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vinylacetate copolymerization under high pressure1
Chemical Engineering and Processing 37 (1998) 55 – 59
Runaway phenomena in the ethylene/vinylacetate copolymerization under high pressure1 Jens Albert, Gerhard Luft * Institut fu¨r Chemische Technologie, Darmstadt Uni6ersity of Technology, Petersenstraße 20, 64287 Darmstadt, Germany Received 17 June 1997; accepted 30 June 1997
Abstract Ethylene is an ‘endothermic’ compound. Above certain pressure and temperature it can decompose explosively into carbon, hydrogen and methane. The high pressure polymerization and copolymerization of ethylene can lead to thermal runaways, ethylene decomposes during polymerization. The influence of vinylacetate on the thermal runaway of ethylene during the copolymerization was investigated in the pressure range of 25 – 125 MPa and temperatures of 250 – 375°C. Decomposition boundaries above which ethylene violently decomposes have been established for the mixture of 10 and 30 wt.% vinylacetate. As well as the decomposition limit, maximum explosion pressure, maximum rate of pressure increase, maximum explosion temperature and composition of the produced gases are reported. The results show that the addition of vinylacetate lowers the decomposition boundary of ethylene. The lowering of the limit depends on initial pressure and composition of the mixture. The maximum rate of pressure increase and maximum explosion temperature are not influenced by vinylacetate. However, the maximum explosion pressure is about 20 MPa higher than for pure ethylene. Carbon monoxide, carbon dioxide and water are additionally produced when vinylacetate is present. The composition of the decomposition products depend on the initial pressure, wall temperature and concentration of vinylacetate. © 1998 Elsevier Science S.A. Keywords: Thermal runaway; Ethylene; Vinylacetate; High pressure; Decomposition
1. Introduction Producers of low density polyethylene (LDPE) have frequently encountered thermal runaway problems because the polymers are produced at high pressure and high temperature, where ethylene is inherently unstable because it is an ‘endothermic’ compound and requires heat (52.33 kJ mol − 1) to be formed from its component elements. Under certain conditions ethylene decomposes explosively into its elements and methane [1,2]. Because of accelerating pressure and temperature increase during decomposition, it is a safety concern and results in loss of production time. The ethylene decomposition conditions are dependent on pressure, temperature, reactor size, mechanism of ignition and additives to ethylene. * Corresponding author. 1 Dedicated to Professor K. R. Westerterp on the occasion of his 70th birthday 0255-2701/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 2 5 5 - 2 7 0 1 ( 9 7 ) 0 0 0 3 6 - 6
Table 1 shows some of the studies carried out by various groups on ethylene decomposition. Most investigations on decomposition of ethylene refer to pure ethylene whereas the influence of additives is rarely investigated. There are some reports on the influence of vinylacetate, but the studies were carried out using a hot spot technique [7,8] and small vessels [5]. This paper not only examines the influence of vinylacetate on the thermal runaway of ethylene, but also determines explosion characteristics and the composition of the gases produced.
2. Experimental The high pressure equipment has been described in previous publications [8,9]. For implementation of thermal runaway experiments with mixtures of ethylene and vinylacetate, the high pressure system was equipped with a mixing autoclave (Fig. 1).
J. Albert, G. Luft / Chemical Engineering and Processing 37 (1998) 55–59
56 Table 1 Studies in ethylene decomposition Pressure (MPa)
Temperature (°C) Volume, shape
10–25 6–27 10–500
120 – 325 150 – 200 25 – 400
6a, cylinder 200b, cylinder 15b, cylinder
11 1–17 5–60
40 350 – 540 25 – 300
10–200
250 – 450
a
Volume expressed in litres,
b
Ignition source
Additives
Reference
— — Vinylacetate, oxygene
[3] [4] [5]
120a, tube 550b, sphere 200b, cylinder
Hot spot (Pt-wire) Hot spot (Cu-wire) Hot spot (Pt-wire), thermal runaway Gun powder Thermal runaway Hot spot (Pt-wire)
[6] [1] [7,8]
200b, cylinder
Thermal runaway
— Nitrogene, oxygene Vinylacetate, methylmethacrylate, acetaldehyde Oxygene
[9,10]
millilitres.
