流化床裂解废塑料

Bed defluidisation during the fluidised bed pyrolysis of plastic waste mixtures *
Maria Laura Mastellone,Umberto Arena )
Department of Environmental Sciences,University of Naples II,Via Vivaldi,43-81100Caserta,Italy
Received 25October 2002;accepted 30April 2003
Abstract
The risk of occurrence of phenomena inducing worsening of fluidisation quality and eventually leading to bed defluidisation is one of the major constraints to an easier utilisation of fluidised beds for the low-temperature pyrolysis of plastic wastes.In order to investigate these phenomena,different mixtures of plastic wastes,obtained by combination of three commercially-available recycled polymers,were fed to a laboratory-scale bubbling fluidised bed reactor.Two mechanisms of defluidisation were identified and the time at which defluidisation occurred was measured and correlated to a key variable of the process.The observed phenomenology was also simulated by means of room-temperature experiments carried out with a selected mineral oil.The results allow us to demonstrate the existence of a synergistic effect between the observed mechanisms of defluidisation.Ó2004Elsevier Ltd.All rights reserved.
Keywords:Defluidisation;Plastic waste;Pyrolysis;Fluidised bed
1.Introduction
The series of advanced thermolytic technologies for plastic recycling (pyrolysis,gasification,hydrogenation),generally known as ‘feedstock recycling’,is economically and environmentally attractive when the waste has low homogeneity and is contaminated by non-polymeric constituents.The polymeric residues are processed into their basic chemical components and these can be used again as raw materials in refineries or in the petrochem-ical industry.In this way new plastics can be produced without any deterioration in their quality and any restriction to their application [1e 3].Fluidised bed pyro-lysis appears to be one of the most promising of feed-stock recycling technologies for a series of reasons.The almost constant temperature provides for uniform pro-
ducts;the suppression of side reactions increases the process controllability;the absence of moving parts in the hot region reduces the maintenance time and cost;the possibility of applying the process on a relatively small scale makes the range of investment alternatives wider.The disadvantages that are not yet completely eliminated are the necessity to limit the chlorine content in the inlet stream and the risk of fluidisation worsening as a con-sequence of solid agglomeration in the bed [4,5].
The latter aspect has been investigated in two recent papers [3,6]which refer to experiments carried out with single polymer feeding of polyethylene (PE),polypro-pylene (PP)and polyethylene terephthalate (PET).The results highlighted that the bed defluidisation can occur during the pyrolysis of plastics as a consequence of the agglomeration between the molten polymer and the inert material (which in turn depends strongly on molten polymer viscosity,and then on bed temperature and poly-mer molecular weight).The defluidisation phenomenon can occur in different ways,depending on the polymer type,and with different rates,depending on the reactor temperature,heat transfer and other operating con-ditions as the polymer feed rate and the bed amount.The sequence of steps that can occur after the injection
*
Based on a paper presented at the 2nd International Conference on Modification,Degradation and Stabilisation of Polymers (MoDeSt),Budapest,30June e 4July 2002.
)Corresponding author.Tel.:C 39-0823-274-603/274-414;fax:C 39-0823-274-605.
E-mail addresses:mlaura.mastellone@unina2.it (M.L.Mastellone),umberto.arena@unina2.it (U.Arena).
0141-3910/$-see front matter Ó2004Elsevier Ltd.All rights reserved.doi:10.1016/j.polymdegradstab.2003.04.002
Polymer Degradation and Stability 85(2004)1051e
1058
/locate/polydegstab
of a polymer particle into a fluidised bed pyrolyser and that can lead to segregation and defluidisation phenom-ena are schematically sketched in Fig.1.
The aim of this work was to extend the investigation to mixtures of different polymeric wastes that should be a most suitable feedstock for fluidised bed pyrolysers.The main scope was to define and quantify the mech-anisms of phenomena that affect the fluidisation quality and to verify the existence of a synergistic effect between them.To this end,three commercially-available recycled polymers were combined to obtain different mixtures of plastic wastes and fed to a laboratory-scale fluidised bed pyrolyser.The phenomenology observed in the experi-mental runs was then simulated by means of room-temperature tests carried out with a selected mineral oil.
