Page 222 - Synthetic Fuels Handbook

208 CHAPTER SEVEN

Natural gas is readily available and offers relatively rich stream of methane with lower
amounts of ethane, propane, and butane also present. The thermocatalytic decomposition of
natural gas hydrocarbons offers an alternate method for the production of hydrogen:
C H → nC + m/2H
n m 2
If a hydrocarbon fuel such as natural gas (methane) is to be used for hydrogen pro-
duction by direct decomposition, then the process that is optimized to yield hydrogen
production may not be suitable for production of high-quality carbon black by-product
intended for the industrial rubber market. Moreover, it appears that the carbon produced
from high-temperature [850–950°C (1562–1742°F)] direct thermal decomposition of
methane is soot-like material with high tendency for the catalyst deactivation. Thus, if
the object of methane decomposition is hydrogen production, the carbon by-product may
not be marketable as high-quality carbon black for rubber and tire applications.
Hydrogen sulfide decomposition is a highly endothermic process and equilibrium yields
are poor. At temperatures less than 1500°C (2732°F), the thermodynamic equilibrium is
unfavorable toward hydrogen formation. However, in the presence of catalysts such as
platinum-cobalt [at 1000°C (1832°F)], disulfides of molybdenum (Mo) or tungsten (W) at
800°C (1472°F) , or other transition metal sulfides supported on alumina [at 500–800°C
(932–1472°F)], decomposition of hydrogen sulfide proceeds rapidly . In the temperature
range of about 800 to 1500°C (1472–2732°F), thermolysis of hydrogen sulfide can be
treated simply:
H S → H + 1/xS x ΔH 298 K =+34,300 Btu/lb
2
2
where x = 2. Outside this temperature range, multiple equilibria may be present depending
on temperature, pressure, and relative abundance of hydrogen and sulfur.
Above approximately 1000°C (1832°F), there is a limited advantage to using catalysts
since the thermal reaction proceeds to equilibrium very rapidly. The hydrogen yield can
be doubled by preferential removal of either hydrogen or sulfur from the reaction environ-
ment, thereby shifting the equilibrium. The reaction products must be quenched quickly
after leaving the reactor to prevent reversible reactions.
Shell Gasification (Partial Oxidation) Process. The Shell Gasification Process is a flex-
ible process for generating synthesis gas, principally hydrogen and carbon monoxide, for
the ultimate production of high-purity, high-pressure hydrogen, ammonia, methanol, fuel
gas, town gas or reducing gas by reaction of gaseous or liquid hydrocarbons with oxygen,
air, or oxygen-enriched air.
The most important step in converting heavy residue to industrial gas is the partial
oxidation of the oil using oxygen with the addition of steam. The gasification process takes
o
place in an empty, refractory-lined reactor at temperatures of about 1400°C (2552 F) and
pressures between 29 and 1140 psi (199–7860 kPa). The chemical reactions in the gasifica-
tion reactor proceed without catalyst to produce gas containing carbon amounting to some
0.5 to 2 percent by weight, based on the feedstock. The carbon is removed from the gas
with water, extracted in most cases with feed oil from the water and returned to the feed oil.
The high reformed gas temperature is utilized in a waste heat boiler for generating steam.
The steam is generated at 850 to 1565 psi (5860–10790 kPa). Some of this steam is used
as process steam and for oxygen and oil preheating. The surplus steam is used for energy
production and heating purposes.

Steam-Naphtha Reforming. Steam-naphtha reforming is a continuous process for the
production of hydrogen from liquid hydrocarbons and is, in fact, similar to steam-methane
reforming that is one of several possible processes for the production of hydrogen from