Jun-2012

Amine plant corrosion reduced by removal of bicine

Bicine, an amino acid, has been found in numerous amine gas treating systems including tail gas treating units, certain refinery services, and some natural gas processing plants.

Gary L Lawson, Arthur L Cummings and Shade Mecum
MPR Services, Inc

Article Summary

Bicines (bicine and other similar amino acids) are formed in amine gas treating systems as a result of amine degradation due to the presence of oxygen and/or sulfur dioxide.  The presence of Bicine alone in the amine system is not necessarily corrosive. However, Bicine is a strong chelator with iron and can initiate, in the presence of H2S, a fast acting corrosion mechanism. Through both laboratory analytical studies and numerous full scale “in plant” Bicine removal projects, it has been demonstrated that amine system corrosion can be reduced when the amine system is cleaned by ion exchange through the removal of Bicine and the precursors of Bicine. Numerous examples and case histories are presented to demonstrate the benefits of removing Bicine from contaminated amine solutions.

Bicine
Bicine is an amino acid that results from amine degradation due to the presence of oxygen and/or sulphur dioxide. The amine solvents experiencing this degradation are MDEA, DEA, TEA and mixed amine solvents containing any of these amines as components.

Chemical formula

H - O - C - C    C
                /     
                 N     C = O
                  /     /
H - O - C - C    H - O

N,N-bis(2-hydroxyethyl)glycine (bicine)

Formation
Bicine has been shown to form in amine systems utilising MDEA-based amines when subjected to O2 contamination. Plant operating experience indicates a slow but continuous formation of bicine in the amine system when low levels of O2 and/or SO2 are present in the amine system feed gas.

Numerous bicine formation mechanisms have been proposed. The generally accepted O2 degradation path involves one of two mechanisms:
•  A disproportionation reaction of two moles of DEA to form one mole each of TEA and MEA followed by oxidation of the TEA to form bicine
•  A disproportionation reaction of MDEA to TEA and other mixed amines followed by further oxidation of the TEA to bicine. Regardless of the actual formation mechanism, the bicine formation reaction can be generally described as:

                                                 ---  Bicine
O2                                            --- Other amino acids
S2O3 + MDEA + heat + time   --- Formate
SO2                                          ---  Acetate
                                                 ---  DEA
                                                 ---  TEA

Accelerated bicine formation has been noted after upsets in the SRU and the hydrogenation section of the TGTU increased the amount of O2, H2S and SO2 entering the TGTU absorber. The resulting elevated levels of thiosulphate (S2O3) contribute to the formation of bicine and other amino acids.1,2 Removal of the thiosulphate, a heat-stable salt anion precursor for bicine, soon after a TGTU upset could prevent much of the resulting bicine formation in the TGTU amine because the bicine formation mechanism may take several weeks to produce a substantial amount of bicine and the subsequent corrosion.¹

Corrosion
The primary corrosion mechanism in an amine system is H2S attack on carbon steel, with the resultant formation of FeS. In an ideal amine system with a clean amine solution, the protective layer of FeS formed on the carbon steel prevents further corrosion. However, when bicine is present, the protective layer is continuously destroyed or not formed at all. Bicine is a strong chelator and will chelate iron, maintaining the iron in a soluble form and preventing or weakening the stable formation of the protective FeS layer. The unprotected carbon steel is once again attacked by the H2S, resulting in an accelerated corrosion rate.^3,4,5

Bicine by itself in an amine solution is not corrosive to carbon steel. As shown in Figure 1, there is no increase in the instantaneous corrosion rate of a 30 wt % MDEA solution when H2S is not present. The corrosion data presented in Figures 1 and 2 were generated using a mini-amine plant designed to simulate, as closely as possible, an actual amine plant in operation. The corrosion probe was placed in the 250°F “reboiler” section.
The corrosion studies were conducted using a linear polarisation probe to measure instantaneous corrosion rates. The triple electrode probe design eliminates interference by solution conductivity on the indicated corrosion rate.³

Bicine prevents the protective FeS passivation layer from forming. The chelating corrosion mechanism involves the removal of the protective FeS layer by dissolution of the Fe++ by the chelant (bicine). The mechanism proposed is as follows: