Today's transportation system is contributing to increasing air pollution and lack of future fuel for a growing number of vehicles. Over the years, many alternate solutions have been proposed to replace or to assist conventional fuels in order to alleviate the environmental damage and future fuel shortage. One such solution is to use hydrogen gas as fuel in an internal combustion engine or a fuel cell. Hydrogen being light, flammable and having very low critical temperature has associated problems of storage, transportation and utilisation. The methylcyclohexane-toluene-hydrogen (MTH)-system is a safe and economical way of storage and 'on-board' hydrogen generation. The dehydrogenation reaction of MCH is highly endothermic and suffers from equilibrium limitations. Therefore, success of the MTH-system for 'on-board' applications lies in the development of a highly active, selective and stable catalyst as well as a reactor supplying high rates of heat transfer to the catalytic bed. A review of the literature has shown that there is a huge disagreement in describing the kinetic mechanism of the dehydrogenation reaction of MCH. There is no consensus on the rate-determining step and the inhibition offered by the products. Moreover, there is no detailed kinetic investigation over a wide range of operating conditions including experiments without H2 in the feed and under integral conditions.The present study is designed to conduct a detailed kinetic investigation over a wide range of operating conditions including experiments without hydrogen in the feed for the most promising catalyst developed to date. The reaction kinetics are incorporated into a two-dimensional pseudo-homogeneous model to predict observed longitudinal temperature profiles. Alternative configurations and schemes for 'on-board' hydrogen generation based on the MTH-system are compared and a prototype reactor, suitable for 'on-board' hydrogen generation, is designed and simulated in detail, exchanging heat with the engine exhaust gas. Kinetic experiments were performed in a laboratory fixed bed tubular reactor under integral conditions. A 1.0 wt% Pt/Al2O3 catalyst was prepared and a wide range of experimental conditions were studied. A number of kinetic models were applied based on the power law, Langmuir-Hinshelwood-Hougen-Watson (LHHW) and Horiuti-Polanyi (HP) mechanisms. A kinetic model based on LHHW single-site mechanism with loss of the first H2 molecule the rate rate-controlling step was found to best fit the data. Analyses of the products show that the dehydrogenation of MCH is very selective towards toluene. As well as the main product toluene, a number of condensable by-products were also identified. Benzene, cyclohexane and ring-closed products (ethylcyclopentane and dimethylcyclopentanes) are the major by-products.Laboratory experimental data for the 12 experimental runs made under varying conditions of pressure, space velocity and feed composition were simulated and good agreement between predicted and observed centreline temperatures was found. A hybrid MTH-gasoline-system is a viable option. Using titanium aluminide as the material of construction, the dynamic (start up) time requirement for the prototype reactor may be halved over that required for a stainless steel construction.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:528135 |
| Date | January 2010 |
| Creators | Usman, Muhammad Rashid |
| Contributors | Garforth, Arthur ; Cresswell, David |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/kinetics-of-methylcyclohexane-dehydrogenation-and-reactor-simulation-for-onboard-hydrogen-storage(dfd62a36-75a7-4de6-96fa-3f30ecdff9a7).html |
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