High-Efficiency Structured-Packing Catalysts with Activated Carbon for

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High-Efficiency Structured-Packing Catalysts with Activated Carbon for SO2 Oxidation from Flue Gas Radu V. Vladea,† Natalia Hinrichs,‡ Robert R. Hudgins,*,† Sam Suppiah,§ and Peter L. Silveston† Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, University of Erlangen, Erlangen, Germany, and Chemical Engineering Branch, Advanced Reactor Development Division, Atomic Energy of Canada Limited (AECL) Research, Chalk River, Ontario, Canada K0J 1J0 Received September 20, 1996. Revised Manuscript Received January 27, 1997X

Coiled screen or Sulzer CY gauze structured packings, loaded with an activated carbon treated to make it partially hydrophobic, were employed to investigate the removal of SO2 from a stack gas. Measurements were made of SO2 removal and its conversion to H2SO4 and gas-side pressure drop as a function of gas and liquid loadings, temperature, and method of operation (continuous liquid flow vs intermittent liquid flow). Both packings exhibited remarkably high catalytic efficiency in terms of SO2 converted per gram of carbon, as well as high conversions of SO2 to H2SO4 and low-pressure drops. Although the Sulzer CY packing displayed the lowest pressure drop per meter of packing depth, the coiled screen provided higher removal of SO2 per volume of packing. The performances of each were close enough that a choice between them would require an economic analysis.

Introduction Environmental regulations impose the removal of SO2 from stack gas. Huge volumes of flue gases containing up to 0.3% SO2 impose severe constraints on industrial flue gas cleanup facilities: the need for (1) minimal pressure drops at high gas space velocities (10 000100 000 h-1), (2) fouling- and poison-resistant catalysts, and (3) processes that do not generate solid or liquid waste. One of several SO2 capture processes that appears to meet these constraints is scrubbing of flue gas over an activated carbon which catalyzes oxidation of SO2 to SO3 to form H2SO4.1-10 A scrubbing system, using a trickle bed with relatively large carbon particles, has been commercialized by Lurgi, and full-scale installations exist. The Lurgi process11 has some specific drawbacks: (1) very low effectiveness factor (0.021 for 2.6 mm particles), (2) high-pressure drop, (3) liquid maldistribution, (4) low concentrations of sulfuric acid in the effluent, and (5) a need for high mechanical strength of the carbon particles. †

University of Waterloo. University of Erlangen. Atomic Energy of Canada Limited. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Komiyama H.; Smith, J. M. AIChE J. 1975, 21, 664-674. (2) Amadeo, N. E.; Laborde, M. A.; Lemcoff, N. O. Chem. Eng. J. 1989, 41, 1-8. (3) Lu, G. Q.; Do, D. D. Sep. Technol. 1993, 3, 106-110. (4) Haure, P. M.; Hudgins, R. R.; Silveston, P. L. Chem. Eng. J. 1990, 43, 121-125. (5) Metzinger, J. V.; Ku¨hter, A.; Silveston, P. L.; Gangwal, S. K. Chem. Eng. Sci. 1994, 49, 4533-4546. (6) Haure, P. M.; Hudgins, R. R.; Silveston, P. L. AIChE J. 1989, 35, 1437-1444. (7) Ju¨ntgen, H. Carbon 1977, 15, 273-283. (8) Richter, E.; Knoblauch, K.; Ju¨ntgen, H. Gas Sep. Purif. 1987, 1, 35-43. (9) Davini, P. Carbon 1991, 29, 321-327. (10) Hartman, M.; Coughlin, R. W. Chem. Eng. Sci. 1972, 27, 867880. (11) Ullman’s Encyclopedia of Industrial Chemistry; Verlag Chemie: Weinheim, Germany, 1994; Vol. A25. ‡ §

