Method for reducing performance variability of multi-layer filters
Multi-layer membrane devices with reduced performance variability, and method of reducing performance variability of such devices. The variability is reduced by combining two or more membranes with similar pore sizes, and carefully selecting the upstream membrane based upon its performance rating, in order to control the performance of the overall device. Selective layering reduces the capacity range of the device when compared to random layering, with the mean capacity greater than overall population mean. The flux range also can be reduced, with the mean near the overall population mean. The LRV range also can be reduced, with the mean LRV near or higher than the overall population mean.
1 . A filtration device comprising a housing having an inlet and an outlet spaced from said inlet, and a plurality of membranes including at least an upstream membrane and at least a downstream membrane, wherein each said membrane has substantially the same rating, and wherein said upstream membrane has a first predetermined performance characteristic selected such that said filtration device has a second predetermined performance characteristic regardless of the performance characteristics of said downstream membrane.
2 . The filtration device of claim 1 , wherein said performance characteristic is capacity.
3 . The filtration device of claim 1 , wherein said performance characteristic is permeability.
4 . The filtration device of claim 1 , wherein said performance characteristic is retention.
5 . A multi-layered membrane filter device made from at least two membrane layers, said layers having at least one higher and one lower capacity rating, said device having a capacity greater than the average of the capacity ratings of the membranes used to make the device, and less than or equal to the higher capacity membrane.
6 . A population of multi-layered membrane filter devices made from at least two membrane layers, said layers having at least one higher and one lower capacity rating, said devices having an population average capacity greater than the average of the capacity ratings of the membranes used to make the device, and less than or equal to the highest capacity membrane.
7 . A filtration device comprising a housing having an inlet and an outlet spaced from said inlet, and a plurality of membranes including at least an upstream membrane and at least a downstream membrane, wherein each said membrane is selected from a population of membranes having a capacity such that the capacities of said population define a distribution, and wherein said upstream membrane has a capacity at about the median of said distribution, and said at least one downstream membrane has a capacity at either the high end or low end of said distribution.
8 . The filtration device of claim 7 , wherein said capacity distribution of said population of membranes is a Gaussian distribution.
9 . The device of claim 7 , wherein there are at least three membranes.
10 . The device of claim 9 , wherein the membrane between said upstream and downstream membranes has a capacity at either the high end or the low end of said distribution.
11 . The device of claim 7 , wherein there are at least four membranes.
12 . A filtration device comprising a housing having an inlet and an outlet spaced from said inlet, and a plurality of membranes including at least an upstream membrane and at least a downstream membrane, wherein each said membrane is selected from a population of membranes having a capacity such that the capacities of said population define a distribution, and wherein said upstream membrane has a capacity at the high end of said distribution, and said at least one downstream membrane has a capacity at the low end of said distribution.
13 . The filtration device of claim 12 , wherein said capacity distribution of said population of membranes is a Gaussian distribution.
14 . The device of claim 12 , wherein there are at least three membranes.
15 . The device of claim 12 , wherein there are at least four membranes.
16 . The filtration device of claim 12 , wherein said high end of said distribution is at least as high as the median of said distribution.
17 . The filtration device of claim 12 , wherein said low end of said distribution is at least as low as the median of said distribution.
18 . A filtration device comprising a housing having an inlet and an outlet spaced from said inlet, and a plurality of membranes including at least an upstream membrane and at least a downstream membrane, wherein each said membrane is selected from a population of membranes having a capacity such that the capacities of said population define a distribution, and wherein said upstream membrane has a capacity higher than the median capacity of said distribution by a certain amount, and said at least one downstream membrane has a capacity lower than the median capacity of said distribution by about said same certain amount.