Applying Proper Dispersion Models for Industrial Accidental Releases

Transcription

1 Applying Proper Dispersion Models for Industrial Accidental Releases Paper No. 726 Prepared By: Weiping Dai, PhD, PE, CM BREEZE SOFTWARE Merit Drive Suite 900 Dallas, TX (972) breeze-software.com Modeling Software for EH&S Professionals

2 Applying Proper Dispersion Models for Industrial Accidental Releases Paper # 726 Weiping Dai Trinity Consultants North Central Expressway, Suite 1200, Dallas, TX wdai@trinityconsultants.com ABSTRACT Even though many industrial facilities have put extra efforts to prevent accidental releases of active and inactive chemicals from processes or storage, accidental releases do happen from time to time due to many reasons. Federal and state regulations require industrial facilities to evaluate and report the impacts of potential accidental releases in terms of distances to toxic endpoint concentrations or radius of exposure at threshold values. When an accidental release occurs in the real world, industrial facilities would also need to evaluate the impact of the release scenario. This kind of industrial accidental releases typically lasts for a limited time period (e.g., from minutes to hours). Many dispersion models (e.g., DEGADIS,, INPUFF, or ALOHA) are available and designed to perform the dispersion analyses for different accidental releases. However, choosing a proper dispersion model for an industrial accidental release is not a straightforward process. It depends on many factors such as the emission source characteristics (e.g., released chemicals, chemical properties, released source orientation, and release conditions), plume characteristics (e.g., dense gas plume or neutrally buoyant plume) and atmospheric conditions. Analyses will be even more complicated if chemical reactions involve upon release. This paper will compare various dispersion models readily available in the public domain for industrial accidental releases. The fundamental theories and dispersion techniques utilized by each model will be analyzed. Strengths and shortcomings of each model will also be listed. Case studies of industrial accidental release scenarios will be presented to help industrial facilities to identify proper models for different accidental release scenarios. 1

3 INTRODUCTION Even though many industrial facilities have put extra efforts to prevent accidental releases of active and inactive chemicals (e.g., ammonia, chlorine, chlorine dioxide, ethylene oxide, or hydrogen sulfide) from processes or storage, accidental releases do happen from time to time due to many reasons. As such, there are regulations at both federal and state levels requiring to assess various accidental release scenarios in order to mitigate or minimize the potential impacts. For example, the Risk Management Plan (RMP) program regulated by the U.S. Environmental Protection Agency (US EPA) requires facilities stored a listed chemical above a threshold quantity to perform an off-site consequence analysis of a worst-case (most conservative) scenario and an alternative (more likely to occur) scenario to determine the distance to a defined toxic endpoint and develop a prevention/emergency response program accordingly. Similar technical analyses can also apply for process safety management and onsite worker protection. Moreover, when an accidental release occurs in the real world, industrial facilities would also need to evaluate the impact of the release scenario. This kind of industrial accidental releases typically lasts for a limited time period (e.g., from seconds to hours). Performing such a technical analysis immediately on an occurred accidental release can help to identify effective measures to provide best protection the health and wealth of both on-site and off-site people. Performing dispersion modeling analyses for accidental releases of toxic chemicals is technically complex in many aspects. First of all, it is not easy to determine the source terms (e.g., release rate, release duration, and release temperature) for an accidental release of toxic chemicals. As discussed in the US EPA s Guidance on the Application of Refined Dispersion Models to Hazardous/Toxic Air Pollutant Releases, chemical releases can typically be categorized into the following classes: two-phase gas release under a choked or unchoked condition, two-phase pressurized liquid release, two-phase refrigerated liquid release, singlephase gas release under a choked or unchoked condition, single-phase liquid release with high or low volatility. 1 For different chemicals stored under different conditions, different release classes will be applied. Emission rates and release conditions should be determined differently for each release class. Typically, accidental releases of toxic chemicals last for a relatively short period of time from several minutes to several hours. The chemicals may be vertically or horizontally released at a ground or elevated level at temperature/ pressure conditions different from the ambient conditions. Second, for many situations, choosing a proper dispersion model and applying appropriate modeling technique for an industrial accidental release is not a straightforward process. It depends on many factors such as the emission source characteristics (e.g., released chemicals, chemical properties, released source orientation, and release conditions), plume characteristics (e.g., dense gas plume or neutrally buoyant plume) and atmospheric conditions. Analyses will be even more complicated if chemical reactions involve upon 2

