Why does diesel hydrogenation lower the freezing point?
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Why does the freezing point of diesel decrease after hydrogenation, and what is the principle behind this?
The freezing point is an important quality indicator for diesel. In winter, or when the air temperature drops to a certain level, the wax crystals that form in diesel cause it to lose its fluidity, posing difficulties in its use, storage, and transportation. For crude oils with high wax content, dewaxing is often required during the production process in order to obtain diesel whose freezing point meets the specified requirements; generally, the lower the boiling range of the diesel, the lower its freezing point.
It’s just that hydrogenation converts unsaturated alkanes into saturated ones, so the freezing point should increase. Is it hydrogen isomerization or hydrocracking that is being carried out?
This is mainly related to the packing scheme of the catalyst in your reactor. Nowadays, many diesel hydrogenation units install a certain amount of hydrocracking catalyst in the last reactor; this catalyst selectively cracks high-freezing-point components such as alkanes, alkanes with short side chains, and cycloalkanes with long side chains into smaller molecules during the oil processing process, thereby reducing the freezing point of the oil. The remaining large-molecule isomeric alkanes, cycloalkanes, and aromatics do not undergo any reaction since they cannot enter the pores of the catalyst. The long and narrow paraffin molecules in diesel fractions can easily enter the micropores of the dewaxing catalyst where they are cracked; therefore, the hydrodewaxing process is also known as catalytic dewaxing. The advantage of this approach is that the hydrogenation saturation reaction takes place in the first few reactors, while the hydrodesulfurization reaction is carried out in the last reactor; some modification reactions may also occur. This allows for ensuring the proper degree of hydrogenation saturation of the diesel, reducing its freezing point, and thereby improving its quality. The above analyses are mainly applicable to diesel derived from petroleum; diesel derived from coal, especially that with a high content of aromatics, basically does not suffer from this problem.
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