Ind. Eng. Chem. Res. 1993,32, 2736-2739
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Chromatographic Separation and Concentration of Sulfur Dioxide in Flue Gases Harvey
G. Stenger, Jr.,'*+Kaihong Hu,+and Dale R. Simpsont
Department of Chemical Engineering and Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015
The ability to separate and concentrate SO2 from a gas stream similar to coal-derived flue gas is accomplished due to the relative adsorption strengths of SO2 and water on a synthetic mordenite, a phenomenon known as rollup. Laboratory experiments using both simulated and real flue gas are reported which demonstrate the potential of this process. The rollup effect is shown to be influenced by temperature, regeneration conditions, and the feed concentrations of water and SO:!. Adsorption at higher temperatures (150 vs 50 "C)result in less desirable, smaller and broader rollup peaks. Air regeneration a t 300 O C was found to irreversibly decrease the rollup effect while regeneration with helium a t temperatures above 200 OC and with air a t 150-200 "C were found to be optimal. The influence of water was strong with the rollup peak being optimal with 7 4 % moisture in the gas. Increasing the feed concentration of SO2 did not decrease the rollup effect, which implies the use of this phenomenon could be cascaded into a multistage process. Introduction The removal of sulfur dioxide from gas streams is not a new problem or a problem short of solutions. Processes in various stages of development and commercialization include throwawayand regenerative wet scrubbing, sorbent injection, dry bed adsorption, oxidation to sulfuric acid and recovery, semipermeable membrane separation, electron beam and plasma destruction, and reduction to hydrogen sulfide and sulfur. The constraints imposed on all of these solutions by the potential customers (e.g., coal fired power plants) include small concentrations of S02, a large number of other gaseous components, large gas flow rates, low pressure drops, minimal heating needs, modest temperatures (80-200 "C), and simple operation. Because of these constraints, the processes being designed and built are relatively large and capital intensive. A significant reduction in the capital cost of these processes could be achieved if several of the constraints were relaxed. In particular if the gas stream had a higher SO2 concentration and a lower flow rate, the size of the unit could be greatly reduced. For example if a process designed to remove 90% of the SO2 in a gas stream could be changed to remove 90% of the SO:! from a stream of one-fifth the original volumetric flow rate and 5 times the original inlet concentration, the volume of the removal system would be reduced by nearly a factor of 5 (assuming the kinetics of the removal process were unaffected by concentration). Because of the capital demands for conventional SO2 removal, there is a large incentive to develop a simple and inexpensive method to separate and concentrate SO2 from the flue gas stream into a smaller gas stream that can then be treated by an available or a developing technology. Several ways which have been considered to accomplish this concentration goal include pressure swing adsorption, temperature swing adsorption, and membrane separation. In this paper we present results from an alternative means to concentrate SO2 into a gas stream of lower flow than the original stream. This technique is best described as chromatographic (sometimes referred to as rollup (Yang, 198711,and utilizes mordenite as an adsorbent that concen-
* To
whom correspondence should be addressed. E-mail:
[email protected]. + Department of Chemical Engineering. t Department of Earth and Environmental Sciences.
