Chemical analysis in amine system operations - EPTQ

1 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 C. hemical analysis opens the door to understand- Fundamental analytical parameters of amine solutions ing, responding to and preventing alkanolamine Parameter Analyte Description amine strength Free amine (FA) amine available for acid gas absorption ( amine ) system operational Acid gas loading (AGL) Rich loading (RL) Acid gas (H2S & CO2) in the solution exiting the absorber difficulties.

2 Unfortunately, the Lean loading (LL) Acid gas (H2S & CO2) in the solution exiting the regenerator link between the analysis Water Water The remainder of the solution (if no contaminants). report and the amine system operation is often unclear. The Table 1. uncertainty results from several factors that can lead to the acid gases, and then the lated in the amine system , this misunderstandings, misinter- amine is cooled as it returns to could be the extent of the pretations, frustrations and the absorber. The amine solu- analytical information required mistrust of the lab. This article tion can continuously circulate.

3 To operate. Unfortunately, attempts to provide clear defi- The simple acid base chemistry contaminants do accumulate in nitions of analytical terms, link of the process can be moni- amine systems and affect them to their operational tored by a few analytical equipment longevity as well as importance, translate terminol- titrations (see Table 1), and the success of the operation of ogy from different sources to a operators need only to monitor the amine system . More unfor- uniform set of terms, and temperatures, pressures and tunate is the fact that the expose analytical methods that flow rates to balance the amine contaminants can affect the can mislead you.

4 This article absorbing capacity with the results of the fundamental also provides questions you acid gas removal requirement analytical methods, misleading can ask your chemist, lab of the incoming petroleum gas the operator, and yet the oper- analyst or amine vendor to or liquid. (The acid gas content ator continues to rely on these avoid operational pitfalls and of the sweetened product is, of few simple tests for day-to-day get the information you need course, the ultimate control operations . from the analysis reports. measure, but is beyond the Increased awareness of the An alkanolamine acid gas scope of this article.)

5 This effects of contaminants on scrubbing system is a simple simplified view of amine operations has led to an concept: a solution of water systems and operational condi- increased understanding of the and alkanolamine absorbs acid tions is the basis on which the variety and identity of contam- gases from petroleum gas or most common analytical meth- inants that exist in amine liquid and is pumped to a ods were developed. systems. Common analytical heated regenerator that releases If no contaminants accumu- methods have been adapted, PTQ Q3 2013 1. true for a pH dead-stop titra- tion. The most common amine 1% formate, 1% acetate strength titration methods were 0% formate, 0% acetate developed for amine solutions Error, wt% free amine with no contaminants.

6 Thus, contaminants may cause errors. Methyl purple indicator range Understanding analyses With knowledge of the preced- Bromphenol blue ing information, the engineer indicator range responsible for the amine system can now ask the analyt- 0. ical chemist or operator, What endpoint indicator do you use for the amine strength titra- pH at endpoint tion? If the chemist responds Bromophenol blue or pH. Figure 1 Endpoint pH affects accuracy of free amine titration , the engineer knows that amine strength results are modified and sometimes the titration to determine free probably higher than their misapplied to contaminant amine can also respond to the actual level, unless there are no analysis .

7 Contaminant-specific weak acid anions. The choice weak acids in the amine analytical methods have multi- of pH for the endpoint of the solution. plied. The amine system titration determines whether The better titration methods operator can now be the free amine titration is accu- require the tracking of pH or confronted with a maze of rate or over-estimates the conductivity during the titra- analytical parameters, a blur of amine strength. A pH or colour tion and determination of the analytical results, and a host of indicator that provides accu- endpoints by inflexion points analyte names and acronyms rate amine strength in a clean in the first or second deriva- that can be ambiguous, confus- amine solution can grossly over- tive, respectively.

8 Both pH and ing and even misleading. estimate the amine strength of conductivity can provide accu- a solution that contains weak rate free amine results, but Better analytical methods acid anions (HSS or LL). Note, conductivity is preferred needed for example, in Figure 1, because it also provides clear A prime example of misleading that Methyl purple and endpoints for the weak ,2. results is illustrated in Figure 1 Bromothymol blue are both Before we seek to understand and Table 2. When weak acids acceptable indicators for the analytical methods, let us clar- (such as formic acid, acetic titration of clean amine solu- ify our understanding of the acid, and so on) have accumu- tions, but fail miserably if the amine solvent itself.

9 Lated in the amine solution, solution contains significant forming heat stable salts (HSS), weak acid HSS. The same is amine system is supposed to make salts Error of fixed pH endpoint titration 45 wt% MDEA The chemistry of alkanolamine solutions is quite simple, but is often described in terms that Acetate, Formate, Sodium, Actual free F ound free amine , wt% at endpoint pH. wt% wt% wt% amine , wt% lead to an incorrect view of 0 0 0 amine interactions with acid 1 1 0 gases and contaminants. 1 0 0 1 1 Consequently, the understand- 1 0 ing of analytical results becomes more difficult. Table 2 Alkanolamines are bases.

10 2 PTQ Q3 2013 Bases react with acids to make Regenerable salts from acids gases in amine solutions salts. Acid gases are absorbed and held in amine solutions Acid + vase cation + anion because the amine forms a salt with the acid gas. The acid gas Acid Base Salt Salt name H2S Free amine ( Am ) AmH+ + HS- Aminium bisulphide becomes an anion (negatively CO2 + H2O H2CO3 Am AmH+ + HCO3- Aminium bicarbonate charged ion) and is associated HCO3- Am 2 AmH+ + CO3= Aminium carbonate CO2 Am + Am AmH+ + AmCO2- Aminium amine carbamate with an amine cation (positively charged ion). The acid gas ion is no longer in a gaseous state and Table 3.