88 Ind. Eng. Chem. Res. 1992, 31, 88-94

Particle Formation by NH3-S02 Reactions at Trace Water Conditions

Hsunling Bai, Pratim Biswas,* and Tim C. Keener Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0071

Parti~le formation from NH3-S02 reactions at trace water vapor concentrations has been studied expenmentally and t~eoretically. Thr~e possible solid products-NH3S02, (NH3hS02, and (NH4)2S205-were considered, and calculatiOns were carried out to predict equilibrium concentrations of t~e r~acta.nts. A gas to pa~ticle con.version model for the NH3-S02 system was developed, cons~der~ng simultaneous chem1cal reactiOn, nucleation, condensation, and coagulation. Potential apphcat10ns of this system of reactions are also discussed.

Introduction The gas to particle conversion process associated with

the reaction of sulfur dioxide and ammonia has attracted the attention of many researchers. It has application in the areas of ammonium sulfate production and for en­hanced capture of sulfur dioxide in flue gas desulfurization processes (Hartley and Matteson, 1975; Scargill, 1971). The direct reaction of these two gases may also be prev­alent in the lower stratosphere where an aerosol layer is expected to form (Scott et al., 1969).

Various reaction products have been proposed in the literature depending on the reactant concentrations, re­action time, moisture content, and reaction temperature. Different products of the anhydrous reaction between NH3 and so2 have been reported in studies dating to the nineteenth century (Divers and Ogawa, 1900; Scott et al.,

• To whom correspondence should be addressed.

1969, historical review). Badar-ud-Din and Aslam (1953) were the first to conclusively report that the products of the reaction were amidosulfurous acid (NH3S02) and am­monium amidosulfite ( (NH3hS02) below a temperature of 10 oc in the absence of water vapor. Hartley and Matteson (1975) conducted room temperature experiments of NH3 and S02 and found by X-ray diffraction that the most likely products were NH3S02 and (NH3) 2S02 at water vapor concentrations approaching those of NH3 and 802•

Ammonium sulfate was found to be the product under excess moisture conditions. Vance and Peters (1976a) studied the anhydrous reactions in a concentric flow re­actor and stated that (NH3) 2S02 being less volatile is the favored solid reaction product, except under excess 802 concentrations. They also measured size distributions using an impactor and a diffusion battery, with particle sizes in the range of 0.01-7.5 ~m. Size distributions were also reported by Carabine et al. (1971) with a mean size of around 0.12 ~m.

0888-5885/92/2631-0088$03.00/0 © 1992 American Chemical Society

A J!!J Mass flow controller e · Bypass either outlet stream

r, Pressure gauge/regulator

ea Filter

I Rotameter

Reactor

~

Ind. Eng. Chern. Res., Vol. 31, No.1, 1992 89

so2 'tank

NH3 Tank

1---~~--~~~

£::7 Computer

B (a). Setup 1

to hood ....

Air

Air Filter

(b). Setup 2 (c). Setup 3

Figure 1. (A) Schematic of the experimental setup to study the NH3-S02 reaction. (B) Schematic diagram of three different setups to check for mixing conditions.

Studies in the literature thus indicate that the products of the anhydrous reaction are primarily NH3S02 and (NH3hS02• However, Hartley and Matteson (1975) and Vance and Peters (1976a) also reported that water vapor acts as a catalyst during the initial particle formation process. Trace quantities of water vapor ( <400 ppm) can also act as a reactant (St. Clair, 1937; Scargill, 1971), and the most likely reaction product is ammonium pyrosulfite (NH4hS20 5• In this paper, we present an analysis of the NH3-S02 reaction under trace water conditions by con­sidering three reaction products: NH3S02, (NH3hS02, and (NH4) 2S20 5• The equilibrium calculations performed at these conditions are compared to experimental data. The evolution of the aerosol size distribution is measured in a laminar flow reactor and compared to the predictions of a lognormal model accounting for simultaneous nu­cleation, condensation, and coagulation.

