WO2011107764A1

WO2011107764A1 - Sorbents

Info

Publication number: WO2011107764A1
Application number: PCT/GB2011/050223
Authority: WO
Inventors: Matthew John Cousins; Kevin Young
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2010-03-04
Filing date: 2011-02-09
Publication date: 2011-09-09
Anticipated expiration: 2012-09-04
Also published as: GB201003549D0

Abstract

A sorbent is described comprising a thiol functionalised silane supported on a shaped support, said sorbent having a mean crush strength ≥ 0.5kg and a tapped bulk density in the range 0.4 to 0.8 kg/litre. The sorbent may be used to remove heavy metals e.g. mercury and/or arsenic, from wastewater streams such as produced water or flue gas scrubber waters.

Description

Sorbents

This invention relates to sorbents suitable for removing mercury and/or arsenic and other heavy metals from aqueous streams and their use.

Mercury and arsenic and other heavy metals such as cadmium and lead are poisonous substances and are often present as soluble compounds in produced water from oil and gas recovery processes, and may be found in waste-water streams from refineries, flue gas washing and coal gasification. There is a need to find an environmentally sound and cost effective heavy metal removal technology.

US 5814226 discloses a process for removing heavy metal ions from aqueous streams by contacting the streams with an inorganic ceramic support such as a silica gel on which has been supported a thiol or amine-functionality by means of a suitably functionalised organic silane or titanizing agent. No examples with mercury or arsenic are given.

Ind. Eng. Chem. Res. 2003, 42, 1955-1964 discloses mercury (II) ion adsorption from wastewaters using a thiol-functionalised adsorbent ("SOL-AD-IV"). The adsorbent is formed by co-hydrolysis of (3-mercaptopropyl) trimethoxy silane and tetraethoxysilane to form an adsorbent with specific physical characteristics.

J. Environ. Monit., 2003, 3, 366-370 discloses that mercury may also be absorbed from aqueous solutions using a thiol-functionalised silica material using a thiol-functionalised silane wherein the thiol-functionalised silane was formed on a silica gel surface by the reaction 3- mercaptopropyl silane and ethylene sulphide.

The physical characteristics of the known products, which were tested as powders, are not ideal for industrial scale use, and large amounts of expensive silane need to be used. We have further found that different physical characteristics in the supports can lead to sorbents with improved heavy metal capture performance at an industrial scale.

We have developed an sorbent that overcomes the disadvantages of the known materials.

Accordingly the invention provides a sorbent is comprising a thiol functionalised silane supported on a shaped support, said sorbent having a mean crush strength > 0.5 kg and a tapped bulk density in the range 0.4 to 0.8 kg/litre.

The invention further provides a method for making a sorbent, comprising the steps of: (i) reacting a thiol-functionalised silane with a shaped support to form a thiol-functionalised support, and (ii) drying the thiol-functionalised support. The invention further provides a process for one or more removing heavy metals from an aqueous stream, comprising contacting the sorbent with an aqueous stream containing one or more heavy metals.

Herein by the term "sorbent" we include "adsorbent" and "absorbent".

The term "heavy metals" includes mercury, arsenic, lead, cadmium, antimony, tin, copper, nickel, zinc, chromium, platinum, palladium and gold.

The thiol functionality is provided by a thiol-functionalised silane. This silane suitably has one or more reactive halide or alkoxy groups attached to the silicon atom that react with the support and one or more non-reactive thiol-containing groups. In a preferred embodiment the silane is of formula (RO)_aSi((CH₂)xSH)_b in which a is 2 or 3, b is 4-a, x is 1-6 and R is CH₃, C₂H₅ or C₃H₇. Especially the silane is (3-mercaptopropyl) trimethoxy silane or (3-mercaptopropyl) triethoxy silane.

The support is in the form of a shaped unit, which may be formed from powders using techniques such as tabletting, extrusion and granulation. Starting with a shaped unit support is preferred because it is better able to provide the necessary bulk physical properties and lessens the risk of reducing the thiol functionality during subsequent processing. The support material preferably has hydroxyl functionality so that it may interact strongly with the thiol- containing compound, such as a silane. The support may be a silica, titania, alumina, zirconia, pillared or anionic clay, or zeolitic material, or a mixture thereof, which may further comprise a binder such as an aluminate cement and optionally a second binder such attapulgite clay. Hydrated oxides may also be used, for example alumina trihydrate or boehmite. The support is preferably a silica material, which may be natural or synthetic (precipitated silica) or a silica gel. We have found the properties of the support can influence the performance of the sorbent.

