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Design of mitigation systems for indoor and outdoor ammonia releases
Journal of Loss Prevention in the Process Industries 16 (2003) 93–101 www.elsevier.com/locate/jlp
Design of mitigation systems for indoor and outdoor ammonia releases M.L. Mastellone ∗, M. Ponte, U. Arena Dipartimento di Scienze Ambientali, Seconda Universita` di Napoli, Via Vivaldi, 43-81100 Caserta, Italy
Abstract The paper describes the design of two mitigation systems adopted by an important food company, which uses ammonia for its refrigeration plant. The results of an accurate risk analysis identified the critical risk areas and the alternative mitigation systems. Two different approaches were used for indoor and outdoor accidents. A specific model was developed to estimate the time concentration of ammonia profile after a release inside an enclosure. It was used to minimise the amount of releasable ammonia by the insertion of automatic valve devices along the pipe containing pressurized ammonia. The approach for outdoor accidents led to the design of a fluid curtain capable of intercepting and absorbing the released ammonia cloud. Another specific model, which takes into account the two-phase character of an ammonia cloud, was devised in order to calculate the water flow rate needed to ensure adequate absorption efficiency. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ammonia release; Mitigation systems; Intercepting valves; Fluid curtain
1. Introduction Anhydrous ammonia is widely used as refrigerant in many industrial facilities, e.g. food processing, dairy and ice cream plants, wineries and breweries, cold storage warehouses, etc. Each of these facilities has to take into account the risk of an ammonia release, sometimes associated with a fire or an explosion (⬍http://www.chemsafety.gov/circ/⬎). Ammonia is considered a high health hazard because it is corrosive to the skin, eyes, and lungs. The Occupational Safety and Health Administration (OSHA) Permissible Exposure Level (PEL) is 50 ppm, 8-h time-weighted average. The effects of inhalation of ammonia range from irritation to severe respiratory injuries, possibly resulting in fatalities at higher concentrations. The National Institute of Occupational Safety and Health (NIOSH) established an Immediately Dangerous to Life and Health (IDLH) level of 300 ppm for the purposes of respirator selection. Ammonia is also flammable in
Corresponding author. Tel.: +1-39-0823-274603; fax: +1-390823-274605. E-mail address: [email protected] (M.L. Mastellone). ∗
the range of concentrations between 15 and 28% by volume in air. Historical analysis of accidental releases of ammonia from refrigeration facilities shows that a number of events resulted in both injuries and deaths to employees, which have come in contact with liquid or vapour ammonia. Moreover, when stored for use in refrigeration systems, anhydrous ammonia is liquefied under pressure: this means that it expands 850 times when released into ambient air, so forming large vapour clouds. In addition, liquid anhydrous ammonia may aerosolise if accidentally released and behave as a dense gas. This dense gas behaviour may increase the potential for exposure for workers and the public (EPA, 2000). It is necessary, for these industries, to undertake an accurate risk assessment of their own facility in order to identify all the potential accidents that could possibly occur, evaluating both their probability and their consequences. This information is assembled into a final risk assessment. In order to reduce the level of risk it is possible to improve the intrinsic safety of the plant (by means of pre-release actions) and/or to adopt adequate mitigation systems (by means of post-release actions) (Crowl and Louvar, 1990; CCPS, 1997). This paper relates to a recent case study: an important food factory using ammonia as refrigerant developed an
0950-4230/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0950-4230(02)00116-X
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accurate risk analysis. On the basis of the obtained results, the factory management decided to adopt a couple of mitigation systems in order to reduce the potential consequences of indoor and outdoor accidents. The related design procedures are defined and described in a step by step approach below.
2. Results of the risk analysis 2.1. The refrigeration facility The refrigeration unit of the factory consists of a series of plant items shown in the simplified diagram, see Fig. 1. The first item is a boiler that provides ammonia up to 150°C and 14 bar. The superheated ammonia passes though the absorption tower and a heat exchanger. The reservoir receives the liquid ammonia that is destined for the separators at different equilibrium temperatures and pressures. The ammonia reaches the different utilization points. As a consequence, a series of pipes containing ammonia at different temperatures (from 45°C to -45°C) and pressures (from 14 bar to ⫺0.5 bar) and a series of tanks where liquid ammonia is stored, are present in the facility. This very short description suggests that the core of the plant is the central services area where the boiler, the absorption tower, the reservoir of liquid ammonia and the heat exchanger are located. This area is characterized by a limited but not negligible risk index and, as a consequence, it must be more thoroughly investigated.
Fig. 1. Schematic diagram of the examined absorption-refrigeration unit.
2.2. Characterization of accident scenarios The risk analysis identified the potential accidents which have the greatest magnitude of damage, expressed as lethal injuries inside and outside the facility, which can occur as a consequence of an ammonia release. Just after the release, the ammonia becomes airborne and it is transported by the wind away from the spill site. While being transported downwind, it mixes with air and disperses. The mixture between gases and two-phase liquidvapour can be classified in three classes: positively, neutrally or negatively buoyant, based on the density difference between the released material and the air. Upon release, a rising plume is formed in the case of less dense material, while a heavier-than-air cloud usually slumps toward the ground. One of the main characteristics of a heavy cloud is that the vertical mixing is suppressed, due to the stable density stratification, while a slowly diluting vapour cloud that hugs the ground is generated. With reference to the damage which could occur outside the factory fence, a series of zones (cities, small towns, sporting centres, highways, railroads, etc.), localized in a certain radius from the point source of the worst accident, has been identified and classified on the basis of the associated damage. The classic tools of the risk analysis (dispersion models, meteorological data of the zone, probit analysis, etc.) were utilized. This identifies the areas inside the facility where the worst accident can occur, i.e. those to be controlled in order to reduce the potential outdoor damage (Crowl and Louvar, 1990). Even though the plant is largely outside, i.e. outside the buildings, it is possible that an accident can occur inside the building. In this case the related damage can be dramatic. The extent is related to the ammonia accumulation in the room volume and to the difficulty in evacuating a large number of people (up to 150 can be simultaneously present in the production hall). For example, the risk index, evaluated for a given accident and having an estimated probability, can increase from 0 up to 1.4·10⫺6 lethality/year, according to whether it occurs outside or inside. The identification of the accident scenarios has been the first step of the analysis. The second was to define and design adequate pre- and post-release mitigation systems able to reduce the risk, by lowering the accident probability or its consequence. In the actual case to which this paper refers, a reduction of the accident frequency has been obtained by introducing suitable maintenance and operating procedures; by substituting some components (relief valves, flanges, etc.) with others having a higher reliability; by extending the technical knowledge of the facility to a larger number of people and by improving the ‘risk consciousness’ of every person (from the top management to the working people) active in the facility. A reduction of the potential consequence has been obtained by introducing two mitigation
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systems: a series of automatic closing valves localised along the ammonia pipes of interest for indoor areas and a fluid curtain intercepting the ammonia cloud formed after the worst potential release.
