Effect of Dissolved Oxygen, Sodium Bisulfite, and Oxygen Scavengers

Sodium bisulfite had negligible effect on hydrate inhibition at low concentrations but showed greater inhibition performance at higher concentrations ...
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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Effect of Dissolved Oxygen, Sodium Bisulfite, and Oxygen Scavengers on Methane Hydrate Inhibition Khalid Alef,*,† Stefan Iglauer,‡ and Ahmed Barifcani† †

WA School of Mines: Minerals, Energy, and Chemical Engineering, Curtin University, Bentley, Western Australia 6102, Australia Petroleum Engineering Department, Edith Cowan University, Joondalup, Western Australia 6027, Australia

‡

ABSTRACT: Numerous chemical additives are added to monoethylene glycol (MEG) injection streams to maintain and protect assets as well as to ensure steady production of hydrocarbons. Oxygen scavengers are injected for the purpose of lowering dissolved oxygen to levels that do not pose the risk of corrosion. In this study, the effect of dissolved oxygen and some oxygen scavengers on gas hydrate inhibition was investigated. Results reveal that high levels of dissolved oxygen may promote the formation of hydrates due to the reaction of dissolved oxygen with impurity components such as iron carbonate that may exist in the MEG solution, thus decreasing overall MEG quality. Sodium bisulfite had negligible effect on hydrate inhibition at low concentrations but showed greater inhibition performance at higher concentrations due to the electrostatic attraction between ions and water molecules. A proprietary oxygen scavenger showed hydrate promotion effect, which suggests that proprietary chemical additives should undergo extensive compatibility and risk analysis. An erythorbic acid-based oxygen scavenger showed minor inhibition performance albeit at small concentration, possibly due to hydrogen bonding between hydroxyl groups of its components with water molecules.

1.0. INTRODUCTION Gas hydrate formation and corrosion are flow assurance issues which adversely affect gas processing and transportation. Chemical additives such as hydrate inhibitors are commonly injected to shift hydrate formation conditions so that pipeline operating conditions are within the hydrate-safe region, or to postpone hydrate nucleation, or to prevent the agglomeration of hydrate particles thus preventing hydrate plugging.1−3 Monoethylene glycol (MEG) is a popular thermodynamic hydrate inhibitor due to its recoverability using close-loop MEG regeneration and reclamation facilities.4 Preventing or lowering the risk of corrosion in gas pipelines is commonly achieved by pH stabilization or the injection of film forming corrosion inhibitors.5,6 The pH stabilization method requires that pH is adjusted using an amine to precipitate a stable protective iron carbonate film.7,8 However, dissolved oxygen (DO) even in small concentrations within lean-MEG injection lines, gas pipelines, downstream and well-head equipment, and MEG regeneration facilities also poses serious corrosion and operational risks.9−14 Dissolved oxygen can cause serious pitting corrosion to carbon steel and certain corrosion resistant alloy (CRA) pipelines especially in the presence of MEG.6,10 Dissolved oxygen also increases the rate of carbon dioxide corrosion of carbon steel.15−17 Furthermore, DO may hinder the effectivity of film forming corrosion inhibitors as well as the stability of iron carbonate films on the inner walls of pipelines.17,18 © XXXX American Chemical Society

Oxygen ingress is typically addressed by either purging using an inert gas for the removal of dissolved oxygen or the injection of specific chemicals known as oxygen scavengers (typically sulfites) to react with dissolved oxygen, lowering levels to 3%).6 A combined approach where nitrogen purging in storage vessels alongside the injection of oxygen scavengers is commonly implemented to not only reduce the dissolved oxygen concentration but to do so in a short duration of time.19 Even small amounts of dissolved oxygen over an extended period could result in nucleation of corrosion pits and consequent autocatalytic propagation.12 In terms of gas hydrate inhibition, it is important to understand how the added chemicals or oxygen scavengers will affect the hydrate inhibition performance of MEG. These chemicals must be assessed to ensure there are no opposing effects on the desired hydrate inhibition performance owing to dissociation products, byproducts of side-reactions, impedance to MEG’s inhibition kinetics, and incompatibilities. The tendency for these side-reactions to occur are further enhanced Received: February 20, 2018 Accepted: April 11, 2018

A

DOI: 10.1021/acs.jced.8b00150 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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oxygen scavenger, and the dosages used in the test solutions are reported in Table 2 and 3.

by the high operational temperatures applied in MEG close loops, and thus buildup of byproducts and chemical additives in the MEG close loop may play a role in hydrate inhibition. In this study, the methane hydrate inhibition performance of MEG combined with various oxygen scavengers was investigated. Oxygen scavengers are required in large concentrations to have an effective result.6 Optimally, oxygen scavengers should be nonvolatile, allowing for removal with salts during the reclamation process, preventing unnecessary build-up and fouling of the MEG close loop.6 However, MEG operations may not have a reclamation stage, or may have slip-stream reclamation depending on the allowable salt tolerance in the final lean-MEG solution to be injected at the well head, so oxygen scavengers may not be removed at all, or are removed from only a portion of the MEG inventory. Thus, knowing whether they perform as hydrate promoters or inhibitors is crucial to a successful hydrate flow assurance program.

