N-Pentane asphaltene

Thus, although the use of both -pcntanc and w-heptanc has been widely advocated, and although n-heptane is becoming the deasphalting liquid of choice (ASTM, 1995), this is by no means a hard-and-fast rule. And it must be recognized that large volumes of solvent may be required to effect a qualitative and quantitative reproducible separation. In addition, whether -pentane or n-heptane is employed, the method effects a separation of the chemical components with the most complex structures from the mixture and this fraction should be correctly identified as n-pentane asphaltenes or as n-heptane asphaltenes. 

Asphaltite a variety of naturally occurring, dark brown to black, solid, nonvolatile bituminous material that is differentiated from bitumen primarily by a high content of material insoluble in n-pentane (asphaltene) or other liquid hydrocarbons. 

Deasphaltening removal of a solid powdery asphaltene fraction from petroleum by the addition of low-boiling liqnid hydrocarbons snch as n-pentane or n-heptane nnder ambient conditions. 

Deasphalting the removal of the asphaltene fraction from petrolenm by the addition of a low-boiling hydrocarbon liqnid snch as n-pentane or n-heptane more correctly, the removal asphalt (tacky, semisolid) from petroleum (as occurs in a refinery asphalt plant) by the addition of liqnid propane or liquid butane under pressnre. 

The material boiling above 470°F is separated into oils, resins, and n-pentane insoluble residue. The residue is separated into asphaltenes and benzene insolubles by extraction with benzene while the oils are separated into aromatics and saturates. The saturates can be further separated into n-paraffins and non-n-paraffins with 5 A molecular sieves... 

Panzer t al. (5) extracted Athabasca tar sand in two steps, the first with compressed n-pentane (Tc = 570 K, Pc = 3.37 MPa) and the second with compressed benzene (Tc = 563 K, Pc = 4.92 MPa). At 533-563 K and 2.0-7.7 MPa, n-pentane extracted 95% of the maltenes and asphaltenes from the tar sand, whereas at atmospheric pressure only 75% was extracted. Further extraction with benzene at 633 K and 2.0 MPa removed the remaining higher molecular weight asphaltenes. This indicates that the chemical nature of the dense gas is important in some applications. 

The asphaltene fraction (ASTM D-893 ASTM D-2007 ASTM D-3279 ASTM D-4124 ASTM D-6560 IP 143) is the highest molecular weight and most complex fraction. In any of the methods for determination of the asphaltene content, the sample is mixed with a large excess (usually >30 volumes hydrocarbon per volume of sample) of low-boiling hydrocarbon, such as n-pentane or n-heptane. For an extremely viscous sample, a solvent such as toluene may be used prior to addition of the low-boiling hydrocarbon, but an additional amount of the hydrocarbon (usually >30 volumes hydrocarbon per volume of solvent) must be added to compensate for the presence of the solvent. After a specified time, the insoluble material (the asphaltene fraction) is separated (by filtration) and dried. The yield is reported as a percentage (% w/w) of the original sample. 

Distillation concentrates the metallic constituents in the residua (Table 3-5) some can appear in the higher-boiling distillates but the latter may, in part, be due to entrainment. Nevertheless, there is evidence that a portion of the metallic constituents may occur in the distillates by volatilization of the organometallic compounds present in the petroleum. In fact, as the percentage overhead obtained by vacuum distillation of reduced crude is increased, the amount of metallic constituents in the overhead oil is also increased. The majority of the vanadium, nickel, iron, and copper in residual stocks may be precipitated along with the asphaltenes by low-boiling alkane hydrocarbon solvents. Thus, removal of the asphaltenes with n-pentane reduces the vanadium content of the oil by up to 95% with substantial reductions in the amounts of iron and nickel. 

