Evaluating a Tunable Filter Spectrometer Online Gas Analyzer Juan Escobar, Measurement Specialist, Saudi Arabian Oil Company (Aramco) 1.0 EXECUTIVE SUMMARY A brief introduction to Aramco’s gas system is given to highlight the large gas volumes managed by the grid and the importance of the gas composition determination to obtain custody measurement quality and to control the quality of the gas delivered to the customers. The technology currently used by Aramco to determine the quality of the gas (composition) is the traditional gas chromatography (GC); however, new and more robust technologies are emerging and Aramco is interested in reviewing and testing these new methods for gas quality measurement. Continuous gas on-line analyser systems, as per API MPMS Chapter 14, Natural Gas Fluids Measurement, are used in gas custody measurement. The incumbent GC technology is the most widely used when compared to other online gas analysers technologies. GCs are known to require complex installation (dedicated shelter house), costly operation and complicated maintenance. GC requires frequent and complex field calibration and continuous carrier gas for effective operation. Some alternatives to GC technology are presented including Fractional Distillation and some forms of spectroscopy technologies like Fourier Transform Infrared spectroscopy (FTIR), Raman and Tunable Diode Laser (TDL). Tuneable Filter Spectrometer (TFS) technology is an emerging spectroscopy technology that offers the following benefits against the online gas chromatograph: • Real-time gas composition response time with fast updated results. Saudi Aramco: Company General Use
Transcript
Evaluating a Tunable Filter Spectrometer Online Gas Analyzer
Juan Escobar, Measurement Specialist, Saudi Arabian Oil Company (Aramco)
1.0 EXECUTIVE SUMMARY
A brief introduction to Aramco’s gas system is given to highlight the large gas volumes managed by the grid and the importance of the gas composition determination to obtain custody measurement quality and to control the quality of the gas delivered to the customers. The technology currently used by Aramco to determine the quality of the gas (composition) is the traditional gas chromatography (GC); however, new and more robust technologies are emerging and Aramco is interested in reviewing and testing these new methods for gas quality measurement.
Continuous gas on-line analyser systems, as per API MPMS Chapter 14, Natural Gas Fluids Measurement, are used in gas custody measurement. The incumbent GC technology is the most widely used when compared to other online gas analysers technologies. GCs are known to require complex installation (dedicated shelter house), costly operation and complicated maintenance. GC requires frequent and complex field calibration and continuous carrier gas for effective operation.
Some alternatives to GC technology are presented including Fractional Distillation and some forms of spectroscopy technologies like Fourier Transform Infrared spectroscopy (FTIR), Raman and Tunable Diode Laser (TDL).
Tuneable Filter Spectrometer (TFS) technology is an emerging spectroscopy technology that offers the following benefits against the online gas chromatograph:
• Real-time gas composition response time with fast updated results. • Easy installation with less space requirements.• Field mounted, robust and rugged; suitable for harsh industrial environments and hazardous area
installation.• Minimal maintenance with no consumables.• No requirement of carrier gases or calibration gases.
A trial was conducted to validate the analyzer readings using a third party laboratory. This ensures a primarily traceability of the analyzer reading. After successful laboratory validation, the analyzer was then tested in the field using a reference GC.
The following deliverables were successfully completed
1. Installation and automatic measurement of gas composite C1 through C4+. 3. Analysis of the results and comparison to existing GC. 4. Evaluate maintenance requirement and ease of functionality/operation.
It is planned to perform another trial with an extended composition, C1-C6+, which is normally used for custody applications and throughout the year to assess the impact of different seasons on instrument performance.
Saudi Aramco: Company General Use
2.0 INTRODUCTION
Saudi Aramco transports large amounts of natural gas throughout the Kingdom of Saudi Arabia using the Master Gas System (MGS). The MGS is comprised of long pipelines running from the East to the West coast and from the North in Safaniyah to the South in Harad. There are seven gas plants processing over 8 BSCFD to feed these pipelines. All customers are attached to the MGS. Gas Plants process sales gas for power generation customers to meet the growing electricity demand in the kingdom of Saudi Arabia. Gas Plants also supply sales gas to refineries and other customers ranging from brick manufacturers to petrochemicals. Sales gas production supplies about 120 customers Kingdom-wide.
Sales gas is analyzed using online Gas Chromatographs (GC) at the customers’ sites for the continuous calculation of the volumes and the heating value which is used to compute the BTUs to bill the customers. Such GCs require extensive maintenance and carrier gas to be available all the time, increasing operation cost. In addition of the high & costly maintenance and operation, it is bulky, requiring a housing shelter and complex operations and support.
A novel and attractive alternative to GC, is a compact online gas analyzer that uses Tuneable Filter Spectrometer (TFS) technology. This TFS-based analyzer offers a fast update rate (down to 1 second), requires no carrier or calibration gases and it is portable (light weight). It is permanently calibrated for the lifetime of the instrument and presents very low zero-drift (re-zeroing is recommended every month or more).
This experiment was conducted by comparing the performance and accuracy of a new inline (real-time) gas analyzer to the existing field mounted GC for gas composition calculation in custody transfer application.
2.1 NATURAL GAS ANALYSES APPLICATIONS
Composition variations in the fuel gas including natural gas exist, which can be detrimental to power generation equipment such as furnaces, turbines, internal combustion engines, fuel cell power plants and boilers. Certain parameters such as heating or calorific value and Wobbe index are important to measure in real time to ensure optimum and safe operation. In some cases, the actual composition information is needed.
Besides power plants, gas analyses are widely used in hydrocarbon processing facilities when high quality product specifications are required.
Natural gas chromatography is also performed in custody measurement applications where the composition of the natural gas is used to compute the calorific value and many other parameters required to calculate total Btu content and volumes. Some parameters calculated from the gas composition besides calorific value are compressibility, density, and viscosity — all of them used for the computation of volume.
