4 Asphaltene Stability
4.4 SEDIMENT FORMATION .1 R EACTIVITY.1 REACTIVITY
It is accepted that the asphaltenes present in crude oils form micelles which are stabilized by resins kept in solution by aromatics. Resins are considered to be natural asphaltene dispersants. The effect of different surfactants on the dispersion of asphaltenes, in crude oils, was reported.
Alkyl phenols were found to show good peptizing properties but were found to partially lose their capacity when oxyethylenic groups were incorporated into the molecule. Long chain aliphatic alcohols and alkylbenzenes were found to be inefficient as dispersing agents. Primary aliphatic amines were found to show some ability to disperse asphaltenes (Gonzales and Middea, 1991). The effect of different molecules on the colloidal stability of asphaltenes, dispersed in pentane, was also reported. Nonylphenol was found to act as a dispersing agent. Stearic acid was found not to affect the colloidal stability of asphaltenes while hexade-cylamine was found to act as a flocculant (Lian et al., 1994). The literature reported on the use of dodecylbenzenesulfonic acid (DBSA) as a dispersant to study the electrophoretic mobility of asphaltene particles in ethanol. DBSA was found to decrease the electrophoretic mobility of asphaltene particles until a constant positive value was reached and the neutralization of the positive charges of the asphaltene particles was achieved by the adsorption of DBSA (Leon et al., 2000). The more recent literature reported that synthetic dispersants can increase the solubility of asphaltenes in crude oils and are more effective than resins when used at high concentrations. Synthetic dispersants were reported to make asphal-tenes soluble in n-heptane and thereby convert them to resins (Wiehe and Jermansen, 2003). The effect of different commercial dispersants on the stability of HCGO and the C7 insolubles precipitation is shown in Table 4.23.
Before the addition of any dispersants, HCGO was found to contain no sediment and have C7 insoluble content of 171 ppm. The repeated C7 insoluble content of 172 ppm indicated a good repeatability of the ASTM D 3279 test procedure. After the addition of 1 wt% of dodecylphenol, no sediment was observed and the C7 insoluble content was found to vary in a range of 128–165 ppm. After the addition of 1 wt% of nonylphenol, no sediment was observed and the C7 content was found to vary in a range of 161–171 ppm. After the addition of 1 wt% of DBSA Na salt also, no sediment was observed and C7 insolubles content was found to vary in a similar range of 118–145 ppm. The literature reported that two types of polymers, dodecylphenolic resin and poly(octadecene maleic anhydride) prevented asphaltenes, obtained from crude oils, from flocculating in n-heptane through the acid–base interaction. At low polymer-to-asphaltene weight ratios, asphaltenesflocculate with themselves and with the polymer while at higher polymer-to-asphaltene weight ratios, asphaltene–polymer aggregates are peptized by the extra polymer and become more stable (Chang and Fogler, 1996). The poly(octadecene maleic anhydride), under the name poly(maleic anhydride-1-octadecene), is produced by reacting maleic anhydride with an olefin, such as 1-octadecene, and is commercially available. The effect of poly(maleic anhydride-1-octadecene), which is polymer, on the stability of HCGO and the C7 insolubles precipitation, is shown in Table 4.24.
While the commercial poly(maleic anhydride-1-octadecene) is advertised as an oil-soluble polymer, some heating and stirring was required to dissolve it in HCGO. Before any addition of a polymer, the HCGO was found to contain no sediment and have a low C7 insoluble content of only 46 ppm. After the addition of 0.1–1 wt% of polymer, no sediment was formed, however, some changes in C7 insolubles precipitation were observed. After the addition of 0.1–0.5 wt% of the polymer, the C7 insolubles precipitation was found to vary in a range of 141–164 ppm, indicating some precipitation increase and its color changed from black to beige. After the addition of 1 wt% of the polymer, the C7 insolubles precipitation
TABLE 4.23
Effect of Dispersants on Stability of HCGO and C7 Insolubles Precipitation
HCGO Sediment C7 Insolubles, ppm C7 Insolubles, ppm (repeated experiments)
Source: From Pillon, L.Z., Petroleum Science and Technology, 19, 863, 2001.
