Process Gas Chromatographs

What Is a Process Gas Chromatograph?

Gas chromatography is a separation and measurement technique in which a sample mixture is carried through a column by a flow of carrier gas. Because each component interacts differently with the column packing, the components leave the column at different times. Every component reaches the detector at its own characteristic retention time, and the peak areas in the chromatogram — the record of the detector signal over time — are used to calculate component concentrations.

A process gas chromatograph takes this technique out of the laboratory and into the field. It draws the sample directly from the process line, repeats the analysis cycle on its own, runs calibration and validation gas at defined intervals and transfers the results continuously to the control system. Composition data is therefore available to operations within minutes and without interruption, instead of arriving hours later in a laboratory report.

In the natural gas industry the chromatograph is the core analyzer for calculating heating value, which defines the energy content of the gas, and hydrocarbon dew point, which is critical for pipeline operation. In refineries, gas processing and petrochemical plants it monitors the composition of reactor feed and product streams, supplying data for process control, product quality verification and tracking of trace impurities. The field-mounted chromatographs covered on this page are designed to be installed close to the measuring point without an analyzer shelter.

How Online Gas Chromatography Works

In an online gas chromatograph, each analysis cycle consists of a sequence of fully automated steps, from selecting the sample through to sending calculated results to the control system:

1.      Stream selection and sampling: The conditioned sample enters the analyzer through an internal stream selection module. The solenoids are driven by inert gas; several sample points can be analyzed in turn, and a double block and bleed arrangement can be chosen to prevent cross-contamination between streams of differing composition.

2.      Injection: A pneumatically actuated diaphragm chromatograph valve injects the gas held in a fixed-volume sample loop into the carrier gas flow. Valve timing is governed by timed event tables in the analysis method; combinations of 6-port and 10-port valves handle tasks such as pre-column backflush and column switching.

3.      Separation in the columns: The carrier gas sweeps the sample through columns housed in a temperature-controlled isothermal oven. Depending on how strongly they interact with the column packing, the components travel at different speeds and separate; light components generally elute first and heavy hydrocarbons later.

4.      Detection: Each component leaving the column produces a peak at the detector. Depending on the application, a thermal conductivity (TCD), flame ionization (FID) or micro flame photometric (µFPD) detector is used, in single- or dual-detector configurations.

5.      Peak identification and integration: Peaks are identified by their retention times, and their areas are calculated by fixed-time or automatic slope-sensing integration. Retention times are updated during calibration or during the analysis itself to compensate for drift caused by temperature and flow.

6.      Calculation and reporting: Peak areas are converted to mole percent using response factors determined from the calibration gas. Physical properties such as heating value and density, as well as the hydrocarbon dew point, are calculated from the composition; results are archived together with averages and alarms and transferred to the control system.

Detector Options: TCD, FID and µFPD

The thermal conductivity detector (TCD) senses the difference between the thermal conductivity of the sample components and that of the carrier gas, and it responds to nearly all components found in natural gas and light hydrocarbon streams. Because the high-sensitivity TCD design can also handle low-ppm measurements in many applications, it reduces the need for more complex detectors.

The flame ionization detector (FID) measures the ion current produced when organic compounds burn in a hydrogen flame and detects trace hydrocarbons at ppb concentrations. The compact FID, less than 76 mm high, is designed to fit inside the flameproof enclosure of the analyzer. The micro flame photometric detector (µFPD) uses a photomultiplier tube to measure the characteristic light emitted by sulfur compounds in a flame, enabling sulfur analysis at ppb levels; integrated in the analyzer oven, this detector does not require instrument air.

Micro-Packed and Capillary Columns

Micro-packed columns combine the speed and sharp peak separation of capillary columns with the robustness of conventional packed columns, while keeping carrier gas consumption low. The oven accommodates up to 27 m of micro-packed column with a 1/16-inch outside diameter or, where the application calls for it, up to 91 m of capillary column. The column set is engineered for each application according to the component list and the target cycle time.

Automatic Calibration, Validation and Peak Integration

The calibration stream is part of the analyzer's stream selection sequence. In a calibration run, a gas of known composition is analyzed, updated response factors and retention times are calculated, and the drift relative to the previous calibration is reported. In a validation run, the measured values are compared with the nominal values, and the percent deviation and the permitted deviation limit are recorded. These records make it easier to document measurement quality during audits.

