Ind Eng Chem Res. 1990. 29, 440-447
440
Stability and Equilibrium Properties of Macroreticular Resins for Flue Gas Desulfurization Ten-Wen Chen and Neville G . Pinto* Department of Chemical Engineering, University of Cincinnati, Cincinnati. Ohio 45221 -01 71
Five basic macroreticular ion-exchange resins have been evaluated as adsorbents for small-scale flue gas desulfurization. The mechanical and thermal stabilities of these resins have been measured, and a weak-base resin with suitable stability characteristics has been identified. The adsorption capacities of this resin for sulfur dioxide, nitrogen dioxide, and carbon dioxide have been determined a t two temperatures and up to a pressure of 120 kPa. The adsorption data have been analyzed with the Langmuir, loading ratio, and vacancy solution models. On the basis of these analyses, the mechanism of adsorption has been discussed, and the adsorption behavior of mixtures of these gases has been predicted. The need to use domestic coal for more than utility power generation has been well recognized for some time. Consequently, in recent years, a significant research effort has focused on the development of advanced, small-scale, coal-fired combustors for industrial and residential applications. Combustors currently undergoing development under the Department of Energy's Advanced Combustion Technology Program will burn beneficiated coal. However, in every case, a postcombustion desulfurization system will be necessary. Postcombustion desulfurization systems for small-scale applications, especially residential applications, have unique requirements. These systems must be compact, efficient, of low cost, mechanically simple, and have virtually no maintenance requirements. Fixed-bed adsorbers have the potential for providing desulfurization systems with these desired characteristics. However, successful practical application is hinged on the correct choice of adsorbent. Adsorbents currently used, such as activated carbon in the BF/FW process and copper oxide in the Shell/UOP process, achieve removal by chemisorption (Dullien, 1989). Consequently, regeneration of the adsorbent is a complex and expensive proposition, and these adsorbents are unlikely to find use in small-scale applications. In contrast, adsorbents that rely on physisorption provide the option of low-cost recycling of the adsorbent, potentially minimizing the overall waste-disposal problem. Previous work with polymeric ion-exchange resins has demonstrated that under suitable conditions these materials have substantial capacities for sulfur dioxide, nitrogen dioxide, and hydrogen sulfide (Pollio and Xunin, 1968; Vaidyanathan and Youngquist, 1973; Avgul et al., 1977). Furthermore, it has been established that high rates of adsorption are attained when macroreticular resins are used (Layton and Youngquist, 1969; Kats et al., 1986). However, because the mechanism of adsorption is physisorption, the resin capacity rapidly decreases with increasing temperature (Layton and Youngquist, 1969). While this has restricted the use of polymeric resins in large-scale utility systems, it presents a less serious problem for domestic applications, since much lower volumetric quantities of flue gas have to be treated, and the combustors operate on an intermittent basis. This paper presents the results of a study to establish the utility of commercial macroreticular ion-exchange resins for domestic desulfurization applications. Five macroreticular anion-exchange resins have been evaluated for their thermal and mechanical stabilities. On the basis of these stability tests,
* Author t o whom
correspondence should be addressed.
one resin was further studied for its adsorption characteristics. The adsorption capacities of this resin for sulfur dioxide, nitrogen dioxide, and carbon dioxide have been determined at a variety of conditions. A number of adsorption models have been used to analyze the adsorption data. On the basis of these models, the effects of carbon dioxide and nitrogen dioxide on the adsorption of sulfur dioxide have been predicted.
