Calculation Of Ecologically Harmful Substances Concentration Fields In The Atmosphere
CALCULATION OF ECOLOGICALLY HARMFUL SUBSTANCES CONCENTRATION FIELDS IN THE ATMOSPHERE
AS A RESULT OF ACCIDENT SPILLS, LEAKS AND RELEASES
FROM STORAGES IN THE SEA COASTAL ZONE
O.M. Prokopova, A.L. Tsykalo
State Academy of Refrigeration of Odessa, Ukraine
In Ukraine as well as in many other countries and regions they widely use large-tonnage storage, transportation and transfer of substances and materials which are very dangerous for people and environment in the case of their accident releases, spills, or leaks. Such substances are ecologically harmful, toxic, chemically aggressive, explosive and fire-hazardous substances and compounds (ammonia, chlorine, methane, liquefied natural gas, etc.). Thus, the evaluation of potential danger caused by the storage of great amounts of the above-mentioned substances is extremely important for the most rational location of industrial enterprises, the development of individual and collective safety measures for the staff of attendants and the population.
Carrying out the appropriate calculations is a rather complicated task. It is necessary to distinguish the estimation of reliability of equipment and technological processes, probability of arising accident situations due to external factors, interaction of various productions and enterprises. Predicting the consequences of accident releases, spills, and leaks of toxic substances into the atmosphere plays the decisive role in the evaluation of the risk and the possible impact of ecologically dangerous chemical substances on the environment. The prediction of concentration fields is complicated by specific features of different substances, sophisticated nature of air mass transfer in relatively small area of the space near the Earth surface, distinctions between physical mechanisms of processes at various stages of their development, etc.
At the Department of Chemistry and Environmental Protection of the Odessa State Academy of Refrigeration we developed a method of predicting affection zones in case of accident spills of low temperature ecologically dangerous liquids. This method was the basis for creating at the Odessa Port
Plant a computer-aided system of modeling and predicting consequences of accident releases and leaks of ammonia as well as warning the personnel of the enterprise and the population of the vicinity.
This method takes into account:
-nonstationarity of the liquid evaporation processes from the underlying surface;
- possibility of the aerosol cloud formation and evolution;
- propagation of a gas - air cloud;
- possibility of changing weather conditions.
However, in this case the affect of water objects on ammonia dissipation processes was not taken into account. But it is known that this affect can be testified by the following:
a) it is known that one volume of water absorbs about 700 volumes of gaseous ammonia;
b) the interaction of ammonia with water is accompanied by the release of a great amount of heat which can result, under appropriate conditions, in considerable increase of the system temperature;
c) the absorption of ammonia by water leads to some reduction of the total atmosphere contamination by vapours of this substance and redistribution of ammonia concentration in the atmosphere.
Besides, enterprises producing ammonia and its large-tonnage storage’s are located, as a rule, near water objects. For example, the Odessa Port Plant on the territory of which are located 4 large-tonnage storage’s for ammonia (30,000 tons each, 120,000 tons total), is situated on the near Black Sea coast of the Grigorievsky liman. But the line of the shortest direction: the plant’s territory - the city of Odessa passes just above the Odessa bay water surface. The Ventspils Port Plant as well as many plants and terminals of the USA and of other countries are in the same situation. The ammonia main pipelines also run in some cases near water objects, and rather often evens cross them. Therefore in the case of accident damage of the ammonia pipeline the interaction of ammonia with water can be unavoidable. The account of ammonia - water interaction is, of course, very important for cases of direct spill on water. But this is another situation.
We are interested in the case of accident spill on a solid underlying surface when the initial boiling and the following evaporation of large amounts of ammonia lead to the formation of a zone of its high concentration in the atmosphere which passes over the water surface moving under the influence of the wind and in such a way interacting with the water object.
