Pyrolysis for the Recycling of Polystyrene Plastic (PSP) Wastes in a Swirling Fluidized-Bed Reactor

159 Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) © 2006 Elsevier B.V. All rights reserved

529

Pyrolysis for the Recycling of Polystyrene Plastic (PSP) Wastes in a Swirling Fluidized-Bed Reactor Suk Hwan Kang\ Sung Mo Son", Pyung Seob Song", Yong Kanga* and Myoung Jae Choib a

School of Chemical Engineering, Chungnam National University, Daejeon 305-764, Korea(South) ( [email protected]) Advanced Chemical Technology Division, KRICT, Daejeon 305-600, Korea(South)

ABSTRACT Pyrolysis of waste polystyrene plastics (PSP) was investigated for the effective recycling. To obtain the kinetic information, a non-isothermal process (TGA/DTA) was used. The thermal decomposition was conducted in a swirling fluidized-bed reactor (0.762 m ID x 2.5 m in height). The apparent activation energy for the thermal decomposition of waste PSP was in the range of 50.4-72.1 kJ/mol according to the conversion level. The mean reaction order was 0.36. In the swirling fluidized-bed reactor, the optimum temperature, gas velocity and swirling ratio were 500cC, 0.4 m/s and 0.3, respectively, for the maximum yields of oil and styrene monomer. 1. INTRODUCTION Recycling to monomers, fuel oils or other valuable chemicals from the waste polymers has been attractive and sometimes the system has been commercially operated [1-4]. It has been understood that, in the thermal decomposition of polymers, the residence time distribution (RTD) of the vapor phase in the reactor has been one of the major factors in determining the products distribution and yield, since the products are usually generated as a vapor phase at a high temperature. The RTD of the vapor phase becomes more important in fluidized bed reactors where the residence time of the vapor phase is usually very short. The residence time of the vapor or gas phase has been controlled by generating a swirling flow motion in the reactor [5-8]. In the present study, the pyrolysis of a waste polystyrene plastic (PSP) has been investigated in a swirling fluidized-bed reactor to develop an effective reactor. Effects of the reaction time, temperature, ratio of the swirling gas and the gas velocity on the yields of an oil and a styrene monomer have been discussed. 2. EXPERIMENTAL Experiments were carried out in a stainless steel fluidized-bed reactor whose diameter was 0.762 m and 2.5 m in height, respectively, as can be seen in Fig. 1. The nitrogen was injected into the reactor through the perforated-type distributor installed between the main column section and the nitrogen box through which the main stream of nitrogen (primary) was fed to the reactor. The distributor contained 147 holes with triangular pitches and it was covered with a 400 mesh screen to prevent the bed material from weeping. Silica sand particle whose density and mean diameter were 2500 kg/m3 and 0.24 mm, respectively, were used as the bed materials. To generate the swirling flow pattern of the gas-solid mixture interior of the reactor, a swirling gas (secondary) was fed tangentially into the reactor at the wall of the reactor. The height of the swirling gas (nitrogen) injection port was 0.2 m from the distributor to avoid the end effect. The waste polystyrene plastic (PSP) was melted at 250°C prior to being injected into the reactor, to create particles with a mean diameter of 2 mm. About fifty grams of feed material (waste PSP) was fed into the reactor at a given operating condition. The oil product or styrene monomer was obtained

530 by means of a condenser. The yields of the oil and the styrene monomer were determined by the following equations [5]. Yield of oil; Yoil = ^ - x 100 , Yield of styrene monomer; YSM = ^ M - x100= WsM * Y°il x 100 (1) The outlet gas, which was obtained at the gas detection port, was analyzed by means of an on-line gas analyzer (GC-MS, HP-5890 plus, column; DB-1HT; GC-TDC; GC-FID). The yield of the gas product was determined by Eq (2). Yield of gas ; YGas =

-xlOO

x 100 =

Wf

(2)

Fig. 1. Experimental apparatus : 19—•—( 19

Vent

\-~12

13

11 8

TC

9

5

1. Fluidized bed reactor 2. Distributor box 3. Distributor 4. Drain 5. Freeboard 6. Electric heater 7. T-controller 8. Cyclone 9. Heat exchanger 10. Condenser 11. Mist filter 12. Gas sample bag 13. GC

18

TC

14. Regulator 15. Preheater 16. Flowmeter 17. Control valve 18. Water jacket 19. Purge gas

10

6 7 PT

1

3. RESULTS AND DISCUSSION

Swirling N2 stream

Typical variations of the temperature and the conversion rate in terms of the weight loss of waste PSP during the non-isothermal 3 TC 16 4 15 pyrolysis process (TGA/DTG) can be seen in 2 TC Fig. 2. In Fig. 2, the waste PSP would be TC N decomposed in the range from 370 °C to 17 485 °C at a heating rate of 10°C/min. The heating rate was either 10, 20 or 30*C/mm. One maximum peak in the conversion rate could be obtained at a reaction temperature of 430 °C. The activation energy was obtained from the plot of ln(dX/dt) versus (1/T) by using Friedman's method [9], The calculated activation energy at different conversion levels of the waste PSP can be seen in Fig. 3. In this Figure, the value of the activation energy tends to increase gradually with an increasing conversion level. The activation energy was distributed in the range from 50.4 to 72.1 kJ/mol according to the change of the conversion level [5]. The mean value of the reaction order could be obtained as 0.36. The melting of the PSP, initially started to be decomposed by cutting the relatively short carbon-hydrogen bond and the benzene radical which is attached to the main chain. With an increasing reaction time, however, the carbon-carbon bond of the main chain could be decomposed gradually. Thus, the activation energy would be distributed. Effects of the reaction time on the yield of the oil in a swirling fluidized-bed reactor can be seen in Fig. 4. In this figure, 82 wt.% of YOJI has been obtained in the reactor at 400 °C during a reaction of 34 min, while it can be increased up to 91 wt.% for a reaction time of 20 min at 550°C. TC

