GB2214447A - Filter element - Google Patents

Abstract

A filter element for a precoat filter has a septum 10 formed with longitudinal pleats having broad roots proximal to a perforate core and narrow tips distal from the core. The element may include circumferential supporting bands 80 spaced axially along the filter element and constraining the septum tips. In use a deposit of mechanical or ion exchange precoat is applied to the element before filtering and later removed by backwashing. <IMAGE>

Description

SEPTUM FOR A BACKWASHABLE PRECOAT FILTER ELEMENT The present invention relates to precoat-type filter elements. Precoat filter units may be used in chemical processing and industrial waste treatment.

For example, diatomaceous earth coated filters may be used to filter varnishes, oils, milk, beer, fruit juices, and water. In particular, coated filters are used where, after treatment, the liquid product must be very high in purity and closely adhere to specified standards of deionization or chemical composition.

Precoat-type filters are used, for example, with ion exchange resin coatings in the treatment of boiler feedwater in nuclear steam generating systems and in the condenser stage of nuclear power plants.

Precoat filter elements generally comprise a porous support structure, termed a septum, which is coated with a filter medium, termed a precoat, which performs the filtration operation and may also interact chemically with the filtrate. A filtering material which is commonly used for many filtering applications is diatomaceous earth. Ion exchange resins may be used in water treatment applications requiring high effluent purities. In each case, the filter material may be built up on the filter septum by forming a slurry, in the same type of liquid as is to be filtered, and passing the slurry through the porous septum to cause the suspended coating material of the slurry to collect on the septum. Where diatomaceous earth is used, the coating material particles are preferably much smaller than the pores of the septum material.They bridge the septum pores to form a dense but permeable layer which, once the coating is built up to sufficient depth and the filtering operation has begun, will serve to filter out the contaminant particles from the filtrate.

Examples of liquids which may be filtered by such coated filters are oils; food products such as milk, syrup, beer, juices; water; liquid waxes; chemicals; and various chemical solutions. The quality of the particulate filtration by the coated filter is dependent on the fineness of the diatomaceous earth or other coating which overlies the coarser septum.

As the filtering operation progresses, the pressure drop over the coated filter will gradually increase until continued filtration is impractical and the coated filter must be cleaned, removing the coating material, for example, diatomaceous earth, and replacing it with a coating of fresh material. The preferred method of cleaning is generally by reversing the flow of liquid through the filter septum by reversing the pressure gradient over the filter element. The reverse fluid flow is generally driven at a higher rate than during the filtering process to remove the filter layer. Previously filtered liquid may be used for this process after which it is put in a holding tank to allow the coating material to settle out. Once the coating material has settled out, the fluid may be reclaimed as product by repeating the filtering process.Frequently, a sudden, violent reverse flow of liquid is utilized to dislodge and carry away the filter coat. Though the pore size of the coat is generally smaller than that of the septum, nonetheless, after a number of successive filtering runs and backwashings, septums will become sufficiently clogged to prevent their effective cleaning and they must be replaced.

Precoat filter elements are often utilized in the treatment of liquids which contain both suspended particulate and dissolved chemical and ionic contaminants where the treated effluent must be of a very high degree of purity and closely adhere to specified standards of deionization or chemical composition. Industrial processes utilizing steam from steam generating systems may produce condensate contaminated by corrosion products, by the inleakage of cooling water, and by various substances used in the process. Contaminants in the condensate or boiler feedwater of a nuclear steam generating system may cause corrosion and scaling of heat transfer equipment in the system, damaging heat exchange surfaces and decreasing their heat transfer efficiency.This, in turn, may result in overheating of tubes, followed by tube failure, further equipment damage, and possibly radioactive pollution of the environment. To prevent corrosion in nuclear steam generating systems, water treatment must include conditioning of the raw water supply, condensate returns from process steam or turbines, and boiler water.

A particular example of water treatment requiring stringent quality control, and in which the use of precoated filters is common, is the condenser stage of nuclear power plants. Two principal types of nuclear power plants are boiling water reactors and pressurized water reactors. Though they use different processes for generating steam to drive turbines in producing electricity and require different water chemistry, both employ similar water purification systems commonly referred to as condensate polishers.

In boiling water reactors, should particles pass through the feedwater system and into the reactor, they may cause degradation and become radioactive.

