REDUCING AMMONIA CONCENTRATIONS IN ATMOSPHERE AFTER ITS UNPLANNED RELEASE

Dep. «Hydraulics and Water Supply», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 273 15 09, e-mail water.supply.treatment@gmail.com, ORCID 0000-0002-8525-7096 Dep. «Hydraulics and Water Supply», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 273 15 09, e-mail water.supply.treatment@gmail.com, ORCID 0000-0002-1531-7882 Dep. «Hydraulics and Water Supply», Dnipropetrovsk National University of Railway Transport named after Academician V. Lazaryan, Lazaryan St., 2, Dnipro, Ukraine, 49010, tel. +38 (056) 273 15 09, e-mail water.supply.treatment@gmail.com, ORCID 0000-0003-3057-9204


Introduction
Unplanned ammonia release as a result of accident or terror act can cause harm for persons and intensive environment pollution [4,6,10,12].From the point view of industrial safety, in case of possible unplanned ammonia release we must solve two important problems.The first problem is prediction of possible contamination zones (their dimensions, intensity and development in time).
To solve the problem of air contamination in case of unplanned toxic chemicals release Gaussian plume model or some analytical models are widely used [2,3,[10][11][12][13]15].These models allow quickly calculating zones of chemical contamination.These models are implemented in some codes, for example, ALOHA, CULLPUFF, etc.
The second problem of unplanned ammonia release is protection environment from pollution and reducing negative influence of toxic chemical after NH 3 emission.In this work we study the problem of neutralizer supply in to ammonia plume to reduce the intensity and dimensions of contamination zones.

Problem statement
We consider the unplanned ammonia release at ammonia pump station which is situated at ammonia pipeline Toliatti-Odessa.To reduce the negative effect on environment in this case we propose to supply neutralizer (NL) to the plume of toxic chemical.As the neutralizer the acid solution can be used.If, for example, we use H 2 SO 4 solution, the process of chemical interaction «ammonia + neutralizer» can be described as follows Application of neutralizer will cause NH 3 reduction in atmosphere.To supply NL into the plume of toxic chemical the special equipment will be used (Fig. 1).But in case of NL supply we face some problems: what amount of neutralizer must be supplied, what will be the effect of the neutralization for different meteorological conditions, etc.To answer these questions and develop the effective strategy of neutralization we can't use physical experiment but only the mathematical one.

Purpose
The purpose of this paper is to develop a mathematical model for quick computing the process of chemical interaction «ammonia + neutralizer» in case of unplanned ammonia release at ammonia pump station.

Mathematical formulation
To simulate the ammonia and acid dispersion in atmosphere 2D transport model is used [5,7] x where С is mean concentration of admixture (ammonia and H 2 SO 4 ); u, v are the wind velocity components;  is the parameter taking into account the process of pollutant washout and gravity fallout [5]; where P is the potential of velocity.The wind velocity components are calculated as follows: Boundary conditions for modeling equations are discussed in [5,7].

Numerical model
The computation of wind pattern and pollutant dispersion is carried out on rectangular grid.To create the form of buildings we use porosity technique or so called «markers method» [1,7].Markers are used to separate the computational cells where flow takes place from the cells which correspond to buildings.Description of the finite difference schemes, used by us, is discussed in [ 7].To solve equation ( 2) we used change-triangle difference scheme, while A. A. Samarskii's and implicit difference scheme was used for numerical integration of Eq. ( 3).
For coding of difference formulae, we used FORTRAN language.

Findings
Computational region is shown in Fig. 2 In this figure one can see position of unplanned ammonia release and position of NL supply equipment.It was supposed that ammonia release takes place at time t=0 and at time t=15 sec neutralizer supply starts.Fig. 2. Sketch of computational region (ammonia pump station): 1, 2, 3, 4 -buildings; 5 -position of neutralizer supply; 6 -position of ammonia release at the territory of pump station Contaminated zone in case of neutralizer supply absence is shown in Fig. 3.We can see that contaminated zone covers all the territory of ammonia pump station.In Fig. 4 the contaminated zone in the case of neutralizer supply is shown.
As we can see from Fig. 4 neutralizer supply allows quickly reducing dimensions of contamination zone in atmosphere.
Worthy of note that computational time was 6 sec.It allows using the model for development of protection measures at chemically dangerous enterprises.The peculiarity of the developed model is the use of standard meteorological information and quick calculation of modeling process in comprehensive computational region.

Conclusions
Numerical model for estimating the efficiency of neutralizer supply to ammonia plume is proposed.Proposed numerical model allows to compute ammonia concentrations and neutralizer concentrations and chemical interaction between substances.The solution of the aerodynamic problem is based on the numerical integration of the equation for the velocity potential.The equation of mass transfer is used to calculate dispersion of ammonia and neutralizer dispersion in atmosphere.The mass transfer equation takes into account the convective and diffusive transport of admixture in atmosphere with account of buildings situated near the source of emission.Emission of ammonia and neutralizer is simulated by a point

Fig. 1 .
Fig. 1.Equipment for neutralizer supply: 1 -pump; 2 -vessel containing neutralizer; 3 -rotatable nozzle  are the diffusion coefficients; Q is intensity of point source emission; point source.We consider ammonia and NL dispersion among buildings it means that we must take into account influence of these obstacles on process of NH 3 and NL dispersion.To simulate the flow pattern in the case of the buildings at the territory of Pump Station the 2D model of potential flow is used[7]

Fig. 4 .
Fig. 4. Computed ammonia concentration, t=28 s (neutralizer supply)Originality and practical valueMathematical model has been developed to compute dispersion and chemical interaction between ammonia and neutralizer which is used to reduce ammonia concentration in atmosphere.The presented mathematical model is based on the application of the fundamental equations of aerodynamics and mass transfer.The peculiarity of the developed model is the use of standard meteorological information and quick calculation of modeling process in comprehensive computational region.