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Ind. Eng. Chem. Res. 1995,34, 1926-1928
RESEARCH NOTES
An On-Line Calibration Method for Process Gas Chromatographs Ming Fang" and Fu Wang Research Centre, The Hong Kong University of Science & Technology, Clearwater Bay, Hong Kong
Calibrating process gas chromatographs (GCs) can be difficult because a wide range of concentration is usually required. Using gas mixtures as standards is cumbersome, costly, and unreliable because the gases may deteriorate during storage. In this paper, we describe a method using pure gases for the calibration of automatic GCs by varying and measuring precisely the pressure of the sample loop, thus sending known quantities of gas into the column. The concentration of the gas is calculated using the ideal gas law, and we are able to calibrate GCs in a range from 1000 ppm to 100 ~ 0 1 % .
Background Gas chromatographs (GCs) used to measure gas concentrations in process streams usually need calibration for a wide range of concentrations. Two approaches are commonly used: (1) permeation tubes for low concentrations (low ppms), if available, or (2) certified or self-prepared gas mixtures using a variety of methods with concentrations in the appropriate concentration range; often a large number of gas mixtures are required if five to six gases are present. Nelson2systematically summarized practically all the common methods found in the literature used to prepare gas mixtures for calibrating analytical equipment. By and large, all these methods are capable of preparing calibration standards and utilize some basic thermodynamic measurements and relationships to determine the concentration of a mixture blended from pure gases. In dynamic systems, where gas mixtures are blended on-line, the deterioration of gases, especially at low concentrations, due to reactions and adsorption during storage is circumvented. This deterioration due storage time is the major disadvantage of static systems. The exception is the permeation tube where a single component is involved. Guiochon et a1.l described the calibration of process GCs using the deferred standard or purchased gas mixture. The same problem described earlier still exists. The gas chromatographs used in process applications are usually equipped with automatic samplers using sample loops. The sampling system can be modified to provide on-line calibration, by adding a vacuum system and a precision pressure gage so that the delivery pressure of the sample to the column can be varied and accurately measured; this process will be described later. The concentrations can be calculated using the ideal gas law.
Operation Principles The sample loop in an automatic gas sampling system of a gas chromatograph delivers a fured volume of gas at a preset condition to the column for analysis, thus guaranteeing repeatability in the measurements. A pure gas injected into the GC at the standard sampling conditions (e.g., 14.7 psia) represents 100 vol % in gas
"ft- , (standard)
Digital pressure
from process
TO
vacuum pump
Figure 1. Calibration circuit diagram.
concentration. By lowering the pressure in the sample loop via evacuation, a lower concentration sample is obtained since fewer gas molecules are injected into the column. This method can be used to calibrate gas chromatographsusing pure gases, and the accuracy and limit of the concentration range are dependent only on the accuracy of the pressure gage and the vacuum system.
The System Figure 1 is a schematic of the calibration-sampling system used on our GC. It is possible to modify or replumb most of the automatic samplers on existing GCs for adaptation of the calibration method to be described. All valves used were stainless steel barrel valves to prevent leakage in and out of the system and to minimize gas adsorption; only high quality or electropolished stainless steel tubings and fittings should be used. The pressure gage used was a Digital Strain Indicator, Series DP 87/88, made by Omega Engineering, Inc., with a range of 0-15 psia. The vacuum pump was manufactured by Edwards High Vacuum International, Model E2M2 and was capable of pulling a vacuum of mbar. The valves, switches, and pressure readout were mounted on a small instrument panel for ease of operation and reading. The gas chromatograph used was an HP 5890 I1 with a thermal conductivity detector (TCD) at 160 "C. A Poraplot Q column of 0.53 mm i.d. and 12 m length was used for SO2 analysis, and a molecular sieve 5A column, 60-80 mesh, 2 mm i.d. and 1.8 m length was used for
0888-5885/95/2634-1926$09.00/0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1927 1 0 0 1 .
0
,
100000
.
I
200000
Peak area
Figure 2. High concentration SO2 calibration curve. (Low GC detector sensitivity was used. The expression “e-nx” is mathematically equivalent to “exp-NLn.)
.
8
300000
-
I
400000
.
