- Email: [email protected]
Atmospheric release tests of monomethylamine
Atmospheric release tests of monomethylamine R. J. Lantzy, R. D. Myers, D. B. Pfenning* and S. B. Millsap* Rohm and Haas, P. 0. Box 584, Bristol, PA 19007, USA *Energy Analysts, Inc., P. 0. Box 1508, Norman, OK 73070, USA
Reduced storage temperature is one preventive measure that can be employed to lessen the consequences from accidental releases. A literature survey indicates that a large reduction in potential cloud formation can be obtained by storing a material below the point at which vapour flashing, due to liquid superheat, will completely atomize the release. To extend and confirm these studies to monomethylamine, 10 atmospheric releases were made using a 6.3 mm (0.25 inch) orifice. The results indicate that flash atomization can be prevented by storing monomethylamine with < 10°C of superheat (below 3°C). The tests generally confirmed earlier studies, but did not fully quantify the transition between hydraulic and flash atomization. Additional experimental work is needed to better understand flash atomization. (Keywords:
release; storage areas; monomethylamine)
To minimize the potential consequences to the surrounding community, hypothetical releases are, modelled to determine the distance of an impact. Where there are likely to be significant effects on the community or plant, steps are taken to reduce the consequences. Continuous releases from pressurized vessels can be classified into four types: ‘all gas’; subcooled liquid with low velocity; subcooled liquid with high velocity; and superheated liquid. For a subcooled liquid released with a low velocity, the liquid will rain out onto the ground, forming a pool. The rate of vaporization from that pool will govern the rate of emission into the vapour cloud. For a small pool size (e.g. due to the presence of a dike), the vaporization rate may be significantly lower than the release rate. In this case rainout will reduce the ‘reach’ of the vapour cloud. As the liquid release velocity is increased. hydraulic atomization will occur (breakup of the liquid stream due to hydraulic shear at the liquid-air interface). As the droplets travel through the air, material will vaporize from them. The amount of vaporization will depend on the physical properties of the liquid, the velocity, the size of the drops, and the time for the droplets to fall to the ground. The liquid that falls onto the ground forms a secondary gas source for the cloud from the subsequent evaporation. The fourth class of release, a superheated liquid, can result in behaviour ranging from an ‘all gas’ release to a subcooled liquid, low velocity release, depending on the amount of superheat (difference between storage temperature and the atmospheric boiling point of the material). Upon release, a superheated liquid Received I2 September 1989; revised 20 October I989 ~~~~~~~~~ at the hst h canf. on massaf Ccmtammenr12-14 September
will lose its superheat by vaporizing a portion of the liquid. Because of the large relative density difference between the vapour and liquid, only a small amount of superheat is needed to create enough vapour to atomize the liquid stream into a fine mist. This effect is called flash atomization. Rapid air entrainment into the atomized jet then causes further evaporation of the remaining liquid. In such a situation there will be little, if any, liquid rainout, and the amount of material in the cloud will equal the amount being released. Unfortunately, flash atomization is not well understood. One key paper is that by Brown and York*. They reported that a critical superheat exists, above which the liquid release atomizes into fine drops. Their results were published in the form of a curve that defined two regions. Below the curve, the liquid would be hydraulically atomized by the velocity of the release into large drops, which would tend to rain out. Above the curve, the material would be atomized into fine drops by flash atomization. Figure I reproduces Brown and York’s results. The bubble growth rate constant, plotted along the ordinate, is defined as follows: C = (CP AT/L) (P,/P~) (rDP5 where C, is bubble growth rate constant (rn~-~.~); C,, is liquid heat capacity (J kg-l K-l); AT is degrees of superheat (K); L is latent heat (J kg-‘); p, is density of liquid (kg m -3); pV is density of vapour (kg m -3); and D is thermal diffusivity of liquid (m* s-l). For any given fluid the ordinate essentially reflects the degrees of superheat. The Weber number is plotted on the abscissa. It is the ratio of aerodynamic forces to cohesive forces and
1999.
London. UK 095CM230/90/010077-05$3.00 0 1990 Butterworth &Co.
J. Loss Prev. Process Ind., 1990, Vol3, January (Publishers)
Ltd
77
Atmospheric
e
release tests of monomethylamine:
R. J. Lantzy et al. o.*o.
O.IO-
y
E \.!I.
