Molten waste plastic pyrolysis in a vertical falling film reactor and the in fluence of temperature on the pyrolysis products☆

Zechen Jin,Dezhen Chen,Lijie Yin*,Yuyan Hu,Huangqing Zhu,Liu Hong

Thermal and Environmental Engineering Institute,Tongji University,Shanghai 200092,China

1.Introduction

With the development of Chinese economy and improvement of people's living standards,plastic production,consumption and the amount of waste rapidly increase.China has become the world's second largest plastic producer,and the yield of plastic products is approximately 75.6 million tons in 2015[1].However,40%of these plastics will be discarded in 1-2 years[2],and ultimately become waste.

Pyrolysis is considered as one of the important methods of recycling waste plastics.It is an irreversible thermo chemical process that breaks down the chemical bonds of the polymers and decomposes them into low molecular weight compounds under oxygen-free condition.The main products of plastic pyrolysis are oil and gas with high calorific values[3].The pyrolysis process is extremely complex,previous studies have shown that the pyrolysis of plastic is divided into several stages including:melting(from solid to liquid),pyrolysis characterised by volatile emission from the molten plastics,which is a process of oilgas mixture “evaporation”(from liquid to gas),and coke formation at the end.During the melting and pyrolysis stages,the heat supply is needed and the ratio of heat required in melting stage to that of volatile“evaporation”stage is approximately 1:4[2].Molten plastics are a kind of non-Newtonian fluid[4],and chara cterised with high viscosity and low thermal conductivity,which make it hard to flow and difficult to be heated up,therefore,how to improve the flow performance,and heat transfer efficiency during the volatile “evaporation”stage are some key factors considered when design pyrolysis reactors.

Liquid film heat transfer is an effective method for heat transfer enhancement[5].This method has been widely applied to various industrial productions.Shi et al.[6]investigated the heat transfer performance of lithium bromide absorption chiller in falling film generator,and found that the overall heat transfer coefficient of falling film generator is 4.37 times higher than that of immersed tube generator.Zhang et al.[7]found that the yield of water desalination by using the falling film evaporation technology is about two to three times more than that by conventional type under the same condition.Vertical falling film reactor based on liquid film heat transfer is appropriate to deal with high viscosity materials[8],and the heat transfer performance of viscous material is similar to that of Newtonian fluid[9].Applying vertical falling film pyrolysis reactor to deal with waste plastics,can not only improve heat transfer efficiency,but also solve the flow problem easily.So far,there are few reports of applying vertical falling film reactor to molten plastics pyrolysis.Additionally,although there have been many reports of waste plastics pyrolysis,there are no special standards for pyrolysis products in China.The reason is that the quality of pyrolysis products vary,which is due to the uneven heating during waste plastics pyrolysis process.One of the most advantages of vertical falling film reactor is that the heating is uniform,which can help to improve the singularity ofproducts.Therefore,itis importantfor achieving high efficiency,high-value recovery of waste plastics.Yin et al.[10]numerically simulated the falling film pyrolysis behaviour of molten plastics in a vertical falling film reactor,examined the feasibility of this type of pyrolysis reactor and predicted the temperature distributions and velocity profiles.However,no work was performed to determine the compositions of the pyrolysis products.

Heating temperature is one of the key operating parameters of a pyrolysis reactor[11,12],and has an important effect on the yield of liquid oil and non-condensable gas.Dong etal.[13]studied the pyrolysis process of five typical waste plasticsin a fluidised-bed reactor and found that as the temperature increases,the yield of non-condensable gas increases,whereas the yield of pyrolysis oil decreases.Demirbas[14]researched the pyrolysis process of mixed plastics in a tubular reactor and found that when the temperature increases from 277 °C to 627 °C,the yield of non-condensable gas increases from 12.3%to 42.4%,and the yield of the pyrolysis oil first increases and then decreases.Elordi et al.[15]found that as the temperature increasing from 500°C to 700°C in a Conical Spouted Bed Reactor(CSBR),the yield of the pyrolysis wax reduces 16%,and the yield of gasoline has maximum 33%at 700°C.Therefore,it is necessary to discuss the influence of heating temperature on the pyrolysis products of waste plastics in a new pyrolysis reactor.

In this paper,a vertical falling film pyrolysis reactor was constructed,and the pyrolysis process of molten PP in the reactor was experimentally investigated.The influence of the heating temperature on the yields and compositions of the oil and non-condensable gas were studied.

