Jul-1990

Analysis method for determining bound and free alkanolamines in HSS contaminated solutions

A conductometric titration method has been developed to determine the true available alkanolamine in solutions used for H2S and CO2 removal from refinery gas

Arthur L Cummings, Fred C Veatch, Alfred E Keller, Shade M Mecum and Robert M Kammiller, MPR Services.

Article Summary

In the presence of heat stable salts (HSS) and sodium contamination, traditional indicator-based titration methods were found to be inaccurate by including weak acid anions as available amine. The new method will help operators to correctly monitor alkanolamine systems and to take corrective action for optimisation.

Introduction
Aqueous amine solutions are used at refineries and well sites to scrub acid gases from gas streams. The trapping of the acid gas in the solution involves an acid-base reaction between the acid gas and the amine, which results in a protonated amine and a deprotonated anion of the acid gas. Once protonated, or “bound”, an amine molecule is no longer available (“free”) to trap another molecule of acid gas. Eventually, the scrubbing efficiency of the amine solution decreases to unacceptable level and/or the corrosivity of the solution becomes intolerable. A method of directly measuring bound and free amine concentrations could be helpful for process control.

Regeneration of used amine for reuse is economically and environmentally desirable. Some acids can be stripped from amine solutions by heat, freeing some of the bound amine and reducing the corrosivity. Sodium hydroxide frees bound amine, renewing the acid scrubbing capacity, but not reducing the concentration of anions from acids absorbed by the solution. Regeneration methods could be more efficiently and beneficially conducted if a method of directly measuring bound and free amine concentrations were available.

Analytical methods are available for determining total amine and total HSS, but these do not address the issue of scrubbing typically performed by titration with strong acid. Bound amine, which can be a sensitive indicator of HSS build-up, is generally not directly determined, but is calculated from the difference of two much larger quantities, total and free amine, and may not be accurate.

Titrations for free amine are often practised in such a way that weak acid anions (such as acetate and formate) interfere, and an inflated free amine concentration is reported. Symptoms of this situation include amine concentrations reported at greater than 100% of the amount added to the system, or amine concentrations from analysis not dropping in accordance with rising corrosion rates. If contactor amine flow is set using an erroneous inflated free amine concentration, acid gas loading may be too high, product H2S will be too high, or system corrosion rates will increase. All these lead to a reduction in profits.

This report describes a method by which free and bound amine are determined directly and separately. Total amine is calculated from the sum of the bound and free. Amine concentrations from 100 ppm to 95 wt% can be determined. Unique to this method is the segregation of free amine from weak acid anions and the determination of excess NaOH.

The method employs common titration equipment, but uses conductance, rather than pH, for endpoint detection. Conductance measurement is more rugged than pH, often provides sharper endpoints and reveals much about the chemistry of the amine solution. The method could be used to determine the amine concentration in sour gas scrubbing solutions, to monitor the amine activity in refinery operations, to detect amine in aqueous waste or to follow amine regeneration processes.

Experimental
Procedural details of the conductometric titration are provided in the Appendix. Amine samples were obtained from refinery operations, which use about 45 wt% methyldielthanolamine (MDEA). Measurement of pH was accomplished with a glass electrode and the potentiometer of an automatic titrator, referenced against pH 4 and pH 7 buffers.

Amine reactions
Amine solutions trap acid gases with the aid of a chemical reaction in which the non-volatile amine “salt” is formed. In the solution, the salt primarily exists as dissociated cation and anion:

“Free Amine” + Acid Gas --> “Bound Amine” Cation + Acid Anion

For example:
Amine + RCOOH --> AmineH+ + RCOO-
or
Amine + H2S --> AmineH+ + HS-
If heating the solution reverses the reaction to a significant degree, the amine salt is a “heat labile salt”, as are those formed from hydrogen sulphide and carbon dioxide. For example, in a steam stripper: