Recycling and Pyrolysis of Plastic
By Melanie Smith
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Plastics are durable and degrade very slowly; the molecular bonds that make plastic so durable
make it equally resistant to natural processes of degradation. Since the 1950s, one billion tons
of plastic has been discarded and may persist for hundreds or even thousands of years.
Unfortunately, recycling plastics has proven difficult. One of the biggest problem with plastic
recycling is that it is difficult to automate the sorting of plastic waste, and so it is labor
intensive. Typically, waste plant workers sort the plastic by looking at the resin identification
code; though common containers like soda bottles can be sorted from memory. Plastic is also
sorted with float/sink water tests and wind tests. There are machines that sort with laser
technology and can differentiate between most of the known plastics. Other recyclable
materials, such as metals, are easier to process mechanically. However, new mechanical sorting
processes are being utilized to increase plastic recycling capacity and efficiency.
While bottles are made from a single type of plastic, making them relatively easy to sort out, a
consumer product like a cellular phone may have many small parts consisting of over a dozen
different types and colors of plastics. In a case like this, the resources it would take to separate
the plastics far exceed their value and the item is discarded. However, developments are taking
place in the field of Active Disassembly, which may result in more consumer product
components being re-used or recycled. Recycling certain types of plastics can be unprofitable,
as well. For example, polystyrene is rarely recycled because it is usually not cost effective.
These unrecycled wastes are typically disposed of in landfills, incinerated or used to produce
electricity at waste-to-energy plants.
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Testing plastic with laser
equipment to determine type
at the MBA Polymer plant in
Richmond, CA. The small
white pipe is held to the laser
sensor and the screen reads
the type, in this case it is PE. |
The only recyclable plastics in curbside or household recycling are number 1, PET, and 2, HDPE.
The majority of this is exported to China where it is reprocessed. Plastic film is gathered at
markets and is remanufactured into decking. In fact it is the only recycled plastic that stays in
the US for reprocessing and reuse in its new form. The majority of other recycled plastics are
shipped to Asia or Canada and then often returned to the US in its new product form.
The biggest threat to the conventional plastics industry is most likely to be environmental
concerns, including the release of toxic pollutants, greenhouse gas, litter, biodegradable and
non-biodegradable landfill impact as a result of the production and disposal of petroleum and
petroleum-based plastics. Of particular concern has been the recent accumulation of enormous
quantities of plastic trash in ocean gyres.
Recycling Solutions
Plastic polymers require greater processing to be recycled than metal or glass... Heating alone
is not enough to dissolve such a large molecule; because of this, plastics must often be of nearly
identical composition in order to mix efficiently otherwise when different types of plastics are
melted together they tend to separate, like oil and water, or bead up preventing them from
being reformed. Even plastic of the same type, such as HPDE 2 if it has been molded as in an
open neck container verses blown for a closed neck container, requires different flow agents
and will not combine. To offset this problem virgin polymers are added to the recycled stock to
create new pellets or beads. There are companies such as MBA Polymers of Richmond CA with
plants in China and Austria that recycle sorted, shredded plastic into new resin beads with an
additive of a small percentage of new resin to strengthen and bind the recycled material. They
are recycling high-impact poly-styrene, polypropylene, ABS from commercial sources.
Another potential option is the conversion of plastics into petroleum by thermal
depolymerization or pyrolysis process. Such a process would be able to accept almost any
polymer or mix of polymers, including rubber tires. Like natural petroleum, the chemicals
produced can be made into fuels as well as polymers.
DEFINITION OF PYROLYSIS
Pyrolysis is the chemical decomposition of condensed organic substances by heating. The word
is coined from the Greek-derived elements pyro "fire" and lysys "decomposition". Pyrolysis is
usually the first chemical reaction that occurs in the burning of many solid organic fuels, like
wood, cloth, and paper, and also of some kinds of plastic. Anhydrous pyrolysis can also be used
to produce liquid fuel similar to diesel from plastic waste.
The pyrolysis process for plastic takes the long chain polymer molecules and breaks or cracks
them into shorter chains through heat and pressure. Essentially the process is mimicking the
natural process of the earth to break down carbon into oil which takes million of years in
nature. The pyrolysis process does this with intense heat in a closed system in a short amount
of time.
Conditions for producing pyrolysis oil are more likely to include virtually no oxygen. The
pyrolysis of plastics produces a liquid product, pyrolysis oil or oil that can be readily stored and
transported. Pyrolysis oil can be used directly as fuel or further refined into diesel or jet fuel.
