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of the circulating solution. Most mechanical amine filters and the downstream activated car-
bon filter are located downstream of the lean amine pump and lean amine cooler. Usually a
10 to 20% slipstream of lean amine is filtered. See Figure 3-1. Filtering the cold, lean solu-
tion is preferred because, if the feed gas contains HzS, filter maintenance is safer at this loca-
tion because H2S is not present in lethal concentrations. Also, the activated carbon filter is
more effective on the lean side because contaminants are less soluble and more surface-
active at lower temperatures (Pauley, 1991; Bacon, 1987).
However, in systems using DEA, MDEA, or MDEA mixtures to treat gases with low
H2S/CO2 ratios, the most effective location for both the mechanical and activated carbon fil-
ter may be downstream of the rich amine flash tank and upstream of the ledrich exchanger
(Smith and Younger, 1972; Perry, 1974). With MDEA and DEA, amine degradation prod-
ucts cannot be removed by online slipstream reclaiming, and polymeric amine degradation
products, which can chelate iron molecules, can build up in the amine solution. These amine
degradation products can dissolve the protective iron sulfide film to form iron chelates which
can, in turn, react with HzS in the feed gas to form insoluble iron sulfide particles in the
amine contactor. For MDEA and DEA solutions or mixed solutions containing MDEA and
another amine, this can lead to plugging of the amine contactor and regenerator trays and
other piping and equipment in contact with the rich amine solution (Smith and Younger,
1972; Perry, 1974). Placing the mechanical filter on the rich amine side minimizes plugging
of the ledrich exchanger and amine regenerator with iron sulfide particles.
Several types of mechanical filters have been used to remove solids and viscous liquids
from alkanolamine solutions. These include string-wound cartridge, cotton sock, molded cel-
lulose, pressed cloth, proprietary cartridge, bag, stacked paper, pcoat, etched disk, and sin-
tered metal fiber filters. Guidelines for selecting filters are provided by Ballard and von Phul
(1991), Fabio (1992), and Pauley (1991). Mechanical filters are generally divided into two
types: surface or depth type filters. Surface filters collect the particulate contaminants on the
surface of the filter media. Depth filters trap particles inside their structure. Depth filters are
usually used in relatively dirty services such as amine filtration as they have more dirt hold-
ing capacity than surface filters. Since depth filters trap suspended solids within the filter
pure structure, they usually cannot be reused and must be disposed of when they are spent.
Depth filters are either flexible (e.g., string-wound cartridges which deform as the differen-
tial pressure increases) or rigid (e.g., proprietary polypropylene cartridge filters). Rigid-depth
filters have either a uniformly sized or a graded structure. The openings in graded structure
depth filters become progressively smaller; whereas, depth filters with a uniformly sized
structure have the same size openings throughout the filter media matrix (Ballard and von
Phul, 1991). In amine service, depth filters with a graded structure usually have greater dirt
holding capacity at a given micron rating than depth filters with a uniform structure.
Filter performance is usually quoted in terms of either a “nominal” or an “absolute” rating,
e.g., 5 microns absolute or 5 microns nominal. Nominal ratings are assigned by the manufac-
turer and are not necessarily indicative of performance unless the manufacturer provides the
removal efficiency in addition to the micron rating. For example, a filter with a nominal rating
of 5 microns may capture only 30% of the particles that size, while another filter with a nomi-
nal 5 micron rating may capture 90%. Therefore, nominal ratings that do not include capture
efficiency do not permit the comparison of one filter with another. Some filters have an
absolute rating. An absolute rating is the diameter of the largest hard spherical particle that
will pass through the filter. Most commercial amine filters do not have an absolute rating and
even those filters that have an “absolute” or a “nominal” rating are not tested with an industry
standard test. Given this lack of standardization and the variety of filter designs that are avail-