Mercaptan scavenging revisited

Jonathan Wylde and Grahame Taylor, Clariant Oil Services, USA, examine the reaction of alkyl mercaptans with hydrogen sulfide scavengers.

Sulfur-containing species found within oil and gas production fluids are far more varied than simply hydrogen sulfide (H2S). Other species include alkyl mercaptans (alkane thiols), dialkyl sulfides (such as dimethyl sulfide), dialkyl disulfides, carbon disulfide, carbon oxysulfide, and many others. While the majority of the industry focuses on the removal of H2S largely for corrosion mitigation and health and safety purposes, the removal of alkyl mercaptans is also of great concern and interest. Mercaptans are the sulfur analogues of alcohols where the oxygen atom is replaced with sulfur. They are much more volatile than their oxygen counterparts; while methanol and ethanol are liquids at room temperature and pressure, methyl mercaptan is a gas and ethyl mercaptan is a very low boiling point liquid. This is attributed to the much weaker hydrogen bonding between sulfur and hydrogen when compared with oxygen and hydrogen atomic species. Mercaptans have very powerful, highly offensive odours and are often removed simply for this reason alone. They are very weakly acidic: methyl mercaptan has a pKa of 10.4 and ethyl mercaptan a pKa of 10.6, whereas H2S has a pKa of 7. The concentration of mercaptans found in oil and gas produced fluids varies but it is most typical for the total mercaptan concentration to be in the range of 0 to 150 ppm. Most cases will be at the lower end of this range (<20 ppm). Further individual mercaptans can vary and typical ranges for methyl mercaptan are 0.5 to 100 ppm, ethyl mercaptan 0.5 to 120 ppm, and butyl mercaptan 0.1 to 8 ppm. To date there has been very little in the way of systematic studies into the removal of

alkyl mercaptans other than the generic statement that: “H2S scavengers also remove mercaptans but with a lower level of efficiency.”

Molecular species containing sulfur

Apart from their reaction with H2S, very few rigorous studies have been carried out regarding the reaction of any of the above sulfur species with scavengers. While it is undoubtedly true, for example, that they all react with hexahydrotriazines (the most widely used scavengers in the industry and often known simply as triazines), there is a great paucity of careful studies. A 1951 study described the reaction of carbon disulfide with 1,3,5 trisubstituted hexahydrotriazines or triazines and found that they produced a cyclic thiadiazine-2-thione (I) by insertion of the carbon sulfur species into the cyclic ring structure (Figure 1).1 Two studies have been carried out which touch on the aspect of sulfur mitigation by removal of mercaptans, but key questions are still left unanswered: perhaps, most notably, the reaction mechanism and chemical identity of the reaction products was missing.2,3 An article published in November 2020 addressed this missing aspect together with a study into the capacity of conventional H2S scavengers to remove mercaptans from a multiphase application.4 Some surprising results were uncovered that challenged conventional thinking on many fronts with respect to these reactions. Hexahydrotriazines are the mainstay of sulfur mitigation and indeed were found to be effective for mercaptan removal. The removal of sulfur species by hexahydrotriazines makes use of a vital feature of the sulfur atom. Being a Group VI or chalcogen element, sulfur has many electrons in its outer valance shell. Since these are further from the nucleus than the oxygen atom they are far more easily displaced, which confers a high degree of nucleophilicity or affinity for positive carbon centres upon this atomic species. Such species are often referred to as soft nucleophiles, indicating that the electron density is very easily displaced and is not held tightly by the nucleus. The reaction mechanism was studied in detail and there is one key aspect whereby it must differ from H2S. The H2S molecule possesses two easily removed hydrogen atoms, and thus during its reaction with hexahydrotriazines this enables an overall insertion of a sulfur atom into the six membered ring, which opens and then closes via two SN2 nucleophilic substitutions. The resulting product is the dithiazine (II). Such a reaction is impossible for alkyl mercaptans, which only possess one easily removed hydrogen atom. The carbon sulfur bond of the mercaptan is very strong and remains unbroken in the reaction products ultimately formed. By means of careful isolation and analysis, the reaction products were shown to be (III), (IV) and (V), as seen in Figure 1. These are the logical products one would expect but were for the first time separated and identified. Not only were the reaction products positively identified for the first time but the capacity of hexahydrotriazines, and in particular

Figure 1. Reaction products of molecular sulfur species with hexahydrotriazines.