Experimental Methods Stability Measurements. In fixed-bed desulfurization, the adsorbent is subjected to cyclic mechanical and thermal stresses. Thus, it is important to evaluate stability characteristics as well as adsorptive capacities in establishing the suitability of macroreticular resins as pollutant adsorbents. A number of accelerated test methods have been developed to evaluate the stability of ion-exchange resins (Kunin, 1958; Dorfner, 1973; Hochmuller, 1984). The automated method of Hochmuller (1984) was adapted for this study. Figure 1 is a schematic representation of the apparatus used. It consists of a 0.02-m-0.d.X 0.305-m filter tube into which approximately 0.03 kg of the resin to be tested is placed. The resin is then subjected repeatedly to a stress cycle, either mechanical or thermal. Between stress cycles, the resin is backwashed with water at a constant pressure. If the resin undergoes degradation during the test, flow rate changes are observed during successive backwashes. The rate of change of the flow rate is a measure of the relative stability of the resin. For the measurement of mechanical stability, a four-step stress cycle was used. Hydrochloric acid (3.0 M) was pumped through the tube at approximately 5 X m3/s for 100 s. The resin was then backwashed with water at a constant upstream pressure of 207 kPa for 200 s. During the backwash, the downstream flow rate of the water was continuously monitored with a flow sensor (Cole Parmer, Model MK 505). This backwash was followed by 5 X m 3 / s of sodium hydroxide solution (3.0 MI for 100 s. Finally, the water backwash step was repeated to complete the cycle. The cyclic exposure of the resin to concentrated acid and alkali causes swelling and shrinking of the resin. which leads t o the generation of osmotic and hydromechanical stresses internally and physical attrition externally. The thermal stress cycle involved a two-step process. Air at 275 "C was fed to the filter tube for a period of 2 h. The hot air maintained the resin at an average temperature of 175 f 3 "C. The air was followed by water at room temperature in a backwash step identical with that used in the mechanical stress cycle. The thermal stress cycle was
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Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 441 Table I. ProDerties of Macroreticular Resins resin MWA-1 MSA-1 IRA-93 IRA-94 IRA-900
functional group trimethylamine on styrene-divinylbenzene dimethylamine on styrene-divinylbenzene tertiary amine on styrene-divinylbenzene tertiary amine on styrene-divinylbenzene quaternary ammonium on styrene-divinylbenzene
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density, k/m3 640 705 608 640 640-688
water content, wt%
55 f 5 60 f 4 58 60 60
ion-exchange capacity, equiv/L 1.0 1.0 1.3 1.2 1.0
Table 11. Relative Mechanical Stability of Resins flow rate (% of initial flow rate) at flow cycles resins 25 50 75 100 125 MSA-1 92.02 91.75 91.11 90.11 88.74 MWA-1 91.21 91.66 91.11 89.58 87.05 IRA-900 85.97 85.46 85.08 84.82 84.70 IRA-93 88.09 88.21 88.17 87.97 87.62 IRA-94 87.01 85.94 84.48 85.62
Water Inlet Valve
-1
particle size, mesh 16-50 16-50 16-50 16-50 16-50
Chemical outlet
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Figure 1. Apparatus for chemical and thermal stability measure__ ment.
designed to measure the resin's stability during prolonged exposure to high temperatures, as well as its resistance to thermal shock. Adnnrntinn Tnntherm Menniirementn
Adsorption isotherms were obtained by using an adaptation of the volumetric apparatus developed by Dorfman and Danner (1975). A schematic representation of the apparatus is shown in Figure 2. The apparatus has two closed loops: an adsorption loop and a recirculation loop. The recirculation loop is the outer loop in Figure 2 and contains a pressure transducer (Sensotec, model Super TJE), a thermocouple, a volumetric micrometer (Volumetrics, Model VM-l), and a recirculating pump (Fluid Metering, Model RP-J). It is isolated from the adsorption cell by valves V3 and V4. With valves V3 and V4 open and valve V2 closed, the adsorption cell is connected to the outer loop, and the resultant closed loop is called the adsorption loop. The adsorption cell is a 0.0064-m-0.d. X 0.457-m glass tube immersed in a constant-temperature oil bath. Both loops are well sealed from the environment, and leak rate tests with helium gave a leak rate of 3.7 X kPa m3/s. Prior to adsorption experiments, the volumes of the adsorption and recirculation loops were determined; these volumes are necessary for computing adsorption isotherms from experimental data. The volumes were determined with helium by using a procedure described in detail by Kaul (1987). Briefly, helium is introduced into the loop of interest, and the pressure is noted. The volume of the
loop is then changed by a measured amount with the micrometer, and the new pressure in the loop is recorded. From the observed change in the pressure, the volume of the loop can be calculated. The adsorption experiments were conducted with apkg of resin. Fresh resin was conproximately 2.5 X ditioned by using standard procedures (Helfferich, 1962), rinsed in deionized water, centrifuged, and weighed before being placed in the adsorption cell. The adsorption loop was then evacuated, and the resin was dried overnight at 75 "C. Subsequently, the resin was exposed to the gas of interest at approximately 138 kPa for a period of about 1 week. The objective here was to ensure that the irreversible capacity of the resin (Layton and Youngquist, 1969) was saturated prior to adsorption isotherm measurements. The resin was then heated to 75 "C and maintained under vacuum for a period of 4 h, to completely regenerate its reversible capacity. At this point, measurements of adsorption isotherms were initiated. A quantity of pure gas was introduced into the recirculation loop, and its temperature and pressure were measured. These measurements were used in conjunction with the loop volume, measured earlier, to calculate the moles of gas introduced. Valves V2-V4 were then switched, and the gas was recirculated through the adsorption loop. The progress of the adsorption was monitored continuously with the pressure gauge. At equilibrium, the temperature and pressure of the gas were noted, and these were used to calculate the moles of the species remaining in the gas phase. From the difference in the number of moles in the gas phase before and after adsorption, the number of moles adsorbed were determined. Prior to the next adsorption experiment, the resin was regenerated at 75 "C for 4 h. Research-grade sulfur dioxide, nitrogen dioxide, and carbon dioxide were used for these measurements.