PYROLYSIS TYPES Defined
Thermal Depolymerization is a process using hydrous pyrolysis for the reduction of
complex organic materials (usually waste products of various sorts, often known as biomass
and plastic) into light crude oil. It mimics the natural geological processes thought to be
involved in the production of fossil fuels. Under pressure and heat, long chain polymers of
hydrogen, oxygen, and carbon decompose into short-chain petroleum hydrocarbons with a
maximum length of around 18 carbons. TDP can mean conversion of biomass to oils using
superheated water, although it more usually is applied to fuel production via pyrolysis.[2][3]
Thermal Conversion Process: A company called Renewable Environmental Solutions
(RES) was formed as a joint venture between ConAgra Foods and Changing World Technologies
to operate the plant at Carthage, Missouri and the name of the process was changed from
Thermal Depolymerization to Thermal Conversion Process.
Hydrothermal Liquifaction: Direct hydrothermal liquefaction involves converting
biomass to an oily liquid by contacting the biomass with water at elevated temperatures (300-
350°C) with sufficient pressure to maintain the water primarily in the liquid phase (12-20 MPa)
for residence times up to 30 minutes.
Hydrous pyrolysis: Refers to the thermal decomposition which takes place when organic
compounds are heated to high temperatures in the presence of water.
Anhydrous Pyrolysis: Simple heating without water, anhydrous pyrolysis has long been
considered to take place naturally in the earth’s crust. It is the process in which organic material
contained in rock is broken down to release the oil and fossil fuels. Some pyrolysis methods
which create hydrocarbons through depolymerization use dry materials (or anhydrous
pyrolysis) which requires expending a lot of energy to remove water.
Pyrolysis Plant Manufactures
Ozmotech, Melbourne-based environmental technology manufacturer, developed its
ThermoFuel system using a pyrolysis chamber, a patented catalytic converter and a series of
specially built condensers to produce energy-rich diesel fuel from unsorted waste plastics.
Plastics that are unsuitable for other recycling purposes because of an undesirable or
contaminated mix of polymers are no problem. Ozmotech has spent two years developing the
original pyrolysis technology into a fully operational system capable of producing over 19,000
litres of diesel fuel per day for less than 30 cents per liter. Several systems are already in
operation in Japan but the fuels produced are used exclusively for power generation through
diesel generators.
The Envion Oil Generator accepts PET, HDPE, PVC, LDPE, PP, PS, and several other plastic
materials, such as GPPS, EPS, HIPS, and PA. Envion originally developed their platform as a joint
effort between South Korea and the US to assist China with the growing problem of trash. The
first plant they built was to be mounted on a barge for use on a major river in China. It would
travel up and down the river to process waste, mostly plastic, and create oil that could be
further refined into fuel. They now have a plant operating in Derborne Maryland and plants in
China.
 |
The Envion Oil
Generator.
Converts waste
plastic into oil
producing 3 to 5
barrels of refined
light or medium oil
per ton of plastic
waste |
Biodegradable (Compostable) Plastics
Research has been done on biodegradable plastics that break down with exposure to sunlight
(e.g., ultra-violet radiation), water or dampness, bacteria, enzymes, wind abrasion and some
instances rodent pest or insect attack are also included as forms of biodegradation or
environmental degradation.
Extensive testing is being done on polymers made from polysaccharides, such as corn glucose
and agroproteins currently known as PLA and PHA. These are possibly the answer to creating a
fully marine degradable plastic. Other biodegradable plastics will only fully degrade if the plastic
is exposed at the surface, with microbial exposure in composting systems and will not degrade
in landfills... Starch powder has been mixed with plastic as a filler to allow it to degrade more
easily, but it still does not lead to complete breakdown of the plastic within the matrix, the
starch breaks sown leaving the polymer molecules intact.
Some researchers have actually genetically engineered bacteria that synthesize a completely
biodegradable plastic, but this material, such as Biopol, is expensive at present. The German
chemical company BASF makes Ecoflex, fully biodegradable polyester for food packaging
applications.
References:
http://www.wme.com.au/categories/waste_managemt/feb6_05.php
http://www.changingworldtech.com/what/problems.asp#environmental
http://web.archive.org/web/20070814144750/http://www1.eere.energy.gov/biomass/pyrolysis.html
http://www.wme.com.au/categories/waste_managemt/feb6_05.php
http://en.wikipedia.org/wiki/Thermal_depolymerization#
http://en.wikipedia.org/wiki/Plastic
http://www.mbapolymers.com/about.htm
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