PARTⅠ PYROLYSISPOLY(METHYL METHACRYLATE) PYROLYSISBY TWO FLUIDIZED BED PROCESS
Akinobu Sasaki,*1 Toshiro Tsuji2
12Mitsubishi Rayon, Otake, Hiroshima, Japan.
Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido, Japan.
E-mail: sasaki_ak@mrc.co.jp

Abstract: PMMA was continuously pyrolyzed by our new process consisting of pyrolyzer
and regenerator and crude MMA of purity 94-95% was recovered with a yield 95%.
Transparent MMA of purity 99.8% was obtained by distillation of the crude MMA. The
characteristics of this process are 1) PMMA is pyrolyzed by sensible heat of circulating sandbetween the two fluidized beds, 2) residual oil of distillation is used as a fuel at regenerator
where sand is heated up and soot derived from PMMA is incinerated, 3) at pyrolyzer, nitrogenis used as fluidization gas which is separated from pyrolysis product and reused. This process
was successfully scaled up from bench plant(15kg/hr) to demonstration plant(300kg/hr). In
comparison with MMA synthesis by natural resources, MMA production by PMMA pyrolysis
can reduce energy usage.
1. Introduction
PMMA pyrolysis has been intensively studied by many researchers1). PMMA pyrolysis
by fluidized bed is relatively new method. Kaminsky2) carried out experiments at alaboratory-scale plant with a capacity 3kg/h. The fluidized bed was heated indirectly fromoutside to temperatures between from 600 to 900 C. Because of the heating system, thismethod can not be done in large-scale plant. We invented a new process consisting ofpyrolyzer and regenerator. In this process, pyrolyzer is not heated from outside. Instead, hotsand is continuously circulating between pyrolyzer and regenerator. PMMA is pyrolyzed onlyby the sensible heat of the circulating hot sand. This enables the new process to be up-scaled.
2. Experiments
Experiments were conducted both at bench plant and demonstration plant. Each plantconsists of pyrolyzer, regenerator, condenser, mist-separator and distillation column.
2.1 Apparatus
Fig.1 shows apparatus. Tab. 1 summarizes the dimensions of pyrolyzer and regeneratorof bench plant. Hot sand is discharged out of pyrolyzer by screw and fed to regenerator. The- 79 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
discharge rate is controlled by the speed of screw rotation. Between the two reactors, twoautomatic valves exist, and these valves open and close by turns so that nitrogen in pyrolyzer
and air in regenerator will not mix with each other. Residue of PMMA discharged out ofpyrolyzer with sand is burned out in regenerator. Residue-free hot sand out of regenerator isfed to pyrolyzer.
Exhaust gas

Regenerator
Condenser

TankPyrolyzer

HotsandPMMAHot airHot sandNitrogen

Fig.1 ApparatusTab.1 Dimensions
Pyrolyzer

Bench Plant

Regenerator

Shape

DiameterHeightShape

DiameterHeighttube

0.35m

1.5m

tube

0.9m

2m

The mixed gas of pyrolysis gas and nitrogen is cooled and condensed by multi tube heatexchanger. This heat exchanger is followed by two mist-separators. The first mist-separator is
cyclone type and the second one is wire-mesh type. Nitrogen separated from pyrolysis
product is pumped up by a blower and fed to pyrolyzer again. A part of the mixed gas whichcontains volatile organic compounds is fed to regenerator and incinerated.
2.2 Material
Homo PMMA was supplied by Mitsubishi Rayon. Its size was 5mm in diameter. Sand
used was river sand and its mean size was 0.3mm in diameter. Minimum fluidization gas
velocity is estimated to be approximately 5cm/s.
2.3 Standard conditions
Tab. 2 shows standard conditions employed at bench plant.

- 80 -PARTⅠ PYROLYSIS
Tab.2 Standard and Conditions

Bench plant

QpQs
kg/s kg/s
0.004 0.04

Wskg
85

θsTs.ins
C
2.1E+03 580

T
Qn2 Tn2.in F1
C Nm3/s C
400 0.005 30
0.002

F20.02

3. Result and Discussion
3.1 Analysis of Pyrolyzer
In this analysis, we define two important factors F1 and F2. F1 is weight ratio of polymer
to sand in the pyrolyzer and defined by Equation 1. For stable operation, low F1 value ispreferred because polymer melt is viscous and may deteriorate the fluidity of sand bed.
WpWs
Wp under stationary condition in pyrolyzer can be estimated by Equation 2.
F1 =(1)dWp= Qp - k ´ Wp = 0(2)dt
k is rate constant of polymer pyrolysis at temperature T.k is estimated bythermogravimetric analysis.
dW= -k ´ W
dtThen, F1 can be expressed by Equation 4.
F1 =(3)

Wp
Qp=Ws k ´ Ws

(4)

