Ind. Eng. Chem. Res. 1993,32, 2447-2454
2447
KINETICS, CATALYSIS, AND REACTION ENGINEERING A Phase-Separation Kinetic Model for Coke Formation Irwin A. Wiehe Corporate Research Laboratories, Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801-0998
Coke formation during the thermolysis of petroleum residua is postulated to occur by a mechanism that involves the liquid-liquid phase separation of reacted asphaltenes to form a phase that is lean in abstractable hydrogen. This mechanism provides the basis of a model that quantitatively describes the kinetics for the thermolysis of Cold Lake vacuum residuum and its deasphalted oil in a n opentube reactor at 400 "C. The previously unreacted asphaltenes were found to be the fraction with the highest rate of thermal reaction but with the least extent of reaction. This not only described the appearance and disappearance of asphaltenes but also quantitatively described the variation in molecular weight and hydrogen content of the asphaltenes with reaction time. Further evidence of the liquid-liquid phase separation was the observation of spherical particles of liquid crystalline coke and the preferential conversion of the most associated asphaltenes to coke. Thermal-cracking processes are commonly used today to convert petroleum vacuum residua into distillable liquid products. Example processes include visbreaking, delayed coking, Fluid Coking, FLEXICOKING, and Eureka. In all of these processes the simultaneous formation of coke limits the conversion to distillable liquid products. For visbreaking and the Eureka processes that require flowing liquid and gaseous products, the conversion has to be held below the onset of coke formation. Coking processes, which are designed to accept this solid byproduct, allow for much higher conversions to distillable liquids. However, the significant fraction (10-30 wt ?6 ) converted to coke is much less valuable than that converted to distillable liquids. Although many aspects of the kinetics of residuum thermolysis and the mechanism of coke formation have been published, there has not been much effort at pulling these together in one comprehensive model. Many previous investigators follow the lead of Levinter et al. (1966,1967) and postulate that coke is produced as a direct byproduct of residuum thermolysis by a sequence of polymerization, condensation steps from the lightest to the heaviest fractions:
- -
-
-
oils resins asphaltenes carbenes coke Present evidence (Wiehe, 1992) shows that this reaction pathway results from aromaticity increase,oligomerization, and combinations of these. However, kinetic models that contain such sequences of direct chemical reaction to coke fail to predict the induction period before coke formation begins. This induction period has been observed experimentally by many previous investigators (Levinter et al., 1966,1967; Magaril and Akseneva, 1968; Valyavin et al., 1979;Takatsukaet al., 1989a)as well as making visbreaking and the Eureka processes possible. Magaril et al. (1968, 1970,1971)were the first to postulate that coke formation is triggered by the phase separation of asphaltenes. Unfortunately, their scattered kinetic data led them to use linear variations of the concentration of each fraction with reaction time, or zeroth-order kinetics, rather than first-order kinetics expected for thermolysis. More recently, Yan (1987)described coke formation in visbreaking as resulting from a phase-separation step but did not include this step in his kinetic model for coke formation. QSSS-5SS5/93/2632-2447$04.0Ql0
This mechanism also appears in the residua hydroconversion literature (Sosnowski et al., 1980). Takatsuka et al. (1989b) even included a phase-separation step to form "dry sludge" in their hydroconversion kinetic model but neglected this step to form coke in their thermal-cracking model (Takatsuka et al., 1989a). Several kinetic studies have concentrated on the covnersion of the asphaltene portion of petroleum residua. Schucker and Keweshan (1980) showed that Cold Lake asphaltenes exhibit a coke-induction period when thermally converted at 400 "C under 6 MPa of hydrogen that is greatly extended by adding 200 ppm of dispersed molybdenum catalyst. However, their kinetic model did not include a coke-formation step. Savage et al. (1985, 1988)observed a coke-induction period for the thermolysis of asphaltenes from an off-shore California crude at 400 "C that disappeared when the thermolysis temperature was raised to 450 "C. Savage and Klein (1989) have developed a very sophisticated Monte Carlo simulation using asphaltene structural information and model compound kinetics. They assigned a product species to toluene-insoluble coke if it had a molecular weight of greater than 300 and an H/C atomic ratio of less than 1. Wiehe (1992) has shown that the use of molecular weight and H/C atomic ratio to assign solubility fractions is a reasonable one but has experimentally determined a different set of values for coke than Savage and Klein. Meanwhile, the Klein group (Neurock et al., 1990;Nigam et al., 1990) has begun to use a regular-solution model for liquid-solid equilibrium prediction to determine how species in their Monte Carlo simulation are divided into maltenes, asphaltenes, and coke. As yet, however, their simulation does not allow for reactions that produce molecular weight increase. Experimental Section Isothermal, batch reactors were used that may be open or closed to the atmosphere. The open reaction was a quartz test tube containing about 3 g of reactant that was heated to within 95 5% of the 400 OC reaction temperature in less than 4 min by inserting it in a preheated, vertical tube furnace. Cold Lake vacuum residuum with normal 0 1993 American Chemical Society
2448 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 VOLATILES A
INSOLUBLE COKE A
INSOLUBLE ASPHALTENES 4
REACTANT C ASPHALTENE FRACTION
THERMAL REACTION
TOLUENE FILTRATION
n FULL RESIDUA
HEPTANE FILTRATION
0 HEPTANE SOLUBLE FRACTION
c HEPTANE SOLUBLES (MALTENES)
Figure 1. Separation scheme for reaction products.
