Tertiary polymer recycling: study of polyethylene thermolysis as a first step to synthetic diesel...

Tertiary polymer recycling: study of polyethylene thermolysis as a firststep to synthetic diesel fuel

Norbert Horvat, Flora T. T. Ng*

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

Received 2 February 1998; received in revised form 1 September 1998

Abstract

High density polyethylene (HDPE) thermolysis and polyethylene derived oil hydrogenation were studied as two steps of a novel process toconvert waste polyethylene (PE) into a hydrocarbon liquid suitable for use as diesel fuel. This report is focused on the thermolysis of PE. Theeffects of reaction conditions on reaction rate as well as on the physical and chemical properties of the products were examined. Thermolysisproducts consisted of liquids (PE oil), polymer residue and gas. The liquid product composition was 80–90 wt. % straight chain alkanes and1-alkenes. Semi-batch reactor thermolysis rates were found to be in good agreement with a theoretical model from the literature. The kineticsof formation of light C6–C11, medium C12–C16 and heavy C17–C20 n-alkane and 1-alkene product fractions from HDPE thermolysis weredetermined in the temperature range of 400–440oC. Subsequent hydrogenation of the PE oil resulted in a diesel fuel with a high cetane indexand low sulphur and aromatic content.q 1999 Elsevier Science Ltd. All rights reserved.

Keywords:Polymer recycling; Polyethylene; Thermolysis; Thermogravimetric analysis; Diesel

1. Introduction

Ever since the first large-scale production of syntheticpolymer (plastic) products began in the 1940s, the use andproduction of polymers as well as their variety has increaseddramatically. Accompanying the increased production ofpolymers is the generation of enormous amounts of polymerwastes; for example, in 1989 the United States produced 21billion lbs of polymer waste [1]. Of the polymeric wastesgenerated, approximately 50% are high and low densitypolyethylenes (HDPE and LDPE) [2]. There are now severalmethods used to recycle polymers and accordingly theyhave been classified into the following categories: primary(re-extrusion), secondary (mechanical recycling), tertiary(chemical or thermal recycling) and quaternary (incinera-tion). Recently, much attention has been paid to thermolysisand catalytic polymer degradation techniques as methods ofproducing various fuel fractions from polymer wastes [2,3].In particular, polyethylene (PE) has been targeted as apotential feedstock for fuel (gasoline) producing technolo-gies [4,5]. There is also growing interest in developing valueadded products such as synthetic lubricants [6] via PE ther-mal degradation. The development of value added recyclingtechnologies is highly desirable as it would increase the

economic incentive to recycle polymers. In this work,some fundamental aspects of a novel two-step process forthe conversion of waste PE to high quality diesel fuel orcetane number enhancing fuel additive was developed [7].In the first step PE is heated, under a nitrogen atmosphere, totemperatures in the range 400–4508C. At these tempera-tures, volatile thermolysis products (PE oil) are producedwhich are then condensed and collected. In the second step,the PE oil is hydrogenated at 30–908C to produce a dieseltype liquid fuel. To our knowledge, the hydrogenation of oilderived from the uncatalysed thermolysis of PE with theobjective of producing a diesel fuel has not been reportedin the literature. This paper will focus on the first step of theprocess, thermolysis.

2. Experimental

2.1. Materials

HDPE beads and LDPE powder were obtained fromAldrich Chemical Company. HDPE had average molecularweight 125 000, polydispersity of 1.8 and density of 0.95 g/cm3. LDPE had density of 0.915 g/cm3. Nitrogen (99.999%N2) gas was purchased from Linde. Shredded slices ofwaste, clear, unwashed PE wrapping were also used in the

Fuel 78 (1999) 459–470

0016-2361/99/$ - see front matterq 1999 Elsevier Science Ltd. All rights reserved.PII: S0016-2361(98)00158-6

* Corresponding author. Tel.:11-519-885-1211-X3979; Fax:11-519-746-4979; e-mail: [email protected]

experiments. The hydrogenation catalyst was type T-2464,0.3% Pd on gamma-alumina from United Catalysts.