2.Materials and Methods

2.1.Materials

The plastic used in this work is analytically pure plastic powder(PP),produced by the Daqing Re fining and Chemical Company(Daqing,China).The average diameter of the powder is 2 mm,with the ultimate and proximate analyses shown in Table 1.The ultimate analysis of the tested sample is carried out in accordance with the ultimate analysis of coal because there are no special municipal solid waste(MSW)ultimate analysis standards.The high heat value(HHV)is 46.28 MJ·kg-1.The higher carbon and hydrogen content in plastic fuels can avoid the need of further upgrading.

Table 1 Ultimate and proximate analyses of the feedstock

2.2.Experimental methods

The pyrolysis plant is shown in Fig.1,and is composed of four parts:the pyrolysis system,the heating and control systems,the condensing and collecting system and the non-condensable gas treatment system.The pyrolysis system consists of a melting tank and a falling film plate.The vertical falling film plate is 0.05 m×1 m.A feed inlet and a nitrogen inlet are located at the top of the melting tank.Three rows of type-K thermocouples with three in each row are mounted from top to bottom at equal distances to monitor the falling film plate temperatures,the liquid film temperatures and the volatile temperatures.A glass window in front of the falling film plate is used to observe the liquid film.The pyrolysis volatile outlet is at the top of the falling film plate.Another nitrogen inlet is at the bottom of the falling film plate to sweep the glass surface.A fractional condensation and collection method is applied in the condensing and collecting system,and the temperatures of the cooling water are 85°C to collect the high pour point oil(HPO)and 0°C ice salt water to collect the low pour point oil(LPO).The temperatures of the melting tank and the falling film plate are controlled separately,and are recorded using an electric recorder.The non-condensable gas treatment system is used to deal with the remaining non-condensable gas.

Prior to experimentation,500 g PP plastic powder was inserted into the melting tank from the feeding inlet,and the entire system was swept with nitrogen for 15 min to ensure an inert atmosphere.Then,the system was heated up.According to the results of the thermogravimetric(TG)and differential thermal analysis(DTA)[16],PP begins to melt at 172 °C and starts to pyrolysis at 385 °C.Therefore,the temperature of the melting tank is kept at 250°C.When plastic is completely melted in the melting tank,the nitrogen pushes the molten plastics out from the melting tank to the top of the falling film plate.Next,the molten plastics are heated,decomposed when flowing down along with the falling film plate.The volatile(mixture of oil and gas) flow out from the top ofthe falling film plate then is condensed and collected.During the experiment,the flow rate of nitrogen at the bottom of the falling film plate is kept with 20 L·h-1to avoid the volatile condensing on the surface of the glass.Before the experiment,200 g of quartz sand was placed at the bottom of the falling film plate to collect dripping molten plastics which did not have enough time to pyrolysis.

An elemental analyser(Vario EL III,Germany Elmentar)was used to perform a proximate analysis of the feedstock.A Gas Chromatography(GC-9160,Shanghai Ouhua analysis instrument factory)was used to determine the compositions of the non-condensable gas.A Gas Chromatography and Mass Spectrometry(GC-MS,SHIMADZU QP2010,ultra-gas chromatograph,quadrupole mass spectrometer with He as the carrier gas,a capillary column of 30 m RESTEK×0.25 mm ID and a 0.25 μm film thickness)was used to analyse the liquid oil products.The GC injector port was at 300°C.The GC analytical program operated at 35 °C for 5 min,35 °C-120 °C at 5 °C·min-1,120 °C-250 °C at 5 °C·min-1,250 °C-300 °C at 10 °C·min-1and 300 °C for 5 min.A viscometer(SV-10,A&D Co.,Ltd.)was used to test the dynamic viscosity(DV)of the pyrolysis oil.A digital oxygen bomb calorimeter(XRY-1A,Shanghai Hu Yue Ming Scientific Instrument Co.,Ltd.)was used to determine the HHV.A thermostatic water bath(HH-4,Jintan City Hongye experimental instrument factory)was used to maintain the temperature of cooling water.