0888-5885f 9312632-2736$04.00/0
trates the SO2 by water-driven desorption. Previously published work using this mordenite as an adsorbent for SO2 (Stenger et al., 1992) reported breakthrough data for dry SO2 in helium. In those experiments, a sharp stepfunction-breakthrough of SO:! was observed, and the SOZ breakthrough was modeled accurately using a single species-single site adsorption model. In that work, one experiment was reported which showed a gas mixture of water, SO2, and helium gave a breakthrough peak (rollup) of SO2 that was almost twice the concentration of the inlet stream. In the present study we focus exclusively on this rollup effect using gas mixtures that are representative of coal derived flue gas (Le., with COZ, 0 2 , HzO, SOZ, and inert). The goal of this paper is to present our experimental findings on this phenomenon and specifically how it is affected by important flue gas variables. Experimental Section Adsorption measurements were made by flowing a multicomponent gas mixture through a packed bed and measuring the exiting concentration of each component as a function of time. This method is relatively simple, if the exiting gas can be rapidly analyzed for all the adsorbing species. Also, if properly designed, this method allows the adsorption kinetics to be determined (Stenger et al., 1993). The apparatus consists of three gas feed lines which carry air, 1.5% SO2 in helium, and 20% COz (all gases are CP grade from MG Industries). The gases are metered using mass flow controllers (MKS, Inc.), and water is added by bubbling the air stream through distilled water contained in a temperature-controlled bath. All the feed and exit lines are heated to prevent water from condensing. The sorbent, typically 2 g, is held in a Pyrex tube by a porous frit at the bottom and the gas flow is upward. The bed temperature is controlled to within 1 OC using a thermocouple placed a t the center of the bed and a temperature controller (Omega CN9000). The bed operates at the pressure drop of the exhaust lines, which is less than 10 kPa. Runs using no sorbent result in immediate breakthrough of sulfur dioxide,thus confirming that the data are not influenced by the sample holder, bubbler, tubing, etc. The exiting gas is sampled and analyzed using a Fourier transform infrared (FTIR) spectrometer (Midac). Details of the analysis procedure and calibration of the FTIR are given elsewhere (Stenger 0 1993 American Chemical Society
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Figure 1. Breakthrough curve of SO2 exiting the mordenite bed. Feed and bed conditions are listed in Table I. Open squares, first run; Solid squares, second run.
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and Meyer, 1992). Adsorption measuremenb are also reported using a larger packed bed (270 g) and flue gas derived from a fluidized bed methane combustor. A description of the combustion apparatus and its procedures is also given in Stenger and Meyer (1992). Mordenite was synthesized in self-bonded masses by reaction of a limited quantity of sodium silicate solution with volcanic glass (perlite) at 200 "C and 1.6 MPa (Stenger et al., 1993). Following synthesis the samples were dried in air at 500 OC, sieved to a particle size between 175 and 300 pm, and washed in dilute hydrochloric acid to remove unreacted sodium silicate. After reaction the synthesis to mordenite was confirmed with X-ray diffraction.
Results and Discussion Figures 1,2, and 3 show the concentration of S02, H20, and C02, respectively, in the exiting gas of two consecutive and identical runs using the same mordenite sample. The conditions for the two runs are listed in Table I. During the first 40 min of both runs the exiting gas contained no measurable S02, as expected for a packed bed containing a strong SO2 sorbent. The retention capacity of the sorbent for SO2 (see area A of Figure 1) in this experiment was 0.016 g of SOdg of mordenite. Integrating the area under the adsorption and desorption sections (A and B) shows that approximately 90% of the SO2 adsorbed between 0 and 40 min is desorbed between
Figure 3. Breakthrough curve of COz exiting the mordenite bed. Feed and bed conditions are listed in Table I. Open squares, first run; Solid squares, second run. Table I bed temp = 50 OC;bubbler temp O C flow = 150 cmS(STP)/min;2.0 g of mordenite feed gas composition (mol % ): COz = 14,02 = 4, HzO = 8, SO2 = 2200 X lo-' (2200 ppm), balance = helium regenerationprior to adsorption: temp = 200 O C ; gas = dry air; flow rate = 100 cmS/min;time = 90 min ~
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42 and 120 min. The subsequent peak of S02, reaching a concentration nearly 3.5 times the feed concentration, is due to rollup of SO2 by the more strongly adsorbed water. Although rollup has been observed and modeled by several investigators, it has not been reported for S02/ H20, nor have others published rollup peaks as sharp or tall as in this work (Kapoor and Yang, 1987; Haas et al., 1987). We have previously evaluated the mass-transfer resistances for this system and found both internal and external transport to be much faster than the adsorption rate process (Stenger et al., 1993). Thus we conclude that the rollup is caused by the competitive adsorption of water and not transport limitations (Kapoor and Yang, 1987). Chromatographic desorption (rollup) provides a breakthrough