2.2. Apparatus/method

Fig. 1 shows the thermolysis apparatus. The reactor (1) isa 25 cm long pyrex test tube which was fitted with a dip tube(to introduce nitrogen) and a thermocouple well (2). Poly-mer beads were placed at the bottom inside the tube. Nitro-gen bubbling was relied upon for mixing the polymer meltand the nitrogen was also used to blanket the reactingmixture from oxidizing gases. The reactor was mountedwithin a 550 W electric tube furnace (3). Volatile polymerdegradation products were directed through a heated trans-fer tube (4) into two sets of condenser test tubes (5) placed inseries. Optionally, catalytic material could be inserted in theheated transfer tube (6). The condenser test tubes wereplaced in ice-water baths kept at 2–58C (7). The non-

condensible degradation products were directed out of thecondenser test tubes past a septum sample point (8) andwere vented through a bubble flow meter. An initial chargeof 15 g of PE was used for almost all experiments. Allpieces of the thermolysis equipment were weighed priorto and after reaction to determine the amounts of PE oiland residue (solidified thermolysis products left in the reac-tor) obtained in the reaction, the amount of gases formedwas calculated from the balance.

The bulk of the analysis of the liquid product wasperformed using gas chromatographs (Perkin-Elmer SigmaIIIB and Autosystem GC) equipped with a DB-1 capillarycolumn.1H-NMR (Bruker 200 MHz) was used to determinethe various types of unsaturation within the PE oil. TGA-DTA analysis on a TA model SDT 2960 instrument was alsoused to provide thermolysis kinetics information. Analysisof the hydrogenated product was provided by the ImperialOil Company, Sarnia, Canada.

N. Horvat, F.T.T. Ng / Fuel 78 (1999) 459–470460

Figure 1. Polyethylene thermolysis reactor apparatus: (1) reactor tube; (2) thermowell; (3) tube furnac;, (4) heated transfer tube; (5) condenser test tubes; (6)catalyst; (7) ice baths; and (8) septum for sample measurement.

Figure 2. Effect of reaction temperature on product composition for HDPE thermolysis in a semi-batch reactor (4 h run, N2 flow rate of 35 ml/min (linesincluded to aid visualization). (O) liquid products, (B) polymer residue,(V) gas products.

3. Results and discussion

3.1. Thermolysis of HDPE in a semi-batch reactor: effect ofreaction conditions on product distribution

The two main thermolysis reaction controllable variableswere temperature and carrier gas flowrate. Reaction pres-sure was approximately equal to atmospheric since thesemi-batch reactor effluent was open to the atmosphere onthe downstream end with no major constrictions betweenthe reactor and the open end. Since the semi-batch reactorwas constructed out of pyrex glass and relatively high reac-tion temperatures were used, no attempts were made toelevate or vary the pressure.

Fig. 2 shows the relative amounts of the solid, gas andliquid products obtained from the thermolysis of HPDEafter 4 h reaction time at temperatures between 400 and4608C. The main effect of increasing temperature withinthis range was to increase the rate of formation of liquidproducts (conversion) and to decrease the formation ofsolid residue as the percentage of gases formed was rela-tively constant at,7 mass%. The increased conversion athigher temperatures probably results from the increase in

the rate of the main chain sigma bond cleavage reactions inthe more thermally energetic high temperature environ-ments. Fig. 3 shows the proposed reaction mechanisms forthe production of alkenes and alkanes from the thermolysisof PE. The nature of the products was also affected byreaction temperature. After 4 h reaction at 4008C, only 5mass% each of liquids and gases were produced. Once itwas cooled to room temperature the 90 mass% residue was asolid white wax that had the colour, texture and smell ofcandle wax. The residue that was produced by thermolysisat higher temperatures was brown, more grease-like andcontained a small amount of evenly dispersed very fineblack particles. It is believed that this product is a mixtureof relatively long chain length (non-vaporizable) thermoly-sis products and coke (black particles). At 4608C, the resi-due contained more black particles and the reactor wallswere covered with a layer of black material believed to becoke. Coke formation was likely the result of secondaryreactions of products (coke precursors) formed during theprimary thermolysis process [8]. Dialkenes, alkenes andaromatics are all coke precursors and have been shown tobe present in the gas phase as products during PE thermo-lysis [9]. Proposed mechanisms for formation of cokeprecursors and coke derived from PE have been reportedin the literature [8-10]. Coke formation would also yieldhydrogen which could be used to cap the radicals generatedduring thermolysis.

Gaseous products consisted mainly of C3 and C4 hydro-carbons as shown in Table 1. The composition of thegaseous products did not change appreciably with tempera-ture as was noted by other researchers [11]. C5 and C6

products were also detected in the gas analysis whichcould be attributed to inefficient gas cooling in the conden-sers. The amount of gases produced stayed essentiallyconstant with an increase in reaction temperature. A likely

N. Horvat, F.T.T. Ng / Fuel 78 (1999) 459–470 461

Figure 3. Proposed mechanism for thermolysis of polyethylene.

