Envlron. Scl. Technol. 1004, 18, 793-796
Behavior of Metal Mercury in Gases Yoshlo Otanl,' Hltoshl Eml, Chlkao Kanaoka, and Saburo Matsuit
Department of Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa, Japan 920 The state of existence of mercury-whether mercury exists as a vapor or particulate in a dust-free atmosphere-was studied with aerosol analyzers based on different measurement principles: diffusion tubes, condensation nuclei counter and piezoelectric microbalance mass monitor, by cooling mercury vapor to a given temperature. It was found that mercury exists mostly as a vapor at the supersaturation of several hundreds but that a few percent of mercury is in the particulate state. The formation of particulate mercury takes place at a supersaturation much less than the critical supersaturation predicted by the conventional nucleation theory. W
Introduction Incineration is an effective treatment of solid wastes and sewage sludges to utilize heat as well as stabilize and lighten the waste materials. In the ordinary incineration system, an absorption tower and an electrostatic precipitator are installed as collectors to remove air pollutants in waste gases. However, it has been reported that most heavy metals with low boiling points such as Hg, Cd, and Sn are released to the atmosphere to a large extent. Among these heavy metals, especially, mercury is found to volatilize from the waste materials by combustion, leaving trace amounts in the chars ( I ) . Since there coexist a large number of soot particles in incineators, some of the volatilized mercury is adsorbed on the particles and subsequently removed by the particle collectors. In fact, Kato ( I ) reported that a few percent of mercury is removed by the particle collectors. At a high temperature, as in incinerators, any mercury compound decomposes to form metal mercury. Since the boiling point of metal mercury is as low as 357 "C, it is likely that the mercury is vaporized at a higher temperature. However, the behaviors of vaporized mercury in various processes such as heat exchanger, scrubbers, and spray towers are not understood yet because of complicated interactions between mercury and particles or reactive gases. To develop an effective collection technique for mercury, the fundamental knowledge on the behaviors of mercury undergoing different processes is needed. In the present work, the physical state of pure metal mercury, i.e., whether mercury exists as a vapor or particulate in the absence of particulate matter, is studied. The experiments are carried out by measuring the fraction of particulate mercury with aerosol analyzers by cooling vaporized mercury with different concentrations from lo4 to lo+ g/cm3 at the standard temperature and pressure. Experimental Section To characterize the physical state of mercury in gases, the concentration and size of mercury in particulate state are measured with aerosol analyzers by cooling vaporized mercury with different concentrations to a given temperature. The analyzers used in the experiments are diffusion tubes (abbreviated as DTs; laboratory made), a conden: sation nuclei counter (CNC; semiautomatic condensation Present address: Department of Construction and Environmental Engineering, Kanazawa University. 0013-936X/84/09 18-0793$01.50/0
Table I. Experimental Conditions aerosol analyzer
DT vaporization temp, Tv, O C temp of plenum chamber, T, " C flow rate, Q, cm3/min mercury vapor concn, g/cm3
e,,
CNC
PB
20-90
20-94
20-120
20
0
0
150-1200
1000
900
1.5 X 3.0
1.5 X X
lo-'
3.5 X
lo-'
1.5 X 8 X lo-'
nuclei counter, Environmental One Model E1012E1, and a piezoelectric microbalance mass mointor (PB; respirable aerosol mass monitor, Kanomax Model 51-1111). The DT is an instrument that determines particle size smaller than about 0.5 pm by measuring the diffusivity of aerosol particles. The CNC measures number concentration of particles with the diameter larger than about 5 nm for the concentration less than 10' particles/cm3. The PB determines mass concentration of aerosol particles less than 30 mg/m3. Figure 1shows the experimental setup. The pure metal mercury is placed in a flask, the temperature of which is controlled by a silicone oil bath. The metal mercury is vaporized by blowing dry, clean nitrogen gas onto the surface. Nitrogen gas is used to prevent formation of mercury oxide film on the surface. The gas containing mercury is heated up to 150 "C by a reheater and cooled down to 0 or 20 "C in a plenum chamber with a volume of 800 cm3. The resulting gas is then led into three different aerosol analyzers. Before the measurements with the aerosol analyzers, the amount of mercury vaporized at different temperatures and flow rates of carrier gas is determined with a flameless atomic absorption spectrophotometer after collecting the mercury with gas washing bottles containing the 200 cm3 of absorbent (0.25 wt 7% KMnO, + 2 N H,SO,) (2). The collection efficiency of the absorbent is at least 99.5% for the mercury concentration studied in the present work. The concentrations of mercury vaporized at 20 and 60 "C are shown in Figure 2 as a function of the flow rate of nitrogen. When the flow rate is higher than 400 cm3/min, the concentrations of vaporized mercury at both 20 and 60 "C are lower than the concentration at saturation, and they remain almost constant regardless of the flow rate. Figure 3 shows the concentration of mercury as a function of the vaporization temperature at a constant flow rate of 400 cm3/min. Since the concentration of vaporized mercury is not influenced by the gas flow rate when the flow rate is higher than 400 cm3/min, Figure 3 is, hereafter, used to obtain the concentrations of mercury vaporized at different temperatures. The experimental conditions for the measurements with aerosol analyzers are shown in Table I. The DTs are used to measure the size of particulate mercury, and the CNC and PB are respectively for number and mass concentrations. The DTs used in the experiments are 0.6-cm diameter copper tubes with lengths of 50,100, and 620 cm. To collect gaseous mercury together with particulate mercury, the DTs made of glass tubes with wetted wall of
0 1984 American Chemical Soclety
Envlron. Sci. Technol., Voi. 18, No. 10, 1984 793
1.0
VACUUM
0.8GAS WASHING
z 0
Figure 1. Experimental setup.
