1. Field of the Invention
This invention relates to a method for separating straight chain hydrocarbons from a mixture of straight chain hydrocarbons, nonstraight chain hydrocarbons and a sulfur compound; and more particularly to the use of a zeolite having large crystals to effect this separation.
2. Description of the Prior Art
Straight chain hydrocarbons are found mixed with nonstraight chain hydrocarbons in some hydrocarbon or petroleum fractions, such as gasoline, kerosine, diesel, gas oil and naphtha, which fractions can have a boiling range of any where between about 40.degree. C. to 350.degree. C. Straight chain hydrocarbons have commercial utility being useful in the manufacture of detergents. Further, the hydrocarbon fraction remaining after removal of the straight chain hydrocarbons has a higher octane number making it more valuable. For these reasons, and others, it is desirable to separate the straight chain hydrocarbons from the hydrocarbon fractions in which they are found.
One method of separating straight chain hydrocarbons comprises the use of a crystalline zeolite as a selective absorbent for the straight chain hydrocarbons. In such a method, which is described in several patents including coassigned U.S. Pat. Nos. 2,818,455; 2,859,256; and 3,373,103, the mixture of straight chain and nonstraight chain hydrocarbons, preferably in the vapor phase, at an elevated temperature, and at a superatmospheric pressure, is contacted with a crystalline zeolite. The pores of the crystalline zeolite are large enough to permit the entry of the straight chain hydrocarbons, however, they are not large enough for the admission of the nonstraight chain hydrocarbons. The crystalline zeolite selectively absorbs the straight chain hydrocarbons from the mixture.
The remaining portion of the hydrocarbon fraction, which has a large concentration of nonstraight chain hydrocarbons, is then purged from the surface of the zeolite and the area surrounding the zeolite. Purging can be accomplished by first stopping the flow of the hydrocarbon fraction to the zeolite, and then passing a purging medium countercurrent to the flow of the hydrocarbon fraction through the zeolite. After purging, the straight chain hydrocarbons are desorbed from the crystalline zeolite. Other purging methods can include the use of vacuum or the use of noncondensible gases as carbon dioxide or nitrogen.
After purging, the straight chain hydrocarbons can be desorbed from the zeolite by contacting the straight chain saturated zeolite with a desorbing medium, which preferably is a fluid comprising straight chain hydrocarbons in the vapor phase, having a molecular weight less than the molecular weight of the lightest absorbed straight chain hydrocarbon, and also preferably having a lower boiling point than the straight chain hydrocarbons. After the straight chain hydrocarbons are desorbed from the zeolite, they are separated from the desorbing medium and are used as desired. Other desorbing methods can include the use of vacuum or the use of noncondensible gases as carbon dioxide or nitrogen.
In many such processes, especially those employed recently, the hydrocarbon fraction was hydrotreated, such as by catalytically reacting the hydrocarbon fraction with hydrogen, prior to being contacted with the zeolite in order to reduce the concentration of thiophene, mercaptan, and other sulfur compounds in the hydrocarbon fraction. Thiophene, mercaptan, and other sulfur compounds found in the hydrocarbon fraction were generally found to be deleterious to the zeolite, rapidly reducing its capacity for selectively absorbing straight chain hydrocarbons. The hydrogen sulfide produced by hydrotreating the sulfur compounds is readily removed from the treated hydrocarbon fraction leaving a low sulfur content fraction for the separation process. However, hydrotreating is a costly proess due to the cost of hydrogen and the cost of the equipment involved in the hydrotreating process.