- Email: [email protected]
Study on the thermal risk of the ethylene-vinyl acetate bulk copolymerization
Thermochimica Acta 671 (2019) 54–59
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Study on the thermal risk of the ethylene-vinyl acetate bulk copolymerization
T
Feng Sun, Guangjian Wang
⁎
College of Chemical Engineering, Qingdao University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Ethylene-vinyl acetate copolymerization Thermal runaway Vent sizing Adiabatic test
The bulk copolymerizaition is an effective method to produce ethylene-vinyl acetate (EVA) copolymer. In this paper, the thermal risk of the EVA copolymerization was studied using vent size package 2 (VSP2) in order to avoid thermal runaway of the reaction. The effect of pressure, residence time, initiator concentration and heat input on the runaway reaction under adiabatic condition was investigated. The emergency relief system of the EVA reactor was sized. The results show that the EVA copolymerization reaction is autocatalytic and greatly influenced by pressure. The increase of the amount of initiator will significantly increase the reaction rate, thus is the worst case for relief system sizing. This study will provide the safety basis for the industrialization of the EVA bulk copolymerization process.
1. Introduction
2. Experimental
The ethylene-vinyl acetate (EVA) copolymer can be prepared by copolymeriztion of ethylene and vinyl acetate, then alcoholized to form ethylene vinyl alcohol (EVOH) copolymer. The EVOH copolymer is an excellent oxygen barrier due to its recyclability and transparent nature, which makes it an ideal film for food packaging [1,2]. There are four main processes for the EVA copolymer production: high pressure continuous bulk polymerization, medium pressure suspension polymerization, solution polymerization and emulsion polymerization [2]. The EVA copolymer [3,4] with 5–40 wt% vinyl acetate (VAc) content can be produced by the method of bulk polymerization. The polymerization process is hazardous because of its high thermal runaway risks [5,6] and the bulk polymerization is most harzardous among which due to the lack of solvent to dilute the heat released in polymerization. The polymerizations of vinyl acetate and ethylene caused many thermal runway accidents because of the high exothermicity involved [7,8]. Although some studies on thermal risk of polymerization have been reported [7–10], there is no research on thermal hazard of the EVA copolymerization. Therefore, we studied the thermal risk of the EVA copolymerization by vent size package 2 (VSP2). The effect of pressure, residence time, initiator concentration and heat input on the runaway reaction under adiabatic condition was investigated. The emergency relief size needed for the EVA reactor was calculated. The purpose of this study is to provide the safety basis for the industrialization of the EVA bulk copolymerization.
2.1. Samples
⁎
Vinyl acetate (99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Ethylene (99.9%) was supplied by Shanghai Shenkai Gas Technology Co. Ltd. Methanol (99.5%) was supplied by Tianjin Guangcheng Chemical Reagent Co., Ltd. Azobisisobutyronitrile (99%) was purchased from Tianjin Bodi Chemical Limited by Share Ltd. Vinyl acetate for the adiabatic test was purified by two times of vacuum distillation in order to eliminate inhibitor in the reagent. 2.2. Adiabatic tests VSP2 [11,12] manufactured by Fauske&Associates, Inc, was used to investigate the runaway reaction of EVA polymerization. The VSP2 makes an adiabatic circumstance by fast temperature and pressure tracing and balance between inside and outside the test cell. The VSP2 has small phi factor (around 1.1) and can simulate upset scenarios as loss of cooling, loss of agitation, mischarge of reagents, etc. So it is suitable to investigate runaway reaction and size the vent of reactor. The experiments were tested with closed cell test. The heat-wait-search (H-W-S) mode and heating mode with constant temperature rise rate were adopted [13]. The thermal inertia factor ϕ can be calculated as follows:
Corresponding author. E-mail address: [email protected] (G. Wang).
https://doi.org/10.1016/j.tca.2018.11.007 Received 7 June 2018; Received in revised form 6 November 2018; Accepted 12 November 2018 Available online 16 November 2018 0040-6031/ © 2018 Elsevier B.V. All rights reserved.
Thermochimica Acta 671 (2019) 54–59
F. Sun, G. Wang
=1+
mcell × Ccell ms × Cs
polymerization process, the typical thermal runaway scenarios were identified as follows:
(1)
Where mcell is the mass of cell, Ccell is the specific heat capacity of cell, ms and Cs are the mass and specific heat capacity of testing sample. About 50 g of vinyl acetate was added into the test cells. The mass fraction of ethylene is around 10%. The Cs of the mixture is assumed to be 1.5 J/(g °C). The mcell and Ccell of the cell are about 27 g and 0.5 J/ (g °C). So ϕ is calculated to be 1.15 using Eq. (1), which is small enough to simulate the reactor with volume of 4.8 m3. Therefore, the test result of VSP2 will not be corrected by thermal inertia factor.
(1) (2) (3) (4)
Improper pressure setting. Cooling failure or high temperature setting. Overcharge of initiator. Excesive heating or external fire.
The thermal risk of the EVA copolymerization was studied around these aspects.
