Upgrading of an Asphaltenic Coal Residue: Thermal Hydroprocessing

Energy & Fuels 1996, 10, 401-408

401

Upgrading of an Asphaltenic Coal Residue: Thermal Hydroprocessing A. M. Benito, M. T. Martıńez,* I. Fernańdez, and J. L. Miranda Instituto de Carboquı´mica, C.S.I.C. P.O. Box 589, 50080-Zaragoza, Spain Received August 1, 1995X

A residue from deasphalting a syncrude obtained by direct coal liquefaction of a subbituminous Spanish coal was processed by thermal hydrotreatment. Kinetic study of the cracking reaction and coke formation reaction has been performed. The viscosity, coke content, boiling point distribution, elemental analysis, and aromaticity of the reaction products have been determined. The experimental data fits with the first-order kinetic model proposed. The main effects observed with the thermal hydrotreatment have been a large decrease of the viscosity that varies from 4608 cSt in the feedstock to 147 cSt in the products. The conversion of the heavy fraction (bp > 350 °C and soluble in toluene) and asphaltenes increased with the temperature and residence time, and the formation of coke was inhibited even at the hardest reaction conditions used. At 425 °C a kinetic control conducts the reaction, the cracking reaction with lower activation energy was more favorable and the coke formation (condensation) remained almost completely inhibited.

Introduction In recent years, the demand for jet fuel, kerosene, and diesel fuels has increased significantly whereas future worldwide crude supply projections indicate a shortage of high-quality, low-sulfur crudes. On the other hand, the availability of heavy oil streams has increased by exploitation of the Canadian tar sands and the heavy oil reservoirs in Venezuela. These trends jointly with more rigorous environmental specifications and the reduced markets for residual fuel have challenged the oil refining industry to increase its heavy oil conversion capabilities. The processes that convert heavier oil fractions into lighter and more valuable products are based either on carbon rejection or on hydrogen addition:

CmHn f C + Cm-1Hn CmHn + H2 f CmHn+2 Modern carbon-rejection processes include Flexicoking, heavy oil cracking, and Dynacracking. These emerging technologies offer considerable advantages over the traditionally applied carbon-rejection processes such as visbreaking, solvent deasphalting, and delayed coking. The numerous hydrogen-addition technologies include Veba-Combi-Cracking, H-Oil, LC-Fining, Shell-HDM/ HCON, Gulf-DRB, CANMET, Auraban, and M-Coke. In principle, hydrogen addition is preferred since carbon “wastage” from the crude oils is avoided. The key challenge in residue hydroprocessing is the conversion of asphaltenes. Asphaltenes are complicated molecular structures in which heteroatoms such as nickel vanadium, sulfur, nitrogen, and oxygen are arranged in a matrix of aromatic structures with a low hydrogen/carbon ratio. Asphaltenes consist of complex mixtures of macromelecular compounds with hybrid structures. They are highly polar substances, chemiX

Abstract published in Advance ACS Abstracts, February 1, 1996.

0887-0624/96/2510-0401$12.00/0

cally active and insoluble in low molecular weight alkanes. A disproportionate quantity of heteroatoms and coke precursors reside in the asphaltene type material.1 Furthermore, asphaltenes tend to form aggregates with greater size and heavier weight and they sediment and deposit into the reservoir matrix. The need for a more efficient exploitation of heavy feedstocks has motivated many research efforts in studying the molecular structure and colloidal stability of asphaltenes.2-5 The knowledge of the chemical structure of asphaltenes is the key for understanding their behavior in thermal and catalytic cracking. The two most important reactions which take place in residue hydrotreating are thermal cracking to lighter products and catalytic removal of feed contaminants.The objectives in the processes of hydrogen addition to residues or to heavy oils are mainly to increase the H/C atomic ratio, to remove the metals, and to reduce the heteroatom content (reaching a good desulfurization is specially important) to improve the product quality and to make the subsequent treatments easier.6 The hydroconversion processes can be thermal or catalytic and consist of several thermal and/or catalytic stages. The hydroprocessing conditions currently used are high temperature and pressure, high hydrogen/ feedstock ratios, and high activity catalysts. Under these conditions, a wide range of reactions are simultaneously produced; hydrocracking, hydrogenation, aro(1) Quann, R.; Ware, R. A.; Hung, C. W.; Wei, J. Adv. Chem. Eng. 1988, 4, 95. (2) Calemma,V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225. (3) Tranth, D. M. Z.; Start, S. M.; Petti, T. F.; Neurock, M.; Yasar, M.; Klein, M. T. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1993, 33, 2, 434. (4) Stornm, D. A.; Shen, E. Y.; Detar, M. M.; Barresi, R. J. Energy Fuels 1994, 8, 567. (5) Chaala, A.; Benallal, B.; Hachelef, S. Can. J. Chem. Eng. 1994, 72, 1036. (6) Speight, J. G. Fuel Science and Technology Handbook; Marcel Dekker: New York, 1990.

