reactions, the remainder being obtained during depressurisation (20–30 wt %). As the temperature increases from 420 to 4608C more material is removed during heating and less is removed during depressurisation. When operating at 0.1 MPa, most of the liquid originates during the heating stage, as there is no depressurisation, meaning that during soak time about 25–30 wt % of the initial feedstock remains in the reactor. At 1.0 MPa, after the heating period and before depressurisation, there exists about 60–70 wt % of the original feedstock in the reactor. Accordingly, the chemis-
try of the pyrolyses, at 0.1 and 1.0 MPa pressures, differs from each other.
4.1.1.3. Solids. The short variation of solid yields with soak time is caused by the limited distillation of liquids (pd) generated during pyrolysis, as well as gases. Mainly, almost all liquids are produced during heating time (fd), and the amount of gases produced during pyrolysis is lower than 5 wt %, even at the highest temperature. So, pressure, by reason of its effect on feedstock distillates (fd), controls yields. The low influence of temperature is due to: (i) the feedstock has little material which distill between 420–4608C (Table 1); and (ii) there is little production of cracking products (gases and liquids). 4.1.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 4.1.2.1. Gas analysis. The gases come principally from the breaking of bonds, a to the aromatic ring: later, during the pyrolysis process, gases come from cleavage of aliphatic compounds with long branched side-chains, of higher molecular weight. The hydrogen comes from the stabilisation of an alkyl radical with formation of a double bond and from breakage of aromatic compounds with the
Fig. 7. Variation of weight of liquid (wt %) with soak time (h) and pyrolysis temperature (8C) for five pyrolysis experiments: R1-420-1.0 / 1; R1-440-1.0 / 1 and R1-460-1.0 / 1. (b) R2-440-1.0 / 1 and R2-480-1.0 / 1.
´ M. Martınez-Escandell et al. / Carbon 37 (1999) 1567 – 1582 Table 4 H 2 evolved as gases per H 2 in the reactor for one pyrolysis experiment (460-1 / 1) Feedstock
H 2 gases / H 2 reactor
R1
R2
R3
1.8
2.9
3.1
formation of a cyclic aromatic radical [14], or aromatisation of naphthenic compounds. For R1, the majority of the aliphatic hydrogen is from H a or H b , from the side-chains, producing gases of low molecular weight. Although the volume percentage of hydrogen is the highest of all the gases, the amount of hydrogen evolved, per total amount of hydrogen in the reactor added as petroleum residue, is the smallest (Table 4). This means that the process of increasing aromaticity is the least extensive of the three feedstocks, mainly because R1 is the most aromatic of the original feedstocks.
4.1.2.2. Liquid analysis. The data indicate that operating at 0.1 MPa, liquids are mainly produced by distillation of original R1 (fd), recovering all liquids of boiling point ,3608C and the light compounds of the heavy fraction (Fig. 2(b)). With increasing temperature there is distillation of heavier compounds. At 1.0 MPa a fraction of the compounds of boiling point 200–3608C does not distill and remains in the reactor. There is also a slight increase of compounds with boiling point 100–2008C compared with the feedstock, indicating the cracking of larger compounds. Yet, the effect of pressure is to suppress liquid yields, increasing slightly the cracking of larger compounds. The additional material retained by pressure within the system, of lower molecular weight, acts as a diluent to the system and retards the formation of mesophase [15–17], giving a small percentage of cracking reactions. The development of the aromaticity of distillates is dependent on pyrolysis conditions. The different behaviours found in the evolution of aromaticity of distillates with increasing soak time at 0.1 and 1.0 MPa are due to the depressurisation. When operating at 0.1 MPa, once distillation has occurred, there is little production of distillates (due to cracking reactions) and their aromaticity is similar or even lower than dis-
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tillates produced during heating. On the other hand, when operating at 1.0 MPa, a fraction of the distillates that distills at 0.1 MPa during heating is now retained in the reaction. This material has more time to eliminate side chains and increases its aromaticity. On depressurisation, part of this light material distills, its aromaticity being higher than that distilled before depressurisation and also higher than operating at 0.1 MPa. The aromaticity of the distillates obtained with depressurisations increases with soak time. Table 5 shows the differences in aromaticity and the percentage of the distillates obtained before depressurisation and after depressurisation. The aromaticity of the whole amount of distillates is also shown. It is seen that aromaticity of distillates obtained with depressurisation is much higher than that obtained before depressurisation. The effect of temperature is to accelerate this phenomena.