Table 1Thermolysis of HDPE in a semi-batch reactor. Average composition of gasproducts for reactions at 4008C, 420oC and 4408C and flow rate of N2 of35 ml/min

Component Avg. mass % St. Dev.

C1 2.8 0.6C2 1-alk 1 n-alk 11.7 1.9C3 1-alk 1 n-alk 28.1 4.3C4 1-alk 1 n-alk 23.2 1.7C5 1-alk 1 n-alk 14 2.3C6 1-alk 1 n-alk 9.6 3.3

explanation is that the polymer degrades sequentially [4]from polymer to degraded polymer which decomposes toliquid products and finally to gas products. Once a fractionof the polymer degrades to a product which is liquid (atroom temperature) it becomes vaporizable and can eitherbe removed from the reactor as a vapour by the carriergas or undergo further thermolysis. It is likely that a nearlyconstant fraction (,10 mass%) of this liquid material doesnot leave the reactor before further thermolysis to gaseousproducts.

A clear liquid product was produced from thermolysis at4008C. Liquid products produced at higher thermolysistemperatures were opaque with yellow colour. These liquidproducts remained as liquids above theTm of PE. A typicalGC analysis (Fig. 4) shows the distribution of straight chainalkane and 1-alkene doublet pairs in the chromatogram asreported previously for the thermolysis of HDPE and linearlow density polyethylene (LLDPE) [6,11]. Based on theretention times of some standard 1-alkene andn-alkanehydrocarbons, the liquid products were found to contain

hydrocarbons with chain lengths between 2 (dissolved gascompounds in the liquid) and 20, the average chain length ofthe liquid was between 11 and 15. Subsequent analysis ofthe liquid using a high temperature DB-1 GC column oper-ating at higher temperatures indicated trace amounts ofcompounds with chain length between 21 and 30.

Comparison of the relative abundance of CH3 to CH2

protons using 1H-NMR analysis indicated an averagechain length of 13.4 for a sample produced from the thermo-lysis of HDPE at 4208C. As well, the NMR data indicatedthe presence of three types of alkenes: 1-alkenes (69 mol%), internal alkenes (17 mol %) and 1,1-disubstitutedalkenes (14 mol %). Only trace amounts of aromaticswere detected. Fig. 5 shows the effect of thermolysistemperature on the relative amounts ofn-alkanes, 1-alkenesand linear products (1-alkenes1 n-alkanes). The composi-tions of these three components in the PE oil were of parti-cular interest because of their importance in determining thequality of diesel fuel produced upon subsequent hydrogena-tion. Total amounts of linear components in the liquid


Figure 4. A typical GC chromatogram from analysis of liquid product from HDPE thermolysis showing distribution of alkane/alkene doublets (420oC, 4 h run,semi-batch reactor, 35 ml/min N2).

Figure 5. Effect of reaction temperature on composition of liquid product obtained from HDPE thermolysis in a semi-batch reactor (4 h run, N2 flow rate of35 ml/min, (V) linear (1-alkenes1 n-alkanes), (B) n-alkanes,(O) 1-alkenes).

products were observed to be independent of reactiontemperature in the range 400 to 4608C and were roughly84 mass% of the PE oil. The balance of the compounds(judging from GC retention times) consisted of branchedalkenes/alkanes and internal alkenes. Statistical analysis ofvariance of the data shown in Fig. 5 indicated that theamount of 1-alkenes increased and the amount ofn-alkanesdecreased slightly at higher thermolysis temperatures.These observations fit the trend observed by others [11-13] and may be related to the fact that at higher thermolysistemperatures the more endothermic beta-scission reaction(which produces 1-alkenes) is favoured over the chain trans-fer reaction (which producesn-alkanes) as the method ofradical propagation, see Fig. 3. Another influence may beattributed to the presence of the higher ratio of radicals tohydrogen at higher thermolysis temperatures, which hindershydrogen abstraction required for chain transfer andn-alkane production.

Nitrogen carrier gas flowrate was varied within the range

5–75 ml/min, corresponding to an average superficial reac-tor residence time between 14 and 1 min. The carrier gasflowrate affected the thermolysis within the reactor in twoways: firstly, by influencing the average residence time ofthe vaporized thermolysis products within the reactor andsecondly, by varying the mixing rate of the polymer melt.Fig. 6 shows the relative amounts of solid, gas and liquidproducts after 4 h reaction time at different carrier gas flow-rates. Statistical analysis of the data did not indicate a signif-icant effect of carrier gas flowrate on the amount of liquidproducts formed. However, there was a slight (5 mass%) butdefinite increase in the amounts of gaseous products as wellas a decrease (10 mass%) in the residue product with anincrease in flowrate from 5 to 75 ml/min. An increase inthe amount of gas products at higher carrier gas flow ratesis probably an artefact resulting from our equipment config-uration since higher carrier gas flow rates would be expectedto reduce the amount of gas products due to the higher rateof removal of the condensible liquid precursors for the gas


Figure 6. Effect of carrier gas flowrate on HDPE thermolysis product composition. (4 h run, semi-batch reactor, 4208C, (O) liquid products, (B) polymerresidue, (V) gas products).

Figure 7. Effect of carrier flowrate on HDPE thermolysis liquid product composition. (4 h run, 4208C, semi-batch reactor, (V) linear products (1-alkenes andn-alkanes), (B;) 1-alkenes, (O) n-alkanes).

products from the reactor at higher carrier gas flow rates.However, higher carrier gas flowrates also decreased thecondensing efficiency in the condenser test tubes whichprobably caused more of the products that would havecondensed as liquids at lower carrier gas flowrates to beremoved from the system as effluent by the carrier gas.Fig. 7 shows the effect of carrier gas flowrate on the amountsof 1-alkenes,n-alkanes and linear (1-alkenes1 n-alkenes)products. While the amount ofn-alkanes did not show anysignificant change with variations in carrier gas flowrate,there was a slight (7 mass%) but definite increase in theamounts of 1-alkenes formed as the carrier flowrate wasincreased from 5 to 75 ml/min. The increased productionof 1-alkenes at higher carrier flowrates was thought to bethe result of better heat transfer to the polymer melt causedby more vigorous mixing of the melt at high carrier gasflowrates since the 1-alkene producing beta-scission reac-tion is more endothermic than the chain transfer reactionwhich produces then-alkanes. The amounts of total linearproducts increased at greater flowrates, reflecting theincrease in 1-alkenes at higher carrier gas flowrates.

Reproducibility of the experiments was tested byrepeated thermolysis runs at the same reaction conditions(4208C, 35 ml/min N2). Values for the bulk product (resi-due, oil, gases) amounts were all within a 2.5 mass% range.Standard deviations were 1.2, 1.1 and 0.2 mass% for solids,liquids and gases respectively. With the composition of theliquid products, the respective mass% values were all within2 mass% range with standard deviations being 1.6, 0.8 and1.0 mass% for the linear products, 1-alkenes andn-alkanes,respectively. The low standard deviation values indicategood reproducibility and satisfactory control over reactionconditions using our thermolysis equipment and procedures.

3.2. Thermolysis kinetics of HDPE, LDPE beads and scrapPE sheet based on TGA data

Since waste PE is composed of 30% HDPE and 70%LDPE [2], investigation of the thermolysis characteristics

of these two types of PE were carried out. For comparison,the thermolysis characteristics of a shredded, clear PE wastepackaging sheet was also determined. Data obtained fromthe TGA thermolysis study is compared to the semi-batchreactor thermolysis data.

TGA thermolysis kinetics were obtained by using Eq. (1)as the reaction rate model,

2dmdt� k p mn �1�

wherem represents the mass of the unvolatilized polymer atreaction timet, n is the reaction order with respect tomandkis thenth order rate constant. This model requires that all ofthe products of thermolysis be vapours and thus be non-contributing to the mass of the reactant. Therefore, itwould be expected that the data obtained would not bewell represented by the model during the initial stages ofthermolysis when most of the products are still of highmolecular weight and non-vaporizable. Eq. (1) wasconverted to a linear form by taking the natural logarithmof both sides as shown in Eq. (2). A typical plot of ln(2 dm/dt) versus ln(m) is shown in Fig. 8.

lndmdt

� �� ln k� �1 n p ln m� �: �2�

Periodic ‘‘bumps’’ on the plot in the higher ln(m) regionare caused by the formation and rapid loss of vapour bubblesin the polymer melts. The deviation at low ln(m) valueslikely corresponds to the end of the thermolysis reactionwhen the remaining mass contains a large fraction of unva-porizable carbonaceous material: a thin, dark layer of mate-rial was found in the sample pan after the TGA analysis.This material could not be removed by increasing thermo-lysis temperature or reaction time, it was removed only afterheating the sample pan in air above a Bunsen burner flame.To reduce the effects of these phenomena, the relativelystraight, middle portion of the plots were used to obtainthe n and ln(k) values which were utilized to obtain thekinetics. TGA plots for the three different types of PEs


