Conversion of polymers to fuels in a refinery stream

Conversion of polymers to fuels in a refinery stream

Suat Ucara, Selhan Karagoza, Tamer Karayildirimb, Jale Yanikb,*aPetrochem. Techn., Dokuz Eylul University, IMYO, 35150 Izmir, Turkey

bDepartment of Chemistry, Ege University, 35100 Izmir, Turkey

Received 12 April 2001; received in revised form 9 July 2001; accepted 17 July 2001

Abstract

The purpose of this study was to investigate the processability of LDPE, PP, PVC/LDPE and PVC/PP in the hydrocracking unitof a refinery. For this, LDPE or PP has been added to vacuum gas oil (VGO). The blends were hydrocracked over different cata-lysts using a batch autoclave at 425–450 �C under hydrogen atmosphere. The catalysts used were HZSM-5, Cobalt loaded activecarbon (Co-Ac) and DHC-8 (commercial silica-alumina catalyst). Addition of polymer to VGO affected the cracking of VGO,

leading to a decrease in the gas yield and an increase in the liquid yield. In hydrocracking over HZSM-5, the product distributionwas similar for two blends. HZSM-5 gave higher gas yields than the other catalysts. When using Co-Ac and DHC-8 catalysts, thegas and liquid yields depend on the polymer type as well as temperature. In the case of the PVC-containing blends (PVC/PP/VGO

or PVC/PE/VGO), the blends were firstly dechlorinated at 350 �C. Then the dechlorinated mixture was hydrocracked in the 400–450 �C range in the presence of DHC-8. HCl evolved by degradation of PVC during the dechlorination step partially degraded PEand PP and these predegraded polymers were more easily cracked in the hydrocracking step. In addition, we observed the effect of

polyene formed from the PVC degradation residue on the thermal hydrocracking. The chlorine compounds in the hydrocrackedfeed affected the catalytic activity of the catalysts. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Hydrocracking; Polyethylene; Polypropylene

1. Introduction

Increased production of polymers leads to generationof enormous amounts of polymer wastes. The develop-ment of viable recycling technologies for plastic wastematerials is becoming increasingly important. There areseveral methods used to recycle polymers and they areclassified into the following categories: primary (re-extrusion), secondary (mechanical recycling), tertiary(chemical or thermal recycling) and quaternary (incin-eration). There is growing interest in thermolysis andcatalytic polymer degradation as methods of producingvarious fuel fractions from polymer wastes.The main components of municipal polymer waste

are polyethylene (PE) and polypropylene (PP). There-fore, their degradation to fuels and valuable chemicalshas been widely studied. These studies may be categor-ized into two groups, thermal and catalytic. Kaminsky[1] studied the thermal cracking of PE in a fixed-bed

reactor over the temperature range 500–600 �C. Attemperatures below 550 �C, high yields of useful pro-ducts with low yields of gas and aromatics wereobtained. William et al. [2] investigated the pyrolysis ofLDPE in a fluidized bed reactor at temperatures of 500–700 �C. A marked decrease in the oil fraction and waxfraction with increasing temperature was observed. Inthe pyrolysis of HDPE at 450–500 �C in both a closedreactor and an open system, the closed system gavehigher yields of gas, distillate and coke at increased tem-peratures, whereas thermal cracking in an open systemresulted in less naphtha but more gas oil. In addition, thedistillates from the open system contained more olefins,less saturates and very little aromatics compared to theclosed system [3]. The ultrapyrolysis of LDPE has beenperformed in an internally circulating fluidized-bedreactor. [4,5]. Using high temperature (860 �C) and shortresidence time, in the millisecond range, total gas yieldsin excess of 90 wt.% were obtained. The olefin ratio ingases was 75 wt.%. McCaffrey [6] has also studiedthermolysis of LDPE at moderate temperatures, 425–450 �C. The major product was a liquid (with yields upto 82.5%) having a high content of olefinic compounds.

0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(01 )00215-4

Polymer Degradation and Stability 75 (2002) 161–171

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* Corresponding author. Fax: +90-232-388-8264.

E-mail address: [email protected] (J. Yanik).

