Solvent fractionation is a process used is the deasphalting process that separates mainly vacuum distillation residue (VDR) by dissolving it into various solvents. The initial step is to place the typically solid VDR in and aromatic solvent such as benzene or toluene which it is soluble in. After this is done an alkane solvent such as n-heptane is added to the mixture which will cause some compounds to become insoluble and precipitate out. Precipitation occurs at this step because the heptane disrupts the gradient solubility of the fractions within the benzene or toluene solvent. As the concentration of non benzene or toluene solvents increase the gradient solubility will deteriorate further. The compounds that initially drop out will be the heaviest molecular weight compounds, known as asphaltenes, of any that come out however they will be the smallest fraction that exits the mixture. There will also be fractions that will not drop out which are called maltenes. To further remove maltenes from the mixture the process can be continually repeated using lighter and lighter alkane solvents such as pentane or propane. These further steps will cause greater amounts of asphalt to precipitate out with the understanding that as more asphalt is precipitated out it will become lighter. This will balance out the heavy fractions that came out initially and lightening the weight of the asphalt overall.

The reason that aromatic solvents are even capable of dissolving the VDR is explained by the Hildebrand Solubility Parameters which measure the strength or power of solvents. The parameters are based on a combination of surface tension and molar volume or latent heat of vaporization and molar volume. As general rules go, the higher the density of a compound (which would mean a lower molar volume) and either higher surface tension or higher latent heat of vaporization will cause a solvent to be more powerful. This explains why aromatic compounds with high density, high surface tension, and latent heat of vaporization can dissolve most compounds while propane is a much poorer solvent.

In a distillation process such as vacuum distillation what forms at room temperatures is a unit contained of quantities of wax. The removal of this wax is necessary for the base stock to have desired low temperature properties. ¹ Two processes to separate the wax from the waxy petroleum fraction is solvent dewaxing and catalytic dewaxing processes. These processes help to prevent corrosion, protect catalyst in subsequent processes and to improve finished products by removing unsaturated, aromatic hydrocarbons from lubricant and grease stocks.² In solvent dewaxing a solvent such as toluene or methyl ethyl ketone are used.² These agents dissolves little wax at low temperatures and acts as a wax predicating agent. When the solvents are mixed with the product stream, the waxy oil and solvent are chilled.² A filter is then used which removes the predicated oil.² This process is much different however to the additional dewaxing method of catalytic dewaxing. This process is completed by selectively hydrodewaxing paraffinic wax contained in liquid petroleum.³ This process starts with a first serial catalyst bed under adiabatic cracking temperatures conditions, while controlling exothermal heat of reation.³ This then produced lighter olefin components, recovering partially hydrodewaxed liquid petroleum from a bottom portion of the first serial catalyst bed.³ The partially hydrocracked liquid petroleum is then further reacted to effect endothermic hydrodewaxing concurrently with exothermic hydrogen transfer causing dewaxing, hydrogenation and cyclization in the presence of hydrogen under adiabatic temperature conditions.³ This finishing process then allows for uniform hydro-dewaxing conditions and the obtaining of a high quality petroleum lubricant product.³ These two processes use very different techniques to finish the product, however complete the same goal of dewaxing the product steam, allows for a more desirable product which can operate under colder temperatures, increasing it usability and range of operation. It also helps to improve a lubricant oil low pour point by decreasing the temperature at which wax forms in the products and by improving its oxidation stability.³

1. http://www.bechtel.com/3868
2. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0CFgQFjAF&url=http%3A%2F%2Fkvbchemicalengg.com%2Fpdf%2FSOLVENT%2520EXTRACTION%2520AND%2520DEWAXING.pdf&ei=SrSlU6e8FISeyATKpoCYCg&usg=AFQjCNHfYhHhEZBls3BktgCc3MynD88vmQ&sig2=d9yAHalTi0rRCLfM9fhFvw&bvm=bv.69411363,d.b2k
3. http://www.google.com/patents/US5246568

Solvent Fractionation is a method of separating petroleum its separate groups of defined liquid, to reach a product of interest, parameters, and need. In a distillation process the incoming feed stock is separated it into different components with respect to defined parameters such as boiling point. ¹ When solvent fractionation is used through deasphalting, the feed is separated into different fractions based on the solubility behavior of the material.¹ This process is often used to separate out vacuum distillation residue or VDR which are present in the feed. This allows for the recovery of asphalt and deasphalted oil from the feed, which then can be catalytically cracked and treated to obtain valuable petroleum products.² This process allows for an improved petroleum treating process, better method for recovery of lubricating oil stocks from petroleum residua, provides a method for the recovery of lubricating oil stocks from a petroleum residuum, and provide a method for improved yield of lubricating oil.² The process behind this technique is solvent extraction. The first step of this process is to have the VDR fed to the deasphalter. The feedstock is then contacted with a solvent in a countercurrent extractor at temperatures and pressures to precipice the asphalt and resin fractions that are not soluble in the solvent.³ This then lead to the final product, with a separated out feed of asphalt. In this process an important concept is the solvent power. This is the ability of the solvent to dissolve asphaltenes.⁴ With non-polar solvents the solvent power can be expressed by the parameter δ which is defined as the ratio between the surface tension and the cubic root of the molar volume.⁴ This parameter allows for the explanation of certain apparent anomalies, such as the insolubility of asphaltenes and complete solubility. ⁵ It also produces an agreement with the derivation of the solubility parameter, for any one series of solvents the relationship between the amount of precipices and the solubility parameter.⁵

1. Course web page
2. http://www.google.com/patents/US3074882
3. http://www.intertek.com/testing/pilot-plants/deasphalting/
4. The Chemistry and Technology of Petroleum  By James G. Speight
5. Petroleum Refining Processes edited by James G. Speight, Baki Ozum