Microwave-assisted pyrolysis of HDPE using an activated carbon bed

Abstract

Microwave assisted pyrolysis of high density polyethylene (HDPE) using a reactor bed of catalytic activated carbon produces a condensed liquid product with a carbon chain length profile matching petrol and diesel. Greater cracking was observed across all operating temperatures, and a lighter liquid product with a narrower range of chain lengths was produced compared to the use of a bed of traditional coke.

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Introduction

In recent years, microwave-assisted pyrolysis (MAP) has been the focus of a rapidly increasing body of research,¹ ranging from treating and recycling toxic wastes such as used motor engine oil,^2,3 to the processing of various forms of biomass (e.g. sewage sludge,⁴ wood,⁵ and algae⁶) into high value outputs such as fuels and other chemicals. Conventional (non-microwave) pyrolysis has long been conceptualised as an effective technique for recycling plastics into valuable petrochemical fuels and monomers,^7,8 and some steps have been taken to apply MAP to the same end.⁹ In this process, polymers are thermally decomposed in the absence of oxygen, yielding useful gases and oils, and a residual carbonaceous char. The use of microwave energy as a heat source for pyrolysis provides several advantages over conventional pyrolysis processes, such as fast heating times, easy control of operating parameters, bulk and targeted heating, and lower operating temperatures and energy requirements.¹ Substances that are poor microwave absorbents, such as plastics, may be processed within a bed of highly microwave-absorbent particles, which absorb the incident microwaves and transmit the energy to the embedded target substance via short-range conduction. Carbonaceous materials have high microwave absorbancy, excellent heat tolerance, and low cost, which have made them widely used in this role;¹⁰ traditionally, this has been in the form of amorphous coke or charcoal. However, the stochastic nature of the thermal decomposition process results in condensed hydrocarbon products (waxes/oils) that span such a wide range of molecular weights that they are unable to be used in most fuelling applications without significant and energy intensive further processing and upgrading such as cracking and separation.⁹ One ideal of a condensed product resulting from the MAP of plastic would be liquid at room temperature, and be of similar makeup to petrol and/or diesel, in order that it could be used directly in the extensive existing applications and infrastructure that exist for these fuels. In this work we use an alternative to the traditional coke bed used in the MAP of plastics, namely, a bed of particles of activated carbon—a high surface-area form of carbon with many catalytic applications,¹¹ which is “activated” from standard amorphous carbon through high temperature physical or chemical means. As activated carbon is an excellent microwave absorbent, in this work it serves two primary functions: as a catalyst in the pyrolytic cracking of polymer (the primary focus of this work), and also as the enveloping and energy transferring agent necessary for processing microwave-transparent material. In other MAP applications, activated carbon has shown promise in the pyrolysis of methane¹² and glycerol¹³ with the aim of producing hydrogen, and syngas (H₂ + CO) respectively. This work represents the first example of MAP using an activated carbon catalytic bed for the pyrolysis of polymeric hydrocarbon materials, and particularly its employment in both maximising the yields of condensed liquid product and accomplishing in situ upgrading of said products.

Methods

Microwave assisted pyrolysis

Fig. 1 shows the apparatus used to conduct the MAP. The reactor itself (1) was a cylindrical cast-iron stirred-bed reactor (250 mm tall, diameter 210 mm). The bed of the reactor (2) comprised of either bituminous coke (FC-250 Coke, TIMCAL; particle size <250 μm), or activated carbon (Aquacarb 207EA, Chemviron; particles sized 0.42 mm to 1.68 mm). The reactor was half filled with carbon—sufficiently high to cover the bed stirrer (3) (a paddle extending 80 mm from the bottom of the reactor with blades slanted at 30° from vertical), which operated at ∼8 revs min⁻¹—a speed sufficiently rapid to maintain an even temperature distribution throughout the carbon bed. The bed was radiated with microwaves using a 3 kW 2.45 GHz magnetron (4, CoberMuegge); an isolator (5) diverted any reflected energy to a water load to prevent damage to the magnetron; a three-stub tuner (6) was used to couple the impedance of the reactor to that of the microwave source, maximising energy transfer and minimising reflected energy. The electromagnetic components were hermetically separated from the reactor with a water-cooled, microwave-transparent silicone window (7). The entire apparatus was pervaded with nitrogen gas (8) to purge any oxygen and maintain an inert atmosphere. The reactor temperature was controlled using a computer (9) that read four thermocouples (10) distributed around the reactor and adjusted the magnetron power accordingly. Studies were conducted with reactor temperatures of 400 °C through 600 °C in 50 °C increments. High density polyethylene (HDPE) (LITEN ML 71, Unipetrol) was chosen as a model input material for this research, as an example of a simple polymer on which MAP research has already been conducted.⁹ Once the reactor had equilibrated at the target temperature, 100 g of HDPE pellets were added using a plunger airlock system (11). The resulting pyrolysis gas exited the reactor and passed through a pyrex condenser (12) maintained at −78 °C using dry ice. Non-condensable gases were continuously sampled using a Millipore 7015 peristaltic pump (13) into a 10 L aluminium-lined gas bag (14) for later analysis. In the event of a blockage, any pressure build up could be vented to the extraction system with a pressure release valve. (15).

