U.S. patent application number 10/168806 was filed with the patent office on 2005-06-16 for cross-flow filtration unit.
Invention is credited to Grummert, Ulrich, Pahl, Ina, Schmidt, Hans-Weddo.
Application Number | 20050126980 10/168806 |
Document ID | / |
Family ID | 7626786 |
Filed Date | 2005-06-16 |
United States Patent
Application |
20050126980 |
Kind Code |
A1 |
Pahl, Ina ; et al. |
June 16, 2005 |
Cross-flow filtration unit
Abstract
An improved cross-flow filtration unit utilizing at least one
two-ply membrane is disclosed that has a particular pore size
relationship in the two membrane layers.
Inventors: |
Pahl, Ina; (Hannover,
DE) ; Schmidt, Hans-Weddo; (Hardegsen, DE) ;
Grummert, Ulrich; (Bad Sooden Allendorf, DE) |
Correspondence
Address: |
Chernoff Vilhauer
McClung & Stenzel
1600 Ods Tower
601 S W Second Avenue
Portland
OR
97204-3157
US
|
Family ID: |
7626786 |
Appl. No.: |
10/168806 |
Filed: |
June 21, 2002 |
PCT Filed: |
December 21, 2000 |
PCT NO: |
PCT/EP00/13073 |
Current U.S.
Class: |
210/321.72 ;
210/321.78; 210/321.87; 210/490 |
Current CPC
Class: |
B01D 2313/44 20130101;
B01D 69/12 20130101; B01D 63/082 20130101; Y02A 20/131 20180101;
B01D 2315/10 20130101; B01D 63/084 20130101 |
Class at
Publication: |
210/321.72 ;
210/321.78; 210/321.87; 210/490 |
International
Class: |
B01D 063/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 5, 2000 |
DE |
100001963 |
Claims
1. A cross-flow filtration device comprising a feed inlet and feed
flow channel; a permeate flow channel and a permeate outlet; a
retentate outlet; and at least one two-ply microporous separation
membrane having a front side ply facing said feed channel and a
back side ply facing said permeate channel wherein the flow of
liquid feed through said separation membrane is from the front side
ply to the back side ply, and wherein the pores of said separation
membrane are sized such that the average pore size of said front
side ply is 1.3 to 5 times the average pore size of said back side
ply.
2. The filtration device of claim 1 containing a plurality of said
separation membranes.
3. The filtration device of claim 1 with a single two-ply
separation membrane.
4. The filtration device of any of claims 1-3 wherein the average
pore size of said back side ply is such as to impart to said back
side ply a molecular weight cutoff of at least 1000 Daltons.
5. The filtration device of claim 4 wherein the maximum average
pore size of said back side ply is 1.2 microns.
6. The filtration device of any of claims 1-3 wherein the average
pore size of said back side ply is from 0.2 to 1.2 microns.
7. The filtration device of claim 6 in the form of a cassette.
Description
[0001] This is a Section 371 application of PCT/EP 00/13073 that
claims priority of DE 100 00 196.3 filed Jan. 5, 2000.
BACKGROUND OF THE INVENTION
[0002] In cross-flow filtration, a liquid feed flows tangentially
over the surface of a filter material and is thereby split into a
concentrate (retentate) stream and a filtrate (permeate) stream.
Generally, microporous membranes are used which fall into the
ultrafiltration and microfiltration classifications.
Ultrafiltration membranes have average pores sizes that are capable
of retaining macromolecules having a molecular weight between 500
and 1,000,000 Daltons, known in the filtration art as having a
molecular weight cutoff (MWCO) of 500 to 1,000,000 Daltons.
Microfiltration membranes exhibit average pore sizes of between
0.01 and 10 microns. See generally Chapter 4.3.3 in Gasper,
Handbook of Industrial Solids/Fluids Filtration (1990).
[0003] Retentate flowing over the surface of the separation
membrane is typically recycled to flow over the membrane's surface
repeatedly. The permeate which penetrates the membrane generally
perpendicular to its surface is removed from the back side of the
membrane. The target substances can be in either the permeate
and/or the retentate. Cross-flow filtration units are often used in
the form of filter cassettes, as described, for example in U.S.
Pat. No. 4,715,955 and in DE PS 34 41 249. Cassettes are comprised
of a multiplicity of adjacent filter arrays, each array generally
consisting of flat custom-cut sections of retentate spacers which
form feed flow channels, a first single membrane layer, a spacer
for the formation of a filtrate collection opening, and a second
single membrane layer. Each feed flow channel is in fluid
communication with a liquid feed inlet and with a retentate outlet,
and each permeate channel is in fluid communication with a permeate
outlet. UK Patent Application 2,236,693 discloses a similar
cross-flow filter, but with either self-supporting porous filter
plates or porous polymeric membranes supported by and bonded to a
porous ceramic layer having larger pores.