© 1996 American Chemical Society

402

Energy & Fuels, Vol. 10, No. 2, 1996

matization, condensation, hydrodesulfurization, hydrodenitrogenation, deoxygenation, demetallization and isomerization. However, the hydroprocessing conditions may be more favorable for some of these reactions and they will be produced to a greater extent than the others. In the range of temperatures applied in hydroconversion processes, the driving force of the conversion reactions is essentially thermal activation. The catalyst, the hydrogen pressure, and the sophistication of the techniques basically limit and control the nondesirable side condensation reactions. Among the thermal processes of hydroconversion, we have focused our attention on hydrovisbreaking which differs from visbreaking in the addition of hydrogen in the thermal treatment. In both cases the activation is thermal, the reaction runs via free radicals and the effects that the operating variables temperature, residence time, and concentration of the different reactants produce are similar. Nevertheless, hydrogen in the usual hydrovisbreaking of petroleum residues or heavy oils reactions has an inhibitor effect in the condensation reactions that leads to coke formation.7,8 In this work, a residue from deasphalting a synthetic crude obtained by direct liquefaction of a subbituminous Spanish coal9 was hydroprocessed under similar conditions to those used in hydrovisbreaking processes in the petroleum industry. The raw syncrude was processed through two upgrading routes, the oils were hydrogenated in two steps,10 and the residue has been upgraded by thermal cracking,11 hydrothermal cracking, and catalytic hydrocracking12 in order to get an integral use of natural resources. The aim of this paper is to study the effect of temperature and residence time during the hydroprocessing experiments by measuring the change produced in the kinetic parameters, viscosity, coke content, elemental analysis, aromaticity, and boiling point distribution as well as to compare these results with those obtained in the thermal process of the same residue previously studied in our laboratory.11 Experimental Section The feedstock used in this work was a residue rich in asphaltenes which do not undergo cracking easily and are coke precursors in thermal cracking. Some feedstock properties are shown in Table 1. Experiments were conducted batchwise in a stainless steel tubular reactor 40 cm long and 1.3 cm i.d. heated in a sand fluidized bed and shacked up by a pneumatic device that provides perfect mixing of the products. The reaction was quenched by plunging the microreactor in a cold water tank. Experiments were carried out in duplicate putting in each reactor 25 g of the sample. A determined initial hydrogen pressure different in each experiment was used in such way that the hydrogen pressure at the operating temperature was the same in all the processes (15 MPa). Thus the pressure (7) Sikoma, J. G. Hydrocarbon Process. 1980, 59 (6), 73. (8) Sanford, E. C. Energy Fuels 1994, 8, 1276. (9) Martıńez, M. T.; Fernańdez, I.; Benito, A. M.; Cebolla, V.; Miranda, J. L.; Oelert, H. H. Fuel Process. Technol. 1993, 33, 159. (10) Fernandez, I.; Martinez, M. T.; Benito, A.; Miranda, J. L. Fuel 1995, 74, 1, 72. (11) Benito, A. M.; Martıńez, M. T.; Fernańdez, I.; Miranda, J. L. Fuel 1995, 74, 6, 922. (12) Benito, A. M.; Martinez, M. T.; Miranda, J. L. Coal Science and Technology 24. In Coal Science; Pajares, J. A., Tascon, J. M. D., Eds.; Elsevier: New York, 1995; pp 1467-1470.

