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.     

INVESTIGATION OF ASPHALTENE PRECIPITATION AT ELEVATED TEMPERATURE

In order to obtain quantitative data on the asphaltene precipitation induced by the addition of n-alkane (heptane) at temperatures above the normal boiling point of the precipitant, a high temperature/ high pressure filtration apparatus has been constructed. Oil and alkane are mixed at the appropriate temperature and the pressure in closed vessels keeping the mixture at the liquid state. The filtration is performed with a small differential pressure over the filter so as to avoid flashing the mixture. The technique requires a low dead volume in the system to minimize the content of maltenes in the extracted fraction, hence there is no room for stirring. The equipment as well as solutions to some of the problems are presented along with precipitation data from 40 to 200° C The asphaltenes separated are analyzed using FT-ir. The filtrate containing the maltenes was cooled to room temperature and the asphaltenes separating upon cooling was collected and analyzed. The oil and selected maltene fractions and extraction/ cleaning solvents were analyzed using GC     

ASPHALTENE PRECIPITATION AND SOLVENT PROPERTIES OF CRUDE OILS

ABSTRACT

Improved prediction of the onset of asphaltene precipitation may be achieved using refractive index (RI) to characterize crude oils and their mixtures with precipitants and solvents. Experimental measurements of RI for mixtures of several crude oils with the precipitant n-heptane, are reported at ambient conditions. Theoretical developments are described that will permit extension of these observations to reservoir conditions

Measurements of RJ at the onset of precipitation have shown that the onset occurs at a characteristic RI for each oil/ precipitant combination, supporting the premise that precipitation is dominated by London dispersion interactions and thus, that RI can be used to predict the onset of precipitation. Reports in the literature showing that the onset of precipitation occurs at constant solvent-to-precipitant ratios provide additional confirmation

The theory is developed on the assumption that London dispersion forces dominate aggregation and precipitation of asphaltenes. The interaction energy of asphaltene molecules or aggregates in a medium of oil can be expressed as a function of the difference between the RI of asphaltene and oil. The RI of live crude oil during pressure depletion can be calculated from the RI of the stock tank oil, the molar refraction of the separator gas, the formation volume factor B_o and the solution gas/ oil ratio R_s     

ASPHALTENE PRECIPITATION AND SOLVENT PROPERTIES OF CRUDE OILS

Improved prediction of the onset of asphaltene precipitation may be achieved using refractive index (RI) to characterize crude oils and their mixtures with precipitants and solvents. Experimental measurements of RI for mixtures of several crude oils with the precipitant n-heptane, are reported at ambient conditions. Theoretical developments are described that will permit extension of these observations to reservoir conditions

Measurements of RJ at the onset of precipitation have shown that the onset occurs at a characteristic RI for each oil/ precipitant combination, supporting the premise that precipitation is dominated by London dispersion interactions and thus, that RI can be used to predict the onset of precipitation. Reports in the literature showing that the onset of precipitation occurs at constant solvent-to-precipitant ratios provide additional confirmation

The theory is developed on the assumption that London dispersion forces dominate aggregation and precipitation of asphaltenes. The interaction energy of asphaltene molecules or aggregates in a medium of oil can be expressed as a function of the difference between the RI of asphaltene and oil. The RI of live crude oil during pressure depletion can be calculated from the RI of the stock tank oil, the molar refraction of the separator gas, the formation volume factor B_o and the solution gas/ oil ratio R_s     

MICROSCOPIC INVESTIGATION OF THE ONSET OF ASPHALTENE PRECIPITATION

ABSTRACT

A microscopic study of the onset of asphaltene precipitation is reported. The onset conditions can be quantified by measurement of mixture refractive index, together with microscopic observations of particulate formation in mixtures of oil and precipitant, with or without added solvents. For isooctane mixtures with a variety of hydrocarbon solvents and a crude oil from Alaska, the onset of precipitation occurs over a narrow range of solution refractive index. Addition of polar solvents or different precipitating agents can shift the refractive index at which precipitation begins. Refractive index decreases when a crude oil is diluted by precipitant, as in this study, or when changes in temperature and pressure alter the relative molar volumes of species in the oil. If it falls below some critical value, resin/asphaltene aggregates that had been in stable dispersion become unstable and precipitate. These observations provide a method of screening solvents to differentiate between those that prevent precipitation mainly by maintaining a higher mixture refractive index and others that may participate in or disrupt asphaltene/resin interactions.     

