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Modeling of visbreaking process: The effect of naphthenic content on yields and VB residue stability
European Symposium on Computer-Aided Process Engineering - 14 A. Barbosa-P6voa and H. Matos (Editors) 9 2004 Elsevier B.V. All rights reserved.
157
Modeling of Visbreaking Process: The Effect of Naphthenic Content on Yields and VB Residue Stability Giulia Bozzano*, Francesco Carlucci, Mario Dente CMIC Dept. "Giulio Natta" Politecnico di Milano, P.zza L. Da Vinci, 32, Milano, Italy
Abstract In the past, visbreaking (VB) process was essentially studied on empirical bases. This oversimplified approach affects the prediction of feedstock and products properties. The present work deals with a quite more sophisticated and flexible mechanistic model. The latter was initially performed by taking into account just two macro-classes of pseudocomponents, paraffins and aromatics (Dente et al. (1997), Bozzano et al. (1998), Dente et al. (1993)). Now naphthenes macro-class has been added and its importance in VB modeling is here described and underlined. Particularly the effect of naphthenes presence on VB residue stability, against asphaltenes flocculation, is discussed.
Keywords: Visbreaking, Naphthenic Components, Fouling, Residues Stability.
1. Introduction Visbreaking is adopted since many years in some refineries, mainly in Europe and Far East areas. It consists of a liquid phase pyrolysis of the atmospheric or vacuum distillation residues of crude oils. It is aimed to reduce the viscosity and increase the production of distillates like gas, gasoline, kerosene, light, heavy and vacuum gasoils. In previous papers (Dente et al. (1997), Bozzano et al. (1998), Dente et al. (1993)) paraffins and aromatics were considered as the only two most important macro-classes of pseudo-components. In fact, initially, the only available data was related to residues mainly containing aromatics and paraffins. More recently, new data on the visbreaking of naphthenic rich feed-stocks imposed to take into account also this macro-class of components. Their importance is connected, once again, to the need of an accurate prediction of the residue stability. The latter is essential for individuating the maximum acceptable severity of the process (and, therefore, the yield of distillates). The present paper recalls the principles used and the results of a more complete VB simulation model that includes the naphthenes (mono- and poly) macro-class and their products in addition to the paraffins and aromatics ones. The kinetic scheme has been extended including the fate of naphthenic pseudo-components and their derivatives; of course feedstock characterisation and products properties have been re-modeled. The difficulties depending on the huge number of new real components and reactions involved have been overcome by using again proper lumping and de-lumping procedures. The number of equivalent reactions is actually 369 involving 401 pseudo-
* Author to whom correspondence should be adressed: [email protected]
158 components. Statistical average of potentially real components has been adopted for the definition of pseudo-components properties and for deducing the "equivalent reactions". Yields, compositions and physical properties of each process effluent are predicted. In particular for the liquid fractions: sulphur amount, kinematic viscosity, specific gravity, bromine number, H/C ratio. For VB residues are also predicted the stability index (like P.V.), the C.C.R. (Conradson Carbon Residue, ASTM-D-189-65) and asphaltenes content. Fouling rate has been also modeled.
