A comprehensive program for visbreaking simulation: product amounts and their properties prediction

Computers chem. Engng Vol.21, No. 10, pp. 1125-1134. 1997

Pergamon PII: S0098-1354(96)00323-7

© 1997ElsevierScienceLtd.All rightsreserved Printedin Great Britain 0098-1354/97$17.00+0.00

A comprehensive program for visbreaking simulation: product amounts and their properties prediction M. Dente, a G. Bozzano a* and G. Bussani b

Chem. Eng. Dept. G. Natta, Politecnico di Milano, P.zza L. Da Vinci, 32, Milano, Italy b K.T.I.S.p.A., Via Ripamonti, 133, Milano, Italy Abstract The visbreaking process is adopted in many refineries, particularly in Europe and the Far East. It consists of a liquid phase pyrolysis of atmospheric or vacuum residues, with the aim to reduce the viscosity of the visbroken residues and, in doing that, to significantly increase the distillate yields. In spite of the importance of the process, mainly empirical models have been proposed. However, rising interest appears to be towards a deeper understanding of the phenomena influencing process performance. The largest difficulties, in principle, are connected with the infinite number of components, reactions involved and, in the mean time, to the relatively poor level of the available data for the feedstock characterisation. Also, the prediction of the product amounts and their properties presents considerable problems. A mechanistic approach to this process is proposed, including aspects such as fouling of the coils, effluent amounts and properties (residue stabilities, sulphur and asphaltene content, viscosity, specific gravity and so on). A set of results is compared with data from literature, research lab tests and commercial units. © 1997 Elsevier Science Ltd Keywords: visbreaking; pyrolysis; fouling; residues stability

Introduction For many years pyrolytic liquid-phase processes (i.e. Thermal Cracking, Delayed Coking and Visbreaking) have been adopted in many refineries. They are mainly aimed to increase the economical value of the oils heavy ends by transforming them into more valuable products. The process defined as thermal cracking is applied to vacuum gasoils; the so-called delayed coking and visbreaking have been preferred for treating atmospheric or vacuum distillation hydrocarbon residues. The selection among these different (and sometimes associated) HC residue pyrolytic treatments historically has depended on: • quality of the available crude • environmental policy of the refinery or of the country • the possibility of integration with the other refinery processes • use and saleability of the products (e.g. fuel oil, asphalt, coke) • specific management decisions, and so on. This paper will be devoted to the description of the * Author to whom correspondence should be addressed. E-mail: dente @ipmch 10.chin.polimi.it; Fax: + 39-2-70638173.

principles used, and the performance, of a model simulating the third of the mentioned processes, i.e. visbreaking. Kinetics aspec~ Part of the rules and criteria adopted when facing the problem of kinetic modelling are that the liquid-phase pyrolysis of HC residues present strong analogies with the ones already successfully adopted in extended gasphase pyrolysis kinetic schemes: several of them have been presented in the literature (Dente and Ranzi, 1983; Clymans and Froment, 1984; Hillewaert et al., 1988; Dente et al., 1992). The complexity of the molecular statistical characterisation of the HC residues and pyrolytic products has been reduced by grouping the infinite number of real components into a relatively limited number of pseudocomponents (Jacob et al., 1976; Vinckier and Froment, 1991; Quann and Jaffe, 1992). The herein adopted internal statistical distribution (in terms of methylation degree, multiple rings etc.) of the possible real components, within the groups pertaining to the different classes (paraffines, aromatics etc.), has been described in a previous paper (Dente et al., 1993). The global kinetic mechanism is usually one of the

