Article pubs.acs.org/IECR
Identification of the Molecular Species Responsible for the Initiation of Amorphous Dithiazine Formation in Laboratory Studies of 1,3,5Tris (hydroxyethyl)-hexahydro‑s‑triazine as a Hydrogen Sulfide Scavenger Grahame N. Taylor,* Philippe Prince, Ron Matherly, Ramakrishna Ponnapati, Rose Tompkins, and Panchalingam Vaithilingam Baker Hughes, Applied Liquids Technology, Tomball and Sugar Land, Texas, United States ABSTRACT: Amorphous dithiazine is produced from a solution of tris-(2-hyroxyethyl)-hexahydro-s-triazine (I) that is heavily consumed by hydrogen sulfide (H2S). Previously, it has been reported that the chemical structure of amorphous dithiazine is a polymeric structure which involves opening of the dithiazine ring. Evidence is presented here that the first step in this polymerization reaction is conversion of the terminal hydroxyl functionality into a terminal thiol. Thereafter, the thiol initiates the ring opening of the dithiazine to yield a polymeric, highly insoluble material. It has been observed that the critical chemical species in the initiation of this chain reaction is the bisulfide anion. This bisulfide anion is produced from the reaction of H2S with ethanolamine liberated in the sulfur insertion reaction undergone by tris-(2-hyroxyethyl)-hexahydro-s-triazine (I). This process has been artificially induced by the reaction of monomeric or crystalline 5-hydroxyethyldithiazine (II) and ethanolammonium hydrosulfide.
■
been described previously.2 When (I) is spent to a high level, or when the reaction with hydrogen sulfide has proceeded very far along, its pathway (II) exceeds its solubility in the aqueous medium and comes out of solution as a lower, highly dense (SG = 1.3) liquid layer. Once formed this lower liquid layer can undergo one of two outcomes. It may simply crystallize to the monomeric species (II) in large cubic crystals as has been observed in dithiazine isolated from field fluids. Under certain conditions it may undergo a secondary, polymerization reaction as has been postulated previously, a reaction which inolves opening of the dithiazine ring.4 The solid material produced has various designations such as “solids”, “scavenger solids” “triazine scale” and “amorphous dithiazine” The formation of two distinctly different types of solid from (I) has been discussed in a recent publication.5 Crystalline dithiazine is simply a different physical form of the monomeric dithiazine species (II). It has good solubility in common organic solvents such as methanol, occurs at low temperatures, and can be removed by melting and/or treating with an appropriate solvent. Amorphous dithiazine was reported to form at elevated temperatures, an observation which is confirmed in this current study. This type of solid is highly insoluble in all organic solvents and does not melt. Avoidance is the best current strategy for amorphous dithiazine, either by preventing the initial phase separation with cosolvency or ensuring that the degree spent does not reach a level sufficient to cause its formation. Using all the individual components present in partially spent scavenger fluids based
INTRODUCTION 1,3,5-Tris(hydroxyethyl)-hexahydro-s-triazine (I) is the most ubiquitous hexahydrotriazine-based hydrogen sulfide scavenger, and its mechanism of reaction with hydrogen sulfide is very well established.1,2 Reaction with two moles of hydrogen sulfide yields 5-hydroxyethyldithiazine (II) and two moles of ethanolamine (Figure 1). Under conditions where hydrogen sulfide is
Figure 1. Reaction of hydrogen sulfide with 1,3,5-tris(hydroxyethyl)hexahydro-s-triazine.
the only acid gas, ethanolamine will be salted to form ethanolaminium bisulfide but in the presence of moderate to high carbon dioxide the majority, if not the entire quantity, of ethanolamine will be ethanolaminium bicarbonate. Thus the stoichiometry of reaction is between 2 and 4 dependent upon the mole ratio of carbon dioxide.3 (II) is the sole reaction byproduct despite the obvious intermediacy of the thiadiazine (III); this component is never isolated in the spent fluids. The isolation and identification of (I) and (II) in spent fluids has © 2012 American Chemical Society
Received: Revised: Accepted: Published: 11613
May 16, 2012 July 23, 2012 August 2, 2012 August 2, 2012 dx.doi.org/10.1021/ie301288t | Ind. Eng. Chem. Res. 2012, 51, 11613−11617
Industrial & Engineering Chemistry Research
Article
Table 1. Effect of Gas Composition on Dithiazine Byproduct hydrogen sulfide content (%) carbon dioxide content (%) conversion of (II) to amorphous dithiazine (%) fully spent hexahydrotriazine after 24 h
100
75
50
25
10
0
25
50
75
90
100
100
40
0
0
Entire container is one solid mass. Both phases have solidified.
