GB2398518A - A filter element - Google Patents

Abstract

A method of producing a filter element for the absorption of hydrophobic water-immiscible liquids, comprises providing a porous filter element component formed of a lignocellulosic material, applying a pressure differential across the component to effect acetylation and causing a vapour phase acetylating agent at an elevated temperature to pass through the component. Preferably the acetylating agent is acetic anhydride which may be mixed with acetic acid. The acetylation process may be effected at a temperature of 130{ to 150{C and a weight gain of up to 30% can be achieved. Preferably the lignocellulosic material is selected from wood or other plant fibres such as straw or sisal. Advantageously the filter is used to remove pollutants such as hydrocarbons from water.

Description

1 2398518

FILTER ELEMENT

The present invention relates to a filter element for use in the removal of organic and other pollutants from liquids and also relates to filter units and filter methods utilizing this filter element. The invention relates more particularly, but not exclusively, to filter elements that may be used for absorbing hydrophobic water-immiscible liquids, particularly from a mixture of such liquids and water.

There are many situations where it is desirable to remove components including organic pollutants (such as hydrocarbons) from produced water and water run-off before this water is released as effluent. One way of separating the pollutant is by passing the polluted water through a f Itering system capable of reducing the level of contamination of the polluted water to a permitted level for discharge into the environment It is known that acetylated lignocellulosic materials have oil absorbing properties.

Thus, WO-A-9206146 discloses a material for use in absorbing hydrophobic water- immiscible liquids. In the preferred embodiment of WO-A-9206146 the material is produced by impregnating thermomechanical fibre pulp (TMP) with liquid acetic anhydride, removing excess liquid, and then heating the impregnated pulp, e.g. at a temperature of 120 C. The acetylated material may then be incorporated within an outer 'covering' such as a net or porous sheet to form a boom or pillow for clean-up of oil-in-water spillages. The Examples of the WO specification test the fibrous material by dispersing it is a mixture of oil and water and evaluating the oil pick-up.

Reference is additionally made to the use of the acetylated material in filters designed to separate and recover hydrophobic water-immiscible liquids from mixtures of such liquids with water. Although the 'loose' acetylated fibrous material is capable of being formed into filter components, it is brittle and thus makes the process of forming filters potentially hazardous due to the dust produced when the material is being processed. Dust removal systems could be used, but these would add to the capital expenditure for the apparatus required. WO-A-9206146 also discloses the possibility of making a sheet of lignocellulosic material more attractive to hydrophobic water-immiscible liquids by the acetylation techniques disclosed on WO-A-9206146. We have however found that it is very difficult to ensure (in a batch process as contemplated in WO-A-9206146) that the anhydride penetrates evenly into the whole of the sheet and that there is a uniform temperature across the sheet.

Methods of acetylating lignocellulosic materials are well known. In the case of wood, the main interest in using such processes has been to stabilise the dimensions of wood articles and/or increasing the resistance to biological attack. Such treatments tended to involve long treatment times. US-A-4804384, discloses a method for catalyst-free acetylation of lignocellulosic material, comprising the steps of: contacting lignocellulosic material in the form of veneer, chips, flakes, fibres or particles with a liquid reactant consisting essentially of acetic anhydride and from about 10 percent to about 55 percent by volume acetic acid; heating the reactant- contacted lignocellulosic material at a temperature of up to 120.degree. C. for a time sufficient to acetylate said material, the time of heating being from 1 to 5 hours; and, removing from the resulting acetylated lignocellulosic material unreacted acetic anhydride and acetic acid. This procedure does however have the disadvantage that it gives rise to 'dead spots' and 'channellings'.

Klinga and Tarkow (Tappi, the Journal, the Technical Association of the Pulp and Paper Industry, Vol. 49, No. 1, January 1966) carried out the stabilization of hardboard by "uncatalyzed vapor-phase acetylation,". The board contained aluminium sulfate which could act as a catalyst. The treatment time was, however, very long and "overnight heating was adopted." This would appear to be due to the fact that the method relies on diffusion.

