Kinetics of asphaltene hydroconversion

Fuel Vol. 76, No. 10, pp. 899-905, 1997 © 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016-2361/97 $17.00+0.00

PIh S0016-2361(97)00107-5

ELSEVIER

Kinetics of asphaltene hydroconversion 1. Thermal hydrocracking of a coal residue

Maria T. Martinez, Ana M. Benito and Maria A. Callejas Instituto de Carboquimica, C.S.I.C., P.O. Box 589, 50080 Zaragoza, Spain (Received 14 March 1997; revised 18 April 1997) An asphaltenic residue from deasphalting a synthetic crude obtained by direct liquefaction of a Spanish subbituminous coal was submitted to several upgrading routes. This paper reports the results from thermal hydroprocessing. Solubility-based lumped kinetics with parallel reactions for oils, gases and coke formation are proposed. Conversion data fit second-order kinetics for asphaltene conversion and for oil, gas and coke formation. Coke formation was inhibited except under the harshest reaction conditions. Two pathways for coke formation are proposed: one as primary reaction product after an induction period and the other as secondary product from a sequence of polymerization steps. The presence of hydrogen strongly inhibits the primary coke formation, Structural analyses show higher aromaticity of the oils from products obtained at 475°C than at 450°C, which supports the proposed condensation mechanism for coke production at high temperatures. © 1997 Elsevier Science Ltd.

(Keywords: asphaitenes; thermal hydrocracking; kinetics)

Environmental concerns and economic incentives have forced petroleum, chemical and material manufacturing companies to place stringent constraints on acceptable product quality and process operations. The need for process models to predict accurate product yields is of great interest in the petroleum industry. Historical limitations in the analytical chemistry of complex feedstocks often necessitated the modelling of their reactions at a global lumped level. Lumping methodology 1-3 reduces complexity by grouping the entire set of molecules into a small, manageable number of lumps which can be correlated with process yield data. The two most commonly used globally lumped models are based on boiling point and solubility characteristics. These global reaction models represent the interconversion of aggregates of many molecules with common attributes. Advances in separation and characterization techniques over the past 20 years have allowed rational formulation of molecular-level reaction models. Quann and Jaffe 4'5 have developed a structure-oriented lumping (SOL) method incorporating molecular detail to predict product composition and properties. Klein et al.6 have developed a stochastic approach for constructing the molecular structure of thousands of molecules and their fundamental reactions, using a set of structural attributes whose frequencies are determined by bulk property data. Liguras and Allen 7 incorporated fundamental chemistry in an FCC (fluid catalytic cracking) model by using several hundred selected archetypal molecules, called pseudocomponents, whose concentration in a feedstock is calculated from mass spectroscop~y and 13C n.m..r spectroscopy. Froment and co-workers °-)° have taken a more detailed approach by using a Boolean matrix representation of a molecule and its

complex sequence of elementary reaction steps involving radicals in thermal reactions or surface intermediates in catalysis. The molecular characterization schemes described have been formulated for complex but volatile feeds. The characterization of heavier fractions such as residua and asphaltenes is difficult. A typical heavy hydrocarbon mixture may comprise a huge number of molecular species. The reactivity of these molecules depends not only on the reaction conditions but also on mixture interactions. The molecular-level modelling requires an experimental database of reaction pathways and rate constants since it is prohibitively expensive to obtain the information experimentally. Molecularly structure-explicit models are difficult to solve analytically and can be computationally intensive. Although the importance of product quality and environmental concerns suggests the use of a molecular-level model, the higher computational costs associated with process calculations can often necessitate the use of a global model. Lumped models could be more convenient for applications such as parameter estimation, optimization and process control. The present paper reports a kinetic study of thermal hydroconversion of an asphaltenic coal residue using solubility-based lumped fractions. EXPERIMENTAL A residue from deasphalting a synthetic crude obtained by direct liquefaction 11 of a Spanish subbituminous coal was processed by hydrothermal cracking. Some properties of the residue are shown in Table 1. Experiments were conducted batchwise in a stainless

Fuel 1997 Volume 76 Number 10

899

Kinetics of asphaltene hydroconversion: 1: M. T. Martinez et al. Table 1 Properties of asphaltenic coal residue a Kinematic viscosity, 65°C (mm2s -i) Boiling point distribution (vol.%) L (b.p. < 350°C) H (b.p. > 350°C) C Distribution by solubility (wt%) Oil Asphaltenes Coke

