United States Environmental Protection Agency Environmental Sciences Research Laboratory Research Triangle Park NC 27711 EPA-600/2-79-205 December 1979 Research and Development Solid Sorbent for Collecting Atmospheric Sulfur Dioxide ------- RESEARCH REPORTING SERIES Research reports of the Office of Research and Development, U.S. Environmental Protection Agency, have been grouped into nine series. These nine broad cate- gories were established to facilitate further development and application of en- vironmental technology. Elimination of traditional grouping was consciously planned to foster technology transfer and a maximum interface in related fields. The nine series are: 1. Environmental Health Effects Research 2. Environmental Protection Technology 3. Ecological Research 4. Environmental Monitoring 5. Socioeconomic Environmental Studies 6. Scientific and Technical Assessment Reports (STAR) 7. Interagency Energy-Environment Research and Development 8. "Special" Reports 9. Miscellaneous Reports This report has been assigned to the ENVIRONMENTAL PROTECTION TECH- NOLOGY series. This series describes research performed to develop and dem- onstrate instrumentation, equipment, and methodology to repair or prevent en- vironmental degradation from point and non-point sources of pollution. This work provides the new or improved technology required for the control and treatment of pollution sources to meet environmental quality standards. This document is available to the public through the National Technical Informa- tion Service, Springfield, Virginia 22161. ------- EPA-600/2-79-205 December 1979 SOLID SORBENT FOR COLLECTING ATMOSPHERIC SULFUR DIOXIDE by R. J. Cotter S. G. Smith Jr. Union Carbide Corporation Chemicals and Plastics Research Laboratories Bound Brook, New Jersey 08805 Contract No. 68-02-1782 Project Officer James Mulik Atmospheric Chemistry and Physics Division Environmental Sciences Research Laboratory Research Triangle Park, North Carolina 27711 ENVIRONMENTAL SCIENCES RESEARCH LABORATORY OFFICE OF RESEARCH AND DEVELOPMENT U.S. ENVIRONMENTAL PROTECTION AGENCY RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711 ------- DISCLAIMER This report has been reviewed by the Environmental Science Research Laboratory, U.S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the U.S. Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. 11 ------- ABSTRACT This research program was initiated with the overall objective of developing a replacement method for the West-Gaeke method presently used to measure 24-hour ambient sulfur dioxide concentrations in ambient air. It was demonstrated that a solid sorbent, consisting of Puramer S coated open cell polyurethane foam, can be used to fix the quantities of sulfur dioxide that would be collected if typical ambient air was filtered for 24 hours at 200 cc/minute. The method of assaying sulfur dioxide collected by the adsorbent consisted of controlled thermal desorption of sulfur dioxide followed by continuous analysis using a Dohrmann micro- coulometric Titration System. Also, it was shown that trouble- some sulfur dioxide decay, occurring during post collection storage, was primarily the result of oxidation. This decay was minimized, to an acceptable level, by properly sealing the spent Puramer S collector devices to prevent oxygen contamination from contacting the adsorbent prior to thermal desorption and subse- quent assaying. This report was submitted in fulfillment of 68-02-1782 by Union Carbide Corporation under the sponsorship of the U.S. Environmental Protection Agency. This report covers a period from May 1, 1975 to November 30, 1977 and work was completed November 30, 1977. 111 ------- CONTENTS Abstract iii Figures vi Tables vii 1. Introduction 1 2. Conclusions and Recommendations 2 3. Prior Program Background 3 4. Puramer S-based Ambient Air Monitoring System ... 5 5. Experimental 9 Sulfur Dioxide Adsorption Characteristics of Puramer S - Polyurethane Foam Adsorbents .... 9 Effect of Desorption Temperature on Sulfur Dioxide Recoveries 11 Effect of Puramer S Content on Sulfur Dioxide Recovery Efficiency 12 Effect of Collector Cycling on the Sulfur Dioxide Recovery Efficiency 14 Effect of Prehydration on the Sulfur Dioxide Recovery Efficiency of Puramer S Adsorbents ... 16 Effect of Storage on Sulfur Dioxide Recovery Efficiency 19 6. Procedures 25 Method of Loading Sulfur Dioxide on Puramer S Adsorbents 25 Calibration of Sulfur Dioxide Purmeation Tubes . 27 Thermal Desorption Dioxide Method for Assaying Sulfur Dioxide Dioxide Adsorbents ... 29 Preparation of Puramer S - Polyurethane Foam Adsorbent and Subsequent Collector Devices ... 32 References 34 ------- FIGURES Number Page 1 Puramer S Based Sulfur Dioxide Monitoring System-Collector System 6 2 Puramer S Based Sulfur Dioxide Monitoring System-Analyzer System 7 3 Puramer S Filter Seal 22 VI ------- TABLES Number Page 1 Sulfur Dioxide Analysis by a Puramer S-based Analytical Method 8 2 Adsorption Characteristics of Puramer S-polyurethane Foam Adsorbents 10 3 The Effect of Desorption Temperature on Sulfur Dioxide Recoveries 11 4 Effect of Puramer S Content on Sulfur Dioxide Recovery Efficiency 13 5 Puramer S Adsorbent Cycling Versus Sulfur Dioxide Recovery Efficiency 15 6 Effect of Prehydration on the Sulfur Dioxide Recovery Efficiency of Puramer S Adsorbents 16 7 Puramer S-based Sulfur Dioxide Analytical System Parameters . 17 8 Sulfur Dioxide Assays Via a Puramer S-based Method ... 