boiling points above 566 "C and its fractions were used as reactants for this study. Thus, the residuum had to be thermally cracked to become volatile under the experimental conditions. Nitrogen flowing over the sample facilitated removal of volatile liquids when they formed while preventing refluxing as well as oxidation of the hot residuum. A thermocouple inserted in the sample was used to monitor and to control the temperature. At the end of each reaction time the quartz tube was removed from the tube furnace and inserted into ice-water to cool the sample to below 200 "C within 20 s. The asphaltene fraction could not be reacted in an open reactor because of its tendency to foam out of the reactor. For this reason, a closed-tube reactor was used. This reactor was a presulfided stainless steel tubing bomb containing about 5 g of asphaltenes and 1.2 MPa of nitrogen. It was heated within 95% of the reaction temperature in less than 3 min by inserting it in a preheated, fluidized sand bath. Again a thermocouple inserted in the sample assured that the rapid heat-up, isothermal condition for the designated reaction time and rapid cooling with cold water was obtained. Separation Scheme. A vacuum residuum contains a complex mixture of thousands of molecules with a wide distribution of molecular sizes and chemical structures. Therefore, measuring the chemical kinetics of each compound is unrealistic. Rather, the reactants and products were divided into a small number of compound classes based on volatility and solubility in solvents. Then kinetics were used to describe the net conversion of compounds from one class to another as a function of reaction time. Figure 1 showsthe separation scheme used to fractionate the reactants and the products. The gas and volatile liquids that are formed during the reaction are allowed to escape the open reactor, resulting in the first class. This is measured by the difference in weight of the reactor before the after reaction. The remaining reaction mixture was separated into toluene-insoluble coke, toluene-solubleheptane-insoluble asphaltenes, and the nonvolatile heptane solubles or maltenes. In each of these solvent separations the samples are allowed to stand overnight in 15 parts of solvent per part heavy oil. This mixture was poured through a fine (4-5.5 pm pores) fritted glass filter. The solids on the filter were washed in stages with at least 25 parts additional solvent and continued with more until the solvent passed the filter without color. The author has found this multistage approach to be more efficient and effective (more reproducible yields, lower H/C ratio) than the more typical dilution with 40 volumes of solvent in one step. The insolubles were vacuum dried on the filter at 100 "C for at least 8 h. The solvent was removed from the soluble oil by rotary evaporation at 50 "C followed by vacuum drying for 8 h at 50 "C. The same procedures was followed for the closed reactor without removing the volatile oils. However,in the process of evaporating solvent the more volatile oils also evaporated. Therefore, the closed reactor was used to measure the rate of change of asphaltenes and the rate of appearance of coke without any attempt to measure the rate of appearance of volatile and nonvolatile heptane solubles separately.
TIME AT 400'C. MIN
Figure 2. Temporal coke formation curves for residuum and residuum fractions.
Previously (Wiehe, 1992), it has been shown that each of these solvent-separation classes: toluene-insoluble coke, asphaltenes, and heptane solubles occupy a unique area on a plot of molecular weight versus hydrogen content, the solvent-resid phase diagram. Generally, the conversion from heptane solubles to asphaltenes to coke is in the direction of higher molecular weight and higher aromaticity (lower hydrogen content). However, the conversion from solvent-soluble to solvent-insoluble class can occur also by molecular weight growth a t equal aromaticity or by aromaticity increase at equal or lower molecular weight. Molecular Weight Measurement. The number average molecular weight of asphaltenes and coke separated from the feed and thermolysis products were measured using a Knauer vapor pressure osmometer. The relatively unassociated molecular weight was measured using odichlorbenzene at 130 "C, and an associated molecular weight of asphaltenes was measured using toluene at 50 "C. In each case three to five molecular weights were measured at varying concentrations in the solvents. The number-average molecular weight reported was obtained by linearly extrapolating the data to zero concentration. Results Cold Lake vacuum residuum was separated into 25.0 wt 5% asphaltenes and 75.0 wt % heptane solubles using the procedure just described. The unreacted residua contains neither toluene insolubles nor volatiles. The heptane solubles and the full residuum were reacted at various times at 400 "C in the open reactor and separated according to the scheme described. The asphaltenes were also reacted at 400 OC for various times in the close reactor, and the products were separated into fractions. For each of these reactants the variation in the concentration of each of the solvent-volatility fractions have been plotted against reaction time. These plots have shown four common features of residuum thermal conversion kinetics that will be described in the following. Coke-Induction Period. Figure 2 shows the formation of coke as a function of reaction time for three reactants: heptane solubles,asphaltenes, and the full residuum. When the reactant is the asphaltene fraction, coke forms immediately at a high rate without an induction period. When the reactant is the heptane-soluble fraction there is a 90min induction period when no coke forms after which it forms at a slow rate. We might expect that the full residuum, composed of 25 wt % asphaltenes, to form coke initially at about a quarter of the rate as the asphaltenes. Instead, we observe a coke-induction period of 45 min. This coke-induction period is our first common feature of residua thermal conversion kinetics. It demonstrates that
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2449
'
Experimental Data
:
:
\
I
0 VOLATILES 0 HEPTANE SOLU0LES
Experlmental Data HEPTANE SOLUBLE REACTANT
TIME AT 400'C. MIN.
