Fate of Liquid Ammonia Spilled onto Water Phani P. K. Raj" Arthur D. Little, Inc., Cambridge, Mass. 02140
Robert C. Reid Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass. 02 139
Anhydrous ammonia is commonly transported in insulated tankers as a bulk liquid cargo at its normal boiling point of 239 K. Should an accident lead to a release of this volatile liquid into water, it is important to estimate the degree of water contamination and the extent of the vapor cloud. In an experimental program with spills of liquid ammonia onto water (maximum size -0.19 m3),the data indicated that about 70% of the spilled ammonia entered the water phase to form a dilute NH40H solution. These results were independent of spill size, method of spill, and water temperature or salinity. A simple thermodynamic model was developed, and the predictions from this model are in excellent agreement with experimental results.
Anhydrous liquid ammonia (LNH3) is frequently transported in bulk on inland or coastal waters. In some cases, the LNH3 is carried as an ambient temperature, high-pressure cargo, but it is becoming common to employ insulated tanks and store the LNH3 a t about one bar where its boiling point is approximately 239 K. Accidents involving the release of this volatile liquid would pose serious health and pollution hazards. The focus of the present paper relates, therefore, to the behavior of atmospheric LNH:I spilled on water. Of particular interest are experimental data and reliable analytical models to indicate the fraction of the spilled LNH3 which dissolves in the water, and, by difference, the fraction vaporized.
Hazards Involved in Liquid Ammonia Spills The toxicity of ammonia is well known. As a vapor the TLV (threshold limit value) is only 50 ppm, although most humans can detect the presence of this material around 20 ppm ( I ) . Thirty-minute exposures to concentrations near 0.5% can be lethal or result in serious injury. Dissolution of LNH3 in water is accompanied by an exothermic process and a concomitant increase in pH. The ammonium hydroxide solutions are destructive to flora and fauna, and the water is also unsafe for human consumption. Less serious is the fire hazard of an ammonia vapor cloud. Although it is flammable in concentrations between 15 and 28%, the ignition temperature is relatively high (1100 K). Ignition tests with LNH3 in an open pan resulted in brief flames but no sustained fire (2). Ammonia flames are also nonluminous with low radiation fluxes. Detonations of ammonia-air mixtures are apparently limited to cases where the gases are confined ( 2 ) . Previous Studies No experiments appear to have been carried out to study the interaction between LNH3 and water. Ball ( 3 ) has reviewed the problems associated with the storage of liquid ammonia and has discussed the dispersion characteristics of ammonia vapor clouds, while Resplandy ( 4 ) conducted experimental spill tests with both pressurized and refrigerated liquid ammonia into earthen dikes. Experimental Program A three-level experimental program was carried out. The 1422
Environmental Science & Technology
principal result of interest was to measure the partition coefficient, Le., the fraction of the spilled LNH3 which dissolved in the water. Laboratory Tests. A small (1.8 m X 0.5 m) glass aquarium tank was used. The tank was about 50 cm high and contained some 25 cm of water. Thermocouples were inserted a t a number of locations and high-speed cameras recorded the events following a spill. LNH3 spills up to 2500 cm3were made. After completion of the tests, the water was recirculated and mixed; titration with acid then allowed one to determine the total dissolved ammonia. Spill rates were varied from 50 to 100 cm3/s with the spill tube both tangential and vertical to the water surface. Both sea water and distilled water were used, and initial water temperatures ranged between 276 and 307 K. In some cases, baffles were set in the tank; in others, the water was agitated. Phenolphthalein dye was normally added to the water to indicate the extent of the dissolution zone. The results of this initial phase of the test program are shown in Figure 1. With few exceptions, partition coefficients were between 0.65 and 0.80 and did not appear to correlate with any of the test variables. High-speed movies showed that the LNH3 evaporated in a rather small area (maximum diameter of 20 cm) and a front of warm, NH40H-rich liquid propagated from this boiling zone. The layer rarely exceeded 3-4 cm in thickness. Intermediate-Size Tests. Up to 0.02-m3 spills of LNH3 were made onto a 6-m diameter pool