OPERATIONS OF ON-LINE CHROMATOGRAPHS Paul E. Kizer Applied Automation, Inc. Pawhuska Road, Bartlesville, Oklahoma, 74005 INTRODUCTION Gas Marketing or Gas Brokering organizations are putting additional demands on the gas measurement systems. On-line Chromatography has at least two definitions When a company can warrant a gas quality, that today. The most common is that the Chromatograph, company can have a leg up on the competition to sell commonly known as a “GC,” is “on-line” when it extracts gas to a particular market. The company that has gas a sample from a continuously flowing line, injects this composition and BTU information via remote telemetry sample and analyzes it for composition, and then will have a competitive advantage over the one who is perhaps calculates BTU, SG, Relative Density, and/or “just guessing.” Wobbe index or other parameters. This data is used on site or at a remote location to calculate volumes for custody transfer of the natural gas. This is the definition that will be used in this paper. The other definition of “on-line” is that the Chromatograph data is fed or downloaded to a flow computer(s) or RTU(s) on the flowing stream that was analyzed at the measurement site. This definition is illustrated in picture, Fig. 1 and the diagram in Fig. 2, seen below. Most gas composition analysis data, however, can be applied to more flow measurement sites than just the one where the gas is extracted from the flowing stream. This is because the gas stream will not change in composition unless there is another source of gas or there is drastic change in temperature or pressure. Some companies use what they call “Zoning” to assign a particular composition or BTU to a set of flow computers. With either definition, the gas analysis information must be applied to the gas FIGURE 1. Picture of a multi-tube meter site with EFM and analysis volume somewhere to produce the energy amount that the meter station is flowing. After all, what the natural gas industry is selling is energy, not volume. When natural To summarize, on-line gas chromatography is today gas is burned, the following chemical reactions are the being chosen more often than any other analytical primary ones that take place: method in the natural gas industry for monitoring of gas quality for the following reasons: ➔ CH4 + 2O2 CO2 + 2H2O + Heat (1010 BTU/CF) 2C H + 7O ➔ 4 CO + 6 H 0 + Heat (1769 BTU/CF) • The calculations of the gas volumes in modern 2 6 2 2 2 5,10,13 ➔ electronic flow meters requires not only BTU C3H8 + 5O2 3CO2 + 4H2O + Heat (2516 BTU/CF) information, but specific gravity, Mol. % CO2 and Mol. % N as well. Even today, most natural gas custody transfer contracts 2 2 • The current AGA-8 compressibility calculation use MMBTU rather than MCF as the accounting units equations also require a complete analysis for of gas transfer. A BTU is the acronym for British Thermal 16 the “detailed method” of calculation of FPV . Unit. One BTU is the quantity of heat required to raise • With remote telemetry, Gas Marketers have the temperature of one pound of water from 58.5°F to 3 nearly instantaneous gas quality information at 59.5°F (about 1055.056 joules (SI.)) The higher the BTU their finger tips. value, the more energy can be obtained from burning • While not nearly as instantaneous as the gas. It doesn’t take as many cubic feet of gas to calorimeters, GCs with modern micro-packed heat the hot water tank if the gas is 1090 BTU instead of columns are providing faster analysis times. 940 BTU per SCF. The BTU, then, is the measure of the • It is well known that the installation requirements actual amount of heat energy contained in a cubic foot 2 for Chromatographs are less stringent than (CF) of this natural gas. An MMBTU is calculated by: calorimetric methods. BTU/CF * MMCF = MMBTU 2001 PROCEEDINGS PAGE 69 AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY LABORATORY VERSUS ON-LINE GCs is that the gas pressure is reduced to 15 PSIG at the probe for transport to the analyzer. This is to drop the There are two general classes of chromatography; effective dew point of the sampled gas in order to prevent laboratory chromatography and “on-line” or process the loss of any part of the heavy component (C6+) prior chromatography. The scope of this paper is limited to to injection into the analyzer, and to minimize the number on-line chromatography. The basics of each are very of molecules in the sample transport tubing (thus similar. The differences are mostly “location,” not only shortening the sample transport lag). The gas sample is the location of the GC but