Dewaxing is the required to remove the hydrocarbons that solidify as temperatures decrease. Removing these hydrocarbons lowers the pour point which is a desired characteristic of a fuel because it can continue to function as lower temperatures. Dewaxing can be done in one of two methods. Solvent dewaxing is a physical process of freezing and removing the waxes. Catalytic dewaxing is a chemical process which removes wax by reaction of long chain n-alkanes or wax.

Solvent dewaxing is done by refrigeration of the feedstock after it is mixed with a solvent. The temperature of the refrigeration process depends on the desired pour point of the product. If the desired pour point is very low then the refrigeration will be very low. The wax crystals are separated by a cloth filter. These wax crystals are called slack wax and can be used for making candles, cosmetics and petroleum jelly. The solvents used in this process are methyl ethyl ketone (MEK) and propane.

Catalytic dewaxing utilizes a process to crack the n-paraffins (wax). This method of selective cracking takes place in the zeiolite catalyst. The small pore size of this catalyst wont allow i-paraffins to react. Increasing the concentration of i-paraffins in the fuel lowers the pour point because there are less n-paraffins to freeze at higher temperatures. Advantages of this method over solvent dewaxing include product stability, lower capital investment, and flexibility to produce both lube oil stock and light distillates.

There are currently two types of dewaxing that are being used commercially. The first dewaxing process is called solvent dewaxing. Solvent dewaxing is a physical process that separates the wax by freezing or solvent transport. The other type of dewaxing, Catalytic dewaxing, is a chemical process, which is unlike the physical process used for solvent dewaxing. Catalytic dewaxing uses a chemical process to remove the wax through a reaction of long chain n-alkenes. First the solvent dewaxing process will be discussed. The solvent is cooled until the wax compound freeze to form crystals. The solvent with the frozen wax crystals is carried into a rotary filter where the wax crystals get caught in a filter cloth. The layer of wax on this cloth is then scrapped by a blade and carried away in a solvent stream. The solution then goes through a steam stripping process to recover the solvent. The chemical process of catalytic dewaxing is much different then the process of solvent dewaxing. This process uses a selective cracking processe to “crack” n-alkanes. This cracking process takes place in pores of a catalyst that has openings about 0.6nm in diameter. This keeps out the i-paraffins because of their larger size. Hydrogen is also introduced into this process to prevent coking on the catalyst. Hydrogen prevents disproportionation which in turn prevents coking. This process has a much higher yield then the solvent dewaxing as well as producing a lube base stock with a much lower pour point. The catalytic dewaxing has many advantages over the solvent dewaxing. The chemical process of catalytic dewaxing has a much lower capital investment then that of the physical process of solvent dewaxing. The cracking of n-paraffins that takes place in catalytic dewaxing produces a by product of distillate fuels such as gasoline. Overall catalytic dewaxing is a more advantageous process then solvent dewaxing.

Solvent fractionation is a very important process with in the refinery. It is a key process in readying some products of the refinery for commercial use. Solvent fractionation is a different from distillate in a variety of ways. Distillation achieves fractionation by using differences in boiling points to separate the components of the crude oil. Solvent fractionation uses the solubility or insolubility of molecular components in a solvent to separate the key components. This process is also called deasphalting and uses vacuum distillation residue as it’s feedstock. During the first step of deasphalting the vacuum distillation residue is completely dissolved in aromatic solvents. Examples of these aromatic solids are benzene and toluene. The highest molecular weight component of VDR is called asphaltene and can be separated by precipitation using a light paraffin solvent. This light solvent is mixed with the VDR previously solubilizied in a aromatic solvent. VDR is just a abbreviation for vacuum distillation residue. Maltenes are a portion of VDR that is soluble in the light solvent. This light solvent also defines the characteristics of the separated asphaltenes. Even a lighter solvent, propane, is used to separate n-pentane solubles. This process yields soft resins and oil products. The power of non-polar solvent is measured by the Hildebrand Solubility Parameters, also know as HSP. The 1st Hildebrand parameter is dependent on two variables, surface tension and molar volume of the solvent. The 2nd Hildebrand Parameter is dependent upon energy vaporization and molar volume. Both these parameters are accurate at expressing a dissolving power of a solvent. These parameters can be calculated using the equations given within the notes. Solubility parameters increase with increasing density with surface tension, or increasing latent heat of vaporization. It is for those properties that aromatic solids have higher solvent power then aliphatic hydrocarbons. In review the strength of nonpolar solvents is described by the two hildebrand solubility parameters.

