Heat Transfer in Vacuum Pyrolysis of Decomposing Hazardous Plastic

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Heat Transfer in Vacuum Pyrolysis of Decomposing Hazardous Plastic Wastes Jujun Ruan, Jiaxin Huang, Baojia Qin, and Lipeng Dong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00255 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Sustainable Chemistry & Engineering

1

Heat Transfer in Vacuum Pyrolysis of Decomposing

2

Hazardous Plastic Wastes

3 4

Jujun Ruan∗, Jiaxin Huang, Baojia Qin, Lipeng Dong

5

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation

6

Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, 135

7

Xingang Xi Road,Guangzhou,510275, People's Republic of China

8 9

ABSTRACT

10

Vacuum pyrolysis was widely used to decompose plastic wastes for obtaining

11

purity chemicals or remove hazardous organic pollutants. Pyrolysis temperature

12

decided the variety of pyrolysis products. The final (highest) temperature was the

13

most investigated factor. However, the temperature in vacuum pyrolysis was not

14

equally distributed. The lowest temperature also determined the variety of the

15

products. Regrettably, this vital factor has not been researched and reported. In this

16

paper, heat transfer process of vacuum pyrolysis was fully analyzed. The models for

17

computing the lowest temperature were constructed. The models were certified by the

18

vacuum pyrolysis of novolac epoxy particles collected from crushed waste printed

19

circuit boards. According to the models, we found the heat transfer coefficient of

20

crucible, inner diameters of crucible and alundum tube were the key factors for

21

controlling the lowest temperature. The models can help us to modify and adjust the

22

lowest and highest (final) temperatures in vacuum pyrolysis. It contributed to recover Corresponding author: Jujun, Ruan Tel:+86 20 84113620; Fax:+86 20 84113620; E-mail: [email protected]. 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

23

purity chemicals or remove the pollutants from plastic wastes by vacuum pyrolysis.

24 25

Key words: waste plastics, vacuum pyrolysis, heat transfer model, the lowest

26 27

temperature

28

INTRODUCTION

29

Since plastics were born, they have been widely used in every area of the world.

30

Plastics have excellent characters of flexibility, corrosion resistance, thermal

31

insulation, non-conductive etc. Thus, plastics are always used to manufacture

32

industrial products such as electronic products, chemical products, and living goods

33

together with metals, glass, or other materials (1). With the discard of the products,

34

abundant waste plastics were generated. Most of waste plastics were reused and

35

remanufactured. Parts of waste plastics were not reused and became hazardous

36

materials. Small size of mixed particles of different plastics, metals, and glass are

37

always considered as hazardous materials (Figure 1) (2, 3).

38 39 40

Figure 1. abundant hazardous tiny particles of different plastics, metals, or glass were generated in many plants of recovering plastics in China

41

If the percent of metals in the mixed particles is small, it will cause no interests to

42

treat or recover them. Most of them were transported to landfill or incineration (4-6). 2


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43

Abundant resources were lost in landfill and incineration process. Meanwhile, the

44

hazardous mixture of different plastics, metals, or glass contained abundant toxic

45

organics (fire retardants) and heavy metals. Environmental risks would be induced in

46

these treatments. Heavy metals or organic pollutants would expose to human and

47

environment (7-9). Thus, environment-friendly technology of recovering tiny particles

48

of different plastics, metals, or glass is pressing needed.

49

Vacuum pyrolysis technology was advised to decompose waste plastics for

50

recovering energy and chemicals (10-11). The pyrolysis process was operated under

51

the condition of vacuum. Without the atmospheric molecules disturbance, the reaction

52

rate could be accelerated, and the required temperature was reduced. Compared to

53

traditional pyrolysis, vacuum pyrolysis was an energy saving method (12). People

54

always pay close attention to the products of vacuum pyrolysis. Therefore, the

55

parameters (final temperature, heating rate, and atmospheric pressure) of vacuum

56

pyrolysis were optimized (13-15). For obtaining purity product or removing pollutants,

57

the relationship between particle characters (size, purity, impurity percentage),

58

catalytic agents, and the products were deeply researched (16-19). Meanwhile,

59

pyrolysis mechanisms or pyrolysis routines of organics were also well concerned (20,

60

21). Beside the above information, another important area of vacuum pyrolysis is

61

worthy to be paid closed attention to. However, little information was published about

62

it. It is how to know the lowest temperature of vacuum pyrolysis in the vacuum

63

furnace. To know the lowest temperature in vacuum pyrolysis is very important for

