Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVED COMP08ITIOH l,HD DSBIGN
OF A FILTER F18RIC UBEBUL IN PULP
AHD PAhE33 ~IN~3 APP~1RATU8
. 1 ~~11C1C~iR0 OF T88 INPE19TIOH
2 1. Meld of the Invention.
V
3 The present invention relates to filter
4 fabric which is particularly useful in a rotary drum
vacuum separator washer or the like for use in pulp
6 and paper making apparatus.
7 2. Descr~otion~,of the Prior Art.
g For many years, rotary drum filters have
9 been used in the pulp and paper industry to separate
suspended and processed pulp particles from the
11 fluids (black liquor) used in the manufacture of
12 paper. These filters operate on the principle of
13 the bottom arc of a rotating drum contacting a
14 reservoir of pulp suspended in liquid and applying
a vacuum to draw the pulp suspension onto the drum
16 and to draw the fluid through a perforated screen
17 surface such as a fabric or wire cloth which covers
18 the drum. The solids are thereby deposited on the
19 covering filter while the process liquors are drawn
into the core of the drum. At a later point in the
21 drum rotation cycle, the deposited cake or layer of
22 pulp is removed from the surface of the filter.
23 Removal is accomplished by means such as a doctor
24 blade or board set in close proximity to or against
the "surface" of the drum (see for example, U.S.
26 Patent No. 4,505,137, the disclosure of which is
27 incorporated by reference and U.S. Patent Nos.
1
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1 3,363,744: 3,403,786; 3,409,139 and 4,138,313). The
2 composition and design of the drum covering plays an
3 important role in separating the object (pulp) from
4 the process liquids. This filtering device must
have sufficient tortuosity to effectively remove the
6 suspended particles while simultaneously allowing a
7 large volume of the liquid fraction to flow through
8 the medium and into the drum core.
9 The continuous cycle of paper-making process
imposes a high degree of fatigue on the filter
11 device covering as a result of the constant
12 flexing/compression of the rotation. In addition,
13 the filter cover is continually abraded by hard
14 deposits in the solids of the filtrate such as
mineral deposits and other foreign material, as well
16 as by the pulp removal mechanism.
17 Further demands are also made on the filter
18 cover since the process takes place at elevated
19 temperatures and the aqueous suspension frequently
contains harsh chemicals such as bleach and
21 chlorides from bleach, peroxides, processing aids
22 and acids.
23 These hostile environments may shorten the
24 useful life of the filter.
The trapping of particles of mineral deposits,
26 lignin-based products or particles from corrosion
27 by-products within the matrix of the filter layer is
28 especially deleterious: such precipitates tend to
2
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1 adhere to the filaments of the filter, thereby
2 "blinding" or plugging the filtration process.
3 When any of these hostile conditions render the
4 filter wrap ineffective, the overall manufacturing
process must be stopped and a replacement filter
6 wrap must be installed. This obviously entails the
7 high cost of lost production as well as the expense
8 to install a new filter cover.
9 It has been a continuing goal of the industry
to extend the useful life of the filtering medium.
11 Moreover, the trend in the pulp processing industry
12 is towards increasingly stressful and stringent
13 operating conditions and temperatures.
14 Thus, advances in both the composition and
design of the filter layer covering these rotating
16 drums is desirable to minimize both the plugging and
17 mechanical and/or corrosive wear, i.e. to extend
18 their useful life.
19 In terms of composition, these filter matrices
are generally made of either stainless steel or
21 extruded polymer. The typical format of the filter
22 materials is a woven textile-type mat or screen.
23 It should also be noted that U.S. Patent No.
24 5,407,736 - McKeon describes the use of a
monofilament formed from the combination of a
26 polyester resin and a fluoropolymer resin for a
27 paper making fabric having improved abrasion
28 resistance.
3
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1 Each of these other prior types of composition
2 has advantages and disadvantages.
