The invention relates to a process to continuously filter highly viscous fluids, in particular molten polymers. It also relates to a filter system to carry out such a process.

The filtration of molten polymers is necessary while producing man-made fibers, film, etc. The production processes require continuous flow of clean molten polymer. Impurities, such as contaminants and gels, tend to cause fiber breakage or film ripping during orientation.

Continuity and uniformity of flow are critical process conditions. Stagnation areas and creeping flow velocities will cause the polymer to degrade due to high residence time at elevated temperatures. Degradation products add impurities to the polymer stream being processed. These impurities have to be removed in order to avoid production problems mentioned earlier. Consequently, the residence time of a molten polymer in a filter system is a critical factor.

Currently, most of the filtration processes are carried out using systems that contain two filter housings and two switch¬ over diverter valves. The molten polymer flows through the inlet valve, then through one of the filter housings and out through the second outlet valve. When the filter elements in the housing become exhausted, the flow gets diverted to the second housing, which has fresh filter elements. The exhausted filter housing is then removed, cleaned, returned with fresh filter elements, and put on stand-by mode until the second housing becomes exhausted.

The switch-over valves regulate the flow at the inlet and at the outlet of the system. When switching from one filter housing to the next the valve at the inlet is moved first, allowing a small portion of the flow (e.g. ten percent) to enter the fresh filter housing. The valve at the outlet side remains unchanged. A vent valve on the fresh filter housing is opened to allow the incoming product flow to fill the filter housing completely (venting or bleeding of the filter housing to come on stream) . When the fresh filter housing is completely filled with product, the switch-over valves at the inlet and at the outlet are moved simultaneously to force the complete product flow through the fresh filter housing, thus taking the first filter housing off stream.

It may be noted, that because of the venting phase, the product first filling the fresh filter housing sees, at an assumed vent flow of ten percent of total flow, an up to ten times longer residence time than the regular product flow. As a consequence, the above described product degradation may occur.

Filter systems with two filter housings and two switch-over valves have an installed filter area equal to twice the filter area normally on stream. Normally at least three complete filter housings are needed for continuous operation: one housing on-line, one housing stand-by, and one housing undergoing external cleaning or replacing of filter elements.

The switch-over valves define the most critical parts of these double filter systems. It is particularly difficult to design and manufacture such valves absolutely free of gaps, dead flow areas, and areas with reduced flow velocity. In the filtration of molten polymers, such gaps and areas with different flow characteristics may lead to product degradation with subsequent production problems as described above.

A different process and a device for the filtration of highly viscous fluids is known from DE 40 18 310. This known device

consists of a rotating filter body with multiple filter chambers, each containing one or more filter elements. The product flow is from an inlet port to one of the filter chambers and then to an outlet port. When the filter chamber on-stream is exhausted, the filter body rotates to remove the exhausted filter chamber from the flow, while simultaneously turning a fresh filter chamber into the stream.

Similar to the above described state-of-the-art systems with two switch-over valves, the changing of filter chamber in a device according to DE 40 18 310 is in two phases: venting (bleeding), followed by the actual change. In the device according to DE 40 18 310 at first the filter body rotates the fresh filter chamber in the venting position, allowing a small portion, e.g. ten percent of the incoming flow to go to the fresh filter chamber. Next the filter body rotates the exhausted filter chamber out and the fresh filter chamber into the product flow. The prolonged residence time for the first product passing through a fresh filter chamber is similar to the prolonged residence time for the first product passing through a fresh filter housing in the above described systems with two housings and two switch-over valves.

Devices according to DE 40 18 310 are, for all practical means, limited in the filter area that can be installed and in the filter area that can be put on line. As the individual filter chambers are being rotated into the product line, for filter chambers substantially larger than the diameter of the product line, proper sealing and avoiding any dead flow areas, become formidable engineering tasks.

Devices according to DE 40 18 310 have, as a minimum, twice the filter area installed relative to the filter area on stream. As a matter of fact, they typically have a filter area installed equal to at least four times the filter area on stream.

The technical problem forming the basis for the invention consists on providing a process to continuously filter highly viscous fluids in which the entire available filter surface is used in the best way possible and providing a filter system for carrying out the process.

This technical problem is solved by a process according to the invention with the features of claim 1 and a filter system according to the invention with the features of claim 4.

The basis for the invention is the idea to have a filter system for highly viscous liquids, in particular molten polymers, that provides continuity, that offers any desired filter area on stream, that eliminates the need for switch¬ over valves, that avoids prolonged residence times typical of bringing fresh filter area on stream, and that needs an amount of installed filter area only marginally larger than the filter area actually in use.

