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.

Another separation process in petroleum refining is, the process of dewaxing. What dewaxing is, is exactly what one might guess, and that is the removal of wax from a feedstock in an oil refinery. That feedstock can either be, deasphalted oil from deasphalting or heavy vacuum gas oil from vacuum distillation. The wax that is removed from the feedstock is long chain paraffin moleculues that solidify readily within the feedstock and once removed can be sold as a marketable by-product. Once a feedstock has undergone dewaxing, it can be used as the basis for lubricating oils, this is the most desired product of dewaxing.

Dewaxing however, can be accomplished using one of two different methods. Those methods are known as solvent dewaxing, and catalytic dewaxing. These two types of dewaxing have their obvious differences.

The first type, solvent dewaxing, is a physical process that includes the refridgeration of a feedstock after it has be mixed with a solvent. What happens is, once the feedstock is refrigerated, the wax within it solidifies and forms crystals. These crystals are then carried to a large rotating drum covered by a filter cloth. It is on the cloth that the wax will accumulate, thus removing it from the feedstock until it is removed from the surface of the drum using a large blade. This product is called slack wax and it then further undergoes steam stripping to recycle and remove the solvent. This wax product can then be used for things like candle wax and petroleum jelly.

Catalytic dewaxing is the second technique used and this is a chemical process. Catalytic dewaxing is the selective cracking of the long chain paraffins or wax into smaller chain alkanes. This is done by way of using a sieve catalyst to filter out the i-paraffins and filter the n-paraffins into cracking reactions. This is accomplished by way of using catalysts with very small pores, as small as 0.6 nm, that only allow n-paraffin’s long chain structure to pass through while blocking the bulky i-paraffins. Catalytic dewaxing is the cheaper of the two processes, as well as it is more flexible because it can be used to produce either lube oil base stock or light distillates.

Solvent fractionation is a further form of distillation that takes place in the Light Ends Unit (LEU). The feedstock for solvent fractionation is the residue that is left over from previous Vacuum Distillation, aptly called, vacuum distillation residue (VDR). VDR is a very heavy viscous compound that is solid at ambient temperatures. The reason vacuum distillation residue is so dense, is because of its high aromaticity along with its high asphaltene concentration. Asphaltenes are the highest molecular weight compounds contained in VDR, and appear in solution. What solvent separation is, is when the asphaltenes within VDR are precipitated from the solution using a light paraffinic solvent. The portion of the VDR that is soluble in the paraffin is called maltenes. The first paraffin used is typically n-heptane. The n-heptane soluables can further be separated using n-pentane, which is a lighter an weaker solvent. The result of this step is an insoluable compound called hard resin and n-pentane soluables. Finally, the lightest of the solvents, propane, is used. This final stage yields soft resin and oil products. In refining, this is done by skipping to the final step and only using propane, which yields asphalt and deasphalted oil.

The solubility of compounds in different solvents can be measured. For non-polar solvents, solvents are measured using Hildebrand Solubility Parameters (HSP). What the Hildebrand Solubility Parameters are, are two parameters used to accurately determine the solubility power for non-polar solvents. The first of the two parameters pertains to the surface tension and the molar volume of the solvent, while the second pertains to the energy of vaporization and the molar volume of the solvent. In general, the solubility parameter increases with increasing density of the solvent, as well as increasing surface tension or energy of vaporization of the solvent. With this knowledge we are able to understand why using too large of a volume of a solvent will interfere with the solubility of asphaltenes.

Dewaxing is a separation process that takes advantage of deasphalted oil (DAO) and heavy vacuum gas oil (HVGO) from the vacuum distillation tower as feedstocks in producing lubricating oil base stock and, to some extent, distillate fuels such as gasoline. The goal of dewaxing is to remove hydrocarbons that would potentially increase the pour point of the lube oil base stock to a desirable range of -9^o – 14^o F. There is two different processes that result in this marketable lube oil: solvent dewaxing and catalytic dewaxing.

Solvent dewaxing is a physical process that uses refrigeration, scraping techniques, and methyl ethyl ketone (MEK) and propane solvents to separate the feedstocks and produce a valuable product. The MEK solvent and the deasphalted oil combine in the first phase and go through a series of refrigeration processes at different temperatures to form wax crystals, which are then transported to a rotary filter where a filter cloth separates the wax from the oil + solvent. The wax goes on to produce candle wax and petroleum jelly while the dewaxed oil purifies into the desirable lube oil base stock. Issues that arise from this process come from choosing which type of solvents to use. Most refineries use MEK even though propane can be used from multiple aspects of the process, save money, increase filtration rates.

Catalytic dewaxing is a chemical process that, by nature, is actually a conversion process. It uses catalytic cracking of n-paraffins, but since the purpose is to remove wax, it is classified as a separation process. There is not much detail to this process although it does use hydrogen addition to prevent coking. It is obviously the best way to dewax the feedstocks because it results in a product with lower pour points, high yield, and high stability. Catalytic dewaxing also allows for the production of light distillates such as gasoline since the n-paraffins are cracking.

In reality, distillation can never achieve absolute separation of the different hydrocarbons liquids that enter the Light Ends Unit (LEU) as feedstock. Theoretically, there is a way to achieve a degree of separation that proves suitable for the refinery’s needs by distilling small fractions of the liquid using a number of ‘theoretical plates’. The feedstock that enters the LEU has components of low molecular weight and high volatility. These characteristics allow refineries to analyze the feedstock on a molecular level and define the degree of separation in terms of specific hydrocarbon concentrations. Once a stage of distillation is complete, the new condensate, enriched in the more volatile component, is taken past the next plate and redistilled in a different section. This process proceeds for however many theoretical plates are calculated until almost-pure components are left. For the Light Ends Unit, the feed is separated into two fractions, the distillate and the bottoms.

Solvent extraction out of the vacuum distilled residue pulls the asphaltenes out of the one-phase material. The strength of a specific compound, determined by the two Hildebrande Parameters, indicates the solubility of said compound in a solvent. The first parameter depends upon the surface tension and molar volume of the non-polar solvent while the second parameter uses the solvent’s energy of vaporization and molar volume. With these different characteristics in mind, it is easy to see how each component of the vacuum distilled residue has their own solubility properties. Aromatic hyrdocarbons have higher density than aliphatic hyrdrocarbons, which means that they also have a lower molar volume. This results in the increase of the second Hildebrande Paramenter (and consequently the increase of compound strength) since molar volume is the denominator in the equation. On the same line, higher carbon number paraffins have a larger molar volume, resulting in a lower strength value. Using this information, refineries add a large amount of low strength paraffin solvent to the vacuum dissolved residue to basically ‘cancel out’ the asphaltene’s solubility and yield solid particles that can be filtered out.