The sorbent desirably is in the form of a shaped unit with maximum and minimum dimensions (i.e. length or width) in the range 1 to 15 mm, with an aspect ratio (longest dimension divided by shortest dimension) < 4. In one embodiment, the shaped units are spherical with a diameter in the range 1-15 mm, preferably 1-10 mm, and more preferably 1-5 mm.

The physical properties of the sorbent, such as the surface area, porosity and density of the particles can have a significant effect on the absorption profile. Thus beds of sorbent of high bulk density and limited porosity, e.g. molded tablets, may exhibit a relatively broad absorption front, whereas bed of material with lower bulk density and higher porosity have been found to have much sharper absorption front. This enables a closer approach to be made to the theoretical absorption capacity. The sorbents of the present invention are particularly suited to large-scale industrial processes. The mean crush strength of the sorbent shaped units, which is typically measured using ASTM D-4179, or a variant thereof, is > 0.5 kg, preferably > 1.0 kg. This allows deep fixed beds to be used to maximize the heavy metal capture and time between sorbent replacements. The tapped bulk density of the shaped units is in the range 0.4-0.8 kg/litre. Shaped units with a tapped bulk density in this range have improved physical properties. The tapped bulk density measurement may be made simply by pouring the sorbent material into a 100ml measuring cylinder and tapping the cylinder until a constant volume of sorbent is observed. The volume and weight of sample may then be used to determine the tapped bulk density.

The attrition loss, i.e. .the weight loss from the sorbent shaped units after a set period of attrition, is preferably < 5% wt, more preferably < 2% wt. For measuring attrition, a drum- tumbling method whereby the sorbent shaped units are rotated at 60 rpm for 30 minutes is preferred. Low attrition rates reduces the amounts of dust or fine material that can be formed during loading which can block flow through the sorbent bed and so increase pressure drop.

The sorbent desirably has a BET surface area in the range 200-500 m²/g, more preferably 250- 450 m²/g. The sorbent also preferably has a pore diameter in the range 70 to150 Angstroms, more preferably 90 to 150 Angstroms, most preferably 100 to 150 Angstroms. The sorbent also desirably has a pore volume > 0.25 cm³/g, more preferably > 0.50cm³/g, most preferably > 0.75cm³. Sorbents with these properties may be obtained by use of suitable support materials having physical properties that provide these characteristics in the final sorbent. Such support materials may be made using known methods, but are also commercially available.

Thiol-functionalised sorbents can be unstable. For example, the SOL-AD-IV adsorbent had to be stored under argon to prevent oxidation of the active thiols to inactive disulphides. Such storage can be difficult on an industrial scale, even if an alternative inert gas was used.

Therefore the sorbent according to the present invention may further comprise a stabilizing amount of an alkaline metal, reacted with the thiol functionality. Without wishing to be bound by theory, we believe the alkaline metal forms a metal sulphide compound that prohibits the oxidation of the thiol to form the less-active disulphide. The alkali metal is subsequently released upon exposure to heavy metal due to the favourable difference in reactivity with these metals and the stability of the resulting alkaline metal salt.

The alkaline metal may be one or more alkali metals or alkaline earth metals, or a mixture thereof. Preferably the alkaline metal is selected from one or more of Li, Na, K, Cs Mg, Ca or Ba, especially Na and/or Ca. The level of thiol functionality is preferably in the range 0.1 to 10.0% wt (as S), more preferably 0.5 to 5.0% wt (as S), most preferably 0.5 to 3.0% wt (as S), which may be achieved, for example, by treating the supports with a suitable amount of (3-mercaptopropyl) trimethoxy silane. We have found surprisingly that highly effective sorbents may be produced with relatively low thiol-levels compared to previous materials by selection of supports with different physical characteristics.

Where used, the alkaline metal content is preferably present in the sorbent in an mount in the range 20-100% on a molar basis to the amount of thiol functionality (as S) present, depending upon the alkaline metal chosen. Desirably > 75%, more desirably > 90% of the thiol functionality is the sorbent is reacted with an alkaline metal.

However, alkaline metal stabilization need not be used if alternative methods for avoiding oxidation of the sorbent, e.g. by storage under inert gas, are used.