It is an object of the present invention to obviate or mitigate the aforementioned disadvantages According to a first aspect of the present invention there is provided a method of producing a filter element for the absorption of hydrophobic water-immiscible liquids, the method comprising providing a porous filter element component formed of a lignocellulosic material, effecting acetylation of said lignocellulosic material by applying a pressure differential across the component and causing a vapour phase acetylating agent at elevated temperature to pass through the component, and if necessary forming the component into the filter element.

Surprisingly, we have found that it is feasible to use vapour phase acetylation to confer pollutant absorbing properties on the filter element component with relatively short treatment times. This is due to the fact that the acetylating agent is caused to pass through the material of the filter element component under the influence of the pressure differential. Thus the present invention uses the physical characteristics required of a filter (porosity, uniformity, lack of alternative flow channels) to facilitate acetylation. The method is particularly convenient because it avoids problems associated with handling 'loose', relatively brittle, acetylated lignocellulosic fibres and using such fibres to produce a filter element According to a second aspect of the present invention there is provided a filter unit comprising a housing and a filter element produced in accordance with the first aspect of the invention provided within the housing.

The filter unit may for example be a filter column, filter cartridge or filter pod.

Filter elements produced in accordance with the first aspect of the invention are particularly useful for separating organic pollutants (particularly hydrophobic water- immiscible liquids, e.g. oil) from mixtures of such pollutants and water. For the purpose of effecting the separation, the mixture may be passed through the filter element (which may of course be provided in a filter unit in accordance with the second aspect of the invention) so that the filter element absorbs, and becomes loaded with, the pollutant. If the pollutant is, for example, a hydrocarbon, the used filter element can be burnt and used as an energy source.

Filter elements produced in accordance with the invention, may, for example, be tubular or any other convenient physical form, and can be produced by acetylation of blanks having the same physical form (e.g. dimensions, pore size etc). We do not however preclude the possibility of the blank being, say, tubular but of greater length than required for the final filter element, the acetylated product then subsequently being cut to the required length. A further possibility is for filter element components to be acetylated individually and then assembled into the final form of the filter element. Thus, for example, in the case of a tubular filter element it is possible to acetylate annular components (of lesser length than the desired tubular filter element) and then assemble the annular components into the final filter element.

The acetylation may conveniently be effected by providing the lignocellulosic material in an enclosed reaction vessel in which the gaseous acetylating agent is introduced or provided and which may be subjected to the required temperature conditions. Additionally the apparatus is provided with means for applying a pressure differential across the filter element component so as to cause the vapour phase acetylating agent to flow therethrough during the acetylation process. If the filter element component is tubular then the vapour phase acetylating agent may be passed along the central tubular space of the blank and 'sucked through' the walls of the filter element component. Alternatively the vapour phase acetylating agent may be caused to pass radially inwardly through the filter element component and be 'sucked-out' along the central space of the filter element component. In all embodiments, the gaseous acetylating agent may be supplied to the component by heating a source of the agent. Excess acetylating agent that has passed through the blank may be recycled to said source.

The acetylating may be acetic anhydride. Improved results may however be obtained using a mixture of acetic anhydride and acetic acid. That said, given that the acetylating agent is provided from a source thereof to which excess agent is recycled, an acetylation process initially using acetic anhydride as the sole acetylating agent will generate acetic acid which is therefore passed to the source (with the excess acetic anhydride) so that ultimately the acetylating agent supplied to the filter element component comprises both acetic anhydride and acetic acid. Most conveniently, the acetylation of the filter element component is effected by passing acetic anhydride vapour through the component at a temperature of 130 to 150 C, e.g. about 140 C.

This treatment may be effected without the need for a catalyst, although a conventional acetylation catalyst may be employed if desired. This acetylation method results in acetylation of the lignin in the lignocellulosic material and it is the acetylation of lignin which enhances the capability of the filter element to absorb organic pollutants, such as hydrophobic water-immiscible liquids (e.g. oil).

The lignocellulosic material (which will generally not be pre-impregnated with a liquid acetylating agent) may if desired be pre-heated prior to the acetylation procedure by passing hot air and/or steam thorough the material. On completion of acetylation, any excess acetylation agent and any by-product acetic acid may be removed by drawing warm air through the element.

The time required for acetylation will depend on a number of factors, e.g. the size of the filter element, the internal surface area and the degree of acetylation required. The necessary treatment times may readily be determined by persons skilled in the art.

Generally the acetylation is effected to achieve a weight gain of up to 30% by weight, e.g. 15 to 25% by weight. A typical weight gain is around 20% by weight.

A wide variety of lignocellulosic materials may be used for producing the filter element component. Examples of suitable lignocellulosic materials include wood, bark, straw, flax, linseed, bagasse, sisal, jute, kenaf, miscanthus, coir, and hemp.

It is particularly preferred that the lignocellulosic material comprises wood fibre.

Conveniently the filter element blank is produced using (at least partly) a wet-forming procedure from a slurry or suspension of the lignocellulosic fibres. Thus, for example, the filter element component may be in the form of a sheet prepared by dewatering a slurry or suspension of the lignocellulosic fibres on a mesh or screen, in a manner akin to the production of paper. The sheet may be acetylated in its 'flat' form. Alternatively, it may be subject to a further forming operation to produce the filter element component, for example by rolling the sheet into a tube. Alternatively a tubular blank may be produced by introducing a slurry of the lignocellulosic fibres into an annular mould having a perforated base through which water is drained to give a tubular form which may be subjected to axial compression to produce the filter element component.

Fibres for use in forming such slurrys may conveniently be prepared by the same techniques as employed in the production of fibres for the manufacture of Medium Density Fibre Board (MDF). More particularly, fibres used for the manufacture of MDF are refined at high temperature. This softens the lignin between the cells which then breaks and gives a fibre with a high lignin content at its surface. The process of the invention results in the acetylation of lignin so fibres as used for manufacturing MDF are particularly suitable.

The fibres will typically have a length of less than lmm to over lOmm.

Alternatively the filter element may be produced from slivers of insulation board that is formed from wood fibres by a wet process and contains no binder. Such board has a low density (usually in the range 200 to 350 kg m3) and with its relatively open structure offers little resistance to the passage of the polluted water when formed into filter components.

The invention will be further described by way of example only with reference to the single figure of the accompanying drawings, in which: Fig 1 schematically illustrates an embodiment of apparatus that may be used for acetylating a tubular filter element component.

Referring to Fig 1, the illustrated apparatus comprises a flask I (for containing acetic anhydride 2) having an upper condenser 3 and a side arm 4 extending into a closed reaction vessel 5. Flask 1 is associated with a heating mantle 6.

The end of the side arm 4 remote from the flask 1 is 'blanked-off' and a portion of the wall of the side arm within the reactor 5 is provided with perforations 7.

A line 8 returns from the reaction vessel 5 to the supply vessel via a pump 9.

The illustrated apparatus is intended for producing a filter element by acetylating a tubular filter blank 10 of lignocellulosic material. For this purpose, the blank is located on the side arm 4 so as to be located within the reaction vessel 5. Acetic anhydride in the supply vessel is heated by means of the heating mantle 6.

Additionally a pump 9 is operated to provide for circulation of acetic anhydride vapour in the direction of the arrows shown in the drawings.

The invention will be further described by the following non-limiting Examples.

Example 1

A sheet of insulation board (1 2mm thick) made of wood fibre, density 224 kg/m3, was cut into annular pieces, outside diameter 75mm, inside diameter 32mm. These were assembled into the form of a filter on a perforated stainless steel tube former, and acetylated by drawing acetic anhydride vapour therethrough for three hours. The temperature of the acetic anhydride vapour was about 140 C and the temperature in the filter typically rose to about 160 C. The pieces were then cleaned by drawing hot air (80 C) through them for 4 hours. This procedure was repeated for several batches of annular pieces of insulation board.

