2110 Ind. Eng. Chem. Res. 1992,31, 2110-2118

Baumgartner, F.; Finsterwalder, L. On the Transfer Mechanism of Uranium(V1) and Plutonium(1V) Nitrate in the System Nitric Acid-Water/Tributylphosphate-Dodecane. J. Phys. Chem. 1970, 74,108-112.

Chilton, T. H.; Colburn, A. P. Mass Transfer (Absorption) Coeffi- cients. Prediction from Data on Heat Transfer and Fluid Friction. Ind. Eng. Chem. 1934,26,1183-1187.

Cox, M.; Flett, D. S. Metal Extractant Chemistry. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hanson, C., Eds.; Wiley: New York, 1983; Chapter 2.2, pp 53-89.

Edwards, J. 0. Correlation of Relative Rates and Equilibria with a Double Basicity Scale. J . Am. Chem. SOC. 1954, 76,1540-1547.

Eigen, M.; Tamm, K. Sound Absorption in Electrolytes as a Conse- quence of Chemical Relaxation. 11. 2. Elektrochem. 1962, 66,

Fuoss, R. M. Ionic Association. III: The Equilibrium Ion Pairs and Free Ions. J . Am. Chem. SOC. 1958,80, 5059-5061.

Gouger, S.; Stuehr, J. Kinetics of Iron(II1) Interactions with Phenol and o-Aminophenol. Inorg. Chem. 1974,13,379-384.

Harada, M.; Miyake, Y. Solvent Extraction with Chelating Agents. In Handbook of Heat and Mass Transfer; Cheremisinoff, N. P., Ed.; Gulf: Houston, 1989; Chapter 21, pp 789-881.

Horner, D.; Mailen, J.; Thiel, S.; Scott, T.; Yates, R. Interphase Transfer Kinetics of Uranium Using the Drop Method. Znd. Eng. Chem. Fundam. 1980,19, 103-109.

Imura, H.; Takahashi, H.; Ueki, Y.; Suzuki, N. Liquid-liquid Ex- traction of Rhodium(II1) in Various Systems: Extraction Behav- ior of Some Rhodium(II1) Species in Aqueous Solutions. R o c . Symp. Solvent Extr. 1988, 1, 1-6.

Matauyama, H.; Miyake, Y.; Izomo, Y.; Teramoto, M. Kinetics and Mechanism of Metal Extraction with Acidic Organophosphorus Extractanta (11): Extraction Mechanism of Fe(II1) with Di(2- ethylhexy1)Phosphoric Acid. Hydrometallurgy 1990,24,37-51.

Miyake, Y.; Takenoshita, Y.; Teramoto, M. Extraction of Copper with SME529. J . Chem. Eng. Jpn. 1983,16, 203-209.

Miyake, Y.; Matauyama, H.; Nishida, M.; Nakai, M.; Nagase, N.; Teramoto, M. Kinetics and Mechanism of Metal Extraction with Acidic Organophosphorus Extractanta (I): Extraction Rate Lim- ited by Diffusion Process. Hydrometallurgy 1990, 23, 19-35.

Niitau, M.; Sekine, T. Solvent Extraction Equilibria of Acids-I. Extraction of Nitric and Perchloric Acid with Trioctylphosphine Oxide. J. Inorg. Nucl. Chem. 1975,37, 1056-1057.

107-121.

Nitsch, W.; Schoor, A. V. The Kinetica of Coextraction in the System Uranylnitrate, Nitric Acid, Tributylphosphate. Chem. Eng. Sci.

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Received for review January 13, 1992 Revised manuscript received June 1, 1992

Accepted June 12, 1992

1983,38, 1947-1957.

Mechanism and Kinetics of the Reaction between Sulfur Dioxide and Ammonia in Flue Gas

Klaus Hjuler and Kim Dam-Johansen* Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark

The reaction between sulfur dioxide and ammonia in flue gas is of importance in understanding scaling on process equipment and monitoring systems in a number of flue gas cleaning processes. The reaction was studied a t atmospheric pressure using simulated flue gas, 1.5-12.0-mm-internal diameter tube reactors, and flows ranging from 0.5 to 4.0 (N L)/min. The reaction temperature, sulfur dioxide concentration, and ammonia concentration were 40-80 "C, e 1 0 0 0 ppm, and 120-2000 ppm, respectively. The water content of the gas was typically 6 vol 9% and the oxygen content 5 vol ?% . The fiist step of the reaction is the formation of liquidlike ammonium salt containing sulfur in oxidation state IV on surfaces. With oxygen and small amounts (5 ppm) of nitrogen dioxide present, S(1V) is irreversibly oxidized to S(VI), and the product becomes solid and thermally stable. This product contains primarily ammonium sulfate. An overall mechanism for the reaction involving two intermediary species is proposed, and a simple three-parameter expression for the reaction rate has been developed from the mechanism.

Introduction Ammonia is generally used for the selective catalytic and

noncatalytic reduction of nitrogen oxides and for the re- moval of sulfur dioxide in wet scrubbing. Scaling on catalyst surfaces or in flue gas ducts or monitoring sample lines by solid or liquid ammonium salt is a potential problem in these processes, and more information on the

0888-5885/92/2631-2110$03.00/0

formation mechanism is needed in order to avoid it. On the other hand, the production of ammonium salt is de- sirable in the electron beam dry scrubbing process, where sulfur dioxide removal is due substantially to the reaction between sulfur dioxide and ammonia.

