Bruce H.Solka Senior Analytical Chemist
and Amir Attari
Associate Director, Chemical Research Services Institute of Gas Technology
Chicago, Illinois 60616
ABSTRACT
Since 1961 the Institute of Gas Technology (IGT) has provided the gas industry with natural gas standards of certified heating value and specific gravity for calibration of calorimeters and gravitometers, respectively. The establishment of this program at IGT was the result of an earlier project initiated by A.G.A. in cooperation with the National Bureau of Standards (NBS), This program was recently expanded
at IGT with the support and the initiative from the Gas Research Institute (GRI) to produce additional natural gas standards including a gas chromatographic calibration standard gas.
INTRODUCTION
For the past 20 years, the Institute of Gas Technology (IGT) has provided the gas industry cylinders of standard gas with certification of heating value and specific gravity of the contents. These cylinders have been widely used in to calibrate their measurement instruments. During this 20 year period, however, users have seen many changes in the requirements of their natural gas characterizations.
One significant change has been the increased utilization (and hence, measurement) of fuel gases having heating values higher than the nominal 1000 Btu/SCF. Another change is the growing interest in obtaining detailed compositional analyses of natural gas by routine gas chromatography (GC).
These developments have resulted in industry demands for reliable calibration standards for gas quality measurements by GC. We at IGT have been made aware of this demand by increasing number of inquiries from our current certified gas customers. As a result of these inquiries, in 1981 we mailed a questionnaire to 55 of the largest customers of the present program. This questionnaire addressed their anticipated use of reference standard mixtures for gas chromatographic analysis of natural gas. Of the 31 respondents, 80%
were using GC at that time. Satisfaction with available standards was mixed. A clear majority desired NBS traceability of standards as well as calorimetric verification of the heating value of the GC standard mixture.
The Gas Research Institute (GRI) also became aware of industry interest in new reference standard mixtures certified by an independent agency. As a result, in August of 1984, GRI initiated the present program at IGT with the ultimate goal of supplying the gas industry with a series of NBS traceable calibration gases.
This talk is a summary of the present status of that program. The program’s goal is to make available several types of calibration standards to the gas industry by the end of 1985. These include a GC calibration mixture with direct NBS traceable certification of composition and similarly traceable calorimetric calibration standards with heating values in range of 800 to over 1200 Btu/SCF.
Selection of GC Mixture Composition
Selection of Mixture Composition, GC Standard.A GC calibration gas will be most useful if its composition approximates that of the test sample, as closely as possible. The survey referred to above gave respondents the opportunity to express their wishes regarding composition of a GC calibration standard. With this information as well as our own knowledge of typical composition of pipeline-quality natural gas, numerous possible gas mixtures were considered as candidates for this program. Given recent interest in the heavier ends of natural gases, careful consideration was given to the advisability of including hexane and heavier components in the standard.
Table 1. lists five mixtures typifying the composition ranges which we considered for a GC calibration gas. A primary requisite for a usable standard is that its composition remain constant during the period of its use. While the presence of heavy components may be desirable for calibration purposes, they increase the possibility of composition changes due to condensation or adsorption. In the northern climates, certified gas cylinders could well be exposed to winter temperatures on the order of −30°F during shipment or storage.
We therefore, first calculated SRK equation-of-state dew points as a function of pressure for some 75 gas mixtures and based on that consideration selected gas mixture D as our primary standard gas.
Dew point curves for the five gas mixtures of Table 1 are shown in Figure 1. Cost and convenience considerations in cylinder purchase and shipping of an adequate volume of gas will necessitate the gas mixture to be pressurized. It can be seen that the dew point of the mixture D will remain below −10°F at pressures up to 300 psig. A gas cylinder with an internal volume of one cubic foot at this pressure will contain approximately 21 SCF of gas, a quantity large enough to provide an extensive number of GC calibration runs.
