Methods and systems for removing mercury

Abstract

The present application pertains to methods and systems for removing mercury from a gas stream. In one embodiment the method comprises providing a gas stream comprising free mercury via a pipe to a mechanical separation device. At least a portion up to about all of the free mercury is removed from the gas stream with the mechanical separation device.

Description

Attorney Docket No. T-I26I0 (037287.0000365)METHODS AND SYSTEMS FOR REMOVING MERCURYCROSS REFERENCE TO RELATED APPLICATION

[0001] This application relates to U.S. Provisional Patent Application Serial No. 63 / 735,797, filed on December 18, 2024, the entire disclosures of which are incorporated herein by reference.FIELD OF THE DISCLOSURE

[0002] The present disclosure relates to methods and systems for removing mercury from, for example, a process gas.BACKGROUND AND SUMMARY

[0003] 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) or natural gas liquids (NGL) facilities, brazed aluminum heat exchangers and / or aluminum turbo expanders 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. Thus, the ability to remove mercury from process streams upstream of the MRU is challenging.

[0004] Figure 1 shows a known-art inlet processing system for an LNG plant. Production fluids first contact a slug catcher (SCI), which does a rough separation of gas and liquid. The gas stream leaving the slug catcher (1) is typically heated in some heat exchanger (El) prior toAttorney Docket No. T-I2610 (037287.0000365) a pressure letdown in valve (VI), which controls the nominal pressure for the downstream portion of the plant. The heater is used to minimize the risk of hydrate formation and to regulate temperature since pressure-letdown will result in Joule-Thompson cooling of the stream.

[0005] In the prior art such as Figure 1, the main method of removing the gas-phase mercury is to install the MRU at the end of the process, where all the gas streams (fresh feed and various recycle streams) commingle. Unfortunately, when the MRU beds decline in performance due to increases in mercury concentrations in the inlet gas, liquid carry over, adsorbent degradation, poisoning by impurities, etc., the bed lifetime may be reduced compared to design. In the case of large LNG facilities, unplanned shutdow ns and / or prolonged maintenance or turnarounds can result in significant monetary loss through loss of production. What’ s more, relying on the MRU as the mam unit for removing mercuries means that mercury has a chance to potentially accumulate and / or drop out in all of the piping and equipment upstream of the MRU. Unexpected mercury’ dropout can result in liquid impingement on equipment, or potential plugging of valves and piping.

[0006] What is needed are new ways of reducing mercury in gas streams that do not rely wholly on an MRU. Furthermore, it would further be advantageous if such new methods and systems removed mercury’ before any MRU processing so that process equipment upstream of the MRU is subjected to less mercury. It would be further beneficial if such new methods and systems were economical and could be implemented in existing facilities without significant process equipment and / or capital expense. Advantageously, the systems and methods described here solve many or all of the aforementioned issues with prior processes and systems.

[0007] In one embodiment, the application pertains to a method which comprises providing a gas stream comprising free mercury via a pipe to a mechanical separation device. At least aAttorney Docket No. T-12610 (037287.0000365) portion up to about all of the free mercury is removed from the gas stream with the mechanical separation device.

[0008] In another embodiment, the application pertains to a system comprising a slugcatcher configured to separate liquids and gases from a production fluid to form a liquid stream and a gas stream, a pipe, and a mechanical separation device for removing free mercury from the gas stream. The pipe is configured to pass the liquid stream from the slugcatcher to the mechanical separation device.

[0009] 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

[0010] 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.

[0011] Figure 1 shows a prior art processing system for an LNG plant.

[0012] Figure 2A shows a process where a mechanical separation device (Ml) is placed downstream of a slug catcher gas outlet in a processing system for an LNG plant.

[0013] Figure 2B shows a process where a mechanical separation device (Ml) is placed downstream of a slug catcher gas outlet in a processing system for an LNG plant with a cooler (E3) added.

