Page 212 - Gas Purification 5E

198 Gas Purijkation

Table 3-1
Removal of Corrosive Components from an FCCU Gas Weam by a Single Stage Water
Wash Followed by Amine Contacting

Removed by Absorbed by Balance to
Water Wash Diethanolamine Solution Gas Plant
Total
Constituents (Pounds (Pounds
(Pounds per (Pounds per Per Per
Constituents Day) Day) (Percent) Day) (Percent) Day) (Percent)

Ammonia 7,400 6,300 85 700 10 400 5
Sulfides 86,000 21,100 24 64,200 75 700 1
Carbonates 32,000 4,000 12 21,500 67 6,500 20
Cyanides and
Thiocyanates 2,000 1 ,OOo 50 400 20 600 30
Phenols 7,100 1,750 25 100 1 5,250 74
Aliphatic Acids 6,500 1,850 28 1,600 25 3,050 47
TOTAL 141,000 36,000 25 88,500 63 16,500 12
Source: Neumaier and Schillmoller (1955)

Upstream Ammonium Polysulfide Wash. Corrosion due to cyanide and ammonium
bisulfide can be greatly reduced by washing the gas upstream of the amine absorber with an
ammonium polysulfide solution (Ehmke, 1981A, B). In the upstream wash, the ammonia is
dissolved in the wash water while the ammonium polysulfde reacts with CN- to form thio-
cyanate ion, SCN-, by reaction 3-12. Thiocyanic acid, HSCN, is a stsong acid and is more
water-soluble than HCN. Ehmlre (1981A, B) presents data that show that ammonium polysul-
fide in the upstream wash can reduce CN- concentrations to less than 10 ppmw, which is low
enough to prevent both HIC and SSC (Neumaier and Schillmoller, 1955; NACE, 1994B).

Sx-z + CN-= S,1-2+ SCN- (3-12)
Ammonium polysulfide is preferred over sodium polysulfide because it reacts faster with
CN- and does not raise the pH of the wash water (Ehmke, 1981A, B). Design and operating
guidelines for ammonium polysulfide wash systems are provided by Ehmke (1981A, B).
Design principles are reviewed by Bucklin and Mackey (1983) who recommend the use of a
multistage counter-current contactor with 8 to 10 actual trays. The water wash system
described in this reference uses stripped sour water containing 50 ppm NH3 to reduce the
ammonia in the washed gas to 0.1 ppm. Ammonium polysulfde is added to the wash water
to convert the cyanide to thiocyanate. Additional polysulfide wash design and operating data
are provided by Carter et al. (1977), Penderleith (1977), Lynch (1982), and Donovan and
Laroche (1981). With an upstream ammonium polysulfide wash to remove both ammonia
and HCN, the amine regenerator overhead system can be all carbon steel.

Regenerafor Reflux Wafer Stripping. Neumaier and Schillmoller (1955) outline another
method for eliminating corrosion by ammonia and cyanide in the amine regenerator over-