Experimental Setup The schematic of the experimental system to study the

NH3-S02 reaction is shown in Figure 1A. The reactor consists of either a 40-mm-i.d. quartz tube or a 6.4-mm-i.d. stainless steel tube. The quartz tube is approximately 80 em long. 1.02% S02 gas in N2 is fed through a 1.5-mm-o.d. tube along the reactor centerline. The length of this feed tube can be adjusted to obtain desired reactor residence times (2-32 s). A mixture of 4.5% NH3 gas (in N2 gas) and clean air are introduced through a side port of the quartz tube reactor. The stainless steel tube reactor was used for shorter residence times ( <1 s). Sulfur dioxide and am­monia gases were mixed in a union tee junction and then reacted in the stainless steel tube reactor. Concentrations

of the gases were controlled by using MKS mass flow controllers. Compressed air was passed through a Balston filter to remove C02, hydrocarbons, and oil and then dried and cleaned by passing through a silica gel air dryer and a 0.2-~m Gelman air filter (AcroFlow II). The water vapor content in both S02 and NH3 tanks was provided by the manufacturer to be less than 5 ppm. The relative humidity (RH) of the dried, clean air was close to 0%, but water vapor content was estimated from the data of Bower (1934) and Trusell and Diehl (1963) to be between 30 (42 ppm, 0.15% RH) and 70 ~g/L (94 ppm, 0.3% RH). The PMS model LAS-X Optical Particle Counter was used as the particle measurement instrument, and the Horiba PIR-2000 Infrared Gas Analyzer was used for measuring the S02 gas concentration. An attempt was made to use the condensation nuclei counter (CNC) to measure the total particle number density of the reaction products. No particles were detected by the CNC, and it is suspected that the anhydrous reaction products evaporate in the CNC's saturation zone where the temperature is about 35 °C. Similar observations were made by Vance and Peters (1976a).

Preliminary experiments were conducted using three different setups (Figure 1B) to ensure that proper mixing was obtained in the reactor. The first setup was as de­scribed above for the quartz tube reactor, with so2 being injected through the center and NH3 through the side port of the reactor. The second setup was same as the first, but the two gas inlet ports were exchanged; that is, NH3 en­tered through the center tube and so2 through the side port. The third setup used was the stainless steel tube reactor with the union tee mixing head, and provided a

90 Ind. Eng. Chern. Res., Vol. 31, No. 1, 1992

Table I. Comparison of the Three Different Reactor Setups (S02,1n = 3000 ppm, NHa,in = 6000 ppm, H201n = 50 ppm)"

802 removal at outlet, o/o

reaction time, s setup 1 setup 2 setup 3

0.4 0.6 1.2 2.2 4.5

32

28 30 30

16 12 15

27 27 28

a The comparison cannot be made at the same reaction residence time due to the geometrical limitations of the reactors. However, it is seen that the 802 concentration reached quasi-equilibrium state in less than 0.4 s. The standard error of the experiment is around ::1:4 o/o of S02 removal.

rapid mixing of the reactant gases. The results (Table I) show that the first and the third setup produce the same S02 removal after a quasi-equilibrium state is attained, and the removal of S02 is higher than that from the second setup. This is because the diffusivity of NH3 in air is about twice the diffusivity of S02 in air. Thus on injecting NH3 from the side port, better mixing is obtained amongst the gases as compared to injecting S02 from the side port. Similar observations have been made by Kodas et al. (1987) in their aerosol reactor studies.

Theory Gas-Phase Equilibria. Past studies (Hartley and

Matteson, 1975; Vance and Peters, 1976a) have assumed NH3S02 and (NH3) 2S02 to be the only products under anhydrous conditions. However, (NH4hS20 5 could also be formed at low water vapor conditions (St. Clair, 1937; Scargill, 1971). In this work, we examine the following three reactions to be simultaneously occurring:

NH3(g) + S02(g) ~ NH3S02(s) (1)

2NHa(g) + S02(g) ~ (NH3hS02(s) (2)

2NHa(g) + 2S02(g) + H20 ;:::t (NH4) 2S205(s) (3)

The equilibrium constants of (1) and (2) have been mea­sured by Scott et al. (1969, 1970) and Landreth et al. (1974, 1985) and are listed in Table II with the appropriate tem­perature regimes. The computed values of the equilibrium constant differ from each other by several orders of mag­nitude (Kp1 = 1.9 X IQ-4 and 4.5 X 10-£ atm2 from data of Landreth et al. (1985) and Scott and Lamb (1970), re­spectively, at 23 °C). The equilibrium constant for reaction 3 has been computed by St. Clair (1937) and Scargill (1971) and is listed in Table II. Reasonable agreement is obtained in the values of the equilibrium constant computed by these two expressions.

