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Alkali-catalysed polymerization of ethylene oxide and propylene oxide—hazard evaluation using accelerating rate calorimetry
Short C83mmunicatbns Alkali-catalysed polymerization of ethylene oxide and propylene oxide- hazard evaluation using accelerating rate calorimetry Brian G Freeder and Timothy J Snee Health and Safety Executive Research and Laboratory Services Division, Harpur Hill, Buxton, Derbyshire SKI 7 9JN, UK An experimental investigation of the tendency of ethylene oxide and propylene oxide to undergo rapid exothermic polymerization in the presence of aqueous alkali is described. The results from accelerating rate calorimetry indicate that the likelihood and severity of thermal explosion are much greater in the case of ethylene oxide. Estimates of the critical temperatures and concentrations that could lead to thermal explosion under industrial conditions suggest that the level of control measures adopted for ethylene oxide is not necessarily appropriate to the storage and processing of propylene oxide. (Keywords: hazard analysis; accelerating rate calorimetry; polymerization of EtOH) Ethylene oxide is known to be susceptible to rapid exothermic polymerization ‘. This can be initiated thermally or by acids and bases and by catalysts such as anhydrous chlorides of iron, aluminium, tin and metal oxides. If a storage vessel is exposed to heat or becomes contaminated with catalyst, rapid polymerization can lead to an explosion. Precautions against contamination or overheating of vessels containing ethylene oxide are set out in a Chemical Industries Association code of practice ’ _ Propylene oxide is chemically similar to ethylene oxide and is capable of polymerization. However, no detailed study has so far been undertaken to determine the recommendations for whether storage and handling of ethylene oxide are also appropriate to propylene oxide. The work described in this communication addresses three related issues: l
If propylene oxide undergoes rapid polymerization, are the consequences likely to be as severe as in the case of ethylene oxide? l What levels of contamination will cause rapid polymerization of propylene oxide compared with those which can cause explosive polycondensation of ethylene oxide? 0 What is the temperature dependence of the polymerization reactions and how can it be used to derive safe storage and handling temperatures? A short survey of industrial applications involving ethylene and propylene oxide revealed a wide range of processes with the possibility of contamination by a large variety of substances. To gain a realistic measure of the relative reac-
Received I3
May
I988
tivity of ethylene oxide and propylene oxide, it was considered appropriate to concentrate on the alkylene oxide-alkali systems. The alkali metal hydroxides are widely used in processes involving ethylene oxide and propylene oxide. Alkylene oxides in the presence of alkali are likely to show the same relative reactivity in the presence of other catalysts. A study of the tendency to polymerize in the presence of heterogeneous catalysts such as iron oxide would be likely to be highly dependent on the particle size and surface condition of the catalyst. Such a study has not been included in the present investigation because of the difficulty in reproducing experimental conditions and interpreting results.
Experimental
method
Equipment This investigation was carried out using an accelerating rate calorimeter (ARC). The ARC is a thermoanalytical instrument in which a sample, in a- closed is subjected to a stepwise system, increase in temperature, but is held in an adiabatic state between each temperature step. Adiabaticity is achieved by matching the temperature of the surroundings to that of the sample, thermal decomposition being detected when, after a temperature step, the temperature of the system continues to rise due to self-heating of the sample. If the rate of temperature rise exceeds a preset value (0.02’C min-‘), stepwise heating is suspended and an adiabatic environment is maintained as temperature and pressure are recorded during the exothermic decomposition of the sample. The sample to be tested in the ARC is loaded into a spherical bomb of 25 mm
0960-423O/S0~030164-tXS3.00 0
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Butterworth
& Co.
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(Publishers)
Process
Ltd
fnd., 1988,
Vol I, July
diameter which has a 3.175 mm diameter tube welded to its top. After addition of the test sample the bomb is attached to the interior of the ARC calorimeter lid assembly by means of a compression tube fitting which has been pre-swaged onto the 3.175 mm diameter tube. Pressure in the bomb is measured by a transducer which is connected to the exterior of the calorimeter lid via a 500 mm length of 1.588 mm diameter stainless steel tube and compression fittings. The temperature of the bomb is measured by means of a thermocouple held under a clip on the exterior of the bomb. Sample preparation ARC analysis was performed on ethylene oxide and propylene oxide mixed with various concentrations of aqueous sodium hydroxide. The sodium hydroxide solutions were prepared by diluting commercially prepared concentrates (British Drug Houses Ltd). The alkylene oxide/alkali mixtures were prepared in situ in the ARC bomb and consisted of =1.8-g of ethylene or propylene oxide mixed with ~0.2 g of sodium hydroxide solution. For those mixtures containing propylene oxide this was accomplished by first weighing the sodium hydroxide solution, of the into the appropriate concentration, bomb using a 1 ml syringe and then adding the propylene oxide in the same manner. Care was necessary to ensure that no bubbles of the materials were deposited in the neck of the ARC bomb, this was found to be best accomplished by using a syringe needle long enough to reach almost to the bottom of the bomb and by withdrawing the needle up the side of the neck. The preparation of the mixtures containing ethylene oxide was made slightly
Short
more difficult by the low boiling point of the ethylene oxide and by its tendency to polymerize at ambient temperatures. The mixtures were prepared in the same general manner however, except that after the addition of the sodium hydroxide the bomb was placed in a cooled incubator at -2O”C, together with a quantity of the ethylene oxide and a syringe, for a period of l-2 h. The ethylene oxide was then added as rapidly as possible and the bomb transferred to the ARC. By having the incubator and balance as close to the ARC as possible it was found practicable to commence the ARC run with a starting temperature of about IO-lS°C in the bomb, a low enough temperature to ensure that little polymerization occurred before the ARC run was started. For both mixtures the contents of the bomb were agitated immediately before the ARC run was commenced by gently swirling the bomb.
temperature dependence constant is given by:
when the rate
k = A exp( - E/RT)
Communications
Examination of Figures I and 2 shows that the initial parts of the plots of self-heat rate versus temperature (against logarithmic and reciprocal scales respectively) are approximately linear. The initial gradients have been evaluated and, within experimental error, have been found to be independent of the concentration of alkali. The average gradient from the ethylene oxide plots corresponds to an activation energy of 81.1 kJmol-‘. This is in reasonable agreement with the results of
(1)
where: k = rate constant; A = preexponential factor; E = activation energy; R = gas constant and T= temperature. In the initial stages of a reaction the rate will not be affected by reactant consumption and Equation (1) implies that a plot of In (self-heat rate) vwsus (l/r) will be a straight line of gradient E/R.