The method of the present invention requires reacting a thiol-functionalised silane with a support to form a thiol-functionalised support and drying the thiol-functionalised support. The reaction may be performed simply by combining the support and the thiol-functionalised silane in a suitable solvent under conditions where a reaction can take place and filtering or removing the solvent by evaporation. Temperatures <100°C and reaction times < 8 hrs have been shown to be effective. The thiol functionalised silane, depending on the leaving groups present, reacts with the support, releasing halide or alkoxy groups to form a stable M-O-Si link (where M = Si. Al, Ti, Zr etc) that anchors the thiol functionality to the surface of the support. Silica supports are preferred as detailed above.

Suitable solvents for industrial scale use are alcohols such as industrial methylated spirit, denatured ethanol, isopropanol and the like. Water is not preferred when using silanes as can lead to competing side reactions that reduce the thiol-functionalisation of the support. Toluene may be used, but is less preferred because of handling difficulties.

If desired, the support may be treated with a modifying material prior to reaction with the thiol- functionalised silane to increase the reactivity of the support with the thiol-functionalised silane or modify the support physical properties. In particular where the support has a low reactivity with the thiol-functionalised silane, the modifying compound may increase the thiol functionality in the resulting sorbent. For example we have found that alumina may be treated with an alkyl silicate, such as tetramethyl orthosilicate (TMOS), and dried to increase the support's reactivity with thiol-funtionalised silanes. If the sorbent is to be stabilized by alkaline metal, the thiol-functionalised support may be treated with an alkaline metal compound to form an alkaline metal-modified thiol-functionalised support. This reaction may be performed by combining the thiol-functionalised support and alkaline metal compound in a suitable solvent and filtering, or removing the solvent by evaporation to recover the product. The solvent in this case may be aqueous. Because of the high solubility of alkaline metal compounds in water and the ease of use, water is the preferred solvent. However organic solvents, such as methanol, may also be used. The alkaline metal compound is preferably a soluble salt or other soluble compounds such as the hydroxide. The alkaline metal compound is preferably NaOH or Ca(OH)₂.

The functionalised support is dried to form the sorbent material. Drying may be performed using conventional means at atmospheric pressure or under vacuum. The drying temperature is preferably <120°C to prevent decomposition of the silane.

The sorbent according to the present invention may be used widely to remove heavy metals such as mercury, arsenic, lead, cadmium, antimony, tin, copper, nickel, cobalt, zinc, chromium, platinum, palladium and gold from aqueous streams. The invention is of particular utility for mercury, and arsenic, especially mercury.

The mercury and/or arsenic and/or other metals may be removed by contacting the aqueous stream containing mercury and/or arsenic and/or other metals with the shaped sorbent either in batch mode, for example in a stirred tank, or preferably, in continuous mode. Batch mode generally requires filtration, which can be time consuming. In continuous mode the aqueous solution is passed through one or more fixed beds of the sorbent in a suitable vessel such as an adsorbent column. Such practices are known and may be used effectively in the present invention.

The aqueous streams from which the heavy metals may be removed include produced water streams produced from natural gas or oil recovery processes, waste water streams from industrial processes such as chlor-alkali processes, refineries, power stations, coal gasifiers, or other contaminated aqueous streams. Thus the present invention may be applied to the removal of heavy metals from contaminated aqueous streams generated by the exploration and/or production of fossil fuels, such as natural gas, crude oil or coal. Alternatively the contaminated aqueous stream may be a by-product of the processing of fossil fuels used to generate chemical products and/or energy. Examples include aqueous streams from the co- production of reservoir water in oil and gas exploration and/or production, oil refining, gas transmission, Natural Gas Liquid (NGL) production, gasification, combustion processes such as Integrated Gasification Combined Cycle (IGCC) processes, or scrubber liquors, e.g. those obtained from flue-gas desulphurisation units. In an alternative embodiment, the sorbent s may be used to remove heavy metals from water destined for use in industrial processes or for domestic/municipal use, e.g. as drinking water. In a further embodiment the sorbent may be used to remove heavy metals from aqueous streams produced by the decontamination of industrial process equipment or transportation equipment, such as tankers, floating, production storage and off-loading (FPSO) facilities, static vessels or pipelines.

Typically the concentration of mercury in the contaminated feed stream is from 0.1 to 20 parts per million by weight (ppm wt).