Fig. 1 MAP apparatus. Numbered components are described in text.

GC-MS analysis

GC-MS analysis was conducted using an Agilent HP 6890 gas chromatograph and HP 5973 mass selective detector, operating with a mass range 1.6–500. Condensed pyrolysis products were analysed with an SGE HT5 30 m × 0.25 mm × 0.1 μm column, with an inlet pressure of 5 psi and a split ratio of 100 : 1. The oven temperature was held at 30 °C for 3 min, then ramped to 350 °C at 8 °C min⁻¹ and held there for 30 min. Pyrolysis gases were analysed with an Agilent GS-Gaspro 60 m × 0.32 mm PLOT column, with an inlet pressure of 5 psi and a split ratio of 20 : 1. Oven temperature was held at 30 °C for 10 min, then ramped to 150 °C at 5 °C min⁻¹. Peaks were identified using the NIST 2005 mass spectral library. The large number of individual compounds present in the pyrolysis products meant that quantitative calibration of the GC-MS analysis was impractical; instead, the percentages given are for the area of each substance's peak as a proportion of the total area of all peaks in the analysis. This method allows a descriptive and quantitative analysis of products that gives a good approximation to mass percentage.

Results/Discussion

Product analysis: condensed oils/waxes

Visual assessment. A dramatic visual difference was obvious in the condensed products from the MAP of HDPE: when using a coke reactor bed, a viscous, waxy product of mixed consistency was collected in the condenser; when an activated carbon bed was used a clear, homogeneous product resulted that was entirely liquid at room temperature.

Mass distribution of products. Fig. 2 shows the size of the compounds in the condensed pyrolysis products produced at 500 °C as measured by carbon number (the number of carbons in each molecule, or Cx where x is the number of carbons). The dramatic difference in the composition of the MAP products when using the activated carbon bed is clearly visible, and similar figures are obtained at other reaction temperatures. The mean, maximum, and spread of the size of the compounds in the pyrolysis oils produced at all reaction temperatures are detailed in Table 1. The pyrolysis oil produced using an activated carbon bed is far lighter, with an average carbon number of 10.0 to 11.5 compared with that produced with a coke bed of 25.0 to 33.0—approximately three times larger. Furthermore, the hydrocarbons produced with an activated carbon bed lie within a much smaller range (C5 to C35 vs. C5 to C55, also reflected by the lower standard deviation). The lighter products over a smaller range of sizes account for the visual differences observed in the products described earlier. The small amount of oil produced at 400 °C using a coke reactor is significantly lighter than that produced at other temperatures. In this case the pyrolysis process was extremely slow—at or near the threshold of it occurring at all, and the process was still not complete even an hour after the HDPE sample was added. The residence time of the pyrolytic products within the hot zone of the reactor has been shown to be an important factor in determining the makeup of condensed MAP products,⁹ and it is this that accounts for the lighter oil produced at this temperature. In practice, the dramatically longer operating time would make working at this temperature unfeasible. It is clear that far more cracking has occurred at the same thermal energy level in the presence of an activated carbon bed. This is particularly evident when the proportion of oil products that fall within the range of liquid transport fuels, petrol (C5–C12¹⁴) and diesel (C8–C21¹⁵) is considered. Indeed, as Table 1 shows, >98% of the oil produced using an activated carbon bed at temperatures of 450 °C and lower falls within this liquid transport fuel range. With the standard coke bed, an increase in the useful C ≤ 21 products is observed as increased reaction temperature (and thus energy expenditure) promotes increased cracking (the 400 °C scenario being a special case). Perhaps most interestingly, as the reaction temperature increases with an activated carbon bed, the proportion of condensed product that lies within the useful and valuable liquid transport fuel range decreases, as hydrocarbons are produced across a wider range of masses/carbon numbers. In other words, activated carbon produces a narrower range of more useful products as less energy is input (with a lower bound of sufficient energy being input for the pyrolysis process to occur in a reasonable amount of time). A full analysis of energy expenditure per unit of product produced is beyond the scope of this communication, though is expected to be rapidly forthcoming. The >98% proportion of pyrolysis oil with C ≤ 21 suggests that it would be very easy to use this process to produce a high value, saleable product from the pyrolysis oil with a minimum of extra processing. Given the breadth of fuels that diesel engines can be run on,^16–19 is should be possible to run a suitably configured engine on the pyrolysis oil directly, or, given further research into meeting the appropriate requisite standards^20,21 it may be possible to directly separate the oil into petrol and diesel for sale on the open market, though these avenues require further research. In either case, it is clear that using an activated carbon bed in the MAP of HDPE produces, with less energy, a much lighter and potentially more useful liquid product than conventional processing.

