Asphaltenes coke formation, reactions

Residue HDP is complicated by the quality of the feed high nitrogen concentration, asphaltenes and metals are the complicating factors. A large number of parallel and simultaneous reactions occur, both thermal and catalytic. Besides contributing to conversion, the thermal reactions contribute to coke formation, as well. 

The results in Table III show that the virgin bitumen that contains the asphaltenes produced relatively more gas and nonhydrocarbon products than did the maltenes. This trend with respect to gases and liquids appears to be confirmed by the results of the run with the asphaltene-enhanced bitumen however, appreciable quantities of coke were formed at the reaction conditions used and good material balances on this run were not achieved. Without essentially complete reduction of coke formation by hydropyrolysis, the significance of results for the asphaltene-enhanced bitumen are suspect. Removal of carbon in the form of coke will have an unknown effect on results that may not be attributable to asphaltenes. These results are included principally as negative results to show the dramatic effects that can result if asphaltenes are not fully dispersed and coke formation is not inhibited during hydropyrolysis. 

The initial coking causes several other undesirable effects. The loss of surface area resulting from filling smaller pore by coke certainly causes loss in activity. At the same time, larger pores in the catalyst become coated with adsorbed asphaltene, at least to the detriment of acid-catalysed coke formation. Such reactions appear not to include hydrodesulphurisation, which has been found not to be affected by initial coke deposition [18]. This presumably reflects the ease of removal of sulphur from the feedstock. 

Generally the mechanism of coke formation can by described be polycondensation reactions of asphaltenes or asphaltenes with aromatic or unsaturated compounds. In the simplest form all these reactions can be represented by the following reactions ... 

A comparison between the activation energies for coke formation from light aromatics (52-58 kJ/mol) and from asphaltenes and resins (34— 47 kJ/mol) shows that the reaction velocity of coke formation from light aromatics grows faster with increasing temperature than for coke formation from asphaltenes or resins. 

Crude oil residues and bitumen are colloidal disperse systems. In these systems, high-molecular solid structure units (asphaltenes) are dispersed in an oily phase (maltenes) (see section 8.2). In industrial thermal cracking processes, these units precipitate as coke. Coke formation is caused by polycondensation reactions of aromatic cores of asphaltenes, which lose the paraffinic periphery. The main objective of a substantial portion of this chapter is to show how deep cracking of bitumen at low temperature can be achieved without coke formation (i.e., without polycondensation of asphaltenes). The main reactions of asphaltenes that lead to coke formation are described. Also described are ways to reduce the negative influence of these reactions on the process. 

In contrast to the results at 400°C, no reduction in asphaltene molecular weight was observed for residence times up to 40 minutes for reactions conducted at 425°C (see Fig. 9.2). This means that at higher temperatures, polycondensation reactions proceed faster than decomposition reactions. At any temperature, the determined molecular weight of asphaltenes shows that it reaches equilibrium as the reaction proceeds. This implies that at a longer residence time, the molecular weight of the asphaltene fraction will not increase any further because after achieving equilibrium molecular weight, they become less soluble in the maltenes. This leads to their flocculation from the maltenes fraction (Fig. 9.2) and, finally, to coke formation. 

On account of the tendency of the asphaltene core to polycondensation, the increase in molecular weight at residence time over 15 minutes and a reaction temperature of 400°C can be explained by polycondensation of aromatic cores of asphaltenes (reaction (9.2)). Polycondensation reactions affect coke formation. This is noticed at 40 minutes residence time. By the thermal treatment of Bitumen 200, approximately 1 wt.% coke (based on water free feed) was formed. The fact that coke formation begins at the temperature of 400°C only if asphaltenes attain a very high molecular weight in comparison to higher temperature levels is caused by the formation of a stable steric colloid with the resins. This stable colloid is not cracked at the temperature of 400°C as deeply as at higher temperature levels. The formation of such a colloid is represented in Figure 9.3 ... 

From the investigation into asphaltene chemistry, it is obvious that paraffin side chains can be readily cracked during the thermal treatment of pure bitumen at a relatively low temperature. Aromatic and heteroatom structures are inclined to polycondensation reactions and finally to coke formation. Especially interesting are bridge-ring structures since these structural elements will only be cracked either at high temperature or by the addition of plastics to the feedstock. 

The investigation into the influence of paraffinic plastics on asphaltene chemistry during thermal cracking showed that pure plastics affect only the equilibrium of alkylation reactions by the increase of the paraffinic radicals in the reaction zone (Figure 9.18). This means that asphaltene decomposition will be slowed down. As such, there will be no decomposition to form aromatic cores without paraffinic periphery. This decelerates polycondensation and coke formation during the thermal treatment of mixtures of vacuum residue and plastics. However, it does not promote the cracking of the asphaltenes. 