Assuming that the three reaction products form an ideal solution with x mole fraction of NH3S02 and y mole fraction of (NH3hS02, the following expressions can be written (Scott and Lamb, 1970)

Kp! = PN,.?s,e/ X

Kp2 = PN/Ps,e/Y

Kp3 = PN,e2Ps,e2PH,e/(1- X- y)

(4)

(5)

(6)

where the subscripts N, S, and H refer to ammonia, sulfur dioxide, and water vapor, respectively, and the subscript e denotes the equilibrium state. Combining (1)-(3), the overall reaction is

(2 - x)NH3 + (2 - x - y)S02 + (1 - x - y)H20 ~ xNH3S02 + y(NH3) 2S02 + (1 - x - y)(NH4hS20 5 (7)

Table II. Equilibrium Constants for Three NH8-802 Reactions

equilib temp possible reactions expressions range, °C

NH3S02(s) = NH3 + 802 Kp1 = exp(22.7 5 to 45 - 9259/T)

Kp1 = exp(42.7 -10 to -70 - 16205/T)

(NHal2S02(s) = 2NH3 + KP2 = exp(43.8 5 to 45 802 - 16606/T)

Kp2 = exp(88.0 -10 to -70 - 31306/T)

(NH4)2S20 5(s) = 2NH3 + Kpa = exp(94.6 0 to 23 2802 + H20 - 39144/T)

Kpa = exp(96.5 60 to 110 - 40767/T)

source•

2

1

2

3

4

a (1) Landreth et al., 1985. (2) Scott and Lamb, 1970. (3) Scar­gill, 1971. (4) St. Clair, 1937.

Using (4)-(7), the equilibrium vapor pressure of each gas is

y Kp2 PN = - - = Pw - m(2 - x) (8)

,e X Kp! ,tn

x2 Kp12 Ps = - - = P8 · - m(2 - x - y) (9)

,e Y Kp2 ,m

1- X- y Kp3 PH,e = 2 K 2 = PH,in - m(1 - X - y) (10)

X pl

where m times the stoichiometric constant of the gas reactant in (7) gives the vapor pressure consumption of that gas when equilibrium is reached. Equations 8-10 are a set of three nonlinear equations in x, y, and m and are solved using a nonlinear equation solver, DNEQNJ (IMSL, 1987).

Particle Formation Mechanisms. The general dy­namic equation for the gas to particle conversion process is (Friedlander, 1977)

an a(Gn) - + -- - l'(v*) o (v-v*) = at au

1 fv 2 Jo {3(v-iJ,iJ) n(v-iJ,t) n(iJ,t) diJ-

n(v,t) J:~ {3(v,v) n(v,t) diJ (11)

The first term on the left-hand side (lhs) is the rate of change of the particle distribution function in the particle volume interval v to v + dv, the second term on the lhs accounts for the effect of condensation at rate G, and the third term on the lhs describes the formation of new particles of critical volume v* at rate I'. The terms on the right-hand side account for the effect of Brownian coag­ulation. A monomer balance for the aerosol growth can be written in terms of the system saturation ratio S (Friedlander, 1983):

dS Rr' l'K* ( k8 T )11

2

- = - - - - (S - 1) - A dt n. n. 211"m1

(12)

where the first term on the right-hand side is the monomer formation rate by chemical reaction, the second and third terms account for the nucleation and condensation effects, respectively, n. is the monomer concentration at saturation, K* is the number of monomers in the critical size nucleus, ks is the Boltzmann constant, m1 is the monomer mass, and A is the total surface area of the particles. The mo-