S0887-0624(96)00164-8 CCC: $14.00

Work undertaken by our laboratory is aimed at improving the activated carbon-based scrubbing process. Our first efforts examined the use of intermittent rather than continuous flushing of the trickle bed and demonstrated that improved removals and conversions to acid can be achieved.4-6 Use of acid in place of water and operating near the boiling point of water were also explored.5 Acid as strong as 8 N can be obtained by operating at 80 °C. Other authors12,13 have reported that the activity of carbon for SO2 oxidation in the presence of water can be enhanced by doping with typical oxidation catalysts or by impregnating the carbon with Pt or Pd. In recent work, we have shown that the Centaur carbon produced by Calgon Carbon of Pittsburgh is active for the oxidation, performing as well as other activated carbon impregnated with noble metals at 0.1 wt %.14,15 In this paper, we report our work on increasing carbon utilization and reducing pressure drop. To achieve the latter, rolled coils to form a coiled screen-type structure or open static mixer configurations were used. In these structures, activated carbon catalyst is deposited as a thin layer on the metal substrate. The carbon is bonded to the metal by a hydrophobic material forming hydrophobic zones on the carbon particles. Experimental Section Catalyst Packings. Two basic types of structured packings of a woven metal fiber (gauze) were used: coiled screens and static mixers. Coiled screens were built up by rolling together a corrugated gauze on which activated carbon was (12) Steiner, P.; Ju¨ntgen, H.; Knoblauch, K. Adv. Chem. Ser. 1975, No. 139, 180-191. (13) Kruel, M. Brennst.-Waerme-Kraft 1971, 23, 91-97. (14) Lu, G. Q.; Gray, P. G.; Do, D. D. Chem. Eng. Commun. 1990, 96, 15-30. (15) Py, X.; Roizard, C.; Midoux, N. Chem. Eng. Sci. 1995, 50, 20692079.

© 1997 American Chemical Society

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Table 1. Catalyst Bed Types and Dimensions catalyst configuration

carbon particle mesh size

mass of Centaur carbon (g)

height (cm)

area of cross section (cm2)

volume (cm3)

coiled screen hydrophilic spacing screen hydrophilic spacing screen hydrophilic spacing screen hydrophilic spacing screen hydrophobic spacing screen static mixers CY

120 20 20 28 28 28

10.23 23.43 22.52 28.44 28.44 23.28

3 × 10 3 × 10 3 × 10 1 × 10 1 × 10 2 × 15

19.63 19.63 19.63 19.63 19.63 15.90

589.1 589.1 589.1 196.3 196.3 476.8

Figure 1. Schematic of the coiled screen packing showing position of screen spacers.

Figure 2. Photograph of the Sulzer CY static mixers cut and bundled together to form the trickle-bed packing. bound using a hydrophobic material. The coated screens in the roll were separated with an uncoated screen that was hydrophilic but could be made hydrophobic. A roll packing is shown in Figure 1. The coiled screen catalytic rolls were supplied by Atomic Energy of Canada Limited (AECL), Chalk River, ON. Figure 2 shows the packing based on Sulzer static mixers cut and bound to fit into our trickle-bed reactor. Static mixers, type Sulzer CY, were supplied by Sulzer Chemtech, Winterthur, Switzerland, and loaded with activated carbon by AECL, using their proprietary binder technology. Properties of the activated carbon and the catalytic packings are given in Table 1. Centaur activated carbon (Calgon Carbon Corp.) with an average particle diameter of 0.3 µm was used to coat all of the packings employed in our study. BET surface areas of the bound carbon were determined using a Micropore Version 2.44 Quantachrome Autosorb unit. Typical values were 139.9 m2/g for overall surface area, 95.7 m2/g for mesopore area, and 44.2 m2/g for micropore area. Apparatus. The SO2 oxidation was carried out in a glass reactor of 600 mm length and 50 mm i.d. Gaseous reactants and water flowed downward through the reactor. Water was supplied by a gear micropump (Model 130/150, Micropump Corp., Vancouver, WA), after preheating (as needed) and

Figure 3. Schematic of the experimental trickle bed. saturation with oxygen in a saturation column filled with polypropylene static mixers (Sulzer Mellapak 125Y). An adjustable height spray nozzle was used to distribute washing liquids evenly over the cross section of the reactor. A schematic of the apparatus appears in Figure 3. Periodic liquid interruptions were achieved by means of a solenoid valve in the feed line to the reactor vessel. The valve was controlled by a microcomputer through a D/A interface. Feed gas was a simulated stack gas with a volumetric composition of 10% CO2, 5% O2, and 0.3% SO2, the balance being N2. The gas was preheated (as needed) and saturated with water at 45 °C corresponding to 10 vol % H2O in the gas mixture. Flow rates of the individual gaseous streams were governed by mass flow controllers (Unit Instruments, Inc., Orange, CA). The reactor was jacketed and a circulating fluid used to minimize heat loss (see Figure 3). Temperatures were measured by 0.2 mm o.d. thermocouples and recorded. Pressure drop in the catalytic bed was measured with a water-filled manometer connected to taps located above and below the bed. The reactor effluent was