2.3. Vent sizing equations
3. Results and discussion
Leung’s method and simplified equilibrium rate model (ERM) were used for calculating the required relief rate W and mass flow rate per unit flow area G [14]. For a vapor pressure system:
3.1. Effect of pressure
dT dt
q = 0.5Cf
+ r
dT dt
m
In the ethylene-vinyl acetate reaction system, ethylene is gaseous and needs to be dissolved in vinyl acetate in order to participate in the reaction. The solubility of ethylene in vinyl acetate varies greatly with pressure. Therefore, the thermal runaway characteristics of the EVA copolymerization with normal initiator concentration under different pressure were studied. The adiabatic environment was maintained from the initial temperature of 65 ° C. The temperature and pressure profiles in relation to time were recorded. Fig. 1 and Table 1 show runaway reaction characteristics of the EVA copolymerization with initial pressure of 3.9 MPa, 5.3 MPa and 6.1 MPa at the initial temperature of 65 °C. It can be seen that the adiabatic temperature rise (ΔTad) of the EVA copolymerization is about 100 °C. The time to maximum reaction rate under adiabatic condition (TMRad) increases with increasing initial pressure from 3.9 MPa to 6.1 MPa. For example, at 3.9 MPa the TMRad is about 20 min, which increases to 67 min at 6.1 MPa. From Fig.1 (c) (d) and Table 1, it can be seen the maximum temperature rise rate ((dT/dt) max) and the maximum pressure rise rate ((dP/dt) max) are 621 °C/min and 46.2 MPa/min respectively at the initial pressure of 3.9 MPa. When the initial pressure increases to 6.3 MPa, the (dT/dt) max and (dP/dt) max decrease to 81 °C /min and 3.6 MPa/min. The results demonstrate that reaction rate of the vinyl acetate polymerization is higher than that of the EVA copolymerization. When the initial pressure increases, the ethylene concentration in liquid increases, the proportion of the EVA copolymerization increases and the reaction rate decreases. For n-order reaction, the rate constant k can be determined based on adiabatic test using equation below:
(2)
Where Cf is average liquid specific heat capacity between the relief pressure and maximum accumulated pressure. (dT/dt)r is temperature rise rate at the relief pressure. (dT/dt)m is temperature rise rate at the maximum accumulated pressure. The mass flow rate (W) can be calculated as follow:
mR q
W= V × hfg mR fg
2
+
Cf T
(3)
Where mR is mass in reactor at the relief pressure, V is volume of reactor, hfg is latent heat of vaporization, vfg is difference between vapour and liquid specific volumes, ΔT is temperature difference between the temperatures at the relief pressure and maximum accumulated pressure. vfg is given by: fg
=
1
1
g
f
(4)
Where ρg is gas density, ρf is liquid density. The two-phase mass flow rate per unit flow area (G) can be determined using equation below:
hfgr
G= fgr
Cfr Tr
(5)
k=
Where subscript r means at relief pressure. The required relief flow area (A) and diameter (d) can be evaluated as follows:
W A= G
d=
4A
(
Tmax Tmax
lnk = lnA
(6)
)
dT / dt
T n (Tmax T0
Ea RT
T0 ) C0n
1
(8) (9)
Where n is the order of reaction, C0 is initial concentration of reactant, A is the pre-exponential factor, Ea is the activation energy, and R is the universal gas constant. The plots of lnk versus 1/T are expected to be a straight line, providing n is chosen correctly. It would be zero order reaction if the curve of ln (dT/dt) vs. -1/T is a straight line. Fig. 2 shows the ln (dT/dt) vs. -1000/T curves of the EVA copolymerization with different pressure. From Fig. 2, the profile of the curves shows that the increasing range of reaction rate decreases with polymerization at the initial pressure of 6.1 MPa. It is approximately zero order reaction of the EVA copolymerization with initial pressure of 5.3 MPa because the curve is almost a straight line. When the initial pressure decreases to 3.9 MPa, the increasing range of reaction rate increases with polymerizaion. The reduction of initial pressure of the EVA copolymerization makes the autocatalytic effect become more significant. Table 2 lists (dT/dt)max, (dP/dt)max, and the maximum pressure (Pmax) of runaway reaction on the EVA copolymerization with
(7)
2.4. Thermal runaway scenarios The ethylene-vinyl acetate copolymerization is initiated by azobisisobutyronitrile at 3–6 MPa and 50–65 °C. The reaction equation is as follows [3].
Methanol and azobisisobutyronitrile is 5 wt% and 0.1 wt% of vinyl acetate respectively under normal conditions. The maximum filling ratio is 80%. A preliminarily hazard analysis was performed for the EVA 55
Thermochimica Acta 671 (2019) 54–59
F. Sun, G. Wang
Fig. 1. Thermal runaway of the EVA copolymerization with different pressure. Table 1 Thermokinetic data of thermal runaway on the EVA copolymerization with different pressure. P0/MPa
T0/°C
Tmax/°C
ΔTad/°C
TMRad/min
Pmax/MPa
3.9 5.3 6.1
65 65 65
164 181 169
99 116 104
20 55 67
8.8 10.3 11.9
Table 2 Data of runaway reaction on the EVA copolymerization with different initial pressure. P0/MPa
(dT/dt)max / °C /min
(dP/dt)max / MPa/min
Pmax /MPa
3.5 3.9 4.5 5.3 6.1
3604 621 477 158 81
42.5 46.2 11.8 7.2 3.6
8.1 8.8 9.2 10.3 11.9
Fig. 3. The temperature versus time curves of the three experiments.
different initial pressure. From Table 2, we can see that with the increase of initial pressure, the (dP/dt)max decreases, the Pmax increases. It indicates the reaction rate of vinyl acetate polymerization is higher
Fig. 2. The ln (dT/dt) vs. -1000/T curves of the EVA copolymerization with different pressure.
56
Thermochimica Acta 671 (2019) 54–59
F. Sun, G. Wang
Fig. 4. Thermal runaway of the EVA copolymerization with normal ratio and 1.8 times of initiator at initiating pressure of 4.5 MPa (a) temperature vs. time, (b) pressure vs. time. Table 3 Thermokinetic data of the EVA runaway reaction with normal ratio and 1.8 times of initiator. sample
T0/°C
Tmax/°C
TMRad/ min
ΔTad/°C
(dT/dt)max/°C /min
Pmax / MPa
(dP/dt)max/ MPa/min
Normal ratio 1.8 times of initiator
80 60
201 206
16 17
121 146
477 997
9.2 9.4
11.8 32.9
3.2. Autocatalytic effect of the EVA copolymerization
sample
intercept
slope
A/ 1/min
Ea / kJ
Autocatalytic reaction is common in fine chemistry. The product of an autocatalytic reaction can act as a catalyst on the reaction course. The autocatalytic reaction has a period of induction. During the induction period, the reaction rate does not increase. But once it is induced, it can react violently in a short time [15], so the reaction with autocatalytic effect is very hazardous. In order to investigate the autocatalytic effect of the EVA copolymerization, three experiments were designed at the initial pressure of 5.3 MPa. In the first experiment, the heating-waiting-searching (H-W-S) mode was used starting from room temperature. In the second experiment, the reactant first be heated to 65 °C, then an adiabatic environment was maintained (W-S mode). In the third experiment, the reactant was maintained at 65 °C for 7 h to finish the reaction, then adiabatic mode was adopted. Fig. 3 shows the curves of temperature versus time at different experiments. In the second experiment, the TMRad from 65 °C is 55 min. But in the third experiment, the TMRad from 65 °C is shortened to 29 min after the 7 h reaction at constant temperature. The reaction rate of the EVA copolymerization increases with the decrease of reactant concentration, indicating that the reaction product acts as a catalyst. In the first experiment, the VSP2 detects the exothermic reaction at 88 °C, and the TMRad is only 4 min, which showing a typical autocatalytic characteristic [12]. Therefore, it is not recommended to control the process temperature refer to the onset temperature of the EVA exothermic reaction. The process temperature should be controlled strictly during the reaction proceeds due to the increase of thermal risk.