To prepare an ethylene/vinylacetate mixture, ethylene was first compressed with a diaphragm compressor (1) into the mixing autoclave (3). The vinylacetate was later added to the ethylene in the mixing autoclave (3) with a screw press (2). The reactor (4) is constructed for maximum pressure of 500 MPa and temperatures of 500°C. The reactor (4) has an inside diameter of 5 cm and a height of 10 cm, equivalent to 200 ml in volume. With a pressure gauge (5) and a Pt – PtRh thermocouple (6), we are able to measure the pressure and the temperature inside the reactor during an experiment. The temperature and pressure data are stored in a computer. To carry out an experiment, the reactor (4) is first evacuated and heated to a pre-set wall temperature. The ethylene/vinylacetate mixture is fed to the reactor (4) from the mixing autoclave (3) by opening a remotecontrolled valve (7). As soon as the mixture is injected, pressure and temperature are recorded continuously for 30 min. The data will show whether the mixture has undergone thermal decomposition.
3. Results By varying the initial pressure and temperature, a boundary above which the mixture becomes unstable can be determined. A thermal runaway occurs in the region where the pressure and the wall temperature of the reactor are too high. Because of the high temperature, the system cannot remove the polymerization heat fast enough, which causes the thermal polymerization to accelerate and finally causes the mixture to decompose. A thermal runaway is characterized by an abrupt increase of pressure and temperature. Fig. 2 shows a pressure and temperature profile of a thermal runaway. The reactor autoclave was set at 300°C. As soon as the ethylene/vinylacetate mixture was fed into the autoclave, it reached 300°C and 120 MPa in a few seconds.
Soon after, the thermal polymerization starts to occur which is evident by a pressure decrease in the reactor. Because of the insufficient removal of polymerization heat, the temperature inside the reactor increases. As soon as the pressure has decreased by 30 MPa and the temperature increases to 320°C the ethylene/vinylacetate mixture starts to decompose, as evident by the abrupt increase of pressure and temperature. Fig. 3 shows decomposition limits for ethylene/vinylacetate mixtures. For comparison the decomposition limit of pure ethylene is shown [9]. Decomposition of ethylene is not affected by vinylacetate below 25 MPa. Above this pressure, the influence becomes larger, shifting the decomposition boundary to lower temperatures. It appears that, above 125 MPa, the influence of pressure has levelled off. The decomposition limit has shifted 30° maximum for the 10 wt.% vinylacetate containing mixture and 60° maximum for the 30 wt.% vinylacetate. While the decomposition boundary is important for the manufacturing of copolymers, one needs maximum explosion pressure, maximum rate of pressure increase or maximum temperatures during the decomposition for technical designs of plants and safety devices. Fig. 4 shows the maximum explosion pressure as a function of the initial pressure at 325, 350 and 375°C.
Fig. 1. High pressure equipment for determination of decomposition boundaries: (1) Diaphragm compressor, (2) screw press, (3) mixing autoclave, (4) reactor, (5) pressure gauge, (6) thermocouple, (7) remote controlled valve.
J. Albert, G. Luft / Chemical Engineering and Processing 37 (1998) 55–59
Fig. 2. Course of pressure and temperature of decomposition during polymerization.
The maximum explosion pressure increases with increasing initial pressure and decreasing wall temperatures. In comparison, the maximum pressures for pure ethylene [9] is 20 MPa higher than that of ethylene/ vinylacetate (10 wt.%) mixtures. As one can see from Fig. 5 the maximum explosion pressure increases with increasing density, meaning that the monomer mixture with a higher density produces more explosive gases. The figure also shows that the curves derived from ethylene, and ethylene with 10 and 30 wt.% vinylacetate fall on top of each other, within accuracy of measurement. The results show that the maximum explosion pressure is solely dependent on the density of the mixture regardless of its composition. Another important safety factor for the design of a reactor is the maximum rate of pressure increase. This number is a measure of the violence of decomposition. Fig. 6 shows the maximum rate of pressure increase as a function of initial pressure. As shown in Fig. 4 for maximum explosion pressure, the maximum rate of pressure increase rises with increasing initial pressure and decreasing wall temperature. The maximum value is 950 MPa s − 1 and is comparable with the values for pure ethylene [9]. During the decomposition of ethylene/vinylacetate mixtures the highest temperatures are between 1200 and 1400°C. This data is critical for the design of a temperature robust reactor.