Reproduced by permission of Taylor & Francis Group, LLC, http:==www.
taylorandfrancis.com.
did not increase any further but actually decreased to 89 ppm and it was beige.
FTIR spectrum of C7 insolubles, precipitated in the presence of 0.3 wt% of poly (maleic anhydride-1-octadecene), indicated the presence of sulfates and an increase in the presence of C¼¼O peaks characteristic of an anhydride group.
The presence of C¼¼O peaks characteristic of an acid type functional group and the presence of a broad peak at 3500 cm1 characteristic of H-bonded OH groups were also observed indicating the presence of unreacted poly(maleic anhydride-1-octadecene) polymer.
The literature reported that in crude oils, the multiple polar anhydride groups on a poly(maleic anhydride-1-octadecene) polymer molecule were found to associate with more than one asphaltene molecule resulting in the hetero-coagulation (Chang and Fogler, 1996). According to the literature on asphaltenes present in crude oils, unsaturated bonding of maleic anhydride can react with unsaturation of multiring aromatic structures and a subsequent hydrolysis can yield a product bearing carboxylic acid function (Speight, 1999). The effect of maleic anhydride on the stability of HCGO and C7 insolubles precipitation is shown in Table 4.25.
After an addition of only 0.1 wt% of maleic anhydride, while no sediment formation was observed, the C7 insolubles content of HCGO drastically increased, from 52 to 635 ppm, and its color changed from black to brown. After an addition of 0.3 wt% of maleic anhydride, while no sediment formation was observed, the C7 insolubles content of HCGO further increased to 926 ppm and its color was brown.
After an addition of 0.5 wt% of maleic anhydride, while no further increase in C7 insolubles was observed, its color changed from brown to beige. With an additional increase in the maleic anhydride content to 1 wt%, while no sediment formation was observed, the C7 insolubles content of HCGO further increased to 1028 ppm and its color was beige. FTIR spectrum of C7 insolubles, precipitated in the presence of 0.1 wt% of maleic anhydride, indicated the presence of aromatics, multiring aromatics and only one C¼¼O peak at 1720 cm1, characteristic of an acid type functional group. The presence of a broad peak at 3100–3300 cm1 indicated the presence of H-bondedOH groups. No presence of C¼¼O peaks TABLE 4.24
Effect of Poly(Maleic Anhydride-1-Octadecene) Polymer on C7 Insolubles
HCGO Sediment C7 Insolubles, ppm C7 Insolubles Color
Neat HCGO 0 46 Black
Polymer (0.1 wt%) 0 146 Beige
Polymer (0.3 wt%) 0 164 Beige
Polymer (0.5 wt%) 0 141 Beige
Polymer (1 wt%) 0 89 Beige
Source: From Pillon, L.Z., Petroleum Science and Technology, 19, 863, 2001. Reproduced by permission of Taylor & Francis Group, LLC, http:==www.taylorandfrancis.com.
characteristic of anhydride and C¼¼C peaks characteristic of unsaturation indicated no presence of any unreacted maleic anhydride left in HCGO. The effect of temperature and the presence of air on the sediment formation and C7 insoluble precipitation from HCGO is shown in Table 4.26.
A fresh sample of HCGO, containing no sediment and having a relatively low C7 insoluble content of 46 ppm, was heated without a nitrogen blanket. After heating for 2 h at 828C, no sediment was formed but some increase in C7 insolubles precipitation was observed. After heating for 3 h at 828C, the sediment was formed and a drastic increase in C7 insolubles precipitation was observed. With an increase in C7 insolubles precipitation, no increase in MCR was observed.
Heteroatom content provides polarity and is responsible for oil incompatibility.