Preferably the sorption of the heavy metals is conducted in the liquid phase at a temperature below 100°C, as at such temperatures the overall capacity for mercury absorption is increased. Temperatures as low as 4°C may be used to good effect in the present invention. The pH of the aqueous stream is preferably in the range 2-10, more preferably 4-10, most preferably 5-9, especially 5-7. For pH's above 10 it maybe desirable to reduce the pH to 10 or below, e.g. by addition of a suitable acid, prior to treatment with the sorbent.

The invention is further described by reference to the following Examples.

Example 1 : Preparation of sorbents

Step 1. (3-Mercaptopropyl) trimethoxysilane (125. Og, 0.64mol) in ethanol (1 L) was added to 500g of a commercially available granular silica material (silica I) in a round bottom flask, and the contents stirred using an overhead mechanical stirrer. The mixture was heated to reflux at a temperature in the range 70-80°C for 1.5 hours to complete the reaction between the silane and the shaped silica support. The granules were then filtered using Buchner filtration apparatus and washed thoroughly with acetone before being dried in an extracted oven at 40°C overnight.

Step 2. The thiol-functionalised silica material was then transferred to a beaker containing an aqueous solution of NaOH (1 L) and this then stirred at ambient temperature for 5 mins to react the NaOH with the thiol functionality. The alkaline metal-modified material was then filtered using Buchner filtration apparatus and washed thoroughly with water before being dried in an oven at 60°C overnight. The resulting product was named sorbent A.

The experiment was repeated using a different commercially available granular silica (silica II) to generate sorbent B.

The shaped silica support and silane-functionalised sorbent materials were analysed using several techniques in order to characterise their bulk properties. The mean crush strength (MCS) of the materials was measured using an Engineering Systems CT-5 instrument which was fitted with a 5kg load cell. Twenty granules of each sorbent material were crushed and the average measurement recorded.

Tapped bulk density (TBD) measurements were performed by pouring the sorbent material into a 100ml measuring cylinder and tapping the cylinder until a constant volume of sorbent was achieved. The volume and weight of sample were then recorded.

Attrition testing was performed on the sorbents using a drum tumbling method according to ASTM D4058-96. 100g of sorbent was weighed into the drum and rotated at a rate of 60 rpm for 30 minutes. The contents of the drum were then passed over a 1 mm sieve and the weight of on-size particles was recorded. Drum tumbling loss (DrTL) was calculated as a percentage of the original mass.

The sulphur content of the material (wt % S) was determined by combustion of the sample at 1300°C and subsequent infra-red analysis to quantify the amount of S0₂ evolved.

Silica support Properties

Surprisingly the MCS and TBD for sorbent B is higher than the parent silica. The DrTL for sorbent A is surprisingly superior to the parent silica.

Example 2: Testing

Samples A and B were tested for mercury removal performance. These tests were performed to closely replicate the fixed bed process condition. In the test, a sorption vessel was charged with functionalised sorbent material (25ml). An aqueous solution of mercuric chloride from a feed vessel (4L capacity) was then pumped through the reactor containing the sorbent material by a circulation pump at a flow rate of 1 ml/min. The solution from the feed vessel was analysed by an atomic fluorescence analysis system to measure the concentration of Hg²⁺. This provided the mercury concentration at the inlet of the absorption vessel. This was maintained as closely as possible to 10ppm by a concentrated mercuric chloride solution from a separate vessel via a dosing pump.

The solution exiting the sorbent vessel was collected in a collection vessel with an overflow line that returned the overflow to the feed line of the adsorption vessel. In this way the process was run continuously.

The solution in the collection vessel was sampled to measure the concentration of mercury exit the sorption vessel by atomic fluorescence detection. The concentration of mercury detected in the exit solution that was required to stop the test, also known as the breakthrough level, was set at 20% of the inlet concentration, so in this case when the exit concentration is greater that 2ppm the test was stopped.

The breakthrough times for both sorbents A and B was in excess of 30 days showing excellent mercury capture behaviour for these materials.

The flow of liquid through the bed of shaped sorbent was fast with no crushing or physical degradation of the granules observed after the testing was completed.

Claims

Claims.

1. A sorbent comprising a thiol functionalised silane supported on a shaped support, said sorbent having a mean crush strength > 0.5 kg and a tapped bulk density in the range 0.4 to 0.8 kg/litre.

2. A sorbent according to claim 1 wherein the silane is of formula (RO)_aSi((CH₂)xSH)_b in which a is 2 or 3, b is 4-a, x is 1-6 and R is CH₃, C₂H₅ or C₃H₇.