10
Results Figures 1 and 2 show the ARC plots of self-heat rate versus temperature for ethylene oxide and propylene oxide in the presence of various concentrations of aqueous sodium hydroxide. The general pattern of behaviour is the same for each of the samples. Exothermic reactions are detected at the onset temperature (TO) ,when the self-heat rate exceeds Above To the rate 0.02”C min-‘. increases rapidly (note logarithmic scale) until it approaches a maximum after which the rate falls off sharply as the reaction goes to completion. In general, higher alkali concentrations correspond to lower onset temperatures and higher self-heat rates until the maximum is reached. Self-heat rates up to and including the maximum are substantially higher for ethylene oxide than for propylene oxide at the same temperature and alkali concentration. Apart from these general trends the traces show considerable complexity and variability. This is partly due to the complexity of the polymerization reactions, but also attributable to experimental difficulties in rapidly assembling well mixed samples in the ARC bombs and loading them into the instrument before significant reaction or evaporation has taken place, particularly in the case of ethylene oxide. Repeat experiments showed poor reproducibility particularly at low alkali concentrations. Some of the variation may be due to intrinsic variability in the initiation of polymerization of small quantities of material in an unstirred system. To characterize the self-heat rate plots and arrive at some measure of the comparative reactivity of ethylene oxide and propylene oxide and the effect of alkali the reactions will be concentration, assumed to follow an Arrhenius-type
1.0
0.011
’
’
0
’
1
’
1
I
I
I
I
I1II1I11111111111111
100
I,,,
200
300
400 Temperature (Y) Figure 1 ARC plots of self-heat rate versus temperature for ethylene oxide/ NaOH mixtures: -, 1 MNaOH; - . - . , 0.5 MNaOH; - - - , 0.25 MNaOH; -, 0.125 M NaOH. (Heat rate = log scale; temperature scale = 1 /T Kelvin)
J
100 200 300 400 Temperature (OC) for propylene oxide/ Figure 2 ARC plots of self-heat rate ~erasus temperature NaOH mixtures: 1 M NaOH; - . - , 0.25 MNaOH; - - - -, 0.125 MNaOH; -, 0.0625 M NaOH. (Heat rate = log scale; temperature scale = l/T Kelvin)
-,
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Table 1
tions
Experimental
conditions
o Factor
Onset temperature f°C)
Final temperature f°C)
2.005
2.082
22.5
222
2.000 2.000
2.082 2.164 2.112
35.5 39.0 55.0
250 178 263
Bomb weight fg)
Sample weight fg)
1 .O
8.069
:::5 0.125
8.069 8.646 8.259
NaOH molarity
and results of ARC analysis on ethylene
a previous investigation’. An activation energy of 83.6 kJmol_’ has been evaluated from the propylene oxide plots. These activation energies have nc detailed chemical significance relating to the reaction mechanisms, but should be regarded as global values which serve to characterize the experimental results and provide a measure of the comparative reactivity of ethylene and propylene oxide. The ARC sample bomb is maintained in an adiabatic environment, except under conditions of very rapid selfheating when some heat losses may occur. Thus the final temperature of the sample and containing bomb is determined by the heat generated by the sample, and the heat of reaction is given by: H=
AT
-oC,
where: AT= difference between onset temperature TO and final temperature Tf; and C, = specific heat of sample. I$ is the thermal dilution factor defined as:
Q=
M,C,
+
hfbcb
M,Cs
where: M, = sample mass; Mb = bomb mass; and Cb = specific heat of the bomb. The 4 factor takes account of the fact that heat generated by the sample is distributed between the sample and the containing bomb. 4 AT is the corrected adiabatic temperature rise which would occur in an isolated sample. The results of ARC analysis on the various ethylene oxide-alkali mixtures are summarized in Tub/e I. The adiabatic temperature rise produced by the polymerization of ethylene oxide in the presence of various concentrations of sodium hydroxide appears, within experimental error, to be independent of
Table 2
Experimental
conditions
NaOH molarity
Bomb weight (gl
Sample weight (g)
1 .o 0.25 0.125 0.0625
8.699 8.444 8.753 8.795
1.999 2.009 2.019 2.012
166
oxide/NaOH
mixtures
Heat of polymerization fkJ mol-‘1
Maximum pressure IkPaf
Temperature at maximum pressure f°C)
415.5
44.2
4465
147.0
456.5 296.5 439.5
48.6 31.6 46.7
3608 3876 4055
152.0 151.8 188.0
Corrected adiabatic temperature rise f°C)
the concentration of alkali apart from the mixture containing 0.5 N NaOH where the heat of reaction appears to be anomalously low. The average value of the heat of polymerization calculated from ARC data is 42.8 kJ mol-‘, which is lower than previously published values’. The discrepancy may be due to failure of the ARC. to follow rapid temperature changes such that the sample temperature exceeds that of the calorimeter. This results in an underestimate of the adiabatic temperature rise and the heat of reaction. The ARC results for the propylene oxide mixtures are summarized in Table 2. Heats of reaction calculated from these results, within experimental error, are independent of the concentration of alkali and yield an average value of 65.53 kJ mol-‘.This is less than the published value’ but the discrepancy is less than in the case of ethylene oxide. Maximum rates of temperature rise are lower for propylene oxide so it should be possible for the ARC to follow the course of the reaction more closely. Ethylene oxide differed from propylene oxide in that it exhibited exothermic polymerization in the ARC, without any NaOH as a catalyst, at an onset temperature of 112OC. This effect was not observed with propylene oxide.
rate on alkali concentration temperature, and the likelihood runaway reaction under industrial ditions.