[0004] As the feed flows over the membrane surface, the retentate
substance, because of its size, is blocked from passage through the
pores of the membrane and is rinsed away from the membrane surface,
so that it will not plug the membrane pores, thus preventing its
permeation through the membrane. In spite of this, for various
reasons, build-up of non-filtered residue is formed on the surface
of the feed side of the membrane, which generally impairs the
filtering capacity, the yield of targeted substances and the
service life of the cross-flow filtration unit.
[0005] Thus a primary object of the invention is to provide an
improved cross-flow filtration unit, which is characterized by an
improved filtration capacity, a longer service life and a high
product yield.
[0006] The foregoing and other objectives, features, and advantages
of the invention will be more readily understood upon consideration
of the following detailed description of the invention, taken in
conjunction with the accompanying drawing.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention provides an improved cross-flow
filtration unit that separates
[0008] the contents in a feed liquid;
[0009] the permeate content in a permeate; and
[0010] the retentate content in a retentate.
[0011] The essence of the invention comprises sizing the pores in
the two layers of a two-ply microporous membrane such that the
pores of the layer facing the feed channel of the cross-flow
filtration unit are on average 1.3 to 5 times the pores of the
layer facing the permeate channel.
[0012] With the inventive cross-flow filtration units, fluids can
be filtered which include liquids, emulsions, suspensions, potable
fluids such as beer, beer flavorants, wine, juice, water, milk and
whey; laboratory grade water; wastewater; fluids in the fields of
pharmaceuticals, medicine, cosmetics, chemistry, biotechnology,
gene technology, environmental protection and in laboratory work.
The inventive cross-flow filtration units can be employed for
recovery of valuable material, for separation of substances such as
macromolecules and biomolecules, for depyrogenation and
sterilization of solutions, for the separation of corrosive
substances from fluids, for the filtration and concentration of
biological solutions, for the separation of microorganisms such as
bacteria, yeasts, virus and cell components and for the
desalination of protein solutions and other biological media.
BRIEF DESCRIPTION OF THE DRAWING
[0013] FIG. 1 is a schematic cross-sectional view of the operative
parts of an exemplary cross-flow filtration unit of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0014] The present invention comprises an improvement in cross-flow
filtration unit that enhances its efficiency and extends its
service life and provides greater yield of target substances.
[0015] Referring to FIG. 1, there is depicted in schematic form the
operative parts of a cross-flow filtration unit of the invention,
consisting of a series of liquid feed inlets 10, feed flow channels
12, permeate flow channels 20, permeate outlets 22, retentate
outlets 30 and two-ply microporous polymeric membranes 40, each
membrane consisting of a front side layer or ply 41 facing feed
flow channel 12, and a back side layer or ply 42 facing permeate
flow channel 20, with the two plys 41 and 42 being joined together
by small spacers 43 in their peripheries. The two plys 41 and 42
physically lie one on top of the other, but are not bonded together
in the area 44 between them. The feed and permeate collection
channels are advantageously held open by spacers (not shown in FIG.
1) which are conventional.
[0016] The flow of fluid through the filtration unit is as depicted
by the arrows in FIG. 1, with the liquid feed initially entering
the unit through feed inlet 10, flowing through feed flow channel
12, where the stream is split into a retentate-containing fluid
flowing out through retentate outlet 30 and a permeate-containing
fluid that permeates membrane 40 substantially perpendicularly from
its front side 41 to its back side 42 into permeate flow channel 20
and permeate flow outlet 22.
[0017] In the case of single ply or supported microporous membranes
suitable for using in microfiltration and ultrafiltration wherein
the pore sizes of the filtration membrane are in the range of a
MWCO of 1000 Daltons to 1.2 microns there is often build-up on the
feed side of the membrane. Rather surprisingly, it has been
discovered that demonstrably greater permeate flow and greater
yields are achieved when the filter material consists of two
microporous polymeric membrane layers situated one on top of the
other but slightly spaced apart and wherein the membrane layer 41
facing the feed flow channel 12 has an average pore size greater by
a factor of from about 1.3 to about 5 than the average pore size of
the membrane layer 42 facing the permeate flow channel 20. The
slight spacing apart in the area 44 of the two polymeric membrane
plys 41 and 42, which are inherently flexible, permits the two
membrane plys to flex during filtration, which tends to retard
blinding of the membrane layers and consequently accelerate
filtration.
[0018] If the pore sizes of the membrane layers differ by a factor
of less than 1.3, then, from a commercial operation standpoint,
either insignificant or no effects on filtration efficiency are
observed. On the other hand if the pore sizes in the membrane
layers differ by a factor of greater than 5, then blinding of the
membrane layer with the greater average pore size occurs very
quickly, especially in the case of particulate-laden feed liquids,
causing a halt to filtration.
[0019] In a preferred embodiment, the cross-flow filtration unit is
constructed as a filter cassette, the membrane layers of which
consist of microfiltration membranes. Particularly preferred is a
filter cassette, in which the membrane layers facing the permeate
collection channels have an average pore diameter in the range of
0.1 to 1.2 .mu.m.