Very little is in fact known about the reaction mecha- nism, its kinetics, or its products. Most investigations of

0 1992 American Chemical Society

Ind. Eng. Chem. Res., Vol. 31, No. 9,1992 2111

the reaction between gaseous sulfur dioxide and ammonia have been performed in the field of atmospheric chemistry. In the atmosphere, the temperature and the concentrations of sulfur dioxide and ammonia are significantly lower than in flue gas, and consequently such investigations are not necessarily relevant. As will be discussed later, some in- formation is available on the thermodynamics of the for- mation of ammonium sulfite from ammonia, water vapor, and sulfur dioxide. However, under flue gas conditions it has been observed that the reaction product primarily contains sulfur in oxidation state VI @(VI)), i.e. sulfate. Thus the thermodynamics, mechanism, and kinetics of the S(IV) oxidation step are of particular interest.

In this study the reaction between SO2, NH3, and H20 was investigated in the temperature range from the water dew point of the gas to the temperature where no reaction was observed. The gas composition was typical of a coal flue gas. The inlet sulfur dioxide, ammonia, and water concentrations were varied to examine the kinetics. The influence of flue gas components such as oxygen, carbon oxidea, nitrogen oxidea, sulfur trioxide, and fly ash was also investigated. S(VI) product composition was studied by X-ray diffraction.

Experimental Setup Preliminary experiments were carried out using an

oven-thermostated, variable volume, 25-mm4.d. mixed- flow reactor with a movable "piston" bottom where a 2-wm membrane filter was placed. These experiments made clear that the reaction was very sensitive to temperature. Because of problems with gas mixing and temperature control, tube reactors operated in the laminar flow range were found to be better suited.

The tube reactors were various lengths of 1.5-12.0-mm- i.d. tubes made of steel, Teflon, nylon, or glass, thermo- stated either in an oven or by a heating jacket. The tube length, gas flow rate, and residence time were typically 20-30 cm, 0.6-0.7 (N L)/min, and 0.2 s, respectively (at 20 "C and atmospheric pressure).

A simulated flue gas was mixed in a manifold from cylinders of the individual components by means of re- duction valves and mass flow controllers. The manifold and the gas lines up- and downstream of the reactor were heated to above 100 OC to avoid condensation of water and undesirable reactions. The inlet gas temperature had little or no influence on the observed conversion in the 40-120 OC temperature range; any reador temperatures quoted in the following are therefore surface temperatures. Water vapor was introduced by humidifying a stream of nitrogen or by evaporating water from an HPLC pump.

Sulfur dioxide was measured continuously using non- dispersive dual wavelength absorption (Hartman & Braun Radas 1G) and multiwavelength differential absorption (OPSIS AR 600) in the UV range. The OPSIS analyzer was used extractively and equipped with a 14 cm long measuring cell thermostated at 150 OC. The OPSIS ana- lyzer was also used for continuous monitoring of ammonia. Nitrogen oxide and oxygen were measured respectively by means of chemiluminescence (Monitor Labs) and a para- magnetic gas analyzer (Hartman & Braun Magnos 5T). In the following, gas concentrations are given as volume fractions for the sake of convenience.

Figure 1 shows the experimental setup with gas-mixing manifold, water vaporizer, and jacket-thermostated tube reactor. Also shown is the setup for the OPSIS analyzer: an oven with a UV lamp and a measuring cell. The light is transmittd to the spectrometer through an optical fiber. The spectrometer was operated by means of a built-in

Figure 1. Experimental setup.

0.8

N

% 0.6 L 0

0 0.2

I

t 0.0 -

45 50 55 60 Surf ace Temperature (deg . C)

Figure 2. Conversion of SO2 versus temperature in a 30-cm reactor (a) and a 435-cm reactor (0). The a data were taken from four similar experiments. Inlet conditions: 930-1100 ppm SO,; NH3/S02 ratio, 1.7-2.0; 5.9 vol % HzO. Flow range: 0.8-0.9 (N L)/mm.

personal computer, which was also used for data acqui- sition and analysis.

Experimental Results (I): Formation of Sulfite The experimental study started with the simplest pos-

sible system, namely, a mixture of SO2, NH3, and H20 in NP The concentrations (volume fractions) of SO2 and NH3 were in the ranges 930-1100 and 1580-2150 ppm, respec- tively. N2 was added to balance; about 5 vol % of O2 was added in some runs. All experiments were carried out at atmwpheric pressure using oven-thermostated Teflon tube reactors (id. 3.2-4.0 mm).

The reaction product was always found on the reactor walls, especially close to the ammonia inlet. It was li- quidlike, yellowish in color, and unstable with respect to decomposition: it sublimed when the reactor was heated or one of the gaseous components was removed, indicating that sulfur in oxidation state IV was a main constituent, as discussed below. No direct influence of oxygen on the reaction rate was found, but the product oxidized slowly in the presence of oxygen, forming a solid, white ammo- nium salt containing S(V1). This is in agreement with results obtained by Landreth et al. (1975), Scargill (1971), and Haber et al. (1974).

Figure 2 shows the conversion of SO2 versus the surface temperature for tube reactors 30 cm (0) and 435 cm (0) in length. Through the transparent Teflon wall of the 435-cm reactor, it was observed that product deposita formed on the first 60 cm only, corresponding to an "active" residence time of about 0.46 s at 50 OC. Conse-

2112 Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992

0.6

6 - v, c 0 0.4

C 0 -

a, > 0.2 C 0 - 0 .

.- P -

0 . 0 -

'9 -Y&, 0 Exp. equi l ibr ium - & \

- o',,', * Eq: (NH4)rSOa . 0 .,"\

0 \' Eq: (NHI)zSO, 0 0:.

\' % ':K x Eq: (NHI)ZSO,.HZO - \',\

le\, \",, A Eq: NH,HSOJ

$ 'i

?\

a Ooo '5

8 0 <, *\

- 4 "0 k',

O \\e\ 0 \ \

\\ \ \ \ \ 0 %

' ' a ' L ' e 3 b

Figure 3. Comparison between experimental equilibrium data and data calculated using the equilibrium constanta listsd in Table I. Inlet conditions: 1070 ppm SO,; NH3/SOZ ratio, 1.7; 6.0 vol % H20.