Table III. Computed Mole Fraction of the Three Solid Products for Different Water Vapor Concentrations•

H20, NH3S02, (NH3)2S02, (NH4)2S20s, ppm X y 1-x-y

802 = 3000 ppm; 0 NH3 = 3000 ppm 50 0.045 0.005 0.950

100 0.042 0.005 0.953 200 0.036 0.004 0.960 400 0.024 0.002 0.974

802 = 3000 ppm; 0 NH3 = 6000 ppm 50 0.086 0.020 0.894

100 0.081 0.019 0.900 200 0.077 0.018 0.905 400 0.057 0.011 0.932

802 = 3000 ppm; 0 NH3 = 9000 ppm 50 0.130 0.046 0.824

100 0.123 0.043 0.834 200 0.117 0.041 0.842 400 0.088 0.029 0.883

• The computation is based on thermodynamic data of Landreth et al. (1985) and Scargill (1971).

ment formulation is employed to obtain the integral properties of the size distribution using:

Mk(t) = foro vkn(v,t) dv (13)

Multiplying (11) by v on both sides, and integrating over all particle sizes and using the lognormal distribution function as an approximation of the entire size spectrum, the following equations are obtained for a laminar flow reactor (Pratsinis, 1988):

2(1 - r 2) dS! dO = R. - IK* - 71(S - 1)M0 (14)

2(1 - r 2) dM0 jdO = I- rM02 (15)

2(1- r 2) dMifdO = IK* + 71(S- 1)M0 (16)

2(1- r 2) dM2fdO = IK* 2 + 2f(S- 1)M1 + 21/;M12 (17)

where M0, M1, and M2 are the total particle number con­centration, total particle volume, and second volume mo­ment, respectively, r is the dimensionless radial distance (r = r*jR), 0 is the dimensionless time (=t/T, Tis the characteristic time for particle growth); 11 and fare coef­ficients for the condensation rate, and ~ and 1/; are coef­ficients for the coagulation rate.

Equations 14-17 form a set of four coupled ordinary differential equations which were solved using a differential equation solver, DIVPAG (IMSL, 1987). The initial con­ditions for the above equations are S = Sin and M0 = M1 = M2 = 0 for all radial positions (Bai and Biswas, 1990). The predominant solid product is (NH4) 2S20 5 (as can be seen in Table III), and this is expected as it is less volatile than the other two products. The saturation ratio for forming the (NH4) 2S20 5 product can be written as

(18)

if chemical equilibrium is attained before particle growth begins, and Sin is equal to 1 if chemical reaction and particle growth occur simultaneously.

Results and Discussion The measured S02 gas-phase concentration at the outlet

of the reactor as a function of the inlet ammonia concen­tration is plotted in Figure 2. The residence time was varied to ensure that the reaction attained a quasi-equi­librium state. Calculations were also done at water vapor

Ind. Eng. Chern. Res., Vol. 31, No. 1, 1992 91

4000

3000

E a. 0.2000 ~:::::

! 0 (f)

1000

0 3000

:::::

"' "'

H20=0 ppm

50 ppm

T 200 ppm

! 400 ppm

"' ...... ......... ~ ~ H2o .. o

"' ...... PPrn so 'PP;n ...... _

4500 6000 7500 9000 NH3,in• ppm

Figure 2. Measured and calculated 802 equilibrium vapor pressure as a function of inlet NH3 concentration. Symbols are experimental data and lines are theoretical results of our model using different equilibrium constants for reactions 1 and 2: (-, Landreth et al., 1985; -- -, Scott and Lamb, 1970).

