METHODS AND SYSTEMS FOR REMOVING MERCURY USING ADDITIVES

In one embodiment the application pertains to a method for removing mercury from a gas stream comprising mercury. The method comprises providing a mercury scavenging agent and a dehydrating agent. The gas stream comprising mercury is then contacted with the provided ingredients under conditions to sequester at least a portion up to about all of the mercury from the gas stream.

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Description
FIELD OF THE DISCLOSURE

The present disclosure relates to methods and systems for removing mercury from, for example, natural gas.

BACKGROUND AND SUMMARY

Often oil and gas must be processed from high mercury-containing wells and unfortunately some process equipment is mercury sensitive. In the case of liquefied natural gas (LNG) facilities, brazed aluminum cryogenic heat exchangers located in the cold box of the plant are particularly sensitive to mercury. Therefore, mercury removal units (MRU) are typically used to remove mercury down to 0.01 micrograms/Nm3. However, upstream of the MRU are entire sections of process equipment that will be in contact with mercury. Depending on temperatures, pressures, and fluid compositions the mercury can partition between the gas, liquid, and aqueous phases. In some cases, mercury can form a separate liquid phase (free mercury) when streams exceed their solubility limit of mercury due to changes in temperature and/or pressure.

What is needed are new ways of reducing mercury in gas streams before gas is processed and/or as close to the wellhead as possible to reduce impact on the downstream equipment. It would further be advantageous if such new methods and systems were capable of removing mercury during, for example, subsea transport of natural gas. It would be further beneficial if such new methods and systems were economical and could be implemented in existing facilities without significant process equipment addition and/or capital expense. Advantageously, the systems and methods described here solve many or all of the aforementioned issues with prior processes and systems. That is, the systems and methods described here offer lower capex, reduced mercury emissions, reduced impact on process equipment, and/or simpler operations.

In one embodiment, the application pertains to a method for removing mercury from a gas stream comprising mercury. The method comprises providing a mercury scavenging agent and a dehydrating agent. The gas stream comprising mercury is then contacted with the provided ingredients under conditions to sequester at least a portion up to about all of the mercury from the gas stream.

In another embodiment, the application pertains to a composition comprising monoethylene glycol; water; a solid mercury complex comprising a reaction product of mercury and copper sulfide; and natural gas.

These and other objects, features and advantages of the exemplary embodiments of the present disclosure will become apparent upon reading the following detailed description of the exemplary embodiments of the present disclosure, when taken in conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure, together with further objects and advantages, may best be understood by reference to the following description taken in conjunction with the accompanying drawing.

FIG. 1 shows the Hg loading to Cu(I) sulfide and Cu(II) sulfide.

 DETAILED DESCRIPTION

This application pertains to processes and systems for removing mercury from a gas stream. While such methods and systems are described with respect to being used, for example, during subsea transport of natural gas prior to processing or liquefication, it should be understood that they may be applicable in any application where mercury is in need of removal from a gas stream. That is, any gas stream comprising mercury may benefit from the concepts described herein. Such gas streams may, for example, comprise one or more hydrocarbons such as, for example, alkanes, alkenes, aromatics, as well as natural gas.

Generally, the methods comprise providing a mercury scavenging agent and a dehydrating agent and then contacting the gas stream comprising mercury with the ingredients under conditions to sequester at least a portion up to about all of the mercury from the gas stream.

The mercury scavenging agent may be any substance that complexes with the mercury in the gas stream and may vary depending upon the gas, other components, and desired results. Useful mercury scavenging agents are often solids at room temperature and may include, for example, a copper sulfide or a mixture of copper sulfides wherein copper sulfide as used herein includes compounds that comprise one or more copper atoms and one or more sulfur atoms. In some embodiments the copper sulfide comprises CuS2, CuS, Cu9S8, Cu39S28, Cu8S5, Cu7S4 (Cu1.75S), Cu9S5, Cu58S32 (Cu1.8S), Cu31S16, Cu2S, or any mixture thereof. If in solid form, the mercury scavenging agents such as a copper sulfide may be in the form of beads, extrudates (cylindrical-shaped or otherwise), pellets, powder, or a combination thereof. Often, if the scavenging agent is being added during transport, pipeline or otherwise, then it is preferred to employ a smaller scavenging agent. That is, the scavenging agent should be sufficiently small to facilitate transport by entrainment. Such sizes may vary but often may be in the range of sub-micron up to about 100 microns in size. Depending upon the specific mercury scavenging agent, a support or substrate may or may not be employed with the mercury scavenging agent.

