Change of asphaltene and resin properties after catalytic aquathermolysis

Petroleum Science, May 2009

Resin and asphaltene were separated from Liaohe heavy oil. Catalytic aquathermolysis of asphaltene and resin was investigated by using water soluble catalysts (NiSO4 and FeSO4) and oil soluble catalysts (nickel naphthenate and iron naphthenate). Before and after aquathermolysis, the properties of the resin and asphaltene was compared by means of ultimate analysis, vapor pressure osmometer (VPO) average molecular weight, X-ray diffraction (XRD), 13C and 1H nuclear magnetic resonance (NMR). The conversion sequence was as follows: No-catalyst<NiSO4<FeSO4<nickel naphthenate<iron naphthenate. In the presence of catalysts, the amount of H2 and CO increased significantly, while H2S in the gas product decreased. The molecular weight of asphaltene and resin increased after reaction without catalyst. But the catalysts restrained this trend. The H/C ratio of asphaltene and resin decreased after reaction. From the results of average structural parameters and molecular weight, it was found that asphaltene and resin were partly aggregated after aquathermolysis.

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Change of asphaltene and resin properties after catalytic aquathermolysis

Pet.Sci. Change of asphaltene and resin properties after catalytic aquathermolysis Yi Yufeng 1 Li Shuyuan 1 Ding Fuchen 0 1 Yu Hang 1 0 School of Chemical Engineering, Beijing Institute of Petrol-chemical Technology , Beijing 102617 , China 1 School of Chemical Science and Engineering, China University of Petroleum (Beijing) , Beijing 102249 , China Resin and asphaltene were separated from Liaohe heavy oil. Catalytic aquathermolysis of asphaltene and resin was investigated by using water soluble catalysts (NiSO4 and FeSO4) and oil soluble catalysts (nickel naphthenate and iron naphthenate). Before and after aquathermolysis, the properties of the resin and asphaltene was compared by means of ultimate analysis, vapor pressure osmometer (VPO) average molecular weight, X-ray diffraction (XRD), 13C and 1H nuclear magnetic resonance (NMR). The conversion sequence was as follows: No-catalyst<NiSO4<FeSO4<nickel naphthenate<iron naphthenate. In the presence of catalysts, the amount of H2 and CO increased significantly, while H2S in the gas product decreased. The molecular weight of asphaltene and resin increased after reaction without catalyst. But the catalysts restrained this trend. The H/C ratio of asphaltene and resin decreased after reaction. From the results of average structural parameters and molecular weight, it was found that asphaltene and resin were partly aggregated after aquathermolysis. Asphaltene; resin; aquathermolysis; molecular weight; catalyst 1 Introduction The supply of conventional light crude oil is inevitably and inexorably decreasing worldwide and its quality becomes poorer than ever. At the same time, extremely large reserves of heavy crude oil and bitumen are found at home and abroad. However, the high contents of sulfur, nitrogen, oxygen and metals, as well as the high viscosity and density of the heavy crude oil and bitumen make its production and utilization difficult. Steam stimulation is the most popular and effective technology to recover heavy oil. During steam injection, a down hole upgrading of heavy crude oil, named aquathermolysis firstly by Hyne, would take place, resulting in great decrease of its viscosity and improvement of its quality (Hyne and Greidanus, 1982) . Since then, underground upgrading of heavy crude oil has been widely studied due to its great advantages compared with the aboveground upgrading. Great progress was made in searching for suitable catalysts (Cesar et al, 2003; Li et al, 2007) , in selecting hydrogen donors (Ovalles, 2003; Ovalles, 1995) , and in interpreting reaction mechanism (Cesar et al, 2003; Richard et al, 2000; Beigrave et al, 1997) . According to the literature, almost all of the scientists have directly used heavy crude oil to carry out their work (Fan et al, 2001; Mujiea et al, 2000; Fan et al, 2007) . Because of the complicated composition of heavy crude oil, it is difficult to understand the reaction mechanism of aquathermolysis and to select appropriate catalysts if the mutual influence of its components is not eliminated. Nevertheless, it is impossible and unnecessary to separate the heavy crude oil into single components for research. In the exploitation and utilization of heavy oil resources, many problems appear due to asphaltene and resin, which tend to associate and precipitate out from the oil system (Nikookar et al, 2008) , resulting in deposition and plugging oil wells, hence causing a number of costly operational problems to oil producers (Abdulrazag et al, 2007; Mansoori et al, 2007) . Asphaltene and resin are also the primary cause of catalyst poisoning and of reducing the activity of hydrotreating catalysts