Plastics are an unexploited resource of energy and materials
The love affair with human-made materials has grown into a nightmare, as millions of tonnes of discarded plastic waste enter the biosphere each year. Its characteristic of durability and non-degradability, so treasured by the manufacturing industry, is a cause of enormous concern for the environment. The natural world does not know what to do with unnatural products.
Some governments have woken up to the potential of plastic as a resource, but many others are still dumping it. So, how can plastic best be exploited?
Plastic is cheap to produce, light and easily stored. It makes airtight, hygienic and tough containers, and strong, lightweight carriers, so is ideal for packaging, storage and transport. Plastic production uses 8% of the world oil production, which is 7 million barrels a day. Half of this oil is for feedstock in the production of plastic, and half is consumed in the production process itself. Reducing the amount of plastic produced will reduce energy consumption and emissions of carbon-dioxide (CO2), nitrogen-oxide (NO) and sulphur-dioxide (SO2) during manufacture, and reduce non-biodegradable landfill waste.
There are three basic ways to ensure the most is obtained out of plastic:
The low cost of plastic has a negative effect on rates of recycling and reuse. Thankfully, the concept of reusing bags for shopping trips is changing the throw-away, single-use habit which has caused environmental contamination for decades.
Similarly, transport crates, storage vessels, and many other plastic items can be reused by industry and commerce, and regulations are being imposed to ensure that economic criteria are not the only factors considered by management. Environmental movements activate consumer power in encouraging, for example, customers to return packaging to the retail outlets, bringing pressure back up the supply line for a more efficient policy which reduces wastage.
The first challenge in recycling plastics is the difficulty of post-use separated collection from the consumer. Ferrous and aluminium cans can be collected together, and easily separated by magnets. However, the public are not sensitive to all the groups of plastics, and low collection rates of recyclable plastic are still the norm. Plastic waste is still widely sorted by hand into polymer category and colour, in the collection or reprocessing facilities. Some technologies are being developed to aid sorting. These include X-ray fluoresence, infrared and near infrared spectroscopy, electrostatics and flotation.
The second hurdle relates to the practicality of cleaning, removal of contaminants (such as paper labels and glue residues), colour mixes, and the economicability of options for the reclaimed material. Mechanical recycling involves melting, shredding or granulation. The sorted plastic is melted down, as it is or after shredding into flakes, and regranulated.
Feedstock recycling involves a process of breaking the polymer into its constituent monomers, which are then used in applications in refineries, petrochemical and chemical industries. The technologies being developed for this purpose include pyrolysis, hydrogenation, gasification, and thermal cracking. The advantage of feedstock over mechanical recycling is greater impurity tolerance. Disadvantages are that it is capital intensive, and scale, whereby very large quantities (in excess of 50 kt p.a.) are required to make the operation economic. One tonne of plastics is equivalent to 20,000 two litre drinks bottles or 120,000 carrier bags.
Plastic can be recycled for use in products as diverse as polyethylene bin liners and carrier bags, piping made of PVC, building elements such as flooring, window frames, and insulating board, garden furniture, clothing (e.g. fleeces), and office accessories.
The first principle is the avoidance of waste generation in the first place. The second is where possible the reuse or recycling of substances, which are separated from the regular waste stream. For the remaining amount, the better option is exploitation of the energy content, and only then should any material that cannot be reused, recycled or which has no energy value, should be carefully landfilled under controlled conditions.
Plastic can be incinerated in waste-to-energy plants, and the energy used to generate electricity. The flue gases need to be scrubbed, as they contain dioxins and other pollutants.
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