Page 246 - Gas Purification 5E

232 Gas PuriJcation

Entrainment

In many cases, the major cause of amine loss is entrainment (Veldman, 1989). Entrain-
ment losses are caused either by inefficient mist extraction or by foaming and subsequent
carry-over of solution. This problem can be minimized by the use of efficient mist elimina-
tion equipment and the application of foam inhibitors. Mist elimination equipment includes
an efficient treated gas knockout drum downstream of the amine contactor as well as the
installation of mist elimination equipment (chevron mist eliminators and wire and fiber mesh
pads) inside the amine contactor or treated gas knockout drum. Pauley (1991) recommends
fiber mesh pads rather than a wire mesh pad or chevron mist eliminator because fiber mesh
pads can capture aerosol particles that are less than 3 microns in diameter. Wire and fiber
mesh pads should, however, be used with caution in amine contactors treating gas streams
that contain olefins or traces of oxygen because the mesh can gradually plug with elemental
sulfur or polymers formed by the reaction of olefinic gas molecules with each other or with
oxygen. In these circumstances, chevron mist eliminators should be considered because they
are more resistant to plugging.
Entrainment losses from an amine absorber vary considerably depending on the mechani-
cal design of both the upper section of the absorber and the mist elimination device. Veld-
man (1989) states that entrainment in a properly designed absorber should average less than
0.5 lb amine/hlhlscf of treated gas, but notes that entrainment of well over 3 lb/MMscf is not
uncommon. Veldman recommends that amine absorbers be designed for 60% or less of
flooding and that demister pads be installed in properly sized treated gas knockout drums.
Entrainment losses from an amine regenerator are generally low because the regenerator
water wash trays limit these losses. In some amine systems LPG liquid-liquid amine treaters
are the major source of amine entrainment losses. For example, losses of up to 500 ppmw of
amine in the treated LPG product have been reported (Veldman, 1989). See Chapter 2 for a
discussion of LPG treater losses and methods to minimize these losses.

Solution Degradation

Reaction with Oxygen

Alkanolamines are subject to degradation by contact with free oxygen. Several mechanisms
have been identified, the principal ones involving the direct oxidation of the amines to organic
acids and the indirect reaction of oxygen with HzS to form elemental sulfur, which then reacts
with the amines to form dithiocarbamates, thiourea, and further decomposition products. A
third route whereby oxygen can degrade amines is the oxidation of H2S to stronger acid
anions such as thiosulfate, which ties up amine as a heat stable amine salt (HSAS).
Monoethanolamine appears to be more vulnerable to oxidation than secondary and tertiary
amines. Hofmeyer et al. (1956) have shown that MEA is subject to oxidative deamination
that results in the formation of formic acid, ammonia, substituted amides, and high molecular
weight polymers.
The role of oxygen in the formation of carboxylic acids was investigated by Blanc et al.
(1982A, B). They conducted experiments in which air was bubbled through aqueous amine
solutions at 194°F (9OOC). After thirty days of operation, the solutions were analyzed and
found to contain the following acid concentrations: DEA-O.88 formic acid, 0.15% oxalic
acid, and 0.02% acetic acid; MDEA-O.3% formic acid; MEA-2.8% formic acid (the
MDEA and MEA apparently were not analyzed for oxalic and acetic acids).