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Visbreaking of an asphaltenic coal residue
FM,/ Vol 74 No. 6. pp. 922-927. 1995 Copyright t’ 1995 Elscvier Science Ltd Printed in Great Britain. All rights reserved 0016-2361.95,$10.00+0.tM~
0016-2361(95)00013-5
Visbreaking Ana M. Benito,
of an asphaltenic
Maria
lnstituto de Carboquimica, (Received 8 April 1994)
coal residue
T. Martinez,
lsaias Fernhndez
CSIC, Apartado
583, 50080 Zaragoza, Spain
and Jose L. Miranda
A residue from deasphalting of liquids obtained by direct liquefaction of a Spanish subbituminous coal was processed by thermal treatment. This residue is rich in asphaltenes which do not undergo cracking easily and are coke precursors in thermal cracking. The kinetics of the cracking and coke formation reactions were studied and the viscosity, coke content, boiling point distribution, elemental analysis and aromaticity of the reaction products were determined. The experimental data fit the first-order kinetic model proposed. The main effects produced by the thermal treatment were a large decrease in the viscosity from 4608 mm2 s-l for the feedstock to 939 mm2 s- ’ for the product. The conversion of the heavy fraction (b.p. > 350°C and soluble in toluene) increased with the temperature and residence time, the conversion to coke being higher than the conversion to light products (b.p. < 350°C and soluble in toluene). (Keywords: coal liquids: asphaltenic residues: vishreaking)
In a simple way, a residue from coal liquefaction or pyrolysis, a petroleum residue or a residual oil can be described as a colloidal system where the disperse phase consists of micelles that contain asphaltenes and other high-molecular-weight aromatic compounds (part of the maltenes) and the continuous phase is formed by the rest of the maltenes. Asphaltenes are mixtures of hydrocarbons and heterocompounds of sulfur, nitrogen and oxygen soluble in benzene (or toluene) and insoluble in paraffins of low molecular weight (n-pentane, n-hexane). Asphaltenes form complex structures of high molecular weight and low hydrogen content’. They have a highly aromatic character and do not undergo cracking easily, but they change character during thermal treatment by breakage of cross-linking bonds and generation of other asphaltenes through condensation reactions. Asphaltenes are precursors of coke formation in thermal and catalytic cracking reactions and confer undesirable properties on heavy fractions, such as high viscosity. In brief, a heavy residue is characterized by a high viscosity, high heteroatom and asphaltene contents, high molecular weight, high mean boiling point and low H/C ratio. Consequently, the refining technology will be strongly influenced by the type of treatment applied to these asphaltenes. Visbreaking is a gentle thermal treatment used mainly to reduce the viscosity of heavy fractions, for the production of light and medium distillates2*3 and for the preparation of the feedstock for catalytic cracking plants4. The technology used in this process is not complicated. Basically, the visbreaking unit consists of a furnace and a fractionation unit. The furnace is the heart of the process and its design depends on the residue characteristics and the yields needed. Thermal cracking at high temperatures is achieved in the furnace. In accordance with the fundamentals of thermal cracking technology, the conversion obtained depends on two operating variables: temperature and residence
922
Fuel 1995 Volume 74 Number 6
time. The cracking severity and the fuel oil stability are the parameters to be taken into account when selecting the operating conditions and the yields to be attained. In general, the process yield improves when the severity increases, that is, when residence time and/or temperature increase. Nevertheless, the severity of the process is limited by coke formation5.‘j and by the instability of the product7. The literature on the thermal cracking of coal liquids is limited, and this process does not seem to be attractive for obtaining compounds of commercial interest, since the high aromatic content of such liquids minimizes the extent of cracking and results in severe coking. Residues from coal liquids processed by thermal treatment could be used as a feedstock for catalytic cracking units. The coke and undesired products in these units can be gasified to produce the hydrogen needed in other refining processes. In this work, a residue from deasphalting of coal liquids was processed by thermal treatment. The aim was to study the effect of temperature and residence time during thermal cracking by measuring the changes produced in viscosity, coke content, elemental analysis, aromaticity and boiling point distribution. EXPERIMENTAL Material
The material used was a residue from deasphalting of liquids obtained by direct liquefaction of a Spanish subbituminous coal. Operating conditions of the liquefaction and deasphalting processes have already been described in detai18,9. Some properties of the residue are shown in Tuble 1. Tlzermul treutment Experiments were conducted batchwise in a steel tubular reactor 40 cm long and of 1.3 cm i.d. heated in a fluidized sand bed and agitated by a pneumatic device
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Visbreaking of a coal residue: A. M. Benito et al. Table 2
Properties of visbreaking products Residence time (min)
Reaction temp. (W)
5 Kinematic viscosity (“C) (mm' s-t)
425
30
40
4240
3445
3210
2238
0.91
0.91
0.91
0.91
0.88
N/C.103
16.42
16.37
16.24
16.15
15.73
s/c, 103
24.10
20.72
19.82
19.00
19.52
.