1125

1126

M. DENTEet al.

radical elementary chain processes in hydrocarbons entropies and energies, for their transposition into the pyrolysis. In the present case it is essentially charac- condensed state (Benson, 1960). The corrective contributions to be applied to Hterised by initiation, fl-scission, H-abstraction, internal isomerisation of radicals, substitutive addition of radi- abstractions and fl-scissions have reasonably been cals onto unsaturated molecules and radical recombina- assumed to be about zero, so that the same figures of the hypothetical reference gas phase are used. On the tion (= termination) reactions. Because of the strong sterical hindrance in the liquid contrary, significant corrections (coherent with the state, molecular rotational movements of large previous criterion) have to be applied to the initiation sequences of C-C segments are so largely reduced that reactions (i.e. the splitting of the C-C bonds of internal isomerization reactions of the radicals can be molecules for generating two radicals). Some examples of the approximate initiation reacpractically neglected. The substitutive-addition reactions can be of impor- tion's kinetic constants adopted in the model are shown tance on methylated (more in general alkylated) poly- below, related to the C-C bond cleavage in the liquid aromatic molecules; it consists of the addition of highly phase: resonant aromatic radicals (i.e. benzyl-like) to the alkyl- 1. kinetic constant for the single n-paraffinic bond substituted positions of the aromatic rings, followed by cleavage: the expulsion of the alkyl radical (e.g. CH3), and consequently the substitution of those (paraffinic) groups with aromatic structures. The net effect is equivalent to k= 10 'agexp T - - C yJSC - a polymerisation of alkylaromatic molecules. These reactions are very important in the Delayed Coking process but, because of the shorter residence time, in the 2. kinetic constant for the single i-paraffinic bond case of Visbreaking, they can generally be disregarded. cleavage: Substantially, H-abstraction plus fl-scission can be considered the only effective propagation steps of the chain. 40500+corr(nc) C In principle, therefore, the H abstraction on the k= 1014"9exp T I pseudo-components is followed only by the resulting radical kinetic competition among their H re-abstraction (on the neighbouring hydrogen of other molecules) and their fl-scission (that produces smaller pseudo-compo- 3. kinetic constant for the resonant (benzyl-like in the mono- or poly-aromatics, allyl-like in the olefins) nents and radicals). However the related activation bond cleavage: energies, typical of H-abstraction reactions, together with the high concentration of neighbouring H atoms in the liquid phase, make the H re-abstractions much faster 36000+corr(nc) k= 10'3Sexp T than the fl-scission rates. Therefore these latter constitute the rate determining step of the components decomposition. Of course, the (smaller) radicals pro--C----C--C 7CC-duced through those fl-scissions, quasi-immediately generate the equivalent molecules before undergoing a new fl-scission. The final and practical consequence of having defined the function c o r r ( n c ) = 5 7 5 ~ J these considerations, regarding the propagation reac- (200+no), where n c is the total number of the paraffinic tions, is that for the decomposition fate of every pseudo- carbon atoms of the component. In the liquid state, the component it can be proposed that a specific stoichio- recombination rates are determined by the oriented metry of the global reaction takes into account the collision of radicals. That means, besides geometrical probability of scission of the different kinds of bonds factors, by molecular diffusion of radicalic positions at (the stoichiometric coefficients, slightly varying with the atomic radius distances: therefore, they are inversely proportional to the viscosity of the liquid phase (Benson, temperature, have been standardised at 700 K). 1960). The presently developed version of the kinetic model contains about 150 grouped components (reactants and products) and 100 equivalent global reactions. The kinetic constants (i.e. frequency factors and Generalities on VB. Reactor modelling activation energies) of all kinds of reaction involving The visbreaking reactor is constituted by long coils paraffines, aromatics, olefins and diolefins etc., have been derived from the equivalent ones of a hypothetical (generally two and sometimes three) horizontally or equivalent gas phase (where consolidated rules for vertically placed in a furnace, where a radiant and a hydrocarbons are already available, see Dente et al., convective section can be distinguished. Frequently two 1992). The corresponding constants of the liquid phase consecutive cells (heating and soaking cell), with have been obtained by applying into the previous ones independent firing, are present. In several plants the coils the additive corrective contributions of the activation are followed by an adiabatic reaction extra-volume,

41300+corr(nc)]

-

]

--CfZC--

-

Reactormodelling

]