Fig. 2 Distribution of compounds in condensed pyrolysis oil produced at 500 °C by number of carbons in molecule.

Table 1 Comparison of the condensed products produced with activated carbon and coke reactor beds using MAP. Units for mean, maximum, and standard deviation (SD) are carbon number/chain length, i.e., the number of carbons in each molecule

	Activated Carbon					Coke
a At 400 °C the pyrolysis process was extremely slow, resulting in a prolonged reactor residence time for the HDPE and its resulting products. Pyrolysis at this temperature in batch operation would not be practical or efficient in reality, and these figures should be read in this context. b As liquid hydrocarbons at room temperature, the pyrolysis oils share a common minimum carbon number of five (e.g. pentane for the alkanes).
Pyrolysis Temperature	400 °C^a	450 °C	500 °C	550 °C	600 °C	400 °C^a	450 °C	500 °C	550 °C	600 °C
Mean	10.3	10.5	10.0	10.6	11.5	25.0	33.0	32.7	32.3	29.9
Maximum^b	30	30	32	32	35	55	54	55	55	55
SD	3.7	3.8	4.0	5.2	6.3	14.6	13.5	14.2	14.5	14.5
C ≤ 21	98.1%	98.3%	97.5%	94.0%	90.8%	51.7%	25.5%	28.9%	30.8%	35.7%

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Chemical makeup of condensed pyrolysis products. Fig. 3 compares the composition of the HDPE pyrolysis oil produced using activated carbon and coke beds. With a standard coke bed the oil is comprised of straight chain alkanes and alkenes (and alkadienes, etc), with a small quantity of aromatics (around three percent or less). GCMS analysis of the oil revealed the classic alkene, dialkene, and alkane tri-peaks observed by Ludlow-Palafox in similar work;⁹ these occur with characteristic regularity along the chromatogram with increasing carbon number, and indeed comprise the bulk of the oil. From 400 °C to 500 °C the proportion of alkanes to alkenes is approximately even and constant, though from 500 °C the proportion of alkenes increases with increasing temperature. The increased thermal energy allows for the greater formation of alkenes in spite of their higher enthalpy of formation: each pyrolytic scission of the polyethylene backbone creates two molecules, one of which will be unsaturated due to the lack of the necessary hydrogen atom. Through either increased reaction energy or decreased reactor residence time, the increased reaction temperature changes the balance of products so that a greater absolute number of compounds with one or more double bonds are created.⁹ The catalytic activity of the activated carbon bed is clearly evident in the pyrolysis oil produced from it. While simple straight chain alkanes and alkenes still make up a significant fraction of the oil, there is a dramatic increase in aromatic compounds (35.5–45.3%), and the presence of other types of hydrocarbons are observed (cyclo- and branched alkanes and alkenes, as well as alkynes), increasing to 10% peak area of the oil at 600 °C. The increase in alkenes at the expense of alkanes as the reaction temperature increases is also observed using an activated carbon bed, likely as a result of the reasons outlined above, and it is exactly this reducing environment coupled with a dearth of hydrogen as the HDPE is progressively cracked that promotes the formation of these more “complex” hydrocarbons. Also notable is the variety of compounds produced by the activated carbon bed relative to the coke bed: more than 150 different compounds are present in the activated carbon derived oil (compared with typically around 100 for coke-bed derived oil), and these compounds occur over a smaller range of carbon lengths. Much of this variety derives from the greater quantity of aromatic compounds present in the activated carbon derived oil. These compounds are primarily benzene derivatives, made up of varying and/or multiple alkyl groups bonded to a central benzene ring. The presence of these compounds in such quantities points to the activated carbon catalysing a reaction pathway in which benzene rings are formed- these then combine with the alkyl groups created through more conventional pyrolysis reactions.