In TGA experiments, we found k to be 0.02(1/s) at 400(degree C). In the case of benchplant, F1 is estimated to be 0.002.
Sand was fed and discharged at the same speed to hold constant Ws. Mean residence
time of sand is defined by Equation 5.

qs =WsQs(5)

As hot sand is circulating between the two reactors, a part of PMMA does not pyrolyze
in pyrolyzer and is discharged out of it. F2 is the weight ratio of unpyrolyzed polymer topolymer fed to the reactor and defined by Equation 6.
F2 =WpWs = Qs ´ F1 = 1Qp
Qpk ´ qsQs ´(6)For higher yield of PMMA pyrolysis product, F2 must be kept very small. Small F2value means that most part of PMMA pyrolyzes in the reactor.
- 81 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
θs is 2100(s) at bench plant. Then, F2 is calculated to be 0.02. It is apparent that lower
F2 value can be obtained at larger rate constant k (higher temperature T) or longer residence
time θs. As stated by Kaminsky2), MMA purity of the crude MMA inevitably decreases athigher temperature T. Preferable T is around 400 degree C in terms of relatively large rate
constant k and high MMA purity.
Scaling-up of pyrolyzer was carried out so that F1 and F2 at demonstration plant arealmost the same as those at bench plant.
3.2 Heat balance in Pyrolyzer
Heat balance in pyrolyzer is expressed by Equation 7.
Qp ´ [H 1 + H 2 + Cp × p ´ (T - Tp × in)] =Cp × s ´ Qs ´ (Ts × in - T ) + Cp × n2 ´ Qn 2 ´ (Tn 2 × in - T ) - EnergyLoss(7)

At bench plant, approximately 80% of sensible heat of hot sand was consumed topyrolyze PMMA. The value at demonstration plant increases to approximately 90% because
the ratio Qn2/Qp decreases from 1.2 at bench plant to 0.6 at demonstration plant. In ourprocess, nitrogen was not heated up before being fed to pyrolyzer. By using hot nitrogen, theenergy efficiency can be increased.
3.3 Properties of recovered crude MMA and distilled MMA
Tab. 3 shows the properties of crude MMA and distilled MMA at bench plant anddemonstration plant. The yield of crude MMA was the same, however its MMA purity
deteriorated by 1%.
Tab.3 Properties of crude MMA and ditilled MMA

Crude MMADistilled MMA

Bench Plant

Demonstration Plant

Yield

95%

95%

Purity95%

94%

Purity

99.8%

99.8%

3.4 Comparison between MMA synthesis and PMMA pyrolysis
There are two major MMA synthesis methods, namely ACH method and C4 method.
According to published books3-4), ACH method and C4 method consume 72MJ/kg-MMA and53MJ/kg-MMA to synthesize MMA, respectively. Based on our experiments at demonstration
plant, the energy consumption is estimated to be 10-13 MJ/kg-MMA when the total yieldfrom PMMA to distilled MMA is 90-70%. The total yield greatly depends on the MMA
purity of distilled MMA being aimed at.
4. Conclusion- 82 -PARTⅠ PYROLYSISPMMA was successfully pyrolyzed and very pure MMA was obtained at relatively
large-scale plant. By taking into consideration the two important factors, namely F1 and F2,
preferable mean residence time of sandθs turned out to be 30min or longer at 400C. PMMA
pyrolysis is advantageous in comparison with other MMA synthesis methods in terms ofenergy usage.
5. Nomenclatures
Qp : feed rate of polymer [kg/s]
Qs : feed rate of sand [kg/s]
Qn2 : feed rate of nitrogen [Nm3/s]
Wp : weight of polymer in pyrolyzer [kg]Ws : weight of sand in pyrolyzer [kg]θs : mean residence time of sand in pyrolyzer [s]k : rate constant of pyrolysis [1/s]H1 : heat of depolymerization [J/kg]
H2 : heat of vaporization [J/kg]
Cp.p : specific heat of polymer [J/(kg.K)]
Cp.s : specific heat of sand [J/(kg.K)]
Cp.n2 : specific heat of nitrogen [J/(Nm3.K)]
Tp.in : temperature of polymer fed to pyrolyzer [C]
Ts.in : temperature of sand fed to pyrolyzer [C]Tn2.in : temperature of nitrogen fed to pyrolyzer [C]T : temperature in pyrolyzer [C]References[1] A. Sasaki JETI, 2008, 56(14), 26.
[2] W. Kaminsky Journal of Analytical and Applied Pyrolysis, 1991, 19, 311.
[3] Eco-profiles of European Plastics Industry, Methyl Methacrylate APME, 2005.
[4] Inventry Analysis of MMA products (in Japanese) Sangyo Joho Knennkyu Center, 2003.

- 83 -