Page 256 - Gas Purification 5E

242 Gas Purification

is also undesirable, and an upper limit of 10 wt% is commonly set. Techniques for removing
sodium salts from amine solutions are discussed in the next section.
Hydrogen cyanide, HCN, is also an acid, and as such, is absorbed in the alkaline amine
solution. Unlike most other acids, however, it is weaker than CO,, and is usually released
from the amine solution in the stripper without degrading the amine. However, if both H2S
and oxygen are present, the HCN can react to form thiocyanate, which is the anion of a
strong acid and forms a heat-stable salt.
Sulfw dioxide is a relatively strong acid and reacts rapidly with amines to form sulfite.
However, the sulfite does not generally remain in solution as a heat-stable salt because of its
high reactivity. For example, SO2 in solution reacts rapidly with H2S by the Claus reaction
(which is catalyzed by liquid water) to form elemental sulfur and polysulfides. Zero valent
sulfur can, in turn, disproportionate to form thiosulfate, which is a heat-stable anion. The sul-
fite may also be oxidized to sulfate which forms a heat-stable amine salt.
Ammonia is a common impurity in gases derived from coal. It is not an acid, but is readily
absorbed by amine solutions because of its extremely high solubility in water. It does not
react chemically with any of the amines, and is generally expelled in the stripper. However,
its presence can cause operating problems such as corrosion in the stripper overhead system
(see prior section on wet acid gas corrosion).
Mercaptans are related to H2S. They are slightly acidic and are, therefore, absorbed to
some extent by the alkaline amine solutions. Rahman et al. (1989) looked for possible irre-
versible reactions between mercaptans and MEA, DEA, DIPA, and MDEA and found none.

PURIFICATION OF DEGRADED SOLUTIONS

As previously discussed, amine solutions are degraded by reaction with CO,, oxygen,
organic sulfur compounds, and other gas impurities to form heat-stable salts and amine
degradation products. These contaminants cause corrosion and lower the treating capacity of
the amine solution. In commercial plant operations, the concentration of amine decomposi-
tion products should not be allowed to exceed about 10% of the active amine concentration.
Once amine contamination has reached this level, contaminants should be removed by solu-
tion purging, solution change-out, or amine solution purification.
The operation of sidestream purification units makes it possible to maintain a constant con-
centration of active amine in the treating solution and prevent the accumulation of undesirable
degradation products. Techniques used to purify amine solutions include distillation under vac-
uum or low pressure (thermal reclaiming), ion exchange, electrodialysis, and adsorption. Ther-
mal reclaiming is the most commonly used process; however, both ion exchange and electro-
dialysis have become more common in recent years. Mechanical filtration and adsorption using
activated carbon are very useful to control foaming; however, their application for solution
purification is generally limited to the removal of solid particles (by filtration) and the removal
of high-boiling or surface active organic compounds (by adsorption).
Table 3-6 provides a generalized comparison of mechanical filtration, activated carbon
adsorption, thermal reclaiming, ion exchange, and electrodialysis for purifying amine solutions.

Amine Solution Mechanical Filtration

Suspended solids and surface-active contaminants can be removed from the treating solu-
tion by continuous mechanical and activated carbon fitration of a side stream. For large sys-
tems, Ball and Veldman (1991) recommend continuous filtration of a minimum of 10 to 15%