Dewaxing is a separation process that removes wax from feedstock such as DAO from deasphalting and HVGO from vacuum distillation in refining process. Wax is a desirable by-product which is necessary to remove for producing lubricating oil base stock with low pour points. Solvent dewaxing and catalytic dewaxing are the two commercial methods of dewaxing that generally used in refining process. Solvent dewaxing is a physical process which involves freezing and solvent transport. Methyl ethyl ketone (MEK) and propane are the two main solvents used in this process. In US, MEK is the most common solvent used in process because it has advantages such as lower capital investment, energy saving and higher filtration rates. The process need to freeze the feedstock in a several stage. Wax crystals will be formed by solidifying wax compounds thought the refrigeration. In a rotary filter, crystals will be dissolved in solvent and separated. Eventually, the layer of wax will go through the steam stripping unit to separate solvent from the product. Unlike solvent dewaxing, catalytic dewaxing is a chemical process which involves reactions of long-chain n-alkenes. A selective catalytic cracking of n-paraffins take place in this process. In the pores of molecular sieve catalysts (zeolites), the pores only open 0.6 nm to filter i-paraffin out. Through this process, the ratio of i-paraffins and n-paraffins will increase to lower the pour point. In order to prevent coking on the surface, hydrogen is used alone the process. This cracking process produces some by-products such as gasoline. Overall, two dewaxing achieve the goal to lower the pour points and separate wax. However, catalytic dewaxing produce lube base stock with even lower pour point and in higher yield. Solvent dewaxing produce lube base stock in a lower yield because to separate the wax from oil in this process is hard.

In lesson 4, we learned that distillation is a separation process which depends on the boiling point of compounds. Compare to distillation, solvent fractionation fractionate the feedstock or vacuum distillation residue (VDR) depend on the solubility of the compound in the given solution. Use graph from lesson 5 on class website as an example, it showed us a simple flow of the process. Asphaltenes, which are compounds that have highest molecular weight of VDR, is soluble in aromatic solution such as benzene and toluene. They can be separated by using precipitation under the condition that paraffin solvent was mixed in. Maltenes is the part of VDR which is soluble in paraffin solvent and also known as a solvent in the separation process. For further separation of n-heptane soluble, a lighter and weaker solvent n-pentane is used. That gives us an insoluble fraction (hard resin) and soluble fraction (n-pentane). Even lighter solvent propane is used to separate soluble fraction (n-pentane) to soft resin and oil products.

Since there is a large difference in structure of asphaltenes and oil fractions in VDR, it is normal for us to see a suspension of discrete asphaltene particles rather than a solution in VDR. However, VDR normally appear as a solution (one-phase material). The gradient solubility model is a general acceptable hypothesis that explains what we observed. In this model, the strength of solvent influences the solubility of a compound in given solvent. Hildebrand Solubility Parameters (HSP) measures the strength of solvent in this model. There two types of parameters used in the measurement and they correlate well to each other. 1^st Hildebrand Parameter depends on surface tension and molar volume of the given solvent. 2^nd Hildebrand Parameter depends on energy of evaporation and molar volume of given solvent. From the equations of two parameters, we can see that solubility increase with increasing surface tension, increasing energy evaporation and decreasing molar volume. As we know, aromatic solvent are stronger solvent than aliphatic hydrocarbon and it is explained by two types of Hildebrand parameters. That also explains why increasing carbon number make the solvent power of paraffin solvent increase.

Write a post comparing the solvent dewaxing and catalytic dewaxing processes.

Wax is made up of long-chain paraffins and it is a desirable by-product, particularly lube oil base stock. Dewaxing is the process of removing wax from feedstocks that would otherwise readily solidify, such as DAO from deasphalting and HVGO from vacuum distillation. There are two commercial methods of dewaxing. On utilizes a physical process known as solvent dewaxing, while the other method of catalytic dewaxing involves a chemical process.

Solvent dewaxing is a physical process which separates the wax with respect to freezing and solvent transport. This method uses stage-wise refrigeration of the feedstock after being mixed with the solvent. Wax crystals are then carried to a rotary filter via the solvent to be separated on a filter cloth. This layer of wax is collected and taken to a steam-stripping unit to recycle the solvent separated from the wax product, known as slack wax. This product has several marketable uses, such as paraffin wax for candles, microwax for cosmetics, and for petroleum jelly. The refrigerator’s temperature can be manipulated to control the desired pour point of the resulting lube oil base stock product.