8 0 0.4 .- P a, > c 0 0.2 0

Table I"

- \ x o\- 4 O\O 0 0

-

ammoniumpbosulfite (a):

ammonium sulfite (b):

hydrated ammonium sulfite (c):

ammonium bisulfite (d):

(NH,),SOS(s) + 2NH3(g) + 2SOAg) + H2O(g)

(NH~)ZSO~(E)

(NH~~~SO~*HZO(S) + 2"3(g) + SOz(g) + 2H20k)

NH4HSO&) + NH,(g) + SOzk) + HzOk)

2NH3(g) + SOz(g) + HzO(g)

Ka p"Q2pS0:pH20 log K, 41.89 - 17705/T Kb = PNH$ISO&~O log Kb = 33.27 - 14171/T K, = p N H s P s O ~ H ~ ~ ~ log K, = 42.00 - 17411/T Kd = P N H ~ P S O ~ H ~ O log Kd = 23.77 - 9958/T

O p is equilibrium partial pressures in atmospheres and T the temperature in kelvin.

quently, equilibrium at the outlet was assumed in this experiment. However, in Figure 2, it is shown that the results from the equilibrium experiment do not differ from the data obtained using the 30-cm reactor, and it was concluded that the formation of sulfite took place rapidly. Notice that the scattering of the data reflects the exper- imental uncertainty.

Thermodynamics of Sulfite Formation Equilibrium constants of the formation of four S(1V)

salts, given by Scargill (1971) and Tock et al. (1979), are listed in Table I. In Figure 3, the temperature profile of the abovementioned equilibrium experiment is compared to profiles calculated from the thermodynamics. Bearing in mind the experimental uncertainty, the composition of the S(IV) salt or salt mixture formed cannot be determined on this basis.

It was found that results obtained from thermodynamic data on ammonium salt formation may vary to a great extent. For example, thermodynamic functions derived by Tock et al. (1979) based on data given by Earhart (1969) were tested but gave significant deviations from both ex- perimental results and calculations using the expressions in Table I. In experiments carried out by Landreth et al. (1975), the enthalpy and entropy for ammonium sulfite formation was determined with about 20% uncertainty.

Experimental Results (11): Formation of Sulfate The reaction product obtained under flue gas conditions

primarily contained sulfur(V1). A search was therefore initiated to find one or more flue gas components which in concentrations typical of coal combustion flue gases effected a rapid oxidation of S(1V) or a direct formation of sulfate. The search was conducted adding one compo-

0.8

0" v) 0.6

O Y-

K 0 0.4 .- P

300 p p m NO

A 3000 pprn NO

45 50 55 60 65 70 S u r f ace Tempera tu re (deg .C)

Figure 4. Temperature profiles obtained with HCI (0) and NO present (0 , A) compared to the case without additive (the area within the broken lines). Inlet conditions: 775-1110 ppm SO,; NH3/SOZ ratio, 1.7-2.4; 5.5-6.0 vol % H,O; 4.5 vol % 0,. Specific surface area of the tube reactor: 3.0-4.4 m2/((N m3)/min) at 0.74.8 (N L)/min.

6 cn 0.6 0 +

n

t 0.0 ' ' ' ' ' ' ' ' ' r * ' ' " " ' ' ' ' '0' 'AI

Su rf ace Tern peratu re (deg .C) 45 50 55 60 65 70

Figure 5. Comparison of temperature profiles obtained with and without NOz. Inlet conditions: 930-1100 ppm SO,; NH3/SOZ ratio, 1.7-2.0; 5.5-6.0 vol % HzO 4.5 vol % OP Reactor specific surface area: 3.0-4.4 m2/((N m3)/min).

nent at a time to a gas containing SO2, NH3, H20, 02, and N2.

Thermodynamics indicate that NH4C1 aerosols may be formed by reaction between NH3 and HC1 in flue gas at or below about 100 OC (Barin, 1989). On the basis of the hypothesis that NH&l particles may act as nuclei for the formation of other solid ammonium salts, HCl(g) was in- troduced. Figure 4 compares the results obtained by in- troducing 15 ppm HCl (0) to results obtained without additive. The broken lines define an area of experimental uncertainty beyond which any effect is assumed to be significant. Evaluated in this way, HC1 has no significant effect. Also shown in Figure 4 are the results obtained by adding 300 and 3000 ppm NO. With 300 ppm NO present (o), the temperature at which reaction was first observed (initiation temperature) was not significantly higher than it was without the additive. With 3000 ppm NO present (A), the initiation temperature was significantly higher, and the slope of the profile is not as steep. In order to check if this was due to the formation of NO2 from NO, NO2 was added.

Figure 5 shows the marked effect of NO2 on the observed conversion of SO2 versus temperature. The data shown are taken from a number of experiments performed under

Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 2113

0.6 1 1 1 1 1 1 1 1 1 , 1 1 1 , , , , , , 1 \a, 0 5 ppm NOz

0.5 10 pprn NO2

A 20 ppm NOz 0" V, 0.4

c 0

0

e, 0.2 > C 0

0.3 .- P

0 0.1

n n -.- 55 60 65 70

Surface Tern peratu re (deg .C) Figure 6. Temperature profiles obtained by varying the concen- tration of NO2. Inlet conditions: 920-1000 ppm SO,; NH3/SO, ratio, 2.0; 5.5-6.0 vol % HzO; 4.5 vol % 0,. Reactor specific surface area: 4.4 mZ/((N m3)/min).

0 1 2 3 4 Oxygen Concentration (vol-n)

Figure 7. Influence of oxygen on the reaction at 59 "C with 16 ppm NO2 present. Notice the starting point of the Y-axis. Inlet condi- tions: 960 ppm SO,; NH3/SOz ratio, 2.0; 5.8 vol % HzO; 4.5 vol % OZ. Reactor specific surface area: 4.4 m2/((N m3)/min).

similar conditions. In the presence of only 15 ppm NO2 (and 5 vol % 02), the initiation temperature of the reaction increased about 15 "C. Moreover, the appearance of the reaction product changed from liquidlike to white and crystalline.