4.1.2.3. Solid analysis. Mesophase content increases with soak time or temperature. There is a rapid increase in aromaticity and a decrease of H / C relation as expected with this residue. With increasing pressure light material which does not distill reduces average molecular weights and further reaction time is then needed to produce mesogens which lead to mesophase, slowing down the process of mesophase formation and aromatisation. This residue produces highly ordered mesophase as it was expected from a highly aromatic feedstock. 4.2. Feedstock R2 4.2.1. Effect of experimental parameters on yields 4.2.1.1. Gases. The effect of experimental parameters on gas yields is similar. An increase of yield of gases from R2 with respect to R1 is due to its higher content of aliphatic side-chains. The behaviours of R1 and R2 are similar in that there is an increase in gas yield with increasing pressure, caused by more material being in the reactor due to pressure. 4.2.1.2. Liquids. The effect of experimental parameters on liquid yields differs from R1. In R1 yields are mainly affected by pressure, effects of temperature and soak time
Table 5 Aromaticity of liquids from R1 and R2 before and after depressurisation Experimental conditions
Yield of distillates before depressurisation (wt %)
Yield of distillates after depressurisation (wt %)
H ar (%) of distillates before depressurisation
H ar (%) of distillates after depressurisation
H ar (%) of total distillates
R1-460-1 / 1.5 R2-440-1 / 6
75 61
25 39
50 16
62 5
52 33
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being of less importance. In R2, the effect of temperature is more pronounced, the use being helpful of simulated distillation histograms of the feedstock (Table 1, Fig. 3(b)). At 3608C, only 17 wt % is distilled from the feedstock; between 440–4808C as much as 52–76 wt % is produced. This is typical of compounds with a homogeneous molecular weight distribution and with a narrow band of boiling points. In this way, temperature (more so at 0.1 MPa) and pressure exert a major influence on production of distillates. Cracking reactions also have an important role; thus, yields vary with soak time. Fig. 7(b) shows the variation of weight of distillates with soak for two temperatures (a) at R2-440-1.0 / 1, and (b) at R2-480-1.0 / 1. The distillations of R2 differ from R1. Operating at 4408C and 1.0 MPa there is no distillation during heating and about 15 wt % of liquids is lost during the soak period as products of cracking reactions. The remainder (|30 wt %) is produced during depressurisation. When operating at 4808C and 1.0 MPa, 25 wt % of the original feedstock is evolved as distillates (fd), about 15 wt % during the soak time, as pyrolysis distillates (pd), the remainder being evolved during depressurisation, |10 wt %. Amounts of distillates from cracking reactions (pd) increase compared with R1. As occurred with R1, amounts of liquids obtained during depressurisation decrease as the temperature increases. With R2, the fraction obtained during the second distillation is less than for R1, but higher yields of solids are obtained when operating at 1.0 MPa, Fig. 7(a–b).
4.2.1.3. Solid. Solid yields of pyrolysis of R2 vary with pressure, temperature and soak time. Variation of solid yield are a consequence of what occurred with liquids and gases. Pressure exerts each effect as it controls distillation, and there is an important fraction that can distill or not depending on the pressure, thus affecting solid yields. Temperature affects in two ways: (i) when increasing temperature there is an increase in liquids distilled during heating; (ii) there is also an increase of distillates due to cracking reactions. Soak time affects more, because there is more production of distillates and gases due to cracking reactions (mainly at 1.0 MPa.) 4.2.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 4.2.2.1. Gas analysis. For R2, it is aliphatic hydrogen which is responsible for the gas composition, and the length of chains of molecules within R2. Because of this, there is an increase in the average molecular weight of the gases with respect to R1. R2 produces larger amounts of hydrogen, methane and hydrocarbons of higher molecular weight than R1, indicating more extensive cracking reactions leading to smaller molecules. R2 evolves a higher percentage of overall hydrogen in the reactor than R1 indicating that it has had to undergo further molecular transformations to give products similar to R1 (Table 4).