Generally, the mercury scavenging agents such as a copper sulfide are provided to the gas stream separately from the dehydrating agent or alternatively the ingredients may be mixed in a convenient manner to provide a mixture comprising copper sulfide and dehydrating agent. Dehydrating agents are generally employed in a type and an amount such that moisture, i.e., water, present in the gas (if any) is changed from a gas phase to a liquid phase. In some embodiments the dehydrating agent is also selected such that in a mixture with water the freezing point of the dehydrating agent and water mixture is lower than the freezing point of water alone. In this manner water is removed from the gas phase, the freezing point is lowered, and/or hydrate formation in the gas stream is reduced or eliminated.

Dehydrating agents may comprise a glycol in some embodiments. Suitable glycols may include, for example, a monoalkylene glycol, a dialkylene glycol, a trialkylene glycol, or any mixture thereof. The alkylene may be substituted or unsubstituted and typically comprises from about 1 to about 6 carbon atoms. In some embodiments, the dehydrating agent comprises monoethylene glycol, triethylene glycol, or any mixture thereof.

The amount of dehydrating agent and the amount of mercury scavenging agent employed in the gas stream may vary depending upon the specific agents, the amount of mercury in the gas stream, the amount of water in the gas stream, and the desired results. Generally, the amount of dehydrating agent employed is an amount such that a significant amount of hydrates are prevented from forming. That is, the amount of dehydrating agent employed may be an amount such that water content in the gas stream is less than about 5%, or less than about 1%, or less than about 0.1% by weight based on total weight of the gas stream.

Similarly, the amount of mercury scavenging agent employed is an amount such that a significant amount of mercury present in the gas stream is sequestered. Such sequestering may involve forming a solid mercury complex that can then be removed from the gas stream by a suitable method. Generally, the amount of sequestering agent employed is sufficient to sequester at least about 80%, or at least about 90%, or at least about 95%, or at least about 99% or more of the mercury present in the gas stream.

In some embodiments a copper sulfide such as Cu2S, CuS, or a mixture thereof are employed as the mercury scavenging agent while monoethylene glycol is employed as the dehydrating agent. An appropriate copper sulfide to monoethylene glycol ratio may be from about 0.01 to about 0.1 mg/gm. In some embodiments, the copper sulfide amount is typically determined by how much mercury is to be scavenged. In some embodiments a Cu to Hg atomic ratio range may be from about 100 to about 10,000. On the other hand the amount of monoethylene glycol employed depends upon the amount of moisture in the natural gas stream.

The dehydrating agent and mercury scavenging agent may be added to the natural gas stream separately or together. If added together they may be mixed and such mixing may be conducted under any suitable conditions. Specific suitable conditions may vary depending upon the specific agents, the amount of each employed, and the gas stream composition and/or other properties. In some embodiments the agents are simply mixed under room temperature and pressure with agitation before being introduced into the gas stream for the contacting step.

In the contacting step the gas stream comprising mercury is contacted with each ingredient (mercury scavenging agent and dehydrating agent) separately and/or in a mixture comprising mercury scavenging agent and dehydrating agent under conditions to sequester at least a portion up to about all of the mercury from the gas stream. The conditions for the contacting may vary depending upon, for example, the specific components and amounts of each ingredient, the composition of the gas stream, and desired results. In general, the conditions are not particularly critical so long as the desired amount of mercury in the gas stream is sequestered and hydrates are prevented to the extent that any hydrates formed interfere with any processing or transporting of the gas stream, e.g., liquefaction of a natural gas stream.

The methods described here may be employed wherein the contacting of the mercury scavenging agent and the dehydrating agent with the gas stream, such as a natural gas stream, occurs prior to processing of a gas, such as prior to liquifying the natural gas stream. That is, in some embodiments the contacting and/or sequestering of mercury and reducing potential of hydrates could come while the gas stream is being transported from a wellhead to, for example, a processing facility. The transportation methods of such a gas stream are not particularly limited. However, advantageously it has been discovered that the methods and compositions described here may be particularly preferable during subsea transport because, for example, of the long residence time for mercury interaction with the scavenger as a result of the long transportation distance. That is, the contacting of the above-described ingredients with a gas stream such as natural gas may occur during a subsea transport of the gas stream. In this manner, hydrates may be reduced and/or eliminated while most, if not almost all or all, mercury in the gas stream is sequestered into a solid mercury complex.

