THERMODYNAMICS OF ASPHALTENE PRECIPITATION AND DISSOLUTION INVESTIGATION OF TEMPERATURE AND SOLVENT EFFECTS

ABSTRACT

Petroleum asphaltenes have been precipitated in solvent mixtures of n-heptane and toluene at various temperatures, likewise n-heptane asphaltenes have been dissolved in under similar conditions. This give added evidence to apparent hysteresis phenomenon between the two processes. The Asphaltenes have been characterized showing that although data is scattered convergence to certain structural parameters as incipient flocculation is approached. The asphaltenes are seen to consist of an associating and a non-associating part. The solubility of asphaltenes has been correlated/modelled using the Flory-Huggins equation using two different terms for the Flory parameter. A process for evaluation of best choice of solubility parameter and molar volume for the asphaltenes is proposed. Dissolution processes are seen to be best fitted by the equations. Based on these findings the asphaltenes are proposed to be formed by a colloidal and a true solution part.     

DIBSOLUTION OF SOLID BOBCAN ASPHALTENES IN MIXED SOLVENTS.

ABSTRACT

Solid petroleum asphaltenes have been fractionated according to solubility in toluene/n-heptane mixtures of increasing toluene content. A large hysteresis was observed between this dissolution and the precipitation from the crude oil. In order to shed light on the solution mechanism, the fractions obtained have been analyzed using size exclusion chromatography (SEC-HPLC-UV-vis), VPO, elemental analysis, UV-vis adsorption spectroscopy and phenol interaction values and methylene content by FTir. Less polar non-associating low molecular weight species are dissolved and a specific extraction of porphyrins is observed. An increased association in the insolubles is indicated. More basic interaction sites are available on the asphaltenes in both fractions relative to the native asphaltene. From the SEC chromatograms it was seen that the soluble fractions did not associate as the insoluble fractions even when making up more than 60 % of the total asphaltenes.     

SEPARATION AND CHARACTERIZATION OF VANADIUM AND NICKEL ORGANOMETALLIC COMPOUNDS IN HEAVY CRUDES

A new separation scheme has been developed to concentrate nickel and vanadium complexes in selected fractions before attempting a molecular characterization of these organometallic species in heavy crude oils. It involves a combination of solubility- (n-heptane deasphalting and pyridine/water extraction of asphaltenes) and polarity- (S0₃-modined silica gel column) based steps The proposed method has been applied to Belayim crude oil.The nature of pyridine/water extract from asphaltenes has been investigated using both preparative gel permeation chromatography (GPC) and reverse phase liquid chromatography (RP-HPLC) combined (off-line) to graphite furnace atomic absorption (GFAA). The fractions obtained after the separation of the extract on a S0₃-modified silica gel column have been submitted to a series of analytical techniques such as GFAA, mass spectrometry using fast atom bombardment lionization technique (MS-FAB),UV-VIS spectroscopy and electron paramagnetic resonance (EPR)

All the analytical evidences, risen from the present study, indicate that vanadium, present in pyridine/water extract obtained from asphaltenes, is mostly bound in porphyrin structures, although the coordination sphere around vanadyl species, present in the most polar fraction obtained from the extract separation by column chromatography, shows different EPR parameter. On the other hand, nickel complexes are smaller and more polar than common Ni-porphyrins, which are concentrated in deasphalted oil.     

Variation in Composition of Subfractions of Petroleum Asphaltenes

Boscan asphaltenes were precipitated from the crude oil using mixtures of toluene and heptane at temperatures of 24, 50, and 80°C. Another process of extracting solid n-heptane asphaltenes (24°C) using the same solvent systems and temperatures was also investigated. Asphaltene yield is different by the two processes at similar conditions although both increases in temperature or toluene content lead to lower solid yield. This way the asphaltene continuum was investigated from incipient flocculation to total n-heptane precipitated asphaltenes. The asphaltenes were analyzed using elemental analysis, fluorescence spectroscopy, vapor pressure osmometry, and HPLC size exclusion chromatography. The distribution of porphyrins was also measured using absor-bance spectroscopy. The asphaltenes were found to follow a regular trend in elemental ratios, H/C and N/C, indicating increased aromaticity and nitrogen content as the solid yield decreases. Also the molecular weight was seen to increase. Size exclusion chro-matograms and fluorescence spectra were found to be different comparing solids from the two separation processes. This indicates that the asphaltene fractions obtained by extraction of solid asphaltenes are altered relative to asphaltenes obtained by ordinary precipitation. The porphyrin concentration was found to diminish rapidly with solid yield decrease in both precipitation and extraction experiments, the latter fractions, however, containing significantly less asphaltenes indicating an adsorption step in the coprecipitation of porphyrins. Soluble fractions were found to exhibit relatively low molecular weights and an apparent lack of indications of association up to a point of solubles exceeding 50 % of the total asphaltenes. The-latter have implications for the further understanding and experimental investigation. of the associating nature of asphaltenes such as concentration effects during analytical characterization.     

