5-7 OCTOBER

ISFR

2011

173

TOLEDO (SPAIN)FEEDSTOCK RECYCLING OF WASTE POLYMERS BY THERMAL CRACKING IN
MOLTEN METAL – THERMODYNAMIC ANALYSIS OF THE PROCESS
M. Stelmachowski,
Faculty of Process and Environmental Engineering, Technical University of Lodz, Lodz, Polande-mail: marek.stelmachowski@p.lodz.plAbstract
Waste plastics contribute to great environmental and social problems due to the loss of natural resources, environmentalpollution, depletion of landfill space on the one hand and demands of environmentally-oriented society on the other hand.
Feedstock recycling of scrap polymers by thermal and chemical methods are well known and environmentally accepted. The
paper presented results of thermodynamic analysis of the process conversion of polyolefins to fuel like mixture ofhydrocarbons by thermal cracking in a new type of the tubular reactor with molten metal. Evaluation of the efficiency of theprocess was based on exergy calculations. The calculated exergy efficiency was equal to 80%. It means that feedstock
recycling of the wastes is the better process from the energetic and ecological point of view than other processes, particularlyincineration of them.
Keywords: plastic wastes, thermodynamic analysis, exergy, pyrolysis, thermal cracking

1. Introduction

2. Methods

Feedstock recycling of scrap polymers by thermaland chemical degradation are well known andenvironmentally accepted. These methods haveovercome a long way from the scientific idea to theindustrial application. At least, 30 commercial
technologies were available for thermal degradation of
post-consumer plastics based on pyrolysis or catalytic
cracking to fuel like liquid mixture of hydrocarbons [1,2].
However, the industrial plants are rare. It means that theproposed solutions were imperfect and their profitability
was weak. The unfavorable situation of feedstock
recycling of waste polymers was mostly based on thehigh investments costs, necessity of frequent cleaning of
the reactor, costs of catalysts and other economic
circumstances e.g. taxes. The paper presents the resultsof the study of conversion of waste polyolefins to theliquid mixture of hydrocarbons that was performed bythermal cracking in a new type of the tubular reactor withmolten metal. The construction of this reactor differs fromthe known basin reactors that have been patented until
now. The final product consists of gaseous stream (8-16
weight % of the input) and liquid (84-92 weight %)stream. The light, “gasoline” fraction of the liquidhydrocarbons mixture (C4-C10) made over 50% of theliquid product. The obtained results indicate that thermal
degradation may be in some cases also as good as
catalytic cracking and the method makes possible
obtaining the high quality liquid product with good yield of
it and good selectivity for “gasoline” and “diesel”
fractions. Based on the laboratory experiments, a pilotscale plant was proposed for feedstock recycling ofwaste polyolefins to fuel like mixture of hydrocarbons orelectricity power. The thermodynamic (based on exergy
calculations) analysis was also performed for the process
to show its energetic efficiency.

The first law of thermodynamics states that for every
process, no energy can disappear or be created; thesecond law notes that the quality of energy decreases.
This quality, expressed as “useful energy”, is calledexergy. Exergy is the maximum amount of work that canbe obtained from a material, taking into account the stateof the surroundings [3, 4]. The loss of exergy is linearly
related to the entropy generated in the process. Every
product is useful, and waste can be seen as potential
resources as long as it has exergy content. A number ofenvironmental indicators can be defined for evaluation ofenergy efficiency of the processes and assessment of
the impact of them on the environment. However, it
seems that indicators based on thermodynamic
considerations based on exergy, can illustrate whether adevelopment is sustainable in the best way. The basic
principles of the methodology used to assess the impact
of industrial processes on the environment and theirenergetic efficiencies are discussed in many articles [3,5]. This type of analysis is used also for the evaluation of
the recycling method of wastes. This paper presents a
simplified assessment of the feedstock recycling process
for waste polyolefins using thermal degradation andconversion to a fuel-like mixture of hydrocarbons.
The description of the laboratory set-up and theresults of the laboratory thermal degradation of waste
polyethylene and polypropylene can be found in aprevious study [6]. The yields of gaseous and liquid
products and their composition indicate that the proposed
reactor and method of scrap polyolefin degradation
meets all demands for profitable technology. Over 90%of waste polyolefins may be converted into a liquid
product. The light (i.e., “gasoline”) fraction makes up 50
to 70% of the liquid product. The fraction of light waxes isbelow 18% for polyethylene (PE) degradation and below

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6th International Symposium on Feedstock Recycling of Polymeric Materials

