Feb 15, 1988 - by accidents such as the cyclohexane explosion at ..... and 14 spills of 13-31 m3 of propane onto water. Some ...... Gas Workshop, Toronto.
4lmosph"ic Environm,n' Vol. 23, No.4, pp. 731-745. 1989.
0004-6981/89 $3'(Xl+O.OO
Printed in Great Britain.
Pergamon Press pic
A REVIEW OF RECENT FIELD TESTS AND MATHEMATICAL MODELLING OF ATMOSPHERIC DISPERSION OF LARGE SPILLS OF DENSER-THAN-AIR GASES RONALD
P.
KOOPMAN, DONALD
L.
ERMAK
and
STEVENS
T.
CHAN
Lawrence Livermore National Laboratory, Box 808, Livermore, CA 94550, U.S.A. (First received 15 February 1988 and in final form 2 November 1988)
Abstract-Large-seale spills of hazardous materials often produce gas clouds which are denser than air. The dominant physical processes which occur during dense-gas dispersion are very different from those recognized for trace gas releases in the atmosphere. Most important among these processes are stable stratification and gravity flow. Dense-gas flows displace the ambient atmospheric flow and modify ambient turbulent mixing. Thermodynamic and chemical reactions can also contribute to dense-gas effects. Some materials flash to aerosol and vapor when released and the aerosol can remain airborne, evaporating as it moves downwind, causing the cloud to remain cold and dense for long distances downwind. Dense-gas dispersion models, which include phase change and terrain effects have been developed and are capable of simulating many possible accidental releases. A number of large-scale field tests with hazardous materials such as liquefied natural gas (LNG), ammonia (NH3)' hydrofluoric acid (HF) and nitrogen tetroxide (N 2 0,,) have been performed and used to evaluate models. The tests have shown that gas concentrations up to ten times higher than those predicted by trace gas models can occur due to aerosols and other dense-gas effects. A methodology for model evaluation has been developed which is based on the important physical characteristics of dense-gas releases. Key word index: Atmospheric dispersion, dense gas, accidental release, dispersion modelling, hazardous materials, gravity flow, aerosol, flashing, complex terrain.
could be developed, This approach involves work in three complementary areas:
INTRODUCTION
Research into the atmospheric dispersion of denserthan-air gases was begun in the early t 970s, prompted by accidents such as the cyclohexane explosion at Flixborough (U.K.) in 1974, the pesticide release in Seveso (Italy) and the ammonia tanker truck spill in Houston (U.S,), both in 1976. Since then, the tragic methyl isocynate (MIC) release in Bhopal, India (1984) killed over 2000 people and the LPG explosions in Mexico City (1984) killed over 400 people. These tragic accidents indicate that further research into the possible consequences of accidental release of hazardous materials coupled with the development of prediction, mitigation and emergency response capabilities is still needed. The purpose of this paper is to review the major advances that have been made over the last 10 years in our understanding of the atmospheric dispersion of large-scale spills and our ability to predict the consequences of an accidental release. Dense gas dispersion phenomena, which are often characteristic of these spills, have been investigated intensively both through theory and experiment (Koopman, 1987). The emphasis of the research at Lawrence Livermore National Laboratory (LLNL) has been on gaining a physical understanding of the many processes involved such that a physically detailed and accurate quantitative predictive capability 731
(1) mathematical modeling based on physical laws
in the form of conservation equations and submodels of the important physical processes, (2) field experiments to validate models and discover important phenomena, (3) scaled simulations in wind tunnels, laboratory or water flume, Work at LLNL has concentrated on the first two of these areas. The dominant physical processes which occur during dense gas dispersion are very different than those recognized for trace gas releases. Some dense gas dispersion models now include these processes and can be evaluated against good quality data from a variety of well-instrumented dense gas dispersion experiments, CAUSES OF DENSE GAS BEHAVIOR
The accidental release of hazardous gases into the environment often results in what has come to be known as dense gas behavior, even when the gases themselves may be nominally less dense than the atmosphere into which they are released. This dense gas behavior can dominate the consequences of the accidental release making them either worse or better
732
RONALD
P.
