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Jul-2013

Chemical analysis in amine system operations

The array of contaminants that can disrupt the operation of alkanolamine systems needs to be precisely characterised and analysed

SCOTT WAITE, ARTHUR CUMMINGS and GLEN SMITH
MPR Services
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Article Summary
Chemical analysis opens the door to understanding, responding to and preventing alkanolamine (amine) system operational difficulties. Unfortunately, the link between the analysis report and the amine system operation is often unclear. The uncertainty results from several factors that can lead to 
misunderstandings, misinterpretations, frustrations and mistrust of the lab. This article attempts to provide clear definitions of analytical terms, link them to their operational importance, translate terminology from different sources to a uniform set of terms, and expose analytical methods that can mislead you. This article also provides questions you can ask your chemist, lab analyst or amine vendor to avoid operational pitfalls and get the information you need from the analysis reports.

An alkanolamine acid gas scrubbing system is a simple concept: 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 can continuously circulate. The simple acid base chemistry of the process can be monitored by a few analytical titrations (see Table 1), and operators need only to monitor temperatures, pressures and flow rates to balance the amine absorbing capacity with the acid gas removal requirement of the incoming petroleum gas or liquid. (The acid gas content of the sweetened product is, of course, the ultimate control measure, but is beyond the scope of this article.) This simplified view of amine systems and operational conditions is the basis on 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 as well as 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, and yet the operator continues to rely on these few simple tests for day-to-day operations.

Increased awareness of the effects of contaminants on operations has led to an increased understanding of the variety and identity of contaminants that exist in amine systems. Common analytical methods have been adapted, modified and sometimes misapplied to contaminant analysis. 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 needed
A prime example of misleading results is illustrated in Figure 1 and Table 2. When weak acids (such as formic acid, acetic acid, and so on) 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 Bromothymol 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.

Understanding analyses
With knowledge of the preceding information, the engineer responsible for the amine system can now ask the analytical chemist or operator, “What endpoint indicator do you use for the amine strength titration?” If the chemist responds “Bromophenol blue” or “pH 4.5”, the engineer knows that amine strength results are probably higher than their actual level, unless there are no weak acids in the amine solution.

The better titration methods require the tracking of pH or conductivity during the titration and determination of 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.1,2
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 an incorrect view of amine interactions with acid gases and contaminants. Consequently, the understanding of analytical results becomes more difficult.

Alkanolamines are bases. Bases react with acids to make salts. Acid gases are absorbed and held in amine solutions because the amine forms a salt with the acid gas. The acid gas becomes an anion (negatively charged ion) and is associated with an amine cation (positively charged ion). The acid gas ion is no longer in a gaseous state and it cannot leave the amine solution as long as it remains as an anion. 

The regenerable salts in Table 3 and the HSS 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 neighbouring 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 they 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 AmH+ cation and an HS- anion, which 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 reacts with it and removes one of its H+ to form another salt.
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