Oct-2007

Troubleshooting tail gas amine treaters

Simulation of a tail gas-treating unit contaminated with sodium and heat-stable salt anions. Comparisons are made between plant performance data and the simulations. Accurate solvent analysis leads to improved simulation accuracy

Nathan A Hatcher and Alfred E Keller, ConocoPhillips Company
Ralph H Weiland and M S Sivasubramanian, Optimized Gas Treating

Article Summary

Simulation can play a key role in troubleshooting the performance of a tail gas-treating unit contaminated with sodium and heat-stable salt anions. Comparisons can be made between plant performance data and simulations that assume clean amine, allow HSSs just to inactivate part of the amine, and fully account for the effect of HSSs and alkali metal ions on the chemistry of the system. The outcome demonstrates unequivocally that simulation accuracy is predicated upon accurate solvent analysis.

Amine plant simulation has traditionally assumed the solvent to be perfectly clean, containing only water, amines and acid gases, along with light hydrocarbons and fixed gases. However, outside the laboratory, clean solvents probably exist in most plants for only a short time immediately following initial system charging and startup.

Over time, solvents accumulate contaminants from the gases being treated, through the use of make-up agents (water and amine) that are not completely pure and by the purposeful addition of chemicals (such as certain mineral acids) intended to improve performance. The anions of organic and inorganic acids are referred to as heat-stable salts (HSSs) and include thiosulphate, oxalate, sulphate, glycolate, propionate, acetate, thiocyanate, formate and chloride. Contaminants may also be cationic such as alkali metal ions (sodium, potassium, calcium and magnesium) that accumulate from the result of make-up water hardness or through deliberate addition in the form of hydroxides or carbonates to deprotonate amine associated with HSS anions. All of these ions can have a profound effect on amine-treating unit performance.

Process and solvent chemistry
Clean solvents are water solutions of one or more amines together with the acid gases CO2 and H2S. The reactions of H2S and CO2 with amines generate ions, and all involve protonation of the amine. The reactions are reversible and the reactants are all volatile species so the solvent can be stripped thermally, an essential characteristic in treating with amines. As an example, the reversible reaction of H2S with amine is illustrated as follows:

H2S + R1R2R3N ÷ R1R2R3NH+ + HS-

After a period of use, especially in treating sour gases generated from refinery cracking operations (cokers, FCCs), trace amounts of acid anion contaminants can build to significant levels in the solvent. The commonly found acid anions are formed by oxidation, hydrolysis and other reactions of contaminants that enter the treating system with the sour gas. These reactions all ultimately produce protonated amine as a reaction product, as generically shown by the following reaction. For a strong acid HnX, where X is an n-valent anion (Cl-, SO4=, etc), the reaction with amine is:

HnX + n R1R2R3N ’ n R1R2R3NH+ + X-n

Thus, HSSs are an additional source of protonated amine, above and beyond the protonation that results from simple acid gas absorption. HSS anions are generated from much stronger acids than the acid gases being treated and their reactions with amines are irreversible. Thermal regeneration is not possible so they permanently tie up part of the amine as R1R2R3NH+ ion.

In a lean MDEA solution at regenerator temperatures (ie, towards the bottom of the column and in the reboiler), the total acid gas loading is usually quite low, so the concentrations of HS-, HCO3- and protonated amine are also small. However, as the solvent becomes increasingly contaminated with acid anions that are stronger than HS- and HCO3-, the concentration of protonated amine at the regenerator’s lean end becomes determined by the extent of contamination rather than the acid gas loading. The higher protonated amine concentration drives the acid gas reactions strongly towards product decomposition. Thus, all other conditions being equal, solvent contaminated with relatively strong acid is always easier to regenerate than a clean one. Of course, this forcing of the reverse reaction also occurs in the absorber; however, the extent of reduction in lean loading usually far outweighs the negative effect on absorption.

With increasing time on-stream, acidic contaminants continue to build until a point is reached where something must be done to recover the permanently neutralised, bound amine to restore free amine circulation capacity or to prevent the amine unit from self-dissolving through corrosion. Some operators employ the addition of a stronger base (typically NaOH or KOH) to “neutralise” the amine HSS; that is, remove the proton from the protonated amine and “attach” the anion to the alkali metal cation. This forms the sodium or potassium salt of the HSS (ie, NaX or KX) and releases R1R2R3NH+ or R1R2NH2+ back into the free amine forms R1R2R3N or R1R2NH. Using NaOH as an example:

NaOH + R1R2R3NHX ’ R1R2R3N + NaX + H2O               
However, unless carried out very carefully, neutralisation with strong bases can have disastrous, unforeseen consequences. The danger lies in the possibility of over-neutralisation. If the solvent becomes over neutralised, the excess caustic irreversibly binds H2S 
and CO2:

NaOH + H2S ’ Na+ HS- + H2O
               
NaOH + CO2 ’ Na+HCO3-                           
The resulting elevated concentrations of HS- and HCO3- remain in solution no matter how hard the solvent is stripped in the regenerator; ie, there is a permanently high, heat-stable lean loading. In some cases, an order-of-magnitude increase in lean loading has been observed. The net result is failure to meet treated gas specifications by a wide margin, which cannot be overcome using more reboiler energy or an increased solvent circulation rate. The excess sodium associates with HS- and HCO3-, and the situation will remain that way until HSSs again build up to a level sufficient to displace these ions fully and consume all the excess caustic.

To understand and simulate quantitatively the effect of HSSs and alkali metal ions on treating performance, one must be able to model the regenerator with just as much confidence and accuracy as the absorber — the regenerator sets lean solution quality, the main controlling factor at the lean end of the absorber where performance is determined. It is equally crucial to account for ionic contaminants and to use a comprehensive solvent analysis.