Although thermal degradation requires high tem-perature and gives a very broad product range, catalyticdegradation provides control of both the product yieldand distribution in polymer degradation as well as sig-nificantly lowering the degradation temperature. Fordegradation of plastics to fuels and valuable products,most research has focused on catalytic degradation.Common plastics such as PE and PP have already beentested extensively. Some of the studies reported in theliterature are reviewed below. Sakata et al. studied thecatalytic degradation of PP and PE by a silica-aluminacatalyst in a semi-batch reactor [7–9]. It was seen thatsilica-alumina was effective in increasing the degrada-tion rate and yield of oil products. They also studied theeffect of catalyst types on polymer degradation [10,11].They observed that catalysts with strong acid sites suchas zeolite (ZSM-5) accelerated the degradation of PPand PE into gases which resulted in low liquid yields.For the catalyst having no acid sites (a non-acidicmesoporous silica catalyst, FSM), the initial rates of PPand PE degradation into liquid were as fast as thoseover an acid catalyst, but liquid yields were higher.Ochoa et al. [12] have investigated the liquefaction ofPE through hydrocracking. A series of silica-aluminacatalysts having different acidity and different Bronstedto Lewis acid site density ratio and the commercialHZSM-5 catalyst were tested. It was found that there isno correlation between total acidity and oil production.They mentioned that the oil yield from degradation ofPE can be correlated with the concentration of Bronstedacidity on the catalyst and not total acidity. Ding et al.[13,14] have also studied hydrocracking of PE usingHZSM-5 and metal loaded hybrid catalysts preparedfrom HZSM-5 and silica-alumina. HZSM-5 producedmore aromatic hydrocarbons, whereas hybrid catalysts(especially Ni loaded) showed higher hydroisomeriza-tion ability. An interesting result was that liquid pro-ducts obtained over hybrid catalysts were clean andwhite or light yellow with a gasoline like smell, while theliquids produced from thermal cracking and overHZSM-5 were brown-red with a strong unpleasantsmell. Sharratt et al. [15] pyrolyzed HDPE over HZSM-5 catalyst in a fluidized-bed reactor. In a pyrolysis attemperature of 350 �C, the conversion to volatile hydro-carbons (especially C3–C5) was more than 90 wt.% offeed. Aguado et al. [16] investigated the effect of zeolitebeta on the degradation of PP, LDPE and HDPE at400 �C in a batch reactor. It was observed that degrada-tion of HDPE affords a high selectivity of C5–C12 pro-ducts (70 wt.%) whereas in the cracking of LDPE andPP, selectivity to gasoline is reduced (around 64 wt.%)and higher proportions of lighter products C1–C4 areobtained. Mordi [17] showed that the activities of zeo-lites in polymer degradation varied with the differencesof their framework structure and composition. Theyperformed the thermolysis of LDPE using catalyzed

zeolites (H-mordenite, Si/Al=8; H-Theta-1, Si/Al=35;HZSM-5, Si/Al=21). It was found that HZSM-5 pro-duced a gasoline range of hydrocarbons containingmore aromatics, whereas over H-mordenite and H-Theta, jet fuel hydrocarbons in the range of C11–C18were obtained. Zhao et al. [18] investigated the effect ofthe type of zeolite on the degradation temperature ofPP. It was found that different zeolites showed differentactivities, depending not only on zeolite frameworkstructure but also on its composition. Cardong et al. [19]evaluated the catalytic cracking of PP with large porezeolites such as amorphous and ordered silica-aluminasand a spent equilibrium FCC commercial catalyst. Theresults demonstrated that amorphous or ordered silica-aluminas are very active catalysts. However, they con-cluded that an FCC catalyst can be a better option froman economic point of view, since it gives a very goodselectivity, and even its activity is lower than that ofsilica-alumina.In degradation of plastics to useful fuels, thermal

cracking may be used in an initial process for producinglow quality hydrocarbons, subsequently, these may betreated in a catalytic reactor to upgrade then into highquality liquid fuels [20–23]. One of the advantages ofthermal plus catalytic cracking is the possibility of pro-cessing chlorine containing plastics, since dechlorinationtakes place during the thermal degradation.Coprocessing waste plastics with coal provides

another alternative to the production of fuels and che-mical feedstocks from waste plastics. In these studies,HZSM-5, metal loaded silica-alumina catalysts havebeen used [24–28].A more interesting approach for processing waste

plastics for conversion into valuable fuels is mixing theplastics with a heavy petroleum fraction then treatingthem conventionally in a refinery unit [29–31]. Ng et al.[30] have evaluated the conversion of PE blended withVGO into fuels by catalytic cracking in a fixed bedreactor at 510 �C. Catalytic cracking resulted in highergasoline yields particularly when 10 wt.% HDPE wasused. Although the coprocessing of coal with plasticshas being extensively studied, little research has focusedon the coprocessing of plastics with a feedstock of arefinery unit. These studies are based on the directaddition of plastics into FCC feedstocks for the processof catalytic cracking, FCC. The hydrocracking process,which basically converts high boiling molecules intomore desirable lower molecular weight products withlow olefin and high iso/n-parafin yields is an importantprocess in modern oil refineries. One of the most pro-mising processes for conversion of waste plastics may bethe coprocessing of waste plastics in a hydrocrackingunit in a refinery.The aim of this study is to investigate the processa-

bility of LDPE, PP, PVC/PP and PVC/PE in a hydro-cracking unit of a refinery. LDPE or PP has been added

162 S. Ucar et al. / Polymer Degradation and Stability 75 (2002) 161–171

into a feed of a hydrocracking unit, VGO. The blendwas catalytically cracked in the presence of H2. Thecatalysts employed were commercial DHC-8 catalyst,HZSM-5 and cobalt loaded active carbon. In the case ofhydrocracking of PVC/PP/VGO or PVC/PE/VGO, theblend was initially thermally treated at 350 �C for elim-ination of chlorine. Then the dechlorinated mixture wascracked using a DHC-8 catalyst.