FULL RESIDUUM REACTANT
Figure 3. Temporal variation in the four product classes from thermolysis of the heptane-soluble fraction. Curves were calculated from the phase-separation kinetic model.
the heptane solubles inhibit the formation of coke by the asphaltenes. As pointed out earlier in this paper, many other investigators have observed this coke-induction period in residua thermal conversion kinetics. Asphaltene Maximum. Figure 3 shows the conversion of the heptane-soluble reactant to the four classes as a function of reaction time. The asphaltene concentration increases from zero to a maximum and then decreases. This maximum occurs at the same reaction time as the end of the coke-induction period. This maximum is the second common feature of residua thermal conversion kinetics. It is a result of heptane solubles reacting to form asphaltenes which in turn react to form coke. This maximum in the asphaltene concentration has also been observed by other investigators (Levinter et al., 1966; Magaril and Aksenova, 1968; Valyvin et al., 1980; Takatsuka et al., 1989). Decrease of Asphaltenes Parallels Decrease of Heptane Solubles. In Figure 3 during the period that coke is formed, the ratio of the asphaltene concentration to the concentration of heptane solubles approaches a constant. This approach to a constant ratio is the third common feature of residua thermal conversion kinetics. Here it is suggested that this ratio is the solubility limit of converted asphaltenes in the heptane solubles. Unlike the other three features, this feature has not previously been reported. Therefore, in this case "common feature" refers only to the author's experience with the thermal kinetics of more than five different vacuum residua. High Reactivity of Unconverted Asphaltenes. A first-order reaction rate constant of 0.013 min-' was obtained by fitting the decrease in heptane solubles with reaction time as shown in Figure 4. While these data include the full residuum and the heptane-soluble fraction as reactants, data for which the coke concentration exceeded 3 w t ?4 were excluded. Once significant coke begins to form, the data deviate from this first-order behavior. As will be discussed later, this has led to the hypothesis of a heptane soluble byproduct for the cokeforming reaction. When the reactant is the asphaltene fraction, the disappearance of the asphaltene fraction, shown in Figure 5, can be described with a first-order kinetic model using a reaction rate constant of 0.026 min.-l This is within experimental error of the value of 0.025 min-l measured by Schucker and Keweshan (1980) at 400 "C under 6 MPa of hydrogen for the same asphaltenes and in the range of 0.025 to 0.030 min-' measured by Savage and Klein (1988) for different asphaltenes under conditions of neat pyrolysis in toluene. Thus, unlike the refractory nature ascribed to asphaltenes by some previous investigators, the uncon-
10
TIME AT 400"C, MIN.
Figure4. Evaluation of first-order rate constant for heptane solubles thermolvsis usina data for which the coke concentration was less than 3 & % . -
2
I L
u)
z F
Y
8
.
'0
20
40
00
80
roo
li0
D
TIME AT 400% MIN.
Figure 5. Evaluation of first-order rate constant for asphaltene thermolysis in the closed-tube reactor.
verted asphaltenes are actually the most thermally reactive fraction of the vacuum residuum but with the least extent of reaction. This high reactivity of unconverted asphaltenes provides the fourth common feature of residua thermal conversion kinetics. Kinetic Model. A kinetic model has been developed that describes these four common features of residua thermal conversion kinetics. It represents the conversion of asphaltenes over the entire conversion range and of heptane solubles in the coke-induction period as firstorder reactions upon the basis of the data in Figures 4 and 5. Each of these reactions give simultaneously lower aromatic and higher aromatic products, on the basis of the evidence described previously (Wiehe, 1992). Also this previous work showed that residua fractions can be converted without completely changing solubility classes. This is represented by having both reactant and product species for asphaltenes and heptane solubles. This model uses the concept of Magaril et al. (1968, 1971) that coke formation is triggered by the phase separation of converted asphaltenes. The maximum solubility of these product asphaltenes is proportional to the total heptane solubles,
2450 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
as suggested by the observation that the decrease in asphaltenes parallels the decrease of heptane solubles. Finally, the conversion of the insolubleproduct asphaltenes into toluene-insoluble coke is pictured as producing a heptane-soluble byproduct as a mechanism for the heptane-soluble conversion to deviate from first-order behavior once coke begins to form. An infinite reaction rate for this coke-forming reaction is used to show that this reaction rate is phase equilibrium controlled: kw
H++aA*
Experimental Data 0 VDLATILES b HEPTANE SOLUBLES
3
"1 \
A ASPHALTEYES TOLUENE INSOLUBLES
+ (1-a)V
h
+ nH* + (1 - rn - n)V solubility limit: A:= = S,(w + H*) A + - , mA*
0
20
40
60
(0
IO0
IPO
140
I40
I80
ZOO
TIME AT 400.C. MIN.