The temperature profiles shown in Figure 6 were ob- tained by varying the NO2 concentration from 5 to 30 ppm. It is apparent that the conversion of SO2 at a given tem- perature increases with the addition of NO2. Sudden "jumps" in the profiles were observed in several cases as shown, which probably indicate a surface dependence of the reaction.

The influence of the oxygen concentration on the re- action at 59 "C with 16 ppm NO2 present is shown in Figure 7. Above about 1 vol %, the reaction order in oxygen is apparently zero. With no oxygen present, a conversion of zero is expected because S(1V) is not stable at the actual conditions. Oxygen was not removed com- pletely, due to the presence as an impurity of dissolved oxygen in the humidifier water and in the nitrogen cylin- ders.

The search was continued with the addition of fly ash, HC1, NO, CO, C02, CHI, and SO3 to a gas containing SO2, H20, NH3, NO2, and 02. Fly ash was introduced onto quartz wml placed in a reador tuh experiments were also conducted with quartz wool alone for comparison. Only NO was found to have a significant effect: with both NO and NO2 preaent, a "tail" was observed on the temperature profile (Figure 8). This may be due to an effect of NO on the absorption of NO2 in a water film layer, as discussed below. Presence of 10-20 ppm of SO3 did not influence

10 ppm NO2

55 60 65 70 75 Surface Tern perat ure (deg r.C)

Figure 8. Temperature profile obtained with the addition of 15 ppm NO, and 300 ppm NO (X) compared to the 10 and 20 ppm NO, profiles shown in Figure 6. Inlet conditions: lo00 ppm SO,; NH3/SOp ratio, 2.0; 6.0 vol % H20; 4.5 vol % 02. Reactor specific surface area: 4.5 m2/((N m3)/min).

0 10 ~01% Hz0

0.5 - 0 6 VOI-S HzO - A 3 VOI-S HzO 0 " -

V, 0.4 O 1.5 VOI-S Hz0 - 0

-

% - -

Figure 9. Observed conversion of SO, versus the reactor surface temperature at water contents between 1.5 and 10 vol %. The up- ward and downward arrows indicate cooling and heating profiles, respectively. The cooling profile with about 3 vol % water was not obtained. The reactor was equipped with a heating jacket. Inlet conditions: 500 ppm SO2; NH3/S02 ratio, 2.0; 5.5-6.0 vol % H,O; 4.5 vol % 0,; 10 ppm NO,. Reactor surface area: 27 cm2. Gas flow: 0.58 (N L)/min.

SO2 removal at all, but formation of ammonium sulfate via condensed sulfuric acid was observed above 100 O C .

Surface Dependence Phenomena indicating a surface dependence of the re-

action have been observed in several experiments, the sudden jumps shown in Figure 6 being an example. An- other example is the temperature hysteresis shown in Figure 9. These results were obtained when starting from a reactor temperature around 70 OC, cooling at a rate of 0.1 "C/min to about 55 "C, and then reheating (without removing the product formed) until no conversion was observed. No product was present initially. As shown, the cooling and heating profiles are not identical. In one ex- periment, the above procedure was repeated three times without cleaning the reactor in between (without removing the product formed). The heating profile remained about the same, while the gap between cooling and heating profiles diminished gradually.

Finally, an influence of the reactor construction material on the initiation step of the reaction was observed. Most significant was the effect when a nylon tube (i.d. 4.5 mm/length 44 cm) was used: at 61 "C, the conversion of SO2 was only 0.08 compared to 0.47 when a Teflon reactor

2114 Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992

n 4- 1.4 1.7 1 .e 2.1 2.3

Inlet NH3/S02 Stoichiometry

F i r e 10. Observed N to S reaction stoichiometry with NO2 preaent calculated from measured inlet and outlet concentrations of NH, and SOI. Inlet conditions: 516560 ppm SO2; NH,/SO, ratio, 1.85-2.10: 5.4-6.1 vol % H,O 4.5 vol % 0,: 10 oom NO,: reaction tempera&, 39-59 oC. 0.6-3.9 (N L)/min.

&iZO cma.-'Gaa flow:

with similar dimensions, inlet conditions, and thermal history was employed. When the nylon reactor was cooled about 10 "C and then reheated to 61 'C, the conversion increased to 0.51. The choice of construction material had no effect once the surface had heen covered with reaction product.

Composition of Sulfur(V1) Product The reaction product obtained in the presence of both

NO2 and 0, was thermally stable above 100 OC. Very little or no generation of SO, and NH3 was detected when product formed in the presence of NO, and 0, at the S ( N formation temperature was flushed with N, or heated moderately. Two samples of product, one obtained at 60 OC and one

at 65 "C, were analyzed by X-ray diffractometry. The characteristic peaks of the samples were compared with reference characteristic peaks of ammonium sulfate, (NH4),S04, and ammonium pyrosulfate, (NH4),S207, and most major peaks of the samples could be accounted for. From the ratio of the height of the highest (NH&SO, peak to the height of the highest (NH4),S,0, peak, the molar N to S ratios of the samples were found to be 1.64 and 1.77, respectively. Taking the uncertainty of the method into consideration, it was concluded that the two samples were identical, with a molar mass of about 146 i 5 g/mol.

The overall reaction stoichiometry, a, was determined on the basis of measured outlet concentrations of SO, and NH,. Figure 10 shows a versus the inlet N to S ratio. The reaction stoichiometry calculated from the 32 data points shown is 1.M (solid line) f 0.15. The result from the X-ray analysis agrees quite well with this result.

