Purification Unit

This invention relates to purification units suitable for the purification of a compressed fluid containing a contaminant, in particular small lightweight units that may be suitable for purification of compressed fluids used as fuels in vehicles.

Fuels such as liquid or gaseous hydrocarbons often contain residual contaminants that can cause damage to exhaust gas catalysts or pollute the atmosphere. Compressed gaseous hydrocarbons such as natural gas or LPG are of growing interest as alternatives to traditional petrol- or diesel-fuelled vehicles. These gases can contain sulphur compounds as contaminants that are desirably removed.

WO2008/007899 discloses a discoloration indicator for checking the life span of a desulfurization adsorbent, able to effectively adsorb and remove organic sulfur compounds from fossil fuels, including natural gas or LPG containing the organic sulfur compound, and to a desulfurization reactor and a desulfurization system including the same. The sorbents used were Mn- and or Ag-based. The designs do not enable ready assessment of the absorption profile through the vessel and are heavy, making them unsuitable for uses such in vehicles, e.g. vehicles powered by compressed natural gas. Moreover the Mn and Ag sorbents are not particularly effective across a range of contaminant compounds.

Accordingly, the invention provides a purification unit, suitable for the purification of a compressed fluid containing a contaminant, comprising a purification vessel having an inlet and an outlet and a particulate purification material capable of undergoing a colour change in reaction with said contaminant, disposed within said vessel such that a fluid entering the vessel through said inlet may pass through said particulate purification material and exit the vessel through said outlet, wherein the vessel has one or more window portions aligned in the direction of fluid flow through the vessel enabling a user to observe a colour change in the particulate purification material, wherein the vessel is fabricated from a fibre-reinforced polymer composite having one or more transparent window portions.

The invention further provides a process for purifying a compressed fluid containing one or more sulphur contaminant compounds comprising passing said compressed fluid though the purification unit.

The vessel is fabricated from a fibre-reinforced polymer composite. The composites are transparent or translucent, or comprise a transparent or translucent portion such that a user is able to observe the contents. Such vessels and methods for fabricating them are described, for example, in EP 0300931. The vessel desirably comprises an outer sheath or protective

casing which may have one or more holes therein suitably positioned to allow the user to observe the particulate material disposed within the vessel. In a preferred embodiment the vessel comprises an inner, fluid-tight liner layer and a pressure supporting layer outside the liner, as well as an outer, protective casing. The said layers consist of transparent or translucent materials, and the casing comprises a middle section having surface portions cutaway so that parts of the actual container, being located inside the casing are visible from the outside. The casing has shock-absorbing properties. Such sheathed fibre-reinforced polymer composite vessels are described in EP0958473, the contents of which are hereby incorporated by reference.

The vessel is preferably an elongate vessel, for example cylindrical in shape, and may have domed ends. The inlet and outlet may be positioned in one end or at opposite ends. A bed of particulate purification material is disposed within the vessel and restrained if necessary by suitable gauzes or meshes. The arrangement of inlet and outlet and particulate material is such that the fluid enters the vessel through the inlet, passes through the particulate purification material and then leaves the vessel via the outlet. Preferably the fluid passes through the purification material in a direction aligned with the longitudinal axis of the vessel. Where both inlet and outlet are positioned in one end, a suitable dip tube or other means should be provided within the vessel to transfer the fluid to the distal end of the bed of particulate material such that the fluid may flow through the bed in an axis aligned with that of the vessel. In a preferred embodiment, the inlet and outlet are disposed in the same end of the vessel. Such arrangements simplify fabrication and improve the strength of the unit, which is desirable where compressed fluids are being treated.

In contrast to the present invention, the use of the vessel disclosed in EP0958473 is for the storage of pressurised fluids such as propane and butane. In the present invention the vessel further contains a particulate absorbent material capable of undergoing a colour change in reaction with a contaminant present in the compressed fluid. One or more purification materials may be evenly dispersed within the vessel to monitor one or more different contaminants. If desired, a non-colour-changing purification material may be included within the vessel in addition to a colour changing absorbent. Any combination of contaminant and purification material may be used as long as there is a reaction between at least one of them that results in a colour change that may be observed by the user. As the fluid passes through the particulate purification material within the vessel a colour change occurs and is observed as a 'front' passing through the vessel in alignment with the fluid flow. Accordingly one or more window portions aligned with the fluid flow allows the user to observe the passage of this front as it progresses with time. In this way, the user may determine when the purification material requires replacement.