Fig. 3. Decomposition boundaries for ethylene [9] and ethylene/vinylacetate mixtures.
57
Fig. 4. Maximum explosion pressure as function of initial pressure and wall temperature for ethylene/vinylacetate (10 wt.%) mixtures.
The main products of ethylene decomposition are carbon black, methane, hydrogen [1,11] and traces of ethane and undecomposed ethylene. The added vinylacetate produces additional carbon monoxide, carbon dioxide and water. GC analysis of the decomposition gases reveals that the composition of gases is dependent on initial pressure, wall temperature and concentration of vinylacetate in the mixture (Fig. 7). It should be pointed out that methane and hydrogen are still the main products. The methane content is about 70–77 vol.%, this increases with increasing initial pressure. The hydrogen content ranges between 17 and 25 vol.%, but it decreases with increasing initial pressure. The wall temperature has a bigger effect on the hydrogen content than on the methane content. The volume concentrations of ethane and ethylene are in the range of one percent. Their production is not affected by pressure and temperature. In the mixture (10 wt.% vinylacetate), the volume concentrations of carbon monoxide and carbon dioxide are about 1 and 0.2%, respectively. Their production is also not affected by the initial parameters. On the contrary, their production is strongly influenced by the initial parameters for the mixture of 30 wt.% vinylacetate. They decrease with increasing initial pressure. At
Fig. 5. Maximum explosion pressure as function of density of monomer mixture.
58
J. Albert, G. Luft / Chemical Engineering and Processing 37 (1998) 55–59
Fig. 6. Maximum rate of pressure increase as function of initial pressure and wall temperature for ethylene/vinylacetate (10 wt.%) mixtures.
Fig. 8. Influence of vinylacetate on the decomposition boundary of ethylene. Below the isobars polymerization occurs, above decomposition during polymerization.
44 MPa, carbon monoxide and carbon dioxide concentrations are 4.5 and 2 vol.%, respectively. Vinylacetate concentration also exerts an influence on the production of methane and hydrogen. For the same initial pressure and wall temperature the methane content is about 1.5 vol.% higher for the 30 wt.% vinylacetate. In contrast the hydrogen concentration is about 4 vol.% lower for 30 wt.% vinylacetate. The addition of 10 wt.% vinylacetate to ethylene has no effect on methane concentration in the decomposition gas. On the other hand the hydrogen content is influenced by vinylacetate. For the same initial parameters the hydrogen content is about 1.5 vol.% higher for pure ethylene than for the mixture with 10 wt.% vinylacetate.
The decomposition boundary and safety factor data are important for technical design and operation of ethylene/vinylacetate copolymer production plants. Fig. 8 shows the decomposition temperature of ethylene as a function of percentage vinylacetate at different pressures. The experimental decomposition boundaries were extrapolated to lower and higher pres-
sures. It can be seen that the decomposition temperature decreases with increasing vinylacetate content and the influence becomes smaller with increasing vinylacetate content. With increasing initial pressure the decomposition temperature moves towards a limiting temperature, below which decomposition does not occur. This corresponds to Fig. 3, where the curvature of the decomposition boundary decreases with increasing pressure. To avoid thermal runaways in high pressure ethylene/vinylacetate copolymerization one should avoid the temperatures above this limit. Besides thermal runaway due to insufficient removal of heat from polymerization, another possible ignition source is hot spot. If one compares the influence of vinylacetate on the decomposition limits of ethylene for thermal runaway and hot spot, there are significant differences (Fig. 9). The addition of vinylacetate shifts the decomposition limit for a thermal runaway to lower temperatures. On the contrary, the decomposition limit for ignition by a hot spot [7] moves to higher temperatures. Because of the high temperature at the hot spot, ethylene decomposes spontaneously. In this case vinylacetate has an inhibiting effect on the decomposition of ethylene and the explosion region becomes smaller.