EFFECT OF PRECIPITATION TEMPERATURE ON THE COMPOSITION OF N-HEPTANE ASPHALTENES

Petroleum asphaltenes were separated from Boscan and Kuwait crude oils by n-heptane at constant temperatures ranging from -2 to 80 ° C. A decreased yield with increase in temperature was observed. The precipitated material was characterized using infrared spectroscopic functional group analysis, HPLC-size exclusion chromatography, vapor pressure osmometry, and elemental analysis. The change in vanadium porphyrin content was estimated for Boscan using UV-Vis. With elevated temperature the asphaltenes are more aromatic (lower H/C) and have higher apparent molecular weight, whereas the pattern of changes in functional groups and heteroatoms (NSO) are more complex and apparently dependent on the crude oil. Porphyrins and low molecular weight types were seen to stay in solution at increased temperature, and a specific extraction of these takes place above 40 ° C. Asphaltene solubility is seen to approximate the Flory-Huggins theory as small molecules go into solution before large molecules. Several other mechanisms may, however, be involved in the phase separation. The molecular size distribution curves obtained by HPLC-SEC were found to explain the trends found in the molecular weights by VPO.     

SEPARATION AND CHARACTERIZATION OF VANADIUM AND NICKEL ORGANOMETALLIC COMPOUNDS IN HEAVY CRUDES

Abstract

A new separation scheme has been developed to concentrate nickel and vanadium complexes in selected fractions before attempting a molecular characterization of these organometallic species in heavy crude oils. It involves a combination of solubility- (n-heptane deasphalting and pyridine/water extraction of asphaltenes) and polarity- (S0₃-modined silica gel column) based steps The proposed method has been applied to Belayim crude oil.The nature of pyridine/water extract from asphaltenes has been investigated using both preparative gel permeation chromatography (GPC) and reverse phase liquid chromatography (RP-HPLC) combined (off-line) to graphite furnace atomic absorption (GFAA). The fractions obtained after the separation of the extract on a S0₃-modified silica gel column have been submitted to a series of analytical techniques such as GFAA, mass spectrometry using fast atom bombardment lionization technique (MS-FAB),UV-VIS spectroscopy and electron paramagnetic resonance (EPR)

All the analytical evidences, risen from the present study, indicate that vanadium, present in pyridine/water extract obtained from asphaltenes, is mostly bound in porphyrin structures, although the coordination sphere around vanadyl species, present in the most polar fraction obtained from the extract separation by column chromatography, shows different EPR parameter. On the other hand, nickel complexes are smaller and more polar than common Ni-porphyrins, which are concentrated in deasphalted oil.     

CHARACTERIZATION OF ASPHALTENE PARTICLES BY LIGHT SCATTERING AND ELECTROPHORESIS

The solubility of asphaltenes in heptane/toluene mixtures was studied at several temperatures. A significant increment in asphaltene solubility was observed when the temperature increases from 0°C to 20°C and a moderate increase when the temperature rose from 20°C to 50°C. These results indicate that asphaltenes behave as a higher consulate temperature system, similar to nonpolar waxes. Examined by photon correlation spectroscopy, diameters from the particles formed a range between 125 and 400 nm, depending on the amount of non-solvent (n-heptane) used for the precipitation process and the initial concentration of asphaltenes. The particles presented a small positive surface potential that was not altered by the addition of resins.     

CHARACTERIZATION OF ASPHALTENE PARTICLES BY LIGHT SCATTERING AND ELECTROPHORESIS

The solubility of asphaltenes in heptane/toluene mixtures was studied at several temperatures. A significant increment in asphaltene solubility was observed when the temperature increases from 0°C to 20°C and a moderate increase when the temperature rose from 20°C to 50°C. These results indicate that asphaltenes behave as a higher consulate temperature system, similar to nonpolar waxes. Examined by photon correlation spectroscopy, diameters from the particles formed a range between 125 and 400 nm, depending on the amount of non-solvent (n-heptane) used for the precipitation process and the initial concentration of asphaltenes. The particles presented a small positive surface potential that was not altered by the addition of resins.     