4 release. Many dispersion models (e.g., DEGADIS,, INPUFF, AFTOX, and ALOHA) are available and designed to perform the dispersion analyses for certain types of accidental releases. In general, two distinctive types of models are applied to an accidental release scenario: dense-gas models (e.g., DEGADIS and ) and neutrally buoyant dispersion models (e.g., INPUFF and AFTOX). Most of these models are readily available in the public domain for industrial accidental releases and recognized by various regulatory agencies such as US EPA and state agencies. 2, 3 In this paper, several dispersion models (e.g., DEGADIS,, INPUFF, and AFTOX) are compared and studied. The fundamental theories and dispersion techniques utilized by each model are analyzed and strengths and shortcomings of each model will be listed. Case studies of industrial accidental releases by utilizing these models are presented to help industrial facilities to identify proper models for different accidental release scenarios. IMPORTANT CONCEPTS IN ACCIDENTAL RELEASE MODELING Dense Gas Release vs. Neutrally Buoyant Release Accidental releases of toxic chemicals heavier than air, mixing in both aerosol and gaseous phases, and/or under a release temperature much colder than the ambient temperature can result in a dense gas release. In a dense gas release with zero momentum, the plume slumps to the ground and then diffuses horizontally. A cloud from a dense gas release behaves very differently than a plume from a lighter-than-air release. For a dense gas cloud, the cloud characteristics are primarily gravity-driven non-gaussian dispersion with negative buoyancy and stable density stratification. A dense gas cloud has a tendency to fall toward the ground. Even if the release is a vertical upward jet, the plume will stop moving upward and turn downward as it moves downwind. When the cloud slumps with the ground, the momentum of the fall causes the center of the cloud to dip while the edges bulge. The cloud then bounces back to resemble a layered flattened cylinder in which the vertical dimension is a minor fraction - typically 1/20th to 1/50th---of the horizontal dimension. This pancakeshaped cloud elongates and spreads as the wind flow moves it along and ambient air becomes entrained in the cloud. A dense gas release often requires modeling a secondary pool of liquid as the source of the dense gas cloud. A dense gas release may begin at a temperature significantly below ambient, either as a result of a release of a gas refrigerated to a liquid or as a result of the sudden depressurization of a container storing gas as a liquid. The cloud is affected by heat transfer from air and the underlying surface. Heat released by condensation of atmospheric water vapor can also be important. 3

5 Release Duration Industrial accidental release events typically last for a short period of time from seconds to minutes or at most hours. In the modeling terminology, the release type can be described as instantaneous, finite, or continuous in terms of the release duration. A finite duration release is one that only exists for a limited period of time. For example, if a relief valve opened and the entire contents of the vessel escaped in 10 minutes, a finite duration release of 10 minutes would have occurred. When a finite duration release lasts for hours, the release can also be treated as continuous release and thus steady-state modeling techniques can be applied. To determine whether a release can be treated as continuous, two factors should be considered: 1) the release duration, and 2) the location of the nearest receptors or the maximum distance to the lowest concentration of interest. The time required for the released chemical to reach a specific downwind distance is compared to the actual emission duration. If the emission duration is longer than the time it took for the released chemical to reach a downwind distance of interest, the release may be considered continuous. Otherwise, the release should be considered instantaneous or finite duration. The travel time (T) to a downwind distance (X) from the release location can be estimated by relating with the ambient wind speed (U a ): T = 2X U a Note that a factor of 2 in the equation reflects the plume meandering during dispersion along the wind direction. Furthermore, a finite duration release should also be compared to the desired averaging time. If the release duration is less than the averaging time of concern, a transient model should be used to reflect average concentrations more accurately. Averaging Time Averaging time applied in the modeling is important because it dictates how long the downwind concentration of a released chemical should be averaged. Accidental release events often last for a short duration while the concentrations of interest are typically shortterm averages based on the hazard concern. Typical concerns from a toxic air release are the maximum short-term concentration and/or the maximum dose. Many accidental release models are designed to provide concentration predictions for averaging times ranging from seconds to an hour and meteorological input is often based on an hourly average. To determine the appropriate averaging time, the user should examine the release duration and the concentration levels of concern while keeping health or other reference levels in mind. For example, Short Term Exposure Limits (STEL) have an implicit time interval of 15 minutes, the Immediately Dangerous to Life and Health limit (IDLH) has an implicit interval of 30 minutes, and the Emergency Response Planning Guideline (ERPG) concentrations have a 60-minute implied exposure time. Defining an appropriate averaging time for dense gas 4