Analysis cycle time depends on the component list, the column set and the number of valves. In typical natural gas applications, a C6+ analysis can be run on a three-minute cycle and a C9+ analysis on a five-minute cycle.

Key Features

·         Modular, airless analytical oven: The dual-zone isothermal oven has a clean architecture with minimal wiring and a swing-out upper plate that gives full access to its components; every part inside the oven can be serviced in the field.

·         Up to six chromatograph valves: The oven takes as many as six diaphragm valves of the 6-port or 10-port type, of which no more than four may be 10-port. This capacity allows complex applications to be handled by a single analyzer and standard analyses to be completed in shorter cycles.

·         Low-maintenance dual-diaphragm valves: Dual-diaphragm valves with no springs, O-rings or lubrication are rated for more than five million cycles; replacing a diaphragm set normally takes less than ten minutes.

·         Multiple detector options: A TCD capable of low-ppm measurement, an FID for ppb-level hydrocarbons and a µFPD for ppb-level sulfur, configured as single or dual detectors to suit the application.

·         Internal stream selection module: An internal four- or eight-stream module removes the need for external stream switching assemblies and can be extended to 20 streams with external control. A double block and bleed option is available for streams of variable composition.

·         Comprehensive data archive: 31,744 analysis records — more than 88 days at a four-minute cycle — plus 370 final calibration and 370 final validation records are stored with time stamps in 2 GB of flash memory.

·         Audit-ready record structure: Data and event logs conform to API 21.1 for metering audits. Event, alarm and maintenance logs together with four-level password protection strengthen traceability.

·         Expandable I/O: Five digital inputs, five digital outputs, two analog inputs and six isolated analog outputs are standard; two expansion slots raise the number of analog outputs to as many as 14.

·         Optional local operator interface: A 12-inch color VGA display with infrared touch keys lets technicians start the analyzer, change calibration gas settings and view chromatograms in the field without a laptop; the interface preserves the analyzer's hazardous-area rating.

·         Front-panel status indicators: Green, yellow and red LEDs show analyzer health, and additional LEDs show valve positions and processor status; every valve can also be switched by hand for troubleshooting and for fast purging after maintenance.

Technical Specifications

The table summarizes hardware and analytical performance data for the field-mounted process gas chromatograph. Because the measured components, ranges and cycle times are set by an application-specific analysis design, final values are confirmed in the application sheet prepared at quotation stage.

The maximum oven operating temperature is given as 120 °C or 150 °C depending on configuration; the value applicable to the application should be confirmed at project stage. The temperature class depends on the selected options (see Hazardous-Area Approvals).

Key Advantages

·         Field mounting that reduces the need for shelters: A design that can be installed in the field without compromising analytical performance, and hazardous-area suitability without purge gas, can significantly cut the cost of shelters, HVAC and purge systems.

·         Comprehensive analysis in one analyzer: Multi-valve and multi-detector capacity lets several analysis tasks be consolidated in one instrument, reducing the number of analyzers and the variety of spare parts at pipeline stations.

·         Low operating consumption: The low carrier gas consumption of micro-packed columns, modest power draw and a µFPD that needs no instrument air keep consumable and infrastructure costs down.

·         Short maintenance windows: Full access to oven components, valve diaphragms that can be changed in minutes and simple wiring shorten the time the analyzer is out of service during planned maintenance.

·         Measurement reliability and traceability: Automatic calibration and validation runs, a time-stamped archive and audit logs make it easier to verify results retrospectively and to answer disputes with documented evidence.

·         Flexible system integration: One of the two Ethernet interfaces can connect to the plant maintenance network and the other to the control network via Modbus TCP; remote access can also be set up through a cellular broadband gateway.

·         Stable analysis across wide ambient temperatures: With the TCD configuration able to operate at –20 to +60 °C ambient, heating value and C9+ dew point calculations remain dependable even at field locations without climate control.

Application Areas

Wherever selected components of a gas or liquid stream have to be tracked continuously, process gas chromatographs find use — in natural gas and refining, in petrochemical plants, in power generation and in environmental monitoring.

