Liquid purification or separation – Filter – Supported – shaped or superimposed formed mediums
Reexamination Certificate
2001-04-27
2004-04-13
Drodge, Joseph (Department: 1723)
Liquid purification or separation
Filter
Supported, shaped or superimposed formed mediums
C210S650000, C210S500220, C096S011000, C264S029100, C264S029600, C423S447100
Reexamination Certificate
active
06719147
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of filtration membranes, generally, and specifically to the field of mesoporous membranes, particularly ultrafiltration and diafiltration membranes. The invention also relates to the novel process of using supported porous carbon membranes for selective adsorption and separation.
BACKGROUND OF THE INVENTION
Membrane filtration technologies are critical to a variety of industrial process applications including cell harvesting, sterilization of biological solutions, clarification of antibiotics, concentration of protein solutions, and particulate filtration. Ultrafiltration is a particular type of membrane separation process that is used to separate macromolecules such as proteins from solutions containing solvents and low molecular weight solutes under the presence of a pressure gradient. Ultrafiltration membranes typically have a pore size from 1 nm to 100 nm. Diafiltration is similar to ultrafiltration, except that changes are made to the solution during processing; in diafiltration the dilution level is typically manipulated during filtration. Membranes used for either of these purposes have the ability to fractionate macromolecular components based upon their individual molecular masses. The typical membrane molecular weight cut-off range is from about 10
3
to 10
7
g/mol.
Ultrafiltration is typically carried out with the solution to be processed (rententate) on one side of the membrane and the purified stream (permeate) exiting the system on the other side. The rejected-stream side is operated under higher pressure than the permeate side creating a pressure gradient that drives the solution through the porous membrane structure. The desired component or components remain behind, blocked or retained by their inability to permeate the membrane.
During operation, membrane throughput—the rate at which solution passes through the membrane—typically diminishes as the membrane surface becomes fouled with the retained component. Accordingly, the membrane must be periodically cleaned to remove fouling agents, i.e., aggregated proteins, bacterial contamination, etc. This is most commonly performed by exposing the membrane surface to a chemical reagent and back-flushing the system.
Traditionally, ultrafiltration membranes have been primarily polymeric in nature. See Zydney and Zeman (1996),
Microfiltration and Ultrafiltration—Principles and Applications
, Marcel Dekker, New York, N.Y. Asymmetric ultrafiltration membranes are commonly synthesized using phase inversion, where a polymer solution of a base and poreformer in a solvent is induced to form two interdispersed liquid phases. After coagulation, a solid membrane gel is produced. Membranes synthesized in this manner include the bilayer type which contains slit-shaped fissures or cracks, see Michaels, U.S. Pat. No. 3,615,024 (1971), and those membranes that contain plasticizers and are stable while dry, see Tweddle and Thayer, U.S. Pat. No. 4,451,424 (1984). See also Cabasso and Levy, U.S. Pat. No. 4,954,381 (1990).
Despite the widespread use of these types of polymeric ultrafiltration membranes, they have several well-known disadvantages. First, the low mechanical stability of polymeric ultrafiltration membranes constrains their maximum operating pressure. The low mechanical stability of polymeric ultrafiltration membranes limits their operating capacity, i.e., maximum permeate flux, because permeate flow is proportional to operating pressure under most conditions. Additionally, the low mechanical stability of polymeric ultrafiltration membranes leads to deformation during operation that can adversely affect membrane performance. Second, polymeric ultrafiltration membranes are particularly sensitive to the harsh reagents and solvents used to remove fouling components. After repeated cleaning, polymeric ultrafiltration membranes typically show signs of degradation. Third, most polymeric ultrafiltration membranes must contain either a humectant, such as glycerol or water, or must be maintained in a saturated state at all times which requires that they be transported and stored in a solvent. Membranes that are unstable with respect to drying or leaching of a humectant are not robust and special considerations, which can be expensive, must be taken during their processing and handling. See Degen et al., U.S. Pat. No. 5,480,554 (1996). Last, mass-produced polymeric ultrafiltration membranes are known to possess cracks and other defects that span the separating layer and limit the performance of these membranes. Curiously, the porous structure of some polymeric ultrafiltration membranes is derived solely from cracking during processing. See Michaels, U.S. Pat. No. 3,615,024 and Degen et al., U.S. Pat. No. 5,480,554.
The supported porous carbon ultrafiltration membranes of the present invention offer many advantages over existing ultrafiltration membranes. The present invention relates to a supported porous carbon membrane having pores in the ultrafiltration range. The carbon membrane is synthesized both within and on top of the macroporous support. The support provides the membrane with high mechanical strength and resists deformation even at high driving force pressures. Deformation due to organic solvent influx, i.e., polymeric swelling, is avoided because the membrane is not polymeric and is strengthened by the rigidity of the support. Because the membrane can operate at higher pressures compared to polymeric membranes, filtration processes using membranes of the present invention can be operated at higher throughput rates.
The carbon membranes of the present invention naturally resist chemical attack during cleaning. In addition to the chemical-based cleaning methods known in the art, the membranes can also be cleaned using either steam sterilization or high temperature desorption because the membranes are stable at high temperatures. Notably, the membranes are stable at temperatures above the melting point of polymeric ultrafiltration membranes.
Carbon membranes are also stable when exposed to air and moisture. The carbon membranes do not require the addition of plasticizing agents or to be handled under a solvent which is necessary for many polymeric ultrafiltration membranes. See Foley (1995),
Carbogenic Molecular-Sieves—Synthesis, Properties and Applications
, Microporous Materials, 4, 6, pp. 407-433.
The present invention relates to a supported mesoporous carbon membrane where the mesoporous carbon exists both within and external to a structural support, such as porous stainless steel. Currently, the only known examples of supported carbon membranes are used in gas phase separations. These gas-phase membranes have pore sizes in the range of from 0.3 to 1 nm (nanoporous range). The nanoporous carbon membranes are synthesized by the pyrolysis of certain organic and natural polymers. Upon unimolecular reaction at high temperatures, the carbonizing polymers decompose, leaving a nanoporous graphite-like carbon solid. See Foley (1995) at 407-433. The porosity of the polymer precursors is not preserved in the final product. Rather, the porosity results from the carbon membrane's metastable, graphite-like structure having atomic-size pores. See Acharya and Strano (1999),
Simulation of Nanoporous Carbons: A Chemically Constrained Structure
, Phil. Mag. B, 79, 10, pp. 1499-1518. Acharya and coworkers have used stainless steel supports to prepare nanoporous gas separation membranes from poly(furfuryl alcohol) resin. See Acharya and Raich (1997),
Metal-Supported Carbogenic Molecular Sieve Membranes: Synthesis and Applications
, Industrial & Engineering Chemistry Research, 36, 8, pp. 2924-2930; Acharya and Foley (1999),
Spray-Coating of Nanoporous Carbon Membranes for Air Separation
, Journal of Membrane Science, 161, pp. 1-5. These particular membranes have a remarkable ability to affect small molecule separations such as oxygen and nitrogen extraction from air. See Shiflett and Foley (1999),
Ultrasonic Deposition of High Selectivity Nanoporous Carbon Membranes
, Scie
Agarwal Hans
Foley Henry C.
Strano Michael
Connolly Bove & Lodge & Hutz LLP
Drodge Joseph
Menon Krishnan S
The University of Delaware
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