Page 81 - Synthetic Fuels Handbook

68 CHAPTER THREE

ground. The separator may be no more than a large vessel that gives a quieting zone for
gravity separation into three phases: gases, crude oil, and water containing entrained dirt.
Desalting is a water-washing operation performed at the production field and at the
refinery site for additional crude oil cleanup (Fig. 3.2). If the petroleum from the separators
contains water and dirt, water washing can remove much of the water-soluble minerals and
entrained solids. If these crude oil contaminants are not removed, they can cause operating
problems during refinery processing, such as equipment plugging and corrosion as well as
catalyst deactivation.

Electrical
Process power
water

Alternate Desalted
crude

Unrefined
Gravity
crude
settler
Effluent
Heater Emulsifier water
FIGURE 3.2 An electrostatic desalting unit.

3.3.2 Distillation
Distillation was the first method by which petroleum was refined. In the early stages of
refinery development, when illuminating and lubricating oils were the main products, dis-
tillation was the major and often only refinery process. At that time gasoline was a minor,
but more often unwanted, product. As the demand for gasoline increased, conversion pro-
cesses were developed because distillation could no longer supply the necessary quantities
of this volatile product.
It is possible to obtain fuels ranging from gaseous materials taken off at the top of the
distillation column to a nonvolatile residue or reduced crude (bottoms), with correspond-
ingly lighter materials at intermediate points. The reduced crude may then be processed by
vacuum or steam distillation in order to separate the high-boiling lubricating oil fractions
without the danger of decomposition, which occurs at high (>350°C, >662°F) tempera-
tures. Atmospheric distillation may be terminated with a lower boiling fraction (cut) if it is
felt that vacuum or steam distillation will yield a better-quality product, or if the process
appears to be economically more favorable. Not all crude oils yield the same distillation
products and the nature of the crude oil dictates the processes that may be required for
refining.
Atmospheric Distillation. The distillation unit is a collection of distillation units but, in
contrast to the early battery units, a tower is used in the modern-day refinery (Fig. 3.3) and
brings about an efficient degree of fractionation (separation).
The feed to a distillation tower is heated by flow through pipes arranged within a large
furnace. The heating unit is known as a pipe still heater or pipe still furnace, and the heating
unit and the fractional distillation tower make up the essential parts of a distillation unit or
pipe still. The pipe still furnace heats the feed to a predetermined temperature—usually a