18 9 Effect of Post-collection Storage on Puramer S Adsorbent Recovery Efficiency 20 10 Puramer S—Sulfur Dioxide Storage Stability in an Inert Atmosphere . . » 21 11 Effect of Proper Sealing on the Storage Stability of Spent Puramer S Adsorbents 23 ------- SECTION 1 INTRODUCTION One of the goals of the Environmental Protection Agency (EPA) is to accurately monitor 24-hour average sulfur dioxide concentrations in ambient air. This monitoring must be as simple as possible because it is carried out at unmanned mon- itoring stations where collector pickup is usually performed by unskilled, volunteer workers. The concentration range of interest is 26 to 2600 yg/m3 (n,.01 to 1 ppm) for sulfur dioxide. At the present time, monitoring is generally accomplished using the West-Gaeke method. This wet chemical method uses a toxic liquid collector containing a mercuric salt dissolved in water. This troublesome collector solution is packaged after use and sent to a central testing laboratory for analysis. During both the storage and analysis of spent collectors, spillage can result in unwanted contamination of important work areas. Also, storage at ambient temperatures consistently results in uncontrollable sulfur dioxide decay which leads to lower than actual sulfur dioxide levels. The handling of liquid systems by unskilled workers at the monitoring sites also leads to poor assay accuracy. The purpose of this investigation, therefore, is to develop a simple, quantitative method for collecting and assaying atmo- spheric levels of sulfur dioxide. The method is to be based on a proprietary Union Carbide Corporation polymeric amine adsorbent called Puramer S. ------- SECTION 2 CONCLUSIONS AND RECOMMENDATIONS It has been demonstrated that a Puramer S-polyurethane foam adsorbent can be used to fix the levels of sulfur dioxide that would be collected if typical ambient ait is filtered for 24 hours at 200 cc/minute. The method of- assaying the sulfur dioxide collected by the Puramer S-based adsorbent, consists of controlled thermal desorption of sulfur dioxide followed by continuous sulfur dioxide analysis using a Dohrmann Micro- coulometric Titration System. It has also been shown that sulfur dioxide decay can be expected during storage of spent collector devices, but that elimination of oxygen during this storage period reduces the degree of decay to a minimum. Therefore a collector sealing method, based on a glass plug, Teflon sleeve end cap, has been shown to be effective in preventing undesirable sulfur dioxide decay during storage of spent Puramer S collectors at 25°-27°C. Collectors were successfully stored for up to 14 days, prior to assaying, without significant loss of sulfur dioxide (>95% S02 recovery). Storage of spent collectors at 40°C were not totally successful. After only 3 days at 40°C, expected assays were reduced by as much as 26 percent. Subsequently, results obtained on spent filters again sealed with the glass rod-Teflon sleeve end cap and stored in an oxygen-free environment showed negligible sulfur dioxide decay. These results indicate the need for an even better collector seal than presently available. All of the data obtained to date has been under controlled laboratory conditions and free of potential interferences from gaseous species normal to ambient air. Therefore, before this unique Puramer S-based sulfur dioxide monitoring system can be subjected to comparative field study with the presently used West-Gaeke method, a study of possible interferences is recom- mended. To totally complete the development of the Puramer S monitoring system will require additional funds. ------- SECTION 3 PRIOR PROGRAM BACKGROUND As previously mentioned, the sulfur dioxide monitoring system under development is to be based on Puramer S, a pro- prietary Union Carbide Corporation polymeric amine adsorbent.1 Puramer S is an efficient, high-capacity sulfur dioxide adsorb- ent which is prepared by heating N-glycidyl piperazine oligomer until it becomes cross-linked and hence water insoluble.2»3 It has the following general structure: —CH — 0 CH2-CH-CH2-N/ \J- PURAMER S It is the tertiary amine and beta aliphatic hydroxyl groups that are believed responsible for the sulfur dioxide chemisorbent properties of this very unique polymer. Hence, sulfur dioxide is removed from a flowing gas stream as a sulfite or bisulfite group. Because of this chemistry, moisture is very important for efficient sulfur dioxide removal. Best results are obtain- ed at a relative humidity of 70-95 percent. ------- Early in our UCC Corporate Research funded program, direct- ed towards developing Puramer S adsorbents for use in sulfur dioxide removal from industrial gas streams, as well as ambient air, it was shown that a wide variety of substrates, coated with Puramer S, could quantitatively remove and collect ambient concentrations of sulfur dioxide from properly humidified gas streams, at high linear flow rates.4 During an ambient air filtration, performed at a New Jersey Environmental Protection Department Air Monitoring Station located in Camden, New Jersey, ambient sulfur dioxide was quantitatively removed using a 2-inch x 1/4-inch Puramer S-polyurethane foam adsorbent, for a continuous period of 70 days. These results convinced us that low levels of sulfur dioxide could be collected from ambient air and fixed by the Puramer S structure, but we did not know exactly how to measure the collected sulfur dioxide. Early thermal analyses, performed by Dr. B. L. Joesten (Union Carbide Corporation, Research and Development Laboratories), indicated that sulfur dioxide could be thermally desorbed and subsequently purged from the Puramer S structure by heating to 100°-110°C in the presence of nitrogen. This fact, coupled with the knowledge the sulfur dioxide can be continuously measured by means of a Dohrmann Microcoulometric Titration System, dictated that initial contract research should be directed towards the optimization of the Dohrmann Sulfur Analyzer as well as defining thermal methods and equipment for thermally generating the collected sulfur dioxide so that it can be quantitatively assayed. The following subsections will be devoted to discussing, in detail, the research that has led to the successful develop- ment of a Puramer S-based method for measuring the low levels (nanogram quantities) of sulfur dioxide required for ambient air monitoring. ------- SECTION 4 PURAMER S-BASED AMBIENT AIR MONITORING SYSTEM Conceptually the Puramer S-based monitoring system consists of two parts: the collector and the analyzer. The collector which is located at the air monitoring site consists of an air mover, probably an air pump, a humidifier to