Entire container is one solid mass. Both phases have solidified.
Entire container is a semisolid mass. Both phases have solidified.
Two distinct liquid phases are observed. The lower organic liquid, excess dithiazine, remains liquid. After extensive storage some crystallization can occur.
Two distinct liquid phases are observed. The lower organic liquid, excess dithiazine, remains liquid. After extensive storage some crystallization can occur.
Figure 2. Samples of chemically induced amorphous dithiazine.
Periodically throughout the experiment the gas column, which had been tared at the start of the experiment, was disconnected from the gas supply line and weighed. Prior to the start of the experiment the theoretical mass uptake was calculated based upon the expected stoichiometry of between 2 and 4 mols hydrogen sulfide per mole of hexahydrotriaizne dependent on the carbon dioxide present. In all cases, the degree spent was judged to be 100% based upon the quantitative absorption of gas by mass increase to constant mass of the gas tower. The chemical yield of amorphous dithiazine is also shown in Table 1. A quantitative conversion of (I) into (II) initially results in all cases since the samples are fully spent. The monomeric species (II) then polymerizes to the amorphous material to the extent shown. At very high hydrogen sulfide levels, the ethanolamine liberated from the sulfur inclusion reaction will be almost entirely ethanolaminium bisulphide (V). However, as the percent of carbon dioxide increases this will be gradually replaced by ethanolaminium bicarbonate (IV). This data strongly suggest that the polymerization to amorphous dithiazine somehow involves the bisulfide anion. There is clearly a critical point in the profile whereby there is insufficient bisulfide anion to rapidly polymerize the dithiazine to amorphous dithiazine. Over prolonged storage a very small amount may occur but nothing compared with the degree seen with very high percent of hydrogen sulfide. Samples spent with very high percent carbon dioxide gas mixture normally remain as a two-phase system with little or no solid formation even after prolonged storage at ambient temperature. If, however, the temperature of this two phase system is raised to 50 °C or even higher to 80 °C the solidification of the lower liquid layer rapidly occurs. After approximately 24 h, most of the monomeric dithiazine has been converted to what we consider to be polymeric amorphous dithiazine. It is postulated that in order to remain consistent with the involvement of the bisulphide anion, a mechanism such as Figure 3 occurs. It is well known that ethanolamine is used in amine recycling units. The salts formed during its exposure to sour produced gas (IV and V) are heated to liberate hydrogen
upon (I) a series of test samples was constructed from the calculated component quantities that would be present in a series of fluids spent from 50% to 100% in 10% incremental steps. After approximately 7 days of equilibration we concluded that amorphous dithiazine is in danger of forming if the fluid reaches 60% or more spent with respect to the original hexahydrotriazine (I). The current work described herein seeks to shed more light upon this later reaction. Recent literature reports describe a proposed mechanism for formation of amorphous dithiazine.4 In particular it was noted that the presence of a terminal hydroxyl functionality in the hexahydrotriaizne molecule is essential for this reaction to occur. If this is blocked with a methoxy group, as was achieved by the synthesis of 2methoxyethyl or 3-methoxypropyl hexahydrotriazine derivatives, no solidification of the separated dithiazine layer was observed at the complete spend condition, whereas 2hydroxyethyl hexahydrotriazine spent to 100% would have solidified within 24 h. This previous study was limited to spending 1,3,5-tris(hydroxyethyl)-hexahydro-s-triazine to completion with pure hydrogen sulfide in laboratory glass towers. Under these conditions, the lower dithazine layer forms very readily and the polymerization process begins, as described in a recent paper.4 Since hydrogen sulfide very commonly occurs with carbon dioxide, this current study was expanded to include this situation.
