Page 93 - Synthetic Fuels Handbook

80 CHAPTER THREE

pressures from 100 to 1000 psi, depending on the particular process, the nature of the feed-
stock, and the degree of hydrogenation required. After leaving the reactor, excess hydrogen
is separated from the treated product and recycled through the reactor after removal of
hydrogen sulfide. The liquid product is passed into a stripping tower where steam removes
dissolved hydrogen and hydrogen sulfide and, after cooling the product is taken to product
storage or, in the case of feedstock preparation, pumped to the next processing unit.

3.3.6 Reforming Processes
When the demand for higher-octane gasoline developed during the early 1930s, attention
was directed to ways and means of improving the octane number of fractions within the
boiling range of gasoline. Straight-run (distilled) gasoline frequently had very low octane
numbers, and any process that would improve the octane numbers would aid in meeting
the demand for higher octane number gasoline. Such a process (called thermal reforming)
was developed and used widely, but to a much lesser extent than thermal cracking. Thermal
reforming was a natural development from older thermal cracking processes; cracking con-
verts heavier oils into gasoline whereas reforming converts (reforms) gasoline into higher
octane gasoline. The equipment for thermal reforming is essentially the same as for thermal
cracking, but higher temperatures are used.

Thermal Reforming. In the thermal reforming process a feedstock such as 205°C (401°F)
end-point naphtha or a straight-run gasoline is heated to 510 to 595°C (950–1103°F) in a
furnace, much the same as a cracking furnace, with pressures from 400 to 1000 psi (27–68 atm).
As the heated naphtha leaves the furnace, it is cooled or quenched by the addition of cold
naphtha. The material then enters a fractional distillation tower where any heavy products
are separated. The remainder of the reformed material leaves the top of the tower to be sepa-
rated into gases and reformate. The higher octane number of the reformate is due primarily
to the cracking of longer chain paraffins into higher octane olefins.
The products of thermal reforming are gases, gasoline, and residual oil or tar, the lat-
ter being formed in very small amounts (about 1 percent). The amount and quality of the
gasoline, known as reformate, is very dependent on the temperature. A general rule is: the
higher the reforming temperature, the higher the octane number, but the lower the yield of
reformate.
Thermal reforming is less effective and less economic than catalytic processes and has
been largely supplanted. As it used to be practiced, a single-pass operation was employed
at temperatures in the range 540 to 760°C (1004–1400°F) and pressures of about 500 to
1000 psi (34–68 atm). The degree of octane number improvement depended on the extent
of conversion but was not directly proportional to the extent of crack per pass. However at
very high conversions, the production of coke and gas became prohibitively high. The gases
produced were generally olefinic and the process required either a separate gas polymeriza-
tion operation or one in which C3 to C4 gases were added back to the reforming system.
More recent modifications of the thermal reforming process due to the inclusion
of hydrocarbon gases with the feedstock are known as gas reversion and polyforming.
Gaseous olefins gases produced by cracking and reforming can be converted into liquids
boiling in the gasoline range by heating them under high pressure. Since the resulting liquids
(polymers) have high octane numbers, they increase the overall quantity and quality of
gasoline produced in a refinery.

Catalytic Reforming. Like thermal reforming, catalytic reforming converts low-octane
gasoline into high-octane gasoline (reformate). When thermal reforming could produce
reformate with research octane numbers of 65 to 80 depending on the yield, catalytic