Selection of Calorimetry Standard Compositions. The composition criteria for low and high Btu calorimetric standards are less severe than for the GC standard. This is because the calorimeter measures the gross heating value of a fuel gas independently of its composition. Thus, simple binary mixtures of methane/
ethane and methane/nitrogen can be selected to attain any desired
Table 1. COMPOSITION OF VARIOUS NATURAL GAS BLENDS (Dew Point Diagrams Appear in Figure 1) ---Mole %---
Components /Sample 1 2 3 4 D (IGT)
Methane 70.55 90.58 88.73 90.65 90.65
Ethane 8.98 3.50 3.50 3.50 4.00
Propane 5.95 1.00 1.00 1.00 1,00
---Mole %---
Components /Sample 1 2 3 4 D (IGT)
i-Butane 3.02 0.50 0.40 0.50 0.30
n-Butane 2.98 0.50 0.40 0.50 0.30
i-Pentane 1.00 0.15 0.15 0.15 0,10
n-Pentane 1.00 0.15 0.15 0.15 0.10
neo-Pentane -- 0.05 0.10 -- --
n-Hexane -- 0.05 0.05 0.05 0.05
n-Heptane -- 0.01 0.02 -- --
n-Octane -- 0.01 -- -- --
Helium 0.46 -- -- -- --
Nitrogen 4.95 2.50 2.50 2.50 2.50
Carbon Dioxide 1,09 1.0 3.00 1.00 1.00
Propylene 0.02 -- -- -- --
Key to Sample Numbers:
1: NGPA calibration gas from Phillips Petroleum 2, 3, and 4: Suggested by gas industry sources.
D: Gas composition selected by IGT for the forthcoming GC calibration gas.
heating value for calibration standards in the range of 800 to 1200 Btu/SCF. However, the criteria of stability still requires a dew point consideration. Calorimetric analysis also consumes several orders of magnitude more gas volume than GC analysis. Thus a calorimeter standard is normally compressed to near 2000 psig in a 1-A size cylinder in order to deliver a volume of about 225 SCF to the user.
Table 2 lists the compositions and heating values of several mixtures which were considered for these standards. Figure 2 illustrates the SRK. dew point vs pressure curves for these mixtures. It can be seen that, while the 812 Btu/SCF gas (mixture B) has a dew point temperature of −144°F, the 1261 Btu/SCF gas (mixture 5) approaches the −5°F dew point region. Thus the mixture represented by curve “C”, a 75%
methane, 25% ethane blend with a heating value of 1185 Btu/SCF was chosen to be as near 1200 Btu/SCF as possible and yet maintain a reasonably low dew point temperature of −26°F/.
Certification Process
Sequence of Events Leading to Certification. Figure 3 diagrams the overall sequence of activities leading to delivery of IGT certified calibration standards with NBS traceability. At this time, the four primary standard mixtures have been analyzed and certified by NBS.
We are currently formalizing the protocol for certification of the GC calibration standard gas at IGT. The traceability of these certified standards to the NBS primary standard will be the analysis of each cylinder containing the certified gas on a GC system which will be calibrated on a daily basis with the NBS primary standard. Protocol has not yet been established but our experience with the long-standing calorimetry standard problem will serve as a guide. In that program a cylinder is tested on three separate days, using two separate calorimeters which are calibrated daily with an NBS primary standard. The results of the three runs must agree to within 1 Btu before the cylinders′ contents are considered certifiable.
We do anticipate offering two types of GC standards; a small cylinder with composition certification only and a larger cylinder with composition certified as above plus a calorimetrically certified heating value.
Table 2. APPROXIMATE COMPOSITION AND HEATING VALUE OF SEVERAL BINARY METHANE MIXTURES CONSIDERED AS CALORIMETER CALIBRATION STANDARDS (Dew Point Diagrams Appear in Figure 2)
--- Mole % ---
Components Sample 5 6 7 A (IGT) B (IGT) C (IGT)
Methane 65,00 70.00 72.50 99.99 81.50 75.00
Ethane 35.00 30.00 27.50 Trace -- 25.00
Nitrogen -- -- -- Trace 18.50 --
Heating Value, Btu/SCF at 60°F, 14.735 psia and Saturated
1261 1223 1204 996 812 1185
Key to Sample Designations
Figure 1. HYDROCARBON DEW POINT DIAGRAMS FOR SEVERAL NATURAL GAS BLENDS, AS PPEDICTED BY SRK EQUATION-OF-STATE
--- Mole % ---
Components Sample 5 6 7 A (IGT) B (IGT) C (IGT)
5, 6, and 7: These methane-ethane blends were rejected because of their higher dew point temperatures as compared to blend C.
