164
6 Waste Plastics Management and Conversion into Liquid …
cracking or pyrolysis is the degradation of polymeric materials in the absence
of oxygen and at high temperature (350–900 °C) and low-to-high pressure (from
vacuum to up to around 70 bar). The liquid products are usually highly olefinic and
aromatic and need upgrading treatment with hydrogen to improve their properties as
liquid fuels. Application of catalysts in thermal cracking results in catalytic cracking,
facilities the chemical reactions and produces more gasoline [11]. Hydrocracking is
a process commonly used in petroleum refineries and involves the conversion of
high boiling point hydrocarbons in petroleum crude oil into lighter fractions such
as diesel gas oils and kerosene [11]. The process uses hydrogen gas to enhance the
hydrogen-carbon ratio in cracked molecules. Different plastic types such as PVC,
PP, PS, PET and all types of mixed plastics can be used for high-quality gasoline
production through hydrocracking. Catalysts such as Pt, Ni, Mo, Fe and supported
acid solids such as silica, alumina, zeolites and sulfated zirconia are usually used to
improve the cracking and hydrogenation reactions for gasoline production.
6.4.1.1 Pyrolysis
Pyrolysis is a type of thermal cracking, involving thermal degradation of polymeric
materials in the absence of oxygen. The process usually takes place at 350–900
°C and produces liquid and gaseous products and solid residues (chars). The liquid
products are crude oils that need to be refined, by hydrotreatment or blending by
another fuel, into transportation fuels [12, 13].
During pyrolysis, de-polymerization reaction takes place, which is opposite to
the plastics formation through polymerization [12]. Different types of catalysts are
used to improve the pyrolysis efficiency and reducing the process temperature and
reaction time. Choosing a suitable catalyst can also facilitate the selective production
of the desired target products from plastics such as diesels, gasoline or jet fuels. The
most common types of catalysts that are widely used in the pyrolysis of plastics are
zeolites, fluid cracking catalyst (FCC) and silica-alumina catalysts. Zeolites have
the ability to achieve higher yields of aromatic hydrocarbon products and thus are
considered as effective catalysts for the conversion of waste plastics into pyrolysis
oils [14]. In the absence of catalysts, waste plastics are converted more into waxes
or tars instead of liquid fuel products. The waxes can be converted further into gases
and light liquids by higher temperature treatments or catalytic cracking. Several
types of reactors such as tube furnace, batch reactor, fixed and fluidized beds have
been used for pyrolysis of plastics. However, fluidized beds are the most common
reactor types used in laboratory- and pilot-scale experiments [10, 15, 16]. In addition
to the catalysts and reactor types, several other parameters such as composition of
the feedstock, reaction temperature, and type and rate of fluidizing gas, pressure
and residence time can affect the liquid oil production. The process parameters can
be optimized for desired product yields. Temperature was found to be the most
important parameter as it controls the thermal cracking of the polymer chains, and
the thermal degradation of the common plastic types such as PET, HDPE, LDPE, PP
and PS was found to start at 350 °C, while degradation of PVC at a lower degradation
6 Waste Plastics Management and Conversion into Liquid …
cracking or pyrolysis is the degradation of polymeric materials in the absence
of oxygen and at high temperature (350–900 °C) and low-to-high pressure (from
vacuum to up to around 70 bar). The liquid products are usually highly olefinic and
aromatic and need upgrading treatment with hydrogen to improve their properties as
liquid fuels. Application of catalysts in thermal cracking results in catalytic cracking,
facilities the chemical reactions and produces more gasoline [11]. Hydrocracking is
a process commonly used in petroleum refineries and involves the conversion of
high boiling point hydrocarbons in petroleum crude oil into lighter fractions such
as diesel gas oils and kerosene [11]. The process uses hydrogen gas to enhance the
hydrogen-carbon ratio in cracked molecules. Different plastic types such as PVC,
PP, PS, PET and all types of mixed plastics can be used for high-quality gasoline
production through hydrocracking. Catalysts such as Pt, Ni, Mo, Fe and supported
acid solids such as silica, alumina, zeolites and sulfated zirconia are usually used to
improve the cracking and hydrogenation reactions for gasoline production.
6.4.1.1 Pyrolysis
Pyrolysis is a type of thermal cracking, involving thermal degradation of polymeric
materials in the absence of oxygen. The process usually takes place at 350–900
°C and produces liquid and gaseous products and solid residues (chars). The liquid
products are crude oils that need to be refined, by hydrotreatment or blending by
another fuel, into transportation fuels [12, 13].
During pyrolysis, de-polymerization reaction takes place, which is opposite to
the plastics formation through polymerization [12]. Different types of catalysts are
used to improve the pyrolysis efficiency and reducing the process temperature and
reaction time. Choosing a suitable catalyst can also facilitate the selective production
of the desired target products from plastics such as diesels, gasoline or jet fuels. The
most common types of catalysts that are widely used in the pyrolysis of plastics are
zeolites, fluid cracking catalyst (FCC) and silica-alumina catalysts. Zeolites have
the ability to achieve higher yields of aromatic hydrocarbon products and thus are
considered as effective catalysts for the conversion of waste plastics into pyrolysis
oils [14]. In the absence of catalysts, waste plastics are converted more into waxes
or tars instead of liquid fuel products. The waxes can be converted further into gases
and light liquids by higher temperature treatments or catalytic cracking. Several
types of reactors such as tube furnace, batch reactor, fixed and fluidized beds have
been used for pyrolysis of plastics. However, fluidized beds are the most common
reactor types used in laboratory- and pilot-scale experiments [10, 15, 16]. In addition
to the catalysts and reactor types, several other parameters such as composition of
the feedstock, reaction temperature, and type and rate of fluidizing gas, pressure
and residence time can affect the liquid oil production. The process parameters can
be optimized for desired product yields. Temperature was found to be the most
important parameter as it controls the thermal cracking of the polymer chains, and
the thermal degradation of the common plastic types such as PET, HDPE, LDPE, PP
and PS was found to start at 350 °C, while degradation of PVC at a lower degradation