Diolefins and their impact on the stability of FCCU naphtha

Gasoline stability is a critical factor for maintaining product quality during storage and use. It is primarily affected by oxidation and polymerization reactions that lead to the formation of gums and undesirable deposits ^[1], particularly under exposure to oxygen, elevated temperatures, and prolonged storage conditions.

Commercial gasoline is produced by blending different naphtha streams, among which FCCU (fluidized catalytic cracking unit) naphtha plays a key role. This stream is characterized by relatively high concentrations of olefins and, in particular, diolefins (dienes), highly reactive compounds that act as precursors for gum formation ^[2].

To meet product specifications, especially sulfur limits, FCCU naphtha is subjected to hydrotreatment. In this context, diolefins are particularly critical due to their direct impact on the catalytic performance of hydrodesulfurization (HDS) units, where they promote coke and deposit formation within the catalyst bed, ultimately reducing catalyst life ^[2].

1. Main factors influencing diolefin formation in FCCU

Diolefin formation can occur in different sections of the FCCU process, predominantly through thermal reactions ^[1]:

• High temperatures (reaction zone and regenerator dense phase)
  • More severe operating conditions, such as elevated temperatures and lower catalyst-to-oil ratios, favor thermal cracking and, consequently, diene formation ^[2];
  • In this context, the increase in operational severity in the FCCU, often adopted to improve the conversion of poorer quality feeds or to maximize the production of light olefins, intensifies the formation of these compounds, negatively impacting naphtha stability ^[1].

• Non-catalytic thermal reactions - Post-riser (disengager vessel and stripper) ^[1]
  • Diene formation via thermal cracking is intensified in high-temperature regions where catalyst is absent, such as the disengager vessel and, to a lesser extent, the stripper;
  • Although dienes are highly reactive compounds, their conversion depends on the presence of a catalyst. In their absence, these compounds are not consumed and are instead carried over into the product, increasing their concentration in the naphtha ^[1];
  • These post-riser thermal reactions are among the primary contributors to diolefin formation in FCCU naphtha.

• Feed composition:
  • Feed composition is generally more influential on diolefin formation than physicochemical properties such as boiling range or Conradson carbon residue (CCR) content ^[1];
  • Even lighter feeds can exhibit lower conversion and poorer naphtha stability when they have a higher degree of aromaticity and higher levels of basic nitrogen, leading to increased diolefin formation. On the other hand, heavier feeds, even if they have higher density or carbon residue, can result in better naphtha stability when they are more paraffinic and contain lower levels of contaminants (Table 1) ^[1]:

^{Table 1: Influence of feed composition on diolefin formation in FCCU naphtha
Source: Adapted from GILBERT (2004)}

• Inefficient catalyst regeneration:
  • Inadequate catalyst regeneration can contribute to increased diolefin formation ^[6];
  • This behavior has been observed in studies involving high-activity catalysts operating at higher delta-coke levels, which impose limitations on effective regeneration under unit operating conditions. As a consequence, there is a reduction in operational flexibility, especially in the processing of heavier or residual feeds, as well as a greater tendency for diene formation through thermal cracking ^[6].

• Butane addition to FCCU naphtha
  • The incorporation of butanes into FCCU naphtha can increase the presence of diolefins in the naphtha ^[2]. In addition, higher vapor pressure may cause sudden vaporization in the HDS catalyst bed ^[3], leading to a decrease in selective hydrogenation temperature and reduced conversion efficiency. Consequently, diolefin concentration in the primary reaction zone increases, promoting gum formation and potentially shortening unit run length ^[3];
  • Therefore, controlling the Reid Vapor Pressure (RVP) of FCCU naphtha is essential.

2. Diolefin monitoring

Brazilian regulatory agencies define gasoline specifications directly related to diene content, as summarized in Table 2:

^{Table 2: Gasoline quality parameters associated with stability and gum formation according to ANP Resolution No. 807/2020 [4]}

ASTM D1319 does not directly quantify diolefins, as these compounds are included within the total olefin fraction. Therefore, conjugated diene content is typically estimated using:

• Maleic Anhydride Value (MAV): parameter used to estimate the content of conjugated dienes, determined by the chemical method based on the Diels-Alder reaction (UOP 326) ^[5]:

3. Control strategies

• Severity adjustment
  • Lower temperatures in the reaction zone, stripper, and disengager vessel reduce thermal reactions responsible for diolefin formation;

• Quench:
  • Injection of quench streams in the post-riser region, particularly in the disengager vessel, reduces temperature and suppresses thermal cracking reactions that generate diolefins ^[3];
  • This strategy is especially important under high-severity operating conditions ^[3].

• Catalyst selection:
  • Rare earth content
    • Increasing rare earth content enhances catalyst acidity and activity, promoting hydrogen transfer reactions. This results in higher feed conversion and reduced olefin content in FCCU naphtha, improving stability;
    • However, lower olefin content may negatively affect gasoline octane number. This effect can be mitigated in more aromatic feeds, where cracking preferentially occurs in alkyl side chains, forming lighter aromatics and isoparaffins that help preserve octane ^[7];
    • Increased catalyst activity may also raise delta-coke levels, imposing operational constraints. Therefore, catalyst selection requires balancing activity, stability, and operational limits.
  • ZSM-5 additives
    • ZSM-5-based additives promote selective cracking of heavier olefins, increasing light olefin production (C3–C4) while reducing reactive compounds in the gasoline range, thereby contributing to improved naphtha stability.

4. Summary

The main impacts of diolefins on naphtha quality and downstream operation are summarized in Table 3.

^{Table 3: Diolefins in cracked naphtha: impacts and control strategies}

Final considerations

FCCU naphtha stability is strongly influenced by the presence of highly reactive compounds, particularly diolefins, which are primarily formed in post-riser regions and are affected by process thermal conditions and feedstock composition.

More severe operating conditions, combined with feeds containing higher levels of contaminants and lower paraffinic character, tend to favor the formation of these unsaturated compounds, which act as gum precursors and negatively impact both naphtha quality and HDS unit performance.

Effective control of diolefins in FCCU operation is essential to prevent catalyst deactivation, increased pressure drop, and reduced run length in HDS units. Consequently, the operational reliability of these units is critical, as failures or unplanned shutdowns can directly impact FCCU performance as well and, in more severe cases, the overall refinery operation.