HYDROPHOBIC ADSORBENTS AND MERCURY REMOVAL PROCESSES THEREWITH

Abstract

A hydrophobic adsorbent composition and process for removal of mercury from a gas phase fluid near the water and/or hydrocarbon dew point is disclosed herein.

Claims

1. A hydrophobic adsorbent composition for removal of elemental mercury from a gas phase fluid, the comprising: a. an adsorbent material having pores therein and a pore volume, wherein the adsorbent material is selected from the group consisting of activated carbon, thiol-modified self-assembled monolayers on mesoporous supports, zeolites, and supported metal sulfides; and b. a fluid immiscible with water at least partially filling the pores of the adsorbent material to form the hydrophobic adsorbent; wherein the hydrophobic adsorbent has at least a 50% lower uptake of water than the adsorbent material without the fluid at least partially filling the pores when exposed to saturated water vapor at room temperature.

2. The hydrophobic adsorbent of claim 1 wherein the hydrophobic adsorbent has at least a 75% lower uptake of water than the adsorbent material without the fluid at least partially filling the pores when exposed to saturated water vapor at room temperature.

3. The hydrophobic adsorbent of claim 1 wherein the hydrophobic adsorbent has at least a 90% lower uptake of water than the adsorbent material without the fluid at least partially filling the pores when exposed to saturated water vapor at room temperature.

4. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water has a solubility for mercury greater than 2 ppb at room temperature.

5. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water has a solubility for mercury greater than 50 ppb at room temperature.

6. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water has a solubility for mercury greater than 100 ppb at room temperature.

7. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water has a solubility for mercury greater than 1000 ppb at room temperature.

8. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water is selected from the group consisting of hydrocarbons, jet fuel, diesel fuel, condensate, alcohols, halocarbons, crude oil, lubricating base stock, formulated lubricants, and white oil.

9. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water occupies 10% or more of the pore volume of the adsorbent material.

10. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water occupies 25% or more of the pore volume of the adsorbent material.

11. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water occupies 50% or more of the pore volume of the adsorbent material.

12. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water occupies 90% or more of the pore volume of the adsorbent material.

13. The hydrophobic adsorbent of claim 1 wherein the fluid immiscible with water occupies 100% or more of the pore volume of the adsorbent material.

14. A hydrophobic adsorbent composition for removal of elemental mercury from a gas phase fluid, the adsorbent comprising: a. an adsorbent material having pores therein, a pore volume and a surface, wherein the adsorbent material is selected from the group consisting of activated carbon, thiol-modified self-assembled monolayers on mesoporous supports, zeolites, and supported metal sulfides; and b. a surface modifier comprising a hydrophobic agent on the surface of the adsorbent material to form the hydrophobic adsorbent; wherein the hydrophobic adsorbent has a pore volume at least 50% lower than the adsorbent material without the surface modifier and wherein the hydrophobic adsorbent has at least a 50% lower uptake of water than the adsorbent material without the surface modifier when exposed to saturated water vapor at room temperature.

15. The hydrophobic adsorbent of claim 14 wherein the hydrophobic adsorbent has a pore volume at least 25% lower than the adsorbent material without the surface modifier.

16. The hydrophobic adsorbent of claim 14 wherein the hydrophobic adsorbent has a pore volume at least 10% lower than the adsorbent material without the surface modifier.

17. The hydrophobic adsorbent of claim 14 wherein the hydrophobic agent is selected from the group consisting of chlorosilanes, fluorosilanes and combinations thereof.

18. A process to remove elemental mercury from a gas phase fluid, the process comprising: a. contacting the gas phase fluid having an first elemental mercury content and having a water dew point with the adsorbent of claim 1 or claim 2 in a vessel at a temperature less than or equal to 28° C. from the water dew point thereby forming a gas phase fluid having a second elemental mercury content.

19. The process according to claim 2 wherein the temperature is less than or equal to 10° C. from the water dew point.

20. The process according to claim 2 wherein the temperature is less than or equal to 5° C. from the water dew point.

21. The process according to claim 2 wherein the temperature is less than or equal to 1° C. from the water dew point.

22. The process according to claim 2 wherein the temperature is less than or equal to the water dew point.

23. The process according to claim 2 wherein liquid water condenses in the vessel.

24. The process according to claim 2 wherein liquid hydrocarbons condense in the vessel.

25. The process according to claim 2 wherein the second elemental mercury content is at least 50% lower than the first elemental mercury content of the gas phase fluid.

26. The process according to claim 2 wherein the second elemental mercury content is at least 90% lower than the first elemental mercury content of the gas phase fluid.

27. A process for preparing a hydrophobic adsorbent useful in a process to remove elemental mercury from a gas phase fluid, the process comprising: a. providing an adsorbent material having pores therein selected from the group consisting of activated carbon, thiol-modified self-assembled monolayers on mesoporous supports, zeolites, and supported metal sulfides; and b. at least partially filling the pores of the adsorbent material with a fluid immiscible with water to form the hydrophobic adsorbent; such that the hydrophobic adsorbent has at least a 50% lower uptake of water than the adsorbent material without the fluid at least partially filling the pores when exposed to saturated water vapor at room temperature.