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.     

MICROSCOPIC INVESTIGATION OF THE ONSET OF ASPHALTENE PRECIPITATION

A microscopic study of the onset of asphaltene precipitation is reported. The onset conditions can be quantified by measurement of mixture refractive index, together with microscopic observations of particulate formation in mixtures of oil and precipitant, with or without added solvents. For isooctane mixtures with a variety of hydrocarbon solvents and a crude oil from Alaska, the onset of precipitation occurs over a narrow range of solution refractive index. Addition of polar solvents or different precipitating agents can shift the refractive index at which precipitation begins. Refractive index decreases when a crude oil is diluted by precipitant, as in this study, or when changes in temperature and pressure alter the relative molar volumes of species in the oil. If it falls below some critical value, resin/asphaltene aggregates that had been in stable dispersion become unstable and precipitate. These observations provide a method of screening solvents to differentiate between those that prevent precipitation mainly by maintaining a higher mixture refractive index and others that may participate in or disrupt asphaltene/resin interactions.     

EFFECTS OF ASPHALTENE PRECIPITATION AND REPRECIPTION ON THE METAL-CONTAINING COMPOUNDS IN HEAVY RESIDUA

ABSTRACT

Boscan vacuum residuum (VR) has been separated into isooctane insoluble asphaltenes and isooctane soluble maltenes. The asphaltenes were dissolved in a minimum of toluene and were further separated by two additional reprecipitations using isooctane as the precipitating solvent. We examined the fractions, including the recovered isooctane soluble material, by size exclusion chromatography (SEC) with inductively coupled plasma (ICP) emission spectroscopy to determine the effects, if any, the reprecipitations have on the size distribution of the metal-containing compounds.

The asphaltene fractions show little change in size upon reprecipitation, but removal of a small amount of the metal components does occur. The first reprecipitation produces isooctane soluble material which is very similar in size to the original maltene fraction. However, the second reprecipitation removes small amounts of material in the size range of the asphaltenes. When the amounts were weight balanced, the resulting separation did little to effect the overall maltene and asphaltene size profiles.     

EFFECTS OF ASPHALTENE PRECIPITATION AND REPRECIPTION ON THE METAL-CONTAINING COMPOUNDS IN HEAVY RESIDUA

Boscan vacuum residuum (VR) has been separated into isooctane insoluble asphaltenes and isooctane soluble maltenes. The asphaltenes were dissolved in a minimum of toluene and were further separated by two additional reprecipitations using isooctane as the precipitating solvent. We examined the fractions, including the recovered isooctane soluble material, by size exclusion chromatography (SEC) with inductively coupled plasma (ICP) emission spectroscopy to determine the effects, if any, the reprecipitations have on the size distribution of the metal-containing compounds.

The asphaltene fractions show little change in size upon reprecipitation, but removal of a small amount of the metal components does occur. The first reprecipitation produces isooctane soluble material which is very similar in size to the original maltene fraction. However, the second reprecipitation removes small amounts of material in the size range of the asphaltenes. When the amounts were weight balanced, the resulting separation did little to effect the overall maltene and asphaltene size profiles.     