of SO2 that can be captured and processed as a more concentrated smaller flow rate stream. For example if the exiting gas between 0 and 45 rnin were exhausted as effluent and the gas from 45 to 65 min were diverted to a conventional SO2 scrubber, the scrubber would be fed a more concentrated stream for a shorter period of time (20 vs 65 min) than if the total stream were fed to the scrubber at the lower feed SO2 concentration. Utilizing this phenomenon could lead to the design and operation of a continuous process that cycles beds through adsorption, desorption, and drying, thus producing an SO2-free gas stream and a smaller, more concentrated (in SO21 gas stream. Further designs can be conceptualized that cascade these systems so that the feed to a second unit would be the concentrated stream from the first and would produce a new effluent that is even further concentrated and reduced in flow. Temperature. The influence of temperature was investigated by operating the bed at 50,100, and 150 "C. The breakthrough curves for water and SO2 are plotted in Figures 5 and 6, respectively. Other than temperature these runs were operated at the same conditions as the runs in Figures 1-3 (see Table I). Figures 4 and 5 show that a higher bed temperature decreases the mordenite's capacity for SOP and water, as evidenced by shorter breakthrough times at the higher temperatures. Of practical importance is the broadening of the SO2 peak
2738 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 I
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Figure 6. Effect of regeneration conditions on the breakthrough of S02. Feed and bed conditions are listed in Table I.
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Figure 5. Breakthrough curve of water exiting the mordenite bed. Feed and regeneration conditions are listed in Table I. Bed tamperatures are labeled on the graph.
and its lower maximum value at higher temperatures, indicating that lower bed temperatures provide more optimal separation and concentration. The lower constraint on temperature would be the flue gas stream's dew point. Regeneration. After the breakthrough of water is complete, the mordenite contains a large quantity of water (at 50 "C nearly 19 g of water per 100 g of mordenite) and some residual SO2 (approximately 0.25 g of SO2 per 100 g of mordenite). To regenerate the mordenite, the bed is heated and purged with dry gas. The temperature of the bed, the composition of the drying gas, and the time of regeneration are three parameters of practical importance. Optimally we would use air, a t a low temperature, for a short period of time. Figure 6 shows the effect of regenerating the bed at 300 O C using air or helium for 90 min. For the helium case, the rollup of SO2 is identical to the case shown in Figure 1when air at 200 OC was used for regeneration; however, when using air at 300 "C the rollup peak height decreases significantly. Thus the presence of oxygen and high temperature decreases the rollup effect, but if one of the two is absent (high temperature or oxygen) the effect is retained. Water Concentration. A third important variable is the inlet water concentration, because water concentrations vary significantly depending on fuel type and because an optimum water concentration may exist. Figure 7 shows the SO2 breakthrough curves from three different inlet
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Figure 7. Effect of water concentration on the breakthrough of S02. Bed and regeneration conditions are listed in Table I. Feed water concentrations are labeled on the graph.
water concentrations. This figure indicates that too much or too little moisture decreased the rollup peak height and sharpness. Increasing moisture also has the effect of decreasing the SO2 breakthrough time by lowering the SO2 capacity of the mordenite. SO1 Feed Concentration. The final variable investigated was the inlet concentration of SO2. These experiments were to test the possibility of cascading the rollup effect into a multistage process. If the concentrated SO2 stream exiting a bed could be sent to a second bed to repeat the phenomenon, the ultimate concentration of SO2 could be quite high. For example, if the rollup peak exiting the bed from the runs shown in Figure 1,which has an average exiting concentration of approximately 4500 ppm, could be further concentrated by the same factor, a concentration of 9000 ppm SO2 would be expected from two beds in series and perhaps 18000 ppm from three beds. Figure 8 shows the results from runs using 2000, 4000, and 6000 ppm SO2 in the feed gas. The other conditions for these three runs are identical to those in Table I. As hoped, the effect is not deteriorated as the inlet SO2 concentration increases. When a feed with 2000 ppm SO2 is used, the exiting peak reaches 5300 ppm, and when a feed with 6000 ppm is used, the exiting peak reaches 13 000 ppm. The final experiment was to test the rollup phenomenon at an increased scale using combustion-derived flue gas. This experiment used our combustion apparatus to generate 5 L(STP)/minflue gas which was sent to a packed
Ind. Eng. Chem. Res., Vol. 32,No. 11,1993 2739
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Figure 9. Breakthrough of SO2 and temperature of center of bed using 5 L/min of combustion gas, a feed concentration of SO2 equal to 2200 ppm, and 270 g of mordenite.