Normal ratio 1.8 times of initiator
30.35 32.51
12.38 12.58
1.516E+13 1.315E+14
102.9 104.6
3.3. Effect of initiator concentration
Fig. 5. The ln [(dT/dt)/(Tmax-T0)] vs. -1000/T curves of the EVA copolymerization with different concentration of initiator. Table 4 The A and Ea of the EVA copolymerization with normal ratio and 1.8 times of initiator at initiating pressure of 4.5 MPa.
The EVA copolymerization is a chain reaction. The initiator decomposes to free radicals, then starts the chain reaction. The concentration of initiator may have great influence on the reaction. Therefore, the effect of initiator concentration on the thermal risk of EVA copolymerization was investigated. Fig.4 and Table 3 show adiabatic test results of the EVA copolymerization with normal ratio and 1.8 times of initiator at initiating pressure of 4.5 MPa. It can be seen that when the initiator concentration
than that of EVA copolymerization. When the initial pressure increases, the ethylene concentration in liquid increases, the proportion of the EVA copolymerization increases and the overall reaction rate decreases.
57
Thermochimica Acta 671 (2019) 54–59
F. Sun, G. Wang
Fig. 6. Thermal runaway of the EVA copolymerization on adiabatic condition and heat input condition with the heating rate of 2 °C/min at initiating pressure of 4.5 MPa (a) temperature vs. time, (b) pressure vs. time. Table 5 Thermokinetic data of the EVA runaway reaction with adiabatic condition and heat input condition. Test condition
T0 /°C
Tmax /°C
TMR/min
ΔT/°C
(dT/dt)max/ °C/min
Pmax/ MPa
(dP/dt)max/ MPa/min
Adiabatic Heat input
80 —
201 208
16(from 80 °C) 17(from 65 °C)
121 —
477 777
9.2 9.5
11.8 12.4
The A and Ea can be determined through the slope and intercept of the fitted linears using Eq. (9). Table 4 shows the A and Ea of the EVA copolymerization with normal ratio and 1.8 times of initiator at initiating pressure of 4.5 MPa. The difference of activation energy is small. The pre exponential factor of the EVA copolymerization with 1.8 times of initiator inceases significantly because the increase of initiator provides more free radicals.
Table 6 The parameters of the EVA copolymerization reactor. Reactor volume / V/m3
Filling ratio
Pr / MPa
Pm /MPa
Cf / kJ/ (kg·K)
hfg / kJ/kg
ρf / kg/m3
ρg / kg/m3
4.8
80%
7.2
7.92
1.5
419.5
920
53
3.4. Effect of heat input
Table 7 The temperature and temperature rise rate at the relief pressure and maximum accumulated pressure of different scenarios using VSP2. Test condition
Tr/°C
(dT/dt)r/°C /s
Tm/°C
(dT/dt)m/°C /s
3.5 MPa 3.9 MPa 4.5 MPa 4.5 MPa, 1.8 times initiator 4.5 MPa, heat 5.3 MPa 6.1 MPa
177 124 144 144 115 105 80
256 64 204 265 69 3.3 0.56
204 139 159 170 151 118 93
55 89 180 474 127 24 2.3
There is a water heating system in the EVA copolymerization system, which can heat the reactants to the synthetic temperature at the start up stage. The misoperation and leakage of the heating system will lead to exceesive heat input on the copolymerization system. And the external fire will also lead to the input of external energy. Therefore, the effect of the heat input on the EVA polymerization was investigated. Fig.6 and Table 5 show the results for the EVA runaway reaction on adiabatic condition and 2 °C/min heat input condition at initiating pressure of 4.5 MPa. It can be seen that the time to maximum reaction rate (TMR) from 65 °C is significantly reduced with the add of heat input. Meanwhile, the (dT/dt)max increases significantly. The results indicate that the external heat input increases the thermal runaway risk of the EVA copolymerization. The experiment results will be used to size the emergency relief system of the EVA copolymerization reactor.
Table 8 Vent sizing parameters of the EVA copolymerization reactor.
4.5 MPa 4.5 MPa, 1.8 times initiator 4.5 MPa, heat input 5.3 MPa 6.1 MPa
W/kg/s
G/kg/(m2 s)
A/m2
d/mm
d/mm
119 308 68 16 1.7
15184 14450 19555 24403 27828
0.012 0.033 0.0054 0.0010 0.00009
124 204 83 36 11
150 250 100 40 15
3.5. Vent sizing of the emergency relief system The parameters of the EVA copolymerization reactor are listed on Table 6. Table 7 shows the temperature and temperature rise rate at the relief pressure Pr and maximum accumulated pressure Pm of different scenarios using VSP2 [14]. The runaway reactions of the EVA copolymerization are vapour pressure systems [14]. For a vapor pressure system, the temperature rise rate at the relief pressure (Pr) and the maximum accumulated pressure (Pm) determines the emergency relief system size. From Table 2 and Table 7, it can be seen that when the initial pressure increases, the (dT/ dt)max decreases, but the temperature rise rate at the relief pressure (dT/ dt)r does not necessarily decrease. When the initial pressure is 4.5 MPa, the temperature rise rate at the Pr and the Pm reach the maximum.
increases from normal ratio to 1.8 times, the onset temperature decreases from 80 °C to 60 °C, and the (dT/dt)max and the (dP/dt)max increase significantly. Excessive initiator will increase the risk of thermal runaway on the EVA copolymerization. Therefore, it is necessary to control the initiator concentration strictly. The emergency relief system should be sized for the scenario of overcharge of the initiator. Fig. 5 shows the ln [(dT/dt)/(Tmax-T0)] vs. -1000/T curves of the EVA copolymerization with different concertration of initiator. Linear fitted curves can be obtained by assuming the reaction order of zero. 58
Thermochimica Acta 671 (2019) 54–59
F. Sun, G. Wang
The temperature rise rate at the Pr and the Pm increases with the increase of initiator concentration, the heat input and the reaction process proceeding. The effect of the initiator concentration on the temperature rise rate is the most significant. The scenario with the initial pressure of 4.5 MPa and 1.8 times of initiators is most dangerous among the EVA copolymerization system. Table 8 shows vent sizing parameters of the EVA copolymerization reactor. It can be seen that the standard nominal diameter is 250 mm according to the most dangerous scenario of the EVA copolymerization. If effective methods are used to limit the flow of the initiator, or the pressure is controlled higher than 5 MPa, the vent size can be reduced.