Figure 6. Temporal variation in the four product classes from thermolysis of the full residuum. Curves were calculated from the phase-separation kinetic model.
experimental data on the thermolysis of the heptane soluble fraction: where a is a stoichiometric coefficient; A+, reactant asphaltenes; A*, asphaltene cores; A:=, maximum asphaltene cores that can be held in solution; A:,, excess asphaltene cores beyond what can be held in solution; H+, reactant, nonvolatile heptane solubles; H*, product, nonvolatile heptane solubles; kA, first-order reaction rate constant for reactant asphaltene thermolysis (min-l); kH, first-order reaction rate constant for the thermolysis of reactant heptane solubles (min-l); m, stoichiometric solubility limit coeffcient;n,stoichiometric coefficient;SL, (wt 5% /wt 5% ); TI, toluene-insoluble coke; and V, volatiles. The first two parallel first-order reactions for the thermolysis of unreacted heptane solubles and of unreacted asphaltenes are the only reactions that occur during the coke-induction period. The stoichiometric coefficients provide the fractional split of the converted reactants among the products. During the coke-induction period the reactant asphaltenes form only lower molecular weight products. As long as the asphaltenes remain dissolved, the heptane solubles can provide sufficient abstractable hydrogen to terminate asphaltene-free radicals, making asphaltene radical-asphaltene radical recombinations infrequent. Langer et al. (1961) have shown that partially hydrogenated refinery process streams provide abstractable hydrogen and as a result, inhibit coke formation during residuum thermal conversion. Here it is suggested that the heptane-soluble fraction of a residuum contains naturally occurring, partially hydrogenated aromatics that also provide abstractable hydrogen. As the conversion proceeds, the concentration of asphaltene cores continues to increase and the heptanesoluble fraction continues to decrease until the solubility limit, SL, is reached. Beyond the solubility limit, the excess asphaltene cores, A:,, phase separate to form a second liquid phase that is lean in abstractable hydrogen. As a result, in this new phase asphaltene radical-asphaltene radical recombination is quite frequent, causing a rapid reaction to form solid coke and a byproduct of a heptanesoluble core. Comparison of the Kinetic Model with Quantity Data. Equations were derived on the basis of the phase separation kinetic model and are presented in the Appendix. According to these equations, four constants are required to describe the temporal variation in the four classes when the reactant is the heptane soluble fraction. While kH was determined as discussed previously (Figure 4), the three remaining constants were evaluated to fit the
It, = 0.013 min-'
a = 0.221
S, = 0.61 (heptane soluble reactant)
y = 0.30
The kinetic model, shown in Figure 3, describes quite well the variation in quantity of each of the four compound classes with reaction time. Since the four curves are all nonlinear, the agreement is more than a curve fit. As discussed previously,the asphaltene thermolysis data was used to evaluate the asphaltene rate constant: It, = 0.026 min-'
Since the closed reactor conversion could not provide the split between volatile and nonvolatile heptane solubles, it could not be used to evaluate stoichiometric Coefficients. Instead, the remaining stoichiometric coefficients and a different solubility limit were evaluated using the full residuum thermolysis data:
m = 0.825
n = 0.02
SL= 0.49 (full residual reactant) Again, the agreement between the experimental data of the variation of the four compound classes with reaction time and the nonlinear kinetic model calculation is quite good as is shown in Figure 6. However, the dependence of the solubility limit on the initial asphaltene concentration indicates that the solubility of asphaltene cores depends on whether they are formed from heptane solubles or reactant asphaltenes. Comparison of the Kinetic Model with Asphaltene Quality Data. The asphaltene concentration varies little within the coke-induction period (Figure 6) and then decreases once coke begins to form. As a result, if the thermolysis data were only collected on a full residuum, one might incorrectly conclude that asphaltenes are unreactive. On the contrary, it is the high reactivity of the asphaltenes down to the asphaltene core that offsets the generation of asphaltene cores from the heptane solubles to keep the overall asphaltene concentration nearly constant. In addition it is only by postulating two asphaltene species, reactant asphaltenes and asphaltene cores, that the kinetic model can simultaneously describe the thermolysis data of the two residuum fractions and the whole residuum. Therefore, we sought further evi-
Ind. Eng. Chem. Res., Vol. 32,No. 11,1993 2461
60
100
I000
160
I
SO
100
160
J
TIME AT 400'C, MIN.
TIME AT 400% MIN.