[0014] Figure 3A shows a process where a mechanical separation device (Ml) is placed downstream of a slug catcher gas outlet just prior to a mercury recovery unit (MRU) in a processing system for an LNG plant.Attorney Docket No. T-12610 (037287.0000365)

[0015] Figure 3B shows a process where a mechanical separation device (Ml) is placed downstream of a slug catcher gas outlet just prior to a mercury recovery unit (MRU) in a processing system for an LNG plant with a cooler (E3) added.

[0016] Figure 4A shows a double pocket vane pack (DPV).

[0017] Figure 4B shows a standard vane pack (STV).

[0018] Figure 5A shows a vertical vessel with vertical flow.

[0019] Figure 5B shows a vertical vessel with horizontal flow.

[0020] Figure 6A shows a horizontal vessel with vertical exit flow.

[0021] Figure 6B shows a horizontal vessel with horizontal exit flow.

[0022] Figure 7A shows a plan view of a horizontal vessel with horizontal lateral flow and double filter banks

[0023] Figure 7B shows an elevation view of a horizontal vessel with horizontal lateral flow and double filter banks.

[0024] Figure 7C shows an end elevation view of a horizontal vessel with horizontal lateral flow and double filter banks.

[0025] Figure 8A shows an elevation view of a horizontal vessel with angled lateral flow and double filter banks.

[0026] Figure 8B shows an end elevation view of a horizontal vessel with angled lateral flow7and double filter banks.DETAILED DESCRIPTION

[0027] 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 in, for example, an LNG processing facility 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 streamAttorney Docket No. T-I2610 (037287.0000365) 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.

[0028] Generally, the methods comprise first providing a gas stream comprising free mercury via a pipe to a mechanical separation device. At least a portion up to about all of the free mercury may then be removed from the gas stream with the mechanical separation device. Generally, the systems involved generally comprise a pipe carrying the gas stream comprising free mercury and the mechanical separation device. In some embodiments, a slugcatcher or other device first separates one or more liquids from one or more gases.

[0029] Free mercury generally comprises mercury that is not dissolved in the gas. Such free mercury is in a liquid form and may be in the form of, for example, entrained droplets (fine or larger), a free flowing separate liquid film, or any combination thereof. The amount of free mercury that may be dissolved versus free may vary depending upon such factors as temperature, pressure, and gas composition. In order to increase the amount of free mercury in some embodiments, it may be desirable to decrease the temperature of the gas comprising dissolved mercury in order to facilitate additional free mercury that may then be removed. Additionally or alternatively, the pressure may be increased to facilitate additional free mercury being formed from the dissolved mercury.

[0030] Any convenient method may be employed to cool the gas comprising dissolved mercury and / or increase the pressure. Generally, cooling is employed prior to the removing with the mechanical separation device to increase the concentration of free mercury. While the specific temperature to cool the gas comprising dissolved mercury may vary, for many hydrocarbon gases such as those compositions comprising natural gas, the cooling generally comprises lowering the gas stream temperature to a temperature lower than about 15 °C, orAttorney Docket No. T-I26I0 (037287.0000365) lower than about 12°C, or lower than about 10°C, or even lower. However, if the gas composition also comprises water and / or other hydrate-forming compounds, then it may be desirable to not cool the gas composition below a temperature at which an amount of hydrates form that may interfere with processing, i.e., significant hydrate formation is typically to be avoided.

[0031] In another process embodiment, a gas stream comprising free mercury or tank of gas comprising free mercury may be cooled below about 0°C by contacting the gas stream or gas tank with a cryogenic gas to form a solid comprising mercury, water, heavy hydrocarbons, or any combination thereof. The solid may then be removed or separated from the gas in any convenient manner. Such a process could be implemented prior to sending gas to an NGL or LNG facility. For example, if being shipped by a vessel the vessel may be equipped with two parallel chillers and one could be chilling while the other is regenerating and routing separating hydrate and other solid products to another tank or vessel.

