production of liquid fuels from recycled plastics using ...production of liquid fuels from recycled...
TRANSCRIPT
Production of liquid fuels from recycled plastics using acidic HNaY catalysts
Maja Jaśkiewicz
ABSTRACT
This study investigates thermal and catalytic cracking of high -density polyethylene and low-density
polyethylene ( HDPE and LDPE) using simultaneously thermogravimetry (TG) and
differential scanning calorimetry (DSC) techniques.
The catalyst chosen for this work were Ultra Stable acidic Y zeolites (USHY). For the purpose of the study,
the catalysts were subjected to ion exchange with sodium nitrate in order to obtain a set of different acid
contents catalysts. The amount of acid sites was calculated by resorting to temperature programmed
desorption (TPD) with ammonia. The relationship between the acidity and activity of the zeolites was
investigated.
The TG-DSC analysis allowed studying the conditions of thermal, as well as catalytic degradations. It was
found that the degradation temperatures decreased during the reactions involving catalysts and also that
the changes in acidity of the zeolites did not have a significant influence on the cracking process. Moreover
it was found that the structure of the polymer itself induces changes in cracking conditions. More
branched molecule – LDPE, presented lower mel ting temperatures as expected, and lower pre-cracking
temperatures than linear HDPE.
A kinetic model was fitted to the signals from TG-DSC in order to describe the kinetics of the process,
allowing the estimation of several kinetic and thermodynamic parameters. The estimated values showed a
decreas e in activation energy for the catalytic processes.
The degradation products were analyzed using gas chromatography (GC). It was observed that there is a
significant difference in product distribution between thermal and catalytic process. The presence of the
catalyst favoured the formation of C4-C6 hydrocarbons, whereas after thermal degradation the majority of
products were in C1-C4 range. The acidity of the zeolites had no significant influence on the product
distribution. Overall it was found that the amount of olefins was higher than paraffins for all of the
analyses.
1. Introduction
One of the problems of modern society
is waste management, and the management of
the plastic wastes became a major concern in
recent years, because of our consumer lifestyle.
The amount of plastics on Earth is enormous,
mostly since plastics have replaced materials
such as glass, ceramic or wood in the
manufacture of everyday use objects. Another
concern of our and future generations is
depletion of natural crude oil resources, and
what’s involved – constant increase of gasoline
prices.
Studies presented in this work present the
possibility of contributing to the solution of
these global problems, by recycling of plastic
wastes by incorporating them in petrochemical
industry, as a feed to FCC process. [1-5, 7-12]
The main objective of this work is to
study the production of liquid fuels by thermal
and catalytic degradation of polyethylene using
acidic HNaUSY zeolites as catalysts.
Furthermore the influence of zeolite acidity on
the catalytic degradation was investigated.
Degradations were performed on a
TG/DSC apparatus and the data collected was
evaluated with the use of kinetic model that
helped to understand better the differences
between degradation parameters for each
catalyst.
Part of this work was used for writing the
extended abstract “Catalytic Cracking of High
Density Polyethylene Using USY zeolites – a
TG/DSC Study” that will be presented at
CHEMPOR 2011 conference, held in Lisbon,
Portugal.
2. Experimental
2.1. Materials
The polymer, used for thermal and
catalytic decomposition in these studies, was
low and high density polyethylene. The material
was supplied by Borealis and it doesn’t possess
any additives.
The low density PE is in the form of small pellets
with the average molecular weight of
Mw =376000. High density PE has a powdery
form and its average molecular weight
Mw=290000.
The parent catalyst used in this work was a
commercial Y-82 zeolite with Si/Al ratio of 4.5
supplied by UOP.
2.2. Catalyst preparation and
characterization
To obtain different acidic strengths of
the catalyst ion exchange was performed. The
original Y-82 was subject to ion-exchange with
0.5M and 1M solutions of sodium nitrate. During
this step some of the protons were replaced with
sodium ions and by this modification the total
amount of Bronsted acid sites was decreased.
The ion exchanges were performed by stirring
the zeolite with sodium nitrate solution at room
temperature for four hours. The amount of
NaNO3 solution used was 4ml per gram of
catalyst. Afterwards the catalyst was filtrated
and washed with distilled water.