KOOPMAN
than might otherwise be I'Y'''l'iftl'rl tion of the consequences of Materials released during ""!I/''''''''''''' can be denserthan-air due to one or a of the following: (l) they are cold or cool evaporation when released; (2) their molecular is higher than that of air; (3) they are contained elevated pressure and flash to aerosol and vapor release; (4) chemical reactions. The first two categories, molecular weight, are discussed further. The of flashing, on the other hand, are not so 0 Many materials, such as NH3 and HF are kept at fIIeva'[ea pressure so that as liquids or efficiently they may be efficiently participate in a chemical If these materials are will result in a mixture accidentally released, of cooled vapor and liquid aerosol. The by flashing, the flash amount of vapor prod fraction, is a function of the storage temperature and pressure and the therm amic properties of the released material, and can calculated from basic thermodynamics. The the liquid aerosol entrained this vapor plume is not easily calculated and is the subject of research. Several competing are being examined but it appears to this author the small droplet size predicted by the boiling theory (Crowe and Comfort, 1978) is most COll.!.tellt with the /-lm-size particles observed in the existing data. Better data on HF aerosol size be available soon and will help identify the approach to modeling aerosols. The presence causes several droplets make the dense-gas effects. First, the cloud denser than air even the vapor may be lighter. Second, the evapo of the liquid droplets cools the surrounding air mixture, which could further increase the cloud Third, turbulence is suppressed within the reducing mixing with ambient air. The importance of this during large-scale NH 3 Nevada Test Site in 1983 and The Fertilizer Institute during HF tests (Goldfish Amoco Oil Co. (Blewitt, 1 mately 20% of the liquid remaining liquid (80%) remained airborne. Figure depression measured 20 m first three Goldfish HF Flashing has created a cold, the aerosols evaporate as ing the cloud to remain cold spill rate, Test 1, the cloud tlnnn",rM same at 60 m (Fig. 1b) downwind (Fig. la). The H temperature. mately 105°F, close to am which increase or Chemical reactions can pie, when N 2 0 4 (a decrease cloud density. For
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Fig. 1. Temperature differences for Goldfish HF Tests 1, 2 and 3 measured at (a) 20 m and (b) 60 m downwind of the spill point.
rocket fuel oxidizer) is released, it dissociates rapidly to N0 2 which undergoes a further rapid endothermic reaction with atmospheric water vapor to produce an HN0 3 mist (McRae, 1984, 1985). This mist behaves differently than does the ambient temperature N0 2 , cooling the cloud and increasing its density which reduces vertical mixing. As another example, cold HF droplets cause water vapor to condense on them and react, releasing heat. This increases the temperature which could be expected to increase the evaporation rate and further cool the droplet. However, the water-HF solution has lower volatility allowing the droplets to persist longer downwind. The net result is that cloud travel distances to concentration levels of concern are expected to increase with increasing humidity up to about 70% for HF releases even though the HF-water reaction is exothermic. Work is planned to include these effects in dense-gas dispersion models so that parameter studies and analysis can be performed. Dense gas effects
The important dense-gas effects which are not observed in the dispersion of trace emissions include turbulence damping and gravity spreading due to
Review of recent field tests density gradients in the horizontal direction. In some situations, the dense cloud actually displaces the ambient wind field (Koopman, 1982a, 1987) in much the same way that wind flows over a solid body. This displacement results in a stably stratified dense gas layer and an interface through which it is difficult for external turbulence to penetrate (Hunt et al., 1983). Further, in many situations, the kinetic energy of the turbulence within the cloud is not sufficient to give fluid elements the large, vertical displacements needed to mix upward with the ambient atmosphere. Thus, stable stratification damps the vertical component of turbulence at the interface. Displacement of the ambient wind field also causes the cloud to become nearly decoupled from the ambient flow and to move downwind at a rate slower than the ambient wind speed. All of these effects are most pronounced when the ambient wind speed is low and the atmospheric conditions are stable. Figure 2 shows data from bivane anemometers both upwind and downwind of the 1980 Burro 8 liquefied natural gas (LNG) test. The wind speed recorded by the anemometer in the cloud drops to near zero when the cloud is present indicating that the cloud is displacing the ambient wind flow. Figure 3 shows vertical turbulence data from a bivane anemometer as it experienced a cold dense LNG cloud during the 1987 Falcon series tests (Brown et al., 1989). The effect of the cloud on turbulence is marked. Turbulence damping is accounted for in the various dense gas dispersion models in a number of different ways (Ermak, 1988a). Currently, FEM3 (Chan, 1988) uses a K-theory model to parameterize turbulence which is dependent on cloud Richardson number, a
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