due to coking on the catalyst surface and pore-mouth blocking (Takeshige et al, 2005). Much work on heavy oil upgrading has been aimed at asphaltene (Zhang et al, 2005; Enkhsaruul and Yasuo, 2003; Fernando and Jorge, 2005; Thierry et al, 2008; Wang and Edward, 2003) . There is no doubt that heavy oil aquathermolysis will ultimately focus on asphaltene and resin. To study the aquathermolysis of heavy oil and its property change, a heavy oil sample, from Liaohe Oilfield, Northeast China, was separated into four groups, namely saturates, aromatics, resin and asphaltene, according to their polarity. In this paper, aquathermolysis of asphaltene and resin, using dissolved nickel and iron compounds as catalyst, were carried out to investigate the reaction behavior and their property change after reaction. 2 Experimental 2.1 Separation of asphaltene and resin The contents of saturates, aromatics, resin and asphaltene in Liaohe heavy crude oil are 25.35%, 19.17%, 53.14%, 2.34%, respectively. The heavy crude oil contains 2.40% of moisture and has a density of 1.0006 g·cm-3 at 20 °C. The ultimate analysis and simulation distillation results are shown in Table 1 and Table 2. Liquid-solid adsorption chromatography was used to separate the heavy crude oil. About 20 grams of the heavy oil was dissolved in 400 mL pentane, and then heated and boiled for 1 hour under reflux. The mixture of heavy oil and pentane was filtered on Whatman filter paper (British Whatman Co., maximum filtering diameter 6-12 μm). Remains on the filter paper were enwrapped by more filter paper and extracted in a Soxhlet extractor using 300 mL of pentane for 12 hrs. Asphaltene was obtained on the filter paper. Neutral alumina with a size of 100-200 mesh was calcined at 550 °C for 4 hours. And then about 1% (by weight) of water was added to the alumina to lower its adsorption ability. The alumina was filled into a glass column shaped like a condensor with a stopcockcontrolled outlet on the bottom like a separating funnel. The column was about 2 cm in diameter and 2 m long. In its interlayer 50 °C water was circulated. The remaining part of the heavy oil was dissolved in light naphtha and poured into the glass column. The column was washed using different solvents, including light naphtha, benzene, and ethanolbenzene mixture. Through the operation, saturates, aromatics and resin were washed down in turn. After removal of solvent through evaporation, saturates, aromatics and resin were obtained. 2.2 Catalytic aquathermolysis of asphaltene and resin About 5 g asphaltene or resin was added into a 500 mL stainless-steel batch reactor equipped with a magnetic stirrer. Fifty milliliters of cyclohexane was used as the hydrogen donor. It contained oil soluble catalyst, such as 0.1 wt% nickel naphthenate (abbreviated as NiN) or iron naphthenate (abbreviated as FeN). Then fifty milliliters water was added, which contained water soluble catalyst, such as 0.1 wt% nickel sulfate (abbreviated as NiSO4) or ferrous sulfate (abbreviated as FeSO4). Air in the reactor was purged with high purity nitrogen to avoid possible explosion risk. The reactor was stirred at 120 rpm for 48 hrs at 280 °C. Then the reactor was cooled, and the oil and gas products were collected for analysis. 2.3 Tests and analysis The obtained oil product sample was vacuum-dried at 80 °C for 24 hrs, and removing cyclohexane at the same time. The amount of gas product was calculated by weight loss (Gas = 100×[asphaltene (or resin) reactant − weight of oil product]/asphaltene (or resin) reactant). The concentration of hydrogen in the gas product was analyzed by using GC7890 gas chromatography equipped with a thermal conductivity detector and 5A zeolite molecular sieve column. Nitrogen was used as carrier. The conditions for analysis were as follows: oven temperature 50 °C, detector temperature 100 °C, injector temperature 80 °C, and bridge electric current of 120 mA. The external standard method was used to calculate the content of hydrogen in the gas product. Other components in the gas product were analyzed on a Agilent 6890/5973 GC-MS with electronic ionization and mass spectroscopy detector. The GC column was an Abel AB-5MS (60m×0.25mm×0.25um) (Abel Co., USA). High purity helium was used as carrier. About 2 g oil product samples were enwrapped in Whatman filter paper and extracted by 80mL toluene in a Soxhlet extractor for 12 h to obtain the toluene insoluble fraction. The oil products were separated by liquid-solid adsorption chromatography as mentioned above to get the different SARA fractions (saturates, aromatics, resin and asphaltene). The separation was carried out on a small-scale glass column as described above. Conversion was calculated as follows: conversion = 100×reacted asphaltene (or resin)/asphaltene (or resin) reactant. Molecular weight was determined using a model 833 Vapor Pressure Osmometer made by UIC Inc., USA. Toluene was used as solvent and sucrose octaacetate as the standard substance with molecular weight of 678.6. Ultimate analysis was carried out by using a FLASH-EA1112 made by Thermo Fisher Scientific Inc., USA. XRD of asphaltene and resin was conducted using an XRD-7000 diffractometer (Shimadzu Co., Japan). 