ISOLATION OF ASPHALTENES USING A CONTINUOUS PRECIPITATION METHOD

ABSTRACT

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.     

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.     

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 OF ASPHALTENES BY POLARITY USING LIQUID-LIQUID EXTRACTION

ABSTRACT

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     

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     

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.     

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 EXPERIMENTAL CONDITIONS AND SOLVENTS ON THE PRECIPITATION AND COMPOSITION OF ASPHALTENES

The ASTM D 3279 test method for “n-Heptane Insolubles” is being currently used to determine the asphaltene content of fuel oils as defined by insolubility in normal (n) heptane solvent. Precipitation of n-heptane insolubles can produce sediments which vary in color, morphology and composition. The SEM analysis of n-heptane insolubles indicated that the sediments were heterogeneous containing some large particles >300 μ and the majority of particles to be small, <10 μ, “chained” together forming agglomerates. Significant fractions of n-heptane insolubles were found insoluble in toluene indicating their non-asphaltenic nature (“solids”). FTIR and XRF analyses of “solids” indicated the presence of multiring aromatics, Ni and carboxylate and sulfate salts. FTIR analysis of toluene soluble asphaltenes showed the presence of multiring aromatics. GC/MS analysis of toluene soluble asphaltenes indicated desorption of alkylbenzenes and O-containing molecules but no desorption of N- and S-containing molecules was observed. The tendency for N- and S-containing molecules to remain in the nonvolatile residue during the GC/MS analysis indicates that the N- and S-containing compounds have higher stability. The XRF analysis of the toluene soluble asphaltenes indicated that the molecules are similar to asphaltenes found in crude oils in terms of C, H, N and S contents, however, no presence of V- and Ni-containing molecules was found.     

PRECIPITATION OF ASPHALTENES AT HIGH PRESSURES; EXPERIMENTAL TECHNIQUE AND RESULTS

Precipitation of asphaltenes in crude oil has been measured both as a function of pressure and composition. The onset of precipitation has been determined by the measurement of conductivity. A specially designed high pressure conductivity cell has been constructed. Due to the low conductivity of most crude oils the electrode spacing has to be very narrow and the effective electrode area as large as practical. The cell has a coaxial design with inlet and outlet on opposite sides on the cylinder. The cell is rated to 700 bar and 120 deg.C but can be constructed for both higher pressure and temperature. The onset of precipitation is detected by a maximum in the weight normalized conductivity, i.e. the conductivity divided by the weight fraction of oil

By using normal alkanes from ethane to pentane the precipitation onset is shown to increase linearly as a function of n-alkane carbon number. A smaller but significant increase in the onset is found by increasing the hydrostatic pressure. For monophasic live crude the onset is shown by a break in the conductivity curve.     

PRECIPITATION OF ASPHALTENES AT HIGH PRESSURES; EXPERIMENTAL TECHNIQUE AND RESULTS

ABSTRACT

Precipitation of asphaltenes in crude oil has been measured both as a function of pressure and composition. The onset of precipitation has been determined by the measurement of conductivity. A specially designed high pressure conductivity cell has been constructed. Due to the low conductivity of most crude oils the electrode spacing has to be very narrow and the effective electrode area as large as practical. The cell has a coaxial design with inlet and outlet on opposite sides on the cylinder. The cell is rated to 700 bar and 120 deg.C but can be constructed for both higher pressure and temperature. The onset of precipitation is detected by a maximum in the weight normalized conductivity, i.e. the conductivity divided by the weight fraction of oil

By using normal alkanes from ethane to pentane the precipitation onset is shown to increase linearly as a function of n-alkane carbon number. A smaller but significant increase in the onset is found by increasing the hydrostatic pressure. For monophasic live crude the onset is shown by a break in the conductivity curve.     

EVALUATION OF PARAFFINIC MATERIAL IN ASPHALTENES ISOLATED BY PRECIPITATION WITH LIGHT ALKANES

ABSTRACT

The precipitation of paraffins by the methods usually employed for isolation and quantification of asphaltenes was verified by a brief characterization of n-alkane insoluble material from a paraffinic petroleum. So, a technique was sought to allow ready detection of these paraffins in n-pentane and n-heptane asphaltenes of heavy petroleum fractions of interest, and that was achieved by the use of differential scanning calorimetry. Liquid chromatography, which might yield material for further characterization, was also studied, with high temperature elutions, but it could not resolve groups with asphaltenes as the initial sample.     