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Table 2. Operating Conditions for the Packing Performance Studies temperature, °C pressure, kPa liquid phases gas flow rate, m3/s, at 25 °C liquid flow rate, m3/s

23-80 101.3 deionized water (DI) DI + Tween 20 (0.4 µmol/L) DI + acetone (20 wt %) (5.5-33) × 10-5 (0-55) × 10-9

cooled, and liquid and gas phases were separated in a disengager just below the trickle bed. SO2 concentration in the gaseous effluent was continuously monitored by a UV spectrometer (Western Research, Model 721 AT). The liquid phase was analyzed for H2SO4 by titration with 0.1 N solution of NaOH. Sulfurous acid and sulfur dioxide dissolved in liquid effluent were determined by means of a conventional iodine titration. Operating Conditions. Typical conditions are summarized in Table 2. As can be seen, variables in the performance study besides packing type and hydrophobicity of the spacing screens were temperature, gas phase space velocity, and the liquid loading. Several runs were made using a surfactant, Tween 20, to explore the influence of the combined internal and external wetting of the packing. Higher internal wetting could increase the SO2 conversion rate. Acetone was also used to alter the surface tension of water and was part of the wetting study.

Results Exploratory investigations were carried out on three catalytic packings: coiled screens coated with 20 mesh carbon particles and with hydrophilic spacing screens, coiled screens coated with 28 mesh carbon particles and with hydrophobic spacing screens, and type CY static mixers coated with 28 mesh carbon particles. The binder used to hold Centaur carbon on the screens was hydrophobic, so the carbon coatings were at least partially hydrophobic. Operation Observations. Coiled screens coated with 20 and 28 mesh particles and hydrophilic spacing screens exhibited high liquid static holdup (equivalent to about 20% of the packing weight). Both periodic operation (flushing periods of 30-45 s) followed by “dry” periods of 255-270 s) and continuous flushing gave nonreproducible results. Periods of high SO2 removal and conversions alternated with periods of lower removal in a random way. The data suggest alternate charging and discharging of a reservoir of SO2. Possibly the reservoir is the liquid holdup on the hydrophilic spacing screens. This was not tested, nor was an explanation sought for this surprising behavior. Despite high SO2 removal and conversions in portions of an experimentsespecially in forced periodic operationsthe influence of process variables could not be measured because of the oscillating SO2 removal and conversions in other portions of the experimental run. In experiments with the coiled screen packing and a hydrophobic spacing screen, the static holdup was smaller and we did not observe any erratic behavior. The Sulzer CY sieve packing also gave reproducible data. Comparison of Packings. Although our experiments were limited, a comparison of the performance for the three packings is possible. The critical performance variables for scrubbing were SO2 removal from the stack gas, conversion to acid, and pressure drop. Removal was based on SO2 fed to the trickle bed through the mass flow controller and UV spectrometric measurement of the gas phase SO2 leaving the bed. A soap

Figure 4. Comparison of the three packings studied (one with periodic flow interruption) for the important performance criteria: (a) % SO2 removal; (b) rate of SO2 removal per unit volume of bed; (c) conversion of SO2 to H2SO4 expressed as acid produced/g of carbon‚h.

bubble meter and the spectrometer were used to check the mass flow controller. Conversion was calculated from the acid formed and the SO2 fed to the trickle bed. The acid formed was measured by titration. Titration also gave sulfurous acid and dissolved SO2. Figure 4 compares the packings for these performance variables. With respect to SO2 removal, the highest removal percentages are obtained with the coiled screen packing with hydrophilic spacers. However, as pointed out above, operation with this packing was erratic and occasional excursions with poor removal were observed. Figure 4a also shows there is considerable variation among experiments for each type of packing. In making our comparison, we used averages of the measurements shown in the figure. Thus, the Sulzer packings provide a mean percent removal about a few percent higher than the removal observed for the coiled screens with hydrophilic spacers. Experiments with the hydrophobic spacers gave removals somewhat higher again. With intermittent flushing of the coiled packing with hydrophilic spacers, the percent SO2 removal was less. The percent removal data given in Figure 4a compare different