concentrations using the equilibrium expressions listed in Table II, and the results are shown in Figure 2. The zero water vapor concentration results are the Landreth et al. (1985) and Scott and Lamb (1970) model of two reactions. Significant difference is obtained between the two pre­dictions, and this may be due to the different temperature regions in which the equilibrium constants were deter­mined. The calculations were then done using the three­reaction model proposed in this work and using equilib­rium constants for reactions 1 and 2 proposed by both Scott and Lamb (1970) and Landreth et al. (1985). When the Scott and Lamb (1970) equilibrium constants are used, not much change in the predicted equilibrium concentra­tion is obtained at 50 ppm water vapor content. Also, as seen in Figure 2, the curve tends to move further away from the experimental data. However, there tends to be better agreement with the measured concentrations when the Landreth et al. (1985) expressions are used for a water vapor content between 200 and 400 ppm. This corresponds to a RH between 0.75 and 1.5%, which is slightly higher than the estimated water content of silica gel dried air streams ("'0.2% RH) (Trusell and Diehl, 1963). The better agreement of predicted and measured concentra­tions when the Landreth et al. (1985) equilibrium constants are used as compared to the Scott and Lamb (1970) ex­pressions may be due to the temperature range of the Landreth et al. (1985) experiment being closer to that used in this work. The equilibrium mole fractions of the dif­ferent solid products at different water vapor contents are summarized in Table III.

Using the Scargill (1971) computed equilibrium con­stant, Kpa, the initial saturation ratio (18) is of the order of 104 for inlet concentrations of NH3, S02, and H20 of 6000, 3000, and 50 ppm, respectively. This is much higher than the saturation ratios of the other two compounds. Therefore we can safely assume that (NH4) 2S20 5 is the primary nucleation product, with subsequent condensation of NH3S02, (NH3) 2S02, and (NH4) 2S20 5 monomers onto those nuclei. Our experimental data indicate particle number density of around 105/ cm3 and geometric mean sizes of around 0.1-o.2 ~m, which compared well with results of Carabine et al. (1971), who also utilized a light-scattering device to detect particles. The size dis­tribution of the aerosol is measured by the OPC and plotted in Figure 3.

The integral properties of the size distribution are computed and compared to the predictions of the log-

92 Ind. Eng. Chern. Res., Vol. 31, No. 1, 1992

......... a. "0 ......... 0'1 .2 <J

........... c 0

:;::; u a ....

I.J...

6.0

4.0

2.0

0.0 0.08

r-

r-

r-

~ h 0.1 0.2

Particle size, I'm

0.3

Figure 3. Size distribution of NH3/S02 reaction products at room temperature with inlet 802 concentration of 3000 ppm and inlet NH3 concentration of 6000 ppm at 0.6-s residence time.

normal model described earlier, and the results are shown in Figure 4. Since (NH4)~205 is the predominant product, we assumed it is the only product in the particle formation model. Both particle density and the surface tension are unknown for carrying out the gas to particle conversion calculation. Particle density was estimated from data of (NH4) 2S04 (pP = 1.77 gjcm3) and NH4HS03 (pp = 2.03 gfcm3) and was varied in the range of 1.8-2.0 gfcm3 in our model. The value for surface tension is not available and was adjusted to fit the experimental results (symbols in Figure 4). Chemical reaction and particle growth may be occurring simultaneously. As indicated by (14), the satu­ration ratio increases due to chemical reaction (till the reaction reaches equilibrium) and decreases due to nu­cleation and condensation of the vapors. Limited infor­mation is available in the literature regarding the chemical reaction order or the reaction rate for the formation of (NH4) 2S20 5• Hence an average value of R. was estimated from our experimental data. The time required to reach equilibrium, t., is between 0 and 0.4 sin our experiments (Bai et al., 1991). The average value of Rr' was thus cal­culated by {Ps,in- P8,.)N8v/82t.T, where Navis Avogadro's number. This approach is better than assigning arbitrary values to reaction rates as done by Kodas et al. (1986).

Three sets of model predictions are plotted in Figure 4. All three sets are plotted by appropriate choice of param­eters so that there are best fits with the number concen­tration, geometric mean particle diameter, and geometric standard deviation. The solid lines are plotted with the results of model predictions accounting for simultaneous chemical reaction and particle growth; the dashed lines are plots for very fast reaction, or assuming that reaction equilibrium is attained before particle growth begins. On assuming very fast chemical reaction and on varying the surface tension, reasonable agreement of experimental data (number concentration, geometric mean particle size, and standard deviation) with model predictions was obtained for a surface tension of 70 dynfcm, this being close to the value of H2S04 (Liu and Levi, 1980). The other dashed line was plotted by using a higher surface tension of 93 dyn/ em, this being less than the value of NH4N03 used in the literature [u = 113-119 dynjcm, Raes et al. (1990)]. The nucleation rate had to be scaled up by a factor of 109

for agreement between model predictions and experimental data. The solid line is plotted using a t. of 0.04 s, and reasonable agreement is obtained with the experimental

10 "!

