Environ. Sci. Technol. 1988, 23, 1420-1422
Sample Vokmre Measurement Errors Caused by C02 Adsorption in Desiccants Scott W. Klamm' and George W. Scheil Midwest Research Institute, 425 Volker Boulevard, Kansas City, Mlssouri 641 10
Silica gel, often used as a desiccant to adsorb water vapor during stack sampling operations, has also been demonstrated to adsorb other gases, among them carbon dioxide. A potential error in volume measurement arises when such adsorption occurs prior to sample entering a dry gas meter. Using silica gel and some other common desiccants, a series of tests was performed to determine the magnitude of such volume measurement errors. Laboratory results showed that for small sample volumes, error could be as high as 12% from a typical combustion source, and that significant response delay may be seen in some continuous monitoring systems.
Introduction Silica gel, often used as a desiccant to adsorb water vapor during stack sampling operations, has also been demonstrated to adsorb other gases, among them carbon dioxide (1). A potential error in volume measurement arises when such adsorption occurs prior to sample entering a dry gas meter. Combustion stack gases typically contain 12% C02, which could result in a large sample volume measurement error should a significant portion of this C02 be adsorbed by the silica gel. Using silica gel and some other common desiccants, a series of tesb was performed to determine the magnitude of such volume measurement errors due to adsorption of C02.
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Method
A test apparatus was set up to allow dry simulated combustion gas (12% C02 in nitrogen) and dry purge gas (N,) to be alternately connected to a desiccant cartridge. Gas flow was directed into an inlet rotameter, through the desiccant cartridge (2.75 cm in diameter by 7.5 cm long), and then through an exit rotameter. The nominal gas flow rate for all tests was 0.75 L/min. COOconcentrations in the exit gas stream were monitored downstream of the second rotameter with a Horiba PIR-2000 S, NDIR analyzer. Data were recorded simultaneously by a Health SR-204 chart recorder and a Wintek MCS data logger with a Zenith 2-181 portable computer. Readings were averaged by the data logger every 10 s. For each desiccant tested, an initial tare weight of the desiccant cartridge was taken. The cartridge was then placed in-line with the flowmeters. The COz adsorption was performed by quickly raising the COPflow from zero to the desired level and allowing flow to continue until stable C02 readings were achieved, indicating that C02 adsorption had ceased. The desiccant cartridge was reweighed to determine the weight of C02 adsorbed. The C02gas was then disconnected and the nitrogen connected. The nitrogen purge cycle was similarly performed by quickly raising the nitrogen flow from zero to the desired value. Flow was continued until stable readings were achieved, indicating that C02desorption had ceased. The desiccant cartridge was weighed to determine the weight of C02 desorbed. 1420 Environ. Sci. Technol., Vol. 23, No. 1 1 , 1989
Test blanks were also run, using glass beads of volume similar to the desiccants being tested. The test blank was used to simulate the dead-space volume of a desiccant cartridge without possessing any adsorbancy characteristics. Thus, test blanks represent the ideal response, allowing a comparison with the actual desiccant test runs.
Results To quantify the C02/desiccant system behavior, three methods were used adsorption and desorption based upon analyzer response delay, weight change of the desiccant cartridge, and volumetric flow change through the system. Table I outlines the data for each desiccant. Duplicate runs were performed on all desiccants except for alumina and spent grade 59 silica gel. Measuring the response delay of the continuous C02 analyzer located at the exit of the test apparatus was the primary quantification method. Using C02 analyzer data as recorded by the logger every 10 s, a typical adsorption/desorption cycle is shown in Figure 1. Calculations were then performed by numerically integrating the area between the curves (ideal vs actual) by using the trapezoidal rule (2). Weight change of the desiccant cartridge, a second method of analysis, allows gravimetric confirmation of C02 adsorbed per 100 g of desiccant. Calculations were based upon the increase in weight of the cartridge and the amount of desiccant used. Calculations for the purge cycle followed the same general procedures, with the result being a weight loss, rather than gain, by the desiccant cartridge. Measurement of volumetric flow change, a third method of analysis, verified the apparent COz adsorption and desorption trends observed. During the C02 adsorption cycle, exit flows were 5-10% lower than at the inlet, indicating removal of gases by the desiccant. This offset decreased, approaching zero, as C02 readings stabilized. The N2 purge cycle also showed a rotameter flow offset. In this case exit flow readings were higher, indicating the release of gases into the stream. Again, the offset approached zero as C02 readings became stable. Discussion A direct comparison of the first method, integration of the COz analyzer data, can be made with the second method, weight change of the desiccant cartridge. Results for the two methods are similar, except for spent silica gel and spent anhydrite (Drierite). In both of these cases, adsorbance measured by weight change of the desiccant cartridge was significantly lower than that measured by integration of the C02 analyzer data. This indicates that moisture was being driven off the spent adsorbent. COz adsorption was greatest with fresh grade 44 silica gel and a mixture of recycled silica gel. Both of these desiccants exhibited an adsorbancy of 0.2 L of C02 adsorbed/100 g of desiccant. Spent silica gel showed much less COz adsorbance, and it also demonstrated a weight loss due to moisture loss. Anhydrite displayed characteristics similar to the silica gel, but to a lesser degree. Alumina pellets, a less common desiccant, showed higher
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Table I. Summary of Results adsomtion cvcle response delay desiccant method C02 test condition net wt, g adsorbed, L/100g fresh silica gel 41.9 0.175 41.9 0.205 (grade 44 indicating) 31.3 0.219 fresh silica gel 31.3 0.192 (recycled mixture) 27.9 0.036 spent silica gel 27.7 0.034 (recycled mixture) 21.5 0.118 fresh silica gel 21.5 0.099 (grade 59) 0.057 20.0 spent silica gel‘ (grade 59) fresh anhydrite 56.1 0.033 0.014 56.1 (Drierite) 57.4 0.012 spent anhydrite 57.4 0.003 (Drierite) 55.6 0.270 alumina‘ 76.1 glass beads 0 76.1 0 Related to fresh grade 44 = 100, glass beads = 0.