3 Stainless steel is heavy, and very difficult
4 and costly to install. Stainless steel can handle
a wide range of operating temperatures and
6 conditions. However, the most serious drawback of
7 stainless steel is that mineral deposits and other
8 sticky contaminants readily adhere to the matrix,
9 causing pluggage and stoppage and resulting process
downtime.
11 Polymeric filters, generally made of woven PVDF
12 monofilament, are lightweight, less costly and easy
13 to handle and install. While not as temperature
14 tolerant as stainless steel, PVDF filter fabrics
nevertheless withstand operating temperatures up to
16 275 degrees F. PVDF filter fabrics are not quite as
17 smooth as stainless steel mesh since the brittle
18 skin of the monofilament is frequently fibrillated
19 during both the weaving as well as in actual use.
As in the case of stainless steel, the most
21 serious disadvantage of PVDF is the adherence of
22 sticky components from the pulp slurry to the
23 monofilament yarns of the fabric. The hairy fibrils
24 may exacerbate this adhesion, which eventually leads
to the plugging and shortened life of the filter.
26 SUMMARY OF THE INVENTION
27 It is an object of the present invention to
28 provide a filtering medium particularly suited for
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1 use in vacuum separators of the type used in pulp
2 and paper making processes and apparatus.
3 It is a further object of the present invention
4 to provide a filtering medium exhibiting an increase
in useful temperature and pH range as compared to
6 prior filtering mediums used in paper making
7 processes.
8 It is a still further object of the present
9 invention to provide a filtering medium for use in
paper making apparatus or the like which exhibits
an
11 extended useful life and is less susceptible to
12 plugging of openings as compared to prior media used
13 for that purpose.
14 In accordance with a preferred embodiment of
the present invention, a filtering medium suited
for
16 use on a rotary drum vacuum filter for processing
17 pulp comprises a fabric comprising extruded
18 monofilament E-CTFE fiber, preferably woven of
19 fibers of approximately 12 mil diameter.
~FSCRIPTION OF DRAWING
21 Figure 1 is a plot of test results for two
22 filter media, a prior art filter made of KYNAR~ PVDF
23 filter material (manufactured by ATOCHEM) and a
24 filter made of HALAR~ E-CTFE fiber according to the
present invention (manufactured by Ausimont,
26 U.S.A.).
27 DESCRIPTION OF PREFERRED EMBODIMENT.
28 A filter medium suitable for use in vacuum
29 separators of the type used in paper making
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1 processes comprises a fabric woven of extruded
2 monofilament E-CTFE. The fabric is wrapped around
3 a conventional vacuum drum assembly.
4 In order to provide an increase in both the
useful temperature and pH range of the filter,
6 fabric constructions similar to PVDF filter fabric
7 was woven of HALAR~ E-CTFE polymer and tested.
8 The HALAR and KYNAR filter fabrics which were
9 tested were made with identical yarn size and fabric
construction. The fabrics were KYNAR and a HALAR
11 Twinflex filtration fabric made by Barrday, Inc. of
12 Cambridge, Ontario, Canada. Twinflex weave is a
13 design developed by Barrday in which face and
14 backing fabrics are woven together. The face side
of the fabric is a plain weave and provides the
16 filtration function. In plain weave, each weft yarn
17 passes successively over and under each warp yarn
18 with each row alternating. The back side of the
19 fabric is a leno weave and it adds strength and
stability to the fabric. The size of the yarn is 12
21 mil (12/1000") in both cases. Mesh opening is the
22 distance between two adjacent yarns expressed in
23 "mils." Open area in a fabric is an indication for
24 straight through drainage. The open area of a
fabric is calculated as follows:
26 open area (%)_ [1-(warp count) x (warp
27 diameter)] x [1-(weft count) x (weft
2g diameter)] x 100.