~ 10 '1 ~ 10 1

'*' g 10' :.:> E c

10 I I

~ 10.

Ua

10 • .... ., .c 10' E ::l z 10.! c 0 10 f-

1 0.0

A __ a•70 ergs/.em', p,~1.8 g/.em: 0 ----- aQ93 ergs;tcm!. p0•2.0 g;tcm,. \,=10 , _ a•93 ergs/em . p0•2.0 g/cm, K,•6 X 10

---------------,------------i·--------------------~-----------------j--

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Average Residence Time (sec)

10 -•...------------------,

E' $ .... 10 _,

~ E

B __ a=70 ergs/.cm:. p,•t.B g/cm!

1 _____ a•93 ergs/.em,. p,~2.0 g/./em,. 1,•10 , _ a-=93 ergs/em, p,•2.0 g em. R, ... s X 10

.!2 10 ... ~ ------------------------------0 /~~------------" . ! ~ / ~ i t:: 10 -• I !! cE ' c:

~ 10 -· :IE 0

·;: ., 10 _, E 0

" (!)

10~+-~---r--~~--.---,--.--,--.--~ 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Average Residence Time (sec)

2.0 ..,.--------------------,

1.9

§ 1.8

~ 1.7 " 0

'E 1.6 0 -g 1.5 .B Ul 1.4 0

~ 1.3 E g1.2

(!)

1.1

c __ a=70 ergs/.em', p0=1.8 g/.em: 0 _____ a•93 ergs/em', p,•2.0 g;tem,. 1,=10 0 _ a•93 ergs/em', p0•2.0 g/cm, R,=6 X 10

1 .0 +---,----.---,:---r-r--.--r----r:--::T"-:-1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Average Residence Time (sec)

Figure 4. (A) Measured and simulated results of cup mixing average total particle concentration as a function of average residence time. The inlet 802 and NH3 concentrations are 3000 and 6000 ppm, re­spectively. (B) Measured and simulated results of cup mixing av­erage geometric mean particle diameter concentration as a function of average residence time. The inlet 802 and NH3 concentrations are 3000 and 6000 ppm, respectively. (C) Measured and simulated results of cup mixing average geometric standard deviation as a function of average residence time. The inlet 802 and NH3 con­centrations are 3000 and 6000 ppm, respectively.

data. Increasing t. (slower reaction) does not yield good agreement with the experiments, and hence the results are not shown. The two dashed curves indicated that there is a burst of nucleation and formation of particles, which then primarily grow by condensation. This is also con­flrmed by the fl.nal geometric standard deviation tending to 1. In the case of simultaneous reaction and particle growth, nucleation is delayed by a fraction of a second (short delay as reaction is still rapid). The fmal geometric

standard deviation is higher, indicating growth occurs by both condensation and coagulation. Also, the computed characteristic time for particle growth (T) is about 0.12 s, much larger than t, (=0.04 s). On comparing the experi­mental data to the three sets of model predictions, it can be assumed that chemical reaction is extremely rapid and completed before particle growth begins.