2.Experimental 2.1.Apparatus
The experimental apparatus is a bubbling fluidised bed made of quartz and high-temperature austenitic stainless steel,described in detail elsewhere [6].In particular,it was equipped with a basket that allowed us to retrieve,at any time after the beginning of the run,agglomerates larger than bed particles.The room-tem-perature tests were carried out in a cold model,made of Plexiglas,which has an internal diameter of 108mm and is equipped with a feeding system allowing the con-trolled injection of a fixed flow rate of mineral oil.A
series of pressure transducers along the bed height allowed us to measure the time profiles of pressure during the experimental tests.2.2.Materials and procedures
The experiments were carried out with three different recycled polymers,polyethylene (PE),polyethylene terephthalate (PET)and polypropylene (PP),all ob-tained as cylindrical pellets from single material collec-tions and a successive process of mechanical shredding,washing and pelleting.Table 1reports some of their main chemical,physical and geometrical properties together with those of the mineral oil used in the experiments at room temperature.The different polymers were com-bined in order to obtain mixtures of plastic wastes with different percentages of polyolefins and polyethylene terephthalate (Table 2).
A fixed amount of inert materials (silica sand with a size range of 0.3e 0.4mm and particle density of 2600kg/m 3)was charged into the reactor at the beginning of each test and fluidised by air at a given velocity.When the desired temperature was reached,nitrogen was used as fluidising gas and a fixed feed rate of one of the prepared mixtures of plastic pellets was injected into the pyrolyser from the top.The standard procedure de-scribed in similar studies [3,6]for defluidisation tests with single-polymer feeding was modified to take into account properties of the different polymers of the mixtures.In particular,the feeding was suddenly stopped when defluidisation occurred.After a
period
Fig.1.Sequence of steps that can occur after the injection of a polymer particle into a fluidised bed pyrolyser and that can lead to segregation and defluidisation phenomena.
1052M.L.Mastellone,U.Arena /Polymer Degradation and Stability 85(2004)1051e 1058
of time sufficient to complete the devolatilisation of the polymers and to verify if fluidisation started again,the whole bed was retrieved by means of the basket.Its size distribution was then measured on both mass and numerical basis.
The tests with the cold model were carried out in a similar way.The mineral oil (AGIP F.1super motor oil 15W-40)was fed from the top of the cold model fluidised bed by means of a tube,1.6mm ID,connected to a volumetric pump.The mineral oil was chosen since its viscosity at ambient temperature lies in the range of the molten polymers tested at 450e 500(C.This allows us to use the experiments performed in the cold model to simulate some conditions occurring during the hot pyrolysis of plastics.
In all the tests with both the apparatus,the pressure at the bottom of the bed was continuously measured and recorded,and used as index of the fluidisation quality.In particular,the pressure vs.time profile indicates the time at which the defluidisation phenomenon occur:this defluidisation time was defined as that at which the whole bed suddenly leaves the fluidisation regime and becomes fixed.
3.Experimental results for mono-waste feed
The defluidisation phenomenon was studied by operating the fluidised bed reactor under conditions typical of low-temperature pyrolytic processes for plastic wastes [4,7].For all the three polymeric wastes,
the results highlighted that,at a fixed reactor temper-ature,the defluidisation time was linearly related to the value of the ratio between the bed solids hold-up and the polymer feed rate,which is a key parameter of the pro-cess.Diagrams in Fig.2summarise all the data obtained in these experiments under different operating condi-tions and report the linear relationships that fit the data very well for the specific temperature [3,6].
In the case of tests with recycled PE and PP,the bed defluidised as a consequence of sintering between sand particles (sequence 1-2-3-4-5-7-9in Fig.1).Some ag-gregates certainly formed (step 4)at early instants [6]but,since the polyolefins does not produce a sticky carbon residue,they crumbled fast (step 5)so setting several sand particles free.These are covered by a layer of polymer that,at the tested feed rate,had no time to completely devolatilise,so becoming progressively larger.The polymer present on the particle surface could be seen as a viscous liquid that completely wets the sand particles.When two of these particles collide,their relative velocity could be too low to overcome the viscous adhesion of the bridge between the surfaces.A recent paper [8]proposed a predictive model to esti-mate the occurrence of defluidisation under different operating conditions.The results agree well with those obtained in experiments with both PE and PP.