The invention includes a filter system basically consisting of an inlet housing, a rotatable filter body with multiple filter housings distributed over one or more circles, and an outlet housing. The product flow enters the inlet housing, which at its inlet side has any desired type of opening and which has an arched opening at its outlet side. The arch- or banana- shaped opening at the outlet side of the inlet housing distributes the flow to the majority of the filter housings in the rotatable filter body of the filter system. The filter housings all contain one or more filter elements. The filter body of the filter system is connected to the outlet housing. The flow passing through the individual filter housings is collected in an arched opening at the inlet side of the outlet housing. The outlet side of the outlet housing may have any desired type of opening.

The invention defines a continuous filtration process. There is no interruption of operation foreseen whenever filter area gets exhausted. By rotating the filter body of the filter

system, a small portion of the filter area is turned off-line, at the same time introducing a fresh filter housing to the flow. During rotation, all but one of the multiple filter housings on stream remain on stream without any changes whatsoever.

The filter system according to the invention forces the product flow from its original flow path into an arch-shaped distributor serving multiple filter housings. The size of the arch-shaped distributor, as well as the size and the number of filter housings can be scaled according to individual needs, putting no fundamental restrictions to the filter area that may be installed.

To change filter housings, the filtration process according to the invention lacks the need to install specific diverter valves. The flow distribution over the multiple housings is by the fixed arch-shaped inlet and outlet housings, which need not to be changed during any phase of operation.

Whatever fresh filter area needs to be put on line, the process is as follows. Through partial rotation of the filter body of the filter system, a fresh filter housing is brought in the venting position. After venting the fresh filter housing, the filter body rotates further to move one filter housing off line, at the same time bringing the fresh filter housing on line.

The particular advantage of the invention for the changing of exhausted filter area is in the circumstances that only a part of the filter area on line at any given time is being replaced by fresh filter area. During normal operation, multiple filter housings, for example ten filter housings, are on stream. All these housings see, on the average ten percent of the incoming flow. Venting takes places for only one fresh filter housing at any given time. Assuming ten percent of the flow is used for venting, then the venting step takes only so much time as is the typical residence time for the product in

the filter during normal operation. After venting, one filter housing is turned off stream, the just vented housing is turned on stream.

Compared to the known filter operation processes as referred to above, the invention offers the possibility to replace the exhausted filter area step by step, after venting the fresh filter area also step by step.

As it is now possible to replace the filter area step by step, the total filter area installed needs to be only marginally larger than the filter area on stream. As an example, with ten filter housings on stream, a total of thirteen filter housings, ten on stream, one in venting position, one stand¬ by, and one in the position in which the complete housing can be removed for off-line cleaning, is all that is required for continuous operation.

The critical design area of the filter system according to the invention is the sealing of the rotatable filter body to the inlet housing and to the outlet housing. Given the process conditions typical of molten polymer filtration, temperatures over 250 or 300 degree C and pressures of e.g. 300 bar, no elastomers or other flexible materials can be used to seal the arched areas typical of the invention. Metal-to-metal seals, with or without the use of temperature resistant coatings are to be applied.

The surface-to-surface seal needs to be a near-perfect one, as the smallest of gaps would, in a typical molten polymer filtration, allow polymer to penetrate the seal. Subsequently, the polymer could degrade and easily damage the seal.

The detailed design of the seal is an optimization of surface treatment and coating on the filter body and on the inlet and outlet housings, the dimensions of the rim around the arched distributors, and the forces that can be applied to seal the

unit, at the same time allowing rotational movements of the filter body at all times.

Preferably a filter system according to the invention is provided in such a way that the system is self-cleaning by means of a reverse flow of clean filtrate through the exhausted filter area. The changing of exhausted filter elements can be avoided by such a reverse flow of clean filtrate.

For further explanation and better understanding of the invention, a more detailed description follows with reference to the enclosed drawings, in which

fig. 1 schematically shows the filter system according to the invention

fig. 2 shows a cross-section of the inlet housing

fig. 3 shows an upstream top-view of the inlet housing with the arched distributor and the inlet port

fig.4 shows a side-view of the rotatable filter body with a cut-away section showing one of the filter housings, in this case with one filter elements

fig. 5 shows the upstream top-view of the rotatable filter body with the regularly distributed filter housings

fig. 6 shows a cross-section of the outlet housing

fig. 7 shows an upstream top-view of the outlet housing with the arched collector, the outlet port, and the vent opening.