2. Feedstock Characterization The feed of VB unit is a complex hydrocarbon mixture. It has been represented by three macro-classes, paraffins, aromatics and naphthenes, constituted by pseudo-components. The latter has been defined through lumping rules (for reference, Bozzano et al. (1995)). Starting from initial boiling point, kinematic viscosity, specific gravity, sulphur content and C.C.R., the distribution of the three macro-classes can be evaluated. The relative amount of each pseudo-component, part of the macro-class, can be deduced from the following statistical distribution:
E nc ncol
df _ 1 exp dnc nc,~ - nc,n~ nc,n~ - no,~
(1)
(f is the fraction of the single component for each class on the total, nc,med and nc,mi, are respectively the average and the minimum number of C atoms). This equation has been derived from a careful elaboration of the data published in the literature (see for instance Ali et al. (1985)). The naphthenic components are of course part of every kind of residue. As a matter of fact, several crude oils, for example those from some North Sea reservoirs (but also from many other sources), contain relatively high amounts of naphthenes. In trying to characterize the feedstock in terms of three classes of components, instead of two like previously, at least one extra-information regarding feedstock has to be given. The easiest and usually available has been identified in the C.C.R. index. The latter is a reliable indicator of the aromatics content. When its value is quite high (e.g. 15-25 in a vacuum residue) the residue can be supposed to be very rich in aromatic components, whilst when it is low (e.g. 1 to 6), the presence of naphthenes is higher in quantity. The coupling of C.C.R. data with specific gravity (which is also deeply affected by the presence of aromatics), enables to evaluate the composition of the feedstock in terms of macromolecules. Molecular weight is strictly connected to the kinematic viscosity. Naphthenes have been distinguished into two subclasses of pseudo-components: 9 Original naphthenes (i.e. contained into the feed): they are mono- and polynaphthenic components with a repartition 60/40 of six and five-membered rings (50% of average methylation degree of the boundaries), and a single alkyl side chain (with 20% of methylation degree). An internal distribution into the molecules is assumed so that length of the side chain, number of rings etc. are available. It is based on an average structure composed by relatively few rings and long side chain.
159 9 Naphthenic components derived from cracking: shorter side chain naphthenes, cyclo-olefines and olefinic naphthenes. These components are characterized by two numbers representing the total number of C atoms and that in the side chain.
3. Kinetics Aspects Most of the typical aspects of VB kinetic modeling initially have been derived as an extension of gas-phase pyrolysis kinetic scheme (Dente and Ranzi, 1983, Clymans and Froment, 1984, Hillewaert et al. 1988, Dente et al., 1992). Also in this case, lumping rules allows to reduce the huge number of real components and their reactions. A radical chain process is involved. The main reactions can be divided into three groups: initiations, propagations (13-cleavage, H-abstraction, substitutive addition onto unsaturated molecules) and terminations (radicals recombination). Paraffins and aromatics decomposition behavior and kinetic constants have been already described in a previous paper (Dente et al. (1997)). Both side chain and naphthenic rings of mono- and poly-naphthenes can be attacked in the radicalic process. In the first situation, a radical position is formed in some place of the chain and two different 13-scissions can occur with the same probability, giving place respectively to a paraffinic radical and a naphthene with olefinic side chain or to a naphthenic radical and an olefin. Main fate of naphthenic radical is to give place to other shorter side chain naphthenes by H-abstraction on the liquid substrate. In the second situation two possibilities can be distinguished: if the radical is adjacent to a methylated position, a methyl radical and a naphthenic cyclo-olefin are preferably formed. Otherwise smaller naphthenes with unsaturation on the side chain or cycloolefines, lacking of some rings, are formed. Of course, cyclo-olefines, and particularly their poly-cyclic structures, are responsible for the formation of some aromatic components.