Discussion The marked effect of NO, on the reaction rate indicates

that the oxidation of SO, is not directly affected by 0, but that some intermediate step is involved. A similar con- clusion has heen drawn from many years of experience with the lead-chamber process used previously for the manu- facture of sulfuric acid (Duecker and West, 1959). In a theory of Berl(1935) it is proposed that the oxidation of SO, actually takes place at the gas-liquid interface and that the reaction is a cyclic process involving the alternate oxidation and reduction of a transiently formed interme- diate species, written as NO-H+304. Due to decomposition of NO.H2SO4, the operation of the cycle is dependent on a continuous supply of NO,(g). A catalytic effect of NO, on sulfite oxidation by 0, was reported by Takeuchi et al. (1977) and Wittig et al. (1988) in agreement with the re- sults obtained in this study, because as little as 5 ppm NO,

Figure 11. Mechanism proposed.

is capable of oxidizing, for example, 500 ppm SO, in the presence of 5 vol % 0,.

The effect of NOz on the initiation temperature of the reaction (Figure 5) may be due to the heterogeneous re- actions of NO, and HzO studied by Jenkin et al. (1988). Jenkin et al. found that adsorbed nitric acid, HNO,(ads), was formed and that the reaction

NO,(adn) + H,O(ads) - NOz-HzO(adn) (1) determined initial kinetics. The study was carried out at room temperature and partial pressures of NO, and HzO of about 4 ppm and 0.5 vol % HzO, respectively.

With dependence on the relative humidity, NOz in probably absorbed in a water film rather than adsorbed prior to reaction. According to Schwartz and White (1981), gaseous NO2 is in equilibrium with NO; and NO3-:

2N02(g) + H,0(1) + 2H+ + NO3- + NOz- (2)

3NOz(g) + HzO(l) % 2H+ + 2N03- + NO@ (3)

NO(g) + NO,(g) + HZO(1) % 2H+ + 2NO2' (4)

Notice in eqs 2-4 that the molar ratio of NOz- to NO&) is l/,, 0, and 2, reapectively. Martin et al. (1981) concluded that NO,- oxidizes S(IV) more rapidly than diasolved NO or NO, does. Thus if NO, is actually absorbed and NOz- is rate-limiting, then the rate of reaction should increase with NO, partial pressure and in the presence of NO (eq 4), as was also shown experimentally (Figures 6 and 8). Martii et al. (1981) also reported that nitrous oxide, N,O, was evolved at pH values below 2. However, no N20 was detected in this study when Samples were analyzed using gas chromatography.

Oblath et al. (1981) found that the initiate product of the reaction between NO2- and HSO, wan hydroxylamine disulfonate (HADS), HON(SO3)?-, at 15-30 "C in the 4.5-7 pH range using approximately equimolar solutions:

NO,- + 2HS03- =, HON(S03)z2- + OH- (5)

The HADS eventually reacts further with SO,?- or HzO to produce S(VI). On the basis of eqs 2-4, an increane in pH is expected to increase the rate of absorption of NOz, while the rate of oxidation of S(IV) decreases, eq 5. In- vestigations carried out by Tanaka et al. (1979) a t 50 "C showed a 300-fold increase in the rate of conversion of NOz- when the pH value was decreased from 7.5 to 5.6.

Proposal for Reaction Mechanism and Kinetics It is proposed that the reaction mechani i can be sim-

p l i ed as outlined in Figure 11. The proceas takes place on a surface, and three phases are considered gas, water film, and solid. Except for the solid, all species situated on the surface are considered as being absorbed in ad- sorbed water (or water film), assuming that a few or eaveral

Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 2115

moles per unit surface area of R1 and & present. kl and kl are defined by k 1 / k l = kl*/kl*Kso *bKN abKWaQ where Krb are absorption Constants, and KwaBis an ad: sorption constant of water. Similarly we have k 3 / k 3 = k?*/k-3*KN0;bS. The absorption constants, Killb, are de- rived below from the pertinent wet-chemical equilibra:

(9)

(10)

(11)

net: HzO(l) + S02(g) + S032- % 2HS03- (12)

where Ks0;" = h&Kl,,/Kll). With respect to NH3 we have h13

NH3(g) 9 NH3(aq) (13)

(14)

(15)

(16)

h¶ HZO(1) + SOz(g) t; H2SO3

HzS03 '=; H+ + HS03-

H+ + S032- + HS03-

K m h

KlO

K i i '

K14 NH3(aq) + H20 - NH4+ + OH-

HSOc * H+ + S032-

OH- + H+ + HzO

KIS

Kw-1

net: K m h 9 NH,(g) + HZO(1) + HSO, H20 + 502- + NH4+

(17)

where K"sabs = h13(K14K15/KW). Making eqs 7 and 8 equal to zero due to the assumption of quasi-statiomrity and rearrangement gives

5 0.2 0 'Bi 0.0

0 20 40 60 80 100 Spec. Surface Areo, m2/(Nm3/min)

Figure 12. Fit of k, and kox (solid line) using data obtained at 60 "C with 15 ppm of NOz present. Inlet conditions: lo00 ppm SO,; NHB/SOz ratio, 2.0; 6.0 vol % H,O; 4.5 vol 7% 0,; 10 ppm NO,.

layers of adsorbed water molecules can provide a medium for fast solution chemistry.