filled with water to a depth of about 0.6 m. The pool surface was a t grade. The fraction of the ammonia which dissolved was again determined by titrating the water a t the end of the experiment. Before samples were taken, however, the pool was well mixed. Also, an independent check was obtained from vapor concentration measurements made in the downwind plume. Small suction flasks were used and the ammonia absorbed in boric acid solutions. Spill rates were varied from essentially instantaneous to continuous (0.02 m3 in 5 min). The partition coefficients calculated both from the water analysis and from the vapor cloud measurements agreed quite well. For simple instantaneous spills, values of about 0.6 were obtained, whereas for continuous spills this coefficient increased to 0.66-0.7. The latter results are quite close to those obtained in the laboratory tests. The difference may have been due to some aerosol or ammonia-fog formation in the rapid spills. The fog could not, however, have been too important as visual observations as well as vapor cloud data indicated that the ammonia vapor was less dense than the ambient air and the plume rose while moving downwind. (The density of saturated ammonia vapor a t 1 bar is about 0.89 kg mP3, whereas, for air a t 300 K, the density is near 1.2 kg m-3.) The agreement between the water analysis and cloud concentrations provided confidence that, in the larger scale tests noted below, only vapor-cloud measurements were necessary to estimate partition coefficients. Large-Size Tests. Spills of LNH3 up to about 0.19 m3 were made on the surface of a natural lake. No water sampling was attempted. The dissolution of the ammonia was inferred from extensive measurements of the total vapor dose downwind at various lateral and vertical positions. Evaluations of partition coefficients were less precise in these tests, but they varied only from 0.5 to 0.7 for most of the 0013-936X/78/0912-1422$01,00/0 @ 1978 American Chemical Society
O
M
0 Y
0
Key:
0 0
J
0 Rate= 50cm3 s = 1OOcm Is 3 Open : tangential spill Filled : vertical spill
, ,,
--
: 3.5%NaCl : baffled t a n k : agitated tank
(
: water temp ( K )
-1-
\I
)
different from 293 K
0.4 0
I
I
400
800
I
1600
1200
experiments and were not significantly affected by the rapidity of the spill. While the three test phases produced other valuable data (e.g., dispersion characteristics of ammonia-vapor clouds), the principal result of interest in this paper was the unexpected constancy of the partition coefficient in all tests, Le., normally from 60 to 70% of the LNH3 spilled remained in the water. This percentage was not significantly affected by any of the experimental variables such as mode of spill, salinity, water depth, spill size, etc. A report describing the experimental program and results is available elsewhere ( 5 ) .
Analytical Models Several thermodynamic and rate models were developed in an attempt to describe the interaction between liquid ammonia and water ( 5 ) .The one described below was chosen as it predicts in a logical fashion that the partition coefficient should be constant and on the order of 0.73, a value which is quite close to that measured experimentally. The physical picture associated with the model considers the spilled LNHBto mix, in an incremental fashion, with the bulk of the water. After each incremental mixing, the LNH3 and water are allowed to equilibrate so as to produce some vapor and a saturated NH40H solution at one bar. Consider Figure 2. This is an enthalpy-concentration diagram for the ammonia-water system at 1 bar (6).The reference enthalpy for both water and liquid ammonia is H = 0 at 273.2 K. Suppose a small quantity of water at 293 K (H= 83.7 kJ/kg) were mixed with the ammonia. Some vapor would be evolved (essentially pure ammonia, Ii = 1217 kJ/kg) and the resultant liquid would lie on the 1-bar curve. A new increment of water is then added to the liquid mixture and the calculations repeated. It is important to note that the enthalpy-concentration locus of the liquid phase follows the 1-bar saturation curve. Until the weight fraction ammonia in the liquid drops below about 40%, it is an excellent approximation to assume that any vapor evolved is pure ammonia.
2000
400
2400
Sarurated Liquid
One Bar
200 I
-2 5
.
z m -y u1 c
0
-200
-400
0.2
0.4
0.6
0.8
1.0
Weight Fraction Ammonia - X N
Figure 2. Ammonia-water liquid enthalpy
A thermodynamic analysis of the mixing process wherein dNwL of liquid water are added to the LNH3 and dNNV of ammonia are vaporized yields: d H = H,dNwL - HNVdNNV
(1)
Volume 12, Number 13, December 1978 1423
100
- 90 I r n.