VAPOR ( D = O . 121 crr?//s)
0"
, I 0.5 I
0 0.1
1
1
90
0
50 100
5 IO
500
L / Q Cs/cmZI
Figure 4. Penetrations of mercury through diffusion tubes. The temperature of the plenum chamber is 20 OC. 8
FLOW R A T E ,
I
I
I
I
1
I
'
T/1
0 CcmVminI
Flgure 2. Vaporized mercury at 20 and 60 OC as a function of flow rate.
I
CALCULATED WITH SATURATED VAPOR /'
/
I VAPORIZATION TEMP., Tv C"C1
Flgure 5. Number concentration of particulate mercury at a different vaporization temperature.
copper DTs. The solid lines in Figure 4 are the penetrations calculated by the following Gormley and Kennedy equation (3):
P = 0.8191 exp(-3.657p)
+
P = 1- 2 . 5 6 ~ + ~ 11.2p ~
+ O.177p4l3+ ...
0.0975 exp(-22.3p) + 0.0325 exp(-57p) + ... p I 0.0312 (1)
0 1 0 -. 0
20
40
60
80
100
VAPORIZATION TEMP., Tv
120 140
C"C 1
Figure 3. Concentration of vaporized mercury as a function of vaporization temperature at the flow rate of 400 cm3/min.
the absorbent are also used. The glass tubes are 0.6 cm in diameter and 10 and 20 cm in length. The concentrations of mercury at the inlet and outlet of the tubes are determined by sampling mercury with the washing bottles. The CNC and PB are connected to the plenum chamber in place of the DTs when the number and mass concentrations of particulate mercury are measured.
Results The penetrations of mercury through the DTs are shown in Figure 4, as a function of the ratio of tube length to flow rate, LIQ, with vaporization temperature, Tv,as a parameter. The temperature of the plenum chamber is maintained at 20 "C. The solid circles are for the DTs with the wetted wall of the absorbent and open ones for the 794
Envlron. Sci. Technol., Vol. 18, No. IO, 1984
p C
0.0312 (2)
where (3) In Figure 4, there are large differences in the penetrations depending on the existence of the absorbent. The penetrations obtained by the copper DTs show that the mercury exists as particles with the diameter of the order of 10 nm, while the data obtained by the DTs with wetted walls indicate that the mercury is in a vapor state. This difference can be explained by accounting that the mercury in the vapor state occupies a large portion of the total mercury. However, the results obtained with the copper DTs do indicate the existence of particulate mercury. Figure 5 shows the number concentration of particulate mercury measured by the CNC, as a function of the vaporization temperature. It is seen from the figure that a very large number of particulate mercury exist and that the number of particulate mercury increases with in-
I
TmooC
-I
-05
i 2 e a (L
c
z -01
z 0
-005
m m
a
I 0 Oll
0
I
I 40
I
I
00
I
I
I
120
100 200 SUPERSATURATION, SI-I
0
VAPORIZATION TEMP., Tv C O C l
Figure 8. Mass concentration of particulate mercury at a different vaporization temperature. Open circles indicate the results obtained with reheater, and solid ones are those without reheater.
creasing the vaporization temperature. Figure 6 shows the mass concentration of particulate mercury measured by the PB as a function of the vaporization temperature. The mass concentration of particulate mercury increases with the vaporization temperature, again indicating the existence of particulate mercury. From this figure and Figure 3, it is possible to make an estimation on the ratio of particulate mercury to the total mercury. It is calculated to be about 2% at the highest concentration of particulate mercury (Tv = 120 "C).In the same figure, the mass concentrations of particulate mercury without reheater are also shown. There is a considerable increase in the mass concentration due to the reheating, indicating that the cooling process may affect the mass concentration of particulate mercury.
Discussion Through the measurements with aerosol analyzers, it is found that the formation of particulate mercury takes place when mercury vapor undergoes cooling. In this section, the formation of particulate mercury from the vapor is theoretically discussed by considering the size and number concentration of the particulate mercury through the Kelvin's equation and the homogeneous nucleation theory. When a saturated vapor condenses on nuclei or forms particles by itself, there is a critical particle diameter at which particles do not grow or stop growing, Le., in equilibrium. The critical particle diameter, d,, is given by the following Kelvin's equation: (4)
where S is the supersaturation defined by
s = CJC,
(5)
Equation 4 indicates that, for a given condensing vapor and temperature, the critical particle diameter is only a function of the supersaturation, S. Thus, the number and mass concentrations of particulate mercury shown in Figures 5 and 6 are replotted in Figure 7 as a function of S. In the figure, both the number and mass concentrations of particulate mercury increase with supersaturation, and these lines are straight and parallel to each other in the range where the supersaturation is higher than 30. Since
300
Figure 7. Number and mass concentrations of particulate mercury as a function of supersaturation.