Variation in Composition of Subfractions of Petroleum Asphaltenes

ABSTRACT

Boscan asphaltenes were precipitated from the crude oil using mixtures of toluene and heptane at temperatures of 24, 50, and 80°C. Another process of extracting solid n-heptane asphaltenes (24°C) using the same solvent systems and temperatures was also investigated. Asphaltene yield is different by the two processes at similar conditions although both increases in temperature or toluene content lead to lower solid yield. This way the asphaltene continuum was investigated from incipient flocculation to total n-heptane precipitated asphaltenes. The asphaltenes were analyzed using elemental analysis, fluorescence spectroscopy, vapor pressure osmometry, and HPLC size exclusion chromatography. The distribution of porphyrins was also measured using absor-bance spectroscopy. The asphaltenes were found to follow a regular trend in elemental ratios, H/C and N/C, indicating increased aromaticity and nitrogen content as the solid yield decreases. Also the molecular weight was seen to increase. Size exclusion chro-matograms and fluorescence spectra were found to be different comparing solids from the two separation processes. This indicates that the asphaltene fractions obtained by extraction of solid asphaltenes are altered relative to asphaltenes obtained by ordinary precipitation. The porphyrin concentration was found to diminish rapidly with solid yield decrease in both precipitation and extraction experiments, the latter fractions, however, containing significantly less asphaltenes indicating an adsorption step in the coprecipitation of porphyrins. Soluble fractions were found to exhibit relatively low molecular weights and an apparent lack of indications of association up to a point of solubles exceeding 50 % of the total asphaltenes. The-latter have implications for the further understanding and experimental investigation. of the associating nature of asphaltenes such as concentration effects during analytical characterization.     

EFFECT OF PRECIPITATION TEMPERATURE ON THE COMPOSITION OF N-HEPTANE ASPHALTENES Part 2

Petroleum asphaltenes precipitated by n-heptane at temperatures between ambient and 80^°C from two crude oils have been characterized regarding hydrocarbon structures using ¹H and ¹³C nmr, and fluorescence spectroscopy. This indicates a very complex and apparently very small change in structural features except for a molecular weight increase as more material stays in solution at elevated temperature. Aromaticity increase and apparently alkyl chains diminish. The latter trend is however affected by the structural equation used. According to the fluorescence spectroscopy the content of large and complex chromophores increases but a significant effect of the presence of porphyrins on the spectra is also observed. The data is analyzed in combination with previous characterizations of the same asphaltenes presented in the first part of this work (Andersen, 1994). This indicates that as smaller molecules are extracted at elevated temperature the precipitated asphaltenes tend to associate more. This implys that these small molecules block association sites which therefore become available for further association at elevated temperature. For Boscan part of the smaller easy to extract porphyrins may have this behavior.     

EFFECT OF PRECIPITATION TEMPERATURE ON THE COMPOSITION OF N-HEPTANE ASPHALTENES Part 2

ABSTRACT

Petroleum asphaltenes precipitated by n-heptane at temperatures between ambient and 80^°C from two crude oils have been characterized regarding hydrocarbon structures using ¹H and ¹³C nmr, and fluorescence spectroscopy. This indicates a very complex and apparently very small change in structural features except for a molecular weight increase as more material stays in solution at elevated temperature. Aromaticity increase and apparently alkyl chains diminish. The latter trend is however affected by the structural equation used. According to the fluorescence spectroscopy the content of large and complex chromophores increases but a significant effect of the presence of porphyrins on the spectra is also observed. The data is analyzed in combination with previous characterizations of the same asphaltenes presented in the first part of this work (Andersen, 1994). This indicates that as smaller molecules are extracted at elevated temperature the precipitated asphaltenes tend to associate more. This implys that these small molecules block association sites which therefore become available for further association at elevated temperature. For Boscan part of the smaller easy to extract porphyrins may have this behavior.     