3. Proposed mitigation systems 3.1. Mitigation of the internal risk by means of automatic valve devices The level of damage consequent to an indoor ammonia release (and related to different accidents like the releases from leaks, flanges, etc.) is proportional to the concentration of ammonia that can be reached in the building. This means that the consequence reduction can be obtained by enhancing the ventilation of the building or by lessening the scale of release. In the examined case study, the first goal was obtained by increasing the number of air changes of each building: new and more powerful fans (with a certain number of spares) were installed. The main mitigation action was, however, focused on the reduction of release entity by inserting a series of automatic valves along the pipes containing pressurized ammonia. The valves were located, when it was possible, just at the exterior of each building in order to avoid the insertion of new risk points inside it. This solution cannot be applied when the amount of ammonia segregated in the internal pipes (which depends on the pipe length and diameter, ammonia stored properties, air flow rate of the fan, etc) is however too high, i.e. able to cause, in a short time, the death of the people present in the building. In this case, the position of closing valves along the pipes has to be optimised to minimise the amount of releasable ammonia. The variation of ammonia concentration as a function of time inside a given building must be known. To this end a simple indoor model has been developed (and described in Appendix 1) and applied to several accident situations. The following important results were reached. Fig. 2 shows the time profile of ammonia concentration in the case of release resulting from a leak of equivalent diameter of 16 mm on a pipe of 52.5 mm ID, 100 m length and containing liquid ammonia at ⫺35°C and 2.5 bar. Fig. 3 reports the profile obtained, for the same scenario as Fig. 2, if the automatic closing of valves is utilised. The time at which the (automatic) valve is closed (tint) is determined by the time at which the ammonia analysers detect a specific concentration (e.g. 140 mg/m3) fixed as a set point. The time (te) necessary to completely empty the pipe downstream of the closing valve depends on the volume of ammonia entrapped in the segregated pipe (and equal to 194s in the examined scenario). A comparative analysis of the figures shows that the utilization of automatic closing valves can strongly reduce tint (from 600 s to 10 s) with a consequent sharp reduction
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of the maximum value of ammonia concentration (from about 9000 to 2500 mg/m3). But it also shows that the value of te was long enough to cause human lethality. This indicates that the automatic interception must be associated with a very fast emptying of the pipe. These considerations, together with the cited advantages related to an installation outside the buildings, lead to the device (drawn in Fig. 4) that couples a vent-holeopening valve (V2) with an automatic closing valve (V1). When V1 is activated, the valve V2 opens by allowing the rapid discharge of ammonia from the pipe to an environment under vacuum. Installing a non-return valve between the end of pipe and the final or intermediate plant items such as reservoirs, freezers, cold tunnels, etc. completed the indoor mitigation actions. This avoids a reverse flow of ammonia that could come from these apparatus as a consequence of the negative pressure gradient caused by an accidental release from a hole or a leak. 3.2. Mitigation of the outdoor risk using fluid curtains The analysis of an accident having repercussions on the outside of the factory implies the definition of a series of aspects, like the type of release (dense or less dense continuous stream, puff, evapourating pool), the type of the surrounding zone (degree of urbanisation, presence of important infrastructures, presence of sensible environmental receptors…) and the meteorological conditions. This information is important to estimate the maximum downwind distance, to choose the suitable model able to calculate the iso-concentration profiles of the cloud and to evaluate the damage level in terms of lethal injures, consequences on flora and fauna, problems caused to the infrastructures. In particular, the interception of an ammonia cloud can be obtained by using water curtain or water-air spray systems. In both cases it is possible to add a strong acid to the water in order to enhance the abatement efficiency by means of absorption with reaction (Fthenakis, 1998; Lees, 2001). In the examined case study, the following list of steps was followed: 앫 Definition of the lay-out of pipes containing ammonia, highlighting all the connected apparatus (condensers, reservoirs, freezers, pumps, etc), in order to estimate the extent of any potential release; 앫 Definition of the possible release points (flanges, connections, valves, container leaks, etc.) and the related accident scenarios, i.e. flow rate, temperature, pressure and density of release, height of release points, evolution along the time of cloud iso-concentration curves, etc.; 앫 Analysis of the effects due to the presence of physical barriers between any release point and the external
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Fig. 2. Time profile of ammonia concentration after a release in the production building (manual closing of valve with estimated time equal to 600 s).
Fig. 3. Time profile of ammonia concentration profile after the same release in the production building to which Fig. 2 refers (automatic closing valve set up when 140 mg/m3 ammonia concentration is reached).
environment. This means taking into account of a possible natural mitigation, since the ammonia can condense on building walls, but it means also to consider a possible deviation from the estimated pattern, which could make more and more difficult the interception by means of the fluid/spray barrier. The design of the barrier can be schematically designed from the following: 앫 Definition of the geometrical parameters: type of the barrier (one-fluid or two-fluid), flux direction (vertical upward or downward, horizontal, inclined), distance from the release point, width;
앫 Definition of the hydrodynamics: dilution/absorbtion factor that is necessary to reduce the ammonia concentration down to an acceptable level; outlet pressure and angle of jet/spray; 앫 Definition of the nozzle characteristics: type, total number, relative distance, and outlet pressure.
In the following, the ammonia release mitigation obtained by a fluid curtain in the case study is described. The number and type of fluid barriers depends on the number and location of areas where largest releases are expected. In the case study only a zone, the energy service area, was identified as critical, due to the high num-
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Table 1 Input and output data of the largest possible accident scenario in the energy area Pipe diameter Ammonia physical state in the pipe Leak equivalent diameter Release physical state Vapor fraction Release height Exit velocity of release Mass flow rate of release Duration of release Released amount Release temperature
Fig. 4.
Sketch of proposed system of automatic valves.
ber of plant items, the large quantity of stored ammonia and its thermodynamic conditions. The area is completely open, i.e. facilities, piping, storage vessels and any kind of apparatus are all outside. This means that it is unnecessary to consider that the concentration of ammonia in a potentially released cloud is inside the flammability range. The largest possible accident scenario was identified as the explosion of the ammonia boiler with the consequent release of the contained gas. The resulting cloud could move and enlarge, becoming a risk for the health of workers and close inhabitants. The barrier has then to be designed and located in order to obtain efficient gas absorption before the cloud size becomes too large. This worst possible accident scenario was thoroughly investigated and the related parameters are reported in Table 1. The resulting dense gas cloud can move at 1.6 m for a very long distance leading to a high level of damage. The location of the barrier and the evaluation of its distance from the release point are crucial in estimating the geometrical and physico-chemical conditions of the ammonia cloud when it hits the barrier. The cloud formed after the worst potential release had an expansion rate so high that it was able to overtake the factory boundaries in few seconds and, in the meantime, to reach a height of 30 m. In order to avoid the cloud, overcoming the boundaries, moving towards other facilities, a highway, and, eventually, a relatively close village, the fluid curtain was located along the supporting structure defining the risk zone.
