PTQ Q3 2025 Issue

Silicon trap unleashed for coker naphtha processing

Coker naphtha catalysts have been enhanced to interact with silicon contaminants and catalyst surfaces, leading to higher HDS and HDN removal

Xavier E. Ruiz Maldonado and Christian Frederik Weise Topsoe

O rganic silicon is recognised as a major contaminant in the refining industry, adversely affecting cata - lytic processes such as hydroprocessing, catalytic reforming, and isomerisation. In hydroprocessing units, processing coker-derived feed streams (especially coker naphtha) represents a formidable challenge due to the high silicon content of these streams, which can vary in concentration from a few wtppm to sev - eral dozen wtppm. Silicon concentration is highly depend - ent on how the anti-foaming agent, polydimethylsiloxane (PDMS), breaks down in the coker and the subsequent dis - tribution into the various fractions upstream of the hydro - treater unit. PDMS is an anti-foaming agent with six- and eight-membered cyclic PDMS molecules (see Figure 1 ). PDMS is widely used in delayed cokers, visbreaking units, pyrolysis, and distillation processes. Upon thermal exposure, PDMS breaks down into smaller cyclic siloxane fragments of various sizes. These fragments deposit on the catalyst surfaces and consequently decrease catalytic activity (see Figure 2 ). In the delayed coker, PDMS (Figure 2) thermally cracks into smaller cyclic-siloxane fragments of various sizes,

ranging from 7-12 Å, depending on their boiling point. Some examples of these molecules are shown in the fol - lowing discussion. Each ‘D’ unit in the previously noted Figure 1 represents one (CH₃)₂SiO group and measures approximately 1.64 Å. This unit serves as a fundamental building block for silox - ane molecules, allowing researchers to define the overall molecular dimensions in a standardised way. By express - ing a molecule’s size in multiples of ‘D’ units, it becomes possible to predict its diffusion behaviour, adsorption char - acteristics, and interaction with the catalyst surfaces. This precise measurement is crucial for designing catalysts with optimised pore sizes and surface properties that are specif - ically tailored to effectively trap these siloxane fragments. Inorganic silicon predominantly originates from fluid cata - lytic cracking (FCC) fines and mineral clays present in crudes. Catalyst fines generated from FCC units are of particular concern because their fine particulate nature makes them difficult to remove completely during the cracking process. Furthermore, crudes derived from oil sands contribute silicon-rich sand particles to the feedstock, further com - plicating separation processes. Suspended solids are also encountered, arising from the degradation or agglomera - tion of contaminants during storage and pipeline transport. The accumulation of these inorganic particles leads to a build-up of particulates within reactor beds, which in turn results in an increased pressure drop across hydrotreating units and can hinder process efficiency. A summary of organic and inorganic silica sources can be found in Table 1 . Understanding the distinct origins and characteristics of these organic and inorganic silicon sources is essential for developing optimal loading strategies.

Siloxane type

D-size

Boiling point

D3 siloxane

273˚F (133˚C) typically in light naphtha

Hexamethylcylotrisiloxane

D 4 siloxane

349˚F (176˚C) typically in heavy naphtha

O n

Octamethylcyclotetrasiloxane

Figure 1 Siloxane typical properties

Figure 2 Representation of PDMS

PTQ Q3 2025