“0 \i
3
P 3
0.00.
3
r; O.OO
?lrah
al
Atordzdon
6
P p:
a O.OO
s 3 2
o.o*
Atmnixdion R.&WJ
-.--
2
I
5
i0
lb
Weber
= ~a V2 I/(2
0.00
a
I
20
6
40
Weber
I
80
I
20
Number
Plot of Brown and York’s data showing flash and Figure 2 hydraulic atomization regimes with the proposed extrapolation to larger Weber numbers: A, sharp orifice, water; 0. rough orifice, water; n , sharp orifice, Freon
Test Design gc4
V is relative velocity where N,, is Weber number; (mssl); pa is density of atmosphere (kgm-3); 1 is characteristic length, taken as the diameter of the liquid jet (m); (T is surface tension (Nm-*); and g, is 1 (kg m N - I s -*). For any given release of a fluid, the two most important parameters in determining Weber number are velocity and liquid jet diameter. Extrapolation of this correlation to diameters larger than those studied by Brown and York (0.55 2 mm, i.e., 0.02-0.08 inch) is a problem. Most consequence analyses consider diameters that are 5-20 times larger. Unfortunately, extrapolation to larger diameters indicates that at some Weber number no superheat would be needed to atomize the liquid stream. This does not make any sense. Others have recognised this same problem and have decided to assume an arbitrary lower limit of 0.01 rn~-O.~ for the bubble growth rate constant (M. Emerson. personal communication, 1987). The resulting modified correlation is shown in Figure 2. A major purpose for these tests was Rohm and Haas’s desire to define more accurately the extrapolation of the Brown and York curve and to extend it to larger diameter releases. Rohm and Haas wanted to store methylamine in a location where it was desirable to reduce the cloud reach. To limit the ‘reach’ it was decided to store the material at a temperature below the transition to flash atomization. Besides changing the mechanism from flash atomization to hydraulic atomization, the reduced temperature also reduces the storage pressure. For a given hole or line size, the resulting reduced pressure driving force also reduces the release rate. The major unknown was the temperature of transition from hydraulic to flash atomization for monomethylamine.
78
Proposed htrmpal~on ___-----
Lber
given by: N,,
1
2’5
Figure 1 Plot of Brown and York’s data showing the Weber number-bubble growth rate constant region in which flash atomization is expected to occur and the region in which hydraulic atomization would occur: A, sharp orifice, water; V, rough orifice, water; n , sharp orifice, Freon is
0.04
9 E ” 0.02 2 %
f
& “, o.os“p
0.00
J. Loss Prev. Process lnd., 1990, Vol3, January
To determine this critical shattering temperature for methylamine, Rohm and Haas contracted Energy Analysts to perform atmospheric release tests at their test site in Oklahoma. The tests were designed to measure the amount of liquid methylamine that rains out as a function of storage temperature. This liquid was captured in a pit of water, to which dilute sulphuric acid was added to chemically bind the methylamine. The release rate was determined by timing the release and measuring the weight difference of the standard 270 kg (600 lb) storage cylinder before and after the release. The amount of capture was determined from the volume of the pit and an analysis of the pit composition for nitrogen before and after each release. The fraction of material in the cloud was then estimated by difference between the amount released and the amount captured in the pit. Figure 3 shows a schematic diagram of the test apparatus. To maintain a constant temperature during each release, nitrogen was added to the vapour space of the cylinder. For each test the saturation pressure of the cylinder was measured and some increment of regulated pressure (approximately 138 kPa, 20 psig) was added above the saturation pressure to maintain liquid conditions at the release orifice. Temperature and pressure measurements were taken at the orifice to
Figure 3
Schematic
diagram
of the experimental
setup
Atmospheric verify liquid conditions. To assure uniform composition in the pit, the pit was agitated via submersible pumps for 10 min before sampling. Samples were drawn from four different locations in the pit. The releases were made via a 6.3 mm (0.25 inch) orifice. The tank temperature was varied by autorefrigeration of the tanks, by releasing vapour to the atmosphere between the tests. Rigorous safety precautions were enforced during the test program. All personnel were instructed in the use of air packs and were issued a self-contained breathing apparatus during the releases. Local emergency personnel were notified when tests were being conducted.
