Page 167 - Gas Purification 5E

Alkanolainines for Hydrogen Sulfide and Carbon Dioxide Removal 15

amines, MDEA and dimethylethanolamene (DMEA), did not give any evidence of reaction
with COS in these tests. The second order rate constants for reactions between COS and
MEA, DEA, and DIPA have been reported to be 16, 11, and 6 Mol-' s-l, respectively at
25°C. (Danckwerts and Sharma, 1966; Shanna and Danckwerts, 1964; Sharma, 1965).

Mercaptans are substituted forms of H2S in which one of the hydrogen atoms is replaced
by a hydrocarbon group. They have the general formula RSH, and their properties are gov-
erned to a large extent by the length of the hydrocarbon chain, R. Like H2S they have acidic
properties, but because of the hydrocarbon radical, they are much weaker acids than H2S.
Mercaptans behave less like acids and more like hydrocarbons as the hydrocarbon chain
length increases.
Because of their acidic properties, mercaptans can react with alkalies to form mercaptides;
however, the salts are weakly bonded and readily dissociated. As a result, the solubility of
mercaptans in alkaline amine solutions tends to be higher than would be expected on the
basis of simple physical solubility, and increases with increased alkalinity while decreasing
with increased temperature. Since the partial pressure of mercaptans in most gases is very
low and the quantity of solution flowing is based on the more reactive acid gases (H2S and
COz), the percent removal of mercaptans in most amine plants is small.
According to Butwell et al. (1982), the approximate removal efficiencies by MEA and
DEA plant solutions are as follows:

Methy mercaptan 45-558
Ethyl mercaptan 2&258
Propyl mercaptan &lo%

Huval and van de Venne (1981) describe several large DGA plants in Saudi Arabia treat-
ing gas streams containing 34% HzS and 8-14% COz at contact pressures as low as 115
psig. The plants produced sweet gas containing 1-2 ppmv H2S and less than 100 ppmv COP
The.y also removed about 90% of the COS and a small fraction of the mercaptans present in
the feed gas. Data on organic sulfur removal in one of these plants are given in Table 2-26.
The absorbers operated at unusually high temperatures with lean amine temperatures as high
as 150°F and much higher (but unreported) temperatures within the contactors. It is possible
that these high temperatures favored COS removal (by increasing the rate of reaction), but
inhibited mercaptan removal by reducing the equilibrium solubility.
Harbison and Dingman (1972) describe a small DGA plant that was reported to remove
about 989 of the mercaptans from a natural gas stream. This unit operated with a very low
rich solution acid gas loading (0.27 mole acid gas per mole amine). a moderate contactor
bottom temperature (rich solution 156"F), a tall absorber (25 perforated trays), and a concen-
trated amine solution (50% DGA), all of which would be expected to enhance mercaptan
removal efficiency. It should be noted, however, that the 98% efficiency value is based on
the absorber inlet and outlet gas analyses. If the mercaptan removal efficiency is based on
the amount of mercaptan in the feed gas to the absorber and the amount found in the acid gas
stream. a removal efficiency of only about 454 is calculated. Kenney et al. (1994) claim that
DGA solutions can achieve mercaptan removal levels as high as 90%.