The first step of the reaction is the absorption of SO2, NH3, 02, and NO2 into the water film. This step is not considered to be rate-limiting: rapid equilibrium exists between the gaseous and absorbed species. The next step is the formation of intermediary soluted species. The existence of two intermediary products (Rl and R2) is proposed, containing sulfur in oxidation states IV and VI, respectively. & is the product of the reaction between R1 and absorbed NOz. The final step of the reaction is the transformation of R1 and R2 into crystalline ammonium salts denoted S(1V) and S(VI), respectively. S(1V) is unstable with respect to decomposition, while S(V1) is thermally stable under the conditions studied. S(V1) is primarily ammonium sulfate. An important feature of the mechanism is that adsorbed NO2 acts as an oxygen carrier, so that & is formed only in the presence of both NOz and 02. Notice that the actual chemical compositions of R1 and Rz are not precisely known and that additional in- termediary products may be formed in the course of the reaction. The rate of oxidation of S(V1) is probably in- fluenced by film conditions such as pH and the presence of trace metals or oxidants, but this wi l l not be considered. A simple mathematical model has been developed on the basis of the mechanism and the following assumptions:

(1) No reaction takes place in the gas phase. (2) The first reaction step is an absorption of gaseous

reactants in a water film on surfaces. The quantity of water adsorbed is approximated, assuming direct pro- portionality with the water partial pressure.

(3) Direct proportionality exists between the partial pressures and the film concentrations of SOz, NH3, and NOz. All equilibra are rapid.

(4) There is quasi-stationarity with respect to the in- termediary products Rl and & and zero-order dependence in solid S(1V) and in 02(abs) (cf. Figure 7). There is first-order dependence in H20(ads), SOz(abs), NH3(abs), and NOz(abs).

(5) The direct oxidation of S(1V) to S(V1) by Oz is slow and does not influence the overall reaction rate. The effect of gaseous NO on NOz absorption is not considered.

(6) The overall reaction rate is not limited by the crystallization process.

Equations 6-8 are derived, where Rj is the rate of for- mation in number of moles of component i per unit surface area and per unit time. N R 1 and N R z are the number of

R S 0 2 klPH20PS0#NH3 - k-lNR1 (6)

~ ~ N O , N R , - k - 3 N ~ ~ = 0 (7) (8)

= k- lNR1 - klPH20PS0#NH3 + k2NR1 - k-2 +

R, = k - 3 N ~ ~ - k $ N 0 2 N R l + k4Nk = O

where

As long as product is formed, it is reasonable to assume that kl << u, and with the definitions

eq 18 can be written as

If pH2O and p N O 2 are considered to be approximately con- stant, the rate of reaction is second order overall. Notice that if pNO and RSo, are both zero, eq 21 takes the form of the equdibrium expression of ammonium bisulfite; thus Ks4 is related to the S(1V) equilibrium. Insertion of eq 21 in the performance equation of a plug flow reactor, ana- lytical integration over the surface area, and rearrangement

2116 Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992

0

Table XI. Pre-Exponential Factor and Activation Energy of the Model Parameters

0.3 0

Q) 0.2 > c - 0

.- !?

* 0.1

f (T ) APE LL,, kJ/mol

Kd 1.8 X loz3 atm3 190 k, 3.6 X (mol/cm2)/(min/atm3) -85

kox 2.2 x lo-* atm-l -50

result in an explicit expression of the conversion of SO2 obtained

- - -

-

where

and

x, =

xz =

The above expressions are valid provided that there are no mass-transport limitations. Mathematical modeling assuming a laminar gas velocity profile and mass transport by independent radial diffusion of NH3 and SOz has shown that this is a reasonable assumption in the range of gas velocities and reactor lengths used (Hjuler, 1991).

Estimation of the Model Parameters The parameters k,, Kd, and kox were all estimated using

a leasbsquarea algorithm (Fletcher, 1971) and the classical Arrhenius temperature dependence, even though Ks4 and kox are defined by several rate constants (eqs 19 and 20). The value of a was kept constant at 1.8 in agreement with the N to S stoichiometries obtained in the presence of NOz (Figure 10). Ks4 is fitted to the experimental equilibrium data depicted in Figure 3 (0) obtained without NO2 present. Proceeding in an optimal way, kl should be de- termined from the S(1V) formation data (Figure 2), but no reliable kinetic information can be extracted from them. Alternatively, the values of both kl and kox at 60 OC with 15 ppm of NO2 present were determined from the data depicted in Figure 12 obtained by varying the length of an 1/8-in.-i.d. Teflon tube. In this way a reasonable fit was obtained, as shown by the solid line.

Finally, freezing the values of kl and kox found at 60 "C, the activation energies of kl and kox were determined by using the 10 and 20 ppm NO2 *cooling" profiles shown in Figure 8. Due to the limited temperature range in- volved, the scattering of the data, and the fact that kl and kox are not independent variables, the values of the ac- tivation energies of k, and kox can be varied without af- fecting the result significantly. Chosen values of the pre-exponential factors, A,, and activation energies, E,, of the parameters are given in Table 11.

0.5 -

0 " - 1/) 0.4

O

- Y-

0.3 - 0

Q) 0.2 - > c 0

.- !?

0 0.1 -

0 0 5 PPm

0 30 ppm ?

0

0

0

O 0 10 VOI-S HzO

0 6 vol--li H20 0.5 1

0 0

VOI%

0 0

o,, 0 0.0 ' , ' I . " * ~ ' ' b ' ~ ' ~ c l '

Surface Temperature (degr.C) 50 55 60 65 70

Figure 14. Cooling parts of the temperature profiles depicted in Figure 9 (symbols) and the corresponding calculated profiles (-).

Comparison of Model Predictions with Experimental Data

In the following, experimental results not used in the parameter estimation are compared with model predic- tions. The model should be able to predict the dependence of the conversion of SOz on various concentrations of NO2, H20, SO2, and NH3 within an experimental uncertainty of about 0.1.

Figure 13 shows the conversion of SOz versus tempera- ture with 5 and 30 ppm of NOz present. The increase in the conversion of SOz and the initiation temperature with increasing NO2 content of the gas is predicted quite well.

In Figure 14, the cooling profiles from Figure 9 are shown together with the theoretical curves. The depen- dence of the initiation temperature on the water content of the gas is predicted reasonably well. However, the "heating" parts of the temperature profiles do not corre- spond well with the model predictions. This is due to the fact that the parameters of the model were fitted using only cooling data and constant temperature data.

