Environmental Engineering Reference
In-Depth Information
through catalyst-filled tubes. Hydrogen and carbon oxides are produced.
Steam reformers are also used in syngas, ammonia and methanol plants.
The hydrogen producing reactions are limited by thermodynamic
equilibrium. The reactions must take place under carefully controlled ex-
ternal firing, with heat transfer taking place from the combustion gas in
the firebox to the process gas in the catalyst-filled tubes. Carbon monox-
ide in the product gas is converted almost completely to hydrogen in the
downstream catalytic reactor.
Hydrocarbon feedstocks for steam reformers include natural gas, re-
finery gas, propane, LPG and butane. Naphtha feedstocks with boiling
points up to about 430°F can also be used. The ideal fuels for steam re-
formers are light hydrocarbons such as natural gas and refinery gas, al-
though distillate fuels are also used. Residual fuels are not used since they
contain metals that can damage reformer tubes.
Reformers are fired to maintain a required process gas outlet tem-
perature. Most modern reformers are top fired. In a top-fired reformer, the
burners are located at the top of the furnace and fire downward. Process
gas flows downward through catalyst-filled tubes. This flow of process
gas and flue gas allows the highest flue gas temperature when the in-tube
process gas temperature is lowest and the lowest flue gas temperature
when the in-tube process gas temperature is highest. This results in tube-
wall temperatures that are uniform over the tube's length and since the
average tubewall temperature is lower this reduces tube cost and increas-
es tube life.
Also, as the flue gas cools, it sinks in the same direction as its nor-
mal flow which results in stable furnace operation. Flue gas back-mixing
is avoided and the flue-gas outlet temperature is closer to the process-gas
outlet for maximum furnace efficiency.
The burners at the reformer's top are in an enclosure called a pent-
house. The flue gas is collected at the bottom in horizontal fire-brick ducts
called tunnels. Flue gas exits horizontally into a waste heat recovery
(WHR) unit. Combustion gas is drawn through the WHR unit by an in-
duced-draft fan and then discharged to the atmosphere through a stack.
The shift reaction is independent of pressure, but the reforming reac-
tion equilibrium is favored by low pressure. At lower pressures, the con-
version of hydrocarbon to hydrogen is higher.
In most hydrogen plants, a pressure-swing adsorption (PSA) system
is used for hydrogen purification. In these plants, a major portion of re-
former fuel is PSA offgas with a hydrocarbon stream for makeup fuel.
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