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Apr-2012

Micromachined gas chromatography for gas plants

Onsite gas chromatography is available to troubleshoot, optimise, debottleneck and help identify energy savings in natural gas plants

Les Alberts
Seala Services
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Article Summary
Gas analysis provides insight on plant performance by identifying components of interest, but most gas plants operate without online analysis or have only hydrogen sulphide analysers. The natural gas matrix can include helium, argon, oxygen, carbon dioxide, hydrogen sulphide, trace sulphur species, hydrogen, methane, ethane, propane and heavier hydrocarbons. Both trace and higher concentration components can be troublesome for operations. Troubleshooting typically involves taking samples for analysis away from the plant, which prolongs identifying the root cause of operation issues. Operators rely on the interpretation of smaller sample sets to identify the root cause, and those sample sets may not be representative. For example, the presence of oxygen is usually first identified during routine glycol or amine analysis as degradation components. Oxygen ingress may be continuous or intermittent, such as from a vapour recovery system. Another example is identifying the presence of different sulphur species and the ability to meet the total sulphur specification that is not normally measured.

Developments in the portable gas chromatograph (GC) provide the opportunity for analysing a wide range of components at rates fast enough to replicate online analysis. A number of applications related to oxygen, carbon dioxide, hydrogen sulphide and trace sulphurs will be presented to show micro-machined gas chromatography’s capability, performance and benefit in gas treating.

Troubleshooting is never easy and often impossible when sources of data are limited, inconsistent or collected through multiple sources. Micromachined gas chromatography (µGC) allows the user to see what is not normally seen in real time, with speed and portability. The µGC can be used to measure many chemical components, given the correct equipment. In gas processing, the components of interest include oxygen, hydrogen sulphide, carbon dioxide, nitrogen, C1-C10 hydrocarbons, carbonyl sulphide, mercaptans, methanol and more.

But what can be done with this information? Seala Services’ onsite gas analysis, combined with the proper interpretation of amine analysis and the use of modelling tools, enables unprecedented operating solutions/ improvements. For example, oxygen identified in a gas stream can degrade amines to form heat-stable salts or bicine at varying rates. The heat-stable salts are non-regenerable, use plant capacity and contribute to corrosion. However, oxygen degradation of tertiary amines such as methyldiethanolamine (MDEA) also leads to secondary amines such as diethanolamine (DEA) and monomethylethanolamine (MMEA), which can negatively impact plant performance through reduced CO2 slip when CO2 slip is important. The second effect is through onsite gas analysis to validate CO2 slip, and with amine analysis and modelling tools to validate additional CO2 pickup from the presence of DEA and MMEA in the amine solution.

Developments in µGC provide the opportunity for analysing a wide range of components at rates fast enough to replicate online analysis through modular design, parallel chromatography, incorporation of a vapouriser and quick swapping of columns when required. Also, µGCs have detection limits in the low ppm range, parallel chromatography and ease of transport to remote locations. Although operation seems simple, the complexities of operating and interpreting chromatography still exist. A number of applications related to oxygen, carbon dioxide, hydrogen sulphide, carbonyl sulphide and mercaptans will be presented to show the capability, performance and benefit of µGC for gas treating facilities.

Basic gas chromatography
Gas chromatography is a means of separating a mixture of components in a vapour state. A gas chromatograph is made up of four main components:
• A column that is a tubular embodiment made of stainless steel or glass coated internally and evenly with either a polymer or an organic or inorganic adsorbent. The column has a pure carrier gas flowing through it constantly at a precisely controlled temperature. Since each of the components of the mixture has different chemical and physical properties, this leads to a different partition co-efficient and, hence, a dissimilar migration speed of the component through the column
• A mobile phase, which is typically a gas, also known as a pure carrier gas that constantly flows through the column at a controlled temperature
• A sample inlet and vapouriser for liquid samples
• A detector as a means to detect the presence of each component. The most common detectors are the flame ionisation detector (FID) and the thermal conductivity detector (TCD). A TCD is used in this application, allowing detection of any component other than the carrier gas.
In addition, a method or set of conditions is established that includes inlet temperature, detector temperature, column temperature, carrier gas and carrier gas flow rates, type of column, diameter and length, inlet type and flow rates, and sample size. Separation of the components is accomplished with different types of absorbents or polymers in the column and different column temperatures, with a lower temperature providing better separation. Columns are unique and have to be appropriately selected to attain the desired separation for the components of interest.
Operation is simple: an aliquot of the sample is injected into one end of the column to determine the different components at the other end. The time it takes to elute the component (retention time) is used to establish a tentative identification of the component, whereas the intensity of the signal from the detector is used to establish the concentration of the component. Known reference standard materials are used to aid in establishing both identification and concentration of the components.

Although operation of a GC is simple, many complexities exist:
• All sub-components can fail, including columns, network cable, tubing and valves, while filters plug and fittings not properly connected will leak. µGC components are 
delicate, requiring care during maintenance and setup. Columns can be swapped, so training is important to avoid costly mistakes
• There is a need for accurate, defendable results. Natural gas is a mix of many components of interest, requiring complex and expensive standards. Components with similar retention times on a column may co-elute, giving a false positive. For example, argon and oxygen may show up as one peak, providing a false positive, as their retention times are similar. Argon is naturally present in natural gas, so limited or no separation will show higher levels of oxygen if argon is present. The method needs to be robust to ensure separation. For example, Figure 1 shows co-elusion of argon and oxygen
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