**7. Concluding remarks**

Active plastic waste catalytic cracking materials involve Brönsted acid sites, present in zeolite catalysts and which we introduced in MCM-41 mesoporous materials by (i) the incorporation of Al and Ga and (ii), by impregnation of the MCM-41 surface with tungstophosphoric acid. The several solid acid catalysts we tested yield gas and liquid products from the LDPE cracking reaction. The gas products consist of a mixture of ethane, propane, butane and pentane, all of them of interest for petrochemical industries or as domestic energy source. Liquid products consist of gasoline, turbosine, kerosene, gas oil, and fuel oil, corresponding to fuel fractions obtained in industrial petroleum refinery plants. The selectivity towards particular products depends mainly on the choice of catalyst and reaction conditions. The study of the catalytic cracking of plastic waste has led to relevant scientific knowledge and to the development of innovative technologies.

The large-scale application of these processes has been limited by economical and profitability reasons. Previous stages involve plastic separation from municipal waste and, since different mixtures of products are obtained, end stages involve product separation processes. Up to now, it is hard to compete with the still cheap option of producing fuels from natural gas and crude oil and to produce plastics from new raw materials. However, worldwide growing concerns about preserving our environment give plenty of room for imaginative ideas on how to scale up these processes to industry level.

#### **8. References**

170 Material Recycling – Trends and Perspectives

bins or drop-off sites. Then, they go to a material recovery facility, where the materials are sorted into broad categories (plastics, paper, glass, etc.). The resulting mixed plastics are sorted by plastic type, baled, and sent to a reclaiming facility. A Mexican study reported than the average cost of one ton of trash is around 350 US Dollars (USD), but if plastics are reduced, the cost would be around 4 USD less (Cortinas, 2009). The investment for recycling

The price of virgin plastic is influenced by the price of oil, which is the principle feedstock for plastic production. As the quality of recovered plastic is typically lower than that of virgin plastics, the price of virgin plastic sets the ceiling for prices of recovered plastic. The net heat generation from the use of crude oil in plastic making is roughly 0.4x1014 kWh from 1939 to 2000. It corresponds to 1.3% of the missing heat and contributes to 0.5% of the global

The profitability of feedstock, chemical recycling methods depends on three key factors: the degree of separation required in the raw wastes, the capital investment involved in the processing facilities and the value of the products obtained (Clark, 1999). For most of feedstock recycling methods, some pretreatment or separation operations are unavoidable. Feedstock recycling methods can be ordered according to the separation steps required

Gasification < thermal cracking < hydrogenation < catalytic cracking < chemical

Whereas the value of the products obtained by the feedstock recycling methods follows the

Thermal oils < synthesis gas < hydrogenation oils < catalytic olefins and paraffins <

Important benefits of the catalytic cracking of polymer waste with respect to other chemical feedstock recycling methods is the possibility of controlling the selectivity towards desired products and the possibility of reducing energy consumption. The use of cheap catalysts is central under the actual circumstances and some of the current best options are natural materials and waste catalysts from other industries. To use existing industrial facilities is a way for much cost reduction. Since the catalysts and the reaction mechanism of the polymer catalytic cracking are about the same as for the hydrocarbon fluid catalytic cracking (FCC), it is possible to incorporate plastic waste into the FCC refinery feed, with the added benefit

Active plastic waste catalytic cracking materials involve Brönsted acid sites, present in zeolite catalysts and which we introduced in MCM-41 mesoporous materials by (i) the incorporation of Al and Ga and (ii), by impregnation of the MCM-41 surface with tungstophosphoric acid. The several solid acid catalysts we tested yield gas and liquid products from the LDPE cracking reaction. The gas products consist of a mixture of ethane, propane, butane and pentane, all of them of interest for petrochemical industries or as domestic energy source. Liquid products consist of gasoline, turbosine, kerosene, gas oil,

that plastic waste has almost no sulfur content and no heavy metals content.

of 150 ton could be 300,000 USD producing a monthly utility of 30,000 USD.

warming (Gervet, 2007).

(Clark, 1999):

depolymerization

opposite order:

**7. Concluding remarks** 

monomers


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**7** 

Kyong-Hwan Lee

*South Korea* 

*Korea Institute of Energy Research* 

**Pyrolysis of Waste Polystyrene and** 

As the rate of consumption of plastic materials in the world is greatly expanded, more waste plastics are generated. In recent years, their generation amount in Korea becomes about four million tons per year, according to data from the National Institute of Environmental Research. The disposal of waste plastic is mostly achieved by conventional ways such as landfill or incineration. However, these methods have a problem of a social resistance due to the air pollution, soil contamination, and the economical resistance caused by an increase of a space and a disposal cost. Thus, the recycling of plastic wastes as a cheap source of raw materials has become a predominant subject over all countries. The development of technologies acceptable from the environmental and economical fields is one of the most

Generally, the recycling methods are classified as the material recycling and chemical recycling. The former is one of the most conventional methods but is limited by difficulties in maintaining the high quality and adequate price of final products, in particular, for the mixture of plastic waste. Thus, application of other procedures such as chemical recycling

The chemical recycling, referred to as an advanced recycling technology, is included in a tertiary recycling. The process is converted from plastic wastes into smaller molecules corresponding to chemical intermediates through the use of heat and chemical treatment, such as liquids, gases and waxes. These chemical intermediates can be used as the fuel oil and feed stocks of petrochemicals processes, etc. The chemical recycling is described by the

The chemical recycling can be mainly explained by the chemical recovery systems, which are classified as a heterogeneous and a homogeneous process. The chemolysis methods as homogeneous process utilize chemical agents as catalysts for depolymerization of polymers to obtain the products with low molecular wieghts. Chemolysis includes the processes such as glycolysis, hydrolysis, methanolysis and alcoholysis. On the other hand, heterogeneous processes are greatly described by gasification and pyrolysis. Gasification as partial oxidation (using oxygen or steam) can generate a mixture of hydrocarbons and synthesis gas (CO and H2), which are dependent on the type of polymer, biomass, coal and co-

**1. Introduction** 

important key factors.

and energy recovery is required (Al-Salem et al., 2009).

mixture, and on quantity of and quality of resulting product.

routes as follows (Kumar et al., 2011).

**High-Density Polyethylene** 