Equations (2) and (3) were fit to experimental data using nonlinear regression to obtain values of the first-order reaction rate constants and the stoichiometric coefficients at each temperature. The conversion data from the 400°C thermal run and the best fit of the kinetic model are shown in Figure 1. It is interesting to note that at the time of incipient coke formation ( 60 minutes) the asphaltene and maltene data deviate from predicted first-order behavior. From this we concluded that both asphaltenes and maltenes were participating in secondary coke-forming reactions. Further separation of the maltenes into resins (polar aromatics) and oils confirmed this to be true and showed that it was the resin fraction that was involved in coke formation. 

Karacan and Kok recently studied the pyrolysis of two crude oils and their SARA fraetions." Differential scanning calorimetry and thermogravimetry techniques were used to evaluate the pyrolysis behaviour of the feedstoeks. The results indicated that the pyrolysis mechanisms depend on the nature of the constituents. Thermogravimetric data showed that asphaltenes are the main contributors to coke formation and that resins are a second contributor. The weight loss for the SARA components was additive. The authors argued that each fraction in a whole crude oil follows its own reaction pathway and there is no interaction or S5mergy between the components. 

Aqua-conversion. Catalytic process that uses catalyst-activated transfer of hydrogen from water added to the feedstock in slurry mode. The homogeneous catalyst is added in the presence of steam, which allows the hydrogen from the water to be transferred to the heavy oil when contacted in a coil-soaker system normally used for the visbreaking process. Reactions that lead to coke formation are suppressed and there is no separation of asphaltene-type material (Houde et al., 1998). 

Decomposition of petroleum asphaltenes has received attention primarily because of its tendency toward coke formation under thermal conditions. For this reason a key parameter for understanding residue processing via coking is to study the chemistry of coke formation at different temperatures (Goncalves et al., 2001 Douda et al., 2004). Various reaction pathways have been proposed for asphaltene thermal decomposition and it has been reported that the main products are alkanes ranging from Cj to C40 and polynuclear aromatics (from 1 to 4 aromatic rings)... 

It is also assumed that heterocyclic nitrogen plays an important role in thermolysis of resins and asphaltenes, by which the first reactions involve thermolysis of aromatic alkyl bonds. Secondary reactions are aromatization of naphthenic species and condensation of aromatic rings that activates the coke formation. Thus, the initial step in the coke formation from heavier fractions (resins and asphaltenes) is the formation of volatile hydrocarbons and nonvolatile heteroatom-containing systems. Products obtained are insoluble in the surrounding medium enhancing the carbonization to finally form coke (Magaril and Aksenova, 1968 ... 

We should caution that the above concept of the genetic relationship between kerogens and asphaltenes differs from the more historic view that asphaltenes are condensation and/or alteration products of hydrocarbons and resins. Certainly, in some petroleum processing treatments and probably at higher maturation levels in nature, various reactions do form new products with asphaltene solubility characteristics. These new condensation products may be regarded as altered asphaltenes and intermediates in the coke or pyrobitumen formation process (62-64)- Contamination of original asphaltenes by subsequently formed or altered products, of course, will result in a less definitive correlation between an asphaltene and its source kerogen. 

Asphaltene and resid pyrolysis provide two relevant examples of global pyrolysis models. The pyrolysis of an isolated asphaltene feedstock typically yields the type of data summarized in Figure 2, a plot of the temporal variation of weight based product fractions as a function of time (7). This figure illustrates the exponential disappearance of asphaltene accompanied by the formation of coke, maltene and gas product fractions. Consideration of the initial slopes for the formation of coke, maltene and gas fractions led to the type of reaction network shown in Figure 3. Since resid and its reaction products can likewise be defined in terms of the solubility and volatility-based product groups asphaltene,... 

Therefore, in a mixture as complex as petroleum, the reaction processes can only be generalized because of the difficulties in analyzing not only the products but also the feedstock as well as the intricate and complex nature of the molecules that make up the feedstock. The formation of coke from the higher molecular weight and polar constituents of a given feedstock is detrimental to process efficiency and to catalyst performance. One method by which the process chemistry can be rationalized is to separate the resid and its conversion products into fractions using solubility/ insolubility in volatile liquids as well as adsorption/ desorption on solids. In this way a number of resids and resid conversion products were separated into coke (toluene insoluble), asphaltenes (toluene soluble/ n-heptane insoluble), resins (n-heptane soluble, adsorbs on alumina), aromatics (n-heptane soluble, does not adsorb on alumina), and saturates (n-heptane soluble, does not adsorb on alumina). 

The asphaltenes are responsible for the formation of coke residue due to their high content of condensed aromatic ring systems. This is shown by the correlation of the coke residues / 600 or RSOO with the contents of asphaltenes (Fig. 4-60). Independent of the origin of the samples, two straight lines result for the distillation bitumens, whereas the data of the blown bitumens fit a third line with a steeper slope. Correlation of the maxima of the reaction rate DTG upon the concentration of asphaltenes presents a similar picture (Fig. 4-61). 

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