Synthesis, Reaction Byproduct Characterization, and Mechanistic Understanding of Hemiformal Hydrogen Sulfide Scavengers: Part 2─Identification of Byproducts, Reaction Mechanisms, and Suppression Thereof

Abstract

A detailed account of the molecular species identified in the synthesis of alcohol hemiformal H 2 S scavengers was recently published by the authors. This study focused on the molecular mass and structural characterization of the ultimate products of the scavenging process but was unable to identity the initially formed molecular species upon reaction with hydrogen sulfide. It was conjectured that the reaction of formaldehyde with H 2 S yielded hydroxymethyl mercaptan but without direct evidence. In this current work, we study the molecular species first present when hemiformal scavengers react with H 2 S before the formation of a solid polymeric precipitate. The current study of early and later times in the hemiformal/H 2 S scavenging process showed surprisingly that the first formed species are multi-sulfur dithiols devoid of oxygen. The critical secondary reaction to form a solid polymer is shown here to be the reaction of these dithiols with further formaldehyde released from unreacted hemiformal. This proves hemiformals to be another example of a scavenger whose ultimate reaction products derive from the initial sulfur-containing species reacting with "unreacted" scavenger in the fluid medium. The multi-sulfur dithiols observed likely derive from thioformaldehyde. This transient species is well-known to be highly reactive and can only have been observed by generation followed by trapping this molecular species as a cyclopentadiene Diels− Alder adduct during the photolysis of thietane. The significance of this work is the contribution to understand the structure and mechanism of formation of the (unwanted) solid byproducts. Using the new results presented here on the mechanism of formation and the specific intermediate species involved, better processes can be designed for removal of H 2 S in both oilfield and other (biogas) systems. In particular, better ways to mitigate the solids that concentrate in field operations can be developed, and this is demonstrated in this paper.

Mechanistic Insights into the Efficiency of Formaldehyde-Based Compounds and Glyoxal as H2S Scavengers

“…39,40 The first process was chosen because the capture of H 2 S by aldehydes occurs in aqueous media, 7,21,49 while the second is a well-known process of releasing of formaldehyde from trioxane. 25,50 The performance of the combination of the functionals B3LYP, CAM-B3LYP, M06-2X, N12SX, OLYP, and ωB97X-D with the basis sets 6-311+G(d,p), 6-311++G(d,p), aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ was assessed by computation of the enthalpy change for both processes and comparing with thermochemical experimental data. Figure 1 displays the absolute deviation from the computed data to the experimental data.…”

“…Wylde and co-workers reported the synthesis and utilization of hemiformals derived from different diols and triols in the H 2 S scavenging process. 25 Inspired by these structures, we investigated to what extent the hemiformals dissociate into formaldehyde and the respective alcohols, by simulating the chemical equilibria shown in Scheme 1 and computing the corresponding enthalpy and Gibbs free energy changes.…”

“…Our computed values are in good agreement with the previous experimental values reported by Wylde. 25,26 Among the assessed aldehyde releasers, the trimeric form of formaldehyde (1,3,5-trioxane), presented the highest positive value of ΔH, but the lowest ΔG, indicating that, although the equilibrium is shifted toward the starting material, more formaldehyde is released relative to the other formals. This is probably due to entropic reasons, given the significant increase in the system's degree of freedom: for the decomposition of the trioxane, each trioxane molecule is decomposed into three formaldehyde molecules in the product, increasing the translational and rotational degrees of freedom.…”

“…20,26,27 Wylde and co-workers investigated the H 2 S scavenging efficiency of hemiformals�hemiacetals derived from formaldehyde, and different alcohols. 25 They noted the formation of oligomeric products, characterized as poly(oxy)methylenesulfide, as a white solid that becomes insoluble after incorporating three sulfur atoms. 25,26 Sperotto and co-workers assessed the performance of a bis-hemiformal H 2 S scavenger under Brazilian presalt conditions, in addition to verifying the concentration of unwanted free formaldehyde.…”

“…25 They noted the formation of oligomeric products, characterized as poly(oxy)methylenesulfide, as a white solid that becomes insoluble after incorporating three sulfur atoms. 25,26 Sperotto and co-workers assessed the performance of a bis-hemiformal H 2 S scavenger under Brazilian presalt conditions, in addition to verifying the concentration of unwanted free formaldehyde. The results indicated that the bis-hemiformal scavenger was obtained with low levels of free formaldehyde (<0.1%), exhibiting 31 high efficiency in H 2 S scavenging (1.75 L scavenger/kg H 2 S), and appropriate for injection under subsea conditions.…”