In refineries, high-octane products are desired since they are used to produce premium gasoline. This production is a highly hazardous operation which requires strict adherence to safety standards (IECEx) and constant monitoring of key process parameters such as the octane number (ON). By providing dependable process data in a timely manner, downstream process units (catalytic reformer) can be optimized quickly, increasing profits while lowering operational costs.

This Process Application Note presents a way to closely monitor the octane number in fuels in «real-time». Near-infrared spectroscopy (NIRS) technology, which fits well within the international standards (American Society for Testing Materials «ASTM»), makes this possible.  This technique offers simple, fast, and reliable online analysis of the octane number, allowing quick adjustments to the process for a better-quality product and higher profitability.

The octane number (ON) is a key parameter measured in the petrochemical refining process which indicates the performance of commercial fuels (e.g., gasoline and jet fuels). It determines the tendency of the fuel to resist auto-igniting in the engine during combustion (knock resistance).

The ON is measured based on the knocking resistance of two reference fuels: iso-octane (C₈H₁₈) and n-heptane (C₇H₁₆). Iso-octane has a high resistance to knocking under harsh conditions and is therefore assigned an ON of 100. Conversely, n-heptane has a low resistance to auto-igniting, thus it is assigned an ON of 0.

There are two main categories of octane numbers, since the knocking resistance varies based on the operating conditions: research octane number (RON) and motor octane number (MON). The RON is measured under lower temperatures and speeds, and the MON is measured under high temperatures and speeds.

Premium gasoline requires high-octane ingredients. The refining process which produces these high-octane products is called catalytic reforming
(Figure 1). 

Catalytic reforming converts heavy naphtha (a paraffin mixture with low octane rating) into a high-octane liquid product called «reformate» (a mixture of aromatics and iso-paraffins C7 to C10). Therefore, catalytic reforming has a significant impact on the refinery profitability.

The octane numbers of the produced reformate must be constantly monitored to ensure high throughput along the refining process. Traditionally, RON values can be measured by two different methodologies: inferred octane models (IOM) and laboratory octane engine analysis. These methodologies do not provide «real-time» results and require constant maintenance and human intervention to adapt to current operation conditions.

Furthermore, calibration of the octane engine for RON values greater than 100 (a common value for reformate) requires specific blends. These calibrations are not always available. Indeed, refineries often use octane engines to analyze and qualify final blended products (gasoline), with RON between 92–98.

After samples are preconditioned, NIR measurements are performed in a flow-through cell. The instruments used in refineries are ATEX or Class 1 Div 1/2 certified.

Instruments are either mounted in the refinery where they will require positive air pressure, or in a pressurized shelter. The distance between the instrument/shelter and the sample points can be hundreds of meters.

Every 30 seconds, RON and MON values are transmitted to the programmable logic controller (PLC) or distributed control system (DCS) depending on the communication protocol used.