25.3
25.3
25.7
26.0
27.2
H (b.p. >35o”C)
57.7
55.4
50.7
49.9
47.8
Coke
17.0
19.3
23.6
24.1
25.1
3721
2524
1815
1094
Kinematic viscosity (65°C) (mm’s_t)
0.92
0.90
0.90
0.86
0.84
N/C.103
16.51
16.19
16.08
16.62
16.47
S/C.lO’
22.08
19.66
18.01
17.91
13.75
28.4
28.7
29.4
30.4
31.2
H (b.p. > 350°C)
52.5
50.2
44.9
39.0
36.8
Coke
19.2
21.0
25.1
30.6
32.0
2820
1330
1264
2385
2657
Kinematic viscosity (65°C) (mm' s-t) Atomic ratios H/C
0.89
0.90
0.86
0.86
0.85
N/C.103
16.61
16.32
16.89
16.44
15.91
s/c.103
21.53
19.72
16.56
14.12
15.38
Product distribution (wt%) L (b.p. < 350°C)
28.8
29.1
30.7
28.6
21.9
H (b.p. >35O”C)
51.5
49.1
38.1
34.1
33.6
Coke
19.8
21.2
31.2
37.4
38.6
kinetics at all temperatures, but at 475°C it reaches a maximum at 20 min of reaction and then decreases slightly. On the other hand, the existence of a linear correlation between C and L (Figure I) indicates that the reaction mechanism could be expressed as follows:
1
.o r
0.8 -
??
425T
0 45OT A 415T
L
k,
20
The condensation of L to give C was disregarded because it is known that coke is produced directly from asphaltenes15 and L by definition consists of oil lighter than asphaltenes. To determine the kinetic parameters for the mechanism studied, first-order kinetics were proposed for two reactions. In agreement with this scheme, the evolution of the various compounds with time is described by the following integrated equations: -ln~=(k,+k,)t 0
L-Lo -=C-Co
k, k,
where H, L and C denote the fractions of heavy and light compounds and coke respectively. The kinetic constants
924
939
Atomic ratios H/C
Product distribution (wt%) L (b.p. < 350°C)
415
20
Atomic ratios H/C
Product distribution (wt%) L (b.p. 1350°C)
450
4459
10
Fuel 1995 Volume 74 Number 6
30
Time (min) Figure 2 Semilogarithmic plot of concentration of heavy compounds versus time in visbreaking
were obtained from the representation In H versus time (Figure 2) and C versus L (Figure 1). The activation energies for these processes were calculated by applying the Arrhenius equation k=Aexp(-EdRT) where the symbols have the usual meanings, and plotting In k against l/RIT; Figure 3. It must be mentioned that k is generally a function of the conversion. In other words, the first-order concept is not a very rigorous approximation and it can be applied only over a narrow range of conversions. The orders of
Visbreaking -6 ??
-1
&I
A k2
-12 = 0.16
0.17
0.18
l/RT Figure 3 Arrhenius plot of rate constants k, for the cracking reaction, kz for coke formation
Table 3
Kinetic parameters” from visbreaking experiments
Temperature (“C)
&,
$1,
42.5 450 47s
1.32,10-5 2.96.10-5 3.20,10-5
7.48.10-s 14.48.10-5 19.50~10~5
E, (kJmol_‘)
61
72
’Rate constants k, for cracking, k, for coke formation
reaction estimated in the literaturei6-‘* for different residues vary from 1 to 1.5. Nevertheless, owing to the small range of temperatures studied and the low conversions obtained, the first-order kinetic model can be considered adequate to describe the thermal process. Experimental studies that confirm the possibility of using this model have been reported”. The kinetic parameters obtained are detailed in Table 3. Acceptable correlations were found. The rate of the condensation reaction was higher than that of the cracking reaction in all instances (k,
of a coal residue: A. M. Benito et al.