Comprehensive program for visbreaking simulation called soaker, which is eventually equipped with an internal, stage-wise structure of perforated plates. The purpose of the coil-plus-soaker configuration is to increase the total residence time by a factor of two-three (compared to that of the simple coil configuration), and therefore reducing the average V.B. temperature of 20-30°C. The relatively low maximum process temperature level (<500°C), suggests that practically no significant reaction is taking place in the vapour phase. Once a reliable liquid phase pyrolysis kinetic scheme and the physical properties of the vapour- and liquidphase are available, quite conventional mass, enthalpy and momentum balance equations for a two-phase flow can be proposed for the reactor modelling. The scheme given below is indicative of the model's logical structure. The problem of the fouling The formation of carbonaceous deposits on the heat transfer walls, which grow with time and local temperature (as in other pyrolytic processes), creates complicated problems, which depend on the feedstock nature and are worth special attention. The fouling phenomena constitute an important aspect since they partially control the reactor on-stream time. The macroscopic mechanisms governing fouling formation are essentially two: a catalytic one and a radicalic one. When the tubes of the coil are relatively clean, as at start-of-run conditions, the reactor metallic walls play the role of a heterogeneous poly-addition catalyst. Starting from vinyl-aromatic molecules, polymeric material is formed (like in conventional heterogeneous catalysis for poly-olefines, poly-diolefines, poly-styrenes and so on). As soon as the wall surface is covered by the polymeric layer, the radicalic mechanism becomes more

1127

and more important, increasing with temperature along the coil. The radicals involved are provided from the surroundings or directly generated from the formed polymers (as a consequence of the cleavage of the weakest and highly resonant C-C bonds). The last radicalic mechanism, in combination with degradation, dehydrogenation and crosslinking, gives place to the formation of more and more amorphous structures. This phenomenological interpretation is confirmed by the theory developed for modelling the fouling in Transfer Line Exchangers (TLE) in the ethylene production plants (particularly for the heavy feedstocks steam cracking, like vacuum gasoils) and the related experimental data (Zimmerman et al., 1990). The mentioned experience has also suggested the guide-lines for the fouling model in the V.B. case (Bozzano et al., 1995a). For instance quasi-asymptotic behaviour after a fast initial growth rate of the coke layer has been observed (this one being connected to the catalytic action of the reactor walls that sometimes can be hindered if the coke layer has only been partially removed during the decoking operation). The empirical knowledge of the process limiting variables has led, with time, to different kinds of solution in order to reduce fouling in the system. These latter are based on the well accepted fact that fouling is conditioned by the temperature level (acting both on radicalic and catalytic mechanism). As a consequence two kinds of intervention have been adopted and can be distinguished: the first is related to process conception, the other one to process managing. The addition of the soaker to the system belongs to the first of them. In fact its presence, giving the possibility for the reactions to have a longer residence time, has allowed lower coil outlet temperatures and, therefore, a reduction of the fouling.

STRUCTURE OF THE MODEL Feedstock Characterisation[

CLUMPING)

Pseudocomponents Properties ]

]

Two phase flow (Fluid Dynamics and Heat Transfer)

KINETIC SCHEME Pressure Drop

I Vavor-Lifluid Equilibria [

MATERIAL, MOMENTUM, ENTHALPY BALANCES FOR THE COIL AND THE SOAKER [

¥oulinE model in the coil [ Asuhaltenes [

[ DE-LUMPING PROCEDURES

!

ASTM Curves L Simulation ~ ~ . .

J-

/

]

Stability

]

i.-1/I (FV.d MD)I

",,,

~'~

Sulphur Content [

[ Kinematic Viscosity ]

M. DENTE et al.

!128

From another side, operative solutions can consist of increasing the internal heat transfer coefficient. This effect can be obtained, for instance, by addition of water to the stream: through its vaporisation and consequent rising of the mixed phase velocity, the residence time and the coil skin temperature can be controlled (something similar to the just described fluid dynamics effects, in some cases this is obtained by adding, directly to the feed, the fluxant of the V.B. tar). At the end it has to be mentioned that, in the recent years, chemical additives have been proposed and sometimes adopted, aimed to reduce the catalytic contribution to the fouling mechanism. However, this latter cannot be easily stopped by means of usual antifoulant agents. Both the effect of the latter and of the previously described interventions against fouling formation are fairly well simulated by the model (Bozzano et al., 1995a). Typical examples of dependent variable evolution, as of the coke layer thickness, of the pressure drop and of the maximum skin coil temperature resulting from simulations with the proposed model are shown in the following figures for both the adopted configurations (Figs 1-4) with and without a soaker. In the simulations, for the sake of simplicity, independent variables have been assumed as constant: feedstock quality and flowrates, coil inlet- and outlet-temperature and pressure. In Fig. 1, the quick initial growth of the fouling layer can be 0.007 -[...~