4608 23.6 61.5 14.9

resonance frequency, with 2.353 s acquisition time, 0.27 s pulsed delay, 3402 Hz spectral width and 50 wt% deuterated chloroform. RESULTS AND DISCUSSION The results from processing the asphaltenic coal residue are indicated in Table 2. Asphaltene conversion ranged from 15.3 to 37.8 wt%. The range of asphaltene conversion was narrower than that obtained in thermal cracking

19.6 65.5 14.9

"L, light fraction; H, heavy fraction; C, coke fraction

Table 2 Distribution by solubility of the hydrocracked products (wt%) Residence time (min) 5

10

20

30

40

Experiments at 425°C Gas

0.7

1.1

2.3

2.4

2.7

Oil

30.1

32.7

35.0

35.6

37.8

Asphaltenes

55.4

51.9

48.6

47.6

43.9

Coke

13.8

14.3

14.1

14.4

15.6

Experiments at 450°C 1.3

1.7

2.7

3.0

3.2

Oil

33.1

34.7

38.4

38.4

38.3

Asphaltenes

51.4

49.4

43.1

42.6

40.7

Coke

14.2

14.2

15.8

16.0

17.8

Gas

Experiments at 475°C Gas

1.8

3.0

3.1

4.1

4.8

Oil

33.5

36.1

40.7

33.9

28.6

Asphaltenes

49.6

45.5

37.5

41.4

41.8

Coke

15.1

15.4

18.7

20.6

24.8

steel tubular reactor 40 cm long and 1.3 cm i.d. heated in a sand fluidized bed and shaken by a pneumatic device that "provides good mixing of the products. The reaction was quenched by plunging the microreactor in a cold water tank. Experiments were carried out in duplicate, with 25 g of the sample in each reactor. A predetermined initial hydrogen pressure, different for each experiment, was used in such a way that the hydrogen pressure at the operating temperature was the same throughout the processes (15 MPa). Thus the pressure remained constant in all the experiments and only studied the temperature and residence time effects were studied. Kinetic experiments were carded 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 placing of the microreactor in the fluidized bed, which gave good results, taking into account the rapid heating of the reactor in this system (temperature was reached in < 2 min). The processed products from each experiment were analysed for coke, oil and asphaltene contents. Coke content was determined by ultrasonic extraction as the material insoluble in toluene with a solvent/product ratio of 5 (w/w). Oil content was also determined by ultrasonic extraction as the material soluble in n-hexane with the same solvent/ product ratio. The asphaltenes were the material soluble in toluene and insoluble in n-hexane. Structural analyses of oils from feed and products were carried out by elemental analysis and tH n.m.r, measurements. Proton n.m.r, spectra were recorded at 300 MHz

900

Fuel 1997 Volume 76 Number 10

(10.6-55.9 wt%) for the same feedstock 12. Nevertheless, oil yields were higher in hydrocracking since gas and coke formation decreased. Complex reaction networks have been proposed for asphaltene thermolysis 13. However, in the presence of hydrogen, asphaltene radical recombinations are less frequent and coke formation is inhibited. The literature shows that no coke is formed under mild conditions but that it is formed in high yields at higher severity. Shucker and Keweshan 14 reported no toluene-insoluble material from hydropyrolysis of Cold Lake asphaltenes at 400°C for 120 min. Savage et a l ) ~ also reported no coke formation from asphaltene hydropyrolysis up to 40 min of reaction at 400°C but coke formation at 10 min at 450°C. It can be seen from Table 2 that at 425°C coke was not produced until beyond 40 min of reaction. The decrease in the tolueneinsoluble content of the reaction products at 425°C and 5 30 rain of reaction was possibly due to preasphaltene conversion. The preasphaltenes are insoluble in toluene but soluble in tetrahydrofuran and they are frequently included in the toluene-insoluble lump with the coke, being capable of conversion by hydrogenation. The data from Table 2 indicate the existence of an induction period that was shorter at 450°C and disappeared at 475°C, Figure 1. This induction period has been described by some authors 16'17 as resulting from a phaseseparation step of asphaltenes in thermolysis and in residue hydroconversion. Magaril et al. 18 were the first to postulate that coke formation is triggered by the phase separation of asphaltenes.

Kinetics of asphaltene hydroconversion: 1: M. T. Martinez et al.

