WO1994011087A1

WO1994011087A1 - Crossflow precoating process and apparatus for carrying out the process

Info

Publication number: WO1994011087A1
Application number: PCT/US1993/009749
Authority: WO
Inventors: Jörg-Thomas LIETZAU; Gerhard Lietzau
Original assignee: Isp Investments Inc.
Priority date: 1992-11-09
Filing date: 1993-10-12
Publication date: 1994-05-26
Also published as: DE4237714C2; DE4237714A1; AU5328894A; CN1088469A

Abstract

In a crossflow precoating process, a filtrate containing prefilt slurry with, optionally, added filter aids is passed into at least one approximately cylindrical filter chamber through an opening at one end thereof (8). Said filter chamber is formed by the filter basket (5) disposed in a pressure vessel. The filtrate flows through a filter medium delimiting the filter chamber, forming a filter cake, and is discharged as permeate. The filter cake is first formed by the filtrate containing the filter aids in the filter chamber, said filtrate permeating said filter medium. Subsequently, in order to extend the useful life of the filter medium, a partial flow is passed through the filter chamber in approximately axial direction thereof (17) to an opening for retained substances (6) at an end of the filter chamber opposite the first end. The partial flow, one discharged from the opening for retained substances (6), is returned outside the filter chamber to the first opening (8) of the filter chamber.

Description

Crossflow precoating process and apparatus for carrying out the process

The invention relates to a crossflow precoating process in accordance with the preamble of Claim 1.

Further aspects of the invention relate to a filter basket for carrying out the crossflow precoating process in accor¬ dance with the preamble of Claim 6, and a crossflow pre- coating device in accordance with the preamble of Claim 10.

A process for filtering beverages and chemical, pharmaceuti¬ cal or similar liquids by separating the insoluble particles from the liquid is already known. In this case, filter aids are added to the untreated prefilt slurry, causing a precoat to form a filter cake. The solid particles deposited in the filter cake are then rinsed out, and the filter aid, together with any stabilising agents added, is regenerated (e.g. EP 0 460 499) . This is intended to enable one and the same filter cake to be used repeatedly for both pre-filtering and post- filtering once conditioned. In detail, pre-filtering the prefilt slurry involves the constant metered addition of a filter aid compound, which causes a filter cake 10 - 50 mm thick to be deposited. When there are no more suspended particles left in the filtering apparatus, or once a maximum permissible pressure differential has been reached, the par¬ ticles from the filtered liquid retained in the filter cake are rinsed out using solvents. Post-filtering involves the pre-filtered liquid being passed through the rinsed and ster- ilised filter cake formed during pre-filtering for filter- medium filtration. This is again followed by a scavenging step with suitable solvents. To enable continuous operation of the process, precoating of the filter cake and pre-filter¬ ing is carried out in a first filter and post-filtering in a second filter downstream. In the filtering apparatus used to carry out this filtration process, which comprises a boiler filter, the filter cake is formed on disk-shaped filter ele¬ ments. The filter cake can be removed completely by rotating the disks using a motor drive. This requires costly sealing. The filtering apparatus is maintenance-intensive and prone to defects.

The present invention involves a different process from coarse and/or fine filtration and can be referred to as the crossflow precoating process. This process only makes use of some of the steps common in a state-of-the-art micro-cross- flow process. In particular, use is made of a filter basket with an additional outlet for the retained substances, which is a prerequisite of the device in micro-crossflow filtering processes. Here, the retained substances are discharged through this outlet and not, as is the case with filter bas¬ kets fitted with a filter bag, by removing the filter bag containing the retained substances. Support baskets with filter bags only serve the purpose of absorbing the pressure exerted on the support basket from the interior of the filter chamber, the volume of which is limited by the filter bag and filter basket.

In other known filtering processes, which involve a rewashing step in combination with a filter basket as such, solid par¬ ticles are discharged through the filtrate opening in the opposite direction of flow to that used in normal filter ope¬ ration.

These filtering processes have one drawback in common, namely that the build-up of deposited particles in the filter medium during filtering is so rapid that thresholds are soon reached where the filtering process has to be aborted and the pre- coated solid particles, or possibly even the complete filter medium, need to be removed from the filter chamber. The problem which the present invention attempts to solve is primarily to provide a filtration process in which the filter cake, once formed, can be used as the filter medium for long¬ er, enabling regeneration of the filter to be delayed. The process according to the invention should also be particular¬ ly suitable for automation.

An apparatus suitable for carrying out this process should also have a minimum of moving parts, and thus low additional energy requirements (apart from that required for pumping the fluids) ; it should be less subject to wear and tear and less susceptible to defects.