the location of the sample then analyzed on the site at regular intervals 24 hours a point or sampling method. Laboratory Chromatographs day, 365 days a year. The analysis data can be stored on are located in the “lab.” Because of this, they must have site for manual retrieval or it can be streamed to a remote their samples brought to them. This process of obtaining location via phone modems, radios, cellular phones and a sample in the field is called “spot” or “grab” sampling even satellite links. The chromatographic process is the or perhaps proportional (to flow or time) or composite same with both, but due to the location, the construction sampling. With spot or composite samples care is taken of the on-line GC must be very rugged. to maintain pipeline conditions. These samples of gas are contained in cylinders that must be carried to the DESCRIPTION OF A TOTAL ENERGY METERING lab. When it is time to analyze the sample, it must be STATION heated to 120°F for several hours before the analysis can be done. The diagram in Fig. 2. shows how an energy metering station might be configured. The gas flow for the metering On-line Chromatographs, Fig. 1, however, are on some station is accumulated in the primary tube flow computer flowing line. In this industry, it would be a natural gas for one, two or all three of the flow computers shown pipe line. The flowing gas is sampled continuously by on the drawing. Each flow computer independently use of a probe installed in the line. A major difference measures and calculates the gas flow on its particular when sampling for an on-line Chromatograph, however, meter tube and maintains its own data base. Comm Link RS-485 TOTALFLOW AIU HCIA TOTALFLOW Proportional Valve Control Signal(s) 6713 FCU Tube No. 1 On/Off Valve Control Signal(s) TOTALFLOW TFLocal.BUS 6410 FCU Tube No. 2 On/Off Valve Control Signal(s) TOTALFLOW 6410 FCU Tube No. 3 FIGURE 2. Typical three tube gas orifice meter station with energy measurement. PAGE 70 2001 PROCEEDINGS AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY The On-line Chromatograph takes a sample of the Transmitter Stream 1 Sample flowing gas from a common header and performs an Probe analysis as fast as once every three minutes. This data F5 F4 F3 F2 F1 RELIEF VALVE PRESSURE can be accumulated as an hourly (or at least daily) Detector vents GAUGE average data base. This type of data base is required to Sample Vent Stream 1 Mtg. Brkt. provide for the analysis audit trail, to match the hourly G1 50 cc/min flow computer data base. In Fig. 2, it is stored in a stand- Purge Coil alone box called an Analyzer Interface Unit (AIU). F6 F7 HV1 Stream 1 In At the top of each hour, the AIU down loads this 400cc/min. Purge Coil average of the Methane, Carbon Dioxide, Nitrogen Liquid Drain concentrations, the Relative Density and BTU data to each of the flow computers at the station. This allows Cal. Gas the gas calculation to be done more precisely in the flow Carrier Gas He computers and allows the digital display on the FCU to Mounting plate show actual energy flow. FIGURE 3.Diagram of Sample Conditioning System The differential pressure in the primary tube is used to calculate which of the other tubes needs to be turned This procedure is as essential for good sampling for on- on or off. The primary tube flow computer sends a line GCs as it is for spot or composite samples. command to tell the secondary units to open the control valve and start calculating flow or to close it and there- Sometimes it is useful to have a sample bypass loop to by stop calculating. reduce sample transport lag times, as is shown in Fig. 3. Note that filtering and liquid coalescing is done if needed Both the flow and analysis information can be sent out on to protect the integrity of the analyzer columns and to communication links using serial digital communication ensure a single phase sample. Sometimes double block protocols to PC based host software programs. This and bleed sample switching is used to prevent cross allows the gas marketers to have not only instant quantity contamination by the standard or other sample streams.8. but also instant quality information as well. Gas Chromatograph Oven: The primary tube flow computer software can either The GC oven provides an isothermal environment for calculate the needed flow rate using the nominations the elements indicated in Fig. 4 below. Once the natural volume from the gas sales contract for this station, or it gas enters the oven, a precise sample is measured by a can be computed by a host software program like WinCCU sample inject valve.