In Figures 15 and 16, the conversion of SOz at 60 OC is plotted against the inlet concentrations of SOz and NH3, respectively. In Figure 15, the NH, inlet concentration is kept constant at about 2000 ppm, while the inlet SO2 concentration is varied. In Figure 16, the SOz concen- tration is kept constant at about lo00 ppm, and the inlet

Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 2117

surface areas such as heat-transfer surfaces and insuffi- ciently insulated ducts. Product formed during plant startup or shutdown may sustain the formation of even more product during operation due to the hygroscopic properties of the salt. Increasing the concentration of NOz, irradiation, or corona dicharge energization (e.g. in elec- trostatic precipitators) significantly favors product for- mation.

When ammonia and sulfur dioxide are simultaneously monitored in flue gas, experience has shown that false measurements can generally be avoided by heating sample lines and gas-contacted surfaces to 100 "C or more.

Acknowledgment We gratefully acknowledge financial support from EL-

SAM, The Jutland-Funen Electricity Consortium. Thanks to Dr. Bo Sander, ELSAMProjekt A/S, and to Bent Agerholm, Slanderjyllands Hlaj speendingsvserk (presently Elkraft), for valuable discussions. Finally, thanks to Dr. Peter F. Binderup Hansen, who carried out a part of the experimental work.

Nomenclature A = surface area of the plug flow tube reactor (cm2) E,,, = activation energy (kJ/mol) hl, hz = Henry's constanta kox = defined by eq 20 (atm-') k1 = reaction rate constant ((mol/cm2)/(min/atm3)) Ki = ionization constant Ktba = absorption constant Ks4 = defined by eq 19 (atm3) Kw = dissociation constant of water KWads = adsorption constant of water M = NH3 to SO2 molar ratio at reactor inlet N = number of moles per unit area (mol/cmz) p = partial pressure, volume fraction (ppm or vol %) 8, = molar flow rate of gas (mol/s) 2 = rate of reaction ((mol/cm2)/min) R = gas constant T = temperature ("C) X i = degree of conversion of component i X1, X 2 = defined in eq 22

Greek Letters a = observed NH3 to SO2 stoichiometry of reaction ,9 = H20 to SO2 stoichiometry of reaction Subscripts i = NH3, H20, SO2, or NOz o = inlet condition Superscript abs = absorbed ads = adsorbed

Registry No. SOz, 7446-09-5; NH3, 7664-41-7; S04(NH1)2r

Literature Cited

7783-20-2; NO^, 10102-44-0.

Barin, I. Thermochemical Data of Pure Substances (ISBN 3-527- 27812-5); VCH Verlagsgesellschaft: Weinheim, Germany, 1989.

Berl, E. Trans. Am. Inst. Chem. Eng. 1935,31,193. Duecker; West. Manufacture of Sulfuric Acid; ACS Monograph

Series No. 144; Rheinhold Publishing Corp.: New York, 1959. Earhart, J. P. Discussion of Gaseous Ammonia for Flue Gas Desul-

furization. Unpublished report from Bureau of Mines, Morgan- town Energy Research Center: Morgantown, WV, 1969 (referred by Tock et al. (1979)).

Fletcher, R. A modified Marquardt subroutine for non-linear least squares; Harwell Laboratory report, AERE R.6799, Subroutine VAO7A/AD, U.K. Atomic Energy Authority: Harwell, U.K., 1971.

Haber, J.; Pawlikowska-Czubak, J.; Pomianowski, A.; Najbar, J. Untersuchungen iiber die Kinetik und den Mechanismus der

q 0.2

5 0.1 0

0 400 800 12w 1600 Inlet Concentration of so2 (ppm)

Figure 15. Experimental (0) and predicted (-) conversion of SOz versus the inlet concentration of SOz at 60 O C . Met Conditions: 20oO ppm NH3; 6.0 vol % HzO 4.5 vol % 0,; 15 ppm NOz. Specific surface area: 4.9 mz/((N m3)/min).

0.3

0.2 .- P t

0.0 0.4 0.8 1.2 1.6 2.0 Inlet NHs/SOz Stoichiometry

Figure 16. Experimental (0) and predicted (-) conversion of SOz versus the inlet concentration of NH3 at 60 "C. Inlet conditions: lo00 ppm SO,; 6.0 vol % H20; 4.5 vol % 0,; 15 ppm NOz. Specific surface area: 4.8 mZ/((N m3)/min).

molar N to S ratio is varied by changing the NH3 con- centration. The model-calculated curves fit well with the experimental data. Notice that no reaction is observed below a threshold value of about 130 ppm SOz X 2000 ppm NH3 (Figure 15) and 300 ppm NH3 X lo00 ppm SOz (Figure 16).

Summary With only SOz, HzO, and NH3 present in the gas, a

thermally unstable, liquidlike or solid ammonium salt containing sulfur(IV) is formed. With Oz and about 5 ppm of NOz also present, the rate of removal of SOz is signif- icantly increased. The product obtained is mainly am- monium sulfate, and it is formed on surfaces. Therefore the reactor surface area rather than the gas residence time determines the reactor performance. The product of the partial pressures of SOz and NH3 must exceed a threshold value for any reaction to be observed. This threshold value depends strongly on reactor surface temperature as well as the concentrations of HzO and NOz. Similarly, a threshold value was reported for the formation of ammo- nium nitrate (Mearns and Ofosu-Asiedu, 1984a,b). A mechanism involving water film reactions was proposed, and a simple three-parameter rate expression for a re- versible reaction of first order in both SO2 and NH3 has been developed from the mechanism. In comparison, Hartley and Matteson (1975) proposed a rate expression for an irreversible reaction. No formation of aerosols was observed under the conditions studied. However, the mechanism and kinetics developed may also be relevant to ammonium sulfate aerosol formation.

