Oct-2006

Troubleshooting a refinery fuel gas treater

The effect of solvent contaminants on treating plant performance and the importance of basing simulations on actual solvent analyses are shown through the successful troubleshooting of an MDEA-based fuel gas H2S-treating system

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

Article Summary

Commercial software packages for amine plant simulation use column models that range from simple equilibrium stages, equilibrium stages modified for reaction kinetics, equilibrium stages with computed stage efficiencies, right through to true mass and heat transfer rate models. Regardless of the underlying principles on which each one is based, simulations of amine plants have traditionally assumed the solvent to be perfectly clean, meaning it contains only water, amines and acid gases. In some cases, the solubility of light hydrocarbons and inert gases may be taken into account. 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 primarily from the gases being treated or through the use of make-up agents (water and amine) that are not completely pure. Contaminants of interest here are frequently the anions of organic and inorganic acids, also called heat-stable salts (HSSs). Anions commonly found in amine solutions include thiosulphate, oxalate, sulphate, glycolate, propionate, acetate, thiocyanate, formate and choride, which usually enter the solution with the gases or liquids being treated. The use of sulphate or phosphate to enhance H2S removal in tail gas-treating units (TGTUs) is becoming increasingly common. 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 of hydroxides to deprotonate amine associated with heat-stable amine salt anions. All of these ions can have profound effects, sometimes positive, sometimes negative, on the perfor-mance of amine-treating units.

Chemistry
Clean solvents consist of water solutions of one or more amines together with the acid gases CO2 and H2S. A number of reaction equilibria are set up, all involving ionic species and all depending on the presence of water. For a system containing a single amine together with H2S and CO2 in aqueous solution, five equilibrium ionic reactions involving amine occur. If the amine is either primary (R2, R3 = H) or secondary (R3 = H), reaction with CO2 will form the carbamate:

R1R2R3N + H2O 1  R1R2R3NH+ + OH-

H2S + R1R2R3N 1  R1R2R3NH+ + HS-

HS- + R1R2R3N 1  R1R2R3NH+ + S=   

CO2 + R1R2R3N + H2O 1  R1R2R3NH+ + HCO3-

HCO3- + R1R2R3N 1  R1R2R3NH+ +CO3=

CO2 + 2 R1R2R3N 1  R1R2NCOO- + R1R2NH2+ (primary or secondary)

It is useful to note that protonated amine is common to all the reactions. The thermal reversibility of these reactions makes treating with amines economically feasible. But things can go awry when gases contain certain types of components, even in seemingly quite small amounts.

After a period of use, especially in treating sour gases generated from refinery cracking operations (cokers and FCC), trace amounts of acid anion contaminants can build to significant levels in the solvent. The most commonly found acid anions, formate and thiocyanate, result from the absorption of hydrogen cyanide, with formate being formed by the hydrolysis of the cyanide ion to ammonium formate, and with thiocyanate being formed from dissolved oxygen reacting with H2S followed by reaction of the oxysulphur anion with cyanide ion. Higher molecular weight organic acid anions come from the hydrolysis of higher molecular weight nitrile compounds. Ammonium ion from the hydrolysis will release H+ to the amine and be steam stripped in the regenerator where it accumulates in the overhead condensing system, leaving the protonated amine/HSS anion pair in the amine solution. Hydrogen from gasoline reformers can contain HCl, which will directly acid-base neutralise the amine.

Thiosulphates generally result from the reaction of dissolved oxygen with H2S or from the SO2 reaction with H2S in Claus tail gas units when no HCN is present. Sulphates can be formed either from the absorption of sulphuric acid or from further oxidation of thiosulphates. These are shown in the following reactions:

RCN + 2 H2O γ NH4+ + RCOO- (R=H, alkyl group)

NH4+ + R1R2R3N 1  R1R2R3NH+ + NH3

2 HCN + O2 + 2 H2S + 2 R1R2R3N γ 2 R1R2R3NH+ + 2 SCN- + 2 H2O

2 H2S + 2 O2 + 2 R1R2R3N γ 2 R1R2R3NH+ + S2O3= + H2O

2 H2S + 4 SO2 + H2O + 6 R1R2R3N γ 6 R1R2R3NH+ + 3 S2O3=

S2O3= + 5/2 O2 γ 2 SO4=
For a strong acid HnX, where X is an n-valent anion (Cl-, SO4=), the reaction with amine is:
HnX + n R1R2R3N ( n R1R2R3NH+ + X-n

Unlike the acid gas-amine reactions, reactions with HSSs are not thermally reversible (thus the term heat-stable salt), so the HSSs permanently tie up part of the amine as R1R2R3NH+ ion.1 The fact that the amine is gradually degraded to HSAS and becomes inactivated is bad enough. But far more worrying is its effect on the ability to regenerate the rich solvent to satisfactory acid gas lean loadings (moles of acid gases per mole of total amine) and to use the regenerated solvent effectively in the absorber.2 Heat-stable salt anions are also known to complex iron ion and accelerate corrosion in the hot, lean section of the amine unit. Iron sulphide particles are formed when the complexed iron contacts higher concentrations of H2S in the absorber. These particles can foul equipment, lead to loss of treating capacity and further exacerbate corrosion by eroding the protective iron sulphide film on the surface of carbon steel equipment.