Figure 8. A kinetic plot for HDPE thermolysis using TGA at 4608C. Range of ln(m) used to estimate linear section is, 21.5 to 1.5.

were obtained at 450, 460 and 4708C in order to obtainactivation energies (Ea) shown in Table 2. Similar valuesfor the reaction orders (n between 0.69 and 0.75) wereobtained for all three PEs. There is no agreement on thevalue ofn in the literature as other researchers have assumedvalues between 0 and 1 to model the thermolysis reaction[11,14]. A recent review [15], has suggested that the reac-tion order is a function of the degree of conversion (at higherconversions, the order is closer to 1). The review also indi-cated that the valid degree of conversion range for modelssuch as Eq. (1) are between 70%–90%. In the TGA workpresented here, the conversion range used in the calculationswas ,90%–99% which may explain why the reactionorders we obtained are not equal to one. In any case, it isexpected that the n values are similar for all three PEs sincethe main thermolysis mechanism is expected to be the same.Differences in the n values were likely the results of experi-mental variations. The fastest thermolysis rates, as indicatedby the rate constants, were observed with LDPE. This resultwas expected since LDPE contains more branch points[15,16]. The tertiary carbon–carbon bonds at the branchpoints are more susceptible to thermolysis than linear C–C sigma bonds in the PE chain. It is expected that thermo-lysis occurs first at these branch points, which would accel-erate the thermolysis of LDPE in comparison to HDPE. Thewaste PE exhibited thermolysis rates that are more similar tothose of HDPE, and it is likely that the waste PE sheet is aform of processed HDPE. The relative rates for the differentPE obtained from TGA thermolysis are in agreement withthe thermolysis data obtained for the three different types ofPE in the semi-batch reactor. The relative rates of thermo-lysis in the semi-batch reactor was estimated based on therelative amounts of gases and liquids formed after a speci-fied reaction time. As shown in Table 3, after 4 h of thermo-lysis reaction time, LDPE produced the most liquid and gasproducts and the results for waste PE were comparable toHDPE. The activation energies, obtained from TGA data,

which ranged from 56.7–66.6 kcal/mol were found to bewithin the range of data reported in the literature by inves-tigators using different thermolysis models [14,15,17].There was no significant difference in the activation energyof LDPE from the other types of PE. This is an indicationthat although the initial polymer thermolysis rate isenhanced in the PE that contains more branch points, theratio of sigma bonds at the branch points to main chainsigma bonds is small and the bulk of the thermolysis occursat main chain sigma bonds. It is likely that the activationenergy is related to the initiation reaction. The slightlyhigher activation energy value of the waste PE sheet maybe the result of the presence of stabilizing (radical suppres-sing) additives.

3.3. Kinetics of HDPE thermolysis using a semi-batchreactor

The thermolysis reaction kinetics as measured by TGAtechniques which use small samples on the order of milli-grams is expected to be different from those determinedfrom a semi-batch reactor due to the refluxing of heavierhydrocarbons within the reactor. It was observed that someheavier products tend to condense at the top of the semi-batch reactor and return back into the polymer melt. Gener-ally, and depending on reaction temperature and carrier gasflowrates, fractions smaller thann-C30 can leave the reactor.To accommodate this phenomena, several reactor thermo-lysis kinetic models are available in the literature [18,19].The general assumption used in these models is that poly-mer thermolysis occurs via random chain scission and thepolymer molecules degrade in the reactor until they reach aminimum chain length for evaporation. The model used inthis work [18] is given in Eq. (3).

W�t� �W0 a 1 1� �exp ks p t

ÿ �2 a p

P0 2 a 2 1P0 2 a

� �P0 2 a� �

P0 p exp a 1 1� � p ks p tÿ �

�3�

whereW(t) is the mass of the polymer in the reactor at timet, W0 is the initial mass of the polymer,P0 is the initialdegree of polymerization,ks and ‘‘a’’ represent the (zeroorder) rate constant for scission and the degree of polymer-ization of the largest molecule that can be vaporized out ofthe reactor respectively. The model in Eq. (3) has been usedrecently to model reactor data with moderate success [6].Differences between observed and model predicted datavalues were reported [6], especially as the reactionprogressed, which were likely caused by the fact thatW(t)was determined indirectly by measuring only the mass ofthe condensible (liquid) fraction. The method employed didnot take into account the mass of the non-condensed ther-molysis degradation product which becomes a larger frac-tion of the total products with increase in thermolysisreaction time. The difficulty described above is avoided in