The catalysts after the exchanges were
designated with the names : Y-82 for the original
zeolite, Y-82_1 for zeolite exchanged with 0.5M
NaNO3 , Y-82_2 for zeolite exchanged with 1M
NaNO3 , Y-82_3 for zeolite two times exchanged
with 1M NaNO3 and Y-82_4 for zeolite three
times exchanged with 1M NaNO3.
To purify the catalyst material from any
trace compounds that might be adsorbed on its
surface, it underwent a calcination process that
was held in a tubular reactor by heating the
zeolite at 10oC/min up to the temperature of
500oC, temperature which was kept for 8 hours.
The calcination took place under a dry air flow of
0.5l/h per gram of catalyst.
The acidity of catalyst used in this work
was measured by ammonia Temperature
Programmed Desorption (TPD). This method
involves saturating the sampl e with an
adsorbate (in this case – ammonia) and heating
it according to a previously established
temperature profile. The procedure uses
ammonia because it is a strong base and a small
molecule that may access most of the acid sites
in zeolites. The TG-DSC apparatus also was used
for desorption measurements.
The saturation step, prior to TG-DSC
experiments, was carried-out by placing the
catalyst sample (20mg) and ammonia in Schlenk
thermostated at 40oC using a water bath for 8
hours.
2.3. TG and DSC analysis
Thermogravimetry and Differencial
Scanning Calorimetry analysis were used in this
study to obtain signals referring to the weight
change of the sample as well as the change in
heat flux, which allows us to followthe heat
changes in the sampl e, including the amount of
heat involved in the brakeage of polymer bonds.
The experiment consisted of two degradation
runs ( 1st and 2nd cycle respectively) and catalyst
regeneration run ( 3rd cycle, only when the
catalyst was present).
During the first cycle, the polymer
sample (pure or mixed with the catalyst) was
placed in a quartz pan and the whole system was
purged for 30 minutes with ni trogen, to wash
out all the oxygen from the heating chamber. The
sample was then heated according to the
temperature profile – 10oC/min up to 600oC.
After the first run was complete, and the sample
was cold again, a second cycle begun without
changing the conditions inside the furnace and
no new polymer added; this 2nd run was
performed to obtain the baseline for the DSC
signal. Both cycles were carried out under
continuous nitrogen flow rate of 80ml/min.
Catalyst regeneration was performed to
measure the amount of coke deposited on the
catalyst. Throughout the 3rd cycle, air, with the
flow of 75ml/min, was used to burn the coke at a
given temperature profile.
Thermogravimetric (TG) and
Differential Scanning Calorimetric (DSC)
analyses were performed in SDT 2960 DSC-TGA
apparatus
The amount of PE used for thermal and catalytic
degradation was around 10 mg, whereas the
amount of zeolite added was around 1mg.
2.4. GC analysis
The gas chromatograph used in the
experiments was Shimadazu GC-9A, working
under the ni trogen pressure of 2 bar, equipped
with flame ionizing detector (FID) and PLOT
column (KCl/Al2O3). The machine was connected
to Shimadazu C-R3A integrator. The gas
Chromatograph is equipped with the column
oven, flow control section, split/splitless injector
system and FID detector.
Gases for the analysis were collected
directly from the outlet of the TA Universal
Analysis 2000 apparatus. A Teflon tube
connected the apparatus to the opening of
volumetric flask filled with water (V=1000ml),
that was set up inverted on water container.
After the collection the gases produced during
polymer degradation were sampled using a
100µl syringe and injected to the GC.
3. Resuls and discussion
3.1. Acidity of catalysts
It was expected to obtain a TPD curve
with peaks only around 200-300oC, however
after running the blank tests and subtracting the
experimental and blank signals, at high
temperatures around 700oC another peak is
visible. Usually this peak corresponds to
dehydroxylation of the zeolite sample. Yet on the
blank test no peaks were observed for the
catalyst at these higher temperatures, which
indicates that this third peak in the ammonia
TPD curve seems to be related to very strong
acid sites.