Benito et al. Table 1. Raw Material Propertiesa kinematic viscosity (cSt, 65 °C) elemental analysis of the fraction soluble in toluene C (%) H (%) N (%) S (%) O (%) H/C atomic ratio N/C × 103 atomic ratio S/C × 103 atomic ratio boiling point distribution (%) L (bp < 350 °C) H (bp > 350 °C) C distribution according to solubility (%) oil asphaltene coke a

4608 81.78 6.38 1.55 4.65 5.64 0.94 16.25 21.32 23.63 61.42 14.95 19.58 65.47 14.95

L ) light fraction; H ) heavy fraction; C ) coke fraction.

remained constant in all the experiments and we only studied the temperature and residence time effects. Kinetic experiments were carried out at different temperatures (425, 450, and 475 °C) and reaction times (5, 10, 20, 30, and 40 min). The reaction times were measured from the putting of the microreactor in the fluidized bed which gives good results taking into account the fast heating rate of the reactor in this system (temperature was reached in less than 2 min). Analysis. The hydrovisbroken products obtained in each experiment were analyzed for viscosity, coke content, boiling point distribution, elemental analysis, and aromaticity (1H NMR). Viscosity was measured at 65 °C with Canon-Fenske viscometers (ASTM D445, -86, ASTM D-446, -85). Coke content was determined by ultrasonic extraction as the material insoluble in toluene with a solvent to product ratio of 5 (w/w). Oil content was also determined by ultrasonic extraction as the material soluble in n-hexane with a solvent to product ratio of 5 (w/w). Boiling point distribution was determined by gas chromatography in a Varian 3400 chromatograph with capillary SE 50 column and FID detector in the conditions: injector temperature 250 °C; detector temperature 250 °C; initial temperature 50 °C (5 min); final temperature 300 °C; and temperature gradient 5 °C/min. Calibration was carried out with several pure compounds (naphthalene, phenanthrene, fluoranthene, and chrysene), and n-decane was used as internal standard for quantification. C, H, N, and S elemental analysis was carried out in an elemental analyzer Carlo Erba CHNS-O Model EA1108. 1H NMR spectra were recorded in a Varian XL 200 MHz at 300 K, 200.057 MHz resonance frequency, 2.353 s acquisition time, 0.27 s pulse delay, 3400.2 Hz spectral width, and 50% of deuterated chloroform.

Results There are three phases in the hydrovisbreaking process: solid (coke), liquid (starting material and obtained liquid products) and gas (hydrogen that has not reacted and gases that are formed during the process, H2S, CO2, C1-C4). Noting that coke is a final product in the reaction there are no limitations in the mass transference fluid-solid that involve coke. There is an excess of hydrogen during the reaction and since the equilibrium between gas and liquid phases is reached rapidly, it is valid to consider, as some other authors have, that hydrogen concentration in the liquid phase remain constant and there are not gas-liquid

Upgrading of an Asphaltenic Coal Residue

Energy & Fuels, Vol. 10, No. 2, 1996 403

Figure 2. Representation of the logarithm of the heavy fraction versus time.

During the course of the process, the heavy fraction content (bp > 350 °C) decreased with time while the concentration of the light fraction (bp < 350 °C), gas, and coke increased with time (Figure 1). It has been observed that the cracking reaction produces light distillates and follows a first-order kinetic for all the experimental temperatures. A linear correlation between coke and distillates has been obtained. Pseudo-first-order kinetics has been proposed for the cracking reaction and a first-order kinetics for the condensation process, as follows: K1

L+G

K2

C

H

Figure 1. Evolution of the different fractions obtained by hydrothermal processing.

diffusion problems.13-15 Then, for our calculations we assume the hydrovisbreaking process to be kinetically better than diffusionally controlled. The feedstock used in these experiments has a heavy nature as can be seen in Table 1 and it is characterized by a high viscosity and high percentage of products insoluble in n-hexane (80,42% asphaltene + coke). The heteroatom content is also too high. (13) Qader, S. A.; Hill, G. R. Ind. Eng. Chem. Process Des. Dev. 1969, 8, 98. (14) Qader, S. A.; Hill, G. R. Ind. Eng. Chem. Process Des. Dev. 1969, 8, 456. (15) Qader, S. A.; Wiser, W. H.; Hill, G. R. Fuel 1972, 51, 54.

