constant boiling point with the degradation products, they are shut in and dumped to
the drain. Proprietary chemical treatments, not covered here, are also available on a
contract basis to remove degradation products. Union Carbide has developed a
well-proven corrosion inhibitor system for MEA that allows solution strengths of
28–32 %. The inhibitor requires payment of a royalty. The inhibitor chemicals are
both expensive and hazardous for personnel to handle. The system does reduce
corrosion problems to nearly zero and allows much higher system capacity for the
same size equipment. In addition to the chemical degradation mentioned above,
MEA oxidizes when exposed to air. Storage and surge tanks must be provided with
inert blanket gases such as N 2 or sweet natural gas to avoid this degradation.
Amine systems foam rather easily resulting in excessive amine carry-over in the
contactor. Foaming can be caused by solids such as carbon or iron sulfide; condensed hydrocarbon liquids from the gas stream; degradation products; and almost
any foreign material introduced to the system such as valve grease, excess corrosion
inhibitor, etc. Some of these items, such as iron sulfide or carbon particles, are
removed by cartridge filters. Hydrocarbon liquids are usually removed by the use of
a carbon bed filter on a lean amine side stream (about 10 % of total flow). Corrosion
by-products are removed by reclaiming as noted above.
High skin temperatures on the reboiler/reclaimer tubes promote amine degradation. Steam or hot oil used for the reboiler should be limited to a maximum of
285
F (140
C) to avoid excessive temperatures. The reclaimer should not see hot
oil or steam above 415
F (213
C).
MEA is nonselective in absorbing acid gases. It will absorb H 2 S faster than CO 2 ,
but the difference is not significant enough to allow its use to separate them. With
the lowest molecular weight of the common amines, it has a greater carrying
capacity for acid gases on a unit weight or volume basis. This generally means
less pure amine circulation to remove a given amount of acid gases.
Because the solvent is in solution with water, the gas with which it comes in
intimate contact will leave the contactor at its water saturation point. If dehydration
is necessary, it must be done after the MEA system.
MEA has a vapor pressure 30 times that of DEA (300 times that of TEA and
DGA) at the same temperature. This causes MEA losses of 1–3 lb/MMscf compared to 1/4–1/2 lb/MMscf for the other amine systems using the same system
design parameters. Entrainment and leakage losses prevent low vapor pressure
amines from attaining the predicted losses.
Diethanolamine (DEA)
DEA does not degrade when contacted with CS 2 , COS, and mercaptans as does
MEA. However, for product gas streams which must meet specifications lower than
1 grain per scf of H 2 S, MEA must be used. Because of this, DEA has been developed
as a preferred solvent when these chemicals are present in the stream to be treated.
The reaction with acid gas for any of the amines is a mole-to-mole reaction. As
shown in Table 1, the molecular weight of DEA is 1.7 times that of MEA. Even
after correcting for density, it requires 1.6 lb of DEA to react with the same amount
of acid gas as 1 lb of MEA.
Refinery Gas Treating Processes
503
the drain. Proprietary chemical treatments, not covered here, are also available on a
contract basis to remove degradation products. Union Carbide has developed a
well-proven corrosion inhibitor system for MEA that allows solution strengths of
28–32 %. The inhibitor requires payment of a royalty. The inhibitor chemicals are
both expensive and hazardous for personnel to handle. The system does reduce
corrosion problems to nearly zero and allows much higher system capacity for the
same size equipment. In addition to the chemical degradation mentioned above,
MEA oxidizes when exposed to air. Storage and surge tanks must be provided with
inert blanket gases such as N 2 or sweet natural gas to avoid this degradation.
Amine systems foam rather easily resulting in excessive amine carry-over in the
contactor. Foaming can be caused by solids such as carbon or iron sulfide; condensed hydrocarbon liquids from the gas stream; degradation products; and almost
any foreign material introduced to the system such as valve grease, excess corrosion
inhibitor, etc. Some of these items, such as iron sulfide or carbon particles, are
removed by cartridge filters. Hydrocarbon liquids are usually removed by the use of
a carbon bed filter on a lean amine side stream (about 10 % of total flow). Corrosion
by-products are removed by reclaiming as noted above.
High skin temperatures on the reboiler/reclaimer tubes promote amine degradation. Steam or hot oil used for the reboiler should be limited to a maximum of
285
F (140
C) to avoid excessive temperatures. The reclaimer should not see hot
oil or steam above 415
F (213
C).
MEA is nonselective in absorbing acid gases. It will absorb H 2 S faster than CO 2 ,
but the difference is not significant enough to allow its use to separate them. With
the lowest molecular weight of the common amines, it has a greater carrying
capacity for acid gases on a unit weight or volume basis. This generally means
less pure amine circulation to remove a given amount of acid gases.
Because the solvent is in solution with water, the gas with which it comes in
intimate contact will leave the contactor at its water saturation point. If dehydration
is necessary, it must be done after the MEA system.
MEA has a vapor pressure 30 times that of DEA (300 times that of TEA and
DGA) at the same temperature. This causes MEA losses of 1–3 lb/MMscf compared to 1/4–1/2 lb/MMscf for the other amine systems using the same system
design parameters. Entrainment and leakage losses prevent low vapor pressure
amines from attaining the predicted losses.
Diethanolamine (DEA)
DEA does not degrade when contacted with CS 2 , COS, and mercaptans as does
MEA. However, for product gas streams which must meet specifications lower than
1 grain per scf of H 2 S, MEA must be used. Because of this, DEA has been developed
as a preferred solvent when these chemicals are present in the stream to be treated.
The reaction with acid gas for any of the amines is a mole-to-mole reaction. As
shown in Table 1, the molecular weight of DEA is 1.7 times that of MEA. Even
after correcting for density, it requires 1.6 lb of DEA to react with the same amount
of acid gas as 1 lb of MEA.
Refinery Gas Treating Processes
503