In one embodiment, especially where the contaminant is one or more sulphur compounds, the purification material is desirably a copper (ll)-containing material. Suitable copper (II) compounds that may form all or part of the purification material are include copper oxide, copper hydroxide and copper hydroxy-carbonate, also called basic copper carbonate. These copper (II) compounds are green and undergo a colour change from green to black as they react with sulphur compounds. In a preferred embodiment, the absorbent material is a composite of one or more copper (II) compounds, one or more zinc (II) compounds and alumina. Preferably the absorbent comprises agglomerates of an intimate mixture of oxides, hydroxides, carbonates and/or basic carbonates of copper, zinc and alumina. Such agglomerates are characterised by a high surface area (as measured by the BET method) and a low density. The BET surface area is preferably at least 80 m ² g ^"1 , more preferably at least 100 m ² g ^"1 . Typically the agglomerates have an average size within the range 1 to 10 mm, preferably in the range 2-5 mm. Such materials also generally have a calcined density of not more than 1.5 gem ³ . The purification material preferably comprises oxides, hydroxides, carbonates and/or basic carbonates, in such proportions that the copper atoms form 30-97, preferably 50-95%, of the total number of copper, zinc, and aluminium atoms in said agglomerates; and said agglomerates having a total copper and zinc compound content such that, after ignition at 900 DEG C, the cupric oxide plus zinc oxide content of the ignited composition is at least 70, preferably at least 80%, by weight. Furthermore, the proportions of zinc and alumina are preferably such that the zinc atoms constitute 0 to 60, particularly at least 5, and more particularly 10 to 40, % and said aluminium atoms constitute 0 to 30, particularly 5 to 20, % of the total copper, zinc and aluminium atoms in the agglomerates. Such purification materials are described for example in EP0243052, the contents of which are hereby incorporated by reference.

As indicated above, such copper(ll)-containing materials are especially suitable for removing sulphur compounds. The contaminant may be one or more sulphur compounds selected from the list consisting of hydrogen sulphide, carbonyl sulphide, mercaptans and thiophene.

An advantage of using copper(ll)-containing materials for purifying sulphur-contaminated fluids is as the copper becomes sulphided, it becomes active for capturing mercury, arsenic and other metal contaminants that may also be present in the fluid.

The unit desirably has a volume in the range 1 litre to 1000 litres, preferably 5 to 30 litres and contains a particulate purification material with a particle size in the range 1 to 10 mm, preferably 2 to 5 mm.

The units may be used as stationary units for the garage forecourt purification of compressed fluid fuels or for mobile vehicle applications. Accordingly the invention further provides a powered vehicle, fuelled by a compressed fluid having a purification unit as described herein.

The vehicle may be an internal combustion engine powered vehicle suitably adapted to operate with compressed hydrocarbon fluid as fuel; in particular a vehicle powered by compressed natural gas.

The unit according to the present invention may be used to purifying any compressed fluid by passing said compressed fluid though the particulate purification material disposed within the vessel. The unit may be effective at temperatures below 200 ⁰ C, and can be at ambient temperatures, or even lower, e.g. as sulphur compounds, acid gases such as hydrogen cyanide, hydrogen halides, e.g. chloride, nitric oxide, nitrogen dioxide, chlorine, sulphur dioxide, and sulphur trioxide can also be absorbed. Preferred operating temperatures are in the range -10 to100°C. The pressure of the compressed fluid may be in the range 1.5 to 60 bar abs, preferably in the range 10 to 50 bar abs, more preferably 15 - 45 bar abs,

The unit of the invention may be used for the removal of sulphur compounds as aforesaid from any fluid stream to which the purification material is inert. Examples of suitable fluid streams include natural gas, substitute natural gas, reforming gases, liquid hydrocarbons, air, nitrogen, argon, helium, chlorinated hydrocarbons, , carbon dioxide, and organic compounds such as alcohols, esters, and oxygenates such as biofuels and polyethers. Hydrocarbon fluids are preferred, in particular compressed hydrocarbons such as compressed natural gas.

The fluid is preferably free of reducing gases such as carbon monoxide or hydrogen, although fluid streams containing such gases may be treated if the absorption is effected at a temperature low enough to avoid substantial reduction of the copper compound to metal, for example at a temperature below 17O ⁰ C, preferably below 15O ⁰ C.

The invention is further illustrated by reference to Figure 1 , which depicts a cross section of a purification unit according to one embodiment of the present invention.

In Figure 1 there is a purification unit 10 having a volume of about 20 litres, comprising a transparent, fibre-reinforced polymer vessel 12 of cylindrical configuration with domed ends, a base 14 attached at its lower end to support the cylinder in an upright position, and a cap 16 over an opening at the opposite, upper end. A sheath member 18 is attached around the body of the cylinder, through which are cut one or more holes 20 that are shaped in alignment with the longitudinal axis of the vessel 12. An inlet tube 22 is provided in the cap that passes through the opening and allows a pressurised fluid to be fed to the upper surface of a bed of particulate purification material 26 disposed within the vessel 12. An outlet tube 24 is also provided in the cap. The outlet tube passes through the opening and extends through the bed of particulate purification material 26 to near the base of the vessel. Process fluid is thus able

to pass longitudinally through the bed from the upper surface to near the base of the vessel where it is collected by the outlet tube 24. The closed arrows depict fluid flow in the unit.

In an example, a compressed natural gas containing one or more sulphur compounds as contaminants at about 30 bar and about 2O ⁰ C, is passed through a bed of 2-5 mm spherical agglomerates of a copper-zinc-alumina material as detailed in EP0243052. The green copper(ll) compound is converted to black CuS by reaction with the sulphur contaminants. The change in colour may be observed through the windows 20 cut in the sheath 18, thereby allowing the user to determine the remaining lifetime of the purification material.