In contrast,the PET pellets showed a large formation of polymer-sand aggregates,which at the lower reaction temperatures (between 450and 650(C)had a lifetime long enough to generate accumulation,due to the presence of a sticky carbon residue.These aggregates
Table 1
Properties of recycled polymers used for experiments Tested material
Pellets of recycled PE Pellets of recycled PP Pellets of recycled PET Mineral oil AGIP 15W-40
Proximate analysis Moisture
e e e Volatile matter 99.9799.9688Ash
0.030.040.04Fixed carbon a e
e 12Viscosity,kg/ms
0.3(at 450(C)e 0.096(at 40(C)Material density,kg/m 310008501300884Softening temperature,(C 100165260e Melting temperature,(C
137183265e Pellets (diameter;length),mm
5;1
4.5;2.5
2.5;3
e
a
Evaluated in a TG system at 450(C.
Table 2
Experimental results of runs carried out with mixtures of different polymers at T Z 500(C Mixture 12345678910PET,%01025405564708090100PE-PP,%67-3360-3050-2540-2030-1524-1220-1013-77-30-0W bed ,g 360360360360360360360360360360Q mix ,g/s 0.060.060.060.060.060.060.060.060.060.06t def ,s >5400386312171144125124123126110x aggr ,%
5
6
8
9
10
12
12
16
17
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M.L.Mastellone,U.Arena /Polymer Degradation and Stability 85(2004)1051e 1058
covered a large fraction of the bed (up to about 25%at the lowest temperature)and had very high minimum fluidisation velocities.They are responsible for the beginning of segregation,with the consequent worsening of the fluidisation quality,and contribute to cause the occurring of defluidisation (sequence 1-2-3-4-6-8-9in Fig.1),as confirmed by the evidence of experiments carried out in the cold model.Some sintering between sand particles however occurred,probably promoted by the reduced momentum of bed material which,in turn,was determined by the segregation effect.An increase of bed temperature causes a progressive change of the defluidisation mechanism for PET pellets:at the lowest temperature,the aggregates played a predominant role since their accumulation inside the bed was remarkable and led to segregation effects,which rapidly led to defluidisation.At this temperature,the sintering be-tween sand particles occurred only when the fluidisation quality became very poor.At higher temperatures a different mechanism led to defluidisation (Fig.3):the accumulation of larger aggregates was progressively less pronounced up to become negligible and the worsening of fluidisation appeared mainly due to the progressive increasing of a polymeric layer on the sand as observed for polyolefins.No predictive model to estimate the occurrence of defluidisation during pyrolysis of PET is at the moment available.
The phenomenology observed in the experimental runs with PET was then simulated by means of room-temperature tests carried out with the mineral oil that,at
2500500075001000012500150001750020000W bed /Q PE , s
050100150200250300350400
4505000
2500500075001000012500150001750020000
05010015020025030035040045050002500500075001000012500150001750020000
50100150200250300350400450500d e f l u i d i z a t i o n t i m e , s
W bed /Q PP , s
W bed /Q PP , s
Fig.2.Experimental defluidisation times obtained with the tested polymers.
1054M.L.Mastellone,U.Arena /Polymer Degradation and Stability 85(2004)1051e 1058
this temperature,persists on the sand particles as a viscous layer.The tests showed a mechanism very similar to that observed for PET at temperature as low as 450e 500(C:each oil drop (or each part of the continuous feeding stream),just after its injection from the top of the reactor,covers a certain number of sand particles and keeps them together as a single agglom-erate.This goes down,whirled into the depression created by the rising of the closest gas bubble of the fluidised bed.As in the case of PET pellets,each agglomerate has a minimum fluidisation velocity larger than the fluidising velocity of the bed:as a consequence,it rapidly segregates at the bottom and progressively fills up the bed.When a critical fraction of the bed is filled by agglomerates,the bed defluidises.On the basis of experi-mental evidence,a descriptive model has been imple-mented in order to predict defluidisation phenomenon when this is activated by segregation of agglomerates.The defluidisation time due to this mechanism is evaluated by assuming that the bed is filled by the non-fluidisable aggregates that,linearly with the time,progressively substitute the individual sand particles.The linearity is experimentally evident and it is quanti-tatively defined by the feed rate of injected oil.On the basis of these considerations the following equation is derived:
A bed Áð1ÿ3bed ÞÁh ðt Þ¼W sand r sand C Q agg
r agg
Át ð1Þ
where:Q agg ¼Q Á
W agg W drop
and
W sand ¼W (sand ÿa ÁQ Át end
ð2Þ
where Q agg is the mass flow rate of produced aggregates;Q is the mass feed rate;r agg is the density of a single
aggregate;W agg is the mass of a single aggregate;W sand and W sand (are the mass of the sand in the bed at a generic time t and at t =0,respectively;a is the ratio of the mass of sand to that of oil in the single aggregate;t end is the time at which the oil feeding is stopped,which always coincides with the defluidisation time.