29
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1 The fabric specifications
were as follows:
2
3 Style MFN-2801 MFN-3901
' 4 Material Kynar Halar
Weave Dual Layer Dual Layer
6 Thread Count 42 Warp x 30 42 Warp x 30
7 Face Cloth Fill Fill
8 Yarn Diameter 12 Warp x 12 12 Warp x 12
9 (mil) Fill Fill
Mesh Opening 12 Warp, 12 Warp,
11 (mil) 21 Fill, 21 Fill,
24 Diagonal 24 Diagonal
12 % Open Area 32 32
13 Shrink 10 to 12% Warp 10 to 12% Warp
2 to 3% Fill 2 to 3% Fill
14 Flow Rate xxxx GPM, xxxx GPM,
CFM > 1000 CFM >~ 1000
Weight 18.5 oz/yd~ 18.5 oz/yd=
16 pH Operating 1 through 14 1 through 14
17 Range
18 Maximum 265F, 129C 300F, 150C
19 Operating
Temperature
21
22 The capability
of higher temperature
range was
23 verified in actual field use, as might be expected
24 from the higher melting point of HALAR~ E-CTFE
polymer (464 degrees F) as compared to PVDF.
26 The useful pH range Was also broadened by -
27 converting to a fabric constructed wholly of HALAR~
28 E-CTFE polymer.
29 To verify proper flow through of process
liquids and filtration efficiency, identical fabric
31 weave constructions of HALAR~ E-CTFE and PVDF were
( 32 also compared in short-term testing and indeed, no
33 significant differences were found.
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1 Thus assured that the conversion to an
2 identical fabric construction made of HALAR~ E-CTFE
3 could be made to take advantage of the higher
4 temperature resistance and the wider pH tolerance
offered by HALAR~ E-CTFE, the change was made and
6 side-by-side long term field testing was initiated.
7 Surprisingly, however, these actual field
8 trials revealed an unexpected phenomena. Referring
9 to Figure 1, the relative liquid flow rates (initial
flow rate taken as 100%) over a period of twenty
11 four weeks was plotted for similar filter made of
12 Ausimont HALAR~ E-CTFE fiber and ATOCHEM KYNAR~ PVDF
13 fiber. As the exposure time for the two fabrics
14 lengthened, the PVDF fabric construction began to
plug, or blind at a rate consistent with typical
16 performance of PVDF fabric weaves in this end use
17 and under these conditions. The HALAR~ fiber,
18 however, maintained a surprisingly higher flow
19 through rate, essentially exhibiting very little
change in performance. These actual field test
21 data, and the slopes of the flow-through performance
22 curves are depicted graphically in Figure 1.
23 This data shows that the same sticking
24 phenomena that plugs PVDF and stainless steel filter
fabrics is substantially minimized or eliminated
26 when a HALARm fiber is employed, even under the same
27 use conditions. This surprising result was not
28 indicated by initial flow-through measurements, nor
8
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1 would this behavior have been predictable from the
2 chemistry of the two fabric constructions.
3 This unexpected degree of less plugging
4 substantially extends the useful life of the E-CTFE
filter fabrics.
6 This result was completely unexpected since
7 both the closely chemically related E-CTFE and PVDF
8 fluoropolymers have very similar surface energy and
9 coefficients of friction (drag).
While the monofilament yarns of the E-CTFE
11 fabrics did have less fibrils on the surface (E-CTFE
12 is less brittle than PVDF), this is believed to have
13 only a minor effect.
14 To better understand this unexpected result,
the tested fabric constructions were then dried and
16 the fiber surface morphology was examined by Atomic
17 Force Microscopy. This very advanced technique of
18 examining surface roughness revealed new information
19 not normally available to those skilled in this art.
It was found that the E-CTFE fiber surfaces were
21 discernibly smoother and they thus provide much
22 fewer sites than the PVDF filaments for particle
23 anchorage and buildup.
24 The reason for the ultra-smooth extruded
surface of HALAR~ E-CTFE monofilament is still not
26 completely understood. However, it is clear that
27 the HALAR yarn surfaces would be much easier to keep
28 free of contamination buildup. In turn, this would
29 maintain the E-CTFE woven fabric filter
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effectiveness while greatly extending its useful
life.
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