Mixing or diluting gases for multipoint calibrations

CAC Gas & Instrumentation
Wednesday, 01 February, 2023

Designed and produced by AlyTech and distributed by CAC Gas & Instrumentation, GasMix^TM products enable multipoint gas calibrations by mixing or diluting several calibration gases.

GasMix provides an on-site gas standard preparation system for analytical laboratories that need to prepare gas standards for all kinds of analytical instruments including gas chromatography and gas spectroscopy systems, refinery gas analysers, on-line gas analysers and more.

The GasMix software is fully automated, allowing the option to run a single injection or a fully automated and pre-programmed sequence. The GasMix Software can be remotely controlled to drive an external injection valve or the Start command of an analyser.

Using GasMix provides the following benefits:

Reduces the number of standard gas cylinders stored (lower cost and greater safety)
Multiple gas standards can be created
Adapts to all gas analysis instruments — being modular, it is a unique and universal gas sampler and injector
Drastically reduces operator bench time

The GasMix products can be used in a variety of industry applications including petrochemical, environmental, R&D, oil and gas, pharmaceuticals, and specialty gases.

Environmental applications

Environment protection is one of the great issues of this century. Many laboratories therefore conduct analyses of air emissions to meet environmental standards. To do so, they need efficient and accurate analysers.

Being able to create standards according to the current application gives more flexibility to the user. Environmental standards such as EN 14212 require multipoint calibration. Gas standards are expensive and certain gas mixtures require a long time to be obtained. These new conditions have created a demand for mixers producing accurate gas concentrations. GasMix meets these new requirements: its accuracy allows the user to produce optimal mixes, even at low concentrations. GasMix reduces gas consumption, the operator time is decreased by automation, and the GasMix software helps with inventory management.

Oil and gas applications

Whether they are control laboratories in production plants or R&D labs for the oil and gas industry, petroleum refineries and gas plants use many analysers (such as GCs) in their laboratories. Those analysers must deliver accurate and reliable measurements to meet product specifications according to ASTM or ISO standards.

By enabling the dilution or mix of 2–16 different gases, GasMix is the right tool for analysts needing certified gas standards at various levels of concentration. This includes traces which are often not stable or not available in bottles on the market.

Application examples

Automatic multipoint calibration of GC

One refinery customer from the Gulf region ordered an analyser based on the UOP 603 method. This method involves the analysis of carbon monoxide, carbon dioxide and methane at trace levels in light hydrocarbon gaseous fractions. The analyser is fitted with a methaniser that converts the carbon oxides into methane, which itself is detected by an FID detector.

The customer has one certified gas standard, whose concentration for each compound is around 10 ppm, and must verify that the analyser can quantify down to 40 ppb.

A GasMix dilutor model Zephyr, fitted for aggressive gases, is installed between the gas cylinder and the GC. A sequence involving five different concentrations, two replicates per level, is built up. Each of these solutions is flown into the analyser for two minutes to purge the lines and take the possible adsorption phenomenon into account. The automatic sequence of the Zephyr turns all gas flows down to zero once the sample has been injected and remains that way during the remaining time of the GC run.

The use of the dilutor allowed the verification that the methaniser was performing well, as well as verifying the linearity of the detector’s response. Additionally we can quantify trace level analysis from calibration solutions which are close to the unknown samples (<100 ppb), starting from only one calibration gas.

To complete this process without the use of the diluter would be to use several calibration gas cylinders, at least four or five, whose concentration might go down to 1 ppm (not below). The supply of such gases may involve some long delivery times and can be expensive. Zephyr allows us to reach low levels (50 ppb and above in this case) using an unlimited number of points, and at a much lower cost. Zephyr can also then be used for other analysers and with other gases.

LPG dilution with Aura

An LPG is a mixture of light hydrocarbon gases and is prepared by refining petroleum or ‘wet’ natural gas. It is almost entirely derived from fossil fuel sources, being manufactured during the refining of petroleum (crude oil), or is extracted from petroleum or natural gas streams as they emerge from the ground. These gases are mostly propane, butane, or a mix of both.

LPG will evaporate quickly at normal temperature and pressure. It is usually supplied in pressurised steel vessels for transportation and commercialisation.

LPG is a mixed phase matrix, meaning that at atmospheric pressure, one can find at the same time liquid and gas. If a component is in a gas phase, it will not have the same behaviour as in a liquid phase or in a mixed phase. The matrix can have a considerable effect on the way the analysis is conducted and on the quality of the results obtained; such effects are called matrix effects. The most common approach for accounting for matrix effects is to build a calibration curve using standard samples with known analyte concentration and trying to approximate the matrix of the sample as much as possible.

The AURA heats and vaporises the LPG containing the compounds of interest into GasMix, which will then dilute it with another gas. The use of a gas mixing system with Aura allows us to check the linearity of the analyser. Using an LPG standard avoids matrix effects, giving more accurate results.

GasMix can prepare low level standards, without limitations in the number of points, with only one LPG standard, saving time and money.

Overall benefits

Reduce the number of necessary gas standards

Components in cylinder mixtures have a fixed and defined ratio. A different ratio automatically implies a different gas cylinder. The use of the GasMix system provides flexibility in the laboratory. Two or more gas standards can be mixed with different ratios. Furthermore, different concentrations are required for a multipoint calibration. That means several gas cylinders must be purchased. GasMix only uses the high concentration standards and dilutes that to the needed lower concentrations.

Optimising the efficiency of gas cylinders

All gas standards can be optimised to their fullest, allowing all gas standards to be used at multiple concentrations. A higher gas concentration can be diluted down to lower levels, which maximises the efficiency of the gas cylinder.

Reduction of bench time

Manual gas injection requires a large amount of user intervention as there is no ‘automatic injector’ for gas samples. GasMix software automates the steps that an operator must do by creating an automated pre-programmed sequence.

For more information on AlyTech or other calibration and specialty gas products for your application, contact CAC GAS at cac@cacgas.com.au.