50

40

0.8"

475oc

• 4250C a 450oc • 4750C

/p

0.6-

• t-

N 30

0.4"

O

ffl

m 0.2" >

m 20 "o

o

E

~

E

0

o ",- 1 0 -

450oc

0.4"

o T

0

10

20

30

40

50 0.2-

t (min)

Figure 1 Coke formation versus time in the hydrocracking experiments

l

0.1

•

,

-

0.2

i' ,

0.3

•

,

0.4

0.5

Conversion

Figure 3 First-rank Delplot for coke formation at 475 and 450°C

1.2' 475"C Oils

0.8' 0.4'

Gases

E

o

"~ 1 . 2 ' 450oc >

Oils 0.8

-o

0.4,

Gases

_ r

_ -

~

-

1.2' 425"C a....._.~......_~......~

0.8

Oils

0.4.

Gases

•0

011

"

012

013

0'.4

0.5

Conversion

Figure 2 First-rank Delplot for oil and gas formation

To establish the reaction network in asphaltene hydroconversion, Delplot analysis was applied 19. This technique for network analysis identified primary and higher-rank products. On the first-rank Delplot, oil and gas formation at 425 and 450°C, Figure 2, showed non-zero intercepts and they were therefore considered primary products. At 475°C, the oil and gas showed two different slopes, both with non-zero intercept, one from 5 to 20 min and the other from 20 to 40 min (the points in Figure 2 have been joined in the sequence of increasing reaction time). This could indicate the formation under these conditions of primary oil and gas by two different reaction pathways. Coke formation at 450 and 475°C and 5, 10 and 20 min of reaction showed a zero intercept, i.e. secondary product, and at 475°C and longer reaction times a non-zero intercept, i.e. primary product, Figure 3. This supports the idea that after the induction period the asphaltenes decompose to form primary coke, oil and gas. According to this, the induction period also exists at 475°C and primary coke was produced only at reaction times beyond 20 min and at 475°C. This seems to indicate that the presence of hydrogen strongly

inhibited primary coke formation. Primary coke formation was observed in thermal cracking of the same feedstock at 425,450 and 475°C at both short and long reaction times 12. Asphaltenes have been considered to contain condensed aromatic rings systems with heteroatomic, alkyl and naphthenic substituents and inter-unit linkages located along their periphery. The thermolysis causes fission of these peripheral entities and this is the likely first step in asphaltene conversion2°. This strictly primary product evidently is transformed from benzene-soluble to benzeneinsoluble material over a very small 'window' of degradation. It would be consistent with this that the primary coke observed is the asphaltenic core stripped of its peripheral substituents. Further thermolysis produces primary products that arise from fission of carbon-carbon bonds, which are hydrocarbon gases, cycloalkanes, paraffins and possibly condensed polycyclic aromatics. These are the primary gases and oils that appear at 475°C and long reaction times in Figure 2. On the other hand, another mechanism of secondary coke formation exists at 425,450 and at 475°C at shorter reaction times, which could be that postulated by Levinter et al. 2~ in which coke is produced by a sequence of polymerization and condensation steps from the lightest to the heaviest fraction. The condensation reactions are slow and can be clearly estimated only at 450 and 475°C. A kinetic model based on parallel reactions for oil, gas and coke formation was used to describe quantitatively the experimental kinetic data on the hydrothermal cracking of asphaltenes. During the course of the reaction, asphaltenes decompose to form primary gases and oils and secondary coke:

kl Asphaltenes

Gases

Oils k~'~""~* Coke ~

Fuel 1997 Volume 76 Number 10

901

Kinetics of asphaltene hydroconversion: 1: M. T. Martinez et al. The primary coke-forming reaction rate is phaseequilibrium-controlled and has not been included in the model that is valid during the induction period before primary coke begins to form. The conditions under which the induction period exists include all those used in the experiments except 475°C and reaction times of 30 and 40 min. After the induction period, oil and gas are produced in addition to coke after phase separation of asphaltenes at a reaction rate that has been considered infinite by Wiehe 16. The rate of asphaltene removal in batch operation and assuming a homogeneous perfectly mixed system can be expressed in the form:

3x1~. 4750C '~. 2x50~,

450"C i

~1x10"s. Q

*C //,

dCA-klC ~ +k2C /A/ 2 +k3C nA3

(1)

dt where CA is the asphaltene concentration, kl, k2 and k3 are the kinetic constants for gas, oil and coke formation respectively, n 1, n2 and n 3 are the respective orders of reaction and t is the reaction time. The rate of gas formation is expressed by: dCG = kl CA' dt