2. Materials and methods

2.1. Materials

The heavy vacuum gas oil (HVGO) which was used isa broad vacuum distillate with a boiling point range242–578 �C, and base feed for the hydrocracking unit ina refinery in Izmir. LDPE, PP (as 3.2 mm extruded pel-lets) and PVC (as a powder) were supplied by ALPET-Izmir. All polymers were supplied in pure form (withoutfilling or stabilizer, etc.) except PVC, which contains astabilizer. The DHC-8 catalyst used is a commercialcatalyst which is used for hydrocracking of VGO in theIzmir refinery. DHC-8 is an amorphous hydrocrackingcatalyst consisting of non-noble hydrogenation metalson a silica alumina base. It is a bifunctional catalystincorporating both hydrotreating and hydrocrackingfunctions. HZSM-5 (SiO2/Al2O3=216 (molar ratio) wassupplied by Toso Chem. Ind, Japan. Cobalt loadedactive carbon catalyst (Co-Ac) used was prepared froman aqueous solution of Co(NO3)2 6 H2O by impregna-tion and heat treatment under an N2 atmosphere at300 �C to give a 3.2 wt.% CoO.

2.2. Methods

Hydrocracking reactions with or without catalystswere carried out by using a 100 ml shaking type batchautoclave. Reactions were carried out at a 6.5 MPainitial hydrogen pressure with 25 g feed. In catalytichydrocracking experiments, 2.5 g of catalyst was loadedin the reactor. Hydrocracking runs were made at tem-peratures of 425, 435 and 450 �C for a reaction time of120 min. At the end of the reaction time, the autoclavewas cooled to room temperature by a fan. The gaseswere released. The reactor content was treated as shownin Fig. 1In the case of a PVC-containing mixture, the mixture

was first dechlorinated, then it was hydrocracked withDHC-8 catalyst or thermally.Dehydrochlorination was carried out in a heated stir-

red tank reactor (V=1 l) under atmospheric pressure.400 g of LDPE or PP (20% by weight)/PVC (5% byweight)/HVGO mixtures were placed in the reactor. Thereactor was gradually heated to 350 �C and held at thistemperature for an hour. An N2 gas flow was used to

remove the volatile products and HCl gas from thereactor to the collection flasks. Following the con-densation of distillate, HCl gas was absorbed in anaqueous solution of NaOH and the remaining gaseswere released. After dechlorination, the cooled reactorcontent (dechlorinated mixture) was withdrawn fromthe reactor.The liquid products from hydrocracking were ana-

lyzed on a glass capillary column (50 m�0.32 mm)coated with a stationary phase of cross-linked methylsiloxane. A flame ionization detector was used in ana-lysis. The data obtained from GC was used to evaluatethe simulated distillation curves [32]. Chlorine contentof the liquid products was analyzed by GC with anatomic emission detector (HP: G2350A-AED). Chlorinecontent in the dechlorinated mixture was measured by aburning method [33].

3. Results and discussion

3.1. Hydrocracking of PE/VGO blend

Table 1 shows a typical product distribution observedin the hydrocracking of PE/VGO blend at three reactiontemperatures with and without catalyst. For compar-ison, VGO was hydrocracked alone thermally and in thepresence of DHC-8 commercial catalyst. The hydro-cracking results of VGO alone are also shown in thesame table. When VGO was cracked in the absence ofcatalyst, the gas yield increased while the liquid yielddecreased with increasing reaction temperature. Whenusing catalyst, the gas and liquid yield did not changewith temperature. By comparing the thermal crackingof VGO with PE/VGO blend, it is seen that addition ofPE to VGO decreased the gas yield obtained in hydro-cracking at 425 �C, while it significantly increased above425 �C. The liquid yield obtained from VGO at 435 and450 �C were 30.1 and 24.4 wt.%, respectively, while theliquid yield from PE/VGO at the same temperatures were68.5 and 56.0 wt.%. Similar results were also obtainedfrom hydrocracking over DHC-8 catalyst. Above 425 �C,the addition of PE to VGO increased the liquid yield.Thus, the presence of PE affected the cracking of VGO atall temperatures. This may be explained by consideringthe reaction between the primary degradation productsof VGO and PE. This synergistic effect was alsoobserved by Ng [30].The temperature and the type of catalyst effected the

degradation of PE. At 425 �C, the degradation ratio ofPE in the presence of DHC-8, Co-Ac and HZSM-5 was73.0, 83.6 and 100 wt.%, respectively, whereas in theabsence of catalysts, it was 78.6 wt.%. When the tem-perature was increased to 435 �C, a degradation valuegreater than 90 wt.% was obtained under both thermaland catalytic conditions.

