The analysis of different gas mixtures is one of the most important challenges for measurement in industry. In process engineering gas analysis has a very important function – the continuous measurement of gas composition enables the successful monitoring of both process efficiency and product quality.Through continuously operating analyzers and gas conditioning systems, a real-time reporting and assessment of different process conditions is possible. The results are used for monitoring, evaluation and control of a process. Gas analyzers provide and document fundamental information on the development of a process, which enables targeted process optimisation and scheduling of actual maintenance needs for the facility.
The composition of a gas mixture within a process, or how a chemical reaction is proceeding, can in principle be determined in two ways:
• Firstly by installing a suitable sensor in contact with the process (in duct or work area)
• which is called In Situ Measurement, or
• Secondly, by the extraction of a sample stream from the process and then transporting it through
• a suitable conditioning system to the analyzer, which is called extractive measurement.
Since its very beginning, M&C has had a focus on the business of providing products for extractive measurement of difficult, complex and challenging gas mixtures, which therefore forms the basis of the following discussion:
In many cases, the sample taken from the process cannot be fed directly to an analyzer. A sample usually has to be prepared by extractive gas conditioning components to ensure that expensive analyzers are not damaged and that their investment is protected. Conditioning may change the sample gas properties, which can result in differences between the process and sampled gas. Interferences in the sample may cause errors in analytical information and knock-on effects for process decisions.
The location of the extraction point has to be chosen carefully by considering several factors. The choice of the measurement site is also influenced by the application.
Important discussion points
• Is it necessary to measure short-term / temporary fluctuations in the gas matrix to make process adjustments
• for optimisation?
• Is it really possible to get representative information needed on the gas matrix
• from the extraction/sample point chosen?
• Have the expected mixing processes and all chemical reactions been completed?
If the first question can be answered positively the extraction point has to be near to the process reaction zone. Also, the sample volumes of gas may need to be as large as possible to speed up the analysis. Extraction points after heat-exchangers or gas treatment stages are more suited for measurements where the average concentrations over time are of higher interest than short-term fluctuations. In most cases the gas to be measured will be in a condition suited better to analyze at these locations.
Fundamental questions when choosing the extraction point
Gas extraction after a heat-exchanger, filter, pre-separator or scrubber facilitates the conditioning of a sample. If the extraction point is before these stages, then this conditioning is considered on a smaller-scale in the sampling system design. Protection against heat, dust, weather, and provision of good accessibility are also important.
Consideration of on-going service requirements reduces overall cost of ownership
These fundamentals are also valid for In-Situ measurements.
It is recommended to perform preliminary tests of previously unused sampling points before making a decision on a fixed extraction site. In stacks or exhaust ducts with large cross-sections, differences in gas concentrations can exist of gases due to non-mixing and varying flow rates. Therefore it is necessary to evaluate a proposed location as a sample point by making multiple measurements across the section at different loads.
For extracting gas from a process conveying the gas mixture, an appropriate tapping is necessary. Normally, this will be a pipe connection with a flange, on to which a sample probe will be mounted. This consists of a sample tube, or pre-filter, with an adjoining heated filter. The length of the sample tube determines the appropriate position to extract the gas mixture from the process. This ensures that the extracted gas mixture represents a valid sample from the process – the extracted sample provides a representative mixture from the core of the flow. Flows close to the duct wall, or gas mixtures from a stagnant zone within the pipe connection, are normally unrepresentative and will therefore lead to misleading results / actions from the analysis.
Gas sample probes.
These are available with a vast number of configurable options, which highlights the importance of choosing the correct solution for accurate analysis. Applying important basic knowledge at the design stage minimizes irreversible errors which could prove costly. Gas sample probes are chosen according to the expected process temperatures, corrosion resistance, inert surfaces, and filtration of different particle sizes. The concentration and nature of process dust (wet, dry or sticky) is considered for specification of back-flush (blow-back) function. Also influencing the choice are different environmental conditions like polar frost, marine saline atmospheres or extremely high temperatures. The sampling location, and therefore the gas sample probe, must also comply with the local operating regulations for electrical safety. An important example design choice is the protection for explosive gas or dust.
In general, gas sampling should be carried out at a lower flow rate than the main flow. In doing so, the majority of the particles will remain in the process and will not over-burden the extraction filter system.
Electric heating of gas sample probes prevents the gas from falling below its dew point around the sample filters. Ensuring the temperature stays above both water-vapour and acid dew-points maintains the gas matrix integrity and minimises measurement errors. This also prevents dust and condensate agglomerating, maintaining filter efficiency and reducing maintenance cost.
Choice of heating options depends on the gas matrix. With knowledge of the approximate gas matrix, appropriate heating options for sampling systems can be configured.
Gas conditioning systems have to condition the extracted gas flow such that the analyzer will not be damaged by temperature, pressure, corrosive particles, dust or condensate. Ideally, the gas matrix should not be modified within the gas conditioning process.
As this is not possible in practice, and as gas conditioning processes have to run continuously, any changes to the sample should be controllable and evaluated. A gas conditioning system can include to following steps:
• Removal of solid particles (i.e. dust)
• Removal of humidity from the gas
• Optimising a minimum / maximum flow through the analyzer
• Cooling of a gas sample (to precipitate the condensable parts of the gas)
• Option: to remove corrosive components
• Option: to remove interferents from the gas matrix
All particles, in practice process derived dust, can be removed by filters. A primary filter will be installed within the gas sample probe. After the cooler, additional filters with lower porosity will remove further particles. A large range of universally adaptable filters are available for this function. Filter material can be ceramic, sintered metal or fabric. The variety of the possible materials makes adaptation to all process requirements possible. Partial flow / bypass filters may also be used.
Condensate, i.e. aqueous or acidic, can be removed by the cooling of the sample gas. Gas conditioning coolers, and the choice their configuration and heat exchanging materials, can be selected according to the relevant process conditions. Preferred materials are glass, stainless steel or PVDF. The removal of the condensate can be effected by different disposal systems, i.e. peristaltic pumps, float-traps or separators.
We have tried to summarise some basic information on extractive measurement and gas conditioning in this tutorial. This list is definitely not exhaustive – it is always possible to discuss this subject in much more detail. If questions remain open, please do not hesitate to get in contact with M&C!
It was the aim of this tutorial to provide an informed initial overview of the subject and to focus only on the basic elements. Both types of measurements, In-Situ, as well as extractive gas measurement and conditioning, have their merits in the market, albeit being suited for different requirements and measurement demands.
Since 1985, M&C has successfully tackled the market requirements from different kinds of industries which have become more and more demanding, complex and difficult – living up to the claim of the company:
M&C - Embracing Challenge