LL
I
I
I
,\
5
10
50
100
500
SUPERSATURATION, S [-I
Figure 8. Comparison of average diameter of particulate mercury with the critical diameter calculated by Keivln's equation.
the average diameter of particulate mercury is given by the following equation
it is plotted in Figure 8 by comparing it with the critical particle diameter calculated from eq 4. The average diameter is in the order of 10 nm which coincides with the diameter obtained with the DTs (Figure 4). When the average diameter is compared with the critical diameter, the trends of these two curves are quite similar despite the difference in these values by an order of magnitude. The difference in the values may result from the following uncertainties: (i) detection of very small particles with the diameter of several nanometers by aerosol analyzers and (ii) actual supersaturation achieved in the plenum chamber. From the homogeneous nucleation theory, the rate of formation of particles with critical particle diameter, dPc, is given by the following equation: (7)
where p and nG are respectively the flux and the concentration of the condensing vapor: ffp = 4UnG
Environ. Sci. Technol., Vol. 18, No. 10, 1984
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Environmental System and Equipment Division, NGK Insulators Ltd.
P8S nG = kT AG is the free energy change due to the formation of particles given by rdp:a
AG = 3
(10)
Under the assumption that the minimal nucleation rate at which particulate mercury is detected by aerosol analyzers is J = l particle/(cm3 s), it requires a supersaturation of S = 1.85 X lo7 and a particle diameter of dPc= 0.73 nm. However, according to the measurement with aerosol analyzers, the particulate mercury with the diameter of 40 nm is detected even at a supersaturation of several tens and the amount of particulate mercury is found to be influenced by the cooling process of the vapor (Figure 6). Becker and Reiss ( 4 ) have pointed out that the nonuniformity in vapor contributes to the formation of particulates at a very low concentration. In the formation of particulate mercury, the nonuniformity of the mercury vapor seems to play an important role at low concentration of mercury vapor. Thus, it can be said that the formation of particulate mercury cannot be predicted by the conventional homogeneous nucleation theory. Conclusions The physical state of mercury undergoing cooling was studied by using aerosol analyzers based on different measurement principles. Through the measurements with the analyzers, the following conclusions were obtained. (1)Particulate mercury exists at fairly low supersaturations, and the dependency of the average size on the supersaturation is described by Kelvin’s equation. However, mercury exists mostly as a vapor even when the mercury vapor concentration is 200 times as high as the saturated vapor pressure. The ratio of the particulate mercury to the total mercury is about 2% at the supersaturation of 260. Furthermore, the cooling process of mercury vapor may affect the fraction of particulate mercury. (2) The homogeneous nucleation theory cannot be applied to the formation of particulate mercury. Acknowledgments We express our appreciation for the useful discussions with T. Kasakura, Assistant to the General Manager,
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Notations Cunningham’s correction factor mercury vapor concentration, g/cm3 number concentration of particulate mercury, cm-3 mass concentration of particulate mercury, g/cm3 saturated vapor concentration at T , g/cm3 diffusion coefficient, cm2/s particle diameter, nm average particle diameter, nm critical particle diameter, nm free energy change due to the formation of nuclei from vapor, erg/mol rate of particle formation from vapor, particles/ (cm3s) Boltzmann constant, erg/K length of diffusion tube, cm molecular weight concentration of condensing vapor, molecules/cm3 penetration through diffusion tube saturated vapor pressure at T, dyn/cm2 flow rate, cm3/min universal gas constant, erg/(mol K) supersaturation temperature, or temperature in plenum chamber, OC
vaporization temperature of mercury, O C average thermal velocity of gas molecules [ ( S R T / T M , ) ~ / cm/s ~], condensation coefficient of vapor flux of condensing vapor toward a particle, molecules/(cm2s) viscosity of carrier gas, g/(Fm s) dimensionless variable defined by eq 3 surface tension, dyn/cm density of particle, g/cm3 Registry No. Hg, 7439-97-6. Literature Cited (1) Kato, T. “Nihon Gaishi Research Report-Kankyo Souchi Tokushu”;NGK Insulators Ltd.: Japan, 1976. ( 2 ) Kitamura, S.; Kondo, M.; Takizawa, Y.; Fujii, M.; Fujiki, M. “Shuiginr; Kodansha Press: Tokyo, 1976, p 84. (3) Gormley, P. G.; Kennedy, M. Proc. R. Ir. Acad., Sect. A 1949,52A, 163-167. (4) Becker, C.; Reiss, H. J. Chen. Phys. 1976,65,2066-2970. Received for review January 17,1984. Accepted May 7,1984. Supported by NGK Insulators Ltd.