Table 2Comparison of TGA thermolysis of different polyethylene types

HDPE LDPE WASTE PE

k (min21), 450oC 0.314 0.406 0.291k (min21), 460oC 0.554 0.720 0.579k (min21), 470oC 0.910 1.259 1.012Ea, kcal/mol 56.7 60.3 66.6n 0.75 0.71 0.69

Table 3Product distribution from the semi-batch reactor thermolysis of HDPE,LDPE and waste PE sheet (420oC, 35 ml/min N2 flow)

HDPE LDPE Waste PE

Liquid (mass%) 53.5 60.2 55.5Gas (mass%) 9.1 12.7 5.3Residue (mass%) 37.3 27.1 39.2

our current work by fitting the model in Eq. (3) to directlymeasured polymer residueW(t) mass data obtained byweighing the mass of the reactor prior to and after eachsemi-batch run. Each semi-batch reaction was performedto a certain length of time after which it was stopped byremoving the reactor from the furnace and rapidly quench-ing with air. Fig. 9 shows that the model accurately repre-sents the experimentalW(t) data at 4208C. The model alsoaccurately represents the experimental data obtained at4108C and 4408C. Time zero was taken as the time afterwhich the reaction melt reached the specified reactiontemperature. Since thermolysis occurred during the heat-up time as well,P0 was taken to be a fitting parameterwhose value was expected to be significantly less than5000 which is the approximate degree of polymerizationof the undegraded polymer. The rate constant for scission,ks, was also a fitting parameter. The constant ‘‘a’’ wasassumed to be equal to 30, which was the value used byothers [6] and which was not expected to be any different inthis work. In all cases, the initial charge of polymer in thereactor,W0, was 15 g. Although the thermolysis started priorto the time at which the reactor reached the reactiontemperature, no measurable signs of weight reduction wasobserved until roughly 30 min after the melt reached thespecified reaction temperature. Thus, no adjustment had tobe made for lowering of the initial mass of the polymerduring the heat-up time. Table 4 shows the estimated para-metersP0 andks for thermolysis at 410, 420 and 4408C. Asexpected, the initial degree of polymerization was lower athigher reaction temperatures, an effect of increased thermaldegradation which occurs during longer heat-up times. This

initial degree of polymerization also gives an estimate of thedegree of polymerization of the non-volatile, white, waxysolid which is the product of the initial thermolysis reaction.These values were found to be between 80 and 225 forsolids produced from reactions at 410 to 4408C. Giventhat P0 values are calculated after 30 min of thermolysis,such values are reasonable when compared to LLDPE [6]which show degree of polymerization values of 30 for ther-molysis at 4258C after 80 min. The rate constant of scissionapproximately doubled for every 108C rise in temperatureand is within an order of magnitude to that reported forLLDPE where ks � 0.0018 min21 at 4258C [6]. Rateconstant values from Table 4 were used to estimate theactivation energy of scission (67.4 kcal/mol) which iswithin the range reported by others [15]. In Fig. 9 the datapoint which corresponds to the region of maximum rate ofweight loss occurs at roughly 110 min. Fig. 10 shows a plotof 2dW(t)/dt as a function of time and at 110 min, theweight loss is 26.3%. This value corresponds to the theore-tical decomposition point, 26.4%, at which a maximum rateof weight loss occurs for a purely random scission reaction[18].

3.4. Kinetics of formation of various product fractions fromHDPE thermolysis in semi-batch reactor

In order to determine the kinetics of formation of variousliquid product fractions in the PE oil, the identifiablelinear components were arbitrarily divided up into light,medium and heavy fractions based on carbon number.These fractions consist ofn-alkanes and 1-alkenes and


Figure 9. A plot of the residue mass as a function of time for the thermolysis of HDPE in a semi-batch reactor at 4208C and a N2 flow rate of 35 ml/min. ((V)and model (2 )).