adjust and control the moisture content of the air prior to its entering the Puramer S collector device, a flow controller for adjusting the air flow to 200 cc/minute and finally a Puramer S filter device consisting of two 1-inch by 1/4-inch I.D. plugs of Puramer S-coated polyurethane foam fitted into a 3-inch by 1/4-inch O.D. stainless tube. These collector tubes are prehumidified with enough moisture to assure quantitative sulfur dioxide adsorption during the initial phase of testing. The collector tubes are held in the system by 1/4-inch Swaglok tubing connectors. All connection tubing are made of Teflon or glass. This system is graphically illustrated in Figure 1. After a 24-hour collection cycle, the Puramer S tube is removed, purged with oxygen-free nitrogen and carefully sealed using an oxygen impermeable end cap (probably just a pair of Swaglok tubing caps). This spent collector is sent to a central testing laboratory where the amount of collected sulfur dioxide is assayed. Presently, this assaying procedure is accomplished using a thermal desorption technique which produces sulfur dioxide at a rate such that continuous gas analysis can be accomplished. The total analysis system consists of an oxygen-free nitrogen purge gas, a gas flowmeter, a Bendix Flasher Unit and a Dohrmann Microcoulometric Titration System, connected in that order. All gas transfer lines are of 1/4 O.D. Teflon tubing and all connections are made with Swaglok tubing fittings. This system is shown' in Figure 2. The spent collector is carefully fitted into the Bendix Flasher Unit and collected sulfur dioxide is desorbed by programing the flasher oven temperature from 65°- 165°C. The desorbed sulfur dioxide is continuously purged from the collector device, using oxygen-free nitrogen, and assayed via the Dohrmann analyzer. Using this two-part method, sulfur dioxide assays were determined under controlled laboratory conditions (S02 loaded via permeation tubes and zero air). As the data in Table I show, excellent results were obtained. ------- PURAMER S COLLECTOR DEVICE AIR FLOW CONTROLLER AIR PUMP HUMIDIFIER FIGURE 1. PURAMER S BASED SULFUR DIOXIDE MONITORING SYSTEM - COLLECTOR SYSTEM. ------- IN Of •• ^m ERT GAS UNDER FLOW i^ •^ i ME ^_ TER 1 Ooo Ooo 1 J 1 V rll O O OO O pOooo o DOHRMANN ci ii n ID i • =0^j 8 oooaoo JBENDIX ANALYZER RECORDER FLASHER FIGURE 2. PURAMER S BASED SULFUR DIOXIDE MONITORING SYSTEM - ANALYZER SYSTEM. ------- TABLE 1. SULFUR DIOXIDE ANALYSIS BY A PURAMER S- BASED ANALYTICAL METHOD Loaded SO2 Assayed S02 S02 Recoveries nanograms - nanograms - % 3782 4633 5254 6521 9994 11138 16870 17546 19385 22774 3559 4479 5233 6413 9547 11288 17346 17728 20208 22435 94 - 97 99 98 96 101 103 101 104 99 It should be mentioned that the total quantities of sulfur dioxide being measured in this study represent analyses of syn- thetic gas streams, containing sulfur dioxide levels from 13 to 80 yg/m3, if collection is carried out for 24 hours at a flow of 200 cc/minute. These sulfur dioxide concentrations are in the range that would be expected in actual ambient air con- ditions. The accuracy of these results, although obtained under controlled laboratory conditions, certainly demonstrates the feasibility of monitoring ambient levels of sulfur dioxide with a Puramer S-based method. To bring the Puramer S-based sulfur dioxide monitoring system to its present level of technical development, it was necessary to study many system variables in detail. These studies are individually discussed in the following sections. ------- SECTION 5 EXPERIMENTAL SULFUR DIOXIDE ADSORPTION CHARACTERISTICS OF Puramer S - POLYURETHANE FOAM ADSORBENTS Once it was demonstrated that a Puramer S-polyurethane foam adsorbent could quantitatively collect sulfur dioxide from a properly humidified air stream, it was necessary to determine the proper size adsrobent bed required to assure efficient collection of sulfur dioxide for a minimum of 24 hours at the maximum expected level of ambient sulfur dioxide (2600 yg/m3). To properly size such a Puramer S-based filter, collector devices were prepared by cutting 1-inch x 1/4-inch plugs of adsorbent from foam pieces coated with a wide range of Puramer S concentrations. For test purposes, these adsorbent plugs were fabricated into filter devices by placing them into 6.4 mm I.D., shrinkable Teflon tubing fitted with 3 inch end pieces of 6 mm glass tubing. Using sulfur dioxide loading equipment as described in Section 6.1, Procedures, sulfur dioxide adsorption characteristics of a number of various Puramer S filter devices were determined at a flow rate of 200 cc/minute and a sulfur dioxide concentration of 3590 yg/m3. During these evalua- tions, the water content of the feed gas stream was maintained at approximately 22 mg H20/1 of air. As the data in Table 2 show, collection efficiency, defined as the percentage of incident sulfur dioxide removed by the adsorbent, was 100 percent regardless of filter length or the amount of active polymer contained on the foam adsorbent. How^ ever, the total collection time for which quantitative removal of sulfur dioxide was realized, was dependent on the particular adsorbent tested.. At least 2 inches of foam adsorbent, con- taining a minimum of 8.6 percent Puramer S will be required if 3590 vig S02/M3 of air is to be collected at 200 cc/minute for a minimum of 24 hours. ------- TABLE 2. ADSORPTION CHARACTERISTICS OF PURAMER S-POLY- URETHANE FOAM ADSORBENTS . Test. No. 1 2 3 4 5 6 Filter Length 2" * 1" 2" 2" 2" 2" Contained Puramer S 2 8 8 8 8 18 .0% .6% .6% .6% .6% .3% Collection Efficiency* 100% 1 1 1 1 1 00% 00% 00% 00% 00% Collection Time** 7 15 26 28 32 68 hrs. hrs. hrs. hrs. hrs. hrs. *Based on the percentage of incident S02 removed. **Time at which zero S02 is measured in the filtered effluent gas (less than 7 yg/m3). ***Test Conditions; 1. Flow Rate - 200 cc/minute. 