Its interaction with oxygen changes functional group composition which means that oxidation will affect the oil stability. The literature reported that an increase in the process temperature and the use of chemical additives can increase the oil instability leading to solid precipitation (Wiehe, 2003). Additives are used during the coking process and their thermal stability and boiling point can vary which might lead to an increase in coke-like deposits affecting the catalyst activity or increase the carbon residue. Silicone antifoaming agents are used during the coking process. Demulsification of the oil-in-water emulsion from the column top of a TABLE 4.25
Effect of Maleic Anhydride on Stability of HCGO and C7 Insolubles Precipitation
HCGO Sediment C7 Insolubles, ppm C7 Insolubles Color
Neat HCGO 0 52 Black
Maleic anhydride (0.1 wt%) 0 635 Brown
Maleic anhydride (0.3 wt%) 0 926 Brown
Maleic anhydride (0.5 wt%) 0 817 Beige
Maleic anhydride (1 wt%) 0 1028 Beige
Source: From Pillon, L.Z., Petroleum Science and Technology, 19, 863, 2001. Reproduced by permission of Taylor & Francis Group, LLC, http:==www.taylorandfrancis.com.
TABLE 4.26
Effect of Temperature and Air on the Sediment Formation and C7 Insolubles Content
HCGO Stability Sediment C7 Insolubles, ppm C7 Insolubles Color
Fresh HCGO 0 46 Black
2 h at 828C=air 0 Increase Black
3 h at 828C=air Yes Drastic increase Black
fiers were reported more effective in breaking the emulsion (Liu et al., 2004). The use of additives, which is more economical than the optimization of the processing conditions, might further affect the oil stability and even increase the carbon residue.
4.4.2 STORAGESTABILITY
The literature reported that olefins are formed during the catalytic and thermal cracking of petroleum oils and many petroleum products become unstable during their storage or handling. Without the use of hydrogen, the catalytically and thermally cracked petroleum products have a tendency to form a sediment (Gary and Handwerk, 2001). One of the most common operating problems when using HC technology is the formation of coke-like sediment. The carbon-aceous sediment was reported to deposit on the reactor, and downstream vessels as well as on the catalyst surface leading to rapid catalyst deactivation and equipment fouling. The addition of aromatic-rich diluents was reported to decrease the sediment formation (Marafi et al., 2005). Popcorn coke was reported to be produced by polymerization of olefins present in thermally cracked oils. The polymerization rate increases with the temperature; however, it was reported to be the greatest in the temperature range of 5008F–5808F (2608C–3048C). The olefins conjugated to aromatics were reported to be the most reactive (Wiehe, 2003). The literature reported that the self-incompatible crude oils, hydrotreated oils, and thermally cracked oils, produced from converted residue, contain insoluble asphaltenes which cause fouling. The oil compatibility model and testing method was developed for detecting self-incompatible oils (Wiehe, 2004). The compos-ition and the storage stability of HCGO, produced under the same processing conditions, are shown in Table 4.27.
TABLE 4.27
Composition and Storage Stability of HCGO
Composition and Stability HCGO #1 HCGO #2
Density at 158C, g=cc 0.9438 0.9414
Carbon, wt% 84.0 82.5
Hydrogen, wt% 11.6 11.8
Sulfur, wt% 2.8 2.8
Nitrogen, wt% 0.16 0.18
Visible sediment No Yes
C7 insolubles, ppm 35 310
MCR, wt% 0.21 0.2
Source: From Pillon, L.Z., Petroleum Science and Technology, 19, 673, 2001. Reproduced by permission of Taylor & Francis Group, LLC, http:==www.taylorandfrancis.com.
Only some HCGO samples were found to contain suspended particles and required filtration to remove the sediment. HCGO samples, stored for 2 years, were found to contain some visible sediment. The presence of sediment, which could be separated using a 0.8mm filter, was found to be below 100–150 ppm.
HCGO, containing sediment, was found to have an increased C7 insoluble content with no significant increase in MCR. Olefins, formed during the thermal cracking, can polymerize leading to sediment formation but also many other molecules. Heteroatom content provides polarity and interaction with other func-tional groups can lead to agglomeration and sediment precipitation. The sediment was separated from HCGO, using a 0.8 mm filter, washed with toluene and analyzed. The FTIR analysis of toluene washed sediment, found in HCGO, is shown in Table 4.28.