Assuming that the defluidisation occurs when the agglomerates completely fill the bed,i.e.when h (t def )=h bed ,it is possible to obtain t def as:t def ¼
A ÿ
B C
ð3Þ
where:A ¼
ð1ÿ3bed ÞÁA bed Áh bed Ár agg ÁW drop
Q ÁW agg
;
B ¼W (sand Ár agg ÁW drop
r sand ÁQ ÁW agg ;
C ¼
1ÿ
a Ár agg ÁW drop r sand ÁW agg
;
where t def is the defluidisation time;W drop is the mass of a single liquid drop;A bed is the bed area;r agg is the density of a single aggregate;3bed is the bed void fraction.
The equation has been used to estimate the time of defluidisation when this is activated by segregation mechanism.Table 3reports the experimentally-obtained parameters for two tests,the first performed in the cold model and the second in the pyrolyser fed with PET and operated at 450(C.The agreement between measured and calculated defluidisation time is excellent for the two kind of experiments,so confirming the experimental evidence of a substantially equal phenomenology.Fig.4shows that this very good agreement between the model and experimental results is confirmed for all the tests of PET pyrolysis at 450(C.Note that the influence of gas
102030405060708090100450
550
600
650
Bed temperature, °C
M a s s f r a c t i o n i n t h e d e f l u i d i z e d b e d , %
0.06-1.18mm >1.18mm 0-0.6mm
Fig. 3.Size distributions of the defluidised beds under different operating temperatures during the test with PET pellets.
Table 3
Values of parameters in the Eq.(3)obtained for a test carried out in the BFB cold model and in the BFB pyrolyser
Cold model
Pyrolyser at 450(C Q ,g/s 0.060.06W (s and ,g 713360W agg ,g 0.220.098W drop ,g 0.0210.0184r agg ,kg/m 31040444r sand ,kg/m 326002600A bed ,m 20.009330.002373bed ,e 0.60.6h bed ,m
0.110.11Experimental defluidisation time,s 303130Calculated defluidisation time,s
301
139
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M.L.Mastellone,U.Arena /Polymer Degradation and Stability 85(2004)1051e 1058
velocity is not considered by the model since the
experimental evidence showed that,in the investigated range of gas velocities,which is that typical of bubbling bed pyrolysers,it does not affect the defluidisation time and slightly influences only the shape and volume of aggregates [6].
4.Experimental results for plastic mixture feed It is reasonable that the behaviour of a mixture made of polyolefins (for which the sintering mechanism is absolutely predominant)and of polyethylene terephthal-ate (for which the segregation induced by aggregates was demonstrated to work together with sintering)will include both the observed mechanisms.A series of tests were therefore carried out with different mixtures of plastic wastes and at a fixed mass rate of 0.06g/s.A summary of experimental conditions tested and the measured defluidisation times is reported in Table 2.It can be seen that at a reaction temperature of 500(C,the behaviour of the two kinds of polymers and the corresponding defluidisation times were substantially different and the presence of just a small fraction of PET pellets induced a faster defluidisation of the mixture.The defluidisation time for test with only PE and PP at 500(C was in fact larger than 5400s,which reduced to about 390and 190s when the mixture contained 10or 25%of PET pellets.When the PET was increased up to 64%the observed defluidisation time became sub-stantially equal to that of the tests carried out with the PET alone.This synergistic interaction between the two mechanisms is also supported by the consideration that
the aggregate fractions found in the defluidised beds were always less than the value obtained for run carried out with only PET pellets.