2. S02 Cone. - 3590 yg/m3 in zero air. 3. Relative Humidity - ^85% at 27°C. Since actual ambient sulfur dioxide levels will be less than 3590 yg F02/M3 of air, a collector device consisting of 2 inches of foam adsorbent, coated with a minimum of 9 percent active polymer, will be more than is required for 24-hour col- lection cycles. Therefore, future laboratory efforts will center around adsorbents containing at least 9 percent active polymer. Also, it should be pointed out that tests numbers 1, 3 and 4 were run at 0, 23 and 48 days after the foam adsorbent was prepared. As the data in Table 2 show, storage of Puramer S- based adsorbent, prior to its use as a sulfur dioxide collec- tor, does not affect its overall collection efficiency and within experimental error, does not significantly reduce col- lection times. 10 ------- EFFECT OF DESORPTION TEMPERATURES ON SULFUR DIOXIDE RECOVERIES Having shown that properly sized Puramer S-polyurethane foam adsorbents can quantitatively collect sulfur dioxide from air, efforts were directed towards developing an analytical method for measuring the actual amount of collected sulfur dioxide. As previously mentioned, this assaying method was to be the result of controlled thermal desorption of sulfur dioxide using a Bendix Flasher and continuous sulfur dioxide Analysis via a Dohrmann MCTS (see complete method in Section 6.3, Procedures). As the data in Table 3 show, sulfur dioxide recovery from a Puramer S-urethane foam collector (2 inches long-M2% Puramer S), previously blanked to 170°C to assure that no sulfur dioxide was present prior to loading, was highly dependent on the max- imum desoprtion temperature used to produce sulfur dioxide for subsequent assay. At 160°-165°C, recoveries were low and quite TABLE 3. THE EFFECT OF DESORPTION TEMPERATURE ON SULFUR DIOXIDE RECOVERIES Loaded SO2 ng. 7,217 9,478 19,537 23,485 35,924 4,633 22,774 Assayed S02 ng. 6,382 6,440 15,040 23,292 34,400 4,479 22,435 Recovery % . 88 68 77 99 96 97 99 Maximum Desorption Temp. , °C 160° 165° 165° 200° 190° 170°* 170°* *Purge gas heated to 160°C prior to entering the Bendix Flasher Unit. 11 ------- variable, while at 190°C and 200°C, recoveries were quantita- tive. However, being concerned about possible filter decom- position at these high temperatures, which would foul the MCTS cell, attempts were made to reduce the maximum desorption temperature by using preheated purge gas. As the data show, quantitative sulfur dioxide recoveries were realized at 170°C when preheated CM60°C) gas was employed. Using a maximum desorption temperature of 170°C, continuous series of sulfur dioxide assays can be run without fouling the MCTS sulfur cell. Until future laboratory studies dictate a change in thermal desorption technique, 170°C will be the final temperature used to regenerate sulfur dioxide for all Dohrmann MCTS analyses. EFFECT OF PURAMER S CONTENT ON SULFUR DIOXIDE RECOVERY EFFICIENCY In order to study the effect of Puramer S content (poly- urethane foam substrate) on the overall sulfur dioxide recovery efficiencies, using thermal sulfur dioxide desorption and sub- sequent Dohrmann MCTS analysis, a series of assays were run using 2-inch foam adsorbents containing various amounts of Puramer S. Exact sulfur dioxide loadings were obtained using a calibrated sulfur dioxide permeation tube (see Section 6.1 for a detailed description of the loading method). As the data in Table 4 show, foams containing 4.3 percent and 8.9 percent active polymer gave sulfur dioxide recoveries of 88-91 percent while filters prepared from 12 percent and 18 percent Puramer S foams consistently gave quantitative sulfur dioxide recoveries over a wide range of sulfur dioxide loadings. The data also indicated that higher loadings result in more quantititative recoveries regardless of Puramer S content. It was fortunate that higher loadings resulted in better recovery efficiencies since these and even higher loadings are more realistic, in actual ambient applications, than the lower levels measured in this study. The poorer recoveries obtained when low Puramer S content adsorbents were tested might be due to sulfur dioxide reaction with the urethane substrate. Using low amounts of active polymer it would be expected that more urethane structure would be exposed to the sulfur dioxide-water than at the higher Puramer S coatings. If sulfur dioxide were to react in such a way as to render the sulfur dioxide stable and no longer thermally regenerable, low recoveries would be realized. More will be said about sulfur dioxide loss due to reaction with the adsorbent in the section discussing adsorbent cycling. Preven- tion of sulfur dioxide loss by reaction can be achieved using foam coated with at least 12 percent Puramer S. 12 ------- TABLE 4. EFFECT OF PURAMER S CONTENT ON SULFUR DIOXIDE RECOVERY EFFICIENCY Puramer S Content 18 18 12 12 12 12 12 12 12 9 9 4 4 Loaded S02 ng. 4,633 22,774 3,782 4,633 6,521 9,994 16,870 17,546 22,774 3,853 5,524 3,790 6,488 Assayed S02 ng. 4,479 22,435 3,559 4,479 6,413 9,547 17,346 17,728 22,435 3,387 4,855 3,335 5,917 Recovery Efficiency 97 99 94 97 98 96 103 101 99 88 88 88 91 *A11 Puramer S collectors treated for 5 minutes at 170°C and a 200 cc/minute N2 purge to assure that no S02 wss contained on the adsorbent prior to S02 loading. 13 ------- EFFECT OF COLLECTOR CYCLING ON THE SULFUR DIOXIDE RECOVERY EFFICIENCY Another parameter that was studied during this laboratory research program was the effect of cycling or re-use of the filters on sulfur dioxide recovery. This study not only showed that reuse was possible, but probably desirable. Using collector devices containing 2-inch lengths of Puramer S-urethane foam adsorbents, containing 4 -percent, 9 percent, and 15 percent active amine polymer, sulfur dioxide was carefully loaded and then analyzed via thermal desorption and subsequent Dohrmann MCTS analysis of the sulfur dioxide in the effluent purge gas (see Sections 6.1 and 6.3 for details of these experimental procedures). After all the collected sulfur dioxide was desorbed and measured, the collector devices were prehydrated by passing moisturized nitrogen ( ^22 mg H20/& through the device for 5 minutes at 200 cc/minute and the analytical cycle repeated.. The data in Table 5 