28. The process of claim 27 wherein the process occurs within a vessel.

29. A process for preparing a hydrophobic adsorbent useful in a process to remove elemental mercury from a gas phase fluid, the process comprising: a. providing an adsorbent material having pores therein, a pore volume and a surface, wherein the adsorbent material is selected from the group consisting of activated carbon, thiol-modified self-assembled monolayers on mesoporous supports, zeolites, and supported metal sulfides; and modifying the surface of the adsorbent material with a hydrophobic agent to form the hydrophobic adsorbent; such that the hydrophobic adsorbent has a pore volume at least 50% lower than the adsorbent material without the surface modifier and the hydrophobic adsorbent has at least a 50% lower uptake of water than the adsorbent material without the hydrophobic agent when exposed to saturated water vapor at room temperature.

Description

DETAILED DESCRIPTION

[0026] Generally, natural gas streams comprise low molecular weight hydrocarbons such as methane, ethane, propane, other paraffinic hydrocarbons that are typically gases at room temperature, etc. Mercury is present in natural gas as volatile mercury, including elemental mercury Hg.sup.0, in levels ranging from about 0.01 μg/Nm3 to 30,000 μg/Nm3. The mercury content may be measured by various conventional analytical techniques known in the art, including but not limited to cold vapor atomic absorption spectroscopy (CV-AAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence, or neutron activation. If the methods differ, ASTM D 6350 is used to measure the mercury content.

[0027] Depending on the source or sources of the natural gas, in addition to mercury, the stream can have varying amount of (produced) water ranging from 0.1 to 90 vol. % water in one embodiment, from 5 to 70 vol. % water in a second embodiment, and from 10-50 vol. % water in a third embodiment. The volume percents are calculated at the temperature and pressure of the pipeline.

[0028] Natural gas is often found in wells located in remote locations and must be transported from the wells to developed locations for use. This can be done by a production line, or by conversion of the methane in the natural gas into a liquefied natural gas (LNG) for transport.

[0029] The commercial mercury adsorbents have problems when condensable hydrocarbons or water is present in the gas. These condensed liquids either block the adsorption of the elemental mercury or cause the adsorbent to lose mechanical strength. The weakened adsorbent can crumble and lead to plugging in the adsorber. In crude and gas production, the mercury-containing gas is often obtained from separators or from compressor-chillers. In both cases the gas can be at or near its water and/or hydrocarbon dew point. To minimize problems from loss of the adsorbent, the gas is often heated to temperatures above its dew point. Alternatively, the gas can be chilled and the water and/or hydrocarbons condensed. The gas is then reheated prior to the mercury adsorption step. In both processes, expensive equipment is required. Also, the condensed water and hydrocarbon liquids from the second alternative can contain mercury and require additional treatment. It is recommended that hydrocarbon gases be heated to 28° C. above their hydrocarbon dew point to assure that no liquids condense.

[0030] Described hereinafter are hydrophobic MRU adsorbents which show reduced water uptake and improved ability to remove mercury when the temperature of the adsorber is less than or equal to 28° C. from the water dew point. The hydrophobic MRU Adsorbent is used under conditions where water would normally adsorb in the pores and cause a loss in performance. The temperature of the adsorber is less than or equal to 28° C. from the water dew point in one embodiment; less than or equal to 10° C. from the water dew point in another embodiment; less than or equal to 5° C. from the water dew point in another embodiment; less than or equal to 2° C. from the water dew point in another embodiment; and equal to or less than the water dew point in a fifth embodiment. In a sixth embodiment, water condenses as a liquid phase in the adsorber.

[0031] In one embodiment, the mercury content of the gas is reduced by 50% or more. In another embodiment, it is reduced by 90% or more. In another embodiment, it is reduced by 95% or more. In another embodiment, it is reduced by 99% or more. In one embodiment, the mercury content of the gas is reduced to at or below 10 μg/m3. In another embodiment, the mercury content of the gas is reduced to at or below 1 μg/m3. In another embodiment, the mercury content of the gas is reduced to at or below 0.1 μg/m3. In another embodiment, the mercury content of the gas is reduced to at or below 0.01 μg/m3.

[0032] In one embodiment, a fluid immiscible with water is added to at least partially fill the pores of a porous adsorbent material. Water has a low solubility for elemental mercury around 2 ppb at room temperature. By contrast, hydrocarbons and other fluids have much higher solubilities for elemental mercury, e.g., on the order of 1000 times the solubility of water for elemental mercury. Thus, partially filling the pores with a fluid immiscible with water permits the elemental mercury in the gas phase to enter the pores of the hydrophobic MRU adsorbent and react with the adsorption sites, e.g., copper sulfide on an adsorbent. It has been known in the state of the art that average pore diameters, Dp, may be calculated from measured pore volumes and surface areas assuming uniform cylindrical pores (Emmett. et al. J. Am. Chem. Soc., 65, 1253 (1943); Hirschler et al, Industr. and Eng. Chem., vol. 47(2), 1955. In one embodiment, the MRU adsorbent is exposed to the fluid immiscible with water prior to loading the adsorbent in the MRU vessel. In another embodiment, the MRU adsorbent is exposed to the fluid immiscible with water after it has been loaded in the MRU vessel.

