Recycling of Polyethene and Polypropene in a Novel Bench-ScaleRotating Cone Reactor by High-Temperature Pyrolysis

R. W. J. Westerhout, J. Waanders, J. A. M. Kuipers,* and W. P. M. van Swaaij

Reaction Engineering Group, Faculty of Chemical Engineering, University of Twente, P.O. Box 217,7500 AE Enschede, The Netherlands

The high-temperature pyrolysis of polyethene (PE), polypropene (PP), and mixtures of thesepolymers was studied in a novel bench-scale rotating cone reactor (RCR). Experiments showedthat the effect of the sand or reactor temperature on the product spectrum obtained is largecompared to the effect of other parameters (for instance, residence time). In general, it can beconcluded that the amount of polymer converted into propene and butene decreases with highercracking severity (higher temperatures or longer residence times), while the fraction methaneincreases. About 80 wt % of the polymer is converted into gas at a reactor temperature of 898K, while 20 wt % is converted into intermediate waxlike compounds or aromatics in the case ofPE. The gas yield increases slightly with the reactor and/or sand temperature to 88 wt % athigher temperatures. The total amount of alkenes decreases with increasing cracking intensity,which suggests that the reactor should be operated at the lowest possible temperature. Ourresults indicate that the reactor offers a few significant advantages compared to other reactors(no fluidization gas necessary, good solid-polymer mixing, no cyclones necessary) and acompetitive product spectrum. However, significant improvements are still possible to makethe reactor concept technically and economically more attractive.

1. Introduction

1.1. General Introduction. Every year largeamounts of mixed plastic waste (MPW), mainly consist-ing of polyethene (PE), polypropene (PP), polystyrene(PS), and poly(vinyl chloride), (PVC) are produced. Atthe moment this waste is usually dumped or incineratedtogether with household waste, but due to environmen-tal concerns, governments, companies, and universitiesare looking at alternatives for the disposal of this waste.One very promising alternative to dumping or incinera-tion is high-temperature pyrolysis of the MPW torecover valuable chemicals, like ethene, propene, andstyrene.

Several high-temperature pyrolysis processes weredeveloped in the past using bubbling fluidized beds(BFB’s; Sinn, 1974; Sinn et al., 1976) or circulatingfluidized beds (CFB’s; Batelle Memorial Institute, 1992).At University of Twente a novel reactor, a rotating conereactor (RCR), has been developed for the pyrolysis ofbiomass (Wagenaar, 1994). A schematic drawing of thisreactor, termed the bench-scale RCR ([B]RCR), is pre-sented in Figure 1. Polymer or biomass is fed into thereactor together with preheated sand, which is used tosupply heat to the reactor, to enhance the heat-transfercharacteristics and to prevent sticking of particles to thereactor wall. The sand and polymer particles arethoroughly mixed on the bottom plate of the reactor andare subsequently transported upward by the rotatingaction of the cone. In the reactor the polymer particlesare heated and pyrolyzed. The sand leaves the reactorat the top, while the gaseous products are removed atthe bottom of the reactor.

This type of reactor has some advantages comparedto conventional reactors for the pyrolysis of biomass andpolymers:

(a) No (external) fluidization gas is required, whichreduces the required reactor volume and also enablesdownsizing or elimination of auxiliary equipment.

(b) Short gas- and solid-phase residence times.(c) Good polymer-sand mixing.(d) High-intensity reactor (high throughput-volume

ratio).(e) No cyclone necessary for gas-solid separation at

the exit of the reactor.* To whom correspondence should be addressed.

Figure 1. Schematic representation of the bench-scale RCR.

2293Ind. Eng. Chem. Res. 1998, 37, 2293-2300

S0888-5885(97)00704-5 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 05/08/1998

The aim of this study is to explore the suitability ofthe rotating cone reactor concept for the high-temper-ature pyrolysis of PE, PP, and mixtures of thesepolymers and to identify the optimal operating condi-tions for this novel reactor.

