Page 255 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

8.1 Fuel Cleaning 231

8.1.2.2 Adsorptive Claus Process
Recently, increasing restrictive low-sulfur fuel regulations challenge the conven-
tional Claus process. Depletion of low-sulfur feedstocks for the petroleum industry
demands more improved or new technologies to maximize the H 2 S conversion.
Elsner et al. [13] proposed an improved process called adsorptive Claus process
based on the Le Chatelier’s principle [47], which implicates that the removal of a
reaction product results in an equilibrium displacement to higher conversion.
Unlike the conventional Claus process, where elemental sulfur is removed by
in situ condensation, the adsorptive Claus process removes water by selective
adsorption. According to the reaction equilibrium constant described in Eq. (8.3),
removal of water vapor would have a greater impact on the conversion rate than
sulfur removal, because there are 2 mol of water per 3/8 mol of sulfur in the
product.
As introduced in Chap. 5, there are several water vapor adsorbents, and one of
them is zeolite which can be regenerated by inert sweep gas. A challenge to this
process is the chemical resistance of the catalyst and the zeolite to the aggressive
gas system, where SO 2 and water may react and produce sulfuric acids.

8.1.2.3 Natural Gas Sweetening

Natural gas sweetening is important to both environment and final product quality.
Natural gas contains a large amount of methane (CH 4 ), and all kinds of impurities
as shown in Table 8.1. The mole amount of a substance in a raw natural gas
depends highly on the gas field.
In industry, sour gas is referred to high contents of H 2 S and CO 2 . These acidic
gases in the raw natural gas are removed at the gas well to reduce technological
challenges to downstream gas transportation in the pipeline, and equally important,
to reduce SOx and CO 2 emissions.
First of all, it prevents the formation of gas hydrate (commonly called dry ice).
CO 2 hydrate can clog the system during the liquefaction of the natural gas. Fur-
thermore, it reduces the corrosion resulted from H 2 S and CO 2 in the presence of

Table 8.1 Typical Gases Mole ratio
composition of raw natural
70–95 %
gas out of the well CH 4
H 2 S 0–15 %
C2+ 0–15 %
0.1–8%
CO 2
N 2 0–0.2 %
Temperature 30–40 °C
Pressure 5–120 atm
Source Ramdin et al. [40]