release tests of monometh
y/amine:
R. J. Lantzy et al.
pit. For tests 9 and 10 (high humidity) it is not possible to say whether there was aerosol formation or condensation. Test 3, as shown in Figure 6. clearly showed little or no aerosol formation. For test 5, some liquid rainout occurred beyond the pit. The observations varied for releases between 9 and 15°C of superheat. Tests 2 and 7 both showed limited jet expansion and significant rainout, but also substantial aerosol formation. Figure 7 is from test 2, which was conducted during relatively low humidity, and
Test operation Atmospheric testing is very frustrating. A steady wind from a given direction, with a minimum speed. was required for capture in the pit. Releases l-6 were run on the first day of testing and 7-10 on the second day of testing, which was distinguished from day 1 by much higher humidity. On the first day of testing, the pressure transducer at the orifice failed. It was replaced by a more robust transducer, with a pneumatic pressure transducer added as a back up. Both transducers were isolated from the methylamine by several feet of small diameter tubing. During test 5, visual observations showed that some liquid was raining-out beyond the far edge of the capture pit. For tests 7-10 the test vessel was moved back from the pit an additional 1.3 m. and the nozzle was inclined downward at an angle of 3”.
Visual
Figure5 Test 1. flash atomization dense mrst (aerosol), with essentially
regime release, showing no rainout into the pit
test results
The atmospheric boiling point of monomethylamine is -6.3”C. For superheats of more than 15°C (tests I and 4) the release expanded sharply upon release and formed a dense mist with little or no apparent material falling to the ground. Figures 4 and 5 show this type of release. Releases with superheats between 3 and 9°C (tests 3, 5, 6, 9 and 10) showed considerable rainout into the
Figure4 Test 4, flash atomization sharp plume expansion immediately
regime release, upon release
showing
Figure 6 Test 3, hydraulic atomization regime release, ing rainout into the pit and very little mist (aerosol)
Figure 7 Test 2, transition regime release, low showing both mist (aerosol) and larger liquid droplets
show-
humidity,
J. Loss Prev. Process Ind., 7990, Vol3, January
79
Atmospheric
release tests of monomethylamine:
R. J. Lantzy et al.
shows the limited jet expansion. For test 8, the jet expanded sharply and appeared to have a large amount of atomization. Atmospheric condensation of water vapour from the high humidity caused the pit to be nearly totally obscured so that rainout could not be observed. Figures 8 and 9 show this test.
Results of the nitrogen analysis Table I summarizes the test data. Figure 10 plots the superheat against the amount estimated to have gone downwind. In general the quantitative results agree well with the observations. Tests 1 and 4 show nearly all the material going downwind with the vapour cloud. Tests 3 and 6 show almost complete capture. As expected, the amount of capture was low for test 5, due to liquid landing beyond the pit. Tests 9 and 10 display a higher than anticipated amount of aerosol formation. Flow rates for these tests were higher than for tests 3 and 6. The increase in hydraulic shear caused by the higher exit velocity may have resulted in the formation of smaller liquid droplets, which would stay airborne longer. Calculations show that the amount of material carried downwind in these two experiments can be accounted for by partial evaporation before the droplets land in the pit. Figure I1 shows the test data plotted against the modified correlation of Brown and York. While there is insufficient data to conclude that a minimum bubble growth rate constant value of 0.01 rn~-“.~ is indeed Table 1
Figure8 Test 8, transition regime release, showing condensation of water vapour
Figure9 plume
As in Figure
8, but showing
high
humidity,
extent
of the visible
Test 9
Test 10
Monomethylamine release test data
Parameter
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Wind speed (m s-l)
7.9
8.4
10.0
8.1
6.7
6.5
