Conversion of Low Density Polyethylene (LDPE) and Polypropylene (PP) Waste Plastics into Liquid Fuel Using Thermal Cracking Process

In every sector of the world today energy is essential. Energy has many forms such as electricity, transportation fuel and so on. A large amount of energy is produced from crude oil, which is used to produce petroleum and petroleum to produce daily usable plastics. The solution to the above mentioned problems can be solved through the utilization of the new develop technology. This new developed technology will remove these hazardous waste plastics from the environment and convert them into eco friendly liquid fuel. The process is used to convert these waste plastics into liquid fuel creates no harmful emissions and can be produced at a very little overall cost. The thermal process utilized to break down the hydrocarbon chains of the polymers and convert them into liquid fuel. A Steel reactor with temperature range from 100 oC to 400 oC is utilized for the plastic thermal degradation process. The process yield about 80-90% liquid product. The experiment is conducted under a fume hood and open air system, no vacuum process is applied in this particular thermal cracking process.


INTRODUCTION
In recent years the production and consumption of plastics have increased drastically; as a consequence the responsible disposal of plastic wastes has created serious social and environmental arguments. At present both landfilling and incineration of plastic wastes are widely practiced. In Japan, the percentage of municipal plastic wastes, as a fraction of municipal solid waste (MSW), that was landfilled in the early 1980s was estimated to be 45%, incineration was 50%, and the other 5% was subjected to separation and recycling (Plastic Waste Management Institute, 1985). In the USA, more than 15% of the total MSW was incinerated in 1990; only about 1% of post-consumer plastics were recycled (Yakowitz, 1990;Curlee and Das, 1991;Andrews and Subramanian, 1992). Landfilling of plastic wastes is expected to decrease in the future as landfill space is depleted and plastic wastes are resistant to environmental degradation. Co-incineration of plastic wastes with other municipal solid wastes may be increasingly practiced, because the high caloric value of plastics can enhance the heating value of MSW and facilitate an efficient incineration, while their energy content can also be recovered. But the potential relationship between plastics fed into an incinerator and the formation of some highly toxic pollutants such as dioxins and furans is still unclear.
It has been suggested that the chlorine content in PVC and other plastics is related to the formation of dioxins and furans, which are chlorinated polynuclear aromatic compounds. And although there is considerable evidence that these pollutants would still be generated in the absence of plastics, environmental pressures against incineration have never completely disappeared. Various techniques for the treatment of waste polymers have been investigated to complement existing landfill and mechanical recycling technologies. Ideally, it would be desirable to convert the waste into a value-added product. In the case of polyethylene waste, a particularly interesting potential product would be synthetic lubricants. Furthermore, they are reported to be less damaging to the environment, because they do not contain aromatic compounds. In contrast, conventional lubricants have some aromatic compounds which can be released into the environment when the lubricant is utilized in a two-stroke engine. The most suitable decomposition products from polyethylene for the production of light gases and hydrocarbon liquid products are compounds close to 1-decene in both chain length and molecular structure. Several thermo chemical techniques have been employed to convert waste PP (Polypropylene) and LDPE (Low density polyethylene) into value-added products. Thermal cracking of plastic waste has been studied at temperature from 510 -520°C to produce light liquid hydrocarbon products (Andras, 2007). Some researchers investigated polyethylene thermolysis, usually at temperature 250°C to 450°C products light gases including liquid hydrocarbon products (WCM et al., 1994). Others investigated continuous processes in which polyethylene (ldpe) was passed through a fixed or fluidized bed of catalyst at elevated temperatures. The reaction products consisted mainly of low-molecular-weight waxes, with some liquid. The use of catalysts increased both liquid production and the formation of aromatic compounds, when compared to the products from non-catalyzed degradation. A study was conducted in utilizing plastic wastes into light hydrocarbon fuel at low temperature with and without the use of catalyst (Sajid et al., 2010) Studies on the degradation of addition polymers and the composition of the products have suggested several possible reaction mechanisms. As a result, several mathematical models have been proposed to describe the degradation of addition polymers. For a purely random process, the theoretical maximum rate of weight loss has been predicted to occur at 26.4% decomposition. The process carried out in this particular experiment uses thermal degradation with temperature ranging from 100 -400°C in a vertical stainless steel reactor.
The Results obtained from the experiment indicated that the liquid product obtained is all light and heavy hydrocarbon compounds.

