PTQ Q3 2025 Issue

Silicon sources

Organic

Inorganic

Origin

Description

Origin

Description

Antifoaming agents e.g, (polydimethylsiloxane,

Frequently used in delayed cokers, visbreaking units,

Catalyst fines from

Fine particulates generated during catalytic cracking

FCC units

PDMS)

pyrolysis, and distillation processes

processes

Flow improvers, drilling

Silicon-based surfactant, lubricant

Clay

Naturally in crude oils, especially heavy or extra-heavy crude (Canada, Venezuela) broken clay filters

fluids additives

Emulsion breakers

Utilised primarily in desalting units or crude stabilisation processes,

Oil sands

Silicon-rich sand particles

(silicone-based demulsifiers)

in crude feedstocks

especially for heavy oils

General suspended solids

Particulates present due to degradation or agglomeration of contaminants during storage and pipeline transport

Table 1

Detection of silicon types Designing silicon guards for naphtha units requires a thor- ough understanding of the sources of silicon contaminants to avoid under-designing or over-designing the protective layers. A realistic average silicon concentration in the feed stream is essential for a proper design. Analytical techniques are critical in differentiating between inorganic silicon and organic silicon. For instance, gas chromatography-atomic emission Detection (GC-AED) can selectively reveal organic silicon with a typical detec- tion limit of 0.1 weight parts per million (wt ppm). On the other hand, inductively coupled plasma–mass spectrom- etry (ICP-MS) and inductively coupled plasma–optical emission spectroscopy (ICP-EOS) are used to determine the total silicon content, which includes both organic and inorganic species. The difference between the total sili- con measurement and the organic silicon value obtained by GC-AED can be used to calculate the inorganic silicon content indirectly. This distinction is particularly significant because organic and inorganic silicon compounds behave differently during the refining process. Organic silicon, often originating from additives like anti-foaming agents (such as polydimethyl- siloxane), can decompose into cyclic siloxanes that deposit on catalytic surfaces. In contrast, inorganic silicon usually derives from mineral sources present in crude oil. The sil- icon guard catalyst needs to be optimised to handle both

types of silicon to ensure reliable and predictable operation of the coker naphtha processing units. In summary, the integration of advanced analytical meth- ods such as GC-AED, ICP-MS, and ICP-EOS forms the foundation for designing effective silicon guards. By accu- rately quantifying both organic and inorganic silicon, these techniques enable engineers to develop targeted solutions that enhance process reliability, extend catalyst life, and improve overall refining efficiency. Mechanisms of silicon deposition The mechanism for silicon poisoning of hydrotreating cata- lysts is generally accepted to be the reaction of PDMS with the surface -OH groups of the alumina present in the carrier of hydrotreating catalysts. The irreversible formation of a Si-O bond instead of a Si-C bond is the driving force (see Figure 3 ). 1 To better manage silicon poisoning, a tailor-made guard is utilised to trap the maximum amount of silicon. As refin - ers are limited by the volume of the reactor, a maximum surface area per reactor volume is desired; in other words, the grading is aimed at having the highest possible acces- sible surface area per volume. Silicon pick-up Silicon in the catalyst carrier does not contribute to sili- con pick-up. Alumina serves as an effective surface area

OH OH OH

O O O O

A l O

4 CH

Figure 3 Silicon deposition mechanism

PTQ Q3 2025