7.4
6.5
6.1
5.5
Stability Sigma theta
D 8.2
C 9.7
C 8.8
C 17.8
D 6.7
D 7.1
D 7.0
D 7.9
D 7.7
D 8.7
58 15.0
49 18.1
48 18.4
56 16.4
61 15.0
66 13.8
86 19.5
86 19.2
88 18.9
89 18.9
Relative humidity (%) Air temperature (“C) Duration of release(s)
300 0.2827
Flow rate (kg s-l) Liquid temperature (“Cl standard deviation Tank pressure (kPa1: strain gauge pneumatic transducer Total pressure (kPa) (includes Nz pad)
12.38 0.17
1.73 0.635 0
Posttest analysis (mg I-’ of N): Northwest 210” Northeast 1908 Southwest 200” Southeast 195” Average % capture 6 Lowest temp. PC) at 1.7 m (tests l-6) 3.0 m (tests 7-l 0) 8 Concentrations b Concentrations
80
300
220
0.2087
0.2223
7.84 0.08
3.29 0.19
158.55 310
Release height(m) Orifice diameter (cm) Angle of release from horizontal (“1
300
-46.7
0.4000 16.32 0.10
300
293
0.2873 0.27 0.33
186 1.73 0.635 0
1.73 0.635 0
1050 1100 1050 1100 35 -45.9
406
3000 3040 3170 3170
3270 3220 3370 3250
or
before this test were 3.0,3.0,3.3.3.0 before this test were 730,730.720.725
5000 4980 4600 4300 5
-44.9
mg I-’ mg I-’
J. LOSS Prev. Process Ind., 1990, Vol3, January
154
1.73 0.635 0
1.73 0.635 0
76 -44.9
172
42 -46.4
1.73 0.635 0
6750 7330 6770 7500 81 -45.8
180
180
180
0.2621
0.2142
3.92 0.31
5.76 0.20
-0.47 0.21
1.81 0.28
108.45 103
99.63 95
106.04 100
152.58 148
248
290
-3.72 0.30
268.54 207
180
0.2508
248
241 1.73 0.635
1.73 0.635 -3
1500b 18206 1 580b 16OOb 45 -53.6
0.31 75
0.2633
1.73 0.635
1.73 0.635
-3
-3
-3
2080 1900 2100 2000
3300 3200 3400 3320
4440 4400 4280 4280
29 -53.6
68 -52.6
46 -54.3
Atmospheric
release tests of monomethylamine: correct, the evidence indicates that, modification might be conservative.
17.J. Lantzy et al. if anything,
the
Conclusions g a al ‘cl !d V
60Flash Atomi%ation
40-
Rs2ima
g 3
26-
0
?I.&
Vmpor
E
- .
2’5
0
Figure 10 Fraction of the monomethylamine that was released and carried downwind as a function of superheat: n , based on visual observation (aerosol); D, (aerosol?); 0, (rainout?); 0, (rainout). Adiabatic flash fraction is shown by the line near the bottom of the figure
80
L
l 3
0
--r--------
90 6 0
1
20
Proposwd mtmpol8uon
7
0
10
5
I
1
40
60
I
60
Weber Number Figure 11 Plot of monomethylamine and York’s curve with the proposed Weber numbers: n , visual observation 0, (rainout?); 0, (rainout)
data relative to Brown extrapolation to larger (aerosol); e (aerosol?);
In general there is a relatively sharp transition between hydraulic and flash atomization. With 19°C of superheat, the methylamine release is totally atomized into a dense mist with essentially nothing landing in the pit. With 9°C of superheat all the material appeared to be large droplets with nearly total rainout into the pit. The transition may have been even sharper, but complete control of test conditions did not allow a better definition of the width of the transition zone. For example, no data were obtained between 14 and 19°C of superheat. The general shape of the experimental data shown in Figure 11 indicates that the Brown and York results apply to monomethylamine. In addition, the larger diameter orifice used in this study indicates that the modification to the bubble growth rate constantWeber number correlation appears to be appropriate. Of practical importance is the fact that the tests confirmed that a reduction in cloud reach could be obtained by reducing the methylamine storage temperature to below 3°C. To realize the full benefit of reduced storage temperature, evaporation of the rained-out material must also be minimized. From the tests conducted, it appears that the transition from hydraulic atomization to flash atomization may be a function of the release velocity and possibly the atmospheric humidity. Additional testing is required to define the effect of these variables. Larger diameter releases are also needed to further extend the knowledge of the transition between hydraulic and flash atomization. More experiments are also needed for a large suite of compounds. Rohm and Haas and Energy Analysts are working through the CCPS to develop a comprehensive test programme to further investigate aerosol formation.