Table 3. Oxygen Scavenger Dosage in Each Test Solution test solution blank blank sodium bisulfite (NaHSO3)

purity (mol %)

supplier

monoethylene glycol methane nitrogen sodium bisulfite

C2H6O2 CH4 N2 NaHSO3

99.477 99.995 99.9959 >99.5

Chem-Supply BOC NGP10+ Sigma-Aldrich

0.01 (100 ppm) 0.1 1 10 0.01 (100 ppm) 0.1 1 10 0.025 (250 ppm) 0.01 (100 ppm)

OS-P

20

IFEox2

20

catalyst (ppm)

dissolved oxygen (ppb)

1

7500 80 wt %). This hydrate promotion may be overlooked, but due to the small dosage of the chemical as well as the various other proprietary chemical additives used in the industry may result in detrimental effects. We conclude that

Figure 4. Hydrate phase boundaries for aqueous NaHSO3 solutions.

Figure 5. Hydrate phase boundaries for aqueous NaHSO3 + MEG solutions.

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DOI: 10.1021/acs.jced.8b00150 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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the remaining hydroxyl groups of erythorbic acid, DEAE, and erythorbate salt. Erythorbate salt is a result of the postneutralization reaction by DEAE with erythorbic acid (Figure 8).

4.0. CONCLUSIONS The influence of dissolved oxygen and various oxygen scavengers on gas hydrate formation was studied. Gas hydrates can cause dangerous consequences, and thus, it is important to understand how the various chemical additives that are injected alongside MEG behave and distort the hydrate inhibition performance. The study produced new hydrate equilibria data for sodium bisulfite solutions (0.01−10 wt %) with and without the presence of MEG. Results show greater inhibition at higher concentrations as opposed to commonly used dosages for oxygen scavenging applications. However, a proprietary oxygen scavenger promoted hydrate formation, which suggests that chemical additives should be thoroughly assessed for compatibility with other chemicals as well as tested to determine any potential negative consequences. A nonsulfite oxygen scavenger showed inhibition performance but may not surmount to any benefit due to the small dosages required. Furthermore, the study has revealed that dissolved oxygen, while it already negatively affects corrosion risk, may have a hydrate promotion effect as well, which increases the risk of gas hydrate formation. Clearly, dissolved oxygen levels should be kept to a minimum.

Figure 6. Hydrate phase boundary of proprietary oxygen scavenger, OS-P (0.025 wt %) in 20 wt % MEG solution.

proprietary chemical additives designed and created for specific purposes may have negative consequences on other flow assurance issues. 3.4. Effect of Nonsulfite-Based Oxygen Scavenger (IFEox2). The nonsulfite oxygen scavenger (IFEox2) developed by Kundu and Seiersten was investigated in this study which comprises erythorbic acid, diethylaminoethanol (DEAE), and a manganese catalyst. It was tested to realize its influence on gas hydrate formation in the presence of 20 wt % MEG solution. The measured hydrate phase boundary is plotted in Figure 7. The results show that the phase boundary has shifted

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Alef.Khalid@ gmail.com; Phone: +61 412 875 860. ORCID

Khalid Alef: 0000-0003-1751-5636 Stefan Iglauer: 0000-0002-8080-1590 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the contribution of an Australian Government Research Training Program Scholarship in supporting this research.

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Figure 7. Hydrate phase boundary of IFEox2 (0.01 wt %) in 20 wt % MEG solution.

ABBREVIATIONS MEG, monoethylene glycol; NaHSO3, sodium bisulfite; OS-P, proprietary oxygen scavenger; IFEox2, nonsulfite oxygen scavenger developed by Kundu and Seiersten; DEAE, diethylaminoethanol; AARE, absolute average relative error; DI, deionized water

to the left by ∼0.1 °C, suggesting this oxygen scavenger acted as a hydrate inhibitor. This slight inhibition performance could be related to hydrogen bonding of some water molecules with

Figure 8. Conversion of erythorbic acid to erythorbate salt by neutralization reaction by DEAE. E

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DOI: 10.1021/acs.jced.8b00150 J. Chem. Eng. Data XXXX, XXX, XXX−XXX