Precipitation petroleum asphaltene

The complexity of petroleum products raises the question of sample validity is the sample representative of the total flow The problem becomes that much more difficult when dealing with samples of heavy materials or samples coming from separations. The diverse chemical families in a petroleum cut can have very different physical characteristics and the homogeneous nature of the cut is often due to the delicate equilibrium between its components. The equilibrium can be upset by extraction or by addition of certain materials as in the case of the precipitation of asphaltenes by light paraffins. 

An additional subtlety in solvent choice is shown by cyclohexane and methylcyclohexane. In these cases, petroleum asphaltenes are completely soluble, while coal-derived asphaltenes are only sparingly soluble. This would imply that the precipitation line for cyclohexane runs completely outside the petroleum field of Figures 5 and 6 but passes through the coal-liquids field in Figure 6. Thus, some care must be shown when considering the equivalence or nonequivalence of various solvents. 

Many questions concerning the nature of petroleum asphaltenes remain unresolved (1) What is the chemical composition of petroleum asphaltenes (2) What are the molecular weights of asphaltene components (3) Why are asphaltenes precipitated from solution in petroleum by the addition of a hydrocarbon solvent such as n-pentane In this chapter we attempt to answer these questions. In addition, we suggest that asphaltene formation is a general phenomenon that is pertinent to the chemistry of coals, tar sand bitumens, shale oil, and other complex solutions of organic compounds. 

The data in Table I are significant because they suggest that a one-to-one relationship of acids and bases does not exist for petroleum asphaltenes. The precipitation of asphaltenes may be attributed to a phenomenon other than precipitation of acid-base complexes or salts. The data strongly imply that the asphaltenes primarily consist of compounds capable of association through the hydrogen bonding mechanism. 

The ratio of low molecular weight compounds to high molecular weight compounds is another factor in maintaining solubility of all compounds in petroleum. When this ratio is upset, large molecules precipitate (form asphaltene). This study shows that the precipitated... 

It is well known that flocculation of asphaltenes in petroleum reservoirs, wells and surface separation-upgrading facilities pose technical problems and increase the cost of production and processing of crudes. Field conditions conducive to precipitation of asphaltenes include natural depletion, miscible flooding, caustic flooding, acid stimulation and gas-lift operations. Asphaltene precipitation is particularly important problem in miscible flooding since it can reduce permeability, affect well injectivities and productivities, alter rock wettability characteristics and even cause plugging of producing wells. ... 

Fractionation by solubility is used to isolate asphaltenes from maltenes, and the standard method (ASTM-3279-97) has been used in this work. Crude petroleum is mixed with 40 volumes of heptane, heated, stirred, and left to cool. The asphaltenes form a precipitate that can be removed by filtration. Normal practice is to clean the asphaltene fraction by Soxhlet in heptane. The proportions of asphaltene in crude petroleums vary widely, from less than 0.1% in the best crudes to over 10% in the heavy crudes. In general, the heavy crudes such as Maya are of most interest. The solubility of petroleum asphaltene in NMP is of interest since the NMP-insoluble material has no fluorescence [61] and was initially assumed to be aliphatic. However, it has UV absorbance and must be aromatic [62]. The separation into NMP soluble and insoluble was achieved for several asphaltenes. A Kuwaiti asphaltene was separated into NMP-soluble and -insoluble fractions [63]. Seven crude oils were fractionated into heptane solubles and asphaltenes for comparison with an asphaltene from a heavy oil [64]. 

Speight, J. G Yen, T.F. Chinligarian, G.V., (1994). Chemical and Physical Studies of Petroleum Asphaltenes. Asphaltenes and Asphalts. Developments in Petroleum Science, v. 40, p. 7-61, Elsevier Science, Amsterdam Speight, J. G. Andersen, S. L, (1999). Thermodynamic Models for Asphaltene Solubility and Precipitation. Journal of Petroleum Science and Engineering, 22,53-66 Speight, J. G., (1999 a). The Chemistry and Technology of Petroleum. New York. 3rd Edition, Marcel Dekker... 

A method of evaluation of lubricating-oil stocks involving precipitation of asphaltenes with petroleum ether, a fuller s earth petroleum ether fractionation to isolate resins, dewaxing with methyl ethyl ketone and benzene to separate wax, and an adsorption fractionation to provide cuts for determining the Viscosity Index-yield relationship has been developed by N. W. Furby. The results of such a study have been successfully correlated with plant performance. 

In part II of the present report the nature and molecular characteristics of asphaltene and wax deposits from petroleum crudes are discussed. The field experiences with asphaltene and wax deposition and their related problems are discussed in part III. In order to predict the phenomena of asphaltene deposition one has to consider the use of the molecular thermodynamics of fluid phase equilibria and the theory of colloidal suspensions. In part IV of this report predictive approaches of the behavior of reservoir fluids and asphaltene depositions are reviewed from a fundamental point of view. This includes correlation and prediction of the effects of temperature, pressure, composition and flow characteristics of the miscible gas and crude on (i) Onset of asphaltene deposition (ii) Mechanism of asphaltene flocculation. The in situ precipitation and flocculation of asphaltene is expected to be quite different from the controlled laboratory experiments. This is primarily due to the multiphase flow through the reservoir porous media, streaming potential effects in pipes and conduits, and the interactions of the precipitates and the other in situ material presnet. In part V of the present report the conclusions are stated and the requirements for the development of successful predictive models for the asphaltene deposition and flocculation are discussed. 

In modern terms, asphaltene is conceptually defined as the normal-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. 

The details of the XANES experimental setup and data analyses have been described previously 3b,8). All model compounds used in this study were obtained from Aldrich Chemical Company and were used without further purification. The asphaltene samples were prepared from their respective petroleum residua by precipitation from n-heptane following the procedure of Corbett (9). A sample of Rasa coal was generously provided by Dr. Curt White of the Pittsburgh Energy Technology Center. 

Nickel and vanadium in petroleum exist as soluble organometallic complexes that fall into two categories metal porphyrins and nonporphyrin metal complexes. Both the porphyrins and the nonporphyrins may be distributed over a wide boiling range (350-650°C+), reflecting significant variations in molecular weight, structure, and polarity. Metal porphyrins and nonporphyrin metal complexes also tend to precipitate as part of the asphaltene materia] to an extent that varies with the source of the crude oil. 

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