References 1 Brown,
R. and York, J. L. AZChEJournal1962.
S(2). 149
J. Loss Prev. Process Ind., 1990, Vol3, January
81
Reduced storage temperature is one preventive measure that can be employed to lessen the consequences from accidental releases. A literature survey indicates that a large reduction in potential cloud formation can be obtained by storing a material below the point at which vapour flashing, due to liquid superheat, will completely atomize the release. To extend and confirm these studies to monomethylamine, 10 atmospheric releases were made using a 6.3 mm (0.25 inch) orifice. The results indicate that flash atomization can be prevented by storing monomethylamine with < 10°C of superheat (below 3°C). The tests generally confirmed earlier studies, but did not fully quantify the transition between hydraulic and flash atomization. Additional experimental work is needed to better understand flash atomization. (Keywords:
release; storage areas; monomethylamine)
To minimize the potential consequences to the surrounding community, hypothetical releases are, modelled to determine the distance of an impact. Where there are likely to be significant effects on the community or plant, steps are taken to reduce the consequences. Continuous releases from pressurized vessels can be classified into four types: ‘all gas’; subcooled liquid with low velocity; subcooled liquid with high velocity; and superheated liquid. For a subcooled liquid released with a low velocity, the liquid will rain out onto the ground, forming a pool. The rate of vaporization from that pool will govern the rate of emission into the vapour cloud. For a small pool size (e.g. due to the presence of a dike), the vaporization rate may be significantly lower than the release rate. In this case rainout will reduce the ‘reach’ of the vapour cloud. As the liquid release velocity is increased. hydraulic atomization will occur (breakup of the liquid stream due to hydraulic shear at the liquid-air interface). As the droplets travel through the air, material will vaporize from them. The amount of vaporization will depend on the physical properties of the liquid, the velocity, the size of the drops, and the time for the droplets to fall to the ground. The liquid that falls onto the ground forms a secondary gas source for the cloud from the subsequent evaporation. The fourth class of release, a superheated liquid, can result in behaviour ranging from an ‘all gas’ release to a subcooled liquid, low velocity release, depending on the amount of superheat (difference between storage temperature and the atmospheric boiling point of the material). Upon release, a superheated liquid Received I2 September 1989; revised 20 October I989 ~~~~~~~~~ at the hst h canf. on massaf Ccmtammenr12-14 September
will lose its superheat by vaporizing a portion of the liquid. Because of the large relative density difference between the vapour and liquid, only a small amount of superheat is needed to create enough vapour to atomize the liquid stream into a fine mist. This effect is called flash atomization. Rapid air entrainment into the atomized jet then causes further evaporation of the remaining liquid. In such a situation there will be little, if any, liquid rainout, and the amount of material in the cloud will equal the amount being released. Unfortunately, flash atomization is not well understood. One key paper is that by Brown and York*. They reported that a critical superheat exists, above which the liquid release atomizes into fine drops. Their results were published in the form of a curve that defined two regions. Below the curve, the liquid would be hydraulically atomized by the velocity of the release into large drops, which would tend to rain out. Above the curve, the material would be atomized into fine drops by flash atomization. Figure I reproduces Brown and York’s results. The bubble growth rate constant, plotted along the ordinate, is defined as follows: C = (CP AT/L) (P,/P~) (rDP5 where C, is bubble growth rate constant (rn~-~.~); C,, is liquid heat capacity (J kg-l K-l); AT is degrees of superheat (K); L is latent heat (J kg-‘); p, is density of liquid (kg m -3); pV is density of vapour (kg m -3); and D is thermal diffusivity of liquid (m* s-l). For any given fluid the ordinate essentially reflects the degrees of superheat. The Weber number is plotted on the abscissa. It is the ratio of aerodynamic forces to cohesive forces and
1999.
London. UK 095CM230/90/010077-05$3.00 0 1990 Butterworth &Co.
J. Loss Prev. Process Ind., 1990, Vol3, January (Publishers)
Ltd
77
Atmospheric
e
release tests of monomethylamine:
R. J. Lantzy et al. o.*o.
O.IO-
y
E \.!I.
“0 \i
3
P 3
0.00.
