Page 69 - Synthetic Fuels Handbook

NATURAL GAS 57

2.7.7 Hydrogenation and Hydrolysis Processes
The reduction of carbonyl sulfide, carbon disulfide, sulfur dioxide, and sulfur vapor in Claus
tail gas to hydrogen sulfide is necessary when sulfur recovery of more than 99.9 percent is
required. Usually the sulfur recovery level is set by the allowable emissions of sulfur from
the tail gas incinerator. In addition, the reduction of carbonyl sulfide is done on raw synthe-
sis gas when the downstream acid gas removal process is unable to remove carbonyl sulfide
to a sufficient extent to meet sulfur emissions regulations from combustion of the cleaned
fuel gas. These sulfur compounds are reduced to hydrogen sulfide by hydrogenation or by
hydrolysis, at a raised temperature, over a catalytic bed.
In these processes, elemental sulfur and sulfur dioxide are reduced mainly via hydro-
genation, while carbonyl sulfide and carbon disulfide are mainly hydrolyzed to hydrogen
sulfide. Sulfur and sulfur dioxide are virtually completely converted to hydrogen sulfide
when an excess of hydrogen is present.
The SCOT (Shell Claus off-gas treating) process was developed in the early 1970s
and consists of a combination of a catalytic hydrogenation/hydrolysis step and an amine
scrubbing unit. The hydrogenation/hydrolysis of the sulfur compounds in the tail gases
from the Claus unit has already been covered above. The early SCOT units consisted of a
hydrogenation/hydrolysis reactor and a conventional amine unit (Fig. 2.7). The Claus tail
gas, after being reduced in the reactor, is cooled in a quench column and scrubbed by a
Sulfinol solution. The clean tail gas goes to a Claus incinerator and the acid gas rich solu-
tion is regenerated in a stripping column. The acid gas off the top of the stripper is recycled
back to the Claus plant for further conversion of the hydrogen sulfide. The absorber is
operated at near atmospheric pressure and the amine solvent is not highly loaded with acid
gases. Because the solution is not highly loaded, unlike high pressure operation, there is no
need for an intermediate flash vessel and the loaded solution goes directly to a stripper.

2.8 USES

Natural gas is approximately currently one quarter of the energy resources of the world with
use projected to increase over the next two decades (Fig. 2.15). However, to understand
the use of natural gas, it is necessary to review the history of natural gas over the past 2000
years as well as the use of natural gas in the United States (Table 2.7).

100%
90% 18
80% 22 26
Other renewables
70%
Hydro
60% 49 Nuclear
50% 44 40
Natural gas
40%
Oil
30%
20% 29 26 24 Coal
10%
0%
1071 1997 2020
FIGURE 2.15 Current and projected use of fossil fuel resources and other fuels until 2020.