The crossflow precoating process according to the invention is distinguished by the features described in the character¬ ising part of Claim 1.

A characteristic feature of this process is that once the filter cake, including precoating aids, has been formed by the approximately radial flow of filtrate, including filter aids, through the filter basket, the undesirable growth of the filter cake with correspondingly reduced throughput is then prevented by setting up an almost axial partial flow in the longitudinal direction of the filter chamber, which is discharged through an opening for retained substances in the filter chamber and is then returned to the inlet opening of the filter chamber. Surprisingly, this partial flow has a substantial cleansing effect on the inner surface of the fil¬ ter cake, which results in the filter cake being operational for the filtration process longer. During this stabilisation process, however, the effectiveness of the filter cake in filtering fine particles as a result of the filtering aids remains intact as these remain effective in the deeper layers of the filter cake. In an ideal situation, returning the partial flow achieves an equilibrium between the precoating of the filter cake and the erosion of its inner surface as a result of the crossflow effect.

The process according to the invention is especially suitable for implementation in processes for making beverages such as beer, wine, fruit juices and sparkling wine. This filtering process is, however, also advantageous in other areas, e.g. the varnish and paint industry. In this case, there is very little waste to be disposed of. It is also suitable for mi- crofiltration, for which crossflow processes were uneconomi¬ cal before.

An advantageous further embodiment of the crossflow precoat¬ ing process, which uses a filter basket with an approximately cylindrical filter chamber, is the addition of a rewashing step in accordance with Claim 2, to completely remove the exhausted filter cake from the filter chamber or filter bas¬ ket. Fluid is introduced into the filter under pressure through a permeate opening in the filter, the liquid flowing into the filter basket from outside, collecting the remains of the filter cake inside and being discharged through the opening for retained substances. In particular, the filter cake is first dissolved in water, enabling it to be pumped out of the filter chamber. This process has none of the disadvantages attached to filtering apparatuses with disk- shaped filters, in which the exhausted filter cake has to be worn away by rotating the disks mechanically. In addition to the advantages arising from the problems which this process sets to solve, it should be noted that, as a result of the filter basket capacity being well adapted to the total vessel capacity of the filter housing, i.e. only very little dead volume, the total structural volume of the filter can be kept small. This crossflow precoating process, including the step of re¬ moving the exhausted filter cake, is particularly well suited for automation. Automation could be implemented using mechan¬ ically operated shut-off valves dependent on known parameters monitored during filtering processes, such as, the flow of permeate and/or the differential pressure in accordance with Claim 3.

As described in Claim 4, continuous operation of the cross- flow precoating process is possible despite the need for con¬ secutive processing steps as the accumulated filtrate is con¬ tinuously filtered. Thus no substantial intermediate storage or buffering is required.

A filter basket suitable for carrying out the crossflow pre¬ coating process described forms the basic structure of the filter which is formed with the precoated filter cake. Said filter basket is characterised by an opening for retained substances in addition to the standard opening in the filter basket (usually at the top) .

In accordance with Claim 7, the rewashing process step re¬ quires the filter basket to have several concentric layers, the middle layer of which, in accordance with Claim 8, con- tributes most to the filtration effect, while the outer layers in both possible radial directions of flow have above all a supporting effect. A middle layer with a mesh width of 50 - 60 microns has proved itself to be particularly suit¬ able.

A step that is not normally found in precoating devices, whereby the opening for retained substances is connected to the filtrate opening in the filter or its pressure vessel via a line with an intermediate shut-off valve connected to an automatic control system, is described in Claim 10. Filters in which only one approximately cylindrical filter body is located in the pressure vessel are particularly ad¬ vantageous with respect to exploitation of the filter volume. Despite this fact, however, Claim 11 describes a variant that is worth consideration, especially in the case of larger fil¬ ters.

Further details can be found in the description below.

Examples of the invention are depicted be means of 8 figures, representing as follows: Fig. 1 schematic diagram of the entire crossflow precoating device for continuous operation. Fig. 2a section through a filter that can be used in this device.

Fig. 2b filter basket comprising three concentric layers

(section) , Fig. 3 section through a variant of the filter, Fig. 4 the part of the whole device that is activated when the filter cake is formed.

Fig. 5 the part of the whole device that is operative when the filter cake is stabilised. Fig. 6 the part of the device that is active when the fil¬ ter cake is regenerated, Fig. 7 the part of the device that is operative in scaveng¬ ing the filter cake, Fig. 8 the part of the whole device used for rewashing.