The investigation was then extended to verify if,even for mixtures,the ratio W bed /Q mix can be assumed as a key variable of the defluidisation phenomenon.Four plastic mixtures,those indicated with #2,#3,#4and #10,were used for tests carried out at 500(C with values of Q mix equal to 0.04,0.06,0.075and 0.09g/s.The measured defluidisation times are reported in Fig.5as a function of W bed /Q mix .The linear relationship which relates the defluidisation times at a fixed temper-ature for a single polymer test with the ratio between bed solids hold-up and mass feed rate appears saved even for plastic mixtures.
The slope of the fitting lines decreases (i.e.the defluidisation time becomes shorter)when the PET content increases from 10to 25,40and 100%.This indicates that the PET pellets play a dominant role in the defluidisation phenomenon.Bearing this in mind,the defluidisation times were reported in Fig.6as a function of the PET content x PET .It appears that,for a given polymer feed rate,it is:
t def ;mix ¼g Áx ÿq
PET
ð4Þ
where g and q are necessarily function of the ratio W bed /Q mix and of the defluidisation behaviour of the PET pellets.
The next step of the study was then to look for a reliable expression of these two functions,taking in mind that the final goal of the research was to find a relationship able to predict the defluidisation time for
W bed /Q PET , s
050
100
150
200
d e f l u i d i z a t i o n t i m e , s
parison between defluidisation times calculated by Eq.(3)and those experimentally evaluated during tests with PET in the pyrolyser at 450(C.
W bed /Q mix , s
0100
200
300
400
500
600
d i z a t i o n t i m
e , s
i u l f e d Fig.5.Defluidisation times of polymers mixtures as a function of ratio between bed solids hold-up and polymer feed rate.
1056M.L.Mastellone,U.Arena /Polymer Degradation and Stability 85(2004)1051e 1058
a plastic mixture when the main operating variables were known and the behaviour of each single compo-nent was defined.The following expressions for g and q were obtained by processing the data in Fig.6:g ¼0:0085ÁW bed
Q mix
ÿ25:085
Át def ;PET
ð5Þ
q ¼ÿ7Á10ÿ5Á
W bed
Q mix
ÿ0:225ð6Þ
Substituting these expressions in the Eq.(4),the defluidisation time is given by:
t def ;mix ¼0:0085ÁW bed
Q mix
ÿ25:085
Át def ;PET
Áx ÿ7Á10ÿ5ÁW bed =Q mix ÿ0:225ðÞPET
ð7Þ
Fig.7describes how this relationship predicts well the measured defluidisation times.It can be used to estimate the defluidisation time of mixtures of polyolefins and polyethylene-terephthalate when the defluidisation time related to only PET is known from experiments or from the above proposed Eq.(3).
5.Conclusions
The defluidisation behaviour of plastic waste mix-tures under conditions of pyrolysis processes was investigated.The worsening of fluidisation quality was demonstrated to be the consequence of two different mechanisms,induced by the two kind of polymers present in the tested mixtures.
The interaction between these mechanisms was characterised and a key variable of the defluidisation phenomenon was identified.The measured defluidisa-tion times were then correlated to this variable by means of relationship of the same type of those found for single-polymer feed.A variation of mixture composition induces a slope variation while the linearity of the relationship remains.
The observed phenomenology was also simulated by means of room-temperature experiments carried out with a selected mineral oil.The results allow us to implement a descriptive model,able to predict the defluidisation time during pyrolysis of PET from plastic waste.
A relationship able to predict the defluidisation time for plastic mixtures when the main operating variables were known was finally obtained.
Acknowledgements
The authors gratefully acknowledge the financial support of the Italian Ministry of Instruction,Univer-sity and Research (MIUR).Recycled material was kindly provided by EUROPACK S.p.A.,a food pack-aging company in Pontinia (Italy)and Co.Re.Pla.,the Italian Consortium for plastic waste recycling.The authors are indebted to Stefania Barra,Valentina D’Aniello and Giuseppe Barbato for their help in performing experimental runs.
X PET ,
0100
200
300
400
500
600
d e f l u i d i z a t i o n t i m e , s
Fig.6.Defluidisation times obtained at 500(C for the PET alone and for mixtures at different W bed /Q mix .
experimental t def, mix , s
0100
200
300
400
500
600
c a l c u l a t e
d t d
e
f , m i x , s
parison between experimental and calculated data.
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M.L.Mastellone,U.Arena /Polymer Degradation and Stability 85(2004)1051e 1058
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