Investigation on thermal degradation of
polyethylene low polymer
Katsuhide Murata
K. Murata Research Lab, Toyohashi 441-8151, Japan
E-mail:[email protected]
1. Introduction
Waste plastics include the plastic product scraps once on the market and
the off-grade in raw material manufacturing plants. This off-grade polymer
is generally disposed as a factory waste because of its poor use. In this
chapter the thermal degradation experiment will be explained conducted
for the purpose of developing a new plant to obtain fuel oil by the thermal
degradation of the low polymerization constituents of high density
polyethylene (an industrial waste which was called “Low Polymer”,
hereinafter referred to as LP ).
LP is a by-product of the high density polyethylene manufacturing plant.
Previously, LP had been incinerated. However, there were the following
difficulties when incinerated, handling and transportation, generation of
black smoke, damage to furnace materials. In order to solve such
difficulties, the thermal degradation of LP was examined and the thermal
degradation experiment was conducted.
Like plastic products, LP is a straight-chain hydrocarbon, and can be
thermally decomposed to give low-molecular-weight hydrocarbon gas and
liquid. If the thermal decomposition process of LP is directly connected to
the polyethylene manufacturing plant, the pollution-free disposal of LP can
be performed, and fuel oil can be recovered at the same time. Furthermore,
the cost of handling and transportation can be reduced.
A lot of studies on the thermal degradation of polyethylene have already
been published [1-5]. However, all of them are due to the experiment of
batch operation in which small amount of samples are processed. Also, no
studies have been found that describe the behavior regarding the formation
of carbonaceous material during the thermal degradation of polyethylene.
In order to realize a continuous decomposition process of polymer, the most
important subject is how to reduce the carbon deposition on the heat
transfer surface.
In this study, a long-time experiment was conducted using a continuous
flow tank reactor equipped with a powerful stirrer. The decomposition rate
of LP and the composition and physical properties of products, and the
carbon formation behavior during the thermal degradation of LP were
investigated.
2. Experimental
2.1 Property and thermal analysis of LP
LP with a low molecular weight is produced as a by-product from
polyethylene manufacturing plant due to the insufficient polymerization
degree. When this LP is mixed in the product, the thermal and mechanical
properties of the product plastics are deteriorated. Therefore, it is
separated using a solvent in the final step. LP is a white solid at room
temperature, but when heated above 150°C, it melts into a viscous liquid
that can be pumped. The elemental composition of LP is almost the same as
the product polymer, and contains a small amount of ash. Table 1 shows the
properties of LP used in the experiment.
Table 1 Properties of LP
Thermal analysis of LP was performed in order to know the behavior of
LP with respect to temperature. Table 2 shows the conditions for the
thermal analysis of LP. And the results of thermal analysis are shown in
Fig. 1.
Table 2 Conditions for the thermal analysis of LP
Heating rate 6.45 ℃/min
Atmosphere N2 gas
Flow rate of N2 gas 150 ml/min
Amount of sample 93 mg
Fig. 1 Thermal analysis of LP
TGA, DTG, and DTA of LP are shown in Fig. 1. An endothermic peak is
seen in DTA at around 120°C, but no change is observed in TGA and DTG.
This indicates that a phenomenon corresponding to the melting or
transition of LP occurs around this temperature. Weight loss gradually
started at around 200°C. This gradual weight loss is considered to be due to
the volatilization of the remaining solvent in LP. There is a slight weight
loss from 300 to 400°C, but the DTA does not change significantly. The
volatilization of the low boiling point components mainly occurs rather than
Fig.2 Arrhenius plot of rate constant
decomposition. When the temperature exceeds 400°C, the weight loss
(TGA) becomes remarkably large and the rate of weight loss (DTG) also
rapidly increases. DTA has a large endothermic peak with the change of
TGA and DTG which indicates that the thermal decomposition of LP is
occurring. Beyond the thermal decomposition, the curves remain constant
and do not change.
Here, the decomposition rate of LP is examined using TGA and DTG
data. When the decomposition rate of polymer is linearly proportional to
the weight of polymer itself, the following equation holds.
(1)
m : weight of polymer, k : rate constant, t : time
Referring to Fig. 1, m was read
from TGA and -dm/dt was read from
DTG at each temperature, the value
of rate constant k was calculated,
and Arrhenius plot of k was
performed. The result of Arrhenius
plot was displayed in Fig. 2.
Fig.2 shows that the Arrhenius
plot of the rate constant k is bent at
about 420°C and 40% weight loss.
This indicates that different reaction
mechanisms work for weight loss of
LP. When the activation energies
were calculated from the slopes of
the two straight lines shown in Fig.
