Page 60 - Gas Purification 5E

50 Gas PuriJication

tion of irreversible reaction products with COS and CS2, resulting in excessive chemical
losses if the gas contains significant amounts of these compounds. Furthermore,
monoethanolamine solutions are appreciably more corrosive than solutions of most other
amines, particularly if the amine concentrations exceed 20% and the solutions are highly
loaded with acid gas. This feature limits the capacity of monoethanolamine solutions in cases
where high partial pressures of the acid gases would permit substantially higher loadings.
However, several systems, using effective corrosion inhibitors, reportedly overcome these
limitations. Such systems include Dow Chemical Company’s GAS/SPEC FT-1 technology,
which is suitable for C02 removal in ammonia and hydrogen plants, as well as from sweet
natural gas streams @ow, 1983), and UOP’s Amine Guard Systems (Butwell et al., 1973,
1979; Kubek and Butwell. 1979). In general, corrosion inhibitors are effective in C02
removal systems, permitting MEA concentrations as high as 302 to be used. However, they
have not proven to be reliable in preventing corrosion with C02/H2S mixtures.
Another disadvantage of MEA is its high heat of reaction with C02 and H2S (about 30%
higher than DEA for both acid gases). This leads to higher energy requirements for stripping
in MEA systems. Finally, the relatively high vapor pressure of monoethanolamine causes
significant vaporization losses. particularly in low-pressure operations. However, this diffi-
culty can be overcome by a simple water wash treatment of the purified gas.

Monoethanolamine-Glycol Mixtures

Mixtures of monoethanolamine with di- or triethylene glycol, as first described by Hutchin-
son (1939), were once used extensively for simultaneous acid-gas removal and dehydration of
natural gases. This process, commonly known as the glycol-amine process, has as its principal
advantages the features of simultaneous purification and dehydration and somewhat lower
steam consumption when compared to aqueous systems. Furthermore, glycol-amine solutions
can be stripped almost completely of H2S and C02, resulting in the capability of producing
extremely high purity treated gas. However, the glycol-amine process has a number of draw-
backs which have seriously limited its usefulness. Probably the most important of these is the
fact that, in order to be effective as a dehydrating agent, the water content of the solution has
to be kept at or below 5%. requiring relatively high reboiler temperatures. At these tempera-
tures rather severe corrosion occurs in the amine to amine heat exchangers, the stripping col-
umn, and, under certain operating conditions, the reboiler. The only practical solution to the
corrosion problem is the utilization of corrosion-resistant ferrous alloys or nonferrous metals.
Another undesirable feature of the glycol-amine process is a high vaporization loss of the
amine. Furthermore, because of the very low vapor pressure of the glycol, a contaminated gly-
col-amine solution cannot be reclaimed by simple distillation as is possible with the aqueous
system. Finally, hydrocarbons, especially aromatics, are substantially more soluble in glycol-
amine than in aqueous amine solutions. This feature is of major importance if the acid gas is
to be further processed in a Claus type sulfur plant, as the presence of high molecular weight
hydrocarbons usually leads to rapid catalyst deactivation and production of discolored sulfur.
As a result of these limitations and problem areas, the glycol-amine process is no longer con-
sidered competitive. Details of this process are discussed in earlier editions of this text.
Diethanolamine

Aqueous solutions of diethanolamine (DEA) have been used for many years for the treat-
ment of refinery gases which nody contain appreciable amounts of COS and CS2, besides