Investigation on thermal degradation of

polyethylene low polymer

Katsuhide Murata

K. Murata Research Lab, Toyohashi 441-8151, Japan

E-mail:afu26980@mars.odn.ne.jp

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