Page 83 - Synthetic Fuels Handbook

70 CHAPTER THREE

To vacuum
system

Vacuum tower Vacuum
gas oil

Lubricating oils

Residuum Vacuum
residuum
Furnace
FIGURE 3.4 A vacuum distillation unit.
vacuum tower, the units are necessarily of a larger diameter than the atmospheric units. Some
vacuum distillation units have diameters on the order of 45 ft (14 m). By this means, a heavy
gas oil may be obtained as an overhead product at temperatures of about 150°C (302°F), and
lubricating oil cuts may be obtained at temperatures of 250 to 350°C (482–662°F), feed and
residue temperatures being kept below the temperature of 350°C (662°F), above which crack-
ing will occur. The partial pressure of the hydrocarbons is effectively reduced still further by
the injection of steam. The steam added to the column, principally for the stripping of asphalt
in the base of the column, is superheated in the convection section of the heater.
The fractions obtained by vacuum distillation of the reduced crude (atmospheric resid-
uum) from an atmospheric distillation unit depend on whether or not the unit is designed
to produce lubricating or vacuum gas oils. In the former case, the fractions include (a) heavy
gas oil, which is an overhead product and is used as catalytic cracking stock or, after suit-
able treatment, a light lubricating oil; (b) lubricating oil (usually three fractions—light,
intermediate, and heavy), which is obtained as a side-stream product; and (c) asphalt (or
residuum), which is the bottom product and may be used directly as, or to produce, asphalt
and which may also be blended with gas oils to produce a heavy fuel oil.

3.3.3 Thermal Processes
Cracking distillation (thermal decomposition with simultaneous removal of distillate) was
recognized as a means of producing the valuable lighter product (kerosene) from heavier
nonvolatile materials. In the early days of the process (1870–1900) the technique was very
simple—a batch of crude oil was heated until most of the kerosene had been distilled
from it and the overhead material had become dark in color. At this point distillation was
discontinued and the heavy oils were held in the hot zone, during which time some of the
high-molecular-weight components were decomposed to produce low-molecular-weight
products. After a suitable time, distillation was continued to yield light oil (kerosene)
instead of the heavy oil that would otherwise have been produced.