In-Line Catalytic Purification of Carbon Dioxide Used in Precision

Article pubs.acs.org/IECR

In-Line Catalytic Purification of Carbon Dioxide Used in Precision Cleaning Applications Michael E. Zorn,*,†,‡ Dean T. Tompkins,‡ Walter A. Zeltner,‡ Marc A. Anderson,‡,§ and John T. Etter‡ †

Department of Natural and Applied Sciences (Chemistry), University of WisconsinGreen Bay, 2420 Nicolet Drive, Green Bay, Wisconsin 54311, United States ‡ Microporous Oxides Science and Technology, LLC, 393 Lake Kegonsa Road, Oregon, Wisconsin 53575, United States § Environmental Chemistry and Technology Program, University of WisconsinMadison, 660 North Park Street, Madison, Wisconsin 53706, United States ABSTRACT: The use of carbon dioxide (CO2) in precision cleaning applications requires a high-purity feed stream of CO2, which can add considerable cost to the overall process. The ability to purchase and clean a lower grade of CO2 would represent a significant cost savings. In this study, an in-line catalytic purification system was employed to clean two low-purity grades of CO2 to very low contaminant levels at flow rates relevant for full-scale precision cleaning applications (3.63 kg/h or 8 lb/h). A “food” grade of CO2 had an unpurified contaminant concentration of 4.0 ng/g, and a “bone dry” grade had an unpurified contaminant concentration of 4.9 ng/g. After catalytic purification, the food grade contaminant concentration was reduced to 0.14 ng/g (a 97% reduction), and the bone dry grade contaminant concentration was reduced to 0.26 ng/g (a 95% reduction).

1. INTRODUCTION In recent years, carbon dioxide has gained popularity for use in a variety of surface cleaning applications.1−10 It has been used to clean a variety of high-technology components, including semiconductors, optics, hard disks, circuits, solar energy devices, vacuum systems, and medical devices, among others. Surface cleaning with CO2 involves passing either gaseous or liquid CO2 through a small orifice. This process creates tiny dry ice particles that are carried in a high flow rate stream that can be directed at a dirty surface. The high velocity gas/solid mixture is effective for removing a wide variety of contaminants (including organic compounds) from small, delicate, intricate surfaces.4,5 A major advantage of precision cleaning with CO2 compared to traditional solvents is that there is no residue left behind (assuming the precursor CO2 is clean). In addition, CO2 is inert, relatively abundant, and nonflammable. In this paper, the use of CO2 in this manner will be referred to in general terms as “precision cleaning”; terms used by others include “CO2 snow cleaning”, “dry ice jet cleaning”, “CO2 aerosol cleaning”, and “cryogenic CO2 cleaning”. Other systems involve the use of supercritical CO2 for various cleaning applications. These systems involve sealing the dirty components inside a high pressure vessel and allowing supercritical CO 2 to contact the components for a predetermined amount of time. While not the main focus of this research, these systems would also benefit from the CO2 purification capabilities described in this study. A significant cost of precision cleaning systems is the purchase of high-purity CO2. High pressure CO2 is an effective solvent for many organic compounds. For that reason, it is often difficult to manufacture CO2 that is free of residual organic compounds. Any organic materials that are used throughout the specialty gas manufacturing process (e.g., pump oils, valve greases, etc.) represent potential contaminants in high pressure CO2. If present, these contaminants could be left © 2012 American Chemical Society

behind by the cleaning process and significantly decrease the effectiveness of CO2-based precision cleaning systems.2,4 Specialty gas suppliers have methods for purifying CO2 prior to pressurizing it in cylinders; however, the added cost of these high grades of CO2 is significant. A number of previous publications have discussed issues related to contaminants present in specialty CO2.2,4,11−20 Two previous publications17,19 identified these contaminants as chlorofluorocarbon grease, a mixture of oligomers of chlorotrifluoroethylene (CTFE). CTFE and other halogenated lubricants have reportedly been used routinely in the specialty gas industry.21 The structure of CTFE is shown in Figure 1, and

Figure 1. Chemical structure of chlorotrifluoroethylene (CTFE) monomer (left) and polymer (right) molecules.