Scott et al. (1969) and Vance and Peters (1976b) have considered the formation of two adducts, NH3S02 and (NH3)~02, under dry conditions in the stratosphere. Such gas-phase reactions have often been neglected in atmos­pheric chemistry. However, in the stratosphere where the temperature is very low, gas-phase reactions could be an­other mechanism of forming ammonium sulfate aerosols in addition to photochemical reactions. The reaction of NH3 and S02 in the presence of water vapor, as described in this work, to form (NH4)zS20 5 is another possible mechanism that has not been considered in atmospheric chemistry. The reaction is fast and requires very low partial pressures of the reactant gases. For example, the equilibrium constant at -70 °C is 4.9 X 10-16 ppm5 for the reaction that forms (NH4)zS20 5 (extrapolation from Scargill (1971)). This implies that at a 10% relative hu­midity, if the vapor pressure product of ammonia and sulfur dioxide exceeds 4 X 10-s ppm2, (NH4) 2S20 5 particles will possibly form. (NH4)zS20 5 is highly soluble and forms ammonium bisulfite (NH4HS03), which is then easily ox­idized to form a stable product-ammonium sulfate (St. Clair, 1937).

Reactions involving ammonia and sulfur dioxide gases could also be applied for industrial sulfur dioxide emission control. Removal of sulfur dioxide is possible when in­jecting ammonia gas into flue gases at lower temperatures (4o--60 °C). For example, assuming (NH4)zS20 5 as the major product, and with initial S02 and NH3 concentra­tions of 3000 ppm at 50 °C at 8% (by volume) H20 vapor concentration, the 802 removal could be over 90% based on the equilibrium calculation. If this equilibrium is truly achievable, then removal of S02 from a high sulfur coal combustion process by ammonia gas injection provides another route for industrial gas cleaning. A bench-scale experiment is currently under way to evaluate the possi­bility of S02 removal by ammonia gas injection.

Conclusions

The gas-phase reactions between sulfur dioxide and ammonia at trace water vapor concentrations were studied. Quasi-equilibrium gas concentrations and particle prop­erties were measured experimentally and then compared to the predictions of a theoretical model. A three-reaction mechanism leading to NH3S02(s), (NH3)zS02(s), and (NH4) 2S20 5(s) as products was used to carry out the equilibrium calculations. A theoretical model of the gas to particle conversion process accounting for chemical reaction, nucleation, condensation, and coagulation was developed. Since (NH4) 2S20 5(s) is the predominant product of the NH3-S02 reaction, it was considered as the source of primary nuclei and further growth of particles by condensation and coagulation. The measured particle size appears to be lognormally distributed with a geometric mean diameter of 0.1-o.2 ~m. geometric standard deviation of 1.1-1.2, and total particle number concentration of around 105/ cm3•

Acknowledgment

This work was funded by Ohio Coal Development Or­ganization Grant RF3-88-014.

Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 93

Nomenclature

A = total particle surface area G = particle growth rate by the condensation effect I = particle formation rate (nucleation rate) I. = nucleation rate scale K* = number of monomers in the critical size nucleus ka = Boltzmann constant J!.I' = equilibrium constant of the NH3-S02 reaction Mk = kth aerosol volume moment (k = 0, 1, 2) m1 = monomer mass n, n(v,t) = particle density function Nav = Avogadro's number (=6.02 X 1023 molecules/mole) n. = monomer concentration at saturation, molecules/cm3

P = partial pressure of the gaseous reactant, atm R = pipe radius R, = monomer formation rate from chemical reaction

(=R/T/n8)

S = saturation ratio T = temperature t = dimensional time t, = time required to reach chemical equilibrium v = monomer volume v* = monomer critical volume r = radial distance x = mole fraction of NH3S02 y = mole fraction of (NH3hS02

Greek Symbols fJ = collision frequency function o = delta function E = dimensionless condensation coefficient ~ = dimensionless coagulation coefficient 71 = dimensionless condensation coefficient 8 = dimensionless time Pp = particle density u = surface tension 1/1 = dimensionless coagulation coefficient T = characteristic time for particle growth [=[n.s1(k8 T/

211'mr)l/2j-1]

Subscripts N =ammonia S = sulfur dioxide H = water vapor e = equilibrium state in = inlet condition

Superscript ' = dimensional property

Registry No. NH3, 7664-41-7; 802, 7446-09-5; H20, 7732-18-5; NH3S02, 25310-87-6; (NH3)S02, 29307-29-7; (NH4)S20 5, 32736-64-4.

Literature Cited

Badar-ud-Din; Aslam, M. Ammonia-Sulfur Dioxide Reaction. Pak. J. Sci. Res. 1953, 5, 6-10.