Fig. 7. Gaseous decomposition products after thermal runaway.
Fig. 9. Influence of vinylacetate on the decomposition boundary of ethylene for thermal runaway and ignition by a hot spot.
4. Discussion and conclusion
J. Albert, G. Luft / Chemical Engineering and Processing 37 (1998) 55–59
A thermal runaway is characterized by the insufficient removal of polymerization heat. The addition of vinylacetate has an influence on polymerization kinetics. The polymerization rate increases, therefore the rate of heat production increases. The faster heat production results in lowering the decomposition limit. For designing robust plants and safety devices one needs to know the maximum stress the equipment can tolerate during decomposition. The experimental values for maximum explosion pressure, maximum rate of pressure increase and maximum temperatures shows only small differences between pure ethylene and mixtures with vinylacetate. For the same initial pressure and wall temperature the maximum explosion pressure is only 20 MPa higher for the ethylene/vinylacetate (10 wt.%) mixture. If one uses the density as criterion for the maximum explosion pressure there is no noticeable difference. The maximum rate of pressure increase and the maximum temperatures show no differences, within accuracy of measurement. Thus for designing plants and safety devices it is only necessary to take the higher maximum explosion pressure of ethylene/vinylacetate mixtures into consideration. Methane and hydrogen in the explosion gases increases and decreases, respectively, with increasing vinylacetate concentration, and they are potential sources of hazard. If they are mixed with air, it is likely to explode violently especially in the presence of an ignition source. Carbon monoxide, which is produced especially at higher concentrations of vinylacetate, is also a potential hazard source, because of its toxicity. The knowledge of decomposition boundaries, safety factors and the influence of vinylacetate on the explosion characteristics should allow one to safely operate ethylene/vinylacetate copolymer manufacturing plants.
Acknowledgements This publication is part of a DECHEMA research
.
.
59
project, sponsored by the German Ministry of Economic Affairs (AIF-No 10497N).
Appendix A. Nomenclature HS P Pmax t TR VA
hot spot pressure (MPa) maximum explosion pressure (MPa) time thermal runaway vinylacetate
References [1] L.G. Britton, D.A. Taylor, D.C. Wobser, Thermal stability of ethylene at elevated pressure, Plant/Oper. Prog. 5 (1986) 238– 250. [2] R. Neumann, G. Luft, Untersuchungen zum thermischen Zerfall von Ethylen unter Hochdruck, Chem. Eng. Sci. 28 (1973) 1505– 1514. [3] D. Conrad, R. Kaulbars, Untersuchungen zur chemischen Instabilita¨t von Ethylen, Chem.-Ing.-Tech. 47 (1975) 265. [4] W.B. Howard, Ethylene behavior related to hot tapping, The American Institute of Chemical Engineers Loss Prevention Symp. 9 (1975) 9 – 14. [5] G. Luft, R. Neumann, Selbstzerfall und fremdgezu¨ndeter Zerfall von verdichtetem Ethylen, Chem.-Ing.-Tech. 50 (1978) 620–622. [6] G.R. Worell, If ethylene decomposes in pipe..., Hydrocarbon Process. 58 (1979) 255 – 258. [7] Th. Zimmermann, Bestimmung sicherheitstechnischer Kenngro¨ßen von verdichtetem Ethylen, Dissertation, TH Darmstadt 1994. [8] Th. Zimmermann, G. Luft, Untersuchungen zum explosiven Zerfall von verdichtetem Ethylen, Chem.-Ing.-Tech. 66 (1994) 1386 – 1389. [9] H. Bo¨nsel, G. Luft, Sicherheitstechnische Untersuchungen des explosiven Zerfalls von verdichtetem Ethen, Chem.-Ing.-Tech. 67 (1995) 862 – 864. [10] H. Bo¨nsel, Sicherheitstechnische Untersuchungen des explosiven Zerfalls von verdichtetem Ethen, Dissertation, TH Darmstadt 1994. [11] S.X. Zhang, N.K. Read, W.H. Ray, Runaway phenomena in low-density polyethylene autoclave reactors, Am. Inst. Chem. Eng. J. 42 (1996) 2911 – 2925.