a sample GC-ECD (gas chromatography with electron capture detection) chromatogram of CTFE contamination is shown in Figure 2. Notice the characteristic clusters of contamination grouped in various regions throughout the chromatogram. Other attempts to purify CO2 at the point of use have been proposed. For example, Va-Tran Systems, Inc. (Chula Vista, CA) markets a CO2 purifier that operates by evaporating liquid CO2 (a good solvent for many contaminants) to gaseous CO2 (a poor solvent for many contaminants) prior to sending the Received: Revised: Accepted: Published: 2882

October 19, 2011 December 21, 2011 January 16, 2012 January 16, 2012 dx.doi.org/10.1021/ie202402w | Ind. Eng.Chem. Res. 2012, 51, 2882−2887

Industrial & Engineering Chemistry Research

Article

carbon contaminants using GC-FID; however, no FID detectible contaminants were observed. For this reason, only GC-ECD analyses are reported in this study. Previous catalytic purification experiments were published using a sol−gel derived particulate catalyst material at much lower flow rates appropriate for bench-scale analytical chemistry applications.19 The experiments reported in this paper were conducted at considerably higher flow rates that are more relevant for larger scale applications, such as precision cleaning.

2. MATERIALS AND METHODS 2.1. Specialty Gases. Two different grades of CO2 were tested in this study: “food” grade and “bone dry” grade. Both grades were obtained from an unspecified specialty gas supplier. The food grade was supplied in an MVE Dewar, and the bone dry grade was supplied in a high pressure cylinder. 2.2. Catalytic Reactor. Purification of the CO2 samples was achieved using a proprietary catalyst (catalog no. CAT-0102) manufactured by MOST, LLC, that was packed into a 500 mL reaction vessel wrapped with heat tape and insulation. The catalyst was prepared using sol−gel processing methods and was coated onto proprietary supports. The reactor temperature was maintained at 250 ± 5 °C during the catalytic experiments. A 500 mL preheater containing 1/4 in. diameter type 302 stainless steel balls (McMaster Carr, Elmhurst, IL) was used to precondition the CO2 to 250 °C before routing it into the reactor. 2.3. Experimental Setup. The setup for this experiment is depicted in Figure 3. The food grade sample had a relatively low supply pressure (6.80 atm or 100 psi) provided by an MVE Dewar, so a booster pump was used to increase the pressure up to an average pressure of 20.4 atm (300 psi). The bone dry grade was supplied from a high pressure cylinder (54.4 atm or 800 psi) and did not require use of the booster pump. As shown in Figure 3, the CO2 was routed from either the supply cylinder or booster pump into the catalytic preheater, then into the reactor, and then into a heat exchanger to cool the CO2 back to room temperature. The flow was then split into a high flow rate channel and a low flow rate channel. The CO2 in the

Figure 2. GC-ECD chromatogram of contamination present in bone dry grade CO2 as reported by Zorn et al.19 This cylinder was equipped with a helium pad and full-length eductor tube and had a cylinder pressure of 100 atm (1470 psi). The numerals between the bottom trace and the x-axis denote the approximate chromatographic regions originally reported by Noll et al.17

CO2 to the precision cleaning orifice. Also, SAES Pure Gas (San Luis Obispo, CA) markets a nonheated “medium” that removes non-methane hydrocarbons (NMHC) to less than 1 ppb from CO2. While they are effective at removing some contaminants, it is often difficult for distillation or adsorptionbased technologies to purify feed streams to very low levels. Trace quantities of the contamination can be left behind by these types of purification methods.2 The purpose of this study was to evaluate the use of a proprietary catalyst manufactured by Microporous Oxides Science and Technology (MOST), LLC (Oregon, WI), to purify CO2 used in precision cleaning applications. The target compounds for degradation were ECD-responsive compounds that are ubiquitous contaminants present in CO2 cylinders and Dewars used for this and similar applications. ECD is extremely sensitive for detecting halogenated compounds, such as CTFE, whereas flame ionization detection (FID) is used for detecting general hydrocarbons. It should be noted that several attempts were made to collect and quantify non-halogenated hydro-

