HYDROCARBON RETENTION AND DISPOSAL CHALLENGES OF IRON OXIDE-BASED SCAVENGERS IN OFFSHORE GAS SWEETENING

Abstract

Hydrogen sulfide is a toxic and corrosive gas that poses significant health, safety, and environmental risks in industrial operations, particularly in natural gas processing. In floating production storage and offloading (FPSO) platforms, iron oxide-based scavengers are commonly used to “sweeten” natural gas streams by removing H₂S, thereby ensuring operational safety, preventing equipment corrosion, and enabling compliance with offshore environmental regulations. However, during their operational use, these scavengers can also adsorb hydrocarbons, which has implications for waste management and environmental safety. This study investigates the capacity of iron oxide-based scavengers (Fe₂O₃/Fe₃O₄) to retain aliphatic and polycyclic aromatic hydrocarbons, rather than focusing on H₂S removal. Gas chromatography analysis revealed hydrocarbon retention levels of up to 62 mg kg^-1 for aliphatic compounds and 32 mg kg^-1 for PAHs (polycyclic aromatic hydrocarbons). These findings are critical for assessing the environmental risks associated with the final handling of spent scavengers and for guiding the development of appropriate end-of-life treatment strategies. A better understanding of hydrocarbon retention, including the types and amounts retained, contributes to safer and more sustainable management of these materials within offshore operations and informs the development of appropriate end-of-life treatment strategies.

Keywords:
FPSO; hydrocarbons; iron oxide-based scavengers; sweetening gas.

INTRODUCTION

Hydrogen sulfide is a hazardous and corrosive gas commonly found in various industrial settings, such as natural gas processing, biogas purification, and petroleum refining.¹,² Its toxicity, coupled with its ability to corrode equipment, makes H₂S a significant health, safety, and environmental concern.²-⁵ Fossil fuels, particularly raw natural gas, often contain high levels of acidic and sour gases, such as H₂S. It is believed that most of the H₂S generated in oil and gas reservoirs is produced by the thermochemical reduction of sulfate by hydrocarbons. This phenomenon involves the oxidation of hydrocarbons coupled with the reduction of sulfate.⁶

The production of H₂S increases substantially when seawater is injected for secondary oil recovery. This occurs because seawater contains approximately 2.8 g L^-1 of sulfate ions.⁷ The phenomenon associated with increased H₂S generation due to seawater injection is known as souring or biogenic acidification. This process has been very common in Brazilian oil fields and results in a significant increase in the corrosivity of the medium.⁶ Several recent reports⁸ have shown a systematic increase in the sulfur content of crude oils over the past 10-20 years and predict further significant increases in H₂S concentrations in both crude oil and natural gas. This trend reinforces the need for technologies capable of efficiently removing H₂S, which must be reduced to meet commercial natural gas specifications, ensure compliance with environmental regulations, protect equipment from corrosion, and maintain operational safety.⁹,¹⁰

The effectiveness of H₂S removal technologies depends on the chemical and operational properties of the scavengers employed. Among the earlier methods, triazine-based compounds have been widely used due to their high reactivity.¹¹ These liquid-phase chemicals react with H₂S in gas or liquid streams, converting it into stable byproducts.⁹ While effective, triazine-based scavengers present certain drawbacks, including the potential for generating corrosive byproducts, such as diathiazines and monoethanolamine (MEA) (Figure 1).

Figure 1
Reaction of triazine with hydrogen sulfide for sweetening natural gas (adapted from reference ¹²)

As an alternative to liquid-phase scavengers, solid-phase materials have gained attention for their stability and reusability under various process conditions.¹³,¹⁴ Iron oxides or hydroxides, such as goethite-type α-FeOOH and hematite-type α-Fe₂O₃ are effective in removing hydrogen sulfide from waste gas streams originating from chemical plants or natural gas.¹³,¹⁵

The exothermic reaction between H₂S and iron oxyhydroxide or ferric oxide results in the formation of iron(II) sulfide, elemental sulfur, and water, as represented in Equations 1 and 2. Additionally, certain studies suggest that iron(III) sulfide may also be produced under specific conditions.¹³,¹⁶

