Enhancement of the pyrolysis process using ultrasound

Abstract

It has been estimated that Botswana has more than 200 billion tons of coal locked up in its underground reserves. The logistics for export purposes simply do not exist to get these vast resources to the markets. There has been huge efforts prove feasibility of develop plans to establish these logistics but to date there is still nothing forthcoming. On the other end of the spectrum there has been interest to develop and commercialize processes in which coal is converted into valuable products (e.g. via coal pyrolysis) which either have local markets or for which the export logistics already exist (or can easily be created). Sonochemistry can be utilized to enhance the coal pyrolysis process. In sonochemistry, ultrasound waves are applied to a liquid or liquid mixture to such an intensity that cavitation takes place. Cavity formation and collapse cause extreme local temperatures (>5000K) and pressures (>100bar) at extreme heating and cooling rates (109K/s). The hot spot theory is widely used to describe this phenomenon. Since chemical kinetics are directly influenced by pressure, temperature and heating/cooling rates, it follows that sonochemistry can be used enhance chemical processes and this has been shown to be true in many other industries (especially the medical industry). It is therefore reasonable to expect increased oil production yields and rates during the pyrolysis of coal as well and this would result in a better return on investment (ROI) for projects. This project, which is still in its incipient stage, aims to determine the sonochemical parameters which will be conducive to the pyrolysis of coal. The kinetics of sonochemical pyrolysis reactions have been studied and these sonochemical principles have been used to establish a framework in which the project objectives can be met. These objectives include the formulation of a mathematical model for sonochemical pyrolysis, the design and construction of sonochemical pilot reactor and the subsequent testing and analysis using the pilot facility. The experimental work will entail attachment of an ultrasonic transducer to a small desktop adiabatic batch reactor (able to contain 1000ml of raw material). The pressure and temperature of this reactor will be set to predetermined values before the experiment starts. The changes in pressure and temperature will be measured accurately and recorded together with product yields for various ultrasonic frequencies and intensities. Gas and vapour products will be sampled during the experiment and analysed while the liquid and solid products will be analysed after completion of the experiment. The experiment will be repeated for a range of operating conditions. The effect of ultrasonic frequency, intensity, temperature and pressure will be analysed, documented together with conclusions and recommendations.