Figure 3. Experimental setup utilized for sampling food grade CO2. Note the use of a booster pump to increase the pressure prior to routing it into the rest of the system. When sampling without catalytic purification, the CO2 was routed directly into the restrictor, contaminant trap, and flow meter of the low flow rate channel. The experimental setup utilized for sampling the bone dry grade carbon dioxide was the same, except that the booster pump was not used. 2883

dx.doi.org/10.1021/ie202402w | Ind. Eng.Chem. Res. 2012, 51, 2882−2887


Article

internal standard for regions 4−7. Internal standard corrected concentrations were converted to units of mass of contamination per mass of CO2.

high flow rate channel was passed through a needle valve to control the flow rate, followed by a digital flow meter (Model ADM-1000, Agilent Technologies, Santa Clara, CA). The CO2 exiting the flow meter was vented into a hood. The low flow rate channel was used to collect samples for analysis as described in section 2.4. The high flow rate channel was set to deliver 3.63 kg/h (8 lb/h), while the low flow rate channel was set to deliver 250 mL/min. During collection of the unpurified CO2 samples, the gas from either the booster (food grade sample) or the cylinder (bone dry grade sample) was routed directly through the low flow rate channel (i.e., restrictor, to trap, to flow meter) for sample collection to avoid the introduction of any additional contaminants. 2.4. Sample Collection. Samples were collected both with and without catalytic purification by routing the flow of CO2 through the low flow rate channel. The CO2 was passed through a high-purity variable restriction device to control the flow rate, followed by a cooled adsorption trap that was filled with precleaned Florisil (60−100 mesh, Fisher Scientific), as previously reported.19 Impurities present in the CO2 were collected on the Florisil trap. The CO2 exiting the trap was then routed into a flow meter and vented into a hood. Samples were collected for 1 h at a trap temperature of 0 °C and a flow rate of 250 mL/min (the flow rate was measured at a pressure of 1 atm and a temperature of 20 °C). After 1 h of collection, CO2 flow was stopped, the trap was heated back to room temperature, and 4 mL of hexane (Optima grade, Fisher Scientific) was passed through the Florisil trap to desorb the trapped contaminants. In addition to the hexane extracts derived from sampling the CO2, procedural blanks were collected for comparison purposes by simply rinsing the Florisil trap with 4 mL of hexane―no CO2 was passed through the trap prior to rinsing with hexane. 2.5. Gas Chromatography. All extracts were subsequently analyzed using a Hewlett-Packard 5890 Series II GC with ECD. The detector temperature was maintained at 330 °C, and the makeup gas (N2) flow rate was 25−30 mL/min. The carrier gas (He) head pressure was maintained at 18 psi. The injection port temperature was 280 °C, and injections (2 μL) were performed in the splitless mode (0.7 min purge delay). The GC column was a Supelco SPB-5 column (30 m × 0.25 mm i.d., 0.25 μm film thickness). The oven program was as follows: initial temperature 90 °C (retained for 1.2 min); 5 °C/min to 240 °C; 10 °C/min to 300 °C; retained for 25 min at 300 °C. As mentioned, attempts were made to quantify general hydrocarbon contamination using a similar GC with FID. However, chromatograms acquired on this instrument did not show the presence of significant hydrocarbon contamination; therefore, only GC-ECD results are discussed in this paper. 2.6. Contaminant Quantification. Quantification of the ECD-responsive contamination was performed as described by Noll et al.17 using the relative response of the ECD-responsive contamination to two internal standards. A CTFE grease standard was prepared by dissolving a sample of the pure grease in isooctane to a final concentration of 108 ng/mL. Two internal standards (PCB No. 30 and No. 204) were added to the sample extracts to allow for quantification. Chromatograms were separated into seven different regions based on elution of ECD-responsive contamination as follows: region 1 = 5−17 min, region 2 = 17−25 min, region 3 = 25−30 min, region 4 = 30−34 min, region 5 = 34−37 min, region 6 = 37−41 min, and region 7 = 41−50 min. PCB No. 30 was used as the internal standard for regions 1−3, and PCB No. 204 was used as the