The FTIR analysis of the toluene insoluble fraction of sediment indicated the presence of aromatics, multiring aromatics, sulfates, CH, CH2CH3, and
CH3 groups and the presence of carbon black particles. A broad peak at 3100–3300 cm1 indicated the presence of hydrogen bonding through the
OH group which might involve OHS, OHN, and OHO inter-actions. The FTIR analysis of the toluene soluble fraction of sediment indicated the presence of aromatics, multiring aromatics, aromatic ethers, carboxylates,
CH, CH2CH3, and CH3 groups. No presence of a hydrogen bonded
OH group was observed. The toluene soluble fraction of sediment was found to contain aromatic ethers and carboxylates, not present in C7 insolubles, indi-cating the presence of oxidation by-products. The optical microscopy analysis of sediment and C7 insolubles, precipitated from HCGO, is shown in Table 4.29.
The optical microscopy of the sediment, separated asfilter residue, indicated the presence of a heterogeneous mixture containing large black, rust, and brown particles. The optical microscopy of the n-heptane insolubles, precipitated from HCGO without filtration and containing sediment, indicated the presence of some small particles and several large black and rust colored particles. After the filtration and the sediment removal, the optical microscopy of the n-heptane insolubles precipitated from HCGO, indicated the presence of a homogeneous and uniform mixture. The color of C7 insolubles was dark and described as
TABLE 4.28
FTIR Analysis of Toluene Washed Sediment Found in HCGO
FTIR of Sediment from
HCGO (Toluene Insoluble Fraction) FTIR of Sediment from HCGO (Toluene Soluble Fraction)
Aromatics Aromatics
Multiring aromatics Multiring aromatics
Carbon black Aromatic ethers
Sulfates Carboxylates
Source: From Pillon, L.Z., Petroleum Science and Technology, 19, 673, 2001. With permission.
between plum and aubergine. SEM, equipped with energy dispersive x-ray spec-troscopy (EDS), can be used to determine the particle size and their composition.
The SEM–EDS analysis of sediment and C7 insolubles, precipitated from HCGO, is shown in Table 4.30.
The SEM–EDS analysis of sediment confirmed the presence of large particles
‘‘chained’’ together forming agglomerates. The main components of sediment were carbon, sulfur, oxygen, and iron. The SEM–EDS analysis of C7 insolubles, precipitated in the presence of sediment, indicated also the presence of some large particle,>300 mm and chained together forming agglomerates. The main components of C7 insolubles were carbon, sulfur, and iron. The large black particles were primarily carbon and found to resemble a thermal coke. The large rust colored particles were primarily iron indicating the presence of rust particles. The SEM–EDS analysis of C7 insolubles, precipitated from HCGO filtrate and after the sediment removal, indicated the presence of a homogeneous mixture containing small particles, <1 mm, also chained together forming TABLE 4.29
Optical Microscopy Analysis of Sediment and C7 Insolubles
Sediment from HCGO
(Filter Residue) C7 Insolubles from HCGO
(Containing Sediment) C7 Insolubles from HCGO (after Sediment Removal) Heterogeneous mixture Heterogeneous mixture Homogeneous mixture Several large particles: Source: From Pillon, L.Z., Petroleum Science and Technology, 19, 673, 2001. With permission.
TABLE 4.30
SEM–EDS Analysis of Sediment and C7 Insolubles from HCGO
SEM–EDS of Sediment
(Filter Residue) SEM–EDS of C7 Insolubles
(Containing Sediment) SEM–EDS of C7 Insolubles (after Sediment Removal) Majority particles>300 mm Majority particles>300 mm Majority particles<10 mm Forming agglomerates Forming agglomerates Forming agglomerates Main components:
Source: From Pillon, L.Z., Petroleum Science and Technology, 19, 673, 2001. With permission.
presence of heteroatoms, metals, and asphaltenes in the feed was reported to cause sludge and coke formation leading to the catalyst deactivation. Even small amounts of iron, copper, and particularly nickel and vanadium can affect the activity of the catalyst (Speight, 1999). The refining of crude oils leads to some products which are unstable and form sediments. The amount of cracking needs to be limited to prevent polymerization and sludge formation during the storage of petroleum products (Gary and Handwerk, 2001). The sediment from HCGO was also found to contain many other components, such as oxidation by-products, particles of coke and rust.
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