4. Asphaltenes and Residues Stability The problem of residue stability against Asphaltenes flocculation has been already discussed in previous papers (Bozzano et al., 1995, Dente et al., 1997). Essential aspects will be here reported. Asphaltenes are defined as those components that precipitate after addition to the residue of specified amounts of n-heptane (IP 143). They can be considered as large poly-aromatic sheets generally methylated on the boundaries, and characterized by short paraffinic side chain. Aromatics decompose during the pyrolysis increasing the content of asphaltenes in the VB residue. The latter can flocculate, in certain conditions, giving place to mud. This affects residue quality and therefore constitutes a limit to VB severity. At a molecular scale the mixture can be represented by two phases. The first one (called malthenic phase) is constituted by aromatics, alkanes, alkenes and naphthenes, the latter by aromatics, and asphaltenes. Reciprocal compatibility is related to solubility parameters. The latter ranges from 7-8 (cal/cm3) 1~ for alkanes and alkenes, 8-9 for naphthenes or cyclo-olefines, 8.8-10 for aromatics and finally to 11-12 for asphaltenes. The more close the solubility parameters of the two phases are, the more stable is the system against flocculation. Every asphaltenic molecule is supposed to be surrounded
160 by aromatics, deposited as "sheets" over the asphaltenic "sheet", constituting a molecular aggregate (-- adsorbed phase). The effect of this coverage is to allow a better compatibility with the malthenic environment thanks to improved solubility parameter of the aggregate with respect to the asphaltenes alone. The non-asphaltenic aromatics are supposed to be distributed between malthenic and adsorbed phase. Their repartition ratio depends on the temperature and composition of the two phases, and can be estimated from the activity coefficients of the aromatic class inside the two phases. The latter are estimated by using the solubility parameters. Precipitation will occur when, as a result of the dilution (e.g. with cetane), the total concentration of "aromatics" in the malthenic phase is lowered so that only the de-sorption of "aromatics" deposited on "asphaltenes" can compensate it. Naphthenic components (or cyclo-olefines) increase, with their presence, the global solubility parameter of the malthenic phase, so that a lower amount of "aromatics" is necessary to cover the asphaltenes.
5. The Problem of the Fouling VB takes place in a furnace frequently followed by a soaker that is equivalent to an adiabatic reactor. Soaker increases total residence time, allowing a reduction of 20-30~ on the Coil Outlet Temperature (C.O.T.) at equal severity. Reactions takes place mainly in the liquid phase since process temperature is usually under 500~ As described in a previous paper of Dente et al. (1997), the formation of carbonaceous deposits on the internal walls of the coils is a very important aspect. The reactor on-stream time is largely influenced by this phenomenon because of the increase of pressure drop and of the tube skin temperature at coil outlet. The main mechanisms governing fouling are catalytic and radicalic ones. Initially, tube internal skin is relatively clean and provides a metallic catalytic surface for poly-additions of vinyl-aromatic molecules. Once the first polymeric layer is formed, a radicalic mechanism prevails into coke deposit growth. The necessary radicals derive from the surroundings or are directly generated by the formed polymers by means of the cleavage of the weakest and highly resonant C-C benzyl-like bonds. Subsequent degradation, dehydrogenation and cross-linking of the deposit produce a more and more amorphous structure. Fouling can be reduced by moderating C.O.T. through soaker introduction and also by means of steam or fluxant dilution. The last ones allow higher velocity inside the coil, improving heat exchange coefficient, residence time and coil temperature control.
6. Results, Comparisons and Conclusions In Table 1 properties and yields derived from the VB of two different vacuum residues in industrial plants are compared. The first one has a prevailing aromatic nature; the second one has a considerable content of naphthenes. Residue 1 is characterized as follow: TBPI 550, specific gravity 15/4 1.0495 g/cm 3, kinematic viscosity at 150~ 320 cSt, CCR 24, sulphur 4.35 wt. %, while for residue 2: TBPI 530, specific gravity 15/4 0.975 g/cm 3, kinematic viscosity at 150~ 46 cSt, CCR 11.8, sulphur 0.79 wt.%. Operating conditions are, for feed 1 and 2: C.O.T. 460 ~ inlet temperature 330 ~ coil outlet pressure 11.5 ata, total residence time 30 and 45 minutes respectively.
161
Table 1 Comparison of experimental and calculated ~,ields and properties I
III
Vacuum Residues
1 I
Exp.
Model
Exp.