Practical Implications of the Results Ammonium salt formed under flue gas conditions con-

tains primarily S(W) due to the presence of oxygen, NO, and NOz. The reaction takes place on relatively cool

2118 Znd. Eng. Chem. Res. 1992,31, 2118-2127

Bindung von Schwefeldioxid und Stickoxiden durch Gasformiges Ammoniak. 2. Anorg. Allg. Chem. 1974, 404, 284-294.

Hartley, E. M.; Matteson, M. J. Sulfur Dioxide Reactions with Am- monia in Humid Air. Ind. Eng. Chem. Fundam. 1975,14,67-12.

Hjuler, K. The Reaction Between Sulphur Dioxide and Ammonia in Flue Gas. Ph.D. Thesis from Department of Chemical Engi- neering, Technical University of Denmark, 1991 (ISBN 87-

Jenkin, M. E.; Cox, R. A.; Williams, D. J. Laboratory Studies of the Kinetics of Formation of Nitrous Acid from the Thermal Reaction of Nitrogen Dioxide and Water Vapour. Atmos. Environ. 1988,

Landreth, R.; de Pena, R. G.; Heicklen, J. Thermodynamics of the Reaction of Ammonia and Sulfur Dioxide in the Presence of Water. J. Phys. Chem. 1975, 79, 1785-1188.

Martin, L. R.; Damschen, D. E.; Judeikis, H. S. The Reactions of Nitrogen Oxides with SOz in Aqueous Aerosols. Atmos. Environ. 1981,15, 191-195.

Mearns, A. M.; Ofosu-Asiedu, K. Kinetics of Reaction of Low Con- centration Mixtures of Oxides of Nitrogen, Ammonia and Water Vapour. J. Chem. Technol. Biotechnol. 1984a, 34A, 341-349.

Mearns, A. M.; Ofosu-Asiedu, K. Ammonium Nitrate Formation in Low Concentration Mixtures of Oxides of Nitrogen and Ammonia. J. Chem. Technol. Biotechnol. 1984b, 34A, 350-354.

Oblath, S . B.; Markowitz, S. S.; Novakov, T.; Chang, S. G. Kinetics of the Formation of Hydroxylamine Disulfonate by Reaction of

983894-0-8).

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Nitrite with Sulfites. J. Phys. Chem. 1981,85,1017-1021. Scargill, D. Dissociation Constants of Anhydrous Ammonium Sul-

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Schwartz, S . E.; White, W. H. Solubility Equilibria of the Nitrogen Oxides and Oxyacids in Dilute Aqueous Solution. In Advances in Environmental Science and Engineering; Pfafflin, J. R., Zie- gler, E. N., Eds.; Gordon and Breach New York, 1981; Vol. 4, pp

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Receioed for review November 20, 1991 Revised manuscript received March 30, 1992

Accepted May 26,1992

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Kinetics of Melt Transesterification of Diphenyl Carbonate and Bisphenol-A to Polycarbonate with LiOH*H20 Catalyst

Yangsoo Kim and Kyu Yong Choi* Department of Chemical Engineering, University of Maryland, College Park, Maryland 20742

Thomas A. Chamberlin Central Research Organic Chemicals and Plastics Laboratory, Dow Chemical Company, Midland, Michigan 48674

The kinetics of batch melt transesterification of diphenyl carbonate with 2,2-bis(4-hydroxy- pheny1)propane to polycarbonate have been studied in the presence and absence of LiOH.H20 catalyst in the temperature range of 180-250 OC. The effects of reaction temperature and catalyst con- centration on conversion and oligomer formation are analyzed. For quantification of the reaction kinetics, a detailed molecular species model has been developed and solved. Quite satisfactory agreement between the model predictions and experimental data has been obtained. The forward and reverse reaction rate constants are estimated, and the difference between the catalyzed and uncatalyzed transesterification kinetics is discussed.

Introduction Polycarbonates derived from the reaction between a

carbonate ester and a diol are important engineering thermoplastics with good mechanical and optical properties as well as electrical and heat resistance useful for many engineering applications. Polycarbonates are produced commercially by interfacial polymerization and melt transesterification processes. Polymerization in an in- terfacial process consists of phosgenation and polycon- densation stages (Schnell, 1964; Oliver, 1969). Phosgen- ation takes place between a bisphenol-A (2,2-bis(4- hydroxypheny1)propane; BPA) salt in an aqueous caustic solution and phosgene in an organic solution. High mo- lecular weight polycarbonate is obtained in the polycon- deneation stage with triethylamine as catalyst. After the polymerization, the organic polymer is isolated and dried. Toxic solvents and reactants such as methylene chloride and phosgene gas used in the interfacial process pose en-

* To whom correspondence should be addressed.

vironmental and safety concerns. In the melt transesterification process, diphenyl carbo-

nate (DPC) and BPA are reacted at high temperature in the presence of basic catalysts such as alkali or alkaline earth metals as their oxides, hydroxides, or phenolates (Schnell, 1964; Oliver, 1969). Phenol is produced as a condensation byproduct and must be removed from the reaction mixture to facilitate the chain growth reaction. During polymerization, the reaction temperature is kept above the melting point of the reaction mixture. In practice, the reaction temperature is increased gradually, and the pressure is reduced to less than 1 mmHg at the final stage of the polymerization.

The melt process offers many unique advantages. Conceptually, the overall process scheme is simpler than the others, and polymer is obtained in undiluted form which can be directly pelletized. Toxic solvents are not used, and thus the process is environmentally more friendly than other existing p~x'~g888. Melt polymerization is also suitable as a continuous process. However, it has been reported that catalyst residue remaining in the

0888-5885/92/2631-2118$03.00/0 0 1992 American Chemical Society