Runaway phenomena in the ethylene/vinylacetate copolymerization under high pressure1 Jens Albert, Gerhard Luft * Institut fu¨r Chemische Technologie, Darmstadt Uni6ersity of Technology, Petersenstraße 20, 64287 Darmstadt, Germany Received 17 June 1997; accepted 30 June 1997
Abstract Ethylene is an ‘endothermic’ compound. Above certain pressure and temperature it can decompose explosively into carbon, hydrogen and methane. The high pressure polymerization and copolymerization of ethylene can lead to thermal runaways, ethylene decomposes during polymerization. The influence of vinylacetate on the thermal runaway of ethylene during the copolymerization was investigated in the pressure range of 25 – 125 MPa and temperatures of 250 – 375°C. Decomposition boundaries above which ethylene violently decomposes have been established for the mixture of 10 and 30 wt.% vinylacetate. As well as the decomposition limit, maximum explosion pressure, maximum rate of pressure increase, maximum explosion temperature and composition of the produced gases are reported. The results show that the addition of vinylacetate lowers the decomposition boundary of ethylene. The lowering of the limit depends on initial pressure and composition of the mixture. The maximum rate of pressure increase and maximum explosion temperature are not influenced by vinylacetate. However, the maximum explosion pressure is about 20 MPa higher than for pure ethylene. Carbon monoxide, carbon dioxide and water are additionally produced when vinylacetate is present. The composition of the decomposition products depend on the initial pressure, wall temperature and concentration of vinylacetate. © 1998 Elsevier Science S.A. Keywords: Thermal runaway; Ethylene; Vinylacetate; High pressure; Decomposition
1. Introduction Producers of low density polyethylene (LDPE) have frequently encountered thermal runaway problems because the polymers are produced at high pressure and high temperature, where ethylene is inherently unstable because it is an ‘endothermic’ compound and requires heat (52.33 kJ mol − 1) to be formed from its component elements. Under certain conditions ethylene decomposes explosively into its elements and methane [1,2]. Because of accelerating pressure and temperature increase during decomposition, it is a safety concern and results in loss of production time. The ethylene decomposition conditions are dependent on pressure, temperature, reactor size, mechanism of ignition and additives to ethylene. * Corresponding author. 1 Dedicated to Professor K. R. Westerterp on the occasion of his 70th birthday 0255-2701/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 2 5 5 - 2 7 0 1 ( 9 7 ) 0 0 0 3 6 - 6
Table 1 shows some of the studies carried out by various groups on ethylene decomposition. Most investigations on decomposition of ethylene refer to pure ethylene whereas the influence of additives is rarely investigated. There are some reports on the influence of vinylacetate, but the studies were carried out using a hot spot technique [7,8] and small vessels [5]. This paper not only examines the influence of vinylacetate on the thermal runaway of ethylene, but also determines explosion characteristics and the composition of the gases produced.
2. Experimental The high pressure equipment has been described in previous publications [8,9]. For implementation of thermal runaway experiments with mixtures of ethylene and vinylacetate, the high pressure system was equipped with a mixing autoclave (Fig. 1).
J. Albert, G. Luft / Chemical Engineering and Processing 37 (1998) 55–59
56 Table 1 Studies in ethylene decomposition Pressure (MPa)
Temperature (°C) Volume, shape
10–25 6–27 10–500
120 – 325 150 – 200 25 – 400
6a, cylinder 200b, cylinder 15b, cylinder
11 1–17 5–60
40 350 – 540 25 – 300
10–200
250 – 450
a
Volume expressed in litres,
b
Ignition source
Additives
Reference
— — Vinylacetate, oxygene
[3] [4] [5]
120a, tube 550b, sphere 200b, cylinder
Hot spot (Pt-wire) Hot spot (Cu-wire) Hot spot (Pt-wire), thermal runaway Gun powder Thermal runaway Hot spot (Pt-wire)
[6] [1] [7,8]
200b, cylinder
Thermal runaway
— Nitrogene, oxygene Vinylacetate, methylmethacrylate, acetaldehyde Oxygene
[9,10]
millilitres.