Model
0.4 2.34
0.44 2.62
0.04 1.6
0.04 1.51
TBP (~ Specific gravity 15~176 3 Sulphur wt% Bromine Number Yield wt%
40-185 0.739 0.85 76 4.12
40-i85 0.734 0.80 79 4.56
10-185 0.724 0.26 90 5.5
10-185 0.721 0.23 88 5.86
TBP (~ Specific gravity 15~176 3 Kin. Visc. at 50~ cSt Sulphur wt% Bromine number Yield wt%
185-385 0.867 2.8 2.0 31 13.62
185-385 0.867 3.1 2.4 32 12.68
185-385 0.823 2.5 0.45 44 17.1
185-385 0.821 2.7 0.39 43 16.78
EFFLUENTS GAS
H2S Yield
wt% wt% I
GASOLINE
GASOIL
II
VISBROKEN RESIDUE
TBP (~ Specific gravity 15~176 cSt Kin. visc. at 150~ Peptisation Value Sulphur wt% Asphaltenes wt% CCR wt% Yield wt%
>385 1.075 280 1.2 4.6 22 28 79.82
>385 1.081 260 1.21 4.4 20 26 80.14
>385 0.985 32 1.25 0.87 14.0 17.0 75.8
>385 0.990 27 1.26 0.88 15.6 17.8 75.85
I
Estimated composition (molar fractions) for residue 1 is: XArom"~- 0.86, XNaphth" ~- 0.01, Xpa~af. = 0.13, while for residue 2: Xmom. = 0.41, XNaphth. = 0.42, Xpa~af. = 0.17. The agreement between experimental data and the model simulations is good. In particular, final stability value (i.e. PV) of both VB residues can be observed to be in a range of 1.2-1.25, while naphthenic feed gives a higher yield in volatile products (24.2% vs. 20.18%). At fixed degree of VB severity, a larger quantity of valuable effluents derives from naphthenic residues. Naphthenes improves the stability through the increase of the malthenic phase solubility parameter and of the aromatic content of the residue, given that part of their decomposition products is constituted by aromatics. Figure 1 reports an extended comparison among calculated and experimental PV for different residues (virgin or thermally treated); about one half of the data referred in this figure belongs to highly naphthenic residues.
Fig. 1. Comparison between experimental and calculated P V
162 T II
Maximum Skin Temperature (~
560
ExperimentalData
[--"--Model Results
540
F
520
%=",
~" 500
,
~
480 460 440 0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
DaysOnStream. . . . . . . . . .
Fig.2. Comparison between exp. and sim. maximum skin temperatures at the coil outlet
Figure 2 shows a comparison between experimental and calculated maximum skin temperature for an industrial plant including both soaker and steam dilution. The data are related to a run characterised by a periodical change of aromatic and naphthenic rich feed. The comparison is satisfactory. In conclusion, a correct modeling of VB needs to include naphthenic components in feedstocks and products characterization, due to the implication of their presence on plant optimization and connected economical aspects. References Ali M.F., M.U. Hasan, A.M. Bukharl and M. Sallem, 1985, Hydrocarbon Processing, February, 83. Bozzano G., M. Dente, C. Pirovano and M. Molinari, 1995, The Characterization of the Residues Stability, AIDIC Conference Series, Eds. S. Pierucci, Eris C.T.S.r.l., Milano, vol. 1,173. Bozzano G., M. Dente, M. Sugaya and C. McGreavy, 1998, The Characterization of Residual Hydrocarbon Fractions with Model Compounds Retaining the Essential Information, preprints of Symposia - vol. 43, No.3, 2162 ACS National Meeting Boston, 653 Clymans, P.J. and G.F. Froment, 1984, Computers chem. Engng., vol. 8, 137. Dente M. and E. Ranzi, 1983, Mathematical Modeling of Hydrocarbon Pyrolysis Reactions, in Pyrolysis: Theory and Industrial Practice. Eds. Albright, Crynes, and Corcoran, Academic Press, 133 Dente M., S. Pierucci, E. Ranzi, and G. Bussani, 1992, Chem. Eng. Sc., Vol. 47, 2629. Dente M., G. Bozzano, 1993, Reactor Modeling of The Visbreaking Process, Proceedings of the First Conference on Chemical and Process Engineering, Florence, 163. Dente M., G. Bozzano and G. Bussani, 1997, Computers chem. Engng., vol. 21, 1125 Hillewaert L.P., J.L. Dierickx and G.F. Froment, 1988, A.I.Ch.E. Journal 34, 17.