Figure 7. Temporalvariation in the hydrogen content of asphaltenes from the thermolysis of the full residuum. Curve was calculated = 7.94 w t % and from the phase-separation kinetic model using P*= 5.95 wt %.
Figure 8. Temporal variation in the molecular weight (vaporpressure osmometry in o-dichlorobenzene at 130 "C) of the asphaltenea from the thermolysis of the full residuum. Curve was calculated from the phase-separation kinetic model using = 2980 and MA= 1611.
dence in the asphaltene quality data for two asphaltene species and high asphaltene reactivity. Previously, Savage et al. (1988)showed that the hydrogento carbon atomic ratio for the asphaltenes decreases rapidly with reaction time for asphaltene thermolysis and then approaches an asymptotic limit at long reaction times. Schucker and Keweshan (1980)had shown a similar result for asphaltenes reacted under hydrogen pressure for both the hydrogen to carbon ratio and the molecular weight as measured in toluene by vapor pressure osmometry. Although this provides qualitative evidence for asphaltenes cracking down to a core that both of these groups also favored, neither attempted comparing this data quantitativelywith a kinetic model. This is possible because the kinetic models provide the relative amount of the different types of asphaltenes and from the properties of each of the asphaltenes one can calculate the overall asphaltene property:
the asphaltenes from the thermolysis of the full residuum. While the properties of the reactant asphaltenes can be directly measured, the properties of the asphaltene core have to be fitted to the data at long reaction times. Nevertheless, having fixed the initial and long-time property of theasphaltenes, Figure 7 showsthat the kinetic model provides a quantitative description of the hydrogen content between these limits. Although the molecular weight data is more scattered, Figure 8 shows a reasonable description of molecular weight versus reaction time by the kinetic model. Asphaltene Association Factor. The measurement of the molecular weight of petroleum asphaltenes is known to give different values depending on the technique, the solvent and the temperature (Dickie and Yen, 1967; Moschopedis et al., 1976;Speight et. al., 1985). As is shown by small-angle X-ray (Kim and Long, 1979)and neutron (Overfield et al., 1989)scattering, this is because asphaltenes tend to self-associate and form aggregates. Eventually, it is this association tendency that causes asphaltenes to phase separate and form coke. Thus, it is of interest to investigate the change in asphaltene association with thermolysis reaction time. In order to measure the relative tendency for asphaltene association, advantage is made of the effect of environment on the number average molecular weight measured by vapor pressure osmometry. Thus, the asphaltene association factor, a,is defined:
6
HA
=
A+% + A*", A+ + A*
MA
=
A+% + A*M', A+ + A*
where A+ is the reactant asphaltene concentration as predicted by the kinetic model (Appendix) (wt 7%); A*, asphaltene core concentration as predicted by the kinetic model (Appendix) (wt % );HA,overall asphaltene hydrogen content, (wt %); G, hydrogen content of reactant asphaltene (wt %); hydrogen content of asphaltene core (wt %); MA,overall asphaltene molecular weight; molecular weight of reactant asphaltene; and MA, molecular weight of asphaltene core. Hydrogen content, rather than hydrogen to carbon ratio was used becasue the carbon content is nearly constant (82-86w t % ) for all residuum fractions during thermolysis (Wiehe, 1992). Molecular weight was measured by vapor pressure osmometry in o-dichlorobenzene, one of the best asphaltene solvents, a t 130"C, the maximum instrument temperature, to minimize the effect of asphaltene association (Wiehe, 1992). Thus, in Figures 7 and 8 the hydrogen content and molecular weight are compared with the kinetic model for
vA,
wA,
mol w t in toluene at 50 "C (3) mol wt in o-dichlorobenzene a t 130 "C This gives the associated molecular size of the asphaltene measured in toluene, the poorest solvent required to dissolve the asphaltene, at a relatively low temperature divided by our best measure of the unassociated molecular weight (Wiehe, 1992). Figure 9 shows how this asphaltene association factor varies with reaction time for the thermolysis of the full residuum. Also the variation of toluene-insoluble coke with reaction time is replotted in Figure 9. During the 45-min coke-induction period, the association factor is nearly constant at 1.9. However, once coke begins to form, the remaining asphaltenes have less tendency to associate. As more and more toluene insolubles are formed, the association factor continually decreases until at 180 min a=
2462 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 Cold b k a
Vacuum Rmlduum
II
r
-
W C udn* T o l w n ~ D I s p . m u i 180 Mln. Inrolubles In Qulnollna
Figure 11. NO On p ltlil dUgm micrographs of toluene-insoluble coke from 40
80
n
80 LW 120 m at 400-0. Mln.
140
180
180
Figure 9. Temporal variation of the asphaltene asanciation factor for aaphaltenes from thermolysia of the full residuum. Shows that asaoeiated asphaltenee preferentidy phase separate to form coke.