[where the heavy fraction (H) is the nonconverted fraction of heavy compounds (fraction with bp > 350 °C and soluble in toluene); light fraction (L) (fraction with bp < 350 °C and soluble in toluene); gas fraction (G) (gases produced); distillate fraction (D) (L + G); coke fraction (C) (fraction insoluble in toluene)]. The condensation reactions of D to give C was laid away because it is known that coke is produced directly from asphaltenes16 and L for its definition consist of oil better than asphaltenes. According to the mechanism proposed, the evolution of the different compounds with time will be described with the following integrated equations:

H -ln ) (k1 + k2)t Ho D - Do k1 ) C - Co k2 The kinetic constants were obtained from the representation ln H versus time (Figure 2) and C versus D (Figure 3). The activation energies for these processes were calculated from the semilogarithmic representation of ln K versus 1/RT (Figure 4) by applying Arrhe(16) Morgaril, R. Z.; Axenova, E. I. Oil Gas J. 1970, 5, 47.

404


Benito et al.

Figure 4. Representation of the Arrhenius equation. Table 2. Kinetic Parameters Obtained in the Hydrovisbreaking Experiments temperature (°C)

k1 (s-1)a

k2 (s-1)a

425 450 475 Ea (kJ/mol)

3.10 × 7.50 × 10-5 9.59 × 10-5 97

1.67 × 10-5 3.71 × 10-5 8.89 × 10-5 145

10-5

a k kinetic constant for the cracking reaction. k kinetic 1 2 constant for the coke formation reaction.

Figure 3. Representation of the percentage of the distillate fraction versus coke obtained in the experiments of hydrovisbreaking.

nius equation,

k ) AeEa/RT [where A is the frequency factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature (K)]. The kinetic parameters obtained are detailed in Table 2. Acceptable correlations have been found. The rate

of the cracking reaction was higher than that of the condensation reaction in all the cases (k1 > k2) and both kinetic constants increased with temperature during the hydrovisbreaking experiments. The condensation reaction, responsible for the coke formation, has an activation energy much higher than the cracking reaction and the coke formation was more important with increasing temperature. Thermodynamic control was observed in that as the temperature increased, the greater the tendency was for the more stable product to be formed. In this case the more stable product was the coke fraction, as L and G fractions condensed to produce H fraction. At the lowest temperature used (425 °C) the coke content hardly was modified with time, but as the temperature increased, an increase of C with time was observed. The maximum coke percentage was reached at the more severe conditions used (475 °C and 40 min) in which a slight decrease in L was observed (Figure 1). Then at 425 °C,the reaction seems to be kinetically controlled; i.e., the process with lower activation energy (cracking reaction) was more favorable and the reaction with higher activation energy (condensation reaction) remained almost completely inhibited. In the hydrothermal process studied here, the activation energy for the condensation reaction was higher than that observed during the thermal treatment studied previously (72 kJ/mol was obtained).11 So, for the same temperature and time conditions the coke production was more important when the process was carried out without hydrogen pressure and a higher conversion to L + G was obtained for the hydrovisbreaking process. Seeing that the limiting factor (coke formation) did not affect the hydroprocess until 20 min



Figure 5. Comparison between the experimental results and the kinetic curves calculated for every pseudocomponent.

Figure 6. Evolution of the oil, asphaltene, and coke fractions obtained in the hydrovisbreaking experiments.

of residence time at 450 and at 475 °C it would be possible to work under more severe conditions to get higher distillates production than in the thermal process. The operating conditions will be chosen depending on the needed conversion to light products and the maximum acceptable coke in the liquids. At the most severe conditions used (475 °C and 40 min), the conversion to light products (Lo - L)/Ho × 100) was 18.8%; the conversion to gases (G/Ho × 100) was 8% and the conversion to coke ((C - Co)/Ho × 100) was

16%. The maximum conversion to light products was 21%, the conversion to coke obtained at the same condition (450 °C and 40 min) being 9%. In the thermal process the maximum conversion to L was 12% and in those conditions a 28% of C was obtained. Then, when hydrogen was used, a higher conversion to L and lower C production were obtained as mentioned above. Figure 5 shows a comparison between the experimental results and the kinetic curves calculated for every

406


Benito et al.