Catalytic dewaxing is a chemical process, which removes the wax by means of selective reactions of long chain n-alkanes. This method is technically a low-severity conversion process, which involves a selective catalytic cracking of n-paraffins. Molecular sieve catalysts, known as zeolites, host selective cracking of n-alkanes while simultaneously keeping out bulky i-paraffins. Therefore, this process increases the ratio of i-paraffins to n-paraffins in the product, in turn lowering its pour point.

In comparison, catalytic dewaxing has an advantage over solvent dewaxing in the fact that it yields a lube oil base stock with a lower pour point as well as a higher yield of this product. Catalytic dewaxing also poses the flexibility to produce both lube oil base stock along with light distillates such as gasoline.

Post a blog to comment on how solvent fractionation works and review the parameters to describe the solvent power for non-polar solvents.

Solvent extraction is a process in which compounds may be separated based on their relative solubilities. It is basically the extraction of a substance from one liquid into another liquid phase. As opposed to distillation which exploits the different boiling points of the feedstock to achieve fractionation, deasphalting utilizes a solvent extraction process that factors in solubility or insolubility of compounds in a certain solvent. Vacuum distillation residue (VDR) is completely dissolved in aromatic solvents, like benzene and toluene. A paraffin solvent (n-heptane) is mixed with VDR in toluene, and the soluble portion of VDR in the n-heptane is called maltenes. The n-heptane solubles can then be further separated using a lighter and weaker solvent, such as n-pentane. These solubles can be separated even further utilizing a lighter solvent like propane. Figure 5 of Lesson 5 shows the overall process of solvent fractionation of VDR.

Rather than being considered a suspension of discrete asphaltene particles in VDR, this residue is actually considered a solution (one-phase material). The gradient solubility model is a widely acknowledged hypothesis which explains this observation. This model declares that asphaltene molecules can dissolve in resins which can then be dissolved in oil, ultimately yielding a single phase solution. The asphaltene is able to be forced out of solution in VDR by solvent extraction. The degree of solubility of a compound in a solvent is dependent upon the dissolving power of the solvent which is measured for non-polar solvents by Hildebrand Solubility Parameters (HSP).

There are two different Hildebrand Solubility Parameters. The first relates solubility to the ratio of surface tension to the cubic root of molar volume, in which solubility increases as surface tension increases and as molar volume decreases. The second parameter equates solubility to the square root of the ratio of latent heat of vaporization to molar volume, where solubility increases with an increasing heat of vaporization. It is clear why aromatic solvents are stronger solvents than aliphatic hydrocarbons.

Solvent Extraction: http://en.wikipedia.org/wiki/Liquid%E2%80%93liquid_extraction

Write a post explaining the need for vacuum distillation and discuss the utility of Watson Characterization for selecting the vacuum distillation temperature.

Vacuum distillation is essential in petroleum refining. Through this method the distillates can be separated into light vacuum oils, heavy vacuum oils, and vacuum residue. The lighter components of the crude can then be removed out of the top of the distillation column so that the heavier non-volatile compounds remain on the bottom. A vacuum is needed for this method in order have properly met temperatures and pressures within the distillation column. It also is needed in order to avoid thermal cracking.

The heaviest and most contaminated portion of the crude oil is known as the vacuum distillation residue, found on the very bottom. This residue is actually able to be upgraded into a usable fuel source through various processes such as deasphalting, visbreaking, and coking. Deasphalting requires hydrotreating whereas both visbreaking and coking require thermal cracking with coking being more severe.

In catastrophic instances a distillation unit is shut down. This puts a refinery in a very bad position. Very major coking cases can actually block the path of flow in a distillation column and force it to be shut down. Because of this reason it is very important to select the proper temperature in a vacuum distillation column. Doing so will control the risk of coking within the column. The Watson Factor, known as Kw, is used to estimate the upper temperature limit for vacuum distillation to avoid coking and ensure that the process within the distillation column occurs smoothly without any clogging. This is essential to keep a refinery up and running and not have to stop for maintenance.