Asphaltene continued

Utilize a continuous-cycle oil flush into the inlet of the bottoms exchanger. This keeps the asphaltenes in solution and increases tube velocity. 

One major question of interest is how much asphaltene will flocculate out under certain conditions. Since the system under study consist generally of a mixture of oil, aromatics, resins, and asphaltenes it may be possible to consider each of the constituents of this system as a continuous or discrete mixture (depending on the number of its components) interacting with each other as pseudo-pure-components. The theory of continuous mixtures (24), and the statistical mechanical theory of monomer/polymer solutions, and the theory of colloidal aggregations and solutions are utilized in our laboratories to analyze and predict the phase behavior and other properties of this system. 

Figure 3. Molecular weight distributions of asphaltenes before and after flocculation predicted by our continuous mixture model.

It appears that the high molecular weight species originally present in the feedstock (or formed during the process) are not sufficiently mobile (or are too strongly adsorbed by the catalyst) to be saturated by the hydrogenation components and, hence, continue to condense and eventually degrade to coke. These deposits deactivate the catalyst sites and eventually interfere with the hydrodesulfurization process. Thus, the deposition of coke and, hence, the rate of catalyst deactivation, is subject to variations in the asphaltene (and resins) content of the feedstock as well as the adsorptive properties of the catalyst for the heavier molecules. 

The bitumen comes as a residue from the refining of conventional or heavy crude oil, or from natural deposits of oil (tar) sand. Bitumen, being a complex mixture of more than 1000 different molecules, is itself a colloidal suspension of asphaltenes in a continuous phase of saturated parrafins, aromatic oils and resins [774], Descriptions of different kinds of asphalts are given in Refs. [775,776], At low asphaltene concentration the suspension is Newtonian. Once the concentration increases above about 8 % v/v, however, the asphaltenes form a three-dimensional network and the suspension can become a viscoelastic gel [774]. The asphaltenes interact through van der Waals forces so that a bitumen containing 15% asphaltenes is solid at room temperature and liquid above about 60-100 °C. 

A survey of the methods used to determine asphaltene structure indicates that there are serious shortcomings in all of the methods because of the assumptions required to derive the molecular formulae. The continued insistence that a complex fraction such as asphaltenes, derived in a one-step process from petroleum as a consequence of its insolubility in nonpolar solvents, has a definitive molecular structure is of questionable value to petroleum technology, and it is certainly beyond the scope of the available methods to derive such formulae. Asphaltenes would best be described in terms of several structural types rather than definite molecular structures. 

An early hypothesis of the physical structure of petroleum (52) indicated that asphaltenes are the centers of micelles formed by adsorption, or even by absorption of part of the maltenes, that is, resin material, onto the surfaces or into the interiors of the asphaltene particles. Thus, most of those substances with greater molecular weight and with the most pronounced aromatic nature are situated closest to the nucleus and are surrounded by lighter constituents of less aromatic nature. The transition of the intermicellular (dispersed or oil) phase is gradual and almost continuous. Continued attention to this aspect of asphaltene chemistry has led to the assumption that asphaltenes exist as clusters within the micelle. This arises mainly because of the tendency for asphaltenes to associate in dilute solution in solvents of low polarity and from possible misinterpretation of viscosity data (58, 64). The presence of asphaltene stacks in the solid phase, as deduced from x-ray diffraction patterns (68), also seemed to support the concept of the widespread existence of asphaltene clusters in the micelle. 

Brule (27) deduced from his experiments on asphalts that isolated asphaltenes form agglomerates which may be dissociated into micelles by simple dilution and that the intensity of the phenomena should characterize the force of interaction. He also noticed that in some cases there is a drift towards small sizes which implies a continual departure by molecules of all dimensions. On different types of material there was a general trend towards molecules in the approximate range of 50 A moreover, our experiments have clearly demonstrated that some of the processes involved may be very slow. Also, the redistribution of material across the chromatogram might be attributable to various types of reassociation once some part of the aggregate dissociated to small MW species. 