1.2. Reaction Scheme for the Pyrolysis of PEand PP. Both PE and PP degrade thermally via arandom degradation mechanism, yielding a broad prod-uct spectrum (C1-C50) (see, for instance, Seeger et al.,1975, 1977). The primary devolatilization reactionyields mainly intermediate waxlike products. In the gasphase these products are cracked further (secondary gasphase reactions) to smaller hydrocarbons (for instance,ethene and propene). However, the low alkenes andalkanes are thermodynamically unstable at these hightemperatures and are converted into aromatic com-pounds like benzene and toluene (ternary gas-phasereactions). At sufficiently high residence times, signifi-cant amounts of coke are formed. Other productsobtained at long residence times are methane andhydrogen. A schematic reaction scheme for the overallconversion process is given in Figure 2.

2. Experimental Equipment and Procedures

2.1. Pyrolysis Experiments. The bench-scale ro-tating cone reactor ([B]RCR) used in this study isidentical with the one used by Wagenaar (1994) to studythe pyrolysis of biomass. The main dimensions of the[B]RCR are shown in parts a and b of Figure 3.

Prior to an experiment the reactor was preheated tothe desired temperature using an electrical oven. Dur-ing the heat-up period the reactor volume was purged

using a small nitrogen stream to remove all oxygen fromthe reactor.

Most experiments were conducted using low-densitypolyethene (LDPE1) with a density of 917 kg/m3 andan average initial molar weight of 350 000 g/mol andpolypropene (PP) with an unknown initial molar weight.In addition, some experiments were performed usinghigh-density polyethene (HDPE) with an average initialmolar weight of 125 000 g/mol and linear low-densitypolyethene (LLDPE) of unknown initial molar weight.The diameter of the polymer particles was typically lessthan 300 µm. The polymer particles were fed to thereactor using a vibratory feeder at a mass flow rate ofapproximately 1 g/s through a water-cooled feeding pipepositioned in the center of the reactor. A small coldnitrogen purge stream was fed through the feeding pipeto prevent hot pyrolysis gases from entering the feedingpipe.

Prior to the actual pyrolysis experiment, 30 kg of sandwas heated to approximately 150 K above the reactortemperature using a small sand bunker surrounded bythree electrical ovens. The preheated sand (diameter0.5-0.8 mm) was introduced in the cone reactor in thecenter of the bottom plate at an angle of π/4 with respectto the polymer feed stream to ensure good mixingbetween polymer and sand. The sand mass flow ratein most experiments was approximately 4-5 g/s. Thedisadvantage of this configuration is that the sandexhibits preferential flow paths through the reactor,which effectively means that only part of the reactor isused. It was not possible to modify the existing reactorto prevent this phenomenon.

In the [B]RCR part of the interior volume was blockedusing an inner cone to limit the gas-phase residencetime to prevent conversion of the desired intermediateproducts (ethene, propene, and butene) to undesired,less valuable products (methane and aromatic com-pounds).

During all experiments a cone rotating frequency of600 rpm was used. By the rotating action of the cone,the sand-polymer mixture was transported to the topof the reactor. The sand particles exited the reactor atthe top of the reactor and were subsequently collectedin an annular space surrounding the cone (see Figure1). There is no sand outlet present in the reactor. Thegaseous products exited the reactor at the bottom andwere separated in the product collection section. Nocyclone was needed, because no sand particles wereentrained in the gaseous product stream. The gaseousproducts were cooled using six ice-cooled bottles inwhich the largest fraction of the liquid and solidproducts could be collected. Aerosols present in the gasflow were collected using a cotton-wool filter. Thecotton-wool filter and the ice-cooled bottles were weighedbefore and after each experiment to determine thefraction of polymer converted to liquid products. Thegas flow rate was measured using a volumetric gasflowmeter. During the experiment gas samples weretaken after the aerosol filter, which were subsequentlyanalyzed using a Varian 3400 GC with a Haysep Qcolumn (temperature 473 K) and a flame ionizationdetector (FID), which unfortunately meant that hydro-gen could not be detected.

An experiment was started by opening the valve inthe sand pipe connecting the sand bunker and thereactor. After ensuring that a proper sand flow wasestablished, the actual pyrolysis experiments were

Figure 2. Reaction scheme for the pyrolysis of polymers.

a b

Figure 3. (a) Dimensions of inner cones. (b) Dimensions ofrotating cones.