2, 5.7 kcal/mol was obtained below
420°C and 60.9kcal/mol above 420°C.
From these results, it is assumed
that the physical process such as evaporation is rate-determining below
420°C, and the chemical process such as decomposition is rate- determining
above 420°C. The thermal degradation of polymer is the latter, and its
temperature sensitivity is extremely high. According to the literature, the
activation energy for thermal degradation of polyethylene is reported to be
68 kcal/mol [6] and 69 kcal/mol [7].
2.2 Experimental setup and operation
The experimental setup is shown in Fig. 3. The raw material LP is put
into the melter (7), heated and melted therein. It becomes a viscous liquid
at about 200° C., and is supplied to the reactor (150φx 500H) by operating
the valve(15 or 16). A turbine-type stirrer is installed in the reactor, and
strong stirring is performed to prevent carbon from depositing on the vessel
wall. The temperature of reactor content can be controlled in the range of
420 to 455°C, and the supplied LP is rapidly heated up to this temperature
and thermally decomposed.
Fig.3 Experimental setup for thermal degradation of LP
Decomposition products are volatilized through a reflux condenser (9).
Some of the higher molecular weight products condense back into the
reactor and decompose again. The decomposition rate is controlled by the
reactor temperature and the outlet temperature of the reflux condenser.
The volatilized product is separated into a gaseous product and a liquid
product by a vent condenser (11). The decomposition products are cooled to
below 30°C by the vent condenser. The gaseous product that still does not
condense goes out of the system after measuring the flow rate through the
seal pot (13). On the other hand, the condensed liquid product is stored in
the receiver and its level is measured at regular intervals. The rate of
volatilization of LP can be calculated from the increased amount of liquid
product and the flow rate of gaseous product. When the receiver is full, the
valve (18) is opened and the liquid product is transferred to another
container.
When such a operation is continued for a long time, the concentration of
carbon (non-volatile product) in the reactor gradually increases, and the
possibility that carbon will precipitate on the heat transfer surface
increases. Therefore, we performed an operation to take out a part of the
reactor content at an appropriate timing. Since the reactor has an internal
pressure of about 500 mmAq due to the seal pot and the valve (19), the
withdrawal of the reactor content can be performed by opening the valve
(17).
The thermal energy required for decomposition was supplied by the
combustion of LPG. The combustion gas makes two laps outside the reactor
and then escapes to the chimney. A proportional gas burner (max. 3 kg/hr)
was used. The temperature was measured at each point shown in Fig. 3.
Since the liquid level in the reactor cannot be directly observed, the level
was detected by the temperature difference between the liquid phase and
the gas phase in the reactor. The raw material LP was supplied so that the
level would be constant.
2.3 Analysis methods of products
(1) Gaseous products were analyzed using a gas chromatograph with a
molecular sieve column and a TCD detector at 40 °C under a helium
carrier.
(2) Liquid products were analyzed by a gas chromatograph with a Golay
column (90 m, SE-30 silicon gum-rubber) using a temperature program
from 50 to 220 °C at a heating rate of 5 °C /min and a FID detector.
(3) Specific gravity was determined by float method.
(4) Kinematic viscosity was determined using a Canon–Fenske capillary
column viscometer.
(5) Average molecular weight was determined by freezing point depression
of benzene.
(6) Flash point was determined by JIS K2274 method.
(7) Calorific value was determined by JIS K2279 method.
(8) Ash was determined by JIS K2272 method.
(9) Iodine number was determined by Wijs method.
(10) Distillation test was performed based on JIS K2254. The average boiling
point was calculated from the result.
(11) Apparent density of the reactor content in working reactor was
measured as follows. Insert two stainless steel pipes (inner dia. 2 mm)
with a height difference of 150 mm, measure the pressure difference
between the two pipes while blowing a small amount of N2 gas into both
pipes, and obtain the apparent density. See Fig. 4.
Fig.4 Measurement of the apparent density
(12) Carbon concentration in the reactor was determined by Soxhlet
extractor.
Soxhlet extraction was performed using xylene as a solvent, and the
insoluble was taken as the carbon concentration. This carbon is defined
as the xylene insoluble and also includes ash. The ash content in the
xylene insoluble was about 10%.
3. Results and Discussion
3.1 Rate of thermal degradation
In all of the experiments, the raw material LP was fed so that the
reactor temperature was kept constant and the liquid level was kept
constant. Therefore, the mass balance for the reactor is expressed by the
following equation.