As it can be noted in figure 1, the most
acidic catalyst, the parent zeolite, presents three
very distinctive peaks with a large area under
the curves, corresponding to the amount of
adsorbed ammonia, therefore the amount of acid
sites. The first two peaks refer to less acidic acid
sites, whereas the last peak seems tocorrespond
to very strong acid sites, because the higher
activation energy is needed for the desorption of
ammonia from those sites.
The behaviour of the ion-exchanged
catalysts is as expected. The higher the sodium
content in the zeolite the lesser the amount of
ammonia absorbed; however the strong acid
sites peak still occurs, albeit smaller, but it is
shifted towards higher temperatures, meaning
the catalyst still possesses strong acid sites.
It might be concluded from the graph that the
USHY family of zeolites present a very strong
catalytic activity due to having acid sites that
perform well at high activation energies.
Figure 1 Comparison between the ammonia TPD model curves obtained for different zeolites
To obtain the acid strength distribution
an activation energy grid was chosen to fit a
model for the desorption to the experimental
using the “Solver” tool in an Excel ( ©Microsoft
Corp.) spreadsheet. Energy values are in kJ/mol
and are the same for all of the catalysts; the grid
is as follows: 50, 60, 70, 80, 90, 110, 150, 160,
180, 200, 250, 280.
Calculating the quantity of ammonia per
each decomposition curve for all of the catalysts,
led to the information about the total number of
acid sites in catalysts. Those values are shown in
table 1. The most acidic catalyst presents the
highest amount of acid sites, as expected. The
zeolites which were exchanged with sodium ions
0,0E+00
5,0E-06
1,0E-05
1,5E-05
2,0E-05
2,5E-05
3,0E-05
150 400 650 900
dm
/dt(
mm
ol.
s-1
)
Temperature(ºC)
Y-82
Y-82_1
Y-82_2
Y-82_3
Y-82_4
in different concentrations, show smaller
number of acid sites than the parent catalyst.
Yet, it needs to be noted that the values of acid
sites do not vary significantly between the
exchanged catalysts, so it might be assumed that
the degree of exchange did not change much
with the exchange procedureand, thus, the
amount of available acid sites for this particular
catalyst.
Table 1 Total amount of acid sites
catalyst
acid sites
(mmol/mgzeolite)
Y-82 3.37x10-3
Y-82_1 1.21x10-3
Y-82_2 1.00x10-3
Y-82_3 1.07x10-3
Y-82_4 9.77x10-4
3.2. Polymer degradation
The DSC curves for degradation both
HDPE and LDPE are very similar: they consist of
one melting peak and two cracking peaks. The
first cracking peak might correspond to pre-
cracking of the polymer taking pl ace at the
surface of the catalyst. The second peak is the
cracking of the species formed during the pre-
cracking, probably taking place in the acid sites
inside the zeolite pores. [13]
The temperature of melting of LDPE is lower
than for HDPE.
Analyzing the data presented in table 2
it may be observed that there is little influence
between of the number of ion-exchanges and the
cracking temperatures of HDPE, because for all
of the zeolites the degradation temperatures are
similar. In relation to the temperatures of 1st and
2nd cracking peaks for LDPE there is a noticeable
trend of increasing the temperature needed for
the cracking process with decreasing acidity.
The Y-82 zeolite cracks LDPE at 402oC and then
at 432oC, the less acidic catalysts present higher
temperatures.
Degradation of LDPE with Y-82_4 zeolite
does not follow the increased temperature for
decreased acidi ty pattern. Although this catalyst
presents the least acidic sites and is the most
exchanged, it presents a cracking temperature of
LDPE comparabl e to those of Y-82_1 zeolite.
The degradation temperature for the
pre–cracking of LDPE is slightly lower than for
HDPE; the 1st cracking occurs at 402oC and
407oC for Y-82 zeolite for LDPE and HDPE
respectively. Neverthel ess, the temperatures of
pre-cracking for both polymers are more or less
in the same range 402-418o C.