0.006 --

© ~

0.005 -

~

0.004

Z rJU.l ~

0.003 0.002 0.001

J <

0.000

I

I

I

I

I

40

80

120

160

200

TIME (DAYS) Fig. 1. Average coke thickness in the coil for the two typical process configurations.

COIL

COIL + SOAKER 650 7 600 ~z

1

5

o

550

'411 400 0

I I 10 20 30 40 50 60 70 80 90 100 % TOTAL COIL LENGTH

Fig. 3. Maximum skin temperature along the coil in a coil plus soaker configuration. observed: after 20--40 days the rate of the phenomenon is decreasing. This behaviour is related to the abovementioned initial predominance of the catalytic mechanism. However, in real situations, all the mentioned dependent variables are functions of the independent ones (feedstock, flowrate, coil outlet temperature, etc.), that are continuously changing with time. This practically unavoidable interference can partially mask the effect of the fouling phenomena. An example of a real case has been made in the case of an industrial plant. In fact it has been possible to follow the maximum skin temperature along a complete run, comparing the experimental data with the simulations of the model. Figure5 shows the obtained results (the experimental points are represented with their own uncertainty error, _+10°C). The comparison can be considered satisfactory, mainly pointing out that during the operation of the plant, for periods of several days, experimental data was not available. The developed fouling model seems to be satisfactory for the prediction of the named variables. However further improvements of these features of the model will be obtained through further industrial experiences. Product amounts

and properties

A rigorous visbreaking simulation model cannot neglect the prediction of the product's properties such as viscosity, specific gravity, sulphur and asphaltenes 650

10 <

~

DAYS 21o Isl}

~.

600

Z

550

E

COIL

DAYS

150 120

9o

3o

8

500 450 6

0

20

40

60

80

100 120

140

I 160

TIME (DAYS) Fig. 2. Evolution of pressure drop during the time in a configuration without soaker.

400

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 % TOTAL COIL LENGTH

Fig. 4. Maximum skin temperature along the coil in a configuration without soaker.

Comprehensive program for visbreaking simulation

1129

COMPARISON WITH INDUSTRIAL DATA 640

"1" ~o~

EXP. DATA

-I"

620 ¢~ ~

600

[...d Z ~

540

~["

520

500 480

I

I

I

I

I

I

I

I

I

0

10

20

30

40

50

60

70

80

TIME IN OPERATION (DAYS) Fig. 5. Comparison between experimental and simulated maximum skin temperatures at the coil outlet for a commercial plant. content, Conradson Carbon Residue and residue stability. The last one is most important.

The asphaltenes and their stability Following the most diffused experimental indication, the asphaltenes are defined as the components that are precipitated by adding a certain amount of n-heptane to a residue (IP 143). This type of determination, not being founded on a molecular basis, obviously gives rise to determination errors. Asphattenes can be viewed as polycatacondensed aromatic rings (the experimental H/C ratio confirms their aromatic nature) with a short paraffinic side chain, whereas the same type of components with a longer side chain can still be defined as aromatics. During the pyrolysis, the last ones are decomposed so that the content of asphaltenes in the residue increases. The final mixture (made up of aromatics, asphaltenes, olefines, diolefines and paraffins) is a typical colloidal solution. Asphaltenes can join and flocculate giving rise to muds; this phenomenon limits the visbreaking severity so that the prediction of residue stability can be of help in determining the best operating conditions. By now the most commonly accepted stability index is the peptisation value (PV): it is determined by adding increasing quantities of cetane (C~6) to a certain amount of residue until the beginning of asphaltene flocculation. Cetane is used in order to simulate the addition of the worst possible fluxant cutter stock to the residue. The preferred operating values to be maintained in visbreaking operations are in the range of 1.1-1.2. The problem of PV modelling can be approached by representing the system with three macroclasses of pseudo-components: aromatics, asphaltenes and paraffines (the last grouping paraffines, olefines and diolefines) (Bozzano et al., 1995b). The Hildebrand method