1
500000
Peak area
Figure 4. High concentration CO calibration curve. (Low GC detector sensitivity was used. The expression “e-nx” is mathematically equivalent to “exp-NLn.)
Peak area
Peak area
Figure 3. Low concentration SO2 calibration curve. (High GC detector sensitivity was used. The expression “e-nx” is mathematically equivalent to “exp-n”’)
Figure 5. Low concentration CO calibration curve. (High GC detector sensitivity was used. The expression “e-nx” is mathematically equivalent to ‘‘exp-w’’.
CO measurements; the column temperature was maintained a t 40 “C and 90 “C, respectively. The sample loop volume was 250 mL, and the injection temperature was 110 “C. A helium leak detector made by Balzers (HLT16O) was used t o leak test the whole system under pressure and vacuum.
tions and to give the sensitivity needed for the ppm measurements. A pure gas sample was injected into the GC under standard sampling conditions to obtain the 100 vol % points; the lower concentration points were obtained by evacuating the sample loop. Care was taken so that the system reached equilibrium after evacuation before measurements were made. The correspondingpressure was measured t o two places after the decimal point on a digital readout. A step-by-step procedure for operating the system is provided in the Appendix.
Calculations Since the operating pressure of the sample loop is at o r below barometric pressure and only pure gases were used, deviation from the ideal gas law was deemed to be insignificant for some of the common gases used in the process industry. Procedure The HP 5890 I1 TCD came with two levels of sensitivity. In our experiments, the low sensitivity level was used for the 15-100 vol % concentration range, while the high sensitivity level was used for the lower concentrations (1000 ppm-20 ~ 0 1 % )This . was needed to prevent saturation of the detector a t high concentra-
Discussion of Results Figures 2-5 are the calibration curves obtained for sulfur dioxide and carbon monoxide. A linear relationship between gas concentrations and peak areas (concentration indices determined by the GC) is obtained for all gases and all concentration ranges. Furthermore, the straight lines go through the zero concentration point. There is an overlap between the two sensitivity levels, making it possible to measure the whole concentration range. The confidence levels for the correlation
1928 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
are over 0.99. Freshly purchased certified standard f302-N~ gas mixture (4255 ppm, Matheson) was used to check the calibration curves, and the average measured concentration was 4083 ppm.
Conclusion The basic principles behind this method are simple and sound; thus the good agreement in the results was to be expected. The key points of the method are (1) the system must be leak-proof, both in and out of the system, and (2) the pressure gage must be accurate and reliable. Aside from being simple and easy to use, the most important thing is that only one pure gas is used at a time, thus eliminating the mixing of gases; and since the calibration hardware is an integral part of the sampling system, very accurate concentration measurement is possible because the sampling conditons are identical.
Acknowledgment The authors wish to thank Dr. Robert Day for his suggestions in the implementation of the system.
Appendix. Calibration Procedure The procedure used in our experiments is as follows: (1)Maintain the pure (standard)gas delivery pressure at 15 psi. (2) Ensure valves V2 and V4 are closed and open valves V1 and V3. This purges the sample loop and related plumbing with the pure gas. Purging time is 1 min. I
(3) Evacuate the sample loop by opening V4 and closing V1 and V3. (4)Refill the sample loop with pure gas by closing V4 and opening V1. (5) Evacuate by opening V4 and closing V1. (6)Repeat steps 4 and 5. Open V1 and close V4 to fill the sample loop with the pure gas until 15 psi is obtained. Adjust to the desired pressure by manipulating V4, if necessary. When the desired pressure has been established, shut off V4 and wait 2 min t o make sure the pressure has reached equilibrium. (7) Inject the sample by switching V5. (8) Go back to step 3 for the calibration of the next concentration.
Literature Cited (1) Guiochon, G.; Guillemin, C. L. Quantitative Gas Chromatography: for Laboratory Analyses and On-line Process Control; Elsevier: Amsterdam, 1988. (2) Nelson, G. 0. Gas Mixtures: Preparation and Control;Lewis Publishers: 1992.
Received for review November 10, 1994 Accepted February 23, 1995 * IE940660A
* Abstract published in Advance ACS Abstracts, April 1, 1995.