10% for polypropylene (PP) degradation in liquidproducts, which means that the liquid product is very
useful as a raw product for fuel production. No solidproduct (i.e., coke) was obtained from the laboratory
experiments for the degradation of pure plastics.
However, impurities that are usually present in genuine
waste of polymers may cause cooking, and a small
amount of the solid product consisting of mineral
impurities and coke may be obtained during theconversion of this type of wastes. The experimental
results were used to design pilot-scale experiments andcreate a mass balance for them
Exergy analysis was performed for the pilot-scale
reactor, in which 50 kg/h of genuine polyethylene (PE)wastes would be utilized by thermal degradation. Hot fluegases from gasification of the biomass are the source ofthe heat demanded to waste plastic degradation. It wasassumed that 28 kg/h of sawdust (11.7 kJ/kg) is gasified
in the gasifier and combusted to continue this process.
Combustion of the recycled gas product is a
supplementary source of energy. Water is required forcooling of the liquid product and to the scrubber for
purifying flue gases. Electric power is also needed: forthe waste and sawdust feeders, fans, water and productpumps, control and acquisition data systems and thelighting system. The experimental reaction rate of thedegradation process was between 951 and 9183
kg/(m3h). The reaction rate in the pilot plant wasassumed to be 2877 kg/(m3h). Assessment andcalculation of the exergy efficiency of the process werebased on the following assumptions:
1. no mechanical disintegration of the waste wasperformed, as no additional energy was needed,
2. generally, the total exergy of the stream is thesum of kinetic exergy, potential exergy, nuclear
exergy, physical (or termomechanical) exergy
and chemical exergy, but only chemical andphysical exergy are taken into account,
3. the reference state was T=298.15 K and
P=0.101325 MPa,
4. the calculations of physical exergy were basedon standard values of enthalpy and entropy,
5. the chemical exergy of the sawdust, the wastes
and coke was calculated using the higher heating
value of the components,
8. the exergy of electricity was equal to energy
power,
9. the exergy of SO2, NOx, construction materials,
labor and capital was not considered.

3. Results and Discussion
The exergy efficiency of the described process wascalculated for two case studies, resulting in 79.9% for thefirst one, in which the liquid mixture of hydrocarbons (C4C24) was the required product and 81,9% for the second
one, in which the electricity power was generated using
total product – mixture of hydrocarbons ((C4-C24)
without condensation of them. The true efficiency may
not be so high after including the exergy of theconstruction materials, labor and capital. However, itinteriores libro.indd 174

should be higher than the efficiency of incineration
processes for waste plastics, which usually haveefficiency levels ranging from 25 to 35%.

4. Conclusions
The method for the thermal decomposition of PE or
PP in a molten metal bed is a promising process ascompared with catalytic cracking in vessel reactors using
a stirrer or tubular flow reactors. In the proposed method,no catalyst and no stirring are needed. Over 90% of
scrap polyolefins may be converted into a liquid product.
The exergy efficiency of the process is high, whichmeans that thermal cracking is a more valuable
technology than the incineration of waste plastics. In thiscontext, gasification of this type of waste may be the only
process that is more environmentally friendly
The well-known technologies for utilizing waste
plastics, particularly incineration, produce high levels ofentropy and require a great deal of attention. At present,
there are numerous better options to use and recycle
waste and waste energy. Depending on local conditions,each specific situation requires the selection of the best
technology that takes into account several implications,such as ecological, social, economical, legal andtechnical consequences. The best way to assess andcompare different technologies for waste utilization may
be performed through thermodynamic (that is, exergy)
analysis because many of these implications can betaken into account. The application of exergy analysis toaccount for the amount of raw materials and waste used
to produce energy is effective in screening alternativetechnologies for sustainable development. This allows us
to identify targets for direct or indirect waste recycling
with respect to a single industrial process and for thewaste management system at the national level.

References[1] Scheirs J., Kaminski W., (editors), Feedstock
recycling and pyrolysis of waste plastics: converting
waste plastics into diesel and other fuels, Wiley Series inPolymer Sciences, John Wiley & Sons, Ltd; (2006).
[2] Aguado J., Serrano D. P., Escola M. J, Ind. Eng. Chem.
Res., 47(21), ,2008,.7982-7992.,[3] Wall G., Gong M., On exergy and sustainable
development – Part 1: Conditions and concepts. Exergy,
an International Journal 1(3), 2001, 128–145; Part 2
Indicators and Methods. Exergy, an International Journal
1(4), 2001, 217–233.
[4] Dewulf J.P., Langenhov van H., Quantitave
Assessment of Solid Waste Treatment Systems in theIndustrial Ecology Perspective by Exergy Analysis. Env.
Sci. Technol. 36, 2002, 1130–1135.
[5] Szargut J., Exergy analysis: technical and ecologicalapplications. WIT-press. Southampton, Boston; 2005.
[7] Stelmachowski M., Thermal conversion of waste
polyolefins to the mixture of hydrocarbons in the reactor
with molten metal bed, Energy Conversion andManagement 51, 2010, 2016–2024.

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