Maximum iemperature and pressures resulting from rapid polymerization Conditions during ARC experiments differ substantially from those during bulk storage and handling. The maximum temperatures and pressures which would occur due to rapid polymerization of ethylene oxide or propylene oxide under industrial storage conditions will depend on factors such as the strength of the vessel, rate of heat loss, degree of venting, volume of ullage space and the mass of the vessel compared with the mass of the contents. Values for the corrected adiabatic temperature rise (4 AT) listed in Tub/es I and 2 represent a theoretical temperature rise which would occur in an isolated sample under adiabatic conditions. A similar temperature rise might be expected in a large insulated storage vessel of low thermal mass but, in general, the temperature rise under industrial conditions would be substantially less than the corrected adiabatic temperature rise. However, if the temperature rise is sufficient to cause autoignition in the vapour phase, much higher temperatures will be achieved. Ethylene oxide is much more likely to auto-ignite than propylene oxide. Flammability limits for ethylene oxide are from 3 to 100% with a standard autoignition temperature of 429”C, the limits for propylene oxide are from 2.1 to 21.5% with a standard auto-ignition temperature of 550°C. At elevated temperatures the fuel concentration in the vapour space of a propylene oxide tank is likely to be too rich for flame propagation. In the absence of auto-ignition,
Discussion The ARC data on the alkylene oxide-alkali mixtures can be used to assess the probabilities and likely consequences of runaway polymerization of ethylene oxide and propylene oxide. As indicated in the introduction, consideration will first be given to the effects of rapid polymerization, followed by separate discussion of the dependence of reaction
and results of ARC analysis on propylene oxide/NaOH
and of a con-
mixtures
$J Factor
Onset temperature 1°C)
Final temperature (OC)
Corrected adiabatic temperature rise 1°C)
Heat of polymerization (kJmol-‘)
Maximum pressure fkPa1
Temperature at maximum pressure 1°C)
2.186 2.132 2.173 2.179
53.0 55.0 68.0 91.0
262 270 296 298
452.5 458.5 495.5 451 .o
63.9 64.7 69.9 63.7
2656 2388 2299 2089
205.8 210.8 217.8 202.7
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Short Tables I and .2 suggest that there would be no significant difference between the maximum temperatures which might result from rapid polymerization of propylene and ethylene oxide. The pressures recorded during ARC experiments cannot be readily corrected Tables I and 2 for thermal dilution. show that maximum pressures are achieved at a temperature below the maximum temperature. This is conbistent with a reduction in the average vapour pressure of the reacting mixture as polymerization takes place. Maximum pressures recorded for ethylene oxide are substantially higher than those recorded for the propylene oxide mixtures. This suggests that the pressure is due largely to the vapour pressure of the monomer. The vapour pressure of ethylene oxide at its critical point (195.8OC) is 7194 kPa4 compared with a vapour pressure of 4924 kPa for Propylene oxide at its critical point (209 C). Polymerization during ARC experiments resulted in pressures very much less than those at the critical points, but in large vessels (with small thermal dilution factors) polymerization will lead to higher temperatures and the pressure will increase, but by an amount which is difficult to predict became of the complex composition of the partially polymerized material. However, in general, pressures resulting from the rapid polymerization of ethylene oxide on an industrial scale will be substantially higher thah those caused by polymerization of propylene oxide. Concentration and temperature dependence of reaction rate of Figures 3 and 4 show the variation self-heat rate at 209C with alkali concentration for ethylene oxide and propylene oxide respectively. As would be expected, self-heat rates diminish with reducing alkali concentration. There is evidence of a rapid reduction in the rate of polymerization of propylene oxide at alkali concentrations below 0.25N. A similar effect has been observed for ethylene oxide’. The effect is not evident in the present results for ethylene oxide. the result for ethylene However, oxide + 0.5N NaOH must be considered anomalous and previous findings are not
Y
20 I&___!
::
-
r7 3 .g
0
16 12
e ulg
8
.G ‘G I"
4 0
0
0 0
0.2
0.4
0.6
Sodium hydroxide
0.6
1.0
molarity
Figure 3 Ethylene oxide - self-heat at 20°C versu.s NaOH concentration
rate
Communications
Table 3 Effect of NaOH concentration on the critical temperatures of ethylene oxide/NaOH and propylene oxide/NaOH mixtures when stored under similar conditions Critical temperature "0
0.2 Sodium
0.4
0.6
hydroxide
0.6
1.0
NaOH concentration
Ethylene oxide/MaOH
kg/m31
(v2
toa
0.88 1.77 2.65
20 8 3
39 34
3.53
0
motarity
Figure 4 Propylene oxide - self-heat at 20°C versus NaOH concentration
rate
necessarily contradicted by the results reported here. Overall activation energies derived from the plots of self-heat rates versus and were 81.1 temperature 83.6 kJmol-’ for ethylene oxide and propylene oxide, respectively. The lower activation energy for the polymerization of ethylene oxide compared with that for propylene oxide implies that, at rates for ethylene 2o?O”c, reaction oxide/NaOH mixtures are between 10 and 20 times greater than those for the propylene oxide/NaOH mixtures, as can be seen from Figures 3 and 4. At higher temperatures this difference becomes progressively more marked.
