Process For Removing Metals From Hydrocarbons

PATENT
Process For Removing Metals From Hydrocarbons
Pub. No.:    WO/2012/046057   
International Application No.:    PCT/GB2011/051906
Publication Date:  12.04.2012
Applicants: 
The Queen's University Of Belfast [GB/GB]; University Road Belfast Antrim BT7 1NN (GB)
Inventors: 
ABAI, Mahpuzah; (GB).
ATKINS, Martin Philip; (GB).
CHEUN, Kuah Yong; (GB).
HOLBREY, John; (GB).
NOCKEMANN, Peter; (GB).
SEDDON, Ken; (GB).
SRINIVASAN, Geetha; (GB).
ZOU, Yiran; (GB) 
Abstract:  
The present invention relates to a process for the removal of mercury from a mercury- containing hydrocarbon fluid feed using specifically selected ionic liquids comprising, contacting the mercury-containing hydrocarbon fluid feed with an ionic liquid having the formula[Cat+][M+][X-] and separating from the ionic liquid a hydrocarbon fluid product having a reduced mercury content compared to the mercury-containing fluid feed.
This invention relates to a process for removing metals, and particularly mercury, from hydrocarbon fluids. More specifically, the invention relates to a process wherein metals are extracted from gaseous or liquid hydrocarbons using an ionic liquid.

Liquid and gaseous hydrocarbons obtained from oil and gas fields are often contaminated with mercury. In particular, liquid and gaseous hydrocarbons obtained from oil and gas fields in and around the Netherlands, Germany, Canada, USA, Malaysia, Brunei and the UK are known to contain mercury. As reported by N. S. Bloom (Fresenius J. Anal. Chem., 2000, 366, 438-443), the mercury content of such hydrocarbons may take a variety of forms. Although elemental mercury tends to predominate, particulate mercury (i.e. mercury bound to particulate matter), organic mercury (e.g. dimethylmercury and diethylmercury) and ionic mercury (e.g. mercury dichloride) may also be found in naturally occurring hydrocarbon sources. The mercury concentration in crude oils can range from below 1 part per billion (ppb) to several thousand ppb depending on the well and location. Similarly, mercury concentrations in natural gas can range from below 1 ng-m"3 to greater than 1000 μg■m"3.

The presence of mercury in hydrocarbons is problematic due to its toxicity. In addition, mercury is corrosive towards hydrocarbon processing equipment, such as that used in oil and gas refineries. Mercury can react with aluminium components of hydrocarbon processing equipment to form an amalgam, which can lead to equipment failure. For example, pipeline welds, cryogenic components, aluminium heat exchangers and hydrogenation catalysts can all be damaged by hydrocarbons contaminated with mercury. This can lead to plant shutdown, with severe economic implications, or, in extreme cases, to uncontrolled loss of containment or complete plant failure, with potentially catastrophic results. Furthermore, products with high levels of mercury contamination are considered to be of poorer quality, with the result that they command a lower price.

A number of approaches to the removal of mercury from hydrocarbons have been proposed. These include: scrubbing techniques using fixed bed columns containing sulfur, transition metal or heavy metal sulfides and iodides on an activated support; reduction of inorganic and organomercury to elemental forms followed by scrubbing or amalgamation; oxidation followed by complexation with sulfur-containing compounds; and oxidation followed by solvent extraction. However, there remains a need in the art for more effective methods for the removal of mercury from hydrocarbons, such as crude oil distillates and natural gas, particularly to remove 'total' mercury from hydrocarbon sources containing two or more forms of mercury.

The term "ionic liquid" as used herein refers to a liquid that is capable of being produced by melting a salt, and when so produced consists solely of ions. An ionic liquid may be formed from a homogeneous substance comprising one species of cation and one species of anion, or it can be composed of more than one species of cation and/or more than one species of anion. Thus, an ionic liquid may be composed of more than one species of cation and one species of anion. An ionic liquid may further be composed of one species of cation, and one or more species of anion. Still further, an ionic liquid may be composed of more than one species of cation and more than one species of anion.