show that sulfur dioxide recoveries, via thermal treatment, were always lower than expected for the first collection-assay cycle. Once again the recoveries were lowest for adsorbents containing the least amount of Puramer S. In every case, however, second, third and even fourth cycle treatment resulted in quantitative recoveries. Cycling the filters through the desorption part of this process showed that the improved recoveries were not the result of residual sulfur dioxide left on the collector and subsequently measured on the next cycle. The low, first cycle results again indicated that sulfur dioxide was reacting with some functional group on the adsorbent which renders it nonregenerable and hence, not available for MCTS detection. Once this small amount of reaction was com- pleted, however, the functional group or reactive site was no longer available for sulfur dioxide attack and, therefore, additional cycling resulted in quantitative assays. Efforts will be needed to develop a pretreatment for the Puramer S adsorbent so that quantitative recoveries can be realized in the first cycle. Also, this study showed that EPA could recycle Puramer S collector devicies without affecting subsequent analysis of ambient sulfur dioxide. 14 ------- TABLE 5. PURAMER S ADSORBENT CYCLING VERSUS SULFUR DIOXIDE RECOVERY EFFICIENCY Cycle 1 2 3 1 2 3 1 2 3 4 Puramer S % 4 4 4 9 9 9 15 15 15 15 Loaded SO2 ng. 6,488 3,683 3,452 3,853 7,359 5,557 24,120 24,228 24,084 24,084 « Assayed S02 ng. 5,917 3,650 3,365 3,387 7,572 5,407 22,320 23,599 24,000 23,441 Recovery % 91 99 98 88 103 97 93 97 100 97 15 ------- EFFECT OF PREHYDRATION ON THE SULFUR DIOXIDE RECOVERY EFFICIENCY OF PURAMER S ADSORBENTS Very early in our research, it was shown that many of the low sulfur dioxide recoveries were the direct result of a low moisture content on the Puramer S-polyurethane foam adsorbent prior to sulfur dioxide loading. This was particularly true for Puramer S adsorbents being used to study filter cycling and its effect on recoveries. The low preloading moisture, results in poor adsorption efficiency early in the loading cycle. Because this laboratory effort used short filter loading periods, using high concentrations of sulfur dioxide (3600 yg S02/M3 of air), the effect of moisture on the filter prior to loading was mag- nified, since there was not enough time, early in the collection cycle, during which water, needed for sulfur dioxide adsorption, could be adsorbed. As the data in Table 6 show, low sulfur dioxide recoveries due to low pre-sulfur dioxide loading moisture could be easily remedied by simply prehydrating the collectors by passing 200 cc/minute of moisturized nitrogen (^22 mg H20/ji) through the device for 5 minutes. (Longer prehydration times did not effect recoveries. Prehydration of Puramer S-based sulfur dioxide collectors is now standard procedure. TABLE 6. EFFECT OF PREHYDRATION ON THE SULFUR DIOXIDE RECOVERY EFFICIENCY OF PURAMER S ADSORBENTS Puramer S Prefilter Loaded Assayed Recovery Cycle Content Hydration S02-ng S02-ng - % 7 8.9 No 3,604 2,771 77 8 8.9 Yes 6,587 6,708 102 3 12.0 No 5,016 4,592 91 4 12.0 Yes 19,385 20,208 104 16 ------- Having completed most of the more important parameter studies, a viable Puramer S-based analytical procedure has been developed. This method can quantitatively assay known amounts of sulfur dioxide which are carefully loaded onto adsorbents under controlled laboratory conditions. The method, however, assumes that actual thermal desorption and subsequent Dohrmann MCTS analysis of the loaded sulfur dioxide is carried out imme- diately after collection. This is not at all practical because there are sometimes many days that elapse between sulfur dioxide collection and final assay. This delay is caused by shipment of spent collectors from the air monitoring site to a central laboratory for final testing. Before a study of the effect of filter storage time and temperature, after sulfur dioxide col- lection, on the subsequent recovery efficiency is detailed, a summary of the parameters required to assay sulfur dioxide, by a Puramer S-based method, is appropriate. This summary can be seen in Table 7. TABLE 7. PURAMER S-BASED SULFUR DIOXIDE ANALYTICAL SYSTEM PARAMETERS Collection of Filter Used - 2" x 1/4" Puramer S-polyurethane foam. % Puramer S - 12-15%. Prehydration - Yes. % Relative Humidity in Gas Stream - 85% at 25°C. Gas Flow Rate - 200 cc/min. Assaying of SO 9 Desorption Temp. - 50° to 170°C. Purge Gas - Oxygen-free nitrogen. Purge Gas Flow - 135 cc/min. S02 Measurement - Dohrmann MCTS or equivalent. 17 ------- Using these parameters, sulfur dioxide carefully loaded on Puramer S-polyurethane foam adsorbents can be assayed over a wide range of concentrations. A representative summary of these results are shown in Table 8. TABLE 8. SULFUR DIOXIDE ASSAYS VIA A PURAMER S-BASED METHOD 24 Hour Average SOo Conc.-ug/M3 13.1 16.1 18.2 22.6 34.7 38.7 58.6 60.9 67.3 78.1 79.1 83.6 83.6 83.6 84.1 Total Collected S0?-ng 3,782 4,633 5,254 6,521 9,994 11 ,138 16,870 17,546 19,385 22,500 22,774 24,084 24,084 24,084 24,228 Assayed S02~ng. 3,559 4,479 5,233 6,413 9,547 11,288 17,346 17,728 20,208 22,164 22,435 23,760 24,072 23,280 23,592 S02 Recovery - % 94 97 99 98 96 101 103 101 104 99 99 99 100 97 97 18 ------- EFFECT OF STORAGE ON SULFUR DIOXIDE RECOVERY EFFICIENCY Having successfully developed a method for assaying ambient levels of sulfur dioxide, collected by a Puramer S-based adsorb- ent under ideal laboratory conditions, efforts were concentrated on determining whether or not the method was applicable under reallife conditions. The first phase of this study was to determine the effect of storage time and temperature of spent adsorbent (sulfur dioxide loaded), on subsequent recovery efficiency. Puramer S-polyurethane foam adsorbents were loaded with various amounts of sulfur dioxide using the procedure outlined in Section 6.1. The spent filters were sealed, using rubber septa and stored at 25°-27°C, 45°C, 0°C and -30°C. At various storage times the amount of loaded sulfur dioxide was