157
Modeling of Visbreaking Process: The Effect of Naphthenic Content on Yields and VB Residue Stability Giulia Bozzano*, Francesco Carlucci, Mario Dente CMIC Dept. "Giulio Natta" Politecnico di Milano, P.zza L. Da Vinci, 32, Milano, Italy
Abstract In the past, visbreaking (VB) process was essentially studied on empirical bases. This oversimplified approach affects the prediction of feedstock and products properties. The present work deals with a quite more sophisticated and flexible mechanistic model. The latter was initially performed by taking into account just two macro-classes of pseudocomponents, paraffins and aromatics (Dente et al. (1997), Bozzano et al. (1998), Dente et al. (1993)). Now naphthenes macro-class has been added and its importance in VB modeling is here described and underlined. Particularly the effect of naphthenes presence on VB residue stability, against asphaltenes flocculation, is discussed.
Keywords: Visbreaking, Naphthenic Components, Fouling, Residues Stability.
1. Introduction Visbreaking is adopted since many years in some refineries, mainly in Europe and Far East areas. It consists of a liquid phase pyrolysis of the atmospheric or vacuum distillation residues of crude oils. It is aimed to reduce the viscosity and increase the production of distillates like gas, gasoline, kerosene, light, heavy and vacuum gasoils. In previous papers (Dente et al. (1997), Bozzano et al. (1998), Dente et al. (1993)) paraffins and aromatics were considered as the only two most important macro-classes of pseudo-components. In fact, initially, the only available data was related to residues mainly containing aromatics and paraffins. More recently, new data on the visbreaking of naphthenic rich feed-stocks imposed to take into account also this macro-class of components. Their importance is connected, once again, to the need of an accurate prediction of the residue stability. The latter is essential for individuating the maximum acceptable severity of the process (and, therefore, the yield of distillates). The present paper recalls the principles used and the results of a more complete VB simulation model that includes the naphthenes (mono- and poly) macro-class and their products in addition to the paraffins and aromatics ones. The kinetic scheme has been extended including the fate of naphthenic pseudo-components and their derivatives; of course feedstock characterisation and products properties have been re-modeled. The difficulties depending on the huge number of new real components and reactions involved have been overcome by using again proper lumping and de-lumping procedures. The number of equivalent reactions is actually 369 involving 401 pseudo-
* Author to whom correspondence should be adressed: [email protected]
158 components. Statistical average of potentially real components has been adopted for the definition of pseudo-components properties and for deducing the "equivalent reactions". Yields, compositions and physical properties of each process effluent are predicted. In particular for the liquid fractions: sulphur amount, kinematic viscosity, specific gravity, bromine number, H/C ratio. For VB residues are also predicted the stability index (like P.V.), the C.C.R. (Conradson Carbon Residue, ASTM-D-189-65) and asphaltenes content. Fouling rate has been also modeled.