100 mm 100% liquid 100 mm Dense phase (liquid–vapour equilibrium) 0.2 6m 1.9 m/s 9.26 kg/s 665 s 6160 kg ⫺33.35°C
The calculation of the water flow rate able to reduce the ammonia concentration down to the safety limit requires the knowledge of its values in the cloud at the impact front with the barrier. Output data obtained from Breeze Hazard Pro/Trinity Consultant software were elaborated to estimate these values. In particular, on the basis of data in Table 1, it was possible to obtain the concentration profile reported in Table 2 at the cloud vertical centreline and for a 6 m distance from the release source (estimated as the minimum one from the location of the barrier). A 12 m high barrier was capable of intercepting the whole cloud front. The mean value of cloud front concentration at the barrier distance, C, can be obtained from elaborating the data in Table 2 or by considering that the mass flow rate of ammonia that intercepts the water barrier is given by: W(NH3) ⫽ C·A·vcloud where A is the surface of the cloud front and vcloud is the cloud velocity. The value of mass flow is known and reported in Table 1, therefore the mean concentration can be obtained on the basis of the surface A, which, in turn, is an output of the cited software. Note that the cloud velocity is often assumed to be equal to the wind velocity. In this case, since the distance between the release point and the barrier is very small (6 m) and the structure is sheltered from the wind, the vcloud was assumed to be equal to the exit velocity of the release. The calculation of the mass flow of water that reduces Table 2 Concentration profile of ammonia cloud at 6 m from release source and at vertical centreline Height (h), m
C, mg/Nm3
1.6 5 8 12
59 400 5900 110 0
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the cloud’s ammonia concentration to a safe level is based on a procedure that involves both energy and mass balances and is reported in detail in the Appendix 2. The calculation is complicated by the presence in the cloud of both liquid and gas ammonia (the estimated vapour fraction is 0.2) that behave differently when contacting the water curtain. The utilized design procedure is:
As a consequence, the theoretical total barrier height resulted equal to 2.32 m. The same result is obtained if the analytical solution, reported in the Appendix 2 as eq. (9), is used. Fig. 5 shows the profile along the barrier of the ammonia concentration inside the water curtain, together with that of the maximum solubility at the step temperature. In order to highlight the strong dependence of the solubility on the temperature, the temperature profile is also shown in the graph. The reduction of the driving force along the barrier appears clearly. Choosing the nozzle type, the diameter and the cone angle of opening completed the design of the barrier. The number and the distance between the nozzles depend on the cone angle. The chosen nozzles were different to those used in traditional sprinkler systems: they produce flat jets of water having a large flow rate and
1. Acquisition of the series of thermodynamic data (the solubility of ammonia in water and in the ammonia– water solution at the temperature of water curtain, specific heats, mixing heat, absorption heat, etc.); 2. Calculation of the accident scenario parameters (flow rate of released ammonia, cloud buoyancy and its vapour fraction, partial pressure of ammonia at the barrier distance, mean temperature of cloud, concentration profiles as a function of the height, maximum height of cloud at curtain distance, etc); 3. Utilization of the absorption model reported in Appendix 2 (once the distance from the release source is fixed and the mean concentration of ammonia is calculated), in order to obtain the value of water flow rate able to obtain the desired mean value of absorption efficiency (that was set equal to 1). Other outputs of the model are the ammonia concentration profile along the water curtain, the corresponding temperature profile and the height of curtain. The model results obtained for the largest possible accident scenario (Table 1) are reported in Table 3, where Qw is the mass flow rate of water (plus the absorbed ammonia) that results from each calculation step (see Appendix 2); T is the temperature reached as a consequence of the released heats and of the cloud– water heat exchange; x is the ammonia concentration in the water (or, more properly, in the ammonia–water solution); xmax is the maximum concentration of ammonia that can be solubilized in the solution at the temperature T; h represents the height of the specific strip of barrier considered in the calculation step, as obtained by discretization of the mass transfer equation.
Fig. 5. Temperature and concentration profiles along the fluid curtain height (0 m is the starting level of curtain).
Table 3 Data obtained by the model calculation for 10 calculation steps and for the scenario shown in Table 1 Step No.
Qw, kg/s
X, kgNH3/m3
T, °C
xmax, kg/kgsol
h, m
1 2 3 4 5 6 7 8 9 10
73.04 73.78 74.52 75.26 76 76.74 77.48 78.22 78.96 79.7
0.013 0.025 0.036 0.047 0.058 0.068 0.078 0.087 0.096 0.104
8 12 15 19 22 24 27 29 32 34
0.210 0.196 0.182 0.168 0.155 0.144 0.134 0.124 0.115 0.105
0.01 0.01 0.02 0.02 0.02 0.03 0.04 0.05 0.10 2.02
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an opening jet angle of almost 180°. This made it possible to cover a larger area and, as a consequence, to reduce the number of nozzles. Moreover, this kind of nozzle has a diameter large enough to avoid a highpressure drop and occlusion problems. The nozzles were installed along two rows, located at two different levels, 6 and 12 m, pointing vertically in the downward direction.
4. Conclusions The design procedure of two mitigation systems adopted for an ammonia refrigeration facility of an important food company has been described. The first refers to accidents that can potentially occur inside buildings. The other refers to outdoor accidents that can lead to extremely dangerous consequences outside of the factory fence. The high level of damage consequent to ammonia release inside a building can be substantially reduced by inserting a certain number of devices formed by two automatic valves. A simple model able to estimate the ammonia concentration profile inside any indoor area as a function of time is described and used to optimise the points where the automatic devices must be located. Also, a design procedure for a fluid curtain able to intercept and absorb the ammonia cloud generated by an accident occurring in open areas has been described. The procedure utilizes a model that was specifically set up to calculate the water flow rate of a curtain able to give the required absorption efficiency. The model takes into account the two-phase feature of the ammonia cloud, as well as all the thermodynamic parameters of the cloud– water curtain system.
Acknowledgements The authors are indebted to Dr Bruno Farina for his technical co-operation during the development of the risk analysis and the designing of mitigation system as well as for his critical discussion during paper preparation. The contribution of the student Alessandro Iadevaia has been also appreciated.
