SEPARATION OF CRUDE OIL INTO CUTS OR FRACTIONS Paraffin Base Oils.—With paraffin base oils where lubricating stock is desired the cuts are not made on temperatures, but upon a specific gravity basis.
The first cut is completed when the product reaches a gravity of 54°Be.
The distillate secured is crude naphtha, which is run to a sepa rate tank and then re-run to give gasoline, naphtha distillate, and other products.
The next cut is at 41°Be. This product is natural lamp dis tillate (kerosene distillate) which is refined.
When the stream reaches 38°Be. the receiver is changed. This distillate is the mineral seal stock, which is later re-run and re fined. At 36.5°Be. the cut is gas and fuel-oil stock. The next cut at 3313e. is the last. This fraction is the lubricating distillate which is cold pressed to obtain the slack wax. The residue is cylinder stock which is filtered and treated as shown in the scheme.
The first flow sheet (see Fig. 89) presented above gives the product obtained from the fractional distillation of a paraffin base petroleum. This sheet shows both the various groups, and their product.
Crude 0.1 Mixed Base Crude.—With mixed base crude, and using destruc tive or dry distillation, the oil is distilled in direct heated stills fired by coal, gas, or oil. The first cut is at to 325°F. The product is crude naphtha. The second cut is at 325 to 475°F. This is the crude heavy naphtha.
The third cut, the natural lamp distillate, is obtained at 475 to 625°F.
After 650° F. up to 700° "cracking" takes place. The fires are slowed down and the distillation takes place slowly.
All the lighter fractions are first taken off without "cracking." Cracking does not take place until after 625°F. All the "cuts" to that point are normal. From 625° on the temperature is raised very slowly to 675 to 700°F. A distillate is produced. This distillate contains gasoline, some lamp oil and much heavier oil, and consists of about 20 per cent of the crude.
The residue in the still is a heavy back tar about 42 per cent of the crude. This tar is further distilled rapidly and gives off
about 22 per cent of paraffin distillate and about 15 per cent of cracked distillate. The residue consists of wax tailings and coke. The flow sheet (see Fig. 90) gives the products of mixed base crude.
Cracking" in its simple analysis means breaking up or disassociating by heat and pressure the molecules of the heavier hydrocarbons of crude oil, and combining them into molecules of the higher elements.
"Cracking" has been considered a dangerous process as the early attempts resulted in some serious explosions. Also, the products obtained had very unpleasant odors.
New Cracking new cracking process called the Bacon Process is in use by the Gulf Refining Company of Pitts burgh. This process depends upon the use of vertical steel tubes or stills 20 ft. long and 6 to 19 in. in diameter. The strength of tubes of this size is much greater than that of stills of larger diameter and they will withstand heavy pressures.
This system is claimed to be .safer and cheaper than any other. The process is continuous, the still being kept full of oil from the bottom to the top of the heat chamber.
Fire and Steam Distillation on Mixed Base fire and steam distillation where cylinder stock is desired the first cut is the crude naphtha at 280°F. Open steam is intro duced at a temperature of 212°. The second cut, the crude heavy naphtha, is at 330°F. The third cut, lamp-oil distillate from which kerosene is obtained, is made at 500°. The last cut is at 620°F., at which temperature the wax distillate is over, leaving the cylinder stock in the still.
This cylinder stock is then treated further. The use of bottom steam in the stills produces a larger quantity of cylinder stock of good quality.
Asphalt Base base crude is refined very much as the paraffin and mixed crudes. The residue is asphalt instead I of cylinder or tar stock. No wax is obtained.