Should radioactive particles be created, they pose a costly disposal problem and may well present a threat of exposing personnel to radioactive materials. The presence of particulate contaminants in pressurized water reactors can cause stress cracks in heat exchanger tubes.

A raw water supply may initially contain many different types of dissolved and suspended matter.

Most commonly, these materials include silica, iron, copper, calcium, magnesium, and sodium compounds. The metallic constituents generally occur in combination with bicarbonate, carbonate, sulfate, and chloride radicals. Many of these materials are ionic in solution, which may be used to advantage in treating the water to achieve a high degree of purity in the effluent.

As the water may contain a wide variety of harmful contaminants, it is common to use more than one technique in its treatment to remove them.

Usually, the water is first filtered to mechanically remove the larger suspended particulate contaminants and then demineralized through an ion exchange. The demineralization process, generally known as condensate polishing, can produce water closely approaching theoretical maximum chemical purity from ionic contaminants. Condensate polishing involves a reversible exchange of ions between the liquid phase and a solid phase which occurs by virtue of the charges carried by the ions. The solid phase typically comprises an ion exchange resin saturated with ionic groups which substitute new ions for ions present in the liquid when the solid is brought in contact with the liquid.In the condensate polishing process, ion exchange may be used to treat the contaminated water in two ways; ionic contaminants in the water may be replaced by relatively harmless products of deionization and, ionic contaminants may be transformed into products which are harmless or less harmful by replacing key ions in their molecular structure with other ions which in turn react with yet other ions present in the liquid to form the relatively harmless products. In the second case, for example, the replacement ions may react with remaining ionic contaminants to form additional water.

In one process for demineralizing boiler feedwater or steam condensate, for example, it is common to provide one tank, containing a resin saturated with hydrogen ions to replace metallic cations in the water, in series with another tank, to replace anions in the water. Each resin continues ionic substitution of contaminants until its deionizing capacity is exhausted. Once exhausted, the resin is either replaced, in the case of cartridgetubular type condensate polishing systems, or regenerated to restore its ion exchange capacity, in the case of deep bed type demineralizer systems.

In condensate polishing systems used in the condenser stage of a nuclear power plant, for example, it is common to utilize a filter unit with precoat filter elements to simultaneously perform the filtration and demineralization steps of the water conditioning process. A filter element of this type of unit is typical of all precoat filter elements as described above and comprises a porous support structure, termed a septum, which is coated with a medium, termed a precoat, which performs both the filtration and ion exchange steps.In condensate polishing systems of the cartridge-tubular type, where the precoat is disposable, when the precoat becomes clogged with particulate contaminants, as evidenced by increase in pressure drop, or its ion exchange capacity is depleted, as evidenced by effluent water chemistry, it is discarded in a backwash operation in which the precoat is stripped off the septum and flushed out of the system. A new resin precoat is then applied to the septum.

A typical backwashable precoated filter unit 100 used in demineralization applications is shown schematically in Figure 1. The filter unit 100 has a housing comprising a pressure vessel 101 which has an inlet 102 and an outlet 103. A tube sheet 104, is secured at its periphery to the inside wall of the vessel 101, and divides the vessel into a lower low pressure chamber, or plenum 105, and an upper high pressure chamber 106. The outlet 103 communicates the plenum 105 with the exterior of the vessel. The upper chamber 106 communicates with the exterior of the vessel via an aperture in the tube sheet 104 and the inlet 102.

Multiple filter elements 109 are located within the upper chamber 106. Each of the filter elements 109 includes a stand-off tube 111 which passes through a hole in the tube sheet. Each filter element 109 is supported by the tube sheet 104 and a seat 110 at the base of its stand-off tube 111. Each hole is circumscribed by a gasket (not shown) to provide a seal between the tube sheet 104 and the stand-off tube 111. Each filter element 109 comprises a hollow core (not shown) and a porous septum 112 which can support a resin precoat. When the pressure in the upper chamber 106 is greater than that in the plenum 105, the porous septum allows water to flow into the core. The stand-off tube 111, extending vertically downward from the core serves as a discharge passage for filtrate from the element 109 into plenum 105.

