coking of recycled heavy oil

A four-lump kinetic model for the cracking/coking of recycled heavy oil Narendra

C. Dave,

Gregory

J. Duffy

and Pinky Udaja

CSIRO Division of Coal and Energy Technology, Lucas Heights Research Laboratories, Private Mail Bag 7, Menai, NSW2234, Australia (Received 14 January 1992; revised 73 November 1992)

In recycled solids processes, shale oil undergoes secondary cracking/coking reactions on the surface of the recycle solids used as the heat carrier. Heavy oil recycle uses these secondary reactions to advantage to convert the heavy oil fraction produced during primary pyrolysis reactions to lighter, more desirable products, thereby reducing the overall severity of hydrotreating which would otherwise be required to upgrade this fraction to a refinery grade feedstock. A four-lump kinetic model has been developed to describe the rate of production of the lighter condensable oil, as well as the coke and non-condensable gases which are also formed. This kinetic model can be used to predict yields from the cracking/coking of heavy oil in any type of reactor. (Keywords: kinetics; heavy oils; coking)

Oil shale retorting processes can produce a crude product in which the heavy oil fraction (boiling point above 420°C) can represent over 35% of the product oil. Previous work’ has shown that the heavy oil fractions have the highest contents of heteroatoms. High severity hydrotreating followed by catalytic cracking would therefore be required to convert this fraction into an acceptable refinery feedstock. Heavy oil recycle is seen as an attractive option to avoid the high costs associated with such processing. When a retorting process is operated in heavy oil recycle mode, the heavy oil fraction separated from the product oil is returned to the retort where cracking reactions catalysed by the minerals in the shale reduce the fraction to lighter products. Critical to the design of such a process are the kinetics and stoichiometry of these reactions. This work has therefore been directed towards developing a kinetic model to describe the cracking/ coking reactions which transform the heavy oil fraction into lighter condensable oil, non-condensable gases and coke. It considers both vapour phase cracking reactions and catalytic cracking reactions which lead to coke deposition when the heavy oil is contacted with the shale minerals. The chemistry of these reactions is discussed in more detail elsewhere’. EXPERIMENTAL The experimental work on which this study is based was performed using a heavy oil fraction distilled from Stuart shale oil produced in the CSIRO Process Development Unit pyrolyse?. A simulated distillation of this oil showed that 57.0% was gas oil (boiling point range 34&538”(Z) and 43% was non-volatiles (boiling point Presented at ‘6th Australian Oil Shale Workshop’, S-6 December 1991, The University of Queensland, Queensland, Australia

0016-2361/93/09/1331X)4 0 1993 Butterworth-Heinemann

Ltd

above 538°C). The cracking/coking experiments were performed in a mini-moving packed bed reactor which has been described in more detail elsewhere2*3. In this reactor, oil vapour was contacted with fully combusted shale ash descending in plug flow countercurrent to the flow of oil vapour. The product oils, gases and coke from these experiments have been characterized by elemental and n.m.r. analyses and the results reported elsewhere’. DEVELOPMENT

OF MODEL

Earlier heavy oil studies of Australian oil shales3 have shown that while coking predominates in the presence of spent shale ash, thermal cracking also takes place simultaneously. Thermal cracking is a homogeneous reaction whereas coking is a heterogeneous reaction catalysed by the surface characteristics of the shale ash. A knowledge of cracking and coking kinetics is therefore important for satisfactory design of an oil shale retort which incorporates heavy oil recycle. The coking of heavy oil is a form of catalytic cracking and, as observed with the conventional cat-cracking of gas oil, the acidic sites on spent shale ash become progressively deactivated as coke build-up proceeds with the reaction. Cat-cracking of gas oil has been modelled by several investigators by lumping various products into three or more general lumps”6. It would be reasonable to expect that a similar analysis could be applied to a heavy fraction of shale oil which has heavy gas oil as a major component. The coking of heavy oil can be considered to produce three lumps: non-condensable gases, condensed liquid product and solid residue (i.e. coke). The condensed liquid product results from primary and secondary cracking reactions of heavy oil. In the course of primary and secondary reactions, this light oil itself undergoes secondary reactions, producing non-condensable gases

Fuel 1993 Volume 72 Number 9

1331

A four-lump kinetic model: N. C. Dave et al. Heavy

oil

)

Light

of light oil gives:

oil

dY2 6Y2 ~+ug,=u,R,(y,,t)-Rz(y~,t)

