Brajendra SHARMA, et al.

Plastic to Oil

See also : ITO : Plastic-to-Oil Conversion *** McNAMARA : Plastic-to-Oil *** ZADGOANKAR : Plastic-to-Oil

Plastic shopping bags make a fine diesel fuel, researchers report

Used plastic shopping bags can be converted into petroleum products that serve a multitude of purposes.


Diana Yates

CHAMPAIGN, Ill. — Plastic shopping bags, an abundant source of litter on land and at sea, can be converted into diesel, natural gas and other useful petroleum products, researchers report.

Brajendra Kumar Sharma, center, a senior research scientist at the Illinois Sustainable Technology Center at the U. of I., with research chemist Dheeptha Murali, left, and process chemist Jennifer Deluhery, converted plastic shopping bags into diesel fuel. Photo by L. Brian Stauffer

The conversion produces significantly more energy than it requires and results in transportation fuels – diesel, for example – that can be blended with existing ultra-low-sulfur diesels and biodiesels. Other products, such as natural gas, naphtha (a solvent), gasoline, waxes and lubricating oils such as engine oil and hydraulic oil also can be obtained from shopping bags.

A report of the new study appears in the journal Fuel Processing Technology.

There are other advantages to the approach, which involves heating the bags in an oxygen-free chamber, a process called pyrolysis, said Brajendra Kumar Sharma, a senior research scientist at the Illinois Sustainable Technology Center who led the research. The ISTC is a division of the Prairie Research Institute at the University of Illinois.

“You can get only 50 to 55 percent fuel from the distillation of petroleum crude oil,” Sharma said. “But since this plastic is made from petroleum in the first place, we can recover almost 80 percent fuel from it through distillation.”

Americans throw away about 100 billion plastic shopping bags each year, according to the Worldwatch Institute. The U.S. Environmental Protection Agency reports that only about 13 percent are recycled. The rest of the bags end up in landfills or escape to the wild, blowing across the landscape and entering waterways.

Plastic bags make up a sizeable portion of the plastic debris in giant ocean garbage patches that are killing wildlife and littering beaches. Plastic bags “have been detected as far north and south as the poles,” the researchers wrote.

“Over a period of time, this material starts breaking into tiny pieces, and is ingested along with plankton by aquatic animals,” Sharma said. Fish, birds, ocean mammals and other creatures have been found with a lot of plastic particles in their guts.

Whole shopping bags also threaten wildlife, Sharma said.

“Turtles, for example, think that the plastic grocery bags are jellyfish and they try to eat them,” he said. Other creatures become entangled in the bags.

Previous studies have used pyrolysis to convert plastic bags into crude oil. Sharma’s team took the research further, however, by fractionating the crude oil into different petroleum products and testing the diesel fractions to see if they complied with national standards for ultra-low-sulfur diesel and biodiesel fuels.

“A mixture of two distillate fractions, providing an equivalent of U.S. diesel #2, met all of the specifications” required of other diesel fuels in use today – after addition of an antioxidant, Sharma said.

“This diesel mixture had an equivalent energy content, a higher cetane number (a measure of the combustion quality of diesel requiring compression ignition) and better lubricity than ultra-low-sulfur diesel,” he said.

The researchers were able to blend up to 30 percent of their plastic-derived diesel into regular diesel, “and found no compatibility problems with biodiesel,” Sharma said.

“It’s perfect,” he said. “We can just use it as a drop-in fuel in the ultra-low-sulfur diesel without the need for any changes.”

The research team also included Bryan Moser, Karl Vermillion and Kenneth Doll, of the USDA National Center for Agricultural Utilization Research, in Peoria, Ill.; and Nandakishore Rajagopalan, of the ISTC at the U. of I.

Excerpts :
Fuel Processing Technology -- Volume 122, June 2014, Pages 79–90

Production, characterization and fuel properties of alternative diesel fuel from pyrolysis of waste plastic grocery bags ?