2. Feedstock Characterization The feed of VB unit is a complex hydrocarbon mixture. It has been represented by three macro-classes, paraffins, aromatics and naphthenes, constituted by pseudo-components. The latter has been defined through lumping rules (for reference, Bozzano et al. (1995)). Starting from initial boiling point, kinematic viscosity, specific gravity, sulphur content and C.C.R., the distribution of the three macro-classes can be evaluated. The relative amount of each pseudo-component, part of the macro-class, can be deduced from the following statistical distribution:
E nc ncol
df _ 1 exp dnc nc,~ - nc,n~ nc,n~ - no,~
(1)
(f is the fraction of the single component for each class on the total, nc,med and nc,mi, are respectively the average and the minimum number of C atoms). This equation has been derived from a careful elaboration of the data published in the literature (see for instance Ali et al. (1985)). The naphthenic components are of course part of every kind of residue. As a matter of fact, several crude oils, for example those from some North Sea reservoirs (but also from many other sources), contain relatively high amounts of naphthenes. In trying to characterize the feedstock in terms of three classes of components, instead of two like previously, at least one extra-information regarding feedstock has to be given. The easiest and usually available has been identified in the C.C.R. index. The latter is a reliable indicator of the aromatics content. When its value is quite high (e.g. 15-25 in a vacuum residue) the residue can be supposed to be very rich in aromatic components, whilst when it is low (e.g. 1 to 6), the presence of naphthenes is higher in quantity. The coupling of C.C.R. data with specific gravity (which is also deeply affected by the presence of aromatics), enables to evaluate the composition of the feedstock in terms of macromolecules. Molecular weight is strictly connected to the kinematic viscosity. Naphthenes have been distinguished into two subclasses of pseudo-components: 9 Original naphthenes (i.e. contained into the feed): they are mono- and polynaphthenic components with a repartition 60/40 of six and five-membered rings (50% of average methylation degree of the boundaries), and a single alkyl side chain (with 20% of methylation degree). An internal distribution into the molecules is assumed so that length of the side chain, number of rings etc. are available. It is based on an average structure composed by relatively few rings and long side chain.
159 9 Naphthenic components derived from cracking: shorter side chain naphthenes, cyclo-olefines and olefinic naphthenes. These components are characterized by two numbers representing the total number of C atoms and that in the side chain.
3. Kinetics Aspects Most of the typical aspects of VB kinetic modeling initially have been derived as an extension of gas-phase pyrolysis kinetic scheme (Dente and Ranzi, 1983, Clymans and Froment, 1984, Hillewaert et al. 1988, Dente et al., 1992). Also in this case, lumping rules allows to reduce the huge number of real components and their reactions. A radical chain process is involved. The main reactions can be divided into three groups: initiations, propagations (13-cleavage, H-abstraction, substitutive addition onto unsaturated molecules) and terminations (radicals recombination). Paraffins and aromatics decomposition behavior and kinetic constants have been already described in a previous paper (Dente et al. (1997)). Both side chain and naphthenic rings of mono- and poly-naphthenes can be attacked in the radicalic process. In the first situation, a radical position is formed in some place of the chain and two different 13-scissions can occur with the same probability, giving place respectively to a paraffinic radical and a naphthene with olefinic side chain or to a naphthenic radical and an olefin. Main fate of naphthenic radical is to give place to other shorter side chain naphthenes by H-abstraction on the liquid substrate. In the second situation two possibilities can be distinguished: if the radical is adjacent to a methylated position, a methyl radical and a naphthenic cyclo-olefin are preferably formed. Otherwise smaller naphthenes with unsaturation on the side chain or cycloolefines, lacking of some rings, are formed. Of course, cyclo-olefines, and particularly their poly-cyclic structures, are responsible for the formation of some aromatic components.
4. Asphaltenes and Residues Stability The problem of residue stability against Asphaltenes flocculation has been already discussed in previous papers (Bozzano et al., 1995, Dente et al., 1997). Essential aspects will be here reported. Asphaltenes are defined as those components that precipitate after addition to the residue of specified amounts of n-heptane (IP 143). They can be considered as large poly-aromatic sheets generally methylated on the boundaries, and characterized by short paraffinic side chain. Aromatics decompose during the pyrolysis increasing the content of asphaltenes in the VB residue. The latter can flocculate, in certain conditions, giving place to mud. This affects residue quality and therefore constitutes a limit to VB severity. At a molecular scale the mixture can be represented by two phases. The first one (called malthenic phase) is constituted by aromatics, alkanes, alkenes and naphthenes, the latter by aromatics, and asphaltenes. Reciprocal compatibility is related to solubility parameters. The latter ranges from 7-8 (cal/cm3) 1~ for alkanes and alkenes, 8-9 for naphthenes or cyclo-olefines, 8.8-10 for aromatics and finally to 11-12 for asphaltenes. The more close the solubility parameters of the two phases are, the more stable is the system against flocculation. Every asphaltenic molecule is supposed to be surrounded
160 by aromatics, deposited as "sheets" over the asphaltenic "sheet", constituting a molecular aggregate (-- adsorbed phase). The effect of this coverage is to allow a better compatibility with the malthenic environment thanks to improved solubility parameter of the aggregate with respect to the asphaltenes alone. The non-asphaltenic aromatics are supposed to be distributed between malthenic and adsorbed phase. Their repartition ratio depends on the temperature and composition of the two phases, and can be estimated from the activity coefficients of the aromatic class inside the two phases. The latter are estimated by using the solubility parameters. Precipitation will occur when, as a result of the dilution (e.g. with cetane), the total concentration of "aromatics" in the malthenic phase is lowered so that only the de-sorption of "aromatics" deposited on "asphaltenes" can compensate it. Naphthenic components (or cyclo-olefines) increase, with their presence, the global solubility parameter of the malthenic phase, so that a lower amount of "aromatics" is necessary to cover the asphaltenes.
