Raffinate Oil is produced in solvent extraction units handling vacuum distillates or propane-deasphalted residual stock. Feed is contacted countercurrently with furfural, N-methyl-2-pyrrolidone, or phenol in a rotating-disc contactor or packed extraction column. The extract phase removes aromatics, resinous material, sulfur-bearing molecules, and a portion of the nitrogen load. The raffinate phase is stripped of solvent in a steam or vacuum recovery section and stored as an unfinished base oil intermediate or sold for downstream conversion. Product models are designated by neutral number—70N, 150N, 250N, 500N—where the number approximates Saybolt Universal Seconds at 100°F. Composition is tested under ASTM D2140 for carbon-type distribution and ASTM D3238 for n-d-M analysis. Density is reported under ASTM D4052, sulfur under ASTM D4294, kinematic viscosity under ASTM D445 at 40°C and 100°C, and aniline point under ASTM D611. Relative to aromatic extract from the same unit, raffinate oil has lower aromatic carbon, lower density, lower refractive index, higher aniline point, and lower solvency for polar additives. Relative to a finished API Group I solvent-neutral base oil, the unfinished raffinate retains a higher aromatic and sulfur burden until dewaxing and hydrofinishing are completed.

For specification purposes, Raffinate Oil 150N is certified against kinematic viscosity at 40°C under ASTM D445, flash point under ASTM D92, pour point under ASTM D97, sulfur under ASTM D4294, aromatic carbon under ASTM D2140, and water and sediment under ASTM D96. Flash point is typically above 180°C for light neutral cuts and above 220°C for heavy neutral cuts, but the measured value depends on solvent carryover and cut point. Published data for this specific configuration is limited; therefore, the certificate of analysis is the controlling document.

How does solvent selection and extraction severity define the composition window?

Extraction severity is not a single set point but a combination of solvent-to-oil ratio, temperature profile, solvent water content, rotor speed, and interface level. Furfural units typically operate with extractor top and bottom temperatures in the 65–120°C range; N-methyl-2-pyrrolidone units run near 60–100°C; phenol extraction is carried out between 50°C and 90°C. Solvent-to-oil volume ratios are commonly specified between 1:1 and 3:1, depending on the target aromatic carbon of the raffinate. Increasing the ratio or widening the top-to-bottom temperature gradient removes more aromatics but transfers more solvent into the extract phase, reduces raffinate yield, and can lower extractor throughput.

When the feed is a heavy vacuum distillate or deasphalted oil, the solvent must dissolve enough low-molecular-weight paraffin to avoid precipitation. A high extraction temperature improves paraffin solubility but reduces aromatic selectivity; a low temperature increases selectivity but can form a stable interface rag layer when the feed contains asphaltenes or high-viscosity tail fractions. In a rotating-disc contactor, rotor speed and interface level determine droplet breakage and residence time. Excessive rotor speed raises extractor differential pressure and can produce solvent-oil emulsions that are difficult to break in the settler. The raffinate phase exits the top of the extractor through a settler; residual solvent is removed by steam stripping or vacuum distillation. Batch-to-batch variation in raffinate aromatic carbon is therefore coupled to crude source, vacuum cut point, solvent carryover, and extractor fouling. Published data for this specific configuration is limited unless the refinery certifies the tank after each rundown.

Solvent water content is a separate control. In furfural service, water addition of 1–5 wt% reduces solvent power, increases raffinate yield, and changes selectivity; excess water depresses extraction efficiency and raises steam demand in solvent recovery. In N-methyl-2-pyrrolidone extraction, water content is held in a narrow band because the solvent-water azeotrope affects recovery column operation. Furfural is also sensitive to oxygen at elevated temperature; nitrogen blanketing and antioxidant injection are used to prevent gum formation. Gum formation in the solvent recovery reboiler can reduce heat transfer and elevate pressure drop, which in turn raises residual solvent in the stripped raffinate.

Bulk storage of raffinate oil requires pour-point-based temperature control because the oil is not yet dewaxed. The pour point measured by ASTM D97 can be significantly higher than that of the corresponding finished neutral oil, especially for 250N and heavier cuts. Storage tanks are equipped with steam coils or hot-oil heating and are set above the batch pour point after measurement; water and sediment are limited by ASTM D96 or ASTM D6304, with water content normally below 0.05 vol% before hydrotreater charging. Residual solvent is monitored by gas chromatography against ASTM D7504 or an agreed in-house method because high residual furfural, N-methyl-2-pyrrolidone, or phenol can depress hydrotreating catalyst activity. Trace water in the raffinate can hydrolyze catalyst support in the hydrotreater or generate acid gases; nitrogen blanketing and dry storage reduce moisture pickup. Pump suction losses are calculated from kinematic viscosity at the storage temperature under ASTM D445 and density under ASTM D4052; line tracing is applied when the calculated cold-start viscosity exceeds the pump manufacturer's suction limit. Positive-displacement pumps are selected when viscosity at the minimum storage temperature exceeds 300–500 mm²/s. The oil is incompatible with strong caustic and with condensed water in tank bottoms; water draw-off is scheduled before transfer to prevent water slugs entering the hydrotreater feed furnace.