6 modeling applications is complicated. Large averaging times allow for more plume meandering, and therefore, lower average concentrations. Relevant time scales for the modeled averaging time to be used in modeling include: 1) the averaging time associated with the hazard being assessed (e.g., a 15-minute short-term exposure limit); 2) the duration of the release event; and 3) the travel time from the release location to a receptor. To reflect the effect of plume meandering, the largest recommended averaging time used in the modeling should be the minimum of these three averaging time values (for steady-state or transient releases). When the averaging time used in the model is less than the averaging time associated with the exposure criterion, the modeling results must be converted to values corresponding to the exposure criterion. For example, if the release duration is 5 minutes and the averaging time for a short-term exposure limit is 15 minutes, the averaging time of 5 minutes should be modeled. To convert the modeled concentrations to concentrations for the averaging time associated with the exposure limit (i.e., 15 minutes for this case), the modeled concentration must be multiplied by 1/3 (5 minutes /15 minutes). COMPARISON OF ACCIDENTIAL RELEASE MODELS DEGADIS The DEnse GAs DISpersion model (DEGADIS) was developed specifically to model heavier-than-air gaseous releases from a single source over flat terrain. A single meteorological condition is specified for the duration of the release. DEGADIS was developed by Tom Spicer and Jerry Havens of the University of Arkansas, upon commission by the U.S. Coast Guard in 1980s. 4, 5 It incorporates the vertical jet plume model developed by Ooms, Mahiu, and Zelis. 6 This is useful for chemical processes requiring the storage of pressurized substances that, if released, produce high velocity emissions. The Ooms jet plume model provides for the prediction of the trajectory and dilution of these types of vertically oriented gas or aerosol jets. It also accounts for ground reflection when the plume s lower boundary reaches the ground. Overall, DEGADIS models the following release types: Continuous Release: A steady-state release of dense gas at a constant rate into the atmosphere over a long period of time. The output from modeling a steady-state release is concentration estimates at various downwind distances determined by the model. Finite Duration Release: A steady-state release of dense gas at a constant rate into the atmosphere over a short period of time. Finite duration model output is organized either by time or distance, depending on which parameter is of greater interest. Transient Release: Release rates vary over time. For example, if a liquid pool boils off or a container of gas depressurizes. As the pool decreases in size, the emission 5

7 rate and radius change. Other transient releases include near-instantaneous releases such as container ruptures. Transient modeling output is organized either by time or distance, depending on which parameter is of most interest. Jet Release: A vertical release of a dense gas or aerosol by using the Ooms mathematical model. The jet plume model requires that the jet be vertical, with a definable exit velocity. If the jet release is such that the plume centerline does not reach the ground before dispersing, the jet plume model is run alone. If this is unclear, or if the plume centerline does reach the ground, the jet plume model is run in conjunction with the regular DEGADIS model as either a continuous or finite duration release. Liquid Spill: A release of a chemical in its liquid state. The liquid is assumed to form a pool at ground level, with the evaporation rate calculated using one of three different evaporation models incorporated into DEGADIS. The results from the evaporation model are run as either a continuous or finite duration release. Note that the liquid spill option will only be available if the chemical s normal boiling point is greater than the ambient temperature. Furthermore, the DEGADIS model has the following assumptions and limitations that govern the model execution: Use of the model is restricted to dense gas releases or liquid spills that evaporate to a dense gas. Both the DEGADIS and the jet plume models assume a flat atmospheric flow field with no obstructions, such as buildings or trees. The model does not consider sloping terrain either. Use of the model is restricted to conditions in which the depth of the dispersing gas layer is much greater than the surface roughness of the surrounding area. The jet model is strictly for vertical releases. No horizontal jet release velocity is incorporated into the model. If the jet release is not perpendicular to the ground, the modeling results will not be accurate. discussed below is an appropriate model for horizontal jet releases. simulates the atmospheric dispersion of denser-than-air releases. 7 The sources may be modeled as either continuous, finite duration, or instantaneous releases. Continuous and finite duration releases are applicable to evaporating pool, horizontal jet, and vertical jet sources. Instantaneous releases are modeled using the instantaneous volume source. The types of releases treated by the model are described as follows: 6