Saf. (2005) 36–46. [9] K.H. Hu, C.S. Kao, Y.S. Duh, Studies on the runaway reaction of ABS polymerization process, J. Hazard. Mater. 159 (2008) 25–34. [10] M. Surianarayanan, S.P. Rao, R. Vijayaraghavan, K.V. Raghavan, Thermal behavior of acrylonitrile polymerization and polyacrylonitrile decomposition, J. Hazard. Mater. 62 (1998) 187–197. [11] M. Das, S.H. Liu, Y.T. Tsai, W.C. Lin, C.M. Shu, Assessment of thermal explosion for an industrial recovery reactor by GC/ MS product analysis combined with calorimetric techniques, Thermochim. Acta 656 (2017) 90–100. [12] F. Sun, F. Zhang, M.P. Jin, N. Shi, W. Xu, Vent sizing of cumene hydroperoxide (CHP) system under fire scenario considering emergency flooding measure, J. Loss Prev. Process Ind. 32 (2014) 230–237. [13] V. Luc, M. Wilfried, B. Jean-Pierre, Vent sizing: analysis of the blowdown of a hybrid non tempered system, J. Hazard. Mater. 191 (2011) 8–18. [14] J. Etchells, J. Wilday, Workbook for Chemical Reactor Relief System Sizing, Health and Safety Executive (HSE), 1998. [15] F. Stoessel, Thermal Safety of Chemical Processes-risk Assessment and Process Design, WILEY-VCH, Weinheim Germany, 2008.
4. Conclusions In this paper, the thermal risk of the EVA bulk copolymerization was studied. The vent of the emergency relief system of the EVA copolymerization reactor was sized. The conclusions are as follows:
Glossary A: cross-sectional flow area of relief system A’: pre-exponential factor Ccell: specific heat capacity of cell Cf: average liquid specific heat capacity between the relief pressure and maximum accumulated pressure Cfr: liquid specific heat capacity at relief pressure Cs: mass of testing sample C0: initial concentration of reactant d: relief system diameter Ea: activation energy G: mass flow rate per unit flow area hfg: average latent heat of vaporization between the relief pressure and maximum accumulated pressure hfgr: average latent heat of vaporization at relief pressure k: reaction rate constant mcell: mass of cell mR: mass in reactor at the relief pressure ms: specific heat capacity of testing sample n: order of reaction P: pressure Pm: maximum accumulated pressure Pmax: maximum Pressure Pr: relief pressure dP/dt: pressure rise rate (dP/dt) max: maximum pressure rise rate P0: initial pressure R: universal gas constant T: temperature Tm: temperature at maximum accumulated pressure Tmax: maximum temperature Tr: temperature at relief pressure dT/dt: temperature rise rate (dT/dt)max: maximum temperature rise rate (dT/dt)m: temperature rise rate at the maximum accumulated pressure (dT/dt)r: temperature rise rate at the relief pressure T0: initial temperature/onset temperature ΔT: temperature difference between the temperatures at the relief pressure and maximum accumulated pressure ΔTad: adiabatic temperature rise TMR: time to maximum reaction rate TMRad: time to maximum reaction rate under adiabatic condition V: volume of reactor W: required relief rate ρf: average liquid density between the relief pressure and maximum accumulated pressure ρg: average gas density between the relief pressure and maximum accumulated pressure ϕ: thermal inertia factor vfg: difference between vapour and liquid specific volumes vfgr: difference between vapour and liquid specific volumes at relief pressure
(1) The probability and maximum reaction rate of runaway reaction of the EVA bulk copolymerization decrease with the increase of initial pressure. When the initial pressure is about 4.5 MPa, the temperature rise rate at the design pressure is highest and the biggest vent size is required. (2) The EVA copolymerization shows a typical autocatalytic characteristic. The risk of thermal runaway increases with the reaction proceeding. (3) The difference of activation energy of the EVA copolymerization with different concentration of initiator is small. The pre exponential factor of the EVA copolymerization inceases significantly with the increase of initiator concentration. (4) The condition with the initial pressure of 4.5 MPa and 1.8 times of initiator concentration is the most dangerous scenario of the EVA copolymerization system. In the most dangerous scenario, the diameter of 250 mm is obtained using the Leung’s method and simplified equilibrium rate model. If effective methods are used to limit the flow of the initiator, or the pressure is controlled higher than 5 MPa, the vent size can be reduced. References [1] C.F. Ge, C. Fortuna, K. Lei, L.X. Lu, Neat EVOH and EVOH/LDPE blend centered three-layer co-extruded blown film without tie layers, Food Packag. Shelf Life 8 (2016) 33–40. [2] I. Poljansek, E. Fabjan, K. Burja, D. Kukanja, Emulsion copolymerization of vinyl cetate-ethylene in high pressure reactor-characterization by inline FTIR spectroscopy, Prog. Org. Coat. 76 (2013) 1798–1804. [3] J. Tanaka, K. Matsumoto, Process for continuous production of ethylene-vinyl acetate copolymer. US4657994. In US, 1987. [4] H. Yanai, T. Kitamura, F. Nakahara, K. Shimizu, A. Aoyama, T. Moritani, Process for producing ethylene-vinyl alcohol copolymers, US5240997. In US, 1993. [5] C.P. Lin, L.T. Wang, C.J. Wang, C.M. Chang, J.M. Tseng, Evaluation of thermal hazards in phenol-formaldehyde polymerization, J. Loss Prev. Process Ind. 49 (2017) 493–508. [6] C.S. Kao, Y.S. Duh, Accident investigation of an ABS plant, J. Loss Prev. Process Ind. 15 (2002) 223–232. [7] S. Copelli, M. Derudi, J. Sempere, E. Serra, A. Lunghi, C. Pasturenzi, R. Rota, Emulsion polymerization of vinyl acetate: safe optimization of a hazardous complex process, J. Hazard. Mater. 192 (2011) 8–17. [8] J.L. Gustin, Understanding vinyl acetate polymerization accidents, Chem. Health
59
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Study on the thermal risk of the ethylene-vinyl acetate bulk copolymerization
T
Feng Sun, Guangjian Wang
⁎
College of Chemical Engineering, Qingdao University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Ethylene-vinyl acetate copolymerization Thermal runaway Vent sizing Adiabatic test
The bulk copolymerizaition is an effective method to produce ethylene-vinyl acetate (EVA) copolymer. In this paper, the thermal risk of the EVA copolymerization was studied using vent size package 2 (VSP2) in order to avoid thermal runaway of the reaction. The effect of pressure, residence time, initiator concentration and heat input on the runaway reaction under adiabatic condition was investigated. The emergency relief system of the EVA reactor was sized. The results show that the EVA copolymerization reaction is autocatalytic and greatly influenced by pressure. The increase of the amount of initiator will significantly increase the reaction rate, thus is the worst case for relief system sizing. This study will provide the safety basis for the industrialization of the EVA bulk copolymerization process.
