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Compurers them. Engng Vol. 19, Suppl., pp. S205-8210.1995

Coavrieht 0 1995 Elsevier Science Ltd

Pergamon

0098-1354 95)ooo73-9

Printed’ in &eat Britain. All rights reserved

0098-1354/95 $9.50 + 0.00

THE VISBREAKING PROCESS SIMULATION: PRODUCTS AMOUNT AND THEIR

PROPERTIES PREDICTION

M. DENTE, G. BOZZANO, G. BUSSANI*

Politecnico di Milan0 - Chem. Eng. Dept. “G. Natta” - P.zza L. Da Vinci, 32 - Milan0 -Italy

* K.T.I. S.p.A. - Via Ripamonti, 133 - Milan0 - Italy

ABSTRACT

The visbreaking process is adopted by many refineries. It consists of a liquid phase pyrolysis of

atmospheric or vacuum residues with the aim of increasing the production of light fractions and

simultaneously reducing the viscosity of the visbroken residues. In spite of the economical

importance of this operation, the literature is lacking of scientific informations; only empirical

models have been presented. Major difficulties are constituted by the huge number of components

and reactions that is characterising the system and by the relatively poor level of the available data

for the feedstock characterisation. Due to the complexity of the feed to the reactor also the

characterisation of the products properties, besides their amount, presents considerable problems.

The results coming from a mechanistic approach are here presented. They are compared with

experimental data from literature, research lab tests and commercial units. Moreover they are

covering different aspects like fouling in the coils, effluents amounts and properties (residues

stability, sulphur and asphaltenes content, viscosity, specific gravity, Conradson carbon residue

and so on). The developed model constitutes a valid support for the understanding and prediction

of the complex phenomena that are occurring during the HC liquid phase pyrolysis and can find a

practical and important application both for improving the visbreaking furnaces design and for

monitoring and control of the V.B. operations.

KEYWORDS

Visbreaking; pyrolysis; fouling; residues stability

KINETICS ASPECTS

Some of the rules and criteria followed facing on this problem are similar to those ones

successfully adopted in an extended gas-phase pyrolysis kinetic scheme. The complexity of the

molecular characterisation of these systems has been resolved (Dente et al., 1993) by lumping the

huge number of real components into a relatively limited number of pseudo-components. For

every pseudocomponent it has been proposed a specific reaction fate taking also into account the

probability of scission of the different kind of bonds. The global kinetic mechanism is

characterised by initiation, P-scission, H-abstraction, substitution and recombination reactions.

Due to the liquid phase state, molecular rotational movements of the C-C segments are limited so

that isomerization reactions of the radicals can be practically neglected.

S205

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The kinetic constants (frequency factors and activation energies) of the reactions involving

paraffines, aromatics, olefines and diolefines, have been derived from the equivalent ones of an

hypothetical equivalent gas phase (where consolidated rules are already available, Dente et al.

1992). The equivalent constants in the liquid phase have been obtained through correction factors

of the activation entropies and activation energies for the transposition into the condensed state

(Benson, 1960).

REACTOR MODELLING: THE PROBLEM OF FOULING

The typical visbreaking reactor is constituted by long coils (generally two) horizontally placed in a

furnace; where a radiant and a convective section can be distinguished. Often two cells, with

independent heating, are present. Water is sometimes injected at the coil inlet to increase the fluid

velocity, thus controlling the residence time. In the latest years in many plants an adiabatic extra-

volume , the so called soaker, has been added after the coils. It allows longer residence times and

therefore lower coil outlet temperatures. As a consequence, the reduced fouling into the coils

allows an extended on stream factor for the visbreaking unit.

The fouling phenomena are an important aspect since, as said before, their are dictating the

reactor on stream time. Their mechanisms of 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 condition, the

reactor metallic walls play the role of an heterogeneous polyaddition 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). The radicalic mechanism becomes more

and more important as the wall surface is covered, increasing with temperature along the coil. The

radicals that are involved in this phase are coming from the surroundings or are provided directly

from the formed polymers (in fact the bonds of these complex molecular structures are weak at

the beginning of the growth before crosslinking and dehydrogenation have taken place). The last

radicalic mechanism, in combination with degradation, dehydrogenation and crosslinking of the

formed polymers, gives place to the formation of more and more amorphous structures; the pure

radicalic mechanism cannot be easily stopped with antifoulants agents.