Brajendra K. Sharma, Bryan R. Moser, Karl E. Vermillion, Kenneth M. Doll, Nandakishore Rajagopalan


Pyrolysis of HDPE waste grocery bags followed by distillation resulted in a liquid hydrocarbon mixture with average structure consisting of saturated aliphatic paraffinic hydrogens (96.8%), aliphatic olefinic hydrogens (2.6%) and aromatic hydrogens (0.6%) that corresponded to the boiling range of conventional petroleum diesel fuel (#1 diesel 190–290 °C and #2 diesel 290–340 °C). Characterization of the liquid hydrocarbon mixture was accomplished with gas chromatography–mass spectroscopy, infrared and nuclear magnetic resonance spectroscopies, size exclusion chromatography, and simulated distillation. No oxygenated species such as carboxylic acids, aldehydes, ethers, ketones, or alcohols were detected. Comparison of the fuel properties to the petrodiesel fuel standards ASTM D975 and EN 590 revealed that the synthetic product was within all specifications after addition of antioxidants with the exception of density (802 kg/m3). Notably, the derived cetane number (73.4) and lubricity (198 µm, 60 °C, ASTM D6890) represented significant enhancements over those of conventional petroleum diesel fuel. Other fuel properties included a kinematic viscosity (40 °C) of 2.96 mm2/s, cloud point of 4.7 °C, flash point of 81.5 °C, and energy content of 46.16 MJ/kg. In summary, liquid hydrocarbons with appropriate boiling range produced from pyrolysis of waste plastic appear suitable as blend components for conventional petroleum diesel fuel.

1. Introduction

Plastic retail bags are ubiquitous in modern society because they represent a convenient means to transport purchased goods from the supermarket to the home. Plastic bags are plentiful, inexpensive to produce, sturdy yet low weight, and easy to store and transport. However, the same properties that make them commercially successful also contribute to their proliferation in the environment. Although they are recyclable, the U.S. EPA noted that only 13% of the approximately one trillion produced in 2009 were recycled in the U.S. [1]. The remainder were disposed of in landfills, released into the environment as litter, or used in secondary applications by end-users eventually ending in landfills. Plastic bags may take centuries to naturally decompose due to their stable chemical composition [2]. In addition to being a source of litter in urbanized areas, plastic bags exacerbate localized flooding by clogging municipal drainage systems and constitute a significant portion of floating anthropogenic marine debris [3], [4] and [5]. In fact, plastic bags contribute to the so-called Great Pacific Garbage Patch of floating refuse in the Pacific Ocean and have been detected as far north and south as the poles [3], [4] and [5]. Once in the environment, plastic bags are lethal to animals that ingest or become entangled in them [6], [7] and [8]. Because of these and other factors, various regional and national governments have banned or are contemplating bans or fees on plastic bags [9].

Standard plastic bags consist of thin polyethylene (PE) sheets produced commercially from polymerization of ethylene. PE is divided into categories based on density and molecular branching frequency. The two types most important to production of plastic bags are low-density PE (LDPE) and high-density PE (HDPE). HDPE is a copolymer with up to 1% 1-butene and is made historically with either Cr or Ziegler catalysts at 1–16 MPa at temperatures as low as 60 °C. More recently, single site catalysts such as metallocenes have been employed [10]. LDPE is produced at high temperatures (200–300 °C) and supercritical ethylene pressures (130–260 MPa) using peroxide-free radical initiators [10]. HDPE is a linear copolymer with a density range of 0.945–0.965 g/cm3 whereas LDPE is branched with densities ranging from 0.915 to 0.925 g/cm3[10]. Due to these differences in structure, the crystalline melting point, softening point and tensile strength of LDPE are considerably lower than the corresponding values for HDPE [10]. However, LDPE shows higher elongation at break and higher impact strength than does the more rigid HDPE [10]. It is also translucent rather than opaque due its lower crystallinity (55%) relative to HDPE (85–95%) [10]. HDPE is more commonly utilized for production of plastic bags due to its greater tensile strength coupled with its less energy-intensive production process. Pyrolysis is defined as the irreversible anaerobic thermochemical decomposition of material at elevated temperature (300 + °C). The principal benefit of pyrolysis is conversion of low energy density substrates into higher density liquid (bio-oil) and solid (biochar) fractions. A low-density volatile (syngas) fraction is also produced. Pyrolysis has been utilized for millennia to produce charcoal and coal. More recently, pyrolysis is used to produce charcoal, activated carbon, coke, carbon fiber, and methanol, among others. The distribution of products (bio-oil, biochar and syngas) is dependent on the type of pyrolysis, reaction conditions and feedstock. Pyrolysis is classified into four categories: slow, fast, flash, and gasification. Of these, fast and flash pyrolysis maximizes bio-oil production, slow pyrolysis augments the yield of biochar and gasification maximizes syngas production. With regard to production of liquid transportation fuels, fast or flash pyrolysis is employed to produce bio-oil [11], [12], [13] and [14]. The properties and composition of bio-oil such as high moisture and heteroatom content, presence of oxygenates such as organic acids, and broad distillation curve prevent its direct use as a transportation fuel; thus upgrading such as hydroprocessing and distillation is necessary [14], [15] and [16].