When the wind goes from the land to the pool surface, there occurs the transition to a surface with new roughness and the wind velocity, being one of the main parameters of the airflow, changes. Knowing the wind velocity on the land one can calculate the wind velocity over the water by the relationship:
vw = k×vl, (1)
where k - coefficient of reduction depending on the distance to the coast and the wind velocity on the coast at the given type of the locality; vw and vl - the wind velocity over the water surface and on the land, respectively.
It is necessary to take into account the type of the locality as the land roughness parameter depends on it. For example, type A (open localities) is characterized by the roughness parameter value of 0.035 m, type B (country-side and city territories with buildings of under 20 m height) - 0.38 m; type C (large cities with buildings of over 20 m height) - 1 m.
Formula (1) is used if an anemometer measures the wind velocity. If the wind velocity is measured by a wind-vane, then relationship (1) has the another form:
vw = k×kwv×vl, (2)
where kwv - coefficient of the data recalculation by wind velocities measured by the wind-vane.
For quantitative estimation of the water objects influence on processes of ammonia dissipation it is possible to use an approximate approach based on the application of the similitude method and the analogy between processes of vapour condensation and absorption of a well-dissolved gas. There are data in literature, which enables the choice of mass-transfer coefficients for the case of absorbing ammonia by a free water surface we are interested in. It is possible to use two approaches for this purpose. First, there is a reason to believe that there exists the analogy between the process of water vapour condensation on the surface of an open pool and absorption of the ammonia cloud by the water surface. In this case the mass-transfer coefficient can be found by using the Parienter equation:
b = 11,9 + 1,6vw (3)
The second approach supposes hydrodynamic analogy of transfer processes and makes it possible to use the Rheinolds’ analogy for the mass-transfer between the air-ammonia mixture and the free water surface. The mass-transfer coefficient in this case is defined by the expression:
b= e vw/8 (4)
where e - fraction of the release from a source of ammonia per one elementary zone of the cloud which is on its boundary turned to the source.
The model of nonstationary diffusion in a still medium through a flat surface can be used for the calculation of the mass transfer process between the cloud and the surface of the pool. The surface concentration at the time of t=0 suddenly changes from c0 to c1 and is kept at this value. The diffusion of the dissolved substance into the surroundings in the “y” direction is defined by the equation:
(c1 - c)/(c1-c0) = erf (y/2/(Dt)0.5), (5)
where с - concentration at the distance of “y” from the surface, D - coefficient of diffusion, t - time.
Knowing the dependence of power of release, wind velocity, cloud boundary coordinates and its height on time as well as the value of ammonia concentration in the cloud before it appears above the water surface one can make numerical estimations of water objects influence on processes of ammonia dissipation in the atmosphere.
The problem of diminishing the danger of ammonia spills from large-tonnage storages is no less important. They are usually located on sites with banking-up which should prevent spontaneous spreading of the stored substance in case of its accident release or leak so that the part of the product coming outside was concentrated within the bunking-up.
Then it is possible not only to localize the spill without harmful substance getting into the sea, the river, on the territory of a populated area, in underground levels, to prevent possible fires and explosions, etc., but in some cases it is possible to pump out this substance into reserve vessels. Nevertheless, in case of serious damages there can be but once a leak from a large-tonnage storage of a great amount of liquid exerting a strong mechanical influence on the banking-up that can result in a complete or partial damage of it. To solve the problem of checking the reliability of appropriate structures, estimating the probability of their failure and predicting possible consequences it is needed to determine the force action on the wall of the banking-up.
The force of the banking-up reaction can be found as:
R = -M(v1 - v2cos a), (6)
M = r S0 e j , (7)
where e - coefficient of compression, v1 and v2 - mean velocities of the jet in appropriate cross-sections, a - angle of changing the jet direction before the contact with the wall of banking-up, r - density of the liquid, g - gravity, Hi - depth of the hole from the free surface of the liquid.
To reduce the load on the banking-up at large spills it is possible to use dampers of liquid flow energy in the form of walls making the “hydraulic jump” as the form of the transition of the liquid flow from the turbulent nonstationary state to the laminar one.
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