FILTRATION OF FLUID MEDIA

Abstract: An elongate tubular porous ceramic filter for use in the filtration of fluid media, e.g. the filtration of particulate matter from hot gases, is formed from a plurality of shorter tubular portions (10, 11, 12, 13, 14, 15, 20, 21) adhered together by means of a high-temperature adhesive at bonding points (30, 31) to provide long filter tubes.

Description: This invention relates to the filtration of fluid media and a filter for use therein. More particularly, the invention relates to the filtration of hot fluid media e.g. filtration of hot gases at temperatures of up to 1600°C or more. One reason for filtering hot fluid media is to remove material which might otherwise harm the environment e.g. promote or contribute to atmospheric pollution e.g. coal gasification plants and incineration plants. Another reason for filtering fluid media is to recover valuable materials such as, for example precious metal particles from high temperature processes e.g. in the smelting of gold, silver, platinum or the like; in the manufacture of chemical compounds or refractory oxides in order to render such processes more efficient. The filters used in these processes have generally been fabricated of a metal which is suitable for use at elevated temperatures. Generally the preferred metal has been a stainless steel or the like nickel containing high temperature resistant alloy. Unfortunately, such filters are extremely expensive to produce and the too frequent replacement thereof can render a manufacturing process uneconomic and labour intensive in terms of the periodic maintenance that is required. In processes where arduous operating conditions exist i.e. where the temperature exceeds about 900°C and/or where the hot gases are corrosive, such metal filters may fail because of erosion and thus have an uneconomically short working life. Reducing the working temperature of e.g. hot gas filtration processes to a lower level is not always practicable since there may be a risk of condensation products forming at the lower temperature employed. In certain cases a reduction in the process temperature may result in an unacceptable loss of energy which might otherwise have been recovered by heat exchangers for reuse. To overcome the above disadvantages it has been proposed to use filters e.g. in tubular form, comprising a major proportion of fibrous material made by a vacuum-forming process from e.g. a slurry. However, this process whilst capable of forming relatively long filter tubes, the overall length is generally limited to 1.5 metres or less. In order to achieve optimum filter efficiency related to a given volume and surface area it is often the case that filters of greater overall length are required. This longer length has been achieved by mechanically mating two or more shorter lengths together and holding them together by a costly and complex supporting structure. Furthermore, the shaped but as yet unhardened filter, i.e. before the binder has set, has proven very difficult to handle particularly if the shape is more than 1 metre in length. Although DE-A-2856471 discloses one means of forming a long tubular ceramic filter by joining together a number of shorter lengths we have now found an improved means of reducing the above disadvantages using a tubular porous ceramic filter having an overall length of several metres and which will withstand higher operating temperatures and mechanical stress such as vibration without any costly or complex mechanical holding or supporting means other than that inherently provided by the filter chamber itself. Accordingly, the present invention provides a tubular porous ceramic filter, for use in the filtration of fluid media, which filter comprises ceramic fibre and a binder and is an elongate tubular filter formed from a plurality of shorter tubular portions bonded together end to end, characterised in that the elongate tubular filter is formed from a first plurality of shorter tubular portions and a second plurality of shorter tubular portions, one plurality being disposed inside the other such that the bonding points of one plurality are axially displaced from those of the other plurality. This arrangement has the benefit that joints and bond points of the inner and outer tubes are not coincident with each other. According to another aspect of the present invention there is provided a method of separating particulate matter from fluid media comprising the step of filtering the media through a porous ceramic filter as hereinbefore defined in the immediately preceding paragraph. A wide range of ceramic fibres may be used but examples of particularly suitable ceramic fibres are alumina fibres, alumino-silicate fibres containing alumina, silica and in some instances a minor proportion of zirconia. The amount of ceramic fibre may be from 35 to 95% by weight, preferably 55 to 85% by weight. The