bed (i.d. = 4.5 cm, length = 28 cm) containing 270 g of mordenite. The resulting breakthrough curve and bed temperature are plotted in Figure 9. At this scale the phenomenon is still effective at concentrating the S02, even though the bed is not isothermal. The temperature rise shown in Figure 9 is due to the water's heat of adsorption which significantly raises the mordenite's temperature. The capacity of the mordenite is lower than in the small-scale experiments due to this rising bed temperature. Conclusions The ability to separate and concentrate SO2 from a flue gas stream is possible by taking advantage of the relative adsorption strengths of SO2 and water on a synthetic
mordenite. The influences of temperature, regeneration conditions, water concentration, and SO2 concentration have been examined experimentally. Higher temperatures (150 vs 50 "C) cause the effect to be less pronounced, resulting in smaller and broader rollup peaks. Regenerating with air at temperatures greater than 200 "C was found to irreversibly decrease the rollup effect; however, regeneration with helium at temperatures above 200 O C did not decrease the effect, thus indicating the surface can be damaged by oxidation after sulfur has been adsorbed. Water concentration was shown to be detrimental to the rollup peak if too high or too low. Higher feed concentrations of SO2 did not decrease the rollup effect. This important result implies that the use of this phenomenon can be cascaded into a multistage process that could increase the concentration of SO2 by severalfold, and thus decrease the amount of SO2-containing gas that needs to be treated. A scaleup of 150 times the benchscale experiments, which were conducted adiabatically rather than isothermally, also showed the rollup effect. These results and conclusions are important to understanding the practicality of using rollup as a method to concentrate a dilute pollutant. In separate work we have begun a model-based analysis of this system (see Stenger et al. (1993) for more details). The effects of water concentration, temperature, and feed sulfur dioxide concentration are shown in the present paper, but as yet they are not interpreted. Interfacing the results of this work with our modeling efforts will provide a meaningful interpretation of the rollup effect and should allow us to determine the dominant causes of our observations. Acknowledgment Support for this project was provided by the Pennsylvania Energy DevelopmentAuthority, Pennsylvania Power and Light, and Allegeny Power Systems. Literature Cited Haas, 0. W.; Kapoor, A.; Yang, R. T. Confirmation of HeavyComponent Rollup in Diffusion Limited Fixed Bed Adsorption. AIChE J., 1987 34 (ll),1913-1915. Kapoor, A.; Yang, R. T. Rollup in Fixed Bed, Multicomponent AdsorptionUnder Pore Diffusion Limitation.AIChE J. 1987,33 (7), 1215-1217. Stenger, H. G.;Meyer, E. C. Laboratory Scale Fluidized Bed Coal Combustor. Energy Fuels 1992,6 (3), 277-286. Stenger, H. G.;Simpson, D. R.; Hu, K. Competitive Adsorption of SO2, H20, and NO on Mordenite Synthesized from Perlite. Gas Sep. Purif. 1993, 7 (l),19-25. Yang,R. T. Gas Separation by Adsorption Processes; Butterworth Publishers: Stoneham, MA, 1987; Chapter 5.
Received for review April 5, 1993 Accepted August 12, 1993. Abstract published in Aduance ACS Abstracts, October 1, 1993.