The filter unit 100 of Figure 1 incorporates a baffle plate 113, typical of such precoat filter units, in the upper chamber 106 of the pressure vessel 101, above the tube sheet 104 and over the inlet aperture 107. The baffle plate 113 is supported in a spaced relationship with the tube sheet to provide an annular space about the periphery of the plate through which water may pass. Thus, as water enters the upper chamber 106 through the inlet 102, the flow encounters the baffle plate 113 and flows through the annular space and throughout the upper chamber 106.

The baffle plate is intended to reduce turbulence in the lower central portion of the chamber near the inlet, in particular, and throughout the chamber, in general.

Three basic, commonly used precoat filter septa are the yarn wound perforated core, coarse mesh, and porous metal cartridge types. Generally, these septa are of cylindrical configuration, though some may be of other configurations. Yarn wound perforated core septa typically comprise a perforated cylindrical stainless steel hollow core wrapped with protective string or yarn windings. Polymeric yarns, such as nylon or polypropylene, are commonly used. These yarns typically have a diameter of 1.6 millimeters (1/16 inch) and are wrapped around the hollow core to provide a septum depth of approximately 1.27 centimeters (1/2 inch). Once the contaminated water has flowed through the precoat, it is intended to pass through the microporous openings of the yarn filaments rather than through any spaces between adjacent strands in the yarn winding.Coarse mesh septa include septa made of polypropylene mesh and wire mesh. Porous metal cartridges generally comprise a filtration medium of fine metal particulates sintered or otherwise bonded together. Coarse mesh septa and porous metal cartridges may be wound with yarn.

Precoats for these filters typically comprise a slurry of ion exchange resin particles suspended in a deionized water base. The suspension is formulated with a predetermined ratio of cation and anion particles, depending on the intended application. The operation of the filter unit 100 of Fig. 1 may be divided into two stages: (1) a precoat stage and (2) a filtration stage. Generally, each filter septum 112 is precoated by introducing a flow of precoat resin slurry through the inlet 102 past the baffle plate 113 and through the filter elements 109 to accumulate a precoat layer on the upstream surface of the septum 112. The resin particle size distribution, flow rate, and proportion of flocculants are optimized to achieve proper precoat.The precoat layer is then compressed on the surface of the septum 112 of each element 109 by continuously circulating deionized water through the coated septum briefly at the process flow rate. A good precoat should exhibit an even thickness along the entire length of each of the septa, experience no erosion of the cake during precoat or treatment cycle, have no radial or axial cracks, and be of uniform thickness on all septa in the filter unit housing.

During the water treatment stage, the contaminated water flows into the vessel 101, through the inlet 102, past the baffle plate 113 and into the upper chamber 106. The water contacts the resin precoat on the surface of each septum 112 and flows radially inward, first through the precoat and then through the septum 112. As the water flows through the resin in precoat, the specially formulated ion exchange resin particles remove or transform minerals and other ionic contaminants in the water by the processes described above. The ion exchange precoat also acts as the filtration medium and typically has a finer pore structure than that of the septum 112.

Ideally, particulate contaminants are captured in the precoat and prevented from penetrating into the septum 112. The treated filtrate then passes through the septum 112 to the core of the filter element 109, flows through the stand-off tube 111, into the plenum 105, and exits the vessel 101 through the outlet 103.

As the precoat continues to capture particulate contaminants during the demineralizing operation, the pressure difference across the filter unit 100 required to maintain a given flow rate increases until it reaches a level at which the filtration operation becomes too inefficient, and, as ion exchange continues, the ion exchange capacity of the resin is depleted, as evidenced by the change in water effluent chemistry. At that time, the treatment stage is discontinued, and the filter unit 100 is subjected to a backwash operation in which water is flushed through the unit in the reverse direction. The exhausted precoat is stripped off each septum 112 and flushed out of the system by the reverse flow. A new cycle is then begun with a new precoating operation.

Known demineralization precoat filter systems of this type have several defects. Many prior art precoat filter units are inefficient because backwash frequency is dictated by occurrence of the maximum allowable pressure differential prior to exhaustion of the precoat. The amount of contaminant which can be held within the precoat while maintaining the high flow rate necessitated by the limited flow area afforded by cylindrical septa within the differential pressure limitation is relatively small. The resin precoat, though clogged with particles, is not exhausted of its demineralizing properties and the precoated filter is utilized to filter less liquid than it could demineralize. Thus, backwashing frequently is dictated by the pressure differential, useful precoat is wasted, and the amount of radioactive waste which must be disposed of is increased.

