Journal of Supercritical Fluids 13 (1998) 247–252

Polyethylene conversion in supercritical water

Masaru Watanabe, Hideyuki Hirakoso, Shuhei Sawamoto, Tadafumi Adschiri,Kunio Arai *

Department of Chemical Engineering, Tohoku University, Sendai 980-8579, Japan

Received 19 July 1997; received in revised form 5 November 1997; accepted 28 November 1997

Abstract

This paper describes pyrolysis of polyethylene (PE) and n-hexadecane (nC16) in supercritical water (SCW ). Batchreactions were conducted at temperatures ranging from 673 to 723 K, at a reaction time of 30 min, and water densitybetween 0 and 0.42 g/cm3. The pyrolysis rate of nC16 in SCW was almost the same as that in 0.1 MPa argon (Ar)atmosphere. Product distribution was also more or less the same for both cases. PE pyrolysis results were clearlydifferent from pyrolysis results in Ar. In SCW, higher yields of shorter chain hydrocarbons, higher 1-alkene/n-alkaneratio, and higher conversion were obtained. This difference of PE pyrolysis in SCW and in Ar could be explained byconsidering the difference in the reaction phase. © 1998 Elsevier Science B.V.

Keywords: n-Hexadecane; Hydrocarbon; Polyethylene; Pyrolysis; Supercritical water

1. Introduction method is not available. For these plastics, thermalcracking is a well known technique to recover oilsand fluidized bed pyrolysis has been under develop-A vast amount of waste plastics is now a signifi-ment [3,4]. Control of product distribution for thecant environmental problem. Thus various treat-oils to be used widely is considered to be anment or decomposition processes for waste plasticsimportant technical requirement.are now under development. In recent years, chem-

The use of a solvent during pyrolysis may beical recycling has been gaining greater attention,one idea for controlling pyrolysis product distribu-since the recovery of valuable products from wastestion, as is reported for the heavy oil cracking.is supposed to lead to the simultaneous solutionSince the reaction temperature is above 673 K inof energy, resources, CO2, and waste problems.most cases, organic solvents will be a supercriticalHydrolysis of polycondensation polymers, suchfluid. Some basic studies for the effect of supercriti-as polyethylene terephthalate (PET), polyure-cal solvent on pyrolysis of heavy oil or its modelthane, nylon etc., in supercritical water is a promis-compounds have been reported [5,6 ].ing method to recover its monomers [1,2].

In general, the use of SCF as a reaction solventHowever, for the polymers of addition polymeriza-changes reaction rate, equilibrium, and main reac-tion, including polyethylene or polypropylene, thistion pathway, specifically around a critical point,because of the significant variation of solvent* Corresponding author. Tel: +81 22 217 7245;

e-mail: karai@arai.scw.che.tohoku.ac.jp properties. We focused on supercritical water

0896-8446/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved.PII S0896-8446 ( 98 ) 00058-8

248 M. Watanabe et al. / Journal of Supercritical Fluids 13 (1998) 247–252

(SCW ) as a solvent, which can dissolve hydro- was analyzed by GC-FID (Hewlett Packard,model 5890 series II ) and GC-MS (Japan Electroncarbons [7] and whose dielectric constant is widely

variable, and started research for the development Optics Laboratory, model Automass 20).Conversion of nC16 (X

nC16) was evaluated asof a new process of conversion of waste plasticsin SCW, expecting the control of productdistribution. X

nC16=A1−amount of nC16 recovered

amount of nC16 loaded BIn this study, the experiments of nC16 andpolyethylene (PE) pyrolysis were conducted in ×100[mol%] (1)SCW and in argon (Ar). The effects of SCW on

The first order rate constant (k) was evaluatedthe pyrolysis are discussed through the comparisonbyof the product distribution and of the rate

constant.k=−

ln(1−XnC16/100)

t(2)

2. Experimental where t is the reaction time of 30 min. Molarselectivity was evaluated as

In this study, n-hexadecane (nC16) of 99.5%purity was purchased from Wako Chemicals and

Molar selectivity=moles of products

total mole of productslow density polyethylene (mean molecular weightof 68 000) was from Aldrich. Experiments were

×100[%] (3)conducted with a stainless steel (SUS 316) tubebomb reactor (6 cm3) with a high pressure valve. 1-Alkenes/n-alkane ratio was defined byA thermocouple was inserted into the reactor forthe measurement of reaction temperature. 1−alkene/n−alkane ratio=

mole of 1−alkene

mole of n−alkaneAround 0.3 g of nC16 was loaded in the reactorwith 0.6 to 2.52 g of water (water density, rw,