Figure 4 compares the experimental results and the kinetic curves calculated for every pseudo-component. The agreement obtained is good. At 475°C the conversion to light compounds was lower than would be expected from first-order kinetics, and more coke was produced. It is generally accepted that coke formation is mainly associated with asphaltene cracking15. The curves in Figure 5 show that asphaltene disappearance reflects increasing production of coke and oil. The chemical transformation of asphaltenes was also studied through the structural changes observed by elemental and ‘H n.m.r. analyses. In the thermal process, the H/C and S/C atomic ratios decreased, and the decrease was greater when the temperature and reaction time increased. The N/C ratio remained almost constant, although a slight increase was observed when the conditions were more severe. The hydrogen distribution and structural parameters obtained by ‘H n.m.r. are detailed in Table 4. A high percentage of the hydrogen in the feedstock was aromatic, and H,, increased with time and temperature. Consequently, the aromaticity (f,) was quite high and increased with time and temperature. At 425”C,f, hardly changed and a slight decrease in alkyl chain (Ho/H,) and in H,,/H,, occurred. At 450 and 475”C,J& increased with
70 425°C
60
- -.I
--.---,
50
??
m--B-_.
+C
30 20
__ .----_
II
c _D 1 c -*
_ -o----___y----
lo,
10
0
Q
----=p===sr _P m_--
20
30
40
70 I450°C
60 G c .o z
s
a p: 05
-.
50
t-,,
-a
,_D’
20
,,_*
II
SC
---m.___
- -.-
40 30
??
? ?L
‘r
____ _-
--*”
- _ _
-(7__==ar~_e’LZ _ -*-
- ‘. = B
10 1
I
0
I
I
I
I
10
20
30
40
475T
0
8 II
I
I
I
I
10
20
30
40
Time (min) Figure 4 Model predictions and experimental pseudo-component at different temperatures
Fuel 1995
Volume
results
74 Number
for each
6
925
Visbreaking
of a coal residue: A. M. Benito et al.
Oil 0 Asph ??
425°C
40 .-m-a
10
0
20
??
3 w 50 2 ‘;I 40 S 5 30 6x
Table 4 Hydrogen distribution and n.m.r. structural parameters of the fractions soluble in toluene
Oil 0 Asph * Coke
450°C
60
40
30
4608 mm2 s- 1 (at 65°C) for the feedstock to 939 mm2 s- ’ under the reaction conditions of 450°C and 40 min. Seeing that the cracking reactions follow first-order kinetics and that the logarithm of the viscosity varies linearly with the concentration of light products22, the representation of ln(viscosity) versus residence time should give a straight line. Reasonable straight lines were obtained (Figure 7); the curvature at 475°C could be due to the condensation reactions that yielded a high-viscosity material owing to the increase in coke content. These reactions became
Reaction temp. (‘C)
Time (min) 5
10
20
30
40
59.8 0.84 0.17 0.78 1.31
58.8 0.85 0.17 0.76 1.40
58.8 0.84 0.18 0.77 1.28
60.6 0.85 0.16 0.77 1.38
59.2 0.84 0.18 0.76 1.11
61.9 0.85
58.5 0.84
60.3 0.85
61.8 0.86
65.2 0.87
0.81 0.17 1.17
0.18 0.76 1.31
0.17 0.77 1.21
0.17 0.75 1.16
0.15 0.74 0.96
&/C,,
60.8 0.85 0.78 0.18
61.6 0.85 0.78 0.16
62.7 0.86 0.74 0.16
64.2 0.87 0.74 0.15
67.2 0.88 0.76 0.13
Ho/H,
1.21
1.19
1.19
1.12
1.07
425
20 10 0
10
20
30
0 H,&, Ho/Hz
40 450 ?? Oil 0 Asph * Coke
O
10
20
30
Far
;;,~lC.~ Ho/Hz 475
Fr
40
Time (min) Figure 5 Oil, asphaltene and coke concentrations reaction time at different temperatures
time whereas 0, Ho/H, and H,,/C,, decreased, this effect being more pronounced at 475°C. The increase in aromaticity could be a consequence of the removal of alkyl chains as well as their breakage to shorter ones. Gases are produced in these ruptures and the global effect is like hydrogen removal. Dehydrogenation to produce more condensed structures (lower H.&Z,,) also contributes to the increase in f,. These processes promote progressive asphaltene insolubilization, which aids condensation reactions and subsequent coke deposition. The aliphatic hydrogen content mainly consisted of H, and H, because the percentage of H, was very low and decreased with increasing time and temperature, so the chains could not have been very long, as is corroborated by the low values of Ho/H,. The values found for H&Z,,, a parameter that provides information about the aromatic hydrogen to aromatic carbon ratio in the hypothetical case in which the rings are not substituted, ranged from 0.7 to 0.8, corresponding to structures with two to three condensed aromatic rings. The viscosity decreased sharply with increasing time and temperature (Figure 6), but the decrease is not as fast as would be expected from first-order kinetics. The thermal treatment reduced the viscosity from
926
Fuel 1995
Volume
74 Number
6
. 425’C 0 450°C A 475’C
as a function of
5001
,
,
,
,
0
10
20
30
40
Time (min) Figure 6 Variation of viscosity with the time and temperature visbreaking
in
‘0 r
5t -0
A 475’C 10
20
30
40
Time (min) Figure 7 Semilogarithmic plot of viscosity versus reaction time in visbreaking
Visbreaking 425°C
fractions, especially that boiling >45O”C. The large decrease in the fraction boiling >45O”C suggests that the condensation reactions producing coke occurred preferentially in this fraction.