X.9
(F + G(NaOH))QA
V
RT,
exp( - E/RT,:)
where: V= volume; S = surface area; X= surface heat transfer coefficient; TE = critical temperature (NaOH) = sodium hydroxide concentration; Q = heat of reaction; and F and G are constants (determined by linear regression on the plots of self-heat rate versus (NaOH)). Ethylene oxide and propylene oxide can be compared by arbitrarily specifying a storage condition (i.e. fixing X, S, V etc.) and calculating corresponding values for Tc and (NaOH). These values are listed in Tables 3 and 4 for a storage condition in which ethylene oxide contaminated with 10% by weight of 0.25M NaOH (i.e. 0.88 kgm-‘) has a critical temperature of 20°C. Tables 3 and 4 must be interpreted with care since they apply to one specific condition. However, the following trends can be identified and are likely to apply over a range of vessel sizes and NaOH concentrations:
rate of polymerization follows temperature Arrhenius-type zpendence characterized by a global activation energy. The rate of polymerization is directly proportional to the concentration of NaOH. (This will appear a drastic assumption in view of the variation of rate with NaOH concentration shown in Figures 3 and 4. However, assessment of the relative stability of the two substances is unlikely to be affected by more detailed analysis of the concentration dependence, and previous studies* suggest that a linear relationship is not unrealistic.) Explosive polymerization takes place via a thermal mechanism governed by Semenov boundary conditions’ in is which reactant temperature assumed to be uniform with thermal resistance only at the surface.
J. Loss Prev.
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Given these assumptions, the critical storage temperature which can lead to explosive polymerization will be related to the concentration of NaOH by an expression of the form:
Probability of rapid polymerization under industrial conditions The small scale experimental studies reported here have demonstrated that ethylene oxide is substantially more prone to rapid polymerization than propylene oxide. However, to assess the implications of this when the materials are stored or processed under industrial conditions, it is necessary to estimate the temperatures and degrees of contamination which can lead to rapid polymerization when the materials are handled in bulk. The following assumptions have to be made in order that the results of laboratory investigations can be extrapolated: The
Propylene oxide/N&H
0
Critical temperatures fall as NaOH The concentration is increased. reduction is more pronounced in the case of ethylene oxide.
Table 4 Comparison of the concentrations of NaOH necessary to produce the same critical temperatures when ethylene oxide/NaOH and propylene oxide/NaOH mixtures are stored under similar conditions NaOH concentration Critical temperature (“C)
Process
Ethylene oxidelNa0t-l (kg mm?
Propylene oxide/NaOH (kgm-?
0.88 1.77 2.65 3.53
10.73 46.84 83.94 121.32
20 8 3 0
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7988,
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Short Communications 0 Critical
l
temperatures are substantially higher for the propylene oxide mixtures (between 19 and 29°C higher over the range of NaOH concentrations investigated). Table 4 shows that, if the critical temperatures and storage conditions are the same. the level of contamination of propylene oxide must be at least 10 times that of ethylene oxide.
Conclusion Ethylene oxide is known to be more reactive than propylene oxide and this study has confirmed that, specifically in the case of alkali-catalysed polymerization, substantial differences in reaction rates can be expected. It has been demonstrated that the consequences of
rapid polymerization of propylene oxide are unlikely to be as severe as for ethylene oxide. Some tentative calculations have indicated margins of safety in terms of storage temperature and degree of contamination which would apply when ethylene oxide and propylene oxide are stored under similar conditions. An exhaustive study using other catalyst systems has not been undertaken. The results of the present Study suggest that the level of control measures recommended for the storage and handling of ethylene oxide’ is not justified for propylene oxide, although a similar design philosophy may be appropriate. More detailed knowledge of the likelihood of contamination or temperature excursions under industrial conditions is
Computerized
hazards
Drive,
References ‘Guidelines for the Bulk Handling of Ethylene Oxide’, Chemical Industries Association, 1983 Gupta, N. K., ‘The Explosive PolyCondensation of Ethylene Oxide’, J.S.C.1. 68, June 1949 Pogany, G. A., ‘On the Safe use of Alkylene Oxides in High-Pressure BenchScale Experiments’, Chem. Ind. (London) January 1979 Yaws, C. L. and Rackley, M. P., ‘Ethylene, Propylene and Butylene Oxides’ Chem. Eng., April 1976 Semenov, N. N., Z. Physik, 1928, Vol48, PP 571 0 British Crown Copyright
analysis
The backbone of a hazards analysis hazards information exchange C. Robert Nelms Failsafe Network, Inc., 4337 Roundhill
required before the relative hazards can be properly assessed.
effort/basis
Chesterfzeld,
for a corporate
VA 23832,
USA
Computerized hazards analysis has many obvious advantages. Criticality codes can be assigned, modified, sorted, grouped, and regrouped. Critical event assignments can be automatically tracked. Graphics capability can show the distribution of various types of hazards dramatically, and the risks involved with each. But, computerized hazards analyses can provide benefits orders of magnitudes beyond those mentioned above. Consider the following: as hazards analyses are performed, i.e., as hazard severity, critical events, and risks are defined for each component and each operator action in your facility, you are relying on the memories of many people for much of your input. The following text includes real life examples of such a situation (Keywords:
hazard analysis: risk; criterion)
For example, you might ask ‘Can this compressor cause downtime? If so. how, how often, and to what extent does it affect the rest of the plant?’ During this questioning, you will be relying on historical records, your own experience, and the experiences of operators and maintenance personnel. The capturing of this information in specifically structured computer form, along with the routine inputting of all unanticipated critical events, is actually the beginnings of an expert system relating to the operating hazards of a plant. If performed uniformly throughout a corporation, this type information could be shared for the benefit of all. Received
I9 May 1988
A crisis avoidance What is a hazards
machine, analysis?
