Coating processes – Foraminous product produced – Filter – sponge – or foam
Reexamination Certificate
2000-12-21
2003-01-14
Pianalto, Bernard (Department: 1762)
Coating processes
Foraminous product produced
Filter, sponge, or foam
C427S365000, C427S366000, C427S371000, C427S385500, C427S395000
Reexamination Certificate
active
06506449
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a method and equipment for making the static filtration media disclosed in U.S. application Ser. No. 09/506,575 filed Feb. 18, 2000, the disclosure of which is hereby incorporated by reference herein. For example the invention is useful in the manufacture of the following preferred embodiment of the substrate from the application Ser. No. 09/506,575:
Polyester, or other fiber, non-woven fabric about 4 to 7 ounces/sq. ft. when compressed and about 8 mm thick
Functional Coating equal to about 100 to 200% of the uncoated fabric weight, comprised of:
FDA compliant vinyl acrylic binder about 10 to 20% of the coating by weight
Coconut shell activated carbon about 60 to 85% of the coating by weight
Zeolite molecular sieve about 0 to 25% of the coating by weight, preferably about 5-20%
A useful term to describe a filter medium, which can be operated in a static manner, is the ratio of “readily deliverable fluid volume” (RDV) to total bed volume (BV). Readily deliverable fluid volume is defined here as the volume of fluid, which will drain from a decanted filter bed without the application of any external force (other than gravity). Static filters typically exhibit RDV/BV ratios from 30 to 80 percent, as measured from the cessation of streaming flow.
Traditional filtration devices cannot be operated effectively in a static manner, because the extra-particle bulk volume in a packed bed is very small relative to the bed volume. The RDV/BV ratio of a granular activated carbon bed packed with 12×30-mesh carbon is typically 9 percent for a cylindrical bed around 8.5 inches in depth and 4.5 inches in diameter. The argument cannot be made that a packed bed overlaid with a column of fluid constitutes static treatment, as the mean distance between a fluid molecule and an adsorptive site is too large to allow for treatment within a reasonable amount of time. In addition, in such a system the tortuosity of the fluid path between the particles of the packed bed would hinder diffusion to the point of making the majority of the bed inaccessible to adsorption.
Compression is used to reduce the average voids which hold the water, to be approximately 6-7 (e.g. about 6.5) *10
−8
liters in volume. This equates to a RDV/BV ratio in the neighborhood of 63% for a cylindrical bed around 8.5 inches in depth and 4.5 inches in diameter, as measured from the cessation of streaming flow. Larger voids are tolerated if residence time between use is not a priority. Controlling the RDV/BV ratio rather than the minimum fill and pour rate, addresses a weakness inherent in EP 0402661.
A significant improvement in the treatment material described in EP 0402661 is obtained by a post impregnation modification of the substrate. The capacity of the media is significantly increased by narrowing the void size distribution about what is believed to be an optimum value. Fill and pour rates are determined to a large degree by the size of the largest voids in the non-woven. Capacity on the other hand is determined by smaller relative percentages of the smallest voids, since capacity is determined by the ability of the fluid to freely flow from the media and not be held by capillary forces. (Capacity in the context of this application refers to the volume the filter is able to deliver on demand, rather than amount of contaminant removal which can occur over the life of the filter). In effect what is desired is to have a medium which has a narrow size distribution of voids just large enough to maintain the rapid fill and pour rates. By employing a base material which has a void size distribution in a range where the smallest voids are excluded, compression can be used to preferentially collapse the larger voids. This yields a product with the narrowest void size distribution, centered around an optimal size value for production capacity, flow rate, and removal rate. This effectively alleviates a static-filter size restriction which exists if the fabric contains a large number of small voids, and creates an improvement in efficiency over a product which contains larger volume voids. Without this technique it becomes more difficult to fabricate a static filter which can still pour effective volumes when in a small size configuration.
The diffusion of material from inside a well mixed sphere of fluid to it's sorptive outer boundary can be modeled by
∂
C
∂
t
=
D
(
∂
2
C
∂
r
2
+
2
r
∂
C
∂
r
)
where C is the concentration within the fluid, t the time the fluid has been within the sphere, D the diffusion coefficient of the chemical dissolved in the fluid, and r the radial dimension within the sphere. A mathematical transformation facilitates the solution of this problem.
By setting
u=Cr
the differential mass balance (Equation 1) becomes
∂
u
∂
t
=
D
∂
2
u
∂
r
2
which is easily solved[1] for the ratio between the total mass of chemical which has left the sphere at a given time and the amount that would eventually be removed
m
t
m
∞
=
1
-
6
π
2
∑
n
-
1
∞
1
n
exp
(
-
Dn
2
π
2
t
a
2
)
where m
t
is the mass removed at time t, m
∞
the mass which would be removed at equilibrium, and a the radial dimension at the sphere surface.
FIG.
1
: The theoretical effects of compression on performance.
TABLE 1
The theoretical effects of compression on performance.
Percent Contaminant Removal
Time (min)
Uncompressed
Compressed
0.0
30.63
32.86
0.5
70.16
75.97
1.0
78.68
85.26
1.5
83.44
90.08
2.0
86.53
92.93
2.5
88.70
94.75
3.0
90.33
95.98
It will be seen from the results of this model (Equation 1) that the size and distribution of void spaces within the filter media has a marked affect on the performance of the filter with respect to contaminant removal. If one assumes a Weibull distribution of void sizes with parameter ‘a’ of 20 and a parameter ‘b’ of 1.05, the kinetics of removal are shown in Table 1 and FIG.
1
. In this figure we see that by shifting just the largest 10% of voids to the median value of the previous distribution by selectively collapsing some of the larger void spaces, we obtain a significant improvement in the amount of contaminant removed in a given time. Table 1 lists the percentage removal for the theoretical system at several time intervals. If the treatment objective is to reach 90% removal of a particular contaminant (as is required for certification by the NSF[2] for lead removal), the figure illustrates that this improvement may be dramatic in terms of ease of use of the filter. In this example the uncompressed media takes twice as long to reach compliance.
TABLE 2
The effects of compression on performance.
Contaminant Concentration (&mgr;g/mL)
Compression Level (%)
Influent
Effluent
12
185
30
37
185
25
51
185
8
Laboratory analysis of media performance under various levels of compression were performed to validate the theoretical model, with these results presented in Table 2. A controlled volume vessel was packed with media compressed from 100% to around 50% of it's original thickness. Media which was midway through it's useful life was used for testing at exposure times of 3 minutes, in order to yield effluent concentrations which were detectable by anodic stripping voltametry. The performance of the media for lead removal was shown to increase dramatically with compression, moving from a low tested value of 84% removal at 12% compression to a maximum tested value of 96% at 50% compression.
The preferred manner of manufacture of coated non-wovens such as those which can be used in static filtration, are to draw rolls of a web (e.g. non-woven fabric) through a dipping bath where the materials the fabric is to be coated with are suspended. For purposes of the invention the bath contains water treatment materials exemplified by: activated carbon, ceramic cation-exchangers such as
Larsen Gerald J.
Mierau Bradley D.
Nohren, Jr. John E.
Reid Henry C.
Innova Pure Water Inc.
Nixon & Vanderhye P.C.
Pianalto Bernard
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