Figure IO. Temporal variation in the molecular weight (vapor preaaureoemornetry ino-dichlorobemneat 130°C)ofthemkeand Mphallanes from the thermolysis of the asphaltene fraction.
the remaining asphaltenes show almost no tendency to asaociate(a= 1). Anerplanationisthatsomeasphaltenes are associated (denoted by circled A's in Figure 9) and someasphaltenesarenot aasociated (denoted by individual A's). Botharedistributed inamatrixofheptanesolubles (denoted by " 8 ) . The associated asphaltenes tend to preferentially phase separate to form coke, as is indicated by the product below the line. Thus, the asphaltenes that remain (those above the line) in solution have, on average, a lower association factor. This process continues with increasing reaction time until all of the associated asphaltenes are phase separated to form coke, causing the association factor to reduce to unity. Coke Molecular Weight. Once sufficient quantity of toluene-insoluble coke is formed from either the thermolysisof the heptane-soluble fraction or the thermolysis of the full residuum, the coke is not completely soluble in o-dichlorobenzeneat 130T used tomeasure themolecular weight. However, since coke forms at high rates at short reaction times during the thermolysis of asphaltenes, the solubility of this coke is high enough to measure its molecular weight as shown in Figure 10. Up through a reaction time of 60 min the molecular weight of the coke continuously increases but at 90 min the coke is not
thermolysis of the full residuum at 400 'C for 180 min and dispersed in quinoline.
completely soluble in o-dichlorobenzene at 130 "C. Meanwhile, the asphaltenes that do not form coke decrease in molecular weightwith increasing readion time by cracking off lower molecular weight byproducts. This is direct evidence thatwhentheasphaltenesareinaseparatephase, they form coke by molecular weight growth that is better characterized as oligomerization than as polymerization. This evidence is consistent with the phase separation mechanism for coke formation. Coke Microscopy. Coke, resulting from the thermolysis of the full residuum for 180 min, was dispersed in quinoline, an excellent solvent for carbonaceous materials, and observed in transmitted light at 600X with an optical microscope (Figure 11). The quinoline insolubles are spheres or agglomerates of spheres and are anisotropic or ordered (appear bright under cross-polarized light). This is direct proof that a t least part of the coke was formed by a liquid-liquid phase separation and that interfacial tension forced them into the spherical shape, much like oil dispersed in water. The liquid crystalline coke, or carbonaceous mesophase, was originally discovered by Brooks and Taylor (1965)and has had an enormous impact on the technology of producing carbon-based products from lower molecular weight, aromatic streams. However, it has not previously been recognized as evidence of a liquid-liquid phase separation in residuum thermolysis. Studies using transmission electron microscopy (Oberlin, 1982)indicate thatthemesophase only growstosizes large enough (0.5 pm and larger) to resolve with an optical microscopeif itsviscosity and the viscosity of the medium remain low. Particles that appear isotropic by optical microscopy often are ordered as measured by electron microscopy but have reached high viscosity before they could coalesce into large enough particles to resolve with an optical microscope. Thus, even quinoline-soluble coke that appears isotropic may be sub-micrometer spheres of partially ordered structures. Summary a n d Conclusions. There are four common features of residuum conversion kinetics: an induction period before coke begins to form, a maximum in the asphaltene concentration at the end of the coke induction period,anapproachtoaconstantratiooftheconcentration of asphaltenes to the concentration of nonvolatile heptane solubles as long reaction times, and high asphaltene reactivity. A mechanism has been proposed to account for thse common features. Asphaltene cores are formed from unreacted asphaltenes and from heptane solubles as a byproduct of forming the more desirable volatile liquids. These asphaltene cores are unreactive as long as they remain dissolved in heptane solubles that provide ab-
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2453 stractable hydrogen. As the conversion is increased, the asphaltene core concentration increases and the nonvolatile heptane solubles concentration decreases until the solubility limit of asphaltene cores in heptane solubles is exceeded. At that point asphaltene cores phase separate to form a second liquid phase that is lean in abstractable hydrogen. The asphaltene cores crack off nonvolatile heptane-solublefragmentsand recombine to form tolueneinsoluble, solid coke. This triggers the formation of coke at end of the induction period and causes a decrease in asphaltene concentration from its maximum value. As the unreactedasphaltenes become depleted, the asphaltene concentration approaches a constant ratio to the nonvolatile heptane solubles concentration because of the solubility limit of the remaining asphaltene cores in nonvolatile heptane solubles. A kinetic model based upon this mechanisms is shown to quantitatively describe experimental kinetic data on the thermolysis of Cold Lake vacuum residuum and ita heptane-soluble fraction in an open tube reactor at 400 "C. The higher reaction rate of unreacted asphaltenes than heptane solubles was substantiated by directly measuring the rate of disappearance of asphaltenes in a closed reactor and comparing with the rate of disappearance of heptane solubles during the coke-inductionperiod in an open reactor. The thermolysis of asphaltenes also indicates that asphaltenes form toluene-insoluble coke without an induction period due to the absence of the heptane-soluble fraction. Molecular weight measurement of the coke showed that the coke was formed by molecular weight growth. Meanwhile, the quantitative description of the decrease in hydrogen content and the decrease in molecular weight of the asphaltenes during the thermolysis of the full residuum validates the use of a two asphaltene species model and provides a measurement of the asphaltene core molecular weight of 1611 and 5.95 wt % for the hydrogen content. If one couples this asphaltene core molecular weight with recent models of asphaltenes (Strausz, et al., 1992) that have only a few polynuclear aromatic rings, one comes to the conclusion that the asphaltene core contains multiple aromatic structures, unlike the large, single aromatic structure previously proposed (Schucker and Keweshan, 1980). The nonvolatile, heptane-soluble byproduct of coke formation from these asphaltene cores is consistent with multiple aromatic structures. Optical microscopy of the coke produced direct evidence for the liquid-liquid phase separation step for coke formation by revealing the presence of spherical particles of liquid crystalline coke. Finally a new method was proposed for measuring the tendency for asphaltenes to associate by the ratio of the molecular weight measured in toluene at 50 OC to that measure in o-dichlorobenzene a t 130 OC. From this measurement it was shown that coke is preferentially formed by the most associated asphaltenes. Again, this is consistent with a phase-separation mechanism for coke formation.