binder used may be organic, inorganic or a mixture of both of these types. The amount of binder used will vary according to its source but in the case of inorganic binders 5 to 35% by weight is satisfactory, preferably 10 to 30% by weight. An organic binder if used may be up to 25% by weight, preferably up to 15% by weight. The inorganic binder may be any of those known for their suitability for bonding ceramic fibres but preferably the binder employed comprises at least one of alumina sol and silica sol. Similarly, the organic binder may be any suitable material such as natural resins, synthetic resins or starch the latter being particularly preferred. Preferably the tubular portions forming the porous ceramic filter according to the present invention are formed by mixing together ceramic fibre with at least one inorganic binder with sufficient water to form an aqueous slurry dewatering the slurry against a perforated mesh walled former having dimensions which correspond to those of the portion to be formed, applying a vacuum source to the interior of the former so as to create a pressure difference and thus cause the solid constituents of the slurry to be deposited against the mesh until the required wall thickness of the portion has been deposited, removing the former from the slurry and drying the portion to remove any residual water and if appropriate, after drying, firing the filter portion to effect a ceramic bond. The portions thus formed may be from about 200 mm up to about 500 mm or more in length and these portions are assembled together to produce long self-supporting filter tubes several metres in length. The adhesives used are those which are suitable for bonding ceramic fibre containing articles together and can also withstand high temperatures. These adhesives possess a high dynamic strength, which is of significance particularly when cleaning the filter. The filters according to the invention can reach lengths in the order of 8 metres and they are usable at temperatures of up to 1600°C or more. It has also been found that the addition to the aqueous slurry of a minor proportion of a flocculating agent such as for example of a cationic starch aids the distribution of the binder during manufacture of the filter which in turn enhances the final mechanical strength of the filter. Optionally, the aqueous slurry may additionally contain a minor proportion of a particulate filler material such as a refractory oxide such as alumina, magnesia, silica, titania, zirconia or a refractory silicate e.g. an alumino-silicate, which has been found to increase the refractoriness of the filter and also optimise its porosity. The porosity of the filter may be between relatively broad limits e.g. between 50 to 95%, more typically between 75 to 85% to encompass its several different possible uses. The permeability of the filter or in other terms the resistance to the flow of air through the material is also an important factor. Filters whose resistance is in the range from about 1 x 10⁵ to about 1 x 10⁸ Nm⁻⁴s. are satisfactory for use in connection with filtering particulate matter from fluid media including particles from hot gases. More typically resistance values from about 5 x 10⁵ to 5 x 10⁶ Nm⁻⁴s. provide particularly efficient filters. A filter according to the present invention is capable of removing particles from fluid media of 0.5 microns or greater. Typically, in use, the filters will be cleaned at frequent intervals, which may be as often as every 10 minutes. Preferably, the cleaning is effected by a reverse pulse method using compressed air. Furthermore, filters of the present invention are not adversely affected by corrosive fluid media of an acidic or alkaline nature, even at elevated temperatures of up to 1600°C or more unlike metal filters which are readily corroded and subject to erosion in such conditions. In addition the filters are not oxidised by oxygen containing fluid media. Other advantages of filters according to the present invention include their resistance to thermal shock, their low overall weight, their ease of manufacture and the fact that they are self-supporting. A further benefit which accrues from the use of a filter according to the invention relates to the purity of the hot gas of a given process after filtration in that it is possible to reuse the hot gas in the same or another process without further treatment of the gas thereby achieving substantial energy savings. Filters according to this invention exhibit high efficiency in conjunction with a minimum pressure drop across the body of the filter following long periods of use of filters which may be up to approximately 4000 hours. The invention is more particularly described with reference to the accompanying drawings in which:- Figure 1 is a part longitudinal section of one embodiment of the invention comprising a plurality of inner and outer tubular