In-situ process gas analysis

Process gas analyzers are used for continuous determination of the concentrations of one or more gases in a gas mixture. Determination of the concentration of gases in a process is used to control and monitor process flows, and is therefore decisive for the automation and optimization of processes and ensuring product quality. In addition, process gas analyzers are used to check emissions, thus making an important contribution to environmental protection, as well as for ensuring compliance with statutory directives.

In-situ analytical procedures feature physical measurements in the flow of process gas directly in the actual process gas line. In contrast to extractive gas analysis, a sample is not taken and routed on to the analyzer via a sample line and sample preparation. Only in exceptional cases, the process conditions make it necessary to condition the sample gas stream in a bypass line with respect to process temperature, pressure and/or optical path length. Further conditioning of the process gas, such as drying or dust precipitation, is unnecessary. The analyzer carrying out in-situ measurements must always take into account changing process conditions (if these occur) and be able to automatically process them in the calibration model. Computed temperature and pressure compensation is frequently required for this. In addition, the analyzer must be extremely rugged since its sensors have direct contact with the process gas. The fast and non-contact measurement of gas concentrations directly in the process is the domain of in-situ diode laser gas analyzers.

The gas analyzer LDS 6 combines the compact and service-friendly design, simple operation and network capability of the Series 6 analyzers with the well-known exceptional performance data of in-situ gas analysis - namely high ruggedness and availability as well as low maintenance - by using diode laser technology and fiber-optics. Up to three CD 6 in-situ cross-duct sensors (which are also optionally available in an intrinsically-safe version for operation in hazardous areas) can be combined with an LDS 6 analyzer in the compact 19" rack unit enclosure. The distance between the analyzer’s control unit - typically in an existing instrument room or the process plant’s control room - and the max. three measuring points can be up to 700 m in each case.

The SITRANS SL gas analyzer for highly sensitive measurement of oxygen and carbon monoxide has a more integrated design without fiber-optic cables and with only one pair of cross-ducts sensors - a transmitter unit and a detector unit. In this case the receiver has a local user interface (LUI) which is controlled using IR remote control.

A maintenance-free reference gas cell integrated in both analyzers drastically reduces the need for recalibration (SITRANS SL) or even makes its superfluous (LDS 6). Remote scanning and diagnostics of the analyzers is possible using the Ethernet interface present as standard.

The list of gas components measurable using NIR diode laser technology already comprises:

• For the LDS 6 analyzer:
  O₂, NH₃, HCl, HF, H₂O, CO, CO₂, ...
• For the SITRANS SL analyzer: O₂, CO

The list is being permanently extended as laser technology is developed further. The LDS 6 O₂ analyzers additionally allow simultaneous non-contact determination of high process gas temperatures.

Gas measurements with diode lasers feature exceptional selectivity and flexibility. Neither high process temperatures nor high and varying concentrations of particles in the gas have an influence on the quality of the result within wide ranges. For example, it is possible with the LDS 6 to determine trace concentrations of NH₃, HCl or HF directly in moist process gases even before any gas purification stage.

These features together with fast measurements free of dead times mean that diode laser gas analysis with the LDS 6 or the SITRANS SL is an extremely interesting alternative to established extractive analyses.