REVERSIBILITY AND INHIBITION OF ASPHALTENE PRECIPITATION IN BRAZILIAN CRUDE OILS

ABSTRACT

A fundamental understanding of the aggregation and precipitation of asphaltenes in petroleum crudes is important for the development of preventive and curative measures for the potential problem of asphaltene deposition occurring during production, transport and refining operations. The question of reversibility of asphaltene precipitation, yet a controversial issue, is crucial for a clear and unequivocal understanding of the precipitation phenomenon, development of mathematical models that describe the behavior of asphaltenes in petroleum fluids, and the design of inhibitors. In this work, the behavior of precipitated asphaltenes in Brazilian crude tank oil samples following flocculant removal and gradual addition of fresh oil was investigated. The results obtained revealed a re-dissolution of precipitated asphaltene particles following flocculant removal and oil addition. On the inhibition of asphaltene precipitation, the capacity of a number of surfactants and block copolymers to inhibit asphaltene precipitation and deposition was also examined. Ethoxylated Nonylphenols and Hexadecyl Trimethyl Ammonium Bromide displayed highest capacity in the inhibition of asphaltene deposition.     

溶剂芳香性对渣油沥青质稳定性的影响

为深入认识固定床渣油加氢-催化裂化双向组合技术溶剂芳香性与沥青质稳定性的关系,采用Flory-Huggins模型考察了沙特轻质原油常压渣油的沥青质稳定性,并用溶解度参数关联了溶剂的芳香性。结果表明,Flory-Huggins模型可用于研究渣油体系的沥青质稳定性。随着溶剂芳香性的增加,其溶解度参数增大,沥青质稳定性增强。密度较大、H/C原子比较小的溶剂具有较大的溶解度参数,对沥青质的稳定性较有利。     

REVERSIBILITY AND INHIBITION OF ASPHALTENE PRECIPITATION IN BRAZILIAN CRUDE OILS

A fundamental understanding of the aggregation and precipitation of asphaltenes in petroleum crudes is important for the development of preventive and curative measures for the potential problem of asphaltene deposition occurring during production, transport and refining operations. The question of reversibility of asphaltene precipitation, yet a controversial issue, is crucial for a clear and unequivocal understanding of the precipitation phenomenon, development of mathematical models that describe the behavior of asphaltenes in petroleum fluids, and the design of inhibitors. In this work, the behavior of precipitated asphaltenes in Brazilian crude tank oil samples following flocculant removal and gradual addition of fresh oil was investigated. The results obtained revealed a re-dissolution of precipitated asphaltene particles following flocculant removal and oil addition. On the inhibition of asphaltene precipitation, the capacity of a number of surfactants and block copolymers to inhibit asphaltene precipitation and deposition was also examined. Ethoxylated Nonylphenols and Hexadecyl Trimethyl Ammonium Bromide displayed highest capacity in the inhibition of asphaltene deposition.     

溶剂芳香性对渣油沥青质稳定性的影响

为深入认识固定床渣油加氢-催化裂化双向组合技术溶剂芳香性与沥青质稳定性的关系，采用Flory-Huggins模型考察了沙特轻质原油常压渣油的沥青质稳定性，并用溶解度参数关联了溶剂的芳香性。结果表明，Flory-Huggins模型可用于研究渣油体系的沥青质稳定性。随着溶剂芳香性的增加，其溶解度参数增大，沥青质稳定性增强。密度较大、H/C原子比较小的溶剂具有较大的溶解度参数，对沥青质的稳定性较有利。     

ON THE MASS BALANCE OF ASPHALTENE PRECIPITATION

In the evaluation of experimental data as well as in calculation of phase equilibria the necessity of the application of mass balances is obvious. In the case of asphaltenes the colloidal nature of these compounds may highly affect the mass balance. In the present paper several experiments are performed in order to check the consistency of mass balances within asphaltene precipitation. Asphaltenes are precipitated in two step processes either by changing temperature or by changes in precipitant with increasing precipitation power. This has been performed for three different oils. The data indicates that in temperature experiments as well as in solvent series experiments the precipitation of heavy asphaltenes affects the following precipitation of lighter asphaltenes. In both cases the mass balance using standard separation techniques cannot be closed, as less material is precipitated in a two step process than in the direct process either at low temperature or by direct precipitation with one precipitant. The different fractions were subject to HPLC size exclusion chromtagraphy showing that the material remaining in solution in the stepwise process was of low molecular weight, and that the material in the second precipitation step was often of higher apparent molecular weight and had an increased overall absorbance coefficient.     

