Textiles: fluid treating apparatus – Machines – Combined
Reexamination Certificate
2001-02-12
2003-10-21
Stinson, Frankie L. (Department: 1746)
Textiles: fluid treating apparatus
Machines
Combined
C068S020000, C068S202000
Reexamination Certificate
active
06634192
ABSTRACT:
FIELD OF INVENTION
The present invention is directed to a system and method for manufacturing filtration media.
BACKGROUND
In the manufacturing process for filtration membrane materials, it is sometimes necessary to flush a continuous web of membrane material wetted with a first solvent in a bath containing a second solvent. This may be done to remove residues of other solvents left over from previous steps in the manufacturing operation. It may also be done to impregnate the web with other chemicals to impart different mechanical or physical properties, such as hydrophilicity, hydrophobicity, surface charge, ion exchange capabilities, strength and appearance, to the membrane material.
Two methods have been predominantly used to flush continuous webs of membrane material. In the conventional “mass transfer” technique, the first solvent-wetted web is simply soaked in a bath containing the second solvent. The period of time required for soaking is dependent upon the diffusion properties of the solvents and the effective area of the membrane-bath interface. The process is relatively slow since no driving force other than diffusion is used to move the first solvent out of the membrane material and replace the first solvent with the second solvent. For thin membrane materials, the diffusion-induced driving force for the mass transfer process can be approximated according to Fick's First Law as:
J
net
=−D&Dgr;C/&Dgr;x
(1)
where: J
net
is the net diffusional flux, &Dgr;C is the difference in concentration between the two regions separated by a distance of &Dgr;x, and D is the “diffusion coefficient”, a proportionality constant with dimensions of cm
2
/sec.
A second process commonly used for web flushing involves the use of a “water” bearing, which is a hollow tube with openings in its exterior through which water or some other flushing chemical may be pumped. In a flushing process of this type, the membrane material web is floatably supported by water flowing from the interior of the water bearing tube through the exterior openings. Because there is no contact between the membrane material and the water bearing, this technique is often used for applications in which it is important that the introduction of surface defects in the membrane web be minimized. Flushing of the first solvent from the web is improved through the use of water bearings, since support of the membrane web on a layer of water produces a differential pressure gradient across the membrane web. The pressure differential can be approximated as:
P=T/R
(2)
where:
P=the trans-membrane differential pressure (psi)
T=the tension on the web (pounds/linear inch)
R=the radius of the pipe (inches)
If it is assumed that the portion of the web floatably supported by a water bearing is supported along roughly one-half of the circumference of a cylindrical roller bearing, the amount of time for which any portion of the web is subjected to water flow at this differential pressure can be calculated as:
t
flush
=ΠR/v
web
(3)
where:
t
flush
=the time of contact between supporting water and a portion of the web
v
web
=the speed at which the web is moving (inches/second)
Therefore, the volume of water flushed through the web can be calculated as
Volume flushed (per unit area of membrane)=&Dgr;
V
water
/&Dgr;t*t
flush
*P
(4)
where:
&Dgr;V
water
/&Dgr;t=the flow rate of water through the membrane web (in
3
/in
2
*psi*sec)
As shown by the above equations, for a system involving water bearings, the volume of water flushed through the web will depend on the tension at which the web is being held, since the pressure differential across the web is directly proportional to the web tension. In many flushing applications, subjecting membrane webs to high tension may cause damage to the membrane web or even contact with the surface of the water bearings.
REFERENCES:
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patent: 6103013 (2000-08-01), Stevens, III et al.
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patent: 6261366 (2001-07-01), Dubs et al.
patent: 55-119407 (1980-09-01), None
Hopkins David H.
Simonetti John A.
PTI Advanced Filtration, Inc.
Stinson Frankie L.
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