The pyrolysis process is commonly used to produce fuel oil products and char. Common problems with the oil products have been identified to be large boiling point ranges and high sulphur content. These problems increased the post process refining requirements of the pyrolysis oils to produce commercial grade products. The high sulphur content is mainly due to the high sulphur content of waste tyre rubber feedstock used for pyrolysis. To reduce the sulphur content of the feedstock, a thermal desulphurization process was developed and implemented prior to the pyrolysis process. To reduce the large boiling point ranges of the oils, a fractional condensation system was developed and implemented with the pyrolysis process. The combination of the desulphurization process and fractional condensation system can produce pyrolysis fuel fractions which have fuel characteristics similar to commercial fuel sources such as diesel, gasoline., naphtha and marine bunker oil.
With reference to the process flow diagram shown above, the overall process could be divided into two stages (Process 1 and 2). The first stage is the continuous ex-situ desulphurization stage, which would take place in a preheating vacuum oven under inert conditions with a specified vacuum applied. During the first stage, the cold feedstock is continuously fed to the vacuum oven and will be heated to the specified desulphurization treatment temperature. The feedstock will be exposed to the desulphurization treatment temperature for a predetermined exposure time. The feedstock was subjected to lower temperatures to break the cross-linking sulphur bonds and sulphur-carbon bonds within the pyrolysis feedstock. The breaking of these bonds without significant cleavage of the carbon-carbon bonds allows the sulphur to be liberated in the form of a gas (H2S) and reducing the sulphur content of the pyrolysis feedstock The liberated sulphur in the form of H2S will be extracted prior to the second stage to avoid significant interactions of the H2S vapours with the feedstock or char generated during the second stage of the process.
During the second stage (process 2) the pre-heated desulphurised feedstock is continuously fed to the second vacuum oven for heating to the pre-set pyrolysis temperature. The vacuum oven also operates under inert conditions with a predetermined vacuum applied. The full degraded feedstock (char product) is removed while the produced pyrolysis vapours migrate through the fractional condensation network. The produced vapours are cooled stepwise in a minimum of three separate stages. Each stage is heated to a specific temperature in order to produce fractions of the pyrolysis oil based on the varying boiling points of the expected compounds within the pyrolysis oil. Condensed oil migrates out of the heated condensers by means of gravity, to cool under ambient conditions, which avoids excessive cracking of the fractionated pyrolysis oils. By varying the condenser operating temperature or number of fractional condensation stages, certain fuel properties (density, viscosity, boiling point range and cetane index) can be manipulated.