PTQ Q3 2025 Issue

Silicon sizes and typical distribution in naphtha fractions

Siloxane size

7–8 Å

8–10 Å

10–12 Å

Optimal catalysts selection

TK-461/467 SiliconTrap

TK-453 SiliconTrap

10000 12000 16000 14000 Norm.

Light naphtha ‘D’ distribution

0 4000 2000 6000 8000

Min.

Norm.

Heavy naphtha ‘D’ distribution

10000

8000

6000

4000

2000

Min.

Table 3 and Figure 4 (Top) Silicon speciation in light naphtha by AED and Figure 5 (Bottom) Silicon speciation in heavy naphtha by AED

because the intrinsic porosity is functional for silicon cap- ture. In contrast, the inclusion of silicon dioxide (SiO₂) in the catalyst carrier will inflate the measured surface area without contributing to the effective silicon uptake. In other words, the additional surface area measured from SiO₂ is not active for capturing silicon, which can lead to misinter - pretations of the material’s capacity. It is therefore critical to formulate and characterise the catalyst carrier so that the reported surface area reflects only the contribution of alumina, excluding any inert area provided by SiO₂, as demonstrated in Table 2 . Topsoe developed two catalyst support formulations for direct comparative evaluation: one comprising high-purity alumina and the other featuring alumina modified with SiO₂. Both supports were incorporated into a parallel, side- by-side reactor configuration and exposed to an identical silicon-rich feedstock. This induced high silicon loadings in each catalyst under the same operating conditions. Following the test, the spent catalysts were character- ised through X-ray fluorescence (XRF) spectroscopy and Brunauer-Emmett-Teller (BET) surface area analysis. These techniques facilitated a detailed assessment of the com - positional changes resulting from silicon uptake. Notably, although the composite support (Al₂O₃ + SiO₂) displayed a modest 6% increase in measured surface area, its silicon

uptake per unit volume was approximately 30% lower than that of the pure alumina support. This discrepancy is attributed to the inherently less acidic hydroxyl (OH) groups provided by SiO₂, which reduce its ability to retain silicon. These findings underscore the importance of designing support with a truly functional surface area, beyond nomi - nal measurements, to ensure optimal performance, particu - larly when tailored for high-silicon environments. For this reason, catalyst systems have been strategically designed to obtain maximum silicon pick-up by using an optimised in-house alumina formulation while providing robust hydrodenitrification (HDN) and hydrodesulphurisa - tion (HDS) activity. Topsoe coker naphtha catalysts do not contain any silicon in the catalyst carrier, so the silicon found on the catalyst is originating from the feed. Most of the other coker naphtha catalysts in the market today contain silicon native to the

Effect of silicon on actual silica capacity

Al₂O₃ Base

Al₂O₃ + SiO₂ Base *1.06

Surface area, m2/g

Si content from manufacture, wt%

Silicon pick-up per volume

10.1

7.3

Table 2

PTQ Q3 2025