8 Ground-level Evaporating Pool: An evaporating pool is a ground-level, area source of finite duration. When the spill duration is sufficiently short, a steady-state plume does not form at any downwind distance. When the model determines that this is the case, it automatically stops the calculation, redefines the source as an instantaneous release, and starts the calculation over. Elevated Horizontal Jet: A horizontal jet is an area source with the source plane perpendicular to the ambient wind direction and source velocity pointing directly downwind. Stack or Elevated Vertical Jet: A vertical jet is an area source with the source plane parallel to the ground and the source velocity pointing directly upward. Instantaneous Release: An instantaneous release is a combination of two sources: 1) an instantaneous volume source with a total mass; and 2) a short duration, groundlevel, area source with a source rate and spill duration. The instantaneous release is a default for the evaporating pool release when the spill duration is so short that steadystate is not reached anywhere within the dispersing cloud. While this type of source can be run directly using the instantaneous release, it is recommended that an evaporating pool release of any finite duration be run as an evaporating pool. If a steady-state cloud is not achieved due to a short spill duration for the evaporating pool, the model automatically changes the source to an instantaneous release. Liquid Spill: A liquid spill is the release of a chemical in its liquid state. The liquid is assumed to form a pool at ground level, and the evaporation rate is calculated using an evaporation model. The results from the evaporation model are then run in the model as an evaporating pool finite duration release. Except for the evaporating pool source, which is assumed to be vapor only, all of the remaining sources are either pure vapor or a mixture of vapor and liquid droplets. Atmospheric dispersion of the release is calculated by solving the conservation equations of mass, momentum, energy, and species. The conservation equations are spatially averaged so that the cloud is treated as a steady-state plume, transient puff, or a combination of the two depending upon the duration of the release. A continuous release is treated as a steady-state plume. In the case of a finite duration release, cloud dispersion is initially described using the steady-state plume mode and remains in the plume mode as long as the source is active. When the source is shut off, the cloud is treated as a puff and subsequent dispersion is calculated using the transient puff mode. For an instantaneous release, the transient puff dispersion mode is used for the entire calculation. The model has several built-in assumptions that govern the model execution. These limitations are: Use of the model is restricted to dense gas releases or liquid spills that evaporate to a 7

9 dense gas. The model assumes a flat atmospheric flow field with no obstructions (buildings, trees, etc.). The model does not take sloping terrain into account either. Use of the model is restricted to conditions where the depth of the dispersing gas layer is much greater than the surface roughness of the surrounding area. INPUFF INPUFF is a Gaussian integrated puff model with a wide range of applications such as simulating the atmospheric dispersion of neutrally buoyant stationary or moving releases. The implied modeling scale is from tens of meters to tens of kilometers. The wind field in INPUFF is assumed to be homogeneous for the entire averaging period. INPUFF is capable of addressing the accidental release of a neutrally buoyant substance over a short period of time, or of modeling the more typical continuous plume from a stack. The model includes the following features: Optional stack-tip downwash Wind speed extrapolated to release height Optional buoyancy induced dispersion In Gaussian-puff algorithms, source emissions are treated as a series of puffs emitted into the atmosphere. Constant conditions of wind and atmospheric stability are assumed during a time interval. The diffusion parameters are functions of travel time. During each time step, the puff centers are determined by the trajectory and the in-puff distributions are assumed to be Gaussian. Thus, each puff has a center and a volume that are determined separately by the mean wind, atmospheric stability, and travel time. Plume rise is calculated using the methods of Briggs. Although plume rise from point sources is usually dominated by buoyancy, plume rise due to momentum is also considered. Building downwash and gradual plume rise are not treated by INPUFF. Stack-tip downwash (optional) can be considered using the methods of Briggs. In such an analysis, a height increment is deducted from the physical stack height before momentum or buoyancy rise is determined. Use of this option primarily affects computations from stacks having small ratios of exit velocity to wind speed. INPUFF model has several built-in assumptions that govern the model execution: Use of the model is restricted to neutrally-buoyant gas releases. The model assumes a flat atmospheric flow field with no obstructions (buildings, trees, etc.). Wind direction constant with height. No consideration of chemical reactions. No consideration of building wake or cavity effects. 8