2294 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998

started. This was done by starting the vibratory feeder.The pressure in the reactor was kept constant by usinga vacuum pump connected to the exit of the productcollection section. The gas samples and temperaturemeasurements in the reactor showed that a stationarysituation was reached after 4 min of operation, whereaseach experiment lasted approximately 10 min.

After each experiment, the integral mass balance wascalculated and the yield of the liquid and gaseousproducts was determined. For most experiments theintegral mass balance was satisfied within 90 wt %. Themain reasons for this deviation are due to coke forma-tion, which was not measured (estimated at approxi-mately 1-2 wt %), and the slip of aerosols through theproduct collection system. In all figures presented belowthe composition of the gas phase is given, which has tobe multiplied by the gas yield to obtain the actual yieldof the gaseous product.

2.2. Residence Time Distribution Experiments.To study the residence time distribution characteristicsof the [B]RCR, propane tracer gas experiments wereconducted (Westerterp et al., 1984). The experimentalequipment used in this study is shown in Figure 4.During the experiments 1 g/s of nitrogen gas was purgedthrough the polymer feed pipe to simulate the genera-tion of gas in the actual pyrolysis experiments. Theeffect of changing the reactor geometry could be simu-lated by blocking the three different dead volumes inthe reactor as shown in Figure 4. Most of the experi-ments were done without sand as our first experimentsshowed that the sand flow had a very limited influenceon the residence time distribution in the reactor. Theparameters for the standard case for the residence timedistribution (RTD) experiments are given in Table 1.

3. High-Temperature Pyrolysis Experiments ofPolyethene

3.1. Definition of the Standard Case. To studythe effect of different parameters, a standard case (I)for experiments with PE was defined based on literaturedata and our own initial experiments. For pyrolysis

experiments involving PP, a second standard case (II)was defined with a lower temperature. The operatingconditions for both standard cases are summarized inTable 2.

The results of the experiments and the influence ofdifferent parameters will be discussed in the nextsections for PE, whereas the results for PP will bepresented in the next paragraph.

3.2. Influence of the Reactor Temperature. Theinfluence of the reactor or sand temperature on theproduct spectrum is larger than that of any otherparameter (Westerhout et al., 1998). The effect of thereactor temperature on the product spectrum obtainedis shown in Figures 5 and 6. The reactor temperatureis defined as the temperature in the reactor volumebefore each experiment.

At elevated temperatures the methane fraction in-creases sharply, while the fractions of propene andbutene decrease. The ethene fraction and the total gasyield are relatively insensitive to the reactor tempera-ture. About 80 wt % of the polymer is converted to gas,while 20 wt % is converted to liquid/solid wax productsat 923 K (see Figure 6). The liquid yield is the sum ofpartially converted intermediate waxlike componentsand aromatic products. The total liquid yield decreasesslightly with an increasing reactor temperature. Athigher temperatures there might also occur a shift inthe composition of the liquids fraction from intermediatewaxlike products to aromatic products. In the case ofPE pyrolysis it might be possible or advantageous torecycle the unconverted intermediate waxlike productsto the reactor to obtain higher gas yields.

Figure 4. Schematic drawing of the experimental setup for RTDexperiments.

Table 1. Standard Case for Residence Time DistributionExperiments

value value

rotation frequency 600 rpm volume I unblockedN2 purge 1 g/s volume II unblockedgas outlet middle volume III unblocked

Table 2. Standard Cases for the Pyrolysis of PE and PPin the Bench-Scale RCR

parameter standard case I standard case II

polymer PE PPTr [K] 998 898Ts [K] 1123 998Qp [g/s] 1 1Qs [g/s] 5 5QN2 [g/s] 0.4 0.4cone angle [rad] π/2 π/2inner cone yes yes

Figure 5. Influence of the reactor temperature on the gascomposition for the pyrolysis of PE.

Figure 6. Gas yield as a function of the reactor temperature forthe pyrolysis of PE.

Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2295

If the reactor temperature is increased, the totalamount of valuable alkenes produced decreases, whilethe total amount of alkanes (especially methane) in-creases. It is therefore advantageous to operate thereactor at the lowest possible temperature to obtain amaximum yield of alkenes. However, if the temperatureof the reactor becomes to low, the yield of unconvertedintermediate waxlike compounds increases sharply,which leads to clogging of the product collection system.The temperature at which this occurs lies between 873and 923 K. Below this temperature the unconvertedintermediate waxlike product is the main productobtained and in this case we essentially deal with a backto fuel (BTF) process, while above this temperature thelower olefins constitute the main product, which meansthat at higher temperatures the process is a back tomonomer (BTM) process. From an energetic point ofview it is also advantageous to operate the reactor atlower temperatures.

In an earlier study experiments were also conductedwith LDPE1 in an isothermal tubular reactor with(near) plug flow characteristics (Westerhout et al.,1998). The yield of alkenes obtained in this reactor issignificantly higher than the yield obtained in either[B]RCR (see Table 3) or BFB reactors, indicating thatsignificant improvements in the product spectra are stillpossible. The decreased yield of alkenes is probablycaused by the nonideal gas-phase residence time dis-tribution and the existence of temperature gradients inthe reactor, resulting in excessive cracking of intermedi-ate products such as ethene, propene, etc., to methaneand coke. In our earlier study it was found that thepolymer throughput or the intermediate waxlike prod-uct concentration does not significantly influence theproduct spectrum obtained in the tubular reactor,indicating that the decreased yield of alkenes is notcaused by the higher throughput and accompanyinghigher product concentration in the bench-scale RCR.Therefore, the gas-phase residence time distribution wasmeasured with propane injection experiments.

The influence of the different dead reactor volumeson the cumulative residence time distribution functionF(t) is presented in Figure 7. The corresponding aver-age residence time and standard deviation are given inTable 4. It can be seen from this figure that in the basecase (no volumes blocked) the cumulative residence timedistribution function F(t) shows significant tailing,which can be reduced by blocking one or more deadvolumes in the reactor. Especially, the blocking ofreactor volume III causes a significant reduction in theextent of tailing of F(t). This implies that during normalreactor operation significant amounts of (product) gasare exchanged with reactor volume III. The relativelylong residence time of the gas exchanged with thisreactor volume probably causes excessive cracking ofintermediate products (ethene and propene), whichcould be one of the main reasons for the relatively highmethane and liquid yields observed in our [B]RCRcompared to the tubular reactor.

3.3. Influence of the Sand Temperature. Theeffect of the sand temperature on the product spectrumof PE is the same as the influence of the reactortemperature, because the sand temperature directlyinfluences the temperature in the reactor. The sandtemperature is defined as the temperature of the sandin the bunker, before it is fed to the reactor.

It can be concluded from Figure 8 that the sandtemperature was chosen too high in the base case, whichleads to excessive formation of methane (39 wt %).

3.4. Influence of the Gas-Phase Residence Time.The gas phase residence time in the reactor was alteredby removing the inner cone from the interior reactorvolume (see Figure 1). In the case where the inner coneis present the gas-phase volume is approximately 1.3× 10-3 m3, while the total inner reactor volume isapproximately 34.7 × 10-3 m3. These reactor volumescorrespond with calculated average gas-phase residencetimes of respectively 0.4 (0.13 s in the bottom-platevolume) and 10 s in the standard case (see Table 2).

It can be seen from Figure 9 that the influence of theaverage gas-phase residence time is minor compared tothe influence of the reactor and/or sand temperature.A higher gas-phase residence time enhances methaneformation, while the fractions of propene and buteneproduced decrease. This effect is similar to the effectof increased reactor and/or sand temperature. Theproduct spectrum is therefore often correlated to the

Table 3. Comparison of the Product Spectra of PEObtained in a Bench-Scale RCR and a Tubular Reactor

product spectrum (wt %)

compound PE (RCR) PE (TR, 923 K) PE (TR, 1073 K)

methane 38.9 6.6 11.0ethene 31.6 41.6 52.5propene 10.0 23.6 19.0butene 6.1 24.1 13.5butadiene 0.2 0.0 0.0other alkanes 11.5 4.1 4.0>C4 1.7 0.0 0.0total alkenes 48 89 85gas yield 87.8 89 99a

a Formation of coke is necessary to satisfy mass balance but isnot measured.

Figure 7. Influence of dead zones on the cumulative residencetime distribution function F(t).

