I
Aerosol Formation Resulting from the Reaction of Ammonia and Sulfur Dioxide James L. Vance and Leonard K. Peters. Department of Chemical Engineering, University of Kentucky, Lexington, Kentucky 40506
The aerosol producing reaction between ammonia and sulfur dioxide was studied experimentally in a concentric flow reactor. The reactions were conducted at NH3to SO2 reactant ratios of 7:1, 1:1, and 1:7,with reactant concentrations ranging from 950 ppm to 16 200 ppm. Only trace amounts of water vapor (510ppm) were present, and all reactions were carried out at 25 OC and atmospheric pressure. The reaction proceeded at significant rates, even at the lower concentrations, apparently producing two solid compounds. Results of chemical analyses showed that changes in the NH3 to SO2 ratio in the solid product roughly corresponded to similar changes in the same ratio in the gas phase. Particle sizes up to 7.5 k m were measured, but the majority of the aerosol particles were in the submicron range. Classical nucleation theory indicates that ammonium amido sulfite ((NH3)2.S02) may be significantly less volatile than amidosulfurous acid (NH3.S02) at these conditions. Thus the sulfite is the favored reaction product except when large excesses of sulfur dioxide are present.
Introduction The process of gas-to-particle conversion has been given increased attention in recent years (cf..Kiang et al. (1973a), Burton (1973), Stauffer and Kiang (1974)). Industrially the concept has been utilized in the production of semiconductor crystals and in certain metallurgical processes, as well as being important in fogging in both the agricultural and chemical industries. In addition, it appears that the process plays an important role in atmospheric chemistry, resulting in significant amounts of both liquid and solid aerosols. In general, gas-phase reactions can lead to aerosol formation if the reaction product itself, or a subsequently formed solution, has a sufficiently low vapor pressure to effect nucleation. Many of the recent nucleation studies have dealt with both photochemical and nonphotochemical reactions between sulfur dioxide and various other atmospheric gases, including water vapor, ozone and oxygen (Vohra et al. (1972),Friend et al. (1973), Wood et al. (1974), Bricard et al. (1972), Payne et al. (1973)). These studies investigated the possible direct ,sulfate formation or, in some cases, the formation of sulfuric acid vapor. The production of this species may be significant in the atmosphere in light of several recent nucleation models (Kiang et al. (1973b), Stauffer et al. (1973), Mirabel and Katz (1974)) which indicate that aqueous acid aerosols may be Torr. formed at sulfuric acid concentrations as low as Although most of the above studies focused upon the formation of aqueous aerosols, there are some gas-phase reactions which are known to result in direct formation of solid products. One is the reaction between sulfur dioxide and ammonia. This reaction was first studied by Dobereiner (1826), and subsequently by several investigators including Divers and Ogawa (1900) and Badar-ud-Din and Aslam (1953). The results of these studies showed that the anhydrous reaction proceeded quickly a t temperatures below 10 "C, but apparently required a t least a trace of water vapor at higher temperatures. The composition of the solid products was found to be dependent upon the relative reactant ratios and temperature. Although a large variety of compounds are present, the primary products of the anhydrous reaction were amidosulfurous acid (NH3802) and ammonium amido sulfite ((NH&S02), the latter product being favored when excess ammonia was present, while the former was dominant when sulfur dioxide was in excess. Although these studies revealed the apparent behavior of 202
Ind. Eng. Chern., Fundarn., Vol. 15, No. 3, 1976
the NHs-SO2 reaction, few thermodynamic or kinetic data were reported. In order to obtain additional information about the reaction, Scott et al. (1969,1970) measured the pressures above the solid reaction products and suggested values for several thermodynamic properties. This work was followed by a calorimetric study of the reaction (McClaren et al. (1974)) which measured heats of formation for the reaction under both wet and dry conditions. Recently, the rates of reaction were studied by Hartley and Matteson (1975) a t SO2 concentrations which would be expected in a stack gas. These workers reported significant production rates of solid product and suggested that the gas-phase reaction may initially form a gaseous adduct of NHpS02, which would subsequently join with ammonia or with itself to yield the two primary products having 2:l and 1:l NH3/SO2 ratios. These reactions would then be followed by nucleation and formation of the two species in the solid phase. The authors concluded that water vapor acts as a catalyst for the initial particle production and that the solid product may be a precursor which subsequently reacts with water vapor and oxygen to form a final sulfate product. The catalytic effect of water vapor on the nucleation process was also suggested previously for the gas-to-solid formation of NH&1 from NHs and HCl vapors (Johnston and Manno (1952)). Twomey (1959) reported that the nucleation rates of NH&l aerosols resulting from gas-phase reaction followed the classical rate equations developed from nucleation theory. In addition, results of a study of the NH3-SO2 reaction by Arrowsmith et al. (1973) led them to conclude that aerosol production rates for their system may also have been following classical theory. In the present paper, we present new results on the gasphase reaction between ammonia and sulfur dioxide. The data on the aerosol formation will be discussed and explained in terms of classical nucleation theory.
