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.

Desirable by-products are ones that come from lube oil base stocks such as waxes that consist of long chain paraffins. There are two ways of “dewaxing” which removes wax from the feedstocks. These methods are solvent dewaxing and catalytic dewaxing.  They are physical and chemical methods respectively. Solvent dewaxing separates waxes using freezing points. Freezing is done in stages after being mixed with the solvent.  This process then forms wax crystals which is then further modified to produce slack wax. Slack wax is found in many everyday common objects such as candles. This method is done in a deasphalting process separates vacuum distillation residue. After placing the residue into an aromatic solvent, an alkane solvent is added which has the effect of some of the compounds to become insoluble. When this happens they precipitate out.

Catalytic dewaxing ultimately increases the ratio of i-paraffins to n-praffins. It does this through the chemical reactions of long chain n-alkanes which will lower the pour point of the wax. When comparing catalytic dewaxing to solvent dewaxing, catalytic waxing is the better method. It gets more lube oil stock because of the variance in pour point as well as its ability to produce both lube oil base stock and light distillates.

Dewaxing is process carried out in oil refineries that takes in deasphalted oil as well as heavy gas oil and attempts to produce a lubricating oil base stock. Lubricating oil is desired to have low pour points, low volatility, moderate viscosity, a high viscosity index, and a high thermal stability however to produce such a substance oils must have long chain paraffinic compounds removed through one of two processes: solvent dewaxing or catalytic dewaxing.

Solvent dewaxing is a physical process where solvents such as methyl ethyl ketone or propane are added to the oil mixture and then the oil mixture is cooled in a refrigeration unit. The temperature of this refrigeration unit will be based on the amount of waxes that need to be removed to reach the desired qualities of the lubricating oil. Once the solvent is mixed in and the solution is cooled, the wax will begin to form crystals and solidify. The solid wax will build up on a cloth within the separation unit then cut off and removed via a solvent stream. As a last step this wax is run through a steam stripper where the solvent can be taken back out and recycled. The lubricating oil is run through a steam stripper to remove and recycle the solvent as well.

Catalytic dewaxing is a chemical process where the long chain paraffins are cracked through the use of a selective catalyst. This catalyst is a molecular sieve with small specifically sized pore openings which allow n-paraffins to enter and be cracked while keeping i-paraffins out of the cracking process. By having a catalyst of this nature it allows the number of n-paraffins to increase while keeping the number of i-paraffins close to the same which increases the n-paraffin to i-paraffin ratio. This is beneficial to the desired lubricating oil product because as the ratio increases the pour point decreases and thus creates a better oil. Due to the selectivity of the catalyst, catalytic dewaxing has a higher yield of more consistent products when compared to solvent dewaxing. Catalytic dewaxing often costs less to install in a refinery system than solvent dewaxing.