3. RESULTS AND DISCUSSION 3.1. Catalytic Degradation. Chromatograms of the food and bone dry grades of CO2 with and without catalytic purification at 250 °C are shown in Figures 4 and 5,

Figure 4. GC-ECD chromatograms of contamination present in food grade CO2 samples collected in this study. The bottom trace is a Florisil/hexane procedural blank (i.e., no CO2 was sampled), the top trace is unpurified food grade CO2, and the middle trace is catalystpurified food grade CO2. The traces have been manually offset on the y-axis so that they do not overlap, but they all have the same relative response scale. The numerals between the bottom trace and the x-axis denote the approximate chromatographic regions originally proposed by Noll et al.17

Figure 5. GC-ECD chromatograms of contamination present in bone dry grade CO2 samples collected in this study. The bottom trace is a Florisil/hexane procedural blank (i.e., no CO2 was sampled), the top trace is unpurified bone dry grade CO2, and the middle trace is catalyst-purified bone dry grade CO2. The traces have been manually offset on the y-axis so that they do not overlap, but they all have the same relative response scale. The numerals between the bottom trace and the x-axis denote the approximate chromatographic regions originally proposed by Noll et al.17

respectively. In each figure, the top trace is from the unpurified CO2, and the middle trace is from the catalyst-purified CO2. The procedural blank (i.e., Florisil trap rinse) is also shown in Figures 4 and 5 (bottom traces) for comparison purposes. The traces have been manually offset on the y-axis so that they do not overlap, but they all have the same relative response scale. For clarity, the region beyond 25 min is not shown because it contained no detectable ECD-responsive contamination, except 2884



Article

assumes a specific heat for CO2 of 1.075 J/(g °C) at 250 °C and 37.4 atm (550 psi), a mass flow rate of 3.63 kg/h (8 lb/h), an inlet CO2 temperature of 20 °C, an outlet CO2 temperature of 250 °C, and an electricity cost of $0.10/kWh. Using these values, the estimated cost to operate a CO2 catalytic purifier would be $0.60 per day. This estimate assumes that the reaction chamber is perfectly insulated and any heat loss to the surrounding ambient air is negligible. The model does not include the effect of any heat generated by the reaction itself. When operated under these conditions, the purification system would likely require electrical costs of less than $1 per day (a conservative estimate) when CO2 is flowing continuously at 3.63 kg/h. The cost would be lower if the CO2 flow was intermittent. 3.2. Comparison to Previous Studies. As mentioned, the concentration of ECD-responsive contamination in several grades of CO2 was previously reported by Noll et al.17 Those grades had a higher purity than the unpurified food or bone dry grades used here. Noll et al. reported the following levels of contamination in these grades of CO2: SFC grade = 1.442 ng/g, SFC/SFE grade = 0.180 ng/g, and SFE grade = 0.004 ng/g. Based on the results reported above, the food and bone dry grades of CO2 used in this study were purified to levels similar to the SFC/SFE grade reported by Noll et al. It should be noted that higher levels of purity could theoretically be achieved by utilizing a larger mass of the MOST catalyst and/or by decreasing the CO2 flow rate through the system. In addition to the observed catalytic degradation of ECDresponsive compounds, some interesting observations can be made with regard to the distribution of contaminants present in the unpurified CO2 used here compared with the CO2 samples used in previous studies. As mentioned above, the food grade CO2 had an unpurified contaminant concentration of 4.0 ng/g, and the bone dry grade had an unpurified contaminant concentration of 4.9 ng/g. Virtually all (99%) of the observed contamination in each grade was due to compounds that eluted within the first 25 min of the chromatograms (regions 1 and 2, as detailed under Materials and Methods), while only 1% eluted after region 2. Previous studies have reported ECD-responsive contamination in all seven regions of the chromatogram, similar to that shown in Figure 2.17,19 For example, Noll et al.17 tested three different grades of CO2 (SFC grade, SFE grade, and SFC/SFE grade) and found that 90−97% of the contamination (depending on the CO2 grade) was due to compounds that eluted after region 2. Zorn et al.19 also tested a “bone dry” grade of CO2 from a different supplier and found that 96% of the contamination was due to compounds that eluted after region 2. For purposes of illustration, Figure 6 shows the full chromatogram of the unpurified bone dry grade of CO2 used in this study. This is the same sample displayed in Figure 5; however, the chromatogram has been expanded to 50 min and a chromatogram of a CTFE standard solution is also depicted in the same figure. As shown, the CTFE standard contains much of the ECD-responsive contamination in regions 3−7, whereas these regions are relatively clean in the unpurified bone dry grade CO2 sample tested here (except for two small peaks that also show up in the procedural blank). There are several possible explanations for this contrast. One explanation derives from the fact that the CO2 samples used in this study had supply pressures of 20.4 atm (food grade) and 54.4 atm (bone dry grade); however, the samples used by Noll et al.17 and Zorn et al.19 had much higher supply pressures of

