IE913487A1 - Filtration and liquid/solid interaction - Google Patents

Abstract

In order to provide long periods of operation without blinding, a dead-end filter (30) has a reversible filter cloth (105) of conical shape. Filtration is performed in one direction, and back-washing is performed in the opposite direction, causing the filter cloth (105) to turn inside out and assisting release of filtered particles. The filter is of particular use in a system for removing nitrate from water using a resin/filter aid mixture, for regenerating the resin. The solid particles are carried as a suspension and are removed using the filter, which is then dried, the particles being regenerated and washed by passing a regenerant and then water through the filter in the same direction, and the particles are returned to a point of use by passing water through the filter in the opposite direction.

Description

FILTRATION AND LI QUID/SOLID INTERACTION Background of the Invention The invention relates generally to filtration and to causing a liquid to interact with a solid. There is an extensive discussion of liquid/solid reaction in WO 91/04791, the disclosure of which is incorporated herein by reference.

The filtration aspect of the invention is completely general, and the filter can be used for any suitable filtration. However, the invention has been developed in the course of ion exchange procedures for removing nitrate from water, but can be particularly useful in reducing the hardness of water, selectively removing or regaining materials such as pollutants or metals from water, and adsorbing colour. A separation or liquid is required after or as part of the liquid/solid interaction, or as part of a regeneration cycle. The mixture is passed to a filter, which can be in a dead-end mode. The advantage of using a dead-end mode is that a much higher flux can sometimes be obtained, possibly twenty times as great. In some cases, the contact stage can occur on or in the filter itself, if appropriate to the particular process.

It is desirable to provide a suitable low cost filtration arrangement that can operate for long periods when filtering solids of small particle size.

The Invention The present invention provides a reversible dead-end filter as set forth in Claim 1, a filtering method as set forth in Claim 9 and a liquid/solid interaction method as set forth in Claim 15.

The invention can enable'one take advantage of the enhanced kinetics of e. g. microresins without the drawbacks of excessive pressure drop and danger of fouling or clogging which is experienced with column arrangements, or the attrition and related mechanical blocking and losses commonly associated with continuous systems.

The filter and method of the invention can be simple and efficient. Plant sizes can be reduced and equipment costs lowered.

The invention can be used for a contacting stage and for The filter can have a large retention capacity. The method of operation keeps the filter clean, without requiring any special cleaning step as such. a regeneration stage, or used only in a regeneration stage or only in a contacting stage. The filter can be used for any suitable filtration.

Normal valving can be incorporated if desired because splitting or comminution of the particles is no longer of great significance as the operating particle size can be very small. Thus the invention allows use of fine material, whether it is originally fine or breaks down to fine material as a result of attrition in the process. The particle size may be smaller than that selected for use, for instance, in packed columns or filter beds. The particle size may be less than about 0. 5, 0. 4 or 0. 3 mm, or less than 0. 25 or 0. 2 mm. A filter aid such as diatomaceous earth can be used, and the particles of the filter aid may be larger.

The retained solid may be regenerated, and this may be done in the filter, or elsewhere. Typical regeneration will be done by immersion in a suitable fluid, eg immersion of ion exchange resin in brine. Further washing or flushing may be required after regeneration, to return the solid to a suitable condition for re-use or disposal.

As a separate idea, the invention provides carrying out the method of wo 91 04791 with the addition of a filter aid, for instance diatomaceous earth. The addition of the filter aid avoids blinding of the filters during long periods of operation.

Preferred Embodiments The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a plant for carrying out the method of the invention; Figures 2a and 2b form a diagram of a pilot plant; Figure 3 is a view, partly in vertical section, of a sock filter in the plant of Figures 2a and 2b; Figure 4 is a section along the line IV-IV of Figure 5, showing a multiple sock filter; and Figure 5 is a section along the line V-V of Figure 4.

Standard components are shown in a conventional manner, including level gauges, one-way valves, stop valves and pressure gauges.

Figure 1 A process liquid is fed through a line 21 and a suspension or slurry of a resin/diatomaceous earth (filter aid) mixture is fed through a line 22 into a feed tank 1. The suspension in the feed tank 1 is drawn off through pipe 23 by means of a feed pump 8 and passes through a crossflow filter 3, which may be as disclosed in US 4755906 or in GB-A-2 185 906. Adequate mixing is required so that sufficient contact occurs, and if desired, a stirrer can be included in the mixing tank 1, though the pumping action will normally give good contact in the pipe 23; both in the etirred mixing tank 1 and in the pipe 23, the solid and the liquid are moving in cocurrent. In the filter 3, the liquid is removed and a suspension or slurry of the solid with reduced liquid content is returned through a recycle line 5 to the feed tank 1 whilst the removed liquid exits at 4.