Natural Gas Transmission and Metering Stations

At pipeline monitoring and custody transfer points, gas composition, heating value (BTU) and hydrocarbon dew point are determined continuously. In the flow computer, the composition data from the chromatograph is combined with data from volumetric meters such as ultrasonic gas flow meters to calculate the energy quantity.

Gas Processing, NGL/LNG and Cryogenic Plants

In natural gas liquids (NGL), liquefied natural gas (LNG) and cryogenic gas plants, the composition of feed and product streams is monitored to check separation performance and product specifications.

Refineries

Reactor feed and product streams are analyzed in catalytic reformer, isomerization and aromatics units. The data is passed to the process control system for unit optimization and continuous verification of product quality.

Petrochemical and Polymer Plants

In ethylene and polymer plants, trace impurities are monitored alongside the main components; ppb-level hydrocarbon measurement with the FID and ppb-level sulfur measurement with the µFPD are important for catalyst protection and product purity.

Power Generation and Combustion Turbines

Monitoring the composition and heating value of the fuel gas supplied to combustion turbines helps adjust combustion settings as gas quality changes.

Environmental Monitoring

FID-based configurations are used for ambient air monitoring and for tracking highly reactive volatile organic compounds (HRVOC) at flares and cooling towers.

How to Select a Process Gas Chromatograph

The success of a process gas chromatograph depends less on the analyzer hardware than on a correct definition of the application. Covering the following points in full is the basis for the right analyzer configuration:

Analysis Requirements

·         List of components to be measured and the expected range for each (percent, ppm or ppb level)

·         Need for C6+, C7+ or C9+ analysis of natural gas; whether heating value and hydrocarbon dew point are to be calculated

·         Components that require a special detector, such as sulfur compounds or trace hydrocarbons

·         Target analysis cycle time and repeatability expectations

Sample and Stream Configuration

·         Number of streams to be analyzed and the arrangement of calibration/validation streams

·         Whether the sample is in the gas or liquid phase; need for liquid sample injection

·         Sample pressure, temperature and dew point; heat tracing requirements

·         Need for double block and bleed on streams of very different composition

Site Conditions and Hazardous Area

·         Hazardous-area classification, gas group and required temperature class (heat tracing and liquid injection options affect the temperature class)

·         Minimum and maximum ambient temperature at the installation point; operating range of the selected detector

·         Preference for floor, wall or pipe mounting and the service access area

·         Available power supply: 120/240 Vac or 24 Vdc

Utility Gases and Infrastructure

·         Carrier gas type (helium, nitrogen or hydrogen) and supply logistics

·         Gas supply and pressure regulation for valve actuation

·         Supply of the auxiliary gases needed for the flame if an FID or µFPD is used

·         Composition and certification requirements for calibration and validation gases

Communication and Data Management

·         Communication method with the DCS, SCADA or flow computer: Modbus TCP, Modbus serial or 4–20 mA

·         Required number of analog/digital inputs and outputs and need for expansion cards

·         Remote access, user authorization levels and cybersecurity requirements

·         Audit records, report types and expected data retention period

However sensitive the chromatograph, the result cannot be correct if the sample reaching it does not represent the process. Sample conditioning systems — covering the sample probe, pressure reduction, filtration, heat tracing and vent lines — should therefore be designed together with the analyzer and with the same care. Local gas detection measures, such as infrared gas detectors, should also be considered for the risk of combustible gas leaks along the sample line and in the analyzer area.

Hazardous-Area Approvals and Communication Standards

The hazardous-area approvals and supported standards of the field-mounted process gas chromatograph are summarized below. The scope of the approvals depends on the selected hardware configuration.

In hazardous areas, components added to the sample system, such as vapor regulators and flow switches, must also be suitably certified. National or regional metrological approval requirements for custody transfer should be clarified at project start according to the regulations that apply to the facility.

Online Gas Analysis Solutions from TLY Enerji

At TLY Enerji we treat a process gas chromatograph not as a stand-alone instrument but as an analysis system that reaches from the sample point to the control system. Our engineers review the component list, measuring ranges, number of streams and target cycle time together with the process data and check that the detector, valve and column configuration suits the application.