100
60
100
CONCLUSIONS
r
60 s 40 20
0
of a coal residue: A. M. Benito et al.
10
20
30
40
As a result of the experiments carried out it can be concluded that conversion to light products is thermally impelled. The proposed first-order kinetic model fits the experimental results obtained. As the reaction conditions became more severe, the conversion to light products and coke formation increased. Nevertheless, the conversion of the residue to light compounds was very low. The rate of the condensation reaction was higher than that of the cracking reaction in all instances (k, >k,) and both rate constants increased with temperature. The activation energies for cracking and condensation reaction were 61 and 72 kJ mol- ’ respectively and the formation of coke was important even at the lower temperatures used. A large decrease in viscosity was obtained by increasing the temperature and the residence time. At the same time, the aromaticity of the liquids increased, which could be due to the removal or breakage of alkyl chains. REFERENCES
60
0
10
20 Time
30
40
(min)
Figure 8 Effect of time and temperature on boiling point distribution of the liquid fraction at different temperatures
8
9 10 II
more important as the temperature and reaction time increased, yielding a smaller viscosity reduction than expected. This viscosity reduction could be explained by the increase in oil and light products content when time and temperature increased. The boiling point distributions of the liquid fractions shown in Figure 8 represent the evolution of the different fractions obtained by g.c. In general, it can be seen that the fraction boiling <4OO”C slightly increased and the fraction boiling >45O”C decreased as the time and temperature increased, attaining 89% reduction at 475°C and 40min residence time. So it can be concluded that increasing the severity of the reaction conditions produced an increase in the yield of fraction boiling <4OO”C as a consequence of the cracking of the other
12 13 14 15 16 17 18 19 20
21 22
Csoklich. C. and Hartner. 0. Eriil, Erdgas, Kohle 1986,102,84 Notarbartolo, M., Menegazzo. C. and Kuhn, J. H.vdrocurb. Process. 1979, 57 (9). I14 Hournac, R., Kuhn, J. and Notarbartolo, M. Hydrocarb. Process. 1979, 58 (12), 97 Savaya, Z. F., Al-Soufi, H. H., Al-Azawi, I. and Mohammed, H. K. Fuel 1989,68, 1064 Singh, V. D. Erdiil Kohle, Erdgas, Peirochem. 1986, 39, 19 Rhoe, A. and deBIignieres, C. Hydrocarb. Process. 1979, 58 (l), 131 Krishna. R., Kuchhal, Y. K., Sarna, G. S. and Sing, I. D. Fuel 1988, 67, 379 Martinez, M. T., Fernandez, I., Benito, A. M., Cebolla, V., Miranda, J. L. and Oelert, H. H. Fuel Process. Technol. 1993, 33, 159 Benito, A. M.. Brower, L., Martinez, M. T., Severin, D. and Fernandex. I. Fuel Sci. Technol. Int. (submitted] Reis, T. Am. Chem. Sot. Dit;. Pet. Chem. Preprints 1981,26,476 Singh, I. D., Kothiyal, V. and Ramaswamy, V. Erdtil Kohle, Erdgas, Perrochem. 1991, 44, 113 Phillips, C. R., Haider, N. 1. and Poon, Y. C. Fuel 1985,64,678 Singh, I. D., Kothyal. V., Ramaswamy, V. and Krishna, R. Fuel 1990, 69, 289 Yan. T. H. Fuel 1990, 69, 1062 Morgaril, R. Z. and Axenova, E. I. Oil Gas J. 1970, (5), 47 Sekhar, M. V. and Ternan, M. Fuel 1979, 58, 92 Camobell. J. H. et cl. FLU/ 1978. 57. 372 Sand& R. et al. ‘Upgrading of crudes by thermal treatments’. Final Report, Contract no. EN3C-0041-L 1990 Qader, S. A. and Hill, G. R. Ind. Eng. Chem. Process Des. Dezs. 1969, 8, 98 White, J. L. In ‘Petroleum Derived Carbons’ (Eds M. L. Deviney and T. M. O’Grady), American Chemical Society Symposium Series, 1976, Ch. 21 Lumin, G.. Silva, A. E. and Denis, J. M. C/tern. Can. 1981. 3Y3). 17 Yan, T. Y. Am. Chem. Sot. Dil:. Per. C%m1. Preprints 1987, 32, 490
Fuel 1995 Volume 74 Number 6
927
0016-2361(95)00013-5
Visbreaking Ana M. Benito,
of an asphaltenic
Maria
lnstituto de Carboquimica, (Received 8 April 1994)
coal residue
T. Martinez,
lsaias Fernhndez
CSIC, Apartado
583, 50080 Zaragoza, Spain
and Jose L. Miranda
A residue from deasphalting of liquids obtained by direct liquefaction of a Spanish subbituminous coal was processed by thermal treatment. This residue is rich in asphaltenes which do not undergo cracking easily and are coke precursors in thermal cracking. The kinetics of the cracking and coke formation reactions were studied and the viscosity, coke content, boiling point distribution, elemental analysis and aromaticity of the reaction products were determined. The experimental data fit the first-order kinetic model proposed. The main effects produced by the thermal treatment were a large decrease in the viscosity from 4608 mm2 s-l for the feedstock to 939 mm2 s- ’ for the product. The conversion of the heavy fraction (b.p. > 350°C and soluble in toluene) increased with the temperature and residence time, the conversion to coke being higher than the conversion to light products (b.p. < 350°C and soluble in toluene). (Keywords: coal liquids: asphaltenic residues: vishreaking)