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Figure 5. Square root of pressure drop vs superficial gas velocity at T ) 22 °C, P ) 105 kPa: (a) coiled screen with hydrophobic spacers; (b) Sulzer CY packing.

depths of packing, as well as different carbon granule sizes. For example, the coiled screen packing with a hydrophobic spacer had a depth about one-third that of most packings with the hydrophilic spacers. If removal is considered in terms of the rate per unit volume of packing, conclusions change somewhat. Figure 4b shows that the coiled screen packing with hydrophobic spacers now becomes the best packing, followed by the packing with hydrophilic spacers. The Sulzer packing and intermittent flushing of the coiled screens with hydrophilic spacers reveal lower removal rates per volume of packing. With respect to acid production, that is, conversion of SO2 to acid, the Sulzer packing shows the highest conversion rate, followed by the coiled screen with the hydrophilic spacer. The rates of acid production for the coild packing with hydrophilic spacers under intermittent flushing are lower still. Pressure drop measurements demonstrate an advantage for the Sulzer packing. Experiments were performed only on coiled screens with hydrophobic spacers. Experimental results are shown in Figure 5. The square root of the pressure drop is plotted against the superficial gas velocity for different liquid loadings.

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Influence of Gas and Liquid Loading on Pressure Drop. Mass flow controllers in the trickle-bed experiments were undersized for the flows needed to obtain accurate pressure drop measurements. Bed depth was limited by the dimensions of the glass cylinder used for the trickle bed. This, in turn, was sized to provide accurate measurements of SO2 removal and its conversion to acid. To measure pressure drop, therefore, air was used and the air mass flow controller was bypassed. Flow rate was measured using a rotameter placed upstream from the trickle bed. Two sets of experiments were carried out: the first used cocurrent liquid and gas flow, while the second set turned off the liquid flow and observed the transition to the pressure drop at a new steady state in the absence of liquid flow. Our intent in observing the transition was to monitor the draining of the packing after the liquid flow was halted and to see if evaporation of the static liquid phase could be observed using pressure drop measurements. It was found that the transition occurred in several seconds and was so fast that it could not be used to measure draining. Pressure drop was, as expected, significantly lower after the liquid flow was turned off. However, even with the much higher gas flow rates used, the evaporation effect on pressure drop could not be measured. Figure 5a shows the pressure drop measured for the coiled screen packing with the hydrophobic spacers. Measurements were not made for screens with hydrophilic spacers. Figure 5b gives the measurements for the Sulzer CY screen packing. The square root of the pressure drop is plotted in these figures vs the superficial gas velocity. Both the Talmadge and Ergun equations16 predict straight lines in this type of plot. Figure 5 shows that the experimental data are indeed well correlated by the plot. Solid lines show the pressure drop in the presence of cocurrent liquid flow. A significant decrease is evident when the liquid flow through the bed is interrupted. As expected, the pressure drop increases with liquid loading but liquid loading is less important than the superficial gas velocity. The experimental measurements can be represented by the following linear least-squares correlations:

coiled screen packing: (∆P/Z)1/2 ) 11.126vg + 167.18vl - 1.287

(1)

Sulzer packing: (∆P/Z)1/2 ) 14.248vg + 55.031vl - 1.865

(2)

The units of ∆P/Z, the pressure drop per unit of column length, are kPa/m; units of the superficial velocities of liquid and gas, vg and vl, respectively, are both m/s. For both packings, vg ranged from 0.145 to 0.178 m/s. For the coiled screen packing, vl ranged from 0 to 1 mm/s, but for the Sulzer packing vl ranged from 0 to 2.8 mm/ s. The correlation coefficient for both correlations was 0.989. These correlations permit an estimate of pressure drop at much higher flow rates. Effect of Gas and Liquid Loadings on SO2 Removal and Conversion to Acid. SO2 scrubbing in trickle beds with catalytically active packings requires (16) Moreno-Castilla, C.; Carasco-Marin, F.; Utrera-Hidalgo, E.; Rivera-Utrilla, J. Langmuir 1993, 9, 1378-1383.