for the study of mixture stability against separation, based on the determination of the mixing free energy minima in order to find the unmixing area, is very sensitive if applied to the considered system. The mixture can be considered as being formed by two phases that are compatible only under certain conditions. The first one consists of aromatics and paraffines, the latter of aromatics and asphaltenes. Aromatics are adsorbed on asphaltenes so that the last ones are kept hidden from the paraffines, with whom they are incompatible. The coverage degree can be inferred (through the molecular structure of aromatics and that of asphaltenes) together with the critical concentration of aromatics, paraffines and asphaltenes after their precipitation starts taking place. The coverage degree is also directly related to the sulphur content in the asphaltenes. In fact S atoms are larger than C atoms so that a large amount of aromatics is needed in order to assure asphaltene coverage. The repartition constant of aromatics is given by the ratio of the activity coefficient of adsorbed aromatics and that present in the maltenic phase. The last one is correlated to the Hildebrand solubility parameters that have been obtained through the analysis of some experimental data available from literature (typically a group contribution method has been specifically developed for this problem). The results obtained by adopting this phenomenological approach, when compared with experimental data, are in very good agreement, both in the prediction of the PV of the feedstocks and of the visbroken residues at different severities. Figure 6a and b presents the comparison between approx 100 experimental and simulated data: the lines are showing the experimental average error in PV determination. Experimental data are referred to industrial plants, to research lab tests (priv. comm.) and to pilot plant (Di Carlo and Janis, 1992).

M. DENTE et al.

1130

The validity of the adopted phenomenological approach is also demonstrated by its application to an other stability index (XE=xylene equivalent) adopted in some refineries, that is coming from the Martin Bailey stability method (Bozzano et al., 1995b). It consists of the precipitation of asphaltenes through the addition of n-heptane and subsequent resolubilisation by xylene-nheptane solution. The amount of xylene necessary to have asphaltenes in solution gives the XE value• The model predictions have shown fairly good agreement with the experimental data obtained on different industrial V.B. units (operating on atmospheric and vacuum residues) as it is possible to observe in Fig. 7. In addition, it has to be pointed out that in the case of the feedstocks the PV determination is considered of minor importance so that experimental data are less accurate and give only an order of magnitude.

Commercial yield data are frequently different in terms of cut points• However, in the field of gases,

(a)

>

e~ o~

I 2

100 100 exp. PV

6 -- (b)

J

5 --

S / /

4 --

•~

/

•

80--

60

fi£ 40 30 20 10 - j

o

I 10

I 20

I 30

I 40

I 50

I 60

I 70

I 80

exp. MB Fig. 7. Comparison between experimental and calculated Martin Bailey index.

Yields data for several fractions

/

90-

/

gasolines, kerosenes, light gasoiis, heavy gasoils and residues, the standard deviations of the predictions of the simulation program are, respectively (wt%): 0.18, 0.35, 0.53, 0.23, 0.82, 1.1 (related both to lab and commercial plant data). Figures 8-13 show the comparison between experimental and calculated effluent yields. The experimental data have been obtained by lab tests at different residence times and feedstocks (seven vacuum residues from several crude blendings or single ones characterised by different kinematic viscosity, specific gravity, sulphur content and initial true boiling point) at the same operating temperature• The lab plant consisted of a CSTR maintained at a constant temperature and pressure of about 442°C and 2 Mpa. The comparison is very satisfactory. Table 1 shows the comparison between experimental data and calculated effluent yields and product properties for some commercial cases• The first two are related to a coil plus soaker configuration of V.B. reactor whereas the third is typical of the same reactor but without soaker•

~

/

•

/

2.3 - -

2.1 --

/

1.9 -<

/

///5

1.7 -1.5 --

<

1.3

--

1.1

--

0.9

--

."

0.7 -- ."