4.2.2.2. Liquid analysis. As occurred with R1 distillates, with R2 distillates there is an enrichment in low boiling point compounds (,3608C). This enrichment indicates that distillates are produced by cracking of larger molecules (higher boiling point). This phenomenon is enhanced when operating at 1.0 MPa because at this pressure more material is available for cracking reactions, because of retention of higher amounts of the .3608C fraction. The percentage of distillates coming from cracking reactions is higher in R2 than in R1 (10–25 versus 3%, respectively). This indicates that the stability of the heavy fraction of R1 is higher than R2 (they are more aromatic). Although the aromaticity of the distillates from R2 is much less than that of R1, the changes with respect to increasing temperature and soak time are similar. The aromaticity of distillates from R1 and R2, at 1.0 MPa, increase with soak time. The reason is that, at 1.0 MPa, liquids from the second distillation are more aromatic than the rest, because they have been in the reactor longer, and so have had more time to aromatise and eliminate sidechains (Table 5). Aromaticity of the distillates increases significantly after depressurisation. Extents of aromaticity before depressurisation are close to those in a pyrolysis at 0.1 MPa pressure. 4.2.2.3. Solid analysis. The behaviour of R2 is different (Fig. 5(c and d)). The anisotropy content increases with soak time and temperature, but higher temperatures (208C) are needed to achieve the same anisotropy contents as with R1. According to the low aromaticity of the feedstock a higher reactivity was expected. Whereas it is generally understood that reactivity in terms of mesophase formation increases with increasing aromaticity of the feedstock, the behaviour of R2 indicates that there can be exceptions to this general rule. It is difficult in such a complex system to separate out the competing influences of radical formation, radical capping via hydrogen transfer reactions, dehydrogenation and dealkylation reactions as well as cyclisation processes. In this way R2 gives typical products for a low reactive system in which low viscosity conditions have occurred during pyrolysis. The influence of pressure is much less than is in R1. It was expected that there exists wider differences in extents of anisotropy when pressure is increased from 0.1 to 1.0 MPa, as occurred in R1 (in R2 yields vary considerably with pressure). The composition does not change much (is more homogeneous than in R1) with pressure and does not affect as much the development of anisotropy. 4.3. Feedstock R3 4.3.1. Effect of experimental parameters on yields 4.3.1.1. Gases. R3 has percentages of gases very similar to R2 (operating at lower temperatures than R2) and more than R1 because it contains a greater proportion of material that cracks into smaller molecules. NMR spectra of the
´ M. Martınez-Escandell et al. / Carbon 37 (1999) 1567 – 1582
feedstocks indicate that the percentage of chains in R2 and R3 is much greater than in R1, the most aromatic of the feedstocks, Table 1. Pressure does not affect gas yields mainly because there is little change in amounts of material in the reactor with pressure (Fig. 1(e–f)).
4.3.1.2. Liquids. In R3 Pyrolysis, no material (fd) is distilled from the feedstock during the heating-up stage, all liquids coming from cracking reactions of the high molecular weight constituent material. Extended soak times and higher reaction temperatures promote cracking reactions. Effects of pressure are not as important as with R1 and R2 because of the lack of sensitivity of cracking reactions to pressure, unlike the distillations of materials from R1 and R2, during heating-up (Fig. 1(e–f)). 4.3.1.3. Solids. Solid yields decrease with temperature and soak time because of the evolution of products of cracking (liquids and gas). Temperature and soak time promote cracking reactions but pressure has a negligible effect (Fig. 1(e–f)). 4.3.2. Effect of pyrolysis conditions on the composition of products ( gases, liquids and solids) 4.3.2.1. Gas analysis. Gases of R3 have a higher molecular weight than for R1 and R2. Generally, the average molecular weight of gases evolved in a pyrolysis increases as the aliphatic nature of the feedstock increases. Amounts of hydrogen evolved per hydrogen in the reactor is the highest of the three feedstocks, indicating that R3 is undergoing the largest molecular transformations of the three feedstocks during formation of semicokes (Table 4). 