Although, n-pentane and n-heptane are the solvents of choice in the laboratory other solvents can be used (Speight, 1979) and cause the separation of asphaltenes as brown-to-black powdery materials. In the refinery, supercritical low molecular weight hydrocarbons (e.g., liquid propane, liquid butane, or mixtures of both) are the solvents of choice and the product is a semisolid (tacky) to solid asphalt. The amount of asphalt that settles out of the paraffin/residuum mixture depends on the size of the paraffin, the temperature, and the paraffin-to-feedstock ratio (Figure 3-10) (Girdler, 1965 Mitchell and Speight, 1973 Corbett and Petrossi, 1978 Speight et al., 1984). 

Deasphaltened oil the fraction of petroleum after the asphaltenes have been removed using liquid hydrocarbons such as n-pentane and n-heptane. 

Bitumens from archaeological asphalts were glassy black pitches that acquired electrical charges when broken into small pieces. They were nearly insoluble in petroleum ether or n-pentane—the classical criterion for the presence of asphaltenes. Because asphaltenes can be formed by oxidizing hot petroleum residues, indications that they were also the end-products of weathering seemed reasonable. 

In modern terms, asphaltene is conceptually defined as the n-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. There are a number of procedures used to isolate asphaltene (2-7), all of which appear to be reproducible (8) but do not necessarily provide equivalent end-products. The similarity between coal- and petroleum-derived asphaltenes begins and ends at the definition of the separation procedure. Puzinauskas and Corbett s (9) comments on asphalt may be paraphrased and applied to asphaltene. They state that the broad solvent classification is unfortunate it leads to misconceptions that petroleum and coal materials are alike, or at least similar. However, these two classes of materials differ not only in their origin, mode of manufacture and uses, but also in their chemical composition and physical behavior. 

Figure 8. Comparison of gel permeation chromatograms of basic fractions (A-15) of (a) Athabasca asphaltenes and (b) resins Bz - benzene MeOH = methanol iPrN - iso-propylamine nC5 - n-pentane PhMe and PS (15,000) - VR of toluene and Vo, respectively...

Sample Preparation. The residua samples were separated into asphaltenes and maltenes by deasphalting the resid with a 25 1 (v/v) amount of n-pentane. After stirring, the mixture was allowed to sit overnight, then filtered through a 0.45-p porous glass filter. The asphaltenes were washed with several portions of pentane and dried under vacuum at 90°C. Pentane was evaporated from the filtrate to yield the maltenes. 

Concentration of Type II Nitrogen or Type II Sulfur in Subfractions. When separating distillation residues of Oficina crude oil into asphaltenes, resins, and deasphalted oils using n-pentane, some drastic changes occur in nitrogen and sulfur distribution, as can be seen in Tables I and II. 

This suggests the existence in residues of molecules having vanadium and nickel bound in a constant ratio to Type II nitrogen and Type II sulfur atoms that could be precursors of coke formation (34). These molecules or structures are more or less selectively precipitated by n-pentane into the asphaltenes, although a certain quantity still remains in the resins. 

Asphaltenes are generally defined as those components in petroleum and coal liquids that under certain conditions are soluble in benzene but insoluble in aliphatic solvents, such as n-pentane, n-heptane, or cyclohexane. This definition obviously includes a broad variety of components, as insolubility in the above solvents can be caused by high molecular weight, high polarity, hydrogen bonding, acid-base complexing, or combinations of these parameters. 

Extraction of tar sands with benzene or toluene yields a bitumen fraction and a mineral fraction. Further treatment of the bitumen with heptane or n-pentane yields asphaltenes and petrolene, which are the insoluble and soluble fractions, respectively. Depending on the source, these fractions contain varying amounts of vanadium (117,118). 

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. 

SiLLCA Gel Chromatotraphy. The acid-, base-, and neutral-nitro-gen-free asphaltene (.126 g) was dissolved in n-pentane (10 mL) and placed on a silica gel column (30 g) that had been wet-packed with n-pentane. The column was eluted with n-pentane (500 mL) to remove the saturate hydrocarbons. Aromatic hydrocarbons were eluted from the column using 85% n-pentane-15% benzene (250 mL) and 60% benzene-40% methanol (250 mL). UV analyses of the saturate fraction indicated that trace amounts of aromatic hydrocarbons were present. The amount of saturates in the aromatic fraction, if any, is unknown. 

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