for small peaks that were also present in the procedural blank. As shown in both Figures 4 and 5, the catalytic purifier significantly reduced the concentration of ECD-responsive contaminants to very low levels. An effort was made to quantify the contamination present in the food and bone dry grades of CO2 before and after catalytic purification using the method described above and by Noll et al.17 These results are presented in Table 1. Chromatograms Table 1. ECD Responsive Contamination in Carbon Dioxide Samples Tested in This Study concentration (ng/g of CO2) carbon dioxide

unpurified

purified

% degraded

food grade bone dry grade

4.0 4.9

0.14 0.26

97 95

were corrected to account for contamination present in the procedural blank. The GC-ECD areas were converted to a concentration of contamination in units of nanograms of ECDresponsive contaminants per gram of CO2. The mass of CO2 passing through the trap during contaminant collection was calculated to be 27 g for a flow rate of 250 mL/min, a time of 60 min, a pressure of 1 atm, and a temperature of 20 °C. Based on this treatment of the data, the food grade CO2 had an unpurified contaminant concentration of 4.0 ng/g, and the bone dry grade had an unpurified contaminant concentration of 4.9 ng/g. After catalytic purification, the food grade contaminant concentration was reduced to 0.14 ng/g (a 97% reduction), and the bone dry grade contaminant concentration was reduced to 0.26 ng/g (a 95% reduction). These results qualitatively and quantitatively show that this catalyst purification method was able to successfully clean low grade CO2 at flow rates that are representative of large-scale applications, such as precision cleaning. Elucidation of the exact catalytic reaction mechanism is beyond the scope of this study; however, a number of possible explanations for the decrease in observed ECD-responsive contamination can be postulated. Possibilities include the following: (1) the CTFE contaminants reacted with residual oxygen in the feed stream resulting in catalytic oxidation, (2) the CTFE polymers underwent catalytic depolymerization to gaseous monomer molecules that were not trapped by the Florisil, (3) the CTFE polymers underwent noncatalytic depolymerization to gaseous monomer molecules that were not trapped by the Florisil, or (4) the CTFE contaminants simply adsorbed onto the surface of the catalyst. These possibilities were originally proposed by Zorn et al.19 In that publication, it was concluded that the CTFE contaminants were either reacting with residual oxygen in the feed stream resulting in catalytic oxidation (explanation 1) or the polymers were possibly undergoing catalytic depolymerization to gaseous monomer molecules that were not trapped by the Florisil (explanation 2). Catalytic depolymerization has been reported to occur at 275−300 °C22 (only slightly above the temperature in this study). Noncatalytic thermal degradation and adsorption onto the catalyst surface were concluded to be less likely. Noncatalytic thermal degradation of CTFE polymers requires temperatures that are significantly higher than 250 °C (i.e., 360−390 °C23,24), and adsorption onto the catalyst surface was not supported by experiments described in that publication.19 A thermodynamic model was used to estimate the cost of operating a catalytic purifier in this manner. The model 2885



Article

responsive contaminants for the two different grades of CO2. Unpurified samples of CO2 used in this study exhibited a different distribution of contaminants than found in previous studies, possibly due to the lower supply pressures used here. Nevertheless, contaminants that were present were successfully removed from the CO2 feed streams at flow rates that are relevant for full-scale applications, such as precision cleaning.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: (920) 465-5758. Fax: (920) 465-2376. E-mail: zornm@ uwgb.edu.