■
EXPERIMENTAL SECTION Dynamic Mixture Spend Study for 1,3,5-Tris(hydroxyethyl)-hexahydro-s-triazine. Dynamic mixtures of hydrogen sulfide and carbon dioxide were produced using two parallel Cole Parmer flow meters such that the combination flow rate was always 0.055 L per minute, as had been used in previous studies. The gas stream was passed through a tall thin glass column measuring 45 cm × 4 cm with a sinter glass plate at the bottom of the inlet tube to diffuse the gas stream into small bubbles. The content of the gas stream was varied as shown in Table 1 and the resulting fully spent fluid was observed and reported as shown in Figure 2. 11614
dx.doi.org/10.1021/ie301288t | Ind. Eng. Chem. Res. 2012, 51, 11613−11617
Industrial & Engineering Chemistry Research
Article
purification of highly spent triazine scavenger field fluids (8.29 g) was dissolved in water (27.75 g) and methanol (5.0 g). This was designated as solution A. Solution B was designated as either (a) aqueous ethanolaminium bisulfide as prepared below, (b) aqueous ethanolaminium bicarbonate as prepared below, or (c) deionized water. The ratio of solution A to solution B or C was that which would be derived from a typical commercial hexahydrotriazine based scavenger solution. Solution A was added to both solutions B and C, and the resulting mixtures were stored at ambient temperature. The concentration of (II) and ethanolamine present as its respective salt was exactly that which would result from spending a 50/50 mixture of HSW700 in water. The results are shown in Table 2 and Figure 4. When solution B was ethanolaminium bicarbonate, the hydrogen
Figure 3. Ethanolamine salt formation.
sulfide and carbon dioxide thus regenerating ethanolamine for reuse. It is thought that the bicarbonate salt readily decomposes at mildly elevated temperatures thus liberating carbon dioxide and that the ethanolamine liberate has to some extent the ability to generate sufficient bisulfide from any free hydrogen sulfide present in the system. By this means, it is possible that sufficient bisulfide can be formed to initiate the polymerization to amorphous dithiazine. Field operations are known to very often involve significant quantities of carbon dioxide and yet amorphous dithiazine formation is certainly a problematic occurrence in these operations. In a recent case history, a gas tower in Oklahoma was found to contain almost one-third of its volume in solidified amorphous dithiazine due to incorrect partial draining of a replacement of a triazine scavenger. Gas scrubber towers invariably run at temperatures above ambient, which may be sufficient to initiate this polymerization reaction. This would explain why amorphous dithiazine is readily formed in field conditions, when carbon dioxide levels are often much higher than hydrogen sulfide, since gas towers and scrubber units very often run at temperatures above ambient. It was also discovered that under conditions where it is disfavored (high carbon dioxide, low hydrogen sulfide gas mixture), the formation of amorphous dithiazine could be accelerated if the gas sparge was continued several hours after it is known, from mass gain studies, that the solution was fully spent. A mechanism similar to that above postulated to occur at elevated temperatures may be responsible for generating sufficient bisulfide concentration to initiate the process, although this does remain uncertain. This situation of exposure to excess hydrogen sulfide is of academic interest only since it is unlikely to be encountered during normal field operations. Model Experiment for Amorphous Dithiazine Formation. The results indicate that under conditions of high carbon dioxide the formation of amorphous dithiazine is severely inhibited. At low carbon dioxide content the polymerization to amorphous dithiazine resumed. This was somewhat surprising and in order to verify this result a model experiment was designed. The exact chemical content of fully spent tris(hydroxyethyl)-hexahydro-s-triazine in both pure hydrogen sulfide and also 10% hydrogen sulfide/carbon dioxide was calculated. Pure crystalline (II) obtained from the
Figure 4. Model study for amorphous dithiazine formation.
sulfide/carbon dioxide condition, the resulting fluid showed dithazine separation but no amorphous dithiazine formation was observed. When Solution B was water, as a control experiment, no amorphous dithiazine formation was observed. If however Solution B was ethanolaminium hydrosulfide, within 48 h of mixing the composite the fluid showed very heavy solid amorphous dithiazine formation. These observations clearly indicate that the key species responsible for the solid amorphous dithiazine formation is an excess of the bisulfide anion. If this is not present, as in Solution B, present as bicarbonate or water, no formation of amorphous dithiazine occurs. Synthesis of Ethanolaminium Bicarbonate (IV). Ethanolamine (30 g) was dissolved in water (30 g) in a gas sparge tower. Pure carbon dioxide was sparged through the tower and the expected mass increase observed. A strong exotherm was observed and the sparge stream was terminated when the tower had reached constant mass. The mass gained calculated to the theoretical quantity of carbon dioxide absorbed based on a stoichiometry of 1:1 as expected. Synthesis of Ethanolaminium Bisulphide (V). Ethanolamine (30 g) was dissolved in water (30 g) in a gas sparge tower. Pure hydrogen sulfide was sparged through the tower and the mass increase was observed. A strong exotherm was observed and the sparge stream was terminated when the tower
Table 2. Model Study Results solution A
5-hydroxyethyldithiazine/water/methanol
solution B solution C fluid characteristics after 72 h
ethanolaminium bisulfide (V) n/a entire two phase system was a solid mass of amorhpous dithiazine 11615
5-hydroxyethyldithiazine/water/ methanol ethanolaminium bicarbonate (IV) n/a two phase liquid system, no solid formation
5-hydroxyethyldithiazine/water/ methanol n/a water two phase liquid system, no solid formation
dx.doi.org/10.1021/ie301288t | Ind. Eng. Chem. Res. 2012, 51, 11613−11617
Industrial & Engineering Chemistry Research
Article
Figure 5. Amorphous dithiazine polymer formation mechanism.
