Page 219 - Synthetic Fuels Handbook

FUELS FROM SYNTHESIS GAS 205

where ΔH is the heat of the reaction. A more general form of the equation that shows the
chemical balance for higher-boiling hydrocarbons is:

C H + nH O → (n + m/2)H + nCO
n m 2 2
The reaction is typically carried out at approximately 815°C (1499°F) over a nickel
catalyst packed into the tubes of a reforming furnace. The high temperature also causes
the hydrocarbon feedstock to undergo a series of cracking reactions, plus the reaction of
carbon with steam:
CH → 2H + C
4
2
C + H O → CO + H 2
2
Carbon is produced on the catalyst at the same time that hydrocarbon is reformed to
hydrogen and carbon monoxide. With natural gas or similar feedstock, reforming predomi-
nates and the carbon can be removed by reaction with steam as fast as it is formed. When
higher boiling feedstocks are used, the carbon is not removed fast enough and builds up,
thereby requiring catalyst regeneration or replacement. Carbon buildup on the catalyst
(when high-boiling feedstocks are employed) can be avoided by addition of alkali com-
pounds, such as potash, to the catalyst, thereby encouraging or promoting the carbon-steam
reaction.
However, even with an alkali-promoted catalyst, feedstock cracking limits the process
to hydrocarbons with a boiling point less than 180°C (356°F). Natural gas, propane, butane,
and light naphtha are most suitable. Pre-reforming, a process that uses an adiabatic cata-
lyst bed, operating at a lower temperature, can be used as a pretreatment to allow heavier
feedstocks to be used with lower potential for carbon deposition (coke formation) on the
catalyst.
After reforming, the carbon monoxide in the gas is reacted with steam to form additional
hydrogen (the water-gas shift reaction):
CO + H O → CO + H 2 ΔH 298 K =−16,500 Btu/lb
2
2
This leaves a mixture consisting primarily of hydrogen and carbon monoxide that is
removed by conversion to methane:
CO + 3H → CH + H O
4
2
2
CO + 4H → CH + 2H O
2 2 4 2
The critical variables for steam-reforming processes are (a) temperature, (b) pressure,
and (c) the steam/hydrocarbon ratio. Steam reforming is an equilibrium reaction, and con-
version of the hydrocarbon feedstock is favored by high temperature, which in turn requires
higher fuel use. Because of the volume increase in the reaction, conversion is also favored
by low pressure, which conflicts with the need to supply the hydrogen at high pressure. In
practice, materials of construction limit temperature and pressure.
On the other hand, and in contrast to reforming, shift conversion is favored by low
temperature. The gas from the reformer is reacted over iron oxide catalyst at 315 to
370°C (599–698°F) with the lower limit being dictated activity of the catalyst at low
temperature.
Hydrogen can also be produced by partial oxidation (POx) of hydrocarbons in which
the hydrocarbon is oxidized in a limited or controlled supply of oxygen:
2CH + O → 2CO + 4H 2 ΔH 298 K =−10,195 Btu/lb
2
4