64

getting the pyrolysis products. It was well known that the temperature decided the 3



65

variety of the products (23, 24). However, the temperature in the pyrolysis furnace is

66

not uniform. Thus, the pyrolysis products are diverse. Mixed products bring little

67

economic benefits and might be considered as hazardous materials. In pyrolysis

68

process, the controllable temperature is the highest (final) temperature that we give it

69

to vacuum furnace to decompose the plastic particles. The lowest temperature which

70

also decides the variety of pyrolysis products is difficult to be controlled. If the

71

highest and lowest temperatures of pyrolysis process are determined, the species of

72

the pyrolysis products are limited. High purity products can be obtained or hazardous

73

pollutants can be concentrated.

74

In this paper, the heat transfer in vacuum pyrolysis process was analyzed and

75

models for computing the lowest temperature of pyrolysis process was constructed.

76

The models were certified by vacuum pyrolysis process of novolac epoxy collected

77

from crushed waste printed circuit boards. The models also told us what parameters

78

would impact the lowest temperature. The models can help us to adjust the lowest and

79

highest temperature in vacuum pyrolysis. It contributed to obtain purity products or

80

remove the hazardous pollutants from plastic wastes by vacuum pyrolysis.

81 82

MATERIALS AND METHODS

83

The employed plastic particles for pyrolysis

84

The novolac epoxy resin particles employed in this study were collected from

85

crushed waste printed circuit boards. Waste printed circuit boards were crushed to

86

liberate the metallic and nonmetallic materials. Then, the mixed particles of crushed 4


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87

waste

printed

circuit

boards

were

separated

by

two-step

high-voltage

88

corona-electrostatic separation. high-voltage corona-electrostatic separation was

89

skilled in separating the mixed particles which had different electrical conductivities.

90

The particles different in electrical conductivity would have different movement

91

trajectories in corona-electrostatic separation and so as to be separated. The first step

92

separation was used to separate metallic fraction from the nonmetallic fraction. The

93

second step separation was used to separate novolac epoxy resin particles from the

94

glass fiber particles (25). Particle size was ranged from 0.5-2 mm (presented in Figure

95

2a). The molecular structure of novolac epoxy resin was drawn by the software of

96

ChemBio 3D ultra and presented as Figure 2b.

97 98 99

Figure 2. Novolac epoxy resin particles collected from crushed waste PCBs

Vacuum pyrolysis equipment

100

Structure of vacuum pyrolysis furnace was presented as Figure 3. The equipment

101

of vacuum pyrolysis furnace was presented as Figure S1. There were three sections

102

(Heating A, Condensing B, Condensing C) controlled in different temperatures in the

103

vacuum furnace. Heating A was used to decompose the plastic particles with high 5



104

temperature and transform the organics to gases. Condensing B and Condensing C

105

were controlled in low temperature to condensate the gases. Mechanical pump and

106

diffusion pump were used to provide vacuum condition. They were also responsible

107

for moving the pyrolysis gases from heating area to condensing areas. Then, the

108

pyrolysis gases were cooled into oil in condensing B and condensing C areas.

109

Small-molecule gases which cannot be condensed were pumped out and collected.

110 111

Figure 3. The schematic diagram of vacuum furnace

112 113

RESULTS AND DISCUSSION

114

Analysis of the heat transfer in pyrolysis process of vacuum tubular furnace

115

The structure of empty vacuum tubular furnace was given in Figure 4a. The heat

116

was transferred from alundum tube to crucible and then to plastic particles. Sketch of

117

filled vacuum tubular furnace was given in Figure 4b. The inner diameters of crucible

118

and alundum tube were marked as r2 and r1 respectively (in Figure 4c). Alundum tube

119

was marked as i and crucible was marked as ii. Due to vacuum condition, there was

120

little fluid (gas) in vacuum tubular furnace before pyrolysis reaction. Thus, convective

121

diffusion of heat was neglected in the construction of heat transfer models. Hazardous

122

plastic particles contacted crucible suffered the solid heat transfer from the crucible

123

and then heat diffused to all particles. Meanwhile, the plastic particles on the top part 6


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124

suffered the radiation heat from alundum tube and then the heat diffused to all

125

particles. Generally, solid has faster heat transfer than radiation heat transfer.