!IJ-----::-::-r,l

Coke

Figure 1

Overall

(Cl-C4

gases)

reaction scheme

as well as coke. Thus, the overall reaction scheme in its most simplified form is as shown in Figure 1. This model therefore divides reactant and product mixtures into four different lumps. The analysis detailed below considers the interactions between the lumps, rather than the interactions between molecular components in the mixture, as a basis for the kinetic study. In developing the model it is assumed that coking of the ‘heavy oil lump’ proceeds by a second-order reaction, whereas the coking of the ‘light oil lump’ is first-order. This is a logical approximation since the light oil lump consists mainly of lighter compounds and therefore will have a smaller range of cat-cracking rates. However, with the heavy oil there will be considerable change in reactivity as the composition of the reacting heavy oil fraction changes, resulting in an increase in the apparent order of the reaction. Therefore, a pseudo-second-order rate should give a better approximation to the nonlinearity of heavy oil coking. It is further assumed that the active acidic sites on spent shale ash follow an exponential first-order decay rule. This implies that the rate of catalytic cracking of heavy oil is a function of concentration in the gas phase as well as the time for which the catalyst particles are exposed to the reaction environment. Hence, at any point in time the rate of reaction of heavy oil is given by:

where R2 =reaction rate for the light oil which is a function of y, and t. It will be noted that a first-order reaction has been assumed for the light oil lump in Equation (3). For a reactor operating under steady state conditions the first terms in Equations (2) and (3) are zero. Since it is likely that both the heavy and light oil molecules will crack on similar sites, it seems reasonable to assume that the decay function will be the same for all reaction paths. It can therefore be shown that the equations describing these reactions can be written in the form:

dy, -= dx

where y, = concentration of heavy oil, k = intrinsic rate constant for the formation of light oil from heavy oil, and a1 =e’-“‘I represents the catalyst decay function, where t = time for which catalyst has been exposed to the oil, a=catalyst decay coefficient. It can be seen that the intrinsic rate constant is simply multiplied by the decay function to yield an effective rate constant following the approach used by WeekmansV6. Assuming that solids and vapour flow through the reactor in an ideal plug flow manner and that there is negligible interparticle diffusion, then a mass balance around an element in an isothermal moving bed reactor for the heavy oil component yields an expression of the form :

S+u$=

-%y* s

(5)

where x = dimensionless reactor length, K. = heavy oil cracking/coking rate constant, K, =light oil cracking/ coking rate constant, a, = mass of light oil produced per mass of heavy oil converted and s = vapour hourly space velocity in the reactor. For a moving bed reactor operating under steady state conditions, the exposure time of the catalyst is simply the catalyst residence time for flow through the entire reactor, t,, multiplied by the fractional distance, x, traversed so that the catalyst decay function becomes: @= e’ - atcx) (6) Similarly, assuming that the C,-C, gaseous products do not crack any further to produce coke, the formation of gas and coke can be described by:

Kl dy, Ko ~=u*-@Y:+u,--Y, i

dn=u&Py:+u,-l~y, s

(7)

Ii S

where u2 =mass of C,-C4 gases produced per mass of heavy oil converted, u3 = mass of coke produced per mass of heavy oil converted, u4=mass of C1-C4 gases produced per mass of light oil converted and a5 =mass of coke produced per mass of light oil converted. Thus, we now have a set of equations which define the instantaneous weight fractions of heavy oil (yl), light oil (yZ), C,-C4 gases (y3) and coke (yJ. The coefficients a, to u3 define the stoichiometry of the reactions for the cracking of heavy oil, while u4 and u5 describe the stoichiometry for the secondary cracking reactions of the light oil product. The solution of these equations5’6 therefore gives the quantities of various product lumps formed during the cracking/coking reactions. The weight fraction of heavy oil remaining unconverted at the reactor outlet is given by:

-R,(y,,t)

where u = the vapour velocity, z= distance along the reactor and R 1= reaction rate for the heavy oil which is a function of y, and t. Now if the stoichiometry of the cracking reactions is such that a, kg of light oil are produced per kg of heavy oil converted, then a similar mass balance for the cracking

1332

s

dy, Ko -=ua,--y: dx s

dy,

(1)

_!5@y~

Fuel 1993 Volume 72 Number 9

while the amount of lighter oil produced is given by 1 y, =r1r2 e-‘2/y’ _ $2 12

_

Y!!! @‘I

_ Ein(r,)

+ Ein

rl

w-v

A four-lump

kinetic model: N. C. Dave et al.

