Page 255 - Gas Purification 5E

Mechanical Design and Operation of Alkanolamine Plants 24

Rahman et al. (1989) examined the reactions of COS with several amines. The protonated
amine thiocarbamate was found in the reaction products of COS with MEA, DEA, and DGA. It
was not found with DIPA, but this was attributed to a very slow rate for the COS-DIPA reaction.
No evidence of reaction was found with the tertiary amines MDEA and dimethyethanolamine
(DMEA), and it is generally assumed that tertiary amines do not react irreversibly with COS.
Carbon disulfide reacts with primary and secondary amines, first forming substituted
dithiocarbamates and, subsequently, thiocarbamides (thioureas):

S
II
2(HOCHzCH2)NH + CS2 = (HOCH2CH2)z NCSH2N(CH2CH20H)2 (3-32)

diethanolamine diethanolammonium N,N-bis-
(2-hydroxyethy1)dithiocarbamate

S S
II II
=
(HOCHzCH2)z NCSH~N(CH~CH~OH)Z (HOCH&!H2)2NCN(CH2CH,OH)2 + H2S
diethanolammonium N,N-bis- N,N,N',N'-tetrakis(2-hydroxyethyl)- (3-33)
(2-hydroxyethy1)dithiocarbamate thiocarbamide
A laboratory study of the reactions of carbon disulfide with monoethanolamine,
diethanolamine, and diisopropanolamine has been reported by Osenton and Knight (1970).
The results of this study indicate that all three amines form dithiocarbamic acid salts quite
rapidly. While the salts formed from diethanolamine and diisopropanolamine appear to be
thermally stable, the monoethanolamine salt is less stable and forms one mole of oxazoli-
done 2-thione for each mole of monoethanolamine. The end product of the CS2-DGA reac-
tion is reported to be bis(hydroxyethoxyethy1)thiourea (Kenney et ai., 1994).
The MEA plant tests reported by Pearce et al. (1961) showed that, under the operating
conditions investigated, more than half of the carbon disulfide was released as COS in the
solution stripper, with the balance being hydrolyzed to H2S and COz. This would indicate
that the rate of reaction of monoethanolamine with carbon disulfide is slow and that amine
losses resulting from this reaction are not serious.
Tertiary amines do not react with carbon disulfide.

Reactions with Other Gas Impurities

The irreversible reactions of amines with 02, C02, COS, and CS2 are discussed in the pre-
ceding sections. The principal other potentially reactive gas phase impurities which may
enter the absorber are organic acids (such as formic or acetic acids), HCl, HCN, SOz, NH3,
and mercaptans. All of the acidic compounds form salts with the alkaline amines. If the
acids are stronger than C02 and H2S, their amine salts are not efficiently decomposed under
the stripping conditions (which are designed to decompose the amine-COz and amine-H2S
salts), and they build up in the solution as heat-stable amine salts (HSAS). This, of course,
lowers the capacity of the solution for C02 and H2S, and can lead to other operational diffi-
culties such as corrosion and foaming. Caustic soda or soda ash is often added to the solu-
tion to tie up the acids as sodium salts and release the amine; however, sodium salt build up