A, B, and C: Gas compositions selected by IGT for the forthcoming new calorimeter calibration standards.
Certification of the three new calorimetry standards will follow the present protocol for the existing program. In the case of the GC standards, concentrations of 10 components (methane through hexane plus nitrogen and carbon dioxide) in the mixture will be certified to within a range of uncertainties based on thorough statistical evaluation of analytical system performance.
Equipment Used. The paticular set of equipment that has been assembled and dedicated to the GC portion of this program consists of the following principal components:
Gas Chromatograph: Hach/Carle AGC Series 400, Model 04192–A Data Aquisition: Perkin-Elmer Sigma 15 Chromatography Data System
Sample Handling: Vacuum manifold with Validyne Model CD 223 Digital Manometer.
Figure 2. HYDROCARBON DEW POINT DIAGRAMS FOR VARIOUS METHANE BINARY MIXTURES, AS PREDICTED BY SRK EQUATION-OF-STATE
The chromatography data station transmits results to an IBM PC computer via RS 232 communications for the purpose of data storage and evaluation.
The calorimetry certification program will continue to use the Cutler Hammer recording calorimeter that has been the workhorse of the existing IGT program. This has been supplemented for the current program expansion by the addition of a third calorimeter and construction of an automated, six-port gas manifold sampling system to permit unattended measurements on batches of five cylinders.
CURRENT STATUS OF PROGRAM GC Calibration Standard
Cylinder Material. A major area of effort of the program has been to verify that the choice of cylinder material will not affect the stability of the calibration mixture. To test this, we filled duplicate sets of aluminum and stainless steel cylinders with the calibration gas mixture from the carbon steel source cylinder. These five cylinders have been analyzed on a monthly basis for the past six months. As expected, no significant concentration changes in any of the 10 components are evident in any of the five cylinders at this point.
Precision of Certified Concentrations. The cylinder stability data, numbering some 150 separate analyses, also form a basis set for statistical evaluation of instrument precision and accuracy. Our preliminary evaluation of the collected data indicates that the componential analysis of the certified gas will produce an overall precision well within the ASTM Method D-1945–81 repeatability requirements.
System Linearity. We are currently studying the overall GC system linearity for all the components over a range of concentrations expected for common natural gases. A principal reason for this study is to determine the validity of the certification procedure for gas compositions that may vary substantially from that of the NBS primary standard.
Pressure Drawdown Tests. This series of experiments were designed to determine the effect of pressure reduction on the stability of certified gas composition between a maximum filling pressure of 300 psig and atmospheric pressure. Two sets of cylinders filled with a 10–component gas blend, similar in composition to our NBS primary standard, will be analyzed before and after each 50–psig pressure reduction at different gas withdrawal rates. The results of these tests will indicate the optimum rate of gas withdrawal as well as the residual cylinder pressure for the GC calibration gas.
Figure 3. OPERATIONAL SEQUENCE OF IGT CERTIFICATION PROGRAM
Calorific Standards
Three new NBS primary standards were obtained under the new program, designated as standards A, B, and C. Composition and approximate heating values for these primary standards are listed in Table 2.
The primary purpose of the new standards was to measure the linearity of calorimeter response over its entire range from 800 to 1200 Btu/SCF. Preliminary tests on three calorimeters show that there may be a bias of +1.5 Btu at the midrange of the calorimeter. That means that when standard B (certified heating value of 813.3 Btu/SCF) or standard C (certified heating value of 1186.1 Btu/SCF) are used to calibrate the calorimeter and the other two standards are used as a test gas, only standard A (certified heating value of 996.6 Btu/SCF) reads high by 1.5 Btu/SCF. This study will continue for other points within the 800–1200 Btu range in order to develop an accurate calibration curve for future use.