It is the fuel that drives our crisis avoidance machine. Without the fuel, the machine stops, exposing us to unexpected catastrophe. Bhopal, Chernobyl, the Challenger, and Three Mile Island have helped highlight the absolute necessity for this machine. Industry can no longer afford to react to its crises. It is time for industry to proact. proactive machinery Fortunately, (approaches) have been successfully implemented in a variety of industries. A close scrutiny of these success stories reveals their dependence on a formal structured approach. Recognizing this, the U.S. government-as well as many
0960-4230/68/030168-OW3.00 0 1988 Butterworth & Co. (Publishers1 Ltd 166 J. Loss Prev. Process lnd., 7988,
Vol
1, July
defence and design contractorsrequire strict adherence to MIL-STD882B, ‘System Safety Program Requirements’ in many of their contracts. Paraphrased from MIL-STD-882B, a for specific structure is necessary developing and implementing a comprehensive system safety program’, ‘to help identify the huzurds of a system and to impose design requirements and management controls to prevent mishaps’, ‘by eliminating hazards or reducing the associated risk to a level acceptable to the managing activity’. (Although the standard is addressing safety, its emphasis can be applied to any proactive effort, i.e., avoiding any crisis large or small). A hazards analysis is nothing more
If propylene oxide undergoes rapid polymerization, are the consequences likely to be as severe as in the case of ethylene oxide? l What levels of contamination will cause rapid polymerization of propylene oxide compared with those which can cause explosive polycondensation of ethylene oxide? 0 What is the temperature dependence of the polymerization reactions and how can it be used to derive safe storage and handling temperatures? A short survey of industrial applications involving ethylene and propylene oxide revealed a wide range of processes with the possibility of contamination by a large variety of substances. To gain a realistic measure of the relative reac-
Received I3
May
I988
tivity of ethylene oxide and propylene oxide, it was considered appropriate to concentrate on the alkylene oxide-alkali systems. The alkali metal hydroxides are widely used in processes involving ethylene oxide and propylene oxide. Alkylene oxides in the presence of alkali are likely to show the same relative reactivity in the presence of other catalysts. A study of the tendency to polymerize in the presence of heterogeneous catalysts such as iron oxide would be likely to be highly dependent on the particle size and surface condition of the catalyst. Such a study has not been included in the present investigation because of the difficulty in reproducing experimental conditions and interpreting results.
Experimental
method
Equipment This investigation was carried out using an accelerating rate calorimeter (ARC). The ARC is a thermoanalytical instrument in which a sample, in a- closed is subjected to a stepwise system, increase in temperature, but is held in an adiabatic state between each temperature step. Adiabaticity is achieved by matching the temperature of the surroundings to that of the sample, thermal decomposition being detected when, after a temperature step, the temperature of the system continues to rise due to self-heating of the sample. If the rate of temperature rise exceeds a preset value (0.02’C min-‘), stepwise heating is suspended and an adiabatic environment is maintained as temperature and pressure are recorded during the exothermic decomposition of the sample. The sample to be tested in the ARC is loaded into a spherical bomb of 25 mm
0960-423O/S0~030164-tXS3.00 0
1998
164
Butterworth
& Co.
J. Loss Prev.
(Publishers)
Process
Ltd
fnd., 1988,
Vol I, July
diameter which has a 3.175 mm diameter tube welded to its top. After addition of the test sample the bomb is attached to the interior of the ARC calorimeter lid assembly by means of a compression tube fitting which has been pre-swaged onto the 3.175 mm diameter tube. Pressure in the bomb is measured by a transducer which is connected to the exterior of the calorimeter lid via a 500 mm length of 1.588 mm diameter stainless steel tube and compression fittings. The temperature of the bomb is measured by means of a thermocouple held under a clip on the exterior of the bomb. Sample preparation ARC analysis was performed on ethylene oxide and propylene oxide mixed with various concentrations of aqueous sodium hydroxide. The sodium hydroxide solutions were prepared by diluting commercially prepared concentrates (British Drug Houses Ltd). The alkylene oxide/alkali mixtures were prepared in situ in the ARC bomb and consisted of =1.8-g of ethylene or propylene oxide mixed with ~0.2 g of sodium hydroxide solution. For those mixtures containing propylene oxide this was accomplished by first weighing the sodium hydroxide solution, of the into the appropriate concentration, bomb using a 1 ml syringe and then adding the propylene oxide in the same manner. Care was necessary to ensure that no bubbles of the materials were deposited in the neck of the ARC bomb, this was found to be best accomplished by using a syringe needle long enough to reach almost to the bottom of the bomb and by withdrawing the needle up the side of the neck. The preparation of the mixtures containing ethylene oxide was made slightly
Short
more difficult by the low boiling point of the ethylene oxide and by its tendency to polymerize at ambient temperatures. The mixtures were prepared in the same general manner however, except that after the addition of the sodium hydroxide the bomb was placed in a cooled incubator at -2O”C, together with a quantity of the ethylene oxide and a syringe, for a period of l-2 h. The ethylene oxide was then added as rapidly as possible and the bomb transferred to the ARC. By having the incubator and balance as close to the ARC as possible it was found practicable to commence the ARC run with a starting temperature of about IO-lS°C in the bomb, a low enough temperature to ensure that little polymerization occurred before the ARC run was started. For both mixtures the contents of the bomb were agitated immediately before the ARC run was commenced by gently swirling the bomb.
temperature dependence constant is given by:
when the rate
k = A exp( - E/RT)
Communications
Examination of Figures I and 2 shows that the initial parts of the plots of self-heat rate versus temperature (against logarithmic and reciprocal scales respectively) are approximately linear. The initial gradients have been evaluated and, within experimental error, have been found to be independent of the concentration of alkali. The average gradient from the ethylene oxide plots corresponds to an activation energy of 81.1 kJmol-‘. This is in reasonable agreement with the results of
(1)
where: k = rate constant; A = preexponential factor; E = activation energy; R = gas constant and T= temperature. In the initial stages of a reaction the rate will not be affected by reactant consumption and Equation (1) implies that a plot of In (self-heat rate) vwsus (l/r) will be a straight line of gradient E/R.