The regeneration procedure is countercurrent regeneration. There is a bleed 24 which is pumped by a variable-stroke displacement pump 25 into a holding tank 26. The pump 25 controls the solid concentration and hence the solid detention time in the system. A regenerant tank 27, a rinse tank 28, an air compressor 29, a service water line 30 and a drain 31 are shown connected by suitable valving to a reversible sock filter 32 (also called a reversible fine fabric filter or a flip-flop filter).

The regeneration procedure is in the following steps: a) pass from holding tank 26, down through filter 32, to line 22, to hold back resin/filter aid mixture in filter 32; b) pass air from compressor 29, down through filter 32, to line 22, to dewater the filter 32; c) recirculate regenerant from brine tank 27, down through filter 32, back to brine tank 27, to regenerate the resin; d) pass air from compressor 29, down through filter 32, to drain 31, to dewater the filter 32; e) pass water from service water line 30, down through filter 32, to drain 31, to rinse the resin; f) pass air from compressor 29, down through filter 32, to drain 31, to dewater the filter 32; g) pass water from service water line 30, up through filter 32, to line 22, to backflush filter 32 and remove resin/filter aid mixture to carry it back to the feed tank 1.

Figures 2a and 2b Figures 2a and 2b should be joined along their respective right and left margins.

The plant of Figures 2a and 2b is based on the plant of Figure 1.

The contact stage (Figure 2a) is described in WO 91/04791, with reference to Figure 5a therein, using the same reference numerals, and the description need not be repeated here.

The regeneration stage (Figure 2b) is generally as in the lower part of Figure 1. A regenerant saturator 82 is shown for feeding regenerant to the regenerant tank 27. The drain 31 leads to a waste tank 86 which overflows into a basin 87 pumped to waste by a pump 88.

Though not shown, the plant can be arranged such that the spent regenerant is run off continuously and in a small stream, so that concentrations around the system are substantially constant.

In operation, the surplus resin/filter aid mixture recycles around the loop or recycle line 5; this surplus would take up any sudden increase in say -NO^ content, and if the -N03 content decreases shortly afterwards, the operation is not affected. If however, no decrease occurs within a limited time, the increase causes the pump 8 to speed up and the recycle rate through the line 5 is increased.

Figure 3 Figure 3 illustrates the sock filter 30 or 44. The filter 30, 44 comprises two cylindrical casings 101 bolted together at flanges 102. The flanges 102 sandwich between them suitable gaskets, flanges 103 on two stainless steel perforated conical cages or baskets 104 which are within the casings 101, and the flange of a flexible filter material or cloth 105. The filter cloth 105 is shown in full lines in one position and in dashed lines in its alternative position. The flanges 102, 103 form annular securing means for the filter cloth 105 and define a filter opening 106, which is preferably circular but could be another suitable shape. The filter cloth 105 is stitched to form a cone of the same size as the conical baskets 104 with a flange on the open end of the cone (in longitudinal section, the filter cloth 105 is of generally triangular shape). The cone forms a reversible bag or pocket, and the whole cone can move in either direction normal to the filter opening 106, reversing or turning inside out as it does so; the tip of the cone can move for instance through a distance from the plane of the filter opening 106 at least equal to twice the diameter of the filter opening 106. On reversing, there can be a type of peeling procedure as each annulus of the cone flexes through a large angle, greater than 90’ and nearly 180’, causing a major distortion of the cloth and tending to shed any solids lodged thereon or therein. The baskets 104 prevent the filter cloth 105 over-extending and bursting, ie. they suspend and restrain the filter cloth 105, and they also prevent the filter cloth 105 tending to pull the gaskets in and destroy the seal. The filter cloth 105 can be formed of polyester and can be of a single ply of the same specification as described with reference to Figures 17 to 18a of US 4 765 906. Pipe connection or ducts 107 are shown at each end of the filter 30, 44.

Figures 4 and 5 Figures 4 and 5 illustrate a multiple-sock filter 30 in which the casings 101' clamp aperture plates 111 and the filter cloth cones and gaskets (not shown); the aperture plates 111 each carry a number of conical cages 104 which will receive as appropriate cones of filter cloth 105. The multiple sock filter 30 increases capacity.

Example The plant of Figures 2a and 2b was used on a pilot plant scale, but with just one cross-flow filter 3, 8m long. The filter 3 was as disclosed in Figures 11, 12, 17, 18a and 18b of US 4 763 906. However, as there were no solids present in the raw water, no cleaning cycle was required. Some of the resin and filter aid settled on the weave of the filter support to form a membrane, but the remainder of the resin and filter aid did not build up a significant layer.

