01 April 2004
Wafer technology has chromatograph
Silicon, modular GC has traditional flexibility, additional analytical capabilities, and a shorter cycle time.
By Ulrich Gokeler
It is common practice to use online process gas chromatographs (GCs) to automatically measure natural gas both qualitatively and quantitatively.
Typical applications involve the determination of the heating value contents and other relevant parameters for quality control and custody transfer purposes.
Further applications involve measurements to control the various steps in the purification and fractionated separation of hydrocarbons in natural gas. Because of the huge natural gas quantity processed and the valuation of the product correlating directly to its energy content, the demand for analytical precision and instrumental reliability are very high.
These performance requirements grow larger by the practice of installing such analytical systems directly at the sampling points, typically without extensive infrastructure. The installation is often remote and operates entirely automatically—from sample preparation, analysis, operation, and calibration to data transmission. Another important consideration is the frequent limitations of the local operating and maintenance personal due to lack of expertise and maintenance experience.
There's a new type of small and compact process gas chromatograph that platforms on silicon wafer technology with modular design and that has the flexibility of traditional process gas for a wide range of applications. One can specifically apply this type of GC to the measurement of heating values. Because of its design and analytical capabilities, it provides very short analytical cycle times. It also eliminates the influence of sample pressure or ambient pressure variations on the measurement accuracy.
Due to the requirements for long-time stability, reproducibility, and independency of environmental changes, the design permits one to routinely monitor analytical results, precision, and hardware performance. Consequently this allows one to recognize performance deviations and intervene proactively before failure. Both are important criteria for analytical online reliability and measurement precision. Furthermore, due to the modular design, providing reproducible and self-contained analytical modules, maintenance reduces to a quick exchange of modules.
Standardized in design
The online process GC uses the latest miniaturization techniques, such as silica wafer technology and microelectromechanical systems (MEMS), and has been designed specifically to be suitable for the environment, requiring no extensive infrastructure. The applied manufacturing technique positively affects the electronics and analytics with regard to size and capability, as well as design, mechanical reproducibility, and cost. Consequently, the analyzer consists of the housing and three modules: analytical, pneumatic, and electronic. Each module is standard in design, connections, and interfacing and can swap out individually. The analytical module is so precise that one can swap a specific separation setup with another module and still generate identical separation and retention times.
All three modules integrate into a small cast alloy housing, making them suitable for installation in a hazardous environment without purge requirements. The design also permits installation into field locations without the need for extensive analyzer shelters. To have the widest possible separation flexibility, narrow bore capillary columns are the basis of the actual separation system. Using valveless, maintenance-free column switching configurations is typically superior to valve switching. Combining these requirements and facts, and applying silicon wafer technology, the necessary injection, valveless column switching, and detection devices reach unsurpassed miniaturization, inertness, and flexibility in online process gas chromatographs.
A diaphragm valve injects a certain sample volume into the carrier gas stream, thereby equilibrating the sample precisely to the carrier gas pressure. By means of a valveless injection technique, only a slice from the initially injected sample volume goes into the first separation column. The unused portion vents. The sample volume size actually injected can be varied by the width of the slice, which is time controlled. Therefore the initial sample pressure and sample pressure stability have no influence on the measurement precision. Further-more, ambient or sample vent backpressure changes no longer influence the injection volume. Another significant advantage of this injection system is if the primary diaphragm valve is leaking, it does not affect the actual injection size and quality, and consequently does not affect the analytical results as long as the sample pressure does not exceed the carrier gas pressure.
In general the separation system consists of at least two columns coupled in series by the valveless switching system capable of performing back-flush operations. The carrier gas flows and column switching operations respond to electronic pressure regulators that permit not only very simple but also remote pressure setup and monitoring.
The flexibility and simplicity of the analytical system is greatly enhanced by a number of micromachined thermal conductivity detectors (TCD) located in-line at various points throughout the analytical system. The injected volume before entering the first separation column, the column inlet and outlets, and all vents are monitored by individual detectors. By means of the multidetection capabilities, important analytical information is available, including the separation progress numbers, exact back flushing, and heart-cut timing information. Specifically, for this natural gas application, within the first column there are actually up to four in-line detectors. Each detector detects the separation progress. The column length and separation phase were selected in such a way that specific separation tasks are performed in the best way, with the shortest cycle time of just 120 seconds for the entire separation from oxygen to hydrocarbon molecules with six or more carbons.
The entire analytical module, with the separation columns, the microinjection and column switching parts, as well as the microdetectors, integrates into an enclosed module. The module also includes electrical heating for isothermal heating of the whole module up to 200°C (392°F).
The analytical module connects to the pneumatic module, which includes the individual miniaturized electronic pressure regulators and the few solenoid valves needed to activate selected vent lines, as well as external stream selection and calibration valves.
Due to the process requirements and the resultant design, the modular system minimizes service requirements and onsite maintenance. Therefore, any of the three modules can swap out in a short period of time. Fundamental application changes happen by changing the entire analytical module.
The Ethernet-based analyzer data network permits communication with the process control system as well as observation for maintenance purposes. It also allows one to continuously archive multichannel chromatograms, results, and status data for every analyzer. A dedicated RS485 or RS232 serial port permits the local interface to a flow computer or control systems.