We support our customers throughout the project with sample conditioning and sample line design, planning of carrier and calibration gas infrastructure, meeting hazardous-area requirements, and integrating analysis results into flow computers, DCS or SCADA systems via Modbus or 4–20 mA. At natural gas metering stations we plan the chromatograph together with the flow measurement so that the energy calculation is consistent end to end, and we stay involved through commissioning, documentation and the briefing of site teams.

Frequently Asked Questions

What is the difference between a process gas chromatograph and a laboratory GC?

Both use the same separation principle; the difference lies in how they operate. A process gas chromatograph takes its sample automatically from the line, repeats the analysis cycle continuously, runs calibration and validation by itself and sends results to the control system in real time. Field-mounted units also have hazardous-area-approved flameproof enclosures and wide ambient temperature ranges, so they can be installed close to the measuring point.

How is the heating value of natural gas determined with a gas chromatograph?

The chromatograph measures the mole percentages of components such as methane, ethane, propane, butanes, pentanes, hexanes and heavier, nitrogen and carbon dioxide. The heating value is calculated by weighting the known heat of combustion of each component by its measured fraction. In a temperature-controlled environment, a C6+ analysis on a three-minute cycle can achieve a heating value repeatability of ±0.0125%.

What is the difference between C6+, C7+ and C9+ analysis?

The difference is how the heavy hydrocarbons are reported. In a C6+ analysis, hexane and heavier components are measured as a single group; a C7+ analysis also determines heptanes separately; a C9+ analysis additionally measures octanes and nonane individually. C6+ is sufficient for heating value in most cases, but an extended C9+ analysis is preferred where the hydrocarbon dew point must be calculated reliably, because that calculation needs detailed knowledge of the heavy components.

Which applications use TCD, FID and µFPD detectors?

The TCD is the most widely used detector for natural gas and light hydrocarbon analysis because it responds to most components, and it can measure down to low-ppm levels. The FID is used to measure trace hydrocarbons at ppb level, and the µFPD to measure sulfur compounds at ppb level. A single- or dual-detector configuration is selected according to the component list required by the application.

Can a gas chromatograph calculate the hydrocarbon dew point?

Yes. The hydrocarbon dew point is calculated from the detailed composition of the heavy hydrocarbons. An extended C9+ analysis, which measures octane and nonane individually, provides the data needed for this calculation. A C9+ analysis typically runs on a five-minute cycle, and the heating value is obtained from the same analysis. Dew point data that is critical for pipeline operation can therefore be monitored continuously and automatically.

Does a field-mounted gas chromatograph need an analyzer shelter?

The chromatographs covered on this page are designed for field installation without a shelter and without compromising analytical performance, and they operate in hazardous areas without purge gas. With a TCD they can operate at ambient temperatures of –20 to +60 °C. The FID and µFPD have narrower ambient ranges, however, so a simple sunshade or protective structure may be needed in extreme climates. The decision should be based on site conditions.

How many sample streams can one gas chromatograph analyze?

The internal stream selection module is available with four or eight streams, and the calibration stream is included in that number. With externally controlled stream selection, up to 20 streams can be analyzed. Because streams are analyzed one after another, each stream is updated less often as the number of streams grows; the number of streams, the cycle time and the update rate required by process control should therefore be evaluated together.

How is a gas chromatograph connected to the control system and flow computer?

The analyzer has two 10/100 Mbps Ethernet interfaces, serial ports selectable for RS-485, RS-422 or RS-232 operation, and isolated 4–20 mA analog outputs. Data can be sent to DCS, SCADA and flow computers via Modbus TCP or Modbus serial; 4–20 mA outputs are preferred where existing cabling is to be reused. Expansion cards increase the number of analog outputs to as many as 14.

Which carrier gases are used in a gas chromatograph?

The choice of carrier gas depends on the application; zero-grade helium, nitrogen or hydrogen is typically used. The selection is based on the components to be measured, the detector type, the required sensitivity and the gas supply options at the site. The low carrier gas consumption of micro-packed columns extends cylinder change intervals and reduces the logistics burden, especially at remote stations.