Just when you thought separation processes could not be more drawn out, dewaxing appears! Undoubtedly another slightly misleading process title, dewaxing is chiefly concerning the lubricating oil base stock product more so than the wax byproduct. The lube oil base stock market price depends on its volatility, viscosity, viscosity index, and thermal stability. Depending on the process employed, the resultant lube oil base stock may not require as many additives to enhance engine performance.

Solvent dewaxing requires a tremendous amount of energy and solvent to remove wax from the lube oil base stock. We discussed the use of solvents such as methyl ethyl ketone and propane along with the need for refrigeration units and steam-stripping to remove wax. Bechtel Corp. displays a general layout of its own solvent dewaxing process. Notably, Bechtel uses inert gas instead of energy-intensive steam for stripping. Regardless, the operating costs illustrate how significant the dewaxing footprint is within the refinery. No wonder those oil leaks cost me a fortune!

You may question the utility of wax on the open market, like I do. Never fear, catalytic dewaxing is here!

Shell Global Solutions’ website illustrates the company’s catalytic dewaxing technology for the work to study (and purchase). This separation process actually involves catalytic cracking of n-paraffins using a selective catalyst. To mitigate fouling of the catalyst, hydrogen is also applied. This process enables the operator to produce even more distillates and lube oil base stock. This is tremendously lucrative since it requires a lower capital investment; the selection of this process relies on the robustness and effectiveness of the catalyst. Perhaps, I should invest in the catalyst industry…

Fig. 1: Shell Global Solutions’ proprietary catalyst (Source)

In the oil and gas industry, the bottom line is the most essential metric of success. We have discussed the complexity of crude oil and the general refinery path which it takes. Certainly, the most energy intensive portion of refining, separation processes, dictates the success or failure of an oil refinery. Along with the increase in light distillates, we observe an increasing need for heavy crude deasphalting capacity. As distillation columns use pressure and temperature gradients to fractionate distillates and bottoms, solvent fractionation is a “carbon rejection” process that uses a “chemical gradient” to separate asphaltenes and resins from deasphalted oil (DAO) in vacuum distillation residue. Additionally, solvent dewaxing involves solvents and temperature gradient to produce wax and lube oil. Ultimately, solvents may enhance the overall refinery profitability while adding product flexibility and utilizing the entire barrel of crude oil!

In order to understand solvent fractionation, we must understand the gradient solubility model. For engineers to able to characterize the seemingly fruitless vacuum distillation residue. Resins in the crude oil dissolve asphaltene molecules in a solution, preventing precipitation. Miscibility, or the “mixabilty” property, must be altered through the use of a solvent. Paul J. Flory, a Standard Oil Development Co. scientist, correlated the difference in molecule sizes (solvent versus VDR) with system entropy; as the difference in molecule size increased, the entropy (or disorder) increased, causing large deviations from ideal miscible behavior. With this understanding, we can attempt to quantify the strength of various solvents to compare.

Fig. 1: Paul J. Flory, 1974 Recipient of Nobel Prize in Chemistry (Source – Nobelprize.org)

At this point in the separation stages, large hydrocarbons such as asphaltenes and waxes can precipitate out of the crude oil solution given a specified paraffinic solvent. The strength of these non-polar solvents are defined by Hildebrand Solubility parameters. While Wikipedia only displays the Hildebrand parameter as a function of vaporization enthalpy, temperature, and molar volume, a similar value may be approximated using surface tension and molar volume. As surface tension increases (or the enthalpy of vaporization increase), the solvent’s strength increases.