conventional plastic vs. pp bags

Topics: Petroleum, Carbon dioxide, Natural gas Pages: 6 (1363 words) Published: November 21, 2013
The following is an analysis of conventional plastic (polymer) bags and its comparison to the emerging “bio-based” carrier bags. The study consists of examining a bio-based bag made from polyhydroxyalkanoate (PHA) in the United States and a polyethylene plastic bag from Singapore. We examine the production processes of both of these bags, from its initial stage of gathering raw materials to the disposal of these products to determine whether Bio-bags are in fact more environmentally friendly than traditional plastic bags. We attempt to follow Kearney’s Life Cycle Approach to Sustainability in order to clearly illustrate the differences that exist between the various stages of the life cycle of these products. Supply & Manufacturing

Polyethylene Plastic bags
The production for plastic begins with crude oil and natural gas extraction, fossil fuel resources which are unattainable in Singapore. These resources are transported from the Middle East, travelling 8000km in an ocean tanker to Singapore. Simultaneously, in the production of plastic bags natural gas is used and is extracted from the Natuna Gas fields in Indonesia and piped to a refinery 730km away in Singapore. Once the crude oil and natural gas arrive at the refinery, the distillation process begins converting crude oil to Naphtha followed by a process of ‘Cracking’ where heavy hydrocarbons are broken down to simpler molecules. At this time the manufacturing of Polypropylene (PP) begins, first by the production of monomers, and then polymerization and blown film extrusion which is using an air jet to create the shape of the plastic bag. Bio-Bag(PHA)

Because PHAs can be produced directly from renewable resources such as crops like corn, the process begins in agricultural areas such as farmlands. Corn is grown by farmers and wet milling is used to produce glucose which is eventually fermented to produce PHA. At this point, similar to Polyethylene plastic bags, blown film extrusion is used to form the shape of the bio-bag. The following tables depict the amount of energy requirements in both of the production processes. Energy requirement for PP Bag production

Fossil Fuels production
24.3 MJ/kg Crude oil
Natural Gas exploration/production
11.34 MJ/kg natural gas
Crude oil distillation & natural gas cracking
4.92 MJ/kg crude oil
Polymerization
19.05 Mj/kg pp monomer

Energy requirements for bio-bag production
Corn production
2.5 MJ/kg corn
Wet milling
4.9 MJ/kg glucose
Fermentation
52.53 MJ/kg PHA

While exerting this energy during the process, there are several environmental implications that follow primarily in the form of air emissions. The following emissions are being examined: carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), mono-nitrogen oxide (NOx), sulfur oxide (SOx), and non-methane volatile organic compounds (NMVOC) The following chart displays the air emissions from energy supplied to PP bag production. Emissions (kg/MJ)

Singapore electricity mix: for supply of energy to refinery and plastic manufacturer Malaysia electricity mix: for supply of energy for natural gas exploration and production Natural gas-fired power: for supply of energy for crude oil exploration and production CO

4.9E-05
1.11E-05
7.50E-07
CO2
1.56E-01
1.60E-01
2.06E-01
CH4
2.23E-06
1.00E-06
5.33E-06
N2O
8.42E-07
n/a
1.41E-06
NOx
2.04E-03
1.94E-03
2.64E-05
SOx
3.31E-04
5.83E-07
5.56E-07
NMVOC
6.03E-06
5.83E-06
2.78E-06

This data is meaningless unless we compare it to the emissions from the production of Bio-bags. The following is the direct emissions to the air from agriculture and wet milling processes. Direct emissions to air from agriculture and wet milling stages Air emissions

Agriculture (kg/kg corn)
Wet milling (kg/kg glucose)
CO2
-0.857 (sequestered)
3.5E-01
N2O
6.51E-04
0

There are enormous amounts of pollution that occur from the energy supplied to produce a bio-bag but rather than...
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