The second catalytic cracking peak
occurs at wider distributions of temperatures,
especially for the degradation of low density PE,
and the values obtained for particul ar catalyst
are hard to compare. In general the 2nd cracking
occurs at lower temperatures for HDPE.
When investigating the TG curves, it
may be noticed that only during thermal
degradation the final weight of the sample was
zero, for the catalytic processes the final mass
equalled mass of the catalyst plus the deposited
coke. Also the weight loss for reactions involving
catalysts started more rapid, comparing with the
thermal degradation, the catalytic curves have
much more rigid appearance when the polymer
starts to vaporize.
Table 2 Degradation temperatures obtained in thermal and catalytic cracking for HDPE and LDPE
Melting
temperature (oC)
1st cracking temperature(oC) 2nd cracking
temperature (oC) E HDPE 134 483 - HDPE+ Y-82 133 407 430
HDPE + Y-82_1 133 411 433 HDPE + Y-82_2 134 406 433 HDPE + Y-82_3 134 418 427 HDPE + Y-82_4 133 410 435
LDPE 113 476 - LDPE + Y-82 113 402 432
LDPE + Y-82_1 112 415 435 LDPE + Y-82_2 112 418 447 LDPE + Y-82_3 113 418 452 LDPE + Y-82_4 113 412 439
3.3. Kinetic model
The degradation process occurs through
successive breakages of bonds and to describe
this process kinetic model was used in this
studies. The model illustrates the reactions
taking place during the pyrolysis in a certain
way – for each broken bond, some energy is
required, and this energy should be seen in the
signal of heat flow. Using only TG analysis it is
only possibile to observe the mass loss of a
sample and this only occurs when the products
are light enough to vaporize. Smaller, shorter
compounds produced during degradation are
volatile, so they will be detected when they leave
the system both by the mass signal and by the
heat flux signal, since they also require energy to
be vaporized. However, breakages leading to
heavier, non-volatile molecules, can be seen only
using the DSC signal. Only when combining the
two analysis, it is possibile to obtain a more
complete information about the degradation
process.
Hence, the model used was constructed
using a material balance, to the number of
bonds, and an energy balance of the sampl e in
time. If we assume that the polymer is a long
alkane chain, that has n carbon atoms, the
number of C-C bonds per unit mass, N, will be
given by the following expression:
N=
Taking into account that n is a very large
number, the amount of C-C bonds at the
beginning of the run will be close to 1/14, and
this value is assumed to be the bond density of
the sample.
(1) (1)
However, the number of the actual bonds that
may be broken is only half of this value,
supposing that the process is very effective.
According to this argument, the total breakable
bond density used for polyethylene in this study
was 1/28.
To perform the bal ance to the number of
breakable bonds we will have to take under
considerations all processes that lead to loss of
breakable bonds. That is, thermal and/or
catalytic cracking, and the evaporation process,
during which small molecules will carry to the
gas phase a certain amount of unbroken bonds.
Additionally to properly balance the number of
bonds, it was assumed that the process of
breaking the bonds is first-order in relation to
bond concentration.
Therefore the balance equation for the number
of breakable bonds is expressed as :
where dm/dt is the rate of weight loss and α is
the average number of bonds lost to the gas
phase.
The k(T), temperature dependent rate constant
is described by the Arrhenius law:
where Tref is a reference temperature (573 K in
this case), Ea is the reaction activation energy
and kref is the kinetic constant at the reference
temperature.
To estimate the heat flow in the experiment an
energy balance to the pan was performed, with
the assumption that the apparatus is capable of
correctly compensating the required flows.
Following this, the heat flow is given as:
Heat Flow =
where m is the weight of the sample at any given
time, obtained experimentally, Cp is the average
heat capacity of the mixture, ΔHC-C is the average
C-C bond enthalpy and ΔHvap the average
vaporization enthalpy per unit mass.
The model was fitted within the range of
the degradation temperatures obtained from the
experiments. The fi tting parameters were kref, Ea,
α, Cp, ΔHC-C, and ΔHvap. The last three parameters
were initially guessed from published data.