Deasphalting is an alternative to distillation. Just like distillation, deasphalting fractionates different components from a feed stock. Unlike distillation, the fractionation is done by using a solvent extraction process with deasphalting. This process is more often used for vacuum distillation residue.  The highest molecular weight compounds (known as asphaltene,) found in vacuum distillation residue can be separated by precipitation once a light paraffin solvent is mixed with the feed stock in an aromatic solvent like toluene. The portion of the feedstock that is soluble in a paraffin solvent is called maltenes. Maltenes can be separated using paraffin solvents like n-heptane. Using n-pentane solvents, which are lighter and weaker, we can fractionate these compounds further. These are separated into hard resin and n-pentane soluble fractions. Using propane, we can further refine the soluble fractions.  Lighter solvents can be used to further separate the feedstock. This process is important because it allows us to separate products based on demand.

In order to understand how asphaltenes can be separated out of a vacuum distillation residue utilizing the solvent extraction method, we must understand the Hildebrand Solubility Parameters (HSP) for non-polar solvents. The Hildebrand Solubility Parameters measure the strength of the solvent that is used for extraction. There are two definitions of HSP. Each are a function of molar volume, but the first definition has a variable of surface tension, where the second definition has a variable of energy of vaporization.  Using these equations we can explain how a large volume paraffin solvent can disrupt the gradient solubility of asphaltenes to allow for the solid particles precipitate and then filtered.

Dewaxing is a process that is carried out to remove wax, long chain paraffins, from a feedstock for production of candles, cosmetics purposes, petroleum jelly and other lubricating oils. There are to different types of dewaxing, one being solvent based and the other involves catalytic cracking of the compounds. Solvent based dewaxing involves refrigeration of the feedstock after it is mixed with the solvent. Depending on the desired production the temperature is set accordingly to allow for a range of lube oils produced based upon a pour point. The solvent/feedstock mixture are chilled to a certain temperature and ran through a rotary filter to allow for the separation of the wax from the feedstock. There are two main solvents used for solvent dewaxing, propane and methyl ethyl ketone (MEK). In most U.S. refineries methyl ethyl ketone is used over propane for its characteristic changes of the feedstock. MEK has a small difference between filtration temperatures and pour points of dewaxed oils, has a fast chilling rate, and although propane is slightly better still has good filtration rates.

Dewaxing is considered a separation process, but because Catalytic dewaxing involves breaking and making bonds it is usually referred to as a conversion process of n-paraffins.  Catalytic dewaxing is still considered dewaxing because of its ability to remove long chain n-paraffins. Contrary to solvent dewaxing, catalytic dewaxing uses a molecular sieve catalysts with a pore opening size small enough that iso- structures, such as i-paraffins, are unable to go through the sieves. The increase of i-paraffins that this causes helps to lower the pour point of the feedstock. When cracking a molecule it is important to supply hydrogen so the radical chains do not bond to the catalyst surface active sites inhibiting there use, also known as coking. Cracking n-paraffins can lead to the production of distillate fuels as a by-product, such as gasoline. Catalytic dewaxing produces lube base stock with a lower pour point and a in higher yield than that of a product from solvent dewaxing. Both dewaxing processes have low capital investments. Catalytic dewaxing is also useful for the production of both lube oil base stock and light distillates.

Distillation is different from deasphalting in the sense that distillation is a separation by boiling points. Deasphalting is a process by which a solvent is used to fractionate feedstock, atmospheric or vacuum distillation with respect to the solubility/insolubility of molecular components in a given solvent. Vacuum Distillation Residue (VDR) is completely dissolved in aromatic solvents such as toluene. VDR is typically a solid at room temperature so the aromatic solvents are used to create a liquid mixture at which point a light paraffin solvent is mixed with the liquid to precipitate the VDR component asphaltene. The light paraffin used will determine the separated asphaltene from the solubilized product based on what portion of the VDR is soluble or not. The VDR component that is soluble in this light paraffin is referred to as a maltene and is classified with a prefix of the paraffin used such as n-heptane maltenes. This separation process can continue by using the product from each step of VDR component mixed with a lighter and lighter solvent to precipitate an insoluble product. This process allows for specific separated products depending on what is needed. This procedure can be carried out to start with a low H/C ratio and eventually make a high H/C ratio by-product, although is not typically high value.

The solubility of the VDR compounds for solvent extraction depends on the strength of the solvent measured by the Hildebrand Solubility Parameters (HSP) for non-polar solvents. There are two parameters that affect this solubility. The first parameter is a relationship between surface tension and molar volume where, increasing surface tension would depend on decreasing molar volume. The second HSP is the relationship of the heat energy required to evaporate a solvent under constant volume conditions and the molar volume. The relation here is a decreasing molar volume will correspond to an increasing energy of vaporization. These two parameters demonstrate why aromatics have a higher solvent power than aliphatic hydrocarbons. An aromatic has a lower molar volume or higher density than an aliphatic hydrocarbon. The molar volume is inversely proportional to the parameter thus the parameter will have a higher value signifying a higher solvent power.