Fast or flash pyrolysis has been reported on biological materials such as wood [13], triglycerides [17], grasses [18], shrubs [19], corn cobs and stover [20], alfalfa [21], oilseed presscakes [22], and pig compost [23], among others. Examples of fast pyrolysis on non-biological feedstocks include scrap tires [24] and [25], sewage sludge [25], general municipal solid waste [26], waste electrical and electronic equipment [27], and various plastics [25], [28], [29], [30], [31], [32], [33], [34], [35] and [36]. The plastics include polystyrene [30], [31] and [32], poly(vinyl chloride) [30] and [31], polypropylene [31], [32], [33] and [34], PE terephthalate [32], acrylonitrile–butadiene–styrene [32], and PE [30], [31], [32], [35] and [36]. In some cases plastics were co-pyrolyzed with other materials such as waste motor oil [32]. With regard to fast pyrolysis of PE, pyrolysis of LDPE [30], HDPE [35] and [36] and various mixtures [31] and [32] was reported. In all PE studies, the properties of the resulting bio-oils were not reported, nor were the upgrading to fuel-grade hydrocarbons and subsequent fuel property determination.

The objective of our study was the production, characterization and evaluation of alternative diesel fuel from pyrolysis of HDPE waste grocery bags. Comparison of our pyrolyzed polyethylene hydrocarbons (PPEH) with conventional petroleum-derived ultra-low sulfur (< 15 ppm S) diesel (ULSD) fuel was a further objective, along with a comparison to petrodiesel standards such as ASTM D975 and EN 590 (Table 1). Blends of PPEH with ULSD and biodiesel were prepared and the resultant fuel properties measured. It is anticipated that these results will further understanding of the applicability and limitations of HDPE as a feedstock for the production of alternative diesel fuel.

Table 1. Fuel properties of pyrolyzed polyethylene hydrocarbons (PPEH) and ULSD along with a comparison to petrodiesel fuel standards.a

2. Materials and methods

2.1. Materials

Plastic HDPE grocery bags were collected from local retailers and represent the typical ones used in grocery stores. Summer grade ULSD was donated by a major petrochemical company. With the exception of conductivity and corrosion inhibitor additives, ULSD contained no performance-enhancing additives. Soybean oil methyl esters (SME) were donated by a BQ-9000 certified commercial producer. All other chemicals were obtained from Sigma-Aldrich Corp (St. Louis, MO). All materials were used as received.

2.2. Pyrolysis of HDPE to produce plastic crude oil

Thermochemical conversion of plastic grocery bags (HDPE) to oils were conducted using a pyrolysis batch reactor in triplicate. Pyrolysis was performed in a Be-h desktop plastic to oil system (E-N-Ergy, LLC, Mercer Island, WA) containing a 2 L reactor and oil collection system using approximately 500 g of plastic grocery bags each time. The pyrolysis reactor has two heating zones (upper and lower); the upper and lower temperatures were set to 420 and 440 °C, respectively. Once the reactor reached the set temperatures, a reaction time of 2 h was employed from that point on. Vapors produced as a result of pyrolysis were condensed over water as plastic crude oil (PCO). The upper oil layer was separated and weighed. The reactor lid was opened once the temperature was below 50 °C to remove the remaining residual solid material and weighed separately. The mass balance yields were calculated as the ratio of the corresponding product phase (liquid and solid) obtained in 12 batch experiments to the initial feedstock mass. Lastly, the gas-phase yields were calculated based on the resulting mass difference.

2.3. Distillation of plastic crude oil to yield diesel-range hydrocarbons

Distillation of PCO was performed in a Be-h desktop plastic to oil system. A known amount of PCO (1 L) was added to the Be-h reactor vessel. The oil collection tank was cleaned by removing the water and dried before starting distillation. For collecting the gasoline equivalent fraction (< 190 °C), the upper and lower temperatures were set to 175 and 190 °C, respectively. Once the liquid stopped dripping into the collection vessel, the gasoline equivalent fraction was removed, filtered, and weighed to provide yield. The upper zone temperature was then raised to 275 °C and lower zone to 290 °C to collect a #1 diesel equivalent fraction (190–290 °C). The # 2 diesel equivalent fraction (290–340 °C) was then collected by setting the upper zone temperature to 330 °C and lower zone to 340 °C. The material remaining in the reactor vessel was an atmospheric residue equivalent fraction (> 340 °C), which was removed using a siphon pump once the reactor temperature was below 50 °C. All fractions except the atmospheric residue equivalent (> 340 °C) were filtered through Whatman filter paper #4 to remove residual solid particles.