(4)ranges from 0.1 to 0.42 g/cm3). Air in the reactorwas displaced with argon (Ar) gas. The reactor The experimental procedure for PE was basicallywas submerged in a molten salt bath (KNO3 the same as for nC16. The experiments were con-50 wt%–NaNO3 50 wt% mixture, T-3, Shinnippo ducted at 693 K, rw of 0.13 and 0.42 g/cm3, andkagaku) whose temperature was controlled to be 30 min reaction time. In the PE experiments, pro-a reaction temperature (from 673 to 723 K), and ducts were first fractionated to THF soluble andthe reaction has started. Temperature of the reactor insoluble (THFI) and mean molecular weight ofwas monitored by the thermocouple. Heat-up time THFI was evaluated by use of high temperaturewas around 1 min. After the reaction time of GPC (Waters, model 150C). Conversion of PE30 min including the 1 min of heat-up time, the (XPE) was defined asreactor was taken out of the bath and rapidlycooled (in several seconds) in a water bath. XPE=

loaded amount of PE−amount of THFI

loaded amount of PEExperiments without water were also conducted.The reactor was connected to a syringe ×100[weight%]. (5)

(200 cm3) to collect produced gas and measure itsvolume. For the produced gas, C1-C4 hydro- In these experiments, pressure in the reactor was

not measured. However for the nC16 experiments,carbons were analyzed by GC-TCD (Shimadzu,model GC-7A) with He carrier, and H2 by the pressure in the reactor is estimated to be

26–27 MPa (25 MPa water pressure and 1–2 MPaGC-TCD (Hitachi, model GC-163) with Ar car-rier. After sampling the produced gases, the reactor for nC16 and products) and 36–37 MPa (35 MPa

water pressure and 1–2 MPa for nC16 and pro-was washed with tetrahydrofuran (THF). TheTHF solution containing C5-C16 hydrocarbons ducts) respective to 0.1 and 0.2 g/cm3 of water

249M. Watanabe et al. / Journal of Supercritical Fluids 13 (1998) 247–252

density at 723 K. The pressure estimated for thePE experiments is also in these ranges.

3. Results and discussion

3.1. Pyrolysis of nC16 in SCW

Since nC16 is miscible with SCW [7], the reac-tion phase is homogeneous. As alcohols, carboxylicacids, and carbon oxide were not detected in theproducts, the main reaction for n-alkane decompo-sition under the present experimental condition isconsidered to be thermolysis, even in SCW. Fig. 2. 1-Alkane/n-alkane ratio of pyrolysis of nC16. Reaction

temperature=723 k, reaction time=30 min, +, in ArFig. 1 shows product distribution (n-alkane plus(XnC16=56%); $, in SCW (rw=0.1 g/cm3, X

nC16=58%); #, in1-alkene) obtained for the pyrolysis in Ar and inSCW (rw=0.2 g/cm3, X

nC16=57%).SCW (rw=0.1 g/cm3 and rw=0.2 g/cm3). Fig. 2shows the 1-alkene/n-alkane ratio for the pyrolysisproducts. The product distribution and 1-alkene/n-alkane ratio obtained for both SCW and Ar pyrol-ysis were almost the same. C2, C3 and C4 gasproducts’ selectivity shows a little difference butthis is just due to the difficulty in complete sam-pling and the accurate analysis of gas products.

Fig. 3 shows the evaluated first order rate con-stant of nC16 pyrolysis in SCW (35 MPa) and inAr (0.1 MPa). Pressure in the reactor for SCWexperiment was estimated to be 35 MPa fromsteam tables [8]. The rate constant of SCW does

Fig. 3. First order rate constant of pyrolysis of nC16.6, in Ar;+, in SCW (Pressure=35 MPa).

not show a significant difference from that forAr, either.

In general, the role of SCW in the reaction canbe (1) a cage effect; (2) water attack in reactions;(3) rate or equilibrium change due to variation ofthe dielectric constant; and (4) change of phase

Fig. 1. Product distribution of pyrolysis of nCl6. Reaction tem-behavior [9]. However, under the present condi-perature=723 k, reaction time=30 min, +, in Artions, the effect of SCW on the nC16 decomposi-(X

nC16=56%); $, in SCW (rw=0.1 g/cm3, XnC16=58%); #, in

SCW (rw=0.2 g/cm3, XnC16=57%). tion was found to be insignificant.

250 M. Watanabe et al. / Journal of Supercritical Fluids 13 (1998) 247–252

3.2. Pyrolysis of PE in SCW

Pyrolysis reaction of PE is considered to proceedmainly in a molten PE phase, and thus the reactionatmosphere is totally different from nC16 pyrolysis.

Conversion of PE (XPE) was around 30% in boththe Ar and SCW experiments (rw of 0.13 and0.42 g/cm3). Fig. 4 shows mean molecular weightof THFI of products for PE pyrolysis in Ar(0.1 MPa) and in SCW (rw=0.13 g/cm3 andrw=0.42 g/cm3). The mean molecular weight ofTHFI decreased with increasing water density.This clearly shows the promotion of PE pyrolysisin SCW. Fig. 5 shows product (n-alkane plus1-alkene) distribution for lighter products (gasproducts plus THF soluble products). Yield of Fig. 5. Product distribution of pyrolysis of PE. Reaction tem-

perature=693 k, reaction time=30 min, +, in Ar; $, in SCWshorter chain hydrocarbons increased with increas-(rw=0.13 g/cm3); #, in SCW (rw=0.42 g/cm3).ing water density. This also suggests the enhance-

ment of PE pyrolysis in SCW. Fig. 6 shows1-alkene/n-alkane ratio for the lighter products.1-Alkene/n-alkane ratio increased with increasingwater density.