EXPERIMENTAL SECTION
Thermal cracking process without catalyst was used in converting waste plastic into liquid fuel. Two types of waste plastic are selected for this particular experiment. By weight 50% of each Low density polyethylene and polypropylene was selected for the experiment. Both waste plastic are solid hard form. Collected waste plastic was cleaned using liquid soap and water. During waste plastics are cleaned is cerates waste water. This waste water is purified for reuse using waste water treatment process. Washed waste plastics are cut into 3-5 mm size to fit into the reactor conservatively. For experimental purpose we used 600 gm sample 300 gm of PP and 300 gm of LDPE. A vertical steel reactor used for thermal cracking and temperature used ranges from 100° C to 400° C (see figure 1). The experiment is carried out under a Labconco fume hood in open air system with no vacuum process applied during this thermal cracking process. We used low density and polypropylene plastics in a batch process system because conversion temperatures for these plastics are relatively low. Heat is applied from 100° C at start to begin melting the waste plastics, the melted waste plastic turn into liquid slurry form when temperature is increased gradually. When temperature is increased to 270° C liquid slurry turns into vapor and the vapor then passes through a condenser unit. At the end we collect liquid fuel. Between 100º C and 250º C around 20 -30% of the fuel is collected and then when raised to 325º C the next 40% is collected and finally when held at 400º C the yield is fully completed. During the thermal cracking process plastic portions are not broken down immediately because plastics have short chain hydrocarbon to long chain hydrocarbon. 1 st stage of heat applied breaks down only the short chain hydrocarbon. When temperature profile is increased the plastic carbon-carbon bond breakdown slowly. As the temperature is increased the long chains are breakdown step by step. During in this thermal cracking process some light gas such as methane, ethane, propane and butane are produced. These compounds are not able to condense because they have negative boiling point. These light gases could be alkane or alkene group and it can also contain CO or CO 2 emissions. Light gas production percentage is about 6%. This gas portion analysis is under consideration. The method which is considered for treating the light gas is an alkali wash system (see figure 1). After experiment is concluded some solid black residue is collected from the reactor. This solid black residue percentage is about 4%. Liquid fuel yield percentage is 90%. To purify the liquid fuel a purification system to remove water portion and ash or fuel sediment is used. Liquid fuel density is 0.75 g. /ml

FUEL ANALYSIS METHODS
Perkin Elmer Differential Scanning calorimeter equipment is use for boiling point measurements of produced fuel. Nitrogen gas we used for carrier. 20 ml gas used for per minutes. Temperature profile setup initial program temperature is 5º C and end temperature 400º C. Initial temperature to final temperatures increase rate 5º C per minutes. 50 µL aluminum pan used for sample holding.

Fig. 1. Process diagram of the waste plastic conversion
Gas chromatography and mass spectrometer (Perkin Elmer) used for fuel analysis. For GC/MS analysis capillary column use and column dimension is 30 meter length, 0.25 mmID, 0.5 um df, maximum temperature capacity is 350ºC. Helium used for carrier gas at 80 psi. GC program setup for sample run initial temperature is 40ºC, 1 minute hold for 40ºC and final temperature setup 325ºC and temperature ramping rate is 10ºC per minutes. Final temperature holds for 15 minutes. MS program set up for mass scan 35.00 to 528.00 EI+. Data format centroid, scan time 0.25sec, inters scan time 0.15 sec.

RESULTS AND DISCUSSION
Analysis of the liquid produces fuel is conducted using a Differential Scanning Calorimeter (DSC) (graph fig.2) to measure the boiling point. The onset temperature is 101.57ºC. 123.99ºC is the peak boiling point of the produced fuel, which means that at 123.99ºC the fuel has the highest concentration of compounds. The Heat flow Endo up is 29.0887 mW and enthalpy for fuel delta H is 14448.0931 J/g.
From FT-IR Spectrum-100 analysis of low density polyethylene (LDPE) and polypropylene (PP) fuel (shown fig.3  -20 J and ultimately wave number 884.03 cm -1 (C=CH 2 ) energy, E=1.34x10 -20 J. These results provide the functional group band energy of the fuel compounds, thus, providing us with the calorific value of the fuel.

Fig. 4. GC/MS chromatogram of produced fuel
Alcoholic group are formed due to the experiment being conducted in a non vacuumed fume hood. Oxygen is absorbed from moisture during condensation time has manipulated the compound of the produced fuel but results obtain from the fuel is obsolete. This fuel has short hydrocarbon compound to long chain hydrocarbon compound with different retention time and all compound are shown in this chromatogram as compound boiling point wise. From this analysis we found long hydrocarbon compound is Heptacosane (C 27 H 56 ) at retention time 26.07 minutes. This compound is a straight chain carbon and a hydrogen single bonding compound with molecular weight 380. This fuel has only aliphatic compound such as alkane group and alkene group compound. We used only low density polyethylene and polypropylene waste plastic, these 2 types waste plastic have carbon and hydrogen straight combination chain and polypropylene has also methyl group compounds. GC/MS analysis did not indicate any aromatic compounds. GC/MS analysis shows that short chain hydrocarbon C 3 and long chain hydrocarbon C 27 is present in this fuel.

CONCLUSION
The products of thermal cracking are mainly paraffin's, olefins as well as hydrocarbon products (carbon number 3-27). Thermolysis is a viable alternative to high temperature degradation for the recovery of products from waste products. The thermolysis of PP & LDPE at moderate temperatures, below 400°C produces a high yield of liquid products. It is important that the liquid products from thermolysis consist mainly of a mixture of straight chain alkanes and alkenes having an average chain length in the range of C 3 -C 27 carbons. The liquids were not contaminated with aromatic compounds. Thus, the products obtained from thermolysis are potentially useful as a feedstock for the production of synthetic lubricants, requiring fewer purification steps than liquids obtained from degradation processes. The product is a high grade liquid fuel that is classified as an alternative source of energy. In the future the demand for alternative energy source will increase, so in the renewable sector this fuel may play an important role.