(1) $${\mathrm{Fe}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{S}\to 2\mathrm{FeS}+1/8{\mathrm{S}}_8+3{\mathrm{H}}_2\mathrm{O}{\mathrm{\Delta }}_{\mathrm{R}}\mathrm{H}=-27\mathrm{kJ}{\mathrm{mol}}^{-1}$$

(2) $$2\mathrm{FeOOH}+3{\mathrm{H}}_2\mathrm{S}\to 2\mathrm{FeS}+1/8{\mathrm{S}}_8+4{\mathrm{H}}_2\mathrm{O}{\mathrm{\Delta }}_{\mathrm{R}}\mathrm{H}=-63\mathrm{kJ}{\mathrm{mol}}^{-1}$$

The solid byproducts of these reactions are stable and straightforward to manage, reducing environmental and operational risks. A key advantage of these materials is their ability to be easily regenerated at low temperatures in the presence of air, allowing for repeated use in H₂S adsorption processes.¹⁶

However, despite the well-established effectiveness of iron oxide-based scavengers in selectively removing hydrogen sulfide from gas streams, it becomes important to investigate whether these materials also interact with other components commonly found in natural gas, particularly hydrocarbons. This issue has practical relevance, as the co-adsorption of hydrocarbons may alter the properties of the spent material, potentially affecting its regeneration, reuse, and disposal. Such changes can impact both the operational performance and the environmental classification of the adsorbent. In this context, the present study aimed to evaluate the retention of hydrocarbons on iron oxide-based scavengers.

EXPERIMENTAL

A sample of iron-based scavenger, retrieved from the fixed-bed reactor vessel used for natural gas sweetening on a floating production storage and offloading platform (FPSO), was evaluated for its ability to adsorb aliphatic hydrocarbons (AHs), ranging from n-C₁₀H₂₂ to n-C₄₀H₈₂, as well as polycyclic aromatic hydrocarbons (PAHs), including the 16 EPA (Environmental Protection Agency)-priority PAHs and their alkylated homologues. The typical reactor from which the sample was retrieved contains approximately 15 m³ of the iron oxide medium, corresponding to a total mass of about 12 t, considering its bulk density of 770 kg m^-3. During operation, the vessel was exposed to an average gas flow of 4.5 Mm³ day^-1,¹⁷ resulting in a cumulative gas throughput of approximately 450 Mm³ over a 100-day usage period. The iron oxide medium employed in the reactor is single-use and not regenerated, being fully discarded after this period. An additional sample of the same virgin material was used for comparison.

The hydrocarbon extraction was carried out following United States Environmental Protection Agency (US EPA) method 3540C.¹⁸ Twenty grams of the iron-based scavenger, both preand post-application samples, were mixed with anhydrous sodium sulfate to remove residual moisture, then spiked with tetradecene and p-terphenyl d₁₄ as surrogate standards. Each sample underwent Soxhlet extraction for 8 h using a solvent mixture of n hexane/dichloromethane (7:3, v/v). The resulting extracts were concentrated to 1 mL using a TurboVap II evaporator (Biotage) and purified by column chromatography on a glass column packed with 3.6 g of deactivated alumina topped with 1 g of anhydrous sodium sulfate. Elution was performed using 30 mL of a n-hexane/dichloromethane mixture (3:7, v/v). The eluates were then reconcentrated to 1 mL and spiked with hexadecene and eicosene as internal standards (IS) for AHs, along with a mixture of deuterated PAHs, naphthalene d₈, acenaphthene d₁₀, phenanthrene d₁₀, chrysene d₁₂, and perylene d₁₂, as IS for PAHs. AHs were analyzed by gas chromatography equipped with a flame ionization detector (GC-FID; Agilent Technologies 6890N), following the USEPA method 8015D¹⁹ protocol. PAHs were analyzed using GC coupled to mass spectrometry (GC-MS; Agilent Technologies 6890/5973N), following USEPA method 8270E.²⁰

RESULTS AND DISCUSSION

Table 1 and Figure 2 present the concentrations of AHs and PAHs in the iron-based scavenger, along with chromatograms, respectively, both before and after its application in the fixed-bed reactor.