■

Figure 6. ECD chromatogram of contamination present in unpurified bone dry grade CO2 collected in this study (same as top trace in Figure 5 but extended to 50 min). The inset shows a chromatogram of a CTFE standard solution for comparison. The numerals between the bottom trace and the x-axis denote the approximate chromatographic regions originally proposed by Noll et al.17

REFERENCES

(1) Sherman, R.; Hirt, D.; Vane, R. Surface cleaning with the carbondioxide snow jet. J. Vac. Sci. Technol., A 1994, 12 (4), 1876−1881. (2) Hill, E. A. Carbon dioxide snow examination and experimentation. Precis. Clean. 1994, No. Feb, 36−39. (3) King, J. W.; Williams, L. L. Utilization of critical fluids in processing semiconductors and their related materials. Curr. Opin. Solid State Mater. Sci. 2003, 7 (4−5), 413−424. (4) Sherman, R. Carbon dioxide snow cleaning. Part. Sci. Technol. 2007, 25 (1), 37−57. (5) Zhang, X.; Han, B. Cleaning using CO2-based solvents. Clean: Soil, Air, Water 2007, 35 (3), 223−229. (6) Yang, S. C.; Huang, K. S.; Lin, Y. C. Optimization of a pulsed carbon dioxide snow jet for cleaning CMOS image sensors by using the Taguchi method. Sens. Actuators, A 2007, 139 (1−2), 265−271. (7) Morris, D. J. Cleaning of diamond nanoindentation probes with oxygen plasma and carbon dioxide snow. Rev. Sci. Instrum. 2009, 80 (12), No. 126102. (8) Kim, P.; Seok, J. Dynamic modelling and simulation of a cryogenic carbon dioxide cleaning process. Proc. Inst. Mech. Eng., Part E 2010, 224 (E4), 213−221. (9) Kang, M. Y.; Jeong, H. W.; Kim, J.; Lee, J. W.; Jang, J. Removal of biofilms using carbon dioxide aerosols. J. Aerosol Sci. 2010, 41 (11), 1044−1051. (10) Liu, Y. H.; Maruyama, H.; Matsusaka, S. Effect of particle impact on surface cleaning using dry ice jet. Aerosol Sci. Technol. 2011, 45 (12), 1519−1527. (11) Nielen, M. W. F.; Sanderson, J. T.; Frei, R. W.; Brinkman, U. A. T. On-line system for supercritical fluid extraction and capillary gas chromatography with electron-capture detection. J. Chromatogr. 1989, 474 (2), 388−395. (12) Onuska, F. I.; Terry, K. A. Supercritical fluid extraction of PCB’s in tandem with high resolution gas chromatography in environmental analysis. J. High Resolut. Chromatogr. 1989, 12 (8), 527−531. (13) Nielen, M. W. F.; Stab, J. A.; Lingeman, H.; Brinkman, U. A. T. Purity of carbon dioxide as limiting factor in trace-level analysis by online supercritical fluid extraction/capillary gas chromatography. Chromatographia 1991, 32 (11−12), 543−545. (14) Wallace, J. C.; Krieger, M. S.; Hites, R. A. Reduction of contamination levels in online supercritical fluid extraction systems. Anal. Chem. 1992, 64 (21), 2655−2656. (15) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Supercritical-fluid chromatography and extraction. Anal. Chem. 1994, 66 (12), R106− R130. (16) Sterzenbach, D.; Wenclawiak, B. W.; Weigelt, V. Optimization of SFE for the determination of chlorinated hydrocarbons at the partper-trillion level. Anal. Chem. 1997, 69 (5), 965−967. (17) Noll, R. J.; Zorn, M. E.; Mathew, J.; Sonzogni, W. C. Interfering contaminants in carbon dioxide solvent used in the supercritical fluid extraction of polychlorinated biphenyls. J. Chromatogr., A 1998, 799 (1−2), 259−264. (18) Hinz, D. C.; Wenclawiak, B. W. Investigation of SFE/SFC grade carbon dioxide. Fresenius’ J. Anal. Chem. 1999, 365 (4), 355−360.