ON THE MASS BALANCE OF ASPHALTENE PRECIPITATION

In the evaluation of experimental data as well as in calculation of phase equilibria the necessity of the application of mass balances is obvious. In the case of asphaltenes the colloidal nature of these compounds may highly affect the mass balance. In the present paper several experiments are performed in order to check the consistency of mass balances within asphaltene precipitation. Asphaltenes are precipitated in two step processes either by changing temperature or by changes in precipitant with increasing precipitation power. This has been performed for three different oils. The data indicates that in temperature experiments as well as in solvent series experiments the precipitation of heavy asphaltenes affects the following precipitation of lighter asphaltenes. In both cases the mass balance using standard separation techniques cannot be closed, as less material is precipitated in a two step process than in the direct process either at low temperature or by direct precipitation with one precipitant. The different fractions were subject to HPLC size exclusion chromtagraphy showing that the material remaining in solution in the stepwise process was of low molecular weight, and that the material in the second precipitation step was often of higher apparent molecular weight and had an increased overall absorbance coefficient.     

ASPHALTENE PRECIPITATION FROM COLD LAKE AND ATHABASCA BITUMENS

The onset of asphaitene precipitation from Cold Lake and Athabasca bitumens was studied using a heat transfer technique. The onset for these two types of bitumen was at a n-heptane to bitumen volume ratio of 1.4 to 1.7. Since it is important to suppress the onset of precipitation, a number of additives (aromatics; hydrogen donor solvents, solvents with the heteroatoms nitrogen, sulphur, and oxygen; and surfactants) were tested for their ability to delay the onset of asphaitene precipitation from Cold Lake bitumen. Phenanthrene, an aromatic, and the surfactant nonyl-phenol were found to be most effective. Cold Lake bitumen maltenes were found to be as effective as the aromatic chemical additives in delaying the onset of asphaitene precipitation. As the maltenes fraction contains saturates, the resins fraction alone should be even more effective at delaying precipitation.     

浅谈单溶剂与双溶剂的脱蜡效果

就溶剂脱过程中溶剂的选择进行了论述。通过用甲异丁基酮和甲乙酮混合溶剂做脱蜡或脱油溶剂，对溶解能力，脱后油收率，等性能方面的综合比较，提出了发展ＭＩＢＫ脱蜡工艺的建议。     

EFFECTS OF ASPHALTENE PRECIPITATION AND A MODIFIED D 2007 SEPARATION ON THE MOLECULAR SIZE OF VANADIUM-AND NICKEL-CONTAINING COMPOUNDS IN HEAVY RESIDUA

Boscan vacuum residuum (VR), and Maya atmospheric residuum (AR), were separated into operationally defined components by an isooctane asphaltene precipitation followed by a modified D 2007-80 separation of the isooctane soluble material. These collected fractions were examined by size exclusion chromatography (SEC) with element specific detection employing inductively coupled plasma emission spectroscopy (ICP) to investigate the effect the separation has on the the V and Ni compounds. The V and Ni SEC-ICP profiles of the asphaltene fractions indicated some of the metal-containing molecules are substantially larger in size than those observed in the residua. The asphaltene precipitation appears to cause an increase in molecular size through agglomeration of smaller constituents, and accomplishes only limited size separation of the metal-containing compounds.

The resin fraction, the polar fraction obtained by modified D 2007-80 chromatography of the isooctane soluble portion, contained much of the smaller size molecules of the feeds. The agglomeration seen in the SEC-ICP profiles of the asphaltene fraction was not apparent in any of the resin fraction profiles.