10
Results Figures 1 and 2 show the ARC plots of self-heat rate versus temperature for ethylene oxide and propylene oxide in the presence of various concentrations of aqueous sodium hydroxide. The general pattern of behaviour is the same for each of the samples. Exothermic reactions are detected at the onset temperature (TO) ,when the self-heat rate exceeds Above To the rate 0.02”C min-‘. increases rapidly (note logarithmic scale) until it approaches a maximum after which the rate falls off sharply as the reaction goes to completion. In general, higher alkali concentrations correspond to lower onset temperatures and higher self-heat rates until the maximum is reached. Self-heat rates up to and including the maximum are substantially higher for ethylene oxide than for propylene oxide at the same temperature and alkali concentration. Apart from these general trends the traces show considerable complexity and variability. This is partly due to the complexity of the polymerization reactions, but also attributable to experimental difficulties in rapidly assembling well mixed samples in the ARC bombs and loading them into the instrument before significant reaction or evaporation has taken place, particularly in the case of ethylene oxide. Repeat experiments showed poor reproducibility particularly at low alkali concentrations. Some of the variation may be due to intrinsic variability in the initiation of polymerization of small quantities of material in an unstirred system. To characterize the self-heat rate plots and arrive at some measure of the comparative reactivity of ethylene oxide and propylene oxide and the effect of alkali the reactions will be concentration, assumed to follow an Arrhenius-type
1.0
0.011
’
’
0
’
1
’
1
I
I
I
I
I1II1I11111111111111
100
I,,,
200
300
400 Temperature (Y) Figure 1 ARC plots of self-heat rate versus temperature for ethylene oxide/ NaOH mixtures: -, 1 MNaOH; - . - . , 0.5 MNaOH; - - - , 0.25 MNaOH; -, 0.125 M NaOH. (Heat rate = log scale; temperature scale = 1 /T Kelvin)
J
100 200 300 400 Temperature (OC) for propylene oxide/ Figure 2 ARC plots of self-heat rate ~erasus temperature NaOH mixtures: 1 M NaOH; - . - , 0.25 MNaOH; - - - -, 0.125 MNaOH; -, 0.0625 M NaOH. (Heat rate = log scale; temperature scale = l/T Kelvin)
-,
J. Loss Prev. Process
Ind.,
1988,
Vol
7, July
165
Short
Communice
Table 1
tions
Experimental
conditions
o Factor
Onset temperature f°C)
Final temperature f°C)
2.005
2.082
22.5
222
2.000 2.000
2.082 2.164 2.112
35.5 39.0 55.0
250 178 263
Bomb weight fg)
Sample weight fg)
1 .O
8.069
:::5 0.125
8.069 8.646 8.259
NaOH molarity
and results of ARC analysis on ethylene
a previous investigation’. An activation energy of 83.6 kJmol_’ has been evaluated from the propylene oxide plots. These activation energies have nc detailed chemical significance relating to the reaction mechanisms, but should be regarded as global values which serve to characterize the experimental results and provide a measure of the comparative reactivity of ethylene and propylene oxide. The ARC sample bomb is maintained in an adiabatic environment, except under conditions of very rapid selfheating when some heat losses may occur. Thus the final temperature of the sample and containing bomb is determined by the heat generated by the sample, and the heat of reaction is given by: H=
AT
-oC,
where: AT= difference between onset temperature TO and final temperature Tf; and C, = specific heat of sample. I$ is the thermal dilution factor defined as:
Q=
M,C,
+
hfbcb
M,Cs
where: M, = sample mass; Mb = bomb mass; and Cb = specific heat of the bomb. The 4 factor takes account of the fact that heat generated by the sample is distributed between the sample and the containing bomb. 4 AT is the corrected adiabatic temperature rise which would occur in an isolated sample. The results of ARC analysis on the various ethylene oxide-alkali mixtures are summarized in Tub/e I. The adiabatic temperature rise produced by the polymerization of ethylene oxide in the presence of various concentrations of sodium hydroxide appears, within experimental error, to be independent of
Table 2
Experimental
conditions
NaOH molarity
Bomb weight (gl
Sample weight (g)
1 .o 0.25 0.125 0.0625
8.699 8.444 8.753 8.795
1.999 2.009 2.019 2.012
166
oxide/NaOH
mixtures
Heat of polymerization fkJ mol-‘1
Maximum pressure IkPaf
Temperature at maximum pressure f°C)
415.5
44.2
4465
147.0
456.5 296.5 439.5
48.6 31.6 46.7
3608 3876 4055
152.0 151.8 188.0
Corrected adiabatic temperature rise f°C)
the concentration of alkali apart from the mixture containing 0.5 N NaOH where the heat of reaction appears to be anomalously low. The average value of the heat of polymerization calculated from ARC data is 42.8 kJ mol-‘, which is lower than previously published values’. The discrepancy may be due to failure of the ARC. to follow rapid temperature changes such that the sample temperature exceeds that of the calorimeter. This results in an underestimate of the adiabatic temperature rise and the heat of reaction. The ARC results for the propylene oxide mixtures are summarized in Table 2. Heats of reaction calculated from these results, within experimental error, are independent of the concentration of alkali and yield an average value of 65.53 kJ mol-‘.This is less than the published value’ but the discrepancy is less than in the case of ethylene oxide. Maximum rates of temperature rise are lower for propylene oxide so it should be possible for the ARC to follow the course of the reaction more closely. Ethylene oxide differed from propylene oxide in that it exhibited exothermic polymerization in the ARC, without any NaOH as a catalyst, at an onset temperature of 112OC. This effect was not observed with propylene oxide.
rate on alkali concentration temperature, and the likelihood runaway reaction under industrial ditions.