2.4. Chemical characterization of plastic oil fractions

2.4.1. Gas chromatography–mass spectroscopy (GC–MS)

2.4.2. Simulated distillation by GC–FID

2.4.3. Size exclusion chromatography (SEC) analysis

2.4.4. NMR and FT-IR spectroscopy

2.5. Fuel properties

2.6. Preparation of PPEH–petrodiesel blends

3. Results and discussion

3.1. Preparation and chemical composition of pyrolyzed plastics

The pyrolysis temperature range of 420–440 °C was chosen based on previous studies [34]. These temperatures resulted in decomposition reactions of HDPE to provide hydrocarbons of different chain lengths. Pyrolysis of waste plastic grocery bags at temperatures of 420–440 °C provided 74% yield of liquid product referred to as PCO, as shown in Fig. 1. Although not determined in the present paper, literature data suggested gaseous product obtained from pyrolysis of PE consisted primarily of ethane and ethene (C2, 52%) and C4 (32%) compounds [34]. The higher solid residue yield (17%) is likely due to the inorganic content and/or char content and/or unconverted HDPE. As the pyrolysis of PE has higher activation energy (280–320 kJ/mol) compared to polypropylene (190–220 kJ/mol), therefore, increasing the pyrolysis temperature to certain extent could result in increased amounts of the liquid fraction [34]. Also, this residue may have been the fraction boiling above 420 °C (analogous to the higher boiling vacuum gas oil fraction, VGO from petroleum distillation). Further thermal cracking of this product could have been achieved by increasing pyrolysis temperature and/or time, which we speculate would have resulted in higher yields of the desired PCO fraction. This residue along with the VGO fraction from PCO have potential to be used as lubricant basestocks, which upon further refining such as dewaxing/wax isomerization may yield API Group II/III lubricant base oils.

The PCO thus obtained after pyrolysis of waste plastic grocery bags was distilled into four fractions (< 190; 190–290; 290–340; and 340 + °C equivalent of motor gasoline (MG), diesel#1 (PPEH-L), diesel #2 (PPEH-H) and VGO respectively. The product yields are represented in Fig. 2. Similar results were obtained from SimDist analysis of PCOs with maximum coefficient of variation ranging from 0 to 7% (Table 2). In the absence of a catalyst, the major product is PPEH-L (41%). The product distribution can be changed with the use of zeolite catalysts such as ZSM-5, which will increase conversion to more low boiling products, such as MG and PPEH-L [34].

Fig. 2. Distillate fractions yield (%) on distillation.

Table 2. Distillate fraction yields (wt.%) of plastic crude oils (PCO) using simulated distillation (HTGC–FID).

Elemental analysis of waste plastic grocery bags and PCO fractions revealed less than 0.5% nitrogen content and less than 0.7% oxygen content (Table 3). As expected, waste plastic grocery bags have an empirical formula of CH2.1N0.005O0.007 quite similar to that of polyethylene (CH2). Higher carbon and hydrogen content and lower oxygen and nitrogen content resulted in a higher calculated HHV [37], [38] and [39] of 49–50 MJ/kg for most of the fractions, making these high energy liquid fuels (Table 3). The calculated values were slightly higher than the actual determinations (Table 1)...

The boiling point distribution of PCO fractions was obtained using high temperature GC–FID. Table 2 shows that the method developed for boiling point distribution was repeatable with a CV of less than 7% for distribution of various fractions in PCO and was similar to actual distillation data. All PCOs contained a large percentage of fraction 2 (PPEH-L) followed by PPEH-H and MG. Around 98% of the PCO was distilled under 400 °C, which is a good range for producing various fuels such as naphtha, gasoline, aviation fuel, diesel, and fuel oil. The boiling point distribution of PCO and its four fractions is shown in Fig. 3. As boiling point and MW distribution of PCO were similar to petroleum fractions and contained negligible heteroatom content, therefore, we speculate that these PCOs will be compatible with petroleum crude oil for refining in a conventional refinery. The compatibility is further depicted in 10–50% blends of PPEH in ULSD as shown in Fig. 4. As the PPEH content increased in ULSD, the boiling point distribution shifted towards the higher boiling range, although overall the mixture remained within the boiling range of diesel fuel...