1'0

2'0

ab

&

Figure 4 Second-order kinetic plot for asphaltene conversion in residue thermal hydroprocessing

(2) 2.0x'104.

where Cc is the gas concentration. The rate equation for oil formation is: dCo = k2CnA2 dt where Co is the oil concentration. The rate equation for coke formation is:

425'~c

(3)

~~ 15xl04"

dCc = k3 C~a3 (4) dt where Cc is the coke concentration. For nl = n2 = n3 = l, the integrated form of Equation (1) would be: CAo ln-~A = (kl + k2 + k3)t

- kl CAo CG -- kl + k2 + k3 {exp[ - (k I + k2 + k3)t] - 1}

T,-

(6)

0.0

(7) where Coo is the initial concentration of oil. Coke formation from Equations (4) and (5) would be given by: - - k3CAo [exp[ - (k I +k2 + k3)t] - 1} kl +k2 +k3

(8) The global rate constant for asphaltene decomposition, k ---k i + k2 + k3, can be obtained from Equation (5) by plotting ln(CAo/CA) versus t. For n ~= n2 = n3 = 2, the second-order kinetic expression for asphaltene conversion would be: 1

1 - -

CA

=

kt

(9)

CAo

and the expressions for gas, oil and coke formation

902

Fuel 1997 V o l u m e 76 N u m b e r 10

5=C

o:1 lit (min1)

o.o

0:2

Figure 5 Second-order kinetic plot for gas formation in residue thermal hydroprocessing respectively:

Oil formation from Equations (3) and (5) would be given by: - k2CAo Co - Coo - kl +k2 +k3 {exp[ - (kl +k2 + k3)t] - 1}

D

5.0x10s'

(5)

where CAo is the initial concentration of asphaltenes. From Equations (2) and (5), the expression for gas formation would be:

~ 4 5 0 , C

1.0xl0.4"

i

Cc - Cco =

50

t (min)

1

k I +k 2 +k 3

C6

kl CAo 1

CO - Coo -

-

1

C C - Cco

1

_kl+k2+k3 k2Cgo --

kl +k2 +k3 k3CAo

1

~- -

1

1

-

k2C2Ao t - -

1

1

"+"k3C2o t

(11)

(12)

The data in Table 2 fit second-order kinetics for all asphaltene conversion and oil and gas formation. By plotting the lefthand sides of Equations (9)-(12) versus t or 1/t the slopes can be obtained and the rate constants kl, k2 and k3 calculated. Figures 4 - 6 show second-order kinetic plots at the three temperatures studied for asphaltene conversion and gas and oil formation respectively. The activation energies were calculated by applying the Arrhenius equation and plotting In k versus 1/T, Figure 7. The values of the rate constants and activation energies are shown in Table 3. Coke formation fits second-order kinetics at 450 and 475°C. At 425°C no coke formation was observed until

Kinetics of asphaltene hydroconversion: 1: M. 7". Martfnez et al.

1.2~1os' 425° (

Table 4 Elemental analyses (wt%) and atomic ratios of oil (hexane-soluble fraction) from the products of hydrotreating experiments Residence time (min)

~.~ 8.~o ~ 'E

475ot

¢n

4.0x10~

0.0

o.oo

o.~

o.~o

o.~5

o.~o

0.25

lit (min1) F i g u r e 6 Second-order kinetic plot for oil formation in residue thermal hydroprocessing -16

-

1

-19

7

~

~

~

Oils

1.35,xlO.3

1.40~10. 3 lIT (K1)

1.45,x10_ 3

1.50x10-3

F i g u r e 7 A r r h e n i u s plot for the rate c o n s t a n t s o f asphaltene c o n v e r s i o n and oil a n d gas formation

Table 3 Rate constants and activation energies for asphaltene conversion and oil and gas formation in thermal hydroprocessing

Asphaltene conversion 425°C 450°C 475°C Kinetic order 2 Activation energy 110 kJ mol -I (r = 0.92) Coke formation 450°C 475°C Kinetic order 2 Oil formation 425°C 450°C 475°C Kinetic order 2 Activation energy 140 kJ mol-I (r = 1.00) Gas formation 425°C 450°C 475°C Kinetic order 2 Activation energy 104 kJ mol-i (r = 0.99)

