Propane dewaxing

Propane dewaxing uses liquid propane as the solvent. Propane is normally a gas at ambient temperature and the vessels in the unit must be pressure vessels. This also ineludes the filter. Filtration in propane dewaxing is pressure filtration vs. ketone dewaxing which uses vacuum filtration. The propane temperature depends on the pressure, so that it is of paramount importance that the pressure be controlled. It is often said that in propane dewaxing propane is always either flashing or condensing. This adds an additional level of complexity that is not present in ketone dewaxing. 

Solvent Treatment. Solvent processes can be divided into two main categories, solvent extraction and solvent dewaxing. The solvent used in the extraction processes include propane and cresyHc acid, 2,2 -dichlorodiethyl ether, phenol (qv), furfural, sulfur dioxide, benzene, and nitrobenzene. In the dewaxing process (28), the principal solvents are benzene, methyl ethyl ketone, methyl isobutyl ketone, propane, petroleum naphtha, ethylene dichloride, methylene chloride, sulfur dioxide, and iV-methylpyrroHdinone. 

Solvents used for dewaxing are naphtha, propane, sulfur dioxide, acetone—benzene, trichloroethylene, ethylenedichloride—benzene (Barisol), methyl ethyl ketone—benzene (benzol), methyl -butyl ketone, and methyl / -propyl ketone. Other solvents in commercial use for dewaxing include /V-methylpyrrolidinone, MEK—MIBK (methyl isobutyl ketone), dichloroethane—methylene dichloride, and propfyene—acetone. 

In the propane process, part of the propane diluent is allowed to evaporate by reducing pressure so as to chill the slurry to the desired filtration temperature, and rotary pressure filters are employed. Complex dewaxing requires no refrigeration, but depends on the formation of a soHd urea—/ -paraffin complex which is separated by filtration and then decomposed. This process is used to make low viscosity lubricants which must remain fluid at low temperatures (refrigeration, transformer, and hydraulic oils) (28). 

Lubricating Oil Manufacture This consists of solvent deasphalting, phenol treating, and solvent dewaxing. In dewaxing, waxy lube is diluted with a solvent such as propane or methyl ethyl ketone (MEK), and cooled to crystallize the wax which is then removed by filtration. 

Dewaxing (Figure 4.17) processes also produce heater stack gas (carbon monoxide, sulfur oxides, nitrogen oxides, and particulate matter) as well as hydrocarbon emission such as fugitive propane and fugitive solvents. Steam stripping wastewater (oil and solvents) and solvent recovery wastewater (oil and propane) are also produced. The fugitive solvent emissions may be toxic (toluene, methyl ethyl ketone, methyl isobutyl ketone). 

Propane dewaxing a process for dewaxing lubricating oils in which propane serves as solvent. 

Solvent dewaxing removes wax from lubricating oil stocks, promoting crystallization of the wax. Solvents include furfural, phenol, cresylic acid-propane (DuoSol), liquid sulfur dioxide (Eleleanu process), B,B-dichloroethyl ether, methyl ethyl ketone, nitrobenzene, and sulfur-benzene. The process yields de-oiled waxes, wax-free lubricating oils, aromatics, and recovered solvents. 

Liquid propane is employed as the solvent in this process. The use of a petroleum fraction as a solvent for dewaxing is desirable from the standpoint of the availability and cost of solvent, and the higher boiling hydrocarbon fraction has been employed many years for dewaxing of residual stocks. 

In present-day commercial practice, waxy oil charge is blended with 1 to 3 volumes of liquid propane at a temperature sufficiently high (120° to 160° F.) to ensure complete solution of the wax. The mixture is first cooled by exchange with cold filtrate and then charged to a batch chilling vessel, in which temperature is reduced to that required to obtain the desired pour point of the dewaxed oil, by evaporation of propane from the solution. Cold propane is injected into the vessel in order to maintain the propane-oil ratio approximately constant. The crystallized wax is removed by filtration on a continuous rotary filter (59) under a pressure of about 4 to 8 pounds per square inch. 

Propane is available in ample quantities and can be produced as a liquid in most refineries. Propane is a highly satisfactory refrigerant and in dewaxing the principle of direct evaporative cooling is employed advantageously. Propane also is cheap, stable, noncorrosive, and nontoxic. 

Nine units have been installed using liquid propane as a dewaxing solvent. 

Several continuous methods of chilling have been proposed to replace the batch chilling method normally employed in propane dewaxing. 

Solvent extraction has become the most widely used method of refining lubricating oils. Selective solvents which extract the less desirable constituents include phenol, furfural, dichloroethyl ether, mixtures of cresylic acid and propane, and liquid sulfur dioxide. Liquid propane precipitates asphaltic constituents and wax and retains the more desirable oil components in solution. Dewaxing may also be accomplished by other solvents such as mixtures of benzene and methyl ethyl ketone. 

Figure 14.5. Representation of solvent extraction behavior in terms of certain properties rather than direct compositions [Dunstan et aL, Sci. Pet., 1825-1855 (1938)]. (a) Behavior of a naphthenic distillate of VGC = 0.874 with nitrobenzene at 10°C. The viscosity-gravity constant is low for paraffins and high for naphthenes, (b) Behavior of a kerosene with 95% ethanol at 17°C. The aniline point is low for aromatics and naphthenes and high for paraffins, (c) Behavior of a dewaxed crude oil with liquid propane at 70°F, with composition expressed in terms of specific gravity.

In solvent dewaxing, the oil is diluted with a solvent that has a high affinity for oil, chilled to precipitate the wax, filtered to remove the wax, stripped of solvent, and dried. The solvents (principally propane, naphtha, methyl ethyl ketone-MEK) act as diluents for the high molecular weight oil fractions to reduce the viscosity of the mixture and provide sufficient liquid volume to permit pumping and filtering. Wax produced by the solvent dewaxing process is used to make (1) paraffins for candle wax, (2) microwax for cosmetics, and (3) wax for petroleum jelly. 

Solvents in use for wax removal include propane, which also serves as an autorefrigerant for the process by solvent evaporation, methyl ethyl ketone (MEK)-toluene mixtures, and methyl isobutyl ketone (MIBK). The last two systems are in widest use at present. MEK-benzene was once a common solvent combination for dewaxing but has now been abandoned from benzene toxicity considerations. 

Wilson, R. E., P. C. Keith, and R. E. Haylett. 1936. Liquid propane Use in dewaxing, deasphalting, and refining heavy oils. Ind. Engl Chem. 28 1065. 

Propane Dewaxing Technology, brochure from ExxonMobil Research and Engineering, Fairfax, Virginia. 

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