S. Ucar et al. / Polymer Degradation and Stability 75 (2002) 161–171 163

The effect of catalyst on product distribution fromPE/VGO blend varied with temperature. At 425 �C, thecatalytic effect was more pronounced. When the tem-

perature reached 435 �C, both thermal and catalyticreactions occurred simultaneously. In the absence ofcatalyst, the increase in the temperature from 425 to

Fig. 1. Product analysis procedure.

Table 1

Product distribution of hydrocracking of PE/VGO blend

Feed HVGO PE/HVGO

– DHC-8 – DHC-8 HZSM-5 Co-Ac

Reaction temperature 425 �C

Catalyst (2.5 g)

Reaction Products (wt.%)

Gasa 47.2 65.2 20.3 36.6 37.2 22.2

Liquid 40.5 30.4 31.2 38.0 53.6 46.2

Wax 11.2 4.5 44.4 25.4 8.5 28.4

Asphaltene – – 3.5 Trace 0.4 0.1

Coke 1.1 Trace – Trace 0.7 –

Undegraded polymerb – – 21.4 27.0 – 16.4


Catalyst (2.5 g) –

Reaction products (wt.%)

Gasa 62.4 59.5 21.7 31.3 50.8 28.8

Liquid 30.1 31.2 68.5 41.4 34.9 54.9

Wax 7.5 9.3 9.3 26.6 13.4 15.9

Asphaltene – 2.3 0.4 0.6 0.9 0.9

Coke 0.5 – 0.1 – 0.6 0.4

Undegraded polymerb – – 2.0 3.6 – –


Catalyst (2.5 g) –


Gasa 73.4 63.5 40.5 32.0 58.0 49.0

Liquid 24.4 29.4 56.0 45.2 26.6 33.8

Wax 2.2 7.1 3.3 21.4 13.9 17.1

Asphaltene – – – 0.3 1.0 0.9

Coke Trace 2.5 Trace 1.5 1.7 0.1

Undegraded polymerb – – – – – –

a Calculated from mass balance.b Based on polymer charge.


435 �C led to a dramatic increase in liquid yields from31.2 to 68.5%, while as no difference was observed in gasyields. However in the case of hydrocracking of VGOalone under identical conditions a decrease of liquidyields was observed due to thermal cracking. It can beclearly seen that at a temperature of 435 �C, the beha-viour of degradation products from PE dominantlyeffected the degradation of the blend rather than thetemperature. It has been reported that when LDPE wasliquefied at 440 �C by thermal hydrocracking, the productcomposition changed dramatically from that obtained at420 �C. Furthermore the oil yield was increased from 10to 59% [34]. It might be said that 435 �C is a critical tem-perature in degradation of LDPE under hydrogen with-out catalyst. In our case, reactive intermediates fromdegradation of PE at 435 �C attacked the hydrocarbonsin VGO (through intramolecular transfer), leading toformation of the liquid product from VGO. However afurther increase in the temperature (from 435 to 450 �C)led to cracking of the liquid product as expected.Different trends in liquid yields were observed with the

catalysts as the reaction temperature increased. Therewas a continuous increase in gas yield over HZSM-5 byincreasing the temperature whereas liquid yielddecreased. Hydrocracking over HZSM-5 gave the high-est gas yields at all temperatures (37.2% at 425 �C and57.9% at 450 �C). At 425 �C, the maximum liquid yieldwas obtained over HZSM-5, whereas HZSM-5 gave thelowest liquid yield above 425 �C. It is well known thatHZSM-5 is highly active for carbon– carbon bond scis-sion to yield gaseous products preferentially, and conse-quently the liquid yield is low.The degradation of PE/VGO blend over Co-Ac and

HZSM-5 involves completely different mechanisms. As iswell known, the C–C bond cleavage on acidic sites ofHZSM-5 proceeds by a carbenium ion mechanism. Incracking over an active carbon catalyst (Ac), hydro-carbons are cracked via a radical mechanism, which issimilar to thermal cracking [35]. It was demonstrated thatthe concentration of free radicals is controlled at a lowerlevel in the presence of Ac catalyst and pressurizedhydrogen [36]. Yang et al. concluded that the role of metalsupported nonacidic catalysts, such as active carbon, is toactivate the hydrogen molecule to hydrogen atoms whichcan hydrogenate free radicals and olefins to suppressovercracking [35]. In the present study, the cracking ofPE/VGO over Co-Ac, proceeded via a radical mechanism,giving a very different product distribution when com-pared with thermal cracking. These differences in pro-duct distribution are reasonably well explained by theabove conclusions. The gas yields over Co-Ac at 425 and435 �C were less than those obtained using HZSM-5 andDHC-8, which are acidic catalysts. This is because, in theCo-Ac cracking, whose intermediate is not a carbeniumion but a hydrocarbon radical, consecutive cracking of ahydrocarbon radical is suppressed by hydroquenching to