(11) ?-t=Ll,

and

r2=-

Kl K,

(12)

The cracked gas and coke yields, y, and y, respectively, can be obtained in terms of heavy oil fraction and light oil yield by dividing Equations (7) and (8) by Equation (4) to give highly non-linear expressions which must be solved by numerical techniques.

2

0.30

.F 0.25 z 0.20 t; , 0.15 -0 F 0.10 a 0.05

I 0

0.2 Heavy

RESULTS AND DISCUSSION The data from the moving packed bed experiments provided values for the yields y,, y,, y3 and y4 of the various lumps over a range of space velocities, s, and catalyst residence times, t,. Using a combination of linear and non-linear regression techniques, similar to those used by Lee et al. ’ , it was then possible to determine the parameters ~1,K,, K,, a,, Q~, u3, a4 and a5 in Equations (4) to (8). Figure 2 compares experimentally measured yields of light oil (y2), gases (y3) and coke (y4) at 886 K with the model predictions. The x-axis shows the fraction of heavy oil (yi) which remains unconverted, so that heavy oil conversion (1 - y i ) decreases in moving from left to right along the x-axis. It can be seen that the theory predicts an initial increase in light oil yield with increasing conversion of the heavy oil. However, there exists a point beyond which light oil yield decreases, i.e. when the rate at which it is being destroyed by secondary cracking/ coking reactions exceeds the rate of its production from the heavy oil. Thus, as conversion of heavy oil increases towards loo%, the fraction of light oil in the product reaches a maximum and then rapidly declines. However, the end-products of all these cracking reactions, i.e. the non-condensable gases and coke, show a continuous increase with increasing heavy oil conversion. In our experimental programme the aim was to maximize the production of light oil, the most commercially valuable product from the cracking of heavy oil. Unfortunately this meant that the data for light oil production given in Figure 2 were obtained in the region showing the least sensitivity to heavy oil conversion. The results from the production of gases and coke in Figure 2 therefore give a better demonstration of the validity of the model. Figure 3 shows that the theory predicts an increase in the maximum yield of light oil product with increasing reaction temperature. It can be seen that this is the general trend of the experimental data shown in Figure 3 for runs at 847 and 886K. Figure 4 shows that the theory predicts heavy oil conversion to within about 10% over a wide range of temperatures (815 to 886 K). This represents as close an agreement as could be reasonably expected considering that a four-lump model provides at best a crude approximation to the complex series of reactions which are occurring in the reactor. This model suggests that coke formation is a function of heavy oil conversion and the parameters ~1,K,, K,, a,, u2, a?, a4 and u5. Actual determination of coke yield was SUbJeCtto a high degree of experimental error as it required the measurement of low levels of carbon

0.6

0.4 oil weight

0.8

fraction

Figure 2 Product distribution from heavy oil cracking experiments at 886K: 0, light oil; A, cracked gas; 0, coke. Theoretical curves: ‘. , light oil; - - - , cracked gas; -, coke

0.50

;

0.15

r’ 9 -I

0.10

•t

0.05 i

I 0

I

a

0.2 Heavy

Figure 3

0

J

I

I

I

0.4

0.6

0.8

oil weight

fraction

1

Yields of light oil at 847 (0) and 886K (+)

20

40

60

Experimental

80

conversion

100

120

(wt%)

Figure 4 Comparison of predicted and measured heavy oil conversions at various temperatures (K): x, 815; 0,833; A, 847; 0, 886

deposition on the shale ash. However, Figure 5 shows a close agreement between predicted and measured values for coke yield, y,, for temperatures ranging from 815 to 886K. It is one of the strengths of this theory that it enables the prediction of this coke formation in terms of the conversion of heavy oil, as combustion of this coke subsequent to the retorting stage is an important element of heat integration around the plant.

Fuel 1993

Volume

72 Number

9

1333

A four-lump kinetic model: N. C. Dave et al. Table 1 Reaction rate constants and activation energies Temperature (K)

: KY aiKo a,Ko %K* %Ki a,Ki

I 0

10

20

30

Experimental

coke yield (wt8)

40

Figure 5 Comparison ofpredicted and measured coke yields at various temperatures (K): *, 815; 0, 833; A, 847; 0, 886

-\

2.6 1

I 2.4 0.00112 0.00114

I

I

I

I

'.

n‘--.,

.