While not wishing to be bound to any particular theory mercury from the gas phase may first dissolve in monoethylene glycol or other dehydrating agent and at least a portion of dissolved mercury will complex with the CuS or other scavenging agent. Mercury may then be further transported to the liquid phase until the CuS is saturated with mercury or the mercury is depleted.

Upon sequestering mercury into a solid mercury complex, then at least a portion up to all of the solid mercury complex and at least a portion up to all of the dehydrating agent employed may be separated from the gas stream in a convenient manner. Further processing, such as liquefaction, can then be undertaken.

The specific separating steps may employ any convenient means of separating. That is, at least a portion up to all of the solid mercury complex may be separated by known means such as, for example, filtering, centrifugal separation, etc. Similarly, in order to remove water, a separation step may comprise distilling the at least a portion up to all of the dehydrating agent to remove water. The aforementioned steps may remove a substantial amount up to all of any solid mercury complex from a natural gas stream, as well as removing monoethylene glycol and/or associated water from the natural gas stream. Once removed the monoethylene glycol/water mixture may be, for example, distilled to remove water. If desired, at least a portion of distilled monoethylene glycol may be further employed in a subsequent contacting step and/or in a provided mixture comprising copper sulfide and monoethylene glycol.

EXAMPLES Example 1

The present example pertains to mercury removal from natural gas using scavenging agents. A method was employed which dissolved elemental mercury in solutions comprising monoethylene glycol (MEG) and water to make the feed using elemental mercury contained in a dialysis bag. Eight scavenging agent materials were screened at room temperature using shake tests at a 50:1 mass ratio of MEG and water to the scavenging agent (25 g to 0.5 g).

TABLE 1 Scavenging agents evaluated Sample # Scavenging agent Name particle size Source 1 Axtrap 273 beads: 2-3 mm From Axens 2 CuS - SSD extrudates ¼″ Prepared cylinder internally 3 CuS - TRIBOTEC powder From Tribotec 4 Cu(I)S - Sigma Aldrich 325 mesh (44 From Sigma micron) Aldrich 5 Cu(II)S - Sigma Aldrich 100 mesh (149 From Sigma micron) Aldrich 6 Silia - Met Diamine (a 40-63 μm (60 A From Silicycle functionalized silica gel) pore size) 7 Silia - Met Thiol (a 40-63 μm (60 A From Silicycle functionalized silica gel) pore size) 8 Silia - Met Thiourea (a 40-63 μm (60 A From Silicycle functionalized silica gel) pore size)

To prepare feedstock for the screening studies, elemental mercury was dissolved in MEG using a dialysis bag. Elemental mercury was first added to the bag, which was dropped in filtered MEG solution from field (see below) and the solution was stirred at 150 rpm overnight to obtain a solution with mercury concentration of 119 ppb, which is consistent with literature data. A filtration step was employed to remove all mercury particles from the field MEG before starting the feed preparation for the experiments. The filtration was performed using a 1-micron, hydrophilic filter until the mercury concentration in the feed was close to zero, then the stirring process was started to obtain a saturated solution with respect to mercury using the bag.

The screening procedure comprised mixing selected scavenging agents with the feedstock and shaking at room temperature for 2 hours. Samples were filtered with 10 μm syringe filter to separate solid particles from the liquid phase. We relied on mercury concentration measured in the solids instead of the liquid because the mercury concentration in the liquid was small.

From the commercial scavenging agents pool, Copper (I) sulfide showed the best performance over the other candidates. The material is 99% pure which may explain its higher performance compared to other commercial products which have an alumina substrate. Alumina samples (no metal sulfide deposited) were also tested and the capacity was pretty low. Results are summarized in Table 2 below.