portions, Figure 2 and Figure 3 are each part longitudinal sections of a filter unit of the invention showing the detail of the upper and lower ends. Referring to Figure 1, a tubular ceramic fibre filter according to the invention consists of two tubes. The inner tube consists of a plurality of individual filter tube portions, two of which are shown as 12 and 13. The filter tube portions 12 and 13 are joined together at 31 by means of a suitable high temperature adhesive. The outer tube is formed of a plurality of slightly larger diameter tube portions 20 and 21 two such filter tube portions are joined together by means of a high temperature adhesive at 30. It can be seen that the two bond points 30 and 31 are displaced in an axial direction with respect of one to the other. The end parts of the formed filter can be designed as shown in Figure 2 and Figure 3. The upper fixing 40, both of the inner and also of the outer filter tubes reveal a flange-like portion for fixing the tube in the filter chamber. In the region of the fixing plate 41 the filter material is specially strengthened by means of an additional treatment with a ceramic binder. In the region 42 the inner and outer tubes are fastened additionally at different lengths. This increases the strength in the case of horizontally arising stresses, which appear particularly during cleaning of the filters. The opposite, free end is formed with a dome-shaped part 50 (Figure 2) which is attached to the inner and the outer tubes also by means of a high temperature adhesive. The part 40 (Figure 3) may be formed in one piece in which case it has regions with differing outside diameter so that it is suitable for joining with the inner and outer tubes. The following example will serve to illustrate the invention still further:- EXAMPLE The tubular components of a porous ceramic fibre filter were made from an aqueous slurry comprising 74 parts alumino-silicate fibre (available commercially under the trade name "FIBERFRAX"), 19.5 parts silica sol, (available commercially under the trade name "SYTON") and 6.5 parts of cationic starch (available commercially under the trade name "PERFECTAMYL PLV). The solids content of the slurry was 2% by weight. A perforated mesh walled former having dimensions of 300 mm in length and 100 mm diameter for the inner tube, and another former also of 300 mm in length but 120 mm diameter for the outer tubes were each immersed in the slurry for five seconds during which time a vacuum was applied to the inside of the formers until a layer 10 mm thick had been deposited on to each former of the solid constituents of the slurry. On removal from the formers the 'green' filter tube portions were dried at 180°C for 4 hours. After drying, the above components were assembled into a 1.8 metre long filter tube in the manner described previously in accordance with Figure 1, using six tubes joined together by high temperature adhesive. The finally assembled tube also features the components illustrated in Figure 2 and Figure 3.

Claims: A tubular porous ceramic filter comprising ceramic fibre and a binder and being an elongate tubular filter formed from a plurality of shorter tubular portions bonded together end to end characterised in that the elongate tubular filter is formed from a first plurality of shorter tubular portions (12, 13) and a second plurality of shorter tubular portions (20, 21), one plurality (12, 13) being disposed inside the other (20, 21) such that the bonding points (31) of one plurality are axially displaced from those (30) of the other plurality. A filter according to Claim 1, characterised in that the ceramic fibre is selected from one or more of alumina fibres and aluminosilicate fibres and present in an amount from 35 to 95% by weight. A filter according to Claim 1 or 2, characterised in that the binder is a colloidal oxide hydrosol. A filter according to Claim 1, 2 or 3, characterised in that the pluralities of tubular portions (12, 13, 20, 21) are vacuum-formed from an aqueous slurry of ceramic fibre and binder. A filter according to Claim 4 characterised in that the aqueous slurry comprises a flocculating agent. A filter according to Claim 5, characterised in that the flocculating agent is a cationic starch. A filter according to any preceding claim, characterised in that the porosity of the filter is between 50 and 95%. A filter according to any preceding claim, characterised in that the resistance to the flow of air through the filter is in the range from about 1 x 10⁵ to about 1 x 10⁸ Nm⁻⁴s. A method of filtering particulate matter from fluid media which comprises passing the media through an elongate tubular porous ceramic filter according to any of Claims 1 to 8.

Inventors: BARLAG, WILHELM, KEIDEL, WOLFRAM, LOOS, ROLF
Applicants: FOSECO INTERNATIONAL LIMITED
Application year: 1988
Main classification: B01D46
IPC codes: B01D46, B01D39, F16L13