126

Therefore, the plastic particles contacted with the crucible face had higher

127

temperature than the particles which were far from the crucible face. We supposed

128

heat transfers direction is rectilinear in both solid heat transfer and radiation heat

129

transfer (presented in Figure 4d). The plastic particles in point O should spend the

130

longest time to receive the heat transferred from the bottom of crucible and the top of

131

alundum tube. Therefore, the particles in point O had the lowest temperature in the

132

heat transfer process. We employed computer software to simulate the temperature

133

density distribution of plastic particles in the vacuum tubular furnace when the final

134

temperature was increased to 720 oC. The physical field was transient and solid heat

135

transfer. The diameter of the alundum tube model was 6 cm and the length was 8 cm.

136

The type of mesh was set as physical field control mesh, the unit size was

137

conventional. The heat transfer mode on the surface of the material and the inner wall

138

of the alundum tube was set as surface radiation heat transfer. The temperature of

139

outer wall of alundum tube was set as 720 oC. The simulation result was presented in

140

Figure 5. Figure 5 indicated that the temperature of plastic particles in the vacuum

141

tubular furnace decreased as the distance increasing from the crucible surface. Point O

142

was prospected had the lowest temperature in the pyrolysis process.

143

7



144 145 146 147 148 149

150 151 152

Figure 4. (a) Stereogram of alundum tube and crucible, (b) Stereogram of alundum tube, crucible, and organic particles, (c) Parameters of the alundum tube and crucible for the construction of models of heat transfer, (d) Heat transfer way from alundum tube to the organic particles, (e) Transfer ways of adsorption and emission energy

Figure 5. The simulation of the thermal-field distributions of the vacuum gasification furnace

153 154

Solid heat transferred between alundum tube, crucible, and plastic particles

155

The temperature of alundum tube was considered as final temperature and was

156

showed in the instrument of the equipment. The temperature of plastic particles

157

increased resulted from the contacting transferred heat and radiation transferred heat. 8


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158

We supposed the temperature of the particles closed to crucible had the same

159

temperature with the internal surface of crucible. Meanwhile, the temperature of the

160

internal surface of alundum tube had the same temperature with the outside surface of

161

the crucible. The shape of crucible is semi-circle of the concentric circle of alundum

162

in Figure 4a. Therefore, according to Fourier's law, the heat flow from the outside

163

surface of crucible to internal surface could be computed as:

164

Qi ,ii =

165

According to the definition of specific heat, the heat flow also can be expressed as:

166

Qi ,ii = ccTii

167

Then, equation (1) and (2) can be combined as:

168

λc ( r1 − r2 )

cc (Tii − Tr ) =

(Ti − Tii )

(1)

(2)

λc ( r1 − r2 )

(Ti − Tii )

(3)

169

Where Ti is the temperature of the outside surface of crucible (K), Tii is the

170

temperature of the inside surface of crucible (K), Tr is the room temperature (K), r1 is

171

the inner diameter of alundum tube (m), r2 is the inner diameter of crucible (m), λc is

172

the heat conductivity coefficient of crucible (W/mK), cc is the specific heat of crucible.

173

Equation (3) showed the relationship between the temperature (Ti) on the instrument

174

and temperature (Tii). The heat transferred from the internal surface of crucible to

175

point O can be computed as:

176

Qii ,o =

177

Where λp is the heat conductivity coefficient of the plastic particles, To is the

178

temperature of point O. Combining equation (3) and (4), we can compute the heat

λp r2

(Tii − To )

(4)

9



Page 10 of 24

179

transferred from the alundum tube to point O by the solid heat transfer and it was

180

expressed as:

181

 λ T + c T (r − r ) λ Qi ,o =  c i c r 1 2 − To  p  cc ( r1 − r2 ) + λc  r2

(5)

182 183

Radiative Heat transferred from the alundum tube to plastic particles

184

In general, Adsorption and emission energy in Figure 4e can be described as:

185

J = E + ρ G = ε Eb + (1 − α ) G

186

Where G is projected total radiant energy (W/m2), J is the effective radiant energy

187

after projecting (W/m2), E is the own radiant energy of the materials (W/m2), Eb is the

188

own radiant energy of black material (W/m2), ε is the surface emissivity of the

189

material, ρ is the surface reflection ratio of the material.