Table 1
Concentrations of aliphatic hydrocarbons and polycyclic aromatic hydrocarbons in the iron-based scavenger preand post-application in the reactor used for natural gas sweetening on an FPSO

Aliphatic hydrocarbons / (mg kg-1)				Polycyclic aromatic hydrocarbons / (μg kg-1)
	Pre-application	Post-application			Pre-application	Post-application
n-C10H22	< LOD	3.81		naphthalene	0.8	1645.2
n-C11H24	< LOD	6.84		C1-naphthalene	0.6	4293.9
n-C12H26	< LOD	9.88		C2-naphthalene	1.4	8435.4
n-C13H28	< LOQ	14.35		C3-naphthalene	2.8	9697.7
n-C14H30	< LOQ	12.71		C4-naphthalene	2.6	3909.6
n-C15H32	< LOQ	8.86		acenaphthylene	0.1	137.2
n-C16H34	< LOD	3.64		acenaphthene	0.2	38.9
n-C17H36	< LOD	1.04		fluorene	0.4	377.7
Pristane	< LOQ	1.00		C1-fluorene	3.7	493.2
n-C18H38	< LOD	0.28		C2-fluorene	4.7	398.1
Phytane	< LOQ	0.28		C3-fluorene	10.4	126.8
n-C19H40	< LOQ	0.12		dibenzothiophene	0.2	205.4
n-C20H42	< LOQ	0.21		C1-dibenzothiophene	0.6	250.9
n-C21H44	< LOD	< LOD		C2-dibenzothiophene	0.8	111.8
n-C22H46	< LOD	< LOQ		C3-dibenzothiophene	0.6	26.6
n-C23H48	< LOQ	< LOD		phenanthrene	2.7	771.3
n-C24H50	< LOD	< LOQ		C1-phenanthrene	3.9	521.0
n-C25H52	< LOD	< LOD		C2-phenanthrene	3.5	206.9
n-C26H54	< LOD	< LOD		C3-phenanthrene	2.5	65.2
n-C27H56	< LOQ	< LOD		C4-phenanthrene	1.6	13.0
n-C28H58	< LOD	< LOD		anthracene	0.1	12.0
n-C29H60	< LOD	< LOD		fluoranthene	5.2	2.9
n-C30H62	< LOD	< LOD		pyrene	2.8	5.0
n-C31H64	< LOD	< LOD		C1-pyrene	2.0	6.1
n-C32H66	< LOD	< LOD		C2-pyrene	7.6	3.9
n-C33H68	< LOD	< LOD		benz[a]anthracene	4.5	0.8
n-C34H70	< LOD	< LOD		chrysene	8.1	1.8
n-C35H72	< LOD	< LOD		C1-chrysene	10.8	< LOQ
n-C36H74	< LOD	< LOD		C2-chrysene	15.4	< LOQ
n-C37H76	< LOD	< LOD		benzo[b]fluoranthene	6.8	0.4
n-C38H78	< LOD	< LOD		benzo[k]fluoranthene	4.2	0.4
n-C39H80	< LOD	< LOD		benzo[e]pyrene	6.3	0.7
n-C40H82	< LOD	< LOD		benzo[a]pyrene	1.8	0.2
∑ n-alkanes	< LOQ	62		perylene	0.5	< LOQ
RPH	4.0	82		indeno[1,2,3-c,d]pyrene	1.2	< LOQ
UCM	< LOD	202		dibenzo[a,h]anthracene	0.7	< LOQ
				benzo[g,h,i]perylene	3.0	0.3
				∑HPA	125	31760

Figure 2
Total ion chromatograms of aliphatic hydrocarbons and polycyclic aromatic hydrocarbons (a,b) pre-application and (c,d) post-application

