September 2011, Vol. 238 No. 9

Features

Smart Microchip-based Gas Chromatographs

Natural gas is one of the most important energy resources1. However, its chemical composition can fluctuate considerably, depending on where it is found. Added to this, the various gas qualities are mixed together due to the networked pipeline system of main and sub branches, receiving and transfer stations and gas storage facilities. This means that the natural gas must be tested qualitatively and quantitatively at strategic points to establish its volume and its quality parameters such as its calorific value (CV or Btu), hydrocarbon dew point and hydrogen sulfide content.

The analysis of the main components is performed with process gas chromatographs (process GC)2 based on international standards such as ISO 6974. According to these standards, the gas is separated into its components, nitrogen, methane, carbon dioxide, ethane (C2), C3, C4, C5 and hydrocarbons with a higher boiling point as a total (C6+) and their concentrations are measured. Based on standards such as ISO 6976 or GPA 2172, the physical characteristics are calculated with the aid of the individual gas components and their compressibility; the energy content of the gas can then be recalculated from process to standard conditions.

When evaluating the performance of a process GC, important criteria are not only the high degree of accuracy, but also the repeatability of the calorific values, the linearity of the measured components, the efficiency in separating the individual components and the detection limits for components with low concentrations, in particular the higher hydrocarbons. One important condition for precise online analysis is that a representative natural gas sample is supplied to the analyzer. To ensure this, the sample is first suitably conditioned, for example with optimized pressure reduction (avoidance of ice formation by the Joule-Thomson effect) and filtering (separating out solid particles, possibly also a moisture trap).

Siemens developed the compact process gas chromatograph Sitrans CV 3 to meet fiscal metering and custody transfer regulations relating to heating values at natural gas transfer stations or for feeding in upgraded biogas. Other applications are fast process measurements during gas conditioning or at major consumers (for example, the glass industry or metallurgy), accurate heating value measurement at mixing stations or extensive quality measurements in the gas network. It identifies not only the gas composition but also all the characteristics relevant when transporting natural gas such as the heating value, calorific value, density and Wobbe index and makes the acquired data available to higher-level systems such as a flow computer or the process control system.

Figure 2: Analyzer configurations for C6+ / C9+ analysis using conventional and MEMS technology

Precision Pays
The micro process GC meets all the demands of heating value analyzers subject to custody transfer regulations in terms of precision and long-term stability. The device consists of a multiple separation and detection system with three capillary columns and inline micro thermal conductivity detectors (TCD). In terms of their design, connector technology and interfaces, all modules are standardized so that they can be replaced quickly within minutes if maintenance becomes necessary.

All the hardware components such as the valveless live injection system, the high-resolution narrow-bore capillary columns, the valveless live column switching system and the TCDs have practically identical inner diameters (normally 0.15 mm) and are therefore so well-matched that no disturbing dead volumes occur that could impair the separation performance due to the effects of diffusion4. Based on the example of the valveless column switching, Figure 1 illustrates the relative sizes of MEMS and conventional technology.

Further micro TCDs at various points within the analytic system monitor the injection peaks, the separation progress following each column and the gas outlets. The polarity and length of the narrow-bore columns are dimensioned so that the analyses can be completed as quickly and as simply as possible. With the information relating to the quality of injection, the precise setting for backflushing and the time of the cut, the measurement system is simple to validate.

Apart from the measured components nitrogen, methane, carbon dioxide, ethane C3, C4, C5 and C6+, the micro-process GC also provides the option of evaluating hydrocarbons with a higher boiling point as a group. The analytical system is largely standardized and implemented with micro-electro-mechanical systems technology (MEMS) with multi-inline detectors following each column. This means that, in contrast to conventional solutions, it requires only one separation system instead of two for expanded highly accurate heating value analysis including the separate identification of C6, C7, C8 and C9+. Figure 2 shows a comparison of the analytical configuration of both gas chromatographs for C6+ and C9+ analysis.

Apart from significantly reduced hardware costs, with its single measurement system (one GC instead of two linked GCs), the C9+ variant also has advantages when it comes to operation and maintenance with the same high-performance parameters as the C6+ option – without any restrictions regarding accuracy (< 0.1% rel.) and repeatability. At central points in a pipeline network, operators often require redundancy in the analysis system so that the measurements can still be made if one of the systems fails. Here, the solution with a micro process GC has significant advantages in terms of integration in the metering station. Such a C9+ analyzer pays its way immediately in many cases for rich natural gas with greater concentrations of higher hydrocarbons as shown in the following example: A pipeline with a diameter of 32 inches transports more than 45 million m³ of natural gas daily over a distance of 305 km. If the heating value is determined according to the C6+ method, a value of 9.6919 kWh/Nm3 is measured. If the heating value is determined according to the C9+ method, this results in a more precise heating value of 9.6952 kWh/Nm3. At a price of 12 cents per kWh and a standard volume of 10 million Nm³ per day, this would result in daily revenue of 11,634,000 Euros - and therefore, 4,000 Euros more per day. Figure 3: Separation power between high nitrogen and low methane using narrowbore capillary columns Long-term Stability Reduces Costs
Repeatable measurement results start with precise sample injection. The patented, valveless live injection system has no moving parts and is therefore completely maintenance-free. Thanks to the perfect interaction of live injection, high-performance capillary columns and multi and inline detection, an extremely short analysis time can be achieved. This means, for example, that the C6+, the C7+, and also the C9+ analysis is executable within 100 seconds and with the optional O2 measurement without the need of additional hardware within 150 seconds.