suppress the yield of gaseous hydrocarbon. For the runsusing a Co-Ac catalyst, the yields of liquid and gas pro-ducts slightly increased when the temperature increasedfrom 425 to 435 �C. However, with further increase ofthe temperature up to 450 �C the liquid yield decreaseddrastically whereas the gas yield increased. This showsthat the cracking activity of Co-Ac increased due to thehigh temperature, leading to more radical formation.In contrast to cracking over HZSM-5 and Co-Ac, the

temperature had no major effects on gas and liquidyields from cracking over DHC-8. It is noteworthy thatwhen VGO was cracked alone with DHC-8, the productdistribution was not significantly changed with tem-perature. This indicates that the catalytic activity ofDHC-8 did not change considerably at the temperatureswithin 425–450 �C.Fig. 2 shows the ASTM curves of liquids obtained

from PE/VGO blends using different reaction processes.The composition of the liquid product varied with thetype of catalysts and the temperature. When usingHZSM-5, the amount of fractions with boiling pointbetween IBP-172 �C (naphtha fraction) was alwayshigher than that of thermal or catalytic reactions overCo-Ac and DHC-8. The increase in the hydrocrackingtemperature generally increased the lighter fractionsboth in the presence and the absence of the catalysts.The liquids obtained at 425 �C by thermal and catalytichydrocracking contained heavier compounds (exceptHZSM-5). At this temperature, yields of naphtha(<172 �C) and middle distillates (172–232 �C) in theliquid product are 12 and 7% (over DHC-8); 33 and19% (over Co-Ac); 60 and 20% (over HZSM-5); 20 and7% (thermal), respectively. An increase in hydrocrack-ing temperature from 425 to 435 �C led to a significantincrease in lighter compounds. At this temperature, theboiling point distributions of liquids from thermally andcatalytically over DHC-8 or Co-Ac are close to eachother. The yields of naphtha and middle distillate arearound 45–50 and 23%, respectively. When the tem-perature was further increased to 450 �C, the naphthayield increased for HZSM-5, DHC-8 and for the ther-mal run. In contrast, the naphtha yield decreased from50 to 35%, whereas the middle distillate yield increasedfrom 15 to 27% for Co-Ac. This result gave the con-clusion that the free radicals in high concentration inhydrocracking over Co-Ac at 450 �C led to formation ofgas products and also formation of middle distillates bya hydrogen-quenching reaction. In the hydrocracking ofVGO over NiMo-Ac at 405 �C [35], it was also shownthat a hydrogen quenching reaction led to the highermiddle distillate yield and lower gas and naphtha yield.

3.2. Hydrocracking of PP/VGO blend

The product distributions from hydrocracking of aPP/VGO blend are given in Table 2. At all temperatures


PP was completely degraded in the presence or absenceof catalyst.By comparing the results from hydrocracking of VGO

alone with the PP/VGO blend using DHC-8 and ther-mally, it is clearly seen that the presence of PP affectedthe cracking of VGO as in the case of PE/VGO. Thepresence of PP in VGO led to an increase in liquid yieldwhereas it decreased the gas yield at all tested tempera-tures. In thermal hydrocracking of PP/VGO, the tem-perature had no considerable effect on the productdistribution. However, the product distribution variedwith temperature in the case of catalytic hydrocracking.It is noteworthy that in the presence of HZSM-5, theproduct distribution was very close to that of the PE/VGO blend. This shows that HZSM-5 has a similarcracking activity to that of PP and PE degradation atthe temperature range 425–450 �C. HZSM-5 gave ahigher gas yield than the other catalyst as expected. The

variations in the gas and liquid yield with temperaturewere similar for HZSM-5 and Co-Ac. Upon increasingtemperature, the liquid yield decreased and the gas yieldincreased. However, for DHC-8 the gas yield increasedslightly at 435 �C and then dramatically decreased witha further increase in temperature. By contrast no majorchange was observed in the liquid yield. This result maybe due to conversion of lighter compounds to heaviercompounds as follows.