1. Introduction
2. Experimental
The ethylene-vinyl acetate (EVA) copolymer can be prepared by copolymeriztion of ethylene and vinyl acetate, then alcoholized to form ethylene vinyl alcohol (EVOH) copolymer. The EVOH copolymer is an excellent oxygen barrier due to its recyclability and transparent nature, which makes it an ideal film for food packaging [1,2]. There are four main processes for the EVA copolymer production: high pressure continuous bulk polymerization, medium pressure suspension polymerization, solution polymerization and emulsion polymerization [2]. The EVA copolymer [3,4] with 5–40 wt% vinyl acetate (VAc) content can be produced by the method of bulk polymerization. The polymerization process is hazardous because of its high thermal runaway risks [5,6] and the bulk polymerization is most harzardous among which due to the lack of solvent to dilute the heat released in polymerization. The polymerizations of vinyl acetate and ethylene caused many thermal runway accidents because of the high exothermicity involved [7,8]. Although some studies on thermal risk of polymerization have been reported [7–10], there is no research on thermal hazard of the EVA copolymerization. Therefore, we studied the thermal risk of the EVA copolymerization by vent size package 2 (VSP2). The effect of pressure, residence time, initiator concentration and heat input on the runaway reaction under adiabatic condition was investigated. The emergency relief size needed for the EVA reactor was calculated. The purpose of this study is to provide the safety basis for the industrialization of the EVA bulk copolymerization.
2.1. Samples
⁎
Vinyl acetate (99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Ethylene (99.9%) was supplied by Shanghai Shenkai Gas Technology Co. Ltd. Methanol (99.5%) was supplied by Tianjin Guangcheng Chemical Reagent Co., Ltd. Azobisisobutyronitrile (99%) was purchased from Tianjin Bodi Chemical Limited by Share Ltd. Vinyl acetate for the adiabatic test was purified by two times of vacuum distillation in order to eliminate inhibitor in the reagent. 2.2. Adiabatic tests VSP2 [11,12] manufactured by Fauske&Associates, Inc, was used to investigate the runaway reaction of EVA polymerization. The VSP2 makes an adiabatic circumstance by fast temperature and pressure tracing and balance between inside and outside the test cell. The VSP2 has small phi factor (around 1.1) and can simulate upset scenarios as loss of cooling, loss of agitation, mischarge of reagents, etc. So it is suitable to investigate runaway reaction and size the vent of reactor. The experiments were tested with closed cell test. The heat-wait-search (H-W-S) mode and heating mode with constant temperature rise rate were adopted [13]. The thermal inertia factor ϕ can be calculated as follows:
Corresponding author. E-mail address: [email protected] (G. Wang).
https://doi.org/10.1016/j.tca.2018.11.007 Received 7 June 2018; Received in revised form 6 November 2018; Accepted 12 November 2018 Available online 16 November 2018 0040-6031/ © 2018 Elsevier B.V. All rights reserved.
Thermochimica Acta 671 (2019) 54–59
F. Sun, G. Wang
=1+
mcell × Ccell ms × Cs
polymerization process, the typical thermal runaway scenarios were identified as follows:
(1)
Where mcell is the mass of cell, Ccell is the specific heat capacity of cell, ms and Cs are the mass and specific heat capacity of testing sample. About 50 g of vinyl acetate was added into the test cells. The mass fraction of ethylene is around 10%. The Cs of the mixture is assumed to be 1.5 J/(g °C). The mcell and Ccell of the cell are about 27 g and 0.5 J/ (g °C). So ϕ is calculated to be 1.15 using Eq. (1), which is small enough to simulate the reactor with volume of 4.8 m3. Therefore, the test result of VSP2 will not be corrected by thermal inertia factor.
(1) (2) (3) (4)
Improper pressure setting. Cooling failure or high temperature setting. Overcharge of initiator. Excesive heating or external fire.
The thermal risk of the EVA copolymerization was studied around these aspects.