In the scavenger recovered from the fixed-bed reactor vessel, after use, the concentrations of n-alkanes reached 61.7 mg kg^-1, resolved petroleum hydrocarbons (RPH) 82.3 mg kg^-1, and the unresolved complex mixture (UCM) 202 mg kg^-1. In contrast, in the virgin (unused) product, n-alkanes and UCM were not detected, and the concentration of RPH was only 4.0 mg kg^-1. Qualitatively, the iron oxide retained n-alkanes ranging from n-C₁₀H₂₂ to n-C₂₀H_42, likely carried by the natural gas,²¹ but no heavier n-alkanes were detected. While natural gas is predominantly composed of light paraffins, trace amounts of mid-chain and heavier hydrocarbons (C₁₀-C₂₀) can be carried as vapors or micro-condensates.²² The presence of a significant UCM in the spent iron oxide scavenger, although unexpected for a gas purification process, can be explained by the partial adsorption of complex hydrocarbons carried in the natural gas stream. As shown in Figure 2c, the observed UCM also matches the retention times of the short-chain aliphatic hydrocarbons, supporting this hypothesis. Nevertheless, the possibility of catalytic conversion of syngas (i.e., a mixture of CO and H₂) derived from natural gas, via Fischer-Tropsch synthesis, in the presence of iron oxide, which can also generate UCM, cannot be ruled out.²³ The high amount of UCM is of environmental concern, as it may include complex polycyclic or alkylated hydrocarbons with potential toxicity, reinforcing the need for careful handling and classification of the spent scavenger as hazardous waste. Regarding PAHs, in the iron oxide retrieved from the reactor, the concentration of PAHs reached 31.8 mg kg^-1, of which 99.9% were 2- and 3-ring PAHs, and 90% of them were alkylated, a distribution pattern typical of petrogenic PAHs.²⁴ Considering the total 12 t of iron oxide used in the reactor, this corresponds to approximately 740 g of n-alkanes, 988 g of RPH, 2.42 kg of UCM, and 382 g of PAHs, representing the cumulative hydrocarbon retention in a single reactor over an average 100-day operational period.

It should be noted that no replicate analyses or additional sample collections were performed in this study. While the physical process of hydrocarbon adsorption on the iron oxide medium is generally uniform, some variability may occur between different wells and over time due to differences in gas composition and well characteristics. This potential variability indicates that the reported concentrations represent a snapshot of typical conditions and should be interpreted accordingly.

These findings demonstrate the high adsorption capacity of iron-based scavengers for a broad range of hydrocarbons. Such materials may therefore play a dual role in gas sweetening processes, not only removing H₂S but also capturing mid-chain n-alkanes and PAHs from natural gas. However, hydrocarbon co-adsorption can alter the physicochemical properties of the scavenger, potentially reducing its regeneration efficiency, operational lifespan, and influencing its environmental classification. Given that spent scavengers retain potentially toxic compounds such as PAHs,²⁴ they must be treated as environmentally hazardous waste. Appropriate management of solid waste is essential to prevent contamination of humans and the environment. According to ABNT NBR 10004 on waste classification,²⁵ the detection of even a single PAH in a sample is sufficient to categorize the waste as hazardous. In the present study, all PAHs listed in the standard were detected in the spent iron oxide, reinforcing its classification as hazardous and highlighting the need for proper handling and disposal.

Thus, understanding the extent and nature of hydrocarbon retention is essential for optimizing scavenger performance and for developing safe handling and disposal strategies for these materials.

CONCLUSIONS

This study demonstrated that iron oxide-based scavengers used in fixed-bed reactors for natural gas sweetening exhibit a significant capacity to adsorb a wide range of hydrocarbons, including mid-chain n-alkanes and PAHs. Determining the capacity of the scavenger to retain AHs and PAHs can assist in decision-making regarding its post-use management, helping to identify appropriate treatment, disposal methods, and disposal locations based on the presence of retained hydrocarbons and sulfur. Such advancements are particularly relevant for natural gas processing operations on FPSO, where efficient and environmentally friendly solutions are critical for managing constraints related to space, safety, and offshore environmental impact.

DATA AVAILABILITY STATEMENT

All data are available within the text.

ACKNOWLEDGMENTS

R. A. L. acknowledges the Brazilian National Research Council (CNPq grants No. 305415/2023-9 and process 408074/2023-0). The authors thanks E. J. M. Fritsch for the iron-based scavenger samples. This study was financed in part by the CAPES, Finance Code 001.

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Edited by

• Associate Editor handled this article:
  Eduardo M. Richter

Publication Dates

• Publication in this collection
  26 Jan 2026
• Date of issue
  2026

History

• Received
  08 Sept 2025
• Accepted
  24 Nov 2025
• Published
  27 Nov 2025