Process boundaries for dewaxing and hydrofinishing of stored raffinate oil

Raffinate oil is rarely a terminal lubricant. In a solvent-neutral train, the raffinate is charged to methyl ethyl ketone-toluene solvent dewaxing or a catalytic dewaxing unit, then hydrofinished over Ni-Mo or Co-Mo fixed-bed catalysts. The hydroprocessing feed is evaluated for sulfur under ASTM D4294, nitrogen under ASTM D4629, olefins under ASTM D1159, and chlorine under ASTM D4929 because nitrogen bases and polar species compete for active sites and raise reactor temperature. In low-pressure hydrofinishing, nitrogen and sulfur in the feedstock determine catalyst life; the liquid hourly space velocity and hydrogen partial pressure are set by the catalyst supplier's loading model, and the reactor pressure drop is monitored for catalyst plugging from particulate iron scale or solvent degradation products. Catalytic dewaxing can lower pour point to -15°C or below in the finished neutral cut, but the exotherm is controlled by quench-gas injection and reactor bed thermocouple mapping. Solvent dewaxing uses methyl ethyl ketone-toluene mixtures; the solvent ratio and chilling rate are adjusted to the wax content of the raffinate. In both routes, the incompatibility is with residual acidic extraction solvent: furfural-derived gums and N-methyl-2-pyrrolidone oxidation products can foul preheat exchangers and increase pressure drop across the hydrotreater. Published data for this specific configuration is limited; therefore, feed quality is set by unit-specific pilot plant tests rather than a universal specification.

When raffinate oil is compared with Group II and Group III base oils, the difference is not solely aromatic carbon. Group II and Group III stocks are produced by severe hydrocracking or wax isomerization followed by hydrofinishing; they have sulfur below 10 mg/kg under ASTM D4294, lower Noack volatility under ASTM D5800, and viscosity indices generally above 95–120. Raffinate oil retains a higher fraction of sulfur, nitrogen, and aromatics. The retained aromatic content provides moderate solvency, which is relevant in some metalworking fluid concentrates, roll oils, and rubber process oil formulations where additive dropout is a field failure mode. In transformer oil feedstock, however, residual aromatics can reduce oxidation stability; the material must be hydrofinished to meet oxidation stability under IEC 61125 or ASTM D2112, and dielectric dissipation factor under IEC 60247. The selection boundary is not viscosity alone but aniline point, sulfur, and aromatic carbon. A low-aromatic paraffinic raffinate with an aniline point above 90°C is unsuitable for dissolving certain high-polarity additives and may require a naphthenic co-blend, whereas an aromatic extract with an aniline point below 50°C may provide excessive solvency but poor oxidative stability.

When raffinate oil replaces aromatic extract in lower-solvency process oil formulations

Substitution of raffinate oil for aromatic extract in a process oil formulation is not a drop-in viscosity adjustment. The formulator must rebalance solvency, viscosity, volatility, and staining tendency. Aromatic extract from furfural extraction of distillate has aromatic carbon measured by ASTM D2140 in the 35–55 wt% range, aniline point measured by ASTM D611 typically below 50°C, and density measured by ASTM D4052 above 0.960 g/cm³. Raffinate oil for the same cut has aromatic carbon 8–18 wt%, aniline point 85–105°C, and density 0.865–0.885 g/cm³. The solubility parameter shifts upward, and the oil can reject polar additives or cause resin separation in rubber. For a rubber extender oil, the critical controls are viscosity under ASTM D445, refractive index under ASTM D1747, and aniline point under ASTM D611; a raffinate with high paraffinicity may reduce tensile strength retention after heat aging. In controlled mixing trials on a production Banbury internal mixer, the substitution may require increased mixing energy or addition of a compatibility agent because the lower aromatic phase cannot hold the same resin load. Published data for this specific configuration is limited, and the substitution limit is established by the compounder's response to the ASTM D3192 or ISO 2322 formulation matrix.

For regulated rubber applications, raffinate oil must be evaluated under IP 346 or EN 16143 for polycyclic aromatic hydrocarbon content. The measured polycyclic aromatic hydrocarbon content depends on crude origin and extraction severity; a raffinate with low aromatic carbon may still fail a regulatory limit if the remaining aromatics include heavy polycyclic aromatics. Therefore, the substitution cannot be approved on the basis of viscosity and aniline point alone. In the European Union, the substance or mixture may require evaluation under EC 1907/2006 and any relevant downstream user exposure scenario. Compared with treated distillate aromatic extract, residual aromatic extract, and mild extraction solvate, Raffinate Oil has a lower polycyclic aromatic burden and lower solvency. It differs from a Group I base oil because it is not dewaxed; it differs from a naphthenic process oil because its paraffin content is higher, giving a higher aniline point and lower gas absorption in rubber. The operational boundary is not the raffinate oil specification alone but the batch-to-batch variation in aromatic carbon and sulfur; a narrower refinery cut with a certificate of analysis from a REACH-registered source is preferable when the product is used in regulated elastomers.