The core of the crossflow precoating device, as shown in Fig. 1, is the two identical, alternately operated filters 1 and 2. The section in figure 2a provides more detailed information about the filters. In the figure, reference numeral 3 desig¬ nates a pressure-proof filter housing sealed with a lid 4, which receives a support basket or filter basket 5.

As can be seen in Figure 2b, the filter basket 5 contains three concentric layers 50, 51 and 52, the innermost 50 and outermost 52 of which, serve as coarse-meshed supporting lay¬ ers, while the middle layer 51 is fine-meshed for filtering purposes.

A special feature of the support basket 5, as shown in Fig. 2a, is the opening for retained substances 6, which opens into the outlet 7 in the filter housing. The support basket is open at the top as is customary, this opening is marked 8.

The lid 4 of the filter holds line 9 which serves. In partic¬ ular, as a standard filtrate supply line. A permeate outlet

10 is located at the bottom of the housing 3.

Using the above filters, the crossflow precoating process can be carried out as described below with particular reference to Fig. 1 and Fig. 4.

Once preliminary precoating has been completed at the start of the work cycle, filter 1 is supplied with liquid via pump

11 and a metered amount of filter aid or stabiliser is added to the flow via metering pump 12. In the case of a separation of, for example, stabilising agents from beverages or good cake-building solid particles from other liquids, the prelim¬ inary precoating and later addition of filter aids can be omitted, if necessary. In this case, shut-off elements 13 and 14 are opened, while all the other shut-off elements to fil¬ ter 1 are closed. When the filter has been completely de- aerated, shut-off element 13 is closed and a shut-off element 15 is additionally opened to allow circulation of the beve¬ rage or filtrate with filter aid via a line 16.

The latter constellation for returning the filtrate contain- ing the filter aid is depicted in Fig. 4. It is in this way that the filter cake is formed inside the support basket 5, as shown in Fig. 2 . The filter cake with the embedded filter aids is formed when the filtrate is returned. The thickness of the filter cake is assumed to be sufficient when the per- meate flowing from permeate outlet 10 in filter 1 is clear. Shut-off element 15 is then closed and filter 1 remains on standby until required because filter 2, which has been fil¬ tering so far, is being regenerated.

The actual filtering step, see Fig. 5, is carried out by opening a shut-off element 17 which allows the permeate to pass from permeate outlet 10 to a filling station (no refe¬ rence numeral). The shut-off element 14 on line 9, which is connected to the lid of the filter, is also open at this stage. Not only filtrate and filter aid taken from the cor¬ responding unmarked storage vessels continue to flow through line 9, but also a partial flow from inside the support bas¬ ket. This partial flow runs essentially in the direction of the longitudinal axis of the support basket and is marked in Fig. 2 at numeral 17. This partial flow is discharged through the opening for retained substances 6 and is returned to pump 11 via a shut-off element 18 located at the opening for re¬ tained substances and via return line 16. The partial flow 17 in the support basket keeps a channel (not shown) open in the support basket and precoated filter cake, enabling the filter to remain effective, so that a radial flow - indicated at 19 - also penetrates the filter medium, in this case the filter cake, since its surface is kept clean and early blockaging is avoided. This return flow via shut-off element 18, and thus the

"crossflow operating mode", is initiated when the flow of permeate falls below a preset value. Previous to this, shut- off element 18 is first kept closed while the filter cake is still being formed with the filter aid during the filtering process; this has the effect that the differential pressure over the filter increases, and the flow of permeate decreases simultaneously.

After the discharged permeate has thus been switched to cir¬ culation and the filter has been kept in operation, a situa¬ tion is finally reached in which a sufficient flow of perme¬ ate is no longer achieved despite the circulating partial flow of concentrate rinsing the top layer off the filter cake. At this point, filtering is switched from filter 1 to filter 2, which was on standby, by closing shut-off elements 14, 17 and 18 and opening shut-off elements 20 and 21 of filter 2, enabling filtrate to continue to reach the filling station.

A particular advantage of regenerable filter aids, such as "Polyclar Plus", is that the filter cake formed last in fil¬ ter 1 can be regenerated. To this purpose, as shown in Fig. 6, lye is fed by pump 22 into intake line 9 of filter 1 via shut-off elements 23, 24 and 14. The lye can be discharged via shut-off elements 17 and 25, if contaminated, or if not contaminated, it can be returned to the storage vessel via shut-off element 26 and reused.