Feed

5

10

20

30

40

89.3 5.6 0.9 1.5 2.3 0.80 8.54 6.42

87.1 6.3 0.8 1.5 4.3 0.87 7.97 6.37

88.3 6.3 0.8 1.6 3.0 0.85 7.96 6.71

89.2 6.2 0.8 1.5 2.3 0.84 7.59 6.31

88.1 6.4 0.8 1.6 2.4 0.86 7.91 6.71

87.9 6.3 0.9 1.4 3.6 0.86 8.78 6.02

89.3 5.6 0.9 1.5 2.3 0.80 8.54 6.42

88.2 6.2 0.9 1.8 3.0 0.85 7.77 7.35

88.3 6.4 0.9 1.8 2.7 0.86 8.54 7.60

88.5 6.2 0.9 1.3 3.0 0.84 9.01 5.64

89.5 6.4 0.8 1.3 2.0 0.86 8.04 5.32

89.3 6.4 0.8 1.7 1.8 0.86 7.58 7.31

89.3 5.6 0.9 1.5 2.3 0.80 8.54 6.42

87.3 6.2 0.9 1.7 3.9 0.86 8.44 7.26

88.5 6.3 0.8 1.6 2.8 0.86 7.94 6.65

89.5 6.4 0.9 1.5 1.6 0.86 8.62 6.32

89.9 6.4 0.8 1.4 1.5 0.85 8.01 6.05

88.4 5.9 0.9 1.3 3.4 0.80 9.21 5.43

Gases

-20 ~

-21

Experiments at 425°C C H N S O H/C 103 N/C 103 S/C Experiments at 450°C C H N S O H/C 103 N/C 103 S/C Experiments at 475°C C H N S O H/C 103 N/C 103 S/C

k (ppm i min-i)

r

1.23 X 10 -8 1.47 X 10 -8 4.38 x 10 -8

0.98 0.95 1.00

7.53 x 10 -~° 6.14 × 10 -I°

0.92 1.00

1.58 × 10 9 4.11 x 10 -9 7.82 x 10 -9

0.99 0.96 0.97

3.94 X 10 -9 8.49 x 10 -9 1.30 X 10 -8

0.99 0.99 0.96

40 min of reaction and the rate constant for coke formation at this temperature has not been obtained. The rate constants for secondary coke formation at 450 and 475°C are indicated in Table 3. It can be seen that the rate constant at 450°C was higher than that at 475°C. This could be due to some contribution from primary coke at 450°C and long reaction times that overestimates the secondary coke formation and makes the rate constant in the kinetic plot increase. The rate constants for asphaltenes removal are somewhat lower than those obtained for thermal cracking of the same feedstock 12, but the values are of the same order of magnitude. The activation energies are also similar, but a little higher in hydroprocessing. The rate constants for coke formation are lower in hydroprocessing. Second-order kinetics provide an excellent fit for the products but could result from a lump of a large spectrum of components with widely different reactivities and first-order parallel reactions. The structural analysis of the oils indicated that the presence of hydrogen led to the hydrogenation of some of the radicals arising by thermal cracking. The H/C ratio, Table 4, was higher for the oils from the products, but with increasing temperature and reaction time it decreased. This is especially true at 475°C and long reaction times, when condensation reactions predominate. The aromatic hydrogen, Table 5, decreased during the process and was lower for oil from the products obtained at 450°C. The lowest aromaticity and the lowest degree of condensation, represented by the H J C a~parameter, Table 6, was obtained at 450°C and 40 min reaction. At 425 and 450°C, Hat decreased with increasing reaction time. However, at 475°C it decreased for reaction times

Fuel 1997 Volume

76 Number

10

903

Kinetics of asphaltene hydroconversion: I: M. T. Martinez et al. Table 5 Hydrogen distribution by IH n.m.r, of oil from the products of hydroprocessing experiments Chemical shift (ppm)

Residence time (min) Feed

5

10

20

30

40

Experiments at 425°C Har (aromatic)

6.0-9.0

69.6

67.1

62.9

63.1

H OH (phenolic)

5.0-6.0

0.3

0.1

0.2

0.1

HF (ring-joining methylene, Ar-CH2-Ar)

3.3-4.5

4.6

H,~ (CH3, CH2 & CH ~ to an aromatic ring)

1.9-3.3

14.1

-12.3

61.9

60.9

--

--

4.3

4.1

4.0

3.8

14.7

14.9

15.3

15.7

H, (CH2 and CH/3 to an aromatic ring)

1.6-1.9

2.4

3.5

2.9

3.0

3.0

3.4

Ha (/3-CH3, CH 2 and CH "/or further from an aromatic ring)