PT

14

2

531 Effects of the reaction temperature on the yields of the oil and styrene monomer can be seen in Fig. 5. In this figure, the increase of the reaction temperature can increase the values of voil yield (Yoy) as well as the SM yield (YSM)- However, the increase of the reaction temperature from 500 to 550 °C leads to a slight decrease of the oil yield. This can be due to the fact that the increase of the reaction temperature leads to an increase of the vapor phase products instead of the liquid phase. It could be stated that the optimum reaction temperature would be 500 °C for the recovery of the oil from the waste PSP within these experimental conditions. Also, the values of the yields increase gradually by increasing the ratio of the swirling gas (Rs)- With an increasing Rs, the size and rising velocity of the bubbles decrease while the number of bubbles increases due to an increase of the turbulence intensity and a more effective periodic gas flow for the breaking up of the bubbles. In other words, the bubble holdup in the bed tends to increase with the increasing Rj. However, the merits due to an increase of R s become marginal with a further increase from 0.3, especially at a high reaction temperature. 0.3

100

80

dX/dt [1/min]

Weight loss [%]

0.2 60

40

0.1

20

Activation energy [kJ/mol]

75

350

400

450

500

65 60 55 50

0.0

0 300

70

550

0.0

0.2

o

0.4

0.6

0.8

1.0 l.O

Conversion [ - ]

Temperature [fC] C]

Fig. 2. Typical TGA curve and rate of conversion for the pyrolysis of waste PSP (heating rate = 10°C/min).

Fig, 3. Calculated activation energies at different conversion levels of waste PSP.

100

100

80

3

60

40 •

Temp. o 400 C o 450 C o 500 C o 550 C

20

0

0

YOil & YSM

Yoil [wt%]

90

5

10 10

15 15

20

25

30 30

YOil

YSM

V2/V1=0 V2/V1=0.1 V2/V1=0.3 V2/V1=0.5

80

70

35

Time [min]

Fig. 4. Typical example of the yield of oil in a fluidizedbed reactor(V,/V]=0, UG=0.3m/s).

60

400 400

450 500 o Temperature [[°C] C]

550 550

Fig. 5. Effects of temperature on the yields of oil and styrene monomer in a swirling fluidized-bed reactor(Uc=0.3m/s).

Effects of the gas velocity (Uo) on the yields of the oil and styrene monomer can be seen in Fig. 6. Note in this figure that the values of the oil yield increase with an increasing UG at a relatively lower

532 gas velocity, but decrease gradually with a further increase in the gas velocity from 0.4 to 0.6 m/s. This could be due to the fact that the time period of the thermal decomposition of the waste PSP decreases with an increasing gas velocity in a swirling fluidized-bed reactor. Since the minimum fluidization velocity of the bed materials is 0.09 m/s, they cannot be fully fluidized at a relatively lower gas velocity. Thus, in these conditions, an increase of the gas velocity could lead to a more effective fluidization condition for a higher value of the oil yield. The optimum gas velocity for the maximum oil yield would be 0.4 m/s 100 within these experimental conditions. It has been understood that, in a relatively lower range of UQ, the fluidization motion 90 of the bed materials becomes more Y Y vigorous with an increasing UG, but in a V /V =0 V /V =0.1 higher range of UG, an increase of UG 80 V /V =0.3 leads to a further higher bed expansion, V /V =0.5 which results in a significant decrease of the holdup of the bed materials. These can 70 directly decrease the heat transfer coefficient and the mixing intensity in the fluidized-bed reactor. 60 YOil & YSM

Oil

0.2

2

1

2

1

2

1

2

1

SM

0.3 0.4 0.5 Gas velocity [m/s]

0.6

4. CONCLUSION

The waste PSP was effectively decomposed in a swirling fluidized-bed reactor. The apparent activation energy for the thermal decomposition of waste PSP, which was calculated by Friedman's method, was distributed in the range from 50.4 to 72.1 kJ/mol according to a change of the conversion level. The mean value of the reaction order was 0.36. The optimum temperature for the maximum yields of the oil and styrene monomer in the swirling fluidized-bed reactor was found to be 500°C. The optimum gas velocity was 0.4 m/s within these experimental conditions. With an increasing swirling ratio of the gas phase (Rs) up to 0.3, the yields of the oil and styrene monomer increase by increasing the turbulence intensity due to the swirling motion of the gas-solid mixture in the reactor. Fig. 6. Effects of gas velocity on the yields of oil and styrene monomer in a swirling fluidized-bed reactor (T=500°C).

ACKNOWLEDGEMENTS This work was supported by the 21C Frontier R&D Program, Industrial Waste Recycling R&D Center (Project 2A-B-1-1).

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