TABLE 2 Summary of screening shake tests with scavenging agents and field Rich MEG for elemental mercury Unfiltered Unfiltered Solids conc. conc. conc. Sample # Scavenging agent Name (ppb) (ppb) (ppb) 1 Axtrap 273 93 39 1547 2 CuS - SSD 72 31 2320 3 CuS - TRIBOTEC 5 8 2713 4 Cu(I)S - Sigma Aldrich 11 −12 11450 5 Cu(II)S - Sigma Aldrich 5 16 6757 6 Silia - Met Diamine 52 31 1457 7 Silia - Met Thiol 1 1 5860 8 Silia - Met Thiourea −15 9 5613 9 Axtrap 283 102 54 622 10 Alumina pellets 41 34 102 11 Glass beads (Blank) 92 54 —

Example 2

To simulate conditions in the field an experiment was designed with a constant concentration of elemental mercury by keeping the dialysis bag with mercury inside throughout the entire run to provide a constant initial condition in the liquid phase. The next step was to analyze the solids at different intervals to monitor capacity over time. This provided both capacity and kinetics data for each scavenging agent. The results are shown in FIG. 1.

Specific Embodiments

Specific embodiments are described in the numbered embodiments below.

1. A method for removing mercury from a natural gas stream comprising mercury wherein the method comprises:

    • providing a mercury scavenging agent and a dehydrating agent; and
    • contacting the natural gas stream comprising mercury with the mercury scavenging agent and the dehydrating agent under conditions to sequester at least a portion up to about all of the mercury from the natural gas stream.

2. The method of embodiment 1 wherein the mercury scavenging agent comprises a copper sulfide.

3. The method of embodiment 2 wherein the copper sulfide comprises CuS2, CuS, Cu9S8, Cu39S28, Cu8S5, Cu7S4 (Cu1.75S), Cu9S5, Cu58S32 (Cu1.8S), Cu31S16, Cu2S, or any mixture thereof.

4. The method of embodiment 1 wherein the dehydrating agent comprises a glycol.

5. The method of embodiment 4 wherein the dehydrating agent comprises a monoalkylene glycol, a dialkylene glycol, a trialkylene glycol, or any mixture thereof.

6. The method of embodiment 5 wherein the alkylene comprises from 1 to about 6 carbon atoms.

7. The method of embodiment 4 wherein the dehydrating agent comprises monoethylene glycol.

8. The method of embodiment 4 wherein the dehydrating agent comprises triethylene glycol.

9. The method of embodiment 1 wherein the conditions comprise conditions under which a significant amount of hydrates are prevented from forming.

10. The method of embodiment 9 wherein an amount of the dehydrating agent is employed under conditions such that a water content in the natural gas stream is less than about 5%.

11. The method of embodiment 9 wherein an amount of the dehydrating agent is employed under conditions such that a water content in the natural gas stream is less than about 1%.

12. The method of embodiment 9 wherein an amount of the dehydrating agent is employed under conditions such that a water content in the natural gas stream is less than about 0.1%.

13. The method of embodiment 1 wherein the natural gas stream comprises carbon dioxide, nitrogen, hydrogen sulfide, helium, or any mixture thereof.

14. The method of embodiment 1 wherein the contacting occurs during a subsea transport of the natural gas stream.

15. The method of embodiment 1 wherein the contacting occurs prior to processing the natural gas to a liquid natural gas.

16. The method of embodiment 1 wherein the sequestered mercury comprises a solid mercury complex.

17. The method of embodiment 16 which further comprises separating at least a portion up to all of the solid mercury complex and at least a portion up to all of the dehydrating agent from the natural gas stream and then liquifying the natural gas stream.

18. The method of embodiment 17 wherein the separating of the at least a portion up to all of the solid mercury complex comprising filtering.

19. The method of embodiment 17 which further comprises distilling the at least a portion up to all of the dehydrating agent to remove water.

20. The method of embodiment 1 wherein the natural gas stream comprises natural gas with from about 10 to about 1000 μg/m3 of mercury.

21. A method for removing mercury from a natural gas stream comprising mercury wherein the method comprises:

    • providing a copper sulfide and monoethylene glycol; and
    • contacting the natural gas stream comprising mercury with the copper sulfide and monoethylene glycol under conditions to form a solid mercury complex;
    • filtering the solid mercury complex from the natural gas stream;
    • removing the monoethylene glycol from the natural gas stream; and
    • distilling the monoethylene glycol to remove water.

22. The method of embodiment 21 which further comprises employing at least a portion of the distilled monoethylene glycol in the contacting step.

23. A composition comprising:

    • monoethylene glycol;
    • water;
    • a solid mercury complex comprising a reaction product of mercury and copper sulfide; and
    • natural gas.