1.0-1.6

8.4

15.7

13.9

13.7

14.7

15.2

H v (CH3 3' or further from an aromatic ring)

0.5-1

0.5

1.3

1.0

1.1

1.1

H~r

6.0-9.0

69.6

65.1

61.6

61.9

61.6

60.0

HoH

5.0-6.0

0.3

0.3

0.0

0.7

0.4

0.3

HF

3.3-4.5

4.6

4.0

4.1

3.4

4.0

3.8

H=

1.9-3.3

14.1

14.2

15.1

~ 14.7

13.8

15.40

H,

1.6-1.9

2.4

6.1

3.1

3.1

4.1

2.5

Ha

1.0-1.6

8.4

9.4

15.1

14.9

14.7

17.0

Hv

0.5-1

0.5

0.9

0.9

1.3

1.4

1.0

H~r

6.0-9.0

69.6

63.9

63.5

63.0

64.7

70.9

HoH

5.0-6.0

0.3

0.3

0.3

0.2

0.3

H~

3.3-4.5

4.6

3.3

3.9

3.5

3.5

3.3

H~,

1.9-3.3

14.1

14.1

14.3

14.4

14.7

13.2

H,

1.6-1.9

2.4

Ha

1.0-1.6

8.4

H~

0.5-1

0.5

0.98

Experiments at 450°C

Experiments at 475°C

Table 6

--

6.0 114 1.0

2.4

3.5

2.3

3.4

15.1

14.5

13.7

8.3

0.7

0.7

0.8

0.7

Structural parameters a by IH n.m.r, of the oil from hydrocracking experiments Residence time (min) Feed

5

10

20

30

40

0.9

0.9

0.9

0.9

0.9

0.9

0.1

0.1

0.1

0.1

0.1

0.1

HJC~

0.7

0.7

0.7

0.7

0.7

0.7

HoPrla

1.1

8.2

1.5

1.5

1.5

1.5

Expefiments~425°C

Expefiments~450°C 0.9

0.9

0.9

0.9

0.9

0.9

a

0.1

0.1

0.1

0.1

0.1

0.1

HJC~

0.7

0.7

0.8

0.7

0.7

0.7

Ho]I--Ia

1.1

1.4

1.5

1.5

1.7

1.6

Experimentsat475°C 0.9

0.9

0.9

0.9

0.9

0.9

a

0.1

0.1

0.1

0.1

0.1

0.1

Ha/Car

0.7

0.7

0.7

0.7

0.7

0.7

Ho/Ha

1.1

1.6

1.6

1.5

1.4

1.2

~fa, aromaticity; o, degree of substitution; Ha/Car, aromatic hydrogen/aromatic carbon ratio; Ho/Ha, average chain length

from 5 to 20 min and then increased at longer reaction times, becoming higher than in the oils from products obtained at 450°C except at 5 min of reaction. This supports the condensation mechanism proposed for secondary coke formation, with reaction rates increasing with increasing temperature. It is important to emphasize the increase in Ht3 in the oils from products obtained at the three temperatures, which indicates /3-scission of the side chains. The highest Hv

g04

Fuel 1997 Volume 76 Number 10

corresponds to the mildest conditions, indicating that breakage of the side chains is favoured by increasing the temperature. CONCLUSIONS The experimental data fitted second-order kinetics for asphaltene conversion and for oil, gas and coke formation. The rate constants for asphaltene conversion were lower

Kinetics of asphaltene hydroconversion: 1: M. T. Martinez et al.

than those obtained in thermal cracking. The rate constants for coke formation were much lower than those corresponding to thermal cracking. By Delplot analysis, two pathways for coke production were discerned. By one of them, primary coke is produced after an induction period; by the other, coke appears as a secondary product from a sequence of polymerization steps. Primary coke formation is strongly inhibited by the presence of hydrogen and it was observed only at 475°C and long reaction times. Secondary coke was produced at 450 and 475°C but only at long reaction times at 425°C. Structural analyses indicated that the aromaticity of the oils from products obtained at 450°C was lower than at 425 and 475°C, indicating an increase in condensation reactions on increasing the temperature from 450 to 475°C. On the other hand the highest Hv corresponded to the mildest reaction conditions, indicating that breakage of the side chains was favoured by increasing the temperature.

5 6

7 8 9 10 11 12 13

ACKNOWLEDGEMENTS This woik was sponsored by the European Union, Contract No. EN3V-0055E, and the Spanish DGICYT, Project CE89007.

14 15 16

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