EVALUATION OF PARAFFINIC MATERIAL IN ASPHALTENES ISOLATED BY PRECIPITATION WITH LIGHT ALKANES

The precipitation of paraffins by the methods usually employed for isolation and quantification of asphaltenes was verified by a brief characterization of n-alkane insoluble material from a paraffinic petroleum. So, a technique was sought to allow ready detection of these paraffins in n-pentane and n-heptane asphaltenes of heavy petroleum fractions of interest, and that was achieved by the use of differential scanning calorimetry. Liquid chromatography, which might yield material for further characterization, was also studied, with high temperature elutions, but it could not resolve groups with asphaltenes as the initial sample.     

PEPTIZATION AND COAGULATION OF ASPHALTENES IN APOLAR MEDIA USING OIL-SOLUBLE POLYMERS

The asphaltene fraction of crude oil contains a variety of acidic and basic functional groups. During oil production and transportation, changes in temperature, pressure or oil composition can cause asphaltenes to precipitate out crude oil through the flocculation among these polar functional groups. In this study, two types of oil-soluble polymers, dodecylphenolic resin and poly (octadecene maleic anhydride), were synthesized and used to prevent asphaltenes from flocculating in heptane media through the acid-base interactions with asphaltenes. The experimental results indicate that these polymers can associate with asphaltenes to either inhibit or delay the growth of asphaltene aggregates in alkane media. However, multiple polar groups on a polymer molecule make it possible to associate with more than one asphaltene molecule, resulting in the hetero-coagulation between asphaltenes and polymers. It was found that the size of the asphaltene-polymer aggregates was strongly affected by the polymer-to-asphaltene weight (or number) ratio. At low polymer-to-asphaltene weight ratios, asphaltenes keep flocculating with themselves and with polymers until the floes precipitate out of solution. On the other hand, at high polymer-to-asphaltene weight ratios, asphaltene-polymer aggregates peptized by the extra polymer molecules can remain fairly stable in the solution.     

PEPTIZATION AND COAGULATION OF ASPHALTENES IN APOLAR MEDIA USING OIL-SOLUBLE POLYMERS

ABSTRACT

The asphaltene fraction of crude oil contains a variety of acidic and basic functional groups. During oil production and transportation, changes in temperature, pressure or oil composition can cause asphaltenes to precipitate out crude oil through the flocculation among these polar functional groups. In this study, two types of oil-soluble polymers, dodecylphenolic resin and poly (octadecene maleic anhydride), were synthesized and used to prevent asphaltenes from flocculating in heptane media through the acid-base interactions with asphaltenes. The experimental results indicate that these polymers can associate with asphaltenes to either inhibit or delay the growth of asphaltene aggregates in alkane media. However, multiple polar groups on a polymer molecule make it possible to associate with more than one asphaltene molecule, resulting in the hetero-coagulation between asphaltenes and polymers. It was found that the size of the asphaltene-polymer aggregates was strongly affected by the polymer-to-asphaltene weight (or number) ratio. At low polymer-to-asphaltene weight ratios, asphaltenes keep flocculating with themselves and with polymers until the floes precipitate out of solution. On the other hand, at high polymer-to-asphaltene weight ratios, asphaltene-polymer aggregates peptized by the extra polymer molecules can remain fairly stable in the solution.     

AN EXPERIMENTAL STUDY OF THE EFFECT OF PARAFFINIC SOLVENTS ON THE ONSET AND BULK PRECIPITATION OF ASPHALTENES

ABSTRACT

Asphaltene onset concentration and bulk deposition were measured for a typical live reservoir oil titrated with n-C₆H₁₄, n-C₅H₁₂, n-C₄H₁₀, C₃H₈, C₂H₆, CH₄ and CO₂ at 100° C (212 ° F) and 29.9 MPa (4340 psia). The concentration of titrant at asphaltene onset was observed to decrease approximately in a linear fashion with decreasing molecular weight of the paraffinic solvent; CH₄ did not induce any asphaltene precipitation. Bulk deposition experiments were performed using a solvent: oil volume ratio of 10:1; the results indicated that the weight percent of asphaltenes precipitated increased exponentially with decreasing molecular weight of the paraffinic solvents. More importantly, the asphaltene molecular weight showed a maximum for n-C₄H₁₀ precipitated asphaltenes. Possible explanations for this unusual result are presented.     

