Solvent dewaxing development

The first commercial installation employing special organic solvents in dewaxing was made by the Indian Refining Co. in 1927 (32). The process as originally used employed a mixture of acetone and benzene as the special solvents, based on the inventions of F. X. Govers. The process as further developed employs a mixture of methyl ethyl ketone and aromatic solvents such as benzene and toluene. It is known as the solvent dewaxing process. 

Dewaxing of paraffinic lube stocks is an essential step in the production of lubricants which will remain fluid and permit machinery to operate at winter temperatures. The next two chapters outline the technologies that have been developed for this purpose (except solvent dewaxing, which was discussed in Chapter 6). These processes reflect the historical development of dewaxing chemical knowledge during this century, first using chemical separation processes and more recently, chemical conversion. 

The first process developed employed an amorphous catalyst reputedly with high fluoride levels to increase acidity and bring about partial hydroisomerization. Since wax conversion was incomplete, the final step was solvent dewaxing to remove unconverted wax and complete achievement of the target pour point. A preliminary hydrotreatment step was used to remove sulfur and nitrogen. 

Dilchill [Dilute, chill] A process for dewaxing petroleum by controlled crystallization, with cooling accomplished by the incremental addition of a cold solvent. Developed by Exxon Research Engineering Company. 

Furby (12) has developed a method for evaluating stocks in the lubricating oil range that results in a breakdown of components into asphaltenes, resins, wax, and dewaxed oil and provides a yield-viscosity index relationship for the dewaxed oil. The author has found such analyses very useful and inexpensive for evaluating a large number of potential lubricating oil stocks. Furby s method utilizes petroleum ether to precipitate asphaltenes, a fuller s earth-petroleum ether fractionation to isolate resins, methyl ethyl ketone-benzene dewaxing on the deasphalted-deresinified material to separate wax, and an adsorption fractionation to provide cuts from which the yield-viscosity index relationship for dewaxed, solvent-refined oil is obtained. 

A hyperfiltration process developed by Mobil Oil, now ExxonMobil, for this separation is illustrated in Figure 5.28(b). Polyimide membranes formed into spiral-wound modules are used to separate up to 50 % of the solvent from the dewaxed oil. The membranes have a flux of 10-20 gal/ft2 day at a pressure of 450-650 psi. The solvent filtrate bypasses the distillation step and is recycled directly to the incoming oil feed. The net result is a significant reduction in the refrigeration load required to cool the oil and in the size and energy consumption of the solvent recovery vacuum distillation section. 

As developed by Dean and Davis, this empirical method expresses viscosity variation with temperature numerically, initially on a simple scale of 0 to 100, based on two sets of reference distillate fractions. These oils were from two crudes whose distillates had not been refined in any manner (i.e., they had not been dewaxed or solvent refined). The viscosity changes with temperature of the 0 reference oil fractions were large, while those of the fractions from the 100 reference were small. These assignments of 0 and 100 were of course arbitrary and reflected experience at that time. It was assumed in developing the method that all distillation fractions from each of these reference crudes had the same VI (and that approximately agreed with the current knowledge) and that this was true for all other crudes and their lubricant fractions. A further assumption was that the Vis of all oils would fall between 0 and 100. 

FIGURE 5.10 Scheme of MAX-DEWAX process. (Reprinted from J. Membr. ScL, 286, White, L.S., Development of large-scale applications in organic solvent nanofiltration and pervaporation for chemical and refining processes, 26-35, Copyright (2006), with permission from Elsevier.)... 

Methyl ethyl ketone (MEK), the most widely used solvent, is used as a mixture with up to 70 per cent toluene to improve the rate of filtration and to raise the temperature required in dewaxing. Two solvents (or three if benzol is also used) are troublesome, and hence the use of larger-molecule ketones is being developed. Methyl n-propyl ketone appears to be superior to toluene-MEK solutions, but several other ketones are also being investigated. 

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