[0032] Convenient cooling methods or devices may be employed and are not particularly limited. Such methods and devices include, for example, an absorption chiller. Useful absorption chillers may be one stage or two stage absorption chillers to provide chilled water and / or other substances, e.g.. 5-10°C, to an area surrounding or in contact with the pipe carrying the gas comprising absorbed mercury. In this manner, the chilled water and / or other substances can be made to stay in thermal contact with the gas comprising absorbed mercury’ for a time sufficient to form a desired amount of free mercury from dissolved mercury.

[0033] Absorption chillers (or the other cooling methods and devices described herein) may, if desired, use waste heat as a source of energy. Such waste heat is particularly available in LNG processes and plants as it may be sourced from hot turbine gas. Additionally or alternatively, other cooling methods or devices may be employed. For example, cooling mayAttorney Docket No. T-12610 (037287.0000365) comprise employing a mechanical chiller and / or a propane cooling loop. A mechanical chiller and / or a propane cooling loop may be particularly suitable in an LNG processing plant. That is, a propane cooling loop could readily employ a refrigerant slipstream from the LNG processing.

[0034] The type of mechanical separation device that may be employed herein is not particularly limited. Generally, the mechanical separation device comprises a passive device configured to allow at least a portion of the free mercury in the gas to coalesce and / or further coalesce, if not already sufficiently coalesced. In this manner, the mechanical separation device can facilitate mercury being removed or drained from the pipe at, near, and / or subsequent to the mechanical separation device in any convenient manner. While not required, at least a portion up to all of the coalesced mercury is typically removed or drained simultaneously or immediately after coalescence to prevent damage to processing equipment from and / or due to the mercury.

[0035] In one embodiment the mechanical separation device comprises one or more vane packs. Exemplary vane packs are shown in figures 4A and 4B. Vane packs may be high velocity droplet separators which can be designed for droplet collection of droplets greater than 8, or greater than 10 up to about 20 microns or larger. Such vane packs may include a good resistance to blockage by solids due to their high voidage construction.

[0036] Standard vane packs shown in Figure 4B are often made using multiple parallel blade profiles assembled into a pack. This facilitates passing the gas to pass vertically or horizontally between the blades. The blades may be relatively widely spaced for a high volume throughput capacity with relatively low pressure loss. These may be employed with any droplet size but may be preferable for droplet sizes of, for example, greater than 10 microns in size. Advantageously, such vane packs may also be highly resistant to plugging. As a person ofAttorney Docket No. T-I2610 (037287.0000365) ordinary skill will appreciate, the spacing of the vane packs’ blades and the number of stages can be varied depending upon the application and desired results. In this manner, to modify the collection efficiency and / or pressure loss of the vane packs such as these standard vane packs they may combined with a knitted mesh layer. The other mechanical separation devices employed herein may also comprise a mesh layer such as a knitted mesh layer. It has been discovered that such a mesh layer may facilitate and / or increase removal of smaller particle sizes of mercury droplets.

[0037] In some applications it may be desirable to employ in addition to or as an alternative to standard vane packs the double pocket vane packs such as those shown in Figure 4A. Double pocket van packs may offer more efficiency and / or higher liquid load handling than standard vane packs. Such double pocket vane packs may comprise one or more built-in liquid catch pockets to facilitate draining and / removing the liquid mercury away from the gas stream more effectively. If desired, the double pocket vane packs may be used vertically in order to handle horizontal gas flows.

[0038] The free mercury in the gas may vary in surface tension depending upon, for example, droplet size and other factors. Therefore, it may be advantageous to modify the surface of a mechanical separation device depending upon the surface tension of the droplets to be captured. It has been discovered that modifying the surface to provide, for example, texturing may facilitate capturing higher surface tension particles. Thus, one or more vane packs employed herein may comprise texturing to facilitate capture of free mercury with a high surface tension.