Appendix 1. The algorithm for ammonia concentration evolution as a consequence of an indoor release The algorithm used to obtain the concentration-time profile has been designed by considering two simplifying hypotheses: (a) the indoor space of the building where the release occurs behaves as a Constant Stirred Tank
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Reactor; (b) there is no ammonia condensation. The obtained equation: dCNH3 ⫽ Qin Vroom NH3⫺QaspCNH3 dt
(1)
where Vroom is the building volume (m3), Qasp is the air ventilation flow rate and Qin NH3 is the flow rate of ammonia released from the leak (m3/min). Assuming the initial condition CNH3 = 0 per t = 0, the following relationship for the ammonia concentration is found b CNH3 ⫽ (1⫺e⫺at) a
(2)
Qin Qasp NH3 and b = . Vroom Vroom If the time t∗ is defined as tint + te, the total amount of ammonia released at the time t∗ is:
where a =
| 冕 t∗
t∗
⫽
WNH3
0
CNH3(t)dt
(3)
0
After a time t∗, the Qin NH3becomes equal to zero and the Eq. (1) has the following solution, valid from t = t∗ until t = ⬁:
冋
CNH3 ⫽ Ct∗ NH3exp ⫺
Qasp (t⫺t∗) Vroom
册
(4)
Appendix 2. The model for the calculation of curtain water flow rate The absorption by water, with the addition or not of a selective reactant, is an effective method of reducing the concentration of a hazardous substance like ammonia. The polarity of NH3 molecules and their ability to form hydrogen bonds explains to some extent the high solubility of ammonia in water. However, a chemical reaction also occurs when ammonia dissolves in water. In aqueous solution, ammonia acts as a base, acquiring hydrogen ions from H2O to yield ammonium and hydroxide ions. NH3 (aq) ⫹ H2O(l)⇔NH+4(aq) ⫹ OH⫺(aq) The production of hydroxide ions when ammonia dissolves in water gives to aqueous solutions of ammonia their characteristic alkaline (basic) properties. Not all of the dissolved ammonia reacts with water to form ammonium ions. A substantial fraction remains in the molecular form in solution. A quantitative indication of this strength is given by its base ionisation constant, Kb = 1.8·10⫺5. Since this value is small enough, the dissociation can be considered as negligible, except in the case when a strong acid is added to the water. The model
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presented in the following is valid for physical absorption of ammonia since it assumes no reaction term in the ammonia mass balance. The ammonia gas absorption efficiency depends on the thermodynamic parameter of the ammonia cloud + water stream system, i.e. cloud temperature, fraction of liquid ammonia, partial pressure of ammonia vapour, vapour pressure of ammonia over the solution, etc. The evaluation of water flow rate that is necessary to absorb the ammonia must take into account a series of different steps occurring along the water curtain. To this end a discrete model has been designed. It considers the mixing of liquid ammonia fraction with the water (by including the non ideality of the solution, the heat of mixing, etc.) and the absorption of the ammonia vapour phase as two processes occurring in several discrete volumes of the water curtain. The mixing in each of these volumes is good enough to be equivalent to that in a CSTR. The liquid ammonia is, in fact, totally absorbed in the water while the ammonia gas solubility changes with its partial pressure and the curtain temperature. The solubility of pure ammonia is, for example, 0.899gNH3/gH2O at 0°C and 0.533gNH3/gH2O at 20°C. The first feature to be taken into account is that when the liquid ammonia drops are mixed with the water the temperature of the non-ideal solution can lead to thermodynamic conditions under which the ammonia becomes gas. At this time, stripping of ammonia gas can occur. These considerations suggest that the mass and energy balance must take into account the following process steps: 앫 A liquid ammonia–water mixture is formed after the cloud impacts the curtain; the heat of mixing is developed. 앫 The gas ammonia–water absorption process occurs at the same time; the heat of solution is developed. 앫 The ammonia gas concentration in the water reaches the maximum equilibrium value: the absorption process ends and the stripping begins. The model requires input data of the ammonia concentration in the cloud as a function of height. It is also reasonable to consider the cloud as perfectly mixed and, as a consequence, to use a constant average value as input. The following equations refer to the scheme of Fig. 6 and represent the mass and energy balances on each discrete curtain volume. Model equations: Qi,liqNH3 ⫹ Qi,vapNH3 ⫹ Qw,i⫺1·xi⫺1 ⫽ Qw,i·xi
⫹ Qi,vapNH3·⌬Hsol ⫽ Qw,i·cpwater·Ti ⫹ Qout i,vapNH3·cpvapNH3·Ti Qw,i⫺1 ⫹ Qi,liqNH3 ⫹ Qi,vapNH3 ⫽ Qw,i
(7)
Conditions to be verified at each step i: (a) xi⬍S(Ti), where S is the ammonia solubility dependent on the temperature, over aqueous ammonia solutions (Perry & Green, 1998) and xi and Ti are the ammonia concentration and the temperature of the water curtain at the step; (b) x always belongs in the liquid area of enthalpy–concentration diagram for the non-ideal water-ammonia solution, i.e. liquid ammonia must remain liquid even if the temperature increases. The solution of the system made by eqs. (5), (6) and (7) together with external conditions (a) and (b) gives the water mass flow rate Qw. Finally, the height of curtain able to absorb the ammonia cloud is given by the solution of the following equation: d(Qw·x) ⫽ kx·(S⫺x)·aAdz which is:
冋
(8)
册
(1⫺x’n)·S Qw ·ln A·kx·a·(1⫺S) (S⫺x’n)
(5)
z⫽
(6)
where x’n is the ammonia concentration at the final step, kx·a is the product between the liquid mass transfer coefficient and the specific surface of contact that has to be experimentally determined or found in the literature
⫹ Qout i,vapNH3 Qi,liqNH3·cpliqNH3·Tcloud ⫹ Qi,vapNH3·cpvapNH3·Tcloud ⫹ Qw,i⫺1·cpwater·Ti⫺1 ⫹ Qi,liqNH3·⌬Hmix
Fig. 6. Variables involved in mass and energy balances related to the generic discrete volume of the curtain.
(9)
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(Perry and Green, 1998, Table. 23-9) and A is the area of the transversal section of fluid curtain.
References CCPS (1997). Guidelines for post-release mitigation technology in the chemical process industry. New York: American Institute of Chemical Engineers.
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Crowl, D. A., & Louvar, J. F. (1990). Chemical process safety: fundamentals with applications. New Jersey: Prentice-Hall. EPA, Chemical Emergency Preparedness and Prevention Office (2000). Report EPA-F-00-005. Fthenakis, V. M. (1998). Mitigation of ammonia aerosol releases via water spraying. In Ammonia Plant Safety and Related Facilities (pp. 155–162). New York: American Institute of Chemical Engineers. Lees, F. P. (2001). Loss prevention in the process industries. Butterworth-Heinemann ch. 15. Perry, R. H., & Green, D. W. (1998). Perry’s chemical engineers’ handbook. New York: McGraw Hill Int 7th edn.
Design of mitigation systems for indoor and outdoor ammonia releases M.L. Mastellone ∗, M. Ponte, U. Arena Dipartimento di Scienze Ambientali, Seconda Universita` di Napoli, Via Vivaldi, 43-81100 Caserta, Italy
Abstract The paper describes the design of two mitigation systems adopted by an important food company, which uses ammonia for its refrigeration plant. The results of an accurate risk analysis identified the critical risk areas and the alternative mitigation systems. Two different approaches were used for indoor and outdoor accidents. A specific model was developed to estimate the time concentration of ammonia profile after a release inside an enclosure. It was used to minimise the amount of releasable ammonia by the insertion of automatic valve devices along the pipe containing pressurized ammonia. The approach for outdoor accidents led to the design of a fluid curtain capable of intercepting and absorbing the released ammonia cloud. Another specific model, which takes into account the two-phase character of an ammonia cloud, was devised in order to calculate the water flow rate needed to ensure adequate absorption efficiency. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ammonia release; Mitigation systems; Intercepting valves; Fluid curtain
1. Introduction Anhydrous ammonia is widely used as refrigerant in many industrial facilities, e.g. food processing, dairy and ice cream plants, wineries and breweries, cold storage warehouses, etc. Each of these facilities has to take into account the risk of an ammonia release, sometimes associated with a fire or an explosion (⬍http://www.chemsafety.gov/circ/⬎). Ammonia is considered a high health hazard because it is corrosive to the skin, eyes, and lungs. The Occupational Safety and Health Administration (OSHA) Permissible Exposure Level (PEL) is 50 ppm, 8-h time-weighted average. The effects of inhalation of ammonia range from irritation to severe respiratory injuries, possibly resulting in fatalities at higher concentrations. The National Institute of Occupational Safety and Health (NIOSH) established an Immediately Dangerous to Life and Health (IDLH) level of 300 ppm for the purposes of respirator selection. Ammonia is also flammable in
Corresponding author. Tel.: +1-39-0823-274603; fax: +1-390823-274605. E-mail address: [email protected] (M.L. Mastellone). ∗
the range of concentrations between 15 and 28% by volume in air. Historical analysis of accidental releases of ammonia from refrigeration facilities shows that a number of events resulted in both injuries and deaths to employees, which have come in contact with liquid or vapour ammonia. Moreover, when stored for use in refrigeration systems, anhydrous ammonia is liquefied under pressure: this means that it expands 850 times when released into ambient air, so forming large vapour clouds. In addition, liquid anhydrous ammonia may aerosolise if accidentally released and behave as a dense gas. This dense gas behaviour may increase the potential for exposure for workers and the public (EPA, 2000). It is necessary, for these industries, to undertake an accurate risk assessment of their own facility in order to identify all the potential accidents that could possibly occur, evaluating both their probability and their consequences. This information is assembled into a final risk assessment. In order to reduce the level of risk it is possible to improve the intrinsic safety of the plant (by means of pre-release actions) and/or to adopt adequate mitigation systems (by means of post-release actions) (Crowl and Louvar, 1990; CCPS, 1997). This paper relates to a recent case study: an important food factory using ammonia as refrigerant developed an
0950-4230/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0950-4230(02)00116-X
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accurate risk analysis. On the basis of the obtained results, the factory management decided to adopt a couple of mitigation systems in order to reduce the potential consequences of indoor and outdoor accidents. The related design procedures are defined and described in a step by step approach below.
2. Results of the risk analysis 2.1. The refrigeration facility The refrigeration unit of the factory consists of a series of plant items shown in the simplified diagram, see Fig. 1. The first item is a boiler that provides ammonia up to 150°C and 14 bar. The superheated ammonia passes though the absorption tower and a heat exchanger. The reservoir receives the liquid ammonia that is destined for the separators at different equilibrium temperatures and pressures. The ammonia reaches the different utilization points. As a consequence, a series of pipes containing ammonia at different temperatures (from 45°C to -45°C) and pressures (from 14 bar to ⫺0.5 bar) and a series of tanks where liquid ammonia is stored, are present in the facility. This very short description suggests that the core of the plant is the central services area where the boiler, the absorption tower, the reservoir of liquid ammonia and the heat exchanger are located. This area is characterized by a limited but not negligible risk index and, as a consequence, it must be more thoroughly investigated.
Fig. 1. Schematic diagram of the examined absorption-refrigeration unit.
2.2. Characterization of accident scenarios The risk analysis identified the potential accidents which have the greatest magnitude of damage, expressed as lethal injuries inside and outside the facility, which can occur as a consequence of an ammonia release. Just after the release, the ammonia becomes airborne and it is transported by the wind away from the spill site. While being transported downwind, it mixes with air and disperses. The mixture between gases and two-phase liquidvapour can be classified in three classes: positively, neutrally or negatively buoyant, based on the density difference between the released material and the air. Upon release, a rising plume is formed in the case of less dense material, while a heavier-than-air cloud usually slumps toward the ground. One of the main characteristics of a heavy cloud is that the vertical mixing is suppressed, due to the stable density stratification, while a slowly diluting vapour cloud that hugs the ground is generated. With reference to the damage which could occur outside the factory fence, a series of zones (cities, small towns, sporting centres, highways, railroads, etc.), localized in a certain radius from the point source of the worst accident, has been identified and classified on the basis of the associated damage. The classic tools of the risk analysis (dispersion models, meteorological data of the zone, probit analysis, etc.) were utilized. This identifies the areas inside the facility where the worst accident can occur, i.e. those to be controlled in order to reduce the potential outdoor damage (Crowl and Louvar, 1990). Even though the plant is largely outside, i.e. outside the buildings, it is possible that an accident can occur inside the building. In this case the related damage can be dramatic. The extent is related to the ammonia accumulation in the room volume and to the difficulty in evacuating a large number of people (up to 150 can be simultaneously present in the production hall). For example, the risk index, evaluated for a given accident and having an estimated probability, can increase from 0 up to 1.4·10⫺6 lethality/year, according to whether it occurs outside or inside. The identification of the accident scenarios has been the first step of the analysis. The second was to define and design adequate pre- and post-release mitigation systems able to reduce the risk, by lowering the accident probability or its consequence. In the actual case to which this paper refers, a reduction of the accident frequency has been obtained by introducing suitable maintenance and operating procedures; by substituting some components (relief valves, flanges, etc.) with others having a higher reliability; by extending the technical knowledge of the facility to a larger number of people and by improving the ‘risk consciousness’ of every person (from the top management to the working people) active in the facility. A reduction of the potential consequence has been obtained by introducing two mitigation
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systems: a series of automatic closing valves localised along the ammonia pipes of interest for indoor areas and a fluid curtain intercepting the ammonia cloud formed after the worst potential release.