Table 4. Calculated Average Residence Time andStandard Deviation for Different Blocked Volumes

blockedvolume

averageresidence time (s)

standarddeviation (s)

none 4.91 22.76I 5.00 23.04I/II 4.46 21.14I/II/III 2.42 13.86

Figure 8. Gas composition as a function of the sand temperaturefor the pyrolysis of PE.

2296 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998

cracking severity of the pyrolysis conditions expressedby an intensity function IF (Sawaguchi et al., 1980,1981):

where a equals 0.03 for LDPE, while Sawaguchi et al.found a value of a of 0.04 for PP which indicates thatthe influence of the residence time is minor comparedto the influence of temperature, which is generally foundfor the pyrolysis or cracking of hydrocarbons (Albrightet al., 1983).

3.5. Influence of Other Parameters. 3.5.1. Typeof PE. In the majority of the tests performed with thebench-scale RCR, LDPE was used, but some experi-ments were conducted under base case I conditionsusing HDPE and LLDPE. Tests were also done withanother LDPE with the same average molecular weightbut a different molecular weight distribution. Earlierpyrolysis experiments conducted in a tubular reactorusing these polymer types (Westerhout et al., 1998)already revealed that the type of PE had no significantinfluence on the product spectrum obtained, a findingwhich was confirmed by the experiments conducted inthe bench-scale RCR. The differences in the productspectra were minimal and certainly within experimentalerror.

This means that the results found for the LDPE usedin this study are also valid for other types of PE.

3.5.2. Cone Angle and Bottom Plate Size. In thisstudy pyrolysis experiments were performed using threedifferent cone angles. In the standard case a cone witha top angle of π/2 was used, while experiments with conetop angles of π/3 and π (i.e., a flat-plate reactor) werealso performed. Experiments were conducted with thebase case reactor temperature (998 K) and a slightlyhigher reactor temperature (1023 K).

The experiments with the π/3 cone yielded slightlymore ethene, propene, and butene and less methanethan the experiments with the π/2 cone (see Figure 10),while the product spectrum with the flat plate did notdiffer significantly.

In the case of the cone reactor with a top angle of π/3the gas-phase volume in the cone reactor is 3.1 × 10-3

m3 if the inner cone is present, while the gas-phasevolume equals 60.2 × 10-3 m3 without an inner cone.The corresponding calculated gas-phase residence timesare respectively 0.89 (0.83 s in the bottom-plate volume)and 17.2 s. The main difference with the cone reactorwith a top angle of π/2 is the much longer residence timein the bottom-plate volume (0.83 versus 0.13 s) andtherefore less gas-phase residence time in the inclinedpart of the reactor. This implies less exchange of gaswith dead volumes in the case of the cone reactor witha top angle of π/3 and therefore less gas-phase residencetime distribution, leading to a slightly improved productspectrum.

No clear trend concerning the effect of the cone anglecould be observed, and therefore it was concluded thatthe cone angle itself has no significant influence on theproduct spectrum but that the variations in the productspectrum were caused by different factors, such as thegas flow patterns, mixing of sand and polymer, etc.

3.5.3. Polymer Mass Flow Rate. The polymermass flow rate was changed from 0.6 to 1.7 g/s understandard case conditions. The results of these experi-ments are shown in Table 5. The polymer flow rate hasa significant effect on the product spectrum obtained,because the polymer mass flow rate directly influencesthe actual temperature distribution in the reaction zone.The reactor temperature decreases with increasingpolymer flow rate, because more heat is consumed bythe endothermic pyrolysis reaction. Moreover, the gas-phase residence time decreases with increasing polymermass flow rate, because more gas is produced. Thetemperature effect is more important than the gas-phase residence effect, as already mentioned earlier,which results in a decreased cracking severity withincreasing polymer mass flow rate. Therefore, the totalalkene yield increases with increasing polymer massflow rate, resulting in an increased yield of propene andbutene and a decreased yield of methane.

No information could be obtained on the maximumpossible throughput of polymer, due to the limitationsof the polymer and sand feeding systems. Cold flowexperiments showed that up to 10 kg/s sand could behandled easily by the reactor, which corresponds to apossible polymer mass flow rate of 0.5 kg/s. The onlymajor problem that would emerge is the large reactorvolume required to achieve the desired average gas-phase residence time of about 0.5 s (Westerhout et al.,1998) for complete conversion of intermediate waxlikeproducts.