[0039] In addition to or as an alternative, to the vane packs, the mechanical separation device may comprise a drip trap, a cyclonic separator coupled to a gravity separator, or any combination thereof. Suitable drip traps are not particularly limited and may include those that are conventionally employed to separate a condensate from steam. Figure 9 shows a suitableAttorney Docket No. T-12610 (037287.0000365) mechanical separating device similar to a drip trap for use herein.

[0040] A cyclonic separator coupled to a gravity separator may additionally or alternatively be employed. The cyclonic separator may concentrate denser liquid mercury droplets in the gas stream to promote mercury coalescence to facilitate removal while the gravity separator facilitates mercury removal by allowing the mercury to settle out.

[0041] The above-described one or more mechanical separation devices may be implemented in line with the pipe carrying the gas steam comprising free mercury (with or without the cooling). That is, the one or more mechanical separation devices may be integral with the pipe or may be a separate device (or devices) attached to the pipe in parallel or in series. The one or more mechanical separation devices may be located anywhere in a process where mercury reduction in a gas stream may be beneficial. As shown in the figures, the one or more mechanical devices may be at the beginning of a process, e.g., after a slugcatcher, and / or near an end of gas processing, e.g., near an MRU, or anywhere in between.

[0042] The above methods and systems may be employed and / or adjusted to remove a desired amount of mercury. For example, in some embodiments the systems and methods may result in at least about 60%, or at least about 65%, or at least about 70%, or at least about 75% of the total mercury being removed from the gas stream by the mechanical separation device.Comparative Example - No mechanical separation device employed as in Figure 1

[0043] The configuration in prior art Figure 1 is employed to process gas from a well high mercury. Table 1 below shows the process conditions and mercury distribution between streams 1 and 2 in Figure 1. Stream 1 is at 12.1C and 7995 kpag while Stream 2 after heating is at 27C and 7695 kpag. Although the total mercury concentration is the same between Stream 1 and Stream 2 at 452 ppbwt, Stream 2 has zero free mercury and 452 ppbwt of dissolved mercury. This means that the gas has sufficient solubility at the temperature and pressure toAttorney Docket No. T-12610 (037287.0000365) completely solubilize all the mercury. In Stream 1, which is at a lower temperature, the gas is only able to dissolve 169 ppbwt of mercury, while the remaining 283ppbwt of mercury is free mercury, forming its separate phase. Free mercury may be in the form of fine entrained droplets, a free flowing separate liquid film, or some combination of the two. Although mercury is very dense (13 g / mL) compared to liquid water (1 g / mL), at sufficiently small particle size and high gas velocity, it can remain entrained with the flowing gas stream and carried further downstream. Some mercury may coalesce and dropout if the gas encounters a low spot, deadleg. Once mercury drops out, it may cause problems due to it accumulating in the piping / equipment over time, getting re-entrained in the gas, and / or some combination of the two depending on conditions.Attorney Docket No. T-12610 (037287.0000365)

[0044] Table 1%Hg Removal by Device 0Example 2 - Mechanical separation device (Ml) employed as in Figure 2A

[0045] Comparative Example 1 was repeated except that the configuration in Figure 2A was employed. Figure 2A shows a process where the mechanical separation device (Ml) is placed downstream of the slug catcher gas outlet, Stream 1. This stream is already at a low arrival temperature (12. EC), and has the highest concentration of free mercury. 62% removal of mercury will result in minimum free mercury in the outlet stream la, leaving only the dissolved mercury as shown in Table 2 below.Attorney Docket No. T-12610 (037287.0000365)

[0046] Table 2%Hg Removal by Device 62Example 3 - Mechanical separation device (Ml) employed with cooler (E3) as in Figure 2B