Fig. 3 Makeup of condensed pyrolysis products using an a) activated carbon and b) coke reactor bed. “Alkenes” includes all compounds with one or more double bond.

Product analysis: gases

The non-condensable gaseous component of the pyrolysis products (Table 2) comprises of alkanes and alkenes, the bulk of which are two or three carbons in length. By virtue of their small molecular size the compounds are simple—almost all straight chains with trace quantities of branched molecules. This limited spectrum also means that the gases produced with both coke and activated carbon beds are relatively similar. In both cases we see increased cracking (more smaller molecules) as the reaction temperature increases (the increased cracking seen at 400 °C with a coke bed is due to the prolonged reaction/residence time as described above). The activated carbon pyrolysis gases are slightly heavier, with longer chains than the coke ones at the same reactor temperature. In spite of this, methane (the only C1 compound) is not produced in large quantities at any temperature, and hydrogen gas is not produced at all (no ions of the appropriate mass were found in searches of the chromatogram). As the hydrocarbon with the highest hydrogen to carbon ratio, the formation of methane is discouraged in the hydrogen-deficient environment of the reactor. Molecular hydrogen is not present, due to the highly reducing carbon bed, and the fact that hydrogen radicals would quickly bond to the myriad of newly-cracked hydrocarbon radicals present (C=C bond formation is more energetically difficult than C–H). The gases produced during MAP of HDPE are economically important as prime components of LPG/natural gas, or, alternatively they could be put through a generator in order to power the MAP and make the process self-sustaining.

Table 2 Comparison of non-condensable gaseous fraction of pyrolysis products produced using an activated carbon and coke reactor bed and MAP

	Activated Carbon					Coke
a At 400 °C the pyrolysis process was extremely slow, resulting in a prolonged reactor residence time for the HDPE and its resulting products. Pyrolysis at this temperature in batch operation would not be practical or efficient in reality, and these figures should be read in this context.
Pyrolysis Temperature	400 °C^a	450 °C	500 °C	550 °C	600 °C	400 °C^a	450 °C	500 °C	550 °C	600 °C
Carbons:
1	2.8%	2.9%	3.1%	5.3%	6.8%	4.6%	3.6%	6.1%	4.5%	5.8%
2	28.8%	27.2%	30.9%	37.6%	40.5%	46.1%	39.9%	42.3%	45.6%	54.0%
3	55.2%	59.0%	55.9%	49.6%	49.0%	45.1%	49.6%	43.3%	36.4%	31.5%
4	13.3%	9.4%	9.2%	4.8%	3.8%	4.3%	6.9%	8.3%	13.5%	8.6%
5		1.6%	1.0%	2.7%
Of which:
n-Alkane	85.0%	69.7%	67.8%	62.7%	56.9%	57.9%	51.6%	55.5%	62.4%	66.9%
n-Alkene	15.0%	26.3%	28.4%	32.8%	39.6%	40.7%	41.5%	38.8%	33.5%	27.8%
iso-Alkene		2.4%	2.9%	1.8%	1.6%		4.3%	2.5%	2.5%	4.0%
iso-Alkane		1.6%	1.0%	2.6%	1.9%	1.4%	2.6%	3.2%	1.6%	1.2%