Maximum iemperature and pressures resulting from rapid polymerization Conditions during ARC experiments differ substantially from those during bulk storage and handling. The maximum temperatures and pressures which would occur due to rapid polymerization of ethylene oxide or propylene oxide under industrial storage conditions will depend on factors such as the strength of the vessel, rate of heat loss, degree of venting, volume of ullage space and the mass of the vessel compared with the mass of the contents. Values for the corrected adiabatic temperature rise (4 AT) listed in Tub/es I and 2 represent a theoretical temperature rise which would occur in an isolated sample under adiabatic conditions. A similar temperature rise might be expected in a large insulated storage vessel of low thermal mass but, in general, the temperature rise under industrial conditions would be substantially less than the corrected adiabatic temperature rise. However, if the temperature rise is sufficient to cause autoignition in the vapour phase, much higher temperatures will be achieved. Ethylene oxide is much more likely to auto-ignite than propylene oxide. Flammability limits for ethylene oxide are from 3 to 100% with a standard autoignition temperature of 429”C, the limits for propylene oxide are from 2.1 to 21.5% with a standard auto-ignition temperature of 550°C. At elevated temperatures the fuel concentration in the vapour space of a propylene oxide tank is likely to be too rich for flame propagation. In the absence of auto-ignition,
Discussion The ARC data on the alkylene oxide-alkali mixtures can be used to assess the probabilities and likely consequences of runaway polymerization of ethylene oxide and propylene oxide. As indicated in the introduction, consideration will first be given to the effects of rapid polymerization, followed by separate discussion of the dependence of reaction
and results of ARC analysis on propylene oxide/NaOH
and of a con-
mixtures
$J Factor
Onset temperature 1°C)
Final temperature (OC)
Corrected adiabatic temperature rise 1°C)
Heat of polymerization (kJmol-‘)
Maximum pressure fkPa1
Temperature at maximum pressure 1°C)
2.186 2.132 2.173 2.179
53.0 55.0 68.0 91.0
262 270 296 298
452.5 458.5 495.5 451 .o
63.9 64.7 69.9 63.7
2656 2388 2299 2089
205.8 210.8 217.8 202.7
J. Loss Prev. Process
Ind.,
1988,
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1, July
Short Tables I and .2 suggest that there would be no significant difference between the maximum temperatures which might result from rapid polymerization of propylene and ethylene oxide. The pressures recorded during ARC experiments cannot be readily corrected Tables I and 2 for thermal dilution. show that maximum pressures are achieved at a temperature below the maximum temperature. This is conbistent with a reduction in the average vapour pressure of the reacting mixture as polymerization takes place. Maximum pressures recorded for ethylene oxide are substantially higher than those recorded for the propylene oxide mixtures. This suggests that the pressure is due largely to the vapour pressure of the monomer. The vapour pressure of ethylene oxide at its critical point (195.8OC) is 7194 kPa4 compared with a vapour pressure of 4924 kPa for Propylene oxide at its critical point (209 C). Polymerization during ARC experiments resulted in pressures very much less than those at the critical points, but in large vessels (with small thermal dilution factors) polymerization will lead to higher temperatures and the pressure will increase, but by an amount which is difficult to predict became of the complex composition of the partially polymerized material. However, in general, pressures resulting from the rapid polymerization of ethylene oxide on an industrial scale will be substantially higher thah those caused by polymerization of propylene oxide. Concentration and temperature dependence of reaction rate of Figures 3 and 4 show the variation self-heat rate at 209C with alkali concentration for ethylene oxide and propylene oxide respectively. As would be expected, self-heat rates diminish with reducing alkali concentration. There is evidence of a rapid reduction in the rate of polymerization of propylene oxide at alkali concentrations below 0.25N. A similar effect has been observed for ethylene oxide’. The effect is not evident in the present results for ethylene oxide. the result for ethylene However, oxide + 0.5N NaOH must be considered anomalous and previous findings are not
Y
20 I&___!
::
-
r7 3 .g
0
16 12
e ulg
8
.G ‘G I"
4 0
0
0 0
0.2
0.4
0.6
Sodium hydroxide
0.6
1.0
molarity
Figure 3 Ethylene oxide - self-heat at 20°C versu.s NaOH concentration
rate
Communications
Table 3 Effect of NaOH concentration on the critical temperatures of ethylene oxide/NaOH and propylene oxide/NaOH mixtures when stored under similar conditions Critical temperature "0
0.2 Sodium
0.4
0.6
hydroxide
0.6
1.0
NaOH concentration
Ethylene oxide/MaOH
kg/m31
(v2
toa
0.88 1.77 2.65
20 8 3
39 34
3.53
0
motarity
Figure 4 Propylene oxide - self-heat at 20°C versus NaOH concentration
rate
necessarily contradicted by the results reported here. Overall activation energies derived from the plots of self-heat rates versus and were 81.1 temperature 83.6 kJmol-’ for ethylene oxide and propylene oxide, respectively. The lower activation energy for the polymerization of ethylene oxide compared with that for propylene oxide implies that, at rates for ethylene 2o?O”c, reaction oxide/NaOH mixtures are between 10 and 20 times greater than those for the propylene oxide/NaOH mixtures, as can be seen from Figures 3 and 4. At higher temperatures this difference becomes progressively more marked.
X.9
(F + G(NaOH))QA
V
RT,
exp( - E/RT,:)
where: V= volume; S = surface area; X= surface heat transfer coefficient; TE = critical temperature (NaOH) = sodium hydroxide concentration; Q = heat of reaction; and F and G are constants (determined by linear regression on the plots of self-heat rate versus (NaOH)). Ethylene oxide and propylene oxide can be compared by arbitrarily specifying a storage condition (i.e. fixing X, S, V etc.) and calculating corresponding values for Tc and (NaOH). These values are listed in Tables 3 and 4 for a storage condition in which ethylene oxide contaminated with 10% by weight of 0.25M NaOH (i.e. 0.88 kgm-‘) has a critical temperature of 20°C. Tables 3 and 4 must be interpreted with care since they apply to one specific condition. However, the following trends can be identified and are likely to apply over a range of vessel sizes and NaOH concentrations:
rate of polymerization follows temperature Arrhenius-type zpendence characterized by a global activation energy. The rate of polymerization is directly proportional to the concentration of NaOH. (This will appear a drastic assumption in view of the variation of rate with NaOH concentration shown in Figures 3 and 4. However, assessment of the relative stability of the two substances is unlikely to be affected by more detailed analysis of the concentration dependence, and previous studies* suggest that a linear relationship is not unrealistic.) Explosive polymerization takes place via a thermal mechanism governed by Semenov boundary conditions’ in is which reactant temperature assumed to be uniform with thermal resistance only at the surface.