3
r; O.OO
?lrah
al
Atordzdon
6
P p:
a O.OO
s 3 2
o.o*
Atmnixdion R.&WJ
-.--
2
I
5
i0
lb
Weber
= ~a V2 I/(2
0.00
a
I
20
6
40
Weber
I
80
I
20
Number
Plot of Brown and York’s data showing flash and Figure 2 hydraulic atomization regimes with the proposed extrapolation to larger Weber numbers: A, sharp orifice, water; 0. rough orifice, water; n , sharp orifice, Freon
Test Design gc4
V is relative velocity where N,, is Weber number; (mssl); pa is density of atmosphere (kgm-3); 1 is characteristic length, taken as the diameter of the liquid jet (m); (T is surface tension (Nm-*); and g, is 1 (kg m N - I s -*). For any given release of a fluid, the two most important parameters in determining Weber number are velocity and liquid jet diameter. Extrapolation of this correlation to diameters larger than those studied by Brown and York (0.55 2 mm, i.e., 0.02-0.08 inch) is a problem. Most consequence analyses consider diameters that are 5-20 times larger. Unfortunately, extrapolation to larger diameters indicates that at some Weber number no superheat would be needed to atomize the liquid stream. This does not make any sense. Others have recognised this same problem and have decided to assume an arbitrary lower limit of 0.01 rn~-O.~ for the bubble growth rate constant (M. Emerson. personal communication, 1987). The resulting modified correlation is shown in Figure 2. A major purpose for these tests was Rohm and Haas’s desire to define more accurately the extrapolation of the Brown and York curve and to extend it to larger diameter releases. Rohm and Haas wanted to store methylamine in a location where it was desirable to reduce the cloud reach. To limit the ‘reach’ it was decided to store the material at a temperature below the transition to flash atomization. Besides changing the mechanism from flash atomization to hydraulic atomization, the reduced temperature also reduces the storage pressure. For a given hole or line size, the resulting reduced pressure driving force also reduces the release rate. The major unknown was the temperature of transition from hydraulic to flash atomization for monomethylamine.
78
Proposed htrmpal~on ___-----
Lber
given by: N,,
1
2’5
Figure 1 Plot of Brown and York’s data showing the Weber number-bubble growth rate constant region in which flash atomization is expected to occur and the region in which hydraulic atomization would occur: A, sharp orifice, water; V, rough orifice, water; n , sharp orifice, Freon is
0.04
9 E ” 0.02 2 %
f
& “, o.os“p
0.00
J. Loss Prev. Process lnd., 1990, Vol3, January
To determine this critical shattering temperature for methylamine, Rohm and Haas contracted Energy Analysts to perform atmospheric release tests at their test site in Oklahoma. The tests were designed to measure the amount of liquid methylamine that rains out as a function of storage temperature. This liquid was captured in a pit of water, to which dilute sulphuric acid was added to chemically bind the methylamine. The release rate was determined by timing the release and measuring the weight difference of the standard 270 kg (600 lb) storage cylinder before and after the release. The amount of capture was determined from the volume of the pit and an analysis of the pit composition for nitrogen before and after each release. The fraction of material in the cloud was then estimated by difference between the amount released and the amount captured in the pit. Figure 3 shows a schematic diagram of the test apparatus. To maintain a constant temperature during each release, nitrogen was added to the vapour space of the cylinder. For each test the saturation pressure of the cylinder was measured and some increment of regulated pressure (approximately 138 kPa, 20 psig) was added above the saturation pressure to maintain liquid conditions at the release orifice. Temperature and pressure measurements were taken at the orifice to
Figure 3
Schematic
diagram
of the experimental
setup
Atmospheric verify liquid conditions. To assure uniform composition in the pit, the pit was agitated via submersible pumps for 10 min before sampling. Samples were drawn from four different locations in the pit. The releases were made via a 6.3 mm (0.25 inch) orifice. The tank temperature was varied by autorefrigeration of the tanks, by releasing vapour to the atmosphere between the tests. Rigorous safety precautions were enforced during the test program. All personnel were instructed in the use of air packs and were issued a self-contained breathing apparatus during the releases. Local emergency personnel were notified when tests were being conducted.