5. The Problem of the Fouling VB takes place in a furnace frequently followed by a soaker that is equivalent to an adiabatic reactor. Soaker increases total residence time, allowing a reduction of 20-30~ on the Coil Outlet Temperature (C.O.T.) at equal severity. Reactions takes place mainly in the liquid phase since process temperature is usually under 500~ As described in a previous paper of Dente et al. (1997), the formation of carbonaceous deposits on the internal walls of the coils is a very important aspect. The reactor on-stream time is largely influenced by this phenomenon because of the increase of pressure drop and of the tube skin temperature at coil outlet. The main mechanisms governing fouling are catalytic and radicalic ones. Initially, tube internal skin is relatively clean and provides a metallic catalytic surface for poly-additions of vinyl-aromatic molecules. Once the first polymeric layer is formed, a radicalic mechanism prevails into coke deposit growth. The necessary radicals derive from the surroundings or are directly generated by the formed polymers by means of the cleavage of the weakest and highly resonant C-C benzyl-like bonds. Subsequent degradation, dehydrogenation and cross-linking of the deposit produce a more and more amorphous structure. Fouling can be reduced by moderating C.O.T. through soaker introduction and also by means of steam or fluxant dilution. The last ones allow higher velocity inside the coil, improving heat exchange coefficient, residence time and coil temperature control.
6. Results, Comparisons and Conclusions In Table 1 properties and yields derived from the VB of two different vacuum residues in industrial plants are compared. The first one has a prevailing aromatic nature; the second one has a considerable content of naphthenes. Residue 1 is characterized as follow: TBPI 550, specific gravity 15/4 1.0495 g/cm 3, kinematic viscosity at 150~ 320 cSt, CCR 24, sulphur 4.35 wt. %, while for residue 2: TBPI 530, specific gravity 15/4 0.975 g/cm 3, kinematic viscosity at 150~ 46 cSt, CCR 11.8, sulphur 0.79 wt.%. Operating conditions are, for feed 1 and 2: C.O.T. 460 ~ inlet temperature 330 ~ coil outlet pressure 11.5 ata, total residence time 30 and 45 minutes respectively.
161
Table 1 Comparison of experimental and calculated ~,ields and properties I
III
Vacuum Residues
1 I
Exp.
Model
Exp.