approximately 100 atm. The higher pressure used in these previous publications was provided by a pad of helium and a full-length eductor tube. It is possible that a higher pressure was required to dissolve the higher molecular weight polymers observed in these previous studies. If that is the case, only the lightest polymers (regions 1 and 2) would have been soluble at 20.4 or 54.4 atm, and the remaining polymers (regions 3−7) would have been left behind in the MVE Dewar or cylinder. It is not likely that the higher molecular weight polymers were condensing elsewhere in the system since the CO2 was routed directly into the restrictor of the low flow rate channel during the experiments without catalytic purification. Previously, Hinz and Wenclawiak18 showed that the concentration of impurities in CO2 increased with increasing head pressure; they concluded that the impurities originated in the CO2 cylinder, and that higher pressures increased the solubility of these impurities. Sherman et al.1,4 also noted that liquid CO2 used for precision or “snow” cleaning had more contamination than a similar gaseous source of CO2. A second explanation has to do with the presence of small amounts of helium in CO2 from a headspace pad. It has been shown by a number of previous researchers that helium in CO2 can affect the solubility of impurities (and analytes).25−27 The helium pad used in the previous studies17,19 might have increased the solubility of the higher molecular weight polymers, resulting in contamination in all seven regions of the chromatogram. These results suggest that there might be an advantage to using cylinders or Dewars with lower pressure for high-purity applications since a portion of the total contamination will be less soluble. However, there is a practical minimum pressure below which portions of the process (e.g., precision cleaning nozzles) will not operate properly. If the supply pressure is below that point, the use of additional mechanisms (i.e., pumps) to increase the pressure might introduce their own set of contaminants to the overall system, decreasing the effectiveness of the precision cleaning process. To avoid these problems, catalytic purification appears to be a viable option for using high pressure, low grade CO2 for high-purity applications.

4. CONCLUSIONS In this study, two low-purity grades of CO2 were successfully purified using a proprietary catalyst material. Catalytic purification resulted in 95 and 97% reductions in ECD2886



Article

(19) Zorn, M. E.; Noll, R. J.; Anderson, M. A.; Sonzogni, W. C. Inline catalytic purification of carbon dioxide used in analytical-scale supercritical fluid extraction. Anal. Chem. 2000, 72 (3), 631−633. (20) King, J. W. Supercritical fluid extraction: present status and prospects. Grasas Aceites (Sevilla, Spain) 2002, 53 (1), 8−21. (21) Vassilaros, D. L. Contamination of electron-capture detectors with volatile and semivolatile halocarbon impurities in GC gas distribution systems. LC-GC 1994, 12 (2), 94−104. (22) Feiring, A. E. Fluoroplastics. In Organofluorine Chemistry; Banks, R. E., Smart, B. E., Tatlow, J. C., Eds.; Plenum Press: New York, 1994. (23) Patrick, C. R. The thermochemistry of organic fluorine compounds. Adv. Fluorine Chem. 1961, 2, 1−34. (24) Sheppard, W. A.; Sharts, C. M. Organic Fluorine Chemistry; W.A. Benjamin, Inc.: New York, 1969. (25) Porter, N. L.; Richter, B. E.; Bornhop, D. J.; Later, D. W.; Beyerlein, F. H. Effects of fluid filling techniques on reproducibility in capillary SFC. J. High Resolut. Chromatogr. Chromatogr. Commun. 1987, 10 (8), 477−478. (26) Raynie, D. E.; Delaney, T. E. Effect of entrained helium on the kinetics of supercritical-fluid extraction with carbon-dioxide. J. Chromatogr. Sci. 1994, 32 (7), 298−300. (27) King, J. W.; Johnson, J. H.; Eller, F. J. Effect of supercritical carbon-dioxide pressurized with helium on solute solubility during supercritical-fluid extraction. Anal. Chem. 1995, 67 (13), 2288−2291.

2887


In-Line Catalytic Purification of Carbon Dioxide Used in Precision

Recommend Documents