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



  • Paul Mason, PTQ, sales@petroleumtechnology.com



  • 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. 


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