3. Proposed mitigation systems 3.1. Mitigation of the internal risk by means of automatic valve devices The level of damage consequent to an indoor ammonia release (and related to different accidents like the releases from leaks, flanges, etc.) is proportional to the concentration of ammonia that can be reached in the building. This means that the consequence reduction can be obtained by enhancing the ventilation of the building or by lessening the scale of release. In the examined case study, the first goal was obtained by increasing the number of air changes of each building: new and more powerful fans (with a certain number of spares) were installed. The main mitigation action was, however, focused on the reduction of release entity by inserting a series of automatic valves along the pipes containing pressurized ammonia. The valves were located, when it was possible, just at the exterior of each building in order to avoid the insertion of new risk points inside it. This solution cannot be applied when the amount of ammonia segregated in the internal pipes (which depends on the pipe length and diameter, ammonia stored properties, air flow rate of the fan, etc) is however too high, i.e. able to cause, in a short time, the death of the people present in the building. In this case, the position of closing valves along the pipes has to be optimised to minimise the amount of releasable ammonia. The variation of ammonia concentration as a function of time inside a given building must be known. To this end a simple indoor model has been developed (and described in Appendix 1) and applied to several accident situations. The following important results were reached. Fig. 2 shows the time profile of ammonia concentration in the case of release resulting from a leak of equivalent diameter of 16 mm on a pipe of 52.5 mm ID, 100 m length and containing liquid ammonia at ⫺35°C and 2.5 bar. Fig. 3 reports the profile obtained, for the same scenario as Fig. 2, if the automatic closing of valves is utilised. The time at which the (automatic) valve is closed (tint) is determined by the time at which the ammonia analysers detect a specific concentration (e.g. 140 mg/m3) fixed as a set point. The time (te) necessary to completely empty the pipe downstream of the closing valve depends on the volume of ammonia entrapped in the segregated pipe (and equal to 194s in the examined scenario). A comparative analysis of the figures shows that the utilization of automatic closing valves can strongly reduce tint (from 600 s to 10 s) with a consequent sharp reduction
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of the maximum value of ammonia concentration (from about 9000 to 2500 mg/m3). But it also shows that the value of te was long enough to cause human lethality. This indicates that the automatic interception must be associated with a very fast emptying of the pipe. These considerations, together with the cited advantages related to an installation outside the buildings, lead to the device (drawn in Fig. 4) that couples a vent-holeopening valve (V2) with an automatic closing valve (V1). When V1 is activated, the valve V2 opens by allowing the rapid discharge of ammonia from the pipe to an environment under vacuum. Installing a non-return valve between the end of pipe and the final or intermediate plant items such as reservoirs, freezers, cold tunnels, etc. completed the indoor mitigation actions. This avoids a reverse flow of ammonia that could come from these apparatus as a consequence of the negative pressure gradient caused by an accidental release from a hole or a leak. 3.2. Mitigation of the outdoor risk using fluid curtains The analysis of an accident having repercussions on the outside of the factory implies the definition of a series of aspects, like the type of release (dense or less dense continuous stream, puff, evapourating pool), the type of the surrounding zone (degree of urbanisation, presence of important infrastructures, presence of sensible environmental receptors…) and the meteorological conditions. This information is important to estimate the maximum downwind distance, to choose the suitable model able to calculate the iso-concentration profiles of the cloud and to evaluate the damage level in terms of lethal injures, consequences on flora and fauna, problems caused to the infrastructures. In particular, the interception of an ammonia cloud can be obtained by using water curtain or water-air spray systems. In both cases it is possible to add a strong acid to the water in order to enhance the abatement efficiency by means of absorption with reaction (Fthenakis, 1998; Lees, 2001). In the examined case study, the following list of steps was followed: 앫 Definition of the lay-out of pipes containing ammonia, highlighting all the connected apparatus (condensers, reservoirs, freezers, pumps, etc), in order to estimate the extent of any potential release; 앫 Definition of the possible release points (flanges, connections, valves, container leaks, etc.) and the related accident scenarios, i.e. flow rate, temperature, pressure and density of release, height of release points, evolution along the time of cloud iso-concentration curves, etc.; 앫 Analysis of the effects due to the presence of physical barriers between any release point and the external
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Fig. 2. Time profile of ammonia concentration after a release in the production building (manual closing of valve with estimated time equal to 600 s).
Fig. 3. Time profile of ammonia concentration profile after the same release in the production building to which Fig. 2 refers (automatic closing valve set up when 140 mg/m3 ammonia concentration is reached).
environment. This means taking into account of a possible natural mitigation, since the ammonia can condense on building walls, but it means also to consider a possible deviation from the estimated pattern, which could make more and more difficult the interception by means of the fluid/spray barrier. The design of the barrier can be schematically designed from the following: 앫 Definition of the geometrical parameters: type of the barrier (one-fluid or two-fluid), flux direction (vertical upward or downward, horizontal, inclined), distance from the release point, width;
앫 Definition of the hydrodynamics: dilution/absorbtion factor that is necessary to reduce the ammonia concentration down to an acceptable level; outlet pressure and angle of jet/spray; 앫 Definition of the nozzle characteristics: type, total number, relative distance, and outlet pressure.
In the following, the ammonia release mitigation obtained by a fluid curtain in the case study is described. The number and type of fluid barriers depends on the number and location of areas where largest releases are expected. In the case study only a zone, the energy service area, was identified as critical, due to the high num-
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Table 1 Input and output data of the largest possible accident scenario in the energy area Pipe diameter Ammonia physical state in the pipe Leak equivalent diameter Release physical state Vapor fraction Release height Exit velocity of release Mass flow rate of release Duration of release Released amount Release temperature
Fig. 4.
Sketch of proposed system of automatic valves.
ber of plant items, the large quantity of stored ammonia and its thermodynamic conditions. The area is completely open, i.e. facilities, piping, storage vessels and any kind of apparatus are all outside. This means that it is unnecessary to consider that the concentration of ammonia in a potentially released cloud is inside the flammability range. The largest possible accident scenario was identified as the explosion of the ammonia boiler with the consequent release of the contained gas. The resulting cloud could move and enlarge, becoming a risk for the health of workers and close inhabitants. The barrier has then to be designed and located in order to obtain efficient gas absorption before the cloud size becomes too large. This worst possible accident scenario was thoroughly investigated and the related parameters are reported in Table 1. The resulting dense gas cloud can move at 1.6 m for a very long distance leading to a high level of damage. The location of the barrier and the evaluation of its distance from the release point are crucial in estimating the geometrical and physico-chemical conditions of the ammonia cloud when it hits the barrier. The cloud formed after the worst potential release had an expansion rate so high that it was able to overtake the factory boundaries in few seconds and, in the meantime, to reach a height of 30 m. In order to avoid the cloud, overcoming the boundaries, moving towards other facilities, a highway, and, eventually, a relatively close village, the fluid curtain was located along the supporting structure defining the risk zone.