Natural gas is bought and sold based on the amount of energy delivered. The quantity of energy delivered is calculated by multiplying the gas volume by the heating value (Btu) per unit volume.
Saudi Aramco: Company General Use
2.2 TECHNOLOGIES
Fractional Distillation: Is the separation of a mixture into its component parts, or fractions, by their boiling point by heating them to a temperature at which the fractions of the compound will vaporize. Once components are separated, they are measured to calculate their volumes and molar percententage.
Gas Chromatography: Is a technique for separating the components parts of a mixture by the use of a column. The sample is carried by a moving gas stream through a tube packed with a finely divided solid or may be coated with a film of a liquid. Same as fractional distillation, once components are separated, they are measured to calculate their volumes and moles percent.
Spectroscopy: Is the analysis of the interaction between matter and any portion of the electromagnetic spectrum. Traditionally, spectroscopy involved the visible spectrum of light, but x-ray, gamma, IR and UV spectroscopy also are valuable analytical techniques. Spectroscopy may involve any interaction between light and matter, including absorption, emission, scattering, etc. Examples of spectroscopy techniques are: Fourier Transform Infrared spectroscopy (FTIR), Raman, Tunable Diode Laser (TDL), Nondispersive Infrared Spectroscopy (NDIR), and Tunable Filter Spectroscopy (TFS).
3.0 TFS MEASUREMENT PRINCIPLE
When a gas sample is introduced into the gas cell, the light radiation provided by a broadband light source is partially absorbed by the gas species present. The light absorption occurs at specific wavelengths and magnitudes depending on the gas components and the concentration of that component. The TFS Gas Analyzer spectrometer module scans the wavelength of the received light, measures the actual absorption spectra of the gas, and compares them with the pre-loaded calibration spectra. The onboard analysis algorithm computes the gas concentrations in real-time, which can then be outputted using the MODBUS protocol running over TCP/IP (Ethernet) or RS-485.
Saudi Aramco: Company General Use
In principle, the absorption spectrum of each component is unique, and acts as a “fingerprint” for identification or speciation analysis. In addition, the magnitude of the absorption is a function of the number of molecules of the gas. With a known path length, pressure, and temperature, the magnitudes of the absorption spectra are then used to compute volumetric concentrations. This principle-based technique provides accurate and robust measurements with minimal span and baseline drifts.
The TFS Gas Analyzer employs an internal pressure transducer to measure the sample pressure in real-time enabling pressure variation corrections. The flow cell is heated to a constant temperature with a sample preheat module to maintain both sample and optical sensor temperature at a constant calibrated temperature, thereby ensuring measurement accuracy and stability despite sample and environmental variations. An advanced spectral processing and chemometric algorithm provides highly accurate and robust speciation performance compensating for various spectral nonlinearities.
4.0 ANALYSER TESTING METHODOLOGY
The trial was conducted by initially validating the analyzer using a third party laboratory facility. This will ensure a primarily traceability of the analyzer reading.
After successful laboratory validation, the analyzer is then field trial using a reference GC. The GC and analyzer reading are synchronized. The GC reading times are taken from the DCS clock while the analyzer times are taken from its own processing unit.
The purpose of these tests was to compare the performance of the TFS with a Gas Chromatograph (GC) used for Natural Gas analysis in a United Kingdom Accreditation Service (UKAS) accredited laboratory.
The TFS is to be compared to the GC and the deviations to the GCs measured concentrations were established. Both analyzers were calibrated using the same reference gas and then differing mixtures (test gases) were supplied to both analyzers multiple times. Between each sample the TFS was flushed through with nitrogen, while the GC was made ready to take the next sample for analysis.
The slightly differing procedure between each analyzer is due the GC being a “batch” sampling (non-continuous) analyzer, while the TFS is a Continuous Gas Analyzer (CGA). Therefore repeatability for the GC was calculated between batches, and the repeatability of the TFS was calculated between “zero” (nitrogen) and “span” (test gas).
The performance of the GC was compared to the certified test gases, while the performance of the TFS was compared to both the certified test gases and the GC.
Tests were performed at an UKAS accredited laboratory utilizing a total of eight reference mixtures consisting of five real natural gas samples and three synthetic mixtures. The reference mixtures were analyzed independently with industry standard laboratory methods.
The main objective of this test is to characterize the accuracy of the analyzer relative to the independently measured reference mixtures. Note that the test analyzer was calibrated at the factory and was not re-calibrated or re-spanned prior to the test. The tables below summarize the results.
CV (Superior, Gross) 36.7314 36.742 0.0106Wobbe Index 45.3103 45.336 0.0257
4.2 FIELD TRIAL TEST
The test took place in one of Aramco’s gas plants on July 15 and 16 of 2014. The TFS was set on a 5 reading moving average filter rate. The unit reports were read every second, so this is equivalent to a 5 second moving average filter.
At the start of each day the GC and TFS were calibrated on the reference gas. The zero of the TFS was checked and corrected. All corrections were recorded.
The test gas (sales gas out of the gas plant) was applied continuously to the TFS and the GC. The Ethernet TCP/IP modbus output of the TFS was recorded by a data logger, set to log data every 1 second. The output of the GC was logged by the gas plant DCS. This allows for further analysis of the GC data to perform CV calculations, which was compared to the TFS. Both outputs are date and time stamped, so the data can be brought together for comparative analysis later.
Some one-hour average results (without normalization) of this test are shown below:
As can be seen, errors obtained when the equipment is tested in a field environment are bigger than the
ones obtained at a lab environment.
It is planned to perform another trial with an extended composition, C1-C6+, which is normally used for custody applications, and throughout the year to assess the impact of different seasons on instrument performance.