To prepare an ethylene/vinylacetate mixture, ethylene was first compressed with a diaphragm compressor (1) into the mixing autoclave (3). The vinylacetate was later added to the ethylene in the mixing autoclave (3) with a screw press (2). The reactor (4) is constructed for maximum pressure of 500 MPa and temperatures of 500°C. The reactor (4) has an inside diameter of 5 cm and a height of 10 cm, equivalent to 200 ml in volume. With a pressure gauge (5) and a Pt – PtRh thermocouple (6), we are able to measure the pressure and the temperature inside the reactor during an experiment. The temperature and pressure data are stored in a computer. To carry out an experiment, the reactor (4) is first evacuated and heated to a pre-set wall temperature. The ethylene/vinylacetate mixture is fed to the reactor (4) from the mixing autoclave (3) by opening a remotecontrolled valve (7). As soon as the mixture is injected, pressure and temperature are recorded continuously for 30 min. The data will show whether the mixture has undergone thermal decomposition.
3. Results By varying the initial pressure and temperature, a boundary above which the mixture becomes unstable can be determined. A thermal runaway occurs in the region where the pressure and the wall temperature of the reactor are too high. Because of the high temperature, the system cannot remove the polymerization heat fast enough, which causes the thermal polymerization to accelerate and finally causes the mixture to decompose. A thermal runaway is characterized by an abrupt increase of pressure and temperature. Fig. 2 shows a pressure and temperature profile of a thermal runaway. The reactor autoclave was set at 300°C. As soon as the ethylene/vinylacetate mixture was fed into the autoclave, it reached 300°C and 120 MPa in a few seconds.
Soon after, the thermal polymerization starts to occur which is evident by a pressure decrease in the reactor. Because of the insufficient removal of polymerization heat, the temperature inside the reactor increases. As soon as the pressure has decreased by 30 MPa and the temperature increases to 320°C the ethylene/vinylacetate mixture starts to decompose, as evident by the abrupt increase of pressure and temperature. Fig. 3 shows decomposition limits for ethylene/vinylacetate mixtures. For comparison the decomposition limit of pure ethylene is shown [9]. Decomposition of ethylene is not affected by vinylacetate below 25 MPa. Above this pressure, the influence becomes larger, shifting the decomposition boundary to lower temperatures. It appears that, above 125 MPa, the influence of pressure has levelled off. The decomposition limit has shifted 30° maximum for the 10 wt.% vinylacetate containing mixture and 60° maximum for the 30 wt.% vinylacetate. While the decomposition boundary is important for the manufacturing of copolymers, one needs maximum explosion pressure, maximum rate of pressure increase or maximum temperatures during the decomposition for technical designs of plants and safety devices. Fig. 4 shows the maximum explosion pressure as a function of the initial pressure at 325, 350 and 375°C.
Fig. 1. High pressure equipment for determination of decomposition boundaries: (1) Diaphragm compressor, (2) screw press, (3) mixing autoclave, (4) reactor, (5) pressure gauge, (6) thermocouple, (7) remote controlled valve.
J. Albert, G. Luft / Chemical Engineering and Processing 37 (1998) 55–59
Fig. 2. Course of pressure and temperature of decomposition during polymerization.
The maximum explosion pressure increases with increasing initial pressure and decreasing wall temperatures. In comparison, the maximum pressures for pure ethylene [9] is 20 MPa higher than that of ethylene/ vinylacetate (10 wt.%) mixtures. As one can see from Fig. 5 the maximum explosion pressure increases with increasing density, meaning that the monomer mixture with a higher density produces more explosive gases. The figure also shows that the curves derived from ethylene, and ethylene with 10 and 30 wt.% vinylacetate fall on top of each other, within accuracy of measurement. The results show that the maximum explosion pressure is solely dependent on the density of the mixture regardless of its composition. Another important safety factor for the design of a reactor is the maximum rate of pressure increase. This number is a measure of the violence of decomposition. Fig. 6 shows the maximum rate of pressure increase as a function of initial pressure. As shown in Fig. 4 for maximum explosion pressure, the maximum rate of pressure increase rises with increasing initial pressure and decreasing wall temperature. The maximum value is 950 MPa s − 1 and is comparable with the values for pure ethylene [9]. During the decomposition of ethylene/vinylacetate mixtures the highest temperatures are between 1200 and 1400°C. This data is critical for the design of a temperature robust reactor.