The present invention provides a filter element for a precoat type filter unit, the filter element comprising a central cylindrical core having an upper end and a lower end, a central drainage space, and a perforate wall to allow filtrate to flow into the drainage space and thence to an end of the core and a septum surrounding and generally coaxial with the core, the septum including longitudinal pleats which support a precoat and have roots proximal to the core wall and tips distal from the core wall.

The present invention also provides a filter element in a precoat type filter unit wherein a precoat slurry can be passed along a flow path through the filter unit, the filter element comprising a septum disposed across the flow path and including longitudinally extending pleats having an upstream surface which accumulates a precoat on the upstream surface of the pleats.

The present invention further provides a method for treating a liquid comprising the steps of passing a precoat slurry in a first direction through a pleated septum to accumulate a precoat on the pleats of the septum, passing a liquid in the first direction through the precoated pleats of the septum to treat the liquid with a precoat material, and passing a liquid in the opposite direction through the precoated pleats of the septum to strip the precoat from the pleats of the septum.

Because the pleated septum has a greater surface area than cylindrical septum of the present art, the filter element and method provided by the present invention have a longer life and greater efficiency than those of the prior art.

The pressure difference across the element is reduced for a given flow rate during filtration treatment operations providing greater precoat contaminant capacity. Thus, the filter element and method of the present invention require less frequent backwashing and effectively utilize a septum while maintaining the pressure difference across the septum during filtering, backwash, and precoat operations within acceptable differential pressure ranges.

The large surface area of the pleated septum provides greatly enhanced performance. Depending on the element diameter envelope, a pleated septum may have twice the area of a cylindrical septum, thus cutting the flow density in half. Because dirt capacity is highly dependent on flow density, cutting the flow density in half nearly doubles the dirt capacity per unit area of the septum and precoat.

Thus, the combined effect of doubling the area of the septum increases the dirt holding capacity of the precoat by a factor of four.

This increased dirt capacity of the element may be traded to allow use of a finer pore size septum without exceeding the pressure differential limitations. This results in important advantages.

With a coarse septum element, low flow rates must be maintained for a prolonged period of time after the precoat cycle to prevent excessive resin leakage from the precoat through the septum. A finer septum allows the initial treatment flow rate to be increased and the vessel brought on line much more quickly. Resin leakage during the continuing treatment cycle is also reduced. Resin leakage results in a build-up of resins downstream of the demineralizer which can cause erosion and reduce efficiency of the heat exchange equipment.

Figure 1 is a half section schematic view of a precoat filter unit of the prior art.

Figure 2 is a cut-away perspective view of a support and drainage core and surrounding septum of an embodiment of the present invention.

Figure 3 is a sectional view of the septum and drainage core of an embodiment of the present invention, taken normal to the axis of the septum and core, and showing a portion of a precoat applied to the septum.

Figure 2 shows an exemplary precoat filter element 1 embodying the present invention. The precoat filter element of the exemplary embodiment generally comprises a septum 10 of longitudinally pleated form which is coaxially mounted on a perforated metal core 20. The core comprises a perforate cylindrical wall 24 defining a drainage space therein. The core 20 is mounted on a base cap 30. A standoff tube 40 extends from the base cap vertically downward and is in fluid communication with the drainage space. The perforated cylindrical metal core 20 may be made of any suitable material, for example, from stainless steel. The core 20 has two caps, an upper end cap 35 and a base cap 30. The upper end cap 35 is disposed at the upper end of the core and the base cap 30 is disposed at the lower end of the core.The caps are fixedly secured to the cylindrical core wall 24 by welding or any other suitable bonding technique, for example, resin bonding.

Figure 3 is a schematic cross section of the septum 10 of Figure 2, from a cutting plane normal to the septum axis, and like reference numbers in Figures 3 and 4 denote the same elements. Generally, the septum 10 includes longitudinal pleats 12 having roots 14 tangential to the cylindrical core wall 24 and tips 16 distal from the core wall. The septum, a portion of which is shown coated with a precoat, denoted 500, just prior to filtering operations, may be formed of any suitable material, for example, porous metal membrane material, PMM, marketed by Pall Corporation.