Table 3. Elemental Analysis of the Products Obtained in the Hydrovisbreaking Experiments (a) Experiments at 425 °C elemental analysis of the fraction soluble in toluene C (%) H (%) N (%) S (%) O (%) H/C atomic ratio N/C × 103 atomic ratio S/C × 103 atomic ratio

5 82.02 6.32 1.60 4.61 5.45 0.92 16.72 21.08

residence time (min) 10 20 30 82.91 6.34 1.66 4.31 4.78 0.92 17.16 19.49

84.90 6.36 1.67 4.20 2.87 0.90 16.86 18.55

83.90 6.22 1.69 3.80 4.39 0.89 17.27 16.98

Table 4. Composition of the Gaseous Fraction Obtained in the Hydrovisbreaking Experiments (Feedstock Basis %) (a) Experiments at 425 °C residence time (min)

40 83.56 6.10 1.71 3.78 4.85 0.88 17.54 16.96

gas

5

10

20

30

40

CH4 CO CO2 C2H6 SH2 C3H8 C4H10

0.00 0.00 0.53 0.03 0.12 0.05 0.00

0.07 0.05 0.59 0.06 0.24 0.07 0.06

0.12 0.06 1.52 0.09 0.44 0.09 0.00

0.16 0.03 1.14 0.16 0.54 0.18 0.17

0.20 0.07 1.37 0.17 0.61 0.18 0.14

(b) Experiments at 450 °C elemental analysis of the fraction soluble in toluene

5

C (%) H (%) N (%) S (%) O (%) H/C atomic ratio N/C × 103 atomic ratio S/C × 103 atomic ratio

82.45 6.34 1.63 4.47 5.11 0.92 16.95 20.33

(b) Experiments at 450 °C


83.83 6.26 1.71 3.93 4.27 0.90 17.48 17.58

83.81 6.19 1.70 3.73 4.57 0.89 17.39 16.69

residence time (min) 40

gas

5

10

20

30

40

84.01 5.92 1.70 3.51 4.86 0.85 17.34 15.67


0.07 0.07 0.75 0.05 0.29 0.05 0.00

0.14 0.07 0.78 0.12 0.34 0.15 0.11

0.28 0.07 1.12 0.28 0.49 0.30 0.13

0.31 0.07 1.35 0.31 0.53 0.25 0.14

0.33 0.05 1.33 0.32 0.71 0.34 0.16

(c) Experiments at 475 °C

(c) Experiments at 475 °C elemental analysis of the fraction soluble in toluene C (%) H (%) N (%) S (%) O (%) H/C atomic ratio N/C × 103 atomic ratio S/C ×103 atomic ratio

5 82.42 6.04 1.67 3.84 6.03 0.88 17.37 17.47


85.72 5.80 1.76 3.15 3.57 0.81 17.60 13.78

84.86 6.00 1.75 3.11 4.28 0.85 17.68 13.74

residence time (min) 40 87.00 5.79 1.75 2.54 2.92 0.80 17.24 10.95

pseudocomponent. The obtained concordance was good and the process fitted with the reaction scheme proposed. Figure 6 shows the evolution of the asphaltene, oil, and coke fractions with time in the hydrovisbreaking process. The asphaltene disappearance reflects an increase in the production of coke and oil which agrees with the accepted idea that coke formation is mainly associated with asphaltene cracking and it is produced as a result of the condensation reactions between the free radicals formed during the asphaltene cracking.16 At the less severe conditions used in our experiments it is probable that hydrogen stabilized these free radicals which explains why the production of coke was inhibited. At the most severe conditions used, the condensation reactions competed with the stabilization of the free-radical reactions, which led to an increase in coke formation. The chemical transformation of asphaltenes has been also studied through the structural changes observed in elemental and 1H NMR analysis. In the hydrothermal process, a decrease of the H/C and S/C atomic ratios with both time and temperature was produced while the N/C atomic ratio was kept almost constant (Table 3). It can be said that nitrogen present in these liquids is integrated in the condensed aromatic structures and difficult to crack and it is concentrated in the asphaltene fraction. Hydrogenation of the nitrogen heterocyclic structures is necessary before nitrogen removal can occur. The H/C and N/C values indicated that hydrogenation and denitrogenation reactions were not produced.