2.3. Vent sizing equations
3. Results and discussion
Leung’s method and simplified equilibrium rate model (ERM) were used for calculating the required relief rate W and mass flow rate per unit flow area G [14]. For a vapor pressure system:
3.1. Effect of pressure
dT dt
q = 0.5Cf
+ r
dT dt
m
In the ethylene-vinyl acetate reaction system, ethylene is gaseous and needs to be dissolved in vinyl acetate in order to participate in the reaction. The solubility of ethylene in vinyl acetate varies greatly with pressure. Therefore, the thermal runaway characteristics of the EVA copolymerization with normal initiator concentration under different pressure were studied. The adiabatic environment was maintained from the initial temperature of 65 ° C. The temperature and pressure profiles in relation to time were recorded. Fig. 1 and Table 1 show runaway reaction characteristics of the EVA copolymerization with initial pressure of 3.9 MPa, 5.3 MPa and 6.1 MPa at the initial temperature of 65 °C. It can be seen that the adiabatic temperature rise (ΔTad) of the EVA copolymerization is about 100 °C. The time to maximum reaction rate under adiabatic condition (TMRad) increases with increasing initial pressure from 3.9 MPa to 6.1 MPa. For example, at 3.9 MPa the TMRad is about 20 min, which increases to 67 min at 6.1 MPa. From Fig.1 (c) (d) and Table 1, it can be seen the maximum temperature rise rate ((dT/dt) max) and the maximum pressure rise rate ((dP/dt) max) are 621 °C/min and 46.2 MPa/min respectively at the initial pressure of 3.9 MPa. When the initial pressure increases to 6.3 MPa, the (dT/dt) max and (dP/dt) max decrease to 81 °C /min and 3.6 MPa/min. The results demonstrate that reaction rate of the vinyl acetate polymerization is higher than that of the EVA copolymerization. When the initial pressure increases, the ethylene concentration in liquid increases, the proportion of the EVA copolymerization increases and the reaction rate decreases. For n-order reaction, the rate constant k can be determined based on adiabatic test using equation below:
(2)
Where Cf is average liquid specific heat capacity between the relief pressure and maximum accumulated pressure. (dT/dt)r is temperature rise rate at the relief pressure. (dT/dt)m is temperature rise rate at the maximum accumulated pressure. The mass flow rate (W) can be calculated as follow:
mR q
W= V × hfg mR fg
2
+
Cf T
(3)
Where mR is mass in reactor at the relief pressure, V is volume of reactor, hfg is latent heat of vaporization, vfg is difference between vapour and liquid specific volumes, ΔT is temperature difference between the temperatures at the relief pressure and maximum accumulated pressure. vfg is given by: fg
=
1
1
g
f
(4)
Where ρg is gas density, ρf is liquid density. The two-phase mass flow rate per unit flow area (G) can be determined using equation below:
hfgr
G= fgr
Cfr Tr
(5)
k=
Where subscript r means at relief pressure. The required relief flow area (A) and diameter (d) can be evaluated as follows:
W A= G
d=
4A
(
Tmax Tmax
lnk = lnA
(6)
)
dT / dt
T n (Tmax T0
Ea RT
T0 ) C0n
1
(8) (9)
Where n is the order of reaction, C0 is initial concentration of reactant, A is the pre-exponential factor, Ea is the activation energy, and R is the universal gas constant. The plots of lnk versus 1/T are expected to be a straight line, providing n is chosen correctly. It would be zero order reaction if the curve of ln (dT/dt) vs. -1/T is a straight line. Fig. 2 shows the ln (dT/dt) vs. -1000/T curves of the EVA copolymerization with different pressure. From Fig. 2, the profile of the curves shows that the increasing range of reaction rate decreases with polymerization at the initial pressure of 6.1 MPa. It is approximately zero order reaction of the EVA copolymerization with initial pressure of 5.3 MPa because the curve is almost a straight line. When the initial pressure decreases to 3.9 MPa, the increasing range of reaction rate increases with polymerizaion. The reduction of initial pressure of the EVA copolymerization makes the autocatalytic effect become more significant. Table 2 lists (dT/dt)max, (dP/dt)max, and the maximum pressure (Pmax) of runaway reaction on the EVA copolymerization with
(7)
2.4. Thermal runaway scenarios The ethylene-vinyl acetate copolymerization is initiated by azobisisobutyronitrile at 3–6 MPa and 50–65 °C. The reaction equation is as follows [3].
Methanol and azobisisobutyronitrile is 5 wt% and 0.1 wt% of vinyl acetate respectively under normal conditions. The maximum filling ratio is 80%. A preliminarily hazard analysis was performed for the EVA 55
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Fig. 1. Thermal runaway of the EVA copolymerization with different pressure. Table 1 Thermokinetic data of thermal runaway on the EVA copolymerization with different pressure. P0/MPa
T0/°C
Tmax/°C
ΔTad/°C
TMRad/min
Pmax/MPa
3.9 5.3 6.1
65 65 65
164 181 169
99 116 104
20 55 67
8.8 10.3 11.9
Table 2 Data of runaway reaction on the EVA copolymerization with different initial pressure. P0/MPa
(dT/dt)max / °C /min
(dP/dt)max / MPa/min
Pmax /MPa
3.5 3.9 4.5 5.3 6.1
3604 621 477 158 81
42.5 46.2 11.8 7.2 3.6
8.1 8.8 9.2 10.3 11.9
Fig. 3. The temperature versus time curves of the three experiments.
different initial pressure. From Table 2, we can see that with the increase of initial pressure, the (dP/dt)max decreases, the Pmax increases. It indicates the reaction rate of vinyl acetate polymerization is higher
Fig. 2. The ln (dT/dt) vs. -1000/T curves of the EVA copolymerization with different pressure.
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Fig. 4. Thermal runaway of the EVA copolymerization with normal ratio and 1.8 times of initiator at initiating pressure of 4.5 MPa (a) temperature vs. time, (b) pressure vs. time. Table 3 Thermokinetic data of the EVA runaway reaction with normal ratio and 1.8 times of initiator. sample
T0/°C
Tmax/°C
TMRad/ min
ΔTad/°C
(dT/dt)max/°C /min
Pmax / MPa
(dP/dt)max/ MPa/min
Normal ratio 1.8 times of initiator
80 60
201 206
16 17
121 146
477 997
9.2 9.4
11.8 32.9
3.2. Autocatalytic effect of the EVA copolymerization
sample
intercept
slope
A/ 1/min
Ea / kJ
Autocatalytic reaction is common in fine chemistry. The product of an autocatalytic reaction can act as a catalyst on the reaction course. The autocatalytic reaction has a period of induction. During the induction period, the reaction rate does not increase. But once it is induced, it can react violently in a short time [15], so the reaction with autocatalytic effect is very hazardous. In order to investigate the autocatalytic effect of the EVA copolymerization, three experiments were designed at the initial pressure of 5.3 MPa. In the first experiment, the heating-waiting-searching (H-W-S) mode was used starting from room temperature. In the second experiment, the reactant first be heated to 65 °C, then an adiabatic environment was maintained (W-S mode). In the third experiment, the reactant was maintained at 65 °C for 7 h to finish the reaction, then adiabatic mode was adopted. Fig. 3 shows the curves of temperature versus time at different experiments. In the second experiment, the TMRad from 65 °C is 55 min. But in the third experiment, the TMRad from 65 °C is shortened to 29 min after the 7 h reaction at constant temperature. The reaction rate of the EVA copolymerization increases with the decrease of reactant concentration, indicating that the reaction product acts as a catalyst. In the first experiment, the VSP2 detects the exothermic reaction at 88 °C, and the TMRad is only 4 min, which showing a typical autocatalytic characteristic [12]. Therefore, it is not recommended to control the process temperature refer to the onset temperature of the EVA exothermic reaction. The process temperature should be controlled strictly during the reaction proceeds due to the increase of thermal risk.