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Figure 6. Effect of superficial liquid velocity on SO2 conversion to H2SO4 for Sulzer CY packing and for the coiled screen packing with hydrophobic spacers in the presence and absence of surfactant. T ) 22 °C, P ) 105 kPa, superficial gas velocity ) 3.4 cm/s.

SO2 and O2 dissolution in the liquid phase. The dissolved species are converted on the activated carbon surface to SO3, which combines instantly with water to form H2SO4. Thus, the liquid phase draining from the scrubber contains H2SO4 as a dilute, fully ionized acid and dissolved SO2, some of which would be present as sulfurous acid (H2SO3), a weak acid only partially ionized. The titrations used measured strong acid and total acid. The strong acid is assumed to be H2SO4 and the remainder is presumed to be split between sulfurous acid and dissolved SO2. Because dissolved SO2 can desorb and shift the sulfurous acid equilibrium toward SO2, the objective of scrubbing is the highest possible conversion of SO2 to H2SO4. Figure 6 shows the variation of SO2 conversion to H2SO4 as a function of superficial liquid velocity for the coiled screen and Sulzer packings. In this experiment, the liquid was saturated with oxygen before entering the bed. An increase in SO2 conversion with liquid flow rate at constant temperature and a constant gas loading is clearly evident. Data for the coiled screen packing with hydrophobic spacers lie below those for the Sulzer packing (also partially hydrophobic) and appear to exhibit a greater slope. This is probably the result of the external carbon surface area, which is higher per unit volume for the coiled screen packing. Figure 6 also compares data points under identical conditions for the coiled screen packing with hydrophobic spacers in the presence and absence of a surfactant. The surfactant was Tween 20 and was used at a concentration of 1 mg/m3. The data points taken with the surfactant exhibit a different slope but nevertheless lie clearly above the points for the measurements made with water alone. With the surfactant, conversions exceeded 80%, suggesting that virtually all of the SO2 absorbed is converted to acid. Our rationale in adding a surfactant was that the hydrophobic binder renders part of the surface hydrophobic and increases thereby access of SO2 and O2 to the carbon surface. Our expectation was that surfactant would reduce conversion rather than increase it. Our explanation for this

Figure 7. Effect of temperature at different liquid loadings on Sulzer CY packing at SO2 inlet concentration ) 3300 mL/ m3, O2 ) 5 vol %, and vg ) 3.8 cm/s: (a) % SO2 removal; (b) conversion of SO2 to acid; (c) dissolved SO2.

unexpected result is that the surface active agent reduces the surface tension of water, increasing the wetting in the interior pore surface of the carbon. It is this surface that affects conversion to acid. Tween 20 apparently did not affect the hydrophobic regions of the external carbon surface. Adding acetone to water also increases conversion. However, much more acetone is needed to give the improvement achieved with parts per million quantities of Tween. Experiments with the Sulzer packing examined the effect of temperature at different liquid loadings. Temperature effects on the coiled screen packings should be similar. Liquid loading appears important, as Figure 7 shows. Temperature has just a small effect on SO2 removal. At each temperature, however, the percent removal depended on the liquid loading. These experiments were performed with 3300 mL/m3 SO2 in the feed, which contained 5 vol % O2. The gas superficial velocity was 3.8 cm/s. In Figure 7b, the variation of SO2 conversion to acid with temperature is shown. The acid yield decreases slightly with increasing temperature but, as can be seen, the effect of the water loading remains important. In Figure 7c, the yield of SO2 in solution decreases sharply with temperature but now the liquid loading has a smaller effect. The temperature trends are surprising as the rate of SO2 oxidation is expected to increase with temperature. The explanation appears to be that increasing the temperature decreases the solubility of both O2 and SO2 in the liquid phase. Lower concentrations of the reactants thus reduce the conversion rate and the yields. Dissolved SO2 and SO2 converted to H2SO4 should add up to the SO2 removal, but as can be seen from Figure 7, a difference of up to 25% occurs in some experiments. There are several sources for the discrepancy. The most important of these is imprecise measurement of the