A 50: 50 (by weight) resin/filter aid mixture was used. The resin was a Purolite A520E ion-exchange resin in powder form having particles in a range from 100 μΐη down to less than ΙΟμίη; attrition of the resin occurred during operation. The filter aid was diatomaceous earth in a range of about 3μιη to about 96μπ> (5% less than 3μΐη, 5% more than 96μΐη (by weight), weight average 22. 3μπ) - probably all particles were in a range of 2 to 120μπι); it is believed that some attrition of the diatomaceous earth occurred during operation. The regenerant was 10% w/w brine.

The resin was dosed by the pump 51 into the feed tank 1 at such a rate that the average resin flowrate (excluding the filter aid) was 0.56% w/w of the raw water flowrate, though the dynamics of the system caused the resin concentration in the feed tank 1 to be significantly higher, namely about 1. 25%. The lengths (in seconds) of the steps in the regeneration procedure noted above were: a) 240, b) 80, c) 60, d) 80, e) 80, f) 80, g) 50.

There was no significant change in filter rate after long periods of operation. Nitrate concentration in ground water was reduced from about 60 to 35 3 mg-NO^/l. The water throughput was 1. 8 m /hour.

The ground water contained no detectable solids.

The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention.

Claims (19)

1. , A reversible dead-end filter, comprising: annular means for securing a filter material, the securing means defining a filter opening therethrough; a flexible filter material secured by securing means in order to filter fluid passing through the filter opening, at least a zone of the filter material being capable of moving a substantial distance in either direction normal to the plane of the filter opening and the filter material being reversible, whereby when the filtrand is passed to the filter in one said direction, the filter material forms a bag extending in the opposite direction from the filter opening, and vice versa; and duct means for conducting filtrand to one side of the filter material and withdrawing filtrate from the other side of the filter material, and vice versa.

2. The filter of Claim 1, wherein said zone of the filter material is capable of moving in each direction normal to the plane of the filter opening by a distance from the plane of the filter opening equal to the maximum dimension of the filter opening.

3. The filter of Claim 1, wherein said zone of the filter material is capable of moving in each direction normal to the plane of the filter opening by a distance from the plane of the filter opening equal to twice the maximum dimension of the filter opening.

4. The filter of any of the preceding Claims, wherein the securing means define a circular filter opening.

5. The filter of any of the preceding Claims, wherein the filter material is of generally triangular shape as seen in longitudinal section.

6. The filter of any of the preceding Claims, wherein there is a perforate filter support on either side of the filter opening, of not greater size than the filter material when the filter material is fully extended/ hut permitting said movement of the filter material, for supporting the filter material.

7. The filter of Claim 6/ wherein the filter support is of generally triangular shape as seen in longitudinal section.

8. The filter of Claim 6 or 7, wherein the filter support is rigid.

9. A method of filtering a suspension of fine solid particles in a liquid, comprising: passing the suspension to a filter material to retain the particles on the filter material, the filter material being in the form of a reversible bag; and removing the particles as a suspension by passing liquid in the opposite direction through the filter material to reverse the bag and carry the particles away from the filter material.

10. The method of Claim 9, wherein the suspension is a suspension of unregenerated solid particles, a regenerant being subsequently passed through the filter.

11. The method of Claim 9 or 10, wherein, after filtering the first-mentioned suspension, a regenerant for regenerating the solid particles is passed into the filter in the same direction as the first-mentioned suspension.

12. The method of any of Claims 9 to 11, wherein, after filtering and regenerating the first-mentioned suspension in the filter, liquid passed in the opposite direction through the filter to carry the regenerated particles back to a point of use of the regenerated particles.

13. The method of any of Claims 9 to 12, wherein, after filtering the first-mentioned suspension, liquid is removed from the filter material by blowing air through the filter material in the same direction as the first-mentioned suspension.

14. The method of any of Claims 9 to 13, wherein the filter is as set forth in any of Claims 1 to 8.

15. A method of causing a liquid to interact with a solid, comprising: forming a suspension of particles of the solid in the liquid and moving the solid particles and the liquid in the same direction while the liquid interacts with the solid particles; removing liquid from the suspension; recycling solid particles from the suspension after liquid has been removed; and regenerating solid particles by a procedure which includes passing the solid particles as a suspension to a filter so that the filter holds the solid particles back, and back-washing the filter to remove the solid particles from the filter.

16. The method of Claim 15, and also being in accordance with the method of any of Claims 9 to 14.

17. A filter substantially as hereinbefore described with reference to the Examples and accompanying Drawings.

18. A method of filtering a suspension substantially as hereinbefore described with reference to the Examples and accompanying Drawings.

19. A method of causing a liquid to interact with a solid substantially as hereinbefore described with reference to the Examples and accompanying Drawings.