1911
Anal. Chem. 1982, 5 4 , 1911-1913
Sampling of Staclk, Gas for Sulfur Dioxide with a Molecular Sieve Adsorbent Colln D. Chrlswell" and Douglas 1.Gjerde' Ames Laboratory USDOE, I'owa State University, Ames, Iowa 5001 1
Atmospheric sulfur dioxide has been shown to aggrevate human respiratory difficulties, to damage vegetation, to accelerate the deterioration of calcareous building materials, and to be a major contributor to acid rain (1-,3). The sources o€ this pollutant include fo!ssil fuel combustion, cement manufacture, metal smelting, and paper pulping (1-3). Various procedures are available for determining sulfur dioxide in gaseous effluents such as stack gas. Most of these rely on the use of impinger trains to trap sulfur dioxide (4). Procedures in which sulfur dioxide iia accumulated with an impinger are effective in yielding accurate and reproducible results. However, even under the best of circumstances the sampling of a stack is inconvenient, and the need to transport a liquid-filled impinger train to a sampling site on the side of a stack does little to enhance the convenience of sampling. In the present work it has been found that a unique, hydrophobic molecular sieve can be used as an adsorbent to accumulate sulfur dioxide from stack gas. A small, pencilsized, adsorption column replaces the impinger train normally used and, thus, greatly increases the convenience of sampling. Sulfur dioxide is quantitatively adsorbed until the capacity of the molecular sieve is exceeded. Major components of stack gas such as nitrogen, oxygen, carbon dioxide, and water vapor do not interfere with the aiccumulation of sulfur dioxide. Many of the trace components found in stack gas are accumulated by adsorption on Silicalite (5)and an obvious extension of the present work would be to couple the accumulation technique with thermal desorption and gas Chromatography to determine other low-molecular-weigh t Components of gaseous effluents. The adsorption properties of Silicalite also suggest potential. applications for scrubbing stack gas and application has been filed for a patent on t h k use.
EXPERIMENTAL SECTION Materials. The adsorbent used is a silica-based,hydrophobic molecular sieve known as Sillicalite (Linde Division, Union Carbide, Tarrytown, NY). It contains 6 A diameter pores as part of the crystal structure and these ]poresoccupy 33% of the crystal volume (6). Various binders are used with this molecular sieve, and it has been found that they can affect the adsorption properties (5). In the present work a material designated as LZ-115 was used. This material is in the form of 20 X 80 mesh granules and is believed to contain an alumina-silicate clay binder. The molecular sieve is heated at 800 "C for 24 h to remove any adsorbed impurities and then sized to yield a 60 X 80 mesh fraction which is used for sulfur dioxide accumulation. Synthetic stack gas (Matheson, East Rutherford, NJ) containing 543 ppm sulfur dioxide, 10.34% carbon dioxide, 9.38% oxygen, and the balance nitrogen was used during the development of the analytical protocol. Other gases used were of a purity of at least 99%. Recommended Procedure for Determining Sulfur Dioxide. Load 4.6 mm i.d. x: 5 cm stainless steel adsorption tubes with about 0.5 g of cleaneld, sized Silicalite. Attach a sampling probe to the inlet of one adsorption tube, a second adsorption tube to the outlet of the first, and an air pump such as the Air Cade$ (Cole Palmer, Chicago, IL) to the outlet of the second adsorption tube. The probes in this instance consists of a 3 ft length of ' 1 4 in. stainless steel tubhy which is inserted through a sampling port with one end in the ;gas stream and the adsorption tubes outside the stack. Draw stack gas through the adsorption beds at a measured flow rate of approximately 100 mL/min. The total volume sampled will be determined by the sensitivity of the 'Present address: Exxon Research and Development, Linden, NJ. 0003-2700/82/0354-1911$01.25/0
Table I. Adsomtion of Gases on Silicalite Molecular Sieve distrbn coeff vs. He gas sulfur dioxide carbon dioxide nitrous oxide methane nitrogen oxygen water carbon monoxide propane carbonyl sulfide dimethyl mercaptan hydrogen sulfide a
50 "C
100 "C
9
130 16 3
24
7
1600a
77 9
3
9 70
4 20
11 1300a
220
>loo0 >loo0 150
4
> 500 > 500 40
Approximate.