100 mm 100% liquid 100 mm Dense phase (liquid–vapour equilibrium) 0.2 6m 1.9 m/s 9.26 kg/s 665 s 6160 kg ⫺33.35°C
The calculation of the water flow rate able to reduce the ammonia concentration down to the safety limit requires the knowledge of its values in the cloud at the impact front with the barrier. Output data obtained from Breeze Hazard Pro/Trinity Consultant software were elaborated to estimate these values. In particular, on the basis of data in Table 1, it was possible to obtain the concentration profile reported in Table 2 at the cloud vertical centreline and for a 6 m distance from the release source (estimated as the minimum one from the location of the barrier). A 12 m high barrier was capable of intercepting the whole cloud front. The mean value of cloud front concentration at the barrier distance, C, can be obtained from elaborating the data in Table 2 or by considering that the mass flow rate of ammonia that intercepts the water barrier is given by: W(NH3) ⫽ C·A·vcloud where A is the surface of the cloud front and vcloud is the cloud velocity. The value of mass flow is known and reported in Table 1, therefore the mean concentration can be obtained on the basis of the surface A, which, in turn, is an output of the cited software. Note that the cloud velocity is often assumed to be equal to the wind velocity. In this case, since the distance between the release point and the barrier is very small (6 m) and the structure is sheltered from the wind, the vcloud was assumed to be equal to the exit velocity of the release. The calculation of the mass flow of water that reduces Table 2 Concentration profile of ammonia cloud at 6 m from release source and at vertical centreline Height (h), m
C, mg/Nm3
1.6 5 8 12
59 400 5900 110 0
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the cloud’s ammonia concentration to a safe level is based on a procedure that involves both energy and mass balances and is reported in detail in the Appendix 2. The calculation is complicated by the presence in the cloud of both liquid and gas ammonia (the estimated vapour fraction is 0.2) that behave differently when contacting the water curtain. The utilized design procedure is:
As a consequence, the theoretical total barrier height resulted equal to 2.32 m. The same result is obtained if the analytical solution, reported in the Appendix 2 as eq. (9), is used. Fig. 5 shows the profile along the barrier of the ammonia concentration inside the water curtain, together with that of the maximum solubility at the step temperature. In order to highlight the strong dependence of the solubility on the temperature, the temperature profile is also shown in the graph. The reduction of the driving force along the barrier appears clearly. Choosing the nozzle type, the diameter and the cone angle of opening completed the design of the barrier. The number and the distance between the nozzles depend on the cone angle. The chosen nozzles were different to those used in traditional sprinkler systems: they produce flat jets of water having a large flow rate and
1. Acquisition of the series of thermodynamic data (the solubility of ammonia in water and in the ammonia– water solution at the temperature of water curtain, specific heats, mixing heat, absorption heat, etc.); 2. Calculation of the accident scenario parameters (flow rate of released ammonia, cloud buoyancy and its vapour fraction, partial pressure of ammonia at the barrier distance, mean temperature of cloud, concentration profiles as a function of the height, maximum height of cloud at curtain distance, etc); 3. Utilization of the absorption model reported in Appendix 2 (once the distance from the release source is fixed and the mean concentration of ammonia is calculated), in order to obtain the value of water flow rate able to obtain the desired mean value of absorption efficiency (that was set equal to 1). Other outputs of the model are the ammonia concentration profile along the water curtain, the corresponding temperature profile and the height of curtain. The model results obtained for the largest possible accident scenario (Table 1) are reported in Table 3, where Qw is the mass flow rate of water (plus the absorbed ammonia) that results from each calculation step (see Appendix 2); T is the temperature reached as a consequence of the released heats and of the cloud– water heat exchange; x is the ammonia concentration in the water (or, more properly, in the ammonia–water solution); xmax is the maximum concentration of ammonia that can be solubilized in the solution at the temperature T; h represents the height of the specific strip of barrier considered in the calculation step, as obtained by discretization of the mass transfer equation.
Fig. 5. Temperature and concentration profiles along the fluid curtain height (0 m is the starting level of curtain).
Table 3 Data obtained by the model calculation for 10 calculation steps and for the scenario shown in Table 1 Step No.
Qw, kg/s
X, kgNH3/m3
T, °C
xmax, kg/kgsol
h, m
1 2 3 4 5 6 7 8 9 10
73.04 73.78 74.52 75.26 76 76.74 77.48 78.22 78.96 79.7
0.013 0.025 0.036 0.047 0.058 0.068 0.078 0.087 0.096 0.104
8 12 15 19 22 24 27 29 32 34
0.210 0.196 0.182 0.168 0.155 0.144 0.134 0.124 0.115 0.105
0.01 0.01 0.02 0.02 0.02 0.03 0.04 0.05 0.10 2.02
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an opening jet angle of almost 180°. This made it possible to cover a larger area and, as a consequence, to reduce the number of nozzles. Moreover, this kind of nozzle has a diameter large enough to avoid a highpressure drop and occlusion problems. The nozzles were installed along two rows, located at two different levels, 6 and 12 m, pointing vertically in the downward direction.
4. Conclusions The design procedure of two mitigation systems adopted for an ammonia refrigeration facility of an important food company has been described. The first refers to accidents that can potentially occur inside buildings. The other refers to outdoor accidents that can lead to extremely dangerous consequences outside of the factory fence. The high level of damage consequent to ammonia release inside a building can be substantially reduced by inserting a certain number of devices formed by two automatic valves. A simple model able to estimate the ammonia concentration profile inside any indoor area as a function of time is described and used to optimise the points where the automatic devices must be located. Also, a design procedure for a fluid curtain able to intercept and absorb the ammonia cloud generated by an accident occurring in open areas has been described. The procedure utilizes a model that was specifically set up to calculate the water flow rate of a curtain able to give the required absorption efficiency. The model takes into account the two-phase feature of the ammonia cloud, as well as all the thermodynamic parameters of the cloud– water curtain system.
Acknowledgements The authors are indebted to Dr Bruno Farina for his technical co-operation during the development of the risk analysis and the designing of mitigation system as well as for his critical discussion during paper preparation. The contribution of the student Alessandro Iadevaia has been also appreciated.