Equations 2 and 4 were solved
numerically, using the Euler method for each
run, and the model parameters were estimated
by least-squares procedure, using the sum of the
squares of the residues on the heat flow as the
objective function (O.F) to be minimized:
O.F =
The optimization was carried out using the
“Solver” tool in an Excel (©Microsoft Corp.)
spreadsheet. [5]
Due to reasons that have not yet been
identified the parameters fitted with the model
show little consistency with the results made in
other works.
Analyzing the data provided in table 3 it
is possible to conclude that presence of catalyst
decreases the activation energy as it was
expected and the values for the less acidic
catalysts are lower.
The second important conclusion is the
relationship between the branching of polymer
and the amount of heat needed for the cracking
to occur. HDPE being more linear polymer
presents higher activation energy than LDPE.
(4)
(5)
(5) (2)
(3)
Table 3 Model parameters obtained by fitting the kinetic model to experimental data for catalytic cracking
of HDPE with catalyst of different acidity
HDPE
Thermal
Degradation
HDPE+
Y-82
HDPE +
Y-82_1
HDPE +
Y-82_2
HDPE +
Y-82_3
HDPE +
Y-82_4
k ref(min-1 ) 3x10-5 3.72x10-7 5.40x10-6 2.92x10-7 5.83x10-6 5.19x10-6
Ea (kJ/mol) 111 74 81 77 82 80
ΔH c-c(kJ/mol) 145 41 51 46 48 61
alfa (Bondmol/g) 1,35x10-4 9.53x10-4 1.40x10-4 2.22x10-4 1.34x10-4 3.21x10-4
Eaα (kJ/mol) 114 89 236 311 153 166
ΔH vap (J/g) 276 431 485 477 399 416
Cp (J/g) 2.99 2.02 4.23 4.30 3.92 3.76
Table 4 Model parameters obtained by fitting the kinetic model to experimental data for catalytic cracking
of LDPE with catalyst of different acidity
LDPE Thermal
Degradation
LDPE +
Y-82
LDPE +
Y-82_1
LDPE +
Y-82_2
LDPE +
Y-82_3
LDPE +
Y-82_4
k ref(min-1 ) 6.27x10-5 6.27 x10-5 3.41 x10-7 5.69 x10-5 9.82 x10-5 6.18 x10-5
Ea (kJ/mol) 89 67 68 67 70 83
ΔH c-c
(kJ/mol) 196 19 20 176 315 192
alfa (Bond
mole/g) 2.25 x10-4 1.19 x10-4 1.28 x10-4 2.41 x10-4 1.47 x10-4 2.37 x10-4
Eaα (kJ/mol) 108 89 111 141 148 139
ΔH vap (J/g) 302 431 471 463 392 409
Cp (J/g) 2.50 3.26 3.37 3.62 3.39 2.90
3.4. Product distribution
It was noticed that the presence of
catalyst shifted the formation of products
towards longer and more branched compounds
with C4-C6 in catalytic cracking of HDPE as well
as LDPE.
Figures 3 and 4 illustrate the product
distribution as a func tion of carbon atoms.
The highest yield for all of the catalysts
was obtained for C4 products. Among them
there can be distinguished paraffin compounds
such as butane and isobutane, as well as olefins
with butene aas the most relevant.
Other products that were produced in
significant amounts for the catalytic processes
were C3 (propane, propene), C5 (pentane and
isopentane, pentene), C6 (hexane and hexane),
C7 (heptane and heptene) and C8 (octane and
octene). Hydrocarbons with more than 8 carbon
atoms consti tuted to less than 5% molar of total
obtained products.
The acidity of the catalysts didn’t
influence in a significant way the product
distribution for HDPE or LDPE cracking.
Figure 3 Product distribution obtained for catalytic cracking of HDPE
Figure 4 Product distribution obtained for catalytic cracking of LDPE
Table 5 presents the olefin to paraffin
and hydrogen to carbon ratio as well as molar
percent of aromatic compounds formed. It is
noticed that thermal processes for HDPE and
LDPE produce higher amounts of olefins than
catalytic processes.
