Page 63 - Synthetic Fuels Handbook

NATURAL GAS 51

The natural gas should be wet when passing through an iron sponge bed as drying
of the bed will cause the iron sponge to lose its capacity for reactivity. If the gas is not
already water-saturated or if the influent stream has a temperature greater than 50°C
(approximately 120°F), water with soda ash is sprayed into the top of the contactor to
maintain the desired moisture and alkaline conditions during operation.
Slurry processes were developed as alternatives to the iron sponge process. Slurries of
iron oxide have been used to selectively absorb hydrogen sulfide (Fox, 1981; Samuels,
1988). The chemical cost for these processes is higher than that for iron sponge process
but this is partially offset by the ease and lower cost with which the contact tower can be
cleaned out and recharged. Obtaining approval to dispose of the spent chemicals, even if
they are nonhazardous, is time consuming.

2.7.4 Methanol-Based Processes
Methanol is probably one of the most versatile solvents in the natural gas processing industry.
Historically, methanol was the first commercial organic physical solvent and has been
used for hydrate inhibition, dehydration, gas sweetening, and liquids recovery (Kohl and
Nielsen, 1997). Most of these applications involve low temperature where methanol’s
physical properties are advantageous compared with other solvents that exhibit high
viscosity problems or even solids formation. Operation at low temperatures tends to sup-
press methanol’s most significant disadvantage, high solvent loss. Furthermore, methanol
is relatively inexpensive and easy to produce making the solvent a very attractive alternate
for gas processing applications.
The use of methanol has been further exploited in the development of the Rectisol
process either alone or as toluene-methanol mixtures to more selectively remove hydrogen
sulfide and slip carbon dioxide to the overhead product (Ranke and Mohr, 1985). Toluene
has an additional advantage insofar as carbonyl sulfide is more soluble in toluene than in
methanol. The Rectisol process was primarily developed to remove both carbon dioxide and
hydrogen sulfide (along with other sulfur-containing species) from gas streams resulting
from the partial oxidation of coal, oil, and petroleum residua. The ability of methanol to
absorb these unwanted components made it the natural solvent of choice. Unfortunately,
at cold temperatures, methanol also has a high affinity for hydrocarbon constituents of the
gas streams. For example, propane is more soluble in methanol than is carbon dioxide.
There are two versions of the Rectisol process (Hochgesand, 1970)—the two-stage and the
once-through. The first step of the two-stage process is desulfurization before shift conver-
sion; the concentrations of hydrogen sulfide and carbon dioxide are about 1 and 5 percent
by volume, respectively. Regeneration of the methanol following the desulfurization of
the feed gas produces high sulfur feed for sulfur recovery. The once-through process is
only applicable for high pressure partial oxidation products. The once-through process is
also applicable when the hydrogen sulfide to carbon dioxide content is unfavorable, in the
neighborhood of 1:50 (Esteban et al., 2000).
Recently, a process using methanol has been developed in which the simultaneous capa-
bility to dehydrate, to remove acid gas, and to control hydrocarbon dew point (Rojey and
Larue, 1988; Rojey et al., 1990). The IFPEXOL-1 is used for water removal and hydro-
carbon dew point control; the IFPEXOL-2 process is used for acid gas removal. The novel
concept behind the IFPEXOL-l process is to use a portion of the water-saturated inlet feed
to recover the methanol from the aqueous portion of the low temperature separator. That
approach has solved a major problem with methanol injection in large facilities, the metha-
nol recovery via distillation. Beyond that very simple discovery, the cold section of the
process is remarkably similar to a basic methanol injection process. Modifications to the
process include water washing the hydrocarbon liquid from the low temperature separator