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oa Kinetic modeling of GTL product distribution over a promoted cobalt catalyst
- Publisher: Hamad bin Khalifa University Press (HBKU Press)
- Source: Qatar Foundation Annual Research Forum Proceedings, Qatar Foundation Annual Research Forum Volume 2012 Issue 1, Oct 2012, Volume 2012, EEO8
Abstract
Qatar is the world leader in fuel production from gas-to-liquid (GTL) technology and home of the largest GTL plant in the world (Pearl GTL, a joint development by Qatar Petroleum and Shell). In the GTL process natural gas is converted into liquid fuels and waxes. Fischer-Tropsch synthesis (FTS) is the key part of that process. FTS is a heterogeneously catalyzed reaction in which a mixture of CO and H₂ is converted into a wide range of hydrocarbon products. Advanced design and optimization of large scale FTS reactors requires a detailed knowledge of reaction chemistry. Kinetic models used for this application need to be robust, physically reasonable and fundamental. This study will present one such a model. Experiments were conducted in a 1-L slurry reactor over 25% Co/0.48% Re/Al₂O₃ catalyst. A broad range of operating conditions was achieved (i.e., temperatures of 478, 493 and 503 K, pressures 1.5 and 2.5 MPa, H2/CO feed ratio 1.4 and 2.1 and gas space velocities of 1.0-22.5 NL/g-cat/h). Rate laws for the kinetic model have been derived using the CO-insertion mechanism and chain-length-dependent 1-olefin desorption concept. The model accounts for the formation of n-paraffins and 1-olefins. CO hydrogenation and insertion of CO into the growing chain are considered to be rate determining, as well as the chain termination steps. Non-isothermal model parameters are estimated by minimization of a multi-response objective function. A global minimum is obtained with the hybrid genetic algorithm and a total of 696 experimental responses used in the estimation. Estimated model parameters are meaningful, considering physicochemical tests and statistical tests. They are also in a good agreement with previously reported values for activation energies. The model fit is in good agreement with experimental data and the mean absolute relative residual (MARR) was 24%. The model also provides a good prediction of CO and H₂ rates of consumption, with a MARR of 17.7 and 16.1%, respectively. The main advantage of the proposed model is its ability to explain and predict the main features of GTL product distribution in a physically meaningful and fundamental way over a wide range of industrially relevant process conditions.