What is Pyrolysis gasoline or «Pygas»?

Pyrolysis gasoline, also known as Pygas, is a byproduct of naphtha cracking during the production of ethylene and propylene. Pyrolysis gasoline is an easily flammable, colorless liquid with high aromatic contents and represents a mixture of light hydrocarbons (Figure 1). It is a high octane number mixture which contains aromatics, olefins, and paraffins ranging from C5 to C12.

Because of its high octane number, Pygas has a high potential for blending in various end-user products.  In addition, pyrolysis gasoline can be used as a component separator for benzene, toluene, and xylene. For this purpose, it is used as a component separation additive.

Pygas contains some undesired conjugated diolefins that when present in high quantities makes them unsuitable as a motor fuel. These conjugated diolefins are highly reactive to polymerization and can plug the downstream refining processes causing unwanted shutdowns and high costs for remediation. These compounds also affect the stability of commercial gasoline. Therefore, the conjugated diolefins content must be controlled.

The content of conjugated diolefins is indirectly measured as the «maleic anhydride value» (MAV), or as the «diene value» (DV). This parameter is usually determined by the Diels-Alder wet chemical method (UOP 326). Furthermore, the determination of Bromine Number (ASTM D1159) in pygas is useful as it indicates the degree of aliphatic unsaturation. These determination methods require several hours and must be analyzed by highly trained analysts. In contrast to using primary methods, near-infrared spectroscopy (NIRS) is a cost-efficient and fast alternative solution for the determination of MAV or DV and Bromine Number in pyrolysis gasoline.

Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of pygas

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, an available solution for the determination of maleic anhydride value (MAV) or diene value (DV) and Bromine number are outlined which have been developed according to the NIRS implementation guidelines of ASTM E1655 (method development), ASTM D6122 (method validation), and ASTM D8340 (results validation).

Did you miss the first parts in this series about NIRS as a QC tool for the petrochemical industry? Find them all below!

Analysis of Diene Value (DV) and Bromine Number (BN) in pygas with the DS2500 Liquid Analyzer

Historically, NIRS analysis of the diene value and Bromine Number in pygas has been considered to be complicated due to the presence of other non-conjugated dienes as well as alkenes that have similar molecular functional groups. In addition, the majority of the samples are a complex mixture of aromatics and alkanes that varies with process conditions in the ethylene production process, as well with as the different feedstocks used to produce ethylene (e.g., alkanes, naphtha, or gas oil). Also, DV is not reliant on one specific conjugated diolefin, but over a dozen different compounds including cyclopentadiene (a ring structure) and straight-chain diolefins with different chain lengths and side chains. As explained earlier, the diene value is usually determined by the Diels-Alder wet chemical method (UOP 326). Bromine Number (BN) is determined by electrochemical titration at 5 °C (ASTM D1159).

Now, spectroscopic analysis of these parameters in such a complicated system is made successful through a combination of stable NIRS measurements with the DS2500 Liquid Analyzer, and the Partial Least-Squares (PLS) modelling capabilities in the Vision Air complete software package.

Metrohm offers a related application note for the proper use of NIRS for pyrolysis gas analysis (Table 1).

Table 1. Metrohm’s NIRS solutions for pygas including application details and benefits.

Application example: determination of DV and BN in pygas using the NIRS DS2500 Liquid Analyzer

The diene value and Bromine Number are key parameters for the quality control of pygas.  According to UOP 326, the maleic anhydride is refluxed with the sample in boiling toluene for three hours. Any unreacted maleic anhydride is hydrolyzed to maleic acid, extracted from the reaction mixture, and then titrated with sodium hydroxide. This wet chemical method requires several hours to perform by highly trained analysts. 

For the determination of the Bromine Number according to ASTM D1159, the sample must be cooled down below 5 °C to minimize side reactions like oxidation or substitution.

In contrast to primary methods, near-infrared spectroscopy (NIRS) is a cost-efficient and fast analytical solution for the determination of DV and BN in pyrolysis gasoline.

Figure 2. Quality control of pygas as performed by the Metrohm NIRS DS2500 Liquid Analyzer.

The obtained Vis-NIR spectra (Figure 2) were used to create a prediction model for the determination of DV and BN. The quality of the prediction model was evaluated using a correlation diagram, which displays the correlation between the Vis-NIR prediction and primary method value. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figure 3).

Figure 3. Correlation plots and figures of merit (FOM) for DV and Bromine Number in pygas.

This solution demonstrates that NIR spectroscopy is excellently suited for the analysis of diene value or maleic anhydride value in pygas in less than one minute without sample preparation or using any chemical reagents. In comparison to the wet chemical method in UOP 326, the time to result is a major advantage of using NIRS since a single measurement is performed within one minute instead of taking 6–7 hours with the primary method. Additionally, Bromine Number is easy to measure with NIRS without requiring any chemicals or sample preparation (such as cooling) as mentioned in ASTM D1159.

Want to learn more? Download our free Application Notes.

Summary

Near-infrared spectroscopy is an excellent choice for measuring MV / DV and BN in pygas. A simple feasibility study using your own pyrolysis gasoline samples will quickly indicate if the NIR spectra will be able to be correlated to the primary method values.  Positive aspects of using NIRS as an alternative technology are the short time to result (less than one minute), no chemicals or other expensive equipment needed, and ease of handling so that even shift workers and non-chemists can perform these analyses in a safe manner.

•  Lubricants
• ASTM Norms