Experimental and modeling study of Pitch Pyrolysis kinetics
Creators: name: Yue, Chengqing
Publisher: name: University of British Columbia
Year: 2009

description: Kinetics of thermal pyrolysis of both CANMET and Syncrude pitches from heavy oil upgrading have been studied with Thermogravimetric Analysis (TGA), and with Pyroprobe-Gas Chromatography (Pyroprobe-GC). In the latter technique samples are pyrolyzed at high heating rates and products analyzed with in-line gas chromatography. Experiments with TGA were carried out at atmospheric pressure and at temperatures between 700 and 950 °C. The heating rates were 25, 50 100 and 150 °C/min. The sample weight was varied between 3 and 17.2 mg. The effects of sample weight, heating rate and final temperature on the weight loss as a function of time were examined. Experiments with Pyroprobe-GC were carried out at atmospheric pressure and at temperatures between 500 and 1000 °C. The heating rates were 600, 3000, 30000, 300000 °C/min, using a sample weight of about 5 mg. The accumulated pyrolysis products were analyzed and lumped into major groups for yield estimation based on number of carbon atoms. The final weight of residue was also determined. The effects of the final temperatures on the yield of each major group were examined. At temperatures below 150 °C, there is little pyrolysis of either pitch. At higher temperatures, the pyrolysis takes place in two following stages, with a first stage of low activation energy barrier and low pre-exponential factor, and the second stage of higher activation energy and pre-exponential factor. Higher conversion to volatiles was achieved with Syncrude pitch than with CANMET pitch. Heating rates had a minor effect on the weight loss. The total weight loss decreased slightly with the increase of sample weight, and final temperatures. The most abundant components of the pyrolysis products were species lighter than C7, which are primarily gases. The C10 group yield was strongly influenced by heating rates. Higher molecular weight components C11, C12, C13, and C14 were also detected. The pyrolysis products from Syncrude pitch consisted of higher yields of lighter components (C7) than those from CANMET pitch. A general first order equation for the kinetics of volatile release under temperature programmed conditions is widely used in the pyrolysis literature. Interpretation of results via the single stage integral method and methods due to Coats-Redfern, Chen-Nuttall and Friedman were tested with the TGA data and found inadequate. The single stage first order model of Anthony and Howard, which incorporates a Gaussian distribution of activation energies also failed. An adequate description of the pitch pyrolysis kinetics was achieved using a 2-stage first order model with the integral analysis method. The 2-stage model reflects changes in the chemical constitution or structure as conversion proceeds using two sets of kinetic parameters. This feature is essential to describe the dependence of devolatilization rates on remaining volatile content. The transition between these two stages is a sharp one, occurring at about 450 °C for both CANMET and Syncrude pitches. The magnitude of the activation energies suggested that both stages are kinetically controlled. The analysis methods of Coats-Redfern, Chen-Nuttall, and Friedman were also tested as two stage methods and found to be inadequate to describe the pitch pyrolysis kinetics in the temperature range studied. The pre-exponential factors and activation energies from the different kinetic methods exhibited the compensation effect, in which the values of the derived pre-exponential factors and activation energies are related. This mutual dependence prompted an the examination of the accuracy of these kinetic parameters, and a search for a single set of parameters for each stage of the pitch pyrolysis. It was found that the accuracy of these kinetic parameters derived by different analysis procedures are not identical, and a single set of kinetic parameters for each stage can be obtained with adequate fitting of the experimental data.