Gas hydrocarbons ��! liquid hydrocarbons

��! waxy compoundsð1Þ

The hydrocracking over DHC-8 at 450 �C gave alower gas yield and a higher liquid yield than HZSM-5and Co-Ac. These results imply that DHC-8 also cata-lyzed recombination and disproportionation reactionsat 450 �C in the case of PP/VGO blend.Fig. 3 shows the results of a simulated distillation of

liquid products obtained from hydrocracking of PP/VGO blend at different reaction conditions. The com-positions of liquids obtained from catalytic and thermalhydrocracking at 435 �C are similar. All liquids con-tained about 70% naphtha and 15% middle distillates.However at 425 and 450 �C, the type of catalyst had aneffect on the composition of the liquids. In contrast tohydrocracking of PE/VGO blend, Co-Ac gave lightercompounds than HZSM-5. It is noted that in the case ofhydrocracking over DHC-8 at 450 �C, the amount ofheavy compounds with boiling point above 300 �C wasmore than that of hydrocracking over HZSM-5 and Co-Ac. This result supports the proposal that DHC-8 cata-lyzed recombination and disproportionation reactionsin the hydrocracking of PP/VGO blend at 450 �C.It can also be noted that although the liquid yield for

hydrocracking of PP/VGO and PE/VGO blends overCo-Ac and HZSM-5 catalysts above 425 �C are thesame, the liquid composition is not same. In addition,the liquids from catalytic and thermal hydrocracking ofPP/VGO at all temperatures contained more lightercompounds than that of PE/VGO blend. This is due tothe differences in degradation mechanism of PP and PE.

3.3. Comparison of hydrocracking of PP/VGO and PE/VGO blends

In thermal hydrocracking, the variations in productdistribution with temperature depend on the polymertype in the blend. In the presence of PP, product dis-tribution did not change much with temperature. How-ever the temperature had an effect in the presence of PE.As we discussed before, the free radicals from degrada-tion of VGO and polymers can react with each other.The amount and type of free radicals formed from

degradation of PP and PE were different. It is wellknown that the degradation of PP occurs at a lower

Fig. 2. Simulated distillation curves of liquids obtained from hydro-

cracking of PE/VGO blend at different temperatures: (a) 425 �C; (b)

435 �C; (c) 450 �C.


temperature than PE and the variety of products fromPP degradation is more complex than PE degradation.Due to the branched structure of PP, the thermal degra-dation mechanism involves tertiary radicals [37]. Howeverthe degradation mechanism of PE must proceed via

primary or secondary radicals. The radicals fromdegradation of polymer (PE or PP) promote cracking ofVGO via extraction of a hydrogen atom from VGO. Athigher temperature, more scissions in PE polymerchains occur leading to a higher number of short pri-mary radicals, which react with hydrocarbons of VGO.Therefore, the gas+liquid yield from PE/VGOincreased with increasing temperature.In catalytic hydrocracking, the product distribution

depends on the catalyst as well as the polymer type andtemperature. In hydrocracking over HZSM-5 catalyst,the product distribution was similar for the two blends.However for DHC-8 and Co-Ac, the catalytic activityvaried with the polymer type in VGO and temperature.In the presence of Co-Ac, PP/VGO blend gave the

higher gas+liquid yield than PE/VGO at 425 �C,whereas no significant differences were observed above425 �C. From these results, we can mention that at lowtemperature (425 �C) the active carbon should moreeasily initiate the cracking reaction by abstraction of thetertiary hydrogen in PP than secondary one in PE.In contrast to HZSM-5 and Co-Ac, DHC-8 catalyst

showed the different activity at all temperatures for thetwo blends. For the PP containing blend, DHC-8 gavehigher liquid yields at all temperatures and lower gasyields at 425 and 450 �C. At 435 �C, DHC-8 showed anexcellent cracking activity (a high liquid yield, a lowamount of waxy compounds) for PP/VGO as comparedto PE/VGO.

3.4. Hydrocracking of PP/PVC/VGO and PE/PVC/VGO

One of the aims of this study was to investigate theprocessability of a PVC-containing mixture in a refinerystream. For this, a feed of PVC/PP or PVC/PE in VGOwas hydrocracked with a commercial catalyst (DHC-8)to simulate a possible future industrial process. Becausethe presence of chlorine in refinery feeds causes someproblems, these mixtures were first dechlorinated as anadditional step before hydrocracking.

Fig. 3. Simulated distillation curves of liquids obtained from hydro-

cracking of PP/VGO blend at different temperatures: (a) 425 �C; (b)

435 �C; (c) 450 �C.

Table 2

Product distribution of hydrocracking of PP/VGO blend

Reaction temperature (�C)

425 435 450

Catalyst (2.5 g) – DHC-8 HZSM-5 Co-Ac – DHC-8 HZSM-5 Co-Ac – DHC-8 HZSM-5 Co-Ac


Gas 38.7 25.6 37.1 23.9 33.1 33.1 46.2 33.9 34.8 19.2 61.2 53.0

Liquid 54.1 50.1 52.3 60.0 59.2 55.2 39.7 50.8 61.2 55.1 24.8 32.5

Wax 7.1 24.3 10.2 15.9 7.7 9.5 12.2 15.4 3.5 25.7 12.7 14.3

Asphaltene 1.4 0.6 0.4 0.6 1.1 0.5 0.9 0.6 1.2 2.1 1.2 0.9

Coke 0.1 – 0.5 0.5 – 2.2 1.8 – 0.5 – 1.8 0.2

Undegraded polymer – – – – – – – – – – – –


It is well known that thermal degradation of PVCunder inert conditions takes place in two steps. In thefirst step (220–350 �C), mainly dechlorination (HClevolution) occurs, leaving a double bond-containingchain (polyene) as a residue. In related studies [38,39] ofthe kinetics of pyrolysis of a PVC/PE mixture, it wasobserved that HCl formation was completed before thebeginning of weight loss of the remaining polymer.Moreover, there was no interaction that influenced thedegradation products in polymer mixtures.In this study, the dechlorination of a PVC/PE/VGO