Figure 4: Analyzer accuracy for calorific value and density using various certified reference gases (CRM)

The high separation performance using narrow-bore columns allows flexible bandwidth for various measurement tasks on site, for example in mixing stations in which the N2 load could be high. This means that the resolution between nitrogen and the methane peak is adequate to allow analysis with concentrations of 25% nitrogen alongside methane. Figure 3 shows the extremely good separation property with baseline separation of nitrogen and methane at TCD 1.

An additional device variant of the micro process GC can be used for custody transfer when feeding conditioned biogas (known as bio natural gas or biomethane) into the natural gas network. This variant allows the full analysis of hydrogen alongside the components oxygen, nitrogen, CO2, C1 to C4. The process GC is not affected by sample pressure fluctuations and guarantees the reliable dosing of the natural gas sample. This results in a relative standard deviation in repeatability of significantly less than 0.01% for the heating value. As a result, the system significantly exceeds the admission requirements for the calorific values and for the measured components (based on ISO 6974-5: 2000) for highly accurate heating value analyzers.

Figure 5: Analyzer cabinet with Micro-Process gas chromatographs for redundant offshore metering

Automatic Calibration
When used as a custody transfer device for gas invoicing, a heating value analyzer must be calibrated regularly, usually weekly. The calibration of a comparative analyzer technology such as gas chromatography involves the use of an external test gas that is connected directly to the analyzer and fed in automatically. Sitrans CV has a high degree of linearity for all measured components over the entire measurement range. This means that a single calibration gas (one-point calibration) is adequate to calibrate the device.

Complicated multi-point calibrations with up to seven test gases are not mandatory. This was confirmed by the German metrological institute (PTB – Physikalisch-Technische Bundesanstalt). Figure 4 shows a typical example of the repeatability of the analyses based on the achieved standard deviations of the calculated heating value (bottom) and the density (middle) over a wide temperature range (top). The device is therefore approved for measurements with mandatory calibration for various gas qualities in the natural gas transport network and also for extreme environmental conditions as found in the Near East or in the Arctic.

Figure 6: An electrically powered natural gas pipeline compressor owned by TransCanada.

Intelligence On Site
In process mode, the micro process GC operates with an automated method setting or auto optimization. The system regularly checks the retention times of the measured components, their evaluation parameters and the switching times of the backflushing and automatically adjusts these as necessary. The optimum pressure setting of the electronic pressure controller (EPC) is calculated once mathematically during production of the device and does not need to be set using a time-consuming empirical method. All this contributes significantly to the long-term stability of the overall analytic system.

The micro-process GC is equipped with a simple, clear and easy to operate software based on Microsoft Windows. Long-term data storage and mean value calculations for all components and calorific values are implemented internally. A log records all events and alarms. The analyzer communicates via the RS-485/Modbus interfaces with the process control systems and flow computers or via Ethernet TCP/IP with an operator PC that allows remote access and remote control and that is connected to the Internet via a modem. If no telephone connection is available, communication is possible via GPRS through additional solution packages. Thanks to firewall and VPN technology, secure bidirectional communication between the control room and deployment location is possible.

The miniaturization allows installation directly at the point of extraction without requiring an analysis house and therefore reduces the investment and operating costs of the overall analytic system. A compact and flameproof housing provides a high degree of protection against moisture, dust and corrosion (IP65, NEMA4X), against extreme environmental temperatures (-20 to +55 °C) and against risk of explosion (EEx d) and is therefore also ideal for offshore applications5,6 as shown in Figure 5.

Conclusion
With the Sitrans CV, a natural gas chromatograph is available that fully utilizes the performance of high-resolution capillary columns along with serial integrated multi-detection options to provide ideal solutions for measurement tasks such as heating value analysis. Beyond the standard C6+ analysis, extended analysis methods such as C7+ or C9+, additional oxygen measurement and the biogas application including hydrogen and oxygen can be implemented.

The performance criteria for fiscal metering are met for all these applications. The standardized construction and design make it easier to introduce chromatographic solutions in process analysis in particular for natural gas metering stations. The deployment of smart microchip-based process gas chromatographs directly at the sample extraction points may well lead to a rethink when considering integration of these analyzers in process plants.

Literature
1 BP, Statistical Review of World Energy, http://www.bp.com

2 Maurer, T., Mahler H. (1999) Prozessgaschromatographie, In: Moderne Prozess-Messtechnik – Ein Kompendium [Process gas chromatography, in: Modern process equipment – a compendium] (Gundelach V, Litz L, Hrsg), Springer, 3.7: pp. 240-274

3 Mahler H. (2004) On-line CV determination – New aspects using multi- and in-line detection capabilities of a micro-machined process gas chromatograph, Gas Analysis Symposium, Amsterdam, Netherlands

4 Mahler H. (2007) Advances in process GC using MEMS technology, ISA AD 2007
52nd Analysis Division Symposium, Houston, TX

5 Olsen, I. (2003) Present and Future Trends in Process Gas Analysis for the
Offshore Oil and Gas Industries, Exploration and Production – The Oil & Gas Review, Vol. 2, p.1-4

6 Mahler H. (2011) Innovative on-line natural gas analysis for offshore process applications, European Oil&Gas, Issue 11, p 158-159