Page 62 - Synthetic Fuels Handbook

50 CHAPTER TWO

The regeneration step, that is, the reaction with oxygen is exothermic and air must be
introduced slowly so the heat of reaction can be dissipated. If air is introduced quickly the
heat of reaction may ignite the bed. Some of the elemental sulfur produced in the regenera-
tion step remains in the bed. After several cycles this sulfur will cake over the ferric oxide,
decreasing the reactivity of the bed. Typically, after 10 cycles the bed must be removed and
a new bed introduced into the vessel.
In some designs the iron sponge may be operated with continuous regeneration by
injecting a small amount of air into the sour gas feed. The air regenerates ferric sulfide
while hydrogen sulfide is removed by ferric oxide. This process is not as effective at regen-
erating the bed as the batch process and requires a higher-pressure air stream (Arnold and
Stewart, 1999).
In the process (Fig. 2.9), the sour gas should pass down through the bed. In the case
where continuous regeneration is to be utilized a small concentration of air is added to the
sour gas before it is processed. This air serves to continuously regenerate the iron oxide,
which has reacted with hydrogen sulfide, which serves to extend the on-stream life of a
given tower but probably serves to decrease the total amount of sulfur that a given weight
of bed will remove. The number of vessels containing iron oxide can vary from one to four.
In a two-vessel process, one of the vessels would be on-stream removing hydrogen sulfide
from the sour gas while the second vessel would either be in the regeneration cycle or
having the iron sponge bed replaced.

Raw feed gas

Water
Air

Sponge Sponge
bed bed

Drain Drain Sweet gas
To next process
FIGURE 2.9 Typical iron oxide process flow sheet. Maddox, R. N. Gas and Liquid Sweetening,
2nd ed., Campbell Petroleum Series, Norman, Okla., 1974.

Generally, the iron oxide process is suitable only for small to moderate quantities of
hydrogen sulfide. Approximately 90 percent of the hydrogen sulfide can be removed per
bed, but bed clogging by elemental sulfur occurs and the bed must be discarded and the use
of several beds in series is not usually economical. Removal of larger amounts of hydrogen
sulfide from gas streams requires a continuous process, such as the Ferrox process or the
Stretford process. The Ferrox process is based on the same chemistry as the iron oxide
process except that it is fluid and continuous. The Stretford process employs a solution
containing vanadium salts and anthraquinone disulfonic acid (Maddox, 1974).