Microwave pyrolisis of polymeric materials

The polymeric materials consumption are growing ceaselessly in the world even in spite of the financial crisis. World rubber demand, for instance, is foreseen to increase up to 4% annually to 26.5 million metric tons in 2011. Therefore the disposal of waste polymers is a serious environmental problem against which public is becoming more aware. The interest of waste polymeric materials disposal is focused on new ways of dealing rather than land filling or incineration. The pyrolysis of polymeric materials or plastic-containing wastes including scrap tires, that is heating in the absence of an oxidative agent, is a possible answer to this problem because it let recover of gas, oil and solid able to be employed as a source of products and energy. Therefore the relevance of the pyrolysis processes of plastic waste disposal has been growing. Regarding to scrap tires, also a strong attention has been paid over the last years to the claims for their recycling or reprocessing. In consideration of their complex composition, calorific power and shape hindrance they may be burned hardly but they cannot send to land fill anymore and an alternative methodology should be eligible in order to dispose scrap tires. Starting from the beginning of the last decade the amount of tires displaced in land fill has been steadily decreased while energy recovery has been increased. Waste plastics and tires are pretty attractive materials as source of renewed raw materials and chemical substances. These products may be achieved by pyrolysis and these processes may be viewed as a promising technology. A plethora of studies over the thermal degradation of polymeric materials are carried out using conventional heating method with internal or external heating source, in inert or oxidizing atmosphere. In this area also the interest to microwave heating technologies has become remarkable since they could represent a charming alternative to current technologies based on conventional heating processes. In this chapter we have planned to report the state of art gathered from the scientific literature and patents in addition of our experience and studies, concerning the pyrolysis process with microwave heating of polymers and complex plastics. Emphasis will be given to the influence of the main process variables on products obtained: apparatus set-up, temperature, heating rate, and so on. Strong attention will be devoted to the pyrolysis process of scrap tires. Tires, due to their high carbon content (up to 30%) are suitable for direct microwave pyrolysis. They are able to absorb microwave and quickly turning it to heat. The chapter consists of two main sections. The first part is exclusively dedicated to microwave pyrolysis of tires. We report a review of literature and patent, we discuss the results on the bases of our own experience and we integrate them together with our recent achievements in this field. The pyrolysis products are: a gas, a liquid and a solid. The liquid fraction is the most attractive because of its high content of valuable hydrocarbons like: benzene, toluene, xilene and limonene. We thoroughly deal with the relation between layout technologies, operating parameters and products composition. We address in detail the problem to maximize the amount of liquid fraction and valuable hydrocarbons. The second section is on a review of literature and patents on traditional and microwave heating pyrolysis of pure materials. These polymers don’t absorb microwave, to carry out the pyrolysis is essential a microwave absorbent mixed with polymeric load. Typical absorbent are: coal or carbon-containing materials (for instance tires). We treat about the main polymeric materials sent to landfill: polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylentereftalate (PET) and polyvinylchloride (PVC). We present the processes in use and the operative parameters employed (cited above) for each polymer in order to obtain renewed raw materials, refinery feedstock or energy source. A separate paragraph will be focused on sulfur removal from pyrolysis products by means of catalytic hydrodesulfurization and other available processes.