3. A sorbent according to claim 1 or claim 2 wherein the silane is (3-mercaptopropyl)

trimethoxy silane or (3-mercaptopropyl) triethoxy silane.

4. A sorbent according to any one of claims 1 to 3 wherein the support comprises a silica, titania, alumina, zirconia, or zeolitic material, or a mixture thereof and optionally one or more binders.

5. A sorbent according to any one of claims 1 to 4 wherein the support comprises silica.

6. A sorbent according to any one of claims 1 to 5 further comprising a stabilising amount of an alkaline metal selected from one or more of Li, Na, K, Cs Mg, Ca or Ba reacted with the thiol functionality.

7. A sorbent according to claim 6 wherein the alkaline metal is Na and/or Ca.

8. A sorbent according to any one of claims 1 to 8 wherein the level of thiol functionality is in the range 0.1-10.0% wt (as S).

9. A sorbent according to any one of claims 6 to 8 wherein the alkaline metal is present in the sorbent in an mount in the range 20-100% on a molar basis to the amount of thiol functionality (as S) present.

10. A sorbent according to any one of claims 1 to 9 wherein the shaped unit has maximum and minimum dimensions in the range 1 to 15 mm, and an aspect ratio < 4.

1 1. A sorbent according to any one of claims 1 to 10 wherein he attrition loss of the sorbent, as measured by a drum tumbling method, is < 5% wt, preferably < 2% wt.

12. A method for making a sorbent according to any one of claims 1 to 1 1 comprising the steps of:

(i) reacting a thiol-functionalised silane with a shaped support to form a thiol- functionalised support, and

(ii) drying the thiol-functionalised support.

13. A method according to claim 12 wherein the silane is of formula (RO)_aSi((CH₂)xSH)_b in which a is 2 or 3, b is 4-a, x is 1-6 and R is CH₃, C₂H₅ or C₃H₇.

14. A method according to claim 12 or claim 13 wherein the silane is (3-mercaptopropyl) triethoxy silane.

15. A method according to any one of claims 12 to 14 wherein the support comprises a silica, titania, alumina, zirconia, clay, or zeolitic material, or a mixture thereof.

16. A method according to any one of claims 12 to 15 wherein the support comprises silica.

17. A method according to any one of claims 12 to 16 further comprising the step of treating the support with a modifying material prior to reaction with the thiol-functionalised silane, to increase the reactivity of the support with the thiol-functionalised silane or modify the support physical properties.

18. A method according to any one of claims 12 to 17 further comprising reacting the thiol- functionalised support an alkaline metal compound selected from one or more compounds of Li, Na, K, Cs Mg, Ca or Ba.

19. A method according to claim 18 wherein the alkaline metal compound is NaOH and/or Ca(OH)₂.

20. A method according to any one of claims 12 to 19 wherein the thiol-functionalised silane is provided in an amount sufficient to provide a level of thiol functionality in the sorbent in the range 0.1-10.0% wt (as S).

21. A method according to any one of claims 12 to 20 wherein the alkaline metal compound is provided in an amount sufficient to provide alkaline metal in an amount in the range 20-100% on a molar basis to the amount of thiol functionality (as S) present.

22. A process for the removal of heavy metals from an aqueous stream contaminated with a heavy metal, comprising contacting a sorbent according to any one of claims 1 to 1 1 or prepared according to the method of any one of claims 12 to 22 with the contaminated aqueous stream.

23. A process according to claim 22 wherein the heavy metal is one or more of mercury, arsenic, lead, cadmium, antimony, tin, copper, nickel, cobalt, zinc, chromium, platinum, palladium and gold, preferably mercury.

24. A process according to claim 22 or claim 23 wherein the stream is a contaminated aqueous stream generated by the exploration and/or production of fossil fuels.

25. A process according to claim 22 or claim 23 wherein the stream is a by-product of the processing of fossil fuels used to generate chemical products and/or energy.

26. A process according to claim 22 or claim 23 wherein the stream is water for use in industrial processes.

27. A process according to claim 22 or claim 23 wherein the stream is water for domestic and/or municipal use.

28. A process according to claim 22 or claim 23 wherein the stream is produced by the decontamination of industrial process equipment or transportation equipment.

29. A process according to any one of claims 22 to 28 wherein the pH of the aqueous stream is in the range 2-10, preferably 4-10.

30. A process according to claim 29 wherein the pH range is obtained by a step of adjusting the pH to 10 or below prior to treatment with the sorbent.