LP supplied = Liquid product + Gaseous product + Withdrawal (2)
As an observed result, the amount of gaseous product was about 4 wt%
of the amount of liquid product. The amount of withdrawal was set to 5 wt%
of the amount of liquid product. That is, the amount of LP supplied was
1.09 times the amount of liquid product. The production rate of the liquid
product was calculated from the slope by plotting the integrated amount of
the liquid product accumulated in the receiver (20) against time. The
volatilization rate of LP is expressed by the following formula in
consideration of the gaseous product and the liquid product.
Volatilization rate of LP [g/cm3/hr] =
rate of liquid product [g/hr] / working volume of reactor [cm3] x 1.04 (3)
In the above formula, the amount of gas is 4 wt% of the amount of liquid.
Equation (3) expresses the decomposition rate on a volume basis, but it is
also possible to convert it to a decomposition rate on a weight basis by using
the apparent density of the reactor content described below. When
discussing the thermal decomposition behavior of a polymer, it is common
to use a decomposition rate on a weight basis. But when designing a
thermal decomposition apparatus for a polymer, it is more practical to use
the decomposition rate on a volume basis, because the reactor volume can
be calculated immediately from the throughput and decomposition rate.
Fig. 5 shows the decomposition rate of LP. And from the Arrhenius plot
of this decomposition rate, its activation energy was calculated to be 48.6
kcal/mol. This value is smaller than the activation energy of 60.9 kcal/mol
obtained by the thermal analysis mentioned above. It is considered that the
apparent density of the reactor content decreases as the decomposition
temperature increases. That is, it is due to the difference between the
volume-based decomposition rate and the weight-based decomposition rate.
Fig.5 Decomposition rate of LP
Fig.6 Apparent density of reactor content in working reactor
3.2 Decomposition product of LP
(1) Gaseous product
Table 2 shows the composition of gaseous product. The average
molecular weight of gaseous product changes little with decomposition
temperature.
Table 2 Composition of the gaseous product
(2) Liquid product
The analysis result of the liquid product is shown in Fig. 7. The liquid
product contains a lot of hydrocarbons with a wide range of boiling points
from room temperature to over 400°C.
Fig.7 Gas-chromatograph analysis of the liquid product (Run 10)
In Fig. 7, they are shown separately for straight chain paraffin, 1-olefin
and other isomers. According to Fig.7, the liquid product consists of C5 to
C25 hydrocarbons, and the ratio of straight chain paraffin, 1-olefin and
other isomers is about 6:3:1. The straight chain paraffin is abundant in C10
to C20, 1-olefin is abundant in C6 to 15, and other isomers have a peak at
C9. It is also noted that the even numbered straight chain paraffins are
more than the odd numbered straight chain paraffins.
Table 3 summarizes the observed results of various properties of the
liquid product. The decomposition temperatures of runs 10 to 14 are 440,
435, 440, 450 and 455°C, respectively. The liquid products of Runs 10 and
11 were liquid at room temperature, and the liquid products of Runs 12 to
14 were waxy. The liquid products of Runs 10 and 11 were liquid at room
temperature, and it was a yellowish transparent liquid. The liquid product
has a wide boiling point range. It contains gasoline, kerosene and gas oil,
and also contains many double bonds. If the average molecular weight and
the average boiling point are compared, it corresponds to kerosene in
petroleum products. When estimated from the iodine number and the
average molecular weight, about 40% of the molecules have one double
bond.
Table 3 Properties of liquid product
The higher calorific value of the liquid product is about 11,000 cal/g,
which is the same as petroleum products. The flash point is low, since it
may be below room temperature. An example of a distillation curve is
shown in Fig.8. The distillation curve shows that the initial boiling point of
liquid product is 80℃ and the decomposition point is reached at a
distillation rate of 95%. As shown in Table 3, the average boiling point
changes significantly from 207℃ to 326℃ depending on the decomposition
temperature.
Fig.8 Distillation curve of the liquid product (Run 10)
(3) Reactor content
The reactor content is a degrading LP that remains in the working
reactor. It is decomposed to reduce its molecular weight, but it is still not
volatile. Table 4 shows the observed results of various properties of the
reactor content. The reactor content appears as a solid brawn wax at room
temperature, but becomes fluid when heated above 60°C.
Table 4 Properties of reactor content (Run 11)
3.3 Formation of carbon
One of the reasons why thermal degradation of polymers is not carried
out on an industrial scale is the carbon deposition on the heat transfer
surface of reactor. The deposition of carbon on the reactor wall impairs heat
transfer and reduces the ability of the plant. If it gets worse, it will lead to a
plant stop or an accident. Therefore, various measures have been taken to
prevent the carbon deposition in any pyrolysis process.