Fig. 3. Decomposition boundaries for ethylene [9] and ethylene/vinylacetate mixtures.
57
Fig. 4. Maximum explosion pressure as function of initial pressure and wall temperature for ethylene/vinylacetate (10 wt.%) mixtures.
The main products of ethylene decomposition are carbon black, methane, hydrogen [1,11] and traces of ethane and undecomposed ethylene. The added vinylacetate produces additional carbon monoxide, carbon dioxide and water. GC analysis of the decomposition gases reveals that the composition of gases is dependent on initial pressure, wall temperature and concentration of vinylacetate in the mixture (Fig. 7). It should be pointed out that methane and hydrogen are still the main products. The methane content is about 70–77 vol.%, this increases with increasing initial pressure. The hydrogen content ranges between 17 and 25 vol.%, but it decreases with increasing initial pressure. The wall temperature has a bigger effect on the hydrogen content than on the methane content. The volume concentrations of ethane and ethylene are in the range of one percent. Their production is not affected by pressure and temperature. In the mixture (10 wt.% vinylacetate), the volume concentrations of carbon monoxide and carbon dioxide are about 1 and 0.2%, respectively. Their production is also not affected by the initial parameters. On the contrary, their production is strongly influenced by the initial parameters for the mixture of 30 wt.% vinylacetate. They decrease with increasing initial pressure. At
Fig. 5. Maximum explosion pressure as function of density of monomer mixture.
58
J. Albert, G. Luft / Chemical Engineering and Processing 37 (1998) 55–59
Fig. 6. Maximum rate of pressure increase as function of initial pressure and wall temperature for ethylene/vinylacetate (10 wt.%) mixtures.
Fig. 8. Influence of vinylacetate on the decomposition boundary of ethylene. Below the isobars polymerization occurs, above decomposition during polymerization.
44 MPa, carbon monoxide and carbon dioxide concentrations are 4.5 and 2 vol.%, respectively. Vinylacetate concentration also exerts an influence on the production of methane and hydrogen. For the same initial pressure and wall temperature the methane content is about 1.5 vol.% higher for the 30 wt.% vinylacetate. In contrast the hydrogen concentration is about 4 vol.% lower for 30 wt.% vinylacetate. The addition of 10 wt.% vinylacetate to ethylene has no effect on methane concentration in the decomposition gas. On the other hand the hydrogen content is influenced by vinylacetate. For the same initial parameters the hydrogen content is about 1.5 vol.% higher for pure ethylene than for the mixture with 10 wt.% vinylacetate.
The decomposition boundary and safety factor data are important for technical design and operation of ethylene/vinylacetate copolymer production plants. Fig. 8 shows the decomposition temperature of ethylene as a function of percentage vinylacetate at different pressures. The experimental decomposition boundaries were extrapolated to lower and higher pres-
sures. It can be seen that the decomposition temperature decreases with increasing vinylacetate content and the influence becomes smaller with increasing vinylacetate content. With increasing initial pressure the decomposition temperature moves towards a limiting temperature, below which decomposition does not occur. This corresponds to Fig. 3, where the curvature of the decomposition boundary decreases with increasing pressure. To avoid thermal runaways in high pressure ethylene/vinylacetate copolymerization one should avoid the temperatures above this limit. Besides thermal runaway due to insufficient removal of heat from polymerization, another possible ignition source is hot spot. If one compares the influence of vinylacetate on the decomposition limits of ethylene for thermal runaway and hot spot, there are significant differences (Fig. 9). The addition of vinylacetate shifts the decomposition limit for a thermal runaway to lower temperatures. On the contrary, the decomposition limit for ignition by a hot spot [7] moves to higher temperatures. Because of the high temperature at the hot spot, ethylene decomposes spontaneously. In this case vinylacetate has an inhibiting effect on the decomposition of ethylene and the explosion region becomes smaller.