3.Results and Discussion

3.1.Effect of temperature on the pyrolysis products

3.1.1.Effect of temperature on the yield of oil and conversion degree

Fig.2 shows the influence of temperature on the conversion degree and yields ofpyrolysis oil and non-condensable gas in the vertical falling film reactor.In this study,the conversion degree is the mass ratio of the decomposed PP to the fed.The yield of pyrolysis oil is the mass ratio of the liquid oil condensed and collected to the decomposed PP.The sum of the yields of pyrolysis oil and non-condensable gas is equal to one.From Fig.2,as the pyrolysis temperature increases,the conversion degree increases slightly.All conversion degrees at different temperatures are very close to one,meaning that the material fed into the reactor is almost completely decomposed.Because the plastic is pure,no coke is generated during the pyrolysis process.When the pyrolysis temperature increases from 550 °C to 625 °C,the yield of oil decreases from 74.4 wt%(±2.2 wt%)to 53.5 wt%(±1.3 wt%),and the yield of noncondensable gas increases from 22.7 wt%(±2.7 wt%)to 46.5 wt%(±1.3 wt%)for the secondary cracking reactions[17].The pyrolysis process of PP pyrolysis mainly consists of initiation,propagation and termination.During the initiation stage,degradation follows the free radical mechanism,which proceeds via a random chain scission to form primary radicals,including exfoliation of the branched chain methyl groups and the breaking of C--C bonds along the polymer backbone.The primary radicals generated during the initiation stage generate random radicals with the same carbon number(secondary random radicals)via intramolecular hydrogen transfer reactions.This is due to the abundance of hydrogen in the polymer backbone[18].Because the energy of C--C bond in the β position is the lowest[19],these random radicals are prone to β-scission reactions,and produce alkenes and tertiary random radicals.The tertiary random radicals can absorb hydrogen,methyl random radicals and generate alkanes.The reaction ends when there are no random radicals.

Fig.1.Vertical falling film reactor for plastics pyrolysis.1-Control panel,2-Nitrogen inlet for pushing the molten plastics,3-Feedstock inlet,4-Melting tank,5-Non-condensable gas collector,6-Thermocouple,7-Glass window,8-Falling film plate,9-Heating tube,10-Nitrogen inlet for sweeping the glass,11-Volatile outlet,12-Condenser tube,13-Digital display thermostatic bath,14-Non-condensable gas treatment,15-Ice salt water.

Fig.2.Effect of temperature on the yield of pyrolysis products.

3.1.2.Effect of temperature on the composition of non-condensable gases

Fig.3 shows the composition distribution of non-condensable gas at different temperatures,as measured by GC.The main composition of the non-condensable gas is low carbon organic gas.The volume fraction of C3H6is more than 50%at 550°C.As the temperature increases,the volume fractions of C3Hnand C4Hndecrease,whereas the volume fractions of C2Hnand CH4increase.This is because C3H6and C4H8convert into CH4and C2H4by thermal decomposition at high temperatures[20].For the inorganic gas,the volume fraction of H2increases with the temperature,whereas the volume fraction of CO changes little.

Fig.3.Effect of temperature on the volume fraction of non-condensable gases.

3.1.3.Effect of the temperature on the composition of liquid oil

Fig.4 shows the composition distribution ofliquid oilatdifferentpyrolysis temperatures.The yield ofalkenes is greaterthan thatofalkanes.According to Levine et al.[21],intermolecular hydrogen transfer between two radicals is the main reason behind the formation of alkanes,and β-scission reactions of the radicals are the main reason behind the formation of alkenes,meaning that β-scission reactions are predominant during the pyrolysis process of PP.

Fig.4.Effect of temperature on the yield of oil according to the nature of the bonds.

As the pyrolysis temperature increases,the yield of alkenes gradually decreases from 54.2 wt%at 550 °C to 49.7 wt%at 625 °C.The yields of naphthenes and aromatics are much less than that of alkanes and alkenes.The yield ofnaphthenes is irregularwith temperature increasing.This is because the alkenes convert to naphthenes via the cycloaddition reaction,and the naphthenes convert into alkenes via the ring-opening reaction.The cycloaddition reaction is an exothermic and reversible process,and the ring-opening reaction is an endothermic process[22].The yield of aromatics increases with temperature increasing,from 0.4 wt%at550 °C to 10.2 wt%at625 °C.This is because the high temperature promotes the conversion of dienes and alkenes by such routes as the Diels-Alder reaction[23].At the same time,the chemical properties of the aromatics are relatively stable.The results of this research are in accordance with those from Demirbas[14].He found thatas the heating temperature increases from 452 °C to 602 °C,the yield of alkenes decreases from 42.1 wt%to 35.5 wt%,the yield of aromatics increases from 1.4 wt%to 10.2 wt%,and the yield of naphthenes first increases from 22.8 wt%to 24.4 wt%at 527°C,and then decreases to 23.5 wt%.

3.2.Comparison of PP pyrolysis products in different reactors

The reported reactors for pyrolysis mainly includefixed-bed(batch)reactors[16,24],rotary kilns[25], fluidised-bed reactors[13],etc.The fixed-bed reactor is characterised by its simple structure but low heat transfer coefficient and non-uniform temperature distribution for reacting materials,causing pyrolysis to occur at different temperatures simultaneously[26,27].The fluidised-bed exhibits good heat and mass transfer performance,but particle agglomeration generally begins with the formation of small agglomerates of bed material,leading to the “de fluidisation”phenomenon[28].The rotary kiln enables good mixing ofwastes and is typically used for slow pyrolysis[26],butwastes with high viscosity tend to stick to internal walls,causing heat transfer deterioration.The vertical falling film reactor can heat the molten PP up to pyrolysis temperature at a short time and have no rotating parts and does not require a fluidised medium,meaning there is no de fluidisation and medium inactivation.