gas

5

10

20

30

40


0.16 0.10 0.76 0.14 0.31 0.17 0.11

0.34 0.08 1.16 0.33 0.57 0.35 0.21

0.45 0.06 1.13 0.41 0.59 0.37 0.18

0.76 0.07 1.25 0.58 0.70 0.48 0.25

1.11 0.07 1.28 0.73 0.80 0.52 0.26

Compared with the thermal process, higher S removal was reached and H/C decreased on a lower scale. This is because the hydrogen contributed to the production of hydrogen-rich volatile compounds and to the stabilization of the free radicals formed. The gas composition is described in Table 4. As shown in this table, CO2 and H2S were found in higher concentration than light hydrocarbons. At the lowest temperature used, a low percentage of C1-C4 fraction was obtained for short reaction times. This percentage increased with time and temperature, which means that the extent of the cracking reaction increased as residence time and temperature increased. The high content of CH4 obtained at 475 °C as compared to that obtained at 425 °C can be particularly observed. Hydrogen distribution and structural parameters obtained by 1H NMR are detailed in Table 5. A great percentage of hydrogen in the feedstock was aromatic and Har increased with time and temperature. Consequently, the aromaticity parameter (fa) was quite high and increased with time and temperature. At 425 °C, fa increased lightly and a light decrease of alkyl chain length (Ho/HR) and Har/Car parameters were produced. At 450 and 475 °C, the changes in the structural parameters went in the same direction, but they were produced in a higher extent than they did at 425 °C. The aromaticity parameter (fa) increased from 0.83 to 0.87 with time at 450 °C and from 0.85 to 0.90 at 475 °C. The parameter σ, degree of substitution of aromatic nuclei, decreased sharply at 475 °C going from 0.18 at 5 min to 0.12 at 40 min of reaction; this fits with the higher percentage of cracking observed as the temperature increased. This cracking would be produced



Table 5. Hydrogen Distribution and Structural Parameters Obtained by 1H NMR of the Fractions Soluble in Toluene

Table 6. Kinematic Viscosity of the Liquids Obtained in the Hydrovisbreaking Experiments (cSt, 65 °C) residence time

(a) Raw Material NMR data

experiments at 425 °C experiments at 450 °C experiments at 475 °C

58.45 0.84 0.18 0.78 1.31

Ηar fa σ Har/Car Ho/HR

5

10

20

30

40

415 348 224

390 233 187

318 224 147

304 216 328

285 210 411

(b) Experiments at 425 °C time (min) NMR data Ηar fa σ Har/Car Ho/HR

5

10

20

30

40

57.42 0.83 0.19 0.79 1.21

57.82 0.84 0.18 0.77 1.26

58.64 0.84 0.15 0.74 1.30

59.65 0.84 0.17 0.76 1.17

59.81 0.85 0.17 0.74 1.24

(c) Experiments at 450 °C time (min) NMR data Ηar fa σ Har/Car Ho/HR

5

10

20

30

40

56.80 0.83 0.18 0.77 1.49

58.44 0.84 0.17 0.75 1.42

59.71 0.84 0.17 0.76 1.20

60.15 0.85 0.17 0.75 1.31

63.58 0.87 0.16 0.74 1.06

(d) Experiments at 457 °C time (min) NMR data Ηar fa σ Har/Car Ho/HR

5

10

20

30

40

59.86 0.85 0.18 0.76 1.31

63.11 0.87 0.16 0.74 1.10

64.99 0.88 0.15 0.70 1.00

65.89 0.88 0.15 0.75 1.01

69.93 0.90 0.12 0.71 1.83

a f aromaticity. σ degree of substitution. H /C a ar ar aromatic hydrogen to aromatic carbon ratio. Ho/HR average chain length.

preferentially in the alkyl chains. Consequently, the parameter Ho/HR that represents a measure of the alkyl chains length decreased with temperature and time. Then the alkyl chains removal as well as the breaking of the alkyl chains bonded to aromatic rings to give shorter ones produced an increase of the aromaticity of the liquids. Gases were produced in these ruptures, but in the hydrothermal process this gas formation produced lower dehydrogenation than that reached in the thermal process due to the presence of hydrogen. The light increase of the condensation degree (Har/Car) also contributed to making fa higher. The condensation reactions produced the progressive insolubilization of the asphaltenes which make more favorable the coke deposition. Aliphatic hydrogen content mainly consisted of HR and Hβ because the percentage of Hγ was very low and decreased with time and temperature, so chains must not be very long, which was corroborated by the low values of Ho/HR obtained. Har/Car, the parameter giving information about the aromatic hydrogen to aromatic carbon ratio in the hypothetical case the rings were not substituted, had values ranging from 0.7 to 0.8. Therefore, the reaction products would have structures with 2 to 3 condensed aromatic rings. With increasingly severe reaction conditions, the structures tended to have 3 condensed aromatic rings.