Figure 1 schematically shows the filter system according to the invention with an inlet housing 1, the rotatable filter

body 2 and the outlet housing 3. The inlet housing 1 has an inlet port 11, shown in the cut-away section, through which the product flow enters the filter system. The inlet port 11 is connected with the arched distributor 10, also shown in the cut-away section. The arched distributor 10 is U-shaped, the open side of the U facing the filter body 2. Inlet housing 1 and filter body 2 are sealed surface-to-surface.

A filter housing 21, with in this case one filter element 20, is shown in the cut-way section of the filter body 2. The product flow coming from the arched distributor 10 passes through the multiple filter housings 21, hence also through the filter elements 20 in the filter housings 21.

The outlet housing 3 is shown with the arched collector 30 and the outlet port 31. The arch-shaped collector 30 is U-shaped as the distributor 10 in the inlet housing 1, with the open side of the U facing the filter body 2. The arched collector 30 is connected to the outlet port 31. The product flow coming from the filter housings 21 in the filter body 2 enters the arched collector 30 and leaves the filter system via outlet port 31.

The cross-section in figure 2 illustrates the inlet housing 1, as well as the inlet port 11 and the U-shaped arched distributor 10 in the cut-away section.

In figure 3, the view on inlet housing 1 from the upstream side shows the arched distributor 10 with the inlet port 11. At the downstream side is the sealing area 13 surrounding the arched distributor 10. The sealing area 13 seals the inlet housing to the rotatable filter body 2 (not shown in figure 3) and is the area that is critical in the design as explained above. The recess 14 provides access to the filter housing 21 in the filter body 2 (21 and 2 not shown in figure 3) due up for replacement .

Figures 4 and 5 illustrate the simple design of the filter body 2. Basically, the filter body 2 is a cylinder with multiple holes, the filter housings 21, in a circular arrangement . Figure 5 shows only one circle with filter housings 21; the invention covers similar designs with multiple concentric circles just the same. Also, figure 4 shows one filter element 20 in a filter housing 21; the invention covers similar designs with multiple filter elements just the same. Moreover, the filter element 20 is shown as a cartridge type filter element; the invention covers filter systems with differently shaped filter elements just the same. For example, a stack of filter discs could be used in every single filter housing 21.

In the design as shown in figure 4, the product flow enters the filter housing 21 on the left hand side, flows through the cylindrical filter element 20 from outside to inside, passing the filter media installed on the filter element 20, and leaves the filter housing 21 on the right hand side.

Figures 6 and 7 show the outlet housing 3, which in many aspects is a mirror copy of the inlet housing 1. The product flow enters the outlet housing 3 via the arched collector 30 and leaves the filter system via outlet port 31. the sealing area 33 seals the outlet housing 3 to the rotatable filter body 2 (not shown on figures 6 and 7) . Recess 32 serves the same goal as recess 14 in the inlet housing 1 (both not shown on figures 6 and 7) , hence, provides access to the filter body 2 to replace a filter housing 21 (both not shown on figures 6 and 7) .

Figure 7 also shows the vent outlet 34 in the outlet housing 3. The vent outlet 34 is needed to vent or bleed fresh filter housings 21 (not shown) prior to be taken on stream. Venting is critical to provide any gas, air or other, to come in contact with the product flow when taking fresh filter area on stream.

To further illustrate the procedure to renew filter area, the procedure for a filter with ten filter housings on stream is described below. When the differential pressure over the filter system indicates that a change of filter area is required, as a first step a fresh filter housing is vented. This is done by rotating the filter body slightly, to link a fresh filter housing to the distributor of the inlet housing and at the same time to the vent outlet in the outlet housing. The fresh filter housing is now one of eleven filter housings being fed from the distributor in the inlet housing. The vent outlet remains open as long as there is still gas escaping from the fresh filter housing. When the fresh housing has been vented completely and product starts to leak from the vent outlet, the vent outlet is closed. Next, the filter body is rotated a little further, to remove one filter housing completely off line, i.e. off the inlet distributor and off the outlet collector, at the same time turning the freshly vented housing on line. The fresh filter housing is now one of ten filter housings being fed from the distributor in the inlet housing. All ten filter housings discharge into the collector in the outlet housing.

To keep up a continuous flow after the filter, the incoming product flow is slightly increased during venting. It is reduced again when only ten housings are being supplied with product from the inlet distributor. Critically important is the residence time of the product used to vent the fresh housing. In the above example, the time the highly viscous fluid needs to pass the fresh filter housing is similar to the time the product flow needs to pass whatever other filter housing.