2 3

I 4

I 5

I 6

exp. PV Fig. 6. Comparison between experimental and calculated PV of (a) the tar and (b) the feedstock•

0.5

I I I " L I 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 EXP. GAS

Fig. 8. Comparison between experimental and simulated gas yield (wt%).

Comprehensive program for visbreaking simulation

I 131

94 -5

--

92 -z O <

90 -+

9

88 -

0.

t¢3

d,..a

< r..)

•0

4 h

•

3 --

e•

•

86 -

•

d,.d

e..

< L)

0•2 ,'"

I

2

3

84 ,o

82 -

•"

8O 80

82

EXP. G A S O L I N E

I 84

I

I

f

I

I

86

88

90

92

94

EXP. 350+RESIDUE

Fig. 9. Comparison between experimental and simulated gasoline yield (wt%).

Kinematic viscosity of HC fractions The problem of the prediction of viscosity for the produced liquid fractions has been considered by starting 14 --

4

Fig. 12. Comparison between exp. and simulated 350+yield (wt%).

from the semi-empirical relations developed by Eyring and Guzman-Andrade. In both cases the dependence is exponential with the inverse of the absolute temperature• By assuming the expression given by Eyring for liquid viscosity prediction, kinematic viscosity can be related to the molecular weight as follows:

13 -0 0

d <

12 -11

exp[r T -I] (Cst)

,7:

--

10

--

9

--

8

--

..6 .'e

¢.: J 6,

I

I

7

8

9

7 ~--

''"'

10

I

f

I

|I

12

13

14

EXP. LGO Fig. 10. Comparison between exp. and simulated light gasoil yield (wt%).

where: Tb=boiling point o f the fraction (K), M W = m o lecular weight of the fraction, T= temperature (K) and o~ a n d / 3 are constants• The correlation proposed by Eyring is valid for small molecules, so that it has been necessary to make an extrapolation for the long molecules that are characterising hydrocarbons fractions• Through the analysis of different hydrocarbon compounds, for which viscosity was available, the following expression has been obtained:

80 -29 27 0

d <

U.1

•0" .o

25 -

70 m

23 -

+ O O

21 -

d,.d

19 -

<

17 --

o'

60 --

o• Q.

.•

e"

.O

15

•o

O. •

-'1 15

I

[

17

19

I 21

23

25

I

I

27

29

EXP. HGO Fig. 11. Comparison between exp. and simulated heavy gasoil yield (wt%).

50 ~ 50

I

I

60

70

80

EXP. 500+RESIDUE Fig. 13. Comparison between exp. and simulated 500+yield (wt%).

M. DENTEet al.

1132

r/= ~ e x p

~

[ f C(x) 357F(asph.,nc)] "q=exp L ~k~ - ) (Cst)

(Cst)

where: T= temperature (K), F(asph., nc)=a function of asphaltene content and of the average number of C atoms, C(x)=linear function of molecular fractions of the residue pseudo-components. The presence of the asphaltenes has been taken into account because of their colloidal state that is sensibly increasing the viscosity. Figure 14 shows the results of the model predictions compared with some experimental

where FI is a function of the composition of the fraction and E,. is related to the boiling point of the different species constituting the fractions, obtained through group contribution methods (for instance Meissner rule). The residue viscosity prediction creates a peculiar case, due to the high molecular weight of constituting molecules. Taking into account the work of Singh et al. (1993), a relation has been obtained:

Table I. Vacuum Residues Operating Conditions

Inlet temp.°C Outlet temp.°C Outlet pressure bar Total residence time min

Effluents Feedstock

Gas

Specific gravity 15°/4°g/cm 3 Kin. visc. at 100°C cSt Kin. visc. at 120°C cSt Asphaltenes wt% CCR wt% Sulphur wt% Yield wt% H2S wt%

1

2

3

326 455 12 24 (with soak.)

330 450 12 26 (with soak.)

326 468 18.3

Exp.

Theor.

Exp.

Theor.

Exp.

Theor.

