Sep-2010

Better alkanolamine system operations through chemical analysis

You can have better operating amine and glycol systems. An alkanolamine acid gas scrubbing system is an elegantly simple concept.

Arthur L Cummings and Glen D Smith
MPR Services, Inc

Article Summary

A solution of water and alkanolamine absorbs acid gases from petroleum gas or liquid, and is pumped to a heated regenerator that releases the acid gases, and then the amine is cooled as it returns to the absorber. The amine solution circulates happily forever and the simple acid base chemistry of the process can be monitored by a few analytical titrations (Table 1). Operators need be concerned with monitoring only temperatures, pressures and flow rates, and with balancing the amine absorbing capacity with the acid gas demand of the incoming petroleum gas or liquid so that the product gas or liquid meets specifications. (The acid gas content of the sweetened product is, of course, the ultimate control measure, but is beyond the scope of this paper.) Such is the design of amine systems and the conditions for which the most common analytical methods were developed.

If no contaminants accumulated in the amine system, this could be the extent of the analytical information required to operate. Unfortunately, contaminants do accumulate in amine systems and affect equipment longevity and the success of the operation of the amine system. More unfortunate is the fact that the contaminants can affect the results of the fundamental analytical methods, misleading the operator, yet the operator continues to rely on these few simple tests for day-to-day operations.

Increasing awareness of the effects of contaminants on operations has been accompanied by increasing understanding of the variety of contaminants that exist in amine systems. Common analytical methods have been adapted for, adjusted for, or misapplied to contaminants.    Contaminant-specific analytical methods have multiplied. The amine system operator can now be confronted with a maze of analytical parameters, a blur of analytical results and a host of analyte names and acronyms that can be ambiguous, confusing and even misleading.

Better analytical methods are needed
A prime example of misleading results is illustrated in Figure 1 and Table 2. When weak acids (such as formic acid and acetic acid) have accumulated in the amine solution, forming heat stable salts (HSS), the titration to determine free amine can also respond to the weak acid anions. The choice of pH for the endpoint of the titration determines whether the free amine titration is accurate or over-estimates the amine strength. A pH or colour indicator that provides accurate amine strength in a clean amine solution can grossly over-estimate the amine strength of a solution that contains weak acid anions (HSS or LL). Note, for example, in Figure 1 that Methyl Purple and Bromthymol Blue are both acceptable indicators for the titration of clean amine solutions, but fail miserably if the solution contains significant weak acid HSS. The same is true for a pH “dead-stop” titration. The most common amine strength titration methods were developed for amine solutions with no contaminants. Thus, contaminants may cause errors.

Engineers and chemists need an understanding of analyses
Armed with the preceding information, the engineer responsible for the amine system can now ask the analytical chemist, “What endpoint indicator do you use for the amine strength titration?” If the chemist responds “bromphenol blue” or “pH 4.5” the engineer knows that amine strength results are probably higher than true, unless there are no weak acids in the amine solution.

The better titration methods require tracking pH or conductivity during the titration and determining the endpoints by inflexion points in the first or second derivative, respectively. Both pH and conductivity can provide accurate free amine results, but conductivity is preferred because it also provides clear endpoints for the weak acids (see Appendix A).

This and other difficulties with amine strength determination, and their effects on operations, are discussed at length in the Amine Best Practices Group presentations at the 2004 and 2005 Brimstone Sulfur Symposia.

Before we seek to understand analytical methods, let us clarify our understanding of the amine solvent itself.

Amine system is supposed to make salts
The chemistry of alkanolamine solutions is quite simple, but is often described in terms that lead to a cloudy or incorrect view of amine interactions with acid gases and contaminants. Consequently, the understanding of analytical results become more difficult.

Alkanolamines are bases. Bases react with acids to make salts. Acid gases are absorbed and held in amine solutions because the amine makes a salt of the acid gas: The amine makes an anion from the acid gas so it cannot be a gas. As long as it is an anion, it cannot leave the amine solution.

The regenerable salts in Table 3 and the heat stable salts in Table 4 are written to emphasis the separateness of the ions. A cation is a positively charged molecule that is physically disconnected from its neighboring anion, which is an independent molecule with a negative charge. Anions and cations must be in equal numbers and uniformly distributed throughout the solution, but are continually changing partners. This view of ions is critical to the understanding of amine acid gas absorption and regeneration.

For example, H2S absorbed in an amine solution is not bound to the amine. Rather, the amine has taken a hydrogen ion (H+) from the H2S, creating an HS- anion that cannot escape from the solution. The amine is bound to the H+ and does not readily release it. The only way for the HS- to escape the solution is to take an H+ from an AmH+, thereby recreating H2S, which has low solubility and high volatility and will exit the solution, unless another amine molecule encounters it and takes one of its H+ away.

Acid gases are readily released from the thin films of liquid amine solution in an amine system regenerator, not because temperature “breaks the salts”, but because at higher temperatures AmH+ more readily releases its H+, and the anions of the acid gases readily take the H+, creating gases that are less soluble and more volatile at elevated temperature.