release tests of monometh
y/amine:
R. J. Lantzy et al.
pit. For tests 9 and 10 (high humidity) it is not possible to say whether there was aerosol formation or condensation. Test 3, as shown in Figure 6. clearly showed little or no aerosol formation. For test 5, some liquid rainout occurred beyond the pit. The observations varied for releases between 9 and 15°C of superheat. Tests 2 and 7 both showed limited jet expansion and significant rainout, but also substantial aerosol formation. Figure 7 is from test 2, which was conducted during relatively low humidity, and
Test operation Atmospheric testing is very frustrating. A steady wind from a given direction, with a minimum speed. was required for capture in the pit. Releases l-6 were run on the first day of testing and 7-10 on the second day of testing, which was distinguished from day 1 by much higher humidity. On the first day of testing, the pressure transducer at the orifice failed. It was replaced by a more robust transducer, with a pneumatic pressure transducer added as a back up. Both transducers were isolated from the methylamine by several feet of small diameter tubing. During test 5, visual observations showed that some liquid was raining-out beyond the far edge of the capture pit. For tests 7-10 the test vessel was moved back from the pit an additional 1.3 m. and the nozzle was inclined downward at an angle of 3”.
Visual
Figure5 Test 1. flash atomization dense mrst (aerosol), with essentially
regime release, showing no rainout into the pit
test results
The atmospheric boiling point of monomethylamine is -6.3”C. For superheats of more than 15°C (tests I and 4) the release expanded sharply upon release and formed a dense mist with little or no apparent material falling to the ground. Figures 4 and 5 show this type of release. Releases with superheats between 3 and 9°C (tests 3, 5, 6, 9 and 10) showed considerable rainout into the
Figure4 Test 4, flash atomization sharp plume expansion immediately
regime release, upon release
showing
Figure 6 Test 3, hydraulic atomization regime release, ing rainout into the pit and very little mist (aerosol)
Figure 7 Test 2, transition regime release, low showing both mist (aerosol) and larger liquid droplets
show-
humidity,
J. Loss Prev. Process Ind., 7990, Vol3, January
79
Atmospheric
release tests of monomethylamine:
R. J. Lantzy et al.
shows the limited jet expansion. For test 8, the jet expanded sharply and appeared to have a large amount of atomization. Atmospheric condensation of water vapour from the high humidity caused the pit to be nearly totally obscured so that rainout could not be observed. Figures 8 and 9 show this test.
Results of the nitrogen analysis Table I summarizes the test data. Figure 10 plots the superheat against the amount estimated to have gone downwind. In general the quantitative results agree well with the observations. Tests 1 and 4 show nearly all the material going downwind with the vapour cloud. Tests 3 and 6 show almost complete capture. As expected, the amount of capture was low for test 5, due to liquid landing beyond the pit. Tests 9 and 10 display a higher than anticipated amount of aerosol formation. Flow rates for these tests were higher than for tests 3 and 6. The increase in hydraulic shear caused by the higher exit velocity may have resulted in the formation of smaller liquid droplets, which would stay airborne longer. Calculations show that the amount of material carried downwind in these two experiments can be accounted for by partial evaporation before the droplets land in the pit. Figure I1 shows the test data plotted against the modified correlation of Brown and York. While there is insufficient data to conclude that a minimum bubble growth rate constant value of 0.01 rn~-“.~ is indeed Table 1
Figure8 Test 8, transition regime release, showing condensation of water vapour
Figure9 plume
As in Figure
8, but showing
high
humidity,
extent
of the visible
Test 9
Test 10
Monomethylamine release test data
Parameter
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Wind speed (m s-l)
7.9
8.4
10.0
8.1
6.7
6.5
7.4
6.5
6.1
5.5
Stability Sigma theta
D 8.2
C 9.7
C 8.8
C 17.8