3.5.4. Sand Mass Flow Rate. A reduction of thesand mass flow fed to the reactor causes a lowertemperature in the reactor zone, because less heat issupplied to the reactor. This implies a decrease in

Figure 9. Influence of the gas-phase residence time on the gascomposition of the pyrolysis of PE.

Figure 10. Gas composition for the pyrolysis of PE in the bench-scale RCR as a function of cone angle.

IF ) Tτa (1)

Table 5. Product Spectrum for the Pyrolysis of PE as aFunction of Polymer Mass Flow Rate

product spectrum (wt %)

compound 0.59 g/s 0.98 g/s 1.65 g/s

methane 49.7 38.9 41.5ethene 27.1 31.6 23.2propene 11.2 10.0 15.8butene 5.6 6.1 8.9butadiene 0 0.2 0.1other alkanes 6.4 11.5 8.8>C4 0 1.7 1.7total alkenes 43.9 48 48

Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2297

cracking severity, resulting in a larger alkene (especiallypropene and butene) yield. This effect was also foundexperimentally.

3.5.5. Gas Outlet Position. During all experimentspresented so far the gas outlet position was located onthe bottom plate of the reactor (see Figure 1). To studythe effect of the location of the gas outlet on the productspectrum, an inner cone with four gas outlets locatedhalfway up the inclined cone wall was inserted into thereactor to change the gas flow pattern in the conereactor. This modification, however, had no significantinfluence on the product spectrum obtained. This resultis in accordance with findings from earlier experiments,which revealed that the influence of the gas-phaseresidence time on the product spectrum is limited.Tracer experiments (volumes I-III not blocked) con-firmed that the influence of this modification on the gas-phase residence time distribution is also limited.

4. High-Temperature Pyrolysis Experiments ofPolypropene

Preliminary experiments with PP conducted understandard case I conditions revealed that the yield ofmethane was very high (see Table 6), with ethene asthe main product and not propene. PP and its mainproduct (propene) apparently degrade significantly fastercompared to PE with its main product ethene (Wester-hout et al., 1997; Kunugi et al., 1969, 1970). Thisimplies that the reaction conditions of standard case Icorrespond to a cracking severity in the reactor, whichis too high for PP and therefore leads to a diminishedyield of alkenes (see experiments with PE). Therefore,a standard case with more favorable conditions wasdefined with a lower reactor (898 K) and sand temper-ature (998 K) compared to the initial experiments.

Under the new standard case conditions, first pyroly-sis experiments of PP were conducted in which thereactor temperature was varied. The results of theseexperiments are presented in Figures 11 and 12. Underbase case conditions the main pyrolysis product of PPis propene, but with increasing reactor temperature theextent of ethene and methane formation increasessharply at the expense of propene and butene formation.In the case of PP approximately 80 wt % of the polymer

is converted into gas at 898 K, while 20 wt % of thepolymer is converted into liquids. The gas yield in-creases with increasing temperature, as is evident fromFigure 12. The gas and liquid yields obtained arecomparable to those found for PE, but the compositionof the liquids is clearly different. Analysis showed thatthe liquid formed during the pyrolysis of PP consistsmainly of aromatics (benzene, toluene), while the liquidformed during PE pyrolysis consists mainly of waxlikeintermediate compounds (long alkanes and alkanes).

The total yield of alkenes is higher at lower reactortemperatures (lower cracking severity), which was alsofound for PE. The total alkene yield drops significantlyfrom 59 wt % (gas composition) at a reactor temperatureof 898 K to 47 wt % at 998 K. The conclusion, whichwas drawn on the basis of the PE pyrolysis experiments,namely, that the reactor and sand temperature shouldbe chosen as low as possible to optimize the total alkeneyield, is confirmed by the PP pyrolysis experiments.However, the product spectrum of PP is more sensitiveto the temperature, compared to the product spectrumof PE, which is in accordance with literature data andresults of earlier kinetic experiments (Westerhout et al.,1997).