Table 4Estimated parameters for the kinetic model (Eq. (3)) for thermolysis ofHDPE in a semi-batch reactor and N2 flow rate of 35 ml/min

Temperature P0 ks, min21

410oC 225 1.24× 1024

4208C 187 2.49× 1024

440oC 80 1.00× 1023

Table 5Liquid product fractions in PE oil

n-Alkanes 1-Alkenes

Light C6–C11 C6–C11

Medium C12–C16 C12–C16

Heavy C17–C20 C17–C20

account for more than 80 mass% of the liquid products.The fractions are listed in Table 5. Although the cut-off points for the fractions were chosen arbitrarily, itmay be noted that the medium and heavy fractions are

the ones suitable for the production of diesel fuel uponhydrogenation.

Following literature examples of kinetic analysison groups of molecular compounds [20], a simple,


Figure 10. A plot of dW(t)/dt as a function of time obtained from numerical differentiation of Eq. (3) on the thermolysis data for the thermolysis of HDPE in asemi-batch reactor at 4208C and 35 ml/min N2 flow.

Figure 11. Logarithmic model for formation of 1-alkene C6–C11 fraction during HDPE thermolysis at 4208C and 35 ml/min N2 flow. (Initial HDPE charge�15 g. Data (B;), model (2 ).

first-order model was used to estimate the rate constantsof formation of the various fractions:

dmi

dt� ki p mp �4�

wheremi andki are the relative mass of theith fraction attime t and the rate constant of formation of theith fractionrespectively andmp is the mass of the polymer residue leftin the reactor at timet. The model assumes that the rate of

formation of each fraction is directly proportional to themass of the degraded polymer residue left in the reactor.mi(t) values were obtained experimentally for each of thefractions; each thermolysis experiment was stopped byrapid air quenching after a predetermined length of reac-tion time and the weighed products were analysed via GC.Themi(t) values are the products of the mass of the PE oilproduced multiplied by the weight fraction determinedfrom GC analysis. A typical plot of the changes ofmi(t)


Figure 12. A plot of kinetic model according to Eq. (4) for the observed data for 1-alkene C6–C11 product fraction obtained from the thermolysis of HDPE at4208C and 35 ml/min N2 flow. Initial HDPE mass� 15 g. {Data (B), model (2 ) Data points 1, 2, 3, 4, and 5 corresponds to reaction times of 1, 2, 4, 6 and 8 h,respectively.}

Table 6Thermolysis rate constants (ki) and activation energies (Ea) for different alkene and alkane fractions for the thermolysis of HDPE in a Semi-batch reactor and aN2 flow rate of 35 ml/min

1-Alkene n-Alkane

C6–C11 C12–C16 C17–C20 C6–C11 C12–C16 C17–C20

kI, 4108C (h21) 0.021 0.024 0.009 0.022 0.026 0.020kI, 4208C (h21) 0.034 0.036 0.018 0.035 0.040 0.038kI, 4408C (h21) 0.144 0.076 0.038 0.134 0.073 0.034Ea (kJ/mol) 265 157 183 247 138 57{st dev, %} {10} {2} {12} {10} {10} {120}

for the8C6– C11) fraction of the product with time isshown in Fig. 11.mp was measured by weighing thesolid residue product in the reactor after the run wascompleted. A differentiable, logarithmic function wasfitted to themi(t) data and upon differentiation provideddmi/dt values. A plot of these values versusmp yielded astraight line with slopeki (Fig. 12). The first data pointobtained from the run was an outlier on all such plots.This discrepancy is likely due to the initial induction timeduring which the thermolysis products are not volatile.Once the thermolysis proceeds to the point where theproducts do become volatile, a burst of volatile productsare removed from the reactor within a short length oftime, causing a relatively high value of dmi/dt for thecorrespondingmp. This effect disappears once the degra-dation reaches a steady state at which time the linearmodel describes the observations quite well. Theki valueswere determined at 410, 420 and 4408C and activationenergies were calculated (Table 6) and the standard devia-tions values ranged from 0.001 h21 to 0.0057 h21 for ki

and 2% to 120% forEa. The activation energy of the C17–C20 n-alkane fraction had an unusually high standarddeviation (120%) and was thus deemed unreliable. Inac-curacies in the gas chromatography analysis for relativelyhigh molecular weight products were the probable causesfor the uncertainty in the concentration of the heavyn-alkane fraction. The maximum GC injector temperatureof 2608C was insufficient for the consistent vaporizationof the heavier fractions thus the concentration values forthe heavier products were relatively uncertain. Althoughthe apparent activation energies reported in this work donot correspond to a specific step in the reaction kinetics,the relative values could be used to indicate the ‘‘ease’’ offormation of the various liquid product fractions. Gener-ally, 1-alkenes have slightly higher apparent activationenergies of formation than n-alkanes. This is not surpris-ing since the mechanism of formation of 1-alkene mole-cules requires an endothermicb-scission step, which is