[0020] In an additional advantageous embodiment the filtration unit
has a single two-ply membrane, a single feed inlet and channel, a
single retentate outlet and a single permeate channel and outlet
wherein the two layers of the microporous membrane are oriented as
noted above, and the average pore sizes of the two layers comply
with the above-noted restriction.
EXAMPLE
[0021] The invention is better understood with the aid of the
following filtration example whereby a cross-flow filtration unit
having the pores of the two-ply membranes sized as noted above is
used to filter a yeast cell suspension (pichia) with a cell
concentration of 6.times.10.sup.6 yeast cells (retentate content)
per mL, which contains a targeted protein having a molecular weight
of 70,000 Daltons (permeate content) in a concentration of 127 mL.
The filtration takes place in a cross-flow mode with recycle of the
retentate. During the filtration, the filtration unit is operated
at a constant transmembrane pressure of 0.64 bar (the transmembrane
pressure is equal to [input pressure+outlet pressure]/2 minus
permeate pressure). The pressure was at 0.9 bar at the feed inlet,
0.4 bar at the retentate outlet and 0.1 bar at the permeate
outlet.
[0022] In order to be able to compare the results and relate these
back to the mode of construction, the cell suspension feed was held
constant for both the Example and the Comparative Example.
[0023] A cross-flow filtration unit fabricated in the form of a
filter cassette was employed with two-ply membranes exposed to the
liquid feed and having a total membrane surface area of 0.4
m.sup.2, and which had 13 feed flow channels and 12 permeate flow
channels. The membrane layers facing the permeate flow channels
were of cellulose acetate with an average pore size of 0.2 .mu.m
and the membrane layers facing the feed flow channels were also of
cellulose acetate, but having an average pore size of 0.45 .mu.m,
larger by a factor of 2.25. A volume of 7.3 L of the cell
suspension was concentrated to a final volume of 1 L. The average
permeate flow rate was 0.875 L/min.multidot.m.sup.2. Filtration was
conducted for 18 minutes. The concentration of the target protein
in the permeate and the retentate was 107 and 238 mg/L,
respectively. The yield of targeted protein was 72%.
Comparative Example 1
[0024] The Example of the invention was repeated with the following
exceptions: both plys of the two-ply membrane had an average pore
size of 0.2 .mu.m, so that the ratio of pore sizes was less than
1.3, i.e., 1; the average permeate flow rate was 0.29
L/min.multidot.m.sup.2; filtration was conducted for 54 minutes;
the concentration of the target protein in the permeate and in the
retentate was 86 and 387 g/L, respectively; and the yield was 58%,
or 14% less than with the inventive filtration device.
Comparative Example 2
[0025] The Example of the invention was repeated with the following
exceptions: the average pore size of the ply facing the feed flow
channel was 1.2 .mu.m, while that of the ply facing the permeate
channel was 0.2 .mu.m, so that the ratio of pore sizes was greater
than 5, i.e., 6; the average permeate flow rate was 0.49
L/min.multidot.m.sup.2; filtration was conducted for 37 minutes;
the concentration of the target protein in the permeate and in the
retentate was 89 and 365 g/L, respectively; and the yield was 61%,
or 11% less than with the inventive filtration device.
Comparative Example 3
[0026] A cross-flow filtration cassette of conventional design
(Sartocon.RTM. from Sartorius AG of Goettingen, Germany) was
employed having a membrane surface exposed to the same feed of 0.7
m.sup.2, which had 17 feed flow channels and 16 permeate flow
channels. The membrane layers facing the permeate flow channels
consisted of a single-ply cellulose acetate membrane with an
average pore size of 0.2 .mu.m. A volume of 12.7 L of the cell
suspension were concentrated to a final volume of 1 L. The average
permeate flow was 0.328 L/min.multidot.m.sup.2. The filtration was
terminated after 51 minutes. The concentration of targeted protein
in the permeate and retentate was found to be 81.3 and 621 mg/L,
respectively. The yield of targeted protein in the filtrate was
thus 59%, or 13% less than with the inventive filtration device.
The permeate flow was increased by a factor of 2.7.
[0027] A further advantage apparent from use of the inventive
cross-flow filtration unit was that a single filtration
simultaneously encompassed both pre-filtration and final
filtration; by way of contrast, in conventional practice two
filtration steps must be carried out to achieve pre- and final
filtration. Thus, use of the cross-flow filtration unit of the
invention clearly results in a reduction of the costs of
filtration.
[0028] The terms and expressions which have been employed in the
foregoing specification are used therein as terms of description
and not of limitation, and there is no intention in the use of such
terms and expressions of excluding equivalents of the features
shown and described or portions thereof, it being recognized that
the scope of the invention is defined and limited only by the
claims which follow.
* * * * *