desorDtion cvcle
wt change method w t gain, C02 adsorbed,
g 0.158 0.187 0.131 0.135 -0.016b -0.010b 0.032 0.040 -0.016b
L/100 g 0.206 0.244 0.229 0.236
0.040 0.043 -0.004 0.003 0.247 -0.004 0.003
0.039 0.042 -0.004 0.003 0.243 -0.003 0.002
0.081 0.102
response delay av re1 C02 method C02 adsorpn: % desorbed, L/100 g 0.189 100 0.209 0.130 106 0.125 0.033 17 0.036 0.100 48 0.102 27 0.022
0.006
2 124
Moisture driven off during test.
o w 0
I
I
I
2
4
6
I
a
g 0.145 0.188 0.135 0.135 0.094b 0.082b 0.038 0.041 0.054b
0.043 0.043 0.009 0.009 0.358 0.003 0.003
g 0.189 0.245 0.236 0.236
0.097 0.104 0.042 0.042 0.009 0.009 0.352 0.002 0.002
’Single run only. Desorption Cycle
r Blank (Ideal)
121
0.003 0.186 -0.002 0.002
0
Adsorption Cycle 13
0.031 0.033
15
wt method w t loss, COz desorbed,
I
1
I
10
0
,
2
,
4
,
6
a
10
Time (min) Flgure 1. COPadsorption/desorptlon cycle for a typical desiccant.
COz adsorbance than silica gel. As seen in Figure 1,the use of silica gel desiccant in a continuous-emission monitoring system leads to an obvious problem of slow response for COz analyzers. Readings of any other gas analyzers present will also be biased because of the continuous changes in sample gas volume, which fluctuates as COzis adsorbed/desorbed from the desiccant. Although it is possible to precondition the system to reduce bias, slow response time will still be evident as new sample is introduced.
Conclusions For precision gas volume measurements used in some sampling methods, COz adsorbance of roughly 0.2 L/100 g of silica gel will influence dry gas volume results. One example of this influence is the VOST (3) system, which allows sample volumes as small as 5 L. A 300-g desiccant cartridge and a sample volume of 5 L could result in a 12%
volume error from a typical 12% COz combustion source. This volume error is difficult to correct, as COzadsorbance depends upon the amount of water absorbed by the desiccant from the wet gas stream. In practical terms, a gas stream of typical moisture content will demonstrate an adsorbance lying somewhere between the “fresh” and “spent” values summarized in Table I. A continuous-emission monitoring system that uses a desiccant as a drying agent will display biased results due to COPadsorption until equilibrium has been established. As seen In Figure 1, this may be a time period of up to 10 min, with new adsorption/ desorption occurring each time the gas stream COz concentration changes. Thus, if the use of a desiccant for continuous-emission monitoring systems is unavoidable, a minimal amount of anhydrite should be used. COz adsorption by anhydrite is -15% of that for silica gel, and water retention is more permanent (4). Silica gel is still acceptable, however, for methods such Environ. Sci. Technol., Vol. 23, No. 1 1 , 1989
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Environ. Sci. Technol. 1989, 23, 1422-1425
as EPA method 10 as part of an Ascarite COPremoval system. Registry No. COz, 124-38-9. Literature Cited (1) Grob, R. Modern Practice of Gas Chromatography, 2nd ed.; John Wiley & Sons: New York, 1985; pp 124-125. (2) Burden, R.; Faires, J.; Reynolds, A. Numerical Analysis; PWS Publishers: Boston, MA, 1981.
(3) VOST Method 0030. Test Methods for Evaluating Solid
Wuste,Field Manual; 3rd ed.; U.S. EPA Washington, DC,
1986; Vol. 11. (4) Mellor, J. W. A Comprehensioe Treatise ofhorganic and
Theoretical Chemistry; Longmans, Green & Co.: Harlow, 1946; Vol. 111.
Essex, U.K.,
Received for review March 27, 1989. Accepted June 13,1989.