INFLUENCE OF TEMPERATURE AND SOLVENT ON THE PRECIPITATION OF ASPHALTENES

Asphaltenes has been precipitated from a Kuwait flash residue using different n-alkanes (n-C5 to n-C8) at various temperatures ranging from 4^°C to reflux temperatures of the used precipitants. Structures in the asphaltene fractions has been revealed using U.V. fluorescence spectroscopy, elemental analysis and to some extent 1H-nmr. These analysis shows that asphaltenes precipitated in the same amount but at different temperature and with different solvents have merely the same composition. For all n-alkanes the curves of precipitated amount versus temperature show maxima at about 25^°C, implying a shift in the solubility of the asphaltenes.The impact of alkane chain length on the aggregation of asphaltenes through hydrogen bonds is discussed using the alkane-alcohol system as a model. The asphaltene solubility is discussed with the help of the Scatchard-Hildebrand equation.     

Deasphalting of crude oils using supercritical fluids

A crude oil has four main SARA constituents: saturates, aromatics, resins, and asphaltenes. The asphaltenes in crude oil are the most complex and heavy organic compounds. The asphaltenes contain highly polar substituents and are insoluble in an excess of n-heptane (or n-pentane). The classic definition of asphaltenes is based on the solution properties of petroleum residuum in various solvents. Asphaltenes are a solubility range that is soluble in light aromatics such as benzene and toluene, but is insoluble in lighter paraffins. The particular paraffins, such as n-pentane and n-heptane, are used to precipitate asphaltenes from crude oil. The effects of four different solvents (water, carbon dioxide, propane, and ethanol) on the deasphalting process under the supercritical conditions were reviewed. Supercritical water is an excellent solvent for removing of high molecular weight organic compounds such as asphaltenes from crude oils under the supercritical conditions.     

离子液体-有机溶剂复合技术处理含油污泥的研究

采用离子液体辅助溶剂萃取法处理胜利油田的含油污泥，回收原油，考察了不同溶剂、剂油比、温度、反应时间、离心机转速等条件对油和沥青质脱附效率的影响。结果表明：最佳的实验条件为在正庚烷+离子液体[Emim][BF4]溶剂体系中，剂油比50/3，温度50 ℃，反应时间20 min，离心机转速2 000 r/min；在此条件下，含油污泥中油的回收率高达90.5%，沥青质的回收率为83.1%；正庚烷和离子液体可以回收重复使用，正庚烷的一次和二次回收率分别为89.2%和74.2%；离子液体几乎没有损失，一次和二次回收率分别高达98.6%和95.4%；使用二次回收的正庚烷和离子液体为溶剂时油和沥青质的回收率分别达到85.1%和78.4%。在此基础上，探讨了含油污泥脱附过程的作用机理。离子液体-有机溶剂复合技术显著提高了油泥萃取的油回收率，对油泥的无害化和资源化工业生产具有重要的参考价值。     

ELECTROPHORETIC MOBILITY AND STABILIZATION OF ASPHALTENES IN LOW CONDUCTIVITY MEDIA

ABSTRACT

The electrophoretic mobilities of asphaltenes in n-heptane and ethanol were determined. The asphaltenes studied show a positive charge in both solvents. However, the magnitude of the charge is considerably lower in n-heptane than in ethanol. It is concluded that although the electrostatic forces are presented in both solvents, the van der Waals’ atractive forces are the main responsable for the flocculation of asphaltene particles. The addition of a well known asphaltene stabilizer, dodecyl benzene sulphonic acid (DBSA), decreases the electrophoretic mobility of asphaltene particles in ethanol until a constant positive value is reached. This means that the neutralization of the positive charges of the asphaltene particles is not complete, probably due to the limited adsorption of the DBSA on asphaltene surface and also, to the presence of charges inside the asphaltene particles that are not accessible to the DBSA.     

ELECTROPHORETIC MOBILITY AND STABILIZATION OF ASPHALTENES IN LOW CONDUCTIVITY MEDIA

The electrophoretic mobilities of asphaltenes in n-heptane and ethanol were determined. The asphaltenes studied show a positive charge in both solvents. However, the magnitude of the charge is considerably lower in n-heptane than in ethanol. It is concluded that although the electrostatic forces are presented in both solvents, the van der Waals' atractive forces are the main responsable for the flocculation of asphaltene particles. The addition of a well known asphaltene stabilizer, dodecyl benzene sulphonic acid (DBSA), decreases the electrophoretic mobility of asphaltene particles in ethanol until a constant positive value is reached. This means that the neutralization of the positive charges of the asphaltene particles is not complete, probably due to the limited adsorption of the DBSA on asphaltene surface and also, to the presence of charges inside the asphaltene particles that are not accessible to the DBSA.     