From several batches, 18 annul) with an apparent weight gain of between 12% and 17% were selected and assembled into a filter on a perforated aluminium former with screwed on ends. The size and shape of the resulting filter matched those of a filter commercially available under the CLERIFY, and it was tested in the same housing.

The filter prepared in accordance with the invention was tested by passing therethrough 10 1itres per minute of water contaminated with about 40ppm of transformer oil. Samples of water were taken for analysis from before and after the filter, and the water pressure before and after the filter was also measured. The concentration of oil passing through the filter was less than lppm for some time, and then rose sharply. 70g of oil had been introduced to the filter at the time that the concentration of oil passing through reached 10% of that flowing in. The water pressure across the filter was 0.16 bar at this point.

In comparison, the commercial filter stopped only 17g of oil before the outflow concentration reached 10% of the inflow. The water pressure across the filter was also 0.16 bar.

The experimental filter therefore performed about 4 times as well as the commercial filter.

Example 2.

260g of wood fibre, as used to make MDF, was made into a slurry with 15 litres of water and tog of sodium hydroxide. The slurry was dewatered on a screen in a wet forming apparatus, and the resulting layer of wet fibre was rolled onto a 34mm diameter plastic tube. The fibre and former were pushed into another plastic tube with 75mm internal diameter. The fibre was then compressed longitudinally to a length of about 250mm. The resulting tubular structure was removed from the outer tube and dried at 55 C. It was then removed from the inner tube and dried at 105 C.

It was then acetylated using the same procedure as disclosed in Example I and achieved an apparent weight gain of 23%.

When tested, as before, it intercepted 90g of oil, with a back pressure of 0.27 bar.

Example 3.

185g of wood fibre was made into a slurry with 10 litres of water. The slurry was poured into the space between an outer tube of 65mm inside diameter and an inner tube of 34mm outside diameter, and allowed to drain through a perforated plate at the end. The wet fire was then compressed by a plunger to a length of 235mm, and the resulting structure was removed from the tubes and dried and acetylated as before.

The apparent weight gain was 21%.

The filter intercepted 47g of oil, at a back pressure of 0.26 bar.

Claims (12)

1. A method of producing a filter element for the absorption of hydrophobic water-immiscible liquids, the method comprising providing a porous filter element component formed of a lignocellulosic material, effecting acetylation of said lignocellulosic material by applying a pressure differential across the component and causing a vapour phase acetylating agent at elevated temperature to pass through the component, and if necessary forming the component into the filter element.

2. A method as claimed in claim 1 wherein the acetylating agent comprises acetic anhydride.

3. A method as claimed in claim 2 wherein the acetylating agent comprises acetic anhydride and acetic acid.

4. A method as claimed in claim 2 or 3 wherein the acetylation is effected at a temperature of 130 to 150 C.

5. A method as claimed in any one of claims l to 4 wherein the acetylation is effected to achieve a weight gain of up to 30% by weight.

6. A method as claimed in claim 5 wherein the acetylation is effected to achieve a weight gain of 15 to 25% by weight.

7. A method as claimed in any one of claims 1 to 6 wherein the lignocellulosic material is selected from wood, bark, straw, flax, linseed, bagasse, sisal, jute, kenaf, miscanthus, coir, and hemp.

8. A method as claimed in any one of claims 1 to 6 wherein the lignocellulosic material comprises wood fibre.

9. A method as claimed in any one of claims 1 to 8 wherein the component is tubular.

10. A filter element produced by the method of any one of claims 1 to 9.

11. A filter unit comprising a housing and a filter element as claimed in claim 10.

12. A method of separating an organic pollutant from a mixture of such a pollutant with water comprising passing the mixture through a filter element as claimed in claim 10.