This is followed by a scavenging step, see Fig. 7, which can be carried out as often as required during the whole process. For this purpose, water is passed through line 9 into filter 1 via pump 27 and shut-off elements 28 to 30 and can be dis¬ charged via shut-off element 17 on permeate outlet 10 and via shut-off element 25. 10

Finally, Fig. 8 shows rewashing of the regenerated filter cake as the final step in the process. This takes place when the filter cake has been rinsed thoroughly enough. To this end, the shut-off elements connected to filter 1 are closed, with the exception of shut-off element 31 at the opening for retained substances 7, which leads to the filter aid contain¬ er. The shut-off elements 32 and 33 are also closed, these two shut-off elements being located in a water line fed by pump 27. The water is used to wash the remains of the filter cake back into the filter aid container for reuse.

The corresponding steps take place analogously for filter 2 but staggered in time, for which reason the shut-off elements concerned are not numbered throughout.

Fig. 3 depicts a variant of the filter, in which the pressure vessel 34 contains three parallel support basket arrangements 35 to 37. Each support basket arrangement may for its part consist of a plurality of layers. The openings 38 - 40 in the support baskets are located above a tray 41. The chamber in the pressure vessel above tray 41 is linked to a connection 42, ba analogy with intake line 9 in Fig. 2. Openings for re¬ tained substances 43, 44 and 45 run into a tray 46 lower down which forms part of a chamber 47 for retained substances. This chamber is provided with an opening 48 for retained substances. A permeate outlet 49 is connected with in the space between trays 41 and 46. This filter, as shown in Fig. 3, is operated in basically the same way as that described for Fig. 2 , though, in the process, the three support baskets or filter units 35, 36 and 37 are active parallel to each other.

Claims

11Claims :

1. Crossflow precoating process wherein a filtrate containing prefilt slurry with, optionally, added filter aids is passed into at least one approximately cylindrical filter chamber disposed in a pressure vessel at the first end (8) of said filter chamber, flows through a filter medium de¬ limiting said filter chamber, forming a filter cake, and is discharged as permeate, characterised in that filtrate containing filter aids is first passed into the filter chamber formed by a filter basket (5) and permeates said filter medium until a filter cake is formed, and that a partial flow is subsequently passed through the filter chamber in approximately axial (17) direction thereof to an opening for retained substances (6) at an end of the filter chamber opposite said first end, and the partial flow discharged from the opening for retained substances (6) is returned outside the filter chamber to said first opening (8) of the filter chamber.

2. Crossflow precoating process as claimed in Claim 1, characterised in that fluid is subsequently passed into said filter under pressure through a permeate opening (10) in the filter, flowing from outside into the filter basket (5) and being discharged through the opening for retained substances (6) together with the retained substances washed out.

3. Crossflow precoating process as claimed in either of

Claims 1 or 2, characterised in that the successions of steps of building up the filter cake in the filter chamber, stabilising the filter cake by means of the axial partial flow in the filter chamber, and the 12

rewashing step by introducing the fluid into the filter through the permeate opening (10) are controlled as a function of the permeate throughput and/or the differen¬ tial pressure between the filtrate inlet (9) and the permeate outlet (10) .

4. Crossflow precoating process as claimed in any one of Claims 1 to 3, characterised in that two filters (1, 2) , each of which comprises at least one filter chamber in a pressure chamber, are operated alter¬ nately in such a way that in each case one of the two filters (1, 2) builds up the filter cake or filters the filtrate, while the other of the two filters is rewashed or remains on standby.

5. Crossflow precoating process as claimed in any of the pre¬ ceding Claims, characterised in that polyvinyl polypyrrolidone in combination with cellulose is used as the filter aid.

6. Filter basket for carrying out the crossflow precoating process in accordance with any one of Claims 1 to 5, which is approximately cylindrical and is open at the top, characterised in that it has an opening for retained substances (6) at the bottom.

7. Filter basket as claimed in Claim 6, characterised in that it comprises three concentric layers (50, 51, 52).

8. Filter basket as claimed in Claim 7, characterised by three layers (50, 51, 52), the middle layer (51) of which has a finer mesh than the innermost layer (50) .

9. Filter basket as claimed in Claim 8, characterised by a mesh width of 50 - 60 microns in the middle layer (51) .

10. Crossflow precoating apparatus with at least one filter basket in a pressure vessel for carrying out the cross¬ flow precoating process in accordance with any one of Claims 1 to 5, using the filter basket in accordance with any one of Claims 6 to 9, characterised in that the pressure vessel (3, 4) has an outlet for retained substances (7) and that a shut-off valve (15) is disposed in a line between the outlet for retained substances and a filtrate inlet (9) of the pressure vessel.

11. Crossflow precoating apparatus as claimed in Claim 10, characterised in that a plurality of filter units (35, 36, 37) are disposed in the pressure vessel (34) , their openings for retained substances (43, 44, 45) running into a common chamber

(47) for retained substances, which is provided with an outlet (48) for retained substances.