FLOCCULATION OF ASPHALTENES AT HIGH PRESSURE. II. CALCULATION OF THE ONSET OF FLOCCULATION

The influence of pressure on the onset of flocculation of asphaltenes was calculated in the region from 1 to 300 bar and from 50 to 100°C. These calculations are the counterpart to our experimental data which, recently, have been reported in part 1 of an equally titled article [9]. As gas component methane and as precipitant i-octane were used. The asphaltene flocculation was considered to be a liquid-liquid equilibrium. For modelling the van der Waals equation of state (vdW-EOS) in the framework of continuous thermodynamics was applied. The composition of the crude oil was described by a continuous distribution function with respect to the solubility parameter δ of the Scatchard-Hildebrand theory. Within the distribution the asphaltenes represent the species with the highest δ-values. For oils with a very low content of asphaltenes the model developed describes the experimental flocculation data reasonably. In accordance to the experimental data the model predicts that, in the considered pressure range, without addition of i-octane asphaltene flocculation does not occur. However, on contrary to the experimental results, the model predicts the asphaltenes to show a higher flocculation tendency with increasing asphaltene content of the crude oil. For very high asphaltene contents the model even completely fails. Probably, the reason of this lack is the disregarding of asphaltene association.     

ISOLATION OF ASPHALTENES USING A CONTINUOUS PRECIPITATION METHOD

A new continuous method allows the isolation of heptane asphaltenes which are nearly identical to asphaltenes isolated by a well-studied batch method. Samples of one-half liter of atmospheric resid can be treated using one to two liters of n-heptane, while keeping heptane/resid ratios at 40:1. Asphaltenes precipitated by a well established batch method were identical within the errors of elemental and NMR characterization methods. Small amounts of colored, heptane-soluble materials can be extracted from samples prepared by the continuous technique, but this extraction does not change the analyses of the extracted solids.     

SEPARATION OF ASPHALTENES BY POLARITY USING LIQUID-LIQUID EXTRACTION

In order to investigate the nature of petroleum asphaltenes in terms of polarity a process was developed using initial liquid-liquid extraction of the oil phase followed by precipitation of the asphaltenes using n-heptane. The liquid-liquid extraction was performed using toluene-methanol mixtures with increasing content of toluene. Although large fractions of the crude oil (Alaska '93) was extracted in the higher polarity solvents (high concentration of methanol), the asphaltene content of the dissolved material was low As the toluene content increased more asphaltenes were transferred to the solvent phase. The asphaltenes were analysed using FTir, Elemental analysis, and HPLC-SEC with a diode array detector. With increasing content of toluene in the methanol the molecular weight distribution of the asphaltenes significantly move to higher molecular weights. The content of nitrogen and sulfur of the mallene phase also increase while H/C decreases. The content of heteroatoms in the asphaltenes are relatively higher and apparently increase with the polarity of the solvent. It is concluded that these asphaltenes are indeed dominated by high molecular weight substances that cannot be extracted in the high polarity solvents     

Deposition and flocculation of asphaltenes from crude oils

A crude oil has four main constituents: saturates, aromatics, resins, and asphaltenes. The asphaltenes in crude oil are the most complex and heavy organic compounds. The classic definition of asphaltenes is based on the solution properties of petroleum residuum in various solvents. Asphaltenes are a solubility range that is soluble in light aromatics such as benzene and toluene, but are insoluble in lighter paraffins. The particular paraffins, such as n-pentane and n-heptane, are used to precipitate asphaltenes from crude oil. Deposition of asphaltenes in petroleum crude and heavy oil can cause a number of severe problems. The precipitation of asphaltene aggregates can cause such severe problems as reservoir plugging and wettability reversal. Asphaltenes can precipitate on metal surface. Cleaning the precipitation site as well as possible appears to slow reprecipitation. To prevent deposition inside the reservoir, it is necessary to estimate the amount of deposition due to various factors. The processes can be changed to minimize the asphaltene flocculation, and chemical applications can be used effectively to control depositions when process changes are not cost effective. Asphaltene flocculation can be controlled through better knowledge of the mechanisms that cause its flocculation in the first place. The processes can be controlled to minimize the asphaltene flocculation, and chemical applications can be used effectively to control depositions when process changes are not cost effective.