Comparing only the catalytic reactions
of HDPE the highest production of olefins is
observed for Y-82_4 zeolite, with relatively high
concentrations of butene. The least amount of
olefins are produced by the Y-82_2 zeolite, O/P
ratio equals 1.4, meaning that there were almost
as much alkanes as alkenes.
The hydrogen to carbon proportion for
all of the catalysts was around 2.1.
Analysis of the aromatic compounds
content leads to the conclusion that there is also
no significant influence of acidity on their
formation. All of the catalysts present rather
0
5
10
15
20
25
30
35
40
1 2 3 4 5 6 7 8 9 10 11 12
Pro
du
ct d
istr
ibu
tio
n (
% m
ola
r)
Number of carbon atoms
HDPE + Y-82
HDPE + Y-82_1
HDPE + Y-82_2
HDPE + Y-82_3
HDPE + Y-82_4
HDPE Thermal Degradation
0
5
10
15
20
25
30
35
40
1 2 3 4 5 6 7 8 9 10 11 12
Pro
du
ct d
istr
ibu
tio
n (
% m
ola
r)
Number of carbon atoms
LDPE + Y-82LDPE + Y-82_1LDPE + Y-82_2LDPE + Y-82_3LDPE + Y-82_4
poor aromatic yields, with no more than 2% of
overall products.
Catalytic cracking of LDPE l eads to the
highest production of olefins for Y-82 zeolite, 2.4
olefin/paraffin ratio, this is connected with high
concentrations of propene, isobutene and
pentene ; and the lowest for Y-82_3 zeolite, with
the an olefin/paraffin ratio of 1.3.
Most of the catalyst produced only small fraction
of aromatics from LDPE during cracking, except
for Y-82_1 zeolite. The molar % of cyclic
aromatic compounds obtained in this latter case
is 8%.
Table 5 Olefin/Paraffin; Hydrogen/Carbon ratios and %molar of Aromatics for thermal and catalytic
degradation of HDPE and LDPE
Sample O/P ratio H/C ratio % aromatic
HDPE 6.1 2.1 1
HDPE +Y-82 1.8 2.3 1.7
HDPE +Y-82_1 1.9 2.1 0.6
HDPE +Y-82_2 1.4 2.1 0.4
HDPE +Y-82_3 1.6 2.1 1.5
HDPE +Y-82_4 2.6 2.1 0.1
LDPE 3.6 2.5 3
LDPE + Y-82 2.4 2.1 0.8
LDPE + Y-82_1 2.3 3.1 8
LDPE + Y-82_2 1.5 2.2 1
LDPE + Y-82_3 1.3 2.2 1
LDPE + Y-82_4 1.9 2.1 1.2
4. Conclusions
The experimental part of this work involved
preparation of Y-82 zeolites with decreasing
acidity and the evaluation of their performance
in catalytic cracking of two polymers – HDPE
and LDPE using TG and DSC analysis as well as
gas chromatography with comparison to thermal
degradation process.
The TG-DSC signals were fitted to previously
made kinetic model, which would calculate the
kinetic and thermodynamic parameters . This
step was done to better understand the kinetics
of the processes i tself and to distinguish the
major differences between catalytic and thermal
cracking.
During the thermogravimetric and differential
scanning calorimetric experiments it was
possible to observe the influence of the polymer
type and presence or absence of catalyst on the
cracking temperatures. First of all it was noticed
that the degradation temperatures for thermal
cracking were different for HDPE and LDPE. This
difference originates in the structure of the
polymers. As expected compounds with higher
degree of branching present lower cracking
temperatures, and what is connected with this,
lower activation energies.
For the catalytic cracking reactions the most
important observation was the occurrence of
two endothermic peaks relating to polymer
cracking. The first cracking peak is assumed to
be the pre-cracking happening on the catalyst
surface, whereas the second cracking is expected
to occur inside the zeolite pores. Another
observation was the expected decrease in
degradation temperatures during the reactions
involving catalysts.
The acidity of the catalyst had some influence on
the cracking temperatures, the zeolite with the
least amount of sodium required the lowest
temperature to crack the polymer. The
temperatures for catalysts with less active sites
were slightly higher, but comparable with each
other.
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