3.2.1.Yields of oil in different reactors

Table 2 presents the comparison of the conversion degree and the yield of oil from pure PP pyrolysis in different reactors.It can be seen that the maximum oil yield is approximately 70 wt%in the tubular reactor[29,30],81 wt%in the medium fluidised-bed[13],and 74 wt%in the rotary kiln reactor at a speed of 1 rad·s-1[25].The yield of oil decreases with the increase of the heating temperature in all reactors[13,29,30].In the fixed-bed reactor,under an atmospheric pressure of 2 kPa,the yields of pyrolysis wax and pyrolysis oil can reach as much as 70 wt%and 25 wt%,respectively[16].Yan et al.[24]found that the yield of oil isapproximately 85 wt%in a semi-batch reactor at atmospheric pressure at 420 °C-460 °C.The yields of oil and non-condensable gas are mainly decided by heating temperature,heating rate(HR),residence time(RT),especially by HR and RT when pyrolysis at same temperature.The short RT and high HR can maximize the yield of pyrolysis oil[26].The if xed walls in a tube shape,and it has some characteristics similar to the fixed-bed,like low HR and low heattransfer coefficient[26].Miranda et al.and Yan et al.have relatively rigour pyrolysis condition for maximum yield of pyrolysis oil/wax.By comparison,it is found that the yield ofoilpyrolysed in the verticalfalling film pyrolysis reactoris slightly higherthan thatachieved in the tubularreactor,equalto thatin the rotary kiln reactor,and slightly lower than that in the medium fluidised-bed.

Table 2 Yields of oil from the different reactors(wt%)

Fig.5.Yields of oil for the different reactors.(a)The distribution of carbon atom numbers in the pyrolysis oilfrom differentreactors.(b)The composition distribution ofpyrolysis oil in different reactors according to the bond types.

Fig.6.Yield of pyrolysis oil collected by fractional condensation at different temperatures.

3.2.2.Comparison of the compositions of the pyrolysis oil in different reactors

Although the pyrolysis of PP produced in different reactors mainly n-alkanes and α-alkenes in liquid products via free radical mechanism,medium fluidised-bed have characteristic with high HR,and the flow rate of 0.3 g·s-1can make the pyrolysis volatile run outof the reactor in time avoiding long RT.The rotary kiln have characteristic with relatively low HR,but the rotating part can adjust its speed to reduce RT,as described by Ye et al.[25],where the speed of 1 rad·s-1has the most yield of liquid oil.The Tubular reactor include a family of reactors with the carbon number of hydrocarbons in liquid product depends heavily on the type ofreactorused[31].Fig.5 showsthe comparison ofthe composition distribution of the pyrolysis oil in differentpyrolysis reactors at 550°C,ofwhich the tubular reactor can refer to the reference[32].From Fig.5(a),we see that for the vertical falling film pyrolysis reactor,the range of the carbon atom numbers in the pyrolysis oil is from C6to C25,and the major constituents are C9,C12and C18,their yields are 28.8 wt%,29.7 wt%,and 24.3 wt%,respectively.For the tubular reactor,the range of the carbon atom numbers of the pyrolysis oil is from C9to C25,the major constituents are C12and C18and their yields are 30.4 wt%and 41.9 wt%,respectively.For the rotary kiln reactor,the range of the carbon atom numbers of the pyrolysis oil is from C11to C27;the major constituents are C12and C18,and their yields are 20.5 wt%and 48.2 wt%,respectively[25].In addition,the yield of light fraction,C6-C12,from the pyrolysis oil is 36.1 wt%in the tubular reactor,58 wt%in the semi-batch reactor[24]and 69.7 wt%in the vertical falling film reactor.Therefore,the carbon atomnumberfrom the pyrolysis oilproduced in the vertical falling film reactor is smaller,meaning that the light fraction content is higher.By comparison,the feedstock in the vertical falling film reactor is decomposed at constant temperatures simultaneously,which can make the compositions of pyrolysis products to be uniform.

Fig.5(b)shows the composition distribution of the pyrolysis oil according to the bond characteristics.The pyrolysis oil from all of the different pyrolysis reactors consists mainly of alkenes and naphthenes.Little aromatics are present at 550°C.Compared to the tubular and rotary kiln reactor,the yield of alkenes in the vertical falling film reactor is greater than that ofnaphthenes for the shorter RT of pyrolysis volatile and weaker cycloaddition reaction.