1.023 2070 620 10.5 18.0 3.8

---11.9 18.9 --

1.042 13400 2800 17 23 3.4

---17.2 25.3 --

1.004 1680 -5.7 17.7 3.47

---8.0 15.4 --

1.8 0.5

2.0 0.4

1.9 0.4

1.9 0.4

2.0 0.3

1.8 0.3

9

Gasoline

TBP (°C) Specific gravity 15°/4°g/cm 3 Sulphur wt% Yield wt% Bromine Number

<200 0.730 0.70 6.9 65

-0.730 0.75 7.0 66

< 195 0.738 0.65 6.8 5 I.

-0.729 0.66 6.6 56

< 175 0.73 0.78 3.9 --

-0.74 0.93 4.0 66

LGO

T B P (°C) Specific gravity 15°/4°g/cm 3 Kin. visc. at 50°C cSt Kin. visc. at 70°C cSt Sulphur wt% Bromine number Yield wt%

<315 0.85 2.0 1.4 1.9 33 7.1

-0.86 t.9 1.4 2.0 31 7.1

<290 0.85 1.9 1.3 2. l 29 5.0

-0.86 1.8 1.3 1.8 32 5.1

< 340 0.86 --2.0 -9.1

-0.88 --2.4 -9.2

HGO

T B P (°C) Specific gravity 15°/4°g/cm ~ Kin. visc. at 50°C cSt Kin. visc. at 70°C cSt Sulphur wt% Bromine number Yield wt%

<390 0.95 6.6 4.2 2.2 22 6.6

-0.96 7.2 4.6 3.1 23 6.9

<385 0.89 5.8 3.9 2.5 21 7

-0.90 6.3 4.1 2.7 20 7.7

395 0.91 --2. l -6.1

-0.92 --2.6 -6.2

Tar + HGO

TBP (°C) Specific gravity 15°/4°g/cm 3 Kin. visc. at 100°C cSt P. value Sulphur wt% Asphaltenes wt% CCR wt% Yield wt%

<305 1.04 530 1.15 3.7 20 24 84.9

-1.06 500 l.ll 3.7 19 22 83.9

>285 1.05 1414 1.27 3.2 25 27 87.0

1.07 1900 1.14 3.3 22 25 86.4

> 510 1.06 -1.2 3.8 17.5 -61.6

-1.06 -1.3 3.6 17.8 -61.5

17.2 2.4 0.94

17.2 2.6 0.94

VGO ( 3 9 5 - 5 1 0 °C)

Yield wt% Sulphur wt% Sulphur gravity 15°/4°g/cm 3

. . .

. . .

. . .

. . .

Table 2.

Standard deviation (g/cm 3)

Gasoline

Light Gasoil

Heavy Gasoil

Tar

5.68 × l 0 -5

9.77 × l0 -3

6.8 × l 0 -3

1.03 × 10 2

Comprehensive program for visbreaking simulation 400 -2200 --

•

~

1133

Exp. ASTM

/e

Calc. A S T M J

2000 --

•

1800 --

•

1600 --

• •

300 --

•

e

.~-

1400 --

.ole

-

~ e "

200-

"~ 1000 G 800 %

-

ht gasoil

"~'. 1200 --

rj

.e "e

0"

Kerosene

o~

600 100

400

200 200

600

[

I

I

I

1000

1400

1800

2200

data (atmospheric and vacuum residues treated in a reactor of a coil followed by a soaker). The specific gravity The specific gravity of the fractions has been deduced on the basis of the additivity rule of molecular volumes of the components and group contributions. It is therefore related to the hydrogen/carbon ratio, to the composition of the considered fraction, to sulphur content and to the total average number of C atoms of the constituting pseudocomponents. The statistical comparison with experimental data coming from lab test and industrial plants is reported, in terms of standard deviation, in Table 2. ASTM and TBP curves The product amounts and their properties are deduced in the model through a de-grouping of the pseudocomponents, representative of a range of C atoms, into components separated by an interval of a single C atom (so including isomers). This operation enables the program to make all possible cuts in terms of boiling point and to build the ASTM curves or the true boiling curves for every kind of product. Figure 15 represents the comparison among the experimental ASTM curve of the most important products and those calculated by the simulation model for an industrial case. Conclusions In this paper the main features and the recent developments of a mathematical model from the simulation of the so-called visbreaking of the refinery hydrocarbon residues have been proposed. Here, in particular, attention has been focused on some of the adopted kinetic concepts of coil fouling mechanisms, on the prediction of the significant properties of the effluent distillates and visbroken tars. The developed model (already validated through industrial tests) can be a valid support for a better understanding and prediction of the phenomena occurring in HC liquid phase pyrolysis and can find practical and important application both for

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Exp. kin. visc. (Cst) Fig. 14. Comparison between experimental and calculated kinematic viscosityof a visbrokenatmosphericresidue (± 10% deviation lines are shown).