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Figure 8. Effect of superficial gas velocity on trickle-bed performance for Sulzer CY packing at T ) 23 °C, SO2 inlet ) 3300 mL/m3, O2 ) 5 vol %: (a) % SO2 removal; (b) yield of acid; (c) SO2 yield in liquid phase.

liquid flow rate due to small variations in holdup, evaporation, and the measurement technique. Acid yields and dissolved SO2 measurements are based on separate titrations, and the latter is obtained from a difference of two titrations. SO2 removal, on the other hand, employs a spectroscopic measurement as well as a mass flow meter reading. The occasional poor closure of material balances probably results from the addition of relatively small errors of these separate measurements. The percent SO2 removal is sometimes greater and sometimes less than the sum of the two percent yields. As a result, the measurements reported in this figure as well as those that follow can be used only to indicate trends. Figure 8 explores the space velocity effect. The percent removal of SO2 decreases with increasing space velocity as expected. Water loading has just a small effect. Thus, the SO2 removal decreases from about 93% at the lowest superficial velocity (3.6 cm/s) to about 75% at the highest velocity used (13.8 cm/s). The yield of acid, as moles of H2SO4 produced per mole SO2 fed to the scrubber, is strongly affected by the space velocity. At the highest liquid and lowest gas loadings, this yield is about 80% but it drops to 32% at a superficial gas velocity of 13.8 cm/s. Liquid loading exerts a strong influence on this yield. Decreasing the liquid loading from 1.05 to 0.42 mm/s reduces the yield from 80% to 45%. These effects are repeated for the yield of SO2 dissolved in the liquid phase. The yield decreases from about 42% at vl ) 1.05 mm/s and vg ) 4 cm/s to 20% at the same liquid loading but at the highest vg used. Decreasing the liquid loading to 0.42 mm/s at vg ) 4 cm/s drops the yield from 42% to about 28%. The influence of SO2 concentration on the trickle-bed performance is examined in Figure 9. Data were collected at about vg ) 4 cm/s, O2 concentration ) 5 vol

Vladea et al.

Figure 9. Effect of SO2 concentration in the feed on tricklebed performance for Sulzer CY packing at T ) 23 °C, vg ) 3.8 cm/s, O2 ) 5 vol %: (a) % SO2 removal; (b) yield of acid; (c) SO2 yield in liquid phase.

%, and T ) 23 °C. The effects of SO2 inlet concentration are small. At 1200 mL/m3 SO2 and at a liquid loading of 1.05 mm/s, the SO2 removal was 98%, but this drops to about 90% at a feed SO2 concentration of 4600 mL/ m3. At this highest feed concentration, increasing the liquid loading by a factor of 2.5 drops the SO2 removal to 70%. This effect carries over, as Figure 9b shows, to the acid yield. As in the previous figures, the liquid flow rate has a larger effect on acid yield and on the yield of dissolved SO2 than it does on SO2 removal from the gas phase. Figure 9c shows the yield of dissolved SO2 increases from 22% at vl ) 0.67 mm/s to about 40% when the SO2 concentration increases from 1200 mL/ m3 to 4600 mL/m3. Figure 10 shows that the effect of an increase in the inlet vol percent O2 in the feed to the trickle bed from 5% to 15% has almost no effect on the percent SO2 removal. The trend for the yield of acid is uncertain, but the dissolved SO2 is suppressed as the O2 in the feed increases. Discussion The observations given in the above figures can be explained with a reaction model which assumes that the pores of the activated carbon catalyst are largely liquidfilled. Results with the surfactant demonstrate that these pores must not be completely filled as the addition of surfactant raises the conversion level. Our model also assumes that mass transfer from the gas phase to the pore surface within the carbon catalyst is rate-controlling. Our basis for this assumption is our analysis of earlier experiments using packed beds of carbon.5,17 In this earlier work, we observed that only the O2 transport rate was important. With the partially hydrophobic (17) Stegasov, A. N.; Kirillov, V. A.; Silveston, P. L. Chem. Eng. Sci. 1994, 49, 3699-3710.