In a simple way, a residue from coal liquefaction or pyrolysis, a petroleum residue or a residual oil can be described as a colloidal system where the disperse phase consists of micelles that contain asphaltenes and other high-molecular-weight aromatic compounds (part of the maltenes) and the continuous phase is formed by the rest of the maltenes. Asphaltenes are mixtures of hydrocarbons and heterocompounds of sulfur, nitrogen and oxygen soluble in benzene (or toluene) and insoluble in paraffins of low molecular weight (n-pentane, n-hexane). Asphaltenes form complex structures of high molecular weight and low hydrogen content’. They have a highly aromatic character and do not undergo cracking easily, but they change character during thermal treatment by breakage of cross-linking bonds and generation of other asphaltenes through condensation reactions. Asphaltenes are precursors of coke formation in thermal and catalytic cracking reactions and confer undesirable properties on heavy fractions, such as high viscosity. In brief, a heavy residue is characterized by a high viscosity, high heteroatom and asphaltene contents, high molecular weight, high mean boiling point and low H/C ratio. Consequently, the refining technology will be strongly influenced by the type of treatment applied to these asphaltenes. Visbreaking is a gentle thermal treatment used mainly to reduce the viscosity of heavy fractions, for the production of light and medium distillates2*3 and for the preparation of the feedstock for catalytic cracking plants4. The technology used in this process is not complicated. Basically, the visbreaking unit consists of a furnace and a fractionation unit. The furnace is the heart of the process and its design depends on the residue characteristics and the yields needed. Thermal cracking at high temperatures is achieved in the furnace. In accordance with the fundamentals of thermal cracking technology, the conversion obtained depends on two operating variables: temperature and residence
922
Fuel 1995 Volume 74 Number 6
time. The cracking severity and the fuel oil stability are the parameters to be taken into account when selecting the operating conditions and the yields to be attained. In general, the process yield improves when the severity increases, that is, when residence time and/or temperature increase. Nevertheless, the severity of the process is limited by coke formation5.‘j and by the instability of the product7. The literature on the thermal cracking of coal liquids is limited, and this process does not seem to be attractive for obtaining compounds of commercial interest, since the high aromatic content of such liquids minimizes the extent of cracking and results in severe coking. Residues from coal liquids processed by thermal treatment could be used as a feedstock for catalytic cracking units. The coke and undesired products in these units can be gasified to produce the hydrogen needed in other refining processes. In this work, a residue from deasphalting of coal liquids was processed by thermal treatment. The aim was to study the effect of temperature and residence time during thermal cracking by measuring the changes produced in viscosity, coke content, elemental analysis, aromaticity and boiling point distribution. EXPERIMENTAL Material
The material used was a residue from deasphalting of liquids obtained by direct liquefaction of a Spanish subbituminous coal. Operating conditions of the liquefaction and deasphalting processes have already been described in detai18,9. Some properties of the residue are shown in Tuble 1. Tlzermul treutment Experiments were conducted batchwise in a steel tubular reactor 40 cm long and of 1.3 cm i.d. heated in a fluidized sand bed and agitated by a pneumatic device
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Visbreaking of a coal residue: A. M. Benito et al. Table 2
Properties of visbreaking products Residence time (min)
Reaction temp. (W)
5 Kinematic viscosity (“C) (mm' s-t)
425
30
40
4240
3445
3210
2238
0.91
0.91
0.91
0.91
0.88
N/C.103
16.42
16.37
16.24
16.15
15.73
s/c, 103
24.10
20.72
19.82
19.00
19.52
.
