Kerogen pyrolysis model

A frequent characteristic of past investigations is excessive complexity of the proposed kerogen pyrolysis models. The works of Fausett et al. (6 ) and Johnson et al. (3) belong in this category. A goal of the present investigation is to keep the model as simple as possible. 

As part of this investigation, kerogen pyrolysis models different from the one proposed here were considered. One such model of theoretical appeal is similar in structure to the one given in Figure 9 but with a pure diffusion process for the heavy oil production. However, this alternative model is incompatible with some experimental findings It predicts lower coke concentrations on the surface of the particle than in the interior, whereas microprobe results indicate a uniform coke distribution. Further, this diffusion model predicts zero coke yield for infinitely small particles, whereas the limited amount of data available for small particle sizes suggest a leveling-off of the coke yield below a particle size of 0.4 mm. 

The proposed model can be compared with both the model of Allred ( ) and that of Campbell et al. (8). Allred s model does not have the feature of competing parallel reactions that is essential to the pyrolysis model proposed here. It does, however, have the intermediate product bitumen which reaches a maximum level almost identical to the one in this work. Allred postulates that all kerogen decomposes into bitumen, whereas bitumen in the present work is the remainder of the kerogen after the light hydrocarbon fraction has been stripped off. 

Higher oil yields and lower coke yields are obtained for smaller particles. Retorting temperature in the range 800-1000°F does not affect coke yield but has an effect on the relative yield of oil and gas. The experimental data are correlated by a simple empirical pyrolysis model where kerogen decomposes by a first-order reaction into a light hydrocarbon product and a heavy intermediate product, "bitumen." Subsequently, the bitumen is subject to two competing processes ... 

Figure 6. Three hypothetical models for the bonding of alkylthiophene units in kerogen and their products on flash pyrolysis and natural maturation.

Kinetic model Labile kerogen Stabile kerogen Kinetic from asphaltenes (34/4-7) MSSV-pyrolysis (Erdmann. 1999) ... 

Therefore we attempted to simulate advanced pyrolysis using a multi-step model (MSM). This model was developed using TGA- and DSC-derived kinetic coefficients, determined for chemically and thermally treated oil shale samples by modelling particular reaction steps. The MSM is based on the reaction scheme shown in Fig. 4-116 which displays a series of parallel and consecutive first order reactions. K and B denote the kerogen and bitumen originally present in the oil shale B, B, and to /Jj are non-volatilized intermediates and products (solids and liquids) to are volatilized products (gases and vapors) and/j to/jg are the stoichiometric coefficients that fulfil the condition ... 

These differences in pyrolysis behavior of the oil shales can be explained by structural differences in the corresponding kerogen types. The kerogens of oil shales Aleksinac, Estonia, and Korea are associated with type I, which is of predominantly paraffinic nature. Oil shale Knjazevac is associated with kerogen type HI, which is of predominantly aromatic nature. Thus the multi-step model appears to be suitable for simulating the pyrolysis of oil shales with kerogen type I, but cannot be properly adjusted for the other kerogen types. 

M. ViUey, A. Oberhn, and A. Combaz. Influence of elemental composition on carbonization. (a) Part I Pyrolysis of sporopollenin and lignite as models of kerogens. Carbon 17, 77-86 (1979) (b) Part II Pyrolysis of kerosen shale and kuckersite. Carbon 18, 347-353 (1980). 

---