Extraction-Hydroconversion

An important aspect of this process is the use of raffinate feeds that are under-extracted relative to the raffinates required to meet typical Group I basestock properties using extraction only. In the combined process this lower severity mode takes best advantage of the extraction step because a higher percentage of the species removed into the extract phase are clustered aromatic ring stmctures with few aliphatic characteristics. In other words, the species that have the least potential to be converted into more highly paraffinic stmctures in the subsequent hydroconversion step are removed more selectively when the extraction treat is mild. 

The combination of extraction and hydroprocessing is a very efficient route to basestocks needed for GF-3 quality. Extraction alone is inappropriate because of an inability to selectively remove multiring naphthenes (these tend to be split evenly between the raffinate and extract phases). Yields by extraction to the same basestock property levels may be less than half of that achieved by RHC . Also, VGO hydrocracking, i.e. with no pre-extraction step, requires more severe conditions and has potentially lower yields than RHC because of the higher conversion needed to offset the highly negative VI characteristics of refiiactory multi-ring species present in the distillate feed. 

A further embellishment to this process results from combining extraction and hydroconversion with a selective hydrodewaxing step, as discussed above. 

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Raffinate hydroconversion (RHC), which can be viewed as a drop-in process upgrade for an existing solvent extraction plant. 

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