The theoretical experience gained in the modeling of fouling phenomena in Transfer Line

Exchangers (TLE) in the ethylene production plants and the experimental data, confirm this

phenomenological interpretation of the fouling mechanism. In fact, it is possible to observe a fast

initial growth, due to the action of walls as catalyst, that subsequently slow down toward a quasi-

asymptotic behaviour. Coke only partially removed during decoking operation may hide the effect

of the catalytic mechanism.

The evolution 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. The developed model seems to be satisfactory for the prevision of the named variables.

However better comparisons and improvements of these features of the model can be obtained

through the knowledge of the complete history of the furnace (in terms of variations of feedstocks

and operating conditions). In fact the last variables influence the different growth rate of the coke

layer. Qualitative and quantitative agreements with some commercial units data have been

encouraging.

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S207

COIL+SOAKER

?

r

$j 0.006 .

5

0.005 .

COIL+SOAKER

0 40 60 120 160 200

DAYS

COIL+SOAKER

0.0 0.1 0.2 o., 0.4 0.5 0.6 0.7 0.6 6.9 1.0

i TOTAL COIL LENGTH

COIL

lD-

6O 2 4

6 8

1 12 14

16

TIME (DAYS)

COIL

DAY:

630 -

610.

590 -

0.0 0.1 0.2

0.3 0.4 0.5

0.6 0.7 0.6 0.9

1.D

Z TOTAL COIL LENGTH

5

PRODUCTS PROPERTIES

A rigorous visbreaking simulation model cannot neglect the prediction of products properties like

viscosity, specific gravity, sulphur and asphaltenes content, Conradson Carbon Residue and

residue stability. The last one is the most important so that it is necessary to spend some words on

it.

The asphaltenes and their stabilitv

The asphaltenes are defined like those components that are precipitated by adding a certain

amount of n-heptane to a residue (IP 143). This type of determination, being not founded on

molecular basis, obviously gives place to determination errors.

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European Symposium on Computer Aided Process Engineering-S

Asphaltenes can be figured like polycatacondensed aromatic rings (the experimental H/C ratio

confirm their aromatic nature) with short pa&ink side chain, whereas still can be defined as

“aromatics” the same type of components with a longer side chain. During the pyrolysis the last

ones are decomposed so that the content of asphahenes in the residue increases. The final

mixture, constituted by “aromatics”,

“asphaltenes”, olefines, diolefines and paraffines, is a typical

colloidal solution. “Asphaltenes” can join and flocculate giving place to muds; this phenomenon

limits the visbreaking severity so that the prediction of residue stability can be of real 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 to a certain amount of residue increasing quantities of cetane (Cl,) till the

beginning of asphaltenes 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

v&breaking operations are in the range of 1. -l .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). The Hildebrand method for the study of mixtures

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 formed by two phases that are compatible only under certain conditions. The first

one is constituted by “aromatics” and “paraf&es”,

the latter by “aromatics” and “asphaltenes”.

“Aromatics” are adsorbed on “asphahenes”

so that the last ones are kept hidden to the

“paraffines” with whom they are incompatibles. The coverage degree can be inferred (through the

molecular structure of “aromatics” and that of “asphaltenes”) together with the critical

concentration of “aromatics”,

“parafIines” and “asphahenes” 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 major amount of “aromatics” is needed in order to

assure “asphaltenes” coverage. The repartition constant of “aromatics” is given by the ratio

between the activity coefficient of adsorbed “aromatics” and that present in the maltenic phase.

The last one is correlated to the Hildebrand solubility parameters that has 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 a very good agreement, both in the prediction of the PV of the

feedstocks and of the visbroken residues at different severity. Next figure presents the comparison

between experimental and simulation 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, 1992).

EXP. PV

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OTHER RESULTS AND COMPARISON

S2O J

The next figures shows the comparison between experimental and calculated effluent yields. The

experimental data have been obtained by lab tests at different residence times and f&stocks

(particularly the latter were characterised by different kinematic viscosity, specific gravity and

sulphur content) at the same operating temperature. The comparison is very satisfactory.

0.5 0.7 0.0 1.1 1 3

1.5 1.7 1.0 2.1 2,

EXP.CAS

14

.4

13

.:

EXP. LGO EXP. HGO

94.

.,:

80 ’ s ’ , ’ .

80 82 84 86 88 90 92 94

EXP. 350f RESIDUE EXP. 500+ RESIDUE

EXP. GASOLINE

Table 1 shows the comparison between experimental data and calculated effluent yields and

products properties for some commercial cases. The first two cases are related to a coil plus

soaker configuration of the reactor whereas the third one is typical of the same reactor without

soaker.