Coal derived materials These products were obtained from our 1 kg h continuous reactor unit (7) as oils (X4 soluble) asphaltenes (tetralin soluble/X4 insoluble) preasphaltene (also known as asphaltol) (tetralin insolubles/tetrahydrofuran (THF) solubles) and THF insoluble materials for subsequent reactivity studies. 

Hydrogenation studies were undertaken on the parent iron-tin treated coal (Drum 289) as well as the THF insolubles, preasphaltene, asphaltene and oil derived from a continuous reactor run as previously discussed. Studies with no additional catalyst added (case A) and with the addition of a sulphided nickel molybdate catalyst supported on alumina (case B) were performed. The results are presented in Table 1. The Ni/Mo catalyst in case B did not increase the conversion of the coal or the THF insolubles beyond that for case A because sufficient amounts of iron and tin materials were already... 

Table 14.1 illustrates the more important properties of vacuum residues from Saudi crude oils [1]. The three most important properties from the upgrading standpoint are sulfur, metals and asphaltenes contents. Sulfur continues to be a problem chiefly because of environmental objections to sulfur dioxide emissions. Therefore, a primary requirement is the removal of at least a major portion of the sulfur in the vacuum residue. 

In many refineries thermal cracking processes are used to convert residues into lighter products. Low value petroleum coke is a product from the more severe cracking processes. The H-Oil process made it possible to convert the asphaltenic carbonizable portion of the residue to higher value liquid products rather than coke. In the H-Oil process an ebullated bed of catalyst is used to convert lower value heavy oil into upgraded higher value products in the presence of hydrogen. The ebullated bed reactor is an expanded bed of catalyst maintained in constant motion by the upward flow of liquid. The reactor behaves as a well mixed continuously stirred tank reactor. 

In moving-bed reactors, both the feed and the catalyst move in cocurrent downflow and the catalysts are continuously renewed. The metal content increases along the bed and metal-rich catalysts are withdrawn from the bottom. It can handle the feed having higher metal content. The moving-bed reactor can be used as a first reactor for demetallization and asphaltene disaggregation. Other conversions (HDN, HDS) can take place in a fixed bed downstream. 

If one cannot diffuse the asphaltenes to the catalyst, why not diffuse the catalyst to the asphaltenes Dispersed catalysts also can be continuously added in sufficiently low enough amounts (i.e., 100 ppm) to consider them throwaway catalysts with the carbonaceous by-product. However, economics usually dictate some form of catalyst recycle to minimize catalyst cost. Nevertheless, by designing the reactor to maximize the solubility of the converted asphaltenes, the conversion of vacuum resids to gas and volatile liquids can be above 95% with greater than 85% volatile liquids. However, the last 5-10% conversion may not be worth the cost of hydrogen and reactor volume to produce hydrocarbon gases and very aromatic liquids from this incremental conversion. The answer depends on the value and use of the unconverted carbonaceous liquid by-product. 

Kodera, Y., Kondo, T., Isaito, S. Y., and Ukegawa, K., Continuous-distribution kinetic analysis for asphaltene hydrocracking, Energy Fuels 16, 291-296 (2000). 

Slurries frequently involve a wide range of particle sizes that include submicron particles in the Brownian diffusional region. When cakes are deposited, the finest particles may diffuse into the filtrate and continuously clog the supporting medium, leading to increasing values of R. Tiller and Leu (1984) showed that clogging was a major problem in the removal of ash and asphaltenes from liquefied coal. 

Reaction (9.10) is thermodynamically possible. However, for the reaction to proceed to a large extent, it is necessary that the methyl radical is produced on a continuous basis with relatively high velocity. Ethane cracking at a temperature under 500°C is thermodynamically not possible, i.e. reaction (9.10) proceeds only very slowly with ethane. But during the common cracking of bitumen and plastics, the methyl radical can form from cracked products of plastics and then enhance deep asphaltene cracking. These reactions can be described by equation (9.12). 

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