Thus, PE pyrolysis in SCW is obviously differentfrom that in Ar, even though any difference couldnot be observed for nC16 pyrolysis. We think thisis due to the difference in the reaction phase. Inthe Ar atmosphere, pyrolysis of PE mainly occursin a molten PE phase. On the other hand, SCWcan dissolve some hydrocarbons produced.Brunner [7] reported that n-hexatricontane (nC36,molecular weight of 146) is miscible in SCW at

Fig. 6. 1-Alkene/n-alkane ratio of pyrolysis of PE. Reactiontemperature=693 k, reaction time=30 min, +, in Ar,; $, inSCW (rw==0.13 g/cm3); #, in SCW (rw=0.42 g/cm3).

693 K and above 30 MPa. Hydrocarbons of lowermolecular weight show the critical locus at a lowerpressure. These results suggest that at least suchhydrocarbons, produced via PE pyrolysis, can bedissolved in SCW and further decomposed intosmaller fragments. The pyrolysis of these hydro-carbons in the supercritical phase must be totallydifferent from that in molten PE phase pyrolysis.Also for the molten PE phase, SCW is consideredto dissolve into it to some extent, which results inFig. 4. Mean molecular weight of THFI. Reaction temper-

ature=693 k, reaction time=30 min. the dilution of the PE phase and thus influences

251M. Watanabe et al. / Journal of Supercritical Fluids 13 (1998) 247–252

the PE pyrolysis. Thus, the total product distribu- dominant against H abstraction (bimolecular reac-tion). Therefore, the product distribution shows ation is affected by the SCW phase pyrolysis.

Therefore, if the reactor was agitated, due to the high yield for smaller hydrocarbons. With increas-ing concentration, contribution of H abstractionpromotion of mass transfer between PE and SCW

phase, we would have observed a more significant becomes significant to b-scission and relatively flatproduct distribution is obtained.effect of SCW on the PE pyrolysis.

The effect of SCW on the product distribution On the basis of this well-known mechanism, wetried to explain the effect of SCW on pyrolysis ofand reaction kinetics of PE pyrolysis is discussed

below, on the basis of well-established theory for PE as follows. In the Ar atmosphere, pyrolysis ofPE occurs in a molten PE phase, where bimolecularthe mechanism and the kinetics of hydrocarbon

pyrolysis. reaction (H abstraction) is dominant. This canexplain the relatively flat product distribution andlow 1-alkene/n-alkane ratio, shown in Figs. 5 and3.3. The effect of reaction phase on pe pyrolysis6. On the other hand, in SCW reaction, as themolten PE phase is diluted with dissolved water,Pyrolysis of hydrocarbon is known as free radi-the contribution of b-scission (unimolecular reac-cal reaction [10]. The initiation reaction istion) increases.

Thus the production of 1-alkene and shorterM�ki 2R

i(6a)

chain hydrocarbons is promoted. The pyrolysis ofproduced hydrocarbons takes place also in theHerein, M is original n-alkane to decompose,supercritical water phase, where b-scission (unimo-R

iis alkyl radicals, and k

iis rate constant for

lecular reaction) must be predominant since theinitiation reaction. Propagation reactions are asconcentration is relatively low. This causes a shiftfollows:of product distribution toward shorter chainhydrocarbon and an increase in 1-alkene yield.H abstraction R

i+M�

kH R

iH(Al)

+R1

(6b)For n-alkane pyrolysis at a relatively high concen-tration, the first-order rate constant can be

b−scission R1�kb

Ri+Ol (6c) expressed by following equation, with a long-chain

steady state approximation for the above freeradical reaction [12].where R1 is the alkyl radical of mother n-alkane,

Ol is 1-alkene (a-olefin), RiH (Al ) is n-alkane, kb

is the rate constant for b-scission, and kH for H k=kb Aki

ktB1/2 [M ]−1/2 (7)

abstraction reaction. Termination reactions are viaradical recombination and their rate constant is

where [M ] is initial concentration of n-alkane.kt.

According to this equation, a first order rateIt is well known that product distribution isconstant and a resulting conversion increases withexplained roughly by considering the relative con-decreasing concentration. Thus, the promotion oftribution of b-scission and H abstraction reactionmolten PE phase pyrolysis in SCW suggested in[11]. This is just because the number of radicalFig. 4 can be explained by this theory.propagation in one life of radical is so large as to

ignore the contribution of alkane formation viatermination upon product distribution. Each b-scission produces one 1-alkene as a product andH abstraction reaction produces an n-alkane. Thus, Acknowledgmentthe 1-alkene/n-alkane ratio and product selectivityof smaller hydrocarbons increases with increasing The authors are grateful for grants from the

New Energy and Industrial Technologycontribution of b-scission. At a low n-alkane con-centration, b-scission (unimolecular reaction) is Development Organisation (NEDO) and the

252 M. Watanabe et al. / Journal of Supercritical Fluids 13 (1998) 247–252

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