or a PVC/PP/VGO mixture was carried out with gra-dual heating to prevent overflow from the reactor. Thus,the mixture was heated to 250 �C at 5 �C min�1 andheld at this temperature for half an hour. At 250 �C, themaximum rate of HCl loss was observed. Then thetemperature was increased to 350 �C and the reactorwas kept at 350 �C for an hour. Pyrolysis yielded aresidue of 90.6 wt.% and a liquid 0.5 wt.% for PE/PVC/VGO and 88.5 and 1.5 wt.% for PP/PVC/VGO. Thechlorine contents of the residue (dechlorinated mixture)from PE/PVC/VGO and PP/PVC/VGO were 700 and600 ppm, respectively. The chlorine contents of theliquid and gases were not measured.The dechlorinated mixture was hydrocracked at a 6.5

MPa initial hydrogen pressure at temperatures of 400,425 and 450 �C with DHC-8 catalyst for 2 h.The product distributions from the hydrocracking

experiments are given in Table 3. For both samples, theliquid yield increased up to 425 �C and decreased athigher temperatures. The effect of temperature on thegas yield varied depending on the polymer type. For thedechlorinated PVC/PE/VGO mixture, the gas yieldsharply decreased at first and then increased as tem-perature increased. However for the dechlorinatedPVC/PP/VGO mixture, the gas yield increased with theincreasing temperature. 425 �C seems the critical tem-perature for hydrocracking of the dechlorinated PVC/PE/VGO. The reason is the decomposition ratio of PEwas increased above 420 �C as we mentioned before.

The use of the catalyst led to a little decrease in thegas yield whereas it dramatically decreased the liquidyield at 425 and 450 �C for both samples. However, theamount of waxy compounds increased in the presenceof the catalyst. This situation shows that the presence ofthe dechlorinated products affected the activity of thecatalyst. By comparing the results of hydrocrackingexperiments between PE or PP/VGO and dechlorinatedmixtures, we can clearly see the effect of the dechlor-ination on the hydrocracking product distribution.There may be two effects of the dechlorination. First,predegradation of polymer may occur during dechlor-ination; second, dechlorination products (organicchlorine compounds and PVC residue-polyene) may bepresent in hydrocracking feed.The effects of the dechlorination varied with the

polymer type (PP or PE). The hydrocracking of thedechlorinated PVC/PE/VGO with and without the cat-alyst produced less gas and more liquid than that of thesamples without PVC. In the case of the hydrocrackingof the dechlorinated PVC/PP/VGO, at 425 �C, the gasyield was less and the liquid yield was more than that ofPP/VGO while at 450 �C this situation was reversed.It must be noted that at 400 �C we obtained waxy

compounds instead of liquid from hydrocracking ofboth PE/VGO and PP/VGO since the temperature wasnot sufficiently high (unpublished results). In addition,no undegraded PE or PP was found at low temperatures(400 and 425 �C). However, in the case of PVC con-taining mixtures, we obtained the product in liquid format 400 �C. This shows that HCl evolved during thedechlorination step degraded partially PE or PP. Thispredegraded PP or PE was more easily cracked. Inaddition the presence of polyene from dechlorinatedPVC (coke-like particles) might also have an additionalcracking effect.By comparing the results of catalytic hydrocracking

of the samples with and without PVC, we can see theeffect of dechlorination products in the feed on the cat-alyst. For example, in the case of the hydrocracking of

Table 3

Product distribution of hydrocracking of dechlorinated PE/PVC/VGO(A) or PP/PVC/VGO(B) mixture

Reaction temperature (�C)

400 425 450

Feed (25 g) A A B B A A B B A A B B

Catalyst DHC-8 (2.5 g) � + � + � + � + � + � +

Reaction products, wt.%

Gas 17.2 16.2 15.9 11.2 7.0 5.1 20.7 21.6 20.5 176 51.4 58.8

Liquid 47.5 47.4 54.8 41.1 77.0 60.8 60.3 47.7 71.8 49.4 35.5 21.0

Wax 35.3 32.9 25.3 44.9 12.4 33.2 16.0 26.8 7.8 26.6 9.1 16.5

Asphaltene 6.0 4.0 4.9 2.4 Trace Trace 1.36 1.6 – 1.3 0.8 1.2

Coke Trace 3.5 3.9 1.7 3.6 0.8 3.0 3.9 – 6.3 3.9 4.8

Undegraded polymer – – – – – – – – – – – –


the dechlorinated PVC/PP/VGO at 450 �C, catalyticruns produced more gas products than thermal runs,whereas in hydrocracking of PP/VGO, thermal runproduced more gas products than catalytic runs.Figs. 4 and 5 give the simulated distillation curves of