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Operating

Conditions

FEEDSTOCK

GAS

GASOLINE

E

LGO

F

F

L

U

E

N

T

HGG

S

TAR+HGO

(39* 5:

C

Table 1

Vacuum Residues I 2 3

Inlet temp. “C 326 330 326

Outlet temp. OC 455 450 468

Outlet bar

ressure

12 12 183

Total residence time min 24 (with soak.) 26 (with soak.) 9

Kin. vise. at 100 “C cSt 2070 - 13400 - 1680 -

Kin. vise. at 120 “C cSt 620 - 2800 - - -

Asphaltenes wt% 10.5 11.9 17 17.2 5.7 8.0

CCR W% 18.0 18.9 23 25.3 17.7 15.4

Sulphur wt% 3.8 - 3.4 - 3.47 -

Yield wt% 1.8 2.0 1.9 1.9 2.0 1.8

HrS wt% 0.5 0.4 0.4 0.4 0.3 0.3

TBP (“0 QOO - Cl95 - <175 -

Specific gravity

1

”/4”gkm3

Sulphur Wt%

Yield Wt%

Bromine Number

TBP (“C)

Specific gravity 15”/4”gkm3

Kin. vise. at 50°C cSt

Kin. vise. at 70°C cSt

Sulphur ti%

Bromine number

0.730

0.70

6.9

65

<315

0.85 0.86 0.85 0.86

2.0 1.9 1.9 1.8

1.4 1.4 1.3 1.3

1.9

2.0

2.1 1.8

33 31 29 32

0.730

0.75

7.0

66

0.738

0.65

6.8

51.

Q90

0.729 0.73

0.74

0.66 0.78

0.93

6.6 3.9

4.0

56 66

<340 -

0.86

0.88

2.0 2.4

Yield wt% 7.1 7.1 5.0 5.1 9.1 9.2

TBP (“C) c390 - <385 - <395 -

Specific gravity

15”/4”&m3 0.95 0.96 0.89 0.90 0.91 0.92

Kin. vise. at 50°C cSt 6.6 7.2 5.8 6.3 - -

Kin. vise. at 70 “C cSt 4.2 4.6 3.9 4.1 -

Sulphur wt% 2.2 3.1 2.5 2.7 2.1 2.6

Bromine number 22 23 21 20 - -

Yield wt% 6.6 6.9 7 7.7 6.1 6.2

TBP (“C) >305 - >285 >510 -

Specific gravity 15”/4”gkm3 1.04

1.06

1.05 1.07 1.06 1.06

Kin. visc.at IOO’CcSt 530 500 1414 1900 - -

P. value 1.15 1.11 1.27 1.14 1.2 1.3

Sulphur wt% 3.7 3.7 3.2 3.3 3.8 3.6

Asphaltenes wt% 20 19 25 22 17.5 17.8

CCR wt% 24 22 27 25 - -

Yield wt% 84.9 83.9 87.0 86.4 61.6 61.5

Yield Wt% 17.2 17.2

Sulphur wt% 2.4 2.6

Specific gravity 15”/4”g/cm3 - - - - 0.94 0.94

REFERENCES

Al So&I, Savaya, Z.F., Mohammed, H.K. and Al-Azawi, 1.A. (1988). Thermal conversion (visbreaking) of heavy

Iraqi residue. Fuel, 67, 1714-1715.

Benson, S.W. (1960).The Foundations of Chemical Kinetics, McGraw Hill, N.Y.

Dente, M., Pierucci, S., Ranzi, E. and Bussani, G. (1992). New improvements in modeling kinetic schemes for

hydrocarbons pyrolysis reactors. Chem.Eng.Sc.. 47, No.9-11, 2629-2634.

Dente, M., Bozzano, G. and Rossi M. (1993). Reactor and kinetic modelling of the visbreaking process.

Proceedings of First Conf. on Chem. and Proc. Eng.,l63-172

Di Carlo, S. and Janis,B. (1992). Composition and visbreakability of petroleum residues. Chem.Eng.Sc., 47, No.9-

1 ,June-Aug.1992, 2695-2700

Zimmerman, G., Dente, M. and Van Leeuwen, C. (1990). On the mechanism of the fouling, AIChE meeting,

Orlando, FL, March 18-22.