The term "ionic liquid" includes compounds having both high melting points and compounds having low melting points, e.g. at or below room temperature. Thus, many ionic liquids have melting points below 200 °C, particularly below 100 °C, around room temperature (15 to 30 °C), or even below 0 °C. Ionic liquids having melting points below around 30 °C are commonly referred to as "room temperature ionic liquids" and are often derived from organic salts having nitrogen-containing heterocyclic cations, such as imidazolium and pyridinium-based cations. In room temperature ionic liquids, the structures of the cation and anion prevent the formation of an ordered crystalline structure and therefore the salt is liquid at room temperature.

Ionic liquids are most widely known as solvents. Many ionic liquids have been shown to have negligible vapour pressure, temperature stability, low flammability and recyclability. Due to the vast number of anion/cation combinations that are available it is possible to fine-tune the physical properties of the ionic liquid (e.g. melting point, density, viscosity, and miscibility with water or organic solvents) to suit the requirements of a particular application.

There are a limited number of reports available which examine the partitioning of metals into ionic liquids from inorganic systems.

For example, the partitioning of mercury ions, in the high 2+ oxidation state, into ionic liquids from water has been reported by Rogers, et al. (Green Chem., 2003, 5, 129-135), who showed that dicationic anionic liquid complexants can be used to partition Hg(ll) from aqueous salt and acid solutions. Prausnitz, et al. (Ind. Eng. Chem. Res., 2008, 47, 5080-5086) have shown that mercuric ions partition preferentially from water into hydrophobic ionic liquids.

Pinto et al. (US Patent Application 2007/0123660) have shown that the combination of metal-complexing ligands and ionic liquids coated onto an inert support may be used as an adsorbent to remove mercury from coal combustion flue gases (see also Ji et al., Water, Air, & Soil Pollution: Focus 2008, 8, 349-358, Ind. Eng. Chem. Res., 2008, 47, 8396-8400, and Main Group Chemistry 2008, 7, 181 -189)

In WO 98/06106, a process is disclosed for the oxidative dissolution of nuclear fuels and nuclear fuel-cladding materials in nitrate-based ionic liquids. The ionic liquids disclosed comprise nitrate anions, and optionally sulphate or tetrafluoroborate anions, and a Bronsted or Franklin acid (such as nitric acid, sulphuric acid, or nitronium cations) which is necessary to increase the oxidising power of the solvent. The ionic liquids disclosed are all water soluble.

Pitner, W. R. et al. have described a similar process for dissolving insoluble uranium oxide (UO2) in which the uranium oxide is oxidised to the soluble species UO22+ in a mixture of nitric acid and an ionic liquid comprising nitrate anions (in Green Industrial Applications of Ionic Liquids, NATO Science Series II: Mathematics, Physics and Chemistry, 2003, 92, 209-226, Kluwer Academic Publishers, Dordrecht.). The dissolution of elemental metals is not described, however.

It will be appreciated that the oxidation processes disclosed in WO 98/06106 and in Pitner et al. require the use of highly corrosive acids containing nitronium ions. Such processes are self-evidently incompatible with hydrocarbon processing since hydrocarbon components such as olefins and aromatic compounds are susceptible to reaction with acids and nitronium ions, forming unwanted by-products. The use of acids also leads to an undesirable increase in the overall acidity of the compositions, especially with respect to hydrocarbon fluids. Furthermore, the high corrosivity of the systems disclosed in WO 98/06106 and in Pitner et al. makes them incompatible with metal hydrocarbon processing, storage and transportation apparatus.

The present invention is based on the surprising finding that certain metal-containing ionic liquids are capable of extracting elemental, ionic and organic forms of mercury from hydrocarbon fluids with high levels of efficiency. It has been found that the ionic liquids have very high capacities for dissolved mercury, for instance the capacity for dissolved mercury in the ionic liquid may be as high as 20 wt%, based on the total weight of ionic liquid and mercury.
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