Thermal coke

Fixed carbon is the material remaining after the determination of moisture, volatile matter, and ash. It is, in fact, a measure of the solid combustible material in coal after the expulsion of volatile matter, and like determination of the carbon residue of petroleum and petroleum products (Speight, 1999, 2001) represents the approximate yield of thermal coke from coal (Zimmerman, 1979). 

In the present context, heavy oils and residua can also be assessed in terms of sulfur content, carbon residue, nitrogen content, and metals content. Properties such as the API gravity and viscosity also help the refinery operator to gain an understanding of the nature of the material that is to be processed. The products from high-sulfur feedstocks often require extensive treatment to remove (or change) the corrosive sulfur compounds. Nitrogen compounds and the various metals that occur in crude oils will cause serious loss of catalyst life. The carbon residue presents an indication of the amount of thermal coke that may be formed to the detriment of the liquid products. 

Figure 2-15 Generalized relationship of (thermal) coke yields for the bulk fractions of feedstocks. 

Figure 4-14 Thermal coke (carbon residue) yield for various feedstock fractions.

Thermal coke the carbonaceous residue formed as a result of a noncatalytic thermal process the Conradson carbon residue the Ramsbottom carbon residue. 

A comprehensive study on coke deposition in trickle-bed reactors during severe hydroprocessing of vacuum gas oil has been carried out. On the basis of results obtained with different catalysts on the one hand, and analytical and catalytic characterisation of the coke deposits on the other, it is argued that coke is formed via two parallel routes, viz. (i) thermal condensation reactions of aromatic moieties and (ii) catalytic dehydrogenation reactions. The catalyst composition has a large impact on the amount of catalytic coke whilst physical effects (vapour-liquid equilibria, VLE) predominate in determining the extent of thermal coke formation. The effect of VLE is related to the concentration of heavy coke precursors in the liquid phase under conditions which promote oil evaporation such as elevated temperatures. A quantitative model which describes inter alinea the distinct maximum of coke deposited as a function of temperature is presented. 

Hereafter we focus on a detailed understanding and model description of coke formation on catalysts in a trickle-bed reactor during hydroprocessing of VGO under the severe conditions mentioned above. Firstly, we will address the nature of the coke deposits in relation to that of the catalyst. A distinction between catalytic and thermal coke is made, based on information obtained from analytical techniques as well as from re-testing of the spent catalysts. Secondly, the extent of coke formation is dealt with on the basis of both experimental and modelling work. In this part the impact of vapour liquid equilibria is shown to be of prime importance. 

In order to explain the effects of the catalyst composition we postulate that two major routes to coke exist, viz. (i) radical reactions giving rise to condensation of the aromatic structures which ultimately lead to thermal coke and (ii) dehydrogenation reactions which... 

It is now suggested that in the absence of Mo 2 as a hydrogen activating phase the thermal coke formation is substantial. A very limited amount of Mo is already sufficient to dissociate hydrogen which leads to termination of radicals, thus preventing their condensation to coke. This explains the strong drop of the coke selectivity going from zero to 0,2% Mo,... 

The catalytic coke is formed via dehydrogenation reactions catalysed by the MoSj phase. Higher Mo loads will not lead to further reduction of thermal coke and to enhanced amounts of catalytic coke. The latter aspect explains the increasing coke formation going from 0,2 to 10 Mo/100 A1203. 

In order to arrive at a sound description of the amount of coke deposited it is essential to build on the insight that two routes to coke exist, viz. catalytic and thermal coke. For the rate of formation of catalytic coke we assume that a simple Langmuir type kinetic expression suffices. 

The description of coke via thermal reactions is somewhat more complicated and has been derived in detail elsewhere [9], The equation for the rate of thermal coke formation, Rlt obtained previously reads ... 

The effect of the H2/oil ratio on the coke content of a NtV/SiOj catalyst (low HDS activity, thermal coke predominates) is shown in Figure 6. The distinct maximum of the coke deposited with the H2/oil ratio is apparent from both experiment and theory. Detailed analysis of the model output indicates that at low gas rates the VGO feedstock is mainly in the liquid phase throughout the reactor, whilst at the highest gas rates the reactor is operated in the gas phase already at the reactor inlet. In both limits the amount of coke deposited is modest. Intermediate gas rates (1000 Nl/kg), however, lead to much higher rates of coke... 

Where either liquid- or gas-phase operation predominates the concentration of Q is moderate and so is the extent of thermal coke formation (Figure 7). 

Thermal coke is characterised by a relatively low extent of unsaturation and graphitisation and a strong degree of agglomeration. The latter aspect limits the poisoning effect of thermal coke. 

Symbol designation Bt (virgin bitumen) S/A (silica-alumina catalyst) MS (molecular sieve catalyst) SB (semibatch Cat A mode) B (batch mode) f (powdered catalyst) T (thermal coking). 

Sometimes the inspection tests attempt to measure these properties, for example, the carbon residue of a feedstock that is an approximation of the amount of the thermal coke that will be formed during refining or a research octane number test devised to measure performance of motor fuel. In other cases the behavior must be determined indirectly from a series of test results. 

The carbon residues of petroleum and petroleum products serve as an indication of the propensity of the sample to form carbonaceous deposits (thermal coke) under the influence of heat. 

The data produced by the nucrocarbon test (ASTM D4530, IP 398) are equivalent to those by the Conradson carbon residue method (ASTM D-189 IP 13). However, this nucrocarbon test method offers better control of test conditions and requires a smaller sample. Up to 12 samples can be run simultaneously. This test method is applicable to petroleum and to petroleum products that partially decompose on distillation at atmospheric pressure and is applicable to a variety of samples that generate a range of yields (0.01% w/w to 30% w/w) of thermal coke. 

The carbon residue of a petroleum product serves as an indication of the propensity of the sample to form carbonaceous deposits (thermal coke) under the influence of heat. In the current context, carbon residue test results are widely quoted in diesel fuel specifications. However, distillate diesel fuels that are satisfactory in other respects do not have high Con-radson carbon residue values, and the test is chiefly used on residual fuels. 

The carbon residue of a petroleum product gives an indication of the propensity for that product to form a carbonaceous residue under thermal conditions. The carbonaceous residue is correctly referred to as the carbon residue but is also often referred to as coke or thermal coke. 

In the Ramsbottom carbon residue test (ASTM D-524, IP 14), the sample is weighed into a glass bulb that has a capillary opening and is placed into a furnace (at 550°C/1022°F). The volatile matter is distilled from the bulb, and the nonvolatile matter that remains in the bulb cracks to form thermal coke. After a specified heating period, the bulb is removed from the bath, cooled in a desiccator, and weighed to report the residue (Ramsbottom carbon residue) as a percentage (% w/w) of the original sample. 

Thermal coke the carbonaceous residue formed as a result of... 

If the thermal coking has to be used to make a product suitable for common carrier pipelining, an overall liquid yield loss of 15% to 20% will be incurred. 

The concept of an asphaltene model that incorporates smaller polynuclear aromatic systems is more in keeping with the types of systems that occur in nature. Indeed, smaller polynuclear aromatic (and pseudoaromatic) systems are capable of producing high yields of thermal coke either because of the heteroatom content (87) or because of the presence of pendant alkyl moieties that have the capability of forming the internuclear cross-links (88) that can lead to coke. In this latter case, it is likely that the indigent alkyl chains can interact in this manner, or shorter alkyl chains, formed by thermolysis, can play the role of cross-linking agents. 

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