Mettler Toledo Petrochemical Industry Solutions

Identifying the Correct Inflection Point in Mercaptan Analysis

One of the important tests applied in the petrochemical industry is the determination of hydrogen sulphide and mercaptan sulphur content in petroleum distillates and other hydrocarbon products. The sulphur content of fuels is an important parameter in quality control and its limits are defined by strict quality standards. In addition to engine damage, atmospheric emissions from sulphur-containing fuels contain high levels of sulphur dioxide which, when dissolved in atmospheric moisture, falls as acid rain. This damages the environment and building structures (Figure 1) and has a negative economic impact. Testing petroleum distillates for sulphur content is therefore mandatory. Many different test methods have emerged, but those proven to be most reliable are based on titration. The UOP 163-05 laboratory test method demonstrates a simple titration with alcoholic silver nitrate solution that enables the determination of sulphur compounds. However, things are rarely that straightforward. The most frequently encountered sulphur compounds in petroleum distillates are hydrogen sulphide (H2S) and alkyl mercaptans (RSH), but some distillates may also contain elemental sulphur. Sulphur reacts with mercaptans to form polysulphide compounds (RSSH). In practice, most real-world samples contain mixtures of these substances which will affect the titration curve profile obtained. When such mixtures are present, the method used to analyse unknown samples should allow correct results to be calculated quickly and easily. The UOP163-05 test method (obtainable from ASTM) explains in practice how different curve types should be evaluated. The first step is to detect the presence or absence of hydrogen sulphide using a qualitative lead acetate strip. According to UOP 41, the "Doctor Test" where a positive test result indicates the presence of H2S. With this information, there are five possibilities to consider.

Interpreting the Titration Curve

The difficulty in automating the procedure is to ensure that only correct results are calculated regardless of curve type. Two variables must be considered: • The number of equivalence points (EQP) in the curve. • The result of the Doctor test (positive or negative). The first of these forms part of the raw data (nEQ) and is very readily accessible. The result of the Doctor test can be entered with the sample data using a correction factor and binary format. For samples with a positive test result f = 1, for those with a negative test f = 0 (see Figure 2) and (see Figure 3). The method contains the full calculations required to cover all five possibilities given in Table 1.

Creative Calculations

The best thing we can hope to do is make useless results zero or NaN (not a number). The calculation formulas themselves must be modified to allow this. The f or (1-f) terms ensure that H2S is only calculated when present. (see Table 2). The results of these calculations will allow the correct RSH value to be identified, as can be seen from the results table (Table 3). The additional (x – EQ) elements in the second and third calculations (Table 2) prevent the calculation of multiple RSH results in scenarios with two or three equivalence points (Figures 5 and 6). There is only one non-zero result for mercaptan content.                    

1- Curve with EQP

a) Positive Doctor test EQP results from titration of H2S.R = Q * C / mb. b) Negative Doctor test EQP results from titration of mercaptan RSH.  

2- Curve with EQP

a) Positive Doctor test The first EQP results from titration of H2S (R = Q * C / m). The second EQP results from titration of mercaptan RSH. (R = Q2 * C / m) b) Negative Doctor test Elemental sulphur is present, so we ignore the first EQP and total consumption gives us the Mercaptan result. (R = (Q + Q2) * C / m)                

3- Curve with EQP

a) Positive Doctor test The first EQP results from titration of H2S (R = Q * C / m). This EQP is not considered. Titration of mercaptan from the third EQP, but the calculation must be based on total titrant consumption between the first and third EQP. (R = (Q2 + Q3) * C / m)

Conclusions

All models in the Titration Excellence series allow the correct calculation selection according to the UOP 163-05 standard test method.

Hatice Gürbüz

Head of Analytical Sales Team Mettler Toledo TR