[0047] Comparative Example 1 is repeated except that the configuration in Figure 2B is employed. Figure 2B shows a similar process as Figure 2A, except that a cooler (E3) is added to lower the temperature to 10°C before the mechanical separation device (Ml). In doing so, the free mercury concentration increases and 68% removal of mercury7will result in minimum free mercury stream in the outlet stream la, leaving only the dissolved mercury as shown in Table 3. Going to a temperature below 10°C may not be desirable, as it can result in hy drate formation.Attorney Docket No. T-12610 (037287.0000365)

[0048] Table 3%Hg Removal by Device 68Example 4 - Mechanical separation device (Ml) employed prior to MRU as in Figure 3A

[0049] Comparative Example 1 is repeated except that the configuration in Figure 3A is employed. Figure 3A shows another alternative where the mechanical separation device (Ml) is placed immediately upstream of the MRU. In this case, the feed stream (3) is heated and will tend to have the lowest amount of free mercury and highest amount of dissolved mercury. The maximum removal rate of 9% of the Hg will result in only dissolved mercury going to the MRU, as shown in Table 4.Attorney Docket No. T-12610 (037287.0000365)

[0050] Table 4%Hg Removal by Device 9Example 5 - Mechanical separation device (Ml) employed prior to MRU with cooler (E3) as in Figure 3B

[0051] Comparative Example 1 is repeated except that the configuration in Figure 3B is employed. Figure 3B shows a similar process as Figure 3A, except that a cooler (E3) is added. That is, in order to promote free mercury formation and facilitate the removal of mercury in a mechanical separation device (Ml), the feed cooler (E3) is added to lower the temperature from 25°C. At 15°C, the free mercury7makes up 57% of the total mercury7, while at 10°C the free mercury7makes up 76% of the total removal. At 8°C, hydrate formation is likely. Tables 5 and 6 show the stream summary for 15°C and 10°C respectively.Atorney Docket No. T-12610 (037287.0000365)

[0052] Table 5 (15°C)%Hg Removal by Device 57

[0053] Table 6 (IO C)%Hg Removal by Device 76Attorney Docket No. T-12610 (037287.0000365)Specific Embodiments

[0054] Additional specific embodiments are described in the numbered embodiments below.

[0055] 1. A method for removing mercury from a gas stream comprising mercury wherein the method comprises: providing a gas stream comprising free mercury via a pipe to a mechanical separation device; and removing at least a portion up to about all of the free mercury from the gas stream with the mechanical separation device.

[0056] 2. The method of any previous embodiment which further comprises cooling the gas stream comprising mercury prior to the removing with the mechanical separation device.

[0057] 3. The method of any previous embodiment wherein the cooling comprises lowering the gas stream temperature to a temperature which increases the concentration of free mercury in the gas and avoids significant hydrate formation.

[0058] 4. The method of any previous embodiment wherein the cooling comprises lowering the gas stream temperature to a temperature between about IO C and about 15"C.

[0059] 5. The method of any previous embodiment wherein the cooling comprises employing an absorption chiller.

[0060] 6. The method of any previous embodiment wherein the absorption chiller employs at least a portion of waste heat.

[0061] 7. The method of any previous embodiment wherein the cooling comprises employing a mechanical chiller.

[0062] 8. The method of any previous embodiment wherein the cooling comprises employing a propane cooling loop with a refrigerant slipstream.Attorney Docket No. T-12610 (037287.0000365)

[0063] 9. The method of any previous embodiment wherein the mechanical separation device comprises a passive device configured to allow free mercury to coalesce and drain from the pipe.

[0064] 10. The method of any previous embodiment wherein the mechanical separation device comprises one or more vane packs.

[0065] 11. The method of any previous embodiment wherein the one or more vane packs comprises a double pocket vane pack.

[0066] 12. The method of any previous embodiment wherein the one or more vane packs comprises texturing to capture free mercury with a high surface tension.

[0067] 13. The method of any previous embodiment wherein the mechanical separation device comprises a mesh layer.

[0068] 14. The method of any previous embodiment wherein the mechanical separation device comprises a drip trap.