A septum material can be chosen for a particular application depending on the nature of the precoat resin mixture which is to be used and the chemistry of the water to be treated.

Circumferential bands 80, as seen in Fig. 2, may wrap around the pleats 12 at intervals along the vertical length of the septum to support the pleats against bending stresses. These bands may be made of any suitable material, for example, stainless steel.

The bands may be attached to the tips 16 by any suitable means, for example, press fitting.

The pleat design must satisfy the requirements of maximizing the filtration area while allowing adequate space between the pleats for deposition of the precoat. The additional requirement of providing adequate mechanical strength to withstand the stresses of the backwash cycle, which may subject the septum to differential pressures of 2.1 to 6.9 bars (30 to 100 psi), must also be satisfied. An analytical optimization routine is used to determine the number of pleats and the pleat height for a preferred embodiment based on meeting the criteria of minimum precoat thickness and maximum allowable bending stress while achieving the maximum pleat area for filtration.

Given the center-to-center spacing of the elements within the upper chamber of a filter unit housing and the desired precoat thickness, the outside diameter envelope within which the pleated element must fit can be determined. The given filter media composite for the particular application will establish the minimum thickness of the resin precoat. As illustrated in Figure 3: t = filtration media, support and drainage composite thickness, meters (inches).

OD = outside diameter envelope within which the pleated element should fit, meters (inches).

CD = core diameter of element consistent with filtrate flow rate and pressure drop criteria, meters (inches).

TR = bend radius at the tip of the pleat as determined by filter media composite physical properties and tooling criteria, meters (inches).

PT = minimum precoat thickness, meters (inches).

PD = maximum pressure drop the element is subjected to, bar (psi).

SS = permissible bending stress in any pleat, bar (psi).

For purposes of optimization of the pleat configuration: N = number of pleats in the element.

CP = circular pitch of pleats = 2 ir/N radians.

b = spacing of circumferential bands along the septum axis, meters (inches).

h = pleat height, meters (inches).

PR = pleat radius, meters (inches).

S = bending stress along the radial direction induced in a pleat h inches high over the distance b between bands, bar (psi).

By interactive methods, the number of pleats N and the core diameter CD can be determined which will achieve a pleat radius PR greater than or equal to the minimum precoat thickness PT and the induced stress along the outward radial direction of the pleat S less than or equal to SS. By modeling the pleat as a simply supported beam, with a beam span equal to the pleat height, h, a beam depth equal to the material composite thickness, t, and a beam width equal to the spacing between the circumferential bands, b, and applying beam flexural theory, the maximum bending stress, S, may be found in terms of known variables as: 3 ~ PD ~ h3 S = 4 . b . t2 The optimization routine gives a pleat shape relatively open at the root of the pleat for optimum precoat application.Though the routine develops a pleat root of generally circular cross section, in consideration of the ease of tooling, the pleat root could be of some other cross section, for example, elliptical or trapezoidal.

As an example of the improvement made possible by the pleated septum, consider a cylindrical element having a 5 centimeter (2 inch) outside diameter and a core diameter of 2.82 centimeters (1.109 inches), which is required to operate with a minimum precoat thickness of 5 millimeters (0.2 inch) and a backwash pressure of 6.9 bar (100 psi) while not exceeding a maximum allowed bend stress of 1034 bar (15,000 psi).

Such an element can be replaced with a pleated element, according to the present invention, having 11 pleats with a pleat height of 1.13 centimeters (0.446 inch), providing a minimum precoat thickness of 5.21 millimeters (0.205 inch) and experiencing maximum induced stress of only 828 bar (12,000 psi). This element provides an increase in surface area of 89% over the cylindrical element.

By providing increased surface area, the pressure drop for a given flow rate is reduced. This allows use of a septum of a finer pore rating. Example of septa materials are fine mesh porous metal membranes such as Pall Corporation's PMM, sintered woven wire meshes Supramesh, Rigimesh, and fiber metal. PMM, Supramesh, and Rigimesh are registered trademarks of Pall Corporation.

The increased surface area of the pleated septum can support a greater volume of precoat than a cylindrical septa. This increases precoat volume and contaminant capacity. As discussed above, flow density is also halved, doubling the dirt capacity.

