Pyrolysis continued rotary kiln

Several processes have been developed [41-43] to overcome the technological drawbacks of plastics incineration cited above. These include continuous rotary-kiln processes a process for glass-reinforced PET a combined system for wood fiber and PET to provide steam to power equipment and a fluidized system for pyrolysis, in combination with silver recovery from photographic film. Incineration of photographic film raises the additional problem of the formation of toxic halogenated compounds due to the presence of silver halides. 

The majority of the cyanuric acid produced commercially is made via pyrolysis of urea [57-13-6] (mp 135°C) primarily employing either directiy or indirectly fired stainless steel rotary kilns. Small amounts of CA are produced by pyrolysis of urea in stirred batch or continuous reactors, over molten tin, or in sulfolane. The feed to the kilns can be either urea soHd, melt, or aqueous solution. Since conversion of urea to CA is endothermic and goes through a plastic stage, heat and mass transport are important process considerations. The kiln operates under slight vacuum. Air is drawn into the kiln to avoid explosive concentrations of ammonia (15—27 mol %). 

The main advantage of rotary kiln pyrolyzers is that the rotation of the pyrolyzing chamber guarantees the permanent turning and mixing of the waste plastic, so that the mixture is continuously homogenized and blended with inert pyrolysis gas. A rotary kiln pyrolyzer is shown in Figure 27.3. 

Pyrolysis of agricultural residue was experimentally assessed as a fuel production process for farm applications. A rotary kiln (3.4 m by 0.165 m I.D.) was used due to its ease of operation, commercial availability, low operating costs and ease of start-up and shutdown. Ground oat straw and corn stover at less than 10% moisture were pyrolysed in an indirectly fired continuous-flow rotary kiln located at the University of Sherbrooke. The principle products were char and gas, less than 1% of the feed mass was converted to tar. Calorific values were about 17 MJ/kg for the feed, 26 MJ/kg for the char, and 12 MJ/m3 for the gas. Calculations indicate that the thermal efficiency of a self-sustaining process would be around 65%. 

Pyrolysis can be used for the thermal decomposition of waste materials that are predominantly organic in nature, e.g. scrap tyres, scrap cables, waste plastics, shredder wastes, and acid sludge. Rotary kilns are particularly suitable as universally applicable pyrolysis units for continuous operation. Highly aromatic pyrolysis oils for use as chemical raw materials are obtained at reactor temperatures of about 700 °G. Such pyrolysis oils could form the basis for the production of aromatics such as benzene, naphthalene, and their homologues, thermoplastic hydrocarbon resins and precursors of industrial carbon, when the proven processes for the refining of coal tar and crude benzene are applied. 

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