SO2 Oxidation from Flue Gas

Figure 10. Effect of O2 concentration on trickle-bed performance for Sulzer CY packing at T ) 23 °C, SO2 inlet ) 3300 mL/m3, vg ) 3.8 cm/s: (a) % SO2 removal; (b) yield of acid; (c) SO2 yield in liquid phase.

structured packing, the O2 transport assumption must be reexamined. Although increasing O2 concentration decreases dissolved SO2, as expected, there is little effect of O2 concentration on SO2 removal. Consequently, SO2 transport cannot be dismissed. The lack of dependence on increasing SO2 concentration on the acid yield (Figure 9) suggests that conversion is kinetically controlled or controlled by SO2 diffusion to the carbon surface. The modest decrease in SO2 removal at high SO2 concentrations, on the other hand, is probably explained by decreased solubility of SO2 because of increasing acid concentrations in the liquid phase within the pores. In our model, SO2 reacts with O2 and forms H2SO4 because of the presence of water in the system. Acid diffuses outward, so that the interior of the carbon particles can have significant acid concentrations. The increase in acid yield with increasing liquid loading in Figure 6 may be explained in the same way. Increasing the liquid loading reduces the acid concentration and increases O2 and SO2 solubility in the liquid phase. Liquid loading increases SO2 removal and dissolved SO2 as well, as may be seen in Figures 7-10. This, too, appears to be a solubility effect. Increasing temperature should increase the rate of mass transfer as well as the rate of oxidation of SO2 to H2SO4, but this is not what Figure 7 shows. SO2 removal and yields of acid and dissolved SO2 decrease with increasing temperature. The decreases can only be explained by reduced solubility at higher temperature. The effect of liquid loading on removal and conversion is contrary to packed bed data for activated carbon5,8 that indicated increasing loading decreased the fraction

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of SO2 oxidized to sulfuric acid. This contradiction is explained by the hydrophobic binder used to fix the activated carbon onto the packing screens. The binder resulted in hydrophobic regions on the carbon coat, permitting mass transfer from the gas phase to the carbon, bypassing dissolution in a liquid film and transport through an external liquid film on the carbon. The gaseous reactants still must dissolve, but in the hydrophobic region dissolution occurs within the porous carbon grains. Granules of activated carbon in randomly packed beds are hydrophilic, so an external liquid film is always present, creating an additional mass transfer resistance. Increasing the liquid loading increases this resistance and thus reduces the rate of conversion and acid yields. The increase of resistance is more important that the reduction in acid concentration probably because carbon utilization is poorer in the randomly packed beds, so that SO2 conversion to acid is lower. The choice between the Sulzer static mixers or the AECL coiled screen packings is still under study. From Figure 4, coiled screens appear to give higher SO2 removal and greater conversion to H2SO4 on a per volume basis, whereas static mixers exhibit a lower pressure drop per depth of packing (Figure 5). Our current focus is on the spacing of the screens as a means of reducing pressure drop. Probably, further data collection alone will not suffice for a decision; the choice between the packing types will likely require economic analysis. Scaleup is also a concern. Although the Sulzer static mixer packing could be devised to fit any column shape or diameter, there may be diameter limits on the coiled screen packing. Conclusions Centaur activated carbon bound as a thin layer either on a coiled screen packing or on a packing made from Sulzer CY static mixer sheets makes an extremely active, low-pressure-drop catalyst for the absorption of SO2 from stack gas and its conversion to sulfuric acid. Up to 92% conversion of SO2, measured as yield on inlet SO2 in the stack gas, has been achieved at a GHSV of 1000 h-1 using a packing 30 cm deep. Carbon utilization in terms of SO2 converted per gram of carbon is an order of magnitude greater than that obtained with packed beds of granular Centaur carbon. The high catalytic activity of the Centaur carbon coatings appears to be due, at least partially, to hydrophobicity introduced by the binding agent. Although the coiled screen packing provides higher removals and conversion to acid per cubic meter of bed volume, the Sulzer CY packings exhibit significantly lower pressure drop at the same gas and liquid loadings. More data and perhaps an economic analysis will be needed to identify the best packing. Acknowledgment. The financial support of the Natural Sciences and Engineering Research Council of Canada in the form of a Strategic Grant to P.L.S., S.S., and R.R.H. is gratefully acknowledged. Sulzer Chemtech generously provided static mixers for gas-liquid mixing and as catalyst support. EF960164U