13C and 1H nuclear magnetic resonance was carried out on a Varian INOVA500 (Varian Co., USA). 3 Results and discussion 3.1 Conversion of asphaltene (or resin) and its product composition Asphaltene and resin are thermally unstable. Different cracking reactions take place at temperatures. And the products would further be converted to coke through secondary reactions. The main reactions are as follows: breaking of C-C, C-H, metal- and heteroatom-linked bonds; aromatization, alkylation, condensation, and hydrogenation and dehydrogenation. The results of aquathermolysis of asphaltene or resin are shown in Fig. 1 and Fig. 2. Aquathermolysis of asphaltene or resin took place even when no catalyst was present. Conversions of asphaltene and resin were about 3.8% and 8.1%, respectively, in the absence of catalyst. In fact, the conversion used here is not the concept itself, since asphaltene would be converted into the other asphaltene with higher polarity. To some extent, it can be used to denote the decomposition degree of asphaltene and resin. The catalysts % % 16 14 12 10 8 6 4 2 0 16 14 12 10 8 6 4 2 0 Conv Gas Sat Res Aro Tol Conv Gas Sat Res Aro Tol were obviously helpful to its conversion. The catalytic ability sequence was as follows: No-catalyst<NiSO4<FeSO4<nickel naphthenate<iron naphthenate. The conversion of asphaltene was 3.8%-14.9%, while the conversion of resin was 8.1%22.9%. Oil soluble catalysts showed better catalytic ability than water soluble ones, which was in accordance with the results of Fan et al (2006 ). Sergio et al (2006) reported that Fe directly interacted with heteroatoms (N, S) leading to the formation of metalasphaltene complexes, and the highest activity was found in C-S bonds. Negative charges, in the presence of H2O, will weaken the Fe-O and Fe-N bonds but reinforce Fe-S ones. Enkhsaruul and Yasuo (2003) reported the cracking of petroleum asphaltene on a catalyst of 10 wt% Fe supported on mesoporous molecular sieve (SBA-15) in a fixed bed reactor at 300 °C under atmospheric pressure He. It is proved that Fe is an effective catalytic component for hydrotreating. Iván et al (2005 ) used a quantum mechanics parametric method to investigate the interaction of metallic nickel, potassium, and water molecule with an asphaltene model molecule. It was found that there is a strong interaction between metallic nickel and nitrogen and sulfur atoms of asphaltene molecules, resulting in the formation of nickelasphaltene intermediate complexes. Nickel-asphaltene complexes could be able to dissociate water. In this case asphaltene molecules may be hydrogenated. Dispersed Ni and Fe participated in the aquathermolysis. The presence of the catalyst decreased the mount of toluene insoluble. Asphaltene would be converted into gas products, saturates, aromatics, resin and toluene insoluble (coke). Resin would be converted into gas product, saturates, aromatics, asphaltene and toluene insoluble residue. According to the literature (Li et al, 2007; Fan et al, 2007) , the amount of asphaltene and resin in heavy oil decreased after aquathermolysis. From this fact people easily make the mistaken assumption that asphaltene and resin would be directly converted into gas product, saturates, aromatics and toluene insoluble residue. Because heavy crude oil is so complex in composition, it is impossible to find that asphaltene and resin could be converted into each other if heavy oil is used to carry out the aquathermolysis reaction. If asphaltene and resin are separately used to carry out the aquathermolysis reactions, their characteristics could be found out. 3.2 Composition of gas product The compositions of gas products of asphaltene and resin aquathermolysis are listed in Table 3 and Table 4. Obviously CO2 26.5 21.4 18.8 3.3 Ultimate analysis The results of ultimate analysis are shown in Table 5. The catalyst used was iron naphthenate. After aquathermolysis, the H/C ratio, oxygen and sulfur contents of asphaltene and resin decreased, which can be attributed to the release of the volatile components including CO, CO2, H2S and light alkanes. The asphaltene and resin had a relatively lower H/ C ratio after the reaction due to dealkylation and increased condensed aromatic structures as well as the decreased branch-chains and aliphatic chain length. the catalyst increased the amount of H2 and CO in the gas product, while the content of H S decreased. Cesar et al (Cesar 2 et al, 2003; Cesar et al, 1998) reported the aquathermolysis of Hamaca oil using dispersed molybdenum and iron catalysts. They found MoS2 and (Fe0.6V0.4)zS (where z ranges from 0.8 to 0.9) in the coke. These two sulfides are the major active components for hydrotreating. It may be interpreted that the formation of sulfides promoted the hydrogen transfer reaction. There was very little hydrogen transfer reaction without catalyst. With the aid of the catalyst, H2 was produced by dissociation of water ( Iván et al, 2005 ). The formation of the sulfides caused the decrease of H2S. Producing CO, CO2 and benzene-insoluble simultaneously indicated that there existed thermal condensations reaction of asphaltene and resin. 