25.3
25.3
25.7
26.0
27.2
H (b.p. >35o”C)
57.7
55.4
50.7
49.9
47.8
Coke
17.0
19.3
23.6
24.1
25.1
3721
2524
1815
1094
Kinematic viscosity (65°C) (mm’s_t)
0.92
0.90
0.90
0.86
0.84
N/C.103
16.51
16.19
16.08
16.62
16.47
S/C.lO’
22.08
19.66
18.01
17.91
13.75
28.4
28.7
29.4
30.4
31.2
H (b.p. > 350°C)
52.5
50.2
44.9
39.0
36.8
Coke
19.2
21.0
25.1
30.6
32.0
2820
1330
1264
2385
2657
Kinematic viscosity (65°C) (mm' s-t) Atomic ratios H/C
0.89
0.90
0.86
0.86
0.85
N/C.103
16.61
16.32
16.89
16.44
15.91
s/c.103
21.53
19.72
16.56
14.12
15.38
Product distribution (wt%) L (b.p. < 350°C)
28.8
29.1
30.7
28.6
21.9
H (b.p. >35O”C)
51.5
49.1
38.1
34.1
33.6
Coke
19.8
21.2
31.2
37.4
38.6
kinetics at all temperatures, but at 475°C it reaches a maximum at 20 min of reaction and then decreases slightly. On the other hand, the existence of a linear correlation between C and L (Figure I) indicates that the reaction mechanism could be expressed as follows:
1
.o r
0.8 -
??
425T
0 45OT A 415T
L
k,
20
The condensation of L to give C was disregarded because it is known that coke is produced directly from asphaltenes15 and L by definition consists of oil lighter than asphaltenes. To determine the kinetic parameters for the mechanism studied, first-order kinetics were proposed for two reactions. In agreement with this scheme, the evolution of the various compounds with time is described by the following integrated equations: -ln~=(k,+k,)t 0
L-Lo -=C-Co
k, k,
where H, L and C denote the fractions of heavy and light compounds and coke respectively. The kinetic constants
924
939
Atomic ratios H/C
Product distribution (wt%) L (b.p. < 350°C)
415
20
Atomic ratios H/C
Product distribution (wt%) L (b.p. 1350°C)
450
4459
10
Fuel 1995 Volume 74 Number 6
30
Time (min) Figure 2 Semilogarithmic plot of concentration of heavy compounds versus time in visbreaking
were obtained from the representation In H versus time (Figure 2) and C versus L (Figure 1). The activation energies for these processes were calculated by applying the Arrhenius equation k=Aexp(-EdRT) where the symbols have the usual meanings, and plotting In k against l/RIT; Figure 3. It must be mentioned that k is generally a function of the conversion. In other words, the first-order concept is not a very rigorous approximation and it can be applied only over a narrow range of conversions. The orders of
Visbreaking -6 ??
-1
&I
A k2
-12 = 0.16
0.17
0.18
l/RT Figure 3 Arrhenius plot of rate constants k, for the cracking reaction, kz for coke formation
Table 3
Kinetic parameters” from visbreaking experiments
Temperature (“C)
&,
$1,
42.5 450 47s
1.32,10-5 2.96.10-5 3.20,10-5
7.48.10-s 14.48.10-5 19.50~10~5
E, (kJmol_‘)
61
72
’Rate constants k, for cracking, k, for coke formation
reaction estimated in the literaturei6-‘* for different residues vary from 1 to 1.5. Nevertheless, owing to the small range of temperatures studied and the low conversions obtained, the first-order kinetic model can be considered adequate to describe the thermal process. Experimental studies that confirm the possibility of using this model have been reported”. The kinetic parameters obtained are detailed in Table 3. Acceptable correlations were found. The rate of the condensation reaction was higher than that of the cracking reaction in all instances (k,
of a coal residue: A. M. Benito et al.
Figure 4 compares the experimental results and the kinetic curves calculated for every pseudo-component. The agreement obtained is good. At 475°C the conversion to light compounds was lower than would be expected from first-order kinetics, and more coke was produced. It is generally accepted that coke formation is mainly associated with asphaltene cracking15. The curves in Figure 5 show that asphaltene disappearance reflects increasing production of coke and oil. The chemical transformation of asphaltenes was also studied through the structural changes observed by elemental and ‘H n.m.r. analyses. In the thermal process, the H/C and S/C atomic ratios decreased, and the decrease was greater when the temperature and reaction time increased. The N/C ratio remained almost constant, although a slight increase was observed when the conditions were more severe. The hydrogen distribution and structural parameters obtained by ‘H n.m.r. are detailed in Table 4. A high percentage of the hydrogen in the feedstock was aromatic, and H,, increased with time and temperature. Consequently, the aromaticity (f,) was quite high and increased with time and temperature. At 425”C,f, hardly changed and a slight decrease in alkyl chain (Ho/H,) and in H,,/H,, occurred. At 450 and 475”C,J& increased with
70 425°C
60
- -.I
--.---,
50
??
m--B-_.