Bai, H.; Biswas, P. Reaction of Sulfur Dioxide with Ammonia to Form Aerosols: Applications in Flue Gas Desulfurization. Ab­stracts of Papers, Annual Meeting of the AAAR, Philadelphia; AAAR: Philadelphia, 1990; P1F-2, p 265.

Bai, H.; Biswas, P.; Keener, T. C. Sulfur Dioxide Removal by Re­action with Ammonia Gas to Form Solid Products. Presented at the 84th Annual Meeting of Air and Waste Management Associ­ation, 1991; paper 91-103.24.

Bower, J. H. Comparative Efficiencies of Various Dehydrating Agents Used for Drying Gases. Bur. Stand. J. Res. 1934, 12, 241-248.

Carabine, M.D.; Maddock, J. E. L.; Moore, A. P. Particle Size Dis­tributions in Aerosols Form from Gaseous Reactants. Nature Phys. Sci. 1971, 231, 18.

Divers, E.; Ogawa, M. Ammonium Amidosulfite. J. Chem. Soc. Trans. 1900, 77, 327-335.

94 Ind. Eng. Chem. Res. 1992, 31, 94-100

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Received for review May 16, 1991 Revised manuscript received August 22, 1991

Accepted September 5, 1991

Productive and Parasitic Pathways in Dilute Acid-Catalyzed Hydrolysis of Cellulose

William Shu-Lai Mok and Michael Jerry Antal, Jr.* Department of Mechanical Engineering and the Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii 96822

Gabor Varhegyi Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budapest, Hungary

Cellulose hydrolysis experiments were conducted in a percolating reactor at 34.5 MPa. A glucose yield of 71% of the theoretical maximum was obtained at 215 °C with 0.05% by weight of sulfuric acid in the percolating solution. The classical model of glucose formation from cellulose followed by secondary sugar degradation did not describe the reaction chemistry under these conditions. A parasitic pathway which leads to the formation of nonhydrolyzable oligomer was discovered in the absence of acid. In the presence of acid, kinetic modeling of the measured, temperature-dependent rates of glucose evolution indicates that an acid-catalyzed parasitic pathway operates in competition with the glucose production pathway. No chemical changes were detected in the solid phase during the course of reaction.

Introduction Although the acid-catalyzed hydrolysis of cellulosic

materials was industrialized almost a century ago (Harris, 1949), the underlying chemistry is still a focus of research interest today. Three approaches characterize the state­of-the-art technology. Dilute acid hydrolysis processes employ flow reactors which only accept a finely ground feedstock. Typical glucose yields of about 55% can be achieved with acid concentrations below 3% and a resi­dence time of a few seconds (Thompson and Grethlein, 1979; Church and Wooldridge, 1981; Brenner and Rugg, 1985). Somewhat higher yields are obtained with perco­lator reactors at the cost of reduced reaction rates, batch operation, and the dilution of sugar products (Faith, 1945; Harris and Beglinger, 1946; Gilbert et al., 1952). Finally, low-temperature, concentrated acid processes are some­times employed (Moore and Barrier, 1987; Sharples, 1957, 1958; Dunning and Lathrop, 1945). In most cases, the yield

of glucose is considerably less than 100%. The goal of this research was to elucidate the underlying chemistry which limits the yield of glucose from cellulose.

The classical explanation for the low yield of glucose was first enunciated by Saeman (1945) and employed more recently by McParland et al. (1982) and Conner et al. (1985). It posits the role of secondary degradation reac­tions in reducing the yield of glucose as the sole primary product of cellulose hydrolysis: cellulose - glucose -degradation products. Contradicting this classical picture, important recent work by Abatzoglou et al. (1986) has shown that the initial products of cellulose hydrolysis are primarily soluble oligosaccharides. Moreover, in the presence of 40 mM H2S04 at 190 °C, Bouchard et al. (1989) detected very significant alterations in the chemical structure of the unconverted "cellulose" after 30 min at reaction conditions. However, Bouchard et al. (1989) did not actually measure the rate of glucose formation from

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