not required in the formation mechanism ofn-alkanes(Fig. 3). It was expected that among the various productfractions in each of the 1-alkenes andn-alkanes, higheractivation energies would be required to produce lowermolecular weight fractions because on the average it takesmore degradative steps to form the lighter products. Table6 shows that the activation energies of then-alkanesfollowed the anticipated pattern. The light C6–C11 and middleC12–C16 1-alkene fractions also adhered to the expectedpattern. The apparent activation energy of the heavy 1-alkenefraction is high and is likely influenced by experimental error.As indicated by the rate constants, the rates of formation of thelightand mediumn-alkane fractionswere relativelyclose to thecorresponding 1-alkene fractions at all three thermolysistemperatures. In the heavy fractions, the rates of formation ofn-alkanes are higher than the corresponding 1-alkenes. Thisphenomenon was also apparent in the GC concentration tracesof the PE oil product (Fig. 4) which showed that while theconcentrations of 1-alkenes andn-alkanes were nearly equalin the light and medium fractions, the concentration of 1-alkenes in the heavy fractions were significantly lower thanthe correspondingn-alkanes.

3.5. Analysis and comparison of hydrogenated PE oil toconventional diesel fuel

The PE oil produced form HDPE thermolysis in a semi-batch reactor at 420–4408C was collected for hydrogenation.Using a 300 ml Parr stirred batch autoclave the PE oil mixturewas hydrogenated at 708C and 600 Psig H2 for over 200 minusing United Catalysts T-2464, 0.3% Pd on gamma-aluminacatalyst. Over 96% conversion of 1-alkenes was observed withgreater than 110% increase in the concentration ofn-alkanes.The PE oil changed gradually from a yellow solution to a clearsolution as the reaction proceeded. Significant double bondshift reaction was also observed with the T-2464 catalyst.Other details of the hydrogenation experiments are given else-where [7] and are not within the scope of this article. Table 7shows the analysis results and comparison of hydrogenated PEoil ‘‘synthetic diesel’’ to conventional diesel fuel. The resultsare very promising. However, refinement of the process is stillnecessary. Synthetic diesel fuel has 41% higher cetane index,50 % more saturates, 1000 times less sulphur, about 14 timesless in aromatic content and lower viscosity than conventionaldiesel fuel. However, due to the high cloud and pour points, thesynthetic diesel could only be used as a motor fuel at tempera-tures above 108C (as a summer diesel fuel). Work is in progressto reduce the cloud and pour points of the synthetic diesel fuel.

4. Conclusions

PE oil can be produced via thermolysis of PE at 420–4608C under an inert, nitrogen atmosphere. The thermolysischaracteristics of clean, unprocessed HDPE and LDPE areessentially the same as unwashed, scrap PE packaging. Itwas also determined that the thermolysis may be performed


Table 7Comparison of conventional diesel fuel to synthetic diesel fuel producedvia hydrogenation of liquid thermolysis products from the thermolysis ofHDPE. N/A � data not available

Synthetic diesel (vol %) 100 5 0Conventional diesel (vol %) 0 95 100

Density at 158C 0.779 0.8515 0.8552Flash point,8C ,22 60 72Sulphur, wppm 3 2700 3000Cloud point,8C 10 210 221Pour point,8C 0 227 230Aniline point,8C 82.6 58.2 56.6Viscosity at 408C, cSt 1.85 2.31 2.34Saturates, wt. % 97.5 N/A 65.6Aromatics, wt. % 2.5 N/A 34.4Crystallizable wax, wt. % 58.16 N/A 10.57Cetane index 58.2 42.2 41.2Cetane number N/A N/A 40.1

at higher reaction temperatures in the range 410–4408Cwith only a small increase in the rate of formation of 1-alkenes overn-alkanes. Synthetic diesel fuel produced byhydrogenation of PE oil has greatly enhanced propertiescompared to conventional diesel fuel, but the processrequires refinement in terms of producing diesel fuel withlower cloud and pour points.

Acknowledgements

The authors would like to thank the Natural Sciences andEngineering Research Council (NSERC-Canada) for aresearch grant as well as the Ontario Ministry of Educationand Training for providing an Ontario Graduate Scholarshipfor N. Horvat. We would also like to thank the assistance ofDr. M. Poirier of the Imperial Oil Research Centre in Sarnia,Ontario, Canada for providing analysis of the syntheticdiesel fuel.

References

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