3.4 Molecular weight The average molecular weight is an important parameter for characterization of asphaltene and resin. However, m e a s u r e m e n t o f m o l e c u l a r w e i g h t s ( M W ) o f t h e s e components is difficult due to their high polydispersity, heterogeneity and easy formation of aggregates. The association state of these molecules is known to be very sensitive to experimental conditions, such as concentration, temperature and solvents. For this reason, interpretation of analytical results might be tedious, and the molecular weight of asphaltene measured by using different techniques can vary by several orders of magnitude (Sophie et al, 2006) . In this study we used the vapor phase osmometer (VPO) method to estimate the MW of asphaltene and resin. The molecular weights of asphaltene and resin before and after aquathermolysis reaction are shown in Fig. 3 and Fig. 4. Generally, dealkylation or shortening of the aliphatic chain would decrease the average molecular weight of asphaltene and resin. But the average molecular weight of asphaltene and resin increased after aquathermolysis (see Figs. 3 and 4). The average molecular weight of resin increased from 1,738 to 2,380 after aquathermolysis reaction without catalyst, while that of asphaltene from 2,710 to 3,510. The reason might be that the partial association of these polar polynuclear aromatics resulted in increased polarity. The presence of catalyst restrained this increase. It might be that the formation of sulfide enhanced the hydrogen transfer reaction, thus holding back the aggregation of asphaltene and resin. Oil soluble catalysts had better restraining ability than CH4 40.2 42.7 41.7 42.4 43.5 C2H6 24.4 22.8 25.2 change in the layer distance between aromatic sheets, while the height of the crystallite increased. This implied the growth of the aromatic sheet after reaction. 3.6 Average structural parameters Asphaltene and resin are very complex in molecular La structure, and consist of polynuclear aromatics with alkyl side chains. It is difficult to know the exact molecular structure of asphaltene and resin because of their chemical complexity. Characterization of the average molecular structure of asphaltene and resin is the basic way to understand their functional properties. 13C and 1H nuclear magnetic resonance data is generally used to describe the total structure of asphaltene and resin (Dong et al, 2004; Hiroyuki and Fumio, 2000) . NMR is used to interpret their structural parameters, such as aromaticity, averaged carbon number of each alkyl side chain, rate of substitution of aromatic rings, and aromatic ring number. Combined with the results of ultimate analysis and molecular weight, the structural parameters of asphaltene and resin shown in Table 7 are calculated by using the 13C and 1H NMR data (Dong et al, 2004) . After aquathermolysis, the aromaticity of asphaltene and resin increased. The aromatic ring number of asphaltene increased from 7.92 to 13.77, while that of resin increased from 7.34 to 12.88, indicating the aggregation of asphaltene and resin. The increase in aromaticity of asphaltene resulted from dealkylation. 4 Conclusions To s t u d y t h e a q u a t h e r m o l y s i s o f h e a v y o i l , i t i s more target-oriented to use its asphaltene and resin than original heavy oil. In aquathermolysis of asphaltene and resin, catalytic activity sequence was as follows: Nocatalyst<NiSO4<FeSO4<NiN<FeN. Oil soluble catalysts had better catalytic ability than water soluble ones. In the presence of catalysts, the amount of H2 and CO increased significantly, while H2S in the gas product decreased. From asphaltene aquathermolysis, gas product, saturates, aromatics, resin and toluene insoluble (coke) could be obtained. Resin could also be converted into gas product, saturates, aromatics, asphaltene and toluene insoluble. The chemical structures of asphaltene and resin after aquathermolysis were different from those of feedstock asphaltene and resin. Resin could be converted into asphaltene by aromatization and by dealkylation. In addition, aromatization and dealkylation of asphaltene could produce asphaltene products with higher polarity. 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Yufeng Yi, Shuyuan Li, Fuchen Ding, Hang Yu. Change of asphaltene and resin properties after catalytic aquathermolysis, Petroleum Science, 2009, 194-200, DOI: 10.1007/s12182-009-0031-y