VI
-80
2 gs a25
..-mC
E
a
-70
1
10
0
20
30
50
40
60
70
80
EE
a
.60
90
Water Added (kg)
Figure 3. Sample results for a 100-kg spill of liquid ammonia on 294 K water
where H is the total solution enthalpy, H w is the specific enthalpy of the added water, and HN”is the specific enthalpy of the ammonia vapor. The phase rule dictates that for a two-component, twophase system in equilibrium, any extensive property is a function of three other variables-at least one of which must be extensive. Choosing P, NwL, and N Nas~these independent variables (the superscript Lrepresents the liquid phase):
H
=
f(P,NwL, NNL)
(2)
Then by differentiation, a t constant pressure: d H = (dH/dNwL)p,N,dNWL
+ ( dH/dNNL)p,NWd””
(3)
The partial derivatives in Equation 3 also contain the restriction that the liquid phase is in equilibrium with the vapor. Such derivatives are, in fact, modified partial molar enthalpies and we will designate them as: (dHIdNwL)p,NN= Rw‘ (dHIdNNL)p,Nw= EN’
(4)
Euler integration (7) of Equation 2 will show that Rw’ and
RN’are the intercepts of any tangent drawn to the 1-bar liquid saturation curve in Figure 2; a t any given ammonia weight fraction Rw‘ is read on the left-hand ordinate ( X N = 0) and RN‘on the right-hand ordinate ( X N = 1.0). Combining Equations 1, 3, and 4, and using the relation (d”’ dNNL) = 0, yields:
+
-d””
_ - (Rw’ - Hw) (5) dNwL (HN’ - RN’) Values of z f ~ and ’ Rr;’were found by approximating the saturation liquid curve in Figure 2 by an analytical equation that expressed the solution enthalpy, H, as a function of composition. Equation 5 was then integrated numerically to obtain NN’ as a function of the quantity of water added, NWL.
To illustrate a typical result, a 100-kg spill of LNH3 is assumed. As water mixes with the liquid ammonia, vapor is evolved. In Figure 3, both the mass of ammonia evolved and that in solution are plotted as a function of the water added. The important conclusion from Figure 3 is that no more am1424
EnvironmentalScience & Technology
monia is evolved after about 86 kg of water has been added. A t this time there remains almost 71.5 kg of dissolved ammonia (weight fraction ammonia -0.45). This behavior becomes clear when Equation 5 is reexamined. As the weight fraction ammonia decreases, Rw‘ increases. When Rw’ equals Hw (-83.7 kJ/kg), then the numerator in Equation 5 becomes zero and, after this point, further addition of water simply dilutes further the ammonium hydroxide solution with no additional vapor evolution. The partition coefficient found in the example calculation (Figure 3) is 0.715, a value quite close to that measured experimentally. Varying the water temperature f10-20 K will affect only slightly the calculated partition coefficient. Other models were also investigated, e.g., one could add “increments” of liquid ammonia to water, but this technique will not lead to a limiting value of the partition coefficient. The model as proposed may also be of possible value for spills of other volatile liquids (e.g., chlorine, S02, NO2) on water. We wish to note that in a large accidental spill of anhydrous ammonia onto water, the dynamics of the spill may play a significant role in determining the fraction of the spilled liquid that is ultimately released into the atmosphere. For example, an instantaneous release of LNH3 onto the water surface results in violent boiling of the LNH3 and this may cause the aerosols of LNH3 to be generated and released into the atmosphere. In such a case, the effective partition coefficient is smaller than that predicted by the thermodynamic analysis. Thermodynamic analysis presented in this paper gives the upper bound for the partition coefficient, Le., a lower bound for the amount of vapor liberated in a spill. Acknowledgment The authors wish to thank the U S . Coast Guard for their support. Nomenclature H = total solution enthalpy, J/kmol HN” = molar enthalpy of ammonia vapor, J/kmol RN’ = modified partial molar enthalpy of ammonia, J/ kmol H w = molar enthalpy of water, J/kmol
Rw’
= modified partial molar enthalpy of water (Equation 41, J/kmol N N L = moles of liquid ammonia, kmol NN” = moles of vapor ammonia, kmol NwL = moles of liquid water, kmol P = total pressure, N/m2 X N = mass fraction of ammonia in ammonia-water aqueous solution
Literature Cited (1) Data Sheet 251A, National Safety Council, Chicago, Ill., 1970. (2) Husa, H. W., Buckley, W. L., Chem. Eng. Prog., 58, (21,814 (Feb
1962).
(3) Ball, W. L., AIChE Ammonia Plant S a f e t y Manual, V (12), 1 (1970). (4) Resplandy, A., Chem. Ind. Gen. Chem., 102,691 (1969). ( 5 ) Raj, P. K., Hagopian, J., Kalelkar, A. S., “Prediction of Hazards of Spills of Anhydrous Ammonia on Water”, U S . Coast Guard Report CG-D-74-74, J a n 1974, NTIS No. AD779400. (6) Research Bulletin No. 34, “Physical and Thermodynamic Properties of Ammonia-Water Mixtures”, Inst. Gas Technology, Chicago, Ill., Sept 1964. (7) Modell, M., Reid, R. C., “Thermodynamics and Its Applications”, Prentice-Hall, Englewood Cliffs, N.J., 1974, Chapter 8. Received for reuiew April 17, 1978. Accepted July 18, 1978. T h e material presented in this paper formed a part of the work performed by Arthur D. Little, Inc., under Contract DOT-CG-22,182-A with the U.S. Coast Guard.