J. Loss Prev.
31 29
Given these assumptions, the critical storage temperature which can lead to explosive polymerization will be related to the concentration of NaOH by an expression of the form:
Probability of rapid polymerization under industrial conditions The small scale experimental studies reported here have demonstrated that ethylene oxide is substantially more prone to rapid polymerization than propylene oxide. However, to assess the implications of this when the materials are stored or processed under industrial conditions, it is necessary to estimate the temperatures and degrees of contamination which can lead to rapid polymerization when the materials are handled in bulk. The following assumptions have to be made in order that the results of laboratory investigations can be extrapolated: The
Propylene oxide/N&H
0
Critical temperatures fall as NaOH The concentration is increased. reduction is more pronounced in the case of ethylene oxide.
Table 4 Comparison of the concentrations of NaOH necessary to produce the same critical temperatures when ethylene oxide/NaOH and propylene oxide/NaOH mixtures are stored under similar conditions NaOH concentration Critical temperature (“C)
Process
Ethylene oxidelNa0t-l (kg mm?
Propylene oxide/NaOH (kgm-?
0.88 1.77 2.65 3.53
10.73 46.84 83.94 121.32
20 8 3 0
Jnd.,
7988,
Vol
7, July
167
Short Communications 0 Critical
l
temperatures are substantially higher for the propylene oxide mixtures (between 19 and 29°C higher over the range of NaOH concentrations investigated). Table 4 shows that, if the critical temperatures and storage conditions are the same. the level of contamination of propylene oxide must be at least 10 times that of ethylene oxide.
Conclusion Ethylene oxide is known to be more reactive than propylene oxide and this study has confirmed that, specifically in the case of alkali-catalysed polymerization, substantial differences in reaction rates can be expected. It has been demonstrated that the consequences of
rapid polymerization of propylene oxide are unlikely to be as severe as for ethylene oxide. Some tentative calculations have indicated margins of safety in terms of storage temperature and degree of contamination which would apply when ethylene oxide and propylene oxide are stored under similar conditions. An exhaustive study using other catalyst systems has not been undertaken. The results of the present Study suggest that the level of control measures recommended for the storage and handling of ethylene oxide’ is not justified for propylene oxide, although a similar design philosophy may be appropriate. More detailed knowledge of the likelihood of contamination or temperature excursions under industrial conditions is
Computerized
hazards
Drive,
References ‘Guidelines for the Bulk Handling of Ethylene Oxide’, Chemical Industries Association, 1983 Gupta, N. K., ‘The Explosive PolyCondensation of Ethylene Oxide’, J.S.C.1. 68, June 1949 Pogany, G. A., ‘On the Safe use of Alkylene Oxides in High-Pressure BenchScale Experiments’, Chem. Ind. (London) January 1979 Yaws, C. L. and Rackley, M. P., ‘Ethylene, Propylene and Butylene Oxides’ Chem. Eng., April 1976 Semenov, N. N., Z. Physik, 1928, Vol48, PP 571 0 British Crown Copyright
analysis
The backbone of a hazards analysis hazards information exchange C. Robert Nelms Failsafe Network, Inc., 4337 Roundhill
required before the relative hazards can be properly assessed.
effort/basis
Chesterfzeld,
for a corporate
VA 23832,
USA
Computerized hazards analysis has many obvious advantages. Criticality codes can be assigned, modified, sorted, grouped, and regrouped. Critical event assignments can be automatically tracked. Graphics capability can show the distribution of various types of hazards dramatically, and the risks involved with each. But, computerized hazards analyses can provide benefits orders of magnitudes beyond those mentioned above. Consider the following: as hazards analyses are performed, i.e., as hazard severity, critical events, and risks are defined for each component and each operator action in your facility, you are relying on the memories of many people for much of your input. The following text includes real life examples of such a situation (Keywords:
hazard analysis: risk; criterion)
For example, you might ask ‘Can this compressor cause downtime? If so. how, how often, and to what extent does it affect the rest of the plant?’ During this questioning, you will be relying on historical records, your own experience, and the experiences of operators and maintenance personnel. The capturing of this information in specifically structured computer form, along with the routine inputting of all unanticipated critical events, is actually the beginnings of an expert system relating to the operating hazards of a plant. If performed uniformly throughout a corporation, this type information could be shared for the benefit of all. Received
I9 May 1988
A crisis avoidance What is a hazards
machine, analysis?
It is the fuel that drives our crisis avoidance machine. Without the fuel, the machine stops, exposing us to unexpected catastrophe. Bhopal, Chernobyl, the Challenger, and Three Mile Island have helped highlight the absolute necessity for this machine. Industry can no longer afford to react to its crises. It is time for industry to proact. proactive machinery Fortunately, (approaches) have been successfully implemented in a variety of industries. A close scrutiny of these success stories reveals their dependence on a formal structured approach. Recognizing this, the U.S. government-as well as many
0960-4230/68/030168-OW3.00 0 1988 Butterworth & Co. (Publishers1 Ltd 166 J. Loss Prev. Process lnd., 7988,
Vol
1, July
defence and design contractorsrequire strict adherence to MIL-STD882B, ‘System Safety Program Requirements’ in many of their contracts. Paraphrased from MIL-STD-882B, a for specific structure is necessary developing and implementing a comprehensive system safety program’, ‘to help identify the huzurds of a system and to impose design requirements and management controls to prevent mishaps’, ‘by eliminating hazards or reducing the associated risk to a level acceptable to the managing activity’. (Although the standard is addressing safety, its emphasis can be applied to any proactive effort, i.e., avoiding any crisis large or small). A hazards analysis is nothing more