Experimental Details For the purpose of this investigation, a concentric tubular flow reactor was designed. It consisted of a 2%-in. i.d. stainless steel tube, approximately 30 in. long, fitted with a 0.25-in. i.d. feed tube along its centerline (see Figure 1).This feed tube was supported by a Plexiglas manifold which allowed the two gas streams to be introduced separately at the upstream end of the reactor. A nitrogen-sulfur dioxide mixture was introduced
Dilution and
rso2
Feed Manifold
Ion
""'""31
-t
Dilution Manifold
Figure 1. Schematic of reactor.
into the annular region formed by the walls of the two stainless steel tubes while an ammonia-nitrogen mixture was fed through the central feed tube. This feed tube was designed so that its position within the reactor could be adjusted to accommodate varying reaction times. The reactor was also fitted with a second manifold located approximately halfway along the length of the tube. This feature served two purposes. First, it allowed for the introduction of a pure nitrogen stream, which was used to dilute the reacting species. Secondly, it was used to feed an ionized stream of nitrogen into the reactor to bring the particles to charge equilibrium. The ionized nitrogen stream was obtained by passing the gas through a Wellsbach T-816 ozone generator. A schematic of the experimental system is shown in Figure 2. The reacting gases were adjusted to 25 "C by passing them through a constant-temperature bath, and flow rates were monitored with rotameters. Tests were conducted for the presence of background nuclei in the gases using a condensation nuclei monitor, and the results indicated no detectable concentrations of foreign nuclei in the gas steams. The moisture concentration in the nitrogen gas was specified by the manufacturer to be less than 10 ppm; this value was accepted and the gases were not additionally dried. Particle size distributions were determined employing an eight stage Anderson Cascade Impactor and a diffusion battery. These instruments, used in conjunction with an absolute membrane filter, allowed for the determination of the aerodynamic equivalent diameters of the aerosols and their production rates. The relative NHs-to-SOz composition of the aerosols was measured. For this determination, samples were obtained by washing either the cascade impactor collection plates or the membrane filter with 100 ml of double-distilled water. A 50-ml aliquot of this solution was mixed with 10 ml of a 6% hydrogen peroxide solution. The peroxide was used to ensure the oxidation of all sulfur oxides to the sulfate form. The presence of sulfite was checked by titration with a standard iodine solution and starch indicator; in all cases, the amount of sulfite was found to be neglibible. A 10-ml portion of the treated sample was then tested for sulfate, while the remaining portion of the original sample was used for the ammonia determination. The Direct Nesslerization Method was utilized for ammonia concentrations and the Thorin Indicator Method for sulfate. Although the latter method is known to be subject to a variety of interferences, determinations carried out on prepared sulfuric acid and ammonium sulfate solutions showed no significant interferences at the concentrations involved in this study. At any set of reaction conditions, the sampling procedure involved three separate steps in order to determine the entire particle spectrum. The first step employed the cascade impactor, connected to a vacuum pump, at the end of the reactor, and samples of the larger particles ( D > 0.5 pm) were collected. After this, the impactor was removed and a probe was inserted into the reactor and a sample stream was drawn
I . N i t r o g e n Tank 2 . NH, T a n k 3. SO, T a n k 4. T e m p e r a t u r e
5. 6. 7. 8.
