Solvent extraction used in petroleum refining is typically used in refining or upgrading various petroleum distillates and deasphalted oil. The presence of aromatic fractions is often undesirable, because such compounds often tend to oxidate or thermally degrade. With respect to diesel and other fuels, government regulations may limit the presence of aromatics. Aromatics also have poor viscometric properties, which is particularly important with respect to the production of lubricating or lube oils. For lube oils, the property of the lube oils that are most often used to indicate lube quality with regard to aromatics is the viscosity index (VI). Oils with high VI (95 or greater) are generally considered acceptable. Oils with a VI below 95 are usually considered inferior. Extracting the aromatics from these oils increases the VI of the oil. As presented herein, viscosity indices are determined pursuant to ASTM D2270.
Typical solvent extraction processes used in the refining of petroleum products and distillates utilize highly polar solvents. These solvents may include such things as phenol, furfural and NMP (N-methyl-pyrolidone), with NMP being the most recently developed solvent system presently in use for removing aromatic compounds. These solvents are highly selective for aromatics and various polar-compounds, but are less selective for saturated hydrocarbons, such as paraffins and cycloparaffins. The aromatic products removed during extraction can be used in fuels production or in specialized applications requiring high aromaticity.
The prior art solvent extraction techniques are usually carried out in a continuous flow process in which the solvent and petroleum product feed stream are maintained in the liquid phase and in countercurrent contact. The solvent is typically recovered, with the aromatics being removed, and the solvent is recycled back into the solvent feed stream. The solvent extraction is usually carried out at elevated temperatures that are well above ambient. Typically, these temperatures are from about 100° F. to 250° F. The elevated temperatures facilitate the flow of the petroleum products, which may contain wax, as well as increase the solubility of the aromatics in the solvent. At these elevated temperatures, however, saturates (i.e. paraffins and cycloparaffins), which may be either oils and/or waxes, may also be extracted by the solvent, resulting in lower yields of these products.
Crude petroleum and partially refined petroleum commonly contain waxes (usually paraffin waxes). These waxes crystallize or solidify at cooler temperatures. This is particularly notable with higher molecular weight n-paraffins, certain branched or iso-paraffins, and cycloparaffins. When petroleum is being refined for use as a lubricating oil, the presence of these materials, which crystallize within the range of temperatures for which the lubricating oils are used, is very deleterious. Thus, these materials are commonly removed in the refining process, which is oftentimes referred to as “dewaxing.” Therefore, after extraction, dewaxing of the petroleum products is usually carried out to improve the oil's low temperature properties.
While conventional solvent refining or extraction techniques may be adequate for many applications, improvements are needed. In particular, extraction techniques that require less energy and processing equipment, and that result in higher purity and greater yields is highly desirous.