190

ρ = 1−α

191

Where α is the surface absorptivity. The radiation heat flow can be described as:

192

q = J −G

193

Meanwhile, q also can be expressed as:

194

q = E −αG

195

According to equation (8) and equation (9), the relationship between radiation heat

196

flow (q) and effective radiant energy (J) can be expressed as:

197

1  J = Eb −  − 1 q ε 

198

Figure 4d showed that the top surface of the filled particles and the alundum tube

199

formed an enclosed space. The equation of describing the radiation heat was:

200

qi ,o = J i X i ,o − J o X o ,i

(6)

(7)

(8)

(9)

(10)

(11) 10


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201

Where qi,o is the heat flow between the internal surface of alundum tube and point O

202

(W), X is the angle coefficient between surface i and point O. According to equation

203

(8), the effective radiant energy between surface i and point O can be expressed as:

204

1  J i = Ebi −  − 1 qi ,o  εi 

(12)

205

1  J o = Ebo −  − 1 qo ,i  εo 

(13)

206

According to energy conservation theorem：

207

qi ,o = −qo ,i

208

Then, combined equation (12), (13), and (14) to equation (11), we can obtain equation

209

(15).

210

qi ,o =

211

When the surface O is flat surface, the value of XI, o is 1. Equation (14) can be

212

simplified as:

213

qi ,o =

(14)

Ebi − Ebo 1 − εi 1 1− εo + + X i ,o εi εo

(15)

( Ebi − Ebo ) 1− εi (1 − ε o ) +1+

εi

(16)

εo

214

According to Stefan-Boltzmann's law, we knew that:

215

E = σ T 4 = 5.67 × 10 −8 T 4

216

Then equation (16) was simplified to:

217

T   T 5.67 × ( i )4 − ( o ) 4  100 100   qi ,o = 1 (1 − ε o ) +

εi

218

(17)

(18)

εo

Where Ti is the temperature of inner wall of alundum tube (K, temperature on the

11



Page 12 of 24

219

instrument), To is the temperature on the surface of filled organic particles (K).

220

According to equation (16), we could get the temperature relationship between the

221

inner wall of alundum tube and the surface of the filled organic particles.

222 223

Total heat transferred from alundum tube to plastic particles

224

The plastic particles suffered the energies from the contacting transferred heat

225

and radiative transferred heat. According to heat conservation law, the total heat flows

226

transferred from alundum tube to plastic particles can be expressed as:

227

(Q

228

Where t is the temperature rise time (min), c is the specific heat capacity of the plastic

229

particle (J/(kgK)) , T is the temperature variation (K). according to equation (5) and

230

equation (18), equation (19) can be expressed as equation (20).

231

 T  T  5.67 × ( i ) 4 − ( o ) 4     λ T + c T (r − r ) λ  100    100   c i c r 1 2 − To  p + × t = c∆T (1 ) − ε 1 ( ) c r r r − + λ  c 1 2  o c  2 +   εi εo  

232

Equation (20) can be used to compute the temperature (lowest temperature) of point O

233

in pyrolysis process of plastic particles in vacuum tubular furnace and guide the key

234

parameters of pyrolysis process.

i ,o

+ qi ,o ) × t = c∆T

(19)

(20)

235 236 237 238

Verification of the models for computing the lowest temperature We used equation (20) to calculate the temperature of point O (lowest temperature) in the vacuum pyrolysis of novolac epoxy resin.

239

12


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240 Table 1. The parameters for the heat transfer in vacuum gasification Parameters Unit Value λc W/(mK) 1.8-2.3 λp W/(mK) 0.22 r1 m 0.025 r2 m 0.022 εi / 0.9 εo epoxy resin / 0.7-0.9 cepoxy resin cc Tr

J/(goC) K

1.0 0.85 298.15

241

According to the parameters of the vacuum pyrolysis furnace and novolac epoxy resin

242

particles presented in Table 1, equation (20) was transferred into:

243

 T  Ti 4  ) − ( o ) 4   × t = To − 298.15  9.985 × Ti − 10To + 3.794 + 4.24 ×  ( 100    100 

244

For verifying the calculation model of lowest temperature, vacuum pyrolysis

245

experience of novolac epoxy resin particles collected from crushed waste PCBs was

246

performed. Firstly, thermogravimetric analysis of novolac epoxy resin particles was

247

investigated and the results were presented in Figure 6.