Results and Discussion Resin Stability. The mechanical and thermal stability tests were performed on five macroreticular anion-exchange resins: Dowex MWA-1, Dowex MSA-1, Amberlite IRA-900, Amberlite IRA-93, and Amberlite IRA-94. All five resins are based on a copolymer matrix of styrene and divinylbenzene. Amberlite IRA-900 and Dowex MSA-1 are strong base resins, and the rest are weak base resins. Physical properties of these resins are summarized in Table I. The results of the mechanical stability tests are reported in Table 11. The relative stability of the resins is reported as a function of the number of stress cycles to which the
442 Ind. Eng. Chem. Res., Vol. 29, No. 3, 1990 F k F 3 Fbwmmr VI MdVS 3-WayVdvcs V1-V4 1-Way Valves
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Thermal Streas Cycles
Figure 3. Relative thermal stability of resins. Rmrculaung Pump
Figure 2. Apparatus for measurement of adsorption isotherms.
resins were subjected. Each resin was subjected to a total of over 100 cycles. The flow rate is expressed as a percentage of the flow rate recorded prior to the first stress cycle. It was observed that in all cases the flow rate decreased as the test progressed. This occurs because with each additional stress cycle a fraction of the resin beads lose their spherical shape due to cracking; cracking of the beads occurs when the stresses induced ultimately break open the the cross-linked polymer matrix. Consequently, since the backwash water is being fed a t a constant upstream pressure, the flow rate through the bed decreases. For all five resins, a rapid initial decrease in flow rate was followed by a much slower decrease. This is postulated to occur because a percentage of the fresh resin beads are defective, and these defective beads degrade rapidly during the initial cycles. From the results reported in Table 11, it is clear that all five resins show good mechanical stability. After more than 100 stress cycles, the packed resin beds exhibit about a 15% decrease in flow rate, which implies that only a small fraction of the beads has degraded. Also, since the mechanical stresses induced in these tests are far more severe than those that can be expected in a fixed-bed desulfurization application, it is felt that all five resins have adequate mechanical stability for the stated application. In comparing the Dowex resins with the Amberlite resins, it appears that the Dowex resins are of a higher initial quality, since they exhibit a smaller initial decrease in flow rate. However, the Amberlite resins show a lower subsequent rate of degradation, and over 125 cycles, both resins exhibit about equal stability. In comparing the weak base and strong base resins, it is clear that no distinction can be made with respect to their mechanical stability. Typical results from the thermal stability tests are shown in Figure 3. All five resins were tested. However, for clarity, data are reported only for three resins. These resins illustrate the three types of behavior that were observed. The strong base resins Dowex MSA-1 and Amberlite IRA-900 showed catastrophic failure within the first two stress cycles. In order to confirm this result, samples of the resin were removed from the filter tube after five cycles and examined under a microscope. The visual examinations confirmed the the beads had indeed lost their integrity. Failure in this case is clearly due to depolymerization of the styrene-divinylbenzene network. The weak base resins Amberlites IRA-93 and IRA-94 showed quite different behavior. For these resins, the flow rate increased as the test progressed. This indicates that
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Figure 4. Correlation of SOz adsorption data.
t h e resin beads maintained their spherical shape, but the average bead diameter decreased; this decrease leads to a decrease in packed column height and, consequently, an increase in flow rate. Visual examinations of the beads under a microscope showed that the after 20 cycles the average bead diameter had decreased by 5 9%. Clearly, depolymerization of the resin is occurring, but unlike the strong base resins, it is restricted to the surface of the resin. The weak base resin Dowex MWA-1 showed essentially no degradation during the test. A small increase in flow rate (