The combined effect is an overall fourfold increase in dirt capacity for the coated element. The increased area allows the optimization of the septum and process parameters to provide that backwashing frequency is determined by depletion of the deionization capacity of the resin rather than by increased pressure drop due to precoat pluggage. The reduced frequency of backwashing which results reduces septum wear and increases operative life of the filter elements. The increase in surface area and resulting reduction in flow density also provides a longer residence time for contaminants in the precoat itself affording more contact time between ionic contaminants and the ion exchange resin. This results in improved precoat utilization.

The longer life of the elements, which reduces the replacement rate of the elements, may reduce overall element replacement cost. More importantly, there is a reduction in rad waste disposal costs for used elements. As a result of full utilization of resin deionization capacity, the reduction in precoat volume which must be disposed of also produces significant savings in radwaste disposal costs.

For example, in the analysis of one particular power plant application, it was found that the cycle life of the precoat could be extended from 15 days up to 60 days which would reduce resin radwaste and resin replacement costs by half. Further, doubling of the surface area of the septa reduces the possibility of septa fouling, ensuring a long life cycle, and eliminates resin leakage. The cost of replacing sepia tubes, as well as the cost of disposing of old tubes, is reduced and radwaste handling further minimized.

In this particular study, it was estimated that a minimum saving over two years of operation could amount to more than half the cost of installing septa comprising the present invention.

While an exemplary precoat filter element embodying the present invention has been shown, it will be understood that, of course, the invention is not limited to that embodiment. Modification may be made by those skilled in the art, particularly in light of the foregoing teachings. It is, therefore, contemplated by the appended claims to cover any such modification which incorporates the essential features of this invention or encompasses the true spirit and scope of the invention.

Claims (16)

1. A filter element for a precoat type filter unit, said filter element comprising: a central cylindrical core having an upper end and a lower end, a central drainage space, and a perforate wall to allow filtrate to flow into the drainage space and thence to an end of the core; and a septum surrounding and generally coaxial with said core, said septum including longitudinal pleats which support a precoat and have roots proximal to the core wall and tips distal from the core wall.

2. A filter element as in claim 1 wherein said roots have a radius of curvature larger than a radius of curvature of said tips.

3. A filter element as in claim 1 or 2 wherein said septum comprises a porous metallic material.

4. A filter element as in claim 3 wherein said metallic material includes a porous metal membrane.

5. A filter element as in claim 1, 2, 3, or 4 further comprising circumferential supporting bands spaced at intervals along the length of the filter element, said bands coaxial with the core and constraining the septum tips.

6. A filter element as in claim 1, 2, 3, 4, or 5 for use with a deionizing precoat in which the shape and number of longitudinal pleats lying about the circumference of said septum is selected to provide an optimum relation between a surface area of said septum and a maximum thickness of a precoat allowed by the space between said pleats to result in a maximum precoat life as determined by exhaustion of precoat deionizing capability.

7. A method for treating a liquid comprising the steps of: passing a precoat slurry in a first direction through a pleated septum to accumulate a precoat on the pleats of the septum; passing a liquid in the first direction through the precoated pleats of the septum to treat the liquid with a precoat material; and passing a liquid in the opposite direction through the precoated pleats of the septum to strip the precoat from the pleats of the septum.

8. A method as in claim 7 further comprising the step of compressing the precoat on the pleats of the septum.

9. A method as in claim 7 or 8 wherein the step of passing a liquid in the first direction through the precoated pleats of the septum includes capturing particulate contaminants.

10. A method as in claim 7, 8, or 9 wherein the step of passing a liquid in the first direction through the precoated pleats of the septum includes treating a liquid with an ion exchange material.

11. In a precoat type filter unit wherein a precoat slurry can be passed along a flow path through the filter unit, a filter element comprising a septum disposed across the flow path and including longitudinally extending pleats having an upstream surface which accumulates a precoat on the upstream surface of the pleats.

12. A filter element as in claim 11 wherein each pleat includes a root having a radius and a tip having a radius smaller than the radius of the root.

13. A filter element as in claim 11 or 12 further comprising a perforate core disposed coaxially within the septum.

14. A filter element as in claim 11, 12, or 13 wherein the root of each pleat is tangential to the core and the tip is distal from the core.

15. A filter element substantially as herein described with reference to the accompanying drawings.

16. A method for treating a liquid substantially as herein described with reference to the accompanying drawings.