measured and recovery efficiencies determined. These initial storage data are summarized in Table 9. These data show that storage of spent Puramer S-based col- lectors resulted in significant reductions in overall sulfur dioxide recoveries. Sulfur dioxide decay was minimized, to an acceptable level, via sub-zero storage at -30°C. Also, these data showed that the rate of decay was closely related to the storage temperature. At 0°C, 84 percent of the col- lected sulfur dioxide could be assayed after 21 days of storage while only 33 percent could be found after 24 days at 45°C. Storage at 25-27°C also resulted in a steady loss of sulfur dioxide, with only 68 percent of the collected sulfur dioxide available for measurement after 21 days. The results seem to suggest that sulfur dioxide was being destroyed via a chemical reaction. It was believed that oxygen present in the free void space of the collector devices could result in oxidation of sulfur dioxide to sulfur trioxide. Sulfur trioxide is not thermally desorbable from Puramer S and if it were, it could not be measured by the Dohrmann analyzer because the sulfur cell employed is specific for sulfur dioxide. Using very simple calculations, it was shown that the 3-inch by 1/4-inch collector tube could contain enough oxygen to convert all the sulfur dioxide, normally loaded, to sulfur trioxide. This contained oxygen could be eliminated via purging with an oxygen-free inert gas, but another calculation^ quickly showed that air permeation through the rubber septum caps, used to seal the spent collector tubes could allow enough oxygen to enter 19 ------- TABLE 9. EFFECT OF POST-COLLECTION STORAGE ON PURAMER S ADSORBENT RECOVERY EFFICIENCY Storage Time-days 0 6 0 5 6 26 26 21 21 21 2 24 2 3 6 12 Storage Temp.-°C 25-27 25-27 25-27 25-27 25-27 25-27 25-27 0 0 0 45 45 -30 -30 -30 -30 Loaded S00 ng. ^ 22,774 8,914 16,840 24,214 10,246 31,982 12,794 25,545 14,458 17,424 13,190 21,874 14,054 22,500 24,036 19,176 Assayed S00 ng. ^* 22,435 7,840 17,346 22,568 8,267 21 ,600 8,400 21,360 11,520 14,640 7,680 7,200 15,240 22,128 23,366 18,408 Recovery % 99 88 103 93 81 68 66 84 80 84 58 33 101 98 97 96 20 ------- the tubes, in 24 hours, to completely convert all the contained sulfur dioxide to trioxide. Having a full appreciation for the effect of oxygen on the decay of sulfur dioxide fixed on a Puramer S filter, a study was run on spent adsorbents stored at 25°-27eC in an oxygen-free atmosphere. To accomplish this study, spent filters were purged for 10 minutes with oxygen-free nitrogen, again sealed with rubber septa and stored at ambient temperature in a container that was continuously purged with oxygen-free nitrogen. As shown in Table 10, sulfur dioxide decay was reduced by min- imizing oxygen during storage. However, some decay still was taking place since only 93-95 percent of the collected sulfur dioxide was measured after 5-7 days. TABLE 10. PURAMER S—SULFUR DIOXIDE STORAGE STABILITY IN AN INERT ATMOSPHERE Storage Time-days 0 6 26 1 4 5 7 Loaded SO -ng. 2 22,774 8,914 31,982 24,084 24,144 24,096 23,092 Assayed SO -ng. 2 22,435 7,840 21,600 22,320 22,046 22,800 21,476 Recovery - % 99 88 68 93 91 95 93 Storage Atmosphere Air Air Air Oxygen Free Oxygen Free Oxygen Free Oxygen Free Again, looking for possible sources of oxygen in our overall test system, it was pointed out that enough oxygen could be dissolved in the rubber end seals, used to cap the spent filters, to convert approximately 38,000 nanograms of sulfur dioxide to undetectable sulfur trioxide.6 This was more sulfur dioxide than normally collected by the Puramer S adsorbents. These storage studies were very encouraging since they suggested that by using a properly designed collector seal to avoid oxygen contamination, sulfur dioxide decay would be eliminated and spent Puramer S filters would have the required stability, regardless of storage temperature. A preliminary program to develop an improved seal for spent Puramer S adsorbents, resulted in a seal consisting of 21 ------- glass rod end butts that were tightly held to the stainless steel collector via small sections of a Teflon sleeve. These seals were firmly held in place using vinyl tape (see Figure 3). PURAMER S TEFLON SLEEVE \ GLASS ROD BUTT SEAL STAINLESS STEEL TUBING FIGURE 3. PURAMER S FILTER SEAL 22 ------- These seals were put in place following a 20-minute collector purge with oxygen-free nitrogen. Using filters fitted with these end seals, storage stability tests were run at 25°-27°C n^n^n0 day?' V, the data in Table 11 snow' this method of preparing spent collectors for ambient storage proved very effective. •* TABLE 11. EFFECT OF PROPER SEALING ON THE STORAGE STABILITY OF SPENT PURAMER S ADSORBENTS Storage Time -days 0 0 0 1 1 3 3 8 8 8 Note — Storage Loaded S02 -ng 27,704 23,114 18,232 18,521 18,259 15,216 30,772 20,683 20,623 20,472 temperature at Assayed S02 -ng 26,170 23,117 18,292 17,905 17,753 14,954 30,609 19,532 19,557 20,391 25°-27°C. SO 2 Recovery 94.5 100.0 100.3 96.7 97.2 98.3 99.5 94.4 94.8 99.6 Although these data show that the glass butt sealing tech- nique was effective for 25°-278C storage, preliminary results at 40°C indicate that sulfur dioxide decay still occurred at an undesirable rate. Sulfur dioxide recoveries of 88 percent and 74 percent were obtained for sealed collectors stored at 40°C for 1 and 3 days respectively. This decay in recovery was probably due to a small amount of oxygen contamination resulting from the loosening of the end butt seals at 40°C. By placing properly sealed spent filters in sealed jars, that were purged with oxygen-free nitrogen, sulfur dioxide decay due to oxygen contamination was minimized and recovery of 95-98 percent was obtained for a small number of samples. 23 ------- This finding again suggested the need for a seal that would completely eliminate oxygen contamination of spent filters. Success in developing such an end seal would bring the Puramer S-based sulfur dioxide monitoring method to a point where it would be functional under real-life ambient condition. This final Puramer S-based sulfur dioxide method would eliminate almost all of the troublesome shortcomings of the presently used West-Gaeke method. 