10 AFTOX AFTOX was developed to model neutrally buoyant gas releases and liquid spills that evaporate as a neutrally buoyant gas. 8 Gas releases are limited to those that are neutrally buoyant (non-dense, non-buoyant) with no velocity, and emanating from a point (non-area) source. Liquid spills are limited to single-phase releases of low volatility liquids. A low volatility liquid is a liquid at ambient conditions. Neither refrigeration nor pressurization is needed to maintain the material in liquid form. A low volatility liquid forms a liquid pool upon release, with the atmospheric emission rate (the rate of vapor cloud formation) dependent on evaporation from the pool. The AFTOX model is a Gaussian puff/plume model designed to model neutrally buoyant gaseous releases. Both gas and liquid (evaporating to a neutrally buoyant gas) sources can be modeled with AFTOX. The sources may be modeled as either continuous, finite duration, or instantaneous releases. The Gaussian puff model in AFTOX uses an equation to describe the dispersion of a puff with time. The equation assumes that the material is conserved during transport and diffusion, that is, there is no decay or deposition. It further assumes that the distribution of concentration within the puff is Gaussian. The concentration at a point in space at a given time depends on the number of nearby puffs, their size, and the amount of material in each puff. The sum effect of all these puffs is given by summing over all emission times. For an instantaneous gas release there is only one emission time and one puff. Therefore, a summation is not necessary. However, for a continuous spill or a spill of finite duration, the summation is performed over puffs whose centers are located within four standard deviations of the puff concentrations upwind and downwind from the location of interest. It is assumed that concentrations from puffs further than four standard deviations contribute little to the concentration at the specified location. In a spill of finite duration, the model assumes 20 puffs/minute out to 300 m from the source, 4 puffs/minute from 300 m to 3 km and then 3 puffs/minute beyond 3 km for winds less than or equal to 4 meters/second. For winds greater than 4 m/sec, the number of puffs per minute will be calculated depending on the distance from the source and the wind speed. The frequency of puffs increases with higher winds because higher winds cause greater distances between puffs, thereby reducing the number of overlapping puffs. By increasing the frequency of puffs, approximately the same number of overlapping puffs are retained no matter how strong the wind. Under steady state, non-inversion conditions, it is not necessary to keep track of the individual puffs. A simple Gaussian plume model will suffice. For liquid spills, the liquid is assumed to form a pool at ground level, with the evaporation rate calculated using one of three different evaporation models incorporated into AFTOX. Furthermore, the AFTOX model has several built-in assumptions that govern the model 9

11 execution: Use of the model is restricted to neutrally buoyant gas releases or liquid spills that evaporate to a neutrally buoyant gas. The AFTOX model assumes a flat atmospheric flow field with no obstructions such as buildings or trees. The model does not take sloping terrain into account either. Use of the model is restricted to conditions in which the depth of the dispersing gas layer is much greater than the surface roughness of the surrounding area. The model converts the wind speed measurement, whose height is specified in the station database, to a 10-m height wind speed. The 10-m wind speed is used in all of the calculations; therefore, note that the plume may move downwind at a faster rate than the measured wind speed would indicate. Choosing Proper a Dispersion Model for a Release Scenario As discussed in the previous sections, each accidental release model was developed with its strength and limitations. Table 1 illustrates proper models for various release scenarios. The Richardson number can be used to determine whether a dense gas model should be selected. A Richardson number greater than 700 for an instantaneous release indicates a dense gas while the Richardson number should be greater than 32 for a continuous dense gas release. Table 1. Proper Models for Various Release Scenarios Release Type Source Type Continuous Finite Transient Instantaneous Ground-level DEGADIS AFTOX* DEGADIS AFTOX* DEGADIS AFTOX* Evaporating Liquid Spill Vertical Jet/Plume DEGADIS AFTOX DEGADIS INPUFF DEGADIS AFTOX DEGADIS INPUFF Horizontal Jet Instantaneous * AFTOX can also model gas release at elevated height. DEGADIS AFTOX 10