24. The composition of embodiment 23 wherein the copper sulfide comprises CuS2, CuS, Cu9S8, Cu39S28, Cu8S4, Cu7S4 (Cu1.75S), Cu9S5, Cu58S32 (Cu1.8S), Cu31S16, Cu2S, or any mixture thereof.

In the preceding specification, various embodiments have been described with references to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded as an illustrative rather than restrictive sense.

Claims

1. A method for removing mercury from a natural gas stream comprising mercury wherein the method comprises:

providing a mercury scavenging agent and a dehydrating agent; and
contacting the natural gas stream comprising mercury with the mercury scavenging agent and the dehydrating agent under conditions to sequester at least a portion up to about all of the mercury from the natural gas stream.

2. The method of claim 1 wherein the mercury scavenging agent comprises a copper sulfide.

3. The method of claim 2 wherein the copper sulfide comprises CuS2, CuS, Cu9S8, Cu39S28, Cu8S5, Cu7S4 (Cu1.75S), Cu9S5, Cu58S32 (Cu1.8S), Cu31S16, Cu2S, or any mixture thereof.

4. The method of claim 1 wherein the dehydrating agent comprises a glycol.

5. The method of claim 4 wherein the dehydrating agent comprises a monoalkylene glycol, a dialkylene glycol, a trialkylene glycol, or any mixture thereof.

6. The method of claim 5 wherein the alkylene comprises from 1 to about 6 carbon atoms.

7. The method of claim 4 wherein the dehydrating agent comprises monoethylene glycol.

8. The method of claim 4 wherein the dehydrating agent comprises triethylene glycol.

9. The method of claim 1 wherein the conditions comprise conditions under which a significant amount of hydrates are prevented from forming.

10. The method of claim 9 wherein an amount of the dehydrating agent is employed under conditions such that a water content in the natural gas stream is less than about 5%.

11. The method of claim 9 wherein an amount of the dehydrating agent is employed under conditions such that a water content in the natural gas stream is less than about 1%.

12. The method of claim 9 wherein an amount of the dehydrating agent is employed under conditions such that a water content in the natural gas stream is less than about 0.1%.

13. The method of claim 1 wherein the natural gas stream comprises carbon dioxide, nitrogen, hydrogen sulfide, helium, or any mixture thereof.

14. The method of claim 1 wherein the contacting occurs during a subsea transport of the natural gas stream.

15. The method of claim 1 wherein the contacting occurs prior to processing the natural gas to a liquid natural gas.

16. The method of claim 1 wherein the sequestered mercury comprises a solid mercury complex.

17. The method of claim 16 which further comprises separating at least a portion up to all of the solid mercury complex and at least a portion up to all of the dehydrating agent from the natural gas stream and then liquifying the natural gas stream.

18. The method of claim 17 wherein the separating of the at least a portion up to all of the solid mercury complex comprising filtering.

19. The method of claim 17 which further comprises distilling the at least a portion up to all of the dehydrating agent to remove water.

20. The method of claim 1 wherein the natural gas stream comprises natural gas with from about 10 to about 1000 μg/m3 of mercury.

21. A method for removing mercury from a natural gas stream comprising mercury wherein the method comprises:

providing a copper sulfide and monoethylene glycol; and
contacting the natural gas stream comprising mercury with the copper sulfide and monoethylene glycol under conditions to form a solid mercury complex;
filtering the solid mercury complex from the natural gas stream;
removing the monoethylene glycol from the natural gas stream; and
distilling the monoethylene glycol to remove water.

22. The method of claim 21 which further comprises employing at least a portion of the distilled monoethylene glycol in the contacting step.

23. A composition comprising:

monoethylene glycol;
water;
a solid mercury complex comprising a reaction product of mercury and copper sulfide; and
natural gas.

24. The composition of claim 23 wherein the copper sulfide comprises CuS2, CuS, Cu9S8, Cu39S28, Cu8S5, Cu7S4 (Cu1.75S), Cu9S5, Cu58S32 (Cu1.8S), Cu31S16, Cu2S, or any mixture thereof.

Patent History
Publication number: 20260226359
Type: Application
Filed: Jan 31, 2025
Publication Date: Aug 6, 2026
Inventors: Raja Ankush Jadhav (Benicia, CA), Marcus Vinicius Dutra e Mello (Moraga, CA)
Application Number: 19/042,748
Classifications
International Classification: C10L 3/10 (20060101);