24 ------- SECTION 6 PROCEDURES METHOD OF LOADING SULFUR DIOXIDE ON PURAMER S ADSORBENTS A. Purpose: The purpose of this procedure is to accurately load known amounts of S02 on Puramer S adsorbents. B. Equipment: 1. A calibrated S02 permeation tube. 2. A cylinder of zero zir fitted with a properly sized pressure regulator. 3. A cylinder of nitrogen fitted with a properly sized pressure regulator. 4. A constant temperature water bath. 5. A flowmeter and needle valve flow regulator. 6. A glass, U-tube holder fitted with inlet and outlet connectors. 7. Two calibrated thermometers (range 7-31°C). 8. A Dynasciences S02 Pollution Monitor fitted with a 0-0.5, 0-1.5, 0-5.0 ppm S02 sensor. 9. A 10 mv. strip-chart recorder. 10. 1/4" I.D. Teflon tubing. 11. 1/4" nylon tubing fittings. 12. Two, Drechsel gas washing bottles. (S.G.A. JB-1370.) 25 ------- C. Equipment Setup: Puramer S Adsorbent Prehydrator. 1. The prehydrator consists of a cylinder of nitrogen, fitted with a properly sized pressure regulator that is connected to a gas flowmeter-regulator using 1/4" I.D. Teflon tubing. The flowmeter-regulator is con- nected to a 250 ml, Drechsel gas washing bottle con- taining approximately 200 ml of distilled water, again using 1/4" I.D. Teflon tubing. The off-side of the gas washing bottle is fitted with a 1/4" nylon tubing fitting into which is placed the device containing the Puramer S-based adsorbent. 2. Using a gas flow of 200 cc/minute, moisture is loaded onto the adsorbent for 5 minutes. D. Equipment Setup: Puramer S Adsrobent S02 Loading System. 1. The S02 loading system consists of a cylinder of zero air, fitted with a properly sized pressure reg- ulator, a flowmeter-regulator, a 250 ml, Drechsel gas washing bottle containing approximately 200 ml of distilled water, a glass U-tube (containing glass beads in the inlet half and an SC>2 permeation tube and thermometer in the off side) immersed in a constant temperature water bath, a Puramer S adsorbent device holder, a Dynasciences SC>2 Pollution Monitor fitted with a 0-0.5, 0-1.5, 0-5.0 ppm SC>2 sensor and a 10 mv. strip-chart recorder. All of this equipment is connected together, in the order listed, using 1/4" I.D. Teflon tubing and 1/4" nylon tubing fittings. 2. At a constant flow of 200 cc/minute, humidified zero air is passed over a calibrated S02 permeation tube (30°4'0.10C) until a constant S02 concentration is measured by the Dynasciences SC>2 monitor. Once a stable SC>2 concentration is realized, Puramer S adsorbents can be accurately loaded with SC>2 by inserting a Puramer S filter device into the gas stream between the permeation tube and the Dynasciences monitor; By measuring the filtered effluent gas, quan- titative collection or removal of S02 by the Puramer S adsorbend can be assured. The loading time is recorded by a stopwatch in seconds. The quantity of loaded SC>2 can be varied by varying the loading time. Loaded Puramer S devices are sealed with a proper sealing device. 3. SC>2 Loading = Permeation Rate (ng S02/min) x Time (sec) 60 26 ------- CALIBRATION OF SULFUR DIOXIDE PERMEATION TUBES A. Purpose: The purpose of this method is to accurately determine the S02 delivery rate of a permeation tube under the exact flow and temperature conditions used when loading SO? onto Puramer S adsorbents. B. Equipment: 1. The exact permeation tube system described in 6.1-D. 2. The S02 permeation tube to be calibrated. 3. A Dohrmann Microcoulometric Titration System equipped with a Model T-300P-oxidative sulfur titration cell. 4. 1/4" I.D. Teflon tubing. C. Equipment Setup and Procedure 1. Using as short a piece of 1/4" O.D. Teflon tubing as possible, connect the exit side of the S02 permeation system to the microcoulometric titration cell. 2. Before actually making this connection, set the Dohr- mann Microcoulometric Titration System parameters as described in the instruction manual. This usually means a Bias setting of 140-150 ma. and a coulometer gain of 200. 3. After the microcoulometer is properly adjusted, connect the S02 permeation system directly to the titration cell and set the permeation conditions of temperature and flow to those to be used in loading Puramer S adsorbents. 4. At a microcoulometer ohm setting of 10, continuously measure the nanograms of sulfur in the synthesized gas stream using the Dohrmann recorder and stroke integrator (each full stroke = 100 counts). Con- tinue this measurement until a stable recorder read- out (constant sulfur level in gas) is realized for a minimum analysis period of 30 minutes. . 27 ------- D. Calculation of S02 Permeation Rate Total Sulfur Conts x 4 Rate _ Kate Analysis Time — (mins. ) x ohm setting Permeation Rate of S02 = nanograms S02/minute. 28 ------- THERMAL DESORPTION METHOD FOR ASSAYING SULFUR DIOXIDE ADSORBED ON PURAMER S-BASED ADSORBENTS A. Purpose: The purpose of this method is to quantitatively desorb S02 from Puramer S adsorbents so that the S02 can be assayed using a Dohrmann Microcoulometric S02 Titration System. B. Equipment: 1. A cylinder of oxygen-free nitrogen, fitted with a properly sized pressure regulator. 2. A Bendix Flasher Unit (Model H/S 10). 3. 3-inch x 0.25-inch stainless steel collector tubes containing Puramer S-polyurethane foam adsorbent (2-inch x 1/4-inch plug). 4. A 25-foot x 1/4-inch O.D. coil of copper tubing filled with small glass beads. 5. A hot plate. 6. A 0-250°C thermometer. 7. A Bell jar containing high-temperature silicon oil stabilized with ionol. 8. A flowmeter. 9 A Dohrmann Microcoulometric Titration System, equipped * with a Model T-300P oxidative sulfur titration cell. 10. 1/16-inch O.D. Teflon tubing and stainless steel Swaglok tubing connectors. 