liquids obtained by hydrocracking of dechlorinatedmixtures at 425 and 450 �C. The hydrocracking tem-perature also had an effect on the composition of liquidsobtained from PVC/PE/VGO. The increase in the tem-perature from 425 to 450 �C led to formation of lightliquid hydrocarbons. However, at this temperaturerange, no the major change in the composition of liquidfrom PVC/PP/VGO was observed. The effect of thecatalyst varied depending on the temperature and thetype of mixtures. In the case of hydrocracking of thedechlorinated PVC/PE/VGO mixture at 425 �C, theliquid produced from the catalytic reaction contained

32% naphtha and 10% middle distillate, whereas in thecase of the thermal run, 5% of the liquid was naphthafraction. In contrast, the thermal run at 450 �C producedlarger amount of hydrocarbons with lower boiling pointsthan that for catalytic hydrocracking. Otherwise, the useof catalyst or the increase in temperature led to no dif-ferences in liquid composition for PVC/PP/VGO. Incatalytic or thermal hydrocracking of the dechlorinatedPVC/PP/VGO, the yields of naphtha and middle dis-tillate are around 60 and 15%, respectively.

4. Conclusion

In this study, the processability of polymer and poly-mer mixtures in a refinery stream was investigated.Coprocessing reactions were performed with model

Fig. 4. Simulated distillation curves of liquids obtained from hydrocracking of dechlorinated mixtures at 425 �C.

Fig. 5. Simulated distillation curves of liquids obtained from hydrocracking of dechlorinated mixtures at 450 �C.


plastics. VGO. LDPE, PP and PVC were selected asmodel plastics because the first two represent a sub-stantial portion of waste plastics and the last causesserious problems during recycling.In the first group of experiments, the blend of LDPE

or PP/VGO was thermally and catalytically cracked inthe presence of H2 at temperatures of 425, 435 and450 �C. The catalysts used were commercial DHC-8,HZSM-5 and cobalt loaded active carbon(Co-Ac).Addition of polymer into VGO affected the cracking

of VGO, leading to a decrease in the gas yield and anincrease in the liquid yield. In thermal hydrocracking,the product distribution from hydrocracking of the PP/VGO blend did not change with temperature whereasthe temperature had an effect for the PE/VGO blend. Itis interesting that the thermal run gave higher gas+li-quid yields than the catalytic run above 425 �C. In cat-alytic hydrocracking, waxy compounds were formed inlarger amounts than in thermal hydrocracking. The useof catalyst affected the product distribution and thecomposition of the liquid product. For two blends,HZSM-5 gave the higher gas yields than the other cata-lyst. When using Co-Ac and DHC-8 catalysts, the gasand liquid yields depend on the polymer type as well astemperature. In hydrocracking over Co-Ac, we obtainedthe maximum liquid yield (54.9%) from the PE/VGOblend at 435 �C, while, in the case of the PP/VGOblend, the maximum liquid yield (60.0%) was obtainedat 425 �C. However for both two blends, the maximumgas yield (53% for PP/VGO and 49% for PE/VGO) wasobtained at 450 �C. In hydrocracking of PE/VGO overDHC-8 commercial catalyst, the gas and liquid yieldswere not changed considerably with the temperature. Inthe case of hydrocracking of PP/VGO over DHC-8, thegas yield decreased dramatically whereas no majorchange was observed in liquid yield.According to the liquid yield and the naphtha content

in liquid, we may mention that HZSM-5 gave the bestresults in the hydrocracking of the PE/VGO blend at425 �C. But in the hydrocracking at 435 �C, Co-Ac wasbetter than the others. For the PP/VGO blend, Co-Acgave the best result at both 425 and 450 �C. Thus,although a high naphtha content is obtained, a hydro-cracking temperature of 450 �C is not appropriatebecause of the low liquid yield.The second group experiments involved the hydro-

cracking of the PVC containing blends (PVC/PP/VGOor PVC/PE/VGO). The conversion of the PVC con-taining mixture of VGO to fuels was carried out by two-step processing in which dechlorination of mixtures wasfollowed by hydrocracking of dechlorinated mixtures inthe presence and absence of the catalyst (DHC-8). HClevolved from dechlorination of PVC partially degradedPE and PP and these predegraded polymers were moreeasily cracked in a hydrocracking step. In addition, weobserved the catalytic effect of polyene formed from

PVC degradation residue on the thermal hydrocrackingprocess. The chlorine compounds in the hydrocrackedfeed affected the activity of the catalysts.

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Conversion of polymers to fuels in a refinery stream

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