+C
30 20
__ .----_
II
c _D 1 c -*
_ -o----___y----
lo,
10
0
Q
----=p===sr _P m_--
20
30
40
70 I450°C
60 G c .o z
s
a p: 05
-.
50
t-,,
-a
,_D’
20
,,_*
II
SC
---m.___
- -.-
40 30
??
? ?L
‘r
____ _-
--*”
- _ _
-(7__==ar~_e’LZ _ -*-
- ‘. = B
10 1
I
0
I
I
I
I
10
20
30
40
475T
0
8 II
I
I
I
I
10
20
30
40
Time (min) Figure 4 Model predictions and experimental pseudo-component at different temperatures
Fuel 1995
Volume
results
74 Number
for each
6
925
Visbreaking
of a coal residue: A. M. Benito et al.
Oil 0 Asph ??
425°C
40 .-m-a
10
0
20
??
3 w 50 2 ‘;I 40 S 5 30 6x
Table 4 Hydrogen distribution and n.m.r. structural parameters of the fractions soluble in toluene
Oil 0 Asph * Coke
450°C
60
40
30
4608 mm2 s- 1 (at 65°C) for the feedstock to 939 mm2 s- ’ under the reaction conditions of 450°C and 40 min. Seeing that the cracking reactions follow first-order kinetics and that the logarithm of the viscosity varies linearly with the concentration of light products22, the representation of ln(viscosity) versus residence time should give a straight line. Reasonable straight lines were obtained (Figure 7); the curvature at 475°C could be due to the condensation reactions that yielded a high-viscosity material owing to the increase in coke content. These reactions became
Reaction temp. (‘C)
Time (min) 5
10
20
30
40
59.8 0.84 0.17 0.78 1.31
58.8 0.85 0.17 0.76 1.40
58.8 0.84 0.18 0.77 1.28
60.6 0.85 0.16 0.77 1.38
59.2 0.84 0.18 0.76 1.11
61.9 0.85
58.5 0.84
60.3 0.85
61.8 0.86
65.2 0.87
0.81 0.17 1.17
0.18 0.76 1.31
0.17 0.77 1.21
0.17 0.75 1.16
0.15 0.74 0.96
&/C,,
60.8 0.85 0.78 0.18
61.6 0.85 0.78 0.16
62.7 0.86 0.74 0.16
64.2 0.87 0.74 0.15
67.2 0.88 0.76 0.13
Ho/H,
1.21
1.19
1.19
1.12
1.07
425
20 10 0
10
20
30
0 H,&, Ho/Hz
40 450 ?? Oil 0 Asph * Coke
O
10
20
30
Far
;;,~lC.~ Ho/Hz 475
Fr
40
Time (min) Figure 5 Oil, asphaltene and coke concentrations reaction time at different temperatures
time whereas 0, Ho/H, and H,,/C,, decreased, this effect being more pronounced at 475°C. The increase in aromaticity could be a consequence of the removal of alkyl chains as well as their breakage to shorter ones. Gases are produced in these ruptures and the global effect is like hydrogen removal. Dehydrogenation to produce more condensed structures (lower H.&Z,,) also contributes to the increase in f,. These processes promote progressive asphaltene insolubilization, which aids condensation reactions and subsequent coke deposition. The aliphatic hydrogen content mainly consisted of H, and H, because the percentage of H, was very low and decreased with increasing time and temperature, so the chains could not have been very long, as is corroborated by the low values of Ho/H,. The values found for H&Z,,, a parameter that provides information about the aromatic hydrogen to aromatic carbon ratio in the hypothetical case in which the rings are not substituted, ranged from 0.7 to 0.8, corresponding to structures with two to three condensed aromatic rings. The viscosity decreased sharply with increasing time and temperature (Figure 6), but the decrease is not as fast as would be expected from first-order kinetics. The thermal treatment reduced the viscosity from
926
Fuel 1995
Volume
74 Number
6
. 425’C 0 450°C A 475’C
as a function of
5001
,
,
,
,
0
10
20
30
40
Time (min) Figure 6 Variation of viscosity with the time and temperature visbreaking
in
‘0 r
5t -0
A 475’C 10
20
30
40
Time (min) Figure 7 Semilogarithmic plot of viscosity versus reaction time in visbreaking
Visbreaking 425°C
fractions, especially that boiling >45O”C. The large decrease in the fraction boiling >45O”C suggests that the condensation reactions producing coke occurred preferentially in this fraction.