Depicted in Table 7 and Table 8 are fuel properties of PPEH-L (Table 7) and PPEH-H (Table 8) blended with ULSD. For each sample, blends of 10 (P10), 20 (P20), 30 (P30), 40 (P40) and 50 (P50) vol.% in ULSD were investigated. With regard to cold flow properties, as the percentage of PPEH-L increased in blends with ULSD, values for CP, CFPP and PP became progressively lower due to the superior low temperature performance of PPEH-L relative to ULSD. In the case of PPEH-H, the opposite trend was elucidated in which cold flow properties (CP and PP) deteriorated as the concentration of PPEH-H in ULSD increased. CFPP of PPEH-H was not measured due to insufficient sample. Similarly, oxidative stability decreased as the percentage of PPEH increased in blends with ULSD. Comparison to the IP specification in EN 590 revealed that only the P10–30 blends of PPEH-H were above the minimum specification of 20 h. Results obtained from measurement of OT corroborated those obtained for IP: deterioration of stability as the concentration of PPEH increased in ULSD as indicated by progressively lower OTs.

Table 7. Fuel properties of pyrolyzed polyethylene hydrocarbons (PPEH-L) blended with ULSD.a

3.4. Influence of blending biodiesel with PPEH

...It was found that diesel obtained from pyrolysis of plastic is as compatible with biodiesel as ULSD due to quite similar hydrocarbon structures and chain length distribution of molecules.

4. Conclusions

Pyrolysis of HDPE waste plastic grocery bags followed by distillation resulted in a major liquid hydrocarbon product (PPEH-L) with average structure consisting primarily of saturated aliphatic paraffinic hydrogens (94.0%) and smaller amounts of aliphatic olefinic hydrogens (5.4%) and aromatic hydrogens (1.0%) that corresponded to the boiling range typical of conventional petroleum diesel fuel (190–290 °C). Negligible heteroatom-containing species were detected from elemental analysis. Also obtained was a heavier boiling fraction (290–340 °C) equivalent of diesel#2 from distillation of the crude pyrolysis product, PPEH-H, which also consisted of paraffinic protons (96.8%), olefinic protons (2.6%) and aromatic protons (0.6%). Based on the results obtained after determination of fuel properties and comparison to petrodiesel standards, the following conclusions were made regarding the applicability of these materials as alternative liquid transportation fuels:

1. PPEH-H is more appropriate as an alternative diesel fuel because it exhibited higher values for FP, IP, KV, DCN, HHV, density, and lubricity than PPEH-L.
2. PPEH-H, after addition of antioxidants, met all ASTM D975 and EN 590 fuel specifications with the exception of density in the case of EN 590.
3. PPEH-L did not meet EN 590 specifications for IP, KV, FP, and density due to its higher content of lower MW constituents.
4. A 1:1 mixture of PPEH-H and PPEH-L met all ASTM D975 and EN 590 specifications with the exception of density and IP in case of EN 590, therefore PPEH-L and PPEH-H distillates can be collected together to provide ~ 64% diesel equivalent fraction from pyrolysis of plastic grocery bags.
5. P10–P30 blends of PPEH-H with ULSD met all ASTM D975 specifications whereas only the P20–P30 blends met all EN 590 limits. P40 and P50 blends require antioxidants to meet the oxidative stability specification listed in EN 590.
6. P10 blend of PPEH-L with ULSD met all ASTM D975 specifications except lubricity, while none of the blends of PPEH-L with ULSD met EN 590 specifications primarily due to poor DCN, IP, FP, and KV.
7. Diesel obtained from pyrolysis of plastic is as compatible with biodiesel as ULSD due to similar hydrocarbon structures and chain length distribution of molecules.
8. Biodiesel blends with PPEH-H met the specifications for lubricity and KV, while PPEH-L blends satisfied the lubricity limits, but not KV limits. PPEH-L improved low temperature properties of SME biodiesel whereas PPEH-H had the opposite effect.

Based on these findings, PPEH-H and a mixture of PPEH-H/PPEH-L are suitable blend components for ULSD in the P10–P50 blend range so long as antioxidants are employed...

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