12 CASE STUDY APPLYING MODELS PROPERLY Dense Gas Modeling Ethylene Oxide Release In this case study, an accidental release of ethylene oxide occurred from a DOT 5P type 55- gallon storage cylinder when it was dropped to the ground during unloading of the cylinder. It was determined that a total of 300 lbs of ethylene oxide was released in 15 minutes. Therefore, the average release rate is 20 lbs/min. The released ethylene oxide initially formed a ground-level vapor cloud and dispersed downwind. Details of the release conditions and representative meteorological conditions during the release event are summarized in Tables 2 and 3, respectively. In addition, the surrounding area is characterized as low crops with occasional large obstacles and thus a typical 0.1 m value for the surface roughness is used. Table 2. Ethylene Oxide Release Parameters Source Parameter Value Release quantity (lbs) 300 Release duration (min) 15 Release rate (lbs/min) 20 Release temperature ( F) 65 Table 3. Meteorological Conditions for the Ethylene Oxide Release Scenario Source Parameter Value Atmospheric stability class D Wind speed (m/s) 4 Anemometer height (m) 10 Average daily temperature ( F) 65 Average relative humidity (%) 50 Since the accidental release occurred at the ground for 15 minutes, the Ground-level source type and the Finite release type are used in the modeling analysis. Since the vapor density of the ethylene oxide cloud upon release is about 1.84 kg/m 3 (which is heavier than the ambient air density of 1.21 kg/m 3 ), a dense gas model such as DEGADIS or should be used. In this study, the dispersion modeling analysis was performed with both DEGADIS and (for comparison purpose) to determine the distance to the toxic endpoint of 50 ppm (which is defined as the level of concern by the risk management program). For DEGADIS, the heat transfer and water transfer are optional. In this case, the heat transfer and water transfer were not used because both effects were not significant for the release occurred at ambient conditions. For toxic endpoint analysis, a receptor height of 1.5 meters is used to correspond with the typical breathing height. Moreover, a 15-minute averaging period is used to match the release duration of 15 minutes. The distance to the 50-ppm toxic endpoint is about 260 meters from DEGADIS and about 170 meters from. In this case study, the 11

13 15-minute average concentrations are used to compare with the toxic endpoint. If necessary, the 15-minute average concentrations can be converted to concentrations corresponding to the averaging time for the exposure limit. Figures 1 and 2 show the ethylene oxide concentrations at the breathing height along the downwind distance from DEGADIS and, respectively. The solid line represents the airborne ethylene oxide concentrations while the dotted line represents the 50-ppm toxic endpoint level. Both figures show a similar pattern of ethylene oxide concentrations along the downwind distance. 12

14 Neutrally Buoyant Dispersion Modeling Ammonia Release In this case, aqueous ammonia was accidentally release during a scheduled maintenance of the ammonia system. The released aqueous ammonia formed a liquid pool in a diked area. Gaseous ammonia evaporated from the liquid pool to the atmosphere. The accidental release occurred at nighttime with a low wind condition. Details of the release conditions and representative meteorological conditions during the release event are summarized in Tables 4 and 5, respectively. Table 4. Ammonia Release Parameters Source Parameter Value Release height (m) 0.01 Dike diameter (m) 14 Release velocity (m/s) 0.01 Release temperature ( F) 54 Release rate (g/s) 30 Release duration (sec)