solid white precipitate. Amorphous dithiazine is certainly not a single molecular species and this is known from the differing elemental analysis for this material dependent upon its source of origin.4 The ultimate goal of these studies is to find a method to inhibit or at least reduce the formation of this troublesome solid byproduct. As a better understanding of the actual chemical mechanism is uncovered this becomes a more realistic goal. Since the key species involved, bisulfide anion, is actually derived from the hydrogen sulfide molecular species and since an alkaline pH is a requirement and feature of hexahydrotriazine systems, this does appear to be a very challenging goal to achieve. Nevertheless it is hoped that a more thorough mechanistic treatment of this problem will lead to that end.
had reached constant mass. The mass gained calculated to the theoretical quantity of hydrogen sulfide absorbed based on a stoichiometry of 1:1 as expected.
■
CONCLUSION The above experimental observations strongly suggest that the bisulfide anion is the chemical species that initiates the formation of polymeric amorphous dithiazine. A suggested mechanism is shown in Figure 5. This has been revised from the previously published version and now no longer requires the involvement of thioformaldehyde generation.4 It is thought that the initial key species in the polymerization process is the generation of 5-thioethyldithiazine (VI) from the reaction of (II) with bisulfide. This step is not possible if the substituent is an alkoxy derivative which does not yield amorphous dithiazine.4 This generates a thiol terminus which then opens the rings of another molecule of (II) to yield (VII). By means of another nucleophilic attack upon (VI) by a bisulfide anion the ethanolamine residue is eliminated and the thiol terminus is generated to yield (VII). In studies conducted upon the upper aqueous layer in samples that are known to favor the formation of amorphous dithiazine we have detected elevated levels of monoethanolamine which supports the suggestion that as the polymer chain grows the monoethanolmine residue is eliminated from the molecule. This process repeats itself to yield (IX) then (X) and (XI). The limiting factor in this process is thought to be solubility in water and at some point in the process (VII), (IX), and (XI) will come out of solution as a
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Bakke, J. M.; Buhaug, J.; Riha, J. Hydrolysis of 1,3,5-Tris(2hydroxyethyl)hexahydro-s-triazine and Its Reaction with Hydrogen Sulfide. Ind. Eng. Chem. Res. 2001, 40, 6051−6054. (2) Taylor, G. N.; Matherly, R. The Gas Chromatography Mass Spectrometric Analysis of Chemically Derivatized Hexahydrotriazine
11616
dx.doi.org/10.1021/ie301288t | Ind. Eng. Chem. Res. 2012, 51, 11613−11617
Industrial & Engineering Chemistry Research
Article
Based Hydrogen Sulfide Scavengers: Part II. Ind. Eng. Chem. Res. 2010, 49, 6267−6269. (3) Taylor, G. N.; Matherly, R. The Laboratory Evaluation and Optimization of Hydrogen Sulphide Scavengers Using Sulphur Specific Flame Photometric Gas Chromatography. SPE Oilfield Chem. Symp. 2011, 140401. (4) Taylor, G. N.; Matherly, R. Structural Elucidation of the Solid Byproduct from the Use of 1,3,5-Tris(hydroxyalkyl)-hexahydro-striazine Based Hydrogen Sulfide Scavengers. Ind. Eng. Chem. Res. 2011, 50, 735−740. (5) Owens, T. R. Formulation for Hydrogen Sulfide Scavenging from Hydrocarbon Streams and Use Thereof. World Patent WO2008049188 20080502, 2008.
11617
dx.doi.org/10.1021/ie301288t | Ind. Eng. Chem. Res. 2012, 51, 11613−11617
