Page 242 - Gas Purification 5E

228 Gas PuriJication

(distillation). Additional, occasionally used solution purification techniques are settling, ion
exchange, and electrodialysis.
Fine particles and sludge are removed from amine solutions by settling or filtration. Set-
tling is quite effective; however, large vessels are required to provide sufficient time for the
very small particles (usually iron sulfide) occurring in amine solution to separate by gravity.
Filtration can be very difficult because the iron sulfide, which is formed as a corrosion prod-
uct or enters the plant with the feed gas, is hard to remove. Designs based on etched disk or
sintered metal fiber elements have proved effective for large amine flowrates, although they
are expensive. Simpler, less expensive equipment such as bag and cartridge type filters are
often used, particularly for the smaller amine flow rates.
High molecular weight organic molecules, such as some amine degradation products, can
be removed from amine solutions by adsorption on activated carbon. A properly designed
activated carbon system should treat a 10 to 20% slip stream of the circulating solution. Hot
rich amine tends to liberate acid gas in response to the pressure drop across the mechanical
and activated carbon filters. The liberated gas forms bubbles around the carbon granules and
pockets of acid gas within the carbon filter that inhibit adsorption from the liquid (Leister,
1996). Therefore, the preferred location for carbon adsorbers is on the cooled lean amine.
Activated carbon systems usually include mechanical filters both upstream and downstream
of the carbon bed. The upstream filter minimizes plugging of the bed by fine phcles, and
the downstream unit catches particles of carbon that may be released from the bed.
Solution reclaiming by distillation (thermal reclaiming) effectively removes all non-
volatile species including particulate matter, heat-stable salts, and high molecular weight
organic compounds. Unfortunately, it is not equally effective for all amines, and is most
commonly used for MEA and DGA solutions, which can be vaporized at pressures slightly
above atmospheric without decomposing. Secondary and tertiary amines generally require
vacuum distillation to avoid serious degradation. Since reclaimers designed to operate under
vacuum are appreciably more complex, they are seldom incorporated into plant systems.
When vacuum reclaiming is employed, it is usually performed by an outside contractor.
Ion exchange and electrodialysis processes have been developed to purify solutions that
cannot readily be reclaimed by distillation. However, both processes are capable of removing
only ionized species (such as heat-stable salts) and are ineffective against non-ionized organ-
ic compounds such as many amine degradation products. Since foaming problems are more
apt to be caused by organic compounds than by heat-stable salts, ion exchange and electro-
dialysis are not common remedies for foaming.
Detailed descriptions of filtration, activated carbon adsorption, thermal reclaiming, ion
exchange, and electrodialysis processes are given in a subsequent section of this chapter enti-
tled “Purification of Degraded Solutions.”
Use of Antifoam Agents

Foaming can in many cases be controlled by the addition of foam inhibitors (commonly
called antifoams). The most widely used foam inhibitors are either silicone compounds or
high-boiling alcohols such as oleyl alcohol or octylphenoxyethanol. The silicones are com-
mercially available either as water emulsions or in their pure form. In amine systems, sili-
cones are generally preferable to the high-boiling alcohols.
Usually antifoams are added batchwise when needed. Ballard (1986A, B) states that typi-
cal batch dosage levels are in the range of 5 to 20 ppm; whereas, Meusburger and Segebrecht