Degradation of Trichloroethylene and trans4 ,2-Dichloroethylene by a Methanotrophic Consortium in a Fixed-Film, Packed-Bed Bioreactor Gerald W. Strandberg, Terrence L. Donaldson,' and Linda L. Farr
Chemical Technology Division, Oak Ridge National Laboratory,+Oak Ridge, Tennessee 37831
A fixed-film, packed-bed bioreactor containing a consortium of microorganisms utilizing methane as the primary carbon source was used to treat a synthetic groundwater containing trichloroethylene (TCE) and trans-1,2-dichloroethylene(DCE). With TCE and DCE influent concentrations of 1 mg/L each and a residence time of -50 min, >50% of the TCE and >90% of the DCE were degraded in a single pass through the bioreactor. Further degradation of TCE was obtained with liquid recycle. The TCE degradation rate appeared to be first order in TCE concentration. The apparent fmt-order rate constant for TCE degradation was -0.02 min-'. Introduction
The ability of methaneutilizing bacteria to cometabolize short-chain chlorinated hydrocarbons such as trichloroethylene (TCE) and tram-1,2-dichloroethylene(DCE) has been reported by several groups (1-6). Little et al. recently reported the mineralization of TCE by a pure culture of a methane-oxidizing organism isolated from TCE-contaminated groundwater (3). It is generally believed that the enzyme methane monooxygenase oxidizes these chloroalkenes to epoxides, which spontaneously degrade to intermediates that can be further metabolized. We have constructed and operated a fixed-film packed-bed bioreactor to evaluate the technical feasibility of bioremediation of TCEcontaminated groundwater using methanotrophic microorganisms. The performance of our bench-scale bioreactor system and observations on the kinetics of degradation of TCE and DCE in the bioreactor are described in this report. Materials and Methods
Bioreactor Design and Operation. The bioreactor (Figure 1) consisted of a 5-cm4.d. X 110-cm-long glass column packed with 0.6-cm ceramic berl saddles (Scientific Products, McGaw Park, IL) as a support matrix for the biofilm. A concentrated feed solution containing mineral salts (3) and TCE, DCE, or both was continuously bled into a stream of process water (nonchlorinated tap water meeting potable water standards). The mixture was distributed over the top of the packing at 10 mL/min unless noted otherwise. The influent concentrations of TCE and DCE (typically 1mg/L each) were controlled by adjusting their 'Operated by Martin Marietta Energy Systems, Inc., for the US. Department of Energy under Contract DE-AC05-840R21400, 1422
Environ. Sci. Technol., Vol. 23, No. 11, 1989
concentrations in the feed concentrate and by varying the dilution with process water. A concurrent gas stream containing methane (4% v/v unless noted otherwise) and air was introduced at the top of the bioreactor at 20 mL/min. This type of bioreactor and mode of operation were chosen to promote transfer of oxygen and methane to the biofilm and to minimize stripping of TCE and DCE into the gas phase. The system was operated at ambient temperature (22-24 "C). Bioreactor performance in terms of TCE and DCE degradation was measured at liquid flow rates of 5,10,20, 35, and 50 mL/min. The mean liquid residence time at each flow rate was estimated by monitoring the effluent conductivity following pulses of NaCl (7). Influent and effluent lines were constructed primarily of glass, Viton tubing, or stainless steel. However, to minimize any errors due to adsorption of TCE and DCE, the feed and effluent streams were sampled at the points where they entered and left the reactor, respectively. Bioreactor Startup. The microbial consortium was obtained from C. D. Little (3). It originated from a TCE-contaminated groundwater monitoring well on the Oak Ridge Reservation. The culture was maintained in a mineral-salts medium (3) under an atmosphere of 20% CHI and 80% air. The microbial population in the bioreactor was established by adding approximately 50 mL of an actively growing culture and 150 mL of fresh medium and then operating the system at -99% liquid recycle at 10 mL/min. Fresh medium containing 1mg/L TCE was introduced at 0.1 mL/min. The gas stream (10 mL/min) contained 20% CHI and 80% air. After 3-4 weeks, a substantial growth of salmon-pink-colored biomass was visible throughout the column. The system was then switched to and routinely operated in a single-pass mode and the methane concentration was reduced to typically 4 % methane in air. Analytical Procedures. Trichloroethylene and DCE were analyzed by gas chromatography using a Varian 3700 gas chromatograph (Palo Alto, CA) equipped with an electron capture detector. Separation was achieved with a DB+1 megabore column (J&W Scientific, Folsom, CA) operated isothermally at 40 "C, with N2as the carrier gas (3-4 mL/min). Samples (20 mL) of liquid influent or effluent were placed in 65-mL amber bottles sealed with Teflon-lined septum closures. The bottles were placed on a rotator (20 rpm, Cole-Palmer, Chicago, IL), and the contents were allowed to equilibrate for 1 h. The headspace gas was then assayed. Samples (5 pL) of the headspace gas or the bioreactor off-gas were injected directly onto the column. Quanti-
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