Acknowledgment The author is grateful to Exxon Research and Engineering Company for permission to publish this paper and to M. Lilga and J. L. Machusak for their assistance in doing the experimental work.
Nomenclature a = stoichiometric coefficient A = heptane insoluble asphaltenes A0 = initial asphaltene concentration, wt 9%
A+ = fraction of reactant asphaltene, wt 7% A* = fraction of asphaltene cores, wt 7% A& = maximum asphaltene cores that can be held in solution, wt 7% A: = excess asphaltenes cores beyond what can be held in solution, wt 7% H = nonvolatile heptane solubles HO= initial concentrationof nonvolatile heptane solubles,wt 9%
H+ = fraction of reactant, nonvolatile heptane solubles, wt ?6
H*= fraction of product, nonvolatile heptane solubles,wt 7% H A = overall asphaltene hydrogen content, wt 7% II+A = hydrogen content of reactant asphaltenes, wt 7% gA = hydrogen content of asphaltene cores, wt 7% &A = first-order reaction rate constant for the thermolysis of reactant asphaltenes, min-l k~ = first-order reaction rate constant for the thermolysis of reactant, nonvolatile heptane solubles, min-l m = stoichiometric coefficient M A = overall asphaltene molecular weight & A = molecular weight of reactant asphaltene MA= molecular weight of asphaltene core n = stoichiometric coefficient SL= solubility limit, wt 7% TI = fraction of toluene insoluble coke, wt ?6 V = volatiles CY = asphaltene association factor Appendix
This section provides the equationsfor the kinetic model described in the text. The same nomenclature as defined in the text is used with addition of the following: A0 = initial asphaltene concentration wt %. HO = initial concentration of nonvolatile heptane solubles,wt % The concentration of reactant asphaltenes, A+, reactant heptane solubles, H+,and volatiles, V, are unaffected by whether coke is forming or not.
.
= HoedHt
V = (1 - a)Ho(l- e&")
+ (1 - m - n)Ao(l-
(A-2)
(A-3)
During the coke-induction period
+
A* = aHo(l - e-kHt) mAo(l -
(A-4)
A=A*+A*
(A-5)
H* = nA,(1- e4At)
(A-6)
H=H++H*
(A-7)
TI = 0
(A-8)
The coke induction period ends when
+ H')
A* = S L ( P
thereafter, A*, is given by the above equation and
(A-9)
2454 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
H* = fl+nAo(l - e-kAt) 1-Y
(A-10)
TI = [l - y] [aHo(l - e-kwt)+ mAO( 1 - e-kAt)-
TI= [I - y / ( i + YS,)I
[ a ~ -, (a +
SL(W
+ H*)]
s
~+ w
( m - nSL)Ao(l -
(A-11)
(A-12)
For Cold Lake vacuum residuum at 400 "C, the following constants are used:
k, = 0.013 min-'
k, = 0.026 min-'
a = 0.221
y = 0.30
m = 0.825
n = 0.02
For the full residua:
A0 = 25.0, HO= 75.0, and SL= 0.49. For the heptane-soluble fraction of the residua as the reactant: A0 = 0, HO= 100, and SL= 0.61.