15 Table 5. Meteorological Conditions for the Ammonia Release Scenario Source Parameter Value Atmospheric stability class F Wind speed (m/s) 3 Anemometer height (m) 10 Average daily temperature ( F) 65 Average relative humidity (%) 50 Since the ammonia vapor cloud at the release conditions is lighter than the ambient air, the release scenario is considered to be a neutrally buoyant release. Thus, a neutrally buoyant model such as AFTOX or INPUFF should be used. In this study, the dispersion modeling analysis was performed with both AFTOX and INPUFF (for comparison purpose) to determine the distance to a 35-ppm level of concern, which corresponds to the time-weighted average STEL value for ammonia. Since INPUFF can only model vertical stack release, the ground-level release in this case is treated as a vertical stack release with a negligible release velocity (e.g., 0.01 m/s). AFTOX gives a distance of about 280 meters to the 35-ppm level of concern while INPUFF gives a distance of about 350 meters. Figures 3 and 4 show the ammonia concentrations at the breathing height along the downwind distance from AFTOX and INPUFF, respectively. The solid line represents the airborne ammonia concentrations while the dotted line represents the 35-ppm STEL value. Although both figures show a similar pattern of ammonia concentrations along the downwind distance, it seems AFTOX generates a peak concentration much higher than INPUFF. One interesting observation is, although the distance to the level of concern predicted by INPUFF and AFTOX are comparable, the peak concentration predicted by AFTOX is much higher than the value predicted by INPUFF. The discrepancy may be caused by the fact that INPUFF is, in theory, a jet dispersion model while AFTOX may be more appropriate for low-momentum releases. Although the initial release conditions (such as the exit velocity and initial dispersion dimensions) to the INPUFF simulation are set to values to reflect the low-momentum ground-level release in this case study, the intrinsic dispersion algorithms implemented in INPUFF may be sensitive to such initial values. 14

16 SUMMARY Accidental releases of toxic chemicals from industrial processes or storage containers can typically be modeled by either dense-gas models (e.g., DEGADIS or ) or neutrally buoyant models (e.g., INPUFF or AFTOX). While each model has its assumptions and limitations, the common limitations among the models discussed in this study include: A single set of meteorological conditions is used to represent the whole dispersion phenomenon. A flat terrain is assumed. In other words, the effects of complex terrain (e.g., rolling or hilly areas) or building downwash are not considered. A single release source is allowed in one simulation. No chemical reactions in the plume are considered. As shown in the case studies, while more than one models can be applied to a specific release scenario, different models may give different results due to inherent assumptions and limitation associated with each model. Choosing an appropriate dispersion model for accidental release scenarios is critical even though it is not always straightforward at some situations. Details of the release scenario should be reviewed carefully in order to reach a 15

17 reasonable decision about which model should be used. More importantly, a model that gives best results may not be the most suitable model for the occasion. Modeling results can be misleading if not interpreted carefully. REFERENCES 1. U.S. Environmental Protection Agency, Guidance on the Application of Refined Dispersion Models to Hazardous/Toxic Air Pollutant Releases (EPA-454/R ), 30 April 1993, Office of Air Quality Planning and Standards (MD-14), Research Triangle Park, NC U.S. Environmental Protection Agency, Guideline on Air Quality Models (Revised), 40 CFR Part 51, Appendix W. 3. American Petroleum Institute, A Guidance Manual for Modeling Hypothetical Accidental Releases to the Atmosphere, Health and Environmental Sciences Department, Public Number 4628, November Spicer, T. and Havens, J., EPA s User s Guide for the DEGADIS 2.1 Dense Gas Dispersion Model, EPA-450/ Spicer, T., Havens, J., Tebeau, P. Key, L., DEGADIS: Heavier-Than-Air Gas Atmospheric Dispersion Model, Paper Presented at the 79th Meeting of the Air Pollution Control Association, Minneapolis, MN. June 22-27, Ooms, G.A., Mahieu A.P., and Zelis, F., The Plume Path of Vented Gases Heavier than Air, First Symposium on Loss Prevention and Safety Promotion in the Process Industries. Elsevier Press, Ermak, D.L., User s Manual for : An Atmospheric Dispersion Model for Denser- Than-Air Releases, DE , Kunkel, B.A., User s Guide for the Air Force Toxic Chemical Dispersion Model (AFTOX), PL-TR , ADA246726,