Precision pending failure
Unattended online analytical measuring systems, frequently installed in remote locations, must measure precisely and not be affected by environmental changes. In addition to the required longevity, it is very desirable to have a certain predictability concerning maintenance requirements and upcoming failure.
Recognizing performance changes and predicting upcoming failure allows for a proactive rather than reactive remedy. It is not uncommon to monitor the retention time repeatability of certain peaks after every analysis. Analyzers are often not capable of monitoring other parameters except during calibration. The analytical configuration makes up for these deficiencies and is capable of validation and correction.
The quantitative analytical precision is a result of the sample parameters from the injection system and of the detection sensitivity. The influence of the sample pressure and temperature and the influence of the ambient pressure and temperature on the injected sample volume no longer exist per the injection system design described previously.
The influence of ambient temperature on component retention time and on the concentration is small. However, there are no traditional means to verify these factors except during calibration.
This is different with the new analytical design. The first detector located between the secondary injection and the separation column measures the injection peak. Based on parameters such as peak area, height, width, and peak tailing, the system determines and registers the injection quality parameters for every analysis. Consequently, by verifying the injection peak parameters, proper injection functionality is determined. Precisely this validation applies for the natural gas measurement, because the unit measures all components present. Therefore the injection peak area is proportional to the sum of the peak areas of each individual peak from every detector. Furthermore, by using individual detectors on every column inlet and outlet as well as on every vent, essentially every component or group of components registers several times on various detectors. Therefore, the response of individual peaks and peak groups on various detectors can correlate with one another and with the injection peak. One can use any changes of ratio data for automatic correction, hence improving precision.
For example, the group consisting of oxygen (O2), nitrogen (N2), and methane (CH4) registers on every detector in the separation system except the vent detectors 1a and 2a. The response of these peaks on a specific detector correlates to the response of these peaks on the other detectors. For any two detectors there is at least one component measured on both. Therefore, any response change on one detector can register on the monitor immediately. One can then use any change detected to correct the measurement of these specific peaks that register on that specific detector. Validation happens by continuously monitoring the relative response of various components on various detectors. Despite changes on one detector, the system can detect a response change, correct it, and proceed with a precise analytical measurement. Repeatable response ratios are a clear confirmation of correct functionality of the analytical system. Changes in response factors are an indication of upcoming failure.
Further data coming
This compact online process gas chromatograph specifically applies to the measurement of the quality of natural gas. The analytical design eliminates known performance problems and sources of failure. Additionally, the sample parameter influences the quantitative precision less. The validation capabilities improve confidence in the generated data.
The compact online process gas chromatograph also recognizes analytical changes and has the capability to compensate in many cases for such changes. Consequently the recognition allows one to react to these changes proactively. Technicians can predict failure and therefore schedule maintenance at less critical process situations, during better environmental conditions or technician availability.
The analytical and associated software possibilities will provide increased analyzer availability associated with a higher degree of confidence in the data generated. As demonstrated, initial tests have shown that the ambient influence on the measurement stability and precision is very limited. Presently, ongoing analytical testing is generating further natural-gas-relevant performance data.
Behind the byline
Ulrich Gokeler is an ISA member. He works at Siemens Applied Automation in Houston, Texas, as product manager. Write him at Ulrich.Gokeler@siemens.com
Heating value, technically speaking
The heating value—or calorific value—of a combustible material is the negative of the standard heat of combustion, usually expressed per unit mass of the material.
The standard heat of combustion of a substance is the heat of the reaction of that substance with oxygen to yield specified products, with both reactants and products at 25°C and 1 atmosphere.
The products and assumptions of this reaction are: all carbon in the fuel forms CO2 (gas), all hydrogen forms H2O (liquid), all sulfur forms SO2 (gas), and all nitrogen forms N2 (gas).
Here are some heating values of common fuels.
Source: Elementary principles of chemical processes
A matter of surface tension
Gas chromatography is a process for the separation of mixtures. This happens by passing a sample mixture (the analyte) in a stream of solvent (the mobile phase) through some form of material (the stationary phase) that will provide resistance by virtue of chemical interactions—not reactions—between the components of the sample and the material.
Chromatography relies on physical interactions between molecules to separate mixtures of substances and their molecules.
Usually, each component has a characteristic separation rate that identifies it and thus the composition of the original mixture.
The International Union of Pure and Applied Chemistry (IUPAC) definition of chromatography is that it is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary while the other moves in a definite direction.
It was the Russian botanist Mikhail Tswett who in 1906 first used the term chromatography. He used the technique to separate differently colored plant pigments—which explains why chroma is part of the name. The modern concept of chromatography has nothing to do with color.
Analytical chromatography serves to determine which chemicals are in a mixture and their concentrations.
A simple and illuminating example of chromatography is that performed with paper.
A small spot of a solution containing the sample goes on to a strip of chromatography paper about 1 centimeter from the base. This sample adsorbs onto the paper. This means that the sample will contact the paper and may form interactions with it. Experimenters then dip the paper into a suitable solvent such as ethanol or water and place it in a sealed container.
As the solvent rises through the paper it meets the sample mixture, which starts to travel up the paper with the solvent. Different compounds in the sample mixture travel different distances according to how strongly they interact with the paper.
The final chromatogram can match against other known mixture chromatograms to identify sample mixes.
Two-way paper chromatography involves using two solvents and rotating the paper 90°. This method is useful for separating complex mixtures of similar compounds.
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