3.3.Distribution of pyrolysis oil collected by fractional condensation

3.3.1.Yields of HPO and LPO at different temperatures

Fig.6 shows the yield of oil for the two condensation stages at different temperatures.When the temperature increases from 525°C to 625 °C,the yield of HPO decreases from 55 wt%(±1.1 wt%)to 17 wt%(±0.46 wt%).This is because that more heat is absorbed by the C--C bonds at high temperatures,which makes a greater number of large molecules decompose into small and low pour point molecules via free radical mechanism.As the temperature increases,the yield of LPO increases,but the rate of growth gradually decreases.When the temperature is greater than 600°C,the yield of oil decreases slightly.This finding is observed because more small and low pour point moleculesconvert into non-condensable gas by secondary cracking.In addition,the high temperature also worsens the condensation effect.

Table 3 The main compositions of HPO and LPO

Fig.7.The composition distributions of HPO and LPO by GC-MS.

3.3.2.Composition distribution of HPO and LPO

Table 3 and Fig.7 show the composition distribution ofHPO and LPO at550°C by GC-MS.The carbon atom numbers of HPO range from C9to C25,and the majorconstituents are C12and C18.Their average molecular weight is 209.69 g·mol-1.Compared to HPO,the carbon atom numbers of LPO are relatively concentrated from C6to C12.The main composition of LPO is 2,4-Dimethyl-1-heptene,which accounts for 35.46%and the average molecular weight is 135.18 g·mol-1.Moreover,the main composition of the pyrolysis oil is α-alkenes,which indicates that β-scission reactions are predominant for PP pyrolysis,which is similar to the results from Miskolczi et al.[33].The short chain α-alkenes can be used as raw materials for polymers such as PP,polyethylene(PE).The long chain α-alkenes can be used as raw material for synthetic lubricants employed in the automotive,mechanical and aerospace industries.A small amount of HPO are condensed in the low temperature condensation section because some pyrolysis oil condensed and attached to the internal face of the condenser tube at the end of the experiment,which weakens the effect of condensation.

3.3.3.Properties of HPO and LPO at different temperatures

Currently,there is no common special criteria for determining the quality of waste plastics pyrolysis oil.Sarker et al.[34]used the sulphur contain and American Petroleum Institute(API)gravity to check for pyrolysis oil of PP.Pinto et al.[35]used the density,research octane number(RON),heat of combustion,etc.to check for pyrolysis oil of mixed plastics.In this paper,physical parameters such as the HHV and DV are tested and the values of chemical analysis allowed the theoretical calculation of RON using the method developed by Lovasic etal.[36].Some properties ofHPOand LPOare compared with commercial gasoline and diesel(GB17930-2013,GB19147-2013),as shown in Tables 4 and 5.

HPO is a dark brown and viscous liquid at ambient temperature.HPO's polycyclic aromatic hydrocarbons(PAHs)meet the requirements for standard diesel,and its density is close to that of commercial diesel.LPO is a liquid with little precipitation at ambient temperature.Compared,as shown in Table 5,the content of benzene and aromatics meet the requirements for standard commercial gasoline and the RON is lower.However,the alkene content is higher,which can be improved by a hydrogenation reaction[37].The DV of LPO decreases as the temperature increases.The HHV of HPO and LPO are very high,with each being greater than 40 MJ·kg-1.

Table 4 Properties of HPO at different temperatures

Table 5 Properties of LPO at different temperatures

4.Conclusions

In this paper,a vertical falling film pyrolysis reactor was constructed;the pyrolysis process of molten PP in the vertical falling film pyrolysis reactor was experimentally studied,and the effects of the heating temperature on the pyrolysis products are discussed.

1.With the temperature increases from 550 °C to 625 °C,the yield of pyrolysis oil decreases from 74.4 wt%(±2.2 wt%)to 53.5 wt%(±1.3 wt%),the major compositions of the pyrolysis oil are C9,C12and C18,and β-scission reactions are predominant.The content of the light fraction C6-C12of pyrolysis oil is 69.7 wt%.

2.The yield of oil from the vertical falling filmpyrolysis reactor is slightly higher than that from the tubular reactor,equal to that from the rotary kiln reactor,and slightly lower than that in the medium fluidised-bed reactor.

3.By fractional condensation,the main compositions of HPO are C12and C18,and the main compositions of LPO are C7,C8and C9.The heating temperature has a great effect on the yield of HPO,and little effect on the yield of LPO.

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