AN EXPERIMENTAL STUDY OF THE EFFECT OF PARAFFINIC SOLVENTS ON THE ONSET AND BULK PRECIPITATION OF ASPHALTENES

Asphaltene onset concentration and bulk deposition were measured for a typical live reservoir oil titrated with n-C₆H₁₄, n-C₅H₁₂, n-C₄H₁₀, C₃H₈, C₂H₆, CH₄ and CO₂ at 100° C (212 ° F) and 29.9 MPa (4340 psia). The concentration of titrant at asphaltene onset was observed to decrease approximately in a linear fashion with decreasing molecular weight of the paraffinic solvent; CH₄ did not induce any asphaltene precipitation. Bulk deposition experiments were performed using a solvent: oil volume ratio of 10:1; the results indicated that the weight percent of asphaltenes precipitated increased exponentially with decreasing molecular weight of the paraffinic solvents. More importantly, the asphaltene molecular weight showed a maximum for n-C₄H₁₀ precipitated asphaltenes. Possible explanations for this unusual result are presented.     

CATALYTIC CRACKING OF QAIYARAH ASPHALTENES USING 5áMOLECULAR SIEVES

Qaiyarah crude oil is one of the heaviest crude oils in the middle east and contains a large proportion of asphattenes (15-17% wt.). Catalytic cracking of hexane-asphaltenes was performed employing 5A molecular sieves. Fractionation of the cracking products was conducted using solvent extraction and column chromatography. 6A molecular sieves in the zeolite form showed a superior cracking behavior, and do not reveal any poisoning by sulphur. Pmr results may indicate production of liquid hydrocarbons from asphaltenes.     

SWELLING OF ASPHALTENES

ABSTRACT

We report x-ray scattering measurements on asphaltenes and bitumen solutions, with attention to the concentrated regimes. We characterize the data in terms of a correlation length which is indicative of proximity to phase separation in a mixture, rather than the radius-of-gyration more indicative of individual colloids. By studying the behavior of the low-angle asphaltene peak on dilution, we show that asphaltenes swell. These results allow the drawing of a phase diagram with two regimes separated by a critical point: a colloidal regime, and a“ swollen asphaltene” regime, the latter being much larger.     

SWELLING OF ASPHALTENES

We report x-ray scattering measurements on asphaltenes and bitumen solutions, with attention to the concentrated regimes. We characterize the data in terms of a correlation length which is indicative of proximity to phase separation in a mixture, rather than the radius-of-gyration more indicative of individual colloids. By studying the behavior of the low-angle asphaltene peak on dilution, we show that asphaltenes swell. These results allow the drawing of a phase diagram with two regimes separated by a critical point: a colloidal regime, and a“ swollen asphaltene” regime, the latter being much larger.     

THE CONCEPT OF ASPHALTENES REVISITED

Petroleum asphaltenes are defined as material insoluble in pentane or heptane and the nature of the molecular species that constitute asphaltenes has been a well researched subject over the past three decades. These investigations have led to a general consensus of the majority of researchers that asphaltenes contain condensed aromatic systems carrying alkyl, cycloalkyl, and heteroatom substituents. A variety of molecular models has been developed which have been used to explain the behavior of asphaltenes in feedstocks during recovery and processing operations

However, an alternate concept introduced the idea that asphaltenes can be defined as a complex mixture of organic compounds of varying molecular weight and polarity. This allows asphaltenes to be described more fully in terms of their behavior during processes and properties in products, such as asphalt

Thus, the concept can be used to explain, even predict, the incompatibility of asphaltenes with the other petroleum constituents as well as the deposition of asphaltic material on reservoir rock and the occurrence of sediments during refinery operations.