100
60
100
CONCLUSIONS
r
60 s 40 20
0
of a coal residue: A. M. Benito et al.
10
20
30
40
As a result of the experiments carried out it can be concluded that conversion to light products is thermally impelled. The proposed first-order kinetic model fits the experimental results obtained. As the reaction conditions became more severe, the conversion to light products and coke formation increased. Nevertheless, the conversion of the residue to light compounds was very low. The rate of the condensation reaction was higher than that of the cracking reaction in all instances (k, >k,) and both rate constants increased with temperature. The activation energies for cracking and condensation reaction were 61 and 72 kJ mol- ’ respectively and the formation of coke was important even at the lower temperatures used. A large decrease in viscosity was obtained by increasing the temperature and the residence time. At the same time, the aromaticity of the liquids increased, which could be due to the removal or breakage of alkyl chains. REFERENCES
60
0
10
20 Time
30
40
(min)
Figure 8 Effect of time and temperature on boiling point distribution of the liquid fraction at different temperatures
8
9 10 II
more important as the temperature and reaction time increased, yielding a smaller viscosity reduction than expected. This viscosity reduction could be explained by the increase in oil and light products content when time and temperature increased. The boiling point distributions of the liquid fractions shown in Figure 8 represent the evolution of the different fractions obtained by g.c. In general, it can be seen that the fraction boiling <4OO”C slightly increased and the fraction boiling >45O”C decreased as the time and temperature increased, attaining 89% reduction at 475°C and 40min residence time. So it can be concluded that increasing the severity of the reaction conditions produced an increase in the yield of fraction boiling <4OO”C as a consequence of the cracking of the other
12 13 14 15 16 17 18 19 20
21 22
Csoklich. C. and Hartner. 0. Eriil, Erdgas, Kohle 1986,102,84 Notarbartolo, M., Menegazzo. C. and Kuhn, J. H.vdrocurb. Process. 1979, 57 (9). I14 Hournac, R., Kuhn, J. and Notarbartolo, M. Hydrocarb. Process. 1979, 58 (12), 97 Savaya, Z. F., Al-Soufi, H. H., Al-Azawi, I. and Mohammed, H. K. Fuel 1989,68, 1064 Singh, V. D. Erdiil Kohle, Erdgas, Peirochem. 1986, 39, 19 Rhoe, A. and deBIignieres, C. Hydrocarb. Process. 1979, 58 (l), 131 Krishna. R., Kuchhal, Y. K., Sarna, G. S. and Sing, I. D. Fuel 1988, 67, 379 Martinez, M. T., Fernandez, I., Benito, A. M., Cebolla, V., Miranda, J. L. and Oelert, H. H. Fuel Process. Technol. 1993, 33, 159 Benito, A. M.. Brower, L., Martinez, M. T., Severin, D. and Fernandex. I. Fuel Sci. Technol. Int. (submitted] Reis, T. Am. Chem. Sot. Dit;. Pet. Chem. Preprints 1981,26,476 Singh, I. D., Kothiyal, V. and Ramaswamy, V. Erdtil Kohle, Erdgas, Perrochem. 1991, 44, 113 Phillips, C. R., Haider, N. 1. and Poon, Y. C. Fuel 1985,64,678 Singh, I. D., Kothyal. V., Ramaswamy, V. and Krishna, R. Fuel 1990, 69, 289 Yan. T. H. Fuel 1990, 69, 1062 Morgaril, R. Z. and Axenova, E. I. Oil Gas J. 1970, (5), 47 Sekhar, M. V. and Ternan, M. Fuel 1979, 58, 92 Camobell. J. H. et cl. FLU/ 1978. 57. 372 Sand& R. et al. ‘Upgrading of crudes by thermal treatments’. Final Report, Contract no. EN3C-0041-L 1990 Qader, S. A. and Hill, G. R. Ind. Eng. Chem. Process Des. Dezs. 1969, 8, 98 White, J. L. In ‘Petroleum Derived Carbons’ (Eds M. L. Deviney and T. M. O’Grady), American Chemical Society Symposium Series, 1976, Ch. 21 Lumin, G.. Silva, A. E. and Denis, J. M. C/tern. Can. 1981. 3Y3). 17 Yan, T. Y. Am. Chem. Sot. Dil:. Per. C%m1. Preprints 1987, 32, 490
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