Plastics waste management

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Contents

Decision situation

Can commercially viable fuel be produced sustainably from plastic waste?

Plastic Waste Management

Plastic waste has a major negative impact on the environment. This impact is increasing as landfill space is depleted and pollution grows.

Types of plastic waste

74% of all plastic waste falls into the following categories:

  1. Polyethylene:
    • Low density polyethylene (LDPE)
    • Linear low density polyethylene (LLDPE)
    • High density polyethylene(HDPE)
  2. Polypropylene (PP)
  3. Polyvinylchloride (PVC)
  4. Polystyrene (PS)
  5. Polyethylene - terephthalate (PET)

End-of-life treatments of plastic waste

Of the options for managing plastic waste, re-use and waste minimisation are seen to have little impact on overall reduction compared with recycling and energy recovery.

Landfill is the other option for managing plastic waste and indeed is by far the most used; for instance in Western Europe in 2005, 53% of plastic was disposed of into landfill[1]

Trends

  1. Landfill:
    • disposal of plastic waste is dropping by 2% annually as available space is dinishing
  2. Recycling:
    • Governments are legislating to promote it
    • Civil society is co-operating in recycling
    • Technological developments are facilitating new ways of recycling plastic

Energy recovery

  1. incineration of plastics
    • emissions of toxic fumes

Recycling

Recycling a PET bottle still costs more than producing a new one, with the result that the recycling rate for PET bottles in Europe is around 25%. The situation is even worse for other plastics. Recycling rates for polystyrene are 2%[2].

Mechanical recycling

'mechanical recycling recovers the plastic material for similar or lower-quality plastics'[1]

Feedstock recycling

'feedstock recycling turns the plastic waste by means of chemical reactions into chemical raw materials or fuels'[1]

  1. represents only 2% of total plastics recycling in Western Europe, for instance
  2. recycling treatments have high investment costs
  3. economics more favourable with larger volumes
  4. requirement for plastic sorting
  5. liquefaction
    • a type of feedstock recycling
    • converts plastics into hydrocarbon mixtures that are useful as fuels via thermal and /or catalytic cracking
    • commercialised by H.Smart Inc and Ozmotech
    • technology being developed to solve the issue of residual PVC in the waste
  6. thermal processes
    • Thermal cracking of these plastics can achieve a worthwhile yield of starting monomers:
      • Polystyrene (PS)
      • Poly(methyl) methacrylate (PMMA)
      • Polytetrafluoroethylene (PTFE)
  7. catalytic processes
  8. comparison

The table below gives some outcomes for various use of reactors, catalysts, heat etc in the cracking of these plastics.

Process / variables Outcome
Reactor:  
Fluidized bed uniform temps -> narrower distribution and more valuable products
  enhancement in polymer/catalysts mass ratio results in smaller amounts of gases and coke and larger amounts of liquid;
  increase of the flow of the fluidizing gas augments yield of primary cracking products (olefins)
Screw kiln recombination of gaseous hydrocarbons favoured ->less gaseous hydrocarbon content & more liquid
Output:  
Temp 500 deg C oils with scarce amounts aromatics
Temp 650-700 deg C gaseous hydrocarbons and olefins increase
Solvent / oils used in reaction viscosity reduced ->heat & mass transfer rates augmented; gas yield decreased
Poor H donating solvents used in reaction: increases liquid hydrocarbons C5 - C32; increases 1-olefins
Catalyst used: decreases cracking temp; increases plastic conversion (vs thermal processes)
Zeolite (HZSM-5) - strong acid sites C3 - C5 olefins
Mesoporous materials (Al-MSM-41, SiO2-Al2O3) - medium acid sites gasolines and middle distillates
Catalysts with highly accessible sites: improved performance as effect of bulk and high viscosity reduced
Nanocrystalline zeolites as catalyst higher conversion than conventional zeolites
hierarchical mesoporous-microporous zeolites higher conversion than conventional zeolites
Deactivation of Catalysts (dependent on acidity and pore structure of catalyst)  
HMOR and HUSY zeolites quicker deactivation because large pore
HZSM-5, SIO2-Al2O3 and Al-MCM-41 slower deactivation because smaller pores
Two Stage process - thermal then catalytic: avoids catalyst deactivation
REY best performing catalyst for yielding gasolines from 2 stage process
HZSM-5, SIO2-Al2O3 and Al-MCM-41 best catalyst for larger share of gaseous hydrocarbons from 2 stage process
Two stage catalytic process i.e. >1 catalysis step: better gasoline
Two catalysts in series (SiO2-Al2O3 + HZSM-5) in a fixed bed high yields of gasoline with good octane number
Above series PLUS increase share of HZSM-5 yield of aromatics and RON increases and yield of liquids decreases
Prepare mixtures of plastic/vacuum gas oil and crack catalytically over fluidized catalytic cracking (FCC) catalysts: reduces viscosity
composition of cracking products depends on plastic share:  
mixture with plastic content of 5% only gaseous products
mixture with plastic content of 10%  
Fast pyrolysis of the plastic then cracking with catalysis Viscosity of reaction medium is decreased
Polyvinyl chloride (PVC) present: serious problem because HCl released above 260deg C
addition of CaO / Ca(OH)2, iron oxides (FeOOH, Fe2O3, or Fe3O4) or CaCO3 Remove released HCl from PVC degradation

References

  1. ↑ 1.0 1.1 1.2 Aguado, J et alFuels from waste plastics by thermal and catalytic processes: A review, Industrial & Engineering Chemistry Research, 2008 47,7982-7992
  2. ↑ Bioplastics get growing: Everyone knows that we should stop depending upon petroleum
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