0.00116 0.00118 0.00120 0.00122 0.00124 lIT(K-')

-

501

E, (kJ mol-‘)

815

833

847

886

01-l)

17.7 12.2 0.897 6.70 2.47 2.75 0.557 0.341

19.7 18.0 1.74 9.34 3.55 3.68 0.852 0.885

20.9 25.0 1.11 12.6 6.62 4.31 0.669 0.436

28.6 40.0 1.52 22.2 10.8 6.92 0.950 0.572

7.24 x 3.10 x 6.57 x 2.22 x 3.80 x 2.39 x 4.42 x 2.19 x

lo3 10’ 10’ 10’ lo* 10’ lo2 lo*

40.9 99.5 44.8 101.6 127.5 76.9 45.4 43.8

‘Rate constant = k, exp( - EJRT)

possible to calculate the stoichiometric coefficients a, to a, for both the primary (heavy oil) and secondary (light oil) cracking/coking reactions. It can therefore be estimated that the primary reactions of heavy oil produce roughly 55g of light oil, 25g of cracked gas, with the remainder as coke for every 100 g of heavy oil converted. Light oil is further cracked due to secondary reactions giving 60 g of cracked gas and the remainder as coke for every 1OOg of light oil cracked.

CONCLUSIONS

I

Figure 6 Arrhenius plot for heavy oil reaction rate and deactivation constants: 0, -, In(a); A, ---, In(K,)

7

ko’

I

The analysis shows that the proposed four-lump model provides an excellent description of the catalytic cracking which a heavy shale oil fraction undergoes when recycled to a retort. In particular, the values of heavy oil conversion predicted by this model are in close agreement with those measured experimentally. One of the advantages of this model is its ability to predict coke formation, an important consideration for the energy balance in the design of an oil shale processing plant.

ACKNOWLEDGEMENTS lYJ 2--Y----s_ ---__ ? ---*--______ -----Y-w 1 F-W.-._._._. -‘-‘-.-.-.-.._._._._._._._._._~, _ P --.-.& ._.-._. _._._. r 0.5 : -‘-.-‘-.~-.-._._._._._._._..._ : b 0.2 1 I I 1 , I 1 0.00112 0.00114 0.00116 0.00118 0.00120 0.00122 0.00124 l/ T(K-I)

This work was partly funded through a collaborative research agreement with Southern Pacific Petroleum NL/Central Pacific Minerals NL, supported by a grant from the Energy Research and Development Corporation. The authors wish to thank M.D. Chensee who performed the cracking/coking experiments, and A. K. Hutchings, A. R. Tibbett and S. B. Weir who performed the chemical analyses.

Figure 7 Arrhenius plots for various cracking/coking rate constants: q , a,K,; A, a,&,; 0, a&,; x, K,; B, G,; A, a&,

REFERENCES Figures 6 and 7 show that the parameters a, Ko, K,, a,KO, a&, asKo, a,K, and a,K, all have an Arrheniustype relationship with temperature. Table I gives values for these parameters, as well as frequency factors and activation energies, for what are effectively the rate constants that define the kinetics for the production of the various lumps. In the development of our model, which follows the analysis of Weekman5v6, we have used the parameters a, to a, to describe the stoichiometry of the cracking/ coking reactions of the heavy and light oil lumps. From the values for the rate constants shown in Table I, it is

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Fuel 1993 Volume 72 Number 9

1 2 3

4 5 6 7

Udaja, P., Duffy, G. J. and Chensee, M. D. Fuel 1990,69,1150 Udaja, P., Duffy, G. J., Fookes, C. J. R. and Chensee, M. D. ‘Proceedings of the Sixth Australian Workshop on Oil Shale’, Brisbane, Australia, 1991, p. 59 Southern Pacific Petroleum NL/Central Pacific Minerals NL, CSIRO Division of Fuel Technology, ‘Integrated Processing of Australian Oil Shales’, National Energy Research Development and Demonstration Program, Project No. 1165, Final Report, Department of Primary Industries and Energy (Australia), 1990 Oliveira, L. L. and Biscaia, E. C. Znd. Eng. Chem. Rex 1989,28, 264 Weekman, V. W. and Nate, D. M. AlChE J. 1970,16, 397 Weekman, V. W. AlChE Monograph Series 1979, 75, 29 Lee, L.-S., Chen, Y.-W. and Huang, T.-N. Can. J. Chem. Eng. 1989,67, 615