[0069] 15. The method of any previous embodiment wherein the mechanical separation device comprises a cyclonic separator coupled to a gravity separator.

[0070] 16. The method of any previous embodiment wherein the gas stream comprises natural gas.

[0071] 17. The method of any previous embodiment which further comprises processing the gas stream in a mercury recovery unit subsequent to the removing.

[0072] 18. The method of any previous embodiment wherein at least about 60% of the total mercury is removed from the gas stream by the mechanical separation device

[0073] 19. A system comprising: a slugcatcher configured to separate liquids and gases from a production fluid to form a liquid stream and a gas stream;Attorney Docket No. T-I2610 (037287.0000365) a pipe; a mechanical separation device for removing free mercury from the gas stream; wherein the pipe is configured to pass the liquid stream from the slugcatcher to the mechanical separation device.

[0074] 20. The system of any previous embodiment wherein the system further comprises a cooling mechanism coupled to the pipe for lowering the gas stream temperature to a temperature between about IO C and about 15°C.

[0075] 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

Attorney Docket No. T-12610 (037287.0000365)WE CLAIM:

1. A method for removing mercury from a gas stream comprising mercury wherein the method comprises: providing a gas stream comprising free mercury via a pipe to a mechanical separation device; and removing at least a portion up to about all of the free mercury from the gas stream with the mechanical separation device.

2. The method of claim 1 which further comprises cooling the gas stream comprising mercury prior to the removing with the mechanical separation device.

3. The method of claim 2 wherein the cooling comprises lowering the gas stream temperature to a temperature which increases the concentration of free mercury' in the gas and avoids significant hydrate formation.

4. The method of claim 2 wherein the cooling comprises lowering the gas stream temperature to a temperature between about 10°C and about 15°C.

5. The method of claim 2 wherein the cooling comprises employing an absorption chiller.

6. The method of claim 5 wherein the absorption chiller employs at least a portion of waste heat.

7. The method of claim 2 wherein the cooling comprises employing a mechanical chiller.

8. The method of claim 2 wherein the cooling comprises employing a propane cooling loop with a refrigerant slipstream.

9. The method of claim 1 wherein the mechanical separation device comprises a passive device configured to allow free mercury7to coalesce and drain from the pipe.

10. The method of claim 9 wherein the mechanical separation device comprises one or more vane packs.Attorney Docket No. T-12610 (037287.0000365)11. The method of claim 10 wherein the one or more vane packs comprises a double pocket vane pack.

12. The method of claim 10 wherein the one or more vane packs comprises texturing to capture free mercury with a high surface tension.

13. The method of claim 9 wherein the mechanical separation device comprises a mesh layer.

14. The method of claim 9 wherein the mechanical separation device comprises a drip trap.

15. The method of claim 9 wherein the mechanical separation device comprises a cyclonic separator coupled to a gravity separator.

16. The method of claim 1 wherein the gas stream comprises natural gas.

17. The method of claim 1 which further comprises processing the gas stream in a mercury recovery unit subsequent to the removing.

18. The method of claim 1 wherein at least about 60% of the total mercury is removed from the gas stream by the mechanical separation device19. A system comprising: a slugcatcher configured to separate liquids and gases from a production fluid to form a liquid stream and a gas stream; a pipe; a mechanical separation device for removing free mercury from the gas stream; wherein the pipe is configured to pass the liquid stream from the slugcatcher to the mechanical separation device.

20. The system of claim 19 wherein the system further comprises a cooling mechanism coupled to the pipe for lowering the gas stream temperature to a temperature between about 10°C and about 15°C.Attorney Docket No. T-12610 (037287.0000365)21. A method for removing mercury from a gas stream comprising mercury wherein the method comprises: cooling a gas stream comprising free mercury below about 0°C by contacting the gas stream with a cry ogenic gas to form a solid comprising mercury7; and removing the solid from the gas stream.

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