Ethene conditions, inert sweep

Our objective was to clarify the reaction mechanisms that determine the observed alkene/alkane ratios under various conditions, and the results are reported here. When oil shale is pyrolyzed either isothermally or nonisothermally, the hydrocarbon and hydrogen concentrations are all time dependent. To determine if the alkene-alkane-hydrogen system is at equilibrium, we heated oil shale at a constant rate and measured the C to C3 hydrocarbons and hydrogen over time. We also measured the effect of an inert sweep gas on the time-dependent ethene/ethane and propene/propane ratios and the integral 1-alkene/n-alkane ratios in the oil. We determined that the C2H4-C2H6-H2 system is not at thermal equilibrium and interpret our results in terms of a nonequilibrium free-radical mechanism proposed by Raley (8). 

The free-radical mechanism also predicts that the ethene/ethane ratio should increase if inert diluent is added (Figure 5). The addition of an inert sweep causes both the instantaneous values above 450°C and the integral values of the ethene/ethane ratio to increase. The integral value of the ethene/ethane ratio increased from 0.21 under autogenous conditions to 0.29 in the slow-sweep experiment and to 0.33 in the fast-sweep experiment. We determined a value of 0.34 at... 

Figure 5. Effect of inert sweep gas on the time-dependent ethene/ethane ratio for oil shale heated at 1.5°C/min under autogenous conditions. The slow-sweep sample size and flow rate were 28 g and 50 cm3/min, respectively. The fast-sweep sample size and flow rate were 14 g and 100 cm3/min, respectively. Most of the ethene and ethane was evolved between 400° and 500°C.

When oil shale is heated at a constant rate, the alkene/alkane ratios in the evolved hydrocarbon gases change with time. In addition, the alkene/alkane ratios in both the gas and the oil are affected by an inert sweep gas. The ethene/ethane ratio is not determined by equilibrium with hydrogen, and we interpret this phenomenon in terms of a free-radical cracking mechanism. The implication is that alkene/alkane ratios, especially the ethene/ethane ratio, can be used as an indicator of retort performance only if the correct relationships are used for each set of retort conditions. 

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