(21)

248 249

Figure 6. Nitrogen thermogravimetric curves of novolac epoxy resin of WPCB 13



250

Thermal decomposition process of weight loss of novolac epoxy resin particles was

251

divided into three stages. At the first stage (30-180 oC), the weight of novolac epoxy

252

resin particles decreased slowly, which is caused by the volatilization of free water in

253

the sample. At the second stage (180-580 oC),the novolac epoxy resin particles

254

suffered a severe weightlessness. Since there were two weight loss peaks in this stage,

255

pyrolysis reactions of novolac epoxy resin particles can be divided into the first

256

reaction (180-360 oC) and the second reaction (360-580 oC). The highest weight loss

257

peak of the DTG curve appeared at 313.6 °C (point a in Figure 6), which meant the

258

particles were decomposed rapidly at this temperature. Another weight loss peak

259

appeared at 449 oC (point b). When temperature was above 500 oC, rates of weight

260

loss became much slower. After 700 oC, TG and DTG curves became smooth, which

261

meant that the rates of weight loss were approximately 0, and the novolac epoxy resin

262

particles had been decomposed completely. The mechanism of novolac resin

263

degradation was discussed. The resin mainly underwent chain scission. The

264

methylene bridge bond (-CH2-) and the ether bridge bond(-C-O-C-) on the main chain

265

were easy to break. Monomer substances were produced. The small molecules

266

released from the resin and caused drastic weight loss. The detail degradation

267

mechanism of novolac resin was complex and was affected by heat and mass transfer

268

characteristics. The residue substance was char. The heat conductivity coefficient of

269

char was about 129 W/(m·K). It indicated that the residue had high thermal

270

conductivity compared to plastic particles. When resin particles were decomposed, the

271

char was formed gradually. The residue char might improve the contacting heat 14


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272

transfer.

273

30 g novolac epoxy resin particles were fed into the crucible placed in section

274

heating A of vacuum furnace. According to the results of TGA of novolac epoxy resin,

275

the temperature of section Heating A was set as 700 oC and the organics began to

276

pyrolysis. Crucibles placed in section Condensing B and Condensing C were used to

277

collect the pyrolysis products. The temperature of Condensing B and Condensing C

278

were set as 180 oC and 80 oC. The heating rate of the vacuum pyrolysis was set as 15

279

o

280

temperature in vacuum pyrolysis of novolac epoxy resin. The results were given in

281

Figure 6. Before the temperature reached 270 oC, there was no pressure change in the

282

furnace. It showed no moisture evaporation existed in novolac epoxy resin. When the

283

temperature increased from 670 oC to 700 oC, the atmospheric pressure in furnace

284

increased rapidly, and the pressure reached 105 Pa at 700 °C. It showed drastically

285

pyrolysis was proceeding and abundant pyrolysis gases were generated. This

286

atmospheric maintained for about 25min, and then pressure dropped sharply to the

287

initial value in vacuumed state. It showed that the pyrolysis reaction of novolac epoxy

288

resin particles was intense and concentrated, which was consistent with its weight loss

289

characteristic curve, and abundant gases were generated. The pyrolysis gases were

290

pumped to condensation area and the gases turned to liquid.

C/min. We investigated the relationship between pressure, pyrolysis time, and

15



291 292 293

Figure 7. The relationship between pressure, time, and temperature in pyrolysis of novolac epoxy resin

294 295

The established models for computing the lowest temperature in the vacuum

296

pyrolysis furnace were verified according to results of Figure 6 and Figure 7. The

297

curve of DTG indicated that novolac epoxy resin particles would be decomposed

298

when the temperature was greater than 275 oC. When the temperature reached about

299

305 oC, novolac epoxy resin particles lost the biggest weight. However, in the vacuum

300

pyrolysis experiments of novolac epovy resin particles, when the temperature reached

301

the related temperature (270-330 oC), only a few novolac epoxy resin particles were

302

decomposed and little gases were produced. The reason was that the mass of novolac

303

epoxy resin particles used in thermogravimetric analysis was 3.5 mg. the particles

304

were decomposed rapidly. In the vacuum pyrolysis experiment, about 30 g particles

305

were feed into the crucible. When the temperature was above 275 oC, only the

306

particles, closed to the crucible, began to be decomposed. Therefore, the mass of the

307

produced gases was small. 16


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308 309 Table 2. The temperatures of pyrolysis furnace for decomposing novolac epoxy resin Time (min)

Given (temperature tube, oC)

Temperature of alundum

Calculated temperatures of point O by the models (oC)

0 2 4 6 8 10 12 14 16

30 60 90 120 150 180 210 240 270

17.18 35.18 52.72 71.04 89.37 105.97 122.13 137.06 151.54

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

300 330 360 390 420 450 480 510 540 570 600 630 660 690 720

165.35 177.6 190.42 202.56 214.26 224.44 235.81 247.17 256.39 267.09 275.11 285.13 293.83 301.04 310.41

Related temperatures in Figure 5 and Figure 6, testing temperature (oC)

275 oC (in Figure 4, began to lose weight) 305 oC (in Figure 4, lost the biggest weight)