Photodegradation of 3,3’-Dichlorobenzidine Sujit Banerjee”, Harish C. Sikka”, Richard Gray, and Christine M. Kelly
Life Sciences Division, Syracuse Research Corporation, Merrill Lane, University Heights, Syracuse, N.Y. 13210 3,3’-Dichlorobenzidine (DCB) is very rapidly photodegraded in aqueous solution to give monochlorobenzidine (MCB), benzidine, and a number of brightly colored waterinsoluble materials. Disappearance quantum yields for DCB and MCB are high, whereas that for benzidine is much lower. Transients are generated upon photolysis of DCB in acidic solutions or on treatment of DCB with chlorine water. The photolability of DCB is markedly lower in nonaqueous solvents, and the mechanism of dechlorination does not appear to involve simple carbon-chlorine bond homolysis. DCB ( l ) ,a widely used pigment intermediate, is known to induce cancer in animals ( I ) , and is regarded by the Occupational Health and Safety Administration as being hazardous to human health ( 2 ) .The material may be introduced to the aquatic environment through effluent discharge from manufacturing plants for the chemical and also from DCB pigment wastes containing unreacted DCB. Consequently, adverse health effects might result from the contamination of drinking water supplies or through bioaccumulation in fish which are subsequently used as food. In a preliminary study on several aspects of the environmental degradation of DCB, we found the material to be extremely photolabile ( 3 ) .We now report more complete data on the photodegradation of DCB and 3-chlorobenzidine (2), and assess the environmental importance of these photoprocesses.
Prior to the initiation of our studies, the maximum water solubility of DCB.2HCl was measured spectrophotometrically in a buffered solution of pH 6.9 and was found to be 3.99 ppm at 22OC. In this medium, A,, (emax) for DCB.2HC1 occurs a t 282 (28 200) and a t 211 nm (57 800). Quantification of the benzidines was carried out on a Waters M6000A HPLC utilizing a pCls Bondapack column and a 1:l mixture of acetonitrile and 5% aqueous acetic acid as the mobile phase. Under these conditions, the retention volumes of DCB, MCB, and benzidine were 11.8,4.7, and 3.7 mL, respectively. Increase of the acetonitrile content of the mobile phase led to a lower retention volume for DCB and an increase in that for MCB. All samples were diluted with acetonitrile before filtration and introduction to the HPLC, in order to minimize sample adsorption onto the filter. The identity of the photoproducts was confirmed by GC, using an H P 5730A FID instrument and a 10% UCW 932 on Chromosorb W column maintained a t 225 “C, and by mass spectrometry. Preparative photolyses were conducted in a 1-L immersion well type photoreactor equipped with a 450-W high-pressure Hanovia lamp fitted with a Pyrex filter. Quantum yields were determined in a Rayonet RMR-400 merry-go-round photoreactor a t 2537 or 3000 A.
Results and Discussion Preparative Photolysis. Irradiation of aqueous solutions of DCB results in a hypsochromic shift of the UV absorption band of the substrate, and is accompanied by about a 50% decrease in intensity. Extraction of the photolyzed solution by ether after basification, and analysis of the ether concentrate by HPLC, GC, and mass spectrometry revealed the presence of benzidine (3) and MCB. In a separate experiment, aliquots were withdrawn periodically and analyzed for DCB, MCB, and benzidine by HPLC. These results are listed in Table I and suggest that DCB is, in part, degraded sequentially to MCB and benzidine. In addition, DCB is also photodegraded to a number of relatively water-insoluble products which adhere to the walls of the photoreactors. These materials are ether soluble, and TLC (silica gel, ether-hexane, 2:l) of an ethereal concentrate resolved the products into more than five brightly colored components. The extent of conversion to these water-insoluble materials was determined through the photolysis of 14C-labeled DCB. Aliquots of the aqueous solut,ion were counted periodically, and after 30 min,
H2N+p-@NHz X1
1.
x,
2
X, = C I . X z = H
=X,=CI
3 . X1 = X z = H
Experimental DCB, as the dihydrochloride, was kindly provided by the Upjohn Company, and MCB was synthesized according to Branch et al. ( 4 ) . I4C-Labeled DCB was obtained from the California Bionuclear Corp. 0013-936X/78/0912-1425$01.00/0
@ 1978 American Chemical Society
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