Model
0.4 2.34
0.44 2.62
0.04 1.6
0.04 1.51
TBP (~ Specific gravity 15~176 3 Sulphur wt% Bromine Number Yield wt%
40-185 0.739 0.85 76 4.12
40-i85 0.734 0.80 79 4.56
10-185 0.724 0.26 90 5.5
10-185 0.721 0.23 88 5.86
TBP (~ Specific gravity 15~176 3 Kin. Visc. at 50~ cSt Sulphur wt% Bromine number Yield wt%
185-385 0.867 2.8 2.0 31 13.62
185-385 0.867 3.1 2.4 32 12.68
185-385 0.823 2.5 0.45 44 17.1
185-385 0.821 2.7 0.39 43 16.78
EFFLUENTS GAS
H2S Yield
wt% wt% I
GASOLINE
GASOIL
II
VISBROKEN RESIDUE
TBP (~ Specific gravity 15~176 cSt Kin. visc. at 150~ Peptisation Value Sulphur wt% Asphaltenes wt% CCR wt% Yield wt%
>385 1.075 280 1.2 4.6 22 28 79.82
>385 1.081 260 1.21 4.4 20 26 80.14
>385 0.985 32 1.25 0.87 14.0 17.0 75.8
>385 0.990 27 1.26 0.88 15.6 17.8 75.85
I
Estimated composition (molar fractions) for residue 1 is: XArom"~- 0.86, XNaphth" ~- 0.01, Xpa~af. = 0.13, while for residue 2: Xmom. = 0.41, XNaphth. = 0.42, Xpa~af. = 0.17. The agreement between experimental data and the model simulations is good. In particular, final stability value (i.e. PV) of both VB residues can be observed to be in a range of 1.2-1.25, while naphthenic feed gives a higher yield in volatile products (24.2% vs. 20.18%). At fixed degree of VB severity, a larger quantity of valuable effluents derives from naphthenic residues. Naphthenes improves the stability through the increase of the malthenic phase solubility parameter and of the aromatic content of the residue, given that part of their decomposition products is constituted by aromatics. Figure 1 reports an extended comparison among calculated and experimental PV for different residues (virgin or thermally treated); about one half of the data referred in this figure belongs to highly naphthenic residues.
Fig. 1. Comparison between experimental and calculated P V
162 T II
Maximum Skin Temperature (~
560
ExperimentalData
[--"--Model Results
540
F
520
%=",
~" 500
,
~
480 460 440 0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
DaysOnStream. . . . . . . . . .
Fig.2. Comparison between exp. and sim. maximum skin temperatures at the coil outlet
Figure 2 shows a comparison between experimental and calculated maximum skin temperature for an industrial plant including both soaker and steam dilution. The data are related to a run characterised by a periodical change of aromatic and naphthenic rich feed. The comparison is satisfactory. In conclusion, a correct modeling of VB needs to include naphthenic components in feedstocks and products characterization, due to the implication of their presence on plant optimization and connected economical aspects. References Ali M.F., M.U. Hasan, A.M. Bukharl and M. Sallem, 1985, Hydrocarbon Processing, February, 83. Bozzano G., M. Dente, C. Pirovano and M. Molinari, 1995, The Characterization of the Residues Stability, AIDIC Conference Series, Eds. S. Pierucci, Eris C.T.S.r.l., Milano, vol. 1,173. Bozzano G., M. Dente, M. Sugaya and C. McGreavy, 1998, The Characterization of Residual Hydrocarbon Fractions with Model Compounds Retaining the Essential Information, preprints of Symposia - vol. 43, No.3, 2162 ACS National Meeting Boston, 653 Clymans, P.J. and G.F. Froment, 1984, Computers chem. Engng., vol. 8, 137. Dente M. and E. Ranzi, 1983, Mathematical Modeling of Hydrocarbon Pyrolysis Reactions, in Pyrolysis: Theory and Industrial Practice. Eds. Albright, Crynes, and Corcoran, Academic Press, 133 Dente M., S. Pierucci, E. Ranzi, and G. Bussani, 1992, Chem. Eng. Sc., Vol. 47, 2629. Dente M., G. Bozzano, 1993, Reactor Modeling of The Visbreaking Process, Proceedings of the First Conference on Chemical and Process Engineering, Florence, 163. Dente M., G. Bozzano and G. Bussani, 1997, Computers chem. Engng., vol. 21, 1125 Hillewaert L.P., J.L. Dierickx and G.F. Froment, 1988, A.I.Ch.E. Journal 34, 17.