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Yields

Table 3 shows the component product yields from the MAP of HDPE. Consistent with other work in the MAP field,^2,9 the operating temperature is the parameter that has the largest single effect on the constituent makeup of the pyrolysis products. As the reaction temperature increases, an increase in the proportion of gaseous products (those cracked the most) is observed accompanied by a corresponding decrease in collected oils/waxes. Approximately similar yields were observed between experiments with activated carbon and coke, though a greater portion of the mass of input HDPE was retained within the reactor (i.e. had not left the reactor by the end of the experiment) when using an activated carbon bed, with a corresponding decrease in the total amount of collected products. As the reaction temperature increases, more cracking occurs: more gaseous products and less condensed products are produced. In spite of the higher percentage of condensable products produced with a coke bed (reaching a maximum of 69.0% of input mass at 450 °C), many of these are less useful long chain waxes; when only products in the liquid transport fuel range (C ≤ 21) are considered, in every case using the activated carbon bed results in significantly higher yields. This is also the case if the same criterion is applied to all products (both gases and condensables). In other words, more useful products are produced using an activated carbon reactor bed than with a coke one, and this is possible at lower temperatures. At low temperatures when using a coke bed, the retained fraction represents un-pyrolysed HDPE. The composition of the fraction retained when using an activated carbon bed requires further investigation, though it may be that the activated carbon catalyses not only hydrocarbon cracking but also the formation of char, which remains within the reactor.

Table 3 Yields of retained, condensed, and non-condensable pyrolysis product. Retained refers to the portion of input mass left in the reactor at the conclusion of the experiment, condensed product is the oil/wax collected in the condenser, condensed C ≤ 21 is the subset of the condensed products with a carbon number less than or equal to 21, non-condensable (gaseous) product accounts for the remaining mass (and is calculated as such). All percentages are expressed on a mass basis as a proportion of the input HDPE

	Activated Carbon					Coke
a At 400 °C the pyrolysis process was extremely slow, resulting in a prolonged reactor residence time for the HDPE and its resulting products. Pyrolysis at this temperature in batch operation would not be practical or efficient in reality, and these figures should be read in this context.
Pyrolysis Temperature	400 °C^a	450 °C	500 °C	550 °C	600 °C	400 °C^a	450 °C	500 °C	550 °C	600 °C
Retained	10.3%	9.0%	6.0%	7.3%	11.3%	2.0%	1.0%	0.0%	0.0%	0.0%
Condensed	54.9%	50.3%	41.0%	30.7%	27.3%	59.1%	69.0%	42.0%	42.3%	31.4%
Condensed C ≤ 21	53.8%	49.5%	40.0%	28.8%	24.8%	30.6%	17.6%	12.1%	13.0%	11.2%
Non-condensable	34.8%	40.7%	53.0%	62.0%	61.3%	38.9%	30.0%	58.0%	57.7%	68.6%

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Catalytic mechanism

While an in-depth study of the catalytic mechanism of activated carbon combined with MAP is beyond the scope of this paper, the subject does deserve some scrutiny. Traditionally the catalytic activity of activated carbon is mediated by chemical reactions occurring at high-energy unsaturated surface functional groups.¹¹ While this mechanism may well explain a portion of the catalytic activity observed, a further microwave-specific mechanism must also be considered. As previous researchers have observed,^22,23 the interaction of the electromagnetic microwave field with particular types of carbon generates “microplasmas”—electrical discharges resulting from the rapidly oscillating electromagnetic microwave field, which create charge imbalances that are restricted by the physical boundaries of the carbon particles. The temperature of these microplasmas is considerably in excess of that of the bulk reactor bed, and hydrocarbon localised in this area will be correspondingly cracked to a greater extent. These microplasmas were observed in abundance during operation with an activated carbon bed, but not at all when using a coke bed, and it is likely that this additional mechanism contributes to the extra cracking observed when using the activated carbon reactor bed.

Conclusions

MAP of HDPE using an activated carbon reactor bed produces a greater yield of useful hydrocarbon products than the equivalent conventional coke bed. Using an activated carbon bed, HDPE undergoes greater cracking, producing a lighter, liquid product, whose constituents have effectively the same range of carbon chain lengths as the transport fuels petrol and diesel—the catalytic reactor bed has, in effect, performed in situ upgrading, resulting in a narrowly targeted pyrolysis product, with a high potential commercial value, across the entire range of operating temperatures examined.

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