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Process analyzer is an analytical instrument used to measure on-line and in real time chemical composition and/or physical properties of gases and liquids in industrial process streams.^[1]

Process analyzers are commonly used in oil refineries, gas processing facilities and petrochemical plants, where they support process control, maintain product specifications, provide safety and enable process optimization.^[2]

Continuous control of quality parameters is required to ensure that products remain within specification throughout the production process. In refinery operation, various physical properties are evaluated together rather than relying on a single measurement. Typical examples include flash point, distillation curve, API gravity and octane number, all of which are used to verify product quality and support process optimisation.

Process analyzers provide real time measurement of key quality parameters. The data is typically transmitted to distributed control systems, where it is used to monitor and adjust process conditions. This allows process streams to remain within defined specifications during operation, although the degree of control depends on both measurement reliability and sampling conditions.

Process analyzers may be divided into two main categories:

• Conventional analyzers: designed to perform measurements in accordance with standard laboratory methods such as ASTM procedures. These analyzers use techniques comparable to laboratory testing and are intended to produce equivalent results under controlled conditions.
• Correlative analyzers: that determine physical properties indirectly by correlating spectral or physical signals with composition. Techniques include near infrared spectroscopy (NIR), nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. These systems rely on mathematical models and chemometric methods to interpret measured data.^[3]

The performance of process analyzers is generally evaluated by comparison with laboratory reference methods. Standards such as ASTM D3764 provide statistical procedures for assessing repeatability, reproducibility and bias of analyzer systems relative to primary test methods.^[1]

Process analyzers are implemented using different installation approaches depending on process requirements:

• Extractive systems: A representative sample is withdrawn from the process stream, conditioned and transported to the analyzer.
• In-situ systems: Measurement is performed directly within the process stream without sample extraction.

Each approach involves trade-offs related to response time, maintenance and suitability for specific operating conditions. In practice, the selection is often influenced by process conditions such as pressure, temperature and the presence of contaminants.^[4]

Industrial facilities such as refineries and petrochemical plants are often distributed over large areas and include hazardous zones where electrical equipment must meet strict safety requirements.

Remote sensing approaches allow measurement to be carried out close to the process while the analytical equipment is located in a safe area. In such configurations, field units containing measuring probes and sample interfaces are installed near the process stream, while the analyzer is located remotely and connected through fibre optic communication links.

In optical systems, the measuring probe may operate without electrical components or moving parts within the hazardous zone. This arrangement enables continuous measurement of process streams while reducing risks associated with installation in hazardous areas. In large installations, multiple measurement points may be connected to a single analyzer system.

Process analyzers are used in a range of industrial applications including:

• Monitoring composition in refining and chemical processing
• Measuring impurities in gas and liquid streams
• Continuous emissions monitoring systems
• Safety-related input to process control systems

In petroleum operations, on-line analyzers are used to perform continuous monitoring of process variables including product quality, for real-time optimization (RTO) and advance process control (APC).^[5]

• Process control
• Analytical chemistry
• Gas analyzer
• Oxygen analyzer