Page 220 - Synthetic Fuels Handbook

206 CHAPTER SEVEN

The shift reaction also occurs and a mixture of carbon monoxide and carbon dioxide
is produced in addition to hydrogen. The catalyst tube materials do not limit the reac-
tion temperatures in partial oxidation processes and higher temperatures may be used that
enhance the conversion of methane to hydrogen. Indeed, much of the design and operation
of hydrogen plants involves protecting the reforming catalyst and the catalyst tubes because
of the extreme temperatures and the sensitivity of the catalyst. In fact, minor variations in
feedstock composition or operating conditions can have significant effects on the life of
the catalyst or the reformer itself. This is particularly true of changes in molecular weight
of the feed gas, or poor distribution of heat to the catalyst tubes.
Since the high temperature takes the place of a catalyst, partial oxidation is not limited
to the lower boiling feedstocks that are required for steam reforming. Partial oxidation
processes were first considered for hydrogen production because of expected shortages of
lower boiling feedstocks and the need to have a disposal method available for higher boil-
ing, high-sulfur streams such as asphalt or petroleum coke.
Catalytic partial oxidation, also known as autothermal reforming, reacts oxygen with a
light feedstock and by passing the resulting hot mixture over a reforming catalyst. The use
of a catalyst allows the use of lower temperatures than in non-catalytic partial oxidation and
which causes a reduction in oxygen demand.
The feedstock requirements for catalytic partial oxidation processes are similar to
the feedstock requirements for steam reforming and light hydrocarbons from refinery
gas to naphtha are preferred. The oxygen substitutes for much of the steam in preventing
coking and a lower steam/carbon ratio is required. In addition, because a large excess
of steam is not required, catalytic partial oxidation produces more carbon monoxide and
less hydrogen than steam reforming. Thus, the process is more suited to situations where
carbon monoxide is the more desirable product, for example, as synthesis gas for chemical
feedstocks.

7.2.2 Processes

In spite of the use of low-quality hydrogen (that contain up to 40 percent by volume hydro-
carbon gases), a high-purity hydrogen stream (95–99 percent by volume of hydrogen) is
required for hydrodesulfurization, hydrogenation, hydrocracking, and petrochemical pro-
cesses. Hydrogen, produced as a by-product of refinery processes (principally hydrogen
recovery from catalytic reformer product gases) often is not enough to meet the total refin-
ery requirements, necessitating the manufacturing of additional hydrogen or obtaining sup-
ply from external sources.

Heavy Residue Gasification and Combined Cycle Power Generation. Heavy residua
are gasified and the produced gas is purified to clean fuel gas . As an example, solvent
deasphalter residuum is gasified by partial oxidation method under pressure of about
570 psi (3930 kPa) and at temperature between 1300 and 1500°C (2372–2732°F). The
high-temperature-generated gas flows into the specially designed waste heat boiler, in
which the hot gas is cooled and high-pressure saturated steam is generated. The gas from
the waste heat boiler is then heat exchanged with the fuel gas and flows to the carbon
scrubber, where unreacted carbon particles are removed from the generated gas by water
scrubbing.
The gas from the carbon scrubber is further cooled by the fuel gas and boiler feed water
and led into the sulfur compound removal section, where hydrogen sulfide (H S) and car-
2
bonyl sulfide (COS) are removed from the gas to obtain clean fuel gas. This clean fuel gas
is heated with the hot gas generated in the gasifier and finally supplied to the gas turbine at
a temperature of 250 to 300°C (482–572°F).