Page 84 - Synthetic Fuels Handbook

FUELS FROM PETROLEUM AND HEAVY OIL 71

Thermal Cracking. One of the earliest conversion processes used in the petroleum indus-
try is the thermal decomposition of higher boiling materials into lower boiling products.
The heavier oils produced by cracking are light and heavy gas oils as well as a residual oil
which could also be used as heavy fuel oil. Gas oils from catalytic cracking were suitable
for domestic and industrial fuel oils or as diesel fuels when blended with straight-run gas
oils. The gas oils produced by cracking were also a further important source of gasoline.
In a once-through cracking operation all of the cracked material is separated into products
and may be used as such. However, the gas oils produced by cracking (cracked gas oils) are
more resistant to cracking (more refractory) than gas oils produced by distillation (straight-
run gas oils) but could still be cracked to produce more gasoline. This was achieved using a
later innovation (post-1940) involving a recycle operation in which the cracked gas oil was
combined with fresh feed for another trip through the cracking unit. The extent to which
recycling was carried out affected the yield of gasoline from the process.
The majority of the thermal cracking processes use temperatures of 455 to 540°C
(851–1004°F) and pressures of 100 to 1000 psi; the Dubbs process may be taken as a
typical application of an early thermal cracking operation. The feedstock (reduced crude)
is preheated by direct exchange with the cracking products in the fractionating columns.
Cracked gasoline and heating oil are removed from the upper section of the column. Light
and heavy distillate fractions are removed from the lower section and are pumped to sepa-
rate heaters. Higher temperatures are used to crack the more refractory light distillate fraction.
The streams from the heaters are combined and sent to a soaking chamber where additional
time is provided to complete the cracking reactions. The cracked products are then sepa-
rated in a low-pressure flash chamber where a heavy fuel oil is removed as bottoms. The
remaining cracked products are sent to the fractionating columns.
Visbreaking. Visbreaking (viscosity breaking) is essentially a process of the post-1940 era
and was initially introduced as a mild thermal cracking operation that could be used to reduce
the viscosity of residua to allow the products to meet fuel oil specifications. Alternatively,
the visbroken residua could be blended with lighter product oils to produce fuel oils of accept-
able viscosity. By reducing the viscosity of the residuum, visbreaking reduces the amount of
light heating oil that is required for blending to meet the fuel oil specifications. In addition to
the major product, fuel oil, material in the gas oil and gasoline boiling range is produced. The
gas oil may be used as additional feed for catalytic cracking units, or as heating oil.
In a typical visbreaking operation (Fig. 3.5), a crude oil residuum is passed through a
furnace where it is heated to a temperature of 480°C (896°F) under an outlet pressure of
about 100 psi. The heating coils in the furnace are arranged to provide a soaking section of
low heat density, where the charge remains until the visbreaking reactions are completed
and the cracked products are then passed into a flash-distillation chamber. The overhead
material from this chamber is then fractionated to produce a low-quality gasoline as an
overhead product and light gas oil as bottom. The liquid products from the flash chamber
are cooled with a gas oil flux and then sent to a vacuum fractionator. This yields a heavy
gas oil distillate and a residual tar of reduced viscosity.

Coking. Coking is a thermal process for the continuous conversion of heavy, low-grade
oils into lighter products. Unlike visbreaking, coking involved compete thermal conversion
of the feedstock into volatile products and coke (Table 3.2). The feedstock is typically a
residuum and the products are gases, naphtha, fuel oil, gas oil, and coke. The gas oil may
be the major product of a coking operation and serves primarily as a feedstock for catalytic
cracking units. The coke obtained is usually used as fuel but specialty uses, such as elec-
trode manufacture, production of chemicals and metallurgic coke are also possible and
increases the value of the coke. For these uses, the coke may require treatment to remove
sulfur and metal impurities.