29 ------- C. Equipment Setup: 1. The system used to assay the SC>2 collected on a Puramer S-based adsorbent consists of a cylinder of oxygen-free nitrogen, fitted with a properly sized pressure regulator, a purge gas preheater made up of 25 feet of 1/4-inch O.D. copper tubing filled with glass beads and immersed in a silicon oil bath heated to 160°C via a stirrer-hot plate, a Bendix Flasher Unit (Model H/S 10) and a Dohrmann Microcoulometric Titration System equipped with a Model T-300P oxidative sulfur cell. All parts of the system are connected via 1/16 inch O.D. Teflon tubing using stainless steel Swaglok fittings. D. Procedure: 1. Using the above described setup, set the nitrogen flow at ^135 cc/minute. 2. Set the gas preheater temperature at 160°C. 3. With the Bendix Flasher Unit in a gas bypass mode, set the Microcoulometer parameters as per the instru- ment instruction manual. Set ohm range at 10. 4. Place the spent Puramer S collector device in the Bendix Flasher Oven and set the oven temperature at 50°C. 5. Change the Bendix Flasher Unit to the analysis mode and continuously measure the amount of S02 in the collector purge gas. 6. Increase the Bendix Flasher Unit oven temperature in increments of 5°C so as to cause thermal generation of S02 at a rate that can be recorded by the S02 analyzer. 7. Program the oven temperature to 170°C. 8. Continue the analysis until no more S02 is measured (Dohrmann recorder back to the original baseline setting). 9. Cool the Flasher oven to 50°C using compressed air. 30 ------- E. Calculation of Assayed S02: 1. Determine the total number of SC>2 integration counts recorded by the disc-integrator. 2. Nanograms SC>2 = 4 x total counts ohm setting 3. % SO, Recovery = assayed SOp x 100 loaded S00 31 ------- PREPARATION OF PURAMER S-POLYURETHANE FOAM ADSORBENT AND SUBSEQUENT COLLECTOR DEVICES A. Materials and Equipment Needed: 1. 45 pores per inch polyurethane foam (Paramount). 2. 15 percent aqueous solution of specification N-glycidyl piperazine oligomer. 3. Vacuum Oven. 4. Buchner funnel and flask. 5. Rubber Dam. 6. Source of vacuum. 7. 8" x 8" x 3" glass dish. 8. Distilled water. B. Procedure: 1. Cut a 6" x 6" x 1" piece of 45 pores per inch polyure- thane foam. 2. Wash thoroughly with distilled water, squeeze out excess water and dry to constant weight in a vacuum oven set at 120°C. 3. After weighing the washed foam, saturate the foam with a 15 percent aqueous oligomer solution by immersing the foam in an 8" x 8" x 3" dish filled with solution. 4. Remove excess oligomer solution by placing the foam piece in a Buchner funnel, covering the funnel with rubber darning and pulling full house vacuum (^28"Hg). The excess solution is collected in the Buchner flask. 5. Place the oligomer-coated foam into a vacuum oven and hold for 16 hours at 120°C and 28" Hg. 32 ------- 6. Remove the Puramer S-coated foam from the oven, cool in a desiccator and weigh to determine the amount of Puramer S. 7. % Puramer S = Wt. of Coated Foam - Wt. of Foam x 1QO Wt. of Coated Foam 8. Cut out 1-inch x 1/4-inch plugs of Puramer S-poly- urethane foam adsorbent using a No. 4 cork borer. 9. Place two 1-inch plugs into a 3" x 1/4" stainless steel tube by carefully pushing one plug into each end of the 3-inch tube. This then is the SC>2 collector device. 33 ------- REFERENCES 1. Cotter, R. J. "Engineered Adsorption Surfaces—Selective Adsorbents for Sulfur Dioxide Based on Polymers Containing Amino and Hydroxyl Groups." Project Report, File No. 3328, June 23, 1972. 2. Keogh, M. J. "Engineered Adsorption Surfaces—Characteri- zation Studies on N-Glycidly Piperazine Oligomer." Project Report, File No. 3892, December 4, 1973. 3. Smith, S. G. Jr. "N-Glycidly Piperazine Oligomer: A Proc- ess Study and Subsequent Scale-Up to Pilot Plant Equipment." Project Report, File No. 4225, November 8, 1974. 4. Heitz, W. D. "SC>2 Adsorbent Fabrication." Project Report, File No. 3819, September 21, 1973. 5. Stenstrom, John. Private communication, September 1976. 6. Stenstrom, John. Private communication, September 1976. 34 ------- TECHNICAL REPORT DATA (rlcasc read Instructions on the reverse before completing! REPORT NO. EPA-600/2-79-205 3. RECIPIENT'S ACCESSION-NO. HTLE AND SUBTITLE OLID SORBENT FOR COLLECTING ATMOSPHERIC SULFUR DIOXIDE 5. REPORT DATE December 1979 6. PERFORMING ORGANIZATION CODE AUTHORiS) 8. PERFORMING ORGANIZATION REPORT NO. R. Cotter and S. Smith Jr. PERFORMING ORGANIZATION NAME AND ADDRESS nion Carbide Corporation ;hetnicals and Plastics Research Laboratories Sound Brook, Hew Jersey 08805 10. PROGRAM ELEMENT NO. 1AD712 BE-03 (FY-77) 11. CONTRACT/GRANT NO. 68-02-1782 2.SPONSORING AGENCY NAME AND ADDRESS Environmental Sciences Research Laboratory - RTP, NC iffice of Research and Development U.S. Environmental Protection Agency Research Triangle Park, NC 27700 13. TYPE OF REPORT AND PERIOD COVERED Final 5/75-11/77 14. SPONSORING AGENCY CODE EPA/600/09 5. SUPPLEMENTARY NOTES 6. ABSTRACT A solid sorbent for collecting atmospheric S02 was evaluated as part of an overall effort to develop a replacement method for the West-Gaeke method presently used to measure 24-hour ambient sulfur dioxide concentrations in ambient air. Research showed that a' solid sorbent, consisting of Puramer S coated open cell polyurethane foam, can be used to fix the quantities of sulfur dioxide that would be collected if typical ambient air was filtered for 24 hours at 200 cc/min. The method of assaying sulfur dioxide collected by the sorbent consisted of controlled thermal desorption of sulfur dioxide followed by continuous analysis using a Dohrmann microcoulometric titration system. Troublesome sulfur dioxide decay, occurring during post collection storage, was primarily the result of oxidation. Decay was minimized, to an acceptable level, by properly sealing the spent Puramer S collector devices to prevent oxygen contamination from contacting the sorbent prior to thermal desorption and subsequent assaying. KEY WORDS AND DOCUMENT ANALYSIS DESCRIPTORS b.lDENTIFIERS/OPEN ENDED TERMS COSATi Field'Group *Air pollution *Su?fur dioxide *Sorption 'Sorbents Foam 'olyurethane resins Evaluation 13B 07B 07D 11G 111 3. DISTRIBUTION STATEMENT RELEASE TO PUBLIC EPA Form 2220-1 (9-73) 19 SECURITY CLASS (ThisRepot UNCLASSIFIED 43 20 SECURITY CLASS (This page! UNCLASSIFIED 22. PRICE 35 ------- |