4.3.2.2. Liquid analysis. The simulated distillation diagrams of R3 liquids (Fig. 2(c)) indicate that all liquids come from cracking reactions. With increasing soak time, low boiling point compounds are produced. With increasing temperature, molecules of higher stability are cracked, resulting in a distillate of higher molecular weight. When increasing pressure, there is more material in the reactor, therefore, cracking reactions are ‘promoted’, increasing amounts of material of low boiling point (,2008C) (simply as a consequence of having increased amounts of material in the reactor). As expected, aromaticity of distillates is almost constant during pyrolysis. In contrast with R1 and R2, the distillates from R3 hardly change in aromaticity with soak time at 1.0 MPa, noting that extents of material obtained in the second distillation are minimal. 4.3.2.3. Solid analysis. Mesophase develops slowly at high temperatures, the rate increasing with temperature. This is the typical behaviour of a reactive system. This behaviour is expected as it is an aliphatic feedstock (Fig. 4(e–f)). Clearly, the amount of aromaticity for the same tem-
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perature in the semicoke from R1, Fig. 5(a–b) is the highest, followed by R2, Fig. 5(c–d) with low values of aromaticity of semicokes from R3, Fig. 5(e–f). The lower aromaticities of the semicokes of R3 may be associated with its higher molecular reactivity, enabling mesogens to be formed early on and these may not have enough time to lose completely some of their aliphatic side-chains. Differences in aromaticity with pressure do not exist as there is no variation in amount of material in the reactor and highly aromatic compounds are not diluted. Mesophase formed from different petroleum residues differ not only in structure but also in H / C content and aromaticity (‘n’ parameter). R1 and R2 develop mesophase in the form of domains or flow domains, which have a low H / C ratio and high ‘n’ parameter, while R3 develops mesophase in form of mosaics of higher H / C ratio and lower aromaticity (‘n’ parameter), Table 3.
5. Conclusions
1. A laboratory pilot plant has been designed to provide a deeper insight into the chemistry of pyrolysis reactions of petroleum residues by following, simultaneously, the formations of solids, liquids and gases during the entire process under a wide range of pyrolysis conditions. 2. Effects of carbonisation parameters (pressure, temperature and soak time) depend on the chemical composition of original feedstock. For semicokes from R1 (the most aromatic), yields are dominantly a function of pressure, with little influence of temperature and soak time. For semicokes from R2 (intermediate aromaticity), yields are dominantly a function of pressure and temperature, with little influence of soak time. For semicokes from R3 (least aromatic), yields are dominantly a function of temperature and soak time, with little influence of pressure. Increasing pressure increases solid yields, thus reducing liquid yields. Increases are larger for R1 and R2 which contains larger amounts of light material which distills during heating. Increases are smaller for R3 which contains larger amounts of material which undergo cracking reactions. 3. The use of pressure release at reaction temperatures causes a second distillation of volatile materials, amounts of material decreasing with increasing temperature and soak time. In the pyrolysis of R1, yields are dominantly controlled by distillation from the system. In the pyrolysis of R2, yields are dominantly controlled by both distillation from the system and by cracking reactions of molecular constituents (elimination of alkyl side-chains). In the pyrolysis of R3 yields are dominantly controlled by cracking reactions of molecular constituents (elimination of alkyl sidechains). 4. Simulated distillation procedures enable a distinction to be made between liquids coming from distillation from
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the original feedstock or from cracking reactions. For example, only 5% of the liquids of R1 come from cracking reactions, 22% from R2, while in R3 all liquids derive from cracking reactions. 5. R1 (most aromatic feedstock) produces semicokes with a high aromaticity and flow domain textures while R3 (most aliphatic) produces semicokes with the lowest aromaticity and an optical texture of coarse mosaics. R2 (intermediate aromaticity) reacts more slowly than expected to give highly ordered mesophase. Differences in aromaticity are not enough to explain differences in reactivity and the resultant optical textures of mesophase. Whereas it is generally understood that reactivity in terms of mesophase formation increases with increasing aromaticity of the feedstock, the behaviour of R2 indicates that there can be exceptions to this general rule. It is difficult in such complex systems to separate out the competing influences of radical formation, radical capping via hydrogen transfer reactions, dehydrogenation and dealkylation reactions as well as cyclisation processes.
Acknowledgements Support from BRITE-EURAM Program, Contract BRE.2CT.0901 is acknowledged. H.M. thanks the Spanish DGICYT (SAB95-0086).
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