670 oC (in Figure 5, began to produce abundant gases)

310

According to Table 2, at this moment, the temperature in point O, computed by the

311

established models, ranged from 151.54-177.6 oC, which was far from 275 oC. It

312

meant that most of the novolac epoxy resin particle did not reach the enough

313

temperature to decompose. When the temperature increased to be above 670 oC,

314

abundant gases were produced in the vacuum furnace. It meant most of the novolac

315

epoxy resin particles began to decompose. According to Table 2, when the

316

temperature reached about 660-690 oC, the temperature of point O (the lowest

317

temperature), computed by the established models, was about 293.83-301.04 oC,

318

which was close to 305

o

C that caused novolac epoxy resin to decompose 17



319

tempestuously. Therefore, when the temperature reached 670 oC, the particles began

320

to decompose rapidly due to the temperature in point O (the lowest temperature) was

321

closed to 305 oC. In this case, most of the novolac epoxy resin particles began to

322

decompose. When the temperature of alundum tube reached 660 oC and 690 oC, the

323

temperature in point O is computed as 293.83 oC and 301.04 oC, which was closed to

324

the temperature of 305 oC presented by the result of Figure 6. This case was similar to

325

the vacuum pyrolysis experiment. When the temperature reached 670 oC, the novolac

326

epoxy resin particles began to decompose rapidly (this phenomenon showed that the

327

temperature in point O is close to 305 oC). in the other hand, it also indicated that the

328

established models for computing the lowest temperature is relatively accurate and

329

correct.

330 331

The application of the established models

332

The constructed models for computing the lowest temperature were important for

333

guiding the industry production of vacuum pyrolysis process. The applications of the

334

models were forecast as the following parts preliminarily. (i) it can be used to

335

optimize the key parameters of vacuum pyrolysis such as final temperature and heat

336

rate for improving the efficiency or obtaining a low-cost pyrolysis process. In general,

337

the final temperature of pyrolysis experiment is decided by the result of

338

thermogravimetric experiment. For the purpose of getting better pyrolysis results and

339

full decomposition of organics, the final temperature is always higher than the

340

temperature got from thermogravimetric experiment. However, if the final

341

temperature is set up at a low level, even higher than the temperature from

342

thermogravimetric experiment, the pyrolysis process will be ineffective and the 18


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343

products are uncontrollable. Meanwhile, some hazardous pollutants will be produced

344

in ineffective pyrolysis process; If the final temperature is set up too high, it can

345

obtain an effective pyrolysis process, however, it will cost so much energy. According

346

to the temperature got from the thermogravimetric experiment, we can set up this

347

temperature as the lowest temperature (temperature of point O) and use the models to

348

compute the final temperature for an effective and low-cost pyrolysis; (ii) the models

349

can be used to guide the design of the vacuum tubular furnace for improving the

350

effective of heat transfer in the pyrolysis process. According to equation (20), we

351

found that heat transfer coefficient of crucible, inner diameter of crucible, inner

352

diameter of alundum tube were the key parameters which impacted heat transfer. The

353

models can guide the design of the vacuum furnace by changing the key parameters

354

for obtaining high level of heat transfer, decomposing the organic particles in an

355

effective final temperature, and saving the energy. (iii) the models also can guide

356

directional pyrolysis process. The models could give accurate range of temperatures

357

(the highest temperature and the lowest temperature) for pyrolysis process. The lowest

358

temperature is the temperature of point O and the highest temperature is the final

359

temperature. It was well known that different pyrolysis temperature would generate

360

different products from organics. The models gave the clear relationship between the

361

lowest and highest temperatures of pyrolysis process. It will contribute to control the

362

accurate pyrolysis temperature for directional pyrolysis to obtain pure product or

363

avoid hazardous pollutants such as dioxin, organic bromide or others.

364 365 366 367

SUPPORTING INFORMATION Contents present the equipment of vacuum pyrolysis furnace. This material is available free of charge via the Internet at http://pubs.acs.org. 19



368 369 370

ACKNOWLEDGMENT

371

This work was supported by the National Natural Science Foundation of China

372

(51308488), the Science and Technology Programs of Guangdong Province

373

(2015B020237005, 2016A020221014), the Pearl River Star of Science and

374

Technology (201710010032), the Fundamental Research Funds for the Central

375

Universities (17lgzd22).

376 377

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TOC art

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Synopsis: Recover pure chemical products from waste plastics in green way was an important work in sustainable development area.

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Heat Transfer in Vacuum Pyrolysis of Decomposing Hazardous Plastic

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