Figure 7. Representation of the logarithm of the viscosity versus time.

At 425 °C, the highest percentage of L + G was produced at 40 min of reaction time and the obtained liquids had higher Har and HR. As the temperature increased the Har and HR contents were higher. Nevertheless, Hγ decreased with time and temperature, reaching the lowest value at 450 °C and 40 min. So the liquids had higher aromaticity, shorter alkyl chains and a lower substitution degree when the reaction conditions became more severe. During the hydrovisbreaking experiments, a sharp decrease of the viscosity with the time and temperature even at the least severe conditions used was produced (Table 6). The viscosity decreased very quickly as the reaction condition became more severe, reaching the lower value (147 cSt) at 475 °C and 20 min. The presence of hydrogen was more effective in the viscosity reduction than the thermal treatment (the lower viscosity found in that case was 939 cSt). Noting that the cracking reaction follows first-order kinetics and that ln of the viscosity is linear with the concentration of light products,18 the representation of the ln(viscosity) versus residence time should give a straight line. Reasonably straight lines were obtained (Figure 7) and the curvature at 475 °C could be due to the condensation reactions which yielded a high-viscosity material due to increasing coke content. These reactions were more important as the temperature and reaction time increased as has already been mentioned, yielding a smaller viscosity reduction than could be expected. This viscosity reduction could be due to an increase in oil and light products content when time and (17) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 2021. (18) Yant, T. Y. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1987, 32(2), 490.

408


Benito et al.

The boiling point distribution shown in Figure 8 represents the evolution of the different fractions obtained by GC. It can be seen that with increased reaction time, there is a moderate increase of the fraction with boiling point under 400 °C. The fraction with boiling point higher than 450 °C decreased when the time and temperature increased. At a temperature of 475 °C and after 40 min of residence time the highest reduction of this fraction is reached. So it can be concluded that an increase in the severity of the reaction conditions produced an increase in the fraction that boils under 400 °C as a consequence of the cracking produced in the other fractions, specially in the one with boiling point above 450 °C. The great decrease produced in the fraction with boiling point above 450 °C indicates that the condensation reactions to produce coke were preferently produced in this fraction. Conclusions

Figure 8. Boiling point distribution obtained at the three temperatures essayed in the hydrovisbreaking experiments.

temperature were increased. The hydrogen contributed to obtain higher conversion to light products and higher percentage of oils than the thermal process. Consequently, the obtained liquids had lower viscosity than that obtained in thermal treatment.

The results obtained in these experiments reflect the benefits of the hydrogen pressure utilization in residue thermal processing. The most obvious and perhaps most important effect of hydrogen has been the inhibition of the condensation reactions to give coke. The proposed first-order kinetic model fits well the experimental results obtained. As the reaction conditions became more severe the conversion of the heavy fraction (H) increased, but the gas and coke production increased too. Nevertheless, the conversions to light compounds with the utilized residue were quite low. The rate of the condensation reaction was lower than that of the cracking reaction in all the cases (k1 > k2) and both kinetic constants increased with temperature. The activation energies for cracking and condensation reactions were 97 and 145 kJ/mol, respectively, and the formation of coke was almost completely inhibited at the lowest temperature applied. Hydrovisbreaking only differs from visbreaking in the utilization of hydrogen with the feedstock and in the working pressure used. Consequently, the hydrogen is responsible for these observed effects. The hydrogen has limited the condensation reactions of asphaltenes and it has allowed higher conversions to light products getting products with lower viscosity (until 6 times lower than the obtained in thermal experiments) The viscosity reduction was mainly due to oil and light compounds increase. The maximum conversion to oil was 34% in hydrovisbreaking and 29% in visbreaking experiments. At the same time, aromaticity of the liquids increased when reaction conditions became more severe. The increase of the aromaticity could be due to the alkyl chains removal or alkyl chain breaking to give shorter ones. Acknowledgment. This study was sponsored by the UE contract No. EN3V-0055E and by the Spanish DGICYT project CE89-0007. EF9501556

Upgrading of an Asphaltenic Coal Residue: Thermal Hydroprocessing

Recommend Documents