Table 1. Capacity of H2S, methyl and ethyl mercaptan reacting with hexahydrotriazines

Scavenger Sulfur species Activity (%) Observed capacity (kg/l) Observed capacity (mole/l) Calculated capacity (kg/l) Calculated capacity (mole/l) Assumed stoicheometry Efficiency (%)

MEA triazine H2S 38 0.12 3.53 0.13 3.68 2 96 MMA triazine H2S 40 0.23 6.76 0.23 6.65 2 101.8

MEA triazine CH3SH 38 0.14 2.92 0.27 5.58 3 52.2 MMA triazine CH3SH 40 0.47 9.79 0.48 9.98 3 98.1

MEA triazine CH3CH2SH 38 0.29 4.68 0.35 5.65 3 82.9 MMA triazine CH3CH2SH 40 0.53 8.55 0.62 10 3 85.5

MMA hexahydrotriazine (or more accurately 1,3,5-trimethylhexahydrotriazine), was shown in fact to be greater than H2S. The experimental determination of mercaptan capacity is a challenging undertaking and H2S detector technology is usually able to also detect alkyl mercaptans but with a lower level of efficiency. The typical technical sensor equipment responds to alkyl mercaptans anywhere from 33% to 50% the extent to which it does for H2S. There are gas detector tubes that can specifically detect alkyl mercaptan, some of which remove the H2S component first to avoid cross-sensitivity. Experimentation of mercaptans can effectively be carried out using recalibrated H2S detection technology but the determination of scavenger efficiencies in the presence of H2S is more challenging. Any H2S detector will give an overall response to all sulfur species but is of little use in and of itself. One study used a sophisticated method of mathematically deconvoluting the absorbance of H2S and methyl mercaptan in composite gas breakthrough studies.2 This allows the use of multiple mercaptans together with H2S in a common gas feed to a contact tower.

By recalibrating the conventional H2S analyser used for the industry-recognised autoclave reactor, multiphase mercaptan capacity numeric values were, for the first time, obtained for hexahydrotriazines, hemiacetals, metal carboxylates, and aldehyde scavengers, which were compared directly with those obtained for H2S. In many instances superior performance was obtained with alkyl mercaptans compared with H2S, which was not expected.

A comparison of the results for molar capacity (this leaves aside the molecular mass differences for the sulfur species) clearly indicates that MMA hexahydrotriazine is, in the data set shown in Table 1, a better scavenger for methyl and ethyl mercaptan than it is for H2S. The same is also true for MEA hexahydrotriazine. Such findings run contrary to conventional wisdom and should be considered when addressing field operations. Reaction rate information can also be drawn from this testing methodology and

indicates that, for both hexahydrotriazines, methyl mercaptan reacts every bit as fast as H2S, if not slightly faster. The reaction rate with ethyl mercaptan is markedly slower, as might be expected; typically the reactivity drops off as the substituent’s size and bulk increases, as shown in Figure 2. In summary, it is clear that when actual numerical capacity data for various Figure 2. Hexahydrotriazines relative reaction kinetics with sulfur species. common scavengers are determined (by an industry standard method), rather than supporting the conventional wisdom the findings show that they scavenge alkyl mercaptan more efficiently than H2S, the most commonly encountered sulfur species in the oil and gas industry. It was also discovered that – contrary to what has been generally communicated – a quaternary ammonium compound, rather than scavenging by an alkyl transfer process to form a sulfonium species, in fact had no measurable chemical interaction with the mercaptan.5 Further studies are certainly needed, but the indications are very positive that the state of the technology may well be better than originally thought.

Conclusions and field significance

The removal of sulfur species from produced oil and gas remains a vitally important consideration for operators and, with the souring of many formations, the importance only grows. While concentrating on the most prevalent species (H2S) it is important not to neglect other entities which must be addressed in many situations. Relying on blanket statements and, in many ways, inaccurate ‘industry myth’ is clearly a poor substitute for rigorous scientific studies. More recently, the removal of mercaptans has finally received this type of attention not only from a numerical capacity data perspective but also from a rigorous organic chemistry perspective, whereby the reaction products have finally been identified and confirmed.

References

1. MERILL SCHNITZER, A., ‘Reaction of 1,3,5-trisubstituted Hexahydro-1,3,5-1 Triazines with Carbon Disulfide’, PhD Thesis, Oklahoma A&M College, (December 1951). 2. CHAKRABORTY, S., LEHRER, S., and RAMACHANDRAN, S., ‘Effective Removal of Sour Gases Containing Mercaptans in Oilfield Application’, paper presented at the Society of Petroleum Engineers International Conference on Oilfield Chemistry, (3 – 5 April 2017), Montgomery, Texas, US. 3. OWENS, T.R., and CLARK, P.D., ‘Triazine Chemistry: Removing H2S and Mercaptans’, ASRL Quarterly Bulletin, 155, Vol. XLVII, No. 3, (October – December 2010), pp. 1 – 21. 4. WYLDE, J.J., TAYLOR, G.N., SORBIE, K.S., and SAMANIEGO, W.N., ‘Scavenging Alkyl Mercaptans: Elucidation of Reaction Mechanisms and By-Product Characterization’, Energy & Fuels, Vol. 34, No. 11, (November 2020), pp. 13 883 – 13 892. 5. WEERS, J. J., and GENTRY, D.R., ‘Quaternary ammonium hydroxides as mercaptan scavengers’, Patent US-6013175, (January 2000).