Normal ratio 1.8 times of initiator
30.35 32.51
12.38 12.58
1.516E+13 1.315E+14
102.9 104.6
3.3. Effect of initiator concentration
Fig. 5. The ln [(dT/dt)/(Tmax-T0)] vs. -1000/T curves of the EVA copolymerization with different concentration of initiator. Table 4 The A and Ea of the EVA copolymerization with normal ratio and 1.8 times of initiator at initiating pressure of 4.5 MPa.
The EVA copolymerization is a chain reaction. The initiator decomposes to free radicals, then starts the chain reaction. The concentration of initiator may have great influence on the reaction. Therefore, the effect of initiator concentration on the thermal risk of EVA copolymerization was investigated. Fig.4 and Table 3 show adiabatic test results of the EVA copolymerization with normal ratio and 1.8 times of initiator at initiating pressure of 4.5 MPa. It can be seen that when the initiator concentration
than that of EVA copolymerization. When the initial pressure increases, the ethylene concentration in liquid increases, the proportion of the EVA copolymerization increases and the overall reaction rate decreases.
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Fig. 6. Thermal runaway of the EVA copolymerization on adiabatic condition and heat input condition with the heating rate of 2 °C/min at initiating pressure of 4.5 MPa (a) temperature vs. time, (b) pressure vs. time. Table 5 Thermokinetic data of the EVA runaway reaction with adiabatic condition and heat input condition. Test condition
T0 /°C
Tmax /°C
TMR/min
ΔT/°C
(dT/dt)max/ °C/min
Pmax/ MPa
(dP/dt)max/ MPa/min
Adiabatic Heat input
80 —
201 208
16(from 80 °C) 17(from 65 °C)
121 —
477 777
9.2 9.5
11.8 12.4
The A and Ea can be determined through the slope and intercept of the fitted linears using Eq. (9). Table 4 shows the A and Ea of the EVA copolymerization with normal ratio and 1.8 times of initiator at initiating pressure of 4.5 MPa. The difference of activation energy is small. The pre exponential factor of the EVA copolymerization with 1.8 times of initiator inceases significantly because the increase of initiator provides more free radicals.
Table 6 The parameters of the EVA copolymerization reactor. Reactor volume / V/m3
Filling ratio
Pr / MPa
Pm /MPa
Cf / kJ/ (kg·K)
hfg / kJ/kg
ρf / kg/m3
ρg / kg/m3
4.8
80%
7.2
7.92
1.5
419.5
920
53
3.4. Effect of heat input
Table 7 The temperature and temperature rise rate at the relief pressure and maximum accumulated pressure of different scenarios using VSP2. Test condition
Tr/°C
(dT/dt)r/°C /s
Tm/°C
(dT/dt)m/°C /s
3.5 MPa 3.9 MPa 4.5 MPa 4.5 MPa, 1.8 times initiator 4.5 MPa, heat 5.3 MPa 6.1 MPa
177 124 144 144 115 105 80
256 64 204 265 69 3.3 0.56
204 139 159 170 151 118 93
55 89 180 474 127 24 2.3
There is a water heating system in the EVA copolymerization system, which can heat the reactants to the synthetic temperature at the start up stage. The misoperation and leakage of the heating system will lead to exceesive heat input on the copolymerization system. And the external fire will also lead to the input of external energy. Therefore, the effect of the heat input on the EVA polymerization was investigated. Fig.6 and Table 5 show the results for the EVA runaway reaction on adiabatic condition and 2 °C/min heat input condition at initiating pressure of 4.5 MPa. It can be seen that the time to maximum reaction rate (TMR) from 65 °C is significantly reduced with the add of heat input. Meanwhile, the (dT/dt)max increases significantly. The results indicate that the external heat input increases the thermal runaway risk of the EVA copolymerization. The experiment results will be used to size the emergency relief system of the EVA copolymerization reactor.
Table 8 Vent sizing parameters of the EVA copolymerization reactor.
4.5 MPa 4.5 MPa, 1.8 times initiator 4.5 MPa, heat input 5.3 MPa 6.1 MPa
W/kg/s
G/kg/(m2 s)
A/m2
d/mm
d/mm
119 308 68 16 1.7
15184 14450 19555 24403 27828
0.012 0.033 0.0054 0.0010 0.00009
124 204 83 36 11
150 250 100 40 15
3.5. Vent sizing of the emergency relief system The parameters of the EVA copolymerization reactor are listed on Table 6. Table 7 shows the temperature and temperature rise rate at the relief pressure Pr and maximum accumulated pressure Pm of different scenarios using VSP2 [14]. The runaway reactions of the EVA copolymerization are vapour pressure systems [14]. For a vapor pressure system, the temperature rise rate at the relief pressure (Pr) and the maximum accumulated pressure (Pm) determines the emergency relief system size. From Table 2 and Table 7, it can be seen that when the initial pressure increases, the (dT/ dt)max decreases, but the temperature rise rate at the relief pressure (dT/ dt)r does not necessarily decrease. When the initial pressure is 4.5 MPa, the temperature rise rate at the Pr and the Pm reach the maximum.
increases from normal ratio to 1.8 times, the onset temperature decreases from 80 °C to 60 °C, and the (dT/dt)max and the (dP/dt)max increase significantly. Excessive initiator will increase the risk of thermal runaway on the EVA copolymerization. Therefore, it is necessary to control the initiator concentration strictly. The emergency relief system should be sized for the scenario of overcharge of the initiator. Fig. 5 shows the ln [(dT/dt)/(Tmax-T0)] vs. -1000/T curves of the EVA copolymerization with different concertration of initiator. Linear fitted curves can be obtained by assuming the reaction order of zero. 58
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F. Sun, G. Wang
The temperature rise rate at the Pr and the Pm increases with the increase of initiator concentration, the heat input and the reaction process proceeding. The effect of the initiator concentration on the temperature rise rate is the most significant. The scenario with the initial pressure of 4.5 MPa and 1.8 times of initiators is most dangerous among the EVA copolymerization system. Table 8 shows vent sizing parameters of the EVA copolymerization reactor. It can be seen that the standard nominal diameter is 250 mm according to the most dangerous scenario of the EVA copolymerization. If effective methods are used to limit the flow of the initiator, or the pressure is controlled higher than 5 MPa, the vent size can be reduced.