t
Ozone Generator N e e d l e Valves Rotometers Reactor
Bath
Figure 2. Schematic of experi$nentalsystem.
through the absolute filter. This procedure gave the total mass production rates. Finally, the sample stream was passed through the diffusion battery before filtering, yielding the total mass penetrating the battery. This procedure provided a limited size distribution analysis of the smaller particles, yielding the total mass of particles having diameter greater than 0.02 pm, which was the effective cutoff diameter for the diffusion battery a t the system velocity. In all cases a typical sampling period lasted for approximately 15 min.
Results The experiments were conducted at approximate ammonia to sulfur dioxide reactant ratios of 7:1, 1:1, and 1:7, with reactant concentrations ranging from 950 to 16 200 ppm. All tests were carried out at 25 "C and atmospheric pressure, with water vapor concentrations less than 10 ppm and residence times before dilution of 5.3 s and after dilution of 5 s. The averaged results of the tests conducted a t the three ratios are presented in Table I with the standard deviations for each set of runs shown in parentheses. Histograms of the particle size distributions for the three reactant ratios are shown in Figure 3. As the results indicate, a bimodal distribution was found in all cases, and for this reason, production rates and N H B / S O product ~ ratios are reported for the total aerosol spectrum and for those particles contained in the larger mode ( D > 0.5 pm). The relatively high standard deviations observed for the composition ratios of the larger particles apparently resulted from the small amount of material available from the cascade impactor for analyses. This was usually less than 10 me,. The size distributions of the larger aerosols ( D > 0.5 pm) appeared to be normal, truncated a t an upper particle size of about 7.5 pm. In all cases these distributions indicated a mass median diameter of about 4..1 pm and standard deviations of about 1.9 pm. A typical plot is shown in Figure 4. Physically, the aerosols appeared to be white in color, the characteristic yellow of the 1:l solid reported by many previous investigators was not evident. All of the aerosols were deliquescent, vigorously absorbing moisture from the atmosphere in the room when left exposed. It should be noted that this affinity for moisture apparently precluded the use of the condensation nuclei counter to measure the actual nucleation rate of the reaction. Since the CN monitor uses a humidification process, this caused t h e aerosols to coagulate and adInd. Eng. Chem., Fundarn., Vol. 15,No. 3,1976
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Table I. Summary of Data for Experimental Runs Reactant ratio 7:l NH, concentration, ppm SO, concentration, ppm
Aerosol production rate, mg/min Total aerosol D > 0.5 pm ",/SO, composition ratio Total aerosol D > 0.5 pm QDenotessample standard deviation.
1:7
1:l
16 200 2 430
6 530 6 160
2 430 16 220
8.5 (0.3p 0.5 (0.1)
4.2 (0.6) LO (0.2)
6.8 (0.4) 1.1 (0.2)
1.91 (0.13) 1.48 (0.37)
1.53 (0.10) 1.52 (0.30)
1.28 (0.09) 1.03 (0.27)
Table 11. Aerosol Production Rates at Dilute Concentrations 0 50
? I Rewant Ratm
Concentration, ppm
so,
NH, ~~~
I .oo 1
0.50
L
I
c
I
I
I
A
I : / Reactant Ratio
1
' 0.20
~
H,O
Aerosol production rate, mg/min __Total D > 0.5 pm 7.80 7.76
-
-
0.03 0.0 0.05
~
2220 950 950 2375 360
410
2100 900 100 350 2375
< 10