Appendix 1. The algorithm for ammonia concentration evolution as a consequence of an indoor release The algorithm used to obtain the concentration-time profile has been designed by considering two simplifying hypotheses: (a) the indoor space of the building where the release occurs behaves as a Constant Stirred Tank
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Reactor; (b) there is no ammonia condensation. The obtained equation: dCNH3 ⫽ Qin Vroom NH3⫺QaspCNH3 dt
(1)
where Vroom is the building volume (m3), Qasp is the air ventilation flow rate and Qin NH3 is the flow rate of ammonia released from the leak (m3/min). Assuming the initial condition CNH3 = 0 per t = 0, the following relationship for the ammonia concentration is found b CNH3 ⫽ (1⫺e⫺at) a
(2)
Qin Qasp NH3 and b = . Vroom Vroom If the time t∗ is defined as tint + te, the total amount of ammonia released at the time t∗ is:
where a =
| 冕 t∗
t∗
⫽
WNH3
0
CNH3(t)dt
(3)
0
After a time t∗, the Qin NH3becomes equal to zero and the Eq. (1) has the following solution, valid from t = t∗ until t = ⬁:
冋
CNH3 ⫽ Ct∗ NH3exp ⫺
Qasp (t⫺t∗) Vroom
册
(4)
Appendix 2. The model for the calculation of curtain water flow rate The absorption by water, with the addition or not of a selective reactant, is an effective method of reducing the concentration of a hazardous substance like ammonia. The polarity of NH3 molecules and their ability to form hydrogen bonds explains to some extent the high solubility of ammonia in water. However, a chemical reaction also occurs when ammonia dissolves in water. In aqueous solution, ammonia acts as a base, acquiring hydrogen ions from H2O to yield ammonium and hydroxide ions. NH3 (aq) ⫹ H2O(l)⇔NH+4(aq) ⫹ OH⫺(aq) The production of hydroxide ions when ammonia dissolves in water gives to aqueous solutions of ammonia their characteristic alkaline (basic) properties. Not all of the dissolved ammonia reacts with water to form ammonium ions. A substantial fraction remains in the molecular form in solution. A quantitative indication of this strength is given by its base ionisation constant, Kb = 1.8·10⫺5. Since this value is small enough, the dissociation can be considered as negligible, except in the case when a strong acid is added to the water. The model
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presented in the following is valid for physical absorption of ammonia since it assumes no reaction term in the ammonia mass balance. The ammonia gas absorption efficiency depends on the thermodynamic parameter of the ammonia cloud + water stream system, i.e. cloud temperature, fraction of liquid ammonia, partial pressure of ammonia vapour, vapour pressure of ammonia over the solution, etc. The evaluation of water flow rate that is necessary to absorb the ammonia must take into account a series of different steps occurring along the water curtain. To this end a discrete model has been designed. It considers the mixing of liquid ammonia fraction with the water (by including the non ideality of the solution, the heat of mixing, etc.) and the absorption of the ammonia vapour phase as two processes occurring in several discrete volumes of the water curtain. The mixing in each of these volumes is good enough to be equivalent to that in a CSTR. The liquid ammonia is, in fact, totally absorbed in the water while the ammonia gas solubility changes with its partial pressure and the curtain temperature. The solubility of pure ammonia is, for example, 0.899gNH3/gH2O at 0°C and 0.533gNH3/gH2O at 20°C. The first feature to be taken into account is that when the liquid ammonia drops are mixed with the water the temperature of the non-ideal solution can lead to thermodynamic conditions under which the ammonia becomes gas. At this time, stripping of ammonia gas can occur. These considerations suggest that the mass and energy balance must take into account the following process steps: 앫 A liquid ammonia–water mixture is formed after the cloud impacts the curtain; the heat of mixing is developed. 앫 The gas ammonia–water absorption process occurs at the same time; the heat of solution is developed. 앫 The ammonia gas concentration in the water reaches the maximum equilibrium value: the absorption process ends and the stripping begins. The model requires input data of the ammonia concentration in the cloud as a function of height. It is also reasonable to consider the cloud as perfectly mixed and, as a consequence, to use a constant average value as input. The following equations refer to the scheme of Fig. 6 and represent the mass and energy balances on each discrete curtain volume. Model equations: Qi,liqNH3 ⫹ Qi,vapNH3 ⫹ Qw,i⫺1·xi⫺1 ⫽ Qw,i·xi
⫹ Qi,vapNH3·⌬Hsol ⫽ Qw,i·cpwater·Ti ⫹ Qout i,vapNH3·cpvapNH3·Ti Qw,i⫺1 ⫹ Qi,liqNH3 ⫹ Qi,vapNH3 ⫽ Qw,i
(7)
Conditions to be verified at each step i: (a) xi⬍S(Ti), where S is the ammonia solubility dependent on the temperature, over aqueous ammonia solutions (Perry & Green, 1998) and xi and Ti are the ammonia concentration and the temperature of the water curtain at the step; (b) x always belongs in the liquid area of enthalpy–concentration diagram for the non-ideal water-ammonia solution, i.e. liquid ammonia must remain liquid even if the temperature increases. The solution of the system made by eqs. (5), (6) and (7) together with external conditions (a) and (b) gives the water mass flow rate Qw. Finally, the height of curtain able to absorb the ammonia cloud is given by the solution of the following equation: d(Qw·x) ⫽ kx·(S⫺x)·aAdz which is:
冋
(8)
册
(1⫺x’n)·S Qw ·ln A·kx·a·(1⫺S) (S⫺x’n)
(5)
z⫽
(6)
where x’n is the ammonia concentration at the final step, kx·a is the product between the liquid mass transfer coefficient and the specific surface of contact that has to be experimentally determined or found in the literature
⫹ Qout i,vapNH3 Qi,liqNH3·cpliqNH3·Tcloud ⫹ Qi,vapNH3·cpvapNH3·Tcloud ⫹ Qw,i⫺1·cpwater·Ti⫺1 ⫹ Qi,liqNH3·⌬Hmix
Fig. 6. Variables involved in mass and energy balances related to the generic discrete volume of the curtain.
(9)
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(Perry and Green, 1998, Table. 23-9) and A is the area of the transversal section of fluid curtain.
References CCPS (1997). Guidelines for post-release mitigation technology in the chemical process industry. New York: American Institute of Chemical Engineers.
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Crowl, D. A., & Louvar, J. F. (1990). Chemical process safety: fundamentals with applications. New Jersey: Prentice-Hall. EPA, Chemical Emergency Preparedness and Prevention Office (2000). Report EPA-F-00-005. Fthenakis, V. M. (1998). Mitigation of ammonia aerosol releases via water spraying. In Ammonia Plant Safety and Related Facilities (pp. 155–162). New York: American Institute of Chemical Engineers. Lees, F. P. (2001). Loss prevention in the process industries. Butterworth-Heinemann ch. 15. Perry, R. H., & Green, D. W. (1998). Perry’s chemical engineers’ handbook. New York: McGraw Hill Int 7th edn.