Question
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How can we improve the cold flow properties in the ULSD from our gasoil hydrotreater?
Mar-2022
Answers
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Chad Perrott, Albemarle Corporation, chad.perrott@albemarle.com
Cold flow properties such as cloud point, pour point, and cold filter plugging point are a function of the diesel paraffin content. Longer carbon chain n-paraffins exhibit poor cold flow properties, and the concentration must be reduced via one or a combination of several solutions to meet desired product specifications.
One avenue is to dilute the n-paraffin content with a lower cloud point material, such as blending kerosene into the diesel pool. Although this is a relatively simple solution, it has the disadvantage of downgrading kerosene to diesel and may impact other diesel properties, such as end point.
Another solution is to adjust the feedstock quality by reducing the end point and removing the high carbon number n-paraffins. This solution has the benefit of potentially improving the performance of the hydrotreating catalyst; however, it also downgrades diesel feed to vacuum gasoil and lowers the yield of diesel products.Finally, cracking or isomerisation dewaxing catalysts enable improvement of cold flow properties without requiring any feed adjustments or blending of other streams. Cracking dewaxing catalysts work by cracking n-paraffins to shorter carbon numbers with lower cold flow properties. Isomerisation dewaxing catalysts isomerise n-paraffins to iso-paraffins, thereby improving cold flow properties while maintaining molecules in the diesel boiling range. When only moderate cold flow improvement is required in trim dewaxing, either catalytic solution can provide comparable performance. However, when high severity cold flow improvement is required, such as the production of winter or arctic diesel, an isomerisation catalyst is the preferred route because cracking catalysts result in high yield loss with the production of naphtha and light ends.
Regardless of the technology selected for catalytic dewaxing, a reduction in treating catalyst volume to make room for dewaxing catalyst is typical. This change can result in higher space velocities and operating severity to ensure dewaxing activity. Higher severities can lead to unit cycle length constraints. The treating section can be rebalanced by using bulk metal catalysts with increased volumetric activity in HDN/HDA reactions to mitigate cycle length concerns when initiating dewaxing operations.
Mar-2022
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