In Table 7 the product spectrum obtained from PPpyrolysis in an isothermal tubular reactor with (near)plug flow reactor characteristics is compared with theproduct spectrum of PP obtained from the bench-scalerotating cone reactor. It can be concluded that the yieldof valuable alkenes is significantly lower in the bench-scale RCR, indicating that improvements in the productspectra are still possible by modifying the reactorcharacteristics (i.e., plug flow behavior and improvedcontrol of the temperature).

5. High-Temperature Pyrolysis Experimentswith Mixtures of Polyethene and Polypropene

In many situations it is very difficult and expensiveto separate two types of polymers from each other. In

Table 6. Comparison of Product Spectra for PE and PPPyrolysis under Standard Case I Conditions

product spectrum(wt %)

product spectrum(wt %)

compound PE PP compound PE PP

methane 38.9 41.5 other alkanes 11.5 8.7ethene 31.6 23.2 >C4 1.7 1.7propene 10.0 15.8 total alkenes 48 48butene 6.1 8.9 gas yield 87.8 86.7butadiene 0.2 0.1

Figure 11. Gas composition as a function of reactor temperaturefor the pyrolysis of PP.

Figure 12. Gas yield as a function of reactor temperature forthe pyrolysis of PP.

Table 7. Comparison of the Gas Product Compositionfor Pyrolysis of PP in the Bench-Scale RCR and aTubular Reactor (Base Case II)

gas product composition (wt %)

compoundPP

(RCR, 898 K)PP

(TR, 923 K)PP

(TR, 1073 K)

methane 19.8 3.9 15.7ethene 14.3 17.7 32.2propene 27.1 47.8 28.5butene 16.6 27.1 16.5butadiene 0.5 0.0 0.0other alkanes 13.1 3.5 7.1>C4 8.6 0.0 0.0gas yield 79.6 99a 99a

total alkenes 59 93 77a Formation of coke must be necessary to satisfy mass balance.

This could not be measured due to the experimental setup.

2298 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998

waste streams encountered in practice the polymers willmost likely be present in a mixed state and willtherefore undergo simultaneous pyrolysis in a reactor.It is therefore important to know whether the productspectrum of the different polymers is influenced by thepresence of a second or third polymer. Kinetic experi-ments (Westerhout et al., 1997) and experiments con-ducted in a tubular reactor (Westerhout et al., 1998)revealed that at very short gas-phase residence timesthe aforementioned mixing effect could not be observed.These earlier results were confirmed by experimentsconducted in the [B]RCR. LDPE and PP were thor-oughly mixed in an extruder, and the mixture wasground to the desired size (<0.3 mm) under cryogenicconditions. Polymer mixtures with respectively 33 and66 wt % PE were pyrolyzed in the cone reactor understandard case II conditions. The results of these experi-ments are presented in Figure 13.

It can be seen from this figure that no significanteffect due to mixing of different polymer types can beobserved and that the conclusions drawn on the basisof the results obtained in a tubular reactor are also validfor the bench-scale RCR. This implies that, for reactorswith a short gas-phase residence time, ternary gas-phase reactions between products do not play a signifi-cant role. However, these results obtained for the conereactor are not necessarily valid for reactors with alarger gas-phase residence time (BFB reactors).

6. Comparison of Experimental Results withLiterature Data

In Figure 14 the product spectrum produced by thebench-scale RCR is compared with product spectrafound in other reactors (BFB and CFB respectivelypublished by Paisley and Litt (1992) and Sinn et al.(1976)). It can be concluded from Figure 14 that theproduct spectra obtained in relatively small reactors aresignificantly better than the product spectra obtainedfrom large-scale reactors. The data for the CFB reactortaken from a patent (Paisley and Litt, 1992) is toolimited to draw any firm conclusions. It appears that

the product spectrum obtained from a BFB is morefavorable compared to the one produced by the [B]RCR(less methane and more ethene and propene), but thedifference is small. However, the use of utilities (steamor nitrogen and energy) in a process based on a BFBreactor leads to larger reactor dimensions and higherseparation costs (larger gas stream). However, thebench-scale RCR is probably more expensive to con-struct (higher investment costs). Which reactor typeoffers an economic advantage over the other is thereforehard to assess without a detailed economical evaluation.It must also be kept in mind that BFB technology hasbeen fully developed during 20 years of experimentationby the Kaminsky group at University of Hamburg, whilethe bench-scale RCR is the first reactor of its kind,which can be developed further.