[0033] In one embodiment, the adsorbent material can be one or more of activated carbon, thiol-modified self-assembled monolayers on mesoporous supports, zeolites, and supported metal sulfides. “Self-assembled monolayers on mesoporous supports” refers to a material developed by the Pacific Northwest National Laboratory and trademarked as SAMMS™, which can be modified by use of thiols. An example of the preparation and use of thiol-modified SAMMS™ for the removal of cationic mercury dissolved in water is described in Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2004, 49 (1), 288, incorporated herein by reference in its entirety.

[0034] Additives to the adsorbent may be utilized to combat problems previously associated with adsorbents. In one embodiment in addition to the adsorbents, at least one of an anti-foam and/or a demulsifier is added. As used herein, the term anti-foam includes both anti-foam and defoamer materials, for preventing foam from happening and/or reducing the extent of foaming. Additionally, some anti-foam material may have binary functions, including but not limited to reducing/mitigating foaming under certain conditions, and preventing foam from happening under other operating conditions. Anti-foam agents can be selected from a wide range of commercially available products such as silicones, e.g., polydimethyl siloxane (PDMS), polydiphenyl siloxane, fluorinated siloxane, etc., in an amount of 1 to 500 ppm.

[0035] In one embodiment, at least a demulsifier is added in a concentration from 1 to 5,000 ppm. In another embodiment, a demulsifier is added at a concentration from 10 to 500 ppm. In one embodiment, the demulsifier is a commercially available demulsifier selected from polyamines, polyamidoamines, polyimines, condensates of o-toluidine and formaldehyde, quaternary ammonium compounds and ionic surfactants. In another embodiment, the demulsifier is selected from the group of polyoxyethylene alkyl phenols, their sulphonates and sodium sulphonates thereof. In another embodiment, the demulsifier is a polynuclear, aromatic sulfonic acid additive.

[0036] In one embodiment, an MRU adsorbent is treated with a hydrophobicity inducing agent that alters the surface properties of the adsorbent such that it no longer adsorbs water. Examples of hydrophobicity inducing agents which functionally achieve this include but are not limited to silanes, including halogenated silanes such as chlorosilanes and fluorosilanes, Exemplary hydrophobic inducing agents and methods for making are seen in US20020114958A1, US20050123739 A1, U.S. Pat. No. 5,354,881 A, U.S. Pat. No. 7,341,706 B2 and U.S. Pat. No. 4,888,309 herein incorporated by reference.

EXAMPLES

Example 1—Comparative Example of Current Operation

[0037] Samples of commercial adsorbents were unloaded from a MRU and analyzed. The MRU had been processing gas from the inlet separator and suffering from short run lives and excessive amounts of mercury slip in the unit. Mercury slip is a high level of mercury in the treated gas. This unit was operating at the dew point of both water and hydrocarbons. Both materials condensed in the bed and liquid water and liquid hydrocarbon were withdrawn at the exit of the MRU.

[0038] Samples from two units (A and B) were obtained from six different depths in the units. Samples 1 were from near the inlet and samples 6 were near the outlet. Samples 2, 3, 4 and 5 were spaced evenly throughout the bed. The samples were analyzed by TGA-MS. The weight loss at 150 and 280° C. were recorded. The MS indicated only water (mass 18) in the vapor product, thus the pores were filled essentially with only water, not hydrocarbons. The loss at 150° C. is attributed to bulk water while the additional loss at 280° C. is attributed to water adsorbed more tightly on the surface of the support. Results are summarized in Table 1.

TABLE-US-00001 TABLE 1 Weight Weight Loss % at loss % at Wt % Unit Sample 150 C. 280 C. Mercury A 1 14.95 18.47 1.13 A 2 13.83 17.38 0.87 A 3 16.38 19.70 0.61 A 4 15.15 18.97 0.49 A 5 13.65 17.75 0.30 A 6 9.45 13.09 0.18 B 1 14.78 18.53 1.16 B 2 14.58 18.27 1.00 B 3 15.40 18.84 0.71 B 4 15.68 19.01 0.47 B 5 14.59 18.01 0.27 B 6 11.26 14.78 0.12

[0039] The mercury levels are significantly below what had been observed historically when the MRU was located after the dehydrator ˜10-20%. JMC reference “Minimizing mercury emissions from Gas Processing and LNG plants” says 10-15%.

Example 2

[0040] In this example, gas phase elemental mercury was dissolved in a white oil which is an example of a fluid immiscible with water. First, five grams of elemental mercury was placed in an impinger at 100° C. and 0.625 SCF/min of nitrogen gas was passed over through the impinger to form an Hg-saturated nitrogen gas stream. This gas stream was then bubbled through 3123 pounds of Superla® white oil held at 60-70° C. in an agitated vessel. The operation continued for 55 hours until the mercury level in the white oil reached 500 ppbw by a Lumex™ analyzer. This illustrates the high solubility of mercury in a fluid immiscible with water. The high solubility will enhance diffusion from the gas phase through the pores of the adsorbent.