analytical procedure used and the the sulfur dioxide content of the stack gas. In the present instance a volume of 1 L was found to be adequate. At the conclusion of sampling, transport the adsorption tubes to the laboratory, unload the molecular sieve into a combustion boat, and determine the total sulfur content using a Fisher Model 470 sulfur analyzer (Fisher Scientific, Chicago, IL) or a similar instrument. The presence of sulfur in the second adsorption tube will indicate that the capacity of the first bed was exceeded and would require resampling of a smaller volume of gas. Blank determinations should be performed on replicate samples of Silicalite not used for sampling, but the sulfur content of cleaned Silicalite and of Silicalite taken from the second adsorption bed has consistently been lower than the detection limit of the sulfur analyzer. ppm SOz =
(6.827)(% S in Silicalite)(wt of Silicalite, mg) (volume of gas sampled, L)
RESULTS AND DISCUSSION Adsorption of Gases on Silicalite. The distribution coefficients obtained for various gases indicate that Silicalite can be used for accumulating different substances from stack gas (Table I). The distribution coefficients given were determined by gas chromatography using pure compounds
D, =
(retention time X flow rate) - dead volume wt of Silicalite
D, has units of mL/g. Of the prevalent components of stack gas sulfur dioxide has the highest distribution coefficient and this provides the basis for its accumulation in the presence of gases such as carbon dioxide, nitrogen, oxygen, water vapor, carbon monoxide, and nitrous oxide. In the case of water vapor and carbon dioxide there was concern that mass effects due to the high concentrations of these species in stack gas would lead to the displacement of the more strongly adsorbed sulfur dioxide. However, this did not occur. Other studies (5, 7) have shown that the capacity of Silicalite for carbon dioxide and for water is relatively low even when these components are accumulated from pure streams. Thus, it appears that there is no competition for adsorption sites between sulfur dioxide and water vapor or carbon dioxide. In the case of water vapor specifically, there is strong evidence (7) that all water retained is adsorbed on the surface possibly by interactions with the binder and that no water enters the pore structure. 0 1982 American Chemical Society
1912
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
Table 111. Sulfur Dioxide Capacity vs. Concentration
so, 543
I
ii
I 20
40 TIME,
60
80
minutes
Flgure 1. Sulfur dloxide breakthrough curve: 543 ppm sulfur dioxide in stack gas; 150 mL/m flow rate; 25 "C; 0.5125 g of 60 X 80 mesh Silicalite LZ-115 in a 4.6 mm X 5 cm adsorption column.
Table 11. Sulfur Dioxide Capacity vs. Temperature 0 25 50 7 5 100 125 150 "C "C "C "C "C "C "C capacity,mg/g
30
ppm in SO, stack gas 6% in N,
1
29
21
17
13
10
5
If other sulfur-containing gases such as hydrogen sulfide, carbonyl sulfide, and dimethyl mercaptan are present in stack gas they will be accumulated and determined as sulfur dioxide using the recommended procedure. Thus, the recommended procedure would in reality give a value for total sulfur as sulfur dioxide. In most instances sulfur dioxide is present at far higher concentrations than other sulfur-containing gases and the differences would be negligible. However, if this interference is unacceptable, gases can be thermally desorbed from Silicalite beds at 350 " C directly into the inlet of a gas chromatograph and can then be separated and individual components quantified. This procedure is rapid and convenient to use but does require specialized instrumentation. Alternatively, adsorbed sulfur dioxide can be stripped from heated adsorption tubes with helium gas, trapped in a peroxide solution, and subsequently determined by titration or ion chromatography. This procedure essentially would be identical with sampling with an impinger train except that the impinger is in the laboratory instead a t the stack. Procedure Tests. The effects of flow rate through the bed were determined by passing a synthetic stack gas through beds a t rates ranging from 37 mL/min up to 150 mL/min. Flow rate was measured with a standard rotameter. No significant differences were observed in the capacity of the Silicalite for sulfur dioxide or in the shape of breakthrough curves obtained by monitoring the concentration of sulfur dioxide in bed effluents. A breakthrough curve obtained a t the highest flow rate, 150 mL/min, is shown in Figure 1. The effects of gas temperature on accumulation of sulfur dioxide were determined by passing the synthetic stack gas at a flow rate of 75 mL/min through beds held at temperatures ranging from 0 O C to 150 "C. As would be expected, the highest capacities measured at the breakthrough equilibrium point were obtained at the lowest temperatures. Acceptable capacities were obtained even a t 150 O C . Stack gas temperatures in excess of 150 "C are frequently encountered, but the gas cools to below 100 O C while passing through the probe. Table I1 shows the effects of gas temperature on capacity. As shown in Table 111, the equilibrium capacity is dependent on the concentration of sulfur dioxide in the gas stream. Even at the lowest concentration studied a capacity of approximately 20 mg/g of Silicalite was determined. As a gas this would occupy a volume of about 7 mL which is far in excess of the pore volume or total bed volume of Silicalite. Sulfur dioxide is obviously accumulated in some condensed state.