D 6.7
D 7.1
D 7.0
D 7.9
D 7.7
D 8.7
58 15.0
49 18.1
48 18.4
56 16.4
61 15.0
66 13.8
86 19.5
86 19.2
88 18.9
89 18.9
Relative humidity (%) Air temperature (“C) Duration of release(s)
300 0.2827
Flow rate (kg s-l) Liquid temperature (“Cl standard deviation Tank pressure (kPa1: strain gauge pneumatic transducer Total pressure (kPa) (includes Nz pad)
12.38 0.17
1.73 0.635 0
Posttest analysis (mg I-’ of N): Northwest 210” Northeast 1908 Southwest 200” Southeast 195” Average % capture 6 Lowest temp. PC) at 1.7 m (tests l-6) 3.0 m (tests 7-l 0) 8 Concentrations b Concentrations
80
300
220
0.2087
0.2223
7.84 0.08
3.29 0.19
158.55 310
Release height(m) Orifice diameter (cm) Angle of release from horizontal (“1
300
-46.7
0.4000 16.32 0.10
300
293
0.2873 0.27 0.33
186 1.73 0.635 0
1.73 0.635 0
1050 1100 1050 1100 35 -45.9
406
3000 3040 3170 3170
3270 3220 3370 3250
or
before this test were 3.0,3.0,3.3.3.0 before this test were 730,730.720.725
5000 4980 4600 4300 5
-44.9
mg I-’ mg I-’
J. LOSS Prev. Process Ind., 1990, Vol3, January
154
1.73 0.635 0
1.73 0.635 0
76 -44.9
172
42 -46.4
1.73 0.635 0
6750 7330 6770 7500 81 -45.8
180
180
180
0.2621
0.2142
3.92 0.31
5.76 0.20
-0.47 0.21
1.81 0.28
108.45 103
99.63 95
106.04 100
152.58 148
248
290
-3.72 0.30
268.54 207
180
0.2508
248
241 1.73 0.635
1.73 0.635 -3
1500b 18206 1 580b 16OOb 45 -53.6
0.31 75
0.2633
1.73 0.635
1.73 0.635
-3
-3
-3
2080 1900 2100 2000
3300 3200 3400 3320
4440 4400 4280 4280
29 -53.6
68 -52.6
46 -54.3
Atmospheric
release tests of monomethylamine: correct, the evidence indicates that, modification might be conservative.
17.J. Lantzy et al. if anything,
the
Conclusions g a al ‘cl !d V
60Flash Atomi%ation
40-
Rs2ima
g 3
26-
0
?I.&
Vmpor
E
- .
2’5
0
Figure 10 Fraction of the monomethylamine that was released and carried downwind as a function of superheat: n , based on visual observation (aerosol); D, (aerosol?); 0, (rainout?); 0, (rainout). Adiabatic flash fraction is shown by the line near the bottom of the figure
80
L
l 3
0
--r--------
90 6 0
1
20
Proposwd mtmpol8uon
7
0
10
5
I
1
40
60
I
60
Weber Number Figure 11 Plot of monomethylamine and York’s curve with the proposed Weber numbers: n , visual observation 0, (rainout?); 0, (rainout)
data relative to Brown extrapolation to larger (aerosol); e (aerosol?);
In general there is a relatively sharp transition between hydraulic and flash atomization. With 19°C of superheat, the methylamine release is totally atomized into a dense mist with essentially nothing landing in the pit. With 9°C of superheat all the material appeared to be large droplets with nearly total rainout into the pit. The transition may have been even sharper, but complete control of test conditions did not allow a better definition of the width of the transition zone. For example, no data were obtained between 14 and 19°C of superheat. The general shape of the experimental data shown in Figure 11 indicates that the Brown and York results apply to monomethylamine. In addition, the larger diameter orifice used in this study indicates that the modification to the bubble growth rate constantWeber number correlation appears to be appropriate. Of practical importance is the fact that the tests confirmed that a reduction in cloud reach could be obtained by reducing the methylamine storage temperature to below 3°C. To realize the full benefit of reduced storage temperature, evaporation of the rained-out material must also be minimized. From the tests conducted, it appears that the transition from hydraulic atomization to flash atomization may be a function of the release velocity and possibly the atmospheric humidity. Additional testing is required to define the effect of these variables. Larger diameter releases are also needed to further extend the knowledge of the transition between hydraulic and flash atomization. More experiments are also needed for a large suite of compounds. Rohm and Haas and Energy Analysts are working through the CCPS to develop a comprehensive test programme to further investigate aerosol formation.
References 1 Brown,
R. and York, J. L. AZChEJournal1962.
S(2). 149
J. Loss Prev. Process Ind., 1990, Vol3, January
81