Tendency to Coke Formation

Matsushita et al. (2004) defined the following relationship that takes into account the H/C atomic ratio of asphaltenes and maltenes, which gives certain information about the solubility of asphaltenes and its influence on coke formation during processing of petroleum  

FIGURE 1.7 Relative solubility of heavy crude oils. 

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The tendency to coke formation also increases again, as might be expected, in the liquefaction of medium and low volatile bituminous coals. 

The rate or velocity of coke formation is, first of all, a function of the feedstock characteristics. Seidel [1] reported about the tendencies of different components of heavy vacuum residues to coke formation. The following sequence shows a descending tendency to coke formation during thermal treatment ... 

The result of 13C-NMR analyses for the asphaltenes and hard resins in Bitumen 200 Elf is presented in table 8.3. From table 8.3, it is evident that asphaltenes (heavier fraction) have a higher aromaticity in comparison to hard resins and at the same time a lesser amount of paraffin side chains. This shows that asphaltenes have a higher tendency to coke formation in comparison with hard resins (see section 8.2). 

In order to minimize the tendency of coke formation within the column, steam is utilized. Steam utilization also helps to reduce the absolute pressure of the system to 10 mmHg or less and can help stabilize the desired unit vacuum levels. By operating at low vacuum pressures, the product yield will increase and the operating costs will typically be reduced. 

It is possible to predict the relative importance of the several modes of decomposition from an analysis of the polymer structure J+1J. Van Krevelen [J+0 ] even showed that the tendency for coke formation is an additive property, which can be computed from the monomer structure. 

The greatest problem during thermal cracking arises from reactions involving aromatic feed. Aromatic compounds in the feed have a very high tendency to undergo polycondensation reactions that lead to coke formation. Coke formation decreases the yields of the desired gasoline and diesel fractions. One example of a polycondensation reaction is shown in reaction (6.7). 

From Figure 9.11, it is seen that dehydrogenation proceeds with a higher probability at a higher temperature. Since the compounds with higher aromaticity have a higher tendency towards coke formation, thermal treatment of bitumen or vacuum residues has to be carried out at the lowest possible reaction temperature. 

The present studies were initiated in order to investigate the effect of the reactor surface on the product distribution and on the tendency for coke formation during the steam cracking of propane in a tubular reactor. Attention has been focused on correlating various effects which can arise in the system. Previous studies of the pyrolysis of propane has been reviewed recently (17, 18), and the findings of the present work are related to these studies later in this paper. 

Another important factor in industrial reactions is coke formation. As expected, the catalyst with the highest content of Si02 (highest acid content) has the most pronounced tendency for coke formation. This is explained by increased formation of carbenium ions, which undergo fast coupling and polymerization reactions that eventually lead to involatile deposits on the surface. This also leads to lower activity and selectivity of the catalyst. 

Hydrocarbon cracking [see Eq. (3.22), Section 3.2] may lead to coke formation. This reaction is well known from nickel catalysts, which form whisker-like coke deposits at high temperature [270]. This type of pyrolytic carbon formation takes place above a critical temperature [81], which is about 600 °C for methane [271]. Precious metal catalysts such as rhodium generally have a lower tendency towards coke formation [272] and are more active, which makes them attractive for hydrocarbon reforming in smaller-scale systems (see Section 4.2). 

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)... 

For the heavier feedstocks, process selection has tended to favor the hydroprocesses that maximize distillate yield and minimize coke formation. However, thermal and catalytic processes must not be ignored because of the tendency for coke production. Such processes may be attractive for processing unconverted residua from hydroprocesses. 

A problem may occur when higher-molecular-weight materials are employed as feedstocks and result in coke formation and deposition. When an alkali-promoted catalyst is employed, corrosion and fouling problems in the reformer (or even in equipment downstream of the reformer because of the tendency of the alkali to migrate) may occur with some frequency. However, coke formation can be eliminated by the use of a proprietary alkali-free catalyst that has an extremely high activity and resistance to poisoning. 

The reactor temperature required to prevent coke formation varies considerably for the different processes. Table 2.1 summarizes the values calculated assuming thermodynamic equilibrium for 2,2,4-trimethylpentane reforming. Generally, the coking tendency increases in the following order at constant O/C ratio SR > ATR > POx. These calculations demonstrate that at steam to carbon ratios (S/C) > 2 and reaction temperatures > 600 °C, which is very common for hydrocarbon fuel processors, coke seems to be an unstable species especially under the conditions of steam reforming. 

Avoiding carbon deposition on the catalyst is a major challenge [2, 3]. Carbon can be present as graphite-like coke and in the form of whiskers, or carbon nanofibers. The latter lead to detachment of the nickel crystallites from the support and breaking of the catalyst pellets. This may cause blockage of the reformer reactor tubes and the formation of hot spots. Higher hydrocarbons exhibit a larger tendency to form... 

In the case gas oils, the seventy of the treatment can always be defined by the ethylene or Cs- yidd However, due to their complex composition, whidi varies widely accending to the source of tiie crude oils Grom whidi they were produced, and due to their pronounced tendency to favor the formation of coke, it is very tfiGhcuH to establish correlations designed to predict the relative ftfoduedon of the other different hydro-carbons, and consequently to define, as for tte naphthas, a value or an index that is sufficiently general smd representative. 

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