capacity, mg/g
20
50
SO, 100%
190
The molecular sieve was preconditioned before use by heating at a temperature of 800 O C . This was done to remove any adsorbed contaminants which might interfere with sulfur dioxide determinations. This preconditioning temperature was chosen solely because muffle furnaces were available continuously at 800 O C . Separate samples of Silicalite were preconditioned a t temperatures of 150 O C and 250 "C with a constant purge with nitrogen or with helium. There were no differences observed in the properties of the Silicalite due to the preconditioning technique and thus considerable latitude is available in choosing a cleanup procedure. A different lot of Silicalite containing a more hydrophobic silica clay binder performed erratically after being heated to any temperatures in excess of 150 "C. In some instances the capacity would be increased slightly and in other instances the material would no longer accumulate any sulfur dioxide. The standard material, LZ-115, does not exhibit this type of erratic behavior and its use is recommended. In laboratory tests of accumulation procedures the sulfur dioxide levels in bed effluents were monitored continuously based on the absorbance of sulfur dioxide at 280 nm. In addition, aliquots of the effluents were collected and sulfur dioxide levels determined by gas chromatography using an instrument equipped with a flame photometric detector. It was not possible to follow this protocol when sampling stack gas. Instead two adsorption tubes were placed in series. No sulfur dioxide was detected in the second tube. In addition, effluents from the pump were passed through an impinger containing a 3% hydrogen peroxide solution. No sulfur anions were detected in these solutions by ion chromatography. In addition to studies performed at different concentrations during the development of the protocol, a series of determinations were performed on the synthetic stack gas certified to contain 543 ppm sulfur dioxide. Results of six determinations ranged from 450 to 541 ppm with an average of 495 ppm and a standard deviation of 43 ppm. In these determinations cylinder pressure was used to maintain flow through adsorption beds. Maintaining a precise flow was difficult and variations in flow rate contributed to variations in values determined. The Ames, IA, municipal power plant burns high-sulfur Illinois coal and low-sulfur Wyoming coal mixed with a very low-sulfur refuse derived fuel. Because of the different boiler fuels used, sulfur dioxide levels vary considerably. During the past year levels determined with impinger trains for accumulation have ranged from 500 ppm sulfur dioxide to 2000 ppm sulfur dioxide in the stack gas. Levels of sulfur dioxide were measured on three different occasions using the molecular sieve adsorption protocol. The levels of sulfur dioxide determined during the first sampling were 870 and 890 ppm, during the second sampling the levels were 1330 and 1310, and during the final sampling were 1210 and 1240. Although no direct comparisons with standard methods are available, the values are in agreement with values normally obtained using standard impinger trains. In each instance a measured volume of gas of approximately 1 L was sampled at a flow rate of 100 mL/min. A backup adsorption tube in series with the first tube did not accumulate any measurable amount of sulfur dioxide. The stack gas temperature at the entrance to the probe was approximately 165 "C but was less than 100 O C outside the stack at the adsorption beds.
Anal. Chem. 1982, 5 4 , 1913-1914
ACKNOWLEDGMENT We appreciate both the advice and assistance offered by Gerda Shultz-Sibbel and John Richard. LITER.ATURECITED (1) Ember, L. R. Chem. Eng. News 1981,59 (37),20. (2) LaBastille, A. Natl. Geogr. 1981, 160 (5),652. (3) Ember, L. R. Chem. Eng. News 1981, 59(38),14. (4) Galeano, S. F.; Tucker, T. w.; Duncan, L. J. Alr POMO~.control ASSOC.
1972,22 (lo), 790. (5) Shultz-Sibbeh G. M. W.; GJerde, D. T.; Chrisweli, C. D.; Fritz, J. S. Talanfa , In
press.
(6) Flanigen, E. M.; Bennet, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R.
1913
L.; Kirchner, R. M.; Smith, J. V. Nature (London) 1978,271 (9), 512. (7) Kleln, S.M. Master of Science Thesls, Iowa State Unlversity, Ames, IA, 1982.
RECEIVED for review March 22, 1982. Accepted June 8, 1982. The Ames Laboratory is operated by Iowa State University for the U.S. Department of Energy under Contract No. W7405-eng-82. This work Was supported in Part by the Assistant Secretary for the Environment, Office of Health and Environmental~ ~contract ~W P A S ~- H A - O ~~-and ~ ~ in , part ~ by the Solar Energy Research Institute.