Fig. 7. Gaseous decomposition products after thermal runaway.
Fig. 9. Influence of vinylacetate on the decomposition boundary of ethylene for thermal runaway and ignition by a hot spot.
4. Discussion and conclusion
J. Albert, G. Luft / Chemical Engineering and Processing 37 (1998) 55–59
A thermal runaway is characterized by the insufficient removal of polymerization heat. The addition of vinylacetate has an influence on polymerization kinetics. The polymerization rate increases, therefore the rate of heat production increases. The faster heat production results in lowering the decomposition limit. For designing robust plants and safety devices one needs to know the maximum stress the equipment can tolerate during decomposition. The experimental values for maximum explosion pressure, maximum rate of pressure increase and maximum temperatures shows only small differences between pure ethylene and mixtures with vinylacetate. For the same initial pressure and wall temperature the maximum explosion pressure is only 20 MPa higher for the ethylene/vinylacetate (10 wt.%) mixture. If one uses the density as criterion for the maximum explosion pressure there is no noticeable difference. The maximum rate of pressure increase and the maximum temperatures show no differences, within accuracy of measurement. Thus for designing plants and safety devices it is only necessary to take the higher maximum explosion pressure of ethylene/vinylacetate mixtures into consideration. Methane and hydrogen in the explosion gases increases and decreases, respectively, with increasing vinylacetate concentration, and they are potential sources of hazard. If they are mixed with air, it is likely to explode violently especially in the presence of an ignition source. Carbon monoxide, which is produced especially at higher concentrations of vinylacetate, is also a potential hazard source, because of its toxicity. The knowledge of decomposition boundaries, safety factors and the influence of vinylacetate on the explosion characteristics should allow one to safely operate ethylene/vinylacetate copolymer manufacturing plants.
Acknowledgements This publication is part of a DECHEMA research
.
.
59
project, sponsored by the German Ministry of Economic Affairs (AIF-No 10497N).
Appendix A. Nomenclature HS P Pmax t TR VA
hot spot pressure (MPa) maximum explosion pressure (MPa) time thermal runaway vinylacetate
References [1] L.G. Britton, D.A. Taylor, D.C. Wobser, Thermal stability of ethylene at elevated pressure, Plant/Oper. Prog. 5 (1986) 238– 250. [2] R. Neumann, G. Luft, Untersuchungen zum thermischen Zerfall von Ethylen unter Hochdruck, Chem. Eng. Sci. 28 (1973) 1505– 1514. [3] D. Conrad, R. Kaulbars, Untersuchungen zur chemischen Instabilita¨t von Ethylen, Chem.-Ing.-Tech. 47 (1975) 265. [4] W.B. Howard, Ethylene behavior related to hot tapping, The American Institute of Chemical Engineers Loss Prevention Symp. 9 (1975) 9 – 14. [5] G. Luft, R. Neumann, Selbstzerfall und fremdgezu¨ndeter Zerfall von verdichtetem Ethylen, Chem.-Ing.-Tech. 50 (1978) 620–622. [6] G.R. Worell, If ethylene decomposes in pipe..., Hydrocarbon Process. 58 (1979) 255 – 258. [7] Th. Zimmermann, Bestimmung sicherheitstechnischer Kenngro¨ßen von verdichtetem Ethylen, Dissertation, TH Darmstadt 1994. [8] Th. Zimmermann, G. Luft, Untersuchungen zum explosiven Zerfall von verdichtetem Ethylen, Chem.-Ing.-Tech. 66 (1994) 1386 – 1389. [9] H. Bo¨nsel, G. Luft, Sicherheitstechnische Untersuchungen des explosiven Zerfalls von verdichtetem Ethen, Chem.-Ing.-Tech. 67 (1995) 862 – 864. [10] H. Bo¨nsel, Sicherheitstechnische Untersuchungen des explosiven Zerfalls von verdichtetem Ethen, Dissertation, TH Darmstadt 1994. [11] S.X. Zhang, N.K. Read, W.H. Ray, Runaway phenomena in low-density polyethylene autoclave reactors, Am. Inst. Chem. Eng. J. 42 (1996) 2911 – 2925.