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~ht g a s • I"me ~g

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20

40

60

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vol % Fig. 15. Comparison between experimental and calculated ASTM curves of some products. improving the visbreaking furnaces design and for monitoring and control of the V.B. operations. References Soufi, A., Savaya, Z.E, Mohammed, H.K. and AIAzawi, I.A. (1988)Thermal Conversion (Visbreaking) of Heavy Iraqi Residue. Fuel 67, 1714-1715. Benson S. W., The Foundations of Chemical Kinetics, McGraw Hill, N.Y., (1960). Bozzano G., M. Dente, M. G. Grottoli and E Macchi, Fouling Phenomena in the Visbreaking Process, AIDIC Conference Series, lCheaP 2 Selected Papers, 1, 269-276 (1995). Bozzano G., M. Dente, C. Pirovano and M. Molinari, The Characterisation of the HC Residues Stability, AIDIC Conference Series, ICheaP 2 Selected Papers, 1, 173-180 (1995). Clymans, P.J. and Froment, G.E (1984) Computer Generation of Reaction Path nd Rate Equations In Thermal Cracking of Normal and Branched paraffins. Chem. Eng. 8(2), 137 Dente M. and E. Ranzi, Mathematical Modeling of Hydrocarbon Pyrolysis Reactions, Pyrolysi: Theory and Industrial Practice. Academic Press, i 33-175 (1983). Dente, M., Pierucci, S., Ranzi, E. and Bussani, G. (1992) New Improvements in Modeling Kinetic Schemes for Hydrocarbons Pyrolysis Reactors. Chem. Eng. Sci. 47(9-11), 2629-2634. Dente M., G. Bozzano and M. Rossi, Reactor and Kinetic Modelling of the Visbreaking Process. Proceedings of ICheaPl, pp. 163-172 (1993). Di Carlo, S. and Janis, B. (1992) Composition and Visbreakability of Petroleum Residues. Chem. Eng. Sci. 47, 2695-2700. Hillewaert, L.P., Dierickx, J.L. and Froment, G.F. (1988) Computer Generation of Reaction Schemes and Rate Equations for Thermal Cracking. A.I.Ch.E. Journal 34(1 ), 17-24. Jacob, S.M., Gross, B., Voltz, S.E. and Weekman, V.M.Jr. (1976) A Lumping and Reaction Scheme for Catalytic Cracking. A.I.Ch.E. Journal 22(4), 701-713.

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Quann, R.J. and Jaffe, S.B. (1992) Structure-oriented Lumping: Describing the Chemistry of Complex Mixtures. Ind. E n g . Chem. R e s . 31(11), 2483-2497. Ranzi E., M. Dente and G. Bozzano, Synthesis of Chemical Reaction Networks for Hydracarbon Pyrolysis. Analysis, Simulation, Dynamics of Chemical Reactors. CUEN, Napoli, pp. 221-253 (1995). Singh B., A. Miadonye, V. R. Puttagunta, Heavy Oil

Viscosity Range from one Test. Hydrocarbon Processing. 157-162 August (1993). Vinckier, E. and G. F. Froment, Modeling of the Kinetics of Complex Processes Based upon Elementary Steps, In Kinetic and Thermodynamic Lumping of Multicomponent Mixtures. Astarita A. and S. I. Sandler, Eds., Elsevier, 131-162 (1991 ). Zimmerman G., M. Dente and C. Van Leeuwen, On the Mechanism of the Fouling. AIChE Meeting. Orlando, FL, 18-22 March (1990).