Comparison of Cointinuous Extractors for the Extraction and Concentration of Trace Organics from Waiter T. L. Peters Analytical Laboratories, The Dow Chemlcal Company, Midland, Michigan 48640
Current and proposed environmental regulations require analysis of a wide variety of organic compounds by gas chromatography/mass spectrometry. Since the required detection limits are a t the parts-per-billion level, an extraction/concentration step in sample preparation is a necessity. While liquid-liquid extraction employing separatory funnel and manual shaking is widely used, large sample volumes and emulsion problems make this a time-consuming procedure at best. Although continuous liquid-liquid extractors (CLLE) are available as an alternative,the time required for an extraction (18-24 h) is usually quobed as a reason for not using them. However, since any number may be run simultaneously, this does not present an insurmountable problem. In addition, the continuous extractors are suited for unattended (i.e., overnight) operation. A great variety of extraction devices have been described in the literature and many are commercially available but little has been done to compaire the relative merits of these extractors. The purpose of this study was to make a comparison and provide direction as to what extractor will best handle a specific problem.
EXPERIMENTAL SECTION Apparatus. The following extractors and solvents were used in this work: (1)Liquid-Liquid Extractor (LLE) (Kontes K-583250 or Ace Glass 6841-10). A commercial extractor in which a heavierthan-water solvent (methylene chloride) is dripped through the water sample. (2) Steam Codistillatiori Extractor (SCDE) (Ace Glass no. 6826-40 or J&W Scientific). This unit allows simultaneous condensation of a steam distillate and an immiscible extraction solvent. In this work, methylene chloride was used. (3) Steam Distillation Extractor (SDE) (Ace Glass no. 6555). This extractor passes sample steam distillate through an immiscible lighter-than-water solvent. Although hexane was used in this study, other lighter-than-watersolvents me also applicable. (4) Flow Under Extractor (FUE) (Custom ScientificGlass, Inc.). In this extractor, the heavier-than-water solvent (methylene chloride) and sample do not inix but only contact at the interface. (5) Flow Over Extractor (FOE).A logical extension of the FUE, this extractor was construlcted by the Dow Glass Fabrication Laboratory (Figure 1). Again, a lighter-than-water solvent, hexane, was used with this extractor. Reagents. All solvenltti used were Burdick & Jackson (Muskegon, MI) spectrograde with no further purification. Procedure. A total of fiive 18-h extractions were made with each extractor. The sampllc was acidified (pH 2) blank water fortified with 50 ppb each of the following chemicals: chloro-
benzene, phenol, 2-chlorophenol, 2-nitrophenol, naphthalene, 4-chlorophenol, 1,2,3,4-tetrachlorobenzene, dimethyl phthalate, 4-nitrophenol, and pentachlorophenol. In each case, a 1-L sample was used. The solvents (either methylene chloride or hexane) were concentrated to 5 mL via Kuderna-Danish evaporative distillation techniques. Before analysis, anthracene-d,, was added as an internal standard. Extracts were analyzed in duplicate by capillary gas chromatography with a flame ionization detector.
RESULTS AND DISCUSSION Table I summarizes the recovery data obtained by using the extractors. Since samples had to be concentrated prior to analysis, values are given for the recovery of the entire procedure, recovery corrected for concentration losses, and the standard deviation. Within experimental error, the LLE and the FUE are equivalent. These extractors can be thought of as general purpose, extracting the widest variety of compounds. The FUE has a slight edge over the LLE in that emulsions with actual samples are nearly impossible to generate using this extractor. The FOE should be considered as complementary to the FUE. The same principle is employed, but more selectivity can be realized using a nonpolar hydrocarbon solvent. Lower recoveries observed in some cases are due to the solvent. In the case of 4-nitrophenol, 4-chlorophenol, 2-chlorophenol, and phenol, and low recoveries are due to a solubility problem. Chlorobenzene losses most likely occurred during the Kuderna-Danish concentration. The higher boiling point of hexane vs. methylene chloride would explain the recovery difference. Use of pentane solvent rather than hexane could partially alleviate this loss. Nonrecovery of 4-nitrophenol indicates that neither hydrocarbon solvents nor distillation techniques can be used to extract this component. Although the SDE and SCDE both rely on steam distillation as a preliminary separation step, the similarity ends there. The SDE recycles the steam condensate through a small (5-10 cm3) volume of lighter-than-water solvent. In the SCDE, the steam condensate mixes with solvent condensate on the condenser walls. The phases are separated and each returned to their respective distillation pots (steam condensate to sample chamber and solvent to solvent chamber). Fresh solvent continually contacts the steam condensate, so marginally soluble species are concentrated in the solvent chamber. The SCDE also has an advantage in that either heavier- or lighter-than-water solvents can be used. In this study, however, only methylene chloride was used.
0003-2700/82/0354-1913$01.25/0 0 1982 American Chemlcal Society
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