7. Conclusions and Future Work

The high-temperature pyrolysis of PE, PP, and mix-tures of these polymers was studied in a bench-scaleRCR, previously used for the flash pyrolysis experimentsof biomass.

Pyrolysis experiments with PE revealed that theinfluence of the reactor and/or sand temperature is largecompared to the influence of other parameters, likeresidence time, cone angle, gas outlet position, etc. Ifthe sand or reactor temperature is increased, moremethane is formed, while the fractions of propene andbutene decrease. The fraction ethene produced isrelatively insensitive with respect to the reactor and/orsand temperature. The yield of gas (approximately 80wt % at a reactor temperature of 998 K) increases withan increasing sand or reactor temperature and couldalso be increased by recycling a part of the liquidproducts (waxlike intermediate compounds) to the reac-tor. The total amount of alkenes (ethene, propene, andbutene) decreases with an increasing temperature,which indicates that the reactor should be operated atthe lowest possible temperature. However, below 873-923 K the formation of waxlike intermediate/liquidproducts increases sharply, which limits the minimumpossible temperature.

In general, it can be said that the formation ofmethane increases, while the fractions of propene andbutene and the total amount of alkenes decrease, withincreasing cracking severity (increasing residence timeand temperature). The effects on the product spectrumof other parameters can be explained by the fact thatby variation of the parameters the residence time and/or temperature is influenced.

The type of PE or the mixing of PE with a secondpolymer has no significant influence on the productspectrum obtained, which confirmed earlier results ofexperiments conducted in other short residence timereactors (for instance, tubular reactors).

PP and its pyrolysis products degrade faster comparedto PE and its pyrolysis products. The influence of thetemperature on the product spectrum of PP is thereforemore pronounced, but the trends found for the pyrolysisof PE are confirmed by PP pyrolysis experiments.

The yield of alkenes in the bench-scale RCR issignificantly lower than the yields found in an isother-mal tubular reactor with (near) plug flow characteris-tics, which probably is caused by the residence timedistribution and temperature gradients inside the larger-scale reactor.

Figure 13. Gas composition as a function of the fraction PE in aPE/PP mixture.

Figure 14. Comparison of the product spectrum of [B]RCR tospectra of other reactors for PE.

Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2299

The effect of the gas-phase residence time distributionwas examined using propane injection experiments. Itwas shown that tailing of the residence time distributionwas caused by exchange of gas with the dead volumebelow the cone. Elimination of these dead volumes willdecrease the tailing in the residence time distributionfunction and is expected to lead to increased yields ofvaluable alkenes.

Other improvements, which do not directly influencethe product spectra but will lead to an increasedeconomical viability of the rotating cone reactor technol-ogy and bring the operation conditions of the reactorcloser to an industrial-scale plant, are as follows:

(a) Continuous operation of the plant by using a sandrecycle loop.

(b) Feeding the polymer as a melt to avoid the use ofsmall particles. These small particles can only beobtained by cryogenic grinding of larger polymer par-ticles, which is very expensive.

(c) Increase of the polymer and sand throughput.It was shown that the yield of valuable products in

the RCR is slightly lower than the yield found in otherreactor types (BFB, CFB), but less utilities (nitrogen andtherefore energy) were used to achieve this productspectrum. However, significant improvements are stillpossible to increase the economical and technical vi-ability of this novel reactor.

Acknowledgment

The authors acknowledge the assistance of MaritSollaart, Olaf Veehof, and Wim Leppink during theexperiments.

Symbols

E(t): differential residence time distribution functionF(t): accumulative residence time distribution functionI: cracking intensityn: rotation frequency (1/s)Q: flow rate (m3/s)t: time (s)T: temperature (K)

Greek Letter

τ: residence time (s)

Subscripts

r: reactors: sand

Abbreviations

BFB: bubbling fluidized bed

CFB: circulating fluidized bedRCR: rotating cone reactor

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Received for review September 11, 1997Revised manuscript received March 24, 1998

Accepted March 25, 1998

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