Literature Cited Brooks, J. D.; Taylor, G. H. The Formation of Graphitizing Carbons for the Liquid Phase. Carbon 1965,2,185-193. Dickie, J. P.; Yen, T. F. Macrostructures of the Asphaltic Fractions by Various Instrumental Methods. Anal. Chem. 1967,39,18471852. Kim, H.; Long, R. B. Characterization of Heavy Residuum by a Small Angel X-ray Scattering Technique. Znd. Eng. Chem. Fundam. 1979, 18, 60-63. Langer, A. W.; Stewart, J.; Thompson, C. E.; White, H. T.; Hill, R. M. Thermal Hydrogenation of Crude Residua. Znd. Eng. Chem. 1961,53,27-30. Levinter, M. E.; Medvedeva, M. I.; Panchenkov, G. M.; Aseev, Y. G.; Nedoshivin, Y. N.: Finkelshtein, G. B.: Galiakbarov, M. F. Mechanism' of Coke Formation in the Cracking of Component Groups in Petroleum Residues. Khim. Tekhnol. Topl. Masel. 1966, NO. 9, 31-35. Levinter, M. E.; Medvedeva, M. I.; Panchenkov, G. M.; Agapov, G. I,; Galiakbarov,M. F.; Galikeev, R.K. The MutualEffectof Group ComDonenta During- Coking. - Khim. Tekhol. Topl. Masel. 1967, No. 4, 20-22. Magaril, R. Z.; Aksenora, E. I. Study of the Mechanism of Coke Formation in the Cracking - of Petroleum Resins. Znt. Chem. Eng. 1968,8, 727-729. Magaril, R. Z.; Aksenora, E. I. Investigation of the Mechanism of Coke Formation During Thermal Decompositionof Asphaltenes. Khim. Tekhnol. Topi.-Masel. 1970, No.-7, 22-24.
~
Magaril, R. Z.; Ramazeava, L. F.; Asenora, E. I. Kinetica of the Formation of Coke in Thermal Processing of Crude Oil Znt. Chem. Eng. 1971,11,250-251. Moschopedis, S. E.; Parkash, S.; Speight, J. G. Investigation of Asphaltene Molecular Weighta. Fuel 1976,55,227-232. Neurock, M.; Libanati, C.; Nigam, A.; Klein, M. T. Monte Carlo Simulation of Complex Reaction Systems: Molecular Structure and Reactivity in Modeling Heavy Oils. Chem.Eng. Sci. 1990,45, 2083-2088. Nigam, A,; Neurock, M.; Libanati, C.; Klein, M. T. Asphaltene Rsaction Paths. Kinetic and Thermodynamic Analyses. Pap. AZChE Spring National Meeting, 1990. Oberlin,A. Carbonizationand Graphitization. Carbon 1982,22,521541. Overfield,R.E.; Sheu, E. Y.; Sinha, S. K.; Liang, K. S. SANS Study of Asphaltene Aggregation. Fuel Sci. Technol. Znt. 1989,7,611624. Savage, P. E.; Klein, M. T. Asphaltene Reaction Pathways - V. Chemicaland Mathematical Modeling. Chem.Eng. Sci. 1989,44, 393-404. Savage, P. E.; Klein, M. T.; Kukes, S. G. Asphaltene Reaction Pathways. 1. Thermolysis. Znd. Eng. Chem. Process Des. Dev. 1985,24,1169-1174. Savage, P. E.; Klein, M. T.; Kukes, S. G. Asphaltene Reaction Pathways. 3. Effect of Reaction Environment. Energy Fuels 1988,2,619-628. Schucker, R. C.; Keweshan, C. F. The Reactivity of Cold Lake Asphaltenes. Prepr. Pap.-Am.Chem. SOC.,Diu. Fuel Chem. 1980, 25,156-164. Sosnowski, J.; Turner, D. W.;Ehg, J. Upgrading Heavy Crudes to CleanLiquidProducta. Pap.88thAZChENatwnal Meeting, 1980. Speight, J. G.; Wernick, D. L.; Gould, K. A.; Overfield, R.E.; b o , B. M. L.; Savage, D. W. Molecular Weight and Association of Asphaltenes: A Critical Review. Rev. Znst. Fr. Pet., 1985, 40, 51-61. S t r a w , 0.P.; Mojelsky,T. W.; Lown, E. M. The Molecular Structure of Asphaltene: An Unfolding Story. Fuel 1992, 71, 1365-1363. Takatauka, T.; Kajiyama, R.;Hashimoto, H.; Matauo, I.; Miwa, S. A Practical Model of Thermal Cracking of Residual Oil. J. Chem. Eng. Jpn. 1989a,22, 304-310. Takatuska, T.; Wada, Y.; Hirohama, S.; Fukui, Y. A Prediction Model for Dry Sludge Formation in Residue Hydroconversion. J . Chem. Eng. Jpn. 1989b, 22,298-299. Valyavin, G. G.; Fryazinov, V. V.; Gimaev, R. H.; Syunyaev, Z.I.; Vyatkin, Y. L.; Mulyukov, S. F. Kinetics and Mechanism of the Macromolecular Part of Crude Oil. Khim. Tekhol. Topl. Masel. 1979, No. 8, 8-11. Wiehe, I. A. A Solvent-Resid Phase Diagram for Tracking Resid Conversion. Znd. Eng. Chem. Res. 1992,31,530-536. Yan, T. Y. Coke Formation in Visbreaking Process. Prepr. Pap.Am. Chem. SOC.,Diu. Pet. Chem. 1987,32,490-496.
~
~
~
~
Received for review July 2, 1993 Accepted July 12, 1993. Abstract published in Advance ACS Abstracts, October 1, 1993.