One of the important purpose of this experiment is to find a condition
for reducing the deposition of carbon on the reactor wall (operationally
acceptable range). Our concept for reducing the deposition of carbon is to
thin the layer on the vessel wall by vigorous stirring and to withdraw a part
of the reactor content to keep the carbon concentration in the reactor
constant.
Since the thermal decomposition naturally involves the reaction of
producing carbon, if this carbon formed is prevented from being deposited
on the vessel wall, the produced carbon will be accumulated in the reactor
content. Therefore, it is necessary to take out the reactor content and keep
the carbon concentration in the reactor constant. In this experiment, an
amount corresponding to 5% of the amount of volatile product was removed
from the reactor content and the concentration of carbon contained therein
was followed.
Here, the reactor system is modeled as shown in Fig.9.
Fig.9 Schematic diagram of the reactor system
The definitions of the symbols used are given below.
B : rate of the reactor content discharged [g/hr]
C : concentration of carbon [g/cm3]
F : feed rate of LP [g/hr]
R : rate of volatilization [g/cm3/hr]
Rc : rate of formation of carbon [g/cm3/hr]
t : time [hr]
V : reactor volume [cm3]
α: rate constant of formation of carbon [-]
ρ: density of the reactor content [g/cm3]
The following equation holds from the carbon balance in the reactor.
(4)
If the production rate of carbon is proportional to the decomposition
rate of LP regardless of the carbon concentration, the following equation
holds.
(5)
“α ” depends on the type of polymer, decomposition temperature,
reactor shape and material, etc. From equations (4) and (5), equation (4)
can be written as equation (6).
(6)
Integrating equation (6) under the initial condition of C = 0 when t = 0
yields equation (7) which represents the change in carbon concentration.
(7)
Further, in equation (7), if t is ∞ , equation (8) expressing the
equilibrium concentration of carbon in the reactor can be obtained.
(8)
C∞ : equilibrium concentration of carbon [g/cm3]
χ : B/RV (ratio of discharge) [-]
As an example, the experimental result of Run12 is applied to the
equation (7), and the relationship between t vs. C/ρ with α as a parameter is
shown in Fig. 10. The observed carbon concentration is also plotted in Fig.
10.
Fig.10 The observed and the predicted carbon concentration over time
(Run 12, decomposition temp. 440℃、discharge ratio 5%)
It is clear from Fig.10 that the carbon reaches the equilibrium
concentration (ca. 5wt%) after about 30 hours and maintains that
concentration. From this result, the value of α was estimated, and
according to Eq. (8) the relationship between the discharge ratio and the
equilibrium concentration of carbon is shown in Fig. 11. The equilibrium
carbon concentration increases as the decomposition temperature decreases.
It is considered that this is because when the decomposition temperature is
low, the decomposition must be repeated until the molecules become
smaller, and thus the chances of carbon production increase. That is, α
increases as the decomposition temperature decreases.
Fig.11 Discharge ratio vs. equilibrium carbon concentration
Table 5 shows the observed results for carbon deposit on the vessel wall.
Both agitation and withdrawal are considered to be related to the carbon
formation on the vessel wall. In Run 13, slightly hard carbon was seen on
the vessel wall near the liquid surface. Care must be taken even in actual
equipment.
Table 5 Observation of carbon deposit on vessel wall
2. Conclusion
Thermal degradation of LP (the low polymerization constituents of high
density polyethylene) was carried out using a continuous flow stirred tank
reactor. Then, the decomposition rate, the apparent density of the reactor
content in working reactor, the composition and properties of products, and
the conditions under which carbon does not deposit on the vessel wall were
investigated. In addition, the carbon concentration over time in the reactor
was modeled. The carbon concentration predicted from the model was
compared with the observed carbon concentration to determine the rate
constant of carbon formation.
The liquid product consists of C5 to C25 hydrocarbons, and the ratio of
straight chain paraffin, 1-olefin and other isomers is about 6:3:1. If the
average molecular weight and the average boiling point are compared, it
corresponds to kerosene in petroleum products. It has a wide boiling point
range containing gasoline, kerosene and gas oil, and also containing many
double bonds. Regarding carbon formation on the vessel wall, both agitation
and withdrawal are thought to be effective in preventing carbon formation
on the vessel wall.
Through this experiment, we were able to understand the behavior of
the thermal degradation of LP, and we were able to establish the prospect
for the industrialization of thermal degradation process of LP. The
remaining issues include the scale-up of apparatus, the carbon deposition
on the vessel wall after scale-up, and the material of the reactor and so on.
In the next chapter, we will explain the design concept to overcome
these issues that still remain before constructing a practical plant.
Literature
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