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Is it feasible to process slack wax in a delayed coker? If so, what are the potential benefits and operational challenges?

Jul-2024

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SRIDHAR BALAKRISHNAN, Bharat Oman Refinery Limited , laksrid@yahoo.com

It is possible to process slack wax in a delayed coker, though it is uncommon and requires careful evaluation. Slack wax, a by-product from lubricating oil refining with high paraffin and oil content, can be thermally cracked at around above 450 °C to produce lighter hydrocarbons such as naphtha, gas oil, and LPG, along with some petroleum coke. The potential benefits include providing additional feedstock to the coker for increased utilization, generating more valuable light products, and reducing storage or disposal challenges of slack wax. However, operational challenges include its low density and high paraffin content, which may cause poor mixing with heavy coker feed and affect heat transfer; production of soft or low-density coke that can impact handling and quality; increased risk of coking drum foaming and operational instability; and the need for proper feed preheating and blending with heavy residues to ensure stable operation.

Aug-2025
TARIQ MALIK, CITGO, tmalik@citgo.com

The high paraffinic content of the waxy feed may be bad news for the coker heater run length.

Sep-2024
Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

The literature presents some reports about slack wax processing in thermal cracking refining units like delayed coking units. It's important to consider that the composition of slack wax is normally high in paraffins (between 80 to 90 % in mass) with low sulfur content (lower 1,0 % in mass), these characteristics will lead to good yields of light compounds like naphtha with high octane index due to the high olefins content which is expected from thermal cracking processes.
The gasoil fractions tends to concentrate paraffinic molecules which will lead to difficulties to meet cold start requirements of diesel pool considering the current quality requirements, but this can be easily solved through adequate hydrotreating/hydrodewaxing processing units.

In my point of view, the main challenge is related to the design of the delayed coking units which are normally designed considering a balance between the bottom and top yield of the main fractionator. Process highly paraffinic feeds like slack waxes will produce high amount of distillates like naphtha and gasoil fractions which will overload the top systems of the main fractionating tower and the pumping systems, on the other hand the bottom systems will operate probably below the minimum flow rate of pumps, heat exchangers and separation sections of the main column, creating significant operating and reliability issues. My advice is to carry out a blending of slack wax with conventional feeds to the delayed coking units like vacuum residue, asphaltic residue, and decanted oil from FCC in order the balance the benefits and side effects to processing highly paraffinic feeds like slack waxes in delayed coking units.

Jul-2024