Saf. (2005) 36–46. [9] K.H. Hu, C.S. Kao, Y.S. Duh, Studies on the runaway reaction of ABS polymerization process, J. Hazard. Mater. 159 (2008) 25–34. [10] M. Surianarayanan, S.P. Rao, R. Vijayaraghavan, K.V. Raghavan, Thermal behavior of acrylonitrile polymerization and polyacrylonitrile decomposition, J. Hazard. Mater. 62 (1998) 187–197. [11] M. Das, S.H. Liu, Y.T. Tsai, W.C. Lin, C.M. Shu, Assessment of thermal explosion for an industrial recovery reactor by GC/ MS product analysis combined with calorimetric techniques, Thermochim. Acta 656 (2017) 90–100. [12] F. Sun, F. Zhang, M.P. Jin, N. Shi, W. Xu, Vent sizing of cumene hydroperoxide (CHP) system under fire scenario considering emergency flooding measure, J. Loss Prev. Process Ind. 32 (2014) 230–237. [13] V. Luc, M. Wilfried, B. Jean-Pierre, Vent sizing: analysis of the blowdown of a hybrid non tempered system, J. Hazard. Mater. 191 (2011) 8–18. [14] J. Etchells, J. Wilday, Workbook for Chemical Reactor Relief System Sizing, Health and Safety Executive (HSE), 1998. [15] F. Stoessel, Thermal Safety of Chemical Processes-risk Assessment and Process Design, WILEY-VCH, Weinheim Germany, 2008.
4. Conclusions In this paper, the thermal risk of the EVA bulk copolymerization was studied. The vent of the emergency relief system of the EVA copolymerization reactor was sized. The conclusions are as follows:
Glossary A: cross-sectional flow area of relief system A’: pre-exponential factor Ccell: specific heat capacity of cell Cf: average liquid specific heat capacity between the relief pressure and maximum accumulated pressure Cfr: liquid specific heat capacity at relief pressure Cs: mass of testing sample C0: initial concentration of reactant d: relief system diameter Ea: activation energy G: mass flow rate per unit flow area hfg: average latent heat of vaporization between the relief pressure and maximum accumulated pressure hfgr: average latent heat of vaporization at relief pressure k: reaction rate constant mcell: mass of cell mR: mass in reactor at the relief pressure ms: specific heat capacity of testing sample n: order of reaction P: pressure Pm: maximum accumulated pressure Pmax: maximum Pressure Pr: relief pressure dP/dt: pressure rise rate (dP/dt) max: maximum pressure rise rate P0: initial pressure R: universal gas constant T: temperature Tm: temperature at maximum accumulated pressure Tmax: maximum temperature Tr: temperature at relief pressure dT/dt: temperature rise rate (dT/dt)max: maximum temperature rise rate (dT/dt)m: temperature rise rate at the maximum accumulated pressure (dT/dt)r: temperature rise rate at the relief pressure T0: initial temperature/onset temperature ΔT: temperature difference between the temperatures at the relief pressure and maximum accumulated pressure ΔTad: adiabatic temperature rise TMR: time to maximum reaction rate TMRad: time to maximum reaction rate under adiabatic condition V: volume of reactor W: required relief rate ρf: average liquid density between the relief pressure and maximum accumulated pressure ρg: average gas density between the relief pressure and maximum accumulated pressure ϕ: thermal inertia factor vfg: difference between vapour and liquid specific volumes vfgr: difference between vapour and liquid specific volumes at relief pressure
(1) The probability and maximum reaction rate of runaway reaction of the EVA bulk copolymerization decrease with the increase of initial pressure. When the initial pressure is about 4.5 MPa, the temperature rise rate at the design pressure is highest and the biggest vent size is required. (2) The EVA copolymerization shows a typical autocatalytic characteristic. The risk of thermal runaway increases with the reaction proceeding. (3) The difference of activation energy of the EVA copolymerization with different concentration of initiator is small. The pre exponential factor of the EVA copolymerization inceases significantly with the increase of initiator concentration. (4) The condition with the initial pressure of 4.5 MPa and 1.8 times of initiator concentration is the most dangerous scenario of the EVA copolymerization system. In the most dangerous scenario, the diameter of 250 mm is obtained using the Leung’s method and simplified equilibrium rate model. If effective methods are used to limit the flow of the initiator, or the pressure is controlled higher than 5 MPa, the vent size can be reduced. References [1] C.F. Ge, C. Fortuna, K. Lei, L.X. Lu, Neat EVOH and EVOH/LDPE blend centered three-layer co-extruded blown film without tie layers, Food Packag. Shelf Life 8 (2016) 33–40. [2] I. Poljansek, E. Fabjan, K. Burja, D. Kukanja, Emulsion copolymerization of vinyl cetate-ethylene in high pressure reactor-characterization by inline FTIR spectroscopy, Prog. Org. Coat. 76 (2013) 1798–1804. [3] J. Tanaka, K. Matsumoto, Process for continuous production of ethylene-vinyl acetate copolymer. US4657994. In US, 1987. [4] H. Yanai, T. Kitamura, F. Nakahara, K. Shimizu, A. Aoyama, T. Moritani, Process for producing ethylene-vinyl alcohol copolymers, US5240997. In US, 1993. [5] C.P. Lin, L.T. Wang, C.J. Wang, C.M. Chang, J.M. Tseng, Evaluation of thermal hazards in phenol-formaldehyde polymerization, J. Loss Prev. Process Ind. 49 (2017) 493–508. [6] C.S. Kao, Y.S. Duh, Accident investigation of an ABS plant, J. Loss Prev. Process Ind. 15 (2002) 223–232. [7] S. Copelli, M. Derudi, J. Sempere, E. Serra, A. Lunghi, C. Pasturenzi, R. Rota, Emulsion polymerization of vinyl acetate: safe optimization of a hazardous complex process, J. Hazard. Mater. 192 (2011) 8–17. [8] J.L. Gustin, Understanding vinyl acetate polymerization accidents, Chem. Health
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